Synthesis and Characterization of Oxide Semiconductor … · especially grateful to Dr. Umair...

i

Synthesis and Characterization of Oxide

Semiconductor Nanostructures for Ultra Violet

and Chemical Sensors

By

Muhammad Amin

CIIT/FA10-PPH-005/ISB

PhD Thesis

In

Physics

COMSATS Institute of Information Technology

Islamabad-Pakistan

Fall, 2014

ii

COMSATS Institute of Information Technology


Semiconductor Nanostructures for Ultra Violet and

Chemical Sensors

A Thesis Presented to

COMSATS Institute of Information Technology, Islamabad

In partial fulfillment

of the requirement for the degree of

PhD (Physics)

By

Muhammad Amin


Fall, 2014

iii



Chemical Sensors

A post Graduate thesis submitted to the Department of Physics as partial

fulfillment for the award of Degree of PhD Physics

Name Registration Number

Muhammad Amin CIIT/FA10-PPH-005/ISB

Supervisor

Dr. Nazar Abbas Shah

Associate Professor

Department of Physics

COMSATS Institute of

Information Technology

Islamabad

Co-Supervisor

Dr. Arshad Saleem Bhatti

Professor


COMSATS Institute of

Information Technology

Islamabad

iv

Final Approval

This thesis titled



Chemical Sensors

By

Muhammad Amin


Has been approved

For the COMSATS Institute of Information Technology, Islamabad

External Examiner: __________________________________________

Dr. Syed Khurshid Hasnain

National Centre of Physics

Quaid-i-Azam University Campus, Islamabad

External Examiner: __________________________________________

Dr. Syed Wilayat Husain

Institute of Space Technology, Islamabad

Supervisor: ________________________________________________


Department of Physics/ Islamabad

HoD: _____________________________________________________

Dr. Sadia Manzoor

HoD (Department of Physics/ Islamabad)

Dean, Faculty of Science______________________________________

Prof. Dr. Arshad Saleem Bhatti

v

Declaration

I Muhammad Amin, CIIT/FA10-PPH-005/ISB, hereby declare that I have

produced the work presented in this thesis, during the scheduled period of study. I

also declare that I have not taken any material from any source except referred to

wherever due that amount of plagiarism is within acceptable range. If a violation of

HEC rules on research has occurred in this thesis, I shall be liable to punishable action

under the plagiarism rules of the HEC.

Date:

Signature of the student:

Muhammad Amin


vi

Certificate

It is certified that Mr. Muhammad Amin, Registration number CIIT/FA10-PPH-

005/ISB has carried out all the work related to this thesis under my supervision at the

Department of Physics, COMSATS Institute of Information Technology, Islamabad

and the work fulfills the requirement for award of PhD degree.

Date:

Supervisor:


Associate Professor


COMSATS, Institute

of Information Technology

Islamabad

Submitted through:

Head of Department:

Dr. Sadia Manzoor

Head, Department of Physics

COMSATS institute of Information

Technology, Islamabad

vii

DEDICATED

to

ALLAH Almighty

The Holy Prophet Muhammad (S.A.W)

My Parents and Family

viii

ACKNOWLEDGEMENTS

All gratitude and praises are to Allah Almighty, the most Gracious, the most Merciful

and the most Compassionate, who is the creator of all knowledge and wisdom. Who

gave me health, thoughts, and capacitates me to achieve this goal. After Almighty

Allah, praises are to His last and the most beloved Prophet Muhammad (S.A.W),

the most perfect and exalted and an everlasting a source of guidance and knowledge

for humanity as a whole.

First of all, I would like to thank my supervisor, Dr. Nazar Abbas Shah, for his

guidance, support and encouragement throughout my PhD research work at the

COMSATS Institute of Information Technology (CIIT). His passion and devotion to

work is a source of inspiration for me. The completion of this thesis has been

possible, only due to his intellectual support.

I would like to express my deepest gratitude to my co-supervisor Prof. Dr. Arshad

Saleem Bhatti (Dean of faculty sciences at COMSATS Islamabad) for his constant

guidance, continuous scientific and technical support. He was always ready to answer

any question and was quite helpful in guiding me during my research work. I

benefited tremendously from his depth of knowledge in many ways. I am also highly

obliged to honorable Prof. Dr. S. M. Junaid Zaidi, Rector, COMSATS Institute of

Information Technology, Pakistan for providing the best possible facilities and ideal

atmosphere for the studies. I would like to thank HoD, all the teachers and staff

members (especially Mr. Tanveer Baig) of the Department of Physics, COMSATS

Institute of Information Technology Islamabad, for their support.

I would also like to express my acknowledgement to the Higher Education

Commission (HEC) of Pakistan for providing MS leading to PhD fellowship under

the Indigenous PhD Scholarship program with pin # 074-2691-Ps4-619 and for six

month research visit to The University of Manchester, UK under International

Research Support Initiative Program (IRSIP). I also acknowledge the support and

facilities provided by Institute of Industrial Control Systems of Pakistan, KRL and my

department (DESTO).

I would like to express my deep gratitude to Prof. Paul O’ Brien and Dr

Mohammad Azad Malik, for providing me an opportunity to work in their group in

ix

the schools of materials and Chemistry, The University of Manchester, UK and for

providing access to use different characterization tools. Special thanks to Dr. Andrew

Thomas from school of Physics and Astronomy, Photon Science Institute,

The University of Manchester, UK for helpful discussions and providing me access to

Raman and XPS.

I am also thankful to Mr. Manzar Abbas advisor at COMSATS for technical

discussions on sensors during my research work. I would like to thank my lab fellows

from Thin Film Research Laboratory who made these last few years a pleasure. I am

especially grateful to Dr. Umair Manzoor, Dr. Awais Ali, Shahid Mehmood, Dr.

Muhammad Hafeez, Fahad Bhopal and Abdul Rehman Malik form CMND, Dr.

Mohsin Rafique and Dr. Amin-ur-Rashid from MMRL, CIIT Islamabad for their co-

operation and support. I would like to thank my dear friend Dr.Toqeer-ud-Din for

helping me through the difficult times, and for all the emotional support, and

encouragement to start PhD.

No gratitude is sufficient to repay the endless love of my parents and family, I wish

to thank my entire family for providing a loving support for me. Words always fail me

to acknowledge the indivisible love, continuous support and encouragement provided by

my dearest Father Muhammad Saeed, beloved Mother (Late Jhulam Jannat). They

raised me, supported me, taught me, and loved me. I give my parents a lot of credit

for my success. My brothers, my sisters, my brother-in-laws and all were particularly

supportive. They have always been a source of courage for me and I deeply appreciate

their care from the core of my heart. Without their support, encouragement and

prayers, it was impossible to achieve what I did till date. My special thanks to my

elder brother Muhammad Idrees who always stands by me whenever I need.

Last but not least, I owe my loving thanks to my wife Nagina and my children

(Ubaid, Fasi, Rafi, Aamna and Haleema) for their love and support. Without their

encouragement and understanding it would have been impossible for me to complete

this work.

Muhammad Amin


x

ABSTRACT

Synthesis and Characterization of Oxide Semiconductor

Nanostructures for Ultra Violet and Chemical Sensors

Zinc oxide (ZnO) is a well suited n-type semiconductor material due to oxygen

vacancies and zinc interstitials, used for a number of applications owing to its

characteristics such as direct and wide bandgap of 3.37 eV, large exciton binding

energy of 60 meV at room temperature. It is mostly used for its simple fabrication and

good biocompatibility. The nanostructures of this material are therefore very

promising due to their high sensitivity to toxic and combustible gases. They show

higher carrier mobility, good chemical and thermal stability at moderately high

temperatures. On the other hand, tin oxide (SnO2) is the most frequently used material

for UV and gas sensing applications and dominant over all metal oxides. The

objective of the current study is to highlight the recent developments and techniques

for chemiresistive gas sensing and ultra violet (UV) sensing properties based on two

1-D oxide semiconductor nanostructures, ZnO and SnO2. Fabricated devices have

been successfully demonstrated as sensors for the detection of UV light and chemical

species.

In this work, ZnO/SnO2 nanostructures have been successfully synthesized by a very

simple vapor transport method using Vapor Liquid Solid (VLS) mechanism on Au

coated Si (100) substrates by using ZnO/SnO2 and carbon mixture as source material.

Morphology tuning is achieved by changing the temperature and the catalyst

thickness. Single crystalline nanowires, nanorods, nanobelts and unique peddle like

nanostructures are formed. The magnesium (Mg) doped ZnO nanobelts and carbon

(C) doped ZnO pedal-like nanostructures showed five times enhanced sensing

properties (compared to undoped) towards 20 ppm of carbon monoxide (CO) gas at

350 °C with good stability, indicating that Mg and C doping is very much effective in

improving the CO sensing of ZnO nanostructures. In addition, Band bending model

has been used to describe CO gas sensing mechanism for both, undoped and doped

ZnO nanostructures. Chemical sensors are successfully fabricated to detect the CO,

CH4 and methanol at different temperatures.

xi

TABLE OF CONTENTS

1 Introduction .................................................................................................. 1

1.1 Nanotechnology and low-dimensional nanostructures ........................ 2

1.2 Wide bandgap oxide semiconductors for sensors ................................ 4

1.3 Synthesis of oxide semiconductors nanostructures .............................. 5

1.4 Applications of oxide semiconductor nanostructures .......................... 7

1.5 Gas sensing properties .......................................................................... 7

1.6 Ultraviolet (UV) sensing properties ..................................................... 8

1.7 Objectives of the thesis ........................................................................ 8

1.8 Outline of the thesis ........................................................................... 10

1.9 Thesis research flow chart .................................................................. 11

1.10 Summary ............................................................................................ 12

2 Literature survey: Fabrication of oxide semiconductor nanostructures

and device processing ................................................................................ 13

2.1 Properties of oxide semiconductors ................................................... 14

2.1.1 Material properties of zinc oxide (ZnO) ................................. 14

2.1.2 Material properties of Tin oxide (SnO2) ................................. 15

2.2 Nanostructures synthesis approaches ................................................. 16

2.2.1 Top-down approach ................................................................ 16

2.2.2 Bottom-up approach ................................................................ 17

2.3 Thermodynamics of VLS growth....................................................... 19

2.4 Optical properties of oxide semiconductors ....................................... 22

2.4.1 Photoluminescence properties of ZnO .................................... 23

2.5 Doping of oxide semiconductors nanostructures ............................... 24

2.6 Gas sensors based on metal oxide semiconductor nanostructures ..... 25

2.6.1 Sensing mechanism of oxide semiconductor gas sensors ....... 25

2.6.2 Gas sensor properties .............................................................. 30

2.7 Ultraviolet (UV) sensors based on metal oxide semiconductor

nanostructures .................................................................................... 32

xii

2.7.1 Photodetection mechanism of oxide semiconductor

nanostructures UV sensors ................................................................. 33

2.8 Summary ............................................................................................ 35

3 Experimental and characterization techniques ...................................... 36

3.1 Catalyst deposition and synthesis experiment ................................... 37

3.2 Detail of samples ................................................................................ 39

3.3 Characterization techniques ............................................................... 39

3.3.1 Scanning electron microscopy (SEM) .................................... 39

3.3.2 Transmission electron microscope (TEM) .............................. 40

3.3.3 X-ray diffraction (XRD) ......................................................... 40

3.3.4 X-Ray photoelectron spectroscopy (XPS) measurement ........ 41

3.3.5 Photoluminescence spectroscopy ............................................ 42

3.3.6 UV-visible diffused reflectance spectroscopy (DRS) ............. 44

3.3.7 Raman spectroscopy ............................................................... 46

3.4 Sensor fabrication/measurement system ............................................ 47

3.5 Summary ............................................................................................ 48

4 Development of highly sensitive UV sensor using morphology tuned

ZnO nanostructures ................................................................................... 49

4.1 Introduction ........................................................................................ 50

4.2 Synthesis of ZnO nanostructures by vapor transport method and

fabrication of sensors ......................................................................... 50

4.3 Results and discussion ........................................................................ 52

4.3.1 Morphology and composition of ZnO nanostructures ............ 52

4.3.2 Structural (XRD) analysis ....................................................... 54

4.4 Optical properties ............................................................................... 55

4.4.1 Photoluminescence (PL) ......................................................... 55

4.4.2 Diffused reflectance spectroscopy .......................................... 57

4.4.3 UV sensing properties ............................................................. 57

4.5 Summary ............................................................................................ 60

5 Effects of Mg-doping on optical and CO gas sensing properties of

sensitive ZnO nanostructures ................................................................... 62

xiii

5.1 Introduction ........................................................................................ 63

5.2 Experimental details ........................................................................... 64

5.2.1 Synthesis of Mg-doped ZnO nanobelts ................................... 64

5.2.2 Gas sensor fabrication and characterization ........................... 65


5.3.1 Morphology and microstructure analysis ................................ 66

5.3.2 XRD and Raman spectroscopic analysis of doped ZnO

nanostructures .................................................................................... 68

5.3.3 Photoluminescence spectroscopy ............................................ 69

5.3.4 Diffused reflectance spectroscopy .......................................... 70

5.3.5 UV sensing .............................................................................. 71

5.3.6 Characterization of CO gas sensors ........................................ 72

5.3.7 Sensing mechanism -Band theory model ................................ 73

5.4 Summary ............................................................................................ 75

6 ZnO on MWCNTs, platform for CO gas sensing ................................... 77

6.1 Introduction ........................................................................................ 78

6.2 Experiment ......................................................................................... 79

6.2.1 Growth of undoped and doped ZnO nanostructures ............... 79

6.2.2 Characterizations of carbon doped ZnO nanostructures ......... 80

6.2.3 Gas sensors fabrication and characterizations ........................ 80


6.3.1 Morphological analysis ........................................................... 81

6.3.2 Structural analysis ................................................................... 84

6.3.3 X-Ray photoelectron spectroscopy (XPS) .............................. 85

6.3.4 Optical properties .................................................................... 86

6.3.5 Carbon doped ZnO nanostructures application as CO gas

sensor 89

6.4 Summary ............................................................................................ 92

7 Dependence of size and surface defects on CO gas sensing response of

SnO2 nanowires .......................................................................................... 94

7.1 Introduction ........................................................................................ 95

xiv

7.2 Experimental ...................................................................................... 95

7.2.1 Synthesis of SnO2 nanowires .................................................. 95

7.2.2 Gas sensors fabrication and characterization .......................... 96


7.3.1 SEM and EDX analysis ........................................................... 97

7.3.2 XRD analysis .......................................................................... 98

7.3.3 Optical properties .................................................................... 99

7.3.4 Gas sensing properties .......................................................... 101

7.4 Summary .......................................................................................... 104

8 Conclusions and Future Work ................................................................ 105

8.1 Summary and Conclusions ............................................................... 106

8.2 Future work ...................................................................................... 108

9 References ................................................................................................. 109

10 List of publications ..................................................................................... 130

11 Conferences, workshop and seminars ...................................................... 131

xv

LIST OF FIGURES

Figure 1.1: Schematic representation of: (a) 0-dimension (0-D), (b) 1-dimension (1-

D), (c) 2-dimension (2-D) and (d) 3-dimension (3-D). The corresponding density of

states (DOS) plots for each type is also presented3........................................................ 3

Figure 1.2: Relative comparison of different oxides used for gas sensing applications5

........................................................................................................................................ 5

Figure 1.3: A collection of different ZnO/SnO2 nanostructures grown by author ........ 6

Figure 1.4: Research flow chart of this thesis ............................................................. 11

Figure 2.1: (a) Crystal structure of hexagonal wurtzite ZnO, ZnO unit cell, including

the tetrahedral-coordination between Zn and its neighboring O. (b) ZnO has a

noncentro-symmetric crystal structure that is made up of alternate layers of positive

and negative ions, leading to spontaneous polarization34

(c) Schematic drawing of

different morphologies4 ............................................................................................... 14

Figure 2.2: (a) Rutile tetragonal unit cell (b) (110) surface structure (c) (101) surface

structure39

..................................................................................................................... 15

Figure 2.3: (a) Schematic illustration of VLS mechanism for ZnO, inset TEM image

shows Au catalyst droplet at the tip of nanowire (b) SEM image of ZnO Nanowires

grown using Au catalyst56

(c) SnO2 Nanowires grown using Au catalyst ................... 18

Figure 2.4: Schematic of VS mechanism illustration of VLS mechanism (a)

impinging of vapors, (b) formation of Seed, (c) formation of nanowires.(d) self-

catalytic growth of ZnO nanowires by Zn droplet (e) ZnO crystal contains no catalyst

and defects59

................................................................................................................. 19

Figure 2.5: Schematic of alloy droplet with first ZnO layer on Si substrate ............. 20

Figure 2.6: Schematic of the process occurring during photoluminescence in a direct

gap semiconductor after excitation at frequency νL (a) the electrons relax at the

bottom of C.B. and holes relax to the top of V.B. by phonon emission before

xvi

recombining by emitting photon (b) Density of states and band occupancies for the

electrons and holes after optical excitation (c) PL spectra of ZnO nanoflowers70

...... 23

Figure 2.7: Schematic showing band bending in n-type semiconductor (a) with O-

adsorbed on surface sites in air (b) After CO gas reaction at the surface42;102

............ 27

Figure 2.8: Schematic of sensing mechanism (a) adsorption of oxygen at surface of

nanowire in air, creation of potential barrier and depletion region (b) modulation of

potential barrier and depletion region after reaction of carbon monoxide (CO) at

surface of n-type semiconductor106

.............................................................................. 28

Figure 2.9: (a) Sensor graph showing response to a reducing gas for an n-type metal-

oxide27

(b) Schematic showing response and recovery times from a plot of sensor

resistance versus time98

................................................................................................ 31

Figure 2.10: (a) UV sensor device (b) A schematic of photo response mechanism of

ZnO nanowires at dark condition (c) during UV illumination (d) Schematic energy

band diagram of photo response process during UV illumination. ............................. 34

Figure 3.1: Furnace setting and placement of the substrates ...................................... 38

Figure 3.2: Furnace temperature mapping when heated at temperature of 1050°C by a

K-type thermocouple ................................................................................................... 38

Figure 3.3: Schematic of XPS working principle ....................................................... 42

Figure 3.4: (a) Schematic illustration of the PL setup (b) Schematic of PL working

principle for direct bandgap material ........................................................................... 43

Figure 3.5: PL spectrum of ZnO nanowires from the sample grown at 800°C,

measured at room temperature with excitation wavelength 325 nm. .......................... 44

Figure 3.6: Schematic illustration of diffused reflectance of light from surface of a

sample .......................................................................................................................... 45

Figure 3.7: (a) Diffused reflectance of ZnO nanobelts (b) Kubelka-Munk transformed

reflectance spectrum: plots of [F(R)] 2

vs. photon energy of ZnO nanobelts ............. 45

xvii

Figure 3.8: Raman effect ............................................................................................. 46

Figure 3.9: Microscopic image of interdigitated Au electrodes (b) Sensor fabrication

...................................................................................................................................... 47

Figure 3.10: The schematic design and UV sensing setup ......................................... 48

Figure 3.11: Schematic diagram of gas sensing setup ................................................ 48

Figure 4.1: (a) Schematic diagram of synthesis setup (b) & (c) Schematic diagram

and actual photograph of UV sensing setup ................................................................ 51

Figure 4.2: SEM and EDX images of different morphologies of ZnO nanostructures

at different synthesis temperatures. (a) SEM nanowires grown at 800°C (b) EDX

analysis of nanowires (c) SEM nanorods grown at 900°C (d) EDX analysis of

nanorods (e) SEM nanobelts grown at 1000°C (f) EDX analysis of nanobelts ........... 52

Figure 4.3: Typical TEM and HRTEM images of ZnO nanorods (a) & (b) TEM of

nanorods showing catalyst droplet (c) HRTEM image showing c-axis growth (d)

Selected area electron diffraction pattern .................................................................... 53

Figure 4.4: Typical XRD pattern of ZnO nanostructures (a) S1, Nanowires (b) S2,

Nanorods (c) S3, Nanobelts .......................................................................................... 55

Figure 4.5: (a) Photoluminescence UV emission peaks in lower range of ZnO

nanostructures (b) Photoluminescence visible emission peaks in higher range of ZnO

Nanostructures (c), (d) & (e) Diffused reflectance spectroscopy of nanobelts,

nanorods and nanowires, respectively. Eg is bandgap energy ..................................... 56

Figure 4.6: (a), (b) &(c) Time dependent photo-response of ZnO nanostructures (d),

(e) & (f) Electrical I-V photo-response of ZnO nanowires, nanorods and nanobelts,

respectively .................................................................................................................. 58

Figure 5.1: Schematic diagram of the experimental setup. (a) Synthesis of ZnO

nanostructures. (b) CO gas sensing setup (c) Actual image of the gas sensing setup.

Inset shows the sensor and holder (d) Optical fluorescent image of the nanobelt sensor

(e) Microscopic image of interdigitated Au electrodes. ............................................... 65

xviii

Figure 5.2: (a) FESEM image of undoped ZnO nanorods; inset shows a magnified

image (b) FESEM image of Mg doped transparent ZnO nanobelts; inset shows a

magnified image (c) & (d) show HRTEM images of undoped nanorods and Mg doped

nanobelts, respectively; insets of (c) & (d) show magnified TEM images and the

corresponding SAED images (e) & (f) show EDX analyses of the undoped ZnO

nanorods and the Mg doped ZnO nanobelts, respectively. .......................................... 67

Figure 5.3: (a) X-ray diffraction analysis of undoped and Mg doped ZnO

nanostructures. The inset image clearly shows slight shifts of the peak positions for

the doped samples (b) Raman spectra of undoped ZnO nanorods and Mg doped ZnO

nanobelts. ..................................................................................................................... 68

Figure 5.4: Room temperature PL spectra of (a) undoped ZnO nanorods deconvoluted

into three Gaussian peaks, and (b) Mg doped ZnO nanobelts fitted with four

Gaussian (c) Schematic representation of the energy transition mechanism in the

undoped and Mg doped ZnO nanostructures (d) Photocurrent response of the undoped

and doped ZnO nanostructures under UV and dark conditions. .................................. 69

Figure 5.5: Plots of F2(R) vs. energy (eV) for (a) undoped and Mg doped ZnO

nanorods and ZnO nanobelts (b) UV sensing responses of Mg doped ZnO nanobelts

and undoped ZnO nanorods. ........................................................................................ 71

Figure 5.6: Signal response and recovery profiles of (a) undoped ZnO nanorods and

(b) Mg doped ZnO nanobelt (c) Schematic of a possible mechanism of how the Mg

doped ZnO nanobelt sensor responds to CO in air and in gas (d) The response-

temperature curve shows that the response increases until 350°C, and then decreases

...................................................................................................................................... 73

Figure 6.1: (a) Schematic diagram of experimental setup for synthesis of ZnO

nanostructure (b) Schematic design of fabricated ZnO/MWCNTs sensor (c)

Schematics of CO gas sensing setup (d) Actual image of gas sensing setup, Inset

shows sensor with pressure contacts ............................................................................ 81

Figure 6.2: (a) SEM image of undoped ZnO tapered belts (b) SEM image of doped

ZnO pedal-like structures, inset shows MWCNTs used for carbon doped ZnO and

xix

schematic drawing for position of MWCNTs and doped ZnO nanostructures (c) & (d)

EDX analysis of undoped ZnO doped ZnO nanostructures. ........................................ 82

Figure 6.3: (a) TEM image of doped sample after sonication shows partial presence

of MWCNTs (b) TEM images of doped pedal-like structures, Inset shows the

magnified TEM image with corresponding EDS spectrum (c) and (d) corresponding

HRTEM and inset SEAD images of undoped and doped ZnO nanostructures,

respectively .................................................................................................................. 83

Figure 6.4: X-ray diffraction patterns of (a) undoped and doped ZnO nanostructures

(b) XRD pattern of the main peaks clearly shows the peak shift, inset shows the XRD

pattern of doped sample after sonication which clearly shows the carbon peak. ........ 85

Figure 6.5: XPS data for the Zn 2p, O 1s and C 1s spectra for undoped (red) and

doped (black) ZnO nanostructures ............................................................................... 86

Figure 6.6: (a) Raman scattering spectra of undoped ZnO (b) Raman scattering

spectra of MWCNTs and C-doped ZnO nanostructures; Inset shows the normalized D

and G bands peak shift (c) Electrical I-V curves represent increased conductance of

carbon doped sample compare to undoped ZnO .......................................................... 87

Figure 6.7: Plots of F2(R) vs. Energy (eV) of (a) undoped ZnO tapered belts and (b)

C-doped ZnO pedal-like structures, the inset shows the respective reflectance spectra

...................................................................................................................................... 88

Figure 6.8: (a) PL spectra of starting material, undoped and C- doped ZnO

nanostructures (b) Room temperature PL spectra of undoped ZnO tapered belts and

doped ZnO pedal-like structures .................................................................................. 89

Figure 6.9: (a) Signal response of undoped ZnO tapered belts (b) signal response of

C-doped ZnO pedal-like structures (c) Response and recovery times of undoped

tapered belts (d) Response and recovery times of C-doped ZnO nanostructures. ....... 90

Figure 7.1: (a) Furnace setting and placement of the substrates (b) Schematics of

fabricated SnO2 Nanowires sensor (c) Schematics of Gas sensing setup (d) Actual

xx

photograph of gas sensing experimental setup. Inset image shows sensor with pressure

contacts ........................................................................................................................ 97

Figure 7.2: SEM images of SnO2 nanowires at 900°C. (a) Nanowires 1 nm Au (b)

nanowires 2 nm Au (c) nanowires 5 nm Au (d) EDX analysis of nanowires (sample 1)

...................................................................................................................................... 98

Figure 7.3: Typical TEM images of SnO2 nanowires (sample 1) (a), (b) & (c)

showing low and high magnification images. (a) Clearly shows catalyst droplet (d)

XRD spectrum of SnO2 nanowires .............................................................................. 99

Figure 7.4: UV-VIS absorption spectroscopy of SnO2 nanowires (b) Room

temperature Photoluminescence spectra of SnO2 nanowires. .................................... 100

Figure 7.5: Room temperature Photoluminescence spectra of SnO2 nanowires. ...... 101

Figure 7.6: Time dependent gas sensing responses of SnO2 nanowires for three

samples (a) 20 ppm of CO gas at 400°C (b) 400 ppm of CH4 gas at 400°C (c) 200

ppm of Methanol at 200°C ......................................................................................... 102

Figure 7.7: (a) Selectivity of SnO2 nanowires (c) & (d) Schematic of possible

mechanism of SnO2 nanowires sensor responses to CO in air and gas, respectively.

.................................................................................................................................... 103

xxi

LIST OF TABLES

Table 2.1: Summary of physical properties of ZnO and SnO227;45-47

.......................... 16

Table 2.2: Reported UV sensors ................................................................................. 33

Table 3.1: Detail of all samples used in morphology tuning and doping of

nanostructures .............................................................................................................. 39

Table 4.1: Summary of the results .............................................................................. 59

Table 4.2: Comparison of the sensor properties of 1-D ZnO nanostructures with

reported semiconductor nanostructures based UV sensors/photodetectors. ................ 60


Table 5.2: Comparison of the CO gas sensor properties of 1-D ZnO nanostructures

with reported semiconductor nanostructures based gas sensors .................................. 75


Table 7.1: Summary of gas sensing results ............................................................... 102

Table 7.2: Comparison with reported SnO2 nanowires CO gas sensors ................... 104

xxii

LIST OF ABBREVIATIONS

1-D One dimensional

DOS Density of states

eV Electron volt

UV Ultra violet

𝜇𝑜 Permeability of free space

ZnO Zinc oxide

SnO2 Tin dioxide

𝑘𝐵 Boltzmann’s constant

λD Debye length

VLS Vapor-liquid-solid

VS Vapor-solid

PCM Phonon confinement model

MWCNTs Multi wall carbon nanotubes

PL Photoluminescence

CMOS Complementary metal oxide semiconductor

ZnOP Vapor pressure of ZnO,

Ω Volume of unit cell

XRD Powder X-ray Diffraction

FE-SEM Field Emission Scanning Electron Microscopy

TEM Transmission electron microscopy

η Vapor phase supersaturation

CO Carbon monoxide

CVD Chemical vapor deposition

Eg Bandgap energy

VO Oxygen vacancy

VZn Zinc vacancy

Zni Zinc interstitial

Λair Width of the depletion region

nо Charge carrier concentration

μ Charge carrier mobility

S Sensitivity

1

Chapter 1

1 Introduction

2

Chapter 1

Introduction

The term nanotechnology was first tossed in 1980 but now it can be found in

almost every field of science around us. Many areas of science have been

revolutionized by Nanotechnology. Applications of nanotechnology can be found in

the fields of electronics, optoelectronics, sensing etc. Development in the field of

nanotechnology is well guessed by having a look at overwhelming publications that

can be found in many journals across different fields. By definition, nanotechnology

is the field of science which deals with the creation and manipulation of functional

materials having dimensions ranges 1-100 nm. Research work and development in the

field of 1-D nanostructures which include their types as nanowires, nanorods,

nanobelts and nanotubes, are one of the best examples in the field of science and

technology. This chapter gives the introduction of the research problem and features

of the nanomaterials studied in this work.

1.1 Nanotechnology and low-dimensional nanostructures

Study and manipulation of small structures or small-sized materials comes

under the scope of nanotechnology. Dimensions of such materials can vary from sub-

nanometers to several hundred nanometers. It is worth noting that materials at the

nano level differ in their physical properties as compared to the bulk. Crystallinity of

these materials is stable at much lower temperature. Nanostructure is defined as the

geometrical shape that’s at least one of the dimensions in the range of 1-100 nm. They

represent structures like wire shape, particle shape, and layer like shapes.1 For basic

understanding, it is well known that reduction in size of an object results in an

increase in its surface to volume ratio. Due to this large surface area, properties like

optical, electrical and chemical etc., are greatly affected. The reduction in the size

(typically 1-10 nm) gives birth to the Quantum confinement in nanostructures. In this

case the object will be of the same size of the critical lengths of the countering

physical processes such as mean free path, coherence length and the de Broglie

wavelength of the electrons in 2-D, 1-D or 0-D structures.2;3

There are many

materials, but specially two materials namely zinc oxide (ZnO) and tin dioxide (SnO2)

3

exhibit interesting properties towards toxic gases and UV sensing at typical nano

scale. They can be synthesized into interesting family of nanostructures like zero-

dimensional (0-D), one dimensional (1-D), two dimensional (2-D), as shown in Figure

1.1 (a-c). Figure 1.1(a) shows a three dimensional (3-D) structure, representing bulk

material. It is notable that in 0-D nanostructures, like quantum dots, carriers are

confined in all three dimensions, whereas in 1-D and 2-D nanostructures, carriers can

easily move in one and two dimensions, respectively, while carriers are free to move

in all three dimensions in bulk materials. Figure 1.1 shows the plots of density of

states (DOS) versus energy and the shapes of the nanostructures. DOS is the number

of electronic states per unit volume and energy. In bulk materials, the DOS has a

square root dependence on energy. In Figure 1.1(b-d) this dependency changes with

the dimensionality of the quantum structures. The variation in DOS with

dimensionality results in fantastic electrical and optical properties of the advanced

nano devices.

As the particle size is greatly reduced in a nanostructure, a number of quantum

mechanical and statistical mechanisms become important giving a huge change in the

electronic and optical properties of such materials. This is due to the reason that at

such a small scale, the de Broglie wavelength of carriers becomes of the same size as

the size of the nanostructures. Recent decades have got special attention in synthesis

and tuning of the morphologies of functional materials for properties of interest.

Specially, the ability to control the size and structure, the physical and chemical

properties of nanomaterials represent best results for planning of the novel

Figure 1.1: Schematic representation of: (a) 3-dimension (3-D) (b) 2-dimension (2-

D) (c) 1-dimension (1-D) and (d) 0-dimension (0-D). The corresponding density of

states (DOS) plots for each type is also presented3

4

optoelectronic and sensing devices with properties better than what was achieved in

the bulk devices.

1.2 Wide bandgap oxide semiconductors for sensors

Two semiconducting materials with wide bandgap ZnO and SnO2 are fabricated

as nanostructures family, namely nanorods, nanowires and nanobelts etc., is presented

in this thesis. Wide bandgap semiconductors are the semiconductor materials which

have energy bandgap much greater than one electron volt (eV) or it is larger than that

of commonly used Si (1.11 eV) and GaAs (1.42 eV) semiconductors. Oxides have

two important structural properties, cations with mixed valence states and anions

with vacancies. With modification of these properties, it is possible to tune the optical,

electrical, magnetic and chemical properties of these materials. Hence it gives the

possibility of fabricating the smart and functional devices.2 Decreasing the physical

dimension for increased surface area is another strategy to improve the sensitivity of

the sensors.

In metal oxide semiconductors, variation in oxygen stoichiometry due to doping

can alter the electrical properties. As a result, carrier concentration, mobility and

conductivity of the material increases.3 So, doping offers another avenue for

expanding the sensing capability of oxide semiconductors.4

Nanomaterials have also wide applications in the field of ultra violet (UV)

radiation sensing and gas sensing. At present, various oxide nanomaterials such as tin

dioxide (SnO2), zinc oxide (ZnO), titanium dioxide (TiO2), gallium oxide (Ga2O3)

indium oxide (In2O3), tungsten oxide (WO3), etc. are synthesized for gas sensing

applications5 as shown in Figure 1.2. Quasi 1-D metal oxide nanostructures are

advantageous over their thin and thick film counterparts, best stability in the form of

single crystal structures, large surface-to-volume ratio, same size as of surface charge

region of nanostructures.6 ZnO and SnO2 have been investigated widely among oxide

semiconductor nanostructures, due to interesting electronic and physical properties.

These properties include wide bandgap (3.37 eV, 3.64 eV), large exciton binding

energy, good chemical and thermal stability.7 These materials are also studied due to

the ease of obtaining large quantities of nanostructures using a vapor-liquid-solid

(VLS) mechanism by thermal evaporation. This method is preferred for its simplicity,

5

cost effectiveness and for large production of nanostructures.8 1-D oxide

semiconductors nanocrystals like zinc oxide9-11

, tin oxide12;13

and titanium oxide14

etc., are considered to be the best sensing materials for sensors. The nanostructures of

these materials have high surface-to-volume ratio, show higher sensitivity and

stability to the surface chemical reactions and Debye length [λD = (εε0kT/e2n0)

1/2]

matching with small size. Consequently, synthesis of 1-D oxide semiconductors

nanostructures having different morphologies and sizes is important for research and

development on sensor devices.15

1.3 Synthesis of oxide semiconductors nanostructures

1-D metal oxide nanostructures can be synthesized using different techniques,

grouped in two major classes: a) “top-down” and b) “bottom-up” approaches. They

are briefly described as, in top-down approach; larger structures are shaped down to

nano size using lithography and etching. These are not effective in terms of cost and

preparation time.

In bottom-up approach atoms and molecules are arranged to build the desired

nanostructure. There are various methods for the growth of 1-D nanostructures. For

Figure 1.2: Relative comparison of different oxides used for gas sensing applications5

6

many years, people have demonstrated variety of chemical methods for “Bottom-up

approach”, few examples of this approach are thermal evaporation, solvo-thermal

technique, and chemical vapor deposition, etc.16-18

Chemical vapor deposition,

thermal transport method and thermal deposition are favored gas phase processes,

owing to the advantage of highly pure crystalline, high quality and high density 1-D

nanostructures in a single step.15

In vapor phase techniques, vapor liquid solid (VLS)

growth or (vapor solid) VS growth is the best due to simplicity and low cost to

produce variety of 1-D nanostructures.19

VLS mechanism is a catalyst assisted growth

which was first reported by Wagner et al. in 1964. An alloy droplet (nano-sized) is

formed due to reaction of the catalyst thin film and substrate at interface, which acts

as a catalyst for nucleation sites for the growth of 1-D nanostructures.20

VS

mechanism is defined as the growth of 1-D nanostructures without any catalyst in

thermal evaporation technique.21

A collection of various kinds of ZnO and SnO2

nanostructures synthesized by author is given in Figure 1.3.

Figure 1.3: A collection of different ZnO/SnO2 nanostructures synthesized by author

7

1.4 Applications of oxide semiconductor nanostructures

Due to multifunctional and tunable properties, nanostructures have very

important practical applications like piezoelectricity, nano-generator, gas sensors,

solar cells, lasers, field emitters and for large and fast photoconductivity, they are

used as optical (UV, IR) sensors. UV sensing and toxic gas sensing applications are

demonstrated in this thesis.

1.5 Gas sensing properties

Toxic gases can affect human life and health even at levels of few parts per

million and therefore, highly sensitive gas sensors are required. CO gas is tasteless,

odourless and toxic and as such may not be easily detected by humans. Therefore,

development of a sensitive CO gas sensor is essential to protect human’s exposure to

CO gas. Gas sensing is a vast scientific area of research that focuses on detecting gas

molecules in the environment for numerous applications like domestic safety,

pollution monitoring, public security, houses, space crafts, air conditioning in

airplanes etc. Various types of gas sensors based on the sensing materials are

available having unique sensor geometries and platforms. The leading class of the gas

sensing materials consists of conducting polymers, composite materials of metal

oxide and polymers and other novel materials.22

Mainly used gas sensor materials

include oxide semiconductors such as ZnO, SnO2, Fe2O3, In2O3, TiO2, etc.23

There are different sensor platforms which include quartz crystal microbalance,

metal oxide semiconductor field effect transistor (MOSFET), chemiresistive and

optical transducers. In addition, other gas detection methods include hybrid fiber

optics 24

, Fourier transforms infrared radiation (FTIR) and gas chromatography etc.

Among these techniques, oxide semiconductor chemiresistive sensors have many

potential benefits including, but not limited to, their high sensitivity, low cost, fast

response/recovery times and low detection limit to a number of gases.25

Most of the

available gas/chemical sensing devices including potentiometric, amperometric, fiber

optic sensor etc., are expensive, bulky and relatively operating at higher temperatures.

Therefore nanoparticles and nanostructures have acquired a great attention to new

sensors for environmental monitoring and gas sensing.26;27

There are different kinds of

8

gas sensors, only chemiresistive gas sensors will be discussed as a part of this

document.

1.6 Ultraviolet (UV) sensing properties

UV sensors are highly desirable in many fields such as environmental studies

for pollution monitoring, water sterilization, space communications, medical and

communication equipment.28;29

A variety of UV detectors mainly based on Si, are

available. These devices can be very sensitive to UV radiation with fast response and

low noise. However there are different limitations for their use, e.g., need of filters,

need of high vacuum and higher operating voltage. To counter these drawbacks oxide

semiconductor UV sensors achieved a great attention due to their visible blindness.30

ZnO is an ideal UV-sensitive semiconductor for optoelectronic applications because

of its wide bandgap (3.37 eV) and large exciton binding energy (60 meV) at 300 K.

ZnO/SnO2 nanostructures are used as UV sensors because of photoconduction

properties.

1.7 Objectives of the thesis

The work presented in this thesis is to discuss the development of

semiconductor oxide nanostructures and address their possible potential for UV and

toxic gas sensing applications. This work has been done using oxide nanomaterials

namely ZnO and SnO2. The major goals of this work are as follows:

1. To develop and optimize UV and toxic gas sensors for improvement of

sensitivity and stability of these sensors by carefully controlling the growth

parameters and the doping levels in oxide semiconductor nanostructures. It is

well-known that the sensitivity of sensors depends on three important factors,

geometry, carrier’s concentration and potential barrier modulation. In

order to achieve these goals, size control and morphology tuning were carried

out by varying growth temperature and thickness of catalyst layer. Doping of

nanostructures was done to achieve the carrier concentration and modulation

of potential barrier.

9

2. Fabrication of individual ZnO and SnO2 nanostructures to get normal control

over nanostructures morphology, best orientation, and high level of

crystallinity.

3. Characterization for structural, optical, electrical and doping effects of

ZnO/SnO2 nanostructures on the above parameters.

Characterization of ZnO and SnO2 nanostructures will be beneficial in explaining

the growth behavior of these nanostructures. The structural, morphological, and

compositional properties of ZnO and SnO2 nanostructures were studied using X-ray

diffraction (XRD), field emission scanning electron microscopy (FESEM),

transmission electron microscopy (TEM), high resolution TEM and energy dispersive

X-ray (EDX) analysis. Optical characterizations were carried out by diffused

reflectance/absorption spectroscopy (DRS) and UV sensing properties based on photo

response electrical I-V curves. Photoluminescence (PL) and Raman spectroscopies

were employed to estimate the bandgap, impurity levels and defects states in the

synthesized ZnO and SnO2 nanostructures. Raman spectroscopy gives information

about the lattice disorder.

4. To understand how the surface area, morphology tuning and doping of metal

oxide nanostructures affected the sensor characteristics to the gases of interest

on gas sensor devices.

The current research work is divided into two sections; section 1 is related to VLS

growth of ZnO (undoped and doped) and SnO2 nanostructures (nanowires, nanobelts,

nanorods etc.). The important factors which can affect the growth rate are growth

temperature, radius, and supersaturation and crystallization rate of the catalyst droplet

at liquid solid interface. Growth conditions including temperature, time, and thickness

of the catalyst layer are also explored to determine the formation of the semiconductor

oxide nanostructures.

Section 2 is related to the applications of these synthesized nanostructures for

ultraviolet light (UV) and toxic gas (CO, CH4) sensors. The synthesized and

optimized nanostructures have been utilized for the enhancement of CO gas sensing

properties. Gas sensor studies revealed that the morphology tuned and doped

nanostructures gave enhanced detection response to a fixed concentration of carbon

10

monoxide compared to pristine nanostructures. Based on the results, it can be inferred

that synthesized nanostructures have unique sensing properties.

1.8 Outline of the thesis

Chapter 1 is related to the introduction of nanotechnology, nanostructure’s

applications, objectives and the outlines of the thesis.

Chapter 2 explains the nanostructure synthesis approaches, different growth

kinetic parameters such as temperature, flow rates, doping of the nanostructures, VLS,

VS mechanisms, nature and thickness of catalyst. At the end sensor fabrication along

with sensing mechanisms of oxide semiconductor nanostructures are presented.

Chapter 3 presents the experimental work performed during the course of this

research. This consists of synthesis procedures, crystal structure, composition and

optical properties, experimental setup and characterization techniques used for

morphological analysis of the grown nanostructures. Finally, the technological steps

involved in sensor fabrication and characterization procedures used for UV and gas

sensing properties are explained.

Chapter 4 describes the development of highly sensitive UV sensor using

morphology tuned ZnO nanostructures. Systematic control of ZnO nanostructural

morphologies by VLS technique, surface defects and a simple cost effective method

for the fabrication of UV sensors without resorting to clean room preparation are

discussed.

Chapter 5 is related to the magnesium doped ZnO nanostructures for CO gas

sensing properties. Effects of doping and optical properties for sensor device are

explained. Very thin and transparent Mg doped ZnO nanobelts using magnesium

acetate in the source material via vapor transport method and their applications as CO

gas sensors with enhanced sensing response has been reported. Gas sensing results of

ZnO nanobelts are presented and compared with other reported gas sensing results for

ZnO nanostructures.

Chapter 6 shows an efficient way of carbon self-doping, using multi-wall

carbon nanotubes (MWCNTs) film into ZnO nanostructures for improved CO gas

11

sensing. The results, based on induced lattice defects produced due to carbon

incorporation into ZnO lattice and their effects on sensing response to CO gas, are

explained in this chapter

Chapter 7 demonstrates the effective control of diameter of SnO2 nanowires by

varying the thickness of catalyst layer by a simple vapor liquid solid (VLS)

mechanism, for sensitive CO gas sensors. Gas sensing properties as a function of

diameters (aspect ratio) and surface defects are evaluated. In addition, the growth and

sensing mechanisms are also presented.

Finally the conclusion and future plans are given in Chapter 8.

1.9 Thesis research flow chart

Road map of current research work is given in Figure 1.4

Figure 1.4: Research flow chart of current research work

12

1.10 Summary

In this chapter brief overview related to the material, physical, chemical, gas

and UV sensing properties of oxide semiconductor nanostructures are given.

Synthesis approaches which are necessary for the growth of these nanostructures are

described. Applications as sensor materials are demonstrated in detail. Motivation and

outline of research thesis along with road map of dissertation are also presented.

13

Chapter 2

2 Literature survey: Fabrication of oxide semiconductor

nanostructures and device processing

14

2.1 Properties of oxide semiconductors

As work done in this thesis mainly deals with ZnO and SnO2 semiconductors,

so structural properties of these materials are presented below:

2.1.1 Material properties of zinc oxide (ZnO)

ZnO have varying properties, which covers its physical, chemical or material

properties. ZnO is a well suited II-VI wide bandgap semiconductor which is naturally

found in three forms: cubic zincblende, hexagonal wurtzite and cubic rocksalt which

is less observed.31

The most common phase of ZnO is hexagonal wurtzite having

space group C6v or P63mc which is found mainly in ambient conditions .32;33

Figure 2.1: (a) Crystal structure of hexagonal wurtzite ZnO, ZnO unit cell, including

the tetrahedral-coordination between Zn and its neighboring O (b) ZnO has a

noncentro-symmetric crystal structure that is made up of alternate layers of positive

and negative ions, leading to spontaneous polarization34

(c) Schematic drawing of

different morphologies4

15

Figure 2.1 (a) shows crystal structure of ZnO which can be explained as number

of alternating planes with tetrahedral coordination of Zn+2 and O-2 ions along the c-axis.

ZnO have polar surfaces which results in spontaneous polarization (Figure 2.1 (b).

Due to different energies and polarity of planes ((0001), = 3.92 J/m2, work

function = 4.25 eV, = 2.01 J/m2, work function = 4.64 eV)

35;36 ZnO exhibit

interesting morphologies as shown in Figure 2.1 (c).

2.1.2 Material properties of Tin oxide (SnO2)

SnO2 is the mostly used metal oxide over the others and is one of the first

considered materials for gas sensing applications. SnO2 has a tetragonal rutile crystal

structure with space-group P42mnm symmetry and lattice constants a = b = 4.74 Å

and c = 3.19 Å. Figure 2.2 (a-c) represent the unit cell of SnO2 the bulk Sn and O

atoms are attached with six-fold and three-fold coordination, respectively. SnO2 is a

functional n-type material having wide bandgap (3.6 eV) and is used for different

application such as UV sensors, optoelectronics and solar cells etc.37;38

Defects like O vacancies and Sn interstitials originate during the synthesis

process of nanostructures of SnO2 which acts as the donor sites in its lattice. Due to

these defects electron density in conduction band increases as a result SnO2 shows n-

type conduction intrinsically.40

Figure 2.2 (b) & (c) show the planes containing O and

Sn atoms arranged in ionic charges as 2-, 4

+ and 2

-.39

SnO2 has two important faces,

Figure 2.2: (a) Rutile tetragonal unit cell (b) (110) surface structure (c) (101) surface

structure39

16

(110) plane have least surface energy when compared to other material surfaces41

and

(101) surface is dominant in nanostructured wires.42;43

The gas sensing mechanism of

SnO2 appears to be dependent on surface composition. Double vacancy of the Sn

atom, surface stiochiometry with Sn2+

, Sn4+

cations depending on oxygen chemical

potential are the key parameters of SnO2 surface properties. Gas sensing properties of

SnO2 have been reported in the literature.44

Detailed summary of the important physical properties of ZnO and SnO2 is given in

the Table 2.1

2.2 Nanostructures synthesis approaches

Synthesis of 1-D nanostructures of metal oxides can be carried out using many

different techniques; two approaches are briefly described below.

2.2.1 Top-down approach

The top-down approaches are based upon the idea of using modern

semiconductor industries to micro prepare nanostructures. Steps involved in these

techniques are first deposition then etching with ion beam on substrates so that of the

nano-sized dimensions of structures can be achieved. Highly ordered nanostructures

Table 2.1: Summary of physical properties of ZnO and SnO227; 45-47

Property ZnO SnO2

Mineral name Zincite Cassiterite

Crystal structure Hexagonal, wurtzite Tetragonal, rutile

Lattice constants [nm] a=b=0.325, c = 0.5207 a=b=0.474, c = 0.319

Melting point [ºC] 2240 1900

Melting point of metal [ºC] 420 232

Bandgap [eV] 3.4 3.6

Heat of formation [eV] 3.6 6.0

Exciton binding energy 60 meV 130 meV

Electron mobility (300K) ~100-200 cm2/V s ~ 250 cm

2/V s

17

can be achieved by using these techniques.48-50

But due to their high cost and

preparation time they are generally not favorable.

2.2.2 Bottom-up approach

For many years, people have demonstrated variety of chemical methods for

“bottom-up approach”; few examples of this approach are thermal evaporation, solvo-

thermal technique, and chemical vapor deposition etc.16-18

Gas phase techniques are

favored due to production of higher density and quality crystalline 1-D nanostructures

in a single step.15

As there are a number of unseen mechanisms involved in this

growth, so it is very complex process. It is difficult to gain control over morphologies

of the growing nanostructures. However, a better control over morphologies can be

achieved by carefully handling the growth parameters like temperature, flow rates

etc., using the thermal evaporation method.51

Wagner and Ellis studied in early 60s

vapor phase growth techniques for the development of micro and nanostructures.20

First step of this technique is the evaporation of source material in a tube furnace. The

source material is evaporated and then transferred by carrier gases to the colder

region. In this colder region, source material condenses at growth sites. First silicon

workers adapted this idea and later were used for other materials like metal oxides.

Two different mechanisms are used to condense the target: vapor liquid solid (VLS)

and vapor solid (VS).

2.2.2.1 Vapor liquid solid (VLS) mechanism

VLS is a catalyst assisted growth of 1-D nanostructures. VLS mechanism52-54

was given the name due to three phases i.e., a) the source material’s vapor phase, b)

the liquid catalyst droplet phase and c) the solid crystalline nanostructure. Many

different techniques like colloidal solution or magnetron sputtering are used to deposit

catalyst droplets on the target substrates.55

Source material is vaporized and vapor

species are transferred through carrier gases towards substrates placed in at lower

temperature in the tube furnace. Energy minimization process takes place due to the

diffusion of vapors from the source material into the catalyst droplet. Surface tension

and accommodation coefficient of the metal catalyst is such that, saturation and then

supersaturation of vapor species starts. This leads to the formation of one dimensional

nanostructure of source material at the liquid-solid interface on the substrate as shown

18

in Figure 2.3 (a). A metal catalyst particle seen at the tip of nanowire is the major

property of the VLS growth, as shown in Figure 2.3 (b).

2.2.2.2 Vapor solid (VS) mechanism

As the growth in this mechanism occurs directly from the source material,

hence named as self-catalyzed growth. This growth mostly anisotropic and is induced

by defects as indicated by SEM studies. In this growth, source material is vaporized at

high temperature and condensed directly on the substrates placed at lower temperature

in tube furnace. As the condensation process takes place, molecules which are

condensed at start to build seed crystals which serves as the nucleation sites57; 58

as

Figure 2.3: (a) Schematic illustration of VLS mechanism for ZnO, inset TEM image

shows Au catalyst droplet at the tip of nanowire (b) SEM image of ZnO Nanowires

grown using Au catalyst56

(c) SnO2 Nanowires grown using Au catalyst

19

shown in Figure 2.4 (a). In this case metal cations act as a catalyst on substrate surface

like VLS growth and leads to the formation of nanowires as shown in Figure 2.4 (b)

& (c).

Impurity free nanostructures were obtained using VS growth as shown by SEM image

in Figure 2.4 (d) & (e).

These methods have many advantages like the simplicity, various

morphologies, good and single crystallinity, cost-efficient, high density and for large

aspect ratio of 1-D nanostructures.19;21

Morphology can be tuned by changing the

growth conditions like reactor tube pressure, temperature, flow rates, deposition time,

composition, and nature of the catalyst on the substrates.54;60-62

Underlying physics

and thermodynamic approach, considering the interface energies and change in the

Gibb’s free energy of nanostructures will be explained in the following sections.

2.3 Thermodynamics of VLS growth

The main key of the 1-D nanostructures growth is the Gibbs free energy in VLS

method. Chemical tension model was used to study morphology of the nanostructure

Figure 2.4: Schematic illustration of VS mechanism (a) impinging of vapors (b)

formation of Seed (c) formation of nanowires (d) self-catalytic growth of ZnO

nanowires by Zn droplet (e) ZnO crystal contains no catalyst and defects59

20

using Gibbs free energy method.63

Consider the growth of ZnO nanostructures using

Au catalyst droplet of radius ro on Silicon substrate. Let us consider the formation

first ZnO nanowire layer. Let lo be initial thickness of the layer and droplet of Au-Si

alloy on the substrate as shown in Figure 2.5.

In this figure β◦ is the contact angle σLS is liquid solid, σVS is vapor solid and σVL is

vapor liquid interface energies. For nano-sized liquid droplet, the relation among

contact angle β◦ , line tension c and surface energies is given by modified Young’s

equation.64

Where and

σcLS and c are effective surface tension component and effective line tension

components, respectively.

In a VLS process vapor species comes to the surface of Au catalyst droplet,

and being energy minimization process it reduces its Gibbs free energy by adsorbing,

diffusing and precipitating at the liquid solid interface. Since the growth in tube

furnace is not an equilibrium process and the reduction in Gibbs free energy is given

by equation (2.2).65

Figure 2.5: Schematic of alloy droplet with first ZnO layer on Si substrate

ln

TK

l BoLS

c

)2.2(2ln2

VSoB

o rlTK

lrG

)1.2( cos o

c

o

VLLSc

VSr

VSo

c l

21

where is a vapor phase supersaturation which act as a driving force for

the formation of nanowires from vapor phase, ZnOP = Vapor pressure of ZnO, ZnO

oP =

equilibrium vapor pressure of ZnO, Ω = volume of unit cell of ZnO, BK = Boltzmann

constant, T = Growth temperature. In equation (2.2) Gibbs free energy consists of two

terms, these are the minimization in Gibbs free energy and the increase in the surface

energy. Tan et al. predicted, by using chemical tension model, various line tension

values can results in nanowire or nano-hillock growth.66

Now, change in Gibbs free

energy from equation (2.2) can be given in the form of effective line and surface

tension components as

Equation 2.3 consists of two important terms which are responsible for nanowires

growth. First term is effective surface tension component which acts along the liquid

solid interface and the second term is the effective line tension component (in plane

vector) acing along the circumference of the liquid solid interface.63

Now the overall

effective chemical tension σc is given by equation 2.4 as

Equation (2.4) shows that σc

depends on radius “r”. It is concluded from above

equation that chemical tension σc gives the geometry of the nanostructure. Hence

morphology of nanostructures depends on the growth temperature “T”,

supersaturation ratio “η” of the ZnO atoms and radius of catalyst droplet “r”. For

nanowire growth, the condition σc

> 0 must satisfy throughout the growth process.

The growth will stop whenever this condition fails during the growth process.

Gibbs Thomson effect showed that, the faster the growth rate the larger will be

the whisker diameter and vice versa. The supersaturation in terms of diameter of the

whisker is given by.54;67

)3.2(22 c

LS

c rrG

ZnO

o

ZnO

P

P

)4.2(

c

cc

rLS

)5.2(4

kTdkTkT

)6.2(4

d

22

According to the experimental data, it was seen that supersaturation is proportional to

the growth rate “V” and is given as

From equation 2.6 we can write growth rate as

Where n and b are fitting parameters equation (2.8) shows that growth rate is

proportional to the vapor solid interface energy, temperature, diameter of nanowire,

and the volume of the unit cell. If the growth is terminated then V=0, from equation

2.8 we have

Equation (2.12) gives a critical limit on diameter dc of nanowires for which growth

terminates. In conclusion the diameter of the nanowires depends on supersaturation

ratio and temperature. Above equations are responsible for the growth of the

morphology tuned nanostructures.

2.4 Optical properties of oxide semiconductors

It is well-known that the optical characterization of ZnO gives information

about reflection, optical absorption, transmission, photoluminescence and so on. In

this section, the focus is on the photoluminescence of ZnO. Both intrinsic and

extrinsic defects contribute to the optical properties of a semiconductor.68;69

)7.2(

n

kTbV

)8.2(14 /1/1/1

db

kTb

kTV nnn

)9.2(14

0 /1/1

d

bkT

bkT

nn

)10.2(14

cdkTkT

)11.2(4

cd

)12.2(ln

4

kTdc

23

2.4.1 Photoluminescence properties of ZnO

Photoluminescence (PL) is a spontaneous process in which light emission

takes place from a material due to optical transition. Excitation of electrons from

valence to conduction band occurs when wavelength of incident light is comparable to

material’s bandgap. As a result electron hole pairs are generated. On de-excitation to

the valence band, photons are emitted in the form of energy as shown in Figure 2.6

(a). In photoluminescence process of direct bandgap materials, before the radiative

emission electrons and holes relax at the top/bottom of conduction/valence bands. On

de-excitation, photons are emitted after recombination of holes and electrons.

Occupancy of the states is shown as shaded region after thermal distribution in Figure

2.6 (b).

Generally the PL spectrum of ZnO consists of two regions one is the UV-emission

band region and the second is the visible-emission band region as shown in Figure 2.6

(c).

At room temperature, the UV-emission band is assigned to the band edge

transition of ZnO which is due to recombination of free excitons. The broad visible

band literally from 420 nm to 650 nm was observed nearly in all samples regardless

of growth conditions is called deep level emission band (DLE). The DLE band has

previously been assigned to the various defects produced in the crystal structure

Figure 2.6: Schematic of the process occurring during photoluminescence in a direct

gap semiconductor after excitation at frequency νL (a) the electrons relax at the

bottom of C.B. and holes relax to the top of V.B. by phonon emission before

recombining by emitting photon (b) Density of states and band occupancies for the

electrons and holes after optical excitation (c) PL spectra of ZnO nanoflowers70

24

include O-vacancy (VO)71-73

, Zn-interstitial (Zni), Zn-vacancy (VZn)74-76

, O-interstitial

(Oi)76

, and extrinsic impurities e.g., substitutional Cu.77

DLE depends upon the

growth conditions like temperature, oxygen flow rate and dwell time etc. Additional

peaks from higher sub-band transition can appear as the lattice temperature or the

carrier density increased due to population electron and a small shift in energy to

emission peak is observed due to Coulomb renormalization and phase filling.78

PL spectroscopy gives the estimation of bandgap, impurity levels, defects states

produced in the bandgap, as a result of radiative recombination/relaxation processes.

In the present thesis, PL was employed to study the shift in energy and intensity of

emission bands both in UV and visible regions.

2.5 Doping of oxide semiconductors nanostructures

Control of optical and electrical properties is an important requirement for

sensing and optoelectronic applications. Doping of nanostructures, a well-established

technique, is used in the fabrication of devices, either it is done by post or in-situ

processing. It provides a favorable method to alter the properties of sensing

materials.79

In recent decades, a lot of work has been reported with a mixed-success,

for doping of nanostructures.80;81

The density of conduction electrons increases due to

the metal ions dopants in the lattice sites. So, tuning of the material properties can be

achieved changing the doping levels. Variation of electrical properties of the doped

oxide semiconductor was observed by changing oxygen stoichiometry or by mixing

other elements. Doping of nanostructures has been proved to be able of sensing toxic

gases with higher sensitivity and stability.

Doping is considered to be an effective method for improving gas sensing

properties of ZnO nanostructure at low temperature. Different researchers have used

doped ZnO nanostructures for the detection of CO by using different techniques. For

example Gaspera et al. showed that the doping of transition metal ions into ZnO

lowers the detection limit to around 1-2 ppm of CO at 300°C.82

Doping effects of Ti,

Fe and Sn were illustrated by Han et al. for gas sensing applications of ZnO.83

Van et

al. used SnO2 functionalized ZnO nanowires for CO sensing with response of 4.6 at

350°C for 300 ppm.84

Changhyun et al. demonstrated that Mg doped ZnO nanowire

25

sensors showed maximum response of 1.04 (4.65%) for 100 ppm of CO gas at

100°C.85

The role of dopant however, is not clear with discussion and presumption based

on defects, surface area, crystal structure and active sensing component by doping.

Crystal defects particularly oxygen vacancies etc., in ZnO play a role as adsorbed site

on the surface nanostructures for gas molecules.85-89

Mg-doped ZnO has been studied

intensively due to its bandgap engineering effect. One of the important features of

ZnO is that by doping with Mg, increases the bandgap of ZnO.85;89

Recently carbon

doped ZnO nanostructures have been reported which were mainly focused on visible

light photo-response90;91

, p-type conduction92

and magnetic93

properties of Carbon

doped ZnO synthesized by vapor phase techniques. Park et al. and Kim et al.

demonstrated the catalyst free ZnO nanostructures on MWCNTs films via thermal

CVD.94;95

2.6 Gas sensors based on metal oxide semiconductor nanostructures

It is important to note that two main types of semiconducting metal-oxides exist

which are used in chemiresistive sensors. The first type is n-type semiconductors

(conductance increases, e.g., TiO2, ZnO, and SnO2) whose majority carriers are

electrons. The other type of metal-oxides used is p-type semiconductors (conductance

decreases, e.g., NiO and CuO) whose majority carriers are holes. The majority of

semiconducting metal-oxides used in chemiresistive sensors are n-type because

electrons are naturally produced via oxygen vacancies at the operating temperature of

the sensors. A typical metal oxide gas sensor can be described as an interactive

material which interacts with the environment and generates a response (as receptor)

plus device which reads the response and converts it into an interpretable and

quantifiable term (as transducer).

2.6.1 Sensing mechanism of oxide semiconductor gas sensors

It is necessary to understand the sensing mechanism of the chemiresistive gas

sensors for the subsequent chapters in this thesis. In order to explain the sensing

mechanism, band theory was applied to the gas sensors which have been the subject

of intense study in literature.96

The analyte to be detected interacts with the surface of

26

the nanostructure through the reactive oxygen ions which give rise to a change in

carrier concentrations of the material. The electrical resistivity of the material changes

due to the change in carrier concentrations. For n-type metal oxide semiconductor,

increase in conductivity occurs upon interaction of reducing gas.97

Thus the sensing

mechanism is based on the principle of change in electrical resistivity /conductivity as

a result of chemical reaction between gas molecules and the reactive oxygen ions on

the surface of nanostructures.

The sensing mechanism is divided into three sections, (a) adsorption of

oxygen at surface, (b) detection of gases by a reaction with adsorbed oxygen, and (c)

change in resistance due to charge transfer at the surface.

2.6.1.1 Adsorption of oxygen at surface

Oxygen interactions with the surface of an oxide are of utmost importance in

gas sensing. Oxygen is a strong electron acceptor on the surface of a metal oxide. A

majority of sensors operate in an air ambient; therefore, the concentration of oxygen

on the surface is directly related to the sensor electrical properties. The conversion to

O2- or

O

- at elevated temperatures are useful in gas sensing since only a monolayer of

oxygen ions are present with these strongly chemisorbed species.98;99

Different forms

of oxygen can be ionosorbed on the surface of metal-oxide.100

At low temperature

ranges (150-200°C), molecular forms of the neutral O2 or charged O2- are adsorbed.

At higher temperatures range above 200°C, atomic form of oxygen as O- ions are

adsorbed.101

In order to increase the reaction kinetics, resistive sensors works better at

temperature of 300°C or above to react with ionosorbed oxygen at the surface.27

At temperature T < 200°C following reactions take place (for physisorption

E < 0.4 eV)

O2(gas) → O2(ad) → (2.13)

O2 (ad) +e → O2-(ad) → (2.14)

At temperature T > 200°C to 400°C following reaction takes (for Chemisorption

E > 0.4 eV)

27

O2-(ad) +e→ 2O

-(ad) → (2.15)

Adsorption energy of oxygen on metals lies in the range of 4-6 eV. Extracted

carriers originate from donor sites of the metal-oxide surface in the material.102

Intrinsic oxygen vacancies and other defects give rise to donor sites and surface

trapped electrons. As a result an ionosorbed oxygen produces a space charge layer on

the surface.103

A build-up charge is created on the surface of metal oxide due to

different event of adsorption and this leads to upward band bending for n-type

semiconductors.104

Figure 2.7 (a) & (b) show a schematic diagram of band bending,

potential barrier denoted by eVsurface for n-type semiconductor.102

In this figure EC,

EV, EF, and +sign represent conduction band, valence band, Fermi energy and donor

sites on the surface, respectively. The width of the space charge layer is denoted by

Λair.

2.6.1.2 Reaction of oxidizing/reducing gases

The detection mechanism of CO gas sensors for n-type material is given by

the following equations.105

2CO+O2-(ad) → 2CO2 (gas) + e

- → (2.16)

CO+2O-→ CO3

2-→ CO2+1/2O2+2e

- → (2.17)

Figure 2.7: Schematic showing band bending in n-type semiconductor (a) with O-

adsorbed on surface sites in air (b) After CO gas reaction at the surface42;102

28

Energy for the formation of CO2 is given by ΔHf˚ = -393.5 kJ/mol and

combustion of CO is ΔHc=-283 kJ/mol for given reaction. These are detection

reactions of a reducing gas CO which reacts with oxygen ions adsorbed on the metal

oxide surface. On exposure of CO gas to a metal oxide nanowire, above chemical

reaction takes place as a result of previously surface-bound electron send back to the

conduction channel. Space-charge layer and potential barrier decrease was observed

which lead to the decrease in the overall resistance of metal oxide. Figure 2.8 (a) &

(b) show a schematic of this process.

2.6.1.3 Charge transfer at the surface

When an electron transfer between the bulk and surface states takes place, the

width of the charge depletion layer decreases due to increase in net carrier density in

the conduction channel. This leads to a band bending near the surface as shown in

Figure 2.7 (b). The thickness of the space charge layer is given by the Debye length

λD (typically 10-100 nm) as given by Schottky approximation107;108

in equation 2.18

and 2.19.

Figure 2.8: Schematic of sensing mechanism (a) adsorption of oxygen at surface of

nanowire in air, creation of potential barrier and depletion region (b) modulation of

potential barrier and depletion region after reaction of carbon monoxide (CO) at

surface of n-type semiconductor106

)18.2(2

1

kT

eVw S

D

29

where w, VS, nо, k, ε, εо and T represent width of the depletion region, induced band

bending, carrier concentration, Boltzmann constant, relative permittivity, permittivity

of the free space and temperature, respectively. The Debye length λD is a quantum

value for the distribution of the space charge region.109

It is defined as the distance to

the surface at which the band bending is decreased to the 1/e-th part of the surface

value.110

Conductance variation takes place due to charge transfer on the surface layer

of the nanostructure channel. The conductance of 1-D metal oxide nanomaterial can

be expressed as111

Where μ, l, D, and w represents mobility, length, diameter of the nanomaterial and

width of space charge region, respectively. Therefore, when nanowires are exposed

to gas atmosphere, change in electrical conductance “∆G” is given by112

The gas sensitivity of sensor S is given by dividing equation (2.21) with equation

(2.20)

Equation 2.22 shows that sensitivity increases with increase in the change in

depletion region due to oxygen adsorption and desorption processes on the surface of

nanowires. Transport behaviour of electrons on surface of ZnO/SnO2 nanostructures

greatly depends upon the width of depletion region when its size is comparable to the

diameter of nanowires. The modulation of depletion region can be achieved by

variation in the surface defects. The gas sensitivity of the chemiresistive gas sensor

according to another report is given by19

)19.2(2

1

2

o

oD

ne

kT

21.2

4

2(12

l

wDen

RG o

20.2

4

2(12

l

wDen

RG o

23.2

442

12

12

1

)()(

SgSa

o

ogDaD

g

gaVV

enDDG

GGS

22.2

n

n

G

G

R

RS

30

Where Ga (Ra) and Gg (Rg) are conductance (resistance) of doped ZnO nanostructures

in air and gas ambient, VSa and VSg represents potential barrier in air and gas,

respectively, λD is the Debye length. According to this relation, CO gas sensing

response depends upon three important factors namely, geometry (4/D), electronic

properties (εεo/eno) and band-bending (VSa1/2

- VSg1/2

) due to adsorption. This control

can be achieved by doping and temperature modulation (Rg = Ra exp (eVs /KbT))

which is desirable in our case.113

The mobility of the charge carriers reduces due to

the potential barrier on the boundaries of the nanostructures and the dominant band

bending effect takes place due to concentration of free charge carriers on the sensing

response. A simplified model of the gas detection and the motion of electrons through

the potential barrier were given in this section. Some of the important sensor

properties will be discussed in the later section.

2.6.2 Gas sensor properties

The resistance or conductance of metal oxide nanostructure change upon

exposure to reducing or oxidizing gases and recovered completely to the initial value

upon removal as described previously. This change in resistance is from the base line

resistance in air (Ra) to a stable level (Rg) on exposure to analyte gas.27

Figure 2.9 (a)

shows a typical plot of an n-type semiconductor for fixed concentration of a reducing

gas. Different important parameters of the gas sensing material are calculated from

this graph are given below.

Generally the sensing response S, called sensitivity of resistive sensor is the

ratio of the resistance in the air (Ra) to the resistance (Rg) in gas environment.

Different forms of the sensitivity reported in literature27;114

are given in equations

(2.24) and (2.25).

25.2100%

X

R

RRS

a

ag

24.2g

a

R

RS

31

Throughout this thesis, the sensitivity, S=Ra / Rg has been used for n-type ZnO/SnO2

oxide semiconductor nanostructures exposed to reducing gases like CO and CH4 etc.

Figure 2.9 (b) shows the response and recovery times from graph of sensor

resistance versus time. Response time is defined as the time taken by resistance

value to 90% of its maximum or minimum.105

Recovery time is defined as the time

taken by resistance value to 10% of its maximum or minimum by sensor to recover its

resistance from exposed condition to the baseline value after target gas is cut out from

the environment. Mainly operating temperature affects the sensitivity and response

and recovery times. Other important sensor properties besides gas sensor response are

given below.

Stability is the capacity of the sensor to produce the same output when

measuring a fixed input over a period of time.

Selectivity of the sensor is defined as the capability of a sensor to measure a

single component of analyte among others. For example, a methane sensor will not

give any response to other gases like CO and NH3; it is called a selective sensor for

methane.

Figure 2.9: (a)Sensor graph showing response to a reducing gas for an n-type metal-

oxide27

(b) Schematic showing response and recovery times from a plot of sensor

resistance versus time98

26.22

1

gasofsenstivity

gasofsestivityySelectivit

32

Gas sensing application of nanostructures, sensing mechanism, sensor parameters and

simple sensing model has been described in this section. UV sensing application of

metal oxides semiconductor nanostructures will be illustrated in the following section.

2.7 Ultraviolet (UV) sensors based on metal oxide semiconductor

nanostructures

UV sensors are highly desirable in many fields such as environmental studies,

space communications, medical and communication equipment.28;29

Oxide

semiconductors nanostructures of ZnO/SnO2 show change in conductivity under the

exposure of UV radiation with wavelength ranges from 325-380 nm. Oxide

semiconductor nanostructures can be used for UV sensors using this property. These

materials can be used as optoelectronic switches as a large change in conductance

value under UV and dark exposure was observed, with the UV exposed state as ON

and dark state as OFF.

Due to faster and efficient photodetection from ZnO nanorods and nanowires

some important works have been reported recently, as summarized in the Table 2.2. A

number of approaches have been reported, e.g., efficient doping, morphology tuning,

formation of heterostructures and other structural modifications. Recently, Park et al.

demonstrated enhanced photo-response from ZnO nanowire treated with isopropyl

alcohol to produce surface roughness. They suggested that enhanced response was due

to an increase in roughening induced by adsorbed oxygen traps.115

Different

researchers have fabricated highly sensitive UV sensors utilizing ZnO

nanostructures.116;117

However, the roles of morphologies, surface defects which have

a great dependence on UV sensing response have not been illustrated in detail. UV

sensors using network of nanostructures show all the properties of single nanowire

devices, including fast response/recovery times and large ON/OFF current ratios.

Significant enchantments of photoresponse current due to large number of nanowires,

easy fabrication for large area nanostructured based sensor arrays and integration with

the flexible plastic substrates are other advantages.118

33

2.7.1 Photodetection mechanism of oxide semiconductor nanostructures UV

sensors

UV sensing is based on the principle of photoconductivity as shown in Figure

2.10. A model of this mechanism is given below using ZnO nanowires. Experimental

work done to date on metal oxide nanostructures indicated that defects like oxygen

vacancies are the key for the modulation of electronic properties as compare to the

bulk systems.4

It is well known that UV photoconductivity arises as due to two pathways:

surface related and bulk related processes.127

Adsorption and desorption of surface

oxygen on ZnO nanostructures generates surface depletion layer which results the

surface related photoconductivity. while deep level defects states are responsible form

the bulk to produce UV photoconduction.128;129

Surface adsorbed oxygen plays a

crucial role in Photosensitivity.130

Table 2.2: Reported UV sensors

Materials/Morphology Device

type

Light of

detection/Bias

voltage (V)

Photoresponse

(IUV/IDark) References

ZnO Nanorods Resistor UV-320 nm /10 V 80 119

Nanowire network Resistor UV-254 nm / 5 V 52 120

Nanowire film Resistor UV-254 nm / 5 V 17.7 121

ZnO Nanorods Resistor UV-325 nm / 2 V 19 122

Nanowire arrays Resistor UV-360 nm / 5 V 7600 123

Nanowire arrays Resistor UV-360 nm / 3 V 104

124

ZnO Nanowire Resistor UV-365 nm / 1 V 1500 125

ZnO Nanorods FET UV-254 nm / 0.2 V 1000 126

ZnO Nanorods FET UV-365 nm / 0.4 V 106 115

34

In dark condition of the sensor, adsorbed oxygen molecules on the surface of

ZnO nanostructure extracts free electron and creates deletion layer which decrease

conductivity,131

as given by the following reaction

O2(gas) → O2(ad) → (2.27)

O2 (ad) +e → O2-(ad) → (2.28)

O2-(ad) +e→ 2O

-(ad) → (2.29)

When sensor is placed under UV light, generation of electron hole pairs take

place by light absorption. Change in conduction is observed when electron and holes

cross the depletion region and recombination after release of oxygen ions. The surface

reaction is given by

hυ → e- + h

+ → (2.30)

O2-(ad) +h

+→ O2(gas) → (2.31)

O2-2

(ad) +2h+→ O2(gas) → (2.32)

This process goes on to increase the concentration of electrons in ZnO

nanostructures thereby increasing the conductivity. Longer response and recovery

times were observed due to defects as a result carrier’s life time extend due to these

trap states.130;132-134

Figure 2.10: (a) UV sensor device (b) A schematic of photo response mechanism of

ZnO nanowires at dark condition (c) during UV illumination (d) Schematic energy

band diagram of photo response process during UV illumination.

35

2.8 Summary

In this chapter, comprehensive background/literature review of the material

properties, synthesis approaches for the morphology tuned and doped oxide

semiconductors are presented. VLS and VS mechanisms have been theoretically

discussed. Doping of nanostructures and optical properties (PL) with special issue of

induced defects produced in processing of nanomaterials are demonstrated.

Applications of the synthesized nanostructures as UV and gas sensors are given.

Theoretical models, mechanisms and properties of UV and gas sensors are discussed.

36

Chapter 3

3 Experimental and characterization techniques

37

In this chapter, experimental conditions for growth and characterizations are

presented. After preparation of the samples, characterization techniques were used to

probe their structural, optical, vibrational and electrical properties. Ultra high vacuum

(UHV) chamber was used for the deposition of the catalysts thin films. Horizontal

tube furnace (with temperature range of 800-1050oC) was used for the growth of

nanostructures by using Ar and N2 as carrier gases. The techniques used for

investigation in this work include: scanning electron microscopy (SEM), transmission

electron microscopy (TEM), Raman spectroscopy, X-ray diffraction (XRD),

photoluminescence (PL), UV visible diffused reflectance spectroscopy (DRS).

3.1 Catalyst deposition and synthesis experiment

N-type Silicon substrates Si (100) with 2-3 nm thick native oxide layer was

used for synthesis of nanostructures. Substrates were cleaned in isopropyl alcohol

(IPA), acetone and deionized water (DI) by sonication to remove the contaminations.

Catalyst gold (Au) was deposited by evaporating gold (99.999 %) using resistive heat

in a tungsten basket under vacuum of 10-7

Torr in UHV chamber. Layers with

different thicknesses (1nm, 2 nm, 5 nm) were obtained according to the deposition

time. Ion sputtering technique was also used for deposition of a thin catalyst layer of

Gold (Au) on Si (100) substrates using the thickness monitor according to deposition

time.

For ZnO nanostructures growth experiments, an equimolar mixture (mixed in

a ball mill for 2 hours with 250 rpm) of ZnO (99.99%) and graphite (99.9%) was

placed in a reactor boat. This boat was placed at the center of quartz tube (length 100

cm and diameter 3.5 cm) as shown in Figure 3.1. Tube furnace was set at a

temperature of 1050°C (0 cm). Catalyst coated substrates labeled as S1, S2 and S3 were

placed at the downstream of the source material at temperature (distance) 800°C (22

cm), 900°C (18 cm) and 1000°C (10 cm), respectively.

38

The synthesis temperature of the VLS process was maintained in high purity

Argon (Ar) atmosphere for 60 min with a constant flow rate of 50 sccm. The other

end of the quartz tube was connected with an open ended flexible tube. After the

reaction, the furnace was cooled to room temperature. The thermocouple (omega K

type) was used for temperature mapping of the tube furnace as shown in the Figure

3.2.

Figure 3.1: Furnace setting and placement of the substrates

Figure 3.2: Furnace temperature mapping when heated at temperature of 1050°C by a

K-type thermocouple

0 5 10 15 20 25 30

600

700

800

900

1000

1100

1200

1050°C

S1= 800°C

S2= 900°C

Tem

pera

ture

(°C

)

Distance from center (cm)

S3= 1000°C

ZnO+C

39

For SnO2 nanostructures all the conditions were same except that the source

material (A mixture of SnO2 (99.99%) and graphite (99.9%) powders with weight

ratio of (4:1)) and carrier gas (high purity N2). Details of all experimental variations

used for tuning and doping of nanostructures are given in respective chapters.

3.2 Detail of samples

Samples containing mainly ZnO and SnO2 nanostructures grown with different

variables were used in our experiments and Table 3.1 presents the selected samples.

Further details are given with respective experiments.

3.3 Characterization techniques

3.3.1 Scanning electron microscopy (SEM)

Investigating the surface morphology at nanoscale dimensions is not an easy

task. Scanning electron microscopy (SEM) is an important technique for

nanomaterials characterization. SEM can be used to see features of dimensions with

resolution of 10 nm or below.

SEM was used to estimate the size, density and to study the morphological

features of the synthesized nanostructures. This characterization was carried out at

S.No. Source Material Catalyst

used

Dopant

material used

Catalyst

Thickness

Substrate

Temperature

T (°C)

1 (S1)

ZnO + C (graphite)

Au

undoped

1 nm

800

2 (S2) 900

3 (S3) 1000

4

900

5

Carbon

(MWCNTs) on

substrate

6 undoped

7 ZnO +C + [Mg

(CH3COO)2 .4H2O]

Magnesium

Acetate

[Mg (CH3COO)2

.4H2O]

8 (S1)

SnO2+C undoped

1 nm

900 9 (S2) 2 nm

10 (S3) 5 nm

Table 3.1: Detail of all samples used in morphology tuning and doping of

nanostructures

40

COMSATs, IIT, Islamabad and at university of Manchester, UK using SEM, Hitachi

SU-1500 and FESEM, Hitachi 8000. The operating voltage of FESEM was 10 kV.

3.3.2 Transmission electron microscope (TEM)

Another microscopy technique is transmission electron microscopy (TEM). In

this particular microscopy technique, sample is bombarded with a beam of electrons

in almost same way as in SEM but in TEM, beam of electrons is allowed to fall on the

sample and transmission takes place after interaction with atoms. As a result of this

interaction of electrons in transmission process through the specimen, an image is

formed. This image is magnified and focused onto an imaging device, such as a

fluorescent screen or CCD camera.

TEM in normal and high resolution modes was used to estimate the size,

shape, crystal structure, growth/plane directions of the synthesized nanostructures. In

general, a high resolution TEM (HRTEM, TECHNAI-20) has an attached X-ray

energy dispersive spectrometer (EDS). Selected area electron diffraction-meter

(SAED) equipped with this particular TEM machine was used to study growth

direction and crystal/Phase purity, etc. TEM characterization was performed at the

School of Materials, University of Manchester, UK with operating voltage at 200 kV.

The samples were prepared by drop casting method on copper grid by dispersing the

nanostructured material in ethanol.

3.3.3 X-ray diffraction (XRD)

X-ray diffraction is another tool which is used for the study of crystal structure

of semiconductor nanostructures. XRD is a non-destructive technique which gives

information about the crystal structure, lattice constants, average crystallite size,

distribution of atoms in the unit cell, chemical composition of materials and thin

films. It also provides information about orientation, quality, defects and stress/strain

of samples. A diffraction pattern is attained as a result of X-rays interaction with a

crystalline solid. Every crystalline solid has its unique characteristic X-ray diffraction

pattern, which is identified by a unique fingerprint. When a collimated beam of

having wavelength λ interacts with the sample at an angle θ, the scattered X-ray beam

is given by Bragg’s law:

41

Where n is an integer and dhkl is the inter-planar spacing (hkl) are the miller indices of

the corresponding lattice planes. The lattice parameters a, b and c are related to the

measured d-spacing for hexagonal crystal system as shown in following equation

(3.2)

In this research work, samples containing synthesized nanostructures were

studied using X- ray diffractometer (X’Pert PRO, PANalytical) using Cu-Kα (λ =

1.5418 Å) as the X-ray radiation source.

3.3.4 X-Ray photoelectron spectroscopy (XPS) measurement

X-ray photoelectron spectroscopy is a surface analytical technique based on

the photoelectric effect.135

A brief working principle of the instrument is given in

Figure 3.3. On illumination of X-ray beam on the surface of a sample, absorption on

surface takes place, an energy decrease in X-ray photon is observed. If the energy of

photon is high, core electron will be emitted with certain kinetic energy. The kinetic

energy Ek of these photoelectrons can be measured with an electron analyzer. If the

kinetic energy and the photon energy (hν) are known, then the Einstein equation can

be used to calculate the binding energy Eb of the electron as given by equation (3.3)

Where Ew is the work function of the material. Every element has more than one

atomic orbital. The electron located at each atomic orbital has a unique binding

energy like a “fingerprint”. With X-ray excitation, each element will show a set of

peaks depending on the kinetic energies in XPS spectrum. XPS spectrum is usually

plotted as a function of binding energy instead of kinetic energy. According to the

working principle of XPS, we can identify almost all elements by measuring the

binding energy of its core electrons.

In the present work, XPS was used for the study of valence states and surface

composition of the grown nanostructures. XPS measurements were taken on a custom

)1.3(sin2 hkldn

)2.3(3

412

222

22

c

lkhkh

ad

)3.3( wkb EEhE

42

system with an Al Kα source (1486.6 eV) and a hemispherical analyzer operated at

pressures of 1 x 10-9

mbar, 10 kV, 20 mA and 100 W. The energy of 100 eV was set

for survey and 25 eV was set for high resolution scans. For this work, XPS was

carried out at University of Manchester, UK (School of Physics and Astronomy,

Photon Science Institute).

3.3.5 Photoluminescence spectroscopy

Photoluminescence is a spontaneous emission of light from a material under

optical excitation148

To date, various types of lasers have been used for PL setup, for

example, He-Cd laser with 325 nm, Ar+ laser with 514 nm/488 nm, solid state lasers

etc. The semiconductor is optically excited by a laser to create electron-hole pairs. A

schematic illustration of the PL setup used during the present work is shown in Figure

3.4. Two laser lights Ar Ion laser, laser line 488 nm and He-Cd laser, laser line 325

nm (PL, DONGWOO Optron, 488 nm Ar laser, PL, DONGWOO Optron, 325 nm Ar

laser) were used for room temperature photoluminescence (PL). When a material is

illuminated with light of sufficient energy, photons are absorbed and atoms of this

particular material get excited. When these excited atoms get relaxed they release

Figure 3.3: Schematic of XPS working principle

43

photons (Figure 3.4 (b)). Emitted photons are used to get PL spectrum which is

further analyzed.

PL spectroscopy in the present work was carried out for the estimation of

bandgap, localized defects and impurity levels of the material. The Laser beam is

projected on the sample with the help of a setup as shown in schematic Figure 3.4 (a).

The excited electron-hole pairs recombine radiatively and emit light which is detected

and dispersed by a double grating monochromator and photomultiplier detectors. The

final spectrum is collected and displayed on a computer.137

The shape of the PL

spectrum gives information of the bandgap, intrinsic properties of the semiconductors,

defects and carrier concentrations. When a material is illuminated with light of

sufficient energy, photons are absorbed and atoms of this particular material get

excited. When these excited atoms get relaxed, they emit a photon. It is well known

that at room temperature PL spectrum consists of two bands for ZnO, a UV emission

band and a broad emission band as shown in Figure 3.5. UV emission band relates to

recombination of free excitons near the band edge termed as (NBE) and it gives

estimated bandgap Eg of ZnO. The broad emission band (420 nm-700 nm) observed

nearly in all samples with similar conditions, was due to presence of luminescence

defects states (DLE). This DLE has been attributed to several defects produced in

crystal structures during the growth process like oxygen vacancies (Vo), Zn vacancies

Figure 3.4: (a) Schematic illustration of the PL setup (b) Schematic of PL working

principle for direct bandgap material

44

(VZn), interstitial Zinc (Zni) and anticite defects (O Zn) have been reported by different

researchers corresponding to the green emission of ZnO.138-140

3.3.6 UV-visible diffused reflectance spectroscopy (DRS)

Optical properties of the synthesized nanostructures were carried out by UV-

VIS diffused reflectance spectroscopy (DRS, lambda 950 UV- VIS

spectrophotometer). DRS is suitable technique for the characterization of unsupported

nanomaterials as compared to the absorption UV-VIS spectroscopy due to the

advantage of larger scattering in powder materials. In this technique powder sample is

not to be dispersed in any liquid medium and hence the material will not consume or

contaminate.

Reflectance of light from the surface of a sample at different angles rather than

a single is known as diffused reflectance. An ideal illuminated diffuse surface has

equal luminescence from all directions which lie in the half space adjacent to the

surface. The most general mechanism by which a surface gives diffused reflectance

Figure 3.5: PL spectrum of ZnO nanowires from the sample grown at 800°C,

measured at room temperature with excitation wavelength 325 nm

350 400 450 500 550 600 6500.0

0.2

0.4

0.6

0.8

1.0

NBE

P

L I

nte

ns

ity

(a

.u.)

Wavelength (nm)

DLE

45

does not involve exactly the surface; most of the light is donated from the scattering

centers below the surface141; 142

as shown in Figure 3.6.

Figure 3.7 shows the diffused reflectance spectroscopy of ZnO nanobelts. The

bandgap of the nanostructures was measured using Kubelka Munk function F(R) as

given by the following relation.

2.8 2.9 3.0 3.1 3.2 3.3 3.4 3.5

0

1x1010

2x1010

3x1010

4x1010

F2(R

)

Energy(eV)

Eg=3.18 eV

(b) Kubelka-Munk Band gap ZnO nanbelts

Where F(R) = α/S (Kubelka Munk function), α = absorption coefficient, S= scattering

coefficient. The diffused reflectance spectrum of the selected sample is in Figure 3.7

(a) and after Kubelka Munk treatment is shown in Figure 3.7 (b). The bandgap can be

calculated by extra- ploting the linear part of graph ( F2(R) vs. Energy) to the photon

Figure 3.6: Schematic illustration of diffused reflectance of light from the surface of a

sample

Figure 3.7: (a) Diffused reflectance of ZnO nanobelts (b) Kubelka-Munk transformed

reflectance spectrum: plots of [F(R)] 2

vs. photon energy of ZnO nanobelts

)4.3(2

)1()(

2

SR

RRF

300 325 350 375 400 425 450 475 500

0

10

20

30

40

50

60

70

80

% (

R)

wavelength(nm)

Diffused reflectance of ZnO nanobelts(a)

46

energy axis143

in electron Volt (eV). Further details will be demonstrated in the

respective chapters.

3.3.7 Raman spectroscopy

Raman spectroscopy is a spectroscopic technique used to study vibrational,

rotational, and other low-frequency modes in a system. Raman is an inelastic

scattering of light from the material. When a beam of light energy is incident on a

material, some of photons are scattered without change in energy and this process is

called Rayleigh scattering. Only a small number of photons are scattered with energy

shift. This change in energy (called the Raman shift) carries the information about

rotational, vibrational and other low frequency modes of the material under analysis.

Classical treatment can be used to explain the Raman process.144

Scattered photons

have two types of frequencies, (ω0 - ωv) and (ω0 + ωv) called stokes and anti-stokes

frequencies, respectively. An intense light source is required (due to weak Raman

Effect) to get the reliable energy of scattered photon. Raman system contains three

components; these are laser excitation source, monochromator and a detector. A laser

beam is focused on the sample and scattered photons are detected with the

spectrometer and detector. In terms of perturbation theory Raman is related to an

effect of absorption and emission of photon from an intermediate energy state.

Figure 3.8 shows the three possibilities described above during Raman process.

Figure 3.8: Raman effect

47

In the experimental work, Raman Effect was used to study the phases, crystal

structures, and doping effects (Defects, Vacancies etc.) of synthesized nanostructures.

Results of these experiments are presented in respective chapters. Raman

spectrometer (Renishaw, wavelength 514) operated at room temperature for 10 min,

was used for analysis of the as grown samples.

3.4 Sensor fabrication/measurement system

In the experimental work, ultraviolet (UV) and gas sensors were fabricated by

depositing a 200 nm thick layer of SiO2 thermally onto the single crystalline Si (100)

wafers. A slurry droplet containing ZnO/SnO2 nanostructures (20 µL) was deposited

on Si substrates (as shown in Figure 3.9) containing a pair of interdigitated electrodes

of gold (Au) (200 nm) having a gap of 50 µm through photolithography technique.

These sensors were heat treated in air for 4 hours at 400°C before performing the

sensing experiments.

The sensing response of the device was measured by resistance/current change

upon exposure to UV light. The schematic design and UV sensing setup is shown in

Figure 3.10. Gas sensing experiments were performed at different temperatures 3/5

min cycles of dry air and test gases. The sensing response (S=Ra / Rg) of the device

was measured by resistance change upon exposure to air (Ra) and gas (Rg) in self-

designed gas chamber connected with Keithly multimeter, gas flow meters and tube

furnace. The schematic design and gas sensing characterization setup have been

shown in Figures 3.11.

Figure 3.9: (a) Microscopic image of interdigitated Au electrodes (b) Sensor

fabrication

48

3.5 Summary

This chapter gives a brief overview of experimental setup, tools and

characterization techniques used along with detailed growth parameters. The details

of the characterization techniques with working principles were given. Optical

characterization techniques were explained in detail. Synthesis procedures for

nanostructures and fabrication of the sensors were presented. Other distinct details

will be discussed in the respective chapters.

Figure 3.10: The schematic design and UV sensing setup

Figure 3.11: Schematic diagram of gas sensing setup

49

Chapter 4

4 Development of highly sensitive UV sensor using

morphology tuned ZnO nanostructures

50

4.1 Introduction

Recently, one-dimensional ZnO nanowires, nanobelts and nanorods have shown

potential as next-generation UV sensors as well as self-powered

Photodetectors.124;130;145

UV sensors are highly desirable in many fields such as

environmental studies, space communications, medical and communication

equipment.28;29

ZnO nanostructures are potential candidate for UV sensors due to their

high surface/volume and ON/OFF current ratios116

Fast response and recovery146

,

visible light blind and low cost fabrication.147;148

Yang et al. reported a single

nanowire based photodetectors130

, a variety of 1-D semiconducting nanostructures

have been fabricated and investigated.149-152

ZnO nanostructures UV sensors have

been fabricated by different research groups.116;117

However, the role of different

morphologies and defects which have great effect on UV sensing, has not been

investigated in detail.

In this Chapter, systematic control of ZnO nanostructural morphologies by

VLS technique and a simple low cost method for fabrication of UV sensors without

resorting to clean room preparation have been presented. The fabricated sensor

showed very quick sensing response to UV light as compared to the single

nanowire/nanobelts based photosensor.153

The photoresponse of the developed sensors

was determined as 1.4 to 3.0 with photocurrent ranging from 4.6 µA to 216.6 µA. UV

response and recovery times were determined and compared with available sensors.

4.2 Synthesis of ZnO nanostructures by vapor transport method and

fabrication of sensors

ZnO nanostructures were synthesized by vapor transport using vapor liquid

solid (VLS) mechanism as shown in Figure 4.1(a). A brief description of procedure is

given, an equimolar mixture (mixed in a ball mill for 2 hours) of ZnO powder

(99.99%) and graphite powder (99.9%) were placed in a ceramic boat. This boat was

placed inside a quartz tube (length 100 cm and diameter 3.5 cm). The set temperature

of the tube furnace was 1050°C. Catalyst coated substrates labeled as S1, S2 and S3

were placed on another boat and were kept downstream from the source material at

800°C, 900°C and 1000°C, respectively. The growth temperature was maintained for

60 min in high purity Argon (Ar) atmosphere with a constant flow of 50 sccm. The

51

furnace was cooled down to room temperature and collected nanostructure samples

were characterized for its properties.

UV sensors were fabricated by using gold coated (200 nm, using ion sputtering

deposition by JFC 1500) quartz substrates. ZnO nanostructures were attached between

the electrodes by the doctor blade method. Doctor blade is a very cheap and well

suited process for mesoscale sensor fabrication.154

Nanomaterial was deposited after

scratching from the as grown samples to Au electrodes coated sensor substrate to get a

thickness of few microns. The resulting substrates were heated in an oven at 400°C

for 2h for removal of methanol and to make best contact of nanostructures. The

experiment was performed by two probe method at room temperature using Keithly

multimeter (Keithly 2100). The sensing response of the device was measured by

resistance/current change upon exposure to UV light (365 nm). The schematic design

and UV sensing setup is shown in Figure 4.1(b) & (c). The inset of the Figure 4.1 (c)

shows the sensor holder with pressure contacts.

Figure 4.1: (a) Schematic diagram of synthesis setup (b) & (c) Schematic diagram

and actual photograph of UV sensing setup

52

4.3 Results and discussion

4.3.1 Morphology and composition of ZnO nanostructures

4.3.1.1 SEM and EDX analysis

Figure 4.3 shows the SEM images of the samples S1 (nanowires), S2 (nanorods) and

S3 (nanobelts) along with their respective EDX spectra.

Figure 4.2(a) shows a large quantity of the cylindrical nanowires randomly oriented

with very high aspect ratio (305) grown at 800°C. The average diameter of nanowires

Figure 4.2: SEM and EDX images of different morphologies of ZnO nanostructures

at different synthesis temperatures (a) SEM nanowires grown at 800°C (b) EDX

analysis of nanowires (c) SEM nanorods grown at 900°C (d) EDX analysis of

nanorods (e) SEM nanobelts grown at 1000°C (f) EDX analysis of nanobelts

53

was 129±30 nm and the average length 39±2.0 µm. Figure 4.2(c) shows large quantity

of the cylindrical nanorods randomly oriented with low aspect ratio (15) grown at

900°C. The average diameter of nanorods was found to be 160±40 nm and average

length 2.4±1.0 µm. At 1000°C, needle like nanobelts were obtained (Figure 4.2(e)).

These also have faceted edges having average thickness, length and width of 425±165

nm, 21±7.0 µm and 3.0±1.0 µm, respectively. The measured aspect ratio was found to

be 23. Figure 4.2 (b), (d) & (f) show corresponding EDX analysis of the ZnO

nanowires, nanorods and nanobelts clearly showing the Zn and O peaks. The

approximate atomic ratios were found to be 66: 34, 68: 32 and 57: 43, respectively.

These ratios show non-stoichiometry (i.e., crystal defects) of the grown

nanostructures during the growth process. Deviation from the stiochiometry is large

due to carbothermal reaction and oxygen deficient environment (Ar gas) during

growth process. Most of the Oxygen is used in CO2 formation.

ZnO + C → Zn + CO → (4.1)

ZnO + CO →Zn + CO2 → (4.2)

Figure 4.3: Typical TEM and HRTEM images of ZnO nanorods, (a) & (b) TEM of

nanorods showing catalyst droplet (c) HRTEM image showing c-axis growth (d)

Selected area electron diffraction pattern

54

The scanning electron micrographs clearly showed that morphologies changed

from nanowires to nanobelts due to change in temperature. High temperature and

supersaturation condition lead to the formation of tapered nanobelts.

The possible reason for this change in morphology is attributed to

supersaturation, growth rate and quick availability of ZnO polar surfaces for the

growth.156; 157

Overall, at different temperatures, the supersaturation conditions are

different which ultimately change the morphology. The growth of one dimensional

structure depends on the rate of adsorption of incoming vapors in the catalyst

particles, their diffusion through the particles and the rate of crystallization at L-S

interfaces.

TEM images shown in Figure 4.3 (a) & (b) clearly show the catalyst particles

on the tips of the nanorods suggesting the VLS growth. HRTEM image (Figure 4.3

(c)) shows the lattice fringes of crystalline nanostructures. The corresponding selected

area electron diffraction (SAED) patterns as shown in Figure 4.3 (d) are also

consistent with the HRTEM lattice fringes. The measured plane spacing was found to

be 0.26 nm between the two adjacent (002) lattice planes and it confirmed the growth

was in the <0001> direction.51

4.3.2 Structural (XRD) analysis

Figure 4.4 (a)-(c) shows X-ray diffraction patterns of the as-synthesized ZnO

nanostructures. All spectra show five peaks at 31.7º (100), 34.5º (002), 36.2º (101),

47.6º (102) and 56.6º (110). The peak at 43.5º (320) is due to Si from substrate. All

diffraction peaks were indexed to the hexagonal wurtizite structure of ZnO (ICDD

PDF-2 entry 01-079-0207). The sharpness of the XRD patterns reflected the

crystalline quality of all ZnO nanostructures. The measured lattice parameters a = b =

3.25 Å and c = 5.21 Å are comparable to the pure ZnO (a = b = 3.25 Å, c = 5.21 Å).

No diffraction peaks for other impurities were observed in the XRD patterns. A

dominant high intensity peak (002) was observed at 34.5º compared to other peaks for

nanowires and nanorods as shown in Figure 4.4 (a) & (b) which confirmed the c-axis

oriented growth, as was confirmed in the HRTEM results. A slight peak shift of 0.12º

to lower angle was observed for nanowires as compared to nanorods and nanobelts

55

which may be due to defect concentration produced in the nanowires.155

Defects were

expected to be more consistent in measurements.

4.4 Optical properties

4.4.1 Photoluminescence (PL)

ZnO nanostructures were characterized for optical properties using

room temperature UV-visible diffused reflectance and photoluminescence

spectroscopy. Figure 4.5 shows the room temperature PL spectrum with laser line 325

nm using He-Cd laser. Figure 4.5 (a) shows the magnified image of PL spectrum

ranging from 360- 390 nm for the UV emission band at 382±2 nm (3.24 eV) for three

samples containing nanowires, nanorods and nanobelts. This UV emission peak is

Figure 4.4: Typical XRD pattern of ZnO nanostructures (a) S1, Nanowires (b) S2,

Nanorods (c) S3, Nanobelts

30 35 40 45 50 55 60

0

20

40

60

80

100

0

200

400

600

800

1000

12000

1000

2000

3000

S3

(3 2

0)

(1 0

1)

(1 0

0)

(0 0

2)

(1 0

1)

(1 0

2)

(1 1

0)

(1 0

0)

(c)

2Degree

Inte

nsit

y(a

.u)

Nanobelts

S2

(1 1

0)

(1 0

2)

(1 0

0)

(0 0

2)

(1 1

0)

(1 0

2)

(1 0

1)

(0 0

2)

(b) Nanorods

(3 2

0)

(a) Nanowires

S1

56

2.6 2.8 3.0 3.2 3.4 3.6

0

5x109

1x1010

2x1010

0

2x1010

4x1010

0

5x108

1x109

2x109

2x109

(e)

Eg=3.20 eV

Nanobelts

(d)

Eg=3.17 eV

Nanorods

(c)

Eg=3.33 eV

Nanowires

F2(R

)

E (eV)

365 370 375 380 385 390 395 400

1x104

2x104

3x104

(a)379 nm

383 nm

Nanowires

Nanorods

Nanobelts

PL

In

ten

sit

y (

CP

S)

Wavelength (nm)

400 450 500 550 600 650

0.0

4.0x106

8.0x106

1.2x107(b)

506

PL

In

ten

sit

y (

CP

S)

Wavelength (nm)

Nanowires

Nanorods

Nanobelts495

511

related to the near band edge (NBE) emission of ZnO which is close to the bandgap of

ZnO. Figure 4.5 (b) shows broad green emission bands in visible spectral range

centered at 495 nm, 511 nm and 505 nm for nanowires, nanorods and nanobelts,

respectively. These peaks were generally attributed to various intrinsic defects like

oxygen vacancies and Zn interstitials, produced during synthesis process of ZnO

nanostructures.158

Nanowires show a very strong and shifted emission peak as

compared to nanorods and nanobelts, which show that more defects, produced in

nanowires sample due to high surface area to volume ratio (L/D).

The degree of non- stoichiometry (crystal defects) in ZnO can be described by

intensity ratio of visible luminescence to ultra-violet luminescence (𝐼𝑣𝑙/𝐼𝑢𝑙) to

evaluate the crystallinity of ZnO crystal: higher the ratio, more the intrinsic defects.159

The calculated ratio of 𝐼𝑣𝑙/𝐼𝑢𝑙 for the nanowires (~440) is very large as compared to

nanorods (~45) and nanobelts (~60), respectively. This reveals that defect

concentration in nanowires is greater as compared to nanorods and nanobelts.

Figure 4.5: (a) Photoluminescence UV emission peaks in lower range of ZnO

nanostructures (b) Photoluminescence visible emission peaks in higher range of ZnO

Nanostructures (c), (d) & (e) Diffused reflectance spectroscopy of nanobelts,

nanorods and nanowires respectively, Eg is bandgap energy

57

4.4.2 Diffused reflectance spectroscopy

Figure 4.5(c), (d) & (e) show graph of [F(R)]2 vs. energy of photon energy for

ZnO nanostructures. Where F(R) = α/S (Kubelka Munk function) given as122

Where α = absorption coefficient, S = scattering coefficient and R = diffused

reflection. The bandgap can be measured by taking x-intercept from the linear part of

the graph on energy axis. The measured optical energy bandgap of ZnO nanowires,

nanorods and nanobelts was found to be 3.33±0.05 eV, 3.17±0.03 eV and 3.20±0.02

eV, respectively. The optical bandgap of nanostructures was found to decrease as

synthesis temperature was increased. The slopes of the plots estimates the defect

concentration, larger slopes showed more defects produced in the nanowires as

compared to other structures. As slight peak shift was also observed in XRD data for

nanorods and nanobelts compared to nanowires, so an increased inter-atomic spacing

decreased the potential seen by electrons in the material, which in turn reduced the

energy bandgap.160

The optical bandgap of nanowires was found to be greater than

nanorods and nanobelts, which might be due to crystallinity, high aspect ratio and

defects like oxygen vacancies as evident from PL results.

4.4.3 UV sensing properties

Figure 4.6 presents the room temperature UV sensing results of the ZnO

nanostructures. Sensing response (ROFF/RON) of the ZnO nanowires, nanorods and

nanobelts was found to be 3.0, 1.6 and 1.4, respectively. The resistance decreased

under UV light and increased again when the UV light is switched off. When the ZnO

nanostructured sensor is exposed to air, space charge region is created due in sorption

of oxygen on the surface nanostructure as a result sensor exhibits high resistance.

When sensor is placed under UV light, generation of electron hole pairs take place by

light absorption. Change in conduction is observed when electron and holes cross the

depletion region and recombination after release of oxygen ions. The enhanced

response in the case of ZnO nanowires was attributed to the photogeneration and

recombination of electron hole pair due to solid-state process. Slow response and

recovery time is due to presence of vacancy-defects as observed in PL spectra and

)3.4(2

)1()(

2

SR

RRF

58

adsorption and photo-desorption of oxygen molecules.161

Moreover the large aspect

ratio of the nanowires makes the life time of the photo-generated carriers longer.162

Soci et al. reported that the high sensing response UV towards ZnO nanowire was due

to oxygen related hole traps present on the surface of nanowires.163

Nanorods and

nanobelts showed relatively low sensing response and very fast response/recovery

times compared to nanowires due to low aspect ratios and low defect concentrations,

respectively. The photoconductivity of the ZnO nanostructures were investigated by

two terminal I-V measurement method using Keithly 4200 multimeter, with a bias

voltage from -10 to +10 V with and without UV illumination. Figure 4.6 (d)-(f) shows

that current under UV light increased 4µA for nanowires, 9 µA for nanorods and 216

µA for nanobelts, respectively, compared to the dark current (0.3 µA). The possible

reason for increased current values is the low defects concentration in nanobelts as

compared to the nanowires and nanorods.

Figure 4.6: (a), (b) &(c) Time dependent photo-response of ZnO nanostructures (d),

(e) & (f) Electrical I-V photo-response of ZnO nanowires, nanorods and nanobelts,

respectively

59

UV sensing is based on the principle of photoconductivity. Briefly, In dark

condition of the sensor, adsorbed oxygen molecules on the surface of ZnO

nanostructure extracts free electron and creates depletion layer which decrease

conductivity.131

Nanowires show longer response and recovery times due to oxygen164

related defects states which stop recombination for a while as a result carrier’ s

lifetime extend.128;130;132-134

Table 4.1 shows the summary of our results. Table 4.2 shows the comparison of the

sensor properties of 1-D ZnO nanostructures with reported semiconductor

nanostructures based UV sensors/photodetectors.164-173

Similar reports on single-

crystal line ZnS/ZnSe nanobelts as ultraviolet-light sensors are reported earlier.51;164

Morphology of

nanostructures

Synthesis

Temp.

(°C)

Nanostructure

size

Aspect

ratio

(L/D)

Bandgap

by UV-VIS

(eV)

UV sensing

response at

room temp.

PL peak

positions

(nm)

Photo-

Current with

UV light (µA)

Nanowires 800 Dav= 128.9 nm

Lav = 39.3 µm 305 3.33 3.05

382

495 5

Nanorods 900 Dav= 160 nm

Lav = 2.4 µm 15 3.17 1.96

382

511 9

Tapered

Nanobelts 1000

Tav = 425 nm

Lav = 21.0 µm

Wav = 2.9 µm

23 3.20 1.45 382

505 216

Table 4.1: Summary of the results

60

4.5 Summary

Morphology tuned ZnO nanostructures were successfully synthesized using

vapor transport method at different temperatures. SEM images showed that the

nanowires were of cylindrical form with high aspect ratio (305). TEM, HRTEM

confirmed the VLS growth and was consistent with XRD results. A high resolution

Table 4.2 Comparison of the sensor properties of 1-D ZnO nanostructures with

reported semiconductor nanostructures based UV sensors/photodetectors.

Materials

Synthesis

Temp.

(°C)

Size Methods UV sensing

response

Dark-

Current

Photo-

Current References

SnO2 Nanowires

(single) 600-650

Dav = 26 nm

Lav = 30-40 µm CVD (VLS)

2.2

UV-320 nm 19.4 nA

2.1 µA

1V bais

165

SnO2 Nanonets

(monolayer porous

film)

600 Dav = 160 nm

Lav = 2.4 µm

polymer

colloid

monolayer

nanofilms

1.3

UV-320 nm 66 µA

232.3 µA

1 V bais

166

Nb2O5 Nanobelts

(single) 650

Tav = 73 nm

Lav = 2-10 µm

Wav = 100-500

nm

hydrothermal 1.16

UV-320 nm 15 pA

52 pA

1 V bais

167

Ga2O3 Nanowires

(multiple) 1000

Dav = tens of nm

Lav = 100 µm CVD (VS)

3x104

UV-25 nm 0.2 pA

6 nA

50 V bais

168

ZnS nanobelt

(single) 1050

Tav = 35 nm

Wav = 1.85 µm CVD (VLS)

3.8

UV-320 nm -

1.05 nA

10 V bais

169

FeS2 nanojunctions

FeS2 QDs/CdS thin

film

220 Dav= 20 nm

Solution

based

2.9

UV-Vis-NIR 0.85 mA

2.27 mA

bais 1V

170

ZnO nanorods

(multiple) 70

Dav = 100 nm

Lav = 1 µm hydrothermal

8

UV-254 nm -

1 µA

2 V bais

171

Reduced grapheme

oxide(rGO)

incorporated ZnO

thin films

623 K Tav = 106 nm dip-coating

2.5

8W Philips

black-light

UV-A lamp

- 20 µA

2 V bais

172

ZnO Nanowires

(multiple) 800

Dav = 128.9 nm

Lav = 39.3 µm CVD (VLS)

3.05

UV-365 nm 0.33 µA

4.6 µA

10 V bais Present work

ZnO nanorods

(multiple) 900

Dav = 160 nm

Lav = 2.4 µm CVD (VLS)

1.96

UV-365 nm 0.2 µA

9.4 µA


ZnO tapered

nanobelts

(multiple)

1000

Tav = 128.3 nm

Lav = 21.0 µm

Wav = 2.9 µm

CVD (VLS) 1.45

UV-365 nm 6.5 µA

216.4 µA


61

TEM image showed the structure’s crystallinity. XRD peaks of nanowires and

nanorods indicated crystalline structures grown along c-axis. The UV sensor

fabricated from multiple networks of the ZnO nanostructures showed very quick

sensing response and fast response (0.7 s) and recovery (0.5 s) times to UV light at

room temperature. The photo response of the developed sensors was found to be 1.4

to 3.0 with photocurrent ranging from 5 µA to 216 µA. UV response and recovery

times were relatively fast compared to the reported sensors. These significant changes

suggested that ZnO nanostructures could be used in gas sensing and optical switching

applications.

62

Chapter 5

5 Effects of Mg-doping on optical and CO gas sensing

properties of sensitive ZnO nanostructures

63

5.1 Introduction

1-D oxide semiconductor nanocrystals are considered to be highly efficient

source for gas sensing due to their high sensitivity to surface chemical reaction, rapid

response and high surface/volume ratios as compared to the thin film gas sensors.

Synthesis of 1-D oxide semiconductor nanostructures with different size and

morphologies are important for research and development of the novel device.15;174; 175

Different techniques have been used to improve sensitivity, stability and reaction

speed.176;177

However, there has been remaining a challenge to increase the detection

limit and sensing response of I-D ZnO nanobelts. Many techniques such as doping178

,

fabrication of hetrostructure179

,

systematic controls of morphologies180

and

functionalization181-183

have been used to enhance the sensitivity, stability, response

and recovery speed of gas sensors using 1-D nanostructure. Different researchers have

used doped ZnO nanostructures for the detection of CO by using different techniques.

For example Gaspera et al. showed that the doping of ZnO by transition metal ions

lowers the CO detection limit by 1-2 ppm at 300°C. Mg-doped ZnO has been studied

intensively due to its bandgap engineering effect.82

One of the important features of

ZnO is that doping with Mg increases the bandgap of ZnO.89

Changhyun et al.

demonstrated that Mg doped ZnO nanowire sensors showed maximum response of

1.04 (4.65%) for 100 ppm of CO gas at 100°C.85

Although Mg doped ZnO

nanostructures have been used for numerous applications, to the best of our

knowledge, there are only a few reports on the synthesis of Mg doped ZnO

nanostructures,184;185

but no reports on very thin and transparent Mg doped ZnO

nanobelts using magnesium acetate in the source material via vapor transport method

and their applications as CO gas sensors with enhanced sensing response.

In this chapter, successful synthesis of very thin and transparent Mg doped ZnO

nanobelts for enhanced CO gas sensing application has been reported. The effects of

Mg-doping on the optical, structural, morphological and gas sensing properties of

ZnO nanobelts are also presented.

64

5.2 Experimental details

5.2.1 Synthesis of Mg-doped ZnO nanobelts

Vapor transport method has been used for the synthesis of undoped and Mg-

doped ZnO nanostructures as shown in Figure 5.1(a). Two experiments were

performed with same conditions for both undoped and doped ZnO. Ion sputtering

technique was used for deposition of a thin catalyst layer (1 nm) of Gold (Au) on Si

(100) substrates. Source material containing a mixture of 99.99% pure ZnO (0.5 g)

and 99.9% Graphite powder (0.5 g) for undoped and a mixture of 0.5 g ZnO

(99.99%), 0.5 g of graphite (99.99%) powder and 0.1 g of magnesium acetate [Mg

(CH3COO)2 .4H2O] for doping of Mg was put in a ceramic boat at the center of quartz

tube (length 100 cm and diameter 3.5 cm). Substrates were placed on a second

ceramic boat at the downstream away from the source material in the quartz tube

which was then mounted inside the tube furnace. The temperature of the furnace was

maintained at 900 °C for 60 min in Argon (Ar) atmosphere with a constant flow rate

of 50 sccm in the two experiments. Furnace was cooled to room temperature after the

reaction. The collected nanostructure samples were characterized for crystallinity,

morphology elemental composition and optical properties. CO gas sensing

characterizations were carried out by measuring respective resistances by two probe

method using a multimeter (Keithly 2100).

65

5.2.2 Gas sensor fabrication and characterization

The CO gas sensors were fabricated by depositing a thick layer (200 nm) of

SiO2 thermally on Si (100) wafers. A slurry droplet (20 µL) of Mg-doped and un-

doped ZnO nanobelts was dropped on a pair of interdigitated electrodes on Si

substrates having a gap of 50 µm through photolithography technique. These sensors

were heat treated in air for 4 hours at 400°C before performing the gas sensing

experiments. The sensing experiment was performed at 350°C with 5 min cycles of

dry air and 20 ppm CO gas. The sensing response (S=Ra/Rg) of the device was

measured by resistance change upon exposure to air (Ra) and CO gas (Rg) in self-

designed gas chamber connected with Keithly multimeter, gas flow meters and tube

furnace. The experimental setup and gas sensing characterization setup have been

shown in Figure 5.1 (b) & (c). The inset of the Figure 5.1 (c) shows the sensor holder

with pressure contacts. Optical fluorescent image of nanobelts sensor and microscopic

image of interdigitated Au electrodes are shown in Figure 5.1(d) & (e).

Figure 5.1: Schematic diagram of the experimental setup (a) Synthesis of ZnO

nanostructures (b) CO gas sensing setup (c) Actual Photograph of gas sensing setup

Inset shows the sensor and holder (d) Optical fluorescent image of the nanobelt sensor

(e) Microscopic image of interdigitated Au electrodes

66


5.3.1 Morphology and microstructure analysis

The morphology of the synthesized undoped and Mg-doped ZnO nanostructures was

investigated by using FESEM, TEM and high resolution TEM. Figure 5.2 (a) shows

undoped ZnO nanorods with uniform diameter and length having average diameter of

168±30 nm and average length of 2.4±1.0 µm. Inset Figure 5.2 (a) shows the

magnified image of the un-doped ZnO nanorods. Figure 5.2 (e) shows corresponding

EDX analysis of the undoped ZnO nanorods, clearly showing the Zn and O peaks.

The approximate atomic ratios found to be 67: 33. Figure 5.2 (b) shows FESEM

image of Mg-doped ZnO nanobelts. Inset Figure 5.2 (b) shows the magnified image

of Mg-doped ZnO nanobelts. The general morphology is identical, i.e. transparent,

smooth edges, uniform width along the entire length and rectangular cross section for

all the nanobelts. The average thickness, width and length of the nanobelts are 34±5.0

nm, 290±160 nm and 3.3±1.0 µm, respectively. The corresponding EDX analysis of

Mg-doped ZnO sample (Figure 5.2 (f)) shows the presence of oxygen, magnesium

and zinc in the ratios O: Mg: Zn was found to be 31 (O): 3 (Mg): 66 (Zn). The aspect

ratio of un-doped and Mg-doped ZnO nanostructures is found to be 14.3 and

8.5(W/T), respectively. The possible reason for the formation of thin and transparent

nanobelts is due to the morphology transition of ZnO from nanorods to nanobelts by

Mg doping because mixing/doping of specific element plays a major role in

modifying the dimensions of nanostructures.186;187

Growth rates and polar surfaces

can induce the asymmetric growth. Formation of nanobelts was explained as

continuous process of 1-D branching and subsequent 2-D interspace filling.188

polar

surfaces of wurtzite crystals of oxide semiconductors can induce asymmetric growth

which leads to the distinct nanostructures59

e.g., nanocombs, nanoburshes, needle like

belts/rods etc.

Figure 5.2 (c) & (d) show the high resolution TEM (HRTEM) images of

undoped and Mg-doped ZnO nanostructures. The inset TEM image of Figure 5.2 (c)

clearly shows the catalyst particles (Au) on the tips of the nanorods suggesting the

VLS growth. HRTEM image (Figure 5.2 (c)) shows the lattice fringes of undoped

crystalline nanostructures where as these fringes are blurred in the Mg-doped

nanobelts Figure 5.2 (d).

67

The corresponding SAED patterns are shown in inset Figure 5.2 (c) & (d)

these are consistent with the HRTEM observations. These results showed single

crystalline nature and confirmed the defect free growth of undoped ZnO nanorods.

The diffused nature of SEAD pattern of the doped sample showed the crystal defects.

J. Singh et al. reported similar results for the Magnesium doped ZnO nanowires.184

Figure 5.2: (a) FESEM image of undoped ZnO nanorods; inset shows a magnified

image (b) FESEM image of Mg doped transparent ZnO nanobelts; inset shows a

magnified image (c) & (d) show HRTEM images of undoped nanorods and Mg doped

nanobelts, respectively; insets of (c) & (d) show magnified TEM images and the

corresponding SAED images (e) & (f) show EDX analyses of the undoped ZnO

nanorods and the Mg doped ZnO nanobelts, respectively

68

5.3.2 XRD and Raman spectroscopic analysis of doped ZnO nanostructures

Figure 5.3 (a) shows XRD patterns of both undoped and the doped ZnO

nanostructures. All peaks match with the hexagonal phase of ZnO (ICDD 01-079-

0207). The measured lattice constants a = 3.26 Å and c = 5.23 Å are comparable to

the pure ZnO; a = 3.25 Å and c = 5.21 Å. Patterns showed the impurity free growth of

nanostructures. A slight shift in peak position of doped ZnO was observed as shown

in the inset of Figure 5.3 (a) which indicated that the Mg doping induced lattice strain

in ZnO nanobelts. The lattice parameter “c” calculated from the (002) plane of the

doped and the undoped samples were found to be 5.23 Å, and 5.18 Å, respectively.

The decrease (ca 0.044 Å/0.83%) of this lattice parameter indicated the lattice

compression along c-axis189

caused by the replacement of Zn by Mg which has a

smaller atomic radius (0.57 Å) as compared to Zn (0.60 Å).68;190

The peak shift

indicates the compression in the lattice structure and overall volume of the lattice is

reduced by Mg doping as well.

Raman spectroscopic analysis using a laser excitation wavelength 514 nm was

performed for analyses of the vibrational modes of the undoped and Mg-doped ZnO

nanostructures as presented in Figure 5.3 (b). A significant difference was observed

between these two spectra. The peaks at 338, 436 and 576 cm-1

were observed for

Figure 5.3: (a) X-ray diffraction analysis of undoped and Mg doped ZnO

nanostructures; The inset image clearly shows slight shifts of the peak positions for

the doped samples (b) Raman spectra of undoped ZnO nanorods and Mg doped ZnO

nanobelts

300 400 500 600 700

0

100

200

300

400

500

600

700

800

Un-doped

Mg-dopedIn

ten

sit

y (

a.u

.)

Raman shift (cm-1)

576.5435.5

338.8

520.1

553.2

(b)

30 40 50 60 70 80

0

200

400

600

800

1000

1200

1400

Inte

nsit

y (

a.u

.)

2Degree)

ZnO

Mg-ZnO

(10

0)

(00

2)

(10

1)

(10

2)

(11

0)

(10

3)

(11

2)

(201)

(004)

(200)

(202)

(a)

30 32 34 36 38

0

200

400

600

800

1000

1200

Un-doped

Mg-doped

(101)(0

02)

Inte

nsit

y (

a.u

.)

2Degree)

(100)

69

undoped ZnO nanorods. The peak at 338 cm-1

is assigned to A1 symmetry mode, the

peak at 436 cm-1

is attributed to non-polar optical phonons E2 (high) mode of ZnO

while the peak at 579 corresponds to E1 (LO) mode which arises due to the defects

formation such as oxygen vacancies.191

Mg-doped ZnO nanobelts showed a shift in

the signals at 335 cm-1

and 438 cm-1

as compared to the pure ZnO. The peak at 438

cm-1

is blue shifted (2.36 cm-1

) with weak intensity and broadened for the Mg-doped

ZnO. Generally, Raman peak shifts due to three reasons: phonon confinement

effects192

, lattice strain193

and the oxygen vacancies.194

The broadening of the peak E2

(high) mode can be assigned to residual stress and size effects in the Mg doped

nanobelts.195

Therefore, in our case the red shift in the Raman active E1 (LO) and E2

(high) modes of the Mg-doped ZnO nanobelts are strong evidence for successful Mg-

doping in ZnO nanobelts.

5.3.3 Photoluminescence spectroscopy

Figure 5.4 (a) & (b) show PL spectrum at room temperature with Ar laser light

of 488 nm. Undoped and doped ZnO show broad green emission bands centered at

527 nm and 530 nm, respectively. The PL results clearly show the shift in Gaussian

fittings at respective bands and presence of new bands at 598 nm and 647 nm for Mg-

doped ZnO nanobelts.

Figure 5.4: Room temperature PL spectra of (a) undoped ZnO nanorods deconvoluted

into three Gaussian peaks, and (b) Mg doped ZnO nanobelts fitted with four

Gaussian peaks

500 525 550 575 600 625 650 675 700

300

400

500

600

700

800

900

2.22 eV

PL

In

ten

sit

y (

a.u

.)

Wavelength (nm)

527.37 nm 558 nm

(a)

2.35 eV

500 525 550 575 600 625 650 675 700

500

1000

1500

2000

2500

3000

2.3 eV,V+o

2.2 eV, Vo

2.0 eV,V++

o

598 nm

567 nm

530 nm

648 nm

(b)

Wavelength (nm)

PL

In

ten

sit

y (

a.u

.)

1.9 eV

70

Green emission is generally attributed to various intrinsic defects such as

oxygen vacancies and Zn interstitial produced during synthesis of ZnO

nanostructures.159

The presence of surface defects such as oxygen vacancies (Vo), Zn

vacancies (VZn), interstitial Zinc (Zni) and anticite defects (O Zn) have been reported

by different researchers corresponding to the green emission of ZnO.138-140

In our experiment, an interesting phenomenon was observed that the intensity

of the visible emission band in Mg-doped ZnO nanobelts is much stronger than the

un-doped ZnO nanorods. This may be attributed to the increase in concentration of

oxygen vacancies and surface recombination effects on the thin nanobelts

system.196;197

5.3.4 Diffused reflectance spectroscopy

The optical properties of undoped ZnO and Mg-doped ZnO nanobelts have

been investigated by UV-visible diffused reflectance spectroscopy. Figure 5.5 (a)

shows the plots of [F(R)]2 vs. photon energy of synthesized nanostructures, where

F(R) is the Kubelka-Munk function which is given by the relation.161

Where R, α, and S are the diffuse reflection, absorption and scattering coefficient,

respectively. The bandgap can be defined by extra- ploting the linear part of the plots

to the photon energy axis.143

The optical bandgap has been calculated and found to increase from 3.19 eV

for undoped ZnO nanorods to 3.31 eV for Mg-doped ZnO nanobelts, because MgO

has a wider bandgap than ZnO.198

Hsu et al. reported that doping with Mg increases

the bandgap of ZnO.89;199

This increase in bandgap might be due to increase in carrier

concentration that blocks the lowest states in the conduction band, this effect is known

as Burstein-Moss effect.143

The schematic diagram of the transition mechanism from

defects states is shown in Figure 5.5 (b). A blue shift in exitonic transition energy was

observed due to Mg doping in ZnO. Cetin et al. reported this blue shift was due to

replacement of Zn+2

ions by Mg+2

ions during the formation of the ZnO: Mg

nanobelts.200

Optical properties of the synthesized ZnO nanostructures are in

agreement with XRD and EDX results.

)1.5(2

)1()(

2

SR

RRF

71

5.3.5 UV sensing

ZnO nanostructures are good materials for ultraviolet light sensors due to its

wide bandgap (UV range for ZnO is about 368-390 nm).201

Figure 5.5 (c) shows the

room temperature UV sensing results of the Mg-doped and undoped ZnO

nanostructures. Sensing response of the Mg-doped ZnO nanobelts and undoped

nanorods was found to be 2.30 and 1.12, respectively. Enhanced (two times) sensing

response was observed for the Mg-doped ZnO nanobelts. The enhanced response was

attributed to energy levels introduced by the dopant into doped ZnO belts. These

doping states increased the excitation probability of an electron to the conduction

band.202

Figure 5.5: Plots of F2(R) vs. energy (eV) for (a) undoped and Mg doped ZnO

nanorods and ZnO nanobelts (b) Schematic representation of the energy transition

mechanism in the undoped and Mg doped ZnO nanostructures (c) UV sensing

responses of undoped and Mg doped ZnO nanostructures (d) Photocurrent response of

doped ZnO nanostructures under UV and dark conditions

2.8 2.9 3.0 3.1 3.2 3.3 3.4 3.5

0.0

0.2

0.4

0.6

0.8

1.0

1.2

Eg=3.31

F

2(R

)

Energy(eV)

Undoped ZnO nanorods

Mg-doped ZnO nanobelts

Eg=3.19

(a)

-10 -5 0 5 10-80µ

-60µ

-40µ

-20µ

0

20µ

40µ

60µ

80µ

Voltage (V)

Cu

rre

nt

(A)

UV ON

UV OFF

75 µA

(d) Mg doped ZnO

0 200 400 600 800

6x105

9x105

1x106

2x106

2x106

Re

sis

tan

ce

()

UV OFF

Time (sec)

Mg-doped ZnO

un-doped ZnO

Response=2.30Response=1.12

UV (365 nm) Sensing

UV ON

72

The optoelectronic properties of the undoped and doped ZnO nanostructures

were investigated by two terminal I-V measurement method using Keithly 4200 meter

with a bias voltage from -10 to +10 V with and without UV illumination. Figure

5.5(d) shows that current under UV light increases 75 µA for Mg-doped nanobelts as

compared to the undoped ZnO.

5.3.6 Characterization of CO gas sensors

Figure 5.6 (a) & (b) show the sensing response signals of the doped and

undoped ZnO nanostructures. Previous reports showed that sensitivity of the resistive

sensors is highly affected by the operating temperature. Therefore, in order to

optimize the operating temperature of Mg-doped ZnO nanobelts, sensors were tested

at temperatures range of 200°C to 400°C for 20 ppm of CO gas as shown in Figure

5.6 (d). It was observed that sensing response increases with increase in working

temperature and reached a maximum at about 350°C and then started decreasing. This

behavior may be interpreted on the basis of adsorption/desorption and reaction

processes taking place on the surface of sensing layer.203

The sensing experiment was

performed at 350°C with 5 min cycles of dry air and 20 ppm CO gas. When CO gas

was passed through the sensor resistance was decreased and it was recovered to initial

value on removal.

Sensing response of the undoped nanorods and Mg-doped ZnO nanobelts was

found to be 1.05 and 5.5, respectively. Enhanced (five times) sensing response was

observed for the Mg-doped ZnO nanobelts. Response and recovery times of both

sensors were nearly equal as shown in Figure 5.6 (a) & (b). Large amount of oxygen

is adsorbed on doped ZnO nanobelts due to larger surface area Abelts~5Arods (greater

number of defects) due to which interaction chance of CO gas increases and fast

chemisorption/desorption properties of the CO gas at 350ºC as compared to undoped

ZnO nanostructures.204;205

N. Hongsith et al. reported that the sensor response is

proportional to the reaction rate constants kco (T) and koxy (T), through oxygen density

which is given by the following equation.206

S = Ra/Rg = (ᴦt kco (T) koxy (T) [Oion

ads] b

[CO] b)/n0+ 1 → (5.2)

73

Where Ra = air resistance, Rg = gas resistance, ᴦt = time constant, [Oion

ads] b

=

chemisorbed Oxygen concentration, [CO] b

= CO concentration, b = charge parameter

and n0= carrier concentration in air. Our results are consistent with this relation due to

oxygen concentration for larger area of the nanobelts.

5.3.7 Sensing mechanism -Band theory model

Band theory was applied to explain the sensing mechanism (shown in Figure

5.6 (c)), of the gas sensors.97;207

The sensing mechanism is based on the principle of

change in electrical resistivity /conductivity as a result of chemical reaction between

gas molecules and the reactive oxygen ions on the surface of ZnO nanostructures. The

change in electrical conductance is given by equation (5.3).208

200 250 300 350 400

1

2

3

4

5

6

(d) CO gas 20 ppm

Re

sp

on

se

( R

a/R

g)

Temperature (C )

0 500 1000 1500 2000840k

855k

870k

885k

900k

915k

20 ppm of CO gas at 350°C

R (

)

Time (sec)

Response = Ra/Rg= 1.05

CO

Air

(a)Undoped

0 500 1000 1500 2000

70k

140k

210k

280k

350k

420k (b)Mg-doped ZnO

20 ppm of CO gas at 350°C

CO

Air

R (

)

Time (sec)

Response = Ra/Rg= 5.41

Figure 5.6: Signal response and recovery profiles of (a) undoped ZnO nanorods and

(b) Mg doped ZnO nanobelt (c) Schematic of a possible mechanism of how the Mg

doped ZnO nanobelt sensor responds to CO in air and in gas (d) The response-

temperature curve shows that the response increases until 350°C, and then decreases

74

Where “l” is the length of nanostructure’s channel, “r” the radius, and “μ” is electron

mobility “e” is electron charge and ∆n◦ represents the change in carrier concentration.

When the surface of the undoped and Mg-doped ZnO nanostructures is exposed to the

air, oxygen is ionosorbed by capturing an electron from conduction band of surface

sites of un-doped and Mg-doped ZnO nanobelts.83;209

At high temperature, reactive

oxygen species are chemisorbed by these ZnO nanostructures as a result electron

transfer takes place. As a result thick depletion layer is formed; a decrease in carrier

concentration takes place which results in increased resistance of the material. In

contrast, when undoped and Mg-doped ZnO is exposed to the CO gas chemical

reaction takes place as adsorbed oxygen ions produce CO2 which leads to increase in

carriers concentration and a decrease in resistance.210

The mechanism can be

summarized by the following chemical reactions.

O2 (ad) +e→ O2-(ad)

→ (5.4)

O2-(ad) + e → 2O

- (ad) → (5.5)

2CO+O2-(ad) +e→ 2CO2 (gas) + e

- → (5.6)

CO+2O-→ CO3

2-→ CO2+1/2O2+2e → (5.7)

Table 5.1 shows the systematic summary of the results. Morphology of the

synthesized ZnO Nanostructures changes by Magnesium doping. PL and Raman peak

positions are found to be shifted and new peaks were observed which confirms Mg

doping in ZnO, bandgap of ZnO nanostructures increases due to defects states as a

result of Mg-doping and hence the CO gas sensing response increases about five

times as compared to the undoped ZnO nanostructures.

)3.5(

12

l

ren

RG

o

75


5.4 Summary

Magnesium doped ZnO nanobelts were successfully synthesized using magnesium

acetate as source material by vapor transport method. A clear change was observed in

the morphology from ZnO (nanorods) to Mg-doped ZnO (nanobelts). Shifts in the

XRD peaks in Mg-doped samples as compared to ZnO were an indication that Mg has

Table 5.2: Comparison of the CO gas sensor properties of 1-D ZnO nanostructures

with reported semiconductor nanostructures based gas sensors

Morphology of

nanostructures

Synthesis

Temp.

(°C)

Bandgap

by UV-

VIS (eV)

Nanostructure

dimensions

Aspect

ratio

20 ppm of

CO gas

Response

at 350°C

PL peak

positions

(nm)

Raman

peak

positions

(cm-1

)

Nanorods

(undoped) 900 3.19

Dav= 168 nm

Lav = 2.4 µm 14.3 1.05±0.01

527.4

558.1

338.8

435.5

576.5

Nanobelts

(Mg-doped) 900 3.31

Tav = 34 nm

Lav = 3.25 µm

Wav= 290 nm

8.5

(W/T) 5.41±0.05

530.2

567.2

598.2

647.7

335.6

437.7

553.2

Sensor description CO

Concentration

Sensing

Response T (ºC) References

ZnO Mg doped nanobelts

(CVD) 20 ppm Ra/Rg ~ 5.41 350 Present work

ZnO/MWCNTs hybrid

nanostructures (CVD) 20 ppm Ra/Rg ~ 5.04 350 Present work

ZnO Mg doped nanobelts

(CVD) 20 ppm Ra/Rg ~ 1.9 200 Present work

ZnO nanorods

(Hydrothermal) 30 ppm Ra/Rg ~ 1.1 400

211

ZnO nanowires Mg doped

(thermal evaporation) 100 ppm Ra/Rg ~ 1.04 100

85

ZnO nanoparticles (sol-gel) 100 ppm Ra/Rg ~ 1.5 350 212

ZnO thin film (MOCVD) 4000 ppm Ra/Rg ~ 1.7 100 213

MWNT/SnO2 hybrid

nanostructures 100 ppm Ra/Rg ~ 2.0 300

214

76

substituted Zn in the ZnO lattice. The doping was also confirmed by EDX,

photoluminescence and Raman spectroscopy. The gas sensor fabricated from multiple

networks of the Mg-doped ZnO nanobelts showed enhanced signal responses to 20

ppm of CO gas at 350°C. The sensitivity of the Mg-doped ZnO nanobelts sensor was

improved five times in comparison to undoped ZnO nanorods sensor. The enhanced

signal response was attributed to the catalytic effect of dopant, defects, like oxygen

vacancies (potential barrier modification) and fast chemisorption and desorption

properties of the CO gas. Band theory model was adopted to explain the possible

sensing mechanism of the Mg-doped ZnO nanostructures.

77

Chapter 6

6 ZnO on MWCNTs, platform for CO gas sensing

78

6.1 Introduction

Carbon monoxide (CO) is highly toxic and one of the most dangerous

component for air pollution. It is a tasteless, odorless and colorless gas, so it is

undetectable to humans.108;215

High concentrations of CO gas can cause dizziness and

confusion and ultimately death and the occupants of a home may not be able to

observe the change.97;216

Fabrication of sensitive gas sensors is essential to prevent

death by inhalation of CO gas.

ZnO nanostructures are considered to be the potential candidates for CO gas

sensors due to the high electron mobility and large surface area.176;183

Different

technical and scientific contributions such as surface functionalization, fabrication of

heterostructures and doping have been used for the improvement in sensitivity,

reaction speed, response/recovery times and thermal stability of nanostructure based

sensors.181;217;218

Carbon Nanotubes are well known candidates for electronics,

photonics and sensing applications due to their superior electronic properties.219-221

Many techniques have been used to synthesize CNTs-ZnO hybrids that have good

optical and photocatalytic applications.222-225

These techniques require modification

with organic functional groups to attach inorganic nanomaterials to the surface of

CNTs via covalent or electrostatic interactions. Several groups have investigated the

direct growth of ZnO nanoparticles and nanowires on CNTs using different

methods.226-228

Park et al. and Kim et al. demonstrated the catalyst free ZnO

nanostructures on MWCNTs films via thermal CVD.94;95

Direct growth of ZnO

nanowires on CNTs promotes carbon doping that is responsible for P-type behavior of

ZnO.229

Doping is considered to be an effective method in order to improve low

temperature gas sensing of ZnO nanostructure. Doping of carbon (C), nitrogen (N) or

sulfur (S) would reduce the bandgap of metal oxide as reported by Ahn et al.199

The

dopant materials into ZnO crystal lattice produce defects in the structure which are

beneficial for gas sensing properties.82

Zhao et al. showed three times enhanced

sensing response to 100 ppm CO gas at 300°C using a hybrid nanomaterial of SnO2

and MWCNTs.214

Van et al. used SnO2 functionalized ZnO nanowires for CO gas

sensing with response of 4.6 at 350°C for 300 ppm.230

In our previous paper we

showed enhanced signal response to 20 ppm of CO gas at 350°C using Mg-doped

ZnO nanobelts.231

79

The role of dopant however, is not clear with discussion and presumption based on

defects, surface area, crystal structure and active sensing component by doping.

Crystal defects, particularly oxygen vacancies etc., in ZnO play a role as adsorbed site

on the surface nanostructures for gas molecules.86-88

Recently carbon doped ZnO

nanostructures via carbon fibers have been reported which were mainly focused on

visible light photo-response90;91;232

, magnetic93

and p-type conduction properties by

vapor phase techniques.92

In this chapter, the synthesis of carbon (MWCNTs) doped ZnO nanostructures

along with their characterization is presented using a simple thermal transport method.

Various applications of Carbon doped ZnO nanostructures have been reported but

still, to the best of our knowledge, carbon doped ZnO nanostructures synthesized

using MWCNT’s by vapor transport method for CO gas sensing have not been

reported. Defects produced due to incorporation of carbon into ZnO matrix and their

effects on sensing properties have been discussed. Pure and carbon doped ZnO

nanostructures were tested as sensors for CO gas and their performance is presented.

The results show five times enhancement in sensitivity for 20 ppm of CO gas at

350°C compared with pure ZnO nanostructures.

6.2 Experiment

6.2.1 Growth of undoped and doped ZnO nanostructures

ZnO nanostructures were synthesized by the vapor transport method as shown

in Figure 6.1(a). MWCNTs (diameter 25-60 nm and length 10-20 µm) were

purchased from Beijing DK nanotechnology (China) and used without further

treatment for carbon doping. MWCNTs have a purity of about 95% and contain

amorphous carbon less than 4% and metal impurities about 0.5%. A uniform

suspension was prepared by mixing 0.02 g of MWCNTs in 50 mL methanol and

sonicated for 30 min. The MWCNTs thick film was transferred on gold coated Si

(100) substrate via a drop coating process. A thin layer (1 nm) of gold (Au) as catalyst

was deposited on Si (100) by ion sputtering for undoped sample. The synthesis

process is similar to our previous research work.180

The process is briefly described as

follows. Two experiments were performed under the same conditions i.e. for undoped

and doped ZnO nanostructures. Source material containing equal amount of ZnO

powder (ZnO + C, 99.99% pure) was ball milled for 2 hours and this mixture was

80

loaded in a ceramic boat. Gold and MWCNTs coated substrates were placed

downstream from the source material. The synthesis experiments were performed at

900°C for 60 min with a constant Argon (Ar) flow of 50 sccm. During the

experiment, C dissociated from the surface of MWCNTs due to etching and peeling of

the outer layers.233

Carbon atoms from the surface of MWCNTs are diffused into the

ZnO nanostructures by an oxidation process of CNTs.

6.2.2 Characterizations of carbon doped ZnO nanostructures

After the reaction sample was taken out from furnace, a white layer was found

on the surface of CNTs. The crystallinity, chemical composition, morphology, and

optical properties of the synthesized ZnO nanostructures were determined by using p-

XRD, X’Pert PRO Difractometer, PANalytical with Cu Kα radiation λ = 1.5418 Å),

X-ray photoelectron microscopy (XPS) equipped with a custom system having an Al

Kα source (1486.6 eV) and a hemispherical analyser at pressures of 1 x 10-9

mbar and

EDX attached with scanning electron microscopy (SEM, Hitachi SU 1500), HRTEM,

Technai G20), UV-Vis diffused reflectance spectroscopy (DRS, lambda 950 UV-VIS

spectrophotometer), PL, DONGWOO Optron, 488 nm Ar laser) and Raman

spectroscopy (Renishaw, 514 nm laser) techniques.

6.2.3 Gas sensors fabrication and characterizations

The CO gas sensors were fabricated by depositing 50 nm thick gold electrodes

(interdigitated) onto the silicon wafers by using ion sputtering deposition (JFC 1500)

and photolithography techniques. For comparison pure and MWCNTs/ZnO

nanostructures were characterized under the same condition. Nanostructures were

attached between the electrodes through the doctor blade method. Thick film sensors

were heat treated in air for 4 hours at 400°C before performing the gas sensing

experiments. CO gas sensing characterizations were carried out by measuring

respective resistances by two probe method using a multimeter (Keithly 2100). The

sensing experiment was performed at 350°C with 5 min cycles of dry air and 20 ppm

CO gas. The sensitivity (S = Ra/Rg) of sensor was determined by change in resistance

upon exposure to air (Ra) and CO gas (Rg) in a home-built gas chamber connected to a

Keithly multimeter, gas flow meters and tube furnace. The schematic design of sensor

and gas sensing characterizations setup has been shown in Figure 6.1(b) & (c). Figure

81

6.1(d) represents the actual image of gas sensing setup, Inset shows sensor with

pressure contacts.


6.3.1 Morphological analysis

Figure 6.2(a) & (b) show magnified SEM micrographs both undoped and

doped ZnO synthesized by vapor transport method using MWCNTs. In Figure 6.2(a)

tapered belts grew on the large area of Si substrate. Average thickness of the undoped

tapered belts was found to be 400 nm, length 15.7 µm and the average width was

found to be 2.87 µm (at bottom) and 420 nm (at tips). Doped ZnO nanostructures

have tapered belts along with pedal-like structures as shown in Figure 6.2(b). Insets of

Figure 6.2(b) show schematic drawing which reveals the position of MWCNTs/ZnO

nanostructures and the MWCNTs used for carbon doping. The average thickness of

the pedal-like structures were estimated 200 nm, average length as 6.9±2.0 µm,

average width found to be 614±160 nm and average diameter of the pedal arm

Figure 6.1: (a) Schematic diagram of experimental setup for synthesis of ZnO

nanostructure (b) Schematic design of fabricated ZnO/MWCNTs sensor (c)

Schematics of CO gas sensing setup (d) Actual photograph of gas sensing setup, Inset

shows sensor with pressure contacts

82

estimated as 203±64 nm. These results show that morphology changed from tapered

belts to pedal-like structures by carbon doping. The reason for the variation in the

morphology of ZnO is due to various factors, e.g., surface diffusion of catalyst

droplet, variation in interface surface energy, catalyst-substrate coverage and presence

of dopants etc. Dopants introduce a distorsion in lattice and growth is guided into a

specific crystallographic dimension.186;187

The chemical compositions of synthesized ZnO nanostructures were analyzed

by EDX. Figure (c) & (d) show EDX spectra for the undoped and doped ZnO

nanostructures. The EDX quantitative analysis confirmed the concentration of carbon

in ZnO. The approximate atomic ratios were found to be 25.69/58.83/15.48 for

oxygen, zinc and carbon, respectively. The carbon peak in the doped sample and

difference in the relative intensities also indicates the carbon doping.

TEM investigations give further insight into the morphology and the structural

features of the doped ZnO nanostructures. TEM analysis of doped sample after

Figure 6.2: (a) SEM image of undoped ZnO tapered belts (b) SEM image of doped

ZnO pedal-like structures, inset shows MWCNTs used for carbon doped ZnO and

schematic drawing for position of MWCNTs and doped ZnO nanostructures (c) & (d)

EDX analysis of undoped and doped ZnO nanostructures, respectively

83

sonication reveals partial presence of MWCNTs in ZnO as shown in Figure 6.3 (a). A

high resolution TEM image in the inset clearly revealed the strain layers produced due

to carbon doping. Average spacing between the strained layers was found to be 4 nm

and the average thickness of the layer is about 20 nm. The corresponding EDS data

(shown in inset of the Figure 6.3(b)) for carbon doped ZnO peddle-like structure

confirms the presence of Zn, O and C elements.

Figure 6.3 (c) & (d) shows high resolution TEM images of undoped and

Carbon doped ZnO nanostructures. HRTEM image (Figure 6.3 (c)) shows the lattice

fringes of undoped crystalline nanostructures where as these fringes are blurred in the

doped peddle-like structures (Figure 6.3 (d)). The corresponding SAED patterns as

shown in the inset of Figure 6.3 (c) & (d) are also consistent with the HRTEM

observations.

Figure 6.3: (a) TEM image of doped sample after sonication shows partial presence of

MWCNTs (b) TEM images of doped pedal-like structures, Inset shows the magnified

TEM image with corresponding EDS spectrum (c) and (d) corresponding HRTEM and

inset SEAD images of undoped and doped ZnO nanostructures, respectively

84

6.3.2 Structural analysis

Figure 6.4(a) shows typical p-XRD patterns of undoped and doped ZnO

nanostructures. All the peaks match with the hexagonal phase of ZnO (ICDD PDF-2

entry 01-076-0704). The calculated lattice parameters (using 1/d2hkl= 4/3

((h2+hk+k

2)/a

2) + l

2/c

2) were found to be a = 3.25 Å and c = 5.21Å are comparable to

the pure ZnO (a = 3.25 Å and c = 5.21Å). The lattice parameter “c” calculated for the

(002) peaks of the doped sample was 5.20 Å. A slight peak shift (0.12° for (002)

plane) was observed as compared with the undoped nanostructures as shown in Figure

6.4(b). The decrease (about 0.02 Å) of the lattice parameter “c” and the peak shift

were due to stresses produced by oxygen deficiency (defects). These changes were

expected with carbon doping as carbon atom is incorporated into the oxygen sites of

ZnO lattice.234

Herng et al. 93

reported similar results for the carbon doping in ZnO

nanoneedles in which the (002) peak, shift to a higher angle by 0.12° when comparing

with the undoped ZnO nanoneedles. Zhang et al. also reported the slight shift of 0.18°

for (002) peak due to Ni doping in ZnO nanostructures235

. Inset of Figure 6.4(b)

shows the XRD pattern of doped sample after sonication for TEM this graph clearly

depicts the peak C (002) at angle 26.4° revealing the presence of MWCNTs into ZnO

nanostructures. An interesting phenomenon was observed which proves the carbon

doping in ZnO pedal-like nanostructures. A small shoulder peak was observed at

34.02° which matches with (111) plane of ZnC2O4 (Zinc Oxalate matched with

JCPDS card No. 00-037-0718) having monoclinic crystal structure. This means that

doped carbon atoms react with ZnO and makes a compound ZnC2O4. Thus one can

conclude that the carbon doping induces lattice strain in the doped sample and

structure consists of strained and un-strained constituents. So in doped sample, the

presence of (111) monoclinic peak along with (002) wurtizite peak indicates the

coexistence of two phases.

85

6.3.3 X-Ray photoelectron spectroscopy (XPS)

XPS measurements were taken on a custom system with an Al Kα source

(1486.6 eV) and a hemispherical analyser at pressures of 1 x 10-9

mbar. Peak fitting

was carried out with CASA-XPS where Shirley backgrounds were applied throughout

and peaks were fitted with a 70:30 Gaussian: Lorentzian lineshape.

Figure 6.5 shows the XPS spectrum of undoped and carbon doped ZnO.

Analysis of carbon-doped ZnO is problematic due to surface contamination with

carbon and oxygen containing species that occurs for all XPS analysis when the

sample is not cleaned in vacuum. Before analysis, the samples were UV-ozone

cleaned in an attempt to remove surface contaminants. Figure 6.5 (a) shows the Zn

2p3/2 and 2p1/2 spectrum consisting of the 3/2 and 1/2 components at 1022.7 eV and

1045.9 eV with a splitting of 23.2 eV associated to the Zn2+

species in ZnO wurtizite

structure236;237

. There is a slight shift as compared to the doped sample which

indicates a change in chemical binding of Zn. O 1s spectrum can be deconvoluted by

using symmetric Gaussian curves (Figure 6.5 (b)) showing two peaks at 530.8 eV and

533.0 eV due to the binding energies of oxygen atom in the C-doped ZnO. The peak

at 530.8 eV is assigned to hexagonal wurtizite crystal structure of Zn2+

ion of Zn-O

bounding.238

A strong O 1s peak at 533.0 eV is believed to be due to the O-C-O

complex. A slight shift (0.3 eV) in O 1s of the doped sample to the higher energy as

Figure 6.4: X-ray diffraction patterns of (a) undoped and doped ZnO nanostructures

(b) XRD pattern of the main peaks clearly shows the peak shift; inset shows the XRD

pattern of doped sample after sonication which clearly shows the carbon peak

30 31 32 33 34 35 36 370

50

100

150

200

250

300

350

400

450

Zn

C2O

4 (1

11

)

(b)

Zn

O (

101)

Zn

O (

100)

Inte

ns

ity

(a

.u.)

2(Degree)

Undoped ZnO

C-doped ZnO

Zn

O (

00

2)

30 35 40 45 50 55 60

0

50

100

150

200

250

300

350

Zn

C2O

4 (111)

Zn

O (

11

0)

SiO

2 (-2

51

)

Zn

O (

10

2)

Zn

O (

10

1)

Zn

O (

00

2)

Zn

O (

10

0)

Inte

ns

ity

(a

.u.)

2(Degree)

Undoped ZnO

C-doped ZnO

(a)

26 28 30 32 34 36 38

ZnO + MWCNTs

C (

002)

Zn

O (

100)

Zn

O (

101)

Zn

O (

002)

86

compared to the undoped sample suggests the O vacancies produced due carbon

doping. Figure 6.5 (c) shows the C 1s spectra. Three peaks can be fit to the C 1s

spectra for both the doped and undoped ZnO films. The undoped sample should be

free of intrinsic carbon and the C species are all related to surface contamination.

There are three peaks at 284.5, 286.3 and 288.6 eV in the ratio of 70:20:9. In the

carbon-doped sample the spectrum is almost identical, with peaks at 284.5, 286.1 and

288.6 eV in the ratio of 73:20:7. Due to the nature of the peaks, their obvious

similarity and the noise of the spectrum, to subtract the spectra to glean any

information regarding the nature of the carbon-doping would be futile.

6.3.4 Optical properties

6.3.4.1 Raman spectroscopy

Raman scattering was utilized to investigate the vibration characteristic of

ZnO nanostructures. Figure 6.6 (a) & (b) show the normalized Raman spectra for

undoped, doped ZnO nanostructures and MWCNTs. According to Group theory A1 +

E1 + 2E2 are the Raman active modes.239

The peaks of ZnO at 326.0, 380.0, 433.5 and

560.2 cm-1

were observed in smaller Raman shift region, on the other hand, the peaks

at 1110 and 1142 cm-1

were also found at larger Raman shift region. Peak at 326 cm-1

denoted A1 symmetry. Peak at 433.5 cm-1

shows high E2 modes which are due to non-

polar optical phonons of ZnO. Peak at 380.0 cm-1

is due to transverse optical mode

showing A1 symmetry. All other peaks have same meaning as reported in literature.240

Figure 6.5: XPS data for the Zn 2p, O 1s and C 1s spectra for undoped and doped

ZnO nanostructures

87

300 600 900 1200 1500 1800

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

(a)

Inte

ns

ity (

a.u

.)

Raman Shift (cm-1)

G

D

2E1

Si

2E2

A1

ZnO

ZnO+MWCNTS

MWCNTS

1000 1500 2000 2500 3000

0.0

0.2

0.4

0.6

0.8

1.0

2D

G

D

Raman Shift (cm-1)

Inte

ns

ity

(a

.u.)

MWCNTS(b)

The peak at E2 (LO) of the doped structures is broader than the same peak of the

undoped structures which is attributed to the residual stress and oxygen deficiency in

the Carbon doped ZnO nanostructures.241

On comparing with undoped ZnO, two new

peaks at 1352 and 1584 cm-1

were also observed in doped sample which are assigned

to the typical signatures of Graphite like materials, with the appearance of the D and

G modes, respectively.242

Raman spectroscopy of the doped ZnO showed the shift in D and G bands and

large full width half maximum (FWHM), which was due to the structural deformation

(stress) produced in the MWCNTs as the number of layers altered after the

processing208;233;243

and simultaneous oxidation process of the MWCNTs. A clear

Figure 6.6: (a) Raman scattering spectra of undoped, doped ZnO nanostructures and

MWCNTs, respectively clearly showing the normalized D and G bands peak shift (c)

Electrical I-V curves represent increased conductance of carbon doped sample

compare to undoped ZnO

-10 -5 0 5 10

-600n

-400n

-200n

0

200n

400n

600n

Cu

rren

t(A

)

Voltage (volts)

Carbon (MWCNTs) doped ZnO

Udoped ZnO

(c)

88

3.0 3.1 3.2 3.3 3.4 3.5

0

1x109

2x109

3x109

4x109

5x109

6x109

7x109

(b)

Eg = 3.23 eV

Carbon doped ZnO

F(R

)2

Energy (eV)

360 380 400 420 440 460 480 500

0

10

20

30

40

50

60

70

%R

Wavelength (nm)

Carbon doped ZnO

3.0 3.1 3.2 3.3 3.4 3.5

0

250

500

750

1000

Undoped ZnO nanobelts

F(R

)2

Energy (eV)

Eg = 3.27 eV

(a)

300 325 350 375 400 425 450 475 500

0

10

20

30

40

50

60

70

80

% (

R)

wavelength(nm)

undoped ZnO nanobelts

change in the two modes was observed in the two spectra obtained from the pristine

MWCNTs due to ZnO growth on MWCNTs as shown in Figure 6.6(a). Thus the

doping of carbon produced defects like oxygen vacancies in ZnO, which act as donors

of electrons for conduction band. This statement is justified by electrical properties as

shown in Figure 6.6(c). I-V curves represent the symmetric shape with increased

conductance due to CNTs in doped sample.

6.3.4.2 Diffused reflectance

Pure and doped ZnO nanostructures were characterized for optical properties

using UV-visible diffused reflectance and photoluminescence spectroscopy. Figure

6.7 (a) & (b) show the graphs of [F(R)]2 vs. energy, where F(R) is the Kubelka Munk

function as given by the following relation.161

X-intercept from the linear part of the graph gives bandgap energy of the

nanostructures. The optical bandgap of carbon doped ZnO nanostructures (3.23 eV)

was found to be less than the undoped ZnO nanostructures (3.27 eV), which might be

due to the defects introduced due to the carbon atoms in ZnO lattice.

6.3.4.3 Photoluminescence (PL) spectroscopy

Room temperature PL in the visible range was performed to determine the role of the

dopant catalysts. It reported that bandgap of ZnO lies in the UV range (3.37 eV);

Figure 6.7: Plots of F2(R) vs. Energy (eV) of (a) undoped ZnO tapered belts and (b)

C-doped ZnO pedal-like structures, the inset shows the respective reflectance spectra

)1.6(2

)1()(

2

SR

RRF

89

500 550 600 650 700220

240

260

280

300

PL

in

ten

sit

y (

a.u

.)

Wavelength (nm)

ZnO+Graphite powder

Carbon doped ZnO

Undoped ZnO

(c)

500 550 600 650

230

240

250

260

270

539

Wavelength (nm)

PL

In

ten

sit

y (

a.u

.) Undoped ZnO

Carbon doped ZnO533

(d)

however, its emission band was observed in visible range. Figure 6.8(a) & (b) show

PL spectra of both doped and undoped ZnO nanostructures. Broad green emission

bands centered at 533 and 539 nm were observed for undoped and doped ZnO,

respectively (Figure 6.6(d)). Green emission is generally attributed to the presence of

surface defects arises during the growth process of ZnO nanostructure.159

Green

emission is attributed to the presence of surface defects like oxygen vacancies (VO),

Zinc (Zni) interstitial, Zn vacancies (VZn) and anticite defects (O Zn) have been

reported.139

Doped ZnO showed broad and low intensity defect induced emission. The

peak at 539 nm (blue shifted as compared to the undoped ZnO) might be originated

from defects and oxygen vacancies (as carbon substitute oxygen into ZnO matrix). PL

results are consistent with XRD, XPS and Raman.

6.3.5 Carbon doped ZnO nanostructures application as CO gas sensor

Figure 6.7 (a) & (b) show the sensing response signals for the undoped and

carbon doped ZnO nanostructures. It was found that the sensing response of the

fabricated sensors was stable and reversible. It was observed that undoped and carbon

doped ZnO nanostructures detect 20 ppm of CO with sensing response (S = Ra /Rg) of

1.17 and 5.06, respectively. Enhanced (five times) sensing response was observed for

the carbon doped ZnO nanostructures. The resistance of the doped sample was

decreased as compared with the undoped sample as carbon (C) atoms substitute

oxygen (O2-

) in doped ZnO sample which give rise to oxygen vacancies and excess

Figure 6.8: (a) PL spectra of starting material, undoped and C- doped ZnO

nanostructures (b) Room temperature PL spectra of undoped ZnO tapered belts and

doped ZnO pedal-like structures

90

900 1000 1100 1200 1300 14000

15k

30k

45k

60k

75k

90k

CO OFF

R (

)

Time (sec)

CO ON

t response= 92 sec

t recovery= 140 sec

Doped ZnO (d)

1000 1100 1200 1300 1400 1500

690k

720k

750k

780k

810k

840k

t recovery= 137 sec

t response= 108 sec

CO OFF

R (

)

Time (sec)

CO ON Undoped ZnO (c)

0 500 1000 1500 2000 2500 3000690k

720k

750k

780k

810k

840kResponse = Ra/Rg= 1.17

CO

Air

R (

)

Time (sec)

(a)Undoped

0 500 1000 1500 2000 2500 3000

15k

30k

45k

60k

75k Response = Ra/Rg= 5.06

CO

Air

R (

)

Time (sec)

(b)

Doped

carriers due to carbon. This gas sensor showed improved performance and reduced

resistance due to superior conductivity of CNTs in MWCNTs/ZnO hybrid

material.210;244

Doped and undoped sensors show response and recovery times to be 108 and

140 seconds, respectively. The thermal stability of the sensors depends upon response

and recovery times which were nearly equal, as shown in Figure 6.7 (c) & (d). These

results show that the response speed and thermal stability of the fabricated sensors are

both good.219

Figure 6.9: (a) Signal response of undoped ZnO tapered belts (b) signal response of

C-doped ZnO pedal-like structures (c) Response and recovery times of undoped

tapered belts (d) Response and recovery times of C-doped ZnO nanostructures

91

6.3.5.1 Sensing mechanism: Band theory model

In order to explain the sensing mechanism, band theory was applied to the gas

sensors which have been the subject of intense study in literature.96

The sensing

mechanism is based on the principle of change in electrical resistivity /conductivity as

a result of chemical reaction between gas molecules and the reactive oxygen ions on

the surface of ZnO nanostructures. The change in electrical conductance is given by

Eq. No (6.2).207

Where l is length of the nanostructure channel, r the radius, μ electron mobility, e

electron charge and ∆n◦ represents the change in carrier concentrations. The gas

sensing response of the resistive sensor is given by Eq. No (6.3) .19

Where Ga (Ra) and Gg (Rg) are conductance (resistance) of carbon doped ZnO

nanostructures in air and gas ambient, VSa and VSg width of Potential barrier in air and

gas, respectively, λD is the Debye length. According to this relation, enhancement of

the CO gas sensing response can be attributed to three important factors namely,

geometry (4/D), electronic properties (εεo/eno) and band-bending (VSa1/2

- VSg1/2

) due

to adsorption. This is particularly done by doping and temperature modulation (Rg =

Ra exp (eVs/KbT)) which is desirable in our case.113

It is found that doped nanomaterial

is capable to adsorb large amount of oxygen due to greater number of defects.205

Doped ZnO nanostructures show high sensing response due to the catalytic effect of

dopant. When the surface of the carbon doped ZnO nanostructures is exposed to the

air, the adsorbed oxygen is ultimately ionized into active oxygen species (O2-, O

-2 and

O-) while moving from point to point by capturing electrons from active surface sites

of carbon doped ZnO.209

The mechanism can be summarized by the following

chemical reactions.

)2.6(

12

l

ren

RG

o

)3.6(

442

12

12

1

)()(

SgSa

o

ogDaD

g

gaVV

enDDG

GGS

92

O2 (gas) → O2 (ad) → (6.4)

O2 (ad) +e → O2-(ad) → (6.5)

O2-(ad) +e→ 2O

-(ad) → (6.6)

2CO+O2-(ad) → 2CO2 (gas) +e

- → (6.7)

CO+2O-→ CO3

2-→ CO2+1/2O2+2e

- → (6.8)

Table 1 shows a summary of the systematic results. Morphology of the

synthesized ZnO nanostructures changes by carbon doping. Energy bandgap of ZnO

nanostructures decreased due to defect states as a result of carbon doping and hence

the CO gas sensing response increased about five times as compared to the undoped

ZnO nanostructures.

6.4 Summary

1-D ZnO pedal-like nanostructures were successfully synthesized on multi-wall

carbon nanotubes (MWCNTs) film by a simple vapor transport method. Powder XRD

pattern of undoped and doped ZnO showed a slight shift in peaks which indicated the

presence of defects due to carbon doping in pedal-like nanostructures. A significant

change in morphology was observed from tapered nanobelts to pedal-like

nanostructures due to carbon doping. TEM, HRTEM, EDX and Raman spectroscopy

results confirmed carbon doping in pedal-like nanostructures and supported the p-


Morphology of

Nanostructures

Synthesis

Temperature

(°C)

Nanostructure

Size

Bandgap

by UV-

VIS (eV)

PL peak

positions

(nm)

20 ppm of

CO gas

Response

(S) at

350°C

Tapered ZnO

nanobelts

(undoped)

900 Tav= 400 nm

Lav = 15.7 µm

Wav = 2.87 µm

Dav= 420 nm

(tip)

3.27 533

(High PL

intensity)

1.17

Pedal-like ZnO

nanostructures

(doped)

900 Tav = 200 nm

Lav = 6.9 µm

Wav= 614 nm

Dav = 203.3 nm

(tail)

3.23 539

(Low PL

intensity)

5.06

93

XRD results. A high resolution TEM image clearly revealed the structural change

produced due to carbon doping. The gas sensor fabricated from multiple networks of

the doped ZnO pedal-like nanostructures showed enhanced signal response to 20 ppm

of CO gas at 350°C. The gas sensing response of the doped pedal-like nanostructures

was improved five times as compared to the undoped ZnO sensor. The increased

signal response was attributed to the catalytic effect of dopant, defects like oxygen

vacancies (potential barrier modification) and fast chemisorption and desorption

properties of the CO gas. Band theory model was adopted to explain the possible

sensing mechanism of the carbon doped ZnO nanostructures.

94

Chapter 7

7 Dependence of size and surface defects on CO gas sensing

response of SnO2 nanowires

95

7.1 Introduction

During the last decades, there have been a demand for sensitive gas sensors in

many fields such as environmental monitoring, security systems and health care

systems.245;246

Sensors based on metal oxides work on the principle of change in

resistance as a result of chemical interaction with test gas247

and have been widely

studied over the years as well as being commercialized. SnO2 is the most dominated

metal oxide over the others and is one of the first considered materials for gas sensing

applications.248;102

Sensitivity of sensors depend upon the thickness of the depletion

layer when the dimension of sensor material is comparable to it.249

Therefore,

different nanostructures such as nanopowders250

, nanowires57;251-253

, nanotubes254

, and

nanobelts231;255

are used for gas sensing owing to high surface to volume ratios.

Yamazoe et al. reported that by reducing the crystallite size of nanomaterial,

improvement in sensitivity and consumption of energy can be achieved.256

Nanowires

with Debye length (thickness of the space charge layer) matching with small diameter

and high surface-to-volume ratio are affected greatly. Sensitivity and response time of

sensors greatly depend on surface processes.257

Sensing models on space charge

region have been confirmed by many works.27

Reports on diameter and aspect ratio

dependence have also been investigated.258;259

In the present chapter, the effective control of the diameter and aspect ratio of

SnO2 nanowires for gas sensing by a simple vapor liquid solid (VLS) mechanism

have been demonstrated. SnO2 nanowire-network-based gas sensor for sensitive CO

gas sensing properties as a function of diameters (aspect ratio) and surface defects

were evaluated. The results show improved sensing response compared to reported

results with respect to sensitivity and limit of detection.

7.2 Experimental

7.2.1 Synthesis of SnO2 nanowires

SnO2 nanowires were synthesized by vapor transport method by the VLS

mechanism shown in Figure 7.1(a). N-type Si (100) substrates were ultrasonically

cleaned in IPA and acetone. After cleaning, Au catalyst deposition (1 nm, 2 nm, 5

nm) was carried by ion sputtering technique. A mixture (mixed in a ball mill for 2

hours) of SnO2 powder (99.99%) and graphite powder (99.99%) with weight ratio

96

(4:1) was taken in a reactor boat. This boat was put in a quartz tube (length 100 cm

and diameter 3.5 cm) at its center. Three experiments were performed using three

substrates having 1 nm, 2 nm, and 5 nm Au catalyst layers, respectively under the

same conditions. The substrate was placed on the boat containing the source material

at 900°C. The synthesis temperature of the VLS process was maintained for 45 min in

high purity N2 atmosphere with a flow of 60 sccm. The furnace was cooled down to

room temperature after the reaction. The collected nanostructure samples were

characterized using different techniques given in chapter 3.

7.2.2 Gas sensors fabrication and characterization

The gas sensors were fabricated using gold coated (thickness 200 nm, using

ion sputtering deposition by JFC 1500) interdigitated silicon substrates. SnO2

nanowires were attached to electrodes through the doctor blade method. Gas sensing

experiments were performed using the two probe method at room temperature using a

Keithly 2100 multimeter. The sensing response (S = Ra/Rg) was measured by the

variation in resistance on exposure to air (Ra) and test gas (Rg) in a home-built gas

chamber connected to a multimeter, gas flow meters and tube furnace. The schematic

design of sensor and gas sensing characterization setup has been shown in Figure 7.1

(b) & (c). Figure 7.1 (d) represents a photograph of gas sensing setup, with an inset

that shows sensor with pressure contacts.

97


7.3.1 SEM and EDX analysis

Figure 7.2 shows the micrographs (SEM) of SnO2 nanowires grown on various

thicknesses of Au catalyst along with associated EDX spectra. Figure 7.2(a) shows

large quantity of the cylindrical nanowires grown at 900°C randomly oriented with

very high aspect ratio (384). The average diameter of nanowires was 104 ± 32 nm and

the average length 40 ± 14 µm. Figure 7.2(b) shows a large quantity of the nanowires

randomly oriented with aspect ratio (209) grown at 900°C. The average diameter of

nanowires was found to be 127±30 nm and the average length 26.6±12.3 µm. Figure

7.2(c) shows nanowires having average diameter and length of 198±19 nm and

20.0±11.8 µm, respectively. The measured aspect ratio was found to be 101. Figure

7.2(d) shows EDX spectrum of SnO2 nanowires clearly showing the Sn and O peaks.

The approximate atomic ratios were found to be 65: 28, with the Si peak due to

substrate. It was observed that nanowires diameter increased with increase in the

catalyst thickness of the catalyst layer. This increase was believed to be due to

increased diameter of nucleation sites.

Figure 7.1: (a) Furnace setting and placement of the substrates (b) Schematics of

fabricated SnO2 Nanowires sensor (c) Schematics of Gas sensing setup (d) Actual

photograph of gas sensing experimental setup. Inset image shows sensor with pressure

contacts

(a)

(b)

98

Figure 7.3 (a), (b) & (c) show TEM images at high and low resolutions of

SnO2 nanowires. The TEM image of Figure 7.3 (a) clearly showed the catalyst

particles on the tips of the nanowire suggesting the VLS growth. HRTEM image

(Figure. 7.3(c)) showed the lattice fringes (inter plane spacing is about 0.26 nm) of

crystalline nanowires suggested the growth along (101) plane of tetragonal rutile

crystal structure of SnO2.

7.3.2 XRD analysis

The X-ray diffraction spectra of the synthesized SnO2 nanowires are shown in

Figure 7.3(d). The identified peaks showed the tetragonal structure of SnO2. Measured

lattice parameters for three samples are as follows: sample 1 a=b=4.75 Å, c= 3.20 Å,

sample 2 a=b=4.74 Å, c=3.20 Å, sample 3 a=b=4.74 Å, c=3.19 Å. All the calculated

parameters were compared with the standard reported reference data (ICDD 00-041-

Figure 7.2: SEM images of SnO2 nanowires at 900°C. (a) Nanowires 1 nm Au (b)

nanowires 2 nm Au (c) nanowires 5 nm Au (d) EDX analysis of nanowires (sample 1)

99

20 30 40 50 600

500

1000

1500

2000

2500

3000

3500

4000 SiO

2

Si (1

10

)

(21

0)In

ten

sit

y (

a.u

.)

2 (Degree)

(21

1)

(00

2)

(22

0)

(11

1)

(20

0)

(10

1)

(11

0)

Sample 1

Sample 2

Sample 3

(d)

1445). The sharpness of the XRD patterns reflected the crystalline quality of all SnO2

nanowires. TEM and XRD results were consistent with each other.

7.3.3 Optical properties

7.3.3.1 UV-visible absorption spectroscopy

The optical characterization of SnO2 nanowires have been investigated by UV-

visible absorption spectroscopy. Figure 7.4 shows the absorption spectrum as obtained

from the three samples having different diameters. The bandgap can be calculated by

taking X-intercept on energy axis from most linear part of the graph. The measured

optical energy bandgap of SnO2 nanowires was found to be 3.77±0.01 eV, 3.74±0.03

eV and 3.68±0.02 eV for 1 nm, 2 nm and 5 nm catalyst thicknesses, respectively.

Optical bandgap was observed to increase with decrease in the thickness of catalyst

Figure 7.3: Typical TEM images of SnO2 nanowires (sample 1) (a), (b) & (c)

showing low and high magnification images (a) Clearly shows catalyst droplet (d)

XRD spectrum of SnO2 nanowires

(a)

100

layer (i.e. decrease in diameter of nanowires) which is in accordance with reported

literature.260

300 400 500 6000

1

2

3

Ab

so

rba

nc

e (

a.u

.)

Wavelength (nm)

Sample 1, 1 nm Au

Sample 2, 2 nm Au

Sample 3, 5 nm Au

sample 1, Eg = 3.77 eV



7.3.3.2 Room temperature photoluminescence (PL) spectroscopy

Figure 7.5 shows room temperature photoluminescence spectra of three

samples using excitation light of He-Cd laser at 325 nm. Spectra showed broad

emission peaks at 567 nm (2.16 eV) and 572 nm (2.19 eV) for three samples

suggesting defect-related (oxygen vacancies and Sn interstitials) electronic states in

the bandgap.261;262

Peaks at 392 nm (3.16 eV), 410 nm (3.02 eV) and 434 nm (2.86

eV) are due to trapped states within the bandgap. Low diameter nanowires (Sample 1)

show a very strong and shifted emission peak as compared to samples 2 & 3, showing

more defects produced owing to high surface/volume ratio.263

The level of defects and

non-stoichiometry in SnO2 was estimated by ratio (𝐼𝑣𝑙/𝐼𝑢𝑙) of visible luminescence to

ultra violet luminescence intensities, the higher this ratio the more will be the intrinsic

defects. The calculated ratio of 𝐼𝑣𝑙/𝐼𝑢𝑙 for the nanowires (sample 1) (4.0) is large as

compared to samples 2 (1.5) and samples 3 (0.46), respectively. This systematic

intensity increase reveals the increase in defects with decrease in diameter of

Figure 7.4: UV-VIS absorption spectroscopy of SnO2 nanowires

101

nanowires. It was found that oxide semiconductors adsorbed large number of oxygen

ions due to greater number of defects.205

7.3.4 Gas sensing properties

Figure 7.6(a), (b) & (c) show the sensing response signals of SnO2 nanowires

for CO gas, CH4 gas and methanol, respectively as a function of catalyst thickness for

three samples. The sensing experiments were performed at 400°C with 3 min cycles

of dry air for 20 ppm CO gas and interference gases (400 ppm CH4 gas and 200 ppm

Methanol) for comparison.264;265

Figure 7.6(a) shows that decrease in resistance in CO

environment and it recovered to initial value of air upon removal of CO gas. Sensing

response of the nanowires was found to be 3.35, 2.58 and 2.32 for aspect ratios 384,

209 and 101, respectively. Sensing response to CH4 gas was found to be 1.20, 1.09

and 1.05 for aspect ratios mentioned above, respectively. Sensing response to

methanol gas was found to be 2.61, 2.33 and 1.80 for aspect ratios mentioned above,

respectively.

Figure 7.5: Room temperature Photoluminescence spectra of SnO2 nanowires

300 350 400 450 500 550 600 6500.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

410 nm

434 nm

392 nm

2.19 eV567 nm

D=198 nm

D=127 nm

D=104 nm

P

L In

ten

sit

y (

a.u

.)

Wavelength (nm)

sample1,D=104 nm

sample2,D=127 nm

sample3,D=198 nm

572 nm2.16 eV

102

Summary of the gas sensing results are shown in Table 7.1. Gas sensing results

showed that sensing response increased with the increase in aspect ratio and decrease

in the diameter of the nanowires.

Gas sensor showed sensitive and a little bit selective response to CO gas compared to

other test gases as shown in Figure 7.7(a). The high sensitivity of SnO2 nanowires was

attributed to the high surface /volume ratio as more space is available for active sites

to adsorb oxygen ions on the surface for this high area and greater interaction with

surrounding gases.266;267

Figure 7.6: Time dependent gas sensing responses of SnO2 nanowires for three

samples (a) 20 ppm of CO gas at 400°C (b) 400 ppm of CH4 gas at 400°C (c) 200

ppm of Methanol at 200°C

Table 7.1: Summary of gas sensing results

Gas Sensing Response (S=Ra/Rg) Properties

Sample

No.

CO

(20 ppm,

400°C)

CH4

(400 ppm,

400°C)

Methanol

(200 ppm,

200°C)

Aspect

Ratio

(L/D)

PL

intensity

ratio

(𝑰𝒗𝒍/𝑰𝒖𝒍)

Average

diameter of

nanowires

(nm)

Sample 1 3.35 1.20 2.61 384 4.0 104±32

Sample 2 2.58 1.09 2.33 209 1.5 127±30

Sample 3 2.32 1.05 1.80 101 0.5 198±19

0 200 400 600 800 1000

480k

490k

500k

510k

Time (Sec)

285k

300k

315k

330k

Air

CH4

Response=1.11Sample 2 =2 nm Au

Sample 3 = 5 nm AuResponse=1.05

Air

CH4

Re

sis

tan

ce

(

)

130k

140k

150k

160k

170k

180k

(b)

Air

CH4

Sample 1 = 1 nm Au

Response=1.20

0 200 400 600 800 1000 1200

240k

320k

400k

480k

560k

Methanol

Air

Response= 1.80Sample 3 = 5 nm Au

Time (Sec)

Re

sis

tan

ce

(

)

25k

50k

75k

100k

125k

Methanol

Air

Response=2.33Sample 2 = 2 nm Au

60k

120k

180k

240k

300k

Methanol

Air

Response=2.61Sample 1 = 1 nm Au(c)

0 200 400 600 800 1000 1200

80k

120k

160k

200k

240k

Time (Sec)

Sample 3 = 5nm Au

(a)Air Response = 3.35

Sample1= 1nm Au

CO

Response = 2.58 Sample2= 2nm Au

Air

CO

Air

Response = 2.32

CO

Re

sis

tan

ce (

)

40k

60k

80k

100k

120k

10k

20k

30k

40k

50k

60k

103

Our results showed improved sensing response as compared to reported results with

respect to gas sensing response and limit of detection as shown in Table 7.2.

7.3.4.1 Gas sensing mechanism

The sensing mechanism is based on the principle of change in electrical resistivity

/conductivity as a result of chemical reaction between gas molecules and oxygen ions

on surface of SnO2 nanowires. When air is flown over the surface of SnO2 nanowires

the oxygen ions are attached at the surface by capturing electrons from conduction

band.83;209

At high temperature reactive oxygen species (O2-

, O2-

and O-) are

chemisorbed by these nanowires as a result electron transfer takes place. At a certain

level of adsorption, a thick depletion layer is formed on the surface due to which

resistance of the material is increased. In contrast, when SnO2 nanowires are exposed

to the CO gas, chemical reaction takes place as adsorbed oxygen ions produce CO2

which results in decreased resistance due to increase in carriers concentration.83

Following chemical reactions can be used to explain this mechanism.

O2 (gas) → O2 (ad) → (7.1)

O2 (ad) +e→ O2-(ad) → (7.2)

O2-

(ad) +e→ 2O-(ad) → (7.3)

2CO+O2-(ad) → 2CO2 (gas) +e

- → (7.4)

CO CH4 Methanol0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

Re

sp

on

se

(S

= R

a/R

g)

sample 1

sample 2

sample 3

(a)

Figure 7.7: (a) Selectivity of SnO2 nanowires (b) & (c) Schematic of possible

mechanism of SnO2 nanowires sensor responses to CO in air and gas, respectively

104

CO+2O-→ CO3

2-→ CO2+1/2O2+2e → (7.5)

7.4 Summary

Size controlled SnO2 nanowires (NWs) were synthesized successfully by using a

very simple vapor transport method by varying the thickness of the gold (Au) catalyst.

XRD peaks indicated the crystalline quality of nanowires grown along c-axis. SEM

images showed that the nanowires are of cylindrical form with a high aspect ratio.

TEM and HRTEM confirmed the VLS growth and are consistent with p-XRD results.

Optical properties provided the information about the bandgap energy and surface

defects of SnO2 nanowires. These results were consistent with gas sensing results. The

gas sensor fabricated from multiple networks of the SnO2 nanowires showed sensitive

and selective response to 20 ppm of CO gas at 400°C compared to other test gases.

Sensing response of the nanowires was found to be 3.35, 2.58 and 2.32 for aspect

ratios 384, 209 and 101, respectively. Gas sensing results showed that sensing response

increased with increase in aspect ratio and decrease in diameter of the nanowires. The

selective and enhanced signal response was attributed to the high aspect ratio of SnO2

nanowires and large surface to volume ratio which provide large number of active sites

for oxygen adsorption. The band theory model was used for explanation of the possible

sensing mechanism SnO2 nanowires.

Table 7.2: Comparison with reported SnO2 nanowires CO gas sensors

Sensor description CO

Concentration

Gas

Response T(ºC) References

Network SnO2 NWs ( CVD) 20 ppm Ra/Rg ~3.35 400 Present work

Network SnO2 NWs ( CVD) 20 ppm Ra/Rg ~2.31 300 Present work

Single SnO2 NWs (CVD)

100 ppm

100 ppm

500 ppm

Ra/Rg ~2.0

Ga/Gg ~1.9

Ra/Rg ~1.2

250-

400

182;268

269

258

Screen printed SnO2 NWs (CVD) 100 ppm Ra/Rg ~2.9 400 270

Network SnO2 NWs 100 ppm Ra/Rg ~1.8 350 264

Network SnO2 NWs 100 ppm Ra/Rg ~1.32 380 271

105

Chapter 8

8 Conclusions and Future Work

106

8.1 Summary and Conclusions

This chapter concludes results and technology investigated from the previous

chapters, particularly chapters 4 to 7. In current research work, systematic study has

been demonstrated on the synthesis, characterization and chemical and UV sensing

applications of ZnO and SnO2 nanostructures. The vapor transport method using

vapor liquid solid (VLS) mechanism has proved to be relatively simple and reliable

technique in acquiring high quality, high density and single crystalline nanostructures

at different temperatures and doping levels. This method has the advantage of low

cost (without vacuum procedures), easy to use and efficient to get morphology tuned

ZnO/SnO2 nanostructures.

Motivation and importance for the experiments/techniques carried out in this thesis is

presented, author was interested in exploring UV and gas sensors using morphology

tuned and doped ZnO/SnO2 nanostructures. The implementation of sensors has been

identified for an area of improvement in sensor’s sensing response, stability, limit of

detection, and response/recovery times. A comprehensive background/literature

review of existing morphology tuned and doped oxide semiconductors with special

issue of induced defects produced in processing of nanomaterials for UV and gas

sensors have been explored.

The focus of this research was to understand how the size, morphology and doping of

the nanostructures of oxide semiconductors affected the sensor performance with

respect to sensitivity, stability, limit of detection and response/recovery times. This

was done through the studies carried out in chapters 4 to 7 as given below

The following goals have been achieved:

1. Morphology tuned ZnO nanostructures are successfully synthesized at different

temperatures. Systematic control of ZnO nanostructural morphologies by a VLS

technique, surface defects and a cost effective method has been given for

fabrication of UV sensors.

2. UV sensor fabricated using multiple networks of morphology tuned ZnO

nanostructures demonstrated high sensitivity, relatively fast response (0.71 sec)

107

and recovery times (0.54 sec) to UV light at room temperature due to low defect

concentration.

3. These significant changes suggested that ZnO nanostructures could be used in

gas sensing and optical switching applications.

4. The photoresponse of the developed sensors can reach as 1.4 to 3.0 with

photocurrent ranging from 4.6 µA to 217 µA.

5. Magnesium (Mg) doped ZnO nanobelts were successfully synthesized using

magnesium acetate in source material (reported first time) by vapor transport

method. Synthesis of very thin and transparent Mg doped ZnO nanobelts with an

average thickness of 34 nm were explored for enhanced CO gas sensing

application.

6. The gas sensing response to 20 ppm of CO gas at 350°C using Mg-doped ZnO

nanobelts sensor was improved ~ 5 times in comparison to undoped ZnO

nanorods sensor. This enhanced sensing response was assigned to increased

surface area (Abelts ~5Arods) and defects like oxygen vacancies.

7. 1-D ZnO carbon (MWCNTs) doped pedal-like nanostructures were successfully

synthesized on MWCNTs film by a simple vapor transport method.

8. Carbon (MWCNTs) doped ZnO nanostructures were used for a numerous

applications but still, to the best of our knowledge carbon doped ZnO

nanostructures synthesized using MWCNT’s by vapor transport method for CO

gas sensing have not been reported.

9. The gas response of the MWCNTs doped pedal-like nanostructures to 20 ppm of

CO gas at 350°C was improved ~ 5 times with good stability as compared to the

pure ZnO nanostructures (reported first time). The results were explained on the

basis of induced defects produced due to carbon doping and their relations to

observed sensing response of gas sensors.

10. Both doped ZnO nanobelts and pedal-like structures showed ~ 3 times enhanced

sensing response towards 20 ppm of CO gas at 350°C with good stability as

compared to reported CO gas sensors.

11. Size controlled SnO2 nanowires (NWs) were synthesized successfully using a

very simple vapor transport method by varying the thickness of the gold (Au)

catalyst.

12. The gas sensor fabricated from multiple networks of the SnO2 nanowires

showed sensitive and selective response to 20 ppm of CO gas at 400°C

108

compared to other test gases. Sensing response of the nanowires was found to be

3.35, 2.58 and 2.32 for aspect ratios 384, 209 and 101, respectively.

13. The selective and enhanced signal response was attributed to the high aspect

ratio of SnO2 nanowires and large surface to volume ratio which provided large

number of active sites for oxygen adsorption.

8.2 Future work

Using the present study, the following goals will be achieved for future work.

There are a small number of reports on the metal oxide semiconductor (MOS)

sensors for detection of solid hazardous chemicals like explosives. Sensitivity,

selectivity and stability of these sensors are still a focus for researchers in this field.

So the focus of future work will be, the detection of toxic solid explosives materials

using these synthesized nanostructures. The existing gas sensing setup can be utilized

to do so.

Room temperature gas sensing will be carried out using metal nanoparticles

decoration or heterostructures of oxide semiconductors nanostructures. This can also

be done using UV light as a catalyst for adsorption and desorption properties of gases,

as ZnO nanostructures showed excellent results for UV sensing so these synthesized

nanostructures can be used for this purpose.

Oxide semiconductors nanostructures can be sensitive and selective chemical

sensors if decorated with functional materials. The main challenge to develop metal-

oxide gas/chemical sensors is high selectivity. Selectivity of these structures could be

achieved by the deposition of functional layer on the surface oxide nanostructures. A

solution based functional layer or the metal catalyst nanoparticles (Pd, Pt, Ag or Au)

have a great potential for selective sensors for various gases. Electronic nose will be

developed using two approaches either by sensor arrays or by modulation of sensor

temperature.

Integrated ZnO/SnO2 nanostructures UV sensors will be fabricated on flexible

polyester (PET) substrates as reports showed that these UV sensors showed

comparable performance and they could be used in flexible electronic devices as

compared to the rigid substrates.

109

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9 List of publications

1. Muhammad Amin, Nazar Abbas Shah, A.S. Bhatti, Mohammad Azad Malik

“Effects of Mg-doping on optical and CO gas sensing properties of sensitive

ZnO nanobelts” CrystEngComm, 16, (2014) 6080-6088

DOI: 10.1039/C4ce00153b

2. Muhammad Amin, Nazar Abbas Shah, Arshad Saleem Bhatti,“Development of

Highly Sensitive UV sensor using Morphology Tuned ZnO nanostructures” Appl

Phys A , 118, (2014), 595-603

DOI: 10.1007/s00339-014-8764-x

3. Muhammad Amin, U. Manzoor, A. S. Bhatti, Mohammad Islam, Nazar Abbas

Shah “Synthesis of ZnO Nanostructures for Low Temperature CO and UV

Sensing” Sensors, 12, (2012) 13842-13851

DOI: 10.3390/s121013842

4. S.Zia, Muhammad Amin, U. Manzoor, A.S. Bhatti “Ultra-long multicolor belts

and unique morphologies of tin-doped zinc oxide nanostructures” Appl Phys A,

115, (2014) 275-28

DOI: 10.1007/s00339-013-7807-z

5. R. Afrin, N.A. Shah, M. Abbas, Muhammad Amin, A.S. Bhatti “Design and

analysis of functional multiwalled carbon nanotubes for infrared sensors” Sens.

Actuators A, 203, (2013) 142-148

DOI:10.1016/j.sna.2013.08.018

6. N.A SHAH, Muhammad Amin, Rahat Afrin, Syed Zafar Ilyas “Synthesis of Cu

Doped ZnO Nanostructures for Ultra Violet Sensing” Sensors & Transducers,

186, (2015) 118-124

7. Muhammad Amin, Nazar Abbas Shah, Hanan-Al-Chaghouri, Mohammad Azad

Malik “Effect of size and photoluminescence properties on CO gas sensing of

SnO2 Nanowires” J Materials Science (Under review)

http://adsabs.harvard.edu/cgi-bin/nph-abs_connect?fforward=http://dx.doi.org/10.1007/s00339-014-8764-x

http://dx.doi.org/10.1016/j.sna.2013.08.018

131

10 Conferences, workshop and seminars

1. Oral Presentation International Conference on Nanotechnology, Nanomaterials

& Thin Films for Energy Applications (NANOENERGY), 19-21 February

2014, London UK

2. POSTER Presentation CIIT-UDEL-NSF Joint International Workshop on

Nanotechnology: Policy Ethics and Science, 25-27 March, 2013, Islamabad,

Pakistan

3. Workshop on thin film Deposition and Characterization, 9-11 April, 2012

Department of physics, CIIT, Islamabad, Pakistan

4. POSTER Presentation COMSATS-ISESCO-NSF International Conference on

Nanomaterials & Nano Ethics, 1-3 December, 2011, Lahore, Pakistan

5. POSTER Presentation12th International Symposium on Advanced Materials

(ISAM) 26-30 September 2011, National Centre for Physics, Islamabad,

Pakistan

6. POSTER Presentation 36th International Nathiagali Summer College on Physics

and Contemporary Needs, 4-8 July, 2011, National Centre for Physics

Islamabad, Pakistan

7. POSTER Presentation12th National Symposium on Frontiers in Physics, 2-4

Feb, 2011, Government College University, Lahore, Pakistan

Synthesis and Characterization of Oxide Semiconductor … · especially grateful to Dr. Umair...

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