13764967 CdS NW Synthesis and Characterization 12
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Transcript of 13764967 CdS NW Synthesis and Characterization 12
Synthesis and Characterization of
Cadmium Sulfide (CdS) Nanowires (NWs)
Edward Bujak and Dr. Ritesh Agarwal
RET Program – University of Pennsylvania
Department of Materials Science and Engineering
and
Laboratory for Research on the Structure of Matter
University of Pennsylvania, PA, 19104-6272
July 27, 2006
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ABSTRACT
Nanostructures have been investigated extensively using various compounds that
exhibit novel, peculiar, and fascinating properties in the nano scale not exhibited in the
bulk materials or superior to their bulk counterparts, such as: optical, electrical,
biological, mechanical, and chemical aspects, with various morphologies such as rods,
belts, ribbons, wire, helices, dots, and tubes. Dramatic progress has been made in the
investigation and application of these structures stimulating further research and
investment.
Semiconductor nanowires have been a focus of attention for nano-electronics and
nano-optics (or nano-optoelectronics). Specifically, cadmium sulfide (CdS) is a
semiconductor with a large and direct bandgap of Eg = 2.42 eV at room temperature
which, upon excitation, emits light of wavelength 517 nm (λ excitation~517nm). Due to
these unique properties, CdS is one of the most promising materials in optics devices.
This study’s main focus is on the synthesis and characterization of cadmium
sulfide nanowires (CdS NWs). Using conventional VLS growth, the NW synthesis was
performed with a custom made horizontal furnace chemical vapor deposition (CVD)
system. Colloidal Au nanoparticles were used as a catalyst with later studies using
sputtered Pt as a catalyst. The optimal condition for nanowire growth was established
varying process temperature, vacuum pressure, gas flow rate, and the diameter of the
catalyst. Characterization on morphology, crystal structure and chemical composition
were done using Optical microscopy, Scanning Electron microscopy (SEM),
Transmission Electron microscopy (TEM), High Resolution Transmission Electron
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microscopy (HRTEM), and X-ray Energy Dispersive Spectroscopy (EDS or EDX) in
STEM mode.
The morphology and the diameter of the nanowires were defined in controlled
fashion using different catalyst deposition methods and different sizes of catalyst (20-
100nm). We conclude that the dominant process parameter for optimal growth were the
temperature of the substrate and the concentration of the precursor. Further
characterization on optical properties is on the way.
INTRODUCTION
The field of electronics continues to grow and expand, but limits to progress are
falling to new and exciting possibilities. Microelectronics revitalized the fields of
telecommunication and technology through the bulk properties of materials in the
production of microchips and integrated circuits that contained millions of linked
semiconducting devices on the scale of µm (10-6 m). In the near future, nanoelectronic
devices may replace microelectronics in communication and computer industries with
nanostructures having one dimension between 1 and 100 nm.5 The emerging field of
nanoelectronics, electronics on the nanoscale, has the potential to take electronics, as well
as other fields, further than ever imagined. 1,11 This is possible because reducing the size
of a semiconductor to nanoscale proportions alters its bulk electronic, magnetic, and
optical properties.10 These enhanced properties enable multiple new applications
including the integration of nanomaterials into nanodevices such as biological imaging
and biolabeling14 , semiconducting nanowire high efficiency photovoltaic (PV) solar
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cells, waveguides, lasers, light emitting diodes (LED), optoelectronic devices, and a wide
array of photosensors, such as: photoresistors, photoconductive devices, photodetectors,
photodiodes, phototransistors, photodarlingtons, and slotted and reflective optical
switches.2 Various nanostructure morphologies have been synthesized such as: rods,
belts, ribbons, spheres, helices, dots, tubes (single walled SWNT and double walled
(DWNT), branches (whiskers or dendritic), and core-shell (coaxial), to name a few, to
capitalize on their unique form and contour. In particular, semiconductor nanowires, in
which one dimension is approximately 100 times the other dimension, represent a broad
class of nanoscale building blocks that have been successfully used to assemble a wide
range of electronic and photonic devices.
We study CdS because it has novel optical properties; namely its high
photoluminescence (PL) quantum efficiency.15 The energy band gap of CdS is direct
and large (wide). An electron will emit energy (E= νh ) in falling from an excited state to
a ground state., but can fall directly or indirectly. With indirect band gap materials, the
electron in the conduction band moves to the point of energy minima at the expense of
Figure 1. Generic energy band gap.
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kinetic momentum. In indirect band gap materials, the electrons in the conductive band
need some source of momentum to reach the minimum and fall into the holes in the
valence band. With indirect energy band gap materials, the electron falls through one or
more intermediate energy bands so the emission of energy is gradual and an inefficient
source of light emission.
In a material with a direct energy band gap, such as CdS, the electron falls in one
step resulting in a faster, more concentrated emission of energy since the conductive band
is directly combined with the valence band, conserving kinetic energy. The energy that is
produced is emitted as a photon (light particle or quanta) and is therefore used in
applications such as solar cells and light-emitting diodes (LED). 1
The energy band gap of CdS is also large (wide) resulting in a relatively large
released energy than materials with a smaller energy band gap. The energy band gap for
CdS is 2.42eV (Eg = 2.42 eV ); corresponding to an excitation wavelength of
approximately 517 nm (λ Excitation~517nm=5170Å). Alternatively, a current can be
Figure 2. Direct band gap (GaAs) and indirect band gap (Si).
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measured when the CdS nanowires are exposed to light of wavelength smaller than 517
nm.
Technically the band gap is the energy difference between the valence band and
the conduction band or it is the energy required to break the chemical bonds thereby
producing free electrons and holes. From a practical point of view, the band gap energy
(Eg ) represents a lower limit on the photon energy necessary to cause a change in
resistance. Photons incident on these materials must have an energy νh > Eg (or a lower
wavelength than its emitted wavelength) in order to cause a change in resistance. Eg is
the band gap in electron volts (eV), h is Planck’s constant (4.13566743 x 10-15 eV·s or
6.626 x 10-34 J·s) and ν is the frequency of the light (s-1). We also know λν=c , where
c is the speed of light (299,792,458 m/s) and λ is the wavelength (m).
Name of Semiconductor Band Gap (eV) at 300K
Wavelength (nm)
Frequency (T Hz)
Cadmium sulphide (CdS) 2.4 517 580 Cadmium Phosphide (CdP) 2.2 564 532 Cadmium Selenide (CdSe) 1.7 729 411 Gallium Arsenide (GaAs) 1.4 886 338 Silicon (Si) 1.1 1127 266 Germanium (Ge) 0.7 1771 169 Indium Arsenide (InAs) 0.43 2883 104 Lead Sulphide (PbS) 0.37 3351 89 Lead Telluride (PbTe) 0.29 4275 70 Lead Selenide (PbSe) 0.26 4769 63 Indium Antimonide (InSb) 0.23 5390 56
Table 1. Photoresistive semiconductor materials. Derived from band gap data presented at
http://www.thiel.edu/digitalelectronics/chapters/apph_html/apph.htm 13
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The peak sensitivity for photoresistors occurs at a frequency somewhat larger than
that determined by the band gap energy or equivalently at a wavelength somewhat
shorter than the wavelength determined by the band gap and falls off on either side. The
wavelength sensitivity for CdS, CdSe and CdTe normalized to a peak of 1 in each case is
shown in Figure 5. Note that the peak wavelength of CdS is at 5180 Å (518 nm) or a low
wavelength green
Figure 3. CdS Photoresistive detectors.
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Figure 4. Electromagnetic/Visible Spectrum. Source: http://en.wikipedia.org/wiki/Electromagnetic_spectrum
Figure 5. Normalized sensitivities of CdS, CdSe, and CdTe as a function of wavelength. Source: http://www.thiel.edu/digitalelectronics/chapters/apph html/apph.htm 13
9
Table 3. Color, wavelength, frequency and energy of light source: http://en.wikipedia.org/wiki/Color
color wavelength interval frequency interval
red ~ 625–740 nm ~ 480–405 THz
orange ~ 590–625 nm ~ 510–480 THz
yellow ~ 565–590 nm ~ 530–510 THz
green ~ 500–565 nm ~ 600–530 THz
cyan ~ 485–500 nm ~ 620–600 THz
blue ~ 440–485 nm ~ 680–620 THz
violet ~ 380–440 nm ~ 790–680 THz
Color nm 1014 Hz 104 cm−1 eV kJ mol−1 Infrared >1000 <3.00 <1.00 <1.24 <120 Red 700 4.28 1.43 1.77 171 Orange 620 4.84 1.61 2 193 Yellow 580 5.17 1.72 2.14 206 Green 530 5.66 1.89 2.34 226 Blue 470 6.38 2.13 2.64 254 Violet 420 7.14 2.38 2.95 285 Near ultraviolet 300 10 3.33 4.15 400 Far ultraviolet <200 >15.0 >5.00 >6.20 >598
For the synthesis of CdS nanowires, it is necessary to understand the
thermodynamics of its formation. Figure 6 is the pseudo-binary phase diagram for gold
(Au) and cadmium sulfide (CdS) that illustrate the thermodynamics of vapor-liquid-solid
(VLS) growth. Note that this phase diagram shows that Au and CdS are partially soluble
in each other. This consists of the phases that pass the Au/CdS interface during the
temperature and time (concentrations) of the reaction. At a process temperature near
Table 2. The colors of the visible light spectrum. Source: source: http://en.wikipedia.org/wiki/Color
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800°C, the CdS in the vapor phase causes the solid nanoparticles (1) to form a liquid
alloy L (Au+CdS), and with an increasing concentration of CdS will cause a
supersaturation in the alloy (2), that will lead nucleation of the solid CdS growing the
nanowires. Figure 7 shows the diffusion process directly from the colloidal nanoparticles
of Au and the interaction with the CdS in the vapor phase.
In this study our major interest was the synthesis and characterization of
nanowires, especially cadmium sulfide. It was necessary to determine the optimal
parameters for the synthesis such as: process temperature, argon (Ar) flow rate, vacuum
1 2 3
Figure 7. Nanowire growth.
1 2 3
Figure 8. CdS vapor diffusion through Au catalyst for nanowire growth.
Figure 6. Pseudo-binary Au-CdS phase diagram.
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pressure, the catalyst and its diameter, and the concentration of the CdS precursor
(Cadmium Dimethylthiocarbonate).
The vapor-liquid-solid
(VLS) method was used for the
fabrication of CdS nanowires.
This method has been reliably
used for over a decade for
producing one dimensional
nanowires. VLS consists of two
main processes: evaporation and
condensation. Evaporation of the powder precursor is accomplished through high heat
(~800°C). Within a sealed quartz tube held at low pressure (~300 torr), the slowly
vaporizing precursor is carried through a by an inert Ar delivery gas to the Si <100>
substrate (~100 SCCM). The substrate is coated with a Au catalyst to stimulate the
nucleation and growth of the
CdS crystalline structure to
form one-dimensional
nanowires. By using colloidal
Au particles as the catalyst in this
technique, the morphology of the CdS nanowires growth is precisely controlled; the
synthesized nanowire diameters are the diameter of the colloidal Au particle. . The
process time was about 15 minutes. For the structural characterization of the nanowires,
we used optical microscopy, scanning electron microscopy (SEM), transmission electron
Quartz Tube
CdS Precursor:Cadmium DimethylthiocarbonateProcess Temp = 780°C
Si (100) SubstrateSubstrate Temp = 680°C
Ar Flow
Tube Furnace
Ar G
as
RP
Tube FurnaceQuartz Tube
Manometers:
Analog Gauge
Main Valve
Venting Valve
Exhaust
MFC LN2 Trap
Digital Gauge
MFC display and control
Pressure displays and control
Figure 9. Horizontal LPCVD (low pressure chemical vapor deposition) schematic.
Figure 10. Loaded quartz tube schematic .
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microscopy (TEM), and high resolution transmission electron microscopy (HRTEM).
For the compositional characterization of the nanowires, we used X-ray Energy
dispersive spectroscopy (EDS).
MATERIALS AND METHODS
Substrate Preparation
• Cut the Si <100> substrate into ~0.5 cm wide strip (Figure 11).
• Rinse the Si substrate with acetone or ethyl alcohol (to remove organic materials).
• Rinse with de-ionized water.
• Blow dry with air.
• Apply poly-L-lysine solution to clean substrate and leave 5-10 minutes. The poly-L-
lysine created a positive charge to aid in the adhesion of the nanoparticle gold (Au)
catalyst.
• Lightly blow dry with air.
• Apply catalyst to substrate: colloidal gold nanoparticle solution to substrate (Au 20-
40 nm) with clean Pasteur pipette (Figure 12).
• Lightly blow dry with air.
Figure 11. Cutting Si <100> substrate. Figure 12. Application of Au catalyst.
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Quartz Tube Preparation
• Under a chemical fume hood:
• Place prepared substrate into end of quartz tube.
• Load precursor into combustion “boat”/ring (Figure 13) and place into
opposite end of quartz tube with a steel bolt.
• Place quartz tube into tube furnace, place glass wool at end of tube (Figures 15,16).
Figure 13. Placing CdS precursor into “boat”/ring.
Figure 14. Placing CdS precursor “boat”/ring into quartz tube.
Figure 15. Placing prepared quartz tube into furnace.
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Fabrication/Synthesis of Nanowires
• Install liquid nitrogen trap into system and fill with liquid nitrogen.
• With vacuum pump:
• Check vacuum of system (assure sustained 20 m torr vacuum test).
• After integrity test (above), set operating low pressure vacuum (~300 torr).
• Start Ar carrier gas flow (~100 SCCM).
• Start tube furnace. The temperature of the process must be at least 750°C (typically
~800°C). The temperature at the edges of the furnace, input where the precursor
“boat”/ring is and output where the substrate is placed is typically 70-100°C less.
• Once operating temperature is reached, slowly push the precursor “boat”/ring into the
furnace with a bolt moved by a magnet.
• After a desired growth time (~15 minutes), stop tube furnace, let cool down.
Glass wool
Ar flow
Ar flow
Tube FurnaceBolt to push precursor in slowly
Precursor powder in “boat”
Substrate
Figure 16. Prepared loaded furnace.
15
• After near room temperature, stop Ar flow, vent vacuum, and disassemble quartz tube
from furnace.
• Under a chemical fume hood, remove substrate with grown nanowires and safely
dispose of all hazardous materials.
• If this is last fabrication of the day, remove the liquid nitrogen trap and place in
chemical fume hood.
Manometers: digital gauge (30-765 torr) and analog gauge (0-100 m torr)
Argon Gas and Regulator
LN2 Trap
Rotary (vacuum) Pump MFC
(100 SCCM)
Valves
Tube Furnace (25-1100°C)
Pressure and vacuumdisplays and controls
Figure 17. LPCVD apparatus.
16
RESULTS AND DISCUSSION
Characterization – Structure - Imaging – Optical Microscope
The nanostructures were first
examined directly on the Si substrate with
optical microscopes (Figure 18). If the
morphology and dimensions were
desirable, we then processed the nanowires
for electron microscopy.
Characterization – Structure - Imaging - Electron Microscopy
(SEM/TEM/HRTEM)
We removed the “good” CdS
nanoparticles from the Si substrate by
scraping the particles off into a small vial,
mixed with acetone, and sonicated it to
disperse the particles uniformly in
suspension. With a Pasteur pipette we
placed drops of the processed nanoparticles
onto a TEM grid. We optionally make a few TEM grids and let air dry. We mounted the
TEM grid into the TEM scanning assembly (Figure 20) and placed it into the TEM
(Figure 21).
Figure 18. Initial inspection of synthesized NWs with optical microscope.
Figure 19. Petri dish with multiple TEM grids.
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The structures of the synthesized products were characterized using scanning
electron microscopy (SEM). Figure 22(a) and 22(b) shows the SEM images of the
nanowires grown on the Si at a temperature of 650°C. The CdS nanowires have diameters
between 50-150nm and lengths up to 30µm as shown in the SEM images.
Figure 20. TEM grid is mounted on tip of TEM assembly.
Figure 21. TEM assembly is inserted into TEM.
Figure 22. SEM images of CdS NWs grown in large scale. NW diameter: 50-150nm, length: up to 30 µm.
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The morphology of CdS nanowires was observed in a transmission electron
microscope (TEM). Figure 23(a) is a typical TEM image, which demonstrates the general
view of the CdS nanowires. Figure 23(b) is a High-Resolution TEM (HRTEM) image
showing the uniformity of the grown nanowires. Figure 23(c) shows an equivalent image
of the CdS nanowires demonstrating the single crystalline nature.
Figure 23. (a) TEM image of CdS NWs (b) HRTEM image (c) Fourier Transform of HRTEM image.
Characterization – Composition – Energy Dispersive X-Ray Spectroscopy
(EDX or EDS)
Energy Dispersive X-Ray Spectroscopy (EDX or EDS) analysis was utilized to
characterize the chemical composition of the nanowires. Figure 24 shows a diffraction
pattern of single-crystalline CdS nanowires. The graphs (counts on the y-axis for a certain
emitted Energy eV on the x-axis) demonstrate that the bodies of the nanowires are
basically composed of Cadmium with a peak between 0.0-5.0 eV and a peak of Sulfide in
the same range. It can be observed, in the second graph, that the tip of the nanowires is
5 nm
002002
19
composed almost completely of gold (Au) with a peak between 0.0-5.0 eV and a wide
peak between 5.0-10.0 eV.
The high peak in the second graph is an artifact from the Molybdenum (Mo) TEM
grid. The peak, between 10.0-15.0 eV, is unknown but didn’t cause any alteration in the
analysis of the graphs.
CONCLUSION
We successfully synthesized CdS nanowires under controlled conditions with an
established protocol utilizing a simple Vapor-Liquid-Solid mechanism at low pressure
(LPCVD) We observed different nanowire growth by varying process parameters such
as: temperature (700°C-780°C), time (5-10 minutes), vacuum pressure (~300 torr),
Molybdenum count spike due to Mo TEM grid
Figure 24. EDX/EDS images
S
??Au Catalyst
CdS nanowire
20 nm
20
carrier-delivery flow (100-300 SCCM), and catalyst (Au ~20-40nm nanocolloidal
particles and sputtered Pt). We observed that the major factors affecting desirable
nanowire morphology and density were concentration of the vapor delivered to the
substrate/catalyst and the process temperature.
Structurally we imaged the fabricated CdS nanowires with scanning electron
microscope (SEM), transmission electron microscope (TEM, and high-resolution
transmission electron microscope (HRTEM) which occasionally showed good nanowire
morphologies of length to width: long and thin.
Compositionally we examined the purity, density, and chemical makeup of the
CdS nanowire, utilizing Energy Dispersive X-Ray Spectroscopy (EDS or EDX) which
showed a uniform and high concentration of Cd and S across the nanowires with little or
no Au or Pt catalyst. Similarly the catalyst at the tip of the nanowires was effectively pure
Au or Pt and do not show any Cd or S peaks.
Optically, CdS is a very interesting photoluminescence (PL) material.
Unfortunately in this time frame we did not have time to investigate these. I understand
what is needed to realize this, but it was a limitation of available equipment.
ACKNOWLEDGEMENTS
I would like to thank the National Science Foundation’s (NSF) funding of the
Research Experience for Teachers (RET) and Dr. Andrew McGhie for this wonderful
opportunity to provide me and other teachers an experience in advanced exploratory
scientific discovery and research. Specifically I would like to thank my advisor Dr.
Ritesh Agarwal and his group of graduate and post doctoral students: Dr. Se-Ho Lee
21
(Lee), Yeonwoong Jung (Eric), Dong-Kyun Ko (Ko), Yu-Han Cheng (Valorie), Xuelian
Zhu (Julian), and undergraduate student Andrew Jennings. Furthermore I would like to
thank fellow visiting individuals in this group: Maria Lòpez (REU-Research Experience
for Undergraduates, University of Puerto Rico), Dr. Spirit Tlali (Collaborative with
Southern Africa – Lesotho), and Dr. Murrell Dobbins (RET-Nanotechnology-Drexel
University) for their assistance and support.
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