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Improved field emission propertiesof carbon nanotubes by dual layerdepositionShama Parveena, Samina Husaina, Avshish Kumara, Javid Alia,Harsha & Mushahid Husaina

a Department of Physics, Jamia Millia Islamia (Central University),New Delhi, IndiaPublished online: 21 Oct 2013.

To cite this article: Shama Parveen, Samina Husain, Avshish Kumar, Javid Ali, Harsh & MushahidHusain (2013): Improved field emission properties of carbon nanotubes by dual layer deposition,Journal of Experimental Nanoscience, DOI: 10.1080/17458080.2013.845914

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Improved field emission properties of carbon nanotubes by dual layerdeposition

Shama Parveen, Samina Husain, Avshish Kumar, Javid Ali, Harsh

and Mushahid Husain*

Department of Physics, Jamia Millia Islamia (Central University), New Delhi, India

(Received 24 April 2012; final version received 15 September 2013)

In this paper, we tried to increase the current density of carbon nanotubes (CNTs)by depositing double layer of CNTs instead of single layer. Both the layers ofCNTs are deposited by the low pressure chemical vapour deposition technique onsilicon substrate with Fe catalyst. Scanning electron microscopic images show thesurface morphology of single and double layer of CNTs. Dual layer deposition ofCNTs is a very simple and easy method to increase the current density of CNTsbased field emitters than other conventional methods. Excellent field emissionproperties of double layer of CNTs are exhibited with large field enhancement fac-tor and low turn-on voltage as compared to those for single layer of CNTs. Highcurrent density of CNTs is required for field-emission-based display devices asso-ciated with field enhancement factor and number of emitting electrons. Therefore,we may say that dual layer deposition of CNTs can be utilised as an alternativeapproach to improve the current density for field emitters. Stability measurementof the samples was also performed for 3 h (180 min) with current at constantapplied voltage, and it was found that the stability of dual layer of CNTs isremarkable than that of single layer of CNTs.

Keywords: field enhancement factor; Fowler–Nordheim equation; low pressurechemical vapour deposition; scanning electron microscope; turn-on field

1. Introduction

Carbon nanotubes (CNTs) have generated an interest amongst scientists and technologists

due to their unique and excellent properties.[1] Large current density, high aspect ratio,sharp tip, high melting point, chemical inertness and vacuum compatibility make the

CNTs a tremendous material for field emitters.[2–6] With these amazing and excellent

properties, the CNTs have potential applications in many nanoscale-based field emission

devices such as screen display, X-rays, cathode ray oscilloscope, scanning electron micro-

scope (SEM) and electron beam evaporators.[7–11] The CNTs based vacuum microelec-

tronic and display devices have already been proposed.[12] Application of CNTs has also

been demonstrated as a field electron emitter sources in display devices due to the high cur-

rent density at low turn-on voltage.[13]Field emission display devices are superior to thermionic-based devices because of low

consumption power, low cost, cold cathode with no heating effect and long life.[14] The

*Corresponding author. Email: mhusain1@jmi.ac.in

� 2013 Taylor & Francis

Journal of Experimental Nanoscience, 2013

http://dx.doi.org/10.1080/17458080.2013.845914

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CNTs based microscopic devices possess single-electron emitter or low-current-densitymaterial, while high current density is required for X-ray tubes and display devices,[15]

and achieving a high current density from CNTs is still a problem due to the limitation of

synthesis technique. A large number of attempts like plasma treatment,[16] laser treat-

ment,[17] oxidation,[18] coating of low-work-function material [19] and improvement of

bonding between the CNTs and substrate [20] have been made, but all experiments had

their limitations due to the need to increase the current density. Seelaboniya et al. [3]

enhanced the current from 14 to 450 mA at 0.4 V/mm by multistage growth of CNTs on

porous silicon substrate. Kim et al. [21] achieved only 500 mA current at same 0.4 V/mmfrom multistage growth of CNTs on carbon fibre. Generally, it is found that aspect ratio

and number of electron emitters should be high to increase the current density. Aspect

ratio is the ratio of length and diameter of CNTs, so the synthesised CNTs should be large

in length. However, it is very difficult to control the geometry of the CNTs such as length

and diameter. In this context, we have adopted a different method to increase the length of

CNTs named as dual layer deposition technique. In this technique, second layer of CNTs

is deposited on the top of first layer of CNTs. This type of deposition technique has advan-

tage by enhancing the length and density of CNTs, which is the basic requirement of thegood field emitters. Our results show enhancement in the field emission current density

from 1.4 to 3.0 mA/cm2 by this dual layer deposition technique.

The main purpose of this work is to increase the current density by dual layer growth of

CNTs using low pressure chemical vapour deposition technique (LPCVD). In general,

there are many methods for the synthesis of CNTs arc discharge, laser ablation and differ-

ent types of chemical vapour depositions such as thermal CVD, plasma CVD, low pressure

CVD and microwave plasma CVD.[22] Out of these, LPCVD is preferred because of low

temperature requirement for large area synthesis of CNTs. The LPCVD techniqueimproves the purity of CNTs by removing unwanted gaseous molecules on the surface of

CNTs during the growth process.[23] Dual layer growth of cold cathode CNTs is a very

simple and easy method to increase the current density of the CNTs field emitters [24,25]

and also for exhibiting high emission stability of CNTs, which is associated with the purity

of CNTs films.

2. Experimental details

The CNTs have been deposited on n-type silicon substrate h100i by using the LPCVD

technique. The Si wafer was cleaned by Radio Corporation of America process which fol-

lows wet oxidation. Cleaning of the substrate is necessary for making a good adhesion

between substrate and catalyst. The best-known catalyst for synthesis of CNTs is iron

(Fe), which was deposited in pattern form on silicon wafer by radio frequency (RF)

magnetron sputtering system (MODEL: 12" MSPT). The thickness of Fe-catalyst film was

5–10 nm.Uniform and patterned deposition of Fe has been obtained in Ar plasma at 10�3

Torr pressure at a power of 150 W. For the synthesis of CNTs, quartz tube of LPCVD

is cleaned by purging nitrogen (N2) gas. Catalyst deposited Si substrate was placed in a

150-cm-long quartz tube at room temperature and was pumped down to 10�3 Torr

using a rotary pump. Rough vacuum is necessary for low pressure growth and to

improve the purity of synthesised CNTs. The substrate was heated to achieve synthesis

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temperature at a rate of 15 �C/min while continuously passing the H2 gas, which worksas carrier gas. After stabilising the synthesis temperature at 750 �C, ammonia (NH3)

gas was passed with a flow rate 250 sccm for pretreatment, which helps in nucleation

of catalyst (Fe). Pretreatment reduces the size of catalyst particle upto nanometre level,

which provides nucleation site for formation of CNTs. Acetylene (C2H2) as the source

gas was introduced with a flow rate 30 sccm for 20 min and the growth time was kept

as 20 min.

The surface morphology of CNTs has been characterised by SEM and hollow type

growth of CNTs is confirmed by transmission electron microscope (TEM). Nature andcrystallinity of CNTs are analysed by Raman spectroscope. For dual layer growth of

CNTs, the Fe catalyst was deposited on the pre-grown CNTs by RF sputtering technique

and then deposited the second layer of CNTs with LPCVD method by keeping the same

parameters as in previous growth process. The surface morphology of this dual layer of

CNTs was studied again. Silicon with grown CNTs is pasted on a copper cathode with sil-

ver paste and dried at 60 �C for field emission measurement. Field emission properties of

single- and dual-layer growth of CNTs were studied in the diode regime. The measure-

ments were performed at room temperature in vacuum of 10�6 Torr. The CNT pasted cop-per cathode was attached in the fixer, and the gap between cathode and stainless steel

anode was fixed at 500 mm.

3. Results and discussions

3.1. SEM characterisation

The SEM images depict the surface morphology of as-grown single and double layer ofCNTs in Figure 1 (a) and 1(b), respectively. The SEM images in Figure 1(a) and 1(b)

reveal that the aligned and uniformly distributed CNTs have been successfully deposited

on silicon substrate with patterned Fe catalyst. The densities and sizes of the Fe-catalytic

nanoparticle played a critical role in the growth of CNTs and strongly depend on the thick-

ness of Fe films. Consequently, synthesis of CNTs is highly correlated with Fe-catalyst film

that was deposited by RF sputtering. The Fe film on Si substrate with appropriate thick-

ness was etched by ammonia, which helps in nucleation of Fe catalyst. Pretreatment

reduces the size of Fe-catalyst particle upto nanometre level, which provides nucleationsite for the formation of CNTs.[12] The Fe catalyst also favours the base growth of CNTs

because of good adhesion with Si substrate. At high temperature, the deposition of hydro-

carbons at the surface of Fe-catalyst nanoparticle occurs and nucleates at their surfaces.

The decomposed carbon atoms diffuse into the Fe-nanoparticle island. Carbon atoms con-

tinue to get deposited on the surface of catalyst particles until the diffusion arrives at the

thermal equilibrium. The accumulation of carbon atoms stops when a thin layer of gra-

phitic film is formed. This graphitic layer quickly encloses the Fe particles and forms the

CNTs.The mechanism of second layer is proposed as a continuous film of Fe catalyst is

deposited on the top of the first layer of CNTs. Pretreatment of this Fe film is done by

ammonia, which forms the nano-island and provides the nucleation sites for the

growth of second layer of CNTs. All growth parameters are repeated as used in the

first layer of CNTs. Second layer of CNTs is not aligned because alignment of CNTs

also depended on the adhesion between catalyst and supporting substrate. In this case

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the supporting material is CNTs, which may not make a good adhesion with the Fe

catalyst. As a consequence, this type of adhesion has not provided much support to

align the CNTs. Hence, a second layer of CNTs is randomly oriented on the first layer

of CNTs.

The length and diameter of as-grown CNTs are in range of 4–5 mm and 20–40 nm,

respectively, in Figure 1(a). It is clear from the SEM image in Figure 1(b) that the density

and length of CNTs have increased after deposition of the second layer. Figure 1(c) and1(d) are the pictorial representation of Figure 1(a) and 1(b), respectively. High density of

CNTs increases the numbers of field emitters and length increases the aspect ratio. Large

length and high density of CNTs make this material suitable for good field-emission prop-

erties. Thus field-emission properties of CNTs are improved. It is also helpful to increase

the maximum current density at low turn-on field (Eto). A high-resolution SEM image of

dual layer growth of CNTs film is shown in Figure 2. The image shows a two-dimensional

array of CNT growth. Two layers of CNTs are clearly depicted by this SEM image. The

interface between the separate growths of nanotubes can also be seen from the micrograph.As seen from this image, the first layer of vertically aligned nanotube film grown by

LPCVD is covered entirely by the surface of the second layer of CNTs and hence increased

the length and density of CNTs. Arrows in the SEM image clearly distinguish these two

layers. Lower arrow indicates single layer and upper arrow indicates second layer. Light

and dark colours of the SEM image are also helpful in differentiating these two layers, i.e.

single layer and dual layer, respectively.

Figure 1. (a) SEM image of single-layer CNTs. (b) SEM image of dual layers of CNTs. (c) Sche-matic diagram of single layer of CNTs. (d) Schematic diagram of dual layer of CNTs.

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3.2. TEM characterisation

Figure 3 shows the TEM image of the as-grown CNTs. The typical length and diameter of

the CNTs are evaluated to be of the magnitude of several microns and less than 50 nm,

respectively. It can be seen that the tube is hollow and several small grains are encapsulated

inside the CNTs. These grains might be iron particles that acted as catalyst and helped in

the further growth of CNTs.

3.3. Raman spectroscopy

Raman spectra provide deep insight into the physical properties as well as structural

qualities by providing us the information of G- and D-band, which stands for graphitic

and defective bands, respectively. Generally, G-mode (tangential mode) in Raman spec-

tra corresponding to the stretching mode in graphitic plane is located around

1580 cm�1. The D-mode (disorder band), located between 1330 and 1360 cm�1, is gener-

ally observed in multi-walled CNTs (MWCNTs). Figure 4 shows a typical Raman spec-

trum obtained by Raman spectrometer (Bruker, RFS 100/s) using neodymium-dopedyttrium aluminium garnet laser with an excitation wavelength of 1064 nm. Our Raman

spectra depict strong peak for G-band at 1585 cm�1 and for D-band at 1358 cm�1, sug-

gesting the formation of graphitised CNTs with some defects. We also observe one extra

peak at 2700 cm�1, which is attributed to the second-order Raman scattering process.

Intensity ratio of defective band and graphite band is a signature of the quality of

CNTs. One important feature, a low-energy radial breathing mode at 248/d (cm�1),

Figure 2. High-resolution SEM of single–double layer of CNTs.

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Figure 3. TEM image of as-grown CNTs.

Figure 4. Raman spectrum of as-grown CNTs.

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where d is tube diameter usually observed in single-walled CNT Raman spectra, whichis not seen in our Raman spectra. This attributes that our as-grown CNTs are

MWCNTs in nature.

3.4. Field emission measurement

Field emission measurement of as-grown CNTs (single and dual layer) was performed at

10�6 Torr as shown in Figure 5. The obtained data were analysed by the Fowler–

Nordheim (FN) equation,[8] a relation between current density and electric field,

J ¼ AE2exp �Bf3=2

E

� �; ð1Þ

where A ¼ 1:54� 10�6AeV=V�6, B ¼ 6:83� 107eV3=2V=cm, E ¼ electric field (V/mm)

and f ¼ work function.

Figure 5(a) shows the plot of emission current density (mA/cm2) vs. the applied macro-

scopic electric field (V/mm) (JE) from single- and dual-layer growth of CNTs. Our results

show that Eto and maximum current density for single layer is 4.0 V/mm and 1.4 mA/cm2,

respectively, and for double layer is 3.6 V/mm and 3.0 mA/cm2, respectively. We find

important results from our measured data that current density improves from 1.4 to3.0 mA/cm2 and Eto decreases from 4.0 to 3.6 V/mm that may be due to the dual layer

deposition of CNTs. This dual layer deposition of CNTs significantly decreases Eto as

compared to the single layer of CNTs, while it increases current density. The field emission

results in our studies are much better than the reported results in the literature of other

workers. Song et al. [26] improved the current density by coating of hafnium oxide on

CNTs and achieved 0.12 mA/cm2 current density at Eto 5.0 V/mm. Wong et al. [27]

increased emission properties of CNTs by hydrogen plasma treatment and achieved

1.0 mA/cm2 at Eto 8.0 V/mm, while Acuna et al. [28] found current density 0.054 mA/cm2

at Eto 12.9 V/mm after oxygen plasma treatment.

The FN plots, ln(I/V2) vs. 1/V for single layer and dual layer are also shown in

Figure 5(b). The field enhancement factor (b) can be calculated with the slope of FN plot

by using following simplified equation [8] derived from FN equation (1),

b ¼ Bf3=2d

m; ð2Þ

where m ¼ slope of FN plot, d ¼ distance between cathode and anode and f ¼ 5 eV as for

carbon. Calculated value of b is 4000 and 13,500 for single and double layer, respectively.

In the case of double layer, the value of b has increased by �3.4 times of single layer. The

high b(�3.4) value may also have resulted in improving the current density in dual layergrowth. In the proposed approach, the dual growth method is advantageous for high cur-

rent density, since there are large numbers of electron field emitters, compared to the sin-

gle-layer CNTs. The double layer of CNTs decreases Eto as compared to the single layer

is, may be due to the increment in the field enhancement factor (b) because of the dual

layer deposition on the aligned CNTs. The SEM images in Figure 1(b) and Figure 2 depict

high density of CNTs, which means large number of field emitters with larger length.

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Figure 5. (a) JE plot for as-grown CNTs with single layer and dual layer. (b) FN plot for as-grownCNTs with single layer and dual layer.

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Length of CNTs is responsible for the increase in the aspect ratio. Therefore, doublelayer increases the length of CNTs and thus enhances field enhancement factor giving

superior emission characteristics. On the tip of CNTs, the field is increased by the factor of

b to the applied electric field, which suggests that low value of the field is sufficient to

emit the electrons from the surface of CNTs. Thus, electrons are emitted at low applied

electric field. Current density is directly related to the number of electrons. Large number

of field emitters or high density of CNTs is responsible for the increase in the number of

emitting electrons. As a consequence, maximum current density increases. The low driving

voltage and high current density are the limiting factors that come out for the practicalapplication of the display devices to increase the number of pixel and less consumption

of power.

3.5. Stability measurement

To study the degradation of CNTs, we plot the graph between emission current density

and time at constant applied voltage on as-grown single layer and dual layer of CNTs and

are also compared. Figure 6 shows the variation in current for dual and single layer at 2

and 1.5 mA/cm2, respectively, for 3 h (180 min) keeping constant voltage at 5 V/mm. We

found that the emission current is rigorously changing till 60 min and is then stabilised

after 80 min. The field emission parameters (Eto, J, b) and stability of both single- and

double-layer CNTs are summarised in Table 1.The emission current of CNTs changes when the CNTs film attached with residual

gases are degraded by joule heating under a high electric field. The out-gassing from the

Figure 6. Variation of emission current with time at constant voltage (5 V/mm).

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CNTs cathode is one of the main reasons of vacuum deterioration, resulting in degrada-

tion of emission currents.[29] In addition, the structural modifications of nanotube tip by

the ion bombardment of residual gases could reduce the emission current at high electricfield region. Therefore, the stability of CNTs would depend on its mechanical strength and

chemical reactivity. More stability of the dual-layer-grown CNTs may be due to the

removal of amorphous carbon and impurities, compared to the single-layer CNTs. During

the growth of dual layer, the sample is heated at high temperature to remove impurities.

Thus it helps to clean the surface of CNTs grown film. However, single-layer CNTs are

not heated at high temperature, so it contains large number of impurities and amorphous

carbon. Impurities presented on the surface of single-layer CNTs also react with the resid-

ual gases and increase the adsorption of gases. Due to this, there are fluctuations in the sta-bility of single-layer CNTs. This is also due to strong electrostatic interaction, local heating

and changes of adsorption energy.[30] The stability of the emission current of dual layer

growth is remarkable compared to the single-layer growth, which attributed that the dual

layer is good for long-term stability of the field emission.

4. Conclusion

To increase the current density and stability of the CNT-based field emitters, we deposited

dual layer of CNTs by LPCVD method on silicon substrate with Fe catalyst. By this dual

layer growth, field emission properties of CNTs are improved due to the large value of field

enhancement factor (b) and large number of electron field emitters. Eto is decreased thatcan give much better current at very low electric field. The first layer of CNTs helps in

increasing the stability of the CNTs and also in providing base to grow the second layer of

CNTs. It may have also helped in increasing the total length of CNTs in obtaining a high

field enhancement factor. Lifetime of CNTs based devices is also increased by high temper-

ature treatment, which changes adsorption energy of single-layer CNTs. As a consequence,

durability of CNT based devices is increased. Our results not only suggest the new way for

synthesis of CNTs, but also offer to achieve high density of field emitters for various nano-

scale electronic devices.

Acknowledgements

Thanks are due to the Department of Electronics and Information Technology (Ministry of Commu-nication and Information Technology), Government of India, for providing financial assistance inthe form of research project. One of the authors (S. Husain) is also thankful to CSIR, India, for pro-viding a research fellowship.

Table 1. Field emission characteristics and stability of single and double layer of CNTs.

Layer of CNTsTurn-on field(Eto) (V/mm)

Current density(J) (mA/cm2)

Field enhancementfactor (b) Stability (min)

Single 4.0 1.4 4000 40Double 3.6 3.0 13,500 90

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