This article was downloaded by: [Tokyo Kogyo University]On: 21 July 2011, At: 19:26Publisher: Taylor & FrancisInforma Ltd Registered in England and Wales Registered Number: 1072954 Registeredoffice: Mortimer House, 37-41 Mortimer Street, London W1T 3JH, UK

Fullerenes, Nanotubes and CarbonNanostructuresPublication details, including instructions for authors andsubscription information:http://www.tandfonline.com/loi/lfnn20

Thin Films of Carbon Nanomaterial fromNatural Precursor by Hot Wire CVDShrikant S. Kawale a b , Sunil Bhardwaj a , D. E. Kshirsagar a , C. H.Bhosale b , Madhuri Sharon c & Maheshwar Sharon aa Nanotechnology Research Centre, Birla College, Kalyan, Indiab Department of Physics, Shivaji University, Kolhapur, Indiac MONAD Nanotech, Pvt. Ltd., Powai, Mumbai

Available online: 31 May 2011

To cite this article: Shrikant S. Kawale, Sunil Bhardwaj, D. E. Kshirsagar, C. H. Bhosale, MadhuriSharon & Maheshwar Sharon (2011): Thin Films of Carbon Nanomaterial from Natural Precursor by HotWire CVD, Fullerenes, Nanotubes and Carbon Nanostructures, 19:6, 540-549

To link to this article: http://dx.doi.org/10.1080/1536383X.2010.494784

PLEASE SCROLL DOWN FOR ARTICLE

Full terms and conditions of use: http://www.tandfonline.com/page/terms-and-conditions

This article may be used for research, teaching and private study purposes. Anysubstantial or systematic reproduction, re-distribution, re-selling, loan, sub-licensing,systematic supply or distribution in any form to anyone is expressly forbidden.

The publisher does not give any warranty express or implied or make any representationthat the contents will be complete or accurate or up to date. The accuracy of anyinstructions, formulae and drug doses should be independently verified with primarysources. The publisher shall not be liable for any loss, actions, claims, proceedings,demand or costs or damages whatsoever or howsoever caused arising directly or indirectlyin connection with or arising out of the use of this material.

Fullerenes, Nanotubes, and Carbon Nanostructures, 19: 540–549, 2011Copyright © Taylor & Francis Group, LLCISSN: 1536-383X print / 1536-4046 onlineDOI: 10.1080/1536383X.2010.494784

Thin Films of Carbon Nanomaterial from NaturalPrecursor by Hot Wire CVD

SHRIKANT S. KAWALE1,2, SUNIL BHARDWAJ1,D. E. KSHIRSAGAR1, C. H. BHOSALE2, MADHURI SHARON3

AND MAHESHWAR SHARON1

1Nanotechnology Research Centre, Birla College, Kalyan, India2Department of Physics, Shivaji University, Kolhapur, India3MONAD Nanotech, Pvt. Ltd., Powai, Mumbai

Thin films of carbon nanotubes were synthesized from natural plant derived precursorssuch as camphor, mustard oil, castor oil, coconut oil, turpentine oil and menthol onquartz substrate by hot wire CVD. Morphological and compositional properties werestudied by using SEM and EDAX. Optical band gap and activation energy were alsodetermined. Type of conductivity and carrier concentration were calculated by usingHall Effect measurement unit. Various experimental parameters like precursor, deposi-tion time, quantity of precursor, current through coil, etc., and their effect on band gapwas studied. Out of the all precursors, camphor was found to be the best precursor forgetting carbon thin films with optimized band gap value.

Keywords Carbon nanotubes, band gap, hot wire CVD, camphor

Introduction

Thin films of carbon nanotubes and their allotropes such as fullerene, carbon nanotubes(CNT), and diamond-like carbon (DLC), etc., have been attracting researchers due to theirunique properties. Carbon atoms can form sp, sp2 and sp3 hybridization for different kindof bonding. The crystalline modifications of diamond with pure sp3 hybridization hasband gap of 5.5 eV (insulator) and graphite with purely sp2 hybridization has band gap of∼0.25 eV (conductor). Thus by synthesizing thin films with controlled composition of sp2and sp3, we can produce semi-conducting carbon with a band gap in the range of 5.5 eV to0.25 eV. Such flexibility does not exist in any other inorganic material. Diamond-like car-bon (DLC) is effective as an anti-reflecting coating on Si solar cell (1,2) and also studied asa solar cell material (3). Fullerenes and polymer composites are effective due to their highelectron affinity and charge transportation capacity (4). CVD is one of the most commonlyused technique due to its simplicity, low cost, higher promise of reproducibility, better con-trol of chemical composition and bonding structure in preparation of thin films. Recentlymuch improved and sophisticated versions of CVD such as hot filament CVD, radio fre-quency (RF) plasma CVD (5,6) and micro-surface (MS)-wave plasma CVD (7,8) havebeen applied for thin film deposition and doping purposes. Hot filament CVD was found tobe useful for rapid and time saving film depositions (9). Pyrolysis of synthetic precursors

Address correspondence to Shrikant S. Kawale, Nanotechnology Research Centre, BirlaCollege, Kalyan, India. E-mail: shri.physics@gmail.com

540

Dow

nloa

ded

by [

Tok

yo K

ogyo

Uni

vers

ity]

at 1

9:26

21

July

201

1

Thin Films of Carbon Nanomaterial 541

such as polyfurfuryl alcohol, acetylene and natural precursors (10) such as different, plantfibers, resins, oils, etc., can give carbon nanomaterial. The main advantage for using natu-ral precursors lies in their original size, shape and structures, which are difficult or nearlyimpossible to produce artificially. The fear of exhausting fossil fuel like petroleum prod-ucts is another important reason for using natural precursors (10). Sharon et al. (11,12) haveproved that in order to form a three dimensional spherical shape, conversion of some of thesp2 carbon atoms to sp3 carbon atom is required. If starting material contains both typesof carbon, then it is easier to make three-dimensional spherical-shaped carbon materials.Camphor (C10H16O) has a three CH3 groups and one C = O group forming pentagonal andhexagonal rings at the time of combustion or pyrolysis. These reactive rings of camphorare the basic building blocks needed for making spherical carbon materials. On the otherhand, conventional graphitic materials require additional energy as well as environmentalconditions to form highly reactive pentagonal rings. Sharon’s group reported the formationof fullerene (13), glassy carbon from camphor-a natural source (14), diamond-like carbon(DLC) (15), carbon nanofibers (16) and multichannel tubules (17) from camphor. Also,the same group recently synthesized semi-conducting diamond-like carbon thin films ofdirect band gap 1.04 eV by HWCVD technique using turpentine oil as a precursor (18).To extend their work we have tried to deposit thin films of carbon with various naturalprecursors. In this work, along with camphor we have utilized some different plant-basedvolatile precursors such as menthol, turpentine oil and few edible oils such as mustard oil(Brassica spp.), castor oil (Ricinus communis) and coconut oil (Eugenia caryophyllata).

Experimental

All the precursors were obtained from local vendors, the list of selected precursorsand there main constituents are given in Table 1. Hot wire chemical vapor deposition(HWCVD): HWCVD was used for preparation of carbon thin films. Basic block diagramof HWCVD is shown in Figure 1.

HWCVD is basically a spherical steel chamber with inbuilt pressure and temperaturesensors. A small quartz boat is solenoid coiled with tungsten wire above which there is asample holder with adjustable distance from the coil. Quartz plate of size 1 cm × 1 cm and0.1 cm in thickness was used as substrate for thin film deposition. The surface of quartzsubstrate was cleaned first with 10% hydrofluoric acid and then washed with acetone anddistilled water. Two terminals of tungsten wire are connected to a high power transformer.The transformer used in this experiment was capable to generate potential difference of15V at a very high current of 100 amps. First, a small amount of precursor (∼1 gm) waskept in the quartz boat and the boat inserted in the coiling of tungsten. After attaching to

Table 1List of selected precursors for hot wire CVD

Sr. No. Precursor Main constituents

1. Camphor (C10H16O) Camphor2. Menthol (C10H20O) Menthol3. Turpentine Oil (C10H16) Units of Pinene (α and β pinene)4. Castor Oil Ricinoleic Acid (90%)5. Mustard Oil Erucic Acid (47%), Oleic Acid (22%)6. Coconut Oil Lauric Acid (52%)

Dow

nloa

ded

by [

Tok

yo K

ogyo

Uni

vers

ity]

at 1

9:26

21

July

201

1

542 S. S. Kawale et al.

Figure 1. Block diagram of hot wire CVD.

quartz substrate to sample holder, the steel chamber was closed with the screws provided.Then this chamber was evacuated to 0.1 bars, and potential difference of 15 volt is appliedacross the tungsten wire. The current passing through the coil was measured by multimeter.This supplied high power makes the tungsten coil heated up to ∼1700oC within a second orless. This high temperature also increase the temperature of substrate up to ∼500–800◦Cdepending on its distance from coil and power supplied to coil. Precursor present in thequartz boat gets heated and sublimates quickly. These vapors get pyrolyzed within a veryshort time on the heated substrate. The so-formed carbon nanostructures are deposited onthe substrate hanging exactly above the heated zone. Chamber was then allowed to cooland thin film so formed was taken out.

Various parameters such as precursor, power applied and distance between heatingcoil and substrate affects on the morphological, electrical and chemical properties of filmsformed. All these parameters are optimized by trial and error method. However, parameterssuch as distance of substrate from heating coil, length of heating coil and vacuum (0.1 bar)were kept constant in all the experiments. The details of experimental parameters are givenin Table 2.

All these samples were analyzed by Scanning Electron Microscopy (SEM). This dataare shown in Figure 2. Optical properties, electrical properties and carrier concentrationswere also determined.

Table 2Preparation conditions for different samples

Deposition Quantity of Voltage Current Powertime precursor applied through applied

Precursors (min.) used (gms) (V) coil (amp) (W)

Camphor 3 0.5 15 60 900Menthol 3 0.5 15 60 900Castor oil 4 0.3 15 60 900Turpentine oil 4 0.5 15 60 900Coconut oil 5 0.5 15 70 1050Mustard oil 6 0.3 15 70 1050

Dow

nloa

ded

by [

Tok

yo K

ogyo

Uni

vers

ity]

at 1

9:26

21

July

201

1

Thin Films of Carbon Nanomaterial 543

Figure 2. SEM images of (a) Camphor, (b) Menthol, (c) Castor oil, (d) Turpentine oil, (e) Coconutoil and (f) Mustard oil.

Results and Discussion

In the present work, we have used two different types of carbon rich compounds (i) volatilecompounds, for example, turpentine, menthol and camphor composed of pure simplehydrocarbons and (ii) lipids composed of complex different types of fatty acids. Thevolatile compounds have low binding energy as compared to the other precursors. Thatis why good quality of thin films can be deposited at low power and in less time, while for

Dow

nloa

ded

by [

Tok

yo K

ogyo

Uni

vers

ity]

at 1

9:26

21

July

201

1

544 S. S. Kawale et al.

the precursors containing higher carbon skeletons like edible oils need higher energy to getfragmented and more time also.

SEM images of all the samples are shown in Figure 2. As the precursors such as cam-phor and menthol contains no impurity, when heated they start degrading at a constanttemperature and form uniform vapor cloud near the substrate and gives thin film of uni-form particles. Camphor (Figure 2a) gives the thin film of carbon nanobeads which are∼20 nm in diameter. The size of the nanobeads are uniform and the film is very smooth.The similar structure (carbon nanobeads) is observed in the case of menthol (Figure 2b);this may be because both the precursors, camphor and menthol, are highly volatile andrequire low temperature for breakdown of the basic structure. Turpentine oil is a volatileoil (drying oil) so it can be evaporated at low temperature but the breakdown of themolecules requires more energy. It also forms the nano beads (Figure 2c) with uniformshape and size.

The nonvolatile oils such as castor oil (Figure 2d), coconut oil (Figure 2e) and mustardoil (Figure 2f) require more energy for the breakdown and they can be pyrolysed at highertemperatures. All these precursors are mixture of different components and the basic car-bon skeletons of each component differ in size and its binding energy; therefore, at thehigher temperature some of the components of precursor degrades but not all of them,this disturbs the uniformity of vapor cloud near the substrate and give mixture of carbonnanofibers, nanobeads and amorphous carbon which can be seen through SEM images(Figure 2). As concerned to control the band-gap of the so formed thin film by mixtureof sp2 and sp3 hybridized carbon atoms, this could be a nice idea to use such a precursorwhich is not a single component. This can give the mixture of different carbon nano mate-rials and the property of the so formed thin film will depend on the actual composition ofthe film.

For determining optical band gap, the absorption spectrum of all the films formed istaken by using spectrophotometer (Systronic double beam spectrophotometer-2203) in thewavelength range of 200–1100 nm. Tauc relation in equation (1) is used to find out bandgap of the carbon thin film from the absorption graph.

αhυ = A(hυ − Eg)n/2 (1)

where α, h, υ and A is absorption coefficient, planks constant, frequency of absorbed radia-tion and is a constant, respectively. n is an integer whose value is n = 1 for direct band gapmaterial, & n = 4 for indirect band gap material. Absorption coefficient α is determinedby measuring absorbance versus wavelength of light. (αhυ)2/n is plotted versus hυ. Fromthe intercept of linear plot at X-axis, band gap is calculated. A typical graph obtained fromthe carbon thin film prepared from camphor (Figure 3) shows a direct band gap (i.e., whenn = 1) of 1.3 eV. No linear plot was obtained at n = 4 which is suggesting the absence ofindirect band gap. In this fashion band gaps of all carbon films were determined, the resultsof these calculations are shown in Table 3.

From the optical band gap of all the samples, it is clear that the camphor and mentholare having band gap of 1.5 eV and 1.8 eV, respectively, which is near the desired band gapfor photovoltaic, that is, 1.4 eV. Higher precursors such as castor oil and mustard oil showlarge band gaps of 2.5 eV and 3.1 eV, respectively.

Electrical properties of carbon thin film are measured by a four-probe resistivitymeasurement technique. All samples showed decrease in resistance with increase intemperature from room temperature to higher temperature (up to 300oC), suggesting

Dow

nloa

ded

by [

Tok

yo K

ogyo

Uni

vers

ity]

at 1

9:26

21

July

201

1

Thin Films of Carbon Nanomaterial 545

Direct band gap study

0

20

40

60

80

100

120

140

160

hv(eV)

(αhv

)^2

0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 5.5 6

(a)

Indirect band gap

0

0.5

1

1.5

2

2.5

3

3.5

4

0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 5.5 6 6.5 7hV(eV)

(αh

V)^

1/2

(b)

Figure 3. Calculation of (a) direct band gap and (b) indirect band gap by absorption spectra ofthin film (color figure available online).

semiconducting behavior. A typical graph of resistivity versus 1/T obtained with carbonfilm from camphor is shown in Figure 4(a).

Activation energies (Ea) for the conductions were calculated from this graph usingequation (2):

Ln (σ ) =(−Ea

2KT

)+ Ln (A) (2)

where σ is conductivity, (Ea) is activation energy and K is Boltzmann constant. Activationenergy (Ea) of all the samples is calculated from the slope of graph of Ln (σ ) versus (1/T)(Figure 4b) which is shown in Table 3. The results clearly suggest that the thin film obtainedby the pyrolysis of camphor having very low band gap (1.5 eV) is due to low activationenergy (1.4 eV). The low activation energy suggests that the material possesses low energy

Dow

nloa

ded

by [

Tok

yo K

ogyo

Uni

vers

ity]

at 1

9:26

21

July

201

1

546 S. S. Kawale et al.

Table 3Optical band gap and activation energy for various precursors

Optical Activation Carrierband energy concentration

Sr. No. Precursor gap (eV) (eV) (cm−3)

1 Camphor 1.5 0.14 3.6 × 1018

2 Menthol 1.8 0.22 2.53 × 1018

3 Castor oil 2.5 0.73 7.48 × 1017

4 Turpentine oil 2.8 0.82 7.17 × 1018

5 Coconut oil 2.2 0.63 8.14 × 1017

6 Mustard oil 3.1 0.89 2.19 × 1017

for the mobility of the carrier. Activation energy is highest for the film deposited frommustard oil (0.89 eV).

The Hall Effect was measured for all the films deposited from different precursors tocalculate the carrier concentration and the nature of the carriers. All films showed negativeHall voltage, which showed the film to be of n-type. Carrier concentration obtained fromthe various films is shown in Table 3. Carbon thin film obtained from the camphor showshighest carrier concentration of 3.6 × 1018 cm−1 which is quite good for the utilization ofthin film in photovoltaic.

A close examination of the molecular formulae of the precursors and type of carbonnanomaterials formed (Figure 5), giving the various optical band gaps and carrier con-centration, suggests that presence of oxygen and nature of molecular structure somehowcontrols the optical band gap of these carbon materials. Camphor possessing one oxy-gen atom attached to hexagonal carbons perhaps helps to produce interconnected carbonnano beads helping in giving lowest band gap of 1.5 eV and high carrier concentration of3.6 × 1018 cm−1. Menthol also possesses one cyclic carbon ring, which produce intercon-nected carbon nanobeads with almost the same band gap and carrier concentration as thatobtained with camphor. Slight increase in the band gap and low carrier concentration maybe due to absence of pure double bond with oxygen (as it contains one OH group). Otherprecursors such as coconut oil, caster oil and mustard oil possess long chain of carbonwhich assists in forming carbon nanofibers instead of pure carbon nanobeads forming amaterial with higher band gap in the range of 2.2–3.1 eV. Interestingly, their carrier con-centration is also lower by multiples of 10. Among these precursors, mustard oil yieldsaggregates of clusters of carbon nanobeads attached with carbon nanofibers inducing thehighest band gap of 3.1 eV. The clusters are separated by some distance; as a result, thecarrier concentration is lowest among all these carbon material. Turpentine oil though pro-duces carbon nanobeads but due to absence of any oxygen atom the band gap is higherthan the carbon nanobeads produced from camphor or menthol. The carrier concentrationis highest of all carbon perhaps because its carbon nanobeads are closely connected, asobserved by their SEM micrograph (Figure 2c).

Conclusion

Carbon thin films were prepared by hot wire CVD technique from natural precursorssuch as camphor, menthol, turpentine oil, castor oil, coconut oil and mustard oil. Of all

Dow

nloa

ded

by [

Tok

yo K

ogyo

Uni

vers

ity]

at 1

9:26

21

July

201

1

Thin Films of Carbon Nanomaterial 547

Temp. Vs resistivity

100000

150000

200000

250000

300000

350000

400000

450000

500000

550000

0.0015 0.0017 0.0019 0.0021 0.0023 0.0025 0.0027 0.0029 0.0031 0.0033

1000/T (K)

ρ (o

hm

.cm

)

(a)

–5.5

–5

–4.5

Ln

(σ)

/oh

m.c

m

–4

–3.50.0015 0.002 0.0025

(b)

1/T Vs. Ln (σ)

1/T (K)

0.003 0.0035

Figure 4. Plot of (a) 1/T Vs Resistivity (b) 1/T Vs Ln (σ ) of carbon thin film (color figure availableonline).

precursors, camphor, menthol and turpentine oil formed uniform carbon nanobeads withdiameter ∼20 nm. Precursors such as castor oil, mustard oil and coconut oil gave a mix-ture of carbon nanobeads, carbon nanofibers and amorphous carbon. Band gap varies in therange of 1.5 eV to 3.1 eV depending on the type of precursor and deposition parameterssuch as deposition time, quantity of precursor and current through coil. Camphor and men-thol had a band gap most suitable for developing photovoltaic solar cell, that is, 1.4 eV.Electrical property and Hall effect study show that films are of a semiconducting naturewith n-type conductivity. These properties suggest that, after doping of impurities, filmsobtained from camphor will be suitable for developing solar photovoltaic.

Dow

nloa

ded

by [

Tok

yo K

ogyo

Uni

vers

ity]

at 1

9:26

21

July

201

1

548 S. S. Kawale et al.

Figure 5. Collection of molecular formula of the precursor and type of carbon, their band gap andcarrier concentration formed by the pyrolysis of these precursors (color figure available online).

Acknowledgments

The authors are thankful for financial support from the Department of Science andTechnology (DST), India.

References

1. Choi, W. S., Kim, K., Yi, J., and Hong, B. (2008) Diamond-like carbon protective anti-reflectioncoating for Si solar cell. Materials Letters, 62(4–5): 577–580.

2. Oliveira Jr., M. H., Silva, D. S., Côrtes, A. D. S., Namani, M. A. B., and Marques, F. C. (2009)Diamond like carbon used as antireflective coating on crystalline silicon solar cells. Diamondand Related Materials, 18(5–8): 1028–1030.

3. Umeno, M. and Adhikary, S. (2005) Diamond-like carbon thin films by microwave surface-wave plasma CVD aimed for the application of photovoltaic solar cells. Diamond & RelatedMaterials, 14: 1973–1979.

4. Thompson, B. C. and Jean, M. J. (2005) Polymer-fullerene composite solar cells. ChemieInternational Edition, 47(1):58–77.

5. Soga, T., Kokubu, T., Hayashi, Y., and Jimbo, T. (2005) Effect of RF power on the pho-tovoltaic properties of boron-doped amorphous carbon/n-type silicon junction fabricated byplasma enhanced chemical vapor deposition. Thin Solid Films, 482: 86–89.

6. Rusop, M., Soga, T., and Jimbo, T. (2005) Structural, bonding and physical characteristics ofphosphorus-doped hydrogenated amorphous carbon films grown by plasma-enhanced chemicalvapor deposition. Thin Solid Films, 482: 280–286.

7. Umeno, M. and Adhikary, S. (2005) Diamond-like carbon thin films by microwave surface-wave plasma CVD aimed for the application of photovoltaic solar cells. Diamond & RelatedMaterials, 14: 1973–1979.

8. Adhikari, S., Adhikary, S., Omer, A. M. M., Rusop, M., Uchida, H., Soga, T., and Umeno, M.(2005) Synthesis of nitrogen incorporated diamond-like carbon thin films using microwavesurface-wave plasma CVD. Diamond & Related Materials, 14: 1824–1827.

9. Nonomura, S., Yoshida, N., and Itoh, T. (2006) The formation of hetero-junction using alloys byhot-wire CVD method. Thin Solid Films, 501: 164–168.

10. Kawale, S. and Sharon, M. (2009) Natural precursors for synthesis of carbon nano mate-rial. In Carbon Nanoforms and Applications, Sharon, M. and Sharon, M., eds., McGraw-Hill:New York, pp. 101–123.

11. Sharon, M., Jain, S., Kichambare, P. D., and Kumar, M. (1998) Effect of pyrolyzing time andtemperature on the bandgap of camphor—pyrolyzed semiconducting carbon films. MaterialsChemistry and Physics, 56: 284–288.

Dow

nloa

ded

by [

Tok

yo K

ogyo

Uni

vers

ity]

at 1

9:26

21

July

201

1

Thin Films of Carbon Nanomaterial 549

12. Sharon, M., Mukhopadhyay, K., Yase, K., Iijima, S., Ando, Y., and Zhao, X. (1998) Spongycarbon nanobeads—a new material. Carbon, 36(5–6): 507–511.

13. Mukhopadhyay, K., Krishna, K. M., and Sharon, M. (1994) Fullerenes: C60 from camphor—anovel approach. Current Science, 67(8): 602–604.

14. Mukhopadhyay, K. and Sharon, M. (1997) Glassy carbon from camphor—a natural source.Materials Chemistry and Physics, 49(2): 105–109.

15. Mukhopadhyay, K., Krishna, K. M., and Sharon, M. (1997) A simple method and new sourcefor getting diamond-like carbon film and polycrystalline diamond film. Materials Chemistry andPhysics, 49(3): 252–257.

16. Rzepkaa, E., Lussona, C., Lévy-Clémenta, M., Kumarb, K., Mukhopadhyay, K., and Sharon,M. (1999) Contribution of sp3 clusters in films and fibers obtained from camphor. Diamond andRelated Materials, 8(2–5): 481–484.

17. Sharon, M., Mukhopadhyay, K., Mukhopadhyay, I., and Krishna, K. M. (1995) Semiconductingmultichannel-multilayer camphoric tubules. Carbon, 33(3): 331–333.

18. Jagadale, P., Sharon, M., and Kalita, G. (2007) Carbon thin films from plant derived precursors.Synthesis and Reactivity in Inorganic, Metal-Organic, and Nano-Metal Chemistry, 37: 467–471.

Dow

nloa

ded

by [

Tok

yo K

ogyo

Uni

vers

ity]

at 1

9:26

21

July

201

1