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Yadav, M K

Surbhi

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Indian J. Phys. 82(1) 117-121(2008) 
Electronic spectral studies of 4-bromo-2-fluoro phenol 
in various solvents 
M K Yadav* and Surbhi 
Molecular Spectroscopy and Biophysics Laboratory, D N (PG) College, Meerut-250 002, Uttar Pradesh, India 
h-mail sap amW'rcdiffmdil com 
Received 22 Novembei 2006, accepted 11 Decembei 200b 
Abstract : Electronic spectra of 4-bromo-2-fluoro phenol in polar solvents (ethanol, methanol and chloroform) 
have been recorded in liquid state in the region 200 to 400 nm The effect of halogen atom on the -n--n\ n-w* and 
n-n* transitions have been studied The n-n*, n-n* and n-n* transitions have been shown the usual behavior 
towards the substitution and polarity of solvent. The effect of pH variation on electronic transitions of this compound 
in ethanol solvent has also been studied 
Keywords : Ultraviolet absorption spectra, electronic absorption bands. 
PACSNo. : 33.20.Lg 
Phenol has been the subject of numerous studies, since it is a good model for the 
investigation of hydrogen bonding and proton transfer in enzymes. Recent spectral 
investigation on halogen phenols find interesting application in agriculture and pharmacy. 
Solvent effect on electronic absorption spectra of halogen derivatives have been 
studied by many workers [1-3]. Experimentally some workers [4,5] have found 
evidence of the presence of electronic transitions other than n-n* and 7r-7r* in the 
spectra of halogen derivatives of phenol and aniline and on substitution n-7r*and n-n* 
system of phenol produce bathochromic (red) shift and hypsochromic (blue) shift 
respectively. But no work appears to have been carried out on the ultraviolet 
absorption spectra of 4-bromo-2-fluoro phenol. The present paper reports the solvent 
effect on the electronic spectra of the said compound. The effect of pH variation has 
also been discussed on the electronic transitions of the said compound. 
•Corresponding Author © 2008 IACS 
118 MKYadavandSurbhi 
The spec-pure compound 4-bromo-2-fluoro phenol was obtained from M/s Aldnch 
Chemie, West Germany used for spectral studies Its purity was confirmed by 
elemental analysis and melting point determination The ultraviolet spectra of the said 
compound were recorded in solvents (ethanoi, methanol and chloroform) on UV-VIS 
double beam spectro-photometer, Model M-1601 in the region of 200-400 nm in liquid 
state The solvents used for preparing the solution of the compound were of 
spectroscopic grade The pH value of solution was measured with Systronic pH digital 
meter (Model 335) This system was standardized a t p H 4 ± 0 1 , 7 ± 0 1 , 9 ± 0 1 
at room temperature with the help of standard buffers The concentration of the 
solution in all cases was kept constant (8 x 10~3 gm/litre) The ratio of pure solvent 
and acid or alkali was maintained at 9 1 by volume 
The molecular structure of 4-bromo-2-fluoro phenol are shown in Figure 1 
The effect of solvents on the electronic transitions of this compound is given in Table 
1 The effect of pH variation in solvent ethanoi on the electronic transitions of this 
compound is given in Table 2 The near ultra-violet absorption spectra of this 
compound are shown in Figure 2 The absorption spectra of this molecule in ethanoi 
solvent at different pH is shown in Figure 3 
OH 
Br 
Figure 1. Molecular structure of 4-bromo-2-fluoro phenol 
Table 1. Effect of solvents on electronic transition of 4-bromo-2-fluoro phenol (all values are in mm) 
Solvent 
Ethanoi 
Methanol 
Chloroform 
Rt 
1 3773 
1 3662 
1 458 
DC 
25 
32 5 
4 5 
n-7r* 
287 
279 
279 
7T-7T* 
-
224 
241 
n-cr* 
209 
206 
203 
Rl = Refractive index, DC * Dielectric constant 
Effect of substftution and solvent polarity on electronic transition : 
In phenol a band observed at 230-245 nm corresponds to A, -+ B2 transition and is 
derived from A1g -> B2u transition on lowering the symmetry from D6h -> C2v. 
Electronic spectral studies of 4-bromo-2-fluoro phenol in various solvents 119 
200 350 250 300 
Wavelength (nm) 
Figure 2. Ultraviolet absorption spectra of 4-bromo-2-fluoro phenol in ethanol (—), methanol ( ) and chloroform 
(-0-) 
Table 2. Effect of pH variation on electronic transition of 4-bromo-2-fluoro phenol (all values are in mm) 
Solvent Rl pH value n-7r* n-a" 
Ethanol 
Ethanol + HCI 
Ethanol + NaOH 
1 3773 8 76 
3 90 
11 98 
287 
280 
297 
-
225 
244 
209 
209 
209 
A 
B 
S 
200 
i 
350 250 300 
Wavelength (nm) 
Figure 3. Ultraviolet absorption spectra of 4-bromo-2-fluoro phenol in ethanol (—-), neutral ( ) acid and (-0-) 
alkaline medium. 
120 MKYadavandSurbhi 
Jeegagal and Rao [6] suggested that 210 nm and 220 nm bands of trisubstituted 
benzene corresponds to A l f l - B1u and A1g - , E1u transitions of benzene. Goyal and 
Gupta [7] have calculated the electronic transition H-TT* corresponds to out of plane 
transition 7r-7r* and n-rr * transitions inplane transitions. 
According to Silverstein [8] the molecules containing non bonding electron 
substituents, there occur a band around 280 nm which corresponds to n-n* transition, 
band 200-215 nm corresponds to n-n* transitions. 
The emergence of n-n* transition only in hydrogen bonding solvent is due to 
the increase in excitation energy. The hydrogen bonding which stabilized the ground 
state will be weakened by the shift of electron density away from the non bonding 
centre on excitation, with a consequent increase in the excitation energy and appearance 
of a blue shift. The higher the polarity of the solvent greater will be the amount of 
solvation. However solvation may be partly due to hydrogen bonding of solvent (if 
protonic) with solute [9]. Methanol being more polar and protonic than ethanol, is 
stronger hydrogen bonding solvent. This explains very well the blue shift of n-n* bands 
observed in 4-bromo-2-fluoro phenol on increasing the polarity of solvent (ethanol —• 
methanol -» chloroform) given in Table 1 which agrees with the literature values 
[10,12]. 
For n-a* transition the ground state is more stabilized than exited states and 
absorption shifts to shorter wavelength and the blue shift increases as the solvent 
polarity increases [9]. The band position may be much influenced by other factors 
such as hydrogen bonding. The shift in hydrogen bond may be explained on the basis 
of solution energies of solute which includes induced dipole-dipole forces between non 
polar solvent and polar solvents [13]. According to Jagmohan [13], on excitation of the 
localized non bonding electron is delocalized on the less electronegative atoms over 
which the exited orbital {-n* or a*) is reduced in the exited state. In present study the 
blue shift in n-o"transition is observed in 4-bromo-2-fluoro phenol on increasing the 
polarity of solvents (ethanol -* methanol -* chloroform) given in Table 1 which is 
similar to the behaviour observed in literature [11,12,14]. 
For TT-TT* transition both ground state and exited stated are stabilised and 
absorption moves to longer wavelengths. The TT-TT* transition undergoes a red shift with 
increasing the polarity of solvents (ethanol -> methanol - * chloroform) [8,9]. This can 
be attributed to the monetary polarization of the solvents by transition dipole of solute. 
From Table 1 it is evident that red shift is observed in the TT-TT* transitions of 
4-bromo-2-fluoro phenol on increasing the polarity of solvents (ethanol -> methanol -> 
chloroform) which is in good agreement with literature values [10-17]. 
Effect of pH variation : 
Since the interpretation of spectral measurement to a considerable extent mainly 
depend on the choice of sites of localized of protons, the question of possible effect 
Electronic spectral studies of 4-bromo-2-fluoro phenol in various solvents 121 
of pH variation on electronic transition of the said compound under study is very 
important. 
In case of dichloro-nitro-aniline there is a blue shift in the position of entire 
band with decrease in pH, the n-n-* and 7r-7r* bands shifted towards shorter wavelength 
m the acidic solution by many workers [14]. KaIsi [9] have suggested a blue shift of 
n-system in strong acidic solution due to the binding of non bonded electrons by 
protonation. In accordance with the above, we have also observed a blue shift in n-n* 
and 7r-7r* bands with the addition of acid in the aqueos solution of 4-bromo-2-fluoro 
phenol as the neutral form changes to acidic form i.e. with the increase in pH value 
a red shift is observed in the n-n* and 7r-7r* bands of the said molecule is given in 
Table 2. This is in good agreement with literature value [10-14]. 
Acknowledgment 
Authors are highly grateful to Shri Abdul Hak, Head Technician, IIT Roorkee for 
arranging necessary facilities for testing and fruitful guidance. 
References 
[1] S P Gupta S Ahmed and A Gupta Acta Physica. Polonica. 68A 637 (1985) 
[2] M S Navati, G B Kalkoti and P R Jeergai Spectrochimica. Acta 51A 909 (1995) 
[3] K Singh, B B B Lai and I S Singh Indian J. Pure Appi Phys. 11 512 (1973) 
[4] P K Mallick and S B Banarjee Indian J. Pure and Appi Phys. 12 296 (1974) 
[5] R B Singh, N P Singh and D K Rai Indian J. Pure Appi. Phys. 19 740 (1981) 
[6] Rekha Rao, P R Jeeragal and N R Katti Indian J. of Phys. 65B 272 (1991) 
[7] R K Goyal, S K Gupta and R M P Jaiswal Indian J. Pure Appi Phys. 21 748 (1983) 
[8] R M Silverstein, G C Bassler and T C Morrill Spectrometric Identification of Organic Compounds (New 
Yark : John Wiley) Ch 4 p305 (1981) 
[9] P S KaIsi Spectroscopy of Organic Compounds (New Delhi: New Age International) Ch.2 (2005) 
[10] B S Yadav ana V Singh Spectrochem. Acta 55A 1261 (1999) 
[11] Vir Singh, Seema, Subhash Chand and B S Yadav Oriental J. Chem. 12 295 (1996) 
[12] B S Yadav, Seema, V Kumar and U K Jetely India J. Pure Appi. Phys. 35 587 (1997) 
[13] Jagmohan Organic Spectroscopy Principles and Applications (New Delhi : Narosa Publishing House) 
(2004) 
[14] B S Adam, M K Yadav, Vipin Kumar and Vir Singh Asian J. Chem. 9 372 (1997) 
[15] B S Yadav, Karuna Gupta and Subash Chand Indian J. Phys. 77B 417 (2003) 
[16] B S Yadav and Nitin Kumar Acta cienica Indica XXXI539 (2005) 
[17] Q Liu D Fang and J Zheng Spectrochimica. Acta 60 1453 (2004) 


English

Electronic Spectral Studies of 4-Bromo-2-Fluoro Phenol in Various Solvents

Indian J. Phys. 82 (1), 127-130 (2008) 
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Choudhury, A

Structural and Optical Properties of 150 MeVTi<SUP>+11</SUP> Irradiated Doped Nanostructures

Borah, Sankar Moni

Pal, Arup Ratan

Bailung, Heremba

Chutia, Joyanti

Indian J Phys 82 (2), 209-213 (2008) 
Titanium nitride nano-structure by DC magnetron 
sputtering plasma 
Sankar Mom Borah*, Arup Ratan Pal, Heremba Bailung and Joyanti Chutia 
Plasma Physics Laboratory, Material Sciences Division 
Institute of Advanced Study in Science & Technology (IASST) 
Vigyan Path, Paschim Boragaon, Gorchuk Guwahati Assam-781 0^5 India 
E mail sankarmonift/ gmail com 
Abstract Titanium nitride is useful as hard coating for surface durability It also provides wear and corrosion 
resistance to the coated material Direct current magnetron sputtering is used for deposition of titanium nitride 
(TiN) on an alloy of copper and tin (bell-metal) substrate Plasma is produced in argon and nitrogen reactive gas 
environment with titanium (cathode) as the target for the sputtering mechanism Qualitative study of the deposited 
titanium nitride film shows the formation of nano-structure XRD study confirms the formation of TiN, SEM and 
AFM analyses are carried out to study the surface morphology of the deposited film The TiN film provides a 
permanent hard protective and anti-corrosive bright gold coloured coating to the bell-metal substrate 
Keywords Magnetron, sputtering, hard coating, nano-structure 
PACS Nos. 47 54 Jk, 52 55 -s 52 80 Sm 
1. Introduction 
Direct current magnetron glow discharge process is used for sputter deposition of different 
compound films in a large number of technological fields [1, 2). Some of the examples 
are as follows - multiple films on magnetic recording media or heads, metallic interconnects 
in microelectronics, thin film solar cells, various coatings namely optical, decorative or 
protective. It is also regarded as an environment friendly process since it fully avoids the 
use of various chemical reactants [3] Sputtering deposition has become a generic name 
for a variety of sputtering processes. Among the various sputtering processes, the one, 
which is most widely used today, involves the use of magnetic field perpendicular to the 
electric field and is known as magnetron sputtering. Magnetron sputtering uses confinement 
of excited plasma in crossed magnetic and electric fields that results in high deposition 
Corresponding Author 
©2008 I ACS 
210 Sankar Mom Borah, Arup Ratan Pal, Heremba Bailung and Joyanti Chutia 
rates and good reproducibility [4, 5] Titanium nitride is a rocksalt structure (NaCI) compound 
consisting of Ti atoms filled in fee based lattice with all octahedral sites filled with nitrogen 
atoms as shown in Figure 1 Some of its physical properties are as follows Lattice 
parameter , 0 424 nm , Melting point 3173 K 
Thermal stability (AhP9*) . 336.6 kJmor1; Young's 
modulus 450 GPa TiN thin films have found 
applications as hard coatings, decorative 
coatings, wear resistant coatings and also 
diffusion barriers in IC technology [6] Here, we 
have presented an analysis of TiN thin film 
deposited on an alloy of copper and tin (bell-
metal) in reactive N^Ar gas environment by direct 
current magnetron sputtering with X-ray diffraction 
(XRD), scanning electron microscope (SEM) and 
Figure 1. Schematic structure of TiN 
atomic force microscope (AFM) techniques 
2. Details 
The experimental cylindrical magnetron device is a stainless steel chamber having 
dimensions of 30 cm in diameter and 100 cm in length A small titanium cylinder is 
placed coaxially inside the chamber which acts as the cathode The length of the cathode 
is 25 cm and its outer diameter is 3 25 cm A schematic diagram of the experimental set 
up is shown in Figure 2. For generation of a steady axial magnetic field, two uniform coils 
are placed around the body of the chamber Low pressure is created inside the chamber 
Pirani Gauge 
Penning Gauge 
Rotary Pump 
Diffusion Pump 
Figure 2. Schematic diagram of experimental set-up E - Electric field, B - Magnetic field, ER - End 
reflectors, Lp - Langmuir probe, MM - Magnetic colls, PS - Power supply 
Titanium nitride nano-structure by DC magnetron sputtering plasma 211 
usmg a combination of rotary and diffusion pumps The base pressure of the chamber is 
of the order of 10"6 Torr and working gas pressure is varied within the range of (1 x10~3 
- 3 x 10~3) Torr A Pirani gauge and an ionization gauge are used for measurement of 
pressure inside the chamber The discharge power is supplied from a stabilized DC power 
supply (1500 V, 5 A) working in the voltage-regulated mode Gas is injected to the chamber 
to raise the neutral pressure up to 10~3 Torr by using a double valve system consisting of 
a stop valve and a needle valve (fine control) Plasma is produced at a pressure of 2 5 x 
10~3 Torr by applying 600 V DC bias between central post electrode as cathode and the 
grounded chamber as anode. Langmuir probe is used as a diagnostics for the estimation 
of various plasma parameters namely the electron temperature, the plasma potential and 
the plasma density 
X-ray diffraction (XRD) pattern of titanium nitride film deposited on bell-metal substrate 
is shown in Figure 3 Thin film XRD pattern is recorded using Cu Ku radiation source of 
wavelength, A = 1 54 A in Bruker AXS make X-ray diffractometer installed at 
Nanotechnology Centre, IIT Guwahati The deposition condition of the TiN film is as follows 
- discharge power is 60 watt, magnetic field is 200 gauss and gas pressure is 2 5 x 
10~J Torr The deposition time is 30 minutes and the substrate is placed at a distance of 
8 cm from the cathode A sharp intense peak corresponding to TiN (200) is observed 
along with another peak TiN (220) of less intensity from the figure The metallic Cu (111) 
and Cu(200) peaks seen corresponds to the bell-metal substrate 
TIN (200) 
Figure 3. XRD pattern of TiN on bell-metal substrate Figure 4 SEM picture of TiN film 
Figure 4 shows SEM picture of the deposited TiN film obtained using LEO-143VP 
make scanning electron microscope installed at Central Instrumentation Facility, IIT 
Guwahati. The picture shows the formation of TiN structures resembling black dots 
distributed throughout the scanned film surface. The dimensions of these structures are 
measured and are found to be in the nano-scale range with an average size of - 180 nm 
The presence of nano-structure in the deposited TiN film is of great importance and utility 
as it will lead a very hard coating to the substrate material No inter-metallic phase peaks 
are found in this study using SEM EDX 
212 Sankar Mom Borah, Arup Ratan Pah Heremba Bailung and Joyanti Chutia 
The surface morphology of the deposited TiN film has been further examined using 
atomic force microscope (AFM) This study is done in a SMENA-B make AFM instrument 
at Physics Department, IIT Guwahati The surface image recording is done by operating 
the instrument in the tapping mode 
Figure 5(a) shows the surface of the deposited TiN film in the presence of both argon 
and nitrogen gases (Ar N^ , = 1 1) which is found to be the optimized deposition condition 
of the film In Figure 5(b), the surface morphology of the film in the presence of nitrogen 
gas only has been shown The maximum roughness of the deposited film is of the order 
of 100 nm But at most places, it has a value of - 40 nm On careful observation of the 
two figures, one can see the formation of dotted net like structures on both the deposited 
TiN films which are found to maintain a distinct pattern throughout the surfaces of both 
films Hardness study by nano-indentation method shows that the deposited TiN film over 
the bell-metal substrate increases the surface hardness by a factor of 13 under the above 
deposition condition The Copper Acetic Acid Salt Spray (CASS) test was done to 
determine the anti-corrosive property of the TiN thin films The CASS test is now covered 
by BS 5466 Part III 1977 (ISO 3770 and ASTM B368) [7] The film exhibited no change 
in its surface morphology for 48 hours This indicates that the TiN coating is able to 
shield the bell-metal surface from the severe and hostile environment and thus, provide a 
good protective cover to it 
Figure 5(a). AFM picture of TiN in argon and Figure 5(b) AFM picture of TiN in nitrogen only 
nitrogen 
3. Summary 
TiN thin film deposited on bell-metal substrate is found to form nano-structures under the 
above mentioned deposition condition The nano-structures possess a definite structural 
pattern and it provides a very hard coating over the substrate material The film coating 
acts as a protective layer and prevents the bell-metal surface from corrosion and getting 
oxidized Further, the golden colour of the TiN film enhances the beauty of the surface 
4 
Acknowledgments 
This work was supported by a grant from the Department of Science and Technology, 
Government of India. The authors greatly acknowledge the facilities provided by the Centre 
Titanium nitride nano-structure by DC magnetron sputtering plasma 213 
tor Nanotechnology, the Central Instrumentation Facility and Prof A Khare, Head, 
Department of Physics of IIT Guwahati for XRD, SEM and AFM analyses of the 
samples. 
References 
[1] F Debal, J Bretagne, M Jumet, M Wautelet, J P Dauchot and M Hecq Plasma Sources Sci Technol 
7 219 (1998) 
[2] A Grill Cold Plasma in Materials Fabrication (New York IEEE) (1994) 
[3] J C Roth Industrial Plasma Engineering Vol 1 (Bristol Institute of Physics) (1995) 
[4] K Wasa and S Hayakawa Handbook of Sputter Deposition Technology Principles, Technology 
and Applications (Westwood New Jersey Noyes Publications) (1992) 
[5] R Behnsck and K Wrttmaack Sputtering by Particle Bombardment III Characteristics of Sputtered 
Particles, Technical Applications (Berlin Heidelberg Sprmger-Verlag) (1991) 
[6] Xu Xuan-qian, H Ye and T Zou J Zhejiang Univ Science A7(3) 472 (2006) 
[7] V E Carter Corrosion testing for metal finishing (Birmingham Institute of Metal Finishing, Butterworth 
Scientific) (1982) 


Titanium Nitride Nano-Structure by DC Magnetron Sputtering Plasma

Ahmed, Sk F

Mitra, M K

Chattopadhyay, K K

Indian J. Phys. 82 (2), 223-228 (2008) 
Electrical characterization of multiwalled carbon nanotubes 
synthesized by DC- plasma enhanced chemical vapor 
deposition technique 
Sk F Ahmed3, M K Mitrab and K K Chattopadhyay' b* 
aThin Film & Nanoscience Laboratory, Department of Physics 
b
 Nanoscience and Technology Center, 
Jadavpur University, Kolkata-700 032, India 
E-mail kalyan_chattopadhyay(</ yahoo com 
Abstract Thermoelectric and electrical conductivity properties of multiwalled carbon nanotubes deposited 
by plasma enhanced chemcal vapor deposition with acetylene as precursor using Ni as a catalyst on Si 
substrates have been studied in detail The substrate temperature has been varied from 750 - 850fC It was 
found that the thermoelectric power of the MWCNT is positive over the whole temperature range, and increases 
with increasing the deposition temperature and showed p-type conduction in the films Room-temperature 
Seebeck coefficients were found in the range 11 12 to 31 24 uVK \ for films deposited with different substrate 
temperature X-ray diffraction studies confirmed the graphitic phase of carbon nanotubes Transmission electron 
microscopy study showed the diameter of the carbon nanotubes lying in the range 15-20 nm The room temperature 
electrical conductivity (oRT) of the carbon nanotubes increased with increase of the substrate temperature, as 
usual for semiconductors and the highest room temperature electrical conductivity obtained was 4 07 x 10J S cm 
' for the sample deposited at 850 °C substrate temperature 
Keywords • Carbon nanotubes; Thermoelectric properties ; p-type conductivity 
PACS Nos. 61 46 Fg, 72 20 Pa, 72 80.Cw 
1. Introduction 
Carbon nanotubes continue to attract much research interest due to their remarkable 
mechanical and electrical properties and promising applications. The carbon nanotubes 
consist of concentric tubes of graphene sheets. Different groups showed that the nanotubes 
have metallic or semiconducting properties depending on their diameter and chirahty [1], 
Nanotubes have been widely used for many applications, such as field emission electron 
guns [2], optical communication device [3], scanning microscope probe tips [4], electronic 
circuits [5], and nanowires [6]. The most common production techniques for carbon 
* Corresponding Author 
© 2008 I ACS 
224 Sk FAhmed, M K Mitra and K K Chattopadhyay 
nanotubes are hydrogen arc discharge, thermal chemical vapor deposition, hot filament-
assisted sputtering, RF magnetron sputtering and Plasma enhanced chemical vapor 
deposition (PECVD). We have chosen plasma enhanced chemical vapor deposition 
(PECVD) for the production of carbon nanotube, because it is a high yield and controllable 
method for production carbon nanotubes with mass production. We have used Ni as a 
catalyst for the formation of carbon nanotubes, because carbon nanotubes formed on Ni 
are more crystalline than those formed using other catalyst [7]. In this paper we report 
the synthesis of carbon nanotubes via DC-PECVD technique. Thermoelectric and other 
electrical properties of the films have been studied. X-ray diffraction patterns and transmission 
electron microscope (TEM) were used to characterize the structure of the films. 
2. Experimental details 
For the deposition of Ni catalyst in thin film form by dc sputtering process we have used 
a Ni plate of thickness - 1 mm with a dimameter 2.5 cm (with purity 99.99%, Aldnch). 
The substrates were 1 0 x 1 0 mm2 cleaned Si (400) wafer. The sputtering was done at a 
pressure 0.2 mbar sending argon as a sputtering gas. The deposition was made for 7 
minutes at room temperature with an inter-electrode distance 1.6 cm. For sputtering, we 
maintained high voltage 2.0 KV and corresponding current density was 19.5 mA cm - 2 
After deposition of the catalyzed film, the sample was immediately transferred to the CVD 
chamber where the synthesis of nanotubes was performed. The deposition chamber was 
initially evacuated by a standard rotary and diffusion pumps arrangement upto a base 
pressure of 10~6 mbar. The silicon substrate was placed on a Molybdenum substrate 
holder, which could be directly heated. When the chamber pressure attained 10~6 mbar, 
the Mo substrate holder was started to heating by sending current thorugh it. The substrate 
temperature was measured by a disappearing filament type Pyrometer (PYROPTO, IT65). 
Acetylene (C2H2) gas was used in PECVD process as a precursor of carbon. Acetylene 
(C2H2) gas was allowed to flow maintaining the CVD chamber pressure 50 mbar. Deposition 
was performed at 2.0 kV dc supply with corresponding current density 25 mA cm"2 for 30 
min. Substrate temperature was varied from 750 to 850°C for different set of samples. 
3. Results and discussion 
3.1 Nanostructural studies and X-ray diffraction: 
The nanostructure of the films was studied by a transmission electron microscope (TEM, 
JEM-2010, JEOL, JAPAN). The micrographs of multiwalled carbon nanotubes have been 
shown in Figure 1. From figure it was found that the diameter of the carbon nanotubes 
decreases with increase the substrate temperature. In our experiment the Ni catalyst 
particles were prepared in a separate DC sputtering unit as described in the experimental 
section. Then these Ni particle-coated Si substrates were placed in the CVD chamber. 
The chances of coalescence of the Ni nanoparticles will depend on two factors : on their 
proximity and also on the rate of substrate temperature increment. In our case the nano 
Ni particles were well dispersed and the substrate temperature was increased very quickly 
Electrical characterization of multiwalled carbon nanotubes synthesized etc 225 
to the desired value (we used Mo thin plate as a heater, hence very large current could 
be made to flow very quickly) Hence in our experiment the Ni nanoparticles could not 
form bigger particles by coalescence With the increase of substrate temperature the 
rate of diffusion of the carbon precursor through the metal catalyst increased, that led to 
smaller diameter but larger length. 
Figure 1 HRTEM micrograph of the MWCNTs film deposited at (a) 800 °C and (b) 850 °C 
X-ray diffraction pattern was recorded by a X-ray diffractometer (Bruker D8 Advance) in 
20 range 20° - 80° using Cu Ka radiation of wavelength \ = 0 15406 nm The films 
deposited on Si substrates showed crystalline structure as shown in Figure 2 The XRD 
spectrum showed various peaks of carbon nanotubes due to diffraction from (022), (100), 
(004) and (110) planes The interplanner spacing (3 40 A, 2 12 A, 1 71A and 1 23A) 
obtained form XRD peaks are well matched with the standard reported data [8] From the 
X ray diffraction of carbon nanotubes (as shown in Figure 2), the (100) and (110) peaks 
have been observed at 42 62° and 77 45° respectiely and the inter-planar spacing (d100 = 
P 12 A and Dl10 = 1 23) is little smaller than that of graphite (d100 = 2 13 A and d11() = 
1 23), which indicates the carbon nanotubes and graphite have similar structures [9] 
CM 
C\J 
20 30 40 50 60 70 80 
20 (degree) 
figure 2. X-ray diffraction pattern of the carbon nanotube film deposited at 850°C 
226 Sk F Ahmed, M K Mitra and K K Chattopadhyay 
3.2 Electrical measurements: 
The electrical conductivity was measured by usual four-probe technique using Keithley 
electrometer (model 6514) to measure the current. All contacts were made by silver paste 
which showed ohmic characteristics over a wide range of voltages and temperatures 
The electrical conductivity of the films can be expressed by the standard relation as 
follows : 
<J - rr0 e x p AT 
where o0 is a temperature independent factor, Ea is the activation energy of the material 
For p-type semiconductor, this is the energy difference between the acceptor level and 
the top of the valence band. Therefore, the slope of the plot of incr vs 1000/T would carry 
the information of the activation energy of the material as shown in Figure 3. The electrical 
conductivity {a ) of the films deposited with different substrate temperature has been 
measured at temperature (T) ranging from 300 K to 450 K. The conductivities of the films 
increase with the increase of temperature as usual for semiconductor. An increase in the 
room-temperature conductivity (oR T ) was observed with the increase in deposition 
temperature (t) of the films. (For example, films with t = 750, 800 and 850 °C, aR1 = 
5.29 x 103, 1.64 x 103, 1.64 x104 and 4.07 x 104 S cm"1). From the slope of the graph 
we calculate the activation energy (Ea). It was found that after 375K the activation energies 
(Eaa) for the CNT films varied from 0.096 to 0.056 eV and below 375K activation energies 
(Eab)varied from 0.776 to 0.513 eV. The lowest activation energy was obtained for the 
sample deposited at 850 °C substrate temperature. The two types of activation energies, 
arises in two different temperature range, are due to two different acceptor levels generated 
during growth of the materials. To activate the carriers in these accetpor levels of our 
MWCNT thin films, two different energies are required. 
4424134 
162755A 
59874-J 
22026" 
8103 
— • — t - 850 °C 
- • - t = 800 °C 
— A - t = 750 °C 
* ^ , , 
****** A 4
- " ^ A 
»
 t • • | . , » ! , | • , 
2 24 26 28 3 0 3 2 3 4 
1000/T (K" 
Flgure 3. In <j vs 1/7 plots of the CNT thin films for different substrate temperature (t). 
Electrical characterization of multiwalled carbon nanotubes synthesized etc 227 
The thermoelectric power of the films has been measured keeping the cold junction at 
room temperature (300 K) and the temperature of the hot junction was varied from 300 to 
450 K. The seebeck coefficients of the films were calculated from the slope of the plots 
of thermo-emf (E) vs. difference of temperature (T) between the hot and cold junctions. 
Figure 4 shows the seebeck coefficient (S) versus 1000/T plots for the MWCNT thin films 
for different substrate temperature during deposition. Seebeck coefficients increased with 
the increase of substrate temperature during deposition of the carbon nanotubes thin 
films. The values of room temperature seebeck coefficients for the films lying in the range 
11 12 to 31.24 JLIVK"1 for different deposition temperature. Room temperature seebeck 
coefficient is 31.24 jiVK""1 for 850°C substrate temperature, which is higher than at TEP 
of CNT reported by Tian et al [10] and always positive. This may imply that the majority 
carriers in the nanotube should be P-type, and hole carriers are dominant. This result is 
in good agreement with the Hall coefficient measurement reported by Song et al [11] 
where the observed Hall coefficients are positive over entire temperature range. From the 
well-known Mott relation it is known that the Seebeck coefficient S depends on the density 
of state function at the Fermi level [12]. Now with the decrease of diameter of the MWCNT, 
the surface to volume ratio increased and hence the density of defect state increased 
near the Fermi level. The Seebeck coefficient for the CNT films deposited at higher 
temperatrue increased as in this case diameter of the MWCNT decreased. 
40 
35 
30 
i 25 
15 
10 
2 2 2 4 26 28 30 3 2 34 
1000/T(K'1) 
Figure 4. Seebeck coefficient (S) vs. 1000/7 of MWCNTs thin films for different substrate temperature 
during deposition (t) 
4. Conclusion 
MWCNTs in the thin films form have been successfully synthesized on Si substrates via 
PECVD using Ni as a catalyst. X-ray diffraction patterns have confirmed the graphitic 
phase of carbon nanotubes. TEM studies showed that the CNTs are multiwalled. The 
diameter of the carbon nanotubes lies in the range 15-20 nm. The room temperature 
conductivity of the carbon nanotubes increased with increase of the substrate temperature, 
as usual for semiconductors and the highest room temperature conductivity obtained was 
228 Sk F Ahmed, M K Mitra and K K Chattopadhyay 
4.07 x104 S cm"1 for the sample deposited at 850 °C substrate temperature. Thermoelectric 
measurement of the carbon nanotubes thin films, showed considerable high value of room-
temperature seebeck coefficients ranging from 11.12 to 31.24 nVK~\ for different substrate 
temperature during deposition of the films with p-type conductivity. This study shows the 
MWCNTs may become a useful candidate for thermoelectric applications. 
Acknowledgment 
The authors wish to acknowledge the financial support by the University Grants Commission 
(UGC), Govt, of India, under the 'University with Potential for Excellence' Scheme during 
the execution of the work. 
References 
[1] J M Mintmire, B I Dunlap and C T White Phys. Rev Lett. 68 631 (1992) 
[2] A G Rinzler, J H Hafner, P Nikolaev, L LouF S G Kim, D Tomanek, P Nordlander, D T Colbert and R 
E Smalley Science 269 1550 (1995) 
[3] S Tatsuura, M Furuki, Y Sato, I Iwasa, M Tian and H Mitsu Advanced Materials 15 534 (2003) 
[4] H J Dai, J H Hafner, A G Rinzler, D T Colbert and R E Smalley Nature 384 147 (1996) 
[5] Y Y Wei and E Gyula AypL Phys. Lett 76 3759 (2000) 
[6] C Dekker Physics Today 52(5) 22 (1998) 
[7] P L Walker, B C Banerjee and T J Hirt Nature 192 450 (1961) 
[8] R Seshadn, A Govindaraj, N H Aiyer, R Sen, G N Subbanna, A R Raju and C N R Rao Current 
Science 66 839 (1994) 
[9] JCPDS data file no. 00-013-0148 
[10] M Tian, F Li, I Chen and Z Mao Phys. Rev. B58 1166 (1998) 
[11] S N Song, X K Wang, R P H Chang and J B Ketteson Phys. Rev. Lett 72 697 (1994) 
[12] M Baxendale, G K Lim and G A J Amaratunga Phys Rev B61 12705 (2000) 


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