IACS Institutional Repository

Sarma, Sidananda

Srinivasan, A

Indian J. Phys. 82 (6), 701-706 (2008) 
Influence of cooling rate on the properties of ferromagnetic 
shape memory alloy 
Sidananda Sarma* and A Snnivasan 
Department of Physics, Indian Institute of Technology, Guwahciti, 
Guwahati-781 039, Assam, India 
E-mail sida(</ utg ernet in 
Abstract Polycrystalline Co4r)Ni^Ga30 alloys were prepared by arc melting method, followed by the 
quenching separately into liquid nitrogen, ice cold water and air, respectively The powder X-ray diffraction 
pattern of slow cool alloys showed a mixed phase structure, whereas the quenched alloys revealed a single 
phase structure No marked structural difference was observed in the X-ray diffraction patterns of the air 
cooled, ice quenched and liquid nitrogen quenched samples However, the martensitic and austenite transformation 
parameters observed by Differential scanning calorimeter (DSC) showed appreciable change in quenched 
samples The slow cooled sample did not show any signature of martensitic transformation The surface relief 
due to martensitic transformation was observed in optical microscope for all three quenched samples The 
thickness of the twin lines increased for samples cooled at higher rates The Curie temperature of the three 
samples shows subtle variations 
K e y w o r d s MaitensiU, Cunc tcmpeiaturt 
PACS Nos. 64 70Kb, 75 50Cc 
1. Introduction 
Ferromagnetic shape memory alloys (FSMA) are excellent candidates for magnetic actuator 
applications since one can invoke large strain in them either through the martensitic 
transformation or rearrangement of martensite variants induced by an external magnetic 
field. The observation of up to 5.7% magnetically induced shear strain in Ni-Mn-Ga [1] 
has intensified the search for other alloys exhibiting FSMA behavior. Co-Ni-Ga [2] and Co-
Ni-AI [3] FSMAs have recently attracted attention due to their good ductility, lower cost 
and stability in preparation. Co-Ni-Ga alloys have a wide range of martensitic transformation 
temperatures and Curie temperatures, which are both sensitive to alloy composition. The 
normal procedure for preparation of these alloys involves preparation of the master alloy 
Corresponding Author 
© 2 0 0 8 I A C S 
702 Sidananda Sarma and A Snnivasan 
ingots by arc melting (or induction melting) followed by homogenization at high temperature 
and quenching into cold temperature. This is done to retain a high temperature phase at 
room temperature and escape some intermediate phase normally present in the system 
In our knowledge, no systematic study on the influence of the cooling rate on the properties 
of these alloys has been carried out. A Differential Scanning Calorimetnc study of 
Co45Ni25Ga30 along with Curie temperature (Tc) of alloy quenched at different cooling rates 
is reported here. 
2. Experimental details 
Polycrystalline ingots (5g weight) of Co45Ni25Ga30 alloys were prepared by arc melting 
appropriate amounts of high purity elemental powders of Co (99 9+%, 100 mesh, Aldrich) 
Ni (99.99%, 100 mesh, Aldrich) and Ga (99.99%, NFC, India) under argon atmosphere, 
homogenized at 1423K for 4 hrs and slowly cooled inside the furnace under a pressure 
~105 mbar. Three small discs were cut out from the slow cooled alloy, sealed in three 
different fused silica ampoules at a pressure ~10'5 mbar, annealed at 1423K for 4 hours 
and then quenched separately into liquid nitrogen, ice cold water and air, respectively In 
this manner, three alloys were prepared under different cooling rates Powder X-Ray 
diffraction (XRD) studies were performed on the three alloys using Seifert 3003 T/T XRD 
(Cu K(x = 1.5406 A). The thermal studies were performed using a differential scanning 
calorimeter (DSC PerkmElmer DSC7) at a constant heating/cooling rate of 20°C/minute 
The surface morphology of the alloys was observed using an optical microscope (Zeiss 
KS 300) in reflection mode. An indigenously developed low-field ac susceptometer (ACS) 
was used to determine the ferromagnetic to paramagnetic transition (Curie) temperature 
(TCY 
3. Results and discussion 
XRD patterns (Figure 1) of both the ice water and liquid nitrogen quenched samples had 
six Bragg reflections at same 20 values All six reflections in Figure 1 were indexed to 
© Liq Nitrogen quenched sample 
c 
•e 
C 
40 _ 50 60 70 80 90 
Ice quenched sample 
S 8 £ = 
Slowly cooled sample 
40 50 60 70 80 90 
26(°) 
Figure 1. XRD pattern of Co^Ni^Ga^ alloys 
Influence of cooling rate on the properties of ferromagnetic shape memory alloy 703 
a body centred tetragonal structure corresponding to a s.ngle martensite phase The 
calculated lattice constants are a » b = 2.72 A, c = 3.23 A The powder X-ray diffraction 
pattern of slow cooled alloys showed a mixed phase structure The extra peaks present 
at 26 = 51° in the XRD pattern is due to the presence of some other phase in the slowly 
cooled alloy. 
The DSC curves of the two quenched samples are shown in Figure 2. The endothermic 
peak on the heating cycle and the exothermic peak on the cooling cycle correspond to 
the structural phase transitions from martensite-austenite (M->A), and austenite-martensite 
(A->M) phases, respectively. The starting temperature of martensitic transformation (Ms), 
the finishing temperature of the forward transformation (M), the starting temperature of the 
reverse martensitic transformation (As) and the finishing temperature of the reverse 
transformation (A) are indicated in Figure 2. The transformation parameters, namely, M 
Mr As 4, and the enthalpy involved in the transformations (AHAan4 AHM) are listed in 
Table 1. No evidence of M->A and/or A->M transformations was observed in slowly-cooled 
sample. This indicates that the martensite phase could be formed in this alloy only by 
rapidly cooling the homogenized master alloy. 
Liq Nitrogen quenched sample 
40 A 
20 i 
0 
_ A A A 
MXV, 
y 
i • i — i " ' i 
100 120 140 160 180 200 
40 
20 
0 
Ice quenched sample 
.Jl_ 
ao ioo i2o 
Air cooled sample 
180 «>O0 
40 
2 0 i 
0 
r 
80 100 120 140 IbO 
Temperature (°C) 
180 200 
Figure 2. D S C curves of Co 4 5 Ni 2 5 Ga 3 0 alloys 
Analysis of the DSC data of the air cooled, ice quenched and liquid nitrogen quenched 
alloys showed higher A and Af temperatures in liquid nitrogen quenched sample. When 
samples were quenched from a high temperature to a very low temperature suddenly, the 
elastic energy of the system can not dissipate by any other way. Thus the system 
accumulate (self accumulation of martensite) the elastic energy by displacive transformation 
of the lattice points. Thus higher quenching rate introduces more elastic energy (or density 
of dislocation) in the system. To change from martensite state to austenite state it has to 
overcome more elastic energy and hence the transformation temperatures are higher for 
samples with higher quenching rate. In short, it can be said that higher transformation 
temperatures can occur due to the presence of a higher concentration of defects in the 
704 Sidananda Sarma and A Srinivasan 
martensite phase [4]. The same phenomena cause more thermal hysteresis. (Af ~ M$ m 
a highly quenched system. The elastic energy around the martensite resists the growth 
of the martensite unless a further driving force (i.e. cooling) is given [5]. The endothermic 
and exothermic peak of air cooled sample is very weak and wide. Thus the martensite <-> 
austenite transformation of air cool sample is not sharp and hence cooling rate of air cool 
sample is not sufficient for the formation of shape memory alloys. The martesite <-> austenite 
transformation of ice quenched and liquid nitrogen quenched samples are very sharp. But 
very high rate of quenching introduces more thermal strain into the system and cracks 
are developed in the bulk sample. Hence ice quenching method is more suitable for the 
preparation of shape memory alloys. 
The surface relief due to martensitic transformation was observed in optical microscope 
for all three quenched samples. The thickness of the twin lines increased for samples 
cooled at higher rates. This is probably due to the larger displacement of lattice points to 
accumulate higher elastic energy for higher quenched rate sample (Liquid Nitrogen 
quenched). The twin lines are not distinct in the air cool sample. Moreover, some black 
spots were seen in the microstructure of the air cool sample. This may be due to the 
presence of small amounts of other intermediate phases which could not be detected in 
the XRD measurement. 
100 urn 50 nm 20 \xm 
Air cooled Ice quenched LN, quenched 
Figure 3. Optical microscope image of Co45Ni25Ga30 alloys. 
Table 1. DSC data of Co^Ni^Ga^ alloys. 
Transformation 
parameter 
A$ ( °C ) 
A, ( °C ) 
AA(=At~As) 
AHA (J/gm) 
Ms ( ° C ) 
H ( ° c ) 
AM (= Mf ~ Ms) 
AHM (J/gm) 
Hysteresis (A^Mg ) 
rc ( ° c ) 
Air cooled 
sample 
148 
179 
31 
2.318 
124 
92 
32 
3.45 
55 
131 
Ice quenched 
sample 
163 
166 
3 
4.588 
133 
129 
4 
4.306 
33 
119 
Liq. N2 
quenched sample 
177 
181 
4 
4.203 
137 
133 
4 
4.917 
44 
98 
Influence of cooling rate on the properties of ferromagnetic shape memory alloy 705 
The ferromagnetic to paramagnetic transition (Curie) temperature (Tc) was measured 
by ACS (Figure 4) and the Tc values are given in Table 1 7cwas lower for samples 
prepared with higher cooling rate (/ e samples quenched in liquid nitrogen) Tc is sensitive 
to local crystallography environment When the sample is quenched from high temperatures, 
the solidification is not under equilibrium conditions and this introduces some crystallography 
defects in the sample These defects create a local disorder in their vicinity Faster cooling 
rates introduce more defects in the sample Thus the degree of disorder in the sample 
prepared by quenching in liquid nitrogen is more than in the other two samples The 
increased crystallography disorder results in decreased grain size in polycrystalline 
materials Tc value show a tendency to shift to lower temperatures with the reduction in 
grain size similar results have been reported in rapidly solidified Nd-Fe-B samples [6] 
Hence, lower Tc values observed in the present work for samples prepared with higher 
quenching rate is in line with the earlier reports on other rapidly solidified ferromagnetic 
materials 
Liquid Nitrogen quenched sample 
- j , , . , . , 1
 r . 1 1 • 1 
40 60 80 100 120 140 160 180 
Temperature (°C) 
Figure 4 Variation of ac susceptibility of Co^Ni^Ga^ alloys with temperature 
4. Conclusions 
Co45Ni25Ga30 alloys exhibiting M-»A and/or A->M transformations were formed by rapidly 
cooling the homogenized master alloy in air, ice water or liquid nitrogen Structurally, the 
alloys prepared under different cooling rates exhibited a single martensite phase However, 
the thermal transformation behavior of these alloys observed using a DSC showed marked 
differences Air cool alloys does not exhibit sharp M-»A and/or A->M transition and very 
high rate of quenching (liquid nitrogen quenched) may develop cracks in the bulk sample 
Thus the ice quenched sample is the best among these samples for shape memory 
application. Moreover, the ice quenched alloy is ferromagnetic in its martensite phase for 
a large temperature range (Tc = 119 °C) Thus it can be considered as a potential candidate 
for ferromagnetic shape memory alloy applications 
706 Sidananda Sarma and A Srinivasan 
References 
[1] S J Murray, M A Marioni, A M Kukla, J Robinson, R C O'Handley and S M Allen J. AppI. Phys 87 
5774 (2000) 
[2] M Wuttig, J Li and C Craciunescu Scr. Mater. 44 2393 (2001) 
[3] K Oikawa, L Wulff, T lijima, F Gejima, T Ohmon, A Fujita, K Fukamichi, R Kainuma and K Ishida 
AppI. Phys. Lett. 79 3290 (2001) 
[4] J S Koehler, F Seitz and J E Bauerle Phys. Rev. 107 1499 (1957) 
[5] K Otsuka and C M Wayman (ed.) Shape Memory Materials (Cambridge, U.K : Cambridge Universitv 
Press) 22 (1998) 
[6] L H Lewisa, M J Kramerb, R W McCallumb and D J Branaganc Proc. 3rd Internal Workshop 
Mater. Sci., Hanoi BNL-66785 (1999)

English

Influence of Cooling Rate on the Properties of Ferromagnetic Shape Memory Alloy

Bhattacharjee, S

Baishya, B

A Study of Superconducting Order Parameter Fluctuation Effects in Dy2 O3, Eu2O3 and Pr6On Substituted Compounds

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Dasgupta, Abhijit

Conference Theme:  Knowledge for All: Role of Libraries & Information Centres | 12‐15 Nov. 2008 
Organiser: Sir Dorabji Tata Memorial Library, Tata Institute of Social Sciences (TISS), Mumbai 
 
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Ontological Semantic Mapping for the Subject Facets 
on Global Warming and Climate Change 
 
Abhijit Dasgupta 
Assistant Librarian, Indian Association for the Cultivation of Science (IACS), Kolkata – 700032  
 
 
ABSTRACT 
 
 
To sustain the life and civilization on this planet ‘Earth’ day by day the term ‘Global Warming’ 
has become one of the most important sought heading. One unique affair of this phenomena 
‘Global Warming’ is that here the concern and danger of the developed nations and developing 
nations remains the same, the economic and technological advantage of the developed nations 
can only show the light on further R & D on this topic and how we can combat this natural 
phenomena which may draw the last paragraph of our 21st century. An effort has been made to 
collect the isolates or facets on the topic ‘Global Warming and Climate Change’ and arrange 
these facets into a semantic and ontological relationship and then transfer these facets into 
SKOS (Simple Knowledge Organization Systems) mapping and lastly translate this SKOS 
mapping into RDF (Resource Descriptive Framework) with the help of XML (Extensible Mark-
up Language). Basically this is an Information System Tool, which can be applied on any 
subject/topic for accessing web literature. In this case this has been developed for accessing web 
literatures on ‘Global Warming and Climatology’. 
Keywords: SKOS, Semantic Web, RDF, XML, Metadata, World Wide Web, W3C, Ontology 
 
 
 


Ontological Semantic Mapping for the Subject Facets on Global Warming and Climate Change

IACS Annual Report contains the yearly proceedings of the each division of the Association for the year 2008-09IAC

Indian Association for the Cultivation of Science

Report of the Association for the Year 2008-09

Behera, D

Biswal, R

Mohapatra, U K

Mishra, N C

Microscopic Inhomogeneity Induced Thermal Fluctuation in High Temperature Superconductors

Hussain, A M Pharhad

Kumar, A

Indian J P/iw 82 (3), 349-356 (2008) 
Electrochemical synthesis and characterization of 
co-polymers of aniline and pyrrole 
A M Pharhad Hussam* and A Kumar 
Department of Physics Tezpur University Napaam Assam 784 928 India 
E mail abiK« tezu ernet in 
Abstract Conducting polymer (CP) films of polyaniline (PAni) polypyrrole (Ppy) and copolymers of aniline 
and pyrrole are synthesized electrochemically in sulfuric acid (H,S04) aqueous electrolyte on indium doped tin 
oxide (ITO) coated glass substrate Cyclic voltammetry (CV) studies are carried out for electrochemical 
characterization and the polymer films showed high electroactivity in acidic media X-ray diffraction study 
revealed polyaniline as completely amorphous and polypyrrole and co-polymer as polycrystallme Electrical (dc) 
conductivity of the co-polymer films was measured by four probe method and found to lie between those of 
polyaniline and polypyrrole UV-VIS spectra of the co-polymer show a peak at 370 nm which is in between the 
characteristic n -n * transition peak of polyaniline polypyrrole suggesting formation of copolymer FTIR spectra 
also suggest formation of copolymer poly (aniline-co-pyrrole) 
Keywords Electrochemistry, co-polymer cyclic voltammetry UV-Vis, FTIR 
PACS Nos. 82 47 Uv, 61 80 x 
1. Introduction 
Conducting polymers (CPs) have drawn much attention in the past decade since 
polyacetyiene was first prepared [1] with a high conductivity [2] of about 103 S/cm They 
are promising materials for supercapacitor, batteries, sensors, electronic displays, smart 
windows, actuators, drug delivery etc because of their processabihty, low cost, high specific 
charge capacity, cycle life and redox property [3,4,9-12] CPs like polyaniline, polypyrrole, 
polythiophene etc. are ideal electrode materials [13] for energy storage and coversion 
systems which satisfy the requirements of high specific power and energy like all polymer 
redox supercapacitors [14] and all polymer rechargeable batteries [15] Most applications 
of the conjugated polymers are based on their electrochemical and optical properties, 
both the properties are closely related to the electrochemical reactions of the polymers 
[6]. Polyaniline is unique because it has a conductivity that can be reversibly controlled 
Corresponding Author 
©2008IACS 
350 A M Pharhad Hussain and A Kumar 
both by charge trnasfer reaction with the conjugated backbone and by protonation of the 
nitrogen [3]. Polypyrrole has higher electrical conductivity, relatively better environmentally 
stability and is polycrystalline [16-18]. However it is infusible and decomposes before 
melting [17J. 
Electrochemical studies on conducting polymers mainly include analysis of growth 
mechanism, doping/dedoping process and charge transfer characteristics. Major problems 
in most conducting polymers are their processability and environmental stability. They are 
isoluble or very poorly soluble in common organic solvents. Sulphonated polyaniline with 
S03 groups attached with the benzene ring is soluble but the conductivity is reduced 
[19]. Copolymers of conducting polymers are expected to show good conductivity as well 
as mechanical stability from two components at the cost of some of their properties such 
as electrical conductivity [20]. Copolymers of pyrrole and N-methyl pyrrole [21], pyrrole 
and phenol [22], pyrrole and terthienyl [23], pyrrole and tetra alkyl ammonium pyrrole 
[24], pyrrole and acrylamide [25] have been prepared by electrolytic oxidation of the mixture 
of monomers. Electro-co-polymerization of aniline and pyrrole have some advantage as 
both the monomers were added to the electrolyte and the oxidative polymerization takes 
place simultaneously. In this study an attempt has been made to synthesize copolymer 
films of aniline and pyrrole in aqueous medium with protonic acid H2S04 as the supporting 
electrolyte. The CV of the polymer films has been carried at different scan rates to 
investigate the redox response. UV-VIS and FTIR spectra of the polymer films have been 
taken for understanding the optical and chemical nature of the polymers. Conductivity 
measurements of the films are carried out with four probe technique. X-ray diffractogram 
has been carried out to study the degree of crystallinity of the polymer films. 
2. Experimental 
Monomers aniline (Aldnch) and pyrrole (SRL-lndia) are distilled under reduced pressure 
and stored in darkness prior to use in inert condition. H2S04 (Qualigen) is used as received 
and double distilled deionized water is used in all experimental purpose. CP films of 
polyaniline, polypyrrole and their copolymer were deposited on ITO (BHEL, India) coated 
glass substrates using stainless steel counter electrode and SCE reference electrode in 
a three electrode electrochemical cell using a potentiostate/galvanostate (AEW2, Sycopel 
Scientific Ltd. UK). Five polymer films are synthesized, two of them are of pure polymer 
films of 0.1 M pyrrole and 0.01 M aniline monomer and other two films with 0.1 M aniline 
and 0.01 M and 0.02M pyrrole monomer. The polymerizations are carried out 
potentiostatically at 1000 mV in an aqueous solution containing the monomers and sulfuric 
acid as the supporting electrolyte for polymerization. The cyclinc voltammogrammes of 
the polymer films have been recorded using the same potentiostate/galvanostate. The dc 
conductivity measurements of the polymer films are carried out by four probe technique 
using a standard four probe setup. The x-ray diffractogram [Philips X-pert pro] of the 
polymer films are recorded. UV-VIS spectra of the polymers are recorded by using a 
double beam UV-VIS spectrometer (Shimadzu) taking ITO coated glass in the reference 
Electrochemical synthesis and characterization of co-polymers etc 351 
beam line. The FTIR spectra of the polyaniline samples have been taken by using Nicolet 
impact 410 FTIR spectrophotometer. 
3. Results and discussion 
3 1 Electrical conductivity: 
The conductivity of the polymer films is measured by four probe method using a standard 
four probe set up at room temperature. The dc conductivity for the polyaniline films was 
found to be 35 S/cm and that for polypyrrole is about 100 S/cm The conductivity of the 
co-polymers films were found to be in the range fo 50-70S/cm Conductivity of the co-
polymer films lies between the conductivity of polypyrrole and polyaniline, which could be 
explained in terms of decreased crystallinity of the copolymer To co-polymer films with 
lower concentrations of pyrrole (0 01 and 0 02M) and higher concentration of aniline 
monomer (0 1M) exhibited lower conductivity, around 50-55 S/cm and the co-polymer films 
with higher concentration of pyrrole monomer (0.1 M) and lower concentration of aniline 
(0 01M) showed higher conductivity on the order of 70 S/cm This behaviour is attributed 
to the fact that polypyrrole films are more conducting as compared to polyaniline 
3 2 Cyclic Voltammetry 
The cyclic voltammetry of the polymer films is carried out at different scan rates of 10, 
20, 40 and 50mV/s in a three electrode electrochemical cell, potentials are applied against 
saturated calomel electrode (SCE) and are shown in Figure 1(a-e) Every CP shows an 
electrochemical window within which the doping/dedoping (redox) process is largely 
reproducible, beyond which oxidative or reductive decomposition of the CP occurs The 
CV is observed to be related with the electrochromism of the CP / e the redox peaks 
observed in the CV usually coincide with the CP's colour changes Polyaniline films changes 
colour from dark green to bottle green with increase in potential in the forward sweep and 
again back to dark green in the reverse sweep The co-polymer films in the forward 
sweep changes colour from blackish green to dark brown and in the reverse sweep it 
becomes again blackish green The results are consistent with the results reported in the 
literature [26-31] The CV of CP depends on several parameters like substrate, electrolyte 
concentration, pH of the electrolyte etc. The cathodic and anodic currents increase with 
increase in scan rates in all CV. The CV of pure polyaniline (Figure 1a) shows distinct 
oxidation reduction peaks at 1 5 V and 0 6V respectively which also depicts a slight 
increase in oxidation potential at higher scan rate The peak current in forward scan in 
case of pure polypyrrole (Figure 1 b) was obtained at 1 8V and the peak in the reverse 
sweep was obtained at 0.2V. In this polymer film a slight shift of the peak towards higher 
and lower potential, in forward and reverse sweep respectively, was also observed with 
increase in scan rate. In the CV of copolymer with 0 1M aniline and 0 01M pyrrole, 
interestingly two peaks are observed in the forward sweep (Figure 1c) The first peak at 
1 55V which coincide with the peak position of pure polyaniline, may be due to the 
oxidation of aniline part of the co-polymer chain and the second one at 1 8V which coincides 
with the peak position of polypyrrole, may be attributed to oxidation of the pyrrole part of 
352 A M Pharhad Hussain and A Kumar 
the copolymer chain. In Figure 1d, the CV of the copolymer with 0.1 M aniline and 0 02M 
pyrolle, at lower scan rate again two oxidation peaks with small potential position difference 
was observed but at higher scan rate instead of two peaks one broad peak was obtained 
This is may be due to overlap of the two independent oxidation peaks of pyrrole and 
o 
1 OH 
0 8-] 
0 6H 
0 4-j 
0 2H 
0 0H 
-0 2-1 
-0 4 J 
-0 6 J 
-0 8j 
-1 OH 
-1 2H 
4Un 
30 
20 
? 10 
D 0 
-20 
-0 5 
8-
6 
4H 
0 0 
— I • I 
0 5 1 0 
Potential (mV) 
" i 
1 5 
i 
2 0 
-30 
< 
^ 2 
3 0, 
-2 
-4-1 
- 0 5 
20 
15H 
10-J 
5 
0 0 
i • ? 
0 5 1 0 
Potential (mV) 
1 5 2 0 
S 0 
-5H 
-10 
- 0 5 0 0 
l ' i 
0 5 1 0 
Potential (mV) 
20-
15-
10-
I * 
i o^ 
o 
-5 . 
-10-
1
 i 
1 5 
I 
2 0 
-15. 
- 0 5 
i 
0 0 
-15 
i • i 
0 5 1 0 
Potential (mV) 
— i — 
1 5 2 0 
i- 10m V/s 
ii- 20mV/s 
in- 30mV/s 
iv-40mV/s 
- 0 5 0 0 0 5 1 0 
Potential (mV) 
i -
1 5 
i 
2 0 
Figure 1. Cyclic Voltammograms of (a) polyaniiine, (b) polypyrrole, (c) poly (aniline-co-pyrrole) [Aniline 
0.1 M + 0.01 M Pyrrole), (d) poly (aniline-co-pyrrole) [Aniline 0.1 M + 0 02M Pyrrole] and poly (pyrrole-co 
aniline) [pyrrole 0 1M + aniline 0 01M] a differnet scan rates in aqueous H2SOA with stainless steel counter 
and SCE as reference electrode. 
Electrochemical synthesis and characterization of co-polymers etc 353 
aniline. The CV of the copolymer with 0.1M pyrrole and 0.01M aniline (Figure 1E) shows 
single broad peak in the forward sweep. As the aniline concentration is low, the dominant 
part is the pyrrole units in this case and hence oxidation of aniline peak is too weak and 
the oxidation of pyrrole unit dominates the current peak. The higher peak height i.e. peak 
current of pyrrole oxidation may be attributed to the higher conductivity of pyrrole in oxidized 
state 
3 3 UV-VIS spectra: 
The UV-VIS spectra of the polymer films of polyaniline, polypyrrole and the copolymer 
deposited on ITO coated glass are presented in Figure 2a-c. The absorption band at 
310nm (Figure 2a) is associated with the n -n * transition of the benzenoid part in the 
polyaniline linear structure or simply the band gap of the polymer and the relatively broader 
band at 620 nm in the absorption spectra of pure polyaniline (Figure 2a) can be attributed 
to the intramolecular transition between benzenoid and quinoid ring in the polymer chain 
Polyaniline is completely transparent in the 200-300 nm region The UV-VIS spectra of 
Dure polypyrrole (Figure 2b) shows absorption at 415nm which may be accounted for the 
r n * electron transition or the characteristic transition of the polypyrrole The theoretical 
study shows that the longer the absorption wavelength of the n - n * electron transition 
of the conjugated chain, the longer will be conjugation length of the chain [32] UV-VIS 
spectrum of the co-polymer film shows the combined features of both polyaniline and 
polypyrrole (Figure 2c) one absorption peak is observed at 370nm which is in between 
the characteristic peaks of polyaniline and polypyrrole and may be the characteristic 
peak of the co-polymer. The second absorption band a broader one at 600nm may be 
due to the n -n * transition of the benzenoid and quinoid of the polyaniline part in the 
co-polymer chain. 
I fL_/\_J 
L dm " • ' '• • ••• • i • • ' » 3 
190 300 400 500 600 700 800 
Xnm 
Figure 2. UV-VIS absorption spectra of (a) polyaniline (b) polypyrrole and (c) poly (aniline-co-pyrrole) 
with 0 1 M aniline and 0 1M pyrrole 
34 FTIRSpectra: 
FTIR spectra of the polymer polyaniline, polypyrrole and the copolymer are presented in 
Figure 3a-c respectively. In Figure 3a peak at 3470 cm"1 is attributed to the N-H stretching 
354 A M Pharhad Hussain and A Kumar 
of the amine group of aniline [33]. The peak near 3000 cm"1 is due to the C-H stretchinc 
absorption. The peak at 1300 cm"1 is due to the C-N stretching of the polymer. The 
strongest band observed near 1100 cm-"1 and the band at 1235 cm"1 are due to C-c 
stretching and C-C twisting, respectively [34]. The peak near 800 cm"1 is due to the N-
H out-of-plane bending absorption. The peak due to N-H bending can be seen in tne 
range of 1560-1640 cm"1. The stretching peak of N-H appears in the range of 3420-345Q 
cm"1. The C = C stretch absorption of aromatic compound (aniline) is obtained at 147? 
cm"1 (Figure 3a) and the same C = C stretch absorption for hetero-aromatic compound 
(pyrrole) is obtained at 1527 cm"1 (Figure 3b). Both the characteristic C = C stretching 
absorption peaks for aromatic and hetero-aromatic compounds are present in the IR spectra 
of the co-polymer (Figure 3c) at around 1480 and 1525 cm"1, and this may be the 
confirmation of formation of co-polymer chain in the electropolymerization. 
' I ' I 1 i—'—r-"*—i ' I 
4000 3500 3000 2500 2000 1500 1000 500 
Wavenumber in cm"1 
Figure 3. FTIR spectra of (a) polyaniline (b) polypyrrole and (c) poly (aniline-co-pyrrole) 
3.5. X-ray diffractogram: 
The XRD spectra of pure polyaniline film, pure polypyrrole and poly (aniline-co-pyrrole) are 
presented in Figures 4a, 4b and 4c respectively. Pure polyaniline deposited on ITO coated 
mm 
*,k. I i l . l^ iUhi 
^L^ikMfU^ 
• i ' 
20 
11 * 
40 60 
]2Q 
80 100 
Figure 4. X-ray diffractogram of (a) polyaniline (b) polypyrrole and (c) poly(aniline-co-pyrrole). 
Electrochemical synthesis and charactenzation of co-polymers etc 355 
glass is completely amorphous (Figure 4a), whereas polypyrrole (Figure 4b) on ITO coated 
glass is polycrystalline. The co-polymer poly (aniline-co-pyrrole) is also found to be 
polycrystalline but the degree of crystaliinity is reduced (Figure 4c) The decrease in 
crystallmity in poly (aniline-co-pyrrole) may be attributed to the incorporation of aniline 
monomer in the co-polymer chain as polyaniline is completely amorphous 
4. Conclusion 
From this study we have observed that free standing films of poly (aniline-copyrrole), 
which can be removed from the substrate, can be synthesized by electro-copolymenzation 
XRD results show that polypyrrole is crystalline and polyaniline is amorphous Though the 
degree of crystallmity of polypyrrole is more as compared to those of co-polymer films, 
the polypyrrole films are more adhesive to the ITO coated glass substrate and are not 
removable from the substrate Conductivity of the co-polymer films lies between the 
conductivity of polypyrrole and polyaniline which could be explained in terms of decreased 
crystallmity of the copolymer. The absorption peak at 370 nm in the UV-VIS spectra of 
the copolymer may be attributed to the characteristic electronic transition absorption of 
poly (aniline-copyrrole). Presence of aromatic and heteroaromatic characteristic C=C 
stretching peaks in the FTIR spectra of copolymer also suggest electro-copolymenzation 
of poly (aniline-co-pyrrole) 
Acknowledgment 
We gratefully acknolwedge UGC, New Delhi, for financial assistance (vide project no 
F10-53/2001 (SR-1) One of the author (AMP Hussain) acknowledges financial assistance 
of CSIR (India) 
References 
M] C K Chiang, C R Fincher (Jr), Y W Park A J Heeger H Shirakawa E J Louis S C Gau A G 
MacDiarmid Phys Rev Lett 39 1098 (1977) 
[2] J Liu and M Wan Synth Met 124 317 (2001) 
[3] Gractela B Blanchet, C R Fincher and F Gao Appl Phys Lett 82 1290 (2003) 
[4] Robert A Huggms Solid State Ionics 134 179 (2000) 
[5) M Mastragostmo, C Arbizzani and F Soavt Solid State Ionics 148 493 (2002) 
[6] S J Choi and S M Park J Electrochem Soc 149 E26 (2002) 
[7] S M Park Handbook of Organic Conductive Molecules and Polymers Vol 3 H S Nalwa (Ed) 
(England John Wiley and Sons, Chichester) (1997) 
[8] A G Macdiarmid Synth Met 84 27 (1997) 
[9] J M Pernaut and J R Reynolds J Phys Chem B104 4080 (2000) 
[10] D K Moon, T Maruyama, K Osakada and T Yamamoto Chem Lett 1739 (1991) 
[11] D K Moon, M Ezuka, T Maruyama K Osakada and T Yamamoto Macromolecules 26 364 (1993) 
[12] Y Sun, A G MacDiarmid and A J Epstein J Chem Soc Chem Commun 529 (1990) 
[13] Peng Weng J Electrochem Soc 149(9) A1171 (2002) 
[14] Maciej Mazur and Pawel Krysinski J Phys Chem B106 10349 (2002) 
[15] Hassan Karami, Mir Fazlollah Mousavi and Mojtaba Shamsipur J Power Sources 124 303 (2003) 
356 A M Pharhad Hussam and A Kumar 
[16] X Zhang and R Bai Langmuir 19 10703 (2003) 
[17] Y Lu, A Pich and H Adler Synth Met 135-136 37(2003) 
[18] R Kostic, D Rakovic, S A Stepanyan, I E Davidova and L A Gribov J Chem Phys 102 3104 
(1995) 
[19] B C Roy, M D Gupta, L Bhoumik and J K Ray Synth Met 130 27 (2002) 
[20] B Zinger, D Kijel Synth Met 41-43 1013 (1991) 
[21] K K Kanazawa, A F Diaz, M T Kround and G B Street Synth Met 4 119 (1981) 
[22] N Kumar, B D Malhotra and S Chandra J Polym Sci Polym Lett Ed23 57 (1985) 
[23] O Inganas, B Leidberg and W Chang-Ru Synth Met 11 239 (1985) 
[24] C J Picket, K S Ryder and J C Moutet J Chem Soc Chem Commun 679 (1992) 
[25] A S Sarac, G Sonmez and B Ustamehmetoglu Synth Met 98 177 (1992) 
[26] M C Bernard, A H L Goff and W Zeng Electrochimica Acta 44 781 (1998) 
[27] M C Bernard, S C Torresi and A H L Goff Electrochimica Acta 44 1989 (1999) 
[28] D W Hatchett, M Josowicz and J Janata J Electrochem Soc 146 4535 (1999) 
[29] V Rajendran, A Gopalan, T Vasudevan and T Wen J Electrochem Soc 147 3014 (2000) 
[30] W Chen, T Wen, C Hu and A Gopalan Electrochimica Acta 47 1305 (2002) 
[31] A L Schemid, L M Lira and S IC Torresi Electrochimica Acta 47 2005 (2002) 
[32] Y Li and Y Fan Synth Met 79 225 (1996) 
[33] A B Samui, A S Patankar, R S Satpute and P C Deb Synth Metals 125 423 (2002) 
[34] A M Pharhad Hussam and A Kumar Bull Mater Sci 26(3) 329 (2003)

Electrochemical Synthesis and Characterization of Co-Polymers of Aniline and Pyrrole

Sarmah, K

Sarma, R

Das, H L

Indian I Phys. 82 (5), 585-594 (2008) 
Effect of wavelength and intensity of illumination on 
the photoelectronic properties of CdSe thin films 
K Sarmah*, R Sarma and H L Das 
Department of Physics. Gauhati University 
Guwdhati-781 014, Assam, India 
E-mail ks_guphys(</»rediffmail com 
Abstract Thermally evaporated CdSe thin films deposited under a vacuum environment of the order of 
10' Torr, using high quality glass as substrate material, which are held at different elevated temperatures, are of 
polycrystalhne nature having hexagonal structure whereas films grown at room temperature are amorphous In 
qap type cell configuration of these films with thermally evaporated aluminium electrodes, the l-V characteristics 
are observed to be linear both under dark and under monochromatic illuminations of different intensities for low 
bias voltages, but such curves show Poole-Frenkel type of conductivity under the same illuminations for high 
bias voltages. In these cases the photocurrents, which are mainly controlled by different types of defects, vary 
widely with variation of intensities and wavelengths of light close to the threshold wavelength The transport 
mechanism is generally not a single activated process and the traps have a very significant role in controlling the 
growth and decay of photocurrents 
Keywords CdSe thin films, Defects, Traps 
PACS Nos. ' 71 55 -i, 72.20 Jv,78.20-e 
1. Introduction 
CdSe is a binary semiconductor and among the ll-VI group of semiconductor compounds, 
it is considered as an important material for the development of different opto- electronic 
devices [ 1,2,3] because of its high photosensitive nature and suitable intrinsic band gap 
(1 74eV) [4]. In recent years special attention has been given to the investigation of 
photoelectronic properties of CdSe thin films in order to improve the performances of 
devices and also for finding new applications [5,6]. Different growing methods for preparation 
of CdSe thin films have been reported [7,8]. Out of these physical vapour deposition 
method is often used because it offers many possibilities to modify the deposition 
parameters and to obtain films with determined structure and properties. Irrespective of 
Corresponding Author 
© 2008 IACS 
586 K Sarmah, R Sarma and H L Das 
preparation conditions such deposited films are associated with different type of defects 
which play dominant role in the photoconductivity processes. The nature and amount o* 
inherent defects, which in turn effect the photoelectronic properties, depend on differed 
deposition parameters such as vacuum inside the chamber, rate of deposition, substrate 
temperature etc. The grain sizes as well as intergrain boundary potential barriers alory 
with these defects also have a prominent role to play. Study of photoelectronic property 
of semiconducting films are essential requirements for proper application in different 
optoelectronic devices. Some optoelectronic properties of thermally deposited CdSe thir 
films, illuminated by monochromatic as well as white light of different intensities are 
reported in this paper. 
2. Experimental 
Thin films of CdSe of various thicknesses were deposited at different substrate temperatures 
on chemically and ultrasonically cleaned glass substrates with the help of Hind Higi 
Vacuum Coating Unit at a vacuum of the order of 10*5 Torr. Pure (99.999%) bulk CdSe 
samples were used as the source material. Prior to deposition of the thin films, higr 
purity aluminium electrodes were vacuum evaporated on glass substrates on which th< 
films were then deposited to obtain a gap type cell configuration of 10mm x 7mm geometry 
Thin tantalum boats of proper size and shape were used as the source heater. A suitably 
designed and assembled multiple beam interferometer was used to measure the thickness 
of the films with an accuracy of ±15A. An ECIL electrometer amplifier (EA815) of input 
impedance of 1014Q (and higher) was used to measure dark and photocurrents. To provide 
bias, a series of highly stable dry cells of emf 9 volt each were used. The sample was 
kept suspended by thin enameled copper wires inside a continuously evacuated glass 
jacket. The entire experimental set up including the observer was housed in a suitably 
fabricated Faraday cage in order to avoid pick-up noises. A set of C-Z metal interference 
filters of different wavelengths starting from 600 nm to 900 nm was used to obtain 
monochromatic radiations. Light intensities were measured by using a highly sensitive 
APLAB luxmeter. 
X-ray diffractograms were obtained using a Phillips X-ray diffractometer (Xpertpro) for 
CuKa radiations. Surface morphology of the films were investigated by using scanning 
electron microscope (LEO 1430VP) with an accelerating potential of 18 kV. 
3. Results and discussion 
3.1. Structural property: 
The X-ray diffraction profiles of the CdSe thin films reveal that films grown at rootr 
temperature were amorphous and those grown at elevated substrate temperatures (TJ 
were (Figure 1) polycrystalline having hexagonal ZnS type structure. In these films reflections 
from the [002] planes were found to be very intense which indicated that the preferential 
growth of crystallites was in this particular direction. However depending upon the Ts of 
Effect of wavelength and intensity of illumination on the photoelectronic properties etc 587 
deposition some percentage orientation of a few other planes were also observed. With 
the increase of Ts the grain sizes of the polycrystalline thin films were found to increase. 
I ,i«o, , j m2) , , U " 
J | tiMC» 
Figure 1. XRD patterns of CdSe thin films of the samo thickness t ~ 2000A and deposited at different Ts 
573K, 473K and 303K respectively 
The film morphology under SEM studies showed that the films deposited at higher Ts 
were fairly uniform, polycrystalline and free from macroscopic defects like cracks or peeling. 
So for the photoconductivity studies the qualities of the films grown at elevated Ts's were 
quite suitable. 
Figure 2. SEM of a CdSe thin film (f = 2000A, Ts = 473K) Magnification 12 14 KX 
3.2. I-V characteristics: 
Variation of dark current, lD, with applied bias voltage for the CdSe thin films are found to 
be linear (ohmic) within the applied bias range [(-108V)-0-(+108V)]. For the low bias voltages 
588 K Sarmah, R Sarma and H L Das 
of both polarities, current under illumination (lL) varies linearly with bias and in high voltage 
range lL increases nonlinearly with the applied bias. In these regions when In J h (j f ,s 
the photocurrent density, photocurrent lph = lL - lD) is plotted against F1'2 (F being the 
field corresponding to the applied bias) the plots (Figures 3 and 4) are found to be linear 
in high field regions and non-linear in low fields. 
-35 
r 
\ 
c 
1 
SO 
f 
i 
t 
r 
r 
X 
F1^ 
75 
i 
X 
r 
l 
r 
\JMI 
X 
X 
f 
X 
X 
X 
r 
I 
m m 
X 
X 
JL 
X 
I 
100 * 125 
*» * * 
r
 r 1 * 1
 r r 
I * r 
a 725 nm 
b 700 nm 
c 750nm 
d 675 nm 
e WL 
1 600 nm 
V 
V 1 / 2m 
(a) (b) 
Figure 3. lnJph vs F1/2 pi at for a CdSe (t = 2000A, Ts = 473K) thin film (a) under different monochromdtK 
illuminations and white light (WL) of constant intensity and (b) under monochromatic illumination of 725nr 
of different intensities 
2 
1 
2 
3 
/I 
-5 
3 
1 E 
.. 1 
; E 
50 
r 
i 
i 
t 
I 
i 
I 
76 
X. 
I * 
l 
i I 
I 
F'" V,/2fr 
2 
X 
X 
T 
,-1/2 
X 
r 
i 
r 
£ 
00 
a 
r 
X 
1 
- V5 
5 • - * 
* * » ? 
a 725 nm 
b 700 nm 
c 750nm 
d 675 n m 
e WL 
f 600nm 
(a) 
i i 
I1 
a 2 0 In \ 
b 4 0 ILIK 
c 6 0 hi \ 
cl 8 0 In \ 
c l ( X ) l u \ 
p1«
 V » / 2 m 1/? 
(b) 
Figure 4. lnJph vs P12 plot for a CdSe (f = 2000A, 7"s = 573K) thin film (a) under different monochromatic 
illuminations and white light (WL) of constant intensity and (b) under monochromatic illumination of 725nm 
of different intensities 
As is apparent from the In J
 h vs F1'2 characteristics depicted in Figures 3 and 4, the 
basic nature of the curves of white and monochromatic illuminations are similar apart from 
the numerical values, both in low and .high field regions. 
This clearly shows that in high field region the conduction mechanism is of Poole-
Frenkel type. This type of conductivity mainly depends upon the grain boundary potentials, 
Effect of wavelength and intensity of illumination on the photoelectronic properties etc 589 
which may be modified by externally applied fields The current density due to such type 
of conduction mechanism is given by [9] 
J = J0exp(/JPFF1/2//c7) (1) 
where CJQF = J0 is the low field current density, pPF is the Poole-Frenkel coefficient and 
other symbols have their usual significance From the slopes of the InJ
 h vs F1'2 plots the 
Poole-Frenkel coefficients are calculated which are found to be in the range (4 4 - 4 9) x 
10A eV V1/2 m1/2 
As the average grain sizes in higher Ts grown films are higher (XRD patterns given in 
Figure 1), the corresponding average grain boundary potentials are lowered The Poole-
Frenkel effect of field lowering of built-in potentials in the body of the thin films decreases 
in the films deposited at higher Ts 
3 3 Intensity dependence of photocurrent 
The nature of intensity (</>) dependence of photocurrent for the thin films is found to 
follow the relation 
I* = 0& (2) 
where C is a constant and y is the power factor The plots of ln/pA7 vsln</> t are linear 
(Figure 5) under different monochromatic illuminations and also under white light (WL) 
From these curves y is found to be close to 0 5 This sublinear dependence of I
 h on </> 
shows that photoconductivity in these thin films are of defect controlled type [10] 
J 
I 
r 
*lh* 
ln<j> 
i ± i 
a 72Snm 
b 700nm 
t fi7Snm 
d (^Onm 
e WI 
f f^^nm 
Figure 5. In/
 h vs I n ^ plot for a CdSe thin film of t = 2000A and deposited at T& = 473K 
3 4 Temperature dependence of conductivity 
The plots of Ina vs 1000/T for the films, both under dark and under illuminations show 
two distinct conductivity regions (Figure 6), which means that in these films the conductivity 
o cannot be represented by a single exponential form and the transport mechanism is 
590 K Sarmah, R Sarma and H L Das 
characterized by a double activated process of the form [11] 
<r = {a0\ exp(-£,/(fcr)) + (a0)2 exp(-E2/(/c7)) (3. 
where E1 and E2 are the relevant activation energies and the other symbols have ther 
usual significance. 
1 5 
0 5 
2 5 
_d Ck 
"
mi.. 
<
*"^»* • « - ^ « i
 mm_ 
' * * • ' * * " • -
• £ 
• • • - • 
• » • ! 
* * 
a 725 nm 
b 7(X)nm 
c 750nm 
d Dark 
t • 
• * 
* * -
• * 
4 
15 
2 2 2 7 
1000/T (K-1) 
(a) 
a 
b 
c 
d 
3 2 
-3 5 
a I 50 lu x 
b l()0lu\ 
c 30 l» \ 
2 2 2 7 
1000/T (K ') 
(b) 
Figure 6. In a vs 1000/T plot for a CdSe thin film (t = 2000A, Tb= 473K) (a) under dark and diffnrpn 
monochromatic illuminations of constant intensity and (b) under monochromatic light of 725 nm at difli'ien 
intensities 
For the experimental film, activation energies (Ea) are calculated (Table 1) from the 
slopes of the In a vs 1000/T plots and the mobility activation energies (Ema) are calculateo 
(Table 1) by subtracting photo activation energies from the corresponding dark activation 
energies. For a film of t = 2000A grown at Ts = 473K, the calculated values of Ea uncle 
dark have been found to be 1.17eV for high temperature region (R-l) and 0.572 eV for low 
temperature region (R-ll), which are in agreement with the already reported results [4] 
Table 1. Calculated values of Activation Energies (Ea) and Mobility Activation Energies (Ema) (eV) for • 
CdSe thin film (f = 2000A and TS=473K) (a) under different monochromatic illuminations of constarv 
intensity (b) under monochromatic illumination of 725 nm of different intensities. 
Wavelength 
Region 
EB 
E™ 
Intensity 
Region 
^ 
$m 
R-l 
0.98 
0 1 9 
R-l 
0.94 
0.23 
750 nm 
30 lux 
R-ll 
0.48 
0.09 
R-ll 
0.43 
0.14 
R-l 
0.94 
0.23 
R-l 
0.89 
0.28 
700 nm 
R-ll 
0 43 
0.14 
100 lux 
R-ll 
0.38 
0.19 
R-l 
0.92 
0.25 
R-l 
0.86 
0.31 
725 nm 
R-ll 
0 37 
0 20 
150 lux 
R-ll 
0.32 
0.25 
Effect of wavelength and intensity of illumination on the photoelectronic properties etc 591 
35 Growth and decay of photocurrent 
The photocurrent at any instant of time during growth is given by the equation 
/,=/0{1-exp(-yrr)} (4) 
where l0 is the maximum photocurrent and rr is the growth time of photocurrent With 
passage of time the photocurrent reaches a steady state and when the light is turned off 
current decreases, because excess number of carrier decreases as a result off rapid 
recombination The decay may be represented by the equation 
/f=/0{expH/rd)} (5) 
where rd is the photocurrent decay time It is observed that the experimental CdSe thin 
films exhibit a rapid growth of photocurrent which is followed by an initial fast decay 
accompanied by a tail (Figure 7) The fast decay is associated with free electron-hole 
recombination and the subsequent region is due to release of electrons from traps 
h4( ) lu \ 
(a) (b) 
Figure 7. Growth and decay of photocurrent for a CdSe thin film (f = 2000A T= 473K) (a) under 
different monochromatic illuminations of intensity 50 lux and (b) under monochromatic illumination of 725nm 
of different intensities 
The trap depth E below the bottom of the conduction band or the top of the valenced 
band may be calculated using the simple decay law 
/ f=/ 0exp(-pf) (6) 
where p is the probability of escape of an electron from the trap per second and is given 
by [12] 
p = Sexp(-E//cT) (7) 
Using these two relations, we get 
E = kT[\r\S-\n{{\nl0/lt)/t}] (8) 
592 K Sarmah, R Sarma and H L Das 
where T is the ambient temperature in K. I0 is the photocurrent at the instant of termination 
of illumination, lt is the photocurrent at any instant of time t after the termination of 
illumination. S is the frequency factor defined as the number per second that the quanta 
from the lattice vibrations attempt to eject the electron from trap, multiplied by the 
probability of transition of the ejected electron to the conduction band [13] and is given 
by [14] 
S=Ne„vthSt (9) 
where Neff is the effective density of states in the conduction band, it is assumed that at 
comparatively low temperature the number of occupied energy in the conduction band 
i.e. n is identical with A/e/f[15]. Now 
n = cj/en (10) 
where a is the conductivity corresponding to photocurrent /0. \i is the mobility of electrons 
in CdSe sample which is taken to be 580 cm2V'1 sec1 [16]. 
St is the capture cross section of electron at temperature T and is given by 
S, =;rr2 (11) 
where r radius of capture center. It is calculated by putting coulomb energy of interaction 
of an electron with the corresponding trap equal to the thermal energy of the electron at 
temperature T [17]. i.e. 
e2/rc = kT (12) 
So St=ne2k2T2e2 (13) 
At300K S, =10 1 0 ,V 2 cm 2 [18] (14) 
where e is the dielectric constant of CdSe sample which is taken to be 5.76 [2]. 
The thermal velocity of an electron vth is calculated by using the relation 
vth={2kT m*)U2 (15) 
where m* is the effective mass of an electron which is taken to be 0.13me [19]. 
For finding S, values of Neff vm and St are separately calculated. Using the calculated 
values of S and p, evaluated from the slopes of ln(/0//f) versus t plots (Figure 8), trap 
depths are calculated (Table 2)at different conditions of illuminations. 
The vertical bars in different figures represent the experimental error calculated on the 
basic of standard deviation of the experimental data. Compared to the ranges of errors 
the resolutions of different results are acceptable. 
Effect of wavelength and intensity of illumination on the photoelectronic properties etc 593 
a 7(X)n m 
b 7 5 0 n m 
c 725nm 
t (socond) 
45 
a 30 lu \ 
h 40 lux 
L GO 111 \ 
12 
1 (saxxxl 
Figure 8. ln(/Q//f) vs time f plot for a CdSe thin film (/ = 2000A, 7"s= 473K) (a) under different monochromatic 
illuminations of constant intensity (b) under monochromatic light of 725nm of different intensities 
Table 2. Calculated values of trap depths for a CdSe thin film sample (/ = 2000A, Ts= 473K) (a) under 
different monochromatic illuminations of constant intensity (b) under monochromatic illumination of 725nm of 
different intensities. 
Illumination 
Intensity Wave 
( lux) ength (nm) 
50 
30 
40 
60 
700 
750 
725 
725 
n(tt 
cm) ' (in 
10 5) 
0 41 
0 35 
0 43 
0 30 
0 39 
0 57 
cm * (in 
1010) 
4 50 
3 77 
4 63 
3 3 
4 28 
6 21 
S 
Sec1 
(in 106) 
2 69 
2 25 
2 76 
1 97 
2 55 
3 71 
E 
0 39 
0 39 
0 40 
0 39 
0 40 
041 
( eV) 
E, 
0 42 
0 42 
0 43 
0 42 
0 43 
0 43 
4. Conclusions 
Thermalfy evaporated polycrystalline CdSe thin films are found to be characterized by 
Poole-Frenkel type of conductivity and barrier modulated photoconductivity. Photoconductivity 
in these polycrystalline films are basically controlled by the grain boundary defects The 
photocurrent is dependent on the wavelength of excitation and increases towards the 
band edge of wavelength. Rise and decay processes of photocurrent are effectively governed 
by different kind of traps. 
References 
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