Synthesis of band gap engineered PbxCd1-xSe thin films: A study on their optical, electrical, structural and localized mechanical properties

Article abstract of DOI:10.1002/pssa.200925286

In this work, we report a novel electrochemical technique to prepare band gap engineered IV-II-VI type ternary solid solution system of Pb _xCd_1-xSe. This technologically important material is prepared using this method for the first time. We have used a self sufficient modified electrochemical cell made up of Pb anode and a transparent conducting oxide (TCO) coated glass cathode, containing Pb(CH₃COO)₂, Cd(CH₃COO)₂, H₂SeO₃ and Na ₂EDTA solutions with proper concentrations as the working electrolyte. X-ray diffraction (XRD) pattern analysis confirms the formation of highly crystalline Pb_xCd_1-xSe, whereas, scanning electron micrograms (SEM) reveals a uniform deposition with compact surface morphology. This ternary system showed band gap energy (E_g) of about 1.4 eV, which is in between CdSe ( 1.77 eV) and PbSe (0.3 eV) and reasonably good for solar energy absorption. Good rectifying behaviour of the TCO/ Pb _xCd_1-xSe hetero-junction was established from the current-voltage measurements, which indicate the p-type conduction nature of the deposited ternary compound. Mechanical properties of such thin films were measured by nano-indentation.

Full text of DOI:10.1002/pssa.200925286

Phys. Status Solidi A 207, No. 8, 1880–1886 (2010) / DOI 10.1002/pssa.200925286
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applications and materials science
Synthesis of band gap engineered
PbXCd1– Se thin films: A study on their optical, electrical,
X
structural and localized mechanical properties
,1
Nillohit Mukherjee1, Gobinda Gopal Khan2, Arijit Sinha2, and Anup Mondal*
1 Department of Chemistry, Bengal Engineering and Science University, Shibpur, Botanic Garden, Howrah 711 103, West Bengal,
India
2 School of Materials Science and Engineering, Bengal Engineering and Science University, Shibpur, Botanic Garden, Howrah 711 103,
West Bengal, India
Received 3 June 2009, revised 11 January 2010, accepted 13 January 2010
Published online 1 March 2010
Keywords optical properties, electrical properties, thin film ternary compound, band-gap engineering, mechanical properties
* Corresponding author: e-mail anupmondal2000@yahoo.co.in, Phone: þ913326684561, Fax: þ913326682916
In this work, we report a novel electrochemical technique to crystalline PbxCd1–xSe, whereas, scanning electron micrograms
prepare band gap engineered IV–II–VI type ternary solid (SEM) reveals a uniform deposition with compact surface
solution system of PbxCd1–xSe. This technologically important morphology. This ternary system showed band gap energy (Eg)
material is prepared using this method for the first time. We of about 1.4 eV, which is in between CdSe (1.77 eV) and PbSe
have used a self sufficient modified electrochemical cell made (0.3 eV) and reasonably good for solar energy absorption. Good
up of Pb anode and a transparent conducting oxide (TCO) rectifying behaviour of the TCO/ PbxCd1–xSe hetero-junction
coated
glass
cathode,
containing
Pb(CH3COO)2, was established from the current–voltage measurements, which
Cd(CH3COO)2, H2SeO3 and Na2EDTA solutions with proper indicate the p-type conduction nature of the deposited ternary
concentrations as the working electrolyte. X-ray diffraction compound. Mechanical properties of such thin films were
(XRD) pattern analysis confirms the formation of highly measured by nano-indentation.
ß 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
1 Introduction Recently, solid solutions with semi- p-PbSe has a too low Eg (ꢀ0.3 eV, room temperature,
conducting properties are of interest due to their potential bulk system) to be used as an absorber material. An easy
applications in the cutting edge science and technology. electrochemical technique has been reported earlier for the
Some well-studied isoelectronic alloys or ternary com- preparation of PbX (X ¼ S, Se and Te) [10] and CdSe [11].
pounds are Pb1–xSnxTe, Pb1–xSnxSe and PbS1–xSex. These However, an appropriate combination of the working
are the series of IV–VI compounds and find applications as electrolytes for the deposition of the two materials can
thermoelectric materials [1], mid-infrared lasers [2], diode lead to form a ternary system, PbxCd1–xSe, which can
lasers [3–5], infrared detectors [6], mechanical alloys [7], make the material to have an optimum Eg of about 1.4–
etc. In this regard, cadmium-lead-chalcogenide (II-IV-VI 1.5 eV, which would be very close to an ideal absorber
system) [8] and cadmium-zinc-chalcogenide (II-II-VI sys- layer. The electrochemical cell we have used for this
tem) [9] ternary compounds have attracted the attention of purpose contained aqueous solutions of Pb(CH3COO)2,
workers for their similar interesting properties. The prepara- Cd(CH3COO)2 and H2SeO3 in appropriate concentrations.
tive methods for these compounds are not well established Na2EDTA solution was added as a complexing agent for
till date, and here an easy electrochemical technique to regulated deposition. The TCO substrate was used as the
deposit band gap engineered PbxCd1–xSe thin films is cathode, while a 99.9% pure metallic Pb strip was used as a
reported for the first time.
self-decaying anode. When the two electrodes were short
It begins with the idea that, the Eg of n-CdSe (ꢀ1.76 eV circuited externally, uniform and highly adherent thin
at room temperature for the bulk system) is not good films of PbxCd1–xSe were deposited on the TCO surface.
enough for a window layer in solid state solar cells. Again, The beauty of this process is that it dose not require any
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Phys. Status Solidi A 207, No. 8 (2010)
1881
external energy for deposition, the overall cell potential and u–2u scanning mechanism. The SEM images of the
serves the need. Optical, morphological, compositional, deposited films were taken by a HITACHI S 3400 N
mechanical and structural characterizations of the films scanning electron microscope fitted with HORIBA 7021-H
were carried out by UV–VIS, SEM, EDX, Nano-indenta- EDX probe. Optical measurements were carried out using a
tion and XRD techniques. Electrical properties were also JASCO V 530 UV–VIS spectrophotometer. The electrical
measured to establish its probable application as a solar (current–voltage) measurements were performed using a
cell material. From UV–VIS spectrum, the onset of a sharp KEITHLEY SCS 4200 Semiconductor Characterization
increase in absorption was evident around ꢀ900 nm; which System and the mechanical properties were studied using
corresponds to an Eg of about 1.4 eV. SEM image indicates CSM Instruments: NHTX 50-0019 nanohardness tester.
a compact surface morphology. Mechanical properties of
such films have been studied and reported for the first time.
The hardness values, Hv (Vickers) and the instrumented
3 Results and discussion
3.1 The chemistry When SeO2 is dissolved in water, it
elastic modulus values EIT (GPa), revealed that both the
easily produces selenous acid (H2SeO3), which dissociates in
hardness and the modulus of the coated surface under
40 mN load was high enough to produce better resistance
solution to 2Hþ and SeO32ꢁ ions. The Pb2þ and Cd2þ ions were
first complexed with the addition of Na2EDTA solution,
towards the deformation caused under thermal stresses
followed by the addition of H2SeO3 solution, to prevent any
while exposed to the nature for practical applications.
precipitation of PbSeO3 and CdSeO3. Glacial CH3COOH was
added to the solution to prevent hydrolysis of the lead and
cadmium salt and also to maintain the optimum pH (ꢀ3) of the
2.1 Deposition procedure To start with, the sub-
2 Experimental
solution for deposition. The controlled dissociation of the
strates were cleaned with detergent and then dipped into
electrolytes Pb(CH3COO)2 and Cd(CH3COO)2 in solution is
concentrated chromic acid solution for about 30 min
and washed thoroughly with distilled water to remove
also achieved by the common ion effect of CH3COOꢁ ion
produced by CH3COOH. When the Pb strip and the TCO
any adhering impurities. They were then boiled in
substrate were dipped into the solution and short circuited
methanol and after drying were degreased in a vapor of
externally by a copper wire, elemental Pb from the anode
dissolves into the solution forming Pb2þ ions, Pb ! Pb2þ þ 2e
trichloroethelene.
Recently, PbSe thin films on TCO coated glass
substrates were deposited by us [10] using a self
through the externally short-circuited path to the TCO
sufficient galvanic cell containing aqueous solutions of
(E0 ¼ þ 0.126 V). The electrons that were released moved
electrode and carry out the required cathodic reduction. The
Pb(CH3COO)2, Na2EDTA and SeO2 at pH 3, whereas,
Murali et al. [11] in 1990 reported the deposition of
Pb2þ and Cd2þ ions present in the solution are attracted by
the TCO cathode and thereby get discharged on the cathode
CdSe using the same technique. Here, a properly cleaned
surface simultaneously, bytaking upelectrons. We propose the
TCO glass substrate and a Pb strip were dipped into a
following overall cathode reactions to take place for this
system:
solution containing 10 ml 0.1 M Pb(CH3COO)2, 5 ml 0.1 M
Cd(CH3COO)2 and 10 ml 0.1 M aqueous SeO2 solutions
(Bath B). 10–15 ml 0.1 M aqueous Na2EDTA solution
was also added to the solution before the addition of
selenous acid, as a complexing agent for regulated
deposition. The overall volume of the solution was made
Pb2þ þ Cd2þ þ SeO32ꢁ þ 6Hþ þ 8e
! PbxCd1ꢁxSe þ 3H2Oð0 < x < 1Þ
up to 100 ml. The schematic of the process is reported in
Since, the amount of Cd2þ ions is lesser than that of Pb2þ
Ref. [10]. We have varied the amount of Cd(CH3COO)2
ions in the solution, and the reduction potential of Cd2þ ion
from 3 ml (Bath A) to 8 ml (Bath C), keeping the amount of
is more negative than that of Pb2þ ion, it is obvious that
Pb(CH3COO)2 fixed, and found that films with excellent
lesser amount of cadmium will contribute to the matrix of
optical, electrical and structural properties were resulted
the deposited film leading to the composition of the film as
when 5 ml Cd(CH3COO)2 was taken (Bath B). The Pb strip
PbxCd1–xSe.
and the TCO glass substrate were short-circuited exter-
nally through a copper wire. The Pb strip served as a self-
3.2 Film growth kinetics In order to study the
decaying anode and the TCO glass substrate as the cathode. growth rate of the films obtained from the three baths, the
The pH of the solution was kept at 3, which was found to be thickness of the films was measured gravimetrically, time to
optimum for PbxCd1–xSe film deposition. The deposition time, using AB54S Mettler balance (fourth decimal). The
was carried out for 60 min (where the saturation in thickness of the films was plotted as a function of time, from
thickness was obtained) at temperature 70 – 75 8C. The which an idea on the film growth rate was obtained. It has
solutions were stirred continuously during deposition.
been found that the saturation thickness at 75 8C was
achieved nearly after 60 min with an almost same growth
2.2 Characterizations XRD pattern of the film was rate of 0.03 mm/min, for the three baths. Since, there was no
taken by a SEIFERT P3000 Parallel Beam X-ray continuous supply of SeO23ꢁ ions solution, the formation of
˚
Diffractometer with Cu Ka (l ¼ 1.540598 A) X-radiation films on cathode surface stops after a certain time, when the
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SeO23ꢁ ions þget exhausted, as the reduction potential of peaks do not match with any diffraction plane of CdSe
SeO23ꢁ þ 6H þ 4e ! Se þ 3H2O (E8 ¼ þ 0.74 V) system (JCPDS ref. 08-0459) and/or PbSe (JCPDS ref. 06-0354);
mainly provides the driving force for the overall cathode hence, we propose that, the diffractions that occur at these
reaction.
angles are from the newly deposited ternary solid solution
compound PbxCd1–xSe. Absence of any diffraction peak
corresponding to CdSe and/or PbSe confirms the fact that
there is no deposition of free CdSe or PbSe along with the
PbxCd1–xSe film. The peaks marked ‘ꢂ’ in the figure are for
the SnO2 of the TCO substrate (JCPDS ref. 21-1250). The
crystallite sizes of the deposited films were calculated from
the most intense XRD peak for each case, using Scherrer
equation; D ¼ 0.9 l/(b cosu), where, l is the wavelength of
3.3 Variation of cathode current density The
variation of cathode current density (I) with respect to time
(t) for the deposition process from all three baths was
measured against a standard resistance of 0.8 V. The current
densities were calculated from the potential values obtained
after short-circuiting the two electrodes. A sharp fall in I was
found to occur during the first hundred seconds. The steady
cathode current density was found to reach nearly after 200 s
and it was ꢀ0.9 mA/cm2. The steady overall cell potential
was calculated to be around 0.72 V, high enough as the
driving force to carry out the desired reactions. The values
were nearly the same for the three different baths; since the
driving force for all the systems was mainly the reduction of
SeO23ꢁ to Se.
˚
the X-radiation (1.54056 A), b is the value of FWHM of the
peak of highest intensity and u is the diffraction angle. The
average crystallite size was calculated to be around 200 and
100 nm for the films obtained from Bath A and Bath C,
whereas, for the PbxCd1–xSe film (from Bath B), it was
around 100 nm.
3.5 Scanning electron micrograph and energy
3.4 X-ray diffraction study The XRD pattern of the dispersive X-ray spectroscopy analysis Figure 2a–c
films deposited at 75 8C from Bath A, Bath B and Bath C are are the representative SEM images of the films obtained from
shown in Fig. 1a–c, respectively. It is evident from Fig. 1a the Baths A, B and C, respectively. The thicknesses of the
that the films obtained from Bath A consists mainly of PbSe films chosen for SEM measurements were in the same range.
(JCPDS ID 06-0354) with the beginning of the formation of Good coverage of the TCO surface by the uniformly
PbxCd1–xSe. On the other hand, the films from Bath C were deposited films from the three baths is evident from the
found to be composed mainly of CdSe (JCPDS ID 08-0459) images. The shape of the grains of this newly deposited
with some traces of PbxCd1–xSe and PbSe, whereas, films material (from Bath B) is found to be cubic with the average
with our desired composition, i.e. PbxCd1–xSe were obtained grain size of about 250 nm, whereas, the shape of the grains
only from Bath B, where we find no traces of other materials of the films with predominantly PbSe (from Bath A) and
like PbSe or CdSe as impurities. Since the reduction potential CdSe (from Bath C) in it were found be nearly spherical and
of Cd2þ ion is more negative than that of Pb2þ ion, and the like coffee beans, respectively. The average grain sizes of the
reaction is carried out under a fixed value of potential, films obtained from Bath A and Bath C were 500 nm and
controlling the amount/concentration of Pb2þ and Cd2þ ions 250 nm, respectively. From XRD measurement it was
in the working solution is crucial in order to obtain the films observed that the crystallite sizes were about 200, 100 and
with desired composition by the competitive migration and 100 nm, respectively for the films obtained from Baths A, B
reduction of the ions at the TCO cathode. Here we see that the and C. It is a well-established fact that small crystallites
electrolytic combination of Bath B is appropriate for the coalesced together to from grains with large size, which is
deposition of PbxCd1–xSe ternary solid solution system. seen by SEM.
Figure 1b reveals the presence of two new peaks at
The compositional details of the film were read out from
2u ¼ 43.664 and 65.653 degrees. These two diffraction the representative EDX spectrum of the ternary material
Figure 1 X-raydiffractionpatternsforthefilmsdepositedfrom(a)BathA,(b)BathB,and(c)BathC.The‘ꢂ’markedpeaksarefortheSnO2
of the TCO substrate.
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Phys. Status Solidi A 207, No. 8 (2010)
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Figure 2 SEM images of the films deposited from (a) Bath A, (b) Bath B, and (c) Bath C on TCO-coated glass substrates.
PbxCd1–xSe, which indicates the presence of the elements Se,
From XRD pattern analyses and EDX measurements it is
Pb and Cd. The result of quantitative EDX analyses shows evident that the films obtained from Bath A contains mainly
that the ratio of atomic percentage of Se, Pb and Cd is PbSe and that from Bath C contains mainly CdSe, which also
51:34:15. From this measurement, we propose the approxi- has influence on the respective optical data. From curve a of
mate stoichiometry of the PbxCd1–xSe film to be Fig. 3, which is for the films obtained from Bath A, we can
Pb0.7Cd0.3Se. However, the films are likely to be slightly locate the sharp rise in the absorption spectrum close from
selenium reach, leading to the p-type conductivity of the 1 400 nm (corresponding to Eg ¼ 0.9 eV), which is moving
deposited material. The proposed stoichiometry (from EDX towards the Eg of PbSe (ꢀ0.3 eV), but is not exactly the
analyses) for the other two types of films obtained from the same, since it has some PbxCd1–xSe ternary compound mixed
Baths A and C are Pb0.8Cd0.2Se and Pb0.2Cd0.8Se, respect- with it, the value is to some extent higher. On the other hand,
ively. These values indicate that, predominantly PbSe and as the films obtained from Bath C contains mainly CdSe and
CdSe were formed from the Baths A and C, which also traces of PbSe and PbxCd1–xSe ternary systems, the rise in the
support the outcome of XRD analyses.
optical absorption curve starts from around 730 nm (curve
‘c’), which corresponds to the Eg ¼ 1.7 eV and close to the
typical value for CdSe (Eg ¼ 1.76 eV). The observations
relating bath composition, film stoichiometry and band gap
energy are summarized in Table 1.
3.6 Optical (UV–VIS) studies The UV–VIS spectral
analysis of the films from Bath B was carried out and a sharp rise
in absorbance was observed at a wavelength near 900 nm
(Fig. 3, curve b), which corresponds to the band gap energy
(1.4 eV) of the deposited PbxCd1–xSe film. A thorough scan
from4 000 nmwas alsoperformedto detect anypresenceof free
PbSe, but no such absorption edge was observed (the result is
not shown here) for such films. The sharp increase in the
absorption spectra indicates good crystalline nature of the films.
The band gap energy (1.4 eV) is close to the ideal value (1.4–
1.5 eV) for application as a photovoltaic material [12].
3.7 Studies on mechanical properties The sus-
tainability/stability of the thin films at adverse conditions
depends on its mechanical properties. The nanoindentation
techniqueis used fortheestimation ofmechanical properties of
the thin film materials [13]. The mechanical properties of
PbxCd1–xSe thin films deposited on TCO glass has been
reported here for the first time, by nanoindentation measure-
ments. In this technique, a load is applied to a triangular
pyramidal shaped diamond indenter tip(Berkovich) which
penetrates the surface of the films being tested. One of the
impressions of the indenter is captured by the inbuilt atomic
force microscope (AFM) of the instrument and is shown in
Fig. 4. The selected scan area was 14 mm ꢃ 14 mm. Analyses
of the load vs. depth of penetration plots yielded the elastic
modulus (EIT) [14] and hardness (HIT) [14], following the
inbuilt Oliver & Pharr algorithm as
EIT ¼ ð1 ꢁ n2s Þ=½1=Er ꢁ fð1 ꢁ ni2Þ=Eigꢄ
HIT ¼ maximum load applied ðin mNÞ=
projected area ðin nm2Þ;
where, ns and ni are the Poisson ratio of the sample and that
of the diamond indenter, respectively. Er and Ei are the
reduced modulus (combined modulus of indenter and
sample) and elastic modulus of the indenter, respectively.
The known value of the elastic modulus and Poisson ratio of
the indenter are 1 141 GPa and 0.07, respectively. The
Figure 3 (online colour at: www.pss-a.com) UV–VIS absorption
spectra of the films deposited from (a) Bath A, (b) Bath B, and
(c) Bath C.
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Table 1 Summary of the composition of the solution, probable formulation and the corresponding band gap energy of the films.
Amount of ions taken in the working solution (0.1 M)
Formulation of the films (as obtained from EDX)
Band gap (eV)
SeO23ꢁ ion (ml)
Bath
Pb2þ ion (ml)
Cd2þ ion (ml)
A
B
C
10
10
10
3
5
8
10
10
10
Pb0.8Cd0.2Se
Pb0.7Cd0.3Se
Pb0.2Cd0.8Se
0.9
1.4
1.7
For the industrial applications of these films in the form
of electronic devices like solar cells, laser diodes etc. there is
a chance of such films being exposed to higher temperatures.
At room temperature also, the films must show good
mechanical properties like adherence with the substrate,
hardness etc. for successful use in devices. If there is a large
difference between the values of co-efficient of thermal
expansion (CTE) of the film and the bottom layer, thermal
stresses would result due to unequal expansion and
contraction. It would deform the surface and hence decreases
the sustainability/stability of the coating. The hardness
values, Hv and the instrumented elastic modulus values, EIT
(in GPa) are presented in the Tables 2a–c, which reveals that
both the hardness and the modulus of the coated surface
decrease significantly with increase in the applied load. The
hardness and modulus values are quite higher, although the
load was increased from 20 to 40 mN. The higher hardness
and elastic modulus refers to better resistance towards the
deformation of the surface under thermal stresses and good
adherence of the deposited film with the bottom layer.
Among the three types of films, the highest hardness and
elastic modulus (under three different loads) were shown by
Figure 4 (onlinecolourat:www.pss-a.com)AFMimageofthethin
film showing the impression of the diamond indenter on the film
(withinthewhitecircle)whichoccurredduringthenano-indentation
measurement at 40 mN load.
nanoindentation characteristics at three different loads the films obtained from Bath A, whereas, the lowest values
(using Berkovich indenter tip) of 20, 30 and 40 mN for the were shown by the films obtained from Bath B. Films from
films grown from Bath A are shown in Fig. 5(a)–(c), for Baths C were found to show values in between the other two
Bath B in Fig. 6 (a)–(c) and for Bath C in the Fig. 7 (a)–(c), compositions. Though the films from Bath B (Pb0.7Cd0.3Se)
respectively. From these figures it is evident that the show the lowest values of hardness and elastic modulus at
maximum depth of penetration obtained in the coating different loads among the films from other two baths, it is
surface is around 400 nm. The curves follow almost the sufficient enough to be used for their practical applications.
same trend, which reveals that the localized mechanical
properties of the films obtained from different baths have
slight differences.
3.8 Electrical measurements DC I–V measure-
ment of the SnO2:F (TCO)/Pb0.7Cd0.3Se hetero-structure
Figure 5 Load vs. depth of penetration plots at three different loads of (a) 20 mN, (b) 30 Mn, and (c) 40 mN for the films deposited from
Bath A.
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Phys. Status Solidi A 207, No. 8 (2010)
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Figure 6 Load vs. depth of penetration plots at three different loads of (a) 20 mN, (b) 30 mN, and (c) 40 mN for the films deposited from
Bath B.
Figure 7 Load vs. depth of penetration plots at three different loads of (a) 20 mN, (b) 30 mN, and (c) 40 mN for the films deposited from
Bath C.
Table 2a Hardness and elastic modulus at three different loads of
was carried out in a voltage range of –1.0 to þ 1.0 V and is
Bath A.
shown in Fig. 8. The upper electrical contact to the film was
made with colloidal Ag-paste with 0.09 cm2 area, while,
Load
(mN)
Hardness:
Hv (Vickers)
Hardness:
EIT (GPa)
Modulus:
EIT (GPa)
TCO itself acted as the bottom contact. From the rectifying
nature of the I–V plot, it can be proposed that the deposited
Pb0.7Cd0.3Se film is p-type in nature. The threshold voltage
of this p–n junction is about þ 0.5 V, whereas, the onset of
Zener effect occurs nearly at –0.5 V. From this plot, the DC
resistance (rdc) which is generally calculated at the point of
20
30
40
858.44
706.21
615.26
9.120
6.970
6.521
143.05
109.80
99.51
Table 2b Hardness and elastic modulus at three different loads of
Bath B.
Load
(mN)
Hardness:
Hv (Vickers)
Hardness:
EIT (GPa)
Modulus:
EIT (GPa)
20
30
40
768.84
686.19
599.22
8.302
6.471
6.210
132.03
100.60
91.21
Table 2c Hardness and elastic modulus at three different loads of
Bath C.
Load
(mN)
Hardness:
Hv (Vickers)
Hardness:
EIT (GPa)
Modulus:
EIT (GPa)
20
30
40
801.47
715.11
625.65
8.521
6.671
6.381
139.07
108.30
94.71
Figure 8 (onlinecolourat:www.pss-a.com)DCI–Vcharacteristic
plot for SnO2:F (TCO)/ PbxCd1–xSe hetero-structure.
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threshold voltage, was found to be nearly 300 V, whereas,
the AC resistance, [dV/dI]rdc was 0.033 V. The resistivity
value at rdc was calculated to be 6.75 ꢃ 105 V-cm.
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TCO coated glass substrates, which involved no use of
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Acknowledgements The authors N. Mukherjee, G. G.
Khan and A. Sinha are thankful to CSIR, India for financial
assistance (grant no. 8/3(0051)/2008-EMR-I, 8/3(0052)/2008-
EMR-I and 8/3(0057)/2008-EMR-I, respectively). The authors
are also thankful to Mr. Debashis Ghosh (M.Sc. final year student
of the Dept. of Chemistry of this university) for his technical
support.
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