CdxZn1−xS–arachidic acid composite LB films
M. Parhizkara,1 , Nigvendra Kumarb , P.K. Nayaka , S.S. Talwara ,
S.S. Majora,∗ , R.S. Srinivasac
a
c
Department of Physics, Indian Institute of Technology Bombay, Mumbai-400076, India
b Maharashtra College, Bombay University, Mumbai-400008, India
Department of Metallurgical Engineering and Materials Science, Indian Institute of Technology Bombay, Mumbai-400076, India
Abstract
Mixed LB multilayers of cadmium arachidate–zinc arachidate (CdA–ZnA) have been exposed to H2 S to form Cdx Zn1−x S alloy nanoclusters
within the multilayers. FTIR and UV–vis spectra of H2 S exposed multilayers showed that the conversion of arachidate salt into arachidic
acid (AA) and sulphide formation begins in the first 5 min and is completed in ∼3 h of H2 S exposure. X-ray reflection studies and the nature
of CH2 scissoring band of the composite multilayers show that the formation of sulphide nanoclusters is accompanied by the formation of
molecular chain domains with different polymorphic phases of AA, depending upon the relative proportions of CdA and ZnA in the precursor
multilayer. Depending upon the CdS and ZnS content, the absorption edge is found to continuously shift from that of pure CdS to that of pure
ZnS nanoclusters, indicating the formation of Cdx Zn1−x S alloy nanoclusters in the arachidic acid matrix. The nanocluster formation is more
facile in the mixed archidate multilayers than in pure CdA or ZnA multilayers.
Keywords: Langmuir–Blodgett; ZnS; CdS; Cdx Zn1−x S alloy; Nanoclusters
1. Introduction
Langmuir–Blodgett (LB) multilayers have been used as
precursors for the growth of semiconducting chalcogenide
nanoclusters within the layered matrix through post deposition treatment with hydrogen sulphide gas [1–7]. The interest
in this approach is primarily because the layered structure
and molecular order present in the LB multilayers are expected to assist in achieving better control over the size,
shape and distribution of nanoclusters. Moreover, the possibility of depositing inorganic–organic nanocomposite films
with molecular level thickness control opens the possibility of fabricating a wide range of nanostructured devices.
LB multilayers of divalent fatty acid salts like cadmium
arachidate/stearate have been most extensively used to develop and understand the growth process of semiconduct-
ing chalcogenide (e.g., CdS) nanoclusters within the layered matrix of LB films [1]. In comparison, there has been
limited work on the growth of ZnS within the LB layered
matrix [8–11]. Studies on the growth of mixed sulphides
such as HgS–CdS within LB matrix have also been very few
[1].
Group II mixed sulphides in thin film forms have been
extensively studied for applications in short wavelength optoelectronics, photovoltaics and photoelectrochemical solar
cells. Synthesis of CdS–ZnS alloy nanoparticles by coprecipitation, and formation in micelles and vesicles has also been
reported [12–16].
In the present work, cadmium arachidate–zinc arachidate
(CdA–ZnA) mixed multilayers have been used as precursors to develop CdS–ZnS alloy nanoclusters in the confined
geometry of LB layered matrix through H2 S exposure. The
formation and growth of CdS–ZnS alloy nanoclusters and the
accompanying changes in the overall structure of the composite multilayer as a function of H2 S exposure have been
investigated by FT-IR, UV–vis spectroscopy and X-ray reflectivity (XR) techniques.
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2. Experimental details
Mixed LB multilayers of cadmium arachidate–zinc
arachidate (CdA–ZnA) were prepared by the conventional
LB deposition technique using a KSV 3000 instrument in
a clean room. A solution of arachidic acid (Aldrich, 99%)
in HPLC grade chloroform (1 mg/ml) was spread on an
aqueous subphase containing CdCl2 and ZnCl2 in varying
proportions and a total salt concentration of 5 × 10−4 M.
Deionised and ultra filtered water (Millipore) having resistivity of 18.2 M cm was used to prepare the subphase. The multilayers transferred were of pure CdA, pure ZnA and mixed
arachidates with 20, 50 and 80 mol% of ZnA. The subphase
temperature was kept constant at 10 ◦ C and the subphase pH
was maintained at 6.5 ± 0.1. The monolayer was compressed
with a constant barrier speed of 3 mm/min and the multilayer
deposition was carried out at a surface pressure of 30 mN/m.
In all the cases, the π–A isotherms (not shown) exhibited a
condensed nature, without a liquid condensed region, that
is indicative of complete ionization of arachidic acid. The
limiting mean molecular area (LMMA) obtained by extrapolating the solid region of the π–A isotherm was found to
be ≈20 Å2 , as expected for fatty acid salts. The details are
reported elsewhere [17].
The compressed monolayer was transferred by vertical
dipping method at a speed of 3 mm/min. Quartz and CaF2
were used as substrates and were cleaned using standard procedures. Typically, 25 monolayers were transferred in each
case. The LB multilayers were exposed for different durations at a constant flow of H2 S gas generated in a Kipp’s
apparatus.
UV–vis spectra were obtained with a Shimadzu UV-160A
spectrophotometer and FT-IR studies were carried out with
a Perkin Elmer make Spectrum 1 instrument. X-ray reflection (XR) studies were performed with Philips X’pert diffractometer using Cu K␣ radiation in the 2θ range 4–20◦ .
3. Results and discussion
The sulphidation of the precursor mixed multilayers have
been studied using FT-IR spectroscopy as a function of
H2 S exposure duration. Typical results are presented in
Fig. 1, which shows the FT-IR spectra in the range of
1350–1800 cm−1 for the mixed CdA–ZnA multilayers with
20, 50 and 80mol% ZnA (in subphase), respectively, on CaF2
substrate in the as-deposited state and after H2 S exposure
of 3 h. In all the cases, the as-deposited multilayers showed
strong absorption bands at ∼1538–1545, 1398–1421 and
1463–1473 cm−1 . These are characteristic bands normally
observed for divalent fatty acid salts [18] and assigned to the
COO− asymmetric and symmetric stretching and the CH2
scissoring vibrations, respectively. The differences in their
nature and positions for mixed multilayers of different compositions are consistent with their characteristic positions for
pure CdA and pure ZnA multilayers [17]. The presence of
Fig. 1. FT-IR spectra of mixed CdA–ZnA LB multilayers with (a) 20 mol%
ZnA, (b) 50 mol% ZnA and (c) 80 mol% ZnA (in the subphase) in the asdeposited state (—) and after H2 S exposure for 3 h (- - -), showing the arachidate salt to arachidic acid conversion process.
COO− band and the complete absence of C O stretching
band of unionized carboxylic acid at ∼1700 cm−1 confirm
that all the as-deposited mixed multilayers consist of arachidate salt and not a mixture of arachidic acid and salt. The
appearance of the CH2 scissoring band as a doublet in the
case of CdA dominated multilayer (Fig. 1(a)) indicates that
the molecular packing in this case is orthorhombic subcell
based close packed (herringbone type) with two molecules
per unit cell [19], the characteristic packing of CdA multilayers [20]. The ZnA dominated multilayer (Fig. 1(c)) in
comparison shows a singlet ∼1468 cm−1 that indicates an
intralayer molecular packing with one molecule per unit cell,
similar to the ‘rotator’ phase like hexagonal layer cell based
packing observed in ZnA multilayers [20]. The mixed multilayer with 50% ZnA (Fig. 1(b)) shows overlapping doublet
and singlet, which indicates the presence of domains with
CdA and ZnA type of molecular packings. These results have
been discussed in detail elsewhere [17].
The FT-IR spectra for the three above described mixed
multilayers were recorded at different stages of H2 S exposures. The spectra in all the three cases after H2 S exposure
for 5 min (not shown here), showed a decrease in the intensity of the absorption peak at ∼1540 cm−1 and the appearance of a new peak at ∼1700 cm−1 , which was attributed to
carbonyl stretching vibration of the arachidic acid. Further
the appearance of a band at ∼1410 cm−1 was seen, which is
characteristic of arachidic acid [21]. Most interestingly, the
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CH2 scissoring band in all the three cases became a doublet. These trends continued with increased H2 S exposure
and were found to saturate in about 3 h of H2 S exposure in
all the cases. In the case of the 3 h H2 S exposed multilayers
(shown for all the three cases), the asymmetric and symmetric stretching peaks due to carboxylate group practically disappear and are replaced by the characteristic peaks at 1700
and 1410 cm−1 , associated with arachidic acid. These results
show that the conversion of arachidate salt to arachidic acid
and hence, the sulphide formation (an indirect inference) begins in the first 5 min of H2 S exposure and is completed in
about 3 h. Interestingly, the appearance of the CH2 scissoring
band as a doublet in all the H2 S exposed multilayers suggests
that the alkyl chains in all these cases are close packed in an
orthorhombic subcell [19].
The changes in the structure of the mixed multilayers that
accompany the formation of sulphide have been studied by
X-ray reflection techniques. Fig. 2 shows the XR patterns of
these multilayers in the as-deposited state, showing third and
higher order Bragg reflections. The bilayer period, in this as
well as all the cases discussed below was calculated using
the modified Bragg equation for refraction [22]. The bilayer
period for CdA and ZnA were found to be 5.5 nm (␣-phase)
and 4.7 nm (␦-phase), respectively, as reported earlier [20].
The XR pattern of mixed multilayer with 20 mol% of ZnA
showed the presence of a single ␣-phase. In comparison, the
multilayer with 50 mol% ZnA shows a broad peak, which on
deconvolution (not shown here) shows the presence of two
types of layered structures corresponding to bilayer periods
of 5.5 nm (␣-phase) and 5.1 nm (called ␤-phase, hereafter).
The reduced bilayer period of 5.1 nm in ␤-phase indicates
that the molecules are tilted by ∼23◦ from the layer normal.
Interestingly, the mixed multilayers with 80 and 90 mol%
of ZnA show a single (␤-phase) layered structure. The XR
studies agree well with the FT-IR studies of δ(CH2 ) band and
indicate that the mixed multilayer with 50% ZnA, consists of
two types of molecular domains; with each domain possibly
containing both CdA and ZnA. However, the other two mixed
multilayers consist of either of the two types of domains.
The XR patterns of the mixed multilayers after 3 h of H2 S
exposure are shown in Fig. 3. All the XR patterns exhibit an
overall decrease in the intensity of the Bragg peaks compared
to Fig. 2, indicating a decrease in the ordered component of
the multilayers as a result of H2 S exposure. The mixed multilayers with 20 and 50 mol% ZnA, on H2 S exposure show
several types of layered structures with bilayer periods of 5.5,
5.1, 4.9, 4.7 and 4.4 nm designated as ␣, ␤, ␥, ␦ and ␧-phases,
respectively. These results are consistent with the earlier studies of sulphide formation in pure CdA and ZnA multilayers
[23,24]. It is however, interesting to note that the decrease in
layered structural order produced by H2 S exposure, appears
to be marginal in the case of 80 mol% ZnA containing mixed
multilayer.
Figs. 4 and 5 show the UV–vis absorption spectra of pure
CdA and pure ZnA as well as those of the mixed CdA–ZnA
Fig. 2. X-ray reflection patterns of as-deposited LB multilayers of (a) pure
CdA, mixed multilayers with (b) 20 mol% ZnA, (c) 50 mol% ZnA, (d)
80 mol% ZnA and (e) pure ZnA. The third order Bragg peaks are indexed.
Fig. 3. X-ray reflection patterns of mixed LB multilayers with (a) 20 mol%
ZnA, (b) 50 mol% ZnA and (c) 80 mol% ZnA after 3 h H2 S exposure. The
third order Bragg peaks are indexed.
408
Fig. 4. UV–vis absorption spectra of (a) pure CdA multilayer and (b) pure
ZnA multilayer in as-deposited state (—) and after H2 S exposed for 5 min
(- - -) and 3 h (· · ·). The enhanced absorption indicates the sulphide formation in the arachidic acid matrix. The bulk bandgaps for CdS and ZnS are
indicated.
multilayers in the as-deposited state and at different stages
of H2 S exposures. In all these figures, the spectra for 5 min
H2 S exposure, exhibit considerable increase in the absorption
as compared to the as-deposited multilayers. The enhanced
absorbance in all these cases indicates sulphide formation in
the LB matrix within the first 5 min of H2 S exposure. This
behaviour is consistent with FT-IR results, which also showed
that the process of conversion of arachidate salt into arachidic
acid is initiated in the early stages of H2 S exposure.
In the case of H2 S exposed CdA (Fig. 4(a)), the absorption
onset is clearly blue shifted with respect to the bulk absorption
edge (515 nm) and a broad hump between 350 and 400 nm is
seen, which is attributed to the excitonic band in CdS [25].
Similarly, in the case of H2 S exposed ZnA (Fig. 4(b)), the
absorption onset is blue shifted with respect to the bulk absorption edge (345 nm) and the absorption spectrum exhibits
a small and relatively sharp hump at ∼280 nm, which is attributed to the excitonic band of ZnS [26]. The blue shift of
the absorption onset and the presence of excitonic peaks in
both the cases are indicative of quantum confinement effects
associated with the formation of CdS and ZnS nanoclusters
in the LB matrix in the respective cases. Similar features are
observed in the cases of the 5 min H2 S exposed mixed multilayers (Fig. 5) in which, enhanced absorbance is observed in
the shorter wavelength region, which is attributed to sulphide
formation in the mixed multilayers. The presence of a hump
in the absorption spectra of mixed multilayers is attributed to
excitonic absorption and hence, indicates the nanocrystalline
nature of the sulphide formed.
With increase in H2 S exposure duration, up to 3 h, the absorbance continues to increase in all the cases and the absorption onset shows a shift towards longer wavelengths, which
may be attributed to the growing size of sulphide nanoparticles with increased H2 S exposure. In all the cases, a saturation behaviour is observed in ∼3 h. of H2 S exposure, which
Fig. 5. UV–vis absorption spectra of the mixed CdA–ZnA LB multilayer
with (a) 20 mol% ZnA, (b) 50 mol% ZnA and (c) 80 mol% ZnA (in the
subphase) in as-deposited state (—) and after H2 S exposed for 5 min (- - -)
and 3 h (· · ·). The enhanced absorption indicates the sulphide formation in
the arachidic acid matrix.
is consistent with the FT-IR results. It is however noteworthy
that in comparison to the pure ZnA and CdA cases, the mixed
multilayers, exhibit only marginal changes in their absorption
spectra after 5 min of H2 S exposure.
In order to analyze the sulphidation behaviour of the mixed
multilayers, the UV–vis spectra of all the LB multilayers
exposed to H2 S for 5 min and 3 h have been re-plotted for
different compositions in Fig. 6. It is seen in Fig. 6(b), in
which the absorption spectra have been plotted for the saturated condition, that the absorption edge and the excitonic
hump continuously and monotonically shift from pure CdS
to pure ZnS spectra indicating the formation of continuous
solid solution of nanocrystalline Cdx Zn1−x S (in ararchidic
acid matrix) across the composition range.
409
sures for different compositions (Fig. 5), which showed that
the sulphidation proceeds much faster in the mixed arachidate
precursors than in pure ZnA and CdA.
The formation of Cdx Zn1−x S alloy nanoclusters in all the
mixed multilayers show that CdA and ZnA are present as
solid solution in not only the mixed multilayers with extreme
compositions but also in the multilayer with 50 mol% ZnA,
which showed the presence of CdA and ZnA type domains.
Since CdA and ZnA have very different equilibrium packing
configurations of molecular packing, the mixed multilayers
are away from a stable configuration, which possibly facilitates the formation of sulphides more than that in pure CdA
or ZnA multilayer systems.
4. Conclusions
Fig. 6. UV–vis absorption spectra of the multilayers of pure CdA (—), mixed
multilayers with 20 mol% ZnA (- - -), 50 mol% ZnA (· · ·), 80 mol% ZnA
(-·-·-) and pure ZnA (-··-··-) after (a) 5 min and (b) 3 h H2 S exposure.
In comparison, Fig. 6(a) shows some unusual features.
The spectra for mixed systems cross over that of pure CdS
and in most of the region show increased absorption. In particular, the spectra for the Cdx Zn1−x S–AA composite film
obtained by 5 min H2 S exposure of a 20 mol% ZnA containing precursor multilayer exhibits an excitonic hump at
∼400 nm and absorption onset ∼450 nm, which is about
the same as that for a pure CdS–AA composite film. It is
however interesting to note that the excitonic hump in the
case of this Cdx Zn1−x S–AA nanocomposite film is stronger
and sharper than that in the CdS–AA nanocomposite film.
These features suggest that compared to the CdS–AA film,
in this Cdx Zn1−x S–AA film, the nanoclusters formed in 5 min
H2 S exposure have a narrower size distribution and are possibly larger in number. Similar features are observed for
the Cdx Zn1−x S–AA composite films formed from precursor multilayers with 50 and 80 mol% ZnA, though both the
absorption spectra are progressively shifted towards the pure
ZnS–AA nanocomposite absorption spectra. These features
of the sulphidation behaviour of mixed arachidates are consistent with the results of the effect of duration of H2 S expo-
H2 S exposure of mixed LB multilayers of CdA–ZnA leads
to the formation of sulphide which begins in the first 5 min
and is completed in ∼3 h of H2 S exposure. X-ray reflection studies and the nature of CH2 scissoring band of the
composite multilayers show that the formation of sulphide
nanoclusters is accompanied by the formation of molecular
chain domains with different polymorphic phases of AA, depending upon the relative proportions of CdA and ZnA in
the precursor multilayer. Depending upon the CdS and ZnS
content, the absorption edge is found to continuously shift
from that of pure CdS to that of pure ZnS nanoclusters, indicating the formation of Cdx Zn1−x S alloy nanoclusters in
the arachidic acid matrix. Addition of ZnA in the precursor
CdA multilayer leads to facile formation of alloy sulphide
with relatively narrow size distribution. This is attributed to
relative metastability of the mixed precursor multilayers.
Acknowledgments
The financial support for this work from the Department
of Science and Technology, Government of India is gratefully
acknowledged.
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