CdxZn1−xS–arachidic acid composite LB films

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 ˚ 2 , as expected for fatty acid salts. The details are be ≈20 A 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.

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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.

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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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