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Thin Solid Films 515 (2

ZnO and Zn1−xCdxO nanocrystallites obtained by oxidation of precursorLangmuir–Blodgett multilayers

Sukhvinder Singh a, R.S. Srinivasa b, S.S. Talwar a, S.S. Major a,⁎

a Department of Physics, Indian Institute of Technology Bombay, Mumbai-400076, Indiab Department of Metallurgical Engineering and Materials Science, Indian Institute of Technology Bombay, Mumbai-400076, India

Available online 6 April 2007

Abstract

ZnO and Zn1−xCdxO nanocrystallites were prepared by oxidation of zinc arachidate–arachidic acid and zinc arachidate–cadmium arachidate–arachidic acid LB multilayers, respectively. The metal content of the multilayers was controlled by manipulation of subphase composition and pH.Precursor multilayers were oxidized in the temperature range of 400 °C–700 °C. The formation of ZnO and Zn1−xCdxO was confirmed by UV–Visible spectroscopy. Uniformly distributed, isolated and nearly mono-dispersed nanocrystallites of ZnO (11±3 nm) and Zn1−xCdxO (18±6 nm)were obtained.© 2007 Elsevier B.V. All rights reserved.

Keywords: ZnO; Zn1−xCdxO; Nanocrystallites; Langmuir–Blodgett

1. Introduction

Zinc oxide, a wide band gap semiconductor is an attractivematerial for a variety of applications [1]. There is a growinginterest in ZnO nanostructures owing to expected improvementsin room temperature lasing due to quantum size effects [2].Formation of ZnO has been reported in low dimensional forms,showing interesting size dependent effects [3]. Alloying ZnOwith CdO allows the engineering of its band gap towards thedesign of novel optoelectronic devices [4].

Langmuir–Blodgett (LB) multilayers of zinc arachidate(ZnA) and cadmium arachidate (CdA) have been used to formzinc oxide and cadmium oxide films by oxidation of thecorresponding precursor multilayers [5–7]. In the present work,the oxidation of ZnA and mixed ZnA–CdA LB multilayers hasbeen carried out to form nanocrystallites of ZnO and Zn1−xCdxOon quartz and freshly cleaved mica substrates.

2. Experimental details

ZnA and ZnA–CdA mixed multilayers were prepared usinga KSV 3000 LB instrument. A solution of arachidic acid (AA)

⁎ Corresponding author. Tel.: +91 22 25767567; fax: +91 22 25767552.E-mail address: syed@phy.iitb.ac.in (S.S. Major).

0040-6090/$ - see front matter © 2007 Elsevier B.V. All rights reserved.doi:10.1016/j.tsf.2007.03.178

in chloroform (1 mg/ml) was spread on subphase containingZnCl2 and CdCl2 in varying proportions and a total concentra-tion of 5×10−4 M in de-ionized and ultra filtered water. Thesubphase temperature was maintained at 10±1 °C and subphasepH was varied in the range 5.0–6.2. Quartz, CaF2 and freshlycleaved mica substrates were used. The as-deposited multilayerswere heat treated for 30 minutes in a stream of oxygen atatmospheric pressure in the temperature range of 400–700 °C.FTIR spectra were obtained with a Perkin Elmer Spectrum1instrument and UV–Visible spectra with a Perkin ElmerLambda 950 spectrophotometer. Atomic force microscopy(AFM) studies were carried out using a Nanoscope IVMultimode SPM.

3. Results and discussion

The zinc content of ZnA multilayers was varied by changingthe subphase pH at which the multilayers were transferred andwas monitored by studying their FTIR spectra. Typical resultsare shown in Fig. 1(a). The spectrum of the multilayertransferred at a subphase pH of 6.2 shows a strong absorptionat∼1540 cm−1, assigned to the asymmetric stretching of COO−

[8]. The presence of this band and the absence of C_Ostretching band of unionized carboxylic acid which is expectedat ∼1700 cm−1, confirm that this multilayer consists of pure

Fig. 1. FTIR transmission spectra of (a) ZnA and ZnA–AA compositemultilayers and (b) ZnA–CdA and ZnA–CdA–AA composite multilayers onCaF2 substrates transferred at different subphase pH (as indicated).

Fig. 2. UV–Visible transmittance spectra of oxidized (a) ZnA and ZnA–AAcomposite multilayers transferred at different subphase pH (as indicated) and(b) ZnA–CdA and ZnA–CdA–AA composite multilayers transferred withdifferent CdA contents in the subphase (as indicated).

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ZnA. As the subphase pH is lowered to 5.8, a peak at∼1700 cm−1 appears and increases in intensity. This shows thatthe ZnA content is reduced and that of AA is enhanced, formingcomposite ZnA-AA multilayers. In case of multilayerstransferred at subphase pH of 5.0, the peak at ∼1700 cm−1

becomes dominant, which shows that the composite multilayerconsists of primarily AA along with a small quantity of ZnA.The spectra of the typical CdA–ZnA multilayers transferred atsubphase pH of 6.2 and 5.0 are also shown (Fig. 1(b)), whichindicate the reduction in metal content of the mixed multilayertransferred at lower subphase pH.

The oxidation of arachidate multilayers and removal oforganic moieties has been reported earlier [7]. XRD studieshave shown that the layered structure of the multilayersdisappeared at ∼150 °C. The corresponding FTIR studieshave shown that all the absorption bands disappeared formultilayers oxidized at 400 °C, indicating the removal of allorganic moieties by oxidation at 400 °C or above. Transmissionspectra of the ZnA and ZnA–AA multilayers transferred onquartz substrates and oxidized at 400 °C are shown in Fig. 2(a).The spectrum of oxidized ZnA multilayer clearly shows anabsorption edge at ∼3.2 eV, close to the bulk band gap of ZnOwhich confirms the formation of ZnO. Subsequent heattreatments up to 700 °C did not show any change in thetransmission spectra. With decrease in subphase pH, theabsorption edge of oxidized ZnA–AA multilayers becomesless pronounced and only a small drop in transmittance on thelower wavelength side is seen for the ZnA–AA multilayertransferred at subphase pH of 5.0. These features are attributedto the formation of ZnO and the decrease in absorption ofoxidized multilayers with decreasing subphase pH is attributedto the reduction in their zinc content.

Fig. 2(b) shows the transmission spectra of the compositeZnA–CdA multilayers transferred at subphase pH of 6.2 fordifferent molar concentrations of CdA (in subphase) and afteroxidation at 400 °C. The spectrum of pure CdA after oxidationis also given as a reference. A monotonic shift in the absorptionedge towards lower energies is clearly seen with increase in theCdA content. This confirms that the oxidation of mixed ZnA–CdA multilayers results in the formation of Zn1−xCdxO alloy.

AFM studies were carried out in tapping mode using etchedsilicon probes to investigate the surface morphology of theoxides. Typical results are shown in Figs. 3 and 4. AFM imagesof ZnO obtained from precursor multilayers consisting of 15monolayers (ML) transferred at subphase pH of 6.2 and 5.0 andoxidized at 400 °C are shown respectively in Fig. 3(a) and (b).Fig. 3(a) shows the presence of a large number of ZnO particleswith a mean size of 1.1±0.7 μm along with particles as large as∼4 μm. On closer observation, the larger particles were foundto be agglomerates, as shown in the inset. Fig. 3(b), incomparison, shows more uniformly distributed particles withlesser spatial density and a smaller mean size of 0.8±0.5 μm.The reduction in size and spatial density is attributed to the factthat the ZnA–AA multilayer transferred at subphase pH of 5.0had the smallest ZnA content and thus the surface density ofzinc atoms available for oxidation was much reduced.

For further oxidation studies at higher temperatures, bothZnA–AA and ZnA–CdA–AA multilayers transferred at a

Fig. 3. AFM images of ZnO particles obtained by oxidation at 400 °C of ZnA–AAmultilayers (15 ML) on quartz, transferred at (a) subphase pH of 6.2 and(b) subphase pH of 5.0. Inset of (a) shows a zoomed in image of an agglomerate.

Fig. 4. AFM images of ZnO and Zn1−xCdxOnanocrystallites obtained by oxidationof (a) 15 ML ZnA–AA, (b) 7 ML ZnA–AA and (c) 7 ML ZnA–CdA–AAtransferred on mica at subphase pH of 5.0 and oxidized at 700 °C.

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subphase pH of 5.0 were taken up. The ZnA–AA multilayer(15ML) transferred on freshly cleaved mica was oxidized at700 °C and the corresponding AFM image is shown in Fig. 4(a).This image shows the presence of uniformly distributed ZnOparticles of much smaller size with a high spatial density, incontrast to the larger size ZnO particles obtained from ZnAprecursor multilayers (Fig. 3). The inset of Fig. 4(a) shows themean size of the ZnO nanocrystallites to be 11±7 nm. No largerparticles or agglomerates were seen in this case even for largearea scans at lower magnifications. The drastic changes in themorphology of particles are attributed to the changes ininterfacial tension between the molten organic moieties andthe substrate surface during oxidation. In order to obtaincrystallites with smaller size and density, the zinc content wasfurther decreased by reducing the number of monolayers in theprecursor. Fig. 4(b) shows the AFM image of a 7ML ZnA–AAmultilayer on mica, oxidized at 700 °C. The image showsuniformly distributed isolated nanocrystallites with a muchreduced spatial density and a mean size of 11±3 nm (shown asinset). The nanocrystallites were nearly mono-dispersed in anarrow range of 10–15 nm. Similar studies were carried out onZnA–CdA–AA composite multilayers. A typical AFM image

(Fig. 4(c)) for a 7 ML ZnA–CdA–AA multilayer (transferredfrom subphase containing 20 mol.% CdA at pH of 5.0) on micasubstrate, oxidized at 700 °C shows the presence of isolatednanocrystallites of Zn1−xCdxO. In comparison to the ZnOnanocrystallites, the Zn1−xCdxO nanocrystallites have a muchreduced spatial density and slightly larger mean size of 18±6 nm.Similar results were obtained on quartz substrates, but the smoothsurface of freshly cleaved mica allows smaller nanocrystallites tobe more clearly seen.

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4. Conclusions

Isolated nanocrystallites of ZnO and Zn1−xCdxO with narrowsize distribution were obtained by controlling the metal contentof precursor LB multilayers through variation of the subphaseconcentration and pH and the number of transferred monolayersas well as by proper selection of oxidation temperature. A 7MLZnA multilayer on mica substrate, oxidized at 700 °C showeduniformly distributed, isolated and nearly mono-dispersed ZnOnanocrystallites of mean size ∼11±3 nm. Zn1−xCdxO alloynanocrystallites formed under similar conditions, showedreduced spatial density and slightly larger mean size of 18±6 nm. Thus, the control of metal content of the precursor LBmultilayers and the oxidation temperature can be effectivelyutilized to obtain isolated nanocrystallites of pure and mixedoxides with a narrow size distribution.

Acknowledgements

The financial support received from MHRD (Govt. of India)for this work is gratefully acknowledged. Sukhvinder Singh is

thankful to CSIR, New Delhi (India) for Senior ResearchFellowship. FIST (Physics)-IRCC Central SPM Facility of IITBombay is acknowledged for providing the facilities for AFMstudies.

References

[1] Ü. Özgür, Ya. I. Alivov, C. Liu, A. Teke, M.A. Reshchikov, S. Doğan, V.Avrutin, S.-J. Cho, H. Morkoç, J. Appl. Phys. 98 (2005) 041301.

[2] Z.K. Tang, M. Kawasaki, A. Ohtomo, H. Koinuma, Y. Segawa, J. Cryst.Growth 287 (2006) 169.

[3] Z.L. Wang, J. Phys., Condens. Matter 25 (2004) R829.[4] T. Makino, Y. Segawa, M. Kawasaki, A. Ohtomo, R. Shiroki, K. Tamura, T.

Yasuda, H. Koinuma, Appl. Phys. Lett. 78 (2001) 1237.[5] M. Seidl, M. Schurr, A. Brugger, H. Voit, Appl. Phys. A 68 (1999) 81.[6] N. Matsuura, D.J. Johnson, D.T. Amm, Thin Solid Films 295 (1997) 260.[7] M. Parhizkar, N. Kumar, P.K. Nayak, S. Singh, S.S. Talwar, S.S. Major, R.S.

Srinivasa, Colloids Surf. A: Physicochem. Eng. Aspects 257–258 (2005)445.

[8] J.F. Rabolt, F.C. Burns, N.E. Schlotter, J.D. Swalen, J. Chem. Phys. 78(1983) 946.