Thin Solid Films 436(2003) 203–207

0040-6090/03/$ - see front matter� 2003 Elsevier Science B.V. All rights reserved.doi:10.1016/S0040-6090(03)00618-7

A hexane solution deposition of SnS films from tetrabutyltin via a2

solvothermal route at moderate temperature

Qing Yang , Kaibin Tang *, Chunrui Wang , Jian Zuo , Daoyuan Zhang , Yitai Qiana a, a a b a

Structure Research Laboratory and Department of Chemistry, University of Science and Technology of China, Hefei, Anhui 230026, PR Chinaa

Astronomy and Applied Physics, University of Science and Technology of China, Hefei, Anhui 230026, PR Chinab

Received 22 March 2003; accepted 14 April 2003

Abstract

SnS films have been deposited on glass and alumina plate substrates by the reactions between an organotin precursor2

wtetrabuyltin,(CH CH CH CH ) Snx and carbon disulfide inn-hexane at the temperature range 180–2008C for 10–40 h. The2 2 2 3 4

reaction system was oxygen free and applied at a moderate temperature. The films so prepared were characterized by techniquesof X-ray diffraction, Scanning electron microscopy, Raman and Mossbauer spectroscopies. The films deposited on glass as well¨as on alumina plate have an average thickness of 30mm, but have different rose-like morphologies, which are influenced by boththe anisotropic growths of crystals and the different substrate structures. Photoluminescence measurements show that the filmshave an emission peak at approximately 590 nm.� 2003 Elsevier Science B.V. All rights reserved.

Keywords: Sulfides; X-ray diffraction; Scanning electron microscopy; Raman scattering

1. Introduction

CdI -type structure SnS , possessing a strong aniso-2 2

tropy of optical propertiesw1x, have been intensivelystudied due to their application in various fields ofscience and technology. SnS thin films have been2

investigated as semiconductors and photovoltaic mate-rials w2–4x. In general, SnS thin films can be prepared2

by conventional non-solution techniques includingorganometallic precursor chemical vapour depositionw5x, molecular beam epitaxyw6x, spray pyrolysisw7,8x,chemical vapor transportw9x and physical vapor trans-port w10x. SnS thin films have also been prepared by2

solution methods such as by the reaction betweenSnCl and sodium thiosulfate, deposited onto glass, tin4

oxide–coated glass and titanium substratesw11x, by theprecipitation mechanism in organic acidyH O baths of2

SnCl , S and Sn(II)-complexing agentsw12x, and by the2

reaction between Sn(IV)-EDTA complex and thioace-tamide onto glass at room temperaturew13x, chemical

*Corresponding author. Tel.:q86-551-360-1600; fax:q86-551-360-1600.

E-mail address: kbtang@ustc.edu.cn(K. Tang).

deposition successive ionic layer adsorption and reactionw14x, and other chemical vapor depositionsw15,16x.

Solvothermal synthesis has been widely used in sci-entific fields and industrial productions. Up to now, alarge number of materials including diamondw17,18x,chalcogenidesw19–22x and othersw23–26x have beenprepared by this effective synthetic technique and it willattract more and more attention in the synthetic com-munity. In order to extend this valuable technique,solvothermal synthesitic method was applied to depositthin films. In this article, we report that the SnS films2

deposited from an organotin precursor inn-hexanesolution through a solvothermal process(similar to theroute for the SnS nanoflakesw20x) on the glass and2

alumina substrates.

2. Experimental

The chemical purity reagents of tetrabutyltinwTBT,(CH CH CH CH ) Snx, CS ,n-hexane, deionized water,2 2 2 3 4 2

microslide glass and alumina plates were used in theexperiments. TBT was purchased from Merck–Schu-chardt Corp. Others were purchased from differentchemical reagent corporations at home. Microslide glass

204 Q. Yang et al. / Thin Solid Films 436 (2003) 203–207

Fig. 1. XRD patterns of SnS films deposited on(a) glass and(b)2

alumina plate.

plate with dimensions of 18=18=0.5 mm and aluminaplate with 20=5=2 mm were used as substrates, whichwere immerged into 0.5 M dilute nitric acid for 2 h toclean the surfaces. The substrates were rinsed well indeionized water and dried in a vacuum. Then the cleanedsubstrates were used for the deposition of the SnS films2

from the solvothermal reaction between TBT and CS2

in n-hexane. Briefly, 1 ml TBT and 2–3 ml CS were2

dissolved in 20 ml inn-hexane, respectively. Thereafter,the two solutions were put into a Teflon–lined stainlesssteel autoclave of 50 ml capacity in sequence. Afterthat, the cleaned substrates were hung vertical in thevessel. Then the autoclave was sealed and maintainedin 180–200 8C for 10–40 h, and cooled to roomtemperature naturally. Finally, the grey–yellow sampleswere obtained and washed with CS , hexane, deionized2

water to remove the possible byproducts of elementalsulfur and other impurities and then dried in a vacuumfor further characterization.

Powder X-ray powder diffraction(XRD) was carriedout on a China Dandong X-ray diffractometer withgraphite–monochromatized Cu Ka radiation (ls1.54178 A.). Scanning electron microscopy(SEM),˚performed on an X-650 scanning electron microscope,was used to observe the morphology of the as-preparedfilms. Raman spectrum was recorded on a Spex 1403Raman spectrometer(ls514.5 nm), which was doneat ambient temperature. The spectrum of the sample wasobtained in the range 100–600 cm at a laser powery1

of 150 mW with slit of 140mm (approx. 1 cm ) andy1

an integration time of 0.5 s per step. Sn Mossbauer119 ¨spectroscopy was measured on an Oxford MS-500 Moss-¨bauer spectrometer with a Ba SnOg-ray source,119 m

3

working at room temperature. Recorded spectra werefitted with Lorentzian profiles by the least-squares meth-od using the program ISOw27x, and the fit quality wascontrolled by the test. All isomer shifts are given relativeto Sn in BaSnO . The photoluminescence(PL) spec-119

3

tra of the samples were obtained from the excitation ofthe 488 nm argon ion laser line from Spex 1403spectrometer.

3. Results and discussion

The films deposited on different substrates wereidentified by XRD. It shows that the films deposited onthe glass substrate are the hexagonal phase SnS(shown2

in Fig. 1a) with lattice parametersas3.6455,cs5.8967A, which accords with the reported dataw28x. The films˚on the alumina plate are also indexed as the hexagonalphase SnS(Fig. 1b) with lattice parametersas3.6463,2

cs5.8970 A. No diffraction peaks of impurities are˚observed in the patterns.

The morphologies of different films are illustrated bySEM images, shown in Fig. 2. The low-magnificationimage for the films deposited on the glass plate reveals

that the films are interesting rose-like morphologies(Fig. 2a). The close-up image(on glass, Fig. 2b) showsthat the rose-like morphologies composed of somesmooth petals of flowers, presumably due to the layeredstructure of SnSw29x. Fig. 2c is the SEM image of the2

samples produced in the procedure after 3–4 h and itshows that there are some scattered islands(particles)on the substrate. Fig. 2d is the SEM image of the filmsdeposited on alumina substrate. It shows the crystals onthe substrate and also has a rose-like morphology, butthe petals are sharper than those on the glass substrate.In contrast with them, we find that the morphologies ofthe two films match the substrates themselves, since theSEM images(not shown) reveal that the alumina sub-strate is rougher than the glass substrate. The thicknessof the two films are approximately 30mm.

The Raman spectra, shown in Fig. 3, demonstrate thetwo SnS films and has a characteristic intense peak2

locating at 313 cm , which corresponds to the Ay11g

mode of the layered–structure tin disulfidew30x. Thevibration of tin (II) sulfide or tin (III ) sulfide has notbeen obviously observed.

Fig. 4 is the Mossbauer spectra of SnS films prepared2¨in the route. The isomer shift(d) of ;1.0–1.05 mms is in the range of the typical values observed fory1

Sn (in octahedral coordination) in sulfides. The fullIV

width at half maximum(G) is 0.98–1.06 mm s ,y1

which coincides with the reported valuew31,32x. Thetypical values for Sn , Sn and Sn oxidation states0 II III

have not been observed in the spectra, confirming thepurity of the films.

The reaction temperature is a critical factor on theformation of SnS films via the solvothermal routew20x.2

When temperature was lower than 180 C, no well-o

crystallized SnS films have been synthesized. A mod-2

205Q. Yang et al. / Thin Solid Films 436 (2003) 203–207

Fig. 2. SEM images of the SnS films:(a) low-magnification image of the films deposited on glass plate;(b) close-up image of the films for2

Fig. 2a;(c) deposition on glass for 3–4 h; and(d) deposited on alumina plate.

erate temperature in the range of 180–2008C is suitablefor the deposition of the films. Other factors such asconcentration and feedstock of the source materials haveno obvious influence on the formation of the films. Theoxygen free system is favorable for the films withoutproducing tin oxides.

It is found that the adhesion of the films to thesubstrate was weak. Strongly tissue-wiping or ultrasonic-washing can remove the films, and a knife can alsoreadily remove them. The adhesion of the film toalumina is stronger than that to glass. It is also foundthat the films are grey–yellow, unlike that of usual tin

disulfide, which is normally yellow. The change of colormight be caused by the absorption and the carbonizationof butyl or organic solvent, considering that the XRD,Raman and Mossbauer results has no any impurities oftin compounds.

In contrast to the films prepared by the non-solutiontechniques noted before (such as in Refs.w5,9,10,15,16x), the films prepared in our work areparticularly interesting for their morphologies. We havedemonstrated that only flake-like powdersw20x wereproduced in the same reaction system without substrate.The special morphologies of the films, therefore, are

206 Q. Yang et al. / Thin Solid Films 436 (2003) 203–207

Fig. 3. Raman spectra of the films deposited on(a) glass and(b)alumina plate.

Fig. 5. PL spectrum of SnS films deposited on glass.2

Fig. 4. Mossbauer spectra of the films deposited on(a) glass and(b)¨alumina plate.

probably due to the anisotropic growth of the layered–structure SnSw29x. In crystallography, the morphologies2

are often determined by the anisotropy of the crystals.In solvothermal process, heat solution is favorable forthe anisotropic growth of the crystalsw20x. When asubstrate is used, the crystal growth onto it may beinfluenced by the surfaces of the substrate and that itselfresult in a film of a rose-like morphology. According tothe view of Burton et al., the atomistic mechanism ofsurface growth could be described as the terrace-step-kink (TSK) model w33x, which has been directly visu-alized by scanning tunneling microscopew34,35x. In thepresent process, SEM image shows a similar TSK modelin a large dimensional scale(see the particles in Fig.2c) instead of the atomic islands in the original TSKmodel. It is thought that the films of the rose-like

morphology were gradually crystallized from the parti-cles on the substrates. Seen from the SEM images ofthe two types of substrates(not shown), the surface ofalumina is rough than that of the glass. Also, the petalsof roses on the alumina substrate are sharper than thoseon the glass substrate, which are in accordance with thesurface roughness of the two substrates. This suggeststhe substrate really affect the morphology of the films.

The optical properties of the films were measured atroom temperature. The spectrum of the films on glassplate (Fig. 5) shows a peak at 590 nm. The film onalumina also shows emission at approximately 590 nm,but the strong fluorescence background of the aluminasubstrate convoluted the peak.

4. Conclusions

SnS films have been successfully deposited on the2

glass and alumina plate substrates from the reactionbetween an organotin precursor TBT and carbon disul-fide undern-hexane solvothermal conditions. The oxy-gen-free reaction system is favorable for the purity ofthe products. The growth mechanism of the films couldbe described as the TSK model. The films deposited onboth glass and alumina plate have rose-like morpholo-gies with approximately 30mm thickness. The filmdeposited on glass is smoother than that deposited onalumina plate due to the different substrate structure.The PL measurements showed that the films displayluminescence at approximately 590 nm, which demon-strates their potential application for photovoltaicmaterial.

Acknowledgments

The project supported by Anhui Provincial NaturalScience Foundation(No. 3044901), the National Natural

207Q. Yang et al. / Thin Solid Films 436 (2003) 203–207

Science Foundation of China, and the 973 Projects ofChina is gratefully acknowledged. We would like toexpress our gratitude to Dr Zhaohui Han for his valuablecomments and Mr Wanqun Zhang and Ke Jiang fortheir help with equipment facility.

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