ARTICLE IN PRESS

Journal of Crystal Growth 269 (2004) 304–309

*Corresp

551-363176

E-mail

wlcai@ust

0022-0248/

doi:10.101

Growth mechanism and photoluminescence of CdS nanobeltson Si substrate

Weifeng Liu*, Chong Jia, Chuangui Jin, Lianzeng Yao, Weili Cai, Xiaoguang Li

Department of Materials Science and Technology, University of Science and Technology of China, Jinzai Road 96,

Hefei, Anhui 230026, PR China

Received 13 January 2004; accepted 25 May 2004

Communicated by M. Schieber

Abstract

Large-scale CdS nanobelts were synthesized using a simple physical evaporation of CdS particles under controlled

conditions on an Si substrate without any catalyst. The synthesized nanobelts are single crystals with a hexagonal

structure growing along the [0 0 1] direction and the growth process follows a vapor–solid mechanism. Two emission

bands around 517 and 735 nm of the nanobelts are observed, and the X-ray photoelectron spectroscopy (XPS) results

reveal that the band at 735 nm originates from the Vs+ vacancies.

r 2004 Elsevier B.V. All rights reserved.

PACS: 81.05.Dz; 81.05.Ys

Keywords: A1. Growth models; A2. Single crystal growth; A3. Physical vapor deposition processes; B1. Nanomaterials; B2.

Semiconducting materials

1. Introduction

Cadmium sulfide, an important-semiconductor,has a typical wide band gap of 2.42 eV at roomtemperature, and displays excellent optical proper-ties. It has been studied extensively due to its wideapplications in laser light-emitting diodes, solarcells and other optical devices based on its

onding authors. Tel.: +86-551-3601702; fax: +86-

0.

addresses: liuweifeng 2002@hotmail.com (W. Liu),

c.edu.cn (W. Cai).

$ - see front matter r 2004 Elsevier B.V. All rights reserve

6/j.jcrysgro.2004.05.093

nonlinear properties [1–4]. CdS, as a famousluminescence material, shows various lumines-cence properties, such as the photoluminescence(PL) and the electroluminescence. It is well knownthat nanoscaled semiconductor materials showfantastic properties, and great efforts have beenmade to control their size, morphology, andcrystallinity in order to obtain desired properties.Thus, varieties of CdS nanomaterials, such asnanorods, nanowires and nanotubes, have beenfabricated and investigated [5–11]. CdS nanorodshave been synthesized using the hydrothemeralmethod [6,10], and the controlled-morphology

d.

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W. Liu et al. / Journal of Crystal Growth 269 (2004) 304–309 305

nanorods have been obtained by controlling theprecursor’s concentration [7]. CdS nanowires havebeen prepared via the electrodeposition into thenanopores of the anodic alumina membranes [5]and the thermal evaporation with Au as a catalyst[8]. CdS nanotubes have been synthesized by usingin situ micelle-template-interface reaction [11].Nanobelts, a new family in the realm of the one-

dimensional nanomaterials, are regarded as anideal system to fully understand dimensionallyconfined transport properties and may act asvaluable units to construct nanodevices owing totheir well-defined geometry. Many kinds of oxidenanobelts, e.g. ZnO and CdO [12,13], and a fewsulfide nanobelts [14] (such as ZnS) have beenprepared so far. Dong et al. [15] have synthesizedCdS nanobelts on a tungsten substrate with an Aucatalyst. In this article, we report the preparationof cadmium sulfide nanobelts on an Si substratewithout any catalyst using a simple thermalevaporation process. The structural characteriza-tion and the photoluminescence (PL) property ofthe nanobelts are studied, and a vapor–solid (VS)growth mechanism of the CdS nanobelts isproposed.

25 30 35 40 45 50 55 60 65 70

(210

)

(203

)

(202

)Si

(004

)(2

01)

(200

)(1

12)

(103

)

(110

)

(102

)

(101

)(0

02)

(100

)

Inte

nsity

(a.

u.)

2θ (degree)

Fig. 1. XRD pattern of CdS nanobelts.

2. Experiments

A silicon wafer (1 cm� 1 cm) was used as asubstrate for the growth of CdS nanobelts. Beforeevaporation, the Si substrate was cleaned using aconventional treatment with HF, HCl/H2O2 andNH4OH/H2O2 solutions in turn, and kept inethanol for use. CdS nanopowder, as a precursor,was synthesized by mixing the saturated Na2S andthe CdSO4 solutions together with a magneticstirring. The deposit was rinsed with deionizedwater, and then dried at 60�C in air. Thetransmission electron microscope (TEM) imagesshow that the diameter of the nanoparticles isabout 5 nm. The CdS nanopowder put in a quartzvessel was then placed into a quartz tube, and acleaned Si substrate was placed next to the powderalong the downstream side of flowing argon. Priorto heating, the air in the quartz tube was pumpedout, and then high-purity Ar was introduced. Thisprocess was repeated for three times to make sure

that the O2 in the quartz tube was eliminated. Atlast, the pressure was kept at 1.6� 10�2MPa, andthe quartz tube was rapidly heated to 900�C (inabout 5min), with a constant Ar flowing rate(0.3 sccm). Held for a certain time, the power wasswitched off and the quartz tube was cooled downto room temperature in furnace. Yellow sponge-like products appeared on the surface of the Sisubstrate. The deposition time was 30 and 90min,respectively, in our experiments.X-ray diffraction (XRD) experiment was per-

formed on an X-ray diffractometer (Rigaku D/MAX-gA) with CuKa radiation to identify thestructure of the products. The morphology of theCdS nanobelts was observed using a field emissionscanning electron microscope (FE-SEM, JEOL,JSM-6700F) and a high-resolution TEM(HRTEM, JEOL-2010). For TEM observation,the synthesized products were ultrasonically dis-persed in ethanol and a drop of the suspension wasplaced on a Cu grid coated with holey carbon film.Absorption, Photoluminescence (PL) and X-rayphotoelectron spectroscopy spectra were carriedout at room temperature.

3. Results and discussion

Fig. 1 shows the XRD pattern of the as-prepared CdS sample. It reveals that the productsare pure hexagonal wurtzite structure of CdS with

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W. Liu et al. / Journal of Crystal Growth 269 (2004) 304–309306

lattice constants of a ¼ 4:123 (A and c ¼ 6:674 (A(JCPDS, No.77-2306), and no byproduct peaksare found.Fig. 2 shows typical SEM images taken from the

as-prepared CdS products after evaporating rawmaterials for 30min (sample A) and 90min(sample B), respectively. The top view of sampleA in Fig. 2a clearly shows that large amounts ofnanostructured materials are formed on the Sisubstrate with a high yield. The lengths are rangingfrom several tens to several hundreds micrometers;

Fig. 2. SEM images of CdS nanobelts of different evaporation ti

magnification of single nanobelts of Sample A; (c) a typical SEM im

nanobelts of Sample B.

some of them are even as long as a few millimeters.Fig. 2b is the magnified SEM image. It can be seenclearly that the products exhibit a belt-likemorphology. Typical nanobelts have a thicknessof around 60 nm and a width of about 400 nm, sothe width-to-thickness ratio is 7 to 10. Some smallpieces and nanowires have also been seen in Fig.2b. As the deposition time lasts longer, the widthof CdS nanobelts gets wider and wider (sample B,shown in Fig. 2c), and even reaches to severalmicrometers, while the thickness is almost kept

me: (a) a typical SEM image of Sample A (30min); (b) the

age of CdS Sample B (90min); (b) the magnification of single

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W. Liu et al. / Journal of Crystal Growth 269 (2004) 304–309 307

unchanged. At the same time, the small pieces andnanowires disappear, only the belt-like nanostruc-tures remain, and no particles are found on the tipsof the nanobelts. Fig. 2d is a typical magnifiedSEM image of a single nanobelt, which has auniform width along its length with sharp edgescompared to the rough ones shown in Fig. 2b.Fig. 3 presents some typical TEM images of

sample B. It can be seen from Fig. 3a that thewidth of the nanobelts ranges in several micro-meters, which is consistent with the SEM imageshown in Fig. 2c. Fig. 3b gives the magnifiedHRTEM image of a single CdS nanobelt and itscorresponding selected area electron diffraction(SAED) pattern. Evidently, the nanobelt is a singlecrystal and structurally uniform without anydislocation. Moreover, its surface is very clean,free of a sheathed amorphous layer. The latticespacing of the fringes is 0.67 nm in agreement withthe (0 0 1) spacing distance calculated from thediffraction spot in the SAED pattern, which

Fig. 3. TEM images, SEAD pattern and structure schematic

diagram of CdS nanobelts: (a) TEM image of several single CdS

nanobelts; (b) the corresponding HRTEM image and SAED

pattern; (c) the structure schematic diagram of a CdS nanobelt.

indicates that the CdS nanobelts grow along the[0 0 1] direction.The nucleation and the growth mechanism of

the belt-like nanostructures are still unclear. Thereare several models that have been proposed for thegrowth of nanomaterials in such an evaporationprocess, including VS, vapor–liquid–solid (VLS)and oxide-assisted mechanisms. The VLS mechan-ism can be ruled out because neither metal is usednor alloy particles are found on the tips of thenanobelts, which is the symbol of the VLS model.The oxide-assisted mechanism is also impossiblefor no oxide is used in this experiment. Theconventional VS mechanism is based on thescrewed dislocation and has a sharp tip, whichexcludes the belt-like structure. Dai et al. [16] havesuggested that the growth might be governed bythe crystal’s surface energy. At the growth zone(about 650–750�C), the CdS molecular species wasadsorbed in the rough growth front, and thendiffused to the low-energy surface, resulting in thefirst formation of a nanobelt. Fig. 3c is thestructure schematic diagram of a CdS nanobelt,which is concluded from the HRTEM image (Fig.3a) and the SAED pattern (Fig. 3b). Obviously,the hexagonal CdS nanobelt has a fastest growthrate along the [0 0 1] direction owing to the closed-packing effect, and the growth rate along the [0 1 0]direction should be faster than that along the[2 1 0] based on the Bravil’s law. The former isconfirmed in our experiment. For short depositiontime, besides the nanobelts, a few nanowires canbe found in Fig. 2b. But, as the deposition goeslonger, only the belt-like materials exist (as shownin Fig. 2d), which means the nanowires haveturned to the belt-like ones. However, the laterdoes not agree with our experiment result.Probably, some other parameters affecting thegrowth process, such as temperature, supersatura-tion, etc., should be taken into account, whichneeds further investigation. Dai et al. haveassumed that the size of the nanobelt cross-sectionis determined by the growth temperature andsupersaturation ratio in the growth process, whichis confirmed by our results. Under a certaincondition, the nanobelts have a width limit ofabout several micrometers. When the roughedges become sharp, it means the newly coming

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W. Liu et al. / Journal of Crystal Growth 269 (2004) 304–309308

molecules can no more accumulate on the edgesurface, and the width growth of the nanobeltsstops (see Fig. 2d).The XPS results, the absorption and the PL

spectra of the CdS nanobelts are displayed in Fig.4. The single S2p peak at 161.35 eV in Fig. 4c isindicative of the sulfur ion. The Cd3d shows twopeaks at 450.0 and 411.8 eV in Fig. 4b correspond-ing to the 3d5/2 and 3d3/2 spin–orbit split compo-nents, respectively. From the spectrum shown inFig. 4a, the oxygen peak at a binding energy of531.5 eV is due to the presence of H2O adsorbedon the sample surface. Deducing from theintensities of the relevant peaks, the ratio of Cd:Son the surface is 1.00:0.75, which means there are alot of Vs

+ on the surface of the nanobelts. It is easyto understand that some S atoms will escape fromthe surfaces of the CdS nanobelts at hightemperatures (over 700�C) resulting in plenty Vs

+

vacancies on the surfaces. The absorption and thePL spectra for the nanobelts at room temperatureare shown in Fig. 4d. The absorption spectrum(curve A) shows an absorption peak around503 nm (B2.47 eV) with about 13 nm blueshiftfrom that of the corresponding bandgap (2.42 eV)of the bulk CdS. Because the size of the CdS

Survey

Rel

ativ

e In

tens

ityR

elat

ive

Inte

nsity

(cp

s)

Rel

ativ

e In

tens

ity

Binding Energy (eV)

Binding Energy (eV)156 158 160 162 164 166 168 170 172

S2p

0 200 400 600 800 1000 4(a) (b)

(c) (d)

Fig. 4. XPS, absorption and PL spectra of CdS nanobelts: (a)–(c) X

absorption and PL spectra of CdS nanobelts.

nanobelts is much larger than the exciton Bohrdiameter of CdS (B6 nm), the quantum confine-ment effect is not as significant as that of othernano-CdS reported in litrrature [7,17–21]. The PLspectrum (curve B) exited at 455 nm shows twoemission bands around 517 and 735 nm, respec-tively. The PL behaviors of the CdS nanostruc-tured materials have been studied intensively [18–22]. Butty et al. [19] have reported that two PLbands are observed at 475 and 690 nm, respec-tively. The band at 475 nm has an intrinsiccharacter, and the other at 690 nm is due to thetrap or the surface states. Chrysochoos et al. [22]have revealed that the energy level of the Vs

+

vacancies is located at about 0.7 eV below theconduction band of the CdS clusters. The bandaround 517 nm is due to the intrinsic emission,which is about 14 nm redshifted from the absorp-tion peak in the absorption spectrum. And, aftercalculation, the PL band at 735 nm (B0.71 eVbelow the conduction band) should be originatedfrom the Vs

+ vacancies, which has been confirmedby the XPS results.In conclusion, the CdS nanobelts with high

quality and in large area have been fabricated via arapid evaporation route on the Si substrate with-

Wavelength (nm)

Binding Energy (eV)

Cd3d

400 500 600 700 800

Abs

orba

nce BA

503

517

735

Em

issi

on In

tens

ity (

a.u.

)

00 405 410 415 420

-ray photoelectron spectroscopy result of CdS nanobelts; (d)

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W. Liu et al. / Journal of Crystal Growth 269 (2004) 304–309 309

out any catalyst. XRD, TEM and SEM investiga-tions reveal that the as-prepared samples aresingle-crystals of CdS nanobelts with a hexagonalwurtzite structure growing along the [0 0 1] direc-tion. The VS model is proposed for the growthmechanism of CdS nanobelts. The PL spectrum ofthe nanobelts shows two emission bands around517 and 735 nm, which should arise from theintrinsic transition and the Vs

+ vacancies, respec-tively. The synthesis method of high-quality CdSnanobelts is very simple with low cost and largeproduction, which will make the CdS nanobeltsgreat opportunities in the optoelectronic devices.

Acknowledgements

This work is supported by the National NaturalScience Foundation of China (NSFC) underGrant No. 50128202. The authors are grateful toDr. Changhui Ye for the PL measurements andhelpful discussion.

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