Materials Chemistry and Physics 94 (2005) 182–184

Materials science communication

Preparation of�-Gd2S3 films by ultrasonic spray pyrolysis

Wei Wanga,b,c, Sheng-Yue Wanga,c,∗, Mei Liu a

a Department of Physics, Southeast University, Nanjing 210096, PR Chinab College of Physics and Technology, Nanjing Normal University, Nanjing 210097, PR China

c National Laboratory of Solid State Microstructures, Nanjing University, Nanjing 210093, PR China

Received 22 July 2004; accepted 17 May 2005

Abstract

Nanophase�-Gd2S3 polycrystalline thin films were deposited by a so-called asynchronous pulse ultrasonic spray pyrolysis method on glasssubstrates at relatively low temperature without any complexing agent. The films were characterized by X-ray diffraction, scanning electronmicroscopy and Raman spectrum. The technical conditions leading to optimal performance were identified. An appropriate ratio of pulse timeto interval time between pulses is found to be necessary for obtaining homogeneous and uniform nanostructural�-Gd2S3 thin films.© 2005 Elsevier B.V. All rights reserved.

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eywords: Spray pyrolysis; Chalcogenides; Thin films; Nanophase

. Introduction

Rare earth sesquisulfides Ln2S3 in the series Ln = La–Dyave three polymorphs at atmospheric pressure[1]. Of all

he known polymorphs of rare earth sesquisulfides, the�odifications with the Th3P4 structure have attracted mostf the attention as promising thermal[2], optoelectronics

3], laser technology and piezoelectric transducers[1,4–8].-Ln2S3 compounds are wide-gap (2–3 eV) high-resistivity∼1010� cm) semiconductors[9] and far-infrared transmit-ing materials[10–12]. Since there is no center of inversionn the Th3P4 structure, they satisfy the necessary conditionor the existence of piezoelectric and electrooptic effects[1].he�-phase is the high-temperature form and crystallizes in

he cubic sysytem (Th3P4 type, space groupI43d). The usualynthesis route for the�-phase consists of reacting the cor-esponding oxide with an H2S flow at 1200◦C for 2 h, in thebsence of moisture or oxygen, in their presence, the�-phase

s mainly obtained[13].In recent 10 years, much effort on�-phase gadolinium

esquichalcogenides has been concentrated on its attractive

materials and to tune their optical and electronic prope[14]. It is worth noting that very few investigations have bcarried on the preparation and characterization of�-phasesesquichalcogenides at low temperatures. The presentdeals with the preparation and characterization of�-Gd2S3nanometer thin films at relatively low temperature byimproved ultrasonic spray pyrolysis method. This noprocess has been found to be very effective in obtainingearth sesquisulfides nanostructural thin films by usinappropriate ratio of pulse time to interval time between pu[15,16].

2. Experimental

The scheme of the set-up of the improved method, achronous pulse ultrasonic spray pyrolysis (APUSP), fopreparation of�-Gd2S3 film was similar to that found in reerence[16]. In this experiment, the process that the misolution of GdCl3 and thiourea were nebulized and pusprayed onto the hot substrates with two separate no

igh-pressure behavior, and techniques to process these

∗ Corresponding author. Tel.: +86 25 83791835; fax: +86 25 83792868.E-mail address: sywang@seu.edu.cn (S.-Y. Wang).

respectively. Typically in the experiments, GdCl3 of 0.05 Mand thiourea [CS(NH2)2] of 0.1 M were dissolved in distilledwater, respectively, for the preparation of�-Gd2S3 films.The prepared solution of GdCl3 and thiourea were nebu-

d.

254-0584/$ – see front matter © 2005 Elsevier B.V. All rights reserveoi:10.1016/j.matchemphys.2005.05.031

W. Wang et al. / Materials Chemistry and Physics 94 (2005) 182–184 183

lized by two commercial ultrasonic humidifiers, respectively,and introduced into the reactor by N2 gas through two sepa-rate nozzles. The flow rate of the N2 gas in each nozzle was2–3 L min−1.

In experiments, prior to the film deposition N2 gas wasfirst introduced to the reaction chamber at relatively low andsteady flow rate for about 30 min to drive the air out. Then,the N2 gas was on in all the later processes. The nebulizedsolution of GdCl3 and thiourea were delivered to the sub-strates in spray pulses through the two nozzles[16], and thespray pulses begin with thiourea mist. Each spray pulse timelasted 5 s for thiourea and GdCl3 solution, respectively, andafter the pulse spray of thiourea was conducted, a delay of2–4 s was employed to insure that the introduced thioureawas completely decomposed before conveying a pulse sprayof GdCl3. The deposition was carried out by repeatedly per-forming these spray processes. It took 12–14 s for each cycleand the deposition time lasted 15–30 min for preparation ofone sample. The substrate temperature was 280–350◦C dur-ing the deposition. It has been indicated that an appropriateinterval between the pulse sprays of thiourea and GdCl3 solu-tion played an important role in depositing high crystallinityfilms.

The phase and crystallinity of as-prepared films werecharacterized by using X-ray diffractometer (XRD, RigakuD/MAX). The surface morphology of the films was observedo M).T laserR ss oft

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Fig. 1. Typical SEM of morphology of�-Gd2S3 films (film thickness750 nm) deposited on glasses at 320◦C by APUSP.

pulse time to 4-s interval time between pulses is shown inFig. 2. The films were identified by XRD to be�-Gd2S3polycrystalline films, which are well consistent to thatof powder XRD card for�-Gd2S3 (JCPDS file 26-1424,gamma-Gd2S3, Cubic, Th3P4 type,a = 8.375A). Mean crys-tallite sizes calculated from the line width of the diffractionpeak (3 1 0) is 26 nm, and the crystallite prepared by thisimproved method is in size range of 23–40 nm. No trace ofgadolinium oxide or other organic, inorganic compoundswas detected by XRD analysis. We also applied the Scherrerformula to the other diffraction peaks in the same pattern,and found that the crystallite size values obtained from thesame pattern were all in agreement. Besides the crystallinityof the films has no obvious improvement when interval timewas lengthen more than 4 s.

In APUSP, thiourea is nebulized, introduced in the reac-tor and decomposed before the deposition started. When theGdCl3 mist is transported into chamber by the carrier gasN2, it must pass through a H2S atmosphere without any otherspecies in the reaction chamber. Therefore, the growth of the

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n a Hitachi X-650 scanning electron microscopy (SEhe Raman spectrum was measured with a SPEX-1403aman spectrometer at room temperature. The thickne

he films measured using SEM was 0.5–1�m.

. Results and discussion

Smooth and homogeneous�-Gd2S3 films were depositen glass substrate. The films had good adherence tubstrates, appeared uniform and shiny, with bright off-wolor. Fig. 1 shows representative SEM image of�-Gd2S3hin film grown with a flow rate of 3 L min−1, deposition timef 25 min (in the ratio of 5s-GdCl3 and 5s-thiourea spraulses time to 4-s interval time between pulses), subs

emperature of 320◦C, the nozzle–substrate distance5 mm and the solution of 0.05 M GdCl3, 0.1 M thioureaEM observation showed that the films consisted of sphearticles of mean size <150 nm, which are in fact comprf densely packed nanocrystallines of mean size <40stimated from Scherrer formula. This may be a consequf the growth of small crystallite. In this case, it was a

ound that short pulse time and relative long interval tould provide enough time for the crystallites to confignd grow as smooth and high dense films. It should be emized that no subsequent heat treatment was used to irystallinity in the spray pyrolysis produced�-Gd2S3 filmsere.

Typical XRD pattern of as-deposited�-Gd2S3 filmseposited in a ratio of 5s-GdCl3 and 5s-thiourea spra

ig. 2. XRD pattern of�-Gd2S3 films deposited on glasses at 320◦C byPUSP (mean crystallite sizes 26 nm).

184 W. Wang et al. / Materials Chemistry and Physics 94 (2005) 182–184

Fig. 3. Raman spectrum of the as-deposited nanophase�-Gd2S3 films.

films proceed via two consecutive steps: (i) the formation ofa H2S atmosphere from thiourea; (ii) followed by a reactionbetween H2S and Gd3+ mist. In traditional techniques, dueto the high vapor pressure of chalcogens, normally it is diffi-cult to obtain stoichiometric crystals. However, in this case,thiourea is completely decomposed and provides enough H2Sbefore Gd3+ mist is introduced into reaction chamber. Thecomplexing agent that played an important role in the chem-ical methods was obviously not necessary in this reactiondevice. Therefore, the improved experimental design over-came the troubles typical of chemical methods employed toprepare metal sulfide thin films.

Laser Raman spectroscopy is an effective method to studythe material structure, so it is suitable for exploring the sur-face layer structure of nanometer-sized crystals of�-Gd2S3.Up to now, there is little work on the Raman characterizationof nanocrystalline�-Gd2S3 films. The Raman spectra of thetypical nanocrystalline�-Gd2S3 film with size of 26 nm atroom temperature is shown inFig. 3. A well-resolved Ramanpeak was seen at 180 cm−1, and the other five broader fea-tures were found at about 153, 280, 320, 363 and 387 cm−1,respectively. For small CdS crystallites with diameters rang-ing from 3 to 100 nm the surface modes and size effects aremanifested by addition to the normal modes of the singlecrystal[17]. When the dimensions become extremely small,only the surface modes persist. The peaks of Raman shiftsa an

spectra of single crystal�-Gd2S3 [1]. It can be concluded thatthey arise from size effects here.

4. Conclusions

An improved ultrasonic spray pyrolysis method was suc-cessfully applied to deposit�-Gd2S3 nanophase polycrys-talline films from GdCl3 and thiourea at relatively lowtemperature. The crystal growth of the rare earth sulfidefilms could be controlled easily by using two nozzles andincrease interval time between spray pulses in the improveddevice. The present experimental design would be expectedto deposit other nanophase polycrystalline rare earth sulfidefilms.

Acknowledgement

Financial support from the Foundation of National Labo-ratory of Solid State Microstructures of Nanjing University.

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