I

AOVANCEO MATERIALS FOR or-nes ANO ELECTRONICSAdv. Mater. Opto Electron. 10,235-240 (2000)

Amorphous Lead Titanate: a New Wide-band Gap Semiconductor with

Photoluminescence at Room Temperature

E. R. t.elte," F. M. Pontes,' E. c. Paris,l c. A. Paskocimas," E. J. H. t.ee,' E. Lonqo,'P. s. Pizani," J. A. Varela" and v. Mastelaro4

1 LlEC, Department of Chemistry, UFSCar, Federal University of São Carlos, CEP 13565-905, São Carlos, SP, Brazil2 Department of Physics, UFSCar, Federal University of São Carlos, CEP 13565-905, São Carlos, SP, Brazil

31nstitute of Chemistry, UNESP, São Paulo State University, Araraquara, SP, Brazil4IFSC, São Carlos Institute of Physics, USP, University of São Paulo, São Carlos, SP, Brazil

This paper describes a new amorphous wide-band gap semiconductor with photoluminescence (PL)at room temperature. The amorphous PbTi03 was prepared by a sol-gel-like process in powder andthin film formoThe optical property and the PL behaviour showed a direct relation to the amorphousstructure. The PL peak energy can be controlled by the change of the exciting surge energy. Copy-right ® 2000 John Wiley & Sons, Ltd.

KEYWORDS luminescence; amorphous materiais; sol-gel process; semiconductors; materiais science

The chemical solution process offers an opportunityto tailor-make the chemistry, structure and micro-structure ofmaterials to achieve specific optical andelectrical properties. This kind of process has beenused to prepare well-known materiaIs as well asnew materiaIs that cannot be produced by othermethods.'

The development of semiconductor materiaIswith active optical properties, such as photo-luminescence (PL), electroluminescence (EL) ornon-linear optic properties, may lead to new opto-electronic devices with superior performance.Amorphous semiconductors may replace single-crystal semiconductors in a variety of opto-elec-tronic applications, particularly in large devices orin cases where cost is an important factor. In

* Correspondence to: E. R. Leite, LIEC, Department of Chem-istry, UFSCar, Federal University of São Carlos, CEP 13565-905, São Carlos, SP, Brazil; e-mail: derl@power.ufscar.br

Contract grant sponsor: FAPESP.Contract grant sponsor CNPq (PADCT III).Contract grant sponsor: FINEP.

Copyright © 2000 John Wiley & Sons, Ltd.

general, it can be stated that the ideal material foropto-electronic or any other electronic applicationis one that can be synthesized at low temperaturesand processed in different ways and shapes, andwhose properties can be modelled to give the bestperformance, preferably at room temperature.Amorphous semiconductors appear to have goodpotential as such materiaIs. A chemically-preparedamorphous semiconductor, using the solution-based method, for example, would naturally be thebest way.' The solution-based method allows forthe use of inexpensive techniques for thin-film depo-sition and microelectronic production, such as spincoating. Although it is no sim pIe matter to find sucha material, we believe we have developed a goodcandidate, i.e., the amorphous semiconductorbased on lead titanates with a PbTi03 formula.

The PbTi03 compound, in its crystalline form, isa typical wide-band gap semiconductor (3.0 eV)with interesting non-linear optical properties,'although other active optical properties, such as PLor EL, are not normally observed."] This report

t Weak PL was observed in BaTi03 only at cryogenic tempera-tures, see Ref. 4.

Received 6 June 2000Accepted 4 July 2000

236 E. R. LEITE ET AL.

describes the PL behaviour of amorphous PbTi03processed by a chemical-solution method. Amorph-ous PbTi03 particles in thin film were processed bya sol-gel process called the polymeric precursormethod ar the Pechini process.'

X-ray diffraction (XRD) analyses and infraredspectroscopy] of the powder and thin film, heattreated at low temperature ( < 400°C) for severalhours, showed the formation of an amorphous inor-ganic phase. However, XRD analysis is insufficientto characterize a non-periodic structure. Thus,high-resolution transmission electron microscopy(HRTEM) and selected-area electron diffraction(SAED) analyses were carried out as comp-lementary structural analyses. The HRTEM of thepowder heat treated at 350°C for 36 hours (Fig.I(A», shows a structure with no long-range order,characterizing an amorphous structure. Fig. 1(B)shows SAED patterns of the particle shown in Fig.1(A), revealing an inner diffuse ring and a seconddiffuse ring. Since the diffraction pattern is relatedto the Fourier transform of the radial distributionfunction p(r), which gives the probability that anatom is surrounded by other atoms at a distance r,a diffuse diffraction pattern suggests that p(r) is notperiodic. The presence of diffuse rings suggests onlythe development of a short-range order (amorphousstructure).

The structure of amorphous PbTi03 (a-PT) wascharacterized using X-ray absorption near edgestructure (XANES) spectroscopy.§ Fig. 2(A) pre-sents the XANES spectrum ofthe a-PT. A pre-edgefeature of a Ti k-edge XANES spectrum located atapproximately 4970 eV can be observed. This peakis related to energy levei transition from Ti ls toTi3dj02p molecular orbital and is usually attri-buted to five-fold Ti co-ordination." This studydemonstrated that amorphous compounds are far-

t Infrared speetroseopy showed only the presenee of metal-oxygen stretehing around 520 em -I.

§Ti R-edge XANES speetra were obtained at the LNLS(National laboratory of synehrotron light) faeilities in Campi-nas, SP, Brazil. The LNLS storage ring was operated at 1.36GeV and 60-100 mA. Data were eolleeted at the Ti R-edge(4966 eV) in the transmission node for powder. The X-ray beamwas monochromatized using an Si(lll) ehannel eut mono-ehromator with an energy step equal to 0.5 eV. Crystal {3-~a2Ti04, Ba2TiSi20S (fresnoite) and r-Ti02 (rutile) powder wereused as struetural referenees. The Ti atoms in this strueture areco-ordinated by 4, 5 and 6 oxygen atoms, respectively. TheXANES analysis was based on the papers in Ref. 6.

Copyright © 2000 John Wiley & Sons, Ltd.

B)

Fig. I. (A) the HRTEM image showing an amorphous PT par-ticle (powder heat treated aI 350°C for 24 hours. (B) Electrondiffraetion pattern displaying broad rings, typical of amorphousmaterial. Nanoparticles were not detected by the HRTEManalysis

med basically through six-fold oxygen Ti co-ordi-nation (Ti06-octahedron) and five-fold oxygen Tico-ordination (TiOs-square-base pyramid). Aquantitative analysis ofthe XANES spectrum" sug-

Adv. Mater. Opt. Electron. 10,235-240 (2000)

237.~\lORPHOC LEAD T1TA:-':ATE

2,0

A)

..,. 1,5:l.!i~'"c:: 1.

~."'"!t;;;EÕz

C

4960 4980 5000 5020

Energy [eV]

a-PT

5040

B) Sixfold Structure Fivefold Structure

Fig. 2. (A) XANES spectrum of a·PT powder (powder heattreated at 350°C for 24 bours). (B) The oxygen Ti co-ordinationin six-fold and five-fold structures

gests that, in the a-PT, ~ 80% of the Ti atoms areco-ordinated by six oxygen atoms and ~ 20% ofthe Ti atoms are co-ordinated by five oxygen atoms.Fig. 2(B) illustrates the six-fold oxygen Ti co-ordi-nation (Ti06-octahedron) and five-fold oxygen Tico-ordination (TiOs-square-base pyramid). Itshould be pointed out that the TrOj-square-basepyramid structure is not found in well-crystallizedtitanáte.

Ab initio quantum mechanics calculations? sug-gest that the chemical bond between titanium andoxygen (Ti-O) has a high covalent character, whilethe bond between the Pb cation and oxygen is basi-cal1y ionic. This result, together with the SAEDand XANES results, suggests that the amorphoustitanate is formed by a Ti-O network. The chargeof the Pb cation must be compensated for by nega-tively-charged non-bridging oxygen (NBO). In

Copyright © 2000 JOl111Wiley & Sons. Ltd.

(a)T= 502 'C(PT Crystal)

(b) T= 473 'C

(c) T= 410'C

(d) T= 345 'C

11 11IA)

(a~)__ ...,

-~.!.CDec:J!oCD

~

300 400 500 600Wavelength [nm]

700

Fig. 3. (A) Spectral dependence of lhe reflectance for the a-PTheat treated at different temperalures and for lhe crystalline PT

addition to NBO, other defects may be present inthe amorphous structure, such as oxygen vacancies.These positively-charged defects stem from possiblydifferent states of valence of the Ti ion (Ti+4 andTi+3).

The a-PT semiconductors displayed exponentialoptical edges (or Urbach edges) (region lI) andatypical tail (region IlI) in the spectral dependenceof the reflectance, as illustrated in Fig. 3. In thisfigure, one can observe the spectral dependence ofthe reflectance for a-PT heat treated at differenttemperatures and for a crystalline PT sample. Theseoptical properties are typical of amorphous sem-iconductors, such as amorphous silicon (a-Si) andAS2S3 glass." It can be observed (Fig. 3) that theexponential optical edges and the tail are sensitiveto the thermal annealing treatment. Increasing theannealing temperature causes the exponential edgesand the tail to disappear (regions II and III of Fig.3) and a typical optical edge of crystalline materialto appear.

Fig. 4(A) shows the PL behaviour observed inthe PbTi03 compound for powder and thin filmo PLcharacterization showed a broad, intense emissionspectrum in the visible region. lt was found that

Adv. Mater. Opt . Electron. 10,235-240 (2000)

238 E. R. LEITE ET AL.

740

720

Ê 700C

:;: 880

~ 660J 640

~ 620

.•• 600

I 580

1560

)( 540

W 520

500 +-"'--'~---r~.-,.,~-.~.-,.,~-,-..,-,....,......,..--l440 460 480 500 520 540 560 580 600 620 640

B)

•

•

Exciting Wavelength (nm)

Fig. 4. (A) PL spectrum of the a-PT particles (powder heattreated at 350°C for 24 hours and a-PT thin film deposited onSi(lOO) substrate (film heat treated at 300°C for 4 hours),obtained with an exciting wavelength of 488.0 nm at room tem-perature. (B) Excited peak wavelength as a function of excitingwavelength

the PL peak energy can be modified by excitmgwavelength energy. Fig. 4(B) shows the excited peakwavelength as a function of exciting wavelength. Alinear relation is observed, with a correlation factorofO.98.

It was ais o found that the PL behaviour of thisclass of compound is sensitive to thermal treatment.As illustrated in the PL spectra of Fig. 5, when theannealing temperature was increased, PL intensity

Copyright © 2000 John Wiley & Sons, Ltd.

(a) T = 345°C

(b)T= 410°C

(e) = 473°C

(d)T= 502°C(PT-Crystal)

2000 4000 6000-1Wavelength [em]

Fig. 5. PL spectrum of the a-PT heat treated at different tem-peratures and for the crystalline PT (same samples used as forthe reflectance measurement (Fig. 3». The peaks observed in thecrystalline PT are the Raman spectra typical for this material

decreased or disappeared. When the material isordered (in crystalline form), PL intensity is vir-tually zero at room temperature. This behaviourmatched the one observed in the spectral depen-dence of the reftectance as a function of the anneal-ing temperature (Fig. 3).

The above-described experimental results stron-gly indicate that the PL is directly related to theexponential optical edges and tail. The nature ofthese exponential optical edges and tail are not yetclear, but may be related to defects promoted bythe disordered structure of the amorphous PbTi03semiconductor, such as NBO or defects promotedby impurities. The reftectance measurement, assoei-ated with the PL characterization of a-PT semi-conductors, suggests a non-uniform band gapstructure with a tail of localized states and possiblymobile edges. We believe that the photo-Juminescence observed in this class of semi-conductor arises from a radiative recombinationbetween trapped electrons and trapped holes in tailand gap states. The PL peak position is mainlydetermined by the separation energy between thosestates. We believe that the PL behaviour measured

Adv. Mater. Opto Electron. 10,235-240 (2000)

~----------------

AMORPHOUS LEAD TlTANATE 239

in the a-PT compound is not related to the quantumconfinement effect due to the presence of theexponential optical edges and the absorption tail.Quantum confinements are usually characterizedby a shift of the refiectance spectrum to high energy(blue shift). The opposite behaviour was observed(red hift) in our experiment.

ln addition to the interesting properties reportedherein a number of aspects remain to be explored,including understanding the origin ofthe exponent-ial optical edges and tail, and what kind of defectsare involved in the PL phenomenon. Another inter-esting point to examine is the infiuence of differentcation substitutions in the electrical and opticalproperties.

The above-described properties of the PbTi03

compound characterize a new wide-band gapamorphous semiconductor material with activeoptical properties, such as PL at room temperature.This material was prepared chemically at a lowtemperature ( < 400°C) in powder and thin-filmformo Processing this material requires no expensiveequipment and reagents, no special atmospherecontrol nor even a high vacuum chamber for thinfilm deposition. The advantages and interestingproperties of this material confirm its strong poten-tial in the development and improvement of opto-electronic applications.

EXPERIMENTAL SECTION

The polymeric precursor method, or the Pechiniprocess, is based on the chelation of cations (metais)by citric acid in a water solution. After synthesis ofthe citrate solution, it is mixed with ethylene glycolto promote polymerization through a polyesteri-fication reaction. The polymeric precursor formedduring the polymerization step can be used to pre-pare thin film or powder. The heat-treatment tem-perature of the polymeric precursor must besufficient to promote polymer pyrolysis, but notcrystallization. For example, amorphous PbTi03thin film with PL characteristics is prepared asfollows.

The first step is to prepare a precursor solutionby dissolving lead(II) acetate trihydrate((CH3C02)2Pb· 3H20) in a titanium citrate aqueoussolution prepared from titanium(IV) isopropoxide{Ti[OCH(CH3)2l4}. The molar ratio between

Copyright © 2000 John Wiley & Sons, Ltd.

L

titanium and lead is 1.00 (this ratio was confirmedafter the pyrolysis step by quantitative analyses,using AAS). ln this step, characterization of thesolution by \3C-NMR spectroscopy (200 MHz,TMS, 25°C) shows the formation of a hetero-metallic acid (CA)-complex, such as (Pb, Ti)-(CA)", as indicated by the resonance at 91 ppm.Ethylene glycol (C2H602) is added to the citratesolution to promote polymerization by a poly-esterification reaction between the heterometallicCA-complex and the ethylene glycol. The citricacid/ethylene glycol ratio is set at 60/40 (mass ratio).After polymerization at 100°C for 6 hours, thepolymeric precursor is dissolved in water to preparethe polymeric precursor solution. Characterizationof the polymer solution by \3C-NMR spectroscopy(200 MHz, TMS, 25°C) shows the presence of thepeak at 91 ppm, suggesting that this heterometalliccomplex has high thermal stability. This analysissuggests that no cation segregation occurs up to thedeposition step and that chemical homogeneity iskept at the molecular level. A polymeric solutionwith a 7 mPa s viscosity is used for spin-coatingdeposition. After deposition (on Si(lOO» thematerial is heat treated at 250°C for 8 hours in anoxygen atmosphere (pyrolysis step). After pyroly-sis, the films are heat treated at 300°C for 4 hours.

The photoluminescence measurements weretaken using a UlOOO Jobin-Yvon double mono-chromator coupled to a cooled GaAs photo-multiplier and a conventional photon countingsystem. The 457.9, 488.0 and 514.5 nm excitingwavelengths of an argon-ion laser were used, withmaximum output power of the laser kept within 20mW. Ali the measurements were taken at roomtemperature.

ACKNOWLEDGEMENTS

This work was supported by FAPESP, CNPq(PADCT III) and FINEP (ali Brazilian researchfunding institutions

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Adv. Mater. Opto Electron. 10,235-240 (2000)

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\I

Adv. Mater. Opt. Electron. 10,235-240 (2000)