X-ray photoelectron spectroscopy and scanning tunneling microscopy investigations of the solid...

2

3

4

5

6

78

9

1011121314151617181920

2122

23

24

25

26

27

SUSC 15015 No. of Pages 16, DTD = 4.3.1

11 June 2004 Disk usedARTICLE IN PRESS

Surface Science xxx (2004) xxx–xxx

www.elsevier.com/locate/susc

ROOFX-ray photoelectron spectroscopy and scanningtunneling microscopy investigations of the solid

solutions TixTa1�xS2 (06 x6 1)

Y. Tison, H. Martinez *, I. Baraille *,1, M. Loudet, D. Gonbeau

LCTPCM/CNRS UMR 5624, Helioparc Pau-Pyr�en�ees, 2 Avenue du Pr�esident Angot 64053 Pau Cedex 9, France

Received 21 January 2004; accepted for publication 27 May 2004

ECTEDP

Abstract

The surface of mixed transition metal sulfur TixTa1�xS2 has been studied both by X-ray photoelectron spectroscopy

and scanning tunneling microscopy over the entire range of stoichiometry 06 x6 1. For compositions close to thenative materials, we highlight according to x an evolution between the existence of a charge density wave (TaS2) and theexistence of structural defects (TiS2), interpreted as metal atoms vacancies or migration within the vacant sites of the

structure. The results obtained by XPS and scanning probe microscopy for intermediate compositions (0:46 x6 0:6)lead us to propose a model in ordered domains, separated by frontier zones where the metal distribution is random.

These various interpretations, supported by our experimental results, find their origin from the different electronic

characteristics of native materials and their evolution according to the composition. On the basis of model systems, we

have undertaken band structure calculations (FP-LAPW method) in order to highlight specific interactions occurred in

TixTa1�xS2 when x varies between 0.4 and 0.6. Those make it possible to rationalize some of our experimental data.� 2004 Published by Elsevier B.V.

Keywords: Solid solutions; Substitution; X-ray photoelectron spectroscopy; Core peaks; Valence bands; STM; Band structure

calculations
R
28

29

30

31

32

33

OR1. Introduction

The purpose of this article is to study spectro-

scopic and structural properties on solid solutions

of TiS2 and TaS2 (1T phase), both layered com-

pounds with different electronic characteristics.

UNC 34

35

36

37

38

39

*Corresponding authors. Tel.: +33-5-59407599; fax: +33-5-

59407588 (H. Martinez), Tel.: +33-5-59407853; fax: +33-5-

59407862 (I. Baraille).

E-mail address: [email protected] (I. Baraille).1 UMR5624, IFR, rue Jules Ferry, 64000 Pau, France.

0039-6028/$ - see front matter � 2004 Published by Elsevier B.V.

doi:10.1016/j.susc.2004.05.134

Numerous works were devoted to the substituted

compounds (i.e. TixTa1�xS2, 0 < x < 1) concerningtheir diagram phases [1], their magnetic [2] elec-

trical and transport properties [3] because of the

different conductive character of the mother com-

pounds [4]. 1T-TiS2 exhibits a high conductivityeven though numerous studies conclude with a

small gap semi-conductor behavior. This can be

explained either by the presence of excess titanium

atoms in the van der Waals gap [5] or by an elec-

tron–hole coupling [6]. On the other hand,

according to resistivity measurements, 1T-TaS2

mail to: [email protected]

40

41

42

43

44

4546

47

48

49

50

51

52

5354

55

56

57

58

59

60

6162

63

64

65

66

67

68

6970

71

72

73

74

75

76

77

78

79

80

81

8283

84

85

86

87

88

89

9091

92

93

94

95

96

97

9899

100

101

102

103

104

105

106107

108

109

110

111

112

113

114115

116

117

118

119

120

121

122

2 Y. Tison et al. / Surface Science xxx (2004) xxx–xxx



RREC

behaves as a metal above 350 K and as a semi-

conductor below this temperature [2]. This

behavior can be explained by the formation of a

charge density wave, observed by low energy

electron diffraction (LEED) [7] or scanning tun-

neling microscopy (STM) [8].Because of the different electronic features of

the mother compounds, the evolution of the

structural properties according to the substitution

rate deserves to be investigated. To our knowl-

edge, most of the studies on substituted TaS2 (with

titanium [9], hafnium [10] or iron [11] for example)

only concerned systems MxTa1�xS2 with low sub-

stitution rates (x comprises between 0 and 0.1).The purpose of these works was to elucidate how

‘‘impurities’’ affect the local properties of the

charge density wave phase using scanning tunnel-

ing microscopy, transport properties measure-

ments or optical reflectivity [3]. In this paper, we

report a systematic study of the substituted com-

pounds TixTa1�xS2 in the entire range 06 x6 1.Our approach of the structural and electronicproperties of these systems is based on X-ray

photoelectron spectroscopy (XPS) and ultra high

vacuum scanning tunneling microscopy experi-

mental techniques, which appeared well suited for

the study of the inserted compounds MxTiS2(M¼Fe, Co, Ni, Ag; x ¼ 1=3, 1/4) [12]. The resultscorresponding to intermediate substitution rate

(0:46 x6 0:6) have been the subject of specialattention due to their specificity. A better under-

standing of the results obtained has required the

implementation of theoretical calculations. A FP–

LAPW–DFT (full potential-linearized augmented

plane wave-density functional theory) approach

on model systems is proposed to elucidate the

specific interactions induced by the simultaneous

presence of the two cations.
O 123
124

125

126

127

128

129

130131

132

UNC2. Experimental and computational details

The elements Ta, Ti and S were mixed in

appropriate proportion and placed inside a quartz

tube which was sealed under vacuum. The mixture

was then heated to 950 �C (gradient 50 �C) during3 weeks and afterwards slowly cooled down to

room temperature. The slow cooling and anneal-

TEDPROOF

ing should ensure that only the stable phase at

room temperature is produced. An intermediate

crushing of the product was necessary to obtain

homogeneity. A small amount of iodine was then

added to favor crystallization [13]. The obtained

single crystals were in platelet shape and could beeasily cleaved. Chemical composition and phase

purity were carried out with an electron micro-

probe mounted on a scanning electron microscope.

X-ray diffraction (XRD) patterns were recorded

on an INEL diffractometer using a curved posi-

tion-sensitive detector (INEL CPS 120) calibrated

with Na2Ca3Al2F14 as standard. The monochro-

matized radiation applied was CuKa (1.5406 �A)from a long fine focus Cu tube operating at 40 kV

and 35 mA. Scans were performed over the 2hrange from 5� to 115�. Accurate unit cell param-eters were determined by a least-squares refine-

ment from data collected by the diffractometer.

For the matrix systems (TiS2 and TaS2) and mixed

samples, we found in every case the 1T polymorph

structure and the cell parameters of TiS2 and TaS2are in agreement with results previously reported

in the literature [14,15]. Note that the substitution

replacement of Ta for Ti in TixTa1�xS2 seems to

stabilize the octahedral 1T phase for 0 < x < 1, asobserved by Thompson [13] and DiSalvo [7]. Sys-

tematic powder XRD studies revealed that the

lattice parameters vary quasi linearly with com-

position (Fig. 1), from 3.35 to 3.41 �A for aparameter and from 5.89 to 5.70 �A for c, when xincreases from 0 to 1. This feature confirms that

the classical Vegard’s law is being followed for the

entire composition range in TixTa1�xS2 (06 x6 1)compounds. Such behavior of lattice parameters aand c was previously reported for TixNb1�xS2(06 x6 1) [1] and MoxW1�xS2 (06 x6 1) solidsolutions [16]. No evidence of structural disorderor superlattice ordering was detected in any dif-

fractogram. However, a widening of the diffraction

peaks (for example, the full width at half maxi-

mum (FWHM) of the (1 0 1) diffraction peak is

0.240� for TaS2 and 0.286� for Ti0:4Ta0:6S2) isobserved for the intermediate compositions

Ti0:4Ta0:6S2, Ti0:5Ta0:5S2 and Ti0:6Ta0:4S2, which

may indicate a loss of crystallinity.The XPS analyses were performed with a Sur-

face Science Instruments Spectrometer (model

133

134

135

136

137138

139

140

141

142

143

144

145146

147

148

149

150

151

152

153

154

155

156157

158

159

160

161

162

163

164165

166

167

168

169

170

171

172173

174

175

176

177

178

179

180181

182

183

184

185

186

187

188189

190

191

192

193

194

195

196197

0.0 0.2 0.4 0.6 0.8 1.0x

5.70

5.90

5.85

5.80

5.75

c(Å)

x0.0 0.2 0.4 0.6 0.8 1.0

a(Å)

3.32

3.44

3.42

3.40

3.38

3.36

3.34

Fig. 1. Variation of the lattice parameters a and c for theTixTa1�xS2 (06 x6 1) solid solutions determined by X-ray

diffraction.

Y. Tison et al. / Surface Science xxx (2004) xxx–xxx 3



UNCORREC301� using a focused (diameter of the irradiatedarea¼ 300 lm) monochromatized AlKa radiation(1486.6 eV) and interfaced to Hewlett Packard

9000 microcomputer. The residual pressure inside

the analysis chamber was in the 5 · 10�8 Pa range.The spectrometer was calibrated using the photo-

emission lines of gold (Au 4f7=2¼ 83.9 eV, withreference to the Fermi level) and copper (Cu

2p3=2¼ 932.5 eV). For the Au 4f7=2 lines, the

FWHM was 0.86 eV. All the compounds were

cleaved in a glove box directly fixed into the

introduction chamber of the spectrometer. The

compounds were sufficiently good conductors topreclude charging effects and the binding energies

were obtained directly.

The fitting peaks of the experimental curves are

defined by a combination of gaussian (80%) and

lorentzian (20%) distribution. For the whole set of
198
TEDPROOF

compounds the curve fits have been obtained using

fixed spin–orbit splits and fixed amplitude

(FWHM) ratios. It is to be noted that a minimum

number of doublets has always been used in order

to fit the experimental curve.

We imaged the samples in UHV using com-mercial (VP from Park Scientific Instrument)

scanning tunneling head, controlled by feedback

electronics and software of conventional design.

The residual pressure inside the analysis chamber

was in the 5 · 10�9 Pa range. Mechanically-sharp-ened Pt/Ir tips were used. Atomic scale images

were recorded in the constant current mode. STM

observed images were filtered by using the fastFourier transform (FFT) procedure to emphasize

the periodic features of the pattern. In this proce-

dure, the pronounced frequency patterns in the

FFT power spectra were saved to construct an

idealized image in the reverse transform. Graphite

used as calibration sample, always gave the correct

periodicity. In order to obtain fresh surfaces, the

samples were cleaved immediately before intro-ducing them in the analysis chamber. The images

were recorded with similar bias voltage and tun-

neling current (typically: )10 mV< V < 10 mVand I ¼ 1:5 nA). Note that the sign of the biasvoltage has no influence on the image features.

We have performed band-structure calculations

using the full linear potential augmented plane

wave (FLAPW) method [17], as implemented inthe WIEN97 code developed by Blaha et al. [18].

Our calculations are based on density functional

theory (DFT) within the generalized gradient

approximation (GGA) [19] which is, in most cases,

an improvement over LSDA (local spin density

approximation). All calculations are well con-

verged in terms of the size of the plane wave basis

set and the k-point sampling within the irreduciblepart of the Brillouin zone. In these studies, we used

52 irreducible Brillouin zone k points, angularmoment lmax ¼ 10, and muffin–tin radius for theatomic species: Ti, Ta¼ 2.2 a.u. and S¼ 2.0 a.u.The unit cell (space group: P�3m1) parameters usedto perform the calculations are those obtained by

X-ray diffraction determined by a least-squares

refinement. In this type of calculations, a moreprecise investigation of inter atomic interactions

needs the analysis of both the projected band

199

200

201

202

203

204

205

206

207208

209

210

211




structures (‘‘fat bands’’) and the charge density

maps [20].

212

213

214

215216

217

218

219

220

221

222

223224

225

3. Experimental results

3.1. XPS results

The XPS analyses were conducted both at the

level of core peaks (local probe) and valence bands

(occupied density of electronic states).

3.1.1. Core peaks

As the crystals have been freshly cleaved beforeanalysis, a very low oxygen percentage (about 2%)

has been observed. For all the TixTa1�xS2

UNCORRECTable 1

Binding energies (eV) and FWHM (full width at half maximum) in b

Ti 2p3=2�1=2 Ta 4f7=2�5=2

TaS2 23.9–25.8

[1.1]–[1.8]

Ti0:05Ta0:95S2 456.2 24.1–26.0

[2.2] [1.1]–[1.8]

Ti0:1Ta0:9S2 456.2 24.1–26.0

[2.2] [1.1]–[1.8]

Ti0:2Ta0:8S2 456.3 24.1–26.0

[2.3] [1.2]–[1.9]

Ti0:4Ta0:6S2 456.3 24.1–26.1, 72%

[2.4] [1.1]–[1.8]

25.1–27.2, 28%

[1.3]–[2.1]

Ti0:5Ta0:5S2 456.3 24.1–26.0, 74%

[2.4] [1.1]–[1.8]

25.1–27.20, 26%

[1.3]–[2.1]

Ti0:6Ta0:4S2 456.2 24.1–26.0, 73%

[2.3] [1.1]–[1.8]

25.1–27.2, 27%

[1.2]–[1.9]

Ti0:8Ta0:2S2 456.1 24.1–26.0

[2.2] [1.2]–[2.0]

Ti0:9Ta0:1S2 456.0 24.1–26.0

[1.9] [1.1]–[1.8]

TiS2 456.0

[1.6]

OOF

(06 x6 1) compounds, the main core peaks: Ti 2p,Ta 4f and S 2p have been characterized (Table 1).

Figs. 2 and 3 exhibit the Ta 4f and S 2p core

spectra. Note that only the Ti 2p3=2 component is

reported in the Table 1, as the Ti 2p1=2 and Ta

4p1=2 components appear around the same bindingenergy.

For the references TiS2 and TaS2, the binding

energies of the core peaks are in good agreement

with those previously published [21]. The Ta 4f

peaks obtained for TaS2 and the main TixTa1�xS2crystals are presented in Fig. 2. We note for TaS2the characteristic strong asymmetry of this doublet

also evidenced in tantalum metal. For the low xvalues (x6 0:2), some changes in the shape of theTa 4f doublet is observed with a slight enhance-

TEDPR

rackets for the main core peaks of TixTa1�xS2

S 2p3=2�1=2

162.0–163.2

[0.9]–[0.9]

162.0–163.2

[1.0]–[1.0]

162.0–163.2

[1.0]–[1.0]

161.9–163.1

[1.0]–[1.0]

161.6–162.9, 71%

[0.9]–[0.9]

162.2–163.4, 29%

[0.9]–[0.9]

161.4–162.6, 69%

[0.9]–[0.9]

162.0–163.2, 31%

[0.9]–[0.9]

161.3–162.5, 72%

[0.9]–[0.9]

161.8–163.1, 28%

[0.9]–[0.9]

160.9–162.1

[1.0]–[1.0]

160.7–161.9

[0.9]–[0.9]

160.8–162.0

[0.8]–[0.9]

CORRECTEDPROOF

226

227

228

229

Fig. 2. Ta 4f7=2�5=2 peaks of TixTa1�xS2 (x ¼ 0, 0.2, 0.4, 0.5, 0.6,0.8).

Fig. 3. S 2p3=2�1=2 peaks of TixTa1�xS2 (x ¼ 0, 0.2, 0.4, 0.5, 0.6,0.8, 1).




UNment of the FWHM of each component. A more

important evolution is observed for Ti0:4Ta0:6S2,

Ti0:5Ta0:5S2 and Ti0:6Ta0:4S2 compounds; in order

to fit the experimental curves, it was necessary to

TEDPROOF

230

231

232

233

234

235236

237

238

239

240

241

242

243244

245

246

247

248

249

250

251252

253

254

255

256

257

258259

260

261

262

263

264

265

266267

268

269

270

271

272

273

274275

276

Fig. 4. XPS valence bands of TaS2 (a), Ti0:2Ta0:8S2 (b),

Ti0:4Ta0:6S2 (c), Ti0:5Ta0:5S2 (d), Ti0:6Ta0:4S2 (e), Ti0:8Ta0:2S2 (f)

and TiS2 (g).




UNCORREC

introduce a second doublet on the high binding

energy side. No significant change occurred for the

binding energy of the main Ta 4f doublet all along

the series. Compared to TiS2, the Ti 2p peak of

each compound is broadened (especially in the

case of Ti0:4Ta0:6S2, Ti0:5Ta0:5S2 and Ti0:6Ta0:4S2)and shows an asymmetric shape.

The analysis of the S 2p peak for TiS2 and TaS2(Fig. 3) has revealed a perfectly resolved doublet,

with a splitting between the two components equal

to 1.1–1.2 eV and a clear difference between their

BE (binding energy) values (TiS2: 160.8/162.0 eV

TaS2: 162.0/163.2 eV). For the TixTa1�xS2 com-

pounds, a progressive and continuous evolution ofthe binding energies of the S 2p peaks is observed

from the values of TaS2 to the ones of TiS2, as the

concentration of titanium atoms increases.

Considering the shape of the S 2p for the mixed

compounds, an evolution is observed with a pro-

gressive widening of the peak and a less resolved

global structure. This is especially evident for the

Ti0:4Ta0:6S2, Ti0:5Ta0:5S2 and Ti0:6Ta0:4S2 com-pounds. For these, in order to fit the experimental

curves, it was necessary to introduce another

doublet on the high binding energy side. It is to be

noted that the relative proportions (�30%–70%) ofthe two S 2p doublets are similar to those of the Ta

4f core peaks.

3.1.2. Valence bands

Fig. 4 shows the valence bands for the main

compositions TixTa1�xS2.

Previous experimental and theoretical studies

[21] on TiS2 and TaS2 allow the following attri-

butions. For both compounds, the bands on the

high binding energy side mainly correspond to the

ionization of S(3s) orbitals. The band on the low

binding energy side exhibit a triplet structure dueto r interactions between S(3p) and M(d) atomicorbitals for a local octahedral environment of

metal atoms is observed for both compounds; this

last phenomenon is clearly evidenced for TiS2(maximum at 3.3 eV), as for TaS2 (maximum at

4.5 eV). Note that TaS2 is also characterized by a

formal ‘‘d1’’ electronic configuration; the identifi-

cation of the associated band is deduced from awell resolved structure just below the Fermi level

(0.9 eV).

TEDPROOF

277

278

279

280

281

282283

284

285

286

287

288

289

290291

292

293

294

295

296

297

298299

300

301

302

303

304305

306

307

308

309

310

311

312313

314

315

316

317

318

319

320321

322

323

324

325

326327

328

329

330

331

332

333

334335

336

337

338

339

340

Fig. 5. STM grey scale images of TiS2 (a) and TaS2 (b).




UNCORREC

For the mixed compounds, a progressive evo-

lution of the whole spectrum is observed from

TaS2 to TiS2. The continuous decrease of the

intensity of the small structure near EF appearswell related with the decrease of tantalum content.

It is to be noted that this structure is always per-ceptible even for the low tantalum content. More

precisely, the spectrum observed for Ti0:2Ta0:8S2 is

similar for the one of TaS2. It is the same for

Ti0:8Ta0:2S2 compared to TiS2. For the other

compounds and especially for Ti0:5Ta0:5S2, broader

bands with less resolved structures are observed.

However, in all cases, the shape of the band on the

low binding energy is preserved and no changefrom a local octahedral environment to a trigonal

prismatic one (as in 2H-TaS2 [22]) is observed.

These results are consistent with our XRD data.

In conclusion of XPS results, the core peak

evolution (related to the chemical environment

around each atom) exhibits a singular behavior for

the highly substituted compounds (0:4 < x < 0:6).A continuous modification is observed at the levelof the valence bands on all the composition range.

3.2. STM results

TiS2 and TaS2 atomic scale images are carried

out to compare and calibrate the substituted

compounds TixTa1�xS2. Typical grey scale images

recorded at room temperature of TaS2 and TiS2are shown in Fig. 5. The white areas in these

images represent surface protrusions while dark

areas indicate depressions. These images are typi-

cal of the many we have obtained at different

locations on several samples using different tips.

As expected for TiS2, the STM scan (Fig. 5a)

exhibits a hexagonal structure, regardless of the

tunneling current (0:5 < I < 2 nA) or the biasvoltage applied to the substrate (5 < V < 25 mV).The lattice spacing of 3.40 ± 0.1 �A (0.1 �A deviationis the typical error observed with our instrument)

measured is in agreement with the literature X-ray

diffraction data [14]. Furthermore, different kinds

of structural defects are detected. They were

interpreted [23,24] as titanium vacancies in octa-

hedral sites.The STM image of native TaS2 (Fig. 5b) exhibit

a prominent hexagonal lattice. The peak spacing

and corrugations are 11.9 ± 0.1 and 6.0 ± 0.1 �Arespectively, much larger than the values expected

for the hexagonal closed pack sulfur plane (3.4 and

less than 1 �A) and each spot has a circular shapewith a radius around 2 �A. These features areassociated either with a charge density wave state

which might result from either affiffiffiffiffi

13p

a�ffiffiffiffiffi

13p

aclustering induced by Ta/Ta multicenter interac-

tions [25,26] or a Fermi surface nesting [7]. As far

as we know, the uncertainty between these two

hypothesis is not raised, as quite recent angle-re-

solved photoemission studies [27] show.Considering the substituted compounds

TixTa1�xS2, we present our results over the entire

range of composition to be comprehensive and

coherent with our systematic study even if STM

images of low Ti contents (x < 0:2) were alreadypublished [9]. The substitution by titanium atoms

CORREC

341342

343

344

345

346

347

348

349350

351

352

353

354

355

356

357358

359

360

361

362

363

364

365366

367

368

369

370

371

372

373374

375

376

377

378

379

380

381382

383

384

385

386

387

388

389390

Fig. 6. STM grey scale images of Ti0:05Ta0:95S2 (a), Ti0:1Ta0:9S2(b) and Ta0:2Ti0:8S2 (c).




UN

at levels x ¼ 0:05 and x ¼ 0:1 (Fig. 6a and b)perturbs the CDW sufficiently so that the atomic

lattice and CDW can be ‘seen’ clearly. However,
391
TEDPROOF

for x ¼ 0:05, the CDW is still prominent as large

peaks (12.8 �A peak to peak, ±0.2 �A separation and5 �A corrugation) even if smaller bright spots (as-signed to the close-packed hexagonal top sulfur

plane) with separation 3.4 ± 0.1 �A are present. Theunderlying atomic lattice remains regular. Forx ¼ 0:1 sample, the atomic lattice (with peak topeak separation 3.4 ± 0.1 �A) is present in the sametime as large peaks from the CDW. The lower

corrugation values (2 �A) indicate that the CDWamplitude decreases with increasing dopant con-

centration. Furthermore, the CDW is distorted

(separation peak to peak: 13.2 �A±0.5 �A) andprovides confirmation of the suggestion that sub-stitution of Ti(3d0) for Ta(5d1) perturbs signifi-

cantly the size of the Fermi surface. The irregular

CDW structure is the result of distortions in direct

response to the random lattice potential associated

with the Ti doping sites. For x ¼ 0:2 (Fig. 6c), theunderlying atomic lattice is now clearly defined

and dominates the STM image. A few incom-

mensurable peaks from CDW state are still presentand exhibit large variations. The CDW is ran-

domly distorted in the wavelength and amplitude

(13.4 ± 2.0 �A peak to peak separation and 0.9 ±

0.2 �A corrugation). Note that the accuracy de-

creases with larger values of x(Ti).These results, where the CDW wavelength

clearly shows large variations with ‘‘doping’’ con-

centration, are consistent with those of C. Lieberet al. [9,11]. The authors discussed the influence of

metal (titanium, iron) ‘‘impurities’’ (x comprisesbetween 0 and 0.2) in the CDW state of TaS2. For

TixTa1�xS2 (x < 0:2) mixed compounds, they

compare their results with those obtained by

LEED measurements [7]. They show that the in-

crease of the CDW wavelength may be associated

with a shrinking of the Fermi surface (and so adecrease of the nesting vector) induced by the

replacement of 5d1 Ta4þ ions by 3d0 Ti4þ ions.

The results for intermediate compositions

(x ¼ 0:4, 0.5, 0.6) are close to each other. Twotypical images (presented in 3D form to emphasize

the features of the pattern) are systematically

present, depending on the location of the analysis.

The first one (Fig. 7a) presents a 3.4 �A · 3.4 �A(±0.1 �A) close-packed hexagonal lattice and thecorrugation amplitude (0.5 �A) is similar to what it

CTEDPROOF

392

393

394

395

396397

398

399

400

401

402

403

404405

406

407

408

409

410

411

412413

414

415

416

417

418

419

420421

422

423

Fig. 7. Typical STM grey scale images of TixTa1�xS2 (0:46 x6 0:6) showing close-packed hexagonal lattice similar to what it wasobserved for TiS2 (a), protrusions with no specific periodicity (b, c) and intermediate case between the two previous ones (d). The

reported intensity profiles correspond to the image (b).




UNCORREwas observed for TiS2. The second ones (Fig. 7b

and c) exhibit protrusions with no specific peri-

odicity. These could not be attributed to CDW

because as we note previously, and in agreement

with many authors [9], the amplitude of CDWdecreases with increasing x(Ti) and the corrugationamplitude measured (2.5 ± 0.6 �A) is higher thanthe expected values for TiS2 or TaS2 (in the ab-

sence of CDW), similar to what it was observed for

lower x(Ti) concentration. Then, the large contrastdifferences and the features of this image should

come from another phenomenon. The lattice

parameters measured (illustrated by the plot ofheight profiles) vary between 2.8 and 3.6 �A.Fig. 7d is presented as an example of an inter-

mediate case where ‘‘bumps’’ and regular atomic

resolution are both present in the image. The

atomic lattice (of TiS2 or TaS2) could be distin-

guished with separation of 3.4 �A±0.4 �A. Thesevariations (±0.4 �A) could be assigned to the per-turbations observed, which reveal that all theheights (and/or the electronic densities around the

Fermi energy) of sulfur atoms are not equal all

over the surface scanned.

For higher concentrations of titanium (x ¼ 0:8,0.9), most of the images are very similar to the

native TiS2 and exhibit a uniform 3.4�A · 3.4 �Ahexagonal lattice (Fig. 8a). However, different

images (Fig. 8b) were recorded from time to time(typically one of ten over a series of fifty analyses);

a modulating intensity is observed which could be

due to different levels from the top sulfur atoms.

424

425

426

427

428

429

430

431

432

433

434435

436

437

438

439

440

441

442

443

444445

446

447

448

449

450

451

452453

454

455

456

457

458

459

460461

462

463

464

465

466

467

468469

Fig. 8. Two kinds of STM images observed for Ti0:8Ta0:2S2 and

Ta0:9Ti0:1S2.




CAnother interesting feature is that we never

observed structural defects (at the levels of the size

and shape) such as TiS2, even for low tantalum

substitution like Ti0:95Ta0:05S2.
E 470
471

472

473

474

475

476477

478

479

480

481

482

483

484485

NCORR4. Discussion

It seems convenient to share the discussion

concerning the effect of substitution over electronic

and structural surface properties into two parts,

according to the composition: low substitution

rate of titanium or tantalum (Ti0:05Ta0:95S2,

Ti0:1Ta0:9S2, Ti0:2Ta0:8S2, Ti0:8Ta0:1S2, Ti0:9Ta0:1S2)and intermediate composition (Ti0:4Ta0:6S2,

Ti0:5Ta0:5S2, Ti0:6Ta0:4S2). For the latter, after

having considered several assumptions, the exper-

imental results lead us to propose a model of

structural arrangement. Specific interactions are

identified for these compositions by the means of a
U
TEDPROOF

theoretical modeling, making it possible to support

some of our XPS and STM results.

4.1. Solid solutions with low substitution rate

In the case of the low substituted solid solu-tions, the results obtained are in agreement with a

random dispersion of the minority metal species

(titanium for x ¼ 0; 05; 0,1; 0,2 or tantalum for

x ¼ 0:8, 0.9) in a compound made up of majoritymetal.

In the composition range 100% TaS2 to 80%

TaS2, the titanium centers cause randomness in the

lattice potential and lead the Fermi surface toshrink. These effects perturb the charge density

wave in different ways. The average CDW wave-

lengths measured (consistent with previous dif-

fraction studies [7]) increase with increasing

titanium concentration, as expected in response to

a decrease in the size of the Fermi surface. For

Ti0:05Ta0:95S2, the charge density-wave is nearly

commensurate at room temperature and the STMimages are closed to TaS2 even if we note a small

difference in wavelength and amplitude of the

CDW state. When the concentration of titanium is

higher (x ¼ 0:1, 0.2) the CDW is incommensurate

with the atomic lattice and the corrugation values

decrease. It appears that the random substitution

of the Ta sites by titanium atoms produces disor-

der in the lattice potential. The increasing disorderas x gets close to 0.2 may be in relation with theslight broadening of the XPS core peaks (espe-

cially for the Ti 2p and Ta 4f) observed for

Ti0:2Ta0:8S2.

Concerning Ti0:8Ta0:2S2 and Ti0:9Ta0:1S2, STM

images and XPS core peaks (binding energy) are

very similar to the matrix TiS2. When the con-

centration of titanium gets close to x ¼ 1, thedisorder decreases: the FWHM of the XPS core

peaks decreases and the STM images, similar to

TiS2, exhibit a 3.4 �A · 3.4 �A hexagonal lattice. Theapproach of a randomly substitution of metal

centers with other metal is consistent with the re-

sults presented above. Furthermore, we note the

absence of defects even for a very weak rate of

substitution. This result is discussed at the end ofthis section.

486

487

488

489490

491

492

493

494

495

496

497498

499

500

501

502

503

504

505506

507

508

509

510

511

512

513514

515

516

517

518

519

520

521522

523

524

525

526

527

528

529530

531

532

533

534

535536

537538

539

540

541

542

543

544

545546

547

548

549

550

551

552

553554

555

556

557

558

559

560

561562

563

564

565

566

567

568

569570

571

572

573

574

575

576

577578

579

580




UNCORREC

4.2. Solid solutions with intermediate composition

(0:46 x6 0:6)

4.2.1. Hypothesis of structural arrangements

The most striking results (both in STM andXPS experiments) of this paper have been ob-

tained for intermediate compositions (Ti0:4Ta0:6S2,

Ti0:5Ta0:5S2, Ti0:6Ta0:4S2). Several structural

arrangements can be invoked starting from a

confrontation with our experimental results.

• Firstly, it was necessary to examine if an inser-tion phenomenon has to be considered. So, we

have synthesized several inserted compoundsTaxTiS2 in order to identify the possible signa-

ture differences between the phenomena of sub-

stitution and insertion. It appears interesting to

compare results for Ta0:4TiS2 (as an example)

with intermediate composition of substituted

samples; it is to be noted that this selected stoi-

chiometry allows having a statistical occupation

of the van der Waals gap. The results obtainedboth from XPS (an unique doublet for the Ta

4f7=2�5=2 and S 2p3=2�1=2 peaks) and STM exper-

iments (no significant feature on the images of

Ta0:4TiS2 with a uniform lattice of sulfur atoms

with a 3.4 �A parameter) are clearly different

from the ones obtained for intermediate solid

solutions. Thus, we can exclude that a large pro-

portion of atoms have migrated in octahedralvacancies in the studied compounds.

• Another possibility is to consider a perfectlyperiodic arrangement of the metal atoms be-

tween two planes of sulfur atoms. In this case,

STM images would be identical and would pres-

ent an ordered succession of intensity maxima

related to sulfur atoms in bimetallic environ-

ment; the results obtained show that thishypothesis is not valid.

• Considering a simple juxtaposition of stackedlayers from TiS2 and TaS2 along the c crystallo-graphic axis is also incompatible with our re-

sults; STM images would reveal in that case

the close-packed hexagonal top sulfur plane

for TiS2 or a regular CDW state for TaS2 as

function of the nature of the top layers. Further-more, the shape of the Ta 4f and S 2p peaks with

two different doublets and the FWHM of Ti 2p

peaks implies the presence of several environ-

TEDPROOF

ments for these atoms. Then, a perfect order

for Ti0:5Ta0:5S2 compound is obviously unlikely.

• Another possible hypothesis for the intermedi-ate compositions may be described as an assem-

bly of ordered domains, separated bydiscontinuities, both with non-equivalent size.

Any disordered array of atoms distributed on

the sites of a regular lattice, whether random

or having a definable degree of short range or-

der, may be divided into regions of ordered lat-

tice, the boundaries between the regions being

usually identifiable as a simultaneous presence

of the two cations (Ti and Ta). In this hypothe-sis, the main doublets (�70%) of Ti0:4Ta0:6S2,Ti0:5Ta0:5S2, and Ti0:6Ta0:4S2 observed for S 2p

and Ta 4f core peaks are associated to the or-

dered lattices TiS2 or TaS2. In the case of the

S 2p core peaks, the binding energy evolution

of the main doublet is characteristic of an aver-

age effect from the ionicity of the Ti–S and Ta–S

bonds. The binding energy of the more intenseTa 4f doublet corresponds to tantalum atoms

in an octahedral sulfur environment. These re-

sults have to be linked to the close-packed hex-

agonal top sulfur plane (from TaS2 or TiS2,

where both have a close a lattice parameter) re-vealed by STM analysis (Fig. 7c), with no influ-

ence from the CDW state. The ordered lattice of

TaS2 (TiS2) may form a small domain surround-ing by TiS2 (TaS2) regions. The statistical distri-

bution of sizes and shapes of these ordered

domains seems to be unordered and are obvi-

ously not uniform. In these conditions, the influ-

ence of the CDW state over corrugation from

the top layers plane would be very low and no

detectable by STM.

The second doublets (whose relative intensitiesare about 30%), located for both S 2p and Ta 4f at

higher binding energy than the previous ones are

more ambiguous. Their binding energies are

characteristic of a loss of electronic charge com-

pared to the atoms associated with the most in-

tense doublet belonging to tantalum or titanium

disulphide domains. As a re-distribution of the

total electronic population has to be considered onthe level of the frontier zones, we propose that they

could be associated to boundaries between the

regions of TiS2 and TaS2.

581

582

583

584

585

586587

588

589

590

591

592

593

594595

596

597

598

599

600

601

602

603

604

605

606

607

608

609

610

611

612

613

614

615616

617

618

619

620

621

622

623624

625

626

627

628

629

630631632

633634

635

636

637

638

639




C

Let us notice as the STM images suggest, that a

bringing together between some sulfur atoms

(2.8 �A instead of 3.4 �A) takes place in these dis-ordered areas. Such a result which supports sulfur-

sulfur interactions, is consistent with the charac-

terization of a second doublet for S 2p peak whosebinding energy is close to the one determined for

S2�2 sulfur pairs in the TiS3 compound [28].

Thus, the images which present a more dis-

turbed profile (Fig. 7) would be associated to the

frontier zones for which the presence of two metals

in the subjacent plane would involve variations of

heights or electronic charge at the level of the

sulfur atoms from the top layer. These results arecoherent with what is observed on the level of the

amorphous compounds [29]: unusual coordinances

and bond length distributions.

The electronic reorganization considered pre-

viously on the level of the frontier zones should

benefit to the titanium atoms in mixed zone via

metal/metal interactions.

4.2.2. Description of specific interactions

As previously mentioned, the hypothesis of

frontier zones between the TiS2 and TaS2 domains

implies the presence of sulfur atoms in bimetallic

environment. As a consequence, the metal/sulfur

but also metal/metal interactions in these bound-

aries regions might be modified. In order to

investigate more deeply these electronic reorgani-

UNCORRE

Fig. 9. Model unit cell used for Ti0:5Ta0:5S2 in the theoretical calculatio

present, either coordinated to two titanium and one tantalum atoms

titanium atoms (S(2Ta, 1Ti). In this case, the second neighbors of the t

reverse proportion should be considered if we look at the second nei

TEDPROOF

zations, we have undertaken a simulation of the

different local environments of sulfur, titanium

and tantalum atoms by performing periodic cal-

culations on three model unit cells (Ti0:25Ta0:75S2,

Ti0:5Ta0:5S2 and Ti0:75Ta0:25S2). The Fig. 9 repre-

sents the unit cell of Ti0:5Ta0:5S2, as an example.These model structures allow simulating different

environments for the titanium or the tantalum

atoms in the metal atom plane.

To characterize more deeply the possible metal/

metal interactions, we report the projected band

structures (‘‘fat bands’’) for the 3d and 5d atomic

orbitals of the metal atoms. In a first step, we find

it necessary to present the results (Fig. 10) ob-tained, in the same computational conditions, on

the TaS2 and TiS2 disulfides taken as reference for

the Ta/Ta and Ti/Ti interactions. The band struc-

tures reported in Fig. 10 show very similar shape

for the first group of two bands (between )15.2and )12.7 eV for TaS2 and between )13.2 and)11.8 eV for TiS2) mainly developed on S(3s)

orbitals. The main contribution to the valenceband located just below the Fermi level (between

)7.2 and )0.8 eV for TaS2 and between )5.1 and0.0 eV for TiS2) comes from S(3p) orbitals with a

non-negligible admixture of M(d) orbitals. Be-

cause of the different electronic configuration of

the metal atoms in their sulfur environment (Ti(d0)

and Ta(d1)), the conduction band is empty for TiS2and partially filled for TaS2. However, in both

ns. In this structure, sulfur atoms in bimetallic environment are

(noted S(2Ti, 1Ta)) or surrounded by two tantalum and one

itanium atoms are constituted by four Ta and two Ti atoms. The

ghbors of the tantalum atoms.

RRECTEDPROOF

640

641642

643

644

645

646

647

648

649650

651

652

653

654

655

656

657658

659

660661

662

663

664

Fig. 10. ‘‘Fat’’ electronic band structures of TiS2 (a) and TaS2(b). The M(d) contributions are set proportional to the thick-

ness of the band. The Fermi level is set at the origin of the

energy.

Fig. 11. ‘‘Fat’’ electronic band structures of Ti0:5Ta0:5S2. The

Ta(5d) (a) and the Ti(3d) (b) contributions are set proportional

to the thickness of the band. The Fermi level is set at the origin

of the energy.




UNCOcases, its shape formed by three t2g plus two egbands clearly reflects the CdI2-type crystal struc-ture based on interacting MS6 octahedra. Note

that the energetic dispersion of the t2g levels is

definitely higher for TaS2 (4.1 eV) than for TiS2(2.1 eV), in agreement with the literature [30]. This

band dispersion can directly be correlated to the

binding metal–metal interactions which take part

in a non-negligible way in cohesion of tantalum

disulphide while such metallic bonds do not existin titanium disulphide.

For the three model structures, the conduction

band energetic levels are progressively occupied as

the tantalum concentration increases. As an

example, the Fig. 11 reports the ‘‘fat bands’’ of

Ti0:5Ta0:5S2 showing the contributions of both

Ti(3d) and Ta(5d). The 12 bands (three bands per

metal atom) which correspond to electronic stateswith a major t2g character, belonging both to

Ti(3d) and Ta(5d) orbitals are localized between

)0.7 and +2.3 eV (energetic dispersion equal to 3eV). The same behavior is observed for the ‘‘fat

bands’’ of the two other model structures. Only the

energetic dispersion differs (2.5 and 3.5 eV

respectively for Ti0:75Ta0:25S2 and Ti0:25Ta0:75S2).

RECTEDPROOF

665

666667

668

669

670

671

672

673

674675

676

677

678679

680

681

682

683

684

685

686687

688

Fig. 12. Charge density map plotted in the (1 0 0) plane in the energetic window corresponding to the occupied t2g bands for TiS2 (a),

Ti0:75Ta0:25S2 (b), Ti0:25Ta0:75S2 (c) and TaS2 (e).




UNCORCompared to the results obtained for the mother

compounds, these results indicative of metallicbinding, highlight Ti/Ta interactions.

The charge density maps plotted in the (1 0 0)

plane (including the metal atoms) in the energetic

window associated to the filled levels of the con-

duction band confirm our results (Fig. 12). The

overlap between the atomic orbitals of the metal

atoms increases from Ti0:75Ta0:25S2 to TaS2-1T

when the titanium atoms are gradually replaced bytantalum ones. As a consequence, the binding

character of the metal/metal interaction increases

in the TixTa1�xS2 series when tantalum concen-

tration increases. Naturally, no effective interac-tion between titanium atoms is observed in the

case of TiS2.

The atomic populations of the Ti(3d) and

Ta(5d) (calculated by integrating the correspond-

ing densities of states, Table 2) in the energy

window corresponding to the occupied t2g levels

show the metallic interaction as a consequence of

the re-distribution of the tantalum electronic den-sity towards the titanium one. The electronic

charge of the titanium atoms increases with the

F

689

690

691

692

693

694695

696

697

698

699

700

701

702703

704

705

706

707

708

709

710711

712

713

714

715

716

717

718719

720

721

722

723

724

725

726

727

728729

730

731732

733

734735

736

737

738

739

740

741

742743

744

745

746

747

748

749

750751

Table 2

Electronic populations (e�) of the Ti(3d) and Ta(5d) atomic orbitals for Ti0:75Ta0:25S2, Ti0:5Ta0:5S2, Ti0:25Ta0:75S2 and TaS2 in the

energetic window corresponding to the occupied t2g bands

Ti0:75Ta0:25S2[)0.4 eV, 0 eV]

Ti0:5Ta0:5S2[)0.7 eV, 0 eV]

Ti0:25Ta0:75S2[)1.0 eV, 0 eV]

TaS2-1T

[)1.3 eV, 0 eV]

Ti(3d) 0.253 0.433 0.716 –

(2Ta, 4Ti) (4Ta, 2Ti) (6Ta)

Ta(5d) 0.210 0.379 0.591 0.756

(6Ti) (2Ta, 4Ti) (4Ta, 2Ti) (6Ta)

The environment of each titanium or tantalum atom in the metallic plane is reported in parenthesis.




CORRECnumber of tantalum atoms located in their second

sphere of coordination. On the other hand, the

tantalum atoms are less and less charged when the

number of titanium atoms increases in their second

sphere of coordination. This phenomenon explains

the observation of the second doublet for the Ta 4fcore peaks of Ti0:4Ta0:6S2, Ti0:5Ta0:5S2 and

Ti0:6Ta0:4S2 toward higher binding energies.

The presence of bimetallic interactions in the

boundaries zones should explain why some defects

were revealed for TiS2 by STM analyses, and why

they were never observed in the case of substituted

compounds. First, TiS2 presents (in the metallic

plane) a very weak metal/metal bonding energywhich can lead to a significant mobility of the

metal atoms and thus to the presence of defects. As

shown in the previous paragraph, the progressive

occupation of the t2g crystalline orbitals in the

TixTa1�xS2 materials reinforces the bonding tita-

nium/tantalum interaction compared to titanium/

titanium one. When x ¼ 1, the tantalum/tantaluminteractions lead to the formation of clusters. Thedemonstration of this phenomenon is a shortening

of the Ta/Ta bond [31] and the creation of a charge

density wave (even if the origin of the phenomenon

is more complex [25–27]). Then, when x increases,the charge transfer of the tantalum atoms towards

the titanium ones makes it possible to fix the metal

atoms, by stabilizing interactions. The existence of

structural defects related to a metal mobility isthus not effective.

752

753

754

755

756
UN5. Conclusion
The study of the solid solutions TixTa1�xS2 was

undertaken with the aim of identifying the struc-

TEDPROOtural and electronic modifications induced by the

simultaneous presence of two cations.

The results obtained by scanning tunneling

microscopy led us to propose two types of struc-

tural arrangements according to the composition:

• a random dispersion of the minority metal cat-

ion in a matrix of the majority species for the

slightly substituted compounds,

• an organization in ordered domains of TiS2 andTaS2 separated by frontier zones for the inter-

mediate compositions.

These two types of structures are in accordancewith the analysis of the experimental results by

XPS, obtained for the slightly substituted com-

pounds and intermediate compositions, respec-

tively. In particular, for the last ones, the images of

the frontier zones highlight a bringing together

between some sulfur atoms, supporting S–S

interaction. This phenomenon would be at the

origin of the presence of the second appearingdoublet with higher binding energy than the main

one and associated to sulfur atoms located in or-

dered domains of disulfides. More, the presence of

two doublets for the Ta 4f core peaks of the

intermediate compositions consolidates the

assumption of ordered domains separated by dis-

ordered frontier zones: the first doublet is associ-

ated with the atoms located in the TiS2 and TaS2domains while the second one should correspond

to those which belong to the frontier zones.

The theoretical study of model structures en-

abled us to highlight metal-metal interactions. On

the basis of these interactions, it is possible to ex-

plain some of our experimental results, as the lack

757

758

759

760761762763764765766767768769770771772773774775776777778779780781782783784785786787788789790791792793794795

796797




of defects characteristic of TiS2 for the solid

solutions, whatever the composition.
798799800801802803804805806807808809810811812813814815816817818819820821822823824825826827828829830831832833834835836837
REC

References

[1] (a) M. Shimikawa, H. Maki, H. Nishihara, K. Hayashi,

Mater. Res. Bull. 32 (1997) 689;

(b) S.J. Furuseth, Alloys Comp. 178 (1992) 211.

[2] P. Fazekas, E. Tosatti, Philos. Mag. B 39 (3) (1979) 229.

[3] J.A. Benda, Phys. Rev. B 10 (4) (1974) 1409.

[4] (a) J.A. Wilson, A.D. Yoffe, Adv. Phys. 18 (1969) 193;

(b) R.H. Friend, A.D. Yoffe, Adv. Phys. 36 (1987) 1;

(c) G.L. Frey, K.J. Reynolds, R.H. Friend, Adv. Mater. 14

(4) (2002) 265.

[5] (a) C.H. Chen, W.A. Fabian, F.C. Brown, K.C. Woo,

B. Davies, B. De Long, A.H. Thompson, Phys. Rev. B 21

(1980) 615;

(b) M. Sch€arli, J. Brunner, H.P. Vaterlaus, F. L�evy,

J. Phys. C 16 (1983) 1527.

[6] F.J. DiSalvo, D.E. Moncton, J.V. Wazczak, Phys. Rev.

B 14 (1976) 4321.

[7] F.J. Di Salvo, J.A. Wilson, B.G. Bagley, J.V. Waszcak,

Phys. Rev. B 12 (6) (1975) 2220.

[8] (a) R.V. Coleman, B. Drake, P.K. Hansma, C.G. Slough,

Phys. Rev. Lett. 55 (4) (1985) 394;

(b) C.G. Slough, W.W. McNairy, R.V. Coleman,

B. Drake, P.K. Hansma, Phys. Rev. B 34 (2) (1975) 994;

(c) X.L. Wu, C.M. Lieber, Science 243 (1989) 1703.

[9] (a) X.L. Wu, P. Zhou, C.M. Lieber, Phys. Rev. Lett. 61

(22) (1988) 2604;

(b0) X.L. Wu, P. Zhou, C.M. Lieber, Nature 335 (1988) 55.

[10] (a) H. Bando, Y. Miyahara, H. Enomoto, H. Ozaki, Surf.

Sci. 381 (1997) L609;

(b) H. Bando, K. Koizumi, Y. Miyahara, H. Ozaki, J.

Phys.: Condens. Matter 12 (2000) 4353.

[11] H. Chen, X.L. Wu, C.M. Lieber, J. Am. Chem. Soc. 112

(1990) 3326.

[12] (a) H. Martinez, S.F. Matar, C. Auriel, M. Loudet,

G. Pfister-Guillouzo, J. Alloys Compd. 245 (1996) 30;

(b) H. Martinez, Y. Tison, I. Baraille, M. Loudet,

D. Gonbeau, J. Electron Spectr. Phenom. 125 (2002) 181;

UNCOR
TED
PROOF

(c) I. Baraille, H. Martinez, Y. Tison, M. Loudet,

D. Gonbeau, Surf. Sci. 517 (2002) 43.

[13] A.H. Thompson, K.R. Pisharody, R.F. Koelher, Phys.

Rev. Lett. 29 (3) (1972) 163.

[14] J.B. Goodenough, in: H. Reiss (Ed.), Progress in Solid

State Chemistry, vol. 5, Pergamon, Oxford, 1971.

[15] G. Haegg, N. Schoenberg, Ark. Kemi. 7 (1954) 371.

[16] S.K. Srivastava, T.K. Mandal, B.K. Samantaray, Synth.

Met. 90 (1997) 135.

[17] D. Singh, in: Planewaves Pseudopotentials and the LAPW

Method, Kluwer Academic Publishers, Boston, 1994.

[18] P. Blaha, K. Schwarz, J. Luitz, WIEN97, Vienna Univer-

sity of Technology 1997. (Improved version of the original

copyrighted WIEN-code, which was published by P. Blaha,

K. Schwarz, P. Sorantin and S.B. Trickey, in Comput.

Phys. Commun. 59 (1990) 399).

[19] J.P. Perdew, S. Burke, M. Ernzherhof, Phys. Rev. Lett. 77

(1996) 3865.

[20] P. Blaha, J. Phys.: Condens. Matter 3 (1991) 9381.

[21] K. Dartigeas, L. Benoist, D. Gonbeau, G. Pfister-Guillo-

uzo, G. Ouvrard, A. Levasseur, Electron Spectrosc. Relat.

Phenom. 83 (1997) 45.

[22] R. Eppinga, G.A. Sawatzky, C. Haas, C.F. Van Bruggen,

J. Phys. C: Solid State Phys. 9 (1976) 3371.

[23] H. Martinez, P. Azavant, M. Loudet, Surf. Sci. 400 (1998)

247.

[24] G.P.E.M. Van Backel, J.T.M. De Hosson, T. Hibma,

Appl. Phys. Lett. 56 (1990) 2402.

[25] M.H. Whangbo, E.J. Canadell, Am. Chem. Soc. 114 (1992)

9587.

[26] M.H. Whangbo, J. Ren, E.J. Canadell, D. Louder,

B.A. Parkinson, H. Bengel, S.N. Magonov, J. Am. Chem.

Soc. 115 (1993) 3760.

[27] P. Aebi, T. Pillo, H. Berger, F. Levy, J. Electron Spectrosc.

Relat. Phenom. 117–118 (2001) 433.

[28] A. Levasseur, P. Vinatier, D. Gonbeau, Bull. Mater. Sci. 22

(1999) 607.

[29] J. Zarzycki (Ed.), Glasses and Amorphous Materials,

Material Science and Technology, vol. 9, 1991.

[30] M.L. Doublet, S. Remy, F.J. Lemoigno, Chem. Phys. 113

(2000) 5879.

[31] R. Brouwer, F. Jellinek, Physica 99B (1980) 51.

X-ray photoelectron spectroscopy and scanning tunneling microscopy investigations of the solid...

Documents

Transcript of X-ray photoelectron spectroscopy and scanning tunneling microscopy investigations of the solid...