UNCORRECTED PROOF 2 Furthertheoreticalanalyses(2Dand3D)ofNi 1=4 TiS 2 3 probedbyXPS/STMstudies 4 I.Baraille * ,H.Martinez,Y.Tison,D.Gonbeau,M.Loudet 5 Laboratoire de Chimie Th eorique et de Physico-Chimie Mol eculaire, UMR CNRS 5624, Facult e des Sciences, BP 1155, 6 64013 Pau Cedex, France 7 Received8February2002;acceptedforpublication15May2002 8 Abstract 9 ThesurfaceofNi 1=4 TiS 2 wasinvestigatedbyX-rayphotoelectronspectroscopy(XPS)andscanningtunnelingmi- 10 croscopy(STM).ElectroniccalculationswereperformedusingtheabinitioHartree–Fockprogramcrystal.TheXPS 11 results (chemical shift of core peaks) are satisfactorily accounted for through a Mulliken population analysis. Con- 12 cerningtheSTMresults,weimagedthetopsulfurplane(001)andinterpretthedataonthebasisofthepartialelectron 13 densityofaslabwhichconsistofseven(001)Ni 1=4 TiS 2 layers.ItwasfoundthatthebrightspotsinexperimentalSTM 14 imagescorrespondtosulfuratomsinasinglemetallicenvironment(Tiatoms).Besidehigherelectronicdensity,these 15 atomsarehighlightedbecauseoftheirproximitytothetipcomparedwithsulfuratomsinadoubleenvironment(Tiand 16 Ni). Ó 2002PublishedbyElsevierScienceB.V. 17 Keywords: Density functional calculations; X-ray photoelectron spectroscopy; Scanning tunneling microscopy; Ab initio quantum 18 chemicalcalculations 19 1. Introduction 20 Forthelastyears,numerousstudieshavebeen 21 devoted to the insertion phenomena in transition 22 metaldichalcogenidessuchasTX 2 (T ¼ Ti,Mo, 23 Ta,VandX ¼ S,Se,Te).Thesecompoundshave 24 layeredstructureswithvanderWaalsgapwhichis 25 advantageoustoinsertvariousatomsormolecules 26 tomodifythephysicalpropertiesofthehostma- 27 terial.Inparticular,theinsertionof3dtransition- 28 metalMintothevanderWaalsgapgivesM x TX 2 29 compounds ð0 < x < 1Þ. 30 AstitaniumdisulfideTiS 2 isnowconsideredas 31 theprototypalcompoundintheseriesoftheTX 2 32 layered materials, the intercalated compounds 33 M x TiS 2 havebeenthesubjectofseveralstudies[1– 34 5].Theyhavebeendevotedpredominantlytothe 35 electronicandstructuralmodificationsinducedby 36 theinsertionofthetransitionmetal.Recently,we 37 have undertaken the analysis of the electronic 38 structures of M 1=4 TiS 2 intercalated compounds 39 (M ¼ Fe, Co or Ni) using both X-ray photoelec- 40 tron spectroscopy (XPS) and scanning tunneling 41 microscopy(STM).Themoststrikingresultshave 42 been obtained in the case of Ni 1=4 TiS 2 with sig- 43 nificantchangescomparedtoTiS 2 .Theaimofthis 44 work is to investigate more deeply these experi- 45 mental data, implementing the periodic ab initio SurfaceSciencexxx(2002)xxx–xxx www.elsevier.com/locate/susc * Correspondingauthor.Address:LCTPCM,IFR,rueJules Ferry,64000Pau,France.Tel.:+33-559-722-063;fax:+33-559- 803-769. E-mail address: isabelle.baraille@univ-pau.fr (I.Baraille). 0039-6028/02/$-seefrontmatter Ó 2002PublishedbyElsevierScienceB.V. PII:S0039-6028(02)01883-6 SUSC 12909 No. of Pages 9, DTD=4.3.1 13 June 2002 Disk used SPS-N, Chennai ARTICLE IN PRESS