Lang, W C

Padalia, B D

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XPS studies o f the charging effects for the non-conducting rare-earth oxidesB. D Pad ALIADpparlrnEiit of Phijsics, JvtdUufr. of TechnoJogi/, Bombaij-AOOOliiAND\V C L a n gDepartment of Mctallunjif, D nine r.sit y of Stmlhrl ydr (Hasgniv, U KIndian J .  Phya. 52A, 463-468 (1978)X-ray inrhipofl plintooiiiiasioii stiaUrM (XPS) of tJip bulk oxides ol the lu«ivy riiro-oarth. (Ooraonts mad(} irsin," a Vaeumn (k^iierator’s ESCA 3 iiistruirKiiit equipped Avitli a X -Y  plotli i^ ami andA1 A"a radiation Hources Cliarcjing elbiots on the eoro-lovel liinding iiiorgies of Tb^O,, Dy^Oa, Ho./3.,, EroOy, Tin/)^, YligOg and Luj,0.j ire debn-miTied It is lomid that the eliai-ging of tln^  Sfiinple" is doptindent on the sample thiekne.ss and also on the radiation souree ised to probe th(i sample. The obarging of th(» samples is (istimatedkVith lesjieet to 4/,/.^  peah for clean gold nietal at 83-7 eV. Tlu^)>:yg(’'n LS^  and carbon LV jx'aks were also monitored These results on bulk oxides are eoniparefl AAuth tl',e sui face oxides ro])or ter I by the authors elsewhere1 IntroductionX-ray induced ph.otoelectron spectroscopy is a poAVorfiil tool for sl-udymg tne binding energies in solids (Siegbahn cl al 1967). Eor non-eonductnig solids, a knoAvledge of charging effects is necessary for obtaining accurate values ol binding energies (j(»haiisson el al 1973) Beeontly, we I’eportiMil the eore-c loctron binding emwgies for the lanthanides (Lang et al 1974, 1975, Padalia d al (1976, 1977)) Inthis pajior, \vc presold, the icsults of our measurements of the charging efhietson the sesquioxides of terbium to lutetium2 E xperimentalPhntoeniission spectra of the hea  ^y lare earth oxides were measured using a Vacuum Generators ESCA 3 machine equipped Avith A1K„ and MgK* radia­tions as (?xeiting sources. Spectroscopically pure oxide powders supplied by Jol>nson-Mattlio3 s Ltd ; England vi^ m-e mounted onto the sample-probe using double-sided scotch-tape. The magnitude of charging was estimat<Ml by imbedd­ing a piece of gold wiro in tli,o poAvdor and rccoiding the gold 4 / peaks by X -7  plotter Details of the experimental method and the equipimnit used in this \Aork wore described in our previous oonimunications (Lang ct o/d975a, 1975b, Padaha cA al (1976, 1977)4634643. R ebuiiTb and  D iboubbionB. D. Padalia and W. C. LangExamples of iJie 4(/ photoemiaeion apectra of th.e sesquioxides of lutetium, ytterbium and holraium arc shown in figureH 1-3. The binding energies expressed in eV aie refon*(Mi to the measured Fermi level for the corresponding pure metals. The (4/ 7/2) for gold imbedded in terbium oxide powder together with carbon and oxygon Is signals are presented in figure 4 Photoomisaion spectra of a blank sellotape and the peak for a piece of gold fixed on this sellotape are given in figure 6. Spectra recorded using AIK* and MgK* radiations are labelled with the alphabets a and h, respectively. Charging of the samples are estimated v^ith respect to the 4/ 7,2 peak for clean gold at 83-7 cV. The values of the charging for the heavy raro-eoi'th oxides are given in table 1,Table 1 Charging of the samples estimated with respect to 4/ 7,2 poak for clean gold metal at 83'7 eVSamples a(oV)b(oV)LU2 O3 2 0 2 - 0Yb^Og 2 1 1-9Tm^Os 2 - 8 2 02 1 1 - 6IT0 2 O3 2 - 8 Mf>y2 0 a 30 1 - 6TbiO, 3 1 1-9Sellotape (blank) 0-9 1 - 0a— A1K„ radiation b—MgK„ radiation.A scnitiny of the data given in table 1, reveals that the magnitudes of charging, in general are higher for AIK* than those obtained for Thisindicates a direct dependence of charging on the penetration power of the radia­tion used to probe the sample. Tt is, however not clear why the charging for MgK, should be more compaj'tMi to AlK j^ in the case of blank sellotape. The measured electron energies for the oxides (figures 1-3) when corrocted for the charging effects (table 1), give the actual value of the binding energies. As on example, take the case of lutetium oxide (figure 1). The corrected values of the binding oiiorgicS for 4^5/2 *j*<*^d 4^3/2 are 196-7 eV and 206-4 eV, respectively. The estimated error in the measurement of these energies is found to be .tO'4 oV, The 4d spectra of the remaining heavy rare earth oxides (figures 2 and 3) are complex because those exhibit multiplot structure (Lang et al 1975).Non-conductirig rare-earth oxides 465Jn our oarlit5r woric, it was roported that tho ovaporatod Hurfooe oxidos of rare earths sliow no indication of tho charging offoots (Padalia et al (in press)). It is interesting to note that the binding energies of the and reportedFig. 1 4rf Photocmiflaion spectrum of lutetium oxide (LujOj).Fig. 2. 4d PhotoomisBion spectrum of ytterbium oxide (Yb2 0 a).for the 8urfoi,o oxide of lutetium are in close agieement nith those determined for tho bulk oxide of lutetium after applying eorreotions due to charging.466 B .  D .  P a d a lia  a n d  W , C . L a n gTli(^  offoci nl’ film thicknoKS on charging wero dotoctccl by iifc) provimisly for tho ovai)orat(i(l buJk oxido of raiM^ earths. Those samples M'ore propai'tw;! byI{ \<J I’hnldomiHsion sponirurn of holmiiim oxulf (Ho2 0 j)Fig. 4. Carbon and oxygon la peaks for toibium oxide (Tb4 0 7 ). N7 (4 /v,2 ) peak for goldimbeddod in TbiOT. Ouvvos labelled with a and h con'cuponding to A1K„ and IMgK. souvooB, roBpoctivoly.evaporating the metal in the pJVHonce of oxygon inKJclo a tiampUi pr(‘paration chamber maintained at a pr(js.ser(' ol If)  ^ R o it s^ aa found that themagnitude of chai'ging increases ^Mlh imjre.asjng Ihiehness ol' tin- oxidi  ^ fdin. in the iirosont case ol Imlk oxides wliice wen» nunoitt i^l on a riellotape, it is ddTi- uult to investigate (iuantilati\'(dy the dependonce ol ehaigmg on the sairple thickness. B.owoAor, tl\o recoi‘d(‘d spectra iiidicati' ilinngtss in llie luagnitudct of charging >Mth change in the tlnckiusss of the saiuxih', using th,o same radiation os oxciting sourceNon-conducting rare-earth oxides 467F j^ . !j Carbon and oxygon li' aignalt. for fjlank holloLapi'. N7  ]joiik for a ])U‘co of gold lixod on fho bollotaiio Tho o u j 'v o h  labollcd wifh u  awl b oorM'Rpoiiding to AllVj, anil MgK„ sourcos, rospoctivolyThe charging effect is also displayed by tJii^  phot/oemission spentia ol' tlu^  oxygen and Larhon present in the samples Since carbon and oxygen ,spectra exhibit the pieSenco of difforont species, tlu'So cannot be taken as staiidiu'ds foi calibration This fact has also boon reali^ o^d by others in recent years. Tho gold peak at 83-7 oV Tiinding energy, however continues to be a roliabk‘ stan­dard for calibration This fact is fuithor supjiortod by tho jirosent results.4. C o n c l u s io n sTho measured binding energies for the nim-iionducting materials must t)o corrected for the charging effects. Tlic chai'ging of the sample is dependent on the radiation source usihI to probe tho sample and also on the sample thieknesa The pcalc at binding cneigy 83-7 eV is recommended as a standard for cali­bration.468 B. D. Padalia and W. C. LangAoknowxedomentsOne of tlio authors (BOP) expresses his sincere thanks to Or. D. J. Fabiau and .Dr. S, Affrosaman for extending gracious Scottish hospitability and for providing necessary facilities. Thanks are also due to Or. L. M. Watson and Mr. Tan Sim for then' help and cooperation. Financial assistance from the Science Research Council for Great Britain is giatofully acknowledged.R e f e b e n o e sSiogbahn K., Nordling C , Fahlman A., Nordberg R , Hamnn E., Hodman J., Johansson G., Borgmavk T., Karlsson S. E., Lingron I. & Lindberg B, 1967 ESCA-Atomic Molectdar and Solid State Structure atudicd by means of electron spectroscopy. Nova Ada, Reg. Soc. So. U]JH Soi IV 20, North Holland Publ Co., Amsterdam.Johansson G , Hedman J., Berndtsson A,, Klosson N. & Nilsson R. 1973 J. Electr. Spedr. 2,293.Lang W. C.. Padalia B D., Fabian D. J & Watson L. M 1974 J. Elccir. Spectr. 5, 207; 197j|7, 357'>Lang W. C , Padalia B. D., Watson L. M., Fabian D. J. & Norris P R. 1975 Faraday discussionNo. 60, 37.Padalia B D., Laiig W. 0., Norris P R., Watson L. M & Fabian D J. 1977 Proc. Boy.Society. A354, 269.Padalia B. D , Gimzowski J K.. Affrossmaii S., Lang W. C., Waton L. M. & Fabian D. J.1976 Surface Science 61, 468.

XPS Studies of the Charging Effects for the Non-Conducting Rare-Earth Oxides

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XPS Studies of the Charging Effects for the Non-Conducting Rare-Earth Oxides

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