XPS studies of ion-bombardment damage of transition metal sulfides

Journal of Electron

Spectroscopy

and Related

Phenomena, 20 (1980) 169-182 - Frmted m The Netherlands

0 Elsevier Sclentlfic Pubhshmg Company, Amsterdam

XPS STUDIES OF ION-BOMBARDMENT METAL SULFIDES

G J COYLE,

T TSANG

DAMAGE OF TRANSITION

and I ADLER

Department of Chemrstry, Unrverszty of Maryland, College Park, Maryland (US A ) and Department of Physzcs and Astronomy, Howard Vnrverszty, Washrngton, D C 20059 (US A )

20742

L YIN

NASA

Goddard

Space Flaght Center,

Greenbelt,

Maryland

20771

(US A )

(First received 2 November 1979, m final form 12 February 1980)

ABSTRACT Radlatlon damage to the surface of transltlon metal sulfides under -l-keV ron bombardment has been mvest~gated by photoelectron spectroscopy Under argon-ion bombardment, decompo&lon to the metal and elemental sulfur was observed However, there are conslderabie differences between the dosages required to Induce decomposition from 70 x IO” lox&cm2 for FeS through the sequenceCoS and NlS to -0 3 x 10” lons/cm2 for CuS On the other hand, under oxygen-ion bombardment, the predommant process appears to be ran-surface chermcal reactlon Agam, the dosages required for oxldatlon differ consIderably In fact, the rate of ion-surface chemical mteractlon ls found to be mversely correlated with the rate of Impact-Induced decomposltlon These observations, together mth previous data, help to clanfy and suggest eystematlce for the behavlour of fwst-row tran&lon metal compounds under ran-bombardment

INTRODUCTION

Radlatlon damage under ion bombardment has been studied by X-ray photoelectron spectroscopy (XPS), electmcal conductivity, electron dlffractlon and optical techniques [l-6]. The radLatlondamage and storchlometlnc changes are espectiy severe on the sample surface. Because of its surface sensltwlty at samphng depths of -10 a, the XPS technique 1s Ideally sulted for surface studies The mltial work of Ym et al. [I, 21 and subsequent XPS studies [7--101 have reveded a remarkable tendency for many of the cations of metal oxldes and hahdes to reduce to the metalhc state and become deficient oh the anion component Furthermore, there are marked

170 differences between the surface reduction-rates of various transltlon metal compounds (e.g. the d9 and d’* compounds) under -l-kV argon-ion bombardment Most dg compounds, such as Cu(II) oxide and Cu(I1) fluoride, are reduced completely to metalhc Cu, with a concomitant loss of the amon species at the rather modest dosage of -2 x 10” lons/cm2. On the other hand, there 1s virtually no reductron of dl* compounds (Cu(1) and Zn) even at dosages as high as 100 x 1017 Ions/cm2 An interesting example ISthe case of CuCl, It was observed that CuC12 (dg ) 1s reduced rapidly to CuCl (die), but further reduction to metalhc Cu was not detected Addltlonally, it has been found that transitron metal compounds with relatively large crystalfield sphttmgs, such as the cyamdes wLththen low-spm configurations, could be reduced at considerably lower dosages than those with high-spin configurations as exemphfled by the fluorides and slhcates These results mdlcate that the reduction process is most hkely “chemical” m nature, although It does take place concurrently with physical sputtering That is, the reduction process may involve the excltatlon and mteractlon of valence electrons, mduced by the incident ions It is evident that there are a number of processes affecting surface changes under ran bombardment Several recurrent phenomena have been observed which are relevant to our descnptlon of “chemical” interactron These can be divided mto the followmg two categories (1) Induced alteration. changes on the surface, other than physical sputtermg, which are due to kmetlc-energy transfer from the xncldent ion. In thrs category there is no chemical mteraction between the surface and the mcident ion These mtracrystalhne changes may be a functron of the covalence of the compound under bombardment. (2) Reactive alteration. changes which appear to be the result of dnect, chemical mteractlon with the incident Ion In order to isolate and study these processes, it was decided to extend our study of Iron-sulfur compounds [ 1 l] to mclude other sulfides of the firstrow transition series This choice was based upon the followmg rationale The chemical shifts of the inner-shell binding energies for sulfur are known [ 121 to be relatively large and readily observable For tram&on metal ions, m addrtlon to binding-energy shifts, the presence or absence of shake-up satelhtes and the hneshapes and widths of the L2,s peaks (hneundths for the metalhc forms are considerably narrower than for the compound forms) are also useful parameters for observmg chemical changes on the sample surface Furthermore, changes m surface stolchlometry can often be inferred from the metal/sulfur photoelectron-intensity ratios However, it should be noted that smce many of the samples are msulators, sample-chargmg may affect the determination of absolute bmdmg energies More nnportantly, it ISconcelvable that ion bombardment may even produce changes m the amount of sample-chargmg, which could lead to binding-energy shifts mdlstmgu&able from chemical shifts Therefore, it is important not to use binding-energy

171

shtits as the sole cntena for momtormg chemical changes. In this respect, sulfides of the first-row tram&on metal series are agam well suited for the present study As wti be seen later, the spectral changes for these compounds under ion bombardment are frequently dramatic, with the appearance of extra structures, sphttmgs of ongmal peaks, and shlftmg of relative mtensltles, as well as completely mdependent behavlour between the spectra of the metal and sulfur These features, coupled mth the parameters mentioned above, make it relat;lvely easy to observe and measure the chemical changes induced by ion bombardment m these transltlon metal sulfides Imtlally, Ar+ was used as the bombardmg ion, to isolate changes of the fust type Because all ion-bombardments were carned out at - 1 kV, the energy unparted to the surface may be considered as constant, and the observed changes, because of the mert character of argon, would be pnmanly a function of the sample The same senes of compounds was subsequently bombarded mth O$, xntroducmg the posslbfity of both reactive and mduced alteration By exammmg the rate at which change was produced as a function of 0: dosage, it was hoped that some mdlcatlon of the degree of competltlon between the two types of alteration might be obtamed

EXPERIMENTAL

As m our previous expenments [8-lo], MgKozlla X-rays (1263,6 eV) were used as the excltmg radlatlon. To mmlmlze chargmg effects, an extremely thm layer of finely-ground sample powder was brushed onto an aluminum-foil or copper-fell substrate coated mth a thm and dilute layer of mtrocellulose adhesive (4% parlo&on or collodlon m amylacetate) which was free from sulfur contammants The glow-discharge or Ion-bombardment of the samples was carned out filth either argon or oxygen, at a potential of - 1 2 kV, an ion-current density of -0.2 mA/cm2, and at a pressure of -0 02 torr The glow-discharge procedure was performed m an antechamber isolated by a gate-valve from the mam, high-vacuum chamber of the electron spectrometer Dunng glow-discharge, the gas was admltted through a controlled-leak valve and then exhausted contmuously, so that the sample surface was bemg swept contmually Photoelectron spectra were recorded at 2 x 10m8 torr after vanous stages of ion bombardment, without exposing the samples to the atmosphere. The spectral dlstnbutlon of the elected photoelectrons was recorded on a multichannel analyzer m the multi-scaler mode, at -0 15 eV per channel. Our previous, me&gas ion-bombardment experunents 181 showed that radlatlon-mduced changes depend mostly on the mtegrated total ion-dosage rather than on the ion-current density (here rv0 2 mA/cm2 ) NIS, CoS and ZnS were prepared as precipitates by mlltmg aqueous sol-

172

utlons (equlmolar quantities of the metallrc chlonde and sodium sulfide To prevent the preciprtatlon of metal hydroxides, acldlfled NlCI,, CoClz or ZnCl, solutions were used, to which the Na,S solution was added slowly so that the pH of the solutlon was kept nearly neutral after mlxmg. After washing with water, the precipitates were dmed under vacuum. These samples together with commercial reagent-grade ZnS and CuS were used for the experunents The photoelectron bmdmg-enermes were calibrated agamst the Au 4fpeaks.

RESULTS

Argon-ion born bardmen t Pnor to bombardment, the sulfur 2p bmdmg energes (BE) for FeS, NlS and CoS were observed to be 162.6 f 0 5, 162 2 f 0.5 and 162 3 * 0 5eV, respectively, these values are shghtly less than our measured value of 163.9 + 0.5 eV for elemental sulfur [ 111 On the other hand, the sulfur 2p binding energy (163 6 f 0.5 eV) observed for CuS 1s higher than those for the other sulfides and 1s almost the same as that for elemental sulfur This somewhat higher value probably reflects the unusual valence of the tram&on metal ion (Cu’) in CuS as compared to its clear dlvalence m FeS, N1S, or CoS. These expenmental values are m f= agreement with the recent results of Furuyama et al [ 131, of Romand et al [ 141, and of Rupp and Weser [15] , as well as with other literature values summarized by Wagner et al [ 161. The Cu 2p and S 2p photoelectron spectra for CuS at V~IXOUS stages of Ar+ ion bombardment are shown m Fig. 1 Although the stolchlometry of thus substance would imply the presence of Cu(II), It has been shown that exclusively Cu(1) 1s bound to sulfur [ 14,151. In fact, CuS was found to be dlamagnetlc rather than pammagnetic Prior to bombardment (Fig lA), the Cu 2p spectrum for CuS 1s quite different from that for Cu(I1) compounds, where the mtensltles of shake-up satellttes are comparable to those of the mam peaks [ 16,171 These satellites are expected to be absent for the dlamagnetic CuS [18] Thus the weak, shake-up satellite structure m Fig 1A mdlcates slight contammatlon of the surface by oxldatlon. Similarly, the weak S 2p peak at 168 eV (characterlstlc of sulfates) as compared to the mam peak at 162 eV (charactenstlc of sulfides) indicates that there 1s not extensive oxldatlon contammatlon With a modest radlatlon-dosage (0 3 x 1017 ions/cm” ) of argon-ion bombardment, the Cu 2p peaks become considerably sharper (Fa 1B) and are qute slmllar to Cu 2p peaks from clean metallic copper (Fg 1D) The S 2p peaks remam very promment at their m&al potiltlons under argon-ion bombardment Although higher radlatlon dosages (5 7 x 1017 lons/cm2) produce httle further change m position (Fig lC), there 1s a definite decrease m the mtenslty ratlo S(2p)/ CU(~P~,~ ) Because of the small hfference m hndmg energy, rt 1s rather dlfflcult to dlstmgulsh between the S 2p photoelectron peaks of CuS and

173

Ar

CuS,Cu. I

950

I

930

I

L

190

100

BE Fig 1 Cu 2p (left) and S 2p (Irght) photoelectron spectra (counts vs bmdmg energy (BE) m eV), of CuS under 1 O-kV argon-lon bombardment (A) Alor to bombardment, (B) 0 3 x 10’ lons/cm2 dosage, (C) 6 7 X 10” ions/cm’ dosage, (D) clean metalhc copper foil The vertical hne mdwates the Cu 2~3,~ peak posrtlon m metalhc copper

those of elemental sulfur. Nevertheless, the considerable increase m sharpness of the Cu 2p peaks observed after a very bnef bombardment with argon ions suggests the formation of metalhc Cu on the sample surface. Furthermore, the decrease m the S(2p)/Cu(2p) photoelectron-peak mtenslty ratio under additional bombardment mdlcates a change m the S/Cu atomic ratio on the sample surface. One possible mterpretatlon of these spectra ISthat under ion bombardment, while some metalhc copper was formed on the sample surface, the CuS substrate was still observable. However, whereas the difference m the bmdmg energy of the Cu 2~ peaks between 0.1~ and Cu+ 1s often small and difficult to measure, the peak-mdth of Cu+ 1s generally greater than that

174 of Cue Therefore, the slmllanty between the widths of the copper peaks m Fig 1B and C and of the copper peak m Fig 1D (Cue m metalhc copper foil) suggests that only metalhc Cu was vlslble under spectroscopy Furthermore, thxs mterpretatlon appears to disagree with our previous experiments [9] wzth CuFz The mltlal Cu*+ spectrum of CuF, consists of pronounced shake-up satelhtes After a brief ion-bombardment, not only did the Cu spectrum become obviously metallic, but also the Fls peak disappeared completely These results suggest that the CuF2 substrate 1s not observable on bombardment. It 1s therefore likely that m the case of the CuS spectra m Fig 1, mltlal argon-ion bombardment caused the sample to decompose mto elemental Cu and S. Additional argon-ion bombardment caused a slow, preferential loss of S, possibly because of Its greater volatility By contrast, previous results [l l] m this laboratory have mdlcated that FeS 1s extremely resistant to radlatlon damage under argon-ion bombardment Slgmflcant decomposltlon to elemental uon and sulfur occurs only after a radlatlon dosage of 70 x 10” ions/cm* The behavlour of CoS and NIS 1s qute sunllar to that of the other transition metal sulfides, and appears to be mtermedmte m rate of decomposltlon between FeS and CuS The results of argon-ion bombardment for NIS are depicted m Fig 2 Although not as rapid as for CuS, the onset of damage for NlS does occur very rapidly (dosage -4 5 x 101’ ions/cm* ) The N12p,,, peak sphts mto two components (Fig 2B) One of these has a BE very surnlar to that pnor to ion bombardment (856 7 f 0 5 eV), that of the other peak 1s less by almost 3 eV On compmon with the N12p photoelectron spectrum from nickel metal foil (Fig. 2E), these results once agam suggest the decomposltlon of the compound mto elemental components on the surface The Ar* bombardments for CoS (not shown) have results quite slmllar to those for NlS, except that CoS 1s less susceptible to radiation damage The Co 2p peak also sphts mto two components under argon-ion bombardment, avmg the mitial Co 2p 5,2 peak at 781.6 + 0.5 eV and a new component at 778 9 f 0.5 eV. These results also suggest the decomposltlon of the compound into elemental components. However, thm sphttmg requires a somewhat higher dosage for CoS (-7.4 x 101’ ions/cm* ) than for NIS Zmc compounds are known to exhibit very small (< 1 eV) chemical-shifts [12]. Furthermore, the 2p spectra of zmc compounds (dlO) normally have no shake-up satelhtes and also show httle differences m hnemdth or shape compared to the 2p spectrum of metalhc zmc It was previously found that ZnF2, m contrast to FeF, , CoF*, NlF2 and CuF2, drd not reduce to metalhc Zn under Ar+ bombardment [9, lo]. The experunental evidence for this conclusion IS the pers&ent presence of the F 1s peak for ZnFz even after prolonged Ar+ bombardment, m contrast to its disappearance m the other fluorides.. In the sulfides, it has been demonstrated that S does not disappear from the sample under reductson by Ar+ bombardment For these reasons, the mterpretatlon of the ZnS spectra m Fig. 3 w rather dlfflcult. It 1s not

175

E

-_i

,,

N&,Nm,Ar

875

855

BE

170 If

Fig 2 NI 2p (left) and S 2p (nght) photoelectron spectra of NIS under 1 0-kV argonson bombardment (A) Pnor to bombardment, (B) 0 4 X 10” ~ons/cm~ dosage, (C) 4 5 X 10” rons/cm’ dosage, (D) 14 x 10” ~ons/cm~ dosage, (E) clean metalllc nickel foil The vertical lme mdlcates the N1 2pw2 peak posrtlon m metab mckel

clear, Judgmg from the spectra alone, whether ZnS 18 reduced However, under Ar+ bombardment the reduction rates both of fluondes [9,10] and of sulfides (Table 1) exhlbrt the same trend. That 16, the dosage requd for reduction decreases m the sequence Fe, Co, Nl, Cu. If tlus gross snmlmty m behavlour 1s used as a guide, then ZnS would be expected to behave Iike ZnFa , 1.e , not to show reduction under Ar+ bombardment. Oxygen-ron bombardment In order to determmetheextent

of mteractlon between the sample surface

176

ZnS,Ar I

I

I

1040

1020 BE

I

I

175 165

Fig 3 Zn 2p (left) and S 2~ (nght) photoelectron spectra of zmc sulfide under 1 0-kV argon-ion bombardment (A) Fkor to bombardment, (B) 0 4 x 10” lons/cm2 dosage, (C) 13 X 101’ lone/cm2 dosage,(D) 37 X 10” rons/cm’ dosage

and the bombardmg con-species, the chemically active oxygen was employed as the bombardmg species, rather than inert argon. The results for CuS are shown m Fig 4. Before bombardment (Fig. 4A), the Cu 2p shake-up structure 16 weak, as noted pre~ously (Fig. IA). fnterestmgly, a very low dosage of O*, bombardment (0 45 x 1O1’ ions/cm* ) appears to produce chemical reductton, as mdlcated (Fig. 4B) by the ngmf~cant sharpenmg of the Cu 2p peaks This reduction ~8 also accompanied by a decrease m mtenslty of the Cu 2p satelhte and of the S 2p peak at - 170 eV However, the processes are reversed with further bombardment of CuS by oxygen ions (Fig 4C and D), resulting m a gradual transfer of the S 2p peak mtenslty from BE 163.6 + 0.5 to BE 169.6 a 0.5eV. As can be seen from Fig 4D, at a dosage of 7.5 x 10” ions/cm*, the S 2p peak at - 164 eV IS greatly dlmu-ushed, at a dosage of

177

cus.0 I

950

I

930

I

I

180

160

BE 4 CU 2p (left) and S 2p (nght) photoelectron

spectra of CuS under 1 O-kV oxygen1on bombardment (A) plnor to bombardment, (B) 0 46 X 101’ ~ons/cm~ dosage, (C) 2 0 x 1o1’ 10ns/cm2 dosage, (D) 7 6 X 1Ol7 ~ons/cm~ dosage The vertical hne mdlcates the Cu 2~312 peak posltlon m metalhc copper

Wg

25 x 10” ions/cm * , this peak disappears completely This mdlcates that the orrgmal, sulfide species on the sample surface has now been completely oxldlzed to a sulfate-like species, smce the 1’70eV peak LSusually mdmatlve of highly oxldlzed sulfur 1121 Further evidence of oxidation of the surface is provided by the Cu 2p spectrum obtamed at this dosage, which now drsplays the very promment shake-up satelhte structure characteristic of Cu(II), along with a shift of the primary peaks by 2 eV to higher BE The shape of the Cu 2p peak at the end of 0: bombardment does not permit unambiguous rdentifmatron of the final oxidized species, its appearance seems to resemble that for CuO rather than that for CuS04 1161. If this were the case, #en the S 2p peak at -170 eV indicates the transformation from sulfide to a type of

178 “isolated” or “decouphng” sulfate-species, sumlar to that observed previously [l&13] on the surface of elemental sulfur samples under oxygen-ion bombardment and under gas adsorption. Such a “sulfate” species was shown to be stable even on exposure to the atmosphere. The overall behavlour of CuS under oxygen-ion bombardment 1s quite different from that of FeS, where the surface oxldatlon appears to be both &ect and prompt (complete at a dosage of -2 x 10” ions/cm2 [ll] ) As m Ar+ bombardment, the behavlour of NIS and CoS IS somewhat mtermediate between those of FeS and CuS. The results of oxygen-ion bombardment for NlS are shown m Fig 5 There 1sa gradual transfer of the S 2p peak intensity from 161 9 f 0.5 to 169 1 f 0.5 eV, and an increase of -2 0 eV m the N12p BE for a dosage of - 5 x 10” ions/cm2 (Fig 5C). The oxldatlon of N1S 1s seen to be complete at a dosage of -19 x 10” rons/cm2 (Frg 5D) The results of oxygen-ion bombardment for CoS are quite slmllar to those for NlS, except that the dosage required for the oxldatlon of CoS

Y”-JL% N&,0

I

a75

I

a55

I

I

170 160 BE

Fig 5 NI 2p (left) and S 2p (nght) photoelectron spectra of lucks1 sulfide under 1 0-kV oxygen-Ion bombardment (A) Rlor to bombardment, (B) 0 4 X 10” ions/cm’ dosage, (C) 4 8 X 10” Ions/cm2 dosage,(D) IQ X 101’ ro~le/cm~ dosage

179

(-5.4 x 1O1’ ions/cm2 ) 16 considerably less than that required for the latter (-19 x 10” ions/cm2 ). As m the case of CuS, although the spectral shapes of FeS, N&i and CoS do not allow unequivocal ldentlflcatlon of the oxidized end-product, they seem to have a closer resemblance to those of metal oxides than to those of metal sulfates. ZnS proved to be rather resistant to 0; bombardment, although oxldatlon did take place, as evidenced by the sphttmg of the S peak and a slow transfer of mtenslty from the ongmal S peak to a peak of high BE. This transfer was eventually completed at a dosage of 21 x 10” ions/cm2 Because of the amblgultles associated with the Zn 2p spectra, ascussed earher, it 1s even more tiflcult to ascertam the final oxidized species m the 0; bombardment of ZnS

DISCUSSION

As mentioned earlier, there are many competing surface-processes under ion bombardment Because of ths complexity, the data from both argonand oxygen-ion bombardment of transltlon metal compounds should be combmed to elucidate the mechamsms of radiation damage as well as the generally complex chemistry of the metal--sulfur compounds The general results are summmzed m Table 1, deduced not only from photoelectron bmdmgener@es, but also from lmeshapes, widths, and shake-up satelhte structures, as well as from the relative transltlon metal/sulfur peak-mtenslty ratios. The mltlal reduction of CuS (to metallic copper and elemental sulfur) by low dosages of either argon- or oxygen-ion bombardment 1s a rather stnkmg mdlcatlon that ion-bombardment reduction may depend on electronic mterTABLE

1

OVERALL

SUMMARY

OF RESULTS

Sample

Ar+ Bombardment

FeS

Reduction completed -70 x 10” 10na/cm2 Reduction completed - 7 4 X 10” Ions/cm2 Reduction completed “4 5 X 10” Ions/cm2 Reduction completed -0 3 x 10” lons/cm2

cos NIS cus

ZnS

0; at at at at

Probably no reduction at “37 X 10’ ’ lon8/cm2

Bombardment

Oxldatlon completed at -2 0 X 101’ ions/cm2 Oxidation completed at -6 4 X 10” ions/cm2 Oxldatlon completed at -19 X 10” ions/cm2 Inrtlal reduction at -0 45 x lol’ lOns/cx& followed $J oxldatlon completed at -25 X 10 ions/cm’ Oxcrdatlon completed at -21 x 10” *one/cm2

180

actions CuS has been reported to consist prunanly of Cu(1) by Romand et al. [ 141 and also by Rupp and Weser [ 153 The Cu(1) valence (dl’ ) u conslstent with the dlamagnetlc susceptlblhty of CuS and also with the vn-tual absence of shake-up satelhtes m the Cu 2p photoelectron spectra. In contrast to CuCI, CuS 1svery easily reduced, probably due to its covalence For all of the transition metal sulfides, It appears that argon-ion bombardment induces decomposltlon mto elemental metal and sulfur At higher dosages of Ar+ bombardment, there 1s comparatively httle further change except for the slight, preferential loss of sulfur The ra&atlon dosage required for decomposltlon, however, IS quite different for the various sulfides, decreasing in the sequence FeS (- 70 x 10” Ions/cm* ), CoS (-7 4 x 10” ions/cm2 ), NlS (-4 5 x 1O1’ rons/cm* ) Whereas sulfur and the respective metal seem to respond to Ar+ bombardment as mdependent elements, the present experlmental data also suggest that 0: bombardment produces separately-oxidized species and “isolated” or “danglmg” sulfate groups rather than metal sulfates If this were the case, It could be related to the requirement of frurly long metal--sulfur distances (3 O-3.381) in first-row transition metal sulfates, as only comparatively cramped condltlons are avsulableon the surface of the parent sulfide (metalsulfur distance 2 3-2 5a) The 2p spectra of the metal after varymg dosages of 0; bombardment seem to resemble those of the metalhc oxides m peak shape and in BE. On the other hand, durmg the course of bombardment each sample exhibits a transfer of sulfur-peak mtenslty from an mltlal BE of -164 eV to a final BE of -170 eV The disappearance of the peak at 164 eV IS unambiguously dlscermble and IS used as an index to determme the approxunate dosage at which oxldatlon 1s complete The dosages required to produce complete oxldatlon are m the sequence FeS (-2.0 x 10” ions/cm* ), CoS (5 4 x 10” ions/cm* ), NlS (-19 x 10” ions/cm* ), ZnS (21 x 10” ions/cm* ), CuS (25 x IO” ions/cm* ) Comparison of this sequence to that reported above for Ar+ bombardment reveals that those sulfides which are most resistant to decomposltlon into metal and sulfur under Ar+ bombardment are most easily oxldlzed under 0’: bombardment, and vice versa While ZnS does not seem to decompose under Ar+ bombardment, its behavlour under 0; bombardment appears to be consistent with that of the other sulfides The reason for this behavlour 1s not clear, but it may be related to structural differences (ZnS has a face-centered cubic lattice, while FeS, NIS and CoS are of the NlAs structure) or to the “closed-shell” ~2” electronic configuration of Zn It IS of particular mterest that CuS IS mltlally reduced to metalhc copper and elemental sulfur by low dosages of oxygen-Ion bombardment This underscores the presence, under Ion bombardment, of concurrent, competitive processes of reactive ion-surface chemical interaction (I e , OXIdatlon) and unpact-mduced decomposrtlon. For the other sulfides studled, the former process appears to predominate The unusual behavlour of CuS

181

m this respect may be due to the “noble” chemical character of copper with respect to the other fust-row transition metals, makmg it more resistant to oxldatlon Conceivably, the radmtlon damage to CuS under 0: bombardment occurs m two steps, reduction to metalhc copper, followed by oxldatlon. Smce the metaJhc form ISnot observed for the other transltlon metal sulfides, it 1s possible that m these cases the first step 1s obscured by the greater reactivity of the metal, or that these substances are damaged m one step with complete absence of the metallic form

CONCLUSIONS

In summary, the present observations may be interpreted as the result of predominantly two competitive processes These processes are (1) induced alteration due to ion impact, which usually takes the form of decomposrtlon mto elemental forms; and (2) reactive alteratron, or Ion-surface chemical mteractlons Only the first process can occur under argon-Ion bombardment In contrast, reactive alteration or oxldatlon IS the dominant process under oxygen-ion bombardment In general, substances which are most susceptible to induced alteration are those most resistant to reactive alteration, and vice versa The relative rates of these two processes appear to be nnportant m charactenzatlon of the chemical aspects of surface radiation-damage This IS especially well demonstrated m the case of CuS, where the induced alteration occurs readily Here, the oxygen-ion bombardment mvolves competltlon between the two processes, unth reduction takmg place at low dosages followed by oxldatlon at higher dosages. For the other sulfides, the reduction step 1s not observed

ACKNOWLEDGMENTS

We gratefully acknowledge fmanclal support under National Science Foundation Grant No EAR-78-16421 at the University of Maryland We thank Dr N Ben-Zvl for her valuable cntlcal comments and D. Evans for his nnportant technical assistance.

REFERENCES 1 2 3 4 5 6

L L H T R R

Ym, S Ghose and I Adler, Appl Spectrosc ,26 (1972) 365 Ym, S Ghose and I Adler, J Geophys Res ,77 (1972) 1360 M Nagulb and R Kelly, Radlat Effects, 25 (1975) 1 E Parker and R Kelly, J Phys Chem Sohds, 36 (1976) 377 Kelly and J B Sanders, Nucl In&urn Methods, 132 (1976) 336 Kelly, Nucl Instrum Methods, 149 (1978) 553

182

7 8 9

10 11 12 13 14 15 16

17 18

K S Kun, W E Bmtmger, J W Amy and N Wmograd, J Electron Spectrosc Relat Phenom ,I5 (1974) 361 L Ym, T Tsang and I Adler, Geophys Res Lett ,2 (1976) 33. L Ym, T Tsang and I Adler, Proc Lunar Scl Conf ,6th, (1976) 3277 L Ym, T Tsang and I Adler, Proc Lunar Scl Conf ,7th, (1976) 891 T Tsang, G J Coyle, I Adler and L Ym, J Electron Spectrosc Relat Phenom ,16 (1979) 389 T A Carlson, Photoelectron and Auger Spectroscopy, Plenum Press, New York, 1975, chap 5 M Furuyama, K Klshl and S Ikeda, J Electron Spectrosc Relat Phenom , 13 (1978) 59 M Remand, M Roubm and J P Deloume, J Electron Spectrosc ReIat Phenom , 13 (1978) 229 H Rupp and W Weser, Blomorg Chem ,6 (1976) 45 C D Wagner, W M Riggs, L E Dam, J F Moulder and G E Mullenberg, Handbook of X-ray Photoelectron Spectroscopy, Perkm-Elmer Carp , Eden Prane, MN, 1979 T Novakov, Phys Rev B, 3 (1971) 2693 L Ym, I Adler, T Tsang, L Matlenzo and S Grun, Chem Phys Lett ,24 (1974) 81