Satellite structure in the x-ray photoelectron spectra of CuO Cu2O

Volume 63. number 1

SATELLITE

STRUCTURE

CHEMICAL

PHYSICS LETTERS

IN THE X-RAY PHOTOELECTRON

1 Bkly 1979

SPECTRA OF 010 AND Cu,O

Marisa SCROCCO

Reeeiwd 13 DeLxmber I978

The satelliie structure in the X-ray photoelectron spectra of CuO and C&O has been measured. The satellites found in the spectra of 3~. 3s subshek of Cu and in the vzdenee-bandreggoon have been compared and discussed on the basis of the t\w Cu configurations d9 and dt”_

I_ Introduction The appearance of satellites on the higher-energy side of valence and core IeveIs in X-ray photoeIectron spectrz (XPS) of solids is a promising tool for investisting the eIectro&c structure of transition metal compounds_ In solids, it is necessary to consider aIi the pos-

sibIe excitations compatibIe with the band structure of the material_ This implies that important information on the Iigands and the unoccupied band sfmcture might be extracted from the sateliite structure and that transition metaIs with incomplete d shelIs play an important role in this satelIite formation_ This note reports the results of an acctirate analysis of the satellites appearing in the 3p, 3s and valence regions of Cu2+ and Cu* in the most stable oxides GLD and Cu20_ This has allowed us to compare the satellite distribution in the configurations dg and c!t”_

on freshIy prepared sampies which were kept in a desiccator to avoid adsorption of water. In a study of the oxidation mechanism of copper foils, Wieder and Czandema [I ] showed that a surface metastable phase formed, of composition Cu302, as a Cu,O gross defect structure. This phase is apparentIy produced when the copper film is oxidised between 100 and 200°C. We have prepared a sampIe under these conditions; the spectrum obtained had virtually the same shape as that of CuzO (as expected) except that the main peaks were intermediate between those of CuO and Cu,O. The energies in the table are referenced to thecentre of the Ozs band, tentatively assigned to 21 A eV. The satellite peak positions relative to the main peak (see table I and figs_ I-5) were deconvoluted via a gaussian fit of the experimenta spectra using a Du Pont 3 IO curve resoIver_

3. ResuIts 2. Eupetimental The XPS spectra of CuO and Cu10 were measured on a Vacuum Generators ESCA 3 photoeIectron spectrometer using an AI Ka, ,2 (izu = 1486.6 eV) source and a pressure of ca_ 1.0 X IO-” Torr- CuO was prepared by heating a Cu foil at 300-350°C in the presence of air_ Cu10 was obtained by reducing the CuO in the instrument by heating it at 200°C in vacua for ca. 2 h. The measurements were repeated several times

The analysis of the spectra obtained in the valence and 3p and 3s regions of Cu is more compIex than those reported to date_ In the spectrum of CuO, sateIIites appear in three regions: (a) a region with L!,E between 8 and 10 eV, observed previously [2,3] on Cuzp and assigned to a Iigand-tometal transition: Ozp + Cuzd (fig. I); (b) a region with LW < 8 eV containing many transi-

Fig_ I_ X-my photoelectron levels in CuO and CuzO.

spectra of CU~~~,~ and Cu_rp,,,

Table 1 Experimental binding energy (eV) of the primary peaks of Cu(3d. 3p, 3s. 2~) and O(?p, 2s) and the separation eneqy the satellites relativeI) to primary XPS peaks of Cu snd 0 in CuO and CuzO

Ic(eVv) of

CUO main peak satellites a)

cu*o main peak satellites

3.8 (1) (1-I (3) (4) (S! (6) (7) (8)

5.6 7.0 84 10.3 1x4

7-l.? 2.5 5.1 7.0

76 6

79.1 2.9

124.0 2.6

130.6 2.6

934.4 3.0

953.4 2-S

7.7

8.4

8.2 9.8

15.6 24.8 3.5

(1) (6) (7) (6)

6.4

6.5

Z-0 2.8

3) The position of the satellites is measured as Ar(eV)

75.6

780 3.0

123.0 2.5

-

931.3

950.7

12.8 19.2 26.4 from the tin

peak.

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Volume 63. number I

Fig_ 3_ X-y CUZO-

CHEMICAL PHYSICS J_ETTERS

phofoeleetron sper%r;lof Cusp level in CuO and

tions which are cIearIy seen in the vaience spectrum (fig_ 2 and table I)_ (c) A series of transitions with AE 7 10 eV. not previousIy reported in the literature, which appear clearly, part in the vaIence spectrum and part in the Cu- spectrum (fig- 5 and table I). q _ \ e wrI1 discuss these transitions on the basis of a comparison of the spectra of CuO and Cu,O which, because of the reduction Cuz+ + Cu+ goes from a dg to a dlo configuration_ Given tbar in Cu,O aII the main peaks are shifted compared to those in CuO by ca- I -3,-3-O eV to lower energy (the same occurs for a possible Cu302 species). we assume that aI1 the compIex structure appearing on the CuO lines at higher binding energy cannot be attributed to partial surface reduction of the sample_ A comparison of the two sampIes aiIows us immediately to make a distinction 54

1 hhy I979

Fig_ 4_ X-y C&O_

photoelectron spectra of Cuss ievei in 010 and

between sateIIites disappearing after reduction and those which remainThe presence in CuO with a partially filled d9 shell presupposes the presence of muItipIet splitting whiIst in the case of Cu20, having a completely filled d sheI1, this is not the case_ Multi-electron transitions are also possible and, for 3s and 3p shells, correlation effects can be importantThe various regions of the spectrum will be discussed separately_ 3.1. 3s regrbn In the 3s spectrum of CuO (dg) there are four transitions (see fig. 4)_ To interpret these transitions, two assumptions are possible:

Voiume 63, num’ber i

CHEhlICAL PHYSICS LETTERS

1 hlay 1979

3.2. 3p region

Fig. s_ X-ray photoelectron spectra of the in 010 and CuaO.

CU~~-CI.I~~

r&on

(A) the two peaks, originate from exchange interaction 3sa + 3dor and 3s.D4 3da. are those marked “A” and “B” in fig_ 4, with AE = 6-4 eV; each of these peaks is accompanied by a satelhte at ca. 2.5 eV which cannot be attributed to multiplet splitting: (b) a more complex structure from the 3s multiplet splitting is induced by electron correlation effects [4_5] _ If we compare the d9 configuration of CuO with the dl” configuration of Cu,O (see fig. 4) we see that the doublet at higher energy disappears (a consequence of the absence of the exchange interaction in this case) whilst a satellite seems to be still present next to peak “A” at ca. 2.6 eV_ This result appears to provide support for assumption (a) rather than (b): the possibility that this satellite can be attributed to multiplet splitting can then be excluded.

Several transitions are clearly observable in the 3p region of CuO (figs. 3 and 5). Comparing the transitions near the main peak appearing in both CuO and Cu,O, a doublet is immediately identifiable with AE = 2.4 eV, due to the spin-orbit coupling of the CU;~ (see fig. 3). Two peaks on the CuO spectrum are still observable: that at higher energy is the satellite at 8.4 eV, observed previously [2,3] on the Cuzp band, and it appears as a doublet. as does the main peak. The other band, at lower energy, is probably a satellite of CU~~,,~ (5e = 2.9 eV); the corresponding band of is covered by the SP,~ band. Note that this CQP3 I* satellite remains after reduction to Cu,O and is also found before and after the reduction, even on 0% (see table 1). It is also present in the Cutp spectrum of CuO (fig- I) but is not observabie in Cu,O. The most probable assignment, proposed by Novakov [6], is that this satellite corresponds to the transition between the ground state and the lowest excited state_ Although shifted a little in enerFf (AE = 2.6 eV) the satellite observed on the 3s line is presumably of the same nature. If we now turn to fig_ 5, which shows a scan spectrum of the CUDS-Cu3r region, a further two (or three) bands at 4~ > 10 eV are observed; all have a marked asymmetry to higher energy. which reproduces the splitting of the main peak. These satellites, here observed for the tirst time, can be interpreted if we put forward two assumptions: (a) that they can be ascribed to multiplet splitting, possibly complicated by electron correlation effects 17231: (b) that they driginate from processes involving escitation of electrons from the valence band to unoccupied states in the conduction band taking place in parallel with the formation of a 3p vacancy [2] _ This structure is rather weak. and a similar structure is not observed in the corresponding spectrum of Cuzp where it is presumably covered by the strong transittons falling in this region. If we again compare the spectrum of CuO with that of Cu?O, it is seen. that in Cu,O the line at S-4 eV (generally-&igned to a O,, - CL&, transition) disappears_ However. the satellites at he > 10 eV are still present, although they have very low intensity (see the arrows in fig. 5) and are at slightly different energies from the corresponding satellites in CuO.

Volume 63. number I

If the assumption of multiplet splitting (a) is v&d, then it must be assumed that the satellites observed on CuO (xvhich are quite intense)-cover a fur:her (weaker) series of satellites, the same in nature as those appearing in Cu,O. The Iatter may be explained only in terms of assumption fb) since both multiplet splitting and the &and-to-Cu(3d) transitions are not possible (CuJ has a 3dJ0 shell). CuO hence should show two series of transitions at roughIy the same energy: one generated by multiplet splitting and the other from an electron shake-up process_ Even allowing for eventual correlation eff’ts, it seems hi-y unlikely that these Jie at the same energy. Kim [9] predicted that the multiplet splitting in the core levels of Cu2+ is less than 4 eV_ It seems more probabie that in both cases we are in the presence of shake-up processes_ In Cu,O :he satelEites are weaker and Iess easily Jocated; & any case their positions are measurably different from those of the corresponding satellites in GO_ This excludes the possibility that they are residual bands of CuO (see table 1). .X3_ VaZeme baizd regiolt

A tentative decvnvoIution of the very broad valence band indicates that the spectrum is quite complex. A comparison between the spectra of c&i and Cu* (fig_ 2) shows that the position of the Cu3d and Ozp bands can be identified [IO, I II as well as that of a transition at S -4 eV_

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I Mny 1979

CHEMICAL PHYSICS LETTERS

This remaining structure can be explained on the basis of the folIowing assumptions: (a) that it derives from multielectron excitations; (b) that, given the considerable d-p overlap and hence strong hybridisation between Ozp and Cujd , at Ieast part of the observed structure may correspond to a “real” valence-band structure, as proposed by Hiifner and N’ertheim [I 2]_ This second suggestion appears attractive for that part of the spectrum tying at AE < 10 eV_ It is less certain whether the transitions lying af 4e > 10 eV are still valence-band structure or are, instead, due to

multielectron

excitations.

References Wiedrr and iLW_ Czmdcmzt. JV Ph)s. Chcm. 66 (1962) 616. [?I T. No’orzkov. Ph)s. Rev. B3 (197111693. 131 KS. Kim. J. Electron Spear\-. 3 (197-Z) 217. 141 P.S. Bogus, AJ- Frecmnnad F. Sauki, Ph)s. Rev. Let-

1I j Ii.

ters 30 (1973) 8.50_ S.P. Ko%ttcz~k. L. Ley. R-A. Polk&. T-R. \lcTi‘ely and DA. Shirley, Phi s_ Ret_ Bf t 1973) 1009. 161 I_ No\&ov and R. Prins. in: Electron spectroscopy, cd. [SJ

D.A. Shirley Worth-Holhnd. Amsterdam. 1971) p_ S11_ 171 S_P_ Ko\~~Ic&L L_ LQ _F-R- Uckel~ and D-X. Shirley, Phys. Rev_ 511 (1975) I72I.

[Sl R-P. Gupta and S 6. Scn. Phys. Rev_ BJO (197-l) 7i_ 191 KS. Kim. Ph>s_ Ret. BI 1 (1975) 2177. [IO! C_K_\Vertheim and S- HXner, Ph>s_ Rev. Letters 18 (1972) 1028.

[I I I A. Rosencwi~ and G-L. \C’ertheim, J. Elcstron Spcctrl_ 1 (2 9731493. [I21 S. 11fifnerand C.K. Wxthcim. Phys_ Rev. BS (1973) 4857.