X-ray photoemission satellites; surface or bulk effect?

Journal of EIectron Spectroscopy and Related Phenomena Elsevier Publishing Company, Amsterdam - Printed in The Netherlands

Short communication

X-ray photoemission

satellites;

surface or bulk effect?

A. ROSENCWAIG and G. K. WERTHEIM Bell Laboratories, Murray Hill, NJ. 07974 (U.S.A.) (First received 20 November 1972; in final form 8 January 1973)

The appearance of satellites on the higher binding energy side of core level X-ray photoelectron spectra (XI%) of solids has been reported by several authors’ - 3, The satellites in question are those which are clearly not due to multiplet coupling or electron energy loss subsequent to photoeffect. The origin of these satellites is not understood in detail, but there appears to be general agreement that they represent a multielectron process, perhaps analogous to the “shake-up” mechanism employed by Carlson et a1.4 to account for similar effects in the rare gases. In a solid one is not, of course, restricted to excitations of the ion from which the core electron is excited, but should consider all possible excitations compatible with the band structure of the material. This immediately implies that valuable information concerning the ligands and unoccupied band structure might reside in these satellites. The empirical fact that satellites were found on the core levels of transition metal and rare earth compounds suggests that an incomplete shell plays an important role in satellite formation. The suggestion that such satellites are due solely to excitation out of an incomplete shell is contradicted by the observation of satellites in LaF, in which the 4f shell is empty 3. This prompts the alternate suggestion that valence band to 4f transitions may also be responsible3. A similar suggestion was recently made* to explain satellites in NiO. The major problem in thinking of these satellites as shake-up satellites resides in their intensity. The strength of the shake-up satellites in the rare gases is well accounted for by theory; that in solids is very much greater and argues against this simple analogy. Recently Novakov and Prins6 have suggested that some satellites are, in fact, a surface rather than a bulk effect. This proposal cannot be dismissed lightly since the photoelectrons do come mainIy from within a - 15 A layer at the surface. Their conclusion was based on the observation that satellites in “off the shelf” Cu,O disappear following heat treatment in vacuum at 200°C for 20 minutes. They further J. Electron S’ectrosc.,

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493

propose that the satellites are intimately connected with the presence of adsorbed water or oxygen. Some confirmation was obtained from similar results on CuCl and CuBr. These conclusions, when verified, would of course add another dimension to the interpretation of satellites data. Data on the 2p electrons of CuO and Cu,O taken in a study of the valence bands of transition metal oxides’ confirmed that the spectrum obtained by Novakov and Prins after heat treatment is indeed that of Cu,O. The pretreatment spectrum, however, had features reminiscent of CuO. In view of the well-known thermal decomposition of CuO to Cu,O and O2 at elevated temperature’ we decided to investigate this process at lower temperatures by XPS. A fresh CuO sample was prepared by heating a Cu sample to 250°C in an atmosphere of oxygen for 5 minutes. An adherent black CuO layer was produced. In Figure la we show three regions from this sample. The O(ls) line is a. single peak indicating that there is no detectable adsorbed water or oxygen. In CuO which has been exposed to the atmosphere for an appreciable length of time, a second line at higher binding energy due to adsorbed water is generally found. The Cu(2p) region in Figure la exhibits strong satellites associated with both the 2p,,, and 2p1,2 lines. There are clearly two satellites associated with the 2p3,2 line, one 8 eV and the other 10 eV from the primary line. On the 2p 1, 2 line the 10 eV satellite is very weak. The valence spectrum of CuO consists of a broad 3d line under which lies an unresolved O(2p) level centered at about 5 eV. There is also a satellite 8 eV from the 3d line. Both

4

1

60

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cu (2p)

70

60

65

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VALENCE

cu (2p)

BINDING

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CuO

ON

(b)

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AFTER TAKEN

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WITH

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Figure l.(a) The O(ls), Cu(2p) and valence regions of a fresh 0.10 on Cu sample; (b) The O(k), Cu(2p) and valence regions of the same sample after thirty minutes at 550 K in the spectrometer.

494

J. Ekctron Spectrosc., 1 (1972173)

the 2p and valence regions shown here are identical to those obtained for other CuO samples .including commercial “off the shelf” material. When the CuO is heated to 250 “C in the IO- 7 Torr vacuum of the spectrometer for only a short time, the Cu spectra change dramatically as shown in Figure 1b, while the 0( Is) line shows no qualitative change. In the Cu(2p) region the satellites are now almost completely gone, while the primary lines themselves have become much sharper and are shifted by about 1.2 eV to lower binding energy. The valence region is also considerably changed, the 3d line now being sharper with a shoulder near the top of the valence band. The O(2p) band located about 5 eV from tl& top of the valence band, is now clearly resolved, while the satellite at 8 eV has disappeared. These changes in the Cu regions can be readiIy accounted for by the observation that the spectra obtained after heat-treatment are identical to those of directly prepared Cu,O. Moreover, upon removal from the spectrometer the sample had the characteristic red color of that material. Other observations are also in agreement with this finding. The 1.2 eV shift to lower binding energy of the core levels has the sign and magnitude expected for reduction of CL?+ to Cu+. The narrowing of the lines in Cu+ relative to those in Cu*’ is due to the absence of multiplet broadening in diamagnetic 3d1 * Cu”. The absence of satellites is consistent with the model which ascribes them to valence band to 3d transitions which are precluded by the filled 3d band. Finally it might be noted that the valence band structure of the Cu,O is in good agreement with the calculations of Dahl and Switendickg, at least in so far as bands close to the Fermi edge are concerned. We now return to a consideration of the results of Novakov and Prins. In comparing their data with the Cu(2p) regions for CuO and Cu,O we see that their spectrum for “off the shelf” Cu,O is not that for Cu,O but rather a combination of both Cu,O and CuO spectra, strongly suggesting that their material was partially oxidized to CuO. The broad satellite structure close to the primary line corresponds to the Cu(2p) lines of CuO, while the satellite structure at N 10 eV contains the unresolved satellites of CuO. The fact that all these features can be assigned to CuO tends to rule out the alternative that they are due to Cu,O with adsorbed water or oxygen. The thermal decomposition of CuO in the vacuum of the electron spectrometer serves as an additional warning that stability of inorganic compounds cannot be taken for granted in XPS. In fact decomposition of CuO to Cu,O has been observed even at room temperature under the influence of X-ray irradiation. Unfortunately the published information on the equilibrium oxygen pressure of Cu08 corresponding to the reaction 2cuo

+ cu,o

+ + 02

in the region 840 to 1080°C J. Ekctron Specfrosc., 1 (1972173)

cannot

be extrapolated

with confidence

to room tem495

perature. As a check on these conclusions we also grew a thin film of NiO on a Ni sample. The O(ls) spectrum consisted of two lines, with the smaller line at higher binding energy due in the main to the oxygen in adsorbed water. The Ni(2p 3/2) region exhibited two satellites, one at 1.5 eV and the other at 7 eV from the primary line. Upon heating the NiO for several hours at 200°C in the spectrometer, we are able to drive off much of the adsorbed water as indicated by the reduced strength of the higher energy O(ls) line. However, the Ni(2p,,,) region was unaltered except for a small charging shift due to the loss of the adsorbed water. It thus appears that the origin of these satellites is not strongly related to the presence of adsorbed water or oxygen*. This result is not surprising since with an escape depth of 15 A, the majority of the photoelectrons come from atoms which are not in the outermost atomic layer. REFERENCES 1

2 3 4

T. Novakov, Phys. Rev. B, 3 (1971) 2693. A. Rosencwaig, G. K. Wertheim and H. J. Guggenheim, Phys. Rev. Lett., 27 (1971) 479. G. K. Wertheim, R. L. Cohen, A. Rosencwaig and H. J. Guggenheim, in D. A. Shirley (editor), EIectron Spectroscopy, North-Holland Publ. CO., Amsterdam, 1972, p. 813. For a review of this work see T. A. Carlson, M. 0. Krause and W. E. Moddeman, .7. Phys. (Paris), 32, colloque C4 (1971) 76. K. S. Kim, preprint. T. Novakov and R. Prim, Solid State Commun., 9 (1971) 1975. G. K. Wertheim and S. Htifner, Phys. Rev. Left., 28 (1972) 1028. F. H. Smyth and H. S. Roberts, J. Amer. Chem. Sot., 42 (1920) 2582; ibid., 43 (1921) 1061. J. P. Dahl and A. C. Switendick, J. Phys. Chem. Solids, 27 (1966) 931.

* It has been brought to our attention that similar results have recently been obtained by

T. Robert, M. Bartet and G. Offergeld, Surface Sci., 33 (1972) 123; and by D. C. Frost, A. Ishitani and C. A. McDowell, Mol. Phys., 24 (1972) 861.

496

J. EIectron

Spectrosc.,

1 (1972173)