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Multi-electron excitations in the optical and X-ray photoelectron spectra of NiO
Solid State Communications, \lol. 96, No. 3, p. 161-165, 1995 ,.$y(p&g?&i;~~&~ 003%1098/95 $9.50 + .oa
0038-1098(95)00344-4
MULTI-ELECTRON EXCITATIONS IN THE OPTICAL AND X-RAY PHOTOELECTRON SPECTRA OF NiO Fulvio Parmigiani Dipartimento
di Fisica, Politecnico di Milano, P.le Leonardo da Vinci 32, 20133 Milano, Italy Giorgio Samoggia
Dipartimento
di Fisica A. Volta dell’Universita di Pavia, Via A. Bassi, 6, 27100 Pavia, Italy and Luigi Sangaletti
Dipartimento
di Chimica e Fisica per i Materiali, Universita’ di Brescia, Via Branze 38, Brescia, Italy (Received 28 March 1995 by C. Calandr)
A study of multi-electron excitations in the 0 1s X-ray photoemission (XP) region of NiO single crystal is reported. The observed spectral lines are discussed and assigned to surface and bulk plasmon losses on the basis of a Kramers-Kronig analysis of the O-30eV range reflectivity data. These results, in agreement with former and present optical studies and electron energy loss data, show for the first time, that the bulk and surface plasmon losses are well detectable, until the third order, in the XP spectra of high quality NiO single crystals. Keywords: A. insulators, D. optical properties, spectroscopies, E. light absorption and reflection.
1. INTRODUCTION RECENTLY, photoelectron spectra of the late transition metal oxides (TM) have been the subject of a large deal of theoretical and experimental researches with the aim to reach a better understanding of the satellite structures deriving from charge transfer screening, multiplet splitting and exchange mechanisms [ 1- 61. On the other hand, minor attention has been paid to structures such as shake-up and plasmon losses which can be present in the X-ray photoelectron (XPS) spectra of these compounds. Furthermore the nature of the plasmon losses in XPS spectra is still open to debate [7-91. They could have an intrinsic, extrinsic or a mixed origin. Extrinsic plasmons are produced by photoexcited electrons during their transport to the surface [lo]. In this case plasmon losses do not depend on the photoemitting species; as a consequence, in ionic crystals, the same loss bands are expected for photoelectrons emitted either from the cation or the anion.
E. photoelectron
On the contrary, in the intrinsic process, where the final state consists of the outgoing electron leaving behind a core hole with possible band-plasmon excitations, we expect that plasmon losses depend on the sort of the emitting atom. However, late TM oxides, being open shell systems, present strong charge fluctuations in the same energy range where plasmon effects are expected; this fact prevents from testing the dependence of plasmon loss lines on the atomic site. As a consequence investigation of the properties of plasmon satellites, in these systems, is possible only in the energy region behind the 0 1s line. XPS studies have usually neglected these features probably because only careful measurements performed on high quality single crystals could have clarified their properties. This letter reports a study where optical and XPS plasmon losses data, obtained on NiO single crystals, are compared. Electron energy loss spectroscopy data (EELS) reported in the literature are also used to 161
OPTICAL AND X-RAY PHOTOELECTRON
162
complete the work. The attention is focused on the satellite features appearing on the high binding energy (BE) side of the 0 1s XP spectrum. 2. EXPERIMENTS XPS and optical experiments were carried out using commercially available NiO single crystals [1 I]. To perform XPS measurements, the crystals, oriented on the base of Laue patterns, were mounted on a sample holder to be cleaved perpendicularly to the (100) direction. A Perkin-Elmer Model 5400 XPS spectrometer with a monochromatic Al-Ka source was used to detect the core line spectra. The analyser pass energy was set to x 6eV, which resulted in an ultimate energy resolution of M 0.4 eV, as measured on the Ag 3d5,* core line. The spectrometer BE scale was calibrated by assigning to the Ag Fermi edge and to the Ag 3djj2 core level the BE values of 0.0 eV and 368.2 eV respectively. Surface charging during the XPS measurements was reduced by an electron flood gun. The crystals were cleaved inside the analysis chamber whose pressure was kept below 3x 10-i’ torr during the whole experiment. Synchrotron radiation from Adone storage ring, available at the optical spectroscopy beam line of the PULS facility (INFN, Frascati National Laboratories) was used for the reflectivity measurements in the 4-35 eV photon energy range. The radiation was dispersed by means of a modified Hilger & Watts onemeter monochromator operating at normal incidence. This system is equipped with two interchangeable gratings of 600 1mm’ and 1440 1mm-‘, working in the 2-10eV and lo-35eV range respectively 1121. 3. RESULTS AND DISCUSSION Figure l(a) shows the 0 1s XP spectrum measured on the NiO (10 0) surface. Only a single line with a full width at half maximum, FWHM, of 0.9 f 0.1 eV and centered at 529.1 f 0.1 eV was detected. The symmetric and relatively narrow line shape suggests, within the sensitivity of the XPS core lines to the local crystallographic structure, a stoichiometric and ordered oxygen structure. Figure l(b) reports the XPS emission in the 530610eV BE region. Several structures with different shapes and labelled A-F are detected. Peak A at 535 f 0.2eV is followed by structure B at 545.9 f 0.2eV, while the most intense structure formed by a shoulder, C, and a main line, D, are found at 547.4 f 0.2eV and 552 f 0.1 eV respectively. In the high-BE spectral region two broad structures are observed at 572 f 1 (E) and 598 f 1 eV (F).
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Vol. 96, No. 3
SPECTRA OF NiO
FWHM=O.9eV -
2 5
I
533
I
I
I
531
I
529
I
I
I
527
I
525
Binding Energy (eV)
610
570
590
550
530
Bmdmg Energy (eV)
Fig. 1. (a) 0 1s XP spectrum; (b) high binding energy region of the 0 1s core line; the main peak centroid at 529 f 0.1 eV is indicated by a dashed line. To obtain information about the origin of these spectral lines a comparison with optical and electron energy loss data is performed. Figure 2 shows the 3.830eV reflectivity spectrum of NiO. For the region below 3.8 eV reflectivity data reported in the literature were used [13, 141. The real, cl(u), and imaginary, e2(w), part of the dielectric function, the effective electron number per unit formula, iV,n, and the energy loss function L(w) = -1m [l/e(w)] were calculated from the proper Kramers-Kronig (K-K) inversion equations [15]. Above 30 eV the reflectivity function was extrapolated on the basis of the trial formula R = R,(wJw)p; where w, and R, are the frequency and the reflectivity at the upper end of the experimental range, and p is an empirical parameter chosen to give a consistent value c2 = 0 in the low energy limit. ei(w) and E*(W)are reported in Fig. 3. Figure 4 shows Nen and the energy loss function L(w). The reflectivity spectrum (Fig. 2) can be approximately divided into two distinct spectral regions. The first region ranges from 3.8 to 1OeV while the
_.-
_.--
-.
.-
.
--.__
Vol. 96, No. 3
OPTICAL AND X-RAY PHOTOELECTRON
SPECTRA OF NiO
163
5 4 z% 3 2 1 0
0
5
10
15 Energy
20
25
30
0
5
CeV)
Fig. 2. NiO reflectivity spectrum.
10
15 20 Energy (ev)
25
30
Fig. 4. Effective electron number per unit formula, N,n, and the energy loss function L(w) = -1m [ 1/c(w)] in the range O-30eV.
second region ranges from 10 to 30eV. In the first region., the main structure, A’, at 4.2 eV is followed by a series of minor structures (B’-D’). In the second region, a shoulder, E’, at 12.5eV, followed by two broad peaks, F’ and G’, at 13SeV and 15.5eV are detected. The most intense peak, H’, is located at 18.3 eV. The optical spectrum of NiO has been widely discussed in the past 116-211. In the framework of Zaanen-Sawatzky-Allen phase diagram [ 1r] and accordingly to direct and inverse photoelectron spectroscopy studies [18], the structures in the 4-8 eV range are attributed to charge-transfer mechanisms. Peak A’ is assigned to an 0 2p -+ Ni 3d charge transfer excitation, as well as structure B’ at 5.8 eV. Similar features have been observed also in Ni dihalides [19]. The structures resulting at photon energies larger than 1OeV are assigned, in agreement with combined photoemission-inverse photoemission results [ 181, to 2p -+ 4~:and 2p -+ 4p transitions. As already observed [21], the latter transitions are made possible by the 2p3p hybridization in the valence band which allows to overcome the dipole selection rules. The energy of
these transitions, reported in Table 1, were obtained calculating the energy difference between the centroid of the 0 2p band in the photoemission spectrum and the energy of the peaks appearing in the BIS (Bremsstrahlung isochromatic spectroscopy) spectrum. The energy of the 2p -+ 4s transition was estimated to be M 12 eV energy, while the 2p + 4p transition energy is larger than 18 eV. More in general, to what concerns the upper and lower energy limits of the band-to-band optical excitations, there is a reasonable agreement between the present experimental data and the theoretical results by Huge1 and Carabatos [20, 211 as reported in Table 1. From the electron effective number function (Fig, 4) it can be observed that the value of N,n at 30eV is smaller than six. This indicates that the oscillator strength of the transition starting from 0 2p and Ni 3d levels is rather low. Therefore a large contribution is expected at higher energies. The data agree with former studies [13, 14, 211 though few
12 I----
Table I. Calculated and measured optical transition energies in the range O-30eV. AN energies are measured in eV. The labels A’ to H’ are referred to Fig. 2
l
-epsl eps2
Optical trans. Present study (ev)
4.0 4.8 5.9 (B’) 5.8 (C’) 7.0 (3d + 4s, 4~) 7.2 8.25 (D’) 8.2 12.8 (E’) 12.5 (2~ ---)4s) 13.6 (F) 13.5 (G’) 15.5 17.8 (H’) 18.3 (2~ --t 4~) (A’) 4.2 (2p + 3d)
Eaergy W) Fig. 3. Real, cl(w), and imaginary, Q(W), part of the dielectric function obtained from the reflectivity data by proper Kramer+Kriinig (K-K) inversion equations [15].
Theor. Optical Trans. [13] [21] (eV) (ev) 5.5 13.0 17.9 20.8
XPS-BIS [181 (eV) 4.3 13.5 17.3 -
164
OPTICAL AND X-RAY PHOTOELECTRON
discrepancies are noticed in the relative intensity of some structures. As Powell and Spicer have already pointed out [13], they can be ascribed to the crystal surface order. The broad peak at about 23 eV observed in the loss function (Fig. 4) is assigned to a bulk plasmon on account of the fact that it occurs where ~2is minimum and cl crosses the abscissa axis (Fig. 3). Furthermore it is possible to compare the energy of this point with that observed from the usual formula E’ = ’ U2 where E. M 15 eV is the average (E;/QJ + Eo) energy of the 2p -+ 4s and 2p --) 4p transitions, J% x 15.5 eV is the plasma energy of the four electrons connected with these transitions, and coo w I is the value of the dielectric constant due to higher lying transitions, as deduced from Fig. 3. From this, one calculates E’ M 21 eV in good agreement with the experimental results. A comparison between the optical and the photoemission results may help to identify the satellite lines on the higher BE side of the 0 1s core line in the XPS spectra of NiO and to give a more consistent picture of the multi-electron excitations. As a matter of fact, line D in the XPS spectrum [Fig. l(b)] is separated from the 0 Is main line by z 23 eV which is close to the energy of the first order plasmon evaluated from the optical data and reported in Fig. 4. Being line D in the XPS spectrum assigned to the first order plasmon loss, the surface plasmon is expected at an energy Esp = Ear/d2 = 16.3 eV which is close to line B (16.8eV) in the XPS spectrum. Second and third order plasmon losses expected at 46 eV and 69eV corresponds to line E and F of the XPS spectrum. So far shoulder C remains unidentified. Peak A observed at 535eV in the XPS spectra [Fig. l(b)] is assigned on the basis of the optical studies, to a shake-up transition involving 0 2p electrons. This line originates from the loss of kinetic energy of the photoelectrons ejected from the 0 1s core level to excite 0 2p electrons from the valence band levels to low-lying levels in the conduction band. The energy separation between the 0 1s main line in the XP spectrum and the A peak is 4-6eV, which corresponds to the 0 2p + Ni 3d optical transitions in the reflectivity spectrum. The XPS plasmon loss spectra can be compared also with electron energy loss (EEL) data [22]. Table 2 reports such a comparison. The D-E lines of the XPS spectra have a counterpart in the EEL spectra within an uncertainty of f 2 eV. On the contrary, a larger discrepancy is found between the F feature and the EEL results. The assignment of peak D is confirmed by recent EEL measurements [23] where a plasmon
SPECTRA OF NiO
Vol. 96, No. 3
Table 2. Measured XPS and EELSplasmon excitations in the range 0-60eV. All energies are reported in eV. The labels B to F are referred to Fig. l(b) XPS satellite lines (ev> (A) 5.0
Shake-up
(B) 16.8 (C) 18.3 (D) 22.9 -
Surface plasmon Bulk plasmon
(E) 42.0
Bulk plasmon
&) 68.0
Bulk plasmon
EELS [22] (eV) 13.8 Bulk plasmon 18.8 Bulk plasmon 23.5 Bulk plasmon 28.5 Bulk plasmon 37.0 Bulk plasmon 44.0 Bulk plasmon 54.0 Bulk plasmon -
peak is reported at 2 1.5 eV. Line C in the XPS spectra could be identified with a bulk plasmon line found by EEL at 18.8 eV [22]. The good quality of NiO single crystal influences the interband electronic excitations in the lower energy region where a fair agreement between the energy-loss XP-derived data and EELS data is found. Indeed, the lack of contaminations, impurities, and the reduced density of defects [24] allows one to detect well-defined structures which otherwise would appear smoothed out or hidden below the signal noise level. In conclusion, for the first time an experimental study of multi-electron excitations in the 0 1s XPS spectrum of NiO single crystal is reported. It is shown that this region is dominated by plasmon induced energy loss lines, whereas lines associated to optical transitions were detected, with a rather low intensity, about 4.2eV from the 0 Is core line. REFERENCES 1.
2. 3. 4. 5. 6. 7. 8.
S. Hiifner, Unfilled Inner Shells: Transition Metals and Cornpow&. In Photoemission in Solti IICase Studies (Edited by M. Cardona and L. Lcy), Chapter 3. Springer-Verlag, Berlin (1979). A. Fujimori & F. Minami, Phys. Rev. B30,957 (1984). J. Zaanen, C. Westra & G.A. Sawatzky, Phys. Rev. B33, 8060 (1986). G. Lee&S.-J. Oh, Phys. Rev. B43,14674 (1991). K. Okada 8c A. Kotani, J. Phys. Sot. Jap. 60, 772 (1991). M.A. van Veenendaal & G.A. Sawatzky, Phys. Rev. Lett. 70,2459 (1993). J.E. Inglefield, J. Phys. C: Solid State Phys. 16, 403 (1983). M. Cini & A. d’Andrea, J. Phys. C: Solid State Phys. 21, 193 (1988).
Vol. 96, No. 3
OPTICAL AND X-RAY PHOTOELECTRON
H. Kuhlenbeck, G. Odorfer, R. Jaeger, G. Illing, M. Menges, Th. Mull, H.-J. Freud, M. Pohlthen, V. Staemmler, S. Witzel, C. Scharfschwerdt, K. Wennemann, T. Liedtke & M. Neumann, Phys. Rev. B43, 1969 (1991) (and references therein). 10. See, e.g. B.K. Agarwal, X-ray Spectroscopy. An Introduction. Second Edition, Chapter 5. Springer Verlag, Berlin, Heidelberg. 11. In the literature several 0 Is XPS spectra report rather intense features beside the main line. High quality NiO single crystals should exhibit only one symmetric line since the crystallographic oxygen sites are equivalent in NiO. Only in such NiO samples it is possible to study properly shake-up and plasmon loss satellites. 12. P. Camagni, G. Samoggia, L. Sangaletti, F. Parmigiani & N. Zema, Phys. Rev. B50,4292 (1994). 13. R.J. Powell & W.E. Spicer, Phys. Rev. B2,2182 (1970). 14. S. Suga, K. Inoue, M. Tan&u&i, S. Shin, M. Seki, K. Sato & T. Teranishi, J. Phys. Sot. Jap. 52, 1848 (1983). 9.
15. 16. 17. 18. 19. 20. 2 1. 22. 23. 24.
SPECTRA OF NiO
165
F. Stem, Solid State Phys. 15, 299 (1963). J.W. Allen, Optical Properties of Magnetic Oxides in Magnetic Oxides (Edited by D.J. Craik). Wiley, New York (1975). J. Zaanen, G.A. Sawatzky & J.W. Allen, Phys. Rev. L&t. 55,418 (1985). G.A. Sawatzky & J.W. Allen, Phys. Rev. Lett. 53, 2339 (1984). G. Guizzetti, L. Nosenzo, I. Pollini, E. Reguzzoni, G. Samoggia & G. Spinolo, Phys. Rev. B14,4622 (1976). J. Huge1 & C. Carabatos, J. Phys. C: Solid State Phys. 16, 6713 (1983). J. Huge1 8~C. Carabatos, J. Phys. C: Solid State Phys. 16, 6723 (1983). Y. Sakisaka, K. Akimoto, M. Nishijima & M. On&i, Solid State Commun. 24, 105 (1977). A. Gorschltiter & H. Merz, Phys. Rev. B49, 17293 (1994). Nickel oxide can easily take up excess oxygen, forming nickel vacancies in the lattice. See, e.g., N. Tsuda, K. Nasu, A. Yanase & K. Siratori, Electronic Conduction in Oxides, Chapter 4. Springer Verlag, Berlin (1991).
0038-1098(95)00344-4
MULTI-ELECTRON EXCITATIONS IN THE OPTICAL AND X-RAY PHOTOELECTRON SPECTRA OF NiO Fulvio Parmigiani Dipartimento
di Fisica, Politecnico di Milano, P.le Leonardo da Vinci 32, 20133 Milano, Italy Giorgio Samoggia
Dipartimento
di Fisica A. Volta dell’Universita di Pavia, Via A. Bassi, 6, 27100 Pavia, Italy and Luigi Sangaletti
Dipartimento
di Chimica e Fisica per i Materiali, Universita’ di Brescia, Via Branze 38, Brescia, Italy (Received 28 March 1995 by C. Calandr)
A study of multi-electron excitations in the 0 1s X-ray photoemission (XP) region of NiO single crystal is reported. The observed spectral lines are discussed and assigned to surface and bulk plasmon losses on the basis of a Kramers-Kronig analysis of the O-30eV range reflectivity data. These results, in agreement with former and present optical studies and electron energy loss data, show for the first time, that the bulk and surface plasmon losses are well detectable, until the third order, in the XP spectra of high quality NiO single crystals. Keywords: A. insulators, D. optical properties, spectroscopies, E. light absorption and reflection.
1. INTRODUCTION RECENTLY, photoelectron spectra of the late transition metal oxides (TM) have been the subject of a large deal of theoretical and experimental researches with the aim to reach a better understanding of the satellite structures deriving from charge transfer screening, multiplet splitting and exchange mechanisms [ 1- 61. On the other hand, minor attention has been paid to structures such as shake-up and plasmon losses which can be present in the X-ray photoelectron (XPS) spectra of these compounds. Furthermore the nature of the plasmon losses in XPS spectra is still open to debate [7-91. They could have an intrinsic, extrinsic or a mixed origin. Extrinsic plasmons are produced by photoexcited electrons during their transport to the surface [lo]. In this case plasmon losses do not depend on the photoemitting species; as a consequence, in ionic crystals, the same loss bands are expected for photoelectrons emitted either from the cation or the anion.
E. photoelectron
On the contrary, in the intrinsic process, where the final state consists of the outgoing electron leaving behind a core hole with possible band-plasmon excitations, we expect that plasmon losses depend on the sort of the emitting atom. However, late TM oxides, being open shell systems, present strong charge fluctuations in the same energy range where plasmon effects are expected; this fact prevents from testing the dependence of plasmon loss lines on the atomic site. As a consequence investigation of the properties of plasmon satellites, in these systems, is possible only in the energy region behind the 0 1s line. XPS studies have usually neglected these features probably because only careful measurements performed on high quality single crystals could have clarified their properties. This letter reports a study where optical and XPS plasmon losses data, obtained on NiO single crystals, are compared. Electron energy loss spectroscopy data (EELS) reported in the literature are also used to 161
OPTICAL AND X-RAY PHOTOELECTRON
162
complete the work. The attention is focused on the satellite features appearing on the high binding energy (BE) side of the 0 1s XP spectrum. 2. EXPERIMENTS XPS and optical experiments were carried out using commercially available NiO single crystals [1 I]. To perform XPS measurements, the crystals, oriented on the base of Laue patterns, were mounted on a sample holder to be cleaved perpendicularly to the (100) direction. A Perkin-Elmer Model 5400 XPS spectrometer with a monochromatic Al-Ka source was used to detect the core line spectra. The analyser pass energy was set to x 6eV, which resulted in an ultimate energy resolution of M 0.4 eV, as measured on the Ag 3d5,* core line. The spectrometer BE scale was calibrated by assigning to the Ag Fermi edge and to the Ag 3djj2 core level the BE values of 0.0 eV and 368.2 eV respectively. Surface charging during the XPS measurements was reduced by an electron flood gun. The crystals were cleaved inside the analysis chamber whose pressure was kept below 3x 10-i’ torr during the whole experiment. Synchrotron radiation from Adone storage ring, available at the optical spectroscopy beam line of the PULS facility (INFN, Frascati National Laboratories) was used for the reflectivity measurements in the 4-35 eV photon energy range. The radiation was dispersed by means of a modified Hilger & Watts onemeter monochromator operating at normal incidence. This system is equipped with two interchangeable gratings of 600 1mm’ and 1440 1mm-‘, working in the 2-10eV and lo-35eV range respectively 1121. 3. RESULTS AND DISCUSSION Figure l(a) shows the 0 1s XP spectrum measured on the NiO (10 0) surface. Only a single line with a full width at half maximum, FWHM, of 0.9 f 0.1 eV and centered at 529.1 f 0.1 eV was detected. The symmetric and relatively narrow line shape suggests, within the sensitivity of the XPS core lines to the local crystallographic structure, a stoichiometric and ordered oxygen structure. Figure l(b) reports the XPS emission in the 530610eV BE region. Several structures with different shapes and labelled A-F are detected. Peak A at 535 f 0.2eV is followed by structure B at 545.9 f 0.2eV, while the most intense structure formed by a shoulder, C, and a main line, D, are found at 547.4 f 0.2eV and 552 f 0.1 eV respectively. In the high-BE spectral region two broad structures are observed at 572 f 1 (E) and 598 f 1 eV (F).
._.li..“_._
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I
’
I
’
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’
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I
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Vol. 96, No. 3
SPECTRA OF NiO
FWHM=O.9eV -
2 5
I
533
I
I
I
531
I
529
I
I
I
527
I
525
Binding Energy (eV)
610
570
590
550
530
Bmdmg Energy (eV)
Fig. 1. (a) 0 1s XP spectrum; (b) high binding energy region of the 0 1s core line; the main peak centroid at 529 f 0.1 eV is indicated by a dashed line. To obtain information about the origin of these spectral lines a comparison with optical and electron energy loss data is performed. Figure 2 shows the 3.830eV reflectivity spectrum of NiO. For the region below 3.8 eV reflectivity data reported in the literature were used [13, 141. The real, cl(u), and imaginary, e2(w), part of the dielectric function, the effective electron number per unit formula, iV,n, and the energy loss function L(w) = -1m [l/e(w)] were calculated from the proper Kramers-Kronig (K-K) inversion equations [15]. Above 30 eV the reflectivity function was extrapolated on the basis of the trial formula R = R,(wJw)p; where w, and R, are the frequency and the reflectivity at the upper end of the experimental range, and p is an empirical parameter chosen to give a consistent value c2 = 0 in the low energy limit. ei(w) and E*(W)are reported in Fig. 3. Figure 4 shows Nen and the energy loss function L(w). The reflectivity spectrum (Fig. 2) can be approximately divided into two distinct spectral regions. The first region ranges from 3.8 to 1OeV while the
_.-
_.--
-.
.-
.
--.__
Vol. 96, No. 3
OPTICAL AND X-RAY PHOTOELECTRON
SPECTRA OF NiO
163
5 4 z% 3 2 1 0
0
5
10
15 Energy
20
25
30
0
5
CeV)
Fig. 2. NiO reflectivity spectrum.
10
15 20 Energy (ev)
25
30
Fig. 4. Effective electron number per unit formula, N,n, and the energy loss function L(w) = -1m [ 1/c(w)] in the range O-30eV.
second region ranges from 10 to 30eV. In the first region., the main structure, A’, at 4.2 eV is followed by a series of minor structures (B’-D’). In the second region, a shoulder, E’, at 12.5eV, followed by two broad peaks, F’ and G’, at 13SeV and 15.5eV are detected. The most intense peak, H’, is located at 18.3 eV. The optical spectrum of NiO has been widely discussed in the past 116-211. In the framework of Zaanen-Sawatzky-Allen phase diagram [ 1r] and accordingly to direct and inverse photoelectron spectroscopy studies [18], the structures in the 4-8 eV range are attributed to charge-transfer mechanisms. Peak A’ is assigned to an 0 2p -+ Ni 3d charge transfer excitation, as well as structure B’ at 5.8 eV. Similar features have been observed also in Ni dihalides [19]. The structures resulting at photon energies larger than 1OeV are assigned, in agreement with combined photoemission-inverse photoemission results [ 181, to 2p -+ 4~:and 2p -+ 4p transitions. As already observed [21], the latter transitions are made possible by the 2p3p hybridization in the valence band which allows to overcome the dipole selection rules. The energy of
these transitions, reported in Table 1, were obtained calculating the energy difference between the centroid of the 0 2p band in the photoemission spectrum and the energy of the peaks appearing in the BIS (Bremsstrahlung isochromatic spectroscopy) spectrum. The energy of the 2p -+ 4s transition was estimated to be M 12 eV energy, while the 2p + 4p transition energy is larger than 18 eV. More in general, to what concerns the upper and lower energy limits of the band-to-band optical excitations, there is a reasonable agreement between the present experimental data and the theoretical results by Huge1 and Carabatos [20, 211 as reported in Table 1. From the electron effective number function (Fig, 4) it can be observed that the value of N,n at 30eV is smaller than six. This indicates that the oscillator strength of the transition starting from 0 2p and Ni 3d levels is rather low. Therefore a large contribution is expected at higher energies. The data agree with former studies [13, 14, 211 though few
12 I----
Table I. Calculated and measured optical transition energies in the range O-30eV. AN energies are measured in eV. The labels A’ to H’ are referred to Fig. 2
l
-epsl eps2
Optical trans. Present study (ev)
4.0 4.8 5.9 (B’) 5.8 (C’) 7.0 (3d + 4s, 4~) 7.2 8.25 (D’) 8.2 12.8 (E’) 12.5 (2~ ---)4s) 13.6 (F) 13.5 (G’) 15.5 17.8 (H’) 18.3 (2~ --t 4~) (A’) 4.2 (2p + 3d)
Eaergy W) Fig. 3. Real, cl(w), and imaginary, Q(W), part of the dielectric function obtained from the reflectivity data by proper Kramer+Kriinig (K-K) inversion equations [15].
Theor. Optical Trans. [13] [21] (eV) (ev) 5.5 13.0 17.9 20.8
XPS-BIS [181 (eV) 4.3 13.5 17.3 -
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OPTICAL AND X-RAY PHOTOELECTRON
discrepancies are noticed in the relative intensity of some structures. As Powell and Spicer have already pointed out [13], they can be ascribed to the crystal surface order. The broad peak at about 23 eV observed in the loss function (Fig. 4) is assigned to a bulk plasmon on account of the fact that it occurs where ~2is minimum and cl crosses the abscissa axis (Fig. 3). Furthermore it is possible to compare the energy of this point with that observed from the usual formula E’ = ’ U2 where E. M 15 eV is the average (E;/QJ + Eo) energy of the 2p -+ 4s and 2p --) 4p transitions, J% x 15.5 eV is the plasma energy of the four electrons connected with these transitions, and coo w I is the value of the dielectric constant due to higher lying transitions, as deduced from Fig. 3. From this, one calculates E’ M 21 eV in good agreement with the experimental results. A comparison between the optical and the photoemission results may help to identify the satellite lines on the higher BE side of the 0 1s core line in the XPS spectra of NiO and to give a more consistent picture of the multi-electron excitations. As a matter of fact, line D in the XPS spectrum [Fig. l(b)] is separated from the 0 Is main line by z 23 eV which is close to the energy of the first order plasmon evaluated from the optical data and reported in Fig. 4. Being line D in the XPS spectrum assigned to the first order plasmon loss, the surface plasmon is expected at an energy Esp = Ear/d2 = 16.3 eV which is close to line B (16.8eV) in the XPS spectrum. Second and third order plasmon losses expected at 46 eV and 69eV corresponds to line E and F of the XPS spectrum. So far shoulder C remains unidentified. Peak A observed at 535eV in the XPS spectra [Fig. l(b)] is assigned on the basis of the optical studies, to a shake-up transition involving 0 2p electrons. This line originates from the loss of kinetic energy of the photoelectrons ejected from the 0 1s core level to excite 0 2p electrons from the valence band levels to low-lying levels in the conduction band. The energy separation between the 0 1s main line in the XP spectrum and the A peak is 4-6eV, which corresponds to the 0 2p + Ni 3d optical transitions in the reflectivity spectrum. The XPS plasmon loss spectra can be compared also with electron energy loss (EEL) data [22]. Table 2 reports such a comparison. The D-E lines of the XPS spectra have a counterpart in the EEL spectra within an uncertainty of f 2 eV. On the contrary, a larger discrepancy is found between the F feature and the EEL results. The assignment of peak D is confirmed by recent EEL measurements [23] where a plasmon
SPECTRA OF NiO
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Table 2. Measured XPS and EELSplasmon excitations in the range 0-60eV. All energies are reported in eV. The labels B to F are referred to Fig. l(b) XPS satellite lines (ev> (A) 5.0
Shake-up
(B) 16.8 (C) 18.3 (D) 22.9 -
Surface plasmon Bulk plasmon
(E) 42.0
Bulk plasmon
&) 68.0
Bulk plasmon
EELS [22] (eV) 13.8 Bulk plasmon 18.8 Bulk plasmon 23.5 Bulk plasmon 28.5 Bulk plasmon 37.0 Bulk plasmon 44.0 Bulk plasmon 54.0 Bulk plasmon -
peak is reported at 2 1.5 eV. Line C in the XPS spectra could be identified with a bulk plasmon line found by EEL at 18.8 eV [22]. The good quality of NiO single crystal influences the interband electronic excitations in the lower energy region where a fair agreement between the energy-loss XP-derived data and EELS data is found. Indeed, the lack of contaminations, impurities, and the reduced density of defects [24] allows one to detect well-defined structures which otherwise would appear smoothed out or hidden below the signal noise level. In conclusion, for the first time an experimental study of multi-electron excitations in the 0 1s XPS spectrum of NiO single crystal is reported. It is shown that this region is dominated by plasmon induced energy loss lines, whereas lines associated to optical transitions were detected, with a rather low intensity, about 4.2eV from the 0 Is core line. REFERENCES 1.
2. 3. 4. 5. 6. 7. 8.
S. Hiifner, Unfilled Inner Shells: Transition Metals and Cornpow&. In Photoemission in Solti IICase Studies (Edited by M. Cardona and L. Lcy), Chapter 3. Springer-Verlag, Berlin (1979). A. Fujimori & F. Minami, Phys. Rev. B30,957 (1984). J. Zaanen, C. Westra & G.A. Sawatzky, Phys. Rev. B33, 8060 (1986). G. Lee&S.-J. Oh, Phys. Rev. B43,14674 (1991). K. Okada 8c A. Kotani, J. Phys. Sot. Jap. 60, 772 (1991). M.A. van Veenendaal & G.A. Sawatzky, Phys. Rev. Lett. 70,2459 (1993). J.E. Inglefield, J. Phys. C: Solid State Phys. 16, 403 (1983). M. Cini & A. d’Andrea, J. Phys. C: Solid State Phys. 21, 193 (1988).
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H. Kuhlenbeck, G. Odorfer, R. Jaeger, G. Illing, M. Menges, Th. Mull, H.-J. Freud, M. Pohlthen, V. Staemmler, S. Witzel, C. Scharfschwerdt, K. Wennemann, T. Liedtke & M. Neumann, Phys. Rev. B43, 1969 (1991) (and references therein). 10. See, e.g. B.K. Agarwal, X-ray Spectroscopy. An Introduction. Second Edition, Chapter 5. Springer Verlag, Berlin, Heidelberg. 11. In the literature several 0 Is XPS spectra report rather intense features beside the main line. High quality NiO single crystals should exhibit only one symmetric line since the crystallographic oxygen sites are equivalent in NiO. Only in such NiO samples it is possible to study properly shake-up and plasmon loss satellites. 12. P. Camagni, G. Samoggia, L. Sangaletti, F. Parmigiani & N. Zema, Phys. Rev. B50,4292 (1994). 13. R.J. Powell & W.E. Spicer, Phys. Rev. B2,2182 (1970). 14. S. Suga, K. Inoue, M. Tan&u&i, S. Shin, M. Seki, K. Sato & T. Teranishi, J. Phys. Sot. Jap. 52, 1848 (1983). 9.
15. 16. 17. 18. 19. 20. 2 1. 22. 23. 24.
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F. Stem, Solid State Phys. 15, 299 (1963). J.W. Allen, Optical Properties of Magnetic Oxides in Magnetic Oxides (Edited by D.J. Craik). Wiley, New York (1975). J. Zaanen, G.A. Sawatzky & J.W. Allen, Phys. Rev. L&t. 55,418 (1985). G.A. Sawatzky & J.W. Allen, Phys. Rev. Lett. 53, 2339 (1984). G. Guizzetti, L. Nosenzo, I. Pollini, E. Reguzzoni, G. Samoggia & G. Spinolo, Phys. Rev. B14,4622 (1976). J. Huge1 & C. Carabatos, J. Phys. C: Solid State Phys. 16, 6713 (1983). J. Huge1 8~C. Carabatos, J. Phys. C: Solid State Phys. 16, 6723 (1983). Y. Sakisaka, K. Akimoto, M. Nishijima & M. On&i, Solid State Commun. 24, 105 (1977). A. Gorschltiter & H. Merz, Phys. Rev. B49, 17293 (1994). Nickel oxide can easily take up excess oxygen, forming nickel vacancies in the lattice. See, e.g., N. Tsuda, K. Nasu, A. Yanase & K. Siratori, Electronic Conduction in Oxides, Chapter 4. Springer Verlag, Berlin (1991).