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surface science ELSEVIER
Surface Science 367 (1996) 196-202
An XPS study of thin NiO films deposited on MgO(100) J . M . S a n z , G . T . T y u l i e v .,1 Departamento de Fisica Aplicada C-XH, UniversidadAutrnoma de Madrid, E-28049, Madrid, Spain
Received 22 December 1995; accepted for publication 20 May 1996
Abstract Auger and core-level photoemission from thin NiO films deposited on an MgO(100) substrate are studied by XAES and XPS, respectively. A detailed analysis of the Ni 2p3/2 main line and Ni L 3VV for submonolayer coverages allows us to give a new assignement to the different spectral features. In terms of a cluster model, the high binding-energy component of the Ni 2p3/2 main line is assigned to the c3dgL(here Lstands for the O 2p ligand hole) final state configuration, and the low binding-energy one to the c3d93d, where 3d denotes a hole localized in the Ni 3d orbitals. The intensities of higher- and lower-energy components of the Ni 2p3/2 main line depend on the number of ions (oxygen anions and nickel cations) surrounding emitting nickel rather on their chemical state. Non-local effects are also observed in the Ni LVV Auger spectra. At low coverages (less than 0.5 ML), a single line at 841 eV kinetic energy is observed. The higher-energy component of Ni LVV spectra at 846 eV is related to the presence of Ni cations as next-nearestest neighbors. Keywords: Low index single crystal surfaces; Nickel oxides; Photoelectron emission; X-ray photoelectron spectroscopy
I. Introduction The preparation and characterization of twodimentional materials like thin oxide films are topics of great interest b o t h experimentally and theoretically [ 1 - 3 ] . In the present w o r k we focus our attention on the non-local effects observed in the p h o t o and Auger emission of very thin N i O films. The effect of next-nearest neighbour on the core-level photoemission of nickel oxide was discussed for the first time by O k u et al. I-4]. The main line of Ni 2p3/2 shows characteristic splitting for x > 0 . 5 in the NixMg~_xO mixed oxides, and the high binding-energy c o m p o n e n t was assigned * Corresponding author. On sabbatical leave from the Institute of General and Inorganic Chemistry, Bulgarian Academy of Sciences, 1113-Sofia, Bulgaria. E-mail:
[email protected]
to the presence of nickel cations as next-nearest neighbours. This double peaked structure in the Ni 2p3/2 main line has been discussed by m a n y authors I-5-10]. Different assignments have been given to the high binding-energy component, including that of satellites caused by multielectron excitations [ 5 ] or multiplet splitting [ 6 ] , the presence of defects like O - and Ni 3+ [ 7 - 9 ] , and nonlocal screening effects in photoemission [ 10]. O u r experiments with thin films of NixMg~_xO obtained by c o e v a p o r a t i o n of Ni and M g in an oxygen atmosphere indicated that the changes are m o r e complex and are not restricted to the simple appearance of a high binding-energy shoulder. The width of the satellite and the main lines in Ni 2p3/2 increase with x, and thus the two spectral groups become less resolvable. At this point it is worthwhile noting, that for a proper description
0039-6028/96/$15.00 Copyright © 1996 Elsevier Science B.V. All fights reserved Pll S0039-6028 (96) 00818-7
J.M. Sanz, G.T. Tyuliev/Surface Science 367 (1996) 196-202
of the changes in the core-level photoemission, with increasing x values in NixMgl_xO, it is important to use a reliable reference line for energy calibration. Below, we use the Mg 2p line as a reference, centered at 50.5 eV binding energy, as suggested in Ref. [ 11 ]. The present communication aims to follow with higher resolution changes in the Ni 2p and Ni LVV spectra when Mg 2+ cations are gradually replaced by Ni z+ cations as nextnearest neighbours of the Ni ion from which the photo and X-ray excited Auger emissions are detected. Bearing in mind the specific nature of XPS experiments, we obtain different next-nearest neighbour coordinations by depositing very thin NiO films on an MgO(100) substrate. In this way, the effects of clustering into Ni- and Mg-rich regions, characteristic for NixMgx_~O mixed oxides, can be avoided.
2. Experimental The XPS measurements were performed in the analysis chamber of the electron spectrometer LHS10 (Leybold-Heraeus) with a base pressure of l x 1 0 - 8 Pa. The spectra excited with MgKQt radiation had an instrumental resolution of 1 eV as determined by the F W H M of the Ag 3d5/2 photoelectron line. MgO(100) single crystals (10 mm x 10 mm x 1 mm in size), obtained from Mateck were mounted on a Ta foil. Before inserting into the vacuum, MgO(100) samples were rinsed in trichloroethylene, acetone and finally in boiling methanol. Heating at 1000 K in vacuum for 1 h was not enough to remove all the carbon from the surface, so a slight bombardment with 3 k e y Ar + ions (3-5 #A beam current) was applied. Finally, the MgO(100) samples were heated in 5 x 10 -s Pa 02 at 900 K for 20 min and then flashed at 1200 K for 1 rain in vacuum. NiO films were deposited by evaporation of nickel and simultaneous admission of oxygen in the preparation chamber. In order to enhance the local pressure on the sample surface during NiO film growth, the oxygen was introduced through metal tube, 35 cm long and 1 cm in diameter, directed to the sample [ 12]. The deposition rate was varied between 0.056
197
and 0.73 A min -1. In order to follow in more detail the spectral changes for submonolayer coverages, a lower deposition rate was used. The oxygen pressure during deposition was 5 x 10 -s Pa. The uptake curves can be fitted by formulae of a type I ~ (NiO) ( 1 - exp ( - d/2)) and I ~ ( M g O ) exp (-d/2), where d is the film thickness and X the inelastic mean free path in NiO film. P°(NiO) and I°~(MgO) are the intensities for the bulk samples of NiO and MgO respectively. An example of this type of fit is shown in Fig. 1. In order to compare spectra acquired at different deposition rates, film thickness in monolayers was used as a parameter instead of deposition time. The thickness was evaluated using the I(Ni 2p)/I(Mg 2p) intensity ratio with Z values of 14.4 and 6.6 A for Mg 2p (Ek = 1200 eV) and Ni 2p (Ek= 390 eV) lines, respectively. The latter values were calculated according to the method of Penn [13] for Ni metal and ,-~10% was added as a correction for oxides [14]. To convert film thickness into monolayers, a value of 1 M L = 2 . 0 8 8 A,
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0 '100 2;0 3;0 4;0 a()0 600 Deposition Time, (min) Fig. 1. Variation of the substrate (Mg2p) and the film (Ni 2pa/2) photoelectron intensities with the time of deposition. The solid curves show fits of the experimental data using an exponential growth and decay functions.
J.M. Sanz, G.T. Tyuliev/Surface Science 367 (1996) 196-202
198
equal to the half of lattice constant for bulk NiO, was used [ 1].
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3. Results Fig. 2 summarizes several experiments performed with different deposition rates. As one can see from Fig. 2, there is a good correlation between the NiO film thickness and the F W H M of the Ni 2P3/2 main line (evaluated after linear background subtraction in the 854-860eV energy range, Fig. 4), and this dependence justifies the use of film thickness as a reproducible parameter. Fig. 3 shows a comparison of the Ni 2p spectra for NiO films with different thicknesses. As can be seen, it is difficult to give a simple description of the changes with film thickness. The extra peakbroadening assigned to new peak features are observed not only in the low binding-energy side of the main lines, but also in the satellite regions. Below, we will focus our attention on the Ni 2p3/2 (main line) and Ni LVV Auger transitions. Figs. 4 and 5 show these spectra for different NiO film
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Fig. 2. FWHM of the Ni 2p3/z main and Mg 2p photoelectron lines versus film thickness for NiO grown at deposition rates between 0.027 and 0.35 ML rain-~.
thicknesses. For coverages < 1 ML the Ni 2p3/2 main line consists of one broad ( F W H M ~2.5-2.8eV) line centered at 857.6eV. When going to higher coverages, an extra feature appears on the low binding-energy side, and its position
J.M. Sanz, G.T. Tyuliev/Surface Science 367 (1996) 196-202
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for bulk NiO is 855.8 eV. A similar dependence on film thickness is observed for the Ni LVV Auger line (see Fig. 5). For thickness less than 0.5 ML, a single line at 841 eV kinetic energy is observed, and with increasing film thickness a second spectral feature at 846 eV appears. The O ls photoelectron spectra for NiO films with different thicknesses are shown in Fig. 6. Despite the fact that there is an overlap of the O ls signal from the NiO film with that from the MgO substrate, the spectral features can be summarized as follows. The O ls photoelectron line for clean MgO(100) substrate consists of a single peak at 531.2 eV, and we assign it to an 0 2- lattice oxygen. If the hot MgO sample is cooled in oxygen, a high binding-energy shoulder appears at 533.3 eV, and this is assigned to the emission from an adsorbed oxygen. This peak disappears after annealing of the MgO sample in vacuo at 1200 K. Increasing the NiO film thickness (room-temperature deposition), the main 0 2- peak preserves its position, but a high binding-energy shoulder (we call it here "excess oxygen") centered at 532.8 eV appears. As one can see from the peak decompositions in Fig. 6, the high binding-energy intensity increases with film thickness. An estimation for the
amount of excess oxygen in the NiO film evaluated by the subtraction of the O ls signal coming from the MgO substrate is presented in Fig. 7. A value of about 35% is reached after the deposition of the third NiO layer. It is worth noting that the O ls line consists of only one symmetrical peak for films 100
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200
J.M. Sanz, G.T. Tyuliev/Surface Science 367 (1996) 196-202
deposited at substrate temperatures higher than 600 K. At this point it is interesting to note the shift of the peaks in the course of NiO film growth. Due to the wide band gap of MgO it is not possible to make it conducting by doping, so the positive charge accumulated on the surface is around 4-5 eV. The energy shift has linear dependence up to 3 ML of NiO, after which a change in the slope is observed (Fig. 8). Comparing with Fig. 2 we note that the change in the slope for F W H M dependence is also observed around this thickness value. The dependence in Fig. 8 is in the opposite direction compared to that reported for the deposition of MgO on NiO/Mo(100) [1].
4. Discussion It was shown recently [12,15], that NiO grows on an MgO(100) substrate in a layer-by-layer mode. Bearing this in mind, the Ni 2p photoemission at submonolayer coverages can be considered as coming mainly from Ni 2÷ ions coordinated with oxygen anions and having ions with no occupied
valence levels close to the O 2p band (in this case Mg 2÷ from the MgO(100) substrate) as nextnearest neighbors. At these coverages, only one broad peak in the Ni 2p3/2 main line is observed, at an energy of 857.6 eV (see Figs. 3 and 4). This peak is assigned to a c3d9Lfinal-state configuration, where L stands for a hole in the O 2p band. It is worth noting that similar spectra were observed for Ni 2+ ions well dispersed in a TiO2 matrix [ 16]. At higher coverages, a second feature on the low binding-energy side of the Ni 2p3/2 main line appears, and we ascribe it as being due to the presence of Ni cations as next-nearest neighbors of the nickel ion from which the photoemission is detected. An attempt to assign this emission to a single peak (as shown in Fig. 9) failed, because the PCA (principal component analysis) of 26 spectra representing film thicknesses from 0.15 to 8 ML gives between 3 and 6 for the number of factors (depending on the criteria). Therefore we assign this low binding-energy feature to the c3d9-3d finalstate configuration, where 3d stands for a hole in the Ni 3d orbitals, localized in the next-nearest neighbor nickel cations. This new assignment is NiO on MgO(100) 17 ML
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J.M. Sanz, G.T. Tyuliev/Surface Science 367 (1996) 196-202
consistent with the peak decomposition used in a previous work [ 17]. The high binding-energy component of the Ni 2p3/2 main line has an F W H M of 2.5 eV, while the value for the low bindingenergy line is 1 eV, and this can be explained by the O 2p (band width of 4-5eV) and Ni 3d (the band width here is less than 1 eV) origin of the corresponding configurations. Both, c3d9L and c3d9-3d final-state configurations contribute to the intensity of Ni 2p3/2 (mail line). Their intensities increase with the number of neighbouring oxygen and nickel ions and this explains the "main line to satellite" ratio dependance on the NiO film thickness presented in Fig. 10. In fact, the role of NiO particle size on the Ni 2p3/2 main line spectral features was studied previously by Espinrs et al. [ 18]. These authors assign the low binding-energy component as being inherent to big NiO particles. Using the new peak assignment, we can give an explanation of the annealing effects on the photoemission. The left part of Fig. 11 represents O ls and Ni 2p3/2 main line spectra for 8 ML NiO films deposited at 300 K. After annealing, the high binding-energy contribution to the O ls line decreases from 35 to 3%, while the corresponding NiO on MgO(100)
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decrease for the Ni 2p3/2 main line is considerably smaller. This fact is not consistent with the description proposed in Ref. [9], according to which the high binding-energy peak is entirely attributed to the presence of oxygen holes in the ground state, i.e. 3d9L configurations. Instead of this, we can simply connect the intensities of the components discussed with the number of neighbouring oxygen and nickel ions. Indeed, the oxygen-to-nickel ratio decreases after annealing by ~ 15%, which corresponds to the changes in intensity ratio of the high and low binding-energy components of the Ni 2p3/z main line. Further support for this picture can be found in experiments with buried NiO layers. Fig. 12 shows changes in the Ni 2p3/2 photoemission after covering 17 ML of NiO/MgO(100) with 14 ML of MgO film. There is a slight transfer of intensity from the low to high binding-energy line, and this can be explained by the increased oxygen coordination of nickel cations after covering the NiO with MgO film. It is worth noting, that changes in the Ni 2p3/2 spectrum similar to that shown for buried NiO layers were reported for NiO covered with ZrO2 films [19]. The nextnearest neighbor contribution to the core-level photoemission depends on the degree of overlapping between c3d 9 and 3d 8 states, while the nonlocality is a result of delocalization in 3d levels.
J.M. Sanz, G.T. Tyuliev/Surface Science 367 (1996) 196-202
202
14 ML MgO / 17 ML NiO / MgO(100)
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highest occupied levels in NiO appear to have Ni3d character, which is in favor of a Mott-Hubbard instead of charge-transfer type for the insulating band gap.
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Such delocalization can be a result of the strong hybridization between Ni 3d and O 2p states in NiO. It is interesting to mention here the spectra of Ni(OH)2 reported by Kim and Winograd [6], where a single peak for the Ni 2p3/2 main line is observed.
5. Summary The non-local effects in Ni 2p photo and X-ray excited Ni LVV Auger emission for thin NiO layers deposited on an MgO(100) substrate are presented. A new peak assignment for the Ni 2p3/2 main line is given. The peak at 857.6 eV binding-energy is related with the c3d9L final-state and that at 855.8 eV with the c3d93d one. The intensity ratio between these two components depends on the stoichiometry of the deposited NiO films. According to the picture proposed above, the
The authors thank Dr. A.R. Gonz~iles-Elipe for helpful discussions and Mr. J.-A. Rodriguez for technical assistance. Financial support from the DGICYT of Spain is gratefully acknowledged.
References [1] M.L. Burke and D.W. Goodman, Surf. Sci. 311 (1994) 17. [2] M.-C. Wu, C.M. Truong and D.W. Goodman, J.Phys. Chem. 97 (1993) 4182. [3] M.D. Towler, N.M. Harrison and M.I. McCarthy, Phys. Rev. B 52 (1995) 5375. [4] M. Oku and K. Hirokawa, J. Electron Spectrosc. Relat. Phenom. 10 (1977) 103. [5] S. Hiifner and G. Wertheim, Phys. Rev. B 8 (1973) 4857. [6] K.S. Kim and N. Winograd, Surf. Sci.43 (1974) 625. [7] M.W. Roberts and R.St.C. Smart, J. Chem. Soc. Faraday Trans. 1 80 (1984) 2957. [8] M. Tomellini, J. Chem. Soc. Faraday Trans. 1 84 (1988) 3501. [9] M. Oku, H. Tokuda and K. Hirokawa, J. Electron Spectrosc. Relat. Phenom. 53 (1991)201. [10] M.A. van Veenendaal and G.A. Sawatzky, Phys. Rev. Lett. 70 (1993) 2459. [ 11 ] J.S. Corneille, J.-W. He, D.W. Goodman, Surf. Sci. 306 (1994) 269. [12] S.D. Peacor and T. Himba, Surf. Sci. 301 (1994) 11. [13] D.R. Penn, J. Electron Spectrosc. Relat. Phenom. 9 (1976) 29. [14] S. Hofmann, Surf. Interface Anal. 9 (1986) 3. [15] D.M. Lind, S.D. Berry, G. Chern, H. Mathias, L.R. Testardi, Phys. Rev. B 45 (1992) 1838. [16] J.P. Espin6s, A.R. Gonz~ilez-Elipe and G. Munuera, Solid State Ionics 63-65 (1993) 748. [17] G. Tyuliev, P. Stefanov and M. Atanasov, J. Electron Spectrosc. Relat. Phenom. 63 (1993) 267. [18] J.P. Espin6s, A.R. Gonz~ilez-Elipe, A. Fernandez and G. Munuera, Surf. Interface Anal. 19 (1992) 508. [19] J.-M. Mariot, S. Hareland and C.F. Hague, Appl. Surf. Sci. 65/66 (1993) 337.