Core and Valence Band Photoemission Study of Highly Strained

Feb 10, 2007 - Stefano Agnoli, Andrea Barolo, Paola Finetti, Francesco Sedona, Mauro Sambi, and. Gaetano Granozzi*. Dipartimento di Scienze Chimiche ...
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J. Phys. Chem. C 2007, 111, 3736-3743

Core and Valence Band Photoemission Study of Highly Strained Ultrathin NiO Films on Pd(100) Stefano Agnoli, Andrea Barolo, Paola Finetti, Francesco Sedona, Mauro Sambi, and Gaetano Granozzi* Dipartimento di Scienze Chimiche and Unita` di Ricerca INFM-CNR, UniVersita` di PadoVa, Via Marzolo, I-35131 PadoVa, Italy ReceiVed: October 30, 2006; In Final Form: December 21, 2006

We discuss the results of an extensive investigation on NiO ultrathin films grown on Pd(100), carried out by means of high-resolution X-ray photoelectron spectroscopy (core levels) and ultraviolet photoelectron spectroscopy (valence band levels). The peculiarity of the investigated system is the rather high lattice mismatch between NiO and the Pd substrate (7.3%). Detailed information on the electronic structure of NiO ultrathin films is obtained as a function of thickness, with a particular emphasis put on the fully wetting single c(4 × 2)-Ni3O4 defective monolayer, up to the bulklike relaxed films, through the intermediate three-dimensional strained islands resulting from a Stranski-Krastanov growth behavior.

1. Introduction Nickel oxide is a material of great technological relevance in many different fields (e.g., catalysis,1,2 sensors,3,4,5 magnetoelectronics, and spintronics6,7,8) because it associates a high structural and chemical stability, which makes it a good candidate in almost every environmental condition, to quite peculiar electronic and magnetic properties. NiO is a strongly correlated charge-transfer insulator9 and the description of its electronic structure also is interesting from a fundamental point of view because long controversial debate concerning its description is still far from being concluded.10 There also are intriguing possibilities based on electronic structure modifications when this material is in the ultrathin film form: both the strain induced by an epitaxial growth over a mismatched substrate and the reduction of Madelung potential at the interface with a metal substrate are potentially important and useful variables to induce specific novel properties. In recent years, the growth of NiO on Ag(100) has been thoroughly studied and the NiO/Ag(100) interface has assumed the status of a reference system for a well matched ultrathin film.11,12,13,14,15 Although the investigation of NiO ultrathin films grown over a metal substrate with some degree of mismatch can shed some light on interesting questions concerning the interplay between structural and electronic properties, we recently have focused our attention toward NiO ultrathin films on Pd(100), which represent an example of a mismatched epitaxial system. We have chosen Pd(100), which differs from Ag(100) both for the significantly larger lattice mismatch (mPd ) +7.3%, mAg ) +2.2%) and for the different nature of its valence band (VB) near the Fermi level (Ef), which predominantly has a 4d character, as compared to the 5sp character of the silver VB. This latter point is important to assess the role and relevance of the hybridization of overlayer and metal states at the interface in determining the properties and the reactivity of the system. Hybridization has been proven to be of limited importance for the NiO/Ag(100) case, as shown by theoretical investigations,16 * Corresponding author. E-mail: [email protected].

but it is expected to be more relevant in the present case, due to the d character of the substrate VB. We have already reported on the effect of the lattice mismatch on the structure and morphology of the NiO/Pd(100) ultrathin films with particular attention toward innovative structures and properties.17,18,19,20,21 In the monolayer (ML) coverage range, a novel defective c(4 × 2)- Ni3O4 structure has been thoroughly investigated by both static and dynamic low-energy electron diffraction (LEED)20 and scanning tunneling microscopy (STM).21 The model of this peculiar ordered defective structure (see Figure 1) has been also validated by theoretical ab initio calculations and an explanation for its kinetic stabilization has been proposed.21 The progressive relaxation of the NiO ultrathin films toward the bulklike structure has been already investigated by STM,18 spot profile low-energy electron diffraction (SPALEED)19 and high-resolution electron energy loss spectroscopy (HREELS).18 The growth is formally of the Stranski-Krastanov type with a fully wetting two-dimensional (2D) interfacial ML followed by the nucleation of three-dimensional (3D) islands, even if the 2D c(4 × 2)-Ni3O4 phase has a different stoichiometry with respect to subsequent layers, which are shown to be stoichiometric NiO starting from the second atomic layer, as evidenced by STM22 and X-ray photoelectron spectroscopy (XPS).17 In the early stages of growth, the first two/three NiO layers in the 3D islands are compressively strained in-plane with a concomitant interlayer distance expansion, as evidenced by the combination of X-ray photoelectron diffraction (XPD) and LEED measurements.17 The onset of strain relaxation is rather abrupt at a coverage around 3-5 MLE17 (see Experimental for definition of MLE) and coincides with the onset of islands coalescence.19 However, full strain relaxation is accomplished only gradually up to 10-12 MLE, where bulklike NiO is observed.17,19 Because of the high lattice mismatch, small angle mosaics at the onset of strain relaxation also were observed both by SPA-LEED and STM.18,19 In the present paper, we report the results of an extensive investigation carried out by using high-resolution X-ray photoelectron spectroscopy (HR-XPS) from core levels and ultraviolet photoelectron spectroscopy (UPS) (using either

10.1021/jp0671244 CCC: $37.00 © 2007 American Chemical Society Published on Web 02/10/2007

Study of Ultrathin NiO Films on Pd(100)

J. Phys. Chem. C, Vol. 111, No. 9, 2007 3737 Ni3O4 ML is obtained with a coverage of 0.75 MLE. All the films have been subjected to a postannealing treatment at 573 K in an oxygen atmosphere at 5 × 10-7 mbar O2 (for 3 min). 3. Results and Discussion

Figure 1. Schematic drawing of the c(4 × 2)-Ni3O4/Pd(100) defective monolayer. The rhombic unit cell is outlined by black lines. Color codes: white ) Pd, red/light gray ) O, blue/dark gray ) Ni.

standard sources or synchrotron radiation) from VB levels to obtain detailed information on the electronic structure of NiO ultrathin films, starting from the defective c(4 × 2) monolayer up to the bulklike relaxed films through the intermediate strained films. 2. Experimental The experiments reported in this paper have been performed in different ultrahigh vacuum (UHV) systems, either in our home laboratory or at the ELETTRA synchrotron radiation facility (Trieste, Italy). The main UHV chamber used to standardize the preparative procedures and to carry out some photoemission experiments (XPS and He(I) and He(II) measurements) is a modified VG ESCALAB MK II (Vacuum generators, Hastings, England) equipment where a four grids rear view LEED, an electron beam evaporator with an integrated flux monitor, a quartz microbalance, a mass quadrupole, a twin (Mg/Al) anode X-ray source, a discharge lamp for noble gas ionization, a cold cathode sputter gun, and a hemispherical electrostatic analyzer ending with a five channeltrons detector are incorporated. The angular acceptance of the analyzer can be varied between 1.5° and 8° (the latter used for UPS experiments). The binding energy (BE) calibration was determined using the Fermi edge (Ef) and 4f peaks of a gold sample. Further photoemission experiments were performed at different beamlines at ELETTRA: UPS was done at the POLAR beamline, while HR-XPS was done at the SUPERESCA beamline. For a detailed description of the two beamlines we address the reader to the pertinent information sources.23 Nickel oxide overlayers were prepared by reactive evaporation of Ni metal in 2 × 10-6 mbar O2 background at room temperature (RT). Typical evaporation rates were 0.5-1.5 MLE/ min (1 monolayer equivalent, MLE, is referred to as the atom density of the Pd(100) surface and corresponds to 1.3 × 1015 Ni atoms/cm2) as measured with a quartz microbalance. All the reported experimental data are labeled referring to such experimental coverage unit. According to this, a complete c(4 × 2)-

In the following, we will first discuss the core and VB photoemission data of the defective c(4 × 2)-Ni3O4 ML phase, and subsequently we will examine the data of the NiO/Pd(100) ultrathin films at different coverages. 3.1. c(4 × 2)-Ni3O4 ML. 3.1.1. Core LeVel Photoemission. Let us start with the core level HR-XPS data (Figure 2) of the defective c(4 × 2)-Ni3O4 phase taken at the SUPERESCAbeamline. They will allow us to put forward some preliminary considerations on its electronic structure and to confirm the defective stoichiometry. Figure 2a reports the Pd 3d lines (normal emission, hν ) 450 eV) both for the clean substrate and for the ML phase. In the case of the clean metal (Figure 2a, bottom), it is possible to clearly detect the presence of a small shoulder on the low-energy side on both 3d components, indicative of a surface shift. The splitting between the surface and bulk components (0.44 eV) obtained after deconvolution of the experimental peak shape is in good agreement with the corresponding literature data.24 After the deposition of the c(4 × 2) ML, the surface component disappears (as evidenced by the reduction of the full width at half-maximum (fwhm) of the Pd 3d5/2 peak from 1.1 to 0.8 eV) and even after a deconvolution procedure it is hard to detect a possible second component under the experimental envelope. Therefore, according to the present data, it is not possible to either confirm or refuse the theoretical picture emerging from ab initio calculations,25 where the topmost Pd atoms are charge depleted because they are involved in a significant charge transfer to the oxygen atoms of the c(4 × 2) phase, which on their own are electron deficient because they share the holes induced by the systematic presence of the Ni vacancies. Figure 2b reports the Pd 3p3/2 and O 1s core levels (normal emission, hν ) 650 eV). In the case of the c(4 × 2) phase, the two lines are well resolved: Pd 3p3/2 is centered at 531.9 eV while O 1s is at 529.3 eV. As it can be seen from the comparison with the corresponding clean substrate spectrum, no significant change occurs on the Pd 3p3/2 line. On the other hand, the O 1s peak exhibits a very narrow fwhm, which is compatible with the presence of just one component. This value is 0.7 eV lower than in bulklike NiO (see section 3.2.1) and is very near to the value observed for the O chemisorption phases on Pd(100).26 The possibility to resolve the O 1s from the Pd 3p peak (Figure 2b) proves to be crucial for an accurate determination of the stoichiometry of the overlayer phase. This task is hampered with standard X-ray photon sources because the two peaks are strongly overlapping (see for example the data reported in section 3.2.1). By fitting the experimental peak envelope with a two-component function and subtracting a Shirley background (see inset of Figure 2b), one can obtain a rather precise evaluation of the integral intensity of the O 1s peak. Renormalizing this signal, as well as the one obtained directly for Ni 2p (Figure 2c), for the photoionisation cross sections and photon intensity, one can obtain a very accurate determination of the Ni/O ratio, which resulted to be 0.7 ( 0.1. It must be noted that with the oxide phase perfectly wetting and with Ni and O atoms almost coplanar the error in the stoichiometry determination deriving from neglecting attenuation length or photoelectron diffraction effects is expected to be negligible. In addition, having used almost the same energy of the outgoing electrons also errors due to instrumental factors, such as the

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Figure 2. HR-XPS data of the c(4 × 2)-Ni3O4/Pd(100) monolayer and of the clean Pd(100) substrate: (a) Pd 3d, (b) Pd 3p3/2 -O 1s, and (c) Ni 2p data. For comparison purposes, in (c) we also report photoemission data taken on a bulklike metallic Ni film (14 MLE of Ni/Pd(100), hν ) 950 eV) and on a bulklike NiO film (20 MLE of NiO/Pd(100), Al KR radiation). The inset in (b) presents a two peak deconvolution of the experimental data.

spectrometer transmittance, are minimized. The stoichiometry determination obtained by this procedure is in very good agreement with the expected value (Ni/O ratio ) 0.75) coming from our model of a Ni-defective Ni3O4(100) surface, derived on the basis of LEED-I/V, STM, and theory.20,21 Figure 2c shows the Ni 2p line (normal emission, hν ) 950 eV) for the ML phase together with the spectrum of the bulklike Ni metal (here obtained as a 14 MLE Ni/Pd(100), hν ) 950 eV) and the spectrum of a NiO bulklike film (20 MLE of NiO/ Pd(100), obtained using a standard Al KR radiation source). The spectrum of the c(4 × 2) ML shows remarkable differences with respect to the metal (i.e., it is shifted to higher BE and the fwhms of both 2p components are higher (i.e., fwhmc(4×2) ) 2.4 eV vs fwhmNi ) 1.7 eV). On the other hand the main components of the spectrum of the NiO bulklike sample are broader and shifted toward higher BE (this is not only a consequence of the used photon source because with Al KR fwhmNiO ) 4.4 eV vs fwhmc(4×2)-NiO ) 3.7 eV; see more data in section 3.2.1). In particular, the shoulder at 1.2 eV from the 2p3/2 peak maximum, which is a typical fingerprint of bulklike NiO, is missing in the spectrum of the c(4 × 2) structure. The Ni 2p BE values of the main components of the defective ML are intermediate between the values for the metallic and of the completely oxidized state (e.g., for Ni 2p3/2: BENiO ) 854.4 eV > BEc(4×2) ) 853.4 eV > BEMetal ) 852.2 eV). Also, the structure of 2p hole-3d satellites is changed in the c(4 × 2) ML with respect to both metallic Ni and bulklike NiO. In the case of the c(4 × 2) phase, the Ni 2p3/2 satellite is shifted to higher BE, and it is structured in two main very low-intensity features, one centered at about 863.2 eV (∆E ) 9.8 eV) from the main peak and the other at 860.8 eV (∆E ) 7.4 eV). In the metallic Ni, the satellite is only 6 eV apart from the main component, but the intensity ratio between the main peak and its satellite is quite similar to the defective ML phase. On the other hand, the satellite structure of the NiO bulklike sample is much more complex and intense and lays at about 7 eV from the main peak. A detailed interpretation of the Ni 2p region would require a complex theoretical treatment, which is beyond the scope of the present paper. We recall here that the interpretation of the Ni 2p XPS data has been the object of an extensive debate in the literature. As an example, the reader can refer to a very recent paper in which the whole matter has been reexamined

and the literature therein reported.27 Actually, several different approaches have been put forward to describe the p shell photoemission spectra of transition metal (TM) oxides, and they could be roughly divided into two main groups. Some authors take into account the many-body effects bringing into play a charge transfer from the ligand to the TM, but neglecting the atomic many-body effects due to the angular momentum (AM) coupling of the TM electrons in the d and ionized core level open shells.28 Other authors have recently adopted a more rigorous treatment of the electron correlation and have calculated the intershell and intrashell AM coupling of p levels in the TM ion.29,30 By adopting this approach, it has been possible to reproduce most of the features in the XPS spectra of ionic compounds without the need to invoke charge-transfer effects. In the following, we will adopt the first, simpler approach to give a qualitative interpretation of our Ni 2p XPS data of the c(4 × 2) ML. A convenient starting point to discuss the features of our defective ML is referring to the Ni 2p XPS data of NiO bulk. Its interpretation is mainly based on the fact that NiO is a chargetransfer insulator with a conductivity gap determined by the charge-transfer energy ∆.9 According to this model, one can consider the localized 3d orbitals as impurity states in a semiconducting host, so that the ground state can be expressed as |Ψg > ) R| 3d8 > + β| 3d9 L > +γ| 3d10L2 >, where R . β, γ and L and L2 indicate one or two holes in the oxygen VB, respectively. In the ground state, the energy separation between the first two states is given by ∆, with the 3d8 having the lowest energy, while in the final state the 3d9L is lowered with respect to the 3d8 one by Q-∆, where Q represents the Coulomb interaction between the 2p core hole and the 3d electron. Hence, the 3d9L has the lowest energy. This explains why the Ni 2p XPS spectrum of NiO displays two peaks split by Q-∆, where ∆ represents the charge-transfer energy associated to the fluctuation 3d8 f 3d9L. For a detailed description of all the features of the NiO bulk spectrum, however, multiplet structure and interaction with neighboring Ni ions must be also taken into account. In particular, to explain the shoulder at 1.2 eV from the Ni 2p3/2 main peak, typical of ordered stoichiometric NiO, Alders et al.31 have introduced a nonlocal screening effect, though recently Godehusen et al.30 have shown that it could arise just from the different coupling of the 3d9 shell with the 2p hole.

Study of Ultrathin NiO Films on Pd(100) In the case of the c(4 × 2)-Ni3O4 phase, one has to consider the factors that might contribute to the explanation of its spectral features (i.e., the shift and the low intensity of the 2p-3d satellite and the absence of the shoulder at 1.2 eV from the main 2p3/2 peak. It is worth mentioning that these peculiarities are typical also of other divalent nickelates that share with the c(4 × 2) phase a 2D confinement.32 On this basis, one is induced to associate most of the experimental peculiarities to the spatial confinement. Let us now discuss qualitatively how the parameters ∆, the d-d on site coulomb repulsion (U) and Q implied by the adopted model could be changed in the c(4 × 2) ML. An obvious factor is the metal screening because of the Pd substrate. Such a factor can induce considerable changes to the parameters. In particular, ∆ and U will be lowered by a factor proportional to the image charge induced by the ML on the substrate.33 However, a reduced ∆ value also could be expected because of the lower Madelung potential predictable for a 2D phase and by the presence of systematic vacancies in the defective ML. On the other hand, the effect of the metal screening on the Q value is expected to be less dramatic than in the case of ∆, because the screening of a core hole is mostly performed by the valence electrons of the same atom and the polarizability of the surroundings is of minor importance. As a proof of this statement, the Q value changes less than ∆ in the series of nickel halides.34 By considering the arguments presented above, it is easy to explain that in the case of the c(4 × 2)-Ni3O4 ML structure the value of Q-∆ must be higher than in bulk NiO, thereby determining a larger energy separation between the main peak and the satellite, as actually observed in the experiment. The absence of the 1.2 eV shoulder in the Ni 2p3/2 peak of the c(4 × 2) ML spectrum also can be interpreted with the mentioned nonlocal screening hypothesis.31 In particular, in the case of our phase the shoulder absence is coherent with the disappearance of the nonlocal screening effect due to the reduced Ni-O-Ni connectivity (because of the presence of Ni vacancies) and to the 2D structure of the oxide. This explanation was proposed also in the case of some 2D nickelates,32 which presented a featureless Ni 2p3/2 peak. The dependence of the satellite intensity from ∆ is more complex to be interpreted; in our energetic range, a decrease of ∆ should lead to an increase of the satellite intensity,28 so to explain our experimental findings other considerations should be made. For example, the highly reduced Madelung potential typical of this ultrathin phase could determine a lower energy separation between the Ni 3d and O 2p bands,35 so introducing the possibility of an increased hybridization, or, in other words, a larger transfer integral T ().28 Ab initio calculations have shown, indeed, that Ni and O valence states lay almost in the same energy region.25 The presence of the strain and of the reduced charge on O atoms also could determine a higher degree of covalency. In conclusion, we think that the peculiar Ni 2p pattern observed in the c(4 × 2)-Ni3O4 structure can be qualitatively discussed within the impurity approximation model using the appropriate values of ∆ and T that in this phase should be considerably different with respect to bulklike NiO. Further theoretical calculations are needed to put this discussion on a firmer basis. As a final consideration, we underline that the reduced value of ∆ determines an increase of the electron occupancy in the Ni 3d levels and a depopulation of the oxygen valence band; therefore the c(4 × 2) structure is more a p-metal rather than a charge-transfer insulator like NiO.9 The study of the hole

J. Phys. Chem. C, Vol. 111, No. 9, 2007 3739 TABLE 1: Photoemission Differential Cross Sections at Different Energies (from ref 36) atomic orbital

hν ) 21.2 eV

hν ) 40.8 eV

hν ) 134.0 eV

Ni 3d O 2p Pd 4d Ni 3d/ O 2p Ni 3d/ Pd 4d O 2p/Pd 4d

0.318 0.804 2.08 0.395 0.153 0.386

0.666 0.492 2.476 1.354 0.269 0.199

4.487 0.5772 0.288 7.774 15.580 2.004

dynamics by means of other techniques (such as X-ray absorption spectroscopy) would be of great importance to understand the physics involved in such important phenomena such as superconductivity or colossal magneto resistance. 3.1.2. Valence LeVel Photoemission. To discuss the VB data, a purely experimental approach will be adopted. We will try to exploit the rich set of experimental data obtained at different photon energies (He I, He II and 134 eV) to build a consistent picture of the nature of the electronic states of the investigated films. Among the several factors that influence the intensity changes at different photon energies, the changes of the photoemission cross sections of the Ni 3d, O 2p, and Pd 4d atomic orbitals is easily predictable (see Table 1).36 According to the values reported in Table 1, the most relevant intensity changes expected on passing from the He I to the He II data is an intensity enhancement of the Ni 3d states when compared to the O 2p ones. On the other hand, when passing to the most energetic photon (134 eV) Ni 2p based states are further strongly enhanced with respect to both O 2p and Pd 4d. Figure 3 reports the He I (21.2 eV) (a) and He II (40.8 eV) (b) VB photoemission spectra of the fully developed and wetting c(4 × 2) ML (i.e., corresponding to 0.75 MLE of NiO) together with that of the clean Pd(100) substrate. Figure 3c reports the VB data taken at the POLAR beamline with a photon of 134 eV, which corresponds to the Cooper’s minimum of Pd 4d orbitals,37 so that the intensity contribution from the substrate is highly depressed (see also Table 1). All the spectra were taken at normal emission at RT. Remarkable spectral changes are evident on adopting the different photon energies. The spectral features associated with the clean Pd(100) substrate span over a variable region which starts from Ef up to 3 eV (He I), 4 eV (He II), and 11 eV (at 134 eV) for the different photon energies. They have been labeled in alphabetical order in the corresponding spectra (Figure 3). When the c(4 × 2) ML is considered, new spectral features become evident in the regions where the substrate does not show its fingerprint. For example, in the case of the He I and He II data, the formation of a well-resolved peak centered at about 3.8 eV (labeled as X in Figure 3a,b) can be seen, which is absent in the case of the clean substrate. Spectral changes are evident also in the regions where the substrate shows its own states. The He I spectrum of the c(4 × 2) phase (Figure 3a) shows remarkable changes just below Ef: the three components typical of clean Pd(100) (labeled as a, b, and c in Figure 3a) change their relative intensity ratios. This is even more evident in the He II spectrum (Figure 3b): on going from the clean substrate to the oxide ML phase, the b and c components show a significant intensity decrease with respect to the a component. Moreover, the component labeled b is shifted toward lower BE in the c(4 × 2) ML. In addition, in the He II spectrum (Figure 3b) there is some evidence for a further low-intensity component in the ML spectrum centered at ca. 6 eV. The VB spectrum of the c(4 × 2)-Ni3O4 ML taken with a photon energy of 134 eV (Figure 3c) is rather distinct from those obtained with the standard UV source. In particular,

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Figure 3. VB spectra of the c(4 × 2)-Ni3O4/Pd(100) defective monolayer and of the clean Pd(100) substrate: (a) He I, (b) He II, and (c) taken with a photon energy of 134 eV. In the latter, the difference spectrum obtained according to the procedure described in the text is reported also.

because of the higher photon energy, we are predominantly probing the Ni 3d contribution to the density of states (DOS) (see Table 1). In this case, two intense peaks and one broader feature are clearly seen, which have their maxima located at 1.1, 5.7, and 10.7 eV (see Figure 3c). Because of the expected reduced contribution from the substrate states, we retain that the VB spectrum reported in Figure 3c is the one that best gives the fingerprint of the electronic structure of the c(4 × 2)-Ni3O4 ML. For this reason, in this case we also have evaluated the difference spectrum with respect to the substrate, which also has been reported in Figure 3c. This has been obtained by subtracting the two spectra after a normalization procedure that takes into account the actual photon flux and by attenuating the substrate spectrum intensity by a factor that considers the inelastic attenuation (due to the overlayer and calculated using the TPP-2M algorithm38). The most important point emerging from the examination of the difference spectrum is that the overlayer VB states can now be associated with four specific features: one close to Ef (0.8 eV), the second at ca. 3.8 eV, a third one at ca. 5-6 eV, and a broad and low intensity one around 10 eV (labeled as Ox1, Ox2, Ox3, and Ox4 in Figure 3c). The picture emerging from the difference spectrum is strongly indicative of a ML having a metal-like behavior with the strong Ni 3d based feature close to Ef and labeled as Ox1. This feature also was visible in the He II spectrum (Figure 3b) and to a lesser extent in the He I spectrum (Figure 3a) because of the unfavorable Ni 3d cross section at the lowest photon energy. Finding a counterpart of the Ox2, Ox3, and Ox4 features in the corresponding He I and He II excited spectra is rather difficult because they become strongly mixed with the O 2p and Pd 4d states. By comparing the He I and He II spectra of the ML with the corresponding ones of the clean substrate, it is clear that the feature labeled as X in Figure 3a,b is mostly related to O 2p based states. This is the region where O 2p states are expected

according to the comparison with the literature data of NiO based systems. In conclusion, it seems reasonable to assign the very intense peak observed at 0.8 eV in the difference spectrum (Figure 3c) mainly to Ni 3d states and the less intense features at 3.8 and 5.7 eV to states where the oxygen π and σ contributions become relevant, as expected on the basis of related NiO literature data.39-43 However, our VB data cannot exclude the presence of oxygen derived states in the Ox1 peak centered at 0.8 eV. On the other hand, the theoretical data recently reported by Ferrari et al.25 on the c(4 × 2) phase seem to support this idea; the calculated projected DOS would indicate the copresence of both anion and cation states in this energy region. A cross check of the proposed assignment can be obtained by looking at the oxidation process of a nickel film grown over the Pd(100) substrate. The growth of NiO ultrathin films by means of a postoxidation procedure of a metal film has been already explored by our group some years ago.44 Following again such deposition route, we have recorded the He I and He II excited spectra of a 1 MLE thick Ni layer deposited on Pd(100) prior and after the exposure to oxygen. It is to be outlined, however, that the c(4 × 2) ML obtained by this procedure is only short-range ordered and with a partial dissolution of Ni metal into the Pd selvedge.45 The corresponding VB data are reported in Figure 4 together with the spectrum of a bulklike 14 MLE thick Ni film on Pd(100). From Figure 4, it appears that the deposition of 1 MLE of metal Ni produces a significant modification of the spectral intensity in the BE region between Ef and 1 eV when the 1 MLE metallic deposit is postoxidized at 250 °C with 270 L of oxygen. In particular, the He II spectrum (Figure 4b) shows a large intensity increase of the spectral feature just below Ef. The most notable change after the postoxidation treatment is the formation of a broad peak at ca. 4 eV, very similar for relative intensity and fwhm to the one observed in the case of

Study of Ultrathin NiO Films on Pd(100)

J. Phys. Chem. C, Vol. 111, No. 9, 2007 3741

Figure 4. He I (a) and He II (b) excited VB spectra of a 1 MLE thick Ni layer deposited on Pd(100) prior and after the exposure to 270 L of oxygen at 250 °C. The data of a bulklike 14 MLE Ni/Pd(100) film also are reported for comparison.

Figure 5. Ni 2p (a) and Pd3p and O 1s (b) core level photoemission spectra of NiO/Pd(100) ultrathin films at different coverages.

the c(4 × 2) phase obtained by reactive deposition (see Figure 3), allowing us to safely assign this feature to oxygen derived states. 3.2. Ultrathin NiO films (2-12 MLE Range). In this section, we discuss the photoemission data (both core and VB levels) of the NiO/Pd(100) films at different coverages from 2 to 12 MLE, i.e., from strained to fully relaxed NiO(100) films. 3.2.1. Core LeVel Photoemission. Figure 5 reports the core level XPS data (Al KR) of the investigated films in the two Ni2p (a) and Pd 3p3/2 -O 1s (b) spectral regions. In all cases, the spectra were recorded with a takeoff angle of 20° from the

surface. For comparison, Figure 5a reports the data for the c(4 × 2) defective ML to be compared with the ones reported in Figure 2c (hν ) 950 eV). It is well evident that the 2 MLE data are very similar to the corresponding c(4 × 2) ones, indicating that at this coverage the contribution of the second NiO layer is either minor or not distinguishable from the one of the defective ML. On the other hand, the experimental Ni 2p pattern stops changing significantly starting from 8 MLE when it reaches the typical structure observed in a NiO-bulklike film (see for example the distinct shoulder in the main Ni 2p3/2 peak). The data at the 4 MLE

3742 J. Phys. Chem. C, Vol. 111, No. 9, 2007

Figure 6. VB photoemission data of NiO/Pd(100) ultrathin films at variable coverage taken with a photon of a134 eV (Pd 4d Cooper’s minimum).

coverage mark a transition region at which the main Ni 2p3/2 peak exhibits an envelope revealing a complex intermediate behavior in which both the partial strain relaxation in the developing 3D Stranski-Krastanov islands and the peculiar growth morphology (leaving portions of the c(4 × 2) regions still uncovered) play a role, which is hardly quantifiable for each separate contribution. Inspection of the Pd 3p3/2 -O 1s spectral region (Figure 5b) reveals that Pd 3p3/2 features are still present even at high coverages. Taking into account the attenuation length and the relative cross sections, the experimental data are perfectly compatible with the actual coverages. As expected, the O 1s contribution becomes progressively predominant and the corresponding BE (530 eV) is perfectly compatible with the value reported in literature for bulk NiO.46 3.2.2. Valence LeVel Photoemission. We now compare the VB spectra of the NiO/Pd(100) ultrathin films, measured as a function of coverage in the 2-12 MLE range, with literature data of similar films on metals and of bulk NiO. To facilitate the comparison with literature data, the observed spectral features have been indicated with labels usually adopted in literature for NiO. Figure 6 shows the NiO film thickness dependence of VB spectra obtained at normal emission with a photon energy of 134 eV (Pd 4d Cooper’s minimum) radiation. For the sake of clarity, this figure includes also the spectra relative to the clean Pd(100) surface and to the c(4 × 2) defective ML previously discussed and reported in Figure 3. The BE scale is referred to Ef. For the data that do not display a well-defined Fermi cutoff, the position of the VB maximum (peak B1 in Figure 6)11 has been located at 1.9 eV. Note that

Agnoli et al. most of the VB literature data relative to bulk NiO report BE scales at which the zero value is positioned at B1.39 From the data of Figure 6, it can be seen that well-defined structures (labeled B1, B2, A1, and S) are present starting from the coverage of 2 MLE. From the observed coverage dependent intensity trend of these peaks and by comparing their absolute (measured from Ef) and relative (measured from B1) energy positions with literature data for NiO thin films11,41 and bulk NiO,39,47 we can assign them to features that are typical of a NiO bulklike VB structure. The details are as follows: B1 and B2 can be assigned to the multiplet structure of the 3d8L photoemission final state of Ni in NiO43 (the absolute BE of B1 is 1.9 eV and the energy difference between B1 and B2 is 1.7 eV), A1 (3.2 eV below B1) corresponds to O 2p derived states, and S (7-9 eV below B1) corresponds to the Ni 3d satellite that is usually attributed to emission from Ni 3d8 states characterized by an unscreened 3d7 final state. We want to outline here that our data at the high coverage taken with a photon of 134 eV are very similar to those obtained on a cleaved NiO(100) surface with a photon of 120 eV.47 This coverage-related trend of the data is consistent with the structure and morphology of the strained NiO/Pd(100) films that was previously determined by XPS, XPD, and STM.17,18 In fact, at coverages exceeding the c(4 × 2) ML, one observes the onset of the growth of strained 3D NiO islands on the Ni defective c(4 × 2) wetting layer. The growth of these highly strained NiO islands continues up to 4 MLE at which the initially scattered 3D islands begin to coalesce and a strain relaxation of the NiO film becomes relevant.18 This particular stage of the growth appears in the VB data in the form of a rather sharp increase of the NiO bulklike VB structure relative to Ef. Above 4 MLE, the 3D growth of the partially relaxed NiO film continues and according to the SPA-LEED data full relaxation of the NiO into its bulklike structural parameters is observed around 10 MLE.19 However, the energy position of the NiO related VB features do not display any appreciable shift with the coverage. This finding is compatible with earlier studies that have shown that even severe reduction of bulk NiO47 does not imply a shift of the NiO derived VB structures whose position appears to be pinned relative to Ef. In fact, it has been demonstrated that oxygen defectivity is rather related to the appearance of states in the NiO band gap. In our case, the trend of the observed residual intensity at Ef and the Ni 2p XPS data discussed in section 3.2.1 suggests that the intensity at Ef is rather due to residual Pd states, observable because of the 3D growth morphology of the overlayer. Actually, the presence of reduced Ni atoms as a consequence of the preparative procedure would show up as residual states at Ef also at higher coverages. Finally, we would like to comment on the possible role of other oxygen defects, namely nonperfectly coordinated oxygen atoms. For instance, it is known from adsorption studies of O2 on bulk NiO that they can enhance the intensity of the S and A1 photoemission structures.40 Variations with coverage of the S and A1 peaks are well evident in the data of Figure 6. The enhanced intensity of these peaks at 2 and 4 MLE compared to the 12 MLE spectrum is consistent with the fact that in the low coverage range, due to the high strain in the film, a higher amount of O-related structural defects are expected. On the other hand, according to some exploratory experiments, we have found that these intensity fluctuations are dependent on the actual film preparation conditions (oxygen pressure and deposition rate at constant temperature); this argument is under further investigation and the results will be reported in a future report.

Study of Ultrathin NiO Films on Pd(100) 4. Conclusions This paper discusses the electronic structure of nickel oxide ultrathin films as a function of thickness from the highly strained 2D c(4 × 2)-Ni3O4 single monolayer up to bulklike NiO, through the intermediate Stranski-Krastanov-like growth of 3D partially strained islands. As far as the c(4 × 2) monolayer phase is concerned, an accurate determination of the Ni/O ratio allowed us to confirm the Ni3O4 stoichiometry proposed on the basis of previously reported structural determinations. HR-XPS data have shown the removal of the Pd 3d surface core level shift on going from the clean substrate to the c(4 × 2) ML. The Ni 2p features of the defective ML have been compared to both metallic Ni and to bulklike NiO and their peculiarities qualitatively discussed within the impurity approximation model.28 Following this procedure, we were able to discuss the role of the 2D spatial confinement and of the substrate. The VB photoemission data obtained with a photon of 134 eV (the energy that has the Pd 4d Cooper’s minimum) allowed us to extract a set of bands that are the actual fingerprinting of the defective monolayer, which points to a metal-like behavior. The energy overlap between the 4d Pd and Ni-oxide derived states, particularly the oxygen π-band (Ox2 peak in Figure 3), evidenced by photoemission data is in good agreement with the hypothesis of a relevant hybridization between the substrate and the overlayer, as predicted by ab initio calculations.25 The strong interaction between the substrate and the overlayer explains the smaller interfacial distance observed by LEED in the NiO/Pd(100) system20 with respect to NiO/Ag(100)13 where on the contrary hybridization effects are not relevant.16 The coverage-dependent trends of both core and VB photoemission data are consistent with the structure and morphology of the strained NiO/Pd(100) films that were previously determined by photoemission, XPD, SPA-LEED, and STM.17,18,19 Partial strain relaxation and 3D islands coalescence, which are known to occur at a coverage of approximately 4 MLE, appear in the photoemission data in the form of a rather sharp increase of the NiO bulklike VB structure. The experimental Ni 2p pattern at the same coverage has shown a complex convolution of features, which eventually lead to the typical structure observed in a bulklike NiO film in the data taken at 8 MLE. Acknowledgment. This work has been funded by the European Community through the STRP GSOMEN (Growth and Supra-organization of Transition and Noble Metal Nanoclusters, Contract No. NMP4-CT-2004-001594) and by the Italian Ministry of Instruction, University, and Research (MIUR) through the fund “Programs of national relevance” (PRIN-2003, PRIN-2005). References and Notes (1) Kung, H. H. Transition Metal Oxides: Surface Chemistry and Catalysis; Elsevier: New York, 1989. (2) Thomas, J. M.; Thomas, W. J. Principles and Practise of Heterogeneous Catalysis; VCH: New York, 1997. (3) Hotovy, I.; Huran, J.; Siciliano, P. Sens. Actuators, B 2004, 103, 300. (4) Cantalini, C.; Post, M.; Buso, D.; Guglielmi, M.; Martucci, A. Sens. Actuators, B 2005, 108, 184192. (5) Dirksen, J.; Duval, K.; Ring, T. A. Sens Actuators, B 2001, 80, 106. (6) Wolf, S. A.; Awschalom, D. D.; Buhrman, R. A.; Daughton, J. M.; von Molnar, S.; Roukes, M. L.; Chtchelkanova, A. Y.; Treger, D. M. Science 2001, 294, 1488. (7) Zhu, W.; Seve, L.; Sears, R.; Sinkovic, B.; Parlin, S. S. P. Phys. ReV. Lett. 2001, 86, 5389.

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