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Femtosecond-Laser Photoemission Spectroscopy of Mo(100) Covered by Ultrathin MgO(100) Films of Variable Thickness Mihai E. Vaida, Tobias Gleitsmann, Robert Tchitnga, and Thorsten M. Bernhardt* Institute for Surface Chemistry and Catalysis, UniVersity of Ulm, Albert-Einstein-Allee 47, 89069 Ulm, Germany ReceiVed: March 25, 2009; ReVised Manuscript ReceiVed: April 21, 2009
Femtosecond-laser photoemission spectra were obtained from a Mo(100) single crystal surface covered with stoichiometric magnesia thin films of well-defined thickness. Depending on the excitation wavelength and the MgO layer thickness, either one or two photon photoemission was detected at 333 and at 402 nm. This photoemission was assigned to originate from the molybdenum d band states even for 10 monolayers MgO coverage. At 263 nm excitation, however, the photoemission power dependence indicated that the signal contained both contributions from the Mo metal d electrons and from the MgO O2p valence band. While one nominal monolayer of MgO only slightly influenced the photoemission from the molybdenum surface, a considerable reduction of the surface electronic work function was observed for ultrathin magnesia overlayers of 2 to 3 monolayers in thickness. No significant change in the work function was measured for more than 3 up to 10 monolayers. I. Introduction Ultrathin metal oxide films on metal single crystal substrates have become an extensively investigated research subject in recent years.1,2 This interest is due to their importance to a wide range of applications, including magnetic tunneling junctions,3 chemical sensors,4 and nanoscale model systems in heterogeneous catalysis.5 Because of their finite thickness of only a few atomic monolayers (ML), electron tunneling through the film into the metal substrate is possible and, hence, scanning tunneling microscopy and electron spectroscopic methods can be applied to investigate the geometric and the electronic structure of these films, even if they are composed of wide band gap insulating materials such as magnesium oxide.6 With respect to surface chemistry and heterogeneous catalysis MgO ultrathin films on Mo(100) are particularly appealing due to their good chemical and thermal stability, their simple rock salt structure and their amenability to high temperature treatment, because of the high refractory metal support.7-9 Nanoparticle model catalysts are prepared by thermal growth10,11 or by mass-selected deposition9,12 of metal clusters on the insulating magnesia film. Most recently, it has been noticed that electronic charging of such supported metal clusters occurs not only if they are attached to MgO defect centers,13,14 but also even on defectfree films, if the thickness of the magnesia thin film ranges around 3 ML.15,16 Most surprisingly, this charging effect was also found to influence the geometric structure of the deposited metal clusters.16-18 Consequently, metal-supported ultrathin oxide film systems were propagated as new materials for the design of supported metal catalysts with the oxide layer thickness as one decisive design parameter.15,16,19,20 The interesting magnesia coverage dependent electronic structure of this support material is reflected, e.g., in the surface electronic work function Φ. Theoretical simulations predict a * To whom correspondence should be addressed. Phone: ++49-73150-25455. Fax: ++49-731-50-25452. E-mail: thorsten.bernhardt@uni-ulm. de.
striking variation in Φ with Θ, the number of magnesia layers, in particular for the first few ML.21 This has, however, not been investigated experimentally in detail so far. The subject of our research is to conduct time-resolved femtosecond(fs)-laser spectroscopy experiments on the MgO(100)/ Mo(100) substrate system, on adsorbate molecules,22 as well as on metal clusters deposited on this substrate.23,24 In this context, we performed a systematic investigation of the kinetic energy resolved photoelectron emission from Mo(100) covered by stoichiometric magnesia layers of variable thickness. The results presented in this contribution confirm the strong variation of Φ with Θ for the first three monolayers and emphasize the importance of the metal d valence band states for the observed fs laser induced photoelectron emission. II. Experimental Section The photoemission experiments were carried out in a µ-metal shielded ultra high vacuum (UHV) surface science apparatus equipped with standard tools for surface preparation and investigation. The Mo(100) single crystal is attached to a liquid nitrogen cryostat manipulator in the center of the UHV chamber (base pressure 1 ML MgO; at 333 nm excitation: 52 nJ for Mo and 1 ML MgO, 4 nJ for 2 ML, 0.3 nJ for 3 ML, and 0.1 nJ for >3 ML MgO.
i. e., primarily the mean free path of electrons in solids as a function of their kinetic energy, is considered.37 Electrons liberated by UPS (He I) excitation from molybdenum valence band states in the energy range of the MgO band gap possess kinetic energies around 14-18 eV. In contrast, the kinetic energy of the electrons assigned to originate from Mo(100) d band states in the present investigation ranges between 0 and 2.5 eV (cf. Figure 1d-f). The mean free path of 14-18 eV electrons is only 2-4 ML, and these electrons emitted by vacuum UV (He I) irradiation are thus most likely stopped within the 10 ML MgO overlayer on Mo. Electrons with kinetic energies around 1-2 eV, however, have a mean free path of more than 100 ML.37 Therefore, these low energy electrons emitted by the fs laser irradiation are able to pass the ultrathin MgO film and are detected in the photoemission spectra of Figure 1d-f. This interpretation does also account for the reduced emission intensity in the “higher kinetic energy” (1-2.5 eV) part of the 10 ML MgO spectrum in Figure 1d compared to the emission spectra from bare Mo(100) in Figure 1b and c that covers a similar electron kinetic energy range because in this energy range, the electron mean free path is a very steep function of the kinetic energy. B. Coverage Dependent Photoemission. Fs photoemission spectra that have been obtained by successively increasing the magnesia film thickness monolayer by monolayer are displayed in Figure 4 for excitation wavelengths of 263 and 333 nm. The spectra recorded from 1 ML MgO on Mo(100) are rather similar to the spectra of the bare metal surface. Also, the surface work function remains almost unchanged as apparent from the width of the spectra. At 2 ML MgO coverage, however, the kinetic energy distribution of the photoemission considerably changes. This
change is due to a reduction of the surface work function by about 1 eV. As a consequence, the 263 nm emission spectrum starts to contain contributions from the two photon excitation of MgO O2p valence electrons as discussed above. In contrast, the 333 nm photoemission spectrum narrows because single photon emission of Mo valence band electrons becomes amenable for excitation with 3.72 eV photons (and dominates in the spectrum) at the reduced work function. For coverages larger than 2 ML up to 10 ML the photoemission spectra are rather similar with only slight changes in appearance for 263 nm excitation (Figure 4). These differences might tentatively be assigned to an increasing contribution of the two photon emission from MgO valence band states with increasing layer thickness, as it has also been observed in UPS measurements on ultrathin magnesia films on Fe(100).38 The relative changes of the surface work function, ∆Φ, as a function of the MgO layer thickness, Θ, deduced from the spectra in Figure 4, are plotted in Figure 5 together with the theoretically predicted values from the work of Pacchioni and co-workers.21 In the theoretical prediction, Φ decreases up to an MgO film thickness of 2 ML by 2.1 eV compared to the bare Mo(100) surface and remains constant afterward. The experimental results of the present investigation confirm the rapid decrease of Φ with the deposition of the first few monolayers. However, in the experiment, Φ decreases up to an MgO film thickness of 3 ML by only about 1.3 eV compared to the bare Mo(100) surface and remains constant afterward. Thus, the observed maximum decrease in the surface work function due to an ultrathin magnesia overlayer is considerably smaller than theoretically predicted (cf. Figure 5). It has to be noted, however, that the work function of metal oxide surfaces is known to be extremely sensitive to the preparation procedure and the analytic method.4 Furthermore, the very first magnesia ML changes Φ only slightly in the experiment. Φ starts to decrease considerably with the second MgO layer, whereas, theoretically, already the first ML is predicted to lower Φ considerably. The reason for the difference in the influence of the first MgO monolayer on the electronic structure of the surface can be explained by an imperfect geometric arrangement of this layer in the experiment. Previous LEED and scanning tunneling microscopy (STM) investigations of MgO(100) on Mo(100) indicate that after deposition of approximately one monolayer, the film still exhibits large holes, confined by nonpolar [100] and polar [110] oriented edges.8,39 The incompleteness of the first monolayer
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(90% fractional coverage) governed by patches with 2 ML thickness has also been observed for MgO on Ag(100).40 After deposition of 2 ML MgO, the metal surface is completely covered with MgO.35,39 With the deposition of the second monolayer, MgO films on Mo(100) were furthermore found to exhibit a distinct moire´-type superperiodic structure comprising tilted MgO facets.39,41 At even larger coverages (>5-7 ML), the formation of a mosaic structure has been observed that is spanned between a dislocation network which relaxes the strain in the oxide layer (due to 5% lattice mismatch between Mo(100) and MgO(100)). A mean domain size of 55 Å has been reported for the mosaic structure.39,41 A similar mosaic pattern has also been observed for MgO growth on Ag(100) (3.1% lattice misfit).42 Above 7 ML film thickness, the oxide was found to gradually flatten and the global roughness to decrease, indicating flat and defect-poor surface structure.39 However, these structural transformations at coverages above 2 ML are not reflected in the photoemission spectra recorded in the present investigation. Our coverage dependent photoemission results are also corroborated by UPS experiments of MgO on Fe(100).38 In these experiments, very little modification is seen in the emission close to the Fermi energy (between 0 and 3.5 eV electron binding energy) for low coverages of 0.5, 1, and 2 ML MgO. The major modification at these coverages consists in the appearance of an intense emission centered at approximately 5.5 eV, which is characteristic of highly hybridized O2p and Mg3s valence band states of the MgO film. Furthermore, it is interesting to note that while photoemission experiments yield information about the integral surface work function, Kelvin probe force microscopy measurements on cleaved single crystal MgO(100) surfaces reveal local work function differences with deviations of up to 1.5 eV for low-coordinated sites like steps, kinks, and corners and, even more importantly, for metal ion vacancies with locally increased electronic charge.43 IV. Conclusions Femtosecond-laser photoemission spectra obtained at photon energies between 3 and 4.7 eV from bare and magnesia covered Mo(100) surfaces have been presented. The major contribution to the photoemission is assigned to electrons originating from the molybdenum valence band states. Due to their low kinetic energy, these electrons can penetrate the ultrathin magnesia film. The electronic transport properties are a strong function of the actual electron kinetic energy and influence the shape of the detected photoemission spectra. Indications for a two-photon excitation of electrons from the MgO O2p valence band have been observed only at 4.71 eV photon energy. The Mo(100) surface work function has been found to be considerably reduced by the first three magnesia overlayers and to stay constant afterward. Work is in progress to obtain information on the carrier dynamics as a function of the magnesia layer thickness employing time-resolved two photon photoemission spectroscopy. A further interesting subject currently under investigation in our laboratory is the influence of surface defect centers on the photoemission dynamics. Acknowledgment. Financial support by the Deutsche Forschungsgemeinschaft (SFB 569 and SPP 1153) and the Fonds der Chemischen Industrie (FCI) is gratefully acknowledged. M.E.V. thanks the DAAD for a fellowship. References and Notes (1) Schintke, S.; Schneider, W.-D. J. Phys.: Condens. Matter 2004, 16, R49.
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