Excited states at interfaces of a metal-supported ultrathin oxide film

Abstract

We report layer-resolved measurements of the unoccupied electronic structure of ultrathin MgO films grown on Ag(001). The metal-induced gap states at the metal/oxide interface, the oxide band gap, and a surface core exciton involving an image-potential state of the vacuum are revealed through resonant Auger spectroscopy of the Mg KL 23 (001) is a model system of the metal/oxide interface at the ultrathin limit. Although the structure and the growth mechanism [4][5][6][7][8][9], as well as changes in electronic properties associated with depositing ultrathin films of MgO on Ag(001), have been investigated [10][11][12][13][14][15][16][17], capturing the physical nature of such a mixed system, and in particular of the interfaces, remains challenging.In resonant auger spectroscopy (RAS), the Auger process can be very different from that occurring with photon energies far above absorption thresholds [18]. Sub-lifetime narrowing effects [19], as well as strong modulations in Auger signals [20], can occur. Furthermore, depending on whether or not the resonantly excited electron delocalizes to the conduction band before the core-hole decay, the decay process can result in a two-hole (2h) final state ("normal" Auger decay) or in a two-hole and one electron (2h1e) final state (spectator channel of the autoionization process), respectively. These two competing decay pathways are both visible in resonant Auger spectra if the time scale of delocalization is comparable to the core-hole lifetime. Thus, information on the screening of the core hole, the degree of localization of excited electrons, or the charge transfer dynamics at interfaces and surfaces can be obtained [21][22][23][24].In this Rapid Communication, we study the evolution of the layer-resolved Mg KL 23 L 23 Auger transition for a 3 monolayer (ML) thick MgO film grown on Ag(001), in a photon energy range corresponding to the Mg K edge. In good agreement with density functional theory (DFT) calculations, we show that the intensity evolution of the resonant Auger spectra with the photon energy allows us to get a layer-by-layer mapping of the local density of empty Mg p states probed by the excited photoelectron in the intermediate dipole transition [25]. We find that, in the pre-edge region, the Auger spectra mostly consist of a single Auger component, the one of the metal/oxide interface, demonstrating the metallic * Corresponding author: thomas.jaouen@unifr.ch character of the oxide interface layer due to the presence of metal-induced gap states (MIGS). We measure the MgO surface band gap and a spectroscopic fingerprint of a surface core exciton involving an image potential state of the vacuum.All experiments were performed at the Photoemission and Atomic Resolution Laboratory (PEARL) beam line situated at bending magnet X03DA of the Swiss Light Source. The MgO films were grown in situ on Ag(001) (for details see Ref.[5]). X-ray absorption was measured by recording the total electron yield (TEY) and RAS was obtained at room temperature using a VG Scienta EW4000 ...

Band Bending in Mg-Colored and O2-Activated Ultrathin MgO(001) Films

“…20 It has been, for example, exploited to access, layer-by-layer, the atomic structure of MgO ultrathin films 22 as well as the unoccupied interface and surface states of the MgO/Ag(001) system. 23 In the present study, the layer-resolved Mg KL 23 L 23 Auger emission further allows us to keep track of band profiles from the oxide surface to the interface. Figure 3a The band bending induced by an Mg exposure of 20 s is clearly revealed in Figure 3a through the evolution of the C 1 , C 2 , and C 3 positions.…”

“…For 3 ML of MgO, the Mg KL 23 L 23 Auger transition is indeed a very powerful spectroscopic probe thanks to its layer-by-layer resolution originating in image potential screening at the metal/oxide interface . It has been, for example, exploited to access, layer-by-layer, the atomic structure of MgO ultrathin films as well as the unoccupied interface and surface states of the MgO/Ag(001) system …”

Surface science at the PEARL beamline of the Swiss Light Source

“…It has been designed by a consortium of Swiss research groups active in surface science for the study of local atomic geometry at the surface of a wide range of novel organic and inorganic systems. Such systems include, for example, functional organic molecules (Fasel et al, 1996;Muntwiler et al, 2005;Pawlak et al, 2012), supramolecular networks (Barth, 2007;Lobo-Checa et al, 2009;Zhang et al, 2014), molecular magnets (Scheybal et al, 2005), chiral recognition (Fasel et al, 2004;Greber et al, 2006;Schillinger et al, 2007), endohedral fullerenes (Treier et al, 2009;Westerströ m et al, 2012Westerströ m et al, , 2014, ultrathin metal oxides (Jaouen et al, 2015), surfaces of ferroelectrics (Despont et al, 2006) and surface alloys (Corso et al, 2010;Pawlak et al, 2015).…”

“…By tuning the photon energy to a particular X-ray absorption line, the photoemission cross section can be resonantly enhanced (Treier et al, 2009), or a particular symmetry of the photoelectron wavefunction can be selected (Morscher et al, 2011;Matsui et al, 2015). Using the core-hole clock method with Raman-active Auger decays, ultrafast delocalization dynamics in the conduction band can be studied (Fö hlisch et al, 2005;Jaouen et al, 2015). In angle-scanned XPD the ratio between forward and backward scattering can be tuned.…”