Ultrathin MgO Films Research Articles

Formation of Gold(I) Edge Oxide at Flat Gold Nanoclusters on an Ultrathin MgO Film under Ambient Conditions

Many active gold catalysts are prepared onreducible oxides, and strong interactions between the supportand the gold particle may create active sites at the peripheryclose to the particle–support interface. These interactionsmay also include charge transfer to or from the particle. Forpurely geometric reasons, small particles have a high propor-tion of low-coordinated edge and corner atoms that might actas reaction centers. Also, thermal effects from localized softphonon modes at particle edges may contribute to thelowering of critical reaction barriers.Lately, a large amount of work has been conducted toelucidate the properties of gold clusters on ultrathin (a fewmonolayers (ML) thick) MgO films supported by metal,typically silver.

Controlling water dissociation on an ultrathin MgO film by tuning film thickness

Controlling water dissociation on an ultrathin MgO film by tuning film thickness

ChemInform Abstract: Ultrathin Magnesia Films as Support for Molecules and Metal Clusters: Tuning Reactivity by Thickness and Composition

AbstractReview: 107 refs.

Polarity of ultrathin MgO(111) films deposited on a metal substrate

We present a theoretical density-functional theory study of the effect of deposition on a metal substrate (Mg, Pt, Ag, and Au) on the phase diagram of ultrathin MgO(111) polar films. By considering various crystallographic structures (rocksalt, zinc-blende, and graphiticlike), layer stackings and lattice registries at the interface, we identify the two principal mechanisms by which oxide films interact with the metal substrate. On one hand, metal provides the charges necessary for polarity compensation at the interface, enables the oxide ions at the interface to recover their usual valence, and induces a strong stabilization of polar films. On the other hand, due to interfacial bonding and band alignment (function of the metal electronegativity), a second type of electron exchange occurs at the interface, dominant in films which are either nonpolar or uncompensated polar. It induces a structural distortion of the oxide film in the interface region and yields a nonvanishing film polarization. The stabilizing effect of the metal substrate is considerably weaker in this case.

Ultrathin magnesia films as support for molecules and metal clusters: Tuning reactivity by thickness and composition

AbstractUltrathin metal oxide films have attracted considerable interest in recent years as versatile substrate for the design of nanocatalytic model systems. In particular, it has been proposed theoretically and confirmed experimentally that the electronic structure of adsorbates can be influenced by the layer thickness and the stoichiometry, i.e., the type and number of defects, of the oxide film. This has important consequences on the chemical reactivity of the oxide surface itself and of oxide supported metal clusters. It also opens new possibilities to influence and to control chemical reactions occurring at the surface of these systems. The present feature focuses on very recent experiments that illustrate the effects of a proper adjustment of layer thickness and composition of ultrathin MgO(100) films on chemical transformations. On the magnesia surface itself, the photodissociation dynamics of methyl iodide molecules is investigated via femtosecond‐laser pump–probe mass spectrometry. Furthermore, the catalytic oxidation of carbon monoxide at mass‐selected Au20 clusters deposited on magnesia is explored through temperature programmed reaction measurements. In the latter case, detailed first principles calculations are able to correlate the experimentally observed reactivity with structural dimensionality changes that are induced by the changing thickness and composition of the magnesia support. magnified image

State-selective dissociation of a single water molecule on an ultrathin MgO film

The interaction of water with oxide surfaces has drawn considerable interest, owing to its application to problems in diverse scientific fields. Atomic-scale insights into water molecules on the oxide surface have long been recognized as essential for a fundamental understanding of the molecular processes occurring there. Here, we report the dissociation of a single water molecule on an ultrathin MgO film using low-temperature scanning tunnelling microscopy. Two types of dissociation pathway--vibrational excitation and electronic excitation--are selectively achieved by means of injecting tunnelling electrons at the single-molecule level, resulting in different dissociated products according to the reaction paths. Our results reveal the advantage of using a MgO film, rather than bulk MgO, as a substrate in chemical reactions.

Gundlach oscillations and Coulomb blockade of Co nanoislands on MgO/Mo(100) investigated by scanning tunneling spectroscopy at 300 K

Ultrathin MgO films on Mo(100) with a thickness up to 12 ML are studied by scanning tunneling microscopy and spectroscopy at room temperature. The spatial variation in the work function within the MgO film is mapped by field emission resonance states (Gundlach oscillations) using $dz/dU$ spectroscopy. We found circular spots with significantly reduced work function $(\ensuremath{\Delta}\ensuremath{\Phi}=0.6\text{ }\text{eV})$, which are assigned to charged defects within the MgO film. On top of the MgO films, small Co clusters are deposited with an average contact area of ${A}_{\text{Co}}\ensuremath{\simeq}4\text{ }{\text{nm}}^{2}$. These islands exhibit Coulomb oscillations in $dI/dU$-spectra at room temperature. Good agreement with orthodox theory is achieved showing variations in the background charge ${Q}_{0}$ for islands at different positions, which are in accordance with the work function differences determined by the Gundlach oscillations.

Investigating the origin of Fermi level pinning in Ge Schottky junctions using epitaxially grown ultrathin MgO films

Fermi level (FL) pinning at the Ge valence band results in a high Schottky barrier height for all metal/n-Ge contacts. The origin of this pinning effect has been ascribed to either metal induced gap states or surface states arise from the native defects at the Ge surface, such as dangling bonds. The discrepancy in the reported results/explanations is mainly due to the lack of an explicit characterization of a high quality metal/Ge or metal/ultrathin oxide/Ge junction, which should be ideally single crystalline, atomically smooth and free of process-induced defects or intermixing. We report the Schottky characteristics of high quality metal/MgO/n-Ge junctions with the ultrathin MgO epitaxially grown on Ge. We find the depinning effect displays a weak dependence on the MgO thickness, indicating the interface states due to the native defects on Ge surface are likely to play the dominant role in FL pinning.

High-Brightness Photocathodes through Ultrathin Surface Layers on Metals

We report how ultrathin MgO films on Ag(001) surfaces can be used to control the emittance properties of photocathodes. In addition to substantially reducing the work function of the metal surface, the MgO layers also favorably influence the shape of the surface bands resulting in the generation of high-brightness electron beams. As the number of MgO surface layers varies from 0 to 3, the emitted electron beam becomes gradually brighter, reducing its transverse emittance to 0.06 mm mrad. We suggest the use of such photocathodes for the development of free-electron x-ray lasers and energy-recovery linac x-ray sources.

Adsorption and activation of O2 at Au chains on MgO/Mo thin films

We have investigated the adsorption of O(2) on Au(n) clusters (n = 1-6) supported by an ultra thin (3ML)MgO(001) film on Mo metal via density functional theory calculations. On thin films, these small clusters have chain like structures and their electronic states resemble 1D quantum well states. The Au(1-3) are charged by one electron whereas the larger Au(4-6) get two electrons from the substrate. This is confirmed both by the symmetries of the HOMO and LUMO states of the clusters and the Bader charge analysis. In contrast to the O(2) adsorption on gas-phase clusters, the adsorption energy of O(2) molecule does not show pronounced oscillations as a function of cluster size. The O(2) is activated (i.e. the O-O bond is elongated and the molecule is charged) in contact with all the cluster sizes. O(2) activation and strong binding is also observed on bare MgO/Mo, whereas the molecule interacts only weakly with bulk MgO. The activation of O(2) is mainly due to charge transferred from the support. The relevance of these results for catalytic activity of small thin-film-supported clusters is discussed.

Topography and work function measurements of thin MgO(001) films on Ag(001) by nc-AFM and KPFM

The surface topography and local surface work function of ultrathin MgO(001) films on Ag(001) have been studied by noncontact atomic force microscopy (nc-AFM) and Kelvin probe force microscopy (KPFM). First principles calculations have been used to explain the contrast formation of nc-AFM images. In agreement with literature, thin MgO films grow in islands with a quasi rectangular shape. Contrary to alkali halide films supported on metal surfaces, where the island heights can be correctly measured, small MgO islands are either imaged as depressions or elevations depending on the electrostatic potential of the tip apex. Correct island heights therefore cannot be given without knowing the precise contrast formation discussed in this paper. KPFM shows a silver work function which is reduced by the MgO islands. The values for the work function differences for one and two layer thin films are -1.1 and -1.4 eV, respectively, in good agreement with recent calculations and experiments.

A DFT study of PtAu bimetallic clusters adsorbed on MgO/Ag(100) ultrathin films

We have performed density functional theory calculations on the properties of small PtAu(n) (n = 1-4) bimetallic clusters adsorbed on ultrathin MgO films deposited over the Ag(100) substrate. The PtAu bimetallic clusters show the occurrence of a charge transfer from the MgO/Ag interface to the supported nanoparticle with the formation of cluster anions. The Pt atom, which exhibits a stronger bonding with the oxide film and a higher diffusion barrier compared to Au, acts as a nucleation site for the bimetallic cluster and limits its diffusion on the surface.

Structural and electronic characterization of the MgO/Mo(001) interface using STM

The nucleation and growth of ultrathin MgO films on Mo(0 0 1) have been investigated with scanning tunneling microscopy and spectroscopy. In the initial growth stage, the MgO forms rather uniform islands with rectangular shapes and defined orientation. This behavior reflects a preferential binding of the oxide O ions to the top positions in the Mo support, which can be realized only in confined areas due to the MgO/Mo lattice mismatch. At monolayer coverage, a characteristic square pattern becomes visible in the STM, indicating the formation of an MgO/Mo coincidence lattice. In the coincidence cell, the interface registry alternates between O and Mg ions being in Mo top positions. The resulting imaging contrast in the STM is dominated by a work-function modulation and not by a topographic effect, as demonstrated with STM-conductance and light-emission spectroscopy. The modulated work function in the coincidence cell is assigned to a small buckling of the oxide film with either O or Mg ions being closer to the Mo surface.

Low Temperature CO Oxidation over Supported Ultrathin MgO Films

Density functional theory is used to investigate CO oxidation over an ultrathin MgO film supported on Ag(100). O(2) is found to be activated on MgO/Ag(100) whereas CO is only weakly bonded to the surface. These adsorption properties together with a low activation barrier render the MgO/Ag system an efficient catalyst for CO oxidation at low temperatures. As the predicted mechanism is general in nature, the result is suggested to have implications for a wide range of oxidation reactions.

X-ray Photoemission Study of the Charge State of Au Nanoparticles on Thin MgO/Fe(001) Films

The charge state of Au nanoparticles deposited on MgO/Fe(001) ultrathin films has been investigated experimentally by means of X-ray photoemission spectroscopy and theoretically by performing density functional theory (DFT) calculations. The binding energy of the Au 4f7/2 core-level peak has been studied as a function of the Au coverage and of the MgO film thickness. The Au 4f7/2 peak presents a high binding energy value (about 600 meV higher than that of bulk Au) for the smallest Au nanoparticles deposited on thick MgO films. The binding energy shift is only 200 meV for Au particles on very thin MgO films, indicating a different electronic configuration in this case. For larger Au coverages, the binding energy of the Au 4f7/2 peak converges to the bulk value for both thick and thin MgO films. In view of the results from DFT calculations, we show that this behavior is consistent with the occurrence of a charge flow from the MgO/Fe(001) interface to the Au nanoparticles. We also show that, on ultrathin MgO films, the Au 4f7/2 photoemission line width is much sharper than that for thicker oxide films, a result which is rationalized based on the different growth mode of Au nanoclusters when a charge-transfer process takes place.

Adsorption of Late Transition Metal Atoms on MgO/Mo(100) and MgO/Ag(100) Ultrathin Films: A Comparative DFT Study

The adsorption properties of Ni, Pd, Pt, Cu, Ag, and Au atoms on MgO ultrathin films deposited on Mo(100) or Ag(100) substrates are determined from first principle DFT calculations and compared with the corresponding adsorption characteristics on the bare MgO(100) surface. The three supports exhibit different behaviors. On MgO/Mo(100) thin films there is an electron transfer from the metal/oxide interface to all considered adatoms. In some cases, like for Pd, the charge transfer is small and the species remains atomic-like, while for other atoms like Cu, Ag, Au or Pt there is a net transfer of one electron with formation of full anions. This has dramatic consequences on the preferred adsorption sites, on the bond strength, and on the magnetic state of the adatoms: all these quantities are totally different from what is found for the same atoms on the bare MgO(100) surface. MgO/Ag(100) films have an intermediate behavior between that of MgO/Mo(100) thin films and of the MgO(100) surface: the charge transfe...

Study of MgO tunnel barriers with conducting atomic force microscopy

The local topographic and conducting properties of ultra-thin MgO films with polycrystalline Ru and amorphous CoFeB as lower electrodes were investigated. The local conductance and topography of the ultra-thin MgO films (from 0.5 to 1.0 nm) were simultaneously measured with a modified conducting atomic force microscope (CAFM). The imaging force was carefully chosen in order to avoid structural damages in the insulating layers. The promising results include the decrease of the density of hotspots with large conductance with the thickness of MgO and show that an insulating barrier is obtained at 1 nm thickness.

Charge-induced formation of linear Au clusters on thin MgO films: Scanning tunneling microscopy and density-functional theory study

Gold deposition onto ultrathin MgO films on Ag(001) results in the formation of linear Au clusters, as revealed from a combined scanning tunneling microscopy and density-functional theory study. The equilibrium structure of small Au clusters containing three to seven atoms is therefore different on thin film and bulk MgO(001) but also deviates from the shape of the respective gas-phase clusters. The peculiar one-dimensional growth regime of gold is stimulated by an electron transfer from the MgO/Ag interface to the deposited Au clusters, resulting in singly and doubly charged cluster anions. Only for larger atom numbers, the formation of compact two-dimensional clusters prevails on the MgO thin films.

Control and Manipulation of Gold Nanocatalysis: Effects of Metal Oxide Support Thickness and Composition

Control and tunability of the catalytic oxidation of CO by gold clusters deposited on MgO surfaces grown on molybdenum, Mo(100), to various thicknesses are explored through temperature-programmed reaction measurements on mass-selected 20-atom gold clusters and via first-principles density functional theory calculations. Au(20) was chosen because in the gas phase it is characterized as an extraordinarily stable tetrahedral-pyramidal structure. Dependencies of the catalytic activities and microscopic reaction mechanisms on the thickness and stoichiometry of the MgO films and on the dimensionalities and structures of the adsorbed gold clusters are demonstrated and elucidated. Langmuir-Hinshelwood mechanisms and reaction barriers corresponding to observed low- and high-temperature CO oxidation reactions are calculated and analyzed. These reactions involve adsorbed O(2) molecules that are activated to a superoxo- or peroxo-like state through partial occupation of the antibonding orbitals. In some cases, we find activated, dissociative adsorption of O(2) molecules, adsorbing at the cluster peripheral interface with the MgO surface. The reactant CO molecules either adsorb on the MgO surface in the cluster proximity or bind directly to the gold cluster. Along with the oxidation reactions on stoichiometric ultrathin MgO films, we also study reactions catalyzed by Au(20) nanoclusters adsorbed on relatively thick defect-poor MgO films supported on Mo and on defect-rich thick MgO surfaces containing oxygen vacancy defects.

F and F+ Centers on MgO/Ag(100) or MgO/Mo(100) Ultrathin Films: Are They Stable?

Using density functional theory (DFT) calculations we have investigated the electronic structure and stability of diamagnetic F0 and paramagnetic F+ centers in perfect and stepped ultrathin MgO films on Ag(100) and Mo(100) supports. The results show that when the one-electron energy of an F0 defect state is very close to the metal Fermi level, like in MgO films on Ag(100), the doubly occupied F0 center is metastable and can transform spontaneously into the paramagnetic F+ center by losing one of its electrons to the metal support. The phenomenon is enhanced when the vacancy is closer to the interface or is located at a step edge. On the MgO/Mo(100) films, where the Fermi level is at higher energy, F0 centers are stable and do not show a tendency to transform into F+, but F+ centers are unstable and capture one electron from the Mo metal to generate the corresponding doubly occupied vacancy.