Ultrathin MgO Films Research Articles

Isophorone on Au/MgO/Ag(001): Physisorption with Electrostatic Site Selection

We report a computational study of isophorone C9H14O adsorption on a Ag(001)-supported ultrathin MgO film with Au adatoms and clusters employing density functional theory calculations. The calculations show that the keto form of isophorone is more stable than the enol tautomers both in gas phase and on the MgO/Ag(001) surface. The interaction between the keto isophorone and step and terrace sites of MgO/Ag(001) displays long interaction distances, relatively weakly exothermic adsorption energies, lack of charge transfer, and minor changes in the density of states, all of which indicate that the molecule merely physisorbs on the surface. The step sites are energetically preferred adsorption sites due to attractive electrostatic interaction between more exposed Mg2+ cations and the polar O=C bond in isophorone. The adsorption on the step sites is 0.6 to 0.8 eV more exothermic compared with terrace sites. The calculations suggest that isophorone can chemisorb on MgO/Ag(001) surface but requires the presence ...

Growth of Magnesium Oxide Thin Films Using Molecular Beam Epitaxy

Ultra thin MgO films (1-2 nm) are grown on Si/SiO2 substrates using molecular beam epitaxy (MBE). Different growth conditions such as substrate temperatures, annealing conditions and growth rates are tested in order to achieve optimum parameters for the smooth and uniform growth. The films were characterized ex-situ using atomic force microscopy and x ray diffractio. Our results demonstrate MBE growth of MgO films with an rms roughness better than 0.5 nm on Si/SiO2 substrates. These results are important for the applications of MgO films as tunnel barriers in spintronic devices.

Splitting methanol on ultra-thin MgO(100) films deposited on a Mo substrate.

The dissociation reaction of methanol on metal-supported MgO(100) films has been studied by employing density functional theory calculations. As far as we know, the dissociation of a single methanol molecule over inert oxide insulators such as MgO has not yet been successfully realized without the introduction of defects or low coordinated atoms. By depositing ultra-thin oxide films on a Mo substrate, we have successfully proposed the dissociative state of methanol. The dissociation reaction is energetically exothermic and nearly barrierless. The lattice mismatch between ultra-thin MgO(100) films and metal substrates plays a crucial role in the heterolytic dissociation of adsorbates, while the electronic effect of the Mo(100) substrate plays a non-ignorable role in the homolytic dissociation of methanol. The metal-supported ultra-thin oxide films studied herein provide a versatile approach to enhance the surface reaction activity and properties of oxides.

A DFT study of (WO3)3nanoclusters adsorption on defective MgO ultrathin films on Ag(001)

The structures and electronic properties of (WO3)3nanocluster adsorption on defective MgO ultrathin films supported on Ag(001) metal surfaces have been investigated by means of density functional theory (DFT) calculations including dispersion interactions.

Reprint of “Theoretical description of metal/oxide interfacial properties: The case of MgO/Ag(001)”

We compare the performances of different DFT functionals applied to ultra-thin MgO(100) films supported on the Ag(100) surface, a prototypical system of a weakly interacting oxide/metal interface, extensively studied in the past. Beyond semi-local DFT-GGA approximation, we also use the hybrid DFT-HSE approach to improve the description of the oxide electronic structure. Moreover, to better account for the interfacial adhesion, we include the van de Waals interactions by means of either the semi-empirical force fields by Grimme (DFT-D2 and DFT-D2*) or the self-consistent density functional optB88-vdW. We compare and discuss the results on the structural, electronic, and adhesion characteristics of the interface as obtained for pristine and oxygen-deficient Ag-supported MgO films in the 1–4ML thickness range.

Ultrathin IBAD MgO films for epitaxial growth on amorphous substrates and sub-50 nm membranes

A fabrication process has been developed for high energy ion beam assisted deposition (IBAD) biaxial texturing of ultrathin (∼1 nm) MgO films, using a high ion-to-atom ratio and post-deposition annealing instead of a homoepitaxial MgO layer. These films serve as the seed layer for epitaxial growth of materials on amorphous substrates such as electron/X-ray transparent membranes or nanocalorimetry devices. Stress measurements and atomic force microscopy of the MgO films reveal decreased stress and surface roughness, while X-ray diffraction of epitaxial overlayers demonstrates the improved crystal quality of films grown epitaxially on IBAD MgO. The process simplifies the synthesis of IBAD MgO, fundamentally solves the “wrinkle” issue induced by the homoepitaxial layer on sub-50 nm membranes, and enables studies of epitaxial materials in electron/X-ray transmission and nanocalorimetry.

Theoretical description of metal/oxide interfacial properties: The case of MgO/Ag(001)

We compare the performances of different DFT functionals applied to ultra-thin MgO(100) films supported on the Ag(100) surface, a prototypical system of a weakly interacting oxide/metal interface, extensively studied in the past. Beyond semi-local DFT-GGA approximation, we also use the hybrid DFT-HSE approach to improve the description of the oxide electronic structure. Moreover, to better account for the interfacial adhesion, we include the van de Waals interactions by means of either the semi-empirical force fields by Grimme (DFT-D2 and DFT-D2*) or the self-consistent density functional optB88-vdW. We compare and discuss the results on the structural, electronic, and adhesion characteristics of the interface as obtained for pristine and oxygen-deficient Ag-supported MgO films in the 1–4ML thickness range.

Properties of two-dimensional insulators: A DFT study of bimetallic oxide CrW2O9 clusters adsorption on MgO ultrathin films

Periodic density functional theory calculations have been performed to study the electronic properties of bimetallic oxide CrW2O9 clusters adsorbed on MgO/Ag(001) ultrathin films (<1nm). Our results show that after deposition completely different structures, electronic properties and chemical reactivity of dispersed CrW2O9 clusters on ultrathin films are observed compared with that on the thick MgO surface. On the thick MgO(001) surface, adsorbed CrW2O9 clusters are distorted significantly and just a little electron transfer occurs from oxide surface to clusters, which originates from the formation of adsorption dative bonds at interface. Whereas on the MgO/Ag(001) ultrathin films, the resulting CrW2O9 clusters keep the cyclic structures and the geometries are similar to that of gas-phase [CrW2O9]−. Interestingly, we predicted the occurrence of a net transfer of one electron by direct electron tunneling from the MgO/Ag(001) films to CrW2O9 clusters through the thin MgO dielectric barrier. Furthermore, our work reveals a progressive Lewis acid site where spin density preferentially localizes around the Cr atom not the W atoms for CrW2O9/MgO/Ag(001) system, indicating a potentially good bimetallic oxide for better catalytic activities with respect to that of pure W3O9 clusters. As a consequence, present results reveal that the adsorption of bimetallic oxide CrW2O9 clusters on the MgO/Ag(001) ultrathin films provide a new perspective to tune and modify the properties and chemical reactivity of bimetallic oxide adsorbates as a function of the thickness of the oxide films.

Strain-induced water dissociation on supported ultrathin oxide films.

Controlling the dissociation of single water molecule on an insulating surface plays a crucial role in many catalytic reactions. In this work, we have identified the enhanced chemical reactivity of ultrathin MgO(100) films deposited on Mo(100) substrate that causes water dissociation. We reveal that the ability to split water on insulating surface closely depends on the lattice mismatch between ultrathin films and the underlying substrate, and substrate-induced in-plane tensile strain dramatically results in water dissociation on MgO(100). Three dissociative adsorption configurations of water with lower energy are predicted, and the structural transition going from molecular form to dissociative form is almost barrierless. Our results provide an effective avenue to achieve water dissociation at the single-molecule level and shed light on how to tune the chemical reactions of insulating surfaces by choosing the suitable substrates.

DFT Study of CO2 Activation on Doped and Ultrathin MgO Films.

The bonding mode of carbon dioxide with the surface of various forms of MgO has been investigated by means of density functional theory calculations. Four supports have been considered: the bare MgO(100) surface, the surface of Al-doped MgO, and ultrathin MgO/Ag(100) and MgO/Mo(100) films. Three forms of adsorbed CO2 have been investigated: physisorbed CO2, chemisorbed carboxylate, CO2–, and carbonate, CO32–. While on MgO(100) CO2 forms either the physisorbed species or the more stable surface carbonate, on Al-doped MgO carboxylate is the preferred species. The adsorption properties of one- or two-layer MgO films differ completely as a function of the metal support. On MgO/Ag(100) the properties of adsorbed CO2 are very similar to those of the MgO(100) surface (formation of carbonate, not of carboxylate); on MgO/Mo(100), on the contrary, both carbonate and carboxylate species can form, depending on the film thickness. On a one-layer film, both species are formed with a comparable stability, while for thic...

Si(100)上のMgO超薄膜における表面ポテンシャルエネルギーのアニールによる変化

Si(100)上のMgO超薄膜における表面ポテンシャルエネルギーのアニールによる変化

Ultra-thin MgO(111)-polar sheets grown onto Ag(111)

An ultra-thin MgO film of two monolayers (ML) was successfully grown onto Ag(111) by repeating twice RT adsorption of Mg atomic monolayer, RT oxidation and 725K thermal annealing under UHV conditions. Using Auger electron spectroscopy, low electron energy diffraction and scanning tunneling microscopy/spectroscopy, we report here formation of MgO(1x1) and MgO(1x1)-R30 epitaxial layers, consistent with the growth of a MgO(111) polar film onto the silver surface and composed of patches having extended slight corrugation oriented along defined directions with respect to the substrate.

Morphology, Work Function, and Silver Ad-Structures of High-Temperature Grown Ultrathin MgO Films on Ag(001)

We characterized the high-temperature growth of ultrathin MgO films on Ag(001) at 783 K through analyzing the film’s morphology and work function by noncontact atomic force microscopy (nc-AFM), scanning tunneling microscopy (STM), Kelvin probe force microscopy (KPFM) and assisting low-energy electron diffraction (LEED). At submonolayer coverage, one-atomic-plane-high MgO islands are formed, which are preferentially embedded into the silver surface and which are larger and more regular than the islands of those films normally grown between 500 and 600 K. KPFM shows that embedded MgO islands decrease the silver work function (WF) by ΔϕMgO–Ag = −50 ± 20 meV, whereas a small amount of supported islands increase the WF by ΔϕMgO–Ag = +50 ± 20 meV. When the MgO film starts to cover entirely the silver surface, ad-islands, nanoparticles (NPs), and much larger particles of silver are formed, which is due to the high mobility of silver and the embedding of the MgO islands into the silver surface. Our work opens the...

Probing the electronic properties and charge state of gold nanoparticles on ultrathin MgO versus thick doped CaO films

Electron transfer into metal nanoparticles on oxide supports is associated with unusual morphological, electronic, and chemical properties of the charged system. Two fundamental charging routes have been identified so far, which are electron tunneling through ultrathin oxide films supported by a bulk metal and charge donation from single-ion impurities embedded in the oxide matrix. In this study, we have investigated whether both routes lead to the formation of metal deposits with identical properties. For this purpose, Au islands have been prepared on $1\ensuremath{-}2\phantom{\rule{0.16em}{0ex}}\mathrm{ML}$ thin $\mathrm{MgO}/\mathrm{Ag}(001)$ layers and on 25-ML-thick CaO films doped with Mo impurities. The morphological and electronic properties of the islands were analyzed with low-temperature scanning tunneling microscopy (STM) and spectroscopy. In both systems, pronounced electron confinement effects are observed in the nanostructures, arising from the quantization of one and the same Au electronic band. Moreover, clear experimental signatures for a charge transfer into the islands are found, such as a layer-by-layer growth of the ad-metal and a negative contrast of the oxide region around the deposits in STM images. Our data provide evidence that the charged nanostructures exhibit comparable properties independent of the origin of the extra electrons. This agreement suggests that ultrathin oxide films may be used as model systems for doped bulk oxides, as used in heterogeneous catalysis.

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

We report layer-resolved measurements of the \textit{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 as well as a surface core exciton involving an image-potential state of the vacuum are revealed through resonant Auger spectroscopy of the Mg $KL_{23}L_{23}$ Auger transition. Our results demonstrate how to obtain new insights on \textit{empty} states at interfaces of metal-supported ultrathin oxide films.

Ultrathin MgO diffusion barriers for ferromagnetic electrodes on GaAs(001)

Ultrathin MgO(100) films serving as a diffusion barrier between ferromagnetic electrodes and GaAs(001) semiconductor templates have been investigated. Using Fe as an exemplary ferromagnetic material, heterostructures of Fe/MgO/GaAs(001) were prepared at 200 °C with the MgO thickness ranging from 1.5 to 3 nm. Structural characterization reveals very good crystalline ordering in all layers of the heterostructure. Auger electron spectroscopy depth-profiling and cross-sectional high-resolution transmission electron microscopy evidence diffusion of Fe into MgO and—for too thin MgO barriers—further into GaAs(001). Our results recommend a MgO barrier thickness larger than or equal to 2.6 nm for its application as a reliable diffusion barrier on GaAs(001) in spintronics devices.

A Combined Effect of Plasmon Energy Transfer and Recombination Barrier in a Novel TiO2/MgO/Ag Working Electrode for Dye-Sensitized Solar Cells

Novel TiO2/MgO/Ag composite electrodes were applied as working electrodes of dye-sensitized solar cells (DSSCs). The TiO2/MgO/Ag composite films were prepared by dip coating method for MgO thin films and photoreduction method for Ag nanoparticles. The MgO film thicknesses and the Ag nanoparticle sizes were in ranges of 0.08–0.46 nm and 4.4–38.6 nm, respectively. The TiO2/MgO/Ag composite films were characterized by X-ray diffraction, scanning electron microscopy, and transmission electron microscopy. The TiO2/MgO/Ag composite electrodes were sensitized by immersing in a 0.3 mM of N719 dye solution and fabricated for conventional DSSCs.J-Vcharacteristics of the TiO2/MgO/Ag DSSCs showed that the MgO film thickness of 0.1 nm and the Ag nanoparticle size of 4.4 nm resulted in maximum short circuit current density and efficiency of 8.6 mA/cm2and 5.2%, respectively. Electrochemical Impedance Spectroscopy showed that such values of short circuit current density and efficiency were optimal values obtained from plasmon energy transfer by 4.4 nm Ag nanoparticles and recombination barrier by the ultrathin MgO film.

Schottky barrier modulation using ultrathin MgO for metal–silicon (100) contacts

In this work, ultrathin MBE-grown MgO has been employed as a thin tunneling interlayer to modulate the Schottky barrier height (SBH) between metal contacts and Si substrates. The ultrathin MgO films were grown with different starting first monolayers (O2, MgO, and Mg) on Si. With an MgO ultrathin film, all contacts show rectifying behavior for both n- and p-type Si. The SBH generally increases (decreases) with increasing metal work function for n-type (and p-type) and is generally lowest for the oxygen first treatment. To our knowledge it is the first time that the role of the first monolayer of an oxide tunnel barrier on the SBH is revealed. These results indicate a novel, interesting way to modulate the barrier height and hence the contact resistivity in CMOS devices.

How Growing Conditions and Interfacial Oxygen Affect the Final Morphology of MgO/Ag(100) Films

In spite of the relevance of ultrathin MgO films for the study of model systems as well as for technological applications, great difficulties have been found so far in the growth of extended, well-ordered, ultrathin films. Combining scanning tunneling microscopy, X-ray photoemission spectroscopy, and high-resolution electron energy loss spectroscopy experiments with ab initio calculations, we demonstrate here that the structure of sub-monolayer MgO films grown on Ag(100) by reactive deposition is strongly affected not only by the growth conditions but also by after-growth treatments. The latter ones allow one to quench the thermodynamically most stable configuration at the deposition temperature or let the system evolve toward the low-temperature equilibrium state. Moreover, we give experimental and theoretical evidence of the accumulation of oxygen atoms at the MgO/Ag interface at the highest deposition temperature, which reduces the stress of the oxide film favoring the formation of extended terraces. The result is the possibility to tune the morphology of the films from small islands with corrugated borders, to perfectly square islands of larger size, to MgO terraces several tens of nanometers wide.

Ab-initio study of the interfacial properties in ultrathin MgO films on O-rich FeO/Fe(001) surfaces

Using ab-initio simulations based on density functional theory, we systematically studied the interfacial properties of MgO films on O-rich FeO/Fe(001) surfaces with increasing number of MgO layers from one to three monolayers (MLs). The structural and the adhesion properties of the MgO/FeO/Fe(001) system were assessed and compared with those of simple MgO/Fe(001) interfaces. Our calculated results showed that the adhesion energy for MgO/FeO/Fe(001) was smaller than that for simple MgO/Fe(001). An analysis of the electronic structures and the charge rearrangements of the MgO/FeO/Fe(001) interfaces was also performed. The work functions of the MgO/FeO/Fe(001) systems upon the deposition of MgO films exhibited smaller decreases (0.45–0.67 eV) than those (1.43–1.74 eV) of the MgO/Fe(001) systems. In addition, the obtained work functions (3.77–3.99 eV) for MgO/FeO/Fe(001) were much larger than those (2.12–2.43 eV) for MgO/Fe(001).