Ultrathin MgO Research Articles

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.

Band Gap Narrowing of 2-D Ultra-Thin MgO Graphene-Like Sheets

Ultra-thin MgO nanosheets obtained by a sol-gel method were found to have cubic crystal structure with a space group of Fm-3m. The thickness of the sheets for the 600°C and 5 h annealing condition sample is 2 nm. Although the MgO sheets are two-dimensional and ultra-thin, unlike graphene, they are polycrystalline. Their bandgap energies are dependent on the thermal annealing conditions (which affect the thickness of the MgO nanosheets). It is found that there was bandgap narrowing of the MgO nanosheets with respect to the bulk values. UV-visible spectroscopy and X-Ray Photoelectron spectroscopy (XPS) enables us to identify the shifts of the valence and conduction bands causing the bandgap narrowing.

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

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...

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.

Effects of interfacial Fe electronic structures on magnetic and electronic transport properties in oxide/NiFe/oxide heterostructures

We report that the magnetic and electronic transport properties in oxide/NiFe(2nm)/oxide film (oxide=SiO2, MgO or HfO2) are strongly influenced by the electronic structure of NiFe/oxide interface. Magnetic measurements show that there exist magnetic dead layers in the SiO2 sandwiched film and MgO sandwiched film, whereas there is no magnetic dead layer in the HfO2 sandwiched film. Furthermore, in the ultrathin SiO2 sandwiched film no magnetoresistance (MR) is detected, while in the ultrathin MgO sandwiched film and HfO2 sandwiched film the MR ratios reach 0.35% and 0.88%, respectively. The investigation by X-ray photoelectron spectroscopy reveals that the distinct interfacial redox reactions, which are dependent on the oxide layers, lead to the variation of magnetic and transport properties in different oxide/NiFe/oxide heterostructures.

Giant thermal spin-torque–assisted magnetic tunnel junction switching

Spin-polarized charge currents induce magnetic tunnel junction (MTJ) switching by virtue of spin-transfer torque (STT). Recently, by taking advantage of the spin-dependent thermoelectric properties of magnetic materials, novel means of generating spin currents from temperature gradients, and their associated thermal-spin torques (TSTs), have been proposed, but so far these TSTs have not been large enough to influence MTJ switching. Here we demonstrate significant TSTs in MTJs by generating large temperature gradients across ultrathin MgO tunnel barriers that considerably affect the switching fields of the MTJ. We attribute the origin of the TST to an asymmetry of the tunneling conductance across the zero-bias voltage of the MTJ. Remarkably, we estimate through magneto-Seebeck voltage measurements that the charge currents that would be generated due to the temperature gradient would give rise to STT that is a thousand times too small to account for the changes in switching fields that we observe.

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.

Interface properties of FeMgOCu-phthalocyanine metal-insulator-organic semiconductor structures

Hybrid interface structures consisting of organic copper-phthalocyanine (CuPc) and ferromagnetic metal Fe(001) with and without a MgO(001) cover were investigated by using surface sensitive techniques of X-ray photoemission spectroscopy and ultraviolet photoemission spectroscopy. A systematic study of the energy level alignment at the interfaces was carried out. For the hybrid interfaces considered here, our results indicate that the insertion of an artificially-grown ultra-thin oxide layer MgO(001) can prevent Femi level pinning and induce a rather large interface dipole, thereby resulting in remarkable CuPc Fermi level shifts when the thickness of the CuPc film is less than 3 nm. This study provides a better understanding of spin filtering in MgO-based organic spin devices and a new way to alter the interface electronic structure of metalorganic semiconductor hybrid systems.

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.

Perpendicular magnetic anisotropy at the interface between ultrathin Fe film and MgO studied by angular-dependent x-ray magnetic circular dichroism

Interface perpendicular magnetic anisotropy (PMA) in ultrathin Fe/MgO (001) has been investigated using angular-dependent x-ray magnetic circular dichroism (XMCD). We found that anisotropic orbital magnetic moments deduced from the analysis of XMCD contribute to the large PMA energies, whose values depend on the annealing temperature. The large PMA energies determined from magnetization measurements are related to those estimated from the XMCD and the anisotropic orbital magnetic moments through the spin-orbit interaction. The enhancement of anisotropic orbital magnetic moments can be explained mainly by the hybridization between the Fe 3dz2 and O 2pz states.

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).

The role of exposed silver in CO oxidation over MgO(0 0 1)/Ag(0 0 1) thin films

The reactivity of MgO(001) films deposited on Ag(001) and Mo(001) in CO oxidation as a function of oxide film thickness was investigated experimentally at ambient pressure reaction conditions. MgO films grown on Mo(001) were found to be inactive in CO oxidation, whereas activity enhancement with decreasing oxide film thickness was observed for MgO(001)/Ag(001). In situ infrared and post-reaction X-ray photoemission data showed that ultra-thin MgO films interact much more strongly with the reactants and residual water than bulk-like MgO. Poisoning of the MgO surfaces by the resultant accumulation of carbonate and hydroxyl species is suggested to inhibit the reactivity in CO oxidation. Detailed investigations of the surface structure of MgO(001)/Ag(001) films indicate that silver ad-islands on Ag(001), which are formed during MgO growth, are responsible for the enhancement of CO oxidation activity over ultrathin MgO(001)/Ag(001) films.

Morphology of monolayer MgO films on Ag(100): switching from corrugated islands to extended flat terraces.

The ability to engineer nearly perfect ultrathin oxide layers, up to the limit of monolayer thickness, is a key issue for nanotechnological applications. Here we face the difficult and important case of ultrathin MgO films on Ag(100), for which no extended and well-ordered layers could thus far be produced in the monolayer limit. We demonstrate that their final morphology depends not only on the usual growth parameters (crystal temperature, metal flux, and oxygen partial pressure), but also on aftergrowth treatments controlling so far neglected thermodynamics constraints. We thus succeed in tuning the shape of the oxide films from irregular, nanometer-sized, monolayer-thick islands to slightly larger, perfectly squared, bilayer islands, to extended monolayers limited apparently only by substrate steps.

Adsorbate-Induced Oxygen Vacancy Mobility in Ultrathin Oxide Films

Oxides at the nanometric scale show a behavior markedly different from that of their bulk counterparts. Ultrathin oxides grown on metals do not reach the full insulator regime, and they cannot decouple the electronic clouds of incoming adsorbates from that of the metal substrate. Although oxygen vacancies control the chemical and physical properties of ultrathin oxide films, the role of intrinsic defects has been overlooked so far. By means of density functional theory methods, we show that the addition of atoms with high electron affinity, such as Au, to ultrathin MgO grown either on a Ag or Mo support, completely reverses the preferential positions of oxygen vacancies, decreases their residual charge, and enhances their mobility. Oxygen vacancies behave completely different for the MgO/M systems compared to the Au/MgO/M systems, which opens a new way to tune the reactivity of adsorbates.