Dynamically stabilized growth of polar oxides: the case of MgO(111)

By using MgO(111) as a model system for polar oxide film growth, we show by first-principles calculations that H acts as a surfactant, i.e., the H changes its position and bonding during the growth process, remaining in the surface region. Continuous presence of H during the growth of MgO(111) film efficiently removes the microscopic dipole moment, thus enabling the growth of perfect fcc-ordered MgO(111) films. These theoretical predictions are confirmed experimentally by molecular beam epitaxy single crystal growth of MgO(111) on SiC(0001).     

Selected growth of cubic and hexagonal GaN epitaxial films on polar MgO(111)

Selected molecular beam epitaxy of zinc blende (111) or wurtzite (0001) GaN films on polar MgO(111) is achieved depending on whether N or Ga is deposited first. The cubic stacking is enabled by nitrogen-induced polar surface stabilization, which yields a metallic MgO(111)-(1 x 1)-ON surface. High-resolution transmission electron microscopy and density functional theory studies indicate that the atomically abrupt semiconducting GaN(111)/MgO(111) interface has a Mg-O-N-Ga stacking, where the N atom is bonded to O at a top site. This specific atomic arrangement at the interface allows the cubic stacking to more effectively screen the substrate and film electric dipole moment than the hexagonal stacking, thus stabilizing the zinc blende phase even though the wurtzite phase is the ground state in the bulk.     

Charging of metal adatoms on ultrathin oxide films: Au and Pd on FeO/Pt(111)

We present a combined experimental (STM/scanning tunneling spectroscopy) and theoretical (density functional theory) study on the deposition of Au and Pd metal atoms on FeO/Pt(111) ultrathin films. We show that while the Pd atoms are only slightly oxidized, the Au atoms form positive ions upon deposition, at variance to a charge transfer into the Au atoms as observed for MgO/Ag(100). The modulation of the adsorption properties within the surface Moiré cell and the charging induce the formation a self-assembled array of gold adatoms on FeO/Pt(111), whereas Pd atoms are randomly distributed.     

Polar oxide interface stabilization by formation of metallic nanocrystals

In situ x-ray photoelectron spectroscopy and ex situ transmission electron microscopy and diffraction studies of a model Fe3O4(111)/MgO(111) polar oxide interface exclude stabilization by interface faceting, reconstruction, or by formation of a continuous interfacial layer with altered stoichiometry, and uncover stabilization by dominant formation of metallic Fe(110) nanocrystals. The iron nanocrystals nucleate both at the interface and within the magnetite film and grow in a Nishiyama-Wasserman orientation relationship with a bimodal size distribution related to twinning. Minority magnetite nanocrystals were also observed, growing in the less polar (100) orientation than the magnetite (111) film. Electron transfer and bond hybridization mechanisms are likely at the metal/oxide and oxide/oxide interfaces and remain to be explored.     

Layer-by-layer epitaxial growth of polar FeO(111) thin films on MgO(111)

We report on the structural properties of epitaxial FeO layers grown by molecular beam epitaxy on MgO(111). The successful stabilization of polar FeO films as thick as 16 monolayers (ML), obtained by deposition and subsequent oxidation of single Fe layers, is presented. FeO/MgO(111) thin films were chemically and structurally characterized using low-energy electron diffraction, Auger electron spectroscopy and conversion electron Mössbauer spectroscopy (CEMS). Detailed in situ CEMS measurements as a function of the film thickness demonstrated a size-effect-induced evolution of the hyperfine parameters, with the thickest film exhibiting the bulk-wüstite hyperfine pattern. Ex situ CEMS investigation confirmed existence of magnetic ordering of the wüstite thin film phase at liquid nitrogen temperature.     

Surface resistance measurements at the metal/electrolyte interface of Ag(100) and Ag(111) thin film electrodes

The conductivity of thin film metal electrodes with a thickness of the order of the mean free path of the conduction electrons (50 nm at 300 K) is sensitive to several processes on the metal surface (e.g. adsorption and desorption of ions). We developed epitaxially grown Ag(100)/MgO(100) and Ag(111)/TiO₂(110) electrodes of 20 nm thickness. The change in the surface resistance of Ag(100) thin film electrodes during adsorption of the halide ions Cl⁻, Br⁻ and I⁻ shows the different strengths of specific adsorption. We investigated the phase transition of thiocyanate (SCN⁻) on Ag(100) electrodes by combining the surface resistance method with voltammetric, capacitance and ex-situ XPS measurements. The influence of adsorbed uracil on the resistance of Ag(100) films was demonstrated. The surface resistance is very sensitive to small concentrations of metal cations (e.g. Tl⁺). The surface resistance of Ag(100) and Ag(111) thin film electrodes shows the typical difference in the stripping potential of Tl⁺ of about 100 mV.     

Ag growth on Si(111) with an Sb surfactant investigated by scanning tunneling microscopy

We have investigated a room-temperature growth mode of ultrathin Ag films on a Si(111) surface with an Sb surfactant using STM in a UHV system. On the Sb-passivated Si surface, small sized islands were formed up to 1.1 ML. Flat Ag islands were dominant at 2.1 ML, coalescing into larger islands at 3.2 ML. Although the initial growth mode of Ag films on the Sb-terminated Si(111) surface was Volmer-Weber (island growth), the films were much more uniform than Ag growth on clean (Si(111) at the higher coverages. From the analysis of STM images of Ag films grown with and without an Sb surfactant, the uniform growth of Ag films using an Sb surfactant appears to be caused by the kinetic effects of Ag on the preadsorbed Sb layer. Our STM results indicated that Sb suppresses the surface diffusion of Ag atoms and increases the Ag-island density. The increased island density is believed to cause coalescence of Ag islands at higher coverages of Ag, resulting in the growth of atomically flat and uniform Ag islands on the Sb surfactant layer.     

Activation of Al2O3 by a long-ranged chemical bond mechanism

On the basis of density functional theory calculations, a novel bond mechanism with chemical strength but of long-ranged character is proposed. As compared to adsorption on a sole oxide or metal, the mechanism is found to enhance the bond strength between an adsorbate and an oxide in contact with a metal. As a model system, NO2 adsorption on alpha-Al2O3(0001) either in contact with Ag(111) or small Agn clusters is used. The observed phenomenon rests on charge transfer and modification of the oxide-metal interface. The mechanism is general in nature and the implication for heterogeneous catalysis is discussed.     

Formation of hydroxyl and water layers on MgO films studied with ambient pressure XPS

To understand the interaction of water with MgO(100), a detailed quantitative assessment of the interfacial chemistry is necessary. We have used ambient pressure X-ray photoelectron spectroscopy (XPS) to measure molecular (H₂O) and dissociative (OH) water adsorption on a 4 monolayer (ML) thick MgO(100)/Ag(100) film under ambient conditions. Since the entire 4 ML metal oxide (Ox) film is probed by XPS, the reaction of the MgO film with water can be quantitatively studied. Using a multilayer model (Model 1) that measures changes in Ox thickness from O 1s (film) and Ag 3d (substrate) spectra, it is shown that the oxide portion of the MgO film becomes thinner upon hydroxylation. A reaction mechanism is postulated in which the top-most layer of MgO converts to Mg(OH)₂ upon dissociation of water. Based on this mechanism a second model (Model 2) is developed to calculate Ox and OH thickness changes based on OH/Ox intensity ratios from O 1s spectra measured in situ, with the known initial Ox thickness prior to hydroxylation. Models 1 and 2 are applied to a 0.15 Torr isobar experiment, yielding similar results for H₂O, OH and Ox thickness changes as a function of relative humidity.     

离轴生长的MgB_2超导薄膜的研究

通过混合物理化学沉积法(hybrid physical-chemical vapor deposition,简称为HPCVD),我们在多种单晶衬底上制备出了MgB2超导薄膜.经测量发现MgB2薄膜在6H-SiC、c-Al2O3、MgO(111)和YSZ(111)衬底上基本是沿c轴外延生长的,形成单晶薄膜;在MgO(211)、MgO(100)和钇稳定氧化锆(YSZ)(110)衬底上,MgB2晶粒的生长表现出明显的择优取向生长方式,并与晶格常数匹配关系所预言的结果一致。     

Structure of Ag(111)-p(4 x 4)-O: no silver oxide

The structure of the oxygen-induced p(4 x 4) reconstruction of Ag(111) is determined by a combination of scanning tunneling microscopy, surface x-ray diffraction, core level spectroscopy, and density functional theory. We demonstrate that all previous models of this surface structure are incorrect and propose a new model which is able to explain all our experimental findings but has no resemblance to bulk silver oxide. We also shed some light on the limitations of current density functional theories and the potential role of van der Waals interactions in the stabilization of oxygen-induced surface reconstructions of noble metals.     

Exploring magnetic properties of ultrathin epitaxial magnetic structures using magneto-optical techniques

We describe magneto-optic Kerr effect studies of ultrathin Fe and Ni films on single crystal surfaces of Ag and Cu. Monolayer Fe films on Ag(100) exhibit the theoretically predicted spin-orbit anisotropy, but also yield some interesting discrepancies between behavior predicted by Kerr effect and by spin-polarized photoemission experiments. Layer-dependent studies of the magnetic moment of Ni on Ag(111) and Ag(100) suggest sp-d hybridization effects quench the first layer magnetic moment on Ag(111) but not on Ag(100). Temperature dependent studies of thin film magnetization obtained from Kerr effect measurements yield thickness dependent Curie temperatures, and critical exponents for several thin film systems.     

Fabrication of transparent p-n junction composed of heteroepitaxially grown p-Li<Subscript>0.15</Subscript>Ni<Subscript>0.85</Subscript>O and n-ZnO films for UV-detector applications

Thin films of high-quality p-type Li_0.15Ni_0.85O (LNO) and n-type ZnO were heteroepitaxially grown on MgO(111) substrate by pulsed laser deposition technique to form transparent wide bandgap heterojunctions. The epitaxial nature of this p-LNO/n-ZnO/MgO heterojunction was confirmed to be (111)LNO||(0001)ZnO||(111)MgO (out-of-plane) and (002)LNO||(1002)ZnO||(002)MgO (in-plane) by X-ray diffraction. Optical transmittance spectrum and I–V characteristics were obtained at room temperature. The heterojunction exhibits reasonable optical transmittance of 50–60% on average in the whole infrared and visible region, and highly asymmetric electrical rectification with a turn-on voltage of about 1.0 V and a small leakage current. The highest photoresponsivity for a wavelength of 350 nm is 3.4×10³ V/W when the junction is irradiated under 5 μW UV illumination. The spectral response peak is obtained in the UV region and a reasonable large responsivity is shown for this p-LNO/n-ZnO/MgO heterojunction, which suggests the possibility of an inexpensive transparent oxide UV detector in a wide variety of electronics applications. PACS 68.55.Jk; 81.05.Dz; 81.15.Fg     

Tuning the growth orientation of epitaxial films by interface chemistry

The support of epitaxial films frequently determines their crystallographic orientation, which is of crucial importance for their properties. We report a novel way to alter the film orientation without changing the substrate. We show for the growth of CoO on the Ir(100) surface that, while the oxide grows in (111) orientation on the bare substrate, the orientation switches to (100) by introducing a single (or a few) monolayer(s) of Co between the oxide and substrate. This tunability of the orientation of epitaxial films by the appropriate choice of interface chemistry most likely is a general feature.     

Intrinsic stress of ultrathin epitaxial films

The present article focuses on the stress developing during the deposition of ultrathin epitaxial films in the thickness range of a few atomic layers. The studied systems exhibit the three well-known modes of film growth: Stranski–Krastanow mode [Ge/Si(001), Ge/Si(111), Ag/Si(111)], Frank–Van der Merwe mode [Fe/MgO(001)] and Volmer–Weber mode [Ag/mica(001), Cu/mica(001)]. The experimental results demonstrate the important role of the misfit strain as well as the contribution of surface stress effects as mechanisms for the stress in single atomic layers. Received: 26 April 1999 / Accepted: 25 June 1999 / Published online: 6 October 1999     

Study on the orientation of silver films by ion-beam assisted deposition

Low energy ion beam assisted deposition (IBAD) was employed to prepare Ag films on Mo/Si (100) substrate. It was found that Ag films deposited by sputtering method without ion beam bombardment were preferred (111) orientation. When the depositing film was simultaneously bombardment by Ar⁺ beam perpendicular to the film surface at ion/atom arrival ratio of 0.18, the prepared films exhibited weak (111) and (200) mixed orientations. When the direction of Ar⁺ beam was off-normal direction of the film surface, Ag films showed highly preferred (111) orientation. Monte Carlo method was used to calculate the sputtering yields of Ar⁺ ions at various incident and azimuth angles. The effects of channeling and surface free energy on the crystallographic orientation of Ag films were discussed.     

Temperature dependence of photo-hole decay in 4d derived Quantum Well States in monolayer Ag(111) films on Pd(111), Ni(111), Mo(110) and Cu(100)

We present experimental data on the temperature dependence of photo-hole decay obtained by Angle Resolved Photoemission (ARPES) measurements from 4d derived Quantum Well States (QWS) on Ag(111) monolayer films deposited on Pd(111), Ni(111), Mo(110) and Cu(100). We have found a significant increase of the Ag 4d electron–phonon (e-ph) coupling strength with respect to the bulk values. The increase is attributed to different mechanisms that are associated with the interaction of the Ag film with under laying substrate. It is proposed that the main channels that contribute to the increased e-ph coupling originate from the inter-band transitions that involve bulk states of the substrates.     

IR absorption enhancement for physisorbed methanol on Ag island films deposited on the oxidized and H-terminated Si(111) surfaces: effect of the metal surface morphology

Infrared absorption measurements using a multiple internal reflection geometry are reported for condensed methanol at 90 K on Ag island films deposited on the oxidized and hydrogen-terminated surfaces of Si(111). The attenuated total reflection (ATR) spectra obtained as a function of methanol exposure (up to 14 L) show that a 1-nm mass thickness of Ag island film on the oxidized Si(111) surface yields an absorption intensity 2–3 times larger than the intensity in the absence of Ag on the oxidized surface. Deposition of the same thickness of Ag on the hydrogen-terminated Si(111) surface results in approximately twice the enhancement. The different magnitudes of the enhancement are discussed based on SEM micrographs for Ag island films formed on the oxidized and H-terminated Si(111) surfaces. Received: 1 March 1999 / Accepted: 8 March 1999 / Published online: 5 May 1999     

Introduction of Ag nanoparticles by picosecond LIFT to improve the photoelectric property of AZO films

The photoelectric properties of conductive films are improved by doping Ag on aluminum-doped zinc oxide(AZO) films by laser induced forward transfer(LIFT).Firstly,the picosecond laser induced transfer mechanism of Ag films was revealed by numerical simulation;then,different-thickness Ag films were deposited on the AZO films by picosecond LIFT.When the film thickness is 30 nm and,50 nm,we have successfully obtained some Ag-AZO films with better optoelectronic properties by adjusting the laser parameters.     

A review of the growth and structures of silicene on Ag(111)

Ag(111) is currently the most often used substrate for growing silicene films. Silicene forms a variety of different phases on the Ag(111) substrate. However, the structures of these phases are still not fully understood so far. In this brief review we summarize the growth condition and resulting silicene phases on Ag(111), and discuss the most plausible structural model and electronic property of individual phases. The existing debates on silicene on Ag(111) system are clarified as mush as possible.