MgO Film Thickness Research Articles

Insight into physical interpretation of high frequency time constant in electrochemical impedance spectra of Mg

Electrochemical impedance spectroscopy measurements were performed to study the evolution of surface condition, which related to the high frequency time constant of Mg, in 0.05 M NaCl solution at three different pH. Based on the assumption that the high frequency time constant originates solely from the surface film, the thickness of MgO film was calculated via fitting EIS results. The deduced results were compared with the actual surface condition, characterized by transmission electron microscopy and local measurements. The results verified that the time constant at the high frequency range originates from the barrier properties of the surface film.

Plane-wave many-body corrections to the conductance in bulk tunnel junctions

The conductance of bulk metal--insulator--metal junctions is evaluated by the Landauer formula using an \textit{ab initio} electronic structure calculated using a plane-waves basis set within density-functional theory (DFT) and beyond, i.e.\ including exact non-local exchange using hybrid functional (HSE) or many-body $G_0W_0$ and COHSEX quasiparticle (QP) schemes. We consider an Ag/MgO/Ag heterostructure model and we focus on the evolution of the zero-bias conductance as a function of the MgO film thickness. Our study shows that the correction of the electronic structure beyond semi-local density functionals goes in the right direction to improve the agreement with experiments, significantly reducing the zero-bias conductance. This effect becomes more evident at larger MgO thickness, that is in increasing tunneling regime. We also observe that the reduction of the conductance seems more related to the correction of wavefunctions rather than energies, and thus not directly related to the correction of the band-gap of bulk MgO. $G_0W_0$ and HSE both provide a correct band-gap in agreement with experiments, but only HSE gives a significant reduction of the conductance. COHSEX, while overestimating the band-gap, gives a reduction of the conductance very close to HSE.

A facile and universal strategy to endow implant materials with antibacterial ability via alkalinity disturbing bacterial respiration.

Multifarious strategies have been proposed to enhance the antibacterial ability of implant surfaces for preventing bacterial infection, however, developing facile and universal modification methods still remains extremely elusive. Herein, inspired by the fact that the electron transfer respiratory chain of bacteria is embedded in the membrane, we proposed a novel strategy of local alkalinity disturbing bacterial respiration to endow implant materials with antibacterial ability. As a demonstration, MgO was deposited on biomedical titanium via magnetron sputtering to regulate surface alkalinity. With the thickness of MgO films increasing, they exhibited an excellent antibacterial rate against both Gram-negative and positive bacteria. The antibacterial mechanism confirmed that the alkaline surface can disturb the bacterial respiration action via weakening the transmembrane proton concentration gradient, resulting in the blockage of energy metabolism and the increase of oxidative stress of bacteria. Cell experiments indicated that MgO films not only have no obvious cytotoxicity to osteoblast cells, but can also selectively kill bacteria and promote cell proliferation in the presence of both bacteria and cells. More importantly, the by-product of MgO was only the biocompatible Mg2+, reducing any concerns about potential toxic effects. Furthermore, sputtering alkaline MgO films was confirmed to work well on polyetheretherketone polymer and zirconia ceramic implants, which indicates that this strategy has broad prospects of clinical application for preventing implant-associated bacterial infection.

Epitaxial growth of Co2FeSi/MgO/GaAs(0 0 1) heterostructures using molecular beam epitaxy

We demonstrate the growth of crystalline Co2FeSi/MgO/GaAs(001) hybrid structures of different MgO interlayer thicknessess by molecular beam epitaxy. The MgO layer thickness was verified by X-ray reflectivity measurements and high-resolution transmission electron microscopy images. Secondary ion mass spectroscopy confirmed that the MgO layer works successfully as a diffusion barrier. Co and Fe defect densities in the GaAs layer could be reduced by three orders of magnitude using a 1.3 nm thick MgO film. High-resolution transmission electron microscopy images together with X-ray diffraction phi scans also revealed an in-plane rotation of the Co2FeSi layer by 45° for samples with MgO interlayers with respect to the GaAs substrate. Investigations of the magnetic properties revealed that the MgO interlayers do not affect the value of the magnetic moment of the Co2FeSi layer. Further, an in-plane rotation of the easy axis of the Co2FeSi layer by 90° for all samples with MgO interlayers was observed and will be discussed.

Exploration of a MgO cathode for improving the intensity of pulsed discharge plasma at atmosphere

With regard to the lower density and energy of electrons in pulsed discharge plasma (PDP) at atmosphere, leading to the lower energy utilization of plasma, we propose a MgO cathode to enhance the plasma intensity according to field emission principle. The MgO cathode is prepared by an electro-depositing MgO film on a stainless steel plate. This way, the positive charges come to the cathode and accumulate on the surface of the MgO film, leading to the enhancement of the electric field intensity between the cathode and MgO film, and result in the strong emission of secondary electrons from the MgO cathode. As a result, the intensity of plasma can be enhanced. Herein, the effect of the MgO cathode on the intensity of PDP is investigated. It was shown that the discharge peak current was improved by 20% compared with that of without the MgO cathode. With increasing the MgO film thickness, discharge intensity, including the peak current, transforming charge and spectrum intensity first increased and then decreased. Higher enhancement of peak current, transforming charge and spectrum intensity were acquired with a higher peak voltage. Compared to a cathode without MgO film, the ozone production is higher with MgO cathode employed. The research proposes a novel approach for improving the intensity of discharge plasma, and also provides a reference for further application of PDP.

Impact of MgO thickness on the perpendicular magnetic anisotropy of Mo/Co2FeAl/MgO/Mo multilayers with improved annealing stability

In this study, an interfacial perpendicular magnetic anisotropy (PMA) is achieved in Mo/Co2FeAl (CFA)/MgO/Mo multilayer with good thermal stability up to 400 °C by tuning the annealing temperature and film thickness of CFA and MgO. For an optimized CFA film thickness (1.0 nm) and annealing temperature (400 °C), the CFA/MgO interface plays a main role in achieving the strong PMA property when the MgO film thickness was tuned from 0.5 nm to 2.0 nm. The effective perpendicular magnetic anisotropy energy density (Keff) is approximately 1.67 × 106 erg/cc when tMgO = 2 nm. Surface morphology and surface roughness changes in the films are investigated by atomic force microscopy (AFM). An enhancement in surface smoothness is observed for the annealed films. X-ray photoelectron spectroscopy (XPS) reveals that the strong PMA in Mo/CFA/MgO is originated from the adequate oxidation of Co and Fe at CFA/MgO interface.

The MgO(111) versus MgO(100) supported Au ultrasmall particles as a model catalyst for carbon monoxide oxidation

Adsorption and reaction of carbon monoxide and oxygen on gold ultrasmall particles deposited on MgO(111) and MgO(100) films in ultra-high vacuum have been studied by surface sensitive spectroscopic and structural techniques. The 3 nm thick MgO(111) and MgO(100) films were formed on Mo(110) and Mo(100) supports, respectively, as templates to grow the corresponding oxide structures. It is found that, unlike MgO(100), and many other oxide surfaces, reported in the literature, the Au particles of and average size of c.a. 2–3 nm, deposited on MgO(111), acquire an effective positive charge as a result of charge transfer from the metal as an effect stabilizing the uncompensated dipolar moment of the polar substrate. Gold nanoparticles on both MgO(111) and MgO(100) are active model catalysts for carbon monoxide oxidation by oxygen, as revealed by temperature desorption studies. However the effect of MgO(111) support is markedly more pronounced due to stronger interaction of precursor CO and O2 induced by specific state of supported Au ultrasmall particles.

Adsorption sites of individual metal atoms on ultrathin MgO(100) films

We use Ca doping during growth of one and two monolayer thick MgO films on Ag(100) to identify the adsorption sites of individual adatoms with scanning tunneling microscopy. For this we combine atomic resolution images of the bare MgO layer with images of the adsorbates and the substitutional Ca atoms taken at larger tip-sample distance. For Ho atoms, the adsorption sites depend on MgO thickness. On the monolayer, they are distributed on the O and bridge sites according to the abundance of those sites, 1/3 and 2/3 respectively. On the MgO bilayer, Ho atoms populate almost exclusively the O site. A third species adsorbed on Mg is predicted by density functional theory and can be created by atomic manipulation. Au atoms adsorb on the bridge sites for both MgO thicknesses, while Co and Fe atoms prefer the O sites, again for both thickness.

Influence of activation parameters on the thickness of MgO thin film on Ag-3Mg alloy and its secondary electron emission property

MgO thin films of different thicknesses were prepared on polished Ag-3wt%Mg alloy by an activation process at 500–650°C under oxygen pressures of 0.5–10.0Pa. The influence of the thickness of MgO film on the secondary electron yield of the Ag-3wt%Mg alloy is investigated. The alloy with an MgO film of moderate thickness of 65±1nm shows the highest secondary electron yield of 10. Oxygen pressure below 10.0Pa is crucial for the formation of intact MgO film on the top surface. Computational simulation analysis reveals that the maximum yield occurs when the penetration depth of primary electrons is 13–18nm less than the thickness of MgO film. Electron supply of this extra MgO film is allowed by an electron tunneling mechanism under certain bias voltage. The replenishment of new electrons to the electron deficient surface is crucial for obtaining optimized secondary electron yields for Ag-Mg alloy cathodes.

MgO on Mo(001): Local Work Function Measurements above Pristine Terrace and Line Defect Sites

We present a local spectroscopy study of thin MgO films on Mo(001), investigated with a combined noncontact atomic force microscope (nc-AFM) and scanning tunneling microscope (STM). The work function of Mo(001) and MgO/Mo(001) was measured with field emission resonance (FER) spectroscopy. The work function of the clean Mo(001) surface ΦMo was determined to be 4.5 eV. After the deposition of an 8 atomic layer thick MgO film, it decreased to ΦMgO/Mo = 3.2 eV (ΔΦFER = −1.3 eV). This work function shift ΔΦFER, introduced by the oxide film on the metal work function, was also investigated with contact potential difference (CPD) measurements. Here, a lowering of the work function by ΔΦCPD = −1.1 eV was measured. Furthermore, the influence of line defects, present in the MgO film, on the local work function was investigated and compared to pristine terrace sites of the oxide film. Here, we measured a slightly higher local work function of +200 meV above line defects as compared to pristine oxide terrace sites.

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.

Performance of the Au/MgO/Ni photovoltaic devices

A 100μm thick MgO film is used to design a metal semiconductor metal device. The device is characterized by means of current voltage characteristics in the dark and under various light energies in the photon energy range of 3.70–2.15eV. A photovoltaic effect presented by an open circuit voltage of 0.12–0.47V, short circuit current density of 3.9–10.5μA/cm2, quantum efficiency of 0.662–0.052, and responsivity of 0.179–0.024A/W under photoexcitation optical power of 2.2–28.2μW is observed. The device was also tested as a UV optical communication component. The test revealed a wide range of tunability and sensitivity for microwave resonant frequencies of 0.5 and 2.9GHz. The differential resistance of the device exhibited different values at each applied ac signal frequency. When the frequency is fixed, the illuminated to the dark current ratio remained constant for all signal powers in the range of 0.00–20.0dBm.

Experimental and Simulation Study of Magnetron Sputtering MgO Film Thickness Distribution

A physical model of magnetron sputtering process was built, the distribution of MgO film thickness on the substrate was deduced, and the data were analyzed by using the Matlab. nanosized MgO thin film was prepared on Si substrate by magnetron sputtering. SGC-10 was used to measure the thickness of MgO thin film. The results of experiment correspond fairly well with the theory. Both experiment and the theory show that the distribution of the film thickness on the substrate is uneven and it is also influenced by the radius as well as the distance between the target and substrate. The physical model provides a theoretical basis for evaluation and estimation of the film thickness.

Interfacial properties in ultrathin MgO films on TiN(001) surfaces: Ab-initio calculations

By using ab-initio electronic structure calculations based on the density functional theory, we systematically investigated the interfacial properties of the MgO/TiN(001) system with increasing MgO film thickness from one to three monolayers (ML). Atomic geometries and adhesion characteristics of the MgO/TiN(001) systems in the range of 1- to 3-ML MgO are presented. O-Ti and Mg-N bonds were formed across the interfaces. Our calculated energetics indicate that the interfacial adhesion of the MgO films to the TiN substrates was stronger than that for the metal-oxide systems of MgO/Ag(001) and MgO/Fe(001). An analysis of the electronic properties of the MgO/TiN(001) systems was also performed. Upon the adsorption of the MgO overlayers, the work function of the TiN(001) surface was found to exhibit a significant decrease due to the large interface dipole formed at the interface of this system. Very interestingly, the obtained work functions of 1.89–1.96 eV for 1- to 3-ML MgO films on TiN(001) were lower than those for other metal-supported MgO films.

MgO/Si(100)構造における仕事関数のMgO膜厚依存性

MgO/Si(100)構造における仕事関数のMgO膜厚依存性

Atomic layer deposition of MgO films on yttria-stabilized zirconia microtubes

Deposition of MgO thin film on nanocrystalline yttria-stabilized zirconia microtubes was investigated. The microtubes were prepared by self-formation from threads drawn directly from zirconium butoxide [Zr(OBu)4] precursor and heat treated at 800°C. The tubes possessed 100% tetragonal phase, their typical outer diameter was 50μm, inner diameter 30μm and length 1cm. MgO films were deposited from β-diketonate-type precursor 2,2,6,6-tetramethyl-heptanedionato-3,5-magnesium(II) at 220°C by atomic layer deposition. Thickness of MgO film on microtubes was 15.8nm and growth rate 0.105Å/cycle.

Preparation of Pd–MgO model catalysts by deposition of Pd from aqueous precursor solutions onto Ag(0 0 1)-supported MgO(0 0 1) thin films

The preparation of Pd–MgO model catalysts via liquid-phase deposition of Pd from aqueous Pd precursor solutions was studied. Thin, single-crystalline MgO(001) films grown on a Ag(001) substrate were used as support and allowed surface science techniques such as Auger electron spectroscopy, X-ray photoelectron spectroscopy, scanning tunneling microscopy, and temperature programmed desorption to be applied for characterization. Thin MgO(001) films were unstable and rapidly dissolved in acidic and neutral environments, but remained stable in thickness in alkaline solutions after an initial dissolution of a few layers of MgO. Pd was deposited by exposure of the thin film MgO substrate to alkaline (pH 12) precursor solutions containing Pd-hydroxide complexes. Scanning tunneling microscopy images taken from ultrathin MgO films revealed the formation of Pd particles 3nm in diameter after thermal decomposition of the precursor at 600K, as well as roughening of the MgO substrate, including the formation of etch pits, which leads to partial exposure of the Ag substrate. For Pd deposited on thick MgO films, the formation of Pd nanoparticles by thermal decomposition of the adsorbed Pd-hydroxide precursor was followed by X-ray photoelectron spectroscopy. Pd–MgO model catalysts with similar Pd coverage prepared either by liquid-phase deposition or physical vapor deposition in UHV exhibited similar properties, as revealed by their comparable behavior in CO adsorption and CO oxidation.

Tuning the Charge State of (WO3)3 Nanoclusters Deposited on MgO/Ag(001) Films

The electronic properties of tungsten oxide (WO3)3 nanoclusters deposited on epitaxial MgO films grown on Ag(001) can be tuned and modified simply by changing the thickness of the oxide film. In particular, we show, on the basis of first principle molecular dynamics calculations, that (WO3)3 clusters on ultrathin (<1 nm) or thick MgO films have different structure, charge state, and bonding mode. On the ultrathin film a net transfer of two electrons occurs from the MgO/Ag(001) film to the oxide cluster via electron tunneling through the thin MgO dielectric barrier. The resulting [(WO3)3]2– cluster stays intact and assumes a geometry similar to that of gas-phase (WO3)3. On bulk MgO or on thick films, on the contrary, electron tunneling is not possible and (WO3)3 is bound via formation of covalent bonds that lead to a strong distortion of the cluster structure. As a consequence, completely different properties and chemical reactivity are expected for (WO3)3 nanoclusters on MgO/Ag(001) as a function of the t...

Promote water photosplitting via tuning quantum well states in supported metal clusters

In an effort to facilitate water photosplitting at surfaces, we identify quantum well states of magic gold clusters supported on ultrathin MgO/Ag(001) as the key to favor sunlight absorption and photocatalytic reactions. Based on density functional theory (DFT) and time-dependent DFT calculations, the adsorption geometry, electronic structures, and excited state properties of supported metal nanoparticles can be precisely controlled. By decreasing the thickness of MgO film, charge transfer to supported gold clusters, and therefore the occupation and energy spacings of quantum well states, can be gradually tuned, leading to redshifted and enhanced plasmonic excitations and optimized energy levels for water splitting.

Growth of Biaxially-Textured MgO Buffer Layers by Inclined Substrate Deposition

MgO buffer layers were grown by Inclined Substrate Deposition (ISD) for superconducting coated conductors. The surface structure, texture, and extended defects were analyzed for films with different MgO layer thicknesses. A roof-tile structure was only observed for films with layer thicknesses exceeding 800 nm. MgO facet widths and amplitudes were determined for different MgO buffer layer thicknesses. At 800 nm the facets height was 18 nm and the facet width was 71 nm. The corresponding values measured for a 5 μm thick MgO film were 64 nm and 234 nm, respectively. For layers with layer thicknesses smaller than 200 nm the films were polycrystalline with a grain size of 20 nm. For larger layer thicknesses the ISD MgO film grew in the form of columns with diameter of 50 to 100 nm. The texture of the layers was analyzed by electron diffraction in the TEM. From radial intensity profiles obtained from electron diffraction patterns the background-corrected (002)/(111) intensity ratio was extracted. It was found that the (002)/(111) intensity ratio allows to quantify the degree of texture.