MgO Overlayer Research Articles

High Fuel Yields, Solar‐to‐Fuel Efficiency, and Excellent Durability Achieved for Confined NiCo Alloy Nanoparticles Using MgO Overlayers for Photothermocatalytic CO2 Reduction

Solar‐energy‐driven CO2 reduction to produce fuel is of great importance for alleviating the greenhouse effect and energy shortage. Here, a composite of MgO overlayers confined NiCo alloy nanoparticles supported on MgO (NiCo@MgO/MgO) is prepared. Extremely high production rates of H2 (rH2) and CO (rCO) (112.98 and 127.13 mmol min−1 g−1, respectively) and large solar‐to‐fuel efficiency (η, 39.3%) are achieved on NiCo@MgO/MgO for photothermocatalytic CO2 reduction by CH4 under ultraviolet–visible–infrared (UV–VIS–IR) illumination. The high fuel production rates and η originate from efficient light–thermal conversion and photoactivation, which can boost the photothermocatalytic activity by reducing the activation energy. Importantly, NiCo@MgO/MgO does not show deactivation during a 90 h photothermocatalytic CO2 reduction by the CH4 durability test, owing to its low carbon deposition rate. Experimental evidence reveals that the NiCo alloy and MgO overlayers can inhibit CO disproportionation and accelerate the oxidation of carbon species, thereby significantly decreasing the carbon deposition rate.

Synergetic Effect of Plasmonic Gold Nanorods and MgO for Perovskite Solar Cells.

We report new structured perovskite solar cells (PSCs) using solution-processed TiO2/Au nanorods/MgO composite electron transport layers (ETLs). The proposed method is facile, convenient, and effective. Briefly, Au nanorods (NRs) were prepared and introduced into mesoporous TiO2 ETLs. Then, thin MgO overlayers were grown on the Au NRs modified ETLs by wet spinning and pyrolysis of the magnesium salt. By simultaneous use of Au NRs and MgO, the power conversion efficiency of the PSC device increases from 14.7% to 17.4%, displaying over 18.3% enhancement, compared with the reference device without modification. Due to longitudinal plasmon resonances (LPRs) of gold nanorods, the embedded Au NRs exhibit the ability to significantly enhance the near-field and far-field (plasmonic scattering), increase the optical path length of incident photons in the device, and as a consequence, notably improve external quantum efficiency (EQE) at wavelengths above 600 nm and power conversion efficiency (PCE) of PSC solar cells. Meanwhile, the thin MgO overlayer also contributes to enhanced performance by reducing charge recombination in the solar cell. Theoretical calculations were carried out to elucidate the PV performance enhancement mechanisms.

Hartman effect for spin waves in exchange regime

Hartman effect for spin waves tunnelling through a barrier in a thin magnetic film is considered theoretically. The barrier is assumed to be created by a locally increased magnetic anisotropy field. The considerations are focused on a nanoscale system operating in the exchange-dominated regime. We derive the formula for group delay τgr of a spin wave packet and show that τgr saturates with increasing barrier width, which is a signature of the Hartman effect predicted earlier for photonic and electronic systems. In our calculations, we consider the general boundary conditions which take into account different strength of exchange coupling between the barrier and its surrounding. As a system suitable for experimental observation of the Hartman effect we propose a CoFeB layer with perpendicular magnetic anisotropy induced by a MgO overlayer.

Atomic layer deposition enabled MgO surface coating on porous TiO2 for improved CO2 photoreduction

In this work, we have prepared porous TiO2 with mixed anatase-rutile phases from a metal-organic framework MIL-125 and employed atomic layer deposition (ALD) method to coat an ultrathin MgO overlayer on porous-TiO2. The CO2 photoreduction performance of porous-TiO2 was more than 4 times higher than that of commercial Degussa P25. The MgO coating on porous-TiO2 by ALD in the range of 1 to 100 atomic layers all improved CO2 photoreduction performance, with 5 layers of MgO being the optimal, delivering 4 times higher CO production than that by pristine porous-TiO2 and 21 times higher than P25. It is demonstrated that the uniform dispersion of MgO on TiO2 surface led to increased concentration of surface Ti3+ species and Mg bonded hydroxyl groups, which are active sites for CO2 adsorption and photoreduction. The ALD layer also served to passivate the TiO2 surface states and hinder surface electron-hole recombination. In addition, a conventional wet-impregnation (WI) method was applied to incorporate MgO on porous-TiO2 to serve as a comparison. The WI modified MgO/TiO2 samples were more active than pristine porous-TiO2 but less active than ALD modified MgO/TiO2, because the ALD samples presented significantly higher amounts of surface Ti3+ than WI samples due to higher and more uniform MgO surface coverage by ALD coating. Findings from this original work on ALD modification to enhance photocatalytic performance may be extended to other photocatalyst systems and other catalytic reactions.

Epitaxial growth of MgO/Ga2O3 heterostructure and its band alignment studied by X-ray photoemission spectroscopy

We have grown an epitaxial MgO/Ga2O3 heterostructure on a MgO(001) substrate by molecular beam epitaxy. Crystallographic studies revealed the out-of-plane and in-plane crystal orientations between the MgO overlayer and the Ga2O3 layer, which were MgO(001) ∥ β-Ga2O3(001) and MgO[100] ∥ β-Ga2O3, respectively. The valence band offset at the MgO/β-Ga2O3 interface was determined to be 0.19 eV (type-II band alignment) by X-ray photoelectron spectroscopy, resulting in a large conduction band offset of 2.7–3.2 eV. These results indicate that MgO is a promising potential barrier for electrons in an epitaxial MgO/Ga2O3 multilayered structure.

Highly efficient perovskite solar cells for light harvesting under indoor illumination via solution processed SnO2/MgO composite electron transport layers

We present new architectures in CH3NH3PbI3 based planar perovskite solar cells incorporating solution processed SnO2/MgO composite electron transport layers that show the highest power outputs ever reported for photovoltaic cells under typical 200–400 lx indoor illumination conditions. When measured under white OSRAM LED lamp (200, 400 lx), the maximum power density values were 20.2 µW/cm2 (estimated power conversion efficiency, PCE = 25.0%) at 200 lx and 41.6 µW/cm2 (PCE = 26.9%) at 400 lx which correspond to a ∼ 20% increment compared to solar cells with a SnO2 layer only (even at standard 1 sun illumination, where the maximum PCE was 19.0%). The thin MgO overlayer leads to more uniform films, reduces interfacial carrier recombination, and leads to better stability. All layers of the cells, except for the two electrodes, are solution processed at low temperatures for low cost processing. Furthermore, ambient indoor conditions represent a milder environment compared to stringent outdoor conditions for a technology that is still looking for a commercial outlet also due to stability concerns. The unparalleled performance here demonstrated, paves the way for perovskite solar cells to contribute strongly to the powering of the indoor electronics of the future (e.g. smart autonomous indoor wireless sensor networks, internet of things etc).

Ab Initio Study of Thin Oxide–Metal Overlayers as an Inverse Catalytic System for Dioxygen Reduction and Enhanced CO Tolerance

Using first-principles density functional theory calculations, we used a thin oxide overlayer, such as MgO, on a metal surface as an inverse catalyst for dioxygen reduction. Surface distortions in the oxide layer, combined with the tunneling of electron from the underneath metal, activated the adsorbed O2 in the form of a superoxo or peroxo. On the other hand, the thin MgO overlayer readily prevents the π-back-bonding between CO and the metal surface, thereby efficiently mitigating the affinity of the metal surface for CO. The operating potential and overpotential for the oxygen reduction reaction (ORR) process have been estimated for various combinations of thin insulators and metals. The strongest binding intermediate in the overall reaction pathway influenced the overpotential. We show that for a Ag(100)-supported MgO surface, the ORR commences with a low overpotential, which is comparable to that of the Pt(111) surface. This suggests that an optimally chosen insulator–metal overlayer structure can yield a sharply tuned free energy profile for ORR.

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.

Influence of thickness of MgO overlayer on the properties of ZnO thin films prepared on c-plane sapphire for H2 sensing

Abstract ZnO/MgO double layered thin films were prepared on Al 2 O 3 substrate by direct current (dc) sputtering at a low power of 40 W. The effect of thickness of MgO overlayer on the properties of ZnO films was investigated. X-ray diffraction patterns revealed that the crystallinity of ZnO films can be improved by addition of a MgO overlayer. The blueshift in the PL peak suggests that the optical bandgap of ZnO can be tuned by varying the thickness of MgO overlayer. SEM micrographs revealed that the surface morphology changes from uniform to dense porous and nano-flake like structure with increase in thickness of MgO layer. Pure ZnO and optimized ZnO/MgO films were tested for their sensing performance towards H 2 at different temperatures. The maximum response of ZnO and ZnO/MgO sensor was observed to be 135 and 260, respectively, at an operating temperature of 140 °C. The very high sensitivity at such a low operating temperature is associated with the fast recovery time of 120 s, which could be achieved due to the promoting effect of MgO on ZnO.

MgO overlayer thickness dependence of perpendicular magnetic anisotropy in CoFeB thin films

The dependence of the magnetization and perpendicular magnetic anisotropy (PMA) of Ta/CoFeB/MgO/Ta on the thicknesses of the CoFeB layer and the MgO overlayer was investigated by vibrating sample magnetometer. The magnetization is found to be small for samples with a thin MgO overlayer. The PMA strongly depends on the MgO overlayer (hereafter, MgO) thickness and its maximum value of 1.74 erg/cm2 is achieved for a 1.0 nm thick MgO overlayer with annealing at 300 °C. The volume anisotropy of the CoFeB layer is found to be independent of the MgO thickness,which suggest that only the interface anisotropy depends on the MgO thickness. The possible mechanisms that may influence the interface magnetization and anisotropy are discussed herein.

Composition Dependence of Perpendicular Magnetic Anisotropy in Ta/CoxFe80-xB20/MgO/Ta (x=0, 10, 60) Multilayers

The perpendicular magnetic anisotropy of sputtered CoFeB thin films covered by MgO was investigated by vibrating sample magnetometry. Three different <TEX>$Co_xFe_{80-x}B_{20}$</TEX> alloys were studied. Under out-of plane magnetic field, the saturation field was found to increase with increasing the Co content. The magnetization and interface anisotropy energy were obtained for all samples. Both showed a marked dependence on the MgO overlayer thickness. In addition, their variations were found to be non-monotonous as a function of the Co concentration.

Reactive growth of MgO overlayers on Fe(0 0 1) surfaces studied by low-energy electron diffraction and atomic force microscopy

Ultra-thin MgO films grown reactively on the Fe(001) surface by evaporation of magnesium in an atmosphere of molecular oxygen have been investigated by low energy electron diffraction and non-contact atomic force microscopy. Preparation of structurally stable crystalline films requires a protocol that prevents an excessive interfacial reaction between oxygen and the Fe(001) surface, but at the same time provides a sufficient amount of oxygen in order to grow a stoichiometric MgO film. The proper ratio between the magnesium deposition rate and the oxygen pressure has been determined, as well as measures to prevent initial interfacial oxidation of the substrate. The effect of both post-annealing and an increased substrate temperature during the growth has been studied. We demonstrate that the reactive deposition method, which gives full control over the gaseous species that reach the surface, can produce terraces that have an average size of 10nm (on an 8 monolayer thick film), which is a significant improvement compared to other preparation methods, such as thermal or electron beam evaporation of MgO.

Epitaxial Growth of Sputtered Ultra-Thin NbN Layers and Junctions on Sapphire

High crystalline quality of ultra-thin NbN layers and of NbN-MgO-NbN tri-layers, epitaxially grown by DC-magnetron sputtering in the superconducting B1-cubic phase has been achieved in a reproducible way on three different orientations of sapphire substrates i.e. R-, A- and M-planes. Significant improvements such as higher Tc, higher Jc and lower resistivity have been obtained by growing un-twined (110) oriented NbN layers on M-plane orientation of sapphire. Uniform, low roughness, 3-5 nm thick films with Tc above 12 K and Jc above 5 MA/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> at 4.2 K were obtained. Characterizations by TEM, AFM and X-Ray diffraction evidence that growth of un-twined NbN on M-plane lead to a better epitaxy in comparison with twinned films observed on other sapphire orientations. We observe that the reduction of the substrate temperature from 600degC to 300degC during the deposition of NbN or NbN-MgO-NbN layers thicker than 20 nm prevents the nucleation of the competing HCP NbN phase. Moreover, 1.5 nm thick AlN or MgO over-layers sputtered in-situ prevent ultra-thin NbN films degradation through aging. The formation of Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> N <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">y</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">5-x</sub> ( ~ 2.2 nm) at the unprotected NbN surface and of interfacial NbO ( ~ 0.7 nm) native oxides has been observed by XPS. It is forecasted that such improvements in ultra-thin NbN films deposited uniformly on 3 and 4 inch sapphire wafers is a key in the future development of superconducting single photon detectors, THz HEB mixers and also in low noise quantum analogical and digital Josephson devices.

Formation of metastable MgO structures on type-III oxide surfaces: Effect of periodic out-of-plane electric dipole moment of substrates

Metastable structures of MgO overlayer on type-III oxide surfaces were achieved by molecular beam epitaxy. Periodic out-of-plane dipole moment of substrates was revealed to play a dominant role in the formation of metastable oxide structures. Under near-equilibrium epitaxial conditions, this dipole moment was found to drive the over-grown MgO to inherit the out-of-plane substrate lattice, resulting in some new structures with different stacking sequences and bonding configurations from those of thermodynamically stable phase. As typical cases, by adopting the substrate dipole moment along the growth direction, MgO overlayers with metastable wurtzite (WZ) and stacking-fault (SF) structures were formed on α-Al 2O 3 (0 0 0 1) and MgAl 2O 4 (1 1 1) surfaces, respectively.

Growth of atomically smooth MgO films on graphene by molecular beam epitaxy

We investigate the growth of MgO films on graphene by molecular beam epitaxy and find that surface diffusion promotes a rough morphology. To reduce the mobility of surface atoms, the graphene surface is dressed by Ti atoms prior to MgO deposition. With as little as 0.5 ML (monolayer) of Ti, the MgO overlayer becomes atomically smooth. Furthermore, no aggregation of MgO is observed at the edges of the graphene sheet. These results are important for the fabrication of nanoscale electronic and spintronic devices.

Influence of Capping Layers on Magnetic Anisotropy in Fe/MgO/GaAs(100) Ultrathin Films

Magnetic anisotropies of Fe/MgO/GaAs(100) hybrid structure with two different nonmagnetic capping materials, Au and MgO have been studied by ferromagnetic resonance (FMR). A uniaxial anisotropy, unexpected from the crystal structure was observed in the ultrathin films for both capping materials. Its global easy axis is along [0-11] direction while two (010) directions are equally magnetic hard regardless of the overlayer material. The in-plane uniaxial anisotropy (in-plane cubic anisotropy) of the Au capped samples is stronger (weaker) than that of the MgO capped ones within the range of t Fe = 7.1 to 28 ML. This suggests that the MgO overlayer suppresses the uniaxial anisotropy faster than the Au overlayer.

MgO∕Fe(100)interface: A study of the electronic structure

Epitaxial MgO(100) films, deposited on the Fe(100) surface by direct evaporation of stoichiometric magnesium oxide under ultrahigh-vacuum conditions, are examined by spin-polarized valence-band photoemission and magnetic linear dichroism in core-level spectroscopy. The excellent quality of the MgO films prepared in this way is clearly revealed by relatively small photocurrent intensity observed above the MgO valence-band maximum and by spectral features in the photoemission data. No evidence of the formation of an interfacial FeO layer is found in this MgO/Fe(100) system. Only a structural reordering is promoted by annealing the interface up to 400{sup o}C , as evident by the minute changes in the photoemission spectra. Further annealing to 500{sup o}C , however, leads to more substantial changes in the spectra, possibly related to further ordering of the interface and/or partial uncovering of the Fe(100) metal due to MgO desorption and/or clustering. The influence of screening effects from the Fe(100) substrate to the photohole created by the photoemission process in the MgO overlayer is also examined.

Effect of MgO interlayer on diamond film growth on silicon (100)

Diamond films were grown, under identical conditions on scratched Si (100) with and without MgO interlayers by using hot filament chemical vapour deposition. Initially both kinds of the substrates were biased for 30 min at 750±50 °C to enhance the nucleation density. A 1:100 vol.% mixture of CH4 and H2 gases was used for deposition at a total pressure of ∼4×103 Pa. Deposition was carried out for total of 2 h and the analysis of the films was carried out using Scanning Electron Microscopy, Atomic Force Microscopy, X-ray Diffraction and Raman Spectroscopy. Both substrates showed predominantly (111) oriented crystals of diamond but with different surface morphology. The crystallites on scratched Si (100) surface are in general (∼1.5–2 μm) and those on MgO overlayer are smaller (<1 μm), but denser film occurs with MgO overlayer on Si (100) substrate.

Electronic structure of the interface of GaAs(1 0 0) with MgO overlayer grown by thermal evaporation of Mg in O 2 under UHV

Thermal evaporation of Mg in oxygen atmosphere under UHV condition was conducted to fabricate ultrathin MgO film over GaAs(1 0 0) substrate at room temperature ( T R). Synchrotron Radiation Photoemission Spectroscopy (SRPES) results show that very weak interaction and interdiffusion exist between the oxide overlayer and substrate indicating that the reaction between Mg and O 2 is much more active than that between Mg with GaAs. The growth of MgO overlayer may be expected in such a mode that Mg reacts with O 2 in gas phase or near surface to form MgO, followed by the deposition of MgO over GaAs(1 0 0).

Epitaxial thin films of MgO on Si using metalorganic molecular beam epitaxy

Epitaxial cubic MgO thin films have been deposited on single crystal Si(001) substrates by metalorganic molecular beam epitaxy. The Mg source was the solid precursor magnesium acetylacetonate and a rf excited oxygen plasma was the oxidant. The growth process involved initial formation of an epitaxial β-SiC interlayer followed by direct deposition of a MgO overlayer. The films were characterized by in situ reflection high energy electron diffraction, x-ray diffraction, conventional and high resolution transmission electron microscopy, atomic force microscopy, Auger electron spectroscopy, and Fourier transform infrared spectroscopy. The β-SiC interlayer had an epitaxial relationship such that SiC(001)∥Si(001) and SiC [110]∥Si [110]. The SiC interlayer showed a columnar grain structure with planar defects including twin bands and stacking faults. The MgO overlayer showed an epitaxial relationship given by MgO(001)∥Si(001) and MgO[110]∥Si[110]. No evidence of twins in the MgO layers was observed.