Pristine MgO Research Articles

Multicenter Bonding Effects in Oxygen Vacancy in the Bulk and on the Surface of MgO

We have studied the electronic structure of F-centers in the bulk and on the (100) surface of the MgO crystal. Chemical bonding analysis was performed using the natural bond orbital (NBO) analysis with periodic boundary conditions and the solid state adaptive natural density partitioning (SSAdNDP) method. The bonding in the pristine MgO was found to be very ionic, i.e., there are four lone pairs (s-, px-, py-, and pz-type) on every oxygen atom with the occupation numbers (ONs) in the range of 1.76–1.86 |e|. Within the O vacancy (Ov) located in the bulk of MgO we found a pair of electrons in the place of the missing oxygen atom that is revealed by SSAdNDP as a six-center two-electron (6c–2e) σ bond delocalized over six Mg atoms with ON = 1.87 |e|. In the other defected MgO system featuring the Ov on the (100) surface we also found a pair of electrons in the place of the missing oxygen atom that is revealed as a 5c–2e σ bond with ON = 1.84 |e|. Inclusion of the four closest Mg atoms of the second layer incr...

Transparent Al+3 doped MgO thin films for functional applications

The present work reports the utilization of a relatively simple, cost effective sol-gel technique based route to synthesize highly transparent, spin coated 4.1 at% Al+3 doped MgO thin films on quartz substrates. The films were characterized by XRD, XPS, Raman spectroscopy, and SIMS techniques. The microstructures were characterized by FESEM and TEM while the nanomechanical properties were assessed by the nanoindentation technique. Finally the optical transmittance was measured by UV–vis technique. The x-ray diffraction (XRD) study suggests the crystal facet (2 0 0) of MgO lattice to be distorted after incorporation of Al+3 into MgO lattice. From FESEM the doped films were found to have a dense microstructure with a crystallite size of about 20 nm as revealed by the TEM studies. Nanoindentation measurements indicated drastic increase of elastic modulus for the Al+3 doped MgO thin films by ~73% compared to that of the pristine MgO thin films along with retaining the nanohardness at ~8 GPa. The transmittance of Al+3 doped MgO thin films in the visible range was significantly higher (~99%) than that of pristine MgO (~90%) thin films. The films also had a relatively higher refractive index of about 1.45 as evaluated from the optical properties. The enhanced transmittance as well as the improved elastic modulus of Al+3 doped MgO thin films suggest its promising candidature in magnetic memory devices and as buffer layers of solar cells.

Facile synthesis and characterization of pH-dependent pristine MgO nanostructures for visible light emission

Herein, we demonstrate a strategy for facile synthesis of pristine MgO nanostructures at different pH values ranging from 7.9, 8.3 and 12.5 to explore their photoluminescence studies. These pH-dependent MgO nanostructures were characterized by various standard techniques such as XRD, SEM, EDS, TEM and photoluminescence (PL) spectroscopy. The obtained PL results clearly demonstrate that the PL emission spectra strongly depend upon growth environment. These nanostructures show a broad PL emission in visible region ranging from 400 to 680 nm at excitation wavelength of 330 nm. Hence, this study provides a unique feature to tailor the PL property of pristine MgO nanostructures which could be potentially used in luminescence harvesting for various optical display devices and sensing applications.

Melting of large Pt@MgO(1 0 0) icosahedra

On the basis of ab initio calculations, we present a new parametrisation of the Vervisch–Mottet–Goniakowski (VMG) potential (Vervisch et al 2002 Phys. Rev. B 24 245411) for modelling the oxide–metal interaction. Applying this model to mimic the finite temperature behaviour of large platinum icosahedra deposited on the pristine MgO(1 0 0), we find the nanoparticle undergoes two solid–solid transitions. At 650 K the ‘squarisation’ of the interface layer, while a full reshaping towards a fcc architecture takes place above 950 K. In between, a quite long-lived intermediate state with a (1 0 0) interface but with an icosahedral cap is observed. Our approach reproduces experimental observations, including wetting behaviour and the lack of surface diffusion.

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.

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.

Modelling the chemistry of Mn-doped MgO for bulk and (100) surfaces.

We have investigated the energetic properties of Mn-doped MgO bulk and (100) surfaces using a QM/MM embedding computational method, calculating the formation energy for doped systems, as well as for surface defects, and the subsequent effect on chemical reactivity. Low-concentration Mn doping is endothermic for isovalent species in the bulk but exothermic for higher oxidation states under p-type conditions, and compensated by electrons going to the Fermi level rather than cation vacancies. The highest occupied dopant Mn 3d states are positioned in the MgO band gap, about 4.2 eV below the vacuum level. Surface Mn-doping is more favourable than subsurface doping, and marginally exothermic on a (100) surface at high O2 pressures. For both types of isovalent Mn-doped (100) surfaces, the formation energy for catalytically important oxygen defects is less than for pristine MgO, with F0 and F2+-centres favoured in n- and p-type conditions, respectively. In addition, F+-centres are stabilised by favourable exchange coupling between the Mn 3d states and the vacancy-localised electrons, as verified through calculation of the vertical ionisation potential. The adsorption of CO2 on to the pristine and defective (100) surface is used as a probe of chemical reactivity, with isovalent subsurface Mn dopants mildly affecting reactivity, whereas isovalent surface-positioned Mn strongly alters the chemical interactions between the substrate and adsorbate. The differing chemical reactivity, when compared to pristine MgO, justifies further detailed investigations for more varied oxidation states and dopant species.

Facile synthesis of defect-induced highly-luminescent pristine MgO nanostructures for promising solid-state lighting applications

Successful demonstration of the facile synthesis of defect-induced highly-luminescent pristine MgO nanostructures for white-light generation and proposed WLED application.

Strengthening and toughening of poly(L-lactide) composites by surface modified MgO whiskers

To improve both the strength and toughness of poly(L-lactide) (PLLA), fibrous-like MgO whiskers with diameters of 0.15–1μm and lengths of 15–110μm were prepared, and subsequently surface modified with L-lactide to obtain grafted MgO whiskers (g-MgO whiskers). The structures and properties of MgO whiskers and g-MgO whiskers were studied. Then, a series of MgO whiskers/PLLA and g-MgO whiskers/PLLA composites were prepared by solution casting method, for comparison, MgO particles/PLLA composite was prepared too. The resulting composites were evaluated in terms of hydrophilicity, crystallinity, dispersion of whiskers, interfacial adhesion and mechanical performance by means of polarized optical microscopy (POM), contact angle measurement, field emission scanning electron microscope (FSEM), transmission electron microscopy (TEM) and tensile testing. The results revealed that the crystallization rate and hydrophilicity of PLLA were improved by the introduction of MgO whiskers and g-MgO whiskers. The g-MgO whiskers can disperse more uniformly in and show stronger interfacial adhesion with the matrix than MgO whiskers as a result of the surface modification. Due to the bridge effect of the whiskers and the excellent interfacial adhesion between g-MgO whiskers and PLLA, g-MgO whiskers/PLLA composites exhibited remarkably higher strength, modulus and toughness compared to the pristine PLLA, MgO particles/PLLA and MgO whiskers/PLLA composites.

Magnetism of Amorphous and Nano-Crystallized Dc-Sputter-Deposited MgO Thin Films.

We report a systematic study of room-temperature ferromagnetism (RTFM) in pristine MgO thin films in their amorphous and nano-crystalline states. The as deposited dc-sputtered films of pristine MgO on Si substrates using a metallic Mg target in an O2 containing working gas atmosphere of (N2 + O2) are found to be X-ray amorphous. All these films obtained with oxygen partial pressure (PO2) ~10% to 80% while maintaining the same total pressure of the working gas are found to be ferromagnetic at room temperature. The room temperature saturation magnetization (MS) value of 2.68 emu/cm3 obtained for the MgO film deposited in PO2 of 10% increases to 9.62 emu/cm3 for film deposited at PO2 of 40%. However, the MS values decrease steadily for further increase of oxygen partial pressure during deposition. On thermal annealing at temperatures in the range 600 to 800 °C, the films become nanocrystalline and as the crystallite size grows with longer annealing times and higher temperature, MS decreases. Our study clearly points out that it is possible to tailor the magnetic properties of thin films of MgO. The room temperature ferromagnetism in MgO films is attributed to the presence of Mg cation vacancies.

Synthesis and characterization of vanadium species coated on alumina, magnesium oxide and hydrotalcite supports to SOx removal

Vanadium oxide catalysts coated on MgO, Al2O3 and mixed oxide (obtained by thermal treatment of hydrotalcite) were synthesized and thoroughly characterized by X-ray fluorescence, textural analysis, temperature-programmed reduction (TPR), UV–vis with diffuse reflectance (DRS), XRD, SEM-EDX and XANES. The V2O5 loading was kept around 5wt.% for all catalysts. The existence of vanadium preferential location in the vicinity of magnesium was ascertained for the mixed oxide. It has been proposed that the vanadium location and its dispersion are governed by the basic magnesium. The DeSOx tests were carried out gravimetrically in a TGA/MS system at similar temperature conditions of those used in FCC units. The vanadium addition to MgO and MgO+MgAl2O4 supports led to an increase of SO3 pick-up efficiency (the absorption rate was multiplied by two to all supports in presence of vanadium) and the amount of SO3 was largely increased upon vanadium addition to both pristine MgO and mixed oxide. The reduction step of sulfate to H2S and water at lower temperature was also largely improved in presence of vanadium; this value increased five and four times for V/MgO and V/MgO+MgAl2O4 catalysts, respectively.

Ab initio study of NH3 and H2O adsorption on pristine and Na-doped MgO nanotubes

We have investigated, on the basis of density functional theory calculations, the structural and electronic properties of chemical modification of pristine and Na-doped MgONTs with NH3 and H2O molecules. We found that the NH3 and H2O molecules can be barrierlessly adsorbed on the Mg atom of the tube sidewall along with a charge transfer from the adsorbate to MgONT. The adsorption is chemical in nature with adsorption energies about −22.3 and −21.5 kcal/mol for H2O and NH3, respectively. The calculated density of state (DOS) shows that the chemical modification of MgONTs with these molecules can be generally classified as certain type of “harmless modification.” In other words, the electronic properties of the MgONT are little changed by the adsorption processes. The substitution of an Mg atom in the tube surface with an Na atom results in a semi-insulator to p-type semiconductor transition based on DOS analysis. It was also found that the doping process reduces the adsorption energies and the electronic properties of Na-doped MgONT is slightly more sensitive toward NH3 and H2O molecules, compared with the pristine one.

Functionalization of Mesoporous Carbon with Superbasic MgO Nanoparticles for the Efficient Synthesis of Sulfinamides

Highly basic MgO nanoparticles with different sizes have been successfully immobilized over mesoporous carbon with different pore diameters by a simple wet-impregnation method. The prepared catalysts have been characterized by various sophisticated techniques, such as XRD, nitrogen adsorption, electron energy loss spectroscopy, high-resolution TEM, X-ray photoelectron spectroscopy, and the temperature-programmed desorption of CO(2). XRD results reveal that the mesostructure of the support is retained even after the huge loading of MgO nanoparticles inside the mesochannels of the support. It is also demonstrated that the particle size and dispersion of the MgO nanoparticles on the support can be finely controlled by the simple adjustment of the textural parameters of the supports. Among the support materials studied, mesoporous carbon with the largest pore diameter and large pore volume offered highly crystalline small-size cubic-phase MgO nanoparticles with a high dispersion. The basicity of the MgO-supported mesoporous carbons can also be controlled by simply changing the loading of the MgO and the pore diameter of the support. These materials have been employed as heterogeneous catalysts for the first time in the selective synthesis of sulfinamides. Among the catalysts investigated, the support with the large pore diameter and high loading of MgO showed the highest activity with an excellent yield of sulfinamides. The catalyst also showed much higher activity than the pristine MgO nanoparticles. The effects of the reaction parameters, including the solvents and reaction temperature, and textural parameters of the supports in the activity of the catalyst have also been demonstrated. Most importantly, the catalyst was found to be highly stable, showing excellent activity even after the third cycle of reaction.

Electron trapping in misfit dislocations of MgO thin films

Misfit dislocations in a thin MgO/Mo(001) film have been investigated by conductance and light-emission spectroscopy using scanning tunneling microscopy and electron-paramagnetic resonance (EPR) spectroscopy. The line defects exhibit a higher work function than the pristine MgO, being explained by their ability to trap electrons. The electron traps are associated with a nonstoichiometric defect composition in thin oxide films and attractive pockets in the Madelung potential in thicker ones. The latter traps can be reproducibly filled by the adsorption of atomic hydrogen, which gives rise to a free-electronlike signal in EPR spectroscopy.

Thickness dependent magnetic transitions in pristine MgO and ZnO sputtered thin films

We report a systematic study of the thickness dependency of room temperature ferromagnetism in pristine MgO (~100-500 nm) and ZnO (~100-1000 nm) thin films deposited by reactive magnetron sputtering technique under the respective identical controlled optimum oxygen ambience. As far as we know this is the first such report on ferromagnetic pure MgO thin films, a result which should be of significance in understanding the functional aspects of magnetic tunnelling characteristics in devices using MgO dielectrics. From the magnetic characterization we observe a distinct variation in the saturation magnetization (MS) with increasing film thickness. In the case of MgO thin films MS values vary in the range 0.04-1.58 emu/g (i.e. 0.0012-0.046 ?B/unit cell) with increasing film thickness showing the highest MS value for the 170 nm thick film. Above this thickness MS is found to decrease and eventually above 420 nm the films show a paramagnetic behaviour followed by the well known diamagnetic property for the bulk (>500 nm). It is obvious that since initially the MS values increase with thickness, there has to be a maximum before the films become diamagnetic at some finite thickness. We also note that the MS values observed for MgO are the highest (more than twice the value observed for ZnO) to be reported for such a defect induced ferromagnetism in a pristine oxide. The origin of ferromagnetic order in both the oxides appears to arise from the respective cat-ion vacancies. The discovery of film thickness dependent ferromagnetic order should be very useful in developing multifunctional devices based on the technologically important materials MgO and ZnO.