Surfaces Of MgO Crystals Research Articles

The micro- to nano-scale dislocation mechanics of (001) MgO crystal hardness

ABSTRACT Micro- to nano-indentation hardness-based stress–strain computations made for Berkovich spherically-tipped impressions put into (001) MgO crystal surfaces show an exceptional plastic strain hardening that is attributed to reacted sessile-type <110> Burgers vector dislocations. Additional evidence for the substantial strain hardening is provided in a compilation of Knoop hardness measurements. The combination of results is compared on the basis of an applied load dependence for an indentation size effect (ISE).

Growth of YBa2Cu3O7 Films with [100] Tilt of CuO Planes with Respect to the Surface on MgO Crystals

Epitaxial YBa2Cu3O7 films with CuO plane tilted with respect to the surface have been grown on crystalline MgO substrates by pulsed laser deposition. Directly before growing a film, a nucleation-center-forming SrTiO3 sublayer with an effective thickness less than the unit-cell size was deposited on the substrate. The tilt angle was varied in the range from 0° (at which CuO planes are oriented parallel to the surface) to 70°. The film lattice rotation is implemented at this tilt due to the substrate rotation around its [100] axis, oriented parallel to the surface. Zero tilt of the CuO plane occurs when the MgO crystal surface is oriented parallel to the (100) plane. It is found that the CuO film planes remain parallel to one or several crystallographic planes {100} of the substrate at any tilt angles. In the range of angles from 0° to 41° the film is a single crystal. At angles larger than 41° the film is transformed into a two-domain texture, and its surface roughness sharply increases.

Carbon Diffusion within Periclase During Melting

Physicochemical possibilities are considered for entry of carbon into the structure of fuzed MgO during crystallization from a melt. It is established that fuzed periclase contains carbon, which during periclase grinding is concentrated at the surface of MgO crystals. With fine milling in a vibration mill carbon concentration is higher at the surface of MgO than with milling in a jet mill, and comprises 41.40 and 35.42 wt.% respectively. Melting MgO in an experimental electric arc furnace confirms the possibility of preparing periclase saturated with carbon.

In Situ TEM Observation of MgO Nanorod Growth

The controlled growth of MgO nanorods was investigated under electron irradiation in transmission electron microscopes at accelerating voltages from 120 to 300 kV in the temperature range from −175 to 610 °C. The nanorod growth on the surface of MgO crystals was induced by Au, Pt, Ni, or Co particles of 2−4 nm in size. The nanorods were grown up to 8 nm after 5−30 min of electron irradiation time with beam current densities of 100−600 A/cm2. The mechanism of nanorod formation and growth is explained on the basis of electrostatic interactions between positive ions of MgO species and negatively charged metal particles.

Theoretical study of the effect of spin-selective adsorption of water molecules on MgO surface

The effect of spin-selective adsorption of water molecules on the surface of MgO crystal is theoretically studied. The study is performed using two different approaches, i.e., quantum-chemical simulation and an analytical calculation in a quasiclassical approximation. The adsorption energy is calculated using the B3LYP density functional and the 6–311G* basis set. The calculated value of the adsorption energy 0.70 eV agrees well with an experimental value of 0.65 eV. It is established that the energy difference of adsorbed ortho- and para-water molecules is negligible and, thus, the difference of the adsorption energies is completely determined by the energy difference of free molecules in ortho- and para-states. It follows from the analytical calculation that this result is essentially general and is related not only to an MgO surface, but to any other surface on which the energy barrier for rotation of the adsorbed molecule is much larger than the corresponding rotational constant. Based on this, the conclusion is reached that the effect of spin-selective adsorption on these surfaces should not be observed under normal conditions.

Influence of MgO substrate treatment on the structure-sensitive properties of epitaxial films (by the example of iron films)

A new technology of growing thin single-crystal MgO films on the MgO crystal surface using the method of chemical transport reactions is developed. The quality of deposited sublayers is checked with the structure-sensitive properties of iron epitaxial films.

Trends in calculation of point and extended defects in wide‐gap solids: periodic models of aperiodic systems

AbstractThe periodic models of aperiodic systems are described. For the point defects supercell and cyclic cluster models are compared for the oxygen vacancy in the corundum Al2O3. The single and repeated slab models of (001) surface of MgO crystal are compared using the results of LCAO Hartree–Fock and DFT calculations. (© 2005 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

In situ study of the recovery of nanoindentation deformation of the (100) face of MgO crystals by atomic force microscopy

Results are described and discussed of an in situ atomic force microscopy study on the evolution of the surface morphology around indentations made on the (100) cleavage face of MgO crystals by square pyramidal silicon tips for loads up to 10 μN. It was found that: (1) nanoindentation does not introduce plastic deformation on the surface; (2) an indented surface always tends to heal with time but the time t of healing increases with penetration depth d (i.e., with applied load); (3) for indentations with d<ca. 17 nm the penetration depth decreases with increasing time following a power-law dependence; (4) with decreasing d, the indentation diameter a first increases and then decreases, exhibiting a peak at a depth of about three to four monolayers; (5) the hill height increases approximately linearly with penetration depth; and (6) indentation leads to the movement of cleavage steps with rates of up to 1.3×10 −11 m s −1. Analysis of the kinetics suggests that the recovery of indented surfaces of MgO crystals occurs by local reorganization, and volume and surface diffusion processes. A model of the formation of an indentation impression, involving the penetration of the indenter under applied load first into the initial surface layer and subsequently deep into the bulk of the crystal, is proposed and used to explain the recovery of nanoindentation deformation.

Correlation between the Madelung field and the reactivity of the MgO low-coordinated surface sites

We study correlations between Madelung constants, charges of surface ions, and chemical activity of low-coordinated sites of the MgO crystal surface with respect to dissociative adsorption of hydrogen. The ab initio Embedded Cluster model [Puchin et al., Phys. Rev. B 47, 6226 (1993)] employed in this study allowed us to reproduce correctly both short range and long range (Madelung) parts of the interaction between ions in a quantum cluster and the rest of the crystal. Our results show that sites having the same coordination numbers may have different properties, depending on values of Madelung constants. Lower Madelung constants correspond to lower ionicity, higher energy of H2 adsorption, stronger O–H and Mg–H bonds, and larger spatial separation of two adsorbed hydrogen atoms.

Preparation and characterization of MgO surfaces by reflection electron microscopy.

We have employed several different methods to prepare (100) and (111) surfaces of MgO crystals. (100) surfaces prepared by simple cleaving give good reflection high energy electron diffraction (RHEED) patterns and surfaces with a high density of coarse steps. Chemical polishing of this surface results in a roughening of the topography whilst annealing in oxygen considerably smoothens the surfaces although they appear to be contaminated. Under certain conditions we find that the MgO crystals will cleave along the (111) plane. Both cleaved and mechanically polished (111) surfaces are atomically flat and reconstructed after oxygen annealing.

The change of optical and structural properties of MgO by the exposition to laser pulse irradiation

en

Computer-simulated electron microdiffraction patterns from MgO crystal surfaces

Dynamical diffraction calculations have been carried out for an incident 100 kV electron wavepacket (STEM case), 12 Å in diameter, running parallel to a flat (100) face of a MgO crystal. The microdiffraction patterns and the electron density distributions have been obtained. A surface potential field, representing the modified image force, was added to the crystal potential to study its effect on the small electron probe. The calculation results show that electron microdiffraction patterns are very sensitive to the surface potential for crystal thickness larger than 120 Å. The streakings of the central disk in the microdiffraction patterns can be accounted for as the contributions from surface channeling electrons and electrons being deflected in the surface potential field. Both contributions have their characteristic features in the streakings of the central spot. The forbidden spots, (100) and ( 100 ), which are experimentally observed, can be obtained only after the replacement of the vacuum potential by the modified surface image potential. With the incident beam outside the surface, the presence of the surface image potential field greatly enhances the electron surface monolayer channeling.

Recovered elastic and plastic strains at residual micro-indentations in an MgO crystal

Abstract Elastic and plastic unloading effects are described for aligned diamond pyramid indentations imposed on a (001) MgO crystal surface. Substantial elastic recovery effects are explained in terms of the theoretical Hertzian behaviour for a ball indentation test. Limited reversal of the plastic deformation field occurs, involving dislocation processes which first produce an extensive amount of slip and cracking during the loading portion of the indentation cycle.

Studies of the scanning ion beam sputter-cleaned MgO crystal surface by scanning reflection electron microscopy

A scanning ion gun system has been installed on the specimen preparation chamber (pressure ∼5xl0-8 torr) of the VG-HB5 STEM microscope. By using the specimen current imaging technique, it is possible to use an ion beam to sputter-clean the preferred surface region on a bulk sample. As shown in figure 1, the X-Y raster-gate control of the scanning unit for the Krato Mini-Beam I is used to minimize the beam raster area down to a 800μm x800μm square region. With beam energy of 2.5KeV, the MgO cleavage surface has been ion sputter-cleaned for less than 1 minute. The carbon film or other contaminant, introduced during the cleavage process in air, is mostly removed from the MgO crystal surfaces.The immediate SREM inspection of this as-cleaned MgO surface, within the adjacent STEM microscope, has revealed the detailed surface structures of atomic steps, which were difficult to observe on the as-cleaved MgO surfaces in the previous studies.

Study of freshly deposited metallic particles on MgO crystal surfaces by scanning reflection electron microscopy

In the newly developed specimen chamber of the VG-HB5 STEM microscope, a fresh monolayer of metallic film can be deposited on the MgO surfaces, and the surface phenomena are studied by the SREM technique. Approximately 6 Å Pd film has been deposited on the heated MgO surfaces ( T s = 250°C). It seems that reactions have taken place between the Pd film and the MgO surfaces. SREM observations show that the surface fringes, formed by the Pd particles of ∼ 10 A ̊ in diameter. have a periodicity of ∼ 25 A ̊ which is equivalent to the periodicity of the periodic array of the MgO surface steps. The beam-induced crystallization of (001)NiO and reduction of (011)Ni from the in-situ-deposited Ni film are reported. After exposure to air and UHV annealing, the only particles observed have 20–30 Å in diameter and appear sitting along the edge of the big surface steps. The pattern shows (001)R45° orientation and ∼ 4.58 A ̊ lattice parameter, but the particle formation is unknown. The NiO particles of ∼ 5 A ̊ are resolved in the SREM images. Nanodiffraction is used to characterize the microstructure of individual particles on the surface regions of less than 10 Å in diameter.

SREM of MgO crystal surface structure and in-situ-deposited metallic particles on MgO surface

Development of an in-situ specimen treatment chamber for a VG-HB5 STEM microscope is reported. Radiative heat cleaning and thermal metal deposition facilitate preparing better crystal surfaces and fresh submonolayer metal films on crystal surfaces. The Scanning Reflection Electron MIcroscopy (SREM) technique is used to study (1) the surface structure of MgO crystals, (2) decoration effects of deposited Cu particles on MgO surfaces, and (3) electron-beam-induced growth of deposited Pd particles on MgO surfaces. The periodic arrays of MgO crystal surface steps show 20–30 Å periodicity. The as-deposited Cu particles tend to align along [001], the incident beam direction, and/or decorate the edges of big surface steps of the MgO crystal, but, after exposure to the air, these particles appear randomly decorating the fine steps. Electron-beam-induced surface diffusion leading to the growth of Pd particles on MgO surface steps is observed. The core region of the particle growth is formed by the concentrated electron beam and the growth process extends to the nearby region as the condensed electron beam further irradiates the sample surface. Nanodiffraction patterns are used to characterize these metallic particles, which are 10–20 Å in diameter.

Epitaxial and electronic structures of ultra-thin copper films on MgO crystal surfaces

The deposition of Cu on MgO(100) and MgO(111) surfaces by electron beam evaporation technique at room temperature in UHV was studied by LEED, AES and EELS. An epitaxial growth was seen, already from the initial deposition stage, of Cu(100) orientation both on freshly cleaved and on Ar + bombarded MgO(100) surfaces and of Cu(111) orientation on freshly cleaved MgO(111) surfaces. The changes in the electron energy loss spectra with copper coverage are discussed and a model for copper epitaxial growth on MgO crystal surfaces is proposed.

Epitaxial and electronic structures of ultra-thin copper films on MgO crystal surfaces

Epitaxial and electronic structures of ultra-thin copper films on MgO crystal surfaces

Interactions of electron beams with surfaces of MgO crystals

The strong interaction of electrons with the flat surfaces of small crystals has been investigated by high resolution CTEM and STEM instruments. When cubic crystals of MgO smoke with edges 20–300 nm are oriented so that the ⇇001↩ or ⇇011↩ zone axis is parallel to the optical axis, then two kinds of external fringes are observed at (100) surfaces. One kind is parallel to the surface, having spacings up to 0.4 nm. These are caused by interference among the electron channelled along the surface. Fresnel-diffracted ones and the remnant of the incident beam. Fringes of the other kind, which appear as fine structure in the first kind of fringes, are perpendicular to the crystal edge. When an electron beam is parallel to the ⇇011↩ axis, the second kind of fringe, whose spacing is 0.3 nm corresponding to d 011, shows the difference of the surface potential between magnesium atoms and oxygen atoms. Selected area diffraction patterns and microdiffraction patterns also show the same periodicities as in the two kinds of fringes. Simulated images, using the scattering amplitudes for ions, are compared with observations.

Sub-threshold [Li]° formation and decoration of strained regions in crystalline MgO

Abstract The effect of surface deformation on [Li]° coloration in lithium-doped MgO crystals is reported. For chemically etched surfaces, the threshold temperature for [Li]° production in oxygen atmospheres is observed to be 1160 K. For mechanically deformed surfaces, such as are produced by SiC abrasives, sub-threshold production of [Li]° defects was observed down to 1050 K. Upon heating the specimens in oxygen at sub-threshold temperatures, decoration of strained regions in the bulk or surfaces of MgO crystals has been produced using [Li]° coloration. These results provide direct evidence that the formation of [Li]° defects is enhanced by dislocations and/or their debris in the crystal. Sub-threshold [Li]° coloration, in conjunction with optical and transmission electron microscopy, indicates that, at elevated temperatures, deformation bands are propagated from surface microcracks to a depth of the order of 1 mm.