A DFT study of substrate effect on the magnetism of V(001) surface

The effect of isoelectric transition metals (TM) Nb and Ta on the magnetism of the V(001) surface is investigated from first principles using Density functional theory (DFT), with the generalized gradient approximation (GGA). Ferromagnetic (FM) moments of 2.5 μ_B and 2.2 μ_B are obtained for the relaxed surface V monolayer (ML) in the V/Nb(001) and the V/Ta(001) systems respectively, at T = 0 K. The values are almost twice of those obtained with Mo and W of group VIB and can be attributed to the comparatively smaller bandwidths of the substrates Nb and Ta. Small induced magnetic moments are present on the Nb and Ta interfacial layers, which are coupled anti-ferromagnetically with the V ML.     

Atomistic simulations for the non-equilibrium surface premelting and melting of Nb(1 1 0) plane

In the present paper molecular dynamics (MD) simulations have been preformed to investigate the surface melting process and microscopic mechanism of Nb(1 1 0) plane in the atomic scale with a modified analytic embedded atom method (MAEAM). On the basis of the MD relaxation dependence of averaged internal energy and layer structure factor at given temperatures, the melting point of the sample has been estimated to be 2510 K. Then by the above results the Nb(1 1 0) plane melting process has been approximately divided into two stages: first the layer-by-layer premelting phase in the surface region and then a simultaneous abrupt melting transition for the inner layers. According to the variation of the averaged internal energy of the inner atomic layer, the melting latent heat has been calculated and the result is in good agreement with the experimental value. The simulated snapshots of atomic configuration for Nb(1 1 0) plane have indicated that the dynamically microscopic mechanism of melting nucleation during the melting transition.     

Growth of Nb thin films on SiO2

Nb thin films have been prepared by electron beam evaporation under ultrahigh vacuum conditions on fused silica substrates at various temperatures, and their structural and morphological evolutions have been investigated using X-ray diffraction and atomic force microscopy. The crystallographic texture of the Nb films is found to depend on the growth temperature. At room temperature, the [1 1 0] texture is dominant. However, at 200°C, the [3 1 0] oriented growth is favored, co-existing with [1 1 0] and [2 0 0] oriented grains. At 400–600°C, a completely [1 1 0] textured film is formed. At even higher temperature (800°C), a complex texture of [1 1 0] (dominant), [2 0 0] and [3 1 0] is observed again. It is also found that the single [1 1 0] textured Nb films have smooth surfaces, and the complex textured Nb films have rough surfaces.     

Chemical properties of structurally well-defined PdRu/Ru(0 0 0 1) surface alloys – Interaction with CO

The chemical properties of structurally well-defined PdRu/Ru(0 0 0 1) monolayer surface alloys [H. Hartmann, T. Diemant, A. Bergbeiter, J. Bansmann, H.E. Hoster, R.J. Behm, Surf. Sci. in press, doi:10.1016/j.sucs.2008.10.055.] and a Pd monolayer on Ru(0 0 0 1) were studied by temperature programmed desorption and infrared reflection absorption spectroscopy using CO as probe molecule. IR experiments on the PdRu/Ru(0 0 0 1) surface alloys demonstrate that CO adsorption on Ru sites resembles that on pure Ru(0 0 0 1) (on-top adsorption), while adsorption on the Pd sites occurs on both multifold coordinated and on-top sites, similar to CO on Pd(1 1 1). A significant destabilization of CO adsorption on Pd sites for both, surface alloys and the Pd monolayer film, compared to pure Pd(1 1 1) surfaces is attributed to a combination of geometric strain and vertical electronic ligand effects; an additional variation in the CO adsorption bond strength in the surface alloys is attributed to changes in the neighboring surface atom shell (lateral ligand effects). The chemical modifications introduced by PdRu surface alloy formation are compared with findings for deuterium adsorption on the same surface alloys; effects of the two-dimensional (2D) distribution of surface atoms are illustrated by comparison with CO adsorption on PtRu/Ru(0 0 0 1) surface alloys, where in contrast to the pronounced 2D phase segregation in PdRu/Ru(0 0 0 1) the surface atoms are essentially randomly distributed.     

Hard magnetic properties of rapidly annealed NdFeB thin films on Nb and V buffer layers

NdFeB thin films of the form A (20 nm)/NdFeB(d nm)/A(20 nm), where d ranges from 54 to 540 nm and the buffer layer A is Nb or V were prepared on a Si(1 0 0) substrate by magnetron sputtering. The hard Nd₂Fe₁₄B phase is formed by a 30 s rapid anneal or a 20 min anneal. Average crystallite size ranged from 20 to 35 nm with the rapidly annealed samples having the smaller crystallite size. These samples also exhibited a larger coercivity and energy product than those treated by a 20 min vacuum anneal. A maximum coercivity of 26.3 kOe at room temperature was obtained for a Nb/NdFeB (180 nm)/Nb film after a rapid anneal at 725°C. Initial magnetization curves indicate magnetization rotation rather than nucleation of reverse domains is important in the magnetization process. A Brown's equation analysis of the coercivity as a function of temperature allowed us to compare the rapidly annealed and 20 min annealed samples. This analysis suggests that rapid annealing gives higher quality crystalline grains than the 20 min annealed sample leading to the observed large coercivity in the rapidly annealed samples.     

Density functional calculations for Ni adsorption on Al(1 1 0)

The adsorption of 0.25, 0.5 and 1 monolayer (ML) of the transition metal Ni on the metal substrate Al(1 1 0) was studied using first-principles calculations at the level of density functional theory. The metal–metal system was analyzed with the generalized gradient approximation. Four stable atomic configurations were considered, and the optimized geometries and adsorption energies of different Ni adsorption sites on the Al(1 1 0) surface at selected levels of coverage were calculated and compared. The four-fold hollow site was determined to be the most stable adsorption site with adsorption energy of 5.101 eV at 0.25 ML, 3.874 eV at 0.5 ML and 3.665 eV at 1 ML. The adsorption energies of the four sites slightly decreased as the Ni coverage increased. Work function analysis showed that when Ni is adsorbed on the Al(1 1 0) surface, the work function decreased as the coverage increased due to depolarization. The Mulliken population and density of states were calculated to determine the charge distribution of the adsorption site, confirming that a chemisorption interaction exists between the adsorbed Ni atom and Al(1 1 0) surface atoms.     

Effect of spin paramagnetism on upper critical field of disordered nanocrystalline PbMo6S8 superconductor

Ignoring the paramagnetic effect Niu and Hampshire [H.J. Niu, D.P. Hampshire, Phys. Rev. Lett. 91 (2003) 027002] have claimed that a disordered nanocrystalline sample of the Chevrel phase superconductor PbMo₆S₈, having a grain size of 20 nm, corresponds to an upper critical field of 110 T. Whether the paramagnetic effect is indeed negligible in this sample or not has been investigated here by using the Werthamer–Helfand–Hohenberg theory of upper critical field of dirty superconductors. The extent of the paramagnetic effect is obtained by matching the experimental values of the upper critical field for various temperatures with those calculated from the Werthamer–Helfand–Hohenberg theory. It has been found that the 20 nm superconductor involves sufficient paramagnetic effect such that the value of the upper critical field cannot be larger than ～70 T.     

Ultrasonically controlled growth of monodispersed octahedral BiVO4 microcrystals for improved photoelectrochemical water oxidation

The synthesis of facet-controlled structures with precise morphology and exposed reactive surface is one of the key research challenges. We effectively endeavoured to obtain the monodisperse octahedral bismuth vanadate microcrystals with exposed {1 0 1},{2 0 0},{3 1 2} and {0 2 1} dominant facets through an optimized sonochemical assisted hydrothermal process. A pulse sonication (5-s ON and 2-s OFF cycle, 21 W ultrasonic power and 20 kHz ultrasonic frequency) for 30 mins followed by 1 h hydrothermal treatment was found to yield the preferred octahedral morphology. The microscopic and X-ray analysis suggested a potent role of ultrasonic waves for the initial seed growth and its evolution into a well-defined monodisperse microcrystals. The density functional theory (DFT) calculations revealed strongly localized bandgap states with a bandgap of ~2.47 eV. The PEC measurements for water oxidation demonstrated the efficacy of these microcrystals as photoanode. Notably, the optimized octahedral BiVO₄ microstructure exhibited a superior performance as evident from photocurrent density ~0.9 mAcm⁻² at 1.23 V vs. RHE and %IPCE value of ~22% compared to analogous photoanodes under visible light irradiation.     

Surface alloy formation,short-range order,and deuterium adsorption properties of monolayer PdRu/Ru(0 0 0 1) surface alloys

The formation and structure of monolayer PdRu/Ru(0 0 0 1) surface alloys and their adsorption properties with respect to deuterium adsorption were investigated by atomic resolution scanning tunneling microscopy and by temperature programmed desorption. Surface alloys, prepared by deposition of up to one monolayer of Pd and flash annealing to 1150 K show (i) negligible loss of Pd by desorption or diffusion into the Ru bulk during this procedure and (ii) dominant phase separation into 2D Pd and Ru islands, in contrast to the random distribution in PtRu/Ru(0 0 0 1) surface alloys [H.E. Hoster, A. Bergbreiter, P.M. Erne, T. Hager, H. Rauscher, R.J. Behm, Phys. Chem. Chem. Phys. 10 (2008) 3812]. 2D short-range order parameters and the abundance of specific adsorption ensembles were evaluated for different Pd contents, effective pair interaction (EPI) energies were derived from Monte Carlo simulations. Deuterium adsorption on Pd monolayer films shows a pronounced weakening of adsorption bond, which is attributed to compressive strain and metal–metal interactions between Pd and underlying Ru atoms (‘vertical ligand effect’). Mixed adsorption ensembles containing both Pd and Ru atoms give rise to D₂ desorption in the intermediate temperature regime. The impact of the specific lateral distribution of the two metal species on the chemical surface properties is illustrated by comparison with deuterium adsorption on dispersed PtRu/Ru(0 0 0 1) surface alloys [T. Diemant, H. Rauscher, R.J. Behm, J. Phys. Chem. C 112 (2008) 8381].     

Phase transition of Sr on Si (0 0 1): First principles prediction and experiment

The ability to understand and predict the phase diagrams of surface phases from first principles can be valuable for developing processes for growth of epitaxial structures. In the growth of epitaxial oxides on Si (0 0 1), a submonolayer phase of Sr plays a key role. The physical structure for this phase, which has 2 × 3 symmetry and occurs at 1/6 monolayer Sr coverage, was recently elucidated using both first principles theory and diffraction experiments [J.W. Reiner, K.F. Garrity, F.J. Walker, S. Ismail-Beigi, C.H. Ahn, Role of strontium in oxide epitaxy on silicon (0 0 1), Phys. Rev. Lett. 101 (10) (2008) 105503.]. Our approach to understanding the broader Sr/Si phase diagram combines density functional theory with a thermodynamic analysis of the phase equilibrium between a Sr lattice gas and the 2 × 3 structure. We use reflection high energy electron diffraction (RHEED) to experimentally determine the phase diagram, finding good agreement with theoretical predictions.     

Room-temperature deposition of group III metals on Si(1 0 0): A comparative study of nucleation behavior

In this paper, we compare and contrast the processes of nucleation and subsequent growth of single-atom wide metal chains formed when group III metals (Al, Ga, In) are deposited onto Si(1 0 0) at room-temperature (RT). Employing Density Functional Theory (DFT) calculations, diffusion pathways on Si(1 0 0) surface are identified and their associated activation barriers are calculated. Then, the relative stabilities of various C-defect-pinned chains are examined by comparing the relevant adsorption energies. We also account for the observation that defect-nucleated chains grow on only one side of a C-defect by showing that the latter’s presence breaks the symmetry between the two previously equivalent binding sites on either side and rendering one much more stable than the other. Next, a growth model tailored for each group III metal/Si(1 0 0) system and incorporating the above results was simulated using Kinetic Monte Carlo (KMC) techniques to show that the surface morphologies generated by this model accurately reflect the observed ratio of homogeneously to heterogeneously nucleated chains. Finally, we examine through KMC simulations the consequences of the contrasting roles of a defect on In/Si(1 0 0) and Al/Si(1 0 0) – it captures adatoms in the former while it merely blocks direct adatom diffusion in the latter – on key quantities such as the mean island density.     

Optical characterization of methanol adsorption on the bare and oxygen precovered Cu(1 1 0) surface

We have studied the adsorption and reaction of methanol on the bare and oxygen precovered Cu(1 1 0) surface at 200 K using reflectance difference spectroscopy (RDS). On the bare and fully oxygen covered surface, the sticking coefficient is close to zero. In contrast, on the partially oxygen covered surface, a sticking coefficient close to unity is obtained. This observation suggests a high mobility of methanol on both bare and oxygen covered Cu(1 1 0) and of methoxy on Cu(1 1 0). Two reaction regimes, an oxygen supply limited and an adsorption site limited regime are identified. The transition between these two regimes occurs for an oxygen coverage of about 0.2.     

The origin of narrowing of the Si 2p coincidence photoelectron spectroscopy main line of Si(1 0 0) surface

The Si 2p photoelectron spectroscopy (PES) main line of Si(1 0 0) surface measured in coincidence with the singles (noncoincidence) Si L_2,3-VV Auger-electron spectroscopy (AES) elastic peak is calculated. The agreement with the experiment is good. The present work is the first many-body calculation of the experimental coincidence PES spectrum of solid surface. The narrowing of the coincidence Si 2p PES main line compared to the singles one is due to the mechanism inherent in the coincidence PES. The inherent mechanism is explained by a many-body theory by which photoemission and Auger-electron emission are treated on the same footing.     

Strong structure sensitivity in the partial oxidation of styrene on silver single crystals

In contrast to the formation of styrene oxide on Ag(1 1 1), phenylacetaldehyde and phenylketene dominate the partial oxidation of styrene on Ag(1 1 0), even though the reactions follow the same mechanism on both surfaces. The origin of this difference is that on Ag(1 1 0) the activation energy for transformation of the oxametallacycle to the combustion intermediate is much lower than on the (1 1 1) surface, so that ring-closure of the oxametallacycle to form styrene oxide is short circuited. Also the combustion intermediate appears more stable on Ag(1 1 0) than on Ag(1 1 1).     

Surface vibrational thermodynamics from ab initio calculations for fcc(1 0 0)

We present vibrational dynamics and thermodynamics for the (1 0 0) surfaces of Cu, Ag, Pd, Pt and Au using a real space approach. The force field for these systems is described by density functional theory. The changes in the vibrational dynamics and thermodynamics from those in bulk are confined mostly to the first-layer. A substantial enhancement of the low-frequency end of the acoustic branch was found and is related to a loosening of the bond at the surface. The thermodynamics of the first-layer also show significant differences (higher heat capacity, lower free energy and higher mean vibrational square amplitudes) from what obtains in bulk. Comparing these results with those calculated using embedded-atom method potentials, we discovered that for Ag(1 0 0) and Cu(1 0 0), the two methods yield very similar results while for Pd(1 0 0), Pt(1 0 0) and Au(1 0 0) there are substantial differences.     

Influence of heat treatment on field emission characteristics of boron nitride thin films

Boron nitride (BN) nanometer thin films are synthesized on Si (1 0 0) substrates by RF reactive magnetron sputtering. Then the film surfaces are treated in the case of the base pressure below 5 × 10⁻⁴ Pa and the temperature of 800 and 1000 °C, respectively. And the films are studied by Fourier transform infrared spectra (FTIR), atomic force microscopic (AFM) and field emission characteristics at different annealing temperature. The results show that the surface heat treatment makes no apparent influence on the surface morphology of the BN films. The transformations of the sample emission characteristics have to do with the surface negative electron affinity (NEA) of the films possibly. The threshold electric fields are lower for BN samples without heat-treating than the treated films, which possibly ascribed to the surface negative electron affinity effect. A threshold field of 8 V/μm and the emission current of 80 μA are obtained. The surface NEA is still presence at the heat treatment temperature of 800 °C and disappeared at temperature of 1000 °C.     

Study on the formation mechanism of isoniazid crystal defects and defect elimination strategy based on ultrasound

In crystallization, crystal growth defects may reduce the strength and purity of crystals, which are not welcomed in the industry. Herein, isoniazid (INH) crystals were chosen as an example to investigate the formation of crystal defects at the molecular scale by combining experiments and molecular dynamics simulations. It was found that the strong interaction between the solvent and the crystal surface, high temperature, small stirring rate, and low supersaturation can lead to more pronounced crystal defects. The bulk severity of INH crystal defects was reflected by N₂ adsorption–desorption measurement. Besides, the single-crystal growth experiments manifested the rough growth mechanism for the (1 1 0) surface in the axial direction and the stepwise growth mechanism for the (0 0 2) surface in the radial direction. For the (1 1 0) surface, cavities occurred under the condition where the growth rate of the crystal edges and corners was greater than that of the surface center due to the starvation phenomenon of diffusion. While for the (0 0 2) surface, when the solvent removal rate was lower than the solute insertion rate, liquid inclusions were formed, which was verified by Raman microscopy. Furthermore, the ultrasonic strategy was successfully proposed to eliminate INH crystal defects and prepare perfect INH crystals. Moreover, the mechanism of ultrasound to reduce the crystal defect was proposed. We believe this work can provide insights into the design and preparation of defect-free crystals in crystallization.     

Irradiation induced modifications in morphology and magnetic property of Mn/Si structure

Mn films of ∼50 nm has been deposited by electron beam evaporation technique on cleaned and etched Si [(1 0 0), 8–10 Ω cm] substrates to realize a Mn/Si interfacial structures. The structures have been irradiated from energetic (∼100 MeV) ion beam from Mn side. The irradiated and unirradiated structures have been characterized from atomic force microscopy, X-ray diffractometry, magnetic force microscopy, and vibrating sample magnetometer facilities. It has been found that surface/interfacial granular silicide phases (of Mn_xSi_y) are formed before and after the irradiation with a irradiation induced modifications of surface morphology and magnetic property. The surface/interface roughness has been found to increase on the irradiation from the atomic force microscopy data. The magnetic property on the irradiation shows an interesting and significant feature of an increased coercivity and a ferromagnetic like behavior in the Mn–Si structure. The observed increased coercivity has been related to the increased roughness on the irradiation. The ferromagnetism after the irradiation is a curious phenomenon which seems due to the formation of Mn–C–Si compound from the carbon dissolved in silicon.     

Thermodynamics of iron at extreme pressures and temperatures

A thermodynamic database on all iron phases (BCC, FCC, HCP and melt) has been created using thermochemical and equations of state data from experiments and theory. The database permits the calculation of the phase diagram of iron to physical conditions of the Earth's core (pressure of 365 GPa and temperature of 6453 K). If the inner core were all iron, its upper temperature would be 6453 (500) K. The average heat capacity of a pure iron HCP inner core is calculated as 29.4 J/mol/K with an entropy of 92 J/mol/K and a gruneisen parameter of 1.81.     

Effects of growth temperature modulated by HCl flow rate on the surface and crystal qualities of thick GaN by HVPE

We studied the influence of the growth temperature and HCl flow rate on the morphological evolution of crack-free thick GaN films by using a home-made horizontal hydride vapor phase epitaxy on sapphire substrates. Optical difference microscopy, scanning electron microscopy (SEM), atomic force microscopy (AFM), and cathodoluminescence (CL) were carried out to reveal the surface property of the GaN epilayer. It was found that the higher growth temperature is a key factor to obtain mirror, colorless and flat GaN surface. However, this key effect of temperature was modulated by HCl flow rate (HCl > 15 sccm). The surface RMS roughness was reduced from 206 to 2.51 nm for 10 μm × 10 μm scan area when GaN was grown at 1070 °C with HCl flow rate up to 30 sccm. These samples also reduced their (0 0 0 2) FWHM result from 1000 to 300 arcsec and showed a strong near-band-edge peak in CL spectra. Results indicated that growth temperature influence growth velocities on different crystalline planes, which will lead to the different morphologies obtained. High growth temperature can improve the lateral growth rate of vertical {1 1 ? 2 0} facets and reduce the vertical growth rate of top {0 0 0 1} facet combined with higher HCl flow rate, which leads to completely coalescence of surface.