Magnetism in boron nitride monolayer: Adatom and vacancy defect

Using the full potential linearized augmented plane wave (FLAPW) method, we have investigated the adatom or vacancy defect induced magnetic properties of hexagonal boron nitride (h-BN) monolayer. It has been observed that the N vacancy defect has no influence on the magnetic property of h-BN, whereas the B vacancy defect caused spin polarization in the nearest three N atoms. The total magnetic moment is about 0.87 μ_B within muffin-tin radius (0.29 μ_B per N atom) and the spin polarized N atoms show metallic feature. In the presence of B adatom defect, we have obtained rather weak spin polarization about 0.1 μ_B. However, the sizable magnetic moment of 0.38 μ_B appears in N adatom defect. Both B and N adatom defect systems preserve very close to semiconducting feature with a finite band gap. We have found that the DOS and the XMCD spectral shapes are strongly dependent on the defect type existing in the h-BN monolayer and this finding may help reveal the origin of magnetism in the h-BN layer if one performs surface sensitive experiment such as spin polarized scanning tunneling microscopy or XMCD measurement in the near future.     

Controllable oxidation of h-BN monolayer on Ir(111) studied by core-level spectroscopies

The effect of atomic oxygen adsorption on the structure and electronic properties of monolayer hexagonal boron nitride (h-BN) grown on Ir(111) has been studied using near edge X-ray absorption fine structure spectroscopy (NEXAFS), photoelectron spectroscopy (PES), and low-energy electron diffraction (LEED). It has been shown that the oxidation of the h-BN monolayer occurs through a gradual substitution of N by O in the h-BN lattice. This process leads to the formation of defect sites corresponding to three different types of the B atom environment (BN_3 ? xO_x with x = 1,2,3). The oxidation of the h-BN monolayer is very different from the case of graphene on Ir(111), where adsorption of atomic oxygen results mainly in the formation of epoxy groups [J. Phys. Chem. C. 115, 9568 (2011)]. A post-annealing of the h-BN monolayer after oxygen exposure results in further destruction of the B–N bonds and formation of a B₂O₃-like structure.     

Fermi surfaces of single layer dielectrics on transition metals

Single sheets of hexagonal boron nitride on transition metals provide a model system for single layer dielectrics. The progress in the understanding of h-BN layers on transition metals of the last 10 years is shortly reviewed. Particular emphasis lies on the boron nitride nanomesh on Rh(1 1 1), which is a corrugated single sheet of h-BN, where the corrugation imposes strong lateral electric fields. Fermi surface maps of h-BN/Rh(1 1 1) and Rh(1 1 1) are compared. A h-BN layer on Rh(1 1 1) introduces no new bands at the Fermi energy, which is expected for an insulator. The lateral electric fields of h-BN nanomesh violate the conservation law for parallel momentum in photoemission and smear out the momentum distribution curves on the Fermi surface.     

Hexagonal boron nitride film substrate for fabrication of nanostructures

The fabrication of material with an atomic scale manipulation requires the suitable advanced substrate for epitaxial growth without the effect by the substrate lattice structure. Hexagonal boron nitride (h-BN) can be the advanced substrate for atomic manipulation due to van der Waals’ gap with little attractive force along to c axis. We have successfully synthesized h-BN layer on the co-deposited Cu/BN film by surface segregation phenomena using helicon wave plasma enhanced radio frequency (rf) magnetron sputtering system. Auger electron spectroscopy (AES) and X-ray photon spectroscopy (XPS) analysis showed that the h-BN composite segregated on the surface of Cu/BN film covered over 95% of the film annealed at 900 K for 30 min. Atomic forces microscopy (AFM) and scanning tunneling microscopy (STM) analysis showed that attractive force on the film surface is uniformly distributed to an extent of 2nN and that the h-BN surface can be a good electric insulator like sintered h-BN plate.     

Strained c(4 × 2) CoO(1 0 0)-like monolayer on Pd(1 0 0): Experiment and theory

We report on an interface-stabilized strained c(4 × 2) phase formed by cobalt oxide on Pd(1 0 0). The structural details and electronic properties of this oxide monolayer are elucidated by combination of scanning tunneling microscopy data, high resolution electron energy loss spectroscopy measurements and density functional theory. The c(4 × 2) periodicity is shown to arise from a rhombic array of Co vacancies, which form in a pseudomorphic CoO(1 0 0) monolayer to partially compensate for the compressive strain associated with the large lattice mismatch (～9.5%) between cobalt monoxide and the substrate. Deviation from the perfect 1:1 stoichiometry thus appears to offer a common and stable mechanism for strain release in Pd(1 0 0) supported monolayers of transition metal rocksalt monoxides of the first transition series, as very similar metal-deficient c(4 × 2) structures have been previously found for nickel and manganese oxides on the same substrate.     

Theoretical and experimental investigation on structural and electronic properties of Al/O/Al,O-doped WS2

Effects of the doping atom (O, Al, and (Al, O)) on structural and electronic properties of the monolayer WS₂ have been studied by using first-principles calculations. Results show that the covalent character of W–S bonding has been enhanced after doping. Meanwhile, W–O, Al–S and W–S bonds of (Al, O) co-doped WS₂ monolayer have higher covalent character compared with O-doped and Al-doped WS₂ monolayer of this work. After doping with Al (or Al, O) atoms, Fermi level moves close to the valence band and the dopant atoms produce the defect energy levels, indicating that Al doped and (Al, O) co-doped WS₂ monolayer both have p-type conductivity. O-doped and (Al, O) co-doped WS₂ ultrathin films was prepared on Si substrates. Results of Raman spectra show the formation of the O-doped and (Al, O) co-doped WS₂ films. Moreover, compared with the pure WS₂, the approximate reduction of 0.43 eV and 0.46 eV for W 4f and S 2p in binding energy after (Al, O) co-doped shows that p-type doping of (Al, O) co-doped WS₂ has been verified.     

Synergetic effects of ultrasound and slightly acidic electrolyzed water against Staphylococcus aureus evaluated by flow cytometry and electron microscopy

This study evaluated the synergetic effects of ultrasound and slightly acidic electrolyzed water (SAEW) on the inactivation of Staphylococcus aureus using flow cytometry and electron microscopy. The individual ultrasound treatment for 10 min only resulted in 0.36 log CFU/mL reductions of S. aureus, while the SAEW treatment alone for 10 min resulted in 3.06 log CFU/mL reductions. The log reductions caused by combined treatment were enhanced to 3.68 log CFU/mL, which were greater than the sum of individual treatments. This phenomenon was referred to as synergistic effects. FCM analysis distinguished live and dead cells as well as revealed dynamic changes in the physiological states of S. aureus after different treatments. The combined treatment greatly reduced the number of viable but nonculturable (VBNC) bacteria to 0.07%; in contrast, a single ultrasound treatment for 10 min induced the formation of VBNC cells to 45.75%. Scanning and transmission electron microscopy analysis revealed that greater damage to the appearance and ultrastructure of S. aureus were achieved after combined ultrasound-SAEW treatment compared to either treatment alone. These results indicated that combining ultrasound with SAEW is a promising sterilization technology with potential uses for environmental remediation and food preservation.     

Γ to Xzelectron transfer times in type-II GaAs/AlAs superlattices due to emission of confined and interface phonons

We calculate the Γ →  X_zelectron transfer times due to the emission of confined and interface LO phonons in type-II GaAs–AlAs and AlGaAs–AlAs superlattices. A dielectric continuum model is employed to describe the electron–phonon interaction, and the electron envelope wavefunctions are obtained from a Kronig–Penney model. The calculated transfer times are in good agreement with available experimental results. We have used two different sets of AlAs X-valley effective masses obtained from different experiments and we show that the transfer times calculated with the heavier masses are in closer agreement with the measured data.     

The geometries of coadsorbate layers of O and H on Ru(0 0 1): How well can quantitative LEED see hydrogen atoms?

Using a CCD LEED system for the collection of IV data with low beam damage, and full dynamical as well as tensor LEED calculations, we have determined the geometries of the (2 × 2)-(O + 3H) and the (2 × 2)-(O + H) coadsorbate structures on Ru(0 0 1). We show that here quantitative LEED can locate the H atoms very well. Not only their sites (hcp in the first, fcc in the second case), but also the Ru–H spacings and changes in the first two substrate layers are clearly determined. We argue that this success is due to the relatively large data range and to the smaller H mobility compared to pure H layers caused by their repulsive lateral interactions with the oxygen atoms.     

Studies of Li intercalation of hydrogenated graphene on SiC(0001)

The effects of Li deposition on hydrogenated bilayer graphene on SiC(0001) samples, i.e. on quasi-freestanding bilayer graphene samples, are studied using low energy electron microscopy, micro-low-energy electron diffraction and photoelectron spectroscopy. After deposition, some Li atoms form islands on the surface creating defects that are observed to disappear after annealing. Some other Li atoms are found to penetrate through the bilayer graphene sample and into the interface where H already resides. This is revealed by the existence of shifted components, related to H–SiC and Li–SiC bonding, in recorded core level spectra. The Dirac point is found to exhibit a rigid shift to about 1.25 eV below the Fermi level, indicating strong electron doping of the graphene by the deposited Li. After annealing the sample at 300–400 °C formation of LiH at the interface is suggested from the observed change of the dipole layer at the interface. Annealing at 600 °C or higher removes both Li and H from the sample and a monolayer graphene sample is re-established. The Li thus promotes the removal of H from the interface at a considerably lower temperature than after pure H intercalation.     

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.     

Crystal structures and band gap characters of h-BN polytypes predicted by the dispersion corrected DFT and GW method

Geometry optimizations are performed for three polytypes of h-BN using density functional theory with dispersion correction for the van der Walls interaction. Quasiparticle band structure calculations are carried out to solve the controversy on band gap type of h-BN. Band energies are corrected by GW method. The h-BN with B_k structure has an indirect band gap of 5.840 eV. Two kinds of h-BN polytypes are shown to be mechanically stable and have quasi-direct band gap type.     

Application of low energy ion blocking for adsorption site determination of Na Atoms on a Cu(111) surface

Ion blocking in the low keV energy range is demonstrated to be a sensitive method for probing surface adsorption sites by means of the technique of time-of-flight scattering and recoiling spectroscopy (TOF-SARS). Adsorbed atoms can block the nearly isotropic backscattering of primary ions from surface atoms in the outmost layers of a crystal. The relative adsorption site position can be derived unambiguously by simple geometrical constructs between the adsorbed atom site and the surface atom sites. Classical ion trajectory simulations using the scattering and recoiling imaging code (SARIC) and molecular dynamics (MD) simulations provide the detailed ion trajectories. Herein we present a quantitative analysis of the blocking effects produced by sub-monolayer Na adsorbed on a Cu(111) surface at room temperature. The results show that the Na adsorption site preferences are different at different Na coverages. At a coverage θ = 0.25 monolayer, Na atoms preferentially populate the fcc threefold surface sites with a height of 2.7 ± 0.1 Å above the 1st layer Cu atoms. At a lower coverage of θ = 0.10 monolayer, there is no adsorption site preference for the Na atoms on the Cu(111) surface.     

Synthesis of boron nitride nanotubes by Argon supported Thermal Chemical Vapor Deposition

Thermal Chemical Vapor Deposition technique is modified with the use of Argon gas flow inside the chamber as an alternative for vacuum and orientation of one end closed quartz test tube. The use of Argon gas not only simplified the experimental set up, but also made it ~ 18 % cost effective compared to the conventional set up. Field Emission Scanning Electron Microscopy micrographs show straight and long BNNTs along with some cotton like morphologies. Transmission electron microscopy revealed bamboo like structure inside the tube and ~0.34 nm interlayer spacing for highly crystalline nature of boron nitride nanotubes. X-ray photon spectroscopy shows B 1s peak at 191.08 eV and N 1s peak at 398.78 eV that represents h-BN. Whereas, Raman spectrum indicates a major peak at ~1379.60 (cm⁻¹) that correspond to E_2g mode of h-BN.     

Strain-dependent electronic and magnetic of Co-doped monolayer of WSe2

We perform first-principles calculation to investigate electronic and magnetic properties of Co-doped WSe₂ monolayer with strains from −10% to 10%. We find that Co can induce magnetic moment about 0.894 μ_B, the Co-doped WSe₂ monolayer is a magnetic semiconductor material without strain. The doped system shows half-metallic properties under tensile strain, and the largest half-metal gap is 0.147 eV at 8% strain. The magnetic moment (0.894 μ_B) increases slightly from 0% to 6%, and jumps into about 3 μ_B at 8% and 10%, which presents high-spin state configurations. When we applied compressive strain, the doped system shows a half-metallic feature at −2% strain, and the magnetic moment jumps into 1.623 μ_B at −4% strain, almost two times as the original moment 0.894 μ_B at 0% strain. The magnetic moment vanishes at −7% strain. The Co-doped WSe₂ can endure strain from −6% to 10%. Strain changes the redistribution of charges and magnetic moment. Our calculation results show that the Co-doped WSe₂ monolayer can transform from magnetic semiconductor to half-metallic material under strain.     

Studies on growth morphology,UV absorbance and luminescence properties of sulphur doped ZnO nanopowders synthesized by the application of ultrasound with varying input power

Sulphur doped ZnO nanopowders have been prepared by sonochemical method in continuous (CS) and pulsed (PS) modes. Precipitation time was found to vary with input power of applied ultrasound. X-ray diffraction studies show the formation hexagonal wurtzite phase of nano ZnO but variation in size, strain and lattice parameters was observed in the samples synthesized through CS and PS modes with different input power. Surface morphology of the samples changed significantly with input power of the pulse as observed from scanning electron microscopic results. Well defined and c-axis oriented nanorods and multipodes were observed with power setting of 187.5 W whereas in all other cases no definite morphology was observed. UV absorption studies show higher value of absorbance for ZnO:S. Further, the absorption band becomes sharp for the samples prepared with power setting of 225 W. PL spectra show several bands due to different transitions in the region of 400–780 nm which have been explained in the light of excitonic and defect induced transitions in ZnO.     

Green synthesis of nanocomposites consisting of silver and protease alpha chymotrypsin

The synergy of ultrasonication and the exposure to light radiation was found to be necessary in the formation of nanocomposites of silver and a protease alpha chymotrypsin. The reaction was carried out in aqueous medium and the process took just less than 35 min. Ultrasonication alone formed very negligible number of nanoparticles of <100 nm size whereas light alone produced enough number but the size of the particles was >100 nm.The effects of pH (in the range of 3–5, 9–10), ultrasonication time periods (0–30 min), ultrasonication intensity (33–83 W cm^?2), energy of light radiation (short UV, long UV and Fluorescent light) and time period of exposure (5–60 min) to different light radiations were studied.The formation of nanocomposites under these effects was followed by surface plasmon resonance (SPR) spectra, dynamic light scattering (DLS), transmission electron microscopy (TEM). Ag–chymotrypsin nanocomposites of sizes ranging from 13 to 72 nm were formed using the synergy of ultrasonication and exposure to short UV radiation. Results show that ultrasonication promoted nuclei formation, growth and reduction of polydispersity by Ostwald ripening.     

Core hole and surface excitation correction parameter for XPS peak intensities

In XPS analysis, surface excitations and excitations originating from the static core hole created during the photoexcitation process are usually neglected. However, both effects significantly reduce the measured peak intensity. In this paper we have calculated these effects. Instead of considering the two effects separately, we introduce a new parameter, namely the Correction Parameter for XPS (or CP_XPS) defined as the change in probability for emission of a photoelectron caused by the presence of the surface and the core hole in comparison with the situation where the core hole is neglected and the electron travels the same distance in an infinite medium. The CP_XPS calculations are performed within the dielectric response theory by means of the QUEELS-XPS software determining the energy-differential inelastic electron scattering cross-sections for X-ray photoelectron spectroscopy (XPS) including surface and core hole effects. This study has been carried out for electron energies between 300 eV and 3400 eV, for angles to the surface normal between 0° and 60° and for various materials, especially metals, semiconductors and oxides. For geometries and energies normally used in XPS, i.e. for emission angle ≤ 60° and photoelectron energy ≤ 1500 eV, we find that CP_XPS values are significantly larger for oxides, (0.55 ? CP_XPS ? 0.75) than for metals and semiconductors (0.45 ? CP_XPS ? 0.6). We show that this behavior is due to the difference in the wave vector dispersion of the energy loss function. This dispersion has been determined from analysis of REELS and is found to be free electron like (α ? 1) for metals but is substantially smaller (α ≈ 0.02–0.05) for materials with a wide band gap. As a result, the group velocity of the valence electrons is very small for oxides with a large band gap. This leads to a reduction in the screening of the core-hole potential before the photoelectron has left the region of interaction and thereby to an increase in the intrinsic excitations caused by the core hole.     

Surface and core hole effects in X-ray photoelectron spectroscopy

In XPS analysis, two effects, which significantly reduce the measured peak intensity, are usually neglected: the core hole left behind in an XPS process which causes “intrinsic” excitations and excitations as the photoelectron pass through the surface region. We have calculated these effects quantitatively for various energies, geometries, and materials. Instead of considering the two effects separately, we introduce a new parameter, namely the correction parameter for XPS or CP_XPS, which takes into account both effects. We define this CP_XPS as the change in probability for emission of a photoelectron caused by the presence of the surface and the core hole in comparison with the situation where the core hole is neglected and the electron travels the same distance in an infinite medium. The calculations are performed within the dielectric response theory by means of the QUEELS–XPS software determining the energy-differential inelastic electron scattering cross-sections for X-ray photoelectron spectroscopy (XPS) including surface and core hole effects. This study has been carried out for electron energies between 300 eV and 3400 eV, for angles to the surface normal between 0° and 60° and for various materials. We find that the absolute effect is a reduction by 35–45% in peak intensities but that the variation in CP_XPS with material, angle and energy are < ± 10% for emission angle ≤ 60° and photoelectron energy ≤ 1500 eV. This implies that when XPS analysis is done using relative intensities, the combined effect of the surface and of the core hole is typically less than ≈ ± 10% for geometries and energies normally used in XPS. In practice, it is however difficult to determine the bare peak intensity without the intrinsic electrons because the two overlap in energy.     

Ga-induced restructuring of Si(5 5 12) − 2 × 1 reconstructed surface at room temperature

Motivated by the need to form 1D-nanostructured dopants on silicon surfaces, we have attempted to grow Ga on the high index Si(5 5 12) surface which has a highly trenched (1D) morphology. The evolution of the interface with Ga adsorption in the monolayer regime has been probed by in situ AES, LEED and EELS. Controlling the kinetics by changing the Ga flux rates shows an interesting difference in the 1.0 to 1.5 ML region. The low flux rate (0.03 ML/minute) results in a Frank van der Merwe (layer by layer) growth mode up to 2 ML, while the higher flux rate (0.1 ML/minute) shows a transient island formation after the completion of 1 ML. The low rate shows the formation of 2 × (3 3 7) and (2 2 5) superstructures, while only the 2 × (3 3 7) is observed in a wide coverage range for the higher rate. The results demonstrate the ability to kinetically control the surface phases with different electronic properties of this technologically important interface.