Surface Review and Letters

Published by World Scientific Publishing
Online ISSN: 0218-625X
Publications
Article
Aiming at the influence of substitution on ferroelectric behaviors, Nd and Mg/Mn cosubstituted bismuth titanate films, Bi3.15Nd0.85Ti2.97Mg0.03O12 (BNTMg) and Bi3.15Nd0.85Ti2.95Mn0.05O12 (BNTMn), have been prepared on Pt/Ti/SiO2/Si(100) substrates at 700°C by a chemical solution deposition technique, respectively. A series of comparisons including structures, surface morphologies, and ferroelectric properties of the as-prepared films were carried out by X-ray diffraction, scanning electron microscope, and a Radiant Technologies Precision Workstation for ferroelectric test system. It was found that the films possess preferred the (117) and (00l)-oriented polycrystalline structures, and the remnant polarization of BNTMg film is larger than that of BNTMn film and the coercive field of BNTMn film is lower than that of BNTMg film. The values of 2Pr and 2Ec of BNTMg and BNTMn are 28 μC/cm2 and 212 kV/cm, and 15 μC/cm2 and 175 kV/cm, respectively. The relevant physical mechanisms were pursued.
 
Article
Bi0.95La0.05FeO3(BLFOx=0.05) film was fabricated on indium tin oxide/glass substrate using a metal organic decomposition method. No secondary phase can be observed in the well crystallized and predominantly (110)-oriented BLFOx=0.05 film. Well-saturated P - E hysteresis loops with the remanent polarization of 60.7 μC/cm2 can be obtained in BLFOx=0.05 film due to the low leakage currents as well as the preferred orientation of the (110) peak. The content of oxygen vacancies in the film is considered to be the dominant factor affecting the asymmetry ferroelectric properties and leakage currents.
 
Article
Carbon nano-coated LiNi0.8Co0.15Al0.05O2/C (LNCAO/C) cathode-active materials were prepared by a sol–gel method and investigated as the cathode material for lithium ion batteries. Electrochemical properties including the galvanostatic charge–discharge ability and cyclic voltammogram behavior were measured. Cyclic voltammetry (2.7–4.8 V) showed that the carbon nano-coating improved the "formation" of the LNCAO electrode, which was related to the increased electronic conductivity between the primary particles. The carbon nano-coated LNCAO/C exhibited good electrochemical performance at high C-rate. Also, the thermal stability at a highly oxidized state of the carbon nano-coated LNCAO was remarkably enhanced. The carbon nano-coating layer can serve as a physical and/or (electro-)chemical protection shell for the underlying LNCAO, which is attributed to an increase of the grain connectivity (physical part) and also to the protection of metal oxide from chemical reactions (chemical part).
 
Article
The recovery behavior of 20% plastically deformed AlSi11.35Mg0.23 in various stages of isochronal annealing is investigated by positron lifetime (LT). Experimental results show that the positron mean lifetime is a function of annealing temperature. The lifetime of the positron annihilating in a perfect lattice is 187.3 ps. It is 229.8 ps in a 20% deformed one. There are two regions in the isochronal annealing, one of them is related to the point defect and the other to the dislocation. The activation enthalpy for the dislocation is calculated from the isothermal study in the dislocation region from 575–675 K by slow and fast cooling and it is 0.16 ± 0.02 and 0.53 ± 0.06 eV, respectively.
 
Article
The discovery of the single-walled carbon nanotubes (SWCNTs) with a diameter of 0.4 nm has attracted extensive attentions. In this paper we report our attempt with two methods to directly observe these SWCNTs by AFM. The first one is to deposit the SWCNTs extracted from the zeolite matrix to a flat surface for AFM observation. While one-dimensional features have been observed, the SWCNT was suspected not to adhere well to the substrate. To overcome the difficulties of weak adhesion, we attempt to expose only part of the SWCNT from the zeolite channel by cutting the zeolite crystal at an angle. This alternative method, in which the SWCNT contained zeolite crystal is polished and etched by HCl, however, did not result in a smooth enough surface and thus no one-dimensional features can be observed. The difficulties in sample preparation and possible improvements are discussed.
 
Article
The (100)-oriented Ca0.4Sr0.6Bi4Ti4O15(C0.4S0.6BTi) thin film was successfully prepared by a sol-gel method on Pt/Ti/SiO2/Si substrate. The orientation and formation of thin films under different annealing schedules were studied using XRD and SEM. XRD analysis indicated that (100)-oriented C0.4S0.6BTi thin film with degree of orientation of I(200)/I(119) = 1.60 was prepared by preannealing the film at 400°C for 3 min followed by rapid thermal annealing at 800°C for 5 min. SEM analysis further indicated that the (100)-oriented C0.4S0.6BTi thin film with a thickness of about 800 nm was mainly composed of equiaxed grains. The remanent polarization and coercive field of the film were 16.1 μC/cm2 and 85 kV/cm, respectively.
 
Article
Lead-free Bi0.5(Na0.4K0.6)0.5TiO3 films have been synthesized by a chemical solution deposition method and deposited on p-Si(111) substrate by spin coating. Powder of the precursor solution heated at 650°C was studied by infrared scattering spectroscopy. The structural characteristics and crystallization of the films were examined by X-ray diffraction. The surface morphology and quality were studied using atomic force microscopy. The films exhibit good insulating properties and resistance to breakdown. The clockwise hysteresis curve is referred to as polarization type switching and the memory window is about 1.5 V.
 
Article
The dielectric properties of Ba0.6Sr0.4TiO3 (BST) thin films deposited using SrRuO3 (SRO) materials as bottom electrode were compared with those of the films grown using La0.5Sr0.5CoO3 (LSCO) materials as bottom electrode. X-ray diffraction scanning revealed that the two kinds of films could be epitaxially grown in pure single-oriented perovskite phases and atomic force microscopy showed that the root mean square roughness of BST/SRO films were similar to BST/LSCO films. The dielectric properties of the BST/SRO and BST/LSCO thin films were measured at 10 kHz and 300 K with a parallel-plate capacitor configuration. Compared with BST/LSCO, the dielectric tunability for BST/SRO films slightly decreased, while the loss decreased synchronously. The figure of merit factor value increases from 25.67 for BST/LSCO films to 48.76 for BST/SRO films under an applied voltage of 6 V. The leakage current density of the thin films at a positive voltage of 2 V decreases from 2.41 × 10-7 A/cm2 for BST/LSCO to 8.41 × 10-8 A/cm2 for BST/SRO. This phenomenon is ascribed to the smaller strain induced in BST/SRO materials.
 
Article
Essential oil from Artemisia herba alba (Art) was hydrodistilled and tested as corrosion inhibitor of steel in 0.5 M H2SO4 using weight loss measurements and electrochemical polarization methods. Results gathered show that this natural oil reduced the corrosion rate by the cathodic action. Its inhibition efficiency attains the maximum (74%) at 1 g/L. The inhibition efficiency of Arm oil increases with the rise of temperature. The adsorption isotherm of natural product on the steel has been determined. A. herba alba essential oil was obtained by hydrodistillation and its chemical composition oil was investigated by capillary GC and GC/MS. The major components were chrysanthenone (30.6%) and camphor (24.4%).
 
Article
The (La0.7Ca0.3MnO3)1-x/(CuFe2O4)x (x = 0–0.15) composites were prepared by conventional solid state reaction method. We have investigated the structural, magnetic and electrical properties of (La0.7Ca0.3MnO3)1-x/(CuFe2O4)x composites using X-ray diffraction (XRD), scanning electron microscopy, field cooled DC magnetization and magnetoresistance (MR) measurements. The resistance measured as a function of temperature demonstrates that pure LCMO samples display metal to semiconductor transition. On the other hand, all the LCMO/CFO composites samples clearly present the electrical behavior of insulator/semiconductor. It indicates that resistivity of the samples increase systemically with the increase of CFO content. The MR was measured in the presence of 0.5 T field. At low temperatures, it is clearly observed that the MR effect is enhanced with x = 0.03 of CFO composition. In summary, the spin-polarized tunneling and the spin-dependent scattering may be helpful to the improved low-field magnetoresistance (LFMR) effect. These phenomena can be explained by the segregation of a new phase related to CFO at the grain boundaries or surfaces of the LCMO grains.
 
Article
We report the electronic structure of the interface of the ultrafilm Fe film with the O-terminated ZnO(000-1) polar surface studied by synchrotron radiation photoemission. It was found that Fe grows in a Stranski–Krastanov mode. The turning point of the growth mode was at the Fe coverage of ~2 Å. At low coverage, about 1 ML (monolayer) of iron (ca. 1.5 Å) was oxidized by the terminative oxygen atoms on the ZnO(000-1) surface. Analysis of the valence band and the Fe3p photoemission spectra indicates that below 2 Å coverage, only Fe2+ species could be detected at the Fe/ZnO interface, after that the formation of a metallic iron overlayer was observed. A sharp Fermi edge appeared at 5.1 Å coverage, indicating the formation of the bulk metallic Fe film. Furthermore, work function of the sample decreased to a minimum of 4.5 eV at 0.2 Å Fe, and then to 4.9 eV with the formation of a dipole layer at the interface.
 
Article
Diffusion of H atom in the Ti(0001) outer-layer and inter-layer surface is studied using density functional theory based on generalized gradient approximation (GGA). The energy barriers for the hydrogen atom diffusion in different interstitial sites at the same layers or between adjacent layers are calculated. It is found that the energy barriers of H atom diffusion in the adjacent interstitial layers are bigger than that in the same interstitial layers. For the diffusion of H atom between adjacent interstitial layers, the diffusion between tetrahedral sites is easier than that between octahedral sites. While for diffusion of H atom between the same interstitial layers, the diffusion between tetrahedral sites is easier than that between tetrahedral and octahedral sites. Moreover, it is found that the most possible inside diffusion from hcp site of a hydrogen atom in the Ti(0001) outer-layer goes through tetrahedral sites.
 
Article
The co-adsorption of carbon monoxide and benzene on Co(0001) has been studied using density functional calculations. We used the ordered $(\sqrt 7 \times \sqrt 7\/)\ R19^{\circ}$ surface unit cell. A comparison of the co-adsorption with CO and benzene two-dimensional networks is also given. The electronic structure reveals that the CO orbitals interact with benzene and Co layer. Regarding the bonding, the Co–Co overlap population decrease 18% after benzene adsorption and increase a little after CO adsorption with a net 14.6% decrease in the co-adsorption system. The CO–benzene interaction is shown by the changes in the C–O (CO) and C–H (benzene) bonds.
 
Article
The success of density functional theory for the description of the adsorption of atoms on surfaces is well established, and based on recent calculations using gradient corrections, it has been shown that it also describes well the dissociative adsorption of molecules at surfaces - admittedly however, the data base for reactions at surfaces is still somewhat small. In the present paper the power of density functional theory calculations is demonstrated by investigations for two different adsorption systems, namely, one with a strongly electropositive adsorbate [Na on Al(111)] and one with a strongly electronegative adsorbate [O on Ru(0001)]. In each case, new hitherto not expected adsorbate phases have been predicted by the theory: For Na on Al(111) the stability of a "four-layer" surface alloy was identified while for O on Ru(0001) it was predicted that the formation of a (1 x 1)-O adlayer should be possible which implies that the apparent saturation coverage of 1/2 is due to kinetic hindering.
 
Article
The adsorption and the growth of ZnO on α-Al2O3(0001) surface at various temperatures are theoretically calculated by using a plane wave pseudopotentials (USP) method based on density functional theory. The average adsorption energy of ZnO at 400, 600, and 800°C is 4.16±0.08, 4.25±0.11, and 4.05±0.23 eV, respectively. Temperature has a remarkable effect on the structure of the surface and the interface of ZnO/α-Al2O3(0001). It is found that the Zn-hexagonal symmetry deflexion does not appear during the adsorption growth of ZnO at 400°C, and that the ${\rm ZnO}[10\overline{1}0]$ is parallel with the $[10\overline{1}0]$ of the α-Al2O3(0001), which is favorable for forming the ZnO film with the Zn-terminated surface. It can be observed from our simulation that there are two surface structures in the adsorption of ZnO at 600°C: one is the ZnO surface that has the Zn-terminated structure, and whose $[10\overline{1}0]$ parallels with the substrate surface $[10\overline{1}0]$, and the other is the ZnO $[10\overline{1}0]$// sapphire $[11\overline{2}0]$ with the O-terminated surface. The energy barrier of the phase transition between the two different surface structures is about 1.6 eV, and the latter is more stable. Therefore, the ideal temperature for the thin film growth of ZnO on sapphire is about 600°C, and it facilitates the formation of wurtzite structure containing Zn-O–Zn-O–Zn-O double layers as a growth unit cell. At 600°C, the average bond length of ZnO is 0.190±0.01 nm, and the electron localization function value indicates that the bond of (substrate) –O–Zn–O has a distinct covalent character, whereas the (Zn)O–Al (substrate) shows a clear character of ionic bond. However, at the temperature of 800°C, the dissociation of Al and O atoms on the surface of the α-Al2O3(0001) leads to a disordered surface and interface structure. Thus, the Zn-hexagonal symmetry structure of the ZnO film is not observed under the condition.
 
Article
Adsorption of oxygen on α-Mo2C(0001) is investigated with Auger electron spectroscopy (AES), low-energy electron diffraction (LEED) and angle-resolved photoemission spectroscopy (ARPES) utilizing synchrotron radiation. It is found that C KLL Auger peak intensity does not change during O2 exposure, indicating that the depletion of C atoms does not proceed. It is deduced from ARPES and LEED results that adsorbed oxygen atoms from a well-ordered (1 × 1) lattice on the α-Mo2C(0001) surface. The ARPES study shows that oxygen adsorption induces a peculiar state around Fermi level (EF). Off-normal-emission measurements prove that the state is a half-filled metallic state.
 
Article
We have performed Density Functional Theory calculations in the Generalized Gradients Approximation for the (001) surface of the intermetallic compound TiFe. We have focused on the interplay between spin polarization and surface relaxations, and the influence of the chemical species at the surface. We found that the surface shows a magnetic ordering. The magnetic moment at the surface layer depends strongly on the surface termination, being much larger for the case of Fe than Ti termination (3.11 and 0.29 μB/atom, respectively). The magnetic moments show an alternating behavior with a slow decaying as we go inside the material. On the other hand, the modification of the atomic positions due to the surface relaxation results in a very small influence on the magnetic moment with respect to the ideal, nonrelaxed configuration.
 
Article
The adsorption of hydrogen atoms on the Ta(001)(1 × 1) surface is studied by first-principles density functional calculations within the generalized gradient approximation. It turns out that at 1 ML, coverage bridge site is energetically preferred over three-fold hollow site. This result does not support a recent LEED and HREELS observation that three-fold hollow site was preferred. The layer separation between the H atom and the topmost Ta atom is 1.12 Å and 0.55 Å for bridge and three-fold hollow site, respectively. Atomic distance between the H atom and the first layer Ta atom is 1.998 Å for bridge and 2.06 Å for three-fold site. The structural and energetic properties of H/W(100)(1 × 1) have been calculated. The results are shown to be in excellent agreement with available experimental and theoretical analysis. We find that the adsorption behavior of hydrogen on Ta(001)(1 × 1) surface is very similar to that of H/W(100) system.
 
Article
Adsorption and diffusion of a Ba adatom on a reconstructed Si(001) surface were studied using first principles density functional calculation. It was found that the lowest energetic configuration is that the Ba atom resides at the T3' (which is displaced along the $[\bar{1}10]$ direction from the T3 site by ~ 0.26 Å). An anisotropic diffusivity of Ba on the reconstructed surface was found. The preferred direction of diffusion was the trough between Si dimer rows.
 
Article
The role of artificial polarization in two different applications of the title color center, namely optical properties of FA2:Sr2+ center and adsorbate–substrate interactions, is investigated by using quantum mechanical CI-singles and DFT ab initio methods. Clusters of variable sizes were embedded in the simulated Coulomb field that closely approximate the Madelung field of the host surface, and ions that were the nearest neighbors to the defect site were allowed to relax to equilibrium. The sensitivity of the calculated transition energies (Stokes shifts) of FA2:Sr2+ center as well as related optical properties, such as optical–optical conversion efficiency, relaxed excited states of the defect-containing surface, orientational destruction of the point defect, recording sensitivity, the Glasner–Tompkins empirical rule, and the artificial polarization were examined. The dependence of NO and ON adsorption energies at RbCl (001) surface on artificial polarization were also examined, and the results were explained in terms of electrostatic potential curves. The coadsorption of NO molecules, and the charge transfer reactions between the NO molecules and the paramagnetic chlorine vacancy on RbCl surface were considered.
 
Article
Surface morphology of the {001} faces of MMTWD crystals grown from by the temperature-lowering method has been studied. Monolayer and multilayer steps elongate along the a direction, which is determined by the crystal structure. Apart from that, the elementary steps have narrower terraces than the bunched ones, which may be resulting from the faster growth rates of the former than the latter. The formation of the protuberances at the step fronts is primarily associated with the uneven growth rates. The hollow cavities also elongate along the a direction, which demonstrates that the formation of them is also restricted by the crystal structure. Cracks are supposed to occur during harvesting, handling, or temperature stress afterwards. Growth of the 3D hillocks in high density can probably cause large stress and induce structure mismatch and serious cracks at the later stage.
 
Article
The electrochemical behaviors of bare/dithiol-modified gold electrode were studied in the PBS solution with/without dithiols. A pair of current peaks between -1.4 and -0.5 V is related to dithiol adsorption or re-adsorption. An oxidative peak at 0.37 V accompanied by a slight peak at 0.03 V is observed between -0.5 and 0.7 V for the bare gold electrode in the thiol solution, which is due to the formation of bilayer. When the potential is scanned to a very positive value at 1.2 V, a new oxidative peak at 1.07 V appears for the bare gold electrode in the dithiol solution or the PBS solution. The assembled dithiols are desorbed, and gold surface is exposed. The study indicates that the 1,3-propanedithiol modified electrodes should be used during the potential range of -0.8 to 0.8 V to keep the assembled layer stable.
 
Article
Ti-48%Al-2%Cr-2%Nb-(0, 0.5, 1) at.%W alloys were synthesized via the powder metallurgical route, and cyclically oxidized at 800, 900, or 1000°C in air for up to 100 h in order to find the effects of W on their oxidation characteristics. At 800°C, they oxidized relatively slowly, and the scales were thin and adherent. At 900°C, the scales began to spall locally. At 1000°C, they spalled repetitively during oxidation. Cr, Nb, and W improved the cyclic oxidation resistance of TiAl alloys. The oxides formed were TiO2 and Al2O3, which contained the alloying elements of Cr, Nb, and W. Nitrides formed were TiN and Ti2AlN.
 
Article
Surface morphology of the {101} freshly cleaved planes of L-arginine trifluoroacetate crystals grown from aqueous solutions have been studied by atomic force microscopy. Various step patterns are described and discussed. Moreover, the dislocations are not easily found and all of them are devoid of hollow cores on the emergence points. The reason may be that the stress field is too weak to generate hollow cores.
 
Article
Single crystals of nonlinear optical material, L-arginine trifluoroacetate (abbreviated as LATF), were grown from an aqueous solution by the low temperature solution growth method. The cell parameters of the grown crystal were determined by the X-ray powder diffraction analysis. Atomic force microscopy (AFM) was used to investigate the surface morphology of {101} cleavage faces of LATF. Straight steps and two-dimensional nuclei were observed. Liquid inclusions and impurities as defects for revealing macrosteps were demonstrated. The molecular structure and crystal structure correlative with surface morphology were discussed.
 
Article
A systematic study of unrelaxed and relaxed surface characters on the TiO2 (101) surface has been carried out by first-principles calculations using plane-wave pseudopotential method. We find that O2c atoms have an inward relaxation of 0.012 Å and Ti5c atoms have an outward relaxation of 0.155 Å by taking a 24-layer slab with 5 Å vacuum width to consider the atomic relaxations, in good agreement with other theoretical values. The slab thickness has significant effect on the quality of band structure and density of states, and 24-layer slab is sufficient to present the electronic properties of TiO2 (101) surface. Atomic relaxations result in a large transfer of surface charges from outermost layer to inner layer, and the surface bonds have a rehybridization, which makes the ionization reduce and the covalence increase; we believe that it causes the surface bond shorten. A fine analysis of band structure and density of states of the TiO2 (101) surface shows that the surface relaxation induces the transformation from semi-metallic to semiconducting characteristic.
 
Favorable CO structures on Mo(110) at θ = 0.25 (a), 1.0 (b), 1.5 (c), and 2.0 (d). Carbon atoms are shown black, O red, and Mo blue. In the first layer, CO molecules are tilted and occupy nearly threefold hollow sites. CO molecules of the second layer are bound to CO of the first layer.
Model TPD spectra obtained by kinetic Monte Carlo simulations for various initial CO coverages. The spectra were simulated with ν = 2 × 10 9 s −1 and binding energies 2.1 and 0.7 eV for CO molecules in the first and second layers, respectively.
Article
The problem of the CO dissociation on Mo(110) has been addressed by means of temperature-programmed desorption (TPD) and density-functional (DFT) calculations. The TPD spectra show a first-order CO desorption, which indicates the desorption from a "virgin" state, not a recombinative form of desorption. The height of the potential barrier for the dissociation (2.75 eV), estimated from DFT calculations, substantially exceeds the energy of CO chemisorption (2.1 eV), which makes the thermally induced CO dissociation on Mo improbable. Monte Carlo simulations of TPD spectra, performed using estimated chemisorption energies, are in good agreement with experiment and demonstrate that the two-peak shape of the spectra can be explained without involving the CO dissociation. Thus, the results of the present study finally refute the concept of a dissociative form of CO adsorption on Mo surfaces.
 
Article
LaNiO3(LNO) thin films were prepared on Si(100) substrates by a chemical solution deposition method. The orientation of LNO films was controlled by changing the acetic acid amount, concentration of the precursor solutions and pre-annealing time. The highly (100)- and (110)-oriented LNO films were obtained by optimizing these processing conditions. The orientation factors of the optimized (100)- and (110)-oriented LNO films were more than 0.99 and 0.93. Their room-temperature resistivities were 5.1 × 10-4Ωcm and 5.4 × 10-4Ωcm, respectively. The growth mechanisms of the (110)- and (100)-oriented LNO films were discussed.
 
Possible dispositions of the (1×1) hydrogen monolayer on the Mo(110) surface. Hydrogen atoms can occupy quasi threefold adsorption sites (a), long-bridge sites (b), or short-bridge sites (c).
Lateral interaction between H adatoms in the threefold sites on the Mo(110) surface.
Potential energy of the interactions vs. distances between hydrogen atoms for three characteristic distances from the Mo(110) surface.
Monte Carlo results for hydrogen desorption from the Mo(110) surface.
Article
Adsorbed hydrogen layers on the Mo(110) surface have been investigated both experimentally by temperature programmed desorption (TPD) method and theoretically by means of DFT-based optimization of surface structures. We suggest a novel microscopic model of the associative hydrogen desorption, which explains essential features of the process. In this model, the process of hydrogen desorption can be described as association of hydrogen atoms on the surface, but molecular formation is actually accomplished while the molecule moves away from the surface. We also suggest a new algorithm for realistic Monte Carlo simulations of associative desorption, which implements the microscopic description of the association of hydrogen adatoms into a molecule with activation energy, found from the DFT calculations. Good agreement between simulated and experimental TPD spectra gives insight into different behavior of the spectra, obtained for low and high hydrogen coverages on the Mo(110) surface.
 
Article
SrBi4Ti4O15 (SBTi) thin films were prepared on (100)- and (110)-oriented LaNiO3(LNO) electrodes by a metalorgranic decomposition (MOD) technique at an annealing temperature of 650°C. c-axis-oriented SBTi thin film with volume fraction of 0.89 can be formed on a (100)-oriented LNO film due to the epitaxial relationship between c-axis-oriented SBTi and LNO(100). In contrast, SBTi film deposited on LNO(110) shows random orientations with strong (119) and (200) peaks. The remanent polarization (Pr) and coercive field (Ec) of the random oriented SBTi film were 18.1 μC/cm2 and 70 kV/cm, respectively. This suggests that (110)-oriented LNO electrode is a better choice for obtaining SBTi films with higher volume fraction of a(b)-axis-orientated grains.
 
Article
The electronic structure of S adsorption on goethite (110) surface has been studied by ASED-MO cluster calculations. For S location, the most exposed surface atoms of goethite surface were selected. The calculations show that the surface offers several places for S adsorption. The most energetically stable system corresponds to S location above H atom. We studied in detail the configurations that correspond to the higher OP values. For these configurations, the H-S and Fe-S computed distances are 2.1 and 3.7 Å, respectively. The H-S and Fe-S are mainly bonding interaction with OP values of 0.156 and 0.034, respectively. The Fe-S interaction mainly involves Fe 3d x2-y2 atomic orbitals with lesser participation of Fe 4p y and Fe 3d yz atomic orbitals. The O-S interaction shows the same bonding and antibonding contributions giving a small OP value. The O-S interaction involves O 2p orbitals. There is an electron transfer to the Fe atom from the S atom. On the other hand, there is an electron transfer to S atom from the H and O atoms, respectively.
 
Article
Periodic, self-consistent, density functional theory calculations have been performed to investigate Li adsorption on Mo(110) surface. It turns out that the long-bridge site is the most stable site and the Li–Mo surface alloy forms easily at high coverage with the substitution of Mo by Li atoms in the outermost layer. Work function analysis showed that the work function decreases dramatically as the coverage from 0 to 0.5 ML, and finally increases again at the coverage of 1 ML, which agrees well with the experimental finding of Kroger et al. (Surf. Sci. 449 (2000) 227–235). Vibrational properties, diffusion barrier of Li along the Mo(110) surface, and the energy of formation of the surface have also been investigated for Li adsorption at various coverages.
 
Phase diagram of surface enrichment of Pt 1−x Nix single crystalline surfaces. This is drawn by the results of the previous works 2-7 and references therein.
Article
The surface segregation, structure, and valence band density of states of Pt3Ni(100), (110), and (111) single crystals are characterized with low energy ion scattering (LEIS), low energy electron diffraction (LEED), and ultraviolet photoemission spectroscopy (UPS). The results of LEIS clearly reveal the complete surface segregation of Pt to the top layer on all crystal alloys. LEED indicates the (5 × 1) surface reconstruction on the Pt3Ni(100), while (110) and (111) surfaces show (2 × 1) and (1 × 1) patterns, respectively, identical to Pt single crystals. The valence bands density of states on Pt3Ni alloys are compared to those of Pt single crystals via UPS measurements.
 
Article
The surface concentrations and concentration depth profiles to the (110) surface of an Au75Pd25 alloy is studied by modified analytical embedded atom method (MAEAM) with the Monte Carlo simulations. The results indicate that Au enriched in the two topmost layers, but depleted in the third layer. The Au concentration in the non-reconstructed surface is less than that in the reconstructed surface. Au concentration in third layer of reconstructed surface, which is more agreement with experimental data in present simulations, is about 63% 61% and 55%, at 800K, 600K and 400K respectively. Thus the present simulations are helpful for a better understanding of surface segregation of AuPd alloys.
 
Article
Based on the Density Functional Theory (DFT) with noncollinear-magnetism formulations, we have calculated the magnetism of single 3d transition-metal atoms and the magnetic anisotropies of supported Ni chains on the Au(110)-(1 × 2) surface. Our results for single absorbed 3d transition-metal atoms show that the surface relaxations enhance the orbital moments of left-end elements (Ti, V) and quenches the orbital moments of right-end elements (Fe, Co, Ni) on the Au(110)-(1 × 2) surface. The magnetic anisotropies of Ni atomic chains on the surface are closely related to orbital quenching. The easy magnetized axes change from the direction parallel to the chains to the direction perpendicular to the Ni chains when they absorb on the surface.
 
Article
The adsorption of Ni atom on the Mo(110) surface has been studied within the density functional theory framework. It turned out that Ni–Mo surface alloy was formed with Ni atoms substituting Mo atom in the outermost layer. The subsurface site adsorption was found to be not preferred. Geometric analysis showed that the rumpling between substitutional Ni and Mo in the first alloy layer was about 0.108 Å at medium and low coverage (Θ). In addition, the diffusion of Ni on bare and Ni-substitutional Mo(110) surface has been investigated. It was shown that the diffusion energy barrier was reduced as the increase of coverage on bare Mo(110) surface, which supports the switch of growth mode layer-by-layer to Stranski–Krastanov as the function of coverage. Substitutional Ni atom only slightly increases the energy barrier for Ni diffusion on Mo(110) surface.
 
Article
This article reports the use of plasma-assisted molecular beam epitaxy (MBE) to grow AlN on Si(111) substrate at 850°C under UHV conditions for 15, 30, and 45 min. The films were characterized by high-resolution X-ray diffraction (HR-XRD) and micro-Raman spectroscopy. XRD measurement revealed that the AlN was epitaxially grown on Si(111). Micro-Raman result showed that all the allowed Raman modes of AlN and Si were clearly visible. Fourier transform infrared (FTIR) spectroscopy has been used to investigate the A1 (LO) and E1 (TO) modes with frequencies (890–899) cm-1 and (668–688) cm-1, respectively. The results are in good agreement with reported phonon frequencies of AlN grown on Si(111).
 
Article
The initial stage of Au adsorption on the Si(111)-7 × 7 surface has been investigated by the first-principles calculations. Different configurations have been considered and the high coordination sites were determined as the most possible places for the Au adsorption. The diffusion barriers for Au atoms on the Si surface have also been evaluated. The results indicated that the Au atoms could diffuse from the top of the Si rest atoms to the high coordination sites only by overcoming a small barrier of 0.11 eV.
 
Article
We studied adsorption and reaction of nitric oxide on Si(111)7 × 7, 5 × 2-Au, $\alpha-\sqrt{3},\times \sqrt{3}-{\rm Au}$, $\beta-\sqrt{3} \times \sqrt{3}-{\rm Au}$, and 6 × 6-Au surfaces with low-energy electron diffraction, Auger electron spectroscopy, and scanning tunneling microscopy (STM). We found that NO gas reacts most strongly with the 7 × 7 surface, strongly with the 5 × 2-Au surface, and little with the other Si(111)-Au surfaces. STM results indicated that the NO exposure removes adatoms and erodes the row structure on the 5 × 2-Au surface at room temperature.
 
Article
High-quality aluminum nitride (AlN) layers with full width at half maximum (FWHM) values of 11 arcmin were grown by plasma-assisted molecular-beam epitaxy on Si (111) substrates. AlN nucleation layers are being investigated for the growth of GaN on Si. Growth using AlN buffer layers leads to Al-polar films, with surfaces strongly dependent on the flux conditions used. Flat surfaces can be obtained by growing as Al-rich as possible, although Al droplets tend to form. Before starting the AlN growth, a few monolayers of Al are deposited on the substrate to avoid the formation of Si3N4. X-ray diffraction (XRD) techniques were employed to determine the surface and structural quality of the layers. XRD revealed that monocrystalline AlN was obtained. Best AlN films were obtained at high substrate temperatures (875°C) and III/V ratios close to stoichiometry.
 
EEL spectra for the Pt 80 Co 20 (111) alloy surface at heating T = 973 K.
The dielectric constant ε depending on energy E 0 for the Pt 80 Co 20 (111) alloy surface at different heating.
Article
Electron energy loss spectroscopy has been used for the investigation of the surface and bulk plasmon excitations depending on the heating in the ultra-thin layers of ordering Pt80Co20(111) alloy from the primary electron beam energies E0 ranging from 200 to 650 eV. Thermo-induced shift of plasmon energy and damping of intensity line of the surface plasmon relative to the bulk plasmon were observed. With an increase in alloy heating, the energy of surface and bulk plasmons is shifted with lowering energy in the whole range E0 and the higher the temperature the higher the shifts of plasmon energy. The physical processes that can influence on the energy shift of plasmon oscillations in the energy loss spectra at heating are considered. The relationship between the damping of oscillating concentration depth profile and the surface plasmon damping at heating was established.
 
Article
Segregation of boron on ${\rm Si}(111)(\sqrt{3} \times \sqrt{3}){\rm R}30^{\circ}$ surface has been studied using the periodical calculations within the local density approximation. The obtained segregation energy (enthalpy) of about -1.9 eV is close to the published data of experimental studies and previous cluster semiempirical calculations. The influence of plane-wave basis set cutoff energy and the slab unit cell depth on the value of segregation energy has been investigated.
 
Article
Surface and bulk plasmon excitations from the ordering Pt80Co20(111) alloy surface are studied by means electron energy loss spectroscopy in the low energy range of the primary electron energy E0. Deviation of the plasmon excitations from the theoretical value was found for Pt, Co metals, and Pt80Co20(111) alloy as calculated in the free-electron gas model. For the ordered alloy, the bulk plasmon energy is 2–3 eV more than for the disordered alloy, whereas the difference for surface plasmon energy is 4–7 eV in the range E0 = 150–800 eV. The ration of intensity lines of plasmons η from E0 was investigated for the (dis)ordered state of the Pt80Co20(111) alloy surface. For the ordered alloy, η has prolonged dependence from energy E0 in comparison with the disordered alloy. The relationship between layer-by-layer surface concentration and surface plasmon damping was observed.
 
Article
The interaction of hydrogen with a platinum (111) cluster using the atom superposition and electron delocalization–higher binding ASED-TB quantum calculation method was studied. The metal surface was represented by a Pt cluster of seven layers. The effect of hydrogen on this metal substrate was studied by the analysis of density of states and crystal orbital overlap populations curves. The energy surface plots allow us to find a possible diffusion path through the cluster from one side to the other. The Pt–Pt metal bond is weakened during H adsorption and diffusion. The main components in the Pt–H bond are the Pt 6s (31%), 6p (26%), and 5 dxz (16%) orbitals.
 
Article
Atomistic simulations of low coverage Sn growth on Au(111) using the Bozzolo–Ferrante–Smith (BFS) method for alloys are presented. Simulated annealing and atom-by-atom analysis of the energetics explains the close competition between the experimentally observed surface alloying and layer-by-layer growth.
 
Article
The addition of hydrogen to the carbon–carbon double bond of 2-butenes adsorbed on Pd(111) was studied within the density functional theory (DFT) and using a periodic slab model. For that purpose, the Horiuti–Polanyi mechanisms for both complete hydrogenation and isomerization were considered. The hydrogenation of cis and trans-2-butene to produce butane proceeds via the formation of eclipsed and staggered-2-butyl intermediates, respectively. In both cases, a relatively high energy barrier to produce the half-hydrogenated intermediate makes the first hydrogen addition the slowest step of the reaction. The competitive production of trans-2-butene from cis-2-butene requires the conversion from the eclipsed-2-butyl to the staggered-2-butyl isomer. As the corresponding energy barrier is relatively small and because the first of these isomers is less stable than the second, an easy conversion is predicted.
 
Article
We report a scanning tunneling microscopy (STM) study of potassium (K)-doped C60 (KxC60) on Cu(111) and Ag(111), with focus on the observation of novel K-induced superstructures. On Cu(111), K-intercalation leads to various superstructures, i.e., p(3 × 3), p(2 × 2), and a disordered "maze-like" structure. Concomitant removal of C60-induced substrate reconstruction was also observed. On Ag(111), K-doping induces C60 reorientation and results in a p(2 × 2) superstructure. All observed superstructures suggest intrinsic inhomogeneity of K-doping in fullerene materials, which must be further understood.
 
Article
The electronic properties of a Ca bilayer deposited on Cu(111) have been investigated by high-resolution electron energy loss spectroscopy. For the as-deposited Ca bilayer, the surface plasmon was recorded at 3.3eV. Temperature induces a surface metal-insulator transition (evidenced by the shift of the surface plasmon energy), leading to the formation of CaO thin layers which are highly reactive towards water molecules. The presence of occupied d band in the calcium overlayer modifies surface plasmon frequency and strongly influences its dispersion relation. Results are well explained within the framework of the s-d polarization theory.
 
Article
The electrochemical formation and reduction of O-layers on gold (111) films in 1 m sulfuric acid under different potentiodynamic routines are investigated utilizing in situ scanning tunneling microscopy. The surface dynamics is interpreted considering the anodic and cathodic reaction pathways recently proposed complemented with concurrent relaxation phenomena occurring after gold (111) lattice mild disruption (one gold atom deep) and moderate disruption (several atoms deep). The dynamics of both oxidized and reduced gold topographies depends on the potentiodynamic routine utilized to form OH/O surface species. The topography resulting from a mild oxidative disruption is dominated by quasi-2D holes and hillocks of the order of 5 nm, involving about 500–600 gold atoms each, and their coalescence. A cooperative turnover process at the O-layer, in which the anion ad-layer and interfacial water play a key role, determines the oxidized surface topography. The reduction of these O-layers results in gold clusters, their features depending on the applied potential routine. A moderate oxidative disruption produces a surface topography of hillocks and holes several gold atoms high and deep, respectively. The subsequent reduction leads to a spinodal gold pattern. Concurrent coalescence appears to be the result of an Ostwald ripening that involves the surface diffusion of both gold atoms and clusters. These processes produce an increase in surface roughness and an incipient gold faceting. The dynamics of different topographies can be qualitatively explained employing the arguments from colloidal science theory. For 1.1 V ≤ E ≅ Epzc weak electrostatic repulsions favor gold atom/cluster coalescence, whereas for E < Epzc the attenuated electrostatic repulsions among gold surfaces stabilize small clusters over the substrate producing string-like patterns.
 
Optimized Si c(2 × 2) structure on the Mo(112) surface. The Si–Si distances are in A. The threefold and bridge-on-row sites, favorable for oxygen adsorption, 16 
Favorable SiO 2 structures on the Mo(112) surface. (a) Oxygen atoms occupy threefold sites favored for oxygen adsorption; Si-O bond length is 1.80Å80Å. (b) Adsorbed linear SiO 2 molecules; oxygen atoms occupy bridge-on-row sites; Strong Si-O interaction results in zigzagging oxygen chains along the Mo(112) surface rows; Si-O bond length in this structure is 2.14Å14Å. The SiO 2 structure with oxygen atoms in the threefold sites (a) is 0.19 eV more favorable than the structure (b).
(a) The c(2 × 2) [SiO 3 ] optimized structure and (b) the c(2 × 2) [SiO 4 ] structure, obtained subsequently by adding the oxygen atoms to the SiO 2 film.
(a) The most favorable c(2 × 2) [SiO 4 ] structure on the Mo(112) surface. (b) The fragment of this structure, used for phonon frequency evaluation. (c) The model [SiO 4 ] structure on the Mo(112) suggested by Chen, Santra and Goodman. 12
Article
Favored structures of SiOn monolayers on Mo(112) surface have been studied using the total energy minimization technique based on DFT semirelativistic approach. In the [SiO4] complexes, which form the c(2 × 2) silica structure on the Mo(112), the bonding of the Si atoms with the surface is accomplished through the oxygen atoms. The structure with a symmetric position of oxygen atoms has been found to be the most favorable. In this structure, two oxygen atoms occupy bridge-on-row sites on the Mo(112) surface, with Si atoms between them, while oxygen atoms in the troughs appear not in expected threefold sites, but adjust their positions along the middle lines of the troughs. Estimated main phonon frequency and density of states for the symmetric [SiO4] structure agree well with experimental results.
 
Top-cited authors
Ernst Bauer
  • Arizona State University
Thomas Schmidt
  • Fritz Haber Institute of the Max Planck Society
Gerhard Lilienkamp
  • Technische Universität Clausthal
Stefan Heun
  • Italian National Research Council
Kevin Charles Prince
  • Sincrotrone Trieste S.C.p.A.