Embedded cluster model studies of impurities at metal surfaces

A knowledge of the electronic properties of impurities at metal surfaces is of great value in the understanding of such important phenomena as chemisorption and surface segregation in alloys. We have adopted here a unified approach based on an Embedded Cluster model to study the properties of surface impurities. We have mainly concentrated on hydrogen impurities either adsorbed above the surface or incorporated into the bulk of metals. We have also considered the case of substitutional metal impurities at the surface of host metals.For hydrogen chemisorption we have considered such substrates as free-electron, transition and noble metals as well as bimetallic substrates composed of a single metal impurity in a host matrix or a metallic overlayer on a metal support. The electronic structure of the chemisorbed system is compared to photoemission data when available, from which interpretation of the details of the experimental spectra may be made. It is found that hydrogen adsorption on transition and noble metals results in the formation of a pair of bonding/antibonding resonances on either side of the metal d-band, while for hydrogen on free-electron metals a single hydrogen induced resonance is observed. One-electron energy differences between the H on jellium and H on metal systems are estimated and trends in such energies across the 3d and 4d transition series are compared to the trends in experimental chemisorption energies for H on these metals. The change in hydrogen chemisorption capacity of an inert substrate due to the introduction of chemically active impurities is investigated. The different properties of Pd overlayers with respect to Pd surfaces are also investigated. Interaction energies between adatoms on surfaces are estimated in order to predict the geometry of ordered structures on surfaces.One-electron heats of segregation for binary alloys are calculated. These show a strong solute surface segregation for noble metal impurities in group VIII metals, which is due to the higher d-band occupancy of the noble metal.     

Surface reactions of TiCl4 and Al(CH3)3 on GaAs(100) during the first half-cycle of atomic layer deposition

GaAs(100) was exposed to pulses of trimethylaluminum (TMA, Al(CH₃)₃) and titanium tetrachloride (TiCl₄) to mimic the first half-cycle of atomic layer deposition (ALD). Both precursors removed the 9.0 ± 1.6 Å-thick mixed oxide consisting primarily of As₂O₃ with a small Ga₂O component that was left on the surface after aqueous HF treatment and vacuum annealing. In its place, TMA deposited an Al₂O₃ layer, but TiCl₄ exposure left Cl atoms adsorbed to an elemental As layer. This suggests that oxygen was removed by the formation of a volatile oxychloride species. A small TiO₂ coverage of approximately 0.04 monolayer remained on the surface for deposition temperatures of 89 °C to 135 °C, but no TiO₂ was present from 170 °C to 230 °C. The adsorbed Cl layer chemically passivated the surface at these temperatures and blocked TiO₂ deposition even after 50 full ALD cycles of TiCl₄ and water vapor. The Cl and As layers desorbed simultaneously at higher temperature producing peaks in the temperature programmed desorption spectrum in the range 237–297 °C. This allowed TiO₂ deposition at 300 °C in single TiCl₄ pulse experiments. On the native oxide-covered surface where there was a higher proportional Ga oxide composition, TiCl₄ exposure deposited TiO₂.     

High-pressure phase equilibria for chlorosilane + carbon dioxide mixtures

Fluid-phase equilibria, including dew points, bubble points, and critical points were measured for four binary systems composed of a chlorosilane and carbon dioxide. The measurements were carried out in a constant-composition, variable-volume cell equipped with a sapphire window, which allowed visual observation of the phases in the cell. A syringe pump was used to inject the CO₂ into the cell and to control its pressure. Methyltrichlorosilane, dimethyldichlorosilane, trimethylchlorosilane, and diethyldichlorosilane up to about 0.14 mol fraction were studied in this apparatus and a total of 243 phase-boundary points were obtained. Displacements in the critical point with respect to pure CO₂ of up to 11.81 MPa and 348.05 K were observed. Modeling of the fluid-phase equilibria for three of the four binary systems was done using the Peng–Robinson equation of state, standard van der Waals mixing rules with two binary interaction parameters, and a φ–φ formulation of the equilibrium. The binary interaction parameters were obtained by fitting the model to the experimental data. The model produced excellent agreement between computed and experimental data. Graphical representations of the modeling results are presented and compared to experimental results. The results indicate that the largest chlorosilane (diethyldichlorosilane) produced the largest shift in critical pressure and critical temperature with respect to pure CO₂.     

Ligand-Controlled Growth of ZnSe Quantum Dots in Water during Ostwald Ripening

A strong ligand effect was observed for the aqueous-phase growth of ZnSe quantum dots (QDs) in the Ostwald ripening (OR) stage. The QDs were made by injecting Se monomer at room temperature followed by a ramp to 100 °C. The ramp produced a second, more gradual increase in the concentrations of both Zn and Se monomers fed by the dissolution of QDs below the critical size. The dissolution process was followed using measurements of the mass of Zn in QDs and in the supernatant by inductively coupled plasma optical emission spectroscopy (ICP-OES). Despite the flux of monomers, there was little growth in the QDs of average size based on UV-vis absorption spectra, until the temperature reached 100 °C, when there was a period of rapid growth followed by a period of linear growth. The linear growth stage is the result of OR as the total mass of Zn in QDs and in the solvent remained constant. The growth data were fit to a continuum model for the limiting case of surface reaction control. The rate is proportional to the equilibrium coefficient for ligand detachment from the QD surface. The ligand 3-mercaptopropionic acid (MPA) was the most tightly bound to the surface and produced the lowest growth rate of (1.5-2) × 10(-3) nm/min in the OR stage, whereas thiolactic acid (TLA) was the most labile and produced the highest growth rate of 3 × 10(-3) nm/min. Methyl thioglycolate (MTG) and thioglycolic acid (TGA) produced rates in between these values. Ligands containing electron-withdrawing groups closer to the S atom and branching promote growth, whereas longer, possibly bidendate, ligands retard it. Mixed ligand experiments confirmed that growth is determined by ligand bonding strength to the QD. Photoluminescence spectroscopy showed that the more labile the ligand, the more facile the repair of surface defects during the exposure of the QDs to room light.     

Hydrogen chemisorption on jellium: A simple interpretation

Existing first principles calculations for H chemisorbed on jellium are interpreted by a simple model which reproduces the main one-body features of the results. The relationship with a picture of the H 1s orbital coupling to substrate wavefunctions is thus brought out.     

Valence electronic structure of alkalis adsorbed on free-electron like and transition metals

Application of a simple wave function-matching technique is made to the problem of alkali adsorption on free-electron like substrates, giving in particular an estimate of the width of the resonance at a given adatom-substrate separation, which is then compared to existing first-principles calculations. The model is then applied to the alkali/transition-metal problem, thus assuming the non-importance of the transition-metal d-bands relative to their sp-bands. The results indicate a fair amount of hybridization between the ns and np valence levels of the alkali adatom.     

Trends in hydrogen chemisorption on transition metals

A systematic study of H adsorption on the close-packed surfaces of the transition metals in the 3d and 4d series is presented. The effect of the TM d band on the chemisorption bond is investigated, by embedding a cluster of TM muffin tins at the surface of an effective jellium-like medium. It is found that the broad and incomplete H/jellium resonance is narrowed, shifted down and made to contain more electrons as a result of hybridization with the TM d states. These effects are larger in the case of the 4d metals, thus indicating a greater participation in the chemisorption bond of the d electrons for these metals than for the 3d metals. Calculation of one-electron energy differences on going from the H on jellium system to that of H on the TM cluster are presented. Trends for the one-electron energy differences are compared to trends in experimental chemisorption energy. H adsorption in the three-fold hollow site with no secondlayer TM atom below the H site is favoured for the hcp metals, while no discernible preference between the two hollow sites is recorded for the fcc metals, with the exception of Rh where the site with no second-layer TM atom below is preferred.     

A new route to self-assembled tin dioxide nanospheres: fabrication and characterization

Nearly monodispersed self-assembled tin dioxide (SnO2) nanospheres with intense photoluminescence (PL) were synthesized using a new wet chemistry technique. Instead of coprecipitating stannous salts, bulk tin (Sn) metal was oxidized at room temperature in a solution of hydrogen peroxide and deionized water containing polyvinylpyrrolidone (PVP) and ethylenediamine (EDA). SnO2 nanocrystals were produced with diameters of approximately 3.8 nm that spontaneously self-assembled into uniform SnO2 nanospheres with diameters of approximately 30 nm. Analysis was performed by X-ray diffraction, scanning electron microscopy, energy-dispersive X-ray spectroscopy, high-resolution transmission electron microscopy, selected area electron diffraction, X-ray photoelectron spectroscopy, Fourier transform infrared spectroscopy, UV-vis absorption spectroscopy, PL spectroscopy, and fluorescence lifetime measurements. The SnO2 nanospheres displayed room-temperature purple luminescence with an intense band at 394 nm (approximately 3.15 eV) and a high quantum yield of approximately 15%, likely as a result of emission from the surface states of SnO2/PVP complexes. The present study could open a new avenue to large-scale synthesis of self-assembled functional oxide nanostructures with technological applications as purple emitters, biological labels, gas sensors, lithium batteries, and dye-sensitized solar cells.