The work functions of various DySi(2-x) nanostructures epitaxially grown on a Si(001) surface were correlated with the structure using high-resolution Kelvin probe force microscopy and scanning tunneling microscopy in ultrahigh vacuum. Dy adatoms induce a surface dipole on Si(001) that increases the surface potential from 0.26 to 0.42 eV with respect to 2 x 1 reconstructed Si(001). DySi(2-x) nanowires showed a 0.2-0.23 eV lower work function than DySi(2-x) nanoislands, which can be attributed to confinement of electrons along the surface normal that induces a surface dipole when the film thickness approaches the Fermi wavelength. The ability to tune the work function of metal nanostructures should be useful for understanding how electronic structure affects catalytic activity.

Driving forces are investigated for assembling low dimensional, metallic, erbium and dysprosium disilicide nanowires on Si(001), using both scanning probe microscopy and density functional theory. Side-by-side comparison between emulated and measured scanning tunneling microscopy images allows establishment of reliable atomic models for complex adatom surface reconstructions of Er/Si(001) and Dy/Si(001) that are precursors to high aspect ratio disilicide nanowires. Peculiar surface reconstructions and relaxation of Si bonds are identified as the key factors for nucleation of these disilicide nanowires in parallel arrays on vicinal Si(001). Stable nanowire widths and heights are calculated with predicted atomic models that are consistent with experimental observations. A clear understanding of the nanowire-substrate interface is determined by correlating adatom reconstruction patterns with nanowire formation that is imperative to the development of unique procedures for massive fabrication of monodisperse nanosystems. © 2010 Elsevier B.V. All rights reserved.

Stability and electronic properties of pristine and metal covered cylindrical yttrium disilicide, YSi2, nanowires were investigated through first principles calculations. The YSi2 nanowire prefers Y-rich surface morphology and is attractive toward metal adsorbates such as silver, gold, palladium, and platinum. Strong charge polarization is found from adsorbate to wires, which reduces the work function and alters the chemical activity of the core-shell structures. © 2008 Elsevier B.V. All rights reserved.

The adsorption patterns of rare earth atoms on Si(001) were investigated using scanning tunneling microscopy measurements and density functional calculations. Stable configurations were systematically determined via calculation of binding energies of various adatom coverage and adsorption geometry. Competition between inter-adatom hybridization and Coulomb repulsion is the mechanism contributing to binding energy minima associated with commonly observed rare earth adsorption geometries. Comparison of stable configurations with experimental scanning tunneling microscopy images demonstrated accuracy of the theoretical models. This paves a way for the understanding of self-assembly of rare earth disilicide nanowires on vicinal Si(001) substrates. © 2009 Elsevier B.V. All rights reserved.

We performed systematic experimental and computational studies to investigate the adsorption geometries of Y atoms on the Si(001) surface. This paves a way for understanding and eventually controlling the growth of rare earth disilicide wires on the Si(001) substrate that are promising for various applications. For a single Y atom, the interrowdn site was found to be at least 400 meV lower in energy than other possible binding sites. The emulated STM images are in good agreement with experimental results of Er on Si(001). The strong bias and coverage dependence indicates the need for theoretical guidance for the correct interpretation of experimental data. We elucidate the Y-Si binding mechanism and provide insights toward the onset of formation of hexagonal rare earth disilicide wires.

Synergistic experimental and theoretical studies of low coverage adsorption geometries of rare earth adatoms on Si(0 0 1) were performed. Density functional calculations showed charge transfer from adatoms to Si(0 0 1) and explained observed bias dependence in scanning tunneling microscopy images. Comparison of STM simulations with empty states STM data revealed a direct correlation between coverage and surface reconstructions; the (4 × 8) reconstruction is a low coverage precursor to (2 × 4) reconstruction. Charge transfer from adatom to substrate was also confirmed by Kelvin probe force microscopy; contact potential difference measurements of Dy/Si(0 0 1) reveal a 0.28 eV higher surface potential than that of (2 × 1) reconstructed Si. © 2008 Elsevier B.V. All rights reserved.

Clear understanding of the relationship between electronic structure and chemical activity will aid in the rational design of nanocatalysts. Core-shell Au-coated dysprosium and yttrium disilicide nanowires provide a model atomic scale system to understand how charges that transfer across interfaces affect other electronic properties and in turn surface activities toward adsorbates. Scanning tunneling microscopy data demonstrate self-organized growth of Au-coated DySi₂ nanowires with a nanometer feature size on Si(001), and Kelvin probe force microscopy data measure a reduction of work function that is explained in terms of charge transfer. Density functional theory calculations predict the preferential adsorption site and segregation path of Au adatoms on Si(001) and YSi₂. The chemical properties of Au-YSi₂ nanowires are then discussed in light of charge density, density of states, and adsorption energy of CO molecules.

The importance of metal oxide photoanodes in solar fuels technology has garnered concerted efforts in photoanode discovery in recent decades, which complement parallel efforts in development of analytical techniques and optimization strategies using standard photoanodes such as TiO2, Fe2O3 and BiVO4. Theoretical guidance of highthroughput experiments has been particularly effective in dramatically increasing the portfolio of metal oxide photoanodes, motivating a new era of photoanode development where the characterization and optimization techniques developed on traditional materials are applied to nascent photoanodes that exhibit visible light photoresponse. The compendium of metal oxide photoanodes presented in the present work can also serve as the basis for further technique development, with a primary goal to establish workflows for discovery of materials that perform better against the critical criteria of operational stability, visible light photoresponse, and photovoltage suitable for tandem absorber architectures.

An efficient photoanode is a prerequisite for a viable solar fuels technology. The challenges to realizing an efficient photoanode include the integration of a semiconductor light absorber and a metal oxide electrocatalyst to optimize corrosion protection, light trapping, hole transport, and photocarrier recombination sites. To efficiently explore metal oxide coatings, we employ a high-throughput methodology wherein a uniform BiVO4 film is coated with 858 unique metal oxide coatings covering a range of metal oxide loadings and the full (Ni-Fe-Co-Ce)Ox pseudoquaternary composition space. Photoelectrochemical characterization of the photoanodes reveals that specific combinations of metal oxide composition and loading provide up to a 13-fold increase in the maximum photoelectrochemical power generation for oxygen evolution in pH 13 electrolyte. Through mining of the high-throughput data we identify composition regions that form improved interfaces with BiVO4. Of particular note, integrated photoanodes with catalyst compositions in the range Fe(0.4-0.6)Ce(0.6-0.4)Ox exhibit high interface quality and excellent photoelectrochemical power conversion. Scaled-up inkjet-printed electrodes and photoanodic electrodeposition of this composition on BiVO4 confirms the discovery and the synthesis-independent interface improvement of (Fe-Ce)Ox coatings on BiVO4.

Deployment of solar fuels technology requires photoanodes with long term stability, which can be accomplished using light absorbers that self-passivate under operational conditions. Several copper vanadates have been recently reported as promising photoanode materials, and their stability and self-passivation is demonstrated through a combination of Pourbaix calculations and combinatorial experimentation.