Measurement of the Band Bending and Surface Dipole at Chemically Functionalized Si(111)/Vacuum Interfaces

The core-level energy shifts observed using X-ray photoelectron spectroscopy (XPS) have been used to determine the band bending at Si(111) surfaces terminated with Si–Br, Si–H, and Si–CH₃ groups, respectively. The surface termination influenced the band bending, with the Si 2p_3/2 binding energy affected more by the surface chemistry than by the dopant type. The highest binding energies were measured on Si(111)–Br (whose Fermi level was positioned near the conduction band at the surface), followed by Si(111)–H, followed by Si(111)–CH₃ (whose Fermi level was positioned near midgap at the surface). Si(111)–CH₃ surfaces exposed to Br₂(g) yielded the lowest binding energies, with the Fermi level positioned between midgap and the valence band. The Fermi level position of Br₂(g)-exposed Si­(111)–CH₃ was consistent with the presence of negatively charged bromine-containing ions on such surfaces. The binding energies of all of the species detected on the surface (C, O, Br) shifted with the band bending, illustrating the importance of isolating the effects of band bending when measuring chemical shifts on semiconductor surfaces. The influence of band bending was confirmed by surface photovoltage (SPV) measurements, which showed that the core levels shifted toward their flat-band values upon illumination. Where applicable, the contribution from the X-ray source to the SPV was isolated and quantified. Work functions were measured by ultraviolet photoelectron spectroscopy (UPS), allowing for calculation of the sign and magnitude of the surface dipole in such systems. The values of the surface dipoles were in good agreement with previous measurements as well as with electronegativity considerations. The binding energies of the adventitious carbon signals were affected by band bending as well as by the surface dipole. A model of band bending in which charged surface states are located exterior to the surface dipole is consistent with the XPS and UPS behavior of the chemically functionalized Si(111) surfaces investigated herein

Magnetic Field Alignment of Randomly Oriented, High Aspect Ratio Silicon Microwires into Vertically Oriented Arrays

External magnetic fields have been used to vertically align ensembles of silicon microwires coated with ferromagnetic nickel films. X-ray diffraction and image analysis techniques were used to quantify the degree of vertical orientation of the microwires. The degree of vertical alignment and the minimum field strength required for alignment were evaluated as a function of the wire length, coating thickness, magnetic history, and substrate surface properties. Nearly 100% of 100 μm long, 2 μm diameter, Si microwires that had been coated with 300 nm of Ni could be vertically aligned by a 300 G magnetic field. For wires ranging from 40 to 60 μm in length, as the length of the wire increased, a higher degree of alignment was observed at lower field strengths, consistent with an increase in the available magnetic torque. Microwires that had been exposed to a magnetic sweep up to 300 G remained magnetized and, therefore, aligned more readily during subsequent magnetic field alignment sweeps. Alignment of the Ni-coated Si microwires occurred at lower field strengths on hydrophilic Si substrates than on hydrophobic Si substrates. The magnetic field alignment approach provides a pathway for the directed assembly of solution-grown semiconductor wires into vertical arrays, with potential applications in solar cells as well as in other electronic devices that utilize nano- and microscale components as active elements

Combined Theoretical and Experimental Study of Band-Edge Control of Si through Surface Functionalization

The band-edge positions of H-, Cl-, Br-, methyl-, and ethyl-terminated Si(111) surfaces were investigated through a combination of density functional theory (DFT) and many-body perturbation theory, as well as by photoelectron spectroscopy and electrical device measurements. The calculated trends in surface potential shifts as a function of the adsorbate type and coverage are consistent with the calculated strength and direction of the dipole moment of the adsorbate radicals in conjunction with simple electronegativity-based expectations. The quasi-particle energies, such as the ionization potential (IP), that were calculated by use of many-body perturbation theory were in good agreement with experiment. The IP values that were calculated by DFT exhibited substantial errors, but nevertheless, the IP differences, i.e., IP_R–Si(111)–IP_H–Si(111), computed using DFT were in good agreement with spectroscopic and electrical measurements

A Mechanistic Study of the Oxidative Reaction of Hydrogen-Terminated Si(111) Surfaces with Liquid Methanol

H–Si­(111) surfaces have been reacted with liquid methanol (CH₃OH) in the absence or presence of a series of oxidants and/or illumination. Oxidant-activated methoxylation of H–Si(111) surfaces was observed in the dark after exposure to CH₃OH solutions that contained the one-electron oxidants acetylferrocenium, ferrocenium, or 1,1′-dimethylferrocenium. The oxidant-activated reactivity toward CH₃OH of intrinsic and n-type H–Si(111) surfaces increased upon exposure to ambient light. The results suggest that oxidant-activated methoxylation requires that two conditions be met: (1) the position of the quasi-Fermi levels must energetically favor oxidation of the H–Si(111) surface and (2) the position of the quasi-Fermi levels must energetically favor reduction of an oxidant in solution. Consistently, illuminated n-type H–Si(111) surfaces underwent methoxylation under applied external bias more rapidly and at more negative potentials than p-type H–Si­(111) surfaces. The results under potentiostatic control indicate that only conditions that favor oxidation of the H–Si(111) surface need be met, with charge balance at the surface maintained by current flow at the back of the electrode. The results are described by a mechanistic framework that analyzes the positions of the quasi-Fermi levels relative to the energy levels relevant for each system

A Comparison of the Behavior of Single Crystalline and Nanowire Array ZnO Photoanodes

The photoelectrochemical behavior of n-type ZnO nanowire arrays was compared to the behavior of single crystalline n-ZnO photoelectrodes in contact with either 0.50 M K₂SO₄(aq) at pH 6.0 or Fe­(CN)₄^3–/4–(aq). The use of a thin film of ZnO as a seed layer produced dense nanowire arrays in which the ZnO nanowires were preferentially oriented perpendicular to the substrate. The average diameter of the ZnO nanowires that were produced by two different growth conditions was ∼125 and ∼175 nm, respectively, with a nanowire length of ∼2–4 μm. Under simulated 1 Sun Air Mass 1.5 illumination conditions, the ZnO nanowire arrays exhibited open-circuit potentials, <i>E</i>_oc, and short-circuit photocurrent densities, <i>J</i>_sc, that were very close to the values observed from single crystal n-type ZnO photoanodes in contact with these same electrolytes. Device physics simulations were in accord with the experimentally observed behavior, indicating that, under certain combinations of materials properties and interface recombination velocities, the use of nanostructured light absorbers can provide an approach to efficient photoelectrochemical solar energy-conversion systems

Vapor Sensing Characteristics of Nanoelectromechanical Chemical Sensors Functionalized Using Surface-Initiated Polymerization

Surface-initiated polymerization has been used to grow thick, uniform poly­(methyl methacrylate) films on nanocantilever sensors. Cantilevers with these coatings yielded significantly greater sensitivity relative to bare devices as well as relative to devices that had been coated with drop-cast polymer films. The devices with surface-initiated polymer films also demonstrated high selectivity toward polar analytes. Surface-initiated polymerization can therefore provide a straightforward, reproducible method for large-scale functionalization of nanosensors

Excitonic Effects in Emerging Photovoltaic Materials: A Case Study in Cu₂O

Excitonic effects account for a fundamental photoconversion and charge transport mechanism in Cu₂O; hence, the universally adopted “free carrier” model substantially underestimates the photovoltaic efficiency for such devices. The quasi-equilibrium branching ratio between excitons and free carriers in Cu₂O indicates that up to 28% of photogenerated carriers during photovoltaic operation are excitons. These large exciton densities were directly observed in photoluminescence and spectral response measurements. The results of a device physics simulation using a model that includes excitonic effects agree well with experimentally measured current–voltage characteristics of Cu₂O-based photovoltaics. In the case of Cu₂O, the free carrier model underestimates the efficiency of a Cu₂O solar cell by as much as 1.9 absolute percent at room temperature

Hydrogen Evolution with Minimal Parasitic Light Absorption by Dense Co–P Catalyst Films on Structured p‑Si Photocathodes

Planar and three-dimensionally structured p-Si devices, consisting of an electrodeposited Co–P catalyst on arrays of Si microwires or Si micropyramids, were used as photocathodes for solar-driven hydrogen evolution in 0.50 M H₂SO₄(aq) to assess the effects of electrode structuring on parasitic absorption by the catalyst. Without the use of an emitter layer, p-Si/Co–P microwire arrays produced a photocurrent density of −10 mA cm^–2 at potentials that were 130 mV more positive than those of optimized planar p-Si/Co–P devices. Champion p-Si/Co–P microwire array devices exhibited ideal regenerative cell solar-to-hydrogen efficiencies of >2.5% and were primarily limited by the photovoltage of the p-Si/Co–P junction. The vertical sidewalls of the Si microwire photoelectrodes thus minimized effects due to parasitic absorption at high loadings of catalyst for device structures with or without emitters

Phase Directing Ability of an Ionic Liquid Solvent for the Synthesis of HER-Active Ni₂P Nanocrystals

An ionic liquid (IL) solvent was used to synthesize small, phase-pure nickel phosphide (Ni₂P) nanocrystals. In contrast, under analogous reaction conditions, substitution of the IL for the common high-boiling organic solvent 1-octadecene (ODE) results in phase-impure nanocrystals. The 5 nm Ni₂P nanocrystals prepared in IL were electrocatalytically active toward the hydrogen evolution reaction. The synthesis in IL was also extended to alloyed Ni_2–<i>x</i>­Co_<i>x</i>P nanocrystals, where 0.5 ≤ <i>x</i> ≤ 1.5

Profiling Photoinduced Carrier Generation in Semiconductor Microwire Arrays via Photoelectrochemical Metal Deposition

Au was photoelectrochemically deposited onto cylindrical or tapered p-Si microwires on Si substrates to profile the photoinduced charge-carrier generation in individual wires in a photoactive semiconductor wire array. Similar experiments were repeated for otherwise identical Si microwires doped to be n-type. The metal plating profile was conformal for n-type wires, but for p-type wires was a function of distance from the substrate and was dependent on the illumination wavelength. Spatially resolved charge-carrier generation profiles were computed using full-wave electromagnetic simulations, and the localization of the deposition at the p-type wire surfaces observed experimentally correlated well with the regions of enhanced calculated carrier generation in the volumes of the microwires. This technique could potentially be extended to determine the spatially resolved carrier generation profiles in a variety of mesostructured, photoactive semiconductors