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 K_(2)SO_4(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, E_oc, and short-circuit photocurrent densities, J_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

Scanning Microwave Microscopy of Aluminum CMOS Interconnect Lines Buried in Oxide and Water

Using a scanning microwave microscope, we imaged in water aluminum interconnect lines buried in aluminum and silicon oxides fabricated through a state-of-the-art 0.13 um SiGe BiCMOS process. The results were compared with that obtained by using atomic force microscopy both in air and water. It was found the images in water was degraded by only approximately 60% from that in air.Comment: 3 pages, 5 figures, conferenc

Photoelectrochemical Behavior of n‑Type Si(111) Electrodes Coated With a Single Layer of Graphene

The behavior of graphene-coated n-type Si(111) photoanodes was compared to the behavior of H-terminated n-type Si(111) photoanodes in contact with aqueous K_3[Fe(CN)_6]/K_4[Fe(CN)_6] as well as in contact with a series of outer-sphere, one-electron redox couples in nonaqueous electrolytes. The n-Si/Graphene electrodes exhibited stable short-circuit photocurrent densities of over 10 mA cm^(–2) for >1000 s of continuous operation in aqueous electrolytes, whereas n-Si–H electrodes yielded a nearly complete decay of the current density within 100 s. The values of the open-circuit photovoltages and the flat-band potentials of the Si were a function of both the Fermi level of the graphene and the electrochemical potential of the electrolyte solution, indicating that the n-Si/Graphene did not form a buried junction with respect to the solution contact

Photoelectrochemical Behavior of n‑type Si(100) Electrodes Coated with Thin Films of Manganese Oxide Grown by Atomic Layer Deposition

Thin (10 nm) films of manganese oxide have been deposited by atomic layer deposition (ALD) onto n-type silicon and onto degenerately doped p-type silicon. The photoelectrochemical properties of the resulting semiconductor/metal-oxide structures were evaluated in contact with aqueous 0.35 M K_4Fe(CN)_6−0.05 M K_3Fe(CN)_6, 1.0 M KOH(aq), as well as in contact with a series of nonaqueous one electron, reversible, outer-sphere redox systems. Under simulated air mass (AM) 1.5 illumination in contact with 0.35 M K_4Fe(CN)_6−0.05 M K_3Fe(CN)_6(aq), MnO-coated n-Si photoanodes displayed open-circuit voltages of up to 550 mV and stable anodic currents for periods of hours at 0.0 V versus the solution potential. In contact with 1.0 M KOH(aq), at current densities of ∼25 mA cm^(−2), MnO|Si photoanodes under 100 mW cm^(−2) of simulated AM 1.5 illuminationyielded stable oxygen evolution for 10−30 min. Variation in the thickness of the MnO films from 4 to 20 nm indicated the presence of a series resistance in the MnO film that limited the fill factor and thus the solar energy-conversion efficiency of the photoelectrodes. Open-circuit photovoltages of 30 and 450 mV, respectively, were observed in contact with cobaltocene^(+/0) or ferrocene^(+/0) in CH_3CN, indicating that the energetics of the MnO-coated Si surfaces were a function of the electrochemical potential of the contacting electrolyte solution

Use of Mixed CH_3−/HC(O)CH_2CH_2−Si(111) Functionality to Control Interfacial Chemical and Electronic Properties During the Atomic-Layer Deposition of Ultrathin Oxides on Si(111)

Silicon surfaces terminated with a mixed monolayer containing both a propyl aldehyde functionality and methyl groups were prepared and used to control the interfacial chemical and electronic properties of Si(111) surfaces during atomiclayer deposition (ALD) of Al_2O_3 or MnO. Si(111) surfaces functionalized only with the aldehyde moiety exhibited surface recombination velocities, S, of 2500 ± 600 cm s^(−1) whereas the mixed CH_3−/HC(O)CH_2CH_2−Si(111) surfaces displayed S = 25 ± 7 cm s^(−1). During the ALD growth of either Al_2O_3 or MnO, both the HC(O)CH_2CH_2−Si(111) and CH_3−/HC(O)CH_2CH_2−Si(111) surfaces produced increased metal oxide deposition at low cycle number, relative to H−Si(111) or CH_3−Si(111) surfaces. As detected by X-ray photoelectron spectroscopy after the ALD process, the CH_3− and mixed CH_3−/HC(O)CH_2CH_2− functionalized Si(111) surfaces exhibited less interfacial SiO_x than was observed for ALD of metal oxides on H−Si(111) substrates

Comparison of the Photoelectrochemical Behavior of H‑Terminated and Methyl-Terminated Si(111) Surfaces in Contact with a Series of One-Electron, Outer-Sphere Redox Couples in CH_3CN

The photoelectrochemical behavior of methyl-terminated p-type and n-type Si(111) surfaces was determined in contact with a series of one-electron, outer-sphere, redox couples that span >1 V in the Nernstian redox potential, E(A/A^−), of the solution. The dependence of the current vs potential data, as well as of the open-circuit photovoltage, V_(OC), on E(A/A^−) was compared to the behavior of H-terminated p-type and n-type Si(111) surfaces in contact with these same electrolytes. For a particular E(A/A^−) value, CH_3-terminated p-Si(111) electrodes showed lower V_(OC) values than Hterminated p-Si(111) electrodes, whereas CH_3-terminated n-Si(111) electrodes showed higher V_(OC) values than H-terminated n-Si(111) electrodes. Under 100 mW cm^(−2) of ELH-simulated Air Mass 1.5 illumination, n-type H−Si(111) and CH_3−Si(111) electrodes both demonstrated nonrectifying behavior with no photovoltage at very negative values of E(A/A^−) and produced limiting V_(OC) values of >0.5 V at very positive values of E(A/A^−). Illuminated p-type H−Si(111) and CH_3−Si(111) electrodes produced no photovoltage at positive values of E(A/A^−) and produced limiting V_(OC) values in excess of 0.5 V at very negative values of E(A/A^−). In contact with CH_3CN-octamethylferrocene^(+/0), differential capacitance vs potential experiments yielded a −0.40 V shift in flat-band potential for CH_3-terminated n-Si(111) surfaces relative to H-terminated n-Si(111) surfaces. Similarly, in contact with CH_3CN-1,1′-dicarbomethoxycobaltocene^(+/0), the differential capacitance vs potential data indicated a −0.25 V shift in the flat-band potential for CH_3-terminated p-Si(111) electrodes relative to H-terminated p-Si(111) electrodes. The observed trends in V_(OC) vs E(A/A^−), and the trends in the differential capacitance vs potential data are consistent with a negative shift in the interfacial dipole as a result of methylation of the Si(111) surface. The negative dipole shift is consistent with a body of theoretical and experimental comparisons of the behavior of CH_3−Si(111) surfaces vs H−Si(111) surfaces, including density functional theory of the sign and magnitude of the surface dipole, photoemission spectroscopy in ultrahigh vacuum, the electrical behavior of Hg/Si contacts, and the pH dependence of the current−potential behavior of Si electrodes in contact with aqueous electrolytes

Photoanodic behavior of vapor-liquid-solid–grown, lightly doped, crystalline Si microwire arrays

Arrays of n-Si microwires have to date exhibited low efficiencies when measured as photoanodes in contact with a 1-1′-dimethylferrocene (Me_2Fc^(+/0))–CH_3OH solution. Using high-purity Au or Cu catalysts, arrays of crystalline Si microwires were grown by a vapor-liquid-solid process without dopants, which produced wires with electronically active dopant concentrations of 1 × 10^(13) cm^(−3). When measured as photoanodes in contact with a Me_2Fc^(+/0)–CH_3OH solution, the lightly doped Si microwire arrays exhibited greatly increased fill factors and efficiencies as compared to n-Si microwires grown previously with a lower purity Au catalyst. In particular, the Cu-catalyzed Si microwire array photoanodes exhibited open-circuit voltages of ~0.44 V, carrier-collection efficiencies exceeding ~0.75, and an energy-conversion efficiency of 1.4% under simulated air mass 1.5 G illumination. Lightly doped Cu-catalyzed Si microwire array photoanodes have thus demonstrated performance that is comparable to that of optimally doped p-type Si microwire array photocathodes in photoelectrochemical cells