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High-resolution electrochemical deposition of silver nanostructures on insulating atomically flat mica surfaces in humid air was achieved with a scanning tunneling microscope (STM) operating in a scanning electrochemical microscopy mode. The current is faradaic and flows between the tip and a surface contact on the mica substrate through a thin water layer on the mica surface. The thickness and conductivity of the water layer governs the magnitude of the faradaic current. Modified low current STM and conventional tungsten STM tips were used for the silver deposition. The tip was held at a negative bias of -1.2 to -1.4 V (tip vs substrate), with setpoint currents of 1 -1.7 pA. The sizes and shapes of the nanostructures were controlled by the tip scan rates and were restricted to the scan areas under the tip.

Selenide and polyselenide UV-vis absorption spectroscopic characteristics were investigated for aqueous polyselenide solutions. From these observations equilibria interrelationships between OH{sup {minus}}, H{sup +}, H{sub 2}Se, HSe{sup {minus}}, Se{sup {minus}2}, Se{sub 2}{sup {minus}2}, Se{sub 3}{sup {minus}2}, and Se{sub 4}{sup {minus}2} were probed. The standard cell potential for the reduction of 1/2 Se{sub 2}{sup {minus}2} to Se{sup {minus}2} was determined to be {minus}0.716 V. The equilibrium reactions of Se{sub 3}{sup {minus}2} and 2 Se{sub 4}{sup {minus}2} with Se{sup {minus}2} to 3 Se{sub 3}{sup {minus}2}, respectively, have pK{sub 23} = {minus}0.7 ({+-}0.1) and pK{sub 34} = {minus}4.0 ({+-}0.1) in 0.5 to 3M aqueous electrolytes. Consistent with these equilibria, the free energy of formation for Se{sub 2}{sup {minus}2}, Se{sub 3}{sup {minus}2}, and Se{sub 4}{sup {minus}2} have been determined as 121, 115, nd 121 kJ/mol, respectively. Polarization studies of these polyselenide solutions demonstrated Pt and CoSe as efficient catalytic surfaces for the oxidation and reduction of selenide species.

The effect of aqueous polyselenide solution modification on the photoelectrochemical behavior of n-GaAs/aqueous polyselenide photoelectrochemical cells (PECS) is investigated. Solution modification is achieved through pH control and changing the ratio of dissolved selenium to selenide. Numerical analysis of speciation in various polyselenide solutions is compared to PEC behavior. It is shown that PEC photocurrent, photovoltage, and fill factor are affected by the distribution of hydroselenide, selenide, and polyselenide in solution. There appears to be an optimum pH ([KOH] = 1 to 2M) providing sufficient concentrations of Se{sup {minus}2} to improve PEC response. A back-wall PEC was designed to investigate the high selenium concentration domain not normally accessible to conventional regenerative solar cells. Photopotential is enhanced by up to 50 mV in high [Se{sup 0}] electrolytes. However, these electrolytes can diminish photocurrent stability.