AFM images were obtained using a NanoScope II or E (Digital Instruments, CA) operating in the constant force repulsive mode. Microfabricated Si3N4 cantilevers 100 pm long with a spring constant of 0.58 N/m were used throughout. Typical force during imaging was less than 1 nN. Calibrations of the piezo scanner for the x and y planes were carried out by imaging mica and a single-crystal Au(l 11) face in Milli-Q water.

p-InSe single crystals grown by the Bridgman-Stockbarger method and GaAs(lOO) single crystals were donated by Dr. Chevy [15] and by the Mitsubishi Chemical Corporation, respectively. A clean surface of InSe was obtained by removing some layers using adhesive tape, and GaAs substrates were cleaned in hot acetone and in ethanol and rinsed with Milli-Q water (Millipore Co.) before each experiment. Ohmic contacts were made by using an In-Zn alloy for p-type and In for n-type substrates.

A glass fluid cell of 0.2 ml volume (Digital Instruments) was used for the electrochemical AFM measurements. The counter electrode and reference electrode were a Pt wire and an Ag/AgCl electrode, respectively. The area of exposed electrode in contact with the solution was about 0.32 cm2. The electrolyte solutions for electrochemical AFM measurements were prepared by using reagent-grade chemicals (Wako Pure Chemicals) and Milli-Q water. The solution was deaerated by passing purified N2 gas for at least 20 min prior to the experiment.

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