Energetics and Kinetics of Interfacial Electron-Transfer Processes at Chemically Modified InP/Liquid Junctions

The electrochemical behavior of freshly etched (111)B-oriented InP surfaces was compared to that of (111)B-oriented InP surfaces that had been chemically modified by reaction with p-BrCH_2C_6H_4CF_3. Differential capacitance versus potential and current density versus potential techniques were used to measure the energetics and kinetics of interfacial electron-transfer reactions in contact with a 70:30 (v:v) mixture of CH_3CN−tetrahydrofuran that contained either 1,1‘-dimethylferrrocene^(+/0) or decamethylferrocene^(+/0). For both the etched and modified (111)B InP contacts, plots of differential capacitance versus potential measurements indicated a linear dependence of the equilibrium voltage drop (V_(bi)) in the semiconductor space-charge region, as a function of the redox potential (E(A/A-)) of the solution, with the slope of V_(bi) vs E(A/A-) ≈ 1.0, as expected for ideal behavior of a semiconductor/liquid junction. The barrier heights calculated for the chemically modified InP/liquid junctions were 100 ± 20 mV higher than the barrier heights of freshly etched, unmodified (111)B InP surfaces in contact with the same electrolyte solutions. The higher barrier heights of the chemically modified surfaces are consistent with a shift in the InP band-edge energies induced by the surface modification process. The modified and etched surfaces both displayed interfacial electron-transfer kinetics that were first order in the concentration of acceptors in solution and first order in the concentration of electrons at the semiconductor surface. The interfacial electron-transfer rate constants for these systems were determined to be ∼10^(-17)−10^(-18) cm^4 s^(-1)