Band edge control of crystalline silicon by chemical functionalization of the surface

Abstract

Methyl-termination of the silicon (111) crystal plane has been shown to yield nearly complete termination of the silicon atop sites with Me groups to yield exceptional stability to oxidn. and low elec. defect densities at the surface. However, Me groups impart a -0.4 eV surface dipole that shifts the semiconductor band-edge positions of p-type silicon unfavorably for the prodn. of fuels, namely hydrogen, from sunlight. Incorporation of electroneg. elements, such as fluorine, into alkyl monolayers can effectively reverse the unfavorable shift on the band-edge positions and maximize the efficiency of solar-fuels devices. Thus, a mixed methyl/4- fluorobenzyl monolayer has been developed herein to shift the band-edge positions on a sliding scale while maintaining low elec. defect densities at the surface. The band-edge positions were detd. using electrochem. measurements and photoelectron spectroscopy to develop a relationship between the band-edge positions and the monolayer compn. Samples with favorable band-edge positions for the prodn. of hydrogen were tested electrochem. to demonstrate the improved efficiency of devices fabricated using mixed methyl/4-fluorobenzyl monolayers compared with homogeneous Me monolayers. This work holds promise to motivate the development of a new class of solar-fuels devices based on chem. functionalization of semiconductor surfaces.