Surface functionalization and passivation of the (111) faces of gallium phosphide for solar energy conversion applications

Abstract

Functionalized benzyl groups and pyridyl groups were reacted onto the gallium-rich (111)A and phosphorus-rich (111)B faces of gallium phosphide (GaP), in the interest of passivating surface recombination sites, probing the chem. states of surface atoms, and adding mol. functionality to the surface for the covalent attachment of catalysts for photoelectrochem. systems, such as pyridine for CO_2 conversion to fuels. The GaP (111)A surface was chlorinated by literature procedures, then functionalized with tris(4-fluorophenyl)phosphine, (P(FC_6H_4)_3. The GaP (111)B was functionalized with either 4-fluorobenzyl bromide, 4-trifluorobenzyl bromide, or 2-(bromomethyl) pyridine hydrobromide. XPS was used to quantify the percentage of surface sites bound to a reagent mol. The signal intensities of the heteroatom and bulk substrate peaks were consistent with a phosphine coverage of 10% on the (111)A face. The phosphine appears to have bound to terminal Ga sites as expected while unfunctionalized sites remained chlorine terminated. Similarly, the 4-fluorobenzyl group coverage was 38%, the 4-trifluorobenzyl group coverage was 30%, and the 2-pyridyl group coverage was 59%. Following published work on indium phosphide, the benzyl bromides were expected to bind to the surface via a surface-bound hydroxyl group displacing the bromine. The benzyl groups are bound as expected, but the detection of ionic bromine on the surface calls the proposed mechanism into question. Further studies of surface reactivity would clarify the chem. states of surface atoms before and after the reaction. Steady-state photoluminescence (PL) measurements show an increase in PL intensity of the functionalized surfaces compared to freshly etched (111) surfaces, indicating partial passivation of electronic trap states corresponding with surface oxide species. This is consistent with XPS spectra of the functionalized surfaces that show less than 1 monolayer of surface oxide coverage.