Surface Passivation and Positive Band-Edge Shift of p-Si(111) Surfaces Functionalized with Mixed Methyl/Trifluoromethylphenylacetylene Overlayers

Chemical functionalization of semiconductor surfaces can provide high-efficiency photoelectrochemical devices through molecular-level control of the energetics, surface dipole, surface electronic defects, and chemical reactivity at semiconductor/electrolyte junctions. We describe the covalent functionalization by nucleophilic addition chemistry of p-Si(111) surfaces to produce mixed overlayers of trifluoromethylphenylacetylene (TFMPA) and methyl moieties. Functionalization of Cl-terminated Si(111) surfaces with TFMPA moieties introduced a positive surface molecular dipole that in contact with CH3CN or Hg produced a positive band-edge shift of the semiconductor relative to junctions with CH3-Si(111) surfaces. Methylation of the Cl/TFMPA surfaces using methylmagnesium chloride resulted in the degradation of the TFMPA moieties, whereas methylation using methylzinc chloride allowed controlled production of mixed TFMPA/methyl-terminated surfaces and permitted reversal of the order of the functionalization steps so that nucleophilic addition of TFMPA could be accomplished after methylation of Cl-Si(111) surfaces. Mixed TFMPA/methyl functionalization resulted in a Si(111) surface with surface recombination velocities of 2 x 10(2) cm s(-1) that exhibited an similar to 150 mV positive band-edge shift relative to CH3-Si(111) surfaces.