Control of the Band-Edge Positions of Crystalline Si(111) by Surface Functionalization with 3,4,5-Trifluorophenylacetylenyl Moieties

Functionalization of semiconductor surfaces with organic moieties can change the charge distribution, surface dipole, and electric field at the interface. The modified electric field will shift the semiconductor band-edge positions relative to those of a contacting phase. Achieving chemical control over the energetics at semiconductor surfaces promises to provide a means of tuning the band-edge energetics to form optimized junctions with a desired material. Si(111) surfaces functionalized with 3,4,5-trifluorophenylacetylenyl (TFPA) groups were characterized by transmission infrared spectroscopy, X-ray photoelectron spectroscopy, and surface recombination velocity measurements. Mixed methyl/TFPA-terminated (MMTFPA) n- and p-type Si(111) surfaces were synthesized and characterized by electrochemical methods. Current density versus voltage and Mott-Schottky measurements of Si(111)-MMTFPA electrodes in contact with Hg indicated that the barrier height, Phi(b), was a function of the fractional monolayer coverage of TFPA (theta(TFPA)) in the alkyl monolayer. Relative to Si(111)-CH3 surfaces, Si(111)-MMTFPA samples with high Phi(TFPA) produced shifts in Phi(b) of >= 0.6 V for n-Si/Hg contacts and >= 0.5 V for p-Si/Hg contacts. Consistently, the open-circuit potential (E-oc) of Si(111)-MMTFPA samples in contact with CH3CN solutions that contained the 1-electron redox couples decamethylferrocenium/decamethylferrocene (Cp*2Fe(+/0)) or methyl viologen (MV2+/+center dot) shifted relative to Si(111)-CH3 samples by +0.27 V for n-Si and by up to +0.10 V for p-Si. Residual surface recombination limited the E-oc of p-Si samples at high theta(TFPA) despite the favorable shift in the band-edge positions induced by the surface modification process.