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, φb, was a function of the fractional monolayer coverage of TFPA (θTFPA) in the alkyl monolayer. Relative to Si(111)-CH3 surfaces, Si(111)-MMTFPA samples with high θTFPA produced shifts in φb of ≥0.6 V for n-Si/Hg contacts and ≥0.5 V for p-Si/Hg contacts. Consistently, the open-circuit potential (Eoc) 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+/+) 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 Eoc of p-Si samples at high θTFPA despite the favorable shift in the band-edge positions induced by the surface modification process.
ASJC Scopus subject areas
- Physical and Theoretical Chemistry
- Electronic, Optical and Magnetic Materials
- Surfaces, Coatings and Films