Journal
JOURNAL OF PHYSICAL CHEMISTRY C
Volume 120, Issue 26, Pages 14157-14169Publisher
AMER CHEMICAL SOC
DOI: 10.1021/acs.jpcc.6b03824
Keywords
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Funding
- National Science Foundation [CHE-1214152]
- National Science Foundation CCI Solar Fuels Program [CHE-1305124]
- Direct For Mathematical & Physical Scien
- Division Of Chemistry [1214152] Funding Source: National Science Foundation
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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.
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