Distance dependence of the electron-transfer rate across covalently bonded monolayers on silicon

Alkyl monolayers covalently bonded directly to n-type Si(111) surfaces have been prepared by UV illumination of the H-Si(111) surface while immersed in CH2=CH-(CH2)(n-3)CH3 (n = 5-8) under high vacuum. The characterization by ellipsometry, infrared spectroscopy, and X-ray photoelectron spectroscopy shows that I-alkenes form dense monolayers on the silicon surface. The electron-transfer rates from the surface of the electrode through the alkyl monolayers; to decamethylferricenium acceptors in tetrahydrofuran have been measured. The rates are proportional to the decamethylferricenium concentration and also to the dopant density as determined from capacitance measurements. The rates show an exponential distance dependence with a decay constant of 1.00 +/- 0.05 per CH2, similar to known behavior with alkanethiol monolayers on metal electrodes. The dependence of the rates on applied potential has a logarithmic slope of about 0.25 F/RT at potentials negative of the flat-band potential derived from capacitance measurements. This slope is qualitatively consistent with the expected potential dependence of the activation energy of electron transfer. The slope increases as expected at potentials positive of the flat-band potential, where a space-charge region forms.