Electron Transfer through Organic Monolayers Directly Bonded to Silicon Probed by Current Sensing Atomic Force Microscopy:  Effect of Chain Length and Applied Force

Electron transfer through organic monolayers directly bonded to a silicon surface has been investigated by using a current sensing atomic force microscope (CSAFM). The research system employs a platinum-coated CSAFM tip in point contact at a confined nanometer size with a set of alkyl monolayers of various chain lengths, which are covalently bonded directly to an n-type Si(111) by immersing a H-terminated silicon substrate in neat CH 2 CH−(CH 2 ) n - 3 CH 3 ( n = 12, 14, 16, and 18) under heating. The current flows were analyzed with a modified electron-tunneling model and showed strong force and chain length dependences. Increasing the applied force resulted in a negative shift of the current−voltage ( I − V ) curves. This observation was mainly explained by the amplified contact area and shortened tunneling distance. In addition, the I − V curves showed chain length dependence, from which the bias-dependent electron tunneling coefficient, β, was analyzed.