Molecularly Controlled Modulation of Conductance on Azobenzene Monolayer-Modified Silicon Surfaces

Controlled modulation of silicon surface properties is of great importance for the development of silicon-based molecular electronic devices because of the ubiquitous role of silicon in microelectronics. In this article, photoresponsive azobenzene molecules were covalently grafted onto hydrogen-terminated Si(111) surfaces via Si−C linkages. These direct Si−C bond linkages are preferred over Si−O linkages at the interfaces because of the higher stability and the better electronic continuation between Si and the alkyl chain. The modified surfaces were characterized by X-ray photoelectron spectroscopy (XPS) and attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy. The reversible photoisomerization effects of the azobenzene molecules were also studied with contact angle measurements, atomic force microscopy (AFM), and conductive atomic force microscopy (C-AFM). The measured conductivity showed a reversible switching behavior by alternate illumination with UV and visible light. Thus, we have demonstrated molecularly controlled modulation of conductance of the Si surface by the photochemical method. Furthermore, the dipole moments of the azobenzene molecules switched accordingly with the alternate illumination. Making use of this characteristic, we have provided a strategy to evaluate the influence of the molecular dipole moments on the conductance of the semiconductor surface.