Molecularly Controlled Modulation of Conductance on Azobenzene Monolayer-Modiﬁed Silicon Surfaces Xingye Zhang, † Yongqiang Wen,* ,† Yingfeng Li, † Guo Li, ‡ Shixuan Du, ‡ Haiming Guo, ‡ Lianming Yang, † Lei Jiang, † Hongjun Gao, ‡ and Yanlin Song* ,† Key Laboratory of Organic Solids, Laboratory of New Materials, Institute of Chemistry, Chinese Academy of Sciences, and Nanoscale Physics and DeVice Laboratory, Institute of Physics, Chinese Academy of Sciences, Beijing 100080, China ReceiVed: December 17, 2007; ReVised Manuscript ReceiVed: March 21, 2008 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. 1. Introduction With the advent of molecular electronics, it is essential to understand thoroughly the inﬂuence of the electronic states of molecules and the molecular aggregate interface on the charge transport character. 1–4 Studies have shown that electronic transport through devices depends critically on the properties of the interfaces through which electrons pass, which is controlled by the interface dipoles and the density and energy distribution of the surface state. 1,5,6 So, it is attractive to seek controllable modulation of device performance through surface modiﬁcations. For which, grafting molecular properties onto metals or semiconductors is an important research approach, which allows the use of the cooperative properties of these solids and the controllable functional versatility of molecules. 6 Over the past decade, researchers have established many different molecule/metal or molecule/semiconductor hybrid systems to learn the impact of interface dipole on the charge transport character. In those studies, series of molecules with different dipole moments were used to regulate the electrical properties of the solid surfaces by introducing π-electron-donating or π-electron-withdrawing functional groups. 2,7–9 In recent years, considerable attention has been paid to the research of covalent attachment of organic monolayers to hydrogen-terminated silicon surfaces. 10–12 Interest in this area is driven by the opportunity to combine the advantages of Si in microelectronics with versatile functionalities of organic mol- ecules. Such silicon surfaces, without the thin native interfacial silicon oxide layer, may have great potential in the development of hybrid molecule/semiconductor electronic devices and bio- compatible devices. 10 In this article, a novel photoresponsive azobenzene-containing molecule, 4-N,N-dimethylamino-4′-ethynyl-azobenzene (DMAE- AB), with an end alkyne group as the binding unit, has been synthesized and grafted directly on hydrogen-terminated Si(111) surfaces through light-promoted hydrosilylation (Scheme 1). These direct Si-C bond linkages are preferred over Si-O linkages at the interfaces because of the higher stability 13,14 and the better electronic continuation between Si and the alkyl chain. 15–17 The DMAEAB-modiﬁed surfaces were characterized by XPS and ATR-FTIR spectroscopy, and the reversible photoisomerization effects of azobenzene molecules were conﬁrmed with water contact angle measurements, AFM, and C-AFM. The measured conductivity showed a reversible switching behavior by alternate illumination with UV and visible light, which provides a promising method to manipulate the chemical and physical properties of silicon surfaces. * Corresponding authors. E-mail: ylsong@iccas.ac.cn; wyq_wen@ iccas.ac.cn. † Institute of Chemistry, Chinese Academy of Sciences. ‡ Institute of Physics, Chinese Academy of Sciences. SCHEME 1: Scheme for the Preparation of Covalently Attached DMAEAB Monolayer on the Si(111) Surface J. Phys. Chem. C 2008, 112, 8288–8293 8288 10.1021/jp711808p CCC: $40.75  2008 American Chemical Society Published on Web 05/10/2008