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2005; Huang et al., 2001; Javey et al., 2007; Lee et al., 2005; McAlpine et al., 2005; Wang et al., 2007; Yerushalmi et al., 2007). One such inorganic material is the crystalline semiconductor nanowire (NW). In this paper, we review recent advances in the assembly and integration of NW arrays on foreign substrates that can be integrated into electronic devices and sensors.

CRYSTALLINE SEMICONDUCTOR NANOWIRES AS BUILDING BLOCKS FOR ELECTRONICS AND SENSORS

To date, a variety of functional NWs have been synthesized and integrated as building blocks of single-component devices, such as field-effect transistors (FETs), sensors, photodiodes, and electromechanical systems, to mention just a few (Ahn et al., 2006; Bryllert et al., 2006; Fan et al., 2008a,b; Ford et al., 2008; Friedman et al., 2005; Huang et al., 2001; Javey et al., 2007; Lee et al., 2005; McAlpine et al., 2005; Wang et al., 2007; Yerushalmi et al., 2007). These chemically derived single-crystalline nanostructures (the majority of them synthesized by chemical vapor deposition [CVD]) have unique advantages over conventional semiconductors. They enable the integration of high-performance device elements on virtually any substrate (including mechanically flexible plastics) with scaled on-currents and switching speeds comparable to or higher than those of state-of-the-art, planar silicon (Si) structures.

For example, p-type FETs based on heterostructured Ge/Si NWs and n-type FETs based on InAs NWs have demonstrated a carrier mobility about 10 times higher than that of Si transistors (Bryllert et al., 2006; Ford et al., 2008; Xiang et al. 2006). These high-mobility NW materials are ideal platforms for high-performance, printable electronics. Uniquely, the electrical properties of NWs are extremely sensitive to their chemical/biological and electromagnetic surroundings because of their miniaturized dimensions, large surface-area-to-volume ratio, and finite carrier concentration. As a result, sensors based on NWs are also highly sensitive. For example, NWs made of Si and In2O3 have been extensively studied for use in biological and chemical sensors capable of detecting analytes down to the level of single molecules (Zhang et al., 2004; Zheng et al., 2005). CdSe and ZnO NWs, which are optically active and have been investigated in the past, have demonstrated a significantly higher photoresponse than their thin-film or bulk counterparts (Fan et al., 2008a; Yu et al., 2008).

Although NWs are obviously promising materials for high-performance nanoelectronics and sensors, a major challenge to their integration into large-scale devices/circuits is perfecting their controlled assembly on substrates. In recent years, many approaches have been investigated with varying degrees of success. These approaches include liquid-flow alignment, Langmuir-Blodgett technique, alternating current (AC) dielectrophoresis, blown-bubble method, contact and roller printing, and others. In this article, we review recent progress on a highly



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