Abstract The formation of large-scale arrays of individually seeded, electrically addressable Si nanowires with controlled dimension, placement, and orientation is demonstrated. E-beam evaporated gold nanoparticles were used for nanowire synthesis by the vapor–liquid–solid growth mechanism. By controlling the lithography and metal deposition conditions, nanowire arrays with narrow size distributions have been achieved. Low-energy postgrowth ion-beam treatment has been utilized to control the orientation of Si nanowires. This process also leads to the attachment of nanowires on the substrate surface. Fabrication of planar devices with robust metal contact formation becomes feasible. Our method, combining bottom-up and top-down approaches, can enable efficient and economical integration of nanowires into device architectures for various applications. Low-dimensional nanomaterials are a new class of advanced materials that have been receiving a lot of research interest in the last decade because of their superior physical and chemical properties. Nanowires (NWs) have been demonstrated to exhibit superior electrical [1], optical [2,3], mechanical [4], piezoelectric [5], and field emission [6] properties, and can be used as fundamental building blocks for nanoscale science and technology, ranging from chemical and biological sensors, field effect transistors to logic circuits. Field effect transistors (FETs) based on NWs have been demonstrated as good candidates for ultrasensitive, miniaturized molecule sensors [7, 8]. Because of the high surface-to-volume ratio of these one dimensional nanostructures, their electronic characteristics may be sensitive enough to a very small amount of charge transfer such that single molecule detection becomes possible [7]. Nanosensors based on Si nanowires (SiNWs) are a promising candidate for label-free, direct, real-time electrical detection of the event of biomolecule binding, because of their several appealing properties including the following: (i) The electrical properties and sensitivity of SiNWs can be tuned by controlling NW diameter and the dopant type and concentration. (ii) The modification of silicon oxide surface, required for the preparation of interfaces selective for binding various analytes of interest, is well established. The vapor–liquid–solid (VLS) growth mechanism, studied in detail in the 1960s and 1970s by Wagner et al. [9], is an ideal growth technique in the gas phase to produce NWs with high crystalline quality, required for sensing applications. Superior performance based on individual SiNW devices has been demonstrated [7, 10]. However, most of the existing studies based on nanostructures assembled using a bottom-up approach are limited by complex integration. Devices have been constructed around single, or several dispersed SiNWs. For practical applications, efficient and precise manipulation and placement of NWs at desired locations needs to be established to allow the integration with microfluidics and CMOS circuits. Methods that can enable reliable contact formation are essential in order for the intrinsic properties of the nanostructures to be realized. We begin this chapter with an overview of prior methods that have been used for nanostructure assembly and device fabrication, including top-down (Sect. 4.1.1) and bottom-up (Sect. 4.1.2) approaches. The advantage and disadvantage of various approaches are discussed. A pathway to fabricating large-scale arrays of individually seeded, electrically addressable SiNWs with controlled dimension, placement, and orientation is described (Sect. 4.2). SiNWs with controlled dimensions and specific placement were produced by the conventional chemical vapor deposition via the VLS process. By controlling the lithography and metal deposition conditions, NW arrays with narrow size distributions have been achieved (Sect. 4.2.2). Low-energy postgrowth ionbeam treatment has been utilized to control the orientation of SiNWs (Sect. 4.2.3). This process also leads to the attachment of NWs on the substrate surface. Fabrication of planar devices with robust metal contact formation becomes feasible. Electrical and structural characterizations were used to assess the quality of NWs after device fabrications (Sect. 4.2.4). Our method, which combines bottom-up and top-down approaches, can enable efficient and economical integration of NWs into device architectures for various applications.