The large-scale assembly of nanowires with controlled orientation on surfaces remains one challenge preventing their integration into practical devices. We report the vapor-liquid-solid growth of aligned, millimeter-long, horizontal GaN nanowires with controlled crystallographic orientations on different planes of sapphire. The growth directions, crystallographic orientation, and faceting of the nanowires vary with each surface orientation, as determined by their epitaxial relationship with the substrate, as well as by a graphoepitaxial effect that guides their growth along surface steps and grooves. Despite their interaction with the surface, these horizontally grown nanowires display few structural defects, exhibiting optical and electronic properties comparable to those of vertically grown nanowires. This paves the way to highly controlled nanowire structures with potential applications not available by other means.

The ability to control the placement of individual molecules promises to enable a wide range of applications and is a key challenge in nanoscience and nanotechnology. Many biological interactions, in particular, are sensitive to the precise geometric arrangement of proteins. We have developed a technique which combines molecular-scale nanolithography with site-selective biochemistry to create biomimetic arrays of individual protein binding sites. The binding sites can be arranged in heterogeneous patterns of virtually any possible geometry with a nearly unlimited number of degrees of freedom. We have used these arrays to explore how the geometric organization of the extracellular matrix (ECM) binding ligand RGD (Arg-Gly-Asp) affects cell adhesion and spreading. Systematic variation of spacing, density and cluster size of individual integrin binding sites was used to elicit different cell behavior. Cell spreading assays on arrays of different geometric arrangements revealed a dramatic increase in spreading efficiency when at least 4 liganded sites were spaced within 60 nm or less, with no dependence on global density. This points to the existence of a minimal matrix adhesion unit for fibronectin defined in space and stoichiometry. Developing an understanding of the ECM geometries that activate specific cellular functional complexes is a critical step toward controlling cell behavior. Potential practical applications range from new therapeutic treatments to the rational design of tissue scaffolds that can optimize healing without scarring. More broadly, spatial control at the single-molecule level can elucidate factors controlling individual molecular interactions and can enable synthesis of new systems based on molecular-scale architectures. Among the candidates for future generation lithography technologies is nanoimprint lithography (NIL), 10,11 which is a high throughput patterning technique in which a pattern is formed in a thin polymer film that has been cast on a substrate by molding it to a relief image in a rigid template (mask). The pattern is then transferred from the polymer by a variety of thin film deposition and/or etching techniques. There is no theoretical limitation to the resolution of the features imprinted by NIL; 12 the practical limit is determined by the size of the features on the NIL template, which is typically patterned by electron beam lithography. We have recently developed a process based on NIL and self-aligned pattern transfer which reduces the imprinted feature size and is capable of creating metallic structures below 5 nm. 13 We have also developed a facile surface chemistry which allows us to functionalize these structures with a broad array of biomolecular species with a high degree of selectivity. 14 Using these techniques, we have fabricated biomimetic surfaces upon which we can control the precise placement of individual biomolecules. We report here how these surfaces can be used to study the role of geometric organization of extracellular matrix (ECM) binding ligand...

The large-scale integration of nanowires into practical devices is hindered by the limited ability to controllably assemble these nanoscale objects on surfaces. Following our first report on the guided growth of millimeter-long horizontal nanowires with controlled orientations, here we demonstrate the generality of the guided growth approach by extending it from GaN nanowires to ZnO nanowires. We describe the guided growth of horizontally aligned ZnO nanowires with controlled crystallographic orientations on eight different planes of sapphire, including both flat and faceted surfaces. The growth directions, crystallographic orientation, and faceting of the nanowires are constant for each surface plane and are determined by their epitaxial relation with the substrate, as well as by a graphoepitaxial effect that guides their growth along surface steps and grooves. These horizontal ZnO nanowires exhibit optical and electronic properties comparable to those of vertically grown nanowires, indicating a low concentration of defects. While the guided growth of ZnO nanowires described here resembles the guided growth of GaN nanowires in its general aspects, it also displays notable differences and qualitatively new phenomena, such as the controlled growth of nanowires with vicinal orientations, longitudinal grain boundaries, and thickness-dependent orientations. This article proves the generality of the guided growth phenomenon, which enables us to create highly controlled nanowire structures and arrays with potential applications not available by other means.

The ability to assemble discrete nanowires (NWs) with nanoscale precision on a substrate is the key to their integration into circuits and other functional systems. We demonstrate a bottom-up approach for massively parallel deterministic assembly of discrete NWs based on surface-guided horizontal growth from nanopatterned catalyst. The guided growth and the catalyst nanopattern define the direction and length, and the position of each NW, respectively, both with unprecedented precision and yield, without the need for postgrowth assembly. We used these highly ordered NW arrays for the parallel production of hundreds of independently addressable single-NW field-effect transistors, showing up to 85% yield of working devices. Furthermore, we applied this approach for the integration of 14 discrete NWs into an electronic circuit operating as a three-bit address decoder. These results demonstrate the feasibility of massively parallel "self-integration" of NWs into electronic circuits and functional systems based on guided growth.T he sustained progress in semiconductor technology introduces new challenges associated with the scaling and functionality of nanosize components. In the face of these challenges, alternative unconventional device and fabrication concepts based on bottom-up assembly of synthetic nanostructures are being intensively explored (1). These nanostructures, such as quantum dots (2), nanotubes (3), and nanowires (NWs) (4), can be chemically synthesized with exquisite control over their structures and properties down to the atomic level. On the other hand, their self-assembly alone is unlikely to produce the arbitrary geometries and long-range order that are required for their integration into functional systems. To realize such systems, bottomup assembly may be used as a complementary step in a sequence of top-down fabrication processes. Such a hybrid top-down/ bottom-up approach can be based on the directed self-assembly of building blocks onto a lithographically produced template to fit the design of an integrated functional system. Thus, the building blocks integrate themselves into the system, as one of the layers in the overall design. Here we demonstrate the feasibility of this "self-integration" concept with the parallel fabrication of large numbers of devices and complex circuits, based on guided growth of horizontal NWs (5).NWs are attractive building blocks for the bottom-up assembly of nanoscale devices and functional systems with potential applications in nanoelectronics (6), photonics (7), renewable energy (8), and biology (9). They can be synthesized with precisely controlled nanoscale dimensions and chemical compositions (10). Moreover, they may be structured to possess unique electronic properties, such as ballistic conductivity due to confinement of a 1D charge-carrier gas in core-shell NWs (11). The potential of NW-based electronics has been demonstrated for various NW materials (12). However, most studies were done at the single-device level. The main obstacle toward NW integration...

function of cells. [8] These forces have different origins, such as actin dynamics, [9] and play important roles at different stages of the lymphocyte immune activity. Initial sampling of antigens on the surface of antigen presenting cells (APCs), as well as activation of immunoreceptors, strongly depends on actin polymerization and dynamics. [10] Moreover, immunoreceptors recognize antigens under mechanical load to discriminate between high-affinity and low-affinity antigens. [11] Once activated, the receptor-antigen complexes on the lymphocyte-APC interface are driven by retrograde actin flow and myosin contraction into highly regulated structures termed immune synapse, whose forces affect the inside-out signaling of lymphocytes. Today, mechanical forces in immune system are a subject of emerging research, which has so far mostly focused on T cells and B cells. [12,13] Studying mechanical forces in cells is challenging, because these forces have relatively low magnitude -mostly at the nanoNewton scale, and often span over miniature regions sized down to the molecular scale. Existing tools include optical traps, [14,15] micropipettes, [16] and atomic force microscopy (AFM), [17,18] which, however, apply and detect forces only at single point on the cell membrane, and do not overview the mechanical behavior of the entire cell. Alternatively, traction force microscopy, which determines the displacement of microbeads embedded in hydrogel surface for cell spreading, maps forces of entire cells, [19][20][21] however, it can hardly detect the exact bead movement since the beads are distributed randomly, and their resting position is unknown. Furthermore, analysis of bead movement requires complex force calculations based on elasticity theory. [22] These constrains can be overcome by elastomeric micropillars for cell spreading, which allow facile mapping of force distribution within cells. [23] Furthermore, micropillars can be functionalized with biomolecules that yield chemical stimuli for various cell functions, such as adhesion [24,25] or immune response, [26] and thereby allow integration of mechanical and biochemical cues. However, the advantages of elastic micropillars come at the expense of their spatial and mechanical resolution. Indeed, poly(dimethyl siloxane) (PDMS) -material of choice for micropillar fabrication -is limited for the fabrication of pillars with micrometerscale size and aspect ratio of 3:1, for which sensing forces below Cells sense their environment by transducing mechanical stimuli into biochemical signals. Commonly used tools to study cell mechanosensing provide limited spatial and force resolution. Here, a novel nanowire-based platform for monitoring cell forces is reported. Nanowires are functionalized with ligands for cell immunoreceptors, and they are used to explore the mechanosensitivity of natural killer (NK) cells. In particular, it is found that NK cells apply centripetal forces to nanowires, and that the nanowires stimulate cell contraction. Based on the nanowire deformation, it is...