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Flexible behaviors over long timescales are thought to engage recurrent neural networks in deep brain regions, which are experimentally challenging to study. In insects, recurrent circuit dynamics in a brain region called the central complex (CX) enable directed locomotion, sleep, and context- and experience-dependent spatial navigation. We describe the first complete electron microscopy-based connectome of the Drosophila CX, including all its neurons and circuits at synaptic resolution. We identified new CX neuron types, novel sensory and motor pathways, and network motifs that likely enable the CX to extract the fly’s head direction, maintain it with attractor dynamics, and combine it with other sensorimotor information to perform vector-based navigational computations. We also identified numerous pathways that may facilitate the selection of CX-driven behavioral patterns by context and internal state. The CX connectome provides a comprehensive blueprint necessary for a detailed understanding of network dynamics underlying sleep, flexible navigation, and state-dependent action selection.

Flexible behaviors over long timescales are thought to engage recurrent neural networks in deep brain regions, which are experimentally challenging to study. In insects, recurrent circuit dynamics in a brain region called the central complex (CX) enable directed locomotion, sleep, and context- and experience-dependent spatial navigation. We describe the first complete electron-microscopy-based connectome of theDrosophilaCX, including all its neurons and circuits at synaptic resolution. We identified new CX neuron types, novel sensory and motor pathways, and network motifs that likely enable the CX to extract the fly’s head-direction, maintain it with attractor dynamics, and combine it with other sensorimotor information to perform vector-based navigational computations. We also identified numerous pathways that may facilitate the selection of CX-driven behavioral patterns by context and internal state. The CX connectome provides a comprehensive blueprint necessary for a detailed understanding of network dynamics underlying sleep, flexible navigation, and state-dependent action selection.

Neural representations of head direction have been discovered in many different species. A large body of theoretical work has proposed that the dynamics associated with these representations might be generated, maintained, and updated by recurrent network structures called ring attractors. Ring attractor models rely on specific assumptions about the structure of excitatory and inhibitory connectivity. These assumptions have been difficult to test directly. We therefore performed electron-microscopy-based circuit reconstruction and RNA profiling of identified cell types in the heading direction system of Drosophila melanogaster to directly examine excitatory and inhibitory synaptic connectivity, generating a dataset that should serve as a reference for future functional studies of the network. Consistent with several theoretical models, we identified network motifs that have been hypothesized to maintain the heading representation in darkness, update it when the animal turns, and tether it to visual cues. Genetically targeted two-photon calcium imaging and thermogenetic perturbation of the constituent neuron types during behavior provided additional support for these functional roles. However, we also discovered network motifs absent in current models, including a surprising degree of recurrence between arbors of different neurons with mixed pre- and post-synaptic specializations. Overall, our results confirm that the Drosophila heading direction network contains the core components of a ring attractor while also revealing unpredicted structural features that might enable the heading system to accurately track the animal9s heading with a small number of neurons.

Seizures induced by visual stimulation (photosensitive epilepsy; PSE) represent a common type of epilepsy in humans, but the molecular mechanisms and genetic drivers underlying PSE remain unknown, and no good genetic animal models have been identified as yet. Here, we show an animal model of PSE, in Drosophila, owing to defective cortex glia. The cortex glial membranes are severely compromised in ceramide phosphoethanolamine synthase (cpes)-null mutants and fail to encapsulate the neuronal cell bodies in the Drosophila neuronal cortex. Expression of human sphingomyelin synthase 1, which synthesizes the closely related ceramide phosphocholine (sphingomyelin), rescues the cortex glial abnormalities and PSE, underscoring the evolutionarily conserved role of these lipids in glial membranes. Further, we show the compromise in plasma membrane structure that underlies the glial cell membrane collapse in cpes mutants and leads to the PSE phenotype.

Selected area Cu-catalyzed vapor–liquid−solid growth of SiGe microwires is achieved using chlorosilane and chlorogermane precursors. The composition can be tuned up to 12% Ge with a simultaneous decrease in the growth rate from 7 to 1 μm min–1. Significant changes to the morphology were observed, including tapering and faceting on the sidewalls and along the lengths of the wires. Characterization of axial and radial cross sections with transmission electron microscopy revealed no evidence of defects at facet corners and edges, and the tapering is shown to be due to in situ removal of catalyst material during growth. X-ray diffraction and transmission electron microscopy reveal a Ge-rich crystal at the tip of the wires, strongly suggesting that the Ge incorporation is limited by the crystallization rate.

Many animals maintain an internal representation of their heading as they move through their surroundings. Such a compass representation was recently discovered in a neural population in the Drosophila melanogaster central complex, a brain region implicated in spatial navigation. Here, we use two-photon calcium imaging and electrophysiology in head-fixed walking flies to identify a different neural population that conjunctively encodes heading and angular velocity, and is excited selectively by turns in either the clockwise or counterclockwise direction. We show how these mirror-symmetric turn responses combine with the neurons’ connectivity to the compass neurons to create an elegant mechanism for updating the fly’s heading representation when the animal turns in darkness. This mechanism, which employs recurrent loops with an angular shift, bears a resemblance to those proposed in theoretical models for rodent head direction cells. Our results provide a striking example of structure matching function for a broadly relevant computation. DOI: http://dx.doi.org/10.7554/eLife.23496.001

Hordes of tourists flock to Washington, D.C. every spring to see the cherry trees blossom. Once in the city, they must find their way to the Tidal Basin where the Japanese trees grow. Fortunately, a number of visual landmarks can help them to navigate. In 1910, the United States Congress passed The Height of Buildings Act, limiting the elevation of commercial and residential structures in D.C. to 130 feet. Thus, the 555-foot-tall Washington Monument often looms large against the horizon, serving as an anchor point to help set the tourists' sense of direction. Once their heading is set, they can lose sight of the monument behind buildings or groups of tall Scandinavian visitors and still use their internal compass to navigate to the Basin. This compass keeps track of their paces and turns and updates their sense of where they are and where they need to go. Yet while their heading informs their actions, it does not dictate them. Tourists who have been to D.C. in the past can, for example, use remembered views to alter their routes to avoid crowds. On an even finer scale, their leg movements also depend on their current state - they might increase the frequency and length of their strides if hunger pangs compete with their desire to see cherry blossoms, for example. The way in which these disparate cues and motivations influence exploration is a neuroscience mystery across creatures large and small.

Silicon microwire arrays have recently demonstrated their potential for low-cost, high-efficiency photovoltaics and photoelectrochemical fuel generation. A remaining challenge to making this technology commercially viable is scaling up of microwire-array growth. We discuss here a technique for vapor–liquid–solid growth of microwire arrays on the scale of six-inch wafers using a cold-wall radio-frequency heated chemical vapor deposition furnace, enabling fairly uniform growth over large areas with rapid cycle time and improved run-to-run reproducibility. We have also developed a technique to embed these large-area wire arrays in polymer and to peel them intact from the growth substrate, which could enable lightweight, flexible solar cells with efficiencies as high as multicrystalline Si solar cells. We characterize these large-area microwire arrays using scanning electron microscopy and confocal microscopy to assess their structure and fidelity, and we test their energy-conversion properties using a methyl viologen (MV $^{2+/+}$ ) liquid junction contact in a photoelectrochemical cell. Initial photoelectrochemical conversion efficiencies suggest that the material quality of these microwire arrays is similar to smaller (∼1 cm $^2$ ) wire arrays that we have grown in the past, indicating that this technique is a viable way to scale up microwire-array devices.

Gallium phosphide exhibits a short diffusion length relative to its optical absorption length, and is thus a candidate for use in wire array geometries that allow light absorption to be decoupled from minority carrier collection. Herein is reported the photoanodic performance of heteroepitaxially grown gallium phosphide on planar and microwire-array Si substrates. The n-GaP|n-Si heterojunction results in a favorable conduction band alignment for electron collection in the silicon. A conformal electrochemical contact to the outer GaP layer is produced using the ferrocenium/ferrocene (Fc^+/Fc) redox couple in acetonitrile. Photovoltages of ∼750 mV under 1 sun illumination are observed and are attributed to the barrier formed at the (Fc^+/Fc)|n-GaP junction. The short-circuit current densities of the composite microwire-arrays are similar to those observed using single-crystal n-GaP photoelectrodes. Spectral response measurements along with a finite-difference-time-domain optical model indicate that the minority carrier diffusion length in the GaP is ∼80 nm. Solid-state current–voltage measurements show that shunting occurs through thin GaP layers that are present near the base of the microwire-arrays. The results provide guidance for further studies of 3D multi-junction photoelectrochemical cells.

Arrays of B-doped p-Si microwires, diffusion-doped with P to form a radial n+ emitter and subsequently coated with a 1.5-nm-thick discontinuous film of evaporated Pt, were used as photocathodes for H_2 evolution from water. These electrodes yielded thermodynamically based energy-conversion efficiencies >5% under 1 sun solar simulation, despite absorbing less than 50% of the above-band-gap incident photons. Analogous p-Si wire-array electrodes yielded efficiencies

A critical shortcoming of current surface functionalization schemes is their inability to selectively coat patterned substrates at micrometer and nanometer scales. This limitation prevents localized deposition of macromolecules at high densities, thereby restricting the versatility of the surface. A new approach for functionalizing lithographically patterned substrates that eliminates the need for alignment and, thus, is scalable to any dimension is reported. We show, for the first time, that electropolymerization of derivatized phenols can functionalize patterned surfaces with amine, aldehyde, and carboxylic acid groups and demonstrate that these derivatized groups can covalently bind molecular targets, including proteins and DNA. With this approach, electrically conducting and semiconducting materials in any lithographically realizable geometry can be selectively functionalized, allowing for the sequential deposition of a myriad of chemical or biochemical species of interest at high density to a surface with minimal cross-contamination.

A comparison of the transport properties of populations of single-crystal, In2O3 nanowires (NWs) grown by unassisted hot-wall chemical vapour deposition (CVD) versus NWs grown by laser-ablation-assisted chemical vapour deposition (LA-CVD) is presented. For nominally identical growth conditions across the two systems, NWs fabricated at 850??C with laser-ablation had significantly higher average mobilities at the 99.9% confidence level, 53.3 ? 5.8?cm2?V?1?s?1 versus 10.2 ? 1.9?cm2?V?1?s?1. It is also observed that increasing growth temperature decreases mobility for LA-CVD NWs. Transmission electron microscopy studies of CVD-fabricated samples indicate the presence of an amorphous In2O3 region surrounding the single-crystal core. Further, low-temperature measurements verify the presence of ionized impurity scattering in low-mobility CVD-grown NWs.

In this paper a statistically significant study of 1096 individual GaN nanowire (NW) devices is presented. We have correlated the effects of changing growth parameters for hot-wall chemically-vapour-deposited (HW-CVD) NW sf abricated via the vapour–liquid–solid mechanism. We first describe an optical lithographic method for creating Ohmic contacts to NW field effect transistors with both top and bottom electrostatic gates to characterize carrier density and mobility. Multiprobe measurements show that carrier modulation occurs in the channel and is not a contact effect. We then show that NW fabrication runs with nominally identical growth parameters yield similar electrical results across sample populations of >50 devices. By systematically altering th eg rowth parameters we were able to decrease the average carrier concentration for these as-grown GaN NWs ∼10-fold, from 2.29 × 10 20 to 2.45 × 10 19 cm −3 ,a nd successfully elucidate the parameters that exert the strongest influence on wire quality. Furthermore, this study shows that nitrogen vacancies, and not oxygen impurities, are the dominant intrinsic dopant in HW-CVD GaN NWs.

Tandem Ga1-xInxP/Si microwire array solar cells are a route towards a high efficiency, low cost, flexible, wafer-free solar technology. Coupled full-field optical and device physics simulations of a Ga0.51In0.49P/Si wire array tandem are used to predict device performance. A 500 nm thick, highly doped “buffer” layer between the bottom cell and tunnel junction is assumed to harbor a high density of lattice mismatch and heteroepitaxial defects. Under simulated AM1.5G illumination, the device structure explored in this work has a simulated efficiency of 23.84% with realistic top cell SRH lifetimes and surface recombination velocities. The relative insensitivity to surface recombination is likely due to optical generation further away from the free surfaces and interfaces of the device structure. To move towards realizing these device structures, GaP and Ga1-xInxP layers were grown heteroepitaxially with metalorganic chemical vapor deposition on Si microwire array substrates. The layer morphology and crystalline quality have been studied with scanning electron microscopy and transmission electron microscopy, and they provide a baseline for the growth and characterization of a full device stack.

A transparent, flexible contact is developed using Ni nanoparticles and Ag nanowires and demonstrated on free-standing, polymer embedded, Si microwire solar cells. Contact yields of over 99% and a series resistance of 14 Ω cm² are demonstrated.

Many proposed next-generation photovoltaic devices have complicated nano- and micro-structured architectures that are designed to simultaneously optimize carrier collection and light absorption. Characterization of the electrical properties of these highly structured materials can be challenging due to the difficulty of creating electrical contacts, as well as the need to decouple the properties of the contact from that of the semiconductor. Regenerative photoelectrochemistry is a powerful technique to characterize the electrical properties of such systems, providing a conformal liquid contact that can be ohmic or rectifying, depending on the system used. We demonstrate the use of the methyl viologen regenerative electrochemical system to characterize different stages of the fabrication of radial junction Si microwire (SiMW) solar cells. Photoelectrochemical characterization, combined with other more traditional measurements allows evaluation of how the different processing steps affect the device performance, without having to construct a fully integrated device. We describe the operating principle of this technique, and demonstrate that it can be applied to semiconductor materials with complex architectures.

Microwire solar cells have demonstrated promising optical and photovoltaic performance in arrays of single junction Si wires. Seeking higher efficiencies, we have numerically investigated III-V on Si 1−x Ge x architectures as candidates for tandem microwire photovoltaics via optical and electronic transport modeling. Optical modeling indicates that light trapping is an important design criterion. Absorption is more than doubled by the presence of Al 2 O 3 scattering particles around the wires, leading to high overall light collection despite low wire packing fraction. Texturing of the microwire outer surface, which was found to occur experimentally for GaP/Si microwires, is also shown to enhance absorption by over 50% relative to wires with smooth surfaces, allowing for the use of thinner layers. Finally, full optoelectronic simulations of GaAs on Ge structures revealed that current matching is attainable in these structures and that wire device efficiencies can approach those of planar cells.

Silicon microwire arrays have recently demonstrated their potential for low cost, high efficiency photovoltaics. These high aspect ratio, radial junction wire arrays allow for the absorption of nearly all the incident sunlight while enabling efficient carrier extraction in the radial direction. One of the remaining challenges to make this technology commercially viable is scaling up of the microwire array growth. We discuss here a technique we have developed for vapor liquid solid growth of microwire arrays over full six-inch wafers using a cold-wall RF-heated chemical vapor deposition furnace. This geometry allows for fairly uniform growth over large areas, rapid cycle time, and improved run-to-run reproducibility. We have studied these large-area microwire arrays using scanning electron microscopy and confocal microscopy to assess their structural fidelity and uniformity. We have also developed a technique to embed these large-area arrays in polymer and peel them off the substrate, which could enable lightweight, flexible solar cells with efficiencies as high as crystalline Si solar cells. We have tested the energy conversion properties of these microwire array samples grown using a liquid junction contact and a photoelectrochemical cell. Initial efficiencies measured in this way suggest that the material quality of these microwire arrays is similar to earlier small-area wire arrays that we have grown, meaning that this technique is a viable way to scale up microwire array devices.

Crystalline Si wires, grown by the vapor–liquid–solid (VLS) process, have emerged as promising candidate materials for lowcost, thin-film photovoltaics. Here, we demonstrate VLS-grown Si microwires that have suitable electrical properties for high-performance photovoltaic applications, including long minority-carrier diffusion lengths (L_n » 30 µm) and low surface recombination velocities (S « 70 cm·s^(-1)). Single-wire radial p–n junction solar cells were fabricated with amorphous silicon and silicon nitride surface coatings, achieving up to 9.0% apparent photovoltaic efficiency, and exhibiting up to ~600 mV open-circuit voltage with over 80% fill factor. Projective single-wire measurements and optoelectronic simulations suggest that large-area Si wire-array solar cells have the potential to exceed 17% energy-conversion efficiency, offering a promising route toward cost-effective crystalline Si photovoltaics.

Si microwire-array solar cells with Air Mass 1.5 Global conversion efficiencies of up to 7.9% have been fabricated using an active volume of Si equivalent to a 4 μm thick Si wafer. These solar cells exhibited open-circuit voltages of 500 mV, short-circuit current densities (J_(sc)) of up to 24 mA cm^(-2), and fill factors >65% and employed Al_2O_3 dielectric particles that scattered light incident in the space between the wires, a Ag back reflector that prevented the escape of incident illumination from the back surface of the solar cell, and an a-SiN_x:H passivation/anti-reflection layer. Wire-array solar cells without some or all of these design features were also fabricated to demonstrate the importance of the light-trapping elements in achieving a high J_(sc). Scanning photocurrent microscopy images of the microwire-array solar cells revealed that the higher J_(sc) of the most advanced cell design resulted from an increased absorption of light incident in the space between the wires. Spectral response measurements further revealed that solar cells with light-trapping elements exhibited improved red and infrared response, as compared to solar cells without light-trapping elements.

Si wire arrays have recently demonstrated their potential as photovoltaic devices [1–3]. Using these arrays as a base, we consider a next generation, multijunction wire array architecture consisting of Si wire arrays with a conformal GaN x P 1−x−y As y coating. Optical absorption and device physics simulations provide insight into the design of such devices. In particular, the simulations show that much of the solar spectrum can be absorbed as the angle of illumination is varied and that an appropriate choice of coating thickness and composition will lead to current matching conditions and hence provide a realistic path to high efficiencies. We have previously demonstrated high fidelity, high aspect ratio Si wire arrays grown by vapor-liquid-solid techniques, and we have now successfully grown conformal GaP coatings on these wires as a precursor to considering quaternary compound growth. Structural, optical, and electrical characterization of these GaP/Si wire array heterostructures, including x-ray diffraction, Hall measurements, and optical absorption of polymer-embedded wire arrays using an integrating sphere were performed. The GaP epilayers have high structural and electrical quality and the ability to absorb a significant amount of the solar spectrum, making them promising for future multijunction wire array solar cells.

Silicon wire arrays, though attractive materials for use in photovoltaics and as photocathodes for hydrogen generation, have to date exhibited poor performance. Using a copper-catalyzed, vapor-liquid-solid–growth process, SiCl_4 and BCl_3 were used to grow ordered arrays of crystalline p-type silicon (p-Si) microwires on p^+-Si(111) substrates. When these wire arrays were used as photocathodes in contact with an aqueous methyl viologen^(2+/+) electrolyte, energy-conversion efficiencies of up to 3% were observed for monochromatic 808-nanometer light at fluxes comparable to solar illumination, despite an external quantum yield at short circuit of only 0.2. Internal quantum yields were at least 0.7, demonstrating that the measured photocurrents were limited by light absorption in the wire arrays, which filled only 4% of the incident optical plane in our test devices. The inherent performance of these wires thus conceptually allows the development of efficient photovoltaic and photoelectrochemical energy-conversion devices based on a radial junction platform.

Solar cells based on arrays of CVD-grown Si nano- or micro-wires have attracted interest as potentially low-cost alternatives to conventional wafer-based Si photovoltaics [1–6], and single-wire solar cells have been reported with efficiencies of up to 3.4% [7]. We recently presented device physics simulations which predicted efficiencies exceeding 17%, based on experimentally observed diffusion lengths within our wires [8]. However, this model did not take into account the optical properties of a wire array device - in particular the inherently low packing fraction of wires within CVD-grown wire arrays, which might limit their ability to fully absorb incident sunlight. For this reason, we have combined a device physics model of Si wire solar cells with FDTD simulations of light absorption within wire arrays to investigate the potential photovoltaic efficiency of this cell geometry. We have found that even a sparsely packed array (14%) is expected to absorb moderate (66%) amounts of above-bandgap solar energy, yielding a simulated photovoltaic efficiency of 14.5%. Because the wire array comprises such a small volume of Si, the observed absorption represents an effective optical concentration, which enables greater operating voltages than previously predicted for Si wire array solar cells.

Silicon Microwires as Photocathodes Solar hydrogen generation will require the development of photocathodes with high surface area, durability, and efficiency. Silicon microwire arrays, which allow for greater light penetration, could achieve this goal if the carrier mobilities are sufficiently high so that surface reactions occur before charges recombine. Boettcher et al. (p. 185 ) report the electronic properties on positively doped silicon microwire arrays that were grown with copper catalysts and used in a methyl viologen redox system. Although equivalent efficiencies for normal solar fluxes were only 2 to 3%, the high internal efficiencies and low use of the available optical flux suggest that further improvements are possible.

The use of silicon nanostructures in solar cells offers a number of benefits, such as the fact they can be used on flexible substrates. A silicon wire-array structure, containing reflecting nanoparticles for enhanced absorption, is now shown to achieve 96% peak absorption efficiency, capturing 85% of light with only 1% of the silicon used in comparable commercial cells. Si wire arrays are a promising architecture for solar-energy-harvesting applications, and may offer a mechanically flexible alternative to Si wafers for photovoltaics1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17. To achieve competitive conversion efficiencies, the wires must absorb sunlight over a broad range of wavelengths and incidence angles, despite occupying only a modest fraction of the array’s volume. Here, we show that arrays having less than 5% areal fraction of wires can achieve up to 96% peak absorption, and that they can absorb up to 85% of day-integrated, above-bandgap direct sunlight. In fact, these arrays show enhanced near-infrared absorption, which allows their overall sunlight absorption to exceed the ray-optics light-trapping absorption limit18 for an equivalent volume of randomly textured planar Si, over a broad range of incidence angles. We furthermore demonstrate that the light absorbed by Si wire arrays can be collected with a peak external quantum efficiency of 0.89, and that they show broadband, near-unity internal quantum efficiency for carrier collection through a radial semiconductor/liquid junction at the surface of each wire. The observed absorption enhancement and collection efficiency enable a cell geometry that not only uses 1/100th the material of traditional wafer-based devices, but also may offer increased photovoltaic efficiency owing to an effective optical concentration of up to 20 times.

Solar cells based on arrays of CVD-grown Si nano- or micro-wires are being considered as a potentially low-cost route to implementing a vertical multijunction cell design via radial p-n junctions. This geometry has been predicted to enable efficiencies competitive with planar multicrystalline Si designs, while reducing the materials and processing costs of solar cell fabrication [1]. To further assess the potential efficiency of cells based on this design, we present here experimental measurements of minority carrier diffusion lengths and surface recombination rates within nanowires via fabrication and characterization of single-wire solar cell devices. Furthermore, we consider a potential Si wire array-based solar cell design, and present device physics modeling of single-wire photovoltaic efficiency. Based on experimentally observed diffusion lengths within our wires, we model a radial junction wire solar cell capable of 17% photovoltaic energy conversion efficiency.

Single-nanowire solar cells were created by forming rectifying junctions in electrically contacted vapor-liquid-solid-grown Si nanowires. The nanowires had diameters in the range of 200 nm to 1.5 microm. Dark and light current-voltage measurements were made under simulated Air Mass 1.5 global illumination. Photovoltaic spectral response measurements were also performed. Scanning photocurrent microscopy indicated that the Si nanowire devices had minority carrier diffusion lengths of approximately 2 microm. Assuming bulk-dominated recombination, this value corresponds to a minimum carrier lifetime of approximately 15 ns, or assuming surface-dominated recombination, to a maximum surface recombination velocity of approximately 1350 cm s(-1). The methods described herein comprise a valuable platform for measuring the properties of semiconductor nanowires, and are expected to be instrumental when designing an efficient macroscopic solar cell based on arrays of such nanostructures.

Semiconducting nanowires have the potential to act as highly sensitive sensors for the detection of pathogenic microorganisms, without the need for a label on the pathogen. Practical miniature sensors would have applications in diagnostics, homeland security and basic research. Current technologies have not been widely adopted for various reasons, including the difficulty of integrating nanoscale devices into practical sensors. Now a team spanning five departments at Yale has developed a new approach to the problem. In a state-of-the-art (CMOS-compatible) system they create miniature, ultra-sensitive sensors that can detect specific unlabelled antibodies at concentrations below 100 femtomolar and are able to monitor the cellular immune response in 'real-time'. A new approach that uses complementary metal oxide semiconductor field effect transistor compatible technology is reported, and demonstrates the specific label-free detection of below 100 femtomolar concentrations of antibodies as well as real-time monitoring of the cellular immune response. Semiconducting nanowires have the potential to function as highly sensitive and selective sensors for the label-free detection of low concentrations of pathogenic microorganisms1,2,3,4,5,6,7,8,9,10. Successful solution-phase nanowire sensing has been demonstrated for ions3, small molecules4, proteins5,6, DNA7 and viruses8; however, ‘bottom-up’ nanowires (or similarly configured carbon nanotubes11) used for these demonstrations require hybrid fabrication schemes12,13, which result in severe integration issues that have hindered widespread application. Alternative ‘top-down’ fabrication methods of nanowire-like devices9,10,14,15,16,17 produce disappointing performance because of process-induced material and device degradation. Here we report an approach that uses complementary metal oxide semiconductor (CMOS) field effect transistor compatible technology and hence demonstrate the specific label-free detection of below 100 femtomolar concentrations of antibodies as well as real-time monitoring of the cellular immune response. This approach eliminates the need for hybrid methods and enables system-scale integration of these sensors with signal processing and information systems. Additionally, the ability to monitor antibody binding and sense the cellular immune response in real time with readily available technology should facilitate widespread diagnostic applications.

We report conformal, epitaxial growth of GaP layers on arrays of Si microwires. Silicon wires grown using chlorosilane chemical vapor deposition were coated with GaP grown by metal-organic chemical vapor deposition. The crystalline quality of conformal, epitaxial GaP/Si wire arrays was assessed by transmission electron microscopy and x-ray diffraction. Hall measurements and photoluminescence show p- and n-type doping with high electron mobility and bright optical emission. GaP pn homojunction diodes on planar reference samples show photovoltaic response with an open circuit voltage of 660 mV.

The effective electron minority-carrier diffusion length, L_(n,eff), for 2.0 µm diameter Si wires that were synthesized by Cu-catalyzed vapor-liquid-solid growth was measured by scanning photocurrent microscopy. In dark, ambient conditions, L_(n,eff) was limited by surface recombination to a value of ≤ 0.7 µm. However, a value of L_(n,eff) = 10.5±1 µm was measured under broad-area illumination in low-level injection. The relatively long minority-carrier diffusion length observed under illumination is consistent with an increased surface passivation resulting from filling of the surface states of the Si wires by photogenerated carriers. These relatively large L_(n,eff) values have important implications for the design of high-efficiency, radial-junction photovoltaic cells from arrays of Si wires synthesized by metal-catalyzed growth processes.

A comparison of methods to create Ohmic contacts to semiconductor nanowires (NWs) and carbon nanotubes (CNTs) is presented. A Ni∕Au lift-off metallization was used to contact GaN and In2O3 NWs and CNTs using electron-beam (e-beam) or optical lithography. In order to render the metal-semiconductor contacts Ohmic, e-beam-processed devices are found to require a postfabrication, high-temperature anneal, whereas the use of an oxygen plasma prior to metallization is found to be crucial for devices defined by optical lithography.

Indium nitride nanowires (NWs) grown by a catalyst-free, vapor-solid method are shown to be high-purity, single-crystal hexagonal wurtzite and intrinsic n type with uniform diameters that range from 70to150nm and lengths that vary between 3 and 30μm. Single NWs were fabricated into field-effect transistors and the electronic material parameters of the wires were extracted and are found to be identical to comparable bulk InN.