Guided Growth of Horizontal GaN Nanowires on Spinel with Orientation-Controlled Morphologies

We report the guided growth of horizontal GaN nanowires (NWs) on spinel (MgAl₂O₄) substrates with three different orientations: (111), (100), and (110). The NWs form ordered arrays with distinct morphologies on the surface of the substrates, controlled by the interaction with the substrate. The geometry of the NWs matches the symmetry of the spinel surfaces: on MgAl₂O₄(111), (100), and (110) the NWs grow in six, four, and two directions, respectively. The epitaxial relations and morphologies of the NW–substrate interface were characterized by cross-sectional transmission electron microscopy. The substrate was found to be mobilized during the growth and either climb up or recede on/under one or two sides of the NW, depending on the substrate orientation. Possible reasons for the similarity and differences between the orientations of the NWs and thin GaN films grown on MgAl₂O₄ are proposed. These results demonstrate the generality and flexibility of the guided growth phenomenon in NWs and specifically show that MgAl₂O₄(111) could be a low-mismatch substrate for the growth of high-quality GaN layers and NWs

Field-Effect Transistors Based on WS₂ Nanotubes with High Current-Carrying Capacity

We report the first transistor based on inorganic nanotubes exhibiting mobility values of up to 50 cm² V^–1 s^–1 for an individual WS₂ nanotube. The current-carrying capacity of these nanotubes was surprisingly high with respect to other low-dimensional materials, with current density at least 2.4 × 10⁸ A cm^–2. These results demonstrate that inorganic nanotubes are promising building blocks for high-performance electronic applications

Bottom-Up Tri-gate Transistors and Submicrosecond Photodetectors from Guided CdS Nanowalls

Tri-gate transistors offer better performance than planar transistors by exerting additional gate control over a channel from two lateral sides of semiconductor nanowalls (or “fins”). Here we report the bottom-up assembly of aligned CdS nanowalls by a simultaneous combination of horizontal catalytic vapor–liquid–solid growth and vertical facet-selective noncatalytic vapor–solid growth and their parallel integration into tri-gate transistors and photodetectors at wafer scale (cm²) without postgrowth transfer or alignment steps. These tri-gate transistors act as enhancement-mode transistors with an on/off current ratio on the order of 10⁸, 4 orders of magnitude higher than the best results ever reported for planar enhancement-mode CdS transistors. The response time of the photodetector is reduced to the submicrosecond level, 1 order of magnitude shorter than the best results ever reported for photodetectors made of bottom-up semiconductor nanostructures. Guided semiconductor nanowalls open new opportunities for high-performance 3D nanodevices assembled from the bottom up

Guided CdSe Nanowires Parallelly Integrated into Fast Visible-Range Photodetectors

One-dimensional semiconductor nanostructures, such as nanowires (NWs), have attracted tremendous attention due to their unique properties and potential applications in nanoelectronics, nano-optoelectronics, and sensors. One of the challenges toward their integration into practical devices is their large-scale controlled assembly. Here, we report the guided growth of horizontal CdSe nanowires on five different planes of sapphire. The growth direction and crystallographic orientation are controlled by the epitaxial relationship with the substrate as well as by a graphoepitaxial effect of surface nanosteps and grooves. CdSe is a promising direct-bandgap II–VI semiconductor active in the visible range, with potential applications in optoelectronics. The guided CdSe nanowires were found to have a wurtzite single-crystal structure. Field-effect transistors and photodetectors were fabricated to examine the nanowire electronic and optoelectronic properties, respectively. The latter exhibited the fastest rise and fall times ever reported for CdSe nanostructures as well as a relatively high gain, both features being essential for optoelectronic applications

Surface-Guided CsPbBr₃ Perovskite Nanowires on Flat and Faceted Sapphire with Size-Dependent Photoluminescence and Fast Photoconductive Response

All-inorganic lead halide perovskite nanowires have been the focus of increasing interest since they exhibit improved stability compared to their hybrid organic–inorganic counterparts, while retaining their interesting optical and optoelectronic properties. Arrays of surface-guided nanowires with controlled orientations and morphology are promising as building blocks for various applications and for systematic research. We report the horizontal and aligned growth of CsPbBr₃ nanowires with a uniform crystallographic orientation on flat and faceted sapphire surfaces to form arrays with 6-fold and 2-fold symmetries, respectively, along specific directions of the sapphire substrate. We observed waveguiding behavior and diameter-dependent photoluminescence emission well beyond the quantum confinement regime. The arrays were easily integrated into multiple devices, displaying p-type behavior and photoconductivity. Photodetectors based on those nanowires exhibit the fastest rise and decay times for any CsPbBr₃-based photodetectors reported so far. One-dimensional arrays of halide perovskite nanowires are a promising platform for investigating the intriguing properties and potential applications of these unique materials

Guided Growth of Horizontal GaN Nanowires on Quartz and Their Transfer to Other Substrates

The guided growth of horizontal nanowires has so far been demonstrated on a limited number of substrates. In most cases, the nanowires are covalently bonded to the substrate where they grow and cannot be transferred to other substrates. Here we demonstrate the guided growth of well-aligned horizontal GaN nanowires on quartz and their subsequent transfer to silicon wafers by selective etching of the quartz while maintaining their alignment. The guided growth was observed on different planes of quartz with varying degrees of alignment. We characterized the crystallographic orientations of the nanowires and proposed a new mechanism of “dynamic graphoepitaxy” for their guided growth on quartz. The transfer of the guided nanowires enabled the fabrication of back-gated field-effect transistors from aligned nanowire arrays on oxidized silicon wafers and the production of crossbar arrays. The guided growth of transferrable nanowires opens up the possibility of massively parallel integration of nanowires into functional systems on virtually any desired substrate

Surface-Guided Core–Shell ZnSe@ZnTe Nanowires as Radial p–n Heterojunctions with Photovoltaic Behavior

The organization of nanowires on surfaces remains a major obstacle toward their large-scale integration into functional devices. Surface–material interactions have been used, with different materials and substrates, to guide horizontal nanowires during their growth into well-organized assemblies, but the only guided nanowire heterostructures reported so far are axial and not radial. Here, we demonstrate the guided growth of horizontal core–shell nanowires, specifically of ZnSe@ZnTe, with control over their crystal phase and crystallographic orientations. We exploit the directional control of the guided growth for the parallel production of multiple radial p–n heterojunctions and probe their optoelectronic properties. The devices exhibit a rectifying behavior with photovoltaic characteristics upon illumination. Guided nanowire heterostructures enable the bottom-up assembly of complex semiconductor structures with controlled electronic and optoelectronic properties

Photoconductive CdSe Nanowire Arrays, Serpentines, and Loops Formed by Electrodeposition on Self-Organized Carbon Nanotubes

Semiconducting nanowires frequently have enhanced properties and unique functionality compared to their bulk counterparts. Controlling the geometry of nanowires is crucial for their integration into nanoscale devices because the shape of a device component can dictate its functionality, such as in the case of a mechanical spring or an antenna. We demonstrate a novel synthetic method for making polycrystalline CdSe nanowires with controlled geometries by using self-organized single-walled carbon nanotubes as a template for the selective electrodeposition of nanowires. Nanowires of up to hundreds of micrometers in length are formed as high-density straight arrays, as well as in the shape of serpentines and loops. These nanowires exhibit significant photoluminescence and photoconductivity applicable to photodetectors and respond to illumination up to 2 orders of magnitude faster than single crystalline CdSe

BCN Nanotubes as Highly Sensitive Torsional Electromechanical Transducers

Owing to their mechanically tunable electronic properties, carbon nanotubes (CNTs) have been widely studied as potential components for nanoelectromechanical systems (NEMS); however, the mechanical properties of multiwall CNTs are often limited by the weak shear interactions between the graphitic layers. Boron nitride nanotubes (BNNTs) exhibit a strong interlayer mechanical coupling, but their high electrical resistance limits their use as electromechanical transducers. Can the outstanding mechanical properties of BNNTs be combined with the electromechanical properties of CNTs in one hybrid structure? Here, we report the first experimental study of boron carbonitride nanotube (BCNNT) mechanics and electromechanics. We found that the hybrid BCNNTs are up to five times torsionally stiffer and stronger than CNTs, thereby retaining to a large extent the ultrahigh torsional stiffness of BNNTs. At the same time, we show that the electrical response of BCNNTs to torsion is 1 to 2 orders of magnitude higher than that of CNTs. These results demonstrate that BCNNTs could be especially attractive building blocks for NEMS

Crystallographic Mapping of Guided Nanowires by Second Harmonic Generation Polarimetry

The growth of horizontal nanowires (NWs) guided by epitaxial and graphoepitaxial relations with the substrate is becoming increasingly attractive owing to the possibility of controlling their position, direction, and crystallographic orientation. In guided NWs, as opposed to the extensively characterized vertically grown NWs, there is an increasing need for understanding the relation between structure and properties, specifically the role of the epitaxial relation with the substrate. Furthermore, the uniformity of crystallographic orientation along guided NWs and over the substrate has yet to be checked. Here we perform highly sensitive second harmonic generation (SHG) polarimetry of polar and nonpolar guided ZnO NWs grown on <i>R</i>-plane and <i>M</i>-plane sapphire. We optically map large areas on the substrate in a nondestructive way and find that the crystallographic orientations of the guided NWs are highly selective and specific for each growth direction with respect to the substrate lattice. In addition, we perform SHG polarimetry along individual NWs and find that the crystallographic orientation is preserved along the NW in both polar and nonpolar NWs. While polar NWs show highly uniform SHG along their axis, nonpolar NWs show a significant change in the local nonlinear susceptibility along a few micrometers, reflected in a reduction of 40% in the ratio of the SHG along different crystal axes. We suggest that these differences may be related to strain accumulation along the nonpolar wires. We find SHG polarimetry to be a powerful tool to study both selectivity and uniformity of crystallographic orientations of guided NWs with different epitaxial relations