Although various device structures based on GaSb nanowires have been realized, further performance enhancement suffers from uncontrolled radial growth during the nanowire synthesis, resulting in non-uniform and tapered nanowires with diameters larger than few tens of nanometres. Here we report the use of sulfur surfactant in chemical vapour deposition to achieve very thin and uniform GaSb nanowires with diameters down to 20 nm. In contrast to surfactant effects typically employed in the liquid phase and thin-film technologies, the sulfur atoms contribute to form stable S-Sb bonds on the as-grown nanowire surface, effectively stabilizing sidewalls and minimizing unintentional radial nanowire growth. When configured into transistors, these devices exhibit impressive electrical properties with the peak hole mobility of B200 cm 2 V À 1 s À 1 , better than any mobility value reported for a GaSb nanowire device to date. These factors indicate the effectiveness of this surfactant-assisted growth for high-performance small-diameter GaSb nanowires.

In recent years, high-mobility GaSb nanowires have received tremendous attention for high-performance p-type transistors; however, due to the difficulty in achieving thin and uniform nanowires (NWs), there is limited report until now addressing their diameter-dependent properties and their hole mobility limit in this important one-dimensional material system, where all these are essential information for the deployment of GaSb NWs in various applications. Here, by employing the newly developed surfactant-assisted chemical vapor deposition, high-quality and uniform GaSb NWs with controllable diameters, spanning from 16 to 70 nm, are successfully prepared, enabling the direct assessment of their growth orientation and hole mobility as a function of diameter while elucidating the role of sulfur surfactant and the interplay between surface and interface energies of NWs on their electrical properties. The sulfur passivation is found to efficiently stabilize the high-energy NW sidewalls of (111) and (311) in order to yield the thin NWs (i.e., <40 nm in diameters) with the dominant growth orientations of ⟨211⟩ and ⟨110⟩, whereas the thick NWs (i.e., >40 nm in diameters) would grow along the most energy-favorable close-packed planes with the orientation of ⟨111⟩, supported by the approximate atomic models. Importantly, through the reliable control of sulfur passivation, growth orientation and surface roughness, GaSb NWs with the peak hole mobility of ∼400 cm(2)V s(-1) for the diameter of 48 nm, approaching the theoretical limit under the hole concentration of ∼2.2 × 10(18) cm(-3), can be achieved for the first time. All these indicate their promising potency for utilizations in different technological domains.

Reliable control in the crystal quality of synthesized III-V nanowires (NWs) is particularly important to manipulate their corresponding electronic transport properties for technological applications. In this report, a "two-step" growth process is adopted to achieve single-crystalline GaAs NWs, where an initial high-temperature nucleation process is employed to ensure the formation of high Ga supersaturated Au7Ga3 and Au2Ga alloy seeds, instead of the low Ga supersaturated Au7Ga2 seeds observed in the conventional "single-step" growth. These two-step NWs are long (>60 μm) and thick (>80 nm) with the minimal defect concentrations and uniform growth orientations. Importantly, these NWs exhibit p-type conductivity as compared to the single-step grown n-type NWs for the same diameter range. This NW conductivity difference (p- versus n-channel) is shown to originate from the donor-like crystal defects, such as As precipitates, induced by the low Ga supersaturated multicrystalline Au7Ga2 alloy seeds. Then the well-controlled crystal quality for desired electronic properties is further explored in the application of large-scale p-type GaAs NW parallel array FETs as well as the integration of both p- and n-type GaAs NWs into CMOS inverters. All these illustrate the successful control of NW crystal defects and corresponding electronic transport properties via the manipulation of Ga supersaturation in the catalytic alloy tips with different preparation methods. The understanding of this relationship between NW crystal quality and electronic transport properties is critical and preferential to the future development of nanoelectronic materials, circuit design, and fabrication.

Using CMOS-compatible Pd catalysts, we demonstrated the formation of high-mobility ⟨111⟩-oriented GaSb nanowires (NWs) via vapor-solid-solid (VSS) growth by surfactant-assisted chemical vapor deposition through a complementary experimental and theoretical approach. In contrast to NWs formed by the conventional vapor-liquid-solid (VLS) mechanism, cylindrical-shaped PdGa catalytic seeds were present in our Pd-catalyzed VSS-NWs. As solid catalysts, stoichiometric PdGa was found to have the lowest crystal surface energy and thus giving rise to a minimal surface diffusion as well as an optimal in-plane interface orientation at the seed/NW interface for efficient epitaxial NW nucleation. These VSS characteristics led to the growth of slender NWs with diameters down to 26.9 ± 3.5 nm. Over 95% high crystalline quality NWs were grown in ⟨111⟩ orientation for a wide diameter range of between 10 and 70 nm. Back-gated field-effect transistors (FETs) fabricated using the Pd-catalyzed GaSb NWs exhibit a superior peak hole mobility of ∼330 cm V s, close to the mobility limit for a NW channel diameter of ∼30 nm with a free carrier concentration of ∼10 cm. This suggests that the NWs have excellent homogeneity in phase purity, growth orientation, surface morphology and electrical characteristics. Contact printing process was also used to fabricate large-scale assembly of Pd-catalyzed GaSb NW parallel arrays, confirming the potential constructions and applications of these high-performance electronic devices.

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Rutile, anatase, and brookite TiO 2 nanorods (TNs) with specific exposed crystal facets have been obtained by a hydrothermal method using peroxide titanic acid (PTA) solution of different pH values. Below pH PTA 10, the condensing complex is uncharged Ti 2 O 5 (OH) 2 (H 2 O) 4 or cations Ti 2 O 5 (H 2 O) 6 2+ , and the corner-shared bonding easily occurs to form rutile phase. Above pH PTA 10, the condensing species is anions Ti 2 O 5 (OH) 4 (H 2 O) 2 2−, resulting in the larger probability of edge-shared bonding which favor the formation of anatase TiO 2 . Brookite whose structure contains both corner-shared and edge-shared bonding can be formed when pH PTA equals to 10. Rutile TN synthesized from PTA solution at pH 4 showed the optimal photocatalytic oxidation activity for MB degradation; Brookite TN exhibited the highest photocatalytic activity for reduction of Cr (VI) despite that it does not exhibit the largest surface area. The presence of a spatial separation of redox sites in faceted rutile TiO 2 nanorod determined the photocatalytic activity of MB decomposition. Compared with rutile and anatase phase, the optimal photocatalytic reduction activity of brookite TN might be resulting from a cathodic shift of the conduction band.

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