SenPo Yip

Anomalous and Highly Efficient InAs Nanowire Phototransistors Based on Majority Carrier Transport at Room Temperature

Core/shell-like n-type InAs nanowire phototransistors based on majority-carrier-dominated photodetection are investigated. Under optical illumination, electrons generated from the core are excited into the self-assembled near-surface photogating layer, forming a built-in electric field to significantly regulate the core conductance. Anomalous high photoconductive gain and fast response time are obtained at room temperature.

Insight into the electrochemical activation of carbon-based cathodes for hydrogen evolution reaction

Recently, carbon nanomaterials with outstanding electrocatalytic performance for the hydrogen evolution reaction (HER) after electrochemical activation have been reported; however, the exact activation mechanism is still under extensive debate. In this study, to better understand the activation, graphite rods and carbon nanohorns, two typical carbon materials in different scales, were electrochemically activated and their catalytic performances in HER were systematically studied, which showed that the HER performance was greatly affected by the counter electrode employed for the activation. An efficient activation was achieved when a platinum wire was used as the counter electrode; simultaneously, Pt transfer from the anode to the cathode was also observed. These results suggest that the improved HER performance was mainly caused by the Pt transfer, rather than the activation of the carbon materials themselves. More importantly, our study implied that the Pt dissolution, although widely ignored, should be taken into consideration during electrochemical tests when Pt metal is utilized as the counter electrode.

Synthesis and characterizations of ternary InGaAs nanowires by a two-step growth method for high-performance electronic devices.

InAs nanowires have been extensively studied for high-speed and high-frequency electronics due to the low effective electron mass and corresponding high carrier mobility. However, further applications still suffer from the significant leakage current in InAs nanowire devices arising from the small electronic band gap. Here, we demonstrate the successful synthesis of ternary InGaAs nanowires in order to tackle this leakage issue utilizing the larger band gap material but at the same time not sacrificing the high electron mobility. In this work, we adapt a two-step growth method on amorphous SiO(2)/Si substrates which significantly reduces the kinked morphology and surface coating along the nanowires. The grown nanowires exhibit excellent crystallinity and uniform stoichiometric composition along the entire length of the nanowires. More importantly, the electrical properties of those nanowires are found to be remarkably impressive with I(ON)/I(OFF) ratio >10(5), field-effect mobility of ∼2700 cm(2)/(V·s), and ON current density of ∼0.9 mA/μm. These nanowires are then employed in the contact printing and achieve large-scale assembly of nanowire parallel arrays which further illustrate the potential for utilizing these high-performance nanowires on substrates for the fabrication of future integrated circuits.

Rational Design of Inverted Nanopencil Arrays for Cost-effective, Broadband and Omnidirectional Light Harvesting

Due to the unique optical properties, three-dimensional arrays of silicon nanostructures have attracted increasing attention as the efficient photon harvesters for various technological applications. In this work, instead of dry etching, we have utilized our newly developed wet anisotropic etching to fabricate silicon nanostructured arrays with different well-controlled geometrical morphologies, ranging from nanopillars, nanorods, and inverted nanopencils to nanocones, followed by systematic investigations of their photon-capturing properties combining experiments and simulations. It is revealed that optical properties of these nanoarrays are predominantly dictated by their geometrical factors including the structural pitch, material filling ratio, and aspect ratio. Surprisingly, along with the proper geometrical design, the inverted nanopencil arrays can couple incident photons into optical modes in the pencil base efficiently in order to achieve excellent broadband and omnidirectional light-harvesting performances even with the substrate thickness down to 10 μm, which are comparable to the costly and technically difficult to achieve nanocone counterparts. Notably, the fabricated nanopencils with both 800 and 380 nm base diameters can suppress the optical reflection well below 5% over a broad wavelength of 400-1000 nm and a wide angle of incidence between 0 and 60°. All these findings not only offer additional insight into the light-trapping mechanism in these complex 3D nanophotonic structures but also provide efficient broadband and omnidirectional photon harvesters for next-generation cost-effective ultrathin nanostructured photovoltaics.

Controllable p-n switching behaviors of GaAs nanowires via an interface effect.

Due to the extraordinary large surface-to-volume ratio, surface effects on semiconductor nanowires have been extensively investigated in recent years for various technological applications. Here, we present a facile interface trapping approach to alter electronic transport properties of GaAs nanowires as a function of diameter utilizing the acceptor-like defect states located between the intrinsic nanowire and its amorphous native oxide shell. Using a nanowire field-effect transistor (FET) device structure, p- to n-channel switching behaviors have been achieved with increasing NW diameters. Interestingly, this oxide interface is shown to induce a space-charge layer penetrating deep into the thin nanowire to deplete all electrons, leading to inversion and thus p-type conduction as compared to the thick and intrinsically n-type GaAs NWs. More generally, all of these might also be applicable to other nanowire material systems with similar interface trapping effects; therefore, careful device design considerations are required for achieving the optimal nanowire device performances.

Surfactant-assisted chemical vapour deposition of high-performance small-diameter GaSb nanowires

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 ~200 cm(2 )V(-1 )s(-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.

Tailoring electromagnetically induced transparency for terahertz metamaterials: From diatomic to triatomic structural molecules

The coupling effects in electromagnetically induced transparency (EIT) for triatomic metamaterials are investigated at terahertz (THz) frequencies both experimentally and theoretically. We observed enhancement and cancellation of EIT with single transparency window, and also two additional ways to achieve double EIT transparency windows. One is from the hybridization between two dark atoms in a bright-dark-dark configuration. Another is from an averaged effect between absorption of the additional bright atom and the EIT from the original diatomic molecule in a bright-bright-dark configuration. It allows us to control EIT and the associated slow-light effect for THz metamaterials with high accuracy.

Surface roughness induced electron mobility degradation in InAs nanowires

In this work, we present a study of the surface roughness dependent electron mobility in InAs nanowires grown by the nickel-catalyzed chemical vapor deposition method. These nanowires have good crystallinity, well-controlled surface morphology without any surface coating or tapering and an excellent peak field-effect mobility up to 15,000 cm(2) V(-1) s(-1) when configured into back-gated field-effect nanowire transistors. Detailed electrical characterizations reveal that the electron mobility degrades monotonically with increasing surface roughness and diameter scaling, while low-temperature measurements further decouple the effects of surface/interface traps and phonon scattering, highlighting the dominant impact of surface roughness scattering on the electron mobility for miniaturized and surface disordered nanowires. All these factors suggest that careful consideration of nanowire geometries and surface condition is required for designing devices with optimal performance.

GaAs nanowire Schottky barrier photovoltaics utilizing Au–Ga alloy catalytic tips

Single GaAs nanowire photovoltaic devices were fabricated utilizing rectifying junctions in the Au–Ga catalytic tip/nanowire contact interface. Current-voltage measurements were performed under simulated Air Mass 1.5 global illumination with the best performance delivering an overall energy conversion efficiency of ∼2.8% for a nanowire of 70 nm in diameter. As compared with metal contacts directly deposited on top of the nanowire, this nanoscale contact is found to alleviate the well-known Fermi-level pinning to achieve effective formation of Schottky barrier responsible for the superior photovoltaic response. All these illustrate the potency of these versatile nanoscale contact configurations for future technological device applications.

Developing controllable anisotropic wet etching to achieve silicon nanorods, nanopencils and nanocones for efficient photon trapping

Controllable hierarchy of highly regular, single-crystalline nanorod, nanopencil and nanocone arrays with tunable geometry and etch anisotropy has been achieved over large areas (>1.5 cm × 1.5 cm) by using an [AgNO3 + HF + HNO3/H2O2] etching system. The etching mechanism has been elucidated to originate from the site-selective deposition of Ag nanoclusters. Different etch anisotropies and aspect ratios can be accomplished by modulating the relative concentration in the [AgNO3 + HF + HNO3/H2O2] etching system. Minimized optical reflectance is also demonstrated with the fabricated nano-arrays. Overall, this work highlights the technological potency of utilizing a simple wet-chemistry-only fabrication scheme, instead of reactive dry etching, to attain three-dimensional Si nanostructures with different geometrical morphologies for applications requiring large-scale, low-cost and efficient photon trapping (e.g. photovoltaics).