Ho-Yuen Cheung

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.

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.

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.

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).

Crystalline GaSb Nanowires Synthesized on Amorphous Substrates: From the Formation Mechanism to p-Channel Transistor Applications

In recent years, because of the narrow direct bandgap and outstanding carrier mobility, GaSb nanowires (NWs) have been extensively explored for various electronics and optoelectronics. Importantly, these p-channel nanowires can be potentially integrated with n-type InSb, InAs, or InGaAs NW devices via different NW transfer techniques to facilitate the III-V CMOS technology. However, until now, there have been very few works focusing on the electronic transport properties of GaSb NWs. Here, we successfully demonstrate the synthesis of crystalline, stoichiometric, and dense GaSb NWs on amorphous substrates, instead of the commonly used III-V crystalline substrates, InAs, or GaAs NW stems as others reported. The obtained NWs are found to grow via the VLS mechanism with a narrow distribution of diameter (220 ± 50 nm) uniformly along the entire NW length (>10 μm) with minimal tapering and surface coating. Notably, when configured into FETs, the NWs exhibit respectable electrical characteristics with the peak hole mobility of ~30 cm(2) V(-1) s(-1) and free hole concentration of ~9.7 × 10(17) cm(-3). All these have illustrated the promising potency of such NWs directly grown on amorphous substrates for various technological applications, as compared with the conventional MOCVD-grown GaSb NWs.

Polymer-confined colloidal monolayer: a reusable soft photomask for rapid wafer-scale nanopatterning.

We demonstrate the repeated utilization of self-assembled colloidal spheres for rapid nanopattern generations. Highly ordered micro-/nanosphere arrays were interlinked and confined by a soft transparent polymer (polydimethylsiloxane, PDMS), which can be used as light-focusing elements/photomasks for area-selective exposures of photoresist in contact. Because of the stiffness of the colloidal spheres, the photomasks do not encounter feature-deformation problems, enabling reliable production of highly uniform patterns over large areas. The geometrical feature of the patterns, including the size, pitch, and even the shape, can be finely tuned by adjusting the mask design and exposure time. The obtained patterns could be used as deposition or etching mask, allowing easy pattern transfer for various applications.

Hierarchical silicon nanostructured arrays via metal-assisted chemical etching

Hierarchically arranged nanostructures, configured in both nanopillars and nanoholes, have been fabricated via a low-cost approach that combines metal-assisted chemical etching (MaCE), nanosphere lithography and conventional photolithography. By manipulating the catalyst morphology as well as the deposition method, different interesting nanostructures like nanowalls and nanograsses were fabricated at the galleries among the nanopillar blocks. Using a similar strategy, hierarchical negative structures (nanoholes) have also been successfully demonstrated. The successful construction of these diversified hierarchical nanostructures illustrates that MaCE could be employed as a feasible, low-cost method for multi-scale silicon micro/nano machining, which is highly desirable for widespread applications, including tissue engineering, optoelectronics, photonic devices and lab-on-chip systems.

Enhanced Self-Assembly of Crystalline, Large-Area, and Periodicity-Tunable TiO2 Nanotube Arrays on Various Substrates

Due to their superior physical properties, titanium dioxide (TiO2) nanotube arrays are one of the most investigated nanostructure systems in materials science until now. However, it is still a great challenge to achieve damage-free techniques to realize controllable, cost-effective, and high-performance TiO2 nanotube arrays on both rigid and flexible substrates for different technological applications. In this work, we demonstrate a unique strategy to achieve self-assemble crystalline, large-area, and regular TiO2 nanotube arrays on various substrates via hybrid combination of conventional semiconductor processes. Besides the usual applications of TiO2 as carrier transport layers in thin-film electronic devices, we demonstrate that the periodic TiO2 nanotube arrays can show the effect of optical grating with large-area uniformity. Specifically, the fabricated nanotube geometries, such as the tube height, pitch, diameter, and wall thickness, as well as the crystallinity can be reliably controlled by varying the processing conditions. More importantly, utilizing these nanotube arrays in perovskite solar cells can further enhance the optical absorption, leading to improved power conversion efficiency. In contrast to other typical template-assisted fabrication approaches, we employ a soft template here, which would enable the construction of nanotube arrays without any significant damage associated with template removal. Furthermore, without the thermal restriction of underlying substrates, these crystalline nanotube arrays can be transferred to mechanically flexible substrates by a simple one-step method, which can expedite these nanotubes for potential utilization in other application domains.