Ruoxue Yan

Semiconductor Nanowire: What’s Next?

In this perspective, we take a critical look at the research progress within the nanowire community for the past decade. We discuss issues on the discovery of fundamentally new phenomena versus performance benchmarking for many of the nanowire applications. We also notice that both the bottom-up and top-down approaches have played important roles in advancing our fundamental understanding of this new class of nanostructures. Finally we attempt to look into the future and offer our personal opinions on what the future trends will be in nanowire research.

Nanowire-based single-cell endoscopy

One-dimensional smart probes based on nanowires and nanotubes that can safely penetrate the plasma membrane and enter biological cells are potentially useful in high-resolution and high-throughput gene and drug delivery, biosensing and single-cell electrophysiology. However, using such probes for optical communication across the cellular membrane at the subwavelength level remains limited. Here, we show that a nanowire waveguide attached to the tapered tip of an optical fibre can guide visible light into intracellular compartments of a living mammalian cell, and can also detect optical signals from subcellular regions with high spatial resolution. Furthermore, we show that through light-activated mechanisms the endoscope can deliver payloads into cells with spatial and temporal specificity. Moreover, insertion of the endoscope into cells and illumination of the guided laser did not induce any significant toxicity in the cells.

Gated proton transport in aligned mesoporous silica films

Modulated proton transport plays significant roles in biological processes such as ATP synthesis as well as in technologically important applications including, for example, hydrogen fuel cells. The state-of-the-art proton-exchange membrane is the sulphonated tetrafluoroethylene copolymer Nafion developed by DuPont in the late 1960s, with a high proton conductivity. However, actively switchable proton conduction, a functional mimic of the ion transport within a cell membrane, has yet to be realized. Herein, we report the electrostatic gating of proton transport within aligned mesoporous silica thin film. It is observed that surface-charge-mediated transport is dominant at low proton concentrations. We have further demonstrated that the proton conduction can be actively modulated by two-fourfold with a gate voltage as low as 1 V. Such artificial gatable ion transport media could have potential applications in nanofluidic chemical processors, biomolecular separation and electrochemical energy conversion.

Direct photonic–plasmonic coupling and routing in single nanowires

Metallic nanoscale structures are capable of supporting surface plasmon polaritons (SPPs), propagating collective electron oscillations with tight spatial confinement at the metal surface. SPPs represent one of the most promising structures to beat the diffraction limit imposed by conventional dielectric optics. Ag nano wires have drawn increasing research attention due to 2D sub-100 nm mode confinement and lower losses as compared with fabricated metal structures. However, rational and versatile integration of Ag nanowires with other active and passive optical components, as well as Ag nanowire based optical routing networks, has yet to be achieved. Here, we demonstrate that SPPs can be excited simply by contacting a silver nanowire with a SnO2 nanoribbon that serves both as an unpolarized light source and a dielectric waveguide. The efficient coupling makes it possible to measure the propagation-distance-dependent waveguide spectra and frequency-dependent propagation length on a single Ag nanowire. Furthermore, we have demonstrated prototypical photonic-plasmonic routing devices, which are essential for incorporating low-loss Ag nanowire waveguides as practical components into high-capacity photonic circuits.

Nanofluidic diodes based on nanotube heterojunctions.

The mechanism of tuning charge transport in electronic devices has recently been implemented into the nanofluidic field for the active control of ion transport in nanoscale channels/pores. Here we report the first synthesis of longitudinal heterostructured SiO(2)/Al(2)O(3) nanotubes. The ionic transport through these nanotube heterojunctions exhibits clear current rectification, a signature of ionic diode behavior. Such nanofluidic diodes could find applications in ion separation and energy conversion.

Self-Organized Ultrathin Oxide Nanocrystals

Sub-2-nm (down to one-unit cell) uniform oxide nanocrystals and highly ordered superstructures were obtained in one step using oleylamine and oleic acid as capping and structure directing agents. The cooperative nature of the nanocrystal growth and assembly resulted in mesoscopic one-dimensional ribbon-like superstructures made of these ultrathin nanocrystals. The process reported here is general and can be readily extended to the production of many other transition metal (TiO2, ZnO, Nb2O5) and rare earth oxide (Eu2O3, Sm2O3, Er2O3, Y2O3, Tb2O3, and Yb2O3) systems.

Synthesis of complex rare earth fluoride nanocrystal phosphors

Complex rare earth fluoride (NaRF(4), R = Ce,Y,Gd) nanocrystals of 30-50 nm were synthesized by hydrothermal and solvothermal techniques. The size, morphology and phase transition of the complex rare earth fluorides are discussed as the effects of different solvents, reaction time and temperature. Hexagonal phase NaRF(4) nanocrystals with good dispersibility can be prepared in the presence of EDTA using ethanol as the solvent. After doping with Yb(3+), Er(3+); Yb(3+), Tm(3+), and Eu(3+), some typical up-conversion and down-conversion photoluminescence was characterized and discussed.

Nanowires for Subwavelength Waveguides

One dimensional semiconducting and metal nanostructures were demonstrated as optical or plasmonic waveguides for subwavelength photonic integration. Model structures built from these subwavelength waveguides are used to illustrate electromagnetic coupling between semiconducting and metallic cavities.