Y. K. Liu

High-quality CdS nanoribbons with lasing cavity

High-density and high-quality CdS nanoribbons were synthesized by a thermal evaporation process. The nanoribbons were characterized by scanning electron microscopy, transmission electron microscopy, x-ray diffraction, and photoluminescence spectroscopy. CdS nanoribbons were found to be single crystals of high phase purity and low defect density. Lasing was observed in the CdS nanoribbons upon optical pumping. The growth of CdS nanoribbons was explained by the vapor–liquid–solid mechanism.

High-Quality Graphenes via a Facile Quenching Method for Field-Effect Transistors

Single- and few-layer graphene sheets with sizes up to 0.1 mm were fabricated by simply quenching hot graphite in an ammonium hydrogen carbonate aqueous solution. The identity and thickness of graphene sheets were characterized with transmission electron microscopy, atomic force microscopy, and Raman spectroscopy. In addition to its simplicity and scalability, the present synthesis can produce graphene sheets with excellent qualities in terms of sizes, purity, and crystal quality. The as-produced graphene sheets can be easily transferred to solid substrates for further processing. Field-effect transistors based on individual graphenes were fabricated and shown to have high ambipolar carrier mobilities.

Manganese doping and optical properties of ZnS nanoribbons by postannealing

Manganese (Mn) doping of ZnS nanoribbons was achieved by simple thermal annealing. Upon heating ZnS nanoribbons with MnS powder up to 700°C, the intrinsic photoluminescence (PL) of the annealed nanoribbons disappeared and a new PL peak at 585nm gradually emerged. Significantly, the annealing process induced no detectable change in the morphology and uniform hexagonal wurtzite 2H structure of the single-crystal ZnS nanoribbons. The PL peak at 585nm is attributed to Mn dopant and confirms Mn incorporation in ZnS because (1) the peak appears only when ZnS ribbons were annealed with MnS, but does not appear without MnS, (2) its intensity increases with increasing annealing temperature, which is consistent with increased incorporation of Mn2+ ions, and (3) its position is similar to that of Mn-related emission in ZnS, and is independent of the measuring temperature and excitation power. This work demonstrates the capability of doping nanostructured materials by simple postannealing treatment.

Continuous near-infrared-to-ultraviolet lasing from II-VI nanoribbons

The authors show that II-VI nanoribbons are capable of room-temperature lasing covering the complete spectral range from near infrared (NIR) to ultraviolet (UV). This is accomplished by simply using nanoribbons of two ternary compositions, namely, CdSXSe1−X and ZnYCd1−YS. Under optical pumping, CdSXSe1−X nanoribbons lase from NIR (710nm) to green (510nm) as X changes from 0 to 1, while ZnYCd1−YS nanoribbons lase from green (510nm) to UV (340nm) as Y varies from 0 to 1. Furthermore, lasing control shows fine-tuning via composition changes that overlap thermally induced tuning. This demonstrates that II-VI materials can enable lasing at any selected wavelength between 710 and 340nm with continuous tuning capabilities.

Photoluminescence and photoconductivity properties of copper-doped Cd1−xZnxS nanoribbons

Copper-doped Cd1?xZnxS (x~0.16) nanoribbons were prepared by controlled thermal evaporation of CdS, ZnS, and CuS powders onto Au-coated silicon substrates. The nanoribbons had a hexagonal wurtzite structure, and lengths of several tens to hundreds of micrometres, widths of 0.6?15??m, and thicknesses of 30?60?nm. Cu doping and incorporation into the CdZnS lattice were identified and characterized by low-temperature photoluminescence (PL) and photoconductivity measurements. Temperature-dependent PL measurement showed that the PL spectra of both Cu-doped and undoped CdZnS nanoribbons have two emission peaks at 2.571 and 2.09?eV, which are assigned to band edge emission and deep trap levels, respectively. In addition, the Cu-doped nanoribbons present two extra peaks at 2.448 and 2.41?eV, which are attributed to delocalized and localized donor and acceptor states in the band gap of CdZnS resulting from Cu incorporation. Photoconductivity results showed the nanoribbons can be reversibly switched between low and high conductivity under pulsed illumination. The Cu-doped CdZnS nanoribbons showed four orders of magnitude larger photocurrent than the undoped ones. The current jumped from ~2 ? 10?12 to ~5.7 ? 10?7?A upon white light illumination with a power density of ~9?mW?cm?2. The present CdZnS:Cu nanoribbons may find applications in opto-electronic devices, such as solar cells, photoconductors, and chemical sensors.

ZnO nanowire arrays grown on Al:ZnO buffer layers and their enhanced electron field emission

Arrays of highly ordered ZnO nanowires have been synthesized on polycrystalline Al-doped ZnO (AZO) buffer layers prepared on p-Si substrates (7–13 Ω cm) with assistance of a thermal deposition method. The diameter and interspacing of the nanowires have been controlled by the growth conditions and properties of AZO films. The optimized array of ZnO nanowires shows low turn-on and threshold fields (∼1.1 and ∼3.0 V/μm, respectively) and displays exceptional time stability of electron field emission. The time-fluctuation instability was found to be less than 0.6% at a current density of 10 mA/cm2, as measured for 500 min. The low turn-on and threshold fields as well as the stable electron emission current suggest that the arrays of ZnO nanowires could be considered in some electron field emission applications.

Dye degradation induced by hydrogen-terminated silicon nanowires under ultrasonic agitations

A method for degradation of environmentally hazardous dyes using silicon nanowires (SiNWs) has been developed. Environmentally unfriendly methyl red was degraded with assistance of H-terminated SiNWs under ultrasonic agitation. The hydrogenated surfaces of SiNWs are shown to be responsible for the surface reaction and decay of methyl red. The rate of degradation increases with the amount of SiNWs and agitation power. SiNWs after their application can be recycled and reactivated for further uses by a simple heating in hydrogen plasmas.

Coaxial nanocables of p-type zinc telluride nanowires sheathed with silicon oxide: synthesis, characterization and properties

Coaxial nanocables with a single-crystalline zinc telluride (ZnTe) nanowire core and an amorphous silicon oxide (SiO(x)) shell have been synthesized via a simple one-step chemical vapor deposition (CVD) method on gold-decorated silicon substrates. The single-crystal ZnTe nanowire core is in zinc-blende structure along the [111] direction, while the uniform SiO(x) shell fully covers the core with no observable pin-hole or crack. Formation mechanisms of the ZnTe-SiO(x) nanocables are discussed. The ZnTe nanowire core shows p-type electrical properties while the SiO(x) shell acts as an effective insulating layer. The ZnTe-SiO(x) nanocables may have potential applications in nanoscale devices, such as p-type FETs and nanosensors.