S. F. Yu

Photoluminescence study of ZnO films prepared by thermal oxidation of Zn metallic films in air

Zinc oxide (ZnO) films were synthesized by thermal oxidation of metallic zinc films in air. The influence of annealing temperatures ranging from 320 to 1000 °C on the structural and optical properties of ZnO films is investigated systematically using x-ray diffraction and room temperature photoluminescence (PL). The films show a polycrystalline hexagonal wurtzite structure without preferred orientation. Room temperature PL spectra of the ZnO films display two emission bands, predominant excitonic ultraviolet (UV) emission and weak deep level visible emission. It is observed that the ZnO film annealed at 410 °C exhibits the strongest UV emission intensity and narrowest full width at half maximum (81 meV) among the temperature ranges studied. The excellent UV emission from the film annealed at 410 °C is attributed to the good crystalline quality of the ZnO film and the low rate of formation of intrinsic defects at such low temperature. The visible emission consists of two components in the green and yellow ...

Selective decoration of Au nanoparticles on monolayer MoS2 single crystals.

We report a controllable wet method for effective decoration of 2-dimensional (2D) molybdenum disulfide (MoS2) layers with Au nanoparticles (NPs). Au NPs can be selectively formed on the edge sites or defective sites of MoS2 layers. The Au-MoS2 nano-composites are formed by non-covalent bond. The size distribution, morphology and density of the metal nanoparticles can be tuned by changing the defect density in MoS2 layers. Field effect transistors were directly fabricated by placing ion gel gate dielectrics on Au-decorated MoS2 layers without the need to transfer these MoS2 layers to SiO2/Si substrates for bottom gate devices. The ion gel method allows probing the intrinsic electrical properties of the as-grown and Au-decorated MoS2 layers. This study shows that Au NPs impose remarkable p-doping effects to the MoS2 transistors without degrading their electrical characteristics.

Upconverting Near‐Infrared Light through Energy Management in Core–Shell–Shell Nanoparticles

Lanthanide-doped upconversion materials, capable of converting low-density (< 1000 W cm ) near-infrared (NIR) excitation to ultraviolet (UV) and visible emissions, have generated a large amount of interests in the areas of information technology, biotechnology, energy, and photonics. Significantly, recent developments in the synthetic and multicolor tuning methods have allowed easy access to upconversion nanoparticles with well-defined phase and size, core–shell structure, optical emission, and surface properties. The technological advances provide promising applications in sensitive biodetection and advanced bioimaging without many of the constraints associated with conventional optical biolabels. Despite the attractions, further progress in using upconversion processes has been largely hindered because upconversion nanoparticles are typically sensitized by Yb ions that only respond to narrowband NIR excitation centered at 980 nm. The absorption of 980 nm light by the water component in biological samples usually limits deep tissue imaging and induces potential thermal damages to cells and tissues. Excitation of conventional upconversion nanoparticles at other wavelengths has been proposed to minimize the effect of water absorption. But the use of this technique is limited mainly by the largely sacrificed excitation efficiency. Efforts have also been devoted to tuning the NIR response of photon upconversion through integration of various sensitizers such as metal ions (e.g.; Nd, V or Cr) and organic dyes. The progress has resulted in visible emission by NIR excitation in the 700–900 nm range where the transparency of biological samples is maximal. However, upconversion emission across a broad range of spectra in these systems have not been demonstrated largely owing to the uncontrollable nonradiative processes. Herein, we describe a novel design, based on nanostructural engineering to separate unwanted electronic transitions for constructing a new class of materials displaying tunable upconversion emissions spanning from UV to the visible spectral region by single wavelength excitation at 808 nm. We also show that these nanoparticles can surpass the constraints associated with conventional upconversion nanoparticles for biological studies. The nanostructure design for management of energy transitions is depicted in Figure 1. A core–shell–shell nanoparticle platform is used to host light-harvesting, upconvert-

Comprehensive study of ZnO films prepared by filtered cathodic vacuum arc at room temperature

Room temperature deposition of high crystal quality zinc oxide (ZnO) films was realized by the filtered cathodic vacuum arc (FCVA) technique. Detrimental macroparticles in the plasma as byproducts of arcing process are removed with an off-plane double bend magnetic filter. The influence of oxygen pressure on the structural, electrical and optical properties of ZnO films were investigated in detail. The crystal structure of ZnO is hexagonal with highly c-axis orientation. Intrinsic stress decreases with an increase of chamber pressure, and near stress-free film was obtained at 1×10−3 Torr. Films with optical transmittance above 90% in the visible range and resistivity as low as 4.1×10−3 Ω cm were prepared at pressure of 5×10−4 Torr. Energetic zinc particles in the cathodic plasma and low substrate temperature enhance the probability of formation of zinc interstitials in the ZnO films. The observation of strong ultraviolet photoluminescence and weak deep level emission at room temperature manifest the high ...

Random laser action in ZnO nanorod arrays embedded in ZnO epilayers

Random laser action with coherent feedback has been observed in ZnO nanorod arrays embedded in ZnO epilayers. The sample was fabricated by depositing a MgO buffer layer and followed by a layer of ZnO thin film onto a vertically well-aligned ZnO nanorod arrays grown on sapphire substrate. Under 355 nm optical excitation at room temperature, sharp lasing peaks emit at around 390 nm with a linewidth less than 0.4 nm has been observed in all directions. In addition, the dependence of the lasing threshold intensity on the excitation area is shown in good agreement with the random laser theory. Hence, it is demonstrated that random laser action can also be supported in ZnO nanorod arrays.

Dynamic analysis of radiation and side-mode suppression in a second-order DFB laser using time-domain large-signal traveling wave model

In this paper, we have developed a relatively simple algorithm to calculate the large-signal dynamic response of DFB lasers by solving the time-dependent coupled wave equations directly in the time domain. The spontaneous emission noise, longitudinal variations of carrier (hole burning) and photon densities as well as that of the refractive index are taken into consideration. To demonstrate the power of this straightforward algorithm, the model shows how the side-mode suppression ratio in devices with high /spl kappa/L and a /spl lambda4: phase shift is significantly affected by the radiation in the second-order DFB laser. The time-dependent radiation pattern in grating-coupled surface-emitting lasers is also calculated for the first time. >

Zinc oxide thin-film random lasers on silicon substrate

Room-temperature ultraviolet lasing is demonstrated in mirrorless zinc oxide thin-film waveguides on (100) silicon substrate. Laser cavities, due to closed-loop optical scattering from the lateral facets of the irregular zinc oxide grains, are generated through the post-growth annealing of high-crystal-quality zinc oxide thin films obtained from the filtered cathodic vacuum arc technique. It is found that the lasing wavelength and linewidth of the zinc oxide random lasers under 355 nm optical excitation are around 390 nm and less than 0.4 nm, respectively. In addition, the lasing threshold characteristics are in good agreement with the random laser theory.

Origin of room temperature ferromagnetism in ZnO:Cu films

Copper-doped ZnO (ZnO:Cu) films were prepared on silicon substrates by filtered cathodic vacuum arc technique at room temperature using a Zn target containing 5at.% of Cu. Room temperature ferromagnetism was observed in the ZnO:Cu films with saturation magnetization of 0.037μB∕Cu atom. The origin of the ferromagnetism in ZnO:Cu was mainly due to Cu ions substituted into the ZnO lattice. X-ray diffraction, x-ray photoelectron spectroscopy, and transmission electron microscopy revealed that no ferromagnetic-related secondary phase could be detected in ZnO:Cu.

Direct Growth of ZnO Nanocrystals onto the Surface of Porous TiO2 Nanotube Arrays for Highly Efficient and Recyclable Photocatalysts

Recently, extensive investigations have been concentrated on the design and synthesis of nanocompositemetal oxides such as ZnO/TiO2 nanocomposite materials to improve the quantum efficiency of photocatalysts for applications in water purification. This is due to the high reactivity of TiO2 and the large binding energy of ZnO, which improve the process of electron and hole transfer between the corresponding conduction and valence bands. As a result, a better separation of photogenerated carriers can be achieved when compared with catalysts from a single metal oxide. A double-layered ZnO/TiO2 system had been proposed to improve thequantumefficiencyofphotocatalysts, however, the photogenerated electrons accumulated in the TiO2 underlayer may be unavailable to participate in the photocatalytic reactions so that the corresponding quantum efficiency could bedegraded.ZnOtetrapods coatedwithTiO2nanoparticles were also suggested for the realization of high efficiency photocatalysts, but the poor control of the total surface exposure area for both metal oxides limited their usefulness as high efficiency photocatalysts. Alternatively, the use of a

An efficient and stable fluorescent graphene quantum dot–agar composite as a converting material in white light emitting diodes

Graphene quantum dots (GQDs) have attracted great attention due to their unique optoelectronic properties. There remains a critical challenge to utilize the water-soluble GQDs for device applications. Here we report a facile method to fabricate a GQD–agar composite. The composite exhibits excellent optical stability and no luminescence quenching is observed. The composite is successfully applied as a colour converting material in blue light-emitting diodes (LEDs) to achieve white light emission. The luminous efficiency and light conversion efficiency of the white LED are 42.2 lm W−1 and 61.1% respectively. The light conversion efficiency of the WLED is stable for over 100 hours of continuous operation.