Xinhai Zhang

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 ...

Photostable fluorescent organic dots with aggregation-induced emission (AIE dots) for noninvasive long-term cell tracing

Long-term noninvasive cell tracing by fluorescent probes is of great importance to life science and biomedical engineering. For example, understanding genesis, development, invasion and metastasis of cancerous cells and monitoring tissue regeneration after stem cell transplantation require continual tracing of the biological processes by cytocompatible fluorescent probes over a long period of time. In this work, we successfully developed organic far-red/near-infrared dots with aggregation-induced emission (AIE dots) and demonstrated their utilities as long-term cell trackers. The high emission efficiency, large absorptivity, excellent biocompatibility, and strong photobleaching resistance of the AIE dots functionalized by cell penetrating peptides derived from transactivator of transcription proteins ensured outstanding long-term noninvasive in vitro and in vivo cell tracing. The organic AIE dots outperform their counterparts of inorganic quantum dots, opening a new avenue in the development of fluorescent probes for following biological processes such as carcinogenesis.

The Physics of ultrafast saturable absorption in graphene

The ultrafast saturable absorption in graphene is experimentally and theoretically investigated in the femtosecond (fs) time regime. This phenomenon is well-modeled with valence band depletion, conduction band filling and ultrafast intraband carrier thermalization. The latter is dominated by intraband carrier-carrier scattering with a scattering time of 8 ( +/- 3) fs, which is far beyond the time resolution of other ultrafast techniques with hundred fs laser pulses. Our results strongly suggest that graphene is an excellent atomic layer saturable absorber.

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 ...

Microelectromechanical Maltese-cross metamaterial with tunable terahertz anisotropy.

Dichroic polarizers and waveplates exploiting anisotropic materials have vast applications in displays and numerous optical components, such as filters, beamsplitters and isolators. Artificial anisotropic media were recently suggested for the realization of negative refraction, cloaking, hyperlenses, and controlling luminescence. However, extending these applications into the terahertz domain is hampered by a lack of natural anisotropic media, while artificial metamaterials offer a strong engineered anisotropic response. Here we demonstrate a terahertz metamaterial with anisotropy tunable from positive to negative values. It is based on the Maltese-cross pattern, where anisotropy is induced by breaking the four-fold symmetry of the cross by displacing one of its beams. The symmetry breaking permits the excitation of a Fano mode active for one of the polarization eigenstates controlled by actuators using microelectromechanical systems. The metamaterial offers new opportunities for the development of terahertz variable waveplates, tunable filters and polarimetry.

Lipid-PEG-Folate Encapsulated Nanoparticles with Aggregation Induced Emission Characteristics: Cellular Uptake Mechanism and Two-Photon Fluorescence Imaging

Folate functionalized nanoparticles (NPs) that contain fluorogens with aggregation-induced emission (AIE) characteristics are fabricated to show bright far-red/near-infrared fluorescence, a large two-photon absorption cross section and low cytotoxicity, which are internalized into MCF-7 cancer cells mainly through caveolae-mediated endocytosis. One-photon excited in vivo fluorescence imaging illustrates that these AIE NPs can accumulate in a tumor and two-photon excited ex vivo tumor tissue imaging reveals that they can be easily detected in the tumor mass at a depth of 400 μm. These studies indicate that AIE NPs are promising alternatives to conventional TPA probes for biological imaging.

Exciton radiative lifetime in ZnO nanorods fabricated by vapor phase transport method

The exciton radiative lifetime in ZnO nanorods is studied. It is found that the exciton radiative lifetime increases with temperature as T2. Furthermore, the spectral linewidth of the photoluminescence of the ZnO nanorods also increases with temperature as T2, suggesting a linear dependence of exciton radiative lifetime on the spectral linewidth. The physics behind is that the oscillator strength of excitons at k=0 is shared equally among all the states within the spectral linewidth and the coherence extension of an exciton decreases with temperature due to the scattering by phonons, defects, or impurities.

Conjugated Oligoelectrolyte Harnessed Polyhedral Oligomeric Silsesquioxane as Light‐Up Hybrid Nanodot for Two‐Photon Fluorescence Imaging of Cellular Nucleus

A water-soluble organic/inorganic hybrid nanodot based on polyhedral oligomeric silsesquioxane (POSS) and conjugated oligoelectrolyte is designed and synthesized for two-photon fluorescence imaging of cellular nucleus, which takes advantage of its small size (∼3.3 nm) that imparts nucleus permeability and substantial DNA-enhanced two-photon excited fluorescence that allows illuminating the nucleus with a high contrast.

Evolution of visible luminescence in ZnO by thermal oxidation of zinc films

Abstract Zinc oxide (ZnO) films were prepared by thermal oxidation of metallic zinc films. The intensities of visible luminescence increase with oxidation time; however the shape and position of the green and yellow luminescence bands are determined by oxidation temperature only and do not vary with oxidation time and excitation laser powers. The green and yellow luminescent bands should originate from different intrinsic defects or defect complexes, which are formed in ZnO at certain temperatures. It suggests that the possibility of donor–acceptor pair recombination as the mechanism responsible for the green and yellow emissions can be excluded.

Observations of nitrogen-related photoluminescence bands from nitrogen-doped ZnO films

We report the synthesis andphotoluminescence (PL) properties of nitrogen-d opedZnO films. These films were synthesized by filtered cathodic vacuum arc technique; nitrogen gas was used as a dopant source. X-ray diffraction results indicated that the ZnO films were highly c-axis oriented. The appearance of the nitrogen-related local vibrational Raman scattering peaks showedthat nitrogen was incorporatedinto the films. In the PL spectrum of the undopedfilms, a near bandedge exciton emission peak at 384 nm anda weak visible bandrelatedto oxygen interstitial at 660 nm were observed. For nitrogen-doped films, besides the two emission bands observed in the undoped samples, two additional PL bands at around 450 and 890 nm were detected. According to the first-principle total energy calculation, nitrogenind ucedacceptor energy level is locatedat 0.4 eV above the valence bandmaximum. Therefore the emission bandat around450 nm may originate from the recombination of photo-generatedelectrons with neutral nitrogen acceptors, andthe 890 nm bandis attributedto electron transition from oxygen interstitial to this neutral nitrogen acceptors. r 2003 Elsevier Science B.V. All rights reserved.