Zhengshan Tian

Facilely synthesized N-doped carbon quantum dots with high fluorescent yield for sensing Fe3+

Nitrogen doped carbon quantum dots (NCQDs) were synthesized using L-glutamic acid as raw material through a facile, effective and green hydrothermal method in one pot. Without any chemical reagents and further surface modifications, the NCQDs emit bright blue fluorescence with a high quantum yield of 17.8% due to the fluorescence enhancement effect of N dopant atoms on a surface of carbon quantum dots. Moreover, the NCQDs present a sensitive response to Fe3+ ions with a detection limit of 4.67 μM (S/N = 3) through the variation in fluorescence with respect to the target concentration and based on electron transfer from the NCQDs to Fe3+ ions.

Self-Assembled Free-Standing Graphene Oxide Fibers

It is a great challenge to directly assemble two-dimensional (2D) graphene oxide (GO) sheets into 1D fibers without any polymer or surfactant for their promising multifunctional applications. Herein, a facile self-assembly strategy is proposed to fabricate neat GO fibers from cost-efficient, aqueous GO suspension at a liquid/air interface based on the repulsive electrostatic forces, attractive van der Waals forces, and π-π stacking. During the self-assembly process and ultrasonic cleaning, the morphology variated from the source graphite powder through GO sheets to GO fibers and finally to neat GO fiber films. It is interesting to note that the electrical property of the GO fiber films was improved dramatically after subsequent low-temperature thermal annealing. The morphological evolution process and formation mechanism were analyzed on the basis of optical microscopy, scanning electron microscopy, and transmission electron microscopy observation, and the electrical characteristics was also discussion.

Ultraviolet electroluminescence from horizontal ZnO microrods/GaN heterojunction light-emitting diode array

ZnO microrods were assembled on p-GaN substrate to form a heterostructural light-emitting diode (LED) array. Ultraviolet (UV) emission was obtained under a low forward bias of 3.5 V. The investigation on the electroluminescence, photoluminescence demonstrated three distinct electron-hole recombination processes. The relative intensity of these three emission bands changed with increase of the forward bias, and resulted in blue shift and spectral narrowing of electroluminescence. The present work provides a facile technique for micro-/nano-devices fabrication besides obtaining UV LED arrays.

Lasing behavior modulation for ZnO whispering-gallery microcavities.

Four configurations of whispering-gallery-mode (WGM) microcavities were designed and fabricated to modulate the optically pumped lasing characteristics by polymer modification on hexagonal ZnO microrod surfaces. On the basis of the total internal reflection (TIR) at the boundary of microcavities, the lasing characteristics were improved by raising the relative refractive index. Considering the different reflective conditions at various side surfaces, the typical lasing mode equation for whispering-gallery microcavity was modified to adapt for general situation even with unsymmetrical structure, and then employed to discuss the observed lasing behaviors, in the polyvinylcarbazole (PVK) modified ZnO microrods, such as mode position, mode numbers and quality factor. The optical field distributions for TE modes of the four configurations were also simulated by 2-dimensional finite difference time-domain (FDTD) method. The simulation agreed well with the experimental results to support the resonance mechanism.

Plasmon-enhanced Electrically Light-emitting from ZnO Nanorod Arrays/p-GaN Heterostructure Devices

Effective and bright light-emitting-diodes (LEDs) have attracted broad interests in fundamental research and industrial application, especially on short wavelength LEDs. In this paper, a well aligned ZnO nanorod arrays grown on the p-GaN substrate to form a heterostructured light-emitting diode and Al nanoparticles (NPs) were decorated to improve the electroluminescence performance. More than 30-folds enhancement of the electroluminescence intensity was obtained compared with the device without Al NPs decoration. The investigation on the stable and transient photoluminescence spectraof the ZnO nanorod arrays before and after Al NPs decoration demonstrated that the metal surface plasmon resonance coupling with excitons of ZnO leads to the enhancement of the internal quantum efficiency (IQE). Our results provide aneffective approach to design novel optoelectronic devices such as light-emitting diodes and plasmonic nanolasers.

Electro-pumped whispering gallery mode ZnO microlaser array

By employing vapor-phase transport method, ZnO microrods are fabricated and directly assembled on p-GaN substrate to form a heterostructural microlaser array, which avoids of the relatively complicated etching process comparing previous work. Under applied forward bias, whispering gallery mode ZnO ultraviolet lasing is obtained from the as-fabricated heterostructural microlaser array. The devices electroluminescence originates from three distinct electron-hole recombination processes in the heterojunction interface, and whispering gallery mode ultraviolet lasing is obtained when the applied voltage is beyond the lasing threshold. This work may present a significant step towards future fabrication of a facile technique for micro/nanolasers.

Synthesis and optical properties of Janus structural ZnO/Au nanocomposites

Zinc oxide quantum dot (ZnO QD)/gold nanoparticle (Au NP) nanocomposites with Janus structure were synthesized through a bridge linking of 3-mercaptopropionic acid (3MPA). The bare ZnO QDs presented a typically weak inherent ultraviolet (UV) emission band and a strong defect-related visible band. After Au NPs were assembled onto the surfaces of ZnO QDs, the ZnO/Au nanocomposites exhibited a dramatic enhancement of UV emission and quenching of visible emission. It was noted that the defect emission of ZnO QDs matched with the localized surface plasmon resonance (LSPR) peak of Au NPs, which resulted in a LSPR-assisted electron transfer process from Au NPs to ZnO QDs, so the defect emission was suppressed. The LSPR-assisted electron transfer process was confirmed by the time-resolved photoluminescence.

A facile preparation route for highly conductive borate cross-linked reduced graphene oxide paper

A facile hydrothermal strategy to synthesize borate cross-linked reduced graphene oxide (B-RGO) sheets with good optical and electrical properties is proposed by using graphene oxide (GO) sheets, boric acid and sodium hydroxide as precursors in one pot without any polymer or surfactant. The resulting B-RGO sheets can be assembled into a highly conductive B-RGO paper by simple filtration. Moreover, the atomic percentage of boron (B) in the B-RGO sheet can be readily controlled, as when it reaches to a maximum of 3.33 at%, about 30.0 mA current passes through the resulting single-layer B-RGO sheet and B-RGO paper at an applied bias of 2.0 V. This current value is much higher than that of the annealed GO paper (∼1.2 mA) and very close to that of the single-layer graphene sheet (∼60.0 mA) synthesized by a chemical vapour deposition method under the same test conditions. The resulting highly conductive B-RGO sheet and B-RGO paper offer a promising transparent conductive material for electronic or optoelectronic applications.

Different wavelength ranges' WGM lasing from a ZnO microrod/R6G:PMMA microcavity

Dual-wavelength or multi-wavelength laser sources have potential application in many areas. The ZnO material is an important candidate for ultraviolet (UV) micro/nanolasers for integrated photonic systems. In this paper, the dual-wavelength whispering-gallery-mode (WGM) laser is fabricated by symmetrically coating Rhodamine 6G (R6G):PMMA on a ZnO microrod, and realizes the UV WGM lasing and orange WGM lasing under optical pumping condition. The performance of the UV and orange WGM lasing, including the lasing threshold, emission intensity and quality factors (Q) are discussed. The resonance mechanism of the dual-wavelength WGM lasing is analyzed in detail. The two-dimensional FDTD simulation on optical-field distribution also confirms the resonance mechanism. This work is important to the design of optically and electrically pumped dual- or multi-wavelength WGM lasers.

Facile assembly of two-dimensional functional ZnO quantum dots/reduced graphene oxide nanocomposites

Without any complex links or multistep process, zinc oxide quantum dots (ZnO QDs) were uniformly assembled onto the surface of the reduced graphene oxide (RGO) sheets to synthesize two-dimensional (2D) functional nanocomposites through a simple and convenient hydrothermal process in one pot. The RGO sheets maintained their previous 2D structure assembled with the well-distributed ZnO QDs. The as-synthesized ZnO QDs/RGO nanocomposites exhibited significantly enhanced photocatalytic activity on methylene blue (MB) under UV illumination. The facile formation mechanism was investigated. It is interesting to note that this strategy is a general method to anchor various nanoparticles (for example Fe3O4, Au and Ag) onto the RGO sheet surface to fabricate 2D functional nanocomposites for potential applications.