Zengliang Shi

Combined whispering gallery mode laser from hexagonal ZnO microcavities

The hexagonal ZnO microrods were employed as whispering gallery mode (WGM) microcavities to obtain ultraviolet laser at room temperature. For each individual ZnO microrod, the laser presented low threshold and high quality Q factor and clear WGM spectral structure. The lasing spectra from different microcavities were combined together by pumping two or more ZnO microrods simultaneously. The resonant process and lasing characteristics, such as lasing mode, threshold, and Q factor, were investigated in experiment and theory.

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.

Surface photoluminescence and magnetism in hydrothermally grown undoped ZnO nanorod arrays

ZnO nanorod arrays were synthesized by a hydrothermal method on the Si substrate with ZnO thin film as seed layer prepared by magnetron sputtering. The presence of -OH ligands on the surface of the as-grown sample was confirmed, and its dominant role in both suppressing the visible emission and boosting the room-temperature ferromagnetism (FM) was revealed. Through alternative H2 and O2 annealing to remove the -OH ligands, reconstruct surface-states and tune the oxygen occupancy in ZnO nanorods, the clear correlation between the characteristic green emission and ferromagnetism was established.

Single Mode ZnO Whispering-Gallery Submicron Cavity and Graphene Improved Lasing Performance

Single-mode ultraviolet (UV) laser of ZnO is still in challenge so far, although it has been paid great attention along the past decades. In this work, single-mode lasing resonance was realized in a submicron-sized ZnO rod based on serially varying the dimension of the whispering-gallery mode (WGM) cavities. The lasing performance, such as the lasing quality factor (Q) and the lasing intensity, was remarkably improved by facilely covering monolayer graphene on the ZnO submicron-rod. The mode structure evolution from multimodes to single-mode was investigated systematically based on the total internal-wall reflection of the ZnO microcavities. Graphene-induced optical field confinement and lasing emission enhancement were revealed, indicating an energy coupling between graphene SP and ZnO exciton emission. This result demonstrated the response of graphene in the UV wavelength region and extended its potential applications besides many previous reports on the multifunctional graphene/semiconductor hybrid materials and devices in advanced electronics and optoelectronics areas.

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.

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.

Plasmon coupled Fabry-Perot lasing enhancement in graphene/ZnO hybrid microcavity

The response of graphene surface plasmon (SP) in the ultraviolet (UV) region and the realization of short-wavelength semiconductor lasers not only are two hot research areas of great academic and practical significance, but also are two important issues lacked of good understanding. In this work, a hybrid Fabry-Perot (F-P) microcavity, comprising of monolayer graphene covered ZnO microbelt, was constructed to investigate the fundamental physics of graphene SP and the functional extension of ZnO UV lasing. Through the coupling between graphene SP modes and conventional optical microcavity modes of ZnO, improved F-P lasing performance was realized, including the lowered lasing threshold, the improved lasing quality and the remarkably enhanced lasing intensity. The underlying mechanism of the improved lasing performance was proposed based on theoretical simulation and experimental characterization. The results are helpful to design new types of optic and photoelectronic devices based on SP coupling in graphene/semiconductor hybrid structures.

SERS-active ZnO/Ag hybrid WGM microcavity for ultrasensitive dopamine detection

Dopamine (DA) is a potential neuro modulator in the brain which influences a variety of motivated behaviors and plays a key role in life science. A hybrid ZnO/Ag microcavity based on Whispering Gallery Mode (WGM) effect has been developed for ultrasensitive detection of dopamine. Utilizing this effect of structural cavity mode, a Raman signal of R6G (5 × 10−3u2009M) detected by this designed surface-enhanced Raman spectroscopy (SERS)-active substrate was enhanced more than 10-fold compared with that of ZnO film/Ag substrate. Also, this hybrid microcavity substrate manifests high SERS sensitivity to rhodamine 6u2009G and detection limit as low as 10−12u2009M to DA. The Localized Surface Plasmons of Ag nanoparticles and WGM-enhanced light-matter interaction mainly contribute to the high SERS sensitivity and help to achieve a lower detection limit. This designed SERS-active substrate based on the WGM effect has the potential for detecting neurotransmitters in life science.

Crystal structure and electron transition underlying photoluminescence of methylammonium lead bromide perovskites

Bromine-based methylammonium lead hybrid perovskites (CH3NH3PbBr3 or MAPbBr3) have exhibited remarkable charge transport and optical properties. Nonetheless, the photoluminescence (PL) behavior and electronic transition state are still obscure. In this paper, the intrinsic emission mechanisms of two peaked CH3NH3PbBr3 microcuboid crystals have been investigated. A systematic analysis of the stable-state, transient-state and temperature-dependent spectra demonstrated the structure–activity relationship between optical properties and crystal phase. The lattice symmetry was also confirmed by the two-photon absorption induced PL. The findings can be assigned to the fact that the two emission states with band-energy ∼2.22 eV and ∼2.31 eV are originated from free exciton and free carrier recombination which are attributed to the coexistence of a non-centrosymmetric tetragonal phase and a centrosymmetric cubic phase for CH3NH3PbBr3 microcrystals at higher temperature (>160 K).