Junfeng Lu

Comparative investigation on temperature-dependent photoluminescence of CH3NH3PbBr3 and CH(NH2)2PbBr3 microstructures

Organic–inorganic lead bromide perovskite is regarded as a highly efficient material for green light devices. In this study, we report on the fabrication of CH3NH3PbBr3 and CH(NH2)2PbBr3 perovskite microstructures. The morphologies and crystal structures of these two types of perovskite nanostructures are characterized and the differences in their photoluminescence behaviour are demonstrated. The temperature-dependent photoluminescence of the CH3NH3PbBr3 microstructure shows that exciton emission shifts continuously with increasing temperature and a trapped charge-carrier emission peak appears in the temperature region from 115 K to room temperature. Temperature-dependent photoluminescence of CH(NH2)2PbBr3 reveals a blue shift of the exciton emission with increasing in temperature, but there is a sudden red shift near 150 K due to phase transition. Temperature coefficients of the band gaps and exciton thermal activation energies of CH3NH3PbBr3 and CH(NH2)2PbBr3 microstructures are calculated. At room temperature, the exciton thermal activation energy of CH3NH3PbBr3 is higher than that of CH(NH2)2PbBr3, which indicates the excitons in CH3NH3PbBr3 are more stable.

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 × 10−3 M) 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 6 G and detection limit as low as 10−12 M 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).

Plasmon-mediated exciton–phonon coupling in a ZnO microtower cavity

Due to its characteristics of altitudinal spatial localization and intense near-field enhancement, the use of surface plasmons (SPs) has been proposed as a promising method to enhance the photoelectric conversion efficiency through light–matter interactions. In order to deeply understand the processes of energy coupling, an enhanced exciton–phonon coupling in the temperature-dependent photoluminescence (TDPL) spectra of a ZnO microtower cavity decorated with Al nanoparticles has been reported and thus reveals the lattice vibration induced by the coupling of SPs. The lasing characteristics such as laser threshold and spectral shift are investigated at room temperature and the SP-mediated energy coupling between excitons and phonons is also further verified.

Plasmon enhancement for Vernier coupled single-mode lasing from ZnO/Pt hybrid microcavities

It is essential to develop a single mode operation and improve the performance of lasing in order to ensure practical applicability of microlasers and nanolasers. In this paper, two hexagonal microteeth with varied nanoscaled air-gaps of a ZnO microcomb are used to construct coupled whispering-gallery cavities. This is done to achieve a stable single mode lasing based on Vernier effect without requiring any complicated or sophisticated manipulation to achieve positioning with nanoscale precision. Optical gain and the corresponding ultraviolet lasing performance were improved greatly through coupling with localized surface plasmons of Pt nanoparticles. The ZnO/Pt hybrid microcavities achieved a seven-fold enhancement of intensity of single mode lasing with higher side-mode suppression ratio and lower threshold. The mechanism that led to this enhancement has been described in detail.

Synergistic graphene/aluminum surface plasmon coupling for zinc oxide lasing improvement

Collective oscillations of free electrons generate plasmons on the surface of a material. A whispering-gallery microcavity effectively confines the light field on its surface based on the total reflection from its internal wall. When these two kinds of electromagnetic waves meet each other, the stimulated emissions from an individual ZnO microrod were enhanced more than 50-fold and the threshold was reduced after the whispering-gallery microcavity was coated with a monolayer of graphene and Al nanoparticles. The improvement of the lasing performance was attributed to the synergistic energy coupling of the graphene/Al surface plasmons with ZnO excitons. The lasing characteristics and the coupling mechanism were investigated systematically.

Burstein-Moss Effect Behind Au Surface Plasmon Enhanced Intrinsic Emission of ZnO Microdisks

In this paper, ZnO microdisks with sputtering of Au nanoparticles were prepared to explore their plasmon/exciton coupling effect. An obvious blue shift and enhanced excitonic emission intensity were observed in the PL spectra of as-grown and Au-sputtered ZnO samples at room temperature. The investigation on the absorption spectra and temperature-dependent PL spectra has been demonstrated the Burstein-Moss effect behind the optical phenomena. These results revealed the coupling dynamics between the metal localized surface plasmon and semiconductor exciton.

Underlying mechanism of blue emission enhancement in Au decorated p-GaN film

Localized surface plasmons (LSPs) excited on metallic structures often play a significant role in mediating the photoluminescence (PL) of semiconductors. For p-GaN film, due to the LSP coupling, blue emission was enhanced while defect-related green emission was quenched to noise level after the decoration with Au nanoparticles (NPs). Why could the Au SP in the green light region enhance the blue and even ultraviolet emission? In this paper, a series of near/far-field spectral analyses and simulations were conducted to understand this process. A clear physical model of LSP-induced electron transfer was proposed to explain the defect-related LSP generation, coupling, electron transfer, and further blue emission increase with green emission reduction. Based on the PL measurement, an insulating SiO2 layer was introduced to confirm the LSP-induced electron transfer between Au and GaN. Additional green light was introduced to observe the LSP-induced PL enhancement, in the same way as for samples with defects. Our study provides a full understanding of the mechanism of PL enhancement in Au decorated GaN and this model should be universal for similar metal/semiconductor systems.