Abstract:We report on the observation of carrier-diffusion-induced defect emission at high excitation density in a-plane InGaN single quantum wells. When increasing excitation density in a relatively high regime, we observed the emergence of defect-related emission together with a significant reduction in bandedge emission efficiency. The experimental results can be well explained with the density-activated carrier diffusion from localized states to defect states. Such a scenario of density-activated defect recombination, as confirmed by the dependences of photoluminescence on the excitation photon energy and temperature, is a plausible origin of efficiency droop in a-plane InGaN quantum-well light-emitting diodes. a cfzhang@nju.edu.cn b jskwak@sunchon.ac.kr c mxiao@uark.edu
Efficiency droop is a major obstacle facing high-power application of InGaN/GaN quantum-well (QW) light-emitting diodes (LEDs). In this paper, we report the suppression of efficiency droop induced by the process of density-activated defect recombination in nanorod structures of a-plane InGaN/GaN QWs. In the high carrier density regime, the retained emission efficiency in a dry-etched nanorod sample is observed to be over two times higher than that in its parent QW sample. We further argue that such improvement is a net effect that the lateral carrier confinement overcomes the increased surface trapping introduced during fabrication.
Graphene quantum-dots (GQDs) have been predicted and demonstrated with fascinating optical and magnetic properties. However, the magnetic effect on the optical properties remains experimentally unexplored. Here, we conduct a magneto-photoluminescence study on the blue-luminescence GQDs at cryogenic temperatures with magnetic field up to 10 T. When the magnetic field is applied, a remarkable enhancement of photoluminescence emission has been observed together with an insignificant change in circular polarization. The results have been well explained by the scenario of magnetic-field-controlled singlet-triplet mixing in GQDs owing to the Zeeman splitting of triplet states, which is further verified by temperature-dependent experiments. This work uncovers the pivotal role of intersystem crossing in GQDs, which is instrumental for their potential applications such as light-emitting diodes, photodynamic therapy, and spintronic devices.
Because of its high strength and high toughness, graphene has been widely used in mechanical reinforced composites. In general, the mechanical enhancement depends mainly on the properties of graphene itself and the number of surface chemical functional groups attached on it. In this paper, we report a method to improve the mechanical performance of polymer by using a kind of functionalized reduced graphene oxide (F-RGO), i.e., the F-RGO is prepared by chemical treatment, and then the F-RGO/polyvinylidene fluoride (PVDF) composite films are obtained by spin coating. Because the chemical treatment can increase the number of functional groups on the surface and edge of F-RGO, these functional groups make the F-RGO sheets strongly coupled with PVDF molecules, so as to achieve the purpose of mechanical enhancement. The experimental results reveal that the mechanical properties of the F-RGO/PVDF composite films are improved for 42% times, when comparing with regular RGO/PVDF composite films.
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