Effect of annealing on the electrical performance of N-polarity GaN Schottky barrier diodes

A nitrogen-polarity(N-polarity)GaN-based high electron mobility transistor(HEMT)shows great potential for high-fre-quency solid-state power amplifier applications because its two-dimensional electron gas(2DEG)density and mobility are mini-mally affected by device scaling.However,the Schottky barrier height(SBH)of N-polarity GaN is low.This leads to a large gate leakage in N-polarity GaN-based HEMTs.In this work,we investigate the effect of annealing on the electrical characteristics of N-polarity GaN-based Schottky barrier diodes(SBDs)with Ni/Au electrodes.Our results show that the annealing time and tem-perature have a large influence on the electrical properties of N-polarity GaN SBDs.Compared to the N-polarity SBD without annealing,the SBH and rectification ratio at±5 V of the SBD are increased from 0.51 eV and 30 to 0.77 eV and 7700,respec-tively,and the ideal factor of the SBD is decreased from 1.66 to 1.54 after an optimized annealing process.Our analysis results suggest that the improvement of the electrical properties of SBDs after annealing is mainly due to the reduction of the inter-face state density between Schottky contact metals and N-polarity GaN and the increase of barrier height for the electron emis-sion from the trap state at low reverse bias.

In situ thermoresponsive supramolecular assembly for switchable circularly polarized luminescence

The dynamic regulation of circularly polarized luminescence(CPL)holds profound significance in various fields,such as highlevel information storage and encryption.Here we developed a chiral amphiphilic molecule,CPSB-GLU-PEG350(CGP),composed of aggregation-induced emission(AIE)chromophores(Z)-4-(1-cyano-2-phenylvinyl)benzoic acid(CPSB),a chiral linker Glutamic acid and polyethylene glycol(PEG)thermoresponsive segments.Within the self-assembled supramolecular system formed by CGP,we have achieved in-situ temperature-responsive chiral structures,facilitating the thermal control switch of the CPL signal.Molecular dynamics simulations demonstrate the distinct behaviors of AIE and PEG units during the temperature-variable assembly process.Furthermore,by co-assembling achiral dye molecules with CGP,we have expanded the color range of the temperature-responsive CPL assembly system in situ and confirmed the occurrence of circularly polarizedF?rster resonance energy transfer(C-FRET)phenomenon in this process,which successfully enriched the strategies for in-situ CPL control in aqueous phases.In addition,the contactless radiative energy transfer of CPL can also be realized in this system,exhibiting more flexible temperature regulation of the CPL signal.This study provides a convenient and universal strategy for the construction of dynamically smart chiroptical materials.

金属有机框架及其衍生纳米负极材料

负极是锂离子电池的重要组成部分之一,较低的离子-电子电导率、明显体积效应以及容易粉化等问题限制着传统负极材料的发展和广泛应用。金属有机框架(MOFs)及其衍生材料的丰富孔隙有利于离子快速迁移,较大的比表面积提供更多的电化学反应活性位点,尤其是能通过改变合成工艺实现结构和化学组成调控,有效提高锂离子电池的可逆容量和循环稳定性。本文系统地总结和归纳了近年来MOFs及其衍生纳米负极材料的研究进展,梳理了不同合成方法、形貌结构与电化学性能间的相互关系,分析了这类负极材料急待解决的关键问题和面临的机遇与挑战。在尽可能充分发挥各自优势的基础上,结合有机配体和金属中心的多样性以及结构的多变性和特殊性,提出了一些改善储锂性能的有效措施和工业化应用的解决方案,展望了这类新型多孔纳米负极材料的未来发展趋势和应用前景。

Demonstration of epitaxial growth of strain-relaxed GaN films on graphene/SiC substrates for long wavelength light-emitting diodes

Strain modulation is crucial for heteroepitaxy such as GaN on foreign substrates.Here,the epitaxy of strain-relaxed GaN films on graphene/SiC substrates by metal-organic chemical vapor deposition is demonstrated.Graphene was directly prepared on SiC substrates by thermal decomposition.Its pre-treatment with nitrogen-plasma can introduce C–N dangling bonds,which provides nucleation sites for subsequent epitaxial growth.The scanning transmission electron microscopy measurements confirm that part of graphene surface was etched by nitrogen-plasma.We study the growth behavior on different areas of graphene surface after pre-treatment,and propose a growth model to explain the epitaxial growth mechanism of GaN films on graphene.Significantly,graphene is found to be effective to reduce the biaxial stress in GaN films and the strain relaxation improves indium-atom incorporation in InGaN/GaN multiple quantum wells(MQWs)active region,which results in the obvious red-shift of light-emitting wavelength of InGaN/GaN MQWs.This work opens up a new way for the fabrication of GaN-based long wavelength light-emitting diodes.

Synthesis of AgInS_2 QDs in droplet microreactors:Online fluorescence regulating through temperature control

For the synthesis of AgInS_2 quantum dots(QDs), a suitable temperature is extremely important for control of the size, shape and fluorescence properties of QDs. Most of synthesis methods for AgInS_2 QDs are based on batch reactors, which bring uneven distribution of temperature, affecting their fluorescence properties and size uniformity. Here we designed a droplet microreactor with a temperature-controllable region, and successfully synthesized water-soluble AgInS_2 QDs. By accurately controlling temperature,we also studied how the reaction temperature affected the fluorescence properties of AgInS_2 QDs. The results showed that with the increasing of reaction temperature, the QDs size increased and the fluorescence peak constantly red-shifted along with enhanced fluorescence intensity. Based on the droplet microreactor, we could achieve more appropriate reaction condition to synthesize AgInS_2 QDs with higher fluorescence quantum yield(QY) and intensity.

Induction and modulation of supramolecular chirality in side-chain azobenzene polymers through the covalent chiral domino effect

Induction,transfer,and modulation of intriguing supramolecular chirality were achieved in a chiral domino-type polymer system using a chiralα-end azobenzene(Azo)moiety as one chiral terminus and achiral Azo repeating units as building blocks.The Azo polymers prepared by atom transfer radical polymerization were imparted with well-controlled chain lengths and a chiral moiety at a well-defined position.One chiral terminus can effectively dictate the helical orientation of the achiral Azo stacks in the aggregation state.The spacer length between the chiral residue and the achiral repeating units dominates the preferred handedness of the side-chain Azo stacks.For instance,the R-configuration moiety with a short alkyl chain(with 0,2,and 3 carbons)induces right-handed supramolecular chirality,whereas those with long alkyl chains(with 4,5,and 6 carbons)induce the opposite helical orientation of the Azo units.Moreover,the chiral regulation of polymer aggregates is successfully achieved using the unique photoisomerization transition of the Azo chromophore and the heat-assisted reassembly approach.Chiral induction and chiral-to-achiral communication are further verified via theoretical simulations.