Core-Shell Semiconductor-Graphene Nanoarchitectures for Efficient Photocatalysis:State of the Art and Perspectives

Semiconductor photocatalysis holds great promise for renewable energy generation and environment remediation,but generally suffers from the serious drawbacks on light absorption,charge generation and transport,and structural stability that limit the performance.The core-shell semiconductorgraphene(CSSG)nanoarchitectures may address these issues due to their unique structures with exceptional physical and chemical properties.This review explores recent advances of the CSSG nanoarchitectures in the photocatalytic performance.It starts with the classification of the CSSG nanoarchitectures by the dimensionality.Then,the construction methods under internal and external driving forces were introduced and compared with each other.Afterward,the physicochemical properties and photocatalytic applications of these nanoarchitectures were discussed,with a focus on their role in photocatalysis.It ends with a summary and some perspectives on future development of the CSSG nanoarchitectures toward highly efficient photocatalysts with extensive application.By harnessing the synergistic capabilities of the CSSG architectures,we aim to address pressing environmental and energy challenges and drive scientific progress in these fields.

Electronic and thermal properties of Ag-doped single crystal zinc oxide via laser-induced technique

The doping of ZnO has attracted lots of attention because it is an important way to tune the properties of ZnO.Postdoping after growth is one of the efficient strategies.Here,we report a unique approach to successfully dope the single crystalline ZnO with Ag by the laser-induced method,which can effectively further post-treat grown samples.Magnetron sputtering was used to coat the Ag film with a thickness of about 50 nm on the single crystalline ZnO.Neodymium-doped yttrium aluminum garnet(Nd:YAG)laser was chosen to irradiate the Ag-capped ZnO samples,followed by annealing at700℃for two hours to form ZnO:Ag.The three-dimensional(3D)information of the elemental distribution of Ag in ZnO was obtained through time-of-flight secondary ion mass spectrometry(TOF-SIMS).TOF-SIMS and core-level x-ray photoelectron spectroscopy(XPS)demonstrated that the Ag impurities could be effectively doped into single crystalline ZnO samples as deep as several hundred nanometers.Obvious broadening of core level XPS profiles of Ag from the surface to depths of hundred nms was observed,indicating the variance of chemical state changes in laser-induced Ag-doped ZnO.Interesting features of electronic mixing states were detected in the valence band XPS of ZnO:Ag,suggesting the strong coupling or interaction of Ag and ZnO in the sample rather than their simple mixture.The Ag-doped ZnO also showed a narrower bandgap and a decrease in thermal diffusion coefficient compared to the pure ZnO,which would be beneficial to thermoelectric performance.

Deep-ultraviolet photonics for the disinfection of SARS-CoV-2 and its variants(Delta and Omicron)in the cryogenic environment

Deep-ultraviolet(DUV)disinfection technology provides an expeditious and efficient way to suppress the transmission of coronavirus disease 2019(COVID-19).However,the influences of viral variants(Delta and Omicron)and low temperatures on the DUV virucidal efficacy are still unknown.Here,we developed a reliable and uniform planar light source comprised of 275-nm light-emitting diodes(LEDs)to investigate the effects of these two unknown factors and delineated the principle behind different disinfection performances.We found the lethal effect of DUV at the same radiation dose was reduced by the cryogenic environment,and a negative-U large-relaxation model was used to explain the difference in view of the photoelectronic nature.The chances were higher in the cryogenic environment for the capture of excited electrons within active genetic molecules back to the initial photo-ionised positions.Additionally,the variant of Omicron required a significantly higher DUV dose to achieve the same virucidal efficacy,and this was thanks to the genetic and proteinic characteristics of the Omicron.The findings in this study are important for human society using DUV disinfection in cold conditions(e.g.,the food cold chain logistics and the open air in winter),and the relevant DUV disinfection suggestion against COVID-19 is provided.

Type-Ⅱ Core/Shell Nanowire Heterostructures and Their Photovoltaic Applications

Nanowire-based photovoltaic devices have the advantages over planar devices in light absorption and charge transport and collection.Recently,a new strategy relying on type-Ⅱ band alignment has been proposed to facilitate efficient charge separation in core/shell nanowire solar cells.This paper reviews the type-Ⅱ heterojunction solar cells based on core/shell nanowire arrays,and specifically focuses on the progress of theoretical design and fabrication of type-Ⅱ Zn O/Zn Se core/shell nanowire-based solar cells.A strong photoresponse associated with the type-Ⅱ interfacial transition exhibits a threshold of 1.6 e V,which demonstrates the feasibility and great potential for exploring all-inorganic versions of type-Ⅱ heterojunction solar cells using wide bandgap semiconductors.Future prospects in this area are also outlooked.

Novel Evolution Process of Zn-Induced Nanoclusters on Si(111)-(737) Surface

A tiny number of Zn atoms were deposited on Si(111)-(797) surface to study the evolution process of Zninduced nanoclusters. After the deposition, three types(type I, II, and III) of Zn-induced nanoclusters were observed to occupy preferably in the faulted half-unit cells. These Zn-induced nanoclusters are found to be related to one, two, and three displaced Si edge adatoms, and simultaneously cause the depression of one, two, and three closest Si edge adatoms in the neighboring unfaulted half-unit cells at negative voltages, respectively. First-principles adsorption energy calculations show that the observed type I, II, and III nanoclusters can reasonably be assigned as the Zn3Si1, Zn5Si2, and Zn7Si3 clusters,respectively. And Zn3Si1, Zn5Si2, and Zn7Si3 clusters are, respectively, the most stable structures in cases of one, two, and three displaced Si edge adatoms. Based on the above energy-preferred models, the simulated bias-dependent STM images are all well consistent with the experimental observations. Therefore, the most stable Zn7Si3 nanoclusters adsorbed on the Si(111)-(797) surface should grow up on the base of Zn3Si1 and Zn5Si2clusters. A novel evolution process from Zn3Si1 to Zn5Si2, and finally to Zn7Si3 nanocluster is unveiled.

全氧化镓薄膜同质p-n结

制备p-n结以及探索其物理机制在发展各种功能器件和推进其实际应用中起到关键作用.超宽禁带半导体在制备高压高频器件上有着巨大的潜力,但是氧化镓p型掺杂困难限制了氧化镓同质p-n结的制备,进而阻碍了全氧化镓基双极型器件的发展.本文通过一种先进的相转变生长技术结合溅射镀膜的方法,成功制备了n型锡掺杂β相氧化镓/p型氮掺杂β相氧化镓薄膜.本工作成功制作了全氧化镓单边突变同质p-n结二极管,并且详细分析了器件机理.该二极管实现了4×10^(4)的整流比、在40 V下9.18 mΩcm^(2)的低导通电阻、4.41 V的内建电势和1.78的理想因子,并在交流电压下表现出没有过冲的整流特性以及长期稳定性.本工作为氧化镓同质p-n结初窥门径,为氧化镓同质双极型器件奠定了基础,为高压高功率器件的应用开创了道路.

Strain-induced direct-indirect bandgap transition and phonon modulation in monolayer WS2

In situ strain photoluminescence （PL） and Raman spectroscopy have been employed to exploit the evolutions of the electronic band structure and lattice vibrational responses of chemical vapor deposition （CVD）-grown monolayer tungsten disulphide （WS2） under uniaxial tensile strain. Observable broadening and appearance of an extra small feature at the longer-wavelength side shoulder of the PL peak occur under 2.5% strain, which could indicate the direct-indirect bandgap transition and is further confirmed by our density-functional-theory calculations. As the strain increases further, the spectral weight of the indirect transition gradually increases. Over the entire strain range, with the increase of the strain, the light emissions corresponding to each optical transition, such as the direct bandgap transition （K-K） and indirect bandgap transition （F-K, ≥2.5%）, exhibit a monotonous linear redshift. In addition, the binding energy of the indirect transition is found to be larger than that of the direct transition, and the slight lowering of the trion dissociation energy with increasing strain is observed. The strain was used to modulate not only the electronic band structure but also the lattice vibrations. The softening and splitting of the in-plane E＇ mode is observed under uniaxial tensile strain, and polarization-dependent Raman spectroscopy confirms the observed zigzag-oriented edge of WS2 grown by CVD in previous studies. These findings enrich our understanding of the strained states of monolayer transition-metal dichalcogenide （TMD） materials and lay a foundation for developing applications exploiting their strain-dependent optical properties, including the strain detection and light-emission modulation of such emerging two-dimensional TMDs.

Engineered tunneling layer with enhaneed impact ionization for detection improvement in graphene/silicon heterojunction photodetectors

Here,an engineered tunneling layer enhanced photocurrent multiplication through the impact ionization effect was proposed and experimentally demonstrated on the graphene/silic on heterojunction photodetectors.With con sidering the suitable band structure of the insulation material and their special defect states,an atomic layer deposition(ALD)prepared wide-bandgap insulating(WBI)layer of AIN was introduced into the interface of graphene/silicon heterojunction.The promoted tunneling process from this designed structure dem on strated that can effectively help the impact ionization with photogain not only for the regular minority carriers from silicon,but also for the novel hot carries from graphene.As a result,significantly enhanced photocurrent as well as simultaneously decreased dark current about one order were accomplished in this graphene/insulation/silicon(GIS)heterojunction devices with the optimized AIN thickness of〜15 nm compared to the conventional graphene/silicon(GS)devices.Specifically,at the reverse bias of-10 V,a 3.96-A W_1 responsivity with the photogain of~5.8 for the peak response under 850-nm light illumination,and a 1.03-A W_1 responsivity with~3.5 photogain under the 365 nm ultraviolet(UV)illumination were realized,which are even remarkably higher than those in GIS devices with either AI2O3 or the commonly employed SiO2 insulation layers.This work dem on strates a universal strategy to fabricate broad ba nd,low-cost and high-performance photo-detecting devices towards the graphene-silicon optoelectronic integration.

Reversing abnormal hole localization in high-Alcontent AlGaN quantum well to enhance deep ultraviolet emission by regulating the orbital state coupling

AlGaN has attracted considerable interest for ultraviolet(UV)applications.With the development of UV optoelectronic devices,abnormal carrier confinement behaviour has been observed for c-plane-oriented AlGaN quantum wells(QWs)with high Al content.Because of the dispersive crystal field split-off hole band(CH band)composed of pz orbitals,the abnormal confinement becomes the limiting factor for efficient UV light emission.This observation differs from the widely accepted concept that confinement of carriers at the lowest quantum level is more pronounced than that at higher quantum levels,which has been an established conclusion for conventional continuous potential wells.In particular,orientational pz orbitals are sensitive to the confinement direction in line with the conducting direction,which affects the orbital intercoupling.In this work,models of Al_(0.75)Ga_(0.25)N/AlN QWs constructed with variable lattice orientations were used to investigate the orbital intercoupling among atoms between the well and barrier regions.Orbital engineering of QWs was implemented by changing the orbital state confinement,with the well plane inclined from 0°to 90°at a step of 30°(referred to the c plane).The barrier potential and transition rate at the band edge were enhanced through this orbital engineering.The concept of orbital engineering was also demonstrated through the construction of inclined QW planes on semi-and nonpolar planes implemented in microrods with pyramid-shaped tops.The higher emission intensity from the QWs on the nonpolar plane compared with those on the polar plane was confirmed via localized cathodoluminescence(CL)maps.

Multiple fields manipulation on nitride material structures in ultraviolet light-emitting diodes

As demonstrated during the COVID-19 pandemic,advanced deep ultraviolet(DUV)light sources(200–280 nm),such as AlGaN-based light-emitting diodes(LEDs)show excellence in preventing virus transmission,which further reveals their wide applications from biological,environmental,industrial to medical.However,the relatively low external quantum efficiencies(mostly lower than 10%)strongly restrict their wider or even potential applications,which have been known related to the intrinsic properties of high Al-content AlGaN semiconductor materials and especially their quantum structures.Here,we review recent progress in the development of novel concepts and techniques in AlGaNbased LEDs and summarize the multiple physical fields as a toolkit for effectively controlling and tailoring the crucial properties of nitride quantum structures.In addition,we describe the key challenges for further increasing the efficiency of DUV LEDs and provide an outlook for future developments.

Facile synthesis of two-dimensional MoS_(2)/WS_(2) lateral heterostructures with controllable core/shell size ratio by a one-step chemical vapor deposition method

Heterostructures based on two-dimensional(2D) transition-metal dichalcogenides(TMDCs) possess unique electronic and optical properties, which open up unprecedented opportunities in nanoscale optoelectronic devices. Synthesizing high-quality 2D TMDC heterostructures with different core/shell size ratios is of great significance for practical applications. Here, we report a simple one-step chemical vapor deposition(CVD) method for fabricating MoS2/WS2 lateral heterostructures with controllable core/shell size ratio. An ultrathin MoO3/WO3 film prepared by thermal evaporation was used as the precursor, and a step-like heating process was adopted to separately grow MoS2 and WS2 monolayers by taking advantage of the different melting points of MoO3 and WO3 sources. High-quality MoS2/WS2 lateral heterostructures with sharp interfaces were fabricated by optimizing the key growth parameters. Furthermore, the core/shell size ratio of heterostructures could be easily controlled by changing the thickness ratio of MoO3/WO3 film, and an approximately linear dependence between them is revealed. Compared with MoS2 or WS2 monolayers, the MoS2/WS2 heterostructure exhibited a shortened exciton lifetime owing to the type-Ⅱ energy band alignment, which is conducive to the application of high-performance devices. This work provides a facile strategy for the synthesis of 2D lateral heterostructures with controllable size ratio.

Determination of the embedded electronic states at nanoscale interface via surface-sensitive photoemission spectroscopy

The fabrication of small-scale electronics usually involves the integration of different functional materials.The electronic states at the nanoscale interface plays an important role in the device performance and the exotic interface physics.Photoemission spectroscopy is a powerful technique to probe electronic structures of valence band.However,this is a surface-sensitive technique that is usually considered not suitable for the probing of buried interface states,due to the limitation of electron-mean-free path.This article reviews several approaches that have been used to extend the surface-sensitive techniques to investigate the buried interface states,which include hard X-ray photoemission spectroscopy,resonant soft X-ray angle-resolved photoemission spectroscopy and thickness-dependent photoemission spectroscopy.Especially,a quantitative modeling method is introduced to extraa the buried interface states based on the film thickness-dependent photoemission spectra obtained from an integrated experimental system equipped with in-situ growth and photoemission techniques.This quantitative modeling method shall be helpful to further understand the interfacial electronic states between functional materials and determine the interface layers.

Tuning the magnetic and electronic properties of strontium titanate by carbon doping

The magnetic and electronic properties of strontium titanate with different carbon dopant configurations are explored using first-principles calculations with a generalized gradient approximation(GGA)and the GGA+U approach.Our results show that the structural stability,electronic properties and magnetic properties of C-doped SrTiO3 strongly depend on the distance between carbon dopants.In both GGA and GGA+U calculations,the doping structure is mostly stable with a nonmagnetic feature when the carbon dopants are nearest neighbors,which can be ascribed to the formation of a C–C dimer pair accompanied by stronger C–C and weaker C–Ti hybridizations as the C–C distance becomes smaller.As the C–C distance increases,C-doped SrTiO3 changes from an n-type nonmagnetic metal to ferromagnetic/antiferromagnetic half-metal and to an antiferromagnetic/ferromagnetic semiconductor in GGA calculations,while it changes from a nonmagnetic semiconductor to ferromagnetic half-metal and to an antiferromagnetic semiconductor using the GGA+U method.Our work demonstrates the possibility of tailoring the magnetic and electronic properties of C-doped SrTiO3,which might provide some guidance to extend the applications of strontium titanate as a magnetic or optoelectronic material.

Direct synthesis of moirésuperlattice through chemical vapor deposition growth of monolayer WS_(2)on plasma-treated HOPG

Vertical van der Waals(vdW)heterostructures composed of two-dimensional(2D)layered materials have recently attracted substantial interests due to their unique properties.However,the direct synthesis of moirésuperlattice remains a great challenge due to the difficulties in heterogeneous nucleation on smooth vdW surfaces.Here,we report a controllable chemical vapor deposition growth of complete monolayer WS_(2)on highly ordered pyrolytic graphite(HOPG)substrates through the plasma pretreatment.The results show that the morphologies of the grown WS_(2)have a strong dependence on the plasma parameters,including gas composition,source power,and treatment time.It is found that the surface C–C bonds are broken in the plasma pretreated HOPG,and the formed small clusters can act as the nucleation sites for the subsequent growth of WS_(2).Moreover,the height of clusters dominates the growth mode of WS_(2)islands.A transition from a 2D mode to three-dimensional(3D)growth mode occurs when the height is higher than the interlayer spacing of the heterostructure.Besides,diverse moirésuperlattices with different twist angles for WS_(2)/HOPG heterostructures are observed,and the formation mechanism is further analyzed by firstprinciples calculations.

Special issue on the 100^(th) anniversary of Xiamen University

This special issue is devoted to the celebration of the century anniversary of Xiamen University(XMU)(6 April 2021)and the establishment of the LSA Editorial Office in Xiamen(3 July 2021),a collection to highlight the recent exciting research works performed in XMU or by XMU alumni,from all aspects of optics and photonics,including basic,applied and engineering research and applications.The guest editors are three XMU alumni who are active researchers in these areas:Professor Minghui Hong from National University of Singapore,Professor Zhongqun Tian and Professor Junyong Kang from XMU.

Multi-step in situ interface modification method for emission enhancement in semipolar deep-ultraviolet light emitting diodes

SemipolarⅢ-nitrides have attracted increasing attention in applications of optoelectronic devices due to the much reduced polarization field.A high-quality semipolar AlN template is the building block of semipolar AlGaN-based deep-ultraviolet light emitting diodes(DUV LEDs),and thus deserves special attention.In this work,a multi-step in situ interface modification technique is developed for the first time,to our knowledge,to achieve high-quality semipolar AlN templates.The stacking faults were efficiently blocked due to the modification of atomic configurations at the related interfaces.Coherently regrown AlGaN layers were obtained on the in situ treated AlN template,and stacking faults were eliminated in the post-grown AlGaN layers.The strains between AlGaN layers were relaxed through a dislocation glide in the basal plane and misfit dislocations at the heterointerfaces.In contrast,high-temperature ex situ annealing shows great improvement in defect annihilation,yet suffers from severe lattice distortion with strong compressive strain in the AlN template,which is unfavorable to the post-grown AlGaN layers.The strong enhancement of luminous intensity is achieved in in situ treated AlGaN DUV LEDs.The in situ interface modification technique proposed in this work is proven to be an efficient method for the preparation of high-quality semipolar Al N,showing great potential towards the realization of high-efficiency optoelectronic devices.