Unveiling strain-enhanced moiréexciton localization in twisted van der Waals homostructures

Moirésuperlattices,arising from the controlled twisting of van der Waals homostructures at specific angles,have emerged as a promising platform for quantum emission applications.Concurrently,the manipulation of strain provides a versatile strategy to finely adjust electronic band structures,enhance exciton luminescence efficiency,and establish a robust foundation for two-dimensional quantum light sources.However,the intricate interplay between strain and moirépotential remains partially unexplored.Here,we introduce a meticulously designed fusion of strain engineering and the twisted 2L-WSe_(2)/2L-WSe_(2) homobilayers,resulting in the precise localization of moiréexcitons.Employing low-temperature photoluminescence spectroscopy,we unveil the emergence of highly localized moiré-enhanced emission,characterized by the presence of multiple distinct emission lines.Furthermore,our investigation demonstrates the effective regulation of moirépotential depths through strain engineering,with the potential depths of strained and unstrained regions differing by 91%.By combining both experimental and theoretical approaches,our study elucidates the complex relationship between strain and moirépotential,thereby opening avenues for generating strain-induced moiréexciton single-photon sources.

Unveiling optical anisotropy in disrupted symmetry WSe_(2)/SiP heterostructures

Two-dimensional(2D)transition metal dichalcogenides(TMDs)have garnered considerable attention for their promising applications in sensors and optoelectronic devices,owing to their exceptional optical,electronic,and optoelectronic properties.However,the inherent high symmetry of TMD lattices imposes limitations on their functional versatility.Here,we present a strategy to disrupt the C_(3)rotational symmetry of monolayer WSe_(2)by fabricating a heterostructure incorporating WSe_(2)and SiP flakes.Through comprehensive experimental investigations and first-principle calculations,we elucidate that in the WSe_(2)/SiP heterostructure,excitons-both neutral and charged-emanating from WSe_(2)exhibit pronounced anisotropy,which remains robust against temperature variations.Notably,we observe an anisotropic ratio reaching up to 1.5,indicating a substantial degree of anisotropy.Furthermore,we demonstrate the tunability of exciton anisotropy through the application of a magnetic field,resulting in a significant reduction in the anisotropic ratio with increasing field strength,from 1.57 to 1.18.Remarkably,the change in heterojunction anisotropy ratio reaches 24.8%as the magnetic field increases.Our findings elucidate that the perturbation of the C_(3)rotational symmetry of the WSe_(2)monolayer arises from a non-uniform charge density distribution within the layer,exhibiting mirror symmetry.These results underscore the potential of heterostructure engineering in tailoring the properties of isotropic materials and provide a promising avenue for advancing the application of anisotropic devices across various fields.

Evidence for interlayer coupling and moiréexcitons in twisted WS_(2)/WS_(2)homostructure superlattices

The formation of moirésuperlattices in twisted van der Waals(vdW)homostructures provides a versatile platform for designing the electronic band structure of two-dimensional(2D)materials.In graphene and transition metal dichalcogenides(TMDs)moirésystems,twist angle has been shown to be a key parameter for regulating the moirésuperlattice.However,the effect of the modulation of the twist angle on moirépotential and interlayer coupling has not been the subject of experimental investigation.Here,we report the observation of the modulation of moirépotential and intralayer excitons in the WS_(2)/WS_(2)homostructure.By accurately adjusting the torsion angle of the homobilayers,the depth of the moirépotential can be modulated.The confinement effect of the moirépotential on the intralayer excitons was further demonstrated by the changing of temperature and valley polarization.Furthermore,we show that a detection of atomic reconstructions by the low-frequency Raman mapping to map out inhomogeneities in moirélattices on a large scale,which endows the uniformity of interlayer coupling.Our results provide insights for an in-depth understanding of the behaviors of moiréexcitons in the twisted van der Waals homostructure,and promote the study of electrical engineering and topological photonics.

Localization-enhanced moiréexciton in twisted transition metal dichalcogenide heterotrilayer superlattices

The stacking of twisted two-dimensional(2D)layered materials has led to the creation of moirésuperlattices,which have become a new platform for the study of quantum optics.The strong coupling of moirésuperlattices can result in flat minibands that boost electronic interactions and generate interesting strongly correlated states,including unconventional superconductivity,Mott insulating states,and moiréexcitons.However,the impact of adjusting and localizing moiréexcitons in Van der Waals heterostructures has yet to be explored experimentally.Here,we present experimental evidence of the localization-enhanced moiréexcitons in the twisted WSe_(2)/WS_(2)/WSe_(2)heterotrilayer with type-II band alignments.At low temperatures,we observed multiple excitons splitting in the twisted WSe_(2)/WS_(2)/WSe_(2)heterotrilayer,which is manifested as multiple sharp emission lines,in stark contrast to the moiréexcitonic behavior of the twisted WSe_(2)/WS_(2)heterobilayer(which has a linewidth 4 times wider).This is due to the enhancement of the two moirépotentials in the twisted heterotrilayer,enabling highly localized moiréexcitons at the interface.The confinement effect of moirépotential on moiréexcitons is further demonstrated by changes in temperature,laser power,and valley polarization.Our findings offer a new approach for localizing moiréexcitons in twist-angle heterostructures,which has the potential for the development of coherent quantum light emitters.

Effect of layered-coupling in twisted WSe_(2) moiré superlattices

Recently,the discovery of a variety of moiré-related properties in the twisted vertical stacking of two different monolayers has attracted considerable attention.The introduction of small twist angles in transition metal dichalcogenide(TMD)heterostructures leads to the emergence of moirépotentials,which provide a fascinating platform for the study of strong interactions of electrons.While there has been extensive research on moiréexcitons in twisted bilayer superlattices,the capture and study of moiréexcitons in homostructure superlattices with layer-coupling effects remain elusive.Here,we present the observation of moiréexcitons in the twisted 1L-WSe_(2)/1L-WSe_(2)and 1L-WSe_(2)/2L-WSe_(2)homostructures with various layer-coupling interactions.The results reveal that the moirépotential increases(~260%)as the number of underlying layers decreases,indicating the effect of layer coupling on the modulation of the moirépotential.The effects of the temperature and laser power dependence as well as valley polarization on moiréexcitons were further demonstrated,and the crucial spectral features observed were explained.Our findings pave the way for exploring quantum phenomena and related applications of quantum information.

Observation of robust anisotropy in WS_(2)/BP heterostructures

Two-dimensional(2D)anisotropic materials have garnered significant attention in the realm of anisotropic optoelectronic devices due to their remarkable electrical,optical,thermal,and mechanical properties.While extensive research has delved into the optical and electrical characteristics of these materials,there remains a need for further exploration to identify novel materials and structures capable of fulfilling device requirements under various conditions.Here,we employ heterojunction interface engineering with black phosphorus(BP)to disrupt the C_(3) rotational symmetry of monolayer WS_(2).The resulting WS_(2)/BP heterostructure exhibits pronounced anisotropy in exciton emissions,with a measured anisotropic ratio of 1.84 for neutral excitons.Through a comprehensive analysis of magnetic-field-dependent and temperature-evolution photoluminescence spectra,we discern varying trends in the polarization ratio,notably observing a substantial anisotropy ratio of 1.94 at a temperature of 1.6 K and a magnetic field of 9 T.This dynamic behavior is attributed to the susceptibility of the WS_(2)/BP heterostructure interface strain to fluctuations in magnetic fields and temperatures.These findings provide valuable insights into the design of anisotropic optoelectronic devices capable of adaptation to a range of magnetic fields and temperatures,thereby advancing the frontier of material-driven device engineering.

Boron nitride microribbons strengthened and toughened alumina composite ceramics with excellent mechanical,dielectric,and thermal conductivity properties

Aluminum oxide(Al_(2)O_(3))ceramics have been widely utilized as circuit substrates owing to their exceptional performance.In this study,boron nitride microribbon(BNMR)/Al_(2)O_(3)composite ceramics are prepared using spark plasma sintering(SPS).This study examines the effect of varying the amount of toughened phase BNMR on the density,mechanical properties,dielectric constant,and thermal conductivity of BNMR/Al_(2)O_(3)composite ceramics while also exploring the mechanisms behind the toughening and increased thermal conductivity of the fabricated ceramics.The results showed that for a BNMR content of 5 wt%,BNMR/Al_(2)O_(3)composite ceramics displayed more enhanced characteristics than pure Al_(2)O_(3)ceramics.In particular,the relative density,hardness,fracture toughness,and bending strength were 99.95%±0.025%,34.11±1.5 GPa,5.42±0.21 MPa·m^(1/2),and 375±2.5 MPa,respectively.These values represent increases of 0.76%,70%,35%,and 25%,respectively,compared with the corresponding values for pure Al_(2)O_(3)ceramics.Furthermore,during the SPS process,BNMRs are subjected to high temperatures and pressures,resulting in the bending and deformation of the Al_(2)O_(3)matrix;this leads to the formation of special thermal pathways within it.The dielectric constant of the composite ceramics decreased by 25.6%,whereas the thermal conductivity increased by 45.6%compared with that of the pure Al_(2)O_(3)ceramics.The results of this study provide valuable insights into ways of enhancing the performance of Al_(2)O_(3)-based ceramic substrates by incorporating novel BNMRs as a second phase.These improvements are significant for potential applications in circuit substrates and related fields that require high-performance materials with improved mechanical properties and thermal conductivities.

Enhanced valley polarization in WSe_(2)/YIG heterostructures via interfacial magnetic exchange effect

Exploiting the valley degrees of freedom as information carriers provides new opportunities for the development of valleytronics.Monolayer transition metal dichalcogenides(TMDs)with broken space-inversion symmetry exhibit emerging valley pseudospins,making them ideal platforms for studying valley electronics.However,intervalley scattering of different energy valleys limits the achievable degree of valley polarization.Here,we constructed WSe_(2)/yttrium iron garnet(YIG)heterostructures and demonstrated that the interfacial magnetic exchange effect on the YIG magnetic substrate can enhance valley polarization by up to 63%,significantly higher than that of a monolayer WSe_(2)on SiO_(2)/Si(11%).Additionally,multiple sharp exciton peaks appear in the WSe_(2)/YIG heterostructures due to the strong magnetic proximity effect at the magnetic-substrate interface that enhances exciton emission efficiency.Moreover,under the effect of external magnetic field,the magnetic direction of the magnetic substrate enhances valley polarization,further demonstrating that the magnetic proximity effect regulates valley polarization.Our results provide a new way to regulate valley polarization and demonstrate the promising application of magnetic heterojunctions in magneto-optoelectronics.

Exploring the regulatory effect of stacked layers on moiréexcitons in twisted WSe_(2)/WSe_(2)/WSe_(2)homotrilayer

Moirésuperlattices in van der Waals structures have emerged as a powerful platform for studying the novel quantum properties of two-dimensional materials.The periodic moirépatterns generated by these structures lead to the formation of flat mini-bands,which alter the electronic energy bands of the material.The resulting flat electronic bands can greatly enhance strong correlative interactions between electrons,leading to the emergence of exotic quantum phenomena,including moiréphonons and moiréexcitons.While extensive research has been conducted on the exotic quantum phenomena in twisted bilayers of transition metal dichalcogenides(TMDs),and the regulatory effect of stacked layers on moiréexcitons remains unexplored.In this study,we report the fabrication of a twisted WSe_(2)/WSe_(2)/WSe_(2) homotrilayer with two twist angles and investigate the influence of stacked layers on moiréexcitons.Our experiments reveal multiple moiréexciton splitting peaks in the twisted trilayer,with moirépotential depths of 78 and 112 meV in the bilayer and trilayer homostructures,respectively.We also observed the splitting of the moiréexcitons at 90 K,indicating the presence of a deeper moirépotential in the twisted trilayer.Moreover,we demonstrate that stacked layers can tune the moiréexcitons by manipulating temperature,laser power,and magnetic field.Our results provide a new physical model for studying moirésuperlattices and their quantum properties,which could potentially pave the way for the development of quantum optoelectronics.

表面增强拉曼光谱在毒品检测中的应用进展

表面增强拉曼光谱(surfaced-enhanced Raman spectroscopy,SERS)作为一种借助贵金属纳米材料可以增强目标分子信号的拉曼光谱技术,由于其具有指纹识别、高灵敏、高准确度、快速无损、不受水分子干扰等特点,在法庭科学领域中的痕量毒品检测方面逐渐受到人们的关注.SERS不仅用于毒品纯品的检测,对于复杂体系的缴获毒品和人体检材毒品的检测也逐渐成为研究热点.本文重点总结了SERS检测毒品的种类和方法,介绍了用于毒品检测的增强基底的发展,以及基于SERS的检测技术的进展,并对SERS毒品检测数据的分析方法做了概括.最后讨论了SERS在毒品检测面临的主要挑战,并展望了基于SERS毒品痕量检测的未来发展趋势.

Evidence for moiréintralayer excitons in twisted WSe_(2)/WSe_(2) homobi layer superlattices

Recent advances in twisted van der Waals heterostructure superlattices have emerged as a powerful and attractive platform for exploring novel condensed matter physics due to the interplay between the moirépotential and Coulomb interactions.The moirésuperlattices act as a periodic confinement potential in space to capture interlayer excitons(IXs),resulting in moiréexciton arrays,which provide opportunities for quantum emitters and many-body physics.The observation of moiréIXs in twisted transition-metal dichalcogenide(TMD)heterostructures has recently been widely reported.However,the capture and study of the moiréintralayer excitons based on TMD twisted homobilayer(T-HB)remain elusive.Here,we report the observation of moiréintralayer excitons in a WSe_(2)/WSe_(2) T-HB with a small twist angle by measuring PL spectrum.The multiple split peaks with an energy range of 1.55-1.73 eV are different from that of the monolayer WSe_(2) exciton peaks.The split peaks were caused by the trapping of intralayer excitons via the moirépotential.The confinement effect of the moirépotential on the moiréintralayer excitons was further demonstrated by the changing of temperature,laser power,and valley polarization.Our findings provide a new avenue for exploring new correlated quantum phenomena and their applications.

Enhanced interlayer neutral excitons and trions in MoSe_(2)/MoS_(2)/MoSe_(2)trilayer heterostructure

Van der Waals heterostructures have recently emerged,in which two distinct transitional metal dichalcogenide(TMD)monolayers are stacked vertically to generate interlayer excitons(IXs),offing new opportunites for the design of optoelectronic devices.However,the bilayer heterostructure with type-II band alignment can only produce low quantum yield.Here,we present the observation of interlayer neutral excitons and trions in the MoSe_(2)/MoS_(2)/MoSe_(2)trilayer heterostructure(Tri-HS).In comparison to the 8 K bilayer heterostructure,the addition of a MoSe_(2)layer to the Tri-HS can significantly increase the quantum yield of IXs.It is believed the two symmetrical type-II band alignments formed in the Tri-HS could effectively promote the IX radiation recombination.By analyzing the photoluminescence(PL)spectrum of the IXs at cryogenic temperature and the power dependence,the existence of the interlayer trions was confirmed.Our results provide a promising platform for the development of more efficient optoelectronic devices and the investigation of new physical properties of TMDs.

Dynamic control of moirépotential in twisted WS_(2)-WSe_(2)heterostructures

Moirésuperlattices are formed by a lattice mismatch or twist angle in two-dimensional materials,which can generate periodical moirépotentials leading to strong changes in the band structure,resulting in new quantum phenomena.However,the experimental engineering of in-situ deformation of moiréheterostructures remains deficient.Here,we demonstrate a dynamic local deformation of the twisted heterostructures using a diamond anvil cell(DAC),enabling in-situ dynamic modulation of moirépotential in twisted WS_(2)–WSe_(2)heterostructures at room temperature.Deformation of the twisted heterostructure increases the moirépotential,causing a red shift of the moiréexciton resonance,and observed the red shift of the intralayer exciton resonance up to 16.3 meV(less than 1.1 GPa).The blue shift of the interlayer excitons of twisted WS_(2)–WSe_(2)heterostructures shows an evident transition of the pressure sensitive exciton,induced by the dominant effect of modifying the band structure on optical properties.Combined with the spectral changes of pressurized Raman,which further demonstrated that the DAC can efficiently regulate the interlayer coupling.Our results offer an effective strategy for in-situ dynamic modulation of moirépotential,providing a promising platform for the development of novel quantum devices.