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Observation of Bragg polaritons in monolayer tungsten disulphide

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Nano Research 2022年第2期15卷 1479-1485页

作者： Xu Wang Lishu Wu Xuewen Zhang Weihuang Yang Zheng Sun jingzhi shang Wei Huang Ting Yu Shaanxi Institute of Flexible Electronics(SIFE) Northwestern Poly technical University(NPU)1 Dongxiang RoadXi’an 710129China Division of Physics and Applied Physics School of Physical and Mathematical SciencesNanyang Technological UniversitySingapore 637371 Ministry of Education Engineering Research Center of Smart Microsensors and Microsystems College of Electronics and InformationHangzhou Dianzi UniversityHangzhou 310018China State Key Laboratory of Precision Spectroscopy East China Normal University500 Dongchuan RoadShanghai 200241China Key Laboratory of Flexible Electronics and Institute of Advanced Materials National Jiangsu Synergetic Innovation Centre for Advanced MaterialsNanjing Tech University30 South Puzhu RoadNanjing 211816China

Strong light-matter interactions involved with photons and quasiparticles are fundamentally interesting to access the wealthy many-body physics in quantum *** emerging two-dimensional(2D)semiconductors with large exciton binding energies and strong quantum confinement allow to investigate exciton-photon coupling at elevated *** we report room-temperature formation of Bragg polaritons in monolayer semiconductor on a dielectric mirror through the exciton-Bragg photon *** the negative detuning energy of -30 meV,angle-resolved reflection signals reveal anti-crossing behaviors of lower and upper polariton branches at±18°together with the Rabi splitting of 10 ***,the strengthened photoluminescence appears in the lower polariton branch right below the anti-crossing angles,indicating the presence of the characteristic bottleneck effect caused by the slowing exciton-polariton energy relaxation towards the band *** extracted coupling strength is between the ones of weak and distinct strong coupling regimes,where the eigenenergy splitting induced by the moderate coupling is resolvable but not large enough to fully separate two polaritonic *** work develops a simplified strategy to generate exciton-polaritons in 2D semiconductors and can be further extended to probe the intriguing bosonic characteristics of these quasiparticles,such as Bose-Einstein condensation,polariton lasing and superfluidity,directly at the material surfaces.

关键词： Bragg polariton exciton-photon coupling two-dimensional(2D)semiconductor Rabi splitting microcavity

Monolayer tungsten disulfide in photonic environment:Angleresolved weak and strong light-matter coupling

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Nano Research 2022年第6期15卷 5619-5625页

作者： Xuewen Zhang Lishu Wu Xu Wang Silin He Hanwei Hu Guangchao Shi Xingwang Zhang jingzhi shang Ting Yu Institute of Flexible Electronics(IFE) Northwestern Polytechnical University(NPU)Xi’an 710129China School of Physical and Mathematical Sciences Nanyang Technological UniversitySingapore 637371Singapore Suzhou Institute of Nano-Tech and Nano-Bionics Chinese Academy of SciencesSuzhou 215123China School of Physics and Technology Wuhan UniversityWuhan 430072China

Light-matter interactions in two-dimensional transition metal dichalcogenides(TMDs)are sensitive to the surrounding dielectric *** on the interacting strength,weak and strong exciton–photon coupling effects can occur when the exciton energy is resonant with the one of *** we report angle-resolved spectroscopic signatures of monolayer tungsten disulfide(1L-WS2)in weak and strong exciton–photon coupling *** optical response of 1L-WS_(2)in the momentum space is uncovered by employing a dielectric mirror as substrate,where the energy dispersion is angleindependent while the amplitudes increase at high detection *** 1L-WS_(2)sits on top of a dielectric layer on silicon,the resonant trapped photon weakly couples with the exciton,in which the minimum of reflection dip shifts at both sides of the crossing angle while the emitted exciton energy remains *** unusual shift of reflection dip is attributed to the presence of Fano resonance under white-light *** embedding 1L-WS_(2)into a dielectric microcavity,strong exciton–photon coupling results in the formation of lower and upper polariton branches with an appreciable Rabi splitting of 34 meV at room temperature,where the observed blueshift of the lower polariton branch is indicative of the enhanced polaritonpolariton *** findings highlight the effect of dielectric environment on angle-resolved optical response of exciton–photon interactions in a two-dimensional semiconductor,which is helpful to develop practical TMD-based architectures for photonic and polaritonic applications.

关键词： light-matter coupling Fano resonance Rabi splitting exciton-polariton dielectric effect

Probing angle-resolved reflection signatures of intralayer and interlayer excitons in monolayer and bilayer MoS_(2)

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Nano Research 2023年第5期16卷 7844-7850页

作者： Hanwei Hu Xuewen Zhang Xinyu Zhang Lishu Wu Vanessa Li Zhang Silin He Guangchao Shi Ting Yu jingzhi shang Institute of Flexible Electronics Northwestern Polytechnical UniversityXi’an 710129China School of Materials Science and Engineering Nanyang Technological UniversitySingapore 639798Singapore School of Physics and Technology Wuhan UniversityWuhan 430072China

Strongly bound excitons in atomically thin transition metal dichalcogenides offer many opportunities to reveal the underlying physics of basic quasiparticles and many-body effects in the two-dimensional(2D)*** reflection investigation on band-edge exciton transitions is essential to exploring wealthy light–matter interactions in the emerging 2D semiconductors,whereas angle-resolved reflection(ARR)characteristics of intralayer and interlayer excitons in 2D MoS_(2)layers remain ***,we report ARR spectroscopic features of A,B,interlayer excitons in monolayer(ML)and bilayer(BL)MoS_(2)on three kinds of photonic substrates,involving distinct exciton–photon *** a BL MoS_(2)on a protected silver mirror,the interlayer exciton with one-third amplitude of A exciton appears at 0.05 eV above the A exciton energy,exhibiting an angleinsensitive energy *** ML and BL MoS_(2)lie on a SiO_(2)-covered silicon,the broad trapped-photon mode weakly couples with the reflection bands of A and B excitons by the Fano resonance effect,causing the asymmetric lineshapes and the redshifted *** transferring MoS_(2)layers onto a one-dimensional photonic crystal,two high-lying branches of B-exciton polaritons are formed by the interactions between B excitons and Bragg photons,beyond the weak-coupling *** work provides ARR spectral benchmarks of A,B,interlayer excitons in ML and BL MoS_(2),gaining insights into the interpretation of light–matter interactions in 2D semiconductors and the design of their devices for practical photonic applications.

关键词： MoS_(2) exciton exciton–photon coupling exciton–polariton angle-resolved reflection

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

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Nano Research 2015年第8期8卷 2562-2572页

作者： Yanlong Wang Chunxiao Cong Weihuang Yang jingzhi shang Namphung Peimyoo Yu Chen Junyong Kang Jianpu Wang Wei Huang Ting Yu Nanyang Technological University-Nanjing Tech Center of Research and Development Nanjing Tech University Nanjing 211816 China Division of Physics and Applied Physics School of Physical and Mathematical Sciences Nanyang Technological University 637371 Singapore Fujian Key Laboratory of Semiconductor Materials and Applications Department of Physics Xiamen University Xiamen 361005 China Key Laboratory of Flexible Electronics (KLOFE) and Institute of Advanced Materials (IAM) Jiangsu National Synergetic Center for Advanced Materials (SICAM) Nanjing Tech University (NanjingTech) Nanjing 211816 China Key Laboratory for Organic Electronics & Information Displays (KLOEID) and Institute of Advanced Materials (IAM) Nanjing University ot Posts & Telecommunications 9 Wenyuan Road Nanjing 210046 China

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.

关键词： monolayer WS2 strain light-emission tuning indirect transition trion crystallographic orientation

Light-emitting devices based on atomically thin MoSe_(2)

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Journal of Semiconductors 2024年第4期45卷 19-35页

作者： Xinyu Zhang Xuewen Zhang Hanwei Hu Vanessa Li Zhang Weidong Xiao Guangchao Shi Jingyuan Qiao Nan Huang Ting Yu jingzhi shang Institute of Flexible Electronics Northwestern Polytechnical UniversityXi'an 710129China School of Physics and Technology Wuhan UniversityWuhan 430072China Wuhan Institute of Quantum Technology Wuhan 430206China

Atomically thin MoSe_(2) layers,as a core member of the transition metal dichalcogenides(TMDs)family,benefit from their appealing properties,including tunable band gaps,high exciton binding energies,and giant oscillator strengths,thus pro-viding an intriguing platform for optoelectronic applications of light-emitting diodes(LEDs),field-effect transistors(FETs),sin-gle-photon emitters(SPEs),and coherent light sources(CLSs).Moreover,these MoSe_(2) layers can realize strong excitonic emis-sion in the near-infrared wavelengths,which can be combined with the silicon-based integration technologies and further encourage the development of the new generation technologies of on-chip optical interconnection,quantum computing,and quantum information ***,we overview the state-of-the-art applications of light-emitting devices based on two-dimensional MoSe_(2) ***,we introduce recent developments in excitonic emission features from atomically thin MoSe_(2) and their dependences on typical physical ***,we focus on the exciton-polaritons and plasmon-exciton polaritons in MoSe_(2) coupled to the diverse forms of optical ***,we highlight the promising applications of LEDs,SPEs,and CLSs based on MoSe_(2) and their ***,we summarize the challenges and opportunities for high-quality emis-sion of MoSe_(2) and high-performance light-emitting devices.

关键词： MoSe_(2) light-matter interaction exciton polariton light-emitting device

Spatial variations of valley splitting in monolayer transition metal dichalcogenide

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InfoMat 2020年第3期2卷 585-592页

作者： Chenji Zou Hongbo Zhang Yu Chen Shun Feng Lishu Wu Jing Zhang Ting Yu jingzhi shang Chunxiao Cong Division of Physics and Applied Physics School of Physical and Mathematical SciencesNanyang Technological UniversitySingaporeSingapore Institute of Flexible Electronics Northwestern Polytechnical UniversityXi'anShaanxiChina State Key Laboratory of ASIC and System School of Information Science and TechnologyFudan UniversityShanghaiChina

In monolayer group-VI transition metal dichalcogenides(TMDs),valley splitting features have received a lot of attention since it can be potentially utilized for information storing and *** the known two-dimensional(2D)TMDs,monolayer WSe2 or MoSe2 has been mostly selected for excitonic and valleytronic physics studies because of its sharp and well-resolved excitonic spectral ***,their high optical quality leads to a tremendous desire for developing promising WSe2-and MoSe2-based valleytronic *** this goal,exploring the uniformity of valley features crossing an entire piece of monolayer becomes necessary and ***,we performed the systematic magnetophotoluminescence mapping measurements on mechanically exfoliated monolayer WSe2 and observed unconventional spatial variations of valley *** observed nonuniformity is attributed to the modulated doping,which is probably due to the different distributions of unintentional absorbates across the *** an unexpected doping effect shows the nonnegligible influence on the valley Zeeman splitting of the trion emission(XT)while affecting that of the neutral exciton emission(X0)trivially,evidencing for the large valleytronic sensitivity of the charged *** work not only enriches the understanding of the doping effect on valley splitting but also is meaningful for developing 2D valleytronics.

关键词： doping effect monolayer WSe2 photoluminescence transition metal dichalcogenide valley splitting

Raman scattering investigation of twisted WS_(2)/MoS_(2) heterostructures: interlayer mechanical coupling versus charge transfer

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Nano Research 2021年第7期14卷 2215-2223页

作者： Lishu Wu Chunxiao Cong jingzhi shang Weihuang Yang Yu Chen Jiadong Zhou Wei Ai Yanlong Wang Shun Feng Hongbo Zhang Zheng Liu Ting Yu Division of Physics and Applied Physics School of Physical and Mathematical SciencesNanyang Technological UniversitySingapore 637371Singapore School of Information Science and Technology Fudan UniversityShanghai 200433China Shaanxi Institute of Flexible Electronics Northwestern Polytechnical UniversityXi’an 710129China Engineering Research Center of Smart Microsensors and Microsystems Ministry of EducationCollege of Electronics and InformationHangzhou Dianzi UniversityHangzhou 310018China School of Materials Science and Engineering Nanyang Technological UniversitySingapore 639798Singapore Key Laboratory of Chemical Lasers Dalian Institute of Chemical PhysicsChinese Academy of SciencesDalian 116023China

Twisted van der Waals homo-and hetero-structures have aroused great attentions due to their unique physical properties,providing a new platform to explore the novel two-dimensional(2D)condensed matter *** robust dependence of phonon vibrations and electronic band structures on the twist angle has been intensively observed in transition metal dichalcogenide(TMD)***,the effects of twist angle on the lattice vibrational properties in the TMD heterostructures have not caused enough ***,we report the distinct evolutions of Raman scattering and the underlying interlayer interactions in the twisted WS_(2)/MoS_(2) *** shifts and linewidths of E_(2g)(Γ)and A_(1g)(Γ)phonon modes are found to be twist angle *** particular,analogous to that of the twisted TMD homostructures,the frequency separations between E_(2g)(Γ)and A_(1g)(Γ)modes of MoS_(2) and WS_(2) in the twisted heterostructures varying with twist angle correlate with the interlayer mechanical coupling,essentially originating from the spacing-related repulsion between sulfur ***,the opposite shift behaviors and broadening of A_(1g)(Γ)modes caused by charge transfer are also observed in the twisted *** calculated interlayer distances and band alignment of twisted WS_(2)/MoS_(2) through density functional theory further evidence our interpretations on the roles of the interlayer mechanical coupling and charge transfer in variations of Raman *** understanding and controlling of interlayer interaction through the stacking orientation are significant for future optoelectronic device design based on the newly emerged 2D heterostructures.

关键词： van der Waals heterostructures interlayer coupling Raman scattering twist angle charge transfer

Defect suppression and energy level alignment in formamidinium-based perovskite solar cells

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Journal of Energy Chemistry 2022年第4期31卷 65-72页

作者： Yi Wang Xiaobing Wang Chenhui Wang Renying Cheng Lanxin Zhao Xu Wang Xuewen Zhang jingzhi shang Huang Zhang Lichen Zhao Yongguang Tu Wei Huang Frontiers Science Center for Flexible Electronics Xi’an Institute of Flexible Electronics(IFE)&Xi’an Institute of Biomedical Materials and EngineeringNorthwestern Polytechnical UniversityXi’an 710072ShaanxiChina Engineering Research Centre of Environment-Friendly Functional Materials Ministry of EducationFujian Engineering Research Centre of Green Functional MaterialsHuaqiao UniversityXiamen 361021FujianChina Honors College Northwestern Polytechnical UniversityXi’an 710072ShaanxiChina State Key Laboratory for Artificial Microstructure and Mesoscopic Physics School of PhysicsFrontiers Science Center for Nano-optoelectronics&Collaborative Innovation Center of Quantum MatterPeking UniversityBeijing 100871China Key Laboratory of Flexible Electronics(KLoFE)&Institution of Advanced Materials(IAM) Jiangsu National Synergetic Innovation Center for Advanced Materials(SICAM)Nanjing Tech UniversityNanjing 211816JiangsuChina Key Laboratory for Organic Electronics&Information Displays(KLOEID)&Institute of Advanced Materials(IAM) Nanjing University of Posts and TelecommunicationsNanjing 210023JiangsuChina

The vast majority of high-performance perovskite solar cells(PSCs) are based on a formamidinium lead iodide(FAPbI_(3))-dominant composition. Nevertheless, the FA-based perovskite films suffer from undesirable phase transition and defects-induced non-ideal interfacial recombination, which significantly induces energy loss and hinders the improvement of device performance. Herein, we employed 4-fluorophenylmethylammonium iodide(F-PMAI) to modulate surface structure and energy level alignment of the FA-based perovskite films. The superior optoelectronic films were obtained with reduced trap density, pure α-phase FAPbI_(3) and favorable energy band bending. The lifetime of photogenerated charge carriers increased from 489.3 ns to 1010.6 ns, and a more “p-type” perovskite film was obtained by the post-treatment with F-PMAI. Following this strategy, we demonstrated an improved power conversion efficiency of 22.59% for the FA-based PSCs with an open-circuit voltage loss of 399 m V.

关键词： Perovskite solar cells Defect suppression Energy level alignment Phase transition 4-Fluorophenylmethylammonium iodide

Probing magnetic-proximity-effect enlarged valley splitting in monolayer WSe2 by photoluminescence

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Nano Research 2018年第12期11卷 6252-6259页

作者： Chenji Zou Chunxiao Cong jingzhi shang Chuan Zhao Mustafa Eginligil Lishu Wu Yu Chen Hongbo Zhang Shun Feng Jing Zhang Hao Zeng Wei Huang Ting Yu Address correspondence to Ting Yu yuting@ntu.edu.sg Chunxiao Cong cxcong@*** Hao Zeng haozeng@bufalo.edu Wei Huang lamwhuang( ***

Possessing a valley degree of freedom and potential in information processing by manipulating valley features (such as valley splitting), group-VI monolayer transition metal dichalcogenides have attracted enormous interest. This valley splitting can be measured based on the difference between the peak energies of σ+ and σ− polarized emissions for excitons or trions in direct band gap monolayer transition metal dichalcogenides under perpendicular magnetic fields. In this work, a well-prepared heterostructure is formed by transferring exfoliated WSe2 onto a EuS substrate. Circular-polarization-resolved photoluminescence spectroscopy, one of the most facile and intuitive methods, is used to probe the difference of the gap energy in two valleys under an applied out-of-plane external magnetic field. Our results indicate that valley splitting can be enhanced when using a EuS substrate, as compared to a SiO2/Si substrate. The enhanced valley splitting of the WSe2/EuS heterostructure can be understood as a result of an interfacial magnetic exchange field originating from the magnetic proximity effect. The value of this magnetic exchange field, based on our estimation, is approximately 9 T. Our findings will stimulate further studies on the magnetic exchange field at the interface of similar heterostructures.

关键词： valley splitting transition metal dichalcogenides magnetic proximity effect heterostructure magnetic exchange field