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High-performance polarization-sensitive photodetector based on a few-layered PdSe2 nanosheet

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Nano Research 2020年第6期13卷 1780-1786页

作者： Jiahong Zhong Juan Yu Lingkai Cao Cheng Zeng Junnan Ding chunxiao cong Zongwen Liu Yanping Liu School of Physics and Electronics Hunan Key Laboratory for Super-microstructure and Ultrafast ProcessCentral South UniversityChangsha410083China School of Electronics and Information Hangzhou Dianzi UniversityHangzhou310018China School of Information Science and Technology Fudan UniversityShanghai200433China School of Chemical and Biomolecular Engineering The University of SydneySydneyNSW2006Australia Shenzhen Research Institute of Central South University Shenzhen518057China State Key Laboratory of High Performance Complex Manufacturing Central South UniversityChangsha410083China

The extraordinary optical and electronic properties of anisotropic two-dimensional materials,such as black phosphorus,ReS2,and GeSe,enable them a promising component of polarization-sensitive ***,these applications are significantly limited by the challenges of air-stability,response time,and linearly dichroic ***,palladium diselenide(PdSe2)with high air stability is an emerging material that has robust in-plane anisotropy induced by its asymmetric pentagonal lattice *** have successfully prepared a few-layer PdSe2 using micromechanical exfoliation,and here we demonstrate the strong linear dichroism behavior of PdSe2 by polarization-resolved absorption spectra *** unique linear dichroism,endows the PdSe2 photodetector powerful ability to detect polarized *** photodetector based on 5L PdSe2,as tested with polarization-dependent photocurrent mapping,exhibited competitive capability to detect polarized light,achieving a significant photocurrent on/off ratio(>10^2),the quite fast response time(<11 ms)and robust linearly dichroic ratios(/max//min≈1.9 at 532 nm).These results are essential advance in the development of polarization-sensitive photodetector,a crucial step towards opening up a new avenue for the application of 2D optoelectronic devices.

关键词： PdSe2 anisotropy polarization-sensitive linear dichroism

Electrical field tuning of magneto-Raman scattering in monolayer graphene

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Nano Research 2015年第4期8卷 1139-1147页

作者： Xiaonan Shen Caiyu Qiu Bingchen Cao chunxiao cong Weihuang Yang Haomin Wang Ting Yu Division of Physics and Applied Physics School of Physical and Mathematical Sciences Nanyang Technological University 637371 Singapore Singapore Wenzhou Institute of Biomaterials and Engineering Wenzhou 325001 China Department of Physics Faculty of Science National University of Singapore 117542 Singapore Singapore Graphene Research Center Faculty of Science National University of Singapore 117546 Singapore Singapore

In this work, we report the electrical field tuning of magneto-phonon resonance in monolayer graphene under magnetic fields up to 9 T. It is found that the carrier concentration can drastically affect the G （E2g） phonon response to a varying magnetic field through a pronounced magneto-phonon resonance （MPR）. In charge neutral or slightly doped monolayer graphene, both the energy and the line width of the E2g phonon show clear variation with magnetic fields. This is attributed to magneto-phonon resonance between magnetoexcitations and the E2g phonons. In contrast, when the Fermi level of the monolayer graphene is far away from the Dirac point, the G band shows weak magnetic dependence and exhibits a symmetric line-shape. This suggests that the magneto-phonon coupling around 4 T has been switched off due to the Pauli blocking of the inter-Landau level excitations. Moreover, the G band asymmetry caused by Fano resonance between excitonic many-body states and the E2g phonons is observed. This work offers a way to study the magnetoexcitation phonon interaction of materials through magneto-Raman spectroscopy with an external electrical field.

关键词： monolayer graphene magneto-phononresonance Raman electrical field suspended graphene

Remarkable anisotropic phonon response in uniaxially strained few-layer black phosphorus

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Nano Research 2015年第12期8卷 3944-3953页

作者： Yanlong Wang chunxiao cong Ruixiang Fei Weihuang Yang Yu Chen Bingchen Cao Li Yang Ting Yu Division of Physics and Applied Physics School of Physical and Mathematical Sciences Nanyang Technological University 637377 Singapore Department of Physics Washington University in St. Louis St. Louis Missouri 63130 USA Department of Physics Faculty of Science National University of Singapore 117542 Singapore

Black phosphorus （BP） is a good candidate for studying strain effects on two- dimensional （2D） materials beyond graphene and transition-metal dichalcogenides. This is because of its particular ability to sustain high strain and remarkably anisotropic mechanical properties resulting from its unique puckered structure. We here investigate the dependence of lattice vibrational frequencies on cry- stallographic orientations in uniaxially strained few-layer BP by in-situ strained Raman spectroscopy. The out-of-plane A1 mode is sensitive to uniaxial strain along the near-armchair direction whereas the in-plane B2g and A2 modes are sensitive to strain in the near-zigzag direction. For uniaxial strains applied away from these directions, all three phonon modes are linearly redshifted. Our experimental observation is explained by the anisotropic influence of uniaxial tensile strain on structural properties of BP using density functional theory. This study demonstrates the possibility of selective tuning of in-plane and out-of-plane phonon modes in BP by uniaxial strain and makes strain engineering a promising avenue for extensively modulating the optical and mechanical properties of 2D materials.

关键词： black phosphorus uniaxial strain Raman spectroscopy anisotrop density functional theory

Molecular interactions induced collapse of charge density wave quantum states in 2H tantalum disulfide nanosheets

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Nano Research 2023年第5期16卷 6960-6966页

作者： Borgea G.M.Ekoya Jinkun Han Junqiang Zhu Yabing Shan Ran Liu Laigui Hu Zhi-Jun Qiu chunxiao cong Micro-Nano System Center School of Information Science and TechnologyFudan UniversityShanghai 200438China Yiwu Research Institute of Fudan University Yiwu 322000China

2H-tantalum disulfide(2H-TaS_(2))is a layered metallic transition metal dichalcogenide(TMD)that has recently been studied from the perspective of new physics phenomena,including simultaneous lattice distortion and charge density modulation known as the charge density wave(CDW)*** we explored the collapse of CDW states in few-layer 2H-TaS_(2)induced by molecular interactions using Raman *** results indicate that the CDW states disappear in few-layer 2H-TaS_(2)with rhodamine 6G(R6G)adsorbed due to the charge transfer,which is reflected by the change of behaviors of lattice vibrational modes in 2HTaS_(2).We observed the 2-phonon mode that signifies the CDW formation in 2H-TaS_(2),becomes a phonon-hardened mode when R6G molecules are absorbed on its surface.R6G adsorption further induces the breakdown of the Raman polarization selection rule in 2H-TaS_(2),which results in the alteration of the A_(1g)phonon mode polarization state of 2H-TaS_(2).This study can shed light not only on the underlying mechanisms of CDW states but also on controlling the CDW states under a variety of environmental conditions.

关键词： charge density wave tantalum disulfide rhodamine 6G charge transfer phonon mode

Unveiling the origin of anomalous low-frequency Raman mode in CVD-grown monolayer WS_(2)

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Nano Research 2021年第11期14卷 4314-4320页

作者： Qian Xiang Xiaofei Yue Yanlong Wang Bin Du Jiajun Chen Shaoqian Zhang Gang Li chunxiao cong Ting Yu Qingwei Li Yuqi Jin Key Laboratory of Chemical Lasers Dalian Institute of Chemical PhysicsChinese Academy of SciencesDalian116023China State Key Laboratory of ASIC&System School of Information Science and TechnologyFudan UniversityShanghai200433China School of Materials Science and Engineering Xi’an Polytechnic UniversityXi’an710048China Division of Physics and Applied Physics School of Physical and Mathematical SciencesNanyang Technological University637371SingaporeSingapore University of Chinese Academy of Sciences Beijing100049China

Substrates provide the necessary support for scientific explorations of numerous promising features and exciting potential applications in two-dimensional (2D) transition metal dichalcogenides (TMDs). To utilize substrate engineering to alter the properties of 2D TMDs and avoid introducing unwanted adverse effects, various experimental techniques, such as high-frequency Raman spectroscopy, have been used to understand the interactions between 2D TMDs and substrates. However, sample-substrate interaction in 2D TMDs is not yet fully understood due to the lack of systematic studies by techniques that are sensitive to 2D TMD-substrate interaction. This work systematically investigates the interaction between tungsten disulfide (WS_(2)) monolayers and substrates by low-frequency Raman spectroscopy, which is very sensitive to WS_(2)-substrate interaction. Strong coupling with substrates is clearly revealed in chemical vapor deposition (CVD)-grown monolayer WS_(2) by its low-wavenumber interface mode. It is demonstrated that the enhanced sample-substrate interaction leads to tensile strain on monolayer WS_(2), which is induced during the cooling process of CVD growth and could be released for monolayer WS_(2) sample after transfer or fabricated by an annealing-free method such as mechanical exfoliation. These results not only suggest the effectiveness of low-frequency Raman spectroscopy for probing sample-substrate interactions in 2D TMDs, but also provide guidance for the design of high-performance devices with the desired sample-substrate coupling strength based on 2D TMDs.

关键词： interface mode monolayer tungsten disulfide low-frequency Raman spectroscopy sample-substrate interaction strain chemical vapor deposition

Intrinsic excitonic emission and valley Zeeman splitting in epitaxial MS2(M=Mo and W)monolayers on hexagonal boron nitride

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Nano Research 2018年第12期11卷 6227-6236页

作者： chunxiao cong Chenji Zou Bingchen Cao Lishu Wu Jingzhi Shahg Haomin Wang Zhijun Qiu Laigui Hu Pengfei Tian Ran Liu Ting Yu State Key Laboratory of ASIC and System School of Information Science and TechnologyFudan UniversityShanghai 200433China Division of Physics and Applied Physics School of Physical and Mathematical SciencesNanyang Technological UniversitySingapore 637371Singapore State Key Laboratory of Functional Materials for Informatics Shanghai Institute of Microsystem and Information TechnologyChinese Academy of Sciences865Changning RoadShanghai200050China

Two-dimensional (2D) semiconductors, represented by 2D transition metal dichalcogenides (TMDs), exhibit rich valley physics due to strong spin-orbit/spin-valley coupling. The most common way to probe such 2D systems is to utilize optical methods, which can monitor light emissions from various excitonic states and further help in understanding the physics behind such phenomena. Therefore, 2D TMDs with good optical quality are in great demand. Here, we report a method to directly grow epitaxial WS2 and MoS2 monolayers on hexagonal boron nitride (hBN) flakes with a high yield and high optical quality; these monolayers show better intrinsic light emission features than exfoliated monolayers from natural crystals. For the first time, the valley Zeeman splitting of WS2 and MoS2 monolayers on hBN has been visualized and systematically investigated. This study paves a new way to produce high optical quality WS2 and MoS2 monolayers and to exploit their intrinsic properties in a multitude of applications.

关键词： transition metal dichalcogenides intrinsic excitonic emission valley Zeeman splitting hexagonal boron nitride chemical vapor deposition

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

Effects of interlayer coupling on the excitons and electronic structures of WS_(2)/hBN/MoS_(2) van der Waals heterostructures

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Nano Research 2022年第3期15卷 2674-2681页

作者： Xudan Zhu Junbo He Rongjun Zhang chunxiao cong Yuxiang Zheng Hao Zhang Songyou Wang Haibin Zhao Meiping Zhu Shanwen Zhang Shaojuan Li Liangyao Chen Key Laboratory of Micro and Nano Photonic Structures Ministry of EducationShanghai Engineering Research Center of Ultra-Precision Optical ManufacturingSchool of Information Science and EngineeringFudan UniversityShanghai 200433China State Key Laboratory of ASIC and System School of Information Science and EngineeringFudan UniversityShanghai 200433China Laboratory of Thin Film Optics Shanghai Institute of Optics and Fine MechanicsChinese Academy of SciencesShanghai 201800China Grating Technology Laboratory Changchun Institute of Optics and Fine Mechanics and PhysicsChinese Academy of SciencesChangchun 130033China State Key Laboratory of Applied Optics Changchun Institute of Optics Fine Mechanics and PhysicsChinese Academy of SciencesChangchun 130033China

Inserting hexagonal boron nitride(hBN)as barrier layers into bilayer transition metal dichalcogenides heterointerface has been proved an efficient method to improve two dimensional tunneling optoelectronic device ***,the physical picture of interlayer coupling effect during incorporation of monolayer(1L-)hBN is not explicit *** this article,spectroscopic ellipsometry was used to experimentally obtain the broadband excitonic and critical point properties of WS_(2)/MoS_(2)and WS_(2)/hBN/MoS_(2)van der Waals *** find that 1L-hBN can only slightly block the interlayer electron transfer from WS_(2)layer to MoS_(2)***,insertion of 1L-hBN weakens the interlayer coupling effect by releasing quantum confinement and reducing efficient dielectric ***,the exciton binding energies in WS_(2)/hBN/MoS_(2)heterostructures blueshift comparing to those in WS_(2)/MoS_(2)*** this exciton binding energies tuning process,the reducing dielectric screening effect plays a leading *** the meantime,the quasi-particle(QP)bandgap remains unchanged before and after 1L-hBN insertion,which is attributed to released quantum confinement and decreased dielectric screening effects canceling each *** QP bandgap as along with blueshift exciton binding energies lead to the redshift exciton transition energies in WS_(2)/hBN/MoS_(2)heterostructures.

关键词： two-dimensional(2D)van der Waals heterostructure interlayer coupling excitons critical points spectroscopic ellipsometry

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