Browsing by Author "Du, Luojun"
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Item 2D proximate quantum spin liquid state in atomic-thin α-RuCl 3(2019-01-01) Du, Luojun; Huang, Yuan; Wang, Yimeng; Wang, Qinqin; Yang, Rong; Tang, Jian; Liao, Mengzhou; Shi, Dongxia; Shi, Youguo; Zhou, Xingjiang; Zhang, Qingming; Zhang, Guangyu; Department of Electronics and Nanoengineering; Zhipei Sun Group; Chinese Academy of Sciences; Renmin University of ChinaTwo-dimensional (2D) atomic crystals have made major inroads into condensed-matter physics and give rise to fascinating phenomena due to quantum confinement. Here we report the first Raman scattering study on phonon-magnetic scattering coupling, proximate quantum spin liquid ground state and collective fractionalized excitations in exfoliated α-RuCl 3 atomic layers. Our results uncover that 2D α-RuCl 3 could harbour the unusual magnetic continuum, serving as a hallmark of the 2D proximate quantum spin liquid state and frustrated magnetic interactions. More importantly, our work demonstrates that the unusual magnetic scattering, as compared with bulk, is more obvious in 2D α-RuCl 3, indicating that the frustrated magnetic interactions are enhanced strongly. Such unusual enhancement of frustrated magnetic interactions may be responsible for the gigantic phonon-magnetic scattering coupling of 2D α-RuCl 3 and play a key role in stabilizing the 2D proximate quantum spin liquid state. Our work establishes a firm basis for exploring and understanding the 2D proximate quantum spin liquid and fractionalized excitations based on the atomically thin α-RuCl 3.Item Chirality logic gates(AMERICAN ASSOCIATION FOR THE ADVANCEMENT OF SCIENCE, 2022-12-07) Zhang, Yi; Wang, Yadong; Dai, Yunyun; Bai, Xueyin; Hu, Xuerong; Du, Luojun; Hu, Hai; Yang, Xiaoxia; Li, Diao; Dai, Qing; Hasan, Tawfique; Sun, Zhipei; Department of Electronics and Nanoengineering; Zhipei Sun Group; Centre of Excellence in Quantum Technology, QTF; National Center for Nanoscience and Technology Beijing; University of CambridgeThe ever-growing demand for faster and more efficient data transfer and processing has brought optical computation strategies to the forefront of research in next-generation computing. Here, we report a universal computing approach with the chirality degree of freedom. By exploiting the crystal symmetry–enabled well-known chiral selection rules, we demonstrate the viability of the concept in bulk silica crystals and atomically thin semiconductors and create ultrafast (<100-fs) all-optical chirality logic gates (XNOR, NOR, AND, XOR, OR, and NAND) and a half adder. We also validate the unique advantages of chirality gates by realizing multiple gates with simultaneous operation in a single device and electrical control. Our first demonstrations of logic gates using chiral selection rules suggest that optical chirality could provide a powerful degree of freedom for future optical computing.Item Comment on “Disentangling Orbital and Valley Hall Effects in Bilayers of Transition Metal Dichalcogenides” ()(American Physical Society, 2021-10-01) Du, Luojun; Department of Electronics and NanoengineeringA Comment on the Letter by 1T.-P. Cysne, Phys. Rev. Lett.126, 056601 (2021).PRLTAO0031-900710.1103/PhysRevLett.126.056601 The authors of the Letter offer a Reply.Item Dual-gated mono-bilayer graphene junctions(Royal Society of Chemistry, 2021-01-21) Du, Mingde; Du, Luojun; Wei, Nan; Liu, Wei; Bai, Xueyin; Sun, Zhipei; Department of Electronics and Nanoengineering; Department of Applied Physics; Zhipei Sun Group; Centre of Excellence in Quantum Technology, QTF; NanoMaterialsA lateral junction with an atomically sharp interface is extensively studied in fundamental research and plays a key role in the development of electronics, photonics and optoelectronics. Here, we demonstrate an electrically tunable lateral junction at atomically sharp interfaces between dual-gated mono- and bilayer graphene. The transport properties of the mono–bilayer graphene interface are systematically investigated with Ids–Vds curves and transfer curves, which are measured with bias voltage Vds applied in opposite directions across the asymmetric mono–bilayer interface. Nearly 30% difference between the output Ids–Vds curves of graphene channels measured at opposite Vds directions is observed. Furthermore, the measured transfer curves confirm that the conductance difference of graphene channels greatly depends on the doping level, which is determined by dual-gating. The Vds direction dependent conductance difference indicates the existence of a gate tunable junction in the mono–bilayer graphene channel, due to different band structures of monolayer graphene with zero bandgap and bilayer graphene with a bandgap opened by dual-gating. Simulation of the Ids–Vds curves based on a new numerical model validates the gate tunable junction at the mono–bilayer graphene interface from another point of view. The dual-gated mono–bilayer graphene junction and new protocol for Ids–Vds curve simulation pave a possible way for functional applications of graphene in next-generation electronics.Item Engineering symmetry breaking in 2D layered materials(Nature Publishing Group, 2021-03) Du, Luojun; Hasan, Tawfique; Castellanos-Gomez, Andres; Liu, Gui Bin; Yao, Yugui; Lau, Chun Ning; Sun, Zhipei; Department of Electronics and Nanoengineering; Centre of Excellence in Quantum Technology, QTF; Zhipei Sun Group; University of Cambridge; CSIC; Beijing Institute of Technology; Ohio State UniversitySymmetry breaking in 2D layered materials plays a significant role in their macroscopic electrical, optical, magnetic and topological properties, including, but not limited to, spin-polarization effects, valley-contrasting physics, nonlinear Hall effects, nematic order, ferroelectricity, Bose-Einstein condensation and unconventional superconductivity. Engineering symmetry breaking of 2D layered materials not only offers extraordinary opportunities to tune their physical properties but also provides unprecedented possibilities to introduce completely new physics and technological innovations in electronics, photonics and optoelectronics. Indeed, over the past 15 years, a wide variety of physical, structural and chemical approaches have been developed to engineer the symmetry breaking of 2D layered materials. In this Technical Review, we focus on the recent progress on engineering the breaking of inversion, rotational, time-reversal and gauge symmetries in 2D layered materials, and present our perspectives on how these may lead to new physics and applications.Item Engineering the Dipole Orientation and Symmetry Breaking with Mixed-Dimensional Heterostructures(WILEY-VCH VERLAG, 2022-07-15) Uddin, Md Gius; Das, Susobhan; Shafi, Abde Mayeen; Khayrudinov, Vladislav; Ahmed, Faisal; Fernandez, Henry; Du, Luojun; Lipsanen, Harri; Sun, Zhipei; Department of Electronics and Nanoengineering; Zhipei Sun Group; Centre of Excellence in Quantum Technology, QTF; Harri Lipsanen GroupEngineering of the dipole and the symmetry of materials plays an important role in fundamental research and technical applications. Here, a novel morphological manipulation strategy to engineer the dipole orientation and symmetry of 2D layered materials by integrating them with 1D nanowires (NWs) is reported. This 2D InSe –1D AlGaAs NW heterostructure example shows that the in-plane dipole moments in InSe can be engineered in the mixed-dimensional heterostructure to significantly enhance linear and nonlinear optical responses (e.g., photoluminescence, Raman, and second harmonic generation) with an enhancement factor of up to ≈12. Further, the 1D NW can break the threefold rotational symmetry of 2D InSe, leading to a strong optical anisotropy of up to ≈65%. These results of engineering dipole orientation and symmetry breaking with the mixed-dimensional heterostructures open a new path for photonic and optoelectronic applications.Item Enhancing and controlling valley magnetic response in MoS2/WS2 heterostructures by all-optical route(NATURE PUBLISHING GROUP, 2019-09-17) Zhang, Jing; Du, Luojun; Feng, Shun; Zhang, Run Wu; Cao, Bingchen; Zou, Chenji; Chen, Yu; Liao, Mengzhou; Zhang, Baile; Yang, Shengyuan A.; Zhang, Guangyu; Yu, Ting; Department of Electronics and Nanoengineering; Zhipei Sun Group; Nanyang Technological University; Singapore University of Technology and Design; Changchun Institute of Optics Fine Mechanics and PhysicsVan der Waals heterostructures of transition metal dichalcogenides with interlayer coupling offer an exotic platform to realize fascinating phenomena. Due to the type II band alignment of these heterostructures, electrons and holes are separated into different layers. The localized electrons induced doping in one layer, in principle, would lift the Fermi level to cross the spin-polarized upper conduction band and lead to strong manipulation of valley magnetic response. Here, we report the significantly enhanced valley Zeeman splitting and magnetic tuning of polarization for the direct optical transition of MoS2 in MoS2/WS2 heterostructures. Such strong enhancement of valley magnetic response in MoS2 stems from the change of the spin-valley degeneracy from 2 to 4 and strong many-body Coulomb interactions induced by ultrafast charge transfer. Moreover, the magnetic splitting can be tuned monotonically by laser power, providing an effective all-optical route towards engineering and manipulating of valleytronic devices and quantum-computation.Item Giant anisotropic photonics in the 1D van der Waals semiconductor fibrous red phosphorus(Nature Publishing Group, 2021-08-10) Du, Luojun; Zhao, Yanchong; Wu, Linlu; Hu, Xuerong; Yao, Lide; Wang, Yadong; Bai, Xueyin; Dai, Yunyun; Qiao, Jingsi; Uddin, Md Gius; Li, Xiaomei; Lahtinen, Jouko; Bai, Xuedong; Zhang, Guangyu; Ji, Wei; Sun, Zhipei; Department of Electronics and Nanoengineering; Department of Applied Physics; Centre of Excellence in Quantum Technology, QTF; Zhipei Sun Group; Surface Science; CAS - Institute of Physics; Renmin University of ChinaA confined electronic system can host a wide variety of fascinating electronic, magnetic, valleytronic and photonic phenomena due to its reduced symmetry and quantum confinement effect. For the recently emerging one-dimensional van der Waals (1D vdW) materials with electrons confined in 1D sub-units, an enormous variety of intriguing physical properties and functionalities can be expected. Here, we demonstrate the coexistence of giant linear/nonlinear optical anisotropy and high emission yield in fibrous red phosphorus (FRP), an exotic 1D vdW semiconductor with quasi-flat bands and a sizeable bandgap in the visible spectral range. The degree of photoluminescence (third-order nonlinear) anisotropy can reach 90% (86%), comparable to the best performance achieved so far. Meanwhile, the photoluminescence (third-harmonic generation) intensity in 1D vdW FRP is strong, with quantum efficiency (third-order susceptibility) four (three) times larger than that in the most well-known 2D vdW materials (e.g., MoS2). The concurrent realization of large linear/nonlinear optical anisotropy and emission intensity in 1D vdW FRP paves the way towards transforming the landscape of technological innovations in photonics and optoelectronics.Item Giant valley coherence at room temperature in 3R WS2 with broken inversion symmetry(AMERICAN ASSOCIATION FOR THE ADVANCEMENT OF SCIENCE, 2019-01-01) Du, Luojun; Tang, Jian; Liang, Jing; Liao, Mengzhou; Jia, Zhiyan; Zhang, Qinghua; Zhao, Yanchong; Yang, Rong; Shi, Dongxia; Gu, Lin; Xiang, Jianyong; Liu, Kaihui; Sun, Zhipei; Zhang, Guangyu; Department of Electronics and Nanoengineering; Department of Applied Physics; Centre of Excellence in Quantum Technology, QTF; Zhipei Sun Group; Chinese Academy of Sciences; Peking University; Yanshan UniversityBreaking the space-time symmetries in materials can markedly influence their electronic and optical properties. In 3R-stacked transition metal dichalcogenides, the explicitly broken inversion symmetry enables valley-contrasting Berry curvature and quantization of electronic angular momentum, providing an unprecedented platform for valleytronics. Here, we study the valley coherence of 3R WS2 large single-crystal with thicknesses ranging from monolayer to octalayer at room temperature. Our measurements demonstrate that both A and B excitons possess robust and thickness-independent valley coherence. The valley coherence of direct A (B) excitons can reach 0.742 (0.653) with excitation conditions on resonance with it. Such giant and thickness-independent valley coherence of large single-crystal 3R WS2 at room temperature would provide a firm foundation for quantum manipulation of the valley degree of freedom and practical application of valleytronics.Item Inducing Strong Light-Matter Coupling and Optical Anisotropy in Monolayer MoS2 with High Refractive Index Nanowire(AMERICAN CHEMICAL SOCIETY, 2022-07-13) Shafi, Abde Mayeen; Ahmed, Faisal; Fernandez, Henry A.; Uddin, Md Gius; Cui, Xiaoqi; Das, Susobhan; Dai, Yunyun; Khayrudinov, Vladislav; Yoon, Hoon Hahn; Du, Luojun; Sun, Zhipei; Lipsanen, Harri; Department of Electronics and Nanoengineering; School common, ELEC; Harri Lipsanen Group; Zhipei Sun Group; Centre of Excellence in Quantum Technology, QTFMixed-dimensional heterostructures combine the merits of materials of different dimensions; therefore, they represent an advantageous scenario for numerous technological advances. Such an approach can be exploited to tune the physical properties of two-dimensional (2D) layered materials to create unprecedented possibilities for anisotropic and high-performance photonic and optoelectronic devices. Here, we report a new strategy to engineer the light-matter interaction and symmetry of monolayer MoS2 by integrating it with one-dimensional (1D) AlGaAs nanowire (NW). Our results show that the photoluminescence (PL) intensity of MoS2 increases strongly in the mixed-dimensional structure because of electromagnetic field confinement in the 1D high refractive index semiconducting NW. Interestingly, the 1D NW breaks the 3-fold rotational symmetry of MoS2, which leads to a strong optical anisotropy of up to ∼60%. Our mixed-dimensional heterostructure-based phototransistors benefit from this and exhibit an improved optoelectronic device performance with marked anisotropic photoresponse behavior. Compared with bare MoS2 devices, our MoS2/NW devices show ∼5 times enhanced detectivity and ∼3 times higher photoresponsivity. Our results of engineering light-matter interaction and symmetry breaking provide a simple route to induce enhanced and anisotropic functionalities in 2D materials.Item Interlayer exciton complexes in bilayer MoS2(American Physical Society, 2022-01-15) Zhao, Yanchong; Du, Luojun; Yang, Shiqi; Tian, Jinpeng; Li, Xiaomei; Shen, Cheng; Tang, Jian; Chu, Yanbang; Watanabe, Kenji; Taniguchi, Takashi; Yang, Rong; Shi, Dongxia; Sun, Zhipei; Ye, Yu; Yang, Wei; Zhang, Guangyu; Department of Electronics and Nanoengineering; Centre of Excellence in Quantum Technology, QTF; Zhipei Sun Group; CAS - Institute of Physics; Peking University; National Institute for Materials ScienceAtomically thin transition metal dichalcogenides can show a rich variety of bound exciton complex states, such as trions, biexcitons, Fermi polarons, and phonon replicas, because of the reduced dielectric screening and enhanced Coulomb interaction. To date, studies have mainly focused on the complexes of intralayer excitons, while the electrically tunable interlayer exciton (IX) complexes remain elusive. Here, we report the observation of IX complexes with large out-of-plane electric dipole, strong emission intensity, and giant valley responses in bilayer MoS2, through on-resonance photoluminescence spectroscopy. In sharp contrast to the small, positive circular dichroism of intralayer excitons, the circular polarization of IX complexes in bilayer MoS2 can basically reach the theoretical limit (100%) but is negative. Such highly unusual light-valley responses of IX complexes in bilayer MoS2 demonstrate the strongly suppressed valley depolarization and spin-preserving scattering of electrons during the formation. Remarkably, by breaking the time-reversal symmetry with an out-of-plane magnetic field, a record level of spontaneous valley polarization (7.7%/Tesla) is identified for IX complexes in bilayer MoS2. The giant valley polarization of IX complexes in bilayer MoS2, together with the feasibility of electrical/optical/magnetic control, provides a firm basis for the development of next-generation electronic and optoelectronic applications with valley functionalities.Item Lattice Dynamics, Phonon Chirality, and Spin–Phonon Coupling in 2D Itinerant Ferromagnet Fe3GeTe2(WILEY-V C H VERLAG GMBH, 2019-11-28) Du, Luojun; Tang, Jian; Zhao, Yanchong; Li, Xiaomei; Yang, Rong; Hu, Xuerong; Bai, Xueyin; Wang, Xiao; Watanabe, Kenji; Taniguchi, Takashi; Shi, Dongxia; Yu, Guoqiang; Bai, Xuedong; Hasan, Tawfique; Zhang, Guangyu; Sun, Zhipei; Department of Electronics and Nanoengineering; Centre of Excellence in Quantum Technology, QTF; Zhipei Sun Group; Chinese Academy of Sciences; National Institute for Materials Science; Songshan Lake Materials Laboratory; University of CambridgeFe3GeTe2 has emerged as one of the most fascinating van der Waals crystals due to its 2D itinerant ferromagnetism, topological nodal lines, and Kondo lattice behavior. However, lattice dynamics, chirality of phonons, and spin-phonon coupling in this material, which set the foundation for these exotic phenomena, have remained unexplored. Here, the first experimental investigation of the phonons and mutual interactions between spin and lattice degrees of freedom in few-layer Fe3GeTe2 is reported. The results elucidate three prominent Raman modes at room temperature: two A(1g)(Gamma) and one E-2g(Gamma) phonons. The doubly degenerate E-2g(Gamma) mode reverses the helicity of incident photons, indicating the pseudoangular momentum and chirality. Through analysis of temperature-dependent phonon energies and lifetimes, which strongly diverge from the anharmonic model below Curie temperature, the spin-phonon coupling in Fe3GeTe2 is determined. Such interaction between lattice oscillations and spin significantly enhances the Raman susceptibility, allowing to observe two additional Raman modes at the cryogenic temperature range. In addition, laser radiation-induced degradation of Fe3GeTe2 in ambient conditions and the corresponding Raman fingerprint is revealed. The results provide the first experimental analysis of phonons in this novel 2D itinerant ferromagnet and their applicability for further fundamental studies and application development.Item Moiré photonics and optoelectronics(American Association for the Advancement of Science, 2023-03-31) Du, Luojun; Molas, Maciej R.; Huang, Zhiheng; Zhang, Guangyu; Wang, Feng; Sun, Zhipei; Department of Electronics and Nanoengineering; Centre of Excellence in Quantum Technology, QTF; Zhipei Sun Group; University of Warsaw; CAS - Institute of Physics; University of California, BerkeleyMoiré superlattices, the artificial quantum materials, have provided a wide range of possibilities for the exploration of completely new physics and device architectures. In this Review, we focus on the recent progress on emerging moiré photonics and optoelectronics, including but not limited to moiré excitons, trions, and polaritons; resonantly hybridized excitons; reconstructed collective excitations; strong mid- and far-infrared photoresponses; terahertz single-photon detection; and symmetry-breaking optoelectronics. We also discuss the future opportunities and research directions in this field, such as developing advanced techniques to probe the emergent photonics and optoelectronics in an individual moiré supercell; exploring new ferroelectric, magnetic, and multiferroic moiré systems; and using external degrees of freedom to engineer moiré properties for exciting physics and potential technological innovations.Item Observation of logarithmic Kohn anomaly in monolayer graphene(American Physical Society, 2020-10-23) Zhao, Yanchong; Du, Luojun; Yang, Wei; Shen, Cheng; Tang, Jian; Li, Xiaomei; Chu, Yanbang; Tian, Jinpeng; Watanabe, Kenji; Taniguchi, Takashi; Yang, Rong; Shi, Dongxia; Sun, Zhipei; Zhang, Guangyu; Department of Electronics and Nanoengineering; Centre of Excellence in Quantum Technology, QTF; Zhipei Sun Group; Chinese Academy of Sciences; National Institute for Materials ScienceElectron-phonon coupling in monolayer graphene breaks the adiabatic Born-Oppenheimer approximation and could lead to exotic logarithmic Kohn anomaly, manifested as logarithmic singularity in optical-phonon energy. However, unraveling unambiguously the fascinating logarithmic Kohn anomaly in monolayer graphene remains challenging due to the large carrier inhomogeneity originating from the unique massless Dirac-like band dispersion and the underneath substrate doping effect. Here we demonstrate a clear signature of intriguing logarithmic Kohn anomaly in monolayer graphene with ultralow carrier inhomogeneity via h-BN encapsulation. Significantly, the magnitude of anomalous phonon softening at 25 K shows an enhancement factor of 2 as compared to that previously observed in bilayer graphene at 12 K, even though bilayer graphene with nearly parabolic band dispersion is more immune to charged impurities. The uncovered unusual logarithmic Kohn anomaly in monolayer graphene can provide a firm basis for the understanding of various peculiar physics and may shed light on the nature of superconductivity in magic-angle graphene superlattices.Item Observation of phonon Stark effect(Nature Publishing Group, 2024-12) Huang, Zhiheng; Bai, Yunfei; Zhao, Yanchong; Liu, Le; Zhao, Xuan; Wu, Jiangbin; Watanabe, Kenji; Taniguchi, Takashi; Yang, Wei; Shi, Dongxia; Xu, Yang; Zhang, Tiantian; Zhang, Qingming; Tan, Ping Heng; Sun, Zhipei; Meng, Sheng; Wang, Yaxian; Du, Luojun; Zhang, Guangyu; Department of Electronics and Nanoengineering; Centre of Excellence in Quantum Technology, QTF; Zhipei Sun Group; CAS - Institute of Physics; CAS - Institute of Semiconductors; National Institute for Materials Science; Institute of Theoretical Physics of the Chinese Academy of SciencesStark effect, the electric-field analogue of magnetic Zeeman effect, is one of the celebrated phenomena in modern physics and appealing for emergent applications in electronics, optoelectronics, as well as quantum technologies. While in condensed matter it has prospered only for excitons, whether other collective excitations can display Stark effect remains elusive. Here, we report the observation of phonon Stark effect in a two-dimensional quantum system of bilayer 2H-MoS2. The longitudinal acoustic phonon red-shifts linearly with applied electric fields and can be tuned over ~1 THz, evidencing giant Stark effect of phonons. Together with many-body ab initio calculations, we uncover that the observed phonon Stark effect originates fundamentally from the strong coupling between phonons and interlayer excitons (IXs). In addition, IX-mediated electro-phonon intensity modulation up to ~1200% is discovered for infrared-active phonon A2u. Our results unveil the exotic phonon Stark effect and effective phonon engineering by IX-mediated mechanism, promising for a plethora of exciting many-body physics and potential technological innovations.Item Optical Control of High-Harmonic Generation at the Atomic Thickness(AMERICAN CHEMICAL SOCIETY, 2022-11-09) Wang, Yadong; Iyikanat, Fadil; Bai, Xueyin; Hu, Xuerong; Das, Susobhan; Dai, Yunyun; Zhang, Yi; Du, Luojun; Li, Shisheng; Lipsanen, Harri; García De Abajo, F. Javier; Sun, Zhipei; Department of Electronics and Nanoengineering; Zhipei Sun Group; Centre of Excellence in Quantum Technology, QTF; Harri Lipsanen Group; Barcelona Institute of Science and Technology; National Institute for Materials ScienceHigh-harmonic generation (HHG), an extreme nonlinear optical phenomenon beyond the perturbation regime, is of great significance for various potential applications, such as high-energy ultrashort pulse generation with outstanding spatiotemporal coherence. However, efficient active control of HHG is still challenging due to the weak light–matter interaction displayed by currently known materials. Here, we demonstrate optically controlled HHG in monolayer semiconductors via the engineering of interband polarization. We find that HHG can be efficiently controlled in the excitonic spectral region with modulation depths up to 95% and ultrafast response speeds of several picoseconds. Quantitative time-domain theory of the nonlinear optical susceptibilities in monolayer semiconductors further corroborates these experimental observations. Our demonstration not only offers an in-depth understanding of HHG but also provides an effective approach toward active optical devices for strong-field physics and extreme nonlinear optics.Item Precise control of the interlayer twist angle in large scale MoS2 homostructures(Nature Publishing Group, 2020-05-01) Liao, Mengzhou; Wei, Zheng; Du, Luojun; Wang, Qinqin; Tang, Jian; Yu, Hua; Wu, Fanfan; Zhao, Jiaojiao; Xu, Xiaozhi; Han, Bo; Liu, Kaihui; Gao, Peng; Polcar, Tomas; Sun, Zhipei; Shi, Dongxia; Yang, Rong; Zhang, Guangyu; Department of Electronics and Nanoengineering; Centre of Excellence in Quantum Technology, QTF; Zhipei Sun Group; Chinese Academy of Sciences; Peking University; Czech Technical University in PragueTwist angle between adjacent layers of two-dimensional (2D) layered materials provides an exotic degree of freedom to enable various fascinating phenomena, which opens a research direction—twistronics. To realize the practical applications of twistronics, it is of the utmost importance to control the interlayer twist angle on large scales. In this work, we report the precise control of interlayer twist angle in centimeter-scale stacked multilayer MoS2 homostructures via the combination of wafer-scale highly-oriented monolayer MoS2 growth techniques and a water-assisted transfer method. We confirm that the twist angle can continuously change the indirect bandgap of centimeter-scale stacked multilayer MoS2 homostructures, which is indicated by the photoluminescence peak shift. Furthermore, we demonstrate that the stack structure can affect the electrical properties of MoS2 homostructures, where 30° twist angle yields higher electron mobility. Our work provides a firm basis for the development of twistronics.Item Probing Electronic States in Monolayer Semiconductors through Static and Transient Third-Harmonic Spectroscopies(WILEY-V C H VERLAG GMBH, 2022-01-20) Wang, Yadong; Iyikanat, Fadil; Rostami, Habib; Bai, Xueyin; Hu, Xuerong; Das, Susobhan; Dai, Yunyun; Du, Luojun; Zhang, Yi; Li, Shisheng; Lipsanen, Harri; García de Abajo, F. Javier; Sun, Zhipei; Department of Electronics and Nanoengineering; Centre of Excellence in Quantum Technology, QTF; Zhipei Sun Group; Harri Lipsanen Group; Barcelona Institute of Science and Technology; KTH Royal Institute of Technology; National Institute for Materials ScienceElectronic states and their dynamics are of critical importance for electronic and optoelectronic applications. Here, various relevant electronic states in monolayer MoS2, such as multiple excitonic Rydberg states and free-particle energy bands are probed with a high relative contrast of up to >= 200 via broadband (from approximate to 1.79 to 3.10 eV) static third-harmonic spectroscopy (THS), which is further supported by theoretical calculations. Moreover, transient THS is introduced to demonstrate that third-harmonic generation can be all-optically modulated with a modulation depth exceeding approximate to 94% at approximate to 2.18 eV, providing direct evidence of dominant carrier relaxation processes associated with carrier-exciton and carrier-phonon interactions. The results indicate that static and transient THS are not only promising techniques for the characterization of monolayer semiconductors and their heterostructures, but also a potential platform for disruptive photonic and optoelectronic applications, including all-optical modulation and imaging.Item Raman fingerprints and exciton-phonon coupling in 2D ternary layered semiconductor InSeBr(AMER INST PHYSICS, 2020-04-20) Hu, Xuerong; Du, Luojun; Wang, Yadong; Lahtinen, Jouko; Yao, Lide; Ren, Zhaoyu; Sun, Zhipei; Department of Electronics and Nanoengineering; Department of Applied Physics; Centre of Excellence in Quantum Technology, QTF; Zhipei Sun Group; Surface Science; Nanomagnetism and Spintronics; Northwestern Polytechnical UniversityCompared to other two-dimensional (2D) crystals with single or binary elements, 2D ternary layered materials have unique physical properties for potential applications due to the stoichiometric variation and synergistic effect. Here, we report the first investigation of lattice dynamics and interactions between the exciton and lattice degrees of freedom in a 2D ternary semiconductor: indium-selenide-bromide (InSeBr). Via linear polarization resolved Raman scattering measurements, we uncover three Raman modes in few-layer InSeBr, including two A(1g) and one E-g modes. Moreover, through the combination of temperature-dependent Raman scattering experiments and theoretical calculations, we elucidate that few-layer InSeBr would harbor strong coupling between excitons and phonons. Our results may provide a firm basis for the development and engineering of potential optoelectronic devices based on 2D ternary semiconductors. Published under license by AIP Publishing.Item Robust circular polarization of indirect Q-K transitions in bilayer 3R-W S2(American Physical Society, 2019-10-23) Du, Luojun; Zhang, Qian; Zhang, Tingting; Jia, Zhiyan; Liang, Jing; Liu, Gui Bin; Yang, Rong; Shi, Dongxia; Xiang, Jianyong; Liu, Kaihui; Sun, Zhipei; Yao, Yugui; Zhang, Qingming; Zhang, Guangyu; Department of Electronics and Nanoengineering; Department of Applied Physics; Centre of Excellence in Quantum Technology, QTF; Zhipei Sun Group; Renmin University of China; Chinese Academy of Sciences; Yanshan University; Peking University; Beijing Institute of TechnologyValley-contrasting Berry curvature and orbital magnetic moment have led to highly selective circular polarization of direct excitons at the K valleys in transition-metal dichalcogenides. In addition to K valleys, Q valleys, another critical point in the conduction band, also possess well-defined but distinct magnetic moment. Being akin to the direct excitons at K valleys, indirect excitons associated with Q (K) valleys in the conduction (valence) band could allow circular polarization in principle. Here, we report an experimental observation of the circular polarization of indirect Q-K transitions in noncentrosymmetric bilayer 3R-WS2. In stark contrast to the circular polarization of direct excitons which depolarizes with increasing lattice temperature, the circular polarization of indirect Q-K excitons is extremely robust and independent on the temperature. Such robust circular polarization can be understood as follows: The spin-orbit coupling in the Q valley is much stronger than that in the K point of the conduction band, significantly suppressing the temperature induced valley depolarization. Our results open up opportunities for exotic valleytronics and quantum information processing applications.
