We review lattice vibrational modes in atomically thin two-dimensional （2D） layered materials, focusing on 2D materials beyond graphene, such as group VI transition metal dichalcogenides, topological insulator bismu...We review lattice vibrational modes in atomically thin two-dimensional （2D） layered materials, focusing on 2D materials beyond graphene, such as group VI transition metal dichalcogenides, topological insulator bismuth chalcogenides, and black phosphorus. Although the composition and structure of those materials are remarkably different, they share a common and important feature, i.e., their bulk crystals are stacked via van der Waals interactions between ＂layers＂, while each layer is comprised of one or more atomic planes. First, we review the background of some 2D materials （MX2, M = Mo, W; X = S, Se, Te. Bi2X3, X = Se, Te. Black phosphorus）, including crystalline structures and stacking order. We then review the studies on vibrational modes of layered materials and nanostructures probed by the powerful yet nondestructive Raman spectroscopy technique. Based on studies conducted before 2010, recent investigations using more advanced techniques have pushed the studies of phonon modes in 2D layered materials to the atomically thin regime, down to monolayers. We will classify the recently reported general features into the following categories： phonon confinement effects and electron-phonon coupling, anomalous shifts in high-frequency intralayer vibrational modes and surface effects, reduced dimensionality and lower symmetry, the linear chain model and the substrate effect, stacking orders and interlayer shear modes, polarization dependence, and the resonance effect. Within the seven categories, both intralayer and interlayer vibrational modes will be discussed. The comparison between different materials will be provided as well.展开更多
In this stud high-performance multilayer WSe2 field-effect transistor （FET） devices with carrier type control are demonstrated via thickness modulation and a remote oxygen plasma surface treatment. Carrier type cont...In this stud high-performance multilayer WSe2 field-effect transistor （FET） devices with carrier type control are demonstrated via thickness modulation and a remote oxygen plasma surface treatment. Carrier type control in multilayer WSe2 FET devices with Cr/Au contacts is initially demonstrated by modulating the WSe2 thickness. The carrier type evolves with increasing WSe2 channel thickness, being p-type, ambipolar, and n-type at thicknesses 〈 3, - 4, and 〉 5 nm, respectively. The thickness-dependent carrier type is attributed to changes in the bandgap of WSe2 as a function of the thickness and the carrier band offsets relative to the metal contacts. Furthermore, we present a strong hole carrier doping effect via remote oxygen plasma treatment. It non-degenerately converts n-type characteristics into p-type and enhances field-effect hole mobility by three orders of magnitude. This work demonstrates progress towards the realization of high-performance multilayer WSe2 FETs with carrier type control, potentially extendable to other transition metal dichalcogenides, for future electronic and oDtoelectronic alpplications.展开更多
In most cases, layered transition metal dichalcogenides （LTMDs）, containing metallic phases, show electrochemical behavior different from their semiconductor counterparts. Typically, two-dimensional layered metallic...In most cases, layered transition metal dichalcogenides （LTMDs）, containing metallic phases, show electrochemical behavior different from their semiconductor counterparts. Typically, two-dimensional layered metallic 1T-MoS2 demonstrates better electrocatalytic performance for water splitting compared to its 2H counterpart. However, the characteristics of low metallic phase concentration and poor stability limit its applications in some cases. Herein, we demonstrate a simple and efficient bottom-up wet-chemistry strategy for the large-scale synthesis of nanoscopic ultrathin Mo1-xWxS2 nanosheets with enlarged interlayer spacing and high metallic phase concentration. Our characterizations, including X-ray absorption fine structure spectroscopy （XAFS）, high-angle annular dark-field- scanning transmission electron microscopy （HAADF-STEM）, and X-ray photoelectron spectroscopy （XPS） revealed that the metallic ultrathin ternary Mo1-xWxS2 nanosheets exhibited distorted metal-metal bonds and a tunable metallic phase concentration. As a proof of concept, this optimized catalyst, with the highest metallic phase concentration （greater than 90%）, achieved a low overpotential of about -155 mV at a current density of -10 ma/cm^2, a small Tafel slope of 67 mV/dec, and an increased turnover frequency （TOF） of 1.3 H2 per second at an overpotential of -300 mV （vs. reversible hydrogen electrode （RHE））, highlighting the importance of the metallic phase. More importantly, this study can lead to a facile solvothermal route to prepare stable and high-metallic- phase-concentration transition-metal-based two-dimensional materials for future applications.展开更多
Two dimensional （2D） nanomaterials are promising fundamental building blocks for use in the next-generation semiconductor industry due to their unique geometry and excellent （opto）-electronic properties. However, ...Two dimensional （2D） nanomaterials are promising fundamental building blocks for use in the next-generation semiconductor industry due to their unique geometry and excellent （opto）-electronic properties. However, large scale high quality fabrication of 2D nanomaterials remains challenging. Thus, the development of controllable fabrication methods for 2D materials is essential for their future practical application. In this review, we will discuss the importance of the space-confined vapor deposition strategy in the controllable fabrication of 2D materials and summarize recent progress in the utilization of this strategy for the synthesis of novel materials or structures. Using this method, various high quality ultrathin 2D materials, including large-area graphene and boron nitride, ReS2/ReSe2 HfS2, pyramid-structured multilayer MoS2, and the topological insulators BiaSe3 and BiaTe3, have been successfully obtained. Additionally, by utilizing van der Waals epitaxy growth substrates such as mica or other 2D materials, patterned growth of 2D nanomaterials can be easily achieved via a surface-induced growth mechanism. Finally, we provide a short prospect for future development of this strategy.展开更多
Layered semiconductors with atomic thicknesses are becoming increasingly important as active elements in high-performance electronic devices owing to their high carrier mobilities, large surface-to-volume ratios, and ...Layered semiconductors with atomic thicknesses are becoming increasingly important as active elements in high-performance electronic devices owing to their high carrier mobilities, large surface-to-volume ratios, and rapid electrical responses to their surrounding environments. Here, we report the first implementation of a highly sensitive chemical-vapor-deposition-grown multilayer MoSe2 field-effect transistor （FET） in a NO2 gas sensor. This sensor exhibited ultra-high sensitivity （S = ca. 1,907 for NO2 at 300 ppm）, real-time response, and rapid on-off switching. The high sensitivity of our MoSe2 gas sensor is attributed to changes in the gap states near the valence band induced by the NO2 gas absorbed in the MoSe2, which leads to a significant increase in hole current in the off-state regime. Device modeling and quantum transport simulations revealed that the variation of gap states with NO2 concentration is the key mechanism in a MoSe2 FET-based NO2 gas sensor. This comprehensive study, which addresses material growth, device fabrication, characterization, and device simulations, not only indicates the utility of MoSe2 FETs for high-performance chemical sensors, but also establishes a fundamental understanding of how surface chemistry influences carrier transport in layered semiconductor devices.展开更多
Optical emission efficiency of two-dimensional layered transition metal dichalcogenides （TMDs） is one of the most important parameters affecting their optoelectronic performance. The optimization of the growth param...Optical emission efficiency of two-dimensional layered transition metal dichalcogenides （TMDs） is one of the most important parameters affecting their optoelectronic performance. The optimization of the growth parameters by chemical vapor deposition （CVD） to achieve optoelectronic-grade quality TMDs is, therefore, highly desirable. Here, we present a systematic photoluminescence （PL） spectroscopic approach to assess the intrinsic optical and crystalline quality of CVD grown MoS2 （CVD MoS2）. We propose the use of the intensity ratio between the PL measured in air and vacuum as an effective way to monitor the intrinsic optical quality of CVD MoS2. Low-temperature PL measurements are also used to evaluate the structural defects in MoS2, via defect-associated bound exciton emission, which well correlates with the field-effect carrier mobility of MoS2 grown at different temperatures. This work therefore provides a sensitive, noninvasive method to characterize the optical properties of TMDs, allowing the tuning of the growth parameters for the development of optoelectronic devices.展开更多
Two-dimensional transition metal dichalcogenide semiconductors have emerged as promising candidates for optoelectronic devices with unprecedented properties and ultra-compact footprints. However, the high sensitivity ...Two-dimensional transition metal dichalcogenide semiconductors have emerged as promising candidates for optoelectronic devices with unprecedented properties and ultra-compact footprints. However, the high sensitivity of atomically thin materials to the surrounding dielectric media imposes severe limitations on their practical applicability. Hence, to enable the effective integration of these materials in devices, the development of reliable encapsulation procedures that preserve their physical properties is required. Here, the excitonic photoluminescence （at room temperature and 10 K） is assessed on mechanically exfoliated WSe2 monolayer flakes encapsulated with SiOx and AlxOy layers by means of chemical and physical deposition techniques. Conformal coating on untreated and non- functionalized flakes is successfully achieved by all the techniques examined, with the exception of atomic layer deposition, for which a cluster-like oxide coating is formed. No significant compositional or strain state changes in the flakes are detected upon encapsulation, independently of the technique adopted. Remarkably, our results show that the optical emission of the flakes is strongly influenced by the stoichiometry quality of the encapsulating oxide. When the encapsulation is carried out with slightly sub-stoichiometric oxides, two remarkable phenomena are observed. First, dominant trion （charged exciton） photoluminescence is detected at room temperature, revealing a clear electrical doping of the monolayers. Second, a strong decrease in the optical emission of the monolayers is observed, and attributed to non-radiative recombination processes and/or carrier transfer from the flake to the oxide. Power- and temperature-dependent photoluminescence measurements further confirm that stoichiometric oxides obtained by physical deposition lead to a successful encapsulation, opening a promising route for the development of integrated two-dimensional devices.展开更多
In the past few years, two-dimensional （2D） transition metal dichalcogenide （TMDC） materials have attracted increasing attention of the research community, owing to their unique electronic and optical properties, ...In the past few years, two-dimensional （2D） transition metal dichalcogenide （TMDC） materials have attracted increasing attention of the research community, owing to their unique electronic and optical properties, ranging from the valley-spin coupling to the indirect-to-direct bandgap transition when scaling the materials from multi-layer to monolayer. These properties are appealing for the development of novel electronic and optoelectronic devices with important applications in the broad fields of communication, computation, and healthcare. One of the key features of the TMDC family is the indirect-to-direct bandgap transition that occurs when the material thickness decreases from multilayer to monolayer, which is favorable for many photonic applications. TMDCs have also demonstrated unprecedented flexibility and versatility for constructing a wide range of heterostructures with atomic-level control over their layer thickness that is also free of lattice mismatch issues. As a result, layered TMDCs in combination with other 2D materials have the potential for realizing novel high-performance optoelectronic devices over a broad operating spectral range. In this article, we review the recent progress in the synthesis of 2D TMDCs and optoelectronic devices research. We also discuss the challenges facing the scalable applications of the family of 2D materials and provide our perspective on the opportunities offered by these materials for future generations of nanophotonics technology.展开更多
基金Q. H. X. gratefully thanks Singapore National Research Foundation via a Fellowship grant （No. NRF-RF2009-06） and an Investigatorship grant （No. NRF-NRFI2015-03）, Ministry of Education via a tier2 grant （No. MOE2012-T2-2-086） and a tier1 grant （No. 2013-T1-002-232）. S. Y. Q. and X. Luo gratefully acknowledge the Singapore National Research Foun- dation （NRF） for funding under the NRF Fellowship （No. NRF-NRFF2013-07）. Z. J. gratefully thanks National Natural Science Foundation of China （Nos. 11574305 and 51527901） and financial support from the National 1000 Talent Plan of China via a Young Project. The computations were performed on the cluster of NUS Graphene Research Centre. S. Y. Q. and X. Luo acknowledge the National Research Foundation, Prime Minister＇s Office, Singapore, under its Medium Sized Centre Programme.
文摘We review lattice vibrational modes in atomically thin two-dimensional （2D） layered materials, focusing on 2D materials beyond graphene, such as group VI transition metal dichalcogenides, topological insulator bismuth chalcogenides, and black phosphorus. Although the composition and structure of those materials are remarkably different, they share a common and important feature, i.e., their bulk crystals are stacked via van der Waals interactions between ＂layers＂, while each layer is comprised of one or more atomic planes. First, we review the background of some 2D materials （MX2, M = Mo, W; X = S, Se, Te. Bi2X3, X = Se, Te. Black phosphorus）, including crystalline structures and stacking order. We then review the studies on vibrational modes of layered materials and nanostructures probed by the powerful yet nondestructive Raman spectroscopy technique. Based on studies conducted before 2010, recent investigations using more advanced techniques have pushed the studies of phonon modes in 2D layered materials to the atomically thin regime, down to monolayers. We will classify the recently reported general features into the following categories： phonon confinement effects and electron-phonon coupling, anomalous shifts in high-frequency intralayer vibrational modes and surface effects, reduced dimensionality and lower symmetry, the linear chain model and the substrate effect, stacking orders and interlayer shear modes, polarization dependence, and the resonance effect. Within the seven categories, both intralayer and interlayer vibrational modes will be discussed. The comparison between different materials will be provided as well.
文摘In this stud high-performance multilayer WSe2 field-effect transistor （FET） devices with carrier type control are demonstrated via thickness modulation and a remote oxygen plasma surface treatment. Carrier type control in multilayer WSe2 FET devices with Cr/Au contacts is initially demonstrated by modulating the WSe2 thickness. The carrier type evolves with increasing WSe2 channel thickness, being p-type, ambipolar, and n-type at thicknesses 〈 3, - 4, and 〉 5 nm, respectively. The thickness-dependent carrier type is attributed to changes in the bandgap of WSe2 as a function of the thickness and the carrier band offsets relative to the metal contacts. Furthermore, we present a strong hole carrier doping effect via remote oxygen plasma treatment. It non-degenerately converts n-type characteristics into p-type and enhances field-effect hole mobility by three orders of magnitude. This work demonstrates progress towards the realization of high-performance multilayer WSe2 FETs with carrier type control, potentially extendable to other transition metal dichalcogenides, for future electronic and oDtoelectronic alpplications.
基金We acknowledge the financial support of the National Basic Research Program of China （Nos. 2014CB848900 and 2016YFA0200602）, the National Natural Science Foundation of China （Nos. U1532112, 11375198, 11574280, and 21573204）, CUSF （No. WK2310000053） and funds from Key Laboratory of Advanced Energy Materials Chemistry （Ministry of Education）. L. S. thanks the recruitment program of global experts, the CAS Hundred Talent Program. We also thank the Shanghai synchrotron Radiation Facility （14W1, SSRF）, the Beijing Synchrotron Radiation Facility （1W1B and soft-X-ray endstation, BSRF）, the Hefei Synchrotron Radiation Facility （MCD and Photoemission Endstations, NSRL） and USTC Center for Micro and Nanoscale Research and Fabrication.
文摘In most cases, layered transition metal dichalcogenides （LTMDs）, containing metallic phases, show electrochemical behavior different from their semiconductor counterparts. Typically, two-dimensional layered metallic 1T-MoS2 demonstrates better electrocatalytic performance for water splitting compared to its 2H counterpart. However, the characteristics of low metallic phase concentration and poor stability limit its applications in some cases. Herein, we demonstrate a simple and efficient bottom-up wet-chemistry strategy for the large-scale synthesis of nanoscopic ultrathin Mo1-xWxS2 nanosheets with enlarged interlayer spacing and high metallic phase concentration. Our characterizations, including X-ray absorption fine structure spectroscopy （XAFS）, high-angle annular dark-field- scanning transmission electron microscopy （HAADF-STEM）, and X-ray photoelectron spectroscopy （XPS） revealed that the metallic ultrathin ternary Mo1-xWxS2 nanosheets exhibited distorted metal-metal bonds and a tunable metallic phase concentration. As a proof of concept, this optimized catalyst, with the highest metallic phase concentration （greater than 90%）, achieved a low overpotential of about -155 mV at a current density of -10 ma/cm^2, a small Tafel slope of 67 mV/dec, and an increased turnover frequency （TOF） of 1.3 H2 per second at an overpotential of -300 mV （vs. reversible hydrogen electrode （RHE））, highlighting the importance of the metallic phase. More importantly, this study can lead to a facile solvothermal route to prepare stable and high-metallic- phase-concentration transition-metal-based two-dimensional materials for future applications.
文摘Two dimensional （2D） nanomaterials are promising fundamental building blocks for use in the next-generation semiconductor industry due to their unique geometry and excellent （opto）-electronic properties. However, large scale high quality fabrication of 2D nanomaterials remains challenging. Thus, the development of controllable fabrication methods for 2D materials is essential for their future practical application. In this review, we will discuss the importance of the space-confined vapor deposition strategy in the controllable fabrication of 2D materials and summarize recent progress in the utilization of this strategy for the synthesis of novel materials or structures. Using this method, various high quality ultrathin 2D materials, including large-area graphene and boron nitride, ReS2/ReSe2 HfS2, pyramid-structured multilayer MoS2, and the topological insulators BiaSe3 and BiaTe3, have been successfully obtained. Additionally, by utilizing van der Waals epitaxy growth substrates such as mica or other 2D materials, patterned growth of 2D nanomaterials can be easily achieved via a surface-induced growth mechanism. Finally, we provide a short prospect for future development of this strategy.
文摘Layered semiconductors with atomic thicknesses are becoming increasingly important as active elements in high-performance electronic devices owing to their high carrier mobilities, large surface-to-volume ratios, and rapid electrical responses to their surrounding environments. Here, we report the first implementation of a highly sensitive chemical-vapor-deposition-grown multilayer MoSe2 field-effect transistor （FET） in a NO2 gas sensor. This sensor exhibited ultra-high sensitivity （S = ca. 1,907 for NO2 at 300 ppm）, real-time response, and rapid on-off switching. The high sensitivity of our MoSe2 gas sensor is attributed to changes in the gap states near the valence band induced by the NO2 gas absorbed in the MoSe2, which leads to a significant increase in hole current in the off-state regime. Device modeling and quantum transport simulations revealed that the variation of gap states with NO2 concentration is the key mechanism in a MoSe2 FET-based NO2 gas sensor. This comprehensive study, which addresses material growth, device fabrication, characterization, and device simulations, not only indicates the utility of MoSe2 FETs for high-performance chemical sensors, but also establishes a fundamental understanding of how surface chemistry influences carrier transport in layered semiconductor devices.
文摘Optical emission efficiency of two-dimensional layered transition metal dichalcogenides （TMDs） is one of the most important parameters affecting their optoelectronic performance. The optimization of the growth parameters by chemical vapor deposition （CVD） to achieve optoelectronic-grade quality TMDs is, therefore, highly desirable. Here, we present a systematic photoluminescence （PL） spectroscopic approach to assess the intrinsic optical and crystalline quality of CVD grown MoS2 （CVD MoS2）. We propose the use of the intensity ratio between the PL measured in air and vacuum as an effective way to monitor the intrinsic optical quality of CVD MoS2. Low-temperature PL measurements are also used to evaluate the structural defects in MoS2, via defect-associated bound exciton emission, which well correlates with the field-effect carrier mobility of MoS2 grown at different temperatures. This work therefore provides a sensitive, noninvasive method to characterize the optical properties of TMDs, allowing the tuning of the growth parameters for the development of optoelectronic devices.
基金The authors would like to thank Georgios Katsaros and Tim Wehling for valuable discussions. Stephan Br~iuer, Albin Schwarz, and Ursula Kainz are ackno- wledged for technical support. A. M. acknowledges the financial support through BES-2013-062593. G. G. acknowledges support from the Austrian Science Fund through project P 28018-B27. I. Z. acknowledges financial support from the Swiss National Science Foundation research grant （No. 200021_165784）. This work was partially funded by the Austrian Science Fund through the projects P24471 and P26830, and by the Spanish Ministry for Economy and Competitiveness trough the project MINECO/FEDER TEC2015-69916- C2-1-R.
文摘Two-dimensional transition metal dichalcogenide semiconductors have emerged as promising candidates for optoelectronic devices with unprecedented properties and ultra-compact footprints. However, the high sensitivity of atomically thin materials to the surrounding dielectric media imposes severe limitations on their practical applicability. Hence, to enable the effective integration of these materials in devices, the development of reliable encapsulation procedures that preserve their physical properties is required. Here, the excitonic photoluminescence （at room temperature and 10 K） is assessed on mechanically exfoliated WSe2 monolayer flakes encapsulated with SiOx and AlxOy layers by means of chemical and physical deposition techniques. Conformal coating on untreated and non- functionalized flakes is successfully achieved by all the techniques examined, with the exception of atomic layer deposition, for which a cluster-like oxide coating is formed. No significant compositional or strain state changes in the flakes are detected upon encapsulation, independently of the technique adopted. Remarkably, our results show that the optical emission of the flakes is strongly influenced by the stoichiometry quality of the encapsulating oxide. When the encapsulation is carried out with slightly sub-stoichiometric oxides, two remarkable phenomena are observed. First, dominant trion （charged exciton） photoluminescence is detected at room temperature, revealing a clear electrical doping of the monolayers. Second, a strong decrease in the optical emission of the monolayers is observed, and attributed to non-radiative recombination processes and/or carrier transfer from the flake to the oxide. Power- and temperature-dependent photoluminescence measurements further confirm that stoichiometric oxides obtained by physical deposition lead to a successful encapsulation, opening a promising route for the development of integrated two-dimensional devices.
文摘In the past few years, two-dimensional （2D） transition metal dichalcogenide （TMDC） materials have attracted increasing attention of the research community, owing to their unique electronic and optical properties, ranging from the valley-spin coupling to the indirect-to-direct bandgap transition when scaling the materials from multi-layer to monolayer. These properties are appealing for the development of novel electronic and optoelectronic devices with important applications in the broad fields of communication, computation, and healthcare. One of the key features of the TMDC family is the indirect-to-direct bandgap transition that occurs when the material thickness decreases from multilayer to monolayer, which is favorable for many photonic applications. TMDCs have also demonstrated unprecedented flexibility and versatility for constructing a wide range of heterostructures with atomic-level control over their layer thickness that is also free of lattice mismatch issues. As a result, layered TMDCs in combination with other 2D materials have the potential for realizing novel high-performance optoelectronic devices over a broad operating spectral range. In this article, we review the recent progress in the synthesis of 2D TMDCs and optoelectronic devices research. We also discuss the challenges facing the scalable applications of the family of 2D materials and provide our perspective on the opportunities offered by these materials for future generations of nanophotonics technology.
