Wei-Ting Hsu

Bandgap tunability at single-layer molybdenum disulphide grain boundaries

Two-dimensional transition metal dichalcogenides have emerged as a new class of semiconductor materials with novel electronic and optical properties of interest to future nanoelectronics technology. Single-layer molybdenum disulphide, which represents a prototype two-dimensional transition metal dichalcogenide, has an electronic bandgap that increases with decreasing layer thickness. Using high-resolution scanning tunnelling microscopy and spectroscopy, we measure the apparent quasiparticle energy gap to be 2.40 ± 0.05 eV for single-layer, 2.10 ± 0.05 eV for bilayer and 1.75 ± 0.05 eV for trilayer molybdenum disulphide, which were directly grown on a graphite substrate by chemical vapour deposition method. More interestingly, we report an unexpected bandgap tunability (as large as 0.85 ± 0.05 eV) with distance from the grain boundary in single-layer molybdenum disulphide, which also depends on the grain misorientation angle. This work opens up new possibilities for flexible electronic and optoelectronic devices with tunable bandgaps that utilize both the control of two-dimensional layer thickness and the grain boundary engineering.

Spectroscopic Signatures for Interlayer Coupling in MoS2–WSe2 van der Waals Stacking

Stacking of MoS2 and WSe2 monolayers is conducted by transferring triangular MoS2 monolayers on top of WSe2 monolayers, all grown by chemical vapor deposition (CVD). Raman spectroscopy and photoluminescence (PL) studies reveal that these mechanically stacked monolayers are not closely coupled, but after a thermal treatment at 300 °C, it is possible to produce van der Waals solids consisting of two interacting transition metal dichalcogenide (TMD) monolayers. The layer-number sensitive Raman out-of-plane mode A(2)1g for WSe2 (309 cm(-1)) is found sensitive to the coupling between two TMD monolayers. The presence of interlayer excitonic emissions and the changes in other intrinsic Raman modes such as E″ for MoS2 at 286 cm(-1) and A(2)1g for MoS2 at around 463 cm(-1) confirm the enhancement of the interlayer coupling.

Second Harmonic Generation from Artificially Stacked Transition Metal Dichalcogenide Twisted Bilayers

Optical second harmonic generation (SHG) is known as a sensitive probe to the crystalline symmetry of few-layer transition metal dichalcogenides (TMDs). Layer-number dependent and polarization resolved SHG have been observed for the special case of Bernal stacked few-layer TMDs, but it remains largely unexplored for structures deviated from this ideal stacking order. Here we report on the SHG from homo- and heterostructural TMD bilayers formed by artificial stacking with an arbitrary stacking angle. The SHG from the twisted bilayers is a coherent superposition of the SH fields from the individual layers, with a phase difference depending on the stacking angle. Such an interference effect is insensitive to the constituent layered materials and thus applicable to hetero-stacked bilayers. A proof-of-concept demonstration of using the SHG to probe the domain boundary and crystal polarity of mirror twins formed in chemically grown TMDs is also presented. We show here that the SHG is an efficient, sensitive, and nondestructive characterization for the stacking orientation, crystal polarity, and domain boundary of van der Waals heterostructures made of noncentrosymmetric layered materials.

Band Gap-Tunable Molybdenum Sulfide Selenide Monolayer Alloy

band gap engineering of TMD has become an important topic. In early studies the TMD solid solutions both in the metal (e.g., Mo x W 1−x S 2 ) and chalcogen (e.g., MoS 2x Se 2(1−x) ) sublattice forms have been realized by the direct vapor transport growth, where the stoichiometric amounts of desired powder elements were introduced into a quartz ampoule for crystal growth. [ 17,18 ] Meanwhile, the growth of MoS 2 , WSe 2 and WS 2 monolayers has been reported recently by using sulfurization or selenization of transition metal oxides with chemical vapor deposition (CVD) techniques. [ 19–21 ] The density-functinoal theory (DFT) calculations show that the single layers of mixed TMDs, such as MoS 2x Se 2(1−x) are thermodynamically stable at room temperature, [ 22 ] so that such materials can be manufactured using chemical-vapor deposition technique. It is therefore useful to know whether it is possible to realize the synthesis of MoS 2x Se 2(1−x) monlayers which exhibit intriguing electronic properties and tunable optical band gaps. Very recently, the transition-metal dichalcogenide monolayer alloys (Mo 1–x W x S 2 ) have been obtained by mechanical cleaving from their bulk crystals, [ 23 ] where the band gap emission ranges from 1.82 eV to 1.99 eV. Note that the mechanical cleavage is valuable for fundamental research; however, a simple and scalable method to obtain TMD monolayers with controllable optical energy gaps is still urgently needed. In this contribution, we report that the MoS 2 monolayer fl akes prepared by CVD can be selenized in the presence of selenium vapors to form MoS x Se y monolayers. The optical band gap of the obtained MoS x Se y , ranging from 1.86 eV to 1.57 eV, is easily controllable by the selenization temperature. It is key demonstration for controlling electronic and optoelectronic structures of TMD monolayers using a simple method, where pproach is straightforward and applicable to the band gap engineering for other TMD monolayers. The CVD-grown MoS 2 monolayers were synthesized based on our previous reports. [ 19 ] In brief, the triangular MoS 2 fl akes are formed by the vapor phase reaction of MoO 3 with S powders, where the MoS 2 monolayers with a lateral size up to tens micron can be obtained and which growth method has been adopted by many other groups . [ 24,25 ] To modulate the electronic structures and optical band gaps of the MoS 2 monolayer, we perform the selenization in a hot-wall furnace at various temperatures. The scheme in Figure 1 a illustrates the experimental set-up for the selenization process, where the inlet gas (a mixture of Ar and H 2 ) carries the vaporized DOI: 10.1002/smll.201302893 2D Materials

Optically initialized robust valley-polarized holes in monolayer WSe2.

A robust valley polarization is a key prerequisite for exploiting valley pseudospin to carry information in next-generation electronics and optoelectronics. Although monolayer transition metal dichalcogenides with inherent spin–valley coupling offer a unique platform to develop such valleytronic devices, the anticipated long-lived valley pseudospin has not been observed yet. Here we demonstrate that robust valley-polarized holes in monolayer WSe2 can be initialized by optical pumping. Using time-resolved Kerr rotation spectroscopy, we observe a long-lived valley polarization for positive trion with a lifetime approaching 1 ns at low temperatures, which is much longer than the trion recombination lifetime (∼10–20 ps). The long-lived valley polarization arises from the transfer of valley pseudospin from photocarriers to resident holes in a specific valley. The optically initialized valley pseudospin of holes remains robust even at room temperature, which opens up the possibility to realize room-temperature valleytronics based on transition metal dichalcogenides.

Controllable Synthesis of Band-Gap-Tunable and Monolayer Transition-Metal Dichalcogenide Alloys

The electronic and optical properties of transition metal dichalcogenide (TMD) materials are directly governed by their energy gap; thus, the band gap engineering has become an important topic recently. Theoretical and some experimental results have indicated that these monolayer TMD alloys exhibit direct-gap properties and remain stable at room temperature, making them attractive for optoelectronic applications. Here we systematically compared the two approaches of forming MoS2xSe2(1-x) monolayer alloys: selenization of MoS2 and sulfurization of MoSe2. The optical energy gap of as-grown CVD MoS2 can be continuously modulated from 1.86 eV (667 nm) to 1.57 eV (790 nm) controllable by the reaction temperature. Spectroscopic and microscopic evidences show that the Mo-S bonds can be replaced by the Mo-Se bonds in a random and homogeneous manner. By contrast, the replacement of Mo-Se by Mo-S does not randomly occur in the MoSe2 lattice, where the reaction preferentially occurs along the crystalline orientation of MoSe2 and thus the MoSe2/MoS2 biphases are easily observed in the alloys, which makes the optical band gap of these alloys distinctly different. Therefore, the selenization of metal disulfide is preferred and the proposed synthetic strategy opens up a simple route to control the atomic structure as well as optical properties of monolayer TMD alloys.

Multidirection Piezoelectricity in Mono- and Multilayered Hexagonal α-In2Se3

Piezoelectric materials have been widely used for sensors, actuators, electronics, and energy conversion. Two-dimensional (2D) ultrathin semiconductors, such as monolayer h-BN and MoS2 with their atom-level geometry, are currently emerging as new and attractive members of the piezoelectric family. However, their piezoelectric polarization is commonly limited to the in-plane direction of odd-number ultrathin layers, largely restricting their application in integrated nanoelectromechanical systems. Recently, theoretical calculations have predicted the existence of out-of-plane and in-plane piezoelectricity in monolayer α-In2Se3. Here, we experimentally report the coexistence of out-of-plane and in-plane piezoelectricity in monolayer to bulk α-In2Se3, attributed to their noncentrosymmetry originating from the hexagonal stacking. Specifically, the corresponding d33 piezoelectric coefficient of α-In2Se3 increases from 0.34 pm/V (monolayer) to 5.6 pm/V (bulk) without any odd-even effect. In addition, we also demonstrate a type of α-In2Se3-based flexible piezoelectric nanogenerator as an energy-harvesting cell and electronic skin. The out-of-plane and in-plane piezoelectricity in α-In2Se3 flakes offers an opportunity to enable both directional and nondirectional piezoelectric devices to be applicable for self-powered systems and adaptive and strain-tunable electronics/optoelectronics.

Negative circular polarization emissions from WSe2/MoSe2 commensurate heterobilayers

Van der Waals heterobilayers of transition metal dichalcogenides with spin–valley coupling of carriers in different layers have emerged as a new platform for exploring spin/valleytronic applications. The interlayer coupling was predicted to exhibit subtle changes with the interlayer atomic registry. Manually stacked heterobilayers, however, are incommensurate with the inevitable interlayer twist and/or lattice mismatch, where the properties associated with atomic registry are difficult to access by optical means. Here, we unveil the distinct polarization properties of valley-specific interlayer excitons using epitaxially grown, commensurate WSe2/MoSe2 heterobilayers with well-defined (AA and AB) atomic registry. We observe circularly polarized photoluminescence from interlayer excitons, but with a helicity opposite to the optical excitation. The negative circular polarization arises from the quantum interference imposed by interlayer atomic registry, giving rise to distinct polarization selection rules for interlayer excitons. Using selective excitation schemes, we demonstrate the optical addressability for interlayer excitons with different valley configurations and polarization helicities.The interlayer coupling in van der Waals heterostructures is sensitive to the interlayer atomic registry. Here, the authors investigate the polarisation properties of epitaxially grown, commensurate WSe2/MoSe2 heterobilayers with well-defined atomic registry, and observe negative, circularly polarized photoluminescence from interlayer excitons.

Corrigendum: Optically initialized robust valley-polarized holes in monolayer WSe2.

Nature Communications 6: Article number: 8963 10.1038/ncomms9963 (2015); Published: November252015; Updated: February012016 The original version of this Article contained a typographical error in the spelling of the author Chang-Hsiao Chen, which was incorrectly given as Chiang-Hsiao Chen. This has now been corrected in both the PDF and HTML versions of the Article.

Fast visible-light phototransistor using CVD-synthesized large-area bilayer WSe 2

P-channel transition metal dichalcogenide ultrathin-body phototransistor (UTB-PT) with a response time as fast as 100 μs has been demonstrated for the first time using the CVD-synthesized large-area bilayer WSe2. Because of its excellent compatibility with mass production, the application of WSe2 UTB-PT for high-speed proximity interactive display has been proposed.