Among group VI transition metal dichalcogenides, MoTe 2 is predicted to have the smallest energy offset between semiconducting 2H and semimetallic 1T′ states. This makes it an attractive phase change material for both electronic and optoelectronic applications. Here, we report fast, nondestructive, and full phase change in Al 2 O 3 -encapsulated 2H-MoTe 2 thin films to 1T′-MoTe 2 using rapid thermal annealing at 900 °C. Phase change was confirmed using Raman spectroscopy after a short annealing duration of 10 s in both vacuum and nitrogen ambient. No thickness dependence of the transition temperatures was observed for flake thickness ranging from 1.5 to 8 nm. These results represent a major step forward in understanding the structural phase transition properties of MoTe 2 thin films using external heating and underline the importance of surface encapsulation for avoiding thin film degradation.
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This content will become publicly available on December 11, 2024
Controllable superconducting to semiconducting phase transition in topological superconductor 2M-WS2
The investigation of exotic properties in two-dimensional (2D) topological superconductors has garnered increasing attention in condensed matter physics, particularly for applications in topological qubits. Despite this interest, a reliable way of fabricating topological Josephson junctions (JJs) utilizing topological superconductors has yet to be demonstrated. Controllable structural phase transition presents a unique approach to achieving topological JJs in atomically thin 2D topological superconductors. In this work, we report the pioneering demonstration of a structural phase transition from the superconducting to the semiconducting phase in the 2D topological superconductor 2M-WS2. We reveal that the metastable 2M phase of WS2remains stable in ambient conditions but transitions to the 2H phase when subjected to temperatures above 150 °C. We further locally induced the 2H phase within 2M-WS2nanolayers using laser irradiation. Notably, the 2H phase region exhibits a hexagonal shape, and scanning tunneling microscopy uncovers an atomically sharp crystal structural transition between the 2H and 2M phase regions. Moreover, the 2M to 2H phase transition can be induced at the nanometer scale by a 200 kV electron beam. The electrical transport measurements further confirmed the superconductivity of the pristine 2M-WS2and the semiconducting behavior of the laser-irradiated 2M-WS2. Our results establish a novel approach for controllable topological phase change in 2D topological superconductors, significantly impacting the development of atomically scaled planar topological JJs.
more » « less- Award ID(s):
- 2228841
- PAR ID:
- 10532435
- Publisher / Repository:
- IOP
- Date Published:
- Journal Name:
- 2D Materials
- Volume:
- 11
- Issue:
- 1
- ISSN:
- 2053-1583
- Page Range / eLocation ID:
- 015018
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Abstract Monolayer ternary tellurides based on alloying different transition metal dichalcogenides (TMDs) can result in new two‐dimensional (2D) materials ranging from semiconductors to metals and superconductors with tunable optical and electrical properties. Semiconducting WTe2
x S2(1‐x )monolayer possesses two inequivalent valleys in the Brillouin zone, each valley coupling selectively with circularly polarized light (CPL). The degree of valley polarization (DVP) under the excitation of CPL represents the purity of valley polarized photoluminescence (PL), a critical parameter for opto‐valleytronic applications. Here, new strategies to efficiently tailor the valley‐polarized PL from semiconducting monolayer WTe2x S2(1‐x )at room temperature (RT) through alloying and back‐gating are presented. The DVP at RT is found to increase drastically from < 5% in WS2to 40% in WTe0.12S1.88by Te‐alloying to enhance the spin‐orbit coupling. Further enhancement and control of the DVP from 40% up to 75% is demonstrated by electrostatically doping the monolayer WTe0.12S1.88via metallic 1T′‐WTe2electrodes, where the use of 1T′‐WTe2substantially lowers the Schottky barrier height (SBH) and weakens the Fermi‐level pinning of the electrical contacts. The demonstration of drastically enhanced DVP and electrical tunability in the valley‐polarized emission from 1T′‐WTe2/WTe0.12S1.88heterostructures paves new pathways towards harnessing valley excitons in ultrathin valleytronic devices for RT applications. -
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Abstract Piezoelectricity in low‐dimensional materials and metal–semiconductor junctions has attracted recent attention. Herein, a 2D in‐plane metal–semiconductor junction made of multilayer 2H and 1T′ phases of molybdenum(IV) telluride (MoTe2) is investigated. Strong piezoelectric response is observed using piezoresponse force microscopy at the 2H–1T′ junction, despite that the multilayers of each individual phase are weakly piezoelectric. The experimental results and density functional theory calculations suggest that the amplified piezoelectric response observed at the junction is due to the charge transfer across the semiconducting and metallic junctions resulting in the formation of dipoles and excess charge density, allowing the engineering of piezoelectric response in atomically thin materials.
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Abstract Monolayer molybdenum disulfide (MoS 2 ) is one of the most studied two-dimensional (2D) transition metal dichalcogenides that is being investigated for various optoelectronic properties, such as catalysis, sensors, photovoltaics, and batteries. One such property that makes this material attractive is the ease in which 2D MoS 2 can be converted between the semiconducting (2H) and metallic/semi-metallic (1T/1T′) phases or heavily n-type doped 2H phase with ion intercalation, strain, or excess negative charge. Using n -butyl lithium (BuLi) immersion treatments, we achieve 2H MoS 2 monolayers that are heavily n-type doped with shorter immersion times (10–120 mins) or conversion to the 1T/1T′ phase with longer immersion times (6–24 h); however, these doped/converted monolayers are not stable and promptly revert back to the initial 2H phase upon exposure to air. To overcome this issue and maintain the modification of the monolayer MoS 2 upon air exposure, we use BuLi treatments plus surface functionalization p-(CH 3 CH 2 ) 2 NPh-MoS 2 (Et 2 N-MoS 2 )—to maintain heavily n-type doped 2H phase or the 1T/1T′ phase, which is preserved for over two weeks when on indium tin oxide or sapphire substrates. We also determine that the low sheet resistance and metallic-like properties correlate with the BuLi immersion times. These modified MoS 2 materials are characterized with confocal Raman/photoluminescence, absorption, x-ray photoelectron spectroscopy as well as scanning Kelvin probe microscopy, scanning electrochemical microscopy, and four-point probe sheet resistance measurements to quantify the differences in the monolayer optoelectronic properties. We will demonstrate chemical methodologies to control the modified monolayer MoS 2 that likely extend to other 2D transition metal dichalcogenides, which will greatly expand the uses for these nanomaterials.more » « less
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Intercalation of alkali metals is widely studied to introduce a structural phase transition from 2H to 1T′ in 2D group VI transition metal dichalcogenides (TMDCs). This highly efficient phase transition method has enabled an access to a library of phases with novel physical and chemical properties attractive for functional devices and electrochemical catalysis. However, despite numerous studies that have predicted that charge doping mainly contributes to the structural phase transition in the intercalation process, a mechanistic understanding of the phase transition at the atomic level has not been fully revealed. Furthermore, the coupled effects of strain and other intrinsic or extrinsic factors on the intercalation‐induced phase transition have not been quantitatively determined. Herein, the progress of the intercalation‐induced phase transition is briefly overviewed and the knowledge gaps in the current understanding of phase transition and intercalation in 2D TMDCs are highlighted. To fully gain the microscopic picture of the intercalation‐induced phase transition, in situ multimodal probes to monitor the real‐time structure−property relationship during intercalation are suggested. The proposed research directions further direct material scientists to efficiently engineer phase transition pathways in 2D materials to explore novel functional phases.
