skip to main content


Title: Dramatically Enhanced Valley‐Polarized Emission by Alloying and Electrical Tuning of Monolayer WTe 2 x S 2(1‐ x ) Alloys at Room Temperature with 1T′‐WTe 2 ‐Contact
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 WTe2xS2(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 WTe2xS2(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.

 
more » « less
PAR ID:
10474442
Author(s) / Creator(s):
 ;  ;  ;  ;  ;  
Publisher / Repository:
Wiley Blackwell (John Wiley & Sons)
Date Published:
Journal Name:
Advanced Science
Volume:
11
Issue:
2
ISSN:
2198-3844
Format(s):
Medium: X
Sponsoring Org:
National Science Foundation
More Like this
  1. null (Ed.)
    Abstract Alloyed transition metal dichalcogenides provide an opportunity for coupling band engineering with valleytronic phenomena in an atomically-thin platform. However, valley properties in alloys remain largely unexplored. We investigate the valley degree of freedom in monolayer alloys of the phase change candidate material WSe 2(1-x) Te 2x . Low temperature Raman measurements track the alloy-induced transition from the semiconducting 1H phase of WSe 2 to the semimetallic 1T d phase of WTe 2 . We correlate these observations with density functional theory calculations and identify new Raman modes from W-Te vibrations in the 1H-phase alloy. Photoluminescence measurements show ultra-low energy emission features that highlight alloy disorder arising from the large W-Te bond lengths. Interestingly, valley polarization and coherence in alloys survive at high Te compositions and are more robust against temperature than in WSe 2 . These findings illustrate the persistence of valley properties in alloys with highly dissimilar parent compounds and suggest band engineering can be utilized for valleytronic devices. 
    more » « less
  2. Abstract

    Crystal phase control still remains a challenge for the precise synthesis of 2D layered metal dichalcogenide (LMD) materials. The T′ phase structure has profound influences on enhancing electrical conductivity, increasing active sites, and improving intrinsic catalytic activity, which are urgently needed for enhancing hydrogen evolution reaction (HER) activity. Theoretical calculations suggest that metastable T′ phase 2D Sn1−xWxS2alloys can be formed by combining W with 1T tin disulfide (SnS2) as a template to achieve a semiconductor‐to‐metallic transition. Herein, 2D Sn1−xWxS2alloys with varyingxare prepared by adjusting the molar ratio of reactants via hydrothermal synthesis, among which Sn0.3W0.7S2displays a maximum of concentration of 81% in the metallic phase and features a distorted octahedral‐coordinated metastable 1T′ phase structure. The obtained 1T′‐Sn0.3W0.7S2has high intrinsic electrical conductivity, lattice distortion, and defects, showing a prominently improved HER catalytic performance. Metallic Sn0.3W0.7S2coupled with carbon black exhibits at least a 215‐fold improvement compared to pristine SnS2. It has excellent long‐term durability and HER activity. This work reveals a general phase transition strategy by using T phase materials as templates and merging heteroatoms to achieve synthetic metastable phase 2D LMDs that have a significantly improved HER catalytic performance.

     
    more » « less
  3. Abstract

    Bandgap engineering plays a critical role in optimizing the electrical, optical and (photo)‐electrochemical applications of semiconductors. Alloying has been a historically successful way of tuning bandgaps by making solid solutions of two isovalent semiconductors. In this work, a novel form of bandgap engineering involving alloying non‐isovalent cations in a 2D transition metal dichalcogenide (TMDC) is presented. By alloying semiconducting MoSe2with metallic NbSe2, two structural phases of Mo0.5Nb0.5Se2, the1Tand2Hphases, are produced each with emergent electronic structure. At room temperature, it is observed that the1Tand2Hphases are semiconducting and metallic, respectively. For the1Tstructure, scanning tunneling microscopy/spectroscopy (STM/STS) is used to measure band gaps in the range of 0.42–0.58 at 77 K. Electron diffraction patterns of the1Tstructure obtained at room temperature show the presence of a nearly commensurate charge density wave (NCCDW) phase with periodic lattice distortions that result in an uncommon 4 × 4 supercell, rotated approximately 4° from the lattice. Density‐functional‐theory calculations confirm that local distortions, such as those in a NCCDW, can open up a band gap in1T‐Mo0.5Nb0.5Se2, but not in the2Hphase. This work expands the boundaries of alloy‐based bandgap engineering by introducing a novel technique that facilitates CDW phases through alloying.

     
    more » « less
  4. Abstract

    Spin‐dependent contrasting phenomena atKandK′ valleys in monolayer semiconductors have led to addressable valley degree of freedom, which is the cornerstone for emerging valleytronic applications in information storage and processing. Tunable and active modulation of valley dynamics in a monolayer WSe2is demonstrated at room temperature through controllable chiral Purcell effects in plasmonic chiral metamaterials. The strong spin‐dependent modulation on the spontaneous decay of valley excitons leads to tunable handedness and spectral shift of valley‐polarized emission, which is analyzed and predicted by an advanced theoretical model and further confirmed by experimental measurements. Moreover, large active spectral tuning (≈24 nm) and reversible ON/OFF switching of circular polarization of emission are achieved by the solvent‐controllable thickness of the dielectric spacer in the metamaterials. With the on‐demand and active tunability in valley‐polarized emission, chiral Purcell effects can provide new strategies to harness valley excitons for applications in ultrathin valleytronic devices.

     
    more » « less
  5. A two-dimensional (2D) topological insulator exhibits the quantum spin Hall (QSH) effect, in which topologically protected conducting channels exist at the sample edges. Experimental signatures of the QSH effect have recently been reported in an atomically thin material, monolayer WTe 2 . Here, we directly image the local conductivity of monolayer WTe 2 using microwave impedance microscopy, establishing beyond doubt that conduction is indeed strongly localized to the physical edges at temperatures up to 77 K and above. The edge conductivity shows no gap as a function of gate voltage, and is suppressed by magnetic field as expected. We observe additional conducting features which can be explained by edge states following boundaries between topologically trivial and nontrivial regions. These observations will be critical for interpreting and improving the properties of devices incorporating WTe 2 . Meanwhile, they reveal the robustness of the QSH channels and the potential to engineer them in the monolayer material platform. 
    more » « less