Abstract:The influence of atomic vacancy defects at different concentrations on electronic properties of MoS 2 and WS 2 monolayers is studied by means of Slater-Koster tight-binding model with non-orthogonal sp 3 d 5 orbitals and including the spin-orbit coupling. The presence of vacancy defects induces localized states in the bandgap of pristine MoS 2 and WS 2 , which have potential to modify the electronic structure of the systems, depending on the type and concentration of the defects. It is shown that although the … Show more
“…4b) is due to the CT-induced decrease in E F,graphene , and can be well described by the Gerischer model (Supplementary Note 2). For WS 2 that is natively n-doped by various defects including S vacancies 31 , its Fermi level is near the conduction band minimum located at −3.93 eV 32 that is 0.64 eV higher than that of graphene. Thus, CT from WS 2 to the redox couples of O 2 /H 2 O is more favorable than from graphene, which is consistent with our finding that the pH threshold for O 2 -induced CT is higher for WS 2 than that for graphene.Fig.…”
Controlling extra charge carriers is pivotal in manipulating electronic, optical, and magnetic properties of various two-dimensional materials. Nonetheless, the ubiquitous hole doping of two-dimensional materials in the air and acids has been controversial in its mechanistic details. Here we show their common origin is an electrochemical reaction driven by redox couples of oxygen and water molecules. Using real-time photoluminescence imaging of WS2 and Raman spectroscopy of graphene, we capture molecular diffusion through the two-dimensional nanoscopic space between two-dimensional materials and hydrophilic substrates, and show that the latter accommodate water molecules also serving as a hydrating solvent. We also demonstrate that HCl-induced doping is governed by dissolved O2 and pH in accordance with the Nernst equation. The nanoscopic electrochemistry anatomized in this work sets an ambient limit to material properties, which is universal to not only 2D but also other forms of materials.
“…4b) is due to the CT-induced decrease in E F,graphene , and can be well described by the Gerischer model (Supplementary Note 2). For WS 2 that is natively n-doped by various defects including S vacancies 31 , its Fermi level is near the conduction band minimum located at −3.93 eV 32 that is 0.64 eV higher than that of graphene. Thus, CT from WS 2 to the redox couples of O 2 /H 2 O is more favorable than from graphene, which is consistent with our finding that the pH threshold for O 2 -induced CT is higher for WS 2 than that for graphene.Fig.…”
Controlling extra charge carriers is pivotal in manipulating electronic, optical, and magnetic properties of various two-dimensional materials. Nonetheless, the ubiquitous hole doping of two-dimensional materials in the air and acids has been controversial in its mechanistic details. Here we show their common origin is an electrochemical reaction driven by redox couples of oxygen and water molecules. Using real-time photoluminescence imaging of WS2 and Raman spectroscopy of graphene, we capture molecular diffusion through the two-dimensional nanoscopic space between two-dimensional materials and hydrophilic substrates, and show that the latter accommodate water molecules also serving as a hydrating solvent. We also demonstrate that HCl-induced doping is governed by dissolved O2 and pH in accordance with the Nernst equation. The nanoscopic electrochemistry anatomized in this work sets an ambient limit to material properties, which is universal to not only 2D but also other forms of materials.
“…[ 21 ] Comparison of the spectral position of with theoretical and experimental data from the literature suggests that the emission originates from bare isolated sulfur vacancies forming a stable intergap state. [ 35,36 ] But this assignment cannot explain our measurements and findings from other groups. This is first that after a laser processing step the emission vanishes, which is not expected for bare sulfur vacancies, where intergap states are formed.…”
Tungsten disulfide is one of the prominent transition metal dichalcogenide materials, which shows a transition from an indirect to a direct bandgap as the layer thickness is reduced down to a monolayer. To use monolayers in devices, detailed knowledge about the luminescence properties regarding not only the excitonic but also the defect‐induced contributions is needed. Herein, monolayers are irradiated with ions with different fluences to create different defect densities. Apart from the excitonic contributions, two additional emission bands are observed at low temperatures. These bands can be reduced or even suppressed, if the flakes are exposed to laser light with powers up to 1.5 mW. Increasing the temperature up to room temperature leads to recovery of this emission, so that the luminescence properties can be modified using laser excitation and temperature. The defect bands emerging after ion irradiation are attributed to vacancy defects together with physisorbed adsorbates at different defect sites.
As a new two-dimensional transition metal carbides, MXene has various potential applications, such as energy storage, catalyst, composite material, and luminescent materials for their excellent physical and chemical properties. The element doping, geometrical defect, surficial functionalization, external electric field, and external strain can be used as effective methods for modulation of their properties. Ti 2 CO 2 , the thinnest Ti-based MXene, exhibits semiconducting character. The effects of electric field on the band structure of perfect primitive Ti 2 CO 2 were explored in this work. The results revealed that the band gap of perfect primitive Ti 2 CO 2 decreased with the increasing electric field. Carbon (C) vacancy in Ti 2 CO 2 MXene was easily produced during the preparation process. Further investigation showed that the tensile strain could be used to regulate the conductivity of this system as the bands around the Fermi energy become smoother with increasing tensile strain. The investigation of charged C vacancy doped 2×2×1 Ti 2 CO 2 indicated that its Fermi energy decreased with the increase of charge state. When it was +2 charged, the C vacancy 无 机 材 料 学 报 第 35 卷 doped 2×2×1 Ti 2 CO 2 exhibited semiconducting character and owned a direct band gap of 0.489 eV.
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