Optical signatures of states bound to vacancy defects in monolayer MoS 2 (original) (raw)
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Electronic and optical properties of 2D monolayer (ML) MoS2 with vacancy defect at S sites
Nano-Structures & Nano-Objects, 2019
Herein, we have studied the electronic and optical properties of S-sites vacancy defect monolayer (ML) MoS 2 from density functional theory (DFT) based on the linear combination of atomic orbitals (LCAO). ML-MoS 2 is an intrinsic semiconductor having direct electronic band gap of ∼1.82 eV. This system is highly sensitive to vacancy defect due to the significant changes in characteristics of fully occupied and unoccupied orbitals near Fermi energy (E F). On increasing the concentration of random vacancy defects ML-MoS 2 exhibits a diminishing semiconducting band gap. Also the profile of electronic band gap changes from direct to indirect as well as the shifting of the E F. The semiconducting behaviour is preserved up to 25% vacancy defects, above which occurs a semiconductor-metal transition. These features arise due to the Mod and S-p states and attributed to the photoluminescence for making MoS 2 a promising material for opto-electronic devices. To investigate the opto-electronic response we have calculated the dielectric function (ϵ), refractive index (n), and absorption coefficient (α) as a function of incident photon energy (hω).
Electronic and magnetic properties of defected MoS2 monolayer
Bibechana, 2021
It is interesting to understand the effect of defects in 2D materials because vacancy defects in 2D materials have novel electronic and magnetic properties. In this work, we studied electronic and magnetic properties of 1S vacancy defect (1Sv-MoS2), 2S vacancy defects (2Sv-MoS2), 1Mo vacancy defect (Mov-MoS2), and (1Mo & 1S) vacancy defects ((Mo-S)v-MoS2) in 2D MoS2 material by first-principles calculations within spin-polarized density functional theory (DFT) method. To understand the electronic properties of materials, we have analyzed band structures and DOS calculations and found that 1Sv-MoS2 & 2Sv-MoS2 materials have semiconducting nature. This is because, 1Sv-MoS2 & 2Sv-MoS2 materials open a small energy band gap of values 0.68 eV & 0.54 eV respectively in band structures. But, in Mov-MoS2 & (Mo-S)v-MoS2 materials, energy bands around the Fermi level mix with the orbital’s of Mo and S atoms. As a result, bands are split and raised around and above the Fermi energy level. Ther...
2 4 M ay 2 01 8 Optoelectronic properties of defective MoS 2 and WS 2 monolayers
2018
We theoretically explore the effect of metal and disulphur vacancies on electronic and optical properties of MoS2 and WS2 monolayers based on a Slater-Koster tight-binding model and including the spin-orbit coupling. We show that the vacancy defects create electronic flat bands by shifting the Fermi level towards the valence band, indicating that both types of vacancies may act as acceptor sites. The optical spectra of the pristine monolayers show step-like features corresponding to the transition from spin split valence band to the conduction band minimum, whereas the defective monolayers exhibit additional peaks in their spectra arising from induced midgap states in their band structures. We find that Mo and W vacancies contribute mostly in the low-energy optical spectrum, while the S2 vacancies enhance the optical conductivity mainly in the visible range of the spectrum. This suggests that depending on the type of vacancy, the atomic defects in MoS2 and WS2 monolayers may increas...
Large Excitonic Effects in the Optical Properties of Monolayer MoS2
The band structure and absorption spectrum of monolayer MoS2 is calculated using the G0W0 approximation and the Bethe-Salpeter equation (BSE), respectively. We find that the so-called A and B peaks in the absorption spectrum arise from strongly bound excitons (0.7-0.8 eV) localized in distinct regions of the Brillouin zone and not from a split valence band as commonly assumed. Furthermore, we find the minimum band gap to be of the indirect type. This seems to conflict with recent experimental results showing stong luminescence in this material. However, our results indicate that the luminescence is a consequence of the large binding energy of the lowest exciton which stabilizes it against thermal relaxation. PACS numbers: 71.20.Nr, 78.20.Bh, 78.60.Lc Nanostructured forms of the semi-conductor MoS 2 have recieved much attention due to their potential as catalysts for desulferization of crude oil and more recently for (photo)-electrochemical hydrogen evolution . Bulk MoS 2 is composed of two-dimensional sheets held together by weak van der Waals forces and individual sheets can be isolated by exfoliation techniques similar to those used to produce graphene . Single layers of MoS 2 therefore comprise highly interesting twodimensional systems with a finite band gap and have recently been proposed for nano-electronics applications .
Tightly bound trions in monolayer MoS2
Nature materials, 2013
Two-dimensional (2D) atomic crystals, such as graphene and transition-metal dichalcogenides, have emerged as a new class of materials with remarkable physical properties 1 . In contrast to graphene, monolayer MoS 2 is a non-centrosymmetric material with a direct energy gap 2-5 . Strong photoluminescence 2,3 , a current on-off ratio exceeding 10 8 in field-effect transistors 6 , and efficient valley and spin control by optical helicity 7-9 have recently been demonstrated in this material. Here we report the spectroscopic identification in doped monolayer MoS 2 of tightly bound negative trions, a quasi-particle composed of two electrons and a hole. These quasiparticles, which can be created with valley and spin polarized holes, have no analogue in other materials. They also possess a large binding energy (~ 20 meV), rendering them significant even at room temperature. Our results open up new avenues both for fundamental studies of many-body interactions and for optoelectronic and valleytronic applications in 2D atomic crystals.
Electronic structures of defects and magnetic impurities in MoS2 monolayers
Nanoscale Research Letters, 2014
We provide a systematic and theoretical study of the electronic properties of a large number of impurities, vacancies, and adatoms in monolayer MoS 2 , including groups III and IV dopants, as well as magnetic transition metal atoms such as Mn, Fe, Co, V, Nb, and Ta. By using density functional theory over a 5 × 5 atomic cell, we identify the most promising element candidates for p-doping of MoS 2. Specifically, we found VB group impurity elements, such as Ta, substituting Mo to achieve negative formation energy values with impurity states all sitting at less than 0.1 eV from the valence band maximum (VBM), making them the optimal p-type dopant candidates. Moreover, our 5 × 5 cell model shows that B, a group III element, can induce impurity states very close to the VBM with a low formation energy around 0.2 eV, which has not been reported previously. Among the magnetic impurities such as Mn, Fe, and Co with 1, 2, and 3 magnetic moments/atom, respectively, Mn has the lowest formation energy, the most localized spin distribution, and the nearest impurity level to the conduction band among those elements. Additionally, impurity levels and Fermi level for the above three elements are closer to the conduction band than the previous work (PCCP 16:8990-8996, 2014) which shows the possibility of n-type doping by Mn, thanks to our 5 × 5 cell model.
Effect of point defects on the optical and transport properties of MoS2 and WS2
Physical Review B, 2014
Imperfections in the crystal structure, such as point defects, can strongly modify the optical and transport properties of materials. Here, we study the effect of point defects on the optical and DC conductivities of single layers of semiconducting transition metal dichalcogenides with the form M S2, where M =Mo or W. The electronic structure is considered within a six bands tight-binding model, which accounts for the relevant combination of d orbitals of the metal M and p orbitals of the chalcogen S. We use the Kubo formula for the calculation of the conductivity in samples with different distributions of disorder. We find that M and/or S defects create mid-gap states that localize charge carriers around the defects and which modify the optical and transport properties of the material, in agreement with recent experiments. Furthermore, our results indicate a much higher mobility for p-doped WS2 in comparison to MoS2.
Optical Constants and Structural Properties of Epitaxial MoS2 Monolayers
Nanomaterials
Two-dimensional layers of transition-metal dichalcogenides (TMDs) have been widely studied owing to their exciting potential for applications in advanced electronic and optoelectronic devices. Typically, monolayers of TMDs are produced either by mechanical exfoliation or chemical vapor deposition (CVD). While the former produces high-quality flakes with a size limited to a few micrometers, the latter gives large-area layers but with a nonuniform surface resulting from multiple defects and randomly oriented domains. The use of epitaxy growth can produce continuous, crystalline and uniform films with fewer defects. Here, we present a comprehensive study of the optical and structural properties of a single layer of MoS2 synthesized by molecular beam epitaxy (MBE) on a sapphire substrate. For optical characterization, we performed spectroscopic ellipsometry over a broad spectral range (from 250 to 1700 nm) under variable incident angles. The structural quality was assessed by optical mi...
The Journal of Physical Chemistry C, 2019
The unique electronic structure of two-dimensional materials paves the way for abundant cutting-edge applications, including electronic devices and optoelectronic devices. The band gap excitations are nonlinearly red-shifted with increasing layer number, leading to intriguing tunable photon response in visible light range. However, there is a lack of systematic studies on the dielectric response of two-dimensional materials in ultraviolet range. Here, we report an anomalous linear layer-dependent blue shift of interband transition with a slope of 18.6±1.0 meV/layer in ultraviolet range on freestanding MoS2 by valence electron energy-loss spectroscopy. Such a method can provide nanometer spatial resolution, far superior to that of traditional optical methods. First-principles calculations reveal that the blue shift of interband transition is due to the change of the second band below the Fermi level around Γ point induced by the increased interlayer van der Waals interaction. The tunable interband transition has a potential application for modulating ultraviolet light devices.