Electrical and thermal properties of atomically thin two-dimensional (2D) materials are affected by their environment, e.g. through remote phonon scattering or dielectric screening. However, while it is known that mobility and thermal conductivity (TC) of graphene are reduced on a substrate, these effects are much less explored in 2D semiconductors such as MoS$_2$. Here, we use molecular dynamics to understand TC changes in monolayer (1L) and bilayer (2L) MoS$_2$ by comparing suspended, supported, and encased structures. The TC of monolayer MoS$_2$ is reduced from ~117 Wm$^{-1}$K$^{-1}$ when suspended, to ~31 Wm$^{-1}$K$^{-1}$ when supported by SiO$_2$, at 300 K. Encasing 1L MoS$_2$ in SiO$_2$ further reduces its TC down to ~22 Wm$^{-1}$K$^{-1}$. In contrast, the TC of 2L MoS$_2$ is not as drastically reduced, being >50% higher than 1L both when supported and encased. These effects are due to phonon scattering with remote vibrational modes of the substrate, which are partly screened in 2L MoS$_2$. We also examine the TC of 1L MoS$_2$ across a wide range of temperatures (300 to 700 K) and defect densities (up to 5$\times$10$^{13}$ cm$^{-2}$), finding that the substrate reduces the dependence of TC on these factors. Taken together, these are important findings for all applications which will use 2D semiconductors supported or encased by insulators, instead of freely suspended.
Transition Metal Dichalcogenide (TMD) monolayers have most widely studied due to their unique physical properties. Recently, Janus TMD Monolayer MoSSe with sandwiched S-Mo-Se structure has been synthesized by replacing the top S atomic layer in $\mathrm{MoS_2}$ with Se atoms. In this work, we systematically investigate the phonon transport and lattice thermal conductivity ($κ_L$) of MoSSe monolayer by first-principles calculations and linearized phonon Boltzmann equation within the single-mode relaxation time approximation (RTA). Calculated results show that the $κ_L$ of MoSSe monolayer is ver…
Intrinsic electrical and excitonic properties of monolayer transition metal dichalcogenides are studied on CYTOP fluoropolymer substrates with greatly suppressed unintentional doping and dielectric screening. Ambipolar transport behavior is observed in monolayer WS2 by applying solid states backdates. The excitonic properties of monolayer MoS2 and WS2 are determined by intricate interplays between the bandage renormalization, Pauli blocking and carrier screening against carrier doping.
Self-assembled topological structures of post-processed two-dimensional materials exhibit novel physical properties distinct from those of their parent materials. Herein, the critical role of desulphurization on self-assembled topological morphologies of molybdenum disulfide ($MoS_2$) monolayer sheets is explored using molecular dynamics (MD) simulations. MD results show that there are differences in atomic energetics of $MoS_2$ monolayer sheets with different desulphurization contents. Both free-standing and substrate-hosted $MoS_2$ monolayer sheets show diversity in topological structures su…
Elastic properties of materials are an important factor in their integration in applications. Chemical vapor deposited (CVD) monolayer semiconductors are proposed as key components in industrial-scale flexible devices and building blocks of 2D van der Waals heterostructures. However, their mechanical and elastic properties have not been fully characterized. Here we report high 2D elastic moduli of CVD monolayer MoS2 and WS2 (~ 170 N/m), which is very close to the value of exfoliated MoS2 monolayers and almost half the value of the strongest material, graphene. The 2D moduli of their bilayer he…
The anharmonic behavior of phonons and intrinsic thermal conductivity associated with the Umklapp scattering in monolayer MoS2 sheet are investigated via first-principles calculations within the framework of density functional perturbation theory. In contrast to the negative Gruneissen parameter occurring in low frequency modes in graphene, positive Gruneissen parameter in the whole Brillouin zone is demonstrated in monolayer MoS2 with much larger Gruneissen parameter for acoustic modes than that for the optical modes, suggesting that monolayer MoS2 sheet possesses a positive coefficient of th…
In this work, we investigated the electronic structure and the quantum capacitance of the functionalized MoS$_2$ monolayer. The functionalizations have been done by using different ad-atom adsorption on Mo$S_2$ monolayer. Density functional theory calculations are performed to obtain an accurate electronic structure of ad-atom doped MoS$_2$ monolayer with a varying degree of doping concentration. The quantum capacitance of the systems was subsequently estimated. A marked quantum capacitance above 200 $μ$F/cm$^2$ has been observed. Our calculations show that the quantum capacitance of MoS$_2$ m…
Various types of defects in MoS2 monolayers and their influence on the electronic structure and transport properties have been studied using the Density-Functional based Tight-Binding method in conjunction with the Green's Function approach. Intrinsic defects in MoS2 monolayers significantly affect their electronic properties. Even at low concentration they considerably alter the quantum conductance. While the electron transport is practically isotropic in pristine MoS2, strong anisotropy is observed in the presence of defects. Localized mid-gap states are observed in semiconducting MoS2 that …
Density functional theory is used to systematically study the electronic and magnetic properties of doped MoS$_2$ monolayers, where the dopants are incorporated both via S/Mo substitution or as adsorbates. Among the possible substitutional dopants at the Mo site, Nb is identified as suitable p-type dopant, while Re is the donor with the lowest activation energy. When dopants are simply adsorbed on a monolayer we find that alkali metals shift the Fermi energy into the MoS$_2$ conduction band, making the system n-type. Finally, the adsorption of charged molecules is considered, mimicking an ioni…
In this work, we have systematically studied the role of point defects in the recombination time of monolayer MoS$_2$ using time-dependent ab initio non-adiabatic molecular dynamics simulations. Various types of point defects, such as S vacancy, S interstitial, Mo vacancy and Mo interstitial have been considered. We show that defects strongly accelerate the electron-hole recombination, especially interstitial S atoms do that by 3 orders of magnitude higher compared to pristine MoS$_2$. Mo defects (both vacancy and interstitial) introduce a multitude of de-excitation pathways via various defect…
Studying the reason, why single-layer molybdenum disulfide (MoS$_2$) appears to fall short of its promising potential in flexible nanoelectronics, we found that the nature of contacts plays a more important role than the semiconductor itself. In order to understand the nature of MoS$_2$/metal contacts, we performed ab initio density functional theory calculations for the geometry, bonding and electronic structure of the contact region. We found that the most common contact metal (Au) is rather inefficient for electron injection into single-layer MoS$_2$ and propose Ti as a representative examp…
Monolayer molybdenum disulphide (MoS$_2$) is a promising two-dimensional (2D) material for nanoelectronic and optoelectronic applications. The large-area growth of MoS$_2$ has been demonstrated using chemical vapor deposition (CVD) in a wide range of deposition temperatures from 600 °C to 1000 °C. However, a direct comparison of growth parameters and resulting material properties has not been made so far. Here, we present a systematic experimental and theoretical investigation of optical properties of monolayer MoS$_2$ grown at different temperatures. Micro-Raman and photoluminescence (PL) stu…
Atom controlled sub-nanometer MoS$_2$ pores have been recently fabricated. Oxidative environments are of particular interest for MoS$_2$ applications in electronics, sensing and energy storage. In this work we carried out first-principles calculations of oxygen adsorption in plain and sub-nanometer MoS$_2$ nanopores. The chemical stability of the layers and pores towards oxygen was verified using density-functional theory. Dissociation and diffusion barriers have been calculated in order to understand surface and pore oxidation and its electronic properties at the atomic scale, which opens the…
Two-dimensional (2D) monolayer pristine MoS$_2$ transition metal dichalcogenide (TMD) is the most studied material because of its promising aspects as nonprecious electrocatalyst for hydrogen evolution reaction (HER). Previous studies have shown that the basal planes of the 2D MoS$_2$ are catalytically inert and hence, they cannot be used directly in desired applications such as electrochemical HER in industries. Here, we have thoroughly studied the defect-engineered Mn-doped 2D monolayer MoS$_2$ (Mn-MoS$_2$) material where Mn was doped in the pristine MoS$_2$ to activate the inert basal plane…
We present a computational study on the impact of line defects on the electronic properties of monolayer MoS2. Four different kinds of line defects with Mo and S as the bridging atoms, consistent with recent theoretical and experimental observations are considered herein. We employ the density functional tight-binding (DFTB) method with a Slater-Koster type DFTB-CP2K basis set for evaluating the material properties of perfect and the various defective MoS2 sheets. The transmission spectra is computed with a DFTB-Non-Equilibrium Greens Function (NEGF) formalism. We also perform a detailed analy…
We utilize density functional theory to calculate the edge energy and edge stress for monolayer MoS$_{2}$ nanoribbons. In contrast to previous reports for graphene, for both armchair and zigzag chiralities, the edge stresses for MoS$_{2}$ nanoribbons are found to be tensile, indicating that their lowest energy configuration is one of compression in which Mo-S bond lengths are shorter than those in a bulk, periodic MoS$_{2}$ monolayer. The edge energy and edge stress is found to converge for both chiralities for nanoribbon widths larger than about 1 nm.
We propose a theoretical model for describing the operation of a transistor with a MoS2 monolayer channel, which allows to obtain an analytical approximation of the potential in the channel. This potential depends on the drain and gate voltages. On this basis we make estimates for the minimum channel lengths due to the fundamental restriction of quantum tunneling through the barrier. It is shown that the relatively large effective mass of electrons in the MoS2 monolayer allows to predict the creation of devices with channels of a significantly shorter (2.5 - 3 nm) length than in traditional si…
We report on the integration of monolayer molybdenum disulphide with silicon nitride microresonators assembled by visco-elastic layer transfer techniques. Evanescent coupling from the resonator mode to the monolayer is confirmed through measurements of cavity transmission. The absorption of the monolayer semiconductor flakes in this geometry is determined to be 850 dB/cm, which is larger than that of graphene and black phosphorus with the same thickness. This technique can be applied to diverse monolayer semiconductors for assembling hybrid optoelectronic devices such as photodetectors and mod…
Understanding the magnetic properties of the various Mn doping configurations that can be encountered in $2H$-MoS$_2$ monolayer could be beneficial for its use in spintronics. Using density functional theory plus Hubbard U (DFT$+$U) approach, we study how a single isolated, double- and triple-substitution configurations of Mn atoms within a MoS$_2$ monolayer could contribute to its total magnetization. We find that the doping-configuration plays a critical role in stabilizing a ferromagnetic state in a Mn-doped MoS$_2$ monolayer. Indeed, the Mn-Mn magnetic interaction is found to be ferromagnt…
Newly measured optical absorption and photoluminescence spectra reveal substantial frequency shifts of both exciton and trion peaks as monolayer MoS2 is cooled from 363 K to 4 K. First-principles simulations using the GW-Bethe-Salpeter Equation approach satisfactorily reproduce these frequency shifts by incorporating many-electron interactions and the thermal expansion of the in-plane lattice constant. Studying these temperature effects in monolayer MoS2 is crucial for rectifying the results of room-temperature experiments with the previous predictions of zero-temperature-limit simulations. Mo…
Very recently, a novel two-dimension (2D) MXene, MoSi$_2$N$_4$, was successfully synthesized with excellent ambient stability, high carrier mobility, and moderate band gap (Science 369, 670, 2020). In this work, the intrinsic lattice thermal conductivity of monolayer MoSi$_2$N$_4$ is predicted by solving the phonon Boltzmann transport equation based on the first-principles calculations. Despite the heavy atomic mass of Mo and complex crystal structure, the monolayer MoSi$_2$N$_4$ unexpectedly exhibits a quite high lattice thermal conductivity over a wide temperature range between 300 to 800 K.…
We investigate electrical gating of photoluminescence and optical absorption in monolayer molybdenum disulfide (MoS$_2$) configured in field effect transistor geometry. We observe an hundredfold increase in photoluminescence intensity and an increase in absorption at ~660 nm in these devices when an external gate voltage is decreased from +50 V to -50 V, while the photoluminescence wavelength remains nearly constant. In contrast, in bilayer MoS$_2$ devices we observe almost no changes in photoluminescence with gate voltage. We propose that the differing responses of the monolayer and bilayer d…
Ab-initio density functional theory calculations are performed to study the electronic properties of a MoS2 monolayer deposited over a SiO2 substrate in the presence of interface impurities and defects. When MoS2 is placed on a defect-free substrate the oxide plays an insignificant role, since the conduction band top and the valence band minimum of MoS2 are located approximately in the middle of the SiO2 band-gap. However, if Na impurities and O dangling bonds are introduced at the SiO2 surface, these lead to localized states, which modulate the conductivity of the MoS2 monolayer from n- to p-…
Monolayers (ML) of Group-6 transition-metal dichalcogenides (TMDs) are semiconducting two-dimensional materials with direct bandgap, showing promising applications in various fields of science and technology, such as nanoelectronics and optoelectronics. These monolayers can undergo strong elastic deformations, up to about 10\%, without any bond breaking. Moreover, the electronic structure and transport properties, which define the performance of these TMDs monolayers in nanoelectronic devices, can be strongly affected by the presence of point defects, which are often present in the synthetic s…
In this letter we show by means of first principle numerical simulations that electron-hole interaction significantly contributes to the second-harmonic generation spectrum of h-BN or MoS$_2$ monolayers. Specifically, it doubles the signal intensity at the excitonic resonances with respect to the contribution from independent electronic transitions. This result hints that the intensity of second-harmonic signal of those materials can be tuned by changing the dielectric screening, that controls the strength of the electron-hole interaction.
Real-time monitoring is essential for understanding and eventually precise controlling of the growth of two dimensional transition-metal dichalcogenides (2D TMDCs). However, it is very challenging to carry out such kind of studies on chemical vapor deposition (CVD). Here, we report the first real time $in-situ$ study on the CVD growth of the 2D TMDCs. More specifically, CVD growth of molybdenum disulfide (MoS$_2$) monolayer on sapphire substrates has been monitored $in-situ$ using differential transmittance spectroscopy (DTS). The growth of the MoS$_2$ monolayer can be precisely followed by lo…
We use micro-Raman and photoluminescence (PL) spectroscopy at 300K to investigate the influence of uniaxial tensile strain on the vibrational and optoelectronic properties of monolayer and bilayer MoS2 on a flexible substrate. The initially degenerate E^1_{2g} Raman mode is split into a doublet as a direct consequence of the strain applied to MoS2 through Van der Waals coupling at the sample-substrate interface. We observe a strong shift of the direct band gap of 48meV/(% of strain) for the monolayer and 46meV/% for the bilayer, whose indirect gap shifts by 86meV/%. We find a strong decrease o…
In this work we present a study on the impact of various intrinsic deformations like ripples, twist, wrap on the electronic properties of ultra-short monolayer MoS2 channels. The effect of deformation (3-7o twist or wrap and 0.3-0.7 buckling amplitude) on a 3.5 nm planar monolayer MoS2 MOSFET is evaluated by the density functional theory and the non-equilibrium Green`s function (DFT-NEGF) approach. We study the channel density of states, transmission spectra and the ID-VD characteristics under the varying conditions, with focus on the negative differential resistance (NDR) behavior. Our result…
We propose an effective lattice Hamiltonian for monolayer MoS$_2$ in order to describe the low-energy band structure and investigate the effect of perpendicular electric and magnetic fields on its electronic structure. We derive a tight-binding model based on the hybridization of the $d$ orbitals of molybdenum and $p$ orbitals of sulfur atoms and then introduce a modified two-band continuum model of monolayer MoS$_2$ by exploiting the quasi-degenerate partitioning method. Our theory proves that the low-energy excitations of the system are no longer massive Dirac fermions. It reveals a differen…
Phonons are essential for understanding the thermal properties in monolayer transition metal dichalcogenides, which limit their thermal performance for potential applications. We investigate the lattice dynamics and thermodynamic properties of MoS2, MoSe2, and WS2 by first principles calculations. The obtained phonon frequencies and thermal conductivities agree well with the measurements. Our results show that the thermal conductivity of MoS2 is highest among the three materials due to its much lower average atomic mass. We also discuss the competition between mass effect, interatomic bonding …
Intrinsic electron spin relaxation due to the D'yakonov-Perel' mechanism is studied in monolayer Molybdenum Disulphide. An intervalley in-plane spin relaxation channel is revealed due to the opposite effective magnetic fields perpendicular to the monolayer Molybdenum Disulphide plane in the two valleys together with the intervalley electron-phonon scattering. The intervalley electron-phonon scattering is always in the weak scattering limit, which leads to a rapid decrease of the in-plane spin relaxation time with increasing temperature. A decrease of the in-plane spin relaxation time with the …
We report circularly-polarized optical reflection spectroscopy of monolayer WS$_2$ and MoS$_2$ at low temperatures (4~K) and in high magnetic fields to 65~T. Both the A and the B exciton transitions exhibit a clear and very similar Zeeman splitting of approximately $-$230~$μ$eV/T ($g\simeq -4$), providing the first measurements of the valley Zeeman effect and associated $g$-factors in monolayer transition-metal disulphides. These results complement and are compared with recent low-field photoluminescence measurements of valley degeneracy breaking in the monolayer diselenides MoSe$_2$ and WSe$_…
Strong correlations between electrons and holes can drive the existence of an electron-hole liquid (EHL) state, typically at high carrier densities and low temperatures. The recent emergence of quasi-2D monolayer transition metal dichalcogenides (TMDCs) provide ideal systems to explore the EHL state since ineffective screening of the out of plane field lines in these quasi-2D systems allows for stronger charge carrier correlations in contrast to conventional 3D bulk semiconductors and enabling the existence of the EHL at high temperatures. Here we construct the phase diagram for the photo-indu…
Molybdenum disulfide (MoS2) is a layered semiconductor which has become very important recently as an emerging electronic device material. Being an intrinsic semiconductor the two-dimensional MoS2 has major advantages as the channel material in field-effect transistors. In this work we determine the electronic structure of MoS2 with the highly accurate screened hybrid functional within the density functional theory (DFT) including the spin-orbit coupling. Using the DFT electronic structures as target, we have developed a single generic tight-binding (TB) model that accurately produces the elec…
Within the framework of many-body perturbation theory (MBPT) integrated with density functional theory (DFT), a novel defect-subspace projection GW method, the so-called p-GW, is proposed. By avoiding the periodic defect interference through open boundary self-energies, we show that the p-GW can efficiently and accurately describe quasi-particle correlated defect levels in two-dimensional (2D) monolayer MoS$_2$. By comparing two different defect states originating from sulfur vacancy and adatom to existing theoretical and experimental works, we show that our GW correction to the DFT defect lev…
We report Raman spectra measurements on a MoS2 monolayer supported on SiO2 as a function of temperature. Unlike in previous studies, the positions of the two main Raman modes, E2g1 and A1g exhibited nonlinear temperature dependence. Temperature dependence of phonon shifts and widths is explained by optical phonon decay process into two acoustic phonons. On the basis of Raman measurements, local temperature change under laser heating power at different global temperatures is derived. Obtained results contribute to our understanding of the thermal properties of two-dimensional atomic crystals an…
We report a systematic study of coherent spin precession and spin dephasing in electron-doped monolayer MoS$_2$. Using time-resolved Kerr rotation spectroscopy and applied in-plane magnetic fields, a nanosecond-timescale Larmor spin precession signal commensurate with $g$-factor $|g_0|\simeq 1.86$ is observed in several different MoS$_2$ samples grown by chemical vapor deposition. The dephasing rate of this oscillatory signal increases linearly with magnetic field, suggesting that the coherence arises from a sub-ensemble of localized electron spins having an inhomogeneously-broadened distribut…
When transistor gate insulators have nanometer-scale equivalent oxide thickness (EOT), the gate capacitance ($C_\textrm{G}$) becomes smaller than the oxide capacitance ($C_\textrm{ox}$) due to the quantum capacitance and charge centroid capacitance of the channel. Here, we study the capacitance of monolayer MoS$_\textrm{2}$ as a prototypical two-dimensional (2D) channel while considering spatial variations in the potential, charge density, and density of states. At 0.5 nm EOT, the monolayer MoS$_\textrm{2}$ capacitance is smaller than its quantum capacitance, limiting the single-gated $C_\text…
The optical susceptibility is a local, minimally-invasive and spin-selective probe of the ground state of a two-dimensional electron gas. We apply this probe to a gated monolayer of MoS$_2$. We demonstrate that the electrons are spin polarized. Of the four available bands, only two are occupied. These two bands have the same spin but different valley quantum numbers. We argue that strong Coulomb interactions are a key aspect of this spontaneous symmetry breaking. The Bohr radius is so small that even electrons located far apart in phase space interact, facilitating exchange couplings to align …
While n-type semiconductor behavior appears to be more common in as-prepared two-dimensional (2D) transition metal dichalcogenides (TMDCs), substitutional doping with acceptor atoms is typically required to tune the conductivity to p-type in order to facilitate their potential application in different devices. Here, we report a systematic study on the equivalent electrical "doping" effect of - single sulfur vacancies (V1S) in monolayer WS2 and MoS2 by studying the interface interaction of WS2-Au and MoS2-Au contacts. Based on our first principles calculations, we found that the V1S can signifi…
By considering driven diffusive dynamics of exciton polaritons in an optical microcavity with an embedded molybdenum disulfide monolayer, we determine experimentally relevant range of parameters at which room-temperature superfuidity can be observed. It is shown that the superfluid transitions occurs in a trapped polariton gas at laser pumping power $P>600$ mW and and trapping potential strength $k > 50$ eV/cm$^2$. We also propose a simple analytic model that provides a useful estimate for the polariton gas density, which enables one to determine the conditions for observation of room temperat…
