Monolayer Molybdenum Disulphide Research Articles

Non-volatile resistive switching mechanism in single-layer MoS2 memristors: insights from ab initio modelling of Au and MoS2 interfaces.

Non-volatile memristive devices based on two-dimensional (2D) layered materials provide an attractive alternative to conventional flash memory chips. Single-layer semiconductors, such as monolayer molybdenum disulphide (ML-MoS2), enable the aggressive downscaling of devices towards greater system integration density. The "atomristor", the most compact device to date, has been shown to undergo a resistive switching between its high-resistance (HRS) and low-resistance (LRS) states of several orders of magnitude. The main hypothesis behind its working mechanism relies on the migration of sulphur vacancies in the proximity of the metal contact during device operation, thus inducing the variation of the Schottky barrier at the metal-semiconductor interface. However, the interface physics is not yet fully understood: other hypotheses were proposed, involving the migration of metal atoms from the electrode. In this work, we aim to elucidate the mechanism of the resistive switching in the atomristor. We carry out density functional theory (DFT) simulations on model Au and ML-MoS2 interfaces with and without the presence of point defects, either vacancies or substitutions. To construct realistic interfaces, we combine DFT with Green's function surface simulations. Our findings reveal that it is not the mere presence of S vacancies but rather the migration of Au atoms from the electrode to MoS2 that modulate the interface barrier. Indeed, Au atoms act as conductive "bridges", thus facilitating the flow of charge between the two materials.

Impact of Interface Traps in Floating-Gate Memory Based on Monolayer MoS

Two-dimensional materials (2DMs) have found potential applications in many areas of electronics, such as sensing, memory systems, optoelectronics, and power. Despite an intense experimental work, the literature is lacking of accurate modeling of nonvolatile memories (NVMs) based on 2DMs. In this work, using technology CAD simulations and model calibration with experiments, we show that the experimental program/erase characteristics of floating-gate (FG) memory devices based on monolayer molybdenum disulphide can be explained by considering bandgap trap states at the dielectric–semiconductor interface. The simulation model includes a classical approach based on drift-diffusion longitudinal channel transport and on nonlocal Wentzel–Kramers–Brillouin (WKB) tunneling for transversal transport (responsible of FG charging/discharging) and for tunneling at contacts. From hysteresis and pulse programming simulations on scaled devices, we find that the long-channel programming window is still maintained at <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\sim 100$ </tex-math></inline-formula> nm and that process improvements aimed at reducing the concentration of interface traps in the semiconducting bandgap could significantly optimize memory operation.

Saturable absorption performance of large area monolayer MoS2 coated planarized optical waveguide

The development of compact waveguide-based pulsed lasers has been of great interest in the past two decades. This is supported by the rapid development of two-dimensional (2D) materials saturable absorbers (SAs). In this paper, we integrate a large area monolayer molybdenum disulphide (MoS2) on a planarized silica optical waveguide and studied its performance as an SA. A single layer MoS2 film on polydimethylsiloxane substrate is mechanically transferred to the planarized optical waveguide. The single layer MoS2 can interact with the evanescent field of the waveguide core mode, thus achieving SA. The MoS2-coated waveguide is then integrated into an erbium-doped fibre laser cavity that operates in the telecommunication wavelength region. Q-switching is achieved with a repetition rate, pulse duration and maximum pulse energy of 22.5 kHz, 5.24 µs and 4.1 nJ, respectively at a pump power of 122.8 mW. The results show that 2D material thin films can be integrated onto a planarized optical waveguide to act as SAs. These findings show the potential of using 2D materials in developing compact, integrated waveguide pulsed laser sources.

Dynamical symmetry and valley-selective circularly polarized high-harmonic generation in monolayer molybdenum disulfide

Circularly polarized high harmonics (CHHs) have been observed in several solid targets under single-color circularly polarized excitation. However, experimental observations show that CHHs cannot be efficiently generated in monolayer molybdenum disulphide (${\text{MoS}}_{2}$) driving by a single-color circularly polarized laser, thus letting its unique valley-selective circular dichroism (VSCD) to remain unexploited in the high harmonic generation process. Here we demonstrate that the efficient generation of CHHs in monolayer ${\text{MoS}}_{2}$ driven by counterrotating bicircular (CRB) fields and the broken inversion symmetry of monolayer ${\text{MoS}}_{2}$ leads to the generation of forbidden $3n$ ($n\ensuremath{\in}\mathbb{N}$) harmonic orders. Interestingly, we find that the VSCD lead to a distinctive valley selection of harmonic orders, while the valley selection caused by the trefoil orientation relative to the lattice is also observed. Dynamical symmetry analyses show that the rotational symmetry of the crystals can be decoded by the combination of a linearly polarized excitation scheme and CRB excitation scheme via the lowest common multiple rule. Additionally, VSCD leaves unique fingerprints in the ellipticity of harmonics.

Quantum weak measurement of Goos–Hänchen shift in monolayer MoS2

We report the direct experimental evidence of the Goos–Hänchen (GH) shift in monolayer molybdenum disulphide ( M o S 2 ) for a Gaussian beam via the quantum weak measurement scheme. We demonstrate the dependence of the GH shift in monolayer M o S 2 on the angle of incidence including the critical angle in the total internal reflection condition. We have also compared the results to the GH shift for single-layer graphene and plane glass–air interface. Our results may open up important applications of M o S 2 in optical devices and sensor industry.

Ultra-narrowband visible light absorption in a monolayer MoS2 based resonant nanostructure.

Enhance light absorption in two-dimensional (2D) materials are of great importance for the development of many optoelectronic devices such as photodetectors, modulators and thermal emitters. In this paper, a resonant nanostructure based on subwavelength gratings of monolayer molybdenum disulphide (MoS2) is proposed. It is shown numerically that the excitation of guided modes in the proposed structure leads to perfect absorption in the visible range. The linewidth of the absorption spectrum can be narrow down to 0.1 nm. The resonance wavelength exhibits an almost linear dependence on the incidence angle. The proposed structure provides a method to design ultra-narrowband absorbers and similar designs can be applied to other 2D materials. It may find applications for optical filters, directional thermal emitters, 2D materials based lasers and others.

Strain-Dependent Polar Optical Phonon Scattering and Drive Current Optimization in Nanoscale Monolayer MoS2 FETs

Recently, microelectromechanical system has been used to dynamically strain atomically thin materials including MoS2. While strain can significantly modulate the electronic and phonon structures of monolayer MoS2, its impact on electron transport, especially in the dissipative regime, has not been well explored. In this paper, using a three-dimensional particle-based quantum-corrected Monte Carlo device simulator, the effects of uniaxial and biaxial strain on room-temperature electron transport in a model monolayer molybdenum disulphide (MoS2) based field-effect transistor have been investigated. In the beginning, the simulator has been validated against recently published experimental results. Overall, strain in monolayer MoS2 strongly affects the polar optical phonon modes as well as the electronic bandstructure. Uniaxial strain breaks the degeneracy of E′ Raman mode and results in phonon softening. In this case, our results show that, for both E+ and E− Raman modes, ON current first increases for up to 3.7% of applied strain and then decreases as the strain is increased further. As for biaxial strain, we consider the effects of both tensile and compressive stresses. We find that the application of biaxial tensile strain boosts the ON current for up to 4% of strain. Especially, biaxial tensile strain leads to ~ 15.56% increase in the ON current, which is highest for any type of applied stress.

Boundary activated hydrogen evolution reaction on monolayer MoS2

Recently, monolayer molybdenum disulphide (MoS2) has emerged as a promising and non–precious electrocatalyst for hydrogen evolution reaction. However, its performance is largely limited by the low density and poor reactivity of active sites within its basal plane. Here, we report that domain boundaries in the basal plane of monolayer MoS2 can greatly enhance its hydrogen evolution reaction performance by serving as active sites. Two types of effective domain boundaries, the 2H-2H domain boundaries and the 2H-1T phase boundaries, were investigated. Superior hydrogen evolution reaction catalytic activity, long-term stability and universality in both acidic and alkaline conditions were achieved based on a multi-hierarchy design of these two types of domain boundaries. We further demonstrate that such superior catalysts are feasible at a large scale by applying this multi-hierarchy design of domain boundaries to wafer-scale monolayer MoS2 films.

Two-dimensional charge carrier distribution in MoS2 monolayer and multilayers

Control of majority carrier type and concentration in transition metal dichalcogenides (TMDs) is an important goal for engineering and improving TMD-based devices. Monolayer and few-layer molybdenum disulphide (MoS2) is an n-type semiconductor due to the presence of electron-donating native defects whose distribution is strongly dependent on the processing history and ambient environment. However, the spatial heterogeneity of the charge carrier concentration has not yet been studied in MoS2 when implemented in devices such as field-effect transistors (FETs). Here, we present a method to extract the spatial distribution of charge carriers using Kelvin probe force microscopy of MoS2 FETs in operando. The carrier concentration in monolayer MoS2 exfoliated on SiO2/Si ranges from 1.2×1012 cm−2 to 2.3×1012 cm−2, corresponding to a three-dimensional concentration of 1018 cm−3 to 2.5×1018 cm−3. A comparable carrier concentration is obtained for few-layer MoS2, while for thicker MoS2 (&amp;gt;50 nm) it is an order of magnitude lower (2×1017 cm−3–4×1017 cm−3). This finding is consistent with an increased concentration of electron-donating sulfur vacancies at surfaces compared to the bulk. Thus, the reported method for measuring the carrier concentration may advance strategies for doping and improve understanding of devices and defects in 2D materials.

Photoluminescence enhancement of monolayer MoS2 using plasmonic gallium nanoparticles.

2D monolayer molybdenum disulphide (MoS2) has been the focus of intense research due to its direct bandgap compared with the indirect bandgap of its bulk counterpart; however its photoluminescence (PL) intensity is limited due to its low absorption efficiency. Herein, we use gallium hemispherical nanoparticles (Ga NPs) deposited by thermal evaporation on top of chemical vapour deposited MoS2 monolayers in order to enhance its luminescence. The influence of the NP radius and the laser wavelength is reported in PL and Raman experiments. In addition, the physics behind the PL enhancement factor is investigated. The results indicate that the prominent enhancement is caused by the localized surface plasmon resonance of the Ga NPs induced by a charge transfer phenomenon. This work sheds light on the use of alternative metals, besides silver and gold, for the improvement of MoS2 luminescence.

Interactions and Magnetotransport through Spin-Valley Coupled Landau Levels in Monolayer MoS_{2}.

The strong spin-orbit coupling and the broken inversion symmetry in monolayer transition metal dichalcogenides results in spin-valley coupled band structures. Such a band structure leads to novel applications in the fields of electronics and optoelectronics. Density functional theory calculations as well as optical experiments have focused on spin-valley coupling in the valence band. Here we present magnetotransport experiments on high-quality n-type monolayer molybdenum disulphide (MoS_{2}) samples, displaying highly resolved Shubnikov-de Haas oscillations at magnetic fields as low as 2T. We find the effective mass 0.7m_{e}, about twice as large as theoretically predicted and almost independent of magnetic field and carrier density. We further detect the occupation of the second spin-orbit split band at an energy of about 15meV, i.e., about a factor of 5 larger than predicted. In addition, we demonstrate an intricate Landau level spectrum arising from a complex interplay between a density-dependent Zeeman splitting and spin- and valley-split Landau levels. These observations, enabled by the high electronic quality of our samples, testify to the importance of interaction effects in the conduction band of monolayer MoS_{2}.

Boosting the Photoluminescence of Monolayer MoS2 on High‐Density Nanodimer Arrays with Sub‐10 nm Gap

AbstractPatterned plasmonic nanodimers are fabricated exploiting an ultrathin porous anodic aluminum oxide membrane as a mask during angle‐resolved shadow deposition. The fabricated nanodimer arrays exhibit consistent sub‐10 nm gaps and a high particle density up to 1.0 × 1010 cm−2 over a large area. The ultrasmall dimer gaps provide highly confined electromagnetic fields, which strongly enhance the photoluminescence (PL) emission and Raman scattering from the surrounding monolayer molybdenum disulphide (MoS2). The ensemble PL intensity from MoS2/dimers is enhanced by up to a factor of ≈160 by resonant excitation of the dimer modes. Anisotropic polarization‐dependent characteristics of PL and Raman from the MoS2/dimers confirm that the dominant enhancement originates from the dimer configuration. These experiments demonstrate a facile approach for the fabrication of low‐cost high‐performance 2D material‐based optoelectronic devices.

The Effective Mass of Dirac Fermions and Spin-Dependent Thermodynamic Properties of Monolayer Ferromagnetic MoS2 in the Presence of Rashba Spin-Orbit Coupling

The k.p Hamiltonian model at low-energy regime and Green’s function technique are carried out to study the effect of Rashba spin-orbit coupling (RSOC) on the spin-dependent thermodynamic properties in monolayer molybdenum disulphide. In particular, we have studied electronic band structure (EBS), electronic heat capacity (EHC), and magnetic susceptibility (MS) of spin-up and spin-down subbands by variation of RSOC strength. An enhancement of the spin splitting can be achieved by increasing the Rashba coupling strength. Electronic subband structure shows that the effective mass of carriers decreases with Rashba coupling while the spin-up and spin-down subband gaps remain constant. The reduced effective mass leads to the increase of both EHC and MS as reasonable results. Spin splitting can be also obtained from the EHC and MS findings. At very strong Rashba couplings, two Dirac-like points appear for spin-up subbands which we can claim that the system gets a new quantum anomalous Hall phase.

Cross-plane enhanced thermoelectricity and phonon suppression in graphene/MoS2 van der Waals heterostructures

The thermoelectric figures of merit of pristine two-dimensional materials are predicted to be significantly less than unity, making them uncompetitive as thermoelectric materials. Here we elucidate a new strategy that overcomes this limitation by creating multi-layer nanoribbons of two different materials and allowing thermal and electrical currents to flow perpendicular to their planes. To demonstrate this enhancement of thermoelectric efficiency ZT, we analyse the thermoelectric performance of monolayer molybdenum disulphide (MoS2) sandwiched between two graphene monolayers and demonstrate that the cross-plane (CP) ZT is significantly enhanced compared with the pristine parent materials. For the parent monolayer of MoS2, we find that ZT can be as high as approximately 0.3, whereas monolayer graphene has a negligibly small ZT. In contrast for the graphene/MoS2/graphene heterostructure, we find that the CP ZT can be as large as 2.8. One contribution to this enhancement is a reduction of the thermal conductance of the van der Waals heterostructure compared with the parent materials, caused by a combination of boundary scattering at the MoS2/graphene interface which suppresses the phonons transmission and the lower Debye frequency of monolayer MoS2, which filters phonons from the monolayer graphene. A second contribution is an increase in the electrical conductance and Seebeck coefficient associated with molybdenum atoms at the edges of the nanoribbons.

Magneto-optical transitions in monolayer molybdenum disulphide on polar substrates

Within the frame of the Huang–Rhys's lattice relaxation model, we theoretically studied the effects of lattice relaxation due to the electrons (holes)-surface optical (SO) phonons coupling on the magneto-optical (MO) transitions in monolayer molybdenum disulphide on three different types of polar substrates. The lattice relaxation resulted in the peak position shifting and the linewidths broadening in MO transitions. The dependences of MO transitions on the magnetic field, temperature and polarization strength of substrates have been discussed in detail. The obtained results may provide a possible method to measure the coupling strength between the electrons (holes) and SO phonons in MO experiments.

The Effect of Twin Grain Boundary Tuned by Temperature on the Electrical Transport Properties of Monolayer MoS2

Theoretical calculation and experimental measurement have shown that twin grain boundary (GB) of molybdenum disulphide (MoS2) exhibits extraordinary effects on transport properties. Precise transport measurements need to verify the transport mechanism of twin GB in MoS2. Here, monolayer molybdenum disulphide with a twin grain boundary was grown in our developed low-pressure chemical vapor deposition (CVD) system, and we investigated how the twin GB affects the electrical transport properties of MoS2 by temperature-dependent transport studies. At low temperature, the twin GB can increase the in-plane electrical conductivity of MoS2 and the transport exhibits variable-range hopping (VRH), while at high temperature, the twin GB impedes the electrical transport of MoS2 and the transport exhibits nearest-neighbor hopping (NNH). Our results elucidate carrier transport mechanism of twin GB and give an important indication of twin GB in tailoring the electronic properties of MoS2 for its applications in next-generation electronics and optoelectronic devices.

Simulation of Longitudinal Mode of Vibration in Piezoelectric Monolayer MoS2

Monolayer molybdenum disulphide (MoS2) is one of the desired materials for the new age piezoelectric devices. The main objective of the paper is to present the finite element (FE) simulation of piezoelectric monolayer MoS2 in COMSOL Multiphysics software. A rectangular MoS2 sheet is simulated in fixed-fixed end and free-free end configuration. The eigenmode analysis is done and the eigen frequency is calculated. Due to the presence of one molecular layer and piezoelectric matrix the vibration should be in longitudinal mode. The longitudinal acoustic velocity is also calculated from the simulation and is verified analytically.

First-principles study of a molecular adsorption of fluorine on monolayer MoS2

We have performed density functional theory (DFT) based first-principles calculations to study the stability, geometrical structures and electronic properties of pure monolayer Molybdenum disulphide (MoS2) and Fluorine (F2) adsorbed monolayer MoS2 within the DFT-D2 level of approximations. Present study shows that a F2 molecule adsorbed MoS2 monolayer system is stable. From the geometry and in- formation of adsorption energy of F2 molecule on the different occupation sites of MoS2 monolayer, it has been found that Fluorine prefers atomic adsorption above sulphur (S) atom. The binding energy per equivalent molecular Fluorine above the sulfur (S) atom is found to be 1.83 eV. The electronic structure calculations of MoS2 and F2 adsorbed MoS2 monolayer shows that there is a band gap of 1.14 eV and 1.01 eV respectively, at the Fermi level. Further, the symmetry of total DOS for up and down spin calculations reveals that the F2 adsorbed MoS2 monolayer is non-magnetic as similar to that of pure MoS2 monolayer. The projected density of states (PDOS) of a F2 adsorbed MoS2 monolayer is studied to understand the weighted contributions of different orbitals.BIBECHANA 13 (2016) 50-59

Low temperature carrier transport study of monolayer MoS2 field effect transistors prepared by chemical vapor deposition under an atmospheric pressure

Large size monolayer Molybdenum disulphide (MoS2) was successfully grown by chemical vapor deposition method under an atmospheric pressure. The electrical transport properties of the fabricated back-gate monolayer MoS2 field effect transistors (FETs) were investigated under low temperatures; a peak field effect mobility of 59 cm2V−1s−1 was achieved. With the assist of Raman measurement under low temperature, this work identified the mobility limiting factor for the monolayer MoS2 FETs: homopolar phonon scattering under low temperature and electron-polar optical phonon scattering at room temperature.

Linear magnetotransport in monolayerMoS2

A momentum balance equation is developed to investigate the magnetotransport properties in monolayer molybdenum disulphide when a strong perpendicular magnetic field and a weak in-plane electric field are applied simultaneously. At low temperature, in the presence of intravalley impurity scattering Shubnikov de Haas oscillation shows up accompanying by a beating pattern arising from large spin splitting and its period may halve due to high-order oscillating term at large magnetic field for samples with ultrahigh mobility. In the case of intervalley disorders, there exists a magnetic-field range where the magnetoresistivity almost vanishes. For low-mobility layer, a phase-inversion of oscillating peaks is acquired in accordance with recent experiment. At high temperature when Shubnikov de Haas oscillation is suppressed, the magnetophonon resonances induced by both optical phonons (mainly due to homopolar and Fr\"ohlich modes) and acoustic phonons (mainly due to intravalley transverse and longitudinal acoustic modes) emerge for suspended system with high mobility. For the single layer on a substrate, another resonance due to surface optical phonons may occur, resulting in a complex behavior of the total magnetoresistance. The beating pattern of magnetophonon resonance due to optical phonons can also be observed. However, for nonsuspended layer with low mobility, the magnetoresistance oscillation almost disappears and the resistivity increases with field monotonously.