Monolayer MoS2 Research Articles

Enhanced carrier mobility and interface charge transfer in Bi–MoS2 heterojunctions induced by point defects

Semimetal Bi in contact with monolayer MoS2 (mMoS2) as a substrate reduced contact resistance. This phenomenon has attracted widespread attention. Presently, systematic research on defects in semimetal-TMDs heterojunctions remains limited. In this paper, nine types of vacancy defects (three S-vacancies at the heterojunction interface (VSI, II, III), three S-vacancies on the top layer (VSI, II, IIIt), one Mo vacancy (VMo), Mo–S double vacancies (VMo–S) and Mo–S triple vacancies (VMo–2S)) and defect-free structure (Perfect) are designed in mMoS2. Their impact on defect formation energy, Interface charge transfer, and carrier mobility in three-layer (3L)Bi(0001)-mMoS2 heterojunctions are studied by the First-Principles. The energy analysis indicates that in the 3LBi-mMoS2 contact, VS are more prone to formation than VMo, VSt, VMo–S and VMo–2S. At the same time VSⅡ has the lowest formation energy under Mo-rich environments. The analysis of electron and charge redistribution in the 3LBi-mMoS2 contact with defects indicates that defect generation introduces peaks at the Fermi level, enhancing the hybridization of Mo_4d and S_3p. The hybridization rate of electron orbitals in VMo surpasses that in VS and VSt, signifying that heterojunction structures with defects exhibit increased electron injection efficiency at the interface compared to Perfect. The charge carrier mobility of S and Mo defects in 3LBi-mMoS2 reveals a significant reduction in the effective mass of charge carriers due to vacancy defects. Due to the location of defects, heterojunctions exhibit anisotropy. Obtain Perfect > VMo > VSII (370.7 > 277>125.1 cm2V−1S−1) for the x-direction carrier mobility (μx), while VMo > Perfect > VSII (440.8 > 279.7>131.6 cm2V−1S−1) for the y-direction carrier mobility (μy). These findings offer practical insights for utilizing defects to modulate the electronic properties of Semimetal-MoS2 heterojunction interfaces.

Photoinduced carrier transfer dynamics in a monolayer MoS2/PbS quantum dots heterostructure

Two-dimensional molybdenum disulfide (MoS2) has been proven to be a candidate in photodetectors, and MoS2/lead sulfide (PbS) quantum dots (QDs) heterostructure has been used to expand the optical response wavelength of MoS2. Time-resolved pump-probe transient absorption measurements are performed to clarify the carrier transfer dynamics in the MoS2/PbS heterostructure. By comparing the carrier dynamics in MoS2 and MoS2/PbS under different pump wavelengths, we found that the excited electrons in PbS QDs can transfer rapidly (<100 fs) to MoS2, inducing its optical response in the near-infrared region, although the pump light energy is lower than the bandgap of MoS2. Besides, interfacial excitons can be formed in the heterostructure, prolonging the lifetime of the excited carriers, which could be beneficial for the extraction of the carriers in devices.

Tailoring amorphous boron nitride for high-performance two-dimensional electronics

Two-dimensional (2D) materials have garnered significant attention in recent years due to their atomically thin structure and unique electronic and optoelectronic properties. To harness their full potential for applications in next-generation electronics and photonics, precise control over the dielectric environment surrounding the 2D material is critical. The lack of nucleation sites on 2D surfaces to form thin, uniform dielectric layers often leads to interfacial defects that degrade the device performance, posing a major roadblock in the realization of 2D-based devices. Here, we demonstrate a wafer-scale, low-temperature process (<250 °C) using atomic layer deposition (ALD) for the synthesis of uniform, conformal amorphous boron nitride (aBN) thin films. ALD deposition temperatures between 125 and 250 °C result in stoichiometric films with high oxidative stability, yielding a dielectric strength of 8.2 MV/cm. Utilizing a seed-free ALD approach, we form uniform aBN dielectric layers on 2D surfaces and fabricate multiple quantum well structures of aBN/MoS2 and aBN-encapsulated double-gated monolayer (ML) MoS2 field-effect transistors to evaluate the impact of aBN dielectric environment on MoS2 optoelectronic and electronic properties. Our work in scalable aBN dielectric integration paves a way towards realizing the theoretical performance of 2D materials for next-generation electronics.

Tuning instability in suspended monolayer 2D materials

Monolayer two-dimensional (2D) materials possess excellent in-plane mechanical strength yet extremely low bending stiffness, making them particularly susceptible to instability, which is anticipated to have a substantial impact on their physical functionalities such as 2D-based Micro/Nanoelectromechanical systems (M/NEMS), nanochannels, and proton transport membrane. In this work, we achieve quantitatively tuning instability in suspended 2D materials including monolayer graphene and MoS2 by employing a push-to-shear strategy. We comprehensively examine the dynamic wrinkling-splitting-smoothing process and find that monolayer 2D materials experience stepwise instabilities along with different recovery processes. These stepwise instabilities are governed by the materials’ geometry, pretension, and the elastic nonlinearity. We attribute the different instability and recovery paths to the local stress redistribution in monolayer 2D materials. The tunable instability behavior of suspended monolayer 2D materials not only allows measuring their bending stiffness but also opens up new opportunities for programming the nanoscale instability pattern and even physical properties of atomically thin films.

Tuning the Electronic Characteristics of Monolayer MoS2‐Based Transistors by Ion Irradiation: The Role of the Substrate

AbstractThis study explores defect engineering in 2D materials using ion beam irradiation to modify the electrical and optical properties with potential in advancing quantum electronics and photonics. Helium and neon ions ranging from 5 to 7.5 keV are employed to manipulate charge transport in monolayer molybdenum disulfide (MoS2). In situ electrical characterization occurs without vacuum breakage post‐irradiation. Raman and photoluminescence spectroscopy quantify ion irradiation's impact on MoS2. Small doses of helium ion irradiation enhance monolayer MoS2 conductivity in field‐effect transistor geometry by inducing doping and substrate charging. Findings reveal a strong correlation between the electrical properties of MoS2 and the primary ion used, as well as the substrate on which the irradiation occurred. Using hexagonal boron nitride (h‐BN) as a buffer layer between MoS2 flake and SiO2 substrate yields distinct alterations in electrical behavior subsequent to ion irradiation compared to the MoS2 layer directly interfacing with SiO2. Molecular dynamics simulations and density functional theory provide insight into experimental results, emphasizing substrate influence on measured electrical properties post‐ion irradiation.

Exploring channel length effects in 2D MoS2-Based memtransistors and their Synaptic behavior

Memtransistors, combining the functionalities of memristors and transistors, signify a substantial advancement with potential applications in areas such as neuromorphic learning and the integration of processing and memory units in current computers. Particularly, memtransistors fabricated using 2D materials have received extensive research attention due to their memory capabilities, energy efficiency, and features enabling compact integration. In this study, we examined the utilization of MoS2 monolayers grown by chemical vapor deposition (CVD), inherently imbued with defects, as memtransistors. Our focus probed into the exploration of how the change in channel length within this imperfect architectural construct exerts influences on the hysteresis loop and hence the operation of the device. In this respect, increasing the gate voltage enhanced the distinction between high and low resistance states and lengthening the channel led to a decrease in the differentiation between resistance states. Despite the shrinkage in hysteresis loop width, the memristive characteristics were preserved even at low gate voltages. Shortening the channel length widened the gap between low and high resistance states but disrupted the loop symmetry. Interestingly, relatively long channel memtransistors exhibited a more symmetrical and closer-to-ideal hysteresis. Experiments in different environments highlighted the impact of photocurrent. We applied consecutive pulses, resulting in an increase in the baseline of the current graph. Short-lived changes in the memtransistors resistance suggested its potential to simulate the short-term potentiation property of synapses. Our study on CVD grown 2D material-based memtransistors provides insights into the potential of using such defected materials in memtransistors wide usage and demonstrating their ability to emulate synaptic behavior.

Magneto-transport in the monolayer MoS2 material system for high-performance field-effect transistor applications

Electronic transport in monolayer MoS2 is significantly constrained by several extrinsic factors despite showing good prospects as a transistor channel material. Our paper aims to unveil the underlying mechanisms of the electrical and magneto-transport in monolayer MoS2. In order to quantitatively interpret the magneto-transport behavior of monolayer MoS2 on different substrate materials, identify the underlying bottlenecks, and provide guidelines for subsequent improvements, we present a deep analysis of the magneto-transport properties in the diffusive limit. Our calculations are performed on suspended monolayer MoS2 and MoS2 on different substrate materials taking into account remote impurity and the intrinsic and extrinsic phonon scattering mechanisms. We calculate the crucial transport parameters such as the Hall mobility, the conductivity tensor elements, the Hall factor, and the magnetoresistance over a wide range of temperatures, carrier concentrations, and magnetic fields. The Hall factor being a key quantity for calculating the carrier concentration and drift mobility, we show that for suspended monolayer MoS2 at room temperature, the Hall factor value is around 1.43 for magnetic fields ranging from 0.001 to 1 Tesla, which deviates significantly from the usual value of unity. In contrast, the Hall factor for various substrates approaches the ideal value of unity and remains stable in response to the magnetic field and temperature. We also show that the MoS2 over an Al2O3 substrate is a good choice for the Hall effect detector. Moreover, the magnetoresistance increases with an increase in magnetic field strength for smaller magnetic fields before reaching saturation at higher magnetic fields. The presented theoretical model quantitatively captures the scaling of mobility and various magnetoresistance coefficients with temperature, carrier densities, and magnetic fields.

Performance Enhancement of Gold Coated D-shaped PCF Sensor Using Monolayer MoS2

Performance Enhancement of Gold Coated D-shaped PCF Sensor Using Monolayer MoS2

O, P, Se, and Cl-doped monolayer MoS2: A new chapter in Co adsorption properties - An exploration based on first principles

This work uses a first-principles approach to thoroughly examine the impact of O, P, Se, and Cl doping of MoS2 on Co adsorption characteristics. Through structure optimization, formation energy calculations, differential charge analysis, and calculations of energy band structure and density of states, we find that P and Cl doping significantly enhances the adsorption capacity of Co on monolayer MoS2, while O and Se doping has a comparatively minor impact. This result reveals that P and Cl doping can significantly improve the adsorption stability of Co through the formation of stronger chemical bonding, which provides a theoretical basis for the design of 2D materials in adsorption applications.

Eight In. Wafer-Scale Epitaxial Monolayer MoS2.

Large-scale, high-quality, and uniform monolayer molybdenum disulfide(MoS2) films are crucial for their applications in next-generation electronics and optoelectronics. Epitaxy is a mainstream technique for achieving high-quality MoS2 films and is demonstrated at a wafer scale up to 4-in. In this study, the epitaxial growth of 8-in. wafer-scale highly oriented monolayer MoS2 on sapphire is reported as with excellent spatial homogeneity, using a specially designed vertical chemical vapor deposition (VCVD) system. Field effect transistors (FETs) based on the as-grown 8-in. wafer-scale monolayer MoS2 film are fabricated and exhibit high performances, with an average mobility and an on/off ratio of 53.5 cm2 V-1 s-1 and 107, respectively. In addition, batch fabrication of logic devices and 11-stage ring oscillators are also demonstrated, showcasing excellent electrical functions. This work may pave the way of MoS2 in practical industry-scale applications.

Recombination dynamics and manybody effect of excitons in large-area monolayer MoS2 capped with (111) NiO epitaxial layer

CVD grown monolayer MoS2 films on c-sapphire substrates are vacuum annealed and capped with (111) NiO epitaxial films using pulsed laser deposition technique. Time, energy and polarization resolved optical techniques are used to understand the effect of capping on the excitonic properties of the monolayer MoS2. It has been observed that trion contribution in the photoluminescence (PL) spectra increases after the capping, suggesting an enhancement of electron concentration in the conduction band. This has been attributed to the capping driven reduction of physisorbed air molecules from the sulphur vacancy ( VS ) sites. Note that the air molecules act as passivating agents for the VS -donors. Low temperature polarization resolved PL spectroscopy and ultrafast pump and probe transient absorption spectroscopy (TAS) show an increase of the biexcitonic population in the system after the encapsulation. The TAS study further reveals longer lifetime for both A and B excitons in capped samples implying a reduction of non-radiative recombination rate of the excitons after the capping. It has also been observed that in the capped samples, K/K′ valleys are populated with trions under sufficiently high pump powers. This has been attributed to the lower non-radiative recombination rates of the high energy photoexcited carriers and the faster transfer of either electrons or holes from the high energy pockets to the K/K′ valleys. The study further reveals different many-body effects in excitons and trions.

Mechanism of adsorption and gas-sensing of hazardous gases by MoS2 monolayer decorated by Pdn (n=1–4) clusters

The adsorption behaviors of hazardous gases (NO2 and SO2) on MoS2 monolayer decorated by Pdn (n=1–4) clusters (Pdn-MoS2) are investigated by first-principles calculations, and the performance of Pdn-MoS2 gas sensors has also been studied in this work. Our results show that Pdn nanocluster can enhance the electrical conductivity of MoS2 monolayer with band gaps of Pdn-MoS2 (n=1–4) sharply decreased, and the adsorption performance of NO2 and SO2 with higher adsorption energy and larger charge transfer. Thus, Pdn-MoS2 show higher sensitive to NO2 and SO2 molecules compared with pristine MoS2. The band gaps of Pdn-MoS2 can be further decreased upon the adsorption of NO2 and SO2 (except for SO2/Pd4-MoS2), indicating Pdn-MoS2 (n=1–3) gas sensor shows improved gas-sensitive response to both NO2 and SO2, and Pd4-MoS2 shows improved gas-sensitive response only to NO2. The recovery time analysis indicates that Pdn-MoS2 (n=1–4) is applicable to be utilized as NO2 and SO2 scavenger at ambient temperature, and the recovery process can be realized simply by heating. Our results indicate that Pdn-MoS2 (n=1–4) monolayer is a favorable candidate as adsorbent for detection and removal of NO2 and SO2, and also provides guidance for further study of the MoS2 monolayer decorated by transition metal clusters.

Ab-initio transport model to study the thermoelectric performance of MoS2, MoSe2, and WS2 monolayers by using Boltzmann transport equation

The potential for thermoelectric applications of two-dimensional materials is quite promising. Using ab-initio calculations, we have investigated the electronic band structure, phonon band structure, electronic density of states, and phonon density of states of monolayers MoS2, MoSe2, and WS2. In order to compute the thermoelectric properties of monolayers MoS2, MoSe2, and WS2, we used the ab-initio model suggested by Faghaninia et al (2015 Phys. Rev. B 91 235123). Within this model, by using inputs from density functional theory and considering all relevant elastic and inelastic scattering mechanisms, we have calculated the thermoelectric properties of monolayers MoS2, MoSe2, and WS2 over various ranges of temperature (T) and carrier concentration (n). The obtained results of Seebeck coefficients (S) and figure of merit (ZT) at T = 300 K for both n/p-types of monolayers MoS2, MoSe2, and WS2 are in good agreement with the findings obtained by other models using the Boltzmann transport equation within a constant relaxation time framework.

A molybdenum-disulfide nanocomposite film-based stretchable triboelectric nanogenerator for wearable biomechanical energy harvesting and self-powered human motion monitoring

Triboelectric nanogenerators (TENGs) are receiving significant interest in the wearable electronics industry owing to their distinct ability to capture ambient energy, particularly from human endeavors. Because they can serve as renewable sources of energy and multifunctional sensing devices. Herein, a stretchable TENG (S-TENG) based on micropatterned fabric-coated molybdenum disulfide (MoS2) and an Ecoflex nanocomposite as the electron-acceptor layer is newly developed for harvesting biomechanical energy to power wearable electronics and enable self-powered sensing. Incorporating the MoS2 monolayer in the composite creates microchannels on the surfaces, improving the conductivity of the composite by facilitating ion transport. Moreover, the incorporation of the monolayer provides an additional triboelectric output by increasing the contact area, enhancing the dielectric constant, and modulating the surface charge density. The use of knitted fabric materials enhances the extensibility (230 %) of the composite film. A thorough investigation was conducted to assess the electric output performance, resulting in the attainment of a maximum peak power density of 11 W/m2. This power density is sufficient to provide electricity to a typical electronic equipment with low power consumption. Regarding passive sensing, the suggested S-TENG may be used as a pressure sensor, with a high sensitivity of 17.18 V/kPa. Furthermore, active sensing enables the identification of dynamic movements in the joints of the human body by establishing a relation between gestures and the corresponding electrical signals. This study presents an innovative approach for developing a wearable S-TENG that exhibits dual-mode energy-harvesting and sensing capabilities, including the use in biomedical applications and human-machine systems.

Enhancing Carrier Mobility in Monolayer MoS2 Transistors with Process-Induced Strain.

Two-dimensional electronic materials are a promising candidate for beyond-silicon electronics due to their favorable size scaling of electronic performance. However, a major challenge is the heterogeneous integration of 2D materials with CMOS processes while maintaining their excellent properties. In particular, there is a knowledge gap in how thin film deposition and processes interact with 2D materials to alter their strain and doping, both of which have a drastic impact on device properties. In this study, we demonstrate how to utilize process-induced strain, a common technique extensively applied in the semiconductor industry, to enhance the carrier mobility in 2D material transistors. We systematically varied the tensile strain in monolayer MoS2 transistors by iteratively depositing thin layers of high-stress MgOx stressor. At each thickness, we combined Raman spectroscopy and transport measurements to unravel and correlate the changes in strain and doping within each transistor with their performance. The transistors displayed uniform strain distributions across their channels for tensile strains of up to 0.48 ± 0.05%, at 150 nm of stressor thickness. At higher thicknesses, mechanical instability occurred, leading to nonuniform strains. The transport characteristics systematically varied with strain, with enhancement in electron mobility at a rate of 130 ± 40% per % strain and enhancement of the channel saturation current density of 52 ± 20%. This work showcases how established CMOS technologies can be leveraged to tailor the transport in 2D transistors, accelerating the integration of 2D electronics into a future computing infrastructure.

Nature of point defects in monolayer MoS2 and the MoS2/Au(111) heterojunction

Deposition of MoS2 on Au(111) alters the electronic properties of MoS2. In this study, we investigate the free-standing MoS2 monolayer and the MoS2/Au(111) heterostructure, with and without strain, as well as defects of interest in memristive and neuromorphic applications. We focus on the so-called atomristor devices based on monolayer materials that achieve resistive switching characteristics with the adsorption and desorption of metal adatoms. Our study confirms that the formation of midgap states is the primary mechanism behind the resistive switching. Our results show that strain lowers the adsorption/desorption energies of Au+defect structures of interest, leading to more favorable switching energies, but simultaneously reduces the switching ratio between states of differing conductivities. The presence of the Au(111) substrate additionally introduces non-uniform amounts of strain and charge transfer to the MoS2 monolayer. We propose that the induced strain contributes to the experimentally observed n- to p-type transition and Ohmic to Schottky transition in the MoS2 monolayer. The charge transfer leads to a permanent polarization at the interface, which can be tuned by strain. Our study has important implications on the role of the electrode as being a source of the observed variability in memristive devices and as an additional degree of freedom for tuning the switching characteristics of the memristor device.

Ultrafast state-selective tunneling in two-dimensional semiconductors with a phase- and amplitude-controlled THz-scanning tunneling microscope

THz-pulse driven scanning tunneling microscopy (THz-STM) enables access to the ultrafast quantum dynamics of low-dimensional material systems at simultaneous ultrafast temporal and atomic spatial resolution. State-selective tunneling requires precise amplitude and phase control of the THz pulses combined with quantitative near-field waveform characterization. Here, we employ our state-of-the-art THz-STM with multi-MHz repetition rates, efficient THz generation, and precisely tunable THz waveforms to investigate a single sulfur vacancy in monolayer MoS2. We demonstrate that 2D transition metal dichalcogenides (TMDs) are an ideal platform for near-field waveform sampling by THz cross-correlation. Furthermore, we determine the THz voltage via QEV scans, which measure the THz rectified charge Q as a function of THz field amplitude E and dc bias Vdc. Mapping the complex energy landscape of localized states with a resolution down to 0.01 electrons per pulse facilitates state-selective tunneling to the HOMO and LUMO orbitals of a charged sulfur vacancy.

Modeling the coverage of MoS2 and WS2 thin films using in-situ spectroscopic ellipsometry

In-situ spectroscopic ellipsometry (SE) was used to analyze monolayer and few-layer samples of molybdenum disulfide (MoS2) and tungsten disulfide (WS2) grown by metalorganic chemical vapor deposition (MOCVD) on sapphire substrates. MoS2 and WS2 film growth times ranged from 5 – 40 min to achieve a range of film coalescence and surface coverage from less than a monolayer to considerable bilayer and additional layers. Post-growth measurements using atomic force microscopy, photoluminescence and Raman spectroscopy were used to assess the evolution of surface morphology and film properties with growth time to determine the areal coverage of monolayer, bilayer, multilayer and void regions. The room temperature SE spectra were modeled using an effective medium approximation to obtain the monolayer and bilayer dielectric functions of MoS2 and WS2. Using these distinct dielectric functions, changes in ellipsometry parameters associated with partial to full coverage of monolayers and bilayers were simulated. Such simulations will enable improved control of the layer-by-layer deposition of MoS2 and WS2 by monitoring in-situ spectroscopic ellipsometry.

Spontaneous polarization dynamics in V-doped monolayer MoS2

Valleytronics has drawn attention both for its fundamental aspects and potential applications in device technology. Lifting the valley degeneracy is an attractive route to achieve stable valley polarization and spontaneous valley polarization. Our first-principles calculations on V-doped monolayer MoS2 reveal a substantial permanent valley splitting of 174 meV, driven by the combined effects of spin-orbit coupling (SOC) and magnetic coupling with the doped atom. Non-adiabatic molecular dynamics simulations show that the spontaneous valley polarization dynamics depends on the SOC, electron-phonon (e-ph) coupling and the scattering with the impurity, and it varies notably with different initial spin orientations of excited holes. For spin-up holes, the process involves e-ph scattering and spin-flip induced by spinor-phonon scattering over several picoseconds. On the other hand, spin-down hole excitation leads to spontaneous polarization through e-ph and impurity scattering on a timescale shorter by an order of magnitude. This study provides insights into the distinct valley polarization dynamics of carriers with different spins, contributing to the design of ultrafast valleytronic and spintronic devices.

Gram-scale microwave-assisted synthesis of high-quality CsPbBr3 nanocrystals for optoelectronic applications

Fully inorganic halide perovskite nanocrystals (NCs) are promising candidates for optoelectronic applications due to their beneficial properties such as their amenability to solution processing, high absorption coefficients, and high quantum yields. Various methods have been developed to prepare perovskite CsPbX3 (X = Cl, Br, and I) NCs; however, large-scale synthesis with high-quality CsPbX3 NCs remains challenging. In the study, we report an advanced method for the gram-scale synthesis of all-inorganic perovskite NCs via specially designed microwave-assisted synthesis, which offers uniform reaction energy transfer, excellent reproducibility, and flexible parameter tuning. The composition, size and uniformity of the CsPbBr3 NCs have been optimized by controlling the surfactant ratio, precursor ratio, and reaction time. This systematic optimization leads to the production of gram-scale (> 0.165 g per a 100 ml reaction mixture for a single synthesis cycle) high-quality, phase-pure CsPbBr3 NCs with a uniform size of ∼ 10 nm. These optimized NCs exhibit a sharp photoluminescence peak centered at 515 nm with a photoluminescence quantum yield of 94 % and produce excellent radioluminescence under X-ray irradiation. To test the feasibility of these NCs for use in real optoelectronic devices, a photodetector consisting of CsPbBr3 NCs and monolayer MoS2 was fabricated, demonstrating enhanced photosensitivity and a faster response time compared with a pure MoS2-based photodetector. The proposed gram-scale microwave-assisted synthesis method thus has the potential to promote the commercialization of metal halide perovskite NCs through cost-effective mass production for use in many optoelectronic applications.