Properties Of Transition Metal Dichalcogenide Monolayers Research Articles

Local Manipulation of the Energy Levels of 2D TMDCs on the Microscale Level via Microprinted Self‐Assembled Monolayers

Abstract2D transition metal dichalcogenides (TMDCs) are atomically‐thick semiconductors with great potential for next‐generation optoelectronic applications, such as transistors and sensors. Their large surface‐to‐volume ratio makes them energy‐efficient but also extremely sensitive to the physical‐chemical surroundings. The latter must be carefully considered when predicting the electronic behavior, such as their energy level alignment, which ultimately affects the charge carrier injection and transport in devices. Here, local doping is demonstrated and thus adjusting the opto‐electronic properties of monolayer TMDCs (WSe2 and MoS2) by chemically engineering the surface of the supporting substrate. This is achieved by decorating the substrate by microcontact printing with patterns of two different self‐assembled monolayers (SAMs). The SAMs posses distinct molecular dipoles and dielectric constants, significantly influencing the TMDCs electronic and optical properties. By analyzing (on various substrtates), it is confirmed that these effects arise solely from the interaction between SAMs and TMDCs. Understanding the diverse dielectric environments experienced by TMDCs allows for a correlation between electronic and optical behaviours. The changes primarily involve alteration in the electronic band gap width, which can be calculated using the Schottky‐Mott rule, incorporating the dielectric screening of the TMDCs surroundings. This knowledge enables accurate prediction of the (opto‐)electronic behavior of monolayer TMDCs for advanced device design.

Valley-resolved quantum anomalous Hall effect in ferromagnetically proximitized monolayer MoTe2

We theoretically investigate the effect of Rashba field and magnetic exchange interaction on the topological properties of monolayer transition-metal dichalcogenides (TMDs) based on a tight-binding model. The tight-binding Hamiltonian incorporates magnetic exchange interaction and the Rashba field which can be induced by proximity to a ferromagnetic (FM) substrate, such as europium oxide (EuO) or similar materials. Through tuning the Rashba field and magnetic exchange interaction, we find a quantum state of matter---the valley-resolved quantum anomalous Hall state---in monolayer ${\mathrm{MoTe}}_{2}$. The most amazing property of this topological quantum state is that the edge states of this phase are all located only at valley $\ensuremath{-}K$ (or $+K$), i.e., they are completely valley resolved. Additionally, the calculated Chern number $\mathcal{C}=\ensuremath{-}2$ indicates that there are two pairs of valley-resolved chiral edge modes at the boundaries of the monolayer ${\mathrm{MoTe}}_{2}$ nanoribbon. The electronic transmission demonstrates the robustness of the edge states of the valley-resolved quantum anomalous Hall effect against defects. This newfound valley-resolved quantum anomalous Hall effect provides an idea for designing dissipationless valleytronics utilizing TMD/FM heterojunctions.

Theoretical Study of Auger Recombination of Excitons in Monolayer Transition-metal Dichalcogenides

Excitons are the most prominent features of the optical properties of monolayer transition-metal dichalcogenides(TMDC). In view of optoelectronics it is very important to understand the decay mechanisms of the excitons of these materials. Auger recombination of excitons are regarded as one of the dominant decay processes. In this paper the Auger constant of recombination is computed based on the approach proposed by Kavoulakis and Baym. We obtain both temperature dependent (from type A, A’ processes) and temperature independent (from type B, B’ processes) contributions, and a numerical estimate of theoretical result yields the value of constant in the order of 10−2 cm2s−1, being consistent with existing experimental data. This implies that Auger decay processes severely limit the photoluminescence yield of TMDC-based optoelectronic devices.

The Dielectric Impact of Layer Distances on Exciton and Trion Binding Energies in van der Waals Heterostructures.

The electronic and optical properties of monolayer transition-metal dichalcogenides (TMDs) and van der Waals heterostructures are strongly subject to their dielectric environment. In each layer, the field lines of the Coulomb interaction are screened by the adjacent material, which reduces the single-particle band gap as well as exciton and trion binding energies. By combining an electrostatic model for a dielectric heteromultilayered environment with semiconductor many-particle methods, we demonstrate that the electronic and optical properties are sensitive to the interlayer distances on the atomic scale. An analytic treatment is used to provide further insight into how the interlayer gap influences different excitonic transitions. Spectroscopical measurements in combination with a direct solution of a three-particle Schrödinger equation reveal trion binding energies that correctly predict recently measured interlayer distances and shed light on the effect of temperature annealing.

Twin Defect Derived Growth of Atomically Thin MoS2 Dendrites.

Morphology management for tailoring the properties of monolayer transition-metal dichalcogenides (TMDCs), that is, molybdenum disulfide (MoS2), has attracted great interest for promising applications such as in electrocatalysis and optoelectronics. Nevertheless, little progress has been made in engineering the shape of MoS2. Herein, we introduce a modified chemical vapor deposition method to grow monolayer MoS2 dendrites by pretreating substrates with adhesive tapes. The as-grown MoS2 crystals are featured with hexagonal backbones with fractal shapes and tunable degrees. By characterizing the atomic structure, it is found that these morphologies are mainly initiated from the twin defect derived growth and controlled by the S:Mo vapor ratio. Due to the accumulated sulfur vacancies in the cyclic twin regions, strong enhancement of photoluminescence emission is localized, which determines the shape dependency of optical property. This work not only enriches the understanding of the twin defects derived crystal growth mechanism and extends its applications from nanomaterials to two-dimensional crystals, but also offers a robust and controllable protocol for shape-engineered monolayer TMDCs in electrochemical and optoelectronic applications.

Simple Chemical Treatment to n-Dope Transition-Metal Dichalcogenides and Enhance the Optical and Electrical Characteristics

The optical and electrical properties of monolayer transition-metal dichalcogenides (1L-TMDs) are critically influenced by two dimensionally confined exciton complexes. Although extensive studies on controlling the optical properties of 1L-TMDs through external doping or defect engineering have been carried out, the effects of excess charges, defects, and the populations of exciton complexes on the light emission of 1L-TMDs are not yet fully understood. Here, we present a simple chemical treatment method for n-dope 1L-TMDs, which also enhances their optical and electrical properties. We show that dipping 1Ls of MoS2, WS2, and WSe2, whether exfoliated or grown by chemical vapor deposition, into methanol for several hours can increase the electron density and also can reduce the defects, resulting in the enhancement of their photoluminescence, light absorption, and the carrier mobility. This methanol treatment was effective for both n- and p-type 1L-TMDs, suggesting that the surface restructuring around structural defects by methanol is responsible for the enhancement of optical and electrical characteristics. Our results have revealed a simple process for external doping that can enhance both the optical and electrical properties of 1L-TMDs and help us understand how the exciton emission in 1L-TMDs can be modulated by chemical treatments.

Tight-binding approach to strain and curvature in monolayer transition-metal dichalcogenides

We present a model of the electronic properties of monolayer transition-metal dichalcogenides based on a tight binding approach which includes the effects of strain and curvature of the crystal lattice. Mechanical deformations of the lattice offer a powerful route for tuning the electronic structure of the transition-metal dichalcogenides, as changes to bond lengths lead directly to corrections in the electronic Hamiltonian while curvature of the crystal lattice mixes the orbital structure of the electronic Bloch bands. We first present an effective low energy Hamiltonian describing the electronic properties near the K point in the Brillouin zone, then present the corrections to this Hamiltonian due to arbitrary mechanical deformations and curvature in a way which treats both effects on an equal footing. This analysis finds that local area variations of the lattice allow for tuning of the band gap and effective masses, while the application of uniaxial strain decreases the magnitude of the direct band gap at the K point. Additionally, strain induced bond length modifications create a fictitious gauge field with a coupling strength that is smaller than that seen in related materials like graphene. We also find that curvature of the lattice leads to the appearance of both an effective in-plane magnetic field which couples to spin degrees of freedom and a Rashba-like spin-orbit coupling due to broken mirror inversion symmetry.

Engineering Bandgaps of Monolayer MoS2 and WS2 on Fluoropolymer Substrates by Electrostatically Tuned Many-Body Effects.

Intrinsic electrical and excitonic properties of monolayer transition-metal dichalcogenides can be revealed on CYTOP fluoropolymer substrates with greatly suppressed unintentional doping and dielectric screening. Ambipolar transport behavior is observed in monolayer WS2 by applying solid-state back gates. The excitonic properties of monolayer MoS2 and WS2 are determined by intricate interplays between the bandgap renormalization, Pauli blocking, and carrier screening against carrier doping.

Effect of Coulomb interactions on optical properties of monolayer transition-metal dichalcogenides

A theoretical investigation is carried out on the effect of Coulomb interactions on the optical properties of monolayer transition-metal dichalcogenides (TMDCs) by solving the Bethe-Salpeter equation under the $k\ifmmode\cdot\else\textperiodcentered\fi{}p$ approximation. The calculated absorption spectrum of TMDCs exhibits strong exciton features, and a peculiar optical selection rule is found for circularly polarized light, in agreement with previous theoretical calculations and experimental observations. The exciton-exciton annihilation rate is evaluated by considering the Auger recombination process, which gives rise to the nonlinear optical response. The calculation results agree well with the experimental data, and confirm strong exciton-exciton interaction effects in monolayer TMDCs.

Quasiparticle band structures and optical properties of strained monolayer MoS2and WS2

The quasiparticle (QP) band structures of both strainless and strained monolayer MoS$_{2}$ are investigated using more accurate many body perturbation \emph{GW} theory and maximally localized Wannier functions (MLWFs) approach. By solving the Bethe-Salpeter equation (BSE) including excitonic effects on top of the partially self-consistent \emph{GW$_{0}$} (sc\emph{GW$_{0}$}) calculation, the predicted optical gap magnitude is in a good agreement with available experimental data. With increasing strain, the exciton binding energy is nearly unchanged, while optical gap is reduced significantly. The sc\emph{GW$_{0}$} and BSE calculations are also performed on monolayer WS$_{2}$, similar characteristics are predicted and WS$_{2}$ possesses the lightest effective mass at the same strain among monolayers Mo(S,Se) and W(S,Se). Our results also show that the electron effective mass decreases as the tensile strain increases, resulting in an enhanced carrier mobility. The present calculation results suggest a viable route to tune the electronic properties of monolayer transition-metal dichalcogenides (TMDs) using strain engineering for potential applications in high performance electronic devices.