Transition‐metal dichalcogenides for spintronic applications

Transition metal dichalcogenides (TMDCs) are ideal candidates to explore the manifestation of spin-valley physics under external stimuli. In this study, we investigate the influence of strain on the spin and orbital angular momenta, effective g-factors, and Berry curvatures of several monolayer TMDCs (Mo and W based) using a full ab initio approach. At the K-valleys, we find a surprising decrease of the conduction band spin expectation value for compressive strain, consequently increasing the dipole strength of the dark exciton by more than one order of magnitude (for strain variation). We also predict the behavior of direct excitons g-factors under strain: tensile (compressive) strain increases (decreases) the absolute value of g-factors. Strain variations of ∼1% modify the bright (A and B) excitons g-factors by ∼0.3 (0.2) for W (Mo) based compounds and the dark exciton g-factors by ∼0.5 (0.3) for W (Mo) compounds. Our predictions could be directly visualized in magneto-optical experiments in strained samples at low temperature. Additionally, our calculations strongly suggest that strain effects are one of the possible causes of g-factor fluctuations observed experimentally. By comparing the different TMDC compounds, we reveal the role of spin–orbit coupling (SOC): the stronger the SOC, the more sensitive are the spin-valley features under applied strain. Consequently, monolayer WSe2 is a formidable candidate to explore the role of strain on the spin-valley physics. We complete our analysis by considering the side valleys, Γ and Q points, and by investigating the influence of strain in the Berry curvature. In the broader context of valley- and strain-tronics, our study provides fundamental microscopic insights into the role of strain in the spin-valley physics of TMDCs, which are relevant to interpret experimental data in monolayer TMDCs as well as TMDC-based van der Waals heterostructures.

Monolayer (ML) transition metal dichalcogenides (TMDs) have emerged as promising channel materials for future transistors due to their atomically thin geometry, enabling aggressive channel length scaling. Their short electrostatic screening length, sizable bandgap, and high carrier mobility make ML TMDs strong candidates for advanced technology nodes. This thesis presents several key advancements that demonstrate the potential of ML TMD transistors based on statistical performance evaluation. First, we fabricated double-gated monolayer WS₂ field-effect transistors (FETs) with a 30 nm channel length, achieving excellent performance, a drive current of 400 µA/µm. Technology Computer-Aided Design (TCAD) simulations were employed to optimize the device structure by introducing an air-gap spacer between the source and top gate, which effectively suppressed off-state degradation and mitigated negative threshold voltage shifts. Second, we significantly reduced contact resistance in ML MoS₂ FETs by using conventional metal, Nickle, bringing it down to approximately 500 Ω·µm through contact interface engineering. Lastly, we developed a contact strategy using metallic Nb_0.3W_0.7Se₂ as a contact for monolayer and bilayer WSe₂ transistors with channel lengths down to 100 nm, the resulting 2D/2D contacted FETs exhibit high on-current densities of up to 375 µA/µm and 1.1 mA/µm on monolayer and bilayer WSe₂ channels, respectively.The pristine, dangling-bond-free nature of monolayer (ML) transition metal dichalcogenides (TMDs) poses a significant challenge for top dielectric formation, which is a key element in enhancing electrical performance. Two different methods have been introduced in this thesis for forming interfacial layers to enable the successful growth of top dielectrics. The first approach involves the formation of a physical metal oxide layer via the natural oxidation of pre-deposited metal atoms. By utilizing a pre-deposited Aluminum seeding layer, an ML WS₂ transistor exhibiting an ideal subthreshold slope of approximately 63 mV/dec has been demonstrated. The second method integrates amorphous boron nitride (aBN), grown at 250^◦C, achieving a seeding-layer-free process for fabricating double-gated field-effect transistors (FETs). This integration results in a low interface trap density of less than 10¹² cm⁻² and an ideal subthreshold slope of around 60 mV/dec over a two-order current range (10^-4 to 10^-6 µA/µm). Furthermore, we investigated the interface trapped density landscape of ML MoS₂, a temperature dependent properties of ML MoS₂. Due to its inherent two-valley degeneracy and reduced phonon scattering, the mobility versus carrier density exhibits a gentler slope near −1, compared to −1.5 in multilayer MoS₂. Notably, ML MoS₂ shows a dielectric-constant-independent mobility behavior, with an extracted mobility in the range of 45 to 55 cm²/V·s.We also demonstrated the first on-chip P-bit core by integrating a P-bit with ML MoS₂ transistors. The low processing temperature of ML MoS₂ FETs enables their fabrication alongside probabilistic magnetic tunnel junctions (MTJs) without degrading MTJ performance. The high on-current of ML MoS₂ allows for effective resistance matching between the MTJs and input resistance, which is critical for the performance of the P-bit core.

Transition metal dichalcogenide (TMDC) monolayers exhibit great photocatalytic performances and tunabilities. By using first-principles simulations based on density functional theory, we demonstrate here that external electric bias can be employed to further improve the photocatalytic hydrogen production efficiencies of the six AB2 (A = Mo, W and B = S, Se, Te) TMDC monolayers since these are influenced by their piezoelectric response. In particular, when subjected to a proper amount of electrically induced tensile biaxial strain, most TMDC monolayers turn into potentially ideal photocatalysts toward the hydrogen evolution reaction (HER). The beneficial effects of introducing tensile biaxial strain on the TMDC monolayers are not limited to the reduction of the band gap and proper adjustment of the band edge positions; the concomitant hydrogen adsorption free energies are also modified in such a way that the HER reaction is noticeably favored.

Nanoscale variations of optical properties in transition metal dichalcogenide (TMD) monolayers can be explored with cathodoluminescence (CL) and electron energy loss spectroscopy (EELS) using electron microscopes. To increase the CL emission intensity from TMD monolayers, the MoSe2 flakes are encapsulated in hexagonal boron nitride (hBN), creating van der Waals (VdW) heterostructures. Until now, the studies have been exclusively focused on scanning transmission electron microscopy (STEM-CL) or scanning electron microscopy (SEM-CL), separately. Here, we present results, using both techniques on the same sample, thereby exploring a large acceleration voltage range. We correlate the CL measurements with STEM-EELS measurements acquired with different energy dispersions, to access both the low-loss region at ultra-high spectral resolution, and the core-loss region. This provides information about the weight of the various absorption phenomena including the direct TMD absorption, the hBN interband transitions, the hBN bulk plasmon, and the core losses of the atoms present in the heterostructure. The S(T)EM-CL measurements from the TMD monolayer only show emission from the A exciton. Combining the STEM-EELS and S(T)EM-CL measurements, we can reconstruct different decay pathways leading to the A exciton CL emission. The comparison with SEM-CL shows that this is also a good technique for TMD heterostructure characterization, where the reduced demands on sample preparation are appealing. To demonstrate the capabilities of SEM-CL imaging, we also measured on a SiO2/Si substrate, quintessential in the sample preparation of two-dimensional materials, which is electron-opaque and can only be measured in SEM-CL. The CL-emitting defects of SiO2 make this substrate challenging to use, but we demonstrate that this background can be suppressed by using lower electron energy.

Wafer-scale epitaxial growth of semiconducting transition metal dichalcogenide (TMD) monolayers such as MoS2, WS2 and WSe2 is of significant interest for device applications to circumvent size limitations associated with the use of exfoliated flakes. Epitaxy is required to achieve single crystal films over large areas via coalescence of TMD domains. Our research has focused on epitaxial growth of 2D semiconducting TMDs on c-plane sapphire substrates using metalorganic chemical vapor deposition (MOCVD). Steps on the miscut sapphire surface serve as preferential sites for nucleation and can be used to induce a preferred crystallographic direction to the TMD domains which enables a reduction in twin boundaries in coalesced films. The step-directed growth is dependent on the surface termination of the sapphire which can be altered through pre-growth annealing in H2 and chalcogen-rich environments. Uniform growth of TMD monolayers with significantly reduced inversion domains is demonstrated on 2” diameter c-plane sapphire substrates enabling large area transfer of monolayers for characterization and device fabrication and testing. Applications for wafer-scale TMD monolayers in nanoelectronics, sensing and photonics will be discussed.

Monolayer transition metal dichalcogenides $MX_2$ ($M$ = Mo,W and $X$ = Te, Se, S) in 1T' structure were predicted to be quantum spin Hall insulators based on first-principles calculations, which were quickly confirmed by multiple experimental groups. For a better understanding of their properties, in particular their responses to external fields, we construct a realistic four-band tight-binding (TB) model by combining the symmetry analysis and first-principles calculations. Our TB model respects all the symmetries and can accurately reproduce the band structure in a large energy window from -0.3 eV to 0.8 eV. With the inclusion of spin-orbital coupling (SOC), our TB model can characterize the nontrivial topology and the corresponding edge states. Our TB model can also capture the anisotropic strain effects on the band structure and the strain-induced metal-insulator transition. Moreover, we found that although $MX_2$ share the same crystal structures and have the same crystal symmetries, while the orbital composition of states around the Fermi level are qualitatively different and their lower-energy properties cannot fully described by a single k $\cdot$ p model. Thus, we construct two different types of k $\cdot$ p model for $M$S$_2$,$M$Se$_2$ and $M$Te$_2$, respectively. Benefiting from the high accuracy and simplicity, our TB and k $\cdot$ p models can serve as a solid and concrete starting point for future studies of transport, superconductivity, strong correlation effects and twistronics in 1T'-transition metal dichalcogenides.

We study theoretically the Coulomb interaction between excitons in transition metal dichalcogenide (TMD) monolayers. We calculate direct and exchange interaction for both ground and excited states of excitons. The screening of the Coulomb interaction, specific to monolayer structures, leads to the unique behavior of the exciton-exciton scattering for excited states, characterized by the non-monotonic dependence of the interaction as function of the transferred momentum. We find that the nontrivial screening enables the description of TMD exciton interaction strength by approximate formula which includes exciton binding parameters. The influence of screening and dielectric environment on the exciton-exciton interaction was studied, showing qualitatively different behavior for ground state and excited states of excitons. Furthermore, we consider exciton-electron interaction, which for the excited states is governed by the dominant attractive contribution of the exchange component, which increases with the excitation number. The results provide a quantitative description of the exciton-exciton and exciton-electron scattering in transition metal dichalcogenides, and are of interest for the design of perspective nonlinear optical devices based on TMD monolayers.

Solid state photovoltaic devices made of monolayer two-dimensional (2D) transition metal dichalcogenide (TMDs) monolayers such as WSe2, MoS2, and MoS2/WSe2have achieved 1-3% power conversion efficiency. However, photo-generated charge carriers in the middle of the device typically traverse tens of micrometers toward end-on electrical contacts at the device edge. The long transport distances parallel to the TMD layers give rise to transport-limited device physics that limit overall device performance. Here we demonstrate ultrathin liquid junction photovoltaic systems that use an iodide-based hole-selective electrolyte contact that facilitates a charge transport pathway perpendicular to the 3-atom thick TMD materials. These proof-of-concept indium doped tin oxide (ITO)|monolayer TMD|I–/I2|Pt cells exhibit peak internal quantum efficiencies (IQE) of 7.1%, 34.5%, and 8.2% and open circuit voltages (V OC) of 0.46 V, 0.26 V, and 0.28 V for WSe2, MoS2, and MoS2/WSe2 heterojunctions. The photocurrent efficiency values are competitive with solid state devices. Surprisingly, we observe a non-linear scaling relation between IQE and layer thickness for homojunction and heterojunction TMDs that can be attributed to either thickness-dependent charge carrier dynamics or a buffer layer screening effect at the ITO/TMD interface. We discuss opportunities to improve the fill factor and V OCby interface engineering and tuning the energy level offset between the TMD composition and redox electrolyte potential.

The "gold-tape" exfoliation technique can deterministically exfoliate macroscopic transition metal dichalcogenide (TMD) monolayers from bulk single crystals, overcoming limitations of the widely used "Scotch-tape" method, but concerns over the quality of the large-area monolayers remain. Here we introduce a critical step improving the gold-tape method by eliminating a previously unknown polymer residue layer on the TMD surface. The resulting pristine and millimeter-scale TMD monolayers exhibit defect density, charge carrier mobility, and excitonic properties intrinsic to the parent crystal. Imaging, transport, and spectroscopy on length scales spanning 6 orders of magnitude demonstrate the viability of gold-exfoliated TMD monolayers for the deterministic fabrication of high-performance van der Waals devices, including moiré interfaces.

Dark excitons in transition metal dichalcogenide (TMDC) monolayers (MLs) are promising candidates for Bose–Einstein condensation and quantum information processing owing to their long lifetime. Previous studies employing nanoparticle‐on‐mirror and tip‐on‐mirror cavities have successfully enabled room‐temperature dark exciton emissions through the Purcell effect. However, these approaches suffer from mode complexity, the geometric variability of the contact facet, and the extreme precision of the tip spacing, which limit further control over emission intensities. Herein, the switching‐on of dark excitons in WSe 2 ML at room temperature is demonstrated using plasmonic Au nanodisk‐on‐mirror cavities with the ML sandwiched in between. The hybrid structures combine simplicity, reproducibility, and flexible control over the dark exciton emission intensity. The dark exciton emissions are controlled by two orthogonal plasmon modes with strongly confined out‐of‐plane‐polarized electric field and can be switched on and off. The emission intensity ratio between dark excitons and bright excitons is enhanced up to 22 times by flexibly controlling the nanodisk diameter to match the plasmon energy to the dark exciton energy. The plasmonic nanodisk‐on‐mirror cavities feature enlarged field enhancement, narrowed mode linewidths, and precise geometry, providing a facile platform for switching on and controlling dark exciton emissions in TMDC MLs under readily accessible conditions.

Many recent studies show that the hole-doped monolayer transition metal dichalcogenides (TMDs) exhibit Ising superconductivity. The Ising-type spin-orbit coupling (SOC) results in spin-momentum locking perpendicular to the two-dimensional crystal near the $K$ and $\ensuremath{-}K$ valleys, strongly protecting the superconducting state against in-plane magnetic fields. In this work, we theoretically study the Kondo effect of a magnetic impurity doped in monolayer TMDs with the Ising SOC protected superconducting state. Based on behaviors of spin-induced Yu-Shiba-Rusinov bound states and localized magnetic susceptibility, we show that the Ising SOC plays an important role in enhancing or suppressing the Kondo screening of the magnetic impurity, depending on the relative position of the Fermi level with respect to the Ising splitting. More importantly, the Ising SOC exhibits opposite influences on the quantum phase transition between magnetic doublet and Kondo singlet ground states in electron- and hole-doped cases. These features in Kondo screening can be tuned by ionic liquid-gating in experiments.

We use a combination of experimental techniques to demonstrate a general occurrence of spin-orbit interaction (SOI) in graphene on transition metal dichalcogenide (TMD) substrates. Our measurements indicate that SOI is ultra-strong and extremely robust, despite it being merely interfacially-induced, with neither graphene nor the TMD substrates changing their structure. This is found to be the case irrespective of the TMD material used, of the transport regime, of the carrier type in the graphene band, and of the thickness of the graphene multilayer. Specifically, we perform weak antilocalization measurements as the simplest and most general diagnostic of SOI, and show that the spin relaxation time is very short in all cases regardless of the elastic scattering time. Such a short spin-relaxation time strongly suggests that the SOI originates from a modification of graphene band structure. We confirmed this expectation by measuring a gate-dependent beating, and a corresponding frequency splitting, in the low-field Shubnikov-de Haas magneto-resistance oscillations in high quality bilayer graphene on WSe$_2$. These measurements provide an unambiguous diagnostic of a SOI-induced splitting in the electronic band structure, and their analysis allows us to determine the SOI coupling constants for the Rashba term and the so-called spin-valley coupling term, i.e., the terms that were recently predicted theoretically for interface-induced SOI in graphene. The magnitude of the SOI splitting is found to be on the order of 10 meV, more than 100 times greater than the SOI intrinsic to graphene. Both the band character of the interfacially induced SOI, as well as its robustness and large magnitude make graphene-on-TMD a promising system to realize and explore a variety of spin-dependent transport phenomena, such as, in particular, spin-Hall and valley-Hall topological insulating states.

We demonstrate strong anisotropic spin-orbit interaction (SOI) in graphene induced by monolayer WS_{2}. Direct comparison between graphene-monolayer WS_{2} and graphene-bulk WS_{2} systems in magnetotransport measurements reveals that monolayer transition metal dichalcogenide can induce much stronger SOI than bulk. Detailed theoretical analysis of the weak antilocalization curves gives an estimated spin-orbit energy (E_{so}) higher than 10meV. The symmetry of the induced SOI is also discussed, and the dominant z→-z symmetric SOI can only explain the experimental results. Spin relaxation by the Elliot-Yafet mechanism and anomalous resistance increase with temperature close to the Dirac point indicates Kane-Mele SOI induced in graphene.

As layered materials beyond graphene discovered after 2004, two-dimensional (2D) transition metal dichalcogenides (TMDs) are vital to fundamental research and practical applications, owing to their unique crystal structures and excellent properties, as well as diversity of the electronic band structures. 2D TMDs have played an important role in electronics, optoelectronics, energy storage and catalysis. To meet with the increasing requirements of programmable and function-integrated devices, property modulation could be concerned as one of the most essential strategies for 2D TMDs. In comparison with conventional external electrical field induction, strain regulation could be much more efficient. In detail, external induction through the electrical field leads to the electron delocalization along the field direction and then induces the transformation of band structures, but electrical field exhibits small modulation for monolayer TMDs. On the contrary, the strain regulation shows excellent tuning efficiency for continuous and reversible modulation. As a result, the strain regulation has become a commonly-used strategy for property tuning of 2D TMDs. On the basis of the lattice transformation, the strain regulation of 2D TMDs can lead to different overlapping of d and p orbits of metal and chalcogen atoms, and then affect the electronic structures of 2D TMDs. Therefore, it can be applied to electronics, optoelectronics, magnetic devices and piezoelectronics. The strategies of introducing strains to 2D TMDs are classified into lattice induction, local deformation, macroscopic regulation and so on. Lattice induction is attributable to the structural distortions and mismatches, including atomic defect induction and lattice mismatch induction. The former one demonstrates that the microenvironment affected by atomic vacancies and doping atoms can introduce strain to 2D TMDs. The latter one means that the lattice mismatches between two materials (between two different TMDs in a heterostructure or between a TMD and the substrate) can result in lattice distortion and then induce the strain. However, strain introduced through lattice induction is fixed and difficult to achieve the reversible modulation. Local deformation refers to the morphological transformation at the scale of several micrometers, which can be induced by the bubbles and wrinkles of 2D TMDs, external forces of tips, as well as patterned substrates. Generally, large but nonuniform strain can be introduced to 2D TMDs through the local deformation, which results in the funnel effects and then induces large property variation. Furthermore, macroscopic regulation introduces the strains to the lattices, including the deformation of flexible substrates (bend, tension and compression), external pressure (applied by the diamond anvil cell) and thermal expansion coefficient mismatch. Macroscopic regulation would be compatible with industrial manufacture to achieve strain regulation on an extremely large scale in the future. There are some other ways to achieve strain regulation such as the design of special stack structures and the induction of the external electrical field. After summarizing the methods of introducing strain to 2D TMDs, we presented the applications based on strain regulation, such as field effect transistors, flexible photodetectors and strain sensors. Finally, we pointed out the further development and challenges of strain regulation of 2D TMDs.

Monolayer transition metal dichalcogenide (TMD) alloys with tunable direct band gaps have promising applications in nanoelectronics and optoelectronics. The composition-dependent band gaps of ternary, quaternary and quinary monolayer TMD alloys have been systematically studied combining density functional theory and machine learning models in the present study. The excellent agreement between the DFT-calculated band gaps and the ML-predicted values for the training, validation and test datasets demonstrates the accuracy of our machine learning based on a neural network model. It is found that the band gap bowing parameter is closely related to the difference between the band gaps of the endpoint material compositions of the monolayer TMD alloy and increases with increasing band gap difference. The band gap bowing effects of monolayer TMD alloys obtained by mixing different transition metals are attributed to the conduction band minimum positions, while those of monolayer TMD alloys obtained by mixing different chalcogen atoms are dominated by the valence band maximum positions. This study shows that monolayer TMD alloys with tunable direct band gaps can provide new opportunities for band gap engineering, as well as electronic and optoelectronic applications.