Coexisting Triferroic and Multiple Types of Valley Polarization by Structural Phase Transition in 2D Materials

Two-dimensional materials, such as transition metal dichalcogenides (TMDCs) in the 2H or 1T crystal phases, are promising (electro)catalyst candidates due to their high surface-to-volume ratio and composition of low-cost, abundant elements. While the edges of elemental TMDC nanoparticles, such as MoS2, can show significant catalytic activity, the basal plane of the pristine materials is notoriously inert, which limits their normalized activity. Here, we show that high densities of catalytically active sites can be formed on the TMDC basal plane by alloying elements that prefer the 2H (1T) phase into a 1T (2H) structure. The global stability of the alloy, in particular, whether it crystallizes in the 2H or 1T phase, can be controlled by ensuring a majority of elements prefer the target phase. We further show that the mixing entropy plays a decisive role in stabilizing the alloy, implying that high-entropy alloying becomes essential. Our calculations point to a number of interesting nonprecious hydrogen evolution catalysts, including (CrTaVHfZr)S2 and (CrNbVTiZr)S2 in the 1T-phase and (MoNbTaVTi)S2 in the 2H-phase. Our work opens new directions for designing catalytic sites via high-entropy alloy stabilization of locally unstable structures.

The preceding decade have witnessed incredible advances in the research of two-dimensional (2D) materials such as graphene, transition metal carbides and nitrides (MXenes), and transition metal dichalcogenides (TMDCs). Since their discovery, 2D materials have enabled the design of nano-scale devices with unique functionalities that are otherwise unavailable in conventional 3D systems. This dissertation focus on the electrical an thermal properties of these materials and presents the study of (1) contribution of the encapsulating layers and (2) surface parameters to the thermal transport at the van der Waals interfaces, (3) synthesis of quasi-binary TMDC alloys through computationally predicted stability maps, and (4) the phase‐dependent band gap engineering in alloys induced by charge density wave (CDW) phases. In the first and second project, power dissipation and thermal management in the nanoscale structures are investigated which is of great importance for the design and operation of energy-efficient 2D nano-devices. Energy transport is heavily dependent of the thermal boundary conductance (TBC) at the van der Waals interfaces, particularly coupling at the interface of 2D channels with their underlying 3D substrates. A low TBC with underlying substrates puts an extrinsic limitation on the ability of 2D materials to conduct heat and dissipate the applied power. In this project a novel self-heating/self-sensing electrical thermometry platform based on atomically thin Ti3C2Tz MXene sheets is developed, which enables experimental investigation of the thermal transport at a Ti3C2Tz/SiO2 interface, with and without an encapsulating layer. Furthermore, the hydrophilic nature and variability of MXene surface terminations together with their metallic nature, provide a new platform to study the effect of the surface parameters on the thermal transport through and along the 2D flakes. In the third project, at theory-guided synthesis approach is employed to achieve 25 unexplored quasi-binary TMDC alloys through computationally predicted stability maps and equilibrium temperature-composition phase diagrams. Compared to other 2D materials, TMDCs exhibit diverse, exciting physical properties, including topological insulator behavior, superconductivity, valley polarization, and superior electrocatalytic activity compared to noble metals. Their properties can be further tuned — or even new properties engineered — by alloying two different elements at either the transition metal or the chalcogen site to form quasi-binary alloys, or by simultaneous alloying at both the sites to form quaternary alloys. The synthesized alloys can be exfoliated into 2D structures, and some of them exhibit: (i) outstanding thermal stability tested up to 1230K, (ii) exceptionally high electrochemical activity for CO2 reduction reaction, (iii) excellent energy efficiency in a high rate Li-air battery, and (iv) high break-down current density for interconnect applications. In the last project, a novel form of bandgap engineering involving alloying non‐isovalent cations in a 2D transition metal dichalcogenide (TMDC) is presented. By alloying semiconducting MoSe2 with metallic NbSe2, two structural phases of Mo0.5Nb0.5Se2, the 1T and 2H phases, are produced each with emergent electronic structure. At room temperature, it is observed that the 1T and 2H phases are semiconducting and metallic, respectively. For the 1T structure, scanning tunneling microscopy/spectroscopy (STM/STS) is used to measure band gaps. Electron diffraction patterns of the 1T structure obtained at room temperature show the presence of a nearly commensurate charge density wave (NCCDW) phase with periodic lattice distortions.

Phase-Controlled 1T Transition-Metal Dichalcogenide-Based Multidimensional Hybrid Nanostructures

A fantastic two-dimensional MoS2 material based on the inert basal planes activation: Electronic structure, synthesis strategies, catalytic active sites, catalytic and electronics properties

Electrical and optoelectronic properties of two-dimensional (2D) transition metal dichalcogenides (TMDCs) can be tuned by exploiting their structural phase transitions. Here semiconducting (2H) to metallic (1T) phase transition is investigated in a strained ${\mathrm{MoWSe}}_{2}$ monolayer using molecular dynamics (MD) simulations. Novel intermediate structures called $\ensuremath{\alpha}$ and $\ensuremath{\beta}$ are found between the 2H and 1T phases. These intermediate structures are similar to those observed in a 2D $\mathrm{Mo}{\mathrm{S}}_{2}$ by scanning transmission electron microscopy. A deep generative model, namely the variational autoencoder (VAE) trained by MD data, is used to generate novel heterostructures with $\ensuremath{\alpha}$ and $\ensuremath{\beta}$ interfaces. Quantum simulations based on density functional theory show that these heterostructures are stable and suitable for novel nanoelectronics applications.

Transition metal dichalcogenides (TMDs) exhibit a wide range of electronic properties due to their structural diversity. Understanding their defect-dependent properties might enable the design of efficient, bright, and long-lifetime quantum emitters. Here, we use density functional theory (DFT) calculations to investigate the 2H, 1T, and 1T' phases of MoS2, WS2, MoSe2, WSe2 and the effect of defect densities on the electronic band structures, focusing on the influence of chalcogen vacancies. The 2H phase, which is thermodynamically stable, is a direct band gap semiconductor, while the 1T phase, despite its higher formation energy, exhibits metallic behavior. 1T phases with spin-orbit coupling show significant band inversions of 0.61, 0.77, 0.24 and 0.78 eV for MoS2, MoSe2, WS2 and WSe2, respectively. We discovered that for all four MX2 systems, the energy difference between 2H, 1T and 1T phases decreases with increasing concentration of vacancies (from 3.13% to 21.88%). Our findings show that the 2H phase also has minimum energy values depending on vacancies. TMDs containing W were found to have a wider bandgap compared to those containing Mo. The band gap of 2H WS2 decreased from 1.81 eV (1.54 eV with SOC included) under GGA calculations to a range of 1.37 eV to 0.79 eV, while the band gap of 2H MoSe2 reduced from 1.43 eV (1.31 eV with SOC) under GGA to a range of 0.98 eV to 0.06 eV, depending on the concentration. Our findings provide guidelines for experimental screening of 2D TMD defects, paving the way for the development of next-generation spintronic, electronic, and optoelectronic devices.

Two-dimensional (2D) Cr-based layered and non-layered materials such as CrI3, Cr2Ge2Te6, Cr2S3, CrSe, and CrOX (X = Cl and Br) have attracted considerable attention due to their potential application in spintronics. Despite few experimental studies, theoretical studies reported that 2D chromium dichalcogenide (CrS2) materials show unique properties such as valley polarization, piezoelectric coupling, and phase dependent intrinsic magnetic properties. Here, we report for the first time the synthesis of 2D layered CrS2 flakes down to the monolayer via the chemical vapor deposition (CVD) method, its phase structures and electronic properties. We observed the 2H, 1T, and 1T' phases coexisting in CVD grown monolayer CrS2. The formation of 1T' phases from 1T phases is described by dimerization of metal atoms at room temperature according to our molecular dynamics studies. The coexistence of 1T and 1T' phases with 2H phases is referred to as the 1T and 1T' puddling phenomenon. We theoretically showed that the monolayer 2H-CrS2 is a direct bandgap semiconductor with a gap of approximately 0.95 eV predicted by the PBE functional, while the 1T- and 1T'-CrS2 are metallic and semi-metallic with approximately 10 meV gap, respectively. Furthermore, 2H CrS2 exhibits nonmagnetic semiconducting properties while for ferromagnetic spin configuration, the 1T and 1T' CrS2 show magnetic characteristics with 0.531μB and 2.206μB magnetic moment per Cr atom respectively, for ferromagnetic spin configuration as predicted from DFT+U calculation. Importantly, CrS2-based field-effect transistors exhibit a p-type behavior. Our study would stimulate further exploration of 2D layered CrS2 with astonishing properties and open up a whole new avenue for the urgent need for developing multifunctional 2D materials for nanoelectronics, valleytronics, and spintronics.

Magnetism in single-layer of Zrse[formula omitted] by substituting 3[formula omitted] transition metals for Zr: Structural symmetry versus exchange splitting

Thermal rectifiers and thermal transistors are expected to be widely used for efficient thermal management and energy cascade utilization due to their excellent directional thermal management. Two-dimensional micro/nano materials have huge potential in the applications of thermal transistors, thermal logic circuits, and thermal rectifiers owing to the phase transition and thermal rectification phenomenon. Herein, a lithium intercalation method was used to transform 2H-MoS2 into the 1T phase with a purity of 76%, and a suspended microelectrode was applied to measure the thermal conductivity and thermal rectification coefficient of the same MoS2 film with 1T and 2H phases in suit. The thermal conductivity and thermal rectification effect of two-phase MoS2 couple with its phase state and structure were also obtained. The results demonstrate that the thermal conductivities of MoS2 in both 1T and 2H phases decrease with increasing temperature. It is also found that the thermal rectification coefficient has no obvious dependence on the temperature and phase change but the asymmetric structure. Furthermore, a thermal rectifier and transistor with a high thermal rectification effect are designed. The direction and magnitude of heat flow through the samples can be effectively controlled and managed by adjusting the phase, size, and structural asymmetry of the different samples. The maximum thermal rectification coefficient of the thermal rectifiers is up to 0.8.

In this work, a novel microwave hydrothermal method is developed to prepare hybrid 1T@2H−MoS2 nanospheres, with the 1T and 2H phases confirmed by high‐resolution transmission electron microscopy (HRTEM), X‐ray photoelectron spectroscopy (XPS) and the stability of the hybrid phase verified by Raman spectroscopy. The efficiencies of methyl orange (MO) degradation and photo‐reduction of Cr(VI) by hybrid 1T@2H−MoS2 are much higher than those by 2H−MoS2, which should be attributed to the synergistic effect of the coexistent 1T and 2H phases. In addition, the hybrid sample prepared by microwave method possesses superior photocatalytic performance than that by conventional hydrothermal method, due to the higher 1T phase concentration and different crystallinity. Free‐radical capture experiments show that .O2− dominates the photocatalytic degradation process with 1T phase producing more electrons. The introduction of 1T−MoS2 into 2H−MoS2 shows great potential for photocatalytic degradation, while microwave treatment could be an effective method to prepare the hybrid phase MoS2.

The magnetic properties of 2H phase of MoS2 (2H-MoS2) and 1T phase of MoS2 (1T-MoS2) were investigated both experimentally and theoretically. Lithium (Li) intercalation method was used to prepare single-layer MoS2 sheets. It was found that pristine MoS2 (2H-MoS2) exhibited weak diamagnetism. After exfoliating by Li intercalation, the crystal structure transformed from 2H to 1T phase, and the magnetism was significantly enhanced from diamagnetism to paramagnetism accordingly. With further annealing in argon atmosphere, the 2H phase recovered gradually from 1T phase, and the magnetism decreased correspondingly. Using crystal field theory and combining the results of first principle calculation, we conclude that the enhanced magnetism can be attributed to the Mo atoms of 1T-MoS2.

The structural, electronic, and magnetic properties of VSSe, VSeTe, VSTe monolayers in both 2H and 1T phases are investigated via first-principles calculations. The 2H phase is energetically favorable in VSSe and VSeTe, whereas the 1T phase is lower in energy in VSTe. For V-based Janus monolayers in the 2H phase, calculations of the magnetic anisotropy show an easy-plane for the magnetic moment. As such, they should not exhibit a ferromagnetic phase transition, but instead, a Berezinskii-Kosterlitz-Thouless (BKT) transition. A classical XY model with nearest-neighbor coupling estimates critical temperatures (T$_{BKT}$) ranging from 106 K for VSSe to 46 K for VSTe.

Molybdenum disulfide (MoS2) is a promising nonprecious catalyst for the hydrogen evolution reaction (HER) that has been extensively studied due to its excellent performance, but the lack of understanding of the factors that impact its catalytic activity hinders further design and enhancement of MoS2-based electrocatalysts. Here, by using novel porous (holey) metallic 1T phase MoS2 nanosheets synthesized by a liquid-ammonia-assisted lithiation route, we systematically investigated the contributions of crystal structure (phase), edges, and sulfur vacancies (S-vacancies) to the catalytic activity toward HER from five representative MoS2 nanosheet samples, including 2H and 1T phase, porous 2H and 1T phase, and sulfur-compensated porous 2H phase. Superior HER catalytic activity was achieved in the porous 1T phase MoS2 nanosheets that have even more edges and S-vacancies than conventional 1T phase MoS2. A comparative study revealed that the phase serves as the key role in determining the HER performance, as 1T phase MoS2 always outperforms the corresponding 2H phase MoS2 samples, and that both edges and S-vacancies also contribute significantly to the catalytic activity in porous MoS2 samples. Then, using combined defect characterization techniques of electron spin resonance spectroscopy and positron annihilation lifetime spectroscopy to quantify the S-vacancies, the contributions of each factor were individually elucidated. This study presents new insights and opens up new avenues for designing electrocatalysts based on MoS2 or other layered materials with enhanced HER performance.

New predicted two-dimensional MXenes and their structural, electronic and lattice dynamical properties

Two-dimensional (2D) molybdenum disulfide $(MoS_2)$ promised a wide range of potential applications. Here, we report the transport investigations on the $MoS_2$ from Armchair (AC) and Zigzag (ZZ) directions with different kinds of leads. The conductance of 2H phase $MoS_2$ depended on the transport directions and lead types (2H phase or 1T phase). System with 1T phase $MoS_2$ as lead can impressively improve the transport properties compared with the 2H phase lead. Moreover, for the system with metal lead, enhanced conductance can be observed, which contrast to the experiment measurements. Further investigation indicated that the conductance sensitively relies on the distance between metal lead and 2D material. The present theoretical results suggested the lead material and interface details are both important for $MoS_2$ transport exploration, which can provide vital insights into the other 2D hybrid materials.