Bridging Quantum Capacitance and Experimental Electrochemical Performance in 2D Materials for Supercapacitors: From Density of States to Device-Level Interpretation

Atomic-Scale Studies of Overlapping Grain Boundaries between Parallel and Quasi-Parallel Grains in Low-Symmetry Monolayer ReS2

Two-dimensional (2D) transition metal dichalcogenides (TMDs) have been made in improving the NRR for the significant effort in catalytic activity due to its unique crystal structures, electronic properties, and promising nonprecious catalysts. However, many studies have provided solutions to activate the inert basal plane with low improvement. With highly asymmetric configuration, Janus monolayer with intrinsic strain and electric field can enable a way to tune the activity in TMDs-based catalysts. Although the first successful experimental synthesis of Janus structure (MoSSe) has gained significant interest and become a fast-growing TMDs-based material, the structure remains limited in the low dimensional and hard to expose the edges site to enhance the catalytic activity. Besides the common benefits of 2D TMDs, such as high surface area, short carrier migration distance, and tunable electronic structure. In this regard, the inherent structural asymmetry of Janus WSSe nanowalls as a new means to enhance the NRR activity was investigated. Janus TMDs nanowalls as the catalyst by taking advantage of its introduction of in-gap states with a shift in the Fermi level in nitrogen adsorbed system because of Janus asymmetry from the origin of stimulating NRR activity. Raman spectra showthe related main modes for WSSe Janus structure as shown in Fig. 1a, for which peaks at 271 and 333 nm can be found, respectively. The linear sweep voltammetric (LSV) curves of the catalysts as shown in Fig. 1b confirm the onset potential of the NRR, which increases with the different materials. Janus WSSe Janus nanowalls exhibit outstanding NRR performance over its parent materials (WS2 and WSe2). The results should propose a new path to design high performance and a novel structure for Janus TMD-based catalysts.Fig1. (a) Evolution of Raman spectra in the WSSe nanowalls as a function of chemical composition (b) LSV curves for WS2, WSe2, WS2-XSe2-2X alloy, and WSSe Janus nanowalls Figure 1

In the past few years, two-dimensional (2D) transition metal dichalcogenide (TMDC) materials have attracted increasing attention of the research community, owing to their unique electronic and optical properties, ranging from the valley–spin coupling to the indirect-to-direct bandgap transition when scaling the materials from multi-layer to monolayer. These properties are appealing for the development of novel electronic and optoelectronic devices with important applications in the broad fields of communication, computation, and healthcare. One of the key features of the TMDC family is the indirect-to-direct bandgap transition that occurs when the material thickness decreases from multilayer to monolayer, which is favorable for many photonic applications. TMDCs have also demonstrated unprecedented flexibility and versatility for constructing a wide range of heterostructures with atomic-level control over their layer thickness that is also free of lattice mismatch issues. As a result, layered TMDCs in combination with other 2D materials have the potential for realizing novel high-performance optoelectronic devices over a broad operating spectral range. In this article, we review the recent progress in the synthesis of 2D TMDCs and optoelectronic devices research. We also discuss the challenges facing the scalable applications of the family of 2D materials and provide our perspective on the opportunities offered by these materials for future generations of nanophotonics technology.

Two-dimensional (2D) transition metal dichalcogenides (TMDs), such as isolated monolayers or few-layers of MoS2 and WSe2, have recently gained intensive interest for their potential in future electronics [1]. The bulk of TMDs usually have a layer structure which is similar to graphene. Because the layers are bound together by weak van der Waals forces, an isolated monolayer of TMDs can be easily obtained by cleaving technique. Unlike graphene, these 2D materials have a significant band gap and exhibit attractive semiconductor properties. Therefore, the 2D TMDs have been widely considered for the ultimately thin channel materials in future Metal-Oxide-Semiconductor Field Effect Transistors (MOSFETs) to suppress short channel effects. Especially, TMD monolayers usually have a direct band gap, very attractive for the mixed electronic and photonic application in future logic and memory devices [2].During the integration of these 2D materials into electronic devices, e.g., as the channel materials in MOSFETs, they will be defined by lithography into nanoribbons, doped into n- or p-type, and stressed by the strain induced from the metal contacts and dielectric interface. In this work, we have studied the effects of these factors on the band structure and electrical properties of MoS2monolayers by using a numerical simulation method based on density functional theory. We found that the doping, strain and size could induce significant variation in the electrical properties of these 2D materials. These will be very challenging issues for the 2D materials.In this work, we first studied the size effect on the properties of MoS2 monolayer nanoribbons. We found that the band gap of MoS2 nanoribbons changed from 1.8 eV of an infinite MoS2 sheet to 1.2 eV of a 10×10 (in molecule) MoS2 nanoribbons or 0.5 eV of a 5×10 MoS2 nanoribbons. We also found that the doping level and dopant position have a significant effect on the electrical properties of MoS2 nanoribbons. The doping level and dopant position can induce large variation (30%) in sheet resistance. This variation in electrical properties may post a serious challenge on the application in logic and memory devices. In addition, we found that the strain will also significantly affect the electrical and carrier transport properties of MoS2 monolayer. Under different strain, the MoS2 will change from direct band-gap to indirect band-gap semiconductor. As the strain further increases to certain value, the MoS2will become metallic.In summary, this work demonstrated that the size, doping and strain have a significant effect on the 2D materials and will induce large variation in electrical properties. Therefore, the future integration of such 2D materials for logic or memory circuit application, the size, doping and strain must be well engineered and controlled in order to minimize the device variation.

The experimental isolation of graphene led to the discovery of an entirely new world of two-dimensional (2D) materials in which the 2D nature often leads to emergent behaviors not seen in bulk systems. 2D transition metal dichalcogenides (TMDs) exhibit physico-chemical properties that depend on the transition metal, polymorph, thickness, and presence and type of defects. Recently, a group of thin (10-100nm) transition metal carbides (TMCs), such as Mo2C, has been synthesized that exhibit a thickness-dependent superconducting critical temperature (Tc). These thin TMCs are different from MXenes, another class of 2D materials consisting of few layers of nitrides or carbides (<5nm) produced by chemical etching and delamination. The goal of this renewal proposal is to combine experiment and computation to synthesize and elucidate the guiding principles that control the growth, orientation and strain of heterostacks of thin TMCs and TMDs composed with Nb, Ti and W. We expect to stabilize metastable hybrid phases of TMCs sandwiched between TMDs (H-TMD/Cs) with unprecedented physico-chemical properties. As part of previous DOE-funded work by the Terrones/Sinnott groups, thin (10-100 nm thick) Mo2C flakes were successfully synthesized by chemical vapor deposition (CVD). By subsequently exposing Mo2C to H2S, partial chalcogenization was achieved, resulting in heterostacks of MoCx phases and MoS2. The formation of MoS2 led to a deficiency of Mo atoms in the underlying Mo2C, resulting in an inhomogeneous phase change from α-Mo2C to γ’-MoCx and then to γ-MoC. The γ’-MoCx is a strained metastable phase and the heterostack of all three phases demonstrated an increased Tc relative to that of α-Mo2C, from 4 to 6K; its interleaved layered structure consisting of superconducting and semiconducting phases is ideal for future studies of Josephson junction series arrays. Moiré patterns in these heterostacked systems could result in new phenomena, as moiré patterns in bilayer graphene showed unconventional superconductivity and moiré excitons have been observed in twisted TMD heterobilayers. The scientific hypothesis of the proposed synergistic computational and experimental research is that orientation and strain control within confined thin metastable TMCs, sandwiched by stable phases of TMCs and layered TMDs, will depend on kinetic and thermodynamic “knobs” that include fast temperature changes, chalcogen diffusion through preferred crystallographic planes, reaction times, pressure, reactive atmosphere, precursors, and surfactants, which will also tailor properties such as superconductivity, magnetism, ferroelectricity, piezoelectricity, and catalytic performance. We will develop the guiding principles for the synthesis and stabilization of metastable H-TMD/Cs based on Nb, Ti and W. In order to validate the hypothesis, four tasks are proposed: The first task will synthesize ultra-thin TMCs based on Nb, W and Ti, by: 1) adapting the CVD method used for Mo2C, 2) plasma assisted CVD, 3) defect-mediated CVD processes, and 4) cryo-milling of carbide powders. The second task will accomplish the synthesis and basic physico-chemical characterizations of H-TMD/Cs by chalcogenization of the materials synthesized in task one, and by carbonization of TMDs. H-TMD/Cs will also be investigated for their suitability in energy conversion applications such as supercapacitors, Li and multivalent ion batteries, and electrocatalysts, topics of interest to DoE. These tasks will be carried out in close conjunction with density functional theory (DFT) calculations with insights into energetics, lattice parameters, stability, phase diagrams, band structures, and density of states of H-TMD/Cs. The third task will characterize and evaluate strain and moiré patterns at the interfaces of different H-TMD/Cs by high-resolution scanning transmission electron microscopy (HR-STEM), scanning tunneling microscopy (STM), and conductive tip atomic force microscopy. Nudged elastic band calculations with DFT will be performed to understand the chalcogen diffusion process, which will provide insights into the interfaces between different phases of TMCs and TMDs. The fourth task aims at quantifying the stability and dynamics of H-TMD/Cs by in-situ TEM and Raman studies under heating, strain, and electrical biasing. Phonon calculations using DFT will provide a basis for interpreting Raman spectra. This coherent framework involving synthesis, characterization, and computation will result in a broad scientific impact for energy related applications. The ability to develop new H-TMD/Cs will enhance a range of applications that include batteries, catalysts, switches, sensors, quantum computing components and smart coatings.

The experimental isolation of graphene led to the discovery of an entirely new world of two-dimensional (2D) materials in which the 2D nature often leads to emergent behaviors not seen in bulk systems. 2D transition metal dichalcogenides (TMDs) exhibit physico-chemical properties that depend on the transition metal, polymorph, thickness, and presence and type of defects. Recently, a group of thin (10-100nm) transition metal carbides (TMCs), such as Mo2C, has been synthesized that exhibit a thickness-dependent superconducting critical temperature (Tc). These thin TMCs are different from MXenes, another class of 2D materials consisting of few layers of nitrides or carbides (<5nm) produced by chemical etching and delamination. The goal of this renewal proposal is to combine experiment and computation to synthesize and elucidate the guiding principles that control the growth, orientation and strain of heterostacks of thin TMCs and TMDs composed with Nb, Ti and W. We expect to stabilize metastable hybrid phases of TMCs sandwiched between TMDs (H-TMD/Cs) with unprecedented physico-chemical properties. As part of previous DOE-funded work by the Terrones/Sinnott groups, thin (10-100 nm thick) Mo2C flakes were successfully synthesized by chemical vapor deposition (CVD). By subsequently exposing Mo2C to H2S, partial chalcogenization was achieved, resulting in heterostacks of MoCx phases and MoS2. The formation of MoS2 led to a deficiency of Mo atoms in the underlying Mo2C, resulting in an inhomogeneous phase change from α-Mo2C to γ’-MoCx and then to γ-MoC. The γ’-MoCx is a strained metastable phase and the heterostack of all three phases demonstrated an increased Tc relative to that of α-Mo2C, from 4 to 6K; its interleaved layered structure consisting of superconducting and semiconducting phases is ideal for future studies of Josephson junction series arrays. Moiré patterns in these heterostacked systems could result in new phenomena, as moiré patterns in bilayer graphene showed unconventional superconductivity and moiré excitons have been observed in twisted TMD heterobilayers. The scientific hypothesis of the proposed synergistic computational and experimental research is that orientation and strain control within confined thin metastable TMCs, sandwiched by stable phases of TMCs and layered TMDs, will depend on kinetic and thermodynamic “knobs” that include fast temperature changes, chalcogen diffusion through preferred crystallographic planes, reaction times, pressure, reactive atmosphere, precursors, and surfactants, which will also tailor properties such as superconductivity, magnetism, ferroelectricity, piezoelectricity, and catalytic performance. We will develop the guiding principles for the synthesis and stabilization of metastable H-TMD/Cs based on Nb, Ti and W. In order to validate the hypothesis, four tasks are proposed: The first task will synthesize ultra-thin TMCs based on Nb, W and Ti, by: 1) adapting the CVD method used for Mo2C, 2) plasma assisted CVD, 3) defect-mediated CVD processes, and 4) cryo-milling of carbide powders. The second task will accomplish the synthesis and basic physico-chemical characterizations of H-TMD/Cs by chalcogenization of the materials synthesized in task one, and by carbonization of TMDs. H-TMD/Cs will also be investigated for their suitability in energy conversion applications such as supercapacitors, Li and multivalent ion batteries, and electrocatalysts, topics of interest to DoE. These tasks will be carried out in close conjunction with density functional theory (DFT) calculations with insights into energetics, lattice parameters, stability, phase diagrams, band structures, and density of states of H-TMD/Cs. The third task will characterize and evaluate strain and moiré patterns at the interfaces of different H-TMD/Cs by high-resolution scanning transmission electron microscopy (HR-STEM), scanning tunneling microscopy (STM), and conductive tip atomic force microscopy. Nudged elastic band calculations with DFT will be performed to understand the chalcogen diffusion process, which will provide insights into the interfaces between different phases of TMCs and TMDs. The fourth task aims at quantifying the stability and dynamics of H-TMD/Cs by in-situ TEM and Raman studies under heating, strain, and electrical biasing. Phonon calculations using DFT will provide a basis for interpreting Raman spectra. This coherent framework involving synthesis, characterization, and computation will result in a broad scientific impact for energy related applications. The ability to develop new H-TMD/Cs will enhance a range of applications that include batteries, catalysts, switches, sensors, quantum computing components and smart coatings.

Monolayer transition metal dichalcogenides (TMDs) with spin-valley coupling are a well-studied class of two-dimensional materials with potential for novel optoelectronics applications. Breaking time-reversal symmetry via an external magnetic field or supporting magnetic substrate can lift the degeneracy of the band gaps at the inequivalent $K$ and ${K}^{\ensuremath{'}}$ high symmetry points, or valleys, in the monolayer TMD Brillouin zone, a phenomenon known as valley splitting. However, reported valley splittings thus far are modest, and a detailed structural and chemical understanding of valley splitting via magnetic substrates is lacking. Here we probe the underlying physical mechanism with a series of density functional theory (DFT) calculations of magnetic atoms with varying coverage on the surface of prototypical monolayer ${\mathrm{WSe}}_{2}$ and ${\mathrm{MoS}}_{2}$ TMDs. Near-valence band edge energies for variable magnetic atom height, lateral registry, and magnetic moment are calculated with DFT, and trends are rationalized with a model Hamiltonian with second-order spin-dependent exchange coupling. From our analysis, we demonstrate how large valley splittings may be achieved and that the valley splitting can be understood with a superexchange mechanism, which strongly depends on overlaps of TMD Bloch states at the valley extrema with the localized $d$ states of the magnetic atom, as well as the out-of-plane component of the magnetic moment of the magnetic atom. Our calculations provide a basis for understanding prior measurements of valley splitting and suggest routes for enhancing valley splitting in future systems of interest.

Low-dimensional materials including carbon nanotubes (CNTs), graphene, transition metal carbides and nitrides (MXenes), transition metal dichalcogenides (TMDs) and beyond have recently become the focus of intense research because of their outstanding physical and chemical properties. Amongst those characteristics, the mechanical properties of the low-dimensional systems play a significant role in exploring their potential applications in industrial sectors. Nevertheless, the mechanical characteristics of the low-dimensional material systems have not yet been fully understood. This is because direct experimental measurements of mechanical properties of these materials face great difficulties due to the critical availability of the high-quality crystals and sophisticated facilities. To this end, the numerical methods become a promising alternative for investigating such properties of the low-dimensional materials. Firstly, this thesis presents the background and objectives of this project (Chapter 1). The recent progresses in numerically exploring the in-plane mechanical properties of low-dimensional material systems, including first-principles density functional theory (DFT), force-field based classical molecular dynamics (MD), and the finite element method (FEM) are then discussed. Some recent cases have been discussed to show the advantages and disadvantages of these multiscale simulation methods (Chapter 2). Since carbon-based materials are considered to be one of the most important family of low-dimensional materials, a comprehensive numerical study was conducted to investigate the mechanical properties (e.g. critical buckling load and vibrational properties) of carbon-based structures, such as carbon nanotubes (CNTs) and their modifications. The FEM was employed because it is able to analyse the systems consisting thousands of atoms (Chapter 3), and investigate macroscopic mechanical properties. Afterward, the mechanical properties of molybdenum disulfides (MoS2), as one of the most important TMDs, were examined by means of DFT calculations. The impacts of the structural polytypes, as well as external pressure on the mechanical response of the layer structured MoS2 were also investigated (Chapter 4). In addition, a comprehensive study was conducted to explore the energy-dependent anisotropic mechanical properties of different types of bulk TMDs using the first principles DFT calculations. Different analyses such as Density of States (DOS) and crystal orbital Hamilton population (COHP) were performed to fully understand the behaviour of the layer-structured systems (Chapter 5). Investigating the in-plane mechanical properties of monolayer two-dimensional (2D) materials was also one of the main objectives of this research. Hence, the mechanical characteristics of the lateral TMDs were thoroughly examined with consideration of the impact of the heterostructure configurations on the stability of the 2D systems (Chapter 6). Finally, the recently discovered MXenes were numerically investigated, and the mechanical properties of the functionalized systems were systematically explored. The DFT results reveals that the studied MXenes exhibit significant mechanical strength, which in-plane Young’s moduli are even larger than that of graphene. It may open the avenue for promising applications of such novel 2D materials (Chapter 7). It is worthy to note that the studies on the mechanical properties of 2D materials are still in early stages. As such, it is vital that future studies focus on exploring plausible numerical approaches for engineering the mechanical characteristics and eventually expanding the application of the low-dimensional materials.

The electronic structure of semiconducting 2D materials such as transition metal dichalcogenides (TMDs) is known to be tunable by its environment, from simple external fields applied with electrical contacts up to complex van der Waals heterostructure assemblies. However, conventional alloying from reference binary TMD compounds to composition-controlled ternary alloys also offers unexplored opportunities. In this work, we use nano-angle resolved photoemission spectroscopy (nano-ARPES) and density functional theory (DFT) calculations to study the structural and electronic properties of different alloy compositions of bulk $\mathrm{W}{\mathrm{S}}_{2(1\ensuremath{-}x)}\mathrm{S}{\mathrm{e}}_{2x}$. Our results demonstrate the continuous variation of the band structure and the progressive evolution of the valence band splitting at the K points from 420 to 520 meV in bulk $\mathrm{W}{\mathrm{S}}_{2(1\ensuremath{-}x)}\mathrm{S}{\mathrm{e}}_{2x}$. We also carried out scanning tunneling microscopy (STM) measurements and DFT to understand the possible S or Se substitutions variants in $\mathrm{W}{\mathrm{S}}_{2(1\ensuremath{-}x)}\mathrm{S}{\mathrm{e}}_{2x}$ alloys, with different local atomic configurations. Our work opens up perspectives for the fine control of the band dispersion in van der Waals materials and demonstrates how the band structure can be tuned in bulk TMDs. The collected information can serve as a reference for future applications.

The phase transitions among polymorphic two-dimensional (2D) transition metal dichalcogenides (TMDs) have attracted increasing attention for their potential in enabling distinct functionalities in the same material for making integrated devices. Electron-injection to TMDs has been proved to be a feasible way to drive structural phase transition from the semiconducting H-phase to the semimetal dT-phase. In this contribution, based on density-functional theory (DFT) calculations, firstly we demonstrate that hole-injection drives the transition of the H-phase more efficiently to the metallic T-phase than to the semimetallic dT-phase for group VI-B TMDs (MoS2, WS2, and MoSe2, etc.). The origin can be attributed to the smaller work function of the T-phase than that of the dT-phase. Our work function analysis can distinguish the T and dT phases quantitatively while it is challenging for the commonly used crystal field splitting analysis. In addition, our analysis provides a unified understanding for both hole- and electron-injection induced phase transitions for 2D materials beyond TMDs, such as the newly synthesized MoSi2N4 family. Moreover, the hole-driven T-phase transition mechanism can explain the recent experiment of WS2 phase transition by hole-doping with yttrium (Y) atoms. Using 1/3 Y-doped WS2 and MoSe2 as examples, we show that the Mo and W valency increases to 5+. These above findings open up an avenue to obtain the metallic T-phase, which expands the possible stable phases of 2D materials.

Two-dimensional materials are among the most scientifically accessible materials in material science at the beginning of the twenty-first century. There has been interest in the monolayer transition metal dichalcogenide (TMDC) family because of its large active site surface area for UV photons of light for wastewater treatment. In the present work, density functional theory (DFT) is utilized to model the optical, structural and electrical properties of TMDCs such as NbS2, ZrS2, ReS2 and NbSe2 using the GGA-PBE simulation approximation. Based on DFT calculations, it is determined that NbS2, ZrS2, ReS2 and NbSe2 have zero energy bandgap (E g). The additional gamma-active states that are generated in NbS2, ZrS2, ReS2 and NbSe2 materials aid in the construction of the conduction and valence bands, resulting in a zero E g. In the ultraviolet (UV) spectrum, the increase in optical conductance peaks from 4.5 to 15.7 suggests that the material exhibits stronger absorption or interaction with UV light due to the excitation of electronic transitions or inter-band transitions. The highest optical conductivity and absorbance of two-dimensional TMDCs NbS2, ZrS2, NbSe2 and ReS2 show 2.4 × 105, 2.5 × 105, 2.8 × 105 and 7 × 105 , respectively. The TMDC family, including two-dimensional TMDCs NbS2, ZrS2, NbSe2 and ReS2, is known for its unique electronic and optical properties. Their layered structure and high surface area make them excellent candidates for applications involving light absorption and photodetection. These materials reduce photon recombination and improve charge transport, making them suitable for photocatalytic and photoanode applications.

The discovery of atomically thin layered materials such as graphene and transition metal dichalcogenides has unveiled the unique exploration of novel fundamental physics and device applications in two-dimensions. Characterization of their crystal symmetry and subsequent electronic properties are prominent to realize the full potential of these reduced dimensional systems, which fundamentally determine the topology, chirality and rich interfacial physics. Second harmonic generation (SHG), a nonlinear optical effect, is sensitive to crystal symmetry and electronic structures, which proves to be one of the most powerful yet simple technique to capture the essence physics. On the other hand, the 2D nature of layered materials enables large tunability in its physical properties with a number of external stimuli, which in turn paves the way for the development of 2D nonlinear optoelectronic applications. In this review, we overview recent efforts employing second harmonic generation spectroscopy and microscopy to probe lattice structures and dipole polarizations in two-dimensional transition metal dichalcogenide and polar materials. In addition, multiple external stimuli used to control SHG as potential optoelectronic devices are covered. We conclude with a perspective on the future directions of exploration on emerging 2D magnetic and topological materials based on SHG spectroscopy.

: A method for formulating the unit cell of arbitrarily stacked, two-dimensional (2D) transition metal dichalcogenides is presented. Geometrical considerations and genetic algorithms are used to minimize the number of unit cells utilized in the construction of a supercell which may accommodate the lattice constants of arbitrary 2D and other close-packed materials. Supercells for various combinations of layered 2D transition metal dichalcogenides are calculated and their electronic band structures are simulated using density functional theory. Results are compared with previously reported density functional theory simulations from the literature.

The technique of plasma processing is beneficial for wafer cleaning and precision etching of integrated circuits and essential in manufacture of advanced semiconductor devices with unmatched perfection. Research on two‐dimensional (2D) materials, such as transition metal dichalcogenides(TMDs), offers a promising solution to the challenges in semiconductor miniaturization. TMDs, with their atomic layer thicknesses and silicon‐like bandgaps, can be integrated using existing plasma systems. Different 2D crystal structures, such as 1T and 2H configurations, exhibit distinctive properties. Computational approaches are also developed to provide guidelines for controlled synthesis and etching of large‐scale and high‐quality 2D materials. Plasma/gas‐surface interactions during the synthesis, etching, and phase transformation of 2D materials are explored using atomistic simulations such as density functional theory and molecular dynamics. The reaction energetics, chemical species, and associated kinetics are discovered in the simulation study. These results decipher various mechanisms of 2D materials processing at the microscopic scale and predict certain optimal process parameters. Plasma/gas‐based semiconductor technologies are crucial in electronics because they enable production of advanced semiconductors. Plasma/gas etching allows precise and selective removal of material and plasma‐enhanced chemical vapor deposition enhances chemical reactions for efficient film deposition; therefore, these processes are majorly important for harnessing 2D materials in electronic applications.

The van der Waals interaction and absorption and electron circular dichroism spectra of two-dimensional bilayer stacked structures