Electronic band structure and bonding in transition metal layered dichalcogenides by atomic orbital methods

Electronic and Crystal Structure of New CrxZrSe2 Intercalation Compounds

New intercalation compounds CrxZrSe2 were synthesized and thoroughly studied. Cr atoms were found to occupy the positions both tetrahedrally and octahedrally coordinated by the Se atoms in the interlayer gap. The magnetic properties and electrical resistivity were studied in the temperature ranges of 2.4-300 K and 80-340 K, respectively. The compounds change their behavior from semiconducting (x = 0.1) to metallic (x > 0.1). The magnetic interaction strongly depends on the Cr content and temperature. The spin-glass state with antiferromagnetic interaction exists at T < Tcrit for CrxZrSe2 with x ≤ 0.2, while at x ≥ 0.3 ferromagnetic contribution arises as well. The single crystals of CrxZrSe2 were grown to study the electronic structure of the materials. A combination of the X-ray photoelectron spectroscopy (including that across Cr 2p-3d and Zr 3p-4d resonance) and X-ray absorption spectroscopy methods allowed to propose the location of the Cr 3d-states in the Se 3p-Zr 4d energy gap.

Unravelling strong electronic interlayer and intralayer correlations in a transition metal dichalcogenide

Electronic correlations play important roles in driving exotic phenomena in condensed matter physics. They determine low-energy properties through high-energy bands well-beyond optics. Great effort has been made to understand low-energy excitations such as low-energy excitons in transition metal dichalcogenides (TMDCs), however their high-energy bands and interlayer correlation remain mysteries. Herewith, by measuring temperature- and polarization-dependent complex dielectric and loss functions of bulk molybdenum disulphide from near-infrared to soft X-ray, supported with theoretical calculations, we discover unconventional soft X-ray correlated-plasmons with low-loss, and electronic transitions that reduce dimensionality and increase correlations, accompanied with significantly modified low-energy excitons. At room temperature, interlayer electronic correlations, together with the intralayer correlations in the c-axis, are surprisingly strong, yielding a three-dimensional-like system. Upon cooling, wide-range spectral-weight transfer occurs across a few tens of eV and in-plane p-d hybridizations become enhanced, revealing strong Coulomb correlations and electronic anisotropy, yielding a two-dimensional-like system. Our result shows the importance of strong electronic, interlayer and intralayer correlations in determining electronic structure and opens up applications of utilizing TMDCs on plasmonic nanolithrography.

HfS2 and TiS2 Monolayers with Adsorbed C, N, P Atoms: A First Principles Study

First principles density functional theory was used to study the energetic, structural, and electronic properties of HfS 2 and TiS 2 materials in their bulk, pristine monolayer, as well as in the monolayer structure with the adsorbed C, N, and P atoms. It is shown that the HfS 2 monolayer remains a semiconductor while TiS 2 changes from semiconductor to metallic behavior after the atomic adsorption. The interaction with the external atoms introduces localized levels inside the band gap of the pristine monolayers, significantly altering their electronic properties, with important consequences on the practical use of these materials in real devices. These results emphasize the importance of considering the interaction of these 2D materials with common external atomic or molecular species.

Lowering the Schottky barrier height of G/WSSe van der Waals heterostructures by changing the interlayer coupling and applying external biaxial strain

Graphene-based van der Waals (vdW) heterostructures composed of two-dimensional transition metal dichalcogenides (TMDs) and graphene show great potential in the design and manufacture of field effect transistors.

Destructive role of oxygen in growth of molybdenum disulfide determined by secondary ion mass spectrometry

The application of secondary ion mass spectrometry (SIMS) in investigation and comparison of molybdenum disulfide (MoS2) films grown on SiO2, Al2O3 and BN substrates is presented. SIMS measurements of the MoS2/substrate interface reveals oxygen out-diffusion from the substrates containing oxygen and the formation of an amorphous MoOS layer in addition to MoS2. The total area of MoS2 domains covering the substrate is directly related to the type of substrate. For SiO2, small triangular domains of MoS2 separated by amorphous MoOS material are observed. For Al2O3, the sizes of the MoS2 domains are drastically improved due to the higher stability of sapphire. For a BN substrate, SIMS measurements reveal a uniform MoS2 coverage over the whole 2-inch wafer. These results show the destructive role of oxygen released from substrates such as SiO2 or Al2O3 during the growth process of MoS2. The fast and cheap growth process on a non-oxide substrate allows large wafer-scale uniform molybdenum disulfide material to be obtained, which is promising for device fabrication.

Electronic structure and magnetic behaviors of exfoliated MoS2 nanosheets

The correlation of electronic structure and magnetic behaviors of layered molybdenum disulfide (MoS₂) nanosheets, mechanically exfoliated from pristine hexagonal crystal (2H-MoS₂) have been studied. Raman spectra show the energy difference (ΔE) between two Raman peaks A _1g and [Formula: see text] was about 20.2 cm^-1, indicating the formation of mono-/bi-layered MoS ₂ nanosheets as obtained after mechanical exfoliation from pristine 2H-MoS ₂ . The absence of the reflection peak (0 0 2) in x-ray diffraction patterns confirms the formation of few-layered and mono-/bi-layered MoS ₂ nanosheets with reduced thickness. Mo 3d and S 2p  XPS core level peaks shifted to higher energy with the reduction of the number of layers in exfoliated MoS₂. As the number of layers decreased, valence band maximum position increased from 1.11 eV (pristine MoS₂) to 1.57 eV (mono-/bi-layered MoS ₂ nanosheets), whereas the surface work function (Ф) reduced from 4.85 eV (pristine MoS₂) to 4.47 eV (mono-/bi-layered MoS₂ nanosheets), as observed from UPS (He-I) measurements. UPS (He-II) spectra, as well as VB-PES spectra of mono-/bi-layered MoS ₂ nanosheets, exhibits an enhanced valence band density of states (DOS) of S 3p -derived states near Fermi level (E _f). Mo L _II-edge and S K-edge x-ray absorption near edge structure spectra of mono-/bi-layered MoS ₂ nanosheets show the splitting of different peaks that cause a noticeable change in their band structure. Magnetic M-H hysteresis loops measurement clearly demonstrates the increase of room temperature ferromagnetism from pristine to mono-/bi-layer MoS₂, due to the existence of defects ('S'-vacancies or defects at the grain boundaries region) and the increase of DOS.

Anomalous orbital structure in two-dimensional titanium dichalcogenides

Generally, lattice distortions play a key role in determining the electronic ground states of materials. Although it is well known that trigonal distortions are generic to most two dimensional transition metal dichalcogenides, the impact of this structural distortion on the electronic structure and topological properties has not been understood conclusively. Here, by using a combination of polarization dependent X-ray absorption spectroscopy (XAS), X-ray photoelectron spectroscopy (XPS) and atomic multiplet cluster calculations, we have investigated the electronic structure of titanium dichalcogenides TiX₂ (X = S, Se, Te), where the magnitude of the trigonal distortion increase monotonically from S to Se and Te. Our results reveal the presence of an anomalously large crystal field splitting. This unusual kind of crystal field splitting is likely responsible for the unconventional electronic structure of TiX₂ compounds and ultimately controls the degree of the electronic phase protection. Our findings also indicate the drawback of the distorted crystal field picture in explaining the observed electronic ground state and emphasize the key importance of trigonal symmetry, metal-ligand hybridization and electron-electron correlations in defining the electronic structures at the Fermi energy.

Pressure-induced phase transition, metallization and superconductivity in ZrS2

Zirconium disulfide (ZrS2) is an exemplary case among layered materials that exhibit unusual electronic and vibrational properties, with applications in potential photovoltaic and single-layer transistor materials. Here, we examine the effect of pressure on the structural stability, phonon dispersion, electronic properties and electron-phonon coupling of ZrS2 using first-principles calculations. Our results unravel that ZrS2 undergoes several pressure-induced phase transformations from the ambient-pressure P3[combining macron]m1 structure to a monoclinic P21/m structure at 2.0 GPa, to an orthorhombic Immm structure at 5.6 GPa, and to a tetragonal I4/mmm structure at 25.0 GPa. The electronic band calculations indicate that the layered P3[combining macron]m1 and P21/m structures are narrow-gap semiconductors. The gaps of the above two phases, which are normal semiconductors, decrease with pressure. Our results show that ZrS2 reaches the metallic state by a P21/m → Immm phase transition and keeps its metallic state in the I4/mmm phase. A pressure-driven evolution of the topological Fermi surface has been uncovered. The electron-phonon coupling results identify superconducting states in both metallic Immm and I4/mmm structures. Our research shows that pressure is efficient in the modulation of the bonding states, crystal structures and electronic properties of ZrS2, which will stimulate further high-pressure structural and conductive measurements.

Cross-Plane Carrier Transport in Van der Waals Layered Materials

The mechanisms of carrier transport in the cross-plane crystal orientation of transition metal dichalcogenides are examined. The study of in-plane electronic properties of these van der Waals compounds has been the main research focus in recent years. However, the distinctive physical anisotropies, short-channel physics, and tunability of cross layer interactions can make the study of their electronic properties along the out-of-plane crystal orientation valuable. Here, the out-of-plane carrier transport mechanisms in niobium diselenide and hafnium disulfide are explored as two broadly different representative materials. Temperature-dependent current-voltage measurements are preformed to examine the mechanisms involved. First principles simulations and a tunneling model are used to understand these results and quantify the barrier height and hopping distance properties. Using Raman spectroscopy, the thermal response of the chemical bonds is directly explored and the insight into the van der Waals gap properties is acquired. These results indicate that the distinct cross-plane carrier transport characteristics of the two materials are a result of material thermal properties and thermally mediated transport of carriers through the van der Waals gaps. Exploring the cross-plane electron transport, the exciting physics involved is unraveled and potential new avenues for the electronic applications of van der Waals layers are inspired.

The symmetry-resolved electronic structure of 2H-WSe2(0 0 0 1)

The orbital symmetry of the band structure of 2H-WSe2(0 0 0 1) has been investigated by means of angle-resolved photoelectron spectroscopy (ARPES) and density functional theory (DFT). The WSe2(0 0 0 1) experimental band structure is found, by ARPES, to be significantly different for states of even and odd reflection parities along both the [Formula: see text]-[Formula: see text] and [Formula: see text]-[Formula: see text] lines, in good agreement with results obtained from DFT. The light polarization dependence of the photoemission intensities from the top of the valence band for bulk WSe2(0 0 0 1) is explained by the dominance of W 5[Formula: see text] states around the [Formula: see text]-point and W 5d xy states around the [Formula: see text]-point, thus dominated, respectively, by states of even and odd symmetry, with respect to the [Formula: see text]-[Formula: see text] line. The splitting of the topmost valence band at [Formula: see text], due to spin-orbit coupling, is measured to be 0.49  ±  0.01 eV, in agreement with the 0.48 eV value from DFT, and prior measurements for the bulk single crystal WSe2(0 0 0 1), albeit slightly smaller than the 0.513  ±  0.01 eV observed for monolayer WSe2.

Oxidation Effect in Octahedral Hafnium Disulfide Thin Film

Atomically smooth van der Waals materials are structurally stable in a monolayer and a few layers but are susceptible to oxygen-rich environments. In particular, recently emerging materials such as black phosphorus and perovskite have revealed stronger environmental sensitivity than other two-dimensional layered materials, often obscuring the interesting intrinsic electronic and optical properties. Unleashing the true potential of these materials requires oxidation-free sample preparation that protects thin flakes from air exposure. Here, we fabricated few-layer hafnium disulfide (HfS2) field effect transistors (FETs) using an integrated vacuum cluster system and study their electronic properties and stability under ambient conditions. By performing all the device fabrication and characterization procedure under an oxygen- and moisture-free environment, we found that few-layer AA-stacking HfS2-FETs display excellent field effect responses (Ion/Ioff ≈ 10(7)) with reduced hysteresis compared to the FETs prepared under ambient conditions. Oxidation of HfS2 occurs uniformly over the entire area, increasing the film thickness by 250% at a prolonged oxidation time of >120 h, while defects on the surface are the preferential initial oxidation sites. We further demonstrated that the stability of the device in air is significantly improved by passivating FETs with BN in a vacuum cluster.

Synthesis and properties of molybdenum disulphide: from bulk to atomic layers

This review paper aims at a general introduction on up-to-date studies on the synthesis and properties of two-dimensional atomic layers of molybdenum disulphide, along with a brief overview on the bulk and three polytypes.

Electronic structures and theoretical modelling of two-dimensional group-VIB transition metal dichalcogenides

Two-dimensional group-VIB transition metal dichalcogenides have extraordinary properties originating from their complex electronic structures.

Symmetry-resolved surface-derived electronic structure of MoS2(0 0 0 1)

We find a wave vector dependence of the band symmetries for MoS(2)(0 0 0 1) in angle-resolved photoemission. The band structures are found to be significantly different for states of even and odd reflection parities, despite the absence of true mirror plane symmetry away from Γ, the Brillouin zone center, along the line to the K point, at the Brillouin zone edge. Our measurements agree with density functional theory (DFT) calculations for each band symmetry, with the notable exception of the Mo 4d(x(2)-y(2)) contributions to the valence band structure of MoS(2)(0 0 0 1). The band structure is indicative of strong S 3p and Mo 4d hybridization. In particular, the top of the valence band is predominantly composed of Mo 4d(3z(2)-r(2)) derived states near Γ, whereas near K Mo 4d(x(2)-y(2)) as well as Mo 4d(xy) dominate. In contrast, the bottom of the valence band is dominated by Mo 5s and S 3p(z) contributions.

Resonant photoemission at the L3 absorption edge of Mn and Ti and the electronic structure of 1T-Mn0.2TiSe2

Resonant valence band x-ray photoelectron spectra (ResPES) excited near the 2p(3/2) core level energies, 2p x-ray photoelectron spectra (XPS) and L(3,2) x-ray absorption spectra (XAS) of Ti and Mn in single crystals of 1T-Mn(0.2)TiSe(2) were studied for the first time. The ionic-covalent character of the bonds formed by the Mn atoms with the neighboring Se atoms in the octahedral coordination is established. From the XPS and XAS measurements compared with the results of atomic multiplet calculations of Ti and Mn L(3,2) XAS, it is found that the Ti atoms are in the ionic state of 4 + and the Mn atoms are in the state of 2 +. In ResPES of Mn(0.2)TiSe(2) excited near the Ti 2p(3/2) and Mn 2p(3/2) absorption edges the Ti 3d and Mn 3d bands at binding energies just below the Fermi level are observed. According to theoretical calculations of E(k) the Ti 3d states are localized in the vicinity of the Γ point and the Mn 3d states are localized along the direction K-Γ-M in the Brillouin zone of the crystal.