Quantifying the thickness of WTe2 using atomic-resolution STEM simulations and supervised machine learning

For two-dimensional (2D) materials, the exact thickness of the material often dictates its physical and chemical properties. The 2D quantum material WTe2 possesses properties that vary significantly from a single layer to multiple layers, yet it has a complicated crystal structure that makes it difficult to differentiate thicknesses in atomic-resolution images. Furthermore, its air sensitivity and susceptibility to electron beam-induced damage heighten the need for direct ways to determine the thickness and atomic structure without acquiring multiple measurements or transferring samples in ambient atmosphere. Here, we demonstrate a new method to identify the thickness up to ten van der Waals layers in Td-WTe2 using atomic-resolution high-angle annular dark-field scanning transmission electron microscopy image simulation. Our approach is based on analyzing the intensity line profiles of overlapping atomic columns and building a standard neural network model from the line profile features. We observe that it is possible to clearly distinguish between even and odd thicknesses (up to seven layers), without using machine learning, by comparing the deconvoluted peak intensity ratios or the area ratios. The standard neural network model trained on the line profile features allows thicknesses to be distinguished up to ten layers and exhibits an accuracy of up to 94% in the presence of Gaussian and Poisson noise. This method efficiently quantifies thicknesses in Td-WTe2, can be extended to related 2D materials, and provides a pathway to characterize precise atomic structures, including local thickness variations and atomic defects, for few-layer 2D materials with overlapping atomic column positions.

Understanding palladium-tellurium cluster formation on WTe2: From a kinetically hindered distribution to thermodynamically controlled monodispersity

A fundamental understanding of the transition metal dichalcogenide (TMDC)-metal interface is critical for their utilization in a broad range of applications. We investigate how the deposition of palladium (Pd), as a model metal, on WTe₂(001), leads to the assembly of Pd into clusters and nanoparticles. Using X-ray photoemission spectroscopy, scanning tunneling microscopy imaging, and ab initio simulations, we find that Pd nucleation is driven by the interaction with and the availability of mobile excess tellurium (Te) leading to the formation of Pd-Te clusters at room temperature. Surprisingly, the nucleation of Pd-Te clusters is not affected by intrinsic surface defects, even at elevated temperatures. Upon annealing, the Pd-Te nanoclusters adopt an identical nanostructure and are stable up to ∼523 K. Density functional theory calculations provide a foundation for our understanding of the mobility of Pd and Te atoms, preferential nucleation of Pd-Te clusters, and the origin of their annealing-induced monodispersity. These results highlight the role the excess chalcogenide atoms may play in the metal deposition process. More broadly, the discoveries of synthetic pathways yielding thermally robust monodispersed nanostructures on TMDCs are critical to the manufacturing of novel quantum and microelectronics devices and catalytically active nano-alloy centers.

Tuning Electronic Properties of 2D Materials Using Metal Adsorbates: Cu at WTe2 Edges

Two-dimensional materials exhibit properties promising for novel applications. Topologically protected states at their edges can be harnessed for use in quantum devices. We use ab initio simulations to examine properties of edges in 1T'-WTe₂ monolayers, known to exhibit topological order, and their interactions with Cu atoms. Comparison of (010)-oriented edges that have the same composition but different terminations shows that, as the number of Cu atoms increases, their thermodynamically preferred arrangement depends on the details of the edge structure. Cu atoms aggregate into a cluster at the most stable edge; while the cluster is nonmagnetic, it spin-polarizes the W atoms along the edge, which removes the topological protection. At the metastable edge, Cu atoms form a chain incorporated into the WTe₂ lattice; the topological state is preserved in spite of the dramatic edge restructuring. This suggests that exploiting interactions of metal species with metastable edge terminations can provide a path toward noninvasive interfaces.

Magnetism arising from Mexican-hat-like band dispersion in the WSe2/SnS2 heterostructure via interlayer strain

Mexican-hat-like band dispersion is extremely critical to the realization of hole-doping-induced magnetism in monolayer metal monochalcogenides. However, it is absent from transition-metal dichalcogenides (TMDCs), i.e., WSe2. Herein, using first-principles calculations, we show that Mexican-hat-like band dispersion can be achieved by applying interlayer strain (ε) in the WSe2/SnS2 van der Waals (vdW) heterostructure when ε exceeds 15%. This is because in the strain-induced distorted trigonal prismatic crystal field, at the valence band edge, the W_dz2 orbitals shift upward around the Γ point, while the double-degenerate W_dxy/dx2-y2 orbitals shift downward at the K point, resulting in Mexican-hat-like band dispersion near the Γ point when the energy level of the Γ point surpasses that of the K point. On account of the appearance of the Mexican-hat-like band edge (MHBE), hole-doping in the strained WSe2/SnS2 heterostructure induces magnetization readily from the nonmagnetized phase. Our findings may provide a new strategy for the realization of magnetized TMDC-based vdW heterostructures.

Nonmetal-Atom-Doping-Induced Valley Polarization in Single-Layer Tl2O

Valleytronics that relies on the valley degree of freedom is attracting growing interest because it provides a new platform for information storage. One obstacle in this field is to realize valley polarization in an efficient route to manipulate the valley physics. Here we propose a strategy to induce valley polarization by nonmetal atom doping in single-layer Tl₂O. Owing to the intrinsic inversion asymmetry and large spin-orbit coupling, there are a two-fold valley degeneracy and an excellent spin-valley independence in single-layer Tl₂O. Upon introducing C/N atoms in single-layer Tl₂O, the intriguing valley polarization successfully appears, and the obtained polarization strengths are considerable. In particular, for N-doped case, the top valence band locates around the Fermi level, and there are no impurity states in the band gap, which is desirable for practical applications. It is predicted that these valley polarizations can be effectively engineered under the magnetic field and external strain, suggesting that the control of valley physics in single-layer Tl₂O is accessible.

A Review of the Characteristics, Synthesis, and Thermodynamics of Type-II Weyl Semimetal WTe₂

WTe₂ as a candidate of transition metal dichalcogenides (TMDs) exhibits many excellent properties, such as non-saturable large magnetoresistance (MR). Firstly, the crystal structure and characteristics of WTe₂ are introduced, followed by a summary of the synthesis methods. Its thermodynamic properties are highlighted due to the insufficient research. Finally, a comprehensive analysis and discussion are introduced to interpret the advantages, challenges, and future prospects. Some results are shown as follows. (1) The chiral anomaly, pressure-induced conductivity, and non-saturable large MR are all unique properties of WTe₂ that attract wide attention, but it is also a promising thermoelectric material that holds anisotropic ultra-low thermal conductivity (0.46 W·m−1·K−1). WTe₂ is expected to have the lowest thermal conductivity, owing to the heavy atom mass and low Debye temperature. (2) The synthesis methods influence the properties significantly. Although large-scale few-layer WTe₂ in high quality can be obtained by many methods, the preparation has not yet been industrialized, which limits its applications. (3) The thermodynamic properties of WTe₂ are influenced by temperature, scale, and lattice orientations. However, the in-plane anisotropy cannot be observed in the experiment, as the intrinsic property is suppressed by defects and boundary scattering. Overall, this work provides an opportunity to develop the applications of WTe₂.

Hidden spin polarization in the 1T-phase layered transition-metal dichalcogenides MX2 (M = Zr, Hf; X = S, Se, Te)

The recent discovery of hidden spin polarization emerging in layered materials of specific nonmagnetic crystal is a fascinating phenomenon, though hardly explored yet. Here, we have studied hidden spin textures in layered nonmagnetic 1T-phase transition-metal dichalcogenides MX₂ (M = Zr, Hf; X = S, Se, Te) by using first-principles calculations. Spin-layer locking effect, namely, energy-degenerate opposite spins spatially separated in the top and bottom layer respectively, has been identified. In particular, the hidden spin polarization of β-band can be easily probed, which is strongly affected by the strength of spin-orbit coupling. The hidden spin polarization of ξ-band locating at high symmetry M point (conduction band minimum) has a strong anisotropy. In the bilayer, the hidden spin polarization is preserved at the upmost Se layer, while being suppressed if the ZrSe₂ layer is taken as the symmetry partner. Our results on hidden spin polarization in 1T-phase dichalcogenides, verifiable by spin-resolved and angle-resolved photoemission spectroscopy (ARPES), enrich our understanding of spin physics and provide important clues to search for specific spin polarization in two dimensional materials for spintronic and quantum information applications.

Half-Metallicity in Co-Doped WSe2 Nanoribbons

The recent development of two-dimensional transition-metal dichalcogenides in electronics and optoelelectronics has triggered the exploration in spintronics, with high demand in search for half-metallicity in these systems. Here, through density functional theory (DFT) calculations, we predict robust half-metallic behaviors in Co-edge-doped WSe₂ nanoribbons (NRs). With electrons partially occupying the antibonding state consisting of Co 3d_yz and Se 4p_z orbitals, the system becomes spin-polarized due to the defect-state-induced Stoner effect and the strong exchange splitting eventually gives rise to the half-metallicity. The half-metal gap reaches 0.15 eV on the DFT generalized gradient approximation level and increases significantly to 0.67 eV using hybrid functional. Furthermore, we find that the half-metallicity sustains even under large external strain and relatively low edge doping concentration, which promises the potential of such Co-edge-doped WSe₂ NRs in spintronics applications.