Polymorphic phases in 2D In2Se3: fundamental properties, phase transition modulation methodologies and advanced applications

Two-dimensional (2D) In2Se3, which is a multifunctional semiconductor, exhibits multiple crystallographic phases, each of which possesses distinct electronic, optical, and thermal properties. This inherent phase variability makes it a promising candidate for a wide range of applications, including memory devices, photovoltaics, and photodetectors. This review comprehensively explores the latest progress of various polymorphic phases of 2D In2Se3, emphasizing their unique properties, characterization methods, phase modulation strategies, and practical applications. Commencing with a rigorous examination of the structural attributes inherent in its various phases, we introduce sophisticated techniques for its characterization. Subsequently, modulation strategies, encompassing variations in temperature, application of electric fields, induced stress, and alterations in pressure, are explored, each exerting an influence on the phase transitions in 2D In2Se3. Finally, we highlight recent advancements and applications resulting from these phase transitions, including homoepitaxial heterophase structures, optical modulators, and phase change memory (PCM). By synthesizing insights into phase properties, modulation strategies, and potential applications, this review endeavours to provide a comprehensive understanding of the significance and prospects of In2Se3 in the semiconductor field.

Phase Engineering of 1T'-MoS2 via Organic Enwrapment

Molybdenum disulfide (MoS2) is a layered material known to show various phases. Most studies on it have focused on its semiconductor phase, but it is known to also have a metallic 1T' phase. This 1T' phase has also drawn attention as a quantum spin Hall phase, but the 1T' phase is metastable, and methods for transforming or stabilizing it are still challenging. This Communication demonstrates a method for effectively transforming the monolayer or the topmost layer of multilayer semiconductor MoS2 (the 1H or 2H phase) into the 1T' phase via ultraviolet-ozone (UVO) treatment, followed by polymer enwrapment of the MoS2 surface. UVO induces the transformation of the 1H (2H) phase into the 1T' phase, but the generated phase is unstable. The enwrapment procedure with the polymer poly-l-lysine was found to be effective in transforming the 1H (2H) phase into the 1T' phase and stabilizing it. Moreover, this procedure transformed only the topmost layer and generated a vertical 1T'/2H heterostructure in multilayer cases. This study shows the high potential of surface organic chemical procedures to control the phases in 2D transition metal dichalcogenides.

Bismuthene Under Cover: Graphene Intercalation of a Large Gap Quantum Spin Hall Insulator

The quantum spin Hall insulator bismuthene, a two-third monolayer of bismuth on SiC(0001), is distinguished by helical metallic edge states that are protected by a groundbreaking 800 meV topological gap, making it ideal for room temperature applications. This massive gap inversion arises from a unique synergy between flat honeycomb structure, strong spin orbit coupling, and an orbital filtering effect that is mediated by the substrate. However, the rapid oxidation of bismuthene in air has severely hindered the development of applications, so far confining experiments to ultra-high vacuum conditions. Intercalating bismuthene between SiC and a protective sheet of graphene, this barrier is successfully overcome. As demonstrated by scanning tunneling microscopy and photoemission spectroscopy, graphene intercalation preserves the structural and topological integrity of bismuthene, while effectively shielding it from oxidation in air. Hereby, hydrogen is identified as the critical process gas that was missing in previous bismuth intercalation attempts. These findings facilitate ex-situ experiments and pave the way for the development of bismuthene based devices, signaling a significant step forward in the development of next-generation technologies.

Strain-Induced Charge Density Waves with Emergent Topological States in Monolayer NbSe2

Emergence of topological states in strongly correlated systems, particularly two-dimensional (2D) transition-metal dichalcogenides, offers a platform for manipulating electronic properties in quantum materials. However, a comprehensive understanding of the intricate interplay between correlations and topology remains elusive. Here we employ first-principles modeling to reveal two distinct 2 × 2 charge density wave (CDW) phases in monolayer 1H-NbSe2, which become energetically favorable over the conventional 3 × 3 CDWs under modest biaxial tensile strain of about 1%. These strain-induced CDW phases coexist with numerous topological states characterized by Z 2 topology, high mirror Chern numbers, topological nodal lines, and higher-order topological states, which we have verified rigorously by computing the topological indices and the presence of robust edge states and localized corner states. Remarkably, these topological properties emerge because of the CDW rather than a pre-existing topology in the pristine phase. These results elucidate the interplay between correlations, topology, and geometry in 2D materials and indicate that strain-induced correlation effects can be used to engineer topological states in materials with initially trivial topology. Our findings may be applied in electronics, spintronics, and other advanced quantum devices that require robust and tunable topological states.

Topological phase transition in monolayer 1[Formula: see text]-[Formula: see text]

1[Formula: see text] phase of the monolayer transition metal dichalcogenides has recently attracted attention for its potential in nanoelectronic applications. We theoretically prove the topological behavior and phase transition of 1[Formula: see text]-[Formula: see text] using k.p Hamiltonian and linear response theory. The spin texture in momentum space reveals a strong spin-momentum locking with different orientations for the valence and conduction bands. Also, Berry curvature distributions around the Dirac points highlight the influence of α parameter demonstrating a topological phase transition in 1[Formula: see text]-[Formula: see text]. For [Formula: see text] the spin Hall conductivity is the only non-zero term [Formula: see text] and [Formula: see text], corresponding to a quantum spin Hall insulator (QSHI) phase, while for [Formula: see text], valley Hall conductivity prevails, indicating a transition to a band insulator (BI). Further analysis explores the spin-valley-resolved Hall conductivity and Chern numbers across varying values of α, V, and Fermi energy, uncovering regions of non-trivial and trivial topological phases (TTP) and the role of the edge modes. The zero total Nernst coefficient across energy ranges suggests strong cancellation between spin and valley contributions, providing insights into the material's potential for thermoelectric applications and spintronic devices.

The Critical Role of Interlayer Charge Transfer and Charge Redistribution Inducing the Formation of Phase-Pure Monolayer 1T'-MoTe2

1T'-MoTe2 exhibits a variety of intriguing physical properties, consisting of nontrivial higher-order topological behavior, ferroelectricity, superconductivity, and reversible phase transition. Hence, 1T'-MoTe2 has emerged as a hot spot in the fields of condensed matter physics and materials science. Nevertheless, the large-area synthesis of phase-pure 1T'-MoTe2 thin films has always been a big challenge for their widespread studies and device applications. In this study, three types of 1T'-MoTe2/XTe heterojunction films are proposed and fabricated by molecular beam epitaxy. The mechanisms of lattice strain and charge transfer influencing the 2H-1T' phase transition are clearly elucidated, while centimeter-size and phase-pure monolayer 1T'-MoTe2 can be successfully fabricated via the choice of XTe functional layers. The results reveal that the substantial charge transfer of 0.005-0.056 e/f.u. at the heterojunction interface and the particular electron accumulation in Mo 4d orbitals (0.010-0.016 e/f.u.) are critical for the formation of 1T'-MoTe2, while, in contrast, the effect from lattice strain that is induced by the underlying XTe layer is negligible. Owing to the most remarkable charge transfer effects, phase-pure monolayer 1T'-MoTe2 is achieved in the 1T'-MoTe2/MnTe heterojunction film among all films. This study lays a solid foundation for the in-depth studies of the important physical properties and functional devices based on 1T'-MoTe2 films and provides valuable suggestions for effective phase control in similar materials utilizing heterojunction engineering.

Transverse Josephson Diode Effect in Tilted Dirac Systems

We theoretically study the transverse charge transport in Josephson junctions based on the tilted Dirac materials with valley-dependent gaps. It is shown that a finite tilt-assisted transverse Josephson Hall current is present under broken time-reversal symmetry. This transverse current is driven by the superconducting phase difference across the junction and exhibits a nonsinusoidal current-phase relation, leading to the transverse Josephson diode effect (TJDE), where the critical currents flowing oppositely along the transverse direction are asymmetric. Compared to the conventional longitudinal Josephson diode effect, the predicted TJDE supports a fully polarized diode efficiency with a 100% quality factor and can completely decouple the input signal path from the output, suggesting potential applications for nonreciprocal superconducting devices.

Polarized optical contrast spectroscopy of in plane anisotropic van der Waals materials

Polarized optical contrast spectroscopy is a simple and non-destructive approach to characterize the crystalline anisotropy and orientation of two-dimensional materials. Here, we develop a 3D-printed motorized polarization module, which is compatible with typical microscope platforms and enables to perform broadband polarization-resolved reflectance spectroscopy. As proof of principle, we investigate the in-plane birefringence of exfoliated [Formula: see text]  thin films and few-layer [Formula: see text]  crystals. We compare the measured spectra to a model based on a transfer matrix formalism. Compared to other polarization sensitive approaches, such as Raman or second harmonic generation spectroscopy, optical contrast measurements require orders of magnitude less excitation power densities, which is particularly advantageous to avoid degradation of delicate van der Waals layers.

Investigation of Composition-Dependent Phonon Spectra in In-Plane Heterostructured Cu(1-x)In(1+x/3)P2S6 by Brillouin Light Scattering

CuInP2S6 (CIPS) is a two-dimensional van der Waals material that is ferrielectric at room temperature (TC of 315 K). This TC can be raised up to 335 K by synthesizing CIPS with Cu deficiencies (Cu1-xIn1+x/3P2S6, CIPS-IPS), which causes the material to self-segregate into separate CIPS and In4/3P2S6 (IPS) domains. Using Brillouin light scattering microscopy, we examine the phonon spectra of CIPS, IPS, and CIPS-IPS (x = 0.2, 0.3, 0.5, 0.6, 0.8) at room temperature and across TC. We observe unique longitudinal acoustic (LA) phonon signatures for pure CIPS and IPS; however, the CIPS-IPS samples host LA phonons corresponding to both CIPS and IPS, due to the formation of the in-plane heterostructures. These phonons soften in CIPS and CIPS-IPS near their respective values of TC, and there are sharp discontinuities in the phonon frequencies at TC, indicative of the ferrielectric-to-paraelectric phase transition. The temperature and width of this transition is dependent on composition, with pure CIPS showing the sharpest transition at 40.0 °C, while reduction in Cu leads to broadening and an increased TC, caused by the strain exerted on the CIPS domains by the IPS domains. This strain also manifests in IPS domains, as the phonons soften to accommodate the structural change in the CIPS domains.

Potassium-stabilized metastable carbides and chalcogenides via surface chemical potential modulation

Metastable carbides and chalcogenides are attractive candidates for wide and promising applications. However, their inherent instability leads to synthetic difficulty and poor durability. Thus, the development of facile strategies for the controllable synthesis and stabilization of metastable carbides is still a great challenge. Here, taking metastable ɛ-Fe2C as a case study, potassium ions (K+) are theoretically predicted and experimentally reported to control the synthesis of metastable ɛ-Fe2C from an Fe2N precursor by increasing the surface carbon chemical potential (μC). The controllable synthesis and improved stability are attributed to the better-matched denitriding and carburizing rates and the impeded spillover of carbon atoms in metastable ɛ-Fe2C with high carbon contents due to the enhanced surface μC. In addition, this strategy is suitable for synthesizing metastable γ'-MoC, MoN, 1T-MoS2, 1T-MoSe2, 1T-MoSe2xTe2(1-x), and 1T-Mo1-xWxSe2, highlighting the universality of the methodology. Impressively, gram-level scalable metastable ɛ-Fe2C remains stable for more than 398 days in air. Furthermore, ɛ-Fe2C exhibits remarkable olefin selectivity and durability for more than 36 h of continuous testing. This work not only demonstrates a facile, easily scalable, and general strategy for accessing various metastable carbides and chalcogenides but also addresses the synthetic difficulty and poor durability challenge of metastable materials.

The quantum valley Hall effect in twisted bilayer silicene and germanene

We show that twisted bilayers of silicene or germanene can be utilized for a novel transistor concept that relies on the quantum valley Hall effect. The application of an electric field normal to the twisted bilayer allows to tailor the size of the bandgap in AB- and BA-stacked domains of the twisted bilayer. In contrast to twisted bilayer graphene, the AB and BA bandgaps in twisted bilayer silicene and germanene are not inverted for small electric fields. However, above a critical electric field, the bandgaps invert, giving rise to a two-dimensional triangular network of topologically protected channels. The possibility to controllably switch these topologically protected states on and off using an electric field, combined with its inherent robustness against defects and impurities, establishes a foundation for a new type of transistor with an exceptional resilience.

Exploring a new topological insulator in β-BiAs oxide

The scarcity of suitable quantum spin Hall (QSH) insulators with a significant bulk gap poses a major challenge to the widespread application of the QSH effect. This study employs first-principles calculations to investigate the stability, electronic structure, and topological properties of a fully oxygenated bismuth arsenide system. Without the influence of spin-orbit coupling (SOC), the valence and conduction bands at the Γ-point exhibit a semimetallic nature. However, introducing SOC leads to a substantial 352 meV band gap, which allows operation at room temperature. The calculation of the topological invariant reveals , and the presence of topologically protected edge states in a Dirac cone at the Γ point confirms the existence of a non-trivial topological state. The epitaxial growth of β-BiAsO2 on a SiO2 substrate maintains the band topology of β-BiAsO2, spin lock with SOC effect. Additionally, the fully oxidized surfaces of β-BiAsO2 are inherently resistant to surface oxidation and degradation, suggesting a promising approach for developing room-temperature topological quantum devices. These findings not only introduce new vitality into the 2D group-VA materials family and enrich the available candidate materials in this field but also highlight the potential of these 2D semiconductors as appealing ultrathin materials for future flexible electronics and optoelectronics devices.

Solution-Processable Electronic-Grade 2D WTe2 Enabled by Synergistic Dual Ammonium Intercalation

Tungsten ditelluride (WTe2) exhibits thickness-dependent properties, including magnetoresistance, ferroelectricity, and superconductivity, positioning it as an ideal candidate for nanoelectronics and spintronics. Therefore, the scalable synthesis of WTe2 with defined thicknesses down to the monolayer limit is crucial for unlocking these properties. Here, we introduce a universal electrolyte chemistry utilizing dual-ammonium compounds to exfoliate WTe2, enabling precise control over the intercalation stages and flake thicknesses. This approach achieves an 86% exfoliation yield, producing high-quality flakes averaging 2.83 nm in thickness, in which approximately 10% are monolayers. A solution-processed, single-flake device (10 nm thick) exhibits a magnetoresistance (MR) of 50% at 2 K and 9 T, and piezo-response force microscopy (PFM) indicates ferroelectricity in WTe2 flakes. Additionally, large-area WTe2 thin films (15 × 15 mm2), fabricated using Langmuir-Schaefer deposition, exhibit metallic behavior with a high conductivity of 2.9 × 104 S/m. Overall, the hybrid electrolyte approach facilitates the scalable synthesis of high-quality, solution-processable, two-dimensional (2D) WTe2 flakes with excellent properties. This versatility of the developed method has been further exemplified through the exfoliation of other transition metal dichalcogenides (e.g., MoS2 and MoSe2), expanding the potential for the extensive application of exfoliated 2D materials in printable and flexible nanoelectronics.

Dissipationless edge transport in single-layer topological insulator Bi4Br4based device under high vacancy concentration

Single-layer Bismuth Monobromide (SL-Bi4Br4) is a recently experimentally confirmed room temperature quantum spin hall insulator with a relatively large bulk band gap. In this paper, we investigate the electronic properties of SL-Bi4Br4and single-layer bismuth monobromide nanoribbon (SL-Bi4Br4NR) introducing different vacancy defects near the nanoribbon edges. With maximally localized wannier function (MLWF) constructed Hamiltonian we show that SL-Bi4Br4NR edge states are protected by bulk topology and robust against disorder. In conjunction with MLWF and non-equilibrium Green's function, we also show that in devices made from SL-Bi4Br4, transmission through the topologically protected edge states do not suffer from degradation when the device is sufficiently wide. Increasing channel length and defect concentration affect only the bulk states transmission leaving edge states transmission perfectly quantized. This resilience against disorder signifies SL-Bi4Br4's promising candidacy for next-generation electronic & spintronics devices application.

MoTe2 Polymorphs: A DFT Approach to Structural, Electronic, Mechanical and Vibrational Properties

Molybdenum ditelluride (MoTe2), a key member of the transition metal dichalcogenides (TMDCs) family, holds significant potential for applications in electronics, energy storage, and catalysis. Despite its importance, the range of MoTe2 structural forms that has been explored is still limited. The primary aim of this research is to identify new stable MoTe2 polymorphs that may exist under zero-temperature and zero-pressure conditions. This study offers an in-depth analysis of 11 different structural variations (polymorphs) of MoTe2 using advanced computational methods based on density functional theory (DFT). By employing the Heyd-Scuseria-Ernzerhof (HSE06) hybrid functional, accurate calculations of electronic properties, such as band structure, are achieved. Bonding analysis, including charge density and electron localization, reveals consistent covalent interactions across the hexagonal and trigonal forms of MoTe2. The study also assesses the mechanical stability of these polymorphs using elastic constants, identifying both stable and metastable forms. Additionally, phonon and thermal properties, including heat capacity and entropy, are calculated for all dynamically stable polymorphs. Raman and infrared spectra provide insights into their distinct vibrational modes. These findings help distinguish structural attributes relevant to layer-specific applications. This comprehensive investigation of MoTe2 polymorphs uncovers new stable structures and provides crucial insights for their potential use in technological applications.

Intrinsic One-Dimensional Moiré Superlattice in Large-Angle Twisted Bilayer WTe2

Moiré effects in two-dimensional (2D) twisted van der Waals structures are attracting great interest owing to the emergence of intriguing physical properties. Despite growing interests, moiré effects with large twist angles remain unexplored because the increase of twist angle is thought to quickly suppress the size of moiré patterns. In this study, we focused on large-angle twisted tungsten ditelluride and discovered two orthogonal one-dimensional (1D) moiré patterns at twist angles around 62° and 58° using transmission electron microscopy. Their diffraction patterns exhibited pair formation features supporting the one-dimensionality of these moiré patterns. We also succeeded in revealing the origin of large-angle moiré patterns as well as the intrinsic nature of 1D moiré patterns. The observed moiré patterns are purely 1D in the sense that the long-range periodicity exists only along one direction, being a promising platform to study 1D phenomena. Our results propose a universal concept for studying large-angle twisted structures.

Unveiling the Stable Semiconducting 1T'-HfCl2 Monolayer: A New 2D Material

Designing novel 2D materials is crucial for advancing next-generation optoelectronic technologies. This work introduces and analyzes the 1T'-HfCl2 monolayer, a novel low-symmetry variant within the 2D transition metal dichloride family. Phonon dispersion calculations reveal no imaginary frequencies, suggesting its dynamical stability. 1T'-HfCl2 exhibits semiconducting behavior with a direct band gap of 1.52 eV, promising for optoelectronics. Strong excitonic effects with a binding energy of 525 meV highlight significant electron-hole interactions typical of 2D systems. Furthermore, the monolayer achieves total reflection of linearly polarized light along the ŷ direction at photon energies above 2.5 eV, showcasing its potential as an optical polarizing filter. Raman spectra calculations also reveal distinct peaks between 96.72 and 270.38 cm-1. The tunable excitonic and optical properties of 1T'-HfCl2 highlight its potential in future functional devices, paving the way for its integration into semiconducting and optoelectronic applications.

Growing and nanomanipulating heterostructures of α-bismuthene in a nearly isolated state

The growth of vertical heterostructures, which incorporate bismuthene with minimal coupling to adjacent materials, is pursued to fully exploit the exceptional properties intrinsic to the 2D allotropic forms of bismuth. Here, the growth of vertical heterostructures of ultrathin α-bismuthene and one-atom-thick layers of hexagonal boron nitride (h-BN) supported on Rh(110) surfaces is reported. Scanning tunneling microscopy (STM) characterization shows that the sample morphology is dominated by the presence of ultrathin α-bismuthene islands, with a lower thickness limit of a paired bilayer, randomly scattered over the h-BN surface. Unlike previous studies on heterostructures combining α-bismuthene with different 2D materials, which only allowed specific relative angles between the atomic lattices of both constituents, the Bi structures grown here can adopt any in-plane orientation relative to the underlying h-BN/Rh(110) surface, although certain twist angles are preferred. The greater rotational variety found in this study suggests a weaker interaction between bismuthene and h-BN, meaning that these islands could be the most weakly coupled 2D Bi nanocrystals to a substrate reported to date. Additionally, in pursuit of precise control over the spatial distribution of the islands on the h-BN/Rh(110) surface, they have been nanomanipulated using the STM tip.

Understanding the activity origin and mechanisms of the oxygen reduction reaction on the tetramethyl metalloporphyrin/MoS2 electrocatalyst

The efficiency of the oxygen reduction reaction (ORR) on the cathode plays a crucial role in determining the performance of proton exchange membrane fuel cells. Porphyrin, distinguished by its cost-effectiveness, eco-friendly nature, and efficient utilization of its metal, stands out as a promising candidate for a metal single-atom catalyst in fuel cell cathodes. The metal and support modifications significantly impact the porphyrin's ORR activity. Nevertheless, the effects of Ni, Co, and Fe metals in tetramethyl metalloporphyrin/MoS2, named MeTMP/MoS2, catalyst on the mechanisms and activity of the ORR remain unknown. This study elucidates the topic using van der Waals dispersion-corrected density functional theory (DFT) calculations and thermodynamic model. Results showed that the rate-limiting step is located at the first and second hydrogenation steps in the associative mechanisms for Ni and Co (Fe) substitutions, respectively. For the dissociative mechanisms, the dissociation of molecular oxygen to two oxygen atoms is the rate-determining step on all the NiTMP/MoS2, CoTMP/MoS2, and FeTMP/MoS2 catalysts. The presence of the MoS2 support significantly reduces the thermodynamic activation barrier of the ORR, and hence improves the ORR activity in the dissociative mechanisms. This activation barrier is 3.45, 0.92, and 1.82 eV for NiTMP/MoS2, CoTMP/MoS2, and FeTMP/MoS2, which is much better compared to 4.85, 3.34, and 2.19 eV for NiTMP, CoTMP, and FeTMP, respectively. CoTMP/MoS2 is the best candidate among the considered catalysts for the ORR. Furthermore, we provide a detailed explanation of the physical insights into the interaction between the ORR intermediates and the catalysts.

Photon-drag photovoltaic effects and quantum geometric nature

The bulk photovoltaic effect (BPVE) generates a direct current photocurrent under uniform irradiation and is a nonlinear optical effect traditionally studied in noncentrosymmetric materials. The two main origins of BPVE are the shift and injection currents, arising from transitions in electron position and electron velocity during optical excitation, respectively. Recently, it was proposed that photon-drag effects could unlock BPVE in centrosymmetric materials. However, experimental progress remains limited. In this work, we provide a comprehensive theoretical analysis of photon-drag effects inducing BPVE (photon-drag BPVE). Notably, we find that photon-drag BPVE can be directly linked to quantum geometric tensors. Additionally, we propose that photon-drag shift currents can be fully isolated from other current contributions in nonmagnetic centrosymmetric materials. We apply our theory explicitly to the 2D topological insulator 1T'-WTe2. Furthermore, we investigate photon-drag BPVE in a centrosymmetric magnetic Weyl semimetal, where we demonstrate that linearly polarized light generates photon-drag shift currents.

In-Plane Anisotropy in van der Waals NiTeSe Ternary Alloy

The anisotropic properties of materials profoundly influence their electronic, magnetic, optical, and mechanical behaviors and are critical for a wide range of applications. In this study, the anisotropic characteristics of Ni-based van der Waals materials, specifically NiTe2 and its alloy NiTeSe, utilizing a combination of comprehensive scanning tunneling microscopy (STM), angle-resolved photoemission spectroscopy (ARPES), and density functional theory (DFT) calculations, are explored. Unlike 1T-NiTe2, which exhibits trigonal in-plane symmetry, the substitution of Te with Se in NiTe2 (resulting in the NiTeSe alloy) induces a pronounced in-plane anisotropy. This anisotropy is clear in the STM topographs, which reveal a distinct linear order of charge distribution. Corroborating these observations, ARPES measurements and DFT calculations reveal an anisotropic Fermi surface centered at theΓ ¯ $\bar \Gamma $ point, which is notably elongated along the ky direction, leading to directional variations in in-plane carrier velocities. Consequently, the Fermi velocity is highest along the kx direction where the linear charge distribution aligns in real space and is lowest along the ky direction. These findings offer valuable insights into the tunability of anisotropic properties in ternary transition metal dichalcogenide systems, highlighting their potential applications in the development of anisotropic electronic and optoelectronic devices.

Realization of a one-dimensional topological insulator in ultrathin germanene nanoribbons

Realizing a one-dimensional (1D) topological insulator and identifying the lower-dimensional limit of two-dimensional (2D) behavior are crucial steps toward developing high-density quantum state networks, advancing topological quantum computing, and exploring dimensionality effects in topological materials. Although 2D topological insulators have been experimentally realized, their lower dimensional limit and 1D counterparts remain elusive. Here, we fabricated and characterized arrays of zigzag-terminated germanene nanoribbons, a 2D topological insulator with a large topological bulk gap. The electronic properties of these nanoribbons strongly depend on their width, with topological edge states persisting down to a critical width (∼2 nm), defining the limit of 2D topology. Below this threshold, contrary to the tenfold way classification, we observe zero-dimensional (0D) states localized at the ends of the ultrathin nanoribbons. These end states, topologically protected by time-reversal and mirror symmetries, indicate the realization of a 1D topological insulator with strong spin-orbit coupling. Our findings establish germanene nanoribbons as a platform for investigating 1D topology and dimensionality effects in topological materials.

Impact of barrier width on topological insulator phase in InN/InGaN quantum wells with moderate strain

We present a theoretical study demonstrating that in InN/InGaN quantum wells, the topological insulator phase depends largely not only on the quantum well width, but also on the width of the barriers. We show that for structures with a large width of the barriers equal to 200 nm, the topological insulator exists only when the quantum well width is less than 4.5 nm. For quantum wells with widths of 4.5 nm, we obtain a unique topological phase transition from the normal insulator phase to the nonlocal topological semimetal via the Weyl semimetal phase. Decreasing the width of the barriers from 200 to 20 nm results in a large increase in the bulk energy gap in the topological insulator phase, which can greatly facilitate experimental verification of the topological insulator in InN/InGaN quantum wells. We reveal that this effect originates from increasing the built-in electric field in the barriers, which remarkably decreases the penetration of the conduction band wavefunction in the barrier. We also demonstrate that the bulk energy gap in the topological insulator phase is larger in the free-standing structures than in the structures grown on the substrates with the same In content as the barriers.

Temperature Effects on the Electronic Structures of Epitaxial 1T'-WSe2 Monolayers

Transition metal dichalcogenides (TMDCs) with a 1T' structural phase are predicted to be two-dimensional topological insulators at zero temperature. Although the quantized edge conductance of 1T'-WTe2 has been confirmed to survive up to 100 K (Wu, S.; Fatemi, V.; Gibson, Q. D.; Watanabe, K.; Taniguchi, T.; Cava, R. J.; Jarillo-Herrero, P., Science 2018, 359, 76-79), this temperature is still relatively low for industrial applications. Addressing the limited studies on temperature effects of 1T'-TMDCs, our research focuses on the crystal and electronic properties of epitaxial 1T'-WSe2 monolayers grown on bilayer graphene (BLG) and SrTiO3(100) substrates at various temperatures. For the 1T'-WSe2 grown on BLG (1T'-WSe2/BLG), we observed a significant thermal expansion effect on its band structures with a thermal expansion coefficient of ∼60 × 10-6 K-1. In contrast, 1T'-WSe2 grown on SrTiO3(100) (1T'-WSe2/SrTiO3) exhibits minimal changes with varied temperatures due to the enhanced stress exerted by the substrate. In addition, a significant Coulomb gap (CG) was observed to be pinned at the Fermi level for both 1T'-WSe2/BLG and 1T'-WSe2/SrTiO3 in the angle-resolved photoemission spectroscopy (ARPES) and scanning tunneling spectroscopy (STS). The CGs show different sizes depending on the different dielectric environments and interfacial doping from the substrates. The CG was also found to decrease with increasing temperatures and can persist up to 200 K for 1T'-WSe2/BLG, consistent with our Monte Carlo simulations. The observation of CG at Fermi level endows the epitaxial 1T'-WSe2 monolayers with a huge potential for realizing quantum spin Hall devices at high temperature and the topological computing designations in the future.

In2F2 monolayer: a new class of two-dimensional materials with negative Poisson's ratio and topological phase

Two-dimensional (2D) materials have garnered significant attention for their exceptional potential in electronic, optical, and flexible nanodevices. In this study, we introduce a novel 2D In2F2 monolayer, revealed through first-principles calculations, and demonstrate its thermal, dynamic, and mechanical stability. Our findings show that the In2F2 monolayer exhibits notable anisotropic mechanical behavior, including auxetic properties characterized by a negative Poisson's ratio. Electronic band structure calculations, using both PBE-GGA and HSE06 functionals, indicate that this monolayer is a semiconductor with a small, nontrivial topological bandgap of approximately 1.58 meV. The observed s-p band inversion and calculated invariant, confirm the presence of a nontrivial topological phase in this material. Furthermore, the optical absorption spectrum reveals strong anisotropy, with significant absorption in the visible to near-infrared range along the y-axis, suggesting potential applications in polarized photodetectors and anisotropic optoelectronic devices. The relatively low work function (3.86 eV) further increases its suitability for electron-emission applications, such as thermionic devices. These mechanical, electronic, and optical properties position the In2F2 monolayer as a promising candidate for next-generation electronics, flexible electronics, and anisotropic optoelectronics.

Topological Exciton Density Wave in Monolayer WSe_{2}

Based on the first-principles calculations coupled with the Bethe-Salpeter equation, the topological exciton density wave is investigated in two-dimensional monolayer WSe_{2}. We find that the topological excitonic insulator phase can exist in monolayer WSe_{2}, and it is robust against in-plane strain. In this system, the energy minimum of exciton bands is shifted to a finite in-plane momentum, forming a Fulde-Ferrell-Larkin-Ovchinnikov-like state. Using the Gross-Pitaevskii equations, stripe-patterned exciton density waves with a nonzero velocity emerge in monolayer WSe_{2}. Our findings pave a new way for exploring the interplay between electron correlation and nontrivial topology.

Constructing the Dirac Electronic Behavior Database of Under-Stress Transition Metal Dichalcogenides for Broad Applications

Discovering and utilizing the unique optoelectronic properties of transition metal dichalcogenides (TMDCs) is of great significance for developing next-generation electronic devices. In particular, research on Dirac state modulations of TMDCs under external strains is lacking. To fill this research gap, it has established a comprehensive database of 90 types of TMDCs and their response behaviors under external strains have been systematically investigated regarding the presence of Dirac cones and electronic structure evolutions. Among all the conditions, 27.3% of the TMDCs are Dirac materials with three distinct types of Dirac cones, which are mainly attributed to the electron localizations induced by external strains. TMDCs based on tellurides with 1H phase favor the formation of Dirac cones under stresses, leading to metallic-like properties and ultra-fast charge transportation. Correlations among Dirac cones, energy, electronic properties, and lattice structures have been revealed, offering critical references for modulating the properties of well-known TMDCs. More importantly, it has confirmed that the phase transition points are not sufficient for the appearance of Dirac cones. This work provides critical guidance to facilitate the development of TMDCs-based superconducting and optoelectronic devices for broad applications.

Exciton Condensation in Landau Levels of Quantum Spin Hall Insulators

We theoretically study the quantum spin Hall insulator (QSHI) in a perpendicular magnetic field. In the noninteracting case, the QSHI with space inversion and/or uniaxial spin rotation symmetry undergoes a topological transition into a normal insulator phase at a critical magnetic field B_{c}. The exciton condensation in the lowest Landau levels is triggered by Coulomb interactions in the vicinity of B_{c} at low temperature and spontaneously breaks the inversion and the spin rotation symmetries. We propose that the electron spin resonance spectroscopy with the ac magnetic field also aligned in the perpendicular direction can directly probe the exciton condensation order. Our results should apply to QSHIs such as the InAs/GaSb quantum wells and monolayer transition-metal dichalcogenides.

Insights on Fabrication Strategies and Energy Storage Mechanisms of Transition Metal Dichalcogenides Cathodes for Aqueous Zn-Based Batteries

Aqueous zinc-based batteries (AZBs) are gaining widespread attention owing to their intrinsic safety, relatively low electrode potential, and high theoretical capacity. Transition metal dichalcogenides (TMDs) have convenient 2D ion diffusion channels, so they have been identified as promising host materials for AZBs, but face several key challenges such as the narrow interlayer spacing and the lack of in-deep understanding energy storage mechanisms. This review presents a comprehensive summary and discussion of the intrinsic structure, charge storage mechanisms, and key fabrication strategies of TMD-based cathodes for AZBs. Firstly, the structural features including phase types and electrical properties of TMDs are underscored. Then, the charge storage mechanisms and activation principles in TMDs are elaborated along with the discussions about their influence on electrochemical performance. Afterward, specific attention is focused on the fabrication strategies of high-performance TMD cathodes, including interlayer expansion, defect creation, phase transition, and heteroatom doping. Finally, the key challenges are considered and potential effective strategies are proposed to design high-performance aqueous Zn-TMDs batteries.

Anisotropic Raman Scattering and Lattice Orientation Identification of 2M-WS2

Anisotropic materials with low symmetries hold significant promise for next-generation electronic and quantum devices. 2M-WS2, which is a candidate for topological superconductivity, has garnered considerable interest. However, a comprehensive understanding of how its anisotropic features contribute to unconventional superconductivity, along with a simple, reliable method to identify its crystal orientation, remains elusive. Here, we combine theoretical and experimental approaches to investigate angle- and polarization-dependent anisotropic Raman modes of 2M-WS2. Through first-principles calculations, we predict and analyze the phonon dispersion and lattice vibrations of all Raman modes in 2M-WS2. We establish a direct correlation between their anisotropic Raman spectra and high-resolution transmission electron microscopy images. Finally, we demonstrate that anisotropic Raman spectroscopy can accurately determine the crystal orientation and twist angle between two stacked 2M-WS2 layers. Our findings provide insights into the electron-phonon coupling and anisotropic properties of 2M-WS2, paving the way for the use of anisotropic materials in advanced electronic and quantum devices.

Defect dependent electronic properties of two-dimensional transition metal dichalcogenides (2H, 1T, and 1T' phases)

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 Transition Metal Dichalcogenides: A Theory and Simulation Perspective

Two-dimensional transition metal dichalcogenides (2D TMDs) are a promising class of functional materials for fundamental physics explorations and applications in next-generation electronics, catalysis, quantum technologies, and energy-related fields. Theory and simulations have played a pivotal role in recent advancements, from understanding physical properties and discovering new materials to elucidating synthesis processes and designing novel devices. The key has been developments in ab initio theory, deep learning, molecular dynamics, high-throughput computations, and multiscale methods. This review focuses on how theory and simulations have contributed to recent progress in 2D TMDs research, particularly in understanding properties of twisted moiré-based TMDs, predicting exotic quantum phases in TMD monolayers and heterostructures, understanding nucleation and growth processes in TMD synthesis, and comprehending electron transport and characteristics of different contacts in potential devices based on TMD heterostructures. The notable achievements provided by theory and simulations are highlighted, along with the challenges that need to be addressed. Although 2D TMDs have demonstrated potential and prototype devices have been created, we conclude by highlighting research areas that demand the most attention and how theory and simulation might address them and aid in attaining the true potential of 2D TMDs toward commercial device realizations.

Tunable Quantum Confinement in Individual Nanoscale Quantum Dots via Interfacial Engineering

Introducing quantum confinement has shown promise to enable control of charge carriers. Although recent advances make it possible to realize confinement from semiclassical regime to quantum regime, achieving control of electronic potentials in individual nanoscale quantum dots (QDs) has remained challenging. Here, we demonstrate the ability to tune quantum confined states in individual nanoscale graphene QDs, which are realized by inserting nanoscale monolayer WSe2 islands in graphene/WSe2 heterostructures via interfacial engineering. Our experiment indicates that scanning tunneling microscope (STM) tip pulses can trigger a local phase transition in the interfacial nanoscale WSe2 islands, which, in turn, enables us to tune discrete quantum states in individual graphene QDs. By using a STM tip, we can also generate one-dimensional (1D) position-tunable domain boundaries in the WSe2 islands. The 1D boundary introduces atomically wide electrostatic barriers that bifurcate quasibound states into two regions in the graphene QD, changing the QD from a relativistic artificial atom to a relativistic artificial molecule.

Nonvolatile Ferroic and Topological Phase Control under Nonresonant Light

Light-matter interaction is a long-standing promising topic that can be dated back to a few centuries ago and has witnessed the long-term debate between the particle and wave nature of light. In modern condensed matter physics and materials science, light usually serves as a detection tool to effectively characterize the physical and chemical features of samples. The light modulation on intrinsic properties of materials, such as atomic geometries, electronic bands, and magnetic behaviors, is more intriguing for information control and storage. This corresponds to a light-induced order parameter switch in the phase space. Most prior works focus on the situation when photon energy is larger than the material band gap, in which the photon is absorbed by the electron subsystem and then transfers its energy into other subsystems such as phonon and spin. This can be described by the imaginary part of the dielectric function. In contrast, recent theoretical predictions and experimental advances have suggested that the real part of dielectric function could also vary the energy landscape in phase space, so that it triggers phase transition in an athermic approach (without direct photon absorption). In this Perspective, we review some recent theoretical, computational, and experimental developments of such a low-frequency light-induced phase transition, focusing on ferroic and topological order parameters. We also elucidate its fundamental mechanisms by comparing it with the optical tweezers technique, and light irradiation could trigger impulsive stimulated Raman phonon excitation. Finally, we propose some further developments and challenges in such a nonresonant light-matter interaction.

Quantum spin Hall states in MX2 (M = Ru, Os; X = As, Sb) monolayers

The quantum spin Hall (QSH) effect has attracted extensive research interest due to its great promise in topological quantum computing and novel low-energy electronic devices. Here, using first-principles calculations, we find that MX2 (M = Ru and Os; X = As and Sb) monolayers are 2D topological insulators (TIs). The spin-orbit coupling (SOC) band gaps for RuAs2, RuSb2, OsAs2, and OsSb2 monolayers are predicted to be 80, 131, 118, and 221 meV, respectively. Additionally, the nontrivial topological states are further confirmed by calculating the topological invariant and the appearance of gapless edge states. More interestingly, for RuSb2 and OsSb2 monolayers, the position of node points in energy can be effectively tuned by applying in-plane strain. Our results consistently indicate that all MX2 monolayers can serve as an effective platform for achieving the room-temperature QSH effect.

Twofold Anisotropic Superconductivity in Bilayer T_{d}-MoTe_{2}

Noncentrosymmetric two-dimensional superconductors with large spin-orbit coupling offer an opportunity to explore superconducting behaviors far beyond the Pauli limit. One such superconductor, few-layer T_{d}-MoTe_{2}, has large upper critical fields that can exceed the Pauli limit by up to 600%. However, the mechanisms governing this enhancement are still under debate, with theory pointing toward either spin-orbit parity coupling or tilted Ising spin-orbit coupling. Moreover, ferroelectricity concomitant with superconductivity has been recently observed in the bilayer, where strong changes to superconductivity can be observed throughout the ferroelectric transition pathway. Here, we report the superconducting behavior of bilayer T_{d}-MoTe_{2} under an in-plane magnetic field, while systematically varying magnetic field angle and out-of-plane electric field strength. We find that superconductivity in bilayer MoTe_{2} exhibits a twofold symmetry with an upper critical field maxima occurring along the b axis and minima along the a axis. The twofold rotational symmetry remains robust throughout the entire superconducting region and ferroelectric hysteresis loop. Our experimental observations of the spin-orbit coupling strength (up to 16.4 meV) agree with the spin texture and spin splitting from first-principles calculations, indicating that tilted Ising spin-orbit coupling is the dominant underlying mechanism.

Double Quantum Spin Hall Phase in Moiré WSe2

Quantum spin Hall (QSH) insulators are topologically protected phases of matter in two dimensions that can support a pair of helical edge states surrounding an insulating bulk. A higher (even) number of helical edge state pairs is usually not possible in real materials because spin mixing would gap out the edge states. Here, we report experimental evidence for a QSH phase with one and two pairs of helical edge states in twisted bilayer WSe2 at Moiré hole filling factor ν = 2 and 4, respectively. We observe nearly quantized (within 10%) resistance plateaus of h ν e 2 and large nonlocal transport at ν = 2 and 4 while the bulk is insulating. The resistance is independent of the out-of-plane magnetic field and increases under an in-plane magnetic field. The results agree with quantum transport of helical edge states in a material with high spin Chern bands protected by Ising spin conservation symmetry.

First principles study of photocatalytic activity in ZnO-Janus van der Waals heterostructures

The design of type-II van der Waals (vdW) heterostructures is regarded as a promising route to produce green hydrogen via photocatalytic water splitting. To this aim, we propose novel vertically stacked vdW heterostructures based on ZnO and Janus VXY (X = Br, Cl, Y = Se, and Te) phases, and investigate their optoelectronic properties and photocatalytic performance by means of density functional theory simulations. The thermal stability of the heterostructures is confirmed by ab initio molecular dynamics simulations at 300 K. The HSE06 calculated band structures show that a specific stacking of ZnO-VBrSe and ZnO-VClSe exhibits an indirect band gap with type-II band alignment, while all other stackings exhibit a direct band gap with type-I band alignment. The type-II band alignment, along with the difference in the work function and the electrostatic potential between the ZnO and VXY monolayer, will result in a built-in electric field direct from the ZnO monolayer to the VXY monolayer which is crucial for photogenerated charge separation, and prevents the charge recombinations. The optical absorption coefficient α of all the considered ZnO-VXY heterostructures displays the first excitonic peak in the energy range required for photocatalysis applications. Based on the band edge potential analysis, all the studied systems are capable of starting an oxygen evolution reaction spontaneously, while some external stimuli will be required to initiate the hydrogen evolution reaction. The reported results suggest that the proposed ZnO-VXY vdW heterostructures have great potential for photocatalysis and optoelectronic device applications.

Recent advances in artificial neuromorphic applications based on perovskite composites

High-performance perovskite materials with excellent physical, electronic, and optical properties play a significant role in artificial neuromorphic devices. However, the development of perovskites in microelectronics is inevitably hindered by their intrinsic non-ideal properties, such as high defect density, environmental sensitivity, and toxicity. By leveraging materials engineering, integrating various materials with perovskites to leverage their mutual strengths presents great potential to enhance ion migration, energy level alignment, photoresponsivity, and surface passivation, thereby advancing optoelectronic and neuromorphic device development. This review initially provides an overview of perovskite materials across different dimensions, highlighting their physical properties and detailing their applications and metrics in two- and three-terminal devices. Subsequently, we comprehensively summarize the application of perovskites in combination with other materials, including organics, nanomaterials, oxides, ferroelectrics, and crystalline porous materials (CPMs), to develop advanced devices such as memristors, transistors, photodetectors, sensors, light-emitting diodes (LEDs), and artificial neuromorphic systems. Lastly, we outline the challenges and future research directions in synthesizing perovskite composites for neuromorphic devices. Through the review and analysis, we aim to broaden the utilization of perovskites and their composites in neuromorphic research, offering new insights and approaches for grasping the intricate physical working mechanisms and functionalities of perovskites.

Unveiling the Role of Electronic, Vibrational, and Optical Features of the 1T' WSe2 Monolayer

Understanding the optoelectronic profile and chemical stability of transition-metal dichalcogenides (TMDs) is crucial for advancing two-dimensional (2D) material applications, particularly in electronics, optoelectronics, and energy devices. Here, we investigate the structural, electronic, optical, and excitonic properties of the 1T' WSe2 monolayer. Phonon dispersion analysis confirmed the thermodynamic stability of this system. The 1T' WSe2 monolayer exhibits a small electronic band gap of 0.17 eV, and its linear optical response suggests the potential use as a polarizing filter due to its strong reflectivity at ŷ light polarization. Unlike the 1T' MoS2 system, 1T' WSe2 does not show an excitonic insulator phase. Instead, its exciton binding energy of 150 meV is consistent with values expected for 2D materials. This distinction underscores the unique electronic and optical properties of 1T' WSe2, positioning it as a promising candidate for advanced technological applications such as flexible electronics, photodetectors, and quantum computing. By exploring these properties, we can unlock the full potential of TMDs in creating innovative high-performance devices.

Revealing the Thermodynamic Mechanism of Phase Transition Induced by Activation Polarization in Lithium-Ion Batteries

Enhancing the phase transition reversibility of electrode materials is an effective strategy to alleviate capacity degradation in the cycling of lithium-ion batteries (LIBs). However, a comprehensive understanding of phase transitions under microscopic electrode dynamics is still lacking. In this paper, the activation polarization is quantified as the potential difference between the applied potential (Uabs) and the zero-charge potential (ZCP) of electrode materials. The polarization potential difference facilitates the phase transition by driving Li-ion adsorption and supplying an electron-rich environment. A novel thermodynamic phase diagram is constructed to characterize the phase transition of the example MoS2 under various Li-ion concentrations and operating voltages using the grand canonic fixed-potential method (FPM). At thermodynamic quasi-equilibrium, the ZCP is close to the Uabs, and thus is used to form the discharge curve in the phase diagram. The voltage plateau is observed within the phase transition region in the simulation, which will disappear as the phase transition reversibility is impaired. The obtained discharge curve and phase transition concentration both closely match the experimental results. Overall, the study provides a theoretical understanding of how polarization affects phase evolution in electrode dynamics, which may provide a guideline to improve battery safety and cycle life.

Charge-neutral electronic excitations in quantum insulators

Experiments on quantum materials have uncovered many interesting quantum phases ranging from superconductivity to a variety of topological quantum matter including the recently observed fractional quantum anomalous Hall insulators. The findings have come in parallel with the development of approaches to probe the rich excitations inherent in such systems. In contrast to observing electrically charged excitations, the detection of charge-neutral electronic excitations in condensed matter remains difficult, although they are essential to understanding a large class of strongly correlated phases. Low-energy neutral excitations are especially important in characterizing unconventional phases featuring electron fractionalization, such as quantum spin liquids, spin ices and insulators with neutral Fermi surfaces. In this Perspective, we discuss searches for neutral fermionic, bosonic or anyonic excitations in unconventional insulators, highlighting theoretical and experimental progress in probing excitonic insulators, new quantum spin liquid candidates and emergent correlated insulators based on two-dimensional layered crystals and moiré materials. We outline the promises and challenges in probing and using quantum insulators, and discuss exciting new opportunities for future advancements offered by ideas rooted in next-generation quantum materials, devices and experimental schemes.

Controlled Synthesis and Phase Transition Mechanisms of Palladium Selenide: A First-Principles Study

Using density functional theory, we carefully calculated the relative stability of monolayer, few-layer, and cluster structures with Penta PdSe2, T-phase PdSe2, and Pd2Se3-phase. We found that the stability of Penta PdSe2 increases with the number of layers. The Penta PdSe2, T-phase PdSe2, and Pd2Se3 monolayers are all semiconducting, with band gaps of 1.77, 0.81, and 0.65 eV, respectively. The formation energy of palladium selenide clusters with different phase structures is calculated, considering the cluster size, stoichiometry, and chemical environment. Under typical experimental conditions, Pd2Se3 phase clusters are found to be dominant, having the lowest formation energy among all of the phases considered, with this dominance increasing as cluster size grows. Adjusting the Pd-Se ratio in the environment allows for controlled synthesis of specific palladium selenide phases, providing theoretical insights into the nucleation mechanisms of PdSe2 and other transition metal chalcogenides.

An elemental ferroelectric topological insulator in ψ-bismuthene

A ferroelectric quantum spin Hall insulator (FEQSHI) exhibits coexisting ferroelectricity and time-reversal symmetry protected edge states, holding exciting prospects for inviting both scientific and application advances, particularly in two-dimensional systems. However, FEQSHI candidates that consist of only one constituent element are rarely reported. Here, we show that ψ-bismuthene, an allotrope of bilayer Bi (110), is a concrete example of a two-dimensional elemental FEQSHI. It is demonstrated that ψ-bismuthene possesses measurable ferroelectric polarization and nontrivial band gap with moderate switching barrier, making it highly suitable for the detection and observation of ferroelectric topologically insulating states. Additionally, the auxetic behavior, quantum transport properties and ferroelectric controllable persistent spin helix in ψ-bismuthene are also discussed. These findings make ψ-bismuthene promising for both fundamental physics and technological innovations.

Doxorubicin-Intercalated Li-Al-Based LDHs as Potential Drug Delivery Nanovehicle with pH-Responsive Therapeutic Cargo for Tumor Treatment

Clinical oncology is currently experiencing a technology bottleneck due to the expeditious evolution of therapy defiance in tumors. Although drugs used in chemotherapy work for a sort of cell death with potential clinical application, the effectiveness of chemotherapy-inducing drugs is subject to several endogenous conditions when used alone, necessitating the urgent need for controlled mechanisms. A tumor-targeted drug delivery therapy using Li-Al (M+/M3+)-based layered double hydroxide (LDHs) family has been proposed with the general chemical formula [M+1-x M3+x (OH)]2x+[(Am-)2x/m. n(H2O)]2x-, which is fully biodegradable and works in connection with the therapeutic interaction between LDH nanocarriers and anticancerous doxorubicin (DOX). Compositional variation of Li and Al in LDHs has been used as a nanoplatform, which provides a functional balance between circulation lifetime, drug loading capacity, encapsulation efficiency, and tumor-specific uptake to act as self-regulatory therapeutic cargo to be released intracellularly. First-principle analyses based on DFT have been employed to investigate the interaction of bonding and electronic structure of LDH with DOX and assess its capability and potential for a superior drug carrier. Following the internalization into cancer cells, nanoformulations are carried to the nucleus via lysosomes, and the mechanistic pathways have been revealed. Additionally, in vitro along with in vivo therapeutic assessments on melanoma-bearing mice show a dimensional effect of nanoformulation for better biocompatibility and excellent synergetic anticancer activity. Further, the severe toxic consequences associated with traditional chemotherapy have been eradicated by using injectable hydrogel placed just beneath the tumor site, and regulated release of the drug has been confirmed through protein expression applying various markers. However, Li-Al-based LDH nanocarriers open up new design options for multifunctional nanomedicine, which has intriguing potential for use in cancer treatment through sustained drug delivery.

Tunability of topological edge states in germanene at room temperature

Germanene is a two-dimensional topological insulator with a large topological band gap. For its use in low-energy electronics, such as topological field effect transistors and interconnects, it is essential that its topological edge states remain intact at room temperature. In this study, we examine these properties in germanene using scanning tunneling microscopy and spectroscopy at 300 K and compare the results with data obtained at 77 K. Our findings show that the edge states persist at room temperature, although thermal effects cause smearing of the bulk band gap. Additionally, we demonstrate that, even at room temperature, applying an external perpendicular electric field switches the topological states of germanene off. These findings indicate that germanene's topological properties can be maintained and controlled at room temperature, making it a promising material for low-energy electronic applications.

Tunable Valley Pseudospin and Electron-Phonon Coupling in WSe2/1T-VSe2 Heterostructures

Heterostructure engineering provides versatile platforms for exploring exotic physics and enhancing the device performance through interface coupling. Despite the rich array of physical phenomena presented by heterostructures composed of semiconductor and metal van der Waals materials, significant gaps remain in understanding their optical, thermal, and electronic properties. Here, we demonstrate that the valley pseudospin and electron-phonon coupling in monolayer WSe2 are significantly influenced by interface coupling with 1T-VSe2. The heterointerface alters the relaxation process of valley excitons, leading to a transition in magnetic-field-dependent valley polarization from a linear to a "V" shape. Furthermore, we uncover that enhanced electron-phonon coupling exacerbates variations in exciton and valley exciton behavior with temperature, involving higher phonon energies and a shift from acoustic to optical phonons. These findings highlight a promising pathway to manipulate valley excitons and investigate electron-phonon coupling through van der Waals interface interactions.

Concentration-Dependent Layer-Stacking and the Influence on Phase-Conversion in Colloidally Synthesized WSe2 Nanocrystals

We report a synthesis of WSe2 nanocrystals in which the number of layers is controlled by varying the precursor concentration. By altering the ratios and concentrations of W(CO)6 and Ph2Se2 in trioctylphosphine oxide, we show that high [Se] and large Se/W ratios lead to an increased number of layers per nanocrystal. As the number of layers per nanocrystal is increased, the nanocrystal ensembles show less phase-conversion from the metastable 2M phase to the thermodynamically favored 2H phase. Density functional theory calculations indicate that the interlayer binding energy increases with the number of layers, indicating that the stronger interlayer interactions in multilayered nanocrystals may increase the energy barrier to phase-conversion. The results presented herein provide insights for directing phase-conversion in solution-phase syntheses of transition metal dichalcogenides.

Intercalating Architecture for the Design of Charge Density Wave in Metallic MA2Z4 Materials

We present a novel approach to induce charge density waves (CDWs) in metallic MA2Z4 materials, resembling the behavior observed in transition metal dichalcogenides (TMDCs). This method leverages the intercalating architecture to maintain the same crystal field and Fermi surface topologies. Our investigation reveals that CDW instability in these materials arises from electron-phonon coupling (EPC) between the d band and longitudinal acoustic (LA) phonons, mirroring TMDC's behavior. By combining α-MA2Z4 with 1H-MX2 materials in a predictive CDW phase diagram using critical EPC constants, we demonstrate the feasibility of extending CDW across material families with comparable crystal fields and reveal the crucial role in CDW instability of the competition between ionic charge transfer and electron correlation. We further uncover a strain-induced Mott transition in β2-NbGe2N4 monolayer featuring star-of-David patterns. This work highlights the potential of intercalating architecture to engineer CDW materials, expanding our understanding of CDW instability and correlation physics.

Large-Area Epitaxial Growth of Transition Metal Dichalcogenides

Over the past decade, research on atomically thin two-dimensional (2D) transition metal dichalcogenides (TMDs) has expanded rapidly due to their unique properties such as high carrier mobility, significant excitonic effects, and strong spin-orbit couplings. Considerable attention from both scientific and industrial communities has fully fueled the exploration of TMDs toward practical applications. Proposed scenarios, such as ultrascaled transistors, on-chip photonics, flexible optoelectronics, and efficient electrocatalysis, critically depend on the scalable production of large-area TMD films. Correspondingly, substantial efforts have been devoted to refining the synthesizing methodology of 2D TMDs, which brought the field to a stage that necessitates a comprehensive summary. In this Review, we give a systematic overview of the basic designs and significant advancements in large-area epitaxial growth of TMDs. We first sketch out their fundamental structures and diverse properties. Subsequent discussion encompasses the state-of-the-art wafer-scale production designs, single-crystal epitaxial strategies, and techniques for structure modification and postprocessing. Additionally, we highlight the future directions for application-driven material fabrication and persistent challenges, aiming to inspire ongoing exploration along a revolution in the modern semiconductor industry.