Hexagonal Transition Metal Research Articles

Topological Polar Networks in Twisted Rhombohedral-Stacked Bilayer WSe2 Moiré Superlattices.

Sliding ferroelectricity enables materials with intrinsic centrosymmetric symmetry to generate spontaneous polarization via stacking engineering, extending the family of ferroelectric materials and enriching the field of low-dimensional ferroelectric physics. Vertical ferroelectric domains, where the polarization is perpendicular to atomic motion, have been discovered in twisted bilayers of inversion symmetry broken systems such as hexagonal boron nitride, graphene, and transition metal chalcogenides. In this study, we demonstrate that this symmetry breaking also induces lateral polar networks in twisted bilayer rhombohedral-stacked WSe2, as determined through symmetry considerations and vector piezoresponse force microscopy (V-PFM) results. Lateral polarization (LP) in saddle point (SP) regions forms head-to-tail triangular vortices, exhibiting elliptical domain shapes with widths up to 40 nm. The LP encloses the vertical polarization (VP), forming a network of Bloch-type merons and antimerons. Our work enhances the understanding of domain distribution and polarization orientation in moiré ferroelectrics.

A General Synthesis Method for Covalent Organic Framework and Inorganic 2D Materials Hybrids.

Two-dimensional (2D) inorganic/organic hybrids provide a versatile platform for diverse applications, including electronic, catalysis, and energy storage devices. The recent surge in 2D covalent organic frameworks (COFs) has introduced an organic counterpart for the development of advanced 2D organic/inorganic hybrids with improved electronic coupling, charge separation, and carrier mobility. However, existing synthesis methods have primarily focused on few-layered film structures, which limits scalability for practical applications. Herein, we present a general synthesis approach for a range of COF/inorganic 2D material hybrids, utilizing 2D inorganic materials as both catalysts and inorganic building blocks. By leveraging the intrinsic Lewis acid sites on the inorganic 2D materials such as hexagonal boron nitride (hBN) and transition metal dichalcogenides, COFs with diverse functional groups and topologies can grow on the surface of inorganic 2D materials. The controlled 2D morphology and excellent solution dispersibility of the resulting hybrids allow for easy processing into films through vacuum filtration. As proof of concept, hBN/COF films were employed as filters for Rhodamine 6G removal under flow-through conditions, achieving a removal rate exceeding 93%. The present work provides a simple and versatile synthesis method for the scalable fabrication of COF/inorganic 2D hybrids, offering exciting opportunities for practical applications such as water treatment and energy storage.

Temperature Dependent Behavior of Elastic and Ultrasonic Proprieties of Transition Metal Carbide Mo2C Superconductor

Abstract: Transition metal carbides exhibit peculiar chemical and physical properties, making them integral to industrial applications that demand performance under high temperatures In the case of the hexagonal transition metal carbide Mo2C superconductor, higher-order elastic constants were calculated to be temperature-dependent using an interactional potential model. Second-order elastic constants are used to determine other allied ultrasonic variables. Second-order coefficients are used to analyze the temperature variation of ultrasonic velocities along the z-direction of the superconductor. Furthermore, the temperature difference of Debye average velocity and thermal relaxation time are considered along the same direction. The temperature dependence of ultrasonic properties is explored in relation to thermal, elastic, and mechanical properties. Ultrasonic attenuation , resulting from phonon–phonon (p–p) interactions, is calculated at different temperatures .The study establishes that thermal conductivity is a core provider of the observed ultrasonic attenuation, particularly at higher temperatures. The mechanical and thermal properties of the Mo2C superconductor are superior at lower temperatures. Keywords:Transition metal carbide superconductor, Thermal conductivity, Ultrasonic properties, Elastic properties, Mechanical properties. PACS numbers: 43. 35. Cg; 62.20.Dc; 63.20.Kr.

Janus layers and electronic structure of 1T-(TiSeS)2.

TiS2-TiSe2 is one of the most studied titanium based solid solution systems. However, so far, all research on it has only focused on its disordered phase. Here, we systematically investigate its ordered phases. Using a structure search method based on the particle swarm optimization (CALYPSO) algorithm, we identify TiSeS-156 and discover a new structure (1T-(TiSeS)2). Based on first principles theory, their phonon spectra, formation energy, mechanical, electronic, thermal, and optical properties, as well as chemical bond analysis and synthetic pathways, have been investigated. The primitive cell of TiSeS-156 has three atoms and has a space group of P3m1 (no. 156). 1T-(TiSeS)2 has six atoms and has P3̄m1 symmetry (no. 164). TiSeS-156 and 1T-(TiSeS)2 are constructed by stacking the S-Ti-Se Janus layer materials. TiSeS-156 and 1T-(TiSeS)2 are narrow-gap semiconductors. The localized nature of the Ti(3d) states of TiSeS-156 and 1T-(TiSeS)2 leads to their semiconductor properties. 1T-(TiSeS)2 and TiSeS-156 have very similar mechanical, electronic, thermal, and optical properties of 1T-TiS2 and 1T-TiSe2, and are members of the 2D hexagonal lattice transition metal dichalcogenide layered material family. However, compared with 1T-TiS2 and 1T-TiSe2, TiSeS-156 and 1T-(TiSeS)2 have a wider range of potential applications, such as photovoltaic devices and photocatalysis, due to their S-Ti-Se Janus layer structure. They also provide a pathway for the preparation of Janus TiSeS monolayer and multi-layer materials. Moreover, our findings provide crucial insights for understanding the rich and complex crystal structures of the TiS2-TiSe2 system, which have broad implications for further exploration of this class of promising materials.

Facilitated fluorination and etching of 2D materials

Precise control of functionalization and etching have been required for surface modification and device fabrication of two-dimensional (2D) materials. Specifically, fluorination of graphene has been used to control the properties of graphene. Recently, xenon difluoride (XeF2) has been used for selective fluorination of graphene and etching of other 2D materials, such as hexagonal boron nitride (hBN) and transition metal dichalcogenides (TMDs). However, there is a lack of studies on key factors that govern the XeF2 treatment, which results in inconsistent behaviors of fluorination and etching. Here, we report control over chemical reactions between XeF2 and 2D materials using a chemical mediator of Si. Even a small amount of Si can accelerate the dissociation of XeF2, leading to the formation of chemically reactive xenon fluoride (XeF) that enhances the fluorination and etching of 2D materials. Additionally, our findings show that defects in 2D materials serve as chemically unstable sites that facilitate the additional dissociation of XeF2, generating F single atoms that easily form covalent bonds on the surface of 2D materials. Our study suggests that Si can be utilized as a reaction mediator to regulate XeF2 treatment on 2D materials, which has important implications for the fabrication of 2D electronic devices.

Recent progress on chemical vapor deposition growth of 2D materials

Remarkable progress has been made to understand the chemical vapor deposition (CVD) of two-dimensional (2D) materials over the last two decades. The review summarized the state-of-the-art experimental synthesis and modelling and simulation on 2D materials CVD growth. Firstly, the family of 2D materials, and their CVD growth processes are introduced. Secondly, the experimental synthesis and modelling and simulation on graphene growth are discussed. In particular, the applications of reactive molecular dynamics methods, kinetic Monte Carlo and density-functional theory in 2D material growth are addressed. Then, the CVD growth of hexagonal boron nitride and transition metal dichalcogenides are further discussed, focusing on the effects of reaction conditions (growth temperature, pressure, vapour-phase composition, etc.) on the domain morphologies, edge structures and grain boundaries of 2D materials. Last, conclusions and outlooks are presented.

High-temperature oxidation kinetics of a metastable dual-phase diboride and a high-entropy diboride

The processing of multicomponent (Ti0.25V0.25Zr0.25Hf0.25)B2 ultra-high temperature hexagonal transition metal diboride in dual-phase and single-phase microstructures and investigation of oxidation behavior in the air at 1000 and 1500 °C are reported. The dual-phase diboride is a metastable phase composed of Hf-Zr-rich and Ti-V-rich phases that undergo phase transformation to a single-phase high-entropy diboride after thermal annealing. At 1000 °C, a B2O3 layer was formed on the material's surface, and the oxidation kinetics followed a para-linear behavior. At 1500 °C, a porous oxide layer was formed, facilitating oxygen diffusion and reaction with the diboride, resulting in linear oxidation kinetics. The prediction of the lifetime of the materials during high-temperature oxidation suggested that the high-entropy material outperforms the dual-phase diboride, making it most suitable for related applications. The superior performance of the high-entropy single-phase diboride was associated with the high-entropy and sluggish diffusion effects.

Energy transport and relaxation of phonon polaritons in two-dimensional polar crystals

Energy transport and relaxation of phonon polaritons (PhPs) are studied, based on a macroscopic phonon model, for atomic layers of hexagonal boron nitride (hBN) and transition metal dichalcogenides (TMDs). The velocity of the energy flow (energy velocity) is derived from the energy flow and density; it equals the group velocity, similar to the results of bulk and surface PhPs. In electrostatic approximation, valid once the frequency is slightly above ω0 (e.g., ω>1.002ω0∼224cm−1 for pentalayer MoTe2; ω0 is the zone-center optical-phonon frequency), simple formulas are obtained for the energy velocity and relaxation rate (ERR). While the energy velocity increases proportionally with the number of layers N, the ERR is independent of N. The ERR equals the phonon damping rate in freestanding layers, but it is slightly decreased in SiO2-supported layers and has a non-monotonic frequency dependence (the decrease is smaller than 5.4% for hBN layers and negligible for TMD layers). The energy velocity decreases significantly with frequency in both freestanding and supported layers. Near ω0, however, the PhP properties should be calculated rigorously, and they all depend on N as well as the dielectric environment. High-frequency screening should be included to study the energy transport. The energy velocity can be engineered by varying N and the dielectric environment; it also can be tuned together with the propagation quality factor by the incident light frequency. The MoTe2 layers should be exploited for a far-infrared PhP material (wavelengths 43–45 μm); this is just an example of application of the proposed model to the considered materials.

Manipulation of Valley and Spin Properties in Two-Dimensional Janus WSiGeZ4 (Z = N, P, As) through Symmetry Control

A class of septuple-atomic-layer two-dimensional (2D) materials, MA2Z4, is sought as an alternative to 2D hexagonal transition metal dichalcogenides in the fields of valleytronics and spintronics. In these materials, the structural symmetry can be varied by changing the stacking of its three parts in the monolayer. We use first-principles calculations to show that in the Janus monolayer WSiGeZ4 (Z = N, P, As), Berry curvature and the Rashba effect are enhanced by modifying the stacking orders. The intrinsic electric field and composition of the d orbitals play a dominant role in determining these properties. Berry curvature is strengthened by up to 300% compared to its ground state through symmetry control, along with a significant increment in the Rashba coefficient. Moreover, monolayers WSiGeP4 and WSiGeAs4 have multiple valleys, implying another valley-dimension. The interesting spin-valley physics tunability in septuple-atomic-layer 2D materials suggests their exceptional potential for spintronic and valleytronic applications.

Thickness Mapping and Layer Number Identification of Exfoliated van der Waals Materials by Fourier Imaging Micro-Ellipsometry

As performance of van der Waals heterostructure devices is governed by the nanoscale thicknesses and homogeneity of their constituent mono- to few-layer flakes, accurate mapping of these properties with high lateral resolution becomes imperative. Spectroscopic ellipsometry is a promising optical technique for such atomically thin-film characterization due to its simplicity, noninvasive nature and high accuracy. However, the effective use of standard ellipsometry methods on exfoliated micron-scale flakes is inhibited by their tens-of-microns lateral resolution or slow data acquisition. In this work, we demonstrate a Fourier imaging spectroscopic micro-ellipsometry method with sub-5 μm lateral resolution and three orders-of-magnitude faster data acquisition than similar-resolution ellipsometers. Simultaneous recording of spectroscopic ellipsometry information at multiple angles results in a highly sensitive system, which is used for performing angstrom-level accurate and consistent thickness mapping on exfoliated mono-, bi- and trilayers of graphene, hexagonal boron nitride (hBN) and transition metal dichalcogenide (MoS2, WS2, MoSe2, WSe2) flakes. The system can successfully identify highly transparent monolayer hBN, a challenging proposition for other characterization tools. The optical microscope integrated ellipsometer can also map minute thickness variations over a micron-scale flake, revealing its lateral inhomogeneity. The prospect of adding standard optical elements to augment generic optical imaging and spectroscopy setups with accurate in situ ellipsometric mapping capability presents potential opportunities for investigation of exfoliated 2D materials.

Single-Photon Emission from Two-Dimensional Materials, to a Brighter Future

Single photons, often called flying qubits, have enormous promise to realize scalable quantum technologies ranging from an unhackable communication network to quantum computers. However, finding an ideal single-photon emitter (SPE) is a great challenge. Recently, two-dimensional (2D) materials have shown great potential as hosts for SPEs that are bright and operate under ambient conditions. This Perspective enumerates the metrics required for an SPE source and highlights that 2D materials, because of reduced dimensionality, exhibit interesting physical effects and satisfy several metrics, making them excellent candidates to host SPEs. The performance of SPE candidates discovered in 2D materials, hexagonal boron nitride and transition metal dichalcogenides, will be assessed based on the metrics, and the remaining challenges will be highlighted. Lastly, strategies to mitigate such challenges by developing design rules to deterministically create SPE sources will be presented.

Impact of vacancies on structure, stability and properties of hexagonal transition metal diborides, MB2 (M = Sc, Y, Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W, Mn, and Fe)

In this study, we have used density functional theory (DFT) calculations to characterize if and how defects influence the stability and electronic/mechanical properties of MB2 (AlB2-type) for different transition metal M. From a point defect analysis including vacancies, interstitials, and anti-sites, we identify vacancies to be most favored, or least unfavored. To provide insight into possible vacancy ordering, we focus on vacancies on M- and B-sublattices for nine metals (M = Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W), modelled both as disordered and ordered. We demonstrate and explain why vacancies have a significant impact for M from Group 4 (Ti, Zr, Hf), Group 5 (Nb, Ta) and 6 (Mo, W) with improved thermodynamical and dynamical stability as well as mechanical properties. This by diverging from the ideal composition through controlled off-stoichiometry in terms of vacancies in M- or B-deficient structures. Line compounds TiB2, ZrB2 and HfB2 account for B-poor or M-rich conditions by forming planar defects comprised of vacant B. This in contrast to the ordered M- and B vacancies identified for MoB2 and WB2, with an optimal result at 33.33% M- and 25% B-vacancies, respectively, which significantly improves the stability and concurrent properties through elimination of antibonding states and minimization of non-bonding states. Similar behavior with enhanced stability and properties is demonstrated for NbB2 and TaB2 with an optimum around 10% M- and 17% B-vacancies, respectively.

Revisiting the origins of super-hardness in TiB2+z thin films – Impact of growth conditions and anisotropy

Hexagonal transition metal diborides embody promising material systems for the purpose of protective thin films. Here, we focus on DC magnetron sputtered TiB2+z coating materials, comprehensively revisiting the impact of the stoichiometry on the structure-mechanical properties, from nearly stoichiometric TiB2.07 (B: 67 at. %) up to super-stoichiometric TiB4.42 (B: 82 at. %). The structural analysis confirmed the apparent correlation between the deposition pressure and the preferred {0001} orientation, which is essential to gain super-hardness (>40 GPa). In contrast, the hardness decreases for >10 GPa for 101¯1 and 1000 oriented thin films, underlining the pronounced anisotropy of TiB2+z. The broad stoichiometry variation revealed no predominant hardness effect based on a B-rich tissue phase. The excess B contributes to a decreasing column size correlating with a decreasing hardness of ≈ 7 GPa (B/Ti ratios >2.5) due to column boundary sliding events. Micro-cantilever bending experiments proved a declining fracture toughness from 3.02 ± 0.13 MPa√m for TiB2.43 to 2.51 ± 0.14 MPa√m for TiB4.42 to be column size dependent.

Hidden breathing kagome topology in hexagonal transition metal dichalcogenides

A kagome lattice, formed by triangles of two different directions, is known to have many emergent quantum phenomena. Under the breathing anisotropy of bond strengths, this lattice can become a higher-order topological insulator (HOTI), which hosts topologically protected corner states. Experimental realizations of HOTI on breathing kagome lattices have been reported for various artificial systems, but not for simple natural materials with an electronic breathing kagome lattice. Here we prove that a breathing kagome lattice and HOTI are hidden inside the electronic structure of hexagonal transition metal dichalcogenides (h-TMD). Due to the trigonal prismatic symmetry, $s{p}^{2}$-like hybrid $d$ orbitals create an electronic kagome lattice with anisotropic intersite and on-site hopping interactions. We demonstrate that HOTI h-TMD triangular nanoflakes host topologically protected corner states, which could be quantum-mechanically entangled with triple degeneracy. Because $\mathrm{h}\text{\ensuremath{-}}\mathrm{TM}{\mathrm{D}}_{\mathrm{S}}$ are easily synthesizable and stable with a large bandgap at ambient conditions, our findings open an avenue for quantum physics based on simple condensed matter systems.

Strong Moiré Excitons in High-Angle Twisted Transition Metal Dichalcogenide Homobilayers with Robust Commensuration.

The burgeoning field of twistronics, which concerns how changing the relative twist angles between two materials creates new optoelectronic properties, offers a novel platform for studying twist-angle dependent excitonic physics. Herein, by surveying a range of hexagonal phase transition metal dichalcogenides (TMD) twisted homobilayers, we find that 21.8 ± 1.0°-twisted () and 27.8 ± 1.0°-twisted () bilayers account for nearly 20% of the total population of twisted bilayers in solution-phase restacked bilayers and can be found also in chemical vapor deposition (CVD) samples. Examining the optical properties associated with these twisted angles, we found that 21.8 ± 1.0° twisted MoS2 bilayers exhibit an intense moiré exciton peak in the photoluminescence (PL) spectra, originating from the refolded Brillouin zones. Our work suggests that commensurately twisted TMD homobilayers with short commensurate wavelengths can have interesting optoelectronic properties that are different from the small twist angle counterparts.

Prospects and challenges of quantum emitters in 2D materials

The search for an ideal single-photon source has generated significant interest in discovering emitters in materials as well as developing new manipulation techniques to gain better control over the emitters' properties. Quantum emitters in atomically thin two-dimensional (2D) materials have proven to be very attractive with high brightness, operation under ambient conditions, and the ability to be integrated with a wide range of electronic and photonic platforms. This Perspective highlights some of the recent advances in quantum light generation from 2D materials, focusing on hexagonal boron nitride and transition metal dichalcogenides. Efforts in engineering and deterministically creating arrays of quantum emitters in 2D materials, their electrical excitation, and their integration with photonic devices are discussed. Finally, we address some of the challenges the field is facing and the near-term efforts to tackle them. We provide an outlook toward efficient and scalable quantum light generation from 2D materials to controllable and addressable on-chip quantum sources.

Structures of bulk hexagonal post transition metal chalcogenides from dispersion-corrected density functional theory

We use dispersion-corrected density-functional theory to determine the relative energies of competing polytypes of bulk layered hexagonal post-transition-metal chalcogenides, to search for the most stable structures of these potentially technologically important semiconductors. We show that there is some degree of consensus among dispersion-corrected exchange-correlation functionals regarding the energetic orderings of polytypes, but we find that for each material there are multiple stacking orders with relative energies of less than 1 meV per monolayer unit cell, implying that stacking faults are expected to be abundant in all post-transition-metal chalcogenides. By fitting a simple model to all our energy data, we predict that the most stable hexagonal structure has P$6_3$/mmc space group in each case, but that the stacking order differs between GaS, GaSe, GaTe, and InS on the one hand and InSe and InTe on the other. At zero pressure, the relative energies obtained with different functionals disagree by around 1-5 meV per monolayer unit cell, which is not sufficient to identify the most stable structure unambiguously; however, multi-GPa pressures reduce the number of competing phases significantly. At higher pressures, an AB$'$-stacked structure of the most stable monolayer polytype is found to be the most stable bulk structure; this structure has not been reported in experiments thus far.

Stacking Relations and Substrate Interaction of Graphene on Copper Foil

AbstractThe crystallinity of graphene flakes and their orientation with respect to the Cu(111) substrate are investigated by means of low‐energy electron microscopy (LEEM). The interplay between graphene and the metal substrate during chemical vapor deposition (CVD) introduces a restructuring of the metal surface into surface facets, which undergo a step bunching process during the growth of additional layers. Moreover, the surface facets introduce strain between the successively nucleated layers that follow the topography in a carpet‐like fashion. The strain leads to dislocations in between domains of relaxed Bernal stacking. After the transfer onto an epitaxial buffer layer, the imprinted rippled structure of even monolayer graphene as well as the stacking dislocations are preserved. A similar behavior might also be expected for other CVD grown 2D materials such as hexagonal boron nitride or transition metal dichalcogenides, where stacking relations after transfer on a target substrate or heterostructure could become important in future experiments.

(Invited) Determining Dielectric, Electronic, Magnetic, and Structural Properties of Van Der Waals Materials from First Principles

Van der Waals materials have recently received great interest in the research community because of the ability to exfoliate and grow mono- or few-layer materials. Some important future possible applications are found in the field of nanoelectronics where extremely scaled transistors or magnetic memory devices are envisioned. However, there are more than 1,000 van der Waals materials known to date and an exhaustive experimental investigation is very expensive. This makes the accurate theoretical prediction of materials properties from first principles, enabling the identification of the most promising van der Waals materials possible, very valuable.We show how the dielectric, electronic, magnetic, and structural properties of various van der Waals materials can be determined from first principles. We exhibit our robust methodology to extract the dielectric response of a monolayer/few-layer structure and calculate the low- and high-frequency dielectric constant of hexagonal boron-nitride and transition metal dichalcogenides (TMDs). We reveal a relatively small dependence of the dielectric constant on the number of layers. Surprisingly, we find a very large ionic contribution the dielectric response in TMDs in the tetragonal phase.[1] We also show how elemental tellurium is a promising material for extremely scaled nanowires and a possible successor for silicon. We predict that when fabricated with extremely small diameter, tellurium nanowires have a bandgap that exceeds 1 eV, while silicon’s bandgap increases excessively to above 4 eV.[2] We also show how the van der Waals bonds in tellurium naturally favor the formation of nanowires. Finally, we show how the Curie temperature can be computed in dilute doped TMDs from first principles, taking into account crystal anisotropy.[1] A. Laturia, M. L. Van de Put, and W. G. Vandenberghe, "Dielectric properties of hexagonal boron nitride and transition metal dichalcogenides: from monolayer to bulk," npj 2D Materials and Applications, vol. 2, no. 1, p. 6, 2018/03/08 2018.[2] A. Kramer, M. L. Van de Put, C. L. Hinkle, and W. G. Vandenberghe, "Trigonal Tellurium Nanostructure Formation Energy and Band gap," in 2019 International Conference on Simulation of Semiconductor Processes and Devices (SISPAD), 2019, pp. 1-4: IEEE.

Anisotropy of resistivity in hexagonal late transition metals and their alloys

Transport properties of hexagonal transition metals Co, Ru, and Os at finite temperatures are studied by means of ab initio electronic structure techniques and the Kubo linear response theory. An alloy analogy model for a quantitative treatment of the electrical conductivities due to temperature-induced lattice vibrations (phonons) and spin fluctuations is applied with focus on anisotropy induced by the hexagonal structure. The resistivity anisotropy in Co is found opposite to that in Ru and Os, in agreement with existing experimental data. This result is ascribed to the strong itinerant ferromagnetism of Co which leads to profound differences in the electronic structure and conductivities in the majority and minority spin channels. A similar sensitivity to spin polarization is predicted for the anisotropy of residual resistivity in random hexagonal Co-rich Co–Ni and Co–Ni–Fe alloys.