Monolayer 1T Research Articles

Homogeneous Mo1−xRexS2 monolayer and lateral 1T’@2H heterostructures for significantly enhanced and highly reproducible SERS responses and mechanistic insights

As a promising surface-enhanced Raman scattering (SERS) substrate, two-dimensional (2D) nanomaterials ensure the uniformity and reproducibility of the SERS response. However, their extremely flat surfaces are not beneficial for high-capacity binding of analytes and significant enhancement of Raman signals. In this contribution, monolayer Mo0.27Re0.73S2 alloys with distorted lattice and modulated optoelectronic structure are facilely fabricated by space-confined chemical vapor deposition (CVD), exhibiting a significantly enhanced SERS performance with an enhancement factor of 2.0 × 1011. As demonstrated, the continuous increment of Re ratio results in a phase transition from 2H to 1T’ for homogeneous Mo1−xRexS2 monolayers. As a result, the 1T’-phase Mo0.27Re0.73S2 alloys achieve a sensitive detection of down to 10−15 M for Rhodamine 6G while the value on MoS2 or ReS2 is only 10−9 M. Additionally, the alloy-based SERS substrate demonstrates excellent SERS detection reproducibility, long-term stability, and universality. Referring to the space-confined CVD, a dynamics-controlled process is further employed for the in-situ fabrication of 1T’@2H Mo1−xRexS2 heterostructures, providing a new approach and insight for facilely building varied phases-based lateral heterostructures by the control of composition ratio. Overall, this work demonstrates the enormous potential of alloy engineering in improving the SERS performance of TMDs and the convenient fabrication of out-of-phase TMD alloys.

Helicity-Sensitive Terahertz Detection in Monolayer 1T'-WTe2.

Valleytronics, identified as electronic properties of the energy band extrema in momentum space, has been intensively revived following the emergence of two-dimensional transition metal dichalcogenides (TMDCs) as their valley information can be controlled and probed through the spin angular momentum of light. Previous optical investigations of valleytronics have been limited to the visible/near-infrared spectral regime through which the carriers of most TMDCs can be excited. Monolayer 1T'-WTe2 with broken time-reversal symmetry provides a fertile platform to study the long-wavelength photonic properties in different valleys. Here, we employed a circularly polarized terahertz (THz) laser to selectively excite the valley of monolayer 1T'-WTe2 and demonstrate that the helicity-dependent photoresponse is generated via the photogalvanic effect (PGE). We also observed that the photocurrent is controlled by circular polarization and the external electric field. Because of the tunable Berry curvature dipole derived from the nontrivial wave functions near the inverted gap edge in monolayer WTe2, the bandgap can be tuned efficiently. Our results provide a versatile venue for controlling, detecting, and processing valleytronics and applications in on-chip THz imaging and quantum information processing.

A first-principles study of bilayer 1T'-WTe2/CrI3: a candidate topological spin filter

The ability to manipulate electronic spin channels in 2D materials is crucial for realizing next-generation spintronics. Spin filters are spintronic components that polarize spins using external electromagnetic fields or intrinsic material properties like magnetism. Recently, topological protection from backscattering has emerged as an enticing feature that can be leveraged to enhance the robustness of 2D spin filters. In this work, we propose and then characterize one of the first 2D topological spin filters: bilayer CrI3/1T’-WTe2. To do so, we use a combination of density functional theory, maximally localized Wannier functions, and quantum transport calculations to demonstrate that a terraced bilayer satisfies the principal criteria for being a topological spin filter: namely, that it is gapless, exhibits spin-polarized charge transfer from WTe2 to CrI3 that renders the bilayer metallic, and has a topological boundary which retains the edge conductance of monolayer 1T’-WTe2. In particular, we observe that small negative ferromagnetic moments are induced on the W atoms in the bilayer, and the atomic magnetic moments on the Cr are approximately 3.2 μB/Cr compared to 2.9 μB/Cr in freestanding monolayer CrI3. Subtracting the charge and spin densities of the constituent monolayers from those of the bilayer further reveals spin-orbit coupling-enhanced spin-polarized charge transfer from WTe2 to CrI3. We demonstrate that the bilayer is topologically trivial by showing that its Chern number is zero. Lastly, we show that interfacial scattering at the boundary between the terraced materials does not remove WTe2’s edge conductance. Altogether, this evidence indicates that BL 1T’-WTe2/CrI3 is gapless, magnetic, and topologically trivial, meaning that a terraced WTe2/CrI3 bilayer heterostructure in which only a portion of a WTe2 monolayer is topped with CrI3 is a promising candidate for a 2D topological spin filter. Our results further suggest that 1D chiral edge states may be realized by stacking strongly ferromagnetic monolayers, like CrI3, atop 2D nonmagnetic Weyl semimetals like 1T’-WTe2.

Novel Janus monolayer 1T'-MoSF features robust stability and superior mechanical flexibility

Janus monolayers of transition-metal dichalcogenides (TMDs) have garnered significant attention in various scientific fields due to their exceptional physical properties. Among the known Janus monolayer TMDs, MoSX holds particular significance, with X typically representing chalcogen atoms. The successful synthesis of MoSH has pushed the boundaries of X beyond the conventional chalcogens. Nevertheless, it remains an open question whether other elements can also yield stable Janus monolayer MoSX. Herein, we employ theoretical methods to undertake a comprehensive evaluation of the stability of Janus monolayer MoSX (X = Li, Be, B, C, N, F, Na, Mg, Al, Si, P, Cl). The analysis of stability encompasses various aspects, including energetic, mechanical, dynamical, and thermodynamic features. Interestingly, our investigations have revealed the existence of a hitherto undiscovered stable 1T'-MoSF structure. Remarkably, this structure exhibits significant anisotropy, providing superior mechanical flexibility due to its low Young’s modulus. In addition, despite the dynamical instability of 2H/1T-MoSN and the mechanical instability of 2H/1T-MoSC/Mg, they could potentially exist under specific conditions. This theoretical study not only provides insights into the structural characteristics and stability of novel Janus monolayer MoSX, but also extends the Janus monolayer MoSX family, which would be valuable to future applications of Janus TMDs.

Understanding the importance of four-phonon scattering in low-symmetry monolayer 1T′-ReS2 using machine learning potential

The importance of higher-order anharmonic effects on thermal transport has recently been demonstrated in highly symmetrical 2D materials with large acoustic–phonon (A–O) gap. However, the phonon scattering and the thermal transport properties in low-symmetry structures remain ambiguous. In this work, we employed moment tensor potential and Boltzmann transport equation to investigate phonon thermal transport properties of 1T′-ReS2 and 2H-WS2. We show that the mechanism of four-phonon scattering in 1T′-ReS2 is quite different from that in 2H-WS2. In 1T′-ReS2, the four-phonon scattering can reduce the thermal conductivity by up to 40.29%, even in the absence of an acoustic-optical phonon bandgap. The strong four-phonon scattering in 1T′-ReS2 is attributed to A–O phonon scattering mediated by abundant flattening optical phonon modes. However, in 2H-WS2, the strong four-phonon scattering is attributed to the presence of a large A–O phonon bandgap. Our work suggests that considering the four-phonon scattering is essential for calculating the thermal conductivity of 2D materials, even in the absence of an A–O phonon gap.

Gate tunable linear dichroism in monolayer 1T'-MoS2.

The linear dichroism demonstrates promising applications in the fields of polarization-resolved photodetectors and polarization optical imaging. Herein, we study the optical properties of monolayer 1T'-MoS2 based on a four-band effective k · p Hamiltonian within the framework of linear response theory. Owing to the anisotropic band structure, the k-resolved optical transition matrix elements associated with armchair(x) and zigzag(y) direction polarized light exhibit a staggered pattern. The anisotropy of the optical absorption spectrum is shown to sensitively depend on the photon energy, the light polarization and the gate voltage. A gate voltage can continuously modulate the anisotropy of the optical absorption spectra, rendering it isotropic or even reversing the initial anisotropy. This modulation leads to linear dichroism conversions across multiple wavelengths. Our findings are useful to design polarized photodetectors and sensors based on monolayer 1T'-MoS2. Our results are also applicable to other monolayer transition metal dichalcogenides with 1T' structure.

First-principles prediction of superconducting properties of monolayer 1T'-WS2 under biaxial tensile strain.

High-purity 1T'-WS2 film has been experimentally synthesized [Nature Materials, 20, 1113-1120 (2021)] and theoretically predicted to be a two-dimensional (2D) superconducting material with Dirac cones [arXiv:2301.11425]. In the present work, we further study the superconducting properties of monolayer 1T'-WS2 by applying biaxial tensile strain. It is shown that the superconducting critical temperature Tc firstly increases and then decreases with respect to tensile strains, with the highest superconducting critical temperature Tc of 7.25 K under the biaxial tensile strain of 3%. In particular, we find that Dirac cones also exist in several tensile strained cases. Our studies show that monolayer 1T'-WS2 may provide a good platform for understanding the superconductivity of 2D Dirac materials.

Microscopic growth mechanism and edge states of monolayer 1T′-MoTe2

Transition metal ditellurides (TMTDs) have versatile physical properties, including non-trivial topology, Weyl semimetal states and unique spin texture. Controlled growth of high-quality and large-scale monolayer TMTDs with preferred crystal phases is crucial for their applications. Here, we demonstrate the epitaxial growth of 1T′-MoTe2 on Au (111) and graphitized silicon carbide (Gr/SiC) by molecular beam epitaxy (MBE). We investigate the morphology of the grown 1T′-MoTe2 at the atomic level by scanning tunnelling microscopy (STM) and reveal the corresponding microscopic growth mechanism. It is found that the unique ordered Te structures preferentially deposited on Au (111) regulate the growth of monolayer single crystal 1T′-MoTe2, while the Mo clusters were preferentially deposited on the Gr/SiC substrate, which impedes the ordered growth of monolayer MoTe2. We confirm that the size of single crystal 1T′-MoTe2 grown on Au (111) is nearly two orders of magnitude larger than that on Gr/SiC. By scanning tunnelling spectroscopy (STS), we observe that the STS spectrum of the monolayer 1T′-MoTe2 nano-island at the edge is different from that at the interior, which exhibits enhanced conductivity.

Observation of an abnormally large bandgap in monolayer 1T′-WS2 grown on SrTiO3(001)

The 1T′ structural phase of monolayer transition metal dichalcogenides MX2 (M = Mo, W; X = S, Se, Te) has attracted broad interest because of an exotic quantum spin Hall insulator state. Among them, the investigation on the electronic structures of the 1T′-WS2 monolayer is still lacking due to the difficulty in obtaining the 1T′-WS2 as a metastable phase. Here, we report the growth of 1T′ phase WS2 monolayer on the SrTiO3 (001) substrate using molecular beam epitaxy. Surprisingly, a large bandgap of 0.65 eV is revealed by angle-resolved photoemission spectroscopy and scanning tunneling spectroscopy, which is abnormally larger than the theoretical expectance. Moreover, an additional near-flatband emerges in the grown monolayer 1T′-WS2. The abnormally large bandgap and the emerging near-flatband are suggested to be the effect of interfacial interactions. Our findings provide important information about electronic structures and the interfacial effect of the epitaxial 1T′-WS2 monolayer on the SrTiO3(001) substrate and would stimulate future theoretical and experimental research on the 1T′-MX2 family.

Interface-enhanced superconductivity in monolayer 1T′-MoTe2 on SrTiO3(001)

Introducing superconductivity into two-dimensional (2D) films with nontrivial topology has been intensively pursued as one of the feasible scenarios to realize 1D topological superconductor. Prevailing endeavors mostly exploit the external gating or proximity effect of a traditional superconductor, by which the critical temperatures (T_{mathrm{c}}) are limited to several Kelvin range. Here, we report on the discovery of interface-enhanced superconductivity in monolayer 1T′-MoTe2 film. A thermally driven phase transition from Mo6Te6 nanowires to 1T′-MoTe2 films, grown on SrTiO3(001) surface by the molecular beam epitaxial methods, is demonstrated. A combined study of scanning tunneling microscopy/spectroscopy, electrical transport and magnetization measurements indicates the T_{mathrm{c}} of MoTe2 film is around 30 K, two orders of magnitude larger than its 3D counterpart crystal. This study shows that interfacial engineering is an efficient way to tune monolayer 1T′-MoTe2 film into superconducting states, and thus may pave the way toward higher-T_{mathrm{c}} 1D intrinsic topological superconductivity.

Tight-binding model and quantum transport with disorder for 1T’ transition metal dichalcogenides

We present a simplified tight-binding (TB) model to describe the low-energy physics of monolayer 1T’ transition metal dichalcogenides (TMDCs). The TB model is constructed by combining symmetry analysis and first-principle calculations. Our TB model accurately reproduces the electronic structures near the Fermi energy and provides a better representation of energy band inversion. By considering spin–orbit coupling (SOC), our TB model successfully reproduces the opening of the bandgap, characterizes nontrivial topology, and predicts corresponding helical edge states. Additionally, using this TB model, we observe that quantized electronic conductance remains robust under significant disorder intensity. However, the robustness of the edge states can be suppressed by the Zeeman fields and SOC strength in the scattering zone. Furthermore, moderate concentrations of vacancy disorder destroy the topological protection and eliminate quantized conductance. Our TB model serves as a starting point for a comprehensive understanding of the properties of these materials and can guide future research on superconductivity, strain, and correlation effects.

Ambipolar Superconductivity with Strong Pairing Interaction in Monolayer 1T'-MoTe2.

Gate tunable two-dimensional (2D) superconductors offer significant advantages in studying superconducting phase transitions. Here, we address superconductivity in exfoliated 1T'-MoTe2 monolayers with an intrinsic band gap of ∼7.3 meV using field effect doping. Despite large differences in the dispersion of the conduction and valence bands, superconductivity can be achieved easily for both electrons and holes. The onset of superconductivity occurs near 7-8 K for both charge carrier types. This temperature is much higher than that in bulk samples. Also the in-plane upper critical field is strongly enhanced and exceeds the BCS Pauli limit in both cases. Gap information is extracted using point-contact spectroscopy. The gap ratio exceeds multiple times the value expected for BCS weak-coupling. All of these observations suggest a strong enhancement of the pairing interaction.

Prediction of mechanical, electronic and optical properties of monolayer 1T Si-dichalcogenides via first-principles theory

This study utilizes first-principles calculations to predict the mechanical, electronic, and optical properties of 1 T Si-dichalcogenides (SiX2; X = S, Se, and Te). The results of dynamic and static stability calculations demonstrate the high stability of all three structure. Due to their mechanical flexibility, 1 T- SiX2 structures can undergo strain quite well compared to other 2D structures. Furthermore, we employ a hybrid functional method to investigate the materials’ energy band structures and electronic configuration. Our findings indicate that while 1 T-SiS2 and 1 T-SiSe2 structures are semiconductors and their electronic properties can be controlled through mechanical strain, while 1 T-SiTe2 expresses as a metallic material at all states. The high absorption light coefficient of 1 T-SiX2 indicates polarization in the ultraviolet and sensitivity under strain, revealing the potential for application in nanoelectronics and optical devices. Overall, the study enhances our understanding of the properties of 1 T-SnX2 structures and highlights their potential for future technological applications.

Topology Hierarchy of Transition Metal Dichalcogenides Built from Quantum Spin Hall Layers.

The evolution of the physical properties of two-dimensional material from monolayer limit to the bulk reveals unique consequences from dimension confinement and provides a distinct tuning knob for applications. Monolayer 1T'-phase transition metal dichalcogenides (1T'-TMDs) with ubiquitous quantum spin Hall (QSH) states are ideal two-dimensional building blocks of various three-dimensional topological phases. However, the stacking geometry was previously limited to the bulk 1T'-WTe2 type. Here, we introduce the novel 2M-TMDs consisting of translationally stacked 1T'-monolayers as promising material platforms with tunable inverted bandgaps and interlayer coupling. By performing advanced polarization-dependent angle-resolved photoemission spectroscopy as well as first-principles calculations on the electronic structure of 2M-TMDs, we revealed a topology hierarchy: 2M-WSe2 , MoS2, and MoSe2 are weak topological insulators (WTIs), whereas 2M-WS2 is a strong topological insulator (STI). Further demonstration of topological phase transitions by tunning interlayer distance indicates that band inversion amplitude and interlayer coupling jointly determine different topological states in 2M-TMDs. We propose that 2M-TMDs are parent compounds of various exotic phases including topological superconductors and promise great application potentials in quantum electronics due to their flexibility in patterning with two-dimensional materials. This article is protected by copyright. All rights reserved.

Strain induced effects on the electronic and phononic properties of 2H and 1T[formula omitted] monolayer MoS2

First-principles calculations have been performed on the 2H and the 1T′ phase of single-layer MoS2. We have addressed the strain-induced tunability of the electronic and the phononic properties of both phases, and compared their stability. For the 2H phase, a direct to indirect band gap transition, followed by the band gap lowering leading to a semiconductor to metal transition, is observed. The applied strain destroys the semimetallic nature of the 1T′ phase. Significant strain-induced changes in the frequency and the dispersion of the phonon branches are observed. With increasing strain, the out-of-plane acoustic mode (ZA) turns imaginary, suggesting a possibility of phase transition or instability of the crystal structure. The 2H phase appears to withstand a larger amount of strain indicating better stability compared to the 1T′ phase. We also highlight the significance of anisotropic strain-engineering in tuning the electronic and phononic properties and the stability limit of monolayer 1T′-MoS2.

Quantum Spin Hall States in 2D Monolayer WTe2/MoTe2 Lateral Heterojunctions for Topological Quantum Computation

The quantum spin Hall (QSH) states in two-dimensional topological insulators (2DTIs) are expected to be applied to future topological quantum computation. We investigate the two-dimensional (2D) lateral heterojunctions of the monolayer 1T′–WTe2 as a 2DTI and the monolayer 2H–MoTe2 as a topologically trivial insulator using density functional theory. This 2D material is expected to have QSH states at each periodically arranged junction as well as properties distinct from the individual properties of each constituent material. At heterojunctions perpendicular to the dimer chains of W atoms in 1T′–WTe2 (in the y direction), two pairs of helical (QSH) states, one at each junction, connect the valence and conduction bands. The strain induced by the large lattice mismatch of the two materials in the y direction widens the bandgap of the 1T′–WTe2 monolayer as a QSH insulator. In the case of the heterojunctions in the x direction, the difference in atomic structure between the two junctions due to low symmetry creates an energy difference between two helical states and a potential gradient in the wide-bandgap 2H–MoTe2 region, resulting in various junction-localized bands. The widening bandgap of the heterojunctions in the y direction is essential for electronic applications of the QSH states, suggesting that this 2D material, namely, 2D WTe2/MoTe2 heterojunctions, can be a promising candidate for integrating Majorana qubits for future topological quantum computation.

Evolution of local edge state braiding and spin topological transport characterization of Te-doped monolayer 1T'-MoS2.

We conducted first-principles calculations to investigate the dynamic braiding of local edge states and the spin topological transport mechanism in a strong topological MoS1.75Te0.25 matrix. The presence of type-II Van Hove singularity in the middle of the X-S path indicates a strong cohesive interaction and a paring condensation mechanism within the matrix. The surface state data of MoS1.75Te0.25 clearly demonstrate the characteristic features of strong regular loop braiding in spin transport. The spin Hall conductivity of the matrix was determined from the anisotropic characteristics of the spin Berry curvature. The phase transition of the spin Hall conductivity was evidenced by the positive sign of local spin polarization strength, primarily contributed by the dz2 orbital of Mo atoms, and the negative sign of spin polarization strength, mainly contributed by the p-px orbitals of S atoms. Moreover, the inclusion of Te selectively tuned the spin transport efficiency of the dz2 and px orbitals. Comprehensive braiding and readout of edge states can be achieved using an artificially designed MoS1.75Te0.25 spintronic device. This 2D fractional braiding holds significant potential for applications in topological quantum computation.

Direct Observation of the Topological Surface State in the Topological Superconductor 2M-WS2.

The quantum spin Hall (QSH) effect has attracted extensive research interest because of the potential applications in spintronics and quantum computing, which is attributable to two conducting edge channels with opposite spin polarization and the quantized electronic conductance of 2e2/h. Recently, 2M-WS2, a new stable phase of transition metal dichalcogenides with a 2M structure showing a layer configuration identical to that of the monolayer 1T' TMDs, was suggested to be a QSH insulator as well as a superconductor with a critical transition temperature of around 8 K. Here, high-resolution angle-resolved photoemission spectroscopy (ARPES) and spin-resolved ARPES are applied to investigate the electronic and spin structure of the topological surface states (TSS) in the superconducting 2M-WS2. The TSS exhibit characteristic spin-momentum-locking behavior, suggesting the existence of long-sought nontrivial topological states therein. We expect that 2M-WS2 with coexisting superconductivity and TSS might host the promising Majorana bound states.

Electrical control of spin-polarized topological currents in monolayer WTe2

We evidence the possibility for coherent electrical manipulation of the spin orientation of topologically protected edge states in a low-symmetry quantum spin Hall insulator. By using a combination of ab initio simulations, symmetry-based modeling, and large-scale calculations of the spin Hall conductivity, it is shown that small electric fields can efficiently vary the spin textures of edge currents in monolayer 1T'-${\mathrm{WTe}}_{2}$ by up to a 90-degree spin rotation, without jeopardizing their topological character. These findings suggest a new kind of gate-controllable spin-based device, topologically protected against disorder and of relevance for the development of topological spintronics.

Strain-induced two-dimensional topological insulators in monolayer 1T′-RuO2

Because of their unique structure and novel physical properties, two-dimensional (2D) transition metal dichalcogenides (TMDs) have received a lot of attention in recent years. In this paper, we propose a new 2D TMD 1T′-RuO2 with tunable topological properties. Based on first-principles calculations, we demonstrate that it has good dynamics, thermodynamic, energetic stability, and anisotropic mechanical properties. Although 1T′-RuO2 is a typical semiconductor with a direct bandgap, it can be transformed into topological insulator by applying uniaxial tensile strains. The topological phase transition is attributed to the d–d band inversion at Γ point. The nontrivial topological property is further validated by the topological edge states. We predict that monolayer 1T′-RuO2 is an excellent material for future electronic devices with tunable topological properties.