Observation of Coulomb gap in the quantum spin Hall candidate single-layer 1T'-WTe2

Direct Visualization of Metal-Induced Gap State Distribution and Valley Band Evolution at Metal Versus Semimetal MoS2 Interfaces

The interlayer coupling between metals and the two-dimensional (2D) semiconductors' conduction band (CB), encompassing metal-induced gap states (MIGS) and valley band modulation, critically influences both the Schottky barrier height (SBH) and intrinsic sheet resistance. Understanding the CB modulation induced by metals/semimetals is, therefore, essential for contact engineering optimization. Given that the MIGS decay length and orbital interactions are spatially confined to the nanoscale region proximate to the 2D semiconductor interface, we employed scanning tunneling microscopy/spectroscopy to quantitatively determine the MIGS decay length and CB minimum on various metal/semimetal substrates. This approach enabled the comprehensive characterization of MIGS distribution, charge neutrality level variation, and SBH properties. Our findings demonstrate that maintaining valley band structure integrity during semimetal interlayer coupling facilitates reduced intrinsic sheet resistance. These results elucidate the mechanism underlying weak interlayer coupling at semimetal-2D semiconductor junctions and their superior contact transport performance, providing insights into the rational design of future 2D-based devices.

A Mixed-Valence and Mixed-Spin Two-Dimensional Ferromagnetic Metal-Organic Coordination Framework

Spin-mixed systems with distinct magnetic sublattices present rich physical behaviors and hold promise for magnetic memory, thermomagnetic recording, and optoelectronics. However, most experimental studies remain confined to molecular magnetic salts rather than monolayer two-dimensional (2D) systems. Here, we report the synthesis and characterization of a 2D metal-organic framework (MOF) of Fe2(Fe-DPyP)3, constructed from 5,15-di(4-pyridyl)-10,20-diphenylporphyrin (DPyP) molecules and iron atoms on a Au(111) substrate. Through scanning tunneling microscopy and density functional theory (DFT) calculations, we reveal dual coordination modes: (1) peripheral iron atoms (Fec) coordinate with the pyridyl substituents, forming a honeycomb lattice, and (2) central iron atoms (Fem) bind within the porphyrin core, creating a kagome lattice. Scanning tunneling spectroscopy highlights distinct spin-flip excitations in the bivalent Fem atoms with a magnetic spin state of S = 1, while the monovalent Fec atoms display a V-shaped dip around the Fermi level, attributed to the quenched spin excitations or a soft Coulomb gap, with a magnetic spin state of S = 3/2. DFT calculations reveal an in-plane ferromagnetic ground state with spin-polarized Fe d orbitals and molecular p orbitals. This work contributes to the understanding of mixed-valence and mixed-spin 2D coordination networks with implications for the development of next-generation quantum materials and spintronic devices.

Realization of fractional-layer transition metal dichalcogenides

Layered van der Waals transition metal dichalcogenides (TMDCs), generally composed of three atomic X-M-X planes in each layer (M = transition metal, X = chalcogen), provide versatile platforms for exploring diverse quantum phenomena. In each MX2 layer, the M-X bonds are predominantly covalent in nature and, as a result, the cleavage of TMDC crystals normally occurs between the layers. Here we report the controllable realization of fractional-layer WTe2 via an in-situ scanning tunneling microscopy (STM) tip manipulation technique. By applying STM tip pulses, hundreds of the topmost Te atoms are removed to form a nanoscale monolayer Te pit in the 1 T'-WTe2, thus realizing a 2/3-layer WTe2 film. Such a configuration undergoes a spontaneous atomic reconstruction, yielding a unidirectional charge density redistribution with the wavevector and geometry quite distinct from that of pristine 1 T'-WTe2. Our results expand the conventional understanding of the TMDCs and are expected to stimulate further research on the structure and properties of fractional-layer TMDCs.

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.

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.

Intrinsic Grain Boundary Structure and Enhanced Defect States in Air-Sensitive Polycrystalline 1T'-WTe2 Monolayer

Monolayer WTe2 has attracted significant attention for its unconventional superconductivity and topological edge states. However, its air sensitivity poses challenges for studying intrinsic defect structures. This study addresses this issue using a custom-built inert gas interconnected system, and investigate the intrinsic grain boundary (GB) structures of monolayer polycrystalline 1T' WTe2 grown by nucleation-controlled chemical vapor deposition (CVD) method. These findings reveal that GBs in this system are predominantly governed by W-Te rhombi with saturated coordination, resulting in three specific GB prototypes without dislocation cores. The GBs exhibit anisotropic orientations influenced by kinks formed from these fundamental units, which in turn affect the distribution of grains in various shapes within polycrystalline flakes. Scanning tunneling microscopy/spectroscopy (STM/S) analysis further reveals metallic states along the intrinsic 120° twin grain boundary (TGB), consistent with computed band structures. This systematic exploration of GBs in air-sensitive 1T' WTe2 monolayers provides valuable insights into emerging GB-related phenomena.

Evolution of Metal Tellurides for Energy Storage/Conversion: From Synthesis to Applications

Metal telluride (MTe)-based nanomaterials have emerged as a potential alternative for efficient, highly conductive, robust, and durable electrodes in energy storage/conversion applications. Significant progress in the material development of MTe-based electrodes is well-sought, from the synthesis of its nanostructures, integration of MTes with supporting materials, synthesis of their hybrid morphologies, and their implications in energy storage/conversion systems. Herein, an extensive exploration of the recent advancements and progress in MTes-based nanomaterials is reviewed. This review emphasizes elucidating the fundamental properties of MTes and providing a systematic compilation of its wet and dry synthesis methods. The applications of MTes are extensively summarized and discussed, particularly, in energy storage and conversion systems including batteries (Li-ion, Zn-ion, Li-S, Na-ion, K-ion), supercapacitor, hydrogen evolution reaction (HER), oxygen evolution reaction (OER), oxygen reduction reaction (ORR), and CO2 reduction. The review also emphasizes the future prospects and urgent challenges to be addressed in the development of MTes, providing knowledge for researchers in utilizing MTes in energy storage and conversion technologies.

Realization of monolayer ZrTe5 topological insulators with wide band gaps

Two-dimensional topological insulators hosting the quantum spin Hall effect have application potential in dissipationless electronics. To observe the quantum spin Hall effect at elevated temperatures, a wide band gap is indispensable to efficiently suppress bulk conduction. Yet, most candidate materials exhibit narrow or even negative band gaps. Here, via elegant control of van der Waals epitaxy, we have successfully grown monolayer ZrTe5 on a bilayer graphene/SiC substrate. The epitaxial ZrTe5 monolayer crystalizes in two allotrope isomers with different intralayer alignments of ZrTe3 prisms. Our scanning tunneling microscopy/spectroscopy characterization unveils an intrinsic full band gap as large as 254 meV and one-dimensional edge states localized along the periphery of the ZrTe5 monolayer. First-principles calculations further confirm that the large band gap originates from strong spin-orbit coupling, and the edge states are topologically nontrivial. These findings thus provide a highly desirable material platform for the exploration of the high-temperature quantum spin Hall effect.

Electron Glass Phase with Resilient Zhang-Rice Singlets in LiCu_{3}O_{3}

LiCu_{3}O_{3} is an antiferromagnetic mixed valence cuprate where trilayers of edge-sharing Cu(II)O (3d^{9}) are sandwiched in between planes of Cu(I) (3d^{10}) ions, with Li stochastically substituting Cu(II). Angle-resolved photoemission spectroscopy (ARPES) and density functional theory reveal two insulating electronic subsystems that are segregated in spite of sharing common oxygen atoms: a Cu d_{z^{2}}/O p_{z} derived valence band (VB) dispersing on the Cu(I) plane, and a Cu 3d_{x^{2}-y^{2}}/O 2p_{x,y} derived Zhang-Rice singlet (ZRS) band dispersing on the Cu(II)O planes. First-principle analysis shows the Li substitution to stabilize the insulating ground state, but only if antiferromagnetic correlations are present. Li further induces substitutional disorder and a 2D electron glass behavior in charge transport, reflected in a large 530 meV Coulomb gap and a linear suppression of VB spectral weight at E_{F} that is observed by ARPES. Surprisingly, the disorder leaves the Cu(II)-derived ZRS largely unaffected. This indicates a local segregation of Li and Cu atoms onto the two separate corner-sharing Cu(II)O_{2} sub-lattices of the edge-sharing Cu(II)O planes, and highlights the ubiquitous resilience of the entangled two hole ZRS entity against impurity scattering.

A Gate-Tunable Ambipolar Quantum Phase Transition in a Topological Excitonic Insulator

Coulomb interactions among electrons and holes in 2D semimetals with overlapping valence and conduction bands can give rise to a correlated insulating ground state via exciton formation and condensation. One candidate material in which such excitonic state uniquely combines with non-trivial band topology are atomic monolayers of tungsten ditelluride (WTe2 ), in which a 2D topological excitonic insulator (2D TEI) forms. However, the detailed mechanism of the 2D bulk gap formation in WTe2 , in particular with regard to the role of Coulomb interactions, has remained a subject of ongoing debate. Here, it shows that WTe2 is susceptible to a gate-tunable quantum phase transition, evident from an abrupt collapse of its 2D bulk energy gap upon ambipolar field-effect doping. Such gate tunability of a 2D TEI, into either n- and p-type semimetals, promises novel handles of control over non-trivial 2D superconductivity with excitonic pairing.

Room-Temperature Ferromagnetism in Epitaxial Bilayer FeSb/SrTiO3(001) Terminated with a Kagome Lattice

Two-dimensional (2D) magnets exhibit unique physical properties for potential applications in spintronics. To date, most 2D ferromagnets are obtained by mechanical exfoliation of bulk materials with van der Waals interlayer interactions, and the synthesis of single- or few-layer 2D ferromagnets with strong interlayer coupling remains experimentally challenging. Here, we report the epitaxial growth of 2D non-van der Waals ferromagnetic bilayer FeSb on SrTiO3(001) substrates stabilized by strong coupling to the substrate, which exhibits in-plane magnetic anisotropy and a Curie temperature above 390 K. In situ low-temperature scanning tunneling microscopy/spectroscopy and density-functional theory calculations further reveal that an Fe Kagome layer terminates the bilayer FeSb. Our results open a new avenue for further exploring emergent quantum phenomena from the interplay of ferromagnetism and topology for application in spintronics.

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.

Visualizing symmetry-breaking electronic orders in epitaxial Kagome magnet FeSn films

Kagome lattice hosts a plethora of quantum states arising from the interplay of topology, spin-orbit coupling, and electron correlations. Here, we report symmetry-breaking electronic orders tunable by an applied magnetic field in a model Kagome magnet FeSn consisting of alternating stacks of two-dimensional Fe3Sn Kagome and Sn2 honeycomb layers. On the Fe3Sn layer terminated FeSn thin films epitaxially grown on SrTiO3(111) substrates, we observe trimerization of the Kagome lattice using scanning tunneling microscopy/spectroscopy, breaking its six-fold rotational symmetry while preserving the translational symmetry. Such a trimerized Kagome lattice shows an energy-dependent contrast reversal in dI/dV maps, which is significantly enhanced by bound states induced by Sn vacancy defects. This trimerized Kagome lattice also exhibits stripe modulations that are energy-dependent and tunable by an applied in-plane magnetic field, indicating symmetry-breaking nematicity from the entangled magnetic and charge degrees of freedom in antiferromagnet FeSn.

Giant Periodic Pseudomagnetic Fields in Strained Kagome Magnet FeSn Epitaxial Films on SrTiO3(111) Substrate

Quantum materials, particularly Dirac materials with linearly dispersing bands, can be effectively tuned by strain-induced lattice distortions leading to a pseudomagnetic field that strongly modulates their electronic properties. Here, we grow kagome magnet FeSn films, consisting of alternatingly stacked Sn₂ honeycomb (stanene) and Fe₃Sn kagome layers, on SrTiO₃(111) substrates by molecular beam epitaxy. Using scanning tunneling microscopy/spectroscopy, we show that the Sn honeycomb layer can be periodically deformed by epitaxial strain for a film thickness below 10 nm, resulting in differential conductance peaks consistent with Landau levels generated by a pseudomagnetic field greater than 1000 T. Our findings demonstrate the feasibility of strain engineering the electronic properties of topological magnets at the nanoscale.

Molecular beam epitaxy growth and scanning tunneling microscopy study of 2D layered materials on epitaxial graphene/silicon carbide

Since the advent of atomically flat graphene, two-dimensional (2D) layered materials have gained extensive interest due to their unique properties. The 2D layered materials prepared on epitaxial graphene/silicon carbide (EG/SiC) surface by molecular beam epitaxy (MBE) have high quality, which can be directly applied without further transfer to other substrates. Scanning tunneling microscopy and spectroscopy (STM/STS) with high spatial resolution and high-energy resolution are often used to study the morphologies and electronic structures of 2D layered materials. In this review, recent progress in the preparation of various 2D layered materials that are either monoelemental or transition metal dichalcogenides on EG/SiC surface by MBE and their STM/STS investigations are introduced.

Phase control and lateral heterostructures of MoTe2 epitaxially grown on graphene/Ir(111)

Engineering the growth of the different phases of two-dimensional transition metal dichalcogenides (2D-TMDs) is a promising way to exploit their potential since the phase determines their physical and chemical properties. Here, we report on the epitaxial growth of monolayer MoTe₂ on graphene on an Ir(111) substrate. Scanning tunneling microscopy and spectroscopy provide insights into the structural and electronic properties of the different polymorphic phases, which remain decoupled from the substrate due to the weak interaction with graphene. In addition, we demonstrate a great control of the relative coverage of the relevant 1T' and 1H MoTe₂ phases by varying the substrate temperature during the growth. In particular, we obtain large areas of the 1T' phase exclusively or the coexistence of both phases with different ratios.

On-Site Synthesis and Characterizations of Atomically-Thin Nickel Tellurides with Versatile Stoichiometric Phases through Self-Intercalation

Self-intercalation of native metal atoms in two-dimensional (2D) transition metal dichalcogenides has received rapidly increasing interest, due to the generation of intriguing structures and exotic physical properties, however, only reported in limited materials systems. An emerging type-II Dirac semimetal, NiTe₂, has inspired great attention at the 2D thickness region, but has been rarely achieved so far. Herein, we report the direct synthesis of mono- to few-layer Ni-tellurides including 1T-NiTe₂ and Ni-rich stoichiometric phases on graphene/SiC(0001) substrates under ultra-high-vacuum conditions. Differing from previous chemical vapor deposition growth accompanied with transmission electron microscopy imaging, this work combines precisely tailored synthesis with on-site atomic-scale scanning tunneling microscopy observations, offering us visual information about the phase modulations of Ni-tellurides from 1T-phase NiTe₂ to self-intercalated Ni₃Te₄ and Ni₅Te₆. The synthesis of Ni self-intercalated Ni_xTe_y compounds is explained to be mediated by the high metal chemical potential under Ni-rich conditions, according to density functional theory calculations. More intriguingly, the emergence of superconductivity in bilayer NiTe₂ intercalated with 50% Ni is also predicted, arising from the enhanced electron-phonon coupling strength after the self-intercalation. This work provides insight into the direct synthesis and stoichiometric phase modulation of 2D layered materials, enriching the family of self-intercalated materials and propelling their property explorations.

Thickness and defect dependent electronic, optical and thermoelectric features of [Formula: see text]

Transition metal dichalcogenides (TMDs) receive significant attention due to their outstanding electronic and optical properties. In this study, we investigate the electronic, optical, and thermoelectric properties of single and few layer [Formula: see text] in detail utilizing first-principles methods based on the density functional theory (DFT). Within the scope of both PBE and HSE06 including spin orbit coupling (SOC), the simulations predict the electronic band gap values to decrease as the number of layers increases. Moreover, spin-polarized DFT calculations combined with the semi-classical Boltzmann transport theory are applied to estimate the anisotropic thermoelectric power factor (Seebeck coefficient, S) for [Formula: see text] in both the monolayer and multilayer limit, and S is obtained below the optimal value for practical applications. The optical absorbance of [Formula: see text] monolayer is obtained to be slightly less than the values reported in literature for 2H TMD monolayers of [Formula: see text], [Formula: see text], and [Formula: see text]. Furthermore, we simulate the impact of defects, such as vacancy, antisite and substitution defects, on the electronic, optical and thermoelectric properties of monolayer [Formula: see text]. Particularly, the Te-[Formula: see text] substitution defect in parallel orientation yields negative formation energy, indicating that the relevant defect may form spontaneously under relevant experimental conditions. We reveal that the electronic band structure of [Formula: see text] monolayer is significantly influenced by the presence of the considered defects. According to the calculated band gap values, a lowering of the conduction band minimum gives rise to metallic characteristics to the structure for the single Te(1) vacancy, a diagonal Te line defect, and the Te(1)-[Formula: see text] substitution, while the other investigated defects cause an opening of a small positive band gap at the Fermi level. Consequently, the real ([Formula: see text]) and imaginary ([Formula: see text]) parts of the dielectric constant at low frequencies are very sensitive to the applied defects, whereas we find that the absorbance (A) at optical frequencies is less significantly affected. We also predict that certain point defects can enhance the otherwise moderate value of S in pristine [Formula: see text] to values relevant for thermoelectric applications. The described [Formula: see text] monolayers, as functionalized with the considered defects, offer the possibility to be applied in optical, electronic, and thermoelectric devices.

Planar Heterojunction of Ultrathin CrTe3 and CrTe2 van der Waals Magnet

The fabrication of planar heterojunctions with magnetic van der Waals ultrathin crystals is essential for constructing miniaturized spintronic devices but is yet to be realized. Here, we report the growth of CrTe₃ and CrTe₂ ultrathin films with molecular beam epitaxy and characterize their morphological and electronic structure through low-temperature scanning tunneling microscopy/spectroscopy. The former is identified as a Mott insulator, and the latter has shown a robust magnetic order previously. Through vacuum annealing, CrTe₃ can be transformed into CrTe₂, whose relative ratio is controlled via the annealing time. This renders the feasibility of constructing CrTe₃-CrTe₂ planar heterojunctions, which express atomically sharp interfaces and smooth band bending. We also identified a superstructure conceivably formed via hybrid units of CrTe₃ and CrTe₂, whose electronic structure exhibits stunning tunability with the length of the superstructure. Our study sets a foundation for the development of magnetic tunneling junctions for building spintronic circuits and engineering electronic states in artificial superlattice structures.

Atomically Thin Quantum Spin Hall Insulators

Atomically thin topological materials are attracting growing attention for their potential to radically transform classical and quantum electronic device concepts. Among them is the quantum spin Hall (QSH) insulator-a 2D state of matter that arises from interplay of topological band inversion and strong spin-orbit coupling, with large tunable bulk bandgaps up to 800 meV and gapless, 1D edge states. Reviewing recent advances in materials science and engineering alongside theoretical description, the QSH materials library is surveyed with focus on the prospects for QSH-based device applications. In particular, theoretical predictions of nontrivial superconducting pairing in the QSH state toward Majorana-based topological quantum computing are discussed, which are the next frontier in QSH materials research.

Tunable electronic structure and magnetic anisotropy in bilayer ferromagnetic semiconductor Cr2Ge2Te6

The emergence of ferromagnetism in two-dimensional van der Waals materials has aroused broad interest. However, the ferromagnetic instability has been a problem remained. In this work, by using the first-principles calculations, we identified the critical ranges of strain and doping for the bilayer Cr₂Ge₂Te₆ within which the ferromagnetic stability can be enhanced. Beyond the critical range, the tensile strain can induce the phase transition from the ferromagnetic to the antiferromagnetic, and the direction of magnetic easy axis can be converted from out-of-plane to in-plane due to the increase of compressive strain, or electrostatic doping. We also predicted an electron doping range, within which the ferromagnetism can be enhanced, while the ferromagnetic stability was maintained. Moreover, we found that the compressive strain can reverse the spin polarization of electrons at the conduction band minimum, so that two categories of half-metal can be induced by controlling electrostatic doping in the bilayer Cr₂Ge₂Te₆. These results should shed a light on achieving ferromagnetic stability for low-dimensional materials.

Strain Tunable Semimetal-Topological-Insulator Transition in Monolayer 1T^{'}-WTe_{2}

A quantum spin hall insulator is manifested by its conducting edge channels that originate from the nontrivial topology of the insulating bulk states. Monolayer 1T^{'}-WTe_{2} exhibits this quantized edge conductance in transport measurements, but because of its semimetallic nature, the coherence length is restricted to around 100 nm. To overcome this restriction, we propose a strain engineering technique to tune the electronic structure, where either a compressive strain along the a axis or a tensile strain along the b axis can drive 1T^{'}-WTe_{2} into an full gap insulating phase. A combined study of molecular beam epitaxy and in situ scanning tunneling microscopy or spectroscopy then confirmed such a phase transition. Meanwhile, the topological edge states were found to be very robust in the presence of strain.

Temperature-Induced Lifshitz Transition and Possible Excitonic Instability in ZrSiSe

The nodal-line semimetals have attracted immense interest due to the unique electronic structures such as the linear dispersion and the vanishing density of states as the Fermi energy approaching the nodes. Here, we report temperature-dependent transport and scanning tunneling microscopy (spectroscopy) [STM(S)] measurements on nodal-line semimetal ZrSiSe. Our experimental results and theoretical analyses consistently demonstrate that the temperature induces Lifshitz transitions at 80 and 106 K in ZrSiSe, which results in the transport anomalies at the same temperatures. More strikingly, we observe a V-shaped dip structure around Fermi energy from the STS spectrum at low temperature, which can be attributed to co-effect of the spin-orbit coupling and excitonic instability. Our observations indicate the correlation interaction may play an important role in ZrSiSe, which owns the quasi-two-dimensional electronic structures.

Quantum spin Hall state in monolayer 1T'-TMDCs

Although the 1T'phase is rare in the transition metal dichalcogenides (TMDCs) family, it has attracted rapid growing research interest due to the coexistence of superconductivity, unsaturated magneto-resistance, topological phases etc. Among them, the quantum spin Hall (QSH) state in monolayer 1T'-TMDCs is especially interesting because of its unique van der Waals crystal structure, bringing advantages in the fundamental research and application. For example, the van der Waals two-dimensional (2D) layer is vital in building novel functional vertical heterostructure. The monolayer 1T'-TMDCs has become one of the widely studied QSH insulator. In this review, we review the recent progress in fabrications of monolayer 1T'-TMDCs and evidence that establishes it as QSH insulator.

Anisotropic Growth and Scanning Tunneling Microscopy Identification of Ultrathin Even-Layered PdSe2 Ribbons

Palladium diselenide (PdSe₂ ) is an emerging 2D layered material with anisotropic optical/electrical properties, extra-high carrier mobility, excellent air stability, etc. So far, ultrathin PdSe₂ is mainly achieved via mechanical exfoliation from its bulk counterpart, and the direct synthesis is still challenging. Herein, the synthesis of ultrathin 2D PdSe₂ on conductive Au foil substrates via a facile chemical vapor deposition route is reported. Intriguingly, an anisotropic growth behavior is detected from the evolution of ribboned flakes with large length/width ratios, which is well explained from the orthorhombic symmetry of PdSe₂ . A unique even-layered growth mode from 2 to 20 layers is also confirmed by the perfect combination of onsite scanning tunneling microscopy characterizations, through deliberately scratching the flake edge to expose both even and odd layers. This even-layered, ribboned 2D material is expected to serve as a perfect platform for exploring unique physical properties, and for developing high-performance electronic and optoelectronic devices.

Manipulating Topological Domain Boundaries in the Single-Layer Quantum Spin Hall Insulator 1T'-WSe2

We report the creation and manipulation of structural phase boundaries in the single-layer quantum spin Hall insulator 1T'-WSe₂ by means of scanning tunneling microscope tip pulses. We observe the formation of one-dimensional interfaces between topologically nontrivial 1T' domains having different rotational orientations, as well as induced interfaces between topologically nontrivial 1T' and topologically trivial 1H phases. Scanning tunneling spectroscopy measurements show that 1T'/1T' interface states are localized at domain boundaries, consistent with theoretically predicted unprotected interface modes that form dispersive bands in and around the energy gap of this quantum spin Hall insulator. We observe a qualitative difference in the experimental spectral line shape between topologically "unprotected" states at 1T'/1T' domain boundaries and protected states at 1T'/1H and 1T'/vacuum boundaries in single-layer WSe₂.

Growth and Thermo-driven Crystalline Phase Transition of Metastable Monolayer 1T'-WSe2 Thin Film

Two-dimensional (2D) transition metal dichalcogenides MX₂ (M = Mo, W, X = S, Se, Te) attracts enormous research interests in recent years. Its 2H phase possesses an indirect to direct bandgap transition in 2D limit, and thus shows great application potentials in optoelectronic devices. The 1T′ crystalline phase transition can drive the monolayer MX₂ to be a 2D topological insulator. Here we realized the molecular beam epitaxial (MBE) growth of both the 1T′ and 2H phase monolayer WSe₂ on bilayer graphene (BLG) substrate. The crystalline structures of these two phases were characterized using scanning tunneling microscopy. The monolayer 1T′-WSe₂ was found to be metastable, and can transform into 2H phase under post-annealing procedure. The phase transition temperature of 1T′-WSe₂ grown on BLG is lower than that of 1T′ phase grown on 2H-WSe₂ layers. This thermo-driven crystalline phase transition makes the monolayer WSe₂ to be an ideal platform for the controlling of topological phase transitions in 2D materials family.

Observation of topologically protected states at crystalline phase boundaries in single-layer WSe2

Transition metal dichalcogenide materials are unique in the wide variety of structural and electronic phases they exhibit in the two-dimensional limit. Here we show how such polymorphic flexibility can be used to achieve topological states at highly ordered phase boundaries in a new quantum spin Hall insulator (QSHI), 1T'-WSe2. We observe edge states at the crystallographically aligned interface between a quantum spin Hall insulating domain of 1T'-WSe2 and a semiconducting domain of 1H-WSe2 in contiguous single layers. The QSHI nature of single-layer 1T'-WSe2 is verified using angle-resolved photoemission spectroscopy to determine band inversion around a 120 meV energy gap, as well as scanning tunneling spectroscopy to directly image edge-state formation. Using this edge-state geometry we confirm the predicted penetration depth of one-dimensional interface states into the two-dimensional bulk of a QSHI for a well-specified crystallographic direction. These interfaces create opportunities for testing predictions of the microscopic behavior of topologically protected boundary states.