Enhancing surface sensing performance of cascaded high contrast gratings using bound states in the continuum.

We proposed the cascaded high contrast grating (CHCG) structure to enhance surface sensing capabilities through bound states in the continuum (BICs). Utilizing the finite element method (FEM) and rigorous coupled-wave analysis (RCWA), we studied the dispersion relations, far-field contribution CHCGs, and near-field distributions of BICs corresponding to resonance peaks at different wavelengths. Results demonstrate the ability to precisely control symmetry-protected BIC (SP-BIC) and Friedrich-Wintgen BIC (FW-BIC) resonance peaks by altering incident angles and structural parameters, enhancing structure robustness and tunability. Significantly, modes 1 and 2 have demonstrated substantial enhancement in surface refractive index sensing, achieving highest sensitivities at 51 nm/RIU and the figure of merit reaching 490.8 RIU-1, indicating notable advancement in detecting subtle surface changes. In contrast, mode 3 has shown robust performance in bulk refractive index sensing, achieving a sensitivity of 602 nm/RIU and a figure of merit of 5189.65 RIU-1. These findings underscore the significant potential of the structure as a high-performance integrated sensor, particularly for precise environmental and biological monitoring in surface refractive index sensing.

Observations of Charge-Density-Wave States in W6Te6 Wires.

Determining the electronic ground state of a one-dimensional system is crucial to understanding the underlying physics of electronic behavior. Here, we demonstrate the discovery of charge-density wave states in few-wire W6Te6 arrays using scanning tunneling microscopy/spectroscopy. We directly visualize incommensurate charge orders, energy gaps with prominent coherence peaks, and the picometer-scale lattice distortion in nearly disorder-free double-wire systems, thereby demonstrating the existence of Peierls-type charge density waves. In the presence of disorder-induced charge order fluctuations, the coherence peaks resulting from phase correlation disappear and gradually transform the system into the pseudogap states. The power-law zero-bias anomaly and quasi-particle interference analysis further suggest the Tomonaga-Luttinger liquid behavior in such pseudogap region. In addition, we explicitly determined the evolution of the CDW energy gap as a function of stacking-wire numbers. The present study demonstrates the existence of electron-phonon interactions in few-wire W6Te6 that can be tuned by disorders and van der Waals stacking.

Realization of Multiple Charge-Density Waves in NbTe2 at the Monolayer Limit.

Layered transition-metal dichalcogenides down to the monolayer (ML) limit provide a fertile platform for exploring charge-density waves (CDWs). Here, we experimentally unveil the richness of the CDW phases in ML-NbTe2 for the first time. Not only the theoretically predicted 4 × 4 and 4 × 1 phases but also two unexpected 28×28 and 19×19 phases are realized. For such a complex CDW system, we establish an exhaustive growth phase diagram via systematic efforts in the material synthesis and scanning tunneling microscope characterization. Moreover, the energetically stable phase is the larger-scale order (19×19), which is surprisingly in contradiction to the prior prediction (4 × 4). These findings are confirmed using two different kinetic pathways: i.e., direct growth at proper growth temperatures (T) and low-T growth followed by high-T annealing. Our results provide a comprehensive diagram of the "zoo" of CDW orders in ML-NbTe2.

Dual-band refractive index sensor with cascaded asymmetric resonant compound grating based on bound states in the continuum.

We propose a cascaded asymmetric resonant compound grating (ARCG) for high-performance dual-band refractive index sensing. The physical mechanism of the sensor is investigated using a combination of temporal coupled-mode theory (TCMT) and ARCG eigenfrequency information, which is verified by rigorous coupled-wave analysis (RCWA). The reflection spectra can be tailored by changing the key structural parameters. And by altering the grating strip spacing, a dual-band quasi-bound state in the continuum can be achieved. The simulation results show that the highest sensitivity of the dual-band sensor is 480.1 nm/RIU, and its figure of merit is 4.01 × 105. The proposed ARCG has potential application prospects for high-performance integrated sensors.

Tuning of the Valley Structures in Monolayer In2Se3/WSe2 Heterostructures via Ferroelectricity.

Recent findings of two-dimensional ferroelectric (FE) materials have enabled the integration of nonvolatile FE functions into device applications based on van der Waals (vdW) heterojunctions (HJs), resulting in versatile technological advances. In this paper, we report the results of direct probing of the electronic structures of In2Se3/WSe2 heterostructures at the single-layer limit, where monolayer (ML)-In2Se3 was found to be either antiferroelectric (AFE, ß') or ferroelectric (ß*) at sufficiently low temperatures. A general type-II band alignment was revealed for this heterostructure. Moreover, we observed significant modulations of the valley structures of WSe2, and in situ transformations between the FE and AFE In2Se3 phases demonstrated the dominant role of the polarizations in the top ML-In2Se3 layer. The observed phenomena can be attributed to the combination of both the linear and quadratic Stark shifts from the out-of-plane electric field, which has only been previously theoretically explored for ML-transition metal dichalcogenides (TMDs).

Tailoring Dirac Fermions by In-Situ Tunable High-Order Moiré Pattern in Graphene-Monolayer Xenon Heterostructure.

We report an experimental study of a high-order moiré pattern formed in graphene-monolayer xenon heterostructure. The moiré period is in situ tuned from few nanometers to +∞, by adjusting the lattice constant of the xenon monolayer through annealing. Using angle-resolved photoemission spectroscopy, we observe that Dirac node replicas move closer and finally overlap with a gap opening, as the moiré pattern expands to +∞ and evolves into a Kekulé distortion. A moiré Hamiltonian coupling Dirac fermions from different valleys explains experimental results and indicates narrow moiré band. Our Letter demonstrates a platform to study continuous evolution of the moiré pattern, and provides an unprecedented approach for tailoring Dirac fermions with tunable intervalley coupling.

PTCDA Molecular Monolayer on Pb Thin Films: An Unusual π-Electron Kondo System and Its Interplay with a Quantum-Confined Superconductor.

The hybridization of magnetism and superconductivity has been an intriguing playground for correlated electron systems, hosting various novel physical phenomena. Usually, localized d or f electrons are central to magnetism. In this study, by placing a PTCDA (3,4,9,10-perylene tetracarboxylic dianhydride) molecular monolayer on ultrathin Pb films, we built a hybrid magnetism/superconductivity (M/SC) system consisting of only sp electronic levels. The magnetic moments reside in the unpaired molecular orbital originating from interfacial charge transfers. We reported distinctive tunneling spectroscopic features of such a Kondo screened π electron impurity lattice on a superconductor in the regime of T_{K}≫Δ, suggesting the formation of a two-dimensional bound states band. Moreover, moiré superlattices with tunable twist angle and the quantum confinement in the ultrathin Pb films provide easy and flexible implementations to tune the interplay between the Kondo physics and the superconductivity, which are rarely present in M/SC hybrid systems.

Precise Tuning of Band Structures and Electron Correlations by van der Waals Stacking of One-dimensional W6Te6 Wires.

Stacking of two-dimensional (2D) van der Waals (vdW) atomic sheets has been established as a powerful approach to fabricating new materials with broad versatilities and emergent functionalities. Here we demonstrate a bottom-up approach to fabricating isolated single W6Te6 wires and their lateral assemblies, offering a unique platform for investigating the elegant role of vdW coupling in 1D systems with atomic precision. We find experimentally and theoretically a single W6Te6 wire is a 1D semiconductor with a band gap of ∼60 meV, and a semiconductor-to-metal transition takes place upon interwire vdW stacking. The metallic multiwires exhibit strong Tomonaga-Luttinger liquid characteristics with the correlation parameter g varying from g = 0.086 for biwire to g = 0.136 for six-wire assemblies, all much reduced from the Fermi liquid regime (g = 1). The present study demonstrates wire-by-wire vdW stacking is a versatile means for fabrication of 1D systems with tunable electronic properties.

Ultrathin two-dimensional superconductivity with strong spin-orbit coupling.

We report on a study of epitaxially grown ultrathin Pb films that are only a few atoms thick and have parallel critical magnetic fields much higher than the expected limit set by the interaction of electron spins with a magnetic field, that is, the Clogston-Chandrasekhar limit. The epitaxial thin films are classified as dirty-limit superconductors because their mean-free paths, which are limited by surface scattering, are smaller than their superconducting coherence lengths. The uniformity of superconductivity in these thin films is established by comparing scanning tunneling spectroscopy, scanning superconducting quantum interference device (SQUID) magnetometry, double-coil mutual inductance, and magneto-transport, data that provide average superfluid rigidity on length scales covering the range from microscopic to macroscopic. We argue that the survival of superconductivity at Zeeman energies much larger than the superconducting gap can be understood only as the consequence of strong spin-orbit coupling that, together with substrate-induced inversion-symmetry breaking, produces spin splitting in the normal-state energy bands that is much larger than the superconductor's energy gap.

Probing Critical Point Energies of Transition Metal Dichalcogenides: Surprising Indirect Gap of Single Layer WSe2.

By using a comprehensive form of scanning tunneling spectroscopy, we have revealed detailed quasi-particle electronic structures in transition metal dichalcogenides, including the quasi-particle gaps, critical point energy locations, and their origins in the Brillouin zones. We show that single layer WSe2 surprisingly has an indirect quasi-particle gap with the conduction band minimum located at the Q-point (instead of K), albeit the two states are nearly degenerate. We have further observed rich quasi-particle electronic structures of transition metal dichalcogenides as a function of atomic structures and spin-orbit couplings. Such a local probe for detailed electronic structures in conduction and valence bands will be ideal to investigate how electronic structures of transition metal dichalcogenides are influenced by variations of local environment.

Direct imaging of band profile in single layer MoS2 on graphite: quasiparticle energy gap, metallic edge states, and edge band bending.

Using scanning tunneling microscopy and spectroscopy, we probe the electronic structures of single layer MoS2 on graphite. The apparent quasiparticle energy gap of single layer MoS2 is measured to be 2.15 ± 0.06 eV at 77 K, albeit a higher second conduction band threshold at 0.2 eV above the apparent conduction band minimum is also observed. Combining it with photoluminescence studies, we deduce an exciton binding energy of 0.22 ± 0.1 eV (or 0.42 eV if the second threshold is use), a value that is lower than current theoretical predictions. Consistent with theoretical predictions, we directly observe metallic edge states of single layer MoS2. In the bulk region of MoS2, the Fermi level is located at 1.8 eV above the valence band maximum, possibly due to the formation of a graphite/MoS2 heterojunction. At the edge, however, we observe an upward band bending of 0.6 eV within a short depletion length of about 5 nm, analogous to the phenomena of Fermi level pinning of a 3D semiconductor by metallic surface states.

van der Waals Engineering of Charge Density Waves in One-Dimensional Nb6Te6 Nanowires.

One-dimensional (1D) systems have played a crucial role in the development of fundamental physics and practical applications. Recently, transition metal monochalcogenide (TMM) wires based on molybdenum (Mo) and tungsten (W) have emerged as promising platforms for investigating 1D physics in pure van der Waals (vdW) platforms. Here, we report on the bottom-up fabrication of Nb6Te6 wires down to the single-wire limit. The unique properties of Nb6Te6 single wire enable the realization of 1D charge density wave (CDW) phases in an isolated single TMM wire. Moreover, we revealed the appealing regulation of 1D CDW orders by van der Waals interactions at either the 1D-2D interface (i.e., rotation of a single wire along its wire axis) or the 1D-1D interface. Two rotation angles (30° and 0°) give rise to 3 × 1 and zigzag chain CDW morphologies, respectively, which exhibit pronounced differences in atomic displacement by a factor of 2. The interwire vdW coupling overwhelms its counterpart at the 1D-2D interface, thus locking the rotation angle (at 0°) as well as the interwire atomic registries. In contrast, interestingly, the phases of the charge oscillations are independent of the adjacent wires. The ability to tailor 1D charge orders provides a crucial addition to the toll set of vdW integrations beyond two-dimensional materials.

Twist-angle dependent pseudo-magnetic fields in monolayer CrCl2/graphene heterostructures.

The generation of pseudo-magnetic fields in strained graphene leads to quantized Landau levels in the absence of an external magnetic field, providing the potential to achieve a zero-magnetic-field analogue of the quantum Hall effect. Here, we report the realization of a pseudo-magnetic field in epitaxial graphene by building a monolayer CrCl2/graphene heterointerface. The CrCl2 crystal structure exhibits spontaneous breaking of three-fold rotational symmetry, yielding an anisotropic displacement field at the interface. Using scanning tunneling spectroscopy, we have discovered a sequence of pseudo-Landau levels associated with massless Dirac fermions. A control experiment performed on the CrCl2/NbSe2 interface confirms the origin as the pseudo-magnetic field in the graphene layer that strongly interacts with CrCl2. More interestingly, the strength of the pseudo-magnetic fields can be tuned by the twist angle between the monolayer CrCl2 and graphene, with a variation of up to threefold, depending on the twist angle of 0° to 30°. This work presents a rare 2D heterojunction for exploring PMF-related physics, such as the valley Hall effect, with the advantage of easy and flexible implementation.

Controllable dimensionality conversion between 1D and 2D CrCl3 magnetic nanostructures.

The fabrication of one-dimensional (1D) magnetic systems on solid surfaces, although of high fundamental interest, has yet to be achieved for a crossover between two-dimensional (2D) magnetic layers and their associated 1D spin chain systems. In this study, we report the fabrication of 1D single-unit-cell-width CrCl3 atomic wires and their stacked few-wire arrays on the surface of a van der Waals (vdW) superconductor NbSe2. Scanning tunneling microscopy/spectroscopy and first-principles calculations jointly revealed that the single wire shows an antiferromagnetic large-bandgap semiconducting state in an unexplored structure different from the well-known 2D CrCl3 phase. Competition among the total energies and nanostructure-substrate interfacial interactions of these two phases result in the appearance of the 1D phase. This phase was transformable to the 2D phase either prior to or after the growth for in situ or ex situ manipulations, in which the electronic interactions at the vdW interface play a nontrivial role that could regulate the dimensionality conversion and structural transformation between the 1D-2D CrCl3 phases.

Two 'braking mechanisms' for tin phthalocyanine molecular rotors on dipolar iron oxide surfaces.

Manipulation of artificial molecular rotors/motors is a key issue in the field of molecular nanomachines. Here we assemble non-planar SnPc molecules on an FeO film to form two kinds of rotors with different apparent morphologies, rotational speeds and stabilities. Both kinds of rotors can switch to each other via external field stimulation and the switch depends on the polarity of the applied bias voltage. Furthermore, we reveal that the molecular fragment has a great influence on the motions of molecules. Combining scanning tunneling microscopy and DFT calculations, two braking mechanisms are addressed for molecular rotors. One is the transformation of adsorption configurations under the external electric field stimulus that enables the molecular rotor to stop/restart its rotation. The other is the introduction of embedded molecular fragments that act as a brake pad and can stop the molecular rotation. We find that the rotation can be recovered by separating the molecule from the fragments. Our study suggests a good system for manipulating molecular rotors' properties in nanophysics and has important value for the design of controllable molecular machines.

Identification of twist-angle-dependent excitons in WS2/WSe2 heterobilayers.

Stacking atomically thin films enables artificial construction of van der Waals heterostructures with exotic functionalities such as superconductivity, the quantum Hall effect, and engineered light-matter interactions. In particular, heterobilayers composed of monolayer transition metal dichalcogenides have attracted significant interest due to their controllable interlayer coupling and trapped valley excitons in moiré superlattices. However, the identification of twist-angle-modulated optical transitions in heterobilayers is sometimes controversial since both momentum-direct (K-K) and -indirect excitons reside on the low energy side of the bright exciton in the monolayer constituents. Here, we attribute the optical transition at ∼1.35 eV in the WS2/WSe2 heterobilayer to an indirect Γ-K transition based on a systematic analysis and comparison of experimental photoluminescence spectra with theoretical calculations. The exciton wavefunction obtained by the state-of-the-art GW-Bethe-Salpeter equation approach indicates that both the electron and hole of the excitons are contributed by the WS2 layer. Polarization-resolved k-space imaging further confirms that the transition dipole moment of this optical transition is dominantly in-plane and is independent of the twist angle. The calculated absorption spectrum predicts that the so-called interlayer exciton peak coming from the K-K transition is located at 1.06 eV, but with a much weaker amplitude. Our work provides new insight into the steady-state and dynamic properties of twist-angle-dependent excitons in van der Waals heterostructures.

Approaching strain limit of two-dimensional MoS2 via chalcogenide substitution.

Strain engineering is a promising method for tuning the electronic properties of two-dimensional (2D) materials, which are capable of sustaining enormous strain thanks to their atomic thinness. However, applying a large and homogeneous strain on these 2D materials, including the typical semiconductor MoS2, remains cumbersome. Here we report a facile strategy for the fabrication of highly strained MoS2 via chalcogenide substitution reaction (CSR) of MoTe2 with lattice inheritance. The MoS2 resulting from the sulfurized MoTe2 sustains ultra large in-plane strain (approaching its strength limit ~10%) with great homogeneity. Furthermore, the strain can be deterministically and continuously tuned to ~1.5% by simply varying the processing temperature. Thanks to the fine control of our CSR process, we demonstrate a heterostructure of strained MoS2/MoTe2 with abrupt interface. Finally, we verify that such a large strain potentially allows the modulation of MoS2 bandgap over an ultra-broad range (~1 eV). Our controllable CSR strategy paves the way for the fabrication of highly strained 2D materials for applications in devices.

Monolayer 1T-NbSe2 as a 2D-correlated magnetic insulator.

Monolayer group V transition metal dichalcogenides in their 1T phase have recently emerged as a platform to investigate rich phases of matter, such as spin liquid and ferromagnetism, resulting from strong electron correlations. Newly emerging 1T-NbSe2 has inspired theoretical investigations predicting collective phenomena such as charge transfer gap and ferromagnetism in two dimensions; however, the experimental evidence is still lacking. Here, by controlling the molecular beam epitaxy growth parameters, we demonstrate the successful growth of high-quality single-phase 1T-NbSe2. By combining scanning tunneling microscopy/spectroscopy and ab initio calculations, we show that this system is a charge transfer insulator with the upper Hubbard band located above the valence band maximum. To demonstrate the electron correlation resulted magnetic property, we create a vertical 1T/2H NbSe2 heterostructure, and we find unambiguous evidence of exchange interactions between the localized magnetic moments in 1T phase and the metallic/superconducting phase exemplified by Kondo resonances and Yu-Shiba-Rusinov­like bound states.

Engineering Point-Defect States in Monolayer WSe2.

Defect engineering is a key approach for tailoring the properties of the emerging two-dimensional semiconductors. Here, we report an atomic engineering of the W vacancy in monolayer WSe2 by single potassium atom decoration. The K decoration alters the energy states and reshapes the wave function such that previously hidden midgap states become visible with well-resolved multiplets in scanning tunneling spectroscopy. Their energy levels are in good agreement with first-principle calculations. More interestingly, the calculations show that an unpaired electron donated by the K atom can lead to a local magnetic moment, exhibiting an on-off switching by the odd-even number of electron filling. Experimentally the Fermi level is pinned above all defect states due to the graphite substrate, corresponding to an off state. The close agreement between theory and experiment in the off state, on the other hand, suggests the possibility of gate-programmable magnetic moments at the defects.

Tuning Band Gap and Work Function Modulations in Monolayer hBN/Cu(111) Heterostructures with Moiré Patterns.

The moiré pattern formed between a two-dimensional (2D) material and the substrate has played a crucial role in tuning the electronic structure of the 2D material. Here, by using scanning tunneling microscopy and spectroscopy, we found a moiré-pattern-dependent band gap and work function modulation in hexagonal boron nitride (hBN)/Cu(111) heterostructures, whose amplitudes increase with the moiré pattern wavelength. Moreover, the work function modulation shifts agree well with the conduction band edge shifts, indicating a spatially constant electron affinity for the hBN layer. Density functional theory calculations showed that these observations in hBN/Cu(111) heterostructures mainly originated from the hybridization of the N 3p z orbital and Cu 4s orbital in different atomic configurations. Our results show that the twist-angle dependence of moiré patterns in hBN/Cu(111) heterostructures can be used to tailor the electronic properties including band gap and work function.