Unexpected catalytic activity of nanorippled graphene.

Graphite is one of the most chemically inert materials. Its elementary constituent, monolayer graphene, is generally expected to inherit most of the parent material's properties including chemical inertness. Here, we show that, unlike graphite, defect-free monolayer graphene exhibits a strong activity with respect to splitting molecular hydrogen, which is comparable to that of metallic and other known catalysts for this reaction. We attribute the unexpected catalytic activity to surface corrugations (nanoscale ripples), a conclusion supported by theory. Nanoripples are likely to play a role in other chemical reactions involving graphene and, because nanorippling is inherent to atomically thin crystals, can be important for two-dimensional (2D) materials in general.

Optically controlled single-valley exciton doublet states with tunable internal spin structures and spin magnetization generation.

Manipulating quantum states through light-matter interactions has been actively pursued in two-dimensional materials research. Significant progress has been made toward the optical control of the valley degrees of freedom in semiconducting monolayer transition-metal dichalcogenides, based on doubly degenerate excitons from their two distinct valleys in reciprocal space. Here, we introduce a type of optically controllable doubly degenerate exciton states that come from a single valley, dubbed as single-valley exciton doublet (SVXD) states. They are unique in that their constituent holes originate from the same valence band, making possible the direct optical control of the spin structure of the excited constituent electrons. Combining ab initio GW plus Bethe-Salpeter equation (GW-BSE) calculations and a theoretical analysis method, we demonstrate such SVXD in substrate-supported monolayer bismuthene-which has been successfully grown using molecular beam epitaxy. In each of the two distinct valleys in the Brillouin zone, strong spin-orbit coupling and [Formula: see text] symmetry lead to a pair of degenerate 1s exciton states (the SVXD states) with opposite spin configurations. Any coherent linear combinations of the SVXD in a single valley can be excited by light with a specific polarization, enabling full manipulation of their internal spin configurations. In particular, a controllable net spin magnetization can be generated through light excitation. Our findings open routes to control quantum degrees of freedom, paving the way for applications in spintronics and quantum information science.

Ultrafast electron localization and screening in a transition metal dichalcogenide.

The coupling of light to electrical charge carriers in semiconductors is the foundation of many technological applications. Attosecond transient absorption spectroscopy measures simultaneously how excited electrons and the vacancies they leave behind dynamically react to the applied optical fields. In compound semiconductors, these dynamics can be probed via any of their atomic constituents with core-level transitions into valence and conduction band. Typically, the atomic species forming the compound contribute comparably to the relevant electronic properties of the material. One therefore expects to observe similar dynamics, irrespective of the choice of atomic species via which it is probed. Here, we show in the two-dimensional transition metal dichalcogenide semiconductor MoSe2, that through a selenium-based core-level transition we observe charge carriers acting independently from each other, while when probed through molybdenum, the collective, many-body motion of the carriers dominates. Such unexpectedly contrasting behavior can be explained by a strong localization of electrons around molybdenum atoms following absorption of light, which modifies the local fields acting on the carriers. We show that similar behavior in elemental titanium metal [M. Volkov et al., Nat. Phys. 15, 1145-1149 (2019)] carries over to transition metal-containing compounds and is expected to play an essential role for a wide range of such materials. Knowledge of independent particle and collective response is essential for fully understanding these materials.

Hot carrier extraction from 2D semiconductor photoelectrodes.

Hot carrier-based energy conversion systems could double the efficiency of conventional solar energy technology or drive photochemical reactions that would not be possible using fully thermalized, "cool" carriers, but current strategies require expensive multijunction architectures. Using an unprecedented combination of photoelectrochemical and in situ transient absorption spectroscopy measurements, we demonstrate ultrafast (<50 fs) hot exciton and free carrier extraction under applied bias in a proof-of-concept photoelectrochemical solar cell made from earth-abundant and potentially inexpensive monolayer (ML) MoS2. Our approach facilitates ultrathin 7 Å charge transport distances over 1 cm2 areas by intimately coupling ML-MoS2 to an electron-selective solid contact and a hole-selective electrolyte contact. Our theoretical investigations of the spatial distribution of exciton states suggest greater electronic coupling between hot exciton states located on peripheral S atoms and neighboring contacts likely facilitates ultrafast charge transfer. Our work delineates future two-dimensional (2D) semiconductor design strategies for practical implementation in ultrathin photovoltaic and solar fuel applications.

ZnPSe3 as ultrabright indirect band-gap system with microsecond excitonic lifetimes.

ZnPSe3 was identified as a two-dimensional material wherein valley and spin can be optically controlled in technologically relevant timescales. We report an optical characterization of ZnPSe3 crystals that show indirect band-gap characteristics in combination with unusually strong photoluminescence. We found evidence of interband recombination from photoexcited electron-hole states with lifetimes in a microsecond timescale. Through a comparative analysis of photoluminescence and photoluminescence excitation spectra, we reconstructed the electronic band scheme relevant to fundamental processes of light absorption, carrier relaxation, and radiative recombination through interband pathways and annihilation of defect-bound excitons. The investigation of the radiative processes in the presence of a magnetic field revealed spin splitting of electronic states contributing to the ground excitonic states. Consequently, the magnetic field induces an imbalance in the number of excitons with the opposite angular momentum according to the thermal equilibrium as seen in the photoluminescence decay profiles resolved by circular polarization.

Transition metal dichalcogenide nanospheres for high-refractive-index nanophotonics and biomedical theranostics.

Recent developments in the area of resonant dielectric nanostructures have created attractive opportunities for concentrating and manipulating light at the nanoscale and the establishment of the new exciting field of all-dielectric nanophotonics. Transition metal dichalcogenides (TMDCs) with nanopatterned surfaces are especially promising for these tasks. Still, the fabrication of these structures requires sophisticated lithographic processes, drastically complicating application prospects. To bridge this gap and broaden the application scope of TMDC nanomaterials, we report here femtosecond laser-ablative fabrication of water-dispersed spherical TMDC (MoS2 and WS2) nanoparticles (NPs) of variable size (5 to 250 nm). Such NPs demonstrate exciting optical and electronic properties inherited from TMDC crystals, due to preserved crystalline structure, which offers a unique combination of pronounced excitonic response and high refractive index value, making possible a strong concentration of electromagnetic field in the NPs. Furthermore, such NPs offer additional tunability due to hybridization between the Mie and excitonic resonances. Such properties bring to life a number of nontrivial effects, including enhanced photoabsorption and photothermal conversion. As an illustration, we demonstrate that the NPs exhibit a very strong photothermal response, much exceeding that of conventional dielectric nanoresonators based on Si. Being in a mobile colloidal state and exhibiting superior optical properties compared to other dielectric resonant structures, the synthesized TMDC NPs offer opportunities for the development of next-generation nanophotonic and nanotheranostic platforms, including photothermal therapy and multimodal bioimaging.

Gate-tunable contact-induced Fermi-level shift in semimetal.

Low-dimensional semimetal­semiconductor (Sm-S) van der Waals (vdW) heterostructures have shown their potentials in nanoelectronics and nano-optoelectronics recently. It is an important scientific issue to study the interfacial charge transfer as well as the corresponding Fermi-level shift in Sm-S systems. Here we investigated the gate-tunable contact-induced Fermi-level shift (CIFS) behavior in a semimetal single-walled carbon nanotube (SWCNT) that formed a heterojunction with a transition-metal dichalcogenide (TMD) flake. A resistivity comparison methodology and a Fermi-level catch-up model have been developed to measure and analyze the CIFS, whose value is determined by the resistivity difference between the naked SWCNT segment and the segment in contact with the TMD. Moreover, the relative Fermi-level positions of SWCNT and two-dimensional (2D) semiconductors can be efficiently reflected by the gate-tunable resistivity difference. The work function change of the semimetal, as a result of CIFS, will naturally introduce a modified form of the Schottky­Mott rule, so that a modified Schottky barrier height can be obtained for the Sm-S junction. The methodology and physical model should be useful for low-dimensional reconfigurable nanodevices based on Sm-S building blocks.

Direct Determination of Torsion in Twisted Graphite and MoS2 Interfaces.

The design space of two-dimensional materials is undergoing significant expansion through the stacking of layers in non-equilibrium configurations. However, the lack of quantitative insights into twist dynamics impedes the development of such heterostructures. Herein, we utilize the lateral force sensitivity of an atomic force microscope cantilever and specially designed rotational bearing structures to measure the torque in graphite and MoS2 interfaces. While the extracted torsional energies are virtually zero across all angular misfit configurations, commensurate interfaces of graphite and MoS2 are characterized by values of 0.1533 and 0.6384 N-m/m2, respectively. Furthermore, we measured the adhesion energies of graphite and MoS2 to elucidate the interplay between twist and slide. The adhesion energy dominates over the torsional energy for the graphitic interface, suggesting a tendency to twist prior to superlubric sliding. Conversely, MoS2 displays an increased torsional energy exceeding its adhesion energy. Consequently, our findings demonstrate a fundamental disparity between the sliding-to-twisting dynamics at MoS2 and graphite interfaces.

On the Origin of the Above-Room-Temperature Magnetism in the 2D van der Waals Ferromagnet Fe3GaTe2.

2D magnetic materials have attracted growing interest driven by their unique properties and potential applications. However, the scarcity of systems exhibiting magnetism at room temperature has limited their practical implementation into functional devices. Here we focus on the van der Waals ferromagnet Fe3GaTe2, which exhibits above-room-temperature magnetism (Tc = 350-380 K) and strong perpendicular anisotropy. Through first-principles calculations, we examine the magnetic properties of Fe3GaTe2 and compare them with those of Fe3GeTe2. Our calculations unveil the microscopic mechanisms governing their magnetic behavior, emphasizing the pivotal role of ferromagnetic in-plane couplings in the stabilization of the elevated Tc in Fe3GaTe2. Additionally, we predict the stability, substantial perpendicular anisotropy, and high Tc of the single-layer Fe3GaTe2. We also demonstrate the potential of strain engineering and electrostatic doping to modulate its magnetic properties. Our results incentivize the isolation of the monolayer and pave the way for the future optimization of Fe3GaTe2 in magnetic and spintronic nanodevices.

Alice (and Bob) in Flatland.

2D quantum materials have opened infinite doors, hosting intriguing phenomena and featuring incredible engineering potential. Whether these qualities can boost the use of 2D crystals for quantum applications remains an open field with yet unexplored paths.

Mapping of Vibrational Modes Revealing a Strong and Tunable Coupling in Two Juxtaposed 3R-WSe2 Nanodrums.

Two-dimensional (2D) material resonators have emerged as promising platforms for advanced nanomechanical applications due to their exceptional mechanical properties, tunability, and nonlinearities. We explored the strong mechanical mode coupling between two adjacent 3R-WSe2 nanodrums at room temperature. Combining a piezoelectric material, as noncentrosymmetric 3R-WSe2, and vibration manipulation is the building block for phononic experiments with 2D materials. By strategically placing gate grids beneath each resonator and mapping the spatial distribution of these modes, we demonstrate the ability to transit between localized modes in individual membranes to delocalized, strongly coupled modes that span the entire suspended region. The coherent coupling is strongly tunable with simple gate voltage, and remarkable resonance splitting was achieved, corresponding to up to 5% of the vibration frequency. These results showcase the potential of 2D material resonators for efficient information exchange, paving the way for novel applications in quantum technologies and nanoscale sensing.

Impact of Thickness-Dependent Nanophotonic Effects on the Optical Response of Color Centers in Hexagonal Boron Nitride.

Among a broad diversity of color centers hosted in layered van der Waals materials, the negatively charged boron vacancy (VB-) center in hexagonal boron nitride (hBN) is garnering considerable attention for the development of quantum sensing units on a two-dimensional platform. In this work, we investigate how the optical response of an ensemble of VB- centers evolves with the hBN thickness in a range of a few to hundreds of nanometers. We show that the photoluminescence intensity features a nontrivial evolution with thickness, which is quantitatively reproduced by numerical calculations taking into account thickness-dependent variations of the absorption, radiative lifetime, and radiation pattern of VB- centers. Besides providing an important resource to optimize the performances of quantum sensing units based on VB- centers in hBN, the thickness-dependent nanophotonic effects discussed in this work generally apply to any type of color center embedded in a van der Waals material.

Substrate Interference and Strain in the Second-Harmonic Generation from MoSe2 Monolayers.

Nonlinear optical materials of atomic thickness, such as non-centrosymmetric 2H transition metal dichalcogenide monolayers, have a second-order nonlinear susceptibility (χ(2)) whose intensity can be tuned by strain. However, whether χ(2) is enhanced or reduced by tensile strain is a subject of conflicting reports. Here, we grow high-quality MoSe2 monolayers under controlled biaxial strain created by two different substrates and study their linear and nonlinear optical responses with a combination of experimental and theoretical approaches. Up to a 15-fold overall enhancement in second-harmonic generation (SHG) intensity is observed from MoSe2 monolayers grown on SiO2 when compared to its value on a Si3N4 substrate. By considering an interference contribution from different dielectrics and their thicknesses, a factor of 2 enhancement of χ(2) was attributed to the biaxial strain: substrate interference and strain are independent handles to engineer the SHG strength of non-centrosymmetric 2D materials.

Emergent Dirac Fermions in Epitaxial Planar Silicene Heterostructure.

Silicene, a single layer of Si atoms, shares many remarkable electronic properties with graphene. So far, silicene has been synthesized in its epitaxial form on a few surfaces of solids. Thus, the problem of silicene-substrate interaction appears, which usually depresses the original electronic behavior but may trigger properties superior to those of bare components. We report the direct observation of robust Dirac-dispersed bands in epitaxial silicene grown on Au(111) films deposited on Si(111). By performing in-depth angle-resolved photoemission spectroscopy measurements, we reveal three pairs of one-dimensional bands with linear dispersion running in three different directions of an otherwise two-dimensional system. By combining these results with first-principles calculations, we explore the nature of these bands and point to strong interaction between subsystems forming a complex Si-Au heterostructure. These findings emphasize the essential role of interfacial coupling and open a unique materials platform for exploring exotic quantum phenomena and applications in future-generation nanoelectronics.

Competition of Moiré Network Sites to Form Electronic Quantum Dots in Reconstructed MoX2/WX2 Heterostructures.

Twisted bilayers of two-dimensional semiconductors offer a versatile platform for engineering quantum states for charge carriers using moiré superlattice effects. Among the systems of recent interest are twistronic MoX2/WX2 heterostructures (X = Se or S), which undergo reconstruction into preferential stacking domains and highly strained domain wall networks, determining the electron/hole localization across moiré superlattices. Here, we present a catalogue of options for the formation of self-organized quantum dots and wires in lattice-reconstructed marginally twisted MoX2/WX2 bilayers with a relative lattice mismatch δ ≪ 1 for twist angles ranging from perfect alignment to θ ∼ 1°. On the basis of multiscale modeling taking into account twirling of domain wall networks, we analyze bilayers with both parallel and antiparallel orientations of their unit cells and describe crossovers between different positioning of band edges for electrons and holes across moiré superlattices when θ < δ and θ > δ.

Emergence of Improper Electronic Ferroelectricity and Flat Band in Twisted Bilayer Tl2S.

Two-dimensional (2D) ferroelectrics possessing out-of-plane (OP) polarization are highly desirable for applications and fundamental physics. Here, by first-principles calculations, we reveal that large-angle interlayer twisting can efficiently stabilize an unexpected ordering of sizable electric dipoles, producing OP polarization out of the centrosymmetric ground-state structure of Tl2S, in great contrast to the recently proposed interlayer-sliding ferroelectricity. The ferroelectricity originates from a nonlinear coupling between a polar order dominantly contributed by electrons and an unstable phonon mode associated with a commensurate k point (1/3, 1/3, 0) in the two constituent monolayers, therefore indicating an improper and electronic ferroelectric nature. More interestingly, a flat band and a van Hove singularity occur in its electronic structures just below the Fermi level in the large-angle twisted bilayer Tl2S. The unusual coexistence of improper electronic ferroelectricity, a flat band, and a van Hove singularity in one 2D material offers exceptional opportunities for exploring novel physics and applications.

Remote Epitaxy: Fundamentals, Challenges, and Opportunities.

Advanced heterogeneous integration technologies are pivotal for next-generation electronics. Single-crystalline materials are one of the key building blocks for heterogeneous integration, although it is challenging to produce and integrate these materials. Remote epitaxy is recently introduced as a solution for growing single-crystalline thin films that can be exfoliated from host wafers and then transferred onto foreign platforms. This technology has quickly gained attention, as it can be applied to a wide variety of materials and can realize new functionalities and novel application platforms. Nevertheless, remote epitaxy is a delicate process, and thus, successful execution of remote epitaxy is often challenging. Here, we elucidate the mechanisms of remote epitaxy, summarize recent breakthroughs, and discuss the challenges and solutions in the remote epitaxy of various material systems. We also provide a vision for the future of remote epitaxy for studying fundamental materials science, as well as for functional applications.

A Universal Approximation for Conductance Blockade in Thin Nanopore Membranes.

Nanopore-based sensing platforms have transformed single-molecule detection and analysis. The foundation of nanopore translocation experiments lies in conductance measurements, yet existing models, which are largely phenomenological, are inaccurate in critical experimental conditions such as thin and tightly fitting pores. Of the two components of the conductance blockade, channel and access resistance, the access resistance is poorly modeled. We present a comprehensive investigation of the access resistance and associated conductance blockade in thin nanopore membranes. By combining a first-principles approach, multiscale modeling, and experimental validation, we propose a unified theoretical modeling framework. The analytical model derived as a result surpasses current approaches across a broad parameter range. Beyond advancing our theoretical understanding, our framework's versatility enables analyte size inference and predictive insights into conductance blockade behavior. Our results will facilitate the design and optimization of nanopore devices for diverse applications, including nanopore base calling and data storage.

Characteristic Plasmon Energies for 2D In2Se3 Phase Identification at Nanoscale.

Two-dimensional (2D) materials with competing polymorphs offer remarkable potential to switch the associated 2D functionalities for novel device applications. Probing their phase transition and competition mechanisms requires nanoscale characterization techniques that can sensitively detect the nucleation of secondary phases down to single-layer thickness. Here we demonstrate nanoscale phase identification on 2D In2Se3 polymorphs, utilizing their distinct plasmon energies that can be distinguished by electron energy-loss spectroscopy (EELS). The characteristic plasmon energies of In2Se3 polymorphs have been validated by first-principles calculations, and also been successfully applied to reveal phase transitions using in situ EELS. Correlating with in situ X-ray diffraction, we further derive a subtle difference in the valence electron density of In2Se3 polymorphs, consistent with their disparate electronic properties. The nanometer resolution and independence of orientation make plasmon-energy mapping a versatile technique for nanoscale phase identification on 2D materials.

Superconducting Cavity-Based Sensing of Band Gaps in 2D Materials.

The superconducting coplanar waveguide (SCPW) cavity plays an essential role in various areas like superconducting qubits, parametric amplifiers, radiation detectors, and studying magnon-photon and photon-phonon coupling. Despite its wide-ranging applications, the use of SCPW cavities to study various van der Waals 2D materials has been relatively unexplored. The resonant modes of the SCPW cavity exquisitely sense the dielectric environment. In this work, we measure the charge compressibility of bilayer graphene coupled to a half-wavelength SCPW cavity. Our approach provides a means to detect subtle changes in the capacitance of the bilayer graphene heterostructure, which depends on the compressibility of bilayer graphene, manifesting as shifts in the resonant frequency of the cavity. This method holds promise for exploring a wide class of van der Waals 2D materials, including transition metal dichalcogenides (TMDs) and their moiré, where DC transport measurement is challenging.