Upconversion electroluminescence in 2D semiconductors integrated with plasmonic tunnel junctions.

Plasmonic tunnel junctions are a unique electroluminescent system in which light emission occurs via an interplay between tunnelling electrons and plasmonic fields instead of electron-hole recombination as in conventional light-emitting diodes. It was previously shown that placing luminescent molecules in the tunneling pathway of nanoscopic tunnel junctions results in peculiar upconversion electroluminescence where the energy of emitted photons exceeds that of excitation electrons. Here we report the observation of upconversion electroluminescence in macroscopic van der Waals plasmonic tunnel junctions comprising gold and few-layer graphene electrodes separated by a ~2-nm-thick hexagonal boron nitride tunnel barrier and a monolayer semiconductor. We find that the semiconductor ground exciton emission is triggered at excitation electron energies lower than the semiconductor optical gap. Interestingly, this upconversion is reached in devices operating at a low conductance (<10-6 S) and low power density regime (<102 W cm-2), defying explanation through existing proposed mechanisms. By examining the scaling relationship between plasmonic and excitonic emission intensities, we elucidate the role of inelastic electron tunnelling dipoles that induce optically forbidden transitions in the few-layer graphene electrode and ultrafast hot carrier transfer across the van der Waals interface.

Energy transfer driven brightening of MoS2 by ultrafast polariton relaxation in microcavity MoS2/hBN/WS2 heterostructures.

Energy transfer is a ubiquitous phenomenon that delivers energy from a blue-shifted emitter to a red-shifted absorber, facilitating wide photonic applications. Two-dimensional (2D) semiconductors provide unique opportunities for exploring novel energy transfer mechanisms in the atomic-scale limit. Herein, we have designed a planar optical microcavity-confined MoS2/hBN/WS2 heterojunction, which realizes the strong coupling among donor exciton, acceptor exciton, and cavity photon mode. This configuration demonstrates an unconventional energy transfer via polariton relaxation, brightening MoS2 with a record-high enhancement factor of ~440, i.e., two-order-of-magnitude higher than the data reported to date. The polariton relaxation features a short characteristic time of ~1.3 ps, resulting from the significantly enhanced intra- and inter-branch exciton-exciton scattering. The polariton relaxation dynamics is associated with Rabi energies in a phase diagram by combining experimental and theoretical results. This study opens a new direction of microcavity 2D semiconductor heterojunctions for high-brightness polaritonic light sources and ultrafast polariton carrier dynamics.

Phase-selective in-plane heteroepitaxial growth of H-phase CrSe2.

Phase engineering of two-dimensional transition metal dichalcogenides (2D-TMDs) offers opportunities for exploring unique phase-specific properties and achieving new desired functionalities. Here, we report a phase-selective in-plane heteroepitaxial method to grow semiconducting H-phase CrSe2. The lattice-matched MoSe2 nanoribbons are utilized as the in-plane heteroepitaxial template to seed the growth of H-phase CrSe2 with the formation of MoSe2-CrSe2 heterostructures. Scanning tunneling microscopy and non-contact atomic force microscopy studies reveal the atomically sharp heterostructure interfaces and the characteristic defects of mirror twin boundaries emerging in the H-phase CrSe2 monolayers. The type-I straddling band alignments with band bending at the heterostructure interfaces are directly visualized with atomic precision. The mirror twin boundaries in the H-phase CrSe2 exhibit the Tomonaga-Luttinger liquid behavior in the confined one-dimensional electronic system. Our work provides a promising strategy for phase engineering of 2D TMDs, thereby promoting the property research and device applications of specific phases.

Engineering the Outcoupling Pathways in Plasmonic Tunnel Junctions via Photonic Mode Dispersion for Low-Loss Waveguiding.

Outcoupling of plasmonic modes excited by inelastic electron tunneling (IET) across plasmonic tunnel junctions (TJs) has attracted significant attention due to low operating voltages and fast excitation rates. Achieving selectivity among various outcoupling channels, however, remains a challenging task. Employing nanoscale antennas to enhance the local density of optical states (LDOS) associated with specific outcoupling channels partially addressed the problem, along with the integration of conducting 2D materials into TJs, improving the outcoupling to guided modes with particular momentum. The disadvantage of such methods is that they often involve complex fabrication steps and lack fine-tuning options. Here, we propose an alternative approach by modifying the dielectric medium surrounding TJs. By employing a simple multilayer substrate with a specific permittivity combination for the TJs under study, we show that it is possible to optimize mode selectivity in outcoupling to a plasmonic or a photonic-like mode characterized by distinct cutoff behaviors and propagation length. Theoretical and experimental results obtained with a SiO2-SiN-glass multilayer substrate demonstrate high relative coupling efficiencies of (62.77 ± 1.74)% and (29.07 ± 0.72)% for plasmonic and photonic-like modes, respectively. The figure-of-merit, which quantifies the tradeoff between mode outcoupling and propagation lengths (tens of µm) for both modes, can reach values as high as 180 and 140. The demonstrated approach allows LDOS engineering and customized TJ device performance, which are seamlessly integrated with standard thin film fabrication protocols. Our experimental device is well-suited for integration with silicon nitride photonics platforms.

Optical Gain Spectrum and Confinement Factor of a Monolayer Semiconductor in an Ultrahigh-Quality Cavity.

Two-dimensional (2D) semiconductors have attracted great attention as a novel class of gain materials for low-threshold, on-chip coherent light sources. Despite several experimental reports on lasing, the underlying gain mechanism of 2D materials remains elusive due to a lack of key information, including modal gain and the confinement factor. Here, we demonstrate a novel approach to directly determine the absorption coefficient of monolayer WS2 by characterizing the whispering gallery modes in a van der Waals microdisk cavity. By exploiting the cavity's high intrinsic quality factor of 2.5 × 104, the absorption coefficient spectrum and confinement factor are experimentally resolved with unprecedented accuracy. The excitonic gain reduces the WS2 absorption coefficient by 2 × 104 cm-1 at room temperature, and the experimental confinement factor is found to agree with the theoretical prediction. These results are essential for unveiling the gain mechanism in emergent, low-threshold 2D-semiconductor-based laser devices.

Single Atomic Defect Conductivity for Selective Dilute Impurity Imaging in 2D Semiconductors.

Precisely controlled impurity doping is of fundamental significance in modern semiconductor technologies. Desired physical properties are often achieved at impurity concentrations well below parts per million level. For emergent two-dimensional semiconductors, development of reliable doping strategies is hindered by the inherent difficulty in identifying and quantifying impurities in such a dilute limit where the absolute number of atoms to be detected is insufficient for common analytical techniques. Here we report rapid high-contrast imaging of dilute single atomic impurities by using conductive atomic force microscopy. We show that the local conductivity is enhanced by more than 100-fold by a single impurity atom due to resonance-assisted tunneling. Unlike the closely related scanning tunneling microscopy, the local conductivity sensitively depends on the impurity energy level, allowing minority defects to be selectively imaged. We further demonstrate subsurface impurity detection with single monolayer depth resolution in multilayer materials.

Room-Temperature Photoluminescence Mediated by Sulfur Vacancies in 2D Molybdenum Disulfide.

Atomic defects in monolayer transition metal dichalcogenides (TMDs) such as chalcogen vacancies significantly affect their properties. In this work, we provide a reproducible and facile strategy to rationally induce chalcogen vacancies in monolayer MoS2 by annealing at 600 °C in an argon/hydrogen (95%/5%) atmosphere. Synchrotron X-ray photoelectron spectroscopy shows that a Mo 3d5/2 core peak at 230.1 eV emerges in the annealed MoS2 associated with nonstoichiometric MoSx (0 < x < 2), and Raman spectroscopy shows an enhancement of the ∼380 cm-1 peak that is attributed to sulfur vacancies. At sulfur vacancy densities of ∼1.8 × 1014 cm-2, we observe a defect peak at ∼1.72 eV (referred to as LXD) at room temperature in the photoluminescence (PL) spectrum. The LXD peak is attributed to excitons trapped at defect-induced in-gap states and is typically observed only at low temperatures (≤77 K). Time-resolved PL measurements reveal that the lifetime of defect-mediated LXD emission is longer than that of band edge excitons, both at room and low temperatures (∼2.44 ns at 8 K). The LXD peak can be suppressed by annealing the defective MoS2 in sulfur vapor, which indicates that it is possible to passivate the vacancies. Our results provide insights into how excitonic and defect-mediated PL emissions in MoS2 are influenced by sulfur vacancies at room and low temperatures.

Probing spin dynamics of ultra-thin van der Waals magnets via photon-magnon coupling.

Layered van der Waals (vdW) magnets can maintain a magnetic order even down to the single-layer regime and hold promise for integrated spintronic devices. While the magnetic ground state of vdW magnets was extensively studied, key parameters of spin dynamics, like the Gilbert damping, crucial for designing ultra-fast spintronic devices, remains largely unexplored. Despite recent studies by optical excitation and detection, achieving spin wave control with microwaves is highly desirable, as modern integrated information technologies predominantly are operated with these. The intrinsically small numbers of spins, however, poses a major challenge to this. Here, we present a hybrid approach to detect spin dynamics mediated by photon-magnon coupling between high-Q superconducting resonators and ultra-thin flakes of Cr2Ge2Te6 (CGT) as thin as 11 nm. We test and benchmark our technique with 23 individual CGT flakes and extract an upper limit for the Gilbert damping parameter. These results are crucial in designing on-chip integrated circuits using vdW magnets and offer prospects for probing spin dynamics of monolayer vdW magnets.

Gate-Tunable Bound Exciton Manifolds in Monolayer MoSe2.

Two-dimensional (2D) semiconductors with point defects are predicted to host a variety of bound exciton complexes analogous to trions and biexcitons due to strong many-body effects. However, despite the common observation of defect-mediated subgap emission, the existence of such complexes remains elusive. Here, we report the observation of bound exciton (BX) complex manifolds in monolayer MoSe2 with intentionally created monoselenium vacancies (VSe) using proton beam irradiation. The emission intensity of different BX peaks is found to exhibit contrasting dependence on electrostatic doping near the onset of free electron injection. The observed trend is consistent with the model in which free excitons exist in equilibrium with excitons bound to neutral and charged VSe defects, which act as deep acceptors. These complexes are more strongly bound than trions and biexcitons, surviving up to around 180 K, and exhibit moderate valley polarization memory, indicating partial free exciton character.

Ferroelectricity in Niobium Oxide Dihalides NbOX2 (X = Cl, I): A Macroscopic- to Microscopic-Scale Study.

2D materials with ferroelectric and piezoelectric properties are of interest for energy harvesting, memory storage and electromechanical systems. Here, we present a systematic study of the ferroelectric properties in NbOX2 (X = Cl, I) across different spatial scales. The in-plane ferroelectricity in NbOX2 was investigated using transport and piezoresponse force microscopy (PFM) measurements, where it was observed that NbOCl2 has a stronger ferroelectric order than NbOI2. A high local field, exerted by both PFM and scanning tunneling microscopy (STM) tips, was found to induce 1D collinear ferroelectric strips in NbOCl2. STM imaging reveals the unreconstructed atomic structures of NbOX2 surfaces, and scanning tunneling spectroscopy was used to probe the electronic states induced at defect (vacancy) sites.

Laser-induced topological spin switching in a 2D van der Waals magnet.

Two-dimensional (2D) van der Waals (vdW) magnets represent one of the most promising horizons for energy-efficient spintronic applications because their broad range of electronic, magnetic and topological properties. However, little is known about the interplay between light and spin properties in vdW layers. Here we show that ultrafast laser excitation can not only generate different type of spin textures in CrGeTe3 vdW magnets but also induce a reversible transformation between them in a topological toggle switch mechanism. Our atomistic spin dynamics simulations and wide-field Kerr microscopy measurements show that different textures can be generated via high-intense laser pulses within the picosecond regime. The phase transformation between the different topological spin textures is obtained as additional laser pulses are applied to the system where the polarisation and final state of the spins can be controlled by external magnetic fields. Our results indicate laser-driven spin textures on 2D magnets as a pathway towards reconfigurable topological architectures at the atomistic level.

Ultrathin quantum light source with van der Waals NbOCl2 crystal.

Interlayer electronic coupling in two-dimensional materials enables tunable and emergent properties by stacking engineering. However, it also results in significant evolution of electronic structures and attenuation of excitonic effects in two-dimensional semiconductors as exemplified by quickly degrading excitonic photoluminescence and optical nonlinearities in transition metal dichalcogenides when monolayers are stacked into van der Waals structures. Here we report a van der Waals crystal, niobium oxide dichloride (NbOCl2), featuring vanishing interlayer electronic coupling and monolayer-like excitonic behaviour in the bulk form, along with a scalable second-harmonic generation intensity of up to three orders higher than that in monolayer WS2. Notably, the strong second-order nonlinearity enables correlated parametric photon pair generation, through a spontaneous parametric down-conversion (SPDC) process, in flakes as thin as about 46 nm. To our knowledge, this is the first SPDC source unambiguously demonstrated in two-dimensional layered materials, and the thinnest SPDC source ever reported. Our work opens an avenue towards developing van der Waals material-based ultracompact on-chip SPDC sources as well as high-performance photon modulators in both classical and quantum optical technologies1-4.

Phase Matching via Plasmonic Modal Dispersion for Third Harmonic Generation.

The overall effectiveness of nonlinear optical processes along extended nonlinear media highly depends on the fulfillment of the phase-matching condition for pump and generated fields. This is traditionally accomplished by exploiting the birefringence of nonlinear crystals requiring long interaction lengths (cm-scale). For nonbirefringent media and integrated photonic devices, modal phase matching can compensate the index mismatch. Here, the various interacting waves propagate in transverse modes with appropriate phase velocities, but they suffer from a low refractive index contrast and cm-scale interaction lengths. This work harnesses modal phase matching for third-harmonic generation (THG) in plasmonic waveguides using an organic polymer (poly[3-hexylthiophene-2,5-diyl]) as the nonlinear medium. One demonstrates experimentally an effective interaction area as small as ≈ 0.11 µm2 and the phase-matched modal dispersion results in THG efficiency as high as ≈ 10-3 W-2 within an effective length scale of ≈ 4.3 µm. THG also shows a strong correlation with the polarization of the incident laser beam, corresponding to the excitation of the antisymmetric plasmonic modes, corroborating that plasmonic modal phase matching is achieved. This large reduction in device area of orders of magnitude is interesting for various applications where space is critical (e.g., device integration or on-chip applications).

Dynamic Tuning of Moiré Superlattice Morphology by Laser Modification.

In artificial van der Waals (vdW) layered devices, twisting the stacking angle has emerged as an effective strategy to regulate the electronic phases and optical properties of these systems. Along with the twist registry, the lattice reconstruction arising from vdW interlayer interaction has also inspired significant research interests. The control of twist angles is significantly important because the moiré periodicity determines the electron propagation length on the lattice and the interlayer electron-electron interactions. However, the moiré periodicity is hard to be modified after the device has been fabricated. In this work, we have demonstrated that the moiré periodicity can be precisely modulated with a localized laser annealing technique. This is achieved with regulating the interlayer lattice mismatch by the mismatched lattice constant, which originates from the variable density of sulfur vacancy generated during laser modification. The existence of sulfur vacancy is further verified by excitonic emission energy and lifetime in photoluminescence measurements. Furthermore, we also discover that the mismatched lattice constant has the equivalent contribution as the twist angle for determining the lattice mismatch. Theoretical modeling elaborates the moiré-wavelength-dependent energy variations at the interface and mimics the evolution of moiré morphology.

Data-driven discovery of high performance layered van der Waals piezoelectric NbOI2.

Using high-throughput first-principles calculations to search for layered van der Waals materials with the largest piezoelectric stress coefficients, we discover NbOI2 to be the one among 2940 monolayers screened. The piezoelectric performance of NbOI2 is independent of thickness, and its electromechanical coupling factor of near unity is a hallmark of optimal interconversion between electrical and mechanical energy. Laser scanning vibrometer studies on bulk and few-layer NbOI2 crystals verify their huge piezoelectric responses, which exceed internal references such as In2Se3 and CuInP2S6. Furthermore, we provide insights into the atomic origins of anti-correlated piezoelectric and ferroelectric responses in NbOX2 (X = Cl, Br, I), based on bond covalency and structural distortions in these materials. Our discovery that NbOI2 has the largest piezoelectric stress coefficients among 2D materials calls for the development of NbOI2-based flexible nanoscale piezoelectric devices.

In-Plane Anisotropic Nonlinear Optical Properties of Two-Dimensional Organic-Inorganic Hybrid Perovskite.

Two-dimensional (2D) organic-inorganic hybrid perovskites (OIHPs) with superior nonlinear optical (NLO) properties show great versatility in frequency upconversion applications. Optical anisotropy plays an indispensable role in interpreting the interactions between incoming photons and crystal structure. Recently, the in-plane anisotropic NLO properties of 2D OIHPs have been reported and attracted much attention. However, the structure-related NLO anisotropy of the 2D OIHP framework is not well-established. Here, NLO properties of (C6H5(CH2)2NH3)2PbI4 (PEPI), (C6H11NH3)2PbI4 (C6H11), and (C4H9NH3)2PbI4 (C4PI) were systematically studied to interrogate the correlation between the in-plane anisotropic NLO responses and its lattice structure. In-plane nonparametric NLO responses, e.g., two-photon photoluminescence (2PPL) and three-photon photoluminescence (3PPL), manifest similar anisotropy configurations for PEPI, C6H11, and C4PI regardless of aromatic, cyclic, or linear organic molecules; however, the anisotropies of THG signals are strongly dependent on the specific crystal structures of the individual flakes, and they are much higher than that of the multiphoton excited photoluminescence.

Impurity-Induced Emission in Re-Doped WS2 Monolayers.

Impurity doping is a viable route toward achieving desired subgap optical response in semiconductors. In strongly excitonic two-dimensional (2D) semiconductors such as transition metal dichalcogenides (TMDs), impurities are expected to result in bound-exciton emission. However, doped TMDs often exhibit a broad Stokes-shifted emission without characteristic features, hampering strategic materials engineering. Here we report observation of a well-defined impurity-induced emission in monolayer WS2 substitutionally doped with rhenium (Re), which is an electron donor. The emission exhibits characteristics of localized states and dominates the spectrum up to 200 K. Gate dependence reveals that neutral impurity centers are responsible for the observed emission. Using GW-Bethe-Salpeter equation (GW-BSE) calculations, we attribute the emission to transitions between spin-split upper Re band and valence band edge.

Hexagonal Boron Nitride Crystal Growth from Iron, a Single Component Flux.

The highest quality hexagonal boron nitride (hBN) crystals are grown from molten solutions. For hBN crystal growth at atmospheric pressure, typically the solvent is a combination of two metals, one with a high boron solubility and the other to promote nitrogen solubility. In this study, we demonstrate that high-quality hBN crystals can be grown at atmospheric pressure using pure iron as a flux. The ability to produce excellent-quality hBN crystals using pure iron as a solvent is unexpected, given its low solubility for nitrogen. The properties of crystals produced with this flux matched the best values ever reported for hBN: a narrow Raman E2g vibration peak (7.6 cm-1) and strong phonon-assisted peaks in the photoluminescence spectra. To further test their quality, the hBN crytals were used as a substrate for WSe2 epitaxy. WSe2 was deposited with a low nucleation density, indicating the low defect density of the hBN. Lastly, the carrier tunneling through our hBN thin layers (3.5 nm) follows the Fowler-Nordheim model, with a barrier height of 3.7 eV, demonstrating hBN's superior electrical insulating properties. This ability to produce high-quality hBN crystals in such a simple, environmentally friendly and economical process will advance two-dimensional material research by enabling integrated devices.

Room-temperature nonlinear Hall effect and wireless radiofrequency rectification in Weyl semimetal TaIrTe4.

The nonlinear Hall effect (NLHE), the phenomenon in which a transverse voltage can be produced without a magnetic field, provides a potential alternative for rectification or frequency doubling1,2. However, the low-temperature detection of the NLHE limits its applications3,4. Here, we report the room-temperature NLHE in a type-II Weyl semimetal TaIrTe4, which hosts a robust NLHE due to broken inversion symmetry and large band overlapping at the Fermi level. We also observe a temperature-induced sign inversion of the NLHE in TaIrTe4. Our theoretical calculations suggest that the observed sign inversion is a result of a temperature-induced shift in the chemical potential, indicating a direct correlation of the NLHE with the electronic structure at the Fermi surface. Finally, on the basis of the observed room-temperature NLHE in TaIrTe4 we demonstrate the wireless radiofrequency (RF) rectification with zero external bias and magnetic field. This work opens a door to realizing room-temperature applications based on the NLHE in Weyl semimetals.

Electron tunneling at the molecularly thin 2D perovskite and graphene van der Waals interface.

Quasi-two-dimensional perovskites have emerged as a new material platform for optoelectronics on account of its intrinsic stability. A major bottleneck to device performance is the high charge injection barrier caused by organic molecular layers on its basal plane, thus the best performing device currently relies on edge contact. Herein, by leveraging on van der Waals coupling and energy level matching between two-dimensional Ruddlesden-Popper perovskite and graphene, we show that the plane-contacted perovskite and graphene interface presents a lower barrier than gold for charge injection. Electron tunneling across the interface occurs via a gate-tunable, direct tunneling-to-field emission mechanism with increasing bias, and photoinduced charge transfer occurs at femtosecond timescale (~50 fs). Field effect transistors fabricated on molecularly thin Ruddlesden-Popper perovskite using graphene contact exhibit electron mobilities ranging from 0.1 to 0.018 cm2V-1s-1 between 1.7 to 200 K. Scanning tunneling spectroscopy studies reveal layer-dependent tunneling barrier and domain size on few-layered Ruddlesden-Popper perovskite.