Physical Review Applied

Embedding 2D materials, namely single-layers of MoSe${}_2$ or WSe${}_2$, between thin layers of hexagonal boron nitride of supreme quality suppresses the structural disorder and avoids surface contamination. As a result, the optical properties of such heterostructures improve drastically with respect to unprotected samples, approaching characteristics expected for ideal 2D crystals. Yet, how can one tell if the spectral line-shape of optical transitions is measured free from the disturbing and mostly irrelevant features introduced by the disorder? Do I really observe the intrinsic (the so-called, homogeneous) linewidth in linear absorption or emission? If yes, what are the spatial extensions on which such optimal conditions can be maintained? The authors here employ methods of nonlinear spectroscopy to accurately address these questions.

Just as the propagation of coherent photons can be manipulated by a diffraction grid, so can propagation of coherent electrons be manipulated by periodic gating in bilayer graphene. Similar behavior should be expected of coherent electrons and photons, the main difference being that for electrons, the electrostatic potential can be modulated externally. The authors’ computations of the resistance map as a function of the voltage applied to the extended bottom gate, and to the periodic top gate, reveal an intriguing pattern reminiscent of Fabry-Perot interferometry. It seems, then, that periodically gated bilayer graphene could function as a distributed Bragg reflector for electrons.

Graphene is a tunable plasmonic material, and the resonance of graphene split-ring resonators (SRRs) is predicted to induce very strong field confinement. Recent years have seen many studies of graphene-based SRRs with analytical calculations and numerical simulations, yet without experimental demonstration, due to practical challenges in sample preparation and measurement. This is where the authors have succeeded, by producing tunable, ultracompact SRR arrays based on graphene. Their work experimentally bridges the gap between graphene and magnetic metasurfaces in the terahertz regime, making significant progress toward multifunctional graphene-based metasurfaces.

Interfaces between topological insulators (TIs) and magnetic materials, including 2D van der Waals magnets, are currently generating enormous interest, due to phenomena such as the quantum anomalous Hall effect. Many of the magnets employed contain reactive and diffusive elements such as Mn, however, raising serious materials challenges. This work explores the interface between Bi${}_2$Se${}_3$ and Mn and MnSe, revealing chemical and electronic changes at the interface depending on how Mn is deposited. Mn deposited without excess selenium shows Se out-diffusion from Bi${}_2$Se${}_3$ to the Mn. Supplying excess Se alongside Mn prevents this Se out-diffusion, but the interface becomes affected by a negative surface dipole. This dipole forms due to preferential creation of $\alpha$-MnSe(111) over the 1T-MnSe${}_2$ phase. This study shows that chemical diffusion and dipole formation are important for Mn-Bi${}_2$Se${}_3$ and MnSe${}_{2-x}$-Bi${}_2$Se${}_3$ interfaces and must generally be considered at TI/Mn chalcogenide interfaces.

Devices based on few-layer transition-metal dichalcogenides are rapidly being developed for various quantum technologies, such as valleytronic qubits and quantum emitters. Gate-defined quantum dots provide an appealing platform for coherent control of individual valley pseudospins, but well-resolved, discrete energy levels are required. The authors report gate-defined quantum dots in monolayer and bilayer WSe${}_2$, small enough to allow observation of transport through discrete levels. These devices thus satisfy an essential requirement for the development of (opto)electronic qubits based on valley-pseudospin states.

2D superconductors exhibit novel aspects which are distinct from those of the 3D counterparts. Vortex matter is one of them. Here, the authors report a dynamical vortex phase diagram at zero magnetic field for ion-gated MoS${}_2$. They find that superconductivity is destroyed by current through the dissociation of vortex-antivortex pairs with multiple steps. They are accounted for by the kinematic vortex flow states, including phase slip line formation. Based on the present result, the authors present a comprehensive vortex phase diagram for clean 2D superconductors.

$Klein$ $tunneling$ is the intriguing ability of massless Dirac electrons to tunnel through potential barriers, distinguishing graphene electronics from conventional electronics. Graphene’s electronic structure also features a valley degree of freedom that can be used as an information carrier. Here the authors discover a remarkable functionality that arises counterintuitively from intervalley scattering: Quantum interference of intervalley backscatterings creates a pseudospin gap in a superlattice barrier, selectively blocking transmission in one valley while permitting Klein tunneling in the other. Thus a sort of valleytronic polarizer of pseudospin current could be realized.

The magnetic van der Waals materials with the highest ordering temperatures are generally metallic ferromagnets, such as Fe${}_5$GeTe${}_2$ with a Curie temperature near 300 K. In this work, the authors have demonstrated an ability to produce an antiferromagnetic state with an enhanced critical temperature by substituting cobalt for iron. While such a change to the magnetism is not uncommon, the important finding here is that the change in magnetic order is coupled not just to the chemical modification but also to a change in layer stacking that is induced by the cobalt substitution.

Ordered borides of group IB and IIB metals (Cu, Ag, Au, Zn, Cd, Hg), as the well-known immiscible materials, are practically unknown due to the small electronegativity difference and large mismatch in their atomic sizes. Nevertheless, such rule may be broken under extreme conditions, i.e., high pressure or low dimensionality. Here two-dimensional (2D) copper borides were predicted from ab initio evolutionary searches, identifying that two structures are metallic whereas another one is strikingly a nodal line semimetal. These results challenge the long lasting puzzle in the immiscible systems and add new members to the 2D materials.

Perfect absorption by a two-dimensional (2D) system allows extreme sensitivity to small modulations in light intensity, enabling a host of applications. The phenomenon typically requires complex photonic structures or multiple coherent beams, but this study demonstrates perfect absorption using just a monolayer of MoSe${}_2$ in front of a flat mirror. Success is due to the strong exciton-photon interaction (compared to loss and inhomogeneity) that is unique to 2D semiconductors. With its robustness, simplicity, and flexibility in exciton control, this system provides a route for ultrafast energy-efficient modulation of perfect absorption on integrated semiconductor platforms.

Monolayer semiconductors have the potential to transform the industry, but their intrinsic properties are difficult to characterize due to strong interaction with the environment. This study uses multidimensional coherent spectroscopy to measure the intrinsic homogeneous linewidth of a monolayer transition-metal dichalcogenide that has been isolated from its environment through encapsulation. The measurements reveal that the linewidths become substantially narrower after encapsulation, indicating reduced sample degradation. These findings have the potential to impact the materials and process-control methods used in the semiconductor industry.

It requires substantial computational resources to design plasmonic circuits such as resonators, topological waveguides, modulators, and photonic switches based on graphene and other conducting 2D materials, so a comprehensive study is called for. The authors propose numerical approaches to solve the problem through the transfer-matrix method using reconstructed numerical solutions of Maxwell equations, and find the limits of the method’s applicability. The results would facilitate the design of plasmonic circuits by means of electrostatic gating, ultrafast photoexcitation, Moire, and substrate engineering at the visible to near-infrared telecommunication frequencies.

This work studies moiré structures formed in homobilayer transition metal dichalcogenides (TMDs) due to twisting and/or uniform strain, where the layer index serves as a pseudospin. The layer pseudospin exhibits vortex/antivortex textures in the moiré supercell. Such spatial texture gives rise to a pseudomagnetic field and a geometric scalar potential on low energy electrons. Strain and interlayer bias are shown to tune the in-plane and out-of-plane pseudospin texture, hence, the landscape of the moiré magnetic field and scalar potential. The findings suggest that TMD moiré structures are promising to build tunable flux lattices for exploration of novel transport and topological phenomena.

Controlling the magnetic and electronic properties of 2D materials is important for their promising applications in low-dimensional spintronics. In this paper, the authors are focused on the control and manipulation of spin-spin interactions in monolayer CrI${}_3$ by applying strain and electric fields. A suitable spin model Hamiltonian is used to compute the isotropic and anisotropic symmetric exchange interactions, Dzyaloshinskii-Moriya interactions, and anisotropy energy of monolayer CrI${}_3$ as a representative of 2D transition metal trihalides.

Electrical switching of valley polarization in 2D semiconductors could enable fast and energy-efficient optoelectronics devices that communicate with photon helicity but its realization remains challenging because the electronic valley degree of freedom does not directly couple to electric fields. In this paper, by proximity coupling 2D semiconductors to 2D magnets, and by electrically switching the spins of the 2D magnet, reversible electrical switching of the valley polarization has been demonstrated. This study also paves the path for high-speed valleytronics devices.

The chiral edge states in the quantum Hall regime in 2D materials propagate without dissipation, providing an attractive platform for electronic interactions, resistance standard and topological quantum computation. In particular, atomically thin black phosphorus is attractive due to its high electron mobility and a direct band gap that is tunable by electric field, layer and strain. This work investigates the interaction among the edge states in atomically thin BP devices by introducing regions with different doping levels. The authors observe gate-tunable filtering of edge state transmission, as well as full and selective equilibration of edge states at the interfaces.

Two-dimensional van der Waals superconductors offer a range of properties advantageous for quantum information processing. Integrating these materials into the well-developed platform of aluminum-based superconducting circuits requires robust zero-resistance contacts. Here the authors introduce a simple method for fabricating such contacts between deposited Al and exfoliated few-layer NbSe${}_2$ and they demonstrate that the Al/NbSe${}_2$ interface exhibits Josephson-junction-like behavior in the presence of magnetic flux. The anomalous periodicity of the interference patterns indicates that the junction phase difference depends on the flux ${\mathrm{\Phi }}^{JJ}$ through adjacent regions of the 2D crystal which are an appreciable fraction of the size of the 2D crystal itself, in striking contrast with 3D-3D Josephson junction behavior.

Analogous to the keen interest in electron, hole, and exciton spin relaxation during the early days of semiconductor spintronics, measurements of valley relaxation in monolayer transition-metal dichalcogenide (TMD) semiconductors such as WSe2 are currently a focus of attention for potential applications in valleytronics. For many notional valleytronic devices, the important parameter is the intrinsic valley relaxation time of the resident electrons and holes that exist in n-type and p-type TMD monolayers. Using optical methods, the authors determine these timescales as a systematic function of carrier density, and study the (important) role of the underlying substrate. Microsecond-long valley relaxation of carriers is revealed at low densities.

A unique dense network of multi-member-ring round hole defects is formed in monolayer WSe${}_2$ from the evolution of multivacancies by suitable control of a scanning focused electron beam, whereas the same process leads predominantly to chalcogen-vacancy line defect array in other trigonal-prismatic transition metal dichalcogenide (TMDC) monolayers. Density functional theory (DFT) calculations track the formation of the observed complex multivacancy structures and find that the underlying atomic-scale processes are quasi-thermodynamic, which elaborates the formation mechanism of dense round hole defects in WSe${}_2$ monolayers. The high-density round holes in WSe${}_2$ hold promise for novel applications such as atomic and molecular sieving.

Defect engineering plays an important role in tailoring the electronic transport properties of van der Waals materials. Methods reported so far mainly rely on the $exsitu$ engineering of defect type and concentration, hindering the realization of new types of device functionalities associated with defect engineering. Here, the authors report temperature-sensitive spatial redistribution of defects in PdSe${}_2$ thin flakes through scanning tunneling microscopy. The spatial characteristics of defect distribution is strongly related to the electronic transport properties such as anisotropic carrier mobility and phase coherent length, indicating a different avenue for creating novel device functionalities based on $insitu$ modulation of defect distribution.

Electrostatic gating with ionic liquids, contributing to the accumulation of high surface charge carriers, has been often exploited in thin-film transistors. However, the intrinsic liquid nature of ionic liquids inhibits them from constituting a practical platform for thin-film devices. To this end, lithium-ion solid electrolytic substrates, often used for battery technology, offer similar benefits as ionic-liquids, with the added advantage of solid-state compatibility. In this work, the authors explore a lithium-ion solid electrolytic substrate for direct growth of MoS${}_2$ by chemical vapor deposition method. A near ideal subthreshold swing around 65 mV/dec with field-effect mobility values of 42-49 cm${}^2$V${}^{-1}$s${}^{-1}$ has been achieved with the devices on as-grown crystal, back-gated by the solid electrolyte.

What happens to the electronic properties of a two-dimensional material when it is not ideal anymore? In order to answer this question, the authors studied more than 150 different structural triangular defects in periodic MoS${}_2$ monolayers by means of density-functional calculations, thereby varying the defect size and their distance within the monolayer. The resulting energetic properties can be viewed as a combination of edge and bulk properties dominated by the defect size. Mid-gap edge-localized states are introduced into the density of states by the defects. The grade of orbital localization was quantified by a newly defined localization parameter.

Low-resistance ohmic contacts are a prerequisite for implementing two-dimensional transition-metal dichalcogenides (2D TMDs) in a host of applications. Edge contacts offer unique advantages, yet their electrical properties are not fully understood. Employing an $ab$ $initio$ framework, the authors find that edge contacts to monolayer MoS${}_2$ are pinned to a charge-neutrality level close to the valence band and are $p$-type—unlike typical $n$-type top contacts. This anisotropy in Fermi-level pinning complicates conduction through metal-TMD interfaces and affects design guidelines for low-resistance contacts. Challenges, potential mitigating solutions, and opportunities that leverage this anisotropy are discussed.

Trichalcogenide semiconductors could provide a platform for quasi-one-dimensional electronic devices, on condition that their tunable doping capability is established. This work presents a synthetic strategy towards degenerately doped titanium trisulfide nanostructures that feature ultralow carrier activation barriers and exceptional ambient stability. The anomalous electrical conductivity of the synthesized nanostructures is revealed both experimentally and computationally to be associated with high-density S22- vacancies, which are generated spontaneously during the chemical vapor deposition process and act effectively as the electron dopants. These results hence portend the tantalizing possibility of constructing nanoscale electronics with defect-engineered trichalcogenide semiconductors.