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van der Waals antiferromagnets with chemical formula MPX3 (M = V, Mn, Fe, Co, Ni, Cd; X = S, Se) are superb platforms for exploring the fundamental properties of complex chalcogenides, revealing their structure-property relations and unraveling the physics of confinement. Pressure is extremely effective as an external stimulus, able to tune properties and drive new states of matter. In this review, we summarize experimental and theoretical progress to date with special emphasis on the structural, magnetic, and optical properties of the MPX3 family of materials. Under compression, these compounds host inter-layer sliding and insulator-to-metal transitions accompanied by dramatic volume reduction and spin state collapse, piezochromism, possible polar metal and orbital Mott phases, as well as superconductivity. Some responses are already providing the basis for spintronic, magneto-optic, and thermoelectric devices. We propose that strain may drive similar functionality in these materials.

One-dimensional (1D) van der Waals materials have emerged as an intriguing playground to explore novel electronic and optical effects. We report on inorganic one-dimensional SbPS4 nanotube bundles obtained via mechanical exfoliation from bulk crystals. The ability to mechanically exfoliate SbPS4 nanobundles offers the possibility of applying modern 2D material fabrication techniques to create mixed-dimensional van der Waals heterostructures. We find that SbPS4 can readily be exfoliated to yield long (>10 μm) nanobundles with thicknesses that range from 1.3 to 200 nm. We investigated the optical response of semiconducting SbPS4 nanobundles and discovered that upon excitation with blue light, they emit bright and ultra-broadband red light with a quantum yield similar to that of hBN-encapsulated MoSe2. We discovered that the ultra-broadband red light emission is a result of a large ∼1 eV exciton binding energy and a ∼200 meV exciton self-trapping energy, unprecedented in previous material studies. Due to the bright and ultra-broadband light emission, we believe that this class of inorganic 1D van der Waals semiconductors has numerous potential applications, including on-chip tunable nanolasers, and applications that require ultraviolet to visible light conversion, such as lighting and sensing. Overall, our findings open avenues for harnessing the unique characteristics of these nanomaterials, advancing both fundamental research and practical optoelectronic applications.

Slow light effects are interesting for telecommunications and quantum photonics applications. Here, the authors use coupled exciton-surface plasmon polaritons (SPPs) in a hybrid monolayer WSe2-metallic waveguide structure to demonstrate a 1300-fold reduction of the SPP group velocity.

The spectrally narrow photoluminescence lines occurring in transition metal dichalcogenides (TMD) heterostructures at low temperature have been attributed to interlayer excitons (IXs) localized by the moiré potential between the TMD layers. Here, the authors show that these lines are present even when the moiré potential is suppressed by inserting an hBN spacer between the TMD layers.

Abstract van der Waals solids are ideal platforms for the discovery of new states of matter and emergent properties under external stimuli. Under pressure, complex chalcogenides like MPS3 (M = Mn, Ni, Co, V) host sliding and structural transitions, insulator-to-metal transitions, the possibility of an orbitally-selective Mott state, piezochromism, and superconductivity. In this work, we bring together diamond anvil cell techniques, infrared and Raman scattering spectroscopies, and X-ray diffraction with a detailed symmetry analysis and first-principles calculations to uncover a series of high-pressure phases in NiPS3. Remarkably, we find five different states of matter between ambient conditions and 39 GPa—quite different than in the other MPS3 materials. Even more strikingly, infrared spectroscopy and X-ray diffraction combined with a symmetry analysis reveal both metallicity and loss of the inversion center above ~23 GPa suggesting that NiPS3 may be a polar metal with a P3m1 space group under these conditions and P1 symmetry under maximum compression. In addition to identifying a candidate polar metal ripe for further inquiry, we suggest that pressure may tune other complex chalcogenides into this elusive state.

Free carriers and electrical polarization coexist in ferroelectric metals. Here, the authors use a capacitive method to probe the electronic compressibility of free carriers in a tunable semimetal, extract the polarized contribution, and study the carrier dependence of the ferroelectric state.

Abstract MoSe2–WSe2 heterostructures host strongly bound interlayer excitons (IXs), which exhibit bright photoluminescence (PL) when the twist angle is near 0° or 60°. Over the past several years, there have been numerous reports on the optical response of these heterostructures but no unifying model to understand the dynamics of IXs and their temperature dependence. Here we perform a comprehensive study of the temperature, excitation power, and time-dependent PL of IXs. We observe a significant decrease in PL intensity above a transition temperature that we attribute to a transition from localized to delocalized IXs. Astoundingly, we find a simple inverse relationship between the IX PL energy and the transition temperature, which exhibits opposite power-dependent behaviors for near 0° and 60° samples. We conclude that this temperature dependence is a result of IX–IX exchange interactions, whose effect is suppressed by the moiré potential trapping IXs at low temperature.

Here, the authors report intrinsic donor bound dark exciton states with associated phonon replicas in monolayer WSe2, and defect control crystal synthesis for the deterministic creation of these states.

When semiconducting transition metal dichalcogenide heterostructures are stacked, the twist angle and lattice mismatch lead to a periodic moiré potential. As the angle between the layers changes, so do the electronic properties. As the angle approaches 0° or 60°, interesting characteristics and properties, such as modulations in the band edges, flat bands, and confinement, are predicted to occur. Here, we report scanning tunneling microscopy and spectroscopy measurements on the bandgaps and band modulations in MoSe2/WSe2 heterostructures with near 0° rotation (R-type) and near 60° rotation (H-type). We find a modulation of the bandgap for both stacking configurations with a larger modulation for R-type than for H-type as predicted by theory. Furthermore, local density of states images show that electrons are localized differently at the valence band and conduction band edges.

In monolayer semiconductors phonons with momentum vectors pointing to the corners of the hexagonal Brillouin zone couple strongly to carriers’ spin and valley degree of freedom. Here, the authors report the observation of multiple valley phonons and the resulting exciton complexes in the monolayer semiconductor WSe2.

Plasmonic modulators have many possible applications in optical-frequency devices. Here the authors report a 2D semiconductor nonlinear plasmonic modulator enabled through strong interaction between the surface plasmon polaritons and excitons in a monolayer semiconductor integrated on top of a metallic waveguide.

Van der Waals heterostructures offer a platform for harnessing the spin-valley degree of freedom for information processing. Here, the authors transfer optically generated spin-valley polarization from one layer to another in a two-dimensional molybdenum diselenide–tungsten diselenide heterostructure.

In this paper, high performance top-gated WSe2 field effect transistor (FET) devices are demonstrated via a two-step remote plasma assisted ALD process. High-quality, low-leakage aluminum oxide (Al2O3) gate dielectric layers are deposited onto the WSe2 channel using a remote plasma assisted ALD process with an ultrathin (∼1 nm) titanium buffer layer. The first few nanometers (∼2 nm) of the Al2O3 dielectric film is deposited at relatively low temperature (i.e. 50 °C) and remainder of the film is deposited at 150 °C to ensure the conformal coating of Al2O3 on the WSe2 surface. Additionally, an ultra-thin titanium buffer layer is introduced at the WSe2 channel surface prior to ALD process to mitigate oxygen plasma induced doping effects. Excellent device characteristics with current on–off ratio in excess of 106 and a field effect mobility as high as 70.1 cm2 V–1 s–1 are achieved in a few-layer WSe2 FET device with a 30 nm Al2O3 top-gate dielectric. With further investigation and careful optimization, this method can play an important role for the realization of high performance top gated FETs for future optoelectronic device applications. [ABSTRACT FROM AUTHOR]