Highly Efficient Spin-Orbit Torque Switching in BiSe/FeGeTe van der Waals Heterostructures.

Topological insulators (TIs) have shown promise as a spin-generating layer to switch the magnetization state of ferromagnets via spin-orbit torque (SOT) due to charge-to-spin conversion efficiency of the TI surface states that arises from spin-momentum locking. However, when TIs are interfaced with conventional bulk ferromagnetic metals, the combination of charge transfer and hybridization can potentially destroy the spin texture and hamper the possibility of accessing the TI surface states. Here, we fabricate an all van der Waals (vdW) heterostructure consisting of molecular beam epitaxy grown bulk-insulating BiSe and exfoliated 2D metallic ferromagnet FeGeTe (FGT) with perpendicular anisotropy.

Emergent Moiré Phonons Due to Zone Folding in WSe-WS Van der Waals Heterostructures.

Bilayers of 2D materials offer opportunities for creating devices with tunable electronic, optical, and mechanical properties. In van der Waals heterostructures (vdWHs) where the constituent monolayers have different lattice constants, a moiré superlattice forms with a length scale larger than the lattice constant of either constituent material regardless of twist angle. Here, we report the appearance of moiré Raman modes from nearly aligned WSe-WS vdWHs in the range of 240-260 cm, which are absent in both monolayers and homobilayers of WSe and WS and in largely misaligned WSe-WS vdWHs.

Spin-Sensitive Epitaxial InSe Tunnel Barrier in InSe/BiSe Topological van der Waals Heterostructure.

Current-generated spin arising from spin-momentum locking in topological insulator (TI) surface states has been shown to switch the magnetization of an adjacent ferromagnet (FM) via spin-orbit torque (SOT) with a much higher efficiency than heavy metals. However, in such FM/TI heterostructures, most of the current is shunted through the FM metal due to its lower resistance, and recent calculations have also shown that topological surface states can be significantly impacted when interfaced with an FM metal such as Ni and Co. Hence, placing an insulating layer between the TI and FM will not only prevent current shunting, therefore minimizing overall power consumption, but may also help preserve the topological surface states at the interface.

Effect of alloying in monolayer niobium dichalcogenide superconductors.

When sulfur and silicon are incorporated in monolayer 2H-NbSe the superconducting transition temperature, T, has been found to vary non-monotonically. This was assumed to be a manifestation of fractal superconductivity. Using first-principles calculations, we show that the nonmonotonic dependence of T is insufficient evidence for multifractality.

Metallic semiconducting properties of quasi-one-dimensional tantalum selenide van der Waals nanoribbons.

We conducted a tip-enhanced Raman scattering spectroscopy (TERS) and photoluminescence (PL) study of quasi-1D TaSe nanoribbons exfoliated onto gold substrates. At a selenium deficiency of ∼ 0.25 (Se/Ta = 2.

Laser-Patterned Submicrometer BiSe-WS Pixels with Tunable Circular Polarization at Room Temperature.

Characterizing and manipulating the circular polarization of light is central to numerous emerging technologies, including spintronics and quantum computing. Separately, monolayer tungsten disulfide (WS) is a versatile material that has demonstrated promise in a variety of applications, including single photon emitters and valleytronics. Here, we demonstrate a method to tune the photoluminescence (PL) intensity (factor of ×161), peak position (38.

Stacking-dependent optical properties in bilayer WSe.

The twist angle between the monolayers in van der Waals heterostructures provides a new degree of freedom in tuning material properties. We compare the optical properties of WSe homobilayers with 2H and 3R stacking using photoluminescence, Raman spectroscopy, and reflectance contrast measurements under ambient and cryogenic temperatures. Clear stacking-dependent differences are evident for all temperatures, with both photoluminescence and reflectance contrast spectra exhibiting a blue shift in spectral features in 2H compared to 3R bilayers.

Direct-Write of Nanoscale Domains with Tunable Metamagnetic Order in FeRh Thin Films.

We have directly written nanoscale patterns of magnetic ordering in FeRh films using focused helium-ion beam irradiation. By varying the dose, we pattern arrays with metamagnetic transition temperatures that range from the as-grown film temperature to below room temperature. We employ transmission electron microscopy, X-ray diffraction, and temperature-dependent transport measurements to characterize the as-grown film, and magneto-optic Kerr effect imaging to quantify the He irradiation-induced changes to the magnetic order.

Exciton Fine Structure in Perovskite Nanocrystals.

The bright emission observed in cesium lead halide perovskite nanocrystals (NCs) has recently been explained in terms of a bright exciton ground state [ Becker et al. Nature 2018 , 553 , 189 - 193 ], a claim that would make these materials the first known examples in which the exciton ground state is not an optically forbidden dark exciton. This unprecedented claim has been the subject of intense experimental investigation that has so far failed to detect the dark ground-state exciton.

Interlayer resistance of misoriented MoS.

Interlayer misorientation in transition metal dichalcogenides alters their interlayer distance, total energy, electronic band structure, and vibrational modes, but its effect on the interlayer resistance is not known. This study analyzes the interlayer resistance of misoriented bilayer MoS as a function of the misorientation angle, and it shows that interlayer misorientation exponentially increases the electron resistivity while leaving the hole resistivity almost unchanged. The physics, determined by the wave functions at the high symmetry points, are generic among the popular semiconducting transition metal dichalcogenides (TMDs).

Phase Engineering of 2D Tin Sulfides.

Tin sulfides can exist in a variety of phases and polytypes due to the different oxidation states of Sn. A subset of these phases and polytypes take the form of layered 2D structures that give rise to a wide host of electronic and optical properties. Hence, achieving control over the phase, polytype, and thickness of tin sulfides is necessary to utilize this wide range of properties exhibited by the compound.

Fundamentals of lateral and vertical heterojunctions of atomically thin materials.

At the turn of this century, Herbert Kroemer, the 2000 Nobel Prize winner in Physics, famously commented that "the interface is the device". This statement has since opened up unparalleled opportunities at the interface of conventional three-dimensional (3D) materials (H. Kroemer, Quasi-Electric and Quasi-Magnetic Fields in Non-Uniform Semiconductors, RCA Rev.

Two step growth phenomena of molybdenum disulfide-tungsten disulfide heterostructures.

Here, we report the first demonstration of atomically thin vertically stacked MoS2/WS2 heterostructures, achieved via a two-step chemical vapour deposition (CVD) growth process. Highly ordered stacking of heterostructure domains and patterned defects have been observed. Computations based on first principles have been performed to understand observed enhanced photoluminescence of the heterostructure.

Direct bandgap transition in many-layer MoS2 by plasma-induced layer decoupling.

We report a robust method for engineering the optoelectronic properties of many-layer MoS2 using low-energy oxygen plasma treatment. Gas phase treatment of MoS2 with oxygen radicals generated in an upstream N2 -O2 plasma is shown to enhance the photoluminescence (PL) of many-layer, mechanically exfoliated MoS2 flakes by up to 20 times, without reducing the layer thickness of the material. A blueshift in the PL spectra and narrowing of linewidth are consistent with a transition of MoS2 from indirect to direct bandgap material.

Tin disulfide-an emerging layered metal dichalcogenide semiconductor: materials properties and device characteristics.

Layered metal dichalcogenides have attracted significant interest as a family of single- and few-layer materials that show new physics and are of interest for device applications. Here, we report a comprehensive characterization of the properties of tin disulfide (SnS2), an emerging semiconducting metal dichalcogenide, down to the monolayer limit. Using flakes exfoliated from layered bulk crystals, we establish the characteristics of single- and few-layer SnS2 in optical and atomic force microscopy, Raman spectroscopy and transmission electron microscopy.

Electronic and thermoelectric properties of few-layer transition metal dichalcogenides.

The electronic and thermoelectric properties of one to four monolayers of MoS2, MoSe2, WS2, and WSe2 are calculated. For few layer thicknesses, the near degeneracies of the conduction band K and Σ valleys and the valence band Γ and K valleys enhance the n-type and p-type thermoelectric performance. The interlayer hybridization and energy level splitting determine how the number of modes within kBT of a valley minimum changes with layer thickness.

Charge density waves in exfoliated films of van der Waals materials: evolution of Raman spectrum in TiSe2.

A number of the charge-density-wave materials reveal a transition to the macroscopic quantum state around 200 K. We used graphene-like mechanical exfoliation of TiSe(2) crystals to prepare a set of films with different thicknesses. The transition temperature to the charge-density-wave state was determined via modification of Raman spectra of TiSe(2) films.