Hydrogen-terminated diamond is a promising platform for diamond-based high-power-frequency electronics and radiation-hardened semiconductor devices exhibiting controllable electrical conduction through an accumulated sub-surface 2D hole-gas layer. The diamond conductivity is highly sensitive to local surface conditions, offering new opportunities for device engineering, quantum sensors, and switches, as well as presenting a challenge for device stability. Here we study the impact of focused electron-beam exposure on hydrogen-terminated transfer-doped diamond, revealing a reversible loss of local conductivity. Using Kelvin probe force microscopy (KPFM), we show that irradiation leads to a local change in surface potential and find that both ultra-violet (UV) light and ambient exposure can act to restore the conductivity. We propose that the electron beam injects negative subsurface charge into deep trap states that electrostatically gate the local hole transport. In some cases, local electron-stimulated desorption of electron-accepting surface species may also contribute.

Achieving low contact resistance is a critical challenge in the development of p-type transistors that use monolayer transition metal dichalcogenides, such as tungsten diselenide (WSe2), as their channel material. Contacts made with high work function metals require deposition at high temperatures, which typically creates defects or strain at the metal-channel interface. One solution is to use metallic two-dimensional (2D) materials that have atomically flat surfaces and can be deposited at low temperatures, as have been reported for n-type semiconductors. However, the comparatively large bandgap of WSe2 has hindered experimental progress with p-type transistors. Here we show that metallic layered Nb0.3W0.7Se2 can be used to create contacts for monolayer and bilayer WSe2 field-effect transistors with channel lengths down to 100 nm. Our 2D-2D contacted field-effect transistors exhibit on-current densities of up to 358 & micro;A & micro;m-1 and 1.1 mA & micro;m-1 on monolayer and bilayer WSe2 channels, respectively. In combination with scaled gate dielectrics (effective oxide thickness of 1.3 nm), the fabricated 2D-2D contacted monolayer WSe2 devices achieve a subthreshold swing of 88 mV dec-1.

We measure the temperature-dependent in-plane thermal conductivity, kappa(parallel to)(T), of high-purity gamma-InSe bulk single crystals and exfoliated thin flakes (30-50 nm) from 50 to 300 K. Our bulk results agree with prior bulk reports and provide a reproducible reference for phonon transport. In the literature, cross-plane thermal conductivity of supported InSe flakes shows relatively modest variation, whereas reported room-temperature in-plane values for supported flakes span a wide range, including outliers that exceed bulk despite much smaller thickness. In our measurements, the flake kappa parallel to at room temperature is lower than bulk, as expected, but exhibits substantial sample-to-sample variability; despite being thinner, intrinsic flakes show higher kappa(parallel to) than doped flakes, consistent with reduced impurity scattering and improved flake uniformity. We analyze our data using a Callaway-type phonon-scattering model in which substrate interactions (including plausible strain-related renormalization of acoustic parameters) contribute to the observed spread near room temperature. However, below similar to 150 K, the measured trends cannot be captured without invoking changes in sound velocity far larger than realistic strain levels, indicating that additional mechanisms beyond simple strain renormalization are required at low temperature. These measurements provide low-temperature kappa parallel to benchmarks for gamma-InSe flakes and constrain how much of the reported room-temperature spread can be explained by thickness, nonuniformity, and substrate effects alone.

Low-dimensional media have exhibited optical anisotropy that is unachievable in traditional 3D media due to the asymmetry of their strong, in-plane covalent bonds and weak out-of-plane van der Waals interactions. As a result, 2D media are promising building blocks for ultrathin devices such as polarimeters, polarized light sources, and active polarizers. III-VI semiconductors possess a rare property in the class of multilayered semiconductors, which is that their fundamental excitons are oriented out-of-plane. This allows them to exhibit phenomena such as transparency in the visible range while also being emissive in the visible and near-infrared ranges. Here, we report the first experimental values for the anisotropic refractive indices of γ-InSe and ε-GaSe, and we observe the effects of the out-of-plane excitons on the c-axis refractive index. It is found that both materials exhibit moderate optical anisotropy for multilayered semiconductors. The complex, anisotropic refractive index of γ-InSe and ε-GaSe enables the accurate simulation of these media, allowing for the design of high-performance, ultra-compact optoelectronic devices.

Multiferroics host simultaneous and coupled ferroic orders, allowing disparate external stimuli to induce abrupt transformations in their structures and properties. Creating multiferroicity in two-dimensional (2D) van der Waals (vdW) platforms would add the elements of strong quantum confinement, enhanced quasiparticle excitations, and wide tunability to the capabilities of these systems. In this work, we constructed vdW heterostructures of ferromagnetic triiron gallium ditelluride (Fe3GaTe2) and ferroelectric copper indium thiophosphate (CuInP2S6) to integrate their respective orders. We observed strong interferroic coupling at room temperature by demonstrating ferroelectrically reconfigurable magnetic anisotropy of 2D Fe3GaTe2. The interferroic magnetoelectricity diminished with increasing Fe3GaTe2 thickness, revealing the interfacial nature of heterostructure multiferroicity. Our discovery of all-vdW heterostructure multiferroicity opens the door to the artificial assembly of vdW layers for designer 2D multiferroics.

Two-dimensional (2D) materials are promising platforms for phonon polaritons (PhPs)—hybrid modes arising from photon-phonon interactions—that enable subwavelength light confinement and optoelectronic functionalities. γ-indium selenide (γ-InSe), an emerging 2D semiconductor with thickness-dependent optoelectronic properties, offers a unique system to explore confined surface phonon polaritons (SPhPs). Here, we employ tilt angle-dependent resonant Raman spectroscopy to measure SPhPs in γ-InSe flakes with thicknesses from 10 nm to 1.2 μm, exfoliated on SiO2 and sapphire substrates. We observe a pronounced thickness-dependent dispersion of the polar A1(TO) phonon mode, with a maximal slope near 100-nm thickness, corresponding to the inverse absorption coefficient. Moreover, differences in the polariton dispersion on SiO2 and sapphire show a dependence on the dielectric environment. The dispersions align with calculations modeling the flakes as Fabry-Perot cavities supporting confined SPhPs. These findings advance our understanding of light-matter interactions in γ-InSe and highlight the potential for their use in nanophotonic devices.

Two-dimensional (2D) 1T-VSe2 has prompted significant interest due to the discrepancies regarding alleged ferromagnetism (FM) at room temperature, charge density wave (CDW) states, and the interplay between the two. We employed a combined Diffusion Monte Carlo (DMC) and density functional theory (DFT) approach to accurately investigate the magnetic properties, CDW states, and their responses to strain in monolayer 1T-VSe2. Our calculations show the delicate competition between various phases, revealing critical insights into the relationship between their energetic and structural properties. We performed classical Monte Carlo simulations informed by our DMC and DFT results and found the magnetic transition temperature (Tc) of the undistorted (non-CDW) FM phase to be 228 K and the distorted (CDW) phase to be 68 K. Additionally, we studied the response of biaxial strain on the energetic stability and magnetic properties of various phases of 2D 1T-VSe2 and found that small amounts of strain can increase the Tc, suggesting a promising route for engineering and enhancing magnetic behavior. Finally, we synthesized 1T-VSe2 and performed Raman spectroscopy measurements, which were in close agreement with our calculated results, validating our computational approach. Our work emphasizes the role of highly accurate DMC methods in advancing the understanding of monolayer 1T-VSe2 and provides a robust framework for future studies of 2D magnetic materials.

Strain engineering is one of the most effective routes for tuning the electrical and optical properties of two-dimensional layered materials. Besides reproducing the results of other groups on the tunability and photoluminescence (PL) enhancement of free and bound excitons under mechanical deformation or thermal effects, we systematically studied the mechanical strain (wrinkle) and local thermal strain (laser heating) effects on the PL lifetime and polarization in GaSe multilayers and their interplay, revealing their coupled impact on excitonic dynamics. The PL intensity of localized excitons in the wrinkle region of GaSe increases superlinearly with the excitation laser power. The polarization effect has been observed for the bound exciton in GaSe under mechanical strain, but not in GaSe under thermal strain. Fluorescence lifetime imaging (FLIM) measurements reveal strain-tunable lifetimes for both free and bound excitons, and the lifetime of the bound exciton is longer than the free exciton in the mechanical strain region, correlating with the enhanced emission of the bound exciton. These results highlight previously uncharacterized optoelectronic tunability in GaSe and broaden the design space for strain (or thermally)-controlled 2D layered devices.

We have fabricated all-thin-film solid oxide fuel cell (SOFC) heterostructures and investigated the transport properties of Sm0.2Ce0.8O2−δ (SDC20) in an out-of-plane measurement geometry. The SOFC heterostructures, consisting of a bottom SrRuO3 or Ba0.93La0.07SnO3 electrode and an SDC20 electrolyte, were fabricated on SrTiO3 (001) substrates via pulsed laser deposition. The hetero-epitaxial nature and atomically sharp interfaces between the electrolyte and the electrode were confirmed by x-ray diffraction and high-resolution scanning transmission electron microscopy. Out-of-plane electrochemical impedance spectroscopy measurements indicate that the electrochemical properties of the SDC20 thin films are quantitatively consistent with those of bulk SDC20 in a wide temperature range from 350 to 550 °C. Our results demonstrate that all-thin-film heterostructures can serve as a platform for direct investigation of intrinsic transport properties of electrochemical materials.

Heterostructures between 2D and 3D electron systems remain critically important in developing novel and efficient optoelectronic and electronic devices. In this study, a vertical heterojunction between monolayer MoS2 and bulk InSe was developed. This heterojunction exhibits a type-I band alignment that facilitates rapid energy transfer from the wide bandgap MoS2 to the narrow bandgap InSe resulting in quenching of the MoS2 photoluminescence (PL) emission and enhancement of the A exciton emission in InSe. Temperature-dependent PL measurements of MoS2 on SiO2, MoS2 on InSe, and bare InSe revealed the critical role of defect trapping and electron-phonon coupling in the optical response of MoS2 on InSe. These results demonstrate that heterostructures combining monolayer MoS2 on bulk InSe, showing marked improvement relative to bare InSe, would be advantageous when incorporated into optoelectronic devices such as photodetectors, light emitters, and color converters and highlights the benefit of creating van der Waals (vdW) heterostructures with tailored properties.

Topological superconductivity (TSC) hosts exotic modes enabling error-free quantum computation and low-temperature spintronics. Despite preliminary evidence of edge modes, unambiguous signatures remain undetected. Here, we report the first observation of protected, non-local transport from the edge modes of the potential Weyl-superconductor FeTe_0.55Se_0.45. Namely resonant charge injection, ballistic transport, and extraction via edge modes. An anomalous conductance plateau emerges only when topological, superconducting, and magnetic phases coexist, with source-drain contacts coupled via the edge. Moving the drain to the bulk switches the non-local transport process to a local Andreev process, generating a zero-bias conductance peak (ZBCP). The edge mode's topological protection is confirmed by its insensitivity to external magnetic fields and increasing temperatures until the spontaneous magnetization is substantially suppressed. Our findings provide a new methodology to demonstrate TSC edge states in FeTe_0.55Se_0.45 via topologically protected non-local transport.

The unique geometry of kagome lattices leads to topological features such as flat bands and Dirac cones. When paired with ferromagnetism and a Fermi level near Dirac points, they offer a platform for realizing topological Chern magnetotransport. This prospect recently drew interest in the ferrimagnetic kagome metal TbMn6Sn6. However, density functional theory (DFT) calculations indicate that its 2D Chern gap lies well above the Fermi energy, raising questions about its role in anomalous Hall conductivity. Here, we study YMn6Sn5.45Ga0.55, a structurally and electronically similar material, and find that its intrinsic anomalous Hall effect is three-dimensional. This demonstrates that the Hall response in such compounds does not originate from 2D Chern gaps. Additionally, we confirm that the newly proposed empirical scaling relation for extrinsic Hall conductivity is universally governed by spin fluctuations.

We present a scanning tunneling microscopy study of room-temperature topological surface states (TSS) on Bi-terminated MnBi2Te4. We found that Bi-termination has a larger exchange gap than Te-termination and a higher surface magnetic ordering temperature, making it a promising system for exploring axion electrodynamics. After compensating local surface charge carriers with an electric field of the tip, we observed nontrivial current plateaus and hysteresis loops on tunnel current-voltage characteristics, which we attributed to the compressibility phase transition of TSS and induction of axion insulator quantum dot (QD). Tunneling data allowed to determine the ratio of magnetic and electric fields in QD and estimate the Chern number of a current vortex as C similar to 10, which suggests the formation of collective rotational resonance. We found that magnetic defects inside QD can suppress rotational resonance, as manifested either in a halt of vortex rotation or the development of Schrodinger-cat-like superpositions of rotating and non-rotating states.

In this work, we report on defect generation in multilayer GaSe through hydrostatic pressure quenching and UV laser irradiation. The Raman line width from the UV 266 nm irradiated sample is much wider than that in pressure-quenched GaSe, corresponding to a wider defect energy distribution range in the former sample than the latter. After quenching from 11.2 GPa, three photoluminescence (PL) peaks from defect states are observed at 657, 681, and 695 nm at a low temperature of 93 K. Defect-related peaks at 649, 694, 750, and 774 nm also appear in low-temperature PL spectra after UV laser irradiation, with a nonmonotonous intensity dependence on irradiation duration. There are common features in defects produced by these two methods: the PL peaks with the lowest energy are sharp, and their PL intensities increase linearly with the excitation laser power and saturate above a certain excitation laser power. These two features are similar to those in defects for single-photon emission (SPE) in other 2D materials at even lower temperatures. Fluorescence lifetime imaging shows distinguished short (2.3 ns) and long (75.6 nm) lifetimes of the 695 nm PL line in pressure-quenched GaSe. The density functional theory predicts defect energy levels related to Se vacancy.

Overcoming the Shockley-Queisser limit in photovoltaic systems is an ongoing focus. Processes such as impact ionization that produce carrier multiplication in bulk materials are typically inefficient due to the requirements of energy and momentum conservation. Transition metal dichalcogenides, which are of interest for numerous reasons, have, additionally, recently been shown to exhibit very efficient carrier multiplication (CM) in thin films of 2H-MoTe2 and 2H-WTe2 formed by chemical vapor deposition (CVD). The photoconductivity in the thin films exhibited ultrafast (<1 ps) dynamics, atypical of most indirect semiconductors, that may be hard to translate to increased photovoltaic efficiency. To provide a window to the impact of material quality on this intriguing CM effect, we used optical pump-terahertz probe (OPTP) techniques to investigate carrier lifetimes and the efficiency for carrier multiplication for bulk single crystals of 2H-MoTe2 produced by chemical vapor transport (CVT). Efficient carrier multiplication is observed in this bulk single crystal, and photocarrier lifetimes are increased by orders of magnitude although there is still evidence of photocarrier localization in the THz frequency response.

Two-dimensional (2D) 1T-VSe$_2$ has prompted significant interest due to the discrepancies regarding alleged ferromagnetism (FM) at room temperature, charge density wave (CDW) states and the interplay between the two. We employed a combined Diffusion Monte Carlo (DMC) and density functional theory (DFT) approach to accurately investigate the magnetic properties and response of strain of monolayer 1T-VSe$_2$. Our calculations show the delicate competition between various phases, revealing critical insights into the relationship between their energetic and structural properties. We went on to perform Classical Monte Carlo simulations informed by our DMC and DFT results, and found the magnetic transition temperature ($T_c$) of the undistorted (non-CDW) FM phase to be 228 K and the distorted (CDW) phase to be 68 K. Additionally, we studied the response of biaxial strain on the energetic stability and magnetic properties of various phases of 2D 1T-VSe$_2$ and found that small amounts of strain can enhance the $T_c$, suggesting a promising route for engineering and enhancing magnetic behavior. Finally, we synthesized 1T-VSe$_2$ and performed Raman spectroscopy measurements, which were in close agreement with our calculated results. Our work emphasizes the role of highly accurate DMC methods in advancing the understanding of monolayer 1T-VSe$_2$ and provides a robust framework for future studies of 2D magnetic materials.

Solid-state magneto-ionic (MI) effects have shown promise for energy-efficient nanoelectronics, where ionic migration may be used to achieve atomic-scale control of interfaces in magnetic nanostructures. To date, magneto-ionics have been mostly explored in oxygen-based systems [1-4], while there is a surge of interest in alternative ionic systems due to their different ionic migration mechanisms and characteristics [5-7]. We have recently demonstrated effective MI control of magnetic functionalities using a variety of ionic species, particularly nitrogen. In nitride-based Ta/CoFe/MnN/Ta films, the chemically induced MI effect is combined with the electric field driving of nitrogen to electrically manipulate exchange bias [8]. Upon field-cooling the heterostructure, ionic diffusion of nitrogen from MnN into the Ta layers occurs. A significant exchange bias is observed, which can be further enhanced by ~ 20% after voltage conditioning (Figs. 1a-1e). This enhancement can be reversed by voltage conditioning with an opposite polarity. Nitrogen migration within the MnN layer and into the Ta capping layer causes the enhancement in exchange bias, which is observed in polarized neutron reflectometry studies. We have also achieved all-nitride-based magneto-ionic systems [9]. Thin films of (001)-ordered Mn 4 N are grown by sputtering Mn onto Mn 3 N 2 seed layer on Si (100) substrate. Nitrogen ion migration across the Mn 3 N 2 /Mn layers leads to a continuous evolution of the layers to Mn 3 N 2 /Mn 4 N, Mn 2 N/Mn 4 N, and eventually Mn 4 N alone (Figs. 1f-1g). Furthermore, we have demonstrated MI control of the exchange bias effect in an all-nitride Mn 4 N/MnN x system, where the field-trained exchange field can be varied up to ten times by introducing or extracting nitrogen from the nitride system. This is achieved by adjusting the nitrogen gas partial pressure during deposition or varying annealing temperature after deposition. These effects demonstrate contrasts with oxygen-based MI effects in terms of operating principles, switching speed, and reversibility. Such MI systems are valuable platforms to gain quantitative understanding at buried interfaces. They also offer potentials for device applications based on electric modulation of magnetic functionalities. This work has been supported in part by the NSF (ECCS-2151809, DMR-2005108, DMR-1828420), SRC/NIST SMART Center, and KAUST. [1] U. Bauer et al. , Nat. Mater. 14 , 174 (2015). [2] C. Bi et al. , Phys. Rev. Lett. 113 , 267202 (2014). [3] D. A. Gilbert et al. , Nat. Commun. 7 , 11050 (2016). [4] G. Chen et al. , Sci. Adv. 6 , eaba4924 (2020). [5] A. J. Tan et al. , Nat. Mater. 18 , 35 (2019). [6] G. Chen et al. , Phys. Rev. X 11 , 021015 (2021). [7] J. de Rojas et al. , Nat. Commun. 11 , 5871 (2020). [8] C. J. Jensen et al. , ACS Nano 17 , 6745 (2023). [9] Z. J. Chen et al. , Appl. Phys. Lett. 123 , 082403 (2023). Figure 1

Reducing the dimensions of materials from three to two, or quasi-two, provides a fertile platform for exploring emergent quantum phenomena and developing next-generation electronic devices. However, growing high-quality, ultrathin, quasi2D materials in a templated fashion on an arbitrary substrate is challenging. Here, the study demonstrates a simple and reproducible on-chip approach for synthesizing non-layered, nanometer-thick, quasi-2D semimetals. In one implementation, this method starts with thin semiconducting InSe flakes of below 20 nm in thickness with nickel deposited on top, followed by a low-temperature annealing step that results in a controlled transformation of the layered InSe to a non-layered, crystalline semimetal via reaction with the laterally diffusing nickel. Atomic resolution microscopy reveals the transformed semimetal to be Ni3In2Se2 with a Kagome-lattice structure. Moreover, it is demonstrated that this synthesis method is generalizable by transforming 2D layered chalcogenides such as SnS and SnSe employing Ni and Co to non-layered semimetals, paving the way for engineering novel types of devices.

The vast high entropy alloy (HEA) composition space is promising for discovery of new material phases with unique properties. This study explores the potential to achieve rare-earth-free high magnetic anisotropy materials in single-phase HEA thin films. Thin films of FeCoNiMnCu sputtered on thermally oxidized Si/SiO2 substrates at room temperature are magnetically soft, with a coercivity on the order of 10 Oe. After post-deposition rapid thermal annealing (RTA), the films exhibit a single face-centered-cubic phase, with an almost 40-fold increase in coercivity. Inclusion of 50 at.% Pt in the film leads to ordering of a single L10 high entropy intermetallic phase after RTA, along with high magnetic anisotropy and 3 orders of magnitude coercivity increase. These results demonstrate a promising HEA approach to achieve high magnetic anisotropy materials using RTA.

Herein, photoluminescence (PL) and fluorescence lifetime imaging (FLIM) in multilayer MoSe 2 are studied. Strain‐activated stimulated emission via defect levels in multilayer MoSe 2 under laser excitation is observed, for the first time in defects of transition metal dichalcogenides. The stimulated emission is indicated by a threshold behavior of PL emission intensity with respect to laser intensity, strong polarization effects, achieved population inversion with a difference in lifetimes of two competing excited states, and localization of the stimulated emission zone as observed in FLIM. The presented results not only demonstrate strain‐activated stimulated emission and highlight the necessity of strain engineering in tailoring 2D layered materials for optoelectronic applications, but also shed light on the design of stimulated emission in transition metal dichalcogenide's defects to tailor for potential single‐photon emission behavior.