Understanding and controlling nonlinear optical generation in transition metal dichalcogenides (TMDs) is critical for developing scalable photonic devices. Here, we investigate second-harmonic generation (SHG) in Au-supported 2H-WSe2 and noncentrosymmetric 3R-WS2 flakes and explore the impact of dielectric capping with hexagonal boron nitride (hBN). Using SHG microscopy and finite-element modeling, we demonstrate that optical-cavity resonances in Au-supported TMDs strongly enhance SHG, with multilayer 2H-WSe2 flakes exhibiting up to a 40-fold increase in SHG intensity at resonant thicknesses compared to monolayer WSe2 on the same Au substrate. Sequential deposition of hBN layers onto multilayer WSe2/Au optical cavities further enhances SHG by an average of 350% across a similar to 70 nm spectral bandwidth by partially impedance matching WSe2 to air and, thereby, improving in and out-coupling at the fundamental and second-harmonic wavelengths. Extending this approach to 3R-WS2, we observe similar optical cavity-mediated enhancement, with SHG intensities up to 700 times that of monolayer WS2 on Au. These results demonstrate cavity engineering and dielectric capping as promising strategies for boosting broadband nonlinear optical responses in both centrosymmetric and noncentrosymmetric TMD flakes, providing design principles for future layered photonic platforms.

We experimentally demonstrate negative characteristic temperature indirect bandgap-based lasing in multimodal WS2 microcavities. Unlike conventional lasers, because of phonon-assisted lasing, our system demonstrates ultrahigh thermal stability and a record high negative characteristic temperature.

This study presents the direct measurement of proton transport along filamentous Desulfobulbaceae, or cable bacteria. Cable bacteria are filamentous multicellular microorganisms that have garnered much interest due to their ability to serve as electrical conduits, transferring electrons over several millimeters. Our results indicate that cable bacteria can also function as protonic conduits because they contain proton wires that transport protons at distances >100 µm. We find that protonic conductivity (σP) along cable bacteria varies between samples and is measured as high as 114 ± 28 µS cm-1 at 25 °C and 70% relative humidity (RH). For cable bacteria, the protonic conductance (GP) and σP are dependent upon the RH, increasing by as much as 26-fold between 60% and 80% RH. This observation implies that proton transport occurs via the Grotthuss mechanism along water associated with cable bacteria, forming proton wires. In order to determine σP and GP along cable bacteria, we implemented a protocol using a modified transfer-printing technique to deposit either palladium interdigitated protodes (IDP), palladium transfer length method (TLM) protodes, or gold interdigitated electrodes (IDE) on top of cable bacteria. Due to the relatively mild nature of the transfer-printing technique, this method should be applicable to a broad array of biological samples and curved materials. The observation of protonic conductivity in cable bacteria presents possibilities for investigating the importance of long-distance proton transport in microbial ecosystems and to potentially build biotic or biomimetic scaffolds to interface with materials via proton-mediated gateways or channels.

While coupling between optical, electronic, and mechanical domains is paramount for high-frequency acoustic devices, materials that offer tunability in the degree of such coupling can be crucially enabling in expanding device functionality. Here, we show that the interaction of photons with coherent acoustic phonons confined in 2D layered semiconducting cavities can be controlled through either modifying the material state via a thermally induced electronic bandgap shift (EBS) or altering the polarization state of the incoming photons when optical birefringence is present in the cavity. We demonstrate temperature-driven EBS as an effective tool to engineer the WSe2 cavity readout as it allows one to sweep the excitonic energy relative to a chosen probe wavelength. We envision the resulting amplitude and phase modulation of the optical readout as a way of enhancing the cavity's functionality, given that the diminishing heat capacity of the ultrathin suspended films implies an upper limit for the rate of thermo-optic phase switching in excess of 100 MHz. For acoustic cavities that must operate at lower temperatures, we demonstrate a multiexciton extension of the approach where the output signal is controlled by selectively accessing different excitonic states in birefringent ReS2. Density functional theory calculations indicate that even though the electronic bands of WSe2 and ReS2 are shaped predominantly by intralayer electronic interactions, the out-of-plane strain-driven deformation potential (DP), dEg/dηzz ∼1 eV (critical for optical transduction of "breathing mode" vibrations), is significant for multiple electronic valleys of interest and is consistent with the experimental results. We anticipate that the demonstrated experimental approach for quantifying the out-of-plane DP in ultrathin films can be extended to heterostructures, in which sophisticated cross-plane interactions can be engineered using combined mechanical and electronic properties of heterogeneous 2D materials.

Transition metal dichalcogenides (TMDs) represent an emerging class of layered materials with applications in microelectronics, optoelectronics, photonics, and catalysis. The confinement of charge carriers within the highly anisotropic, quasi-two-dimensional geometry of one-layer (1L) TMDs leads to reduced, variable dielectric screening, giving rise to a quasiparticle band gap that is highly susceptible to the surrounding dielectric environment. Exploiting the contrasting external dielectric environments of gold-supported and suspended 1L WS2, we show how the electronic states of WS2 under the effective and ineffective screening environments align at a junction made within the same sheet of material. Photoelectron spectra point to the close alignment of the charge neutrality levels of WS2 in both environments, and the breakdown of rigid shifts between the valence states and core levels with the core levels shifting more than twice as much as the valence states. Furthermore, the effectively screened WS2 exhibits a valence state with the photoemission linewidth twice as large as the ineffectively screened suspended WS2, presumably originated from the locally varying WS2-Au distance and substrate disorders. Collectively, these findings provide key insights into the electronic behavior of WS2 and its photoemission process with the electronic states renormalized according to the external screening environments.

Advanced MaterialsVolume 36, Issue 7 2470054 FrontispieceFree Access Hexagonal Boron Nitride Slab Waveguides for Enhanced Spectroscopy of Encapsulated 2D Materials (Adv. Mater. 7/2024) Samuel W. LaGasse, Samuel W. LaGasse Electronics Science and Technology Division, US Naval Research Laboratory, Washington, DC, 20375 USASearch for more papers by this authorNicholas V. Proscia, Nicholas V. Proscia NRC Postdoctoral Fellow residing at the US Naval Research Laboratory, Washington, DC, 20375 USASearch for more papers by this authorCory D. Cress, Cory D. Cress Electronics Science and Technology Division, US Naval Research Laboratory, Washington, DC, 20375 USASearch for more papers by this authorJose J. Fonseca, Jose J. Fonseca Electronics Science and Technology Division, US Naval Research Laboratory, Washington, DC, 20375 USASearch for more papers by this authorPaul D. Cunningham, Paul D. Cunningham Electronics Science and Technology Division, US Naval Research Laboratory, Washington, DC, 20375 USASearch for more papers by this authorEli Janzen, Eli Janzen Department of Chemical Engineering, Kansas State University, Manhattan, KS, 66506 USASearch for more papers by this authorJames H. Edgar, James H. Edgar Department of Chemical Engineering, Kansas State University, Manhattan, KS, 66506 USASearch for more papers by this authorDaniel J. Pennachio, Daniel J. Pennachio Electronics Science and Technology Division, US Naval Research Laboratory, Washington, DC, 20375 USASearch for more papers by this authorJames Culbertson, James Culbertson Electronics Science and Technology Division, US Naval Research Laboratory, Washington, DC, 20375 USASearch for more papers by this authorMaxim Zalalutdinov, Maxim Zalalutdinov Acoustics Division, US Naval Research Laboratory, Washington, DC, 20375 USASearch for more papers by this authorJeremy T. Robinson, Jeremy T. Robinson Electronics Science and Technology Division, US Naval Research Laboratory, Washington, DC, 20375 USASearch for more papers by this author Samuel W. LaGasse, Samuel W. LaGasse Electronics Science and Technology Division, US Naval Research Laboratory, Washington, DC, 20375 USASearch for more papers by this authorNicholas V. Proscia, Nicholas V. Proscia NRC Postdoctoral Fellow residing at the US Naval Research Laboratory, Washington, DC, 20375 USASearch for more papers by this authorCory D. Cress, Cory D. Cress Electronics Science and Technology Division, US Naval Research Laboratory, Washington, DC, 20375 USASearch for more papers by this authorJose J. Fonseca, Jose J. Fonseca Electronics Science and Technology Division, US Naval Research Laboratory, Washington, DC, 20375 USASearch for more papers by this authorPaul D. Cunningham, Paul D. Cunningham Electronics Science and Technology Division, US Naval Research Laboratory, Washington, DC, 20375 USASearch for more papers by this authorEli Janzen, Eli Janzen Department of Chemical Engineering, Kansas State University, Manhattan, KS, 66506 USASearch for more papers by this authorJames H. Edgar, James H. Edgar Department of Chemical Engineering, Kansas State University, Manhattan, KS, 66506 USASearch for more papers by this authorDaniel J. Pennachio, Daniel J. Pennachio Electronics Science and Technology Division, US Naval Research Laboratory, Washington, DC, 20375 USASearch for more papers by this authorJames Culbertson, James Culbertson Electronics Science and Technology Division, US Naval Research Laboratory, Washington, DC, 20375 USASearch for more papers by this authorMaxim Zalalutdinov, Maxim Zalalutdinov Acoustics Division, US Naval Research Laboratory, Washington, DC, 20375 USASearch for more papers by this authorJeremy T. Robinson, Jeremy T. Robinson Electronics Science and Technology Division, US Naval Research Laboratory, Washington, DC, 20375 USASearch for more papers by this author First published: 15 February 2024 https://doi.org/10.1002/adma.202470054AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onEmailFacebookTwitterLinkedInRedditWechat Graphical Abstract 2D Materials In article number 2309777 by Samuel W. LaGasse and co-workers, optical coupling of monolayer transition metal dichalcogenides (TMDs) to hexagonal boron nitride (hBN) slab waveguides is studied. The results provide a direct route for waveguide-based interrogation of layered materials, as well as a way to integrate layered materials into future photonic devices at arbitrary positions whilst maintaining their intrinsic properties. Volume36, Issue7February 15, 20242470054 RelatedInformation

Harnessing electronic excitations involving coherent coupling to bosonic modes is essential for the design and control of emergent phenomena in quantum materials. In situations where charge carriers induce a lattice distortion due to the electron-phonon interaction, the conducting states get "dressed", which leads to the formation of polaronic quasiparticles. The exploration of polaronic effects on low-energy excitations is in its infancy in two-dimensional materials. Here, we present the discovery of an interlayer plasmon polaron in heterostructures composed of graphene on top of single-layer WS2. By using micro-focused angle-resolved photoemission spectroscopy during in situ doping of the top graphene layer, we observe a strong quasiparticle peak accompanied by several carrier density-dependent shake-off replicas around the single-layer WS2 conduction band minimum. Our results are explained by an effective many-body model in terms of a coupling between single-layer WS2 conduction electrons and an interlayer plasmon mode. It is important to take into account the presence of such interlayer collective modes, as they have profound consequences for the electronic and optical properties of heterostructures that are routinely explored in many device architectures involving 2D transition metal dichalcogenides.

Mechanical stacking of two dissimilar materials often has surprising consequences for heterostructure behavior. In particular, a two-dimensional electron gas (2DEG) is formed in the heterostructure of the topological crystalline insulator Pb0.24Sn0.76Te and graphene due to contact of a polar with a nonpolar surface and the resulting changes in electronic structure needed to avoid polar catastrophe. We study the spintronic properties of this heterostructure with non-local spin valve devices. We observe spin-momentum locking at lower temperatures that transitions to regular spin channel transport only at ~40 K. Hanle spin precession measurements show a spin relaxation time as high as 2.18 ns. Density functional theory calculations confirm that the spin-momentum locking is due to a giant Rashba effect in the material and that the phase transition is a Lifshitz transition. The theoretically predicted Lifshitz transition is further evident in the phase transition-like behavior in the Landé g-factor and spin relaxation time.

Harnessing electronic excitations involving coherent coupling to bosonic modes is essential for the design and control of emergent phenomena in quantum materials [1]. In situations where charge carriers induce a lattice distortion due to the electron-phonon interaction, the conducting states get "dressed". This leads to the formation of polaronic quasiparticles that dramatically impact charge transport, surface reactivity, thermoelectric and optical properties, as observed in a variety of crystals and interfaces composed of polar materials [2-6]. Similarly, when oscillations of the charge density couple to conduction electrons the more elusive plasmon polaron emerges [7], which has been detected in electron-doped semiconductors [8-10]. However, the exploration of polaronic effects on low energy excitations is still in its infancy in two-dimensional (2D) materials. Here, we present the discovery of an interlayer plasmon polaron in heterostructures composed of graphene on top of SL WS$_2$. By using micro-focused angle-resolved photoemission spectroscopy (microARPES) during in situ doping of the top graphene layer, we observe a strong quasiparticle peak accompanied by several carrier density-dependent shake-off replicas around the SL WS$_2$ conduction band minimum (CBM). Our results are explained by an effective many-body model in terms of a coupling between SL WS$_2$ conduction electrons and graphene plasmon modes. It is important to take into account the presence of such interlayer collective modes, as they have profound consequences for the electronic and optical properties of heterostructures that are routinely explored in many device architectures involving 2D transition metal dichalcogenides (TMDs) [11-15].

The layered insulator hexagonal boron nitride (hBN) is a critical substrate that brings out the exceptional intrinsic properties of two-dimensional (2D) materials such as graphene and transition metal dichalcogenides (TMDs). In this work, the authors demonstrate how hBN slabs tuned to the correct thickness act as optical waveguides, enabling direct optical coupling of light emission from encapsulated layers into waveguide modes. Molybdenum selenide (MoSe2 ) and tungsten selenide (WSe2 ) are integrated within hBN-based waveguides and demonstrate direct coupling of photoluminescence emitted by in-plane and out-of-plane transition dipoles (bright and dark excitons) to slab waveguide modes. Fourier plane imaging of waveguided photoluminescence from MoSe2 demonstrates that dry etched hBN edges are an effective out-coupler of waveguided light without the need for oil-immersion optics. Gated photoluminescence of WSe2 demonstrates the ability of hBN waveguides to collect light emitted by out-of-plane dark excitons.Numerical simulations explore the parameters of dipole placement and slab thickness, elucidating the critical design parameters and serving as a guide for novel devices implementing hBN slab waveguides. The results provide a direct route for waveguide-based interrogation of layered materials, as well as a way to integrate layered materials into future photonic devices at arbitrary positions whilst maintaining their intrinsic properties.

Engineering the transition metal dichalcogenide (TMD)-metal interface is critical for the development of two-dimensional semiconductor devices. By directly probing the electronic structures of WS2-Au and WSe2-Au interfaces with high spatial resolution, we delineate nanoscale heterogeneities in the composite systems that give rise to local Schottky barrier height modulations. Photoelectron spectroscopy reveals large variations (>100 meV) in TMD work function and binding energies for the occupied electronic states. Characterization of the composite systems with electron backscatter diffraction and scanning tunneling microscopy leads us to attribute these heterogeneities to differing crystallite orientations in the Au contact, suggesting an inherent role of the metal microstructure in contact formation. We then leverage our understanding to develop straightforward Au processing techniques to form TMD-Au interfaces with reduced heterogeneity. Our findings illustrate the sensitivity of TMDs' electronic properties to metal contact microstructure and the viability of tuning the interface through contact engineering.

While heterostructures are ubiquitous tools enabling new physics and device functionalities, the palette of available materials has never been richer. Combinations of two emerging material classes, two-dimensional materials and topological materials, are particularly promising because of the wide range of possible permutations that are easily accessible. Individually, both graphene and Pb1-xSnxTe (PST) are widely investigated for spintronic applications because graphene's high carrier mobility and PST's topologically protected surface states are attractive platforms for spin transport. Here, we combine monolayer graphene with PST and demonstrate a hybrid system with properties enhanced relative to the constituent parts. Using magnetotransport measurements, we find carrier mobilities up to 20 000 cm2/(V s) and a magnetoresistance approaching 100%, greater than either material prior to stacking. We also establish that there are two distinct transport channels and determine a lower bound on the spin relaxation time of 4.5 ps. The results can be explained using the polar catastrophe model, whereby a high mobility interface state results from a reconfiguration of charge due to a polar/nonpolar interface interaction. Our results suggest that proximity induced interface states with hybrid properties can be added to the still growing list of behaviors in these materials.

Photoelectron emission microscopy enables us to probe the materials properties by harnessing light-matter interactions. [1]. Because the photoelectron yield is a convolution of the electric field strength and the occupied electron density of states, photoelectron emission microscopy (PEEM) has the potential to explore the electronic structure and the optical response at the nano-meter length scale [2]. In this talk we will highlight two exemplars focusing on the nanoscale heterogeneity at transition metal dichalcogenide (TMD)-gold interfaces [3] and the optical response of TMD-embedded dielectrics cavities [4]

To pinpoint the electronic and structural mechanisms that affect intrinsic and extrinsic performance limits of 2D material devices, it is of critical importance to resolve the electronic properties on the mesoscopic length scale of such devices under operating conditions. Herein, angle‐resolved photoemission spectroscopy with nanoscale spatial resolution (nanoARPES) is used to map the quasiparticle electronic structure of a twisted bilayer graphene device. The dispersion and linewidth of the Dirac cones associated with top and bottom graphene layers are determined as a function of spatial position on the device under both static and operating conditions. The analysis reveals that microscopic rotational domains in the two graphene layers establish a range of twist angles from 9.8° to 12.7°. Application of current and electrostatic gating lead to strong electric fields with peak strengths of 0.75 V/μm at the rotational domain boundaries in the device. These proof‐of‐principle results demonstrate the potential of nanoARPES to link mesoscale structural variations with electronic states in operating device conditions and to disentangle such extrinsic factors from the intrinsic quasiparticle dispersion.

There is an intensive effort to control the nature of attractive interactions between ultrathin semiconductors and metals and to understand its impact on the electronic properties at the junction. Here, we present a photoelectron spectroscopy study on the interface between WS2 films and gold, with a focus on the occupied electronic states near the Brillouin zone center (i.e., the Γ point). To delineate the spectra of WS2 supported on crystalline Au from the suspended WS2, we employ a microscopy approach and a tailored sample structure, in which the WS2/Au junction forms a semi-epitaxial relationship and is adjacent to suspended WS2 regions. The photoelectron spectra, as a function of WS2 thickness, display the expected splitting of the highest occupied states at the Γ point. In multilayer WS2, we discovered variations in the electronic states that spatially align with the crystalline grains of underlying Au. Corroborated by density functional theory calculations, we attribute the electronic structure variations to stacking variations within the WS2 films. We propose that strong interactions exerted by Au grains cause slippage of the interfacing WS2 layer with respect to the rest of the WS2 film. Our findings illustrate that the electronic properties of transition metal dichalcogenides, and more generally 2D layered materials, are physically altered by the interactions with the interfacing materials, in addition to the electron screening and defects that have been widely considered.

We describe a method for phase separating and transferring block copolymer (BCP) nanoscale patterns to arbitrary substrates for surface-independent nanolithography. The enabling technology is a hydrogenated or oxidized graphene thin film that only weakly adheres to its substrate. BCPs are applied to these graphene-based materials and solvent annealed to effect nanoscale phase separation. Then, taking advantage of the weak interaction of the graphene film and its substrate, the BCP/graphene stack is delaminated easily in water. A target substrate is then used to retrieve the stack, which can then serve as a lithographic mask. The use of water as a lift-off agent allows for chemically mild retrieval of the phase-separated BCP, extending the BCP lithography technique to essentially arbitrary substrates. We demonstrate this graphene-enabled BCP lithography on silicon nitride and polyethylene. We also show that using reduced graphene oxide (RGO) as a thin film enables the transfer of wafer-scale BCP films and lithography on SiO x and Si. We use an RGO support to produce phase-separated BCP solvent-annealed patterns on polystyrene, a result which is not possible using standard BCP solvent annealing and which shows the utility of this technique. Finally, we demonstrate the ability to create nanopatterns of higher complexity by stacking multiple BCP masks, a capability that is not possible using conventional BCP lithography. This technique may have applications in fabricating nanoporous membranes and photonically active coatings.

Two-dimensional (2D) materials offer unique opportunities in engineering the ultrafast spatiotemporal response of composite nanomechanical structures. In this work, we report on high frequency, high quality factor ( Q ) 2D acoustic cavities operating in the 50–600 GHz frequency ( f ) range with f × Q up to 1 × 10 14 . Monolayer steps and material interfaces expand cavity functionality, as demonstrated by building adjacent cavities that are isolated or strongly-coupled, as well as a frequency comb generator in MoS 2 /h-BN systems. Energy dissipation measurements in 2D cavities are compared with attenuation derived from phonon-phonon scattering rates calculated using a fully microscopic ab initio approach. Phonon lifetime calculations extended to low frequencies (<1 THz) and combined with sound propagation analysis in ultrathin plates provide a framework for designing acoustic cavities that approach their fundamental performance limit. These results provide a pathway for developing platforms employing phonon-based signal processing and for exploring the quantum nature of phonons.

As the need for ever greater transistor density increases, the commensurate decrease in device size approaches the atomic limit, leading to increased energy loss and leakage currents, reducing energy efficiencies. Alternative state variables, such as electronic spin rather than electronic charge, have the potential to enable more energy-efficient and higher performance devices. These spintronic devices require materials capable of efficiently harnessing the electron spin. Here we show robust spin transport in Cd3As2 films up to room temperature. We demonstrate a nonlocal spin valve switch from this material, as well as inverse spin Hall effect measurements yielding spin Hall angles as high as θSH = 1.5 and spin diffusion lengths of 10-40 μm. Long spin-coherence lengths with efficient charge-to-spin conversion rates and coherent spin transport up to room temperature, as we show here in Cd3As2, are enabling steps toward realizing actual spintronic devices.

Near-infrared-to-visible second harmonic generation from air-stable two-dimensional polar gallium and indium metals is described. The photonic properties of 2D metals, including the largest second-order susceptibilities reported for metals (approaching 10 nm/V), are determined by the atomic-level structure and bonding of two-to-three-atom-thick crystalline films. The bond character evolved from covalent to metallic over a few atomic layers, changing the out-of-plane metal-metal bond distances by approximately ten percent (0.2 Å), resulting in symmetry breaking and an axial electrostatic dipole that mediated the large nonlinear response. Two different orientations of the crystalline metal atoms, corresponding to lateral displacements <2 Å, persisted in separate micrometer-scale terraces to generate distinct harmonic polarizations. This strong atomic-level structure-property interplay suggests metal photonic properties can be controlled with atomic precision.