Nonvolatile Modulation of Light Emission from Delocalized and Defect-Bound Excitons in Monolayer MoS₂ Using the Remnant Polarization in the Ferroelectric Polymer P(VDF-TrFE): Implications for Optical Memory

Excitons in semiconductors are a potential alternative to charge for manipulation and storage of information. The reduced electrostatic screening in monolayers of two-dimensional (2D) semiconductors increases the binding energies of delocalized (free) excitons, while also encouraging the formation of defect-bound excitons that can be leveraged in devices for neuromorphic and quantum computing. We report the nonvolatile modulation of light emission from delocalized and defect-bound excitons in monolayer MoS2 using the remnant polarization of P(VDF-TrFE) ferroelectric polymer islands. Photoluminescence emission intensities of delocalized excitons at room-temperature and bound excitons at cryogenic temperatures are enhanced by polarization that depletes free electrons and reduces Coulomb screening. The opposite polarization suppresses emission, providing a simple means of electrically encoding and optically reading information. Two distinct defect-bound exciton bands are identified by their spectral positions and their distinct variations with temperature and polarization, suggesting that they have distinct origins or interactions with the surrounding environment. The selective reconfiguration of free and bound excitonic emission suggests a novel device scheme toward electrically reconfigurable, optically accessible optical memory devices

Long-Term Stability and Reliability of Black Phosphorus Field-Effect Transistors

Black phosphorus has been recently suggested as a very promising material for use in 2D field-effect transistors. However, due to its poor stability under ambient conditions, this material has not yet received as much attention as for instance MoS₂. We show that the recently demonstrated Al₂O₃ encapsulation leads to highly stable devices. In particular, we report our long-term study on highly stable black phosphorus field-effect transistors, which show stable device characteristics for at least eight months. This high stability allows us to perform a detailed analysis of their reliability with respect to hysteresis as well as the arguably most important reliability issue in silicon technologies, the bias-temperature instability. We find that the hysteresis in these transistors depends strongly on the sweep rate and temperature. Moreover, the hysteresis dynamics in our devices are reproducible over a long time, which underlines their high reliability. Also, by using detailed physical models for oxide traps developed for Si technologies, we are able to capture the channel electrostatics of the black phosphorus FETs and determine the position of the defect energy band. Finally, we demonstrate that both hysteresis and bias-temperature instabilities are due to thermally activated charge trapping/detrapping by oxide traps and can be reduced if the device is covered by Teflon-AF

Flexible Black Phosphorus Ambipolar Transistors, Circuits and AM Demodulator

High-mobility two-dimensional (2D) semiconductors are desirable for high-performance mechanically flexible nanoelectronics. In this work, we report the first flexible black phosphorus (BP) field-effect transistors (FETs) with electron and hole mobilities superior to what has been previously achieved with other more studied flexible layered semiconducting transistors such as MoS₂ and WSe₂. Encapsulated bottom-gated BP ambipolar FETs on flexible polyimide afforded maximum carrier mobility of about 310 cm²/V·s with field-effect current modulation exceeding 3 orders of magnitude. The device ambipolar functionality and high-mobility were employed to realize essential circuits of electronic systems for flexible technology including ambipolar digital inverter, frequency doubler, and analog amplifiers featuring voltage gain higher than other reported layered semiconductor flexible amplifiers. In addition, we demonstrate the first flexible BP amplitude-modulated (AM) demodulator, an active stage useful for radio receivers, based on a single ambipolar BP transistor, which results in audible signals when connected to a loudspeaker or earphone. Moreover, the BP transistors feature mechanical robustness up to 2% uniaxial tensile strain and up to 5000 bending cycles

Large-Area Dry Transfer of Single-Crystalline Epitaxial Bismuth Thin Films

We report the first direct dry transfer of a single-crystalline thin film grown by molecular beam epitaxy. A double cantilever beam fracture technique was used to transfer epitaxial bismuth thin films grown on silicon (111) to silicon strips coated with epoxy. The transferred bismuth films retained electrical, optical, and structural properties comparable to the as-grown epitaxial films. Additionally, we isolated the bismuth thin films on freestanding flexible cured-epoxy post-transfer. The adhesion energy at the bismuth/silicon interface was measured to be ∼1 J/m², comparable to that of exfoliated and wet transferred graphene. This low adhesion energy and ease of transfer is unexpected for an epitaxially grown film and may enable the study of bismuth’s unique electronic and spintronic properties on arbitrary substrates. Moreover, this method suggests a route to integrate other group-V epitaxial films (i.e., phosphorus) with arbitrary substrates, as well as potentially to isolate bismuthene, the atomic thin-film limit of bismuth

Thermal Oxidation of WSe₂ Nanosheets Adhered on SiO₂/Si Substrates

Because of the drastically different intralayer versus interlayer bonding strengths, the mechanical, thermal, and electrical properties of two-dimensional (2D) materials are highly anisotropic between the in-plane and out-of-plane directions. The structural anisotropy may also play a role in chemical reactions, such as oxidation, reduction, and etching. Here, the composition, structure, and electrical properties of mechanically exfoliated WSe₂ nanosheets on SiO₂/Si substrates were studied as a function of the extent of thermal oxidation. A major component of the oxidation, as indicated from optical and Raman data, starts from the nanosheet edges and propagates laterally toward the center. Partial oxidation also occurs in certain areas at the surface of the flakes, which are shown to be highly conductive by microwave impedance microscopy. Using secondary ion mass spectroscopy, we also observed extensive oxidation at the WSe₂–SiO₂ interface. The combination of multiple microcopy methods can thus provide vital information on the spatial evolution of chemical reactions on 2D materials and the nanoscale electrical properties of the reaction products