Dependence of h-BN Film Thickness as Grown on Nickel Single-Crystal Substrates of Different Orientations.

Chemical vapor deposition (CVD) of two-dimensional materials has been an active area of research in recent years because it is a scalable process for obtaining thin films that can be used to fabricate devices. The growth mechanism for hexagonal boron nitride (h-BN) on metal catalyst substrates has been described to be either surface energy-driven or diffusion-driven. In this work, h-BN is grown in a CVD system on Ni single-crystal substrates as a function of Ni crystallographic orientation to clarify the competing forces acting on the growth mechanism.

Interface States in Bilayer Graphene Encapsulated by Hexagonal Boron Nitride.

The threshold voltages at the onset of conduction for electron and hole branches can provide information on band gap values or interface states in a gap. We measured conductivity of bilayer graphene encapsulated by hexagonal boron nitride as a function of back and top gates, where another bilayer graphene is used as a top gate. From the measured conductivity the transport gap values were extracted assuming zero interface trap states, and they are close to the theoretically expected gap values.

Growth of lateral graphene/h-BN heterostructure on copper foils by chemical vapor deposition.

The synthesis of lateral heterostructures assembled by atomically-thin materials with distinct intrinsic properties is important for future heterojunction-embedded two-dimensional (2D) devices. Here we report an etching-assisted chemical vapor deposition method to synthesize large-area continuous lateral graphene/hexagonal boron nitride (Gr/h-BN) heterostructures on carbon-containing copper foils. The h-BN film is first synthesized on the copper foil, followed by hydrogen etching, and then epitaxial graphene domains are grown to form continuous lateral heterostructures.

Improved Room Temperature NO Sensing Performance of Organic Field-Effect Transistor by Directly Blending a Hole-Transporting/Electron-Blocking Polymer into the Active Layer.

Over the past decades, organic field-effect transistor (OFET) gas sensors have maintained a rapid development. However, the majority of OFET gas sensors show insufficient detection capability towards oxidizing gases such as nitrogen oxide, compared with the inorganic counterpart. In this paper, a new strategy of OFET nitrogen dioxide (NO) gas sensor, consisting of poly(3-hexylthiophene-2,5-diyl) (P3HT) and poly(9-vinylcarbazole) (PVK) blend, is reported.

Dual Functions of V/SiO/AlO/pSi Device as Selector and Memory.

This letter presents dual functions including selector and memory switching in a V/SiO/AlO/pSi resistive memory device by simply controlling compliance current limit (CCL). Unidirectional threshold switching is observed after a positive forming with low CCL of 1 μA. The shifts to the V-electrode side of the oxygen form the VO layer, where the threshold switching can be explained by the metal-insulation-transition phenomenon.

Spin-Conserving Resonant Tunneling in Twist-Controlled WSe-hBN-WSe Heterostructures.

We investigate interlayer tunneling in heterostructures consisting of two tungsten diselenide (WSe) monolayers with controlled rotational alignment, and separated by hexagonal boron nitride. In samples where the two WSe monolayers are rotationally aligned we observe resonant tunneling, manifested by a large conductance and negative differential resistance in the vicinity of zero interlayer bias, which stem from energy- and momentum-conserving tunneling. Because the spin-orbit coupling leads to coupled spin-valley degrees of freedom, the twist between the two WSe monolayers allows us to probe the conservation of spin-valley degree of freedom in tunneling.

Graphite-based selectorless RRAM: improvable intrinsic nonlinearity for array applications.

Selectorless graphite-based resistive random-access memory (RRAM) has been demonstrated by utilizing the intrinsic nonlinear resistive switching (RS) characteristics, without an additional selector or transistor for low-power RRAM array application. The low effective dielectric constant value (k) layer of graphite or graphite oxide is utilized, which is beneficial in suppressing sneak-path currents in the crossbar RRAM array. The tail-bits with low nonlinearity can be manipulated by the positive voltage pulse, which in turn can alleviate variability and reliability issues.

Topologically Protected Helical States in Minimally Twisted Bilayer Graphene.

In minimally twisted bilayer graphene, a moiré pattern consisting of AB and BA stacking regions separated by domain walls forms. These domain walls are predicted to support counterpropogating topologically protected helical (TPH) edge states when the AB and BA regions are gapped. We fabricate designer moiré crystals with wavelengths longer than 50 nm and demonstrate the emergence of TPH states on the domain wall network by scanning tunneling spectroscopy measurements.

Hubbard Model Physics in Transition Metal Dichalcogenide Moiré Bands.

Flexible long period moiré superlattices form in two-dimensional van der Waals crystals containing layers that differ slightly in lattice constant or orientation. In this Letter we show theoretically that isolated flat moiré bands described by generalized triangular lattice Hubbard models are present in twisted transition metal dichalcogenide heterobilayers. The hopping and interaction strength parameters of the Hubbard model can be tuned by varying the twist angle and the three-dimensional dielectric environment.

Utilization of Inexpensive Carbon-Based Substrates as Platforms for Sensing.

Gold (Au) has been widely used as a material for Surface Enhanced Raman Spectroscopy (SERS) due to its plasmonic properties, stability and biocompatibility. Conventionally for SERS application, Au is deposited on a rigid substrate such as glass or silicon. The rigid substrates severely limit analyte collection efficiency as well as portability.

Quantum Confinement in Oxide Heterostructures: Room-Temperature Intersubband Absorption in SrTiO/LaAlO Multiple Quantum Wells.

The Si-compatibility of perovskite heterostructures offers the intriguing possibility of producing oxide-based quantum well (QW) optoelectronic devices for use in Si photonics. While the SrTiO/LaAlO (STO/LAO) system has been studied extensively in the hopes of using the interfacial two-dimensional electron gas in Si-integrated electronics, the potential to exploit its giant 2.4 eV conduction band offset in oxide-based QW optoelectronic devices has so far been largely ignored.

Zero-static power radio-frequency switches based on MoS atomristors.

Recently, non-volatile resistance switching or memristor (equivalently, atomristor in atomic layers) effect was discovered in transitional metal dichalcogenides (TMD) vertical devices. Owing to the monolayer-thin transport and high crystalline quality, ON-state resistances below 10 Ω are achievable, making MoS atomristors suitable as energy-efficient radio-frequency (RF) switches. MoS RF switches afford zero-hold voltage, hence, zero-static power dissipation, overcoming the limitation of transistor and mechanical switches.

Strongly Enhanced Tunneling at Total Charge Neutrality in Double-Bilayer Graphene-WSe_{2} Heterostructures.

We report the experimental observation of strongly enhanced tunneling between graphene bilayers through a WSe_{2} barrier when the graphene bilayers are populated with carriers of opposite polarity and equal density. The enhanced tunneling increases sharply in strength with decreasing temperature, and the tunneling current exhibits a vertical onset as a function of interlayer voltage at a temperature of 1.5 K.

Scaling Effect on Silicon Nitride Memristor with Highly Doped Si Substrate.

A feasible approach is reported to reduce the switching current and increase the nonlinearity in a complementary metal-oxide-semiconductor (CMOS)-compatible Ti/SiN /p -Si memristor by simply reducing the cell size down to sub-100 nm. Even though the switching voltages gradually increase with decreasing device size, the reset current is reduced because of the reduced current overshoot effect. The scaled devices (sub-100 nm) exhibit gradual reset switching driven by the electric field, whereas that of the large devices (≥1 µm) is driven by Joule heating.

Autonomous Thermal-Oxidative Composition Inversion and Texture Tuning of Liquid Metal Surfaces.

Droplets capture an environment-dictated equilibrium state of a liquid material. Equilibrium, however, often necessitates nanoscale interface organization, especially with formation of a passivating layer. Herein, we demonstrate that this kinetics-driven organization may predispose a material to autonomous thermal-oxidative composition inversion (TOCI) and texture reconfiguration under felicitous choice of trigger.

Tunable Γ-K Valley Populations in Hole-Doped Trilayer WSe_{2}.

We present a combined experimental and theoretical study of valley populations in the valence bands of trilayer WSe_{2}. Shubnikov-de Haas oscillations show that trilayer holes populate two distinct subbands associated with the K and Γ valleys, with effective masses 0.5m_{e} and 1.

Replica molding-based nanopatterning of tribocharge on elastomer with application to electrohydrodynamic nanolithography.

Replica molding often induces tribocharge on elastomers. To date, this phenomenon has been studied only on untextured elastomer surfaces even though replica molding is an effective method for their nanotexturing. Here we show that on elastomer surfaces nanotextured through replica molding the induced tribocharge also becomes patterned at nanoscale in close correlation with the nanotexture.

Photonic-crystal exciton-polaritons in monolayer semiconductors.

Semiconductor microcavity polaritons, formed via strong exciton-photon coupling, provide a quantum many-body system on a chip, featuring rich physics phenomena for better photonic technology. However, conventional polariton cavities are bulky, difficult to integrate, and inflexible for mode control, especially for room-temperature materials. Here we demonstrate sub-wavelength-thick, one-dimensional photonic crystals as a designable, compact, and practical platform for strong coupling with atomically thin van der Waals crystals.

Electrodeposition of Gold Nanostructures Having Controlled Morphology.

Gold (Au) nanostructures exhibit unique electronic, optoelectronic and plasmonic properties. This makes them potential candidates for applications in areas including biosensing, catalysis, optics, and electronics. These unique properties are governed by the precise control over their morphologies and size.

Exploring the Efficacy of Platinum and Palladium Nanostructures for Organic Molecule Detection via Raman Spectroscopy.

Noble transition metals, like palladium (Pd) and platinum (Pt), have been well-known for their excellent catalytic and electrochemical properties. However, they have been considered non-active for surface enhanced Raman spectroscopy (SERS). In this work, we explore the scattering contributions of Pd and Pt for the detection of organic molecules.

Atomristor: Nonvolatile Resistance Switching in Atomic Sheets of Transition Metal Dichalcogenides.

Recently, two-dimensional (2D) atomic sheets have inspired new ideas in nanoscience including topologically protected charge transport,1,2 spatially separated excitons,3 and strongly anisotropic heat transport.4 Here, we report the intriguing observation of stable nonvolatile resistance switching (NVRS) in single-layer atomic sheets sandwiched between metal electrodes. NVRS is observed in the prototypical semiconducting (MX, M = Mo, W; and X = S, Se) transitional metal dichalcogenides (TMDs),5 which alludes to the universality of this phenomenon in TMD monolayers and offers forming-free switching.

Carrier Trapping by Oxygen Impurities in Molybdenum Diselenide.

Understanding defect effect on carrier dynamics is essential for both fundamental physics and potential applications of transition metal dichalcogenides (TMDs). Here, the phenomenon of oxygen impurities trapping photoexcited carriers has been studied with ultrafast pump-probe spectroscopy. Oxygen impurities are intentionally created in exfoliated multilayer MoSe with Ar plasma irradiation and air exposure.

Enhanced Electron Mobility in Nonplanar Tensile Strained Si Epitaxially Grown on SiGe Nanowires.

We report the growth and characterization of epitaxial, coherently strained SiGe-Si core-shell nanowire heterostructure through vapor-liquid-solid growth mechanism for the SiGe core, followed by an in situ ultrahigh-vacuum chemical vapor deposition for the Si shell. Raman spectra acquired from individual nanowire reveal the Si-Si, Si-Ge, and Ge-Ge modes of the SiGe core and the Si-Si mode of the shell. Because of the compressive (tensile) strain induced by lattice mismatch, the core (shell) Raman modes are blue (red) shifted compared to those of unstrained bare SiGe (Si) nanowires, in good agreement with values calculated using continuum elasticity model coupled with lattice dynamic theory.

On the theoretical maximum achievable signal-to-noise ratio (SNR) of piezoelectric microphones.

A theoretical maximum achievable signal to noise ratio (SNR) for piezoelectric microphones is identified as a function of only microphone volume irrespective of architecture and construction details. For a given piezoelectric material, microphone SNR can be reduced to an expression containing only a dimensionless coupling coefficient and microphone volume. For a given material, the coupling coefficient has a theoretical upper bound defined by the most favorable deformation geometry.