Monolayer WSe Field-Effect Transistor Performance Enhancement by Atomic Defect Engineering and Passivation.

Monolayer two-dimensional (2D) transition metal dichalcogenides (TMDs) have emerged as leading candidates for next-generation electronic devices beyond silicon, owing to their atomically thin structure and superior electrostatic control. However, their integration into industrial applications remains limited due to high densities of lattice defects and challenges in achieving stable and effective doping. In this work, we present a passivation and doping technique that significantly recovers and enhances the electrical properties of monolayer tungsten diselenide (WSe).

Infrared Signatures for Phase Identification in Hafnium Oxide Thin Films.

Phase identification in HfO-based thin films is a prerequisite to understanding the mechanisms stabilizing the ferroelectric phase in these materials, which hold great promise in next-generation nonvolatile memory and computing technology. While grazing-incidence X-ray diffraction is commonly employed for this purpose, it has difficulty unambiguously differentiating between the ferroelectric phase and other metastable phases that may exist due to similarities in the -spacings, their low intensities, and the overlapping of reflections. Infrared signatures provide an alternative route.

Spectral and polarization based imaging in deep-ultraviolet excited photoelectron microscopy.

Using photoelectron emission microscopy, nanoscale spectral imaging of atomically thin MoS buried between AlO and SiO is achieved by monitoring the wavelength and polarization dependence of the photoelectron signal excited by deep-ultraviolet light. Although photons induce the photoemission, images can exhibit resolutions below the photon wavelength as electrons sense the response. To validate this concept, the dependence of photoemission yield on the wavelength and polarization of the exciting light was first measured and then compared to simulations of the optical response quantified with classical optical theory.

Emergent interface vibrational structure of oxide superlattices.

As the length scales of materials decrease, the heterogeneities associated with interfaces become almost as important as the surrounding materials. This has led to extensive studies of emergent electronic and magnetic interface properties in superlattices. However, the interfacial vibrations that affect the phonon-mediated properties, such as thermal conductivity, are measured using macroscopic techniques that lack spatial resolution.

Thermal Conductivity of Aluminum Scandium Nitride for 5G Mobile Applications and Beyond.

Radio frequency (RF) microelectromechanical systems (MEMS) based on AlScN are replacing AlN-based devices because of their higher achievable bandwidths, suitable for the fifth-generation (5G) mobile network. However, overheating of AlScN film bulk acoustic resonators (FBARs) used in RF MEMS filters limits power handling and thus the phone's ability to operate in an increasingly congested RF environment while maintaining its maximum data transmission rate. In this work, the ramifications of tailoring of the piezoelectric response and microstructure of AlScN films on the thermal transport have been studied.

Photophysics and Electronic Structure of Lateral Graphene/MoS and Metal/MoS Junctions.

Integration of semiconducting transition metal dichalcogenides (TMDs) into functional optoelectronic circuitries requires an understanding of the charge transfer across the interface between the TMD and the contacting material. Here, we use spatially resolved photocurrent microscopy to demonstrate electronic uniformity at the epitaxial graphene/molybdenum disulfide (EG/MoS) interface. A 10× larger photocurrent is extracted at the EG/MoS interface when compared to the metal (Ti/Au)/MoS interface.

Uncertainty in linewidth quantification of overlapping Raman bands.

Spectral linewidths are used to assess a variety of physical properties, even as spectral overlap makes quantitative extraction difficult owing to uncertainty. Uncertainty, in turn, can be minimized with the choice of appropriate experimental conditions used in spectral collection. In response, we assess the experimental factors dictating uncertainty in the quantification of linewidth from a Raman experiment highlighting the comparative influence of (1) spectral resolution, (2) signal to noise, and (3) relative peak intensity (RPI) of the overlapping peaks.

Semiconductor Hyperbolic Metamaterials at the Quantum Limit.

We study semiconductor hyperbolic metamaterials (SHMs) at the quantum limit experimentally using spectroscopic ellipsometry as well as theoretically using a new microscopic theory. The theory is a combination of microscopic density matrix approach for the material response and Green's function approach for the propagating electric field. Our approach predicts absorptivity of the full multilayer system and for the first time allows the prediction of in-plane and out-of-plane dielectric functions for every individual layer constructing the SHM as well as effective dielectric functions that can be used to describe a homogenized SHM.

Tunable dual-band graphene-based infrared reflectance filter.

We experimentally demonstrated an actively tunable optical filter that controls the amplitude of reflected long-wave-infrared light in two separate spectral regions concurrently. Our device exploits the dependence of the excitation energy of plasmons in a continuous and unpatterned sheet of graphene on the Fermi-level, which can be controlled via conventional electrostatic gating. The filter enables simultaneous modification of two distinct spectral bands whose positions are dictated by the device geometry and graphene plasmon dispersion.

Nanoscale Carbon Modified α-MnO Nanowires: Highly Active and Stable Oxygen Reduction Electrocatalysts with Low Carbon Content.

Carbon-coated α-MnO nanowires (C-MnO NWs) were prepared from α-MnO NWs by a two-step sucrose coating and pyrolysis method. This method resulted in the formation of a thin, porous, low mass-percentage amorphous carbon coating (<5 nm, ≤1.2 wt % C) on the nanowire with an increase in single-nanowire electronic conductivity of roughly 5 orders of magnitude (α-MnO, 3.

Graphene-Insulator-Semiconductor Junction for Hybrid Photodetection Modalities.

A sensitive optical detector is presented based on a deeply depleted graphene-insulator-semiconducting (DGIS) junction, which offers the possibility of simultaneously leveraging the advantages of both charge integration and localized amplification. Direct read-out and built-in amplification are accomplished via photogating of a graphene field-effect transistor (GFET) by carriers generated within a deeply depleted low-doped silicon substrate. Analogous to a depleted metal-oxide-semiconducting junction, photo-generated charge collects in the potential well that forms at the semiconductor/insulator interface and induces charges of opposite polarity within the graphene film modifying its conductivity.

Designing graphene absorption in a multispectral plasmon-enhanced infrared detector.

We have examined graphene absorption in a range of graphene-based infrared devices that combine either monolayer or bilayer graphene with three different gate dielectrics. Electromagnetic simulations show that the optical absorption in graphene in these devices, an important factor in a functional graphene-based detector, is strongly dielectric-dependent. These simulations reveal that plasmonic excitation in graphene can significantly influence the percentage of light absorbed in the entire device, as well as the graphene layer itself, with graphene absorption exceeding 25% in regions where plasmonic excitation occurs.

Chelant Enhanced Solution Processing for Wafer Scale Synthesis of Transition Metal Dichalcogenide Thin Films.

It is of paramount importance to improve the control over large area growth of high quality molybdenum disulfide (MoS) and other types of 2D dichalcogenides. Such atomically thin materials have great potential for use in electronics, and are thought to make possible the first real applications of spintronics. Here in, a facile and reproducible method of producing wafer scale atomically thin MoS layers has been developed using the incorporation of a chelating agent in a common organic solvent, dimethyl sulfoxide (DMSO).

Experimental Determination of the Ionization Energies of MoSe, WS, and MoS on SiO Using Photoemission Electron Microscopy.

The values of the ionization energies of transition metal dichalcogenides (TMDs) are needed to assess their potential usefulness in semiconductor heterojunctions for high-performance optoelectronics. Here, we report on the systematic determination of ionization energies for three prototypical TMD monolayers (MoSe, WS, and MoS) on SiO using photoemission electron microscopy with deep ultraviolet illumination. The ionization energy displays a progressive decrease from MoS, to WS, to MoSe, in agreement with predictions of density functional theory calculations.

Visibility of dielectrically passivated graphene films.

The visibility of monolayer graphene is dependent on its surrounding dielectric environment and the presence of any contamination associated with 2D layer transfer. Here, the optical contrast of residually contaminated monolayer graphene encased within a range of dielectric stacks characteristic of realistic devices is examined, highlighting the utility of optical microscopy for a graphene assessment, both during and after lithographic processing. Practically, chemical vapor deposited graphene is encapsulated in dielectric stacks of varying thicknesses of SiO.

Dynamic Wavelength-Tunable Photodetector Using Subwavelength Graphene Field-Effect Transistors.

Dynamic wavelength tunability has long been the holy grail of photodetector technology. Because of its atomic thickness and unique properties, graphene opens up new paradigms to realize this concept, but so far this has been elusive experimentally. Here we employ detailed quantum transport modeling of photocurrent in graphene field-effect transistors (including realistic electromagnetic fields) to show that wavelength tunability is possible by dynamically changing the gate voltage.

Self-Heating and Failure in Scalable Graphene Devices.

Self-heating induced failure of graphene devices synthesized from both chemical vapor deposition (CVD) and epitaxial means is compared using a combination of infrared thermography and Raman imaging. Despite a larger thermal resistance, CVD devices dissipate >3x the amount of power before failure than their epitaxial counterparts. The discrepancy arises due to morphological irregularities implicit to the graphene synthesis method that induce localized heating.

Electrodeposited Ni(x)Co(3-x)O4 nanostructured films as bifunctional oxygen electrocatalysts.

Nanostructured Ni(x)Co(3-x)O4 films serve as effective electrocatalysts for both the oxygen reduction and oxygen evolution reactions in alkaline electrolyte.

Thermal boundary conductance between Al films and GaN nanowires investigated with molecular dynamics.

GaN nanowires are being pursued for optoelectronic and high-power applications. In either use, increases in operating temperature reduce both performance and reliability making it imperative to minimize thermal resistances. Since interfaces significantly influence the thermal response of nanosystems, the thermal boundary resistance between GaN nanowires and metal contacts has major significance.

Rotational disorder in twisted bilayer graphene.

Conventional means of stacking two-dimensional (2D) crystals inevitably leads to imperfections. To examine the ramifications of these imperfections, rotational disorder and strain are quantified in twisted bilayer graphene (TBG) using a combination of Raman spectroscopic and low-energy electron diffraction imaging. The twist angle between TBG layers varies on the order of 2° within large (50-100 μm) single-crystalline grains, resulting in changes of the emergent Raman response by over an order of magnitude.

Single element Raman thermometry.

Despite a larger sensitivity to temperature as compared to other microscale thermometry methods, Raman based measurements typically have greater uncertainty. In response, a new implementation of Raman thermometry is presented having lower uncertainty while also reducing the time and hardware needed to perform the experiment. Using a modulated laser to excite the Raman response, the intensity of only a portion of the total Raman signal is leveraged as the thermometer by using a single element detector monitored with a lock-in amplifier.

Electronic hybridization of large-area stacked graphene films.

Direct, tunable coupling between individually assembled graphene layers is a next step toward designer two-dimensional (2D) crystal systems, with relevance for fundamental studies and technological applications. Here we describe the fabrication and characterization of large-area (>cm(2)), coupled bilayer graphene on SiO(2)/Si substrates. Stacking two graphene films leads to direct electronic interactions between layers, where the resulting film properties are determined by the local twist angle.

Experimental investigation of size effects on the thermal conductivity of silicon-germanium alloy thin films.

We experimentally investigate the role of size effects and boundary scattering on the thermal conductivity of silicon-germanium alloys. The thermal conductivities of a series of epitaxially grown Si(1-x)Ge(x) thin films with varying thicknesses and compositions were measured with time-domain thermoreflectance. The resulting conductivities are found to be 3 to 5 times less than bulk values and vary strongly with film thickness.

Evidence for interlayer coupling and moiré periodic potentials in twisted bilayer graphene.

We report a study of the valence band dispersion of twisted bilayer graphene using angle-resolved photoemission spectroscopy and ab initio calculations. We observe two noninteracting cones near the Dirac crossing energy and the emergence of van Hove singularities where the cones overlap for large twist angles (>5°). Besides the expected interaction between the Dirac cones, minigaps appeared at the Brillouin zone boundaries of the moiré superlattice formed by the misorientation of the two graphene layers.

Lithographically defined three-dimensional graphene structures.

A simple and facile method to fabricate 3D graphene architectures is presented. Pyrolyzed photoresist films (PPF) can easily be patterned into a variety of 2D and 3D structures. We demonstrate how prestructured PPF can be chemically converted into hollow, interconnected 3D multilayered graphene structures having pore sizes around 500 nm.