We have studied the surface structure of a single crystal (beta-Ga2O3(010) using quantitative Low Energy Electron Diffraction (LEED) and X-ray photoelectron spectroscopy (XPS). The XPS measurements show spectra typical of stoichiometricGa(2)O(3) with a clean surface. LEED consistently shows a p(1x1) pattern, free of surface reconstruction. Quantitative LEED I(V) curves are acquired for 41 distinct diffraction spots. The experimental I(V) curves are compared to simulations over the first five layers. The best fits to the experimental LEED I(V) curves acquired at all diffraction spots are then used to calculate the interplanar relaxation and atomic rumpling. Significant atomic rumpling and interplanar relaxation are found over the first 5 atomic layers. As a result of rumpling a polarization of similar to 2 mu C/cm(2) develops in the topmost surface layer. The structural results are in good agreement with previous density functional theory calculations and experimental X-ray photoelectron diffraction.

In this work, we report quantitative morphometry of freeze-dried graphene-based aerogels (i.e., graphene aerogel-GA, nitrogenated GA-NGA, graphene-carbon nanotube hybrid-Gr-MWCNTs, carbon foam-CF, and CF-GA hybrid-CF-GA) and monoliths, prepared by hydrothermal and organic sol-gel methods, respectively. X-ray micro-computed tomography (XMCT) in combination with scanning and transmission electron microscopy allowed visualization of internal microstructures in three-dimensional space. Quantitative morphometry analysis through the reconstructed volume renderings from two-dimensional sliced images revealed hierarchical structures possessing interlaced thin sheets, honeycomb organization, and topological interconnected pore background domains. The influence of small-diameter functionalized multi-walled carbon nanotubes (MWCNTs) inclusions to graphene-like sheets and integration with CF is assessed through quantitative morphometry analysis in terms of volume-weighted pore size, wall thickness, and porosity levels. Hybrid composite porous solids elucidated cross-linking reinforced by a homogeneous distribution of CNTs into complex sheets of GA and CF matrices. A consistent trend impacting porosity and interconnectedness was found following NGA ≥ GA > CF > Gr-MWCNT2:1 > CF-GA > Gr-MWCNT3:1 > Gr-MWCNT5:1, from XMCT image processing and analyses in corroboration with physical properties and reliability. The experimental results provide insights and guide the design of characteristic porous carbonaceous and graphene-based functional nanomaterials for energy sciences, environmental engineering, and fundamental reactive transport of fluids.

Here, we demonstrate the incorporation of micron-sized, iron-doped chalcogenide (Fe:ZnSe) in laboratory-synthesized tellurite glass (TeO2 60%, BaO 20%, and ZnO 20% molar basis), which emits photoluminescence (PL) in the MIR region (3600 to 4200 nm) at different temperatures (7 K to 200 K). These results are similar to bulk Fe:ZnSe (3400 to 4500 nm) and to Fe:ZnSe in a chalcogenide glass matrix (3520 to 5200 nm). PL confirmed the emission of Fe:ZnSe and Fe:ZnSe in tellurite glass in the MIR region at various temperatures (7 K to 200 K), which matches literature reports.

Field electron emission cathodes were constructed from knitted fabrics comprised entirely of carbon nanotube (CNT) fibers. The fabrics consisted of a top layer array of ∼2 mm high looped structures and a bottom layer that was 1 mm thick with a flat underlying surface. Field emission (FE) experiments were performed on 25.4 mm diameter CNT fabric cathodes in both direct current (DC) and pulsed voltage (PV) modes, and the results were compared to those obtained from a CNT film cathode. The DC measurements were performed at a maximum voltage of 1.5 kV. The CNT fabric cathode emitted 20 mA, which was an 8× increase over the emission current from the CNT film cathode. The DC results were analyzed using the corrected form of the Fowler–Nordheim FE theory initially developed by Murphy and Good, which allows for the determination of the formal emission area and effective gap-field enhancement factor. The PV experiments resulted in Ampere level emission currents from both CNT fabric and CNT film cathodes. For a 25 kV, 500 ns voltage pulse, the CNT fabric cathode emitted 4 A, which was 2× more current than the CNT film cathode. Scanning electron microscopy imaging after PV testing revealed that the fibers remained intact after >5000 pulses. These results indicate that knitted CNT fabrics offer a promising approach for developing large area, conformable, robust FE cathodes for vacuum electronic devices.

A reverse micelle microemulsion assisted hydrothermal approach has been developed for synthesizing iron (Fe2+) doped ZnSe nanocrystals (NCs). The synthetic process contains a two-step procedure, a chemical reaction between Zn2+ and Se using a reducing agent NaBH4 in a Triton X-100 surfactant contained microemulsion, followed by a hydrothermal treatment. The ZnSe NCs have a well-defined crystallinity and the Fe2+ ions have been effectively incorporated into the ZnSe matrix as shown by the compositional mappings from scanning transmission electron microscope (STEM). As revealed by transmission electron microscope (TEM) and powder X-ray diffraction (XRD), the as-synthesized nanocrystals have a cubic zinc blende ZnSe structure with high purity and homogeneity. The sizes of the as-synthesized ZnSe:Fe NCs can be well controlled by changing temperature in the hydrothermal treatment procedure. If the temperature in the hydrothermal system is set lower, the syn-thesized NCs can be rendered smaller. The size of the ZnSe NCs can be controlled over a wide range from 3.6 nm to 10.4 nm with the temperature range from 90 degrees C to 140 degrees C, respectively. The correlation of NC size versus temperature is nearly linear, which can be used to predict and tune NC sizes. This manuscript reports for the first time that the size of Fe2+ doped ZnSe nanocrystals can be controlled by hydrothermal treatment after synthesis via a microemulsion process. Controlling size of the ZnSe:Fe nanocrystals is critical for developing new sensor materials and mid-infrared laser media in the infrared region based on tunable optical properties of the nanomaterials.

Microscopy by Achromatic X-rays With Emission of Laminar Light (MAXWELL) is a new X-ray/visible technique with attractive characteristics including isotropic resolution in all directions, large-volume imaging and high throughput. An ultrathin, laminar X-ray beam produced by a Wolter type I mirror irradiates the sample stimulating the emission of visible light by scintillating nanoparticles, captured by an optical system. Three-dimensional (3D) images are obtained by scanning the specimen with respect to the laminar beam. We implemented and tested the technique with a high-brightness undulator at SPring-8, demonstrating its validity for a variety of specimens. This work was performed under the Synchrotrons for Neuroscience-an Asia-Pacific Strategic Enterprise (SYNAPSE) collaboration.

Although there has been progress in studying the electronic and optical properties of monolayer and near-monolayer (two-dimensional, 2D) MoS2 upon adatom adsorption and intercalation, understanding the underlying atomic-level behavior is lacking, particularly as related to the optical response. Alkali atom intercalation in 2D transition metal dichalcogenides (TMDs) is relevant to chemical exfoliation methods that are expected to enable large scale production. In this work, focusing on prototypical 2D MoS2, the adsorption and intercalation of Li, Na, K, and Ca adatoms were investigated for the 2H, 1T, and 1T' phases of the TMD by the first principles density functional theory in comparison to experimental characterization of 2H and 1T 2D MoS2 films. Our electronic structure calculations demonstrate significant charge transfer, influencing work function reductions of 1-1.5 eV. Furthermore, electrical conductivity calculations confirm the semiconducting versus metallic behavior. Calculations of the optical spectra, including excitonic effects using a many-body theoretical approach, indicate enhancement of the optical transmission upon phase change. Encouragingly, this is corroborated, in part, by the experimental measurements for the 2H and 1T phases having semiconducting and metallic behavior, respectively, thus motivating further experimental exploration. Overall, our calculations emphasize the potential impact of synthesis-relevant adatom incorporation in 2D MoS2 on the electronic and optical responses that comprise important considerations toward the development of devices such as photodetectors or the miniaturization of electroabsorption modulator components.

Defect engineering of graphene is attractive for a wide range of applications. Here, we present a mask-less, resist-free, and fully reversible process to engineer defects in graphene using electron-beam (e-beam) chemistry with radiolyzed water. This process was performed inside a variable pressure scanning electron microscope by generating radiolysis products using reactions between the e-beam and water vapor, which in turn reacted with the graphene at the location of the probe. These reactions enabled controlled chemistry on the graphene surface at a resolution of ∼60 nm and hence created defects in precise locations defined by the e-beam. Detailed characterization and theoretical analyses suggested the presence of sp3-type defects, the density of which was tuned by varying the e-beam dose. In addition, these sp3-type defects were cycled in and out of graphene by alternating e-beam chemistry and thermal annealing. This reversibility promises future applications of e-beam chemistry in reconfigurable plasmonics and electronics.

We have studied the surface structure of single-crystal, wide-gap semiconductor $\ensuremath{\beta}\text{\ensuremath{-}}{\mathrm{Ga}}_{2}{\mathrm{O}}_{3}(010)$ using x-ray photoelectron diffraction (XPD), low-energy electron diffraction (LEED), and x-ray photoelectron spectroscopy (XPS). The XPS measurements show typical spectra for stoichiometric ${\mathrm{Ga}}_{2}{\mathrm{O}}_{3}(010)$. Annealing in vacuum produced a sharp ($1\ifmmode\times\else\texttimes\fi{}1$) LEED pattern, characteristic of the monoclinic structure. The XPD angular anisotropies were collected for the Ga $2{p}_{3/2}$ and O $1s$ core levels. Surface interlayer relaxation up to 8% of the bulk interplanar distance and 0.11--0.14 $\AA{}$ rumpling are observed at the $\ensuremath{\beta}\text{\ensuremath{-}}{\mathrm{Ga}}_{2}{\mathrm{O}}_{3}(010)$ surface. At the surface, the oxygen atoms shift toward the vacuum with respect to the gallium atoms. The rumpling decreases to zero and and the interplanar distance reaches the bulk value of 1.52 $\AA{}$ by the sixth atomic layer. The surface structure agrees with that predicted by first-principles density functional theory calculations which, in addition, suggest a significant band gap narrowing of $\ensuremath{\approx}1$ eV in the surface layer, due to surface states spatially localized on surface oxygen atoms of ${\mathrm{O}}_{\mathrm{II}}$ type.

Battery research has recently diverged into solid-state chemistry and flexible features to address the increasing demands in electric vehicles and novel electronics. In this study, we successfully fabricate 4-inch sized thin freestanding lithium-ion conducting composite electrolyte membrane. The solid electrolytes are made up of polyethylene oxide (PEO) lithiated with lithium bis(trifluoromethylsulphonyl) imide (LiTFSI) in which submicrometer sized crystalline Li1.4Al0.4Ge1.6(PO4)(3) (LAGP) particles are homogeneously distributed. The impacts of the LAGP loading (20-60 wt%) on the thermal, electrical, and mechanical properties of the composite electrolytes are systematically assessed. The composite membranes exhibit similar conducting behavior of dry polymer electrolytes with two distinct ionic conduction mechanisms transitioned around the melting temperature. The conductivities of the composites are marginally lower than the polymer electrolyte with no LAGP. Addition of LAGP, compared with PEO/LiTFSI, has slightly increased thermal transition temperatures (both glass transition temperature and melting temperature) as well as the crystallinity of PEO. Increasing the amount of LAGP ceramic fillers increased the elastic modulus, reduced the strain to failure point, but has less impacts on the yielding strength. The freestanding ceramic/polymer composite electrolytes with optimal LAGP loading can result appropriate electrical, thermal and mechanical properties and hence, have potential applications to flexible all solid-state lithium-based batteries. (C) 2019 The Electrochemical Society.

We demonstrate the ability to synthesize graphitic carbon sheets around cubic silicon carbide nanowires via an alloy-mediated catalytic process. The transmission electron microscopy analysis shows multilayer graphitic carbon sheets with a large interatomic layer distance of ∼0.45 nm, suggesting the presence of oxygen in the graphitic system. Oxygen-related peaks observed by energy-dispersive X-ray spectroscopy, Raman spectroscopy, and Fourier-transform infrared spectroscopy further confirm the oxidation of the graphitic carbon layers. A detailed investigation of the Raman spectra reveals a turbostratic stacking of the graphitic carbon layers. The turbostratic nature and the presence of oxidation in the graphitic carbon surrounding the silicon carbide nanowires make them a suitable platform for further functionalization, of particular interest for biosensing, as both graphitic carbon and silicon carbide are biocompatible.

Graphene quantum dots (GQDs) - zero-dimensional materials - are sheets of a few nanometers in lateral dimension and exhibit quantum confinement and edge site effects where sp2-bonded carbon nanocore surrounded with edged plane functional moieties is promising as advanced electroactive sensing platforms. In this work, GQDs are synthesized by solvothermal and hydrothermal techniques, with optimal size of 5 nm. Their potential in fundamental (direct electron transfer) and applied (enzymatic glucose biosensor) electrochemistry are demonstrated. Glucose oxidase (GOx) immobilized on glassy carbon (GC) electrodes modified with GQDs are investigated by means of cyclic voltammetry, differential pulse voltammetry, and amperometry. Well-defined quasi-reversible redox peaks observed under various electrochemical parameters helped to determine diffusion coefficient (D) and first-order electron transfer rate (kET). The cyclic voltammetry curves showed homogeneous ion transport for GQD with D ranging between 8.45 × 10−9 m2 s−1 and 3 × 10−8 m2 s−1 following GO < rGO < GQD < GQD (with FcMeOH as redox probe) < GOx/rGO < GOx/GO < HRP/GQDs < GOx/GQDs. The developed GOx-GQDs biosensor responds efficiently and linearly to the presence of glucose over concentrations ranging 10 μM and 3 mM with limit of detection 1.35 μM and sensitivity 0.00769 μA μM−1·cm−2 as compared with rGO (0.025 μA μM−1 cm−2, 4.16 μM) and GO (0.064 μA μM−1 cm−2, 4.82 μM) nanosheets. The high performance and stability of GQDs is attributed to sufficiently large surface-to-volume ratio, excellent biocompatibility, abundant hydrophilic edge site density, and partially hydrophobic planar sites that favors GOx adsorption on the electrode surface and versatile architectures to ensure rapid charge transfer and electron/ion conduction (<10 ms). We also carried out similar studies with other enzymatic protein biomolecules on electrode surfaces prepared from GQD precursors for electrochemical comparison, thus opening up potential sensing applications in medicine as well as bio-nanotechnology.

We have pioneered a platform technology able to harness the properties of graphene directly from silicon carbide on silicon substrates for integrated on chip or in-package applications, ranging from sensing and nanophotonics to integrated energy storage. The graphene synthesis is transfer -free and site-selective, leading to straightforward wafer -level fabrication and yielding sufficient adhesion for subsequent processing. This approach among others can pave the way towards miniaturized energy sources in SiP systems for smart nodes of the Internet of Everything.

This paper presents the heteroepitaxial growth of β‐Ga2O3 thin films on off‐axis (0001) c‐sapphire substrates by low pressure chemical vapor deposition (LPCVD). (−201) oriented β‐Ga2O3 thin films are grown using high purity metallic gallium (Ga) and oxygen (O2) as the precursors. N‐type conductivity in silicon doped β‐Ga2O3 thin films is demonstrated. It is found that the film crystalline quality, surface morphology, and electrical conductivity are remarkably sensitive to the off‐axis angles. X‐ray phi‐scan measurements of the β‐Ga2O3 film grown on on‐axis c‐sapphire indicate the presence of six in‐plane rotational domains due to the substrate symmetry. With the increase of off‐axis angle toward <11–20> of sapphire, one of the in‐plane orientations is strongly favored. The use of off‐axis substrate also reduced the X‐ray rocking curve full width at half maximum and increased the intensities of the Raman peaks. The best electrical properties of the β‐Ga2O3 film are exhibited by the film grown on 6° off‐axis c‐sapphire. The room temperature electron Hall mobility was 106.6 cm2 V−1 s−1 with an n‐type carrier concentration of 4.83 × 1017 cm−3. The results from this study demonstrate high electrical quality β‐Ga2O3 thin films grown on off‐axis c‐sapphire substrates, which are promising for high power electronic and short wavelength optoelectronic device applications.

Growing graphene on SiC thin films on Si is a cheaper alternative to the growth on bulk SiC, and for this reason it has been recently intensively investigated. Here we study the effect of hydrogen intercalation on epitaxial graphene obtained by high temperature annealing on 3C-SiC/Si(111) in ultra-high vacuum. By using a combination of core-level photoelectron spectroscopy, low energy electron diffraction, and near-edge x-ray absorption fine structure (NEXAFS) we find that hydrogen saturates the Si atoms at the topmost layer of the substrate, leading to free-standing graphene on 3C-SiC/Si(111). The intercalated hydrogen fully desorbs after heating the sample at 850 °C and the buffer layer appears again, similar to what has been reported for bulk SiC. However, the NEXAFS analysis sheds new light on the effect of hydrogen intercalation, showing an improvement of graphene's flatness after annealing in atomic H at 600 °C. These results provide new insight into free-standing graphene fabrication on SiC/Si thin films.

Memristor crossbars are capable of implementing learning algorithms in a much more energy and area efficient manner compared to traditional systems. However, the programmable nature of memristor crossbars must first be explored on a smaller scale to see which memristor device structures are most suitable for applications in reconfigurable computing. In this paper, we demonstrate the programmability of memristor devices with filamentary switching based on LiNbO3, a new resistive switching oxide. We show that a range of resistance values can be set within these memristor devices using a pulse train for programming. We also show that a neuromorphic crossbar containing eight memristors was capable of correctly implementing an OR function. This work demonstrates that lithium niobate memristors are strong candidates for use in neuromorphic computing.

The inelastic mean free path (IMFP) for carbon-based materials is notoriously challenging to model, and moving from bulk materials to 2D materials may exacerbate this problem, making the accurate measurements of IMFP in 2D carbon materials critical. The overlayer-film method is a common experimental method to estimate IMFP by measuring electron effective attenuation length (EAL). This estimation relies on an assumption that elastic scattering effects are negligible. We report here an experimental measurement of electron EAL in epitaxial graphene on SiC using photoelectron spectroscopy over an electron kinetic energy range of 50-1150 eV. We find a significant effect of the interface between the 2D carbon material and the substrate, indicating that the attenuation length in the so-called 'buffer layer' is smaller than for free-standing graphene. Our results also suggest that the existing models for estimating IMFPs may not adequately capture the physics of electron interactions in 2D materials.

Layered oxide materials having alternating repeated layer thicknesses of 10 nm or less are difficult to make, especially with sharp interfaces. Nanostructured thin films having repeated layers of two different oxide materials were obtained by using pulsed laser deposition and two independent stationary targets consisting of Al2O3 and BaTiO3. Desired thicknesses were achieved by using a specific number of pulses from a 248-nm KrF excimer laser, at an energy of 450 mJ/pulse, a galvanometer mirror system, and a background pressure of oxygen. Trends in material properties were identified by systematically varying the number of pulses for multiple nanostructured thin films and comparing the resulting properties measured using in-situ spectroscopic ellipsometry and ex-situ capacitance measurements, including relative permittivity and loss. Four films were deposited with a goal of having 0.25-, 1-, 4-, and 10-nm thick layers, and each similar to 220 nm thick. Ellipsometry data were modeled in situ to calculate thickness, n and k. A representative transmission electron microscopy measurement was also collected for the 10-nm sample with corresponding x-ray photoelectron spectroscopy and energy disperive x-ray spectroscopy. Ellipsometry and capacitance measurements were all performed on each of the samples, with one sample having calculated impedance greater than 30 GOhm at 0.001 Hz. (C) 2017 Society of Photo-Optical Instrumentation Engineers (SPIE)

With its low work function and high mechanical strength, the LaB 6 /VB 2 eutectic system is an interesting candidate for high performance thermionic emitters. For the development of device applications, it is important to understand the origin, value, and spatial distribution of the work function in this system. Here we combine thermal emission electron microscopy and low energy electron microscopy with Auger electron spectroscopy and physical vapor deposition of the constituent elements to explore physical and chemical conditions governing the work function of these surfaces. Our results include the observation that work function is lower (and emission intensity is higher) on VB 2 inclusions than on the LaB 6 matrix. We also observe that the deposition of atomic monolayer doses of vanadium results in surprisingly significant lowering of the work function with values as low as 1.1 eV. © 2017 Elsevier B.V. All rights reserved.