Thomas H. Kosel

Anchoring Semiconductor and Metal Nanoparticles on a Two-Dimensional Catalyst Mat. Storing and Shuttling Electrons with Reduced Graphene Oxide

Using reduced graphene oxide (RGO) as a two-dimensional support, we have succeeded in selective anchoring of semiconductor and metal nanoparticles at separate sites. Photogenerated electrons from UV-irradiated TiO(2) are transported across RGO to reduce silver ions into silver nanoparticles at a location distinct from the TiO(2) anchored site. The ability of RGO to store and shuttle electrons, as visualized via a stepwise electron transfer process, demonstrates its capability to serve as a catalyst nanomat and transfer electrons on demand to adsorbed species. These findings pave the way for the development of next generation catalyst systems and can spur advancements in graphene-based composites for chemical and biological sensors.

InAlN/AlN/GaN HEMTs With Regrown Ohmic Contacts and

We report 30-nm-gate-length InAlN/AlN/GaN/SiC high-electron-mobility transistors (HEMTs) with a record current gain cutoff frequency (fT) of 370 GHz. The HEMT without back barrier exhibits an extrinsic transconductance (gm.ext) of 650 mS/mm and an on/off current ratio of 106 owing to the incorporation of dielectric-free passivation and regrown ohmic contacts with a contact resistance of 0.16 Ω·mm. Delay analysis suggests that the high fT is a result of low gate-drain parasitics associated with the rectangular gate. Although it appears possible to reach 500-GHz fT by further reducing the gate length, it is imperative to investigate alternative structures that offer higher mobility/velocity while keeping the best possible electrostatic control in ultrascaled geometry.

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The band edge optical characterization of solution-synthesized CdS nanowires (NWs) is described. Investigated wires are made through a solution-liquid-solid approach that entails the use of low-melting bimetallic catalyst particles to seed NW growth. Resulting diameters are approximately 14 nm, and lengths exceed 1 microm. Ensemble diameter distributions are approximately 13%, with corresponding intrawire diameter variations of approximately 5%. High-resolution transmission electron micrographs show that the wires are highly crystalline and have the wurtzite structure with growth along at least two directions: [0001] and [1010]. Band edge emission is observed with estimated quantum yields between approximately 0.05% and 1%. Complementary photoluminescence excitation spectra show structure consistent with the linear absorption. Carrier cooling dynamics are subsequently examined through ensemble lifetime and transient differential absorption measurements. The former reveals unexpectedly long band edge decays that extend beyond tens of nanoseconds. The latter indicates rapid intraband carrier cooling on time scales of 300-400 fs. Subsequent recovery at the band edge contains significant Auger contributions at high intensities which are usurped by other, possibly surface-related, carrier relaxation pathways at lower intensities. Furthermore, an unusual intensity-dependent transient broadening is seen, connected with these long decays. The effect likely stems from band-filling on the basis of an analysis of observed spectral shifts and line widths.

of 370 GHz

The current-voltage characteristics of AlGaSb/InAs staggered-gap n-channel tunnel field-effect transistors are simulated in a geometry in which the gate electric field is oriented to be in the same direction as the tunnel junction internal field. It is shown that this geometry can also support low-voltage operation and low subthreshold swing. In the absence of a simple analytic theory for this transistor to allow direct analytic comparisons, two-dimensional numerical simulations are used to explore the electrostatic and geometrical design considerations including dependence on gate length, gate underlap, gate undercut, and equivalent oxide thickness.

Band-Filling of Solution-Synthesized CdS Nanowires

We demonstrate the solution-phase synthesis of CdS/CdSe, CdSe/CdS, and CdSe/ZnTe core/shell nanowires (NWs). On the basis of bulk band offsets, type-I and type-II heterostructures are made, contributing to the further development of low-dimensional heteroassemblies using solution-phase chemistry. Core/shell wires are prepared by slowly introducing shell precursors into a solution of premade core NWs dispersed in a noncoordinating solvent at moderate temperatures (215-250 degrees C). Resulting heterostructures are characterized through low- and high-resolution transmission electron microscopy, selected area electron diffraction, and energy dispersive X-ray analysis. From these experiments, initial shell growth appears to occur through either Stranski-Krastanov or Volmer-Weber island growth. However, beyond a critical shell thickness, nucleation of randomly oriented nanocrystals results in a polycrystalline coat. In cases where overcoating has been achieved, corresponding elemental analyses show spatially varying compositions along the NW radial direction in agreement with expected element ratios. Electronic interactions between the core and shell were subsequently probed through optical studies involving UV-vis extinction spectroscopy, photoluminescence experiments, and transient differential absorption spectroscopy. In particular, transient differential absorption studies reveal unexpected shell-induced changes in core NW Auger kinetics at high carrier densities. Previously seen three-carrier Auger kinetics in CdS (bimolecular in CdSe) NWs were suppressed by the presence of a CdSe (CdS) shell. These observations suggest the ability to influence NW optical/electrical properties by coating them with a surrounding shell, a method which could be important for future NW optical studies as well as for NW-based applications.

Performance of AlGaSb/InAs TFETs With Gate Electric Field and Tunneling Direction Aligned

Abstract High chromium white cast irons display a number of advantages in abrasive wear applications. Their composition may be adjusted so that they contain moderate amounts of chromium yet solidify to produce massive chromium-rich carbides in an austenitic matrix of sufficient hardenability to be transformed to martensite with relatively simple heat treatment. In this study the role of carbide volume fraction (CVF) in developing abrasion resistance was investigated using a series of alloys with varying CVF but with constant matrix and carbide compositions. The low stress abrasion resistance of the alloys against quartz and Al 2 O 3 was measured with a dry abrasive rubber wheel abrasion test. The abrasion resistance passed through a maximum at an intermediate CVF, near the eutectic composition, for the softer quartz, while the abrasion resistance increased monotonically with the CVF for tests against the harder Al 2 O 3 . Scanning electron microscopy showed that for quartz abrasion the reversal in the trend of wear versus CVF which occurred at the eutectic composition was caused by spalling and pitting of the massive primary M 7 C 3 carbides in the hypereutectic alloys. This phenomenon did not occur in the Al 2 O 3 abrasive tests, in which a monotonic increase in wear resistance with CVF was observed.

Solution-Based II−VI Core/Shell Nanowire Heterostructures

One dimensional (1D) nanowires (NWs) have gained considerable attention due to their interesting charge transport properties, dielectric contrast effects, and intrinsic polarization sensitivities. As a consequence, they have become viable candidates for use in solar cells, lasers, sensors for biological applications, and even as components in electronic circuits. Common methods for producing NWs include template-based growth, oriented attachment, and ‘‘catalyst’’-based (i.e., seeded) mechanisms. In the latter category, variants include vapor–liquid–solid (VLS), solution–liquid–solid (SLS), and supercritical-fluid– liquid–solid (SFLS) growth. Whereas VLS generally employs gold nanoparticle (NP) catalysts at temperatures exceeding 500 8C, SLS uses lower temperatures (T< 400 8C) and low melting NPs often made of Bi (bulk Tm 271 8C). We have previously demonstrated that this approach can be used to grow high quality CdE (E1⁄4 S, Se, and Te) NWs using Au/Bi core/shell NPs. In tandem, Buhro and coworkers have employed pure Bi particles to synthesize analogous quality CdSe and CdTe NWs. Likewise, III–V wires were made by Korgel and coworkers using Bi. Finally, Mews and coworkers have recently developed a simple way to prepare Bi nanocrystals using BiCl3 in trioctylphosphine and have subsequently synthesized CdSe NWs from them. A number of notable features characterize solution-based NWs. For example, complementary research in the area of colloidal quantum dots (QDs) has shown that both kinetic and

The effect of carbide volume fraction on the low stress abrasion resistance of high Cr-Mo white cast irons☆

We report on 30-nm-gate-length InAlN/AlN/GaN/SiC high-electron-mobility transistors (HEMTs) with a record current gain cutoff frequency (fT) of 400 GHz. Although the high drain-induced barrier lowering (DIBL) value is indicative of significant short-channel effects, more than seven orders of magnitude in the current on/off ratio was observed. The high fT is a result of minimized parasitic effects and at the expense of a low power gain cutoff frequency (fMAX). The gate length dependence and temperature dependence of fT were also measured.

Facile Synthesis and Size Control of II–VI Nanowires Using Bismuth Salts

High-quality compositionally tunable ternary PbSe(x)S(1-x) (x = 0.23, 0.39, 0.49, 0.68, and 0.90) nanowires (NWs) and their binary analogues have been grown using solution-liquid-solid growth with lead(II) diethyldithiocarbamate, Pb(S(2)CNEt(2))(2), and lead(II) imido(bis(selenodiisopropylphosphinate)), Pb((SeP(i)Pr(2))(2)N)(2), as single-source precursors. The alloyed nature of PbSe(x)S(1-x) wires was confirmed using ensemble X-ray diffraction and energy dispersive X-ray spectroscopy (EDXS). Single NW EDXS line scans taken along the length of individual wires show no compositional gradients. NW compositions were independently confirmed using inductively coupled plasma atomic emission spectroscopy. Slight stoichiometric deviations occur but never exceed 13.3% of the expected composition, based on the amount of introduced precursor. In all cases, resulting nanowires have been characterized using transmission electron microscopy. Mean diameters are between 9 and 15 nm with accompanying lengths that range from 4 to 10 μm. Associated selected area electron diffraction patterns indicate that the PbSe(x)S(1-x), PbSe, and PbS NWs all possess the same <002> growth direction, with diffraction patterns consistent with an underlying rock salt crystal structure.

Ultrascaled InAlN/GaN High Electron Mobility Transistors with Cutoff Frequency of 400 GHz

Abstract A study of the effect of carbide size on the abrasion resistance of two cobalt-base powder metallurgy alloys, alloys 6 and 19, was conducted using low stress abrasion with a relatively hard abrasive, A1 2 O 3 . Specimens of each alloy were produced with different carbide sizes but with a constant carbide volume fraction. The wear test results show a monotonie decrease in wear rate with increasing carbide size. Scanning electron microscopy of the worn surfaces and of wear debris particles shows that the primary material removal mechanism is micromachining. Small carbides provide little resistance to micromachining because of the fact that many of them are contained entirely in the volume of micromachining chips. The large carbides must be directly cut by the abrasive particles. Other less frequently observed material removal mechanisms included direct carbide pull-out and the formation of large pits in fine carbide specimens. These processes are considered secondary in the present work, but they may have greater importance in wear by relatively soft abrasives which do not cut chips from the carbide phase of these alloys. Some indication of this is provided by limited studies using a relatively soft abrasive, rounded quartz.