Homogeneous AlGaN/GaN superlattices grown on free-standing (1(1)over-bar00) GaN substrates by plasma-assisted molecular beam epitaxy

Two-dimensional and homogeneous growth of m-plane AlGaN by plasma-assisted molecular beam epitaxy has been realized on free-standing (1 (1) over bar 00) GaN substrates by implementing high metal-to-nitrogen (III/N) flux ratio. AlN island nucleation, often reported for m-plane AlGaN under nitrogen-rich growth conditions, is suppressed at high III/N flux ratio, highlighting the important role of growth kinetics for adatom incorporation. The homogeneity and microstructure of m-plane AlGaN/GaN superlattices are assessed via a combination of scanning transmission electron microscopy and high resolution transmission electron microscopy (TEM). The predominant defects identified in dark field TEM characterization are short basal plane stacking faults (SFs) bounded by either Frank-Shockley or Frank partial dislocations. In particular, the linear density of SFs is approximately 5 x 10(-5) cm(-1), and the length of SFs is less than 15 nm. (C) 2013 AIP Publishing LLC

Nanoalloying and phase transformations during thermal treatment of physical mixtures of Pd and Cu nanoparticles

Nanoscale alloying and phase transformations in physical mixtures of Pd and Cu ultrafine nanoparticles are investigated in real time with in situ synchrotron-based x-ray diffraction complemented by ex situ high-resolution transmission electron microscopy. The combination of metal-support interaction and reactive/non-reactive environment was found to determine the thermal evolution and ultimate structure of this binary system. At 300 degrees C, the nanoparticles supported on silica and carbon black intermix to form a chemically ordered CsCl-type (B2) alloy phase. The B2 phase transforms into a disordered fcc alloy at higher temperature (\u3e 450 degrees C). The alloy nanoparticles supported on silica and carbon black are homogeneous in volume, but evidence was found of Pd surface enrichment. In sharp contrast, when supported on alumina, the two metals segregated at 300 degrees C to produce almost pure fcc Cu and Pd phases. Upon further annealing of the mixture on alumina above 600 degrees C, the two metals interdiffused, forming two distinct disordered alloys of compositions 30% and 90% Pd. The annealing atmosphere also plays a major role in the structural evolution of these bimetallic nanoparticles. The nanoparticles annealed in forming gas are larger than the nanoparticles annealing in helium due to reduction of the surface oxides that promotes coalescence and sintering

The Chiral MagnetoHydroDynamics of QCD fluid at RHIC and LHC

The experimental results on heavy ion collisions at RHIC and LHC indicate that QCD plasma behaves as a nearly perfect fluid described by relativistic hydrodynamics. Hydrodynamics is an effective low-energy Theory Of Everything stating that the response of a system to external perturbations is dictated by conservation laws that are a consequence of the symmetries of the underlying theory. In the case of QCD fluid produced in heavy ion collisions, this theory possesses anomalies, so some of the apparent classical symmetries are broken by quantum effects. Even though the anomalies appear as a result of UV regularization and so look like a short distance phenomenon, it has been realized recently that they also affect the large distance, macroscopic behavior in hydrodynamics. One of the manifestations of anomalies in relativistic hydrodynamics is the Chiral Magnetic Effect (CME). At this conference, a number of evidences for CME have been presented, including i) the disappearance of charge asymmetry fluctuations in the low-energy RHIC data where the energy density is thought to be below the critical one for deconfinement; ii) the observation of charge asymmetry fluctuations in Pb-Pb collisions at the LHC. Here I give a three-page summary of some of the recent theoretical and experimental developments and of the future tests that may allow to establish (or to refute) the CME as the origin of the observed charge asymmetry fluctuations.Comment: 4 pages, talk at Quark Matter 2011 Conference, Annecy, France, 23-28 May 201

Determining the optimal contact length for a metal/multiwalled carbon nanotube interconnect

A focused ion beam (FIB) is used to sequentially reduce the contact length of an evaporated metal film to a multiwalled carbon nanotube (MWCNT). By using this FIB contact cutback technique, the contact resistance between an individual MWCNT and evaporated thin films of Au, Au/Ti, and Ag are accurately determined. The data permit a rational way to specify the minimum contact length of a metallic thin film to a MWCNT

Rapid synthesis of few-layer graphene over Cu foil

We report a unique process for rapid synthesis of few-layer graphene films on Cu foil by microwave plasma chemical vapor deposition (MPCVD). We show that the plasma/metal interaction can be advantageous for a rapid synthesis of such thin films. The process can produce films of controllable quality from amorphous to highly crystalline by adjusting plasma conditions during growth processes of similar to 100 s duration and with no supplemental substrate heating. Films have been characterized using Raman spectroscopy, scanning electron microscopy, transmission electron microscopy and X-ray photoelectron spectroscopy. The results help to identify the stages involved in the MPCVD deposition of thin carbon films on Cu foil, and the findings open new pathways for a rapid growth of few-layer graphene films. (C) 2011 Elsevier Ltd. All rights reserved

Controlled thin graphitic petal growth on oxidized silicon

Factors influencing the formation and structure of graphitic petals grown by microwave plasma-enhanced chemical vapor deposition on oxidized silicon substrates are investigated through process variation and materials analysis. Unlike the spatially homogeneous growth mechanisms reported previously, some graphitic petals are found to grow at an accelerated rate, often growing similar to 20 times faster than other petals located only a fraction of a micrometer away. Using scanning electron microscopy and atomic force microscopy, the rapid growth rate of these fast-growing petals is attributed to the formation of nanoscale cones in the plasma etched SiO2 layer. Electron energy loss spectroscopy reveals that the formation of these nanoscale cones is associated with a localized roughening of the oxidized silicon substrate-a process that depends on plasma power. Raman spectroscopy and transmission electron microscopy are used to confirm the graphitic nature of the as-grown petals. Insights gained into the growth mechanism of these graphitic petals suggest a simple scribing method can be used to control both the location and formation of petals on flat Si substrates. Experiments performed to test this hypothesis show that controlled petal growth can be achieved, a development that enables an exploitation of the graphitic petal properties in many practical applications. (C) 2012 Published by Elsevier B.V

Structural TEM study of nonpolar a-plane gallium nitride grown on (112_0) 4H-SiC by organometallic vapor phase epitaxy

Conventional and high resolution electron microscopy have been applied for studying lattice defects in nonpolar a-plane GaN grown on a 4H-SiC substrate with an AlN buffer layer. Samples in plan-view and cross-section configurations have been investigated. Basal and prismatic stacking faults together with Frank and Shockley partial dislocations were found to be the main defects in the GaN layers. High resolution electron microscopy in combination with image simulation supported Drum s model for the prismatic stacking faults. The density of basal stacking faults was measured to be ~;1.6_106cm-1. The densities of partial dislocations terminating I1 and I2 types of intrinsic basal stacking faults were ~;4.0_1010cm-2 and ~;0.4_1010cm-2, respectively. The energy of the I2 stacking fault in GaN was estimated to be (40+-4) erg/cm2 based on the separation of Shockley partial dislocations. To the best of our knowledge, the theoretically predicted I3 basal stacking fault in GaN was observed experimentally for the first time