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Open Access Publications from the University of California

Scholarly Works (7 results)

  • Article
LBL Publications (2003)
Thin film heteroepitaxy of polar materials such as GaN grown by MOCVD, MBE or HVPE Molecular Beam Epitaxy (MBE), and grown on SiC or Al2O3 is frequently hampered by the formation of structural defects - mostly dislocations, nanotubes and pinholes. The nanotubes and pinholes are empty areas either extending along the growth direction (tubes) or forming v-shape defects on {101?1} planes. In most cases they are associated with dislocations, but in some cases these defects are formed in dislocation-free areas. In this presentation it will be shown that the screw dislocations present in HVPE samples are decorated by pinholes arranged on top of each other ("bamboo" structure), but these defects are not found in MBE samples grown on the top of HVPE samples. These might suggest either different core structure of a screw dislocation in HVPE and MBE grown material or suggest a different purity of these two materials. By applying a direct reconstruction of the phase and amplitude of the scattered electron wave from a focal series of high resolution images, the core structures of screw dislocations in both materials have been studied in order to see if the core structure in HVPE template layer and the MBE overlayer is different. This will be discussed in detail during the presentation.
    Cover page: Dislocations in GaN grown by different methods
    • Article
    LBL Publications (2005)

    Transmission electron microscopy (TEM) studies show formation of different types of Mg-rich defects in GaN. Types of defects strongly depend on crystal growth polarity. For bulk crystals grown with N-polarity, the planar defects are distributed at equal distances (20 unit cells of GaN). For growth with Ga-polarity (for both bulk and MOCVD grown crystals) a different type of defects have been found. These defects are three-dimensional Mg-rich hexagonal pyramids (or trapezoids) with their base on the (0001) plane and six walls formed on 1123 planes. The defects appear in [1120] and [1100] cross-section TEM micrographs as triangular and trapezoidal with sides inclined at 43 and 47 degrees to the base depending on the above observation directions, respectively. The dimension of these pyramids varies depending on growth method (50-1000 Angstrom), but the angle between the base and their sides remain the same. The direction from the tip of the pyramid to its base (and from the shorter to the longer base for trapezoidal defects) is along the Ga to N matrix bond direction. Analysis of the reconstructed exit wave phase image from the pyramid side indicates a shift of Ga atomic column positions from the matrix to the N position within the pyramid. In this way a 0.6+-0.2 Angstrom displacement can be measured on the pyramid side between Ga positions in the matrix and within the pyramid.

      Cover page: Pyramidal Defects in GaN:Mg Grown with Ga Polarity
      • Article
      LBL Publications (2005)

      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.

        Cover page: Structural TEM study of nonpolar a-plane gallium nitride grown on (112_0) 4H-SiC by organometallic vapor phase epitaxy
        • Article
        • Peer Reviewed
        UC Berkeley Previously Published Works (2017)
        Bimetallic, nanostructured materials hold promise for improving catalyst activity and selectivity, yet little is known about the dynamic compositional and structural changes that these systems undergo during pretreatment that leads to efficient catalyst function. Here we use ozone-activated silver-gold alloys in the form of nanoporous gold as a case study to demonstrate the dynamic behaviour of bimetallic systems during activation to produce a functioning catalyst. We show that it is these dynamic changes that give rise to the observed catalytic activity. Advanced in situ electron microscopy and X-ray photoelectron spectroscopy are used to demonstrate that major restructuring and compositional changes occur along the path to catalytic function for selective alcohol oxidation. Transient kinetic measurements correlate the restructuring to three types of oxygen on the surface. The direct influence of changes in surface silver concentration and restructuring at the nanoscale on oxidation activity is demonstrated. Our results demonstrate that characterization of these dynamic changes is necessary to unlock the full potential of bimetallic catalytic materials.
          Cover page: Dynamic restructuring drives catalytic activity on nanoporous gold–silver alloy catalysts
          • Article
          • Peer Reviewed
          Materials Sciences (2022)
          SignificanceDynamically understanding the microscopic processes governing ordering transformations has rarely been attained. The situation becomes even more challenging for nanoscale alloys, where the significantly increased surface-area-to-volume ratio not only opens up a variety of additional freedoms to initiate an ordering transformation but also allows for kinetic interplay between the surface and bulk due to their close proximity. We provide direct evidence of the microscopic processes controlling the ordering transformation through the surface-bulk interplay in Pt-Fe nanoalloys and new features rendered by variations in alloy composition and chemical stimuli. These results provide a mechanistic detail of ordering transformation phenomena which are widely relevant to nanoalloys as chemical ordering occurs in most multicomponent materials under suitable environmental bias.
            Cover page: Composition-dependent ordering transformations in Pt–Fe nanoalloys
            • Article
            • Peer Reviewed
            UC Irvine Previously Published Works (2020)
            Dealloying typically occurs via the chemical dissolution of an alloy component through a corrosion process. In contrast, here we report an atomic-scale nonchemical dealloying process that results in the clustering of solute atoms. We show that the disparity in the adatom-substrate exchange barriers separate Cu adatoms from a Cu-Au mixture, leaving behind a fluid phase enriched with Au adatoms that subsequently aggregate into supported clusters. Using dynamic, atomic-scale electron microscopy observations and theoretical modeling, we delineate the atomic-scale mechanisms associated with the nucleation, rotation and amorphization-crystallization oscillations of the Au clusters. We expect broader applicability of the results because the phase separation process is dictated by the inherent asymmetric adatom-substrate exchange barriers for separating dissimilar atoms in multicomponent materials.
              Cover page: Atomic-scale phase separation induced clustering of solute atoms
              • Article
              • Peer Reviewed
              UC Berkeley Previously Published Works (2019)
              Activating pretreatments are used to tune surface composition and structure of bimetallic-alloy catalysts. Herein, the activation-induced changes in material properties of a nanoporous Ag0.03Au0.97 alloy and their subsequent evolution under steady-state CH3OH oxidation conditions are investigated. Activation using O3 results in AgO and Au2O3, strongly enriching the near-surface region in Ag. These oxides reduce in the O2/CH3OH mixture, yielding CO2 and producing a highly Ag-enriched surface alloy. At the reaction temperature (423 K), Ag realloys gradually with Au but remains enriched (stabilized by surface O) in the top few nanometers, producing methyl formate selectively without significant deactivation. At higher temperatures, bulk diffusion induces sintering and Ag redistribution, leading to a loss of activity. These findings demonstrate that material properties determining catalytic activity are dynamic and that metastable (kinetically trapped) forms of the material may be responsible for catalysis, providing guiding principles concerning the activation of heterogeneous catalysts for selective oxidation.
                Cover page: Evolution of steady-state material properties during catalysis: Oxidative coupling of methanol over nanoporous Ag0.03Au0.97Creative Commons 'BY-NC-ND' version 4.0 license
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