S. Kodambaka

University of California, Los Angeles, Los Ángeles, California, United States

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Publications (73)382.77 Total impact

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    ABSTRACT: Using in situ electron microscopy-based nanomechanical testing, we show that sub-micron-scale ZrC(100) and ZrC(111) single crystals exhibit size- and orientation-dependent room-temperature plasticity under compression. We identify {} and {0 0 1} as the active slip systems operating in ZrC(100) and ZrC(111) crystals, respectively. For both the orientations, yield strengths increase with decreasing crystal size. ZrC(111) is found to be up to 10× softer than ZrC(100). Using density functional theory calculations, we attribute the observed anisotropy to surprisingly facile shear along {0 0 1} compared to {}. Based upon our results, which provide important insights into plastic deformation modes operating in ZrC, we expect that slip systems other than {} can be softer and operate at low temperatures in NaCl-structured refractory transition-metal carbides and nitrides.
    Philosophical Magazine 02/2015; 95(9):1-13. DOI:10.1080/14786435.2015.1012568 · 1.43 Impact Factor
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    ABSTRACT: Using in situ electron microscopy based uniaxial compression and density functional theory calculations, we investigated the room-temperature mechanical responses of sub-micron-scale cylindrical TaC(100) and TaC(011) pillars. The TaC(100) and TaC(011) pillars deform plastically via shear along {1 (1) over bar0} (1 (1) over bar0) and {111} (110), respectively. Interestingly, both TaC(100) and TaC(011) exhibit size-independent yield strengths, with average values of 9 +/- 2.4 and 11 +/- 3.4 GPa, respectively. Our results provide new insights into the role of crystal anisotropy on room-temperature plasticity in TaC.
    Scripta Materialia 12/2014; 100. DOI:10.1016/j.scriptamat.2014.11.036 · 2.97 Impact Factor
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    ABSTRACT: In situ transmission electron microscopy observations of uniaxial compression of sub-300 nm diameter, cylindrical, single-crystalline 6H-SiC pillars oriented along 〈0001〉 and at 45° with respect to 〈0001〉 reveal that plastic slip occurs at room-temperature on the basal {0 0 0 1} planes at stresses above 7.8 GPa. Using a combination of aberration-corrected electron microscopy, molecular dynamics simulations and density functional theory calculations, we attribute the observed phenomenon to basal slip on the shuffle set along 〈11¯00〉. By comparing the experimentally measured yield stresses with the calculated values required for dislocation nucleation, we suggest that room-temperature plastic deformation in 6H-SiC crystals is controlled by glide rather than nucleation of dislocations.
    Acta Materialia 11/2014; 80:400–406. DOI:10.1016/j.actamat.2014.07.066 · 3.94 Impact Factor
  • Y. Murata, S. Kodambaka
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    ABSTRACT: In situ microscopy studies with high spatial and temporal resolutions are ideally-suited for the quantitative description of factors controlling morphological, structural, and compositional evolution in materials and often reveal surprising and previously unknown aspects about the materials. This article showcases scanning tunneling microscopy as a viable and powerful characterization technique for in situ studies of mass transport phenomena controlling solid/vacuum, solid/gas, and solid/solid interfacial stabilities at elevated temperatures up to 1400 K. As representative examples, we present results from our recent studies of the kinetics of: TiO2(110) surface structural evolution as a function of gas chemistry and SiC(0001) surface graphitization in ultra-high vacuum. We expect that similar studies carried out on other technologically-important materials can help the design and development of better materials with improved functionalities.
    Surface and Coatings Technology 10/2014; 257:348–354. DOI:10.1016/j.surfcoat.2014.08.004 · 2.20 Impact Factor
  • Journal of Heat Transfer 08/2014; 136(8):080910. DOI:10.1115/1.4027528 · 2.06 Impact Factor
  • K W Schwarz, J Tersoff, S Kodambaka, F M Ross
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    ABSTRACT: Nanowire growth is generally considered a steady-state process, but oscillatory phenomena are known to often play a fundamental role. Here we identify a natural sequence of distinct growth modes, in two of which the catalyst droplet jumps periodically on and off a crystal facet. The oscillatory modes result from a mismatch between catalyst size and wire diameter; they enable growth of straight smooth-sided wires even when the droplet is too small to span the wire tip. Jumping-catalyst growth modes are seen both in computer simulations of vapor-liquid-solid growth, and in movies of Si nanowire growth obtained by in situ microscopy. Our simulations also provide new insight into nanowire kinking.
    Physical Review Letters 08/2014; 113(5):055501. DOI:10.1103/PhysRevLett.113.055501 · 7.73 Impact Factor
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    ABSTRACT: Using in situ low-energy electron microscopy and density functional theory calculations, we follow the growth of monolayer graphene on Pd(111) via surface segregation of bulk-dissolved carbon. Upon lowering the substrate temperature, nucleation of graphene begins on graphene-free Pd surface and continues to occur during graphene growth. Measurements of graphene growth rates and Pd surface work functions establish that this continued nucleation is due to increasing C adatom concentration on the Pd surface with time. We attribute this anomalous phenomenon to a large barrier for attachment of C adatoms to graphene coupled with a strong binding of the non-graphitic C to the Pd surface.
    Applied Physics Letters 03/2014; 104(10):101606-101606-4. DOI:10.1063/1.4868386 · 3.52 Impact Factor
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    ABSTRACT: Using in situ high-temperature (1073-1173 K) transmission electron microscopy, we investigated the thermal stability of Pt and Mo in contact with polycrystalline ZrB2 thin films deposited on Al2O3(0001). During annealing, we observed the diffusion of cubic-structured Pt1-xMox (with x = 0.2 +/- 0.1) along the length of the ZrB2 layer. From the time-dependent increase in diffusion lengths, we determined that the Pt1-xMox does not react with ZrB2, but diffuses along the surface with a constant temperature-dependent velocity. We identify the rate-limiting step controlling the observed phenomenon as the flux of Mo atoms with an associated activation barrier of 3.8 +/- 0.5 eV. (C) 2013 AIP Publishing LLC.
    Applied Physics Letters 09/2013; 103(12-12). DOI:10.1063/1.4820581 · 3.52 Impact Factor
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    ABSTRACT: Using in situ transmission electron microscopy, we investigated the kinetics of liquid Ga droplet decay on thin amorphous carbon films during annealing at 773 K. The transmission electron microscopy images reveal that liquid Ga forms spherical droplets and undergo coarsening/decay with increasing time. We find that the droplet volumes change non-linearly with time and the volume decay rates depend on their local environment. By comparing the late-stage decay behavior of the droplets with the classical mean-field theory model for Ostwald ripening, we determine that the decay of Ga droplets occurs in the surface diffusion limited regime.
    Applied Physics Letters 04/2013; 102(16). DOI:10.1063/1.4802758 · 3.52 Impact Factor
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    ABSTRACT: An apparatus and test procedure for fabrication and loading of single crystal metal nanopillars under extremely high pressures (>1 GPa) and strain rates (>10(7) s(-1)), using laser-generated stress waves, are presented. Single-crystalline Cu pillars (∼1.20 μm in tall and ∼0.45 μm in diameter) prepared via focused ion beam milling of Cu(001) substrates are shock-loaded using this approach with the dilatational stress waves propagating along the [001] axis of the pillars. Transmission electron microscopy observations of shock-loaded pillars show that dislocation density decreases and that their orientation changes with increasing stress wave amplitude, indicative of dislocation motion. The shock-loaded pillars exhibit enhanced chemical reactivity when submerged in oil and isopropyl alcohol solutions, due likely to the exposure of clean surfaces via surface spallation and formation of surface steps and nanoscale facets through dislocation motion to the surface of the pillars, resulting in growth of thin oxide films on the surfaces of the pillars.
    Journal of Applied Physics 02/2013; 113(8):84309. DOI:10.1063/1.4793646 · 2.19 Impact Factor
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    ABSTRACT: Pulsed laser operated high rate charging of Fe-doped LiNbO3 crystal for electron emission J. Appl. Phys. 112, 073107 (2012) Formation of nanostructured TiO2 by femtosecond laser irradiation of titanium in O2 J. Appl. Phys. 112, 063108 (2012) Finite element calculations of the time dependent thermal fluxes in the laser-heated diamond anvil cell An apparatus and test procedure for fabrication and loading of single crystal metal nanopillars under extremely high pressures (>1 GPa) and strain rates (>10 7 s À1), using laser-generated stress waves, are presented. Single-crystalline Cu pillars ($1.20 lm in tall and $0.45 lm in diameter) prepared via focused ion beam milling of Cu(001) substrates are shock-loaded using this approach with the dilatational stress waves propagating along the [001] axis of the pillars. Transmission electron microscopy observations of shock-loaded pillars show that dislocation density decreases and that their orientation changes with increasing stress wave amplitude, indicative of dislocation motion. The shock-loaded pillars exhibit enhanced chemical reactivity when submerged in oil and isopropyl alcohol solutions, due likely to the exposure of clean surfaces via surface spallation and formation of surface steps and nanoscale facets through dislocation motion to the surface of the pillars, resulting in growth of thin oxide films on the surfaces of the pillars. V C 2013 American Institute of Physics. [http://dx.doi.org/10.1063/1.4793646]
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    ABSTRACT: Using in situ high-temperature (700-1000 K) scanning tunneling microscopy (STM), we studied the influence of ethylene on the surface dynamics of oxygen-deficient, rutile-structured TiO2(110). STM images were acquired during annealing the sample as a function of time, oxygen and ethylene pressures, and temperature. With increasing oxygen pressure and/or decreasing temperature, TiO2(110) surface mass increased, consistent with previous results. Interestingly, annealing the sample in ethylene with traces of oxygen also results in the growth of TiO2 at higher rates than those observed during annealing in pure oxygen. Our results indicate that ethylene promotes oxidation of TiO2(110).
    Applied Physics Letters 11/2012; 101(21). DOI:10.1063/1.4767954 · 3.52 Impact Factor
  • Microscopy and Microanalysis 07/2012; 18(S2):1098-1099. DOI:10.1017/S1431927612007349 · 1.76 Impact Factor
  • H. Ye, Z. Y. Yu, S. Kodambaka, V. B. Shenoy
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    ABSTRACT: The axial composition profiles in two-component alloy semiconductor nanowires are theoretically studied based on a comprehensive transient growth model which accounts for both surface diffusion and direct impingement of atoms to catalyst. The composition variation derives from the different growth rates contributed by each component. Our simulations reveal that the component with larger (smaller) diffusivity will segregate near the bottom (top) of the nanowire. In the presence (absence) of direct deposition on nanowire sidewalls, the steady state alloy composition is determined by the ratio of effective diffusion lengths (impingement rates to the catalyst).
    Applied Physics Letters 06/2012; 100(26). DOI:10.1063/1.4731628 · 3.52 Impact Factor
  • Yuya Murata, V. Petrova, I. Petrov, S. Kodambaka
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    ABSTRACT: Using in situ high-temperature (1395 K), ultra-high vacuum, scanning tunneling microscopy (STM), we investigated the growth of bilayer graphene on 6H-SiC(0001). From the STM images, we measured areal coverages of SiC and graphene as a function of annealing time and found that graphene grows at the expense of SiC. Graphene domains were observed to grow, at comparable rates, at (I) graphene-free SiC step edges, (II) graphene–SiC interfaces, and (III) the existing graphene domain edges. Based upon our results, we suggest that the rate-limiting step controlling bilayer graphene growth is the desorption of Si from the substrate.
    Thin Solid Films 06/2012; 520(16):5289–5293. DOI:10.1016/j.tsf.2012.03.040 · 1.87 Impact Factor
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    ABSTRACT: Using in situ low-energy electron microscopy and density functional theory, we studied the growth structure and work function of bilayer graphene on Pd(111). Low-energy electron diffraction analysis established that the two graphene layers have multiple rotational orientations relative to each other and the substrate plane. We observed heterogeneous nucleation and simultaneous growth of multiple, faceted layers prior to the completion of second layer. We propose that the facetted shapes are due to the zigzag-terminated edges bounding graphene layers growing under the larger overlying layers. We also found that the work functions of bilayer graphene domains are higher than those of monolayer graphene, and depend sensitively on the orientations of both layers with respect to the substrate. Based on first-principles simulations, we attribute this behavior to oppositely oriented electrostatic dipoles at the graphene/Pd and graphene/graphene interfaces, whose strengths depend on the orientations of the two graphene layers.
    Physical Review B 05/2012; 85:205443. DOI:10.1103/PhysRevB.85.205443 · 3.66 Impact Factor
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    ABSTRACT: In-situ high-temperature scanning tunneling microscopy was used to follow the coarsening (Ostwald ripening) and decay kinetics of single and multiple two-dimensional TiN islands on atomically flat TiN(001) terraces and in single-atom deep vacancy pits at temperatures of 750–950°C. The rate-limiting mechanism for island decay was found to be surface diffusion rather than adatom attachment/detachment at island edges. We have modeled island-decay kinetics based upon the Gibbs–Thomson and steady state diffusion equations to obtain a step-edge energy per unit length of 0.23±0.05 eV/Å and an activation energy for adatom formation and diffusion of 3.4±0.3 eV.
    Surface Review and Letters 04/2012; 07(05n06). DOI:10.1142/S0218625X00000816 · 0.37 Impact Factor
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    ABSTRACT: Idealized nanowire geometries assume stable sidewalls at right angles to the growth front. Here we report growth simulations that include a mix of nonorthogonal facet orientations, as for Au-catalyzed Si. We compare these with in situ microscopy observations, finding striking correspondences. In both experiments and simulations, there are distinct growth modes that accommodate the lack of right angles in different ways--one through sawtooth-textured sidewalls, the other through a growth front at an angle to the growth axis. Small changes in conditions can reversibly switch the growth between modes. The fundamental differences between these modes have important implications for control of nanowire growth.
    Physical Review Letters 12/2011; 107(26):265502. DOI:10.1103/PhysRevLett.107.265502 · 7.73 Impact Factor
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    ABSTRACT: We investigated the effect of high flow rates (> 10−5mol/min) of metalorganic precursors on compositional evolution in indium phosphide antimonide (InP1−xSbx) nanowires grown via chemical vapor deposition in the presence of indium droplets as catalysts on InP(111)B substrates maintained at ∼360°C. The as-grown nanowire morphology, composition, and crystallinity are determined using scanning and transmission electron microscopies, selected-area electron diffraction, and x-ray energy dispersive spectroscopy. For all of the precursor flow rates, we obtain zinc blende structured InP1-xSbx wires that are tapered with wider tops, narrower bases, and In-rich In–Sb alloy tips—characteristic of the vapor–liquid–solid process. The Sb content within the InP1−xSbx wires is found to increase non-linearly with increasing Sb precursor flow rate. At the interfaces between the In–Sb alloy tips and the InP1-xSbx nanowires, we observe single-crystalline wurtzite-structured InSb segments whose volumes depend on the Sb precursor flow rate. We attribute this phenomenon to the rapid crystallization of InSb during cooling of the Sb-rich In–Sb alloy droplets.
    Journal of Crystal Growth 12/2011; 336(1):14-19. DOI:10.1016/j.jcrysgro.2011.09.043 · 1.69 Impact Factor
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    ABSTRACT: Nanowire growth in the standard <111> direction is assumed to occur at a planar catalyst-nanowire interface, but recent reports contradict this picture. Here we show that a nonplanar growth interface is, in fact, a general phenomenon. Both III-V and group IV nanowires show a distinct region at the trijunction with a different orientation whose size oscillates during growth, synchronized with step flow. We develop an explicit model for this structure that agrees well with experiment and shows that the oscillations provide a direct visualization of catalyst supersaturation. We discuss the implications for wire growth and structure.
    Physical Review Letters 07/2011; 107(2):025503. DOI:10.1103/PhysRevLett.107.025503 · 7.73 Impact Factor

Publication Stats

2k Citations
382.77 Total Impact Points

Institutions

  • 2008–2015
    • University of California, Los Angeles
      • Department of Materials Science and Engineering
      Los Ángeles, California, United States
  • 2012
    • University of Pennsylvania
      • Department of Mechanical Engineering and Applied Mechanics
      Filadelfia, Pennsylvania, United States
  • 1999–2012
    • University of Illinois, Urbana-Champaign
      • • Department of Materials Science and Engineering
      • • Department of Physics
      Urbana, Illinois, United States
  • 2006
    • IBM
      Armonk, New York, United States