S. Kodambaka

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

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Publications (104)573.34 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
  • Scripta Materialia 12/2014; 100. DOI:10.1016/j.scriptamat.2014.11.036 · 2.97 Impact Factor
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    ABSTRACT: Colloidal iron pyrite nanocrystals (or FeS2 NC inks) are desirable as active materials in lithium ion batteries and photovoltaics and are particularly suitable for large-scale, roll-to-roll deposition or inkjet printing. However, to date, FeS2 NC inks have only been synthesized using the hot-injection technique, which requires air-free conditions and may not be desirable at an industrial scale. Here, we report the synthesis of monodisperse, colloidal, spherical, and phase-pure FeS2 NCs of 5.5 +/- 0.3 nm in diameter via a scalable solvothermal method using iron diethyldithiocarbamate as the precursor, combined with a postdigestive ripening process. The phase purity and crystallinity are determined using X-ray diffraction, transmission electron microscopy, far-infrared spectroscopy, and Raman spectroscopy techniques. Through this study, a hypothesis has been verified that solvothermal syntheses can also produce FeS2 NC inks by incorporating three experimental conditions: high solubility of the precursor, efficient mass transport, and sufficient stabilizing ligands. The addition of ligands and stirring decrease the NC size and led to a narrow size distribution. Moreover, using density functional theory calculations, we have identified an acid-mediated decomposition of the precursor as the initial and critical step in the synthesis of FeS2 from iron diethyldithiocarbamate.
    Chemistry of Materials 11/2014; 26(23-23):6743-6751. DOI:10.1021/cm5030553 · 8.54 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
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    ABSTRACT: We report a mild, effective, room-temperature method for brominating and functionalizing colloidal hydrogen-terminated silicon quantum dots (H-SiQDs) using N-bromosuccinimide (NBS) as the bromination reagent. This post-synthesis bromination overcomes a long-standing challenge of producing emissive SiQDs through the functionalization of directly synthesized halogen-terminated colloidal SiQDs.
    RSC Advances 10/2014; 4(93). DOI:10.1039/C4RA08477B · 3.71 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
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    ABSTRACT: The design and synthesis of shape-directed nanoscale noble metal particles have attracted much attention due to their enhanced catalytic properties and the opportunities to study fundamental aspects of nanoscale systems. As such, numerous methods have been developed to synthesize crystals with tunable shapes, sizes, and facets by adding foreign species that promote or restrict growth on specific sites. Many hypotheses regarding how and why certain species direct growth have been put forward, however there has been no consensus on a unifying mechanism of nanocrystal growth. Herein, we develop and demonstrate the capabilities of a mathematical growth model for predicting metal nanoparticle shapes by studying a well known procedure that employs AgNO3 to produce {111} faceted Pt nanocrystals. The insight gained about the role of auxiliary species is then utilized to predict the shape of Pd nanocrystals and to corroborate other shape-directing syntheses reported in literature. The fundamental understanding obtained herein by combining modeling with experimentation is a step toward computationally guided syntheses and, in principle, applicable to predictive design of the growth of crystalline solids at all length scales (nano to bulk).
    Nanoscale 08/2014; DOI:10.1039/c4nr02755h · 6.74 Impact Factor
  • Journal of Heat Transfer 08/2014; 136(8):080910. DOI:10.1115/1.4027528 · 2.06 Impact Factor
  • Microscopy and Microanalysis 08/2014; 20(S3-S3):1530-1531. DOI:10.1017/S1431927614009386 · 1.76 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 density functional theory calculations with van der Waals corrections, we investigated how the interlayer orientation affects the structure and electronic properties of MoS2-graphene bilayer heterostructures. Changing the orientation of graphene with respect to MoS2 strongly influences the type and the value of the electronic bandgap in MoS2, while not significantly altering the binding energy between the layers or the interlayer spacing. We show that the physical origin of this tunable bandgap arises from variations in the S–S interplanar distance (MoS2 thickness) with the interlayer orientation, variations which are caused by electron transfer away from the Mo–S bonds.
    Applied Physics Letters 07/2014; 105(3):031603-031603-5. DOI:10.1063/1.4891430 · 3.52 Impact Factor
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    ABSTRACT: The motion of Au between AuSi liquid eutectic droplets, both before and during vapor-liquid-solid growth, is important in controlling tapering and diameter uniformity in Si nanowires. We measure the kinetics of coarsening of AuSi droplets on Si(001) and Si(111), quantifying the size evolution of droplets during annealing in ultra high vacuum using in situ transmission electron microscopy. For individual droplets, we show that coarsening kinetics are modified when disilane or oxygen is added: coarsening rates increase in the presence of disilane but decrease in oxygen. Matching droplet size measurements with coarsening models confirms that Au transport is driven by capillary forces, and that the kinetic coefficients depend on the gas environment present. We suggest that the gas effects are qualitatively similar whether transport is attachment limited or diffusion limited. These results provide insight into manipulating nanowire morphologies for advanced device fabrication.
    Nano Letters 07/2014; 14(8). DOI:10.1021/nl501582q · 12.94 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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    Thin Solid Films 12/2013; 549:1. DOI:10.1016/j.tsf.2013.11.018 · 1.87 Impact Factor
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    Surface and Coatings Technology 12/2013; 237:1. DOI:10.1016/j.surfcoat.2013.11.021 · 2.20 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: We report a template-free, halide-free, efficient wet chemical method to synthesize defect-rich ZnO nanostructures with exposed {101} facets. The self-assembled ZnO nanostructures provide an active playground for catalytic reactions, such as CO2 hydrogenation, and exhibits great potential in alternative energy technologies.
    CrystEngComm 04/2013; 15(19):3780-3784. DOI:10.1039/C3CE40167G · 3.86 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

Publication Stats

2k Citations
573.34 Total Impact Points

Institutions

  • 2008–2014
    • University of California, Los Angeles
      • Department of Materials Science and Engineering
      Los Angeles, California, United States
  • 2012
    • University of Pennsylvania
      • Department of Mechanical Engineering and Applied Mechanics
      Filadelfia, Pennsylvania, United States
  • 2001–2012
    • 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