Konstantinos P. Giapis

California Institute of Technology, Pasadena, California, United States

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Publications (71)258.19 Total impact

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    ABSTRACT: Carbon nanotubes have been explored as interconnects in solid acid fuel cells to improve the link between nanoscale Pt catalyst particles and macroscale current collectors. The nanotubes were grown by chemical vapor deposition on carbon paper substrates, using nickel nanoparticles as the catalyst, and were characterized using scanning electron microscopy and Raman spectroscopy. The composite electrode material, consisting of CsH2PO4, platinum nanoparticles, and platinum on carbon-black nanoparticles, was deposited onto the nanotube-overgrown carbon paper by electrospraying, forming a highly porous, fractal structure. AC impedance spectroscopy in a symmetric cell configuration revealed a significant reduction of the electrode impedance as compared to similarly prepared electrodes without carbon nanotubes.
    Physical Chemistry Chemical Physics 08/2013; · 3.83 Impact Factor
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    ABSTRACT: We describe a hybrid mass-mobility instrument in which a continuous-flow ion mobility classifier is used as a front-end separation device for mass spectrometric analysis of ions generated with an electrospray ionization source. Using nitrogen as a carrier gas, the resolving power of the nano-radial differential mobility analyzer (nRDMA) for nanometer-sized ions is 5-7 for tetraalkylammonium ions. Data are presented demonstrating the ability of the system to resolve the different aggregation and charge states of tetraalkylammonium ions and protonated peptides using a quadrupole ion trap (QIT) mass spectrometer to analyze the mobility-classified ions. Specifically, data are presented for the two charge states of the decapeptide Gramicidin S. A key feature of the new instrument is the ability to continuously transmit ions with specific mobilities to the mass spectrometer for manipulation and analysis.
    Analytical Chemistry 04/2013; · 5.70 Impact Factor
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    Xiaoliang Zhang, Konstantinos P. Giapis, Dimos Poulikakos, Ming Hu
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    ABSTRACT: Nonequilibrium molecular dynamics (NEMD) simulations were performed to investigate schemes for enhancing the energy conversion efficiency of thermoelectric nanowires (NWs), including (1) roughening of the nanowire surface, (2) creating nanoparticle inclusions in the nanowires, and (3) coating the nanowire surface with other materials. The enhancement in energy conversion efficiency was inferred from the reduction in thermal conductivity of the nanowire, which was calculated by imposing a temperature gradient in the longitudinal direction. Compared to pristine nanowires, our simulation results show that the schemes proposed above lead to nanocomposite structures with considerably lower thermal conductivity (up to 82% reduction), implying ~5X enhancement in the ZT coefficient. This significant effect appears to have two origins: (1) increase in phonon-boundary scattering and (2) onset of interfacial interference. The results suggest new fundamental–yet realizable ways to improve markedly the energy conversion efficiency of nanostructured thermoelectrics.
    Journal of Heat Transfer 08/2012; 134(10):102402. · 1.72 Impact Factor
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    ABSTRACT: Microdischarges in xenon have been generated in a pressure range of 400–1013 mbar with a fixed flow rate of 100 sccm. These microdischarges are obtained from three metallic capillary tubes in series for excimer emission. Total discharge voltage is thrice as large as that of a single capillary discharge tube at current levels of up to 12 mA. Total spectral irradiance of vacuum ultraviolet (VUV) emission also increases significantly compared to that of the single capillary discharge. Further, the irradiance of the VUV emission is strongly dependent on pressure as well as the discharge current.
    Physics of Plasmas 08/2011; 18(8):083506-083506-3. · 2.38 Impact Factor
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    ABSTRACT: The transfer functions and penetrations of five differential mobility analyzers (DMAs) for sub-2 nm particle classification were evaluated in this study. These DMAs include the TSI nanoDMA, the Caltech radial DMA (RDMA) and nanoRDMA, the Grimm nanoDMA, and the Karlsruhe-Vienna DMA. Measurements were done using tetra-alkyl ammonium ion standards with mobility diameters of 1.16, 1.47, and 1.70 nm. These monomobile ions were generated by electrospray followed by high resolution mobility classification. Measurements were focused at an aerosol-to-sheath flow ratio of 0.1. A data inversion routine was developed to obtain the true transfer function for each test DMA, and these measured transfer functions were compared with theory. DMA penetration efficiencies were also measured. An approximate model for diffusional deposition, based on the modified Gormley and Kennedy equation using an effective length, is given for each test DMA. These results quantitatively characterize the performance of the test DMAs in classifying sub-2 nm particles and can be readily used for DMA data inversion.
    Aerosol Science and Technology 04/2011; 45(4):480-492. · 2.78 Impact Factor
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    ABSTRACT: We report on the effect of germanium (Ge) coatings on the thermal transport properties of silicon (Si) nanowires using nonequilibrium molecular dynamics simulations. Our results show that a simple deposition of a Ge shell of only 1 to 2 unit cells in thickness on a single crystalline Si nanowire can lead to a dramatic 75% decrease in thermal conductivity at room temperature compared to an uncoated Si nanowire. By analyzing the vibrational density states of phonons and the participation ratio of each specific mode, we demonstrate that the reduction in the thermal conductivity of Si/Ge core-shell nanowire stems from the depression and localization of long-wavelength phonon modes at the Si/Ge interface and of high frequency nonpropagating diffusive modes.
    Nano Letters 02/2011; 11(2):618-23. · 13.03 Impact Factor
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    Ming Hu, Xiaoliang Zhang, Konstantinos P. Giapis, Dimos Poulikakos
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    ABSTRACT: Nanostructuring of thermoelectric materials bears promise for manipulating physical parameters to improve the energy conversion efficiency of thermoelectrics. Using nonequilibrium molecular dynamics, we investigate how the thermal conductivity can be altered in core-shell nanocomposites of Si and Ge. By calculating the phonon vibrational density of states and performing normal mode analysis, we show that the thermal conductivity decreases when phonon-transport becomes diffusion-dominated and unveil a competition between modes from the various regions of the nanocomposite (core, interface, and shell). The effects of nanowire length, cross section, and temperature on thermal conductivity are explicitly considered. Surprisingly, the thermal conductivity variation with nanowire length is much weaker than in pure nanowires. Also, the thermal conductivity is almost independent of temperature in the wide region between 50 and 600 K, a direct result of confinement of the core by the shell. These results suggest that core-shell nanowires are promising structures for thermoelectrics.
    Physical review. B, Condensed matter 01/2011; 84.
  • Ming Hu, Konstantinos P. Giapis, Dimos Poulikakos
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    ABSTRACT: The process of forming a junction between crystalline zinc oxide (ZnO) nanoparticles during pulsed thermal annealing in liquid tetradecane is studied using molecular dynamics simulation. Pairs of equal and unequal size particles are considered with emphasis on neck growth and atom mixing. The contact area and interface width of the junction are found to increase with heat pulse power albeit at different rates. The results suggest that it is possible to increase the junction area without significant mixing of atoms across the junction interface by tailoring the heat pulse power.
    Applied Physics Letters 01/2011; 98. · 3.79 Impact Factor
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    ABSTRACT: Stable, porous, nanostructured composite electrodes were successfully fabricated via the inexpensive and scalable method of electrospray deposition, in which a dissolved solute is deposited onto a substrate using an electric field to drive droplet migration. The desirable characteristics of high porosity and high surface area were obtained under conditions that favored complete solvent evaporation from the electrospray droplets prior to contact with the substrate. Solid acid (CsH_2PO_4) feature sizes of 100 nm were obtained from electrosprayed water–methanol solutions with 10 g L^(−1) CsH_2PO_4 and 5 g L^(−1) Pt catalyst particles suspended using polyvinylpyrrolidone (PVP). Alternative additives such as Pt on carbon and carbon-nanotubes (CNTs) were also successfully incorporated by this route, and in all cases the PVP could be removed from the electrode by oxygen plasma treatment without damage to the structure. In the absence of additives (Pt, Pt/C and CNTs), the feature sizes were larger, 300 nm, and the structure morphologically unstable, with significant coarsening evident after exposure to ambient conditions for just two days. Electrochemical impedance spectroscopy under humidified hydrogen at 240 °C indicated an interfacial impedance of ~1.5 Ω cm^2 for the Pt/CsH_2PO_4 composite electrodes with a total Pt loading of 0.3 ± 0.2 mg cm^(−2). This result corresponds to a 30-fold decrease in Pt loading relative to mechanically milled electrodes with comparable activity, but further increases in activity and Pt utilization are required if solid acid fuel cells are to attain widespread commercial adoption.
    Journal of Materials Chemistry 07/2010; · 5.97 Impact Factor
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    ABSTRACT: Achieving stable single-digit nanometer inverted domains in ferroelectric thin films is a fundamental issue that has remained a bottleneck for the development of ultrahigh density (≫1 Tbit / in .<sup>2</sup>) probe-based memory devices using ferroelectric media. Here, we demonstrate that such domains remain stable only if they are fully inverted through the entire ferroelectric film thickness, which is dependent on a critical ratio of electrode size to the film thickness. This understanding enables the formation of stable domains as small as 4 nm in diameter, corresponding to 10 unit cells in size. Such domain size corresponds to 40 Tbit / in .<sup>2</sup> data storage densities.
    Applied Physics Letters 02/2010; · 3.79 Impact Factor
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    Ming Hu, Konstantinos P. Giapis, Javier V Goicochea, Dimos Poulikakos
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    ABSTRACT: The surface segregation of Pt atoms in liquid bimetallic alloys confined in carbon nanotube cavities was studied using molecular dynamics simulations. Considerable enrichment in the Pt-atom surface density was found to occur in Pt alloys, when the complementary metal has surface energy higher than Pt and simultaneously metal-wall interaction strength lower than that of Pt with the confining wall. The results suggest that solidification of liquid binary alloys in nanochannels could produce core-shell nanorods with the shell enriched in one of the components for catalytic and other applications.
    Applied Physics Letters 01/2010; · 3.79 Impact Factor
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    Michael J Gordon, Xiangdong Qin, Alex Kutana, Konstantinos P Giapis
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    ABSTRACT: Most gas-surface chemical reactions occur via reaction of adsorbed species to form a thermal-energy ( approximately kT) product; however, some instances exist where an energetic projectile directly reacts with an adsorbate in a single-collision event to form a hyperthermal product (with a kinetic energy of a few eV). Here we show for the first time that 30-300 eV F(+) bombardment of fluorinated Ag and Si surfaces produces "ultrafast" F(2)(-) products with exit energies of up to 90 eV via a multistep direct-reaction mechanism. Experiments conclusively show that the projectile F atom ends up in the fast molecular product despite the fact that the impact energy is far greater than typical bond energies.
    Journal of the American Chemical Society 03/2009; 131(5):1927-30. · 10.68 Impact Factor
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    01/2009;
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    Alex Kutana, Konstantinos P. Giapis
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    ABSTRACT: Using ab initio plane wave pseudopotential calculations, we study the energetics and structure of adsorbed linear arrays of oxygen and aziridine on carbon nanotubes, graphitic ribbons, and graphene sheets. Chemisorption of arrays of O or NH causes splitting of the CC bond and local deformation of the graphitic structures. The (3,3) nanotube cross section assumes a teardrop-like shape, while graphene sheets warp into a new local geometry around the chemisorbed molecules. The interior of a (3,3) nanotube is less prone to oxidation than the exterior because of steric effects. A zigzag (6,0) nanotube is less reactive and thus chemically more stable than an armchair (3,3) nanotube. The results suggest a partial explanation for the experimentally observed selective etching of metallic carbon nanotubes.
    01/2009;
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    ABSTRACT: We have fabricated high aspect ratio, hydrophilic nanoelectrodes from individual single-walled carbon nanotubes (SWNTs) mounted on conductive atomic force microscope (AFM) tips for use as electrochemical probes. Individual SWNTs with an average diameter of 5 nm and up to 1.5 μm in length were passivated with nanometer-thick SiO_2 films, deposited conformally in an inductively coupled plasma reactor. The electrically insulating SiO_2 films improved the nanotube rigidity and stabilized the nanotube−AFM tip contact to enable use in aqueous environments. The nanotube tip was successfully exposed by subjecting the probe to nanosecond electrical pulse etching but only after electron beam irradiation in a transmission electron microscope (TEM). Probe functionality was verified by electrodepositing gold nanoparticles from aqueous solution only at the exposed tip.
    01/2009;
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    ABSTRACT: A dielectric-sheathed carbon nanotube probe, resembling a “nanopencil,” has been fabricated by conformal deposition of silicon-oxide on a carbon nanotube and subsequent “sharpening” to expose its tip. The high aspect-ratio nanopencil probe takes advantage of the small nanotube electrode size, while avoiding bending and buckling issues encountered with naked or polymer-coated carbon nanotube probes. Since the effective electrode diameter of the probe would not change even after significant wear, it is capable of long-lasting read/write operations in contact mode with a bit size of several nanometers.
    Applied Physics Letters 10/2008; · 3.79 Impact Factor
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    A Kutana, K P Giapis
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    ABSTRACT: We present a reactive empirical potential with environment-dependent bond strengths for the carbon-oxygen (CO) system. The distinct feature of the potential is the use of three adjustable parameters characterizing the bond: the strength, length, and force constant, rather than a single bond order parameter, as often employed in these types of potentials. The values of the parameters are calculated by fitting results obtained from density functional theory. The potential is tested in a simulation of oxidative unzipping of graphene sheets and carbon nanotubes. Previous higher-level theoretical predictions of graphene unzipping by adsorbed oxygen atoms are confirmed. Moreover, nanotubes with externally placed oxygen atoms are found to unzip much faster than flat graphene sheets.
    The Journal of Chemical Physics 07/2008; 128(23):234706. · 3.16 Impact Factor
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    ABSTRACT: Intense xenon vacuum ultraviolet (VUV) emission is observed from a high-pressure capillary cathode microdischarge in direct current operation, by superimposing a high-voltage pulse of 50 ns duration. Under stagnant gas conditions, the total VUV light intensity increases linearly with pressure from 400 to 1013 mbar for a fixed voltage pulse. At fixed pressure, however, the VUV light intensity increases superlinearly with voltage pulse height ranging from 08 to 2.8 kV . Gains in emission intensity are obtained by inducing gas flow through the capillary cathode, presumably because of excimer dimer survival due to gas cooling.
    Applied Physics Letters 07/2007; · 3.79 Impact Factor
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    A Kutana, K P Giapis
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    ABSTRACT: Pure bending of single-walled carbon nanotubes between (5,5) and (50,50) is studied using molecular dynamics based on the reactive bond order potential. Unlike smaller nanotubes, bending of (15,15) and larger ones exhibits an intermediate deformation in the transition between the buckled and fully kinked configurations. This transient bending regime is characterized by a gradual and controllable flattening of the nanotube cross section at the buckling site. Unbending of a kinked nanotube bypasses the transient bending regime, exhibiting a hysteresis due to van der Waals attraction between the tube walls at the kinked site.
    Physical Review Letters 01/2007; 97(24):245501. · 7.94 Impact Factor
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    A Kutana, K P Giapis
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    ABSTRACT: Optimized model potentials for mercury-mercury and mercury-carbon interactions are used in molecular dynamics simulations to study wetting and solidification of liquid mercury encapsulated in single-walled carbon nanotubes. The contact angle of mercury in the nanotube cavity increases linearly with wall curvature. The solid-liquid transition becomes less well defined as nanotube diameter decreases, while the melting temperature drops exponentially. A concentric cylindrical-shell structure is predicted for solidified mercury in small (20,20) nanotubes, while a polycrystalline structure appears in larger (40,40) nanotubes.
    Physical Review B 01/2007; · 3.77 Impact Factor

Publication Stats

411 Citations
130 Downloads
3k Views
258.19 Total Impact Points

Institutions

  • 1994–2013
    • California Institute of Technology
      • Division of Chemistry and Chemical Engineering
      Pasadena, California, United States
    • Massachusetts Institute of Technology
      • Department of Chemical Engineering
      Cambridge, Massachusetts, United States
  • 2011
    • ETH Zurich
      • Department of Mechanical and Process Engineering
      Zürich, ZH, Switzerland
  • 2009
    • University of California, Santa Barbara
      • Department of Chemical Engineering
      Santa Barbara, CA, United States