V. Holý

Charles University in Prague, Praha, Praha, Czech Republic

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Publications (306)869.17 Total impact

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    ABSTRACT: Nanosized particles of ω phase in a β-Ti alloy were investigated by small-angle X-ray scattering using synchrotron radiation. We demonstrated that the particles are spontaneously weakly ordered in a three-dimensional cubic array along the 〈100〉-directions in the β-Ti matrix. The small-angle scattering data fit well to a three-dimensional short-range-order model; from the fit we determined the evolution of the mean particle size and mean distance between particles during ageing. The self-ordering of the particles is explained by elastic interaction between the particles, since the relative positions of the particles coincide with local minima of the interaction energy. We performed numerical Monte Carlo simulation of the particle ordering and we obtained a good agreement with the experimental data.
    Acta Materialia 12/2014; 81:71–82. · 3.94 Impact Factor
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    ABSTRACT: The methodology of single crystal growth of metastable β-Ti alloy TIMETAL LCB in an optical floating zone furnace is presented in this paper. Chemical compositions of both precursor material and single crystals were checked. It was found that the concentration of base alloying elements did not change significantly during the growth process, while the concentrations of interstitial elements O and N increased. DSC measurement determined that this concentration shift has a slight impact on ongoing phase transformations, as in the single-crystalline material peak associated with α phase precipitation moves by a few degrees to a lower temperature and peak attributed to diffusion controlled growth of ω particles shifts to a higher temperature. X-ray reciprocal space maps were measured and their simulation showed that the single crystal has a mosaic structure with mean size of mosaic blocks of approximately 60 nm.
    Journal of Crystal Growth 11/2014; 405:92–96. · 1.55 Impact Factor
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    ABSTRACT: We report on arrangement of iron oxide nanoparticles deposited on flat substrate, on and below graphene, respectively. We combined grazing incidence small angle X-ray scattering (GISAXS) and atomic force microscopy (AFM) to obtain the mean size of the particles and the mean inter-particle distance. While GISAXS provides statistically relevant information averaged over large area, AFM serves to support and clarify the results of GISAXS observations by inspection of the representative area of the sample.
    physica status solidi (b) 08/2014; · 1.49 Impact Factor
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    ABSTRACT: We report on the structure and arrangement of particles created in the Fe2O3/TiO2 + SiO2 multilayers. X-ray diffraction and extended x-ray absorption fine structure spectroscopy reveal the presence of crystalline rutile-TiO2 while the iron oxide remains either amorphous or forms very small clusters of Fe2O3. The Fe3 + oxidation state of iron atoms has been confirmed by Mössbauer and x-ray spectroscopy. The degree of the particle ordering has been studied by grazing-incidence small-angle x-ray scattering. It was demonstrated that with increasing temperature partially-ordered nanoparticles are created and grow up to a critical temperature when the ordering is destroyed. Both particle sizes and inter-particle distances depend strongly on the thickness of the Ti/Fe containing layer.
    Thin Solid Films 08/2014; · 1.87 Impact Factor
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    ABSTRACT: Silicon nanocrystals (SiNCs) smaller than 5 nm are a material with strong visible photoluminescence (PL). However, the physical origin of the PL, which, in the case of oxide-passivated SiNCs, is typically composed of a slow-decaying red-orange band (S-band) and of a fast-decaying blue-green band (F-band), is still not fully understood. Here we present a physical interpretation of the F-band origin based on the results of an experimental study, in which we combine temperature (4-296 K), temporally (picosecond resolution) and spectrally resolved luminescence spectroscopy of free-standing oxide-passivated SiNCs. Our complex study shows that the F-band red-shifts only by 35 meV with increasing temperature, which is almost 6 times less than the red-shift of the S-band in a similar temperature range. In addition, the F-band characteristic decay time obtained from a stretched-exponential fit decreases only slightly with increasing temperature. These data strongly suggest that the F-band arises from the core-related quasi-direct radiative recombination governed by slowly thermalizing photoholes.
    Nanoscale 03/2014; · 6.73 Impact Factor
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    ABSTRACT: Abstract The structure and morphology of uncapped and capped InGaN quantum dots formed by spinodal decomposition was studied by AFM, SEM, XRD, and EXAFS. As result of the spinodal decomposition, the uncapped samples show a meander structure with low Indium content which is strained to the GaN template, and large, relaxed Indium-rich islands. The thin meander structure is responsible for the quantum dot emission. A subsequently deposited low-temperature GaN cap layer forms small and nearly unstrained islands on top of the meander structure which is a sharp interface between the GaN template and the cap layer. For an InGaN cap layer deposited with similar growth parameters, a similar morphology but lower crystalline quality was observed. After deposition of a second GaN cap at a slightly higher temperature, the surface of the quantum dot structure is smooth. The large In-rich islands observed for the uncapped samples are relaxed, have a relatively low crystalline quality and a broad size distribution. They are still visible after capping with a low-temperature InGaN or GaN cap at 700 °C but dissolve after deposition of the second cap layer. The low crystalline quality of the large islands does not influence the quantum dot emission but is expected to increase the number of defects in the cap layer. This might reduce the performance of complex devices based on the stacking of several functional units.
    Journal of Alloys and Compounds 01/2014; 585:572-579. · 2.73 Impact Factor
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    ABSTRACT: Cerium titanate CeTi2O6 has been investigated recently for its photocatalytic activity and as a safe analogue to actinide-containing brannerite-like titanates (UTi2O6, PuTi2O6, e.g.) which are intensively studied because of their use for storing nuclear waste. In this paper we report on the monoclinic phase CeTi2O6 obtained from the Ti–Ce oxide mixture prepared by a reverse micelles directed sol–gel method and subsequently annealed. The kinetics of the isothermal crystallization process is investigated by means of Johnson–Mehl–Avrami–Kolmogorov equation. The effective activation energy of the formation of CeTi2O6 particles, which is an important parameter for its synthesis, is estimated.
    Journal of Physics and Chemistry of Solids 01/2014; 75(2):265–270. · 1.53 Impact Factor
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    ABSTRACT: We present a comprehensive study of graphene grown by chemical vapor deposition on copper single crystals with exposed (1 0 0), (1 1 0) and (1 1 1) faces. Direct examination of the as-grown graphene by Raman spectroscopy using a range of visible excitation energies and microRaman mapping shows distinct strain and doping levels for individual Cu surfaces. Comparison of results from Raman mapping with X-ray diffraction techniques and atomic force microscopy shows it is neither the crystal quality nor the surface topography responsible for the specific strain and doping values, but it is the Cu lattice orientation itself. We also report an exceptionally narrow Raman 2D band width caused by the interaction between graphene and metallic substrate. The appearance of this extremely narrow 2D band with full-width-at-half maximum (FWHM) as low as 16 cm−1 is correlated with flat and undoped regions on the Cu(1 0 0) and (1 1 0) surfaces. The generally compressed (∼0.3% of strain) and n-doped (Fermi level shift of ∼250 meV) graphene on Cu(1 1 1) shows the 2D band FWHM minimum of ∼20 cm−1. In contrast, graphene grown on Cu foil under the same conditions reflects the heterogeneity of the polycrystalline surface and its 2D band is accordingly broader with FWHM >24 cm−1.
    Carbon 01/2014; 68:440–451. · 6.16 Impact Factor
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    ABSTRACT: The structure of magnetron-sputtered Co/SiO2 multilayers has been investigated using grazing-incidence small-angle X-ray scattering, X-ray diffraction, transmission electron microscopy and ion scattering techniques. A theoretical description of diffuse X-ray scattering from three-dimensional self-assembled ensembles of nanoparticles is also presented. The data revealed that Co-rich nanoparticles self-organize in a three-dimensional lattice and a dependence of the lattice parameters as well as the mean particle size on the nominal layer thickness was observed. Originally amorphous Co-rich layers crystallize readily during deposition, creating both pure Co and Co oxide particles. The results presented are important for controlled production and reliable characterization of metallic nanoparticles in solid amorphous matrices, aiming to obtain a well ordered monodisperse ensemble of nanoparticles.
    Journal of Applied Crystallography 12/2013; 46(6). · 3.34 Impact Factor
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    ABSTRACT: We report on a systematic study of the stress transferred from an electromechanical piezo-stack into GaAs wafers under a wide variety of experimental conditions. We show that the strains in the semiconductor lattice, which were monitored in situ by means of X-ray diffraction, are strongly dependent on both the wafer thickness and on the selection of the glue which is used to bond the wafer to the piezoelectric actuator. We have identified an optimal set of parameters that reproducibly transfers the largest distortions at room temperature. We have studied strains produced not only by the frequently used uniaxial piezostressors but also by the biaxial ones which replicate the routinely performed experiments using substrate-induced strains but with the advantage of a continuously tunable lattice distortion. The time evolution of the strain response and the sample tilting and∕or bending are also analyzed and discussed.
    The Review of scientific instruments 10/2013; 84(10):103902. · 1.52 Impact Factor
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    ABSTRACT: Strain engineering and the crystalline quality of semiconductor nanostructures are important issues for electronic and optoelectronic devices. We report on defect-free SiGe island arrays resulting from Ge coverages of up to 38 monolayers grown on prepatterned Si(001) substrates. This represents a significant expansion of the parameter space known for the growth of perfect island arrays. A cyclic development of the Ge content and island shape was observed while increasing the Ge coverage. Synchrotron-based x-ray diffraction experiments and finite element method calculations allow us to study the strain behavior of such islands in great detail. In contrast to the oscillatory changes of island shape and average Ge content, the overall strain behavior of these islands exhibits a clear monotonic trend of progressive strain relaxation with increasing Ge coverage.
    Nanotechnology 08/2013; 24(33):335707. · 3.84 Impact Factor
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    ABSTRACT: Recent studies have demonstrated the potential of antiferromagnets as the active component in spintronic devices. This is in contrast to their current passive role as pinning layers in hard disk read heads and magnetic memories. Here we report the epitaxial growth of a new high-temperature antiferromagnetic material, tetragonal CuMnAs, which exhibits excellent crystal quality, chemical order and compatibility with existing semiconductor technologies. We demonstrate its growth on the III-V semiconductors GaAs and GaP, and show that the structure is also lattice matched to Si. Neutron diffraction shows collinear antiferromagnetic order with a high Néel temperature. Combined with our demonstration of room-temperature-exchange coupling in a CuMnAs/Fe bilayer, we conclude that tetragonal CuMnAs films are suitable candidate materials for antiferromagnetic spintronics.
    Nature Communications 08/2013; 4:2322. · 10.74 Impact Factor
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    ABSTRACT: Coherent high-amplitude precession of the magnetization and spin waves with frequencies up to 40 GHz are generated by injecting picosecond compressive and shear acoustic pulses into nanometer-sized galfenol (Fe81Ga19) films. The magnetization modulation is due to the picosecond inverse magnetostrictive effect. The oscillations of the magnetization measured by magneto-optical Kerr rotation last for several nanoseconds, and the maximum modulation of the in-plane effective magnetic field is as high as 40 mT. These results in combination with a comprehensive theoretical analysis show that galfenol films possess excellent properties for ultrafast magnetization control based on the picosecond inverse magnetostrictive effect.
    Applied Physics Letters 07/2013; 103(3). · 3.52 Impact Factor
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    ABSTRACT: Epitaxial growth of topological insulator bismuth telluride by molecular beam epitaxy onto BaF2 (111) substrates is studied using Bi2Te3 and Te as source materials. By changing the beam flux composition, different stoichiometric phases are obtained, resulting in high quality Bi2Te3 and Bi1Te1 epilayers as shown by Raman spectroscopy and high-resolution X-ray diffraction. From X-ray reciprocal space mapping, the residual strain, as well as size of coherently scattering domains are deduced. The Raman modes for the two different phases are identified and the dielectric functions derived from spectroscopic ellipsometry investigations. Angular resolved photoemission reveals topologically protected surface states of the Bi2Te3 epilayers. Thus, BaF2 is a perfectly suited substrate material for the bismuth telluride compounds.
    Crystal Growth & Design 07/2013; 13(8):3365–3373. · 4.69 Impact Factor
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    ABSTRACT: Simple processes for the preparation of semiconductor quantum dot lattices embedded in dielectric amorphous matrices play an important role in various nanotechnology applications. Of particular interest are quantum dot lattices with properties that differ significantly in different directions parallel to the material surface. Here, a simple method is demonstrated for the fabrication of an anisotropic lattice of Ge quantum dots in an amorphous Al2O3 matrix by a self‐assembly process. A specific deposition geometry with an oblique incidence of the Ge and Al2O3 adparticles was used during magnetron sputtering deposition to achieve the desired anisotropy. The observed Ge quantum dot ordering is explained by a combination of directional diffusion of adparticles from the Ge and Al2O3 targets and a shadowing process which occurs during deposition as a result of the specific surface morphology. The prepared material shows a strong anisotropy of the electrical conductivity in different directions parallel to the sample surface.
    Journal of Applied Crystallography 06/2013; 46(3). · 3.34 Impact Factor
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    ABSTRACT: Hydrogen implanted silicon has been studied using high resolution X-ray scattering. Strain induced by implantation has been measured as a function of implantation dose. The dependence of strain with implanted dose shows different regimes starting from linear to quadratic and saturation. The observed strain is consistent with ab-initio and elasticity calculations. Strain rate changes can be associated to the predominant location of hydrogen in bond center location.
    Journal of Applied Physics 04/2013; 113(15). · 2.21 Impact Factor
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    ABSTRACT: We present a transmission electron microscope (TEM) and x-ray diffraction (XRD) study of metalorganic vapor phase epitaxy grown GaN $(1\,0\,\bar{{1}}\,3)$ layers on m-sapphire with Al(Ga)N interlayers. The interlayers are demonstrated to be beneficial in reducing the stacking fault (SF) density by more than 2.5 orders of magnitude from >2 × 106 cm−1 to <5 × 103 cm−1 as determined by TEM. XRD measurements along the GaN $(1\,0\,\bar{{1}}\,2)$ to the GaN $(1\,0\,\bar{{1}}\,5)$ reflection reveal a diffraction component originating in the basal plane SFs. Fitting the XRD signal enables a fast and simple determination of SF type and density which can even distinguish between the GaN buffer and the upper GaN layers and which is in reasonable agreement with TEM measurements.
    Journal of Physics D Applied Physics 02/2013; 46(12):125308. · 2.53 Impact Factor
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    ABSTRACT: Multiferroic composite materials, consisting of coupled ferromagnetic and piezoelectric phases, are of great importance in the drive towards creating faster, smaller and more energy efficient devices for information and communications technologies. Such devices require thin ferromagnetic films with large magnetostriction and narrow microwave resonance linewidths. Both properties are often degraded, compared to bulk materials, due to structural imperfections and interface effects in the thin films. We report the development of single crystal thin films of Galfenol (Fe81Ga19) with magnetostriction as large as the best reported values for bulk material. This allows the magnetic anisotropy and microwave resonant frequency to be tuned by voltage-induced strain, with a larger magnetoelectric response and a narrower linewidth than any previously reported Galfenol thin films. The combination of these properties make the single crystal thin films excellent candidates for developing tunable devices for magnetic information storage, processing and microwave communications.
    Scientific Reports 02/2013; 3. · 5.08 Impact Factor
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    ABSTRACT: The synthesis of TiO2–CeO2 mixed oxides based on the sol–gel process controlled within reverse micelles of non-ionic surfactant Triton X-114 in cyclohexane is reported. The crystallization, phase composition, trends in nanoparticles growth and porous structure properties are studied as a function of Ti:Ce molar composition and annealing temperature by in-situ X-ray diffraction, Raman spectroscopy and physisorption. The brannerite-type CeTi2O6 crystallizes as a single crystalline phase at Ti:Ce molar composition of 70:30 and in the mixture with cubic CeO2 and anatase TiO2 for composition 50:50. At Ti:Ce molar ratios 90:10 and 30:70 the mixtures of TiO2 anatase, rutile and cubic CeO2 appear. In these mixtures TiO2 rutile is formed at higher temperatures than conventionally. Additionally, the amount of a present amorphous phase in individual mixtures was estimated from diffraction data. The porous structure morphology depends both on molar composition and annealing temperature. This is correlated with the presence of carbon impurities of different character.
    Journal of Solid State Chemistry 02/2013; 198:485–495. · 2.04 Impact Factor
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    ABSTRACT: We analyze X-ray diffraction data used to extract cell parameters of ultrathin films on closely matching substrates. We focus on epitaxial La2/3Sr1/3MnO3 films grown on (001) SrTiO3 single crystalline substrates. It will be shown that, due to extremely high structural similarity of film and substrate, data analysis must explicitly consider the distinct phase of the diffracted waves by substrate and films to extract reliable unit cell parameters. The implications of this finding for the understanding of strain effects in ultrathin films and interfaces will be underlined
    Applied Physics Letters 01/2013; 99(22). · 3.52 Impact Factor

Publication Stats

2k Citations
869.17 Total Impact Points

Institutions

  • 2004–2014
    • Charles University in Prague
      • • Department of Condensed Matter Physics
      • • Faculty of Mathematics and Physics
      Praha, Praha, Czech Republic
  • 2013
    • Universität Ulm
      • Institute of Optoelectronics
      Ulm, Baden-Wuerttemberg, Germany
    • Karlsruhe Institute of Technology
      • Laboratory for Electron Microscopy
      Carlsruhe, Baden-Württemberg, Germany
    • Durham University
      Durham, England, United Kingdom
  • 2012
    • Ruđer Bošković Institute
      • Ruder Bosković Institute
      Zagrabia, Grad Zagreb, Croatia
  • 2011
    • University of Cambridge
      Cambridge, England, United Kingdom
  • 2010
    • Czech Technical University in Prague
      • Department of Microelectronics (FEL)
      Praha, Praha, Czech Republic
  • 2009
    • Ruder Boskovic Institute
      Zagrabia, Grad Zagreb, Croatia
  • 1990–2008
    • Masaryk University
      • • Ústav fyziky kondenzovaných látek
      • • Fakulta Přírodovědecká
      Brno, South Moravian Region, Czech Republic
  • 1995–2007
    • Johannes Kepler University Linz
      • Institut für Halbleiter- und Festkörperphysik
      Linz, Upper Austria, Austria
  • 2001
    • University of Houston
      • Department of Physics
      Houston, Texas, United States
  • 1999
    • Universität Potsdam
      • Institute of Physics and Astronomy
      Potsdam, Brandenburg, Germany
  • 1998
    • Slovak Academy of Sciences
      • Institute of Physics
      Bratislava, Bratislavsky Kraj, Slovakia
  • 1997
    • SAS Institute
      North Carolina, United States
  • 1981
    • Jan Evangelista Purkyně University
      • Faculty of Science
      Aussig, Ústecký, Czech Republic