V. Holý

Charles University in Prague, Praha, Praha, Czech Republic

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Publications (268)695.83 Total impact

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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; · 6.23 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. · 2.39 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: 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. · 10.02 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). · 3.79 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. · 2.93 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). · 3.79 Impact Factor
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    ABSTRACT: We present tensile and compressive strains realized within the same Si capping layer on an array of SiGe islands grown on pit-patterned (001) Si substrates. The strain distributions are obtained from synchrotron X-ray diffraction studies in combination with three-dimensional finite element calculations and simulations of the diffracted intensities. For barn-shaped islands grown at 720 °C with average Ge contents of 30%, the Si cap layer is misfit- and threading-dislocation free and exhibits compressive strains as high as 0.8% in positions between the islands and tensile strains of up to 1% on top of the islands.
    Applied Physics Letters 01/2013; 102(3). · 3.79 Impact Factor
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    ABSTRACT: Determining atomic positions in thin films by X-ray diffraction is, at present, a task reserved for synchrotron facilities. Here an experimental method is presented which enables the determination of the structure factor amplitudes of thin films using laboratory-based equipment (CuK�radiation). This method was tested using an epitaxial 130 nm film of CuMnAs grown on top of a GaAs substrate, which unlike the orthorhombic bulk phase forms a crystal structure with tetragonal symmetry. From the set of structure factor moduli obtained by applying this method, the solution and refinement of the crystal structure of the film has been possible. The results are supported by consistent high-resolution scanning transmission electron microscopy and stoichiometry analyses.
    Journal of Applied Crystallography 01/2013; 46:1749. · 3.34 Impact Factor
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    ABSTRACT: Surface and interface morphology of cerium oxide/carbon bilayers used as thin-film catalysts is studied by grazing-incidence small-angle x-ray scattering, scanning electron microscopy, and atomic-force microscopy, and the dependence of the structural parameters on the thicknesses of the constituting layers is investigated. The applicability of x-ray scattering and its advantages over standard analytical methods are discussed.
    Journal of Applied Physics 01/2013; 113(2). · 2.21 Impact Factor
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    Journal of Applied Crystallography 01/2013; 46(1):120-127. · 3.34 Impact Factor
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    ABSTRACT: We present investigations on the strain properties of silicon capping layers on top of regular SiGe island arrays, in dependence on the Si-layer thickness. Such island arrays are used as stressors for the active channel in field-effect transistors where the desired tensile strain in the Si channel is a crucial parameter for the performance of the device. The thickness of the Si cap was varied from 0 to 30 nm. The results of high resolution x-ray diffraction experiments served as input to perform detailed strain calculations via finite element method models. Thus, detailed information on the Ge distribution within the buried islands and the strain interaction between the SiGe island and Si cap was obtained. It was found that the tensile strain within the Si capping layer strongly depends on its thickness, even if the Ge concentration of the buried dot remains unchanged, with tensile strains degrading if thicker Si layers are used.
    Nanotechnology 10/2012; 23(46):465705. · 3.84 Impact Factor
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    ABSTRACT: Doping silicon nanocrystals (SiNCs) provides a new way to modify their luminescence properties and tailor them for a particular application. We prepared Li-doped SiNCs and characterized them by neutron depth profiling and x-ray diffraction. Our SiNC samples are doped with around 10–100 Li atoms per one nanocrystal and their lattice slightly expands after lithium insertion. We show that the photoluminescence (PL) properties of Li-doped SiNCs are distinctly modified compared to the undoped case. The PL maximum shifts to shorter wavelengths and the PL decay time decreases, both these features being favorable for applications in photonics. The spectral blue-shift is attributed to the tensile strain in SiNCs induced by doping with lithium.
    Journal of Applied Physics 09/2012; 112(6). · 2.21 Impact Factor
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    ABSTRACT: We report on structural, morphological and ordering properties of Fe2O3/TiO2 nanoparticles embedded in SiO2-based multilayers. We investigated the structure of these systems by X-ray diffraction and grazing incidence small angle X-ray scattering after post-growth annealing. We found that the presence of TiO2 promotes the growth and crystallization of the nanocrystals of Fe2O3. In multilayers containing both Fe2O3 and TiO2, crystalline nanoparticles create partially ordered three-dimensional arrays.
    Thin Solid Films 05/2012; · 1.60 Impact Factor

Publication Stats

442 Citations
695.83 Total Impact Points

Institutions

  • 2004–2014
    • Charles University in Prague
      • • Department of Condensed Matter Physics
      • • Matematicko-fyzikální fakulta
      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
  • 1991–2008
    • 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