V. Holý

Charles University in Prague, Praha, Praha, Czech Republic

Are you V. Holý?

Claim your profile

Publications (314)827.4 Total impact

  • [Show abstract] [Hide abstract]
    ABSTRACT: We report on the formation of Ge/Si quantum dots with core/shell structure that are arranged in a three-dimensional body centered tetragonal quantum dot lattice in an amorphous alumina matrix. The material is prepared by magnetron sputtering deposition of Al2O3/Ge/Si multilayer. The inversion of Ge and Si in the deposition sequence results in the formation of thin Si/Ge layers instead of the dots. Both materials show an atomically sharp interface between the Ge and Si parts of the dots and layers. They have an amorphous internal structure that can be crystallized by an annealing treatment. The light absorption properties of these complex materials are significantly different compared to films that form quantum dot lattices of the pure Ge, Si or a solid solution of GeSi. They show a strong narrow absorption peak that characterizes a type II confinement in accordance with theoretical predictions. The prepared materials are promising for application in quantum dot solar cells.
    Nanotechnology 02/2015; 26(6):065602. · 3.67 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: A common way of speeding up powder diffraction measurements is the use of one or two dimensional detectors. This usually goes along with worse resolution and asymmetric peak profiles. In this work the influence of a straight linear detector on the resolution function in the Bragg-Brentano focusing geometry is discussed. Due to the straight nature of most modern detectors geometrical defocusing occurs which heavily influences the line shape of diffraction lines at low angles. An easy approach to limit the resolution degrading effects is presented. The presented algorithm selects an adaptive range of channels of the linear detector at low angles, resulting in increased resolution. At higher angles still the whole linear detector is used and the data collection remains fast. Using this algorithm a well-behaved resolution function is obtained in the full angular range, whereas using the full linear detector the resolution function varies within one pattern which hinders line shape and Rietveld analysis.
    01/2015;
  • [Show abstract] [Hide abstract]
    ABSTRACT: Weshow that in manganese-doped topological insulator bismuth telluride layers,Mnatoms are incorporated predominantly as interstitials in the van der Waals gaps between the quintuple layers and not substitutionally on Bi sites within the quintuple layers. The structural properties of epitaxial layers withMnconcentration of up to 13% are studied by high-resolution x-ray diffraction, evidencing a shrinking of both the in-plane and out-of plane lattice parameters with increasingMncontent. Ferromagnetism sets in forMncontents around3%and the Curie temperatures rises up to 15K for a Mn concentration of 9%. The easy magnetization axis is along the c-axis perpendicular to the (0001) epilayer plane. Angle-resolved photoemission spectroscopy reveals that the Fermi level is situated in the conduction band and no evidence for a gap opening at the topological surface state with the Dirac cone dispersion is found within the experimental resolution at temperatures close to the Curie temperature. From the detailed analysis of the extended x-ray absorption fine-structure experiments (EXAFS) performed at theMnK-edge, we demonstrate that theMnatoms occupy interstitial positions within the van der Waals gap and are surrounded octahedrally by Te atoms of the adjacent quintuple layers.
    New Journal of Physics 01/2015; 17(1):013028. · 3.67 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: Layered iridates have been the subject of intense scrutiny on account of their unusually strong spin-orbit coupling, which opens up a narrow gap in a material that would otherwise be a metal. This insulating state is very sensitive to external perturbations. Here, we show that vertical compression at the nanoscale, delivered using the tip of a standard scanning probe microscope, is capable of inducing a five orders of magnitude change in the room temperature resistivity of Sr2IrO4. The extreme sensitivity of the electronic structure to anisotropic deformations opens up a new angle of interest on this material, and the giant and fully reversible perpendicular piezoresistance makes iridates a promising material for room temperature piezotronic devices.
    Nanoscale 01/2015; · 6.74 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: The spatial strain distribution in and around a single axial InAs1–xPx hetero-segment in an InAs nanowire was analyzed using nano-focused X-ray diffraction. In connection with finite-element-method simulations a detailed quantitative picture of the nanowire's inhomogeneous strain state was achieved. This allows for a detailed understanding of how the variation of the nanowire's and hetero-segment's dimensions affect the strain in its core region and in the region close to the nanowire's side facets. Moreover, ensemble-averaging high-resolution diffraction experiments were used to determine statistical information on the distribution of wurtzite and zinc-blende crystal polytypes in the nanowires.
    Journal of Synchrotron Radiation 01/2015; 22(1). · 2.19 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: The structure and composition of Bi2Te3−δ topological insulator layers grown by molecular beam epitaxy is studied as a function of beam flux composition. It is demonstrated that, depending on the Te/Bi2Te3 flux ratio, different layer compositions are obtained corresponding to a Te deficit δ varying between 0 and 1. On the basis of X-ray diffraction analysis and a theoretical description using a random stacking model, it is shown that for δ≥ 0 the structure of the epilayers is described well by a random stacking of Te–Bi–Te–Bi–Te quintuple layers and Bi–Bi bilayers sharing the same basic hexagonal lattice structure. The random stacking model accounts for the observed surface step structure of the layers and compares very well with the measured X-ray data, from which the lattice parameters a and c as a function of the chemical composition were deduced. In particular, the in-plane lattice parameter a is found to continuously increase and the average distance of the (0001) hexagonal lattice planes is found to decrease from the Bi2Te3 to the BiTe phase. Moreover, the lattice plane distances agree well with the linear interpolation between the Bi2Te3 and BiTe values taking the strain in the epilayers into account. Thus, the chemical composition Bi2Te3−δ can be directly determined by X-ray diffraction. From analysis of the X-ray diffraction data, quantitative information on the randomness of the stacking sequence of the Bi and Te layers is obtained. According to these findings, the layers represent random one-dimensional alloys of Te–Bi–Te–Bi–Te quintuple and Bi–Bi bilayers rather than a homologous series of ordered compounds.
    Journal of Applied Crystallography 12/2014; 47(6). · 3.95 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    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
  • [Show abstract] [Hide abstract]
    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. · 1.69 Impact Factor
  • [Show abstract] [Hide abstract]
    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; · 1.61 Impact Factor
  • [Show abstract] [Hide abstract]
    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
  • Source
    [Show abstract] [Hide abstract]
    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; · 6.74 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    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 03/2014; 68:440–451. · 6.16 Impact Factor
  • [Show abstract] [Hide abstract]
    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 02/2014; 75(2):265–270. · 1.59 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    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 02/2014; 585:572-579. · 2.73 Impact Factor
  • [Show abstract] [Hide abstract]
    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). · 3.95 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: Three‐dimensional reciprocal space mapping of semipolar () GaN grown on stripe‐patterned r‐plane () sapphire substrates is found to be a powerful and crucial method for the analysis of diffuse scattering originating from stacking faults that are diffracting in a noncoplanar geometry. Additionally, by measuring three‐dimensional reciprocal space maps (3D‐RSMs) of several reflections, the transmission electron microscopy visibility criteria could be confirmed. Furthermore, similar to cathodoluminescence, the 3D‐RSM method could be used in future as a reliable tool to distinguish clearly between the diffuse scattering signals coming from prismatic and from basal plane stacking faults and from partial dislocations in semipolar () GaN. The fitting of the diffuse scattering intensity profile along the stacking fault streaks with a simulation based on the Monte Carlo approach has delivered an accurate determination of the basal plane stacking fault density. A reduction of the stacking fault density due to the intercalation of an SiN interlayer in the GaN layer deposited on the sidewall of the pre‐patterned sapphire substrate has led to an improvement of the optoelectronic properties, influenced by the crystal quality, as has been demonstrated by a locally resolved cathodoluminescence investigation.
    Journal of Applied Crystallography 10/2013; 46(5). · 3.95 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    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. · 1.58 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    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. · 3.67 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    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
  • [Show abstract] [Hide abstract]
    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

Publication Stats

3k Citations
827.40 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
    • Karlsruhe Institute of Technology
      • Laboratory for Electron Microscopy
      Carlsruhe, Baden-Württemberg, Germany
    • Universität Ulm
      • Institute of Optoelectronics
      Ulm, Baden-Wuerttemberg, Germany
    • Durham University
      Durham, England, United Kingdom
    • University of Nottingham
      • School of Physics and Astronomy
      Nottingham, ENG, United Kingdom
  • 2011
    • University of Cambridge
      Cambridge, England, United Kingdom
  • 2010
    • Czech Technical University in Prague
      • Department of Microelectronics (FEL)
      Praha, Praha, Czech Republic
  • 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