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Träfflista för sökning "WAKA:ref ;pers:(Eriksson Olle);pers:(Rusz Jan)"

Sökning: WAKA:ref > Eriksson Olle > Rusz Jan

  • Resultat 1-10 av 36
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  • Bhowmick, Somnath, et al. (författare)
  • X-ray absorption spectra : Graphene, h-BN, and their alloy
  • 2013
  • Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 87:15, s. 155108
  • Tidskriftsartikel (refereegranskat)abstract
    • Using first-principles density functional theory calculations, in conjunction with the Mahan-Nozieres-de Dominicis theory, we calculate the x-ray absorption spectra of the alloys of graphene and monolayer hexagonal boron nitride on a Ni (111) substrate. The chemical neighborhood of the constituent atoms (B, C, and N) inside the alloy differs from that of the parent phases. In a systematic way, we capture the change in the K-edge spectral shape, depending on the chemical neighborhood of B, C, and N. Our work also reiterates the importance of the dynamical core-hole screening for a proper description of the x-ray absorption process in sp(2)-bonded layered materials.
  • Edström, Alexander, et al. (författare)
  • Magnetic properties of (Fe1-xCox)(2)B alloys and the effect of doping by 5d elements
  • 2015
  • Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 92:17
  • Tidskriftsartikel (refereegranskat)abstract
    • We have explored, computationally and experimentally, the magnetic properties of (Fe1-xCox)(2)B alloys. Calculations provide a good agreement with experiment in terms of the saturation magnetization and the magnetocrystalline anisotropy energy with some difficulty in describing Co2B, for which it is found that both full potential effects and electron correlations treated within dynamical mean field theory are of importance for a correct description. The material exhibits a uniaxial magnetic anisotropy for a range of cobalt concentrations between x = 0.1 and x = 0.5. A simple model for the temperature dependence of magnetic anisotropy suggests that the complicated nonmonotonic behavior is mainly due to variations in the band structure as the exchange splitting is reduced by temperature. Using density functional theory based calculations we have explored the effect of substitutionally doping the transition metal sublattice by the whole range of 5d transition metals and found that doping by Re or W elements should significantly enhance the magnetocrystalline anisotropy energy. Experimentally, W doping did not succeed in enhancing the magnetic anisotropy due to formation of other phases. On the other hand, doping by Ir and Re was successful and resulted in magnetic anisotropies that are in agreement with theoretical predictions. In particular, doping by 2.5 at.% of Re on the Fe/Co site shows a magnetocrystalline anisotropy energy which is increased by 50% compared to its parent (Fe0.7Co0.3)(2)B compound, making this system interesting, for example, in the context of permanent magnet replacement materials or in other areas where a large magnetic anisotropy is of importance.
  • Hedlund, Daniel, et al. (författare)
  • Magnetic properties of the Fe5SiB2−Fe5PB2 system
  • 2017
  • Ingår i: Physical Review B. - American Physical Society. - 2469-9950 .- 2469-9969. ; 96:9
  • Tidskriftsartikel (refereegranskat)abstract
    • The magnetic properties of the compound Fe5Si1−xPxB2 have been studied, with a focus on the Curie temperature TC, saturation magnetization MS, and magnetocrystalline anisotropy. Field and temperature dependent magnetization measurements were used to determine TC(x) and MS(x). The saturation magnetization at 10 K (300 K) is found to monotonically decrease from 1.11MA/m (1.03MA/m) to 0.97MA/m (0.87MA/m), as x increases from 0 to 1. The Curie temperature is determined to be 810 and 615 K in Fe5SiB2 and Fe5PB2, respectively. The highest TC is observed for x=0.1, while it decreases monotonically for larger x. The Curie temperatures have also been theoretically determined to be 700 and 660 K for Fe5SiB2 and Fe5PB2, respectively, using a combination of density functional theory and Monte Carlo simulations. The magnitude of the effective magnetocrystalline anisotropy was extracted using the law of approach to saturation, revealing an increase with increasing phosphorus concentration. Low-field magnetization vs temperature results for x=0,0.1,0.2 indicate that there is a transition from easy-axis to easy-plane anisotropy with decreasing temperature.
  • Kislitsyn, Dmitry A., et al. (författare)
  • Communication : Visualization and spectroscopy of defects induced by dehydrogenation in individual silicon nanocrystals
  • 2016
  • Ingår i: Journal of Chemical Physics. - 0021-9606 .- 1089-7690. ; 144:24
  • Tidskriftsartikel (refereegranskat)abstract
    • We present results of a scanning tunneling spectroscopy (STS) study of the impact of dehydrogenation on the electronic structures of hydrogen-passivated silicon nanocrystals (SiNCs) supported on the Au(111) surface. Gradual dehydrogenation is achieved by injecting high-energy electrons into individual SiNCs, which results, initially, in reduction of the electronic bandgap, and eventually produces midgap electronic states. We use theoretical calculations to show that the STS spectra of midgap states are consistent with the presence of silicon dangling bonds, which are found in different charge states. Our calculations also suggest that the observed initial reduction of the electronic bandgap is attributable to the SiNC surface reconstruction induced by conversion of surface dihydrides to monohydrides due to hydrogen desorption. Our results thus provide the first visualization of the SiNC electronic structure evolution induced by dehydrogenation and provide direct evidence for the existence of diverse dangling bond states on the SiNC surfaces. Published by AIP Publishing.
  • Kislitsyn, Dmitry A, et al. (författare)
  • Mapping of Defects in Individual Silicon Nanocrystals Using Real-Space Spectroscopy.
  • 2016
  • Ingår i: Journal of Physical Chemistry Letters. - 1948-7185 .- 1948-7185. ; 7:6, s. 1047-1054
  • Tidskriftsartikel (refereegranskat)abstract
    • The photophysical properties of silicon semiconductor nanocrystals (SiNCs) are extremely sensitive to the presence of surface chemical defects, many of which are easily produced by oxidation under ambient conditions. The diversity of chemical structures of such defects and the lack of tools capable of probing individual defects continue to impede understanding of the roles of these defects in SiNC photophysics. We use scanning tunneling spectroscopy to study the impact of surface defects on the electronic structures of hydrogen-passivated SiNCs supported on the Au(111) surface. Spatial maps of the local electronic density of states (LDOS) produced by our measurements allowed us to identify locally enhanced defect-induced states as well as quantum-confined states delocalized throughout the SiNC volume. We use theoretical calculations to show that the LDOS spectra associated with the observed defects are attributable to Si-O-Si bridged oxygen or Si-OH surface defects.
  • Kocevski, Vancho, 1984-, et al. (författare)
  • Band alignment switching and the interaction between neighbouring silicon nanocrystals embedded in a SiC matrix
  • 2015
  • Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 91, s. 165429-165435
  • Tidskriftsartikel (refereegranskat)abstract
    • We present results from density functional theory of the electronic properties of silicon nanocrystals (Si NCs) embedded in a silicon carbide (SiC) matrix, considering different combinations of various NCs and host matrix sizes. We show that the NC and the host matrix form a type-II band alignment, with the states at the top of the valence band being in the Si NC and the states at the bottom of the conduction band in the host matrix. Moreover, this band alignment can be interchanged with introducing oxygen at the interface. We demonstrate that the charge densities of some valence band states can overlap with the charge densities of the neighbouring NCs. We also demonstrate that this leakage of states is significant when the distance between the neighbouring NCs is less than ~1.6 nm.
  • Kocevski, Vancho, 1984-, et al. (författare)
  • First-principles study of the influence of different interfaces and core types on the properties of CdSe/CdS core-shell nanocrystals
  • 2015
  • Ingår i: Scientific Reports. - 2045-2322 .- 2045-2322. ; 5
  • Tidskriftsartikel (refereegranskat)abstract
    • With the expanding field of nanoengineering and the production of nanocrystals (NCs) with higher quality and tunable size, having reliable theoretical calculations to complement the experimental results is very important. Here we present such a study of CdSe/CdS core-shell NCs using density functional theory, where we focus on dependence of the properties of these NCs on core types and interfaces between the core and the shell, as well as on the core/shell ratio. We show that the density of states and the absorption indices depend rather weakly on the type of interface and core type. We demonstrate that the HOMO wavefunction is mainly localised in the core of the nanocrystal, depending primarily on the core/shell ratio. On the other hand the LUMO wavefunction spreads more into the shell of the nanocrystal, where its confinement in the core is almost the same in each of the studied structural models. Furthermore, we show that the radiative lifetimes decrease with increasing core sizes due to changes in the dipolar overlap integral of the HOMO and LUMO wavefunctions. In addition, the electron-hole Coulomb interaction energies follow a similar pattern as the localisation of the wavefunctions, with the smaller NCs having higher Coulomb interaction energies.
  • Kocevski, Vancho, 1984-, et al. (författare)
  • Influence of dimensionality and interface type on optical and electronic properties of CdS/ZnS core-shell nanocrystals : a first-principles study
  • 2015
  • Ingår i: Journal of Chemical Physics. - 0021-9606 .- 1089-7690. ; 143:16
  • Tidskriftsartikel (refereegranskat)abstract
    • Semiconducting nanocrystals (NCs) have become one of the leading materials in a variety of applications, mainly due to their size tunable band gap and high intensity emission. Their photoluminescence properties can be notably improved by capping the nanocrystals with a shell of another semiconductor, making core-shell structures. We focus our study on the CdS/ZnS core-shell nanocrystals, that are closely related to extensively studied CdSe/CdS NCs, albeit exhibiting rather different photoluminescence properties. We employ density functional theory to investigate the changes in the electronic and optical properties of these nanocrystals with size, core/shell ratio and interface structure between the core and the shell. We show that the band gap depends on the size of the NCs and the core/shell ratio. We suggest that the differences in the density of states and absorption are mainly governed by the core/shell ratio. We present that both the LUMO and the HOMO wavefunctions are localised in the core of the NCs, with the distribution of the LUMO wavefunction being more sensitive to the size and the core/shell ratio. We also demonstrate that the Coulomb interaction energies closely follow the behaviour of the localisation of the HOMO and LUMO wavefunctions, and are decreasing with increasing NC size. Furthermore, we investigated the electronic and optical properties of the NCs with different interfaces between the core and the shell, and different core types. We find that the different interfaces and core types have rather small influence on the band gaps and the absorption indices, as well as on the confinement of the HOMO and LUMO wavefunctions. In addition, we compare these results with the previous results for CdSe/CdS NCs, reflecting the different PL properties of these two types of NCs. We argue that the difference in their Coulomb interaction energies is one of the main reasons for their distinct PL properties.
  • Kocevski, Vancho, 1984-, et al. (författare)
  • Size dependence of the stability, electronic structure, and optical properties of silicon nanocrystals with various surface impurities
  • 2015
  • Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 91:12
  • Tidskriftsartikel (refereegranskat)abstract
    • We present a comprehensive, ground-state density functional theory study of the size dependence of the optical and electronic properties and the stability of spherical silicon nanocrystals (NCs) with different impurities on the surface. We vary the size of the NCs from 1.0 to 3.5 nm, considering single-bonded (CH3, F, Cl, OH) and double-bonded (O, S) impurities and bridged oxygen. We show that the density of states (DOS) and absorption indices of the NCs with single-bonded impurities are very similar to each other and the fully hydrogenated NCs, except for the 1.0-nm NCs, where a slight difference is present. In the case of the NCs with double-bonded impurities, the DOS and absorption indices exhibit a significant difference, compared to the fully hydrogenated NCs, for sizes up to 2.5 nm. We argue that this difference arises from the difference in the contribution from the impurity to the states around the gap, which can considerably change the character of the states. We demonstrate that the double-bonded impurities contribute significantly to the states around the gap, compared to the single-bonded impurities, causing changes in the symmetry of these states. This observation was further supported by analyzing the changes of the Fourier transform of the charge densities of the highest occupied and lowest unoccupied eigenstate. We also show that the formation energies of NCs with bridged oxygen and fluorine are the lowest, regardless of the size. Furthermore, we show that high hydrogen concentration can be used to suppress the addition of oxygen and fluorine on the surface of the Si NCs.
  • Kocevski, Vancho, et al. (författare)
  • Transition between direct and indirect band gap in silicon nanocrystals
  • 2013
  • Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 87:24, s. 245401
  • Tidskriftsartikel (refereegranskat)abstract
    • Using ground-state density functional theory we study the transition from indirect to direct band gap in hydrogen-terminated silicon nanocrystals (NCs) as a function of decreasing diameter. The studied range, from 1.0 to 4.6 nm diameter of nanocrystals, with spherical and Wulff-shape NCs, covers the transition from nano-to bulk regime. A change in the symmetry of the lowest unoccupied state as a function of decreasing NC diameter is observed, gradually increasing the oscillator strength of transitions from the highest occupied to the lowest unoccupied state. Real space and Fourier space characteristics of highest occupied and lowest unoccupied states are explored in detail and linked to a smooth transition from nano-to bulk regime.
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