| 11. |
- Kocevski, Vancho, 1984-, et al.
(author)
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Influence of dimensionality and interface type on optical and electronic properties of CdS/ZnS core-shell nanocrystals : a first-principles study
- 2015
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In: Journal of Chemical Physics. - : AIP Publishing. - 0021-9606 .- 1089-7690. ; 143:16
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Journal article (peer-reviewed)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.
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| 12. |
- Kocevski, Vancho, 1984-, et al.
(author)
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Size dependence of the stability, electronic structure, and optical properties of silicon nanocrystals with various surface impurities
- 2015
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In: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 91:12
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Journal article (peer-reviewed)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.
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| 13. |
- Kocevski, Vancho, et al.
(author)
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Transition between direct and indirect band gap in silicon nanocrystals
- 2013
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In: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 87:24, s. 245401-
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Journal article (peer-reviewed)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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| 18. |
- Lidbaum, Hans, et al.
(author)
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EMCD in the TEM - Optimization of signal acquisition and data evaluation
- 2008
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Conference paper (peer-reviewed)abstract
- The method of electron magnetic circular dichroism (EMCD) was recently proposed anddemonstrated by Peter Schattschneider et al. [I]. The EMCD signal consists in differences of L3 andL2 edge intensities of ferromagnetic materials at specific diffraction vectors. EMCD is thereforeelement specific. Furthermore. it was shown recently that sum rules apply to the EMCD methodwhich is an essential progress to obtain quantitative magnetic information from this method [2-3].Though. the theoretical derivations of sum rule suppose that the obtained spin- to orbital magneticmoments do not depend on the choice of scattering vector. This is a simplification which must beverified both by simulations and experiments to make EMCD a quantitative method for the study ofmagnetic moments.In the experiment. the dichroic signal. i.e. the change ofLJ and L2 edges at different diffractionvectors is small. i.e. of the order of 5-15% of the total signal intensity. Therefore. the acquisitionconditions must be optimized to both approach a di ffraction geometry where sum rules can beapplied and to maximize the signal to noise ratio.In this work. we optimize both. the signal and the signal/noise ratio. The experiments were carriedout on a FEI Tecnai F30 equipped with a Gatan GIF2002 energy filter. Instead of recording k-vectordependent single spectra as in the original work [I], we acquire energy filtered diffraction patternsin the energy interval around the transition metal L-edge. This allows for extraction of the EMCDsignal at k-vectors where the signal contains quantitative magnetic information. All measurementsof2 dimensional k-space maps of the EMCD signal are compared with simulations of the EM CDsignal. We find very good agreement between theoretical predictions and experimental values forboth. the k-space evolution and quantitative agreement the EMCD signal. The data evaluationincludes a careful normalization procedure. a statistical optimization of the signal to noise ratio aswell as the consideration of the entire edge intensity. Finally. we demonstrate the quantitativeprecision of the proposed method at the example ofa slightly textured iron layer (figures I and 2).References[I] P Schattschneider et aJ.. Nature 44 I (2006) 486.[2] J. Rusz, O. Eriksson, P. Novak, P. M. Oppeneer. Phys. Rev. B 76 (2007) 060408.[3] L. Calmels et aJ.. Phys. Rev. B 76 (2007) 060409.
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| 19. |
- Lidbaum, Hans, et al.
(author)
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Experimental conditions and data evaluation forquantitative EMCD measurements in the TEM
- 2008
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In: European Microscopy Conference.
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Conference paper (peer-reviewed)abstract
- The recently demonstrated technique electron energy-loss spectroscopy (EMCD) [I)opens new routes for characterization of magnetic materials using transmission electronmicroscopy. The technique enables quantitative measurements of orbital to spinmagnetic moments with element specificity, according to the recently derived sum rules[2). Electron energy-loss spectra is obtained at well defined scattering geometries, seefigure I.The principle of the technique having been demonstrated, further progress isrequired to obtain reliable quantitative information about the magnetic properties of thesample. By using energy filtered diffraction patterns, the distribution of the EM CDsignal in reciprocal space is obtained. The ava,lability of these data sets from a fullreciprocal plane allows for the optimisation of the data treatment. We study the theinfluence of experimental geometries on the EMCD signal and optimise data analysis ofthe probed reciprocal plane. This is essential to obtain correct and reliable magneticinformation. Especially normalization, signal to noise optimization and consideration ofthe entire edge intensities are important. The data cubes consisting of the reciprocalplane and energy-loss were acquired using a FEI Tecnai F30ST microscope equippedwith a Gatan GIF2002 spectrometer. In figure 2, two spectra that were extracted at theP+ and P- positions are shown. The experimental results are compared with calculationsof the EMCD signal for a thin Fe film, showing very good agreement.I. P. Schattschneider, S. Rubino, C. H~bert, J. Rusz, J. Kune~, P Novak, E. Carlino,M. Fabrizioli, G. Panaccione and G. Rossi, Nature 441 (2006), p. 486-488.2. J. Rusz, O. Eriksson, P. Novak and P.M. Oppeneer, Phys. Rev. B 76 (2007),060408(R).
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| 20. |
- Lidbaum, Hans, et al.
(author)
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Quantitative magnetic information from reciprocal space maps in transmission electron microscopy
- 2009
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In: Physical Review Letters. - 0031-9007 .- 1079-7114. ; 102:3, s. 037201-
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Journal article (peer-reviewed)abstract
- One of the most challenging issues in the characterization of magnetic materials is to obtain a quantitative analysis on the nanometer scale. Here we describe how electron magnetic circular dichroism (EMCD) measurements using the transmission electron microscope can be used for that purpose, utilizing reciprocal space maps. Applying the EMCD sum rules, an orbital to spin moment ratio of mL/mS=0.08±0.01 is obtained for Fe, which is consistent with the commonly accepted value. Hence, we establish EMCD as a quantitative element-specific technique for magnetic studies, using a widely available instrument with superior spatial resolution.
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