| 1. |
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- 2019
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Tidskriftsartikel (refereegranskat)
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| 2. |
- Bi, Zhaoxia, et al.
(författare)
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Self-assembled InN quantum dots on side facets of GaN nanowires
- 2018
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Ingår i: Journal of Applied Physics. - : AIP Publishing. - 0021-8979 .- 1089-7550. ; 123:16
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Tidskriftsartikel (refereegranskat)abstract
- Self-assembled, atomic diffusion controlled growth of InN quantum dots was realized on the side facets of dislocation-free and c-oriented GaN nanowires having a hexagonal cross-section. The nanowires were synthesized by selective area metal organic vapor phase epitaxy. A 3 Å thick InN wetting layer was observed after growth, on top of which the InN quantum dots formed, indicating self-assembly in the Stranski-Krastanow growth mode. We found that the InN quantum dots can be tuned to nucleate either preferentially at the edges between GaN nanowire side facets, or directly on the side facets by tuning the adatom migration by controlling the precursor supersaturation and growth temperature. Structural characterization by transmission electron microscopy and reciprocal space mapping show that the InN quantum dots are close to be fully relaxed (residual strain below 1%) and that the c-planes of the InN quantum dots are tilted with respect to the GaN core. The strain relaxes mainly by the formation of misfit dislocations, observed with a periodicity of 3.2 nm at the InN and GaN hetero-interface. The misfit dislocations introduce I1 type stacking faults (...ABABCBC...) in the InN quantum dots. Photoluminescence investigations of the InN quantum dots show that the emissions shift to higher energy with reduced quantum dot size, which we attribute to increased quantum confinement.
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| 3. |
- Khanbabaee, B., et al.
(författare)
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Band bending at the heterointerface of GaAs/InAs core/shell nanowires monitored by synchrotron X-ray photoelectron spectroscopy
- 2016
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Ingår i: Journal of Applied Physics. - : AIP Publishing. - 0021-8979 .- 1089-7550. ; 120:14
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Tidskriftsartikel (refereegranskat)abstract
- Unique electronic properties of semiconductor heterostructured nanowires make them useful for future nano-electronic devices. Here, we present a study of the band bending effect at the heterointerface of GaAs/InAs core/shell nanowires by means of synchrotron based X-ray photoelectron spectroscopy. Different Ga, In, and As core-levels of the nanowire constituents have been monitored prior to and after cleaning from native oxides. The cleaning process mainly affected the As-oxides and was accompanied by an energy shift of the core-level spectra towards lower binding energy, suggesting that the As-oxides turn the nanowire surfaces to n-type. After cleaning, both As and Ga core-levels revealed an energy shift of about -0.3 eV for core/shell compared to core reference nanowires. With respect to depth dependence and in agreement with calculated strain distribution and electron quantum confinement, the observed energy shift is interpreted by band bending of core-levels at the heterointerface between the GaAs nanowire core and the InAs shell.
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| 4. |
- Kjær, Kasper S., et al.
(författare)
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Finding intersections between electronic excited state potential energy surfaces with simultaneous ultrafast X-ray scattering and spectroscopy
- 2019
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Ingår i: Chemical Science. - : Royal Society of Chemistry (RSC). - 2041-6520 .- 2041-6539. ; 10:22, s. 5749-5760
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Tidskriftsartikel (refereegranskat)abstract
- Light-driven molecular reactions are dictated by the excited state potential energy landscape, depending critically on the location of conical intersections and intersystem crossing points between potential surfaces where non-adiabatic effects govern transition probabilities between distinct electronic states. While ultrafast studies have provided significant insight into electronic excited state reaction dynamics, experimental approaches for identifying and characterizing intersections and seams between electronic states remain highly system dependent. Here we show that for 3d transition metal systems simultaneously recorded X-ray diffuse scattering and X-ray emission spectroscopy at sub-70 femtosecond time-resolution provide a solid experimental foundation for determining the mechanistic details of excited state reactions. In modeling the mechanistic information retrieved from such experiments, it becomes possible to identify the dominant trajectory followed during the excited state cascade and to determine the relevant loci of intersections between states. We illustrate our approach by explicitly mapping parts of the potential energy landscape dictating the light driven low-to-high spin-state transition (spin crossover) of [Fe(2,2′-bipyridine)3]2+, where the strongly coupled nuclear and electronic dynamics have been a source of interest and controversy. We anticipate that simultaneous X-ray diffuse scattering and X-ray emission spectroscopy will provide a valuable approach for mapping the reactive trajectories of light-triggered molecular systems involving 3d transition metals.
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| 5. |
- Kjær, Kasper S., et al.
(författare)
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Solvent control of charge transfer excited state relaxation pathways in [Fe(2,2′-bipyridine)(CN)4]2-
- 2018
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Ingår i: Physical Chemistry Chemical Physics. - : Royal Society of Chemistry (RSC). - 1463-9076 .- 1463-9084. ; 20:6, s. 4238-4249
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Tidskriftsartikel (refereegranskat)abstract
- The excited state dynamics of solvated [Fe(bpy)(CN)4]2-, where bpy = 2,2′-bipyridine, show significant sensitivity to the solvent Lewis acidity. Using a combination of optical absorption and X-ray emission transient spectroscopies, we have previously shown that the metal to ligand charge transfer (MLCT) excited state of [Fe(bpy)(CN)4]2- has a 19 picosecond lifetime and no discernable contribution from metal centered (MC) states in weak Lewis acid solvents, such as dimethyl sulfoxide and acetonitrile.1,2 In the present work, we use the same combination of spectroscopic techniques to measure the MLCT excited state relaxation dynamics of [Fe(bpy)(CN)4]2- in water, a strong Lewis acid solvent. The charge-transfer excited state is now found to decay in less than 100 femtoseconds, forming a quasi-stable metal centered excited state with a 13 picosecond lifetime. We find that this MC excited state has triplet (3MC) character, unlike other reported six-coordinate Fe(ii)-centered coordination compounds, which form MC quintet (5MC) states. The solvent dependent changes in excited state non-radiative relaxation for [Fe(bpy)(CN)4]2- allows us to infer the influence of the solvent on the electronic structure of the complex. Furthermore, the robust characterization of the dynamics and optical spectral signatures of the isolated 3MC intermediate provides a strong foundation for identifying 3MC intermediates in the electronic excited state relaxation mechanisms of similar Fe-centered systems being developed for solar applications.
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| 6. |
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| 7. |
- Meier, Stefan C., et al.
(författare)
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Access to the Bis-benzene Cobalt(I) Sandwich Cation and its Derivatives : Synthons for a “Naked” Cobalt(I) Source?
- 2018
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Ingår i: Angewandte Chemie - International Edition. - : Wiley. - 1433-7851. ; 57:30, s. 9310-9314
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Tidskriftsartikel (refereegranskat)abstract
- The synthesis and structural characterization of the hitherto unknown parent Co(bz)2 + (bz=benzene) complex and several of its derivatives are described. Their synthesis starts either from a CoCO5 + salt, or directly from Co2(CO)8 and a Ag+ salt. Stability and solubility of these complexes was achieved by using the weakly coordinating anions (WCAs) [Al(ORF)4]− and [F{Al(ORF)3}2]− {RF=C(CF3)3} and the solvent ortho-difluorobenzene (o-DFB). The magnetic properties of Co(bz)2 + were measured and compared in the condensed and gas phases. The weakly bound Co(o-dfb)2 + salts are of particular interest for the preparation of further CoI salts, for example, the structurally characterized low-coordinate 12 valence electron Co(PtBu3)2 + and Co(NHC)2 + salts.
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| 8. |
- Troian, Andrea, et al.
(författare)
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Nanobeam X-ray Fluorescence Dopant Mapping Reveals Dynamics of in Situ Zn-Doping in Nanowires
- 2018
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Ingår i: Nano Letters. - : American Chemical Society (ACS). - 1530-6984 .- 1530-6992. ; , s. 6461-6468
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Tidskriftsartikel (refereegranskat)abstract
- The properties of semiconductors can be controlled using doping, making it essential for electronic and optoelectronic devices. However, with shrinking device sizes it becomes increasingly difficult to quantify doping with sufficient sensitivity and spatial resolution. Here, we demonstrate how X-ray fluorescence mapping with a nanofocused beam, nano-XRF, can quantify Zn doping within in situ doped III-V nanowires, by using large area detectors and high-efficiency focusing optics. The spatial resolution is defined by the focus size to 50 nm. The detection limit of 7 ppm (2.8 × 1017 cm-3), corresponding to about 150 Zn atoms in the probed volume, is bound by a background signal. In solar cell InP nanowires with a p-i-n doping profile, we use nano-XRF to observe an unintentional Zn doping of 5 × 1017 cm-3 in the middle segment. We investigated the dynamics of in situ Zn doping in a dedicated multisegment nanowire, revealing significantly sharper gradients after turning the Zn source off than after turning the source on. Nano-XRF could be used for quantitative mapping of a wide range of dopants in many types of nanostructures.
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| 9. |
- Yong, Zhihua, et al.
(författare)
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Unravelling uniaxial strain effects on electronic correlations, hybridization and bonding in transition metal oxides
- 2019
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Ingår i: Acta Materialia. - : Elsevier BV. - 1359-6454. ; 164, s. 618-626
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Tidskriftsartikel (refereegranskat)abstract
- The interplay among spin, lattice, charge and orbit is of central importance for several rich and fascinating properties of oxides, and is the subject of intense research at present. Here, we present an approach to manipulate this interplay by Sn doping to effectively apply uniaxial strain on the TiO2 lattice. The evolution of this interplay in pseudo-homoepitaxial Ti1-xSnxO2 films is measured using a combination of X-ray absorption near edge spectroscopy at the O K and Ti L3,2-edges. Supported by various theoretical calculations, we find that the multiplet-type electronic correlations, long-range bonding and hybridization in the system can be controlled by independently modifying uniaxial strain, thereby allowing us to establish the correlations among these effects, doping concentration, and strain. This significantly widens the phase space for experimental exploration of predictive models and leads to new possibilities for manipulation over materials’ functional properties. The methodology presented here can be applied in general to study the nature of the multiplet-type electronic correlations and bonding properties in octahedral-coordinated 3dN transition metal oxides.
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