| 1. |
- Filianina, M. V., et al.
(författare)
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Spin and electronic structure of the topological insulator Bi1.5Sb0.5Te1.8Se1.2
- 2018
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Ingår i: Materials Chemistry and Physics. - : Elsevier BV. - 0254-0584. ; 207, s. 253-258
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Tidskriftsartikel (refereegranskat)abstract
- Electronic and spin structure of the Dirac-cone-like topological surface and valence band states were studied experimentally and theoretically for topological insulator with fractional stoichiometry Bi1.5Sb0.5Te1.8Se1.2 which is considered as one of the best candidates for efficient spin-polarized current generation. By means of spin- and angle-resolved photoelectron spectroscopy we demonstrate the separation of the Dirac point from the bulk states and the helical spin structure of the Dirac cone. For the freshly cleaved surface the Fermi level is located in the bulk band gap and an exposure in residual gases shifts the Fermi level towards the bulk conduction band. Results of the theoretical calculations are in a good agreement with the experimental data. Surface morphology study shows a well-structured atomically sharp surface after cleavage. The transport measurements confirm that this topological insulator has relatively high resistance with semiconductor-like temperature dependence at low temperatures. The studied Bi1.5Sb0.5Te1.8Se1.2 crystals demonstrated a quite large Seebeck coefficient values reaching −400 μV/K at room temperature.
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| 2. |
- Walls, B., et al.
(författare)
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Nanodomain structure of single crystalline nickel oxide
- 2021
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Ingår i: Scientific Reports. - : Nature Portfolio. - 2045-2322. ; 11:1
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Tidskriftsartikel (refereegranskat)abstract
- In this work we present a comprehensive study of the domain structure of a nickel oxide single crystal grown by floating zone melting and suggest a correlation between point defects and the observed domain structure. The properties and structure of domains dictate the dynamics of resistive switching, water splitting and gas sensing, to name but a few. Investigating the correlation between point defects and domain structure can provide a deeper understanding of their formation and structure, which potentially allows one to tailor domain structure and the dynamics of the aforementioned applications. A range of inhomogeneities are observed by diffraction and microscopy techniques. X-ray and low-energy electron diffraction reveal domains on the submicron- and nanometer-scales, respectively. In turn, these domains are visualised by atomic force and scanning tunneling microscopy (STM), respectively. A comprehensive transmission electron microscopy (TEM) study reveals inhomogeneities ranging from domains of varying size, misorientation of domains, variation of the lattice constant and bending of lattice planes. X-ray photoelectron spectroscopy and electron energy-loss spectroscopy indicate the crystal is Ni deficient. Density functional theory calculations-considering the spatial and electronic disturbance induced by the favourable nickel vacancy-reveal a nanoscale distortion comparable to STM and TEM observations. The different inhomogeneities are understood in terms of the structural relaxation induced by ordering of nickel vacancies, which is predicted to be favourable.
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| 3. |
- Wu, Han-Chun, et al.
(författare)
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Large positive in-plane magnetoresistance induced by localized states at nanodomain boundaries in graphene
- 2017
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Ingår i: Nature Communications. - : Springer Science and Business Media LLC. - 2041-1723. ; 8
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Tidskriftsartikel (refereegranskat)abstract
- Graphene supports long spin lifetimes and long diffusion lengths at room temperature, making it highly promising for spintronics. However, making graphene magnetic remains a principal challenge despite the many proposed solutions. Among these, graphene with zig-zag edges and ripples are the most promising candidates, as zig-zag edges are predicted to host spin-polarized electronic states, and spin-orbit coupling can be induced by ripples. Here we investigate the magnetoresistance of graphene grown on technologically relevant SiC/Si(001) wafers, where inherent nanodomain boundaries sandwich zig-zag structures between adjacent ripples of large curvature. Localized states at the nanodomain boundaries result in an unprecedented positive in-plane magnetoresistance with a strong temperature dependence. Our work may offer a tantalizing way to add the spin degree of freedom to graphene.
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| 4. |
- Ualibek, O., et al.
(författare)
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Manipulating and probing the growth of plasmonic nanoparticle arrays using light
- 2013
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Ingår i: Nanoscale. - : Royal Society of Chemistry (RSC). - 2040-3372 .- 2040-3364. ; 5:11, s. 4923-4930
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Tidskriftsartikel (refereegranskat)abstract
- Highly ordered self-assembled silver nanoparticle (NP) arrays have been produced by glancing angle deposition on faceted c-plane Al2O3 templates. The NP shape can be tuned by changing the substrate temperature during deposition. Reflectance anisotropy spectroscopy has been used to monitor the plasmonic evolution of the sample during the growth. The structures showed a strong dichroic response related to NP anisotropy and dipolar coupling. Furthermore, multipolar resonances due to sharp edge effects between NP and substrate were observed. Analytical and numerical methods have been used to explain the results and extract semi-quantitative information on the morphology of the NPs. The results provide insights on the growth mechanisms by the glancing angle deposition. Finally, it has been shown that the NP morphology can be manipulated by a simple illumination of the surface with an intense light source, inducing changes in the optical response. This opens up new possibilities for engineering plasmonic structure over large active areas.
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| 5. |
- Fleischer, K., et al.
(författare)
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Formation of plasmonic nanoparticle arrays -rules and recipes for an ordered growth
- 2016
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Ingår i: Physica Status Solidi (B): Basic Research. - : Wiley. - 1521-3951 .- 0370-1972. ; 253:2, s. 198-205
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Tidskriftsartikel (refereegranskat)abstract
- We review a self-assembled growth method for plasmonic nanoparticle arrays, based on glancing angle deposition. We produced ordered Ag, Au, and Cu nanoparticle arrays over large areas on different stepped oxide templates. Precise control over the final geometry can be difficult and we provide recipes to obtain macroscopically ordered structures. We discuss the influence of the adsorbate diffusion length and facet termination on the shape and size distributions of metallic nanoparticle arrays and show that an increased ad-atom mobility leads to more regular arrays of spherical nanoparticles. We also show how in-situ Reflectance Anisotropy Spectroscopy (RAS) can be used to extract indirect information on the nucleation and ripening the nanoparticles, as well as measure the plasmonic resonance. Schematics of the RAS setup used to in-situ monitor the growth of plasmonic nanoparticle arrays on stepped oxide templates.
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| 6. |
- Fleischer, K., et al.
(författare)
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Reflectance anisotropy spectroscopy of magnetite (110) surfaces
- 2014
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Ingår i: Physical Review B - Condensed Matter and Materials Physics. - 2469-9950 .- 2469-9969. ; 89:19
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Tidskriftsartikel (refereegranskat)abstract
- Reflectance anisotropy spectroscopy (RAS) has been used to measure the optical anisotropies of bulk and thin-film Fe3O4(110) surfaces. The spectra indicate that small shifts in energy of the optical transitions, associated with anisotropic strain or electric field gradients caused by the (110) surface termination or a native oxide layer, are responsible for the strong signal observed. The RAS response was then measured as a function of temperature. A distinct change in the RAS line-shape amplitude was observed in the spectral range from 0.8 to 1.6 eV for temperatures below the Verwey transition of the crystal. Finally, thin-film magnetite was grown by molecular beam epitaxy on MgO(110) substrates. Changes in the RAS spectra were found for different film thickness, suggesting that RAS can be used to monitor the growth of magnetite (110) films in situ. The thickness dependence of the RAS is discussed in terms of various models for the origin of the RAS signal.
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| 7. |
- Lubben, Olaf, et al.
(författare)
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Self-assembly of Fe nanocluster arrays on templated surfaces
- 2012
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Ingår i: Applied Physics Reviews. - : AIP Publishing. - 1931-9401. ; 111:7
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Tidskriftsartikel (refereegranskat)abstract
- The growth of Fe nanoclusters on the Ge(001) and MoO2/Mo(110) surfaces has been studied using low-temperature scanning tunneling microscopy (STM) and X-ray magnetic circular dichroism (XMCD). STM results indicate that at low coverage Fe atoms self-assemble on both surfaces into well-separated nanoclusters, which nucleate at equivalent surface sites. Their size, shape, and the observed spatial separation are dictated by the substrate and depend on preparation conditions. Annealing the Fe nanoclusters on Ge(001) at 420 K leads to the formation of linear nanocluster arrays, which follow the Ge dimer rows of the substrate, due to cluster mobility at such temperature. In turn, linear Fe nanocluster arrays are formed on the MoO2/Mo(110) surface at room temperature at a surface coverage greater than 0.5 monolayer. This is due to the more pronounced row pattern of the MoO2/Mo(110) surface compared to Ge(001). These nanocluster arrays follow the direction of the oxide rows of the strained MoO2/Mo(110) surface. The Fe nanoclusters formed on both surfaces show a superparamagnetic behavior as measured by XMCD. (C) 2012 American Institute of Physics. [doi:10.1063/1.3676207]
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| 8. |
- Murphy, B. E., et al.
(författare)
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Growth and ordering of Ni(II) diphenylporphyrin monolayers on Ag(111) and Ag/Si(111) studied by STM and LEED
- 2012
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Ingår i: Journal of Physics: Condensed Matter. - : IOP Publishing. - 1361-648X .- 0953-8984. ; 24:4
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Tidskriftsartikel (refereegranskat)abstract
- The room temperature self-assembly and ordering of (5,15-diphenylporphyrinato) nickel(II) (NiDPP) on the Ag(111) and Ag/Si(111)-(root 3 x root 3)R30 degrees surfaces have been investigated using scanning tunnelling microscopy and low-energy electron diffraction. The self-assembled structures and lattice parameters of the NiDPP monolayer are shown to be extremely dependent on the reactivity of the substrate, and probable molecular binding sites are proposed. The NiDPP overlayer on Ag(111) grows from the substrate step edges, which results in a single-domain structure. This close-packed structure has an oblique unit cell and consists of molecular rows. The molecules in adjacent rows are rotated by approximately 17 degrees with respect to each other. In turn, the NiDPP molecules form three equivalent domains on the Ag/Si(111)-(root 3 x root 3)R30 degrees surface, which follow the three-fold symmetry of the substrate. The molecules adopt one of three equivalent orientations on the surface, acting as nucleation sites for these domains, due to the stronger molecule-substrate interaction compared to the case of the Ag(111). The results are explained in terms of the substrate reactivity and the lattice mismatch between the substrate and the molecular overlayer.
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| 9. |
- Syrlybekov, A., et al.
(författare)
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Induced morphological changes on vicinal MgO (100) subjected to high-temperature annealing: Step formation and surface stability
- 2015
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Ingår i: Surface and Interface Analysis. - : Wiley. - 1096-9918 .- 0142-2421. ; 47:10, s. 969-977
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Tidskriftsartikel (refereegranskat)abstract
- Highly regular step and terrace structures have been produced on surfaces of single crystalline MgO, miscut from the low-index (001) plane, upon annealing in air. Here, the evolution of the surface morphology of such surfaces is investigated. We demonstrate that the periodicity of these structures can be widely tuned in the submicron range by controlling the annealing conditions. Surface faceting resulted from annealing in the temperature range 1100-1580 °C. The surfaces were characterized by atomic force microscope, X-ray diffraction, transmission electron microscope, and X-ray photoelectron spectroscopy to assess the role of contamination, temperature, and miscut angle in the final morphology. The presence of Al contamination in the post-annealed samples was found to be essential for the formation of the step and terrace structure. The stability of the resultant structures when exposed to ambient conditions is discussed. The cause of the apparent destruction of the surface morphology upon long-term atmospheric exposure has been identified, and a method to recover the faceted morphology is proposed. Overall, the study further facilitates the growth of nanostructures on such faceted surfaces.
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| 10. |
- Verre, Ruggero, 1985, et al.
(författare)
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General approach to the analysis of plasmonic structures using spectroscopic ellipsometry
- 2013
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Ingår i: Physical Review B - Condensed Matter and Materials Physics. - 2469-9950 .- 2469-9969. ; 87:23
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Tidskriftsartikel (refereegranskat)abstract
- In this article a route to analyze the full optical response of plasmonic structures is developed. First, the simple case of an anisotropic thin plasmonic layer supported on a transparent substrate is analyzed by introducing a quantity named anisotropic surface excess function (ASEF). The spectral features are analyzed in terms of effective dielectric function, demonstrating a more direct relation with the plasmonic response of the layer. The formalism is then generalized using a transfer matrix method. The formalism developed is supported by experimental evidence obtained by measuring the response of anisotropic nanoparticle arrays grown at a glancing angle. The agreement between theory and experiment is clear, suggesting that SE can be conveniently employed to measure the spectroscopic response of plasmonic structures. It is also demonstrated that the figure of merit of the plasmonic resonance for refractive index sensing can be greatly improved, with optimized measurement configurations, using polarized spectroscopy.
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| 11. |
- Verre, Ruggero, 1985, et al.
(författare)
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Polarization conversion-based molecular sensing using anisotropic plasmonic metasurfaces
- 2016
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Ingår i: Nanoscale. - : Royal Society of Chemistry. - 2040-3364 .- 2040-3372. ; 8:20, s. 10576-10581
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Tidskriftsartikel (refereegranskat)abstract
- Anisotropic media induce changes in the polarization state of transmitted and reflected light. Here we combine this effect with the refractive index sensitivity typical of plasmonic nanoparticles to experimentally demonstrate self-referenced single wavelength refractometric sensing based on polarization conversion. We fabricated anisotropic plasmonic metasurfaces composed of gold dimers and, as a proof of principle, measured the changes in the rotation of light polarization induced by biomolecular adsorption with a surface sensitivity of 0.2 ng cm(-2). We demonstrate the possibility of miniaturized sensing and we show that experimental results can be reproduced by analytical theory. Various ways to increase the sensitivity and applicability of the sensing scheme are discussed.
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| 12. |
- Zhussupbekova, Ainur, et al.
(författare)
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An in Situ Study of Precursor Decomposition via Refractive Index Sensing in p-Type Transparent Copper Chromium Oxide
- 2022
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Ingår i: Chemistry of Materials. - : American Chemical Society (ACS). - 1520-5002 .- 0897-4756. ; 34:7, s. 3020-3027
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Tidskriftsartikel (refereegranskat)abstract
- Oxide semiconductors are penetrating into a wide range of energy, environmental, and electronic applications, possessing a potential to outrun currently employed semiconductors. However, an insufficient development of p-type oxides is a major obstacle against complete oxide electronics. Quite often oxide deposition is performed by the spray pyrolysis method, inexpensive to implement and therefore accessible to a large number of laboratories. Although, the complex growth chemistry and a lack of in situ monitoring during the synthesis process can complicate the growth optimization of multicomponent oxides. Here we present a concept of plasmonic, optical sensing that has been applied to spray pyrolysis oxide film growth monitoring for the first time. The proposed method utilizes a polarization based refractive index sensing platform using Au nanodimers as transducing elements. As a proof of concept, the changes in the refractive index of the grown film were extracted from individual Cu(acac)2 and Cr(acac)3 precursors in real time to reveal their thermal decomposition processes. Obtained activation energies give insight into the physical origin of the narrow temperature window for the synthesis of high performing p-type transparent conducting copper chromium oxide CuxCrO2. The versatility of the proposed method makes it effective in the growth rate monitoring of various oxides, exploring new candidate materials and optimizing the synthesis conditions for acquisition of high performing oxides synthesized by a high throughput cost-effective method.
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