| 1. |
- Thersleff, Thomas, 1980-, et al.
(författare)
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The interface and defect structure of superconducting Ba(Fe1-xCox)2As2 thin films
- 2011
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Ingår i: Microscopy Conference 2011, 28 August – 02 September 2011, Kiel, Germany.
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Konferensbidrag (refereegranskat)abstract
- Essential to the understanding of key fundamental properties in the newly discovered iron-based superconductors as well as critically assessing their potential for applications and devices is their fabrication into very high quality thin films. Thin films of the Ba(Fe1-xCox)2As2 phase – discovered by Sefat et al. [1] and commonly referred to as “Ba-122” – are particularly interesting since this compound appears to be resilient against oxidation and degradation due to water vapor [2], it is considerably easier to deposit compared to other iron-based superconductors, and it has a relatively high critical temperature of 25.5 K. The search for optimal thin film growth parameters is currently the subject of a number of international research groups. [2 – 11]. However, difficulties overcoming the poor metal/oxide bond at the interface of many substrates has necessitated the need for significant optimization of the deposition parameters [9,10] as well as the use of various intermediate layers [8] to produce well-textured films. In spite of these efforts, nearly all of these films contain an unintentional amorphous or iron-containing layer at the interface. While the nature of this interface is not yet fully understood, the disruption of local crystallographic ordering associated with it precludes the use of these films for interface-sensitive applications such as multilayers or heterostructures where coherent and chemically inert phase boundaries are required. A detailed investigation into the nature of this interface on both STO and LSAT substrates reveals that it is characterized by the presence of an interdiffusion layer. Ba diffuses approximately 2 nm into both substrates and is mass-balanced by the previously mentioned amorphous layer of unknown composition (figure 1). This amorphous layer is rich in iron and, on LSAT, even contains significant amounts of textured iron [9]. The nature of this interface is the first topic of this contribution. Additionally, the current transport properties of these thin films are observed to be strongly dependent on the type and concentration of defects present. An overview of the defects observed in this system constitutes the second topic of this contribution. Finally, it has been shown that, by using a metallic iron buffer layer, a highly coherent and chemically inert interfacial bond can take place [12-14]. The effect of such an interface on the defect landscape and, subsequently, the macroscopic superconducting properties of the Ba-122 thin films will be discussed in detail. [1] A. Sefat, et al., Phys. Rev. Lett. 101, 117004 (2008)[2] H. Hiramatsu, et al., Phys. Rev. B 80, 052501 (2009)[3] H. Hiramatsu, et al., Appl. Phys. Express 1, 101702 (2008)[4] E. -M. Choi, et al., Appl. Phys. Lett. 95, 062507 (2009)[5] T. Katase, et al., Solid State Commun. 149, 2121 (2009)[6] S. Lee, et al., Appl. Phys. Lett. 95, 212505 (2009)[7] K. Iida, et al., Appl. Phys. Lett. 95, 192501 (2009)[8] S. Lee, et al., Nature Mater. 9, 397 (2010)[9] K. Iida, et al., Phys. Rev. B 81, 100507(R) (2010)[10] T. Katase, et al., Appl. Phys. Lett. 96, 142507 (2010)[11] T. Katase, et al., Appl. Phys. Express 3, 063101 (2010)[12] T. Thersleff, et al., Appl. Phys. Lett. 97, 022506 (2010)[13] K. Iida, et al., Appl. Phys. Lett. 97, 172507 (2010)[14] K. Iida, et al., Appl. Phys. Express 4, 013103 (2011)
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| 2. |
- Ali, Hasan, 1985-, et al.
(författare)
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Noise-dependent bias in quantitative STEM-EMCD experiments revealed by bootstrapping
- 2024
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Ingår i: Ultramicroscopy. - : Elsevier. - 0304-3991 .- 1879-2723. ; 257
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Tidskriftsartikel (refereegranskat)abstract
- Electron magnetic circular dichroism (EMCD) is a powerful technique for estimating element-specific magnetic moments of materials on nanoscale with the potential to reach atomic resolution in transmission electron microscopes. However, the fundamentally weak EMCD signal strength complicates quantification of magnetic moments, as this requires very high precision, especially in the denominator of the sum rules. Here, we employ a statistical resampling technique known as bootstrapping to an experimental EMCD dataset to produce an empirical estimate of the noise-dependent error distribution resulting from application of EMCD sum rules to bcc iron in a 3-beam orientation. We observe clear experimental evidence that noisy EMCD signals preferentially bias the estimation of magnetic moments, further supporting this with error distributions produced by Monte-Carlo simulations. Finally, we propose guidelines for the recognition and minimization of this bias in the estimation of magnetic moments.
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| 3. |
- Ali, Hasan, 1985-, et al.
(författare)
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Single scan STEM-EMCD in 3-beam orientation using a quadruple aperture
- 2023
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Ingår i: Ultramicroscopy. - : Elsevier BV. - 0304-3991 .- 1879-2723. ; 251
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Tidskriftsartikel (refereegranskat)abstract
- The need to acquire multiple angle-resolved electron energy loss spectra (EELS) is one of the several critical challenges associated with electron magnetic circular dichroism (EMCD) experiments. If the experiments are performed by scanning a nanometer to atomic-sized electron probe on a specific region of a sample, the precision of the local magnetic information extracted from such data highly depends on the accuracy of the spatial registration between multiple scans. For an EMCD experiment in a 3-beam orientation, this means that the same specimen area must be scanned four times while keeping all the experimental conditions same. This is a non-trivial task as there is a high chance of morphological and chemical modification as well as non-systematic local orientation variations of the crystal between the different scans due to beam damage, contamination and spatial drift. In this work, we employ a custom-made quadruple aperture to acquire the four EELS spectra needed for the EMCD analysis in a single electron beam scan, thus removing the above-mentioned complexities. We demonstrate a quantitative EMCD result for a beam convergence angle corresponding to sub-nm probe size and compare the EMCD results for different detector geometries.
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| 4. |
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| 5. |
- Bejjani, Roland, et al.
(författare)
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Multi-scale study of initial tool wear on textured alumina coating, and the effect of inclusions in low-alloyed steel
- 2016
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Ingår i: Tribology International. - : Elsevier BV. - 0301-679X .- 1879-2464. ; 100, s. 204-212
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Tidskriftsartikel (refereegranskat)abstract
- When turning low-alloyed steel with hard inclusions, scores and grooves have been observed in the wear of the alumina coating in CVD coated cutting tools. This study focuses on detailed understanding of flank wear in the textured alumina coating and the relation to the workpiece's inclusions. For chip formation studies, a quick stop device has been used. A topography analysis for the worn coating was performed in the micro scale. Studies at lower scales were performed using TEM. This allowed the study of wear on the coating crystalline structure and the embedment of workpiece material on its surface. Based on the results, the mechanism behind the initial wear was analyzed and an abrasion wear model is proposed.
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| 6. |
- Bletsa, Eleni, et al.
(författare)
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Highly durable photocatalytic titanium suboxide–polymer nanocomposite films with visible light-triggered antibiofilm activity
- 2023
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Ingår i: Chemical Engineering Journal. - : Elsevier BV. - 1385-8947 .- 1873-3212. ; 454, part 1
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Tidskriftsartikel (refereegranskat)abstract
- Bacterial biofilms on medical devices may result in infections with significant societal burden. One drug-free strategy against biofilms is photocatalysis, in which a semiconducting coating is applied on the medical device and irradiated with light to generate reactive oxygen species providing an on-demand disinfection approach. However, most photocatalytic materials are active in the harmful UV range rendering them unsuitable for biomedical applications. Furthermore, the main manufacturing bottleneck today for antibiofilm coatings is their poor durability. To address these challenges, here we produced silver/titanium-suboxide nanoparticles that are photocatalytically active in the visible-light range. Moreover, we directly deposited the nanoparticles as porous coatings on substrates in situ during their aerosol synthesis. To enhance their durability, we infused the fabricated porous coatings with a polymer solution barely covering the photocatalytic particle film, resulting in the formation of polymer nanocomposite coatings. The optimized polymer nanocomposite films exhibit several cycles of triggered, on-demand biofilm eradication activity under short visible light illumination of 15–90 min with no significant intrinsic cytotoxicity to mammalian cells. The developed films can be considered as a suitable coating material for medical devices, such as catheters, ventilators, wound meshes, and others, that may require repeated disinfection during use.
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| 7. |
- Das, Biswanath, et al.
(författare)
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Bifunctional and regenerable molecular electrode for water electrolysis at neutral pH
- 2023
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Ingår i: Journal of Materials Chemistry A. - : Royal Society of Chemistry (RSC). - 2050-7488 .- 2050-7496. ; 11:25, s. 13331-13340
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Tidskriftsartikel (refereegranskat)abstract
- The instability of molecular electrodes under oxidative/reductive conditions and insufficient understanding of the metal oxide-based systems have slowed down the progress of H2-based fuels. Efficient regeneration of the electrode's performance after prolonged use is another bottleneck of this research. This work represents the first example of a bifunctional and electrochemically regenerable molecular electrode which can be used for the unperturbed production of H2 from water. Pyridyl linkers with flexible arms (–CH2–CH2–) on modified fluorine-doped carbon cloth (FCC) were used to anchor a highly active ruthenium electrocatalyst [RuII(mcbp)(H2O)2] (1) [mcbp2− = 2,6-bis(1-methyl-4-(carboxylate)benzimidazol-2-yl)pyridine]. The pyridine unit of the linker replaces one of the water molecules of 1, which resulted in RuPFCC (ruthenium electrocatalyst anchored on –CH2–CH2–pyridine modified FCC), a high-performing electrode for oxygen evolution reaction [OER, overpotential of ∼215 mV] as well as hydrogen evolution reaction (HER, overpotential of ∼330 mV) at pH 7. A current density of ∼8 mA cm−2 at 2.06 V (vs. RHE) and ∼−6 mA cm−2 at −0.84 V (vs. RHE) with only 0.04 wt% loading of ruthenium was obtained. OER turnover of >7.4 × 103 at 1.81 V in 48 h and HER turnover of >3.6 × 103 at −0.79 V in 3 h were calculated. The activity of the OER anode after 48 h use could be electrochemically regenerated to ∼98% of its original activity while it serves as a HE cathode (evolving hydrogen) for 8 h. This electrode design can also be used for developing ultra-stable molecular electrodes with exciting electrochemical regeneration features, for other proton-dependent electrochemical processes.
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| 8. |
- De Sloovere, Dries, et al.
(författare)
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Phase Engineering via Aluminum Doping Enhances the Electrochemical Stability of Lithium-Rich Cobalt-Free Layered Oxides for Lithium-Ion Batteries
- 2024
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Ingår i: Small. - 1613-6810 .- 1613-6829.
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Tidskriftsartikel (refereegranskat)abstract
- Lithium-rich, cobalt-free oxides are promising potential positive electrode materials for lithium-ion batteries because of their high energy density, lower cost, and reduced environmental and ethical concerns. However, their commercial breakthrough is hindered because of their subpar electrochemical stability. This work studies the effect of aluminum doping on Li1.26Ni0.15Mn0.61O2 as a lithium-rich, cobalt-free layered oxide. Al doping suppresses voltage fade and improves the capacity retention from 46% for Li1.26Ni0.15Mn0.61O2 to 67% for Li1.26Ni0.15Mn0.56Al0.05O2 after 250 cycles at 0.2 C. The undoped material has a monoclinic Li2MnO3-type structure with spinel on the particle edges. In contrast, Al-doped materials (Li1.26Ni0.15Mn0.61-xAlxO2) consist of a more stable rhombohedral phase at the particle edges, with a monoclinic phase core. For this core-shell structure, the formation of Mn3+ is suppressed along with the material's decomposition to a disordered spinel, and the amount of the rhombohedral phase content increases during galvanostatic cycling. Whereas previous studies generally provided qualitative insight into the degradation mechanisms during electrochemical cycling, this work provides quantitative information on the stabilizing effect of the rhombohedral shell in the doped sample. As such, this study provides fundamental insight into the mechanisms through which Al doping increases the electrochemical stability of lithium-rich cobalt-free layered oxides.
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| 9. |
- Donzel-Gargand, Olivier, et al.
(författare)
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Deep surface Cu depletion induced by K in high-efficiency Cu(In,Ga)Se2 solar cell absorbers
- 2018
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Ingår i: Progress in Photovoltaics. - : Wiley. - 1062-7995 .- 1099-159X. ; 26:9, s. 730-739
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Tidskriftsartikel (refereegranskat)abstract
- In this work, we used K‐rich glass substrates to provide potassium during the coevaporation of Cu(In,Ga)Se2 (CIGS) absorber layers. Subsequently, we applied a postdeposition treatment (PDT) using KF or RbF to some of the grown absorbers. It was found that the presence of K during the growth of the CIGS layer led to cell effi- ciencies beyond 17%, and the addition of a PDT pushed it beyond 18%. The major finding of this work is the observation of discontinuous 100‐ to 200‐nm‐deep Cu‐ depleted patches in the vicinity of the CdS buffer layer, correlated with the presence of K during the growth of the absorber layer. The PDT had no influence on the forma- tion of these patches. A second finding concerns the composition of the Cu‐depleted areas, where an anticorrelation between Cu and both In and K was measured using scanning transmission electron microscopy. Furthermore, a steeper Ga/(In+Ga) ratio gradient was measured for the absorbers grown with the presence of K, suggesting that K hinders the group III element interdiffusion. Finally, no Cd in‐diffusion to the CIGS layer could be detected. This indicates that if CdCu substitution occurs, either their concentration is below our instrumental detection limit or its presence is contained within the first 6 nm from the CdS/CIGS interface.
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| 10. |
- Gordeeva, Alisa, 1994-, et al.
(författare)
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Electronic structure characterization of TiO2-II with the α-PbO2 structure by electron-energy-loss-spectroscopy and comparison with anatase, brookite, and rutile
- 2023
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Ingår i: Journal of Solid State Chemistry. - : Academic Press. - 0022-4596 .- 1095-726X. ; 322
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Tidskriftsartikel (refereegranskat)abstract
- TiO2-II is a high pressure form of titania with a density about 2% larger than that of rutile. In contrast to the common polymorphs anatase, brookite and rutile its electronic structure and optical properties are poorly characterized. Here we report on a comparative electron-energy-loss-spectroscopy (EELS) study for which high resolution valence-loss and core-loss EELS data were acquired from nanocrystalline (<75 nm sized) titania particles with an energy resolution of about 0.2 eV. Electronic structure features revealed from titanium L3,2 and oxygen K electron energy loss near-edge structures show a strong similarity of TiO2-II with both rutile and brookite, which is attributed to similarities in the connectivity of octahedral TiO6 units with neighboring ones. From combined valence-loss EELS and UV-VIS diffuse reflectance spectroscopy data the band gap of TiO2-II was determined to be indirect and with a magnitude of ∼3.18 eV, which is very similar to anatase (indirect, ∼3.2 eV), and distinctly different from rutile (direct, ∼3.05 eV) and brookite (direct, ∼3.45 eV).
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