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Abstract: Non-graphitic carbons (NGCs) represent the most abundant class of \(sp^2\)-hybridized carbon materials (coal, char coal, activated carbon, etc.). These carbons consist of small graphene layer stacks possessing significant structural disorder both in the single graphene sheets and the stacking. In this study an advanced evaluation approach for the wide-angle neutron scattering (WANS) was developed, based on the method introduced by Ruland and Smarsly (2002). In particular, we elucidated if and how the enhanced WANS data quality and larger values of the modulus of the scattering vector \(s\)-range affect the accuracy and the values of the size and disorder parameters - being fitting parameters by themselves - in comparison to wide-angle x-ray scattering (WAXS), which is usually performed by laboratory equipment. We find a reasonable agreement for the parameters \(L_\mathrm{a}\) and \(L_\mathrm{c}\), i.e. the lateral dimension and stack height, within the error bars, while for the disorder parameters different results for WAXS and WANS were found, the origin of which is discussed. Thus, this study addresses the general issue of how reliably microstructural parameters can be determined from WAXS/WANS, by fitting simulated WAXS and WANS curves, which are quality-impaired by added Gaussion noise at different levels and cut-off at different \(s\)-values. From this analysis we estimated the minimal data quality required for a reliable NGC microstructural analysis based on WAXS/WANS. As an important finding, these simulations show that typical, standard WAXS laboratory setups are sufficient to provide reliable values for the most relevant structural parameters. Furthermore, pair-distribution function (PDF) analyses were performed on WAXS data obtained from a Synchrotron facility. Comparing PDF and WAXS/WANS fitting analysis thus suggests the presence of small highly ordered oligoaromatic domains embedded in the larger graphene sheets, questioning the classical view on the NGC microstructure.

Open Access

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Abstract: Combining X-ray photoelectron spectroscopy with the standing wave technique we investigated adsorption of a monolayer of water on Ti-oxide terminated SrTiO3(001) in ultra-high vacuum (UHV). At room temperature, the surface is water free but hydroxylated. A quarter monolayer of hydroxyl is tightly bound (1.85 ± 0.06) Å above the TiO2 surface. Deposited at low temperature, a monolayer of water adsorbs with the oxygen located (2.55 ± 0.2) Å above the surface, apparently close to atop Ti, but H2O is unstable at 200K. A fraction desorbs, in part under the X-ray beam, but a major fraction of H2O dissociates immediately, with the liberated hydrogen most likely attaching to a surface oxygen. The produced hydroxyls bind only loosely to the surface, are unstable at 200K and rapidly desorb once the surface is water-free. Although our study was conducted in UHV, the presented results suggests that Ti-oxide terminated SrTiO3(001) may possess a high catalytic activity toward hydrolysis under realistic conditions.

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Abstract: We study the effect of alkali metal intercalation (Cs and Li) on the geometry of graphene on Ir(111) using the x-ray standing waves technique. For both alkali metals, the increase in the mean height of the carbon layer does not depend on the lateral structure or the density of the intercalated layer. For Li, full delamination of graphene from the metal substrate is found already for a small amount of intercalant. Even though Lithium lifts graphene to a smaller height, it is much more efficient in ironing out the corrugation of pristine graphene on Ir(111).

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Abstract: We expose significant changes in emission color of carbazole-based thermally activated delayed fluorescence (TADF) emitters that arise through the presence of persistent dimer states in thin films and organic light-emitting diodes (OLEDs). Direct photoexcitation of this dimer state in 1,2,3,5-tetrakis(carbazol-9-yl)-4,6-dicyanobenzene (4CzIPN) reveals the significant influence of dimer species on the color purity of its photoluminescence and electrolumines-cence. The dimer species is sensitive to the sample preparation method and its enduring presence contributes to the widely reported concentration-mediated redshift in the photoluminescence and electroluminescence of evaporated thin films. This discovery has significant implications on the usability of these, and similar, molecules for OLEDs and explains disparate electroluminescence spectra presented in the literature for these compounds. The significant dimer-ization-controlled changes observed in the TADF process and photoluminescence efficiency mean that careful consid-eration of dimer states is imperative in the design of future TADF emitters and the interpretation of previously reported studies of carbazole-based TADF materials.

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Abstract: Surface X-ray diffraction (SXRD) has been employed to quantitatively determine the geometric structure of an X-ray-induced superhydrophilic rutile-TiO2(110)(1×1) surface. A scatterer, assumed to be oxygen, is found at a distance of 1.90 ± 0.02 Å above the fivefold co-ordinated surface Ti atom, indicating surface hydroxylation. Two more oxygen atoms, situated further from the substrate, are also included to achieve the optimal agreement between experimental and simulated diffraction data. It is concluded that these latter scatterers are from water molecules, surface localised through hydrogen bonding. Comparing this interfacial structure with previous studies suggests that the superhydophilicity of titania is most likely to be a result of the depletion of surface carbon contamination, coupled with extensive surface hydroxylation.