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- ItemAero-TiO2 Prepared on the Basis of Networks of ZnO Tetrapods(Basel : MDPI, 2022) Ciobanu, Vladimir; Ursaki, Veaceslav V.; Lehmann, Sebastian; Braniste, Tudor; Raevschi, Simion; Zalamai, Victor V.; Monaico, Eduard V.; Colpo, Pascal; Nielsch, Kornelius; Tiginyanu, Ion M.In this paper, new aeromaterials are proposed on the basis of titania thin films deposited using atomic layer deposition (ALD) on a sacrificial network of ZnO microtetrapods. The technology consists of two technological steps applied after ALD, namely, thermal treatment at different temperatures and etching of the sacrificial template. Two procedures are applied for etching, one of which is wet etching in a citric acid aqua solution, while the other one is etching in a hydride vapor phase epitaxy (HVPE) system with HCl and hydrogen chemicals. The morphology, composition, and crystal structure of the produced aeromaterials are investigated depending on the temperature of annealing and the sequence of the technological steps. The performed photoluminescence analysis suggests that the developed aeromaterials are potential candidates for photocatalytic applications.
- ItemCore–Shell Structures Prepared by Atomic Layer Deposition on GaAs Nanowires(Basel : MDPI, 2022) Ursaki, Veaceslav V.; Lehmann, Sebastian; Zalamai, Victor V.; Morari, Vadim; Nielsch, Kornelius; Tiginyanu, Ion M.; Monaico, Eduard V.GaAs nanowire arrays have been prepared by anodization of GaAs substrates. The nanowires produced on (111)B GaAs substrates were found to be oriented predominantly perpendicular to the substrate surface. The prepared nanowire arrays have been coated with thin ZnO or TiO2 layers by means of thermal atomic layer deposition (ALD), thus coaxial core–shell hybrid structures are being fabricated. The hybrid structures have been characterized by scanning electron microscopy (SEM) for the morphology investigations, by Energy Dispersive X-ray (EDX) and X-ray diffraction (XRD) analysis for the composition and crystal structure assessment, and by photoluminescence (PL) spectroscopy for obtaining an insight on emission polarization related to different recombination channels in the prepared core–shell structures.
- ItemThe Role of Al2O3 ALD Coating on Sn-Based Intermetallic Anodes for Rate Capability and Long-Term Cycling in Lithium-Ion Batteries(Weinheim : Wiley-VCH, 2022) Soltani, Niloofar; Abbas, Syed Muhammad; Hantusch, Martin; Lehmann, Sebastian; Nielsch, Kornelius; Bahrami, Amin; Mikhailova, DariaThe electrochemical performances of CoSn2 and Ni3Sn4 as potential anode materials in lithium-ion batteries (LIBs) are investigated using varying thicknesses of an alumina layer deposited by the atomic layer deposition (ALD) technique. Rate capability results showed that at high current densities, Al2O3-coated CoSn2 and Ni3Sn4 electrodes after 10-ALD cycles outperformed uncoated materials. The charge capacities of coated CoSn2 and Ni3Sn4 electrodes are 571 and 134 mAh g−1, respectively, at a high current density of 5 A g−1, while the capacities of uncoated electrodes are 363 and 11 mAh g−1. When the current density is reduced to 1 A g−1, however, the cycling performances of Al2O3-coated CoSn2 and Ni3Sn4 electrodes fade faster after almost 40 cycles than uncoated electrodes. The explanation is found in the composition of the solid-electrolyte interface (SEI), which strongly depends on the current rate. Thus, X-ray photoelectron spectroscopy analysis of SEI layers on coated samples cycles at a low current density of 0.1 Ag−1, revealed organic carbonates as major products, which probably have a low ionic conductivity. In contrast, the SEI of coated materials cycled at 5 Ag−1 consists mostly of mixed inorganic/organic fluorine-rich Al-F and C-F species facilitating a higher ionic transport, which improves electrochemical performance.
- ItemInfluence of Alumina Addition on the Optical Properties and the Thermal Stability of Titania Thin Films and Inverse Opals Produced by Atomic Layer Deposition(Basel : MDPI, 2021) Waleczek, Martin; Dendooven, Jolien; Dyachenko, Pavel; Petrov, Alexander Y.; Eich, Manfred; Blick, Robert H.; Detavernier, Christophe; Nielsch, Kornelius; Furian, Kaline P.; Zierold, RobertTiO2 thin films deposited by atomic layer deposition (ALD) at low temperatures (<100 °C) are, in general, amorphous and exhibit a smaller refractive index in comparison to their crystalline counterparts. Nonetheless, low-temperature ALD is needed when the substrates or templates are based on polymeric materials, as the deposition has to be performed below their glass transition or melting temperatures. This is the case for photonic crystals generated via ALD infiltration of self-assembled polystyrene templates. When heated up, crystal phase transformations take place in the thin films or photonic structures, and the accompanying volume reduction as well as the burn-out of residual impurities can lead to mechanical instability. The introduction of cation doping (e.g., Al or Nb) in bulk TiO2 parts is known to alter phase transitions and to stabilize crystalline phases. In this work, we have developed low-temperature ALD super-cycles to introduce Al2O3 into TiO2 thin films and photonic crystals. The aluminum oxide content was adjusted by varying the TiO2:Al2O3 internal loop ratio within the ALD super-cycle. Both thin films and inverse opal photonic crystal structures were subjected to thermal treatments ranging from 200 to 1200 °C and were characterized by in- and ex-situ X-ray diffraction, spectroscopic ellipsometry, and spectroscopic reflectance measurements. The results show that the introduction of alumina affects the crystallization and phase transition temperatures of titania as well as the optical properties of the inverse opal photonic crystals (iPhC). The thermal stability of the titania iPhCs was increased by the alumina introduction, maintaining their photonic bandgap even after heat treatment at 900 °C and outperforming the pure titania, with the best results being achieved with the super-cycles corresponding to an estimated alumina content of 26 wt.%.