Grain orientation dependent Nb-Ti microalloying mediated surface segregation on ferritic stainless steel

Surface segregation and oxide formation anisotropy on Ti-Nb stabilized ferritic stainless steel (EN 1.4521) were studied by XPS and Electron Backscatter Diffraction. Competitive surface segregation of Si, Nb and Ti was initiated at ∼550. °C, and segregation was favored to the open surface sites of 〈111〉 oriented grains. Furthermore, the surface segregation of Cr was strongly limited at the locations of stable Ti(CN)- and (NbTi)C-type precipitates. Consequently, the oxidation resistance of stainless steels can be enhanced cost-efficiently, without alloy additions, by optimizing the microstructure to facilitate the fast and uniform growth of protective oxide scale.acceptedVersionPeer reviewe

High-Power 1180-nm GaInNAs DBR Laser Diodes

We report high-power 1180-nm GaInNAs distributed Bragg reflector laser diodes with and without a tapered amplifying section. The untapered and tapered components reached room temperature output powers of 655 mW and 4.04 W, respectively. The diodes exhibited narrow linewidth emission with side-mode suppression ratios in the range of 50 dB for a broad range of operating current, extending up to 2 A for the untapered component and 10 A for the tapered component. The high output power is rendered possible by the use of a high quality GaInNAs-based quantum well gain region, which allows for lower strain and better carrier confinement compared with traditional GaInAs quantum wells. The development opens new opportunities for the power scaling of frequency-doubled lasers with emission at yellow–orange wavelengths.publishedVersionPeer reviewe

Charge carrier dynamics in tantalum oxide overlayered and tantalum doped hematite photoanodes

We employ atomic layer deposition to prepare 50 nm thick hematite photoanodes followed by passivating them with a 0.5 nm thick Ta2O5-overlayer and compare them with samples uniformly doped with the same amount of tantalum. We observe a three-fold improvement in photocurrent with the same onset voltage using Ta-overlayer hematite photoanodes, while electrochemical impedance spectroscopy under visible light irradiation shows a decreased amount of surface states under water splitting conditions. The Tadoped samples have an even higher increase in photocurrent along with a 0.15 V cathodic shift in the onset voltage and decreased resistivity. However, the surface state capacitance for the Ta-doped sample is twice that of the reference photoanode, which implies a larger amount of surface hole accumulation. We further utilize transient absorption spectroscopy in the sub-millisecond to second timescale under operating conditions to show that electron trapping in both Ta2O5-passivated and Ta-doped samples is markedly reduced. Ultrafast transient absorption spectroscopy in the sub-picosecond to nanosecond timescale shows faster charge carrier dynamics and reduced recombination in the Ta-doped hematite photoanode resulting in the increased photoelectrochemical performance when compared with the Ta2O5-overlayer sample. Our results show that passivation does not affect the poor charge carrier dynamics intrinsic to hematite based photoanodes. The Ta-doping strategy results in more efficient electron extraction, solving the electron trapping issue and leading to increased performance over the surface passivation strategy.Peer reviewe

Low-Temperature Route to Direct Amorphous to Rutile Crystallization of TiO2Thin Films Grown by Atomic Layer Deposition

The physicochemical properties of titanium dioxide (TiO2) depend strongly on the crystal structure. Compared to anatase, rutile TiO2 has a smaller bandgap, a higher dielectric constant, and a higher refractive index, which are desired properties for TiO2 thin films in many photonic applications. Unfortunately, the fabrication of rutile thin films usually requires temperatures that are too high (>400 °C, often even 600-800 °C) for applications involving, e.g., temperature-sensitive substrate materials. Here, we demonstrate atomic layer deposition (ALD)-based fabrication of anatase and rutile TiO2 thin films mediated by precursor traces and oxide defects, which are controlled by the ALD growth temperature when using tetrakis(dimethylamido)titanium(IV) (TDMAT) and water as precursors. Nitrogen traces within amorphous titania grown at 100 °C inhibit the crystal nucleation until 375 °C and stabilize the anatase phase. In contrast, a higher growth temperature (200 °C) leads to a low nitrogen concentration, a high degree of oxide defects, and high mass density facilitating direct amorphous to rutile crystal nucleation at an exceptionally low post deposition annealing (PDA) temperature of 250 °C. The mixed-phase (rutile-brookite) TiO2 thin film with rutile as the primary phase forms upon the PDA at 250-500 °C that allows utilization in broad range of TiO2 thin film applications.publishedVersionPeer reviewe

Pinhole-resistant nanocrystalline rutile TiO2 photoelectrode coatings

Atomic layer deposited (ALD) TiO2 thin films have a wide range of applications in photonics which are, however, limited by the chemical instability of the amorphous as-deposited TiO2. Post-deposition annealing is required for improving the performance by inducing phase transitions and oxide defects. ALD precursor traces remaining in the TiO2 film affect the thermally-induced processes but the understanding of the effect of growth temperature on precursor traces in the film as well as on the thermally-induced processes is weak. In this study 30 nm ALD TiO2 was grown on Si wafer from tetrakis(dimethylamido)titanium and water at 100–200 °C. TiO2 was subsequently annealed in vacuum at 200–500 °C. Increasing the growth temperature decreased the amount of N bearing precursor traces and thus makes the TiO2 more easily reducible. The reduction takes place simultaneously with the crystallization and formation of O1− defects. Vacuum annealing of TiO2 with less than 0.3 at% of N results in nanocrystalline rutile whereas samples with more N containing traces crystallized as microcrystalline anatase. Nanocrystalline rutile TiO2 was chemically stable and resistant to the dissolution at the grain boundaries under alkaline conditions making it a suitable material for protective photoelectrode coatings used in artificial photosynthesis.publishedVersionPeer reviewe

Progress in development of a new luminescence setup at the FinEstBeAMS beamline of the MAX IV laboratory

The main funding for the FinEstBeAMS beamline has been obtained from the European Union through the European Regional Development Fund (project “Estonian beamline to MAX-IV synchrotron”, granted to the University of Tartu) and from the Academy of Finland through the Finnish Research Infrastructure funding projects ( FIRI2010 , FIRI2013 , FIRI2014 ). The authors also acknowledge the funding contributions of the University of Oulu , University of Turku , Tampere University of Technology , the Estonian Research Council ( IUT 2-25 , IUT 2-26 , PRG-111 ), as well as the Estonian Centre of Excellence in Research “Advanced materials and high-technology devices for sustainable energetics, sensorics and nanoelectronics” TK141 (2014-2020.4.01.15-0011). The strategic funding of Finnish FIMAX consortium coordinating university , University of Oulu , for beamline personnel is also acknowledged. The authors thank the MAX IV Laboratory for financial and infrastructural support as well as for assistance during the construction of the FinEstBeaMS beamline.FinEstBeAMS is a new materials science beamline at the 1.5 GeV storage ring of the MAX IV Laboratory in Lund, Sweden. It has been built based on grazing incidence monochromatization of synchrotron light, which allows to cover a remarkably wide excitation energy range from ultraviolet to soft x-rays (4.5–1450 eV). A new mobile luminescence spectroscopy end station has been commissioned with design benefitting from the advantages of a high flux elliptically polarizing undulator light source. We report on the design of the luminescence end station, its technical realization and performance achieved so far. Special attention is paid to the experimental challenges for luminescence spectroscopy under grazing incidence excitation conditions. The first luminescence results obtained demonstrate a reliable performance of the advanced setup at FinEstBeAMS.Eesti Teadusagentuur IUT 2-25,PRG-111,IUT 2-26; European Commission; University of Tartu; Academy of Finland FIRI2010,FIRI2013,FIRI2014; Tampereen Teknillinen Yliopisto; Turun Yliopisto; European Regional Development Fund; Institute of Solid State Physics, University of Latvia as the Center of Excellence has received funding from the European Union’s Horizon 2020 Framework Programme H2020-WIDESPREAD-01-2016-2017-TeamingPhase2 under grant agreement No. 739508, project CAMART

Gas-phase endstation of electron, ion and coincidence spectroscopies for diluted samples at the FinEstBeAMS beamline of the MAXIV 1.5GeV storage ring

Since spring 2019 an experimental setup consisting of an electron spectrometer and an ion time-of-flight mass spectrometer for diluted samples has been available for users at the FinEstBeAMS beamline of the MAXIV Laboratory in Lund, Sweden. The setup enables users to study the interaction of atoms, molecules, (molecular) microclusters and nanoparticles with short-wavelength (vacuum ultraviolet and X-ray) synchrotron radiation and to follow the electron and nuclear dynamics induced by this interaction. Test measurements of N-2 and thiophene (C4H4S) molecules have demonstrated that the setup can be used for many-particle coincidence spectroscopy. The measurements of the Ar3p photoelectron spectra by linear horizontal and vertical polarization show that angle-resolved experiments can also be performed. The possibility to compare the electron spectroscopic results of diluted samples with solid targets in the case of Co2O3 and Fe2O3 at the Co and Fe L-2,L-3-absorption edges in the same experimental session is also demonstrated. Because the photon energy range of the FinEstBeAMS beamline extends from 4.4eV up to 1000eV, electron, ion and coincidence spectroscopy studies can be executed in a very broad photon energy range

Design aspects of all atomic layer deposited TiO2–Fe2O3 scaffold-absorber photoanodes for water splitting

Iron and titanium oxides have attracted substantial attention in photoelectrochemical water splitting applications. However, both materials suffer from intrinsic limitations that constrain the final device performance. In order to overcome the limitations of the two materials alone, their combination has been proposed as a solution to the problems. Here we report on the fabrication of an atomic layer deposited (ALD) Fe2O3 coating on porous ALD-TiO2. Our results show that successful implementation requires complete mixing of the TiO2 and Fe2O3 layers via annealing resulting in the formation of a photoactive iron titanium oxide on the surface. Moreover, we found that incomplete mixing leads to crystallization of Fe2O3 to hematite that is detrimental to the photoelectrochemical performance. IPCE and transient photocurrent measurements performed using UV and visible light excitation confirmed that the iron titanium oxide extends the photocurrent generation to the visible range. These measurements were complemented by transient absorption spectroscopy (TAS), which revealed a new band absent in pristine hematite or anatase TiO2 that we assign to charge transfer within the structure. Taken together, these results provide design guidelines to be considered when aiming to combine TiO2 and Fe2O3 for photoelectrochemical applications.Peer reviewe

Functionalization of TiO2 inverse opal structure with atomic layer deposition grown Cu for photocatalytic and antibacterial applications

TiO2 inverse opal (IO) structure surfaces were functionalized with a sub-monolayer amount of Cu by atomic layer deposition (ALD) and tested for photocatalytic and antimicrobial applications. Decomposition of acetylene (C2H2) into CO2 and reduction of CO2 into CH4 were tested in the gas phase and photodegradation of methylene blue (MB) was tested in the liquid phase. Antimicrobial activity was tested against Gram-positive Staphylococcus aureus (S. aureus) bacteria. ALD Cu without any post-deposition heat treatment (HT) decreased the photo degradation rate of both C2H2 and MB but improved the activity towards CO2 reduction. ALD Cu increased MB photodegradation rate and antimicrobial activity only after HT at 550 C, which was linked to the improved chemical stability Cu after the HT. The same HT decreased the activity towards CO2 reduction and decomposition of C2H2. The HT induced desorption of loosely bound ALD Cu+/2+ from the TiO2 IO surface and the remaining Cu+/2+ was reduced to Cu+. The photocatalytic and antimicrobial activity of TiO2 IO can be tailored by the addition of a sub-monolayer amounts of Cu with performance depending on the targeted reaction

Is carrier mobility a limiting factor for charge transfer in TiO2/Si devices? A study by transient reflectance spectroscopy

TiO2 coatings are often deposited over silicon-based devices for surface passivation and corrosion protection. However, the charge transfer (CT) across the TiO2/Si interface is critical as it may instigate potential losses and recombination of charge carriers in optoelectronic devices. Therefore, to investigate the CT across the TiO2/Si interface, transient reflectance (TR) spectroscopy was employed as a contact-free method to evaluate the impact of interfacial SiOx, heat-treatments, and other phenomena on the CT. Thin-film interference model was adapted to separate signals for Si and TiO2 and to estimate the number of transferred carriers. Charge transfer velocity was found to be 5.2 × 104 cm s−1 for TiO2 heat-treated at 300 °C, and even faster for amorphous TiO2 if the interfacial SiOx layer was removed using HF before TiO2 deposition. However, the interface is easily oversaturated because of slow carrier diffusion in TiO2 away from the TiO2/Si interface. This inhibits CT, which could become an issue for heavily concentrated solar devices. Also, increasing the heat-treatment temperature from 300 °C to 550 °C has only little impact on the CT time but leads to reduced carrier lifetime of ¡3 ns in TiO2 due to back recombination via the interfacial SiOx, which is detrimental to TiO2/Si device performance.publishedVersionPeer reviewe