Nanoporous Gold Thin Films as Substrates to Analyze Liquids by Cryo-atom Probe Tomography.

Cryogenic atom probe tomography (cryo-APT) is being developed to enable nanoscale compositional analyses of frozen liquids. Yet, the availability of readily available substrates that allow for the fixation of liquids while providing sufficient strength to their interface is still an issue. Here, we propose the use of 1-2-µm-thick binary alloy film of gold-silver sputtered onto flat silicon, with sufficient adhesion without an additional layer. Through chemical dealloying, we successfully fabricate a nanoporous substrate, with an open-pore structure, which is mounted on a microarray of Si posts by lift-out in the focused-ion beam system, allowing for cryogenic fixation of liquids. We present cryo-APT results obtained after cryogenic sharpening, vacuum cryo-transfer, and analysis of pure water on the top and inside the nanoporous film. We demonstrate that this new substrate has the requisite characteristics for facilitating cryo-APT of frozen liquids, with a relatively lower volume of precious metals. This complete workflow represents an improved approach for frozen liquid analysis, from preparation of the films to the successful fixation of the liquid in the porous network, to cryo-APT.

Opportunities of combinatorial thin film materials design for the sustainable development of magnesium-based alloys.

Magnesium-based lightweight structural materials exhibit potential for energy savings. However, the state-of-the-art quest for novel compositions with improved properties through conventional bulk metallurgy is time, energy, and material intensive. Here, the opportunities provided by combinatorial thin film materials design for the sustainable development of magnesium alloys are evaluated. To characterise the impurity level of (Mg,Ca) solid solution thin films within grains and grain boundaries, scanning transmission electron microscopy and atom probe tomography are correlatively employed. It is demonstrated that control of the microstructure enables impurity levels similar to bulk-processed alloys. In order to substantially reduce time, energy, and material requirements for the sustainable development of magnesium alloys, we propose a three-stage materials design strategy: (1) Efficient and systematic investigation of composition-dependent phase formation by combinatorial film growth. (2) Correlation of microstructural features and mechanical properties for selected composition ranges by rapid alloy prototyping. (3) Establishment of synthesis-microstructure-property relationships by conventional bulk metallurgy.

Kinetically Limited Phase Formation of Pt-Ir Based Compositionally Complex Thin Films.

The phase formation of PtIrCuAuX (X = Ag, Pd) compositionally complex thin films is investigated to critically appraise the criteria employed to predict the formation of high entropy alloys. The formation of a single-phase high entropy alloy is predicted if the following requirements are fulfilled: 12 J∙K-1 mol-1 ≤ configurational entropy ≤ 17.5 J∙K-1 mol-1, -10 kJ∙mol-1 ≤ enthalpy of mixing ≤ 5 kJ∙mol-1 and atomic size difference ≤ 5%. Equiatomic PtIrCuAuX (X = Ag, Pd) fulfill all of these requirements. Based on X-ray diffraction and energy-dispersive X-ray spectroscopy data, near-equiatomic Pt22Ir23Cu18Au18Pd19 thin films form a single-phase solid solution while near-equiatomic Pt22Ir23Cu20Au17Ag18 thin films exhibit the formation of two phases. The latter observation is clearly in conflict with the design rules for high entropy alloys. However, the observed phase formation can be rationalized by considering bond strengths and differences in activation energy barriers for surface diffusion. Integrated crystal orbital Hamilton population values per bond imply a decrease in bond strength for all the interactions when Pd is substituted by Ag in PtIrCuAuX which lowers the surface diffusion activation energy barrier by 35% on average for each constituent. This enables the surface diffusion-mediated formation of two phases, one rich in Au and Ag and a second phase enriched in Pt and Cu. Hence, phase formation in these systems appears to be governed by the complex interplay between energetics and kinetic limitations rather than by configurational entropy.

Electrochromic Behavior and Phase Transformation in Li4+xTi5O12 upon Lithium-Ion Deintercalation/Intercalation.

The impact of phase transformation from spinel-structured Li4Ti5O12 to rocksalt-type Li7Ti5O12 on the electrochromic properties of the material is studied. Thin films of Li4Ti5O12 are deposited on platinum-coated substrates using radio-frequency-ion beam sputtering. In situ and ex situ optical spectroscopy (in reflectance geometry) is performed along with electrochemical characterization. In situ measurements demonstrate the reversible electrochromic behavior of the deposited thin films and the effect of the change of lithium content on the reflectance spectrum. Ex situ measurements quantify the optical constants of thin films for different charge states by modeling the reflectance spectrum with a Clausius-Mossotti relation. The model reveals the presence of one or two dominant resonant frequencies in the case of Li4Ti5O12 or Li7Ti5O12, respectively, in the UV/visible/NIR region of light. The single strong resonance in the case of Li4Ti5O12 is assigned to transition from O 2p to Ti t2g, that is, across the band gap, whereas for the Li7Ti5O12 phase, the two resonances correspond to the electronic transitions from O 2p to empty Ti t2g and from filled Ti t2g to empty Ti eg. The concentration dependence of the derived dielectric constants points out a fast lithium ion transport through the grain boundaries, thereby segregating a conductive lithium-rich phase at the grain boundaries. This increases the electronic conductivity of the thin films in the initial stages of intercalation and explains the debated mechanism of the fast discharge/charge capability of Li4Ti5O12 electrodes.

Metastable phase formation of Pt-X (X = Ir, Au) thin films.

The dependence of phase formation and mechanical properties on the chemical composition has been investigated for Pt-Ir and Pt-Au combinatorial thin films. The formation of a single, metastable Pt-Ir solid solution has been observed for all experimental compositions and temperatures. Upon Ir addition to Pt the experimentally determined changes in lattice parameter and Young's modulus display rule of mixture behavior which is in good agreement with our ab initio data. Whereas, in the Pt-Au system, the single metastable solid solution decomposes into two phases as the growth temperature is raised to ≥600 °C. The lattice parameters in the dual phase region are independent of chemical composition. The substrate temperature and chemical composition dependent phase formation in Pt-Ir and Pt-Au thin films can be rationalized based on CALPHAD (CALculation of PHAse Diagrams) results combined with estimations of the activation energy required for surface diffusion: The metastable phase formation during film growth is caused by kinetic limitations, where Ir atoms (in Pt-Ir) need to overcome an up to factor 6 higher activation energy barrier than Au (in Pt-Au) to enable surface diffusion.