The potential energy landscape of mobile ions in solid-state materials and the atomic scale structure are intimately interrelated. This interrelation and the resultant properties, e.g. the mobility of the ions, is of paramount interest in contemporary material science with direct applications in energy storage and conversion. Understanding the interplay of structure, energy landscape and ionic transport of ionic solids is of crucial importance for a knowledge based development of improved and new functionalities of these materials.

Here we demonstrate for the first time combined experimental and theoretical efforts in quantifying such landscapes and its resulting transport function. We describe the results of unique transport experiments, where native mobile Lithium ions in a Lithium Borate glass are depleted and replaced unidirectionally by foreign alkali ions M⁺ = K⁺, Rb⁺ and Cs⁺ in a charge attachment induced transport (CAIT) experiment. The resulting depletion/replacement profiles extend down to several hundred nm into the material as measured by time-of-flight secondary ion mass spectrometry (ToF-SIMS) resembling time-dependent macroscopic transport with characteristic transport coefficients. The experimental data reveal that most of the native Lithium ions are indeed mobile but a fraction of several percent of the ions appears to be immobile.

In order to understand and rationalize these experimental findings a combination of both macroscopic and microscopic transport theories is employed. The macroscopic approach is based on the frame work of the Nernst-Planck-Poisson theory focusing on the time dependent macroscopic flux of ion density on a spatial grid with effective diffusion coefficients. The microscopic approach on the other hand is based on the hopping of native and foreign ions in the potential energy landscape of the material for specific concentration ratios reflecting time slices of the experiment.

Both macroscopic and microscopic approach unanimously reveal that the diffusion coefficient of the native mobile Lithium ions decreases by approximately 3 orders of magnitude as the molar fraction of the Lithium is decreased from 1 to 0.2 (the molar fraction of the foreign ion increases concomitantly). Both approaches correctly predict that a finite fraction of the native Lithium ions is effectively immobile because the diffusion coefficient of some native Lithium ions becomes significantly smaller than that of the foreign alkali ions, in which case the concentration profiles exhibits a time-independent plateau. The key to the successful description is the combination of a concentration dependent Fermi energy in the macroscopic approach with the concept of an element specific and concentration dependent miss match in the microscopic approach.

The combination of a CAIT / ToF-SIMS experiment with macroscopic and microscopic transport theories addresses the old challenge of bridging the understanding from the hopping in the local structural landscape all the way to long range DC transport as relevant for technical applications. Progress along this line is being presented. Still, important questions remain to be tackled in the future, but the road map to a new level of understanding transport in ion conducting solids has now been laid out.

Metal-oxide (MO) surfaces have successfully been modified to elicit specific responses attributed to either structural variations and/or chemical functionality. Structural modification of a bulk surface is achieved by exploiting the combined effects of top-down and bottom approaches during electrochemical anodization (EA). Imparting nanoscale geometries has reportedly improved biointegration as a result of structural similarities to corresponding biological environments namely, the extracellular matrix.^[1] Additionally, an influence on surface reactivity due to chemical variations has also been observed.^[2] In this context, nanostructured bulk surfaces are highly relevant as hybrid materials, that are capable of eliciting multi-functional responses. This work describes the use of EA to fabricate MO nanotubular layers that are isolated from the parent substrate and made available for deposition onto bulk ceramic. Herein a free-floating zirconia nanotubular layer is deposited onto a bulk zirconia ceramic preform in an acetone bath.^[3] These nanostructures can further be modified via facile application of self-assembled monolayers (SAM) to subsequently seal or ‘cap’ the nanotube mouths^[4] making them ideal for triggered release applications in the biomedical field. Such bulk surfaces modified with nanotubes may be used to store drugs or therapeutics that can selectively release under appropriate conditions as this hybrid assembly can readily transform static bulk surfaces to those capable of dynamic elution from the overlying nanotubular reservoirs. The hybrid material was thoroughly characterized using scanning electron microscopy (SEM), ToF-SIMS, water contact angle measurements (WCA) etc. Such nanotubular reservoirs developed on the implant surface would be capable of facilitating ‘smart’ developmental strategies towards controlled multi-drug release models that can even elicit sequential release of drugs to limit clotting, inhibit infection and ultimately promote healing.

REFERENCES

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[2] S.N. Vakamulla Raghu, M.S. Killian, Wetting behavior of zirconia nanotubes, RSC Adv. 11 (2021) 29585–29589. https://doi.org/10.1039/D1RA04751E.

[3] S.N.V. Raghu, P. Hartwich, A. Patalas, M. Marczweski, R. Talar, C. Pritzel, M.S. Killian, Nanodentistry aspects explored towards nanostructured ZrO2: Immobilizing zirconium-oxide nanotube coatings onto zirconia ceramic implant surfaces, Open Ceram. (2023) 100340. https://doi.org/10.1016/J.OCERAM.2023.100340.

[4] S.N.V. Raghu, G. Onyenso, S. Mohajernia, M.S. Killian, Functionalization strategies to facilitate multi-depth, multi-molecule modifications of nanostructured oxides for triggered release applications, Surf. Sci. 719 (2022) 122024. https://doi.org/10.1016/j.susc.2022.122024.

OrbiSIMS is a secondary ion mass spectrometer with dual mass analysers; a time of flight mass spectrometer for high-speed imaging and an Orbitrap for high mass resolving power and mass accuracy. It was originally designed for biological imaging but the dual analyser configuration combined with multiple ion beams for analysis has proved to be useful for a wide range of analytical measurements from identification of deposits on fuel injectors to organic electronics [1-4]. More recently, there has been interest in the application to semiconductor materials using the Orbitrap to resolve peak interferences that obfuscate analysis in depth profiling experiments [5]. An Sb delta layer in silicon sample was used to determine the depth resolution and optimise parameters and an Sb implant in silicon sample was used to measure the useful yield. The depth resolution and useful yield performance were compared with a magnetic sector instrument (SC Ultra, CAMECA, France) and time-of-flight instruments (IONTOF, Germany) typically used in the semiconductor industry. Each instrument type has advantages and disadvantages, which are discussed. Guidance is provided for analysts using OrbiSIMS for semiconductor analysis.

1. M. K. Passarelli. et al, I. S. Gilmore, Nat. Methods, 14(2017)12, 1175-1183.

2. C. L. Newell, et al, A. P. Gould, Angew. Chem, 59(2020)41, 18194-18200.

3. J. Zhang, et al, I. S. Gilmore, P. D. Rakowska, Anal. Chem. 92(2020)13, 9008-9015.

4. S. Sul, G. Trindade, J. Kim et al , https://www.researchsquare.com/article/rs-1279729/v1 (2022).

5. A. Franquet, et al., Vacuum 202(2022)4, 111182.

Sub-nanometer thin film surface modifications have emerged as a promising technique to enhance the performance and stability of mixed ionic electronic conducting (MIEC) materials, such as the perovskite La_0.6Sr_0.4CoO_3-δ (LSC), particularly in terms of oxygen exchange kinetics and degradation resistance. However, the underlying interaction mechanism between these decorative layers and MIEC materials remains largely speculative, forming the basis of ongoing research endeavors.

By employing a novel ToF-SIMS approach, this research provides valuable insights into the complex interplay between LSC and sub-nm oxidic decorations. Even less than one monolayer of each oxide decoration exhibited discernible differences in the interaction with LSC, as evidenced by alterations in the secondary ion (SI) signal during analysis. Moreover, experiments revealed noticeable stoichiometric distortions of the charge carriers within the LSC, indicating a competitive relationship for lattice sites within the LSC perovskite.

The results shed light on the underlying electronic-chemical interactions, demonstrating the significance of variations in the oxide thickness. These findings contribute to the ongoing efforts to optimize the design and fabrication of MIEC materials for enhanced oxygen exchange kinetics and improved stability, paving the way for potential advancements in various energy-related applications.