Methods of Preparing Nanoscale Vitreous Ice Needles for High-Resolution Cryogenic Characterization

Nanoscale Analysis of Frozen Water by Atom Probe Tomography Using Graphene Encapsulation and Cryo-Workflows

There has been an increasing interest in atom probe tomography (APT) to characterize hydrated and biological materials. A major benefit of APT compared to microscopy techniques more commonly used in biology is its combination of outstanding three-dimensional (3D) spatial resolution and mass sensitivity. APT has already been successfully used to characterize biominerals, revealing key structural information at the atomic scale, however there are many challenges inherent to the analysis of soft hydrated materials. New preparation protocols, often involving specimen preparation and transfer at cryogenic temperature, enable APT analysis of hydrated materials and have the potential to enable 3D atomic scale characterization of biological materials in the near-native hydrated state. In this study, samples of pure water at the tips of tungsten needle specimens were prepared at room temperature by graphene encapsulation. A comparative study was conducted where specimens were transferred at either room temperature or cryo-temperature and analyzed by APT by varying the flight path and pulsing mode. The differences between the analysis workflows are presented along with recommendations for future studies, and the compatibility between graphene coating and cryogenic workflows is demonstrated.

A Versatile and Reproducible Cryo-sample Preparation Methodology for Atom Probe Studies

Repeatable and reliable site-specific preparation of specimens for atom probe tomography (APT) at cryogenic temperatures has proven challenging. A generalized workflow is required for cryogenic specimen preparation including lift-out via focused ion beam and in situ deposition of capping layers, to strengthen specimens that will be exposed to high electric field and stresses during field evaporation in APT and protect them from environment during transfer into the atom probe. Here, we build on existing protocols and showcase preparation and analysis of a variety of metals, oxides, and supported frozen liquids and battery materials. We demonstrate reliable in situ deposition of a metallic capping layer that significantly improves the atom probe data quality for challenging material systems, particularly battery cathode materials which are subjected to delithiation during the atom probe analysis itself. Our workflow design is versatile and transferable widely to other instruments.

Scalable substrate development for aqueous sample preparation for atom probe tomography

Reliable and consistent preparation of atom probe tomography (APT) specimens from aqueous and hydrated biological specimens remains a significant challenge. One particularly difficult process step is the use of a focused ion beam (FIB) instrument for preparing the required needle-shaped specimen, typically involving a 'lift-out' procedure of a small sample of material. Here, two alternative substrate designs are introduced that enable using FIB only for sharpening, along with example APT datasets. The first design is a laser-cut FIB-style half-grid close to those used for transmission electron microscopy (TEM) that can be used in a grid holder compatible with APT pucks. The second design is a larger, standalone self-supporting substrate called a 'crown', with several specimen positions, which self-aligns in APT pucks, prepared by electrical discharge machining (EDM). Both designs are made nanoporous, to provide strength to the liquid-substrate interface, using chemical and vacuum dealloying. Alpha brass, a simple, widely available, lower-cost alternative to previously proposed substrates, was selected for this work. The resulting designs and APT data are presented and suggestions are provided to help drive wider community adoption.