Atomic-Scale Analytical Tomography

Graphene encapsulation enabled high-throughput atom probe tomography of liquid specimens

A new method for imaging liquid specimens with atom probe tomography (APT) is proposed by introducing graphene encapsulation. By tuning the encapsulation speed and the number of encapsulations, controllable volumes of liquid can be encapsulated on a pre-sharpened specimen tip, with the end radius less than 75 nm to allow field ionization and evaporation. Encapsulation of liquid has been confirmed by using various characterization techniques, including electron microscopy and stimulated emission depletion microscopy. The graphene-encapsulated liquid specimen was then directly frozen at the cryogenic stage inside the atom probe instrument, followed by APT imaging in laser-pulsed mode. Using water as a test example, water-related ions have been identified in the acquired mass spectrum, which are spatially correlated to a reconstructed three-dimensional volume of water on top of the base specimen tip, as clearly revealed in the chemical maps. In addition, the proposed method has also been shown to produce multiple liquid specimens simultaneously on a pre-sharpened silicon micro-tip array for high-throughput APT imaging of liquid specimens. It is expected that the proposed lift-out-free method for preparing APT specimens in their hydrated state will open a new avenue for obtaining insights into various materials at atomic resolution.

Reciprocal space tomography of 3D skyrmion lattice order in a chiral magnet

It is commonly assumed that surfaces modify the properties of stable materials within the top few atomic layers of a bulk specimen only. Exploiting the polarization dependence of resonant elastic X-ray scattering to go beyond conventional diffraction and imaging techniques, we have determined the depth dependence of the full 3D spin structure of skyrmions-that is, topologically nontrivial whirls of the magnetization-below the surface of a bulk sample of Cu₂OSeO₃ We found that the skyrmions change exponentially from pure Néel- to pure Bloch-twisting over a distance of several hundred nanometers between the surface and the bulk, respectively. Though qualitatively consistent with theory, the strength of the Néel-twisting at the surface and the length scale of the variation observed experimentally exceed material-specific modeling substantially. In view of the exceptionally complete quantitative theoretical account of the magnetic rigidities and associated static and dynamic properties of skyrmions in Cu₂OSeO₃ and related materials, we conclude that subtle changes of the materials properties must exist at distances up to several hundred atomic layers into the bulk, which originate in the presence of the surface. This has far-reaching implications for the creation of skyrmions in surface-dominated systems and identifies, more generally, surface-induced gradual variations deep within a bulk material and their impact on tailored functionalities as an unchartered scientific territory.