Atom probe tomography (APT) is considered as the only technique to provide both the chemical composition and three-dimensional (3D) imaging at a near-atomic resolution [1]. However, conventional specimen preparation technique by focused ion beam (FIB) lift-out method is conducted in vacuum and limits APT imaging to solid specimens only, while preparing a single specimen still suffers from the excessive time required and high failure rate.
In this study, we provide a new method of high-throughput APT imaging of liquid specimens. The specimens were prepared by encapsulating the solution containing proteins (ferritins) on a pre-sharpened silicon micro-tip array with graphene membranes. Protein solution has been confirmed to be immobilized on multiple Si tips by scanning electron microscopy (SEM), and each of the final tip diameters has been observed to be less than 100 nm. The Si micro-tip array was then transferred to a laser-pulsed atom probe instrument, and graphene-encapsulated tips were individually analyzed.
In order to confirm the proteins in their hydrated state, heavy water (D2O) was added inside the protein solution. APT experiments were performed on a LEAP 5000XR instrument (Cameca Instruments, USA) in pulsed-laser mode under ultra-high vacuum (<1×10-11 Torr) with the set-point temperature of 35 K. The acquired mass spectrum has identified a large amount of D2O+ and H2O-related ions (O+, OH+ and H2O+), which was spatially correlated to a significant liquid volume on the tip volume in the reconstructed 3D chemical maps. Compared to FIB/cryo-FIB lift-out method [2], the proposed method demonstrates a new route for APT investigations within a controlled liquid environment.