# Atom Probe Tomography for Catalysis Applications: A Review
## 2 Atom Probe Tomography
### Atom Probe Tomography: Specim Preparation  
The high values of electric field, necessary to initiate and sustain the field evaporation process, are obtained by using field enhancement around highly curved surfaces. The specimens must therefore be prepared as sharp tips, with an apex diameter below \(\approx\)150 nm. The two main methods used to obtain such sharp specimens are the electropolishing method, and focused ion beam (FIB) milling, which are described at length in the literature [58; 59; 60; 61; 62; 63; 64; 65; 66; 67; 68; 69].  
The electrochemical etching approach is used for conductive materials that can be prepared as wires, and is based on the controlled dissolution of the wire. Depending on the metal, different solutions can be used, either aqueous solutions (containing mostly HClO\({}_{4}\), H\({}_{2}\)CrO\({}_{4}\), H\({}_{2}\)SO\({}_{4}\), H\({}_{3}\)PO\({}_{4}\), KCN, HF or KOH) or molten salt mixtures for noble metals. At present, electropolishing is generally  
Figure 1: Schematic principle of the atom probe tomography with the specimen tip facing a position-sensitive detector. The evaporation of the specimen can be initiated by voltage or laser pulses. Reprinted from [74], Copyright (2009), with permission from Elsevier.  
used to pre-sharpen tips, followed by micro-electropolishing at lower voltages while imaging by optical microscopy.  
For samples that cannot be etched electrochemically, FIB-based methods constitute a powerful alternative using the lift-out procedure and ion milling. An example is given in Figure 2. From a bulk sample, a specific region of interest is selected, and a thin protective layer is deposited to minimize specimen damage during the sample preparation (Figure 2a). The regions surrounding the region of interest (ROI) are removed by ion milling, including an undercut below the sample, resulting in a rectangular lift-out bar that is still attached to the rest of the sample. A micromanipulator is then attached to the specimen by FIB-deposition of Pt, and the lift-out bar cut free from the surrounding sample. The lift-out bar is transferred onto prefabricated micro-post (or carrier tips, or flat-top-post) by FIB-deposition and ion milling (Figure 2b); five to 10 specimens can be fabricated with a single lift-out bar. The last steps consist of sharpening the specimen by annular milling with decreasing annulus diameters to a final size suitable for APT analysis (Figure 2c-e). Alternatively, rectangular cuts and so-called \"pyramid milling\" can be used to decrease the annular milling time [79]. The final milling steps are performed at low kV to minimize Ga-ion implantation. With the FIB lift-out procedure, it is possible to select the orientation of analysis, allowing for damage-free specimen preparation [78], and also to perform site-specific specimen preparation [79; 80]. Finally, the addition of correlative techniques further improves specimen preparation by targeting specific features that cannot be easily imaged by FIB-SEM (focused ion beam-scanning electron microscopy) [81; 82; 83; 84].  
Figure 2: Procedure for Atom Probe Tomography (APT) specimen preparation by focused ion beam (FIB)-SEM: (**a**) lift-out method used to extract the lift-out bar made of the material of interest; (**b**) transfer of a portion of the bar onto a micro-post; (**c-e**) successive annular milling of decreasing size allow to obtain a sharp tip with a radius of curvature below 50 nm. Reprinted from [77] under the terms of the Creative Commons Attribution License.  
Specific specimen preparation procedures have also been developed for the analysis of unsupported and supported nanoparticles, powders and porous materials; methods include electrophoresis, electrodeposition and encapsulation. These will be discussed on a case by case basis in Section 3.