Preparation of Particles for Full-Range Tomography 1151


understood in terms of possible artifacts and failure condi- tions (Jiang et al., 2014). Thus, these algorithms warrant caution and further study, and should not be considered a replacement for more complete experimental data. Experimentally, partial reduction of the missing wedge


has been demonstrated with specialized tilting schemes (Dahmen et al., 2017) including dual axis tomography (Mastronarde, 1997; Arslan et al., 2006), which reduces the missing wedge to a missing pyramid, and conical tomo- graphy (Lanzavecchia et al., 2005), which reduces it to a missing cone. Unfortunately, these tilting schemes add to the complexity, duration, and radiation dose of tomography experiments and complicate the process of image alignment and reconstruction. Whenever possible, imaging the speci- men over a full 180° rotation is preferable to these approa- ches as it eliminates the missing wedge without significantly increasing the complexity of the tomography experiment. Full-range electron tomography has been accomplished


using micropipette specimens (Barnard et al., 1992), half- grid specimens (Kawase et al., 2007; Ito et al., 2010, 2011) in custom-built holders or in commercial on-axis and micro- pillar tomography holders (Biermans et al., 2010; Ke et al., 2010; Saghi et al., 2016; Andrzejczuk et al., 2017; Wang et al., 2017), available from vendors including Fischione, Hitachi, and Mel-Build. Glass or carbon micropipettes can be


effective containers for biological specimens where a large field of view (microns or hundreds of nanometers) is desired (Barnard et al., 1992; Palmer & Löwe, 2014). Most full- range tomography experiments for materials have been accomplished by using a focused ion beam(FIB) to extract and mill the specimen into a needle geometry (Hernández-Saz et al., 2014). This is ideal for bulk or interface specimenswhere


for full-range tomography using carbon nanofibers (CNF) as a one-dimensional (1D) sample support to provide an unobstructed view of the sample through a complete rota- tion, as illustrated in Figure 1. This approach is suitable for


FIB lift-out and thinning is a routine approach to sample preparation.Unfortunately, thesemethods are poorly suited to an important class of nanomaterials studied via electron tomography: fine powders, such as catalyst particles. Such materials are important for a wide variety of applications— including chemical processing and energy conversion and storage—and the details of their 3D nanostructure are often critical in determining materials properties. While micropip- ettes can be used to contain particulate specimens for electron tomography, their large size and thickness is incompatiblewith the high resolution often needed for nanomaterials. Fine powders could, in principle, be prepared by FIBmilling if they can be embedded without changing their structure, although even then FIB preparation is slow, expensive, and risks damage to specimens and thus should be avoided if possible. Some full rotation tomography specimens have been prepared by col- lecting powders on ametal point ormicro-pillar sharpened by FIB milling (Jarausch & Leonard, 2007; Ito et al., 2010; Jinnai et al., 2010), however these specimens typically extend unob- structed only a short distance beyond the non-electron- transparent support, providing a very limited field of view for tomography. Good clearance from a metal point can be achieved in tomography of elongatedmaterials such as carbon nanotubes (Kizuka et al., 2001). The objective of thiswork is to apply the clearance provided by nanotubes and nanofibers to enable practical preparation of full-range tomography speci- mens for a variety of fine powder material samples. Here we present a new sample preparation method


Figure 1. a: Annular-dark-field scanning transmission electron microscope image of Au/strontium titanate (STO) particle supported on carbon nanofiber for full-range electron tomography. Red square indicates region selected for tomography reconstruction. b,c: Volume rendering of Au/STO tomogram, where orange indicates Au particles, magenta indicates STO, and purple indicates the carbon fiber. Dimensions of rendered volume are 280×192×327 nm.


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