1154 Elliot Padgett et al.
each tilt, averaging out scan noise and limiting distortions to around 1–2 pixels. Images were taken with 1,024×1,024 pixels per frame and a 0.36nm pixel size. Because our stage goniometer is unable to tilt through a
complete 180° range, the full-range tilt series must be acquired in two parts, with an internal rotation by the tomography holder in between.We acquired each full-range tomography tilt series by recording a half-series tilting with the microscope goniometer from approximately −45° to +45° in 1° or 2° steps, then incrementing the sample tilt by ~90°, using the holder’s internal manual rotation, and then recording a second half series. The holder’s manual rotation was found to be imprecise, so additional images were recorded at each end of each half series, to ensure full cove- rage of the angular range. For Pt/CNF, images were recorded at 1° intervals from −46° to +47° at the first holder position, and −47° to +48° at the second. For Au/STO, images were recorded at 2° intervals from −54° to +54° at the first holder position and −54° to +54° at the second. Images in each tilt series were aligned to a common
coordinate system then shifted and rotated to center the axis of rotation. For Au/STO, the image alignment was done manually using a gold nanoparticle in the sample as a fiducial marker. For Pt/CNF, the images were aligned along the tilt axis using cross-correlation of the 1D image signal projected onto the tilt axis, then aligned perpendicular to the tilt axis using the image center of mass. A shear operation was then applied to correct a ~1%nonorthogonality in the image scan directions. Shear and rotation operations used linear inter- polation. The shear correction magnitude and the precise angular offset between the half series in each data set were determined by minimizing visually apparent artifacts in the reconstruction. Prior to tomography reconstruction, images were binned by two, resulting in a 0.71-nm pixel size. Tomograms were reconstructed using the weighted back- projection (WBP) algorithm, implemented using the iradon function in Matlab. 3D visualizations were rendered using the open-source tomography platform tomviz. Surface cur- vature calculations were done in Matlab using the Patch Curvature and Smooth Triangulated Mesh packages (Kroon, 2014, 2010) by Dirk-Jan Kroon.
RESULTS AND DISCUSSION
The composite STEM images in Figure 3 show large areas of each sample that are suitable for tomography. The outermost extent of unobstructed fiber contains several microns of material in the Pt/CNF sample, and multiple complete spe- cimen agglomerates in the Au/STO sample. This provides the opportunity to choose the region of interest for tomo- graphy or include a larger area to improve statistical sam- pling in quantitative studies. Tomograms for Pt/CNF and Au/STO are presented in
Figures 4 and 5, respectively, with 3D volume renderings shown alongside tomogram cross-sections from across the volume to demonstrate the consistent, high quality of the reconstruction. In each tomogram, the small metal
Figure 4. Visualization of Pt/CNF reconstruction. a: Three- dimensional volume rendering with light blue indicating high intensity (platinum) and dark blue–black indicating low intensity (carbon). b–d: Two-dimensional slices through reconstruction that are perpendicular to specimen tilt axis selected from the top, middle, and bottom of the reconstruction. Slices are shown with 0.85 gamma correction and full intensity range. Dimensions of rendered volume are 251×216×337 nm.
presence of theNafion binder in the Au/STO
sample.Nafion was observed in the sample primarily at points of contact between carbon nanofiber supports, but not coating the sample particles or wide areas of the nanofibers. Thus, despite the sensitivity of Nafion to the electron beam, no instability of the nanofiber support structure occurred during the tomography experiment. Furthermore, no accu- mulation of carbon contamination was observed, indicating that the Nafion was not mobile. Eliminating the missing wedge significantly improves
quantitative analysis of electron tomograms, as discussed in previous works (including Kawase et al., 2007; Biermans et al., 2010). Here we will focus on the segmentation of
nanoparticles are well resolved and round in appearance, showing no elongation due to the missing wedge or artifacts caused by motion of the nanoparticles. This suggests that the carbon nanofiber supports are mechanically stable and robust to beam-induced damage. Occasionally fibers in the samples have insufficient mechanical connection to the tungsten needle and exhibit high frequency vibration during imaging. However, this problem is easily recognized during sample screening in the (S)TEM, as the vibrations prevent formation of an acceptable image. Some mild streak artifacts are visible around the metal nanoparticles, likely due to dif- fraction or channeling contrast from high-density metals, although these do not interfere with either qualitative inter- pretation or quantitative analysis of the tomograms. Figure 5 shows no evidence of problems caused by the
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