J. Holm, B. Caplins, R. Keller
National Institute of Standards and Technology,
United States
Keywords: STEM, SEM, detector, transmission imaging, diffraction
Summary:
Demand for effective, accessible, and affordable nanoscale material characterization techniques is growing steadily. Today, scanning electron microscopes (SEMs) are ubiquitous in materials analysis labs and serve many characterization needs because the focused electron probe can provide useful information about diverse samples including nano- and 2D-materials. Detectors designed to collect many signals (e.g., secondary electrons, X-rays, backscattered electrons, cathodoluminescence, etc.) are also commercially available and sufficiently well-developed that non-specialists can obtain meaningful information. One underutilized signal available in the SEM, however, is electrons transmitted through the sample. Although advances have been made to better utilize this signal [1, 2], part of the reason this signal has not been diligently pursued is that for thicker samples, plural- and multiple-scattering makes transmission signals challenging to interpret quantitatively. For nanomaterials though, single-scattering is probable for many samples at typical SEM beam energies, and theories developed for conventional transmission electron microscopy could be utilized if a detector capable of selecting specific signals was available. To that end, this presentation will describe a programmable scanning transmission electron microscope (p-STEM) detector that enables imaging and diffraction in an SEM [3,4]. Detector operation is demonstrated with diverse samples including thin foils, nanoparticles, and 2D materials. The schematic in Figure 1 shows one embodiment of the detector. A key component is the digital micro-mirror device (DMD). Here, an array of mirrors serves as an electronically reconfigurable objective aperture that can take on any user-defined shape on the fly. The mirrors can be programmed to select any part of the transmitted electron signal and direct it to either the digital camera to collect an image of the transmitted electron diffraction pattern, or to the photomultiplier tube to collect a real-space image. This way, almost any conventional transmission imaging mode can be implemented in an SEM, and new imaging modes can also be easily explored. As an example of the distinct advantage of this detector compared to other commercially-available STEM-in-SEM detectors, Figure 2 shows a diffraction pattern and an image of (001) oriented gold foil, both collected with the p-STEM detector. The diffraction pattern shows many satellite spots due to {111} twin boundaries. Rather than selecting one of the satellite spots to form a dark field image of the faults as is commonly done in conventional TEM, some, or all of the spots can be selected as was done with the pSTEM detector for the image shown here. Figure 3 shows STEM-in-SEM images of carbon nanotube synthesis byproducts. The secondary electron image (3a) shows mostly amorphous carbon. The marginal bright-field image (3b) shows strong contrast that can help identify residual materials. For example, amorphous carbon is easily discerned from the catalyst particles. Diffraction patterns from individual particles (3c) can be used to differentiate their composition and optimize synthesis recipes. Note that structure is visible within individual particles, and higher magnification images can reveal more information.