Scanning transmission electron microscopy for nanostructure characterization

The scanning transmission electron microscope (STEM) is an invaluable tool for the characterization of nanostructures, providing a range of different imaging modes with the ability to provide information on elemental composition and electronic structure at the ultimate sensitivity, that of a single atom. The STEM works on the same principle as the normal scanning electron microscope (SEM), by forming a focused beam of electrons that is scanned over the sample while some desired signal is collected to form an image [1]. The difference with SEM is that thin specimens are used so that transmission modes of imaging are also available. Although the need to thin bulk materials down to electron transparency can be a major task, it is often unnecessary for nanostructured materials, with sample preparation requiring nothing more than simply sprinkling or distributing the nanostructures onto a commercially available thin holey carbon support film. No long and involved grinding, polishing, or ion milling is required, making the STEM a rapid means for nanostructure characterization. As in the SEM, secondary or backscattered electrons can be used for imaging in STEM; but higher signal levels and better spatial resolution are available by detecting transmitted electrons. A bright field (BF) detector includes the transmitted beam and so the holes appear bright, whereas a dark field detector excludes the transmitted beam and holes appear dark. Each detector provides a different and complementary view of the specimen. It is one of the key advantages of the STEM to have multiple detectors operating simultaneously to collect the maximum possible information from each scan. Although transmitted electron detectors are usefully fitted to conventional SEM instruments working at relatively low voltages, there are major advantages in increasing the accelerating voltage. Increased specimen penetration means that thicker specimens can be tolerated; but more importantly, the decreasing electron wavelength leads to higher spatial resolution and the ability to see the actual atomic configurations within the nanostructure. Thus the STEM can take many forms: a simple add-on detector to a standard low-voltage SEM; a dedicated, easy-to-use, intermediate voltage STEM with rapid throughput; or an instrument more comparable to a high-resolution transmission electron microscope (TEM), which is able to provide the ultimate spatial resolution and analytical sensitivity. All have important and complementary roles in nanostructure characterization. Rapid feedback is critical to synthesis, and commercially available SEMs with subnanometer resolution at 30 kV have the ability to image tens of samples within a few hours. Similar throughput is also available with dedicated STEMs giving at best around 0.2-nm resolution at 200-kV accelerating voltage. Such instruments can be used to guide the synthesis on a day-to-day basis and represent an invaluable first step in characterization. An example of the imaging of a gold nanocatalyst supported in mesoporous silica is shown in Fig. 6.1. These microscopes are ideal for determining size distributions of nanoparticles at the level of 1 nm and above, but they lack sensitivity at the atomic level. For understanding the functionality of nanostructures, it is highly desirable to examine individual nanostructures with atomic level sensitivity. This requires a more sophisticated instrument at the forefront of what is technologically achievable in electron optics, electronics, and environmental stability. STEMs of this type are generally based on high-resolution TEM columns, operating at 200-300 kV. Ideally, we would like to see every atom in its 3D location, a dream that Feynman first laid forth in his famous lecture "There's Plenty of Room at the Bottom," where he not only forecast the nanotechnology era but also explicitly called for 100-fold improvement in the resolution of the electron microscope [3]. If we could see the atoms clearly, then surely we would be able to see how the nanostructure functioned. Today we are well on the way along this path. We are able to see individual atoms of high atomic number (Z), either on surfaces or inside bulk materials [4,5]. We can even identify single atoms spectroscopically and analyze their local electronic environment by electron energy loss spectroscopy (EELS) [6]. The advances in STEM capability in the past few years have been quite dramatic, comparable to progress in the previous two decades, through the successful realization of another innovation that Feynman called for in his lecture, the correction of lens aberrations. Feynman explicitly called for the incorporation of nonround lens elements to improve the resolution beyond the limit imposed by the unavoidable spherical aberration of the traditional round microscope objective lens. These revolutions in nanoscience and in electron microscopy are going on at the same time, and indeed one is fuelling the other. The ability of aberrationcorrected microscopes to image nanostructures more clearly than ever is significantly increasing the demand for these instruments. It is often said that a picture is worth a thousand words, but in the area of nanoscience a picture can sometimes be a revelation, showing up structures or phenomena that were totally unexpected. Such is the importance of feedback to synthesis, but at the same time a picture can just be a starting point into a quantitative insight into atomistic processes. Catalysis provides a perfect example where images can furnish information on the size and shape of nanoparticles, and how they change with processing conditions. From such knowledge, it becomes possible to perform theoretical modeling of the atomistic processes themselves, calculations of the binding energy of molecules onto the clusters observed, calculation of migration energies and diffusion pathways. It is also possible to calculate actual reaction pathways, processes that can never be observed directly in any microscope. Of course, it is always possible to calculate such things without any image, but then all possible configurations of a nanostructure need to be calculated, which is a vast number of trial structures. Without an image for guidance, it is certain that large numbers of irrelevant structures will be calculated, and there is the possibility that the right structure might be missed. Poised between synthesis and atomistic processes, the STEM fills an exciting and central area of nanotechnology. In this chapter we will cover the basics of the technique, including probe formation, image resolution and contrast in different modes, and analytical techniques. More details on imaging theory have been given elsewhere [7-11]. Here, we will illustrate the discussion with a number of examples in different areas of nanotechnology, including nanocatalysis, nanocrystals, nanotubes, nanostructured magnetic materials, and nanoscale phase separation in complex oxides, pointing out how this level of characterization can provide new insights into the functionality at the nanoscale [12,13]. Other applications in material science have recently been reviewed by Varela et al. [14]. In addition, we present a new possibility that is opened up by the new aberrationcorrected STEMs. Correcting the lens aberrations allows the objective aperture to be opened up, thereby obtaining higher resolution. At the same time, as in an optical instrument like a camera, the depth of field is reduced. Present-day aberrationcorrected STEMs have a depth of field of only a few nanometers, and so it becomes possible to effectively depth slice through a sample and to reconstruct the set of images into a 3D representation of the structure. The technique is comparable to confocal optical microscopy, but provides a resolution that is on the nanoscale.