The need for characterizing the atomic-scale structure of nanoparticles is highly important for understanding the behavior of such structures and for optimizing the conditions utilized in their synthesis. Subtle changes in chemistry, especially at the very surface of nanoparticles can have major impacts on the properties which they subsequently exhibit. For true structure-property correlations to be developed for these materials, careful measurement of the position and identity of the various constituent atoms becomes essential.
The FEI Titan 80-300 analytical electron microscope in the Advanced Measurement Laboratory at NIST was utilized to carry out high spatial-resolution characterization of the CdSe/ZnS quantum dot structure in a compositionally sensitive mode known as high-angle annular dark-field (HAADF) imaging. Since the current in the electron probe in this instrument is sufficient to significantly alter or even destroy the underlying material, it is necessary to limit the dose rate imparted to the sample. To this end, a time-series approach to imaging these quantum dots was employed. This involved the collection of several, low-dose (i.e. fast scan) images from the same area of the sample and then correcting any drift between them. The individual images are inherently noisy due to the low signal they contain; however, the summation of the entire series produces an atomic-scale resolution image with improved informational content.
As shown in Figure (a), the internal structure of the quantum dots is clearly resolved. Most importantly, the structural information associated with the outermost surface layers is enhanced greatly compared to similar images acquired using a single, high-dose scan. By limiting the probe dwell time at each pixel to a few microseconds, the time-series acquisition technique allows for energy dissipation from the sample during the scan, thus mitigating some of the effects of beam damage, as demonstrated in Figures (b) and (c). The former is a more detailed image of the surface region of the quantum dot shown in Figure (a), while the latter depicts the same surface region after a much longer time-series acquisition. There is an entire row of surface atoms clearly visible in Figure (b) which has disappeared by the time Figure (c) was acquired. In addition, the surface is now completely faceted after electron damage and the structure no longer represents the as-synthesized quantum dot.
The compositional information afforded by the HAADF technique is demonstrated in Figure (d). The quantum dot in this case is oriented in such a way that the crystalline projection consists of an array of closely spaced atom-pairs. The individual atoms in these pairs exhibit significantly different contrast, as further illustrated by the intensity trace across the atoms-pair in Figure (e). Due to the atomic-number dependence of the HAADF signal intensity, it can be surmised that the brighter atom is Cd while the less intense atom is Se.