Statistical analysis of support thickness and particle size effects in HRTEM imaging of metal nanoparticles.

High-resolution transmission electron microscopy (HRTEM) examination of nanoparticles requires their placement on some manner of support - either TEM grid membranes or part of the material itself, as in many heterogeneous catalyst systems - but a systematic quantification of the practical imaging limits of this approach has been lacking. Here we address this issue through a statistical evaluation of how nanoparticle size and substrate thickness affects the ability to resolve structural features of interest in HRTEM images of metallic nanoparticles on common support membranes. The visibility of lattice fringes from crystalline Au nanoparticles on amorphous carbon and silicon supports of varying thickness was investigated with both conventional and aberration-corrected TEM. Over the 1-4nm nanoparticle size range examined, the probability of successfully resolving lattice fringes differed significantly as a function both of nanoparticle size and support thickness. Statistical analysis was used to formulate guidelines for the selection of supports and to quantify the impact a given support would have on HRTEM imaging of crystalline structure. For nanoparticles ≥1nm, aberration-correction was found to provide limited benefit for the purpose of visualizing lattice fringes; electron dose is more predictive of lattice fringe visibility than aberration correction. These results confirm that the ability to visualize lattice fringes is ultimately dependent on the signal-to-noise ratio of the HRTEM images, rather than the point-to-point resolving power of the microscope. This study provides a benchmark for HRTEM imaging of crystalline supported metal nanoparticles and is extensible to a wide variety of supports and nanostructures.

Recent developments and applications of electron microscopy to heterogeneous catalysis.

Transmission electron microscopy (TEM) and scanning transmission electron microscopy (STEM) are popular and powerful techniques used to characterize heterogeneous catalysts. Rapid developments in electron microscopy--especially aberration correctors and in situ methods--permit remarkable capabilities for visualizing both morphologies and atomic and electronic structures. The purpose of this review is to summarize the significant developments and achievements in this field with particular emphasis on the characterization of catalysts. We also highlight the potential and limitations of the various methods, describe the need for synergistic and complementary tools when characterizing heterogeneous catalysts, and conclude with an outlook that also envisions future needs in the field.