On the detection and size classification of nanometer-size metal particles on amorphous substrates

Abstract

The detection and size analysis of small metal aggregates supported on amorphous substrates becomes increasingly difficult when the particle size approaches that of the phase contrast background structures of the support. Standard conventional transmission electron microscopy (CTEM), even with subsequent analog image analysis, becomes inconclusive or fails for particles less than 1 nm in diameter, and clear differentiation of the particles from the background can only be made with considerable effort, typically involving several micrographs taken from the same specimen area under different imaging conditions. The TEM image contrast transfer mechanism is briefly reviewed with emphasis on practical conclusions. In the 1–2 nm particle size range, particles can be distinguished from the amorphous background only when focus, astigmatism, and specimen drift are optimally controlled. Operating at 100 keV and with C s = 2.2 mm, a deviation of 100 nm from the optimum focus condition was, for example, found to increase the “apparent” diameter of 1.3 nm particles by as much as 0.4 nm. Furthermore, the electron exposure required to perform such microscopy may severely complicate the situation by radiation- induced changes in specimen and support. These changes may be intensified by specimen contaminants and an improperly controlled environment. Compared to standard high resolution imaging practice, operation at lower magnifications, lower acceleration voltage, larger illumination aperture, and closer to the Gaussian focus is recommended for routine imaging of very small particles on amorphous substrates.