Coherent Nanoprobes. STEM. Defects and Amorphous Materials

Abstract

In this chapter we discuss the special effects and new kinds of information which can be obtained in the coherent nanoprobe mode. This is followed by a discussion of the interpretation of conventional CBED patterns from stacking faults, dislocations, and multilayers. The chapter therefore falls into two parts: In the first sections (8.1–8.4) we are concerned with the smallest possible probes and their use for extracting information on the atomic arrangement at the core of a defect. This subject has been reviewed by Brown et al. (1988), Cowley (1978, 1992), and Spence and Carpenter (1986). In Section 8.5 on conventional CBED the aim is to extract information on the long-range, slowly varying part of the strain field. We will frequently be discussing microdiffraction from defects, so it is important to bear in mind that an emission point within the effective source (corresponding to illumination of the sample by a plane wave) no longer gives rise to a point in the diffraction pattern (as for a perfect crystal), but rather will produce a diffuse streak of scattering. If the illumination is coherent, the streaks generated by adjacent source points must then be added coherently, and the resulting intensity then depends on the aberrations and focus setting of the probe-forming lens. If the sample is weakly scattering (or consists of a small particle on a very thin substrate), the microdiffraction pattern becomes an in-line Gabor electron hologram.