Transmission Electron Imaging AND Diffraction in the SEM: What, Why, and How You Can Do This in Your Microscope

Scanning electron microscopes equipped with solid-state transmission electron detectors are widely available and generally easy to use, making the collection of imaging and diffraction techniques referred to as Scanning Transmission Electron Microscopy in a Scanning Electron Microscope (STEM-in-SEM) more accessible than ever. These techniques are well-suited to a host of applications such as nanoparticle metrology, imaging and diffraction of beam-sensitive samples, grain texture studies of conventional and emerging 2D materials, defect analyses, and Z-contrast imaging for example. This tutorial provides an overview of what can be done with currently available technology on a range of samples from atomically thin 2D materials to conventionally prepared lift-out samples extracted from a bulk sample.

The first part of the tutorial provides a brief summary of STEM-in-SEM pros and cons. Different transmission electron imaging modes that can be accessed in a conventional SEM, and some of the information that can be gleaned from those modes will be described. Basics like the importance of beam convergence angle and detector acceptance angle are emphasized, and practical operating tips that aren’t usually included in transmission detector user manuals are provided. For detectors with limited acceptance angle options, a straightforward mask/aperture system will be briefly described. This approach can be used to obtain nonconventional imaging modes and to quantify transmission electron diffraction patterns for material systems amenable to this type of signal collection.

The second part of the tutorial emphasizes conventional imaging and diffraction methods that utilize “on-axis” transmission electron detection schemes. Different experimental approaches that can be implemented in almost any SEM are described. As an example of how these can be used, and to introduce a relatively new electron microscopy method, 4D STEM is described. Here, a large set of diffraction patterns is obtained from a sample and then processed offline to extract diverse information. For example, user-specified virtual apertures of almost any geometry can be applied to the dataset to obtain real-space images using almost any imaging mode, diffuse scattering between diffraction spots can be analyzed as a local temperature probe, and strain or grain orientation maps can be obtained from spot patterns. Pros and cons inherent to the different experimental setups and the 4D STEM technique will be discussed.

We then demonstrate how to extend one of the on-axis configurations into a programmable detector that enables live imaging and diffraction in one apparatus. Here, a digital micromirror device (DMD) directs scattered electron signals to a camera to record the diffraction pattern, or to a photomultiplier tube synchronized with the microscope scanning system to generate a real space using any part of the diffraction pattern defined by the user. This approach has the distinct advantage that arbitrary user-definable patterns (i.e., virtual apertures) can be implemented in real time, meaning that conventional imaging and diffraction modes can be used to obtain interpretable image contrast and that non-conventional imaging modes can easily be explored. Moreover, all of the 4D STEM capabilities described above are still accessible. Detector operation is demonstrated with diverse samples including steel, thin films, monolayer graphene, and a layered Ni/Cr structure.