Methods
KTH node
Introduction
In a UEM of KTH node, we can operate in two modes: in-situ mode and time-resolved mode. In in-situ mode, a fs-laser pulse is directed towards the sample to initiate the desired change in state (chemical reaction, phase transition, melting, etc.), then a continuous electron beam thermally-emitted from LaB6 cathode illuminates the sample under or after laser excitation.
Then, the information of the sample state is in-situ recorded by the CCD camera. In time-resolved mode (shown in figure1), a femtosecond laser pulse (in UV wavelength) is steered into a modified column of a TEM and directed onto the cathode to generate a few electrons ultra-short electron pulse through a photoemission process. The femtosecond electron pulse travels through the column to the sample. A second laser pulse (from the same source) is directed through an adjustable optical path towards the sample to trigger ultrafast dynamics (phase transition, mechanical motion, spin dynamics etc.), thus enabling a temporal reference point (time zero) for the changes that occur.
By fine-tuning, at the femtosecond time scale, the relative arrival time of the laser pulse (pump) and electron pulse (probe pulse), a series of micrographs (or diffraction patterns) can be recorded during the process of change in state and information on many aspects of its dynamic or transient properties can be obtained.
The UEM in this design can produce nanometer resolution in the imaging mode, a precision of a few thousandths of an Ångström in diffraction mode, and that with a time resolution of sub picosecond. This is 10 orders of magnitude better than conventional microscopes, which are limited by the video-camera rate of recording.
The schematic illustration of the UEM in KTH.
Stockholm University Node
Introduction
SU is internationally leading in electron crystallography, and has developed a number of techniques for 3D electron diffraction (3DED). 3DED data collection by rotation electron diffraction (RED) or continuous RED (cRED) has been established and used on a routine basis for phase and structural analysis. Compared to X-rays, electrons interact with matter a few magnitudes stronger, and ED can study the sub-micron or nano-sized crystals. In addition, ED requires a lower electron dosage on the sample than conventional high-resolution TEM imaging does and therefore causes less specimen damage.
RED method combines goniometer tilt and electron beam tilt to collect 3D diffraction data set, illustrated in Figure X. A series of ED patterns of a single crystal was recorded at different tilt angles by tilting the holder done by the goniometer and in a parallel electron beam condition. In this stepwise acquisition, the tilt angle step can be 0.5º – 2º. This goniometer tilt step is followed by electron beam tilt, in order to fill the gap between the tilt. The electron beam tilt is done by the microscope deflector, and the step can be <0.1º. Another approach is to collect ED patterns while the crystal is being tilted by the goniometer continuously. This is known as continuous RED (cRED), as well as MicroED. Such strategy can ensure the sample having minimum total dose that is most commonly used for study electron beam sensitive materials, e.g. organic molecules, proteins, etc. The reconstructed 3D data set can already provide the information of unit cell, and the processed results can be input into different existing packages for structure refinement, e.g. SHELXS-97 or SIR2011.
Furthermore, a high-throughput technique – SerialED allows automated particles searching (>3000 particles/hour) and automated cRED data collections. In this fully automated method , automatically crystal screening and collecting 3D ED data from hundreds of crystals in a product are able to perform phase analysis and structure determination of products with multiple phases and various sized crystals without human intervention.
The schematic illustration of the Rotation Electron Diffraction (RED) and SerialRED.