3D ED / MicroED

Three dimensional electron diffraction (3D ED), also known as MicroED, is a set of powerful data collection techniques that enable structure elucidation of sub-micron sized particles. The high dynamic range, radiation hardness and ability to count every electron with zero noise, make detectors like the MerlinEM ideal for 3D ED.

Figure 1: Schematic of continuous rotation electron diffraction
Figure 1: Schematic of continuous rotation electron diffraction

What is 3D ED / MicroED?

3D ED (Micro ED), is a powerful technique that enables structure elucidation of sub-micron sized particles. Comparable and complementary to single crystal X-ray diffraction (SC XRD), the simplest data collection strategy of 3D ED is the continuous rotation method. This is where a crystal is continuously rotated under a static parallel electron beam, and both diffraction patterns and the relevant metadata for structural analysis are recorded. Continuous rotation 3D ED is achievable in a transmission electron microscope, with data collection taking only a few minutes.

Find out more about 3D ED in this webinar from Quantum Detectors and Read Crystal > 

What are the benefits of 3D ED?

A consequential benefit of the strong interaction of electrons with matter and the nanosized probe forming capabilities of transmission electron microscopes (TEMs), is that intense diffraction data, from which structure elucidation is possible, can be collected from nanosized (< 500 nm) particles. This can be beneficial in the cases where it is difficult to grow single crystals that are sufficiently large for SC XRD experiments and for the elucidation of rare phases in a polyphasic nanocrystalline powder. Moreover, TEMs are generally more accessible, require fewer nanoparticles to solve a structure and are comparatively cheaper than synchrotron X-ray free-electron lasers.

3D ED workflow

What is a typical 3D ED workflow?

In general, a conventional 3D ED workflow starts with sample preparation, which includes crystal growth and grid preparation. Grown crystals should be small enough to permit electron transmission and to minimise the effect of dynamic scattering but large enough to ensure sufficient signal-to-noise in high resolution shells. The specific requirements for grid preparation are material dependent: some materials require only dry dispersion on the grid while others might require dispersion in a liquid, crushing and/or cryo-cooling or vitrification.

Once the sample is inserted, the next steps are to screen the grid for appropriate crystals and then collect data. The microscope camera length should be configured to maximise the resolution of the data while preventing reflection overlap. The exposure conditions should be chosen to achieve good signal-to-noise while the rotation speed should be chosen to prevent improper integration of reflection intensities. Since 3D ED is an analog of SC XRD, the electron diffraction dataset can be readily processed by established software, including XDS, DIALS, SHELX, SIR2019 and Olex2.

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MerlinEM for 3D ED / MicroED

The MerlinEM, a modern hybrid pixel detector, can provide an outstanding and unprecedented quality of electron diffraction datasets suitable for crystal structure determination by maximising the information collected during data acquisition. This maximisation of information is achieved by MerlinEM’s radiation tolerance, dynamic range, detection mechanism and readout speeds. 

The MerlinEM is radiation-tolerant for electrons with energies between 30 – 300 kV, ensuring that data collection requires no beam stop that would otherwise obscure reflections usable in structure solution. The dynamic range of the MerlinEM enables the simultaneous measurement of the direct beam and weak high-resolution reflections. These high-resolution reflections are often important for accurate structure determination, as they reveal the fine structure of the material. Moreover, MerlinEM’s detection mechanism -electron counting- improves the signal-to-noise ratio and the detective quantum efficiency at all spatial frequencies, thereby further aiding measurement of these weaker reflections, especially at low electron fluxes.  Continuous readout at high frame rates guarantees complete coverage of the reciprocal space volume within the goniometer tilt range.

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