Abstract text (incl. figure legends and references)
Introduction

Electron radiation sensitive materials are subject of vivid fundamental research using transmission electron microscopy. But, organic materials [1] and hybrid inorganic/organic systems [2] suffer from high electron dose. There are several means to avoid radiation damage by e.g., low-dose, specimen cooling, and scanning TEM [3]. In order to tailor TEM measurements to the needs of both, specimen and TEM technique the electron dose has to be quantified and a good compromise has to be identified.

This work aims at the quantification of the electron dose in a JEOL TEM/STEM JEM 2200FS running in TEM mode. Experimental parameters shall be given for preserving the TEM specimen from radiation damage and for optimum signal-to-noise ratio of the particular investigation technique.

Measurement of beam current is performed with an in-built viewing screen connected to ground instead of using a Faraday cup. A large hole in the specimen enables electrons passing vacuum exclusively. Parallel beam is ensured by sharp diffraction spots of a GaAs single crystal.

The electron dose mainly depends on two parameters: spot size and alpha selector setting. Spot size is controlled by condenser lens (CL) 1. It defines the number of electrons passing the CL aperture (Fig. 1). Alpha is controlled by CL2 and defines the area illuminated by the identical number of electrons passing the CL aperture (Fig. 2).

Subsequently, the measured beam current I is correlated with the illuminated area A. Finally, the electron dose D is calculated with q being the elementary charge:

D = I / (A×q)

Beam current measurement and dose calculation are performed for all combinations of spot size and alpha.

In Fig. 3 the beam current is exemplarily given for the combination of spot size 3 and alpha 3 as a function of magnification and CL aperture size. Filled dots mark measurements where the electron beam covers an area smaller than the viewing screen. Hence, the measured current does not follow the inverse quadratic dependence on the magnification anymore and can be directly related to the illuminated area. The error bars of the measurement range within the size of the markers.

Fig. 4 gives the full data set of experimentally derived electron dose on logarithmic scale. Lowest dose is found for spot size 5 and alpha 3 with about 0.3 e-/Å2×s. Highest dose is found for spot size 1 and alpha 1 with about 42 e-/Å2×s. There is no overlap of electron dose values between the different alpha settings. The dose goes down with gradually changing from spot size 1 to spot size 5 for fixed alpha. To enter lower dose level, one has to switch to the next alpha value (e.g. from 1 to 2) and start with spot size 1 again.

This work delivers quantitative values of the electron dose. Spot size and alpha setting for minimum and maximum dose are found. A good compromise for suppression of radiation damage but still good performance of TEM technique seems to be the combination of spot size 5 and alpha 1 giving a dose of about 16 e-/Å2×s.

Ref:

[1] Z.J.W.A. Leijten et al., J. Phys. Chem. C 121 (2017) 10552.

[2] S. Chen et al., Science Bulletin 65 (2020) 1643.

[3] R.F. Egerton, Ultramicroscopy 127 (2013) 100.

[4] Fig. adapted from the instruction manual of the JEOL JEM 2200FS.

Fig. legends:

Fig. 1: Spot size control with CL1.

Fig. 2: Alpha control by CL2 [4].

Fig. 3: Beam current density (spot 3, alpha 3).

Fig. 4: Electron dose.