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• Evaluating local temperature changes during liquid cell transmission electron microscopy by in situ parallel beam electron diffraction

Evaluating local temperature changes during liquid cell transmission electron microscopy by in situ parallel beam electron diffraction

Session

PST.6 - In-situ and in-operando microscopy

Authors

Birk Fritsch (1), Dr. Andreas Hutzler (1), Dr. Mingjian Wu (3), Lilian Vogl (3), Dr. Michael P.M. Jank (2), Prof. Dr. Martin März (1, 2), Prof. Dr. Erdmann Spiecker (3)

Affiliations

1. Electron Devices (LEB), Department of Electrical, Electronic and Communication Engineering, Friedrich-Alexander University Erlangen-Nürnberg (FAU)
2. Fraunhofer Institute for Integrated Systems and Device Technology IISB
3. Institute of Micro- and Nanostructure Research (IMN) & Center for Nanoanalysis and Electron Microscopy (CENEM), Interdisciplinary Center for Nanostructured Films (IZNF), Department of Materials Science and Engineering, Friedrich-Alexander University Erlangen-Nürnberg (FAU)

Keywords

Elastic Filtering, Gold Nanoparticles, in situ Microscopy, Liquid Cell Transmission Electron Microscopy, Python, Temperature Measurement

Abstract text

We demonstrate a direct in situ measurement of local temperature changes within a TEM liquid cell by evaluating the subtle, thermally induced changes of the diffraction pattern of gold nanoparticles under parallel electron beam illumination. An algorithm is presented for automated data analysis.

Liquid cell transmission electron microscopy (LCTEM) provides unique insights into dynamic processes in liquid media down to atomic resolution [1]. Understanding and quantifying the impact of the electron beam on the system under observation, however, is crucial for planning LCTEM experiments and interpreting their results. Due to the inherent dynamics of liquid phases, the electron beam dominates the experimental conditions during LCTEM because of radiolysis and field effects which have been intensively investigated in the past [1–3]. Electron-beam induced heating is an important factor guiding reaction dynamics, as free energy is a function of temperature. Although Grogan et al. predicted that this effect should merely have a minor impact during LCTEM at typical beam conditions [3], an experimental validation has not yet been provided. To do so, a direct in situ measurement method to monitor local temperature changes T within the irradiated area is required.

A T of 10 ^oC would result in lattice expansion and thus contraction of the radius R of a diffraction ring of typical metals of about 0.01%. As this signal is 100 times smaller than typical image aberrations, the demonstrated method relies on a perfectly aligned parallel electron beam during the experiment to exclude R variations caused by bulging and particle movement along the optical axis of the microscope. Albeit this is only providing a relative measure of temperature, a respective in situ measurement of local temperature changes of gold nanoparticles on thin SiN_x membranes was demonstrated in vacuum to an accuracy of ±2.8 ^oC [4]. Applying this approach to LCTEM imposes additional challenges on the experiment, namely the larger penetration depth required, the very strong interaction of the surrounding liquid phase and the additional membrane materials with electrons (in particular inelastic scattering). Furthermore, a dedicated algorithm is required to analyze the significantly poorer datasets in LCTEM with respect to vacuum measurement.

As a proof-of-principle experiment, temperature ramps from 30 °C to 100 °C were applied to deionized water using a Poseidon Select Heating E-Chip (Protochips, Inc.). In advance, gold nanoparticles were grown by sputter-deposition of 10 nm of gold on the inner membrane surface of the heating chip and subsequent dewetting for 10 min at 300°C. The E-Chip was mounted together with a bottom chip with 150 nm thick spacers, as provided by Protochips. We monitored the diffraction pattern at 1 s time interval using a TITAN³ Themis operated at 300 kV. A Gatan Image Filter with 10 eV energy slit around the zero-loss peak was applied to obtain elastically filtered diffraction patterns. A parallel beam with veryfied convergence angle of -6±9 µrad was applied [4] for these experiments.

Elastically filtered electron diffraction is beneficial especially at thick liquid pockets by enhancing the signal-to-noise ratio (gold diffraction vs. diffuse scattering by water and SiN_x). In Figure 1 (a) and (b) it is evident that the diffraction rings are much more pronounced compared to conventional (unfiltered) diffraction analysis. The 222 ring, for example, is not sufficient for evaluation without elastic filtering. Filtering, however, comes at the cost of additional optical distortions, which have to be corrected by image post-processing.
To automatically analyze the data, a dedicated, python algorithm based on the SciPy ecosystem [5] was developed (Figure 1 (c)). The code relies on automatically guessing the center of the diffraction pattern using Hough transform, subsequent non-iterative least-square ellipse fitting [6, 7] to accurately determine the center, and modelling the image distortions up to fourth order [4].            

Figure 1: (a) Conventional, and (b) elastically filtered SAED pattern of polycrystalline gold nanoparticles in liquid (both images are grayscale-inverted, sharpened and contrast-enhanced with 0.3% pixel saturation using FIJI). (c) Flowchart of the used Python-based data analysis routine.

We were able to map the extracted relative change in R to T, which is shown for the 220 diffraction ring data in Figure 2. It was demonstrated before that the 220 diffraction ring yields the best resolution for both, this method and this kind of particles [4]. To stabilize the initial temperature provided by the heating chip, the pattern recording was started 30 s after the applied temperature profile (orange line in Figure 2b). It is clearly visible that the measured temperature profile is in good agreement with the applied temperature range. After about 5 min, a breakdown of the heating chip is observed by a sudden increase in the resistance measurement of the device. It is noteworthy that the relative temperature measured by diffraction continues to decline, which proves the independency of the method. This approach is, thus, applicable for direct investigation of electron-beam induced heating effects during LCTEM within the irradiated area.

Figure 2: (a) Evolution of the Radius change of the 220 ring and (b) temperature profile over time t, based on the Protochips reading and the 220 diffraction data. The error bars labels the 2 precision of the data analysis routine.

In summary, we demonstrate that the in situ measurement of local temperature changes during LCTEM is accessible via parallel beam adjustment and elastically filtered SAED using gold nanoparticles as local temperature probes. The extraction of subtle changes in the diffraction pattern was possible by development of a dedicated data analysis routine [8].

References

[1]    A. Hutzler et al., Nano Letters (2018), pp. 7222-7229

[2]    T.J. Woehl, P. Abellan, Journal of microscopy (2017), pp. 135-147

[3]    J.M. Grogan et al., Nano Letters (2014), pp. 359-364

[4]    F. Niekiel et al., Ultramicroscopy (2017), pp. 161-169

[5]    K.J. Millman and M. Aivazis, Comput. Sci. Eng. (2011), pp. 9-12

[6]    R. Halir and J. Flusser, in Conference proceedings: WSCG 98, the Sixth International Conference in Central Europe on Computer Graphics and Visualization 98, University of West, ed. by V. Skala (Plzen, 1998), p. 125

[7]    B. Hammel and N. Sullivan-Molina, bdhammel/least-squares-ellipse-fitting: Initial release (2019), https://zenodo.org/record/2578663. Accessed 27 February 2019, doi:10.5281/zenodo.2578663

[8]    Financial support by the German Research Foundation (DFG) via the Research Training Group GRK 1896 “In situ microscopy with electrons, X‐rays and scanning probes” is gratefully acknowledged.