Abstract text (incl. figure legends and references)

Introduction

In order to achieve the desired data quality in transmission electron microscopy (TEM) measurements, careful sample preparation forms a crucial step for these experiments. For many samples, dispersing the specimens in solvent and applying the sample as a solution on the grid is a well-suited method. One big obstacle is the formation of carbon contamination on the specimens which can result from organic residues, e.g. solvents.

Objectives

We investigate the impact of different solvents on TEM measurements and apply cleaning and preparation strategies in order to minimize the carbon contamination during TEM investigations. The amount of accumulated contamination resulting from different solvents was evaluated by high-angle annular dark field (HAADF) measurements and electron energy loss spectroscopy (EELS). The sample preparation in our sample cleaning setup was customized based on the specimens" properties and utilized solvent in order to prevent contamination.

Material & Methods

Thickness measurements by evaluating HAADF image contrast and EELS analysis were performed with a Thermo Scientific Talos F200X transmission electron microscope. For specific sample pretreatments, a self-made heatable vacuum setup was utilized in order to perform specialized sample preparation methods. All samples were prepared by applying 1 µL of solvent to a holey carbon copper grid and dried for 10 minutes before sample insertion into the TEM.

Results

HAADF and EELS measurements can be utilized for determining the thickness of deposited carbon contamination. The grids were exposed to the electron beam for different durations immediately after sample insertion and again 30 minutes later. For both toluene and THF as solvents, the determined amount of accumulated carbon showed a linear increase with longer exposure times. When the samples were exposed after 30 min in the TEM vacuum again, the observed carbon layer was significantly less (Fig. 1).

In order to prevent solvent outgassing in the microscope environment, a dedicated sample cleaning setup with a cold trap was utilized to prepare the TEM samples by removing solvent residues before sample insertion. Evaluation of decrease in pressure while pumping with a freshly prepared sample grid inserted, gives insight in the necessary drying times in order to reduce contamination during TEM investigations (Fig. 2).

Conclusion

Our experiments emphasize the crucial impact of the sample preparation in order to achieve best data quality and to keep a clean microscope environment. Based on our experiments, we can prepare our samples in a way resulting in less contamination on the specimen surface and introducing less volatile organic molecules to the microscopes.

Fig. 1: Carbon contamination depending on the beam exposure times of toluene (black) and THF (red) immediately after sample insertion (filled in dots) and after 30 min (circles) in the TEM. For each measurement, an area of 173 nm x 173 nm was exposed to the electron beam. Less volatile solvents like toluene lead to far more contamination, though the solvent can outgas in the TEM vacuum.

Fig. 2: Pressure measurements of pumping a toluene sample compared to a reference measurement (empty TEM grids). The pressure values are measured above and below a baffle cooler. The pressure difference between the sample and the reference above of the cooler indicates the presence of outgassing molecules.