Abstract text (incl. figure legends and references)

Introduction

With the use of organic materials in electronic devices, as well as in biological and bioinspired functional nano-materials, the need for novel characterization methods becomes paramount. Such materials are in general highly beam sensitive and common electron microscopic techniques cannot be applied in general. In particular metal sputtering, heavy metal staining and beam induced degradation of the sample are obstacles hampering the direct observation of organic materials.

Objectives

In our study we want to overcome obstacles listed above using ultra-low voltage Scanning Electron Microscopy (SEM) for direct imaging of organic materials with high spatial resolution, surface sensitivity, and significantly reduced beam-damage. For electron energies below 1 keV the SEM image contrast and imaging quality in particular for low-dose imaging of organic materials is significantly improved.

In addition, we introduce Electron Energy Loss Spectroscopy (EELS) to SEM. Applying electron spectroscopic imaging of backscattered electrons (BSE) we can for the first time in a SEM identify excited states (e.g. plasmon and fluorescence excitations) and use them for low-dose material characterization.

Materials and Methods

We use a prototype of an electron-spectroscopic SEM, i.e. Zeiss Delta-SEM [1] for recording backscattered EEL spectra (bsEELS). As prerequisite for spectroscopic data collection we can achieve sub-nanometer imaging resolution in a wide energy range from a few keV (as in conventional SEM) down to ultra-low landing energies around 20 eV.

Results

As initial model system we studied QDots, which – as inorganic material – showed a strong bsEELS shoulder at the corresponding excitation energy (data not shown). As first organic system we obtain spectral data from graphene on silicon wafer (SiO₂), materials where TEM-EELS measurements can be used as reference [2,3]. Fig. 1 shows measured bsEEL spectra together with a correlating naïve model fit derived from TEM-EELS data. Although the energy resolution of our prototype is limited (worse than about 5eV), we find a good agreement of the SEM spectral data to e.g. the known characteristic surface plasmon signal of graphene. We show here for the first time that backscattered electrons carry specific material information from their elastic and inelastic interaction with the sample.

Next we study organic samples such as DNA and DNA labelled with fluorophores (Fig. 2). The results show that beam damage is minimized and contrast heavily increased for organic materials at ultra-low electrons energies. In Fig. 2 we show the first results of bsEEL spectra of DNA origamis and fluorophores. Although the spatial and energy resolution still needs to be improved, the bsEEL spectra show a pronounced signal of the UV DNA absorption as well as the excitation signal of the bound fluorophores.

Conclusion

bsEELS provides a new, powerful characterization tool for the direct visualization and spectroscopy of organic materials.

Acknowledgements: The authors thank Enrico Lemma (KIT Karlsruhe, Germany), Yannick Dreher and Kerstin Göpfrich (MPI Heidelberg, Germany) for samples. Research funded by DFG (German Research Foundation) via the Excellence Cluster "3D Matter Made to Order" (EXC-2082/1-390761711).

[1] RR Schröder et al., Microsc. Microanal. 24 (Suppl 1), 2018

[2] Wachsmuth et al, Physical Review B 90, 235434, 2014

[3] Park et al, Ultramicroscopy 109, 9, 2009