Backscattered Electron Energy Loss Spectroscopy with an Analytical High-Resolution Ultra-Low-Voltage SEM

<br/>In scanning transmission electron microscopy (STEM) electron energy loss spectroscopy (EELS) is a well-established high resolution analytical characterization technique for a large range of materials and applications. However, the required sample dimension (i.e. &lt; 100 nm thickness) is a limiting factor and often makes sample preparation rather challenging. Furthermore, beam-induced sample degradation at typical TEM electron energies is hampering the observation of beam-sensitive materials. The realization of EELS at very low electron landing energies on bulk materials, however, would definitely provide a completely new characterization tool – from simple materials identification to dynamic in-situ studies, such as plasmon excitations by electrons or light.<br/><br/>Using a prototype of an aberration corrected, ultra-low-energy SEM (Zeiss Delta-SEM [1]) modified with an electrostatic energy-selective detector, we show for the first time that it is possible to identify the excitation of states (e.g., plasmon and fluorescence excitations) in a SEM by their typical excitation energy loss signature in the energy spectrum of the backscattered electrons. We call this new analytical signal backscattered electron energy loss spectroscopy (bsEELS).<br/><br/>As initial model system we analyze the spectral data obtained from graphene on silicon wafer, materials where TEM-EELS measurements can serve as a reference [2,3]. The measured bsEEL spectra can be simulated in the framework of a naïve model which convolves TEM-EELS data with the energy response of the energy-selective detector. Although the energy resolution of our current experimental setup is limited (about 5 eV), we find good agreement of the experimental SEM-bsEELS spectral data to, e.g., the known characteristic surface plasmon signal of graphene. This is the first experimental evidence that backscattered electrons carry specific material information resulting from their elastic and inelastic interaction with the sample. Applying this to an organic PBDB-T:ITIC blend for solar cells we can use the plasmon loss signals in the bsEEL spectra to identify and distinguish the two materials inside the blend to create a surface map of the material distribution.<br/>Using bsEELS relaxes constraints on the sample preparation (thickness, size) while aberration correction allows to use ultra-low electron landing energies, comparable to energies usually used for high-resolution EELS. We show that contrast and imaging quality are significantly improved for electron energies below 500 eV (as far down as 20 eV) – while retaining high spatial resolution (&lt; 1 nm at 100 eV).<br/>To prove the high sensitivity of this novel imaging modality we study organic samples such as DNA and DNA labelled with fluorophores. The results show that at ultra-low electron energies beam damage is minimized and that contrast is strongly increased for organic, low atomic number materials. First results of bsEELS on DNA origamis with 12 covalently bound fluorophore molecules reveal a pronounced signal for the DNA’s UV absorption as well as the excitation signal of the fluorophores.<br/><br/>Acknowledgements: The authors thank Yannick Dreher and Kerstin Göpfrich (MPI Heidelberg, Germany), and Wolfgang Köntges (Heidelberg University) for samples. Research funded by DFG (German Research Foundation) via the Excellence Cluster "3D Matter Made to Order" (EXC-2082/1-390761711).<br/><br/>[1] RR Schröder et al., Microsc. Microanal. 24 (Suppl 1), 2018<br/>[2] Wachsmuth et al, Physical Review B 90, 235434, 2014<br/>[3] Park et al, Ultramicroscopy 109, 9, 2009