Inelastic and Elastic Scattering Cross-sections for Carbon at 20-30 keV

Electron microscopy (EM) is an indispensable tool to probe matter at the nanoscale. However, specimen damage has long been known to limit the information that can be obtained from a variety of electron beam sensitive materials. Damage can still be alleviated combining cryo-conditions and low-electron fluence illumination for some modes of EM operation. It has also been recognized that lower acceleration voltages (∼100 keV) increase the amount of information obtained per unit of damage at the thicknesses of typical specimens observed in cryo-EM [1]. A large variety of 2D materials and structural information of biological proteins below 100 kDa [2] would potentially provide higher contrast at the energy ranges commonly provided by scanning electron microscopes (SEM) (<30 keV), with the additional benefits in terms of cost and ease of use. However, experimental confirmations of the expected trend of increasing information at these low energies < 30 kV, by way of elastic and inelastic scattering cross-sections measurements, have until recently been limited by the availability of suitable detectors at these low energies. Here we use a new pixelated detector [3] and electron-energy loss spectrometer [4] with peak performances in the energy ranges provided by the SEM, to measure the inelastic and elastic scattering cross-sections for carbon in a similar fashion as carried out by Peet et at. [2]. This allows us to experimentally investigate the trends in the information coefficient at lower (20-30 keV) beam energies. Multiple spectra of both the zero-loss peak and the low-loss window of about 100 eV were collected altogether in various spatial sites on a 6.5 nm amorphous carbon layer. Integration of the areas provide the ratio Is-inelastic/I0 as depicted in Fig.1. The inelastic scattering at these lower (30 and 20 keV) energies follows the same linear relation found in the range 80-300 keV when plotted against 1/β2, where β is the electron speed divided by the speed of light. The elastic information of carbon was obtained from graphene at low currents and short exposure times (0.25-0.5 ms) to avoid saturation of the forward scattered beam. 400 to 600 frames were then added to obtain diffraction spots. The projector lens was adjusted at each energy as to obtain similar camera lengths. The elastic scattering cross-section was calculated using the ratio Is-elastic/I0, where I0 and Is-elastic were determined integrating the intensity of the forward scattered beam and 4 to 6 first order diffraction peaks subtracting local background counts, respectively. The experimental certainty was limited by large variations in the intensities collected from different spatial sites, as seen in Fig. 2. The intensity variations do not seem to come from thicknesses variations of the graphene, but may have been influenced by overlying contaminating material, which may be less mobile and have stronger local influences at lower beam energies. Increased aberrations at the energies studied in this work could also be the culprit, having a stronger effect in the elastic scattered electrons. Further exploration of the effects of momentum transfer from the electron beam to the graphene in the studied energy range is also required, as to rule out possible effects of diffraction peak broadening observed in other diffraction experiments in high quality graphene [5]. Inelastic scattering cross sections for amorphous carbon at 30 and 20 keV. (a) Inelastic scattering (Is) to zero loss peak (I0) intensity ratio at 30 and 20 keV versus 1/β2. (b) 20 keV example EEL spectra used to obtain the ratios in (a). Elastic scattering cross sections for graphene in the 30-20 keV range. (a) Elastic (Is) to forward scattered (I0) intensity ratio in the range of 30 to 20 keV versus 1/β2. (b) Example of an electron diffraction pattern of graphene at 24 keV adding 600 frames.

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