Ultra-high Resolution EELS Analysis and STEM Imaging at 20 keV

Abstract

Aberration-corrected scanning transmission electron microscopy (STEM) at primary energy (Eo) of 60 keV and below allows imaging and analysis of light elements largely without knock-on damage [1]. Even though other damage mechanisms exist, per unit of beam damage elastic and inelastic scattering cross-sections increase as Eo is decreased [2], driving interest in operation at the lowest Eo. Modern aberration correctors enable correction of geometric probe aberrations to high-enough angles to allow atomic-scale imaging at Eo=30 keV and below, assuming that chromatic aberration can be overcome too, either by Cc correction or monochromation [3]. The latter option brings the additional benefit of simultaneously enabling high- and ultra-high energy resolution EELS studies such as vibrational spectroscopy [4, 5]. The absolute energy resolution of the Nion U-HERMES system becomes smaller roughly as √Eo, from ∼12 meV at 200 kV to <3 meV for 30 keV and below [6]. Spectra acquired with <3 meV energy resolution reveal many new features, including acoustic phonons near the Г point in the Brillouin Zone [7] motivating operation at the lowest-possible Eo. However, the utility of a STEM is significantly diminished if it cannot form an atom-sized probe. Here we show that Eo = 20 keV is a viable choice for atomic-resolution imaging and ultra-high energy resolution analysis. Figure 1 shows a monochromated zero loss peak (ZLP) from the U-HERMES system at Eo = 20 keV. The full width at half of maximum (FWHM) is 2.6 meV. The same spectrum is also plotted on a log scale to see the tails of the distribution, showing that the FW at 10-4 of maximum is ∼110 meV. In vibrational EELS, weak loss features tend to be on the scale of 10-3 to 10-4 of the ZLP, meaning that weak loss features at ΔE around ∼110/2=55 meV should be easily observable. Figure 2 shows a high-angle annular dark field (HAADF) image of monolayer graphene acquired at Eo = 20 keV, with illumination semi-angle α/2=55 mrad. The energy selecting slit was set to a width of 100 meV to mitigate the loss of contrast due to chromatic aberration. The individual carbon atoms are well resolved, and the absolute modulation between the atoms and center of the 6-member carbon rings is ∼25%. Removing the slit and using the full ∼350 meV wide distribution from the cold field emission source (including tails) increases the beam current and average signal level by ∼4.3x, and decreases the modulation to ∼13%. In other words, the slit-in image gives twice better modulation at less than a quarter of the total beam dose, providing a significant benefit for both imaging and spectroscopy in terms of information gained per unit of dose. . Monochromated ZLP at Eo = 20 keV on a linear scale (left) showing full width at half maximum (FWHM) of 2.6 meV, and log scale (right) showing FW at 10–4 M of ∼110 meV. [3ms x 1,000 aligned spectra, α/2=β/2=10 mrad] (left) HAADF image of monolayer graphene with inset low-pass filtered section showing the location of a line profile (bottom right) to show the absolute contrast, and (upper right) Fourier transform of the image showing 1.07 Å contrast in 3 directions. Eo=20kV, monochromated to δE=100 meV, Ibeam=3 pA, α/2=55 mrad, β/2=86 mrad, aligned sum of 60 images, each 8 seconds and 512 x 512 scan pixels.