High Energy and Spatial Resolution EELS Band Gap Measurements Using a Nion Monochromated Cold Field Emission HERMES Dedicated STEM

Abstract

Local measurement of band gaps in nanomaterials is an intriguing experimental possibility, but until recently not possible because both high spatial resolution and high EELS energy resolution with small zero loss peak (ZLP) tails are required. The addition of a monochromator to a Nion probe corrected, cold FEG source D-STEM microscope (denoted HERMES) fitted with a Gatan Enfinium spectrometer has produced the EELS performance required for band gap measurements down less than 1 eV. The microscope, and its ultrastable installation site have been described [1,2]. ZLPs (normalized to the same intensity) from this microscope (in red) and another probe corrected Schottky FEG source but nonmonochromated AEM located in our laboratory are compared in Figure 1 and the data are summarized in Table 1. The 37 meV energy resolution of the monochromated cold FEG source Nion is significantly better than the typical Schottky FEG 950 meV resolution as shown in Table 1 and the energy tails at lower intensity are reduced. The resolution and beam tails depend somewhat on microscope alignment and collection time. The Nion ZLP in Figure 1 is the sum of 15 one second spectra. Shorter collection times (0.1 second, 1 spectrum) result in higher energy resolution (14 meV at FWHM) and smaller extended tails (54 meV FW at 1% maximum intensity). Kimoto et al have shown that both CFEG and a monochromated Schottky source are useful for measuring band gaps, but both of these have more extended low loss tails than the HERMES monochromated CFEG, particularly the tunneling tail for the CFEG, which complicates small gap measurement [3]. Some first measurements are shown below.