Ultra-high Energy Resolution EELS and 4D STEM at Cryogenic Temperatures

The capabilities of the STEM have advanced tremendously in the past decade. The latest generation of ultra-high energy resolution monochromated EELS-STEM (U-HERMES) instruments can give a probe size of 1.1 Å at 20kV operating voltage and energy resolution of 2.6 meV, also at 20 kV. See Fig. 1(a). These milestones were attained through advances in electron-optical instrumentation: monochromators, correctors, and spectrometers. While these performance benchmarks continue to march steadily forward, microscope users are turning attention to combining advanced electron-optical capabilities with new developments in operating modes, detectors, and software. Here we summarize our efforts to enable, facilitate, and accelerate this trend. Vibrational spectroscopy in the electron microscope has come a long way from the first glimpses of lattice vibrations 9 years ago [1, 2] to mapping isotopes with nm-level resolution [3] and resolving vibrational modes of a single atom [4]. Still, the applicability of this technique has been limited by the availability of different sample environments in combination with the requisite ultra-high energy resolution. As a new generation of side-entry stage [5] becomes more commonplace, this limitation is going away. Fig. 1(b) shows a proof-of-principle experiment using both ultra-high energy resolution and cryogenic operation. The sample is trilayer MoS2 encapsulated (top and bottom) with hexagonal boron nitride. Spectra of the exciton peaks at ∼1.9 and ∼2.1 eV are acquired from the same region at room temperature and 100 K with ∼16 meV energy resolution (chosen to be ∼1.5x narrower that the intrinsic peak width at 100 K). As the sample warms, the exciton spectra change significantly: a small feature between the two peaks, the interlayer exciton, becomes difficult to resolve as the peaks broaden, and there is a 44 meV red shift. This demonstrates the power of ultra-high energy resolution EELS in combination with sample cooling and motivates further work in this direction. Other capabilities also hold much promise if they can become more widespread, such as light- and gas-injection at the sample. Detectors and software are also advancing rapidly. The latest direct detection cameras achieve high DQE and dynamic range with speed approaching that of traditional single-channel detectors [6, 7]. Combining these detectors with powerful open-source software for live processing and smart compression makes multi-frame (time series) 4D-STEM imaging possible, bringing significant advantages when sample stimuli such as heating or cooling excite sample drift. Fig. 2 shows an excerpt from an aligned, summed, live-compressed, 20-frame 4D-STEM multi-scan acquisition across an interface in a GaAs nanowire between areas of zincblende and wurtzite phase. Center of mass analysis on the compressed data gives the beam deflection (proportional to the electric field-center) and its divergence (proportional to the projected charge density-right). At lower resolution the strong atomic potentials are washed out and the charge at the interface can be seen (not shown). Work is in progress to extend the 4D-STEM capability to samples held at liquid N2 temperatures and lower, and examples will be shown at the meeting [8]. (a) Monochromated zero loss peak (ZLP) acquired at 20 kV showing full-width at half maximum (FWHM) of 2.6 meV. [3ms x 1,000, aligned] (b) exciton spectra from BN-encapsulated 3-layer MoS2 acquired at room temperature and 100 K (extracted from a series every 20 K between these two end points). Compared to the cold spectrum, at room temperature the interlayer exciton (the small peak between the two larger peaks) becomes less apparent, and the exciton peaks become wider and red shift by 44 meV. [6] [80kV, Ibeam=4 pA, 2.7 min. total, 160 ms x ,1000 aligned]. A 170-pixel wide excerpt from a 1k x 0.5k scan pixel multi-frame 4D-STEM acquisition of a GaAs nanowire interface between regions of zincblende and wurtzite phase with (left) traditional HAADF, (middle) center-of-mass analysis showing the local electric field, and (right) projected charge density map calculated from the electric field. Live processing was used to reduce the data size to 156 MB (from 660 GB) during acquisition. [200kV, a/2=26 mrad, 56 us/pixel, 20 frames, aligned].