High-temperature phonons in h-BN: momentum-resolved vibrational spectroscopy and theory

Vibrations in materials and nanostructures at sufficiently high temperatures result in anharmonic atomic displacements, which leads to new phenomena such as thermal expansion and multiphonon scattering processes, with a profound impact on temperature-dependent material properties including thermal conductivity, phonon lifetimes, nonradiative electronic transitions, and phase transitions. Nanoscale momentum-resolved vibrational spectroscopy, which has recently become possible on monochromated scanning-transmission-electron microscopes, is a unique method to probe the underpinnings of these phenomena. Here we report momentum-resolved vibrational spectroscopy in hexagonal boron nitride at temperatures of 300, 800, and 1300 K across three Brillouin zones (BZs) that reveals temperature-dependent phonon energy shifts and demonstrates the presence of strong Umklapp processes. Density-functional-theory calculations of temperature-dependent phonon self-energies reproduce the observed energy shifts and identify the contributing mechanisms.Comment: 21 pages, 4 figures, 2 tables, 3 supplemental figures, 3 supplemental table

Automated processing of parallel-detection EELS data

An algorithm for automatic detection and identification of edges in an EELS spectrum is presented. It has the following features: 1) it compresses the dynamic range of EELS spectra and enhances the ionization edge signals via difference transforms, 2) it removes residual background, thereby isolating sharp features associated with the edge thresholds and noise, 3) it distinguishes true edge-threshold features from noise via statistical analysis. In addition to paving the way for rapid, automated EELS elemental analysis, the algorithm is capable of detecting edges which are easily overlooked by human analysts

Developments in EELS instrumentation for spectroscopy and imaging

Recent developments in instrumentation for electron energy loss spectroscopy (EELS) at Gatan R&D are reviewed. A 10-channel intrinsic Si detector with single-electron detection capability is being developed for fast energy-filtered imaging and elemental mapping in the STEM mode. A new type of an imaging filter suitable for attachment at the end of the electron-optical column of a transmission electron microscope has been designed and built. The design of the filter is described, and its electron-optical properties are compared with the properties of an optimized Ω-filter. Finally, an unconventional design of an ultra-high resolution electron spectrometer which does not use any retardation and yet should be able to attain an energy resolution of a few meV at primary energies around 200 keV is proposed

EELS quantification near the single-atom detection level

Parallel-detection electron energy loss spectrometers are able to detect the EELS signal originating from only a few atoms on thin substrates. The instrumental requirements for attaining this level of performance, and the methodology for quantifying the results are described. For the case of small thorium clusters on a thin carbon film, the detection limit with currently available instrumentation is shown to be one atom

Design and first applications of a post-column imaging filter

We have designed and built an imaging filter which can be attached to most standard TEMs, and is capable of operating at primary energies of up to 400 keV. The filter uses a 90° magnetic sector prism, a piezoelectrically controlled energy-selecting slit, and 6 quadrupole and 5 sextupole lenses. It fully corrects second-order aberrations and distortions in images formed with electrons of selected energies, and it also produces second-order aberration-corrected spectra of variable dispersion. We show the first applications of the filter at 200 keV primary energy, which indicate that the filter will excel in chemical mapping and spectroscopy, in improving contrast of electron images and diffraction patterns, and in making high resolution electron microscopy and diffraction more quantitative. We conclude the paper with a discussion about atomic resolution image formation using inelastically scattered electrons, and the relationship between energy-filtered diffraction patterns and images formed with electrons of the same energy