Electron Energy Loss Spectroscopy (EELS) is a versatile STEM technique for analyzing the inelastically scattered fast electrons carrying the beam-specimen interaction information and can determine chemical composition, oxidation states, plasmonic resonance, and electronic structure of materials.
Due to the insufficient energy resolution (e.g., 300 meV or 2420 cm-1 for cold field emission gun), conventional EELS cannot resolve vibrational signals in the energy range of 10-300 meV (80-2420 cm-1). In 2014, a Nion dedicated STEM achieved sub-10 meV energy resolution after paradigm-shifting improvements of both the monochromator and spectrometer. Since then, vibrational spectroscopy in the electron microscope has emerged as a cutting-edge field of research that involves the imaging of vibrational properties of materials with high spatial, energy, and momentum resolutions. Pan group is actively improving the capability of vibrational spectroscopy and exploiting state-of-the-art methodologies in the fields of physics, materials science, and chemistry. We can obtain an energy resolution of 5.7 meV (46 cm-1) at 60 keV and 4.2 meV (34 cm-1) at 30 keV with spatial resolutions of 1.5 Å and 2 Å, respectively. Additionally, by varying the convergent semi-angle of the electron probe, we can continuously adjust the spatial resolution, energy resolution and even momentum resolution to meet various experimental requirements, such as measurement of total phonon density states, phonon dispersion relations or dipole-coupled polariton modes. We are interested in investigating vibration-related properties and performance of emerging materials and devices including phonon-defect interactions, nanoscale thermal transport and thermometry, origin of ferroelectric polarization, tailored polariton modes, and identification of chemical bonds.
Defect Phonon Imaging:
Recently, we have revealed localized defect phonon modes associated with a single stacking fault in SiC, a wide-bandgap semiconductor, which has remained elusive in experimental study. Defect phonons possess a blue-shifted acoustic branch with a flatter dispersion relation, compared to bulk SiC. This observation can explain the defect-induced reduction of thermal conductivity.
Vibrational Mapping of Phonon Reflections from Quantum Dot Interfaces
SiGe quantum dots have shown to have excellent performance in thermoelectric applications that efficiently convert heat to electricity. Typically, thermoelectric research seeks to impede thermal transport while enhancing electron mobility. To this end, it becomes necessary to develop materials that include nanostructures and interfaces that can scatter thermal carriers, or phonons. However, there have been no correlative experiments that spatially track the modulation of phonon properties in and around nanostructures due to spatial resolution limitations of conventional optical phonon detection techniques.
Pan group has demonstrated two-dimensional spatial mapping of phonons in a single silicon-germanium (SiGe) quantum dot (QD) using monochromated electron energy loss spectroscopy (EELS) in the transmission electron microscope (TEM). Tracking the variation of the Si optical modes in and around the QD, we observed the nanoscale modification of the composition induced redshift. We observed nonequilibrium phonons that only exist near the interface and furthermore, developed a novel technique to differentially map phonon momenta providing direct evidence that the interplay between diffusive and specular reflection largely depend on the detailed atomistic structure –a major advancement in the field.
Nanoscale Thermometry by Vibrational Mapping
Vibrational microscopy in STEM also allows us to measure nanoscale temperature distribution inside the nanostructures. Pan group demonstrates a phonon-based thermometry based on the linear temperature-dependence of phonon energy. In the typical semiconductors (Si and SiC) with small coefficients of thermal expansion, both surface phonon mode and bulk phonon mode exhibit an exceptional strong energy redshift as temperature increases due to the anharmonicity contribution. In comparison to the delocalized surface phonon polariton signal, bulk optical phonon modes show high spatial resolution of their spectral signals and can be applied to identify the local temperature. This method provides a new avenue to measure local temperature and temperature gradient in nanodevices and to assist the temperature calibration during in-situ studies of catalytic performance and phase transition of perovskite oxides. [hyperlink to publications tab]
Additionally, we extend this state-of-the-art vibrational microscopy in STEM to a broad range of fundamental research topics of molecular vibration, phonon, polariton, exciton, and other intriguing quasiparticles. There are many ongoing projects, including investigating defect-phonon interactions, understanding the phonon propagation at 2D and 3D hetero-interfaces, pursuing the phonon-related origin of ferroelectric polarizations, and even detecting the Moiré excitations in twisted 2D materials.
Related Publications
- Yan, X., Liu, C., Gadre, C.A. et al. Single-defect phonons imaged by electron microscopy. Nature 589, 65–69 (2021). https://doi.org/10.1038/s41586-020-03049-y