As structural inhomogeneities at the nanoscale or even atomic scale usually play a crucial role in tuning the macroscopic properties, electron energy-loss spectroscopy (EELS) in aberration-corrected scanning transmission electron microscopy (STEM) is of fundamental significance to uncover the origin of functionalities in materials by implementing atom-by-atom analysis. Core-loss EELS, which takes advantage of the interaction between the incident high-energy electrons and the inner-shell atomic electrons, not only offers indispensable atomic insights into the element identification but also demonstrates tremendous potential for probing electronic structure and coordination environment on the single-atom level. Similarly, the outer-shell atomic electrons, which are tightly bonded with local dielectric constant, bandgap and inter/intraband transitions can be revealed by low-loss EELS. Recently, the emerging monochromator in the latest Nion HERMES system ensures the comparable energy-resolving power (the meV-scale energy resolution) to Fourier transform infrared (FTIR) spectroscopy for detecting the lattice vibration with atomic resolution and allows for the direct correlation of vibrational properties with local structural configurations such as point defects, stacking faults and interfaces. Moreover, the flexible electron optics in STEM enable us to realize multiple functional measurements such as the momentum-resolve EELS to detect the phonon dispersion curves in bulk or across interfaces. As phonon vibration is related to thermal conductivity and heat capacity, the high spatial and energy resolution EELS in STEM would open the door to comprehending the phonon-mediated properties and uncover the thermology-involved physics at the atomic scale.

Electron diffraction patterns, which are determined by the interactions between the high energy electrons and imaged materials, encode rich information about materials such as structures, chemical bonds, electron orbitals, electromagnetic fields and so on. In conventional transmission electron microscopy (TEM)/scanning TEM (STEM), only a part of the diffraction pattern is recorded to unveil the atomic-scale structures. As a result, the atomic-scale electromagnetic fields, chemical bonds, electron orbitals and other functional information are lost when adopting conventional TEM/STEM. The recent advances in fast pixelated electron cameras made the recording of full diffraction patterns at each scanning position possible, and the scanning diffraction imaging technique was proposed to simultaneously deliver the structural and functional information of materials using ptychography reconstructions. The appearing of this novel technique is now bringing electron microscopy to a new era of functional imaging, and a high impact of it on the research of functional materials is foreseeable. Our group devotes to optimizing experimental configurations of this technique and improving ptychography reconstruction methods to quantitatively study functionalities of materials with single-atom accuracy.

Two-dimensional (2D) materials are an emerging class of nanostructured low-dimensional materials in fabricating the next generation of miniaturized electronics and optoelectronics devices, relying on their intriguing and unexpected physics exhibited when single or few atomic layers are isolated. The easy cleavability of the 2D materials allows us to study the new emerging properties from their pure form or even on the heterostructures fabricated by stitching, stacking or twisting. The roles of reduced dimensionality and the structure of interfaces and defects are critical to the macroscopic properties of the materials but remain obscure in many cases. Equipped with the world’s leading aberration-corrected and monochromated scanning transmission electron microscopes, in our lab we are able to establish synergistically combined research of high spatial and energy resolution microscopy and materials physics, directly revealing the structure-property relationship of the 2D materials. For example, chemical mapping at the atomic level for assessing element diffusion and atomic bonding, understanding the strong electronic correlations among twisted layers, and investigating the impact of defects on the vibrational properties are some of the hot points in the current microscopy analysis. In addition, these capabilities can be combined with new customized holders for in-situ electron microscopy, interrogating the novel properties of 2D devices by subjecting them to external stimuli such as electric field, temperature or gas.

Developing high-performance energy-related materials is a promising field in the energy industry, on account of their broad application prospect in industrial production, energy sustainability, environmental protection, etc. Although people usually joke that “chemistry” means “chem is try”, designing novel materials with specific performance should be definitely based on rational thought, which calls for a comprehensive understanding of the structure-property relationship. For example, the catalytic performance of a catalyst is tightly dependent on the configuration of active sites, including the location and bonding characteristic of the supported metal atoms on the substrate, and the resultant metal-support interaction. An aberration-corrected scanning transmission electron microscope (STEM) coupled with a series of imaging detectors and spectrometers serves as a versatile platform for atomic resolution characterization, which can offer a full picture of the structure of materials in combination with averaged measurements. In our studies, besides visualizing the atomic structure by high-angle annular dark-field (HAADF) imaging and electron energy-loss spectroscopy (EELS) chemical mapping, we focus on developing atomic-scale EELS core-loss fine structure analysis to probe the valence state and coordination environment of the active sites in energy-related materials. Additionally, in-situ techniques can be integrated with these STEM characterization approaches, offering atomic-scale insights into the evolution and failure mechanism of the active sites under realistic working conditions.