Probing molecular vibrations by monochromated electron microscopy

Abstract

State-of-the-art monochromated electron energy‐loss spectroscopy in the advanced electron microscope achieves a few millielectronvolts energy resolution, enabling the observation of both vibrational spectra in organic molecules and phonon band structures in crystalline materials. Molecular vibrational states can be measured in a nondestructive manner by placing the electron beam outside the sample. The vibrational signals possess high spatial resolution of several tens of nanometers through dipole scattering and even atomic resolution via impact scattering, both of which are superior to most conventional spectroscopic methods. Vibrational spectroscopy in the electron microscope is capable of identifying isotopically labeled atoms at different sites and is sensitive to the polarization of vibrational states. A variety of samples can be investigated by this method, including two-dimensional materials and liquids. Chemical bonds fundamentally determine molecular properties and are prevalently characterized by various spectroscopic means such as infrared and Raman spectroscopies. However, the spatial resolution of these conventional approaches is insufficient to reveal nanoscale features. Recently, monochromated electron energy-loss spectroscopy (EELS) in the transmission electron microscope achieved a groundbreaking energy resolution of a few millielectronvolts and enabled direct observation of molecular vibrational spectrum with unmatched spatial resolution. Vibrational EELS is widely applicable to both organic and inorganic matter in the solid state or liquid phase. In this review, we introduce recent advancements and key concepts of this method, compare with other spectroscopic techniques, and discuss future developments for potential applications in research fields centered on catalysts, polymers, and live cells. Chemical bonds fundamentally determine molecular properties and are prevalently characterized by various spectroscopic means such as infrared and Raman spectroscopies. However, the spatial resolution of these conventional approaches is insufficient to reveal nanoscale features. Recently, monochromated electron energy-loss spectroscopy (EELS) in the transmission electron microscope achieved a groundbreaking energy resolution of a few millielectronvolts and enabled direct observation of molecular vibrational spectrum with unmatched spatial resolution. Vibrational EELS is widely applicable to both organic and inorganic matter in the solid state or liquid phase. In this review, we introduce recent advancements and key concepts of this method, compare with other spectroscopic techniques, and discuss future developments for potential applications in research fields centered on catalysts, polymers, and live cells. condition where even though electron beam is placed outside the sample, it still feels the dipole motion of nearby materials due to long-range Coulomb interactions. Only dipole scattering–induced vibrational modes are measurable in this condition. In this condition, the valence-loss excitations are significantly suppressed to abate the radiation damage for beam-sensitive materials. scattering originating from the long-range Coulomb interaction between the electron beam and dipole motion in materials. The typical scattering angle of dipole scattering is extremely small with nearly zero momentum exchange. This information can be recorded under the regular on-axis geometry. The signal is delocalized in real space and measures the surface loss function. the capability of resolving two adjacent energy-loss peaks with the minimal energy difference. It is defined as the full width at half maximum (FWHM) of the ZLPs. The value of energy resolution is reduced as the primary energy of electron beam decreases. distance of the parked electron beam from the edge of the sample. The signal intensity of the aloof signal exponentially decays with increasing impact parameter. By changing its value, one can modify the received strength of dipole scattering–induced modes. scattering arising from the short-range Coulomb interaction between electron beam and atomic nucleus in materials. The vibrational signals originating from this mechanism are highly localized in real space and only occur under the transmission configuration. Such signals include high-spatial-resolution features. characterizes the inelastic scattering process and can be used to calculate the EELS intensity. It is defined as the imaginary part of negative reciprocal of the material dielectric function, which is a function of both momentum exchange and energy loss. It describes the response of the sample to the swift electron beam. refers to the minimal distance necessary to resolve two atoms or localized vibrational states, mathematically obeying the Rayleigh criterion. The spatial resolution of STEM imaging is determined by the size of the electron probe (about 0.1 nm at 60 keV after aberration correction). However, the spatial resolution of vibrational signal is determined by the intrinsic physical extent of corresponding states. condition where the electron beam intersects the interior of the sample and simultaneously excites both dipole scattering and impact scattering vibrational modes. This condition will lead to inevitable beam damage for most organic samples.