Functional Group Mapping by Electron Beam Vibrational Spectroscopy from Nanoscale Volumes

Abstract

Vibrational spectroscopies directly record details of bonding in materials, but spatially resolved methods have been limited to surface techniques for mapping functional groups at the nanoscale. Electron energy loss spectroscopy (EELS) in the scanning transmission electron microscope presents a route to functional group analysis from nanoscale volumes using transmitted subnanometer electron probes. Here, we now use vibrational EELS to map distinct carboxylate and imidazolate linkers in a metal–organic framework (MOF) crystal–glass composite material. Domains <100 nm in size are observed using vibrational EELS, with recorded spatial resolution <15 nm at interfaces in the composite. This nanoscale functional group mapping is confirmed by correlated EELS at core ionization edges as well as X-ray energy dispersive spectroscopy for elemental mapping of the metal centers of the two constituent MOFs. These results present a complete nanoscale analysis of the building blocks of the MOF composite and establish spatially resolved functional group analysis using electron beam spectroscopy for crystalline and amorphous organic and metal–organic solids.

Simulations of angle- and spatially-resolved vibrational electron energy loss spectroscopy for a system with a planar defect

“…The (scanning) transmission electron microscope [(S)TEM] routinely allows to reach sub-Å spatial resolution [5][6][7][8][9][10] and thanks to recent advances in monochromators it offers an energy resolution of electron energy loss spectroscopy (EELS) down to 4.2 meV [11,12]. Unprecedented experiments such as mapping of bulk and surface modes of nanocubes [13], investigations of the nature of polariton modes in Van der Waals crystals [14], temperature measurement at the nano-scale [15,16], identification and mapping of isotopically labeled molecules [17], position-and momentum-resolved mapping of phonon modes [18][19][20], atomic resolution phonon spectroscopy [21,22], functional group mapping [23], single stacking fault [24], and single-atom vibrational spectroscopy [25] were enabled by these advances in STEM instrumentation.…”

Two Distinct Stages of Structural Modification of ZIF-L MOF under Electron-Beam Irradiation

“…The bend contours visible in individual ZIF-L particles gradually fade away and completely disappear at an accumulated dose of 50 e/Å 2 (Figure a) pointing to the loss of crystallinity. At this stage, while there are no obvious visible changes in the particle morphology in HAADF-STEM images (Figure b), a few % particle shrinkage along both lateral directions is detectable (see the SI, Figure S2) consistent with an earlier prediction of pore collapse …”

Properties of Dipole-Mode Vibrational Energy Losses Recorded From a TEM Specimen

“…We have concentrated on vibrational modes in thin slabs of material; modes in nanostructures are being explored by other researchers (Lagos et al, 2017; Tizei et al, 2020). Vibrational EELS of molecular structures could provide a powerful technique for exploring chemical bonds (Collins et al, 2020), the main problem being radiation damage.…”