Single-atom Vibrational Spectroscopy with Chemical Bonding Sensitivity

Vibrational spectroscopy in scanning transmission electron microscopy (STEM) has allowed for the exploration of phonon properties at the nanometer [1,2] and even atomic scale [3,4], including the isotope-induced vibrational frequency shifts [5,6] and the phonon response to single defects in materials [7,8]. However, to unveil the impact of chemical bonding on the local phonon modes requires extreme sensitivity of the STEM-EELS experiment, and remains challenging. Here, we push the detection limit of single-atom STEM-EELS vibrational spectroscopy to the chemical-bond level and successfully unveil the impact of chemical-bonding configurations and atomic mass on local vibrational properties in defective graphene, complementary with density-functional-theory (DFT) calculations [9]. Fig. 1 demonstrates an atom-by-atom analysis of the vibrational spectroscopy of two types of point defects in graphene, namely the four-fold coordinated planar Si-C4vs. the three-fold coordinated buckled Si-C3. In Figs. 1b and 1d, the vibrational spectra of the Si atoms in the two structures show similar features with a dominant peak around ∼55 meV, while the spectra of the first-neighboring carbon (1st C) atoms feature distinct characters. Even though the 1st C atoms are bonded to the same neighboring atoms in the two defects, i.e., one Si atom and two C atoms, the 1st C in Si-C4 shows a broad feature in the energy-loss range of 80-110 meV as compared to that in Si-C3. As the Si atoms in Si-C3 and Si-C4 adopt the sp3 and sp2d-like hybridization [10], respectively, the variation of the vibrational states of the 1st C atoms can be reasonably attributed to the Si-C bonding difference, including the geometry configurations and bond angles/lengths. Besides the Si point defects, the vibrational properties of the three-fold coordinated N-C3 defects in graphene have also been investigated, as shown in Fig. 2. Unlike the vanishing intensities in the optical mode range (labelled as P2) of the spectra of Si atoms in Figs. 1b and 1d, the vibrational spectrum of N atom in N-C3 features a prominent intensity of P2 at 157 meV, because the low-mass impurity N causes only a small perturbation to the phonon modes of pristine graphene. In addition, the frequency ratio of P2 between the N and 1st C atoms is calculated to be 0.918, which is very close to the ratio mC/mN of 0.926 based on the simple harmonic oscillator model, presenting the correlation of local vibration with the atomic mass. Across the N-C3 defect structure, the carbon atoms with different distances from the N impurity atom, namely the 1st C with two C-C bonds plus one C-N bond vs. the 2nd C with three C-C bonds, show a frequency shift of P2 and can be attributed to their structural difference on the chemical-bonding level. In summary, our atom-by-atom vibrational spectroscopy analysis of defective graphene unveil the impact of chemical-bonding configurations and atomic mass on the phonon response to point defect structures, demonstrating that the vibrational measurement in STEM has stretched to the level of chemical-bonding sensitivity [11]. Vibrational spectra of point defects in graphene with different bonding configurations. (a) The schematic of Si-C4 configuration. (b) Vibrational spectra of the Si atom and 1st C atoms in the Si-C4 structure. (c) The schematic of Si-C3 configuration. (d) Vibrational spectra of the Si atom and 1st C atoms in the Si-C3 structure. The vibrational spectrum of pristine graphene (Gr) is provided as reference. Vibrational spectra of the N-C3 defect in graphene. (a) The schematic of N-C3 configuration. (b) Vibrational spectra of the N atom, 1st C atoms and 2nd C atoms in the N-C3 structure, compared with the vibrational spectrum from pristine graphene (Gr).