The seminar of the ISAN Condensed Matter Spectroscopy Department was held on January 26.
Speaker: Mikhail Popov (FSBI TISNUM)

Topic of the report: RAMAN spectroscopy of nanodiamonds 2-5 nm: the role of Tamm states.

Abstract: Resonance effects have been experimentally detected in the study of raman spectra of 2-5 nm nanodiamonds when excited at a wavelength of 1064 nm (1.165 eV). The resonance spectrum consists of widened bands in the range of 1100-1720 cm-1, which corresponds to the results of DFT modeling of the phonon spectrum of nanodiamonds. The presence of high-frequency (relative to the frequency of 1333 cm-1 cattle for a single crystal of diamond) The mode in accordance with the lattice dynamics indicates the presence of 2-5 nm bonds in nanodiamonds with force constants higher than in a single diamond crystal. This is also confirmed by experiment: the volume compression modulus of 2-5 nm nanodiamonds is 607 GPa, which exceeds the value for a single diamond crystal (447 GPa). When excited away from resonance (405 nm, 3.1 eV), the RAMAN spectrum consists of only 3 bands at 1325 cm-1, 1500 cm-1, and 1600 cm-1. It is significant that additional lines (1500 cm-1 and 1600 cm-1) are observed for nanodiamonds that are not contaminated with impurities and belong to sp3 bonds. In particular, the 1600 cm−1 band is observed up to a pressure of at least 70 GPa, although sp2-bound carbon does not exist at pressures above 40 GPa. The detected resonant phenomena of the Raman and the increase in the volume compression modulus are explained by the presence of Tamm states with an energy of electronic transitions of about 1 eV, previously observed on the surface of single crystals of diamond.