High energy-resolution electron energy-loss spectroscopy study of electronic structures of barium titanate nanocrystals

High energy-resolution electron energy-loss spectra were obtained from barium titanate (BaTiO₃) nanocrystals (BTNCs), which were synthesized by chemical vapor deposition using inductively coupled plasma. Onset energies of spectral intensities were 3.2 eV for 34 nm BTNCs and 3.5 eV for 6 nm BTNCs. This indicates an increase in the bandgap energy of BaTiO₃ with a decrease in crystal sizes. Those onset energies obtained from 90 nm specimen areas showed an excellent agreement with those estimated by previously reported optical measurements. Volume plasmon peaks were observed at 26.5 eV for 34 nm BTNCs and 25 eV for 6 nm BTNCs. Dielectric functions of the BTNCs were derived from loss-functions by Kramers-Kronig analysis using the refractivity of bulk BaTiO₃ (= 2.4). The peaks of O 2p → Ti 3d (t_2g) transitions in the imaginary part of dielectric function (ε₂) were observed at 3.3 eV for 34nm BTNCs and 3.8 eV for 6 nm BTNCs. Those of O 2p → Ti 3d (e_g) transitions were at 7.8eV for 34nm BTNCs and 6.6eV for 6nm BTNCs