To understand Raman
spectra shifts of nanocrystals, the top-down
phonon confinement approach and the bottom-up quantum chemical approach
were developed. The former is suitable for large-sized nanocrystals,
and the latter is suitable for clusters containing fewer atoms. Here,
we find that a simpler chemical bond model based on the bond dispersion
feature can demonstrate Raman spectra shift either in normal size
II–VI semiconductor nanocrystals or in atomically precise clusters.
According to the bond dispersion model, the Raman spectral line of
the II–VI semiconductor nanocrystal (AIIBVI) is expressed as the sum of the Lorentz subpeaks of the AII(i)BVI(j) bonds with different
coordinates i and j. The calculated
Raman lines of CdSe, CdS, CdTe, ZnS, and ZnSe nanocrystals are in
agreement with the measured Raman spectral lines. The origin of the
red shift and asymmetric broadening of the peak position of nanocrystals
may be revealed as well. Results provide insight into how different
bonds contribute to different vibrational spectra.