Absorption Features of CdTe Nanoclusters: Aspect Ratio Dependency of the Singlet/Doublet from First-Principles Calculations

Two categories of absorption features,
singlet and doublet, have
been observed in semiconductive II–VI magic-size clusters (MSCs)
experimentally; however, the underlying mechanism resulting in the
two absorption features has seldom been addressed. Here, the relationship
between morphology and absorption features is studied in CdTe nanoclusters
using first-principles calculations. The explorations focus on CdTe
isomers and nonisomers at first to simulate the isomerization and
layer growth pattern in MSCs. Similar conclusions are reached that
the absorption spectra exhibit a gradual transition from singlet to
doublet as the aspect ratios increase. We further suppose that the
shape of CdTe dMSC-371 may be that of sMSC-371 being stretched in
one dimension. Density functional theory absorption spectra show significant
agreement with the predicted spectra from the Mie–Gans theory,
which indicates that the shape of clusters affects the dielectric
function directly and results in different absorption features. The
major transition orbitals of these optical excitations are assigned
by transition dipole moment, oscillator strengths, and density of
states. Furthermore, the aspect-ratio-dependent singlet and doublet
absorption features are found to be independent of the structural
types, including the zinc blende, hollow nanotubes, and hexagonal
wurtzite structures. The present study reveals the aspect-ratio-determined
absorption behaviors of II–VI MSCs and provides a promising
method for predicting the morphology of MSCs with absorption spectroscopy.