posted on 2017-10-26, 00:00authored byLakshay Jethi, Timothy G. Mack, Patanjali Kambhampati
The
size-dependent optical and electronic properties of semiconductor
nanocrystal (NC) have been exploited over decades for various applications.
This size dependence involves a transition from the regime of bulk
colloids of ∼100 nm radius to quantum dots (QDs) of ∼10
nm radius, the details of which are material specific. To understand
the transition from the QD regime (∼10 nm) to the molecular
cluster regime (∼1 nm) of nanocrystals, we have carefully synthesized
a set of CdSe nanocrystals with sizes ranging from 0.89 to 1.66 nm
in radius. As the nanocrystals become small, the surface emission
strongly increases in amplitude, and the core emission broadens and
red-shifts. These effects are rationalized in terms of coupling to
ligands via electron transfer theory. The core emission spectra arise
from increased vibrational coupling of ligands for very small NC.
The surface emission amplitudes arise from a size-dependent surface
free energy. The transition from the QD to the molecular cluster regime
is found to be at 1.2 nm radius, in contrast to the transition from
the bulk to QD transition at the Bohr radius of 5.4 nm in CdSe. These
size-dependent surface electronic phenomena may be used for light
emission applications.