  Coupled colloidal quantum dot (CQD) dimers represent a new class of
artificial molecules composed of fused core/shell semiconductor nanocrystals.
The electronic coupling and wavefunction hybridization is enabled by the
formation of an epitaxial connection with a coherent lattice between the shells
of the two neighboring quantum dots where the shell material and its dimensions
dictate the quantum barrier characteristics for the charge carriers. Herein we
introduce a colloidal approach to control the neck formation at the interface
between the two CQDs in such artificial molecular constructs. This allows the
tailoring of the neck barrier in pre-linked homodimers formed via fusion of
multifaceted wurtzite CdSe/CdS CQDs. The effects of reaction time, temperature
and excess ligands is studied. The neck filling process follows an
intraparticle ripening mechanism at relatively mild reaction conditions while
avoiding inter-particle ripening. The degree of surface ligand passivation
plays a key role in activating the surface atom diffusion to the neck region.
The degree of neck filling strongly depends also on the initial relative
orientation of the two CQDs, where homonymous plane attachment allows for
facile neck growth, unlike the case of heteronymous plane attachment. Upon
neck-filling, the observed red-shift of the absorption and fluorescence
measured both for ensemble and single dimers, is assigned to enhanced
hybridization of the confined wavefunction in CQD dimer molecules, as supported
by quantum calculations. The fine tuning of the particle interface introduced
herein provides therefore a powerful tool to further control the extent of
hybridization and coupling in CQD molecules.
