  Coupling of atoms is the basis of chemistry, yielding the beauty and richness
of molecules. We utilize semiconductor nanocrystals as artificial atoms to form
nanocrystal molecules that are structurally and electronically coupled.
CdSe/CdS core/shell nanocrystals are linked to form dimers which are then fused
via constrained oriented attachment. The possible nanocrystal facets in which
such fusion takes place are analyzed with atomic resolution revealing the
distribution of possible crystal fusion scenarios. Coherent coupling and
wavefunction hybridization are manifested by a red shift of the band gap, in
agreement with quantum mechanical simulations. Single nanoparticle spectroscopy
unravels the attributes of coupled nanocrystal dimers related to the unique
combination of quantum mechanical tunneling and energy transfer mechanisms.
This sets the stage for nanocrystals chemistry to yield a diverse selection of
coupled nanocrystal molecules constructed from controlled core/shell
nanocrystal building blocks. These are of direct relevance for numerous
applications in displays, sensing, biological tagging and emerging quantum
technologies.
