posted on 2016-01-26, 00:00authored byHuashan Li, David Zhitomirsky, Shreya Dave, Jeffrey C. Grossman
Colloidal
quantum dots (CQDs) are highly versatile nanoscale optoelectronic
building blocks, but despite their materials engineering flexibility,
there is a considerable lack of fundamental understanding of their
electronic structure as they couple within thin films. By employing
a joint experimental–theoretical study, we reveal the impact
of connectivity in CQD assemblies, going beyond the single CQD picture.
High-resolution transmission electron microscopy (HR-TEM) demonstrates
connectivity motifs across different CQD sizes and length scales and
provides the necessary perspective to build robust computational models
to systematically study the achievable degree of connectivity in these
materials. We focused on state-of-the-art surface ligand treatments,
taking into account both the degree of connectivity and nanocrystal
orientation, and performed ab initio simulations
within the phonon-assisted hopping regime. Importantly, both the TEM
studies and our simulation results revealed morphological and electronic
defects that could dramatically reduce optoelectronic performance,
and yet would not have been captured within a single CQD model that
neglects connectivity. We calculate carrier mobility in the presence
of such defect states and conclude that the best-achievable CQD assemblies
for optoelectronics will require a modest degree of fusing via the {001} facet, followed by atomic ligand passivation
to generate a clean band gap and unprecedentedly high charge transport.