posted on 2022-01-26, 18:36authored byCaroline Qian, Alex Abelson, Anneka Miller-Casas, Robert Capp, Ilya Vinogradov, Nina S. Udagawa, Nien-Hui Ge, Matt Law
Highly
ordered epitaxially fused colloidal quantum dot (QD) superlattices
(epi-SLs) promise to combine the size-tunable photophysics of QDs
with the efficient charge transport of bulk semiconductors. However,
current epi-SL fabrication methods are crude and result in structurally
and chemically inhomogeneous samples with high concentrations of extended
defects that localize carriers and prevent the emergence of electronic
mini-bands. Needed fabrication improvements are hampered by inadequate
understanding of the ligand chemistry that causes epi-SL conversion
from the unfused parent SL. Here we show that epi-SL formation by
the conventional method of amine injection into an ethylene glycol
subphase under a floating QD film occurs by deprotonation of glycol
by the amine and subsequent exchange of oleate by glycoxide on the
QD surface. By replacing the amine with hydroxide ion, we demonstrate
that any Brønsted–Lowry base that creates a sufficient
dose of glycoxide can produce the epi-SL. We then introduce an epi-SL
fabrication method that replaces point injection of a base with contactless
and uniform illumination of a dissolved photobase. Quantitative mapping
of multilayer (3D) films shows that our photobase-made epi-SLs are
chemically and structurally uniform and have much lower concentrations
of bulk defects compared to the highly inhomogeneous and defect-rich
epi-SLs produced by amine point injection. The structural–chemical
uniformity and structural perfection of photobase-made epi-SLs make
them leading candidates for achieving emergent mini-band charge transport
in a self-assembled mesoscale solid.