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Reversible Surface Electronic Traps in PbS Quantum Dot Solids Induced by an Order–Disorder Phase Transition in Capping Molecules
journal contribution
posted on 2012-05-09, 00:00 authored by Cory A. Nelson, X.-Y. ZhuThe electronic properties of semiconductor quantum dots
(QDs) are
critically dependent on the nature of the ligand molecules on their
surfaces. Here we show the reversible formation of surface electronic
trap states in the model system of solid thin films of PbS QDs capped
with thiol molecules. As the temperature was increased from cryogenic
to room temperature, we discovered a phase transition in the fluorescence
spectra from excitonic emission to trap emission. The
critical temperature (Tc) of the phase
transition scales with molecular length and in each case is close
to the bulk melting temperature of the capping molecules. We conclude
that an order–disorder transition in the molecular monolayer
above Tc introduces surface mobility and
the formation of a disordered atomic lead layer at the QD/capping
molecule interface, leading to electronic trap formation.