Understanding and controlling the electronic structure of nanomaterials
is the key to tailoring their use in a wide range of practical applications.
Despite this need, many important electronic states are invisible to conventional
optical measurements and are typically identified indirectly based on their
inferred impact on luminescence properties. This is especially common and important
in the study of nanomaterial surfaces and their associated defects. Surface trap
states play a crucial role in photophysical processes yet remain remarkably poorly
understood. Here we demonstrate for the first time that broadband electronic
sum frequency generation (eSFG) microspectroscopy can directly map the
optically bright and dark states of nanoparticles, including the elusive below
gap states. This new approach is applied to model cadmium selenide (CdSe)
quantum dots (QDs), where the energies of interfacial trap states have eluded
direct optical characterization for decades. Our eSFG measurements show clear
signatures of electronic transitions both above the band gap, which we assign
to previously reported one- and two-photon transitions associated with the CdSe
core, as well as broad spectral signatures below the bandgap that are attributed
to interfacial trap states. In addition to the core states, this analysis
reveals two distinct distributions of below gap states providing the first
direct optical measurement of both shallow and deep trapping sites on this
system. Finally, chemical modification of the surfaces via oxidation results in
the relative increase in the signals originating from the interfacial trap states.
Overall, our eSFG experiments provide an avenue to directly map the entirety of
QD bulk and interfacial electronic structure, which is expected to open up
opportunities to study how these materials are grown <i>in situ</i> and how surface states can be controlled to tune
functionality.
Energetics at the Surface: Direct Optical Mapping of Core and Surface Electronic Structure in CdSe Quantum Dots Using Broadband Electronic Sum Frequency Generation Microspectroscopy
