Exploring size and state dynamics in CdSe quantum dots using two-dimensional electronic spectroscopy

Abstract

Development of optoelectronic technologies based on quantum dots depends on measuring, optimizing, and ultimately predicting charge carrier dynamics in the nanocrystal. In such systems, size inhomogeneity and the photoexcited population distribution among various excitonic states have distinct effects on electron and hole relaxation, which are difficult to distinguish spectroscopically. Two-dimensional electronic spectroscopy can help to untangle these effects by resolving excitation energy and subsequent nonlinear response in a single experiment. Using a filament-generated continuum as a pump and probe source, we collect two-dimensional spectra with sufficient spectral bandwidth to follow dynamics upon excitation of the lowest three optical transitions in a polydisperse ensemble of colloidal CdSe quantum dots. We first compare to prior transient absorption studies to confirm excitation-state-dependent dynamics such as increased surface-trapping upon excitation of hot electrons. Second, we demonstrate fast band-edge electron-hole pair solvation by ligand and phonon modes, as the ensemble relaxes to the photoluminescent state on a sub-picosecond time-scale. Third, we find that static disorder due to size polydispersity dominates the nonlinear response upon excitation into the hot electron manifold; this broadening mechanism stands in contrast to that of the band-edge exciton. Finally, we demonstrate excitation-energy dependent hot-carrier relaxation rates, and we describe how two-dimensional electronic spectroscopy can complement other transient nonlinear techniques.

Polarization-Controlled Two-Dimensional White-Light Spectroscopy of Semiconducting Carbon Nanotube Thin Films

“…Peak D at (650 nm, 1006 nm) most likely corresponds to excitation of S12 excitons, interband excitons where the electron and hole are in different valence and conduction sub-band levels, on (6,5) nanotubes, which relax to the S1 state and fluoresce. 76 (Fig 2c), we see a strong absorption at Peak I = (992 nm, 1001 nm) corresponding to ground state bleaching and stimulated emission from the S1 exciton on the (6,5) nanotubes. An excited state absorption is observed at Peak II = (985 nm, 948 nm).…”

“…12, 94 Although we cannot conclusively rule out contributions from other states, we tentatively assign Peaks III and IV in the polarized 2D-WL spectrum to cross peaks between hybridized S1+RBM and the S1 transition on (6,5) tubes.…”

“…The absorption spectrum for this film is shown in Figure 3a. (6,5) nanotubes. The peaks near 1000 nm corresponds to the (6,5) nanotube S1 absorption.…”

“…Because there is no clear correspondence to a different chirality S2 excitation, we assign 14 these to states on (6,5) CNTs. Peak B at (575 nm, 1120 nm) has previously been assigned to the S1 -state in (6,5) nanotubes and reflects defects on the CNTs. 75 Free carrier doping of carbon nanotubes produces new spectral features associated with trions (excitons bound to a free carrier) that emit light at 1170 nm in intentionally doped (6,5) CNT films, 52 thus we assign Peak C at (575, 1160) to trions on the (6,5) CNTs.…”

“…We tentatively assign the peak at 928 nm in the absorption spectrum to the S1 exciton on (9,1) CNTs, although the S2 excitation, S1 emission in the photoluminescence map is surprisingly weak. From the photoluminescence map, we see contributions from majority CNT species (6,5) at Peak A = (λexcitation, λemission)=(575 nm,1005 nm) as well as three features at Peak B = (575 nm, 1120 nm); Peak C = (575 nm, 1160 nm); and Peak D = (650 nm,1006 nm). Because there is no clear correspondence to a different chirality S2 excitation, we assign 14 these to states on (6,5) CNTs.…”