Article pubs.acs.org/JPCC
Ultrafast Dynamics within the 1S Exciton Band of Colloidal PbSe Quantum Dots Using Multiresonant Coherent Multidimensional Spectroscopy Daniel D. Kohler, Stephen B. Block, Schuyler Kain, Andrei V. Pakoulev, and John C. Wright* Department of Chemistry, University of WisconsinMadison, 1101 University Avenue, Madison, Wisconsin 53706, United States S Supporting Information *
ABSTRACT: The simple particle-in-a-sphere model of quantum dot excitons is the basis for understanding the excitonic peak positions, line widths, and relaxation dynamics in many spectroscopic experiments. Recent multiresonant coherent multidimensional spectroscopy (CMDS) with picosecond excitation pulses measured the two-dimensional spectra of PbSe quantum dots and successfully used this simple model of an inhomogeneous distribution of spherically confined exciton and biexciton states and rate constants to describe the dephasing and population relaxation dynamics. The long excitation pulses prevented resolution of faster dynamics. This work reports the development of multiresonant CMDS with femtosecond excitation pulses to resolve the spectra and dynamics associated with the 1S exciton line shape of PbSe quantum dots. The experiments use different combinations of excitation frequencies, excitation pulse time delays, and a monochromator to display and measure correlations between the spectral features and their dynamics. Line-narrowing of the inhomogeneous distribution occurs at short time delays where the excitation excites a subset of the quantum dots within the 1S line shape and the last pulse probes this subset. The line-narrowing disappears at longer delay times. Three pulse photon echo peak shifts (3PEPS) also occur when the line-narrowing is present, but the shifts disappear as the correlation between the first and last coherence frequencies disappears. Wigner plots reveal the spectral dynamics accompanying the peak shift and the disappearance of the line-narrowing. This work shows there is rapid relaxation dynamics occurring within the line profile of the quantum confined excitonic states that is not consistent with current understanding of the excitonic line broadening. The data suggest that the relaxation dynamics play a more dominant role in defining the excitonic line widths than the inhomogeneous broadening of the quantum dot size distribution. These observations are consistent with other spectroscopic experiments on CdSe and PbS quantum dots. The experiments also show the presence of a higher energy feature that lies outside the 1S line shape and undergoes very rapid relaxation.
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INTRODUCTION Quantum dots (QDs) play an important role in developing nanotechnology applications that depend on the quantum effects that occur when the size of nanostructures becomes comparable to their electronic wave functions.1,2 QDs serve as model systems because they offer the simplest examples of the excitonic states that emerge from quantum confinement. They may also play an important role in applications such as sensitizing nanostructures for photovoltaic and photocatalytic applications. These applications require a fundamental understanding of the electronic states and their dynamics. Extensive spectroscopic work on CdSe, PbS, and PbSe colloidal quantum dots has resulted in a deep enough understanding of the electronic states that these materials have become models for the electronic states and dynamics of quantum confined nanostructures.3−45 Although extensive work has been conducted on the electronic structure of quantum dots, these studies have shown that the simple particle-in-asphere (PIAS) model for the excitonic and multiexcitonic states can describe the major features in the absorption spectra.46 The PIAS model is also used to describe the Coulombic coupling in multiexcitionic states, population relaxation, and surface © 2014 American Chemical Society
trapping dynamics between excitonic and multiexcitonic states and to understand the mechanism of relaxation dynamics.34,45−48 Despite the breadth of applicability, there are numerous properties quantum dots possess that require more sophisticated treatments.8,34,49−54 Recent coherent multidimensional spectroscopy (CMDS) experiments on CdSe colloidal quantum dots show the PIAS model is not adequate for describing relaxation within the PIAS excitonic line shapes. Three photon echo peak shift (3PEPS) measurements had a rapid relaxation component within the line shape of the 1S(e)−1S3/2(h) exciton as well as a static component from the inhomogeneous broadening.14,22 The rapid relaxation component was unexpected because the spectral width in the simple PIAS picture is caused by static inhomogeneous broadening from the quantum dot size distribution. The rapid relaxation component experiment dominated over the static inhomogeneous broadening component and was attributed to spectral diffusion Received: December 9, 2013 Revised: February 13, 2014 Published: February 14, 2014 5020
dx.doi.org/10.1021/jp412058u | J. Phys. Chem. C 2014, 118, 5020−5031
The Journal of Physical Chemistry C
Article
arising from relaxation between the different fine structure states.22 Fine structure states are predicted in the effective mass approximation (EMA) for CdSe because a combination of spin−orbit coupling, exchange interactions, the intrinsic crystal field in wurtzite structures, and shape anisotropy perturbations mix the PIAS states and lift the state degeneracies.5,38,46 PbS and PbSe colloidal quantum dots do not have the same fine structure states as CdSe,8,55 but CMDS experiments show that these materials also exhibit ultrafast dynamics within the line shapes of PIAS states. Stimulated photon echo (SPE) measurement of the coherence dephasing time on PbS QDs at 5 K has a two-component dephasing time of
