Investigating the electronic structure of confined multiexcitons with nonlinear spectroscopies.

Strong confinement in semiconductor quantum dots enables them to host multiple electron-hole pairs or excitons. The excitons in these materials are forced to interact, resulting in quantum-confined multiexcitons (MXs). The MXs are integral to the physics of the electronic properties of these materials and impact their key properties for applications such as gain and light emission. Despite their importance, the electronic structure of MX has yet to be fully characterized. MXs have a complex electronic structure arising from quantum many-body effects, which is challenging for both experiments and theory. Here, we report on the investigation of the electronic structure of MX in colloidal CdSe QDs using time-resolved photoluminescence, state-resolved pump-probe, and two-dimensional spectroscopies. The use of varying excitation energy and intensities enables the observation of many signals from biexcitons and triexcitons. The experiments enable the study of MX structures and dynamics on time scales spanning 6 orders of magnitude and directly reveal dynamics in the biexciton manifold. These results outline the limits of the simple concept of binding energy. The methods of investigations should be applicable to reveal complex many-body physics in other nanomaterials and low-dimensional materials of interest.

Investigating exciton structure and dynamics in colloidal CdSe quantum dots with two-dimensional electronic spectroscopy.

Two-Dimensional Electronic Spectroscopy (2DES) is performed on CdSe colloidal quantum dots. These experiments reveal new observations on exciton structure and dynamics in quantum dots, expanding upon prior transient absorption measurements of excitonics in these systems. The 2DES method enables the separation of line broadening mechanisms, thereby better revealing the excitonic lineshapes and biexcitonic interactions. 2DES enables more information rich spectral probing of coherent phonons and their coupling to excitons. The data show spectral modulations and drifts, with differences based upon whether one monitors the excitation energy (E1) or emission energy (E3). These measurements reveal both homogeneous and inhomogeneous broadenings, as well as static and dynamic line broadening. The longitudinal optical phonon modulates the dynamic absorption spectrum both in energy and linewidth. These experiments enable measurement of hot exciton cooling with improved resolution in energy and time. These 2DES results are consistent with prior excitonic state-resolved transient absorption measurements, albeit with the addition of contributions due to coherent phonons. Finally these 2DES experiments enable disentangling of coupling versus relaxation contributions to the signals, further offering a test of electronic structure theory.

Temperature behavior of the Kohlrausch exponent for a series of vinylic polymers modelled by an all-atomistic approach.

The Kohlrausch-Williams-Watt (KWW) function, or stretched exponential function, is usually employed to reveal the time dependence of the polymer backbone relaxation process, the so-called α relaxation, at different temperatures. In order to gain insight into polymer dynamics at temperatures higher than the glass transition temperature T(g), the behavior of the Kohlrausch exponent, which is a component of the KWW function, is studied for a series of vinylic polymers, using an all-atomistic simulation approach. Our data show very good agreement with published experimental results and can be described by existing phenomenological models. The Kohlrausch exponent exhibits a linear dependence with temperature until it reaches a constant value of 0.44, at 1.26T(g), revealing the existence of two regimes. These results suggest that, as the temperature increases, the dynamics progressively change until it reaches a plateau. The non-exponential character then describes subdiffusive motion characteristic of polymer melts.