Atomic fluctuations in electronic materials revealed by dephasing

Abstract

The microscopic origin and timescale of the fluctuations of the energies of electronic states has a significant impact on the properties of interest of electronic materials, with implication in fields ranging from photovoltaic devices to quantum information processing. Spectroscopic investigations of coherent dynamics provide a direct measurement of electronic fluctuations. Modern multidimensional spectroscopy techniques allow the mapping of coherent processes along multiple time or frequency axes and thus allow unprecedented discrimination between different sources of electronic dephasing. Exploiting modern abilities in coherence mapping in both amplitude and phase, we unravel dissipative processes of electronic coherences in the model system of CdSe quantum dots (QDs). The method allows the assignment of the nature of the observed coherence as vibrational or electronic. The expected coherence maps are obtained for the coherent longitudinal optical (LO) phonon, which serves as an internal standard and confirms the sensitivity of the technique. Fast dephasing is observed between the first two exciton states, despite their shared electron state and common environment. This result is contrary to predictions of the standard effective mass model for these materials, in which the exciton levels are strongly correlated through a common size dependence. In contrast, the experiment is in agreement with ab initio molecular dynamics of a single QD. Electronic dephasing in these materials is thus dominated by the realistic electronic structure arising from fluctuations at the atomic level rather than static size distribution. The analysis of electronic dephasing thereby uniquely enables the study of electronic fluctuations in complex materials.

Observing Multiexciton Correlations in Colloidal Semiconductor Quantum Dots via Multiple-Quantum Two-Dimensional Fluorescence Spectroscopy

“…We discuss separately acquired 1Q−1Q 2D spectra in Section S1 in the Supporting Information. These 2D spectra feature coherent phonon oscillations that can be attributed to the longitudinal optical (LO) phonons of CdSe 24,33,37,38,40 and the ZnS shell, 63 in agreement with previous reports.…”

“…Note that we rationalize our results in terms of effective mass theory states and thus use a rather simplistic picture featuring a coarse manifold of discrete biexciton states. Indeed, theoretical treatment using the atomistic approach delivers a more realistic description of the electronic structure of quantum dots. , However, atomistic calculations predict a complicated quasi-continuum of fine-structure electron and hole states. On the other hand, at least for the lowest exciton states, both atomistic and effective mass theory yield comparable results. ,, Especially for well-passivated quantum dots with a ZnS shell like in our case, it turns out that lowest electron and hole levels are well separated, which justifies the use of a model with the first three single-exciton states (and the biexciton states resulting from these states) as discrete states.…”

“…Indeed, theoretical treatment using the atomistic approach delivers a more realistic description of the electronic structure of quantum dots. 24,74 However, atomistic calculations predict a complicated quasi-continuum of fine-structure electron and hole states. On the other hand, at least for the lowest exciton states, both atomistic and effective mass theory yield comparable results.…”

“…Since multiexcitonic states strongly dictate the nonlinear optical response of quantum dots, coherent two-dimensional (2D) spectroscopy is an excellent tool to study them. , Coherent 2D spectroscopy disentangles the nonlinear response over several frequency axes. Thus, 2D spectroscopy reveals correlation functions with high spectral and high temporal resolution, being further ideally suited to probe coherent superpositions of single excitons. − Coherent 2D spectroscopy was previously applied to colloidal quantum dot systems by detecting coherent third-order nonlinear signals, which correlate the single-exciton states of the system to each other. − ,,− This method enables one to probe multiexciton states indirectly by resolving the transitions from single-exciton states to the respective multiexciton states. Conceptually, this is similar to transient absorption spectroscopy, however with the advantage of an additional spectrally resolved pump axis .…”

Dynamic lattice distortions driven by surface trapping in semiconductor nanocrystals

“…A hot carrier is created at the expense of an annihilated exciton via an Auger process, hence substantially curbing the performance of NC lasers 18 and LEDs 19 . Although earlier works focused on identifying optical signatures associated with such nonradiative processes in NCs 20,21 , more recent works have begun to point to the fundamental role of dynamic structural fluctuations interrelated with nonradiative relaxation in NCs [22][23][24][25][26][27][28][29] . In this context, neutron…”