Nanoplatelet-Like Ultrafast Dephasing Behavior of CdS/CdSe/CdS Spherical Quantum Wells

Carrier dynamics in colloidal nanostructures has been the subject of scrutiny for several decades, given the possibility of tuning these materials’ optical and electronic properties by the proper choice of size, shape, and composition. The ever-growing possibilities regarding architecture and composition still translate into a great demand for a comprehensive understanding of the dynamics of the many-body phenomena involved in nanomaterial systems, especially coherent exciton dynamics, with potential applications to emerging quantum systems.<br/><br/>In this work we perform optical multidimensional coherent spectroscopy (MDCS) on different samples of CdS/CdSe/CdS colloidal spherical quantum wells (SQWs), probing their exciton dephasing times and population dynamics, at a series of cryogenic temperatures.<br/>Colloidal nanostructures, in general, have inherently large inhomogeneous spectral broadening due to the size distribution of the probed ensemble, hindering the characterization of the system’s homogeneous responses. In addition to the temporal resolution, common on nonlinear optical spectroscopies, MDCS allow the probing of homogeneous responses even in the presence of large size inhomogeneity. Additionally, by spectrally resolving the signal, one can measure coupling strengths between quantum states.<br/><br/>Spherical quantum wells are nanoparticles consisting of a nucleus, enveloped by a layer of different composition that, in turn, is covered by another layer of distinct composition. The middle layer (well) material (CdSe) has an optical gap smaller than the nucleus and the outer shell (CdS) subjecting the excitons to confinement in the middle layer, effectively creating a quantum well with spherical symmetry.<br/>The SQW architecture results in higher quantum-yields and absorption cross-sections when compared to their CdSe/CdS quantum dots counterparts. Additionally, our results indicate that the strong quantum confinement of the hole wave-function in the CdSe well indeed renders to excitons in such structures dephasing times comparable to those of CdSe nanoplates (NPLs).<br/><br/>Our results show that excitons confined in SQWs can become spatially separated along the spherical surface of the well analogous to the spatial separation of excitons across the lateral dimensions of CdSe NPLs, reducing the Auger recombination rates. The SQWs examined in this work differ in the width of the outer CdS shell, which translates in different strengths of the electron wave-function confinement, giving extra insight to the role of electron confinement on the exciton dephasing times.<br/><br/>We measured sub-ps dephasing times, at 6K, for our samples. These values are, at least, one order of magnitude shorter than those for spherical CdSe/CdS core/shell quantum dots. Additionally, we observe a decrease in dephasing time with increasing the electronic wavefunction delocalization by increasing the outer shell thickness, the opposite of what is reported on core/shell systems. Finally, our temperature series measurements corroborate with acoustic phonon population increase and coupling behaviors similar to core/shell quantum dots, indicating a similar (additional) dephasing route.