Publication date: 27th June 2014
Colloidal CdSe nanoplatelets of zincblende-type crystalline structure represent a novel material system of two-dimensional colloidal nanocrystals, i.e. colloidal quantum wells. In contrast to MBE grown quantum wells, those platelets show a much higher quantum yield and have thus a high potential for usage in photonic devices, like quantum well lasers or sensors. We study excitonic properties, electroabsorption, optical nonlinearities and exciton-phonon coupling in colloidal 2D semiconductor nanocrystals, i.e. CdSe nanoplatelets. Single-particle spectroscopy, temperature-dependent emission and ultrafast recombination dynamics have been measured along with numerical simulations of energy states to understand the influence of 2D quantization and lateral size variation on linear optical properties and exciton-phonon interaction. Single particle spectroscopy is applied to overcome inhomogeneous broadening in ensembles and reveals an excitonic substructure in the photoluminescence emission spectra (PL). Temperature-dependent measurements of the PL spectra show a small exciton-LO-phonon coupling strength in confined ZB systems and therefore a small phonon broadening in the single CdSe platelet emission. We present numerical calculations of the 2D-nanoplatelets quantum well exciton energies including Coulomb interaction and compare the obtained energies with experiments. Due to a large surface to volume ratio, the exciton energies show a strong impact of dielectric confinement. Coulomb interaction corrected numerical simulations reproduce this effect. The thickness quantization to integer monolayers leads to an only lateral size distribution, resulting in narrow PL emission.
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