Femtosecond Cooling of Hot Electrons in CdSe Quantum-Well Platelets

Abstract

Semiconductor quantum wells are ubiquitous in high-performance optoelectronic devices such as solar cells and lasers. Understanding and controlling of the (hot) carrier dynamics is essential to optimize their performance. Here, we study hot electron cooling in colloidal CdSe quantum-well nanoplatelets using ultrafast two-photon photoemission spectroscopy at low excitation intensities, resulting typically in 1-5 hot electrons per platelet. We observe initial electron cooling in the femtosecond time domain that slows down with decreasing electron energy and is finished within 2 ps. The cooling is considerably faster at cryogenic temperatures than at room temperature, and at least for the systems that we studied, independent of the thickness of the platelets (here 3-5 CdSe units) and the presence of a CdS shell. The cooling rates that we observe are orders of magnitude faster than reported for similar CdSe platelets under strong excitation. Our results are understood by a classic cooling mechanism with emission of longitudinal optical phonons without a significant influence of the surface.

Two‐Photon Photoemission Spectroscopy for Studying Energetics and Electron Dynamics at Semiconductor Interfaces

“…Relaxation is almost complete within 1 ps. Using a decay model described by Sippel et al the carrier temperature (not shown) at different times can be extracted providing the basis for the calculation of the energy loss rate (ELR) of hot electrons in dependence of temperature, see Figure (right). The temperature of the hot electron distribution drops from 3500 K at ≈150 fs to ≈650 K within 1 ps followed by slower relaxation on the picosecond time scale.…”

“…The temperature of the hot electron distribution drops from 3500 K at ≈150 fs to ≈650 K within 1 ps followed by slower relaxation on the picosecond time scale. The data can be fitted assuming a constant LO phonon scattering rate with a typical phonon scattering time of 33 fs, phonon energy ≈26 meV and a lattice temperature of 700 K according to the temperature dependent relaxation model described by Sippel et al At the lattice temperature of 700 K the displayed curve drops drastically indicating that the cooling via LO phonon emission is suppressed. The explanation for this is the buildup of a hot phonon distribution responsible for a retardation of carrier cooling .…”

Tuning the Optical Properties of Perovskite Nanoplatelets through Composition and Thickness by Ligand‐Assisted Exfoliation

“…Such quantum‐confined nanostructures have garnered significant interest since the first report on colloidal CdSe platelets and subsequent reports highlighting fascinating properties such as a giant oscillator strength transition, thickness‐dependent exciton binding energies of several hundreds of meV, narrow emission spectra and extremely short radiative decay times . The high quality of such NPls has already led to their promising application in light‐emitting applications . Despite these initial reports, there is still a need for the development of universal fabrication methods enabling high yield, high quality, and tunability of the emerging perovskite NPls.…”

Synthesis of Hybrid to Inorganic Quasi 2D‐Layered Perovskite Nanoparticles

“…Interestingly, when the concentration of CsI reaches to X=1, island‐ like structures appeared as presented in Figure S2f (supporting information). The folding in these sheets and weaker contrast suggests their thin structures which are in agreement with previous reports,, . The nanosheets are not separated, but are stacked on each other which can be seen by varying contrast in the images.…”