title: 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.
authors.affiliation: †Institute for Solar Fuels, Helmholtz-Zentrum Berlin für Materialien und Energie, Hahn-Meitner-Platz 1, D-14109 Berlin, Germany.
authors.forename: Philipp
authors.initials: P
authors.lastname: Sippel
authors.forename: Wiebke
authors.initials: W
authors.lastname: Albrecht
authors.forename: Johanna C
authors.initials: JC
authors.lastname: van der Bok
authors.forename: Relinde J A
authors.initials: RJ
authors.lastname: Van Dijk-Moes
authors.affiliation: ∥Institut für Physik, Technische Universität Ilmenau, Postfach 100565, D-98684 Ilmenau, Germany.
authors.forename: Thomas
authors.initials: T
authors.lastname: Hannappel
authors.affiliation: †Institute for Solar Fuels, Helmholtz-Zentrum Berlin für Materialien und Energie, Hahn-Meitner-Platz 1, D-14109 Berlin, Germany.
authors.forename: Rainer
authors.initials: R
authors.lastname: Eichberger
authors.forename: Daniel
authors.initials: D
authors.lastname: Vanmaekelbergh
keywords: CdSe; TR-2PPE; hot electron relaxation; photoemission; platelet; quantum well