title: Observation of size dependence in multicolor upconversion in single Yb3+, Er3+ Codoped NaYF4 nanocrystals.
abstract: In this Letter we report on the investigation of the upconversion emission of single NaYF(4) nanocrystals codoped with Yb(3+) and Er(3+). Single nanocrystals on a coverslip are excited with continuous wave laser light at 973 nm in a confocal setup and the upconversion fluorescence is analyzed with a spectrometer. With the help of an atomic force microscope the size of the nanocrystals is simultaneously determined. A strong size-dependence of the spectral properties of the upconversion signal of individual nanocrystals is observed. We attribute this to a differing number of available phonons in the individual crystals for multiphonon relaxation processes, depending on their size. We believe that this result provides a new strategy in the synthesis of upconversion nanoparticles with different spectral properties by changing only their size as it is well-known from the case of semiconductor quantum dots.
authors.affiliation: Nano-Optics, Institute of Physics, Humboldt-Universität zu Berlin, Hausvogteiplatz 5-7, D-10117 Berlin, Germany. schietinger@physik.hu-berlin.de
authors.forename: Stefan
authors.initials: S
authors.lastname: Schietinger
authors.forename: Leonardo de S
authors.initials: Lde S
authors.lastname: Menezes
authors.forename: Björn
authors.initials: B
authors.lastname: Lauritzen
authors.forename: Oliver
authors.initials: O
authors.lastname: Benson