Phonon-assisted up-conversion photoluminescence of quantum dots

Abstract

Phonon-assisted up-conversion photoluminescence can boost energy of an emission photon to be higher than that of the excitation photon by absorbing vibration energy (or phonons) of the emitter. Here, up-conversion photoluminescence power-conversion efficiency (power ratio between the emission and excitation photons) for CdSe/CdS core/shell quantum dots is observed to be beyond unity. Instead of commonly known defect-assisted up-conversion photoluminescence for colloidal quantum dots, temperature-dependent measurements and single-dot spectroscopy reveal the up-conversion photoluminescence and conventional down-conversion photoluminescence share the same electron-phonon coupled electronic states. Ultrafast spectroscopy results imply the thermalized excitons for up-conversion photoluminescence form within 200 fs, which is 100,000 times faster than the radiative recombination rate of the exciton. Results suggest that colloidal quantum dots can be exploited as efficient, stable, and cost-effective emitters for up-conversion photoluminescence in various applications.

Synthesis of Weakly Confined, Cube-Shaped, and Monodisperse Cadmium Chalcogenide Nanocrystals with Unexpected Photophysical Properties

“…Monoexponential PL decay lifetime increases upon increasing thickness of the CdS shells and reaches ∼200 ns for CdSe 12 /20CdS nanocrystals, indicating delocalization of the electron wavefunction into the CdS shells due to the quasi type-II band offsets between CdSe and CdS. 47 For the cube-shaped CdSe (Figure S8, Supporting Information) and CdSe/CdS core/shell (Figure 2c) nanocrystals, PL spectra excited by a photon with its energy either higher or lower than the peak energy�respectively known as downconversion PL and up-conversion PL 48 �overlap with each other. First, this suggests that the excellent control on the edge length, shape, and facet structure of the weakly confined cadmium chalcogenide nanocrystals results in monodisperse optical properties.…”

Anti-Stokes Photoluminescence Regulated by Lattice Distortion of CsPbX3 Nanocrystals in Glasses

“…The energy differences between the XB bands and PB bands are found to be ∼21 and ∼18 meV for the NG and WG specimens, respectively, corresponding to the exciton binding energy reported previously. , It is also noted that the spectral positions of these XB bands are independent of the SGE pump wavelength, and as a result, the XB bands are induced by the free exciton, instead of the localized exciton. The direct conversion of these localized excitons into free carriers is unlikely to occur since much more energy and involvement of multiple phonons are required to convert these distorted [PbBr 6 ] octahedron- trapped excitons into free carriers . However, these localized excitons can easily transfer to normal [PbBr 6 ] octahedra through phonon coupling and form free excitons.…”

Current Status and Challenges of Solar Cells Based on Semiconductor Nanocrystals

“…Under solar radiation, the illumination is insufficient to generate multiexcitons, and Auger nonradiative decay can thus be excluded usually. Different from organic dyes, phonon-assisted nonradiative decay of the excitons in QDs does not need to be considered, because the phonon energy of inorganic semiconductors is very low (usually ∼25 meV). , Overall, the competing processes involved in exciton decay within QD solar cells are mainly charge separation, radiative recombination, and charge-trapping-related nonradiative decay (Figure B, bottom panel). The quantitative measure of each process can be described by the rate constant k (or lifetime τ, k = 1/τ).…”