Exciton Recombination and Upconverted Photoluminescence in Colloidal CdSe Quantum Dots

Abstract

We studied photoluminescence (PL) of colloidal CdSe quantum dots (QDs) synthesized by a single-step method using cadmium oxide (CdO) and tri-n-octylphosphine selenide (TOPSe) as the Cd and Se sources, respectively, and tri-n-octylphosphine (TOP) as the reaction medium and subsequently dispersed in hexane. The QDs were excited by a narrow-band incoherent cw light source with photon energies near or below the first absorption maximum of the sample, and the resulting luminescence was dispersed and recorded. In the PL spectra, we identified two displaced by ∼16−38 meV strongly overlapping components with excitation energy dependent intensities. We also investigated lifetimes of the nonresonant PL decay at various excitation and emission energies using a frequency-domain method and observed multiexponential decay with three lifetimes approximately equal to 11.5(1.0), 41(2), and 155(15) ns. The low-energy component with the full width at half-maximum (fwhm) ranging from 135−155 meV was assigned to the charged exciton emission correlating with the shortest lifetime. The narrower high-energy component with the fwhm ranging from 80−90 meV was attributed to the band edge exciton emission with the middle lifetime. Photoexcitation of the QD sample in the onset of the absorption tail yielded blue-shifted (upconverted) luminescence. The blue-shifted PL was determined to result from a single-photon excitation. Therefore, we claimed that the upconversion process was thermally assisted. Numerical modeling showed the blue shift to be consistent with room temperature thermal phonon distribution in the colloidal QDs.