ARTICLE
Photoluminescence of Charged CdSe/ZnS Quantum Dots in the Gas Phase: Effects of Charge and Heating on Absorption and Emission Probabilities Collin R. Howder, Bryan A. Long, David M. Bell, Kevin H. Furakawa, Ryan C. Johnson, Zhiyuan Fang, and Scott L. Anderson* Department of Chemistry, University of Utah, Salt Lake City, Utah 84112, United States
ABSTRACT Gas phase spectral measurements for CdSe/ZnS core/shell nano-
crystal quantum dots (QDs) before and after heating with both infrared (CO2) and visible lasers are reported. As-trapped QDs are spectrally similar to the same QDs in solution; however their photoluminescence (PL) intensities are very low, at least partly due to low absorption cross sections. After heating, the PL intensities brighten by factors ranging from ∼4 to 1800 depending on the QD size and pump laser wavelength. The emission spectra no longer resemble solution spectra and are similar, regardless of the QD diameter. Emission extends from the pump laser wavelength into the near-IR, with strong emission features above the band gap energy, between 645 and 775 nm, and in the near-infrared. Emission spectra from brightened QD ensembles, single QD aggregates, and single QD monomers are similar, showing that even single QDs support PL from a wide variety of states. The heating and cooling processes for QDs in this environment are analyzed, providing limits on the magnitudes of the absorption cross sections before and after thermal brightening. A model, based on absorption bleaching by extra electrons in the conduction band, appears to account for the changes in absorption and emission behavior induced by charging and heating. KEYWORDS: quantum dots . ion trap . single particle . mass spectrometry
A
ccurate, nondestructive mass determination for trapped nanoparticles (NPs) enables a number of interesting experiments, such as measuring kinetics for NP surface reactions by monitoring mass vs time as an NP is heated or exposed to reactants1 or studying spectral properties of single NPs in the gas phase. Several groups have reported experiments wherein single NPs were trapped in a quadrupole (Paul) trap,14 with continuous determination of the mass (M) and charge (Q) by monitoring the motional frequency of the trapped NP. For large NPs, motion is easily monitored by scattering of a low-power laser focused through the trap, and NPs that are too small for detection by light scattering (
