Photoluminescence of Charged CdSe/ZnS Quantum Dots in the Gas Phase: Effects of Charge and Heating on Absorption and Emission Probabilities

Abstract

Gas phase spectral measurements for CdSe/ZnS core/shell nanocrystal 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.

Single‐molecule mass spectrometry

“…This technology has enabled the Anderson group to study a number of interesting features of single isolated QDs, such as fluorescence blinking (Bell et al, ) as well as intense brightening and drastic changes to emission spectra upon heating with a laser (Howder et al, ). They have also developed a method to use a trapped QD to probe the m / z of a co‐trapped, dark particle of comparable size (Howder et al, ).…”

Higher Resolution Charge Detection Mass Spectrometry

“…To access this high mass regime, several MS-based approaches have been developed where masses are determined for individual ions instead of the ensemble approach of conventional MS . These methods fall into two main categories: charge shifting where the m / z is determined for a single ion, the charge is shifted through interaction with a laser beam or a chemical reagent, and then the m / z is remeasured. − These approaches are not well suited to mass measurements for the thousands of ions needed to construct a mass distribution. In the other category, the m / z and charge of each ion are measured simultaneously.…”

Dramatic Improvement in Sensitivity with Pulsed Mode Charge Detection Mass Spectrometry

“…1 These techniques include nanomechnical oscillators 2,3 and a variety of approaches based on more traditional mass spectrometry methods. [4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21] Charge detection mass spectrometry (CDMS), which offers a good compromise between resolution, accuracy, and measurement speed, is one of the more promising approaches. [20][21][22][23][24][25][26][27][28][29][30][31] In particular, CDMS has recently been used to measure the molecular weight distributions for a number of heterogeneous, high-mass samples including amyloid fibrils, 32 synthetic polymers, 33 nanoparticles, 34 gene therapy products, 35 and viruses and virus assembly intermediates.…”

“…Interest in measuring accurate masses for species with molecular weights much greater than 1 MDa has led to the development of a number of specialized single-particle techniques where masses are directly measured for individual molecules . These techniques include nanomechnical oscillators , and a variety of approaches based on more traditional mass spectrometry methods. − Charge detection mass spectrometry (CDMS), which offers a good compromise between resolution, accuracy, and measurement speed, is one of the more promising approaches. − In particular, CDMS has recently been used to measure the molecular weight distributions for a number of heterogeneous, high-mass samples including amyloid fibrils, synthetic polymers, nanoparticles, gene therapy products, and viruses and virus assembly intermediates. − …”