Plasmonic Enhancement of Mn2+ Luminescence and Application of Temperature-Dependent Mn2+ Luminescence

Abstract

Doping semiconductor nanocrystals, commonly known as quantum dots (QDs), has attracted significant attention from the scientific community due to the highly tunable nature of the physical properties, such as optical, electrical, opto-magnetic properties, with respect to both size and dopant type/concentration. In this dissertation, Mn-doped CdS/ZnS (core/shell) QDs were used as a model system to study the characteristics of dopant luminescence coupled with plasmonic metal nanoparticles (MNPs) and its application as a nano-thermometer using temperature dependent Mn luminescence.

In the first part of this dissertation, plasmon-enhanced Mn luminescence from the Mn-doped CdS/ZnS QDs near plasmonic MNPs was studied. Rapid intraparticle energy transfer between exciton and Mn, occurring on a few picoseconds time scale, separates the absorber (exciton) from the emitter (Mn), whose emission is detuned far from the plasmonic absorption of the MNP. The rapid temporal separation of the absorber and emitter combined with the reduced spectral overlap between Mn and plasmonic MNP suppresses the quenching of the luminescence while taking advantage of the plasmon-enhanced excitation. The plasmon enhancement of exciton and Mn luminescence intensities in undoped and doped QDs were simultaneously compared as a function of the distance between MNP and QD layers in a multilayer structure to examine the expected advantage of the reduced quenching in the sensitized luminescence. At the optimum MNP-QD layer distance, Mn luminescence exhibits stronger net enhancement (ca. twice) than that of the exciton, which can be explained with a model incorporating fast sensitization along with reduced emitter-MNP spectral overlap.

In the second part, ratiometric thermometry on Mn luminescence spectrum was performed using Mn-doped CdS/ZnS core/shell QDs that have a large local lattice strain on Mn site, which results in the enhanced temperature dependence of the bandwidth and peak position. Mn luminescence spectral lineshape is highly robust with respect to the change in the polarity, phase and pH of the surrounding medium and aggregation of the QDs, showing great potential in temperature imaging under chemically heterogeneous environment. The temperature sensitivity (ΔIR/IR = 0.5%/K at 293 K, IR = intensity ratio at two different wavelengths) is highly linear in a wide range of temperatures from cryogenic to above-ambient temperatures. Surface temperature imaging was demonstrated on a cryo-cooling device showing the temperature variation of ~200 K (77–260 K) by imaging the luminescence of the QD film formed by simple spin coating, taking advantage of the environment-insensitive luminescence.