We present a study showing cooperative behavior of light emitting quantum dots at room temperature, with large increases in radiative decay rates and efficiencies, in the presence of small gold nanoparticles (1.5 - 4 nm radii) in low fractions. This is a size-regime of metal particles where the expected effect on emission from independent emitters is vain non-radiative loss. But the addition of such metal particles in low fractions induces a strong evolution of the super-radiant modes of emission among quantum dots and aids their survival of thermal fluctuations; exhibiting a phase transition. While an increase of size of metal particles results in an increase in local thermal fluctuations to revert to the behavior of apparently independent emitters. Our theoretical evaluations of their possible collective modes of emission in the presence of metal nanoparticles predict such experimental observations. Two different types of self-assembled nanoscale structures containing quantum dots were experimentally studied. This included the effect of the fractions and size of metal particles on the collective modes of emission in each type of structure; each type of structure had samples of different nominal sizes (and emission energies) of dots to establish generality. First, quantum dots collected in cylindrical cavities surrounded by randomly located gold particles were experimentally studied in large ensembles using polymer templates. The other type of nanostructure was a colloidal monolayer of quantum dots closely packed along with small gold nanoparticles. A cross-over between collective and independent regimes is observed based on the size of metal particles, and also at larger number fractions in the closely packed structure. Time-resolved photoluminescence measurements were also used to confirm this increase in the quantum efficiency and radiative decay rates of the dots.
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