Briefly, Au NPs with an average diameter of ∼100 nm were synthesized following a published protocol. For these reasons, the experimental data at the single QD level should be treated integratively with mathematical rigor.ĭetails on the fabrication of the Au NP-QD hybrid system have been described elsewhere 3 and also in Sec. 13 Consequently, conclusions drawn from a few “representative” QDs, a strategy many single QD studies utilized, may not depict the true characteristics of the whole hybrid system. The dot-to-dot variations are further magnified by the geometry of hybrid systems as how QDs are positioned near metal NPs will substantially affect the plasmon–exciton interaction. Beyond the single QD level, the PL characteristics of QDs exhibit dot-to-dot variations arising from the inherent inhomogeneities in their crystal structure and surfaces. 11,12 The non-ergodic blinking combined with correlated intensity and PL decay could yield varied results of the PL decay curve of a single QD with finite acquisition time. 10 The intensity and PL decay of a single QD are usually correlated, that the PL decay corresponding to a higher intensity level is typically slower. Single QDs exhibit notorious fluorescence intensity intermittency or “blinking,” 9 and this phenomenon is non-ergodic. The missing of the studies at the single QD level probably comes from the difficulties in analyzing and interpreting the data. The ensemble averaged results may lead to ambiguity 8 in interpreting the plasmonic effect on the PL decay mechanism. So far, the published studies on this excitation wavelength dependence were conducted at the ensemble level and a mechanistic understanding on the phenomenon is lacking. 4–7 This phenomenon seems to break the well-known Kasha’s rule however, the mechanism is unclear. Several groups have reported this excitation wavelength dependence at the ensemble level that the PL decay of QDs near plasmonic nanostructures tends to be faster when excited spectrally close to the LSPR peak. 3 Changing the excitation wavelength also has an impact on the PL decay of QDs near plasmonic nanostructures. For example, it was found that when the excitation wavelength overlaps with the LSPR peak of gold (Au) NPs, the statistics of the photons emitted from nearby single QDs could switch from anti-bunched to bunched. Alternatively, when the geometry and composition of a plasmonic metal NP-QD hybrid system are fixed, changing the excitation wavelength provides a simple but effective strategy to modulate the photophysical characteristics of QDs since the plasmonic effect is highly excitation wavelength-dependent. Plasmonic modulation of QD PL is often realized via the geometry and composition of the hybrid structure, which requires much synthetic effort. Thus, the overall effect of plasmonic NPs on PL intensity and lifetime of QDs is a result of multiple factors. Moreover, metal NPs could also accept energy or charges from the excited QDs, providing additional non-radiative recombination pathways to quench the photoluminescence (PL) of QDs. At the meantime, a plasmonic metal NP could also accelerate the radiative excitonic recombination of nearby QDs through Purcell effect. 2 The created electric field near a metal NP will then enhance the absorption of fluorophores, such as QDs in the vicinity. 1 Plasmonic metal NPs are known for localized surface plasmon resonance (LSPR) that describes the collective oscillation of the surface conduction electrons upon electromagnetic excitation. Hybrid nanosystems consisting of plasmonic metal nanoparticles (NPs) and quantum dots (QDs) have been of tremendous interest for QD-related optical and optoelectronic applications, largely because plasmonic NPs offer the flexibility of modulating the photophysical properties of nearby fluorescent QDs for desired applications.
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