Insights on heterogeneity in blinking mechanisms and non-ergodicity using sub-ensemble statistical analysis of single quantum-dots.

Photo-luminescence (P-L) intermittency (or blinking) in semiconductor nanocrystals (NCs), a phenomenon ubiquitous to single-emitters, is generally considered to be temporally random intensity fluctuations between "bright" ("On") and "dark" ("Off") states. However, individual quantum-dots (QDs) rarely exhibit such telegraphic signals, and yet, a vast majority of single-NC blinking data are analyzed using a single fixed threshold which generates binary trajectories. Furthermore, while blinking dynamics can vary dramatically over NCs in the ensemble, the extent of diversity in the exponents (mOn/Off) of single-particle On-/Off-time distributions (P(tOn/Off)), often used to validate mechanistic models of blinking, remains unclear due to a lack of statistically relevant data sets. Here, we subclassify an ensemble of QDs based on the emissivity of each emitter and subsequently compare the (sub)ensembles' behaviors. To achieve this, we analyzed a large number (>1000) of blinking trajectories for a model system, Mn+2 doped ZnCdS QDs, which exhibits diverse blinking dynamics. An intensity histogram dependent thresholding method allowed us to construct distributions of relevant blinking parameters (such as mOn/Off). Interestingly, we find that single QD P(tOn/Off)s follow either truncated power law or power law, and their relative proportion varies over subpopulations. Our results reveal a remarkable variation in mOn/Off amongst as well as within subensembles, which implies multiple blinking mechanisms being operational amongst various QDs. We further show that the mOn/Off obtained via cumulative single-particle P(tOn/Off) is distinct from the weighted mean value of all single-particle mOn/Off, evidence for the lack of ergodicity. Thus, investigation and analyses of a large number of QDs, albeit for a limited time span of a few decades, are crucial to characterize the spatial heterogeneity in possible blinking mechanisms.

Ultranarrow and widely tunable Mn2+-Induced photoluminescence from single Mn-doped nanocrystals of ZnS-CdS alloys.

Extensively studied Mn-doped semiconductor nanocrystals have invariably exhibited photoluminescence over a narrow energy window of width ≤150 meV in the orange-red region and a surprisingly large spectral width (≥180 meV), contrary to its presumed atomic-like origin. Carrying out emission measurements on individual single nanocrystals and supported by ab initio calculations, we show that Mn PL emission, in fact, can (i) vary over a much wider range (∼370 meV) covering the deep green--deep red region and (ii) exhibit widths substantially lower (∼60-75 meV) than reported so far, opening newer application possibilities and requiring a fundamental shift in our perception of the emission from Mn-doped semiconductor nanocrystals.

Solvent Polarity-Induced Pore Selectivity in H-ZSM-5 Catalysis.

Molecular-sized micropores of ZSM-5 zeolite catalysts provide spatial restrictions around catalytic sites that allow for shape-selective catalysis. However, the fact that ZSM-5 has two main pore systems with different geometries is relatively unexploited as a potential source of additional shape selectivity. Here, we use confocal laser-scanning microscopy to show that by changing the polarity of the solvent, the acid-catalyzed furfuryl alcohol oligomerization can be directed to selectively occur within either of two locations in the microporous network. This finding is confirmed for H-ZSM-5 particles with different Si/Al ratios and indicates a general trend for shape-selective catalytic reactions.

Native defects determine phase-dependent photoluminescence behavior of Eu(2+) and Eu(3+) in In2O3 nanocrystals.

We demonstrate the coexistence of Eu(2+) and Eu(3+) in corundum and bixbyite-type colloidal In2O3 nanocrystals. The emission properties of dopants in both oxidation states are determined by their interaction with native defects, and are dramatically different in the two nanocrystal phases. This difference arises from the smaller nanocrystal size and higher defect density in metastable corundum-type nanocrystals.

Spectrally Resolved Photoluminescence Imaging of ZnO Nanocrystals at Single-Particle Levels.

The intrinsic spectral line widths of defect-related transitions in quantum-confined semiconductor nanocrystals are often difficult to estimate using ensemble measurements because the extent of inhomogeneous broadening due to particle size distributions is not known precisely. To address this problem, we performed spectrally resolved photoluminescence (PL) microscopy of individual ZnO NC by directly populating the defects states using low-energy laser excitation. The temporal evolution of PL intensities shows discrete blinking behaviors, suggesting that the NCs are detected near single-particle levels. The transition energies of individual NCs are found to fluctuate around their mean position (2.25 eV) by ∼0.130 eV, which is attributed to particle size distribution and defects densities associated with each NC. The spectral line width associated with defect emission envelope of ZnO NCs is found to be inherently broad (200-400 meV), which further establishes the presence of multiple closely spaced defect energy levels within every ZnO NC.