Ultra-narrow and widely tunable Mn^(2+) Emission from Single Nanocrystals of ZnS-CdS alloy

Extensively studied Mn-doped semiconductor nanocrystals have invariably exhibited photoluminescence (PL) over a narrow energy window of width <= 149 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.Comment: 5 pages, 5 figure

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

Photo-luminescence intermittency (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 signal, and yet, the vast majority of single-NC blinking data are analyzed using a single fixed threshold, which generates binary trajectories. Further, blinking dynamics can vary dramatically over NCs in the ensemble, and it is unclear whether the exponents (m) of single-particle On-/Off-time distributions (P(t)-On/Off), which are used to validate mechanistic models of blinking, are narrowly distributed or not. Here, we sub-classify an ensemble based on the emissivity of QDs, and subsequently compare the (sub)ensemble behaviors. To achieve this, we analyzed a large number (>1000) of intensity 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 m). Interestingly, we find that single QD P(t)-On/Off s follow either truncated power law or power law, and their relative proportion vary over sub-populations. Our results reveal a remarkable variation in m(On/Off) amongst as well as within sub-ensembles, which implies multiple blinking mechanisms being operational among various QDs. We further show that the m(On/Off) obtained via cumulative single-particle P(t)-On/Off is clearly distinct from the weighted mean value of all single-particle m(On/Off), an evidence for the lack of ergodicity. Thus, investigation and analyses of a large number of QDs, albeit for a limited time-span of few decades, is crucial to characterize possible blinking mechanisms and heterogeneity thereinComment: 29 pages including supporting information (single file), 7 main figures, 10 supporting figures and table

Correlated Super-Resolution Fluorescence and Electron Microscopy Reveals the Catalytically Active Nanorods within Individual H-ZSM-22 Zeolite Particles

Nanocrystalline zeolite particles are applied in a wide range of catalytic reactions. Their size, shape, and the distribution of catalytically active sites, significantly varies throughout zeolite batches and within individual particles. This variability leads to a heterogeneous distribution of catalyst performance. Directly investigating the structure-activity relationship at the nanoscale is essential for the rational improvement of catalyst materials. In this work, integrated fluorescence and electron microscope is employed to correlatively study the structure and performance of individual H-ZSM-22 particles. The needle-shaped morphology of these zeolite particles originates from the lateral fusion of multiple elementary nanorods. Indirect bulk scale experiments have suggested that this process converts catalytically inactive external Al into catalytically active internal Al. The correlative investigation performed in this research provides direct evidence that this conversion takes place, as an inactive shell of 20 - 40 nm thickness is observed and reactivity is confined to the crystal core. Furthermore, within the catalyst particles nanometer scale catalytic hotspots have been revealed and they are assumed to result from the presence of structural imperfections that locally increase accessibility into the microporous structure. Linear polarized excitation light experiments confirmed that catalytic transformations exclusively occurred on acid sites confined within the microporous structure.status: accepte

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

Distance-Dependent Energy Transfer between Ga₂O₃ Nanocrystal Defect States and Conjugated Organic Fluorophores in Hybrid White-Light-Emitting Nanophosphors

We report a quantitative analysis and development of hybrid white-light-emitting nanoconjugates, prepared by functionalizing colloidal γ-Ga₂O₃ nanocrystals with selected organic fluorophores. Using the Förster resonance energy transfer (FRET) formalism, we studied the coupling of native defect states in Ga₂O₃ nanocrystals, as energy donors, with different orange-red-emitting fluorophores bound to nanocrystal surfaces, as energy acceptors. Variations in the average nanocrystal size and dye surface coverage were used to characterize the efficiency of the energy transfer process and the corresponding donor–acceptor separations. The results show that for approximately three rhodamine B molecules per nanocrystal the energy transfer efficiency increases from 23% to 49% by decreasing the NC size from 5.3 to 3.6 nm. These FRET efficiencies correspond to the estimated donor–acceptor distances of 3.55 ± 0.02 and 2.99 ± 0.03 nm, respectively. Similar trends were observed for ATTO 590-conjugated Ga₂O₃ nanocrystals, although ATTO 590 proved to be a more effective energy acceptor owing to a larger molar extinction coefficient in the conjugated form. The size-dependent luminescence of Ga₂O₃ nanocrystals and the control of FRET parameters through the variations in the bound dye molecules allow for the generation of tunable blue-orange emission, ultimately resulting in white light with targeted chromaticity and high color rendition

Hybrid ZnO-Based Nanoconjugate for Efficient and Sustainable White Light Generation

Developing new ways of generating white light is of paramount importance for the design of the next generation of smart, energy-efficient lighting sources. Here we report tunable white light emission of hybrid organic–inorganic nanostructures based on colloidal ZnO nanocrystals conjugated with organic fluorophores. These materials act as single nanophosphors owing to the distance-dependent energy transfer between the two components. The defect-based size-tunable ZnO nanocrystal blue-green emission coupled with complementary color emission from different fluorophores allows for the generation of white light with targeted chromaticity, color temperature, and color rendering index. We further show that silane layer-protected nanoconjugates result in increased stability of white light emission over a long period of time. The results of this work demonstrate an inexpensive, green, and sustainable approach to general solid-state lighting, without the use of rare earth or heavy metals. Colloidal form of the reported hybrid nanoconjugates allows for their further functionalization or incorporation into light-emitting devices. More broadly, size dependence of the electronic structure of native defects in transparent metal oxide nanocrystals and their electronic coupling with conjugated organic species could also represent a vehicle for introducing and manipulating new properties in these hybrid nanostructures

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.status: publishe

Dual Europium Luminescence Centers in Colloidal Ga₂O₃ Nanocrystals: Controlled <i>in Situ</i> Reduction of Eu(III) and Stabilization of Eu(II)

Introducing multiple luminescent centers into colloidal nanocrystals is an attractive way to impart new optical properties into this class of materials. Doping disparate ions into specific nanocrystals is often challenging, due to the preferential incorporation of one type of dopant. Here, we demonstrate the coexistence of europium dopants as divalent and trivalent ions in colloidal Ga₂O₃ nanocrystals, achieved by controlled <i>in situ</i> reduction of Eu³⁺ to Eu²⁺. The two dopant species exhibit distinctly different steady-state and time-resolved photoluminescence, and their ratio can be modified via doping concentration, reaction temperature, or thermal treatment of as-synthesized NCs. The Eu²⁺ ions are proposed to be stabilized internally owing to the attractive interaction with oxygen vacancies, while Eu³⁺ dopants partly reside in the nanocrystal surface region. The relationship between the electronic structure of the native defects and the dopant centers is discussed in the context of the overall emission properties. The exposure of these samples to X-ray radiation leads to the reduction of Eu³⁺ to Eu²⁺, demonstrating an alternative way of manipulating the oxidation state and suggesting the potential application of this material as an X-ray storage phosphor. The coexistence of Eu²⁺ and Eu³⁺ and the ability to control their relative fraction over the full oxidation state range in group III oxide nanocrystals allow for the design and preparation of new photonic and light emitting materials

Manifestations of Varying Grading Level in CdSe/ZnSe Core–Shell Nanocrystals

Graded interface core–shell nanocrystals (NCs) are being investigated to attain highly luminescent systems and are also anticipated to be “blinking-free” at the single-particle level. In the present work, CdSe/ZnSe-graded core–shell NCs with varying confinement potential profile are studied at the single-particle level. The internal structure of NCs is determined with the aid of optical, structural, and chemical probes. Notably, the radiative lifetime for different nanostructures decays monoexponentially. The variation in the radiative lifetime due to differing internal structure is understood on the basis of recently reported first-principles study on different interfaces of core–shell NCs, in particular attributed to varying overlap of electron–hole wave functions. The single NC measurements reveal that the percentage of nonblinking NCs is higher for slowly varying confinement potential. Statistically suppressed blinking (∼80%) and a single exponential PL decay curve, accompanied by a very narrow (∼30 meV) emission line at the single NC level, are observed in graded CdSe/ZnSe core–shell NCs