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Principles Of Fluorescence Spectroscopy

Core–shell nanoparticles (CSNs) are a class of nanostructured materials that have recently received increased attention owing to their interesting properties and broad range of applications in catalysis, biology, materials chemistry and sensors. By rationally tuning the cores as well as the shells of such materials, a range of core–shell nanoparticles can be produced with tailorable properties that can play important roles in various catalytic processes and offer sustainable solutions to current energy problems. Various synthetic methods for preparing different classes of CSNs, including the Stober method, solvothermal method, one-pot synthetic method involving surfactants, etc., are briefly mentioned here. The roles of various classes of CSNs are exemplified for both catalytic and electrocatalytic applications, including oxidation, reduction, coupling reactions, etc.

Core–shell nanoparticles: synthesis and applications in catalysis and electrocatalysis

This review captures the synthesis, assembly, properties, and applications of copper chalcogenide NCs, which have achieved significant research interest in the last decade due to their compositional and structural versatility. The outstanding functional properties of these materials stems from the relationship between their band structure and defect concentration, including charge carrier concentration and electronic conductivity character, which consequently affects their optoelectronic, optical, and plasmonic properties. This, combined with several metastable crystal phases and stoichiometries and the low energy of formation of defects, makes the reproducible synthesis of these materials, with tunable parameters, remarkable. Further to this, the review captures the progress of the hierarchical assembly of these NCs, which bridges the link between their discrete and collective properties. Their ubiquitous application set has cross-cut energy conversion (photovoltaics, photocatalysis, thermoelectrics), en...

Compound Copper Chalcogenide Nanocrystals

Photoluminescence blinking—random switching between states of high (ON) and low (OFF) emissivities—is a universal property of molecular emitters found in dyes, polymers, biological molecules and artificial nanostructures such as nanocrystal quantum dots, carbon nanotubes and nanowires. For the past 15 years, colloidal nanocrystals have been used as a model system to study this phenomenon. The occurrence of OFF periods in nanocrystal emission has been commonly attributed to the presence of an additional charge, which leads to photoluminescence quenching by non-radiative recombination (the Auger mechanism). However, this ‘charging’ model was recently challenged in several reports. Here we report time-resolved photoluminescence studies of individual nanocrystal quantum dots performed while electrochemically controlling the degree of their charging, with the goal of clarifying the role of charging in blinking. We find that two distinct types of blinking are possible: conventional (A-type) blinking due to charging and discharging of the nanocrystal core, in which lower photoluminescence intensities correlate with shorter photoluminescence lifetimes; and a second sort (B-type), in which large changes in the emission intensity are not accompanied by significant changes in emission dynamics. We attribute B-type blinking to charge fluctuations in the electron-accepting surface sites. When unoccupied, these sites intercept ‘hot’ electrons before they relax into emitting core states. Both blinking mechanisms can be electrochemically controlled and completely suppressed by application of an appropriate potential.

https://absimage.aps.org/image/MAR12/MWS_MAR12-2011-002227.pdf

Two types of luminescence blinking revealed by spectroelectrochemistry of single quantum dots

Quantum dots are a unique class of emitters with size-tunable emission wavelengths, saturated emission colors, near-unity luminance efficiency, inherent photo- and thermal- stability and excellent solution processability. Quantum dots have been used as down-converters for back-lighting in liquid-crystal displays to improve color gamut, leading to the booming of quantum-dot televisions in consumer market. In the past few years, efficiency and lifetime of electroluminescence devices based on quantum dots achieved tremendous progress. These encouraging facts foreshadow the commercialization of quantum-dot light-emitting diodes (QLEDs), which promises an unprecedented generation of cost-effective, large-area, energy-saving, wide-color-gamut, ultra-thin and flexible displays. Here we provide a Progress Report, covering interdisciplinary aspects including material chemistry of quantum dots and charge-transporting layers, optimization and mechanism studies of prototype devices and processing techniques to produce large-area and high-resolution red-green-blue pixel arrays. We also identify a few key challenges facing the development of active-matrix QLED displays.

Quantum-Dot Light-Emitting Diodes for Large-Area Displays: Towards the Dawn of Commercialization.

A series of Mn2+-doped CsPbCl3 nanocrystals (NCs) was synthesized using reaction temperature and precursor concentration to tune Mn2+ concentrations up to 14%, and then studied using variable-temperature photoluminescence (PL) spectroscopy. All doped NCs show Mn2+ 4T1g → 6A1g d–d luminescence within the optical gap coexisting with excitonic luminescence at the NC absorption edge. Room-temperature Mn2+ PL quantum yields increase with increased doping, reaching ∼60% at ∼3 ± 1% Mn2+ before decreasing at higher concentrations. The low-doping regime is characterized by single-exponential PL decay with a concentration-independent lifetime of 1.8 ms, reflecting efficient luminescence of isolated Mn2+. At elevated doping, the decay is shorter, multiexponential, and concentration-dependent, reflecting the introduction of Mn2+–Mn2+ dimers and energy migration to traps. A large, anomalous decrease in Mn2+ PL intensity is observed with decreasing temperature, stemming from the strongly temperature-dependent exciton l...

Photoluminescence Temperature Dependence, Dynamics, and Quantum Efficiencies in Mn2+-Doped CsPbCl3 Perovskite Nanocrystals with Varied Dopant Concentration

Temperature-dependent photoluminescence (PL) properties of inorganic perovskite CsPbBr3 nanocrystal (NC) films were studied by using steady-state and time-resolved PL spectroscopy. The closely packed solid films were obtained by dropping NC solution on silicon substrates. It was found that the PL intensities of the NC films, which are dependent on the size of NCs, slightly decreased with increasing temperature to 300 K, while the PL intensities dropped rapidly with increasing temperature above 300 K and were nearly quenched at 360 K. Further the corresponding average PL lifetimes increased significantly with increasing temperature below about 320 K and then significantly became shorter. The PL quenching mechanisms were demonstrated through heating and cooling experiments. The experimental results indicated inorganic perovskite NCs exhibited a thermal PL quenching in the temperature range of 80–300 K and a thermal degradation at temperatures above 300 K. The linewidths, peak energies, and lifetimes of PL emissions for the NC films as a function of temperature were discussed in detail.

Temperature-dependent photoluminescence of inorganic perovskite nanocrystal films

The luminescence properties of inorganic perovskite CsPbBr3 nanocrystals (NCs) with emissions of 492 and 517 nm under thermal annealing treatment were studied by temperature-dependent photoluminescence (PL) spectroscopy. The CsPbBr3 NCs were annealed in vacuum at various temperatures. It was found that the NCs exhibited significant thermal degradation of PL at thermal annealing temperatures above 320 K. The transmission electron microscopy, X-ray diffraction and PL spectroscopy demonstrated that the size of NCs almost kept constant at thermal annealing temperatures below 360 K while it significantly enlarged at higher thermal temperatures above 380 K. The PL intensities, peak energies and linewidths of the annealed NCs, as a function of temperature, are discussed in detail. The PL degradation of the NCs was related to the formation of nonradiative recombination centers due to the loss of ligands and growth of NCs under thermal annealing.

Thermal degradation of luminescence in inorganic perovskite CsPbBr3 nanocrystals

We have studied the mechanism of photoluminescence (PL) from MnS/ZnS core/shell quantum dots (QDs) synthesized via hot solution phase chemistry using a nucleation−doping strategy. Efficient PL of the Mn2+ ions with a quantum yield (QY) of over 35% is demonstrated in the resulting QDs coated with a thick ZnS shell on the MnS core. The MnS/ZnS core/shell QDs with a thick shell exhibit PL enhancement with increasing temperature in the range between 140 and 300 K, resulting from the thermal activation of charge carriers localized at the interface between the MnS core and the ZnS shell. The PL decays of the Mn2+ ions in the core/shell QDs consist of three exponential components with time constants on the scales of 1−2 ms, hundreds of μs, and tens of μs. Surprisingly, the PL lifetimes of Mn2+ ions show a very weak dependence on the shell thickness, which is clearly different from that of the PL QY of the QDs. The experimental results indicate that the mechanism for improving the PL QY in MnS/ZnS QDs can be unde...

Efficient Photoluminescence of Mn2+ Ions in MnS/ZnS Core/Shell Quantum Dots

The thermal stability of luminescence is important for the application of quantum dots (QDs) in light-emitting devices. The temperature-dependent photoluminescence (PL) intensities and decay times of Mn-doped ZnS, ZnSe, and ZnSeS alloyed core–shell QD films were studied in the temperature range from 80 to 500 K by steady-state and time-resolved PL spectroscopy. It was found that the thermal stability of Mn-doped QD emissions was significantly dependent on the shell thickness and the host bandgap, which was higher than that of workhorse CdSe QDs. Nearly no PL quenching took place in Mn:ZnS QDs with a thick ZnS shell, which kept a high PL quantum yield (QY) of ∼50% even at 500 K; and the thermally stable PL was also observed in highly luminescent Mn:ZnSe and Mn:ZnSeS QDs with a quenching temperature over 200 °C. Further, the stability of Mn-doped QDs with different shell thickness at high temperature was also examined through heating–cooling cycling experiments. The PL quenching in the thick shell-coated Mn-doped QDs was almost totally recovered. The PL quenching mechanisms of the Mn2+ ion emissions were discussed.

Xi Yuan

Papers

Photoluminescence Temperature Dependence, Dynamics, and Quantum Efficiencies in Mn2+-Doped CsPbCl3 Perovskite Nanocrystals with Varied Dopant Concentration

Temperature-dependent photoluminescence of inorganic perovskite nanocrystal films

Thermal degradation of luminescence in inorganic perovskite CsPbBr3 nanocrystals

Efficient Photoluminescence of Mn2+ Ions in MnS/ZnS Core/Shell Quantum Dots

Thermal stability of Mn2+ ion luminescence in Mn-doped core–shell quantum dots