Following growth doping technique, highly luminescent (quantum yield > 50%) Mn-doped ZnS nanocrystals are synthesized via colloidal synthetic technique. The dopant emission has been optimized with varying reaction parameters and found the ratio of Zn to S as well as the percentage of introduced dopant in the reaction mixture are key factors for controlling the intensity. The method is simple, hassle free, and can be scalable to gram level without hindering the quality of nanocrystals. These nanocrystals retain their emission during various ligand exchange processes and aqueous dispersion.

Following growth doping strategy and using dopant oxides nanocrystals as dopant sources, we report here two different transition-metal ions doped in a variety of group II-VI semiconductor nanocrystals. Using manganese oxide and copper oxide nanocrystals as corresponding dopant sources, intense photoluminescence emission over a wide range of wavelength has been observed for different host nanocrystals. Interestingly, this single doping strategy is successful in providing such highly emissive nanocrystals considered here, in contrast with the literature reports that would suggest synthesis strategies to be highly specific to the particular dopant, host, or both. We investigate and discuss the possible mechanism of the doping process, supporting the migration of dopant ions from dopant oxide nanocrystals to host nanocrystals as the most likely scenario. SECTION Nanoparticles and Nanostructures

We report a reversible phase transformation of platelet-shaped ZnS nanostructures between wurtzite (WZ) and zinc blende (ZB) phases by reversible insertion/ejection of dopant Mn(II) ions induced by a thermocyclic process. In a reaction flask loaded with WZ ZnS platelets and Mn molecular precursors, during heating Mn ions are incorporated and change the phase of the host nanostructures to ZB; during cooling Mn ions are spontaneously ejected, returning the host nanoplatelets to the original WZ phase. These reversible changes are monitored for several cycles with PL, EPR, XRD, and HRTEM. Interestingly, the (0001) WZ platelets transform to (110) ZB following a nucleation and growth process triggered by a local increase/depletion of the Mn(2+) concentration in the nanocrystals.

The photoluminescence (PL) of metal nanoclusters (NCs), originating from their molecule-like electronic structure, is one of the most intriguing properties of NCs. Although various strategies such as tailoring the size, structure, and chemical environment of NCs have shown to improve the PL, their quantum yields (QYs) are still lagging far behind those of conventional luminescent materials, including quantum dots and organic fluorophores. Herein, we report the synthesis of highly luminescent gold cluster assembly (GCA) from Zn2+-ion-mediated assembly of Au4(SRCOO–)4 clusters using mercaptocarboxylic acid as a protective ligand and reductant as well as a growth suppressor. The synergetic combination of unique aurophilic interactions among Au4 clusters and the rigidified chemical environment induced by metal ion chelation through carboxylate groups is responsible for the ultrabright greenish-blue fluorescence with a QY up to 90%. Furthermore, the unique flexibility of dis/reassembly and the aggregation-dependent strong fluorescence of GCA offer a great potential for applications in biodegradable and trackable drug delivery systems.

Synthesis of light-emitting dispersed semiconductor nanocrystals with tunable emission has been widely studied in the last two decades because of their potential applications in photovoltaics, optoelectronics, and biology.[1] Soon after the development of high-quality CdSe nanocrystals with spectacular size-dependent tunable excitonic emission in the entire visible window, [2] simplification of the synthetic method,[3]-stabilization of the emission, [4] surface functionalization of the nanocrystals, [5] design of non-cadmium nanocrystal emitters, [6] fabrication of different kinds of composition-tunable multifunctional alloy nanocrystals, [7] and related photophysical properties [8] have been widely investigated for both fundamental understanding and their implementation in day-to-day developing technology. Analysis of up to date literature reports reveals that biological applications require strongly emitting, small and nontoxic nanocrystals preferably with excitation in the visible window, [6e, 9] light-emitting diodes require nanocrystals having large Stokes shift and high quantum efficiency, [1f-g, 10] and for solar cells nanocrystals having visible/near-IR (NIR) absorption and/or ternary/ quaternary nanocrystals with excess of either of the charge carriers (electron or hole) [1a-d, 11] are preferred. So far, no nanocrystal emitters having all such required properties have been reported, and thus further investigations are required to obtain new materials with new properties that would be suitable for versatile applications.We have now designed a new series of ultrasmall (< 2.5 nm), nearly fixed size, alloyed nanocrystals composed of Cu I -Zn II -In III -Se VI ions which show composition-dependent tunable emissionover most of the visible window. In addition, these nanocrystals are cadmium-free and have aqueous dispersibility, photostability, large Stokes shifts, and high emission intensity (quantum yield (QY) = 25-30 %), which makes them a versatile light-emitting nanoscale materials providing one-step solutions for various applications. The fundamental designing principle of these nanocrystals involves a mechanism whereby composition-variable alloy formation tunes the optical bands from lower to higher energy and vice versa. Here we report details of the synthesis, chemistry of formation, and composition-variable optical tuning of these fixed-size alloy nanocrystals. In addition, aqueous dispersibility and photovoltaic properties of these nanocrystals were investigated.The alloy nanocrystals were synthesized by simultaneous precipitation and surface cation-exchange protocols. Injection of a selenium precursor into a mixture of Zn II , In III , and Cu II salts at 220 8C (see Experimental Section) results in copperdoped zinc indium selenide alloy nanocrystals whose absorption and emission wavelengths are determined by the In:Zn ratio of the reaction mixture. Further addition of Zn with continuous annealing slowly shifts both absorption and emission bands to the blue in a surface ion-exchange process. Succe...

Nonradiative Auger recombination is the primary exciton loss mechanism in colloidal nanocrystals and an impediment for prospective optoelectronic applications. Recent development of new core/shell nanocrystals with suppressed Auger recombination rates has opened the possibility for studying multicarrier states using time-resolved photoluminescence (PL) spectroscopy. An important aspect not addressed in previous works is the scaling of radiative and nonradiative decay rates with the increasing number and type of excitons in individual nanocrystals. Here we conduct extensive single-dot PL spectroscopy of emissive states in PL blinking trajectories of giant silica-coated CdSe/CdS nanocrystals. At low fluences, we observe the appearance of neutral and charged exciton (trion) states. Both negative and positive trions show strongly suppressed Auger recombination rates resulting in PL quantum yields close to 50%. At higher excitation powers, we observe consecutive emergence of lower efficiency states, indicative of higher order excitons. We employ a scaling model for Auger and radiative decay rates and attribute these states to doubly charged excitons, biexcitons, and a triexciton. Simultaneous analysis of the second-order correlation statistics proves that the biexciton Auger recombination channel can be represented in terms of the superposition of independent recombination channels of trions. Analysis of the PL emission of the triexciton state suggests nonstatistical scaling, likely due to the involvement of the transitions between different symmetries. Finally, measurements at high excitation fluence of nanocrystals with low trion quantum yields does not reveal any higher order excitonic states, corroborating the validity of the scaling model and confirming Auger-related mechanisms responsible for blinking behavior in such core/shell nanocrystals.

We performed time-correlated single-photon counting experiments on individual silica coated CdSe/CdS core/thick-shell nanocrystal quantum dots (a.k.a., giant NQDs [g-NQDs]), placed on the plasmonic gap-bar antennas. Optical properties were directly correlated with the scanning electron microscopy (SEM) images of g-NQD-plasmonic antenna coupled structures. The structures, in which the g-NQDs are located in the gap of the antenna, afford a coupling with up to 9.6 fold enhancement of radiative recombination rates. These coupled g-NQDs are also characterized by a strong enhancement of bi-exciton emission efficiency that increases with their radiative enhancement factor. By analysing these findings with a simple model, we show that the plasmonic field of the antenna does not alter the Auger recombination processes of the bi-exciton states. As a result, enhancements of the single and bi-exciton radiative recombination rates lead directly to bi-exciton emission enhancement. These findings suggest that a plasmonic field can be utilized effectively in achieving a strong bi-exciton emission that is needed for photon pair generation and plasmon-assisted lasing.

Doped semiconductor nanocrystals such as Mn doped ZnS, Mn doped ZnSe and Cu doped InZnS, are considered as new classes of fluorescent biological probes with low toxicity. Although the synthesis in high quality of such nanomaterials is now well established, transforming them into functional fluorescent probes remains a challenge. Here we report a fluorescent cellular imaging probe made of high quality doped semiconductor nanocrystals. We have identified two different coating approaches suitable for transforming the as synthesized hydrophobic doped semiconductor nanocrystals into water-soluble functional nanoparticles. Following these approaches we have synthesized TAT-peptide- and folate-functionalized nanoparticles of 10-80 nm hydrodynamic diameter and used them as a fluorescent cell label. The results shows that doped semiconductor nanocrystals can be an attractive alternative for conventional cadmium based quantum dots with low toxicity.