Simple cubic self-assembly of PbS quantum dots by finely controlled ligand removal through gel permeation chromatography

The geometry in self-assembled superlattices of colloidal quantum dots (QDs) strongly affects their optoelectronic properties and is thus of critical importance for applications in optoelectronic devices. Here, we achieve the selective control of the geometry of colloidal quasi-spherical PbS QDs in highly-ordered two and three dimensional superlattices: Disordered, simple cubic (sc), and face-centered cubic (fcc). Gel permeation chromatography (GPC), not based on size-exclusion effects, is developed to quantitatively and continuously control the ligand coverage of PbS QDs. The obtained QDs can retain their high stability and photoluminescence on account of the chemically soft removal of the ligands by GPC. With increasing ligand coverage, the geometry of the self-assembled superlattices by solution-casting of the GPC-processed PbS QDs changed from disordered, sc to fcc because of the finely controlled ligand coverage and anisotropy on QD surfaces. Importantly, the highly-ordered sc supercrystal usually displays unique superfluorescence and is expected to show high charge transporting properties, but it has not yet been achieved for colloidal quasi-spherical QDs. It is firstly accessible by fine-tuning the QD ligand density using the GPC method here. This selective formation of different geometric superlattices based on GPC promises applications of such colloidal quasi-spherical QDs in high-performance optoelectronic devices.

Gel permeation chromatography can finely control ligand coverage of PbS quantum dots. Self-assembly of these QDs with different ligand density leads to the formation of 2D square, hexagonal and 3D simple cubic and face-centered cubic superlattices.     

Self-selective recovery of photoluminescence in amphiphilic polymer encapsulated PbS quantum dots

Self-selected recovery of the photoluminescence (PL) of amphiphilic polymer encapsulated PbS quantum dots (QDs) was observed in water for the first time and possible mechanisms were proposed based on investigations by means of transmission electron microscopy, X-ray photoelectron spectroscopy (XPS), X-ray diffraction and fluorescence spectroscopy. Water-soluble PbS QDs were synthesized by transferring monodispersed QDs capped with hydrophobic ligands of oleylamine from an organic solvent into water via amphiphilic polymers poly(maleic anhydride-alt-1-octadecene-co-poly(ethylene glycol)). The water transfer process leads to a double size distribution (5.6 ± 0.9 nm and 2.7 ± 0.4 nm), attributed to ligand etching together with Ostwald ripening, as well as the fast decay of PL. The automatic recovery of the PL in PbS QDs stored in water in the dark for 3 months was only observed for the subset of smaller QDs and is largely due to the removal of surface defects with aging, as evidenced by the decreased percentage of unpassivated surface atoms from XPS studies. In contrast, the PL of the subset of larger QDs in the same sample does not self-recover in water and can only be slightly recovered by transferring them into environments with less external quenches. The results strongly suggest that it is the surface defect in the larger QDs themselves, introduced during Ostwald ripening, that is primarily responsible for their non-emitting status or rather low PL intensity under different conditions. The increase of unpassivated Pb atoms in larger PbS QDs after the 3 month aging has been confirmed by XPS, which explains their non-recovery behavior in water. The PL-recovered QD sample in water is very stable and shows comparable photostability to the initial QDs dispersed in an organic phase.     

Synthesis of single‐core and multiple‐core core‐shell nanoparticles by RAFT emulsion polymerization: Lead sulfide‐copolymer nanocomposites

We report here a simple and direct route for the preparation of lead sulfide (PbS) quantum dots (QDs) embedded into polymeric nanospheres by emulsion polymerization. In this process, QDs are first dispersed in an aqueous solution containing a statistical oligomer constituted of five butyl acrylate and ten acrylic acid units prepared by reversible addition fragmentation chain transfer (RAFT) polymerization using a trithiocarbonate as RAFT agent. Then, the dispersion of PbS QDs is engaged into an emulsion polymerization process to form core‐shell nanoparticles. Transmission electron microscopy reveals the presence of single‐core core‐shell particles at low concentration of PbS QD, whereas multiple‐core core‐shell particles containing either well separated or aggregated PbS QDs are formed at high concentration of PbS QDs. © 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2012     

Self-Assembly of Semiconductor Quantum Dots using Organic Templates

Colloidal semiconductor nanocrystals, known as quantum dots (QDs), are regarded as brightly photoluminescent nanomaterials possessing outstanding photophysical properties, such as high photodurability and tunable absorption and emission wavelengths. Therefore, QDs have great potential for a wide range of applications, such as in photoluminescent materials, biosensors and photovoltaic devices. Since the development of synthetic methods for accessing high-quality QDs with uniform morphology and size, various types of QDs have been designed and synthesized, and their photophysical properties dispersed in solutions and at the single QD level have been reported in detail. In contrast to dispersed QDs, the photophysical properties of assembled QDs have not been revealed, although the structures of the self-assemblies are closely related to the device performance of the solid-state QDs. Therefore, creating and controlling the self-assembly of QDs into well-defined nanostructures is crucial but remains challenging. In this Minireview, we discuss the notable examples of assembled QDs such as dimers, trimers and extended QD assemblies achieved using organic templates. This Minireview should facilitate future advancements in materials science related to the assembled QDs.     

Colloidal Quantum Dot Arrangement Assisted by Perylene Bisimide Self-Assembly

Colloidal semiconductor nanocrystals, so-called quantum dots (QDs), are attractive as molecular-like smart nanomaterials, and their emission and optoelectronic properties in the dispersed state have been actively studied. The construction of supramolecular structures composed of multiple QDs, however, is still challenging. Here, a new strategy to form supramolecular QD structures via self-assembly of perylene bisimide (PBI) dyes is demonstrated. In a mixed solution, QDs and PBI undergo time-dependent fusion to form an isolated colloidal QD-PBI complex or a unique QD-PBI co-aggregate composed of QDs arranged along a sheet-like PBI nanostructure, and these dramatically different supramolecular structures can be controlled by the solvent polarity.     

Oscillating fluorescence in an unstable colloidal dispersion of CdSe/ZnS core/shell quantum dots

Fluorescence oscillation is observed in an ensemble of colloidal CdSe/ZnS core/shell quantum dots (QDs) dispersed in nonpolar solvent under continuous irradiation. The QDs dispersed in toluene gradually aggregate and change their fluorescence intensity, even in the dark. During the aggregation, the QD/toluene suspension is unstable, that is, overdispersed. The fluorescence oscillation is found only in this unstable state before the system reaches steady state. In addition, the aggregation rate is promoted by irradiation and strongly correlates with the oscillation amplitude. Our experimental results indicate that the dispersion instability plays an important role in both linear and nonlinear dynamics of the fluorescence. It is inferred from the experimental results and previous studies that the complex time evolution of fluorescence in the QD/toluene dispersion is possibly due to adsorption and desorption of surface ligand molecules over the course of QD aggregation.     

Langmuir-Blodgett thin films of quantum dots: synthesis, surface modification, and fluorescence resonance energy transfer (FRET) studies

We describe herein studies on as-prepared hydrophobic ZnS-CdSe quantum dots (QDs) at the air-water interface. Surface pressure-area (pi-A) isotherms have been used to study the monolayer behavior. Uniform, lamellar multilayer thin films of QDs were deposited by the Langmuir-Blodgett (LB) technique. The role of two different surfactant systems commonly employed in the synthesis of these QDs (trioctylphosphine oxide-octadecylamine (TOPO-ODA) system and trioctylphosphine oxide-tetradecylphosphonic acid (TOPO-TDPA) system) on the monolayer behavior and the quality of thin films produced has been investigated. The thin films were characterized by quartz crystal microgravimetry (QCM), contact angle measurements, fluorescence spectroscopy, and transmission electron microscopy (TEM). These QD films were further modified by an amphiphilic polymer, poly(maleic anhydride-alt-1-tetradecene) (PMA). The hydrophobic interaction between the polymers and the surfactants attached to the QDs drove the self-assembly process. The carboxylic acid functional groups in the polymer were also used to immobilize avidin. We have demonstrated a proof of concept for the biosensing strategy wherein the avidin-coated QD films attracted biotinylated gold nanoparticles, resulting in fluorescence resonance energy transfer (FRET) quenching of the thin films.     

Copolymer nanosphere encapsulated CdS quantum dots prepared by RAFT copolymerization: synthesis,characterization and mechanism of formation

Cadmium sulfide (CdS) quantum dots (QDs) encapsulated in block copolymer spheres were synthesized by an aqueous emulsion polymerization process. First, stable dispersions of CdS QDs in water were prepared using a polymer dispersant, either poly(acrylic acid) or a random copolymer having an average of ten acrylic acid and five butyl acrylate units. These polymer dispersants were prepared by reversible addition-fragmentation chain transfer polymerization. Then, the CdS QDs dispersed in water were encapsulated in a polystyrene shell using an emulsion polymerization process. Spectroscopic and microscopic techniques were used to characterize the resulting nanocomposites. Optical properties of QDs in polymer microspheres were investigated by UV-vis and fluorescence spectroscopic studies. Particle sizes of all CdS QD samples were calculated from absorption edges using Henglein's empirical curve. Transmission electron microscopy was used to determine the size and morphology of CdS QD samples. These observations were used to elucidate the mechanism of formation of the resulting well-defined polymer-encapsulated CdS nanoparticles.     

Surface-tunable photoluminescence from block copolymer-stabilized cadmium sulfide quantum dots

The static and time-resolved photoluminescence properties of polystyrene-b-poly(acrylic acid) (PS-b-PAA)-stabilized cadmium sulfide quantum dots (CdS QDs) have been characterized for the first time, demonstrating tunable emission spectra and quantum yields via different chemical treatments of the PAA layer. Samples with the PAA layer in its cadmium carboxylate form showed more-intense band-edge emission and relatively high quantum yields compared with samples in which the PAA layer was in its acid form. This activation effect is explained in terms of passivation of trap sites on the QD surface by specific interactions between the QD and the cadmium-neutralized PAA layer. Lifetimes of band-edge and trap state emission for the various samples ranged from 40 to 61 ns and 244 to 360 ns, respectively. Impressive long-term stability was also shown for a sample of cadmium-neutralized PS-b-PAA-stabilized QDs dispersed in toluene, which maintained 90% of its photoluminescence over 57 days aging under ambient conditions. It is also shown that Cd2+ activation of photoluminescence does not occur when Mg2+ ions are added to similar QD solutions, indicating potential of these block copolymer-stabilized QDs as Cd2+-selective sensors. Irrespective of chemical treatment of the PAA layer, the external PS brush layer effectively stabilized all samples in various organic solvents, resulting in clear CdS colloids with no observed precipitation over several months. Dynamic light scattering and gel permeation chromatography revealed differences in the aggregation numbers and hydrodynamic radii of colloidal QDs for different treatments of the PAA layer, attributed to the lower solubility of the poly(cadmium acrylate) blocks compared to the PAA blocks in the acid form. Finally, it was demonstrated that the PS-b-PAA-stabilized QDs could be well dispersed in PS homopolymer, producing optically transparent photoluminescent films which retained the emission features of the colloidal QDs. Stable and surface-tunable optical properties via the PAA layer and polymer solubility and processability via the PS layer make these PS-b-PAA-stabilized CdS QDs exciting "building blocks" for the bottom-up assembly of functional hierarchical materials for photonics, sensors, and bio-labeling applications.     

Very high third-order nonlinear optical activities of intrazeolite PbS quantum dots

Zeolite-intercalated semiconductor quantum dots (QDs) have long been proposed to give very high third-order nonlinear optical (3NLO) responses. However, measurements of their 3NLO responses have not been possible due to the lack of methods to prepare optically transparent QD-incorporating zeolite films supported on optically transparent substrates and to confine QDs only within zeolite interiors. We found that the zeolite-Y films grown on indium-tin-oxide-coated glass plates (Ygs) remain firmly bonded to the substrates during ion exchange with Pb2+ ions, drying, and formation of PbS QDs by treating Pb2+ ions with H2S. A series of Ygs encapsulating different numbers (n = 0, 8, 14, 23, and 33) of PbS in a unit cell [(PbS)n-Yg] were prepared. The PbS QDs were expelled by adsorbed moisture to the external surfaces, and the expelled QDs formed large QDs. Coating of the (PbS)n-Ygs with octadecyltrimethoxysilane results in effective confinement of the QDs within the internal pores. The zeolite-encapsulated PbS QDs showed remarkably high 3NLO activities at 532 and 1064 nm which are unparalleled by other PbS QDs dispersed in other matrixes.     

Controlled self-assembly of hydrophobic quantum dots through silanization

We demonstrate the formation of one-, two-, and three-dimensional nanocomposites through the self-assembly of silanized CdSe/ZnS quantum dots (QDs) by using a controlled sol-gel process. The self-assembly behavior of the QDs was created when partially hydrolyzed silicon alkoxide monomers replaced hydrophobic ligands on the QDs. We examined systematically self-assembly conditions such as solvent components and QD sizes in order to elucidate the formation mechanism of various QD nanocomposites. The QD nanocomposites were assembled in water phase or on the interface of water and oil phase in emulsions. The partially hydrolyzed silicon alkoxides act as intermolecules to assemble the QDs. The QD nanocomposites with well-defined solid or hollow spherical, fiber-like, sheet-like, and pearl-like morphologies were prepared by adjusting the experimental conditions. The high photoluminescence efficiency of the prepared QD nanocomposites suggests partially hydrolyzed silicon alkoxides reduced the surface deterioration of QDs during self-assembly. These techniques are applicable to other hydrophobic QDs for fabricating complex QD nanocomposites.     

Highly photoluminescent multilayer QD-glass films prepared by LbL self-assembly

A novel and facile preparation method for layer-by-layer (LbL) self-assembled films incorporating quantum dots (QDs) and having intense photoluminescence (PL) from blue to red is presented. Functional sol-gel-derived glass layers prepared by the hydrolysis of 3-aminopropyltrimethoxysilane (APS) or 3-mercaptopropyltrimethoxysilane (MPS) have been used as a linkage between QD layers. Absorption, PL spectroscopy, transmission electron microscopy, and atomic force microscopy were employed for characterization, which revealed that the QDs in the prepared films had a nearly close-packed coverage and were not aggregated. The PL efficiencies of the QDs (CdTe or ZnSe, both are thioglycolic acid-stabilized) dispersed in the films were roughly half that of the initial colloidal solutions but reached 24% before a refractive index correction. The thickness of the red-emitting film with 10 CdTe QD layers was approximately 50 nm. The concentration of QDs in the film derived from the first absorption peak was approximately 0.01 M. Because the PL starts to show a red shift, the obtained concentration is practically the ultimate one in the glass matrix. The mercapto, amino, and carboxyl groups play important roles in LbL self-assembling processes.     

Effects of evaporation velocity and film thickness on poly(3‐hexylthiophene) thin films processed from aggregate dispersions in binary solvent mixtures

Aggregate dispersions of P3HT in two series of solvent mixtures, chloroform:dichloromethane and toluene:dichloromethane, are used to study the impact of the evaporation velocity and film thickness on the P3HT films processed using two spin‐coating speeds (1000 rpm and 2000 rpm). The structural order and surface morphology were investigated with UV/Vis absorption spectroscopy and atomic force microscopy techniques. There is no evidence that the characteristics of the liquid phase P3HT dispersions impact the structures of the films, which is in agreement with a previous study of drop cast P3HT films that were dried over much longer time periods. An association is observed between the extent of aggregation in the liquid phase and the thickness and surface roughness parameters of the films. However, the structural order does not correlate with the thickness of the films, which was previously reported for polymer films processed from amorphous polymer solutions in pure organic solvents. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2017 , 55, 330–343     

A hierarchical self-assembly route to three-dimensional polymer-quantum dot photonic arrays

We demonstrate a new hierarchical self-assembly strategy for the formation of photonic arrays containing quantum dots (QDs), in which sequential self-assembly steps introduce organization on progressively longer length scales, ranging from the nanoscale to the microscale regimes. The first step in this approach is the self-assembly of diblock copolymers to form block ionomer reverse micelles (SA1); within each micelle core, a single CdS QD is synthesized to yield the hybrid building block BC-QD. Once SA1 is completed, the hydrophobic BD-QD building blocks are blended with amphiphilic block copolymer stabilizing chains in an organic solvent; water addition induces secondary self-assembly (SA2) to form quantum dot compound micelles (QDCMs). Finally, aqueous dispersions of QDCMs are slowly evaporated to induce the formation of three-dimensional (3D) close-packed arrays in a tertiary self-assembly step (SA3). The resulting hierarchical assemblies, consisting of a periodic array of hybrid spheres each containing multiple CdS QDs, exhibit the collective property of a photonic stop band, along with photoluminescence arising from the constituent QDs. A high degree of structural control is possible at each level of organization by judicious selection of experimental variables, allowing various parameters governing the collective optical properties, including QD size, nanoparticle spacing, and mesocale periodicity, to be independently tuned. The resulting control over optical properties via successive self-assembly steps should provide new opportunities for hierarchical materials for QD lasers and all-optical switching.     

The effects of altered reaction conditions on the properties of anionic poly(urethane-urea) dispersions and films cast from the dispersions

Aqueous anionic polyurethane (PU) dispersions were synthesised from a polyester polyol, isophorone diisocyanate and α,α-dimethylol propionic acid using the prepolymer mixing process. Samples were neutralised by the addition of triethyl amine. The polymer chains were dispersed in water and extended with 1,2-ethylene diamine. The differences in the dispersion characteristics and the mechanical properties of the polymer films cast and dried from the dispersions caused by altered reaction conditions were determined.The reaction conditions proved to affect both the colloidal properties of the dispersions as well as the mechanical properties of the films. The neutralisation, the dispersion and the chain extension methods had all an influence on the average size of the formed PU particles. For the films, a change in the mechanical properties and probably in the amount of hard and soft domain separation was also observed. A good control over the properties was obtained by selecting the reaction parameters carefully. In particular, the dispersion method in which the prepolymer solution was added to water and not vice versa led to a considerably lower viscosity during the dispersion process. Thus a wider choice of raw materials was facilitated.     

Reversible surface electronic traps in PbS quantum dot solids induced by an order-disorder phase transition in capping molecules

The electronic properties of semiconductor quantum dots (QDs) are critically dependent on the nature of the ligand molecules on their surfaces. Here we show the reversible formation of surface electronic trap states in the model system of solid thin films of PbS QDs capped with thiol molecules. As the temperature was increased from cryogenic to room temperature, we discovered a phase transition in the fluorescence spectra from excitonic emission to trap emission. The critical temperature (T(c)) of the phase transition scales with molecular length and in each case is close to the bulk melting temperature of the capping molecules. We conclude that an order-disorder transition in the molecular monolayer above T(c) introduces surface mobility and the formation of a disordered atomic lead layer at the QD/capping molecule interface, leading to electronic trap formation.     

High‐density quantum dots composites and its photolithographic patterning applications

With the growing interest in quantum dots (QDs), many applications are emerging recently. In particular, the display industry has shown widespread interest in using QDs as the next generation colorants. One application is to replace conventional color filters with QD‐based color conversion films to significantly improve color purity and luminous efficiency. However, QD blending which is capable of photolithographic patterning is a very challenging problem due to its low dispersion property and aggregations in polar medias. Herein, we report a photo‐patternable QD dispersion that can produce fine patterns through a photolithography process. First, the QDs dispersed in a nonpolar solvent, for example, chloroform or hexane, were separated and dried to obtain a QD powder. And then, the dispersion characteristics of the QD powders were investigated after mixing commercial dispersants and UV curable oligomers. Furthermore, the QD dispersion was investigated up to 30 wt.% of QDs by mixing with various commercial additives. We have studied the optical property changes of QDs during the photocuring process and the heating process prior to actual application. And, we have studied the surface characteristics of the fine QDs patterns after patterning process. As a result, it was confirmed that QDs are able to be well dispersed up to 30 wt.%.     

Luminescence and stability of aqueous thioalkyl acid capped CdSe/ZnS quantum dots correlated to ligand ionization.

The spectroscopic properties of CdSe/ZnS quantum dots (QDs) were observed to change as a function of thioalkyl acid ligand. Experiments were performed using 2, 3, 6, and 11-carbon linear thioalkyl acids, as well as mercaptosuccinic acid (MSA) and dihydrolipoic acid (DHLA). Bathochromic shifts of up to 14 nm in the emission spectra of QDs capped with these ligands were observed. Similarly, hypsochromic or bathochromic shifts up to 7 nm were observed for a specific ligand in acidic or basic solution, respectively. These shifts could be correlated to the number of ionized ligands and the ability of the ligands to act as hole acceptors. It was also found that differences in quantum yield between the ligands were primarily due to variations in radiative decay rate and not nonradiative decay rate. This indicated that different degrees of QD surface passivation were not responsible for the differences, and that the radiative system must be considered as the sum of the ligands and the QD nanocrystal. The stability of QDs capped with mercaptoacetic acid, MSA, and DHLA towards aggregation at low pH was found to correlate with the pK(a) of the ligands. Spectral shifts were also observed during aggregation. Overall, the luminescence of thioalkyl acid capped QDs appears to be a complex function of dielectric constant, electrostatic or hole-acceptor interactions with ionized ligands, and, to a lesser extent, passivation.     

Increase of third-order nonlinear optical activity of PbS quantum dots in zeolite Y by increasing cation size

The third-order nonlinear optical (3NLO) activity of PbS quantum dots (QDs) encapsulated in zeolite Y has been expected to depend sensitively on the countercation of the zeolite host. However, ion exchange of the pristine countercation, H(+), with other cations has not been possible because the framework decomposes and the QDs aggregate immediately when the PbS QD-incorporating zeolite Y with H(+) as the countercation is exposed to the atmosphere. We now report that when H(+) is transformed to NH(4)(+), the framework of PbS QD-containing zeolite Y does not undergo decomposition and the PbS QDs do not undergo aggregation to form larger QDs during the aqueous ion exchange of NH(4)(+) with alkali-metal ions (M(A)(+) = Li, Na(+), K(+), Rb(+)). The 3NLO activity of the M(A)(+)-exchanged PbS QD-incorporating zeolite Y film increases with increasing size of M(A)(+). The stabilization of the surface-bound exciton by the electron-rich framework oxide and electron-poor cation is proposed to be responsible for the increase. This is the first example of a method for systematically increasing the 3NLO activity of QDs dispersed in a dielectric matrix by systematically changing its properties. These results will serve as a guideline for future research and also promote applications of QD-incorporating zeolites in various fields.     

Highly crosslinked poly(dimethylsiloxane) microbeads with uniformly dispersed quantum dot nanocrystals

This study demonstrates how luminescent semiconductor nanocrystals (quantum dots or QDs) can be dispersed uniformly in a poly(dimethylsiloxane) (PDMS) matrix by polymerizing a mixture of the prepolymer oligomers and the nanocrystals with a relatively large concentration of crosslinking molecules. A microfluidic device is used to fabricate PDMS microbeads embedded with the QDs by using flow focusing to first form monodisperse droplets of the prepolymer/crosslinker/nanocrystal mixture in a continuous aqueous phase. The droplets are subsequently collected, and heated to polymerize them into solid microbead composites. The degree of aggregation of the nanocrystals in the matrix is studied by measuring the nonradiative resonance energy transfer (RET) between the nanocrystals. For this purpose, two quantum dots are used with maxima in their luminescence emission spectrum at 560 nm and 620 nm. When the nanocrystals are within the F?rster radius (approximately 10 nm) of each other, exciton energy cascades from the QDs which emit at the shorter wavelength to the QDs which emit at the longer wavelength. This energy transfer is quantified, for two concentration ratios of the prepolmer to the crosslinker, by measuring the deviation of the microbead luminescence spectrum from a reference spectrum obtained by dispersing the QD mixture in a solvent (toluene) in which the nanocrystals do not aggregate. For a low concentration of crosslinking molecules relative to the prepolymer (5:1 by weight prepolymer to crosslinker), strong RET is observed as the emission of the 620 nm QDs is increased and the 560 nm QDs is decreased relative to the reference. In the emission spectrum for a higher concentration of crosslinkers (2:1 by weight prepolymer to crosslinker), the resonance energy transfer is less relative to the case of the low concentration of crosslinkers, and the spectrum more closely resembles the reference. This result indicates that the increase in the crosslinker concentration has reduced the nanocrystal aggregation in the cured polymer. The use of crosslinking can serve as a general paradigm for forming, from a prepolymer/nanoparticle mixture, a composite in which the particles are not aggregated. Under the usual conditions the entropic cost to a linearly growing polymer chain of surrounding nanoparticles forces them to aggregate; crosslinking kinetically entraps the particles and circumvents this aggregation driving force. The QD/polymer composite microbeads fabricated in this study find applications in bead-based platforms for high-throughput, multiplexed screening, where the emission spectrum of the QD luminescence can be used as a spectral barcode to label the beads. For microbeads in which the nanocrystals are uniformly dispersed, this barcode is undistorted by energy transfer, and is easily read.