Quantum dots are ideally suited for color conversion in light emitting diodes owing to their spectral tunability, high conversion efficiency and narrow emission bands. These properties are particularly important for display backlights; the highly saturated colors generated by quantum dots justify their higher production cost. Here, we demonstrate the benefits of a hybrid remote phosphor approach that combines a green-emitting europium-doped phosphor with red-emitting CdSe/CdS core/shell quantum dots. Different stacking geometries, including mixed and separate layers of both materials, are studied at the macroscopic and microscopic levels to identify the configuration that achieves maximum device efficiency while minimizing material usage. The influence of reabsorption, optical outcoupling and refractive index-matching between the layers is evaluated in detail with respect to device efficiency and cost. From the findings of this study, general guidelines are derived to optimize both the cost and efficiency of CdSe/CdS and other (potentially cadmium-free) quantum dot systems. When reabsorption of the green and/or red emission is significant compared to the absorption strength for the blue emission of the pumping light emitting diode, the hybrid remote phosphor approach becomes beneficial.

We analyze the stability of the photoluminescence efficiency of flash core/shell quantum dots (QDs). We show that shell design is crucial to suppress thermal quenching and long‐term photodegradation, where a radial composition change from CdS to ZnS results in QDs that surpass the stability requirements of display applications.

Colloidal I-III-VI2 semiconductor nanocrystals form a versatile family of nanomaterials with similar properties as Cd-containing quantum dots (QDs), but with lower toxicity. Their interesting and tunable optical properties have resulted in their use in a variety of applications, ranging from biolabeling over solar cells to white LEDs. For an optimal performance each of these applications requires the QDs to show specific optical properties. The relatively narrow emission of Cd-based QDs can easily be tuned over the visible range at a high chemical yield by adjusting specific synthesis conditions[1,2]. However, the typical broad emission of Cu-In-Zn-S QDs has proven to be more challenging to tune and requires a different strategy. We propose a study on the one-pot synthesis and characterization of Cu-In-Zn-S QDs with efficient and broad emission. A careful preparation of the precursors and control of the reaction time is required to synthesize CuInS2 and Cu-In-Zn-S QDs reproducibly. We discuss the limited emission tuning obtained by changing reaction conditions (e.g. precursor concentrations). More successfully, we have expanded the emission range by composition tuning, where In can for instance be replaced by Ga or Al and Cu by Ag. Finally, we address the potential of the resulting materials as alternative color convertors for white LEDs[3]. 1) S. Abe et al., ACS Nano, 2012, 6, 42. 2) S. Abe et al., ACS Nano, 2013, 7, 943. 3) P.F. Smet et al, J. Electrochem. Soc., 2011, 158, R37

Quantum dots (QDots) are explored in biomedicine as highly fluorescent, photostable nanomaterials, but their use is impeded by their hydrophobic nature. In the present work, we evaluate the potential biomedical use of QDots that have been transferred into the aqueous phase by means of inorganic ligands. CdSe/CdS QDots were prepared and transferred to water upon ligand exchange to S(2-) ions. However, a multiparametric evaluation of the effect of these QDots on multiple cell types revealed significant QDot cytotoxicity. Using optimized methods, the QDots were found to rapidly degrade under endosomal pH, resulting in leached Cd(2+). Together with the induction of oxidative stress, this significantly affected cell viability. Using proliferation-restricted cells, QDot degradation was found to augment cytotoxicity with time resulting in mitochondrial and DNA damage, effects on cell morphology and cell functionality. The final non-cytotoxic concentration was defined at 2 nM, enabling cells to be tracked up to 2 cell divisions. A direct comparison with other QDots and fluorescent particles studied resulted in similar concentrations; however, the functionality of previously analyzed particles was much higher. These data reveal that comparing NP toxicity based on particle concentrations is extremely difficult. A comparison of NPs is better obtained by evaluating NP functionality using a straightforward approach, such as follow-up of QDot fluorescence in dividing cells. These data highlight the importance of (1) considering QDot stability in the intracellular microenvironment, (2) the protective nature of the QDot-stabilizing coating, (3) the need for comparison of particle functionality to understand any observed effects.

We investigate the relation between the chain length of ligands used and the size of the nanocrystals formed in the hot injection synthesis. With two different CdSe nanocrystal syntheses, we consistently find that longer chain carboxylic acids result in smaller nanocrystals with improved size dispersions. By combining a more in-depth experimental investigation with kinetic reaction simulations, we come to the conclusion that this size tuning is due to a change in the diffusion coefficient and the solubility of the solute. The relation between size tuning by the ligand chain length and the coordination of the solute by the ligands is further explored by expanding the study to amines and phosphine oxides. In line with the weak coordination of CdSe nanocrystals by amines, no influence of the chain length on the nanocrystals is found, whereas the size tuning brought about by phosphine oxides can be attributed to a solubility change. We conclude that the ligand chain length provides a practical handle to optimize the outcome of a hot injection synthesis in terms of size and size dispersion and can be used to probe the interaction between ligands and the actual solute.

Quantum dots (QDs) or colloidal semiconductor nanocrystals attract extensive interest in both industry and science nowadays due to their opto-electronic properties that strongly depend on their size, structure, shape and composition. To further develop applications, providing a rational basis to explore and understand how hot injection synthesis parameters affect these properties is key. Various literature studies indicate that the composition of the reaction mixture in which nanocrystals are formed is related to the size the nanocrystals attain at the end of the reaction. We analyze several of these ‘reaction chemistry/nanocrystal property relations’ by combining reaction simulations with an experimental investigation on CdSe quantum dot syntheses. We find that increasing the free acid concentration in the reaction mixture has the same effect on a real synthesis as raising the solute solubility in the simulations and explain the increase of the QD size as result ing from an enhanced consumption of the solute by nanocrystal growth. Similarly, we address the effect of increasing precursor concentrations to an increasing monomer formation rate, which results in a higher nanocrystal concentration and therefore a smaller nanocrystal size. Finally, we relate size tuning by the ligand chain length to the coordination of the solute by these ligands.

Current white LEDs (wLEDs) are composed of a blue pumping LED and color convertors – phosphors – which are typically rare earth doped inorganic materials. As backlight in displays these wLEDs often have the disadvantage that a substantial fraction of the emitted light needs to be filtered out. In general lighting, efficiency losses are often generated by an emission extending into the infrared region, while also color rendering can be suboptimal. Recently, the use of quantum dots (QDs) as color convertor materials in wLEDs has gained attention as a solution to these problems. Here, we discuss our recent research results obtained to improve both performance and cost-efficiency of wLEDs for display and lighting applications. The focus lies on developing color convertor materials and implementing these in a remote phosphor configuration, thereby improving efficiency, (color) homogeneity and stability compared to conventional designs where the phosphor material is deposited directly on the LED chip. We explore the use of hybrid phosphor layers, consisting of a combination of rare earth doped phosphors with CdSe or InP-based QDs. We provide an overview of synthesis strategies to obtain the required QDs and configurations to implement them in a remote phosphor layer. Characterization of optical and scattering properties of these layers using e.g. emission decay measurements to asses self-absorption is discussed. Finally, we show relevant characteristics of a benchmark wLED device.

Light-emitting diodes (LEDs) have successfully outgrown their indicator lamp stage to become increasingly used as white LEDs (wLEDs) in both lighting and display markets replacing incandescent and fluorescent lamps. Technically, current wLEDs are composed of a blue pumping LED and one or more color convertors, usually called phosphors, which are typically rare earth doped inorganic materials. As a backlight in displays these wLEDs often have the disadvantage that a substantial fraction of the emitted light needs to be filtered out. In general lighting, efficiency losses are often generated by an emission extending into the infrared region, while also color rendering can be suboptimal. Recently, the use of colloidal semiconductor nanocrystals – quantum dots (QDs) – as color convertor materials in wLEDs has gained attention as a technological solution to these problems. These luminescent materials with broad absorption mostly owe this interest to their tunable, narrow and efficient emission which enable emission spectra to be adjusted to the needs of different applications. In this presentation, we will discuss the recent research results obtained within NB-Photonics to improve both the performance and cost-efficiency of wLEDs for display and lighting applications. The focus lies on developing color convertor materials and implementing these in a remote phosphor configuration, thereby improving efficiency, (color) homogeneity and stability compared to conventional designs where the phosphor material is deposited directly on the LED chip. Currently, the use of hybrid phosphor layers is explored, consisting of a combination of rare-earth doped phosphors with quantum dots which are either based on nanocrystals of CdSe, InP or chalcopyrites. This work is related to the LumiCoR project, in which also other issues are tackled, including large scale production of high quality quantum dots, the development of economical synthesis routes for InP QDs and the study of cheaper lanthanide ion-free inorganic phosphors.

Colloidal semiconductor nanocrystals or quantum dots (QDs) are a very interesting class of nanomaterials since their properties can be tuned by their size due to quantum confinement. QDs are typically produced via a hot injection synthesis (HIS), which involves the injection of precursors in a hot mixture of a non-coordinating solvent and coordinating ligands such as carboxylic acids, thiols or phosphonic acids. For QDs to be implemented in a broad range of applications, a precise control over their size is essential. As a result, recent developments in the hot injection synthesis focus on producing QDs with predefined sizes, sharp size distributions and high reaction yields. In this study, we show that efficient size tuning at high reaction yield is possible by changing the chain length of the carboxylic acid. By combining an extended experimental reaction screening with reaction simulations, we demonstrate that the acid chain length affects the nanocrystal size by changing the diffusion coefficient and the solubility of the reactive monomers. In addition, we show that the relation between chain length and nanocrystal size can be used to assess the interaction of different coordinating species – including amines and phosphine oxides – with the reactive monomers. In this way, this work contributes to an enhanced, rational understanding of the widely used hot injection methods for the synthesis of colloidal nanocrystals.

We report on the synthesis of core-shell CuInS2/ZnS quantum dots (QDs) in organic solution, their encapsulation with a PEG-containing amphiphilic polymer, and the application of the resulting water-soluble QDs as fluorescent label in quantitative immunoassay. By optimizing the methods for core synthesis and shell growth, CuInS2/ZnS QDs were obtained with a quantum yield of 50% on average after hydrophilization. After conjugation with an aflatoxin B1-protein derivative, the obtained QDs were used as fluorescent labels in microplate immunoassay for the quantitative determination of the mycotoxin aflatoxin B1. QDs-based immunoassay showed higher sensitivity compared to enzyme-based immunoassay.

The synthesis of metal selenide nanocrystals based on the injection of selenium powder in a hot mixture containing the metal cation precursor complexed by a carboxylic acid is demonstrated by means of the formation of CdSe and ZnSe nanocrystals. In both cases, the synthesis can reach reaction yields of 80-85% within 5 min. In the case of CdSe nanocrystals, a more extensive study shows that even without protective atmosphere, the synthesis leads to state-of-the-art nanocrystals with low size dispersion. Importantly, the size of the nanocrystals at close to full yield can be changed by varying the carboxylic acid chain length, whereas the solid loading, that is, the amount of nanocrystals formed over the reaction volume, of the synthesis can be the 10-fold of typical literature syntheses. The potential of this reaction for the scaled up production of metal selenide nanocrystals is discussed and supported by the automated, parallel synthesis of CdSe nanocrystal batches using this heterogeneous selenium precursor where the standard deviation on the nanocrystal diameter is less than 1.5%.

During the last decade, the colloidal synthesis of especially Cd – and Pb chalcogenide nanocrystals has been well-developed. Monodisperse sols of nanocrystals over a large size range have become available and are widely used as building blocks for electronic and optical devices. However, the issue of toxicity remains a major obstacle towards large-scale integration of semiconductor nanocrystals in applications. I-III-VI materials (Cu(In,Ga)(S,Se) 2 (CIGS) and related compounds) are interesting candidates towards greener chemistry and offer the possibility of band gap engineering both by using quantum confinement and altering material composition. Moreover, CIGS is a well-known absorber material for high efficiency thin-film photovoltaics, and the use of nanocrystals as precursor inks is an interesting route to decrease the production cost of CIGS solar cells. Up to now, the synthesis of CIGS nanocrystals is mostly developed on an empirical basis. In this work, our aim is to understand the relation between reaction parameters (temperature, concentration, reaction time,..) on one hand and nanocrystal properties (size, shape, composition) on the other hand. We demonstrate that by altering reaction parameters in a rational way (1), CuInS 2 and CuGaS 2 nanocrystals with sizes from 5 to 20 nm can be obtained which vary in shape from quasi-spheres to flat hexagonal prisms. The ability to control the size and shape of CIGS nanocrystals is of particular importance for the deposition of crack-free thin films by, e.g., inkjet printing that can be incorporated in thin film solar cells. (1) S. Abe, R. Capek, B. De Geyter, Z. Hens (2012), "Tuning the Postfocused Size of Colloidal Nanocrystals by the Reaction Rate: From Theory to Application", ACS Nano, 6, 1: 42-53.

Over the last 15 years, colloidal semiconductor nanocrystals (NCs) or quantum dots (QDs) have proliferated as an alternate opto-electronic material used in photovoltaics, photodetection, color conversion or fluorescent labeling. This application development requires materials with predefined sizes and low size dispersion in larger quantities. As a result, research on the hot injection synthesis - which is typically used to synthesize these materials - is focusing on optimization and scale-up. This involves the high throughput screening of various synthesis parameters to achieve a desired end result, preferably at close to full chemical yield. Essentially, this involves a search for relations between the reaction conditions and the properties of the synthesized NCs, such as size and size dispersion. Here, we use a combination of reaction simulations and experimental synthesis screening using an established CdSe NCs synthesis to map different reaction conditions/nanocrystal property relations and link them to specific simulation parameters. We first show that the simulation model, which combines established expressions for the rate of NC nucleation and growth with the finding that a monomer formation reaction precedes the formation of NCs, captures the main features of the time development of the CdSe synthesis used.[1] Based on the simulations, we predict that the size NCs obtain at close to full yield can be tuned either by changing the rate of monomer formation or by changing the time span of nucleation. For both predictions, we find experimental evidence using the synthesis of CdSe NCs. First, we show that increasing the precursor concentrations results in a faster reaction and a decrease of the NC size, in line with the predicted link between monomer formation rate and nanocrystal size. Second, we find that increasing the concentration of free carboxylic acid leaves the reaction rate constant, yet results in larger NCs. In accordance with the simulations, we interpret this in terms of a reduction of the time span of nucleation linked to an enhanced monomer solubility. We demonstrate that these findings are not limited to the specific CdSe NC synthesis used. We therefore conclude that the enhanced understanding of reaction conditions/nanocrystal property relations following from this work provides a conceptual basis for developing size tuning strategies for the hot injection synthesis. [1] S. Abe et al., ACS Nano 2012, 6, 42-53

Various literature studies show that increasing the concentration of free acid in the hot Injection synthesis of colloidal nanocrystals raises the diameter of the resulting nanocrystals. We analyze this reaction chemistry/nanocrystal property relation by combining reaction simulations with an experimental study on a particular CdSe nanocrystal synthesis. We find that increasing the free acid concentration has the same effect on a real synthesis as raising the solute solubility in the simulations. Both lead to larger sizes and a deterioration of the size dispersion at constant reaction rate. Since free acids are used to coordinate the cation precursors in these syntheses, this leads to a meaningful link between a parameter in reaction simulations and the composition of an experimental reaction mixture. We thus explain the increase of the nanocrystal size with the acid concentration as resulting from an enhanced consumption of the solute by nanocrystal growth, which reduces the number of nanocrystals formed. This link between a simulation parameter and the composition of the reaction mixture provides a rational basis to further explore and understand reaction chemistry/nanocrystal property relations in the hot injection synthesis.

The absorption cross section of colloidal quantum dots in close-packed monolayers shows a 4 (CdSe) to 5-fold (PbS) enhancement compared to quantum dots in a dilute dispersion. Quantitative agreement is demonstrated between the value and the size dependence of the enhancement and theoretical model predictions based on dipolar coupling between neighboring quantum dots. This collective optical behavior offers a new degree of freedom in the custom design of optical properties for electro-optical devices.

Following their increasing use, the supply of larger quantities of monodisperse colloidal nanocrystals(NC) necessitates a scaling up of their production. As a result, synthesis cost, tuneability of the NC size at full yield and synthesis reproducibility have become key issues. Finding an optimal approach in this respect is a matter of methodology, involving the use of larger scale or automated batch reactors or continuous flow-line approaches, yet it also concerns a reassessment of the reagents used and the reaction conditions needed. This is especially true for selenium precursors used to synthesize metal selenide nanocrystals such as CdSe. These involve selenium dissolved in either tri-octylphosphine (TOP) [1]- which is expensive and oxygen sensitive - or 1-octadecene, which leads to a precursor (homogeneous ODE-Se) with a low reactivity and a limited reaction yield [2]. Here, we propose an alternative approach to synthesize metal selenide NC’s using a Se precursor that adds a high reactivity to the advantages of homogeneous ODE-Se. The method involves the direct injection of a heterogeneous mixture of selenium powder dispersed in a carrier liquid in a hot solvent containing a metal carboxylate as the cation precursor and excess carboxylic acid. Both in the case of cadmium and zinc carboxylates, we find that the injection of this heterogeneous ODE-Se precursor is followed by the formation of monodisperse nanocrystals, reaching chemical yields up to 80-90% within a few minutes. Moreover, the reaction can be run under ambient conditions without compromising the quality of the end product and the NC diameter reached at close to full yield can be tuned by changing the carboxylic acid chain length. In addition, since the amount of selenium injected is not limited by the solubility of selenium, the reaction can be executed with a high solid load, thus minimizing the amount of solvent needed. Finally, we demonstrate that syntheses involving the heterogeneous Se precursor as proposed here can be reproducibly executed on an automated synthesis platform, thus showing the potential of this novel approach for scaling up the production of colloidal metal selenide nanocrystals. [1] J. Am. Chem. Soc., 2001, 123 (1), pp 183-184 [2] J. Phys. Chem. B, 2005, 109 (44), pp 20665-20668