Synthesis Of Colloidal Nanocrystals Research Articles

Design and Synthesis of Multi-Component Colloidal Nanocrystals for Catalysis and Sensing

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Nanocrystal Diffusion Doping

A diffusion-based synthesis of doped colloidal semiconductor nanocrystals is demonstrated. This approach involves thermodynamically controlled addition of both impurity cations and host anions to preformed seed nanocrystals under equilibrium conditions, rather than kinetically controlled doping during growth. This chemistry allows thermodynamic crystal compositions to be prepared without sacrificing other kinetically trapped properties such as shape, size, or crystallographic phase. This doping chemistry thus shares some similarities with cation-exchange reactions, but proceeds without the loss of host cations and excels at the introduction of relatively unreactive impurity ions that have not been previously accessible using cation exchange. Specifically, we demonstrate the preparation of Cd(1-x)Mn(x)Se (0 ≤ x ≤ ∼0.2) nanocrystals with narrow size distribution, unprecedentedly high Mn(2+) content, and very large magneto-optical effects by diffusion of Mn(2+) into seed CdSe nanocrystals grown by hot injection. Controlling the solution and lattice chemical potentials of Cd(2+) and Mn(2+) allows Mn(2+) diffusion into the internal volumes of the CdSe nanocrystals with negligible Ostwald ripening, while retaining the crystallographic phase (wurtzite or zinc blende), shape anisotropy, and ensemble size uniformity of the seed nanocrystals. Experimental results for diffusion doping of other nanocrystals with other cations are also presented that indicate this method may be generalized, providing access to a variety of new doped semiconductor nanostructures not previously attainable by kinetic routes or cation exchange.

Highly Luminescent, Size- and Shape-Tunable Copper Indium Selenide Based Colloidal Nanocrystals.

We report a simple, high-yield colloidal synthesis of copper indium selenide nanocrystals (CISe NCs) based on a silylamide-promoted approach. The silylamide anions increase the nucleation rate, which results in small-sized NCs exhibiting high luminescence and constant NC stoichiometry and crystal structure regardless of the NC size and shape. In particular, by systematically varying synthesis time and temperature, we show that the size of the CISe NCs can be precisely controlled to be between 2.7 and 7.9 nm with size distributions down to 9–10%. By introducing a specific concentration of silylamide-anions in the reaction mixture, the shape of CISe NCs can be preselected to be either spherical or tetrahedral. Optical properties of these CISe NCs span from the visible to near-infrared region with peak luminescence wavelengths of 700 to 1200 nm. The luminescence efficiency improves from 10 to 15% to record values of 50–60% by overcoating as-prepared CISe NCs with ZnSe or ZnS shells, highlighting their potential for applications such as biolabeling and solid state lighting.

Nb-Doped Colloidal TiO2 Nanocrystals with Tunable Infrared Absorption

We report a new colloidal synthesis of niobium-doped TiO2 anatase nanocrystals (NCs) that allows for the preparation of ∼10 nm NCs with control over the amount of Nb doping up to ∼14%. The incorporation of niobium ions leads to the appearance of a tunable, broad absorption peak that ranges from the visible range to the mid-infrared. This optical behavior is attributed to the substitution of Nb5+ on Ti4+ sites generating free carriers inside the conduction band of the TiO2 NCs as supported by optical and electron paramagnetic resonance spectroscopic investigations. At the same time, the incorporation of progressively more niobium ions drives an evolution of the shape of the NCs from tetragonal platelets to “peanutlike” rods.

ChemInform Abstract: Advances in the Colloidal Synthesis of Two‐Dimensional Semiconductor Nanoribbons.

AbstractReview: emphasis on CdSe; 84 refs.

Nanotoxicology using the sea anemone Nematostella vectensis: from developmental toxicity to genotoxicology

Concomitant with the fast-growing advances in the synthesis and engineering of colloidal nanocrystals, an urgent evaluation of their toxicity on human beings and environment is strongly encouraged by public health organisations. Despite the in vitro approaches employed for toxicological screening of hazardous compounds, the use of simple and cost-effective living organisms may enormously contribute to solve unanswered questions related to embryotoxic and teratogenic effects of nanomaterials. Here, the sea anemone Nematostella vectensis (Cnidaria, Anthozoa) is presented as a novel model organism to profile bio/non-bio interactions and to show a comprehensive toxicological analysis performed on embryos, larvae and adults treated with fluorescent cadmium-based nanocrystals. Spanning from in vivo biodistribution to molecular investigations, different behaviours and effects depending on the composition and surface coatings are showed. Rod-shaped cadmium selenide/cadmium sulfide (CdSe/CdS) nanocrystals resulted in excellent imaging probes to track N. vectensis development with negligible adverse effects, while spherical CdTe nanocrystals severely impaired embryogenesis, resulting in aberrant phenotypes and deregulation of developmental genes, which raise severe worries for a safe use of this type of nanoparticles for human purposes and environmental contamination.

Codoping n- and p-Type Impurities in Colloidal Silicon Nanocrystals: Controlling Luminescence Energy from below Bulk Band Gap to Visible Range

We present a novel synthesis of ligand-free colloidal silicon nanocrystals (Si-NCs) that exhibits efficient photoluminescence (PL) in a wide energy range (0.85–1.8 eV) overcoming the bulk Si band gap limitation (1.12 eV). The key technology to achieve the wide-range controllable PL is the formation of donor and acceptor states in the band gap of Si-NCs by simultaneous doping of n- and p-type impurities. The colloidal Si-NCs are very stable in an ordinary laboratory atmosphere for more than a year. Furthermore, the PL spectra are very stable and are not at all affected even when the colloids are drop-cast on a substrate and dried in air. The engineering of the all-inorganic colloidal Si-NC and its optical data reported here are important steps for Si-based optoelectronic and biological applications.

Enhancing the Efficiency of Solution-Processed Polymer:Colloidal Nanocrystal Hybrid Photovoltaic Cells Using Ethanedithiol Treatment

Advances in colloidal inorganic nanocrystal synthesis and processing have led to the demonstration of organic-inorganic hybrid photovoltaic (PV) cells using low-cost solution processes from blends of conjugated polymer and colloidal nanocrystals. However, the performance of such hybrid PV cells has been limited due to the lack of control at the complex interfaces between the organic and inorganic hybrid active materials. Here we show that the efficiency of hybrid PV devices can be significantly enhanced by engineering the polymer-nanocrystal interface with proper chemical treatment. Using two different conjugated polymers, poly(3-hexylthiophene) (P3HT) and poly[2,6-(4,4-bis(2-ethylhexyl)-4H-cyclopenta[2,1-b;3,4-b']-dithiophene)-alt-4,7-(2,1,3-benzothiadiazole)] (PCPDTBT), we show that treating the polymer:nanocrystal hybrid film in an ethanedithiol-containing acetonitrile solution can increase the efficiency of the hybrid PV devices by 30-90%, and a maximum power conversion efficiency of 5.2 ± 0.3% was obtained in the PCPDTBT:CdSe devices at 0.2 sun (AM 1.5G), which was slightly reduced to 4.7 ± 0.3% at 1 sun. The ethanedithiol treatment did not result in significant changes in the morphology and UV-vis optical absorption of the hybrid thin films; however, infrared absorption, NMR, and X-ray photoelectron spectroscopies revealed the effective removal of organic ligands, especially the charged phosphonic acid ligands, from the CdSe nanorod surface after the treatment, accompanied by the possible monolayer passivation of nanorod surfaces with Cd-thiolates. We attribute the hybrid PV cell efficiency increase upon the ethanedithiol treatment to the reduction in charge and exciton recombination sites on the nanocrystal surface and the simultaneous increase in electron transport through the hybrid film.

ChemInform Abstract: Biomolecular Specificity Controlled Nanomaterial Synthesis

AbstractReview: 67 refs.

Seed-Mediated Growth of Shape-Controlled Wurtzite CdSe Nanocrystals: Platelets, Cubes, and Rods

Prior investigations into the synthesis of colloidal CdSe nanocrystals with a wurtzite crystal structure (wz-CdSe) have given rise to well-developed methods for producing particles with anisotropic shapes such as rods, tetrapods, and wires; however, the synthesis of other shapes has proved challenging. Here we present a seed-mediated approach for the growth of colloidal, shape-controlled wz-CdSe nanoparticles with previously unobserved morphologies. The synthesis, which makes use of small (2-3 nm) wz-CdSe nanocrystals as nucleation sites for subsequent growth, can be tuned to selectively yield colloidal wz-CdSe nanocubes and hexagonal nanoplatelets in addition to nanorod and bullet-shaped particles. We thoroughly characterize the morphology and crystal structures of these new shapes, as well as discuss possible growth mechanisms in the context of control over surface chemistry and the nucleation stage.

Colloidal Synthesis of InSb Nanocrystals with Controlled Polymorphism Using Indium and Antimony Amides

We report a new synthetic pathway for growing monodisperse colloidal indium antimonide nanocrystals. We propose that highly reactive element-nitrogen bonded precursors, such as In and Sb amides, may provide required nucleation and growth kinetics for the formation of uniform colloidal nanocrystals of InSb. Size-dependent absorption and emission spectra of InSb nanocrystals in the near-infrared region of 1500–2000 nm point to semiconductor behavior with quantum confinement. Furthermore, we demonstrate zinc blende/wurzite polymorphism and polytypism of InSb nanocrystals, conveniently controlled by the In/Sb molar ratio of precursors. This amide-based synthesis route may open new opportunities for designing near- and mid-IR active III–V semiconductor nanostructures.

Shape-Programmed Nanofabrication: Understanding the Reactivity of Dichalcogenide Precursors

Dialkyl and diaryl dichalcogenides are highly versatile and modular precursors for the synthesis of colloidal chalcogenide nanocrystals. We have used a series of commercially available dichalcogenide precursors to unveil the molecular basis for the outcome of nanocrystal preparations, more specifically, how precursor molecular structure and reactivity affect the final shape and size of II-VI semiconductor nanocrystals. Dichalcogenide precursors used were diallyl, dibenzyl, di-tert-butyl, diisopropyl, diethyl, dimethyl, and diphenyl disulfides and diethyl, dimethyl, and diphenyl diselenides. We find that the presence of two distinctively reactive C-E and E-E bonds makes the chemistry of these precursors much richer and interesting than that of other conventional precursors such as the more common phosphine chalcogenides. Computational studies (DFT) reveal that the dissociation energy of carbon-chalcogen (C-E) bonds in dichalcogenide precursors (R-E-E-R, E=S or Se) increases in the order (R): diallyl<dibenzyl<di-tert-butyl<diisopropyl<diethyl<dimethyl<diphenyl. The dissociation energy of chalcogen-chalcogen (E-E) bonds remains relatively constant across the series. The only exceptions are diphenyl dichalcogenides, which have a much lower E-E bond dissociation energy. An increase in C-E bond dissociation energy results in a decrease in R-E-E-R precursor reactivity, leading to progressively slower nucleation and higher selectivity for anisotropic growth, all the way from dots to pods to tetrapods. Under identical experimental conditions, we obtain CdS and CdSe nanocrystals with spherical, elongated, or tetrapodal morphology by simply varying the identity and reactivity of the dichalcogenide precursor. Interestingly, we find that precursors with strong C-E and weak E-E bond dissociation energies such as Ph-S-S-Ph serve as a ready source of thiol radicals that appear to stabilize small CdE nuclei, facilitating anisotropic growth. These CdS and CdSe nanocrystals have been characterized using structural and spectroscopic methods. An intimate understanding of how molecular structure affects the chemical reactivity of molecular precursors enables highly predictable and reproducible synthesis of colloidal nanocrystals with specific sizes, shapes, and optoelectronic properties for customized applications.

Advances in the Colloidal Synthesis of Two-Dimensional Semiconductor Nanoribbons

The shape-controlled synthesis of colloidal semiconductor nanocrystals has attracted a lot of interest because of their shape-dependent physical and chemical properties. With the growing understanding of the shape evolution, several successful syntheses of two-dimensional (2D) nanocrystals have been reported. In this review, we focus on the recent advances in the colloidal synthesis of 2D CdSe nanoribbons with a wurtzite structure. 2D lamellar assemblies of CdSe clusters are generated in the early stage of the synthesis, and then they direct the formation of 2D structure. The dense organic layers on the {112̅0} facets significantly stabilize the 2D nanoribbons. The nanoribbons exhibit unique optical properties originating from the strong 1D quantum confinement within their extremely uniform and thin thickness. Interestingly, a large amount of manganese(II) ions can be incorporated into CdSe nanoribbons via a nucleation-controlled doping process. A high doping concentration and the strong confinement effect of the nanoribbons result in enhanced magneto-optical properties that are suitable for spintronic device applications. CdS nanoribbons have a very similar formation pathway, which suggests the cluster-assembled approaches can potentially be extended to various other materials.

Low-Temperature Processed Ga-Doped ZnO Coatings from Colloidal Inks

We present a new colloidal synthesis of gallium-doped zinc oxide nanocrystals that are transparent in the visible and absorb in the near-infrared. Thermal decomposition of zinc stearate and gallium nitrate after hot injection of the precursors in a mixture of organic amines leads to nanocrystals with tunable properties according to gallium amount. Substitutional Ga(3+) ions trigger a plasmonic resonance in the infrared region resulting from an increase in the free electrons concentration. These nanocrystals can be deposited by spin coating, drop casting, and spray coating resulting in homogeneous and high-quality thin films. The optical transmission of the Ga-ZnO nanoparticle assemblies in the visible is greater than 90%, and at the same time, the near-infrared absorption of the nanocrystals is maintained in the films as well. Several strategies to improve the films electrical and optical properties have been presented, such as UV treatments to remove the organic compounds responsible for the observed interparticle resistance and reducing atmosphere treatments on both colloidal solutions and thin films to increase the free carriers concentration, enhancing electrical conductivity and infrared absorption. The electrical resistance of the nanoparticle assemblies is about 30 kΩ/sq for the as-deposited, UV-exposed films, and it drops down to 300 Ω/sq after annealing in forming gas at 450 °C, comparable with state of the art tin-doped indium oxide coatings deposited from nanocrystal inks.

Colloidal Branched Semiconductor Nanocrystals: State of the Art and Perspectives

Colloidal inorganic nanocrystals are versatile nanoscale building blocks. Advances in their synthesis have yielded nanocrystals with various morphologies including spheres, polyhedra, rods, disks, sheets, wires, and a wide range of branched shapes. Recent developments in chemical methods have allowed the synthesis of colloidal nanocrystals made of sections of different inorganic materials connected together. Many research groups are investigating these nanocrystals' structural and photophysical properties experimentally and theoretically, and many have examined their prospects for commercial applications. Branched nanocrystals, in particular, are gaining attention, in part for their potential applications in solar cells or electronic devices. In this Account, we review recent developments in synthesis and controlled assembly of colloidal branched nanocrystals. Synthesis of branched nanocrystals builds on previous work with spherical nanocrystals and nanorods, but a unique factor is the need to control the branching event. Multiple arms can branch from a nucleus, or secondary branches can form from a growing arm. Branching can be governed by mechanisms including twinning, crystal splitting, polymorphism, oriented attachment, and others. One of the most relevant parameters is the choice of appropriate surfactant molecules, which can bind selectively to certain crystal facets or can even promote specific crystallographic phases during nucleation and growth. Also, seeded growth approaches recently have allowed great progress in the synthesis of nanocrystals with elaborate shapes. In this approach, nanocrystals with a specified chemical composition, size, shape, crystalline habit, and phase act as seeds on which multiple branches of a second material nucleate and grow. These approaches yield nanostructures with improved homogeneity in distribution of branch length and cross section. Ion exchange reactions allow further manipulation of branched nanocrystals by transforming crystals of one material into crystals with the same size, shape, and anion sublattice but with a new cation. Combining seeded growth with ion exchange provides a method for greatly expanding the library of branched nanocrystals. Assembly of morphologically complex nanocrystals is evolving in parallel to developments in chemical synthesis. While researchers have made many advances in the past decade in controlled assembly of nanocrystals with simple polyhedral shapes, modeling and experimental realization of ordered superstructures of branched nanocrystals are still in their infancy. In the only case of ordered superstructure reported so far, the assembly proceeds by steps in a hierarchical fashion, in analogy to several examples of assembly found in nature. Meanwhile, disordered assemblies of branched nanocrystals are also interesting and may find applications in various fields.

Reaction Chemistry/Nanocrystal Property Relations in the Hot Injection Synthesis, the Role of the Solute Solubility

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.

A new mild synthesis and optical properties of colloidal ZnO nanocrystals in dimethylformamide/ethanol solutions

ABSTRACTA green and mild synthesis of colloidal zinc oxide nanocrystals in ethanol/dimethylformamide mixtures was introduced which allows to produce stable crystalline ZnO particles and tailor their average size in the range of 2.8−4.5 nm by varying temperature and duration of post-synthesis ageing. An increase in dimethylformamide fraction in the mixture results in acceleration of ZnO nanocrystals ripening. Colloidal ZnO nanocrystals emit broadband photoluminescence in the range of 2−3 eV with the quantum yields of up to 12 %.

A general single-pot heating method for morphology, size and luminescence-controllable synthesis of colloidal ZnO nanocrystals

Here we demonstrate a single-pot heating approach for controllable synthesis of colloidal zinc oxide nanocrystals from zinc acetylacetonate. Such single-pot heating approaches are inherently amenable to large scale production. We have systematically studied the crystallization process, investigating the growth of nanocrystals and the influence of ligands on the morphology and luminescence of ZnO nanoparticles. We show that the morphology can be tuned to produce dendritic structures, nano-needles, nano-pinecones, nanoclusters, hexagonal pyramid nanoparticles and irregularly-shaped nanoparticles by varying the surfactants and co-surfactants used in synthesis. Moreover, we investigated the effect of ligands on the defect-related photoluminescence of ZnO NPs and demonstrated blue, green, white, yellow, and orange emission. This study opens up new possibilities for the practical use of ZnO nanomaterials for optoelectronic devices and bio-imaging.

Biomolecular specificity controlled nanomaterial synthesis

Biomolecules capable of fabricating complex nanomaterials with required functions in nature have been exploited to artificially control nanomaterial synthesis in all aspects. This tutorial review provides an overview of recent efforts in biomimetic synthesis and the relevant mechanistic studies on biomolecular specificities toward material surfaces. It starts with a discussion of the state-of-the-art progress in colloidal nanocrystal synthesis, wherein the importance of the interfacial control over nanoscale building blocks discloses the potential of exploiting biomolecular recognition properties in nanostructure synthesis. Continued discussions will review the progress in biomimetic syntheses of different classes of nanoscale materials. In vitro biomimetic syntheses with both biomolecules isolated from organisms and material-specific peptide sequences selected from combinatorial molecular evolution processes will be demonstrated. The final part of the review presents the recent research efforts and advances in understanding biomolecule-inorganic material interactions. It is believed that with continued experimental efforts and fundamental understanding of biomolecule-material interactions scientists can one day harvest the ability to rationally design molecules to produce intricate material structures with a similar level of sophistication, precision, and superior functions as those found in nature.

Colloidal synthesis and characterizations of wittichenite copper bismuth sulphide nanocrystals

Wittichenite (Cu(3)BiS(3)) nanocrystals have been synthesized utilizing a hot-injection method. They have been characterized in detail. Cu(3)BiS(3) nanocrystals possessed a band gap of 1.56 eV, and the corresponding nanocrystal-film shows a clear photoresponse in I-V measurements. Our work illustrates that Cu(3)BiS(3) nanocrystals have potential in the application of solar cells.