Luminescent CdSe/CdS core/shell nanocrystals in dendron boxes: superior chemical,photochemical and thermal stability

The surface ligands, generation-3 (G3) dendrons, on each semiconductor nanocrystal were globally cross-linked through ring-closing metathesis (RCM). The global cross-linking of the dendron ligands sealed each nanocrystal in a dendron box, which yielded box-nanocrystals. Although the dendron ligands coated CdSe nanocrystals (CdSe dendron-nanocrystals) were already quite stable, the stability of CdSe box-nanocrystals against chemical, photochemical, and thermal treatments were dramatically improved in comparison to that of the original dendron-nanocrystals. Furthermore, the box structure of the ligands monolayer coupled with the stable inorganic CdSe/CdS core/shell nanocrystals resulted in a class of extremely stable nanocrystal/ligands complexes. The band edge photoluminescence of the core/shell dendron-nanocrystals and box-nanocrystals were partially remained, and could be further brightened through controlled chemical oxidation or photooxidation. Practically, the stability of the box-nanocrystals is sufficient for most fundamental studies and technical applications. The box-nanocrystals may represent a general solution for the commonly encountered instability for many types of colloidal nanocrystals. The size distribution of the empty dendron boxes formed by the dissolution of the inorganic nanocrystals in concentrated HCl was very narrow. The empty boxes as new types of polymer capsules are soluble in solution, mesoporous, and with a very thin but stable peripheral. Those nanometer-sized cavities should be of interest for many purposes in the field of solution host-guest chemistry.     

Seed-mediated growth method for high-quality noble metal nanocrystals

This review highlights work from the authors’ laboratory on the recent development of seed-mediated growth method for noble metal nanocrystals. The seed-mediated growth method has become one of the most efficient and versatile methods for synthe-sizing high-quality noble metal nanocrystals. The seed-mediated growth method can separate the nucleation and growth stages of metal nanocrystals, and thus provide better control over the size, size distribution, and crystallographic evolution of metal nanocrystals. Because of its high controllability, the seed-mediated growth method is especially promising in providing mechanistic insights into the growth mechanisms of noble metal nanocrystals. In this review, the thermodynamic and kinetic parameters for the nucleation and growth of noble metal nanocrystals are systematically summarized. Mechanistic understanding of these parameters is provided. These studies provide useful guidelines for the rational design and synthesis of novel noble metal nanocrystals with high quality.     

Highly luminescent, stable, and water-soluble CdSe/CdS core-shell dendron nanocrystals with carboxylate anchoring groups

A dendron ligand with two carboxylate anchoring groups at its focal point and eight hydroxyl groups as its terminal groups was found to efficiently convert as-synthesized CdSe/CdS core-shell nanocrystals in toluene to water-soluble dendron-ligand stabilized nanocrystals (dendron nanocrystals). The resulting dendron nanocrystals retained 60% of the photoluminescence value of the original CdSe/CdS core-shell nanocrystals in toluene and were significantly brighter than the similar dendron nanocrystals with thiolate (deprotonated thiol group) as the anchoring group which retained just 10% of the photoluminescence value of the original CdSe/CdS core-shell nanocrystals in toluene. The carboxylate-based dendron nanocrystals survived UV irradiation in air for at least 13 days, about 9 times better than the thiolate-based dendron nanocrystals (35 h) and similar to that of the thiolate-based dendron-box stabilized CdSe/CdS core-shell nanocrystals (box nanocrystals). Upon UV irradiation, the dendron nanocrystals became even 2 times brighter than the original CdSe/CdS core-shell nanocrystals in toluene, and the UV-brightened PL can retain the brightness for at least several months. These stable and bright dendron nanocrystals were soluble in various aqueous media, including all common biological buffer solutions tested, for at least 1.5 years. In addition to their superior performance, the synthetic chemistry of carboxylate dendron ligands and the corresponding dendron nanocrystals is relatively simple and with high yield.     

Hybrid organic-inorganic nanomaterials based on polythiophene dendronized nanoparticles

In this work, the synthesis, characterization, and applications of branched oligothiophene dendrons that act as electroactive surfactants for the capping of Au metal nanoparticles and CdSe quantum dots are described. Two distinct methods have been employed for synthesis: a ligand exchange process and a direct-capping synthesis approach. The coverage of the dendrons per nanocrystal, the nature of the surface coordination interactions, and energy transfer interactions were studied in detail using UV-vis absorbance, FT-IR, AFM, TEM, and photoluminescence spectroscopy. The competition/displacement in ligand metathesis is highlighted by the size of the dendron and nature of binding on semiconductor nanocrystals. In the other system using the direct capping method, the size of the Au nanoparticle is mediated by the dimensions of the ligand, i.e. alkyl chain spacer and dendron branching or size. These hybrid dendron/nanoparticle complexes are generally very soluble and stable in non-polar solvents. They exhibit energy transfer, surface plasmon resonance effects, and photoinduced charge transfer interactions between the metal/semiconductor and conjugated ligands. Adsorption on mica and graphite surfaces was observed. A one-layer photovoltaic cell was fabricated to demonstrate the potential for device applications.     

Green chemical approaches toward high-quality semiconductor nanocrystals

Green chemistry principles have gradually been implemented into the development of the synthetic chemistry of high-quality semiconductor nanocrystals. In comparison with the original organometallic approach, the resulting alternative routes are safe, simple, inexpensive, reproducible, versatile, "user friendly", and yield nanocrystals with well-controlled size, shape, and size/shape distribution. Further developments in this direction will promote the understanding of crystallization in general.     

Symmetry-controlled colloidal nanocrystals: nonhydrolytic chemical synthesis and shape determining parameters

Since inorganic nanocrystals exhibit unique shape-dependent nanoscale properties and can be utilized as basic building blocks for futuristic nanodevices, a systematic study on the shape control of these nanocrystals remains an important subject in materials and physical chemistry. In this feature article, we overview the recent progress on the synthetic development of symmetry-controlled colloidal nanocrystals of semiconductor and metal oxide, which are prepared through nonhydrolytic chemical routes. We describe their shape-guiding processes and illustrate the detailed key factors controlling their growth by examining various case studies of zero-dimensional spheres and cubes, one-dimensional rods, and quasi multidimensional structures such as disks, multipods, and stars. Specifically, the crystalline phase of nucleating seeds, surface energy, kinetic vs thermodynamic growth, and selective adhesion processes of capping ligands are found to be most crucial for the determination of the nanocrystal shape.     

A facile "dispersion-decomposition" route to metal sulfide nanocrystals

In this paper, we demonstrate a simple and general "dispersion-decomposition" approach to the synthesis of metal sulfide nanocrystals with the assistance of alkylthiol. This is a direct heating process without precursor injection. By using inorganic metal salts and alkylthiol as the raw materials, high-quality Ag(2)S, Cu(2)S, PbS, Ni(3)S(4), CdS, and ZnS nanocrystals were successfully synthesized. The mechanism study shows that the reaction undergoes two steps. A key intermediate compound, metal thiolate, is generated first. It melts and disperses into the solvent at a relatively low temperature, and then it decomposes into metal sulfide as a single precursor upon heating. This method avoids using toxic phosphine agent and injection during the reaction process. The size and shape of the nanocrystal can be also controlled by the concentration of the reactant and ligands. Furthermore, the optical properties and assembly of the nanocrystals have also been studied. This report provides a facile, direct-heating "dispersion-decomposition" approach to synthesize metal sulfides nanocrystals that has potential for future large-scale synthesis.     

Effective octadecylamine system for nanocrystal synthesis

New chemical reactions and synthetic systems are of key importance for materials fabrication. In this work, we reported a facile and effective octadecylamine (ODA) synthetic system for various nanocrystals including metals, mixed metal oxides, metal/metal oxide heterostructured nanocrystals, intermetallics, and alloys. We found that the products were mainly determined by metal ions used in our synthetic system: noble metal ions led to the formation of metals; two kinds of non-noble metal ions led to the formation of mixed metal oxides; silver ions and non-noble metal ions led to the formation of metal/metal oxide heterostructured nanocrystals; non-noble metal ions and noble metal (excluding Ag) ions led to the formation of intermetallics and alloys. The difference was attributed to different ability to attract electrons from ODA solvent among these metal ions. This effective system provides a general strategy for various nanocrystals which would find potential applications in many significant fields.     

Poly(ethylene glycol)-based multidentate oligomers for biocompatible semiconductor and gold nanocrystals

We have developed a new set of multifunctional multidentate OligoPEG ligands, each containing a central oligomer on which were laterally grafted several short poly(ethylene glycol) (PEG) moieties appended with either thioctic acid (TA) or terminally reactive groups. Reduction of the TAs (e.g., in the presence of NaBH(4)) provides dihydrolipoic acid (DHLA)-appended oligomers. Here the insertion of PEG segments in the ligand structure promotes water solubility and reduces nonspecific interactions, while TA and DHLA groups provide multidentate anchoring onto Au nanoparticles (AuNPs) and ZnS-overcoated semiconductor quantum dots (QDs), respectively. The synthetic route involves simple coupling chemistry using N,N-dicylohexylcarbodiimide (DCC). Water-soluble QDs and AuNPs capped with these ligands were prepared via cap exchange. As prepared, the nanocrystals dispersions were aggregation-free, homogeneous, and stable for extended periods of time over pH ranging from 2 to 14 and in the presence of excess electrolyte (2 M NaCl). The new OligoPEG ligands also allow easy integration of tunable functional and reactive groups within their structures (e.g., azide or amine), which imparts surface functionalities to the nanocrystals and opens up the possibility of bioconjugation with specific biological molecules. The improved colloidal stability combined with reactivity offer the possibility of using the nanocrystals as biological probes in an array of complex and biologically relevant media.     

Surface-functionalization-dependent optical properties of II-VI semiconductor nanocrystals

We report a study of the surface-functionalization-dependent optical properties of II-VI zinc-blende semiconductor nanocrystals on the basis of ligand-exchange chemistry, isomaterial core/shell growth, optical spectroscopy, transmission electron microscopy, and X-ray powder diffraction. Our results show that the transition energy and extinction coefficient of the 2S(h3/2)1S(e) excitonic band of these nanocrystals can be strongly modified by their surface ligands as well as ligand associated surface atomic arrangement. The oleylamine exchange of oleate-capped zinc-blende II-VI nanocrystals narrows the energy gap between their first and second excitonic absorption bands, and this narrowing effect is size-dependent. The oleylamine exchange results in the quenching, subsequent recovery, and even enhancing of the photoluminescence emission of these II-VI semiconductor nanocrystals. In addition, the results from our X-ray powder diffraction measurements and simulations completely rule out the possibility that oleate-capped zinc-blende CdSe nanocrystals can undergo zinc-blende-to-wurtzite crystal transformation upon ligand exchange with oleylamine. Moreover, our theoretical modeling results suggest that the surface-functionalization-dependent optical properties of these semiconductor nanocrystals can be caused by a thin type II isomaterial shell that is created by the negatively charged ligands (e.g., oleate and octadecyl phosphonate). Taking all these results together, we provide the unambiguous identification that II-VI semiconductor nanocrystals exhibit surface-functionalization-dependent excitonic absorption features.     

Metal-free inorganic ligands for colloidal nanocrystals: S2-, HS-, Se2-, HSe-, Te2-, HTe-, TeS3(2-), OH-, and NH2- as surface ligands

All-inorganic colloidal nanocrystals were synthesized by replacing organic capping ligands on chemically synthesized nanocrystals with metal-free inorganic ions such as S(2-), HS(-), Se(2-), HSe(-), Te(2-), HTe(-), TeS(3)(2-), OH(-) and NH(2)(-). These simple ligands adhered to the NC surface and provided colloidal stability in polar solvents. The versatility of such ligand exchange has been demonstrated for various semiconductor and metal nanocrystals of different size and shape. We showed that the key aspects of Pearson's hard and soft acids and bases (HSAB) principle, originally developed for metal coordination compounds, can be applied to the bonding of molecular species to the nanocrystal surface. The use of small inorganic ligands instead of traditional ligands with long hydrocarbon tails facilitated the charge transport between individual nanocrystals and opened up interesting opportunities for device integration of colloidal nanostructures.     

Single-phase and gram-scale routes toward nearly monodisperse Au and other noble metal nanocrystals

Single-phase approaches are introduced for the synthesis of nearly monodisperse Au and other noble metal nanocrystals. The new approaches possess all the advantages of the popular Brust method. With weak ligands or surfactants for the metal ions, the control of the size and size distribution of the nanocrystals in synthesis in the size range between 1 and 15 nm was achieved via maintaining balanced nucleation and growth by tuning the activities of the metal precursors and reducing reagents. Because only weak ligands are employed in the new synthetic schemes, surface modification and functionalization of the resulting nanocrystals can be readily carried out.     

Constraints in post-synthesis ligand exchange for hybrid organic (MEH-PPV)–inorganic (CdSe) nanocomposites

For an optimum charge/energy transfer performance of hybrid organic–inorganic colloidal nanocrystals for applications such as photonic devices and solar cells, the determining factors are the distance between the nanocrystal and polymer which greatly depends upon nanocrystal size/nanocrystal ligands. Short chain ligands are preferred to ensure a close contact between the donor and acceptor as a result of the tunnelling probability of the charges and the insulating nature of long alkyl chain molecules. Short distances increase the probability for tunnelling to occur as compared to long distances induced by long alkyl chains of bulky ligands which inhibit tunnelling altogether. The ligands on the as-synthesized nanocrystals can be exchanged for various other ligands to achieve desirable charge/energy transfer properties depending on the bond strength of the ligand on the nanocrystal compared to the replacement ligand. In this work, the constraints involved in post-synthesis ligand exchange process have been evaluated, and these factors have been tuned via wet chemistry to tailor the hybrid material properties via appropriate selection of the nanocrystal capping ligands. It has been found that both oleic acid and oleylamine (OLA)-capped cadmium selenide (CdSe) quantum dots (QDs) as compared with trioctylphosphine oxide (TOPO)-passivated CdSe QDs are of high quality, and they provide better steric stability against coagulation, homogeneity, and photostability to their respective polymer:CdSe nanocomposites. CdSe QDs particularly with OLA capping have relatively smaller surface energies, and thus, lesser quenching capabilities show dominance of photoinduced Forster energy transfer between donors (polymer) and acceptors (CdSe nanocrystals) as compared to charge transfer mechanism as observed in polymer:CdSe (TOPO) composites. It is conjectured that size quantization effects, stereochemical compatibility of ligands (TOPO, oleic acid, and oleyl amine), and polymer MEH-PPV stability greatly influence the photophysics and photochemistry of hybrid polymer–semiconductor nanocomposites.     

Bioactivation and cell targeting of semiconductor CdSe/ZnS nanocrystals with phytochelatin-related peptides

Synthetic phytochelatin-related peptides are used as an organic coat on the surface of colloidal CdSe/ZnS semiconductor nanocrystals synthesized from hydrophobic coordinating trioctyl phosphine oxide (TOPO) solvents. The peptides are designed to bind to the nanocrystals via a C-terminal adhesive domain. This adhesive domain, composed of multiple repeats of cysteines pairs flanked by hydrophobic 3-cyclohexylalanines, is followed by a flexible hydrophilic linker domain to which various bio-affinity tags can be attached. This surface coating chemistry results in small, buffer soluble, monodisperse peptide-coated nanoparticles with high colloidal stability and ensemble photophysical properties similar to those of TOPO-coated nanocrystals. Various peptide coatings are used to modulate the nanocrystal surface properties and to bioactivate the nanoparticles. CdSe/ZnS nanocrystals coated with biotinylated peptides efficiently bind to streptavidin and are specifically targeted to GPI-anchored avidin-CD14 chimeric proteins expressed on the membranes of live HeLa cells. This peptide coating surface chemistry provides a novel approach for the production of biocompatible photoluminescent nanocrystal probes.     

Colloidal Noble‐Metal and Bimetallic Alloy Nanocrystals: A General Synthetic Method and Their Catalytic Hydrogenation Properties

A general single‐step strategy has been developed for the direct thermal decomposition of noble‐metal salts in octadecylamine to synthesize octahedron‐ and rod‐shaped noble‐metal aggregates and monodisperse noble‐metal or bimetallic alloy nanocrystals without introducing any additive into the system. It has presented a facile and economic way to fabricate these nanocrystals, especially alloy nanocrystals, which does not require a post‐synthesis solid‐state annealing process. The morphology of the nanocrystals can be easily controlled by tuning the synthetic temperature. Their ability to catalyze heterogeneous Suzuki coupling reactions has been investigated and showed satisfactory catalytic activity. The catalytic performance of the monometallic and bimetallic alloy nanocrystals were also evaluated in the selective hydrogenation of citral in a conventional organic solvent (toluene) and a green solvent (supercritical carbon dioxide, scCO₂). Interestingly, the catalysts performed differently to each other when they were in scCO₂ owing to the different morphology, which should be readily optimized for further use.     

Effect of metal ions on photoluminescence, charge transport, magnetic and catalytic properties of all-inorganic colloidal nanocrystals and nanocrystal solids

Colloidal semiconductor nanocrystals (NCs) provide convenient "building blocks" for solution-processed solar cells, light-emitting devices, photocatalytic systems, etc. The use of inorganic ligands for colloidal NCs dramatically improved inter-NC charge transport, enabling fast progress in NC-based devices. Typical inorganic ligands (e.g., Sn(2)S(6)(4-), S(2-)) are represented by negatively charged ions that bind covalently to electrophilic metal surface sites. The binding of inorganic charged species to the NC surface provides electrostatic stabilization of NC colloids in polar solvents without introducing insulating barriers between NCs. In this work we show that cationic species needed for electrostatic balance of NC surface charges can also be employed for engineering almost every property of all-inorganic NCs and NC solids, including photoluminescence efficiency, electron mobility, doping, magnetic susceptibility, and electrocatalytic performance. We used a suite of experimental techniques to elucidate the impact of various metal ions on the characteristics of all-inorganic NCs and developed strategies for engineering and optimizing NC-based materials.     

Noble metal-free catalysts for oxygen reduction reaction

Developing noble metal-free catalysts with low cost, high performance and stability for oxygen reduction reaction(ORR) in fuel cells is of great interest to promote sustainable energy devices. In this review, we summarized noble metal-free catalysts for ORR,including non-noble metal-based and heteroatom-doped carbon nanomaterials. Mesoporous structure, homogeneous distribution of nanocrystals and synergistic effect of carbon base and nanocrystals/doped heteroatoms have great effect on the ORR property.The noble metal-free nanomaterials showed comparable catalytic property, better stability and methanol tolerance than commercial platinum(Pt)-based catalysts, showing great potential as substitutes for noble metal-based catalysts. In addition, the challenges and chances of developing noble metal-free ORR catalysts are also discussed.     

Methods for the Synthesis of μ-Hydrocarbon Transition Metal Complexes without Metal–Metal Bonds

Transition metal complexes in which hydrocarbons serve as σ,σ-, σ,π- or π,π-bound bridging ligands are currently of great interest. This review presents efficient and directed syntheses for such compounds, which often have very aesthetic structures. These reactions are among the most important reaction types in modern organometallic chemistry. They can be a useful aid for the synthesis of tailor-made compounds, for example, for models of catalytic processes and, specifically, for the construction of heterometallic compounds. We will discuss reactions of electrophilic complexes with nucleophilic ones, numerous transformations of (functionalized) hydrocarbons with metal complexes, the currently very topical complexes with bridging acetylide and carbide ligands, and organometallic polymers, which can be expected to have interesting and novel materials properties. Chisholm

1 M. H. Chisholm, Polyhedron 1988 , 7, 757–1077.

has described the importance of these complexes as follows: “Central to the development of polynuclear and cluster chemistry are bridging ligands and central to organometallic chemistry are metal–carbon bonds. Thus bridging ligands hold a pivotal role ins the development of Binuclear and polynuclear organometallic chemistry”.     

Shape-Controlled Nanocrystals for Catalytic Applications

The activity, selectivity, and long-term stability of catalyst nanoparticles can be enhanced by shape modulation. Such shaped catalytic nanocrystals have well-defined surface crystalline structures on which the cleavage and recombination of chemical bonds can be rationally controlled. Metal and metal oxide nanocrystals have been synthesized in various shapes using wet chemistry techniques such as reducing metal precursors in the presence of the surface-capping agents. The surface-capping agents should be removed prior to the catalytic chemical reaction, which necessitates clean catalytically active surface. The removal process should be performed very carefully because this removal often causes shape deformation. A few examples in which the surface-capping agents contribute positively to the chemical reactions have been reported. The examples described in this review include shaped metal, metal composite, and metal oxide nanocrystals that show enhanced catalytic activity, selectivity, and long-term stability for various gas-phase, liquid-phase, or electrocatalytic reactions. Although most of the studies using these shaped nanocrystals for catalytic applications have focused on low-index surfaces, nanocrystals with high-index facets and their catalytic applications have recently been reported. By bridging surface studies with nanoparticle catalysts using shape modulation, catalysts with improved properties can be rationally designed.     

无机纳米材料的光化学合成

无机纳米材料的合成是纳米科学发展的前提和基础之一。区别于传统的高温湿化学合成法,光化学方法在无机纳米材料的合成中表现出许多优点,并在近年来受到了广泛关注。本文分三个部分综述了近年来光化学方法在无机纳米材料合成中的应用,具体包括贵金属纳米材料的光化学合成与负载,半导体纳米材料的光化学合成以及表面等离子体共振诱导的各向异性金属纳米晶合成。最后,在总结光化学方法在无机纳米材料合成中体现出的优势及目前研究仍存在不足的基础上,我们对其未来可能的发展方向进行了展望。