Influence of particle architecture on the photoluminescence properties of silica-coated CdSe core/shell quantum dots.

Light-emitting nanoparticles like semiconductor nanocrystals (termed quantum dots, QDs) are promising candidates for biosensing and bioimaging applications based on their bright and stable photoluminescent properties. As high-quality QDs are often synthesized in organic solvents, strategies needed to be developed to render them water-dispersible without affecting their optical properties and prevent changes in postmodification steps like the biofunctionalization with antibodies or DNA. Despite a large number of studies on suitable surface modification procedures, the preparation of water-soluble QDs for nanobiotechnology applications still presents a challenge. To highlight the advantages of surface silanization, we systematically explored the influence of the core/multishell architecture of CdSe/CdS/ZnS QDs and the silanization conditions on the optical properties of the resulting silanized QDs. Our results show that the optical properties of silica-coated CdSe/CdS/ZnS QDs are best preserved in the presence of a thick CdS (6 monolayers (ML)) intermediate shell, providing a high photoluminescence quantum yield (PL QY), and a relatively thick ZnS (4.5 ML) external shell, effectively shielding the QDs from the chemical changes during silica coating. In addition to the QD core/shell architecture, other critical parameters of the silica-coating process, that can have an influence on the optical properties of the QD, include the choice of the surfactant and its concentration used for silica coating. The highest PL QY of about 46% was obtained by a microemulsion silica-coating procedure with the surfactant Brij L4, making these water-dispersible QDs to well-suited optical reporters in future applications like fluorescence immunoassays, biomedicine, and bioimaging.

Temperature-Dependent Photoluminescence of Silver-Indium-Sulfide Nanocrystals in Aqueous Colloidal Solutions.

The temperature dependence of the photoluminescence (PL) intensity of colloidal semiconductor nanocrystals (NCs) makes them an appealing option in bio-sensing applications. Here, we probed the temperature-dependent PL behavior of aqueous glutathione (GSH)-capped Ag-In-S (AIS) NCs and their core/shell AIS/ZnS heterostructures. We show that both core and core-shell materials reveal strong PL quenching upon heating from 10 to 80 °C, which is completely reversible upon cooling. The PL quenching is assigned to the thermally activated dissociation of complexes formed by ligands with the metal cations on the NC surface and the introduction of water into the NC coordination sphere. This unique mechanism of the thermal PL quenching results in a much higher temperature sensitivity of the aqueous colloidal AIS (AIS/ZnS) NCs as compared with previously reported analogs capped by covalently bound ligands. Our results are expected to stimulate further studies on aqueous ternary NCs as colloidal luminescent nano-thermometers applicable for ratiometric temperature sensing.

Photoluminescence properties of self-assembled chitosan-based composites containing semiconductor nanocrystals.

The photoluminescence (PL) properties of composites obtained by embedding green-emitting semiconductor nanocrystals (NCs) of two different types (thiol-capped CdTe and CdSe/ZnS) into chitosan-based biopolymer particles were investigated. The synthesis of self-assembled particles from oppositely charged polysaccharides involved a preliminary electrostatic binding of positively charged chitosan chains by negatively charged functional groups of NC stabilizing ligands. The amount of NCs and the acidity of the solution were found to be important parameters influencing the PL. The PL properties were mainly discussed in terms of the colloidal stability of the particles and changes in energy gap of NCs. Generally, the obtained biocompatible composites with NCs randomly distributed within a biopolymer particle demonstrated a higher PL resistance to the solution acidity that expands the applicability range of thiol-capped NCs.

Influence of the average molar mass of poly(N-vinylpyrrolidone) on the dimensions and conductivity of silver nanowires.

We investigate the influence of the average molar mass (Mw) of the capping agent poly(N-vinylpyrrolidone) (PVP) on the conductivity of a silver nanowire (AgNW) network. During the polyol process, the chain length of PVP is known to influence the AgNW diameters and lengths. By altering the reaction temperature and time and using PVP of different chain lengths, we synthesized AgNWs with varying diameters, lengths and PVP coverage. The obtained plethora of AgNWs is the basis for conductivity investigations of networks made of AgNWs with a diameter of either 60 nm or 80 nm. The results show a negative influence of long-chain PVP on the conductivity of the subsequent network if 60 nm thick AgNWs are employed. Overall, we obtain well performing AgNW transparent electrodes on glass with RS = 24.4 Ω sq-1 at 85.5%T550nm.

Mercury-indium-sulfide nanocrystals: A new member of the family of ternary in based chalcogenides.

A general synthesis approach of aqueous glutathione-capped ternary Ag-In-S, Cu-In-S, and Hg-In-S nanocrystals (NCs) is introduced, allowing the NC composition to be varied in a broad range. Ternary Hg-In-S (HIS) NCs are reported for the first time and found to have the same tetragonal chalcopyrite motif as Cu-In-S and Ag-In-S NCs, corroborated by phonon spectra, while X-ray photoelectron spectroscopic data indicate mercury to be present as Hg+ in the Hg-In-S NCs. Colloidal HIS and Hg-In-S/ZnS NCs showed little or no variations of the spectral width of the photoluminescence band upon NC size selection, temperature variation in a broad range of 10-350 K, deposition of a ZnS shell, or postsynthesis annealing. All these observations are similar to those reported earlier for Ag-In-S and Ag-In-S/ZnS NCs and allowed us to assume a general photoluminescence mechanism for all three ternary compounds, based on the model of radiative self-trapped exciton recombination.

Solar light harvesting with multinary metal chalcogenide nanocrystals.

The paper reviews the state of the art in the synthesis of multinary (ternary, quaternary and more complex) metal chalcogenide nanocrystals (NCs) and their applications as a light absorbing or an auxiliary component of light-harvesting systems. This includes solid-state and liquid-junction solar cells and photocatalytic/photoelectrochemical systems designed for the conversion of solar light into the electric current or the accumulation of solar energy in the form of products of various chemical reactions. The review discusses general aspects of the light absorption and photophysical properties of multinary metal chalcogenide NCs, the modern state of the synthetic strategies applied to produce the multinary metal chalcogenide NCs and related nanoheterostructures, and recent achievements in the metal chalcogenide NC-based solar cells and the photocatalytic/photoelectrochemical systems. The review is concluded by an outlook with a critical discussion of the most promising ways and challenging aspects of further progress in the metal chalcogenide NC-based solar photovoltaics and photochemistry.

Modern Inorganic Aerogels.

Essentially, the term aerogel describes a special geometric structure of matter. It is neither limited to any material nor to any synthesis procedure. Hence, the possible variety of materials and therefore the multitude of their applications are almost unbounded. In fact, the same applies for nanoparticles. These are also just defined by their geometrical properties. In the past few decades nano-sized materials have been intensively studied and possible applications appeared in nearly all areas of natural sciences. To date a large variety of metal, semiconductor, oxide, and other nanoparticles are available from colloidal synthesis. However, for many applications of these materials an assembly into macroscopic structures is needed. Here we present a comprehensive picture of the developments that enabled the fusion of the colloidal nanoparticle and the aerogel world. This became possible by the controlled destabilization of pre-formed nanoparticles, which leads to their assembly into three-dimensional macroscopic networks. This revolutionary approach makes it possible to use precisely controlled nanoparticles as building blocks for macroscopic porous structures with programmable properties.

Versatile Tri(pyrazolyl)phosphanes as Phosphorus Precursors for the Synthesis of Highly Emitting InP/ZnS Quantum Dots.

Tri(pyrazolyl)phosphanes (5R1,R2 ) are utilized as an alternative, cheap and low-toxic phosphorus source for the convenient synthesis of InP/ZnS quantum dots (QDs). From these precursors, remarkably long-term stable stock solutions (>6 months) of P(OLA)3 (OLAH=oleylamine) are generated from which the respective pyrazoles are conveniently recovered. P(OLA)3 acts simultaneously as phosphorus source and reducing agent in the synthesis of highly emitting InP/ZnS core/shell QDs. These QDs are characterized by a spectral range between 530-620 nm and photoluminescence quantum yields (PL QYs) between 51-62 %. A proof-of-concept white light-emitting diode (LED) applying the InP/ZnS QDs as a color-conversion layer was built to demonstrate their applicability and processibility.

Methods to Characterize the Oligonucleotide Functionalization of Quantum Dots.

Currently, DNA nanotechnology offers the most programmable, scalable, and accurate route for the self-assembly of matter with nanometer precision into 1, 2, or 3D structures. One example is DNA origami that is well suited to serve as a molecularly defined "breadboard", and thus, to organize various nanomaterials such as nanoparticles into hybrid systems. Since the controlled assembly of quantum dots (QDs) is of high interest in the field of photonics and other optoelectronic applications, a more detailed view on the functionalization of QDs with oligonucleotides shall be achieved. In this work, four different methods are presented to characterize the functionalization of thiol-capped cadmium telluride QDs with oligonucleotides and for the precise quantification of the number of oligonucleotides bound to the QD surface. This study enables applications requiring the self-assembly of semiconductor-oligonucleotide hybrid materials and proves the conjugation success in a simple and straightforward manner.

Noble metal aerogels-synthesis, characterization, and application as electrocatalysts.

CONSPECTUS: Metallic and catalytically active materials with high surface area and large porosity are a long-desired goal in both industry and academia. In this Account, we summarize the strategies for making a variety of self-supported noble metal aerogels consisting of extended metal backbone nanonetworks. We discuss their outstanding physical and chemical properties, including their three-dimensional network structure, the simple control over their composition, their large specific surface area, and their hierarchical porosity. Additionally, we show some initial results on their excellent performance as electrocatalysts combining both high catalytic activity and high durability for fuel cell reactions such as ethanol oxidation and the oxygen reduction reaction (ORR). Finally, we give some hints on the future challenges in the research area of metal aerogels. We believe that metal aerogels are a new, promising class of electrocatalysts for polymer electrolyte fuel cells (PEFCs) and will also open great opportunities for other electrochemical energy systems, catalysis, and sensors. The commercialization of PEFCs encounters three critical obstacles, viz., high cost, insufficient activity, and inadequate long-term durability. Besides others, the sluggish kinetics of the ORR and alcohol oxidation and insufficient catalyst stability are important reasons for these obstacles. Various approaches have been taken to overcome these obstacles, e.g., by controlling the catalyst particle size in an optimized range, forming multimetallic catalysts, controlling the surface compositions, shaping the catalysts into nanocrystals, and designing supportless catalysts with extended surfaces such as nanostructured thin films, nanotubes, and porous nanostructures. These efforts have produced plenty of excellent electrocatalysts, but the development of multisynergetic functional catalysts exhibiting low cost, high activity, and high durability still faces great challenges. In this Account, we demonstrate that the sol-gel process represents a powerful "bottom-up" strategy for creating nanostructured materials that tackles the problems mentioned above. Aerogels are unique solid materials with ultralow densities, large open pores, and ultimately high inner surface areas. They magnify the specific properties of nanomaterials to the macroscale via self-assembly, which endow them with superior properties. Despite numerous investigations of metal oxide aerogels, the investigation of metal aerogels is in the early stage. Recently, aerogels including Fe, Co, Ni, Sn, and Cu have been obtained by nanosmelting of hybrid polymer-metal oxide aerogels. We report here exclusively on mono-, bi- and multimetallic noble metal aerogels consisting of Ag, Au, Pt, and Pd and their application as electrocatalysts.

Chiroptical activity in colloidal quantum dots coated with achiral ligands.

We studied the chiroptical properties of colloidal solution of CdSe and CdSe/ZnS quantum dots (QDs) with a cubic lattice structure which were initially prepared without use of any chiral molecules and coated with achiral ligands. We demonstrate circular dichroism (CD) activity around first and second excitonic transition of these CdSe based nanocrystals. We consider that this chiroptical activity is caused by imbalance in racemic mixtures of QDs between the left and right handed nanoparticles, which appears as a result of the formation of various defects or incorporation of impurities into crystallographic structure during their synthesis. We demonstrate that optical activity of colloidal solution of CdSe QDs with achiral ligands weakly depends on the QDs size and number of ZnS monolayers, but does not depend on the nature of achiral ligands or polarity of the solution.

Excitonic improvement of colloidal nanocrystals in salt powder matrix for quality lighting and color enrichment.

Here we report excitonic improvement in color-converting colloidal nanocrystal powders enabled by co-integrating nonpolar green- and red-emitting nanocrystal energy transfer pairs into a single LiCl salt matrix. This leads to nonradiative energy transfer (NRET) between the co-integrated nanocrystals in the host matrix. Here we systematically studied the resulting NRET process by varying donor and acceptor concentrations in the powders. We observed that NRET is a strong function of both of the nanocrystal concentrations and that NRET efficiency increases with increasing acceptor concentration. Nevertheless, with increasing donor concentration in the powders, NRET efficiency was found to first increase (up to a maximum level of 53.9%) but then to decrease. As a device demonstrator, we employed these NRET-improved nanocrystal powders as color-converters on blue light-emitting diodes (LEDs), with the resulting hybrid LED exhibiting a luminous efficiency >70 lm/W(elect). The proposed excitonic nanocrystal powders potentially hold great promise for quality lighting and color enrichment applications.

Alloying Behavior of Self-Assembled Noble Metal Nanoparticles.

The atomic redistribution processes occurring in multiparticle nanostructures are hardly understood. To obtain a more detailed insight, we applied high-resolution microscopic, diffraction and spectroscopic characterization techniques to investigate the fine structure and elemental distribution of various bimetallic aerogels with 1:1 compositions, prepared by self-assembly of single monometallic nanoparticles. The system Au-Ag exhibited a complete alloy formation, whereas Pt-Pd aerogels formed a Pd-based network with embedded Pt particles. The assembly of Au and Pd nanoparticles resulted in a Pd-shell formation around the Au particles. This work confirms that bimetallic aerogels are subject to reorganization processes during their gel formation.

5-(2-Mercaptoethyl)-1H-tetrazole: Facile Synthesis and Application for the Preparation of Water Soluble Nanocrystals and Their Gels.

A facile method for the preparation of the novel capping ligand 5-(2-mercaptoethyl)-1H-tetrazole for the stabilization of water-soluble nanocrystals was developed. This effective synthetic procedure is based on the cycloaddition of sodium azide to 3,3'-dithiobis(propionitrile) followed by the reductive cleavage of a S-S bond with triphenylphosphine. The structure of the synthesized compound was confirmed by single-crystal X-ray analysis. A target tetrazole was successfully applied for the direct aqueous synthesis of CdTe and Au nanocrystals. CdTe nanocrystals capped with 5-(2-mercaptoethyl)-1H-tetrazole were found to reveal high photoluminescence efficiencies (up to 77 %). Nanocrystals capped with this tetrazole ligand are able to build 3D structures in a metal-ion-assisted gelation process in aqueous solution. Critical point drying of the as-formed hydrogels allowed the preparation of the corresponding aerogels, while preserving the mesoporous structure.

Probing Absolute Electronic Energy Levels in Hg-Doped CdTe Semiconductor Nanocrystals by Electrochemistry and Density Functional Theory.

The absolute electronic energy levels in Hg-doped CdTe semiconductor nanocrystals (CdHgTe NCs) with varying sizes/volumes and Hg contents are determined by using cyclic voltammetry (CV) measurements and density functional theory (DFT) -based calculations. The electrochemical measurements demonstrate several distinct characteristic features in the form of oxidation and reduction peaks in the voltammograms, where the peak positions are dependent on the volume of CdHgTe NCs as well as on their composition. The estimated absolute electronic energy levels for three different volumes, namely 22, 119 and 187 nm(3) with 2.7±0.3 % of Hg content, show strong volume dependence. The volume-dependent shift in the characteristic reduction and oxidation peak potential scan can be attributed to the alteration in the energetic band positions owing to the quantum confinement effect. Moreover, the composition (Cd/Hg=98.3/1.7 and 97.0/3.0) -dependent alteration in the electronic energy levels of CdHgTe NCs for two different samples with similar volumes (ca. 124±5 nm(3) ) are shown. Thus obtained electronic energy level values of CdHgTe NCs as a function of volume and composition demonstrate good congruence with the corresponding absorption and emission spectral data, as well as with DFT-based calculations. DFT calculations reveal that incorporation of Hg into CdTe NCs mostly affects the energy levels of conduction band edge, whereas the valence band edge remains almost unaltered.

Cyclic voltammetry as a sensitive method for in situ probing of chemical transformations in quantum dots.

The application of electrochemical methods for the characterization of colloidal quantum dots (QDs) attracts considerable attention as these methods may allow for monitoring of some crucial parameters, such as energetic levels of conduction and valence bands as well as surface traps and ligands under real conditions of colloidal solution. In the present work we extend the applications of cyclic voltammetry (CV) to in situ monitoring of degradation processes of water-soluble CdTe QDs. This degradation occurs under lowering of pH to the values around 5, i.e. under conditions relevant to bioimaging applications of these QDs, and is accompanied by pronounced changes of their photoluminescence. Observed correlations between characteristic features of CV diagrams and the fluorescence spectra allowed us to propose mechanisms responsible for evolution of the photoluminescence properties as well as degradation pathway of CdTe QDs at low pH.

3D Assembly of All-Inorganic Colloidal Nanocrystals into Gels and Aerogels.

We report an efficient approach to assemble a variety of electrostatically stabilized all-inorganic semiconductor nanocrystals (NCs) by their linking with appropriate ions into multibranched gel networks. These all-inorganic non-ordered 3D assemblies benefit from strong interparticle coupling, which facilitates charge transport between the NCs with diverse morphologies, compositions, sizes, and functional capping ligands. Moreover, the resulting dry gels (aerogels) are highly porous monolithic structures, which preserve the quantum confinement of their building blocks. The inorganic semiconductor aerogel made of 4.5 nm CdSe colloidal NCs capped with I(-) ions and bridged with Cd(2+) ions had a large surface area of 146 m(2) g(-1) .

Simultaneous Identification of Spectral Properties and Sizes of Multiple Particles in Solution with Subnanometer Resolution.

We report an unsurpassed solution characterization technique based on analytical ultracentrifugation, which demonstrates exceptional potential for resolving particle sizes in solution with sub-nm resolution. We achieve this improvement in resolution by simultaneously measuring UV/Vis spectra while hydrodynamically separating individual components in the mixture. By equipping an analytical ultracentrifuge with a novel multi-wavelength detector, we are adding a new spectral discovery dimension to traditional hydrodynamic characterization, and amplify the information obtained by orders of magnitude. We demonstrate the power of this technique by characterizing unpurified CdTe nanoparticle samples, avoiding tedious and often impossible purification and fractionation of nanoparticles into apparently monodisperse fractions. With this approach, we have for the first time identified the pure spectral properties and band-gap positions of discrete species present in the CdTe mixture.

Tetrazoles: Unique Capping Ligands and Precursors for Nanostructured Materials.

Capping agents play an important role in the colloidal synthesis of nanomaterials because they control the nucleation and growth of particles, as well as their chemical and colloidal stability. During recent years tetrazole derivatives have proven to be advanced capping ligands for the stabilization of semiconductor and metal nanoparticles. Tetrazole-capped nanoparticles can be prepared by solution-phase or solventless single precursor approaches using metal derivatives of tetrazoles. The solventless thermolysis of metal tetrazolates can produce both individual semiconductor nanocrystals and nanostructured metal monolithic foams displaying low densities and high surface areas. Alternatively, highly porous nanoparticle 3D assemblies are achieved through the controllable aggregation of tetrazole-capped particles in solutions. This approach allows for the preparation of non-ordered hybrid structures consisting of different building blocks, such as mixed semiconductor and metal nanoparticle-based (aero)gels with tunable compositions. Another unique property of tetrazoles is their complete thermal decomposition, forming only gaseous products, which is employed in the fabrication of organic-free semiconductor films from tetrazole-capped nanoparticles. After deposition and subsequent thermal treatment these films exhibit significantly improved electrical transport. The synthetic availability and advances in the functionalization of tetrazoles necessitate further design and study of tetrazole-capped nanoparticles for various applications.

Colloidal semiconductor nanocrystals: the aqueous approach.

This article summarizes the main achievements and challenges in the field of the aqueous synthesis of semiconductor quantum dots in colloidal solutions. Developments in the last two decades demonstrate the great potential of this approach to synthesize nanocrystalline materials with superior properties such as strong photoluminescence, long time stability and compatibility with biological media, and the variability in assembling and self-assembling into larger structures or on surfaces. Being relatively straightforward, the aqueous approach provides some advantages such as versatility, scalability, environmental friendliness and cost effectiveness, leading in summary to very attractive application perspectives.