Nanocrystals (NCs) discussed in this Review are tiny crystals of metals, semiconductors, and magnetic material consisting of hundreds to a few thousand atoms each. Their size ranges from 2-3 to about 20 nm. What is special about this size regime that placed NCs among the hottest research topics of the last decades? The quantum mechanical coupling * To whom correspondence should be addressed. E-mail: dvtalapin@uchicago.edu. † The University of Chicago. ‡ Argonne National Lab. Chem. Rev. 2010, 110, 389–458 389

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Prospects of Colloidal Nanocrystals for Electronic and Optoelectronic Applications

http://sites.psu.edu/cremer/wp-content/uploads/sites/3050/2013/04/66-Yanjie.pdf

Interactions between macromolecules and ions: The Hofmeister series.

Three-dimensional nanofibrous aerogels (NFAs) that are both highly compressible and resilient would have broad technological implications for areas ranging from electrical devices and bioengineering to damping materials; however, creating such NFAs has proven extremely challenging. Here we report a novel strategy to create fibrous, isotropically bonded elastic reconstructed (FIBER) NFAs with a hierarchical cellular structure and superelasticity by combining electrospun nanofibres and the fibrous freeze-shaping technique. Our approach causes the intrinsically lamellar deposited electrospun nanofibres to assemble into elastic bulk aerogels with tunable densities and desirable shapes on a large scale. The resulting FIBER NFAs exhibit densities of >0.12 mg cm(-3), rapid recovery from deformation, efficient energy absorption and multifunctionality in terms of the combination of thermal insulation, sound absorption, emulsion separation and elasticity-responsive electric conduction. The successful synthesis of such fascinating materials may provide new insights into the design and development of multifunctional NFAs for various applications.

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Ultralight nanofibre-assembled cellular aerogels with superelasticity and multifunctionality

Two-dimensional (2D) nanomaterials, such as graphene and transition metal dichalcogenides (TMDs), receive a lot of attention, because of their intriguing properties and wide applications in catalysis, energy-storage devices, electronics, optoelectronics, and so on. To further enhance the performance of their application, these 2D nanomaterials are hybridized with other functional nanostructures. In this review, the latest studies of 2D nanomaterial-based hybrid nanostructures are discussed, focusing on their preparation methods, properties, and applications.

25th anniversary article : hybrid nanostructures based on two-dimensional nanomaterials

The study of the interactions of salts and osmolytes with macromolecules in aqueous solution originated with experiments concerning protein precipitation more than 100 years ago. Today, these solutes are known to display recurring behavior for myriad biological and chemical processes. Such behavior depends both on the nature and concentration of the species in solution. Despite the generality of these effects, our understanding of the molecular-level details of ion and osmolyte specificity is still quite limited. Here, we review recent studies of the interactions between anions and urea with model macromolecular systems. A mechanism for specific ion effects is elucidated for aqueous systems containing charged and uncharged polymers, polypeptides, and proteins. The results clearly show that the effects of the anions are local and involve direct interactions with macromolecules and their first hydration shell. Also, a hydrogen-bonding mechanism is tested for the urea denaturation of proteins with some of these same systems. In that case, direct hydrogen bonding can be largely discounted as the key mechanism for urea stabilization of uncollapsed and/or unfolded structures.

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Chemistry of Hofmeister anions and osmolytes.

Chalcogenide aerogels based entirely on semiconducting II-VI or IV-VI frameworks have been prepared from a general strategy that involves oxidative aggregation of metal chalcogenide nanoparticle building blocks followed by supercritical solvent removal. The resultant materials are mesoporous, exhibit high surface areas, can be prepared as monoliths, and demonstrate the characteristic quantum-confined optical properties of their nanoparticle components. These materials can be synthesized from a variety of building blocks by chemical or photochemical oxidation, and the properties can be further tuned by heat treatment. Aerogel formation represents a powerful yet facile method for metal chalcogenide nanoparticle assembly and the creation of mesoporous semiconductors.

http://science.sciencemag.org/content/sci/307/5708/397.full.pdf

Porous Semiconductor Chalcogenide Aerogels

Aerogels composed of interconnected networks of CdS building blocks have been prepared from controlled aggregation of discrete nanoparticles followed by supercritical fluid extraction The resultant materials have surface areas up to 242 m 2 /g and average pore diameters in the range of 15–25 nm Powder X-ray diffraction indicates that the average crystallite size is on the order of 2 nm in 100 °C heated aerogels, but increases to nearly 16 nm upon heating at 500 °C This increase in size is accompanied by a phase change from the cubic to the hexagonal structure The optical bandgap (measured by absorption onset) is also tunable with heat treatment, and was found to range from 22–28 eV The photoluminescence excitation spectrum ( λ ex  = 400 nm) of a 100 °C-heated CdS aerogel is similar to that of the nanoparticulate starting materials, exhibiting both a sharp excitonic peak near 450 nm corresponding to band edge emission, and a broad feature in the region 500–700 nm, attributed to electron–hole recombination at trap states

A new addition to the aerogel community: unsupported CdS aerogels with tunable optical properties

A systematic study of the effects of pH, precursor salt, and temperature on the surface area, particle size, and copper speciation in 3 wt % CuO in silica aerogels and xerogels has been conducted. The materials were synthesized by copolymerization sol−gel reactions, followed by either supercritical solvent extraction using CO2 (aerogels) or conventional drying (xerogels) and annealing in air. Among the six different copper precursors examined, copper(II) acetate was found to be the best in terms of its stability, solubility, and binding capacity of copper in the silica matrix. Aerogels prepared with copper(II) acetate were found to have considerably higher surface areas than the corresponding xerogels, and the observed formation of CuO particles occurred at higher temperatures (900 vs 650 °C, by TEM analysis). X-ray diffraction of gels annealed at 900 °C likewise revealed the presence of nanocrystalline CuO, with larger particles observed in acid-catalyzed aerogels vs base-catalyzed aerogels (∼26 vs ∼15 n...

Influence of Synthetic and Processing Parameters on the Surface Area, Speciation, and Particle Formation in Copper Oxide/Silica Aerogel Composites

The synthesis and characterization of porous nanostructured inorganic polymers (gels and xerogels) of CdS, ZnS, PbS, and CdSe is described. Primary particles are synthesized via water-in-oil microemulsions, surface complexed with thiolate ligands, and dispersed in acetone. Oxidation of the thiolate ligands with H2O2 leads to particle aggregation and the formation of wet gels, which are then dried under ambient conditions to produce xerogels. The xerogels exhibit optical band-edges that are intermediate between bulk precipitates and the more porous wet gels and aerogels, suggesting the extent of quantum confinement in such colloidal networks is intimately related to the pore structure and surface area (dimensionality). Heating of the xerogels in vacuo results in a continued decrease in the energy of absorption onset due to sintering of the nanoparticle network, and this is also reflected in a growth in crystallite size as probed by powder X-ray diffraction. The resulting xerogel networks demonstrate modest...

Sol−Gel Processing of Semiconducting Metal Chalcogenide Xerogels: Influence of Dimensionality on Quantum Confinement Effects in a Nanoparticle Network

Porous aerogels of CdS that exhibit quantum confined optical properties have been prepared by supercritical drying of wet CdS gels prepared by oxidative stripping of nanoparticle capping groups. The water-to-surfactant ratio (W) employed in the preparation of the nanoparticles is found to have a large impact on the surface areas of resultant aerogels, with W=5 samples producing mesoporous samples with the highest surface areas and cumulative pore volumes. In contrast, the concentration of the CdS sol employed in the gelation has only a small effect on the porosity characteristics, but a large impact on the apparent crystallite and chromophore sizes in as-prepared aerogels. Higher concentrations of the sol result in larger crystallite sizes and lower degrees of quantum confinement, as evidenced by a red shift in the absorbance spectra.

Jaya L. Mohanan

Papers

Porous Semiconductor Chalcogenide Aerogels

A new addition to the aerogel community: unsupported CdS aerogels with tunable optical properties

Influence of Synthetic and Processing Parameters on the Surface Area, Speciation, and Particle Formation in Copper Oxide/Silica Aerogel Composites

Sol−Gel Processing of Semiconducting Metal Chalcogenide Xerogels: Influence of Dimensionality on Quantum Confinement Effects in a Nanoparticle Network

CdS aerogels: Effect of concentration and primary particle size on surface area and opto-electronic properties