No abstract

Colloidal crystals, periodic structures typically self-assembled from monodisperse colloidal building blocks, are one class of photonic band gap materials that can be fabricated at low cost and on a large scale. [1][2][3] They have attracted much attention because of their promise in optoelectronic applications that require the manipulation of photons, for example, as photonic components in telecommunication devices, lasers, and sensors. [4][5][6][7][8][9][10][11][12][13] It is highly desirable for the envisioned applications that a photonic crystal possesses a tunable stop band that can be conveniently controlled by external stimuli. Considerable effort has been devoted to achieving this goal by changing the refractive indices of the materials and the lattice constants or spatial symmetry of the crystals through the application of chemical stimuli, mechanical forces, electrical fields, or light. [14][15][16][17][18][19][20][21][22][23] However, the wide use of these systems as optoelectronic devices is hampered by the limited tunability of the stop band (changes in peak position are typically in the range of tens of nanometers), the slow response to the external stimuli, and the difficulty of integration into existing photonic systems.Adding magnetic components to the colloidal building blocks provides an opportunity for convenient and precise control of the properties of photonic crystals through an external magnetic field. Asher and co-workers explored this approach by fabricating colloidal photonic crystals using highly charged polystyrene microspheres containing superparamagnetic nanoparticles. [24,25] In this case, the magnetic forces exerted on the colloids are weak relative to interparticle electrostatic forces because of the low loading of the magnetic materials, leading to a limited tuning range, a long response time, and consequently limited practical applications. Preparation of polymer microspheres with increased loading of magnetic materials is practically difficult within the typically used emulsion polymerization schemes. In principle, colloidal particles of pure magnetic materials such as iron oxide can be directly used as the building blocks for constructing colloidal photonic crystals. Such effort, however, is limited by the superparamagnetic-to-ferromagnetic transition that occurs as particles are grown into larger domains. [26] Recently, we developed a method for the preparation of polyacrylate-capped superparamagnetic magnetite (Fe 3 O 4 ) colloidal nanocrystal clusters (CNCs) with tunable sizes from 30 to 180 nm by a high-temperature hydrolysis process. [27] Each cluster is composed of many magnetite crystallites of approximately 10 nm, thus retaining the superparamagnetic properties at room temperature. Herein, we report that such superparamagnetic clusters can be directly employed for constructing colloidal photonic crystals with highly tunable stop bands that can be moved across the entire visible spectral region owing to the highly charged polyacrylate-capped surfaces and the strong int...

Hydrophobic inorganic nanocrystals have been transferred from organic solvent to aqueous solution through a robust and general ligand exchange procedure. Polyelectrolytes such as poly(acrylic acid) and poly(allylamine) are used to replace the original hydrophobic ligands on the surface of nanocrystals at an elevated temperature in a glycol solvent and eventually render the nanocrystals highly water soluble. The physical properties of the nanocrystals, such as superparamagnetism, photocatalytic activity, and photoluminescence, are maintained or improved after ligand exchange.

We report that fully alloyed Ag/Au nanospheres with high compositional homogeneity ensured by annealing at elevated temperatures show large extinction cross sections, extremely narrow bandwidths, and remarkable stability in harsh chemical environments. Nanostructures of Ag are known to have much stronger surface plasmon resonance than Au, but their applications in many areas have been very limited by their poor chemical stability against nonideal chemical environments. Here we address this issue by producing fully alloyed Ag/Au nanospheres through a surface-protected annealing process. A critical temperature has been found to be around 930 °C, below which the resulting alloy nanospheres, although significantly more stable than pure silver nanoparticles, can still gradually decay upon extended exposure to a harsh etchant. Nanospheres annealed above the critical temperature show a homogeneous distribution of Ag and Au, minimal crystallographic defects, and the absence of structural and compositional interfaces, which account for the extremely narrow bandwidths of the surface plasmon resonance and may enable many plasmonic applications with high performance and long lifetime, especially for those involving corrosive species.

Magnetit in Form monodisperser superparamagnetischer kolloidaler Nanokristallcluster (CNCs; 30–180 nm groß) entsteht bei der Hydrolyse von FeCl3 bei hoher Temperatur und in Gegenwart eines Tensids (siehe Schema). Die Kombination aus Superparamagnetismus, hoher Magnetisierung und guter Dispergierbarkeit in Wasser empfiehlt diese CNCs für Anwendungen wie Wirkstofftransport und die Trennung biologischer Verbindungen.