Dedicated to Professor Gerhard Wegner on the occasion of his 68th birthdayThe tuneable NIR absorbance of gold in conjunction with its low cytotoxicity has fueled research in the synthesis of rodlike gold nanocrystals for a wide range of biomedical applications such as sensing, imaging, and photothermal therapy.[1] However, a fundamental problem in the realization of these technologies is the need for (cytotoxic) surfactants-such as cetyltrimethylammonium bromide (CTAB)-in order to induce the anisotropic particle growth in aqueous solution.[2]Herein we present an alternate synthetic strategy based on ionic liquid solvents that alleviates the need for shaperegulating surfactants.Ionic liquids (ILs) have attracted interest as benign solvent systems for the synthesis of nanomaterials as they combine several attractive characteristics, for example inherent conductivity, stability over a broad range of electrochemical potentials, and environmental benefits deriving from the low vapor pressure and straightforward separation procedures. [3,4] Two major strategies have been pursued for the synthesis of metal nanoparticles in IL solution: 1) the addition of auxiliary capping agents (in analogy to the reactions in aqueous solution) to stabilize the formation of nanosized particles, and 2) the use of modified ILs capable of acting both as solvent and capping agent. For example, thioland alcohol-substituted ILs were applied for the synthesis of Au and Pt nanoparticles by reduction of the respective metal salts with a strong reducing agent (NaBH 4 ). [5,6] Common to these prior studies is the use of strong reducing agents and the (predominantly) covalent linkage of the capping agent to stabilize the growing metal nuclei.We report herein that under conditions of decelerated particle growth (by use of weak reducing agents) the stabilization of gold nanocrystals is facilitated by solvent coordination in unmodified imidazolium ionic liquids. [7,8] Imidazolium cations are particularly intriguing stabilizing agents for gold nanocrystals since related aromatic heterocycles have been shown to preferentially bind to high-energy crystallographic orientations of gold surfaces such as the {100}, {110}, and {311} orientations. [8][9][10] This suggests that imidazolium-based ILs may stabilize non-equilibrium particle shapes (such as nanorods) that exhibit fewer low-energy {111} facets than the equilibrium (Wulff) shape.We demonstrate herein that anisotropic gold nanocrystals can be synthesized in 1-ethyl-3-methylimidazolium ethylsulfate ([EMIM][ES]) in very high yield in the absence of auxiliary shape-regulating surfactants. Dependent on the amount of Ag I present in the reaction mixture, the particle aspect ratio can be controlled within the range a = L/d = 1-15 (where L and d denote the particle length and thickness); this is comparable to the range of shape anisotropy that has been demonstrated in aqueous solutions.The synthetic approach is based on the seeded-growth method originally developed by the Murphy group for the synthesis of go...

The evolution of grain size and shape as well as type and frequency of grain boundary structures during thermal annealing of lamellar diblock copolymer microstructures is established using large area image reconstruction and analysis. Grain coarsening is found to proceed via an initial transient stage that is characterized by the rapid relaxation of unstable "frozen-in" defects such as kink boundaries and the subsequent quasi-stationary coarsening that is dominated by the continuous relaxation of low-angle symmetric tilt boundaries. The particular relevance of low-angle symmetric tilt boundaries to grain coarsening is interpreted as the consequence of both the associated decrease of boundary energy as well as the availability of favorable kinetic pathwayssuch as grain boundary splittingto facilitate the coarsening process. The inverse relation between grain boundary energy and frequency suggests that the reduction of boundary energy is a relevant governing parameter for the evolution of grain boundary structuresas it is in inorganic materials. The existence of "inert" boundary types (such as asymmetric tilt and twist) thatwithin the experimental windowdo not participate in the coarsening process is expected to have dominant influence on the final morphology that can be attained by thermal annealing of the microstructure. The reduction of the density of inert boundaries during the film preparation process should therefore provide a strategy for increasing the coarsening kinetics in block copolymer films during thermal annealing and thus a path toward a higher degree of order in block copolymer microstructures.

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The effect of filler addition on the grain coarsening characteristics of block copolymer materials is analyzed for the particular case of a lamellar poly(styrene-b-isoprene)-type block copolymer and polystyrene as well as polystyrene-grafted nanoparticle fillers. Filler addition is shown to reduce the rate of grain growth and to induce grain size distributions that deviate from the log-normal type that is characteristic for pristine block copolymer systems. The retardation of grain growth is shown to be associated with the segregation of filler additives into high energy grain boundary defectsa process that bears similarities to the segregation of impurity atoms within grain boundary structures in ceramics or metals. The analysis of grain boundary energy, grain size distribution, and grain coarsening kinetics suggests two major mechanisms for the interference of filler additives with grain coarsening: First, the segregation of fillers into boundary regions lowers the relative grain boundary energy and hence the driving pressure for grain growth. Second, the formation of particle aggregates along grain boundaries gives rise to a "pinning pressure" that counteracts grain growth and that limits the ultimate grain size during thermal annealing. This is in contrast to pristine block copolymer systems in which continuous grain growth is observed during thermal annealing. The results highlight the fundamental differences between structure evolution in pristine and mixed block copolymer systems and suggest that thermal annealing (in the absence of structure-guiding fields) is an inefficient path to facilitate the controlled growth of large grains in athermal block copolymer blend materials.

The (relative) energies of symmetric tilt grain boundaries in a strongly segregated lamellar block copolymer are determined by analysis of the dihedral angles at grain-boundary triple junctions. The analysis reveals two regimes: at low and intermediate misorientations (corresponding to a tilt-angle range 0 85 ) the grain-boundary energy is found to depend on the tilt angle as EðÞ $ x , with 2:5 > x ! 0. At large misorientations the grain-boundary energy is found to be independent (within the experimental uncertainty) of the angle of tilt. The transition between the two scaling regimes is accompanied by the transition of the grain-boundary structure from the chevron to the omega morphology. Grain-boundary energy and frequency are found to be inversely related, thus suggesting boundary energy to be an important parameter during grain coarsening in block-copolymer microstructures, as it is in inorganic polycrystalline microstructures. The ability to self-organize into periodically ordered microdomain morphologies makes block-copolymerbased materials intriguing platforms to facilitate transformative technological breakthroughs in a range of areas, including dynamic photonic sensors, solid state ion conductors, and bulk heterojunction materials for polymer photovoltaics [1][2][3][4][5]. In these applications, nonequilibrium structural defects such as grain boundaries (GB) are expected to play an important role in determining material performance because of their impact on the long-range order and tortuosity of diffusion pathways. The occurrence of GB defects is inherent in quiescent organized blockcopolymer (BCP) microstructures and can be related to the nucleation and growth of ordered grains during the structure evolution process, the interaction of disclinations, or mechanically induced kinking because of inhomogeneous solvent evaporation during the late stages of film formation. The basic phenomenology of GB structures in BCP materials was first discussed by Gido and Thomas who classified and evaluated the various GB types associated with tilt and twist deformations of lamellar BCPs [6][7][8][9]. Subsequent experimental studies revealed the relevance of process parameters (such as the application of shear fields), molecular architecture, and composition on GB formation as well as the implications of GB defects on, for example, the permeability of BCP materials [10][11][12][13][14][15][16][17].Despite the abundance of GB defects and their demonstrated relevance on the physical properties of BCP materials, very little is known about the governing parameters that determine the formation of the various types of GB structures in BCPs or the evolution of GB structures during, for example, thermal annealing. One parameter that is of particular interest is the energy penalty associated with GB formation as it provides insight into both the mechanism and driving force of grain coarsening during annealing. The current understanding of GB energies in BCPs is limited to simulation studies, for example, by Schick and co-worke...