Despite the increasing interest in the applications of functional nanoparticles, a comprehensive understanding of the formation mechanism starting from the precursor reaction with subsequent nucleation and growth is still a challenge. We for the first time investigated the kinetics of gold nanoparticle formation systematically by means of a lab-based in situ small-angle X-ray scattering (SAXS)/wide-angle X-ray scattering (WAXS)/UV-vis absorption spectroscopy experiment using a stopped-flow apparatus. We thus could systematically investigate the influence of all major factors such as precursor concentration, temperature, the presence of stabilizing ligands and cosolvents on the temporal evolution of particle size, size distribution, and optical properties from the early prenucleation state to the late growth phase. We for first time formulated and numerically solved a closed nucleation and growth model including the precursor reaction. We observe that the results can be well described within the framework of classical nucleation and growth theory, including also results of previous studies by other research groups. From the analysis, we can quantitatively derive values for the rate constants of precursor reaction and growth together with their activation free enthalpies. We find the growth process to be surface-reaction limited with negligible influence of Ostwald ripening yielding narrow disperse gold nanoparticles.

A new, extremely efficient method for the fabrication of films comprised of gold nanoparticles (GNPs) crosslinked by organic dithiols is presented in this paper. The method is based on layer-by-layer spin-coating of both components, GNPs and crosslinker, and enables the deposition of films several tens of nanometers in thickness within a few minutes. X-ray diffraction and conductance measurements reveal the proper adjustment concentration of the crosslinker solution of the critical is in order to prevent the destabilization and coalescence of particles. UV/vis spectroscopy, atomic force microscopy, and conductivity measurements indicate that films prepared via layer-by-layer spin-coating are of comparable quality to coatings prepared via laborious layer-by-layer self-assembly using immersion baths. Because spin-coated films are not bound chemically to the substrate, they can be lifted-off by alkaline underetching and transferred onto 3d-electrodes to produce electrically addressable, freely suspended films. Comparative measurements of the sheet resistances indicate that the transfer process does not compromise the film quality.

To enable applications of nanoparticle films in flexible electronics, actuators, and sensors, their mechanical properties are of critical concern. Here, we demonstrate that the elastic and viscoelastic properties of covalently cross-linked gold nanoparticles (GNPs) can be probed using AFM bulge tests. For this purpose 30−60 nm thick films consisting of 1,9-nonanedithiol (NDT) cross-linked GNPs (3.8 nm core diameter) were transferred onto substrates with ∼100 μm circular apertures. The resulting freestanding membranes were bulged by applying pressure differences of up to 10 kPa, and the deflection was measured by intermittent contact atomic force microscopy (AFM). Analyzing the pressure-deflection data using the spherical cap model, either by taking into account the peak deflection values or the measured arc profiles of the bulge, yielded 2.3 ± 0.3 and 2.7 ± 0.4 GPa for Young's modulus, respectively. When cycling the stress−strain measurements at overpressures up to 2.4 kPa, hysteresis was observed and assigned to viscoelastic effects. Creep tests performed at a pressure of 2 kPa revealed both viscoelastic retardation (time constant: 3.3 × 10 −3 s −1 ) and nonrecoverable relaxation (creep rate: 9.0 × 10 −8 s −1 ). Several membranes resisted pressures up to 10 kPa without fracturing, indicating that the ultimate biaxial tensile strength of the films was above ∼30 MPa. ■ INTRODUCTIONThin films consisting of ligand-stabilized or cross-linked gold nanoparticles (GNPs) have received considerable scientific attention during the past two decades, and various applications have been demonstrated. For example, transduction elements based on thin GNP films have enabled the fabrication of novel resistive strain gauges, 1−4 touch sensors, 5,6 and chemiresistors. 7 In these sensors the transduction mechanism is based on changes in the interparticle distances, due to either forceinduced strain or sorption-induced swelling. Because the tunneling current between neighboring nanoparticles is exponentially related to their distance, these sensors can afford extremely high sensitivities. Further, nanoparticle networks have great potential for the implementation in next-generation flexible electronics. Very recently, Kotov and co-workers 8 reported on stretchable conductors made from GNP−polyurethane composites enabling electrical tunability of mechanical properties by dynamic self-organization of the nanoparticles under stress.Obviously, the performance of sensors and flexible electronics based on nanoparticle composites critically depends on their specific mechanical properties, e.g., elasticity, viscoelasticity, and ultimate strength. To some extent, these properties have been studied by nanoindentation 9 and forcedeflection measurements employing atomic force microscopes (AFMs). 10−12 In conventional nanoindentation experiments the indenter tip is pressed into the substrate-supported specimen, and the force−distance data are analyzed to extract the material's hardness and reduced elastic modulus. 13 Recently, the micromechanical p...

Dedicated to Professor Werner Uhl on the occasion of his 60th birthdayAmong all known polyhalides, the chemistry of the iodine compounds has long been known and has been investigated extensively. Already in 1870 Jørgensen [1] discovered that triiodide was formed by mixing potassium iodide with iodine in an alcoholic solution. To date many examples of different mono-, di-, tri-, and tetraanions are known. [2] Focusing on the polyiodide monoanions, only compounds up to [X 9 ] À have to the best of our knowledge been structurally characterized. In 2011 Groessl et al. observed the first higher polyiodides [I 11 ] À , [I 13 ] À , and [I 15 ] À by mass spectrometry. [3] They mixed [PMIM]I (1-propyl-3-methylimidazolium) with the appropriate amount of iodine, diluted it in acetonitrile, and analyzed this mixture by ESI-MS. To confirm the observed fragmentation pathways, they calculated the stable structures for [I 2n+1 ] À (n = 1-7) at the DFT level. The observed compounds in this gas-phase experiment have the highest ratio of elements to charge of any previously studied polyhalides.Regarding the lighter and more reactive homologue of the halogens, bromine, many fewer examples are known. Linear [Br 4 ] 2À , [4] Z-shaped [Br 8 ] 2À , [5] ring-shaped [Br 10 ] 2À , [6] and the recently found [Br 20 ] 2À [7,8] are examples of known polybromide dianions. [9] Among the polybromide monoanions, only [Br 3 ] À has been known for a long time. Indeed, Chattaway and Hoyle [10] discussed already in 1923 the existence of higher polybromide monoanions but until 2010 there has been neither vibrational spectroscopy nor X-ray diffraction evidence for higher polybromides, such as [Br 5 ] À , [Br 7 ] À , and [Br 9 ] À . [11] In 2011 our group reported the evidence for the structure of a higher polybromide monoanion [NPr 4 ][Br 9 ]. [12] Shortly thereafter Feldmann et al. synthesized and characterized [P(Ph) 3 Br][Br 7 ] in ionic liquids, [8] and very recently Himmel et al. reported on the structure of [Br 5 ] À . [13] Beyond these polybromide anions also polychloride and fluoride anions have been characterized by IR and Raman spectroscopy. In the case of polychloride monoanions, [Cl 3 ] À , [14] [Cl 3 ···Cl 2 ] À , [15] [Cl 5 ] À , [16] and [Cl 9 ] À[16] are known, whereby the latter exists only under cryogenic conditions. This holds also for the only example of a polyfluoride anion [F 3 ] À , which

Herein a convenient one-pot route to a sterically demanding superbasic pyridine is presented. Functionalization of the 2- and 6-positions with the strongly σ-donating boryl-groups shifts the calculated gas phase basicity of the pyridine nitrogen atom to 1012 kJ mol , which outperforms the "proton sponge" 1,8-bis(dimethylamino)naphthalene (996 kJ mol ). The diazaboryl groups are oriented orthogonally to the pyridine ring and do not block the N-position, which resembles the geometry of commonly used N-heterocyclic carbenes. This allows the substituted pyridine to be used as a neutral N-donor ligand in coordination chemistry that is demonstrated herein with the Lewis adducts of haloboranes. Contrary to NHCs, which can form extraordinarily stable adducts, the pyridine ligand is intended to act as a weaker-coordinating alternative and could allow for alternative ligand chemistry.