We study the phase behavior of bowl-shaped (nano)particles using confocal microscopy and computer simulations. Experimentally, we find the formation of a wormlike fluid phase in which the bowl-shaped particles have a strong tendency to stack on top of each other. However, using free energy calculations in computer simulations, we show that the wormlike phase is out-ofequilibrium and that the columnar phase is thermodynamically stable for sufficiently deep bowls and high densities. In addition, we employ a novel technique based on simulated annealing to predict the crystal structures for shallow bowls. We find four exotic new crystal structures and we determine their region of stability using free energy calculations. We discuss the implications of our results for the development of materials consisting of molecular mesogens or nanoparticles.KEYWORDS Nanobowls, crystal structures, computer simulations, phase diagram, columnar liquid crystals I n recent years, a whole variety of bowl-shaped nanoparticles and colloids have been synthesized and characterized, 1-6 and possible applications of these systems have been put forward. Most of these applications are of a single particle nature, such as a nanocontainer, 4 or for metallic nanobowls depend on the tendency of these particles to form foamlike structures upon aggregation. Applications of the latter kind include superhydrophobic 5 and infrared-blocking 3 coatings. Metallic nanoparticles can be stabilized, for example, by applying a capping layer, 7 which should prevent aggregation and thus reveal the natural tendency of bowl-shaped particles to form stacks or columns. In the molecular liquid crystal community, this tendency has been thought to decrease the number of defects in columnar phases, which is important if these columnar phases are to replace 8 the crystalline ferroelectrics (materials with a permanent electric polarization) in (future) applications, such as sensors, electromechanical devices, and nonvolatile memory. 9 Several bowl-shaped molecules have already been synthesized and found to form columnar phases. 10 In addition, buckybowlic molecules, that is, fragments of C 60 whose dangling bonds have been saturated with hydrogen atoms, have been shown to crystallize in a columnar fashion. 11-15 An aligned phase of metallic nanobowls could also be a promising novel material, since the individual particles have strongly anisotropic optical properties. 1,3 However, no systematic experimental studies of the structure of nanobowls in solution exist to our knowledge, and therefore it remains an open question whether or not bowlshaped nanoparticles and colloids can form stacks and ordered structures. This issue is also not easily resolved using theory or simulations as it is difficult to model the complicated particle shape, although a recent simulation study 8 exists in which the attractive-repulsive Gay-Berne potential was generalized to a bowl-shaped particle. In another very recent simulation study 16 of hard contact lenses (infinitely thin, shall...

The electrical impedance spectrum of simple ionic solutions is measured in a parallel plate capacitor at small applied ac voltage. The influence of the ionic strength is investigated using several electrolytes at different concentrations in solvents of different dielectric constants. The electric double layers that appear at the electrodes at low frequencies are not perfectly capacitive. At moderate ionic strength, ion transport agrees with a model based on the Poisson–Nernst–Planck (PNP) equations. At low ionic strength, double layer dynamics deviate from the PNP model, and the deviation is well described by an empirical function with only one fit parameter. This deviation from the PNP equations increases systematically with increasing Debye length, possibly caused by the long-range Coulomb interaction.

The permanent electrical dipole moment of colloidal quantum dots is important for their optoelectronic properties and can be determined by dielectric spectroscopy. Until now, however, colloidal interactions have not been taken into account in the interpretation of the spectra. Here, dielectric spectra of PbSe and CdSe colloidal quantum dots dispersed in an apolar liquid are measured from 1 Hz to 10 MHz. At frequencies of 10 kHz−1 MHz, Brownian rotation of nanoparticles with a permanent electric dipole moment is detected. At the lowest concentrations (∼0.1 vol %), the nanoparticles rotate independently of each other, and their dipole moment, for both PbSe and CdSe, is on the order of 40−50 D. At higher concentrations (≥0.3 vol %), the dipolar relaxation becomes slower, indicating the presence of nanoparticle structures. A simple model is used to estimate the interaction strength, which appears to be stronger than expected from the weak dipole moment, and has possibly also contributions from electrical moments of higher order. Our results indicate that nanoparticle interactions in liquid media lead to small equilibrium structures that affect dielectric measurements of the dipole moment already at concentrations of a few tenths of a volume percent.

Cryogenic transmission electron microscopy (cryo-TEM) is utilized to determine the second virial coefficient of osmotic pressure of PbSe quantum dots (QDs) dispersed in apolar liquid. Cryo-TEM images from vitrified samples provide snapshots of the equilibrium distribution of the particles. These snapshots yield radial distribution functions from which second virial coefficients are calculated, which agree with second virial coefficients determined with analytical centrifugation and small-angle X-ray scattering. The size dependence of the second virial coefficient points to an interparticle interaction that is proportional to the QD surface area. A plausible cause for this attraction is the interaction between the surface ions on adjacent QDs.

A sensitive dielectric spectroscopy setup is built to measure the response of nanoparticles dispersed in a liquid to an alternating electric field over a frequency range from 10(-2) to 10(7) Hz. The measured complex permittivity spectrum records both the rotational dynamics due to a permanent electric dipole moment and the translational dynamics due to net charges. The setup consists of a half-transparent capacitor connected in a bridge circuit, which is balanced on pure solvent only, using a software-controlled compensating voltage. In this way, the measured signal is dominated by the contributions of the nanoparticles rather than by the solvent. We demonstrate the performance of the setup with measurements on a dispersion of colloidal CdSe quantum dots in the apolar liquid decalin.