Phase Behavior and Structure of a New Colloidal Model System of Bowl-Shaped Particles

Abstract

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...

Phase behaviour of polarizable colloidal hard rods in an external electric field: A simulation study

“…(12). We note that the integration path was smooth and no hysteresis was observed along the integration path.…”

“…1,2 Additionally, a body-centered orthogonal crystal phase is observed for intermediate field strength and density in the case of soft repulsive spheres. [1][2][3] The phase behaviour of anisotropic colloidal particles, such as rods, [4][5][6] dumbbells, 7-9 snowman particles, 10,11 and bowls, 12,13 has received significantly less attention, even though great progress has been made in the synthesis of such particles in recent times (see, e.g., Ref. 14).…”

Phase diagram of octapod-shaped nanocrystals in a quasi-two-dimensional planar geometry

“…[1][2][3][4][5][6][7][8] Such samples were used to experimentally study self-assembly and mesophase behavior. [6][7][8][9][10][11] Concurrently, new simulation techniques were developed to explain the experimentally observed phenomenology and to tackle the complex numerical problems that such investigations bring about. 8,[12][13][14][15][16][17][18][19][20][21][22][23] Most of these simulation studies focussed on convex particles in two-and three-dimensional (2D and 3D) systems, see Refs.…”

Crystal-structure prediction via the Floppy-Box Monte Carlo algorithm: Method and application to hard (non)convex particles

“…Only recently were the first attempts made to study systems that contain relatively complex curved particles. Nonconvex shapes with sharp edges and smooth surfaces, such as superdisks, 79 bowls, 80 and curved triangles, 71 have also come under investigation. For bowls 80 an overlap algorithm was devised unique to this shape.…”