Patchy Particles

Surface modified particles, so-called patchy particles, have been recognized as important building blocks in the directed assembly of particles into desired target structures.1 Various methods employing shadow evaporation and templating have been used to create spherical particles with reactive anchor patches of controllable size and position. Subsequently, patchy particles can be directed to assemble into interesting structures using electrical and magnetic fields or can be linked chemically via molecular modification of the patches. We will report on the use of the “Glancing angle vapor deposition” technique developed in our group2 combined with stamping for the fabrication of patchy particles with two orthogonal patches. The tilting and rotation of a colloidal monolayer with respect to the evaporation source allows the controlled deposition of patches as small as 4% of the overall particle surface. The stamping enables access to the opposite side of the particles and the deposition of a second patch. A simple geometrical model is used to predict the patch geometry and relative orientation of the patches. Preliminary data on the field directed assembly of these particles and their behavior in oxidative environments will also be reported. (1) Glotzer, S. C.; Solomon, M. J. Anisotropy of Building Blocks and Their Assembly into Complex Structures Nature Mater. 2007, 6, 557-562. (2) Pawar, A. B.; Kretzschmar, I. Patchy Particles by Glancing Angle Deposition Langmuir 2008, 24, 355-358

Guided ion beam and theoretical studies of the reaction of Ru+ with CS2 in the gas-phase: thermochemistry of RuC+, RuS+, and RuCS+

Journal ArticleAbstract: The gas-phase reactivity of the atomic transition metal cation, Ru+, with CS2 is investigated using guided-ion beam mass spectrometry (GIBMS). Endothermic reactions forming RuC+, RuS+, and RuCS+ are observed. Analysis of the kinetic energy dependence of the cross sections for formation of these three products yields the 0 K bond energies of D0(Ru+-C) = 6.27 ? 0.15 eV, D0(Ru+-S) = 3.04 ? 0.10 eV, and D0(Ru+-CS) = 2.59 ? 0.18 eV, and consideration of previous data leads to a recommended D0(Ru+-C) bond energy of 6.17 ? 0.07 eV. A detailed reaction coordinate surface for these processes is determined by quantum chemical calculations and shows that all three reactions take place by insertion to form a S-Ru+-CS intermediate. Although multiple spin states are available, the reaction appears to occur primarily on the quartet ground state surface, although coupling to a sextet surface is required to form the RuS+(6?+) + CS(1?+) ground state products. Calculations are used to locate the approximate crossing points between the quartet and sextet surfaces, finding them in both the bending coordinate of the S-Ru+-CS intermediate and in the exit channel. Elimination of S2 to form RuC+ follows a much more complicated pathway involving a cyclic RuCSS+ intermediate, consistent with the energetic behavior of the experimental RuC+ cross section

Guided ion beam and theoretical studies of the reaction of Ag+ with CS2: gas-phase thermochemistry of AgS+ and AgCS+ and insight into spin-forbidden reactions

Journal ArticleThe gas-phase reactivity of the atomic transition metal cation, Ag+, with CS2 is investigated using guided-ion beam mass spectrometry. Endothermic reactions forming AgS+ and AgCS+ are observed but are quite inefficient. This observation is largely attributed to the stability of the closed shell Ag+(1S,4d10) ground state, but is also influenced by the fact that the reactions producing ground state AgS+ and AgCS+ products are both spin forbidden. Analysis of the kinetic energy dependence of the cross sections for formation of these two products yields the 0 K bond energies of D0(Ag+uS)=1.40±0.12 eV and D0(Ag+uCS)=1.98±0.14 eV. Quantum chemical calculations are used to investigate the electronic structure of the two product ions as well as the potential energy surfaces for reaction. The primary mechanism involves oxidative addition of a CS bond to the metal cation followed by simple AguS or AguCS bond cleavage. Crossing points between the singlet and triplet surfaces are located near the transition states for bond activation. Comparison with analogous work on other late second-row transition metal cations indicates that the location of the crossing points bears directly on the efficiency of these spin-forbidden processes

Impact of particle shape on electron transport and lifetime in zinc oxide nanorod-based dye-sensitized solar cells

Owing to its high electron mobility, zinc oxide represents a promising alternative to titanium dioxide as the working electrode material in dye-sensitized solar cells (DSCs). When zinc oxide is grown into 1-D nanowire arrays and incorporated into the working electrode of DSCs, enhanced electron dynamics and even a decoupling of electron transport (τd) and electron lifetime (τn) have been observed. In this work, DSCs with working electrodes composed of solution-grown, unarrayed ZnO nanorods are investigated. In order to determine whether such devices give rise to similar decoupling, intensity modulated photocurrent and photovoltage spectroscopies are used to measure τd and τn, while varying the illumination intensity. In addition, ZnO nanorod-based DSCs are compared with ZnO nanoparticle-based DSCs and nanomaterial shape is shown to affect electron dynamics. Nanorod-based DSCs exhibit shorter electron transport times, longer electron lifetimes, and a higher τn/τd ratio than nanoparticle-based DSCs

Molecular dynamics simulations of the evaporation of particle-laden droplets

We use molecular dynamics simulations to study the evaporation of particle-laden droplets on a heated surface. The droplets are composed of a Lennard-Jones fluid containing rigid particles which are spherical sections of an atomic lattice, and heating is controlled through the temperature of an atomistic substrate. We observe that sufficiently large (but still nano-sized) particle-laden drops exhibit contact line pinning, measure the outward fluid flow field which advects particle to the drop rim, and find that the structure of the resulting aggregate varies with inter-particle interactions. In addition, the profile of the evaporative fluid flux is measured with and without particles present, and is also found to be in qualitative agreement with earlier theory. The compatibility of simple nanoscale calculations and micron-scale experiments indicates that molecular simulation may be used to predict aggregate structure in evaporative growth processes

Using the Discrete Dipole Approximation and Holographic Microscopy to Measure Rotational Dynamics of Non-spherical Colloidal Particles

We present a new, high-speed technique to track the three-dimensional translation and rotation of non-spherical colloidal particles. We capture digital holograms of micrometer-scale silica rods and sub-micrometer-scale Janus particles freely diffusing in water, and then fit numerical scattering models based on the discrete dipole approximation to the measured holograms. This inverse-scattering approach allows us to extract the the position and orientation of the particles as a function of time, along with static parameters including the size, shape, and refractive index. The best-fit sizes and refractive indices of both particles agree well with expected values. The technique is able to track the center of mass of the rod to a precision of 35 nm and its orientation to a precision of 1.5$^\circ$, comparable to or better than the precision of other 3D diffusion measurements on non-spherical particles. Furthermore, the measured translational and rotational diffusion coefficients for the silica rods agree with hydrodynamic predictions for a spherocylinder to within 0.3%. We also show that although the Janus particles have only weak optical asymmetry, the technique can track their 2D translation and azimuthal rotation over a depth of field of several micrometers, yielding independent measurements of the effective hydrodynamic radius that agree to within 0.2%. The internal and external consistency of these measurements validate the technique. Because the discrete dipole approximation can model scattering from arbitrarily shaped particles, our technique could be used in a range of applications, including particle tracking, microrheology, and fundamental studies of colloidal self-assembly or microbial motion.Comment: 11 pages, 9 figures, 2 table

Floor- or ceiling-sliding for chemically active, gyrotactic, sedimenting Janus particles

Surface bound catalytic chemical reactions self-propel chemically active Janus particles. In the vicinity of boundaries, these particles exhibit rich behavior, such as the occurrence of wall-bound steady states of "sliding". Most active particles tend to sediment as they are density mismatched with the solution. Moreover Janus spheres, which consist of an inert core material decorated with a cap-like, thin layer of a catalyst, are gyrotactic ("bottom-heavy"). Occurrence of sliding states near the horizontal walls depends on the interplay between the active motion and the gravity-driven sedimentation and alignment. It is thus important to understand and quantify the influence of these gravity-induced effects on the behavior of model chemically active particles moving near walls. For model gyrotactic, self-phoretic Janus particles, here we study theoretically the occurrence of sliding states at horizontal planar walls that are either below ("floor") or above ("ceiling") the particle. We construct "state diagrams" characterizing the occurrence of such states as a function of the sedimentation velocity and of the gyrotactic response of the particle, as well as of the phoretic mobility of the particle. We show that in certain cases sliding states may emerge simultaneously at both the ceiling and the floor, while the larger part of the experimentally relevant parameter space corresponds to particles that would exhibit sliding states only either at the floor or at the ceiling or there are no sliding states at all. These predictions are critically compared with the results of previous experimental studies and our experiments conducted on Pt-coated polystyrene and silica-core particles suspended in aqueous hydrogen peroxide solutions.Comment: Total number of pages: 33, Number of figures: 18. The video files, as mentioned in the supplementary material will be provided by the corresponding author upon reques