Colloidal clusters

Clusters of metal atoms can form colloidal suspensions. Colloidal clusters of copper, silver, and gold in glass are responsible for some of the vivid colors of stained glass in medieval cathedrals. Even aqueous suspensions of metal clusters are known (Fig. 8.45). 

M. Carpineti and M. Giglio Spinodal-Type Dynamics in Fractal Aggregation of Colloidal Clusters. Phys. Rev. Lett 68, 3327 (1992). 

Any study of colloidal crystals requires the preparation of monodisperse colloidal particles that are uniform in size, shape, composition, and surface properties. Monodisperse spherical colloids of various sizes, composition, and surface properties have been prepared via numerous synthetic strategies [67]. However, the direct preparation of crystal phases from spherical particles usually leads to a rather limited set of close-packed structures (hexagonal close packed, face-centered cubic, or body-centered cubic structures). Relatively few studies exist on the preparation of monodisperse nonspherical colloids. In general, direct synthetic methods are restricted to particles with simple shapes such as rods, spheroids, or plates [68]. An alternative route for the preparation of uniform particles with a more complex structure might consist of the formation of discrete uniform aggregates of self-organized spherical particles. The use of colloidal clusters with a given number of particles, with controlled shape and dimension, could lead to colloidal crystals with unusual symmetries [69]. 

D. Zerrouki, B. Rotenberg, S. Abramson, J. Baudry, C. Goubault, F. Leal-Calderon, D.J. Pine, and J. Bibette Preparation of Doublet, Triangular, and Tetrahedral Colloidal Clusters by Controlled Emulsification. Langmuir 22, 57 (2006). 

Recent developments in attaining chemical control in the arrested precipitation of semiconductor particles from solution have made it possible to prepare colloidal clusters of narrow size distribution It is now known that the electronic spectra... 

Figure 10.3. Schematic experimental set-up for single-molecule SERS. Insert (top) shows a typical Stokes and anti-Stokes Raman spectrum. Insert (bottom) shows an electron microscope image of SERS-active colloidal clusters. (With permission from Ref. 21.)...

M in concentration. This is in the range required for single-molecule detection. These sensitivity levels have been obtained on colloidal clusters at near-infrared excitation. Figure 10.3 is a schematic representation of a single-molecule experiment performed in a gold or silver colloidal solution. The analyte is provided as a solution at concentrations smaller than 10-11 M, Table 10.1 lists the anti-Stokes/Stokes intensity ratios for crystal violet (CY) at 1174 cm-1 using 830-nm near-infrared radiation well away from the resonance absorption of CY with a power of 106 W/cm2 [34]. CV is attached to various colloidal clusters as indicated in the table. Raman cross sections of 10-16 cm2/molecule or an enhancement factor of 1014 can be inferred from the data. 

Kneipp et al. [34] showed that enhancement is independent of cluster sizes ranging from 100 nm to 20 pm. The data and the electron microscope images of the SERS particles are depicted in Figure 10.8 together with the nonresonance SERS spectrum of 10 6 M crystal violet. SERS enhancement is estimated to be on the order of 106 for the spatially isolated cluster and up to 108 for the colloidal clusters. The isolated silver clusters were made by the laser ablation technique mentioned earlier. 

C6o molecules in water also form colloidal clusters based on 3.4 nm diameter (carbon atoms) icosahedral arrangements of thirteen CWI molecules.5 Here the Ceo molecules are necessarily separated by water molecules to form clusters with this diameter.10 Such an arrangement is shown in Figure 2 within an expanded, but now strain-free, cluster of water icosahedral clusters. The water network is formed by tetrahedral tricyclo decamer (H20)io structures connecting groups of four Cm molecules. The modeled diameter of the cluster... 

Markel VA, Shalaev VM, Zhang P, Huynh W, Tay L, Haslet TL, Moskovits M (1999) Nearfield optical spectroscopy of individual surface-plasmon modes in colloid clusters. Phys Rev B 59 10903... 

Tsai DP, Kovacs J, Wang Z, Moskovits M, Shalaev VM, Suh JS, Botet R (1994) Photon scanning tunneling microscopy images of optical excitations of fractal metal colloid clusters. Phys Rev Lett 72(26) 4149... 

Imbibition - consisting of spreading off the liquid solvent into the colloidal cluster... 

The doping of noble metal catalyst precursors is not restricted to the early transition metal series. Toshima has reported significant promotion of catalytic activity in the hydrogenation of acrylic acid by the addition of neodymium ions to palladium particles. Recently, Liu and coworkers have studied the influence of metal ions on the hydrogenation of o-chloronitrobenzene over platinum colloidal clusters, and the metal complex effect on the catalytic performance of metal... 

V.A. Markel, V.M. Shalaev, P. Zhang, W. Huynh, L. Tay, T. L. Haslett, and M. Moskovits, Near-Field Optical Spectroscopy of Individual Sutface-Plasmon Modes in Colloid Clusters, Phys. Rev. B 59, 10903 (1999)... 

Fig. 15 (a) Top SEM images for the structural evolution of bimodal colloidal clusters of silica microspheres and nanospheres for n = 2. Bottom Surface Evolver simulated structural evolution for = 2 as a function of the amount of silica ntinospheres. (b) SEM images of silica-silica composite clusters for n = 2 8. Scale bar. 2 tm. The size ratio of large and small silica particles was... 

Macromonomer-Mediated Synthesis of Polymer-Silica Colloidal Clusters... 

Tirado-Miranda, M., Schmitt, A., CaUqas-Femandez, J. and Femandez-Barbero, A. (1999). Colloidal clusters with finite binding energies fractal structure and growth mechanism. Langmuir, 15, 3437—3444. 

FIGURE 26.20 (a) Clusters of colloidal beads (dark gray) found by MNN algorithm for the two-level MD-SPH particle system. The gray particles represent a solvent, (b) Zoom-in of the system. Only colloidal clusters are shown. The largest one is colored in light gray. 

Imbibition, consisting in spreading off the liquid solvent into the colloidal cluster, and reducing the cohesive forces between the coUoidal beads. This process corresponds to the wetting of a dry, porous solid by the liquid. 

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