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Iss 11 (2000), 1344-1345
Hollow fibers with nanozeolite walls are firstly fabricated through layer-by-layer (LbL) technique, using carbon fibers as templates, coupled with removal of the templates by calcination. The thickness and the composition of the walls can be readily controlled by varying the number of the deposition cycles and the zeolite types used, respectively.

[38] In-Situ Nucleation and Growth of Gamma-FeOOH Nanocrystallites in Polymeric Supramolecular Assemblies

A. K. Dutta, G. Jarero, L. Q. Zhang and P. Stroeve

Chem. Mater. 12(1) (2000), 176-181

This paper reports the nucleation and growth of lepidocrocite (gamma-FeOOH) crystallites in supramolecular polymer multilayers. Organized, polymeric thin films were formed by the layer by layer deposition technique that consisted of adsorbing alternate layers of a polycation and a polyanion on a quartz substrate. Nucleation of nanoparticles was initiated by absorbing ferric nitrate in the thin film polymer matrix, followed by hydrolysis with ammonium hydroxide-1 Repeating the above process resulted in an increase in the density of the nanoparticles initially formed and further growth in the dimensions of the crystallites with the number of absorption and hydrolysis-cycles;Analysis of the UV-visible absorption spectra of the films revealed the formation of lepidocrocites, which was confirmed by FTIR and selected area electron diffraction (SAED) studies. An important feature of this work is that lepidocrocite (gamma-FeOOH) is formed instead of akaganeite (beta-FeOOH),which is generated when ferrous chloride is used as the starting material and is converted to iron oxyhydroxide through oxidative hydrolysis as reported in our previous work (Langmuir 1999, 15, 2176). It therefore seems likely that the initial starting material plays a key role in determining the structural and morphological characteristics of the iron oxyhydroxides although their chemical compositions are the same.

[39] Surface Sol-Gel Synthesis of Ultrathin Semiconductor-Films

N. I. Kovtyukhova, E. V. Buzaneva, C. C. Waraksa, B. R. Martin and T. E. Mallouk

Chem. Mater. 12(2) (2000), 383-389

Ultrathin films of ZnS, Mn-doped ZnS, ZnO, and SiO2 were grown on silicon substrates using surface sol-gel reactions, and the film growth process was characterized by ellipsometry, atomic force microscopy, X-ray photoelectron spectroscopy, UV-visible absorbance, and photoluminescence (PL) spectroscopy. The, Si substrates were pretreated by chemical oxidation, or by derivatization with 4- (dimethylmethoxy)silyl)butylamine. On the oxidized Si/SiOx surface, nanoparticulate films of ZnS and Mn-doped ZnS were grown by sequential immersion in aqueous metal acetate and sodium sulfide solutions. During the first four adsorption cycles, there was little film growth, but thereafter the amount of material deposited was linear with the number of adsorption cycles. This behavior is consistent with the formation of ZnS nuclei at low coverage, followed by particle growth in subsequent cycles. PL spectra are consistent with incorporation of Mn2+ into the ZnS nanoparticles. In contrast, the growth of SiO2 films from nonaqueous SiCl4 an the same Si/SiOx substrates was regular from the first adsorption cycle, indicating a high density of nucleation sites. On amine- derivatized substrates, ZnO thin films grew as relatively smooth islands, suggesting that the interaction of Zn2+ ions or primary ZnO clusters with the amine surface priming layer was sufficiently strong to prevent the formation of isotropic nanoparticles upon exposure to aqueous base.

[40] Nucleation and Growth of Lead Sulfide Nanocrystallites and Microcrystallites in Supramolecular Polymer Assemblies

A. K. Dutta, T. T. Ho, L. Q. Zhang and P. Stroeve

Chem. Mater. 12(4) (2000), 1042-1048

Multilayer thin film assemblies fabricated by the sequential adsorption of polyelectrolytes on a quartz substrate were used as a supramolecular reaction template to study the in situ nucleation and growth of PbS nano- and microparticles. Chemical reaction within the polymer film was initiated by absorbing pb(2+) from an aqueous solution of Pb(NO3)(2) followed by exposing the film to H2S gas. Electron microscopic examination of the films revealed that while nanoparticles are formed in films that were subject to one or two reaction cycles, large crystallites were formed when these films were exposed to a large number (10) of reaction cycles. In the latter case, a broad distribution of particle sizes is observed and may be attributed to Ostwald ripening. Detailed studies show the nucleation and growth of the PbS particles into crystallites of different shapes. UV-vis absorption studies reveal that the absorption spectral profiles of the films are dependent on the size of the PbS crystallites. The broadened absorption spectral profile observed for films subject to a large number of reaction cycles may be attributed to the superposition of the spectral profiles of the small clusters that tend to be blueshifted due to quantum confinement effects and the large clusters that are redshifted.

[41] Electrostatically Controlled Organization of Carboxylic-Acid Derivatized Colloidal Silver Particles on Amine-Terminated Self- Assembled Monolayers

A. Gole, S. R. Sainkar and M. Sastry

Chem. Mater. 12(5) (2000), 1234-1239

The formation of self-assembled monolayers (SAMs) of an aromatic bifunctional molecule, 4-aminothiophenol (4-ATP) on gold and the subsequent organization of carboxylic acid derivatized silver colloidal particles is described. Quartz crystal microgravimetry (QCM) measurements have been used to follow the formation of 4-ATP SAMs as well as electrostatic assembly of the colloidal silver particles on the SAM surface. It is shown that the electrostatic interaction between the negatively charged colloidal particle surface-bound carboxylic acid groups and the terminal amine groups in the SAM can be modulated by variation of the colloidal solution pH. This enables control over the surface coverage of the colloidal particles on the SAM surface with a maximum surface coverage of 18% being attained. The SAMs as well as the colloidal particle covered SAM films were further characterized with X-ray photoemission spectroscopy (XPS) and energy-dispersive analysis of X-rays (EDAX) measurements.

[42] Layer-by-Layer Assembled Films of HgTe Nanocrystals with Strong Infrared-Emission

A. L. Rogach, D. S. Koktysh, M. Harrison and N. A. Kotov

Chem. Mater. 12(6) (2000), 1526

Layer-by-layer sequential adsorption (LBL) has been applied to the preparation of composite multilayer thin films of thiol- capped HgTe nanocrystals with strong IR luminescence. The films can be used as active coatings for novel optical sources for telecommunications.

[43] Layered Polyelectrolyte Films as Selective, Ultrathin Barriers for Anion Transport

J. J. Harris, J. L. Stair and M. L. Bruening

Chem. Mater. 12(7) (2000), 1941-1946

Synthesis of high-flux composite membranes requires methods for deposition of ultrathin, defect-free films on highly permeable supports. Layer-by-layer deposition of polyelectrolytes on porous alumina (0.02 mu m pore diameter) produces such membranes. Electron microscopy shows that five bilayers (<25 nm) of poly(allylamine hydrochloride) (PAH)/ poly(styrenesulfonate) (PSS) are sufficient to cover porous alumina and that underlying pores are not clogged during the deposition process. The selectivity of anion transport through these membranes increases with the number of bilayers until the substrate is fully covered. Five-bilayer PAH/PSS membranes have Cl-/SO42- and Cl-/Fe(CN)(6)(3-) selectivity values of 7 and 310, respectively. PAH/poly(acrylic acid) membranes show selectivity values similar to those of PAH/PSS membranes but with a 3-fold decrease in anion flux. Selectivity in both of these systems likely results from Donnan exclusion.

[44] Sol-Gel Processing of Ordered Multilayers to Produce Composite Films of Controlled Thickness

J. H. Rouse, B. A. Macneill and G. S. Ferguson

Chem. Mater. 12(8) (2000), 2502-2507

Sol-gel processing of a multilayered film formed by the alternate adsorption of poly(diallyldimethylammonium chloride) and a synthetic silicate sheet mineral, Laponite RD, resulted in an interpenetrating composite network. Kinetic data indicate that the amount of material added due to sorption and gelation of tetraethyl orthosilicate (TEOS) reached a steady-state value within a day. Sol-gel growth occurred within the multilayer, with very little silicate being deposited at the surface. The resulting composite film showed the same order as an untreated multilayer, indicating that sorption and gelation did not disrupt the structural order of the multilayer. Curing the TEOS- treated films at 175 degrees C for 24 h to drive off excess water and complete the gelation process again resulted in an ordered composite structure. Cured films exposed to various levels of relative humidity demonstrated a dramatic decrease in swellability compared to an untreated multilayer.

[45] Ultrathin Composite Films Incorporating the Nanoporous Isopolyoxomolybdate Keplerate (NH4)(42)(Mo132O372(Ch3Coo)(30)(H2O)(72))

D. G. Kurth, D. Volkmer, M. Ruttorf, B. Richter and A. Muller

Chem. Mater. 12(10) (2000), 2829

Ultrathin Composite Films Incorporating the Nanoporous Isopolyoxomolybdate ''Keplerate'' (NH4)(42)[Mo132O372- (CH3COO)(30)(H2O)(72)] The giant, nanoporous Keplerate (NH4)(42)- [Mo132O372(CH3COO)(30)(H2O)(72)] was incorporated into a permeable ultrathin film using a stepwise self-assembly strategy. Alternating adsorption of the Keplerate anions and PAH results in single layers with an apparent surface coverage of approximate to 50%. Such permeable composite coatings may lead into a practical approach to utilize nanoporous polyoxometalate clusters in sensors or heterogeneous catalysts.

[46] Hierarchical Assembly of Zeolite Nanoparticles into Ordered Macroporous Monoliths Using Core-Shell Building-Blocks

K. H. Rhodes, S. A. Davis, F. Caruso, B. J. Zhang and S. Mann

Chem. Mater. 12(10) (2000), 2832

Hierarchical Assembly of Zeolite Nanoparticles into Ordered Macroporous Monoliths Using Core-Shell Building Blocks Latex beads are used as a combined template and porogen in the fabrication of monolithic silica containing a hierarchy of pores. Prefabricated core/shell particles, prepared by the layer-by-layer assembly of zeolite (silicalite) nanoparticles onto spherical latex templates, are assembled into macroscopic close-packed structures. Calcination removes all organic components and causes densification of the inorganic structure producing a macroporous zeolite in which both the pore size and wall thickness can be varied.

[47] Metal-Films Prepared by Stepwise Assembly - 2 - Construction and Characterization of Colloidal Au and Ag Multilayers

M. D. Musick, C. D. Keating, L. A. Lyon, S. L. Botsko, D. J. Pena, W. D. Holliway, T. M. Mcevoy, J. N. Richardson and M. J. Natan

Chem. Mater. 12(10) (2000), 2869-2881

This manuscript describes the stepwise, ligand-directed assembly, characterization, and prospective applications of three-dimensional Au and Ag nanoparticle, multlilayered films. Films were prepared by successive treatments of a Au nanoparticle monolayer with a bifunctional cross-linker and colloidal Au or Ag solutions. Changes in film electrical and optical properties are reported for a series of bifunctional cross-linkers of varying molecular lengths. Interestingly, these films exhibit Beer's law behavior despite the presence of strong interparticle optical coupling. Multilayer films with greater than six exposures to 2-mercaptoethylamine and Au colloid were highly conductive and resembled bulk Au in appearance. In contrast, films of similar particle coverage generated using a longer cross-linker (1,6-hexanedithiol) exhibited higher transmission in the near-infrared region and exhibited a reduced conductivity. Measurement of the multilayer morphology with atomic force microscopy, electrostatic force microscopy, and field emission scanning electron microscopy revealed a porous, discontinuous morphology composed of large, continuous regions of aggregated nanoparticles. This, in turn, results in a surface roughness contribution to surface plasmon scattering and surface-enhanced Raman scattering observed for Au, Au/Ag, and Ag colloid multilayers. Particulate multilayer films made using horseradish peroxidase as a cross-linker remained enzymatically active, even beneath three layers of colloidal Au. Multilayers could also be prepared on surfaces patterned by microcontact printing. These data show how Au colloid multilayers grown in solution are a viable alternative to evaporated metal films for a number of applications.

[48] Buildup of Polymer/Au Nanoparticle Multilayer Thin-Films Based on Hydrogen-Bonding

E. C. Hao and T. Q. Lian

Chem. Mater. 12(11) (2000), 3392-3396

We report two new hydrogen-bonding-based routes for layer-by- layer fabrication of polymer/Au nanoparticle multilayer thin films. Two types of Au nanoparticles surface-modified with carboxyl groups or pyridine groups were prepared in nonaqueous solvents. In the first assembly route, we consecutively adsorbed poly(4-vinylpyridine) (PVP) and Au nanoparticles with carboxyl group tailored surfaces. In the second route, we alternatively deposited poly(acrylic acid) (PAA) and Au nanoparticles with pyridine group tailored surfaces. The multilayer buildup was monitored by UV-vis spectroscopy, which showed a linear increase of the film absorbance with the number of adsorbed Au layers. FTIR spectroscopy was used to verify hydrogen bonding between the pyridine and carboxyl groups, which is believed to be the driving force for the formation of polymer/Au multilayer thin films.

[49] Electron-Transfer Between Adjacent Layers in Self-Assembled Films

V. B. P. Leite, M. Ferreira and O. N. Oliveira

Chem. Phys. Lett.