On one-dimensional self-assembly of surfactant-coated nanoparticles

Nanometer-sized metal and semiconductor particles possess novel properties. To fully realize their potential, these nanoparticles need to be fabricated into ordered arrays or predesigned structures. A promising nanoparticle fabrication method is coupled surface passivation and self-assembly of surfactant-coated nanoparticles. Due to the empirical procedure and partially satisfactory results, this method still represents a major challenge to date and its refinement can benefit from fundamental understanding. Existing evidences suggest that the self-assembly of surfactant-coated nanoparticles is induced by surfactant-modified interparticle interactions and follows an intrinsic road map such that short one-dimensional (1D) chain arrays of nanoparticles occur first as a stable intermediate before further assembly takes place to form higher dimensional close-packed superlattices. Here we report a study employing fundamental analyses and Brownian dynamics simulations to elucidate the underlying pair interaction potential that drives the nanoparticle self-assembly via 1D arrays. We find that a pair potential which has a longer-ranged repulsion and reflects the effects of surfactant chain interdigitation on the dynamics is effective in producing and stabilizing nanoparticle chain arrays. The resultant potential energy surface is isotropic for dispersed nanoparticles but becomes anisotropic to favor the growth of linear chain arrays when self-assembly starts.     

Dielectric properties of self-assembled layers of octadecylamine on mica in dry and humid environments

Atomic force microscopy operating in noncontact electrostatic force mode was used to study the interaction of water with films of alkylamines and alkylsilanes on mica. The films efficiently block water adsorption except in exposed mica areas, where it strongly modifies the mobility of surface ions. We also studied the molecular orientation of octadecylamine molecules forming monolayers and multilayer islands. In monolayer films the molecules bind to mica through the amino group, producing a positive contact potential relative to mica (dipole pointing up). In multilayer films the methyl and amino group terminations are exposed in alternating layers that give rise to alternating values of the contact potential. These findings correlate with low and high friction forces measured in the methyl termination and amino terminations.     

PVT properties of dodecane/polystyrene systems

The PVT properties of crosslinked polystyrene samples containing various amounts of dodecane were measured. The Tait equation was used to describe the PVT behavior of each system in both the glassy and rubbery regions. The glass transition temperature was determined from the abrupt change of the thermal expansion coefficient. Increase in the dodecane content in the samples resulted in a significant decrease of the difference between the expansion coefficients in the glassy and rubbery regions. Addition of dodecane lowered the glass transition temperature linearly. However, the dependence of the glass transition temperature on pressure was not affected by the presence of dodecane in the polymer samples. Above the glass transition temperature, the volume of the swollen polymer, V_m, could be determined by simple addition of the volumes of the pure components at the appropriate temperature and pressure; the volume change of mixing, δV_m, was independent of temperature and pressure. Below the glass transition temperature, volume additivity of the two components was also applicable after appropriate adjustment of the glass transition temperature of the polymer to that of the dodecane/polymer samples. © 1994 John Wiley & Sons, Inc.     

Synthesis of novel rod-shaped and star-shaped fluorescent phosphane oxides--nonlinear optical properties and photophysical properties

The design of a new class of fluorophores is presented. Some push-pull chromophores (D-pi-A) containing polyphenylethynyl units and a phosphane oxide moiety were efficiently prepared from common intermediates. Straightforward syntheses gave novel one-armed, rod-shaped and three-armed, star-shaped fluorophores. The optical properties of the resulting star-shaped derivatives were evaluated, showed high fluorescence quantum yields, and their excitation induces very efficient charge redistribution. Moreover, thanks to their push-pull character, the molecules exhibited significant second-order NLO properties with good transparency, up to 67x10(-30) esu at 1907 nm, with an absorption lambdamax at 369 nm. The effect of the donor group and of the number of phenylethynyl arms have been studied in this work.     

Breath figures in polymer and polymer blend films spin-coated in dry and humid ambience

We investigate effects of two spin-coating parameters, relative humidity (5% < or = RH < or = 80%) in ambient atmosphere and water content (3 wt % < or = f(H2O) < or = 20 wt %) in solution (rich in tetrahydrofuran), on the structure of breath figures (BF) formed in spin-cast films of polar poly(methyl methacrylate) (PMMA) and PMMA mixed with nonpolar polystyrene (PS). Film morphologies, examined with atomic and lateral force microscopy, are analyzed with integral geometry analysis to yield morphological BF measures. In PMMA, water added to solution has much stronger impact than that from moisture on formed BFs, which could be ordered (with conformational entropy S approximately 0.9-1.0). In PMMA/PS, BFs decorate exclusively polar PMMA domains, resulting in morphologies with two length scales (sub-micrometer BFs and domains >10 microm). This suggests a novel strategy for herarchic structure formation in multicomponent polymer films. In PS/PMMA, BFs are better developed than in pure PMMA spin-coated in identical conditions. These observations show that the air boundary layer facing the spin-cast polymer film (region) is more important than the ambient atmosphere.     

A polarised ATR study of structural orientation in dry and humid thin HPMC films

The structure of thin films of the polymer hypromellose (HPMC) have been investigated under dry and ambient humidity conditions using polarised attenuated total reflectance (ATR) infrared spectroscopy. In particular spectra were recorded in the C–H and O–H stretching regions for spin coated films deposited on silicon, germanium and zinc selenide internal reflection elements (IREs). A recent development in the field of polarised ATR has demonstrated a simple quantitative relationship between complementary s- and p-polarised spectra of orientationally ordered monolayer films, which yields the spectrum uniquely in the direction perpendicular to the surface, the “z-polarised” spectrum. As well as recording s- and p-polarised spectra this work examines the z-spectra derived from the experimental s- and p-polarisation spectra. For the C–H band all three polarisation spectra reveal no change in the preferred orientation of the transition dipole between humid and dry films on germanium or silicon but with a marginally increased orientation parallel to the surface on zinc selenide. On the other hand the O–H band spectra show an increased orientation of the transition dipole parallel to the surface for both humid and dried films for all three IREs when compared to the corresponding C–H band spectra. This effect was greater in the dried film, i.e. with free water removed, than in the humid film.     

Preparation and characterization of gold nanoparticles and nanowires loaded into rod-shaped silica by a one-step procedure

Rod-shaped mesoporous silica nanoparticles (RMSN) with built-in gold nanoparticles or thin gold nanowires in the pore channels were in situ synthesized via a one-step procedure. The insertion of a hydrophobic gold precursor into the mesopores of RMSN was reached through a micellar solubilization mechanism and gold nanoparticles were achieved through a thermal reduction. The resulting RMSN and Au-RMSN samples were characterized by using X-ray diffraction, transmission and scanning microscopies (TEM and SEM), X-ray photoelectron spectroscopy (XPS), nitrogen physisorption and solid-state Nuclear Magnetic Resonance (NMR). The interaction of Au precursor (a carbene complex) with the thiol group at the silica surface was identified and found to play a crucial role in the dispersion of the uniform metal nanoparticles at the internal surface of RMSN. Moreover, TEM micrographs revealed the absence of large gold particles outside the mesopore network. The shape of Au nanoparticles and their loading amount in the mesoporous silica could be easily tuned by altering the concentration of gold precursor.     

New fluorophores with rod-shaped polycyano pi-conjugated structures: synthesis and photophysical properties

[reaction: see text] Novel rod-shaped polycyano-oligo(phenyleneethynylene)s were synthesized by Pd cross-coupling reaction. Polycyano groups were found to greatly improve the emission efficiency (Phi(f)) of OPEs. By the end donor modification, we achieved the creation of very intense blue light-emitting fluorophore with the SMe group (Phi(f) = 0.972, log epsilon 4.89, lambda(em) 455 nm) and very intense yellow light-emitting fluorophore with the NMe(2) group (Phi(f) = 0.999, log epsilon 4.75, lambda(em) 555 nm). Contrasting Phi(f) solvent dependency of 6 and 7 and a linear relationship between Phi(f) and sigma(p)-X over the whole region of sigma(p)-X were also found.     

Structure of microemulsions of AOT and water in dodecane

The dielectric properties of the system water/AOT/dodecane are studied as a function of volume fraction of the dispersed phase and molar ration (water/surfactant). Data shows that the spherical model is valid only at lown values or low values. At high concentrations of dispersed phase, one has to consider micellar aggregation or deformation.     

Thermodynamic and acoustic properties of (heptane + dodecane) mixtures under elevated pressures

The speed of sound in (heptane + dodecane) mixtures was measured over the whole concentration range at pressures up to 101 MPa and within the temperature range from (293 to 318) K. The density of (heptane + dodecane) was measured in the whole composition range under atmospheric pressure and at temperatures from (293 to 318) K. The densities and heat capacities of these binaries at the same temperatures were calculated for pressures up to 100 MPa from the speeds of sound under elevated pressures together with the densities and heat capacities at atmospheric pressure. The effects of pressure and temperature on the excess molar volume and the excess molar heat capacity are discussed.     

Synthesis and properties of 1,4,9-triazaspiro[5,5]undecane and 3,7,11-triazaspiro[5,6]dodecane

Bromination of 1-methylpiperidine-4-carboxaldehyde gave 4-bromo-1-methylpiperidine-4-carboxaldehyde hydrobromide which was used to synthesize 1,4,9-triazaspiro[5,5]undecane and 3,7,11-triazaspiro[5,6]dodecane. Cyanoethylation and alkylation with various alkyl halides gave their mono- and di-substituted derivatives. The side chain nitrile group was reduced and the hydrazide was obtained from the ester analog.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 12, pp. 1673–1676, December, 1986.     

Phase transitions and surface morphology of surfactant-coated aerosol particles

Probe molecule spectroscopy and hygroscopic growth curves characterize the morphology of surfactant-coated aerosol particles as a function of relative humidity (RH). This study focuses on particles composed of either potassium iodide or sodium chloride and sodium dodecyl sulfate (SDS). At high RH, these mixed particles assume a reverse micelle type structure, and at low RH, they comprise a solid core of either KI or NaCl coated with SDS and water. The deliquescence relative humidity (DRH) and efflorescence relative humidity (ERH) of the inorganic fraction of the mixed particles are very similar to those of the pure salts. The surface polarity and morphology sampled by the coumarin 314 probe molecule ranges from that of a water-organic interface to that of an ionic surface and depends strongly on the RH and the amount of SDS. When the SDS coverage of the droplet just prior to efflorescence reaches approximately one monolayer, a thin soap film persists on the surface to values of RH much lower than the ERH. Both the electronic spectroscopy and photoelectric charging efficiency show a separate efflorescence for this layer at RH < 5%. The spectroscopy further reveals that there is a hysteresis associated with this low RH phase transition for both KI and NaCl cores.     

Preconcentration and catalysis in reduction of aliphatic organohalides using surfactant-coated electrodes

Voltammetric reductions of several organohalide pollutants in films of didodecyldimethylammonium bromide (DDAB) and clay-DDAB cast onto pyrolytic graphite electrodes were examined. Direct reduction data show that the amount of accumulation in these liquid crystal films was larger for relatively nonpolar analytes such as trans-1,2-dibromocyclohexane than for chlorinated acids. The vicinal dibromides are probably taken up by hydrophobic regions of the films. Chlorinated acids were accumulated and reduced in DDAB films, but their direct reduction was partly blocked by the clay-DDAB films. Catalytic reductions using films containing metal phthalocyanines had good efficiencies for all substrates studied and shifted reduction potentials positive by 200-550 mV.     

Electrical properties of dry and wet polyglycine

Real and imaginary parts of the dielectric constant of wet and dry polyglycine have been measured for several temperatures over a wide range of frequencies. A simple, rather crude model, proposed earlier for nylon 66, fits the experimental data rather well, but the blocking coefficient p is larger and adsorption effects are more important.     

Rotation in suspension of a rod-shaped colloid

The simple strategy of coating a closely packed colloid monolayer with a nanometer-thick metal film to connect colloidal spheres and then gently sonicating produces a series of colloid metastructures, including rods and planar sheets. These structures can be fluorescently labeled, which can serve as a probe to monitor their dynamics in complex environments. The metal coating modulates fluorescence emission as these structures rotate in suspension. By analyzing the time sequence of fluorescence images using single-particle tracking techniques, here we measure the rotational dynamics of a rodlike tetramer, quantifying rotation along the long axis.     

Adsorption properties of nanoparticles

Scanning tunneling microscopy and spectroscopy, Auger spectroscopy, and mass spectrometry were used to study the physicochemical properties of nanoparticles of several metals and their oxides. The adsorption properties of the nanoparticles were determined for their interaction with hydrogen, oxygen, and nitrogen.     

Synthesis and properties of lead nanoparticles

Lead nanoparticles were synthesized by the thermal decomposition of lead stearate in octanol. The nanoparticles were characterized by X-ray powder diffraction, thermal analysis, IR spectroscopy, and electron microscopy. The lead particle size can be controlled by both varying the concentration of lead stearate in octanol and changing the thermolysis time. Lead nanoparticles have an organic coat composed of decomposition products of lead stearate. This coat prevents the particles from oxidation in air and favors their dissolution in organic solvents.     

Boron nanoparticles: Reactivity and properties

The electron affinity of small boron clusters and boron nanoparticles and fullerenes (B _n ) has been calculated as function of their size. The energetic stability of the small clusters is considered, and a mechanism for their assembling into large nanosystems is suggested.     

Redox properties of inorganic nanoparticles

The redox potentials E°(M₁₄₇) (M = Au, Cu) were calculated on the basis of the shell model with inclusion of hybridized electronic configurations of atoms. The prospects for studying E°(M _N ) for nanoparticles of other d metals were outlined.