Can reverse micelle shells limit nanoparticle growth?

The possibility to limit the growth of nanoparticles synthesized in reverse microemulsions by the shells of reverse micelles is analyzed theoretically. At surface tension higher than about 30 mJ m⁻², the reverse micelle shells cannot inhibit the particle growth. At lower surface tension values, the particle growth can be limited, but the synthesized particles should be much larger than the initial micelles.     

Weak surface anchoring energy of 4-cyano-4'-pentyl-1,1'-biphenyl on perfluoropolyether Langmuir-Blodgett films

Functional director alignment layers are needed for high performance liquid crystal displays (LCDs). Reported herein is a novel polymer material for LC alignment, namely, perfluoropolyether (PFPE), which exhibits a weak surface anchoring energy for bend deformation and is amenable to simple fabrication of grooved surfaces by soft lithography, a surface topography desired for multistable LCDs. Liquid crystal optical cells fabricated using Langmuir-Blodgett films of PFPE (of variable thickness) exhibited weak surface anchoring energies on the order of 10(-5) Jm2 for the nematic liquid crystal 4-cyano-4'-pentyl-1,1'-biphenyl with no dependence on film thickness.     

Micellar transitions in solvent-annealed thin films of an amphiphilic block copolymer controlled with tunable surface fields

We investigated the response of symmetric poly(styrene-block-4vinylpyridine) P(S-b-4VP) diblock copolymer micelles to surface fields of variable strength at free surfaces and substrate interfaces when the micelles as spun were subjected to solvent annealing. Free surface interactions were controlled with solvent annealing in solvents of varied selectivity. On exposure to vapors of a solvent strongly selective for PS, the micelles retained their spherical shape but grew into cylindrical micelles or lamellar nanostructures via fusion on exposure to slightly selective or neutral solvent vapors. Giant 2D disks that completely wetted PS-grafted substrates resulted when spherical micelles were exposed to vapors of a highly selective solvent for P4VP. The interfacial interactions were controlled through subjecting them to UV/ozone (UVO) substrates initially coated with an end-grafted layer of short PS chains, with which the grafted PS chains became oxidized, degraded, or totally removed through UVO treatment for a controlled duration. When thin films were annealed in vapors of THF, the structural transition from spherical to cylindrical micelles depended on the interfacial field. On applying selective UVO exposure of optimal duration, we fabricated a substrate with two interfacial chemistries that promoted varied micellar species (spherical and cylindrical micelles) with a sharp boundary developed within thin films through solvent annealing for a controlled duration.     

Wetting and spreading of a surfactant film on solid particles: influence of sharp edges and surface irregularities

In addition to particle size and surface chemistry, the shape of particles plays an important role in their wetting and displacement by the surfactant film in the lung. The role of particle shape was the subject of our investigations using a model system consisting of a modified Langmuir-Wilhelmy surface balance. We measured the influence of sharp edges (lines) and other highly curved surfaces, including sharp corners or spikes, of different particles on the spreading of a dipalmitoylphosphatidyl (DPPC) film. The edges of cylindrical sapphire plates (circular curved edges, 1.65 mm radius) were wetted at a surface tension of 10.7 mJ/m2 (standard error (SE) = 0.45, n = 20) compared with that of 13.8 mJ/m2 (SE = 0.20, n = 20) for cubic sapphire plates (straight linear edges, edge length 3 mm) (p < 0.05). The top surfaces of the sapphire plates (cubic and cylindrical) were wetted at 8.4 mJ/m2 (SE = 0.54, n = 20) and 9.1 mJ/m2 (SE = 0.50, n = 20), respectively, but the difference was not significant (p > 0.05). The surfaces of the plates showed significantly higher resistance to spreading compared to that of the edges, as substantially lower surface tensions were required to initiate wetting (p < 0.05). Similar results were found for talc particles, were the edges of macro- and microcrystalline particles were wetted at 7.2 mJ/m2 (SE = 0.52, n = 20) and 8.2 mJ/m2 (SE = 0.30, n = 20) (p > 0.05), respectively, whereas the surfaces were wetted at 3.8 mJ/m2 (SE = 0.89, n = 20) and 5.8 mJ/m2 (SE = 0.52, n = 20) (p < 0.05), respectively. Further experiments with pollen of malvaceae and maize (spiky and fine knobbly surfaces) were wetted at 10.0 mJ/m2 (SE = 0.52, n = 10) and 22.75 mJ/m2 (SE = 0.81, n = 10), respectively (p < 0.05). These results show that resistance to spreading of a DPPC film on various surfaces is dependent on the extent these surfaces are curved. This is seen with cubic sapphire plates which have at their corners a radius of curvature of about 0.75 microm, spiky malvaceae pollen with an even smaller radius on top of their spikes, or talc with various highly curved surfaces. These highly curved surfaces resisted wetting by the DPPC film to a higher degree than more moderately curved surfaces such as those of cylindrical sapphire plates, maize pollens, or polystyrene spheres, which have a surface free energy similar to that of talc but a smooth surface. The macroscopic plane surfaces of the particles demonstrated the greatest resistance to spreading. This was explained by the extremely fine grooves in the nanometer range, as revealed by electron microscopy. In summary, to understand the effects of airborne particles retained on the surfaces of the respiratory tract, and ultimately their pathological potential, not only the particle size and surface chemistry but also the particle shape should be taken in consideration.     

聚合物三维微图案加工的转移微模塑新方法

光刻蚀技术是微电子加工技术中最成功的一种,但由于受到光学衍射等的限制,100nm是光刻蚀的极限,为此人们探索了许多先进的刻蚀技术,如超紫外线刻蚀(EUV)、软X射线刻蚀、电子束刻蚀和聚焦离子束刻蚀(FIB)等,可制作尺寸小于100nm的图形,但普遍存在加工速度慢及成本高等缺点.     

Small-angle X-ray scattering (SAXS) study on nonionic fluorinated micelles in aqueous system

We have investigated the self-organization structures of perfluoroalkyl sulfonamide ethoxylate, C(8)F(17)SO(2)N(C(3)H(7))(CH(2)CH(2)O)(10)H, a nonionic fluorinated surfactant in aqueous system by small-angle X-ray scattering (SAXS) technique. Structural modulation of the nonionic fluorinated micelle induced by temperature change, surfactant concentration, and the added fluorinated oils have been systematically studied. The SAXS data were analyzed by the indirect Fourier transformation (IFT), and the generalized indirect Fourier transformation (GIFT) depending on the volume fraction of the surfactant. Various plausible classical model calculations have been performed to confirm the consistency of the GIFT analysis of the SAXS data. Upon successive increase in temperature, the cylindrical micelles formed at lower temperatures undergo a continuous one-dimensional growth and ultimately near the cloud point an indication of flat planar like structural pattern is observed. The evolution in structure of particle near the demixing temperature may be due to onset of attractive interactions. The shape and size of the micelle is apparently unaffected by changing the surfactant concentration from 1 to 5 wt% at 25 degrees C. Nevertheless, addition of small amount of perfluoropolyether (PFPE) oil, of structure F(CF(2)CF(2)CF(2)O)(n)CF(2)CF(2)COOH (n approximately 21) modulate the micellar shape and size. Long cylindrical micelles eventually transform into globular like particles. The onset cylinder-to-sphere transition in the structure of micelles in the surfactant/water/oil system is probably due to amphiphilic nature of the oil, which tends to increase the spontaneous curvature. The lipophilic part of the oil tends to reside in the micellar core, whereas, the hydrophilic part goes close to the polar head group of the surfactant so that effective cross-sectional area per surfactant molecules increases and as a result spherical micelles tend to form. Perfluorodecalin (PFD) also decreases size of the micelles but its effect is poor compared to the PFPE oil.     

Effect of particle surface selectivity on composite nanostructures in nanoparticle/diblock copolymer mixture dilute solution

In this study the phase behavior of nanoparticle/diblock copolymer composites in dilute solution has been investigated by the hybrid particle-field (HPF) method. We focus on the influence of particle surface selectivity (i.e. hydrophobic and hydrophilic) on the distribution of nanoparticles in the micelles formed by the diblock copolymers. These two types of particle surface selectivity are simulated systematically. The different competition between the energy from enthalpy and the energy from entropy has been observed in the two kinds of composite systems. Our simulation results show that the particle surface selectivity is a crucial factor for determining the thermodynamic properties in the complex dilute solution, and the morphologies of micelles are controlled by the volume fraction of the nanoparticles. The change of particle distribution in various micelles enriches the composite microstructures that can be formed by nanoparticle and diblock copolymer.     

Micellar aggregation in blends of linear and cyclic poly(styrene-b-isoprene) diblock copolymers

The morphology of micelles formed from blends of linear and cyclic poly(styrene-b-isoprene) (PS-b-PI) block copolymers has been investigated in solution using dynamic light scattering (DLS) and in thin solid deposits by atomic force microscopy (AFM) and transmission electron microscopy under cryogenic conditions (cryo-TEM). Micelles of the pure cyclic PS(290)-b-PI(110) copolymers are wormlike cylindrical objects built by unidirectional aggregation of 33 nm wide sunflower micelles, while the linear block copolymer having the same volume fraction and molar mass forms spherical micelles 40 nm in diameter. The DLS, AFM, and cryo-TEM results consistently show that the addition of the linear copolymer (even for amounts as low as 5% w/w) to the cyclic copolymer rather favors the formation of spherical micelles at the expense of the cylindrical aggregates. Those results clearly show that the linear block copolymer chains can be used to stabilize the thermodynamically unstable elementary sunflower micelle. The thermal stability of the micelles (from the pure copolymers and from the blends) has been examined in solid deposits with in situ AFM measurements. Coalescence starts at about 70 degrees C, and the surface roughness shows a two-step decrease toward a fully homogeneous and flat structure.     

Characterization of gas-expanded liquid-deposited gold nanoparticle films on substrates of varying surface energy

Dodecanethiol-stabilized gold nanoparticles (AuNPs) were deposited via a gas-expanded liquid (GXL) technique utilizing CO(2)-expanded hexane onto substrates of different surface energy. The different surface energies were achieved by coating silicon (100) substrates with various organic self-assembled monolayers (SAMs). Following the deposition of AuNP films, the films were characterized to determine the effect of substrate surface energy on nanoparticle film deposition and growth. Interestingly, the critical surface tension of a given substrate does not directly describe nanoparticle film morphology. However, the results in this study indicate a shift between layer-by-layer and island film growth based on the critical surface tension of the capping ligand. Additionally, the fraction of surface area covered by the AuNP film decreases as the oleophobic nature of the surfaces increases. On the basis of this information, the potential exists to engineer nanoparticle films with desired morphologies and characteristics.     

Modulating the pattern quality of micropatterned multilayer films prepared by layer-by-layer self-assembly

Patterned multilayer films composed of poly(allylamine hydrochloride) (PAH) and poly(sodium 4-styrenesulfonate) (PSS) were prepared using dip and spin self-assembly (SA) methods. A silicon substrate was patterned with a photoresist thin film using conventional photolithography, and PAH/PSS multilayers were then deposited onto the substrate surface using dip or spin SA. For spin SA, the photoresist on the substrate was retained, despite the high centrifugal forces involved in depositing the polyelectrolytes (PEs). The patterned multilayer films were formed by immersing the PE-coated substrates in acetone for 10 min. The effect of ionic strength on the pattern quality in dip and spin multilayer patterns (line-edge definition and surface roughness of the patterned region) was investigated by increasing the salt concentration in the PE solution (range 0-1 M). In dip multilayer patterns, the presence of salt increased the film surface roughness and pattern thickness without any deformation of pattern shape. The spin multilayer patterns formed without salt induced a height profile of about 130 nm at the pattern edge, whereas the patterns formed with high salt content (1 M) were extensively washed off the substrates. Well-defined pattern shapes of spin SA multilayers were obtained at an ionic strength of 0.4 M NaCl. Multilayer patterns prepared using spin SA and lift-off methods at the same ionic strength had a surface roughness of about 2 nm, and those prepared using the dip SA and lift-off method had a surface roughness of about 5 nm. The same process was used to prepare well-defined patterns of organic/metallic multilayer films consisting of PE and gold nanoparticles. The spin SA process yielded patterned multilayer films with various lengths and shapes.     

Imaging of crystal morphology and molecular simulations of surface energies in pentacene thin films

We have investigated the crystal growth of the organic semiconductor pentacene by complementing molecular simulations of surface energies with experimental images of pentacene films. Pentacene thin films having variations in thickness and grain size were produced by vacuum sublimation. Large (approximately 20 microm) faceted crystals grew on top of the underlying polycrystalline thin film. The films were characterized using optical microscopy (OM), X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). Single crystals most commonly grew in a truncated diamond shape with the largest crystal face, (001), growing parallel to the substrate. Crystal morphologies and surface energies were calculated using force field-based molecular simulations. The (001) surface was found to have the lowest energy, at 76 mJ/m(2), which was consistent with experimental observations of crystal face size. It was demonstrated that the morphology of the large faceted crystals approached the equilibrium growth shape of pentacene. From contact angle measurements, the critical surface tension of textured pentacene thin films in air was determined to be 34 mJ/m(2).     

Practical approach for macroporous structure embedded microfluidic system and the catalytic microchemical application

We present a low cost and practical approach to integrate 3D ordered macroporous polyfluoropolyether (PFPE) patterns into a microchannel by a series of porous pattern fabrication processes and subsequent photolithography in a site- and shape-selective manner. The 3D ordered macroporous patterns with high-resolution edges were firstly fabricated by microtransfer molding (μ-TM) of the sacrificial polystyrene (PS) template infiltrated with PFPE as a non-adhesive and solvent-resistant skeletal material. The resulting robust PFPE porous structures with high solvent resistance on a silicon wafer can easily be embedded into the microchannel with the aid of conventional photolithography, leading to a microfluidic system with a built-in microstructure. Moreover, catalytic Pd nanoparticles implanted on the surface of the porous structure were obtained by use of Pd nanoparticle deposited PS spheres, the porous structure embedded channel was utilized to perform a Suzuki coupling reaction.     

Synthesis and surface treatment of magnetite nanoparticles for tuning hydrophobicity and colloidal stability

Post-synthetic surface modification of magnetite nanoparticles synthesized by a modified co-precipitation process was carried out with triethoxy-terminated perfluoropolyether (PFPE) oligomers. The chemisorption of PFPE oligomers on the surface of magnetites was confirmed by ATR-FTIR and TGA analyses. The efficiency of surface modification of the oligomer to prevent the aggregation of magnetite nanoparticles was studied with the dynamic light scattering technique by measuring the hydrodynamic diameter and polydispersity index of the surface treated nanoparticles, together with their zeta potential. Aggregation kinetics profiles were constructed for surface treated nanoparticles. The obtained data was compared with magnetite nanoparticles treated with critic acid, to assess the efficiency of the surface modification with the PFPE oligomers. The comparison showed that the bifunctional PFPE oligomer treated nanoparticles are characterized by improved colloidal stability and hydrophobicity.     

Surface properties and structures of diblock copolymer and homopolymer with perfluoroalkyl side chains

The surface free energy of diblock copolymer, composed of methyl methacrylate and 2-perfluorooctylethyl methacrylate (PMMA-b-PFEMA), was compared with that of PFEMA homopolymer (P-PFEMA) in correlation with their structures in the solid state and in the solution using dynamic contact angle, X-ray photoelectron spectroscopy, X-ray diffraction, and dynamic light scattering. The PMMA-b-PFEMA film cast from chloroform solution was found to possess very low surface free energy (7.8 mJ/m(2)) compared with the surface free energies of the P-PFEMA (8.5 mJ/m(2)) and the PMMA-b-PFEMA (9.8 mJ/m(2)) films cast from CF(3)CF(2)CHCl(2) solutions. These differences in the surface free energy were brought about by the variations in their surface structures. The very low surface free energy was considered to have originated from the surface segregation of the PFEMA segments highly self-assembled by the presence of chloroform.     

A novel electromagnetism‐assisted imprinting technology to replicate microstructures onto a large‐area curved surface using a flexible magnetic mold

Replication of microstructures from a mold onto a curved surface is difficult. The conformal contact between the mold and the substrate has to be ensured. The present study proposes an innovative mechanism, which employs an electromagnetic disk to provide magnetic force and a PDMS flexible mold with a layer compounded magnetic powder. This mechanism provides not only the gradual contact from center to edge to avoid air entrapment but also conformal contact between the mold and the substrate during the imprinting operation. A system based on this electromagnetic soft imprinting technology has been implemented, and imprinting to replicate microstructures from the mold onto a curved surface has been carried out. The results reveal that the PDMS magnetic mold and the electromagnetic disk‐controlled magnetic force can successfully perform the imprinting and accurately replicate the microstructures onto the large‐area, curved surface glass. The PDMS flexible magnetic mold incorporated with the magnetic disk can be employed to achieve the conformal contact between the mold and the substrate. In addition, due to the low surface free energy of the PDMS, the de‐molding without sticking can be easily accomplished. Copyright © 2008 John Wiley & Sons, Ltd.     

Thermoresponsive poly(N-isopropylacrylamide) copolymers: contact angles and surface energies of polymer films

Surface properties of poly(N-isopropylacrylamide) (PNIPAM) copolymer films were studied by contact angle measurements and optical and atomic force microscopy. We prepared a series of copolymers of N-isopropylacrylamide with N-tert-butylacrylamide (NtBA) in order of increasing hydrophobicity. The measurements of the advancing contact angle of water at 37 degrees C were hampered by the observation of a distinct stick/slip pattern on all polymers in the series with the exception of poly(NtBA) (PNtBA). We attributed this behavior to the film deformation by the vertical component of liquid surface tension leading to the pinning of the moving contact line. This was confirmed by the observation of a ridge formed at the pinned contact line by optical microscopy. However, meaningful contact (without the stick/slip pattern and with a time-independent advancing contact angle) angles for this thermoresponsive polymer series could be obtained with carefully selected organic liquids. We used the Li and Neumann equation of state to calculate the surface energy and contact angles of water for all polymers in the series of copolymers and van Oss, Chaudhury, and Good (vOCG) acid-base theory for PNtBA. The surface energies of the thermoresponsive polymers were in the range of 38.9 mJ/m2 (PNIPAM) to 31 mJ/m2 (PNtBA) from the equation of state approach. The surface energy of PNtBA calculated using vOCG theory was 29.0 mJ/m2. The calculated contact angle for PNIPAM (74.5 +/- 0.2 degrees ) is compared with previously reported contact angles obtained for PNIPAM-modified surfaces.     

Macroscopic deformation of a substrate as a new method for controlling geometry and plasmon-resonant properties of highly ordered planar ensembles of metal nanoparticles

A new approach to creating highly ordered two-dimensional ensembles of nanoparticles with variable geometric parameters is proposed. It combines diblock copolymer micellar lithography and controlled deformation of a polymer substrate. The key feature of the approach is the formation of a monolayer of hexagonally packed metal precursor-containing micelles of an amphiphilic diblock copolymer on the surface of an isotropically stretched polymer plate. The average distance between micelle centers is 140 nm. Subsequent thermal treatment (or isotropic stretching) of the sample results in the shrinkage (or elongation) of the substrate, which enables one to vary the distance between micelle centers in a range of 80–200 nm while retaining hexagonal packing of the micelles in the monolayer. At the final stage, ensembles of hexagonally ordered gold nanoparticles are obtained by exposing the micellar films to air plasma. It is demonstrated that gold nanoparticles in these ensembles can be enlarged by seeded growth. The systematic study of the plasmon-resonant properties of the resulting ensembles shows that the gradual increase in the distance between 35-nm gold particles from 80 to 200 nm leads to an unexpected nonmonotonic shift of the maximum of localized surface plasmon resonance, which is, from our point of view, caused by the high degree of organization of nanoparticles on the substrate.     

Shape effect on nanoparticle solvation: a comparison of morphometric thermodynamics and microscopic theories

Conventional wisdom for controlling the nanoparticle size and shape during synthesis is that particle growth favors the direction of a facet with the highest surface energy. However, the particle solvation free energy, which dictates the particle stability and growth, depends not only on the surface area and surface free energy but also on other geometric measures such as the solvent excluded volume and the surface curvature and their affiliated thermodynamic properties. In this work, we study the geometrical effects on the solvation free energies of nonspherical nanoparticles using morphometric thermodynamics and density functional theories. For idealized systems that account for only molecular excluded-volume interactions, morphometric thermodynamics yields a reliable solvation free energy when the particle size is significantly larger than the solvent correlation length. However, noticeable deviations can be identified in comparison to the microscopic theories for predicting the solvation free energies of small nanoparticles. This conclusion also holds for predicting the potential of mean force underlying the colloidal "key-and-lock" interactions. Complementary to the microscopic theories, morphometric thermodynamics requires negligible computational cost, therefore making it very appealing for a broad range of practical applications.     

Size-dependent catalytic behavior of platinum nanoparticles on the hexacyanoferrate(III)/thiosulfate redox reaction

Platinum nanoparticles prepared in reverse micelles have been used as catalysts for the electron transfer reaction between hexacyanoferrate(III) and thiosulfate ions. Nanoparticles of average diameter ranging between 10 and 80 nm have been used as catalysts. The kinetic study of the catalytic reaction showed that for a fixed mass of catalyst the catalytic rate did not increase proportionately to the decrease in particle size over the whole range from 10 to 80 nm. The maximum reaction rate has been observed for average particle diameter of about 38 nm. Particles below diameter 38 nm exhibit a trend of decreasing reaction rate with the decrease in particle size, while those above diameter 38 nm show a steady decline of reaction rate with increasing size. It has been postulated that in the case of particles of average size less than 38 nm diameter, a downward shift of Fermi level with a consequent increase of band gap energy takes place. As a result, the particles require more energy to pump electrons to the adsorbed ions for the electron transfer reaction. This leads to a reduced reaction rate catalyzed by smaller particles. On the other hand, for nanoparticles above diameter 38 nm, the change of Fermi level is not appreciable. These particles exhibit less surface area for adsorption as the particle size is increased. As a result, the catalytic efficiency of the particles is also decreased with increased particle size. The activation energies for the reaction catalyzed by platinum nanoparticles of diameters 12 and 30 nm are about 18 and 4.8 kJ/mol, respectively, indicating that the catalytic efficiency of 12-nm-diameter platinum particles is less than that of particles of diameter 30 nm. Extremely slow reaction rate of uncatalyzed reaction has been manifested through a larger activation energy of about 40 kJ/mol for the reaction.     

Nanostructures of functionalized gold nanoparticles prepared by particle lithography with organosilanes

Periodic arrays of organosilane nanostructures were prepared with particle lithography to define sites for selective adsorption of functionalized gold nanoparticles. Essentially, the approach for nanoparticle lithography consists of procedures with two masks. First, latex mesospheres were used as a surface mask for deposition of an organosilane vapor, to produce an array of holes within a covalently bonded, organic thin film. The latex particles were readily removed with solvent rinses to expose discrete patterns of nanosized holes of uncovered substrate. The nanostructured film of organosilanes was then used as a surface mask for a second patterning step, with immersion in a solution of functionalized nanoparticles. Patterned substrates were fully submerged in a solution of surface-active gold nanoparticles coated with 3-mercaptopropyltrimethoxysilane. Regularly shaped, nanoscopic areas of bare substrate produced by removal of the latex mask provided sites to bind silanol-terminated gold nanoparticles, and the methyl-terminated areas of the organosilane film served as an effective resist, preventing nonspecific adsorption on masked areas. Characterizations with atomic force microscopy demonstrate the steps for lithography with organosilanes and functionalized nanoparticles. Patterning was accomplished for both silicon and glass substrates, to generate nanostructures with periodicities of 200-300 nm that match the diameters of the latex mesospheres of the surface masks. Nanoparticles were shown to bind selectively to uncovered, exposed areas of the substrate and did not attach to the methyl-terminal groups of the organosilane mask. Billions of well-defined nanostructures of nanoparticles can be generated using this high-throughput approach of particle lithography, with exquisite control of surface density and periodicity at the nanoscale.