Highly oriented tunable wrinkling in polymer bilayer films confined with a soft mold induced by water vapor

The authors report the formation of highly oriented wrinkling on the surface of the bilayer [polystyrene (PS)/poly(vinyl pyrrolidone) (PVP)] confined by a polydimethylsiloxane (PDMS) mold in a water vapor environment. When PVP is subjected to water vapor, the polymer loses its mechanical rigidity and changes to a viscous state, which leads to a dramatic change in Young's modulus. This change generates the amount of strain in the bilayer to induce the wrinkling. With a shape-controlled mold, they can get the ordered wrinkles perfectly perpendicular or leaned 45 S to the channel orientation of the mold because the orientation of the resultant force changes with the process of water diffusion which drives the surface to form the wrinkling. Additionally, they can get much smaller wrinkles than the stripe spacing of PDMS mold about one order. The wrinkle period changes with the power index of about 0.5 for various values of the multiplication product of the film thicknesses of the two layers, namely, lambda similar to (h(PS)h(PVP))(1/2).

Phase separation of PS/PVME blend films induced by capillary force

The phase behavior of a miscible PS/PVME (80/20, w/w) blend film in a confined geometry has been investigated at the annealing temperature much lower than the low critical solution temperature (LCST) of the blend. When the annealing temperature (52degreesC) is near the glass transition temperature of the blend (51.2degreesC), PVME-rich phase at the air-film surface under a microchannel forms smaller protrusion. When the annealing temperature is increased to 70degreesC, the protruding stripes, which are almost developed, are mainly composed of the mobile PVME-rich phase. These results reveal that the capillary force lead to the enrichment of PVME-rich phase at the air-polymer interface of a PDMS microchannel, that is, the capillary force lithography (CFL) can induce the phase separation of PS/PVME blend films.

Surface charge influence on the surface plasmon absorbance of electroactive thiol-protected gold nanoparticles

Gold nanoparticles (3.1-5.0 nm in size) surface-derivatized with both electroactive and nonelectroactive self-assembled monolayers were synthesized. The surface-derivatized electroactive particles can be easily oxidized/reduced at an electrode surface based on the diffusion-controlled current-voltage curve observed in cyclic voltammetry measurements. Spectroelectrochemical investigation demonstrated that the maximum absorbance of the nanoparticles in their oxidized state red-shifted compared with their reduced state to a different extent according to their size distribution. In the case of the particles surface-derivatized with nonelectroactive monolayers, much less shift was observed. This study showed that surface plasmon absorbance of gold nanoparticles was not only related to core charge states but was also influenced by surface charge states as well.

Micropatterning of metal films coated on polymer surfaces with epoxy mold and its application to organic field effect transistor fabrication

In this letter, a simple and versatile approach to micropatterning a metal film, which is evaporated on a Si substrate coated with polymer, is demonstrated by the use of a prepatterned epoxy mold. The polymer interlayer between the metal and the Si substrate is found important for the high quality pattern. When the metal-polymer-Si sandwich structure is heated with the temperature below T-m but above T-g of the polymer, the plastic deformation of the polymer film occurs under sufficiently high pressure applied. It causes the metal to crack locally or weaken along the pattern edges. Further heating while applying a lower pressure results in the formation of an intimate junction between the epoxy stamp and the metal film. Under these conditions the epoxy cures further, ensuring adhesion between the stamp and the film. The lift-off process works because the adhesion between the epoxy and the metal film is stronger than that between the metal film and the polymer. A polymer field effect transistor is fabricated in order to demonstrate potential applications of this micropatterning approach.

Solvent-induced microphase separation in diblock copolymer thin films with reversibly switchable morphology

We have studied the surface morphology of symmetric poly(styrene)-block-poly(methyl methacrylate) diblock copolymer thin films after solvent vapor treatment selective for poly(methyl methacrylate). Highly ordered nanoscale depressions or striped morphologies are obtained by varying the solvent annealing time. The resulting nanostructured films turn out to be sensitive to the surrounding medium, that is, their morphologies and surface properties can be reversibly switchable upon exposure to different block-selective solvents.

Pattern formation in a confined polymer film induced by a temperature gradient

We have studied a morphological instability of a double layer comprising the polymer film and air gap confined between the two plates set to different temperatures. The temperature gradient across the double layer causes the breakup of the polymer film into well-defined columnar, striped or spiral structures spanning the two plates. The pattern formation mechanisms have been discussed. The formed patterns can be transferred to produce PDMS stamp, a key element of soft lithography for future microfabrication.

Tunable ordered droplets induced by convection in phase-separating P2VP/PS blend film

Ordered hexagonal droplets patterns in phase-separating polymeric blend films of polystyrene and poly(2-vinylpyridine) (PS/PVP) formed due to the convection effect by solvent evaporation. The influences of PS molecular weight, solvent evaporation rate, and the weight ratio of PS to PVP on the PVP-rich domains pattern formation and distributions were investigated by atomic force microscope (AFM). Only in an appropriate range of molecular weight of PS, can the ordered pattern form. Too low or too high molecular weight of PS led no ordered pattern due to the viscosity effects. The increase of solvent evaporation rate decreased the mean radius of the PVP-rich domains and the intervals between the centers of the domains due to the enhancement of the viscosity on the top layer of the fluid film. The increase of the weight ratio of PS to PVP decreased mean radius of the PVP-rich domains whereas the intervals between the centers of droplets remained constant. Therefore, the size and the distributions of ordered patterns can be tuned by the polymer molecular weight, the weight ratio of the two components and the solvent evaporation rate.

Patterned self-adaptive polymer brushes by "grafting to" approach and microcontact printing

Patterned self-adaptive PS/P2VP mixed polymer brushes were prepared by "grafting to" approach combining with microcontact printing (muCP). The properties of the patterned surface were investigated by lateral force microscopy (LFM), XPS and water condensation figures. In the domains with grafted P2VP, the PS/P2VP mixed brushes demonstrated reversible switching behavior upon exposure to selective solvents for different components. The chemical composition of the top layer as well as the surface wettability can be well tuned due to the perpendicular phase segregation in the mixed brushes. While in the domains without grafted P2VP, the grafted PS did not have the capability of switching. The development and erasing of the pattern is reversible under different solvent treatment.

Direct patterning of polymer-based photo luminescent structures with a mask

We report a simple method to directly pattern polymer-based photo luminescent material, i.e. a prepatterned mask is placed a close distance above it. The final structure is a positive replica of the lateral structures in the mask with submicrometer resolution. The comparison of luminescence efficiency before and after patterning indicates almost no degradation in optical property of the material during the experiments. The mechanism of pattern formation is also discussed.

Direct patterning of rhodamine 6G molecules on mica by dip-pen nanolithography

Dip-pen nanolithography (DPN) has been developed to pattern monolayer film of various molecules on suitable substrate through the controlled movement of ink-coated atomic force microscopy (AFM) tip, which makes DPN a potentially powerful tool for making the functional nanoscale devices. In this paper, the direct patterning of rhodamine 6G on mica by dip-pen nanolithography was demonstrated. R6G features patterned on the mica was successfully achieved with different tip movement which can be programmed by Nanoscript(TM) language. From the AFM image of R6G patterns, we know that R6G molecule is flatly binding to the mica surface through electrostatic interaction, thus stable R6G nanostructures could be formed on mica. The influence of translation speed and contact time on DPN was discussed. The method can be extended to direct patterning of many other organic molecules, and should open many opportunities for miniaturized optical device and site-specific biological staining.

Kinetics of atomic force microscope-based scanned probe oxidation on an octadecylated silicon(111) surface

Atomic force microscope (AFM)-based scanned probe oxidation (SPO) nanolithography has been carried out on an octadecyl-terminated Si(111) surface to create dot-array patterns under ambient conditions in contact mode. The kinetics investigations indicate that this SPO process involves three stages. Within the steadily growing stage, the height of oxide dots increases logarithmically with pulse duration and linearly with pulse voltage. The lateral size of oxide dots tends to vary in a similar way. Our experiments show that a direct-log kinetic model is more applicable than a power-of-time law model for the SPO process on an alkylated silicon in demonstrating the dependence of oxide thickness on voltage exposure time within a relatively wide range. In contrast with the SPO on the octodecysilated SiO2/silicon surface, this process can be realized by a lower voltage with a shorter exposure time, which will be of great benefit to the fabrication of integrated nanometer-sized electronic devices on silicon-based substrates. This study demonstrates that the alkylated silicon is a new promising substrate material for silicon-based nanolithography.

A polymer composite film with reversible responsive behaviors

A responsive polymer composite film was generated by the use of reversibly switchable Surface morphology of polystyrene-block-poly(methyl methacrylate) (PS-b-PMMA) films in response to different block selective solvents on the rough isotactic poly(propylene) (i-PP) substrate. The Maximum difference of the water contact angle of the composite films increased from 22.6 degrees of PS-b-PMMA films on the smooth substrate to 42.6 degrees when they were treated by PS and PMMA selective solvents, respectively. The mechanisms of the responsive extent enhanced and the superhydrophobicity of the composite films were discussed in detail.

Scanned probe oxidation on an octadecyl-terminated silicon (111) surface with an atomic force microscope: kinetic investigations in line patterning

Scanned probe oxidation (SPO) nanolithography has been performed with an atomic force microscope (AFM) on an octadecyl-terminated silicon (111) surface to create protuberant oxide line patterns under ambient conditions in contact mode. The kinetic investigations of this SPO process indicate that the oxide line height increases linearly with applied voltage and decreases logarithmically with writing, speed. The oxide line width also tends to vary with the same law. The ambient humidity and the AFM tip state can remarkably influence this process, too. As compared with traditional octadecylsilated SiO2/Si substrate, such a substrate can guarantee the SPO with an obviously lowered voltage and a greatly increased writing speed. This study demonstrates that such alkylated silicon is a promising silicon-based substrate material for SPO nanolithography.

Direct patterning of negative nanostructures on self-assembled monolayers of 16-mercaptohexadecanoic acid on Au(111) substrate via dip-pen nanolithography

Both bare and self-assembled monolayer (SAM) protected gold substrate could be etched by allyl bromide according to atomic force microscopy (AFM), x-ray photoelectron spectroscopy (XPS) and inductively coupled plasma mass spectrometric (ICPMS) analysis results. With this allyl bromide ink material, negative nanopatterns could be fabricated directly by dip-pen nanolithography (DPN) on SAMs of 16-mercaptohexadecanoic acid (MHA) on Au(111) substrate. A tip-promoted etching mechanism was proposed where the gold-reactive ink could penetrate the MHA resist film through tip-induced defects resulting in local corrosive removal of the gold substrate. The fabrication mechanism was also confirmed by electrochemical characterization, energy dispersive spectroscopy (EDS) analysis and fabrication of positive nanopatterns via a used DPN tip.

Synthesis using electrospinning and stabilization of single layer macroporous films and fibrous networks of poly(vinyl alcohol)

We demonstrated in this paper an electrospinning technique could be employed to prepare the single layer macroporous films and fibrous networks of poly(vinyl alcohol) (PVA). A crucial element using electrospinning on the development of these electrospun structures was to shorten the distance of from the needle tip to the collector (L), which resulted in the bond of the wet fibers deposited on the collector at the junctions. The morphologies and average pore size of electrospun structures of PVA were mainly predominated by L and the time of collecting wet fibers on the collector. In addition, experimental results showed that an increase of the PVA concentration or a decrease of the applied voltage could also diminish slightly the average pore size of electrospun productions. Furthermore, a 60 degrees C absolute ethanol soak to PVA electrospun production led them to be able to stabilize in water for 1 month against disintegration. Differential scanning calorimetry (DSC) demonstrated that the 60 degrees C ethanol soak enhanced the degree of crystallinity of PVA production. The structural characteristic of macroporous films and networks in combination with their easy processability suggests potential utility in issue engineering applications.