Surface-induced structure formation of polymer blends on patterned substrates

Ordered polymeric microhole array made by selective wetting and applications for electrochemical microelectrode array

In this paper, we report the microelectrode array fabrication using selective wetting/dewetting of polymers on a chemical pattern which is a simple and convenient method capable of creating negative polymeric replicas using polyethylene glycol (PEG) as a clean and nontoxic sacrificial layer. The fabricated hole-patterned polypropylene film on gold demonstrated enhanced electrochemical properties. The chemical pattern is fabricated by microcontact printing using octadecanethiol (ODT) as an ink on gold substrate. When PEG is spin-cast on the chemical pattern, PEG solution selectively dewets the ODT patterned areas and wets the remaining bare gold areas, leading to the formation of arrayed PEG dots. A negative replicas of the PEG dot array is obtained by spin-coating of polypropylene (PP) solution in hexane which preferentially interacts with the hydrophobic ODT region on the patterned gold surface. The arrayed PEG dots are not affected the during PP spin-coating step because of their intrinsic immiscibility. Consequently, the hole-patterned PP film is obtained after PEG removal. The electrochemical signal of the PP film demonstrates the negligible leakage current by high dielectric and self-healing of defects on the chemical pattern by the polymer. This method is applicable to fabrication of microelectrode arrays and possibly can be employed to fabricate a variety of functional polymeric structures, such as photomasks, arrays of biomolecules, cell arrays, and arrays of nanomaterials.

Influence of film thickness on the phase separation mechanism in ultrathin conducting polymer blend films

The film morphology of thin polymer blend films based on poly[(1-methoxy)-4-(2-ethylhexyloxy)-p-phenylenevinylene] (MEH-PPV) and poly(N-vinylcarbazole) (PVK) is probed as a function of film thickness. Blend films are prepared with spin-coating of polymer solutions with different concentrations on top of solid supports. The blending ratio of both conducting polymers is kept constant. The film and surface morphology is probed with grazing incidence ultrasmall-angle X-ray scattering (GIUSAXS) and atomic force microscopy (AFM). A linear dependence between the film thickness and the averaged phase separation is found. In addition, X-ray reflectivity measurements show an enrichment of PVK at the substrate interface. UV/vis spectroscopy measurements indicate a linearly increasing amount of both homopolymers in the blend films for increasing film thicknesses. The generalized knowledge about the influence of the film thickness on the phase separation behavior in conducting polymer blend films is finally used to describe the phase separation formation during the spin-coating process, and the results are discussed in the framework of an adapted Flory-Huggins theory for rodlike polymers.

Domain coalescence-induced nucleation and anomalous growth of holes in thin polymer blend film

The phase evolution of a thin polymer blend film of polystyrene (PS) and poly(2-vinyl pyridine) (P2VP) triggered by solvent annealing is examined at both the bulk and single-(macro)molecule levels using wide-field microscopy (WFM). The transitions between different evolutionary stages in the nucleation and growth process are clearly visualized in real time and without intermittent breaks. The nucleation of PS holes arises from the coalescence and growth of P2VP domains and the holes expand in a complex manner involving the dewetting of PS and the absorption of P2VP domains into the holes.

Vertical phase separation and liquid-liquid dewetting of thin PS/PCL blend films during spin coating

Thin films of an amorphous polymer, polystyrene (PS), and a crystalline polymer, poly(ε-caprolactone) (PCL), blend were prepared by spin coating a toluene solution. Surface chemical compositions of the blend films were measured by X-ray photoelectron spectroscopy (XPS), and the surface and interface topographical changes were followed by atomic force microscopy (AFM). By changing the PS concentration and keeping the PCL concentration of the solution at 1 wt %, a great variety of morphologies were constructed. The results show that the morphology of the blend films can be divided into three regions with increasing PS concentration. In region I, PS island domains are embedded in PCL crystals when the PS concentration is lower than 0.3 wt % and the size of the PS island increases with increasing PS concentration. In region II, holes with different sizes surrounded by a low rim are obtained when the concentration of PS is between 0.35 and 0.5 wt %. After selectively washing the PS domains, we studied the interface morphology of PS/PCL and found that the upper PS-rich layer extended into the bottom PCL layer, forming a trench surrounding the holes. In region III, an enriched two-layer structure with the PS-rich layer on top of the blend films and the PCL-rich crystal layer underneath is obtained when the concentration of PS is higher than 0.5 wt %. Last, the formation mechanism of the different surface and interface morphologies is further discussed in terms of the vertical phase separation to a layered structure, followed by liquid-liquid dewetting and crystallization processes during spin coating.

Pressure/electric-field-assisted micro/nanocasting method for replicating a lotus leaf

An electric field-aided process was introduced for a curable casting process. As a micro/nanosized pattern mask, a lotus leaf, which has a hierarchical structure, was used. The process consists of two steps: (1) applying an electric field to a liquid polymer and solidifying the polymer for use as a negative mold, and (2) using the negative polymer mold to fabricate a replicated poly(ethylene oxide) (PEO) surface in the original shape of the lotus leaf. In this process, the applied electric field induces unstable vibration of the liquid polymer, due to electrokinetic phenomena. The electrokinetic fluid motion resulted in well-replicated PEO surfaces. The quality of the fabricated surface was highly dependent on the applied field and pressure. We believe that this technique improves the quality of the standard nanocasting method and will be useful for fabricating micro/nanosized structures.

Engineering 3D ordered molecular thin films by nanoscale control

This perspective aims to report on experimental preparation and investigation tools for engineering molecular thin films with a three-dimensional (3D) nanoscale control that is of relevant interest for different emerging applications as well as for the development of calibration standards. Such thin films may be obtained by man-made methods, self-assembly or spatio-temporal self-organization and/or by the combination of these last approaches with external tools. Understanding the main features and the physical-chemistry underlying the related ordering phenomena is in due course and a theoretical framework is under development. In this respect it is of fundamental importance to achieve the ability to get 3D structural images with a nanoscale detail. This issue is at the early stage and novel techniques like electron tomography and scanning transmission X-ray microscopy are very promising.

Patterning dewetting in thin polymer films by spatially directed photocrosslinking

In this report we examine the dewetting of spin-cast poly (styrene) films in a confined geometry. We designed a platform for laterally confining PS by photo-patterning crosslinks in spin-coated thin films. Heating the patterned film above the glass transition temperature of PS results in localized dewetting patterns in regions that were not crosslinked, while the crosslinked pattern serves as a rigid barrier that confines the retraction of the uncrosslinked polymer in micron-sized domains. The barriers also provide a favorable surface that the liquid PS wets onto, forming a rim at the boundary of crosslinked and uncrosslinked polymer. The resulting patterns are shown to be dependent on the irradiation and annealing time, the dimensions of the uncrosslinked region and the thickness of the film.

Surface-wetting effects on the liquid-liquid transition of a single-component molecular liquid

Even a single-component liquid may have more than two liquid states. The transition between them is called a 'liquid–liquid transition' (LLT). Such LLTs have recently attracted considerable attention mainly because of the fundamental interest in the physical origin of this counter-intuitive phenomenon. In this study, we report the first observation of wetting effects on LLT for a molecular liquid, triphenyl phosphite. We find a transition from partial to complete wetting for nucleation-growth-type LLT when approaching the spinodal temperature of LLT. Some features unique to LLT are also revealed, reflecting for example the non-conserved nature of its order parameter. We also find that the wetting behaviour is not induced by dispersion forces, but by weak hydrogen bonding to a solid substrate, implying its important role in the LLT itself. Using wetting effects may open a new possibility to control kinetics and spatial patterns of nucleation-growth-type LLT.

A phase transition between two liquid states is a counterintuitive phenomenon, but one that is known to happen in certain materials. Murata and Tanaka now show in tryphenyl phosphite that this can also produce a change in the wetting of a surface, from partial to complete, at the transition temperature.

Patterning block copolymer aggregates via Langmuir-Blodgett transfer to microcontact-printed substrates

We demonstrate a new strategy for producing hierarchical polymer nanostructures, which combines nanoscale self-assembly of amphiphilic block copolymers at the air-water interface with microscale templated assembly of the resulting aggregates on chemically patterned substrates. Aggregates are formed via interfacial self-assembly of 141k polystyrene-b-poly(ethylene oxide) (PS-b-PEO, MW = 141k, 11.4 wt % PEO) or a blend of 185k PS-b-PEO (MW = 185k, 18.9 wt % PEO) and PS-coated CdS nanoparticles to form strandlike copolymer or copolymer-nanoparticle aggregates. Using Langmuir-Blodgett (LB) techniques, the aggregates are then transferred to patterned substrates possessing alternating hydrophilic/hydrophobic stripes, obtained by microcontact printing octadecyltrichlorosilane (OTS) on glass. The aggregates are transferred under various conditions of surface pressure, orientation of the patterned substrate, and withdrawal speed. Templated assembly of aggregates into the hydrophilic substrate domains is achieved when the hydrophilic/hydrophobic stripes are oriented perpendicular to the water surface during LB transfer; this is explained by surface energy heterogeneities along the subphase-substrate contact line, which induce selective dewetting and concomitant monolayer rearrangement at the drying front. In contrast, parallel orientation of stripes results in nonselective transfer of the monolayer without registration to the underlying surface pattern. By studying the effect of surface pressure, we show that packing constraints imposed by compression of aggregates to high surface densities prevent the formation of patterned LB films that match the established periodicity of the OTS-patterned glass. As well, it is shown that efficient transfer of aggregates to the patterned glass requires slower substrate withdrawal speeds compared to transfer to unpatterned hydrophilic glass.

A solution concentration dependent transition from self-stratification to lateral phase separation in spin-cast PS:d-PMMA thin films

Thin films with a rich variety of different nano-scale morphologies have been produced by spin casting solutions of various concentrations of PS:d-PMMA blends from toluene solutions. During the spin casting process specular reflectivity and off-specular scattering data were recorded and ex situ optical and atomic force microscopy, neutron reflectivity and ellipsometry have all been used to characterise the film morphologies. We show that it is possible to selectively control the film morphology by altering the solution concentration used. Low polymer concentration solutions favour the formation of flat in-plane phase-separated bi-layers, with a d-PMMA-rich layer underneath a PS-rich layer. At intermediate concentrations the films formed consist of an in-plane phase-separated bi-layer with an undulating interface and also have some secondary phase-separated pockets rich in d-PMMA in the PS-rich layer and vice versa. Using high concentration solutions results in laterally phase-separated regions with sharp interfaces. As with the intermediate concentrations, secondary phase separation was also observed, especially at the top surface.

Phase-separated thin film structures for efficient polymer blend light-emitting diodes

We report laterally and vertically phase-separated thin film structures in conjugated polymer blends created by polymer molecular weight variation. We find that micrometer-scale lateral phase separation is critical in achieving high initial device efficiency of light-emitting diodes, whereas improved balance of charge carrier mobilities and film thickness uniformity are important in maintaining high efficiency at high voltages. The optoelectronic properties of these blend thin films and devices are strongly influenced by the polymer chain order/disorder and the interface state formed at polymer/polymer heterojunctions.

Nanoimprinting using self-assembled ceramic nanoislands

We have combined self-assembled ceramic nanoislands with nanoimprinting to demonstrate a novel, simple, low-cost method for polymer surface patterning. The nanoislands are easy to make and inexpensive, and can produce different distinct island morphologies. With a similar stiffness to steel, the nanoislands have superior durability to silicon, glass, polydimethylsiloxane (PDMS), and other common nanoimprinting materials. The nanoislands are stable up to 1000 degrees C and resist acids, bases, and solvents. We have demonstrated nanoimprinting with PDMS, ethyleneglycol dimethacrylate, and polystyrene polymers. The combination of desirable properties, ease of making, and low cost suggests a useful nanopatterning platform for a wide array of research fields.

Templating membrane assembly, structure, and dynamics using engineered interfaces

The physical and chemical properties of biological membranes are intimately linked to their bounding aqueous interfaces. Supported phospholipid bilayers, obtained by surface-assisted rupture, fusion, and spreading of vesicular microphases, offer a unique opportunity, because engineering the substrate allows manipulation of one of the two bilayer interfaces as well. Here, we review a collection of recent efforts, which illustrates deliberate substrate-membrane coupling using structured surfaces exhibiting chemical and topographic patterns. Vesicle fusion on chemically patterned substrates results in co-existing lipid phases, which reflect the underlying pattern of surface energy and wettability. These co-existing bilayer/monolayer morphologies are useful both for fundamental biophysical studies (e.g., studies of membrane asymmetry) as well as for applied work, such as synthesizing large-scale arrays of bilayers or living cells. The use of patterned, static surfaces provides new models to design complex membrane topographies and curvatures. Dynamic switchable-topography surfaces and sacrificial trehalose based-substrates reveal abilities to dynamically introduce membrane curvature and change the nature of the membrane-substrate interface. Taken together, these studies illustrate the importance of controlling interfaces in devising model membrane platforms for fundamental biophysical studies and bioanalytical devices.

Modulated self-assembly of 4,4'-diphenyltetrathiafulvalene molecules on highly oriented pyrolytic graphite by n-tetradecane solvent

We report the formation of a binary-component self-assembled monolayer (SAM) comprising 4,4'-diphenyltetrathiafulvalene (DP-TTF) and n-tetradecane (n-C(14)H(30)) molecules with periodic strip-like phase separation structures on a highly oriented pyrolytic graphite (HOPG) surface. Scanning tunneling microscopy (STM) imaging reveals that ordered DP-TTF single- and double-lamella are periodically tuned by ordered n- C(14)H(30) single- and double-lamella, respectively. This finding can be qualitatively understood in terms of a phase field model, in which the interplay of three ingredients, including free energy of the binary-component solution monolayer, phase boundary energy and surface stress, determines the final equilibrium sizes of the ordered DP-TTF and n- C(14)H(30) phases in the binary-component SAM. Furthermore, anisotropy of the surface stress breaks the symmetry of the substrate and causes the n- C(14)H(30) molecules to arrange along preferential substrate 010 directions. The orientation of the n-C(14)H(30) molecule stripes further guides the directions of the DP-TTF lamellar structures. In addition, scanning tunneling spectra (STS) of the individual DP-TTF and n- C(14)H(30) molecules in the ordered monolayer show a remarkable difference in I(V) curves on the HOPG substrate.

Size- and density-controlled synthesis of TiO2 nanodots on a substrate by phase-separation-induced self-assembly

This work presents a facile way, i.e. phase-separation-induced self-assembly, to prepare TiO(2) nanodots on a substrate. This method induces convective flow in a spin-coated titanium tetrabutoxide (TBOT)/polyvinyl pyrrolidone (PVP)/ethanol liquid film through the Marangoni effect and turns TBOT into crystalline TiO(2) nanodots on a substrate after calcination. The size and density of the TiO(2) nanodots can be finely tailored by controlling the concentrations of TBOT and PVP in the precursor sol. The TiO(2) nanodot-deposited surface showed a hydrophilic characteristic and the wettability was obviously improved by increasing nanodot size.

Nanophase separation in polystyrene-polyfluorene block copolymers thin films prepared through the breath figure procedure

The amphiphilic block copolymer formed by a hydrophobic body of polystyrene and a hydrophilic head of poly[9,9-di(2-(2-tetrahydropyranyl-oxy)hexyl)fluorene-alt-9,9-dioctylfluorene] was synthesized, and its solution was used to create thin films with ordered pattern of holes, by means of the breath figure technique. These porous films, after a thermal treatment, were found to show ordered aggregates of the pi-conjugated blocks in the place of the cavities. This is probably due to a preorganization of the two different blocks of the copolymer occurring during the breath figure formation, which is driven by the condensation of water microdroplets on the polymer solution, and to a following phase segregation occurring during the thermal annealing. This approach is a promising tool to be employed for the organization of organic materials at the micro and nanoscale.

Controlling film morphology in conjugated polymer:fullerene blends with surface patterning

We study the effects of patterned surface chemistry on the microscale and nanoscale morphology of solution-processed donor/acceptor polymer-blend films. Focusing on combinations of interest in polymer solar cells, we demonstrate that patterned surface chemistry can be used to tailor the film morphology of blends of semiconducting polymers such as poly-[2-(3,7-dimethyloctyloxy)-5-methoxy-p-phenylenevinylene] (MDMO-PPV), poly-3-hexylthiophene (P3HT), poly[(9,9-dioctylflorenyl-2,7-diyl)-co-benzothiadiazole)] (F8BT), and poly(9,9-dioctylfluorene-co-bis-N,N'-(4-butylphenyl)-bis-N,N'-phenyl-1,4-phenylendiamine) (PFB) with the fullerene derivative, [6,6]-phenyl-C(61)-butyric acid methyl ester (PCBM). We present a method for generating patterned, fullerene-terminated monolayers on gold surfaces and use microcontact printing and Dip-Pen Nanolithography (DPN) to pattern alkanethiols with both micro- and nanoscale features. After patterning with fullerenes and other functional groups, we backfill the rest of the surface with a variety of thiols to prepare substrates with periodic variations in surface chemistry. Spin coating polymer:PCBM films onto these substrates, followed by thermal annealing under nitrogen, leads to the formation of structured polymer films. We characterize these films with Atomic Force Microscopy (AFM), Raman spectroscopy, and fluorescence microscopy. The surface patterns are effective in guiding phase separation in all of the polymer:PCBM systems investigated and lead to a rich variety of film morphologies that are inaccessible with unpatterned substrates. We demonstrate our ability to guide pattern formation in films thick enough to be of interest for actual device applications (up to 200 nm in thickness) using feature sizes as small as 100 nm. Finally, we show that the surface chemistry can lead to variations in film morphology on length scales significantly smaller than those used in generating the original surface patterns. The variety of behaviors observed and the wide range of control over polymer morphology achieved at a variety of different length scales have important implications for the development of bulk heterojunction solar cells.

Tunability of mobility and conductivity over large ranges in poly(3,3'''-didodecylquaterthiophene)/insulating polymer composites

Semiconducting polymers are currently being considered as active layers in field-effect transistors, in which high charge carrier mobility and low off conductivity are important. For other applications, such as certain spintronic mechanisms, the opposite characteristics are desirable. Blending such polymers with insulating polymers would be expected to lower the mobility. In this paper, we report that the use of hydrocarbon polymers such as polystyrene as insulators generally raises the mobility when the semiconducting polymer is poly(bisdodecylquaterthiophene). A high mobility value of nearly 0.1 cm(2)/V.s was obtained for an optimal blend. While this is counterintuitive, it is consistent with a few other recent reports. In order to lower the mobility significantly, a much more polar and irregular blending agent is needed. The further addition of tetrafluorotetracyanoquinodimethane as a dopant gave a rare low mobility/high conductivity combination of properties, with a charge carrier density on the order of 10(19) cm(-3). Thus, mobility and conductivity were tuned somewhat independently over 3 and 4 orders of magnitude, respectively.

Generation of surface energy patterns by single pulse laser interference on self-assembled monolayers

Single pulse laser interference lithography is used to structure self-assembled monolayers of thiols on gold. This structuring process is investigated by attenuated total reflection measurements, and a demixing process of a binary polymer blend is used to visualize the produced surface energy pattern. The lithography can be realized with different wavelengths (266, 532, and 1064 nm) which shows that the structuring is a thermal process. As a first demonstration of this process, structures down to 800 nm period and 300 nm width are fabricated.

Optoelectronic and charge transport properties at organic-organic semiconductor interfaces: comparison between polyfluorene-based polymer blend and copolymer

We report detailed studies of optoelectronic and charge transport properties at the organic-organic semiconductor interfaces formed between polymer chains (interchain) and within a polymer chain (intrachain). These interfaces are fabricated using poly(9,9-di-n-octylfluorene-alt-N-(4-butylphenyl)diphenylamine) (TFB [f8-tfb]) (electron-donor) and poly(9,9-di-n-octylfluorene-alt-benzothiadiazole) (F8BT [f8-bt]) (electron-acceptor) conjugated polymers, by blending them together or by covalently attaching them via a main polymer backbone (copolymer). For optoelectronic properties, when a bulky and twisted tfb molecule is incorporated into a rigid F8BT conjugated backbone, it disturbs the conjugation of F8BT polymer, leading to a blue-shift in the lowest absorption transition. However, by acting as an effective electron donor, it assists the formation of an intrachain singlet exciton that has a strong charge-transfer character, leading to a red-shifted and longer-lived emission than that of F8BT. An extremely efficient and fast energy transfer from tfb donor to bt acceptor is observed in the copolymer (<1 ps) compared to transfer from TFB to F8BT in the blend (tens of ps). This efficient energy transfer in the copolymer is found to be associated with its low fluorescence efficiency (40-45% vs 60-65% for blend) because of the migration of radiative singlet excitons to low-energy states such as triplet and exciplex states that are nonemissive or weakly emissive. The presence of molecular-scale tfb-f8-bt interfaces in the copolymer, however, does not hinder an efficient transport of charge carriers at high drive voltages. Instead, it provides a better balance of charge carriers inside the device, which leads to slower decay of the device efficiency and thus more stable light-emitting diodes with increasing voltage than the blend devices. These distinctive optoelectronic and charge transport properties observed at different organic-organic semiconductor interfaces will provide useful input for the design rules of conjugated polymers required for improved molecular electronics.

Periodic porous stripe patterning in a polymer blend film induced by phase separation during spin-casting

A periodic striping pattern with microscale pore size is observed on the surface of thin films prepared by spin-casting from a polystyrene (PS) and polyethylene glycol (PEG) blend solution. The pattern is created by the convection generated by thermal gradients in the solution between the substrate and film solution during solvent evaporation, the radial flow of the spin-coated solution, and the primary and secondary phase separation of the PS and PEG solutions. The formation mechanism of the periodic porous stripe pattern is discussed, wherein the effects of the polymer blend weight ratio, polymer concentration, and drying rate on the formation of the periodic porous striping pattern are investigated using scanning electron and atomic force microscopy.

Nanopatterning of thin polymer films by controlled dewetting on a topographic pre-pattern

We develop a non-lithographic method for fabricating ordered micro/nanostructures of polymer thin films based on controlled dewetting of the films on topographically pre-patterned substrates with a large area. An ordered nanopattern of polystyrene (PS) is accomplished by thermal treatment of a thin PS film above its Tg spin coated on a topographically patterned substrate. We investigate the influence of pattern geometry on the final morphology of the dewetted polymer films using both mesa and indent patterned substrates. The controlled dewetting, initiated preferentially at the edges of individual pre-patterned mesas, in particular gives rise to spherical cap domains located at the center of the mesas. The domains are much smaller than the individual mesas as a consequence of the significant pattern reduction to nearly 300%. The arrays of 70 nm PS nano-sphere caps are obtained from arrays of 200 nm square pre-patterned mesas. Our method is also applicable for other polymers such as a poly(4-vinyl pyridine) (P4VP) containing Rhodamine 6G (Rh6G) dye on a pre-patterned PS substrate and successfully produced highly fluorescent stable nanopatterned films.

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.

Effect of lateral morphology formation of polymer blend towards patterning silane-based SAMs using selective dissolution method

A number of strategies have been developed including soft lithography and photolithography for patterning various surfaces. Here we have discussed a customized strategy for surface patterning of nanosized, silane-based SAMs and monolayer thickness measurement investigated using atomic force microscope (AFM). We have utilized the versatile morphology of a binary polymer blend to generate patterned SAMs over silicon substrate by employing a selective dissolution procedure. This method was confirmed with different organosilanes with varying number of C-atoms and to other polymer blend. The samples were imaged both in tapping mode and pulsed force mode AFM.