Nanoporous hard etch masks using silicon-containing block copolymer thin films

Nanoporous hard etch masks with various pore sizes were fabricated using a new type of silicon-containing block copolymers, polystyrene-block-poly(4-(tert-butyldimethylsilyl)oxystyrene) with different molecular weights. Since organic-inorganic block copolymers have a large difference in etch resistance between the organic and inorganic blocks, a hard etch mask of silicon oxide can be directly produced upon oxygen plasma treatment. Orientation and hexagonal arrays of cylindrical nanodomains were manipulated simply by adjusting the relative composition of selective and non-selective solvents in the annealing solvent. When the cylindrical nanostructures aligned perpendicular to the substrate surface were exposed to an oxygen plasma, hexagonally arranged nanopore arrays of silicon oxide with controlled pore sizes were fabricated. These nanoporous hard etch masks can be applied to the nanopatterning processes that require high aspect ratio structures.     

Morphology of semicrystalline oxyethylene/oxybutylene block copolymer thin films on mica

The morphology of as-cast and annealed thin films of four symmetric semicrystalline block copolymers on mica was investigated by tapping mode atomic force microscopy (AFM) and grazing incidence X-ray diffraction (XRD). It is found that the morphology of the thin films is dependent on chain length of oxyethylene/oxybutylene block copolymers. The as-cast thin films of the shorter E_mB_n block copolymers on mica exhibit a multi-layered lamellar structure parallel to the surface, in which the stems of the E crystals in the first half polymer layer contacting mica are parallel to the mica surface and perpendicular to the mica surface in the upper polymer layers. In contrast, the as-cast thin film of longer E₂₂₄B₁₁₄ exhibits a structure with mixed orientations of lamellar microdomains on a half polymer layer parallel to the surface. After annealing, the multi-layered structure on mica is transformed into a half-layered, densely branched structure, which is formed following a diffusion-limited aggregation mechanism, opposed to the featureless half-layered structure on silicon. Upon annealing, the upper polymer layers gradually retreat and the remaining area becomes thicker, but in contrast the first half polymer layer contacting mica becomes thinner due to wetting and the parallel orientation of the E crystal stems. The densely branched structure and the different chain orientations of the E crystal stems in the first half polymer layer contacting mica are attributed to the strong interaction between the E block and mica, as revealed by our previous work. The width of branches was employed to analyze the kinetics of secondary crystallization. It is also found that the width of the branches and the velocity of crystal front decrease as the chain length increases.     

Structure and phase transition in thin films of block copolymer micelles complexed with inorganic precursors

Block copolymer micelle can be used as nano-reactor where separated domains serve as a compartment for the production of nanomaterials, ultimately creating nanocomposite materials. In this work, thin nanocomposite films generated from polystyrene-b-poly(acrylic acid) (PS-b-PAA) micellar solution in which small amount of inorganic precursor was added were investigated. The films were prepared by spin coating onto silicon substrate, and then solvent-annealed. As-spun films exhibit typical micellar structure with spherical shape along which inorganic nanoparticles are dispersed. Such morphology remains unchanged after solvent annealing for micellar films with small amount of inorganic precursor. However, further increase in the amount of inorganic precursors brings about the morphological changes, producing different organization of inorganic nanoparticles in composite films. This behavior was found to strongly depend on the types of precursors and solvents used for annealing. These results illustrate a simple yet useful route to generate the polymeric nanocomposites with diverse structure and composition.     

Effects of additives on the morphologies of thin titania films from self-assembly of a block copolymer

The effects of additives of poly(methyl methacrylate) (PMMA) and HAuCl₄ on the morphologies of hybrid titania films formed via co-assembly of polystyrene-block-poly(ethylene oxide) (PS-b-PEO) copolymers, titania sol-gel precursor in a selective solvent were investigated. The results show that addition of PMMA or HAuCl₄ has an important influence on the morphologies of hybrid titania films. Addition of PMMA or HAuCl₄ can induce the morphology transition of the PS-b-PEO/titania sol-gel mixture from spherical micelles to vesicles. Therefore, the morphologies of the hybrid films formed on silicon substrate surfaces by spin-coating can be controlled by the addition of homopolymer (PMMA) or inorganic precursor (HAuCl₄) into the PS-b-PEO/titania sol-gel mixtures, allowing access to nanoparticles or nanoporous films. After removing the polymer matrix, nanoparticle aggregates or nanobowl-like structures are left behind on the substrate surfaces.     

Effect of nanoparticle surface functionality on microdomain orientation in block copolymer thin films under electric field

The electric field induced microdomain orientations has been an interesting research topic. In this article, the effect of nanoparticle surface functionality on microdomain alignments in block copolymer/nanoparticle hybrid thin films was investigated with transmission electron microscopy experiments. The presence of gold nanoparticles influenced the microdomain orientation behaviors of block copolymer/nanoparticle thin films. The possibility for complete alignment normal to the substrate was illustrated by controlling electric field strength, concentration, and surface ligands of nanoparticles. This work provides basic and essential data to understand the properties and behaviors of emerging block copolymer/nanoparticle hybrid thin films.     

Characterization of parylene deposition process for the passivation of organic light emitting diodes

The chemical vapor condensation process of Parylene-N thin films was investigated and applied to the passivation of the organic light emitting diodes (OLEDs). The effects of process variables on the deposition rate were studied, and it was found that the deposition rate of Parylene increases with increasing precursor sublimation temperature but decreases with increasing substrate temperature. The dependence of deposition rate was well explained by the condensation polymerization model of the monomer on the surface. The Parylene film was used as a passivation layer for OLEDs, and as a result, the lifetime of the passivated OLEDs was increased by a factor of about 2.3 compared with that of non-passivated OLEDs. This paper is dedicated to Professor Wha Young Lee on the occasion of his retirement from Seoul National University.     

Perpendicular oriented cylinders via directional coalescence of spheres embedded in block copolymer films induced by solvent annealing

Polystyrene-b-poly(methyl acrylate) (PS-b-PMA) block copolymer with PS volume fraction of 25.2 vol% was synthesized by atom transfer radical polymerization. Non-pretreated silicon wafers were used as the substrates to prepare perpendicular oriented PS cylinders in PMA matrix via solvent annealing which could induce the transformation of spheres to vertically oriented and hexagonally packed cylinders. The spherical microdomains were formed after the evaporation of solvents from the solutions of the block copolymer in selective solvents mixed from methanol, acetone and dichloromethane. The thickness of films could be as thick as 1000 nm, which were much thicker than usual cases and the cylinders came from the directional coalescence of the spheres, thus any pre-treatments of the substrates were not required for perpendicular orientation. The structures were characterized by small angle X-ray scattering (SAXS), transmission electron microscope (TEM), atom force microscopy (AFM) and grazing incidence small angle X-ray scattering (GISAXS).     

Restructuring in block copolymer thin films: In situ GISAXS investigations during solvent vapor annealing

Block copolymer (BCP) thin films have been proposed for a number of nanotechnology applications, such as nanolithography and as nanotemplates, nanoporous membranes and sensors. Solvent vapor annealing (SVA) has emerged as a powerful technique for manipulating and controlling the structure of BCP thin films, e.g., by healing defects, by altering the orientation of the microdomains and by changing the morphology. Due to high time resolution and compatibility with SVA environments, grazing-incidence small-angle X-ray scattering (GISAXS) is an indispensable technique for studying the SVA process, providing information of the BCP thin film structure both laterally and along the film normal. Especially, state-of-the-art combined GISAXS/SVA setups at synchrotron sources have facilitated in situ and real-time studies of the SVA process with a time resolution of a few seconds, giving important insight into the pathways and mechanisms of SVA induced restructuring. We give a short introduction to the GISAXS method and review recent theoretical studies, experimental techniques such as sample preparation and in situ chambers together with SVA protocols, and we review and discuss experimental results. We conclude by giving an outlook on emerging developments of the in situ real-time GISAXS scattering technique in combination with new approaches to control BCP thin film structures using SVA.     

Ordering of block copolymer/nanoparticle composite thin films

The ordering behavior of polymer nanocomposites composed of gold nanoparticles confined in the polystyrene (PS) domains of PS based block copolymers was investigated. The results reveal that the self‐assembly of nanoparticles in the PS domains improved the ordering of microdomains. This is attributed to the presence of nanoparticles that reduced the degree of segregation of the system, causing slow phase separation. This facilitates the packing of the cylindrical microdomains, leading to a well‐ordered structure of the composite. When particles were incorporated into the major domains of cylindrically ordered block copolymer, the connectivity of the domains allowed particles to move to the top of the film to gain additional entropy of the system. In contrast, when particles were organized in parallel cylinders in the block copolymer, they were confined in the cylinders which prevented them from diffusing in the depth direction. The aggregation of nanoparticles was amplified when the composite was annealed in air. We believe that the results from this study will enable more understanding of the effects of nanoparticles on the ordering of block copolymer/nanoparticle composite thin films and will provide a tool in the fabrication of composite thin films. Copyright © 2012 Society of Chemical Industry     

Water-induced morphology evolution of block copolymer micellar thin films

Morphology evolution of diblock copolymer polystyrene-block-poly(4-vinylpyridine) (PS-b-P4VP) micellar thin film in the presence of water was investigated. Surface holes with nanoscale cavities in hexagonal order could be induced by water treatment for certain periods. The nanoscale surface cavities could be transformed into isolated nanospheres in a dry environment or back to protruding densely packed spheres by toluene (a selective solvent for PS coronae) treatment. The morphology evolution of micellar thin film strongly depended on the slow evaporation of toluene solvent, the swelling of P4VP cores in the humid environment, and the subsequent movement of PS chains induced by air and toluene. The incompatibility between solvent and block, and that between the unlike blocks also played an important role in the morphology evolution.     

Design and stabilization of block copolymer micelles via phenol-pyridine hydrogen-bonding interactions

A new approach for the preparation of block copolymer micelles in non-selective solvent is introduced. Phenol-pyridine hydrogen-bonding interactions are used for the first time to prepare core-shell micelles in non-selective solvents using block copolymers and bifunctional low-molecular-weight hydrogen-bonding crosslinkers. Poly(styrene-b-4-vinylphenol)/Bis-pyridyl ethane and poly(styrene-b-4-vinylpyridine)/Bisphenol A were investigated as micelle formation due to phenol-pyridine hydrogen bond crosslinking. The influence of several factors such as temperature, concentration, solvent and pH in micellization-demicellization process was analyzed by Fourier transform infrared spectroscopy (FTIR), differential scanning calorimetry (DSC), dynamic light scattering (DLS) and atomic force microscopy (AFM). This method opens new possibilities to the generation of block copolymer micelles in non-selective solvents.     

Mechanisms of ordering in block copolymer sub-monolayer films upon selective solvent annealing

The solvent annealing induced two-dimensional ordering in poly(styrene–ethylene/butylenes–styrene) (SEBS) triblock copolymer sub-monolayer films with a thickness of 16 nm equaling half of the bulk domain thickness l₀ was investigated by time-resolved ex-situ atomic force microscopy (AFM). Cyclohexane, a selective solvent for majority poly(ethylene/butylenes) block was used. The detailed pathway information on the ordering of highly regular hexagonal spheres was obtained by repeatedly taking images of the same marked area on the sample surface after ex-situ annealing treatments. Two different ordering mechanisms were observed under two different solvent annealing conditions: 1) under a well-sealed environment with a slow cyclohexane evaporation rate, poorly-ordered short cylinders first break into spheres, then the evolution of spherical phase takes place to gradually improve the orderliness of spheres, and finally well-ordered hexagonal spheres are formed; 2) under a poorly-sealed environment with a fast cyclohexane evaporation rate, “holes” and “islands” are instantaneously formed on the sample surface first, and then the surface gradually get even annihilating hole and island areas. Within the hole areas, the transition from short cylinders to hexagonal spheres takes place via a not well-defined microphase separation structure, while in the island areas, the transitions between cylinders and spheres take place to improve the orderliness of cylinders, and finally the enough-ordered cylinders transform into hex-spheres.     

Hexagonally ordered arrays of metallic nanodots from thin films of functional block copolymers

We demonstrate a new and simple route to fabricate highly dense arrays of hexagonally close packed inorganic nanodots using functional diblock copolymer (PS-b-P4VP) thin films. The deposition of pre-synthesized inorganic nanoparticles selectively into the P4VP domains of PS-b-P4VP thin films, followed by removal of the polymer, led to highly ordered metallic patterns identical to the order of the starting thin film. Examples of Au, Pt and Pd nanodot arrays are presented. The affinity of the different metal nanoparticles towards P4VP chains is also understood by extending this approach to PS-b-P4VP micellar thin films. The procedure used here is simple, eco-friendly, and compatible with the existing silicon-based technology. Also the method could be applied to various other block copolymer morphologies for generating 1-dimensional (1D) and 2-dimensional (2D) structures.     

Polymerization-induced self-assembly of block copolymer nanoparticles via RAFT non-aqueous dispersion polymerization

There is considerable current interest in polymerization-induced self-assembly (PISA) via reversible addition–fragmentation chain transfer (RAFT) polymerization as a versatile and efficient route to various types of block copolymer nano-objects. Many successful PISA syntheses have been conducted in water using either RAFT aqueous dispersion polymerization or RAFT aqueous emulsion polymerization. In contrast, this review article is focused on the growing number of RAFT PISA formulations developed for non-aqueous media. A wide range of monomers have been utilized for both the stabilizer and core-forming blocks to produce diblock copolymer nanoparticles in either polar or non-polar media (including supercritical CO₂ and ionic liquids) via RAFT dispersion polymerization. Such nanoparticles possess spherical, worm-like or vesicular morphologies, often with controllable size and functionality. Detailed characterization of such sterically stabilized diblock copolymer dispersions provides important insights into the various morphological transformations that can occur both during the PISA synthesis and also on subsequent exposure to a suitable external stimulus (e.g. temperature).     

Ordering in thin films of block copolymers: Fundamentals to potential applications

The ordering of block copolymers in thin films is reviewed, starting from the fundamental principles and extending to recent promising developments as templates for nanolithography which may find important applications in the semiconductor industry. Ordering in supported thin films of symmetric and asymmetric AB diblock and ABA triblock copolymers is discussed, along with that of more complex materials such as ABC triblocks and liquid crystalline block copolymers. Techniques to prepare thin films, and to characterise ordering within them, are summarized. Several methods to align block copolymer nanostructures, important in several applications are outlined. A number of potential applications in nanolithography, production of porous materials, templating, and patterning of organic and inorganic materials are then presented. The influence of crystallization on the morphology of a block copolymer film is briefly discussed, as are structures in grafted block copolymer films.     

Perpendicular lamellae induced at the interface of neutral self-assembled monolayers in thin diblock copolymer films

We obtained perpendicular lamellar orientations in thin films of symmetric polystyrene-block-poly(methyl methacrylate), PS-b-PMMA, on self-assembled monolayers (SAMs) of 3-(p-methoxyphenyl)propyltrichlorosilane (MPTS) prepared on silicon wafers. In contrast to completely parallel lamellae on silicon wafers having a native oxide layer, perpendicular lamellae at the MPTS interface with parallel lamellae at the air interface were directly observed by transmission electron microscopy (TEM) in cross-sectional view. The perpendicular lamellae at the MPTS interface were attributed to the non-preferential (neutral) MPTS-covered substrate to both PS and PMMA blocks. The neutrality of the SAMs of MPTS was confirmed by the similar interfacial tension values of the SAMs of MPTS with PS and PMMA, estimated by contact angle measurements.     

Solvent annealing assisted self-assembly of hydrogen-bonded interpolymer complexes of AB block copolymer/C homopolymer in thin film

A simple process of solvent annealing has been shown to produce ordered self-assembly structures of poly(styrene)-block-poly(4-vinylpyridine) (PS-b-P4VP)/poly(4,4′-oxydiphenylenepyromellitamic acid) (POAA) block copolymer/homopolymer blends in thin film, where POAA chains selectively interact with P4VP blocks by strong interpolymer hydrogen-bonding. By simply exposing the thin film to benzene/NMP (0.97/0.03, in volume) vapor mixture, ordered microphase-separated structures with PS spherical microdomains distributed within P4VP/POAA complexes matrix were obtained. The formation of the microphase-separated structures could be attributed to the substantial mobility of PS blocks and P4VP/POAA complexes and enhanced repulsion between them under the benzene/NMP mixture vapor. When the volume ratio of benzene to NMP increased to 0.98/0.02, the increasing benzene in the mixture vapor induced the adhesive collision of spherical microphase-separated structures to form long “pearl necklaces”. With increasing volume ratio of benzene to NMP to 0.99/0.01, an ordered “pearl necklace” array oriented parallel to the film surface formed. The self-assembly structures were studied by FTIR spectroscopy, atomic force microscopy (AFM), and transmission electron microscopy (TEM). Finally, possible mechanism of self-assembly and formation of microphase morphology was proposed.     

Orientational order and mechanical properties of poly(amide-block-aramid) alternating block copolymer films and fibers

Mechanical properties and orientational order of a series of uniaxially oriented block copolymer films and fibers comprised of alternating rigid aramid blocks of poly(p-phenylene terephthalamide) (PPTA) and flexible blocks of polyamide 6,6 (PA 6,6) have been investigated. The prepared block copolymer films differ in aramid content and average block length. The films were prepared by shearing the polymer solutions (in sulphuric acid) followed by rapid coagulation of the solutions in water. From wide angle X-ray scattering (WAXS) and optical polarisation microscopy (OPM) it was found that films with a mole fraction of PPTA of at least 0.5 show a liquid crystalline (LC) phase. It was found that the mechanical properties of LC block copolymer films are similar to the properties of isotropic films, as determined with dynamical mechanical analyses (DMAs) and from tensile tests. This was attributed to the relative low parameter of the LC films obtained by using WAXS. Copolymerisation of the PPTA blocks with the flexible polyamide blocks resulted in an increase of storage and Young's modulus, a decrease of the elongation at break while the tensile strength was unaffected compared to normal PA 6,6. Block copolymer fibers have been spun from liquid crystalline solutions by means of a dry-jet wet spinning process. The only variable parameter was the imposed draw-ratio in the air-gap of the spinning process. Increasing the draw-ratio resulted in an increased molecular orientation, Young's modulus and tensile strength of the fibers while its effect on the maximum elongation at break was small. Heat treatment at 300 °C of the fibers resulted in an increase of the Young's modulus, a minor increase of the strength and a decrease of the elongation at break. Scanning electron microscopic (SEM) photographs of the fractured surfaces of the block copolymer fibers do not show a fibrillar fracture surface, which is typically observed for pure PPTA fibers.     

One step fabrication and characterization of platinum nanopore electrode ensembles formed via amphiphilic block copolymer self-assembly

A novel one-step approach to Pt nanopore electrode ensembles (NEEs) has been developed using an amphiphilic block copolymer [polystyrene-block-poly (acrylic acid)] self-assembly. This procedure is simple and fast, and requires only conventional, inexpensive electrochemical instrumentation. Electrochemical methods are used to characterize the Pt nanopore electrode ensembles prepared using this new procedure. And the capacitance and voltammetric characteristics for the NEEs have been examined. At lower scan rates, it remains the features of a single nanoelectrode, while at high scan rates the nanoelectrodes act independently. This is an important feature for vivo sensing and other electroanalytical applications.     

Improved light extraction of InGaN/GaN blue LEDs by GaOOH NRAs using a thin ATO seed layer

ABSTRACT: We investigated the effect of gallium oxide hydroxide (GaOOH) nanorod arrays (NRAs) on the light extraction of InGaN/GaN MQW blue light-emitting diodes (LEDs). The GaOOH NRAs were prepared on the indium tin oxide electrode (ITO) layer of LEDs by the electrochemical deposition method. The GaOOH NRAs with preferred orientations were grown on the ITO surface by sputtering a thin antimony-doped tin oxide (ATO) seed layer, which enhances heterogeneous reactions. The surface density and coverage were also efficiently controlled by the different growth voltages. For the LEDs with GaOOH NRAs grown at -2 V, the light output power was increased by 22% without suffering from any serious electrical degradation and wavelength shift, compared to the conventional LEDs.