Ha Soo Hwang

Acid-sensitive semiperfluoroalkyl resorcinarene: an imaging material for organic electronics.

An acid-sensitive semiperfluoroalkyl resorcinarene was synthesized, and its lithographic properties were evaluated. Its solubility in segregated hydrofluoroether solvents enables the patterning of delicate organic electronic materials.

Inverse Opal Nanocrystal Superlattice Films

We describe the single-step self-organization of nanocrystal superlattice films infused with spatially ordered arrays of micrometer-size pores. In a humid atmosphere, water droplets condense on the surface of evaporating thin-film solutions of nanocrystals. Nanocrystals coated with the appropriate ligands stabilize the water droplets, allowing them to grow to uniform size and ultimately pack into very ordered arrays. The droplets provide a temporary template that casts an ordered macroporous nanocrystal film. This process could serve as a reliable bottom-up self-assembly approach for fabricating two-dimensional waveguides with tunable optical properties for single-chip integration of photonic and electronic technologies.

Dry photolithographic patterning process for organic electronic devices using supercritical carbon dioxide as a solvent

The particular challenge of micropatterning organic materials has stimulated numerous approaches for making effective and repeatable patterned structures with fine features. Among all the micropatterning techniques photolithography, being the preferred method for the inorganic semiconductor industry, did not create much impact due to its incompatibility with the majority of organic electronic materials. Here we introduce a novel, chemically benign approach to dry photolithographic patterning of organic materials using super-critical carbon dioxide (scCO2) as a green developing solvent. We illustrate the possible applications of the new technique by patterning conducting polymers and light emitting polymers for organic light emitting diodes.

Role of water on PMMA/clay nanocomposites synthesized by in situ polymerization in ethanol and supercritical carbon dioxide

Poly(methylmethacrylate) (PMMA)/montmorillonite clay nanocomposites were synthesized via the free radical polymerization of MMA in the presence of alkyl ammonium substituted polysilsesquioxane surfactant-modified clay and AIBN initiator in supercritical CO(2) and ethanol. The reactions were also conducted by adding a small amount of water to observe the intercalation and exfoliation behavior of the clay and the properties of the nanocomposites. Initially, clay was cation exchanged with the surfactant to enhance its hydrophobicity and to expand the interlamellar spaces of silicate platelets. Organophilization with the three dimensional surfactant and a small amount of water molecules in the solvent reduced the surface energy of clay dramatically, which promoted the miscibility of polymer/clay nanocomposites. The morphology of the nanocomposites was characterized by scanning electron microscopy. The intercalation and dispersion of the clay were quantified by both X-ray diffraction and transmission electron microscopy. Due to the three dimensional structure, alkyl ammonium substituted polysilsesquioxane surfactant gives stable clay separation and dimension stability of the nanocomposites. Different distribution of the clay also plays an important role in physical properties. Thermogravimetric analysis and differential scanning calorimetry were employed to investigate the thermal properties and glass transition temperature of the nanocomposites.

Preparation and properties of semifluorinated block copolymers of 2-(dimethylamino)ethyl methacrylate and fluorooctyl methacrylates

Semifluorinated block copolymers of poly(2-(dimethylamino)ethyl methacrylate) (PDMAEMA) and poly(fluorooctyl methacrylates) (PFOMA) were prepared using group transfer polymerisation via sequential monomer addition. Wide ranges of copolymers were obtained with good control over both molecular weight and composition by adjusting the monomers/initiator ratio. The micellar characteristics of the copolymers in water and chloroform were investigated by quasi-elastic light scattering and transmission electron microscopy. The size and morphologies of micelles were greatly influenced by copolymer composition, pH, and temperature. In addition, the solubility of copolymers and the formation of water-in-carbon dioxide (W/C) microemulsions were described in terms of the cloud points. The block copolymers exhibited the excellent ability of stabilizing W/C microemulsions.

Formation of TiO2 nanoparticles in water-in-CO2 microemulsions

Titanium dioxide nanoparticles can be produced by the controlled hydrolysis of titanium tetraisopropoxide in water-in-CO2 (w/c) microemulsions stabilized with the surfactants ammonium carboxylate perfluoropolyether (PFPE-NH4) and poly(dimethyl amino ethyl methacrylate-block-1H,1H,2H,2H-perfluorooctyl methacrylate) (PDMAEMA-b-PFOMA); the greater control of hydrolysis and particle growth with PDMAEMA-b-PFOMA is consistent with the differences in the stabilities and interactions for these two microemulsions.

Acid-diffusion behaviour in organic thin films and its effect on patterning

Acid and its formation and placement is one of the most important aspects in the chemically amplified photolithographic process. The choice of photoacid generator (PAG) in the photolithographic patterning of acidic substrates, such as PEDOT:PSS, has consequences for the resolution and overall quality of the patterned image. In this study, an acid exchange and diffusion mechanism is proposed for the undesired decomposition of the unexposed photoresist layer containing ionic PAGs. The use of non-ionic PAGs has been shown to be a solution to this decomposition problem. In addition, the acidic nature of the PEDOT:PSS substrate is employed to produce patterned images of a cross-linkable light-emitting polymer. With further optimization and development, this is potentially a fast and simple method to introduce patterns in various organic electronic devices.

Orthogonal lithography for organic electronics

Organic electronics has recently gained attention as a new field promising cheaper, flexible, and large-scale devices. Although photolithography has proven to be a high-resolution and high-throughput patterning method with excellent registration capabilities, the emerging field of organic electronics has been largely unsuccessful in adapting this well-established method as a viable approach to patterning. Chemical compatibility issues between organic materials and the processing solvents and chemicals required by photolithography have been the main problem. This challenge has led us to identify a set of non-damaging processing solvents and to develop alternative imaging materials in order to extend photolithographic patterning methods to organic electronics. We have identified supercritical carbon dioxide and hydrofluoroether (HFE) solvents as chemically benign to organic electronic materials and which are also suitable as processing solvents. We refer to these solvents as orthogonal in that they do not substantially interact with traditional aqueous and organic solvents. Multi-layered devices are easily realized by exploiting this orthogonality property; subsequent layers are deposited and patterned without damaging or otherwise adversely affecting previously deposited underlying layers. We have designed and synthesized novel photoresists, which are processible in these benign solvents.

Polymerization of methyl methacrylate in supercritical carbon dioxide with PDMS based stabilizers : A study on the effect of stabilizer anchor groups

The effect of different type of polydimethysiloxane (PDMS) based stabilizers on the dispersion polymerization of MMA in supercritical carbon dioxide (scCO 2 ) has been investigated in terms of their anchor group architecture. PDMS chain was used in each case as a CO 2 -philic portion of the stabilizer and five different PMMA-philic endgroups were investigated: an alcohol, an acetate group, a methacrylate unit, and two block copolymers containing acrylate groups. A trifunctional block copolymer, PDMS-b-(PMMA(l.lK)-co-PMA(0.5K) was found to be very effective stabilizer due to the efficient stabilizer anchor-soluble balance (ASB) which produced excellent yield, high molecular weight, and uniform particles of micron sized PMMA.

Photolithographic Patterning in Supercritical Carbon Dioxide: Application to Patterned Light-emitting Devices

Photolithography is a high-throughput, cost-effective patterning technology. However, the application to organic electronic devices has been restricted because of its chemical compatibility issue with delicate organic materials. In this study we propose a new photolithography technique which utilizes supercritical carbon dioxide (scO2) as an organic electronic materials-benign development solvent. Supercritical carbon dioxide is a fluid that exists above its critical pressure and temperature (Pc=73.8 bar and Tc=31.1degC). Apart from the environmental advantages, useful process benefits result from the fact that most non-fluorinated organic materials are not damaged in contact with scCO2. A polymeric material composed of perfluorodecyl methacrylate (FDMA) and tert-butyl methacrylate (TBMA) was synthesized as a negative-tone photoresist, processible in scCO2. Micron-sized resist patterns on a Si wafer were successfully processed under i-line (lambda=365 nm) exposure conditions. The resist showed a capability to make patterns on the conductive poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT: PSS) film. The resulting resist pattern was finally applied to fabricate patterned polymer light-emitting devices.