Block Copolymer Assembled Antireflective Coatings with Self-Cleaning Properties

www.advopticalmat.de Inevitable Fresnel refl ection from the interface between two optical media with different refractive indices ( η ) can decrease light absorption in photovoltaic devices and can cause undesirable glare in fldisplays. Thus, an effective antirefl ection coating (ARC) for a higher optical transmittance can contribute to the accomplishment of improved contrast and brightness in display devices and higher levels of energy conversion effi ciency in solar panels. Traditional ARCs improve optical transmittance by introducing destructive interference between refl ected light beams from the air-fi lm and fi lm-substrate interfaces. An ideal ARC should satisfy the requirements of a quarter-wavelength fi lm thickness and a fi lm η of ( η s ) 0.5 , where η s denotes the η of the substrate. [ 1 ] However, an ARC with a homogeneous η can achieve high transmittance only in narrow wavelength and incidence angle ranges, which are fi xed by its thickness and refractive index. In contrast, an ARC with a gradually changing η , also known as a graded index, can avoid the formation of an interface with a sharp contrast in η . As a result, the Fresnel refl ection can be effectively minimized in a wide wavelength range and high optical transparency can be achieved. In an ideal gradedindex anti-refl ection coating (GIARC), the η should gradually vary from that of the substrate material, onto which the GIARC is coated, to the η of air ( ∼ 1), necessitating the use of a material with a widely tunable η . Theoretical studies showed that a quintic profi le of η varying from that of the substrate (typically 1.5‐2) to 1 is best for achieving the lowest refl ectance (<0.1% in the visible range). [ 2 ] While dense materials generally have an η larger than ∼ 1.35, the extensive variability of indices has been demonstrated by nanoporous materials for the applications for GIARC. [ 2‐6 ] Various methods, such as lithography, obliqueangle deposition, catalytic etching, chemical vapor deposition, and a sol-gel process, have been reported. [ 2‐5 ] However, these methods may lack cost-effectiveness and high-throughput capabilities due to the requirements of vacuum processing, multiple etching steps, or because of the limited controllability of η . Recently, highly porous block copolymer (BCP) thin fi lms were applied to the fabrication of ARC due to the excellent low-cost solution processability and facile tunability of η when controlling the fraction of the pores generated by the selective removal of one block. [ 7‐11 ] Kim and co-workers reported a wide tuning range of η between 1.17 and 1.42 in nanoporous polystyrene (PS) thin fi lms obtained by removing poly(methyl methacrylate) (PMMA) in PS-PMMA BCPs. The stacking of three PS layers with gradually changing porosity achieved a minimum refl ectance of less than 0.1%. Han and coworkers demonstrated that a mixture of PS-PMMA BCPs and PMMA homopolymers led to a graded distribution of PMMA along the vertical direction of the thin fi lms and that a gradient η can be obtained for the remaining PS nanostructures after the removal of PMMA, which can reduce the number of processing steps including spin-coating and acid etching. However, these approaches may be associated with challenges related to the low chemical and mechanical stability of the porous organic thin fi lms and the

The directed assembly of block copolymer nanostructures with large periods exceeding 100 nm remains challenging because the translational ordering of long-chained block copolymer is hindered by its very low chain mobility. Using a solvent-vapor annealing process with a neutral solvent, which was sequentially combined with a thermal annealing process, we demonstrate the rapid evolution of a perpendicularly oriented lamellar morphology in high molecular weight block copolymer films on neutral substrate. The synergy with the topographically patterned substrate facilitated unidirectionally structural development of ultrahigh molecular weight block copolymer thin films-even for the structures with a large period of 200 nm-leading to perfectly guided, parallel, and highly ordered line-arrays of perpendicularly oriented lamellae in the trenched confinement. This breakthrough strategy, which is applicable to nanolithographic pattern transfer to target substrates, can be a simple and efficient route to satisfy the demand for block copolymer assemblies with larger feature sizes on hundreds of nanometers scale.

Graded refractive index layer systems for antireflective coatings and rugate filters deposited by reactive pulse magnetron sputtering

Constructing a molecular theory of self-assembly: Interplay of ideas from surfactants and block copolymers

The incorporation of porous structures into films and coatings can transform their properties for applications in optics, separation, electronics, and energy generation and storage. Packing nanoparticles (NPs) is a versatile approach for fabricating nanoporous films with a tunable structure and properties. The mechanical fragility of NP packing-based films and coatings, however, significantly impedes their widespread utilization. Although infiltrating a polymer into the interstices of these NP packings has been shown to enhance their mechanical durability, this method completely eliminates the porosity of the structures, compromising their properties and functionality. This study presents a new approach to fabricate highly loaded porous nanocomposite films with a gradient in the refractive index by infiltrating subsaturating amounts of poly(methyl methacrylate) (PMMA) into disordered packings of hollow silica NPs. We demonstrate that dual porosity is a critical feature that enhances their antireflection (AR) and mechanical properties. The hollow cores of NPs prevent a substantial increase in the refractive index of the resulting films. Moreover, the interparticle voids allow for mechanical reinforcement to occur when the NP packings are infiltrated with PMMA, making them even more suitable for AR coatings. The refractive index and gradient across the nanocomposites can be tailored by adjusting the amount of PMMA infiltrated into the NP packing, the shape of hollow NPs, and the annealing time. The nanocomposite coatings with a continuous gradient in refractive index exhibit excellent AR properties and enhanced mechanical durability. Combined with the unique structural tunability afforded by the dual porosity, this approach provides a scalable and effective way to create robust and graded nanoporous structures for various applications.

Minimizing efficiency losses caused by unwanted light reflection at the interface between lenses, optical instruments and solar cells with the surrounding medium requires antireflective coatings with adequate refractive index and coating thickness. We describe a new type of antireflective coating material with easily and independently tailorable refractive index and coating thickness based on the deposition of colloidal MgF2 nanoparticles. The material synthesis employs micelles of amphiphilic block copolymers as structure directing agent to introduce controlled mesoporosity into MgF2 film. The coatings thickness can be easily adjusted by the applied coating conditions. The coatings refractive index is determined by the materials porosity, which is controlled by the amount of employed pore template. The refractive index can be precisely tuned between 1.23 and 1.11, i.e., in a range that is not accessible to nonporous inorganic materials. Hence, zero reflectance conditions can be established for a wide range of substrate materials.

We explore for the first time the direct growth of high quality InAs nanowires (NWs) on flexible plastic substrates by MOVPE. We have recently reported on self-catalyzed growth of InAs NWs using a new low-temperature growth regime, where NWs were obtained at temperatures as low as 280°C. We showed that low-temperature growth is beneficial for a variety process flows and growth substrates where excessive heat is detrimental, particularly for plastic substrates. In this work, we synthesize InAs nanowires in-situ on flexible polyimide substrates without any need for transfer techniques. Presently semiconductor nanowires are routinely grown on high-priced crystalline substrates as it is extremely challenging to grow directly on plastics and flexible substrates due to high temperature requirements and substrate preparation. At the same time plastic substrates can offer many advantages such as extremely low price, light weight, mechanical flexibility, shock and thermal resistance and biocompatibility. We show that the fabricated NWs are optically and electrically active with strong light emission in the infrared range. Overall, we demonstrate that light-emitting InAs nanowires can be synthesized directly on flexible plastic substrates inside a MOVPE reactor, and we believe that our results will further advance the development of the nanowires-based flexible electronic devices.

Conventional self-assembly methods of block copolymers in cosolvents (i.e., usually water and organic solvents) has yet to produce a pure and monodisperse population of nanocubes. The requirement to assemble a nanocube is for the second block to have a high molecular weight. However, such high molecular weight block copolymers usually result in the formation of kinetically trapped nanostructures even with the addition of organic cosolvents. Here, we demonstrate the rapid production of well-defined polymer nanocubes directly in water by utilizing the thermoresponsive nature of the second block (with 263 monomer units), in which the block copolymer was fully water-soluble below its lower critical solution temperature (LCST) and would produce a pure population of nanocubes when heated above this temperature. Incorporating a pH-responsive monomer in the second block allowed us to control the size of the nanocubes in water with pH and the LCST of the block copolymer. We then used the temperature and pH responsiveness to create an adaptive system that changes morphology when using a unique fuel. This fuel (H2O2 + MnO2) is highly exothermic, and the solution pH increases with the consumption of H2O2. Initially, a nonequilibrium spherical nanostructure formed, which transformed over time into nanocubes, and by controlling the exotherm of the reaction, we controlled the time for this transformation. This block copolymer and the water-only method of self-assembly have provided some insights into designing biomimetic systems that can readily adapt to the environmental conditions.

The silica-based multilayer films exhibiting both self-cleaning property and antireflection in the visible and near infrared regions have been deposited onto glass substrate by layer-by-layer deposition of PAH/PAA polyelectrolyte bilayers, followed by sequential deposition of PAH/SiO2 nanocomposite layers to create the nanoporous structure in the film, and finally treating with fluorosilane. To obtain the appropriate porosity and surface roughness so as to satisfy both antireflection and self-cleaning properties, the deposition of PAH/SiO2 nanocomposite layers was varied from 2 to 8 layers. Scanning electron microscopy and atomic force microscopy analysis revealed that a highly nanoporous structure was obtained from the more deposition of loosely agglomerated SiO2 nanoparticles which increased with the increased cycle of PAH/SiO2 deposition. The film consisting of 6 and 8 PAH/SiO2 layers exhibited a very low reflection of only 1-2% in the visible region and approximately 2-4% in the near infrared region, a maximum transmittance of approximately 93% in the visible region and approximately 94% in the near infrared region, and a high advancing contact angle of approximaetly 150 degrees. It is anticipated that the film which exhibited both self-cleaning and antireflection properties enables the application of the antireflection coatings under humid environments.

With a high-refractive-index mixed-oxide dielectric material of ZrTiO(4) and ZrO(2) [Substance H2 (Sub2) from E. Merck, Darmstadt, Germany], in combination with magnesium flouride (MgF(2)), design optimization and experimental production of low-loss antireflection (AR) coatings are carried out. Design-optimization studies that make use of these materials as constituents of a seven-layer coating system demonstrate that when the useful bandwidth of an AR coating is extended to cover a wider spectral range, the designs are in general found to have increased integrated reflection loss, higher ripple, and increased spectral instability. The experimental studies on Sub2 material show that the films have excellent optical performance over a wider process window, the advantage of which is demonstrated in the production of different AR coatings on a variety of glasses with refractive indices that range from 1.45 to 1.784 and different mechanical, thermal, and chemical properties. The manufacturing process of AR coatings shows a consistency better than 99% with respect to optical properties and durability.

A self-assembled benzimidazole-copper complex (SABCC) is utilized as the gate insulating material for pentacene organic thin film transistors (OTFTs) fabricated on a flexible polyethylene terephthalate plastic substrate. The fabrication features of SABCC are self-assembling, water-based processing, and selectivity of deposition. The pentacene OTFTs exhibit reasonable device characteristics. Threshold voltage, carrier mobility, on/off current ratio, and subthreshold swing are determined to be ∼−4.2 V, ∼0.08 cm2 V−1 s−1, ∼4×103, and ∼4.4 V/decade, respectively. The water-based fabrication process of the SABCC gate insulator on copper electrodes may provide an opportunity for low cost OTFTs on flexible plastic substrates.

Narrow line-width filters based on rugate structure and antireflection coating

Infrared (IR) optical systems tend to make use of IR lens and window materials with high values of refractive index, such as germanium, zinc selenide and zinc sulphide. Consequent Fresnel reflection is sufficient to significantly degrade effective transmission and IR imaging capability. To maximize IR optical performance in such systems, Barr and Stroud has for over 20 years produced high efficiency IR anti-reflection (AR) coatings for lenses and windows. Typically these coatings provide average reflectance below 0.25 percent per surface with sufficient durability to meet internal surface environmental conditions. Design constraints on these AR coatings require a low refractive index outer material, with recent technology and practice relying on thorium fluoride to fulfill this role. The main disadvantage of thorium fluoride usage is its associated radioactivity and high toxicity. Moreover, present IR coating requirements insist upon replacement of thorium fluoride with a non-radioactive, low toxicity, low index film material. A research program at Pilkington Optronics (Barr and Stroud) has resulted in the identification and characterization of yttrium fluoride as a suitable non- radioactive replacement film material with reduced toxicity for use in the 3 to 5 micrometer and the 8 to 12 micrometer IR spectral regions. As described in this paper, this material has been successfully incorporated into high efficiency single and dual band AR coatings for germanium, zinc selenide and zinc sulphide, without compromising optical and durability performance.

Preparation of broadband antireflective coatings with ultra-low refractive index layer by sol-gel method

Investigation on antireflection coatings for Al:ZnO in silicon thin-film solar cells