Tunable Mesoporous Bragg Reflectors Based on Block Copolymer Self-Assembly

Abstract

Distributed Bragg reflectors (DBRs) consist of periodically alternating layers of high and low refractive index materials, also known as one-dimensional photonic crystals. Interference of light reflected at the interfaces of the dielectric layers leads to strong reflection in a well defined wavelength range resulting in pronounced structural colouration. The spectral response can be finely tuned by varying the refractive index, the thickness, and the number of the alternating layers.1 Simple configurations are realized by sequential coating of thin solid films of alternating refractive index. Direct access to a 1D dielectric lattice via solution processing has been achieved by the self-assembly of block copolymers into a lamellar morphology. 2, 3A interesting recent area of research is the development of mesoporous DBRs (MDBRs) which access new fields of applications due to their porosity on the sub-optical length scale. MDBRs are promising as sensing materials. The adsorption and desorption of gas phase molecules in the pores leads to reversible changes in the refractive index and thereby in the photonic properties of the stack.4-7 Bonifacio et al. introduced the concept of a MDBR-based "photonic nose".8 Other applications include the use of MDBRs as environmentally responsive resonance cavities 10, 11 and their coupling to mesoporous surface layers.12 MDBRs also have great potential in optoelectronic devices. When infiltrated with light emitting polymers, MDBRs have exhibited distributed feedback lasing.13 Efficiency enhancements have been demonstrated by integrating a MDBR into a dye-sensitized solar cell.14 Recently, the coupling of plasmonic particles to MDBR-based resonant cavities has been shown. 15Typically, the refractive index contrast is realized by the alternating deposition of mesoporous layers of TiO 2 and SiO 2 . This concept was first introduced by Ozin and coworkers, who manufactured a TiO 2 /SiO 2 stack with ordered mesoporous morphologies, following a block copolymer directed sol-gel route.4 While the MDBRs exhibited an interconnected pore network, the stack fabrication was extremely time consuming with fabrication times of 4 days or more for each consecutive layer.4, 5 A less complex alternative is the sequential deposition of nanoparticle-based solutions to form mesoporous films. The group of Cohen introduced this concept for a homopolymer mediated nanoparticle approach following numerous dip-coating cycles.9 Miguez et al. subsequently presented

Robust Operation of Mesoporous Antireflective Coatings under Variable Ambient Conditions

“…The structure formation principle via dense packing of sol-loaded block copolymer micelles allows tuning the RI over a wide range by adjusting the I/O ratio. 15 As shown in the Supporting Information (Figure S3), the measured porosity and RI values compare favorably with similar samples that were prepared from higher molecular weight poly-(isoprene)-block-poly(ethylene oxide) (PI-b-PEO) block copolymers. 20 These low RI values provide the potential for incorporating functional materials into the porous network while still maintaining AR conditions.…”

Ordered Mesoporous to Macroporous Oxides with Tunable Isomorphic Architectures: Solution Criteria for Persistent Micelle Templates

“…Crystalline transition metal oxides with nanoscale porous structures are in high demand for energy and catalytic applications that take advantage of their high surface area and interconnected structures. For example, electrochemical devices such as dye-sensitized solar cells, mesoscopic perovskite solar cells, batteries, supercapacitors, fuel cells, and photoelectrochemical devices for solar fuels all take advantage of access to crystalline metal oxides with interconnected mesopores. − The improvement of such devices relies upon judicious optimization of the nanoscale morphology to match the transport limitations of the inorganic materials. Furthermore, many device designs rely on subsequent material deposition ,− by atomic layer deposition or electrodeposition, for example, and will benefit from architectures that are continuously variable beyond the typical 10–30 nm feature sizes from block copolymers up toward larger 100 nm feature sizes more commonly found from colloidal template techniques. − Please note the use of IUPAC pore-size naming convention for 2–50 nm mesopores and >50 nm macropores. , The extension of block copolymer coassembly continuously from the mesoporous regime into the macroporous regime opens new opportunities for ideal comparative studies that use a single synthesis method.…”

Thermally Induced Structural Evolution and Performance of Mesoporous Block Copolymer-Directed Alumina Perovskite Solar Cells

“…14,15,18,19,28,29,34 The photovoltaic device architecture provides an alternative approach to enhance device performance. 39À41 In particular, block copolymer (BCP) self-assembly directed materials have improved charge transport and light management of mesoscopic solar cells via control of morphology, 37,38,41À46 porosity and pore size, 46À50 material crystallinity, 51,52 electronic 37,52 and optical 47,53 properties. For example, wellordered bicontinuous BCP gyroid morphologies enable easy backfilling of the hole transport materials and complete mesopore interconnectivity in solidstate dye-sensitized solar cells.…”