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

Triblock‐Terpolymer‐Directed Self‐Assembly of Mesoporous TiO2: High‐Performance Photoanodes for Solid‐State Dye‐Sensitized Solar Cells

“…See ref. 39 for details. These porosity and surface area estimations indicate that structurally, the triblock terpolymer exhibits a higher accessible surface are for dye loading as well as enhanced porosity compared to the standard nanoparticle based films.…”

Low temperature crystallisation of mesoporous TiO2

“…A subsequent high temperature step degrades the organic material and crystallises the TiO 2 to result in an inorganic nanocrystalline network of high morphological order. This approach has attracted significant interest 11 because it allows detailed control over pore size 12 and porosity 13 of the resulting inorganic thin films.…”

Mesoporous Bragg reflectors: block-copolymer self-assembly leads to building blocks with well defined continuous pores and high control over optical properties

“…In its final calcined state, "titania 3:1" films have shrunk to 42%, "titania 1:2" films to 33% compared to its initial thickness after spin-coating. 20 In order to prevent cracking and delamination, time-and temperature-dependent shrinkage was studied in detail. Results led to the establishment of a 30 min annealing protocol with temperature steps at 100…”

“…The use of optical means enabled us to not only study the temperature dependence but also resolve the shrinkage kinetics in detail. 20 Therefor, we collected the reflection spectra of the films during the annealing on a hotplate.The development of the optical film thickness (n · h) can then be resolved by the position of the interference fringes. Figure 2a and b show the time resolved development of the reflection spectra (1 min intervals, normalized to that of a blank silicon wafer) in a contour plot for "titania 3:1" and "titania 1:2", respectively.…”