Morgan Stefik

A 3D Optical Metamaterial Made by Self-Assembly

Optical metamaterials have unusual optical characteristics that arise from their periodic nanostructure. Their manufacture requires the assembly of 3D architectures with structure control on the 10-nm length scale. Such a 3D optical metamaterial, based on the replication of a self-assembled block copolymer into gold, is demonstrated. The resulting gold replica has a feature size that is two orders of magnitude smaller than the wavelength of visible light. Its optical signature reveals an archetypal Pendry wire metamaterial with linear and circular dichroism.

Enhanced photocatalytic properties in well-ordered mesoporous WO3

We used polyisoprene-block-ethyleneoxide copolymers as structure-directing agents to synthesise well-ordered and highly-crystalline mesoporous WO(3) architectures that possess improved photocatalytic properties due to enhanced dye-adsorption in absence of diffusion limitation.

Improved conductivity in dye-sensitised solar cells through block-copolymer confined TiO2 crystallisation

Anatase n TiO2 is typically a central component in high performance dye-sensitised solar cells (DSCs). This study demonstrates the benefits of high temperature synthesised mesoporous titania for the performance of solid-state DSCs. In contrast to earlier methods, the high temperature stability of mesoporous titania is enabled by the self-assembly of the amphiphilic block copolymer polyisoprene-block-polyethylene oxide (PI-b -PEO) which compartmentalises TiO2 crystallisation, preventing the collapse of porosity at temperatures up to 700 °C. The systematic study of the temperature dependence on DSC performance reveals a parameter trade-off: high temperature annealed anatase consisted of larger crystallites and had a higher conductivity, but this came at the expense of a reduced specific surface area. While the reduction in specific surface areas was found to be detrimental for liquid-electrolyte DSC performance, solid-state DSCs benefitted from the increased anatase conductivity and exhibited a performance increase by a factor of three.

Tunable Mesoporous Bragg Reflectors Based on Block-Copolymer Self-Assembly

Mesoporous distributed Bragg reflectors (MDBRs) exhibit porosity on the sub-wavelength scale. They are promising device components for biological and chemisal sensing as well as for light management in optoelectronic devices. In this chapter a new route for the fabrication of MDBRs is presented which relies on the structure directing properties of the block copolymer poly(isoprene-block-ethylene oxide) in combination with sol-gel chemistry. The interplay between structure directing organic host and coassembled inorganic guest allows the fine tuning of refractive index in the outcome material. Stacking high and low refractive index films in sequential order enables the fast and reliable construction of MDBRs which exhibit a continuous TiO2 network with large accessible pores and high optical quality.

Tunable 3D Extended Self‐Assembled Gold Metamaterials with Enhanced Light Transmission

The optical properties of metamaterials made by block copolymer self-assembly are tuned by structural and environmental variations. The plasma frequency red-shifts with increasing lattice constant and blue-shifts as the network filling fraction increases. Infiltration with dielectric liquids leads also to a red-shift of the plasma edge. A 300 nm-thick slab of gyroid-structured gold has a remarkable transmission of 20%.

Networked and chiral nanocomposites from ABC triblock terpolymer coassembly with transition metal oxide nanoparticles

Multicomponent materials with ordered nanoscale networks are critical for applications ranging from microelectronics to energy conversion and storage devices which require charge transport along 3-dimensional (3D) continuous pathways. The network symmetry can facilitate additional properties such as macroscopic polarization for piezoelectric, pyroelectric, and second-order nonlinear optical properties in non-centrosymmetric morphologies. Although pure block copolymers are able to form multiple network morphologies, network tunability remains a challenge for coassembled systems. Here we report the coassembly of niobia nanoparticles with a poly(isoprene-b-styrene-b-ethylene oxide) (ISO) which resulted in multiple network morphologies, one of which was chiral and non-centrosymmetric. Detailed small-angle X-ray scattering (SAXS) and transmission electron microscopy (TEM) measurements were most consistent with the alternating gyroid (GA) morphology at low nanoparticle loadings and a transition to a centrosymmetric network morphology at higher loadings. This is the first report of multiple network morphologies from coassembly with a single polymer over a ∼10 vol% composition range. The nanoparticle spatial distribution was tomographically reconstructed. Nanocomposite calcination resulted in mesoporous networks. This general approach was further demonstrated with amorphous and anatase titania.

Layer-by-Layer Formation of Block-Copolymer-Derived TiO2 for Solid-State Dye-Sensitized Solar Cells

Morphology control on the 10 nm length scale in mesoporous TiO(2) films is crucial for the manufacture of high-performance dye-sensitized solar cells. While the combination of block-copolymer self-assembly with sol-gel chemistry yields good results for very thin films, the shrinkage during the film manufacture typically prevents the build-up of sufficiently thick layers to enable optimum solar cell operation. Here, a study on the temporal evolution of block-copolymer-directed mesoporous TiO(2) films during annealing and calcination is presented. The in-situ investigation of the shrinkage process enables the establishment of a simple and fast protocol for the fabrication of thicker films. When used as photoanodes in solid-state dye-sensitized solar cells, the mesoporous networks exhibit significantly enhanced transport and collection rates compared to the state-of-the-art nanoparticle-based devices. As a consequence of the increased film thickness, power conversion efficiencies above 4% are reached.

Nanostructured carbon–crystalline titania composites from microphase separation of poly(ethylene oxide-b-acrylonitrile) and titania sols

A simple one-pot method utilizing a graphitic carbon source containing poly(ethylene oxide-b-acrylonitrile) diblock copolymer as a structure directing agent was used to synthesize carbon-crystalline titania composites as well as crystalline mesoporous titania materials after oxidative carbon removal.

Low temperature crystallisation of mesoporous TiO2

Conducting mesoporous TiO2 is rapidly gaining importance for green energy applications. To optimise performance, its porosity and crystallinity must be carefully fine-tuned. To this end, we have performed a detailed study on the temperature dependence of TiO2 crystallisation in mesoporous films. Crystal nucleation and growth of initially amorphous TiO2 derived by hydrolytic sol-gel chemistry is compared to the evolution of crystallinity from nanocrystalline building blocks obtained from non-hydrolytic sol-gel chemistry, and mixtures thereof. Our study addresses the question whether the critical temperature for crystal growth can be lowered by the addition of crystalline nucleation seeds.

Self-assembly as a design tool for the integration of photonic structures into excitonic solar cells

One way to successfully enhance light harvesting of excitonic solar cells is the integration of optical elements that increase the photon path length in the light absorbing layer. Device architectures which incorporate structural order in form of one- or three-dimensional refractive index lattices can lead to the localization of light in specific parts of the spectrum, while retaining the cells transparency in others. Herein, we present two routes for the integration of photonic crystals (PCs) into dye-sensitized solar cells (DSCs). In both cases, the self-assembly of soft matter plays a key role in the fabrication process of the TiO2 electrode. One approach relies on a combination of colloidal self-assembly and the self-assembly of block copolymers, resulting in a double layer dye-sensitized solar cell with increased light absorption from the 3D PC element. An alternative route is based on the fact that the refractive index of the mesoporous layer can be finely tuned by the interplay between block copolymer self-assembly and hydrolytic TiO2 sol-gel chemistry. Alternating deposition of high and low refractive index layers enables the integration of a 1D PC into a DSC.