Self-assembled photonic crystals infiltrated with nanoplatelets and nanotubes.

As we move into the next century, photonics will play a significant role in the exploration of the frontiers of science. Photonic materials have the ability to control the flow and generation of photons, and offer greater control over material properties, which can potentially provide solutions to the optoelectronics industry, i.e. improving the current limit in the speed and the capacity of optoelectronics devices. In 1987, a novel class of artificial structures named as ‘photonic crystal’ (PhC), was invented for the inhibition of spontaneous emission and the localisation of photons, which offers control on absorption, emission and propagation of light. Photonic crystals are long-range periodic materials with a periodicity of the order of the wavelength of light. It is the periodicity in refractive index, which determines the allowed and forbidden bands for the light frequency in the photonic crystals. This periodic structure generate Bragg diffractions which result in forbidden frequency in specific propagation directions, so called photonic stop bands. When light propagation is forbidden in a specific range of frequencies in any direction inside the crystal and polarization a complete photonic bandgap is achieved. There are three types of photonic crystals: one-dimensional (1D), two-dimensional (2D) and three-dimensional (3D) photonic crystals, which depend upon the periodic modulation of the dielectric constant i.e. either created in one, two or three dimensions. This thesis deals with the fabrication and analysis of 3D photonic crystals which shows strong confinement of light in three dimensions. In this thesis, we will introduce two different types of 3D photonic crystals i.e. pristine (undoped) and infiltrated (doped) with different nanomaterials fabricated by soft lithographic method i.e. self-assembly. The fabrication 3D photonic crystal is very challenging task, one need to build up high-quality 3D photonic crystals environments. By 1991, Yablonovitch had demonstrated the first three-dimensional photonic band-gap in the microwave regime by drilling an array of holes in a transparent material, where the holes of each layer form an inverse diamond structure – today it is known as Yablonovite. Over the years state-of-the-art fabrication technologies have been developed to fabricate 3D photonic crystals operating in different range of electromagnetic spectrum ranging from near-infrared to visible wavelength ranges. However, these sophisticated fabrication techniques are expensive and time consuming. Soft lithography is another inexpensive versatile route to fabricate photonic structures. The research conducted in this thesis targets building up a solid and comprehensive study on the fabrication of 3D photonic crystals in the technically important visible wavelength range. This project revolves around the fabrication of undoped photonic crystals (pristine) and in-filled photonic crystals with two dimensional layered nanomaterials such as graphene and boron nitride and 1-dimensional materials (single walled carbon nanotubes). Natural gravitational sedimametaion method has been used to fabricate photonic crystals using latex polymer as a 3D template. Despite potential advantages, there are hardly any reports concerning layered nano-filler based photonic crystals (PhCs). In particular, layered two-dimensional based carbon (Graphene), transition metal dichalcogenides (TMDs: Molybdenum disulphide, Tungsten disulphide) and one dimensional materials such as carbon nanotubes are of particular interest due to the high level of optoelectronic functionality they can impart. One of the biggest issues is to produce large quantity of these nano-fillers and, at the same time maintain the quality as well. Once a stable source of nanoparticles is established achieving a homogenous and controlled distribution of these fillers within a polymer matrix is still an obstacle commonly encountered in the fabrication of nanostructures. To overcome this problem self-assembly of latex particles has been used to fabricate two and one dimensional based photonic crystals. During the self-assembly process the individual polymer particles deform into rhombic dodecahedra, due to capillary forces as the polymer dries. Highly ordered polymeric crystals can be produced by this novel technique. This dodecahedra assembly of polymer particles act as a template to assemble nano-fillers, by forcing the nanoparticles to fill the interstitial sites and create three-dimensional, hexagonal patterns. This assembly technique generates a highly uniform distribution of the filler throughout the polymer matrix. One of the key features of our fabricated photonic crystals is the preparation technique i.e. natural, gravitational, sedimentation which makes it very cost effective and efficient. In this thesis, for the first time, colloidal photonic crystals, embedded with such nano- fillers have been fabricated using a novel and facile latex technology. We also propose that this technique is general and can be applied for a range of other two-dimensional and one-dimensional materials. Critically it is demonstrated that the choice of filler influences the optical and mechanical properties of the resultant crystals. This thesis also demonstrates that the optical properties can also be manipulated mechanically , post-processing, using stretching, compression and humidity, demonstrating their potential as sensors and visual indicators which will greatly extend their applications in various fields such as optical sensing materials and various optical devices.