Sajjad Husain Mir

Hybrid Block Copolymer-Ag Self-assembly to Fabricate Subwavelength Nanostructures for Antireflective Applications

There are lots of problems associated with the low absorption and transmission of light, crucial for the performance of optical and electro-optical devices. Recently, there has been huge developments in the area of Anti-reflective surfaces (ARS) subjecting to sub-wavelength ARS with a tapered morphology. Which is a more practical method for ultrabroadband and omnidirectional anti-reflection. These tapered morphologies similar to the pattern of a moth eye, has overcome the drawbacks of coating AR layered films, that have been used extensively to reduce surface reflection, but typically validated to suppress reflection at specific wavelength, and at specific incidence angles. The reflectance from a selected sub-wavelength or gradient index structures have come down to below 1% in the visible region of the spectrum and efforts are on to achieve broader bands of such enhanced AR regime. Despite the obvious advantage of nano-patterning devices such as solar cells, flat panel displays and LED lighting, the lack of scalability and capital/running cost of fabrication such as lithography are a major technological hurdle for true exploitation. There are also major technical limitations such as the inability to pattern curved surfaces of optical lenses. In recent years roll-to roll processing using nanoimprinting have also been examined but is limited to soft materials and mainly flat substrates. Other existing surface texturing techniques use harsh chemicals with adverse effects on material surface crystallinity. Therefore, it is imperative to develop a new technology to address the current problems with ARS.

State-of-the-art block copolymer (BCP) technology has been applied to achieve “broader-bandwidth” ARS, important for efficient functioning of the photonic devices. These BCP derived nanostructures have been employed for the fabrication of 1D photonic crystals based on lamellar systems, to 2D and 3D structures based on bicontinuous gyroid structures, photonic gels, moth-eye structures, antireflective coatings some with improved self-cleaning properties and metamaterials. However, advancing the technology beyond 1D and 2D photonic crystals in the range of visible light to make AR surfaces has been challenging. This is because in order to modulate photons in the visible light range (400-700 nm), domain sizes must exceed 100 nm. There are two major challenges: (a) With a few exceptions, block copolymers do not easily phase separate above 100 nm domains. The size limitation of BCPS self-assembly is due to significant kinetic penalties that arise from higher molecular entanglement in high molecular weight polymers, (b) synthesizing ultra-high molecular weight BCPs above 500,000 kg/mol (for larger domains) of the required monodispersity remains very challenging. In order to broaden the AR spectrum of moth’s eye structure from (UV-Vis) to near infrared (NIR), the feature size obtained through current BCP nanopatterning needs to be increased even further above 100 nm. Moreover, the etch resistance and selectivity for BCP patterns-crucial for pattern transfer is generally low. To circumvent this problem, thorugh OptSurfTech I aim to extend the current state-of-the-art technology to increase antireflection spectral range by employing the hybrid BCP nanostructures.

This hybrid class of organic-inorganic system can be used to fabricate masks for creating nanopatterns to eliminate the unwanted light reflection. I will also fabricate hybrid BCP system with appropriate dimension of patterns for nano-patterning non-plannar objects such as optical lenses. Moreover, nanopatterning is a hot topic not just in optics industry, but also in automotive industry for injection moulding of nanopatterned plastic parts and also biomedical devices.