Design and fabrication of near perfect ultra black broadband absorber

Abstract

A near perfect absorber is characterized by zero reflection (R) and zero transmission (T) with unit emissivity (ε = 1), which absorbs all the incoming radiation independent of angle and frequency. The need to fabricate perfectly black surfaces is essential for several applications including developing standards for absorption and emission spectra, solar absorber, stray light suspension, thermal detectors, satellites, thermal management of micro/nanosystems, energy storage devices. Reducing the refractive index (n) of coatings close to unity is the key challenge to develop an ideal black body. Various processes as well as materials, have been developed to bring down the refractive index of objects to unity and thus increase their absorption capacity. It is reported that the introduction of hierarchal micro-nano structures leads to a near perfect absorber. The absorbing frequency range of these hierarchal structures depends on several factors including refraction index, texture shape, size and their arrangement in the material microstructure. Near perfect black absorbers are fabricated using gold-black coatings, ultra‐black nickel‐phosphorus (ub-NiP), textured aluminium doped silicon films, vertically aligned carbon nanotubes (VACNTs), graphene and graphite nanocomposites. Moreover, recently, plasmonic-metamaterial have been used to fabricate light trapping materials. With the use of plasmonic-metamaterials efforts have been made to harness the energy in a wavelength range of 1000 nm to 1400 nm for the next generation solar cells. Despite these advancements, most of the designed metamaterial absorbers have a narrow bandwidth, typically only a single absorption peak. Consequently, there remains a need to develop a near perfect absorber which efficiently works in a broader wavelength range of 300 nm to 2000 nm and at variable incident beam angles. The present work is focused on developing near perfect broadband rigid as well as flexible absorbers. While rigid absorber is fabricated on Si substrate, PDMS is used as a base material for the flexible absorber. The development of rigid absorber starts with the creation and optimization of texture geometry on Si wafers. To achieve this, different types of texture geometries are etched on Si wafers (both n- and p-type) using HNA and KOH etching solutions. Later on, the absorption capacity of etched Si wafer is studied as a function of texture size and vertex angle. It is observed that KOH textured silicon wafer shows a maximum scattering at 10 minutes of etching at 80 °C. Further, optically graded assembly is designed and fabricated on textured silicon wafer followed by thin film deposition of iron, zinc oxide and carbon nanotubes. The fabricated assembly with graded optical index led to multiple scattering along with the total internal reflection of the incident wave, which resulted in ultra-high absorption of more than 99 % in UV-Vis-NIR wavelength range. Moreover, to further improve the absorption capacity in UV-Vis-NIR wavelength range, a novel carbon nanotube structure is fabricated using a dual growth step. Fabricated structure name as Flower Carbon Nanotubes (FCNTs) showed more than 99.98 % of light trapping independent of incident angle in entire UV-Vis-NIR wavelength range. The hierarchical dandelion-like flower carbon nanostructure having emissivity of 0.98 confirmed that the designed structure is blackest manmade structure till date. The observed behaviour originates from multiple scattering and subsequent trapping of the incident beam inside FCNT microstructure. Moreover, the contact angle (172°) and roll-off angle (2°) of water droplets reveal the superhydrophobic and self-cleaning behaviour of fabricated FCNT structure. However, in real-life applications, optical absorbers are being mounted on substrates having different curvatures and contours. Due to this, the need for a flexible absorber that can be mounted on any surface with similar absorption capacity as rigid absorbers is highly in demand. To fabricate such an absorber vertically aligned carbon nanotubes (VACNT) are arranged in Polydimethylsiloxane (PDMS) which shows an absorption capacity of more than 98 % in UV-Vis wavelength and more than 94 % in NIR wavelength range. The PDMS-VACNT composite shows ultra-high absorption capacity along with better thermal and electrical conductivity. It is observed that the fabricated flexible absorber using VACNTs in PDMS has lower absorption compared to the rigid absorbers. To further improve the absorption, a new flexorb is designed by texturing the polydimethylsiloxane (PDMS) matrix reinforced by ZnO nanorods, Fe particles and carbon nanotubes (CNTs). The fabricated multifunctional flexible absorber (flexorb) shows the ultra-high absorbing capacity of > 99% in the whole UV-Vis-NIR region. The near perfect absorbing behaviour of flexorb emanates from two factors; (1) the multiple scattering and subsequent trapping of incident light induced by texturing and fillers having variable geometry and aspect ratio and (2) reduced reflection losses at flexorb/surrounding interface due to impedance matching provided by CNTs. The flexibility of flexorb ensures that it could be deformed in variety of shapes without any damage or creasing failure and is readily mounted on surfaces made of stainless steel, glass or acrylic polymer. Based on the above mentioned findings of the present thesis work, it is concluded that the modification in design and material composition leads to near perfect broadband absorbers. Moreover, carbon nanotubes are an ideal candidate to significantly enhance the absorption capacity in both rigid as well as flexible absorbers. It is envisaged that the present study is a significant step ahead to design near perfect broadband absorbers as per the need of a particular application.