Hemant Kumar Raut

Anti-reflective coatings: A critical, in-depth review

Anti-reflective coatings (ARCs) have evolved into highly effective reflectance and glare reducing components for various optical and opto-electrical equipments. Extensive research in optical and biological reflectance minimization as well as the emergence of nanotechnology over the years has contributed to the enhancement of ARCs in a major way. In this study the prime objective is to give a comprehensive idea of the ARCs right from their inception, as they were originally conceptualized by the pioneers and lay down the basic concepts and strategies adopted to minimize reflectance. The different types of ARCs are also described in greater detail and the state-of-the-art fabrication techniques have been fully illustrated. The inspiration that ARCs derive from nature (‘biomimetics’) has been an area of major research and is discussed at length. The various materials that have been reportedly used in fabricating the ARCs have also been brought into sharp focus. An account of application of ARCs on solar cells and modules, contemporary research and associated challenges are presented in the end to facilitate a universal understanding of the ARCs and encourage future research.

A review on self-cleaning coatings

This review article summarizes the key areas of self-cleaning coatings, primarily focusing on various materials that are widely used in recent research and also in commercial applications. The scope of this article orbits around hydrophobic and hydrophilic coatings, their working mechanism, fabrication techniques that enable the development of such coatings, various functions like Anti-icing, Electro-wetting, Surface switchability and the areas where selfcleaning technology can be implemented. Moreover, different characterization techniques and material testing feasibilities are also analyzed and discussed. Though several companies have commercialized a few products based on self-cleaning coating technology, much potential still remains in this field.

Multiscale Ommatidial Arrays with Broadband and Omnidirectional Antireflection and Antifogging Properties by Sacrificial Layer Mediated Nanoimprinting

Moths eye inspired multiscale ommatidial arrays offer multifunctional properties of great significance in optoelectronic devices. However, a major challenge remains in fabricating these arrays on large-area substrates using a simple and scalable technique. Here we present the fabrication of these multiscale ommatidial arrays over large areas by a distinct approach called sacrificial layer mediated nanoimprinting, which involves nanoimprinting aided by a sacrificial layer. The fabricated arrays exhibited excellent pattern uniformity over the entire patterned area. Optimum dimensions of the multiscale ommatidial arrays determined by the finite-difference time domain simulations served as the design parameters for replicating the arrays on glass. A broadband suppression of reflectance to a minimum of ∼1.4% and omnidirectional antireflection for highly oblique angles of incidence up to 70° were achieved. In addition, superhydrophobicity and superior antifogging characteristics enabled the retention of optical properties even in wet and humid conditions, suggesting reliable optical performance in practical outdoor conditions. We anticipate that these properties could potentially enhance the performance of optoelectronic devices and minimize the influence of in-service conditions. Additionally, as our technique is solely nanoimprinting-based, it may enable scalable and high-throughput fabrication of multiscale ommatidial arrays.

Superhydrophobic fluorinated POSS–PVDF-HFP nanocomposite coating on glass by electrospinning

Fluorinated polyhedral oligomeric silsesquioxanes–poly(vinylidene fluoride-co-hexafluoro propylene) (fluoroPOSS–PVDF-HFP) nanocomposite mixtures are prepared by individually mixing two fluorinated POSS materials (FP8 and FPSi8) with PVDF-HFP solution and transparent superhydrophobic coatings on a glass substrate are produced by electrospinning. The fabricated superhydrophobic surface exhibits continuous, uniform and non-beaded nanofibers with a high water contact angle (157.3°) and a low sliding angle (SA < 5°). The superhydrophobic properties including surface energy, water contact angle, and contact angle hysteresis as well as the optical properties are studied by altering the wt% of fluoroPOSS in PVDF-HFP solution. It is observed that due to the increase in the viscosity of the solution, the diameter of the nanofibers increases with increase in the concentration of fluoroPOSS. It is also observed that as the concentration of fluoroPOSS in PVDF solution increases, the amount of fluorine content increases thereby the surface energy of the coating decreases (to around 10 mN m−1) leading to a superhydrophobic surface with low contact angle hysteresis (<5°). The optical properties of the coatings produced using FPSi8 fluoroPOSS–PVDF-HFP mixture are compared with those of FP8 fluoroPOSS, and the former is found to be more transparent due to its relatively high solubility in PVDF-HFP.

Photocatalytic superhydrophilic TiO2 coating on glass by electrospinning

A photocatalytic, superhydrophilic, transparent, porous TiO2 film consisting of rice-shaped nano/mesostructures deposited on glass substrates by electrospinning has been developed. Optical properties such as transmittance and absorbance, as well as the superhydrophilicity and self-cleaning properties (photocatalysis) of the deposited TiO2 film are studied. The water contact angle decreases with increase in the thickness of the TiO2 resulting in superhydrophilic transparent coatings which may be used for applications in window coatings and photovoltaic modules. The self-cleaning property of the TiO2 film consisting of rice-shaped nano/mesostructures is compared with that of commercially available Degussa P-25 in the photodegradation of Alizarin red dye and found to be more effective.

Electrospun SiO2 nanofibers as a template to fabricate a robust and transparent superamphiphobic coating

A one-dimensional morphology of electrospun nanofibers has been used as a template to fabricate a robust and transparent superamphiphobic coating. The template is created by the deposition of a thick layer of SiO2 nanofibers on glass. The developed template (SiO2 nanofibers) is coated with an ultrathin (25 nm) porous silica membrane by the vapor deposition technique. After heat treatment (600 °C), a transparent, superhydrophilic coating consisting of a hybrid silica network (SiO2 nanofibers enclosed by the silica membrane) is obtained. It is observed that during the heat treatment process, the coated silica membrane reinforces the SiO2 nanofibers and prevents the fibers from disintegrating into nanoparticles, resulting in the formation of a hybrid silica network. The fiber morphology assisted the hybrid silica network to keep its roughness and surface texture. After silanization, the coating exhibited superamphiphobic property with surface contact angles achieved using water and hexadecane are 161° and 146.5°, respectively.

Robust and durable polyhedral oligomeric silsesquioxane-based anti-reflective nanostructures with broadband quasi-omnidirectional properties

Polymer-based anti-reflective coatings (ARCs) on glass pose major challenges for outdoor photovoltaic applications due to their incompatible mechanical and thermal properties. Here we demonstrate durable, chemically and thermally stable polyhedral oligomeric silsesquioxane-based (POSS) anti-reflective moths eye nanostructures on glass fabricated by double-side nanoimprint lithography. These anti-reflective nanostructures exhibited excellent broadband and quasi-omnidirectional anti-reflective properties. An optimum resist composition for nanoimprinting was obtained by mixing a methacryl POSS cage mixture with 1,6-hexanediol diacrylate in a 1 : 12 molar ratio. Thermal free radical co-polymerization during nanoimprint lithography produced a uniform array of moths eye nanostructures on both sides of a glass substrate with yields ∼90 to 100%. The transmittance of the resulting glass was enhanced to 98.2% (reflectance 1.26%) with excellent quasi-omnidirectional transmittance observed from −50° to +50° of angles of incidence. Furthermore, a series of ASTM-based tests on the imprinted ARC structures showed strong adherence to glass, better hardness and mechanical strength with superior chemical and thermal stability, thus suggesting their strong potential for commercial applications.

Electrospun ZnO Nanowire Plantations in the Electron Transport Layer for High-Efficiency Inverted Organic Solar Cells

Inverted bulk heterojunction organic solar cells having device structure ITO/ZnO/poly(3-hexylthiophene) (P3HT):[6,6]-phenyl C61 butyric acid methyl ester (PCBM) /MoO3/Ag were fabricated with high photoelectric conversion efficiency and stability. Three types of devices were developed with varying electron transporting layer (ETL) ZnO architecture. The ETL in the first type was a sol-gel-derived particulate film of ZnO, which in the second and third type contained additional ZnO nanowires of varying concentrations. The length of the ZnO nanowires, which were developed by the electrospinning technique, extended up to the bulk of the photoactive layer in the device. The devices those employed a higher loading of ZnO nanowires showed 20% higher photoelectric conversion efficiency (PCE), which mainly resulted from an enhancement in its fill factor (FF). Charge transport characteristic of the device were studied by transient photovoltage decay and charge extraction by linearly increasing voltage techniques. Results show that higher PCE and FF in the devices employed ZnO nanowire plantations resulted from improved charge collection efficiency and reduced recombination rate.

Fabrication of highly uniform and porous MgF2 anti-reflective coatings by polymer-based sol–gel processing on large-area glass substrates

Despite recent progress in the fabrication of magnesium fluoride (MgF2) anti-reflective coatings (ARCs), simple, effective and scalable sol-gel fabrication of MgF2 ARCs for large-area glass substrates has prospective application in various optoelectronic devices. In this paper, a polymer-based sol-gel route was devised to fabricate highly uniform and porous MgF2 ARCs on large-area glass substrates. A sol-gel precursor made of polyvinyl acetate and magnesium trifluoroacetate assisted in the formation of uniformly mesoporous MgF2 ARCs on glass substrates, leading to the attainment of a refractive index of ~1.23. Systematic optimization of the thickness of the ARC in the sub-wavelength regime led to achieving ~99.4% transmittance in the case of the porous MgF2 ARC glass. Precise control of the thickness of porous MgF2 ARC glass also resulted in a mere ~0.1% reflection, virtually eliminating reflection off the glass surface at the target wavelength. Further manipulation of the thickness of the ARC on either side of the glass substrate led to the fabrication of relatively broadband, porous MgF2 ARC glass.

Gecko-Inspired Dry Adhesive Based on Micro–Nanoscale Hierarchical Arrays for Application in Climbing Devices

The unusual ability of geckos to climb vertical walls underlies a unique combination of a hierarchical structural design and a stiffer material composition. While a dense array of microscopic hierarchical structures enables the gecko toe pads to adhere to various surfaces, a stiffer material (β-keratin) composition enables them to maintain reliable adhesion over innumerable cycles. This unique strategy has been seldom implemented in engineered dry adhesives because fabrication of high-aspect-ratio hierarchical structures using a stiffer polymer is challenging. Herein, we report the fabrication of high-aspect-ratio hierarchical arrays on flexible polycarbonate sheets (stiffness comparable to that of β-keratin) by a sacrificial-layer-mediated nanoimprinting technique. Dry-adhesive films comprising the hierarchical arrays showed a formidable shear adhesion of 11.91 ± 0.43 N/cm2. Cyclic adhesion tests also showed that the shear adhesion of the adhesive films reduced by only about 20% after 50 cycles and remained nearly constant until about 200 cycles. Most importantly, the high-aspect-ratio hierarchical arrays were integrated onto the feet of a miniature robot and the locomotion on a 30° inclined surface was demonstrated.