Optimization of microsphere optical lithography for nano-patterning

We present an original approach to realistic modeling of light focusing by microsphere systems to form the photonic jets for nano-patterning of the substrates with high refractive index. In simulations we analyze the photonic jets produced by a single sphere and close-packed array of microspheres on the photoresist layer and Si substrate. We show how the lithographic profiles can be controlled by varying the exposure dose and system geometry in wide ranges of photoresist layer thicknesses and microsphere sizes. The modeling covers the entire lithographic system and accounts for the interference of focused light transmitted through the microlenses and reflected from the Si substrate. We use our approach to optimize the size of the lithographic pattern and confirm the simulation results experimentally. The suggested set of methods is rather universal and may be applied to other microlens and resist materials to minimize lithography lateral resolution.

Nano-patterning on multilayer MoS2 via block copolymer lithography for highly sensitive and responsive phototransistors

Indirect bandgap of multilayer molybdenum disulfide has been recognized as a major hindrance to high responsivity of MoS2 phototransistors. Here, to overcome this fundamental limitation, we propose a structural engineering of MoS2 via nano-patterning using block copolymer lithography. The fabricated nanoporous MoS2, consisting of periodic hexagonal arrays of hexagon nanoholes, includes abundant edges having a zigzag configuration of atomic columns with molybdenum and sulfur atoms. These exposed zigzag edges are responsible for multiple trap states in the bandgap region, as confirmed by photo-excited charge-collection spectroscopy measurements on multilayer nanoporous MoS2 phototransistors, showing that in-gap states only near the valence band can result in a photogating effect. The effect of nano-patterning is therefore to significantly enhance the responsivity of multilayer nanoporous MoS2 phototransistors, exhibiting an ultra-high photoresponsivity of 622.2 A W−1. Our nano-patterning of MoS2 for photosensing application paves a route to structural engineering of two-dimensional materials for highly sensitive and responsive optoelectronic devices.

Surface Nano-Patterning for the Bottom-Up Growth of III-V Semiconductor Nanowire Ordered Arrays

Ordered arrays of vertically aligned semiconductor nanowires are regarded as promising candidates for the realization of all-dielectric metamaterials and artificial electromagnetic materials, whose properties can be engineered to enable new functions and enhanced device performances with respect to naturally existing materials. In this review we account for the recent progresses in substrate nanopatterning methods, strategies and approaches that overall constitute the preliminary step towards the bottom-up growth of arrays of vertically aligned semiconductor nanowires with a controlled location, size and morphology of each nanowire. While we focus specifically on III-V semiconductor nanowires, several concepts, mechanisms and conclusions reported in the manuscript can be invoked and are valid also for different nanowire materials.

Cool, Dry, Nano-scale DIC Patterning of Delicate, Heterogeneous, Non-planar Specimens by Micro-mist Nebulization

Background: Application of patterns to enable high-resolution Digital Image Correlation (DIC) at the small scale (μm/nm) is known to be very challenging as techniques developed for the macro- and mesoscale, such as spray painting, cannot be scaled down directly. Moreover, existing nano-patterning techniques all rely on harsh processing steps, based on high temperature, chemicals, physical contact, liquids, and/or high vacuum, that can easily damage fragile, small-scale, free-standing and/or hygro-sensitive specimens, such as MEMS or biological samples. Objective: To present a straightforward, inexpensive technique specially designed for nano-patterning highly delicate specimens for high-resolution DIC. Methods: The technique consists in a well-controlled nebulized micro-mist, containing predominantly no more than one nanoparticle per mist droplet. The micro-mist is subsequently dried, resulting in a flow of individual nanoparticles that are deposited on the specimen surface at near-room temperature. By having single nanoparticles falling on the specimen surface, the notoriously challenging task of controlling nanoparticle-nanoparticle and nanoparticle-surface interactions as a result of the complex droplet drying dynamics, e.g., in drop-casting, is circumvented. Results: High-quality patterns are demonstrated for a number of challenging cases of physically and chemically sensitive specimens with nanoparticles from 1 μm down to 50 nm in diameter. It is shown that the pattern can easily be scaled within (and probably beyond) this range, which is of special interest for micromechanical testing using in-situ microscopic imaging techniques, such as high-magnification optical microscopy, optical profilometry, atomic force microscopy, and scanning electron microscopy, etc. Conclusions: Delicate specimens can conveniently be patterned at near-room temperature ($\sim $ ∼ 37 ∘C), without exposure to chemicals, physical contact or vacuum, while the pattern density and speckle size can be easily tuned.

Lithography-free and Highly Angle Sensitive Structural Coloration Using Fabry–Perot Resonance of Tin

Recently, there has been much interest in applying the color changes of nano-patterned structures to sensor technology. However, the lithographic nano-patterning process is not environmentally friendly, and it is difficult to fabricate large areas of color due to limitations associated with this approach. In this study, we realized a highly tunable structural coloration based on a Fabry–Perot interferometer design that does not require nano-patterning processes. To increase the reflected color change according to the angle, a color element using an asymmetric metal–insulator–metal structure was applied, fabricated using silver–silicon dioxide–tin (Sn), respectively. Using the optical properties of Sn, we maximized the change in reflection color and realized three primary colors of subtractive color of cyan, magenta and yellow according to the angle of designed MIM structure. Theoretical and experimental results revealed that the color and intensity of the reflectance depended strongly on the angle of the reflective surfaces. The manufacturing process is simple and yields large surfaces of high quality. Based on this premise, we fabricated a soft robot capable of color camouflage, and an angle-detecting color sensor for inspecting the three-dimensional shape quality of curved glass with a high sensitivity of 1.8 nm/degree. In addition, we propose a shape evaluation method by color, spectrometry, and monochromatic light.

Temperature Dependence of Stress and Optical Properties in AlN Films Grown by MOCVD

AlN epilayers were grown on a 2-inch [0001] conventional flat sapphire substrate (CSS) and a nano-patterned sapphire substrate (NPSS) by metalorganic chemical vapor deposition. In this work, the effect of the substrate template and temperature on stress and optical properties of AlN films has been studied by using Raman spectroscopy, X-ray diffraction (XRD), transmission electron microscopy (TEM), UV-visible spectrophotometer and spectroscopic ellipsometry (SE). The AlN on NPSS exhibits lower compressive stress and strain values. The biaxial stress decreases from 1.59 to 0.60 GPa for AlN on CSS and from 0.90 to 0.38 GPa for AlN on NPSS sample in the temperature range 80–300 K, which shows compressive stress. According to the TEM data, the stress varies from tensile on the interface to compressive on the surface. It can be deduced that the nano-holes provide more channels for stress relaxation. Nano-patterning leads to a lower degree of disorder and stress/strain relaxes by the formation of the nano-hole structure between the interface of AlN epilayers and the substrate. The low crystal disorder and defects in the AlN on NPSS is confirmed by the small Urbach energy values. The variation in bandgap (Eg) and optical constants (n, k) with temperature are discussed in detail. Nano-patterning leads to poor light transmission due to light scattering, coupling, and trapping in nano-holes.

Non-equilibrium thermal annealing of a polymer blend in bilayer settings for complex micro/nano-patterning

Fast morphological evolution of polymer blend dictated by underlying polymer thin film upon rapid thermal annealing at high temperature.