Fabrication of Ordered 2D Colloidal Crystals on Flat and Patterned Substrates by Spin Coating

Solution processing for colloidal nanoparticle thin film: From fundamentals to applications

Colloidal nanoparticles (NPs) are widely used as building blocks to construct thin film devices owing to their numerous advantages and unique size-dependent properties. The performance of NP-based devices is highly dependent on the film fabrication method and structure. Therefore, understanding the various solution-based thin film fabrication methods is critical for maximizing the device performance by controlling the NP film structures. This review article surveys eleven representative solution processes (dip coating, blade coating, slot-die coating, Mayer rod coating, inkjet printing, roll-to-roll printing, brush coating, drop casting, spin coating, spray coating, and electrophoretic deposition) using colloidal NPs as building blocks. The merits/limitations and basic deposition mechanisms of these processes are discussed in this review for a broad audience to facilitate their customization to individual industrial or laboratory conditions. This review article also aims to provide insights into how solution processing affects the NP thin film device properties by introducing recent achievements and providing the readers with in-depth information that can aid future research.

Technique for Creating 3D Ordered Colloidal Crystals with Hexagonal Close Packing and Uniform Thickness over a Large Area

Traditional approaches to creating colloidal crystals do not simultaneously achieve uniform thickness, three-dimensional ordering, and large areas of defect-free hexagonal close-packed domains. Only the realization of all these conditions will allow the use of colloidal crystals as templates for fabricating inverse opals with a tunable photonic band gap. Therefore, we propose a novel approach for creating 3D colloidal crystals. It combines the use of the Langmuir-Blodgett (LB) process to form the first layer and sequential spin-coating processes to form all following layers. The original automated LB trough, equipped with a feedback control system for surface pressure control, allowed the formation of a close-packed monolayer across the entire area of a 76 mm substrate, obtaining a defect-free domain area of 3000 μm2. As a result of the developed spin-coating technique, bilayer and three-layer colloidal crystals based on polystyrene spheres (1.25 and 1.8 μm) were obtained. Three-dimensional HCP structure covered ≈96.5% of the substrate, and a defect-free domain area was obtained at least 1000 μm2. The high degree of 3D ordering was confirmed by the presence of stop bands in the transmission spectra at wavelengths corresponding to Bragg diffraction from parallel planes and a 2D array of spherical particles.

Large-Scale Formation of a Close-Packed Monolayer of Spheres Using Different Colloidal Lithography Techniques

The possibility of using colloidal lithography at the industrial level depends on the ability to form defect-free coatings over large areas. The spin-coating method has not yet shown acceptable results, but a more detailed studying of the regularities of this process may improve the quality of masks. The Langmuir-Blodgett method is expected to be the most preferable for forming high-quality large-scale monolayers. Real-time controlling the surface pressure of the monolayer can allow to obtain close-packed arrays with long-range order. In this work, to develop the spin-coating technology, the influence of technological parameters (spin-coating speed and time, concentrations of components in suspension) on the substrate coverage area with a monolayer of polystyrene spheres (1.25 μm) was studied. An original automated Langmuir-Blodgett system was developed to study the influence of the monolayer surface pressure on its quality using polystyrene spheres (1.25, 1.8, 2.1 μm). The developed spin-coating technology resulted in a record coverage area (90%) of Si substrate (76 mm) and a defect-free hexagonally ordered domain area of 500 μm2. As a result of the developed Langmuir-Blodgett technique, a close-packed monolayer coating was obtained over the entire substrate area (coverage area 99.5%, defect-free domain area 3000 μm2) without the use of any surfactants.

Enhancement of infrared absorption through a patterned thin film of magnetic field and spin-coating directed self-assembly of gold nanoparticle stabilised ferrofluid emulsion

Molecular vibration signals were amplified by the gold strip gratings as a result of grating resonances and nearby electric field hotspots. Colloidal gold island films exhibit similar enhancement; however, the uneven geometrical characteristics of these films restrict the tunability of the vibrational enhancement. Infrared absorption is enhanced by regular metallic patterns such as arrays of strips fabricated using a top-down approach such as nanolithography, although this technology is expensive and difficult. The significant infrared absorption may serve as tuneable antenna sensitization to improve the sensor performance. In this article, we present a simple one-step process for fabricating optically sensitive ordered arrays of a gold nanoparticle ferrofluid emulsion in polyvinyl alcohol (PVA) using a magnetic field-directed and spin-coating self-assembly (MDSCSA) process. Techniques such as UV-visible absorption, scanning electron microscopy, and grazing-angle infrared spectroscopy were used to evaluate various parameters associated with the nanostructures. Unlike the gold strips, the chain-like features in the iron oxide nanoparticle arrays were discontinuous. The fabricated chain-like ordered arrays have been shown to increase the local field to enhance the infrared absorption corresponding to the symmetric vibration of the -CH2 (2918 cm-1) group present in PVA by ∼667% at a 45° grazing angle, as the chain thickness (CT) increased by 178%. This scalable and simple method can potentially generate low-cost patterns for antenna sensitisation.

Development of controlled nanosphere lithography technology

This work is devoted to the development of nanosphere lithography (NSL) technology, which is a low-cost and efficient method to form nanostructures for nanoelectronics, as well as optoelectronic, plasmonic and photovoltaic applications. Creating a nanosphere mask by spin-coating is a promising, but not sufficiently studied method, requiring a large experimental base for different sizes of nanospheres. So, in this work, we investigated the influence of the technological parameters of NSL by spin-coating on the substrate coverage area by a monolayer of nanospheres with a diameter of 300 nm. It was found that the coverage area increases with decreasing spin speed and time, isopropyl and propylene glycol content, and with increasing the content of nanospheres in solution. Moreover, the process of controllably reducing the size of nanospheres in inductively coupled oxygen plasma was studied in detail. It was determined that increasing the oxygen flow rate from 9 to 15 sccm does not change the polystyrene etching rate, whereas changing the high-frequency power from 250 to 500 W increases the etching rate and allows us to control the decreasing diameter with high accuracy. Based on the experimental data, the optimal technological parameters of NSL were selected and the nanosphere mask on Si substrate was created with coverage area of 97.8% and process reproducibility of 98.6%. Subsequently reducing the nanosphere diameter lets us obtain nanoneedles of various sizes, which can be used in field emission cathodes. In this work, the reduction of nanosphere size, silicon etching, and removal of polystyrene residues occurred in unified continuous process of plasma etching without sample unloading to atmosphere.

Conformal Self-Assembly of Nanospheres for Light-Enhanced Airtightness Monitoring and Room-Temperature Gas Sensing

The fabrication of conformal nanostructures on microarchitectures is of great significance for diverse applications. Here a facile and universal method was developed for conformal self-assembly of nanospheres on various substrates including convex bumps and concave holes. Hydrophobic microarchitectures could be transferred into superhydrophilic ones using plasma treatment due to the formation of numerous hydroxyl groups. Because of superhydrophilicity, the nanosphere suspension spread on the microarchitectures quickly and conformal self-assembly of nanospheres can be realized. Besides, the feature size of the conformal nanospheres on the substrates could be further regulated by plasma treatment. After transferring two-dimensional tungsten disulfide sheets onto the conformal nanospheres, the periodic nanosphere array was demonstrated to be able to enhance the light harvesting of WS2. Based on this, a light-enhanced room-temperature gas sensor with a fast recovery speed (<35 s) and low detecting limit (500 ppb) was achieved. Moreover, the WS2-covered nanospheres on the microarchitectures were very sensitive to the changes in air pressure due to the formation of suspended sheets on the convex bumps and concave holes. A sensitive photoelectronic pressure sensor that was capable of detecting the airtightness of vacuum devices was developed using the WS2-decorated hierarchical architectures. This work provides a simple method for the fabrication of conformal nanospheres on arbitrary substrates, which is promising for three-dimensional microfabrication of multifunctional hierarchical microarchitectures for diverse applications, such as biomimetic compound eyes, smart wetting surfaces and photonic crystals.

Spin coating mediated morphology modulation in self assembly of peptides

Controlling the morphology and nanostructure of self-assembled peptide molecules is of fundamental importance to chemistry and material science due to their bioactivity in both in vivo and in vitro settings, ability to act as templates for conjugating bio-recognition elements, hybrid supramolecular assembly, possible detection and treatment of diseases and so on. In this article, we show that spin coating, a widely utilized method for obtaining ultra-thin polymer films, has been utilised to modulate the self-assembly of peptide molecules, which has traditionally been achieved by chemical functionalisation of the molecules. With the specific example of diphenylalanine-based peptide molecules, we show that a variety of self-assembled architectures such as long fibrils, short fibrils, globules, nanodots, and so on, spanning over large areas can be obtained by simultaneously varying the spinning speed (RPM) and the solution concentration (Cp) during spin coating. We correlate the variation in morphology to a transition from spin dewetting at very low Cp (or high RPM) to the formation of continuous films at high Cp (or low RPM) during the initial stage of spin coating. We further show the generality of the approach by achieving distinct self-assembled morphologies with diphenylalanine analogues with different C-terminal and N-terminal groups by modulation of spin coating parameters, though the exact morphology obtained under identical coating conditions depends on the chemical nature of the peptide molecules. The work opens up a new possible route for creating complex peptide assemblies on demand by simultaneous control of molecular functionalisation and spin coating parameters vis - a - vis the applied centrifugal force.

Substrate wettability guided oriented self assembly of Janus particles

Self-assembly of Janus particles with spatial inhomogeneous properties is of fundamental importance in diverse areas of sciences and has been extensively observed as a favorably functionalized fluidic interface or in a dilute solution. Interestingly, the unique and non-trivial role of surface wettability on oriented self-assembly of Janus particles has remained largely unexplored. Here, the exclusive role of substrate wettability in directing the orientation of amphiphilic metal-polymer Bifacial spherical Janus particles, obtained by topo-selective metal deposition on colloidal Polymestyere (PS) particles, is explored by drop casting a dilute dispersion of the Janus colloids. While all particles orient with their polymeric (hydrophobic) and metallic (hydrophilic) sides facing upwards on hydrophilic and hydrophobic substrates respectively, they exhibit random orientation on a neutral substrate. The substrate wettability guided orientation of the Janus particles is captured using molecular dynamic simulation, which highlights that the arrangement of water molecules and their local densities near the substrate guide the specific orientation. Finally, it is shown that by spin coating it becomes possible to create a hexagonal close-packed array of the Janus colloids with specific orientation on differential wettability substrates. The results reported here open up new possibilities of substrate-wettability driven functional coatings of Janus particles, which has hitherto remained unexplored.

Light Trapping-Mediated Room-Temperature Gas Sensing by Ordered ZnO Nano Structures Decorated with Plasmonic Au Nanoparticles

An ordered array of 1D ZnO nanorods obtained by colloidal templating is shown to dramatically enhance the sensing response of NO _x at room temperature by confining light and creating periodic structures. The sensitivity is measured for a concentration varying from 2 to 10 ppm (response 53% at 10 ppm) at room temperature under white light illumination with ≈225 nm hole diameter. In contrast, structures with ≈450 nm hole size show better sensing under (response 98% at 10 ppm) elevated temperatures in dark conditions, which is attributed to the increased surface chemical interactions with NO _x molecules due to the porous nature and enhanced accessible surface area of ZnO nanorods. Further, the decoration of ZnO Nanorods with gold nanoparticles shows enhanced sensor performance (response 130% at 10 ppm) due to localized surface plasmon resonance under white light illumination. The findings may lead to new opportunities in the visible light-activated room-temperature NO _x sensors for healthcare applications.