Fabrication of large scale two-dimensional colloidal crystal of polystyrene particles by an interfacial self-ordering process

Simple and Rapid Monolayer Self-Assembly of Nanoparticles at the Air/Water Interface

Colloidal crystals and their two-dimensional (2D) monolayers, which have been commonly applied in nanosphere lithography, have the potential to revolutionize many engineering disciplines; however, current production techniques are hampered by a restricted preparation area, laborious procedures, and the need for advanced equipment. We propose a self-assembly-driven, simple, and low-cost method to prepare 2D colloidal nanosphere monolayers with excellent repeatability across wide regions. The self-assembly capability of colloidal polystyrene (PS) nanospheres at the air/water interface was utilized to transfer the assembled monolayers onto a substrate. This innovative method combines the advantages of methods that permit deposition at the air/water interface, such as Langmuir and drop coating, in order to deliver defect-free, simple-to-install, and simple-to-apply deposition across vast regions. Using field emission scanning electron microscopy and atomic force microscopy, the resultant coatings were characterized. The size of the nanospheres was reduced using an oxygen plasma etch process in an inductively coupled plasma reactive ion etching system, and the reflectance properties of the substrates for various nanosphere sizes were investigated. By evaporation of a thin gold capping layer on the templates, their optical properties were compared using surface-enhanced Raman scattering spectroscopy. This work has the potential to expand the use of nanosphere lithography by offering a simple and reproducible method that eliminates the need for complicated experimental setups and reduces the amount of material required for monolayer coating, thus lowering the cost.

Capillary Transfer of Self-Assembled Colloidal Crystals

Colloidal self-assembly has attracted significant interest in numerous applications including optics, electrochemistry, thermofluidics, and biomolecule templating. To meet the requirements of these applications, numerous fabrication methods have been developed. However, these are limited to narrow ranges of feature sizes, are incompatible with many substrates, and/or have low scalability, significantly limiting the use of colloidal self-assembly. In this work, we study the capillary transfer of colloidal crystals and demonstrate that this approach overcomes these limitations. Enabled by capillary transfer, we fabricate 2D colloidal crystals with nano-to-micro feature sizes spanning 2 orders of magnitude and on typically challenging substrates including those that are hydrophobic, rough, curved, or structured with microchannels. We developed and systemically validated a capillary peeling model, elucidating the underlying transfer physics. Due to its high versatility, good quality, and simplicity, this approach can expand the possibilities of colloidal self-assembly and enhance the performance of applications using colloidal crystals.

Metallic Plasmonic Array Structures: Principles, Fabrications, Properties, and Applications

The vast development of nanofabrication has spurred recent progress for the manipulation of light down to a region much smaller than the wavelength. Metallic plasmonic array structures are demonstrated to be the most powerful platform to realize controllable light-matter interactions and have found wide applications due to their rich and tunable optical performance through the morphology and parameter engineering. Here, various light-management mechanisms that may exist on metallic plasmonic array structures are described. Then, the typical techniques for fabrication of metallic plasmonic arrays are summarized. Next, some recent applications of plasmonic arrays are reviewed, including plasmonic sensing, surface-enhanced spectroscopies, plasmonic nanolasing, and perfect light absorption. Lastly, the existing challenges and perspectives for metallic plasmonic arrays are discussed. The aim is to provide guidance for future development of metallic plasmonic array structures.

Convex-Meniscus-Assisted Self-Assembly at the Air/Water Interface to Prepare a Wafer-Scale Colloidal Monolayer Without Overlap

Self-assembly at the air/water interface (AWI) has proven to be an efficient strategy for fabricating two-dimensional (2D) colloidal monolayers, which was widely used as the template for nanosphere lithography in nanophononics, optofluidics, and solar cell studies. However, the monolayers fabricated at the AWI usually suffer from a small domain area and quasi-double layer structure caused by submerged particles. To overcome this, we proposed an improved protocol to prepare 2D colloidal monolayers free of overlapping nanospheres at the AWI. Utilizing the stable suspension infusion to the water surface, a convex meniscus, whose height is related to viscous force, was formed adjoining the three-phase boundary. As a result of the resistance of the convex meniscus, the polystyrene nanospheres in the initial suspension directly self-assembled into a preliminary monolayer, which proved effective in preventing nanospheres' sinking and increasing the colloidal crystal domain size. An optimal parameter for transferring the monolayer was also developed based on the numerical simulation results. Finally, a wafer-scale monolayer, covered with less than one nanosphere per 100 μm × 100 μm area, was achieved on the desired substrate with an average domain size attaining centimeter scale. The high-quality 2D colloidal crystal may further promote the application of nanosphere lithography, especially in the fields that require a defect-free template.

Fabrication of floating colloidal crystal monolayers by convective deposition

HYPOTHESIS

Well-defined two-dimensional colloidal crystal monolayers (CCM) have numerous applications, such as photonic crystal, sensors, and masks for colloidal lithography. Therefore, significant effort was devoted to the preparation of preparing CCM. However, the fabrication of CCM that can float in the continuous phase and readily transfer to other substrate remains an elusive challenge.

EXPERIMENTS

In this article a facile approach to prepare floating CCM from polymeric colloids as building blocks is reported. The key to obtain floating CCM is the selection of an appropriate solvent to release the formed CCM from the substrate. There are two steps involved in the preparation of floating CCM: formation and peeling off.

FINDINGS

First, colloids are dispersed in a solvent. Evaporation of this solvent results in the formation of a meniscus structure of the air-liquid interface between the colloids that are on the substrate. The deformation of the meniscus gives rise to capillary attraction, driving the colloids together in a dense monolayer. Once a crystallization nucleus is formed, a convective flow containing additional colloids sets in, resulting in the formation of CCM on the substrate. Second, the remaining bulk dispersion is replaced by an extracting solvent that wets the substrate and peels the formed CCM off. The influence of the several solvents, the substrate materials, and the types of colloids on the CCM formation are investigated systematically. The robustness of the approach facilitates the preparation of CCM. Furthermore, the floating feature of the CCM in principle makes transfer of the CCM to other substrates possible, which broadens its applications.

A Facile Interfacial Self-Assembly of Crystalline Colloidal Monolayers by Tension Gradient

Many self-assembly approaches of colloidal monolayers have flourished but with some shortages, such as complexity, time-consumption, parameter sensitivity, and high-cost. This paper presents a facile, rapid, well-controlled, and low-cost method to prepare monolayers by directly adding silica particle suspensions containing water and ethanol to different liquids. A detailed analysis of the self-assembly process was conducted. The particles dove into water firstly, then moved up under the effect of the buoyancy and the tension gradient. The tension gradient induced the Marangoni convection and the relative motion between the water and the particles. At last, the particles were adsorbed at the air-water interface to minimize the free energy. The quality of the monolayers depended on the addition of sodium dodecyl sulfonate or ethanol in the water subphase. An interfacial polymerization of ethyl 2-cyanoacrylate was used to determine the contact angles of the particles at different subphase surfaces. The value of the detachment energy was positively associated with the contact angle and the surface tension. When the detachment energy decreased to a certain value, some particles detached from the surface, leading to the formation of a quasi-double layer. We also observed that the content of ethanol in suspensions influenced the arrangement of particles.

Crosslinking of floating colloidal monolayers

Abstract

Crosslinked colloidal monolayers are promising as templates, lithographic masks, filtration membranes, or membranes for controlled release rates in drug delivery. We demonstrate assembly of monodisperse micron-sized polystyrene (PS) beads at an air/water interface, which are transformed into crystalline monolayers using addition of surface-active agents. Vapor annealing methods with solvents (toluene and xylene) and crosslinking agents (divinylbenzene) were investigated regarding their ability to crosslink these floating monolayers directly at the interface, generating crosslinked membranes with crystal size up to 44 cm², domain size up to 1.9 mm², and nano-sized pores (100–300 nm). The demonstrated fabrication method emphasizes short fabrication time using a simple setup.

Graphical abstract

Electronic supplementary material

The online version of this article (doi:10.1007/s00706-017-1997-6) contains supplementary material, which is available to authorized users.

Approaches to self-assembly of colloidal monolayers: A guide for nanotechnologists

Self-assembly of quasi-spherical colloidal particles in two-dimensional (2D) arrangements is essential for a wide range of applications from optoelectronics to surface engineering, from chemical and biological sensing to light harvesting and environmental remediation. Several self-assembly approaches have flourished throughout the years, with specific features in terms of complexity of the implementation, sensitivity to process parameters, characteristics of the final colloidal assembly. Selecting the proper method for a given application amidst the vast literature in this field can be a challenging task. In this review, we present an extensive classification and comparison of the different techniques adopted for 2D self-assembly in order to provide useful guidelines for scientists approaching this field. After an overview of the main applications of 2D colloidal assemblies, we describe the main mechanisms underlying their formation and introduce the mathematical tools commonly used to analyse their final morphology. Subsequently, we examine in detail each class of self-assembly techniques, with an explanation of the physical processes intervening in crystallization and a thorough investigation of the technical peculiarities of the different practical implementations. We point out the specific characteristics of the set-ups and apparatuses developed for self-assembly in terms of complexity, requirements, reproducibility, robustness, sensitivity to process parameters and morphology of the final colloidal pattern. Such an analysis will help the reader to individuate more easily the approach more suitable for a given application and will draw the attention towards the importance of the details of each implementation for the final results.

Particle contact angles at fluid interfaces: pushing the boundary beyond hard uniform spherical colloids

Micro and nanoparticles at fluid interfaces have been attracting increasing interest in the last few decades as building blocks for materials, as mechanical and structural probes for complex interfaces and as models for two-dimensional systems. The three-phase contact angle enters practically all aspects of the particle behavior at the interface: its thermodynamics (binding energy to the interface), dynamics (motion and drag at the interface) and interactions with the interface (adsorption and wetting). Moreover, many interactions among particles at the interface also strongly depend on the contact angle. These concepts have been extensively discussed for non-deformable, homogeneous and mostly spherical particles, but recent progress in particle synthesis and fabrication has instead moved in the direction of producing more complex micro and nanoscale objects, which can be responsive, deformable, heterogenous and/or anisotropic in shape, surface chemistry and material properties. These new particles have a much greater potential for applications and new science, and the study of their behavior at interfaces has only very recently started. In this paper, we critically review the current state of the art of the experimental methods available to measure the contact angle of micro and nanoparticles at fluid interfaces, indicating their strengths and limitations. We then comment on new particle systems that are currently attracting increasing interest in relation to their adsorption and assembly at fluid interfaces and discuss if and which ones of the current techniques are suited to investigate their properties at interfaces. Based on this discussion, we will finally try to indicate a direction in which new experimental methods should develop in the future to tackle the new challenges posed by the novel types of particles that more and more often are used at interfaces.

Immobilization of Colloidal Monolayers at Fluid⁻Fluid Interfaces

Monolayers of colloidal particles trapped at an interface between two immiscible fluids play a pivotal role in many applications and act as essential models in fundamental studies. One of the main advantages of these systems is that non-close packed monolayers with tunable inter-particle spacing can be formed, as required, for instance, in surface patterning and sensing applications. At the same time, the immobilization of particles locked into desired structures to be transferred to solid substrates remains challenging. Here, we describe three different strategies to immobilize monolayers of polystyrene microparticles at water⁻decane interfaces. The first route is based on the leaking of polystyrene oligomers from the particles themselves, which leads to the formation of a rigid interfacial film. The other two rely on in situ interfacial polymerization routes that embed the particles into a polymer membrane. By tracking the motion of the colloids at the interface, we can follow in real-time the formation of the polymer membranes and we interestingly find that the onset of the polymerization reaction is accompanied by an increase in particle mobility determined by Marangoni flows at the interface. These results pave the way for future developments in the realization of thin tailored composite polymer-particle membranes.

Under-Liquid Self-Assembly of Submerged Buoyant Polymer Particles

The self-assembly of submerged cold-plasma-treated polyethylene beads (PBs) is reported. The plasma-treated immersed millimetrically sized PBs formed well-ordered 2D quasicrystalline structures. The submerged floating of "light" (buoyant) PBs is possible because of the energy gain achieved by the wetting of the high-energy plasma-treated polymer surface prevailing over the energy loss due to the upward climb of the liquid over the beads. The capillary "immersion" attraction force is responsible for the observed self-assembly. The observed 2D quasicrystalline structures demonstrate "dislocations" and "point defects". The mechanical vibration of self-assembled rafts built of PBs leads to the healing of point defects. The immersion capillary lateral force governs the self-assembly, whereas the elastic force is responsible for the repulsion of polymer beads.

Quantitative Metrics for Assessing Positional and Orientational Order in Colloidal Crystals

Although there are numerous self-assembly techniques to prepare colloidal crystals, there is great variability in the methods used to characterize order and disorder in these materials. We assess different kinds of structural order from more than 70 two-dimensional microscopy images of colloidal crystals produced by many common methods, including spin-coating, dip-coating, convective assembly, electrophoretic assembly, and sedimentation. Our suite of analysis methods includes measures for both positional and orientational order. The benchmarks are two-dimensional lattices that we simulated with different degrees of controlled disorder. We find that translational measures are adequate for characterizing small deviations from perfect order, whereas orientational measures are more informative for polycrystalline and highly disordered crystals. Our analysis presents a unified strategy for comparing structural order among different colloidal crystals and establishes benchmarks for future studies.

Ultra-high ordered, centimeter scale preparation of microsphere Langmuir films

Controlling the preparation of nano/microsphere monolayers on large areas remains a difficult task but is crucial for several fabrication methods of highly-ordered periodic nanostructures. We demonstrate the preparation of ordered monolayers of few square centimeters with an extremely high coverage ratio (>98%) by implementing a modified protocol (MP) Langmuir Blodgett (LB) technique. We use octadecyl type hydrocarbon (C18) functionalized spherical particles (polystyrene and silica) with diameters in the range 1-5 μm, and a selected mixture of solvents for accurate control of the surface tension and particles' mobility at the water surface. This leads to a delicate growth of crystal-like monolayers which are subsequently transferred to glass or silicon substrates. While operating the Langmuir-Blodgett trough, a key enabling the quality enhancement resides not only on surface tension measurements but also on simple visual inspections of the water surface supporting the monolayer. The protocol yields a strong reduction of sensitivity to thermodynamical and mechanical disturbances leading to a robust method that could be automated by adding a feedback on the operated system based real-time image processing. A simple analytical approach is used to explain why this MP-LB technique is more appropriate in growing micrometric-sized objects in comparison to standard protocols optimized for the preparation of molecular films.

Well-defined colloidal crystal films from the 2D self-assembly of core–shell semi-soft nanoparticles

Silica core–polymer shell particles are obtained from surface mediated RAFT polymerisation and assembled into ordered 2D colloidal crystals.

Topology assisted self-organization of colloidal nanoparticles: application to 2D large-scale nanomastering

Our aim was to elaborate a novel method for fully controllable large-scale nanopatterning. We investigated the influence of the surface topology, i.e., a pre-pattern of hydrogen silsesquioxane (HSQ) posts, on the self-organization of polystyrene beads (PS) dispersed over a large surface. Depending on the post size and spacing, long-range ordering of self-organized polystyrene beads is observed wherein guide posts were used leading to single crystal structure. Topology assisted self-organization has proved to be one of the solutions to obtain large-scale ordering. Besides post size and spacing, the colloidal concentration and the nature of solvent were found to have a significant effect on the self-organization of the PS beads. Scanning electron microscope and associated Fourier transform analysis were used to characterize the morphology of the ordered surfaces. Finally, the production of silicon molds is demonstrated by using the beads as a template for dry etching.

Direct visualization of the interfacial position of colloidal particles and their assemblies

A method for direct visualization of the position of nanoscale colloidal particles at air-water interfaces is presented. After assembling hard (polystyrene, poly(methyl methacrylate), silica) or soft core-shell gold-hydrogel composite (Au@PNiPAAm) colloids at the air-water interface, butylcyanoacrylate is introduced to the interface via the gas phase. Upon contact with water, an anionic polymerization reaction of the monomer is initiated and a film of poly(butylcyanoacrylate) (PBCA) is generated, entrapping the colloids at their equilibrium position at the interface. We apply this method to investigate the formation of complex, binary assembly structures directly at the interface, to visualize soft, nanoscale hydrogel colloids in the swollen state, and to visualize and quantify the equilibrium position of individual micro- and nanoscale colloids at the air-water interface depending of the amount of charge present on the particle surface. We find that the degree of deprotonation of the carboxyl group shifts the air-water contact angle, which is further confirmed by colloidal probe atomic force microscopy. Remarkably, the contact angles determined for individual colloidal particles feature a significant distribution that greatly exceeds errors attributable to the size distribution of the colloids. This finding underlines the importance of accessing soft matter on an individual particle level.

Preventing bacterial colonization using colloidal crystals

A monolayer of polystyrene particles inhibits colony formation of Pseudomonas aeruginosa, and causes adsorption in particular locations.