Focused laser-induced marangoni dewetting for patterning polymer thin films

Steerable mass transport in a photoresponsive system for advanced anticounterfeiting

Numerous anticounterfeiting platforms using photoresponsive materials have been designed to improve information security, enabling applications in anticounterfeiting technology. However, fabricating sophisticated micro/nanostructures using bidirectional mass transport to achieve advanced anticounterfeiting remains challenging. Here, we propose one strategy to achieve steerable mass transport in a photoresponsive system with the assistance of solvent vapor at room temperature. Upon optimizing the host-guest ratio and the width of photoisomerized areas, wettability gradient is acquired just photo-patterning once, then bidirectional mass transport is realized due to the competition of mass transport induced by surface energy gradient of the material itself and flow of the solvent on the film surface with wettability gradient. Taking advantage of the interaction between solvent and film surface with wettability gradient, this bidirectional polymer flow has been successfully applied in multi-mode anticounterfeiting. This work paves a promising avenue toward high-level information storage in soft materials, demonstrating the potential applications in anticounterfeiting.

Universal Approach to Integrating Reduced Graphene Oxide into Polymer Electronics

Flexible electronics have sparked significant interest in the development of electrically conductive polymer-based composite materials. While efforts are being made to fabricate these composites through laser integration techniques, a versatile methodology applicable to a broad range of thermoplastic polymers remains elusive. Moreover, the underlying mechanisms driving the formation of such composites are not thoroughly understood. Addressing this knowledge gap, our research focuses on the core processes determining the integration of reduced graphene oxide (rGO) with polymers to engineer coatings that are not only flexible and robust but also exhibit electrical conductivity. Notably, we have identified a particular range of laser power densities (between 0.8 and 1.83 kW/cm2), which enables obtaining graphene polymer composite coatings for a large set of thermoplastic polymers. These laser parameters are primarily defined by the thermal properties of the polymers as confirmed by thermal analysis as well as numerical simulations. Scanning electron microscopy with elemental analysis and X-ray photoelectron spectroscopy showed that conductivity can be achieved by two mechanisms-rGO integration and polymer carbonization. Additionally, high-speed videos allowed us to capture the graphene oxide (GO) modification and melt pool formation during laser processing. The cross-sectional analysis of the laser-processed samples showed that the convective flows are present in the polymer substrate explaining the observed behavior. Moreover, the practical application of our research is exemplified through the successful assembly of a conductive wristband for wearable devices. Our study not only fills a critical knowledge gap but also offers a tangible illustration of the potential impact of laser-induced rGO-polymer integration in materials science and engineering applications.

Photochemically Induced Marangoni Patterning of Polymer Bilayers

Surface-tension gradients created along a polymer film by patterned photochemical reactions are a powerful tool for creating surface topography. Here, we use mathematical modeling to explore a strategy for patterning photochemically inactive polymers by coupling a light-sensitive and light-insensitive polymer to form a polymer bilayer. The light-sensitive polymer forms the top layer, and the most dominant surface-tension gradients are introduced at the interface between this layer and air. Lubrication theory is used to derive nonlinear partial differential equations describing the heights of each layer, and linear analysis and nonlinear simulations are performed to characterize interface dynamics. Patterns form at both the polymer-air and polymer-polymer interfaces at early thermal annealing times as a result of Marangoni stresses but decay on prolonged thermal annealing as a result of the dissipative mechanisms of capillary leveling and photoproduct diffusion, thus setting a limit to the maximum individual layer deformation. Simulations also show that the bottom-layer features can remain "trapped", i.e., exhibit no significant decay, even while the top layer topography has dissipated. We study the effects of two key parameters, the initial thickness ratio and the viscosity ratio of the two polymers, on the maximum deformation attained in the bottom layer and the time taken to attain this deformation. We identify regions of parameter space where the maximum bottom-layer deformation is enhanced and the attainment time is reduced. Overall, our study provides guidelines for designing processes to pattern photochemically inactive polymers and create interfacial topography in polymer bilayers.

Multicomponent Copolymer Planar Membranes with Nanoscale Domain Separation

Domain separation is crucial for proper cellular function and numerous biomedical technologies, especially artificial cells. While phase separation in hybrid membranes containing lipids and copolymers is well-known, the membranes' overall stability, limited by the lipid part, is hindering the technological applications. Here, we introduce a fully synthetic planar membrane undergoing phase separation into domains embedded within a continuous phase. The mono- and bilayer membranes are composed of two amphiphilic diblock copolymers (PEO₄₅-b-PEHOx₂₀ and PMOXA₁₀-b-PDMS₂₅) with distinct properties and mixed at various concentrations. The molar ratio of the copolymers in the mixture and the nature of the solid support were the key parameters inducing nanoscale phase separation of the planar membranes. The size of the domains and resulting morphology of the nanopatterned surfaces were tailored by adjusting the molar ratios of the copolymers and transfer conditions. Our approach opens new avenues for the development of biomimetic planar membranes with a nanoscale texture.

Thermocapillary dewetting-based dynamic spatial light modulator

Dynamic spatial light modulators (SLMs) are capable of precisely modulating a beam of light by tuning the phase or intensity of an array of pixels in parallel. They can be utilized in applications ranging from image projection to beam front aberration and microscopic particle manipulation with optical tweezers. However, conventional dynamic SLMs are typically incompatible with high-power sources, as they contain easily damaged optically absorbing components. To address this, we present an SLM that utilizes a viscous film with a local thickness controlled via thermocapillary dewetting. The film is reflowable and can cycle through different patterns, representing, to the best of our knowledge, the first steps towards a dynamic optical device based on the thermocapillary dewetting mechanism.

Laser photonic nanojets triggered thermoplasmonic micro/nanofabrication of polymer materials for enhanced resolution

Micro/nanofabrication of polymer materials is of interest for micro/nanofluidic systems. Due to the optical diffraction limit, it remains a challenge to achieve nanoscale resolution fabrication using an ordinary continuous-wave laser system. In this study, we therefore propose a laser photonic nanojet-based micro/nanofabrication method for polymer materials using a low-power and low-cost continuous-wave laser. The photonic nanojets were produced using glass microspheres. Moreover, a thermoplasmonic effect was employed by depositing a gold layer beneath the polymer films. By applying the photonic nanojet triggered thermoplasmonics, sub-micrometer surface structures, as well as their arrays, were fabricated with a laser power threshold value down to 10 mW. The influences of the microsphere diameters, and thicknesses of gold layers and polymer films on the fabricated microstructures were systematically investigated, which aligns well with the finite-difference time-domain simulation results.

Self-limiting electrospray deposition on polymer templates

Electrospray deposition (ESD) applies a high voltage to liquids flowing through narrow capillaries to produce monodisperse generations of droplets down to hundreds of nanometers in diameter, each carrying a small amount of the delivered solute. This deposition method has been combined with insulated stencil masks for fabricating micropatterns by spraying solutions containing nanoparticles, polymers, or biomaterials. To optimize the fabrication process for micro-coatings, a self-limiting electrospray deposition (SLED) method has recently been developed. Here, we combine SLED with a pre-existing patterned polymer film to study SLED’s fundamental behavior in a bilayer geometry. SLED has been observed when glassy insulating materials are sprayed onto conductive substrates, where a thickness-limited film forms as charge accumulates and repels the arrival of additional charged droplets. In this study, polystyrene (PS), Parylene C, and SU-8 thin films of varying thickness on silicon are utilized as insulated spraying substrates. Polyvinylpyrrolidone (PVP), a thermoplastic polymer is sprayed below its glass transition temperature (T_g) to investigate the SLED behavior on the pre-deposited insulating films. Furthermore, to examine the effects of in-plane confinement on the spray, a microhole array patterned onto the PS thin film by laser dewetting was sprayed with dyed PVP in the SLED mode. This was then extended to an unmasked electrode array showing that masked SLED and laser dewetting could be used to target microscale regions of conventionally-patterned electronics.

Macroscopic Alignment of Block Copolymers on Silicon Substrates by Laser Annealing

Laser annealing is a competitive alternative to conventional oven annealing of block copolymer (BCP) thin films enabling rapid acceleration and precise spatial control of the self-assembly process. Localized heating by a moving laser beam (zone annealing), taking advantage of steep temperature gradients, can additionally yield aligned morphologies. In its original implementation it was limited to specialized germanium-coated glass substrates, which absorb visible light and exhibit low-enough thermal conductivity to facilitate heating at relatively low irradiation power density. Here, we demonstrate a recent advance in laser zone annealing, which utilizes a powerful fiber-coupled near-IR laser source allowing rapid BCP annealing over a large area on conventional silicon wafers. The annealing coupled with photothermal shearing yields macroscopically aligned BCP films, which are used as templates for patterning metallic nanowires. We also report a facile method of transferring laser-annealed BCP films onto arbitrary surfaces. The transfer process allows patterning substrates with a highly corrugated surface and single-step rapid fabrication of multilayered nanomaterials with complex morphologies.

Subnanometer imaging and controlled dynamical patterning of thermocapillary driven deformation of thin liquid films

Exploring and controlling the physical factors that determine the topography of thin liquid dielectric films are of interest in manifold fields of research in physics, applied mathematics, and engineering and have been a key aspect of many technological advancements. Visualization of thin liquid dielectric film topography and local thickness measurements are essential tools for characterizing and interpreting the underlying processes. However, achieving high sensitivity with respect to subnanometric changes in thickness via standard optical methods is challenging. We propose a combined imaging and optical patterning projection platform that is capable of optically inducing dynamical flows in thin liquid dielectric films and plasmonically resolving the resulting changes in topography and thickness. In particular, we employ the thermocapillary effect in fluids as a novel heat-based method to tune plasmonic resonances and visualize dynamical processes in thin liquid dielectric films. The presented results indicate that light-induced thermocapillary flows can form and translate droplets and create indentation patterns on demand in thin liquid dielectric films of subwavelength thickness and that plasmonic microscopy can image these fluid dynamical processes with a subnanometer sensitivity along the vertical direction.

Application of Plasmon-Induced Lithography for Creation of a Residual-Free Pattern and Simple Surface Modifications

Here, we propose a plasmon-induced redistribution of a thin polymer layer as a unique way for a residual layer-free lithographic approach. In particular, we demonstrate an ultrafast area-selective fabrication method using a low-intensity visible laser irradiation to direct the polymer mass flow, under the plasmon-active substrates. Plasmon-supported substrates were created by thermal annealing of Ag thin films and covered by thin polystyrene layers. Then, laser beam writing (LBW) was applied to introduce a surface tension gradient through the local plasmon heating. As a result, polystyrene was completely removed from the irradiated place, without any residual layer. The proposed approach does not require any additional development steps, such as solvent or plasma treatment. To demonstrate the advantages of the proposed technique, we implemented the LBW-patterned structures for further spatially selective surface functionalization, including the metal deposition, spontaneous thiol grafting, and electrochemical deposition of ordered polypyrrole array.

Photoinitiated Marangoni flow morphing in a liquid crystalline polymer film directed by super-inkjet printing patterns

Slight contaminations existing in a material lead to substantial defects in applied paint. Herein, we propose a strategy to convert this nuisance to a technologically useful process by using an azobenzene-containing side chain liquid crystalline (SCLCP) polymer. This method allows for a developer-free phototriggered surface fabrication. The mass migration is initiated by UV-light irradiation and directed by super-inkjet printed patterns using another polymer on the SCLCP film surface. UV irradiation results in a liquid crystal-to-isotropic phase transition, and this phase change immediately initiates a mass migration to form crater or trench structures due to the surface tension instability known as Marangoni flow. The transferred volume of the film reaches approximately 440-fold that of the polymer ink, and therefore, the printed ink pattern acts as a latent image towards the amplification of surface morphing. This printing-aided photoprocess for surface inscription is expected to provide a new platform of polymer microfabrication.

Micropatterning of Metal Nanoparticle Ink by Laser-Induced Thermocapillary Flow

Selective laser sintering of metal nanoparticle ink is a low-temperature and non-vacuum technique developed for the fabrication of patterned metal layer on arbitrary substrates, but its application to a metal layer composed of large metal area with small voids is very much limited due to the increase in scanning time proportional to the metal pattern density. For the facile manufacturing of such metal layer, we introduce micropatterning of metal nanoparticle ink based on laser-induced thermocapillary flow as a complementary process to the previous selective laser sintering process for metal nanoparticle ink. By harnessing the shear flow of the solvent at large temperature gradient, the metal nanoparticles are selectively pushed away from the scanning path to create metal nanoparticle free trenches. These trenches are confirmed to be stable even after the complete process owing to the presence of the accompanying ridges as well as the bump created along the scanning path. As a representative example of a metal layer with large metal area and small voids, dark-field photomask with Alphabetic letters are firstly created by the proposed method and it is then demonstrated that the corresponding letters can be successfully reproduced on the screen by an achromatic lens.

Laser patterning of transparent polymers assisted by plasmon excitation

Plasmon-assisted lithography of thin transparent polymer films, based on polymer mass-redistribution under plasmon excitation, is presented. The plasmon-supported structures were prepared by thermal annealing of thin Ag films sputtered on glass or glass/graphene substrates. Thin films of polymethylmethacrylate, polystyrene and polylactic acid were then spin-coated on the created plasmon-supported structures. Subsequent laser beam writing, at the wavelength corresponding to the position of plasmon absorption, leads to mass redistribution and patterning of the thin polymer films. The prepared structures were characterized using UV-Vis spectroscopy and confocal and AFM microscopy. The shape of the prepared structures was found to be strongly dependent on the substrate type. The mechanism leading to polymer patterning was examined and attributed to the plasmon-heating. The proposed method makes it possible to create different patterns in polymer films without the need for wet technological stages, powerful light sources or a change in the polymer optical properties.

Exploiting the Marangoni Effect To Initiate Instabilities and Direct the Assembly of Liquid Metal Filaments

Utilization of the Marangoni effect in a liquid metal is investigated, focusing on initiating instabilities to direct material assembly via the Rayleigh-Plateau instability. Thin (2 nm) copper (Cu) films are lithographically patterned onto thick (12 nm) nickel (Ni) strips to induce a surface energy gradient at the maximum wavelength of the filament instability predicted by Rayleigh-Plateau instability analysis. The pattern is irradiated with an 18 ns pulsed laser such that the pattern melts and the resultant Ni-Cu surface tension gradient induces Marangoni flows due to the difference in surface energies. The experimental results, supported by extensive direct numerical simulations, demonstrate that the Marangoni flow exceeds the capillary flow induced by the initial geometry, guiding instabilities such that final nanoparticle location is directed toward the regions of higher surface energy (Ni regions). Our work shows a route for manipulation, by means of the Marangoni effect, to direct the evolution of the surface instabilities and the resulting pattern formation.

Laser Writing Block Copolymer Self-Assembly on Graphene Light-Absorbing Layer

Recent advance of high-power laser processing allows for rapid, continuous, area-selective material fabrication, typically represented by laser crystallization of silicon or oxides for display applications. Two-dimensional materials such as graphene exhibit remarkable physical properties and are under intensive development for the manufacture of flexible devices. Here we demonstrate an area-selective ultrafast nanofabrication method using low intensity infrared or visible laser irradiation to direct the self-assembly of block copolymer films into highly ordered manufacturing-relevant architectures at the scale below 12 nm. The fundamental principles underlying this light-induced nanofabrication mechanism include the self-assembly of block copolymers to proceed across the disorder-order transition under large thermal gradients, and the use of chemically modified graphene films as a flexible and conformal light-absorbing layers for transparent, nonplanar, and mechanically flexible surfaces.