Improved development procedure to enhance the stability of microstructures created by two-photon polymerization

Natural functional surfaces often rely on unique nano- and micropatterns. To mimic such surfaces successfully, patterning techniques are required that enable the fabrication of three-dimensional structures at the nanoscale. It has been reported that two-photon polymerization (TPP) is a suitable method for this. However, polymer structures fabricated by TPP often tend to shrink and to collapse during the fabrication process. In particular, delicate structures suffer from their insufficient mechanical stability against capillary forces which mainly arisein the fabrication process during the evaporation of the developer and rinsing liquids. Here, we report a modified development approach, which enables an additional UV-treatment to post cross-link created structures before they are dried. We tested our approach on nanopillar arrays and microscopic pillar structures mimicking the moth-eye and the gecko adhesives, respectively. Our results indicate a significant improvement of the me- chanical stability of the polymer structures, resulting in fewer defects and reduced shrinkage of the structures

Controlled Self-Assembly of Hexagonal Nanoparticle Patterns on Nanotopographies

Diblock copolymer micelle nanolithography (BCML) is a versatile and efficient method to cover large surface areas with hexagonally ordered arrays of metal nanoparticles, in which the nanoparticles are equally spaced. However, this method falls short of providing a controlled allocation of such regular nanoparticle arrays with specific spacing into micropatterns. We present here a quick and high-throughput method to generate quasi-hexagonal nanoparticle structures with well-defined interparticle spacing on segments of nanotopographic Si substrates. The topographic height of these segments plays a dominant role in dictating the spacing between the gold nanoparticles, as the nanoparticle arrangement is controlled by immersion forces and by their self-assembly within the segments. Our novel strategy of employing a single-step BCML routine is a highly promising method for the fabrication of regular gold nanopatterns in micropatterns for a wide range of applications

Bio-inspired photonic surfaces by enhanced Two-Photon Lithography

2D and 3D photonic crystals active in the visible wave range are highly interesting for applications, such as waveguiding elements, sensors, or counterfeiting features. However, the tuneable production of such crystals with the current processes is challenging. Two-photon lithography (TPL), which is mainly used to manufacture microstructures, offers this versatility but currently suffers from insufficient structure resolution and mechanical stability for sub-micrometre structures. In the course of this work several novel approaches including an improved development and standing wave enhanced two-photon lithography are presented. These approaches improve the structure resolution and quality, and thus, allow stable features sizes down to 120 nm in horizontal and 45 nm in vertical direction. The new capabilities were used to fabricate distinct photonic crystals inspired by the 2D-pillar grating found on the moth eye and the 3D ‘Christmas tree’-like structures covering the wings of the Morpho-butterflies. Resulting structures were analysed in detail regarding their sizes and optical properties, showing highly effective diffraction, promising anti-reflection properties, and outstanding angle independent iridescence. The experimental work is supported by correlated simulations investigating the optical properties, structures sizes, and the influences of different experimental parameter settings relevant for the fabrication.2D und 3D photonische Kristalle, die mit sichtbarem Licht interagieren, finden zunehmend Verwendung in Wellenleitelementen, bei Sensoren oder Sicherheitsmerkmalen. Jedoch ist die Herstellung solcher Kristalle mit den derzeitigen Methoden herausfordernd und komplex. Eine vielversprechende Technik wäre die Zweiphotonenlithografie, die gegenwärtig für die flexible Herstellung von Mikrostrukturen eingesetzt wird. Für die Fertigung von photonischen Kristallen, die hochqualitative Strukturen im Nanometerbereich voraussetzen, besitzt diese Technik jedoch keine ausreichende Strukturauflösung. In dieser Arbeit werden neue Verfahren wie ein verbesserter Entwicklungsprozess oder die Zweiphotonenlithograpie mit integrierter stehender Welle präsentiert. Diese Methoden erlauben die Herstellung von Submikrometerstrukturen mit einem Limit von 120 nm in horizontaler und 45°nm in vertikaler Richtung. Diese verbesserte Auflösung wurde genutzt, um zwei natürliche photonische Kristalle, die 2D optischen Gitter der Mottenaugen und die geschichteten 3D Strukturen der Morpho-Schmetterlinge, künstlich nachzuahmen. Die Untersuchung der optischen Eigenschaften dieser Strukturen zeigten hocheffektive Beugungs- und vielversprechende Antireflexeigenschaften sowie eine herausragende, winkelunabhängige Strukturfarbe. Die experimentelle Arbeit wurde durch Simulationen wie die der optischen Eigenschaften, der erwarteten Strukturgrößen und der Einflüsse von Prozessparametern auf die Herstellung unterstützt

Switchable Adhesion in Vacuum Using Bio-Inspired Dry Adhesives

Suction based attachment systems for pick and place handling of fragile objects like glass plates or optical lenses are energy-consuming and noisy and fail at reduced air pressure, which is essential, e.g., in chemical and physical vapor deposition processes. Recently, an alternative approach toward reversible adhesion of sensitive objects based on bioinspired dry adhesive structures has emerged. There, the switching in adhesion is achieved by a reversible buckling of adhesive pillar structures. In this study, we demonstrate that these adhesives are capable of switching adhesion not only in ambient air conditions but also in vacuum. Our bioinspired patterned adhesive with an area of 1 cm² provided an adhesion force of 2.6 N ± 0.2 N in air, which was reduced to 1.9 N ± 0.2 N if measured in vacuum. Detachment was induced by buckling of the structures due to a high compressive preload and occurred, independent of air pressure, at approximately 0.9 N ± 0.1 N. The switch in adhesion was observed at a compressive preload between 5.6 and 6.0 N and was independent of air pressure. The difference between maximum adhesion force and adhesion force after buckling gives a reasonable window of operation for pick and place processes. High reversibility of the switching behavior is shown over 50 cycles in air and in vacuum, making the bioinspired switchable adhesive applicable for handling operations of fragile objects

Nanopillar Diffraction Gratings by Two-Photon Lithography

Two-dimensional photonic structures such as nanostructured pillar gratings are useful for various applications including wave coupling, diffractive optics, and security features. Two-photon lithography facilitates the generation of such nanostructured surfaces with high precision and reproducibility. In this work, we report on nanopillar diffraction gratings fabricated by two-photon lithography with various laser powers close to the polymerization threshold of the photoresist. As a result, defect-free arrays of pillars with diameters down to 184 nm were fabricated. The structure sizes were analyzed by scanning electron microscopy and compared to theoretical predictions obtained from Monte Carlo simulations. The optical reflectivities of the nanopillar gratings were analyzed by optical microscopy and verified by rigorous coupled-wave simulations