Patrick Mueller

Wavelength selective polymer network formation of end-functional star polymers.

A wavelength selective technique for light-induced network formation based on two photo-active moieties, namely ortho-methylbenzaldehyde and tetrazole is introduced. The network forming species are photo-reactive star polymers generated via reversible activation fragmentation chain transfer (RAFT) polymerization, allowing the network to be based on almost any vinylic monomer. Direct laser writing (DLW) allows to form any complex three-dimensional structure based on the photo-reactive star polymers.

Cleaving Direct-Laser-Written Microstructures on Demand

Using an advanced functional photoresist we introduce direct-laser-written (DLW) 3D microstructures capable of complete degradation on demand. The networks consist exclusively of reversible bonds, formed by irradiation of a phenacyl sulfide linker, giving disulfide bonds in a radical-free step-growth polymerization via a reactive thioaldehyde. The bond formation was verified in solution by ESI-MS. To induce cleavage, dithiothreitol causes a thiol-disulfide exchange, erasing the written structure. The mild cleavage of the disulfide network is highly orthogonal to other, for example, acrylate-based DLW structures. To emphasize this aspect, DLW structures were prepared incorporating reversible structural elements into a non-reversible acrylate-based standard scaffold, confirming subsequent selective cleavage. The high lateral resolution achievable was verified by the preparation of well-defined line gratings with line separations of down to 300 nm.

Molecular Switch for Sub-Diffraction Laser Lithography by Photoenol Intermediate-State Cis–Trans Isomerization

Recent developments in stimulated-emission depletion (STED) microscopy have led to a step change in the achievable resolution and allowed breaking the diffraction limit by large factors. The core principle is based on a reversible molecular switch, allowing for light-triggered activation and deactivation in combination with a laser focus that incorporates a point or line of zero intensity. In the past years, the concept has been transferred from microscopy to maskless laser lithography, namely direct laser writing (DLW), in order to overcome the diffraction limit for optical lithography. Herein, we propose and experimentally introduce a system that realizes such a molecular switch for lithography. Specifically, the population of intermediate-state photoenol isomers of α-methyl benzaldehydes generated by two-photon absorption at 700 nm fundamental wavelength can be reversibly depleted by simultaneous irradiation at 440 nm, suppressing the subsequent Diels-Alder cycloaddition reaction which constitutes the chemical core of the writing process. We demonstrate the potential of the proposed mechanism for STED-inspired DLW by covalently functionalizing the surface of glass substrates via the photoenol-driven STED-inspired process exploiting reversible photoenol activation with a polymerization initiator. Subsequently, macromolecules are grown from the functionalized areas and the spatially coded glass slides are characterized by atomic-force microscopy. Our approach allows lines with a full-width-at-half-maximum of down to 60 nm and line gratings with a lateral resolution of 100 nm to be written, both surpassing the diffraction limit.

Photo-Induced Click Chemistry for DNA Surface Structuring by Direct Laser Writing

Oligonucleotides containing photo-caged dienes were prepared and shown to react quantitatively in a light-induced Diels-Alder cycloaddition with functional maleimides in aqueous solution within minutes. Due to its high yield and fast rate, the reaction was exploited for DNA surface patterning with sub-micrometer resolution employing direct laser writing (DLW). Functional DNA arrays were written by direct laser writing (DLW) in variable patterns, which were further encoded with fluorophores and proteins through DNA directed immobilization. This mild and efficient light-driven platform technology holds promise for the fabrication of complex bioarrays with sub-micron resolution.

Direct Laser Writing of 3D Nanostructures Using a 405nm Laser Diode

Direct laser writing (DLW) is a well-known and established technology for fabricating 3D micro- and nanostructures. Usually, red femtosecond laser sources with wavelengths around 800 nm are used. Here, we use a laser diode with a wavelength of 405 nm as the exciting laser source and thus improve structures in terms of decreasing feature size and line distance by exploiting the linear wavelength dependence of the Sparrow resolution limit. A nonlinear multi-photon polymerization process is necessary for manufacturing true 3D structures. We investigate different photoresists and measure their nonlinearities by variation of the electronic pulse scheme of the laser. We observe an adequately high nonlinearity in a resist system based on the monomer pentaerythritol triacrylate. To benefit from the improved theoretical resolution of the smaller wavelength, it is necessary to achieve a close to diffraction-limited focal spot which we have confirmed by measuring the point spread function of the objective lens and comparing it to numerical simulations. In order to prove the performance of the system, we fabricate benchmark structures and characterize them with different experimental methods. Line gratings and point arrays are written to investigate 2D resolution and feature sizes. To characterize the capabilities in 3D, we fabricate woodpile photonic crystals that show a photonic stop band in the visible. The achievable lattice constants in both 2D and 3D are considerably smaller than in previous work with red femtosecond lasers, proving the success of the wavelength-reduction approach. Previous work using the conceptually diffraction-unlimited STED technology is also outperformed.