Pixelated Physical Unclonable Functions through Capillarity-Assisted Particle Assembly.

Recent years have shown the need for trustworthy, unclonable, and durable tokens as proof of authenticity for a large variety of products to combat the economic cost of counterfeits. An excellent solution is physical unclonable functions (PUFs), which are intrinsically random objects that cannot be recreated, even if illegitimate manufacturers have access to the same methods. We propose a robust and simple way to make pixelated PUFs through the deposition of a random mixture of fluorescent colloids in a predetermined lattice using capillarity-assisted particle assembly. As the encoding capacity scales exponentially with the number of deposited particles, we can easily achieve encoding capacities above 10700 for sub millimeter scale samples, where the pixelated nature of the PUFs allows for easy and trustworthy readout. Our method allows for the PUFs to be transferred to, and embedded in, a range of transparent materials to protect them from environmental challenges, leading to improved stability and robustness and allowing their implementation for a large number of different applications.

Controlling liquid crystal boojum defects on fixed microparticle arrays via capillarity-assisted particles assembly.

HYPOTHESIS: Colloidal particles in nematic liquid crystals (LCs) are of high interest for self-assembly of soft matter systems. When two free particles approach within a uniaxially-oriented nematic LC, an elastic force is generated due to the distorted nematic director configuration around them, allowing particles to self-assemble by an attractive force. We hypothesize that if particles are immobilized, repulsive forces emerge instead, causing the deflection of the interacting defects to compensate for the energy increase. EXPERIMENTS: We fabricated tailored arrays of spherical silica microparticles via capillarity-assisted particle assembly (CAPA) to investigate the interactions of defects as a function of particle separation. By transferring the particle arrays from the CAPA templates to a glass substrate, we studied interacting boojum defect textures within thin LC films sandwiched between two substrates using polarized optical microscopy (POM). FINDINGS: We observed deflected boojum defects on arrays of fixed silica particles, confirming our hypothesis that the elastic repulsive force between the particles affects the defect orientation. The nematic director configuration is reconstructed by Landau-de Gennes q-tensor modeling, and simulated POM images are obtained by the Jones-Matrix method. Our results provide a new platform for controlling defect interactions and pave the way for future work to study topology and implement new defect based applications in LC films.

Chiral Optoelectronic Functionalities via DNA-Organic Semiconductor Complex.

We fabricate the bio-organic field-effect transistor (BOFET) with the DNA-perylene diimide (PDI) complex, which shows unusual chiroptical and electrical functionalities. DNA is used as the chirality-inducing scaffold and the charge-injection layer. The shear-oriented film of the DNA-PDI complex shows how the large-area periodic molecular orientation and the charge transport are related, generating drastically different optoelectronic properties at each DNA/PDI concentration. The resultant BOFET reveals chiral structures with a high charge carrier mobility, photoresponsivity, and photosensitivity, reaching 3.97 cm2 V-1 s-1, 1.18 A W-1, and 7.76 × 103, respectively. Interestingly, the BOFET enables the definitive response under the handedness of circularly polarized light with a high dissymmetry factor of approximately +0.14. This work highlights the natural chirality and anisotropy of DNA material and the electron conductivity of organic semiconducting molecules to be mutually used in significant chiro-optoelectronic functions as an added ability to the traditional OFET.

Hierarchically Fabricated Amyloid Fibers via Evaporation-Induced Self-Assembly.

Multiscale hierarchical nano- and microstructures of amyloid fibrils are fabricated by evaporation-induced self-assembly combined with topographic surface patterning techniques. The continuous stick-and-slip motion induces uniaxial alignment of amyloid fibrils characterized by high orientational order during the drying process. The optical textures of the resultant amyloid aggregates are directly observed by polarized optical microscopy (POM) and atomic force microscopy (AFM). The resulting fiber structure can be tuned by varying the width of the topographic pattern, e.g., the microchannel width, inducing different separation between the deposited amyloid fibers on the glass substrate. Additionally, amyloid fibrils are decorated with gold nanoparticles to produce conductive microwires showing good conductivity (∼10-3 S/m). The finely controlled deposited amyloid fibers presented here can show a way to use naturally-abundant biomaterials for practical applications such as nanowires and sensors.

Fabrication of Bilayer Dichroic Films Using Liquid Crystal Materials for Multiplex Applications.

A bilayer dichroic-doped liquid crystal (BDLC) film is fabricated via the uniaxial alignment method and a photopolymerization process. It is found to be useful in dichroic color filters, dual-mode circular polarizers, and chirality detectors. Two kinds of dichroic films with different absorbing wavelengths are cross-stacked to show various colors and contrasts depending on the polarization direction of the incident linearly polarized light, which is comparable with the conventional single-layer dichroic dye-doped (SDLC) film that only shows the contrast difference. This platform can be used in many other applications beyond the applications presented in this study, such as multicolor holograms, optical signal encryption, and electrically tunable devices.