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1. Super-resolution interference lithography enabled by non-equilibrium kinetics of photochromic monolayers

   https://doi.org/10.1039/C9RA05864H  

   Vijayamohanan, Harikrishnan; Kenath, Gopal S.; Palermo, Edmund F.; Ullal, Chaitanya K. (September 2019, RSC Advances)

   Highly parallelized optical super-resolution lithography techniques are key for realizing bulk volume nanopatterning in materials. The majority of demonstrated STED-inspired lithography schemes are serial writing techniques. Here we use a recently developed model spirothiopyran monolayer photoresist to study the non-equilibrium kinetics of STED-inspired lithography systems to achieve large area interference lithography with super-resolved feature dimensions. The linewidth is predicted to increase with exposure time and the contrast is predicted to go through a maximum, resulting in a narrow window of optimum exposure. Experimental results are found to match with high quantitative accuracy. The low photoinhibition saturation threshold of the spirothiopyran renders it especially conducive for parallelized large area nanopatterning. Lines with 56 and 92 nm FWHM were obtained using serial and parallel patterning, respectively. Functionalization of surfaces with heterobifunctional PEGs enables diverse patterning of any desired chemical functionality on these monolayers. These results provide important insight prior to realizing a highly parallelized volume nanofabrication technique. 

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2. How periodic surfaces bend without stretching

   Nassar, Hussein; Weber, Andrew (August 2024, International Association for Shell and Spatial Structures)

   Many compliant shell mechanisms are periodically corrugated or creased. Being thin, their preferred deformation modes are inextensional, i.e., isometric. Here, we report on a recent characterization of the isometric deformations of periodic surfaces. In a way reminiscent of Gauss theorem, the result builds a constraint that relates the ways in which the periodic surface stretches, effectively but isometrically, to the ways in which it bends and twists. Several examples and use cases are presented. 

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3. Generation of Amorphous Silica Surfaces with Controlled Roughness

   https://doi.org/10.1021/acs.jpca.3c04955  

   Nguyen, Nuong P.; Laird, Brian B. (November 2023, The Journal of Physical Chemistry A)

   Amorphous silica (a-SiO2) surfaces, when grafted with select metals on the active sites of the functionalized surfaces, can act as useful heterogeneous catalysts. From a molecular modeling perspective, one challenge has been generating a-SiO2 slab models with controllable surface roughness to facilitate the study of the effect of surface morphology on the material properties. Previous computational methods either generate relatively flat surfaces or periodically corrugated surfaces that do not mimic the full range of potential surface roughness of the amorphous silica material. In this work, we present a new method, inspired by the capillary fluctuation theory of interfaces, in which rough silica slabs are generated by cleaving a bulk amorphous sample using a cleaving plane with Fourier components randomly generated from a Gaussian distribution. The width of this Gaussian distribution (and thus the degree of surface roughness) can be tuned by varying the surface roughness parameter α. Using the van Beest, Kramer, and van Santen (BKS) force field, we create a large number of silica slabs using cleaving surfaces of varying roughness (α) and using two different system sizes. These surfaces are then characterized to determine their roughness (mean- squared displacement), density profile, and ring size distribution. This analysis shows a higher concentration of surface defects (under-/overcoordinated atoms and strained rings) as the surface roughness increases. To examine the effect of the roughness on surface reactivity, we re-equilibriate a subset of these slabs using the reactive force field ReaxFF and then expose the slabs to water and observe the formation of surface silanols. We observe that the rougher surfaces exhibit greater silanol concentrations as well as bimodal acidity. 

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4. Excitation of surface plasmon polaritons by diffraction-free and vector beams

   https://doi.org/10.1364/AO.465853  

   Diouf, Mbaye; Burrow, Joshua_A; Krishna, Krishangi; Odessey, Rachel; Abouraddy, Ayman_F; Toussaint, Kimani_C (August 2022, Applied Optics)

   Surface plasmon polaritons (SPPs) are traditionally excited by plane waves within the Rayleigh range of a focused transverse-magnetic (TM) Gaussian beam. Here we investigate and confirm the coupling between SPPs and two-dimensional Gaussian and Bessel–Gauss wave packets, as well as one-dimensional light sheets and space-time wave packets. We encode the incoming wavefronts with spatially varying states of polarization; then we couple the respective TM components of radial and azimuthal vector beam profiles to confirm polarization-correlation and spatial-mode selectivity. Our results do not require material optimization or multi-dimensional confinement via periodically corrugated metal surfaces to achieve coupling at a greater extent, hereby outlining a pivotal, yet commonly overlooked, path towards the development of long-range biosensors and all-optical integrated plasmonic circuits. 

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5. Discontinuous growth of DNA plectonemes due to atiomic scale friction

   https://doi.org/10.1039/C8M00852C  

   Min, Yifei; Purohit, Prashant K. (September 2018, Soft matter)

   We develop a model to explain discontinuities in the increase of the length of a DNA plectoneme when the DNA filament is continuously twisted under tension. We account for DNA elasticity, electrostatic interactions and entropic effects due to thermal fluctuation. We postulate that a corrugated energy landscape that contains energy barriers is the cause of jumps in the length of the plectoneme as the number of turns is increased. Thus, our model is similar to the Prandtl-Tomlinson model of atomic scale friction. The existence of a corrugated energy landscape can be justified due to the close proximity of the neighboring pieces of DNA in a plectoneme. We assume the corrugated energy landscape to be sinusoidal since the plectoneme has a periodic helical structure and rotation of the bead is a form of periodic motion. We perform calculations with different tensile forces and ionic concentrations, and show that rotation-extension curves manifest stair-step shapes under relatively high ionic concentrations and high forces. We show that the jump in the plectonemic growth is caused by the flattening of the energy barrier in the corrugated landscape. 

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