The Magical Power of Water: A Reminiscence of Herman Johan Christiaan Berendsen.

A personal perspective on the scientific and social dimensions of interactions with Herman Berendsen from 1969 through 2019 shows how his intellect and character had an influence beyond his laboratory.

All Structures Great and Small: Nanoscale Modulations in Nematic Liquid Crystals.

The nature of the nanoscale structural organization in modulated nematic phases formed by molecules having a nonlinear molecular architecture is a central issue in contemporary liquid crystal research. Nevertheless, the elucidation of the molecular organization is incomplete and poorly understood. One attempt to explain nanoscale phenomena merely "shrinks down" established macroscopic continuum elasticity modeling.

Irreversible Shear-Activated Gelation of a Liquid Crystalline Polyelectrolyte.

We report irreversible, shear-activated gelation in liquid crystalline solutions of a rigid polyelectrolyte that forms rodlike assemblies (rods) in salt-free solution. At rest, the liquid crystalline solutions are kinetically stable against gelation and exhibit low viscosities. Under steady shear at, or above, a critical shear rate, a physically cross-linked, nematic gel network forms due to linear growth and branching of the rods.

Direct Measurement of the Angular Pair Correlation Coefficients in Molecular Liquids Using NMR. Benchmarking Force Fields for Atomistic Simulations.

High-field deuterium NMR spectroscopy is used to characterize a number of molecular liquids and their mixtures in order to probe the directional part of the intermolecular interactions through the orientational ordering induced in the isotropic liquid phase by the spectrometer magnetic field. The systems studied include benzene, chloroform, hexafluorobenzene, and thiophene at various concentrations and in mixtures. Dilution with the magnetically isotropic tetramethylsilane provides quantification of ordering at "infinite magnetic dilution", that is, in the absence of magnetic intermolecular correlations, and thereby allows identification of the contribution of these correlations to the orientational ordering in neat phases and at various degrees of magnetic dilution.

Search for microscopic and macroscopic biaxiality in the cybotactic nematic phase of new oxadiazole bent-core mesogens.

The possibility of biaxial orientational order in nematic liquid crystals is a subject of intense current interest. We explore the tendencies toward local and global biaxial ordering in the recently synthesized trimethylated oxadiazole-based bent-core mesogens with a pronounced asymmetric (bow-type) shape of molecules. The combination of x-ray diffraction and optical studies suggests that the biaxial order is expressed differently at the short- and long-range scales.

Molecular ordering in the high-temperature nematic phase of an all-aromatic liquid crystal.

We report the structural characterization of the nematic phase of 2,6-biphenyl naphthalene (PPNPP). This lath-like all-aromatic mesogen provides a valuable benchmark for classical theories of nematic order. PPNPP exhibits a very high temperature nematic phase (417-489 °C) above an enantiotropic smectic A phase.

Liquid-State Structure via Very High-Field Nuclear Magnetic Resonance Discriminates among Force Fields.

Deuterium nuclear magnetic resonance ((2)H NMR) spectra of labeled molecular liquids obtained at high fields, for example, |B| = 22.3 T (950 MHz proton NMR), exhibit resolved quadrupolar splittings that reflect the average orientation of the molecules relative to B. Those residual nuclear spin interactions exhibited by benzene and chloroform provide an experimental determination of the leading tensor component of the pair correlation function for these two molecular liquids.

Benzene at 1GHz. Magnetic field-induced fine structure.

The deuterium NMR spectrum of benzene-d6 in a high field spectrometer (1GHz protons) exhibits a magnetic field-induced deuterium quadrupolar splitting Δν. The magnitude of Δν observed for the central resonance is smaller than that observed for the (13)C satellite doublets Δν'. This difference, Δ(Δν)=Δν'-Δν, is due to unresolved fine structure contributions to the respective resonances.

Additive manufacturing. Continuous liquid interface production of 3D objects.

Additive manufacturing processes such as 3D printing use time-consuming, stepwise layer-by-layer approaches to object fabrication. We demonstrate the continuous generation of monolithic polymeric parts up to tens of centimeters in size with feature resolution below 100 micrometers. Continuous liquid interface production is achieved with an oxygen-permeable window below the ultraviolet image projection plane, which creates a "dead zone" (persistent liquid interface) where photopolymerization is inhibited between the window and the polymerizing part.

The cybotactic nematic phase of bent-core mesogens: state of the art and future developments.

The molecular clustering observed in the fluid nematic phase of nonlinear liquid crystal molecules underlies exaggerated field effects that portend unique technological advances in next-generation liquid crystal displays. However, the detailed nature of the molecular organization within the clusters and the temporal and spatial persistence of the organization remain unclear. Herein we review the evolution of structural studies of this unique nematic phase.

Solution processed Al-doped ZnO nanoparticles/TiOx composite for highly efficient inverted organic solar cells.

We investigated the electrical properties of solution processed Al-doped ZnO (AZO) nanoparticles, stabilized by mixing with a TiOx complex. Thin solid films cast from the solution of AZO-TiOx (AZOTi) (Ti/Zn ∼0.4 in the bulk and ∼0.

Modifications in morphology resulting from nanoimprinting bulk heterojunction blends for light trapping organic solar cell designs.

Nanoimprinting the photoactive layer of bulk heterojunction (BHJ) organic solar cells is a promising technique for enhancing device performance via improved light absorption. Here, we demonstrate that imprinting poly(3-hexylthiophene) (P3HT) and fullerene BHJ blends leads to adverse morphological changes within the photoactive nanopattern which have been previously overlooked. In particular, nanoimprinting induces a factor of 2 difference in polymer:fullerene composition between the nanopattern posts and interconnecting flash layer that inadvertently moves the composition outside the range for optimal performance.

Surface patterning of mesoporous niobium oxide films for solar energy conversion.

An array of periodic surface features were patterned on mesoporous niobium oxide films by a soft-lithographic technique with the goal of constructing a photonic crystal (PC) structure on the back side of the oxide. The oxide films, fabricated by mixing sol-gel derived niobium oxide nanoparticles and hydroxypropyl cellulose, were employed as photoelectrodes in dye-sensitized solar cells (DSSCs), and their performance evaluated against their flat counterparts. The surface patterns were imprinted using a photocurable perfluoropolyether (PFPE) soft-replica of a silicon master with a two-dimensional array of cylindrical posts (200 nm (D) × 200 nm (H)) in hexagonal geometry.

Role of thin n-type metal-oxide interlayers in inverted organic solar cells.

We have investigated the photovoltaic properties of inverted solar cells comprising a bulk heterojunction film of poly(3-hexylthiophene) and phenyl-C(61)-butyric acid methyl ester, sandwiched between an indium-tin-oxide/Al-doped zinc oxide (ZnO-Al) front, and tungsten oxide/aluminum back electrodes. The inverted solar cells convert photons to electrons at an external quantum efficiency (EQE) exceeding 70%. This is a 10-15% increase over EQEs of conventional solar cells.

Extraordinary magnetic field effect in bent-core liquid crystals.

A bent-core mesogen that forms a cybotactic nematic phase exhibits a giant magnetic field-induced shift of its nematic-isotropic and smectic-C-nematic transition temperatures: ΔT(H) = 4 K for H = 10 kOe. In contrast with molecular nematics, in cybotactic nematics the field couples with the anisotropic susceptibility of clusters containing several hundred partially ordered molecules. X-ray diffraction data corroborate a quantitative estimate of inferred cluster size (∼300 molecules).

Nanoforest Nb2O5 photoanodes for dye-sensitized solar cells by pulsed laser deposition.

Vertically aligned bundles of Nb(2)O(5) nanocrystals were fabricated by pulsed laser deposition (PLD) and tested as a photoanode material in dye-sensitized solar cells (DSSC). They were characterized using scanning and transmission electron microscopies, optical absorption spectroscopy (UV-vis), and incident-photon-to-current efficiency (IPCE) experiments. The background gas composition and the thickness of the films were varied to determine the influence of those parameters in the photoanode behavior.

High-resolution PFPE-based molding techniques for nanofabrication of high-pattern density, sub-20 nm features: a fundamental materials approach.

Several perfluoropolyether (PFPE)-based elastomers for high-resolution replica molding applications are explored. The modulus of the elastomeric materials was increased through synthetic and additive approaches while maintaining relatively low surface tension values (<25 mN/m). Using large area (>4 in.

Photonic crystal geometry for organic solar cells.

We report organic solar cells with a photonic crystal nanostructure embossed in the photoactive bulk heterojunction layer, a topography that exhibits a 3-fold enhancement of the absorption in specific regions of the solar spectrum in part through multiple excitation resonances. The photonic crystal geometry is fabricated using a materials-agnostic process called PRINT wherein highly ordered arrays of nanoscale features are readily made in a single processing step over wide areas (approximately 4 cm(2)) that is scalable. We show efficiency improvements of approximately 70% that result not only from greater absorption, but also from electrical enhancements.

Absorption and quasiguided mode analysis of organic solar cells with photonic crystal photoactive layers.

We analyze optical absorption enhancements and quasiguided mode properties of organic solar cells with highly ordered nanostructured photoactive layers comprised of the bulk heterojunction blend, poly-3-hexylthiophene/[6,6]-phenyl-C61-butyric acid methyl ester (P3HT:PCBM) and a low index of refraction conducting material (LICM). This photonic crystal geometry is capable of enhancing spectral absorption by approximately 17% in part due to the excitation of quasiguided modes near the band edge of P3HT:PCBM. A nanostructure thickness between 200 nm and 300 nm is determined to be optimal, while the LICM must have an index of refraction approximately 0.

The pursuit of a scalable nanofabrication platform for use in material and life science applications.

In this Account, we describe the use of perfluoropolyether (PFPE)-based materials that are able to accurately mold and replicate micro- and nanosized features using traditional techniques such as embossing as well as new techniques that we developed to exploit the exceptional surface characteristics of fluorinated substrates. Because of the unique partial wetting and nonwetting characteristics of PFPEs, we were able to go beyond the usual molding and imprint lithography approaches and have created a technique called PRINT (Particle [or Pattern] Replication In Nonwetting Templates). PRINT is a distinctive "top-down" fabrication technique capable of generating isolated particles, arrays of particles, and arrays of patterned features for a plethora of applications in both nanomedicine and materials science.

The Patterning of Sub-500 nm Inorganic Oxide Structures.

Elastomeric perfluoropolyether molds are applied to pattern arrays of sub-500 nm inorganic oxide features. This versatile soft-lithography technique can be used to pattern a wide range of materials; in this work inorganic oxides including TiO2 , SnO2 , ZnO, ITO, and BaTiO3 are patterned on a variety of substrates with different aspect ratios. An example of TiO2 posts is shown in the figure.

Synthesis of water soluble graphene.

A facile and scalable preparation of aqueous solutions of isolated, sparingly sulfonated graphene is reported. (13)C NMR and FTIR spectra indicate that the bulk of the oxygen-containing functional groups was removed from graphene oxide. The electrical conductivity of thin evaporated films of graphene (1250 S/m) relative to similarly prepared graphite (6120 S/m) implies that an extended conjugated sp (2) network is restored in the water soluble graphene.