Confinement-Induced Ordering and Self-Folding of Cellulose Nanofibrils.

Cellulose is a pervasive polymer, displaying hierarchical lengthscales and exceptional strength and stiffness. Cellulose's complex organization, however, also hinders the detailed understanding of the assembly, mesoscopic properties, and structure of individual cellulose building blocks. This study combines nanolithography with atomic force microscopy to unveil the properties and structure of single cellulose nanofibrils under weak geometrical confinement. By statistical analysis of the fibril morphology, it emerges that confinement induces both orientational ordering and self-folding of the fibrils. Excluded volume simulations reveal that this effect does not arise from a fibril population bias applied by the confining slit, but rather that the fibril conformation itself changes under confinement, with self-folding favoring fibril's free volume entropy. Moreover, a nonstochastics angular bending probability of the fibril kinks is measured, ruling out alternating amorphous-crystalline regions. These findings push forward the understanding of cellulose nanofibrils and may inspire the design of functional materials based on fibrous templates.

Visualizing Local Morphology and Conductivity Switching in Interface-Assembled Nanoporous C60 Thin Films.

Carbon materials promise a revolution in optoelectronics, medical applications, and sensing provided that their morphology can be controlled down to the nanometer scale. Nanoporous materials are particularly appealing as they offer a drastically enlarged interfacial area compared to the corresponding planar materials. Entire fields such as organic solar cells, catalysis, or sensing may profit from an enlarged interface and facilitated molecular interaction between a carbon material and the environment. Nanoporous fullerene thin films obtained by the deposition of suspended nanoclusters of fullerene were already reported but suffered from the limitation of the size of these particles to over 100 nm. We study here a complementary method based on interfacial self-assembly forcing C60 clusters to spontaneously form 2D percolating monolayers with most morphological features in the 5-20 nm range. Analysis of these films by means of electron microscopy and scanning probe microscopy proved their morphology to be a nanocomposite of crystalline beads embedded in an amorphous matrix of fullerenes. When contacted between two gold electrodes, these films show an intrinsic conductivity switching behavior. Their electrical conductivity could be reversibly switched on by applying a threshold electrical current and switched off by exposure to oxygen. Interestingly, the on-state exhibits an astonishing conductivity of over 10-3 S/m. Kelvin probe force microscopy (KFM) was used to observe local changes in the distribution of electrical potential upon switching, on the relevant length scale of a few nanometers.

Monitoring the transformation of aliphatic and fullerene molecules by high-energy electrons using surface-enhanced Raman spectroscopy.

We report on using 100 keV electrons to obtain amorphous carbon from aliphatic and fullerene molecules, and study this process by monitoring their Raman signal. In this regard, we use self-assembled monolayers of gold nanoparticles to provide high electromagnetic field enhancement, which allows the detection of the Raman signal from even nanometer-thick layers of analyte. Our results show different dynamics in the amorphization process of the two molecules, although both show suppression of their original vibrational resonances even at low exposure doses. We have also used atomic-force microscopy to evaluate the sensitivity of the C60 molecules to electron beams in forming networks of amorphized molecules that are less soluble in the development process. This method allows precise patterning possibilities as well as in situ functionalization of carbonaceous thin films in the perspective of using in electronic device applications.

Ternary semitransparent organic solar cells with a laminated top electrode.

Tinted and colour-neutral semitransparent organic photovoltaic elements are of interest for building-integrated applications in windows, on glass roofs or on facades. We demonstrate a semitransparent organic photovoltaic cell with a dry-laminated top electrode that achieves a uniform average visible transmittance of 51% and a power conversion efficiency of 3%. The photo-active material is based on a majority blend composed of a visibly absorbing donor polymer and a fullerene acceptor, to which a selective near-infrared absorbing cyanine dye is added as a minority component. Our results show that organic ternary blends are attractive for the fabrication of semitransparent solar cells in general, because a guest component with a complementary absorption can compensate for the inevitably reduced current generation capability of a high-performing binary blend when applied as a thin, semitransparent film.

Biofuel cell operating on activated THP-1 cells: A fuel and substrate study.

It is known that electrochemical energy can be harvested from mammalian cells, more specifically from white blood cells (WBC). This study focuses on an improved biofuel cell operating on phorbol myristate acetate (PMA) activated THP-1 human monocytic cells. Electrochemical investigation showed strong evidence pointing towards hydrogen peroxide being the primary current source, confirming that the current originates from NADPH oxidase activity. Moreover, an adequate substrate for differentiation and activation of THP-1 cells was examined. ITO, gold, platinum and glass were tested and the amount of superoxide anion produced by NADPH oxidase was measured by spectrophotometry through WST-1 reduction at 450nm and used as an indicator of cellular activity and viability. These substrates were subsequently used in a conventional two-compartment biofuel cell where the power density output was recorded. The material showing the highest cell activity compared to the reference cell culture plate and the highest power output was ITO. Under our experimental conditions, a power density of 4.5µW/cm2 was reached. To the best of our knowledge, this is a threefold higher power output than other leukocyte biofuel cells.

A transparent, solvent-free laminated top electrode for perovskite solar cells.

A simple lamination process of the top electrode for perovskite solar cells is demonstrated. The laminate electrode consists of a transparent and conductive plastic/metal mesh substrate, coated with an adhesive mixture of poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate), PEDOT:PSS, and sorbitol. The laminate electrode showed a high degree of transparency of 85%. Best cell performance was achieved for laminate electrodes prepared with a sorbitol concentration of ~30 wt% per milliliter PEDOT:PSS dispersion, and using a pre-annealing temperature of 120°C for 10 min before lamination. Thereby, perovskite solar cells with stabilized power conversion efficiencies of (7.6 ± 1.0)% were obtained which corresponds to 80% of the reference devices with reflective opaque gold electrodes.

Resistive switching of alkanethiolated nanoparticle monolayers patterned by electron-beam exposure.

Carbon-based electronic devices are promising candidates for complementing silicon-based electronics in memory device applications. For example, sputtered thin films of amorphous carbon exhibit memristive behavior. The reported devices, however, have a minimal active area of about 50 nm diameter, leading to large set currents in the µA range. Although power efficiency would benefit from reduced drive currents, resistive switching of amorphous carbon confined to a few cubic nanometers has remained largely unexplored. Here, we investigate resistive switching in 30 nm long and 25 nm wide monolayer arrays of 10 nm gold nanoparticles patterned by direct electron-beam exposure followed by a purpose-designed emulsion-based development process. Electron-beam irradiation transforms the alkanethiol ligands of the gold nanoparticles into a solvent-resistant amorphous carbonaceous matrix allowing pattern development and imparting electronic function. We measure changes in conductivity of up to five orders of magnitude for set currents in the nA range.

Solvent-mediated conductance increase of dodecanethiol-stabilized gold nanoparticle monolayers.

Gold nanoparticle monolayers provide convenient templates to study charge transport in organic molecules beyond single junction techniques. Conductance is reported to increase by several orders of magnitude following immersion of alkanethiol-stabilized gold nanoparticle monolayers in a solution containing conjugated thiol-functionalized molecules. Typically, this observation is attributed to molecular exchange. Less attention has been paid to the role of the solvent alone. Here, we report on an increase in conductance of dodecanethiol-stabilized gold nanoparticle monolayers on Si/SiO2 by an average factor of 36 and 22 after immersion in pure ethanol (EtOH) and tetrahydrofuran (THF), respectively. Analysis by scanning electron microscopy (SEM) and small-angle X-ray scattering (SAXS) reveals a solvent-induced decrease in lattice constant of close-packed monolayers. We compare the conductance of the monolayer after molecular exchange with two different oligophenylenes to shed light on the respective contribution of the solvent-induced structural change and the molecular exchange itself on the conductance increase.

Water-Mediated Assembly of Gold Nanoparticles into Aligned One-Dimensional Superstructures.

This Article shows that water in ethanol colloids of gold nanoparticles enhances the formation of linear clusters and, more important for applications in electronics, determines their assembly on surfaces. We show by dynamic light scattering that ethanol colloids contain mainly monomers and dimers and that wormlike superstructures are mostly absent, despite UV-vis evidence of aggregation. Water added to the colloid as a cosolvent was found to enhance the number of clusters as well as their average size, confirming its role in linear self-assembly, on the scale of a few particles. Water adsorbed from the atmosphere during coating was also found to be a powerful lever to tune self-assembly on surfaces. By varying the relative humidity, a sharp transition from branched to linear superstructures was observed, showing the importance of water as a cosolvent in the formation of cluster superstructures. We show that one-dimensional superstructures may form due to long-range mobility of precursor clusters on wet surfaces, allowing their rearrangement. The understanding of the phenomenon allows us to statistically align both clusters and resulting superstructures on patterned substrates, opening the way to rapid screening in molecular electronics.

Resonance light scattering in dye-aggregates forming in dewetting droplets.

Small organic semiconducting molecules assembling into supramolecular J- and H- aggregates have attracted much attention due to outstanding optoelectronic properties. However, their easy and reproducible fabrication is not yet sufficiently developed for industrial applications, except for silver halide photography. Here we present a method based on aggregate precipitation during the phase separation and dewetting of the evaporating dye precursor solution. The smaller the precursor droplets, the more pronounced the J-aggregation. The aggregates cause the films to resonantly scatter incoming light. Because the dye aggregate extinction resonances have narrowest bandwidths, a wavelength selectivity is observed that exceeds the selectivity of localized surface plasmon resonances. The aggregation mechanism can be easily applied to periodically structured substrates, making the method appealing for photonic applications. We demonstrate this point with a 2D grating, where the narrow absorption range of the aggregates leads to wavelength specific (one color only) scattering.

Growth and alignment of thin film organic single crystals from dewetting patterns.

Studying and understanding the conditions under which organic semiconductors can be engineered to form aligned single crystals in thin films is of primary importance owing to their unique orientation-dependent optoelectronic properties. Efforts to reach this goal by self-assembly from solution-processed films have been rewarded only with limited success. In this article we present a new method to grow single crystalline thin films via solvent annealing. We identify solvate crystal growth in combination with a specific film dewetting morphology as key to successful fabrication of single crystals. Furthermore, these 2D single crystals can align on chemically patterned substrates to minimize their interfacial energy. We explore in situ the conditions for crystal formation and alignment.

Self-organised microdots formed by dewetting in a highly volatile liquid.

Dewetting induced self-organisation was used to prepare an ordered microstructure from a highly volatile liquid. Dewetting of an evaporating iron oxide precursor solute on silicon substrate resulted in arrays of microdots with nearly hexagonal and tetragonal symmetries. Ordered structures form either by stick-slip motion or fingering instability at the receding contact line of evaporating droplets. Subsequent thermal treatment at 550 °C yields crystalline Fe(2)O(3) microdots with a diameter range of 1-4 µm. The size, density and shape of the microdots can be changed by using patterned substrates with different surface energies.