Chris Bower

Mechanically Durable Superhydrophobic Surfaces

Development of durable non-wetting surfaces is hindered by the fragility of the microscopic roughness features that are necessary for superhydrophobicity. Mechanical wear on superhydrophobic surfaces usually shows as increased sticking of water, leading to loss of non-wettability. Increased wear resistance has been demonstrated by exploiting hierarchical roughness where nanoscale roughness is protected to some degree by large scale features, and avoiding the use of hydrophilic bulk materials is shown to help prevent the formation of hydrophilic defects as a result of wear. Additionally, self-healing hydrophobic layers and roughness patterns have been suggested and demonstrated. Nevertheless, mechanical contact not only causes damage to roughness patterns but also surface contamination, which shortens the lifetime of superhydrophobic surfaces in spite of the self-cleaning effect. The use of photocatalytic effect and reduced electric resistance have been suggested to prevent the accumulation of surface contaminants. Resistance to organic contaminants is more challenging, however, oleophobic surface patterns which are non-wetting to organic liquids have been demonstrated. While the fragility of superhydrophobic surfaces currently limits their applicability, development of mechanically durable surfaces will enable a wide range of new applications in the future.

A Nanostructured Electrochromic Supercapacitor

We report the first successful application of an ordered bicontinuous double-gyroid vanadium pentoxide network in an electrochromic supercapacitor. The freestanding vanadia network was fabricated by electrodeposition into a voided block copolymer template that had self-assembled into the double-gyroid morphology. The highly ordered structure with 11.0 nm wide struts and a high specific surface to bulk volume ratio of 161.4 μm(-1) is ideal for fast and efficient lithium ion intercalation/extraction and faradaic surface reactions, which are essential for high energy and high power density electrochemical energy storage devices. Supercapacitors made from such gyroid-structured vanadia electrodes exhibit a high specific capacitance of 155 F g(-1) and show a strong electrochromic color change from green/gray to yellow, indicating the capacitors charge condition. The nanostructuring approach and utilizing an electrode material that has intrinsic electrochemical color-change properties are concepts that can be readily extended to other electrochromic intercalation compounds.

Reversible switching between superhydrophobic states on a hierarchically structured surface

Nature offers exciting examples for functional wetting properties based on superhydrophobicity, such as the self-cleaning surfaces on plant leaves and trapped air on immersed insect surfaces allowing underwater breathing. They inspire biomimetic approaches in science and technology. Superhydrophobicity relies on the Cassie wetting state where air is trapped within the surface topography. Pressure can trigger an irreversible transition from the Cassie state to the Wenzel state with no trapped air—this transition is usually detrimental for nonwetting functionality and is to be avoided. Here we present a new type of reversible, localized and instantaneous transition between two Cassie wetting states, enabled by two-level (dual-scale) topography of a superhydrophobic surface, that allows writing, erasing, rewriting and storing of optically displayed information in plastrons related to different length scales.

Properties of graphene inks stabilized by different functional groups

Different graphene inks have been synthesized by chemical methods. These uniform dispersions were stabilized by various functional groups such as room temperature ionic liquid, polyaniline, polyelectrolyte (poly[2,5-bis(3-sulfonatopropoxy)-1,4-ethynylphenylene-alt-1,4-ethynylphenylene] sodium salt) and poly(styrenesulfonate) (PSS). The dispersions can be easily cast into high-quality, free-standing films but with very different physiochemical properties such as surface tension and adhesion. SEM and AFM methods have been applied to have a detailed study of the properties of the inks. It is found that graphenes modified by p-type polyaniline show the highest surface tension. Diverse surface adhesive properties to the substrate are also found with various functional groups. The different viscoelasticities of graphene inks were related to the microscopic structure of their coating layer and subsequently related to the configuration, chemistry and molecular dimensions of the modifying molecules to establish the property-structure relationship. Modifications of graphene inks made from chemical reduction cannot only enable cost-effective processing for printable electronics but also extend the applications into, for example, self-assembly of graphene via bottom-up nano-architecture and surface energy engineering of the graphenes. To fabricate useful devices, understanding the surface properties of graphene inks is very important. It is the first paper of this kind to study the surface tension and adhesion of graphene influenced by different functional groups.

Flexible solid state lithium batteries based on graphene inks

Different formulations of solution-processable graphene have been characterised as electrode materials for use in electrochemical energy storage devices. Graphene was fabricated by chemical reduction of exfoliated graphene oxide (GO), and modified with either p-type (e.g. polyaniline) or n-type anionic groups (poly(styrenesulfonate) (PSS−) and poly[2,5-bis(3-sulfonatopropoxy)-1,4-ethynylphenylene-alt-1,4-ethynylphenylene] sodium salt (PPE-SO3−) anion). Solutions of these graphene compounds were deposited on charge collecting electrodes and used as battery cathodes. Electrodes using the anionically-modified graphene inks containing anatase titanate (TiO2) nanoparticles show improved performance over pristine graphene ink as well as the p-type conducting polymer modified ones. In addition, the open circuit voltage of batteries based on TiO2 has been boosted over 3 V with good cyclability when mixed with the graphene ink. Combined with a polymer electrolyte, this work suggests a feasible route towards fully printable rechargeable lithium batteries based on graphene inks. This approach is both versatile and scalable and is adaptable to a wide variety of applications.

Compound Quantum Dot–Perovskite Optical Absorbers on Graphene Enhancing Short-Wave Infrared Photodetection

Colloidal quantum dots (QDs) combined with a graphene charge transducer promise to provide a photoconducting platform with high quantum efficiency and large intrinsic gain, yet compatible with cost-efficient polymer substrates. The response time in these devices is limited, however, and fast switching is only possible by sacrificing the high sensitivity. Furthermore, tuning the QD size toward infrared absorption using conventional organic capping ligands progressively reduces the device performance characteristics. Here we demonstrate methods to couple large QDs (>6 nm in diameter) with organometal halide perovskites, enabling hybrid graphene phototransistor arrays on plastic foils that simultaneously exhibit a specific detectivity of 5 × 1012 Jones and high video-frame-rate performance. PbI2 and CH3NH3I co-mediated ligand exchange in PbS QDs improves surface passivation and facilitates electronic transport, yielding faster charge recovery, whereas PbS QDs embedded into a CH3NH3PbI3 matrix produce spatially separated photocarriers leading to large gain.

Formation of Patterned Arrays of Polystyrene Colloidal Crystal Structures on Flexible Functional Substrates

This contribution presents the first preparation of patterned arrays of highly ordered polystyrene colloidal crystal structures on flexible functional substrates. The formation of patterned arrays of colloidal crystals over large areas (5x1 cm) with periodic line patterns ranging in pitch from 25 to 450 microm is demonstrated. The protocol developed to achieve this is applicable to a wide-range of substrates and is inherently scalable. Interestingly, directed colloidal deposition was found to be more susceptible to fluctuations in the deposition conditions than bulk deposition. The conditions required for directed deposition were systematically investigated, and the success of the optimized protocol was illustrated by the deposition of ordered structures on a range of functionalized rigid and flexible substrates. These advances-low-cost production on flexible functional substrates and the fabrication of structures of controlled geometry-address two of the major challenges in developing devices using colloidal crystal structures.

Identification of mechanisms competing with self-assembly during directed colloidal deposition

For many proposed applications of colloidal crystals it is desirable to obtain arrays with specific geometries. In this contribution, mechanisms competing with self-assembly during area specific deposition are identified and discussed. Specifically, flat chemically patterned substrates are created by micro-contact printing of alkanethiols and silanes. Polystyrene spheres (200–460 nm) are then deposited onto these patterned substrates by evaporative vertical deposition. The presence of a Rayleigh–Plateau instability and a stick–slip mechanism during area specific colloidal deposition are reported. It has also been demonstrated that when using the vertical deposition method, with patterned features of the scale 25–400 μm, difference in surface energy rather than electrostatic interactions dominate the direction of colloidal self-assembly.