Drew Evans

Semi-metallic polymers

Polymers are lightweight, flexible, solution-processable materials that are promising for low-cost printed electronics as well as for mass-produced and large-area applications. Previous studies demonstrated that they can possess insulating, semiconducting or metallic properties; here we report that polymers can also be semi-metallic. Semi-metals, exemplified by bismuth, graphite and telluride alloys, have no energy bandgap and a very low density of states at the Fermi level. Furthermore, they typically have a higher Seebeck coefficient and lower thermal conductivities compared with metals, thus being suitable for thermoelectric applications. We measure the thermoelectric properties of various poly(3,4-ethylenedioxythiophene) samples, and observe a marked increase in the Seebeck coefficient when the electrical conductivity is enhanced through molecular organization. This initiates the transition from a Fermi glass to a semi-metal. The high Seebeck value, the metallic conductivity at room temperature and the absence of unpaired electron spins makes polymer semi-metals attractive for thermoelectrics and spintronics.

Condensation and freezing of droplets on superhydrophobic surfaces

Superhydrophobic coatings are reported as promising candidates for anti-icing applications. Various studies have shown that as well as having ultra water repellency the surfaces have reduced ice adhesion and can delay water freezing. However, the structure or texture (roughness) of the superhydrophobic surface is subject to degradation during the thermocycling or wetting process. This degradation can impair the superhydrophobicity and the icephobicity of those coatings. In this review, a brief overview of the process of droplet freezing on superhydrophobic coatings is presented with respect to their potential in anti-icing applications. To support this discussion, new data is presented about the condensation of water onto physically decorated substrates, and the associated freezing process which impacts on the freezing of macroscopic droplets on the surface.

Significant Electronic Thermal Transport in the Conducting Polymer Poly(3,4‐ethylenedioxythiophene)

Suspended microdevices are employed to measure the in-plane electrical conductivity, thermal conductivity, and Seebeck coefficient of suspended poly(3,4-ethylenedioxythiophene) (PEDOT) thin films. The measured thermal conductivity is higher than previously reported for PEDOT and generally increases with the electrical conductivity. The increase exceeds that predicted by the Wiedemann-Franz law for metals and can be explained by significant electronic thermal transport in PEDOT.

Structure-directed growth of high conductivity PEDOT from liquid-like oxidant layers during vacuum vapor phase polymerization

Vapor phase polymerization (VPP) is at the forefront for synthesizing high conductivity poly(3,4-ethylenedioxythiophene) (PEDOT) as an alternative to indium tin oxide (ITO). Little attention, however, has been directed to the oxidant layer used in the polymerization process. In this study the observation of an oxidant layer (oxidant + PEG–PPG–PEG) possessing liquid-like properties during the vacuum synthesis of PEDOT is reported. This is in contrast to the other oxidant layer variants studied which are observed as solid (pristine oxidant) or gel-like (oxidant + pyridine). Tailoring of the liquid-like properties leads to confluent PEDOT films with a conductivity of 2500 S cm−1, placing this PEDOT within the conductivity range of commercially available ITO. Building on the liquid-like observation, XPS and ToF-SIMS experiments reveal that PEDOT growth is via a bottom-up mechanism with transportation of new oxidant up to the forming PEDOT layer.

Acido-basic control of the thermoelectric properties of poly(3,4-ethylenedioxythiophene)tosylate (PEDOT-Tos) thin films

PEDOT-Tos is one of the conducting polymers that displays the most promising thermoelectric properties.

Ultrathin polymer films for transparent electrode applications prepared by controlled nucleation.

The vacuum vapor phase polymerization (VPP) technique is capable of producing conducting polymer films with conductivities up to 3400 S cm(-1). However, the method is not able to produce robust nano-thin films as required for transparent conducting electrode (TCE) applications. We show that with the addition of aprotic solvents or chelating agents to the oxidant mixture, it is possible to control the polymerization rate, and nucleation, in the VPP process. This provides the opportunity of altering the grain size and depositing conducting polymer films with a thickness of 16 to 200 nm with resulting optical transmission within the range 50-98% that are robust enough to endure the post polymerization processing steps. The figure of merit (FoM), which is used to quantify a films suitability for TCE applications, results in values from 12 to 25. This result indicates that the nano-films outperform most of the previously reported graphene films and approaches the accepted industry standard for TCE applications.

Metal-free oxygen reduction electrodes based on thin PEDOT films with high electrocatalytic activity

Oxygen reduction reaction (ORR) electrodes play an important role in the development of new battery and fuel cell technologies. However most of the presented electrode materials cannot provide the efficiency required for these applications, and/or they are based on economically unfavourable noble metals. In this article, multi-layer electrodes of high conductivity PEDOT prepared by vacuum vapour phase polymerization in the presence of a PEG–PPG–PEG triblock copolymer are used to fabricate a metal-free oxygen reduction electrode. After optimizing the main production parameters, measuring ORR performance of the metal-free PEDOT based electrodes confirms that they have the ability to deliver a stable electrocatalytic activity. A chemical treatment is also used for further enhancing the electrocatalytic activity of these electrodes. Depending on pH, the electrocatalytic activity of these treated electrodes reaches a higher or the same level as platinum based electrodes.

Effect of oxidant on the performance of conductive polymer films prepared by vacuum vapor phase polymerization for smart window applications

Conductive polymers synthesized by vacuum vapour phase polymerization (VPP) were investigated and optimized by changing the oxidant solution and VPP chamber parameters for their incorporation into ?smart window? electrochromic devices. Additionally, the interaction of two oxidant solutions with typical electrode materials (aluminium and indium tin oxide) were examined with respect to material etching, device cosmetics and long term device degradation (over 10 000 switch cycles). Devices made with conducting polymers synthesized with the oxidant Fe(Tos)3 rather than FeCl3 produced superior device performance with respect to optical switching range (%T), switch speed and optical relaxation.

Laser Actuation of Cantilevers for Picometre Amplitude Dynamic Force Microscopy

As nanoscale and molecular devices become reality, the ability to probe materials on these scales is increasing in importance. To address this, we have developed a dynamic force microscopy technique where the flexure of the microcantilever is excited using an intensity modulated laser beam to achieve modulation on the picoscale. The flexure arises from thermally induced bending through differential expansion and the conservation of momentum when the photons are reflected and absorbed by the cantilever. In this study, we investigated the photothermal and photon pressure responses of monolithic and layered cantilevers using a modulated laser in air and immersed in water. The developed photon actuation technique is applied to the stretching of single polymer chains.

Charge transport and structure in semimetallic polymers

ABSTRACT Owing to changes in their chemistry and structure, polymers can be fabricated to demonstrate vastly different electrical conductivities over many orders of magnitude. At the high end of conductivity is the class of conducting polymers, which are ideal candidates for many applications in low‐cost electronics. Here, we report the influence of the nature of the doping anion at high doping levels within the semi‐metallic conducting polymer poly(3,4‐ethylenedioxythiophene) (PEDOT) on its electronic transport properties. Hall effect measurements on a variety of PEDOT samples show that the choice of doping anion can lead to an order of magnitude enhancement in the charge carrier mobility > 3 cm2/Vs at conductivities approaching 3000 S/cm under ambient conditions. Grazing Incidence Wide Angle X‐ray Scattering, Density Functional Theory calculations, and Molecular Dynamics simulations indicate that the chosen doping anion modifies the way PEDOT chains stack together. This link between structure and specific anion doping at high doping levels has ramifications for the fabrication of conducting polymer‐based devices.