Compressively Stressed Silicon Nanoclusters as an Antifracture Mechanism for High-Performance Lithium-Ion Battery Anodes.

Silicon has been considered a good candidate for replacing the commonly used carbon anodes for lithium-ion batteries (LIBs) due to its high specific capacity, which can be up to 11 times higher than that of carbon. However, the desirable advantage that silicon brings to battery performance is currently overshadowed by its stress-induced performance loss and high electronic resistivity. The induced stress arises from two sources, namely, the deposition process (i.e., residual stress) during fabrication and the volume expansion (i.e., mechanical stress) associated with the lithiation/delithiation process. Of the two, residual stress has largely been ignored, underestimated, or considered to have a negligible effect without any rigorous evidence being put forward. In this contribution, we produced silicon thin films having a wide range of residual stress and resistivity using a physical vapor deposition technique, magnetron sputtering. Three pairs of silicon thin-film anodes were utilized to study the effect of residual stress on the electrochemical and cyclability performance as anodes for LIBs. Each set consisted of a pair of films having essentially the same resistivity, density, thickness, and oxidation amount but distinctly different residual stresses. The comparison was evaluated by conducting charge/discharge cycling and cyclic voltammetry (CV) experiments. In contrast to the fixed belief within the literature, higher compressive residual-stress films showed better electrochemical and cycle performance compared to lower residual-stress films. The results, herein, present an informed understanding of the role that residual stress plays, which will help researchers improve the development of silicon-based thin-film anodes.

Influence of Postsynthesis Heat Treatment on Vapor-Phase-Polymerized Conductive Polymers.

The effect of thermal treatment on the structure and electrical/optical properties of vapor phase-polymerized poly(3,4-ethylenedioxythiophene):tosylate (PEDOT:Tos) and polypyrrole:tosylate (PPy:Tos) polymer films was investigated. Thermal treatment was applied postpolymerization but prior to washing the embedded oxidant layer out of the polymer film. Structural and chemical changes arising from the treatment were studied in the context of their conductive and electrochromic behavior. Spectroscopic analysis indicated a rise in the doping levels of both conductive polymers when exposed to thermal treatment. Additionally, an increase in the film thickness was recorded after the oxidant and other unbound species were removed from the polymer layer using an ethanol rinse. As such, a strong indication that polymerization continued even in the absence of (external) monomer vapor was present. This film thickness increase was most pronounced for PPy:Tos but also present in the PEDOT:Tos film. Heat-treated films exhibited enhanced cohesion, making them more robust and therefore increasing the viability for the material to be used in the optoelectronics area. This robustness, due to additional (cross-linking) oligomer growth, came at the expense of lower conductivity relative to their untreated counterparts.

Flexible Polymer-on-Polymer Architecture for Piezo/Pyroelectric Energy Harvesting.

An all polymer piezo/pyroelectric device was fabricated using ß phase poly(vinylidene fluoride) (PVDF) as the active material and vapor phase polymerized (VPP) poly(3,4-ethylenedioxythiphene) (PEDOT) as the flexible electrode overlay material. Inherent problems usually associated with coating polymeric electrodes onto the low surface energy PVDF were overcome by air plasma treating the film in conjunction with utilizing the VPP technique to simultaneously synthesize and in situ deposit the PEDOT electrode. Strain measurements up to the breaking-strain of PVDF (approximately 35%) indicated that the change in R/Ro was significantly smaller for the PEDOT based electrodes compared to the platinum electrode. Plasma treatment of the PVDF film increased the level of surface oxygenated carbon species that contributed to increased surface energy, as confirmed by confirmed by contact angle measurement. The enhanced adhesion between the two polymers layers contributed to a significant increase in the measured piezoelectric output voltage from 0.2 to 0.5 V for the same strain conditions. Pyroelectric voltage outputs were obtained by placing the film onto and off of a hotplate, for temperatures up to 50 °C above ambient. Finally, as a proof of concept, a simple energy harvesting device (plastic tube with slots for mounting multiple piezo/pyro films) was fabricated. The device was able to generate a usable level of peak output current (>3.5 µA) from human inhalation and exhalation "waste energy".

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.

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.

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 film's 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.

Cell attachment and proliferation on high conductivity PEDOT-glycol composites produced by vapour phase polymerisation.

High conductivity poly(3,4-ethylene dioxythiophene) (PEDOT) was synthesised using vacuum vapour phase polymerization (VVPP). The process produces PEDOT composites which incorporate glycol within the polymer. To assess biocompatibility, a suite of analytical techniques were utilised in an effort to characterise the level of glycol present and its impact on cell attachment and proliferation. A small decrease in fibroblast cell attachment and proliferation was observed with increasing glycol content within the PEDOT composite. Keratinocyte cell attachment and proliferation by comparison showed an increase. As such, the results herein indicate that cell attachment and proliferation depends on the individual cell lines used and that the impact of glycol within the PEDOT composite is negligible. This positive outcome prompted investigation of this polymer as a platform for electro-stimulation work. Application of oxidising and reducing potentials to the PEDOT composite were utilised to examine the effect on biocompatibility. Significant effects were seen with altered protein presentation on the reduced surface, and lower mass adsorbed at the oxidised surface. Keratinocytes interacted significantly better on the reduced surface whereas fibroblasts displayed dependence on protein density, with significantly lower spreading on the oxidised surface. Understanding how proteins interact at electrically biased polymer surfaces and in turn affect cell behaviour, underpins the utilisation of such tunable surfaces in biomedical devices.

Influence of PEG-ran-PPG Surfactant on Vapour Phase Polymerised PEDOT Thin Films.

The oxidant, Fe(III) tosylate, was used in the vapour phase polymerisation (VPP) of PEDOT. The amphiphilic co-polymer poly(ethylene glycol-ran-propylene glycol) was added and its influence examined. Both the PEDOT conductivity and optical contrast range increased with the inclusion of the co-polymer, with the maximum being recorded at 4 wt.-%. Loadings higher than this resulted in a systematic decrease in both conductivity and optical contrast. Evidence indicates that in addition to the beneficial anti-crystallisation effect to the oxidant layer, the co-polymer also reduces the effective reactivity of the oxidant, as demonstrated by slower polymerisation rates. Confirmation of the change in polymerisation rate was obtained using a quartz crystal microbalance (QCM). The slower polymerisation rate results in higher conductivity and optical contrast; however, XPS data confirmed that the co-polymer remained within the PEDOT film post-washing and this result explains why the performance decreases at high surfactant loadings.