Pejman Hojati-Talemi

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 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.

Advancement in liquid exfoliation of graphite through simultaneously oxidizing and ultrasonicating

Layered crystals, once exfoliated in liquids, create nanosheets with large surface area and likely generate electron band gaps. The current liquid exfoliation of graphite is performed by either oxidation, ultrasonication or the oxidation followed by ultrasonication; these methods are respectable but have limitations in general: the oxidation actually produces graphene oxide while the sonication is time-consuming with a low yield. In this paper we report a highly effective yet simple approach for the fabrication of high-quality graphene; the approach consists of simultaneously oxidizing and ultrasonicating graphite for merely 60 min, followed by washing and filtration. Exfoliation was markedly promoted by the simultaneous treatment, where 80% of the sheets comprise single or few layers with lateral dimensions ranging 50 nm to over 100 nm; their carbon to oxygen ratio is at 8.85; the ratio of Raman D- to G-band intensity is as low as 0.211; and the sheets can be stably dispersed in acetone for at least 48 hours and they have an electrical conductivity over 600 S cm−1. A thin graphene film made by casting exhibited a sheet resistance of ∼1000 Ω square−1 with 80% transparency at 550 nm.

High performance bulk metallic glass/carbon nanotube composite cathodes for electron field emission

We report the preparation of new nanocomposites based on a combination of bulk metallic glass and carbon nanotubes for electron field emission applications. The use of bulk metallic glass as the matrix ensures high electrical and thermal conductivity, high thermal stability, and ease of processing, whilst the well dispersed carbon nanotubes act as highly efficient electron emitters. These advantages, alongside excellent electron emission properties, make these composites one of the best reported options for electron emission applications to date.

Large Area Nanostructured Arrays: Optical Properties of Metallic Nanotubes

In this study, large area metallic nanotube arrays on flexible plastic substrates are produced by templating the growth of a cosputtered alloy using anodized aluminum oxide membranes. These nanotube arrays are prepared over large areas (ca. squared centimeters) by reducing the residual stress within the thin multilayered structure. The nanotubes are approximately 20 nm in inner diameter, having walls of <10 nm in thickness, and are arranged in a close packed configuration. Optically the nanotube arrays exhibit light trapping behavior (not plasmonic), where the reflectivity is less than 15% across the visible spectra compared to >40% for a flat sample using the same alloy. When the nanotubes are exposed to high relative humidity, they spontaneously fill, with a concomitant change in their visual appearance. The filling of the nanotubes is confirmed using contact angle measurements, with the nanotubes displaying a strong hydrophilic character compared to the weak behavior of the flat sample. The ability to easily fabricate large area nanotube arrays which display exotic behavior paves the way for their uptake in real world applications such as sensors and solar energy devices.