Dr James Ball

Abstract:

The electrical stability of low-voltage organic transistors based on phosphonic acid self-assembled monolayer (SAM) dielectrics is investigated using four different semiconductors. The threshold voltage shift in these devices shows a stretched-exponential time dependence under continuous gate bias with a relaxation time in the range of 10(3)-10(5) s, at room temperature. Differences in the bias instability of transistors based on different self-assembled monolayers and organic semiconductors suggest that charge trapping into localized states in the semiconductor is not the only mechanism responsible for the observed instability. By applying 1-5 s long programming voltage pulses of 2-3 V in amplitude, a large reversible threshold voltage shift can be produced. The retention time of the programmed state was measured to be on the order of 30 h. The combination of low voltage operation and relatively long retention times makes these devices interesting for ultra-low power memory applications.

Abstract:

Solution exfoliation of graphite holds promise for large-scale bulk synthesis of graphene. Non-covalent exfoliation is attractive because the electronic structure of graphene is preserved but the yield is low and the lateral dimensions of the sheets are small. Chemical exfoliation via formation of graphite oxide is a highly versatile and scalable route but the covalent functionalization of graphene with oxygen significantly alters the properties. Here, we report a new method for large-scale facile synthesis of micron-sized partially oxidized graphene (POG) sheets with dramatically improved electronic properties compared to other solution-phase exfoliated graphene. Due to low initial oxygen content (∼12%), POG requires only mild annealing (<300 °C) to achieve a sheet resistance of 28 kΩ sq-1 at the neutrality point, only a factor of ∼4 larger than the intrinsic sheet resistance of pristine graphene (∼6 kΩ sq-1) and substantially lower than graphene exfoliated by other methods. Such a partial oxidation approach opens up new promising routes to solution based high-performance, low temperature, transparent and conducting graphene-based flexible electronics. © The Royal Society of Chemistry.

Abstract:

The family of soluble fullerene derivatives comprises a widely studied group of electron transporting molecules for use in organic electronic and optoelectronic devices. For electronic applications, electron transporting (n-channel) materials are required for implementation into organic complementary logic circuit architectures. To date, few soluble candidate materials have been studied that fulfill the stringent requirements of high carrier mobility and air stability. Here we present a study of three soluble fullerenes with varying electron affinity to assess the impact of electronic structure on device performance and air stability. Through theoretical and experimental analysis of the electronic structure, characterization of thin-film structure, and characterization of transistor device properties we find that the air stability of the present series of fullerenes not only depends on the absolute electron affinity of the semiconductor but also on the disorder within the thin-film. © 2011 American Institute of Physics.