Yuvaraj Sivalingam

Low voltage electrolyte-gated organic transistors making use of high surface area activated carbon gate electrodes

In electrolyte-gated transistors, the exceptionally high capacitance of the electrical double layer forming at the electrolyte/transistor channel interface permits current modulations of several orders of magnitude, at relatively low gate voltages. The effect of the nature of the gate electrode on the performance of electrolyte-gated transistors is still largely unclear, despite recent intensive efforts. Here we demonstrate that the use of high surface area, low cost, activated carbon gate electrode enables low voltage (sub-1 V) operation in ionic liquid-gated organic transistors and renders unnecessary the presence of an external reference electrode to monitor the channel potential, thus dramatically simplifying the device structure. We used the organic electronic polymer MEH-PPV (poly[2-methoxy-5-(2′-ethylhexyloxy)-p-phenylene vinylene), as the channel material, and the high ionic conductivity, low viscosity ionic liquid [EMIM][TFSI] (1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide), as the electrolyte gating material. We believe that this will prove to be the first of a new generation of low voltage electrolyte-gated transistors for applications in organic printable electronics.

The influence of film morphology and illumination conditions on the sensitivity of porphyrins-coated ZnO nanorods.

ZnO and porphyrins have complementary properties that make their combination attractive for diverse applications such as photovoltaic and chemical sensing. Among the other features, the organic layer morphology is supposed to influence both the chemical sensitivity and the charge transfer processes. In this paper, we studied the influence of the film morphology on the sensing properties by comparing porphyrins coated ZnO nanorods obtained with two different methods. In the first approach, each porphyrin unit is grafted onto preformed ZnO nanorods by a carboxylic group as linker. The second method is a one-pot procedure, where ZnO nanorods growth occurs in the presence of the water soluble tetrakis-(4-sulfonatophenyl)porphyrin. In both cases the macrocycles share the same Zn-tetraphenylporphyrin core structure, but decorated with different peripheral groups, necessary to comply with the material growth conditions. The adsorption of volatile organic molecules has been monitored measuring the contact potential difference between the sensitive surface and a gold electrode, by means of a Kelvin probe setup. Sensitive signals have been measured both in dark and under visible light. The results show that material preparation affects both the sensitivities to gases and light. A chemometric analysis of four sensors (first and second growth method, measured in dark and in light) shows two main evidences: (a) the interaction between volatile compounds and the sensing layer is largely dominated by non-specific dispersion interaction and (b) the signal of the four sensors becomes rather uncorrelated when the contribution of the dispersion interaction is removed. These results indicate that the differences due to film morphology are enough to differentiate the sensor behaviour, even when the same porphyrin nucleus is used as sensing element. This feature provides an additional degree of freedom for the development of gas sensor arrays.

The influence of gas adsorption on photovoltage in porphyrin coated ZnO nanorods

Recent studies evidence the interplay between the photosensitivity and the gas sensitivity in porphyrin-functionalized ZnO nanorods. These effects are critically governed by the transport phenomena of electronic charge across the interfaces of organic and inorganic structures. Then the surface potential is a useful quantity to investigate the mutual relationship between these two phenomena. For this scope, Kelvin probe measurements of porphyrin-ZnO structures were performed in the dark, under visible light and exposed to organic vapors. Results show a synergic effect of gas sensitivity and photosensitivity for free base porphyrins, while in the case of metalloporphyrins, the coordinated metal ion alters the cooperation between the two effects, as observed in the Zn complex, where the exposure to light increases the adsorption of gases, but the photovoltage is reduced during the exposure to the gas.

Structural and optical correlation of Ni doped ZnO nanorods

Tuning of morphological, structural, optical properties of semiconductor metal oxides often provides a significant strategy for increasing the performance of electronic, optoelectronic, and photocatalytic devices. Here, we study the morphological, structural and optical behavior of zinc oxide nanorods doped with different concentration of nickel by the low-cost wet-chemistry method, before and after annealing treatment. The results are explained using FESEM, XRD and room temperature photoluminescence spectroscopy. These characterizations show that the Ni doping increases the deep level defect and also the structural strain that is caused by the annealing treatment. Overall, this study shows how Ni doping and annealing treatment tune the ZnO properties that can be useful for some applications.

Photo-assisted chemical sensors

The interplay between photosensitivity and chemical sensitivity can give rise to a number of properties that can extend the sensitivity in each separate context. Here we illustrate how the sensitivity to visble light of porphyrins coated ZnO nanorods can modify the gas detection enhancing the sensitivity in particular towards electron donor species.