ABSTRACT
A solution processed two terminal organic bistable memory device was fabricated utilizing films of polymethyl methacrylate PMMA/ZnO/PMMA on top of ITO coated glass. Electrical characterization of the device structure showed that the two terminal device exhibited favorable switching characteristics with an ON/OFF ratio greater than 1 x 10(4) when the voltage was swept between - 2 V and +3 V. The device maintained its state after removal of the bias voltage. The device did not show degradation after a 1-h retention test at 120 degrees C. The memory functionality was consistent even after fifty cycles of operation. The charge transport switching mechanism is discussed on the basis of carrier transport mechanism and our analysis of the data shows that the charge carrier trans- port mechanism of the device during the writing process can be explained by thermionic emission (TE) and space-charge-limited-current (SCLC) mechanism models while erasing process could be explained by the FN tunneling mechanism. This demonstration provides a class of memory devices with the potential for low-cost, low-power consumption applications, such as a digital memory cell.
ABSTRACT
A comparative study has been made on the mechanisms of toxicities of trivalent and hexavalent forms of chromium in Neurospora crassa. Of the two forms, Cr6+ is more toxic than Cr3+. The toxicity of Cr3+ was found to be due to its specific antagonism with iron uptake. Fe3+ was found to be very effective in reversing the toxicity of Cr3+ by concomitantly suppressing its uptake. That the Cr3+ toxicity caused a conditional intracellular iron deficiency was indicated by the decrease in the activities of catalase and uricase and a progressive increase in the excretion of iron binding compound into the medium. The toxicity of Cr6+ (as Cr2O7(2-)) was found to be due to its specific antagonism of sulfate uptake. Methionine was found to be more effective in reversing dichromate toxicity than sulfate, probably by repressing the synthesis of sulfate permeases responsible for dichromate (Cr6+) uptake. Maximal uptake of Cr6+ was nearly tenfold lower and Vmax much higher than that of Cr3+. Evidence has been adduced to show that Cr6+ and Cr3+ were toxic by themselves and that interconversion between the tri- and hexavalent forms of chromium did not occur to any detectable extent.
