Reversible charge trapping/detrapping in a photoconductive insulator of liquid crystal zinc porphyrin

A molecular crystal of zinc octakis(beta-decoxyethyl) porphyrin (ZnODEP) is an insulator in the dark and becomes conductive under irradiation. An externally controllable charge trapping and detrapping within ZnODEP thin films (similar to 1 mu m) occurs when symmetrical sandwich cells of ITO/ZnODEP/ITO are irradiated under a proper bias voltage between two parallel ITO (indium-tin oxide) electrodes. The trapping and detrapping rise time is on the nanosecond time scale. Detrapping of charge stored previously in the cell could be accomplished with pulse irradiation under short-circuit conditions and gives rise to a discharge current spike. Trapped charge induced by a 10 ns laser pulse or by longer time irradiation with a conventional light source could be sensed by a voltage measurement at open circuit. No loss of stored charge was detectable at a 1 pA level for a period of 11 months under open circuit conditions in the dark. After charge trapping with 550 nm light irradiation (10 mu W/cm(2)) under a bias of 0.5 V, the stored charge induced a voltage difference of similar to 20 mV between the two ITO electrodes. This voltage difference was stable for at least 2000 h with no evidence of decay. These results suggest that ZnODEP as a thin film photoconductive insulator might serve as a memory medium for electrooptical information storage in the form of charge. Such a data storage system would be nonvolatile and rewritable. We have shown that a memory element could be subjected to write (trapping)/erase (detrapping) 1.5 billion times with a readout signal that was essentially identical with the first without any evidence of deterioration. To find attainable resolution, charge was injected with a scanning tunneling microscope tip under different bias. For a 6 V bias, charge was trapped in an element of 40 nm diameter, equivalent to a storage density of 8 x 10(10) bits/cm(2).