Optoelectric Charge Trapping/Detrapping in Thin Solid Films of Organic Azo Dyes:  Application of Scanning Tunneling Microscopic Tip Contact to Photoconductive Films for Data Storage

Charge trapping/detrapping within thin solid films of the organic photoconductive materials Sudan I and Disperse Red I was investigated. Charge trapping took place under an external bias upon irradiation. Trapped charges could be stored for long times in the dark under open-circuit or short-circuit conditions and could be removed (detrapped) with irradiation at short-circuit. These substances show good photoconductivity and high dark resistance and are promising for electrooptical information storage. A sharp tip of a scanning tunneling microscope was used as a liftable electrode for the optoelectric characterization. Such a point contact with the tip/organic thin film/ITO configuration provided for more reproducible measurements, since they were essentially unaffected by factors such as structural defects, isolated impurity domains (e.g., dust), nonuniform thickness, and point electrical shorts that frequently perturb measurements with sandwich thin-film cells of larger area. Moreover, the tip can easily penetrate into the film to obtain thickness-dependent information, which is often time-consuming to obtain with conventional thin-layer cells. The fact that charge can be trapped and detrapped in a highly localized area under a sharp tip within the organic thin films demonstrates their potential application as an alternative memory media for high-density data storage (as charge). Information could be written, read, and erased by controlling irradiation and electrical bias as previously reported for zinc octakis(doceoxyethyl)porphyrin thin solid films.