Orientational Dependence of the Color and Photoconductivity of 1,4-Di- p -toluidinoanthraquinone Single Crystals

Bard

1998

Chem. Mater.

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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 highdensity 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.

Section: Resultssupporting

confidence: 76%

Section: Introductionmentioning

confidence: 99%

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

Bard

1998

Chem. Mater.

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“…In this case, electrons from the excited state of ZnODEP molecules are injected into the irradiated ITO contact ͑termed an anodic current͒ while positive charges move to the opposite electrode. 1,5,[7][8][9][10][11][12][13] The relation between I sc as a function of time without bias ͑the short-circuit photocurrent͒ from pixel 4D is shown in Fig. 2b.…”

mentioning

confidence: 99%

Addressing of Optoelectronic Memory of Thin Film Zinc Porphyrin with Crossed 5 μm Indium Tin Oxide Arrays

Electrochem. Solid-State Lett.

Bard²

2001

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We report here the preparation and characterization of an optoelectronic memory device based on a single layer of organic thin film ͑ϳ0.9 m thick͒ of zinc octakis͑␤-decoxyethyl͒porphyrin, sandwiched between two crossed indium tin oxide ͑ITO͒ arrays. The ITO lines in the array were 5 m wide and were separated from each other by a 5 m gap. Data ͑in the form of an electric charge͒ could be independently stored at and retrieved from an intersection of the crossed ITO lines with irradiation. Each intersection defined one memory pixel (5 ϫ 5 m) and there was no cross talk with nearby pixels under the test conditions, clearly demonstrating its potential application as an information storage device using a molecular thin film.Zinc octakis͑␤-decoxyethyl͒porphyrin ͑ZnODEP͒ is a flat molecule with eight long hydrocarbon chains. 1 In the crystal, molecules are regularly stacked to form individual columns and the nonionic alkyl tails serve as an insulating coating, leading to a bundle of electrically insulated one-dimensional molecular ''wires.'' 1 Even within the same molecular column, the intermolecule separation is ϳ4 Å, 1 significantly larger than those in inorganic substances. This produces an intrinsic characteristic of a crystal structure consisting of discrete molecules that can be considered an ''orientated molecular gas.'' 2,3 This structure has an inherent advantage over inorganic materials as a medium for high-density charge trapping and, therefore, information storage, because charge carriers are localized on individual molecules and can be trapped for long times on structural defects or impurity sites. [2][3][4] In principle, the theoretical size of one memory element could be as small as a single molecular column representing the ultimate density in data storage. 5, 6 We have demonstrated previously that an individual pixel could be as small as a few tens of nanometers with a configuration of a sharp scanning tunneling microscopy ͑STM͒ tip/organic film/indium tin oxide ͑ITO͒. 5,7 While this tip-type movable structure is simple and convenient for testing the possibility of ultrahigh density data storage, it is not useful for most memory applications where a fixed rigid device is more desirable. Nor is it useful in examining cross talk among different pixels. This led us to the present study of an alternative configuration as shown in Fig. 1, in which each 5 m wide ITO line served as an independent electrode and was individually externally addressable.When a bias voltage was applied to two of the crossed ITO fingers ͑such as column 3 and row D͒, the organic film was subjected to the electric field at only that particular intersection ͑desig-nated as pixel 3D͒. Thus, Fig. 1 illustrates a total of 25 memory elements. Figure 2a shows the current as a function of bias voltage from pixel 3D with and without irradiation. The dark current is negligible over the potential range of Ϫ1 to ϩ1 V. Essentially the same result was seen with the other pixels. Note that in this and other measurements discussed below, the whole fi...

“…Consequently, the inherent properties of the individual molecules are retained in the OMC to a large extent, e.g., as shown by the strong similarity between the optical spectra of OMCs and those of their constituent molecules. − This characteristic of OMCs, sometimes considered as oriented molecular gases, offers interesting opportunities to explore their potential for molecular electronics, since the unique properties of isolated molecules can be a useful guide to macroscopic properties when the molecules are arranged in the molecular crystal. For example, as discussed below, the orientational dependence of light absorption, detectable only with difficulty for polymer-supported solvent green 3 molecules, can be seen as sharply different colors with the naked eye in the single crystal . The weak and long-distance intermolecular interactions cause only a slight perturbation in the electronic orbitals of the individual molecules in most OMCs, leading to a strong tendency for excitons and charge carriers to be localized on individual molecules − ,, as compared to the complete delocalization in the bands of inorganic semiconductors with strongly bound atomic or ionic lattices, like Si and GaAs.…”

mentioning

confidence: 99%

Optoelectronic Properties and Memories Based on Organic Single-Crystal Thin Films

Bard

1998

Acc. Chem. Res.

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Chong-yang Liu was born in Shaanxi, China, and educated at Northwestern University, the Dalian Institute of Chemical Physics, the Chinese Academy of Sciences, and the Hahn-Meitner-Institut fu ¨r Kernforschung Berlin, Germany, where he carried out research with Chen Yixuan, Li Wenzhao, Wang Hongli, and H. Tributsch, respectively. He was a Postdoctoral Fellow with Allen J. Bard and is presently a Research Scientist Associate at the University of Texas at Austin. Allen J. Bard holds the Hackerman-Welch Regents' Chair in Chemistry at the University of Texas at Austin. His principle research interests are in the application of electrochemical techniques to the study of chemical problems.