ABSTRACT
When organic materials are in contact with metals, electronic charges are displaced from one to the other and organic materials are charged [1-3]. Similar interfacial phenomena are observed at the organic-organic, organicinorganic, and organic-liquid interfaces. All of these are electrostatic phenomena and have been known for a long time, i.e., since the discovery of "electricity." However, the details of the electrostatic phenomena have not been clarified, e.g., possibly owing to either the difficulty in the preparation of sophisticated interface or the lack of techniques to gain information on nanometer-scale interfacial electrostatic phenomena. As such, the issue of electrostatic phenomena is still under continuous study. Fortunately, the situation has been improved due to the remarkable progress in science and high technologies in recent years. For example, we can easily obtain organic ultrathin films with a controlled thickness onto substrates by means of various techniques such as organic molecular beam epitaxy (OMBE) [4], Langmuir-Blodgett (LB) technique [5,6], self-assembly technique [7,8], and others. Furthermore, we have many advanced techniques, e.g., scanning tunneling microscopy (STM) [9], ultraviolet photoemission spectroscopy (UPS) [10], and so on. And many organic materials have been newly synthesized [11-13]. Thus, there is growing interest in the interfacial phenomena from the viewpoint of science and technology [14-16]. Furthermore, in the field of electronics and electrical engineering, it is believed that a full
understanding of the interfacial phenomena is essential to make organic devices such as organic electroluminescent (EL) devices and molecular diodes because the interfacial phenomena will directly affect the device operation, such as current-voltage (7-V) and capacitance-voltage (C-V) characteristics [17-19]. Moreover, the interfacial phenomena are believed to give a deteriorative effect on the electrical properties of commercially used insulator films such as polyethylene [20]. For a profound understanding of the interfacial electrostatic phenomena, it is very helpful to use ultrathin films whose thickness is less than the electrostatic double layer formed at the interface, and then to gain information on the distribution of the electronic density of states as well as the space-charge distribution of excess charges at the interfaces [21,22]. After that, we need to discuss the matters related to interfacial energy surface states, e.g., the energy level alignment in association with interfacial electronic structures at organic-metal and organic-organic interfaces [10]. LB films will be suitable because they can be easily prepared onto solid substrates by the layer-by-layer deposition with an order of monolayer thickness [5,6,23]. For example, polyimide (PI) LB films with the electrical insulating property are favorable and they are especially interesting from the viewpoint of electrical insulation engineering [24,25]. Phthalocyanine LB films are also suitable and interesting in electronics because phthalocyanines are widely used in organic electronic devices such as solar cells, gas sensors, photoconductors, and others [6,26]. Furthermore, electrically conductive LB films, such as charge-transfer salts and fullerenes, are interesting [13]. Obviously, organic-metal, organic-organic, and organic-inorganic interfaces produced by the LB technique may not be sophisticated. There are many possibilities that lead to destruction of the sophisticated interface in the fabricating process possibly because LB films are prepared by the transfer of floating monolayers on a water surface. Among them are the inclusion of water molecules, the destruction of the texture of monolayers on a water surface, and so on [5,6]. One must keep these in mind for the discussion on the matters related to interfaces. However, the interfacial phenomena observed in these films are also very informative for a full understanding of the interfacial electrostatic phenomena, especially for practical use. In this chapter, we focus on the interfacial electrostatic phenomena in insulator and semiconductor monolayer films prepared by the LB technique. It is instructive here to note that basic physics underlying the interfacial phenomena does not rely on the nature of materials, i.e., whether they are insulators or semiconductors. In addition, we may carry a similar discussion for organic films prepared by other methods such as OMBE and so on.
