Organic molecular electronics devices utilizing the recently developed nanotechnology have a bright prospect of being used as the electronics devices of the next generation. In this paper, advantages of utilizing organic molecules instead of the common Si and inorganic compounds are described first. Then, recent advancement in this research field, the novel devices studied by the author's research group, the future information system consisting of monomolecular units, and problems in the development of this molecular nano-electronics are discussed.
We have examined electronic and structural interactions at the interfaces of organic semiconductors and their insulation layers or electrodes with regard to organic electric field effect transistors. It was revealed that the crystal structure of an organic semiconductor layer depends on its preparation conditions and the surface energy of the substrate, especially near field region of the interface. Also it was revealed that the control of the surface smoothness of the insulator layer, contact condition between the organic semiconductor layer and the insulator layer or electrode are essential to obtain an excellent FET characteristic.
This article presents the concept and some fundamental research on ‘self-assembled’ nanoscale transistors using charge transfer (CT) complexes. First, TTF-TCNQ CT complex wires of organic metals having high conductivity, are formed by applying electric field during vacuum deposition. Then, a self-aligned active part is formed in the gap between a pair of grown wires. The semiconductive part spontaneously formed at the point of wire contact can be used as an active part; besides, we can employ the metal-semiconductor (metal-insulator) phase transition devices consisting of CT complex bilayers of which electronic structure is modulated by external electric field. On the CT complex wires and CT complex bilayer transistors, some experimental results are presented.
Nanoscale patterning, electrical conduction measurements and observation of nanomotion of organic self-assembled monolayers (SAMs) on Au surface were studied for the purpose of realizing single molecular devices. We successfully obtained well-controlled nanostructure in the SAMs by simple phase-separation techniques. The electrical conduction through SAMs were evaluated by SPM and found that electrical currents through the monolayers exponentially decreased with the increase in the molecular length, showing the current mechanism is determined by a direct tunneling. Finally, we observed molecular motion induced by the polarity change of electric fields by scanning tunneling microscopy (STM), when small amounts of mobile asymmetrical disulfides with terphenyl moiety were embedded into pre-assembled dodecanethiol SAMs. We believe that such a nanomotion is one of the demonstrations for the single molecular devices.
Fundamental study of thin film formation utilizing inkjet printing and its application to electronic devices are described. As an application development of an organic electroluminescence (EL) display is demonstrated, where the control of the surface free energy is very effective. Properties of inkjet droplets and the diversity of the films prepared by the inkjet printing are shown. Fundamentally, the evaporation process can be divided into the interface kinetics and the diffusion process. The unique behavior of inkjet droplets is discussed based on the relationship between the droplet size and the evaporation time.
We have investigated the adsorption of benzene C2H6 on Si(100)(2×1) surface at room temperature, using infrared absorption spectroscopy (IRAS) with multiple internal reflection geometry (MIR). To determine the detailed adsorption structure of benzene on the surface, we measured IRAS spectra in the C-H stretching vibration region and compared the measured C-H vibration frequencies of the adsorbed benzene molecule with the calculated ones obtained on the basis of the so-called hybrid density-functional theory (DFT). We confirmed that the benzene molecule nondissociatively adsorbs onto dimers on the surface. We found that benzene adsorbs in different manners, depending on the surface coverage of benzene. At low coverage the molecule adsorbs on the surface to favor the formation of benzene adsorbed on two adjacent dimers. On the other hand, at high coverage, the molecule adsorbs on the Si surface to generate benzene that adsorbs on a single dimer.
We have investigated processes of structural changes taking place while depositing In onto the InSb(111)B-(2×2) surface by using scanning tunneling microscopy. The results obtained are as follows: Structural changes from the (2×2) to the (3×3) surface at 200oC are mediated by a disordered structure formed during In deposition. However, at a higher temperature of 250oC the (2×2)-(3×3) change directly occurs without the mediation of disordered structure. This suggests that the disordered structure plays a crucial role in relaxing a strain stored in this system during the change at 200oC. Furthermore, at the very early stage of In deposition at 250oC, the (3×3) structure is initiated by assembling three identical rings, presumably resulting in minimizing the number of dangling bond.
We have investigated the processes of Au-In alloy formation in Au/InSb(111)A system by using TED (transmission electron diffraction) and HR-TEM (high-resolution transmission electron microscopy) in the plan-view and profile geometries. At room temperature, the Au9In4 alloy with a γ-brass structure emerges on the InSb(111)A surface with an Au coverage of 3 ML. Subsequent annealing at 150oC leads to the structural and stoichiometric changes from Au9In4 to AuIn2 that fits the substrate better than Au9In4 (0.6% for AuIn2 and 1.3% for Au9In4). Such changes are promoted by In-rich conditions and high mobility of In atoms induced by high temperature annealing.