aDept.of Electrical, Information & Control Eng., Hongik Univ.
Seoul 121-791, Korea bResearch Institute of Science & Technology, Hongik Univ. cCenter for Organic Materials & Information Devices, Hongik Univ. dDept. of Chemical Eng., Hongik Univ.
In this study, we have investigated the possibility of redox-active organic monolayers as molecular-scale information storage systems. The memory elements are porphyrin molecules; information is stored in the oxidation state of these molecule. While both porphyrin anions and cations can be formed electrochemically, in this study we have used the cation state due to its greater chemical stability. Monolayer films of 5,10,5,20-Tetrakis-Octadecyloxymethylphenyl-Porphyrin-Zn(II) (C120H184O4N4Zn, molecular weight =1744.754) were prepared by Langmuir-Blodgett(LB) method and characterized by using UV/vis Absorption Spectroscopy, Cyclic Voltammetry(C-V), and Current-Voltage measurement(I-V). A limiting area per molecule was about 135 Å2 and relatively low collapse pressure of 45 mN/m was shown. Because the limiting areas for face-on and edge-on orientations of porphyrin ring arranged in a monolayer are known to be about 160 Å2 and 70 Å2, respectively . Therefore this value implies that porphyrin rings are tilted away from the air-water interface. Starting points of oxidation/reduction potentials were measured to be +0.59 V and -1.98 V, respectively. The C-V measurement showed that the ionization potential was 5.39 eV and the electron affinity 2.82 eV; the band gap of oxidation/reduction potential was 2.57 eV. The I-V characteristics of the solid state device can be understood in terms of the molecular properties observed in C-V. Current flow in both the solid-state devices and solutions is reversible at positive potentials and irreversible at negative ones. This correspondence between the solution and solid-state results suggests that the fundamental molecular electronic properties are retained in the solid-state devices. If this is the case, the forward bias current flow should be determined by the HOMO state, whereas the reverse bias current should be determined by LUMO state. Further details of the electrical properties of Porphyrin-Zn(II) derivative films will be discussed.
1. X. Qian, A. Tai, X. Sun, S. Xiao, H. Wu, Z. Lu, and Y. Wei, Thin Solid Films, 285, 433 (1996).
Young Kwan Kim
Dept. of Chemical Eng., Hongik Univ.
72-1, Sangsoo-Dong, Mapo-Ku, Seoul 121-791 Korea, Republic of (South)