Theoretical Study Of Polyphenylene-based Molecular Electronic Systems

Ultrahigh integrated circuits have led the dimension of electronic device to nano-scale. However, it is very difficult to lay out at 0.1μm area and fabricate hundreds of thousand gate circuits into 1 mm 2 silicon film. It is the modern high technology that promote the foundation of molecular electronic, which is a cross-discipline involved with chemistry, physics, biology, material science and electronics. The aim of it is to replace the solid state components such as silicon-based transistor with single molecule, super molecule or molecular cluster, and then assemble them into molecular computer. The study involves designing, fabrication, testing and assembling of various molecular devices. This paper is mainly about theoretical study on the polyphenylene-based molecular electronic systems. At first, this paper gives a brief introduction on the polyphenylene-based molecules, comparing their electrical properties, and demonstrates the mechanisms of conductance of the organic molecules. Then, some polyphenylene-based devices-molecular wire, molecular resistor and molecular diode (rectifier and RTD) are introduced in short. Some theoretical studies are given on some key and difficult problems in molecular electronics: As solid state electronics, molecular rectifier is also one of basic devices of molecular electronics.The electronic structure of the polyphenylene-based molecules with a donor-acceptor structure is studied by quantum mechanical calculation, which applied the ab initio Hartree-Fock self-consistent field molecular orbital method using STO 3-21 basis set, as implemented in the Gaussian98 quantum chemistry simulation program. The result of theoretical calculation demonstrated the structure of the better molecular rectifier had significant rectifying properties. The calculation of I-V characteristic is one of the key problems in the theoretical study on molecular device. A three terminal molecular device model is constructed from common organic donor, acceptor and πelectron conjugated spacer structure. A theoretical formalism based on Buttiker-Landauer formula has been developed and applied to calculate the I-V characteristics of the modeled transistor. The calculated curve shows switching behavior of the transistor on application of an external potential to the gate terminal. The study of electron transport in metal-molecule-metal interfaces is the first step in the development of molecular electronics. In this paper, we proposed a quantum chemical calculation for investigating electron transport across single molecular units sandwiched between metal electrodes. Au-SH ? ( pC 6 H 4 )3? SH-Au system is built for validating this theory. At last, we attain the density of state, transmission spectrum and I-V characterization of the molecular device through precise calculation and systematically analyze the electron transport characteristics of three-benzene-ring polyphenylene-based molecules. The polyphenylene-based molecular devices, Tour wire and molecular diode, have been demonstrated that have the analogical function to solid-state ones in experiments and simulations. So, based on the principle of solid-state electronic circuit design, a binary subtractor based on polyphenylene proposed in this paper. The NOT, OR and XOR gates of combinational logic are replaced by the polyphenylene-based molecular device whileTour-type molecular wire constitute the framework in the molecular circuit accounting for the geometry and steric constraints imposed by the bonding and shapes of the organic molecules. The resulting structure of this circuit should have a dimension of about one millionth of the solid state one.