Electrical Characteristics Of Several 3-methylthiophene Derivatives Devices

The development of silicon-based inorganic semiconductor electronic devices was quickly toward the direction of miniaturization in the last twenty years, and getting closer to its size limit according to the Moore Law. While the size and performance of electronic products have become increasingly demanding with the development of human society. Therefore, aiming at molecular devices and computers through single molecule has been the most active field. Taking this as background, the molecular electronics in terms of new molecule devices, breakthrough in terms of experimentation and fabrication techniques and research progress in the theory are summaried. The progress in both experiment and theory establishes the advantages and disadvantages of each, and shows the importance of the combination of the experimental and theoretical studies. In this thesis,3-Methylthiophene derivatives are taken as objections in both experimental and theoretical aspects.Using a soft stamp-printing method, we fabricated addressable self-assembled monolayer molecular crossbar junctions and studied the charge transport of two long chain 3-Methylthiophene derivatives (TMTP and THPT). The characteristics of asymmetric I-V, the decrease of current ratio with increasing temperature and the differential conductance spectra divided by temperature in dark, are found in both junctions, while the THPT junction also shows obvious reversible optical switching and negative differential resistance (NDR) at low temperatures. Analysis indicates that the charge transport mechanisms in THPT junction are dominated by none temperature dependent direct and Fowler-Nordheim tunneling respectively, in addition, temperature dependent Schottky and Poole-Frankel emission also play key roles in TMTP junction. These differences arise from the structure difference of two molecules and the different effects by temperature and illumination on two junctions. The first principle calculation (using the combination of density functional theory (DFT) and non-equilibrium Green’s function method (NEGF)) indicates that the ideal single TMTP and THPT junctions both show NDR effect, which excludes the reason of NDR in experiment from the extra collateral of THPT. The NDR behavior is attributed to the resonant tunneling caused by the change of coupling degree under different biases.To further study the TMTP molecule from a side view, four ideal single molecular junctions with increasing 3-Methylthiophene rings (TMTP-1-TMTP-4) were built and the transport properties were studied using DFT+NEGF method. The calculation results indicate that, as the number of 3-Methylthiophene ring increase, the Ⅰ-Ⅴ asymmetries (rectification effect) of the systems are strengthened, and the NDR behaviors are enhanced. The appearance of rectification effect depends on the asymmetry and size swap of integral area in bias window at positive and negative biases. The NDR effects of TMTP-2-TMTP-4 are concluded as the results of resonant tunneling caused by the appearing flush and dislocation between the HOMO or LUMO orbital and the Fermi level.Taking into account the pyridine ring in TMTP, three Au/molecule/Au junctions included both 3-Methylthiophene and pyridine ring (TMTP-5-TMTP-7) were built and the transport properties were studied using first principle calculation. The calculations show that the system conductivities decrease as the length increase; the conductive ability of TMTP-7 is better than that of TMTP-6 caused by the different pyridine position; influenced by both 3-Methylthiophene and pyridine, TMTP-5-TMTP-7 exhibit switching characteristics. It is found that the Ⅰ-Ⅴ characteristics of three junctions are contributed by both occupied and unoccupied orbital; the NDR effects are mainly caused by LUMO in negative direction, while those are the common results of the orbitals near the Fermi level.Comparing TMTP-1-TMTP-4 with TMTP-5-TMTP-7 systems, influenced by the pyridine ring, we observe that the conductivity is decreased, the Ⅰ-Ⅴ asymmetry is enhanced, the rectification effect becomes more obvious, and the NDR is changed with the bias direction. It is concluded that pyridine ring changes the molecular electronic structure, and the bias position and direction of resonant tunneling are changed due to the different effect of molecular orbital influenced by biases. Comparing the TMTP junction with seven short junctions, we can conclude that the electrical characteristics of TMTP molecule are decided by the electrical characteristics variation of 3-Methylthiophene with ring number changing and the properties variation after connected with pyridine ring.