The Investigation Of Electron And Spin Transport Properties Based On Porphyrin Molecular Device

The idea that atoms could be used to construct nanoscale devices was first raised by Fernman, a winner of the Nobel Prize for physics. Aviram and Ratner developed a vision for the ultimate miniaturization of electronic devices and the first proposed molecular rectifier. This proposed device uses an asymmetrical combination of electron donar and electron acceptor separated by an insulating electron bridge. The desire of using molecules as functional units in electronic circuits then motivated many researchers in the next few decades. Molecular electronics, not only meets the increasing technical demands of the miniaturization of traditional Si-based electronic devices, but also provides an ideal window of exploring the intrinsic properties of materials at the molecular level. Many experimental techniques, including self assembled monolayer(SAMS), scanning tuueling microscope(STM), or mechanically controllable break junctions(MCBJ) have been employed to fabricate the molecular devices such as molecular wires, molecular switches, molecular rectifiers and molecular storages. Howerer, how to construct the stable molecular devices is one of the most difficult problems in experimental works. Especially, the experimental results for different contact sturctures may show complete contrary characteristics. Therefore, the theoretical computational modeling can be used as a pre-assessment method and the guide for actual reseach. With the rapid development of computer functions, simulation calculations play important roles in developing molecular materials and designing molecular devices. Porphyrin-based molecules have attracted much attention in nanoelectronic and photovotatic devices due to highly conjugated structure, rigid planar geometry, the chemical stability, and unique optoelectronic properties. In particular, porphyrin can coordinate with various metal atoms which may affect the quantumn transport properties through it. In this thesis, we investigated the effect of the molecule-electrode contact configrations, different coordinated metal atoms and functional groups on the electron and spin transport properties of porphyrin-based molecule junction systematically by using the non-equilibrium Green's function in combination with the density functional theory. The main aspects concerned in this thesis include:(1) The electronic transport properties of benzene–porphyrin–benzene(BPB) molecules coupled to gold(Au) electrodes were investigated. By successively removing the front-end Au atoms, several BPB junctions with different molecule-electrode contact symmetries were constructed. The calculated current–voltage(I–V) curves depended strongly on the contact configurations between the BPB molecules and the Au electrodes. In particular, a significant low-voltage negative differential resistance effect appeared at-0.3 V in the junctions with pyramidal electrodes on both sides. Along with the breaking of this tip-contact symmetry, the low-bias negative differential resistance effect gradually disappeared. Moreover, the asymmetric configuration with pyramidal electrodes on the left side and planar electrode on the right side with the top S–Au binding can be regarded as a good candidate for multifunctional molecular devices with both low-bias NDR and rectifying performances. Analysis indicate the tip Au electrodes could dramatically improve the transport properties. And low-bias NDR behavior mainly came from the shift in the frontier molecular orbitals and the narrow features of the transmission peaks.(2) The electron transport properties of M-BPB(M=Zn, Ni, Cu, Mg, Co, Ca)coupled to gold(Au) nanowires electrode in a wide voltage range(0–3.0 V) are investigated.By successively removing the front-end Au atoms, we firstly construct Co-BPB molecular junctions with different S-Au binding modes. Multiple negative differential resistance(NDR) peaks emerge at different bias voltage regions. In our simulations, low-bias regime NDR behavior around 0.4 V can only be seen in S-Au top binding configurations(C1, C4, C6). Besides, at moderate voltages between 1.0 and 2.0 V, two other NDR regions with different characteristics can be seen. NDR peak at 1.2 V only emerges in top configurations and the NDR effect at 1.6 V is shown in different contact modes. The high-bias regime NDR around 2.8 V can be seen in all the six structures.Our results indicate that the change in the coupling between the molecular orbitals and the electrodes due to the shift of the molecular orbitals caused by the contact mode is the origin of the NDR appearing at low bias(0.4 V). The NDR appearing at high bias voltage(2.8 V) originates from the intrinsic frontier molecular orbital of Co-BPB. The combination and competition of these two mechanisms lead to the moderate voltage region NDR effects. In all the M-BPB junctions, Ca-BPB and Co-BPB have better electron transport properties because of the delocalized electron states.(3) The spin polarized transport properties of Co-BPB molecule coupled to gold(Au) nanowires electrodes are investigated by the nonequilibrium Green's function method combined with the density functional theory. The results demonstrate that the top-binding may result in spin dependent transport properties and will be the priority selection in the design of molecular devices.The spin polarized transport properties of Co-BPB molecule coupled to gold(Au) nanowires electrodes with different functional groups(amino and nitro) are also investigated. The calculated results show that spin polarized transport properties can be tuned through substituting H atoms in central porphyrin ring or side benzene ring with different electron-donating/withdrawing functional groups. Both enhanced low-bias NDR effect and high spin-filter effect can be acquired in our proposed molecular junctions. The spin-polarized transferred charges lead to the shift of the frontier molecular orbitals. The changes in the coupling between the molecular orbitals and the electrodes due to the shift of the HOMO levels of spin-down channel of Co-BPB with NH2 and NO2 groups are the origin of enhanced spin polarized transport properties. This system may be a promising candidate for realistic applications in molecular spintronics.Our findings might be useful in the design of multi-functional molecular devices in the future.