First Principles Studies On Transport Properties Of Molecular Devices And Electronic Structures Of Magnetic Adatoms On Metal Surface

With the development of nanotechnology and micro-processing, it is now possi-ble to manipulate atoms and molecules, detect properties of devices. Researches on nano and surface fields develop rapidly. The development enriches researches on solid state physics, materials, quantum chemistry and so on, and it also profoundly affects information techniques, life science, clean energy, environmental science, etc.. With the miniaturization of the system, devices become more and more complex, and the quantum effect become more and more important. First principles calculation becomes indispensable in nanotechnology and surface science. In these researches, it can help to interpret of the experimental phenomenons, to reveal the underlying physics, and also to guide the experiment, even to achieve design of devices.This thesis is about first principles studies on design and characterization of sin-gle molecular devices and magnetic materials. For the formation of single molecular devices, we explain and characterize the problems via first principles calculation, and do some theoretical designs and predictions. We also perform some calculations on magnetic materials. From the first principles calculations, we perfectly interpret the experimental observations. The specific arrangements of this thesis are as the follows:In Chapter1, we simply introduce the basic framework of first principles theory, its implementation in softwares and commonly used approximate methods. Since the birth of quantum mechanics, people describe the properties of systems in nano fields by quantum mechanics. In the electronic structure theory, approximate methods based on quantum theory are used, Hopefully, they are computationally affordable and in the mean time give acceptable results to describe the real world. In this chapter, we first introduce three basic approximations which are widely used in density functional cal-culation, i.e. non-relativistic approximation, Born-Oppenheimer approximation, and the independent particle approximation; As the cornerstones of the modern electronic structure theory, Hartree-Fock approximation and self-consistent field method are also introduced. At the end, several software packages based on density functional theory are introduced.In Chapter2, the background of molecular electronics is introduced, as well as the method to construct single molecular devices and some recent progresses. Then, we briefly introduce Landauer-Biitiker theory and Green’s function method based on tight binding approximation, including the surface Green’s function method, retarded Green’s function method and nonequilibrium Green’s function method. At last, we also introduce commonly used software packages for electron transport properties cal-culations.As the linkage of molecule and electrodes, archoring groups play a critical role in the formation of molecular junctions. In Chapter3, taking butane molecular wire as an example, based on density functional theory and the nonequilibrium Green’s function technique, we systemically compare the effects of three kinds of anchoring groups (amine, sulfide, and phosphide) with two side groups (hydrogen and amide) on the transport properties of the corresponding molecular junctions, and the results are in consistent with available experimental data. The transport properties of the six molecular junctions can be understood by using the tunneling barrier model, and the HOMO level alignment dominated by the p-orbital energy of N, S and P gives a rough indicator to the barrier height, thus affects the transport properties of the six molecular junctions. Our calculation results demostrate the role of archoring group chemistry in molecular electronic devices.In Chapter4, using benzene molecule as an example, we perform a first principles investigation on electron transportation through several carbon based junctions (using (5,5) metallic carbon nanotube (CNT), two dimensional (2D) grapnene and quasi-one dimensional graphene nanoribbon (quasi-ID GNR) as electrodes). The transmission spectra of these devices differ a lot. The geometric structures of quasi1D and2D junctions are nearly in plane, it seems that the electronic structures are conjugated. However, it is not the conjugated electronic structure but the edge states of quasi-1D GNR electrode dominates the transport properties, and there is a significant negative differential resistance (NDR) effect due to the changing of local states under different applied bias. Within a certain distance between the two electrodes, the transmission is independent on the isolated molecule,and the electronic structure of electrodes domi- nate. The transmission of2D graphene electrode based junction is similar to its energy band structure, which show a linear behavior with a Dirac point at the Fermi energy, and the curvature of the CNT electrode causes the difference with the CNT based junc-tion.In Chapter5, we mainly study the Kondo effect of single cobalt (Co) atoms and clusters on Ru(0001) surface. We find out the most stable adsorption structures of sin-gle Co atom and clusters on Ru(0001) surface, and some analyses are done. According to the results of our calculation, we find that the magnetic moment of Co clusters re-mains, and are mainly dominated by the d-orbital with the magnetic quantum number [m]=1, and they are ferromagnetic coupled between any two Co atoms. The dis-tances between Co atoms and Co atom to the Ru(0001) surface are different for all the systems. We suppose that with the formation of close packed triangle trimer, the ferromagnetic coupling is more strong than that in the Co dimer and linear trimer, then the Kondo temperature is low and can’t be detected in the experiment. As a compar-ison, we also calculate the adsorption of single Co atom on the Ag(111) surface. The magnetic properties of single Co atom and clusters on the metal surface can be further understood.