First Principles Investigations Of The Transport Properties Of Homoatomic Clusters And Binary Clusters

Based on a recently developed Density-Function method(DFT) and ab initio non-equilibrium Green's function (NEGF) formalism , we have performed first-principles calculations on the transport properties of small silicon and aluminum clusters: Al₂,Si₂,Al₄ and AlSi. Investigations on these clusters are primarily concentrated on the e?ect of contact distance between electrodes and clusters ,and the e?ect of gate voltage.In Chapter one, following the introduction of the development of the electronic devices, we introduces the research background of molecular conductors, the experimental methods and main theoretical simulation methods which are used for the study of molecular conductors, and what we will study in this thesis and methods. In the last part of this chapter, the contents and importance of this thesis are listed.In Chapter two, we focuses on some of first-principles calculation methods, e.g. Density-Function method(DFT), including the theoretical basis and the applications.Then we show clearly how to combinate the density functional theory(DFT) and the nonequilibrium green function to calculate the molecular electric transport properties . Finally, the programs we used is presented in brief at the end of this chapter.In Chapter three, based on a recently developed ab initio non-equilibrium Green's function (NEGF) formalism,we investigate the effects of the separation distance between the electrodes and the central molecule and gate-voltage on the transport properties of cluster Al4 coupled to two Al(100) electrodes. Our results show that the equilibrium conductance doesn't change monotonously with the increase of the distance, but increases firstly and then decreases. The gate-voltage affects the equilibrium conductance in an oscillatory manner, and this manner is influenced greatly by the distance. And at a fixed distance, there exists high conductivity and low conductivity with the variation of gate-voltage, which demonstrates a molecular switch behavior. Under the analysis of Density of states (DOS) at the Fermi level for all the cases, we find that reason for variation of the equilibrium conductance is the variation of the Density of states (DOS) at the Fermi level. The charge transfer calculation analysis shows that the conductances for all the cases have nothing with the charge transfer.In Chapter four, based on a recently developed Density-Function method(DFT) and ab initio non-equilibrium Green's function (NEGF) formalism,we present first-principles studies on the transport properties of small silicon and aluminum clusters: Al₂,Si₂,Al₄ and AlSi sandwiched between two Al (100) electrodes. The variation of the equilibrium conductance as a function of contact distance for these two-probe systems is probed. Our results show that the transport properties are dependent on both the special nanostructure and the separation distance between the central molecule and the electrodes. For equilibrium transport properties, the clusters with the similar structure show similar transmission spectra at large distances. For current-voltage characteristics, all the clusters show the metallic behaviour at lower bias, however very different non-linear behaviour can be observed at higher bias. For AlSi and Al2, when the distance between the central cluster and the electrodes is 3.5?, large negative differential resistance (NDR) can be found in the bias range 0.8V¹.4 V.All these results implicate that the determination of the contact structure and the special nanostructure is very essential to correctly predict the transport properties of the molecular conductors.