Theoretical Research Of Electronic Transport Properties For Three-dimensional Boron Nitride Molecular Junctions T-B_xN_y{(x,y)=(5,6),(6,5),(11,11)}

With the rapid development of nanotechnology and molecular electronics,electronic components possessing smaller size,higher integration,more functions and faster running speed are becoming the trend of the investigation.Meanwhile,due to the progress of theoretical method and experimental technique,a lot of carbon-based molecular devices with novel properties have been explored and reported in recent years.While boron and nitrogen are closely neighbor to carbon in the periodic table of elements,the binary compound composed of nitrogen and boron can also organize honeycomb or tetrahedral nanostructures,which is similar to carbon-based nanostructures.The investigations for nanostructures based on boron nitride（BN）,of course,have aroused great interests for researchers.In this thesis,firstly,several molecular junctions composed with three-dimensional（3D）BN were constructed,then the electronic transport properties of these 3D BN molecular junctions and their doped or decorated configurations were simulated by density functional theory combined with non-equilibrium Green function approach,and the mechanism of these properties was also investigated.The main contents of this thesis are as follows:The electronic transport properties for T-B_xN_y（x,y=5,6,11）molecular junction sandwiched between two gold electrodes with sulfur atoms were investigated.The calculated results reveal that the current-voltage（I-V）curve of T-B₅N₆ system present strong negative differential resistance（NDR）behavior near ±1.3V bias,the I-V curve of T-B6N5 system presents a significant switch behavior under a lower bias voltage.However,the I-V curve for T-（BN）11 system is asymmetrical in the whole bias range,and the tunneling current is very weak except a delicate NDR behavior appears at higher bias,which is obviously different from the two above systems.These peculiar properties were investigated by analyzing their transmission spectra and molecular projected self-consistent Hamiltonian（MPSH）eigenstates.Comparing the transmission spectrum with projected density of state（PDOS）,we find that the different PDOSs of the left and the right components of the junctions are responsible for the various characteristics of the I-V curves.The sp³ bonded atom within T-Bx Ny molecular junction was substituted by aluminum,phosphorus,carbon,or silicon atom respectively,which realized the dopingof T-B_xN_y molecular systems,and the doped structures were optimized to stable configurations.The properties of I-V for these structures were calculated with the same theoretical method and the same setting of parameters.The calculated results reveal that the tunneling currents of doped T-B_xN_y molecular systems evidently decreased,it is the weakening for transmission peaks and PDOSs on core molecule near Fermi level that results in the evident decrease of the currents.However,the I-V curves for Al@B4N6 and C@B4N6 molecular system present yet significant NDR behaviors,it is the weakening of delocalization for their highest occupied molecular orbitals（HOMO）at high bias that results in the presence of NDR behaviors.In comparison,the NDR behavior presented in the Si@B4N6 system is quite a faintness and appears in high bias region,which leads to little value for practical application.The weak current of the P@B6N4 molecular system only occurs at a few high biases,furthermore,the I-V curve exhibits obvious asymmetry characteristic.The fundamental cause of the asymmetry for I-V curve of the P@B6N4 system is that the spatial distributions of their frontier molecular orbital are asymmetry.Meanwhile,the I-V curve of C@ B6N4 system presents linear growth at lower bias and also presents quite faintness NDR behaviors at higher bias region.In contrast,the tunneling current of Si@ B6N4 system also increase linearly at lower bias,but it demonstrates switch effect at higher bias.The HOMO levels of C@ B6N4 and Si@ B6N4 systems gradual shift to Fermi level at lower bias,which eventually leads to the linear increases for their currents at lower bias.One-dimensional mono-atomic Cn（n=4,5,6,7）chains were connected to one end of the T-B₅N₆ molecule,which was used to explore the modulation of transport properties of the T-B₅N₆ molecule by C atom chains.Analyzing their transport properties at an equilibrium state and finite bias,one can find that the carbon-carbon（C-C）included in C4 and C6 chains demonstrate the rule of long-short alternation at equilibrium state.However,it is the C5 and C7 chains that couple strongly with a T-B₅N₆ molecule and their HOMOs demonstrate delocalization entirely at equilibrium state,which is responsible for their lower threshold voltage,and indicate that the NDR behaviors will emerge in these two systems at lower bias.At finite bias,moreover,the systems composed of C4 or C6 chain and T-B₅N₆ molecule demonstrate significant NDR behaviors at higher bias and demonstrate some negative rectifying performance in thewhole bias region.In contrast,the systems composed by C5 or C7 chain and T-B₅N₆ molecule show preferable positive rectifying effect due to the asymmetry of spatial distribution for their HOMOs at finite bias.Analyzing frontier molecular orbital levels as a function of applied bias for these four systems,the results are consistent with the properties of calculated I-V.All the mentioned results above are a theoretical investigation of electronic transport properties for three-dimensional BN molecular devices,and it is hoped that our results could provide valuable reference information for the practical fabrication and designing of molecular devices based on three-dimensional BN molecules.