Scanning Tunneling Microscopy Study Of Electron Transport Properties In Nanostructures

With the advent of the scanning tunneling microscope (STM), for the first time it has been possible to explore the atomic-realm of surfaces in real space. With the help of STM, we can not only resolve the surface or surface-adsorbed nanostructure in atomic scale, but also carry out in situ spectroscopic studies. Moreover, manipulation of single atoms, molecules and clusters has become possible. STM has been widely utilized in physical science, chemistry, material science and biology in recent decades. In this thesis, we studied the electron transport properties of surface-adsorbed nanostructures by taking advantage of UHV-LT-STM.In Chapter1, we briefly introduced the fundamental principles and methods of STM both in theoretical and experimental aspects. We also reviewed the recent progress in surface and surface-adsorbed nanostructures study with STM as well as the STM techniques employed.In Chapter2, we studied both parallel coupled gold nanoparticles and two dimensional gold nanoparticle assemblies in double barrier tunneling junction (DBTJ). We observed strengthened conductance peak in dIdV spectra when place the tip between the two nanoparticles. The strengthened peak has a magnitude more than twice of the other peaks, which is attributed to constructive electron interference between the two nanoparticles. We employ peak ratio to describe the coupling of a nanoparticle with its neighbors in gold nanoparticle assemblies. It is found that peak ratio decreases with increasing neighbors and the multilayer has a smaller peak ratio than the monolayer.In Chapter3, STM/STS experiments have been performed on Si(111)-(?)×(?)-Ag surface at low temperature. The surface on lightly doped wafer at78K and on heavily doped wafer at5K show similar electron transport properties, whereas the surface on lightly doped wafer at5K shows different electron transport behaviors, which are attributed to limited carrier transport in silicon substrate and band-bending change beneath the surface. Furthermore, we demonstrate that these electron transport behaviors can be tuned by light irradiation.In Chapter4, we have studied the electron transport behaviors of single CoPc molecules on Si(111)-(?)×(?)-Ag surface using STM/STS at5K and80K. The CoPc molecule shows NDR effect at its center, irrespective of measuring temperature and doping type and doping concentration of the silicon substrate. On the basis of our DFT calculations, the NDR observed at the CoPc center is attributed to the resonant tunneling between the surface-state band S1and the localized dz2orbital of central Co2+ion. The NDR position of the Co2+ion center of CoPc on Si(111)-(?×(?)-Ag surface with light doped silicon substrate locates at much high negative bias voltage at5K, which is explained by considering the voltage drop at the space-charge layer. Moreover, the NDR position can be tuned through light illumination with various light intensities. By taking advantage of the intrinsic surface-states of Si(111)-(?)×(?)-Ag surface and the proper molecular orbital of CoPc molecule, we demonstrate a way to build single-molecular NDR device on silicon, which may be used to fabricate hybrid silicon-molecular electronics.In Chapter5, we studied single Dy@Cg2molecule on Si(111)-(?)×(?)-Ag surface by using STM/STS. We find NDR effect in the Ⅰ-Ⅴ curves of the molecule and the NDR effect is sensitive to the intromolecular position and the molecular orientation. We demonstrated that the position of the NDR either in positive or negative bias voltage can be tuned by the STM tip manipulation. Theoretical studies reveal that the NDR effect is determined by the different moving route of Dy3+inside the carbon cage when the tip locates over the different sites of the molecule.