The Characterization And Electronic Transport Research On Self-assembled Layers By Scanning Probe Microscopy

The self-assembled monolayers are wildly used in electrochemistry, surface modification, molecular device, nanolithography, bio-X and so on. The basic research involving self-assembled monolayer has been the hot spot, in which the research of electronic and charge transport properties becomes important and challengeable. In this dissertation, we choose the molecules with different chain length and end groups, and give them a in-depth exploratory research by conductive atomic force microscopy (CAFM) and scanning tunneling microscopy (STM). The main content is as follows:In chapter 1, we describe some important principle molecular devices, and then we introduce the category and self-assembled principle of the monolayer and display some applications in biology, chemistry, nanolithography, organic field effect transistor and other fields. Finally we summarize the present research of self-assembled monolayer characters and the intention of this dissertation.In chapter 2, we brief the principle, modes, functions and some important applications of SPM, which are very important in our experiments,In chapter 3, the charge transport characters of monolayers self-assembled on Au (111) surface are investigated detailedly by CAFM. We study the electrical properties of the self-assembled monolayer under different conditions, such as different molecular chain length, various loads, the results show that current through self-assembled monolayer decreases exponentially with increasing chain length; and the increasing load, the decay factor decreased at low loads, and then get stable under large load. Combining the different theory modes analysis, we verified the rationality of theory models,which clarify two charge tunneling mechanisms in self-assembled monolayer: the through-bond and chain-to-chain; especially, the chain-to-chain current increases more distinctly in self-assembled monolayer with longer molecular chain length.In chapter 4, we further study the charge transport characters of the self-assembled monolayers with different end groups. The results show that the symmetry ofâ… -â…¤and surface potential is influenced by the end groups. Based on the research of load-dependence characters of the conductance of self-assembled monolayers, we find that the end group has strong influence on the through-bond tunneling, but almost no influence on chain-to-chain. With the X-ray photoelectron spectroscopy analysis, we find that the high binding energy would induce the strong electronic localization in the terminal group of alkanethiol molecule, which results in heightening the effective barrier of the electron injecting into the molecular chains and lessening the transport ability of the electrons in the molecule as well.In chapter 5, we study the 4-aminothiophenol (4-ATP) molecule which has strong polarity for self-assembled technique as well as the influence of the topography. We observe different surface topography induced by different STM scan bias, and the surface character remains even the bias is removed. We also observe a reversible change of the 4-ATP film topography by the circular STM scanning bias, however, such change has a hysteresis effect. Otherwise, we find with different scanning bias, the 4-ATP films show different conductive ability. These characters indicate the 4-ATP films may be used as memory materials. In additional, we used an original mix-assembled method to assemble the 4-ATP molecules domain (about dozens of nanometers in diameter) in the alkanethiol films, to improve the storage performance of the film.