Scanning Tunneling Microscopy Study For Detecting And Manipulating Quantum States On The Molecular Scale

The properties of matters in the single-atom and sigle-molecule regime and the interactions among them are quantum mechanical in nature. Detecting and manipulating the quantum states on the molecular scale are important and important and essential for future applications in designing and constructing novel artificial structures and functional nanodevices in the "bottom-up" scheme. As a local probe, scanning tunneling microscope (STM) has shown powerful ability in physical sciences and allows us to directly detect and manipulate single atoms, molecules and clusters as well as their quantum states at the very basic level of matters.In Chapter I, we briefly introduce the fundamental principles and methods of STM in both experimental and theoretical aspects. We also discuss recent progresses in single-molecule study with STM as well as the STM techniques employed.In Chapter II, we studied the properties of single cobalt phthalocyanine (CoPc) molecules adsorbed on Au(111) surfaces and demonstrated how to manipulate the magnetism in individual CoPc molecules through single-molecule bond-selective chemistry. Cutting away eight hydrogen atoms from the molecule with STM allowed the four orbitals of this molecule to chemically bond to the substrate. The localized spin was recovered in this artificial molecular structure and resulted in a clear Kondo resonance with a high Kondo temperature. We also found that the Kondo effect of the dehydrogenated CoPc can be tuned by its chemical environment.In Chapter III, using STM we investigated systematically the initial stages of single Ge atoms adsorbed on a Si(111)-(7×7) surface. When the deposition is at an elevated temperature of 420 K, single Ge atoms are found to substitute for the Si adatoms randomly. When the deposition is at room temperature, single Ge atoms do not replace the Si adatoms but hop among the high coordination sites near Si adatoms. The precise adsorption sites of Ge atoms were determined by first-principles calculations and comparisons with measured STM images.In Chapter IV, using STM we studied the electronic and transport properties of the smallest self-assembled Ag cluster on Si(111)-(7×7) surfaces and found that the Ag cluster serves as a high-performance rectifier with a rectification ratio larger than ~40. We also found that the rectification behavior can be efficiently controlled by tunning the tip-sample distance.