Mapping the Diffusion Potential of a Reconstructed Au(111) Surface at Nanometer Scale with 2D Molecular Gas

The adsorption and diffusion behaviors of benzene molecules on an Au(111) surface are investigated by lowtemperature scanning tunneling microscopy.A herringbone surface reconstruction of the Au(111) surface is imaged with atomic resolution,and significantly different behaviors are observed for benzene molecules adsorbed on step edges and terraces.The electric field induced modification in the molecular diffusion potential is revealed with a 2D molecular gas model,and a new method is developed to map the diffusion potential over the reconstructed Au(111) surface at the nanometer scale.

Adsorption and rotational barrier for a single azobenzene molecule on Au(111)surface

The orientation switching of a single azobenzene molecule on Au(111)surface excited by tunneling electrons and/or photons has been demonstrated in recent experiments.Here we investigate the rotation behavior of this molecular rotor by first-principles density functional theory(DFT)calculation.The anchor phenyl ring prefers adsorption on top of the fcc hollow site,simulated by a benzene molecule on close packed atomic surface.The adsorption energy for an azobenzene molecule on Au(111)surface is calculated to be about 1.76 eV.The rotational energy profile has been mapped with one of the phenyl rings pivots around the fcc hollow site,illustrating a potential barrier about 50 meV.The results are consistent with experimental observations and valuable for exploring a broad spectrum of molecules on this noble metal surface.

Raman Spectrum of Epitaxial Graphene Grown on Ion Beam Illuminated 6H-SiC （0001）

Patterning SiC substrates with focused ion beam for growth of confined graphene nanostructures is interesting for fabrication of graphene devices. However, by imposing an ion beam, the morphology of illuminated SiC substrate surface is inevitably damaged, which imposes significant effects on the subsequent growth of graphene. By using confocal Raman spectroscopy, we investigate the effects of ion beam illumination on the quality of graphene layers that are grown on 6H-SiC （0001） substrates with two different growth methods. With the first method, the 6H-SiC （0001） substrate is flash annealed in ultra-high vacuum. Prominent defects in graphene grown on illuminated areas are revealed by the emergence of Raman D peak. Significant changes in D peak intensity are observed with Ga＋ ion fluence as low as 10^5 μm^-2. To eliminate the damage from the ion beam illumination, hydrogen etching is employed in the second growth method, with which prominent improvement in the quality of crystalline graphene is revealed by its Raman features. The defect density is significantly reduced as inferred from the disappearance of D peak. The Raman shift of G peak and 2D peak indicates strain-released graphene layers as grown in such a method. Such results provide essential information for patterning graphene nano-devices.