Xiangmin Fei

Reversible achiral-to-chiral switching of single Mn--phthalocyanine molecules by thermal hydrogenation and inelastic electron tunneling dehydrogenation.

Induction of chirality in planar adsorbates by hydrogenation of phthalocyanine molecules on a gold surface is demonstrated. This process merely lowers the molecular symmetry from 4- to 2-fold, but also breaks the mirror symmetry of the entire adsorbate complex (molecule and surface), thus rendering it chiral without any realignment at the surface. Repositioning of single molecules by manipulation with the scanning tunneling microscope (STM) causes interconversion of enantiomers. Dehydrogenation of the adsorbate by means of inelastic electron tunneling restores the mirror symmetry of the adsorbate complex. STM as well as density functional theory (DFT) calculations show that chirality is actually imprinted into the electronic molecular system by the surface, i.e., the lowest unoccupied orbital is devoid of mirror symmetry.

Epitaxial growth of large-area bilayer graphene on Ru(0001)

Large-area bilayer graphene (BG) is grown epitaxially on Ru(0001) surface and characterized by low temperature scanning tunneling microscopy. The lattice of the bottom layer of BG is stretched by 1.2%, while strain is absent from the top layer. The lattice mismatch between the two layers leads to the formation of a moire pattern with a periodicity of ∼21.5 nm and a mixture of AA- and AB-stacking. The √3 × √3 superstructure around atomic defects is attributed to the inter-valley scattering of the delocalized π-electrons, demonstrating that the as-grown BG behaves like intrinsic free-standing graphene.

Construction of two-dimensional hydrogen clusters on Au(111) directed by phthalocyanine molecules

Low-dimensional H2 aggregates have been successfully fabricated on Au(111) surfaces and investigated by means of low temperature scanning tunneling microscopy. We use manganese phthalocyanine (MnPc) molecules anchored on the Au(111) surface to efficiently collect and pin hydrogen molecules. A two-dimensional (2D) molecular hydrogen cluster is formed around the MnPc. The hydrogen cluster exhibits bias-dependent topography and spatial-dependent conductance spectra, which are rationalized by the exponentially decreasing threshold energy with distance from the central MnPc to activate the motion of the H2 molecules. This exponential drop reveals an interfacial phase behavior in the 2D cluster.

Controlled Synthesis of Nitrogen-Doped Graphene on Ruthenium from Azafullerene

The controlled synthesis of high-quality nitrogen (N) doped single layer graphene on the Ru(0001) surface has been achieved using the N-containing sole precursor azafullerence (C59NH). The synthesis process and doping properties have been investigated on the atomic scale by combining scanning tunneling microscopy and X-ray photoelectron spectroscopy measurements. We find for the first time that the concentration of N-related defects on the N-doped graphene/Ru(0001) surface is tunable by adjusting the dosage of sole precursor and the number of growth cycles. Two primary types of N-related defects have been observed. The predominant bonding configuration of N atoms in the obtained graphene layer is pyridinic N. Our findings indicate that the synthesis from heteroatom-containing sole precursors is a very promising approach for the preparation of doped graphene materials with controlled doping properties.

Modulation of Fermi velocities of Dirac electrons in single layer graphene by moiré superlattice

Study of electronic properties of graphene on an anisotropic crystal substrate including boron nitride (BN) has raised significant interest recently due to the application of graphene based vertical hetero-devices. We have performed scanning tunneling microscopy (STM) and scanning tunneling spectroscopy studies of single-layer graphene on hexagonal BN (h-BN) substrates with an applied perpendicular magnetic field at low temperature. Different periodic moire superlattices can be resolved with STM, and their quantized Landau levels in high magnetic field are investigated. The renormalized Fermi velocities of massless Dirac fermions in graphene are revealed to show dependent on the moire superlattice. Density functional theory calculation verifies that the interlayer interaction between graphene and h-BN is stronger with smaller twisting angle between lattices of graphene and h-BN, thus, leading to a reduction in the velocity of charge carriers. Our results should provide valuable insight of electronic prope...

Tuning the Proximity Effect through Interface Engineering in a Pb/Graphene/Pt Trilayer System

The fate of superconductivity of a nanoscale superconducting film/island relies on the environment; for example, the proximity effect from the substrate plays a crucial role when the film thicknesses is much less than the coherent length. Here, we demonstrate that atomic-scale tuning of the proximity effects can be achieved by one atomically thin graphene layer inserted between the nanoscale Pb islands and the supporting Pt(111) substrate. By using scanning tunneling microscopy and spectroscopy, we show that the coupling between the electron in a normal metal and the Cooper pair in an adjacent superconductor is dampened by 1 order of magnitude via transmission through a single-atom-thick graphene. More interestingly, the superconductivity of the Pb islands is greatly affected by the moiré patterns of graphene, showing the intriguing influence of the graphene-substrate coupling on the superconducting properties of the overlayer.