Research

Surface-supported Magnetic molecules - Kondo effect

Molecular spintronics that utilizes both spin and charge of magnetic molecule is actively studied because of possible quantum-bit and memory applications. The advantage of using magnetic molecules lies in their small sizes, self-assembly, and adaptability. The key factor of magnetic molecule is the unpaired electron spin that in many cases, locates at the hybridized states between d and p-orbitals of magnetic atoms and organic ligands. Controlling spin states of magnetic molecules is essential for spintronic molecular device applications. An easy way to implement spin-state control in magnetic molecules is to provide another molecule to form a new chemical bond. Here, we study the methods to switch spin state of molecules at the single molecule level. We exploit the Kondo effect, the screening interaction between the unpaired spin of Co-porphyrin and the Fermi electrons of substrate, to sense the spin state of magnetic molecules on metal surfaces. We use voltage ramp and pulse methods in scanning tunneling microscope junctions not only to control spin state of molecules but also to disclose their reaction mechanisms.

On-Surface Reactions and Supramolecular Structures

Molecular self-assembly is ubiquitous in biological systems due to its high efficiency and preciseness. It can be used to fabricate functional nano-structures for electronic devices and sensors. Understanding its mechanism based on weak intermolecular interactions, such as hydrogen (H) bonds, van der Waals (vdW) interactions, and dipole-dipole interactions, is a big concern in current nano-science research. On crystal surfaces, molecular layers grow with ordered structures guided by the intermolecular interactions.The intermolecular interactions have a common origin in electrostatic force, but have different strength and directionality, resulting in various ordering shapes. Here we study characters of intermolecular interactions in two-dimensional supramolecular systems on surfaces using scanning tunneling microscopy. We focus on the rather unawared intermolecular interactions of halogen bonds formed between covalently bonded halogen-ligands that possess unusual charge distributions, and nucleophilic molecular ligands. Due to their favorable lipophilicity, halogen ligands are used in a substantial portion of antibiotic inhibitors and drug candidates to enhance membrane penetration.

Epitaxial Graphenes and Layered Structures

Graphene, the single sheet of honeycomb carbon lattices, has inspired remarkable advances in nano-science due to its unprecedented electronic band structures represented by massless Dirac cones. Graphene can be epitaxially grown on metal substrates using chemical vapor deposition (CVD) of hydrocarbons such as methane and ethylene, which enable one to have large scale uniform graphene layers, with incorporated atomic impurities applicable for charge doping. However, there are always nonnegligible interactions between graphene and metal substrates that hinder the epitaxial graphene to maintain its unique electronic properties. To weaken the interactions, various atoms such as Au, Si, hydrogen, and oxygen have been deposited, during or after the growth of epitaxial graphene at elevated temperatures. Another way to weaken the interactions is to insert hexagonal BN between graphene and substrates. Here we study the atomic and electronic structures of graphene grown on metal substrates with additional oxygen molecules or hexagonal BN layers. We focus on the recovery of Dirac cone, strain effect, and chiral electron tunneling near domain boundaries.

Topological Materials

Topological materials are a new class of materials that shows exotic electronic properties such as absent backscattering by atomic defects and non-magnetic impurities, which originates from strong spin-orbit coupling and broken inversion symmetry, thereby are considered as promising materials for spintronic and quantum computing applications. A binary compound Bi2Se3 has been actively studied as a model topological insulator because of its large bulk energy gap (~0.3 eV), topological surface state with a linear dispersion, and helical spin structure. In ultra-thin films of Bi2Se3, the linear dispersion of surface state is significantly modified not only by the additional potential gradient caused by the substrate but also by the coupling between the electronic states of the top and the bottom surfaces. Here, we study standing waves near the steps of several quintuple layer Bi2Se3 films using scanning tunneling microscopy and spectroscopy. The Bi2Se3 films were in-situ grown with molecular beam epitaxy on the Au(111) substrate.