抄録
In recent years, molecular two-dimensional structures on metal surfaces have been actively investigated in the field of organic molecular electronics and spintronics. Organic molecules on metal surfaces forms self-assemble structures that differ from the bulk due to the presence of molecular-substrate interactions in addition to intermolecular interactions. In other words, molecular two-dimensional structures with arbitrary symmetry can be constructed by regulating the symmetry and intermolecular interactions of the organic molecules used and the molecule-substrate interactions. This is expected to lead to the development of novel properties and functions. In particular, the honeycomb lattices are two-dimensional structures with strongly isotropic lattices proposed by graph theory, and their physical properties have been evaluated in various fields.
In this study, we discuss about the structural and electronic properties of self-assembled organic molecular monolayers on Ag(111), focusing on the C3 symmetric triptycene derivative: Trip-Phz (see Fig.1 insert) by means of scanning tunneling microscopy (STM) measurements, tight binding (TB) calculations and density functional theory (DFT) calculations.
Trip-Phz have been found to form a honeycomb lattice in crystals precipitated from TTF solution [1]. The three-fold symmetry of the Trip-Phz molecule and the symmetry of the honeycomb lattice shows that they form a Kagome lattice when the bonding points between the molecules are taken as vertices (red dash line in Fig.1). The band structure of the Kagome lattice has not only in the Dirac band, but also a flat band [2]. This band structure is expected to show strongly correlation phenomena such as magnetism, superconductivity, and fractional quantum Hall effect.
In this presentation, we comprehensively discuss the film structure and electronic state of the self-assembled molecular structure by combining experimental results from STM measurements and theoretical results from TB calculations and DFT calculations.
In the STM measurement, we deposited a very small amount of Trip-Phz on Ag(111) and observed the molecular self-assembled structure and its electronic state in the sub-monolayer region. In this region, three self-assembled structures were observed: a one-dimensional chain structure, a two-dimensional stripe structure, and a two-dimensional structure with honeycomb lattice. In particular, the honeycomb lattice showed 7x7R21.8° /R38.2° superstructures on the Ag(111) surface. These two domains exhibited a chiral structure with each other.
As noted above, this honeycomb lattice shows that they form a Kagome lattice due to the three-fold symmetry of Trip-Phz. Moreover, Dirac band and flat band are confirmed in the TB calculation for this Kagome lattice. Therefore, we performed DFT calculations to confirm whether this electronic states are maintained even on the Ag(111) substrate. As a result, it was confirmed that the Trip-Phz honeycomb lattice maintains Dirac-like band dispersion and flat band even on the Ag(111) substrate for both HOMO and LUMO states.
It is presumed that this is achieved by the π-conjugated plane of the Trip-Phz molecule adsorbing perpendicularly to the Ag(111) surface, thereby suppressing the molecule-substrate interaction.
Details will be reported in the presentation.
References
[1] R. Ushiroguchi, Y. Shuku, R. Suizu, and K. Awaga, Cryst. Growth Des., 20, 7593 (2020)
[2] T. Niziguchi, M. Maruyama, S, Okada, and Y Hatsugai, Phys. Rev. Mat., 3, 114201 (2019)
