| AVS 62nd International Symposium & Exhibition | |
| Scanning Probe Microscopy Focus Topic | Thursday Sessions |
| Session SP+BI+NS+SS+TF-ThA |
| Session: | Probing Material Growth on the Surface |
| Presenter: | Seokmin Jeon, Oak Ridge National Laboratory |
| Authors: | S. Jeon, Oak Ridge National Laboratory P. Doak, Oak Ridge National Laboratory P. Ganesh, Oak Ridge National Laboratory B. Sumpter, Oak Ridge National Laboratory J.I. Cerda, Instituto de Ciencia de Materiales de Madrid, Spain P. Maksymovych, Oak Ridge National Laboratory |
| Correspondent: | Click to Email |
TTF-TCNQ (TTF = tetrathiafulvalene; TCNQ = 7,7,8,8-tetracyanoquinodimethane) is a prototypical organic charge-transfer complex providing with a metallic conductivity (up to 900 ohm-1cm-1 at 300K). It also represents a broad class of organic electronic compounds that exhibit strong electron correlations and a rich gamut of phase transitions involving charge ordering, Mott and Peierls metal-insulator transitions and superconductivity, etc. Despite decades of research in this area, quantitative understanding of this compound is still elusive and their low-dimensional form is barely explored. We investigated ultrathin films of TTF-TCNQ on silver surfaces using scanning tunneling microscopy/spectroscopy (STM/STS) at 4.3 K. TTF-TCNQ forms self-assembled molecular lattices on noble metal surfaces with a few different TTF-to-TCNQ ratios depending on evaporation condition. Among them the islands with 1 to 1 stoichiometric ratio are found ubiquitously in less dependent on the evaporation condition. Structures of the monolayer islands are elucidated from sub-molecular resolution STM topography images. Single-point conductivity spectroscopy and conductivity mapping elucidate new electronic states which do not stem from their molecular orbital states are spatially located in the void areas of the TTF-TCNQ molecular lattices. We propose the sp-derived metal surface states are confined in the molecular lattices. Due to small periodicity of the lattice, the band minimum of the sp-derived metal surface states is shifted by as much as 1eV. This shift is much more significant than the ones normally observed in organic self-assemblies. As a result, we can also infer the height of the potential barrier within a 1D potential well model, which in turn is directly related to strong molecular dipole associated with large charge transfer and bent molecular geometry due to metal-molecule interactions.
Acknowledgement: A portion of this research (SJ, PD, PG, BS, PM) was conducted at the Center for Nanophase Materials Sciences, which is a DOE Office of Science User Facility.