Paper SS+AS-ThA12
Thermodynamic Control of TTF-TCNQ Molecular Layers on Metallic Surfaces (Invited Talk)

Thursday, November 10, 2016, 6:00 pm, Room 103C

Bulk molecular ionic solids exhibit a fascinating diversity of electronic ground states, including unconventional superconductivity. The electronic properties of these systems have historically avoided surface analytical studies due to the intrinsic difficulty with surface preparation. We are therefore pursuing epitaxial growth of charge-transfer compounds, toward understanding their fundamental properties and creating new kinds of metal-organic or organic-organic interfaces [1,2]. A primary challenge is to assure compatibility of supported epilayers with the redox processes so as to enable, enhance but not eliminate significant charge transfer and electron correlations with supported structures.

In this talk, I will discuss 2D molecular structures of TTF and TCNQ molecules supported on metal and graphitic surfaces. We established that these molecules self-organize into a "zoo" well-ordered structure with a wide-range of TTF:TCNQ ratios [3] - a marked deviation from a single stable 1:1 ratio in the bulk. We propose that the formation these structures is governed by a surface phase diagram that exhibits at least four distinct stable compositions. The diagrammatic picture explains many of the properties of such systems that often seem incidental: the morphology of epilayers, governed by nucleation and growth; coexistence and abundance of various phases, and the distinct molecular structure of phase boundaries. Using coupled image and computational analysis, we conclude that the observed shapes are dictated by the preferential formation of a well-defined “quad”-motif involving TTF(TCNQ) molecules coordinated by at least four of its matching neighbors. Thus enabled deterministic control is beneficial to both electronic properties of surface phases (which can develop local magnetic moments) and the formation of quasi-2D TTF-TCNQ solids, which develop decidedly non-bulk Mott-insulating state despite having 1:1 bulk ratio. We anticipate that these properties are quite general for multicomponent molecular compounds, providing new opportunities for self-organized and electronically interesting molecular systems.

This research was conducted at the Center for Nanophase Materials Sciences, which is a DOE Office of Science User Facility.

[1] G. A. Rojas et. al, and P. Maksymovych, "Ionic Disproportionation of Charge Transfer Salt Driven by Surface Epitaxy", J. Phys. Chem. C, 117 (2013) 19402.

[2] C. Park et. al, and P. Maksymovych, "Weak competing interactions control assembly of strongly bonded TCNQ ionic acceptor molecules on silver surfaces", Phys. Rev. B 90 (2014) 125432.

[3] S. Jeon et. al and P. Maksymovych, ACS Nano in review (2016).