A Two-Dimensional Supramolecular Carpet with Widely Uniform Interfacial Structure
Authors: Kim Ju-Hyung , Kawai Maki , Jung Jaehoon , Fukushima Takanori , Kim Yousoo 
Published: February 2020

Thin films composed of organic molecules are receiving much attention for their practical uses in various organic devices such as organic light-emitting diodes (OLEDs), organic thin film transistors (OTFTs), and organic photovoltaics (OPVs). Particularly, it has been recognized that interfaces between organic thin films and metal electrodes play crucial roles in operation characteristics of organic devices in recent years [1-3]. The interfaces between organic thin films and metal electrodes are where charge-carriers are injected into organic active layers, and various interactions at the interfaces (i.e., surface-molecule and intermolecular interactions) are of great importance in the formation of intermolecular networks and organic epitaxy which have a strong correlation with their electronic structures [1,3]. Both geometric and electronic structures of the interfaces between organic thin films and metal electrodes have profound effects on the performance of organic devices, and it is necessary to understand and control such interfacial structures for further development and enhancement of organic devices. Despite extensive studies, however, the formation of organic thin films with widely uniform interfacial structures on metal surfaces has been suffered from the limitation of molecular ordering imposed by irregular surface structures, due to the relatively weak intermolecular interactions between organic molecules compared to metallic, covalent, and coordinate bonds. Thus, a new approach is required to achieve the formation of widely uniform organic thin films without breaking structural integrity and to enhance the film uniformity.
Here we demonstrate that well-balanced interfacial interactions, based on a careful design of the system, enable the formation of a two-dimensional supramolecular carpet with widely uniform interfacial structure and high adaptability on a metal surface, which can be extended over multiple surface steps and terraces without breaking structural integrity [4]. Strong intermolecular interactions in a planar fashion, symmetry of compounds, and localized surface-molecule interactions were considered in our strategies and principles of the system design. For this work, bis([1,2,5]thiadiazolo)-tetracyanoquinodimethane (BTDA-TCNQ) was used as a key element to form the two-dimensional supramolecular carpet [5], and its geometric and electronic structures were investigated using scanning tunneling microscopy and scanning tunneling spectroscopy under ultrahigh-vacuum conditions. Tetracyanoquinodimethane (TCNQ) is one of the strongest organic electron acceptors which is a widely used class in organic electronics, and it forms a strongly bonded donor-acceptor complex with tetrathiafulvalene (TTF). TCNQ and TTF are self-organized into ordered domains of the strongly bonded donor-acceptor complexes if TCNQ and TTF satisfy a 1:1 stoichiometry. Based on these molecular properties, we designed a model system of BTDA-TCNQ, which integrates both structural properties of TCNQ and TTF with a single molecule. The strong non-bonding intermolecular interactions induced by two electrostatically opposite symmetry axes of the molecule, balanced with surface-molecule interactions and site-specific rearrangements of the BTDA-TCNQ molecules near surface step edges, enables “seamless” growth mode for the formation of supramolecular carpet. It results in widely uniform interfacial structures covering over multiple surface steps and terraces even on the pre-annealed amorphous gold (Au) surface prepared on a glass substrate. In addition, the electronic structures projected onto the supramolecular carpet on the Au surface show different types and dimensionalities corresponding to the energy region and the local position of the system. These result suggest that the supramolecular carpet has great potential for applications in organic electronics, and also provide important guidelines to develop novel materials for seamless growth mode on various surfaces.
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