Article pubs.acs.org/JPCC
Ordered and Robust Ionic Surface Networks from Weakly Interacting Carboxyl Building Blocks Daniel Skomski and Steven L. Tait* Department of Chemistry, Indiana University, Bloomington, Indiana 47405, United States S Supporting Information *
ABSTRACT: Application of self-assembly strategies to technologically relevant materials requires robust interactions between complex organic building blocks at surfaces. However, complex molecules of practical interest are not necessarily in registry with a surface, which may impede self-organization into a desired architecture. In this work, we demonstrate that for an organic system in which neither adsorbate−substrate nor adsorbate−adsorbate interactions are sufficient to yield self-assembly by the organic species alone, a self-organized network possessing high thermal stability is achieved via an ionic bonding protocol. The self-assembled ionic surface network is formed by reaction of tetra-anionic carboxylate species and sodium chloride and achieves a degree of thermal stability not previously demonstrated in self-assembled organic nanostructures at metal surfaces. Thermal stability at 180 °C is demonstrated by molecular-resolution, high-temperature imaging of the supramolecular architectures for several hours. The excellent structural stability is achieved through a cooperative effect of ionic interactions and metal−organic adsorption. Thermodynamically controlled conversion into a single phase, in spite of the nondirectional nature of ionic bonding, is demonstrated here for the first time in ionically bonded networks at the metal surface. The degree of sodium incorporation into the surface networks can be precisely tuned, allowing some structural control in the ionic networks. These results present a strategy for achieving atomically precise supramolecular architectures using molecular carboxylate species that do not show a tendency toward long-range self-organization alone but can be driven toward twodimensional, highly robust self-assembly by the ionic surface reaction discussed herein.
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INTRODUCTION With the growing importance of programing the functionality of organic materials at the nanoscale, new methods are needed to control individual molecules in order to fabricate nanostructures with atomic precision. Current “top-down” methods for nanoscale fabrication in the electronics industry will reach fundamental limits soon.1 Surface-assisted selfassembly is an alternative, “bottom-up” strategy to produce nanoarchitectures that can achieve atomic control of structure. Self-assembled nanostructures have potential uses in organic electronics2 and sensors.3,4 In particular, organic photovoltaic devices may benefit from self-assembly strategies because the ordering of units into a preferred configuration is key to device performance.5 Self-assembly of two-dimensional surface networks has been illustrated with many organic building blocks through a variety of supramolecular interactions.6−8 A method toward ionic selfassembly by cation complexation has been investigated with small organic building blocks like terephthalic acid (TPA)9,10 and tetracyanoquinodimethane (TCNQ),11 but several open questions remained as to the utility of this method for ordering nanostructures at surfaces. In the studies of Cs-TPA, the large cationic species and the nondirectional nature of ionic bonding led to multiple structural phases occurring simultaneously.9 © 2013 American Chemical Society
That was overcome to some extent in our recent study of NaTPA by using a smaller cation and careful annealing treatments.10 Na-TCNQ studies were conducted on the relatively inert Au(111) surface to demonstrate ionic bonding networks when “decoupled” from the surface.11 Here, we demonstrate that ionic bonding to the smaller Na cations, combined with direct interaction between the organic anion and the underlying metal surface, allows excellent ordering into a single structural phase that is thermally robust. For the first time we illustrate that self-assembled ionic alkali carboxylate surface networks at the metal surface can be converted into a single, thermodynamically favored phase via rigorous annealing treatments. Another key issue with regard to the utility of ionic bonding in surface nanostructures is that each of the prior studies showed ionic bonding using molecules that do not require additional complexating species to self-organize. That is, the small TCNQ and TPA molecules readily self-assemble alone (without ionic interactions) into well-ordered surface structures.12,13 Self-assembly of larger molecules requires a very delicate balance between molecule−molecule and molecule− Received: January 7, 2013 Published: January 14, 2013 2959
dx.doi.org/10.1021/jp400213a | J. Phys. Chem. C 2013, 117, 2959−2965
The Journal of Physical Chemistry C
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substrate interactions.14 New tool sets are required to achieve self-organization of complex organic species, including many of interest in various applications. An ionic bonding protocol holds promise for the self-assembly of complex organic materials of technological interest in which adsorbate− adsorbate interactions are insufficient to compensate for unfavorable interactions with a substrate. We utilize a tetraanionic ligand that does not have a good adsorption registry on the metal surface and does not form a stable self-assembled structure by itself. Self-assembly is achieved only via the introduction of electrostatic interactions with alkali cations. While self-assembling capabilities are of great importance, they are not the only properties desirable for nanoscale fabrication. Strong intermolecular interactions and fabrication of thermally robust nanonetworks are required for many potential uses of self-assembled architectures in molecular electronics, sensors, and catalysts.15 Ionic interactions hold promise in this area because not only do they form robust networks10 and are resistant to interfacial diffusion16 but they also maintain long-range ordering and error correction,9 unlike many networks formed via strong covalent interactions.15 A first, and common, test of thermal stability of selfassembled organic systems at surfaces is to conduct spectroscopic or microscopic measurements before and after annealing treatments, i.e., a test of their robustness against decomposition, and has yielded stability measurements in excess of 200 °C for organic molecules adsorbed at metal surfaces.17,18 However, in order to verify that entire selfassembled, supramolecular networks are indeed stable at elevated temperatures, and do not disintegrate during annealing treatments just to re-form again upon cooling, high-temperature experimental work is necessary. Examples of hightemperature low-energy electron diffraction (LEED) of organic structures have been presented in the literature for studies of organic and metal−organic networks.19,20 LEED does not provide local information about structural stability (e.g., resolution of individual molecules and local defect sites). For evidence of the stability of nanostructures at the molecular level, molecular resolution, high-temperature microscopy is needed. Examples of high-temperature STM investigations of stable organic networks in the literature are rare10,21 and have insofar not yielded cases of thermal stability of organic surface networks at or above 180 °C, nor molecular-resolution images of such networks acquired over many-hour time intervals. Here, we illustrate atomic stability of ionic surface nanostructures at elevated temperatures via molecular-resolution, high-temperature STM data acquired at a sample temperature of 180 °C over a period of more than 6 consecutive hours. The interactions of a complex model system, BTA (biphenyl3,3′,5,5′-tetracarboxylic acid), are investigated on the copper (100) surface with scanning tunneling microscopy (STM) and X-ray photoelectron spectroscopy (XPS). While BTA molecules are mobile on the surface without the cation, they can react with an inexpensive and safe reagent, NaCl, to form a highly stable network by Na +···COO − ionic bonding. Conversion of a disordered system exhibiting room-temperature instabilities toward a highly robust, atomically precise, self-assembled architecture is a promising strategy to achieve self-organization of molecules that would not normally be possible. Ionic self-assembly of complex carboxyl ligands holds promise for future applications, where technologically relevant molecular systems may require this level of robust bonding to order on a desired surface.
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EXPERIMENTAL SECTION
Experiments were carried out in an ultrahigh-vacuum (UHV) system of base pressure
