Department of Chemistry, Indiana University, Bloomington, Indiana 47405, USA.
J Am Chem Soc. 2012 Aug 29;134(34):14165-71. doi: 10.1021/ja3053128. Epub 2012 Aug 21.
Ionic bonding in supramolecular surface networks is a promising strategy to self-assemble nanostructures from organic building blocks with atomic precision. However, sufficient thermal stability of such systems has not been achieved at metal surfaces, likely due to partial screening of the ionic interactions. We demonstrate excellent stability of a self-assembled ionic network on a metal surface at elevated temperatures. The structure is characterized directly by atomic resolution scanning tunneling microscopy (STM) experiments conducted at 165 °C showing intact domains. This robust nanometer-scale structure is achieved by the on-surface reaction of a simple and inexpensive compound, sodium chloride, with a model system for carboxylate interactions, terephthalic acid (TPA). Rather than distinct layers of TPA and NaCl, angle resolved X-ray photoelectron spectroscopy experiments indicate a replacement reaction on the Cu(100) surface to form Na-carboxylate ionic bonds. Chemical shifts in core level electron states confirm a direct interaction and a +1 charge state of the Na. High-temperature STM imaging shows virtually no fluctuation of Na-TPA island boundaries, revealing a level of thermal stability that has not been previously achieved in noncovalent organic-based nanostructures at surfaces. Comparable strength of intermolecular ionic bonds and intramolecular covalent bonds has been achieved in this surface system. The formation of these highly ordered structures and their excellent thermal stability is dependent on the interplay of adsorbate-substrate and ionic interactions and opens new possibilities for ionic self-assemblies at surfaces with specific chemical function. Robust ionic surface structures have potential uses in technologies requiring high thermal stability and precise ordering through self-assembly.
在超分子表面网络中，离子键合是一种很有前途的策略，可以通过原子精度的有机构建块自组装纳米结构。然而，在金属表面上，这些系统还没有达到足够的热稳定性，这可能是由于离子相互作用的部分屏蔽。我们证明了在高温下金属表面上自组装离子网络具有优异的稳定性。通过在 165°C 下进行的原子分辨率扫描隧道显微镜（STM）实验直接对结构进行了表征，显示出完整的结构域。这种稳定的纳米尺度结构是通过简单廉价的化合物氯化钠与羧酸相互作用的模型体系对苯二甲酸（TPA）在表面上的反应实现的。角度分辨 X 射线光电子能谱实验表明，不是 TPA 和 NaCl 的明显层，而是在 Cu(100)表面上发生取代反应，形成 Na-羧酸盐离子键。核心电子态的化学位移证实了直接相互作用和 Na 的+1 电荷态。高温 STM 成像显示 Na-TPA 岛边界几乎没有波动，这表明在表面的非共价有机纳米结构中以前没有达到的热稳定性水平。在这个表面体系中，实现了分子间离子键和分子内共价键的同等强度。这些高度有序结构的形成及其优异的热稳定性取决于吸附剂-基底和离子相互作用的相互作用，并为具有特定化学功能的表面上的离子自组装开辟了新的可能性。在需要通过自组装实现高热稳定性和精确有序性的技术中，稳定的离子表面结构具有潜在的用途。
