Extended One-Dimensional Supramolecular Assembly on a Stepped Surface

2,6-naphthalene-dicarboxylic acid was adsorbed on a Ag(110) surface with an average terrace width of only some tens of a nm. Scanning tunneling microscopy shows that the adsorbates self-assemble into one-dimensional mesoscale length chains. These extend over several hundred nanometers and thus the structure exhibits an unprecedented tolerance to monatomic surface steps. Density functional theory and x-ray photoelectron spectroscopy explain the behavior by a strong intermolecular hydrogen bond plus a distinct template-mediated directionality and a high degree of molecular backbone flexibility.

Figure 1

STM and XPS results. (a) $620\times 700{\AA }^2$, $I=-0.39\mathrm{nA}$, $V=-220.9\mathrm{mV}$, 0.1 ML $\mathrm{NDCA}/\mathrm{Ag}(110)$. (b) $35\times 35{\AA }^2$, $-0.25\mathrm{nA}$, $-185.8\mathrm{mV}$, $\sim 0.75\mathrm{ML}$. The molecular model stems from the optimization of a gas phase network (see text). (c) $60\times 60{\AA }^2$, $-0.79\mathrm{nA}$, $-883.8\mathrm{mV}$, $\sim 0.3\mathrm{ML}$. (d) $195\times 195{\AA }^2$, $-0.204\mathrm{nA}$, $-262.8\mathrm{mV}$, $\sim 0.45\mathrm{ML}$. The lines indicate the traces of the line scans in the inset. Inset: line scan over an adsorbate-covered step edge (black) and a free step edge (red). The relative alignment of the scans is arbitrary. (e), (f) O $1s$ XPS, (e) $\sim 0.3\mathrm{ML}$ NDCA deposited onto Ag(110) at room temperature and not treated any further, (f) 1 ML NDCA deposited onto a room temperature Ag(110) surface and annealed to $200°\mathrm{C}$ for 20 min.

Figure 2

DFT results. The numbers refer to Table . (a) Head-to-head hydrogen-bonded NDCA on top of a substrate trough (left) and a close-packed Ag row (right). (b) Gas phase net of two species $A$ and $B$ with different orientations corresponding to the adsorbate orientations observed experimentally. (c)–(f) NDCA dimers at a step edge. (c) Most stable configuration of a protonated NDCA dimer at a step edge. (d) Two protonated NDCA without NDCA-NDCA hydrogen bonds. (e) Two deprotonated NDCA molecules in registry cross the step edge. (f) Two deprotonated NDCA molecules out of registry across the step edge.