Formation of one- and two-dimensional molecular networks on metal surfaces

Abstract

The on-surface synthesis of one- and two-dimensional (1D and 2D) organic nanostructures on metal single crystal surfaces was studied under ultra-high Vacuum conditions. Two different organic molecules were investigated on metal substrates using scanning tunneling microscopy (STM). The first study focuses on the adsorption of Br4Py molecules (a molecule with a pyrene core and four bromine atoms in each corner) onto the Au(111) surface at room temperature. The STM images reveal that the molecules formed a 2D self-assembled network which is stable after stepwise annealing up to 400 K. After annealing at 425 K, two different structures are resolved by STM, one of which indicates an on-surface reaction. The second organic molecule, 4,4',4",4"'-(Pyrene-1,3,6,8-tetrayl) tetra-benzaldehyde (PTTA), also has a pyrene core, with four benzaldehyde groups as arms. PTTA was studied on Au(111) and Ag(111). It should be noted that the low coverage and high coverage experiments on Au(111) were performed in two different ultra-high vacuum systems at and imaged at different temperatures. At low coverage, room-temperature deposition yields a mixture of 1D and 2D molecular arrangements. The 2D networks consist of a combination of a “stacked” 2D close-packed selfassembled structure as well as those with the addition of molecular vacancies. At high coverage, room-temperature deposition leads to a “staggered” 2D close-packed self-assembled network. Density functional theory calculations yield a total stabilization energy of - 4.57 eV/molecule for the staggered 2D structure and - 3.81 eV/molecule for the stacked 2D network, formed at high and low coverage, respectively. The energies indicate that the stacked 2D structure is thermodynamically the more preferred 2D structure. Annealing the high coverage sample to 350 K leads to the formation of a molecular bilayer. After post-annealing low coverage samples to 450 K, STM images indicate a change in the appearance of the resolved features in the 1D structures, by a reduction in the intermolecular spacing along the chain direction. This transformation suggests a chemical change in the 1D structure affording (short range) molecular chains. Similarly, STM image analysis reveals a change in the appearance of 2D molecular features. A reduction in the unit cell dimensions and/or change in the ordering of the stacked and staggered 2D close-packed structures is observed. The changes in the 1D and 2D structures suggests chemical changes in the network in line with the McMurry-like condensation reaction of aldehyde on Au(111). On Ag(111), room temperature deposition of PTTA yields a staggered 2D close-packed selfassembled network. After post-annealing to 375 K, STM images reveal a change in appearance of some of the features in the 2D network and the molecules shifted closer to one another and at some regions intermolecular linkages were observed. After annealing at 400 K, a change in the ordering of the staggered 2D close-packed structure was observed. As on Au the changes in the 2D structure suggests chemical changes in the network.