Molecularly Resolved Dynamics for Two-Dimensional Nucleation of Supramolecular Assembly

Abstract

Nucleation growth of a platinum porphyrin derivative adsorbed on a Cu (100) surface was observed with a scanning tunneling microscope. The molecule attached with four di-tert-butylphenyl groups allowed equilibrium between diffusional gas phase and two-dimensional nuclei. To minimize the free energy for nucleation, the registry of the molecules into the kink sites resulted in formation of nuclei with singular, rectangular contours. Dependence of the equilibrium conditions on the sample-tip bias voltages was also presented.Being faced with the limitation of top-down technology with semiconductors soon, a new paradigm based on molecular assembling with finite nanometer dimensions has attracted attention for high-density and high-speed devices. 1 Development of scanning tunneling microscopy (STM) 2 has realized imaging and manipulation of individual atoms and small molecules. 3-5 Molecule-based materials can ultimately be designed by assembling single molecules, the closed quantum system of which basically defines optoelectronic functions in their solid states. Structures as well as electronic properties of molecular aggregates are strongly dependent on intermolecular interactions with weak van der Waals (VDW) forces. Substrate-molecule interactions are also crucial parameters in case of constructing molecular devices in thin films. Molecular design and synthesis enable control of those interactions by modification of molecular frameworks and substitution of peripheral groups. For example, a metalloporphyrin derivative, Cu-tetra-(3,5-di-tert-butylphenyl)-porphyrin (Cu-TBPP), was the first supramolecule which attained room-temperature manipulation 6 and conformational identification 7 by STM. The four di-tert-butylphenyl (tBP) groups maintained moderate interactions with a Cu (100) surface to prevent unfavorable diffusional motion of adsorbed molecules. This molecule was further provided to demonstrate conductance switching caused by rotation of the tBP group under low-temperature STM manipulation. 8 Recently, substitution of these peripheral groups of metal-free (H 2 -) TBPP with cyanophenyl terminations dramatically modified intermolecular interactions between adjacent adsorbates on a Au (111) surface, resulting in controlled assembling from monomers to tetramers or one-dimensional (1D) wires. 9 At higher coverage of unsubstituted H 2 -TBPP on the surface, nonplanar adsorption and orientational ordering occurred in its two-dimensional (2D) islands. 10 Those reported findings suggest potential versatility of this class of supramolecules for bottom-up assembling from single molecules to higher dimensional aggregates by molecular modifications.In this study, we have synthesized TBPP substituted with a heavy central metal of platinum (Pt-TBPP, Figure 1a) according to the literature 11 and investigated dynamic processes of its 2D nucleation growth on a Cu (100) surface by low-temperature STM. The molecular structure of Pt-TBPP optimized by the ab initio molecular orbital calculation based on the density functional theory...

Surface chemistry of porphyrins and phthalocyanines

“…The islands had rectangular shape, which was attributed to the favourable nucleation at kink sites of the islands, resulting in a minimization of the nucleation free energy. Nucleation of bilayer islands started before the first layer was complete [408].…”

Studying the dynamic behaviour of porphyrins as prototype functional molecules by scanning tunnelling microscopy close to room temperature

“…Such molecules are not directly visible as individual molecules in the STM under the actual imaging conditions due to their high mobility. [61][62][63] The observed effective phase separation of 2HTPP and CoTPP on Cu (111) results from the fact that the adsorption of the free base 2HTPP is dominated by molecule-substrate interactions, while the supramolecular arrangement of CoTPP is caused by attractive molecule-molecule interactions. Fig.…”

Characterization of light emission from subphthalocyanine monolayers using scanning tunneling microscopy

“…The two-dimensional gas phase of large molecules such as porphyrin and phthalocyanine derivatives on Ag (111) and Cu (100), respectively [20,21] suggests that the molecules particularly in submonolayer easily diffuse on metal surfaces. Furthermore, STM operations may induce molecular rotation and orientational change in monolayers [22,23].…”