Self-assembled patterns and Young's modulus of single-layer naphthalocyanine molecules on Ag(111)

Structural and mechanical properties of self-assembled metal-free naphthalocyanine (H₂Nc) films on a Ag(111) surface are studied. Six self-assembled domains are observed by scanning tunneling microscopy (STM). Combining the high-resolution STM images and density functional theory (DFT) based calculations, we found that molecules adsorbed flatly on the substrate by forming six different interlocked square-like unit cells with different lattice parameters. DFT calculations indicated comparable adsorption energies for all the configurations. Six domains with different lattice parameters present different strain states, giving us a possibility to evaluate the Young's modulus of the metal-free naphthalocyanine films on the Ag(111) surface. We found that the Young's modulus of H₂Nc is comparable to those of typical conjugated organic-molecule-based crystals (e.g., naphthalene), providing useful information for future applications when the elastic properties should be concerned.