Distinct Assembly Behaviors and Thermal Stabilities of Vanadyl Phthalocyanine on the Cu₂Bi and Bi(110) Monolayers

Fabrication of well-organized molecular films on metallic monolayers is crucial for the development of molecular-based electronic and optoelectronic nanodevices. In this study, we investigated the assembly behaviors and thermal stabilities of vanadyl phthalocyanine (VOPc) on Cu₂Bi and Bi(110) monolayers by using low-temperature scanning tunneling microscopy. The molecular configurations, orbitals, and thermal stability were elucidated at a submolecular level. On Cu₂Bi, a VOPc single layer with the 4-fold symmetry was fabricated, exhibiting an O-down configuration that can be transformed into an O-up configuration through thermal annealing. Conversely, on the Bi(110) monolayer, a VOPc bilayer structure with a square-shaped unit cell formed, and annealing treatments resulted in the retention of a single layer characterized by a parallelogram-shaped unit cell along with two distinct molecular alignment orientations. Thermal stability assessments indicated that the O-up configuration was much more stable for both types of VOPc single layers supported by Cu₂Bi and Bi(110) monolayers. Conductance spectra revealed that the VOPc single layer supported by Cu₂Bi exhibited an orbital distribution similar to that of the VOPc bilayer fabricated on Bi(110). For both types of VOPc films, thermal annealing significantly reduced the energy gap width of VOPc molecules, possibly due to the enhanced molecule-substrate coupling or strengthened π-π intermolecular interactions. This study would contribute to a deeper understanding of assembly behaviors and stability during molecule-metal contacts, which are essential for fabricating high-speed molecule-based nanodevices.