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ISSN 1998-9539

Molecular Imaging of Macroheterocycles for Demonstration of Classical Nucleation and Quantum Switching

Hisao Yanagi
Graduate School of Science and Technology, Nara Institute of Science and Technology, 8916-5 Takayama, Ikoma, 630-0192 Nara, Japan
E-mail: yanagi@ms.naist.jp
 
Dedicated to Professor T. Torres on the occasion of his Birthday
 
Single molecular imaging by scanning tunneling microscopy (STM) was carried out for platinum tetra(3,5-di-tert-butylphenyl)porphyrin (Pt-TBPP) and chloro[subphthalocyaninato]boron(III) (SubPc) adsorbed on Cu (001) surface under ultrahigh vacuum. In the former, two-dimensional nucleation growth of Pt-TBPP assemblies was demonstrated according to classical thermodynamic mechanism. The registry of Pt-TBPP molecules into the kink sites minimized the free energy for nucleation resulting in formation of singular nuclei with rectangular shapes. In the latter, STM-induced flip-flop switching was molecularly visualized for epitaxially adsorbed SubPc arrays. The asymmetric SubPc molecules initially adsorbed with its axial chlorine atom either upwards or downwards. After scanning at a negative bias, the upward molecules turn upside down while all molecules switched to the upward orientation at a positive bias. This orientational switching is not consistent with that expected from the polarity of the SubPc molecule along the Cl-B bond. Moreover, similar switching phenomena were observed during continuous scanning at a constant bias. Molecular energy calculation and statistical thermodynamic evaluation concluded that the tip-triggered fluctuation in the close-packed SubPc array induced the molecular rearrangement with stochastic flip-flop switching. This stochastic behavior suggests that the switching is not a single molecular event but is more related to intermolecular entanglement among arrayed molecules which envisages us to consider molecular-based quantum phenomena.