## 3 Results and Discussion  
Normally, PCDYol monomolecular films prepared by quasi-LB method are 2D polycrystals composed of small domains of H and P arrangements with an average size of 80 nm. Figure 2 shows typical STM images of PCDYol monomolecular films obtained by quasi-LB method. In the film, the domains of H and P arrangements coexist. Both domains exhibit lamellar structures, where the periods coincide with the PCDYol dimers. In H arrangement, the extended PCDYol chains exhibit a feather-like pattern, as shown in Figure 2a, and the lamellar interval determined by STM observation is 6.2 \(\pm\) 0.2 nm. On the other hand, in P arrangement, the PCDYol molecules align in the same direction as shown in Figure 2b. The period of the lamellar structure in P arrangement is estimated to be 6.8 \(\pm\) 0.2 nm, which is larger than that in H arrangement. For STM and AFM images of PCDYol monomolecular films, we can easily identify H and P arrangement domains by the difference of the period. Figure 1: Molecular structure of PCDYol. At room temperature, the structures of PCDYol monomolecular films are frozen. After keeping the samples at room temperature for two weeks, no distinguishable changes in the film structures were observed. To investigate thermodynamic stability of the polymorphs, we annealed the PCDYol monomolecular films above room temperature. We found that above 40 \({}^{\circ}\)C, relaxation of the film structure is effectively induced. Figure 3 shows the structural changes of the PCDYol monomolecular film observed after annealing at 50 \({}^{\circ}\)C. On the as-grown monomolecular film shown in Figure 3a, the area fraction of the P arrangement region is 40%. Figure 3b,c shows the structures after annealing for 300 and 3600 s, respectively. In addition to the grain growth by Ostwald ripening, P arrangement domains are observed to be predominant after annealing. On the monomolecular film annealed for 300 s, the area ratio of P arrangement region increases up to 80%. After 3600 s annealing, the domains of P arrangement cover almost the entire surface, as shown in Figure 3c. These results demonstrate that P arrangement is thermodynamically stable, while H arrangement is metastable. Figure 2: STM height images of a PCDYol monomolecular film formed on a HOPG surface by quasi-LB method. The dimer lengths are suggested by pairs of white lines. High-resolution images of (**a**) H arrangement (\(I_{\text{set}}\) = 20 pA, \(V_{\text{bias}}\) = \(-\)800 mV, 7.5 Hz) and (**b**) P arrangement (\(I_{\text{set}}\) = 10 pA, \(V_{\text{bias}}\) = 800 mV, 8.1 Hz). The adsorption model is superimposed at the bottom. (**c**) A large-area image showing the coexistence of H and P domains (\(I_{\text{set}}\) = 10 pA, \(V_{\text{bias}}\) = \(-\)100 mV, 3.9 Hz). The \(Z\) range is presented on the right side. Similar behavior is observed in structural changes of monomolecular films induced by STM scanning. We found that continuous STM scanning at room temperature causes morphological changes of the PCDYol monomolecular films. The detailed mechanism of the STM-induced morphological changes is not clear at present, although the reordering effect of the STM tip due to electrostatic interaction has been reported for monolayers of a DA compound 10,12-pentacosadiynoic acid [11]. Figure 4 shows snapshots during continuous STM scanning of the same region of 300 nm \(\times\) 300 nm. In the first STM image shown in Figure 4a, P and H arrangement domains coexist. In the 8 th scan, shown in Figure 4b, the H arrangement domain indicated in Figure 4a has shrunk from 8.3 \(\times\) 10\({}^{3}\) nm\({}^{2}\) down to 5.0 \(\times\) 10\({}^{3}\) nm\({}^{2}\). In the 16 th scan, the H arrangement domain has completely disappeared, as shown in Figure 4c. In the two consecutive images shown in Figure 5, we can see the process by which the P arrangement domain invades the neighboring H arrangement domain.