Part II: The study of the surface structures and phase transitions
Chapter 3 Results and Discussions of the self-assemble layers of
3.5 Oblique row phase
In addition to the dot array, the dicationic viologen molecules stack also in a different arrangement called the “oblique row phase” on the Cl/Cu(100)-c(2×2) surface as shown in the right region of Figure 3-12(a). The left part of Figure 3-12(a) corresponds to the regular dot array and the black curve indicates the domain boundary. Compared to the dot array, the molecular orientation between neighboring rows of the oblique row phase is rotated by a 466 degree. The potential range in which the oblique row phase is observed on the surface is the same as with the dot array in the region Ⅰ of the CV. The black arrow of Figure 3-12(b) points to the region of the oblique row phase. By increasing the potential to -130 mV, the dot array and oblique row phases simultaneously transfer to the metastable phases as shown in Figure 3-12(c). The images of Figure 3-12(b) and (c) are measured at the same regions according to the bottom left island and corresponding defect positions. Phase transitions of the oblique row phase also follow the same sequences as the dot array, namely the metastable phases, a stripe pattern, the closed stacking stripe pattern, a chloride desorption, and the dimer phase.
The surface structure of the oblique row phase is shown in Figure 3-13(a) and clearly displays corresponding rotation of viologen molecules between neighboring rows and the alkyl chain orientations. By removing the surface molecules, the
underlying Cl-c(2×2) surface appeared in Figure 3-13(b) which does not exactly match with Figure 3-13(a) due to the thermal drift. The drift direction can be corrected by comparison of the drift direction of the larger area image in continuous images.
The structure model is sketched in Figure 3-13(c). The positions of the bipyridinium cores remain favorably on the two-fold or the four-fold sites. The carboxylic acid groups of viologen molecules are considered to form the hydrogen bonds of O-H with carboxylic acid groups of viologen molecules of the next row, not with the nearest molecules resulting in the oblique row phase. The nearest molecule orientations show locally a mirror symmetry relation. The network feature arises from the hydrogen bonding of O-H of carboxylic acid groups for the oblique row phase. For the dot array, however the organization force is the hydrogen bonding of N-H between the nitrogen atoms and carboxylic acid groups. Normally, the strength of the hydrogen bonding of H...:N (N-H) is stronger than that of H...:O (O-H).151-153 Therefore, that effect influences the observable rate of the oblique row phase formation. From more than one hundred experiments only three times the oblique row phase was found. The carboxylated viologens in the dicationic state prefer to form the dot array rather than the oblique row phase.
Figure 3-13. (a) shows the detailed structure of the oblique row phase and (b) is the Cl/Cu(100)-c(2×2) structure. (c) is the structure model of the oblique row phase. (a):
Ubias = 388 mV, It = 1.90 nA, and at 0 mV. (b): Ubias = 11 mV, It = 5 nA, and at 0 mV.
The sizes of (a) and (b) are 7.09×7.09 nm2.
( ) a ( ) b
( ) c
For the dicationic viologen molecules, the two possible rearrangements on the Cl/Cu(100) surfaces are drawn in Figure 3-14.
Figure 3-14. Two possible arrangements of the dicationic viologens on the Cl/Cu(100)-c(2×2) surface.
Each model shows that the stacking direction of the molecules is along the same direction with that of the dimer phase and the intermolecular interaction is the hydrogen bonding of O-H between carboxylated viologen molecules in the transverse direction. The model (a) shows is more compact along the black arrow direction then the model (b). In the model (a), the distance between neighboring viologen cores is too close to form a stabile structure due to electrostatic repulsion forces among the dicationic viologen molecules. Compared to the model (a), the arrangement of the model (b) is looser along the black arrow direction to reduce the repulsion forces. The arrangement of the model (b) reduces the repulsion strength, but it still has many sites for molecules to adsorb. For instance, the middle positions between neighboring rows indicated by the black rectangle region. At these positions the repulsion forces can also be avoided and hence more viologen molecules are able to absorb on these positions by electrostatic attraction forces between the dicationic viologens and the anion layer. Both rearrangements are not found from the STM images because there are the effects of the repulsion forces and a loose stacking formation. As viologen molecules absorb on the middle positions, the surface structure becomes the oblique row phase by comparison of model (b) of Figure 3-14 and model (c) of Figure 3-13.
The oblique row phase needs two steps to form. In the initial step, the molecules absorb on the surface to form a loose structure. It is assumed that the hydrogen bonding of O-H between carboxyl viologens have formed in the solution phase.154 Therefore, the solution phase is maintained in the formation on the Cl/Cu(100)-c(2×2)
( ) b
( ) a
surface in loosing stacking configuration. Finally, the extra molecules from the solution species absorb again on the surface and make the original surface molecules to rearrange and to form the oblique row phase. Furthermore, the carboxylic acid group is strongly influenced by the cores of bipyridiniums and then the surface structure rearranges again resulting in the structure transition to the dot array phase as illustrating in Figure 3-15. The intermolecular interaction is the hydrogen bonding of N-H to form the phase as indicated by the solid arrow of Figure 3-15(a). The position of the nitrogen atom is near that of the carboxylic acid group. Therefore, the oblique row phase is easier to rearrange to the dot array phase by rotating about 45 degree of the bipyridinium orientation, which results in the observable rate of the oblique row phase lower than that of the dot array. The easier rearrangement phenomenon also reflects the hydrogen bonding of N-H indicated by the dashed line of Figure 3-15(b) stronger than that of O-H in Figure 3-15(a).
Figure 3-15. (a) and (b) are the structure models of the oblique row phase and the dot array phase. The solid lines and the dashed lines are the orientations of the viologen cores and the alkyl chains, respectively. The solid arrow of (a) and the dashed arrow of (b) describe the intermolecular interactions of O-H and N-H, respectively.