1 m (c)

1 m (d)

1 m

1 m

Figure 5.11. SEM images of colloidal spheres before irradiation (a) and after irradiation for different time periods: (b) 5 min, l/d=1.31; (c) 12 min, l/d=2.03; (d) 15 min, l/d=2.35. Source: From Li et al., 2005b.

0.8 0 3 6 9 12 Irradiation time (min) 15

Figure 5.12. Relationship between the average major-to-minor ratio of the colloidal particles and the irradiation time. Source: From Li et al., 2005b.

An important difference can be seen by comparing the colloid deformation and SRG formation. SRGs can only be induced by light irradiation of interfering laser beams, which produce periodic fringes with variation in the light intensity or light polarization direction (Natansohn and Rochon, 2002; Delaire and Nakatani, 2000). As mentioned earlier, the colloid deformation can also be induced by the irradiation of the interfering laser beams. However, it was somewhat surprising to observe that the deformation of the colloidal sphere can also be induced by a linearly polarized Ar+ laser single beam (Li et al., 2006b). The samples used for the single laser beam irradiation experiments were the same as those discussed in Section 5.4.1. An Ar+ laser at 488 nm was used as the light source, and the linearly polarized laser beam was spatially ltered, expanded, and collimated. The intensity of the laser beam was B150 mW/cm2, and the incident laser beam was perpendicular to the wafer surfaces containing the colloids. The experiments were carried out at room temperature under an ambient condition. In a typical case, colloidal spheres of BP-AZ-CA were used for the test, and the result is as follows. Figure 5.13 gives some typical TEM and SEM images before and after the light irradiation. Figure 5.13a shows a typical TEM image of the colloidal spheres before light irradiation. The average size of the colloidal spheres is 212 nm, with a

Polarization direction (a) (b)

1 m

Figure 5.13. (a) TEM image of the azo polymer colloidal spheres with an average diameter of 212 nm. (b) SEM image of the colloidal spheres after exposure to a linearly polarized Ar+ laser beam for 10 min.(c) TEM image of the deformed colloids after irradiation with the laser beam for 15 min and released from the surface of the silicon wafer by sonication. Source: From Li et al., 2006b.

polydispersity index of 0.03 obtained from the DLS measurement. Figure 5.13b shows an SEM image of the colloidal spheres after being irradiated by the Ar+ laser beam for 10 min. It can be observed that the colloidal spheres are signi cantly elongated in the polarization direction of the laser beam. To avoid possible in uence of the sample preparation, the casting direction is selected to be perpendicular to the polarization direction of the laser beam. Figure 5.13c shows the TEM image of the deformed colloids irradiated by the laser beam for 15 min. The TEM observation con rms that the spheres are deformed to ellipsoids after the irradiation. Figure 5.14 shows SEM images of the colloidal particles observed before irradiation and after irradiation for different time periods. The colloids are continuously stretched along the polarization direction as the irradiation time increases. The average axial ratio (l/d ) of the colloids (estimated statistically from SEM images of 100 colloidal particles) almost linearly increases as the irradiation time increases. Because the stretching relies on the polarization direction, the colloidal spheres can be elongated in multiple directions by adjusting the polarization direction. For example, the colloids can be rst stretched in one direction and then in the orthogonal directions by turning the polarization direction around 901. Figure 5.15 gives a typical SEM image of the colloids irradiated by two laser beams with orthogonally polarized directions, each for 10 min. It can be seen that the colloids are stretched in two orthogonal directions.

Polarization direction (a) (b) (e)