Nano-rubbing of a liquid crystal alignment layer by an atomic force microscope: a detailed characterization

Abstract

Locally rubbing the surface of a polymer using the stylus of an atomic force microscope (AFM nano-rubbing) has recently found extensive applications in prototyping novel micro-and nano-structured liquid crystal (LC) electro-optic devices. We report here on the detailed characteristics of the AFM nano-rubbed polyimide in terms of the friction force and the LC alignment capability. A unidirectionally rubbed polyimide by the contact mode AFM showed anisotropic friction, along and against the rubbing direction, even at a rubbing load as small as 1 nN and yielded a finite pretilt angle reflecting the asymmetry. Rubbing at high loads generated conspicuous scratches on the polyimide surface; when annealed at temperatures well below the glass transition or soaked into an organic solvent, however, these scratches quickly relaxed and completely disappeared while maintaining the capability of aligning LCs. When the load was small, the LC alignment also disappeared altogether. These experimental observations suggest that although the top thin layer might be sufficient to initially orient LCs, persistent surface alignment entails deeper cultivation of anisotropic structures.

Bifunctional ITO layer with a high resolution, surface nano-pattern for alignment and switching of LCs in device applications

“…The maximum anchoring energy of patterned ITO is 3.96Â10 À4 N m À1 , a value that is comparable to those of conventional rubbed PIs (10 À4 -10 À5 N m À1 ) and surfaces rubbed by using AFM (10 À6 N m À1 ). 26 The minimum anchoring energy is 7.75Â10 À8 N m À1 , which is 5100 times lower than the maximum value. Maximum and minimum anchoring energies were calculated using the line with l ca 200 nm and A ca 155 nm (Supplementary Figure S5) and the line with l ca/ 1000 nm and A ca 25 nm (Supplementary Figure S7), respectively.…”

The effects of varied imidization conditions on rubbed polyimide film surface morphology

“…2). The much larger-scale morphological relaxation of the surface scratches seen by Kim et al 12 with low-temperature annealing shows that significant relaxation occurs at the surface with heating. The relaxation seen here, though at a much smaller scale, may be of a similar origin.…”

“…10 Higher temperatures during rubbing have also resulted in increased orientation of the polyimide films. 11 Kim et al 12 have shown morphological relaxation in scratches in the polyimide surface due to low-temperature annealing, after which the scratched areas retained their ability to align liquid crystals. Kikuchi et al 1 have found that BPDA-PDA, the polyimide used in this study, will even align smectic liquid crystals when imidized or cured at low temperatures.…”

Semi-Alicyclic Polyimides: Insights into Optical Properties and Morphology Patterning Approaches for Advanced Technologies

“…As observed in Figure 15 from height profiles the nanochannels created in PI4 film surface are of 45 nm depth and 50 nm wide, while for the PI5 film they have smaller dimensions, namely 10 nm depth and 20 nm wide. The surface ordering induced by DPL in case of Epiclon-derived PIs is more pronounced comparatively with other commercial PIs [71], making them more suitable for LCDs applications. The differences regarding the size of the nanochannels for PI4 and PI5 can be correlated with their surface mechanical properties.…”