Liquid‐Crystal Patterning: Lithographic Alignment of Discotic Liquid Crystals: A New Time–Temperature Integrating Framework (Adv. Mater. 46/2009)

Abstract

Liquid crystals are a successful example of how the control of self-assembly via chemical design leads to novel applications. Massimiliano Cavallini, Fabio Biscari, and co-workers report that direct patterning of a continuous metal-free mesogenic phthalocyanine fi lm (discotic LCs) is achieved by spatial control of column orientation using unconventional lithography. These patterned fi lms work as time-temperature integrators at the columnar rectangularhexagonal transition temperature.

Discotic Liquid Crystals

“…On the other hand, if the phthalocyanine film is confined between two identical substrates, whatever their surface energy or roughness, homeotropic alignment was detected in most cases. A lithographic alignment of phthalocyanine-derived liquid crystals as a time–temperature integrating framework, which is a crucial element for device performance, was reported by Cavallini et al…”

Organic Materials for Time–Temperature Integrator Devices

“…Dewetting is an irreversible process that requires activation energy and displays a temperature- and time- dependent characteristic length scale; the ideal prerequisites for TTI applications. An example was reported by Calò et al, in which it is demonstrated how a patterned thin film of polyisobutylene acts as a TTI device . The polymeric film was patterned by electron irradiation of spatially defined zones by an electron beam (Figure a–f).…”

“…An example was reported by Calòet al, in which it is demonstrated how a patterned thin film of polyisobutylene acts as a TTI device. 33 The polymeric film was patterned by electron irradiation of spatially defined zones by an electron beam (Figure 3a−f). Upon controlled irradiation, polymer chains fragment, generating zones with controlled (reduced) molecular weight.…”

“…Another efficient approach for TTI devices consists of measurement of the change in birefringence, as reported by our group, a TTI based on discotic liquid crystals (DLCs). 33 Birefringence allows one to see colored crystals when observed by a polarized optical microscope with crossed polars, and it is associated with an ordered structure of the sample. Under crossed polars, birefringent materials appear colored while amorphous or nonbirefringent materials appear dark.…”

“…On the other hand, if the film is confined between two surfaces and treated by a melting and quenching process, the columns orient perpendicular to the surfaces, forming the so-named homeotropic phase, which is almost not birefringent. 39 Recently, our group demonstrated that, using a stamp instead of a surface, is also possible to locally modify the alignment of DLC columns in the film, 33 obtaining a homeotropic phase in correspondence with the stamp protrusions and an edge-on phase in correspondence with the stamp recesses. Figure 5 shows the scheme of the process and the resulting patterning.…”