Directed molecular self-assembly via photo-thermal effect

Abstract

Molecular self-assembly, which is spontaneous organization of building block molecules into typical structures, is generally influenced by thermal fluctuation to move forward its thermodynamic minima. In the case of polymeric self-assembly with block copolymers (BCPs) or supramolecules, chain mobility enhancement is essential to overcome the high diffusion barrier for large size molecules. Typically, annealing at a high temperature or under volatile solvent vapor is used to promote the mobility of polymers. Unfortunately, inherent limitations of the conventional methods greatly hamper the potential of polymeric self-assembly for more diverse applications. For thermal annealing, time-consuming heating/cooling process commonly leads to excessive processing time and inert atmosphere is required to avoid thermal degradation of organic molecules. For solvent vapor annealing, finding a suitable solvent with less toxicity and compatibility with a desired processing condition raises a practical challenge. Laser processing is an emerging material fabrication method relying on localized surface photothermal reaction and is highly motivated from the enormous advantages, such as rapid continuous processibility, area selectivity, and minimal influence on the neighboring structures. High energy laser photothermal processes with excimer laser [XeCl, wavelength (λ) = 308 nm] are well established for the area-selective crystallization of silicon and other inorganic materials. Controlled instantaneous irradiation by laser beams enables effective phase transformations of the inorganic materials at highly confined desired locations. Suitability for continuous processing makes this approach attractive for the manufacture of flexible devices. Motivated by the unique advantages, laser processing has been also introduced for organic materials, such as chemically amplified resists, polymer direct patterning, and molecular self-assembly. In this dissertation, the effective molecular self-assembly processes are presented by introducing the photo-thermal process from diverse light sources. Particularly, the chemically modified graphene (CMG) thin film is introduced as an effective light-absorbing, photo-thermal conversion layer. Firstly, photo-thermal process with near-infrared (NIR) quasi-CW laser is introduced for effective molecular self-assembly process. By the focused laser beam, area-selective writing of self-assembly on even flexible and three dimensional non-planar geometries are enabled even in atmospheric condition. Above all, the lateral scanning with extremely high thermal fields originated from local laser heating enables the directed self-assembly of vertically aligned lamellae and cylinders. Millisecond level rapid assembly of block copolymers by intensive flash light driven photo-thermal effect is also demonstrated. Extremely high, but rapid heating/quenching process enables the sufficient self-assembly of high-χ block copolymers without thermal degradation or dewetting. Compatible with the graphoepitaxial patterns, directed self-assembly of block copolymer within millisecond level time scale also attainable.