Investigation of Fractal Formation Phenomena in TIPS/Pentacene System Solution-Sheared on Kapton Substrate

Kapton is a semi-crystalline polyimide tape developed by Dupont that is commonly used for electronic applications. TIPS-Pentacene (6,13-Bis(triisopropylsilylethynyl)pentacene) is an organic molecule commonly used for charge transport when crystallized in solution via confinement through the method of solution-shearing in the evaporative regime of flow onto a smooth substrate. In this investigation, a continuation of previous studies of solution-shearing thin-film organic semiconductors composed of solutions of TIPS-Pentacene in Toluene on various substrates was extended to using Kapton as a primary substrate. Trials performed with Kapton as a substrate resulted in fractal formation approximating Sierpinski Triangles in the films, a phenomenon that was unknown to the author’s knowledge. The investigation so far has involved parameter searches to evaluate the sensitivity of the system, ImageJ analysis of fractal dimension and whether it correlates significantly with any shearing parameters including shearing temperature, shearing rate, and solution concentration. Further, recent experiments have focused on controlling fractal formation and analyzing the role of crystallization molecule and fluid dynamic effects in inducing this phenomenon. In situ high-speed cameras have been employed as a means of attempting to capture and observe the motion that drives the fractal formation. Varying the shearing molecule among TIPS-Pentacene, TCNQ (Tetracyanoquinodimethane), and Caffeine was additionally employed to determine the dependence of fractal formation on the shearing molecule, though no consistent fractals have been observed for films that do not include TIPS-Pentacene. FTIR was conducted to measure the thickness of the TIPS-Pentacene films and appeared to display variations in thickness corresponding to the fractal locations, suggesting the fractals were essentially void of film. The methods used in this study derive from previous strategies of Giri et al. in regards to using a solution-shearing method to yield aligned crystalline films of organic semiconductors with TIPS-Pentacene as the molecule and Silicon Oxide as the substrate.¹ Control samples of TIPS-Pentacene sheared on Silicon Oxide wafers as well as glass and aluminum foil resulted in no fractal formation. Results and current understanding of the system suggest surface energetic effects as well as fluid dynamics phenomena as the primary factors for the fractal formation, though further investigation is required for comprehensive understanding.

1. Giri, G.; Park, S.; Vosgueritchian, M.; Shulaker, M. M.; Bao, Z. Advanced Materials 2013, 26 (3), 487–493.