Phase transitions and morphology control of Langmuir-Blodgett (LB) films of graphene oxide

Hypothesis: Understanding the Langmuir film formation process of flexible and soft materials like graphene oxide (GO) is essential, as it shows different trends compared to the conventional surface pressure-area (π-A) and compressional modulus (ε) isotherms of hard materials. Additionally, the size distribution and mechanical properties of the GO are assumed to affect the distinctive Langmuir-Blodgett (LB) film morphologies, such as overlaps and wrinkles. Experiment: To gain a deeper insights of phase transitions in GO LB films, we propose a novel analysis of elastic tensile modulus versus surface pressure (|ε|-π) isotherms. This approach involves applying adequate compression to the GO sheets to generate an elastic force, followed by measuring the |ε|-π isotherms during the film's expansion. Additionally, we compared the surface morphology of GO LB films deposited under identical conditions using various GO colloidal solutions, each containing sheets with different size distributions. Finding: Upon expanding the GO Langmuir film after sufficient compression, a rapid stress release is observed leading to a clear inflection in the |ε|-π isotherms, indicating a phase transition from solid to liquid. Therefore, this study provides comprehensive insights into the behavior of GO Langmuir films. Furthermore, we demonstrated that morphological features, such as wrinkles and overlaps in GO LB films, can be effectively controlled by adjusting the size distribution and mechanical properties of GO sheets in the colloidal solution.