Title:  Exploring the Wetting Property of Selected 2D Materials

Abstract: The wettability of 2D materials is of paramount importance since it is critical to the interaction of 2D materials with liquids, which significantly impacts the performance in a wide array of applications. The fundamental understanding of wettability enables the optimization of 2D materials' functionality by enhancing their capacity to either repel or attract liquids and modulating adhesion properties. In this dissertation, we investigated the wettability of graphene and hexagonal boron nitride (hBN).

The previously proposed wetting transparency of graphene provides a compelling opportunity for multifunctional device design, allowing precise modulation of wettability by selecting an appropriate substrate. However, the graphene's wetting transparency on liquid substrates, which is promising for real-time wettability control, has been rarely studied before; possibly due to serious experimental challenges. To this end, we have developed a method using contact angle measurement, based on Neumann's Triangle model, to directly characterize the wetting transparency of graphene on liquid substrates for the first time. The experimental data suggests that the graphene is nearly wetting transparent on some liquid substrates. We also demonstrated the real-time wettability control with graphene on water-ethanol substrates.

Although the wettability of hBN has garnered significant attention, the accurate measurement of water contact angle (WCA) remains challenging; possibly due to sample defect/quality and airborne contamination. We have systematically investigated the effect of airborne hydrocarbons and defects on both static and dynamic WCAs of hBN. Importantly, we have demonstrated that the presence of defects significantly affects the static WCA, indicating previously reported static WCA values do not represent hBN's intrinsic water wettability. Instead, our results showed that the advancing WCA on freshly exfoliated hBN, measured at ~79°, provides the most accurate representation of hBN's intrinsic water wettability since it is not impacted by the defects. We have proposed a qualitative model that elucidates the impact of airborne hydrocarbons and defects on the static and dynamic WCAs of hBN, which aligns well with our experimental findings.

Chair:

Dr. Lei Li

Department of Chemical and Petroleum Engineering, University of Pittsburgh

Committee Members:

 Dr. Susan Fullerton

Department of Chemical and Petroleum Engineering, University of Pittsburgh

 Dr. Sachin Velankar

Department of Chemical and Petroleum Engineering, University of Pittsburgh

Dr. Haitao Liu

Department of Chemistry, University of Pittsburgh