Modulating the Adhesion, Friction and Lubrication Characteristics of Few-Atom Thick Materials in Aqueous Environment over Several Length Scales

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Two-dimensional (2D) materials are drawing significant attention because their atomic size results in unique material properties. Although water is often present in these systems and it significantly affects the microelectromechanical system or device performance, very little is known about the interfacial properties of 2D materials in the presence of water. This is partially due to the challenges involved in contamination-free, controlled processing of high quality 2D materials, which limits fundamental studies and applications. This project will study the science associated with the interfacial properties of 2D materials. Since the technology has direct applicability to micro and nano-scale devices, the research has the potential to impact the automotive, consumer electronic, aerospace and defense sectors, and therefore directly impacts economic welfare and national security. The research will contribute to the development of work force in the U.S. by training two graduate students research assistants. Further, the PIs? integrated education plan is designed to spark the early interest of K-12 students in STEM and inspire undergraduate and graduate students to further advance their interests in the intersection of 2D materials and surface science and engineering. In addition, the PIs will focus on broadening nano-engineering education by engaging student veterans and high school and minority students via summer research opportunities and field trips. The PIs will also actively pursue outreach activities, including interactive lectures and hands-on activities.In order to fill the knowledge gap of interfacial properties of 2D materials in aqueous environment, two major lines of research will be followed: (1) establish manufacturing methods for single- and few-layer graphene and molybdenum disulfide (MoS2) that afford control of their quality, the number of layers and substrate-induced doping characteristics over several length scales and (2) fundamentally comprehend, measure and model friction, adhesion, and lubrication by few-atomic thick materials in aqueous environment. The research hypothesis is that substrate-induced doping can be a means to modulate the interactions of few-atomic thick materials with water and ions, as well as their interfacial mobility, and therefore, it can be used to control electrical double layer, friction, adhesion and lubrication mechanisms. Preparation of contamination-free, single-crystalline graphene and MoS2 layers with controlled substrate-induced doping will enable careful study of interfacial properties of 2D materials in aqueous environment. Measurements with a surface forces apparatus (SFA) will provide thorough data of interfacial forces, which will be precisely modeled to quantify the effects of the substrate-induced doping. The ease of preparing samples for atomic force microscopy (AFM) will enable to also investigate substrate-induced doping via metal thin films, a higher number of ionic compositions and also to qualitatively compare the interfacial behavior of MoS2 and graphene with several other (exfoliated) 2D materials and with the bulk crystals. Ultimately, a theory for the relation between substrate-induced doping of 2D materials and their adhesive and frictional characteristics in the presence of water will be established.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.