Collaborative Research: Control of Contact Friction of Van der Waals Heterostructures

Two-dimensional materials enable creating a new class of nanoscale material systems by vertical, layer-by-layer stacking, resulting in ‘van der Waals’ heterostructures. This project aims to investigate the structural-mechanical-electronic coupling of van der Waals heterostructures with the ultimate goal of enabling friction control at nanoscale contacts. This knowledge will advance the design of nano- and microelectromechanical devices serving commercial and U.S. security needs by removing constraints of high friction as a long-standing hurdle for their functionality, as well as emerging manufacturing methodologies based on van der Waals assembly. The progress in this field will improve the sustainability and efficiency of manufacturing processes and thus increase U.S. industrial productivity and competitiveness. The researched collaborative project will contribute to the development of the work force in the U.S. by training two graduate student research assistants. The student demographics at University of California Irvine provide an ideal opportunity for broadening participation in mechanics and nanotechnology and contributing to the education of a diverse STEM work force. The team will take advantage of this opportunity in recruitment of students for engagement with the research at all levels and will provide opportunities for student exchanges in both labs. Findings of this research will be integrated as part of graduate courses at the two universities. It is hypothesized that tuning the structural-mechanical-electronic coupling in ‘van der Waals’ heterostructures will afford control of friction. This is expected because the charge transferred-induced interlayer excitons between the targeted transition metal dichalcogenides monolayers influence the corrugation of the potential energy landscape at the sliding interface. The objective of this hypothesis-driven project is thus to establish experimental and theoretical foundation for van der Waals heterostructures with tunable and controllable friction. The team will investigate (1) how the intrinsic structure (two-dimensional materials combination, stacking order, twist angle, interlayer coupling) of van der Waals heterostructures influences the coupling and friction; and (2) determine how extrinsic factors like strain and electric field affect structural, mechanical and electronic properties of van der Waals heterostructures, and thereby friction. The experimental toolset relies on a scalable approach to assemble van der Waals heterostructures with twist angle control; characterization by photoluminescence/Raman spectroscopy and second harmonic generation; and nanoscale friction measurements correlated with surface topography, adhesion, and stiffness maps. It is also expected that the effect of interlayer charge transfer will make friction less sensitive to oxidation, which will be tested on deliberately oxidized samples. 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.