New phenomena at the interface between 2D materials and liquids

Vanderbilt University, Nashville TN

Investigators

Abstract

NONTECHNICAL DESCRIPTION This project explores the interface between two very different systems: two-dimensional materials (2DMs, e.g.: graphene, single monolayer of carbon; phosphorene, a single layer phosphorus; monolayer molybdenum disulfide) and liquids (e.g.: water, biofluids, ethanol). The project pursues two main scientific goals. First, interactions between electrons in 2DMs are modified and studied by controllably changing the chemical composition, dielectric constant and flow velocity of liquids in contact with 2DMs. This line of investigation is expected to reveal the approaches to suppress electron scattering in 2DMs. Better understanding of carrier scattering, in turn, should advance potential applications of these materials in future electronic devices. The second thrust of the project employs 2DMs as ultrasensitive probes of chemical composition, flow velocity, and nanoscale structure of liquids. Better understanding of these parameters can pave the way towards more efficient water filters and biosensors capable of recording electrical activity of individual neurons. The project includes a significant educational component. The PIs continue their participation in the Bridge program, one of the most successful efforts in the nation to involve underrepresented minorities in cutting-edge research leading to the PhD degree. The PIs continue to involve undergraduates and high school students in their research through their participation in Vanderbilt's Research Experience for Undergraduates and Vanderbilt's Summer Science Academy programs. Finally, the multidisciplinary nature of the project spanning the fields from condensed matter and computational physics to chemistry, biology, and imaging helps preparing PhD students for successful careers in science and industry. TECHNICAL DESCRIPTION This project explores the interface between two-dimensional materials (2DMs, e.g. graphene, MoS2, phosphorene) and liquids (e.g. water, hexane) to (i) study and engineer the effects of the microenvironments of 2DMs on their properties, and (ii) understand the behavior of nanometer-thick interfacial liquid layers. To tackle these problems, the PIs developed a flexible setup that allows placing electrically contacted suspended 2DMs into liquids with controlled chemical composition, electrical potential, ionic concentration, and flow velocity. The 2DMs are interrogated via electrical transport, Hall effect, and optical measurements and studied through state-of-art electron microscopies and computational modeling. In the first part of the project, the setup is used to controllably "tune" multiple parameters in the Hamiltonians of 2DMs. The effective strength of interactions between electrons in 2DMs is tuned by controlling the dielectric constant of liquids surrounding the devices. Hitherto unattainable carrier densities up to >1015 cm-2 are achieved by applying an electric potential between ionic liquids and 2DMs suspended in them. Defects are induced in 2DMs by applying large electric potentials between devices and the liquid. These approaches are expected to reveal multiple exotic phenomena in 2DMs including gate-controlled superconductivity and excitonic collapse. In the second part of the project, the 2DMs are used as ultrasensitive sensors to study properties of a nanometer-thick interfacial layer of liquid in contact with a solid. By measuring the current induced by the Coulomb drag between ions in the flowing liquid and electrons in graphene, the "slip" velocity of the interfacial layer is determined. By measuring the 2DM/liquid capacitance, the dielectric constant of the interfacial liquid layer is probed.