A New Window into 2D Materials: In Situ Control of Density of States and Reactivity

The Science 

Measuring electronic properties, which control key microscopic behaviors of 2D van der Waals (vdW) materials, has generally required cryogenic temperatures and/or ultrahigh vacuum conditions. These measurement constraints have limited researchers’ ability to understand how the electronic structure of these materials evolves under device-relevant operating conditions. A research team developed a new electrochemical quantum capacitance spectroscopy (EQCS) technique that enables scientists to measure the absolute energies of defect states and band edges of 2D vdW materials under ambient conditions. They were also able to manipulate electronic states in real time, adding in-gap states to molybdenum disulfide and increasing its catalytic activity for hydrogen production by nearly a factor of three.

The Impact 

2D vdW materials exhibit highly tunable, unique properties that can be leveraged for applications in complex devices and energy-related technologies. Measuring and controlling the density of states and defect charge configurations of these materials are essential for understanding their physical properties. The EQCS technique developed in this work paves the way toward an extendable, in situ electrochemical platform for understanding how the electronic structure of 2D materials evolves under operating conditions and for probing the initial stages of nucleation at defect sites.

Summary 

Monitoring and controlling the electronic structure of 2D materials are essential for understanding their functional properties. Researchers developed a new EQCS technique to measure the absolute energies of defect states and band edges in ambient environments, rather than at cryogenic and/or ultrahigh vacuum conditions. They achieved an energy resolution of 116 meV for the density of states near the Fermi level at room temperature, approaching the theoretical measurement limit of 91 meV (3.5k_BT). The ambient conditions of measurement enable researchers to probe materials throughout a reaction process at device-relevant conditions. The team used EQCS to probe the evolution of the electronic structure during defect formation and nucleation in a variety of model 2D materials interfacing with the liquid phase. The in situ EQCS platform enabled them to monitor and manipulate the density of states in real time, facilitating the controlled enhancement of electrochemical reactions. For example, creating new gap states by forming sulfur vacancies activates the otherwise inert basal plane of monolayer molybdenum disulfide, increasing nucleation propensity and its catalytic activity for hydrogen generation by nearly threefold. The EQCS platform provides a powerful new method for probing and manipulating the intrinsic density of states and defect states of 2D materials in ambient environments.

Contact 

Maria Sushko, Pacific Northwest National Laboratory, maria.sushko@pnnl.gov

Funding 

This research was funded by the Department of Energy, Office of Science, Basic Energy Sciences, Division of Materials Sciences and Engineering, Synthesis and Processing Science program under Award FWP12152. Pacific Northwest National Laboratory is operated by Battelle for the Department of Energy under contract no. DE-AC05-76RLO1830. M. Kaiser acknowledges the state of Washington for providing a 1.5-year fellowship during the period of this work through the University of Washington Clean Energy Institute.