Measuring and Manipulating Density of States in Two‐Dimensional Materials With Electrochemical Capacitance

Measuring and controlling the density of states (DOS) and defect states of 2D van der Waals materials is essential for understanding their unique physical properties and for advancing their future practical applications. However, this typically requires experiments performed at cryogenic temperatures and/or in ultra‐high vacuum conditions, severely constraining efforts to monitor the electronic structure evolution of these materials under useful device operating conditions. Here, a new electrochemical quantum capacitance spectroscopy (EQCS) technique for detecting the absolute energies of defect states and band edges in an ambient environment is developed. Its applicability is demonstrated using a variety of 2D material systems, with the ability to easily extend to many more. The highest energy resolution achieved at room temperature, 116 meV, approaches the theoretical limit of 91 meV (3.5 k B T ). The in situ EQCS platform can monitor and manipulate the DOS in real‐time, enabling controlled enhancement of electrochemical reactions. Notably, new in‐gap states resulting from the formation of sulfur vacancies can activate the relatively inert basal plane of monolayer molybdenum disulfide and increase the catalytic activity for hydrogen generation by nearly threefold. The EQCS platform provides a powerful new method for probing and manipulating the intrinsic DOS and defect states of 2D materials in ambient environments.