Optical Voltage Imaging in 2D Devices: Overcoming Contact Resistance to Probe Metal-Insulator Transitions

Two-dimensional (2D) semiconductors often suffer from high contact resistance, complicating accurate resistance measurements via conventional four-probe techniques. We introduce a novel optical method to map the electrical voltage profile in 2D devices, bypassing these contact limitations. By flowing current through source-drain contacts on a target layer (e.g., bilayer MoSe2) and detecting the voltage-induced optical response in a proximal sensing layer (monolayer MoSe2 separated by few-nm hBN), we achieve high-resolution voltage imaging. This approach enables measurement of intrinsic resistances below 1 kΩ, even with MΩ-level contacts, as long as current flows.

Applied to MoSe2, we imaged voltage profiles as a function of carrier density, revealing the insulator-to-metal transition. In wide samples, we observed percolative behavior; in narrow samples, temperature-dependent resistance crossovers characteristic of metal-insulator transitions. The critical carrier density matches values from traditional transport measurements, validating our technique.

Versatile applications include characterizing resistance anisotropies in anisotropic materials without cross-axis coupling and separating bulk from edge conductance in topological insulators, enhancing accuracy over conventional methods. This optical voltage profiling offers a powerful tool for 2D device characterization.