Counter-doping in two-dimensional transition-metal dichalcogenides: flipping native polarity and beyond

Two-dimensional (2D) transition-metal dichalcogenides (TMDs) hold immense promise for next-generation nanoelectronics and optoelectronics, yet their technological viability hinges on reliable control over the carrier type and concentration. In practice, most TMDs exhibit their own characteristic native polarity set by intrinsic point defects and unintentional impurities, which fix the Fermi level and frustrate subsequent extrinsic doping approaches. That is, such native donors and acceptors not only define the as-grown electronic ground state, but also complicate attempts at deliberate carrier modulation, often yielding unstable, hysteretic, or spatially non-uniform doping profiles. This mini-review first clarifies the defect-driven origin of native polarity in representative semiconducting TMDs by connecting characteristic vacancy and impurity states to experimentally observed conduction behaviors. We then survey main extrinsic doping strategies, including surface and remote charge transfer, chemical intercalation, and substitutional incorporation. While each approach presents distinct trade-offs regarding stability, controllability and device compatibility, we argue that substitutional doping, where dopant atoms replace host lattice sites, stands out as the most robust route for stable polarity control. We specifically highlight such substitutional counter-doping, where intentional dopants override the native-defect-imposed Fermi level and flip the intrinsic carrier type, and discuss how it enables diverse device applications ranging from complementary logic and low-resistance contacts to emerging optoelectronic and neuromorphic functionalities. We conclude by outlining the key remaining issues, such as dopant activation efficiency, interfacial coupling, and wide-range carrier modulation, to guide the future developments of 2D semiconductor platforms.