**8. Conclusion**

In summary, a universal memristive phenomenon has been observed in 2D materials. These 2D-based memristors exhibit low switching voltage (<1 V), large on/off ratio (>106 ), fast switching speed (<20 ns), and forming-free characteristics. A mechanism based on metal atoms/ions adsorption into intrinsic vacancies producing an atomic-level conductive-point effect, has been proposed and supported by first-principle calculations and STM measurements. Constant voltage stress has been applied on the 2D-based memristors at high resistance state (HRS), revealing an additional higher resistance state that has not been discovered in conventional metal-oxide devices. Current sweeping method unveils the details hidden in the commonly used voltage-sweep curves, in which the transition step number could be attributed to the number of defects/vacancies. These open up a new materials space that might advance diverse applications including high-density neuromorphic computing, non-volatile memory fabrics, and zero-power RF switches.
