▎ 摘　　要

In order to ensure that vacuum electronic devices work with high overall efficiency, it is required to use materials with low secondary electron emission to fabricate or coat collectors, grids, and envelope walls of the devices. We report that the secondary electron yields of monolayer graphenes are ultralow, comparable with the lowest yields of the materials currently used In this practical application. This offers a pathway for the application of light graphene with only one-atom thickness and good electronic and thermal conductivities in vacuum electronic devices.