(color online) The gate dependence of Joule heating power under different current densities in the graphene heater (a) with a channel width of 1 μ m or (b) with a zigzag channel (the channel has a width of 100 nm and a length of 10 μ m ). The inset shows schematics of the corresponding devices. Panels (c) and (d) show the SEM images of the real devices. The scale bar is 1 μ m .