Comparison of common field emitting materials. (a) Polar plot for the various common-place electron emitters. CNT and graphene based field emitters out-perform such sources across most metrics, where Jmax the current density, Eon is the turn-on electric field, ‘Vacuum’ denotes the operating vacuum, T the typical operating temperature, Φ the emitter work function, τ the lifetime, ‘Stability’ is the temporal stability, η the electron-optical brightness, ∆E the energy spread of the emitted electrons, ds the virtual source size. Adapted from . (b) Overview of the on and threshold electric fields (Eon and Ethr, respectively) and maximum current density, Jmax, for various materials used for field emission to date, in order of dimensionality (1D, 2D and bulk) and increasing work function (Φ), including 1D nanowires - AlQ3 [35,36], Si [37–39], MgO [40,41], AlN [42–45], CdS [46–49], W [50–52], ITO , CuPC [54,55], InGaN [56–58], CNTs [59–63] Cu [64–66], ZnO [67–72], GaN [73,74], ZnMgO [70,75], WO [76–79] ), MoO2 [80–82], and ZnS [37,83] -, the 2D platelets - MoS2 , h-BN [85–88], graphene [89–97], C nanowall [98–100], WS2 RGO  -, and the bulk materials - a-diamond [71,102], LaB6 [103–106], nanodiamond [107,108], DLC [109,110], a-C [111–113], ta-C [114–116], W , Si [118–120], diamond [121–125], Cu [65,66,126], Ni [127–130], and CVD diamond [112,131–133].