“…Electron source plays an important role in the development of advanced instruments and functional devices, such as electron microscopes [ 1 ], microwave amplifiers [ 2 ], field emission display (FED) [ 3 , 4 ], etc. Since de Heer demonstrated the excellent field emission (FE) properties of carbon nanotubes (CNTs) in 1995 [ 5 ], CNT emitters have received wide attention for their large aspect ratio, nanoscale tip, chemical stability, and excellent thermal conductivity [ 6 ], and are expected to be the ideal material for the next generation of electron sources [ 7 , 8 , 9 , 10 , 11 ]. The CNT-based electron source shows excellent properties, especially in high brightness [ 1 ], narrow energy distribution [ 12 , 13 ], and stable emission current [ 14 ].…”