A method for making an emitter is disclosed. A number of carbon nanotubes in parallel with each other are provided. The carbon nanotubes have a number of first ends and a number of second ends opposite to the number of first ends. The first ends are attached on a first electrode and the second ends are attached on a second electrode. The first electrode and the second electrode are spaced from each other. A voltage is supplied between the first electrode and the second electrode to break the carbon nanotubes.