Method of reducing work function in carbon coated LaB6 cathodes

A method to prepare a LaB6 cathode, comprising:

• preparing an emitter structure of a LaB6 single crystal,
• the emitter structure comprising:
• a lower cylindrical or rectangular portion,
• an upper tapered portion having a conical surface, and
• a flat surface truncated tip;
• applying a carbon coating to at least the conical surface of the upper tapered portion while retaining the flat surface of the truncated tip free of carbon coating to obtain a precursor LaB6 cathode;
• initially heating the precursor LaB6 cathode to a temperature of from 1700 K to 2000K under vacuum of 1×10−7 to 1×10−8 Torr;
• cooling the first heat treated precursor LaB6 cathode to ambient temperature;
• exposing the ambient temperature precursor LaB6 cathode to an atmosphere comprising water vapor;
• and
• performing a second heating of the precursor LaB6 cathode at an emission temperature of from 1700 K to 2000K under vacuum of 1×10−7 to 1×10−8 Torr to obtain the LaB6 cathode wherein a difference between a Work Function value of the precursor LaB6 (WF1) and a Work Function value of the LaB6 cathode (WF2) satisfies the following relationship:
• WF1\\WF2≧0.01 eV.