||The goal of this project is to investigate the thermionic electron emission properties of (i) carbon nanotubes (CNTs) coated with nitrogen and phosphorus doped nanocrystalline diamond (NCD) films and (ii) NCD films implanted with metal ions. Thermionic emission provides direct energy conversion of thermal energy into electric energy by providing the energy required for electrons to escape from a material's surface into vacuum, inducing a flow of current. Hence, enough charge carriers need to be present and the work function of the material should be as low as possible, facilitating an easy emission of electrons. Therefore diamond is an excellent candidate as hydrogen terminated diamond surfaces have a negative electron affinity, allowing electrons to escape from the surface without any potential barrier present. However, to provide appropriate conductivity levels, the diamond should be n-type doped, possibly inducing an emission barrier through band bending. The N and P-doped NCD films are grown on CNTs using CH4/H2/N2 and CH4/H2/PH3 gas mixtures in a microwave plasma enhanced chemical vapor deposition reactor. In this project the thermionic properties of the doped NCD films on CNTs and metal ion implanted NCD films will be studied, together with their transport and surface properties, whose understanding is indispensable to understand the emission process in detail.