Experimental evidence for large negative electron affinity from scandium-terminated diamond

Negative electron affinity (NEA) or low-work function conditions of wide-band gap materials play a crucial role in developing effective electron-emission devices, field-effect transistors (FETs), and energy converters. Single-crystal diamond with electropositive surface terminating groups can exhibit NEA and has been proposed for possible thermionic emission devices. Here, a report on the in situ observation of large NEA from scandium-terminated diamond is presented. A quarter monolayer of Sc was deposited via electron beam evaporation onto bare diamond (100) and (111) surfaces. The variations of surface structure, electron affinity (EA) and work function (WF) were measured following each annealing step in vacuo at temperatures up to 900 °C. The magnitudes of the EA were found to be dependent upon the surface orientation and annealing temperature, the most negative measured being −1.45 eV and −1.13 eV for the diamond (100) and (111) surfaces, respectively. These values show that these two Sc-diamond surfaces have the highest negative EA for a metal adsorbed onto bare diamond measured to date, as well as being thermally stable up to 900 °C. This study unveils structural and electronic insights into tuning the adsorbate-diamond interface and further expands the potential candidate material map for effective electron-emission applications. Two Sc-diamond (100) and (111) surfaces have the highest negative electron affinity for a metal adsorbed onto bare diamond measured to date, as well as being thermally stable up to 900 °C.