Negative-ion production on carbon materials in hydrogen plasma: influence of the carbon hybridization state and the hydrogen content on H- yield

Highly oriented polycrystalline graphite (HOPG), boron-doped diamond (BDD), nanocrystalline diamond, ultra-nanocrystalline diamond and diamond-like carbon surfaces are exposed to low-pressure hydrogen plasma in a 13.56MHz plasma reactor. Relative yields of surface-produced H- ions due to bombardment of positive ions from the plasma are measured by an energy analyser cum quadrupole mass spectrometer. Irrespective of plasma conditions (0.2 and 2 Pa), HOPG surfaces show the highest yield at room temperature (RT), while at high temperature (HT), the highest yield (similar to 3-5 times compared to HOPG surface at RT) is observed on BDD surfaces. The shapes of ion distribution functions are compared at RT and HT to demonstrate the mechanism of ion generation at the surface. Raman spectroscopy analyses of the plasma-exposed samples reveal surface modifications influencing H- production yields, while further analyses strongly suggest that the hydrogen content of the material and the sp(3)/sp(2) ratio are the key parameters in driving the surface ionization efficiency of carbon materials under the chosen plasma conditions.