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

Abstract

Highly oriented poly crystalline graphite (HOPG), boron-doped diamond (BDD), nanocrystalline diamond (NCD), ultra-nano-crystalline diamond (uNCD), fullerenes C60 and C70 and Diamond Like Carbon (DLC) surfaces are exposed to low pressure hydrogen plasma in a 13.56 MHz plasma reactor. Relative yields of surface produced Hions due to bombardment of positive ions from the plasma are measured by an energy analyzer 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 (~ 5 times compared to HOPG surface at room temperature) is observed on BDD surfaces. The shapes of ion distribution functions (IDFs) 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 Hproduction yields, while further analyses strongly suggest that the hydrogen content of the material and the sp3/sp2 ratio are the key parameters in driving surface ionization efficiency of carbon materials under the chosen plasma conditions.

Investigations on caesium-free alternatives for H− formation at ion source relevant parameters

“…This also is mandatory for suitable materials for efficient H − formation in negative hydrogen ion sources. It is scheduled to extend the present investigations on materials for which negative ion surface formation has been reported like highly ordered pyrolytic graphites (HOPG) and different diamond samples [14].…”

“…Numerous investigations on materials suitable for H − formation at a material surface or close to it are present focusing on various applications and are performed within a wide range of fields and parameters [5][6][7][8][9][10][11][12][13][14]. For example have tantalum and tungsten revealed an enhancing effect on highly vibrationally excited hydrogen molecules H 2 (X,ν) in gas phase experiments [15][16][17].…”

“…diamond like carbon (DLC) negative ion yields have been measured in beam experiments [7,19] and in low-pressure hydrogen plasmas with low electron density (n e ∼ 10 14 m -3 ) and high electron temperatures (T e ∼ 5 -6 eV) [14]. In the latter experimental setup negative ion formation has been reported due to DLC, highly ordered pyrolytic graphites (HOPG) and boron doped diamond (BDD) showing a dependency of the H − yield on the sample temperature which is related to a variation in the sp 3 /sp 2 hybridisation ratio [13,14,20]. Furthermore utilizing the pure surface production of H − at materials with inherent low work function like LaB 6 or BaO may also be suitable for an application at ion sources.…”

Efficiency of Cs-free materials for negative ion production in H2 and D2 plasmas

“…• Carbon materials, in particular non-doped and boron-doped diamond (BDD) have also shown to produce negative hydrogen ions (see e. g. [10,11]). However, in contrast to low work function materials, the formation mechanism at the insulating diamond surface (work function above 4 eV) and thus the actually required conditions for efficient H − production are not fully understood at present.…”

In‐plane and out‐of‐plane defects of graphite bombarded by H, D and He investigated by atomic force and Raman microscopies

“…Recently, it has been demonstrated that carbon materials are interesting candidates to enhance negative ion production in hydrogen plasmas . In a series of papers, negative ion surface production mechanisms on hydrogenated carbon materials have been investigated, pointing out how Raman spectroscopy can be a useful tool for relating surface modification and ion production.…”