Sam Zhang¹, Peter Hing² and Hong Xie³

1. School of Mechanical and Production Engineering, Nanyang Technological University

50 Nanyang Avenue, Singapore 639798

2. Advanced Materials Research Centre c/o School of Applied Science Nanyang Technological University Nanyang Avenue, Singapore 639798

3. Gintic Institute of Manufacturing Technology, 71 Nanyang Drive, Singapore 638705

E-mail: msyzhang@ntu.edu.sg

Raman spectroscopy, electron energy loss spectroscopy and nano indentation are some most commonly used techniques in characterizing of thin solid films of diamond, amorphous diamond-like carbon (DLC) or amorphous carbon. In Raman spectra, amorphous DLC films are characteristic of a broad single peak centered around 1530 cm^-1 (G-band) with a "shoulder" at around 1350 cm^-1 (D-band). The "shoulder" is the signature of the DLC film (otherwise a sharp peak at around 1580 cm^-1 would be that for highly oriented pyrolytic graphite). The peak intensity ratio and peak positions have been commonly used in the characterization of diamond-like carbon or amorphous carbon films. The peak position and the intensity vary from film to film. This variation comes from the structural change or disturbance of otherwise full three-fold coordination of carbon network (sp²) as four-fold coordinated structures (sp³) are introduced through vapor deposition. Parallel Electron energy loss spectroscopy (PEELS) analysis can distinguish atomic elements and differentiate between graphite, amorphous carbon, diamond and the variety of diamond-like carbon materials and enables quantification of the sp² and sp³ bonding in carbon films. The aim of this paper is to clarify for DLC or amorphous films the relationship between the Raman peak intensity ratio (I_d/I_g) and four-fold coordination fraction (sp³) through EELS studies.

Amorphous carbon coatings (a-C, a-C:H) of less than 100 nm thick were deposited on KBr pellets and silicon wafer substrate via magnetron sputtering of graphite target in argon, argon/hydrogen and argon/nitrogen atmosphere. PEELS analysis was used to quantify the sp² / sp³ bonding in carbon films. Sputtering onto compressed KBr pellets and then floating off in distilled water for PEELS study produced stand-alone films of amorphous carbon. Raman spectroscopy was used to measure the peak intensity ratio of D-band to that of the G-band (I_d/I_g). It shows that higher sp³ fraction often associates with lower Raman peak ratio I_d/I_g. At the same time, G-band peak position P_g decreases while sp³ fraction increases.