Chemical structure analysis of diamond-like carbon by Raman spectroscopy

Abstract

The chemical structure of diamond-like carbon (DLC) films was analyzed by Raman spectroscopy. The samples were DLC films synthesized by photoemission-assisted Townsend discharge (PATD). Group theory of the fundamental molecular structures suggested that the Raman spectrum consists of five bands with specific vibration modes, and the lineshape was represented by a modified Voigt-type formula. Analysis of the areas and positions of the bands resulted in the chemical structure of the DLC films with the sp2 cluster model. The model comprises conjugated and conductive clusters of sp2 carbons (sp2 clusters) floating in a non-conjugated and dielectric matrix of sp2 carbon, sp3 carbon, and hydrogen. The sp2 clusters were rather aliphatic for DLC films formed in low concentration of methane. The clusters grew to become aromatic with increasing methane concentration. The number of defects or dangling bonds increased similarly but were terminated with hydrogen for the films formed in a high methane concentration. The essential structure of DLC is the result of the development of random conjugation represented by the sp2 cluster model. We consider that DLC is a carbonaceous material in which conjugation increases slowly with time during the deposition process and which exhibits dielectric characteristics.