I am working with several carbon materials with different morphology such as carbon nanotubes by nanocasting, graphene oxide and graphite. I have observed that band D is broader than band G. What can this be attributed to?
It could be due to chemical modification but also check the acquisition parameters (laser power and acquisition time). High laser power or long acquisition time may cause the sample damage and consequently changes in the spectrum.
The D band originates from a hybridized vibrational mode associated with
graphene edges and it indicates the presence of some disorder to the graphene structure. This band is often referred to as the disorder band or the defect band and its intensity relative to that of the G band is often used as a measure of the quality with nanotubes.
D mode is present in all graphite-like carbons and originates from structural defects. Therefore, the ratio of the G/D modes is conventionally used to quantify the structural quality of carbon nanotubes. High-quality nanotubes have this ratio significantly higher than 100. Also,this ratio gives an idea of the functionalisation of a nanotube. At a lower functionalisation of the nanotube, the G/D ratio remains almost unchanged.
As can be seen from the work of some researchers that made simulations of Raman spectra of giant polyaromatic hydrocarbons - see "Origin of the D line in the Raman spectrum of graphite: A study based on Raman frequencies and intensities of polycyclclic aromatic hydrocarbon molecules", J. Chem. Phys. 114 (NO12) pp.963-974, 2001 and "Graphite and graphitic compounds: vibrationasl spectra from oligomers to real materials", C. Mapelli et al, J. Molec. Struct. 480-481, pp. 615-620, 1999, the real carbons can be seen as having a mixture of "equivalent PAHs" and thus , the resulted D band is a convoluted signal arriving from different shapes and forms of PAHs or graphenes, fact that induce a broadening of D band. Some years ago I synthesized using Hot fillament-PECVD some few wall distorted dense carpet of carbon nanotubes which presented a quite narrow D Raman band due to the size-confinement of defects in the distorted single/few wall nanotubes [see F. Le Normand, C.T. Fleaca, M. Gulas, A. Senger, O. Ersen, I.N. Mihailescu, G. Socol, D. Muller, M.C. Marco de Lucas “Growth of vertically oriented films of carbon nanotubes by activated catalytic chemical vapor deposition on Fe–Co/TiN/Si(100) substrates” J. Mater. Res. 23 (2008) 619-631]
The more prominent D band is to be expected to a certain extent given the multilayer configuration and indicates more disorder in the structure.
Read attached article for detailed analysis
Article Raman spectroscopy study of growth of multiwalled carbon nan...
That is because D band is sensitive to laser excitation energy and spot size on large defective region could rise 2 bands D on around 1350 cm-1. Please see Fig 4 b on https://www.repository.cam.ac.uk/bitstream/handle/1810/279354/1306.5856v1.pdf?sequence=1&isAllowed=y
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