Graphene is currently a hot research topic within the Materials Science community. Graphene is one isolated planar carbon sheet composed of a sp2 bonded honeycomb lattice. Graphite is bulk multilayer stacked graphene. The term graphene is often generically used in reference to several stacked planar sheets, i.e. multilayer graphene. The room temperature properties of high electron mobility, high thermal conductivity and high mechanical strength offer significant potential for graphene applications. Another graphitic carbon form is carbon nanotubes (CNT) or rolled graphene sheets. CNT have two categories, single-walled CNT or one rolled graphene sheet and multi-walled CNT or multiple rolled graphene sheets. CNT research has filled literature over the past two decades since their discovery in 1991. Raman spectroscopy is a quick, nondestructive analytical technique that can clearly distinguish the graphitic carbon forms of graphene, CNT and bulk graphite.
The primary phonon G band is sourced from the carbon lattice stretching within the graphene plane. G band peak features are typically located at ~1580 cm-1 Raman shift. Figure 1 has Raman spectra of graphene, bulk graphite, multi-walled CNT, single-walled CNT obtained with a 785 nm excitation laser wavelength. Visual observation of spectra reveals graphene and bulk graphite both have similar G bands. Graphene and bulk graphite sheets are both planar in orientation. The doublet G band peak for the 30-50 nm diameter multi-walled CNT is attributed to a slight carbon sheet curvature. Single-walled CNT G bands have a unique shape due to the high curvature sourced from the small 1-2 nm diameter. In addition, single-walled CNT have unique radial breathing mode phonons located in the Raman shift range of 75 to 300 cm-1 as seen in Figure 1.
D band is often referred to as the disorder or defect mode. D band peaks have two optical sources. Multi-layer graphitic carbon is composed of irregular stacked graphene (carbon rings do not directly align sheet to sheet) forming disorder. The other D band signal source is defects. D band Raman shift is located within the 1330 to 1360 cm-1 range. As seen in Figure 1, the D band is observed in all graphitic spectra. One variation among spectra is the lower relative D band peak amplitude for single-walled CNT. Composed of a single graphene sheet, single-walled CNT are incapable of stacking disorder. Typically, the D band also has relatively low amplitude in graphene spectra. The high D band amplitude suggests our graphene is defect laden. The 2D band is a secondary phonon sourced from the same physical effects creating the D band. Graphene and single-walled CNT have 2D bands at ~2600 cm-1 where thicker multi-layer graphitic materials are found ~25 cm-1 higher.
Figure 1: 785 nm laser excitation wavelength Raman spectra of various graphitic carbon forms. Black spectrum is 1-2 layer graphene. Blue spectrum is bulk graphite. Green spectrum is 30-50 nm diameter multi-walled CNT. Red spectrum is 1-2 nm diameter single-walled CNT.