Characterizing Carbon Materials with Raman Spectroscopy

1 Characterizing Carbon Materials with Raman SpectroscopyJoe Hodkiewicz, Thermo Fisher Scientific, Madison, WI, USAI ntroductionCarbon nanomaterials have revolutionized the field ofmaterial science in recent years. Individual carbonnanomaterials offer a wide range of useful propertiespertaining to electrical conductance, thermal resistance,and exceptional strength, making them very interestingmaterials to a broad range of industries. The high-level ofinterest in the processing, modification, and customizationof these Materials has created a strong demand fortechniques that can be used to characterize carbonnanomaterials. Raman Spectroscopy is one technique that has proven to be very well suited to many of thecharacterization needs with these Spectroscopy is most sensitive to highlysymmetric covalent bonds with little or no natural dipolemoment.

2 The Carbon - Carbon bonds that make up thesematerials fit this criterion perfectly and as a result Ramanspectroscopy is highly sensitive to these Materials and ableto provide a wealth of information about their structure. Aswe shall see, Raman Spectroscopy is capable of discerningeven slight changes in structure making it a very valuabletool in the characterization of Carbon Nanomaterials DefinedThere are a wide variety of different Carbon nanostructures,however they all have a few basic things in common. First,all of these Materials are predominantly made up of purecarbon, and as such can be called Carbon allotropes. Therange of these Materials starts with the well knownallotropes of diamond and graphite , and continues on toencompass fullerenes, graphene and more complexstructures such as Carbon nanotubes.

3 From a molecularperspective, these Materials are all entirely composed of C-C bonds, although the orientation of these bonds isdifferent in the different Materials and therefore, tocharacterize their molecular structure in a meaningfulmanner, it is necessary to have a technique which is highlysensitive to even slight changes in orientation of C-C Highly Sensitive to MorphologyRaman Spectroscopy is particularly well suited tomolecular morphology characterization of carbonmaterials. Every band in the Raman spectrum correspondsdirectly to a specific vibrational frequency of a bondwithin the molecule. The vibrational frequency and hencethe position of the Raman band is very sensitive to theorientation of the bands and weight of the atoms at eitherend of the bond.

4 Figure 2 shows an example in which theRaman spectrum of diamond is compared to the Ramanspectra of crystalline silicon and germanium. Thesespectra show us several things. First, note that in the caseof diamond, where the material consists of highly uniformC-C bonds in a tetrahedral crystal structure, the Ramanspectrum is very simple. It consists of only a single bandbecause all of the bonds in the crystal are of the sameorientation and strength resulting in a single vibrationalfrequency. We also see that the spectrum of diamond iseasily distinguished from the spectra of silicon andgermanium by the frequency (cm-1position) of the bandeven though they share the same tetrahedral crystalconfiguration. The heavier atoms of silicon andgermanium slow the vibrational frequency and shift thecorresponding Raman band to lower frequency as Words CarbonNanomaterials Carbon Nanotubes D Band Fullerenes G Band graphene MolecularMorphology Multi-wall Carbon Nanotubes(MWCNT) RBM Bands Single-wallCarbon Nanotubes(SWCNT)ApplicationNote: 51901 Figure 1.

5 Structure of some representative Carbon allotropesDiamondFullerene (C60)GrapheneSWCNTS imilarly in Figure 3, when wecompare the Raman spectra of twocarbon allotropes diamond andgraphite again we can easilydistinguish the two Materials by theirRaman spectrum even though bothare composed entirely of C-C graphite spectrum has severalbands in the spectrum and the mainband has shifted from 1332 cm-1indiamond to 1582 cm-1in reason for this is that graphite is composed of sp2bonded Carbon in planar sheets in which the bondenergy of the sp2bonds is higher thanthe sp3bonds of diamond. The higherenergy of the sp2bonds in graphitepushes the vibrational frequency ofthe bonds and hence the frequency ofthe band in the Raman spectrum tohigher frequency. The 1582 cm-1bandof graphite is known as the G presence of additional bands inthe graphite spectrum indicate thatthere are some Carbon bonds withdifferent bond energies in the graphitesample and this is in fact the case, asgraphite is not quite as uniform instructure as good example to showthe remarkable sensitivity of Raman tomolecular morphology is to comparethe Raman spectra of diamond withthat of nanocrystalline diamond as canbe seen in Figure 4.

6 The small crystalsize of nanocrystalline diamond resultsin a finite-size effect in which thelattice is somewhat distorted. This ismanifested in the Raman spectrum bya slightly downshifted tetrahedral sp3band. The additional band at 1620 cm-1and the shoulders on the 1620 cm-1and tetrahedral sp3band are alsoindicative of sp2bonded Carbon thatrepresents surface defect modes,which would be insignificant in largerdiamond crystals. Finally, the verybroad band around 500 cm-1isindicative of some amorphous sp3bonded Carbon . This exampleillustrates just how sensitive Ramanspectroscopy is to even very slightdifferences in molecular 3: Raman spectra of diamond and of a sample of graphite on a silicon substrateFigure 4: Raman spectra of diamond and nanocrystalline diamondFigure 2: Raman spectra of diamond, crystalline silicon, and crystalline germanium1332520300 Diamond bandG band known as the graphite ortangential band Silicon1620 AmorphousCWhat Raman Can Reveal AboutMore Complex Carbon StructuresFullerenes: Fullerenes are essentiallyhollow Carbon shells of various most well known of these is a60- Carbon unit called Buckminsterfullerene or C60.

7 There are manyother fullerenes, from a few to manyhundreds of Carbon atoms. Figure 5compares the Raman spectra of C60and C70. The main feature in the C60spectrum is a relatively sharp line ataround 1462 cm-1, known as thepentagonal pinch mode. This tells usseveral things. Firstly, it tells us thatC60 is composed of sp2bondedcarbon. The sharpness of the bandalso tells us that the bonds are for themost part very uniform in nature. Infact, the Carbon atoms in C60 areequivalent and indistinguishable. Incontrast, the spectrum of C70 islittered with numerous bands. This isdue to a reduction in molecularsymmetry which results in more Ramanbands being active. AdditionallyRaman can also be very sensitive todoping and stress due to temperatureor pressure. graphene : graphene is thefundamental building block of manyimportant Carbon Materials includinggraphite.

8 graphite consists of stacksof sp2bonded planar graphene comparing Raman spectra ofgraphene and graphite , at first glancethe spectra look very similar. This isnot too surprising as graphite is juststacked graphene . However, there aresome significant differences as we cansee in Figure 6. The most obviousdifference is that the band at 2700 cm-1,which is known as the G' band, ismuch more intense than the G bandin graphene compared to may have heard the G' bandreferred to as the 2D band; both 2 Dand G' are accepted names for thisband. Figure 7a allows us to take acloser look at the G' band of thesetwo Materials where we can see thatboth the shape of the band and thepositionare different and both tell peak shift in graphiteis a result of interactions between thestacked graphene layers which has atendency to shift the bands to higherfrequency.

9 Figures 7b and 7c showthe same spectra as in 7a with theaddition of curve fitting that has beenused to provide a better picture of theunderlying structure of these G' band in a single layer graphenespectrum fits to a single bandwhereas curve fitting reveals severalFigure 5: Raman spectra of two fullerenes C60 and C70 Figure 6: Comparison of Raman spectra of graphene and graphiteFigure 7a: G' band of graphite and grapheneFigure 7b: G' band of graphene with curve fittingresults overlaid in redFigure 7c: G' band of graphite with curve fittingresults overlaid in redGraphiteGraphiteGrapheneGrapheneunder lying bands in the graphitespectrum. These bands are a resultof the different interlayerinteractions that occur at differentdepths within the Nanotubes: Carbon nanotubes are essentiallyrolled up graphene sheets thathave been sealed to form hollowtubes.

10 Single-wall carbonnanotubes (SWCNT) are cylindrical tubes with a singleouter wall with diameters that are usually only 1 2 are also double-wall Carbon nanotubes (DWCNT)which have a second layer of graphene wrapped aroundan inner single-wall Carbon nanotube. These are a subsetof the larger category of multi-wall Carbon nanotubes(MWCNT) that have many layers of graphene wrappedaround the core tube. Refer to Figures 8a and 8b. Due totheir unique mechanical, electrical and thermal properties, Carbon nanotubes are one of the most active areas in thefield of Carbon nanotechnology today. The Raman spectrum of a SWCNT bears a lot ofsimilarity to graphene , which is not too surprising as it issimply a rolled up sheet of graphene . Figure 9 shows us aRaman spectrum of a SWCNT in which we can see welldefined G and G' bands as there were in graphene andgraphite.