Characterizing Carbon Materials with Raman SpectroscopyJoe Hodkiewicz, Thermo Fisher Scientific, Madison, WI, USA

Introduction

Carbon 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 materials.

Raman spectroscopy is most sensitive to highlysymmetric covalent bonds with little or no natural dipolemoment. 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.

Carbon Nanomaterials Defined

There 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. 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 bonds.

Raman Highly Sensitive to Morphology

Raman 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. 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-1 position) 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 well.

Key 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: Structure of some representative carbon allotropes

Diamond

Fullerene (C60)

Graphene

SWCNT

Similarly 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 bonds.The graphite spectrum has severalbands in the spectrum and the mainband has shifted from 1332 cm-1 indiamond to 1582 cm-1 in graphite.The reason for this is that graphite is composed of sp2 bonded carbon in planar sheets in which the bondenergy of the sp2 bonds is higher thanthe sp3 bonds of diamond. The higherenergy of the sp2 bonds in graphitepushes the vibrational frequency ofthe bonds and hence the frequency ofthe band in the Raman spectrum tohigher frequency. The 1582 cm-1 bandof graphite is known as the G band.The 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 diamond.

Another 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. 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 sp3

band. The additional band at 1620 cm-1

and the shoulders on the 1620 cm-1

and tetrahedral sp3 band are alsoindicative of sp2 bonded carbon thatrepresents surface defect modes,which would be insignificant in largerdiamond crystals. Finally, the verybroad band around 500 cm-1 isindicative of some amorphous sp3

bonded carbon. This exampleillustrates just how sensitive Ramanspectroscopy is to even very slightdifferences in molecular morphology.

Figure 3: Raman spectra of diamond and of a sample of graphite on a silicon substrate

Figure 4: Raman spectra of diamond and nanocrystalline diamond

Figure 2: Raman spectra of diamond, crystalline silicon, and crystalline germanium

1332

520

300

Diamond band

G band – known as the graphite ortangential band Silicon

1620 AmorphousC

What Raman Can Reveal AboutMore Complex Carbon Structures

Fullerenes: Fullerenes are essentiallyhollow carbon shells of various sizes.The most well known of these is a60-carbon unit called Buckminsterfullerene or C60. 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 sp2 bondedcarbon. 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. Graphite consists of stacksof sp2 bonded planar graphene sheets.When 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 graphite.You may have heard the G' bandreferred to as the 2D band; both 2D

and 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 theposition are different and both tell ussomething. The peak shift in graphiteis a result of interactions between thestacked graphene layers which has a

tendency to shift the bands to higherfrequency. 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 bands.The G' band in a single layer graphenespectrum fits to a single bandwhereas curve fitting reveals several

Figure 5: Raman spectra of two fullerenes – C60 and C70

Figure 6: Comparison of Raman spectra of graphene and graphite

Figure 7a: G' band of graphite and graphene Figure 7b: G' band of graphene with curve fittingresults overlaid in red

Figure 7c: G' band of graphite with curve fittingresults overlaid in red

Graphite GraphiteGraphene Graphene

underlying bands in the graphitespectrum. These bands are a resultof the different interlayerinteractions that occur at differentdepths within the graphene.

Carbon Nanotubes:Carbon nanotubes are essentiallyrolled up graphene sheets thathave been sealed to form hollowtubes. Single-wall carbonnanotubes (SWCNT) are cylindrical tubes with a singleouter wall with diameters that are usually only 1 – 2 nm.There 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. We also see a prominent band around 1350 cm-1.This band is known as the D band. The D band originatesfrom a hybridized vibrational mode associated withgraphene edges and it indicates the presence of somedisorder to the graphene structure. This band is oftenreferred to as the disorder band or the defect band and its

intensity relative to that of the Gband is often used as a measure of thequality with nanotubes. There isanother a series of bands appearing atthe low frequency end of thespectrum known as Radial BreathingMode or RBM bands. The RBMbands are unique to SWCNTs and astheir name suggests, correspond tothe expansion and contraction of thetubes. The frequency of these bandscan be correlated to the diameter ofSWCNTs and they can provideimportant information on theiraggregation state.

MWCNTs have very similarspectra to those of SWCNTs, as isapparent from Figure 10. The primarydifferences are the lack of RBM modesin MWCNTs and a much moreprominent D band in MWCNTs. TheRBM modes are not present becausethe outer tubes restrict the breathingmode. The more prominent D band inMWCNTs is to be expected to acertain extent given the multilayerconfiguration and indicates moredisorder in the structure. Figure 10compares the spectra of samples ofSWCNTs, DWCNTs, and MWCNTs.Notice that the RBM mode completelydisappears as we go to a double-walltube and that the D bands and G'bands get proportionately larger aswe add layers to the walls of the tubes.

Figure 8a: Diagram depicting the structure of graphene and a SWCNT Figure 8b: Diagram depictingthe structure of a MWCNT

Graphene SWCNT

Figure 9: Raman spectrum of SWCNTs with prominent bands labeled

Figure 10: Comparison of Raman spectra of SWCNTs, DWCNTs, and MWCNTs

MWCNT

What is Involved in Recording a Raman Spectrum

It is relatively easy to collect a Raman spectrum of acarbon nanomaterial. The materials are generally recordedneat under standard atmospheric conditions which makefor very easy sampling and provide a lot of flexibility as towhat can be sampled. Although if samples are in the formof a loose powder, then the powder is usually compressedto provide a denser sample. This can be done by simplypressing a small amount of sample between two microscopeslides. If the powders are loose, the density is often toolow to get a measureable spectrum. Many times CNTs arecast onto slides within a surfactant. This works well as thedensity is generally much higher than the powdered formof the material, although it is worth noting that there maybe some bands present from the surfactant. These can beeasily identified by measuring a blank sample of thesurfactant. Films are typically measured in their nativestate on whatever substrate they happen to be on.

In terms of time of analysis, Raman measurements aregenerally short compared to other techniques. Themeasurement time typically ranges anywhere from a fewseconds to five minutes depending primarily on the densityof the sample and on the laser power at the sample. Thedenser a sample, the shorter the necessary measurementtime will be; and the higher the laser power, the shorterthe necessary measurement time will be. Note that whenwe talk about laser power – it is the laser power at sampleand really the optical density of the laser power at thesample that is most critical. If a lot of laser energy isdispersed over a large area it will not have the sameimpact as a lesser amount of energy tightly focused on thearea of interest. It is also worth noting that many carbonnanomaterials are sensitive to laser power, so the amountof laser power you can use is often limited by the samplesand it is important to have good control of the laserpower to insure reproducible measurements.

Conclusions

Raman is a very powerful and valuable technique that canbe of great benefit to characterization of carbonnanomaterials. Raman is particularly well suited to detectsmall changes in structural morphology of carbonnanomaterials making it an indispensable tool for manymaterial scientists working with carbon nanostructures.Raman instruments are very fast and provide a great dealof flexibility in samples that can be accommodated. Every lab that is characterizing carbon nanomaterials willbenefit from having access to Raman instrumentation.

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Figure 11: View of compressed CNTs on a microscope slide (500× magnification)

Figure 13: Thermo Scientific DXR SmartRaman Spectrometer

Figure 12: Thermo Scientific DXR Raman Microscope