PhysRevB.64.075414
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ResonantRamanspectroscopyofdisordered,amorphous,anddiamondlikecarbon
A.C.Ferrari*andJ.Robertson
DepartmentofEngineering,UniversityofCambridge,CambridgeCB21PZ,UnitedKingdom͑Received23October2000;revisedmanuscriptreceived15February2001;published26July2001͒
TheRamanspectraofawiderangeofdisorderedandamorphouscarbonshavebeenmeasuredunder
excitationfrom785to229nm.Thedispersionofpeakpositionsandintensitieswithexcitationwavelengthis
usedtounderstandthenatureofresonantRamanscatteringincarbonandhowtoderivethelocalbondingand
disorderfromtheRamanspectra.Thespectrashowthreebasicfeatures,theDandGaround1600and1350
cmϪ1forvisibleexcitationandanextraTpeak,forUVexcitation,atϳ1060cmϪ1.TheGpeak,duetothe
stretchingmotionofsp2pairs,isagoodindicatorofdisorder.Itshowsdispersiononlyinamorphousnetworks,
withadispersionrateproportionaltothedegreeofdisorder.Itsshiftwellabove1600cmϪ1underUV
excitationindicatesthepresenceofsp2chains.ThedispersionoftheDpeakisstrongestinorderedcarbons.
Itshowslittledispersioninamorphouscarbon,sothatinUVexcitationitbecomeslikeadensity-of-states
featureofvibrationsofsp2ringlikestructures.TheintensityratioI(D)/I(G)fallswithincreasingUVexci-
tationinallformsofcarbon,withafasterdecreaseinmoreorderedcarbons,sothatitisgenerallysmallforUV
excitation.TheTpeak,duetosp3vibrations,onlyappearsinUVRaman,lyingaround1060cmϪ1forH-free
carbonsandaround980cmϪ1inhydrogenatedcarbons.Inhydrogenatedcarbons,thesp3C-Hxstretching
modesaround2920cmϪ1canbeclearlydetectedforUVexcitation.Thisassignmentisconfirmedbydeute-
riumsubstitution.
DOI:10.1103/PhysRevB.64.075414PACSnumber͑s͒:78.30.Ly,63.50.ϩx,61.43.Dq,81.05.Tp
I.INTRODUCTION
Carbonisuniqueinthatsimplechangesinitslocalbond-
ingcangiverisetomaterialsasdiverseasdiamond,graph-
ite,fullerenes,carbonnanotubes,anddisordered,nanostruc-
turedandamorphouscarbons.Thesematerialshavea
remarkablerangeofmechanical,electronic,andelectro-
chemicalpropertiesandmanypossibleapplications.1–3Itis
thusveryusefultodevelopfast,reliable,andnondestructive
techniquestoprobethekeyparametersthatcontroltheir
physicalbehavior.
Weareinterestedinamorphouscarbons.Wedefinedia-
mondlikecarbon͑DLC͒asanamorphouscarbon(a-C)or
anhydrogenatedamorphouscarbon(a-C:H)withasignifi-
cantfractionofsp3bonds.a-C:Hoftenhasarathersmall
C-Csp3content.DLC’swithhighestsp3contentarecalled
tetrahedralamorphouscarbon͑ta-C͒anditshydrogenated
analogta-C:H.Thekeyparametersofinterestinsuchmate-
rialsare͑1͒thesp3content;͑2͒theclusteringofthesp2
phase;͑3͒theorientationofthesp2phase;͑4͒anycross-
sectionalstructure;and͑5͒theHcontent.Thesp3content
alonemainlycontrolstheelasticconstants,butfilmswiththe
samesp3andHcontentbutdifferentsp2clustering,sp2
orientationorcross-sectionalnanostructurecanhavediffer-
entoptical,electronic,andmechanicalproperties.
Ramanspectroscopyisapopular,nondestructivetoolfor
structuralcharacterizationofcarbons.4–11Itistraditionally
carriedoutatwavelengthsintheblue-greenspectralregion͑488–514.5nm͒,butmultiwavelengthRaman͑MWRaman͒
studiesarebecomingincreasinglyused.Indeed,Ramanscat-
teringfromcarbonsisalwaysaresonantprocess,inwhich
configurationswhosebandgapsmatchtheexcitationenergy
arepreferentiallyexcited.Anymixtureofsp3,sp2,andsp1
carbonatomsalwayshasagapbetween0and5.5eV,and
thisenergyrangematchesthatofIR-vis-UVRamansystems.ThisimpliesthatunderstandingtheresonantRamanprocess
incarbonsystemswillgiveapowerful,fastmeansfortheir
structuralandelectroniccharacterization.Forexample,MW
Ramanhasrecentlybeenusedtodistinguishthemetallicand
semiconductingformsofsingle-wallcarbonnanotubes.12,13
Ithasalsobeenusedtoinvestigatetheoriginofthepeaksatϳ1150cmϪ1andϳ1450cmϪ1innanocrystallinediamond.14
Inamorphousanddisorderedcarbonsthereareveryfew
studiesusingmultiplewavelengths.Graphite,disordered
graphite,andglassycarbonwerestudiedbyMWRamanby
Vidanoetal.16andothers,17–22Ramsteinerandco-workers,23
Yoshikawaetal.,24Tamoretal.,25andPocsiketal.26studied
diamondlikea-C:H.Callejaetal.15madethefirstresonant
Ramanstudyofdiamond,Wagnerandco-workers23,27gave
theMWRamanspectraofmicrocrystallinediamondand
Bormettetal.28extendedthistotheUV.
TheRamanspectraofallcarbonsshowseveralcommon
featuresinthe800–2000cmϪ1region,theso-calledGandD
peaks,whichlieataround1560and1360cmϪ1forvisible
excitation,andtheTpeak,seenforUVexcitationataround
1060cmϪ1.TheGandDpeaksareduetosp2sitesonly.The
Gpeakisduetothebondstretchingofallpairsofsp2atoms
inbothringsandchains.9TheDpeakisduetothebreathing
modesofsp2atomsinrings.TheTpeakisduetotheC-C
sp3vibrationsandappearsonlyinUVexcitation.
AlthoughRamanspectroscopyofcarbonshascontinued
for30years,therehavebeensignificantadvancesinour
understandingrecently.Firstly,C-Csp3vibrationsweredi-
rectlydetectedbyUVRamanspectroscopy.29,30Thisshould
