Raman Spectroscopy of Carbon Materials: Structural Basis of Observed Spectra
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Chem. Mater. 1990,2,557-563 557 Raman Spectroscopy of Carbon Materials: Structural Basis of Observed Spectra Yan Wang, Daniel C. Alsmeyer, and Richard L. McCreery" Department of Chemistry, The Ohio State University, 120 W. 18th Ave., Columbus, Ohio 43210 Received April 17, 1990 The first- and second-order Raman spectral features of graphite and related sp2carbon materials were examined with laser wavelengths ranging from 293 to 1064 nm. A wide range of carbon materials was considered, including highly ordered pyrolytic graphite (HOPG), powdered and randomly oriented graphite, and glassy carbon prepared at different heat-treatment temperatures. Of particular interest is boron-doped highly ordered pyrolytic graphite (BHOPG), in which boron substitution decreases local lattice symmetry but does not disrupt the ordered structure. New second-order bands at 2950,3654, and -4300 cm-' are reported and assigned to overtones and combinations. The D band at 1360 cm-l, which has previously been assigned to disordered carbon, was observed in ordered boronated HOPG, and its overtone is strong in HOPG. The observed Raman shift of the D band varies with laser wavelength, but these shifts are essentially independent of the type of carbon involved. It is concluded that the D band results from symmetry breaking occurring at the edges of graphite planes in sp2carbon materials or at boron atoms in BHOPG. The observations are consistent with the phonon density of states predicted for graphitic materials, and the fundamental and higher order Raman features are assignable to theoretically predicted lattice vibrations of graphite materials. The laser wavelength dependence of the D band frequency appears to result from scattering from different populations of phonons, perhaps through a resonance enhancement mechanism. However, the results are inconsistent with resonance enhancement of graphite microcrystallites of varying size. Introduction Table I. Reported Vibrational Modes for Graphitic Carbon The lattice dynamics and vibrational spectroscopy of sp2 Materials carbon materials have been the subject of numerous in- carbon type Av, cm-' re1 int" assgnt ref vestigations, and representative references are included natural single 1575 vs E, 1 Raman spectroscopy has been an important tool crystal in such investigations, because the Raman spectrum is HOPG 1581 vs Ezr 5 particularly sensitive to the microstructure of the carbon. HOPG 1582 vs E% 3 The a- and c-axis coherence lengths, and L,, and the HOPG 1582 vs E, 15 La HOPG 127 2, 41 dW2interplanar spacing all affect the Raman spectrum of W B, HOPG 42 W Ezr 2 graphitic materials, thus providing useful diagnostics for HOPG 868 IR only AZu 14 carbon structure and pr~perties.'~~~~~~J~Despite the sig- HOPG 1588 IR only E,, 14 nificant research effort invested by many laboratories on HOPG -1620 S E', 42 understanding the Raman spectroscopy of graphitic ma- HOPG 3248 vw 2 X 1624 cm-' 5 terials, the assignments and behavior of several Raman HOPG 2710 S 2 X 1355 cm-' 5 HOPG - 2450 W 2 X 1225 cm-'? 5 spectral features remain unclear. Our own laboratory has HOPG, ion 2970 vw E'Zg + D 24 correlated the Raman spectroscopy of sp2carbon materials implanted with their electrochemical behavior'6,21v22and the same glassy carbon 1582-1600b vs E2r 9, 17 questions arise about the structural basis of Raman fea- GC-20 1605 vs Ezr 16 tures. The current work was initiated to provide new GC 1355c vs Ah 1 information about the physical basis of the observed Ra- GC 136OC 16 GC 1355c vs 9 5 man spectra, with particular emphasis on relationships of GC, PG, HOPG 2640-2732c s 2 X 1360 7 Raman spectra with carbon microstructure. GC, PG 132O-136Oc s D 7 Single crystal graphite belongs to the D& symmetry Ovs = very strong, s = strong, w = weak, vw = very weak. group, and vibrational modes are of the types 2Ezg,2Bzg, *Depends on heat-treatment temperature. Depends on &,. Elu,and A2u,193fiJ219shown in Figure 1. The two E, modes are Raman active and have been identified with the Raman calculations of the phonon density of states, Al-Jishi and band at 1582 cm-' and a low-frequency neutron scattering Dresselha~s~~~~~have attributed the second-order bands feature at 47 cm-l, while the E,, (1588 cm-') and AzU(868 cm-') are IR active and observable with IR refle~tance.'~ The Bag modes are optically inactive, but one has been (1) Tuinstra, F.; Koenig, J. L. J. Chem. Phys. 1970, 53, 1126. observed by neutron scattering at 127 cm-'. Highly or- (2) Nicklow, R.; Wakabayashi, N.; Smith, H. G. Phys. Reu. B 1972,5 4951. dered pyrolytic graphite (HOPG)exhibits only the 1582- (3) Nemanich, R. J.; Lucovsky, G.; Solin, S. A. Mater. Sci. Eng. 1977, cm-' band in the fundamental region between 1100 and 31, 157. 1700 cm-',' but also shows second order features between (4) Mani, K. K.; Ramani, R. Phys. Status Solidi 1974, 61, 659. (5) Nemanich, R. J.; Solin, S. A. Phys. Rev. B 1979, 20, 392. 2400 and 3300 cm-1.5*9923 Nemanich and Solin have at- (6) Nakamizo, M.; Kammereck, R.; Walker, P. L. Carbon 1974,12,259. tributed the strong feature at ca. 2720 cm-' and weaker (7) Vidano, R. P.; Fischbach, D. B.; Willis, L. J.; Loehr, T. M. Solid peaks at N 2450 and 3248 cm-' to overtones of fundamental State Commun. 1981,39,341. (8) Nakamizo, M.; Tamai, K. Carbon 1984,22, 197. modes and accounted for their presence using phonon (9) Vidano, R.; Fischbach, D. B. J. Am. Ceram. SOC.1978, 61, 13. dispersion relationship^.^ On the basis of theoretical (10) Tau, R.; Gonzales, J.; Hernandez, I. Solid State Commun. 1978, 27, 507. (11) Dresselhaus, M. S.; Dresselhaus, G. Adu. Phys. 1981, 30, 139. *Author to whom correspondence should be addressed. (12) Reference 11, pp 290-298. 0897-4756/90/2802-0557$02.50/0 0 1990 American Chemical Society 558 Chem. Mater., Vol. 2, No. 5, 1990 Wang et al. Table I1 &, nm lase? spectrometer* detector ref 292.8 QCWc ISA 640 CCDd 40 350.7 Krt Spex 1403 PMT (RCA 31034) 36, 37 406.7 Kr+ Spex 1403 PMT 36, 37 457.9-514.5 Art Spex 1403 PMT 36, 37 568.2 Kr+ Spex 1403 PMT 36, 37 647.1 Kr+ Spex 1403 PMT 36, 37 infra-Red 782 Diode ISA 640 CCD 38, 39 Active 1064 YAG' interferome- germaniume 34 tere "Krt was a Coherent Model 100, Art was a Coherent 90-5;the diode laser was Liconix "Diolite" AlGaAs. * Spex 1403 is an addi- tive dispersion double monochromator; ISA 640 is a 640 mm single spectrograph. 'Quasi-CW YAG pumped dye laser; see ref 32. Optically dCharge couple device array detector. "Located at DuPont, Wil- inactive mington, DE; see refs 26 and 27. 127 cm" -870 cm" or even C,.19251n New vibrational modes of the lattice may then become active, such as an Al mode proposed by Tuinstca and Koenig.' A related mecianism is breakdown of the k = 0 selection rule for optical phonons near crys- Figure 1. Vibrational modes for single-crystal graphite. Drawings tallite edges.'~2~5~23~24Such breakdown permits phonons were adapted from ref 3; vibrational assignments are from refs other than 1582 and 47 cm-' to become active, and the 3,5,and 20. The B,(" frequency is predicted but not yet observed spectra reflect the density of phonon states in the lattice. experimentally. A very different explanation for the 1360-cm-' mode is the existence of specific vibrations at the edges, e.g., oxides or at -2450, 2730, and 3248 cm-' to overtones of Raman- C=C groups which are present only at the edge.25 Unlike forbidden fundamentals, with good agreement between the proposed A', mode, these edge vibrations are not di- predicted and observed second order peaks. They have rectly related to the hexagonal graphite lattice but are also theoretically predicted the dependence of fundamental analogous to functional groups. All reports concur that intensities on La for partially graphitized carbon, on the the 1360-cm-' mode is related to structural disorder (and basis of breakdown of the wave vector selection rule.'* The it will be referred to as the D band hereafter), but there HOPG spectrum will be discussed in more detail below, is not a clear consensus on its origin. but one can conclude from the available literature that the A further complication of the D band is its unusual assignments for known Raman features in pristine HOPG dependence on laser wavelength X@7J9925 Vidano et al. are uncontroversial. Representative frequencies and as- reported that the position (AP) of the D band shifted from signments for carbon vibrational features from the liter- 1360 to 1330 cm-' when A,, was increased from 488 to 647 ature are listed in Table I. nm,7 and several possible explanations for this effect have The spectrum changes significantly for finite-sized mi- been proposed. Variations in X, will change the Raman crocrystallites (La < lo00 A), and the origin of the changes sampling depth, possibly leading to variation in the is not clear.'-5.6J7~23*25~26The most obvious effect is the spectrum if the carbon structure varies with depth.25 appearance of a band at ca. 1360 cm-' with the 1360 Alternatively, the D band may be resonance enhanced, so cm-l/ 1582 cm-' intensity ratio (I13eo) increasing with de- that different subpopulations of crystallites (with different creasing microcrystallite size.' The empirical linear rela- Aij) may be sampled at different hP7 Polyacetylene ex- tionship between l/La and I1360 is a useful means to de- hibits such behavior,28s29and perhaps graphite is acting like termine the average La for a given carbon sample.