Diamond and diamond films studied by raman spectroscopy

81

Journal of Molecular Structure, 292 (1993) 81-88 Elsevier Science Publishers B.V., Amsterdam

Diamond and Diamond Films studied by Raman Spectroscopy Pham V. Huong Laboratoire de Spectroscopic Moleculaire et Cristalline, U.R.A. 124 C.N.R.S., Universite de Bordeaux I, 351, Cours Liberation, 33405 Talence, France - Fax (33) 56 84 84 02. The structure of cubic, hexagonal diamonds and diamond like films are analyzed by comparison of their Raman spectra to those of other carbon structures including fullerenes C6o and C70. Two kinds of “diamond films” are found, One of them has the cubic diamond structure, with nevertheless a slight compressive distorsion which can be evaluated from the increase in frequency and band-width of the C-C stretching mode. The second series of films have a new structure named 'diamite “presenting a pseudo-hexagonal network, similar to that ofgraphite but with longer C-C bonds in the ‘2ayer” and having inter-layer C-C bonds. 1 - INTRODUCTION

The research in diamonds is sharply activated the last few years because of the easy obtention of new carbon structures which present not only attractive academic interests but also precious properties leading to very wide application fields. More than other characterization techniques, Raman spectroscopy offers the possibility to determine not only the chemical nature of samples, but also their “physical” structure either for ordered or amorphous solids. In this paper, we shall present our results on the spectroscopic study of all existing carbon structures, in particular on diamond bulk crystals and “diamond films”. 2 - EXPERIMENTAL The diamond and diamond films are of different origins [l-3]. The diamond films were obtained by microwave plasma assisted chemical vapour deposition while the “diamite” films were deposited mostly by magnetron sputtering. 0022-2860/93/$06.00

C

tb i

a

Fd3m

( Oh7)

Fig.1 -Diamond

cubic structure.

The fullerenes or footballenes C(jo and C7o were acquired from many sources and recrystallized and separated in carbon disulfide. The Raman spectra were recorded on a Raman microspectrometer Dilor OMARS with a spatial resolution of 1 pm2 [4-51 and equipped with a multichannel detector of 1024 photodiodes and an ion-argon laser Spectra-Physics 165.

0 1993 Elsevier Science Publishers R.V _. ‘. A11 rights reserved,

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3 - RESULTS AND DISCUSSION By its relatively weak atomic weight, carbon is not easily detected by current elementary analytical techniques. On the other hand, the carbon solid network can change tremendously from totally amorphous to perfect cubic structure passing through well known graphites of different graphitization temperatures, or fascinating architectures like hexagonal diamond and footballene &I-J. Vibrational spectroscopy, especially Raman spectroscopy, is very suitable for the study of all these structures. It can inform on the nature of the element, on the chemical bond between elements and on the geometry of their networks.

distorsion from the ideal cubic structure. This disorsion is conceivable

as the f.c.c. lattice of diamonds is not very compact : the ei ht carbon atoms occupy only 46 % of the cubic unit volume. 3.2 - Hexagonal diamond Under pressure, synthetized diamond can appear under the hexagonal structure [6-71, with space !p

DIAMONDS

3.1- Cubic diamond

CUBIC

Like silicon and germanium, diamond lattice is all-face-centered cubic with space group Fd3m (OTh) and has point group m3m(Ch)(Fig.l). The high covalent character of their C-C bonds confers to their stretching vibration, active in Raman scattering, at 1331 cm-l. The phonon band-width at half intensity Av+ is less than l&m-l (Fig.2). For natural diamonds WI*.-

*) mn omm c-c

and synthesized diamonds of different origins, the frequency varies from 1331 to 1345 cm-l and their half width varies from 1.8 to 15 cm-l [ 1,31. The increase in C-C stretching frequency is due to a compressive

DIAMOND

WAVENUMBERICM-’

Fig.2 - Raman spectra of diamond and distorted cubic diamonds A : Perfect cubic diamond B,C : Compressive distorted diamonds.

k

HEXAGONAL

Ii25 1350 WAVENUMBERICM-’

WAVENUMRERICM-1

Fig.3 - Raman spectra of hexagonal artifical diamond.

group P63/mmc (D4 ) having four atoms per unit ce fi! . Its cr sta knTafhlc par eters are a z A 4 42? and ‘t deTs$ is 3.51 g.cz-3 s the same t’hin for cubic diamond.’ This corn actness is certain1 the reason for % e location of its C- 8 stretching frequency at 1320 + 5 cm-l (Fig.S), very close tothat of cubic diamond. When referred to the graphite hexagonal structure, Bundy and Kasper [61 indicate that the (100) plane of hexagonal diamond can be superimposed to the basal plane

83

0

0

0

0

0

0

0

0

0

0

0

a

a

0

o

&‘bC

0

modes are Raman active ; they both involve motions within the layer plane (Fig.5) ; one of them appears at 1581 cm-1 and represents the C-C “stretching” vibration ; the other, around 40 cm-l, could correspond to the “layers gliding motion ‘! In fact, the structure of graphite depends on the degree of graphitation and several intermediaries can be detected by Raman spectroscopy (Fig.61 from perfect graphite to

Fig.4 - (100) plane of hexagonal diamond (0) and basal plane ofgraphite (Cl o : in the plane ; o out of the plane.

of aphite but only a half number of car r on atoms are in the plane ; the other carbon atoms are dis laced normal to the plane (Fi .4). T Ris can low c-c explain the relative 7 ;y&r;irg frequency o P hexagonal .

3.3 carbon

-

Graphite

and amorphous

Graphite lattice has hexagonal D46h symmetry with 2 = 4 per unit cell. The optic modes at zone center is given by r OP = 2E2, + El,

+ 2B2, + 2Au

The El, and Afu modes are infrared active while 2B2, are silent. 2E,

( 1700

1600

1500

WAVENUMBER

Fig.5 - Raman active normal modes in graphite.

1400

KM-

Fig.6 - Raman spectra ofgraphite and carbon with different graphitization degrees.

,, 1300

84

amorphous carbon ; the latter having two very broad Raman features centered at 1573 and 1350 cm-l. 3.4 - Fullerenes c60 and c70 Very recently, new carbon structures have been found 171. CSO has a foot-ball (soccer-bail) structure with icosahedral Ih symmetry (Fig.7).

FULLERENE > ‘60

I-

Fig.7 - Truncated icosahedral structure of footballene C60.

. :.

m Z

W l-

Z

LASER ANNEALED

I

> Iv) Z W lz 400

800

1200 WAVENUMBER

1600

2000

( CM-‘]

1800

Fig.8 - Raman spectra of C60 and C70 (recorded on a Jobin-Yvon spectrometer equipped with a CCD detector).

1600 1400 WAVENUMBER (CM-‘)

1200

Fig.9 - Raman spectra (detail) of footballene C60 before and afier laser annealing.

85

For concrete image, the term “footballene” is prefered [8]. The troncated icosahedral structure of C6o is limited by 32 faces : 12 regular pentagons and 20 hexagons. The Cbond length of the pentagons is 1.47 E while that common to hexagons is 1.41 A. Its cavity diameter is about 7 A [9,10]. The irreducible representations for an isolated C60 footballene give 46 normal vibrations :

weak. Among them, the 268 cm-1 band can be used for identification purposes. Of course, C70, with a much lower symmetry, gives rise to a great number of Raman bands (Fig.8b). For analytical purposes, bands at 1568, 1126,118O and 260 can be used. We must mention that the footballene structure is very fragile and sensitive to the laser impact. With high laser illumination, the structure can be reduced into amorphous carbon [15] and it is naturally easy to follow the transformation by Raman spectroscopy (Fig.9).

2Ag + 3T1, + 4T3, + 6G, + 8H, + A, + 4T1, + 5T3, + 6G, + 7HU

I’ =

among which 10 are Raman active (2A,, 8H,) and 4 infrared active

3.5 - Diamond films

(4’hJ.

With C6o recrystallized in carbon disulfide, the Raman spectrum gives two strong lines at 1466 and 495 cm-l (Fig.8a). These two bands could correspond to the expected A, modes, the first one may represent the tangential C = C stretching [g-14]. It is interesting to notice that this frequency, 1466 cm-l, far below the frequency range characteristic of C = C double bonds stretching, around 1600 cm-l, could reflect a partial

For “diamond films” prepared by various methods, and based on their Raman spectra, three types of films can be distinguished. One of them presents only the Raman feature characteristic of amorphous carbon or aphitized carbon : this category of Fllms does not interest us. 3.5.1

The other Raman lines are very

1700

1600

1500

1900

WAVENUMBERIChi’

1300

I

”

I

I

1600

diamond films

Many “diamond films” exhibit a sharp Raman line around 1333 cm-1 and nothing else. This frequency is characteristic of cubic diamond.

double bond character of the C-C bonds in footballene C60.

7-t

-Real

1500

1100

WAVENUMBERICM-

Fig.10 - Raman spectra of diamond-like

1300

1600

1500

1400

WAVENUMBERICM-’

fdms prepared by CVD.

1300

86

DIAMOND

1700

1600

1500

1400

1300

1700

1600

WAVENUMBERId

Fig.1 1 - Ramun spectra of diamond-like

Nevertheless, depending on the preparation method, the Raman band can vary in frequency and band-width, attesting their slight distorsion from the ideal cubic structure. With the studied samples, diamond films containing only the diamond Raman peak are obtained generally with CVD from low methane precursor concentrations and the less distorted diamond is generated with the lowest pressure.

1500

LIKE

1400

WAVENUMBER

FILM

1300

/CM- ’

films with the presence ofgraphitited

carbon.

We must in consequence, admit that this frequency range is characteristic of a new carbon structure different

DIAMITE DIAMOND

LIKE

3.5.2 -Diamond-like films Many other samples, prepared by CVD as well as by magnetron sputtering or other methods, give complex Raman features. Some of them give a peak at 1333 cm-1 overlapped with a broad band around 1540 cm-1 (Fig.10). Of course, the 1333 cm-1 is due to diamond. Some other samples give a peak at 1540 cm-l overlapped with the double band of graphitizatized carbon (Fig.11). Finally, a series of samples give a single band around 1530 + 20 cm-1 (Fig.12). These three series of Raman profiles give rise the individuality of the Raman band at 1540 + 20 cm-l.

I

1800

I

1

I

1600

1400

WAVENUMBER

( CM-11

1

Fig.12 - Raman spectra of diamite films.

12t

87

from diamond and graphiteil-3,163. Remember that in graphite, the only EQ mode active in this frequency range appears at 1580+ 3 cm-l. In the graphite lattice, this mode can be assimilated to the C-C stretching vibration in the hexagonal layers. In fact, this band is the Raman component of a Davydov splitting of a C-C vibration, the other component being only infrared active [171. For disordered graphite a second band appears at much lower frequency around 1350 cm-l, but the first one is always located between 1580 and 1610 cm-l. The fact that the diamondlike frequency is systematically lower than the graphite C-C stretching frequency can be correlated with a lengthening of the C-C bond, in thenetwork. A distorsion from the hexagonal graphite structure leading to bonding between layer could correspond to diamond like structure. The name of “diamite” has been recently suggested [l-3] for this “pseudo hexagonal and helico’idal” structure. 4 - CONCLUSION Raman spectroscopy is very helpful in the characterization of all diamond and diamond-like structures, as well as of all other carbon lattices. We should like to point out the following possibilities : a) Easy identification of graphites of various graphitization degrees by measuring the relative intensity of the two bands at 1580 and 1350 cm-l. b) Determination of the quality of diamond or distorsion from the cubic structure by measuring the frequency shift (from 1331 cm-l) and the broadening of the Raman band. c) Evaluation of the double bond character and correlation spectrumstructure in new carbon networks such as footballene C6o and diamite.

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