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Diamond films studied by surface-enhanced Raman scattering
ELAMOND RELATED MATERIALS Diamond and Related Materials 5 (1996) 608-612
Diamond films studied by surface-enhanced Raman scattering T. Lbpez-Rios Laboratoire d’Etude des Prop-i&% Electroniques des Solides, CNRS, 25 rue des Martyres, BP166, 38042 Grenoble Cedex, France
Abstract Diamond films made from CH4 and Hz by hot-filament-assisted chemical vapour deposition (CVD) were studied in situ by surface-enhanced Raman scattering (SERS) by depositing Ag on the diamond films. The potential of SERS to investigate in situ thin films is discussed. Evidence is given that very thin carbon deposits are easily observed by SERS. Raman spectra of carbon films deposited during a few seconds on Ag surfaces are presented. Keywords:
SERS; Raman; Carbon monolayers; Microcrystalline diamond
1. Introduction Carbon is a simple element with multiple allotropic forms (diamond, graphite, the recently discovered fullerenes and nanotubes, etc). The crystallography of these forms is governed by the stereochemistry of the permitted hybridization of the 2s and 2p orbitals. It is of considerable interest to understand the formation of the different types of carbon and the possible transformation of one type into another. It is of particular interest to understand the mechanisms involved in low pressure diamond formation. Many puzzling experimental observations, such as the preferential growth induced by macroscopic roughness or the existence of a precursor state before diamond growth, are still not fully understood. A realistic study of the growth process necessarily implies an analysis of the problem at the microscopic level. Consequently, it should be interesting to investigate the early stage of diamond formation at monolayer coverages using surface techniques. A difficulty usually encountered in these investigations is that most of the techniques that are sensitive to surfaces need an ultrahigh vacuum (UHV) environment, which does not match the environment needed for carbon deposition. Analytical techniques, such as X-ray diffraction, neutron scattering, Raman and IR spectroscopy, which are usually used to determine the structure of solids, are not sensitive to small amounts of material (monolayers), especially for light elements like carbon. These considerations indicate that it is not easy to determine the coordination of very thin carbon deposits. 0925-9635/96/$15.00 0 1996 Elsevier Science S.A. All rights reserved SSDZ 0925-9635(95)00457-2
In a special case, Raman scattering is very sensitive to surface phenomena and the spectra of monolayers can be easily obtained. This special case is surfaceenhanced Raman scattering (SERS). This effect occurs at rough surfaces of free electron metals for a broad range of laser excitation frequencies [l-4]. The main characteristic of this effect is a very large amplification ( 104-106) of the Raman scattering of molecules adsorbed on these surfaces. Even if the origin of this effect is still a matter of discussion, there is general agreement that surface plasmons specific to rough surfaces are, at least to some extent, responsible for the Raman amplification [l-4]. From a practical point of view, the rules governing SERS are well known and, when pertinently employed, this effect can be a very sensitive analytical technique. SERS has a very appreciable advantage: it combines the large spectral resolution of Raman spectroscopy with an extremely high surface sensitivity. Unfortunately, SERS suffers from a very drastic inconvenience: the studies necessarily involve surfaces of uncontrolled structure at the atomic scale, usually presenting a selective enhancement of the adsorbed species. SERS has been extensively employed to determine the phonons of adsorbates [l-4] and very thin Si films [ 51, but has never, to my knowledge, been used in situ to investigate chemical vapour deposition (CVD). In this paper, the possible application of SERS for investigating in situ diamond films is analysed and several examples are given to illustrate the potential of the technique. This preliminary study, motivated by a wish to know whether CVD can be investigated by SERS, must be
T. Upez-RiosJDiamond
and Related Materials 5 (1996) 608-612
taken as a test of the method rather than a thorough investigation of diamond films.
2. Experimental details Diamond and carbon were deposited by hot-filamentassisted CVD in a small UHV compatible chamber evacuated to lo-‘Torr with a turbomolecular pump prior to CH4 and Hz exposure. Pumping was stopped before the successive introduction of the gases. The relative concentration in the volume (c) was determined from the ratio of the partial pressures P of each gas (c = P,,,/P,,). The total pressure of the mixture was always about 20 Torr. The chamber was equipped with a sample holder that could be resistively heated to 900 “C, a rhenium filament located about 10 mm from the sample holder and a silver source used to make the deposits needed for SERS experiments. Silver was evaporated at grazing incidence which, for thick deposits, is known to produce SERS [6,7]. In the present experiments, the Ag deposits were thin and discontinuous and the grazing geometry itself probably had little influence on SERS. The temperature of the filament was measured during the experiment with an e:xtinction filament pyrometer. The temperature of the isample was measured with a thermocouple placed on the sample holder which was calibrated prior to each experiment with a pyrometer. The temperature of the filament was about 1800 “C and that of the substrate was about 700 “C. Two windows were used for Raman experiments: one to focus a Kr+ laser beam on the sample at an incidence of about 75” and another located parallel to the sample to collect the scattered light normal to the sample. The impinging laser was polarized with an electric field perpendicular to the surface and the scattered light was analysed, without distinction of the polarization state, by a Jobin Yvon UlOOO spectrometer equipped with a photomultiplier working in the counting mode. The experiments shown here were made with the 530.9 nm line and a power of 50 mW on the sample except when indicated otherwise.
609
ing a thin Ag film in vacuum. They prepared a diamond film on an Si substrate by the hot filament method and afterwards deposited Ag on part of the sample in order to produce SERS. A clear signal of diamond superimposed on broader bands, probably due to pollution of the sample, was observed for the region covered by the Ag deposit. SERS has also been used successfully to investigate different types of carbon materials [ 121, carbon at electrochemical interfaces [ 13,141 and carbon-silicon compounds [ 151. 3.1. Diamondjlms The diamond films used in the present experiments were deposited on Si( 111) substrates polished with 1 pm diamond powder. Before diamond deposition, the substrates were heated to 850 “C for 30 min in a vacuum of about lo-‘Torr in order to eliminate the native oxide of Si. Fig. 1 shows the Raman spectrum of a diamond film deposited during 60 min with a methane concentration c=2%. Spectrum (a) was obtained after evacuating the CH4/H2 mixture and spectrum (b) after the deposition of a Ag film on the carbon film. No structures are observed before Ag deposition. The second-order Raman scattering of the Si substrate at about 1000 cm-’ is not observed in the presence of the diamond film using nonoptimized optics to collect the scattered light. The Ag deposits are no thicker than a few nanometres. Electron microscopy observations show that the structure of the silver deposits is an assembly of spherical particles with diameters of several hundred nanometres. Spectrum (b) exhibits an inelastic background. This background, characteristic of SERS, is probably due to Raman scattering by electron-hole pairs [16,17]. The intensity of the background was systematically used to 120
3. Use of thin silver films to reveal carbon deposits The pioneering work of Creighton et al. [S] for revealing the Raman scattering of molecules with Ag colloids was the starting point of many applications of SERS in chemistry and biology [ 91. Metallic deposits, rather than colloids in solution, were also successfully employed to obtain Raman spectra of small amounts of materials [l-4]. In very interesting work, Knight et al. [ 10,111 obtained the Raman spectrum of diamond, unobserved in standard Raman experiments, by deposit-
Roman
Shift
( cm-l
)
Fig. 1. Spectra of a diamond film grown during 60 min (c=2%) before (a) and after (b) Ag deposition.
610
T. Ldpez-RioslDiamondand
Related Materials 5 (1996) 608-612
control the deposition of Ag. In practice, Ag deposition is interrupted when a maximum in this background is observed. Fig. 1 clearly shows that thin Ag deposits largely enhance the Raman scattering of diamond films. The first-order Raman scattering mode at 1332 cm-‘, which is characteristic of diamond [lS], is only distinguished after Ag deposition. The eventual contribution of seed diamond crystals must be very small; SERS of diamond drastically depends on the deposition time and other experimental conditions. The asymmetry of the peak at 1332 cm-’ and the structure at 1242 cm-’ are due to the excitation of phonons with non-vanishing wavevectors as recently discussed in detail in Ref. [ 191. An asymmetric shape of the Raman line is usually observed for disordered systems for which the momentum selection rules are no longer valid. The general shape of spectrum (b) is similar to microcrystalline Si [20] and diamond [21,22], but the asymmetric broadening is much more pronounced in our samples. For microcrystalline samples, the shift and asymmetric broadening of the Raman line are usually attributed to the confinement of phonons in the microcrystals. Work in progress indicates that the shape of spectrum (b) is due to phonon confinement in diamond microcrystals. 3.2. Diamond-like films To determine the potential application of SERS, we prepared deposits at lower temperatures heating the substrates by the hot filament only. Under such conditions, the substrate temperature is estimated to be 500 “C. A carbon film was deposited (c=2.3%) during 170 min. Before Ag deposition, no Raman signal was observed (Fig. 2(a)). After deposition of Ag, the spectrum in Fig. 2(b) was obtained (note that the intensity in spectrum (a) is multiplied by a factor of ten). After heating the sample to about 500 “C for 5 min, the intensity of the spectrum increased (Fig. 2(c)). This is probably due to a modification of the Ag islands leading to a higher electromagnetic enhancement. These spectra are characteristic of diamond-like carbon. The relative intensity of the band centred at 1300 cm-’ with respect to that at 1530cm-’ is related to the ratio of sp3 and sp’ carbon bonds [23-251. This ratio depends on the excitation frequency due to resonant Raman scattering with rc-rc* electronic transitions. The very high intensity of the Raman spectra should be emphasized. The ordinary Raman scattering of amorphous materials, spread over a large spectral range, never exhibits peaks of high intensity. Fig. 3 shows two spectra for two different lines of the laser, which indicate that the observed signal is only due to Raman scattering.
4. Carbon on Ag island films A brief discussion is now given of the observation of very thin carbon films deposited on discontinuous silver
6000
9
6000
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e
Y f 2
4000
x .?L E? al ‘.-
2000
i
OL
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1000
1200
Roman
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Shift
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( cm-l
)
Fig. 2. Carbon deposited during 170 min (c=2.3%) at a temperature of about 500 “C before Ag deposition (a), with an Ag deposit (b) and after being heated to 500 “C for 5 min (c). 10000
so00
0
400
800
Raman
1200
Shift
1600
( cm-l
2000
)
Fig. 3. Same as Fig. 2(c) with excitation wavelengths of 568.3 nm (a) and 530.9 nm (b).
films. In this configuration, it is convenient to deposit Ag on a transparent substrate in order to avoid damping of the plasma resonance in the particles. The carbon films to be analysed are deposited on silver films,
T. Lbpez-RiosjDiamond and Related Materials 5 ( 1996) 608-612
previously prepared on a diamond film which acts, for the Raman experiments, as a substrate. Diamond is a good substrate for SERS since it is transparent. In Fig. 4, a diamond film grown during 110 min (c = 12%) on a scratched Si( 1.11) surface was taken as the substrate. After diamond growth, the gases were evacuated to a pressure of 1O--7Torr and a silver film was deposited. The sample was then exposed once again to CH4 and Hz (c =4%) and spectrum (a) was recorded. The filament was heated to 1400 “C for 30 s and another spectrum was obtained (Fig 4(b)). This operation was prolonged for a further 3 min (Fig. 4(c)). In this case, the Raman, and probably inelastic background, noticeably increase after carbon deposition. This may be due to a change in the Ag structure when heating the sample. It should be noted that the intensity of the peak of the diamond substrate (1332 cm-‘) is almost constant, but the signal from the carbon deposit in contact with the Ag particles is significantly enhanced. It should also be noted that a larger signal is observed with thinner deposits as often happens with SERS [ 51. Spectra (b) and (c), with 1.~0 bands at about 1500 cm-’ and 700 cm-‘, are different from those of Fig. 3, but similar to those of carbon films deposited on substrates at 77 K using laser ablation [26]. The structure at low frequency could be due to diamond-like carbon with four-, three- and two-fold bonding, with a predominant quantity of four-fold bonds as computed by Wang and Ho [27]. The unusual band at about 700 cm-’ may be due to delocalized modes of this type of carbon [27]. A systematic investigation is necessary to unravel the phenomena presented here. An experiment is in progress to determine the best conditions for studying carbon deposits on Ag films, as well as the influence of impurities, which are known to substantially reduce SERS [28].
611
The preliminary results presented here lead to the conclusion that SERS is well adapted to study extremely thin carbon films.
5. Conclusions Experiments carried out in situ using a CVD reactor with a hot filament demonstrate that SERS can be used successfully in such an environment and new information can be obtained from this method. Provided that a careful study is made to determine the optimum conditions for the use of SERS, this method may be an interesting means of investigating chemical bonding at surfaces or in very thin carbon deposits. Experiments demonstrate the sensitivity of SERS and suggest that Raman scattering by thin CVD films on rough Ag surfaces can be easily observed by SERS.
Acknowledgements I thank Mr. J. Leroy for technical assistance. It is also a pleasure to thank Dr. E. Bustarret, Dr. E. SandrC and Dr. E. Sauvain for useful discussions.
References Cl1 R.K. Chang and T.E. Furtak (eds.), Surface Enhanced Raman Scattering, Plenum, New York, 1982. c21 M. Moskovits, Rev. Mod. Phys., 57 (1985) 783. c31 T. LClpez-Rios, in M. Cardona and G.R. Castro (eds.), Lectures on Surface Science, Springer, Berlin, 1987. c41 A. Otto, I. Mrozek, H. Grabhorn and W. Akemann, J. Phys: Condensed Matter, 4 (1992) 1143.
5000
I
I
1
4000 0 Y c zl
b
3000
4 0 ;I x
2000
2 2 c
1000
a
Raman
Shift
(cm-l)
Fig. 4. Diamond film (110 min, c = 12%) with an Ag deposit (a), and after exposure to CH,/H2 (c=:4%) @lament at 1400 “C) for 30 s (b) and 110 s (c).
c51 Y. Gao and T. L6pez-Rios, Solid State Commun., 60 (1986) 55. C61J.L. Martinez, Y. Gao and T. Lbpez-Rios, Phys. Rev. B, 33 (1986) 5917. c71 J.L. Martinez, Y. Gao, T. Lbpez-Rios and A. Wirgin, Phys. Rev. B, 35 (1987) 9481. PI J.A. Creighton, C.G. Blatchford and M.G. Albrecht, J. Chem. Sot., Faraday Trans. II, 75 (1979) 790. c91 W. Kiefer, M. Cardona, G. Schaack, F.W. Scheider and H.W. Schriitter (eds.), Proc. 13th Znt. Conf. on Raman Spectroscopy, Wiley, 1992. Cl01 D.S. Knight, R. Weimer, L. Pilione and W.B. White, Appl. Phys. Lett, 56 (1990) 1320. Cl11 D.S. Knight, L. Pilione and W.B. White, MRS Int. Conf. Proc. New Diamond Science and Technology, 753 (1991). Cl21 M.A. Tadayyoni and N.R. Dando, Appt. Spectrosc., 45 (1991) 1613. v31 J.C. Rubim, G. Kannen, D. Schumacher, J. Diinnwald and A. Otto, Appl. Surf. Sci., 37 (1989) 233. Cl41 Y.W. Alsmeyer and R.L. McCreery, Langmuir, 7 (1991) 2370. [I51 M.B. Tzolov, N.V. Tzenov, D.I. Dimiva-Malinovska and D.Y. Yankov, Appl. Phys. Lett., 62 (1993) 2396. Cl61 E. Burstein, S. Lundqvist and D.L. Mills, in R.K. Chang and T.E. Furtak (eds.), Surface Enhanced Raman Scattering, Plenum, New York, 1982, p. 67.
T. Lopez-RioslDiamond and Related Materials 5 (1996) 608-612
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[ 171 R. Monreal, F. Flores, Y. Gao and T. Lopez-Rios, Europhys. Lett., 4 (1987) 115. [18] S.A. Solin and A.K. Ramdas, Phys. Rev. B, 1 (1970) 1687. [19] T. Lopez-Rios and J.M. Gomez-Rodriguez, C.R. Acad. Sci. Paris, 321 (Strie IIb) (1995) 47. [20] P.M. Fauchet and LH. Campbell, CRC Crit. Rev. Solid State
Mater. Sci., 14 (1988) S79. [21] S.A. Solin and R.J. Kobliska, in J. Stuke and W. Brening (eds.), Proc.
5th Int. Conf. Amorphous and Liquid Semiconductors,
Taylor and Francis, 1974, p. 1251. [22] Y.M. LeGrice, R.J. Nemanich, J.T. Glass, Y.H. Lee, R.A. Rudder and R.J. Markunas, in J.T. Glass, R. Messier and N. Fujimori
(eds.), Diamond, Boron Nitride, Silicon Carbide and Related Wide Bandgap Semiconductors, MRS Symp. Proc., 162 (1990) 219. [23]
M. Yoshikawa, G. Katagiri, H. Ishida, A. Ishitani and T. Akamatsu, J. AppJ. Phys., 64 (1988) 6464. [24] J. Wagner, M. Ramsteiner, Ch. Wild and P. Koidl, Phys. Rev. B, 40 (1989) 1817.
[25]
J. Wagner, C. Wild and P. Koidl, Appl. Phys. L&t., 59 (1991) 779. [26] W.S. Bacsa, J.S. Lannin, D.L. Pappas and J.J. Cuomo, Phys. Rev. B, 47 (1993) 10{16931. [27] C.Z. Wang and K.M. Ho, Phys. Rev. Lett., 71 (1993) 1184. [28] Y. Gao and T. Lopez-Rios, Phys. Rev. Lett., 53 (1984) 2583.
Diamond films studied by surface-enhanced Raman scattering T. Lbpez-Rios Laboratoire d’Etude des Prop-i&% Electroniques des Solides, CNRS, 25 rue des Martyres, BP166, 38042 Grenoble Cedex, France
Abstract Diamond films made from CH4 and Hz by hot-filament-assisted chemical vapour deposition (CVD) were studied in situ by surface-enhanced Raman scattering (SERS) by depositing Ag on the diamond films. The potential of SERS to investigate in situ thin films is discussed. Evidence is given that very thin carbon deposits are easily observed by SERS. Raman spectra of carbon films deposited during a few seconds on Ag surfaces are presented. Keywords:
SERS; Raman; Carbon monolayers; Microcrystalline diamond
1. Introduction Carbon is a simple element with multiple allotropic forms (diamond, graphite, the recently discovered fullerenes and nanotubes, etc). The crystallography of these forms is governed by the stereochemistry of the permitted hybridization of the 2s and 2p orbitals. It is of considerable interest to understand the formation of the different types of carbon and the possible transformation of one type into another. It is of particular interest to understand the mechanisms involved in low pressure diamond formation. Many puzzling experimental observations, such as the preferential growth induced by macroscopic roughness or the existence of a precursor state before diamond growth, are still not fully understood. A realistic study of the growth process necessarily implies an analysis of the problem at the microscopic level. Consequently, it should be interesting to investigate the early stage of diamond formation at monolayer coverages using surface techniques. A difficulty usually encountered in these investigations is that most of the techniques that are sensitive to surfaces need an ultrahigh vacuum (UHV) environment, which does not match the environment needed for carbon deposition. Analytical techniques, such as X-ray diffraction, neutron scattering, Raman and IR spectroscopy, which are usually used to determine the structure of solids, are not sensitive to small amounts of material (monolayers), especially for light elements like carbon. These considerations indicate that it is not easy to determine the coordination of very thin carbon deposits. 0925-9635/96/$15.00 0 1996 Elsevier Science S.A. All rights reserved SSDZ 0925-9635(95)00457-2
In a special case, Raman scattering is very sensitive to surface phenomena and the spectra of monolayers can be easily obtained. This special case is surfaceenhanced Raman scattering (SERS). This effect occurs at rough surfaces of free electron metals for a broad range of laser excitation frequencies [l-4]. The main characteristic of this effect is a very large amplification ( 104-106) of the Raman scattering of molecules adsorbed on these surfaces. Even if the origin of this effect is still a matter of discussion, there is general agreement that surface plasmons specific to rough surfaces are, at least to some extent, responsible for the Raman amplification [l-4]. From a practical point of view, the rules governing SERS are well known and, when pertinently employed, this effect can be a very sensitive analytical technique. SERS has a very appreciable advantage: it combines the large spectral resolution of Raman spectroscopy with an extremely high surface sensitivity. Unfortunately, SERS suffers from a very drastic inconvenience: the studies necessarily involve surfaces of uncontrolled structure at the atomic scale, usually presenting a selective enhancement of the adsorbed species. SERS has been extensively employed to determine the phonons of adsorbates [l-4] and very thin Si films [ 51, but has never, to my knowledge, been used in situ to investigate chemical vapour deposition (CVD). In this paper, the possible application of SERS for investigating in situ diamond films is analysed and several examples are given to illustrate the potential of the technique. This preliminary study, motivated by a wish to know whether CVD can be investigated by SERS, must be
T. Upez-RiosJDiamond
and Related Materials 5 (1996) 608-612
taken as a test of the method rather than a thorough investigation of diamond films.
2. Experimental details Diamond and carbon were deposited by hot-filamentassisted CVD in a small UHV compatible chamber evacuated to lo-‘Torr with a turbomolecular pump prior to CH4 and Hz exposure. Pumping was stopped before the successive introduction of the gases. The relative concentration in the volume (c) was determined from the ratio of the partial pressures P of each gas (c = P,,,/P,,). The total pressure of the mixture was always about 20 Torr. The chamber was equipped with a sample holder that could be resistively heated to 900 “C, a rhenium filament located about 10 mm from the sample holder and a silver source used to make the deposits needed for SERS experiments. Silver was evaporated at grazing incidence which, for thick deposits, is known to produce SERS [6,7]. In the present experiments, the Ag deposits were thin and discontinuous and the grazing geometry itself probably had little influence on SERS. The temperature of the filament was measured during the experiment with an e:xtinction filament pyrometer. The temperature of the isample was measured with a thermocouple placed on the sample holder which was calibrated prior to each experiment with a pyrometer. The temperature of the filament was about 1800 “C and that of the substrate was about 700 “C. Two windows were used for Raman experiments: one to focus a Kr+ laser beam on the sample at an incidence of about 75” and another located parallel to the sample to collect the scattered light normal to the sample. The impinging laser was polarized with an electric field perpendicular to the surface and the scattered light was analysed, without distinction of the polarization state, by a Jobin Yvon UlOOO spectrometer equipped with a photomultiplier working in the counting mode. The experiments shown here were made with the 530.9 nm line and a power of 50 mW on the sample except when indicated otherwise.
609
ing a thin Ag film in vacuum. They prepared a diamond film on an Si substrate by the hot filament method and afterwards deposited Ag on part of the sample in order to produce SERS. A clear signal of diamond superimposed on broader bands, probably due to pollution of the sample, was observed for the region covered by the Ag deposit. SERS has also been used successfully to investigate different types of carbon materials [ 121, carbon at electrochemical interfaces [ 13,141 and carbon-silicon compounds [ 151. 3.1. Diamondjlms The diamond films used in the present experiments were deposited on Si( 111) substrates polished with 1 pm diamond powder. Before diamond deposition, the substrates were heated to 850 “C for 30 min in a vacuum of about lo-‘Torr in order to eliminate the native oxide of Si. Fig. 1 shows the Raman spectrum of a diamond film deposited during 60 min with a methane concentration c=2%. Spectrum (a) was obtained after evacuating the CH4/H2 mixture and spectrum (b) after the deposition of a Ag film on the carbon film. No structures are observed before Ag deposition. The second-order Raman scattering of the Si substrate at about 1000 cm-’ is not observed in the presence of the diamond film using nonoptimized optics to collect the scattered light. The Ag deposits are no thicker than a few nanometres. Electron microscopy observations show that the structure of the silver deposits is an assembly of spherical particles with diameters of several hundred nanometres. Spectrum (b) exhibits an inelastic background. This background, characteristic of SERS, is probably due to Raman scattering by electron-hole pairs [16,17]. The intensity of the background was systematically used to 120
3. Use of thin silver films to reveal carbon deposits The pioneering work of Creighton et al. [S] for revealing the Raman scattering of molecules with Ag colloids was the starting point of many applications of SERS in chemistry and biology [ 91. Metallic deposits, rather than colloids in solution, were also successfully employed to obtain Raman spectra of small amounts of materials [l-4]. In very interesting work, Knight et al. [ 10,111 obtained the Raman spectrum of diamond, unobserved in standard Raman experiments, by deposit-
Roman
Shift
( cm-l
)
Fig. 1. Spectra of a diamond film grown during 60 min (c=2%) before (a) and after (b) Ag deposition.
610
T. Ldpez-RioslDiamondand
Related Materials 5 (1996) 608-612
control the deposition of Ag. In practice, Ag deposition is interrupted when a maximum in this background is observed. Fig. 1 clearly shows that thin Ag deposits largely enhance the Raman scattering of diamond films. The first-order Raman scattering mode at 1332 cm-‘, which is characteristic of diamond [lS], is only distinguished after Ag deposition. The eventual contribution of seed diamond crystals must be very small; SERS of diamond drastically depends on the deposition time and other experimental conditions. The asymmetry of the peak at 1332 cm-’ and the structure at 1242 cm-’ are due to the excitation of phonons with non-vanishing wavevectors as recently discussed in detail in Ref. [ 191. An asymmetric shape of the Raman line is usually observed for disordered systems for which the momentum selection rules are no longer valid. The general shape of spectrum (b) is similar to microcrystalline Si [20] and diamond [21,22], but the asymmetric broadening is much more pronounced in our samples. For microcrystalline samples, the shift and asymmetric broadening of the Raman line are usually attributed to the confinement of phonons in the microcrystals. Work in progress indicates that the shape of spectrum (b) is due to phonon confinement in diamond microcrystals. 3.2. Diamond-like films To determine the potential application of SERS, we prepared deposits at lower temperatures heating the substrates by the hot filament only. Under such conditions, the substrate temperature is estimated to be 500 “C. A carbon film was deposited (c=2.3%) during 170 min. Before Ag deposition, no Raman signal was observed (Fig. 2(a)). After deposition of Ag, the spectrum in Fig. 2(b) was obtained (note that the intensity in spectrum (a) is multiplied by a factor of ten). After heating the sample to about 500 “C for 5 min, the intensity of the spectrum increased (Fig. 2(c)). This is probably due to a modification of the Ag islands leading to a higher electromagnetic enhancement. These spectra are characteristic of diamond-like carbon. The relative intensity of the band centred at 1300 cm-’ with respect to that at 1530cm-’ is related to the ratio of sp3 and sp’ carbon bonds [23-251. This ratio depends on the excitation frequency due to resonant Raman scattering with rc-rc* electronic transitions. The very high intensity of the Raman spectra should be emphasized. The ordinary Raman scattering of amorphous materials, spread over a large spectral range, never exhibits peaks of high intensity. Fig. 3 shows two spectra for two different lines of the laser, which indicate that the observed signal is only due to Raman scattering.
4. Carbon on Ag island films A brief discussion is now given of the observation of very thin carbon films deposited on discontinuous silver
6000
9
6000
5 2
e
Y f 2
4000
x .?L E? al ‘.-
2000
i
OL
800
1000
1200
Roman
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Shift
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( cm-l
)
Fig. 2. Carbon deposited during 170 min (c=2.3%) at a temperature of about 500 “C before Ag deposition (a), with an Ag deposit (b) and after being heated to 500 “C for 5 min (c). 10000
so00
0
400
800
Raman
1200
Shift
1600
( cm-l
2000
)
Fig. 3. Same as Fig. 2(c) with excitation wavelengths of 568.3 nm (a) and 530.9 nm (b).
films. In this configuration, it is convenient to deposit Ag on a transparent substrate in order to avoid damping of the plasma resonance in the particles. The carbon films to be analysed are deposited on silver films,
T. Lbpez-RiosjDiamond and Related Materials 5 ( 1996) 608-612
previously prepared on a diamond film which acts, for the Raman experiments, as a substrate. Diamond is a good substrate for SERS since it is transparent. In Fig. 4, a diamond film grown during 110 min (c = 12%) on a scratched Si( 1.11) surface was taken as the substrate. After diamond growth, the gases were evacuated to a pressure of 1O--7Torr and a silver film was deposited. The sample was then exposed once again to CH4 and Hz (c =4%) and spectrum (a) was recorded. The filament was heated to 1400 “C for 30 s and another spectrum was obtained (Fig 4(b)). This operation was prolonged for a further 3 min (Fig. 4(c)). In this case, the Raman, and probably inelastic background, noticeably increase after carbon deposition. This may be due to a change in the Ag structure when heating the sample. It should be noted that the intensity of the peak of the diamond substrate (1332 cm-‘) is almost constant, but the signal from the carbon deposit in contact with the Ag particles is significantly enhanced. It should also be noted that a larger signal is observed with thinner deposits as often happens with SERS [ 51. Spectra (b) and (c), with 1.~0 bands at about 1500 cm-’ and 700 cm-‘, are different from those of Fig. 3, but similar to those of carbon films deposited on substrates at 77 K using laser ablation [26]. The structure at low frequency could be due to diamond-like carbon with four-, three- and two-fold bonding, with a predominant quantity of four-fold bonds as computed by Wang and Ho [27]. The unusual band at about 700 cm-’ may be due to delocalized modes of this type of carbon [27]. A systematic investigation is necessary to unravel the phenomena presented here. An experiment is in progress to determine the best conditions for studying carbon deposits on Ag films, as well as the influence of impurities, which are known to substantially reduce SERS [28].
611
The preliminary results presented here lead to the conclusion that SERS is well adapted to study extremely thin carbon films.
5. Conclusions Experiments carried out in situ using a CVD reactor with a hot filament demonstrate that SERS can be used successfully in such an environment and new information can be obtained from this method. Provided that a careful study is made to determine the optimum conditions for the use of SERS, this method may be an interesting means of investigating chemical bonding at surfaces or in very thin carbon deposits. Experiments demonstrate the sensitivity of SERS and suggest that Raman scattering by thin CVD films on rough Ag surfaces can be easily observed by SERS.
Acknowledgements I thank Mr. J. Leroy for technical assistance. It is also a pleasure to thank Dr. E. Bustarret, Dr. E. SandrC and Dr. E. Sauvain for useful discussions.
References Cl1 R.K. Chang and T.E. Furtak (eds.), Surface Enhanced Raman Scattering, Plenum, New York, 1982. c21 M. Moskovits, Rev. Mod. Phys., 57 (1985) 783. c31 T. LClpez-Rios, in M. Cardona and G.R. Castro (eds.), Lectures on Surface Science, Springer, Berlin, 1987. c41 A. Otto, I. Mrozek, H. Grabhorn and W. Akemann, J. Phys: Condensed Matter, 4 (1992) 1143.
5000
I
I
1
4000 0 Y c zl
b
3000
4 0 ;I x
2000
2 2 c
1000
a
Raman
Shift
(cm-l)
Fig. 4. Diamond film (110 min, c = 12%) with an Ag deposit (a), and after exposure to CH,/H2 (c=:4%) @lament at 1400 “C) for 30 s (b) and 110 s (c).
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