- Email: [email protected]
Raman spectroscopy of ion irradiated amorphous carbons
.-__ Iii! TYT.U
w
Nuclear Instruments
and Methods in Physics Research B 116 (1996) 19% 199
NCIMB
Beam Interactions with Materials 6 Atoms
ELSEVIER
Raman spectroscopy of ion irradiated amorphous carbons G.A. Baratta ay*, M.M. Arena b, G. Strazzulla ‘, L. Colangeli E. Bussoletti e
d, V. Mennella d,
a Osservatorio Astrojisico di Catania, Citt& Uniuersitaria, I-95125 Catania, Italy b Istituto Nazionale per la Microelettronica, CNR Catania, Catania, Italy ’ Istituto di Astronomia, Catania, Italy a Osseruarorio Astronomico di Capodimonte, Napoli, Italy e Istituto Vniuersitario NavaLe, Napoli, Italy
Abstract
We have’studied, by “in situ” Raman spectroscopy, the modifications induced by 3 keV He+ ions on thin amorphous carbon grain deposits. Previous results obtained with our experimental apparatus show that in the case of carbon-containing frozen targets (such as benzene and butane) for doses greater than about 100 eV/mol, ion irradiation induces the formation of an hydrogenated amorphous carbon. In this paper, the bombarded material is already an amorphous carbon with a relatively high order degree. In this case ion irradiation progressively decreases the order degree in the amorphous carbon. This result is in agreement with analogous ion irradiation experiments carried out on highly ordered pyrolitic graphite crystals. These studies are important to understand physical characteristics and evolution of refractory carbon grains in astrophysical environments.
1. Introduction The interaction between a fast colliding ion and solid targets produces several effects, many of which have been studied in recent years in view of their astrophysical applications [l-6]. Raman spectroscopy can provide valuable information on the effects induced by ions impinging on solids. Indeed this technique is used to distinguish among chemical species and can provide valuable evidence of the structural properties of materials and, in particular, of carbonaceous materials. Raman spectroscopy has been used to gain insight into the structural lattice damage of graphite resulting from ion bombardment [7-91. Among the studied effects the formation of an organic residue evolving at higher doses towards an ion-produced HAC (Hydrogenated Amorphous Carbon) is particularly interesting. The HAC formation has been observed in several kinds of carbon-containing targets (polystyrene, polypropylene, graphite, diamond etc.) and even on frozen gases (C,H,, CH,, C,Hlo etc.). This occurs for a combination of bombarding ions (H, He, Ar, Kr etc.) and ion energies (ranging between a few keV and MeV). Both fast ions (low energy cosmic rays, galactic protons, solar pro-
* Corresponding author. Tel. +39 95 7332213, fax +39 95 330592, e-mail [email protected]. 0168-583X/96/$15.00 Copyright PII SOl68-583X(96)00124-3
tons, solar wind particles) and carbon-containing solid targets, i.e. frozen (interstellar grains in dense regions, comet mantles etc.) and refractory species (interstellar grains in the diffuse medium, interplanetary dust particles etc.), are supposed to be present in space. Thus materials produced in the laboratory may simulate to some extent those present in space. IDPs (Interplanetary Dust Particles) collected in the Earth atmosphere have sizes from 1 to 50 pm and provide a unique opportunity to study extraterrestrial material in the laboratory. In many IDPs, poorly graphitized carbonaceous compounds are arranged in a granular (fluffy) structure where single grains can be as small as 100 A [lo]; in some cases amorphous carbonaceous material surrounds tub micron mineral grains with a covering layer up to 300 A of thickness [ 1I]. A Rarnan study of a representative set of 20 IDPs [ 121 has shown that 6 different groups of spectra can be identified. AlI particles, except those belonging to the last group, show the Raman feature characteristic of amorphous carbon with different degree of order. Since IDPs are exposed to the fast solar proton (100 keV) and solar wind particles (1 keV/amu) bombardment before collection in the Earth atmosphere, it is interesting to compare the Raman spectra of IDPs with those of ion-irradiated fluffy aggregate of sub micron amorphous carbon grains. In this paper we present new experimental results obtained by “in situ” Raman spectroscopy carried out on
0 1996 Elsevier Science B.V. All rights reserved
GA. Baratta et al./Nucl. Ins@. andMeth. in Phys. Res. B 116 (1996) 195-199
1%
amorphous carbon grains irradiated with 3 lceV He+ ions. Some astrophysical implications are also discussed.
2. Characterisation
of the AC samples
The amorphous carbon (AC) studied in this work has been produced by arc discharge between two amorphous carbon electrodes in an inert Ar atmosphere. The dust deposit has been collected onto a LiF substrate. The AC samples have been fully characterised in [ 131.TEM studies carried out on AC samples ([13] and references therein) showed that they are ctnstituted of spheroidal grains with average radii of 50 A arranged in a fluffy structure. Infrared spectroscopy of the AC samples shows the aliphatic (sp’) C-H stretching structure in the 3.4 pm region. Both -CH, and -CH,- symmetric and asymmetric stretches are present. From Fig. 5 in Ref. [13] and by using average areal peak cross-sections for the -CH, and -CH,- groups [ 141it is possible to estimate the fraction of hydrogen atoms bound to sp3 hybridized carbon atoms. The resulting ratio with respect to the total amount of carbon atoms contained in the AC sample is H/C = 4%, and the ratio between the number of -CH, and -CH,groups is: N(-CH,)/N(-CH,-) = 2.3. No C-H aromatic stretching is present (within the noise) in the 3.3 pm region, this points out that most of the hydrogen atoms are bound to sp3 carbon atoms although some H atoms must be bound also to sp’ aromatic C atoms since in the 1 l-14 pm region the C-H aromatic out of plane bending with 1, 2 and 3 adjacent H are observed. From Fig. 5 in Ref. [ 131 it is also evident the presence of a weak C-O stretching feature due to oxygen contamination, the corresponding estimated O/C atomic ratio is less than = 0.5%.
3. Experimental
(vertical axis) and is azimutally rota&l by 45” with respect to the ion beam direction. Hence before, during and after irradiation, spectm can be obtained without tilting the sample. The Raman spectrometer is a SPEX 1877 triple monochromator. The used detector is an OMA III (EG & G Princeton Applied Research) intensified reticon.
4. Experimental results The sample has been irradiated with 3 keV helium ions at different doses. Raman spectra obtained “in situ” at different doses are shown in Fig. 1. It is evident that the shape of the Raman spectrum varies with the dose. In order to analyse the variation quantitatively we adopted a line shape fitting procedure. For both the G and D lines (see the next section) we used a line profile given by the relation [ 161:
(1) where dI/do is proportional to the detector signal at o, o0 is the undamped mode frequency, w is the frequency shift from the laser line (Raman shift), r is a damping constant and C is a constant of proportionality. Non-linear least square routines have been used to fit the experimental
apparatus
Optical microscopy performed on the AC samples showed that they are not uniform in the micron scale. Since the Raman laser spot size is about 20 pm, we used an apparatus to obtain “in situ” Raman spectra, to be sure to analyse always the same portion of the sample during ion bombardment. The experimental apparatus used to perform “in situ” Raman spectroscopy has been described in detail elsewhere [15]. It consists of a vacuum chamber (P = 10e7 mbar) where the targets are mounted on a cold finger (lo-300 K) and irradiated with 3 keV ions. The 514 nm argon laser beam is focalized through a glass window on the sample and the scattered light is collected by a collimating lens on the entrance slit of the monochromator. We used a 90” scattering geometry where the directions of the laser beam, ion beam and the collected Raman scattered light are mutualIy perpendicular. The sample holder is inclined by 30” with respect to the incident laser light
1000
1200
Raman
1400
shifts
1600
1800
[cm-‘]
Fig. 1. Raman spectra of amorphous carbon grains deposited on a LiF subs&ate irradiated at different doses with 3 keV of He+ ions. The contim~ous lines are theoretical fits to the data given by the sum of two line profiIe (dashed lines) representing the G and D lines (see the text).
GA. Baratta et al./Nucl.
Instr. and Meth. in Phys. Res. B 116 (1996) 195-199
1600 _
2 s g 2 2 3I?
1590
G
1500 1570 1560
o-4 A,,. 0
20
40
60
a0
400
600
Dose [eV/C-atom] Fig. 2. Some fitting parametersof the Raman spectra in Fig. 1 are reported versus the irradiation dose. From top to bottom: peak position of the G line, FWHM of D and G lines, ratio between the peak intensities of D and G lines. spectra by minimising the x2 as a function of six free parameters (C, w. and r for each of the lines). The resulting theoretical curves are reported together with the experimental spectra in Fig. 1. The estimated values for the peak frequency of the G line, the PWHM of both G and D lines and the ratios between their peak intensities are reported versus the dose in Fig. 2. Here the error bars correspond to an increase of x2 by 1 (one standard deviation errors). Our analysis evidences: (1) the increasing of both D and G line widths, (2) the downward shift of the G line peak frequency, (3) the decrease (except for the lowest irradiation dose) of the In/Z, intensity ratio with the dose. As it will be explained in the next section, these results are consistent with an increasing disorder induced by ion-irradiation.
5. Discussion Carbonaceous materials can widely vary in chemical composition and structure (diamond, graphite, glassy carbon, hydrogenated amorphous carbon etc.>. Depending on the order degree of graphitic (sp* hybridization) materials, one or two fiit order Raman bands are observed. Accord-
197
ing to the momentum conservation selection rule, only one first order Raman band at about 1582 cm-’ is observed in HOPG (Highly Ordered Pyrolitic Graphite) or natural graphite with large (> 1 ,um) micro crystals [17]. This band is known as the G (graphitic) line and is due to a doubly degenerate deformation vibration of the hexagonal ring corresponding to the E,, mode of graphite with D64, crystal symmetry [18]. However, spectra of microcrystalline graphite and disordered carbons show an additional 1360 cm-’ line. This D (disordered) line is attributed to phonons near the Brillouin zone boundary active in small crystallites or on the boundaries of larger crystallites. It was found [18] that the intensity ratio (fn/io) of the D and G lines varies as the inverse of the crystal planar domain size L, (In/& a l/L,). In amorphous carbons and hydrogenated amorphous carbons, both G and D bands are present. These bands are quite broader than those observed in disordered graphite: the broader the bands, the more disordered the amorphous carbon is. In amorphous carbon obtained by sputtering deposition the two bands cannot be distinguished, the G line is usually shifted at lower Raman shifts ( 1560 cm-,’ ) and the D line becomes a shoulder of the G line. In particular the line width of both G and D bands is related to the bond angle disorder, and to the relative amount of crystallites versus the amorphous matrix. The observed line width increase points out that the amorphous carbon sample becomes more and more disordered during ion irradiation. Computer simulations [19] of the Raman spectra of amorphous carbons, modelled by mixtures of threefoldand fourfold-coordinated atoms, show that both coordination and bond angle disorder produce a G line shift at lower frequencies. Following Ref. [19], the G line is shifted to 1528 cm- ’ when the bond angle is changed from the ideal 120” to a disordered average of 117.7”. This seems to be the only mechanism capable of producing this effect. The G line shift observed in our experiment from 1592 cm-’ down to 1554 cm-’ (see Fig. 2) is in agreement with an enhanced bond-angle disorder induced by ion-irradiation, and may indicate that the fourfold-coordinated atoms increase in number with respect to the threefold-coordinated atoms. Annealing experiments carried out on poor ordered amorphous carbon films [20,21] show that the I,/& ratio vs. the annealing temperature initially increases, then develops a maximum at some temperature. between 800 and 9O@C, and finally decreases down to zero at high annealing temperature when graphitization has occurred [21]. The initial increase of the I,,/& has been interpreted as evidencing that the crystallites grow in size and/or in number. When effects of momentum conservation becomes important (momentum is conserved in large crystals), the In/Io ratio starts decreasing. In the hypothesis that annealing induces a monotone increase in the sp* average cluster size, the initial increasing trend of the &,/lo ratio is in
198
GA. Baratta et aL/Nucl.
Instr. and Meth. in Phys. Res. B 116 (1996) 195-199
agreement with other experiments carried out on amorphous carbon films with different order degree. In fact, Yoshikawa et al. [22] found that the In/& ratio is related to the size distribution of the sp’ cluster in poor ordered amorphous carbon films, as in this case larger clusters contribute preferentially to the D line. On the other hand, the relation found by [ 181, Zn/Zo a l/L,, should be valid in the decreasing part of the curve, when crystal-size effects cause the decrease in the intensity ratio. If the initial increase of the Zn/Zo ratio observed in our irradiated samples is real, in analogy with Ref. [20] we can apply the relation In/Z, a l/L, to the first point in Fig. 2 (lowest panel). This provides (through a comparison with Fig. 3 of Ref. [la]) a micro crystal size of L, = 40 A,. If the crystal size found by Raman spectroscopy is correct, this should imply that the spheroidal grains are rather ordered within their physical size of 50 A (in average) given by TEM analysis. “In situ” Raman spectroscopy of frozen films (T = 10 K) of benzene and butane irradiated with 3 keV of He+ ions [6], showed that the irradiated targets were converted into a a-H:C with a relatively low degree of order. Further irradiation of those a-H:C at room temperature showed an opposite behaviour with respect to that found for the AC sample. In particular the In/Z, ratio was found to increase with the dose up to a saturation value; while the optical gap together with the H/C ratio, measured by ERDA, decreased. These results were interpreted in terms of an increasing average sp* cluster size attributed to the preferential hydrogen loss from sp2 sites with respect to sp3 sites as evidenced by infrared spectroscopy [6]. Analogous results have been obtained bombarding a polystyrene film with 240 keV of Ar+ ions at room temperature [23]. The results obtained for the AC and a-H:C samples are similar to those found by Compagnini et al. [24] on two set of samples: a high temperature annealed (900°C) amorphous carbon called HGC (Hydrogenated Graphitic Carbon) and a conventionally ion produced a-H:C carbon. The HGC possesses an optical energy gap below 0.1 eV, an sp2/sp3 carbon fraction of about 80% (by EELS) and a H content of about 15 at.%. The a-H:C is a highly hydrogenated sample (40 at.%) with an energy gap of 1.5 eV and a sp2/sp3 carbon fraction of about 30%. Ion beam irradiation with 300 keV Ar+ showed that while in a-H:C the optical energy gap decreases reaching about 0.5 eV, the H content decreases to about 18% and the sp2/sp3 carbon fraction increase up to 65%. In HGC the process is reversed, in particular the sp2/sp3 carbon fraction decreases down to a saturation value of about 70%. The final result was an amorphous structure with the same physical properties independently of the starting sample. This similarity is also observed in our case, in fact the Raman spectrum of the a-IX from frozen benzene irradiated at the highest dose (1000 eV/C-atom) [6] is very similar to the Raman spectrum of the AC sample at 660 eV/C-atom (the most disordered).
During ion-beam irradiation, two competitive processes must be considered: graphitization and amorphization. The first one can be related to the energy release in form of heat (“thermal spikes”) inside the collision cascade (sp3 to sp2 conversion with or without H loss), while the latter is caused by the displacement collisions and depends on the size of the crystallites. It should be also considered that the characteristic time for relaxation of the excitation generated by an ion is of the order of one picosecond or less; thus equilibrium processes (like annealing) are not possible (see Ref. [25] for a comparison between laser irradiation, ion and electron bombardment and thermal annealing on a-H:C). According to Ref. [24] the saturation effect is due to the competition between these two processes, being the saturation reached when they are equal. In analogy with Ref. [24], this should imply that ion irradiation of the AC sample produces a strong declustering of the atomic configuration of the sp2 carbon component: the amorphization process (also observed in ion irradiated HOPG [9]) prevails on graphitization. This probably because the initial sp2 on sp3 carbon ratio is relatively high in the AC sample as testified both by the low H content and by the initial G line peak position. Ion irradiation of conventional a-HC, in contrast, produces a clustering of the sp2 carbon fraction suggesting that graphitization, accompanied by the H loss, is the driving process for the structural transformation. Indeed in this case the initial sp2 on sp3 carbon ratio is low [6] and the structure is highly disordered (as testified by the corresponding Raman spectra), so that graphitization prevails on amorphization.
6. Astrophysical
applications
In the Raman study by Wopenka [ 121(see Introduction) the 20 IDPs have been classified in 6 different groups where the order degree decreases from group 1 to group 6, as testified by the increasing band width and by the decreasing Z&Z, ratio. The Raman spectrum of the two most ordered groups is very similar to those of AC grains before irradiation. This result indicates that AC grains are good analogues of the carbonaceous phase of the IDPs belonging to the most ordered groups 1 and 2 (at least from a structural point of view). It was shown that IDPs are exposed to the penetrating (E = 100 keV) solar proton flux up to a total dose of the order of 10 to 100 eV/mol, during the 104-lo5 yr spent in the interplanetary medium before collection on Earth [3]. The flux of solar wind particles provides a much greater dose (i04-lo5 eV/mol), but only to a depth of = 0.02 j.4,m. In this scenario we are investigating the possibility that the different order degree exhibited by IDPs, could be explained (see Fig. 2) by a different ion-irradiation dose corresponding to the time spent in the interplanetary medium before collection in Earth atmosphere.
GA. Baratta et al./NwA.
Instr. and Meth. in Phys. Res. B II6 (1996) 195-199
References [l] R.E. Johnson, Energetic Charged-Particle Interaction with Atmospheres and Surfaces (Springer, Berlin, 1990). [2] W.L. Brown, L.J. Lanzerotti, J.M. Poate and W.M. Augustiniak, Phys. Rev. Lett. 40 (1978) 1027. [3] G. Strazzulla and R.E. Johnson, in: Comets in the Post-Halley Era, eds. R. Newbum Jr., M. Neugebauer and J. Rahe (Kluwer, Dordrecht, 1991) p. 243. [4] G. Strazzulla, G.A. Baratta, R.E. Johnson and B. Donn, Icarus 91 (1991) 101. [5] G. Strazzulla, G.A. Baratta and A. Magazzh, in: Solid State Astrophysics, eds. E. Bussoletti and G. Strazzulla (NorthHolland, Amsterdam, 1991) p. 403. [6] G. Strazzulla and G.A. Baratta, Astron. Astrophys. 266 (1992) 434. [7] B.S. Elmau, M. Shayegan, M.S. Dresselhaus, H. Mazurek and G. Dresselhaus, Phys. Rev. B 25 (1982) 4142. [8] B.S. Elman, M.S. Dresselhaus, G. Dresselhaus, E.W. Maby and H. Mazurek, Phys. Rev. B 24 (1981) 1027. [9] G. Compagnini and G.A. Baratta, Appl. Phys. Lett. 61 (1992) 1796. [lo] I.D.R. Mackimron and F.J.M. Rietmeijer, Rev. Geophys. 25 (1987) 1527. [ll] J.P. Bradley, D.E. Brownlee and D.R. Veblen, Science 223 (1984) 65. [12] B. Wopenka, Earth. Planet Sci. 88 (1988) 221. [13] L. Colangeli, V. Mennella, P. Palumbo, A. Rotundi and E. Bussoletti, Astron. Astrophys. Suppl. 113 (199.5) 561.
199
[141 S.A. Sandford, L.J. Allamaudola, A.G.G.M. Tielens, K. Selgren, M. Tapia and Y. Pendleton, Astrophy. J. 371 (1991) 607. [151F. Spinella, G.A. Baratta and G. Strazzulla, Rev. Sci. Instr. 62 (1991) 1743. [161 M. Di Domenico jr., S.H. Wemple and P.S. Porto, Phys. Rev. 174 (1968) 522. [171J. Robertson, Adv. Phys. 35 (1986) 317. h31 F. Tuinstra and J.L. Koenigh, J. Chem. Phys. 53 (1970) 1126. [191 D. Beeman, J. Silverman, R. Lynds and M.R. Anderson, Phys. Rev. B 30 (1984) 876. DO1R.O. Dillon, J.A. Wollam and V. Katkanant, Phys. Rev. B 29 (1984) 3482. ml R.P. Vidano and D.B. Fishbach, Extended Abstract of the 15th Biennial Conf. on Carbon, eds. F.L. Vogel and W.C. Foresmau, (American Carbon Society, University Park, PA, 19811, p. 468; R.P. Vidano, Ph.D. thesis, University of Washington (1980). ml M. Yoshikawa, G. Katagiri, H. Ishida, A. Ishitani and T. Akamatsu, J. Appl. Phys. 64 (1988) 6464. 1231G. Foti and R. Reitano, Nucl. Instr. and Meth. B 46 (1990) 306. 1241 G. Compagnini, L. Calcagno and G. Foti, Phys. Rev. I&t. 69 (1992) 454. [25] R. Kalish and M.E. Adel, Mat. Sci. Forum 52 (1989) 427.
w
Nuclear Instruments
and Methods in Physics Research B 116 (1996) 19% 199
NCIMB
Beam Interactions with Materials 6 Atoms
ELSEVIER
Raman spectroscopy of ion irradiated amorphous carbons G.A. Baratta ay*, M.M. Arena b, G. Strazzulla ‘, L. Colangeli E. Bussoletti e
d, V. Mennella d,
a Osservatorio Astrojisico di Catania, Citt& Uniuersitaria, I-95125 Catania, Italy b Istituto Nazionale per la Microelettronica, CNR Catania, Catania, Italy ’ Istituto di Astronomia, Catania, Italy a Osseruarorio Astronomico di Capodimonte, Napoli, Italy e Istituto Vniuersitario NavaLe, Napoli, Italy
Abstract
We have’studied, by “in situ” Raman spectroscopy, the modifications induced by 3 keV He+ ions on thin amorphous carbon grain deposits. Previous results obtained with our experimental apparatus show that in the case of carbon-containing frozen targets (such as benzene and butane) for doses greater than about 100 eV/mol, ion irradiation induces the formation of an hydrogenated amorphous carbon. In this paper, the bombarded material is already an amorphous carbon with a relatively high order degree. In this case ion irradiation progressively decreases the order degree in the amorphous carbon. This result is in agreement with analogous ion irradiation experiments carried out on highly ordered pyrolitic graphite crystals. These studies are important to understand physical characteristics and evolution of refractory carbon grains in astrophysical environments.
1. Introduction The interaction between a fast colliding ion and solid targets produces several effects, many of which have been studied in recent years in view of their astrophysical applications [l-6]. Raman spectroscopy can provide valuable information on the effects induced by ions impinging on solids. Indeed this technique is used to distinguish among chemical species and can provide valuable evidence of the structural properties of materials and, in particular, of carbonaceous materials. Raman spectroscopy has been used to gain insight into the structural lattice damage of graphite resulting from ion bombardment [7-91. Among the studied effects the formation of an organic residue evolving at higher doses towards an ion-produced HAC (Hydrogenated Amorphous Carbon) is particularly interesting. The HAC formation has been observed in several kinds of carbon-containing targets (polystyrene, polypropylene, graphite, diamond etc.) and even on frozen gases (C,H,, CH,, C,Hlo etc.). This occurs for a combination of bombarding ions (H, He, Ar, Kr etc.) and ion energies (ranging between a few keV and MeV). Both fast ions (low energy cosmic rays, galactic protons, solar pro-
* Corresponding author. Tel. +39 95 7332213, fax +39 95 330592, e-mail [email protected]. 0168-583X/96/$15.00 Copyright PII SOl68-583X(96)00124-3
tons, solar wind particles) and carbon-containing solid targets, i.e. frozen (interstellar grains in dense regions, comet mantles etc.) and refractory species (interstellar grains in the diffuse medium, interplanetary dust particles etc.), are supposed to be present in space. Thus materials produced in the laboratory may simulate to some extent those present in space. IDPs (Interplanetary Dust Particles) collected in the Earth atmosphere have sizes from 1 to 50 pm and provide a unique opportunity to study extraterrestrial material in the laboratory. In many IDPs, poorly graphitized carbonaceous compounds are arranged in a granular (fluffy) structure where single grains can be as small as 100 A [lo]; in some cases amorphous carbonaceous material surrounds tub micron mineral grains with a covering layer up to 300 A of thickness [ 1I]. A Rarnan study of a representative set of 20 IDPs [ 121 has shown that 6 different groups of spectra can be identified. AlI particles, except those belonging to the last group, show the Raman feature characteristic of amorphous carbon with different degree of order. Since IDPs are exposed to the fast solar proton (100 keV) and solar wind particles (1 keV/amu) bombardment before collection in the Earth atmosphere, it is interesting to compare the Raman spectra of IDPs with those of ion-irradiated fluffy aggregate of sub micron amorphous carbon grains. In this paper we present new experimental results obtained by “in situ” Raman spectroscopy carried out on
0 1996 Elsevier Science B.V. All rights reserved
GA. Baratta et al./Nucl. Ins@. andMeth. in Phys. Res. B 116 (1996) 195-199
1%
amorphous carbon grains irradiated with 3 lceV He+ ions. Some astrophysical implications are also discussed.
2. Characterisation
of the AC samples
The amorphous carbon (AC) studied in this work has been produced by arc discharge between two amorphous carbon electrodes in an inert Ar atmosphere. The dust deposit has been collected onto a LiF substrate. The AC samples have been fully characterised in [ 131.TEM studies carried out on AC samples ([13] and references therein) showed that they are ctnstituted of spheroidal grains with average radii of 50 A arranged in a fluffy structure. Infrared spectroscopy of the AC samples shows the aliphatic (sp’) C-H stretching structure in the 3.4 pm region. Both -CH, and -CH,- symmetric and asymmetric stretches are present. From Fig. 5 in Ref. [13] and by using average areal peak cross-sections for the -CH, and -CH,- groups [ 141it is possible to estimate the fraction of hydrogen atoms bound to sp3 hybridized carbon atoms. The resulting ratio with respect to the total amount of carbon atoms contained in the AC sample is H/C = 4%, and the ratio between the number of -CH, and -CH,groups is: N(-CH,)/N(-CH,-) = 2.3. No C-H aromatic stretching is present (within the noise) in the 3.3 pm region, this points out that most of the hydrogen atoms are bound to sp3 carbon atoms although some H atoms must be bound also to sp’ aromatic C atoms since in the 1 l-14 pm region the C-H aromatic out of plane bending with 1, 2 and 3 adjacent H are observed. From Fig. 5 in Ref. [ 131 it is also evident the presence of a weak C-O stretching feature due to oxygen contamination, the corresponding estimated O/C atomic ratio is less than = 0.5%.
3. Experimental
(vertical axis) and is azimutally rota&l by 45” with respect to the ion beam direction. Hence before, during and after irradiation, spectm can be obtained without tilting the sample. The Raman spectrometer is a SPEX 1877 triple monochromator. The used detector is an OMA III (EG & G Princeton Applied Research) intensified reticon.
4. Experimental results The sample has been irradiated with 3 keV helium ions at different doses. Raman spectra obtained “in situ” at different doses are shown in Fig. 1. It is evident that the shape of the Raman spectrum varies with the dose. In order to analyse the variation quantitatively we adopted a line shape fitting procedure. For both the G and D lines (see the next section) we used a line profile given by the relation [ 161:
(1) where dI/do is proportional to the detector signal at o, o0 is the undamped mode frequency, w is the frequency shift from the laser line (Raman shift), r is a damping constant and C is a constant of proportionality. Non-linear least square routines have been used to fit the experimental
apparatus
Optical microscopy performed on the AC samples showed that they are not uniform in the micron scale. Since the Raman laser spot size is about 20 pm, we used an apparatus to obtain “in situ” Raman spectra, to be sure to analyse always the same portion of the sample during ion bombardment. The experimental apparatus used to perform “in situ” Raman spectroscopy has been described in detail elsewhere [15]. It consists of a vacuum chamber (P = 10e7 mbar) where the targets are mounted on a cold finger (lo-300 K) and irradiated with 3 keV ions. The 514 nm argon laser beam is focalized through a glass window on the sample and the scattered light is collected by a collimating lens on the entrance slit of the monochromator. We used a 90” scattering geometry where the directions of the laser beam, ion beam and the collected Raman scattered light are mutualIy perpendicular. The sample holder is inclined by 30” with respect to the incident laser light
1000
1200
Raman
1400
shifts
1600
1800
[cm-‘]
Fig. 1. Raman spectra of amorphous carbon grains deposited on a LiF subs&ate irradiated at different doses with 3 keV of He+ ions. The contim~ous lines are theoretical fits to the data given by the sum of two line profiIe (dashed lines) representing the G and D lines (see the text).
GA. Baratta et al./Nucl.
Instr. and Meth. in Phys. Res. B 116 (1996) 195-199
1600 _
2 s g 2 2 3I?
1590
G
1500 1570 1560
o-4 A,,. 0
20
40
60
a0
400
600
Dose [eV/C-atom] Fig. 2. Some fitting parametersof the Raman spectra in Fig. 1 are reported versus the irradiation dose. From top to bottom: peak position of the G line, FWHM of D and G lines, ratio between the peak intensities of D and G lines. spectra by minimising the x2 as a function of six free parameters (C, w. and r for each of the lines). The resulting theoretical curves are reported together with the experimental spectra in Fig. 1. The estimated values for the peak frequency of the G line, the PWHM of both G and D lines and the ratios between their peak intensities are reported versus the dose in Fig. 2. Here the error bars correspond to an increase of x2 by 1 (one standard deviation errors). Our analysis evidences: (1) the increasing of both D and G line widths, (2) the downward shift of the G line peak frequency, (3) the decrease (except for the lowest irradiation dose) of the In/Z, intensity ratio with the dose. As it will be explained in the next section, these results are consistent with an increasing disorder induced by ion-irradiation.
5. Discussion Carbonaceous materials can widely vary in chemical composition and structure (diamond, graphite, glassy carbon, hydrogenated amorphous carbon etc.>. Depending on the order degree of graphitic (sp* hybridization) materials, one or two fiit order Raman bands are observed. Accord-
197
ing to the momentum conservation selection rule, only one first order Raman band at about 1582 cm-’ is observed in HOPG (Highly Ordered Pyrolitic Graphite) or natural graphite with large (> 1 ,um) micro crystals [17]. This band is known as the G (graphitic) line and is due to a doubly degenerate deformation vibration of the hexagonal ring corresponding to the E,, mode of graphite with D64, crystal symmetry [18]. However, spectra of microcrystalline graphite and disordered carbons show an additional 1360 cm-’ line. This D (disordered) line is attributed to phonons near the Brillouin zone boundary active in small crystallites or on the boundaries of larger crystallites. It was found [18] that the intensity ratio (fn/io) of the D and G lines varies as the inverse of the crystal planar domain size L, (In/& a l/L,). In amorphous carbons and hydrogenated amorphous carbons, both G and D bands are present. These bands are quite broader than those observed in disordered graphite: the broader the bands, the more disordered the amorphous carbon is. In amorphous carbon obtained by sputtering deposition the two bands cannot be distinguished, the G line is usually shifted at lower Raman shifts ( 1560 cm-,’ ) and the D line becomes a shoulder of the G line. In particular the line width of both G and D bands is related to the bond angle disorder, and to the relative amount of crystallites versus the amorphous matrix. The observed line width increase points out that the amorphous carbon sample becomes more and more disordered during ion irradiation. Computer simulations [19] of the Raman spectra of amorphous carbons, modelled by mixtures of threefoldand fourfold-coordinated atoms, show that both coordination and bond angle disorder produce a G line shift at lower frequencies. Following Ref. [19], the G line is shifted to 1528 cm- ’ when the bond angle is changed from the ideal 120” to a disordered average of 117.7”. This seems to be the only mechanism capable of producing this effect. The G line shift observed in our experiment from 1592 cm-’ down to 1554 cm-’ (see Fig. 2) is in agreement with an enhanced bond-angle disorder induced by ion-irradiation, and may indicate that the fourfold-coordinated atoms increase in number with respect to the threefold-coordinated atoms. Annealing experiments carried out on poor ordered amorphous carbon films [20,21] show that the I,/& ratio vs. the annealing temperature initially increases, then develops a maximum at some temperature. between 800 and 9O@C, and finally decreases down to zero at high annealing temperature when graphitization has occurred [21]. The initial increase of the I,,/& has been interpreted as evidencing that the crystallites grow in size and/or in number. When effects of momentum conservation becomes important (momentum is conserved in large crystals), the In/Io ratio starts decreasing. In the hypothesis that annealing induces a monotone increase in the sp* average cluster size, the initial increasing trend of the &,/lo ratio is in
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agreement with other experiments carried out on amorphous carbon films with different order degree. In fact, Yoshikawa et al. [22] found that the In/& ratio is related to the size distribution of the sp’ cluster in poor ordered amorphous carbon films, as in this case larger clusters contribute preferentially to the D line. On the other hand, the relation found by [ 181, Zn/Zo a l/L,, should be valid in the decreasing part of the curve, when crystal-size effects cause the decrease in the intensity ratio. If the initial increase of the Zn/Zo ratio observed in our irradiated samples is real, in analogy with Ref. [20] we can apply the relation In/Z, a l/L, to the first point in Fig. 2 (lowest panel). This provides (through a comparison with Fig. 3 of Ref. [la]) a micro crystal size of L, = 40 A,. If the crystal size found by Raman spectroscopy is correct, this should imply that the spheroidal grains are rather ordered within their physical size of 50 A (in average) given by TEM analysis. “In situ” Raman spectroscopy of frozen films (T = 10 K) of benzene and butane irradiated with 3 keV of He+ ions [6], showed that the irradiated targets were converted into a a-H:C with a relatively low degree of order. Further irradiation of those a-H:C at room temperature showed an opposite behaviour with respect to that found for the AC sample. In particular the In/Z, ratio was found to increase with the dose up to a saturation value; while the optical gap together with the H/C ratio, measured by ERDA, decreased. These results were interpreted in terms of an increasing average sp* cluster size attributed to the preferential hydrogen loss from sp2 sites with respect to sp3 sites as evidenced by infrared spectroscopy [6]. Analogous results have been obtained bombarding a polystyrene film with 240 keV of Ar+ ions at room temperature [23]. The results obtained for the AC and a-H:C samples are similar to those found by Compagnini et al. [24] on two set of samples: a high temperature annealed (900°C) amorphous carbon called HGC (Hydrogenated Graphitic Carbon) and a conventionally ion produced a-H:C carbon. The HGC possesses an optical energy gap below 0.1 eV, an sp2/sp3 carbon fraction of about 80% (by EELS) and a H content of about 15 at.%. The a-H:C is a highly hydrogenated sample (40 at.%) with an energy gap of 1.5 eV and a sp2/sp3 carbon fraction of about 30%. Ion beam irradiation with 300 keV Ar+ showed that while in a-H:C the optical energy gap decreases reaching about 0.5 eV, the H content decreases to about 18% and the sp2/sp3 carbon fraction increase up to 65%. In HGC the process is reversed, in particular the sp2/sp3 carbon fraction decreases down to a saturation value of about 70%. The final result was an amorphous structure with the same physical properties independently of the starting sample. This similarity is also observed in our case, in fact the Raman spectrum of the a-IX from frozen benzene irradiated at the highest dose (1000 eV/C-atom) [6] is very similar to the Raman spectrum of the AC sample at 660 eV/C-atom (the most disordered).
During ion-beam irradiation, two competitive processes must be considered: graphitization and amorphization. The first one can be related to the energy release in form of heat (“thermal spikes”) inside the collision cascade (sp3 to sp2 conversion with or without H loss), while the latter is caused by the displacement collisions and depends on the size of the crystallites. It should be also considered that the characteristic time for relaxation of the excitation generated by an ion is of the order of one picosecond or less; thus equilibrium processes (like annealing) are not possible (see Ref. [25] for a comparison between laser irradiation, ion and electron bombardment and thermal annealing on a-H:C). According to Ref. [24] the saturation effect is due to the competition between these two processes, being the saturation reached when they are equal. In analogy with Ref. [24], this should imply that ion irradiation of the AC sample produces a strong declustering of the atomic configuration of the sp2 carbon component: the amorphization process (also observed in ion irradiated HOPG [9]) prevails on graphitization. This probably because the initial sp2 on sp3 carbon ratio is relatively high in the AC sample as testified both by the low H content and by the initial G line peak position. Ion irradiation of conventional a-HC, in contrast, produces a clustering of the sp2 carbon fraction suggesting that graphitization, accompanied by the H loss, is the driving process for the structural transformation. Indeed in this case the initial sp2 on sp3 carbon ratio is low [6] and the structure is highly disordered (as testified by the corresponding Raman spectra), so that graphitization prevails on amorphization.
6. Astrophysical
applications
In the Raman study by Wopenka [ 121(see Introduction) the 20 IDPs have been classified in 6 different groups where the order degree decreases from group 1 to group 6, as testified by the increasing band width and by the decreasing Z&Z, ratio. The Raman spectrum of the two most ordered groups is very similar to those of AC grains before irradiation. This result indicates that AC grains are good analogues of the carbonaceous phase of the IDPs belonging to the most ordered groups 1 and 2 (at least from a structural point of view). It was shown that IDPs are exposed to the penetrating (E = 100 keV) solar proton flux up to a total dose of the order of 10 to 100 eV/mol, during the 104-lo5 yr spent in the interplanetary medium before collection on Earth [3]. The flux of solar wind particles provides a much greater dose (i04-lo5 eV/mol), but only to a depth of = 0.02 j.4,m. In this scenario we are investigating the possibility that the different order degree exhibited by IDPs, could be explained (see Fig. 2) by a different ion-irradiation dose corresponding to the time spent in the interplanetary medium before collection in Earth atmosphere.
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