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Inelastic neutron scattering of amorphous hydrogenated carbon
Diamoml and Related Materials, l (1992) 293 297 Elsevier Science Publishers B.V.. Amsterdam
293
Inelastic neutron scattering of amorphous hydrogenated carbon P. J. R. Honeybone and R. J. Newport Physics Department, The University, Canterbury, Kent C T2 7N R ( U K )
W. S. Howells Neutron Science Division. RutherJbrd Appleton Laboratory, Chillon, Dideot, Oxon. OX I I OQX ( U K )
J. Franks* Ion Tech Ltd., 2 Park Street, Teddington, Middlesex T W I 1 0 L T (U K j
Abstract The hydrogen bonding environment of three samples of a-C : H have been extensively studied by means of inelastic neutron scattering. The observed inelastic neutron scattering spectra can be modelled by approximately equal quantities of sp 3 CH and CH2 groups, i.e. approximately two-thirds of the hydrogen incorporated in a-C : H is in the form of CH 2 groups.
1. Introduction Amorphous hydrogenated carbon may be prepared harder, denser and more resistant to chemical attack than any other solid hydrocarbon. These properties, along with optical properties, such as optical gap and refractive index may be varied by changing the deposition parameters, which has led to a large number of potential applications [1]. The structure giving rise to these useful properties is not yet fully understood, with current models involving clusters of s p 2 carbon linked by chains of sp 3 carbon. The reviews of Robertson [2] and Angus et al. [3] give a fuller account of these and other models. The role played by hydrogen in determining the properties of a-C:H is crucial to a full understanding of the material. Zou et al. [4] have shown that high hydrogen content films (>40 at% H) are of polymeric nature (high sp 3 content, soft and with low density) and low hydrogen content films are of graphitic character (soft films consisting of large clusters of sp 2 carbon). It should be noted however that McKenzie et al. [5] have produced a low hydrogen content, hard, high density amorphous carbon film. Within the Robertson model, the hydrogen is seen to stabilise the sp 3 regions, reducing the size of any sp 2 clusters, but at the same time increasing the number of network terminating bonds, leading to a maximum hardness at intermediate hydrogen concentrations. An extensive study of the hydrogen environments in * Present address: Diavac ACM Ltd., 2 Brookfield Avenue, Ealing, London W5 I LA, UK.
a-C:H has been performed by Dischler [6], who proposes frequency assignments for all observed bonding environments. This, together with other infra-red measurements r7] and NMR [8] have suggested that the hydrogen is bonded primarily to sp 3 carbon sites. Unfortunately, neither of these techniques has been able to investigate the degree of hydrogen clustering, although Vandentop et al. [9] have used infra-red spectroscopy to estimate CH2:CH 3 ratios but this depends upon assumptions for the matrix elements of each vibration. Unlike electromagnetic radiation, neutrons interact with matter in a straightforward manner [10]. The incoherent neutron-scattering cross-section for hydrogen is much greater than that for any other element. Incoherent inelastic neutron scattering (IINS) can therefore be used to focus on the hydrogen vibrations in amorphous hydrogenated carbon. Since there is a direct relationship between the observed intensity and the eigenvector of the vibration (unlike infra-red and Raman spectroscopy, where complex matrix elements are present), it is possible to model the dominant hydrogen bonding environment in a direct way.
2. Experimental details The amorphous hydrogenated carbon used in these experiments was produced using a saddle-field ion-beam source [11]. Samples 1 and 2 were deposited onto copper substrates from acetylene and propane gases respectively (as a-C:H does not adhere to copper, this proved an excellent means of producing the large powder samples
0925-9635 92/$05.00 (~ Elsevier Science Publishers B.V. All rights reserved
294
P. J. R. Honeybone et al.
Inelastic' neutron scattering of amorphaus hydrogenated carbon
required for neutron scattering experiments). Sample 3 was collected, over a period of time, from within the source chamber from a mixture of propane, butane and acetylene gases. The inelastic neutron scattering experiments were carried out on the TFXA inverse-geometry spectrometer at the ISIS spallation neutron source [12]. The density measurements were performed using a residual volume technique and the compositions were determined using a Carlo-Erba C H N combustion analyser. These results are summarised in Table 1.
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The CLIMAX program [13] produces a least squares fit of a calculated inelastic neutron scattering spectrum to the observed inelastic neutron scattering spectrum. Tomkinson [14] has provided a good introduction to the inelastic neutron scattering theory employed by this method. Infra-red spectroscopy on amorphous hydrogenated carbon, has shown the presence of many hydrogen bonding environments [6], each with a set of vibrational frequencies. These are used as a basis for constructing models (i.e. the symmetry of each vibration in the model used must correspond to the symmetry of the vibration simulated). The model vibrations are constructed from individual internal coordinates, each of which is assigned a force constant. These force constants are fitted to the observed frequencies, and then to the observed spectrum.
,
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3. Data analysis
i
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10'00 1s'0o 20'00 25'00 3o'o0 3500 Energy T r a n s f e r ( e m - l )
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1000 1500 2000 2500 Energy Transfer (cm-1)
3000
3500
500
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3500
(b)
4. Results n e
The inelastic neutron scattering spectra from all three samples show many broad similarities in shape in each of the three regions of the spectra (see Fig. 1). The part of the spectrum that is of most interest, and that which provides most useful information is that between 500 and 2000cm 1; it is in this region that fundamentals of the C - C stretch and all C - H bending vibrations occur. Testing of various structural models has allowed us to eliminate the possibility that one dominant hydrogen environment could explain the observed neutron scatterTABLE I. Compositionsand densities of samples (densities are lower bound values)
Sample 1 Sample 2 Sample 3
C (at.%)
H (at.%)
p (gcm -3)
65 68 71
35 32 29
1.65 1.81 1.80
!O / t
0
1500
2000
2500
Energy Transfer (cm-l) (c) Fig. 1. Inelastic neutron scattering spectrum for (a) sample 1, (b) sample 2 and (c) sample 3.
ing intensity. However, by constructing a model for a combination of CH and CH2 groups, it has been possible to explain almost all of the observed intensity (see Fig. 2). This does not exclude small quantities of other hydrogen bonding environments, which have been detected by infra-red [6], but the concentrations must be very low.
P. J. R. ftoneybone et al.
Inelastic neutron .scattering of amorphous hydrogenated carbml
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Fig. 2. Observed and calculated inelastic neutron scattering spectrum for (a) sample 1, (by sample 2 and (c) sample 3,
295
P. J. R. Honeybone et al. / Inelastic neutron scattering of amorphous hydrogenated carbon
296
The assignments made by Dischler [6] were used initially, however, by modification of the assignments in accordance with Dollish et al. [15], good agreement with the observed spectra has been obtained. Table 2 compares these assignments with those of Dischler and Dollish et al. [6,15]. The differences between our assignments and those of Dischler are the absence of a CH2 rocking peak at 700cm -1 (we find this mode at 1030cm-1), the CH 2 twist at 1300cm 1 and wag at 1030 cm - 1 (instead of 1170 cm - 1 and 1030 cm- ~ respectively) and the CH bend at l l 9 0 c m -1, not 1370cm -1. (An analogous situation occurs in a-Si:H [16], and has been used to fit inelastic neutron scattering data for silicon rich a - S i : C : H [17]). The hydrogen in all a - C : H samples is seen to be predominantly incorporated in CH2 groups (see Tables 3 and 4), with at least two-thirds of all hydrogen bonded in this way. This contrasts with infra-red analysis of the C - H stretch [6,7], which suggests that CH groups dominate. Problems occur, however, in obtaining intensities from infra-red spectroscopy, due to the complex matrix elements involved. Analysis of inelastic neutron scattering intensities, where the scattering relationship is straightforward, is the more likely to be correct. A purely random distribution would lead to CH groups dominating. However, there are two considerations which would lead to CH2 group formation. The first TABLE 2. Comparison of frequency assignments for sp 3 CH and CH2 groups Assignment
Dischler [6]
Dollish et al. [15]
Best fit
C - C stretch CHbend CH 2 bend Twist Rock Wag
885cm i 1370cm -1 1440 cm -~ ll70cm ~ 700cm 1 1030 cm a
1132_885 cm-1 ll60cm a 1473-1446 cm -~ 1310 1175cm -~ 1060 719cm -1 1411-I174 cm -~
875cm l l 9 0 c m -~ 1470 cm -~ 1300cm 1 1030cm 1330 cm
TABLE 3. Comparison of hydrogen content with CH : CH2 ratio
Sample 1 Sample 2 Sample 3
Hydrogen content (at.%)
CH : CH: ratio
35 32 29
0.80 0.97 0.90
TABLE 4. Comparison of carbon content with degree of hydrogenation I
Sample I Sample 2 Sample 3
Carbon content (at.%)
Unhydrogenated carbon
CH
CH 2
65 68 71
66 69 73
15 15 12
19 16 14
arises from steric hinderance. Unhydrogenated carbon is more densely packed than hydrogenated carbon, which means that hydrogen diffusion can take place more easily through hydrogenated regions. (Koidl [18] has shown that a lower hydrogen content and a higher network density reduces diffusion in a-C:H.) It is possible, therefore to envisage a situation where, during film formation, the hydrogen is more likely to be in a position to react with hydrogenated, rather than unhydrogenated carbon. Also, when adding hydrogen, each CH 2 group formed reduces the number of CH groups by one. It is striking that there appears to be no direct relationship between the hydrogen content and CH : CH 2 ratio. The relatively high CH 2 content in sample 1 can easily be understood on a purely statistical basis in terms of an increased overall hydrogen content increasing the probability of a hydrogenated carbon atom being bonded to a second carbon atom. Another consequence of increased hydrogen content is a reduction in the unsaturated (sp 2) carbon content (it is expected that this will be borne out by a complete analysis of our neutron diffraction data [ 193). Addition of hydrogen across a double bond is energetically favoured over addition across a single bond. In a situation, as is the case for a-C:H, where the hydrogen is predominantly bonded to sp 3 carbon, CH 2 groups will only be formed when addition occurs across a single bond, i.e. the energetically unfavoured situation. Therefore, in amorphous carbon with low hydrogen content (and high sp z carbon content) the hydrogen will be predominantly in the form of CH groups. This corresponds with the observation of hydrogen incorporation increasing the sp 3 carbon fraction [2,3]. A natural limit occurs from the number of sp 2 sites available. This might help explain the apparent maximum fraction of carbon present in the form of CH groups being 0.15. The final result to emerge from this analysis is the proportion of unhydrogenated carbon in each of the films (see Table 4). It is interesting to note that the fraction of carbon that is unhydrogenated closely mirrors the carbon atomic fraction. Diffraction data is required to understand the nature of the bonding of this carbon, although further computer modelling techniques will probably be required to illuminate the clustering of CH and CH2 groups (i.e. are they randomly distributed throughout the a-C:H or are chains the dominant feature?). The first region of the spectrum (0-500 cm 1) also shows differences between the samples, with the most obvious difference being in terms of intensity relative to the rest of the spectrum. The intensity in this region for sample 2 is much greater than for either of the other two samples, yet the shape is identical to that for sample 1. No explanation has yet been found for this, although both the carbon backbone vibrations and CH2-CH2 torsions are expected to give intensity in this region [15].
P. J. R. Honeyhone el al.
hwlastie neutron scatterin~ ql amorphous hydro[zenated carbon
Sample 3 has an a d d i t i o n a l feature superimposed on the same basic shape in this region. This has been assigned to the molecular hydrogen r o t a t i o n [20], and d e m o n s t r a t e s the presence of small quantities of H2. The tinal region is 2000 3 5 0 0 c m 1; there are no obvious differences between the samples, although i n s t r u m e n t a l resolution in this region is low. The overtone of the CH and CH2 b e n d i n g vibrations can be seen at 2 5 0 0 c m ~. as can the CH stretching vibrations at 3000 cm i
5. Conclusions The hydrogen b o n d i n g e n v i r o n m e n t in a m o r p h o u s h y d r o g e n a t e d c a r b o n has been extensively studied by means of inelastic n e u t r o n scattering. It has been found that the majority of hydrogen is b o n d e d in the form of CH2 groups in all cases, with the CH : CH2 ratio varying with hydrogen c o n t e n t in a n o n - l i n e a r m a n n e r . It has been suggested that a c o m b i n a t i o n of s a t u r a t i o n of u n s a t u r a t e d b o n d s together with the availability of hydrogenated c a r b o n sites is responsible for this. It is hoped that a complete analysis of n e u t r o n diffraction data, coupled with extensive c o m p u t e r modelling will give further i n f o r m a t i o n on the hydrogen b o n d i n g e n v i r o n m e n t . In particular, it is i m p o r t a n t to k n o w the extent of n o n - h y d r o g e n a t e d c a r b o n clustering and whether it includes sp 3 or just sp 2 carbon.
Acknowledgments The a u t h o r s would like to thank M r A. Evans and Dr P, J. Revell of Ion Tech Ltd. for their help in depositing the samples, M r A. Fassam {Chemistry Depar{ment, U K C ) for his help in d e t e r m i n i n g compositions, Dr D. Kearley (ILL, Grenoble) and Dr J. T o m k i n s o n (RAL) for allowing use of the C L I M A X p r o g r a m and one of the a u t h o r s (P.J.R.H.) acknowledges financial support from SERC.
297
References I A. Lettington, in R. E. Clausing, L. L. Horton, J. C. Angus and P. Koidl (eds.), Diamond and Diamond-like I-'ilms aml Coatings, Plenum Press, New York, 1991. p. 481. 2 J. Robertson, Adv. Phys.. 35 (1986) 317. 3 J. C. Angus, P. Koidl and S. Domitz, in J. Mort and F. Jansen (eds.), Plasma Deposited Thin Films, CRC Press, Boca Raton, 198f~,p. ~9. 4 J. ',~. Zou, K. Reichelt, K. Schmidt and B. Dischler,J. Appl. Phys.. 65 (1989) 3914. 5 I). R. McKenzie, D. Muller, B. A. Pailthorpe, Z. H. Wang, 1:,. Kravtchinskaia, D. Segal. P. B. Lukins, P. D. Swift, P. J. Martin, G. Amaratunga, P. H. Gaskell and A. Saeed, Diamond and Related Materials, 1 II991l 51. 6 B. Dischler, in P. Koidl and P. Oelhafen (eds.), ,4morphous Hydro~,,em;ted Carbon Films, Les Editions de Physique. Paris, 1987, p. I{q9, 7 B. Dischler. A. Bubenzer and P. Koidl, Sol. State Comm.. 48 (1983) 105. 8 M. A. Petrich, Materials Science Fornmm, 52 (1989) 377. 9 (}. J. Vandentop. M. Kawasaki, K. Kobayashi and G. A. Somorjai, J. l~ae. S{i. Teehnol. A, 9 (1991) 1157. I0 J. M. F. Gunn, in R. J. Newport, B. D. Rainford and R. Cywinski (eds.), Neutron Scattering,, at a Pulsed Source, Adam Hilger, Bristol, 198S. p. 36. I1 J. Franks, Vaeumn, 34 {19841259. 12 ISIS ,4nnual Report. Rutherford Appleton Laboratory Report, RAL-90-041. 1990. 13 G. J. Kearley and J. Tomkinson, Inst. Phys. Conl. Set'., 107 (1990) 245. 14 J. Tomkinson, ira R. J. Newport, B. D. Rainford and R. Cywinski {eds.i, Neutron Seatterin~ at a Pulsed Source, Adam-Hilger, Bristol, 198~,, p. 324. 15 F. R. Dollish, W. G. Fately and F, F. Bentley,Characteristic Raman Frequem'ies, Wiley, New York, 1974. 16 M. Cardona, Phys. Star. Sol (hi, 11,"¢(1983) 463. 17 P. J. R. Honeybone, R. J. Newport, W. S. Howells, J. Tomkinson and C. Hotham. J. Non-Crystalline Solids, submitted. 18 P. Koidl, C. Wild, R. Locher and R. E. Sah, in R. E. Clausing, L. 1.. Horton, J. C. Angus and P. Koidl (eds.), Diamond and Diamondlike Fihns and Coatings, Plenum, New York, 1991, p. 243. 19 P. J. R. Honeybone, R. J. Newport, D. W. Huxley. W. S. Howells and J. Franks, ira preparation. 20 P. J. R. Honeybone, R. J. Newport, W. S. Howells, J. Tomkinson, S. B. Bennington and P. J. Revell, Chem. Phys. Letts., 180 {1991) (45.
293
Inelastic neutron scattering of amorphous hydrogenated carbon P. J. R. Honeybone and R. J. Newport Physics Department, The University, Canterbury, Kent C T2 7N R ( U K )
W. S. Howells Neutron Science Division. RutherJbrd Appleton Laboratory, Chillon, Dideot, Oxon. OX I I OQX ( U K )
J. Franks* Ion Tech Ltd., 2 Park Street, Teddington, Middlesex T W I 1 0 L T (U K j
Abstract The hydrogen bonding environment of three samples of a-C : H have been extensively studied by means of inelastic neutron scattering. The observed inelastic neutron scattering spectra can be modelled by approximately equal quantities of sp 3 CH and CH2 groups, i.e. approximately two-thirds of the hydrogen incorporated in a-C : H is in the form of CH 2 groups.
1. Introduction Amorphous hydrogenated carbon may be prepared harder, denser and more resistant to chemical attack than any other solid hydrocarbon. These properties, along with optical properties, such as optical gap and refractive index may be varied by changing the deposition parameters, which has led to a large number of potential applications [1]. The structure giving rise to these useful properties is not yet fully understood, with current models involving clusters of s p 2 carbon linked by chains of sp 3 carbon. The reviews of Robertson [2] and Angus et al. [3] give a fuller account of these and other models. The role played by hydrogen in determining the properties of a-C:H is crucial to a full understanding of the material. Zou et al. [4] have shown that high hydrogen content films (>40 at% H) are of polymeric nature (high sp 3 content, soft and with low density) and low hydrogen content films are of graphitic character (soft films consisting of large clusters of sp 2 carbon). It should be noted however that McKenzie et al. [5] have produced a low hydrogen content, hard, high density amorphous carbon film. Within the Robertson model, the hydrogen is seen to stabilise the sp 3 regions, reducing the size of any sp 2 clusters, but at the same time increasing the number of network terminating bonds, leading to a maximum hardness at intermediate hydrogen concentrations. An extensive study of the hydrogen environments in * Present address: Diavac ACM Ltd., 2 Brookfield Avenue, Ealing, London W5 I LA, UK.
a-C:H has been performed by Dischler [6], who proposes frequency assignments for all observed bonding environments. This, together with other infra-red measurements r7] and NMR [8] have suggested that the hydrogen is bonded primarily to sp 3 carbon sites. Unfortunately, neither of these techniques has been able to investigate the degree of hydrogen clustering, although Vandentop et al. [9] have used infra-red spectroscopy to estimate CH2:CH 3 ratios but this depends upon assumptions for the matrix elements of each vibration. Unlike electromagnetic radiation, neutrons interact with matter in a straightforward manner [10]. The incoherent neutron-scattering cross-section for hydrogen is much greater than that for any other element. Incoherent inelastic neutron scattering (IINS) can therefore be used to focus on the hydrogen vibrations in amorphous hydrogenated carbon. Since there is a direct relationship between the observed intensity and the eigenvector of the vibration (unlike infra-red and Raman spectroscopy, where complex matrix elements are present), it is possible to model the dominant hydrogen bonding environment in a direct way.
2. Experimental details The amorphous hydrogenated carbon used in these experiments was produced using a saddle-field ion-beam source [11]. Samples 1 and 2 were deposited onto copper substrates from acetylene and propane gases respectively (as a-C:H does not adhere to copper, this proved an excellent means of producing the large powder samples
0925-9635 92/$05.00 (~ Elsevier Science Publishers B.V. All rights reserved
294
P. J. R. Honeybone et al.
Inelastic' neutron scattering of amorphaus hydrogenated carbon
required for neutron scattering experiments). Sample 3 was collected, over a period of time, from within the source chamber from a mixture of propane, butane and acetylene gases. The inelastic neutron scattering experiments were carried out on the TFXA inverse-geometry spectrometer at the ISIS spallation neutron source [12]. The density measurements were performed using a residual volume technique and the compositions were determined using a Carlo-Erba C H N combustion analyser. These results are summarised in Table 1.
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(a)
The CLIMAX program [13] produces a least squares fit of a calculated inelastic neutron scattering spectrum to the observed inelastic neutron scattering spectrum. Tomkinson [14] has provided a good introduction to the inelastic neutron scattering theory employed by this method. Infra-red spectroscopy on amorphous hydrogenated carbon, has shown the presence of many hydrogen bonding environments [6], each with a set of vibrational frequencies. These are used as a basis for constructing models (i.e. the symmetry of each vibration in the model used must correspond to the symmetry of the vibration simulated). The model vibrations are constructed from individual internal coordinates, each of which is assigned a force constant. These force constants are fitted to the observed frequencies, and then to the observed spectrum.
,
n
0
3. Data analysis
i
i
10'00 1s'0o 20'00 25'00 3o'o0 3500 Energy T r a n s f e r ( e m - l )
2.0-
1.5-
1.o-
0.5-
500
1000 1500 2000 2500 Energy Transfer (cm-1)
3000
3500
500
lOOO
3000
3500
(b)
4. Results n e
The inelastic neutron scattering spectra from all three samples show many broad similarities in shape in each of the three regions of the spectra (see Fig. 1). The part of the spectrum that is of most interest, and that which provides most useful information is that between 500 and 2000cm 1; it is in this region that fundamentals of the C - C stretch and all C - H bending vibrations occur. Testing of various structural models has allowed us to eliminate the possibility that one dominant hydrogen environment could explain the observed neutron scatterTABLE I. Compositionsand densities of samples (densities are lower bound values)
Sample 1 Sample 2 Sample 3
C (at.%)
H (at.%)
p (gcm -3)
65 68 71
35 32 29
1.65 1.81 1.80
!O / t
0
1500
2000
2500
Energy Transfer (cm-l) (c) Fig. 1. Inelastic neutron scattering spectrum for (a) sample 1, (b) sample 2 and (c) sample 3.
ing intensity. However, by constructing a model for a combination of CH and CH2 groups, it has been possible to explain almost all of the observed intensity (see Fig. 2). This does not exclude small quantities of other hydrogen bonding environments, which have been detected by infra-red [6], but the concentrations must be very low.
P. J. R. ftoneybone et al.
Inelastic neutron .scattering of amorphous hydrogenated carbml
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Fig. 2. Observed and calculated inelastic neutron scattering spectrum for (a) sample 1, (by sample 2 and (c) sample 3,
295
P. J. R. Honeybone et al. / Inelastic neutron scattering of amorphous hydrogenated carbon
296
The assignments made by Dischler [6] were used initially, however, by modification of the assignments in accordance with Dollish et al. [15], good agreement with the observed spectra has been obtained. Table 2 compares these assignments with those of Dischler and Dollish et al. [6,15]. The differences between our assignments and those of Dischler are the absence of a CH2 rocking peak at 700cm -1 (we find this mode at 1030cm-1), the CH 2 twist at 1300cm 1 and wag at 1030 cm - 1 (instead of 1170 cm - 1 and 1030 cm- ~ respectively) and the CH bend at l l 9 0 c m -1, not 1370cm -1. (An analogous situation occurs in a-Si:H [16], and has been used to fit inelastic neutron scattering data for silicon rich a - S i : C : H [17]). The hydrogen in all a - C : H samples is seen to be predominantly incorporated in CH2 groups (see Tables 3 and 4), with at least two-thirds of all hydrogen bonded in this way. This contrasts with infra-red analysis of the C - H stretch [6,7], which suggests that CH groups dominate. Problems occur, however, in obtaining intensities from infra-red spectroscopy, due to the complex matrix elements involved. Analysis of inelastic neutron scattering intensities, where the scattering relationship is straightforward, is the more likely to be correct. A purely random distribution would lead to CH groups dominating. However, there are two considerations which would lead to CH2 group formation. The first TABLE 2. Comparison of frequency assignments for sp 3 CH and CH2 groups Assignment
Dischler [6]
Dollish et al. [15]
Best fit
C - C stretch CHbend CH 2 bend Twist Rock Wag
885cm i 1370cm -1 1440 cm -~ ll70cm ~ 700cm 1 1030 cm a
1132_885 cm-1 ll60cm a 1473-1446 cm -~ 1310 1175cm -~ 1060 719cm -1 1411-I174 cm -~
875cm l l 9 0 c m -~ 1470 cm -~ 1300cm 1 1030cm 1330 cm
TABLE 3. Comparison of hydrogen content with CH : CH2 ratio
Sample 1 Sample 2 Sample 3
Hydrogen content (at.%)
CH : CH: ratio
35 32 29
0.80 0.97 0.90
TABLE 4. Comparison of carbon content with degree of hydrogenation I
Sample I Sample 2 Sample 3
Carbon content (at.%)
Unhydrogenated carbon
CH
CH 2
65 68 71
66 69 73
15 15 12
19 16 14
arises from steric hinderance. Unhydrogenated carbon is more densely packed than hydrogenated carbon, which means that hydrogen diffusion can take place more easily through hydrogenated regions. (Koidl [18] has shown that a lower hydrogen content and a higher network density reduces diffusion in a-C:H.) It is possible, therefore to envisage a situation where, during film formation, the hydrogen is more likely to be in a position to react with hydrogenated, rather than unhydrogenated carbon. Also, when adding hydrogen, each CH 2 group formed reduces the number of CH groups by one. It is striking that there appears to be no direct relationship between the hydrogen content and CH : CH 2 ratio. The relatively high CH 2 content in sample 1 can easily be understood on a purely statistical basis in terms of an increased overall hydrogen content increasing the probability of a hydrogenated carbon atom being bonded to a second carbon atom. Another consequence of increased hydrogen content is a reduction in the unsaturated (sp 2) carbon content (it is expected that this will be borne out by a complete analysis of our neutron diffraction data [ 193). Addition of hydrogen across a double bond is energetically favoured over addition across a single bond. In a situation, as is the case for a-C:H, where the hydrogen is predominantly bonded to sp 3 carbon, CH 2 groups will only be formed when addition occurs across a single bond, i.e. the energetically unfavoured situation. Therefore, in amorphous carbon with low hydrogen content (and high sp z carbon content) the hydrogen will be predominantly in the form of CH groups. This corresponds with the observation of hydrogen incorporation increasing the sp 3 carbon fraction [2,3]. A natural limit occurs from the number of sp 2 sites available. This might help explain the apparent maximum fraction of carbon present in the form of CH groups being 0.15. The final result to emerge from this analysis is the proportion of unhydrogenated carbon in each of the films (see Table 4). It is interesting to note that the fraction of carbon that is unhydrogenated closely mirrors the carbon atomic fraction. Diffraction data is required to understand the nature of the bonding of this carbon, although further computer modelling techniques will probably be required to illuminate the clustering of CH and CH2 groups (i.e. are they randomly distributed throughout the a-C:H or are chains the dominant feature?). The first region of the spectrum (0-500 cm 1) also shows differences between the samples, with the most obvious difference being in terms of intensity relative to the rest of the spectrum. The intensity in this region for sample 2 is much greater than for either of the other two samples, yet the shape is identical to that for sample 1. No explanation has yet been found for this, although both the carbon backbone vibrations and CH2-CH2 torsions are expected to give intensity in this region [15].
P. J. R. Honeyhone el al.
hwlastie neutron scatterin~ ql amorphous hydro[zenated carbon
Sample 3 has an a d d i t i o n a l feature superimposed on the same basic shape in this region. This has been assigned to the molecular hydrogen r o t a t i o n [20], and d e m o n s t r a t e s the presence of small quantities of H2. The tinal region is 2000 3 5 0 0 c m 1; there are no obvious differences between the samples, although i n s t r u m e n t a l resolution in this region is low. The overtone of the CH and CH2 b e n d i n g vibrations can be seen at 2 5 0 0 c m ~. as can the CH stretching vibrations at 3000 cm i
5. Conclusions The hydrogen b o n d i n g e n v i r o n m e n t in a m o r p h o u s h y d r o g e n a t e d c a r b o n has been extensively studied by means of inelastic n e u t r o n scattering. It has been found that the majority of hydrogen is b o n d e d in the form of CH2 groups in all cases, with the CH : CH2 ratio varying with hydrogen c o n t e n t in a n o n - l i n e a r m a n n e r . It has been suggested that a c o m b i n a t i o n of s a t u r a t i o n of u n s a t u r a t e d b o n d s together with the availability of hydrogenated c a r b o n sites is responsible for this. It is hoped that a complete analysis of n e u t r o n diffraction data, coupled with extensive c o m p u t e r modelling will give further i n f o r m a t i o n on the hydrogen b o n d i n g e n v i r o n m e n t . In particular, it is i m p o r t a n t to k n o w the extent of n o n - h y d r o g e n a t e d c a r b o n clustering and whether it includes sp 3 or just sp 2 carbon.
Acknowledgments The a u t h o r s would like to thank M r A. Evans and Dr P, J. Revell of Ion Tech Ltd. for their help in depositing the samples, M r A. Fassam {Chemistry Depar{ment, U K C ) for his help in d e t e r m i n i n g compositions, Dr D. Kearley (ILL, Grenoble) and Dr J. T o m k i n s o n (RAL) for allowing use of the C L I M A X p r o g r a m and one of the a u t h o r s (P.J.R.H.) acknowledges financial support from SERC.
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References I A. Lettington, in R. E. Clausing, L. L. Horton, J. C. Angus and P. Koidl (eds.), Diamond and Diamond-like I-'ilms aml Coatings, Plenum Press, New York, 1991. p. 481. 2 J. Robertson, Adv. Phys.. 35 (1986) 317. 3 J. C. Angus, P. Koidl and S. Domitz, in J. Mort and F. Jansen (eds.), Plasma Deposited Thin Films, CRC Press, Boca Raton, 198f~,p. ~9. 4 J. ',~. Zou, K. Reichelt, K. Schmidt and B. Dischler,J. Appl. Phys.. 65 (1989) 3914. 5 I). R. McKenzie, D. Muller, B. A. Pailthorpe, Z. H. Wang, 1:,. Kravtchinskaia, D. Segal. P. B. Lukins, P. D. Swift, P. J. Martin, G. Amaratunga, P. H. Gaskell and A. Saeed, Diamond and Related Materials, 1 II991l 51. 6 B. Dischler, in P. Koidl and P. Oelhafen (eds.), ,4morphous Hydro~,,em;ted Carbon Films, Les Editions de Physique. Paris, 1987, p. I{q9, 7 B. Dischler. A. Bubenzer and P. Koidl, Sol. State Comm.. 48 (1983) 105. 8 M. A. Petrich, Materials Science Fornmm, 52 (1989) 377. 9 (}. J. Vandentop. M. Kawasaki, K. Kobayashi and G. A. Somorjai, J. l~ae. S{i. Teehnol. A, 9 (1991) 1157. I0 J. M. F. Gunn, in R. J. Newport, B. D. Rainford and R. Cywinski (eds.), Neutron Scattering,, at a Pulsed Source, Adam Hilger, Bristol, 198S. p. 36. I1 J. Franks, Vaeumn, 34 {19841259. 12 ISIS ,4nnual Report. Rutherford Appleton Laboratory Report, RAL-90-041. 1990. 13 G. J. Kearley and J. Tomkinson, Inst. Phys. Conl. Set'., 107 (1990) 245. 14 J. Tomkinson, ira R. J. Newport, B. D. Rainford and R. Cywinski {eds.i, Neutron Seatterin~ at a Pulsed Source, Adam-Hilger, Bristol, 198~,, p. 324. 15 F. R. Dollish, W. G. Fately and F, F. Bentley,Characteristic Raman Frequem'ies, Wiley, New York, 1974. 16 M. Cardona, Phys. Star. Sol (hi, 11,"¢(1983) 463. 17 P. J. R. Honeybone, R. J. Newport, W. S. Howells, J. Tomkinson and C. Hotham. J. Non-Crystalline Solids, submitted. 18 P. Koidl, C. Wild, R. Locher and R. E. Sah, in R. E. Clausing, L. 1.. Horton, J. C. Angus and P. Koidl (eds.), Diamond and Diamondlike Fihns and Coatings, Plenum, New York, 1991, p. 243. 19 P. J. R. Honeybone, R. J. Newport, D. W. Huxley. W. S. Howells and J. Franks, ira preparation. 20 P. J. R. Honeybone, R. J. Newport, W. S. Howells, J. Tomkinson, S. B. Bennington and P. J. Revell, Chem. Phys. Letts., 180 {1991) (45.