Mater. Res. Soc. Symp. Proc. Vol. 851 © 2005 Materials Research Society
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Au Ion Induced Modification of C60 Thin Film Samples
Navdeep Bajwa1, 3, Alka Ingale2, 4, D.K. Avasthi3, Ravi Kumar3, A.Tripathi3 K. Dharamvir1, V.K.Jindal1∗, Michael Schmitt4, W.Kiefer4 1 Department of Physics, Panjab University, Chandigarh-160014, India 2 Laser Physics Division, Center For Advanced Technology, Indore-452013 India 3 Nuclear Science Center, Aruna Asaf Ali Marg, New Delhi-110067, India 4 Institut fuer Physikalische Chemie, Universitaet Wuerzburg, D-97074 Wuerzburg, Germany The present work reports phase transformations of thin films of C60 irradiated with 100 MeV 197Au8+ ions. This work is in continuation with our earlier work using 58Ni10+ and 16O6+ ions, to study the modification in C60 thin films. The study of C60 thin films using 197Au8+ ions, provides us enough additional data to investigate thoroughly the role of Se in causing phase transformations under different ion fluences. The Raman spectra indicate that swift heavy ion (SHI) irradiation results in several transformations of crystalline C60. At low fluences along with the fragmentation of C60 there is dimer/polymer formation. As fluence increases the dimer/polymer content first rises, optimizes, decreases and finally vanishes at very high fluences. At high fluences, all the C60 molecules as well as the polymer C60 break up, possibly resulting in nano-crystalline graphite embedded in amorphous carbon (a-C). INTRODUCTION The third form of carbon C60 (fullerene) has been a subject of detailed investigation since its discovery in 1985 [1]. These materials have been irradiated by various low and high energy ions to study the modifications taking place in these materials and their resulting applications. Researchers like Kastner et.al. [2] have reported transformation of C60 to amorphous carbon on irradiation by 30 keV K+ ions. Later, a detailed investigation done by Palmetshofer and Kastner [3] using keV ions showed polymerization of C60 at low fluences before it finally transforms to amorphous carbon. The intensity of polymerization was observed to increase at low fluences which reaches a maximum and finally decreases at high fluences. They also showed that amongst H, He, C and Ar (i.e., ions with increasing Se value), maximum of polymerization for H is at the highest fluence ~ 1016 ions/cm2 and is most intense followed by He, C and then Ar. Thus it can be concluded that for low energy ion irradiations (i.e., in keV energy range) the maximum of polymerization is strongly dependent on the Se value of the ion. Researchers like Itoh et. al. [4] have reported fragmentation of C60 induced by impacts of 2 MeV Si++ ions. However, recently Lotha et. al. [5] have reported polymerization of C60 prior to its transformation to a-C. A detailed ∗
Author with whom correspondence be made,
[email protected]
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work is needed to study the trend of the phase transformations taking place in thin films of C60 using SHI’s. The area of investigation using SHI has still remained incomplete. To study the modifications in C60 thin films using SHI, some work has been already performed by us [6, 7] using 58Ni10+ and 16O6+ ions. The present work is in continuation of our earlier work to investigate the effect of SHI. With this detailed investigation, results of phase transformation in C60 thin films in all energy ranges are now available. In this paper we report the modifications of C60 on irradiation by 100 MeV 197 Au8+ ions investigated using Raman spectroscopy. EXPERIMENTAL DETAILS Thin films of C60 (~ 230 nm) were deposited on Si substrate using resistive heating method. The deposition was performed in high vacuum (2×10-6 Torr) environment by sublimation of pellets at a rate of ~ 0.1 nm/sec and passing a current of ~ 75A in a Ta boat. The thickness of the films was kept very small compared to the range of the ions used. These films were irradiated using 15UD Pelletron accelerator at NSC. The Se and Sn values for 100 MeV 197Au8+ ions acquired using the code SRIM 2000 (Ziegler et. al. [8, 9]) were 1.292× 103 eV/Å and 1.895x101 eV/Å respectively. Since the Se value is 2 orders of magnitude higher than the Sn value, the electronic energy loss is dominant for the irradiation of 197Au8+ ion at 100 MeV. Raman spectra of the pristine films and those irradiated with 197Au8+ ions were measured using LABRAM Raman spectrometer at Universität Würzburg, Germany, in a range of 750 – 2150 cm-1. MicroRaman data was recorded at room temperature with Ar ion laser excitation, 514.5 nm (10X objective). Power and exposure time was adjusted so as to avoid any phototransformed polymer peak. We have also made investigations on pristine and 197Au8+ ion irradiated thin films of C60 using optical absorption spectroscopy and in-situ conductivity, details of which will be published separately. DISCUSSION Raman spectra of pristine and 197Au8+ ion irradiated C60 film is shown in Fig. 1. The Raman results indicate that on irradiation all the modes of C60 decrease in intensity with increasing fluence including the most intense mode, the Ag mode at ~ 1468 cm-1. All the three modes broaden progressively with increase in fluence. The region around the prominent Ag mode of C60 i.e., around 1468 cm-1 (shown in Fig. 2), was monitored carefully and it was observed that it becomes asymmetric on swift Au ion irradiation and a new peak appears at 1458 cm-1. This feature has been identified as the dimer/polymer [5, 6, 10] mode of C60. This feature was observed at low fluences which maximizes and finally vanishes at very high fluences (see Fig. 2). Another new feature which arises due to SHI irradiation is the peak around 1447 cm-1. Yogo et. al. [10] have also made a detailed analysis of the peaks around the Ag mode. They conclude that the region around 1444cm-1 (in our case, observed at 1447cm-1) may correspond to tetragonal plains. This
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feature is also observed to increase with increase in fluence, maximize and finally vanish at high fluences (see Fig. 2).
Raman Intensity (arb. units)
P r is tin e
1150
1350
3x10
10
io n s /c m
2
1x10
11
io n s /c m
2
3x10
11
io n s /c m
2
9x10
11
io n s /c m
2
2x10
12
io n s /c m
2
4x10
12
io n s /c m
2
1x10
13
io n s /c m
2
1550
1750 -1
R a m a n S h if t ( c m )
Figure 1. Raman spectra of pristine and 197Au8+ ion irradiated thin film samples of C60 on Si substrate at various fluences. P r is tin e 1 3 0 0
C
6 0
7 0 0 1 0 0 6 0 0
3 x 1 0
1 0
io n s /c m
2
io n s /c m
2
Raman Intensity
1 0 0 5 0 0
3 x 1 0
1 1
2 0 0 5 0 0
9 x 1 0
1 1
2
io n s /c m
2 0 0 1 4 4 0
1 4 6 0 1 4 8 0 R a m a n S h ift (c m
-1
1 5 0 0 )
Figure 2. A plot showing region around the pentagonal pinch mode of C60 at 1468 cm-1 at various fluences on irradiation of swift 197Au8+ ions on thin films of C60 (deposited on Si substrate). The figure shows the rise and fall of the polymer phases of C60 at 1458 cm-1 and 1447 cm-1.
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At high fluences C60 and all the polymer modes of C60 vanish and broad features around 1380 cm-1 and at 1566 cm-1 are observed which are identified as the D and G peaks [11], characteristic of amorphous carbon (a-C). The broad G peak observed at high fluences for Au ion irradiation covers a whole range from 1500 cm-1 to 1650 cm-1 including the position of the characteristic bulk graphite peak appearing in a-C at ~ 1560 cm-1. Further, along with these other new features that are observed at high fluences were one around 1090 cm-1, 1230 cm-1 and 1455 cm-1 (see Fig. 3). The peak around 1090 cm-1 and 1230 cm-1 have been attributed to sp3 rich carbon network [6, 12, 13]. The origin of the peak around 1455 cm-1 has been reported as due to SiC formation [14].
1 0 0 M e V , A u (4 x 1 0
1 2
2
io n s /c m
)
100
0 Raman Intensity
1 0 0 M e V , A u (6 x 1 0
1 2
io n s /c m
2
))
100
0 200
1 0 0 M e V , A u (1 x 1 0
1 3
io n s /c m
2
)
100 980
1130 1280 1430 -1 R a m a n S h if t ( c m )
1580
Figure 3. Plots showing emergence of new features around 1090 cm-1, 1230 cm-1, 1380 cm-1 1450 cm-1 and 1566 cm-1 at high fluences for 197Au8+ ion irradiated C60 films on Si substrate. Plots in Fig. 4, show data for damage of thin films of C60 with increasing fluence on irradiation of 16O6+ and 58Ni10+ ions along with the present work of 197Au8+ ion irradiation on thin films of C60. The data shows that the decrease of the Ag mode of C60 is fastest for 197Au8+ ion with increasing fluence. We also observe that damage of C60 is maximum by the 197Au8+ ion followed by 58Ni10+ ion (Se = 7.3x102 eV/Å) and 16O6+ ion (Se = 81 eV/Å). Thus the phase transformations taking place at different fluence values for different ions used is strongly correlated with the Se value of the ion. For all the ion irradiations (using 16O6+ , 58Ni10+ and 197Au8+ ions) the destruction of C60 and the emergence of various phases of C60 are two competing processes. It has been observed that the ratio of the number of C60 molecules in the unirradiated film (denoted by No) to the number of C60 molecules in the film after a fluence ϕ (denoted by N) is well represented by a superposition of two exponential decays given by the equation
N / N 0 ≈ (1 − A )e−σ 1φ + Ae−σ Pφ
(1)
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This equation has been derived on the basis of phenomenological equations governing modification of C60 thin films with fluence, around the ion path. The details of this analysis will be published separately. A comparative study of the modifications in C60 thin films due to irradiation of ions (16O6+, 58Ni10+ and 197Au8+) having successively higher Se indicate that the vanishing of C60, vanishing of band gap, onset of a-C, rise in conductivity, as well as maximization and vanishing of polymer all take place at successively lower fluence (see Fig. 5). The results of optical absorption spectroscopy and conductivity measurements are consistent with those of Raman measurements. The onset of conductivity almost coincides with the appearance of a-C. Similarly, closing of band gap coincides with the total damage of C60. O; F lo a
1 .0
0
N/N
t g la s
s sub s tra te Si s ubs trat e N i; F lo a t g las s s u b s tr a te
N i;
0 .5
A u ; S i s u b s t r a te
0 .0 0
10
11
10
12
10
13
10
14
2
F lu e n c e ( io n s /c m )
Figure 4. Plot showing the content of C60 (area under the prominent peak of Ag mode of C60) in 197Au8+, 58Ni10+ and 16O6+ ion irradiated thin film sample of C60 with increase in fluence. Note: The lines are drawn for a guide for the eye. 18
10
17
10
16
10
15
10
14
10
13
10
12
10
11
Ni
Au
2
Fluence (ions/cm )
H O
10
V an is hi
D am a ge of C 60
ng of
On s co n et o f duc t iv
ol ym er
O ns et o f aC M ax
0
C los ing of B a nd G ap
P it y
im a
of P o ly m
er
4 00 80 0 12 00 S e (e V /An g s tro m )
Figure 5. Plot showing fluence values for the vanishing of C60 , vanishing of band gap, onset of conductivity, appearance of polymer, its maximization and disappearance, for the four ions (H, 16O6+, 58Ni10+ and 197Au8+) having successively increasing Se values. The data of Hydrogen has been read from reference 10. For 16O6+ ion, those fluence values shown in this plot that lie above 1014 ions/cm2 have been extrapolated from data.
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CONCLUSION The present work supplements the uncovered high energy range data (large Se part) thus providing a whole range of Se to see the role of ion energies in causing phase transformations of C60. This will help in consolidating an overview of the effect of ion induced modifications in C60 thin films. It has been observed that swift heavy ion irradiation on C60 thin films result in aggregation of C60 molecules at low fluences. High fluences result in complete destruction of C60 and polymer C60 and their transformation to amorphous carbon. SiC has also been observed to be formed at the interface. Overall damage of C60 material as a function of fluence can be fitted as a sum of two exponentials with two different cross-sections, one at low fluences and the other at high fluences. A comparison of the results of Raman spectroscopy with the previous work using 58Ni10+ and 16O6+ ions indicate that the various phase transformations taking place at different fluence values for different ions used is strongly correlated with the Se value of the ion. Lower the Se, larger the range of fluence for which C60 persists. It seems that the transformation of C60 to polymerized C60 as well as amorphous carbon is dependent mainly on the total energy deposited in the film. The critical value of energy is responsible for onset and vanishing of polymerized C60. REFERENCES 1. Original information can be obtained at this site : http://www.nobel.se/chemistry/laureates/1996/press.html 2. J.Kastner, H.Kuzmany, L.Palmetshofer, P.Bauer, G.Stingeder, Nucl. Instrum. Methods Phys. Res. B 80/81, 1456 (1993). 3. L.Palmetshofer and J.Kastner, Nucl. Instrum. Methods Phys. Res. B 96, 343 (1995). 4. A.Itoh, H. Tsuchida, K.Miyabe, M. Imai, B.Imanishim Nucl. Instrum. Methods Phys. Res. B 129, 363 (1997). 5. S.Lotha, A.Ingale, D.K.Avasthi, V.K.Mittal, S.Mishra, K.C.Rustagi, A.Gupta, V.N.Kulkarni and D.T.Khathing, Solid State Communications 111, 55 (1999). 6. Navdeep Bajwa, Alka Ingale, D.K.Avasthi, Ravi Kumar, K.Dharamvir and V.K.Jindal J. Appl. Phys. 94, 326 (2003) and references therein. 7. N.Bajwa, A.Ingale, D.K.Avasthi, R.Kumar, A.Tripathi, K.Dharamvir, V.K.Jindal Radiation measurements 36, 737 (2003). 8. J.F.Ziegler,SRIM2000.39, http://www.research.ibm.com/ionbeams/home.htm#SRIM. 9. J.F.Ziegler, in: J.F.Ziegler, J.P.Biersack and U.Littmark (Eds.), The Stopping and Range of Ions in Matter, Vol. I, Pergamon Press, New York, 1985. 10. A.Yogo, T.Majima and A.Itoh, Nucl. Instrum. Methods Phys. Res. B 193, 299 (2002). 11. A.C.Ferrari and J.Robertson, Phys. Rev. B 64, 075414 (2001) and references therein. 12. J.Schwan, S.Ulrich, V.Batori, H.Ehrhardt and S.R.P.Silva, J. Appl. Phys. 80(1), 440 (1996) and references therein. 13. T.Meyuro, A.Hida, Y.Koguchi, S.Miyamoto, Y.Yamamoto, H.Takai, K.Maeda, Y.Aoyagi, Nucl. Instrum. Methods Phys. Res. B 209, 170 (2003). 14. S.K.Srivastava, D.Kabiraj, B.Schattat, H.D.Carstanjen, D.K.Avasthi, Nucl. Instrum. Methods Phys. Res. B 219-220, 815 (2004).
