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Ion irradiation induced surface modification studies of polymers using SPM
Nuclear Instruments and Methods in Physics Research B 236 (2005) 186–194 www.elsevier.com/locate/nimb
Ion irradiation induced surface modification studies of polymers using SPM A. Tripathi a
a,*
, Amit Kumar a, F. Singh a, D. Kabiraj a, D.K. Avasthi a, J.C. Pivin b
Nuclear Science Centre, P.O. Box 10502, Aruna Asaf Ali Marg, New Delhi 110067, India b CSNSM, IN2 P3-CNRS, Batiment 108, F-91405 Orsay Campus, France Available online 16 June 2005
Abstract Various types of scanning probe microscopy (SPM) techniques: atomic force microscopy (AFM) (contact and tapping in height and amplitude mode), scanning tunnelling microscopy (STM) and conducting atomic force microscopy (C-AFM) are used for studying ion beam induced surface modifications, nanostructure/cluster formation and disintegration in polymers and similar soft carbon based materials. In the present study, the results of studies on four materials, namely, (A) methyltriethoxysilane/phenyltriethoxysilane (MTES/PTES) based gel, (B) triethoxisilane (TH) based gel, (C) highly oriented pyrolytic graphite (HOPG) bulk and (D) fullerene (C60) thin films are discussed. In the case of Si based gels prepared from pre-cursors containing organic groups (MTES/PTES), hillocks are observed at the surface and their size decreases from 70 to 25 nm with increasing fluence, whereas, in the case of a gel with a stoichiometry SiO1.25H1, prepared from TH, an increases in the size of hillocks is observed. Hillocks are also formed at the surface of HOPG irradiated with 120 MeV Au beam at a low fluence, whereas, formation of craters and a re-organisation of surface features is observed at a higher fluence. In the case of C60 films, 120 MeV Au ion irradiation induces the formation of conducting ion tracks, which is attributed to the transformation from insulating C60 to conducting graphite like carbon. 2005 Elsevier B.V. All rights reserved. PACS: 68.37. d; 61.82.Pv; 79.20; 6180.Jh Keywords: SPM; Polymers; HOPG; Fullerene; Swift heavy ions
1. Introduction *
Corresponding author. Tel.: +91 11 26893955; fax: +91 11 26893666. E-mail address: [email protected] (A. Tripathi).
Creation of swift heavy ion (SHI) induced latent tracks and structural changes in these tracks constitute topics of active research in polymers.
0168-583X/$ - see front matter 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2005.04.059
A. Tripathi et al. / Nucl. Instr. and Meth. in Phys. Res. B 236 (2005) 186–194
The correlation between the track dimensions and linear density of electronic energy loss has been extensively studies by many authors [1–3]. Fabrication of porous membranes by track etching for their industrial applications as filters, collimators, channel plates and templates of nano- and microwires is also studied by Ferain and Legras [4], Toimil Molares et al. [5] and others [6–8]. Swift heavy ion impacts on polymer surface result in the formation of nanometer sized craters and crater rims [9–11]. The study of these structures is interesting, as they are one of the important phenomena taking place in early stages of track formation, and their study gives information about the diameter of latent tracks in the bulk of the material. Most of the previous studies on polymers have been performed using SEM and very few by means of scanning probe microscopy [12]. The present paper reports preliminary results of the application of SPM techniques to the study of modifications induced by ion beams in polymer and soft carbon based surfaces. Various forms of scanning probe microscopy (SPM) are used depending on the nature of expected transformations: atomic force microscopy (AFM) (contact and tapping in height and amplitude mode), scanning tunnelling microscopy (STM) and conducting atomic force microscopy (C-AFM). The surface modifications induced by swift heavy ions (SHI) in various carbon allotropes, such as highly oriented pyrolytic graphite (HOPG), amorphous carbon (a-C), diamond-like carbon (DLC) etc. have been previously widely studied using surface/near surface tools such as scanning tunneling microscope (STM) and atomic force microscope (AFM) [13–18]. Biro et al. have extensively studied SHI irradiation of HOPG and showed that irradiation at MeV energies induces the formation of tracks, craters and carbon nanotubes (CNTs) by Ne, Kr and U ions [14]. Liu et al. have extensively studied formation of nanometer dimension tracks due to MeV Ar [15] and MeV to GeV Ni, Zn, Xe and U ions [16]. In an earlier study performed in our laboratory, HOPG samples irradiated with 200 MeV Au beam were studied in situ with UHV STM [17]. The atomic structure of the surface was clearly resolved before irradiation, and in situ observations permitted to
187
assess the increase of damage during irradiation. Spectroscopic measurements of the electrical characteristics (1-V) showed a change from a semiconducting to an ohmic behaviour, indicating a local increase in the population of sp2 bonds [18]. There have been few studies on the irradiation effects of low and high energy ion beams on fullerenes [19–27]. LeBrun et al. studied the 420–625 MeV Xe ion induced ionisation and fragmentation of C60 [23]. The irradiation by 189 MeV Ag, 110 MeV N and 50 MeV Si seems to result in polymerisation and damaging of C60 film [24– 27]. However, in these studies, the surface morphology and transverse conductivity on the nanometer scale have not been investigated using SPM techniques. Krauser et al. [28] have recently shown evidence of the formation of narrow conducting ion tracks due to SHI impact in diamond-like carbon (DLC) film with conducting AFM. In the present work, we have studied the formation of conducting ion tracks in C60 films bv conducting AFM.
2. Experimental 2.1. MTES–PTES and TH gels A mixture of methyltriethoxysilane (MTES) and phenlyriethoxysilane (PTES) was used for synthesizing a gel of composition Si1C3.5O1.5H5, constituted of Si-O chains more or less branched and of CH3C6H5 side groups. Another gel containing no organic group was prepared from triethoxysilane (TH). Gel films (500 nm) were spun on Si wafers. MTES–PTES films were irradiated with 900 keV Xe ions (electronic stopping power Se = 0.7 keV/nm and nuclear stopping power Sn = 1.7 keV/nm) using the low-energy ion beam facility at NSC, at fluences varying from 2 · 1013 ions/cm2 to 1 · 1015 ions/cm2. TH films were irradiated by 120 MeV Au ions produced by the 15UD Pelletron accelerator of NSC at a fluence of 1 · 1013 ions/cm2. The surface morphology of these samples was studied with a Veeco Digital Nanoscope IIIa SPM at NSC. Since the polymers are soft materials, tapping mode was used with RTESP tips and a low force constant.
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2.2. HOPG The HOPG samples used in the study were obtained from SPI, USA. The HOPG samples were of type ZYH with typical grain size of approximately 40 nm. The samples were irradiated with 120 MeV Au ions (Se = 18.2 keV/nm). A fresh surface was prepared by cleaving the samples before irradiation. HOPG being a semi-metal with a conducting surface, the samples were studied in STM mode. Pt/Ir tip was used for scanning the samples. For the measurement the bias current was kept at 500 mV whereas a preamplifier gain of 1 · 108 V/A was selected. To obtain the images with atomic resolution a 400-nm scanner was used. 2.3. Fullerene Fullerene thin films were deposited on Si(1 0 0) substrate in a vacuum of 1 · 10 6 Torr by resistive heating of commercially available 99.9% pure C60 in a Ta boat. The thickness of the film, as measured by a quartz crystal thickness monitor, was 150 nm. Since Si substrate does not provide a perfect contact for conducting AFM measurement, a thin layer of Au was first deposited on the Si substrate to provide the contact. The arrangement allowed the measurement of conductivity across the C60 layer. The samples were irradiated with a 120 MeV Au ion beam (Se = 18.2 keV/nm, Sn = 215 eV/nm) from the NSC Pelletron with fluences of 2 · 1010, 6 · 1010 and 2 · 1011 ions/cm2. Results
concerning samples irradiated with higher fluence are not discussed here so as to emphasize single ion induced effects. The properties of these channels were studied by means of conducting atomic force microscopy (C-AFM) using a diamond coated silicon nitride (DDESP) tip.
3. Results and discussion 3.1. MTES–PTES and TH films The PL and Raman studies of these samples are reported elsewhere in this conference, and only the AFM results are discussed here. A typical AFM image in height mode is shown in Fig. 1(A). Since the polymers are soft materials, the images were recorded in the tapping mode with a low force constant. The images recorded with amplitude mode (Fig. 1(B)) exhibit a better contrast facilitating the observation of small features. The typical rounded asperities at the surface of an unirradiated sample have a size of about 70 nm and the root mean square (rms) surface roughness is of the order of 0.9 nm. Fig. 2(A) and (B) show the surface relief after irradiation with 900 keV Xe ions at a fluence of 5 · 1013 and 1 · 1015 ions/cm2 respectively. The dimension of the surface features, decreases to 60 and 25 nm whereas the rms surface roughness increases to 3.4 and 9.6 nm as shown in Table 1. In contrast to that, an increase in the size of surface features, from 18 to 30 nm, is observed
Table 1 Summary of the study on MTES/PTES and TH films Sample
Beam and energy
Fluence (ions/cm2)
Typical grain size
Sample roughness
MT11
500 keV Xe
Unirradiated 2 · 1013 1 · 1015
80 nm 60 nm 25 nm
0.9 nm 3.4 nm 9.0 nm
TH
120 MeV Au
Unirradiated 2 · 1013
18 nm 30 nm
0.6 nm 1.3 nm
Table 2 Summary of the study on HOPG Sample
Beam and energy
Fluence
Se
Sn
Features
HOPG Type ZYH
120 MeV Au
1 · 1011 ions/cm2 1 · 1013 ions/cm2
18.2 keV/nm 18.2 keV/nm
215 eV/nm 215 eV/nm
Hillock diameter 3.2 nm Crater formation, surface reconstruction
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Fig. 1. (A) AFM image (400 nm · 400 nm) of MTES/PTES sample in height mode (unirradiated sample). (B) AFM image (400 nm · 400 nm) of MTES/PTES sample shown in (A) in amplitude mode. Amplitude mode image shows better contrast for surface features.
on TH samples irradiated with swift heavy ions (Fig. 3(A) and (B)), together with an increase in surface roughness from 0.6 to 1.3 nm. These preliminary results can be explained as follows. Gels are made of polymer-like islands floating in a residual solvent, and the preliminary stage of their conversion into glasses involves the condensation of these islands. Thus, there are good reasons to believe that the observed structures at the surface of pristine films correspond
to these islands and that they become more interconnected in TH films submitted to electronic excitations. Two reasons may be invoked for explaining the fragmentation of these islands in the MTES–PTES samples submitted to Xe irradiation. First, the gels with organic side groups are converted into composite ceramics made of silica containing C:H clusters. Secondly, nuclear collisions are known to promote chain scissions in contrast to electronic excitations. The observation of
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Fig. 2. (A) AFM image (200 nm · 200 nm) of MTES/PTES sample after irradiation with 500 keV Xe ions with a fluence of 5 · 1013 ions/cm2. (B) AFM image (200 nm · 200 nm) of MTES/PTES sample after irradiation with 500 keV Xe ions with a fluence of 1 · 1015 ions/cm2. A decrease in grain size with fluence can be seen.
similar samples irradiated with swift heavy ions should permit to discriminate between the two possible origins of the structure subdivision. 3.2. HOPG The STM study of an unirradiated HOPG sample showed atomically resolved images. The samples irradiated with 120 MeV Au ions at a low fluence (1 · 1011 ions/cm2) show the formation of hillocks with a mean diameter of about 3 nm (Fig. 4(A)). With increasing fluence
(1 · 1012 ions/cm2 and 2 · 1013 ions/cm2) craters with a diameters of 2–4 nm are progressively formed (Fig. 4(B)) and a reconstruction of the surface is seen by STM. Section diagram of one of the craters showing typical dimension is shown in Fig. 4(C). From these results it seems that at low values of electronic energy transfer a swelling of the surface occurs locally, which may be attributed to amorphisation of the material in isolated tracks. The formation of craters surrounded by a rim may be explained by the increase of sputtering and a partial re-deposition of sputtered atoms with a
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Fig. 3. (A) AFM image (200 nm · 200 nm) of unirradiated TH sample. Spherical features with typical diameter of 18 nm are seen. (B) AFM image (200 nm · 200 nm) of TH sample irradiated with 100 MeV Au ions (1 · 1013 ions/cm2). Elongated features with typical diameter of 30 nm on surface are seen.
low kinetic energy or the surface diffusion of atoms from the transient disordered phase. The results of study are summarized in Table 2. 3.3. Fullerene In our earlier study with AFM, we have shown that irradiation with 110 MeV Ni ion beam at fluences of 1 · 1012–1 · 1014 ions/cm2 results in a decrease of grain size from 120 nm to 20 nm [24]. It seems reasonable to ascribe this result to the disintegration of the C60 units or their amorphisation/transformation into nanometer sized graphite particles. In the present work, the images were recorded in conducting mode using a diamond coated DDESP tips. In the case of unirradiated sample, no current is observed for bias of up to
10 V, confirming the semi-conducting nature of the fullerene film. In contrast to that, a current of the order of 5 nA was observed for a bias of 500 mV between the tip and Au underlayer after irradiation of the samples. A typical 3-D map of conducting paths, in a sample irradiated with 120 MeV Au ions at a fluence of 2 · 1010 ions/ cm2 is shown in Fig. 5. The conducting sites are characterised by local increase of current. The increase in conductivity is ascribed to the transformation of semi-conducting C60 into conducting graphite like carbon under the effect of electronic energy loss. The number of conducting zones does not match with the fluence and instead a ratio of 0.4 zone per ion is observed. Some of the tracks appear more conducting than others. Given that tracks cannot overlap for the fluence used by us,
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Fig. 4. (A) STM image of HOPG sample (50 nm · 50 nm) irradiated with 120 MeV Au ions with a fluence of 1 · 1011 ions/cm2. A single ion impact induced hillock can be seen. (B) STM image of HOPG sample (20 nm · 20 nm) irradiated with 120 MeV Au ions with a fluence of 2 · 1013 ions/cm2. Formation of craters and reconstruction on surface is seen. (C) STM image of HOPG sample irradiated with 120 MeV Au ions with a fluence of 2 · 1013 ions/cm2. Section diagram of one of the craters showing typical dimension (approximately 4.2 nm) is shown in figure.
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from insulating C60 into conducting graphite like carbon induced by electronic energy loss.
Acknowledgments Two of the authors (D.K.A. and J.C.P.) are thankful to IFCPAR, New Delhi for financial support. The authors are also thankful to DST, New Delhi for providing support for procuring the scanning probe microscope (SPM). Fig. 5. Current image (1 lm · 1 lm) of fullerene film irradiated with 100 MeV Au ions with a fluence of 1 · 1012 ions/cm2. The three dimensional image obtained using Conducting AFM shows zones of high conductivity, representing the ion impact sites.
these two effects are probably due to the probing of several isolated tracks simultaneously by the tip. Besides this, amorphisation around the ion path leads to increased conductivity and hence the current image has a much larger diameter as compared to that of ion track.
4. Conclusion Methyltriethoxysilane, phenyltriethoxysilane films irradiated with 900 keV Xe beam at fluences varying from 5 · 1012 ions/cm2 to 1 · 1015 ions/ cm2 exhibit surface features of size decreasing from 70 nm to 25 nm. In contrast to this result, irradiation with swift heavy ions seems to induce a coalescence of islands constituting the gel prepared from triethoxysilane and the island size increases from 18 to 30 nm after irradiation with a 100 MeV Au beam (1 · 1013 ions/cm2). The STM images of HOPG graphite irradiated with a 120 MeV Au beam show the formation of hillocks/craters (diameter 2–3 nm) at lower fluence, whereas crater formation and re-organisation of surface is observed at higher fluence (up to 2 · 1013 ions/cm2). The images recorded in conducting mode of C60 films irradiated with 120 MeV Au ions show the formation of conducting ion tracks. The increase in conductivity is attributed to the transformation
References [1] A. Alda, V. Buschmann, H. Fuess, C. Trautmann, Nucl. Instr. and Meth. B 185 (2001) 210. [2] M. Dobeli, F. Ames, C.R. Musel, L. Scandella, M. Suta, Nucl. Instr. and Meth. B 143 (1998) 503. [3] C. Trautmann, K. Schwartz, T. Steckeneiter, Nucl. Instr. and Meth. B 156 (1999) 162. [4] E. Ferain, R. Legras, Nucl. Instr. and Meth. B 208 (2003) 115. [5] M.E. Toimil Molares, J. Brotz, V. Buschmann, D. Dobrev, R. Neumann, R. Scholz, I.U. Schuchert, C. Trautmann, J. Vetter, Nucl. Instr. and Meth. B 185 (2001) 192. [6] R. Spohr, in: K. Bethge (Ed.), Ion Tracks and Microtechnology, Basic Principles and Applications, Vieweg, 1990. [7] D. Fink, R. Klett, Braz. J. Phys. 25 (1995) 54. [8] A. Biswas, D.K. Avasthi, Benoy K. Singh, S. Lotha, J.P. Singh, D. Pick, B.K. Yadav, B. Battacharya, S.K. Bose, Nucl. Instr. and Meth. B 151 (1999) 84. [9] R.M. Papaleo, L.S. Farenzena, M.A. De Araujo, R.P. Livi, M. Alurralde, G. Bermudez, Nucl. Instr. and Meth. B 148 (1999) 126. [10] L.S. Farenzena, R.P. Livi, M.A. De Araujo, G.G. Bermudez, R.M. Papaleo, Phys. Rev. B 63 (2001) 104108. [11] F. Ohnesorge, R. Neumann, Europhys. Lett. 50 (2000) 742. [12] R.M. Papaleo et al., Nucl. Instr. and Meth. B 191 (2002) 674. [13] L.T. Chadderton, D. Fick, Y. Gamaly, H. Moeckel, L. Wang, H. Omichi, F. Hosoi, Nucl. Instr. and Meth. B 91 (1994) 71. [14] L.P. Biro, B.S. Szabo, G.I. Mark, J. Gyulai, K. Havancsak, J. Kurti, A. Dunlop, L. Frey, H. Ryssel, Nucl. Instr. and Meth. B 148 (1999) 1102, and references therein, namely: L.P. Biro et al., Nucl. Instr. and Meth. B 112 (1996) 270; Nucl. Instr. and Meth. B 122 (1997) 476; Nucl. Instr. and Meth. B 147 (1999) 142. [15] J. Liu, M.D. Hou, C.L. Liu, Z.G. Wang, Y.F. Jin, P.J. Zhai, S.L. Feng, Y. Zhang, Nucl. Instr. and Meth. B 146 (1998) 356.
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[16] J. Liu, R. Neumann, C. Trautmann, C. Muller, Phys. Rev. B 64 (2001) 184115. [17] A. Tripathi, J.P. Singh, R. Ahuja, R.N. Dutt, D. Kanjilal, Ayan Guha, Amlan Biswas, A.K. Roychaudhari, Rev. Sci. Instr. 72 (2001) 3884. [18] J.P. Singh, A. Tripathi, D. Kanjilal, Vacuum 57 (2000) 319. [19] A. Itoh, H. Tsuchida, K. Miyabe, M. Imai, B. Imanishim, Nucl. Instr. and Meth. B 129 (1997) 363. [20] K.L. Naraynan, M. Yamaguchi, N. Dharmarasu, N. Kojima, D. Kanjilal, Nucl. Instr. and Meth. B 178 (2001) 301. [21] C.E. Foerster, F.C. Serbena, M. Lepienski, D.L. Baltista, F.C. Zawislak, Nucl. Instr. and Meth. B 148 (1999) 634. [22] F.C. Zawislak, D.L. Baptista, M. Behar, D. Fink, P.L. Grande, J.A.H. da Jornada, Nucl. Instr. and Meth. B 149 (1999) 336.
[23] T. LeBrun, H.G. Berry, S. Cheng, R.W. Dunford, H. Esbensen, D.S. Gemmell, E.P. Kanter, Phys. Rev. Lett. 72 (1994) 3965. [24] S. Lotha, A. Ingale, D.K. Avasthi, V.K. Mittal, S. Mishra, K.C. Rustagi, A. Gupta, V.N. Kulkarni, D.T. Khathing, et al., Solid State Commun. 111 (1999) 55. [25] Navdeep Bajwa, Alka Ingale, D.K. Avasthi, Ravi Kumar, A. Tripathi, K. Dharamvir, V.K. Jindal, J. Appl. Phys. 94 (2003) 326. [26] S. Ghosh, D.K. Avasthi, A. Tripathi, S.K. Srivastavta, S.V.S. Nageswara Rao, T. Som, V.K. Mittal, F. Fruner, W. Assman, Nucl. Instr. and Meth. B 190 (2002) 169; Nucl. Instr. and Meth. B 212 (2003) 431. [27] A. Tripathi et al., in: Proceedings of DAE Solid State Physics Symposium, Gwalior, India, 2003. [28] J. Krauser, J.H. Zollondz, Weidinger, C. Trautmann, J. Appl. Phys. 94 (2003) 1959.
Ion irradiation induced surface modification studies of polymers using SPM A. Tripathi a
a,*
, Amit Kumar a, F. Singh a, D. Kabiraj a, D.K. Avasthi a, J.C. Pivin b
Nuclear Science Centre, P.O. Box 10502, Aruna Asaf Ali Marg, New Delhi 110067, India b CSNSM, IN2 P3-CNRS, Batiment 108, F-91405 Orsay Campus, France Available online 16 June 2005
Abstract Various types of scanning probe microscopy (SPM) techniques: atomic force microscopy (AFM) (contact and tapping in height and amplitude mode), scanning tunnelling microscopy (STM) and conducting atomic force microscopy (C-AFM) are used for studying ion beam induced surface modifications, nanostructure/cluster formation and disintegration in polymers and similar soft carbon based materials. In the present study, the results of studies on four materials, namely, (A) methyltriethoxysilane/phenyltriethoxysilane (MTES/PTES) based gel, (B) triethoxisilane (TH) based gel, (C) highly oriented pyrolytic graphite (HOPG) bulk and (D) fullerene (C60) thin films are discussed. In the case of Si based gels prepared from pre-cursors containing organic groups (MTES/PTES), hillocks are observed at the surface and their size decreases from 70 to 25 nm with increasing fluence, whereas, in the case of a gel with a stoichiometry SiO1.25H1, prepared from TH, an increases in the size of hillocks is observed. Hillocks are also formed at the surface of HOPG irradiated with 120 MeV Au beam at a low fluence, whereas, formation of craters and a re-organisation of surface features is observed at a higher fluence. In the case of C60 films, 120 MeV Au ion irradiation induces the formation of conducting ion tracks, which is attributed to the transformation from insulating C60 to conducting graphite like carbon. 2005 Elsevier B.V. All rights reserved. PACS: 68.37. d; 61.82.Pv; 79.20; 6180.Jh Keywords: SPM; Polymers; HOPG; Fullerene; Swift heavy ions
1. Introduction *
Corresponding author. Tel.: +91 11 26893955; fax: +91 11 26893666. E-mail address: [email protected] (A. Tripathi).
Creation of swift heavy ion (SHI) induced latent tracks and structural changes in these tracks constitute topics of active research in polymers.
0168-583X/$ - see front matter 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2005.04.059
A. Tripathi et al. / Nucl. Instr. and Meth. in Phys. Res. B 236 (2005) 186–194
The correlation between the track dimensions and linear density of electronic energy loss has been extensively studies by many authors [1–3]. Fabrication of porous membranes by track etching for their industrial applications as filters, collimators, channel plates and templates of nano- and microwires is also studied by Ferain and Legras [4], Toimil Molares et al. [5] and others [6–8]. Swift heavy ion impacts on polymer surface result in the formation of nanometer sized craters and crater rims [9–11]. The study of these structures is interesting, as they are one of the important phenomena taking place in early stages of track formation, and their study gives information about the diameter of latent tracks in the bulk of the material. Most of the previous studies on polymers have been performed using SEM and very few by means of scanning probe microscopy [12]. The present paper reports preliminary results of the application of SPM techniques to the study of modifications induced by ion beams in polymer and soft carbon based surfaces. Various forms of scanning probe microscopy (SPM) are used depending on the nature of expected transformations: atomic force microscopy (AFM) (contact and tapping in height and amplitude mode), scanning tunnelling microscopy (STM) and conducting atomic force microscopy (C-AFM). The surface modifications induced by swift heavy ions (SHI) in various carbon allotropes, such as highly oriented pyrolytic graphite (HOPG), amorphous carbon (a-C), diamond-like carbon (DLC) etc. have been previously widely studied using surface/near surface tools such as scanning tunneling microscope (STM) and atomic force microscope (AFM) [13–18]. Biro et al. have extensively studied SHI irradiation of HOPG and showed that irradiation at MeV energies induces the formation of tracks, craters and carbon nanotubes (CNTs) by Ne, Kr and U ions [14]. Liu et al. have extensively studied formation of nanometer dimension tracks due to MeV Ar [15] and MeV to GeV Ni, Zn, Xe and U ions [16]. In an earlier study performed in our laboratory, HOPG samples irradiated with 200 MeV Au beam were studied in situ with UHV STM [17]. The atomic structure of the surface was clearly resolved before irradiation, and in situ observations permitted to
187
assess the increase of damage during irradiation. Spectroscopic measurements of the electrical characteristics (1-V) showed a change from a semiconducting to an ohmic behaviour, indicating a local increase in the population of sp2 bonds [18]. There have been few studies on the irradiation effects of low and high energy ion beams on fullerenes [19–27]. LeBrun et al. studied the 420–625 MeV Xe ion induced ionisation and fragmentation of C60 [23]. The irradiation by 189 MeV Ag, 110 MeV N and 50 MeV Si seems to result in polymerisation and damaging of C60 film [24– 27]. However, in these studies, the surface morphology and transverse conductivity on the nanometer scale have not been investigated using SPM techniques. Krauser et al. [28] have recently shown evidence of the formation of narrow conducting ion tracks due to SHI impact in diamond-like carbon (DLC) film with conducting AFM. In the present work, we have studied the formation of conducting ion tracks in C60 films bv conducting AFM.
2. Experimental 2.1. MTES–PTES and TH gels A mixture of methyltriethoxysilane (MTES) and phenlyriethoxysilane (PTES) was used for synthesizing a gel of composition Si1C3.5O1.5H5, constituted of Si-O chains more or less branched and of CH3C6H5 side groups. Another gel containing no organic group was prepared from triethoxysilane (TH). Gel films (500 nm) were spun on Si wafers. MTES–PTES films were irradiated with 900 keV Xe ions (electronic stopping power Se = 0.7 keV/nm and nuclear stopping power Sn = 1.7 keV/nm) using the low-energy ion beam facility at NSC, at fluences varying from 2 · 1013 ions/cm2 to 1 · 1015 ions/cm2. TH films were irradiated by 120 MeV Au ions produced by the 15UD Pelletron accelerator of NSC at a fluence of 1 · 1013 ions/cm2. The surface morphology of these samples was studied with a Veeco Digital Nanoscope IIIa SPM at NSC. Since the polymers are soft materials, tapping mode was used with RTESP tips and a low force constant.
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2.2. HOPG The HOPG samples used in the study were obtained from SPI, USA. The HOPG samples were of type ZYH with typical grain size of approximately 40 nm. The samples were irradiated with 120 MeV Au ions (Se = 18.2 keV/nm). A fresh surface was prepared by cleaving the samples before irradiation. HOPG being a semi-metal with a conducting surface, the samples were studied in STM mode. Pt/Ir tip was used for scanning the samples. For the measurement the bias current was kept at 500 mV whereas a preamplifier gain of 1 · 108 V/A was selected. To obtain the images with atomic resolution a 400-nm scanner was used. 2.3. Fullerene Fullerene thin films were deposited on Si(1 0 0) substrate in a vacuum of 1 · 10 6 Torr by resistive heating of commercially available 99.9% pure C60 in a Ta boat. The thickness of the film, as measured by a quartz crystal thickness monitor, was 150 nm. Since Si substrate does not provide a perfect contact for conducting AFM measurement, a thin layer of Au was first deposited on the Si substrate to provide the contact. The arrangement allowed the measurement of conductivity across the C60 layer. The samples were irradiated with a 120 MeV Au ion beam (Se = 18.2 keV/nm, Sn = 215 eV/nm) from the NSC Pelletron with fluences of 2 · 1010, 6 · 1010 and 2 · 1011 ions/cm2. Results
concerning samples irradiated with higher fluence are not discussed here so as to emphasize single ion induced effects. The properties of these channels were studied by means of conducting atomic force microscopy (C-AFM) using a diamond coated silicon nitride (DDESP) tip.
3. Results and discussion 3.1. MTES–PTES and TH films The PL and Raman studies of these samples are reported elsewhere in this conference, and only the AFM results are discussed here. A typical AFM image in height mode is shown in Fig. 1(A). Since the polymers are soft materials, the images were recorded in the tapping mode with a low force constant. The images recorded with amplitude mode (Fig. 1(B)) exhibit a better contrast facilitating the observation of small features. The typical rounded asperities at the surface of an unirradiated sample have a size of about 70 nm and the root mean square (rms) surface roughness is of the order of 0.9 nm. Fig. 2(A) and (B) show the surface relief after irradiation with 900 keV Xe ions at a fluence of 5 · 1013 and 1 · 1015 ions/cm2 respectively. The dimension of the surface features, decreases to 60 and 25 nm whereas the rms surface roughness increases to 3.4 and 9.6 nm as shown in Table 1. In contrast to that, an increase in the size of surface features, from 18 to 30 nm, is observed
Table 1 Summary of the study on MTES/PTES and TH films Sample
Beam and energy
Fluence (ions/cm2)
Typical grain size
Sample roughness
MT11
500 keV Xe
Unirradiated 2 · 1013 1 · 1015
80 nm 60 nm 25 nm
0.9 nm 3.4 nm 9.0 nm
TH
120 MeV Au
Unirradiated 2 · 1013
18 nm 30 nm
0.6 nm 1.3 nm
Table 2 Summary of the study on HOPG Sample
Beam and energy
Fluence
Se
Sn
Features
HOPG Type ZYH
120 MeV Au
1 · 1011 ions/cm2 1 · 1013 ions/cm2
18.2 keV/nm 18.2 keV/nm
215 eV/nm 215 eV/nm
Hillock diameter 3.2 nm Crater formation, surface reconstruction
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Fig. 1. (A) AFM image (400 nm · 400 nm) of MTES/PTES sample in height mode (unirradiated sample). (B) AFM image (400 nm · 400 nm) of MTES/PTES sample shown in (A) in amplitude mode. Amplitude mode image shows better contrast for surface features.
on TH samples irradiated with swift heavy ions (Fig. 3(A) and (B)), together with an increase in surface roughness from 0.6 to 1.3 nm. These preliminary results can be explained as follows. Gels are made of polymer-like islands floating in a residual solvent, and the preliminary stage of their conversion into glasses involves the condensation of these islands. Thus, there are good reasons to believe that the observed structures at the surface of pristine films correspond
to these islands and that they become more interconnected in TH films submitted to electronic excitations. Two reasons may be invoked for explaining the fragmentation of these islands in the MTES–PTES samples submitted to Xe irradiation. First, the gels with organic side groups are converted into composite ceramics made of silica containing C:H clusters. Secondly, nuclear collisions are known to promote chain scissions in contrast to electronic excitations. The observation of
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Fig. 2. (A) AFM image (200 nm · 200 nm) of MTES/PTES sample after irradiation with 500 keV Xe ions with a fluence of 5 · 1013 ions/cm2. (B) AFM image (200 nm · 200 nm) of MTES/PTES sample after irradiation with 500 keV Xe ions with a fluence of 1 · 1015 ions/cm2. A decrease in grain size with fluence can be seen.
similar samples irradiated with swift heavy ions should permit to discriminate between the two possible origins of the structure subdivision. 3.2. HOPG The STM study of an unirradiated HOPG sample showed atomically resolved images. The samples irradiated with 120 MeV Au ions at a low fluence (1 · 1011 ions/cm2) show the formation of hillocks with a mean diameter of about 3 nm (Fig. 4(A)). With increasing fluence
(1 · 1012 ions/cm2 and 2 · 1013 ions/cm2) craters with a diameters of 2–4 nm are progressively formed (Fig. 4(B)) and a reconstruction of the surface is seen by STM. Section diagram of one of the craters showing typical dimension is shown in Fig. 4(C). From these results it seems that at low values of electronic energy transfer a swelling of the surface occurs locally, which may be attributed to amorphisation of the material in isolated tracks. The formation of craters surrounded by a rim may be explained by the increase of sputtering and a partial re-deposition of sputtered atoms with a
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Fig. 3. (A) AFM image (200 nm · 200 nm) of unirradiated TH sample. Spherical features with typical diameter of 18 nm are seen. (B) AFM image (200 nm · 200 nm) of TH sample irradiated with 100 MeV Au ions (1 · 1013 ions/cm2). Elongated features with typical diameter of 30 nm on surface are seen.
low kinetic energy or the surface diffusion of atoms from the transient disordered phase. The results of study are summarized in Table 2. 3.3. Fullerene In our earlier study with AFM, we have shown that irradiation with 110 MeV Ni ion beam at fluences of 1 · 1012–1 · 1014 ions/cm2 results in a decrease of grain size from 120 nm to 20 nm [24]. It seems reasonable to ascribe this result to the disintegration of the C60 units or their amorphisation/transformation into nanometer sized graphite particles. In the present work, the images were recorded in conducting mode using a diamond coated DDESP tips. In the case of unirradiated sample, no current is observed for bias of up to
10 V, confirming the semi-conducting nature of the fullerene film. In contrast to that, a current of the order of 5 nA was observed for a bias of 500 mV between the tip and Au underlayer after irradiation of the samples. A typical 3-D map of conducting paths, in a sample irradiated with 120 MeV Au ions at a fluence of 2 · 1010 ions/ cm2 is shown in Fig. 5. The conducting sites are characterised by local increase of current. The increase in conductivity is ascribed to the transformation of semi-conducting C60 into conducting graphite like carbon under the effect of electronic energy loss. The number of conducting zones does not match with the fluence and instead a ratio of 0.4 zone per ion is observed. Some of the tracks appear more conducting than others. Given that tracks cannot overlap for the fluence used by us,
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Fig. 4. (A) STM image of HOPG sample (50 nm · 50 nm) irradiated with 120 MeV Au ions with a fluence of 1 · 1011 ions/cm2. A single ion impact induced hillock can be seen. (B) STM image of HOPG sample (20 nm · 20 nm) irradiated with 120 MeV Au ions with a fluence of 2 · 1013 ions/cm2. Formation of craters and reconstruction on surface is seen. (C) STM image of HOPG sample irradiated with 120 MeV Au ions with a fluence of 2 · 1013 ions/cm2. Section diagram of one of the craters showing typical dimension (approximately 4.2 nm) is shown in figure.
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from insulating C60 into conducting graphite like carbon induced by electronic energy loss.
Acknowledgments Two of the authors (D.K.A. and J.C.P.) are thankful to IFCPAR, New Delhi for financial support. The authors are also thankful to DST, New Delhi for providing support for procuring the scanning probe microscope (SPM). Fig. 5. Current image (1 lm · 1 lm) of fullerene film irradiated with 100 MeV Au ions with a fluence of 1 · 1012 ions/cm2. The three dimensional image obtained using Conducting AFM shows zones of high conductivity, representing the ion impact sites.
these two effects are probably due to the probing of several isolated tracks simultaneously by the tip. Besides this, amorphisation around the ion path leads to increased conductivity and hence the current image has a much larger diameter as compared to that of ion track.
4. Conclusion Methyltriethoxysilane, phenyltriethoxysilane films irradiated with 900 keV Xe beam at fluences varying from 5 · 1012 ions/cm2 to 1 · 1015 ions/ cm2 exhibit surface features of size decreasing from 70 nm to 25 nm. In contrast to this result, irradiation with swift heavy ions seems to induce a coalescence of islands constituting the gel prepared from triethoxysilane and the island size increases from 18 to 30 nm after irradiation with a 100 MeV Au beam (1 · 1013 ions/cm2). The STM images of HOPG graphite irradiated with a 120 MeV Au beam show the formation of hillocks/craters (diameter 2–3 nm) at lower fluence, whereas crater formation and re-organisation of surface is observed at higher fluence (up to 2 · 1013 ions/cm2). The images recorded in conducting mode of C60 films irradiated with 120 MeV Au ions show the formation of conducting ion tracks. The increase in conductivity is attributed to the transformation
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