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Carbon nanowires generated by ion irradiation of hydrocarbon ices
Nuclear Instruments and Methods in Physics Research B 326 (2014) 2–6
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Nuclear Instruments and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb
Carbon nanowires generated by ion irradiation of hydrocarbon ices O. Puglisi a,⇑, G. Compagnini a, L. D’Urso a, G.A. Baratta b, M.E. Palumbo b, G. Strazzulla b a b
Dipartimento di Scienze Chimiche, Università di Catania, Viale A. Doria 6, Catania 95125, Italy Istituto Nazionale di Astrofisica, Osservatorio Astrofisico di Catania, Via S. Sofia 78, Catania 95123, Italy
a r t i c l e
i n f o
Article history: Received 28 June 2013 Received in revised form 19 September 2013 Accepted 2 October 2013 Available online 24 January 2014 Keywords: Carbon nanowires Ion irradiation Raman spectroscopy Solid hydrocarbons
a b s t r a c t In this paper we present the formation of carbon nanowires (polyynes and polycumulenes) in the solid state by ion irradiation of frozen hydrocarbons (C6H6 and C2H2). Irradiations have been performed using H+ ions in the 100’s keV energy regime using fluences up to 5 1014 ions/cm2. Beyond the intrinsic significance of these results in the field of material science, this work has been motivated by the fact that ion beam irradiation of hydrocarbon ices is one of the most important process thought to happen in several extraterrestrial environments where many spectroscopic features of polyyne molecules have been identified. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction The physics and chemistry of sp-coordinated all-carbon skeletons (carbynes) is actually a challenging topic in carbon science. Formed as linear chains of carbon atom pairs with alternating single and triple bonds, polyynes represent unique, truly one-dimensional molecular systems with intriguing optical and electronic properties [1,2]. There are a few ways to synthetically assemble polyyne chains by chemical routes, and the most popular method remains the electrochemical reductive carbonisation of poly(tetrafluoroethylene) and the CuI/II-catalysed oxidative homocoupling protocols [3,4]. While these techniques have been successfully used to form extended polyynes, one of the greatest problems remains the inability to produce and isolate significant quantities of the longest compounds, such as C16 and C20. At the same time a number of physical methods have been employed, all of them characterised by the formation and manipulation of hot carbon vapour generated in different ways [5–10]. The interests in these last approaches are manifold. First of all plasma generation methods allow to easy produce large amount of short and long chains (Carbon NanoWires, CNWs); hot plasma well reproduce the physical and chemical conditions of circumstellar environments, explaining the abundance in space of sp-hybridized carbon species variously end-capped [5,6]. Additionally, these methods represent a step towards the understanding in the formation of other carbon nano-phases, that is Single-Walled or Multi-Walled Carbon Nano-Tubes (SWNTs, ⇑ Corresponding author. Tel.: +39 095 7385074; fax: +39 095 580138. E-mail address: [email protected] (O. Puglisi). 0168-583X/$ - see front matter Ó 2014 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.nimb.2013.10.065
MWNTs), fullerenes, carbon onions and others, bridging the gap between the olygomers (polyynes, R–(–C„C–)n–R or polycumulenes, (R1,R2)@(C@C)n@(R3,R4) and the structure of others allotropic forms of carbon. For instance, it has been demonstrated that carbon deposits containing a relevant percentage of polyynes species easily transform to nano-scale carbon tubules by heating and electron irradiation [11,12]. Recently the observation of CNW-CNT hybrid systems has been reported after arc discharge experiments [13,14] or by separate formation of CNTs and polyynes [15,16]. The existence of these linear structures in hot carbon vapour has been firstly observed by Kroto during the experiments leading to the observation of fullerene C60 [17]. Today it is quite clear that the formation of polyynes and polycumulenes happens when a hot carbon vapour is rapidly quenched in a low temperature environment such as in the case of cluster condensation sources [5,6] or in experiments conducted by ablating carbon targets in liquid environments [7,8] or by the ignition of arc discharges [9,10]. Recently Hu et al. have reported the first ‘‘all solid’’ formation of carbon chains by femtosecond pulsed laser deposition [18] in which carbon ion energies can be as high as 0.5–2 keV, one or two orders of magnitude larger than the energies involved during the ablation with nanosecond laser pulses. In a previous study [19] we have shown that ion irradiation of hydrocarbon ices (namely C2H2, C2H4, and C2H6) in the inelastic collision regime leads to the formation of polyynes with definite lengths. This argument is particularly appealing in order to understand processes and materials of astrophysical relevance. Radiolysis of hydrocarbons in solid, liquid and gaseous phase was in the past the subject of much investigation. A number of reactive species, including positive and negative ions as well as
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radicals in addition to electronically excited states have been identified, and several interconversion paths for the formation of unsaturated compounds were proposed [20–23]. Interestingly, favourable conditions for the formation of polyynes and polycumulenes occur near some carbon stars, in some objects in the Solar System (e.g. in comets), and interstellar clouds. Concerning the interstellar medium (ISM), the Infrared Space Observatory (ISO) observations (in the wavenumber range 220–4200 cm 1) of the protoplanetary nebula CRL618 showed infrared absorption bands of two polyynes, C4H2 and C6H2, and two cyanopolyynes, HC3N and HC5N [24]. Furthermore, radio survey towards several molecular clouds revealed the presence of longer cyanopolyynes (H–(C„C)n–N) up to HC11N ([25]). Both infrared and radio observations strongly suggest the formation of longer polyynes in the ISM, although no direct evidence can be obtained by radio detection due to the absence of a permanent electric dipole [26]. Evans et al. [27] presented an observation of the very late thermal pulse object V4334 Sgr (Sakurai’s Object) with the Infrared Spectrometer (IRS) on the Spitzer Space Telescope. They were able to attribute a number of features seen in absorption against the dust shell, to HCN and polyyne molecules. Polyynes have been also detected in Titan’s atmosphere, where photolysis and radiolysis of mixtures containing methane and many other organic species lead to a rich organic chemistry [28]. Here we present new experiments of ion irradiation of simple hydrocarbons deposited on suitable substrates and analysed in situ after irradiation by infrared and Raman spectroscopy. Our specific motivation has been the search for linear carbon species generated by the ion-matter interaction.
2. Experimental methods and simulations C6H6 and C2H2 ices have been deposited at 16 K under High Vacuum conditions (<10 7 mbar base pressure) onto a gold coated silicon (for the Raman analysis) and KBr (for the IR) substrates separately. The final film thickness has been measured using the interference fringes generated by a He–Ne laser beam reflecting at the vacuum-film and film-substrate interfaces, giving values around 2 lm. Ion irradiation has been performed with a 200 keV H+ beam with ion current density around 0.1 lA/cm2 and fluences up to 5 1014 ions/cm2. Further details of the experimental set-up and procedures can be found elsewhere [29]. Simulations of ion-matter interaction (SRIM2008.04 code by James F. Ziegler, version available at http://www.srim.org) are reported in Fig. 1. The figure reports the energy loss suffered by the ions during their slowing down inside matter. It is quite clear that the energy loss by ionisations is predominant and nearly constant through the film thickness (full line, left scale), with the maximum values equal to 110 eV/nm. The energy loss by phonons creation into the sample is also reported (dashed line, right scale), showing values at least two orders of magnitude lower. The inset of Fig. 1 also shows a picture with the collision cascades for 100,000 ions during their slowing down into a C6H6 film. Traces represent the ion path inside the solid and account for the (few) produced recoils. From the figure it is possible to estimate that the average stopping power in a 2 lm thick sample is about 95 eV/nm which corresponds to 2.6 10 14 eV cm2/16u where 16u is a small molecule [30]. In order to compare experimental results obtained with different ions and mixtures and to extend laboratory results to astrophysical environments the dose (eV/16u) is often used. In the present case a fluence of 1 1014 ions/cm2 corresponds to a dose of 2.6 eV/16u. Raman and infrared spectroscopies are techniques appropriate to observe the ice before and after irradiation. Indeed the Raman
3
Fig. 1. Energy loss suffered by 200 keV protons in benzene (C6H6) by ionization and phonon creation as a function of the depth inside the sample. The inset shows a picture of the collision cascades for 100,000 ions.
cross sections for skeleton stretching in sp2 and sp hybridized carbons are predominant under visible or near-infrared excitations [31–33]. Particularly interesting is the ability of Raman spectroscopy to directly observe the modes for the carbon backbone, to account for order/disorder features of the structure and to evaluate many geometrical parameters of a carbon nanostructure such as the radius of the tube in the case of the observation of CNTs or the length of the chain for polyynes or polycumulenes. On the other hand, IR absorption is able to give a clear indication about the presence of different functional groups from simple hydrogenation (CH signals) to complex functionalities and radicals. The present experiments have been conducted with the acquisition of Raman and IR absorption spectra in situ during the irradiation at constant temperature and after annealing of the sample from 16 to 300 K. In many previous studies a limiting factor for the application of Raman spectroscopy to irradiated frozen hydrocarbons has been the appearance of a strong luminescence after irradiation, when visible laser excitations (i.e. 514 nm) are used [34]. In this work this limit has been overcome by using a NIR 785 nm excitation wavelength which strongly reduces this effect. 3. Results and discussion 3.1. Evidences of polyynes formation by Raman spectroscopy Fig. 2a shows the Raman spectra of C6H6 as deposited at 16 K (the band assignment can be found elsewhere [35]), after irradiation at 16 K with 200 keV H+ and the spectrum taken after annealing at 180 K. Fig. 2b shows the Raman spectrum of C2H2 as deposited, after ion bombardment with 200 keV H+ and after in situ annealing at 110 K [19]. As previously mentioned, a high background level has been observed for all the irradiated samples due to the formation of conjugated species with extended p electron systems. Nevertheless the irradiated samples before and after the annealing show a series of new features. These can be grouped into two regions: the sp2 hybridized carbon vibrational field (1100–1600 cm 1) and the sp hybridized carbon vibrational one (1800–2200 cm 1). As expected, the appearance of a broad signal between 1100 and 1600 cm 1 demonstrates the formation of amorphous carbon with a decreasing of the H/C atomic ratio as a function of ion beam fluence, as it has been shown in several other papers in which organic substances are subjected to particle irradiation [36].
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Fig. 2. Raman spectra of C6H6 and C2H2 as deposited and after ion irradiation with 200 keV H+ are shown in the panel (a) and (b) [19] respectively. The samples have been irradiated at 16 K and annealed to sublimate volatile species. The inset in both panels shows the features assigned to polyynes. In figure (b) a peak derived from unidentified impurities was also detected, and it is labelled with an asterisk.
3.2. Infrared spectroscopic studies Infrared absorption analysis of irradiated C2H2 confirms the above mentioned view. In particular Fig. 3 reports the evolution of the CH stretching absorption region during the annealing for an irradiated acetylene sample. Fig. 3 clearly shows the disappearance of the asymmetric CH acetylene stretching feature (3230 cm 1), while the feature at 3280 cm 1 assigned to R–(–C„C–)n–H polyyne chains, that has been formed by ion irradiation, remains unaltered, with only a mild decrease in intensity [40]. These R–(–C„C–)n–H
T=16K irradiated C2H2
T=70 K
Transmittance (a.u.)
New information are available looking at the Raman spectra after irradiation thanks to the use of near infrared exciting radiation (k = 785 nm). One of the most relevant findings is the observation of sharp features in the region of the sp-hybridized signals. Spectra in Fig. 2 show that C6H6 and C2H2 irradiation induces the formation of linear carbon chains (2100, 2170 and 2190 cm 1). The line width of these features is small, indicating that the chains themselves have well-defined lengths. Considering the existing literature data on the correlation between the chain length and the vibrational frequency for hydrogen capped polyynes, it is straightforward to assign the three signals mentioned to C12H2, C10H2 and C8H2 respectively [16]. First principle calculation combined with a new scaling scheme have also suggested that the chains might be even longer [37] and that an exact correlation between the chain length and the signal position is strongly influenced by the surrounding environment. The carbon chains’ features are more evident when the samples are annealed at 180 and 110 K. This process causes a reduction of the above mentioned luminescence and the disappearance of the features due to the pristine C2H2 because of its sublimation while C6H6 remained partially trapped in the refractory residue. Other sharp bands are observed in Fig. 2b superimposed to the broad D and G bands in the graphite-like region. These are at 1150, 1300 and 1480 cm 1. They are not clearly assigned even though the three frequencies have been reported in a recent paper by Dunlop et al. [38], where swift heavy ions are used to irradiate a graphite target. At a first glance features at 1150 and 1480 cm 1 were attributed to transpolyacetylene [39]. Another possibility is that the signal at 1150 cm 1 is representative of nanodiamond, while the 1480 cm 1 signal is due to the presence of cumulenes or carbyne crystals.
T=110 K
T=150 K T=180 K T=240 K
3150
3200
3250
3300
3350
-1
Wavenumbers (cm ) Fig. 3. Infrared transmittance spectra in the 3150–3350 cm 1 region of C2H2 after irradiation with 200 keV H+ at 16 K and after warm up at 70, 110, 150, 180, and 240 K.
polyyne chains support the identification of H–(–C„C–)n–H polyynes observed in Raman spectra. Fig. 4 shows the IR transmittance spectrum in the 1800–2300 cm 1 range of C2H2 irradiated at 16 K and annealed at 240 K. The occurrence of two broad, asymmetric and well separated bands centred respectively at about 1950 cm 1 and 2100 cm 1 is ascribed to a mixture of both sp isomer chains, polycumulene and polyyne (energetically preferred due to a stabilization by the Peierls distortion) [41]. As shown in a previous study [29] these features are also present in the residue left over after ion irradiation of frozen C6H6. It is necessary to point out that the cumulenic component is very difficult to be detected by Raman spectroscopy because the longitudinal mode consisting of the bond length alternation (BLA) oscillation, which is responsible for the strongest Raman transition of polyynes, becomes very weak being BLA ideally zero in the case of cumulene species [42]. Moreover the presence of a luminescence background, partially overlie vibrational features of the already weak sp-Raman signals making
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Note that these equations have to be considered merely as indicative and do not have any stoichiometric relevance. Concerning the astrophysical relevance of the present results it is interesting to note that C2H2, C6H6 polyynes and cyanopolyynes along with other hydrocarbons have been detected in different extraterrestrial environments such as cometary comae, planetary atmospheres, interstellar and circumstellar regions [47–49]. These detections refer to species in the gas phase. However it is largely accepted [50,51] that many complex molecules are formed after energetic processing in the solid phase and are released to the gas phase after sublimation of frozen species. As an example, comets are subjected to a prolonged irradiation by cosmic ions that alters their composition down to a significant depth [30]. The present results support the hypothesis that polyynes could have been synthesized in the cometary ices by cosmic ion irradiation and could be released in the gas evaporating from the cometary nucleus near the Sun.
Fig. 4. IR transmittance spectrum in the 1800–2300 cm at 16 K and annealed at 240 K.
1
range of C2H2 irradiated
cumulene bands undetectable. It should be considered that unlike to the Raman case, H–(–C„C–)n–H polyynes do not show any evident infrared feature in the 2100 cm 1 spectral region [43]. Thus the features observed at about 2100 cm 1 in the IR and Raman spectra are not due to the same polyynes chains. In particular, polyynes giving rise to the 2100 cm 1 IR feature could be either R–(–C„C–)n–H or R–(–C„C–)n–R chains. 4. Discussion and conclusions Up to now the production of carbon nanowires has been explained on the basis of a significant proportion of initially formed Cn radicals with linear carbon chain structures. These can readily add H, N or CN at the ends of their chains to form relatively stable polyynes and cyanopolyynes. Compagnini et al. [41] have studied the effects of ion irradiation (200 keV Ar+) on sp rich cluster assembled films and have found that ion irradiation transforms relatively long sp carbon chains in shorter ones but still retaining a significant amount of sp bonds. Compagnini et al. [19] have shown that polyynes are formed after ion irradiation of frozen C2H2, C2H4, and C2H6; here we have shown that polyynes are also formed after ion irradiation of C6H6. Furthermore the absorption features of polycumulenes are present in the IR spectra after ion irradiation of frozen C2H2. The formation of selected carbon nanowires by irradiation of hydrocarbons could in principle be due to different mechanisms with respect to those considered in the formation and evolution of hot carbon vapours. According to early research studies on the product of radiolysis of saturated and unsaturated hydrocarbons [44,45] Compagnini et al. [19] have shown that after ion irradiation of C2H4 the Raman features of C2H2 are present in the spectra along with the bands due to polyynes. Similarly, after irradiation of C2H6 the Raman features of C2H4 and C2H2 are present. Strazzulla and Baratta [46] have shown by IR spectroscopy that after ion irradiation of frozen benzene C2H2 is formed although it was below the detection limits in the Raman experiments here reported. The effect of ion irradiation in the keV regime for simple hydrocarbon ices can be summarised: C2H2 ? R (–C„C–)n R C2H4 ? C2H2 + R (–C„C–)n R C2H6 ? C2H4 + C2H2 + R (–C„C–)n R C6H6 ? C2H2 + R (–C„C–)n R
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Nuclear Instruments and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb
Carbon nanowires generated by ion irradiation of hydrocarbon ices O. Puglisi a,⇑, G. Compagnini a, L. D’Urso a, G.A. Baratta b, M.E. Palumbo b, G. Strazzulla b a b
Dipartimento di Scienze Chimiche, Università di Catania, Viale A. Doria 6, Catania 95125, Italy Istituto Nazionale di Astrofisica, Osservatorio Astrofisico di Catania, Via S. Sofia 78, Catania 95123, Italy
a r t i c l e
i n f o
Article history: Received 28 June 2013 Received in revised form 19 September 2013 Accepted 2 October 2013 Available online 24 January 2014 Keywords: Carbon nanowires Ion irradiation Raman spectroscopy Solid hydrocarbons
a b s t r a c t In this paper we present the formation of carbon nanowires (polyynes and polycumulenes) in the solid state by ion irradiation of frozen hydrocarbons (C6H6 and C2H2). Irradiations have been performed using H+ ions in the 100’s keV energy regime using fluences up to 5 1014 ions/cm2. Beyond the intrinsic significance of these results in the field of material science, this work has been motivated by the fact that ion beam irradiation of hydrocarbon ices is one of the most important process thought to happen in several extraterrestrial environments where many spectroscopic features of polyyne molecules have been identified. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction The physics and chemistry of sp-coordinated all-carbon skeletons (carbynes) is actually a challenging topic in carbon science. Formed as linear chains of carbon atom pairs with alternating single and triple bonds, polyynes represent unique, truly one-dimensional molecular systems with intriguing optical and electronic properties [1,2]. There are a few ways to synthetically assemble polyyne chains by chemical routes, and the most popular method remains the electrochemical reductive carbonisation of poly(tetrafluoroethylene) and the CuI/II-catalysed oxidative homocoupling protocols [3,4]. While these techniques have been successfully used to form extended polyynes, one of the greatest problems remains the inability to produce and isolate significant quantities of the longest compounds, such as C16 and C20. At the same time a number of physical methods have been employed, all of them characterised by the formation and manipulation of hot carbon vapour generated in different ways [5–10]. The interests in these last approaches are manifold. First of all plasma generation methods allow to easy produce large amount of short and long chains (Carbon NanoWires, CNWs); hot plasma well reproduce the physical and chemical conditions of circumstellar environments, explaining the abundance in space of sp-hybridized carbon species variously end-capped [5,6]. Additionally, these methods represent a step towards the understanding in the formation of other carbon nano-phases, that is Single-Walled or Multi-Walled Carbon Nano-Tubes (SWNTs, ⇑ Corresponding author. Tel.: +39 095 7385074; fax: +39 095 580138. E-mail address: [email protected] (O. Puglisi). 0168-583X/$ - see front matter Ó 2014 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.nimb.2013.10.065
MWNTs), fullerenes, carbon onions and others, bridging the gap between the olygomers (polyynes, R–(–C„C–)n–R or polycumulenes, (R1,R2)@(C@C)n@(R3,R4) and the structure of others allotropic forms of carbon. For instance, it has been demonstrated that carbon deposits containing a relevant percentage of polyynes species easily transform to nano-scale carbon tubules by heating and electron irradiation [11,12]. Recently the observation of CNW-CNT hybrid systems has been reported after arc discharge experiments [13,14] or by separate formation of CNTs and polyynes [15,16]. The existence of these linear structures in hot carbon vapour has been firstly observed by Kroto during the experiments leading to the observation of fullerene C60 [17]. Today it is quite clear that the formation of polyynes and polycumulenes happens when a hot carbon vapour is rapidly quenched in a low temperature environment such as in the case of cluster condensation sources [5,6] or in experiments conducted by ablating carbon targets in liquid environments [7,8] or by the ignition of arc discharges [9,10]. Recently Hu et al. have reported the first ‘‘all solid’’ formation of carbon chains by femtosecond pulsed laser deposition [18] in which carbon ion energies can be as high as 0.5–2 keV, one or two orders of magnitude larger than the energies involved during the ablation with nanosecond laser pulses. In a previous study [19] we have shown that ion irradiation of hydrocarbon ices (namely C2H2, C2H4, and C2H6) in the inelastic collision regime leads to the formation of polyynes with definite lengths. This argument is particularly appealing in order to understand processes and materials of astrophysical relevance. Radiolysis of hydrocarbons in solid, liquid and gaseous phase was in the past the subject of much investigation. A number of reactive species, including positive and negative ions as well as
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radicals in addition to electronically excited states have been identified, and several interconversion paths for the formation of unsaturated compounds were proposed [20–23]. Interestingly, favourable conditions for the formation of polyynes and polycumulenes occur near some carbon stars, in some objects in the Solar System (e.g. in comets), and interstellar clouds. Concerning the interstellar medium (ISM), the Infrared Space Observatory (ISO) observations (in the wavenumber range 220–4200 cm 1) of the protoplanetary nebula CRL618 showed infrared absorption bands of two polyynes, C4H2 and C6H2, and two cyanopolyynes, HC3N and HC5N [24]. Furthermore, radio survey towards several molecular clouds revealed the presence of longer cyanopolyynes (H–(C„C)n–N) up to HC11N ([25]). Both infrared and radio observations strongly suggest the formation of longer polyynes in the ISM, although no direct evidence can be obtained by radio detection due to the absence of a permanent electric dipole [26]. Evans et al. [27] presented an observation of the very late thermal pulse object V4334 Sgr (Sakurai’s Object) with the Infrared Spectrometer (IRS) on the Spitzer Space Telescope. They were able to attribute a number of features seen in absorption against the dust shell, to HCN and polyyne molecules. Polyynes have been also detected in Titan’s atmosphere, where photolysis and radiolysis of mixtures containing methane and many other organic species lead to a rich organic chemistry [28]. Here we present new experiments of ion irradiation of simple hydrocarbons deposited on suitable substrates and analysed in situ after irradiation by infrared and Raman spectroscopy. Our specific motivation has been the search for linear carbon species generated by the ion-matter interaction.
2. Experimental methods and simulations C6H6 and C2H2 ices have been deposited at 16 K under High Vacuum conditions (<10 7 mbar base pressure) onto a gold coated silicon (for the Raman analysis) and KBr (for the IR) substrates separately. The final film thickness has been measured using the interference fringes generated by a He–Ne laser beam reflecting at the vacuum-film and film-substrate interfaces, giving values around 2 lm. Ion irradiation has been performed with a 200 keV H+ beam with ion current density around 0.1 lA/cm2 and fluences up to 5 1014 ions/cm2. Further details of the experimental set-up and procedures can be found elsewhere [29]. Simulations of ion-matter interaction (SRIM2008.04 code by James F. Ziegler, version available at http://www.srim.org) are reported in Fig. 1. The figure reports the energy loss suffered by the ions during their slowing down inside matter. It is quite clear that the energy loss by ionisations is predominant and nearly constant through the film thickness (full line, left scale), with the maximum values equal to 110 eV/nm. The energy loss by phonons creation into the sample is also reported (dashed line, right scale), showing values at least two orders of magnitude lower. The inset of Fig. 1 also shows a picture with the collision cascades for 100,000 ions during their slowing down into a C6H6 film. Traces represent the ion path inside the solid and account for the (few) produced recoils. From the figure it is possible to estimate that the average stopping power in a 2 lm thick sample is about 95 eV/nm which corresponds to 2.6 10 14 eV cm2/16u where 16u is a small molecule [30]. In order to compare experimental results obtained with different ions and mixtures and to extend laboratory results to astrophysical environments the dose (eV/16u) is often used. In the present case a fluence of 1 1014 ions/cm2 corresponds to a dose of 2.6 eV/16u. Raman and infrared spectroscopies are techniques appropriate to observe the ice before and after irradiation. Indeed the Raman
3
Fig. 1. Energy loss suffered by 200 keV protons in benzene (C6H6) by ionization and phonon creation as a function of the depth inside the sample. The inset shows a picture of the collision cascades for 100,000 ions.
cross sections for skeleton stretching in sp2 and sp hybridized carbons are predominant under visible or near-infrared excitations [31–33]. Particularly interesting is the ability of Raman spectroscopy to directly observe the modes for the carbon backbone, to account for order/disorder features of the structure and to evaluate many geometrical parameters of a carbon nanostructure such as the radius of the tube in the case of the observation of CNTs or the length of the chain for polyynes or polycumulenes. On the other hand, IR absorption is able to give a clear indication about the presence of different functional groups from simple hydrogenation (CH signals) to complex functionalities and radicals. The present experiments have been conducted with the acquisition of Raman and IR absorption spectra in situ during the irradiation at constant temperature and after annealing of the sample from 16 to 300 K. In many previous studies a limiting factor for the application of Raman spectroscopy to irradiated frozen hydrocarbons has been the appearance of a strong luminescence after irradiation, when visible laser excitations (i.e. 514 nm) are used [34]. In this work this limit has been overcome by using a NIR 785 nm excitation wavelength which strongly reduces this effect. 3. Results and discussion 3.1. Evidences of polyynes formation by Raman spectroscopy Fig. 2a shows the Raman spectra of C6H6 as deposited at 16 K (the band assignment can be found elsewhere [35]), after irradiation at 16 K with 200 keV H+ and the spectrum taken after annealing at 180 K. Fig. 2b shows the Raman spectrum of C2H2 as deposited, after ion bombardment with 200 keV H+ and after in situ annealing at 110 K [19]. As previously mentioned, a high background level has been observed for all the irradiated samples due to the formation of conjugated species with extended p electron systems. Nevertheless the irradiated samples before and after the annealing show a series of new features. These can be grouped into two regions: the sp2 hybridized carbon vibrational field (1100–1600 cm 1) and the sp hybridized carbon vibrational one (1800–2200 cm 1). As expected, the appearance of a broad signal between 1100 and 1600 cm 1 demonstrates the formation of amorphous carbon with a decreasing of the H/C atomic ratio as a function of ion beam fluence, as it has been shown in several other papers in which organic substances are subjected to particle irradiation [36].
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Fig. 2. Raman spectra of C6H6 and C2H2 as deposited and after ion irradiation with 200 keV H+ are shown in the panel (a) and (b) [19] respectively. The samples have been irradiated at 16 K and annealed to sublimate volatile species. The inset in both panels shows the features assigned to polyynes. In figure (b) a peak derived from unidentified impurities was also detected, and it is labelled with an asterisk.
3.2. Infrared spectroscopic studies Infrared absorption analysis of irradiated C2H2 confirms the above mentioned view. In particular Fig. 3 reports the evolution of the CH stretching absorption region during the annealing for an irradiated acetylene sample. Fig. 3 clearly shows the disappearance of the asymmetric CH acetylene stretching feature (3230 cm 1), while the feature at 3280 cm 1 assigned to R–(–C„C–)n–H polyyne chains, that has been formed by ion irradiation, remains unaltered, with only a mild decrease in intensity [40]. These R–(–C„C–)n–H
T=16K irradiated C2H2
T=70 K
Transmittance (a.u.)
New information are available looking at the Raman spectra after irradiation thanks to the use of near infrared exciting radiation (k = 785 nm). One of the most relevant findings is the observation of sharp features in the region of the sp-hybridized signals. Spectra in Fig. 2 show that C6H6 and C2H2 irradiation induces the formation of linear carbon chains (2100, 2170 and 2190 cm 1). The line width of these features is small, indicating that the chains themselves have well-defined lengths. Considering the existing literature data on the correlation between the chain length and the vibrational frequency for hydrogen capped polyynes, it is straightforward to assign the three signals mentioned to C12H2, C10H2 and C8H2 respectively [16]. First principle calculation combined with a new scaling scheme have also suggested that the chains might be even longer [37] and that an exact correlation between the chain length and the signal position is strongly influenced by the surrounding environment. The carbon chains’ features are more evident when the samples are annealed at 180 and 110 K. This process causes a reduction of the above mentioned luminescence and the disappearance of the features due to the pristine C2H2 because of its sublimation while C6H6 remained partially trapped in the refractory residue. Other sharp bands are observed in Fig. 2b superimposed to the broad D and G bands in the graphite-like region. These are at 1150, 1300 and 1480 cm 1. They are not clearly assigned even though the three frequencies have been reported in a recent paper by Dunlop et al. [38], where swift heavy ions are used to irradiate a graphite target. At a first glance features at 1150 and 1480 cm 1 were attributed to transpolyacetylene [39]. Another possibility is that the signal at 1150 cm 1 is representative of nanodiamond, while the 1480 cm 1 signal is due to the presence of cumulenes or carbyne crystals.
T=110 K
T=150 K T=180 K T=240 K
3150
3200
3250
3300
3350
-1
Wavenumbers (cm ) Fig. 3. Infrared transmittance spectra in the 3150–3350 cm 1 region of C2H2 after irradiation with 200 keV H+ at 16 K and after warm up at 70, 110, 150, 180, and 240 K.
polyyne chains support the identification of H–(–C„C–)n–H polyynes observed in Raman spectra. Fig. 4 shows the IR transmittance spectrum in the 1800–2300 cm 1 range of C2H2 irradiated at 16 K and annealed at 240 K. The occurrence of two broad, asymmetric and well separated bands centred respectively at about 1950 cm 1 and 2100 cm 1 is ascribed to a mixture of both sp isomer chains, polycumulene and polyyne (energetically preferred due to a stabilization by the Peierls distortion) [41]. As shown in a previous study [29] these features are also present in the residue left over after ion irradiation of frozen C6H6. It is necessary to point out that the cumulenic component is very difficult to be detected by Raman spectroscopy because the longitudinal mode consisting of the bond length alternation (BLA) oscillation, which is responsible for the strongest Raman transition of polyynes, becomes very weak being BLA ideally zero in the case of cumulene species [42]. Moreover the presence of a luminescence background, partially overlie vibrational features of the already weak sp-Raman signals making
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Note that these equations have to be considered merely as indicative and do not have any stoichiometric relevance. Concerning the astrophysical relevance of the present results it is interesting to note that C2H2, C6H6 polyynes and cyanopolyynes along with other hydrocarbons have been detected in different extraterrestrial environments such as cometary comae, planetary atmospheres, interstellar and circumstellar regions [47–49]. These detections refer to species in the gas phase. However it is largely accepted [50,51] that many complex molecules are formed after energetic processing in the solid phase and are released to the gas phase after sublimation of frozen species. As an example, comets are subjected to a prolonged irradiation by cosmic ions that alters their composition down to a significant depth [30]. The present results support the hypothesis that polyynes could have been synthesized in the cometary ices by cosmic ion irradiation and could be released in the gas evaporating from the cometary nucleus near the Sun.
Fig. 4. IR transmittance spectrum in the 1800–2300 cm at 16 K and annealed at 240 K.
1
range of C2H2 irradiated
cumulene bands undetectable. It should be considered that unlike to the Raman case, H–(–C„C–)n–H polyynes do not show any evident infrared feature in the 2100 cm 1 spectral region [43]. Thus the features observed at about 2100 cm 1 in the IR and Raman spectra are not due to the same polyynes chains. In particular, polyynes giving rise to the 2100 cm 1 IR feature could be either R–(–C„C–)n–H or R–(–C„C–)n–R chains. 4. Discussion and conclusions Up to now the production of carbon nanowires has been explained on the basis of a significant proportion of initially formed Cn radicals with linear carbon chain structures. These can readily add H, N or CN at the ends of their chains to form relatively stable polyynes and cyanopolyynes. Compagnini et al. [41] have studied the effects of ion irradiation (200 keV Ar+) on sp rich cluster assembled films and have found that ion irradiation transforms relatively long sp carbon chains in shorter ones but still retaining a significant amount of sp bonds. Compagnini et al. [19] have shown that polyynes are formed after ion irradiation of frozen C2H2, C2H4, and C2H6; here we have shown that polyynes are also formed after ion irradiation of C6H6. Furthermore the absorption features of polycumulenes are present in the IR spectra after ion irradiation of frozen C2H2. The formation of selected carbon nanowires by irradiation of hydrocarbons could in principle be due to different mechanisms with respect to those considered in the formation and evolution of hot carbon vapours. According to early research studies on the product of radiolysis of saturated and unsaturated hydrocarbons [44,45] Compagnini et al. [19] have shown that after ion irradiation of C2H4 the Raman features of C2H2 are present in the spectra along with the bands due to polyynes. Similarly, after irradiation of C2H6 the Raman features of C2H4 and C2H2 are present. Strazzulla and Baratta [46] have shown by IR spectroscopy that after ion irradiation of frozen benzene C2H2 is formed although it was below the detection limits in the Raman experiments here reported. The effect of ion irradiation in the keV regime for simple hydrocarbon ices can be summarised: C2H2 ? R (–C„C–)n R C2H4 ? C2H2 + R (–C„C–)n R C2H6 ? C2H4 + C2H2 + R (–C„C–)n R C6H6 ? C2H2 + R (–C„C–)n R
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