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Tailoring carbon nanostructures via temperature and laser irradiation
Chemical Physics Letters 407 (2005) 254–259 www.elsevier.com/locate/cplett
Tailoring carbon nanostructures via temperature and laser irradiation C. Kramberger a
b
a,*
, A. Waske a, K. Biedermann a, T. Pichler a, T. Gemming a, B. Bu¨chner a, H. Kataura b
Leibniz Institute for Solid State and Materials Research, Institute of Solid State Research, Dresden, Helmholzstrasse 20, 01069 Dresden, Sachsen, Germany Nanotechnology Research Institute, National Institute of Advanced Industrial Science and Technology (AIST) Central 4, Higashi 1-1-1, Tsukuba, Ibaraki 305-8562, Japan Received 5 February 2005; in final form 5 February 2005 Available online 13 April 2005
Abstract We present an experimental study on transformation processes in peapods. Local and quick annealing of freestanding films as well as conventional annealing in the furnace are applied. Raman spectroscopy and transmission electron microscopy are used for characterization of the samples. While laser irradiation beyond the optimum transformation of peapods into double walled carbon nanotubes effectively removes the formerly formed inner tubes, too hot furnace annealing preserves the inner tubes but causes the outer nanotubes to coalesce with each outer. These differences are due to resonant photo processes that render laser irradiation and furnace annealing complementary means of tailoring carbon nanostructures. 2005 Elsevier B.V. All rights reserved.
1. Introduction Carbon nanostructures such as fullerenes and single wall carbon nanotubes (SWCNT) are because of their unique properties in the focus of research in physics, chemistry, biology and material science. However, their formation process is still under discussion. Especially, SWCNT have been shown to be ideal probes to study formation processes of novel carbon nano-structures. Since they are archetypes of one dimensional systems and besides their electronic and vibronic properties, their interior is a unique quasi one dimensional confined space. This confinement can stabilize phases that do not exist outside. One dimensional salt crystals and peapods (a linear arrangement of fullerenes) inside a SWCNT are well known examples [1–3]. The coalescence of the fullerene chain into an inner carbon nanotube (INT) using local irradiation in transmission electron microscopy *
Corresponding author. E-mail address: [email protected] (C. Kramberger).
0009-2614/$ - see front matter 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2005.03.089
(TEM) [4] and annealing at high temperatures inside an evacuated furnace [5] was reported recently. The INT obtained via fullerene coalescence are of fundamental interest since they are quite different from any other SWCNT available currently. There is absolute control on their diameter distribution in that the final diameter distribution of the INT can be controlled by the diameter distribution of the starting peapods [6]. Another appealing aspect is that they are made from atomically pure carbon. There is no need for catalytic agents in the templated growth of the INT. Indeed, the structural purity of the INT was proven by the observation of extremely narrow line widths in Raman spectroscopy [7]. Recently, isotope engineering of peapods [8] giving rise to a further tool to a selective study of isotope enriched INT was also reported. In addition, by applying higher temperatures the formation of ÔbicablesÕ, one oval shaped nanotube containing two parallel inner nanotubes [9], was observed. However, the new chemistry inside the confined one dimensional nanospace and the annealing induced transformation of SWCNT into
C. Kramberger et al. / Chemical Physics Letters 407 (2005) 254–259
new carbon nano-structures is still not completely understood. Especially, the manner in which C60 molecules inside a SWCNT coalesce to form an INT and how these bicables are formed is still unclear. In this Letter, we address this point by presenting a new alternate route to produce clean peapods and to induce fullerene coalescence using a focused laser beam to control the temperature of the peapods. In contrast to conventional furnace annealing, there are almost instant heat up and cool down ramps in laser annealing. In addition, our freestanding samples are suitable for in situ characterisation by Raman spectroscopy and allow ex situ analysis of the structure by TEM on the same position. Thus, enabling a closer observation of the dynamics of fullerene coalescence. We show that different ways of annealing have a tremendous impact on the final product. Understanding these differences is a crucial prerequisite for tailoring annealing processes for the efficient production of distinctive carbon allotropes and for a deeper understanding of the formation mechanism of these carbon nanostructures.
2. Experimental Opened and purified SWCNT from laser ablation with a mean diameter of about 1.4 nm [10], and commercial C60 from Ho¨chst AG were used as starting materials. The nanotubes were dispersed in acetone. Thin (grey) films of nanotubes were deposited on KBr single crystals by drop coating from an ultra sonicated dispersion. The films were detached from the substrates by dissolving the surface of the crystal in distilled water. Floating films were caught on either TEM grids or sapphire crystals. In order to obtain peapods from thin films of SWCNT, the samples were mounted inside an UHV chamber and held at a constant temperature of 300 C. The nanotubes were then covered by a black film of vapor deposited C60. The material was evaporated at 650 C. After deposition, the samples were held at 300 C for a minimum of 3 h. Then, the excess C60 that did not enter the SWCNT during equilibration time was removed by annealing the sample at 650 C. Films of peapods from the same batch were prepared on TEM grids and sapphire crystals. TEM micrographs were obtained with a high resolution analytical TEM Tecnai F30 (FEI). The micrographs were recorded using an acceleration voltage of 100 kV to reduce specimen damage caused by the electrons. FT Raman experiments were carried out with a Bruker FT Raman spectrometer equipped with an infrared microscope. The samples are mounted inside a Janis ST-500H microscope cryostat behind a quartz window. The 180 backscattered light is analyzed. The size of the laser spot on the sample is approximately 5 lm in diameter. In addition to acting as a probe, the 1064 nm laser of the FTR spectrometer can also be used to locally anneal the sam-
255
ple. Spectra were measured at 20 mW output power and for annealing as much as 500 mW could be applied. The output power is controlled electronically and is set within a few seconds. Since there is no need to touch the whole setup for (i) taking spectra, (ii) annealing the laser spot, (iii) waiting for cool down, (iv) and taking another spectrum, a whole sequence of different stages of annealing can be characterized in situ and on the same spot. Spectra were recorded in between annealing steps. Different annealing steps were used. Firstly, annealing times of 10 s with incremental increases in power for consecutive anneals were applied. Secondly, this was repeated but with annealing times of 60 s. Finally, runs with increasing annealing times at constant power were applied. After annealing, the spots were investigated by TEM. Peapods on a sapphire crystal were annealed in a horizontal high temperature tube furnace at various temperatures ranging from 1000 to 1550 C and characterized by FTR spectroscopy. The samples were successively annealed at increasing temperatures for 1 h. The furnace was set to reach the target temperature within 1 h and after holding it for another hour switched off. Before any annealing either by laser irradiation or in the furnace, the samples were evacuated to a base pressure of 5 · 10 7 mbar in order to suppress uncontrolled oxidation.
3. Results and discussion The production of peapods is confirmed by TEM. A TEM micrograph of an individual peapod, obtained by UHV filling, described above, is shown in the upper left of Fig. 1. At an excitation wavelength of 1064 nm, carbon nanotubes with appropriate optical transitions exhibit a very strong resonance enhancement in Raman scattering [11]. Therefore, the spectra are dominated by the response of a selected fraction of suitable chiralities in the sample. Since C60 does not have an optical transition suitable for such a resonance enhancement, the response of the peapods is the same as of the pristine nanotubes. The spectrum of the peapods is shown in Fig. 1 (pristine peapods). The total symmetric radial breathing mode (RBM) is well observed in the low frequency region between 150 and 180 cm 1. There is a linear relation between RBM frequency and inverse diameter xRBM = C1/d + C2, where C1 describes the RBM frequency of an individual freestanding nanotube and C2 takes into account the interaction with the local environment. Using values of 234 and 13 cm 1 [12], this frequency region corresponds to diameters between 1.7 and 1.3 nm1. Around 1590 cm 1, there is the response of the tangential modes. These modes are directly de-
1 Within the given digits the range of diameters does not change when using other commonly used combinations of C1 and C2.
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Fig. 1. The inset in the upper left shows a high resolution TEM micrograph of an individual peapod obtained by UHV filling. The second inset shows the glowing laser spot in the microscope. The irradiating laser light is invisible. The shown FT Raman spectra are from bottom to top of: (i) pristine peapods, (ii) 1 min laser annealed material at the optimum content of INT (260 mW), and (iii) laser annealed material at the highest power (500 mW).
rived from in plane modes in graphite. The graphitic line (G-line) shows a slight dispersion with the diameter [13,14]. The G-line of very narrow nanotubes is downshifted by several wave numbers, while larger diameters are approximating the line position of regular graphite. At high output powers of the laser, the material in the laser spot starts glowing immediately. A picture of a glowing sample is shown in the upper center of Fig. 1. The camera that was used is not sensitive to the incident infrared laser light. Thus, the image shows the visible range of the black body radiation. The peapods were successively annealed for 60 s at different laser powers. The first annealing step was performed at 100 mW and then the power was successively increased by 20 mW. The spectra taken after annealing at 260 and 500 mW are also shown in Fig. 1 (laser 260 mW, laser 500 mW). With the stepwise transformation of C60 to inner nanotubes (INT), there are significant changes in the observed Raman spectra. The RBM of the INT arises between 300 and 345 cm 1. This corresponds to INT diameters between 0.7 and 0.8 nm. At the same time, the dip before the maximum of the G-line is covered by the downshifted response of the additional small diameter INT, resulting in one broadened peak with a downshifted center. If the laser power is further increased beyond the optimum transformation, the response assigned to the INT is stepwise decreased and finally vanishes. At the same time, the other lines are diminished in intensity and this is illustrated by the relative increase in noise of the spectra in Fig. 1 (spectra normalized to the G-line). This diminishing of the lines intensities saturates with longer irradiation times and so does not originate from burning the carbon. We
assign this behavior to slight changes in the surface morphology that influence the light scattering. With traditional annealing in the furnace, and consistent with previous results [15], a very different behavior is observed. When annealing up to 1300 C, there is a similar increase in the formation of INT as compared to laser annealing. Between 1300 and 1450 C, only very minor increases in INT are observed. Above 1450 C, a new peak arises on the high frequency side of the G-line. This is accompanied by a slight broadening of the outer tubes RBM. At its low frequency side the background is significantly raised. The RBM of the INT is preserved but significantly broadened as compared to laser annealing or furnace annealing at lower temperatures (e.g. furnace 1300 C). The spectra obtained after furnace annealing are shown in Fig. 2 (furnace 1300 C and furnace 1550 C). As discussed below, these changes in the spectra are assigned to so called bicables. In this context, the observed broadening of the RBM of the INT arises from symmetry breaking of the INTs local environment. While in DWCNTs every INT is in the center of its outer counterpart. In bicables, another arbitrary INT is lying parallel and the individual INT are off center an oval shaped outer nanotube. This changed environment does not any longer fit to the cylinder symmetry of the RBM. Thus, the environmental up-shift of the RBM frequency is no longer a uniform constant and the individual frequencies are smeared out. In addition, the sample morphology of the DWCNT obtained by laser annealing was characterized by TEM. Typical TEM micrographs are depicted in the insets of Fig. 2. The overall morphology shown in the left inset is typical for purified bundled carbon nanotubes. The right inset represents a typical
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Fig. 2. The inset in the upper left shows a TEM overview of annealed peapods. The second inset shows a high resolution micrograph of a DWCNT obtained by laser annealing. Below there are FT Raman spectra of furnace annealed DWCNT (1550 and 1300 C) and of pristine peapods.
high resolution image of the DWCNT clearly showing the success of the laser induced process. We now turn to a detailed analysis of the evolution of the transformation process as a function of laser power and temperature. The left hand side of Fig. 3 depicts contour plots of the evolution of Raman modes in a series of successive annealing steps via increased laser power. Three different observations can be made: (i) a maximum content
of INT (laser 260 mW in Fig. 1), (ii) a lowered content at even higher temperatures, and (iii) their removal (laser 500 mW in Fig. 1). The right hand side of Fig. 3 is the corresponding contour plot for furnace annealing. In this case, we observe a gradual increase of the RBM of the INT concomitant with the characteristic downshift in the G-line. At the highest temperatures, there arise the features mentioned above on the high fre-
Fig. 3. Contour plots of FT Raman spectra with successively increasing power in 1 min laser annealing (left) and increasing temperature in 1 h furnace annealing (right). The individual panels show the evolution of the RBM of the starting nanotubes (130–210 cm 1), the RBM of the fullerene derived nanotubes (290–360 cm 1), and their common G-line (1530–1650 cm 1). Spectra are normalized to the G-line. Color levels in corresponding panels are set identically.
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quency side of the G-line. When following the evolution of the RBM of the INT with furnace annealing, one clearly sees that narrower INT are formed earlier than wider ones. The very fast transformation of encapsulated C60 to an INT in the first seconds is a remarkable finding in the case of laser annealing. Annealing series with just 10 s (not shown) irradiation times still exhibit the same overall behavior. This is not possible in a furnace and is an interesting feature of laser annealing. In addition to this fast transformation, there is also a fast decay of the INT at very high irradiation densities. While the formation of INT is also possible with furnace annealing their destruction is only observed in case of laser annealing. However, in furnace annealing the outer tubes are affected at very high temperatures forming the previously described bicables. These are not observed with laser annealing, even for long periods of time (2 h). From this comparison, it appears that temperature does not seem to be responsible for the selective destruction of INT. This selective removal is ascribed to destructive photo induced processes. Considering the strongly diameter-dependent optical transitions responsible for absorption, this laser annealing effect might be a possible means to tailor the electronic properties of carbon nanotubes. In fact, all INT that were in resonance and exhibited resonant Raman scattering were successfully removed, while the outer shell withstood this treatment. Photo induced processes may also be responsible for the accelerated formation of the INT. The observed faster formation of narrower INT relative to wider INT in furnace annealing is in good agreement with the growth model proposed in [16]. Therein, the authors investigate the behavior of annealing peapods in the furnace at temperatures between 800 and 1200 C for long periods. They also observe a faster formation of narrower INT. Following their model, the C60 fullerenes are forming INT with a diameter just below 0.7 nm before the interlayer distance between the two walls is gradually relaxed. They used the same excitation of 1064 nm for their Raman experiments. Therefore, the same INT are in resonance, but we never observed an INT RBM peak at 360 cm 1. In their experiments, this peak always appeared. This peak, which we do not see, is associated with the first generation of INT with diameters just below 0.7 nm. The most likely explanation for the observed differences between the present RBM of the INT and the one from [16] is a slightly different diameter distribution of the outer shell nanotubes [6]. Assuming, each INT is formed with its optimum diameter from the beginning would explain why our peapods do not form INT with a RBM frequency of 360 cm 1, i.e. our starting SWCNT have a slightly larger diameters. Another striking finding is the effect of very high temperature furnace annealing. The observed changes start at
1450 C. The effect of high temperatures on SWCNT has been investigated before in [17]. Therein, the temperature range between 1600 and 2800 C was investigated. For temperatures between 1600 and 2200 C, the authors observed the coalescence of parallel nanotubes to nanotubes with twice the diameter. Above 2200 C, the bundles of SWCNT are turned into MWCNT. The formation of MWCNT can be definitely ruled out in our case, but still the coalescence of two parallel nanotubes is possible, if there are enough defects to initiate the zipping mechanism at 1450 C. Since purification is always in some way harmful to SWCNT, a rather high defect concentration can be assumed in our opened and purified SWCNT. Then, two adjacent DWCNT can form a bicable as was also observed in [9]. Obviously, the two outer shells can coalesce in this way but the INT stay unaffected. This is in agreement with their much lower concentration of defects as proposed in [7].
4. Summary We present a new approach for tailoring carbonaceous species using annealing of C60 peapods, which were synthesized on different substrates under UHV conditions. A combination of laser induced annealing of freestanding samples, Raman spectroscopy and TEM offers new insight on the transformation rate and light induced processes. We have found major differences between traditional furnace annealing and fast laser annealing of peapods. In the case of laser annealing, the transformation to INT occurs almost instantly. Furthermore the selective removal of INT from DWCNT is possible. Photo processes are believed to be responsible for this selective removal of nanotubes. Considering the diversity of their electronic properties, this photo processes could pave the way for a new means to tailor the electronic properties of bulk CNT material. In case of furnace annealing the earlier formation of narrower INT can be confirmed, still we cannot confirm the growth model proposed by Bandow et al. [16]. We propose that the diameter of the INT is fully determined by the outer nanotube and stays as it is once the INT has been formed. In furnace annealing above 1450 C, we observe the parallel coalescence of outer nanotubes of DWCNT. Due to their much lower defect concentration, the INT do not follow this path and carbon bicables are formed. The very different outcome of these two ways of annealing gives an insight into the role of photo processes and thermal processes in the reshaping of sp2 carbon. Hence, fast laser annealing opens a way to the efficient production of novel carbon nanostructures and is an additional tool to increase the knowledge about the formation mechanism in these sp2 hybridized carbon species.
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Acknowledgments The authors acknowledge the DFG PI-440 project for funding. Dr. Mark Ru¨mmeli is acknowledged for valuable and fruitful discussion.
References [1] R. Meyer, J. Sloan, R. Dunin-Borkowski, A. Kirkland, M. Novotny, S. Bailey, J. Hutchison, M. Green, Science 289 (5483) (2000) 1324. [2] B. Smith, M. Monthioux, D. Luzzi, Nature 396 (6709) (1998) 323. [3] A. Khlobystov, D. Britz, A. Ardavan, G. Briggs, Phys. Rev. Lett. 92 (2004) 245507. [4] D. Luzzi, B. Smith, Carbon 38 (2000) 1751. [5] S. Bandow, M. Takizawa, K. Hirahara, M. Yudasaka, S. Iijima, Chem. Phys. Lett. 337 (1–3) (2001) 48. [6] F. Simon, A. Kukovecz, C. Kramberger, R. Pfeiffer, F. Hasi, H. Kuzmany, H. Kataura, Phys. Rev. B (in print). [7] R. Pfeiffer, H. Kuzmany, C. Kramberger, C. Schaman, T. Pichler, H. Kataura, Y. Achiba, J. Ku¨rti, V. Zo´lyomi, Phys. Rev. Lett. 90 (22) (2003) 225501.
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[8] F. Simon, C. Kramberger, R. Pfeiffer, H. Kuzmany, V. Zo´lyomi, J. Kurti, P. Singer, H. Alloul, Phys. Rev. Lett. (submitted). [9] M. Endo, T. Hayashi, H. Muramatsu, Y. Kim, H. Terrones, M. Terrones, M. Dresselhaus, Nano Lett. 4 (8) (2004) 1451. [10] H. Ishii, H. Kataura, H. Shiozawa, H. Yoshioka, H. Otsubo, Y. Takayama, T. Miyahara, S. Suzuki, Y. Achiba, M. Nakatake, T. Narimura, M. Higashiguchi, K. Shimada, H. Namatame, M. Taniguchi, Nature 426 (2003) 540. [11] H. Kuzmany, W. Plank, M. Hulman, C. Kramberger, A. Gru¨neis, T. Pichler, H. Peterlik, H. Kataura, Y. Achiba, Eur. Phys. J. B 22 (3) (2001) 307. [12] C. Kramberger, R. Pfeiffer, H. Kuzmany, V. Zo´lyomi, J. Ku¨rti, Phys. Rev. B 68 (2003) 235404. [13] O. Dubay, G. Kresse, H. Kuzmany, Phys. Rev. Lett. 88 (23) (2002) 235506. [14] A. Jorio, M. Pimenta, A. Souza Filho, G. Samsonidze, A. Swan, ¨ nlu¨, M. Goldberg, R. Saito, G. Dresselhaus, M. Dresselhaus, M. U Phys. Rev. Lett. 90 (10) (2003), 107403 (1-4). [15] M. Holzweber, C. Kramberger, F. Simon, R. Pfeiffer, M. Mannsberger, F. Hasi, H. Kuzmany, AIP Conf. Proc. 723 (2004) 234. [16] S. Bandow, T. Hiraoka, T. Yumura, K. Hirahara, H. Shinohara, S. Iijima, Chem. Phys. Lett. 384 (4–6) (2004) 320. [17] K. Meteruier, S. Bonnamy, F. Beguin, C. Journet, P. Bernier, M. Lamy de la Chapelle, O. Chauvet, S. Lefrant, Carbon 40 (2001) 1765.
Tailoring carbon nanostructures via temperature and laser irradiation C. Kramberger a
b
a,*
, A. Waske a, K. Biedermann a, T. Pichler a, T. Gemming a, B. Bu¨chner a, H. Kataura b
Leibniz Institute for Solid State and Materials Research, Institute of Solid State Research, Dresden, Helmholzstrasse 20, 01069 Dresden, Sachsen, Germany Nanotechnology Research Institute, National Institute of Advanced Industrial Science and Technology (AIST) Central 4, Higashi 1-1-1, Tsukuba, Ibaraki 305-8562, Japan Received 5 February 2005; in final form 5 February 2005 Available online 13 April 2005
Abstract We present an experimental study on transformation processes in peapods. Local and quick annealing of freestanding films as well as conventional annealing in the furnace are applied. Raman spectroscopy and transmission electron microscopy are used for characterization of the samples. While laser irradiation beyond the optimum transformation of peapods into double walled carbon nanotubes effectively removes the formerly formed inner tubes, too hot furnace annealing preserves the inner tubes but causes the outer nanotubes to coalesce with each outer. These differences are due to resonant photo processes that render laser irradiation and furnace annealing complementary means of tailoring carbon nanostructures. 2005 Elsevier B.V. All rights reserved.
1. Introduction Carbon nanostructures such as fullerenes and single wall carbon nanotubes (SWCNT) are because of their unique properties in the focus of research in physics, chemistry, biology and material science. However, their formation process is still under discussion. Especially, SWCNT have been shown to be ideal probes to study formation processes of novel carbon nano-structures. Since they are archetypes of one dimensional systems and besides their electronic and vibronic properties, their interior is a unique quasi one dimensional confined space. This confinement can stabilize phases that do not exist outside. One dimensional salt crystals and peapods (a linear arrangement of fullerenes) inside a SWCNT are well known examples [1–3]. The coalescence of the fullerene chain into an inner carbon nanotube (INT) using local irradiation in transmission electron microscopy *
Corresponding author. E-mail address: [email protected] (C. Kramberger).
0009-2614/$ - see front matter 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2005.03.089
(TEM) [4] and annealing at high temperatures inside an evacuated furnace [5] was reported recently. The INT obtained via fullerene coalescence are of fundamental interest since they are quite different from any other SWCNT available currently. There is absolute control on their diameter distribution in that the final diameter distribution of the INT can be controlled by the diameter distribution of the starting peapods [6]. Another appealing aspect is that they are made from atomically pure carbon. There is no need for catalytic agents in the templated growth of the INT. Indeed, the structural purity of the INT was proven by the observation of extremely narrow line widths in Raman spectroscopy [7]. Recently, isotope engineering of peapods [8] giving rise to a further tool to a selective study of isotope enriched INT was also reported. In addition, by applying higher temperatures the formation of ÔbicablesÕ, one oval shaped nanotube containing two parallel inner nanotubes [9], was observed. However, the new chemistry inside the confined one dimensional nanospace and the annealing induced transformation of SWCNT into
C. Kramberger et al. / Chemical Physics Letters 407 (2005) 254–259
new carbon nano-structures is still not completely understood. Especially, the manner in which C60 molecules inside a SWCNT coalesce to form an INT and how these bicables are formed is still unclear. In this Letter, we address this point by presenting a new alternate route to produce clean peapods and to induce fullerene coalescence using a focused laser beam to control the temperature of the peapods. In contrast to conventional furnace annealing, there are almost instant heat up and cool down ramps in laser annealing. In addition, our freestanding samples are suitable for in situ characterisation by Raman spectroscopy and allow ex situ analysis of the structure by TEM on the same position. Thus, enabling a closer observation of the dynamics of fullerene coalescence. We show that different ways of annealing have a tremendous impact on the final product. Understanding these differences is a crucial prerequisite for tailoring annealing processes for the efficient production of distinctive carbon allotropes and for a deeper understanding of the formation mechanism of these carbon nanostructures.
2. Experimental Opened and purified SWCNT from laser ablation with a mean diameter of about 1.4 nm [10], and commercial C60 from Ho¨chst AG were used as starting materials. The nanotubes were dispersed in acetone. Thin (grey) films of nanotubes were deposited on KBr single crystals by drop coating from an ultra sonicated dispersion. The films were detached from the substrates by dissolving the surface of the crystal in distilled water. Floating films were caught on either TEM grids or sapphire crystals. In order to obtain peapods from thin films of SWCNT, the samples were mounted inside an UHV chamber and held at a constant temperature of 300 C. The nanotubes were then covered by a black film of vapor deposited C60. The material was evaporated at 650 C. After deposition, the samples were held at 300 C for a minimum of 3 h. Then, the excess C60 that did not enter the SWCNT during equilibration time was removed by annealing the sample at 650 C. Films of peapods from the same batch were prepared on TEM grids and sapphire crystals. TEM micrographs were obtained with a high resolution analytical TEM Tecnai F30 (FEI). The micrographs were recorded using an acceleration voltage of 100 kV to reduce specimen damage caused by the electrons. FT Raman experiments were carried out with a Bruker FT Raman spectrometer equipped with an infrared microscope. The samples are mounted inside a Janis ST-500H microscope cryostat behind a quartz window. The 180 backscattered light is analyzed. The size of the laser spot on the sample is approximately 5 lm in diameter. In addition to acting as a probe, the 1064 nm laser of the FTR spectrometer can also be used to locally anneal the sam-
255
ple. Spectra were measured at 20 mW output power and for annealing as much as 500 mW could be applied. The output power is controlled electronically and is set within a few seconds. Since there is no need to touch the whole setup for (i) taking spectra, (ii) annealing the laser spot, (iii) waiting for cool down, (iv) and taking another spectrum, a whole sequence of different stages of annealing can be characterized in situ and on the same spot. Spectra were recorded in between annealing steps. Different annealing steps were used. Firstly, annealing times of 10 s with incremental increases in power for consecutive anneals were applied. Secondly, this was repeated but with annealing times of 60 s. Finally, runs with increasing annealing times at constant power were applied. After annealing, the spots were investigated by TEM. Peapods on a sapphire crystal were annealed in a horizontal high temperature tube furnace at various temperatures ranging from 1000 to 1550 C and characterized by FTR spectroscopy. The samples were successively annealed at increasing temperatures for 1 h. The furnace was set to reach the target temperature within 1 h and after holding it for another hour switched off. Before any annealing either by laser irradiation or in the furnace, the samples were evacuated to a base pressure of 5 · 10 7 mbar in order to suppress uncontrolled oxidation.
3. Results and discussion The production of peapods is confirmed by TEM. A TEM micrograph of an individual peapod, obtained by UHV filling, described above, is shown in the upper left of Fig. 1. At an excitation wavelength of 1064 nm, carbon nanotubes with appropriate optical transitions exhibit a very strong resonance enhancement in Raman scattering [11]. Therefore, the spectra are dominated by the response of a selected fraction of suitable chiralities in the sample. Since C60 does not have an optical transition suitable for such a resonance enhancement, the response of the peapods is the same as of the pristine nanotubes. The spectrum of the peapods is shown in Fig. 1 (pristine peapods). The total symmetric radial breathing mode (RBM) is well observed in the low frequency region between 150 and 180 cm 1. There is a linear relation between RBM frequency and inverse diameter xRBM = C1/d + C2, where C1 describes the RBM frequency of an individual freestanding nanotube and C2 takes into account the interaction with the local environment. Using values of 234 and 13 cm 1 [12], this frequency region corresponds to diameters between 1.7 and 1.3 nm1. Around 1590 cm 1, there is the response of the tangential modes. These modes are directly de-
1 Within the given digits the range of diameters does not change when using other commonly used combinations of C1 and C2.
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Fig. 1. The inset in the upper left shows a high resolution TEM micrograph of an individual peapod obtained by UHV filling. The second inset shows the glowing laser spot in the microscope. The irradiating laser light is invisible. The shown FT Raman spectra are from bottom to top of: (i) pristine peapods, (ii) 1 min laser annealed material at the optimum content of INT (260 mW), and (iii) laser annealed material at the highest power (500 mW).
rived from in plane modes in graphite. The graphitic line (G-line) shows a slight dispersion with the diameter [13,14]. The G-line of very narrow nanotubes is downshifted by several wave numbers, while larger diameters are approximating the line position of regular graphite. At high output powers of the laser, the material in the laser spot starts glowing immediately. A picture of a glowing sample is shown in the upper center of Fig. 1. The camera that was used is not sensitive to the incident infrared laser light. Thus, the image shows the visible range of the black body radiation. The peapods were successively annealed for 60 s at different laser powers. The first annealing step was performed at 100 mW and then the power was successively increased by 20 mW. The spectra taken after annealing at 260 and 500 mW are also shown in Fig. 1 (laser 260 mW, laser 500 mW). With the stepwise transformation of C60 to inner nanotubes (INT), there are significant changes in the observed Raman spectra. The RBM of the INT arises between 300 and 345 cm 1. This corresponds to INT diameters between 0.7 and 0.8 nm. At the same time, the dip before the maximum of the G-line is covered by the downshifted response of the additional small diameter INT, resulting in one broadened peak with a downshifted center. If the laser power is further increased beyond the optimum transformation, the response assigned to the INT is stepwise decreased and finally vanishes. At the same time, the other lines are diminished in intensity and this is illustrated by the relative increase in noise of the spectra in Fig. 1 (spectra normalized to the G-line). This diminishing of the lines intensities saturates with longer irradiation times and so does not originate from burning the carbon. We
assign this behavior to slight changes in the surface morphology that influence the light scattering. With traditional annealing in the furnace, and consistent with previous results [15], a very different behavior is observed. When annealing up to 1300 C, there is a similar increase in the formation of INT as compared to laser annealing. Between 1300 and 1450 C, only very minor increases in INT are observed. Above 1450 C, a new peak arises on the high frequency side of the G-line. This is accompanied by a slight broadening of the outer tubes RBM. At its low frequency side the background is significantly raised. The RBM of the INT is preserved but significantly broadened as compared to laser annealing or furnace annealing at lower temperatures (e.g. furnace 1300 C). The spectra obtained after furnace annealing are shown in Fig. 2 (furnace 1300 C and furnace 1550 C). As discussed below, these changes in the spectra are assigned to so called bicables. In this context, the observed broadening of the RBM of the INT arises from symmetry breaking of the INTs local environment. While in DWCNTs every INT is in the center of its outer counterpart. In bicables, another arbitrary INT is lying parallel and the individual INT are off center an oval shaped outer nanotube. This changed environment does not any longer fit to the cylinder symmetry of the RBM. Thus, the environmental up-shift of the RBM frequency is no longer a uniform constant and the individual frequencies are smeared out. In addition, the sample morphology of the DWCNT obtained by laser annealing was characterized by TEM. Typical TEM micrographs are depicted in the insets of Fig. 2. The overall morphology shown in the left inset is typical for purified bundled carbon nanotubes. The right inset represents a typical
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Fig. 2. The inset in the upper left shows a TEM overview of annealed peapods. The second inset shows a high resolution micrograph of a DWCNT obtained by laser annealing. Below there are FT Raman spectra of furnace annealed DWCNT (1550 and 1300 C) and of pristine peapods.
high resolution image of the DWCNT clearly showing the success of the laser induced process. We now turn to a detailed analysis of the evolution of the transformation process as a function of laser power and temperature. The left hand side of Fig. 3 depicts contour plots of the evolution of Raman modes in a series of successive annealing steps via increased laser power. Three different observations can be made: (i) a maximum content
of INT (laser 260 mW in Fig. 1), (ii) a lowered content at even higher temperatures, and (iii) their removal (laser 500 mW in Fig. 1). The right hand side of Fig. 3 is the corresponding contour plot for furnace annealing. In this case, we observe a gradual increase of the RBM of the INT concomitant with the characteristic downshift in the G-line. At the highest temperatures, there arise the features mentioned above on the high fre-
Fig. 3. Contour plots of FT Raman spectra with successively increasing power in 1 min laser annealing (left) and increasing temperature in 1 h furnace annealing (right). The individual panels show the evolution of the RBM of the starting nanotubes (130–210 cm 1), the RBM of the fullerene derived nanotubes (290–360 cm 1), and their common G-line (1530–1650 cm 1). Spectra are normalized to the G-line. Color levels in corresponding panels are set identically.
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quency side of the G-line. When following the evolution of the RBM of the INT with furnace annealing, one clearly sees that narrower INT are formed earlier than wider ones. The very fast transformation of encapsulated C60 to an INT in the first seconds is a remarkable finding in the case of laser annealing. Annealing series with just 10 s (not shown) irradiation times still exhibit the same overall behavior. This is not possible in a furnace and is an interesting feature of laser annealing. In addition to this fast transformation, there is also a fast decay of the INT at very high irradiation densities. While the formation of INT is also possible with furnace annealing their destruction is only observed in case of laser annealing. However, in furnace annealing the outer tubes are affected at very high temperatures forming the previously described bicables. These are not observed with laser annealing, even for long periods of time (2 h). From this comparison, it appears that temperature does not seem to be responsible for the selective destruction of INT. This selective removal is ascribed to destructive photo induced processes. Considering the strongly diameter-dependent optical transitions responsible for absorption, this laser annealing effect might be a possible means to tailor the electronic properties of carbon nanotubes. In fact, all INT that were in resonance and exhibited resonant Raman scattering were successfully removed, while the outer shell withstood this treatment. Photo induced processes may also be responsible for the accelerated formation of the INT. The observed faster formation of narrower INT relative to wider INT in furnace annealing is in good agreement with the growth model proposed in [16]. Therein, the authors investigate the behavior of annealing peapods in the furnace at temperatures between 800 and 1200 C for long periods. They also observe a faster formation of narrower INT. Following their model, the C60 fullerenes are forming INT with a diameter just below 0.7 nm before the interlayer distance between the two walls is gradually relaxed. They used the same excitation of 1064 nm for their Raman experiments. Therefore, the same INT are in resonance, but we never observed an INT RBM peak at 360 cm 1. In their experiments, this peak always appeared. This peak, which we do not see, is associated with the first generation of INT with diameters just below 0.7 nm. The most likely explanation for the observed differences between the present RBM of the INT and the one from [16] is a slightly different diameter distribution of the outer shell nanotubes [6]. Assuming, each INT is formed with its optimum diameter from the beginning would explain why our peapods do not form INT with a RBM frequency of 360 cm 1, i.e. our starting SWCNT have a slightly larger diameters. Another striking finding is the effect of very high temperature furnace annealing. The observed changes start at
1450 C. The effect of high temperatures on SWCNT has been investigated before in [17]. Therein, the temperature range between 1600 and 2800 C was investigated. For temperatures between 1600 and 2200 C, the authors observed the coalescence of parallel nanotubes to nanotubes with twice the diameter. Above 2200 C, the bundles of SWCNT are turned into MWCNT. The formation of MWCNT can be definitely ruled out in our case, but still the coalescence of two parallel nanotubes is possible, if there are enough defects to initiate the zipping mechanism at 1450 C. Since purification is always in some way harmful to SWCNT, a rather high defect concentration can be assumed in our opened and purified SWCNT. Then, two adjacent DWCNT can form a bicable as was also observed in [9]. Obviously, the two outer shells can coalesce in this way but the INT stay unaffected. This is in agreement with their much lower concentration of defects as proposed in [7].
4. Summary We present a new approach for tailoring carbonaceous species using annealing of C60 peapods, which were synthesized on different substrates under UHV conditions. A combination of laser induced annealing of freestanding samples, Raman spectroscopy and TEM offers new insight on the transformation rate and light induced processes. We have found major differences between traditional furnace annealing and fast laser annealing of peapods. In the case of laser annealing, the transformation to INT occurs almost instantly. Furthermore the selective removal of INT from DWCNT is possible. Photo processes are believed to be responsible for this selective removal of nanotubes. Considering the diversity of their electronic properties, this photo processes could pave the way for a new means to tailor the electronic properties of bulk CNT material. In case of furnace annealing the earlier formation of narrower INT can be confirmed, still we cannot confirm the growth model proposed by Bandow et al. [16]. We propose that the diameter of the INT is fully determined by the outer nanotube and stays as it is once the INT has been formed. In furnace annealing above 1450 C, we observe the parallel coalescence of outer nanotubes of DWCNT. Due to their much lower defect concentration, the INT do not follow this path and carbon bicables are formed. The very different outcome of these two ways of annealing gives an insight into the role of photo processes and thermal processes in the reshaping of sp2 carbon. Hence, fast laser annealing opens a way to the efficient production of novel carbon nanostructures and is an additional tool to increase the knowledge about the formation mechanism in these sp2 hybridized carbon species.
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Acknowledgments The authors acknowledge the DFG PI-440 project for funding. Dr. Mark Ru¨mmeli is acknowledged for valuable and fruitful discussion.
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