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Thermal stability studies of CVD-grown graphene nanoribbons: Defect annealing and loop formation
Chemical Physics Letters 469 (2009) 177–182
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Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett
Thermal stability studies of CVD-grown graphene nanoribbons: Defect annealing and loop formation J. Campos-Delgado a, Y.A. Kim b, T. Hayashi b, A. Morelos-Gómez a,f, M. Hofmann d,e, H. Muramatsu c, M. Endo b,c, H. Terrones a, R.D. Shull f, M.S. Dresselhaus f, M. Terrones a,* a
Laboratory for Nanoscience and Nanotechnology Research (LINAN) and Advanced Materials Department, IPICYT, Camino a la Presa San José 2055, Lomas 4a. Sección, San Luis Potosí 78216, Mexico b Faculty of Engineering, Shinshu University, 4-17-1, Wakasato, Nagano-Shi 380-8553, Japan c Institute of Carbon Science and Technology, Shinshu University, 4-17-1 Wakasato, Nagano-Shi, Japan d Department of Electrical Engineering and Computer Science, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, MA 02139-4307, USA e Department of Physics, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, MA 02139-4307, USA f National Institute of Standards and Technology, 100 Bureau Drive, Stop 8552 Gaithersburg, MD 20899, USA
a r t i c l e
i n f o
Article history: Received 10 November 2008 In final form 22 December 2008 Available online 30 December 2008
a b s t r a c t We present a high temperature heat treatment study of CVD-grown graphene nanoribbons annealed up to 2800 °C, demonstrating a progressive annihilation of lattice defects as the heat treatment temperature is raised. Starting at 1500 °C, single and multiple loop formation were observed on the ribbons edges as the temperature was increased. The structural changes of the samples are documented by X-ray diffraction, Raman spectroscopy, TGA, SEM, and HRTEM. This work indicates that nanoribbon annealing eventually leads to defect-free samples, through graphitization and edge loop formation. The annealed material exhibits structural differences that could be tailored for a variety of specific applications. Ó 2009 Elsevier B.V. All rights reserved.
1. Introduction Before 1985 researchers believed that all the allotropes of carbon were already known: amorphous carbon, graphite, and diamond. Nevertheless, in that year the discovery of C60 started a revolution in carbon nanostructured materials [1], leading to the synthesis of new carbon nano-materials, such as single-wall carbon nanotubes [2,3] and more recently graphene [4–7]. A lot of attention has recently been drawn to graphene (single and few layered graphite) and also to graphene ribbons because of their predicted unique and theoretically interesting electronic properties, critically depending on edge direction (zigzag or armchair) and width [8–10]. So far, chemical production methods of graphene yield low quantities of sample, uncontrolled edge shapes, and in most cases, vacuum, high temperatures or multi-step processes are needed for its production. Historically, post-synthesis thermal treatments of carbon materials have been used with different objectives, including the achievement of higher degrees of graphitization [11], elimination of amorphous carbon in carbon nanotubes [12,13], graphitization of diamond particles [5], transformation of SWNT bundles into graphite nanoribbons [14], and the elimination of metal particles from the starting materials [15,16].
* Corresponding author. E-mail address: [email protected] (M. Terrones). 0009-2614/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2008.12.082
During these thermally-induced structural transformations, a process called loop formation at the edges of graphitic materials occurs [17–21], whereby the chemically reactive open edges of adjacent graphene sheets join with one another to form a semicylindrical loop with a radius larger than the sheet-to-sheet separation distance. These loops which serve to eliminate the reactive edges and satisfy the dangling bond requirements of the edge structures, have been observed for heat treated filamentous graphite [17], graphite polyhedral crystals [18] and cup-stacked nanofibers [19,20]. José-Yacamán et al. previously reported an elongated graphitic structure with bent ends [21], and although closed ends were also observed in this prior report, their bent ends do not resemble the loops observed in the present work. Here we report the observation of the annihilation of structural point defects and of open edges (dangling bonds) of graphite nanoribbons, caused by thermal annealing at very high temperatures. In addition, we note that at heat treatment temperatures (THT) in the range 1500–2000 °C, single-loop formation appears between adjacent open graphene layers. At higher annealing temperature, multi-loop formation is observed as well as the total absence of bare edges. 2. Experimental details The samples used in the present heat treatment experiments were produced by the pyrolysis of a solution containing ethanol (C2H6O), ferrocene (FeCp2) and a small concentration of thiophene
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Fig. 1. Scanning electron microscopy images of carbon nanoribbons heat treated to various THT.
(C4H4S). Further details on the synthesis and properties of the asproduced nanoribbon material can be found elsewhere [22]. The as-produced graphene nanoribbons were heat treated in a graphite furnace at THT of 1000 °C, 1500 °C, 2000 °C, 2500 °C and 2800 °C for 30 min under an Ar atmosphere to produce heat treated samples that are investigated in the present work, and are denoted by ‘HT °C’. SEM imaging (FEI-field emission SEM-XL30 operated at 1– 15 keV),1 TEM imaging (JEOL JEM-2010F operated at 200 kV), Xray powder diffraction (Philips Diffractometer model 12014424, Cu Ka radiation, k = 1.54 Å), Raman spectroscopy (Kaiser HoloLab 5000 system with a laser excitation source of 532 nm (2.33 eV) obtained with a Nd:YAG laser line) and thermogravimetric analysis (TGA) (Thermo Haake, Cahn VersaTherm HS system heating the samples at 5 °C/min to 900 °C in air), were used to characterize our ribbon samples upon increasing the heat treatment temperature. 3. Results and discussion The morphology of the ribbon samples heat treated to different THT values do not seem to show dramatic changes, as can be observed in Fig. 1, which shows SEM micrographs for the pristine sample, HT 1000 °C, HT 2000 °C and HT 2800 °C samples. Intermediate THT samples (HT 1500 °C and HT 2500 °C, not shown here) share the same basic morphology characteristics. Fig. 2 depicts the edge structure of the heat treated (HT) samples using high-resolution TEM (HRTEM). It is clear that the pris-
1 The identification of specific equipment manufacturers and tradenames in this text is only for the purpose of fully describing the experiments and does not constitute an endorsement by the authors or their organizations.
tine sample contains many active end planes (edges). As the samples are annealed, interesting features can be pointed out: (1) Small structural changes are observed for the pristine and the 1000 °C THT sample which both exhibit a well-developed lattice fringe structure. We attribute these small structural changes to the fact that the synthesis temperature (950 °C) of the pristine material is very close to the first annealing temperature (1000 °C). (2) At 1500 °C, the lattice fringes straighten up and single-loops start to appear, although open edges are still present. Here, it is clear that point defects present in the individual layers start to anneal and disappear. (3) At 2000 °C, the disappearance of open edges and doubleloop formation are observed. (4) For the highest annealing temperatures (2500–2800 °C), we observe increasingly straighter lattice fringes, multi-loop formation covering an increasing number of layers and the absence of open edges. Loop formation at high annealing temperatures has already been reported in carbon systems [17–20], and the zipping mechanism proposed by Rotkin [23] seems to elegantly describe the loop formation phenomenon. According to this model, two adjacent graphene sheets will tend to join their edges to form a loop, and the radius of curvature will systematically be greater than the separation of the sheets. When single and double loops are formed (see Fig. 2) an increase in the radius of curvature is evident, confirming this aspect of the proposed model [23]. This local increase in the interlayer distances at the loops may well explain the presence of slightly higher interlayer spacings observed by analyzing the graphite (0 0 2) peak from the X-ray powder diffraction pat-
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Fig. 2. Transmission electron micrographs of carbon nanoribbons heat treated at various THT focusing especially on the structural changes of the edges as THT is increased (scale bar = 5 nm).
Fig. 3. (a) Micro Raman spectra for Elaser = 2.33 eV on bulk heat treated nanoribbons for various THT and (b) the dependence on THT of the ID/IG ratio (open circles), the ID0 =IG ratio (solid squares), and the FWHM linewidth for the G band (right hand y-axis, solid circles). The growth temperature for the pristine sample is 950 °C.
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terns of samples treated at different temperatures (see below). Note that as the loop radius is increased, the loops start forming more facetted structures, as is commonly seen in other carbon nanostructures [18,24]. The Raman spectra of the HT samples (Fig. 3a), reveal various important features which are further developed in the plots shown in Fig. 3b. This figure summarizes the dependence on THT of the intensity ratios ID/IG and ID0 =IG (open circles and solid squares, respectively). A main conclusion of Fig. 3 is that the intensity ratios ID/IG and ID0 =IG clearly decrease as THT increases. Here, the D-band near 1350 cm1 and the D0 band near 1620 cm1 are, respectively, the disorder-induced features associated with the intervalley K?K0 and intravalley K (or K0 ) double resonance scattering processes [25]. The decrease of the intensity of the disorder-induced D and D0 peaks as THT is increased from 1000 °C to 2000 °C and their very small intensity for HT 2500 °C and 2800 °C confirm the increase in structural order and crystallinity. We believe that one of the reasons for observing this decrease in the D and D0 peaks is the annealing of the open edges, which together with lattice defects (vacancies, interstitial atoms and Stone–Wales transformations) are responsible for a large D-band intensity. As the temperature increases, the edges merge with their adjacent counterparts so as to create a high population of loops on the side surface of all nanoribbons. The large number of loops (for which the loop curvature is symmetry-breaking) and the local strains experienced by the graphene layers at these points are responsible for observing a residual D-band intensity in samples heat treated at 2800 °C, thus, indicating that the ribbons are not perfectly graphitizable due to the widespread formation of edge loops. In addition, the FWMH linewidth of the G band decreases as the THT increases (Fig. 3b, solid circles). The sharpening of this band induced by the tangential vibration of two carbon atoms also confirms the establishment of larger crystalline areas with a larger in-plane coherence distance
La within the graphene layers (caused by the dramatic decrease of point defects and interstitials) with increasing THT. At high Raman frequencies, second order and combination modes appear: the G* feature (2440 cm1), the G0 peak (2700 cm1), the D+G feature (2940 cm1) and the 2D0 feature (3240 cm1). We observe that the G0 peak intensity increases as the THT is increased. Both characteristics, the reduction of the Dband intensity and the increase of G0 band intensity have also been observed in the annealing of cup-stacked nanofibers [19]. As stated above, the presence of disorder-induced features (D, D0 and D+G) and the symmetric shape of the G0 peak in our high THT samples can be explained by the limited stacking order due to the loop termination of edges. However, a more detailed Raman study consisting of different laser excitation energies would give more information about the physical property changes occurring in the HT materials, and would allow us to investigate whether the loops might be resonant with specific excitation wavelengths. X-ray powder diffraction patterns (Fig. 4a) reveal a match to the graphite pattern for several reflections, not just for the (0 0 2) reflection, as is done in most studies of CVD-produced MWNTs [26]. The analysis of the (0 0 2) peak position revealed an average interlayer spacing of 3.36 Å (close to perfect graphite) for the pristine sample, HT 1000 °C, HT 2500 °C and HT 2800 °C samples, and 3.37 Å for the HT 1500 °C and HT 2000 °C samples. Small changes are seen in all the reflections as the annealing temperature increases (see Fig. 4b). It is noteworthy that in the HT samples no iron carbide peaks are present, as has been reported for the pristine ribbons [22]. However, we could not rule out the presence of minute Fe content within the original samples, which could well be below the detection limits of the X-ray microprobe instrument (e.g. <1%). In Fig. 4c, we show the first derivative with temperature (DTA) of the weight loss curves obtained from the TGA analysis. Interesting phenomena can be deduced from these measurements:
Fig. 4. (a) X-ray diffraction characterization of each HT treated sample, the numbers in parenthesis indicate the crystallographic reflections, (b) interlayer distances between the (0 0 2) planes as a function of THT, (c) first derivative with respect to temperature of the weight loss (DTA) curves of the heat treated samples, and (d) Td vs. THT plot for the peak temperature in (c) for the different HT samples (the growth temperature for the pristine sample is 950 °C).
J. Campos-Delgado et al. / Chemical Physics Letters 469 (2009) 177–182
As THT is increased, the TGA analysis reveals an up-shift in the decomposition temperature Td of the ribbon materials in air, from 702 °C for the pristine sample to 780 °C for the HT 2800 °C sample; this behavior suggests a restructuring taking place during the heating process (replacement of open edges by loops), which makes the material less reactive, thereby increasing the decomposition temperature. The HT 1000 °C sample, however, has a lower Td (665 °C) than the pristine sample (702 °C). As suggested by previous work in carbon nanotubes [27], defect migration and vacancy coalescence induced by heat treatment effects could be taking place at this stage, forming large area defects which turn the sample into a more reactive material. It is also possible that at 1000 °C in an Ar atmosphere, the ribbon sample detaches some passivation atoms (e.g. H, O, H2O) that recombine with C atoms at defective areas; during the heat treatment, this recombination might lead to the loss of such carbon atoms in the form of CO or CO2, thus enlarging the defective regions leaving a more reactive holey structure. The above effects, combined with the presence of open edges passivated by oxygen or hydrogen groups, could contribute further to the decrease of the decomposition temperature. It is noteworthy that at 1000 °C the ribbon sample is not able to completely anneal defects into energetically stable sp2 hybridized carbon regions, thus leaving a metastable carbon network that is still quite reactive. A difference of 107 °C in Td for the HT 1500 °C and HT 1000 °C samples suggests that the number of defects and reactive sites decreases significantly in this temperature range. We suggest that competing processes are here in play. Firstly, the high temperature promotes the annealing of defects as revealed by the straightening of the lattice fringes observed in Fig. 2, and the termination of the reactive edges by loop formation are responsible for the Td change. For the HT 1500 °C samples and up, the Td range is from 772 °C to 780 °C (see Fig. 4d). This stability in Td suggests that no major restructuring of the sample is occurring among these different temperature stages, which agrees with our TEM observation in that the edges form single to multiple loops in this temperature range, thereby reducing chemically active sites and reducing the overall reactivity. In order to have a clear picture of the restructuring taking place during annealing treatments, we have identified the transformations that occur for each THT. Fig. 5 shows a proposed diagram imaging such transformations. For THT below 1500 °C the sample
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consists of relatively well ordered planes and well-developed crystallinity (confirmed by XRD and electron diffraction [22]) but the sample still contains point defects (vacancies), out-of-plane interstitial atoms, Stone–Wales and other defects, in addition to the bare edges of the ribbons where some oxygen groups are present [22]. By 1500 °C, defect annealing starts to take place and we observe straighter lattice fringes, approaching a defect-free planar structure, in good agreement with the first stages of the graphitization process of bulk graphite under heat treatment [24]. Moreover, in the case of graphene ribbons, some open edges become passivated by single-loop formation driven by surface energy minimization. For HT 2000 °C, double-loop formation is evident, and as the temperature is increased further, multi-loops with increasing loop diameters are observed and open edges are no longer present. A more detailed study of the thermal annealing of carbon nanoribbons, in which the THT steps are spaced more closely in the range 1000 °C < THT < 2000 °C, will further clarify the exact temperature range of single, double and multiple loop formation. Such a study would also help determine the dynamics of furnace annealing in contrast to annealing by the Joule heating of nanoribbon samples [28], since very different edge behavior is observed for the two types of heat treatment. These differences open widely the application possibilities of these nanoribbons, and suggest a variety of studies at both the fundamental and applied levels. 4. Conclusion We have demonstrated that heat treatments up to 2800 °C of as-produced carbon nanoribbons anneal point defects and interstitials within individual graphene layers, thus leading to a material with a higher degree of crystallinity, in which the reactive edges of the ribbons are transformed into loops in a self-assembly process that can be strongly controlled by the variation of THT. The decrease of the D peak intensity in the Raman spectra as THT increases, the disappearance of the D0 peak, the decrease of the FWHM of the G band, all support the annealing of defects. However, the large number of loops present at the edges of all the ribbons is responsible for observing some residual D-band intensity. The general up-shift of the decomposition temperature in the TGA analysis also reveals a more stable and less reactive material as THT increases in the range above which loop formation takes over. More detailed studies are necessary to understand the physics and chemistry of the loops in more depth and how to use such information to tailor the structural and perhaps the magnetic and electrical properties of these materials for specific applications. Acknowledgements The authors thank Dr. Gene Dresselhaus, D. Ramirez and G. Ramirez for valuable and fruitful discussions. This work is supported by NSF Grants NIRT CTS-05-06830 and DMR-07-04197. We also thank CONACYT-Mexico for Grants: 56787 (Laboratory for Nanoscience and Nanotechnology Research-LINAN), 45772 (MT), 58899-Inter American Collaboration (MT), 2004-01-013/SALUDCONACYT (MT) and PhD scholarships (JCD, AMG). References
Fig. 5. Schematic model for the restructuring process achieved by the thermal annealing treatment (blue balls = carbon atoms, red balls = oxygen atoms, white balls = hydrogen atoms), see text. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
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Contents lists available at ScienceDirect
Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett
Thermal stability studies of CVD-grown graphene nanoribbons: Defect annealing and loop formation J. Campos-Delgado a, Y.A. Kim b, T. Hayashi b, A. Morelos-Gómez a,f, M. Hofmann d,e, H. Muramatsu c, M. Endo b,c, H. Terrones a, R.D. Shull f, M.S. Dresselhaus f, M. Terrones a,* a
Laboratory for Nanoscience and Nanotechnology Research (LINAN) and Advanced Materials Department, IPICYT, Camino a la Presa San José 2055, Lomas 4a. Sección, San Luis Potosí 78216, Mexico b Faculty of Engineering, Shinshu University, 4-17-1, Wakasato, Nagano-Shi 380-8553, Japan c Institute of Carbon Science and Technology, Shinshu University, 4-17-1 Wakasato, Nagano-Shi, Japan d Department of Electrical Engineering and Computer Science, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, MA 02139-4307, USA e Department of Physics, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, MA 02139-4307, USA f National Institute of Standards and Technology, 100 Bureau Drive, Stop 8552 Gaithersburg, MD 20899, USA
a r t i c l e
i n f o
Article history: Received 10 November 2008 In final form 22 December 2008 Available online 30 December 2008
a b s t r a c t We present a high temperature heat treatment study of CVD-grown graphene nanoribbons annealed up to 2800 °C, demonstrating a progressive annihilation of lattice defects as the heat treatment temperature is raised. Starting at 1500 °C, single and multiple loop formation were observed on the ribbons edges as the temperature was increased. The structural changes of the samples are documented by X-ray diffraction, Raman spectroscopy, TGA, SEM, and HRTEM. This work indicates that nanoribbon annealing eventually leads to defect-free samples, through graphitization and edge loop formation. The annealed material exhibits structural differences that could be tailored for a variety of specific applications. Ó 2009 Elsevier B.V. All rights reserved.
1. Introduction Before 1985 researchers believed that all the allotropes of carbon were already known: amorphous carbon, graphite, and diamond. Nevertheless, in that year the discovery of C60 started a revolution in carbon nanostructured materials [1], leading to the synthesis of new carbon nano-materials, such as single-wall carbon nanotubes [2,3] and more recently graphene [4–7]. A lot of attention has recently been drawn to graphene (single and few layered graphite) and also to graphene ribbons because of their predicted unique and theoretically interesting electronic properties, critically depending on edge direction (zigzag or armchair) and width [8–10]. So far, chemical production methods of graphene yield low quantities of sample, uncontrolled edge shapes, and in most cases, vacuum, high temperatures or multi-step processes are needed for its production. Historically, post-synthesis thermal treatments of carbon materials have been used with different objectives, including the achievement of higher degrees of graphitization [11], elimination of amorphous carbon in carbon nanotubes [12,13], graphitization of diamond particles [5], transformation of SWNT bundles into graphite nanoribbons [14], and the elimination of metal particles from the starting materials [15,16].
* Corresponding author. E-mail address: [email protected] (M. Terrones). 0009-2614/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2008.12.082
During these thermally-induced structural transformations, a process called loop formation at the edges of graphitic materials occurs [17–21], whereby the chemically reactive open edges of adjacent graphene sheets join with one another to form a semicylindrical loop with a radius larger than the sheet-to-sheet separation distance. These loops which serve to eliminate the reactive edges and satisfy the dangling bond requirements of the edge structures, have been observed for heat treated filamentous graphite [17], graphite polyhedral crystals [18] and cup-stacked nanofibers [19,20]. José-Yacamán et al. previously reported an elongated graphitic structure with bent ends [21], and although closed ends were also observed in this prior report, their bent ends do not resemble the loops observed in the present work. Here we report the observation of the annihilation of structural point defects and of open edges (dangling bonds) of graphite nanoribbons, caused by thermal annealing at very high temperatures. In addition, we note that at heat treatment temperatures (THT) in the range 1500–2000 °C, single-loop formation appears between adjacent open graphene layers. At higher annealing temperature, multi-loop formation is observed as well as the total absence of bare edges. 2. Experimental details The samples used in the present heat treatment experiments were produced by the pyrolysis of a solution containing ethanol (C2H6O), ferrocene (FeCp2) and a small concentration of thiophene
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Fig. 1. Scanning electron microscopy images of carbon nanoribbons heat treated to various THT.
(C4H4S). Further details on the synthesis and properties of the asproduced nanoribbon material can be found elsewhere [22]. The as-produced graphene nanoribbons were heat treated in a graphite furnace at THT of 1000 °C, 1500 °C, 2000 °C, 2500 °C and 2800 °C for 30 min under an Ar atmosphere to produce heat treated samples that are investigated in the present work, and are denoted by ‘HT °C’. SEM imaging (FEI-field emission SEM-XL30 operated at 1– 15 keV),1 TEM imaging (JEOL JEM-2010F operated at 200 kV), Xray powder diffraction (Philips Diffractometer model 12014424, Cu Ka radiation, k = 1.54 Å), Raman spectroscopy (Kaiser HoloLab 5000 system with a laser excitation source of 532 nm (2.33 eV) obtained with a Nd:YAG laser line) and thermogravimetric analysis (TGA) (Thermo Haake, Cahn VersaTherm HS system heating the samples at 5 °C/min to 900 °C in air), were used to characterize our ribbon samples upon increasing the heat treatment temperature. 3. Results and discussion The morphology of the ribbon samples heat treated to different THT values do not seem to show dramatic changes, as can be observed in Fig. 1, which shows SEM micrographs for the pristine sample, HT 1000 °C, HT 2000 °C and HT 2800 °C samples. Intermediate THT samples (HT 1500 °C and HT 2500 °C, not shown here) share the same basic morphology characteristics. Fig. 2 depicts the edge structure of the heat treated (HT) samples using high-resolution TEM (HRTEM). It is clear that the pris-
1 The identification of specific equipment manufacturers and tradenames in this text is only for the purpose of fully describing the experiments and does not constitute an endorsement by the authors or their organizations.
tine sample contains many active end planes (edges). As the samples are annealed, interesting features can be pointed out: (1) Small structural changes are observed for the pristine and the 1000 °C THT sample which both exhibit a well-developed lattice fringe structure. We attribute these small structural changes to the fact that the synthesis temperature (950 °C) of the pristine material is very close to the first annealing temperature (1000 °C). (2) At 1500 °C, the lattice fringes straighten up and single-loops start to appear, although open edges are still present. Here, it is clear that point defects present in the individual layers start to anneal and disappear. (3) At 2000 °C, the disappearance of open edges and doubleloop formation are observed. (4) For the highest annealing temperatures (2500–2800 °C), we observe increasingly straighter lattice fringes, multi-loop formation covering an increasing number of layers and the absence of open edges. Loop formation at high annealing temperatures has already been reported in carbon systems [17–20], and the zipping mechanism proposed by Rotkin [23] seems to elegantly describe the loop formation phenomenon. According to this model, two adjacent graphene sheets will tend to join their edges to form a loop, and the radius of curvature will systematically be greater than the separation of the sheets. When single and double loops are formed (see Fig. 2) an increase in the radius of curvature is evident, confirming this aspect of the proposed model [23]. This local increase in the interlayer distances at the loops may well explain the presence of slightly higher interlayer spacings observed by analyzing the graphite (0 0 2) peak from the X-ray powder diffraction pat-
J. Campos-Delgado et al. / Chemical Physics Letters 469 (2009) 177–182
179
Fig. 2. Transmission electron micrographs of carbon nanoribbons heat treated at various THT focusing especially on the structural changes of the edges as THT is increased (scale bar = 5 nm).
Fig. 3. (a) Micro Raman spectra for Elaser = 2.33 eV on bulk heat treated nanoribbons for various THT and (b) the dependence on THT of the ID/IG ratio (open circles), the ID0 =IG ratio (solid squares), and the FWHM linewidth for the G band (right hand y-axis, solid circles). The growth temperature for the pristine sample is 950 °C.
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terns of samples treated at different temperatures (see below). Note that as the loop radius is increased, the loops start forming more facetted structures, as is commonly seen in other carbon nanostructures [18,24]. The Raman spectra of the HT samples (Fig. 3a), reveal various important features which are further developed in the plots shown in Fig. 3b. This figure summarizes the dependence on THT of the intensity ratios ID/IG and ID0 =IG (open circles and solid squares, respectively). A main conclusion of Fig. 3 is that the intensity ratios ID/IG and ID0 =IG clearly decrease as THT increases. Here, the D-band near 1350 cm1 and the D0 band near 1620 cm1 are, respectively, the disorder-induced features associated with the intervalley K?K0 and intravalley K (or K0 ) double resonance scattering processes [25]. The decrease of the intensity of the disorder-induced D and D0 peaks as THT is increased from 1000 °C to 2000 °C and their very small intensity for HT 2500 °C and 2800 °C confirm the increase in structural order and crystallinity. We believe that one of the reasons for observing this decrease in the D and D0 peaks is the annealing of the open edges, which together with lattice defects (vacancies, interstitial atoms and Stone–Wales transformations) are responsible for a large D-band intensity. As the temperature increases, the edges merge with their adjacent counterparts so as to create a high population of loops on the side surface of all nanoribbons. The large number of loops (for which the loop curvature is symmetry-breaking) and the local strains experienced by the graphene layers at these points are responsible for observing a residual D-band intensity in samples heat treated at 2800 °C, thus, indicating that the ribbons are not perfectly graphitizable due to the widespread formation of edge loops. In addition, the FWMH linewidth of the G band decreases as the THT increases (Fig. 3b, solid circles). The sharpening of this band induced by the tangential vibration of two carbon atoms also confirms the establishment of larger crystalline areas with a larger in-plane coherence distance
La within the graphene layers (caused by the dramatic decrease of point defects and interstitials) with increasing THT. At high Raman frequencies, second order and combination modes appear: the G* feature (2440 cm1), the G0 peak (2700 cm1), the D+G feature (2940 cm1) and the 2D0 feature (3240 cm1). We observe that the G0 peak intensity increases as the THT is increased. Both characteristics, the reduction of the Dband intensity and the increase of G0 band intensity have also been observed in the annealing of cup-stacked nanofibers [19]. As stated above, the presence of disorder-induced features (D, D0 and D+G) and the symmetric shape of the G0 peak in our high THT samples can be explained by the limited stacking order due to the loop termination of edges. However, a more detailed Raman study consisting of different laser excitation energies would give more information about the physical property changes occurring in the HT materials, and would allow us to investigate whether the loops might be resonant with specific excitation wavelengths. X-ray powder diffraction patterns (Fig. 4a) reveal a match to the graphite pattern for several reflections, not just for the (0 0 2) reflection, as is done in most studies of CVD-produced MWNTs [26]. The analysis of the (0 0 2) peak position revealed an average interlayer spacing of 3.36 Å (close to perfect graphite) for the pristine sample, HT 1000 °C, HT 2500 °C and HT 2800 °C samples, and 3.37 Å for the HT 1500 °C and HT 2000 °C samples. Small changes are seen in all the reflections as the annealing temperature increases (see Fig. 4b). It is noteworthy that in the HT samples no iron carbide peaks are present, as has been reported for the pristine ribbons [22]. However, we could not rule out the presence of minute Fe content within the original samples, which could well be below the detection limits of the X-ray microprobe instrument (e.g. <1%). In Fig. 4c, we show the first derivative with temperature (DTA) of the weight loss curves obtained from the TGA analysis. Interesting phenomena can be deduced from these measurements:
Fig. 4. (a) X-ray diffraction characterization of each HT treated sample, the numbers in parenthesis indicate the crystallographic reflections, (b) interlayer distances between the (0 0 2) planes as a function of THT, (c) first derivative with respect to temperature of the weight loss (DTA) curves of the heat treated samples, and (d) Td vs. THT plot for the peak temperature in (c) for the different HT samples (the growth temperature for the pristine sample is 950 °C).
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As THT is increased, the TGA analysis reveals an up-shift in the decomposition temperature Td of the ribbon materials in air, from 702 °C for the pristine sample to 780 °C for the HT 2800 °C sample; this behavior suggests a restructuring taking place during the heating process (replacement of open edges by loops), which makes the material less reactive, thereby increasing the decomposition temperature. The HT 1000 °C sample, however, has a lower Td (665 °C) than the pristine sample (702 °C). As suggested by previous work in carbon nanotubes [27], defect migration and vacancy coalescence induced by heat treatment effects could be taking place at this stage, forming large area defects which turn the sample into a more reactive material. It is also possible that at 1000 °C in an Ar atmosphere, the ribbon sample detaches some passivation atoms (e.g. H, O, H2O) that recombine with C atoms at defective areas; during the heat treatment, this recombination might lead to the loss of such carbon atoms in the form of CO or CO2, thus enlarging the defective regions leaving a more reactive holey structure. The above effects, combined with the presence of open edges passivated by oxygen or hydrogen groups, could contribute further to the decrease of the decomposition temperature. It is noteworthy that at 1000 °C the ribbon sample is not able to completely anneal defects into energetically stable sp2 hybridized carbon regions, thus leaving a metastable carbon network that is still quite reactive. A difference of 107 °C in Td for the HT 1500 °C and HT 1000 °C samples suggests that the number of defects and reactive sites decreases significantly in this temperature range. We suggest that competing processes are here in play. Firstly, the high temperature promotes the annealing of defects as revealed by the straightening of the lattice fringes observed in Fig. 2, and the termination of the reactive edges by loop formation are responsible for the Td change. For the HT 1500 °C samples and up, the Td range is from 772 °C to 780 °C (see Fig. 4d). This stability in Td suggests that no major restructuring of the sample is occurring among these different temperature stages, which agrees with our TEM observation in that the edges form single to multiple loops in this temperature range, thereby reducing chemically active sites and reducing the overall reactivity. In order to have a clear picture of the restructuring taking place during annealing treatments, we have identified the transformations that occur for each THT. Fig. 5 shows a proposed diagram imaging such transformations. For THT below 1500 °C the sample
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consists of relatively well ordered planes and well-developed crystallinity (confirmed by XRD and electron diffraction [22]) but the sample still contains point defects (vacancies), out-of-plane interstitial atoms, Stone–Wales and other defects, in addition to the bare edges of the ribbons where some oxygen groups are present [22]. By 1500 °C, defect annealing starts to take place and we observe straighter lattice fringes, approaching a defect-free planar structure, in good agreement with the first stages of the graphitization process of bulk graphite under heat treatment [24]. Moreover, in the case of graphene ribbons, some open edges become passivated by single-loop formation driven by surface energy minimization. For HT 2000 °C, double-loop formation is evident, and as the temperature is increased further, multi-loops with increasing loop diameters are observed and open edges are no longer present. A more detailed study of the thermal annealing of carbon nanoribbons, in which the THT steps are spaced more closely in the range 1000 °C < THT < 2000 °C, will further clarify the exact temperature range of single, double and multiple loop formation. Such a study would also help determine the dynamics of furnace annealing in contrast to annealing by the Joule heating of nanoribbon samples [28], since very different edge behavior is observed for the two types of heat treatment. These differences open widely the application possibilities of these nanoribbons, and suggest a variety of studies at both the fundamental and applied levels. 4. Conclusion We have demonstrated that heat treatments up to 2800 °C of as-produced carbon nanoribbons anneal point defects and interstitials within individual graphene layers, thus leading to a material with a higher degree of crystallinity, in which the reactive edges of the ribbons are transformed into loops in a self-assembly process that can be strongly controlled by the variation of THT. The decrease of the D peak intensity in the Raman spectra as THT increases, the disappearance of the D0 peak, the decrease of the FWHM of the G band, all support the annealing of defects. However, the large number of loops present at the edges of all the ribbons is responsible for observing some residual D-band intensity. The general up-shift of the decomposition temperature in the TGA analysis also reveals a more stable and less reactive material as THT increases in the range above which loop formation takes over. More detailed studies are necessary to understand the physics and chemistry of the loops in more depth and how to use such information to tailor the structural and perhaps the magnetic and electrical properties of these materials for specific applications. Acknowledgements The authors thank Dr. Gene Dresselhaus, D. Ramirez and G. Ramirez for valuable and fruitful discussions. This work is supported by NSF Grants NIRT CTS-05-06830 and DMR-07-04197. We also thank CONACYT-Mexico for Grants: 56787 (Laboratory for Nanoscience and Nanotechnology Research-LINAN), 45772 (MT), 58899-Inter American Collaboration (MT), 2004-01-013/SALUDCONACYT (MT) and PhD scholarships (JCD, AMG). References
Fig. 5. Schematic model for the restructuring process achieved by the thermal annealing treatment (blue balls = carbon atoms, red balls = oxygen atoms, white balls = hydrogen atoms), see text. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
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