NANO LETTERS
Microscopic and Spectroscopic Characterization of Paintbrush-like Single-walled Carbon Nanotubes
2006 Vol. 6, No. 7 1408-1414
Davide Bonifazi,† Christophe Nacci,‡ Riccardo Marega,† Stephane Campidelli,† Gustavo Ceballos,‡ Silvio Modesti,*,‡,§ Moreno Meneghetti,*,| and Maurizio Prato*,† Dipartimento di Scienze Farmaceutiche and INSTM UdR Trieste, UniVersita` degli Studi di Trieste, Piazzale Europa 1, I-34127 Trieste, Italy, Laboratorio Nazionale TASC-INFM, I-34012 Trieste, Italy, Dipartimento di Fisica, UniVersita` di Trieste, I-34127 Trieste, Italy, and Dipartimento di Scienze Chimiche, UniVersita` degli Studi di PadoVa, Via Marzolo 1, I-35131 PadoVa, Italy Received February 20, 2006; Revised Manuscript Received May 5, 2006
ABSTRACT Understanding and controlling the chemical reactivity of carbon nanotubes (CNTs) is a fundamental requisite to prepare novel nanoscopic structures with practical uses in materials applications. Here, we present a comprehensive microscopic and spectroscopic characterization of carbon nanotubes which have been chemically modified. Specifically, scanning tunneling microscopy (STM) investigations of short-oxidized single-walled carbon nanotubes (SWNTs) functionalized with aliphatic chains via amide reaction reveal the presence of bright lumps both on the sidewalls and at the tips. The functionalization pattern is consistent with the oxidation reaction which mainly occurs at the nanotube tips. Thermogravimetric analysis (TGA), steady-state electronic absorption (UV−vis−NIR), and Raman spectroscopic studies confirm the STM observations.
Since the discovery of carbon nanostructures,1 single-walled nanotubes (SWNTs)2 have attracted much attention as one of the most promising nanomaterials with exceptional electronic and structural properties which led to a variety of applications such as field-emission displays,3 nanoscale sensors,4 nanocomposite materials,5 and electronic circuits.6-8 However, the lack of solubility and difficult manipulation both in solution and in the solid state have been the main limitation toward the extensive use of such tubular carbon frameworks. Therefore, to take advantage of the remarkable physical properties of such carbon species, the nanotubes have been functionalized with organic pendant groups, which can enhance both the solubility in organic solvents and the number of reactive sites for further covalent integration into multicomponent organic materials.9-11 The main chemical approaches for the modification of such carbon structures can be grouped into two categories: (i) covalent and (ii) noncovalent functionalization. The covalent derivatization of SWNTs has been mainly focused to modify the sidewalls using oxidizing acids,12,13 fluorine,14,15 alkyl* To whom correspondence should be addressed. E-mail:
[email protected] (M.P.);
[email protected] (S.M.);
[email protected] (M.M.). † Dipartimento di Scienze Farmaceutiche and INSTM UdR Trieste, Universita` degli Studi di Trieste. ‡ Laboratorio Nazionale TASC dell’ INFM. § Dipartimento di Fisica, Universita ` di Trieste. | Dipartimento di Scienze Chimiche, Universita ` degli Studi di Padova. 10.1021/nl060394d CCC: $33.50 Published on Web 06/24/2006
© 2006 American Chemical Society
lithium and Grignard reagents, aryl diazonium salts,16 azomethine ylides,17 nitrenes,18 and organic radicals.19 These products of functionalization, however, are difficult to characterize with classical analytical techniques such as those used in standard organic chemistry. The most popular techniques are scanning and transmission electron microscopy (SEM and TEM), atomic force microscopy (AFM), thermogravimetric analysis (TGA), and Raman and nearinfrared (NIR) spectroscopies. Raman spectroscopy is commonly used as an analytical method to determine the extent of sidewall functionalization by following the occurrence and magnitude of the D mode at 1330 cm-1. However, the distribution of the addend groups along the tube has not been detected yet. Despite the importance as imaging techniques, TEM and SEM are not capable of evaluating the covalent modification of SWNTs because the image does not visualize the presence of organic pendant groups. The TGA technique is capable of measuring the amount of organic material in the sample but does not provide an unambiguous separation between the presence of covalent or noncovalent attached organic species. With these problems in mind, a few authors have started to employ other characterization techniques, such as scanning tunneling microscopy (STM) to directly image the organic modification introduced on the SWNTs.20,21 Specifically, the first STM study of fluorinated SWNTs
Scheme 1.
Purification (p-SWNT), Oxidative Cutting (sh-SWNT), and Synthesis of sh-SWNT-C8 and sh-SWNT-C18
revealed a banded structure which indicates the regions of fluorination.22 Other investigations have been performed on SWNTs derivatized via the Bingel reaction,23 showing a patterned functionalization. During our studies toward the preparation of suitable CNTs as molecular scaffolds for biological applications,24,25 we became interested in the reactivity of short-oxidized SWNTs.13,26 The oxidation reaction, which leads to the formation of carbon pipes bearing carboxylic acid groups, can take place at different sites, namely, at the nanotube ends and at the sidewalls.13,26 For this reaction, it is generally accepted that the nanotube extremities are highly functionalized compared with the sidewalls.26 The most extensively explored covalent modification of oxidized SWNTs is the linkage of organic groups via amide formation between the carboxylic groups and amines.27 So far, only indirect AFM imaging of such substituents upon binding of gold nanoparticles has been reported to support this addition pattern.26,28 In this paper, we show that STM is a versatile technique to morphologically distinguish and localize the presence of organic functions which are covalently attached to the nanotube framework. Functionalization of SWNTs, shortened via oxidative methods and amidated with primary aliphatic amines, that is, 1-octylamine and 1-octadecylamine (Scheme 1), will be described.29 Comprehensive characterization of the functionalized carbon pipes shows that the SWNTs are covalently linked to the aliphatic chains, with a percentage of functionalization of ca. 30% in weight as determined from TGA experiments. Our STM studies on functionalized shSWNTs-C8 and sh-SWNTs-C18 unambiguously show for the first time the presence of the attached aliphatic moieties, which are preferentially located at the nanotube terminations. Purified and shortened single-walled carbon nanotubes shSWNTs were obtained by reaction of pristine HiPco SWNTs (Carbon Nanotechnologies Incorporated, CNI, lot R0496) with HNO3 (2.6 M) and by oxidation with H2SO4-H2O2 at 308 K for 1 h, following a slightly modified protocol described by Smalley and co-workers (Scheme 1).13,26 Both purification and oxidation processes cause the formation of defects on the sidewall and the formation of open ends which are both terminated by carboxylic acid groups. The resulting oxidized material was refluxed with SOCl2 under N2 in the presence of a catalytic amount of DMF to convert the carboxylic acid groups into acyl chloride functions. The acid chloride derivative was then isolated and heated under reflux Nano Lett., Vol. 6, No. 7, 2006
with the corresponding amine derivative to afford functionalized carbon nanotubes sh-SWNT-C8 and sh-SWNTC18.26,27,29 Both sh-SWNT-C8 and sh-SWNT-C18 revealed to be easily dispersible in 1,2-dichlorobenzene yielding stable suspensions. All SWNT intermediates as well as the target compounds sh-SWNTs-C8 and sh-SWNTs-C18 were fully characterized via TGA, UV-vis-NIR, and Raman spectroscopies, TEM (see the Supporting Information), AFM (see the Supporting information), and STM microscopies. Thermogravimetric analysis (TGA, TA Instruments Q500 with a heating ramp of 10 °C‚min-1 until 1000 °C and under N2 flow of 40 mL‚min-1; all of the samples have been heated at 100 °C for 30 min previous measurement) has been used as the first technique to evaluate the purified (p-SWNTs), oxidized (sh-SWNTs), and functionalized materials (sh-SWNTs-C8 and sh-SWNTs-C18) (Figure 1). As seen from Figure 1, a steep weight loss, corresponding to 33% for sh-SWNTs-C18 is observed in the temperature interval of 200-500 °C. We assume that this weight loss occurring during fragmentation is due to pyrolysis of the aliphatic residues. On the basis of this assumption, we estimate that the degree of functionalization as one functional group for every ca. 100 nanotube carbons. Similar results have been also obtained with sh-SWNTs-C8, which showed a weight loss of ca. 35% (see Supporting Information Figure S1).
Figure 1. Thermogravimetric curves for SWNTs after HNO3 treatment (2.30 mg (s)), sh-SWNTs (0.93 mg (- - -)), and sh-SWNTs-C18 (1.63 mg (‚‚‚)). The temperature interval (200500 °C) represents the steepest weight loss due to organic decomposition. 1409
Figure 2. UV-vis-NIR spectra of sh-SWNTs (- - -) in water and sh-SWNTs-C18 (s) in DMF at 298 K.
Figure 3. Micro-Raman spectra of p-SWNTs (black), sh-SWNTs (red), and sh-SWNTs-C18 (blue). Parts (a) and (b) show the RBM and G regions exciting at 633 nm, while parts (c) and (d) show the same spectral regions exciting at 488 nm. All the spectra were normalized using the peak intensity of the G band at about 1600 cm-1.
The modification of the sidewalls in sh-SWNTs was also evident in the steady-state absorption spectra in the UVvis-NIR regions because of the partial loss of the sharpest van Hove singularities typical for pristine SWNTs.13,27,30 Notably, the broad features of the spectrum of nanotube derivatives sh-SWNTs-C18 are similar to that of sh-SWNTs showing that the amidation reaction step does not appreciably affect the carbon framework. Raman spectra of the SWNTs were also measured to investigate the effect of the derivatization. The spectra of the oxidized nanotubes, sh-SWNTs, and of sh-SWNTs-C18 are reported in Figure 3. The spectrum of sh-SWNTs-C8 was found very similar to that of the C18-bearing derivative. Assignment of the radial breathing modes (RBMs) in the low-frequency region (see parts a and c of Figure 3) can be obtained on the basis of experimental Kataura plots31,32 which allow the interpretation of the resonance Raman activity of these bands. Excitation at 633 nm (Figure 3a) leads to the identification of metallic tubes with larger diameter (d) at lower frequencies (wRBM ∝ 1/d), whereas semiconducting nanotubes with smaller diameters are detected at higher frequencies. The opposite is found at excitation at 488 nm 1410
(Figure 3c), where nanotubes with large and small diameters are observed at high and low frequencies, respectively. The comparison between the spectrum of the pristine long nanotubes and that of sh-SWNTs shows that the oxidation reaction severely affects both the RBM spectral region (see parts a and c of Figure 3) and the G-band region at about 1600 cm-1 (see parts b and d of Figure 3). In the spectra of the sh-SWNTs, part of the RBM-centered bands with high frequency are absent (283 and 297 cm-1 exciting at 633 nm and 307 cm-1 exciting at 488 nm). This is the consequence of the larger reactivity of the small diameter SWNTs, which are more easily destroyed than those with larger diameters under such oxidative conditions.33 Notably, the intensity of the RBM modes corresponding to the large-diameter shSWNTs are modified (196 and 203 cm-1 exciting at 633 and 488 nm, respectively), but they partially recover their original intensity after the amidation reaction. The latter observation reveals that such alteration of the signal intensity may be induced by a perturbation of the electronic structure as a consequence of the oxidation and not to a mass destruction of the nanotubes as in the case of sh-SWNTs. After the chemical functionalization, the Raman spectrum becomes more similar to that of the pristine nanotubes, although some upshifts of the frequencies are generally observed and the RBM modes with higher frequencies are absent. On the basis of these assignments, the SWNTs with diameters smaller than 0.90 nm are selectively destroyed. Confirmation of this finding can be obtained from the STM observations (see below). In the G-band frequency region, the band is upshifted by 8-10 cm-1 and broad if compared with that observed with SWNT before the oxidation treatment. Both frequency and width recover after the amidation reaction. Furthermore, the relative intensity of the D band, with respect to the G band, is strongly increased in the spectra of sh-SWNTs and partially recovers its original intensity after the amide functionalization. The D band acquires intensity when defects are present on the structure of the nanotube.34,35 As expected, its strong intensity reveals that the oxidation of the nanotubes was obtained as confirmed by the STM investigations (see below), which show an enhancement of defects on the sidewall of sh-SWNT. A larger number of defects is found for the functionalized nanotubes with respect to the oxidized ones (see below). Although this can be traced back to the ease with which the defects can be observed when an aliphatic chain is present, the smaller intensity of the D band in the spectra of sh-SWNT-C8 and sh-SWNT-C18 (see parts b and d of Figure 3) suggests that the density of defects not only affects such intensity but also affects their structural characteristics. For instance, the high intensity of the D band shown in the spectra of sh-SWNT can be correlated to the presence of a high number of localized charges surrounding the defect sites than in the case of the amidate SWNTs, shSWNT-C8 and sh-SWNT-C18. The structural modification of the nanotube framework caused by the oxidation and amidation steps has been investigated by means of STM. All topographic images of p-SWNTs, sh-SWNTs, sh-SWNTs-C8, and sh-SWNTs-C18 Nano Lett., Vol. 6, No. 7, 2006
Figure 4. Constant current STM images (Vbias ) 200-1000 mV, It ) 0.2-1 nA, T ) 298 K, under UHV) of purified p-SWNTs showing the carbon lattice on the sidewall (a), the terminals (b), and some irregular structures (c) which are probably attributed to amorphous carbon or catalyst particles adhered on the carbon surface.
Figure 5. Constant current STM images (Vbias ) 700-1000 mV, It ) 0.2-0.5 nA, T ) 298 K, in air) of oxidized carbon nanotubes sh-SWNTs showing both regular tubes (a) and tubes bearing some irregularities both at the ends or/and on the sidewall (b)-(d).
were acquired in the constant current mode with microscopes operating both in ultrahigh vacuum (UHV) and in air. All samples for STM analysis have been prepared by deposition in air via spin-coating of a suspension of the nanotubes onto Au(111) films grown on mica. We first studied the STM images of purified carbon nanotubes (p-SWNTs, after HNO3 treatment). A typical high-resolution STM image of a single SWNT nanotube, reported in Figure 4a, clearly shows the hexagonal lattice of the nanotube carbon atoms. The majority of the imaged p-SWNTs show a mean diameter of ca. 1 nm, smooth sidewall surfaces with a corrugation of about 0.05 nm (Figure 4a), and rounded endcaps (Figure 4b). Among 200 analyzed p-SWNTs, less than 25% of the units bear irregularities either on the sidewalls or at the terminations (Figure 4c). The linear density (F) of such irregularities on the sidewalls is 2.4 ( 0.6 µm-1 (n ) 70, n ) number of analyzed carbon nanotubes). A certain number of irregularities found on the nanotubes can be reasonably attributed to impurities of metal nanoparticles and amorphous carbon, which have also been observed as separated objects in the analyzed samples. As expected, most of the p-SWNTs are aggregated in bundles, which severely limits a precise statistical treatment of the nanotube diameter and length. In contrast to the precursor material, sh-SWNTs were very difficult to disperse in any solvents so that the preparation of suitable samples for STM characterization was more problematic. The STM images of samples containing shSWNT are shown in Figure 5. The sh-SWNTs maintain the same structural features, that is, a smooth sidewall and extremities, as those of p-SWNTs although with a much Nano Lett., Vol. 6, No. 7, 2006
Figure 6. STM images (Vbias ) 700-1100 mV, It ) 0.2-0.5 nA, T ) 298 K, in air) of the nanotube terminations (a and b) and of the lumps on the sidewall (c and d) of functionalized nanotubes sh-SWNTs-C8.
shorter length (less than 500 nm as measured by AFM and TEM). The distribution of the nanotube length measured by STM has a mean value of 180 nm, with a variance of 160 nm. The nanotube units bear more irregularities, mainly visible as bright small lumps on the sidewalls (F ) 4.2 ( 0.6 µm-1), which we attribute to the additional carbon lattice defects introduced by the oxidative treatment. The STM images of the functionalized material containing sh-SWNTs-C8 are displayed in Figure 6. The most striking characteristic observed in these images is the presence of bright protuberances at the nanotube ends which have not been observed in the purified and oxidized materials. As clearly shown in Figure 6a, some of these protuberances recall the structure of a brush, the bristles of which are associated with the aliphatic C8 chains. In images taken in proximity of the functionalized regions, that is, near to the nanotube terminations, atomic resolution has been never reached. The latter observation is evidenced in Figure 6b where a nonstructured protuberance is linked to a regular graphene-like termination. Another important characteristic is the presence of broad and bright lumps that form on the tubular sidewalls of sh-SWNTs-C8 (parts c and d of Figure 6). The percentage of SWNTs bearing such protuberances both on nanotube sidewalls and at terminations is 55 ( 8% (n ) 89). The length distribution is similar to that of nonfunctionalized oxidized nanotubes, while the linear density of the lumps (F ) 11 ( 2 µm-1) on the sidewall rises by about a factor of 3 with respect to the irregularities detected on the sh-SWNTs. The images of the nanotubes functionalized with the C18 chains, sh-SWNT-C18, are quite similar to those functionalized with the shorter chains (Figure 7). The main difference is the average linear density of the visible lumps on the sidewall surface (parts c and d of Figure 7) that, with C18 chains, reaches a value of 17 ( 4 µm-1 (n ) 30). About 20% (n ) 20) of the observed carbon tube ends terminate with nonstructured protuberances which are appreciably wider than the mean nanotube diameter (Figure 7, see also Supporting Information Figure S4). The large increase of the density of the visible lumps on both sidewall and 1411
Figure 7. Constant current STM images (Vbias ) 500-1000 mV, It ) 0.2-0.5 nA, T ) 298 K) of sh-SWNTs-C18 showing irregularities at the terminations (a) and (b) and on the sidewalls (c) and (d).
termination derivatized with aliphatic C18 chains indicates that most of the observed lumps are originated by such organic groups. It is likely that the chains on the side of the tubes facing the substrate cannot be detected in our STM images. Therefore, the actual linear density of the functionalized sites should be about twice the measured values. The larger lump density detected on samples containing shSWNT-C18 with respect to sh-SWNT-C8 is attributed to the better detectability of the long chains in STM. There is a discrepancy between the density of the functionalized sites estimated by STM (20-40 µm-1) and that measured by TGA (2000 µm-1). This is most probably due to the large number of aliphatic chains present at the SWNT ends, the number of which cannot be estimated correctly by STM, and to the fact that each lump is originated by several chains bound to the same defect on the sidewall. The mean diameter of 100 observed lumps (70 for shSWNT-C8 and 30 for sh-SWNT-C18) is ca. 3 nm, and their height is ca. 0.3 nm. While their diameter is compatible with twice the length of the aliphatic chains, their height is much shorter. However, it is not possible to experimentally determine the relative orientation of the chains with respect to the SWNT sidewall from our data for the following two reasons: (i) the measured diameters of the lumps are affected by the convolution effect caused by the finite size of the tip which therefore leads to overestimations and (ii) even if the chains were perpendicular to the tube, we could not observe a lump 1-2 nm high. This is because the bias voltages that we could apply without displacing or modifying the functionalized carbon nanotubes were lower than 1.5 V. At these bias voltages, an aliphatic C18-carbon chain conducts less than 10-11 A along its length,36,37 since both the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) are far from the Fermi level.38 Our tunneling current was much higher, namely, 10-10 to 10-9 A. Under these conditions, a STM tip placed on top of a chain protruding toward the vacuum should approach the surface of the tube appreciably before it can find a position where the tunneling current reaches values on the order of 1 nA. Therefore, the STM tip will mechanically affect the spatial conformation of the chain, with the ultimate effect of tilting the aliphatic moieties. 1412
Figure 8. Tunneling spectra of a functionalized region (a) and of an intact region (b) of the same sh-SWNTs-C8 nanotube. The arrows point to the van Hove singularities energies closer to the Fermi level that corresponds to V ) 0. The gap of the intact tube is filled in the functionalized region, and the van Hove singularities are no longer detectable.
Besides this effect, the electrostatic and the van der Waals interactions between the aliphatic chains and the STM tip can severely affect the chain conformation, thus bending the aliphatic moieties during the scanning operation. These interactions tend to tilt the chains toward the tip, bringing them in contact before and after the STM probe reaches the local site where the chain is bound. Since the aliphatic chains give a small contribution to local density of states (DOS) in the energy region of the electron tunneling,38 our STM measurements should be more sensitive to the local changes of the CNTs-centered DOS which are supposedly induced by the amide bonds rather than the aliphatic-centered DOS. These changes are delocalized over distances of a few nanometers.39 The local DOS measured by tunneling spectroscopy on the lump and at an unperturbed position of the same CNT is shown in Figure 8. Whereas ca. 10 nm away from the lump the spectra of the semiconducting nanotube show a clear energy gap of ca. 0.6-0.7 eV across the Fermi level (Figure 8b), in the lump region the gap is filled (Figure 8a). It is known that defect sites on the sidewall, tube terminations, and sp3-hybridized C-atoms introduce additional levels in the gap of CNTs. If more chains are bound to nonequivalent C-atoms surrounding a defect site on the sidewall, it is likely that several energy levels contribute to fill the gap with a rather smooth background. A similar background in the nanotube band-gap region has been predicted by calculating the DOS of SWNTs functionalized via 1,3-dipolar cycloaddition.40 The local DOS of the defectNano Lett., Vol. 6, No. 7, 2006
Table 1. Structural Properties of the Functionalized Nanotubes sample p-SWNTs sh-SWNTs sh-SWNTs-C8 sh-SWNTs-C18 a
length (nm) (variance)
Fa (µm-1)
not functionalized (%)
functionalized (%)
180 (160) 181 (110) 160 (170)
1.4 ( 0.6 3.6 ( 0.7 11 ( 2 17 ( 4
71 ( 44 ( 7 37 ( 7 22 ( 9
26 ( 3b 55 ( 8 61 ( 9 78 ( 17
6b
Flump ) linear density of protuberances. b Irregularities as originated from the purification step.
induced levels in the gap decays to zero as a function of the distance from the defect within 1-2 nm.39 A similar decay was also observed in our samples. In conclusion, the sp3 C-atoms on the nanotube surface that bind the aliphatic chains cause an increase of the local DOS in the SWNT gap that is localized within 1-2 nm. Such DOS enhancement causes a displacement of the tip far from the tube surface in a region of similar size around the site where the chain is bound. All these effects can explain why the nanotube sites containing the chains appear as fuzzy, wide, and flat lumps. Besides the filling of the nanotube gap caused by the presence of sp3-hybridized C-atoms surrounding a defect on the nanotube sidewall, a second effect could contribute to making the aliphatic groups visible under our tunneling conditions, even if their HOMO and LUMO levels are well outside the gap, far from the Fermi energy level. The aliphatic chains can be visible because, even though a resonance of an atom or a molecule on a surface lies far above the Fermi level, it may nevertheless make a contribution to an STM image taken at low bias voltage if the size of the orbital associated with that resonance is such that it extends considerably further out into the vacuum than the bare-surface wave functions.41,42 Even if the center of the LUMO resonance is more than 1 eV higher than the Fermi level (and the center of the HOMO and LUMO resonances is well outside the energy range of the tunneling spectra)38 its tail can still substantially contribute to the DOS in the energy region (as observed in the gap region in Figure 8a) of the tunneling electron close to the Fermi level.41 The shSWNTs-C18 show a linear density of the lumps that is four times greater than that of sh-SWNTs, while the number of the defects on the sidewall of the tubes caused by the oxidation processes should be the same in the two cases. This indicates that most of the contrast mechanism that makes the lumps visible is caused by the C-atoms bound to the chains and by the chains themselves. At the termination of the functionalized tubes, the aliphatic chains lie nearly flat with respect to the Au substrate. For these chains, the second mechanism discussed above is expected to make them conducting and thus visible in our STM images. As discussed for the functional groups on the sidewalls, the organic chains attached at the nanotube termination can be severely affected by the electrostatic and van der Waals interactions with the STM tip, which can consecutively bend them during the scanning operation making their imaging fuzzy. The energy gaps between the filled and empty van Hove singularities closer to the Fermi level measured by tunneling spectroscopy on sh-SWNT-C18 (n ) 10) were smaller than Nano Lett., Vol. 6, No. 7, 2006
0.8 and 1.8 eV for semiconducting and metallic nanotubes, respectively. These values correspond to nanotube diameters larger than 0.9 nm. Therefore, the tunneling spectroscopy data confirm the Raman results on the selective destruction of small diameter nanotubes. In conclusion, we have reported the first comprehensive investigation of amido-functionalized SWNTs. In particular, the first STM investigations performed on short-oxidized SWNTs functionalized via the amidation reaction with aliphatic amines display the aliphatic moieties covalently attached to the carbon pipes as bright lumps on both CNT ends and sidewall. The functionalization pattern is consistent with the oxidation reaction mechanism which mainly occurs at the nanotube tips. This powerful microscopy probe, in combination with other characterization techniques, such as TGA, Raman, UV-vis-NIR, AFM, and TEM, has provided new insights into the structural aspects of chemical reactions occurring on CNTs. Experimental Section Characterization. The spectra in the UV-vis-NIR range were taken using a Varian CARY5 UV/vis/NIR spectrometer. The thermal degradation studies were performed with a TGA Q500 instrument from TA instruments. The micro-Raman spectra were collected with an inVia Renishaw system equipped with 633 and 488 nm laser sources. Sample Preparation for STM Analysis. All carbon nanotube materials were deposited on Au(111) substrates in the air. Flat Au(111) terraces (roughness
