Chemical Functionalization of Single-Walled Carbon Nanotubes

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J. Phys. Chem. C 2008, 112, 13141–13149

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Chemical Functionalization of Single-Walled Carbon Nanotubes (SWNTs) by Aryl Groups: A Density Functional Theory Study Jing-xiang Zhao†,‡ and Yi-hong Ding*,† State Key Laboratory of Theoretical and Computational Chemistry, Institute of Theoretical Chemistry, Jilin UniVersity, Changchun 130023, People’s Republic of China and Department of Chemistry, Harbin Normal UniVersity, Harbin 150080, People’s Republic of China ReceiVed: April 9, 2008; ReVised Manuscript ReceiVed: June 4, 2008

Considering the wide applications of aryl diazonium functionalized single-walled carbon nanotubes (SWNTs) and the rapid progress in experiment, in this article we report the first theoretical study of the adsorption of aryl groups on a series of SWNTs using density functional theory (DFT) calculations. Tube diameter, chirality, and the number of aryl groups play important roles on the tube-aryl group interaction. A single aryl group adsorption on metallic tubes is significantly stronger than that on semiconducting tubes, and the adsorption energy decreases with increased tube diameter. The addition of more aryl groups is considered for an (8,0) tube, and the results indicate that the aryl groups prefer the pair arrangement with stronger adsorption energy; the effects of the number of the adsorbed aryl groups on adsorption energy and band gap of SWNTs are further addressed. Our calculations are useful not only to deeply understand available experimental results, but also to further investigate the functionalization of SWNTs. 1. Introduction 1991,1

there has been a great deal of Since their discovery in interest in chemical functionalization of single-walled carbon nanotubes (SWNTs) to facilitate their separation2 and manipulation, enhance their solubility, and make them useful in different kinds of applications in areas such as versatile building blocks for new materials.3 Experiments on nanotube functionalization start from the fluorination of SWNTs followed by a substitution reaction of fluorinated SWNTs in solutions.3a,4 Direct functionalization to the surface of SWNTs by various functional groups has been reported.4 Among these functionzalizations, SWNTs5 functionalized by aryl diazonium salts have important applications such as in separating metallic and semiconducting SWNTs,2d fabricating nanobiolectronics, and incorporation into polymer composite materials. Interestingly, both aryl diazonium salts and SWNTs have been shown to be aromatic compounds. Thus, merging two various kinds of aromatic compounds into a new avenue of research can generate great enthusiasm and interest. Experimental studies have indicated that single or multiple layers of aryl groups can be successfully attached to the SWNT surfaces via grafting of electrochemically reduced aryl diazonium salts.5a In these experiments, changes in physical properties of SWNTs upon aryl groups have been observed by analyzing the Raman spectra before and after aryl group adsorption, and the shifting of Raman spectra has also provided evidence of charge transfer between the functional group and the nanotube. In light of the rapid progress of the aryl diazonium saltsfunctionalized SWNTs in experiments and their potential applications, we feel that it is highly desirable to investigate the interactions between SWNTs and aryl groups from a theoretical point of view. Such knowledge should be useful not only to explain the nature of interactions between SWNTs and * Corresponding author e-mail: [email protected]. † Jilin University. ‡ Harbin Normal University.

functional groups, but also to further functionalize SWNTs by other functional groups as well as building nanodevices for new materials. Unfortunately, to the best of our knowledge, there have been no theoretical reports on the aryl group-functionalized SWNTs. In this article we report the first systematic investigation of the chemical functionalization of aryl groups onto various SWNTs using density functional theory (DFT) methods. The following questions will be explored: (i) what is the stable geometry and the nature of the bonding for different numbers of aryl groups adsorbed on SWNTs? (ii) What are the effects of tube diameter, chiralities, and number of the adsorbed aryl group on tubes? (iii) How are the electronic properties of SWNTs modified by functionalization with different numbers of aryl groups? Our results might be helpful for understanding the experimental results and for further investigations of the functionalization of the tube wall. 2. Models and Methods In this work, we use DFT methods that are implemented in the Dmol3 package6 to study the interactions between aryl groups and various SWNTs. All-electron calculations are employed with the double numerical basis sets plus polarization functional (DNP), which is the most complete set available in the DMol3 code and has been widely used in the theoretical calculations of SWNT systems, including covalent functionalizations of SWNTs.7 In this basis set, the 2s and 2p carbon orbitals are represented by two wave functions each, and 3d (2p) type wave functions on each carbon (hydrogen) atom are used to describe the polarization. Our total energy calculations and corresponding structures optimizations of the most stable geometries are based on the generalized-gradient approximation (GGA) with the Perdew-Burke-Ernzerhof (PBE) correction.8a The GGA/PBE method can reduce the overbinding effects caused by the local density approximation (LDA) and may also compensate the overestimation of the binding energy based on

10.1021/jp8030607 CCC: $40.75  2008 American Chemical Society Published on Web 08/06/2008

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Figure 1. Optimized geometries of a single aryl adsorbed on SWNT structures: (a) the plane of the aryl group perpendicular to the SWNT (8,0) axis, (b) the plane of the aryl group parallel to the SWNT (8,0) axis, (c) the plane of the aryl group perpendicular to the SWNT (6,6) axis, and (d) the plane of the aryl group parallel to the SWNT (6,6) axis. All bond lengths are in units of Ångstro¨ms.

the DNP basis sets, compared with the planar-wave basis set. Moreover, GGA/PBE is known to adequately predict the density of states as compared to experiment.8b Mulliken population analysis is used to obtain the charge on each atom. Spinunrestricted DFT calculations are performed for an odd number of aryl groups adsorbed on a SWNT in a periodically repeating tetragonal supercell with lattice constants of a ) b ) 20 Å, and c at 8.52 and 9.84 Å is adjusted to the periodicity along the zigzag and armchair SWNTs in order to minimize the interaction between the aryl groups in adjacent cells. The Brillouin zone of the supercell is sampled by 1 × 1 × 3 k-points within the Monkhorst-Pack scheme.9 The total adsorption (or binding) energy of n aryl groups adsorbed on SWNTs is defined by eq 1:

Eads(n) ) ESWNT@n-aryl - ESWNT - nEaryl

(1)

where Eads(n) stands for the total adsorption energy of n aryl

TABLE 1: C-C Bond Length (d) between Different Carbon Nanotubes and a Single Aryl Group, Adsorption Energy Eads, and Charge Transfer (QZ) from Carbon Nanotubes to the Aryl Group SWNT

diameter (Å)

d(C-C) (Å)

QZ

Eads (eV)

(4,4) (5,5) (6,6) (7,7) (8,8) (9,9) (10,10) (8,0) (9,0) (11,0) (12,0) (14,0) (15,0) (16,0)

5.42 6.78 8.14 9.49 10.85 12.20 13.56 6.26 7.05 8.61 9.39 10.96 11.74 12.53

1.55 1.55 1.56 1.56 1.57 1.57 1.57 1.55 1.56 1.56 1.56 1.57 1.57 1.57

0.13 0.13 0.13 0.13 0.13 0.13 0.13 0.15 0.14 0.14 0.14 0.14 0.14 0.15

-1.74 -1.43 -1.21 -1.16 -1.12 -0.92 -0.81 -1.48 -1.53 -1.10 -1.23 -0.95 -1.19 -0.70

Chemical Functionalization of SWNTs by Aryl Groups

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Figure 2. (a) The individual aryl group adsorption energy, (b) bond length of Ctube-Caryl nanotube diameter.

groups on SWNT, whereas ESWNT@n-aryl, ESWNT, and Earyl stand for the total energy of the n-aryl-adsorbed SWNT, the nonaryl-adsorbed SWNT, and the single aryl group, respectively. A negative Eads(n) corresponds to a stable adsorption structure. If we suppose that the aryl groups are adsorbed one by one, then the adsorption energy of the nth aryl group on SWNT can be written as follows.

Eads(nth) ) Eads(n) - Eads(n - 1)

(2)

) (ESWNT@n-aryl - ESWNT - nEaryl) - (ESWNT@(n-1)-arylESWNT - (n - 1)Earyl) (3) ) ESWNT@n-aryl - ESWNT@(n-1)-aryl - Earyl

(4)

3. Results and Discussion 3.1. Adsorption of One Aryl Group. We first search the stable adsorption geometry of one aryl group onto SWNTs. Two initial configurations are considered for the individual aryl group adsorbed on the SWNTs sidewall: the group plane is (i) parallel and (ii) perpendicular to the tube axis. Figure 1 shows the side view of the two stable geometries of the isolated aryl group adsorbed on the zigzag (8,0) SWNT. According to the calculated adsorption energies, the most stable configuration can be determined. As shown in Figure 1a, the aryl group plane is perpendicular to the tube axis and the C-C bond length between the C atom on the tube and that of the group is about 1.55 Å, whereas those of the C atom and its three nearest neighbors on the nanotubes are 1.54, 1.53, and 1.51 Å, respectively, which are close to the C-C distance in the sp3-hybridized diamond

group,

and (c) pyramidalization angle, as a function of

phase and are significantly longer than the C-C bond length of 1.42 Å in the perfect nanotube with sp2-hybridization. The adsorption energy in Figure 1a is -1.48 eV, which is significantly larger than that on the graphene sheet with an adsorption energy of 0.25 eV.10 Rotating the plane of the aryl group in Figure 1a along the C1-C2 bond by 90°, another stable configuration is obtained (Figure 1b), and the corresponding energy is raised by 0.32 eV with a bond length of 1.58 Å. Similar to the case of the zigzag nanotube, we also study the adsorption of an individual aryl group on an armchair (6,6) tube and find that the aryl group prefers an orientation with the aryl group plane perpendicular to the tube axis, as shown in Figure 1c, with a adsorption energy of -1.21 eV. According to the results of the isolated aryl group adsorbed on the (8,0) and (6,6) SWNT, we choose the configuration shown in Figure 1, panels a and c, for the rest of the calculations to investigate the adsorption of the aryl group on other zigzag and armchair nanotubes. Table 1 summarizes the adsorption energies (Eads) and C-C bond length between the aryl group and the tubes with various diameter and chirality. In Figure 2a we plot the adsorption energies of various SWNTs corresponding to the most stable configurations versus their diameter. One of the most important results from Table 1 and Figure 2a is the dependence of adsorption energy on the electronic structures. Regardless of diameters, the adsorption on metallic nanotubes is consistently stronger than that on semiconducting tubes. Note that the adsorption energies of an individual aryl group on (3n,0) zigzag nanotubes is close to those of armchair nanotubes, which may

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Figure 3. The calculated geometries of an additional aryl group adsorbed structure of (a-d) the (8,0) zigzag nanotubes and (e-i) the (6,6) armchair nanotubes. All bond lengths are in units of Ångstro¨ms.

Chemical Functionalization of SWNTs by Aryl Groups TABLE 2: Some Calculated C-C Bond Length d(C-C), Adsorption Energy Eads of the Second Aryl Group Adsorbed on Different SWNTs, and Charge Transfer from the SWNT to the Second Aryl Group SWNT (4,4) (5,5) (6,6) (7,7) (8,8) (9,9) (10,10) (8,0) (9,0) (11,0) (12,0) (14,0) (15,0) (16,0)

d(C-C) (Å) 1.549,a

b

1.526 1.541, 1.525 1.537, 1.522 1.537, 1.514 1.538, 1.514 1.533, 1.514 1.530, 1.512 1.618, 1.547 1.617, 1.545 1.615,1.545 1.615, 1.542 1.613, 1.542 1.612, 1.541 1.606, 1.536

d(C-C) (Å)

QZ

Eads (eV)

1.55c

0.13d

1.55 1.56 1.56 1.56 1.56 1.57 1.55 1.55 1.55 1.56 1.56 1.56 1.56

0.14 0.14 0.14 0.14 0.14 0.14 0.11 0.11 0.11 0.11 0.11 0.11 0.12

-2.31 -2.10 -1.95 -1.85 -1.84 -1.76 -1.70 -2.43 -2.31 -2.08 -2.01 -1.94 -1.88 -1.71

a For the axial C-C bond near the adsorption sites. b For the zigzag or armchair C-C bond near the adsorption sites. c The average values of the C-C bond lengths between different SWNTs and aryl group. d The average values of the charge transfer from different SWNTs to aryl group.

Figure 4. Schematic illustration for two patterns of additional aryl groups on an (8,0) tube. (a) Ortho-pattern and (b) para-pattern adsorption.

be due to the semimetallic character of these tubes. In short, it is obvious that the adsorption of the aryl group is dependent on the tube chirality, which is the prerequisite of separation of metallic and semiconducting SWNTs. An equally important result from Table 1 and Figure 1a is that the adsorption energy is strongly dependent on the tube diameters. The adsorption becomes weaker with increasing diameters in both metallic and semiconducting nanotubes. Figure 2b shows the bond length between the aryl group and various SWNTs as a function of diameter. As can be seen, the bond length between the aryl group and the armchair tubes is in the range between 1.55 and 1.57 Å, consistent with the trend of changes in adsorption energy. In zigzag nanotubes, however, the bond length does not increase with increasing diameters, which is determined by the metallicity. It is also interesting to find the changes of pyamidalization angle (θp), as a measure of degree of hybridization, defined as (θσπ - 90)°, where θσπ is the bond angle between σ and π bonds.4c As shown in Figure 2c, the θp monotonically decreases with increasing diameters,

J. Phys. Chem. C, Vol. 112, No. 34, 2008 13145 indicating that the bigger the diameters of nanotubes, the lower the reactivity toward adsorbates. 3.2. Adsorption of the Second Aryl Group on a SWNT. Next, we study another aryl group adsorbed on SWNTs. As shown in Figure 3, various possible configurations are considered based on the above results of individual aryl group adsorption. We find the adsorption of the second aryl group is strongly site-dependent, and the possible adsorption manner of the second aryl group on zigzag and armchair tubes is different. For the zigzag (8,0) tube, four sites are considered for the adsorption of the second aryl group as shown in Figure 3, panels a-d, respectively. Among the locally stable configurations, the most energetically favored configuration is shown in Figure 3a, in which the second aryl group is located at ortho positions in the same hexagon. The adsorption energy of the most stable configuration is -2.43 eV, which is significantly larger than that of the most stable configuration for the adsorption of individual aryl group (-1.48 eV). It is obvious that the interaction can be significantly strengthened with two aryl groups attached to the zigzag SWNTs. This is easily understandable, that is., the adsorption of a single aryl group with an unpaired electron activates these carbon atoms near adsorption sites, and thus these carbon sites can be considered as the “defective” of the tube, and further additions of aryl group can easily take place at these sites.11 As shown in Figure 3a, some C-C bonds adjacent to aryl group adsorption sites are severely weakened, and the two aryl groups are tilted along the axis, which may be due to the steric repulsion between the two aryl groups. In addition to the lowest-energy configuration, there are three metastable adsorption configurations, that is, the second aryl group adsorbed on meta sites in the same hexagon (Figure 3b, adsorption energy of -1.52 eV) and the configuration with two aryl groups adsorbed on para positions (Figure 3c, adsorption energy of -2.24 eV). The configuration of two symmetric C sites as far away as possible is the least energetically preferred one with the adsorption energy of -1.72 eV, in Figure 3d. These results are similar to the adsorption of two NO2 molecules on SWNTs.7c,11 Different from the case of the second aryl group adsorption on the zigzag tubes, Figure 3, panels e-h, presents the several possible configurations of the second aryl group adsorbed on the armchair (6,6) nanotube. The para-configuration is the most favorable with an adsorption energy of -1.95 eV (Figure 3e), which is larger than that of the most stable configuration of the first aryl group adsorbed on armchair (6,6) tube (-1.21 eV). The adsorption energies and the corresponding bond distance of other metastable adsorption configurations are given in Figure 3, panels f-h. We further examine the second aryl group adsorbed on other zigzag and armchair SWNTs as displayed in Table 2, again suggesting that the interaction between the aryl group and zigzag or armchair SWNT can indeed be significantly strengthened when a pair of aryl groups is adsorbed on the tube surface. 3.3. Adsorption of More Aryl Groups on an (8,0) SWNT. The next important question is: can more than two aryl groups be adsorbed on the (8,0) SWNT? To study this issue, we perform further investigations on an (8,0) tube by increasing the number of the adsorbed aryl groups from 3 to 8. On the basis of the above results of two aryl groups adsorption, we consider two isomers for the adsorption of more aryl groups on an (8,0) SWNT, namely, (a) the para-pattern, where other aryl groups are attached to the tube in the sequence that the successive aryl group is adsorbed at the para-position relative to the previous along the circumferential direction (see

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Figure 5. Variation of (a) adsorption energy, (b) band gap, and (c) value of electron charge transfer as a function of the number of aryl groups for ortho-pattern adsorption.

Figure 6. TDOS values and some partial DOS values for a single aryl group adsorbed on the (a) (8,0) SWNT and (b) (6,6) SWNT.

Chemical Functionalization of SWNTs by Aryl Groups

Figure 7. Spin density distribution of a single aryl group adsorbed on (8,0) SWNT.

Figure 4a); and (b) the ortho-pattern (see Figure 4b), which is deduced from the most stable configuration of two aryl groups adsorbed at the ortho-site in the (8,0) tube. The variation as a function of number of aryl groups for an (8,0) tube is displayed in Figure 5. An important result from 5 is that the adsorption of aryl groups favors the form of a pair because naryl group ) 2, 4, 6 exhibits a larger adsorption energy than that of naryl group ) 1, 3, 5. This can be understood by the following explanation; the chemisorption of an odd number of aryl groups leaves radicals on the tube and “activates” the carbon atoms near the adsorption sites, thus the addition of the next aryl group is very desirable. The formation of aryl group pairs is similar to the case of NO2 molecules adsorbed on SWNTs.11 Moreover, we find that the bond length between aryl groups and the (8,0) SWNT are shorter than that of adsorption of the first aryl group. The tube diameter of the (8,0) tube is significantly increased when the eighth aryl group is bonded onto the tube. For the para-site adsorption (Figure 4b), the calculated values of Eads for the number of the adsorbed aryl groups from 3 to 8 are -1.60, -2.18, -1.35, -1.93, -1.62, and -1.23 eV, respectively. Compared to the ortho-pattern adsorption, this kind of adsorption is less favorable. 3.4. Electronic Structures of SWNT after Aryl Groups Adsorption. One of the purposes of functionalization is to modify the electronic structures of the system, especially for chemical sensors and nanobioelectronic devices. Because there is an unpaired electron on the individual aryl group, the adsorption of one aryl group induces a half-occupied impurity states around the Fermi level of the pristine SWNT. To gain an in-depth understanding of the changes of electronic properties of SWNTs, it is essential to calculate the electronic structures of SWNTs before and after aryl group adsorption. Figure 6a shows the total electron density of states (TDOS) of one aryl group-functionalized SWNT (8,0); we note a sharp peak in the TDOS near the Fermi level, indicating a significant increase in conductivity of SWNT (8,0) due to the aryl group adsorption. Moreover, the local density of states (LDOSs) in Figure 6a shows that this sharp peak is contributed by the carbon atoms of the hexagon where the aryl group is adsorbed and of

J. Phys. Chem. C, Vol. 112, No. 34, 2008 13147 the aryl group. Herein, the carbon atoms of the same hexagon where aryl is adsorbed are labeled as ortho-C, meta-C, and paraC, respectively, such that we can distinguish the degree of the contribution of these carbon atoms. From 6a, we observe that the peak at the Fermi level in TDOS is mainly originated from the 2p orbital of ortho-C and para-C atoms, whereas the contribution of meta-C and the aryl group near the Fermi level can be neglected. Therefore, these carbon atoms are more active and thus facilitate the subsequent chemical reactions, such as the adsorption of the next functional group. As shown in Figure 6b, we present the TDOS and some LDOS values for a single aryl group adsorbed on the SWNT (6,6). Increasingly, the tube with metallic character is modified to a narrow gap semiconductor material when the aryl group is attached to it. Similar to the case of SWNT (8,0), the ortho-C and para-C sites have higher reactivity toward the additional adsorbate than other carbon atoms (Figure 4b). Charge-transfer is the main mechanism in changing the electronic structures of the tube. Thus, we study the charge transfer of adsorption of a single aryl group on SWNTs as summarized in Table 1. Using the (8,0) SWNT case as an example, the Mulliken population analyses show that about 0.15 electron is transferred from the (8,0) tube to the aryl group. Note that the attachment of an individual aryl group causes an unpaired electron on the nanotube surface. By obtaining local magnetic moments on each atom of the whole system, we find that the unpaired electron is not centered on one specific carbon atom but is distributed among many carbon atoms. The atomic spin density at the ortho, meta, and para sites and the aryl group is 0.18, -0.04, 0.08, and 0.04 e, respectively. Figure 7 plots the spin density distribution of the aryl group adsorbed-(8,0) tube, and it is obviously seen that the spin densities mainly focus on the orto and para atoms, implying that the second aryl group is preferably attached to the two atoms. Among other SWNTs considered, significant electrons (about 0.14 e) transferred from the tube to the aryl group (Table 1). It is interesting to note that the value of the charge transfer of the aryl group does not depend on the electronic structures. Similar to the case of the (8,0) tube, the spin densities mainly distribute at ortho and para sites. Next, we also examine the density of states when the second aryl group is adsorbed on the SWNTs. As shown in Figure 8, after the adsorption of the second aryl group on SWNT (8,0) and SWNT (6,6), a new energy level is found in the band gap, which comes from the contributions of the carbon atoms near the adsorption site, whereas the aryl groups do not contribute to the new level. The calculated charge transfer for the second aryl group adsorption on various SWNTs is given in Table 2. Obviously, the charge transfer is also independent of the electronic structures of SWNTs. The above results indicate that the double adsorption of the aryl groups will further enhance the electrical conductivity of the tube and obvious electron transfers from the tube to adsorbates. When more than two aryl groups are linked to the tube wall, how the number of aryl groups affecting the electronic properties of SWNTs is an important and unanswered question in experiment. In Figure 5b, we present the band gap of aryl groupmodified (8,0) SWNT as a function of the number of aryl groups adsorped. The pure (8,0) SWNT is semiconducting with a 0.64 eV band gap; as more aryl groups are adsorbed on the tube, more energy bands are introduced within the band gap of the pure tube (see Supporting Information); thus, the electronic properties could be significantly increased. It is worth noting that the degree of enhancement in electronic properties of the

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Figure 8. The density of states of the double-adsorbed (a) SWNT (8,0) and (b) SWNT (6,6). TDOS and LDOS for carbon atoms near the adsorption sites and for aryl groups, respectively.

(8,0) SWNTs is dependent on the number of adsorbed aryl groups: when the number of aryl groups is odd, the tube exhibits a small band gap; whereas a relatively larger gap is found for the attachment of an even number of aryl groups. Contrary to the changes of band gap, the value of charge transfer has a maximum when two aryl groups adsorbed on the tube. Considering the above calculated results, we find that the number of the adsorbed functional groups on the tube plays an important role in the chemical functionalization of SWNTs: the odd number of aryl groups adsorbed could make the tube exhibit a higher electronic property than that of the even number of aryl groups absorbed, whereas the even-number-adsorption of aryl groups has a stronger binding energy and, thus, is more stable. This gives us an inspiration that one can control the degree of functionalization of functional groups to design new nanocomposites with different properties to the demand. We would like to point out that, in principle, the binding energy of aryl groups with SWNTs should arise from two factors, that, (a) covalent bonding between aryl groups and SWNTs through the formation of a C-C bond and (b) noncovalent bonding between aryl groups through the π-π stacking interaction. For a description of the π-π stacking interaction, the localized DNP basis set used in the present work may suffer from the so-called “basis set superposition errors” (BSSEs).12 Because the C-C covalent bonding is significantly stronger than the π-π stacking interaction, the influences of BSSEs over the overall binding energy of aryl(s) on SWNT can be neglected. So, we did not attempt to calculate the BSSEs for the present work. Similarly, due to the negligible importance of π-π stacking interaction between aryl rings relative to the C-C covalent bonding between aryl rings and SWNT, we

adopted the GGA method instead of LDA to study the aryl-SWNT system, although LDA may better describe the π-π stacking interaction. 4. Conclusions In this article, we investigate the chemical functionalization of various SWNTs by aryl groups through DFT calculations. The results indicate that the interaction between aryl groups and SWNTs is dependent on the tube diameter, chirality, and the number of adsorbed aryl groups. When more aryl groups are attachment to an (8,0) SWNT in different patterns, we find that the aryl groups prefer the pair arrangement due to a stronger binding energy. Upon the addition of an odd number of aryl groups, the aryl group-SWNTs systems exhibit radical properties and a smaller band gap; thus, they have a stronger conductivity than that of an even number of aryl groups adsorbed. The present calculation results are useful to not only explain the unanswered question in experiment, but also to provide a pathway for applications of carbon nanotubes in chemical sensors, nanobiolectronics, and other nanoscale materials. Acknowledgment. This work was supported by the National Natural Science Foundation of China (Nos. 20103003, 20573046, 20773054), Doctor Foundation by the Ministry of Education (20070183028), Excellent Young Teacher Foundation of Ministry of Education of China, Excellent Young People Foundation of Jilin Province (20050103), and Program for New Century Excellent Talents in University (NCET). We are very grateful for the reviewers’ invaluable comments and suggestions for improving the manuscript.

Chemical Functionalization of SWNTs by Aryl Groups Supporting Information Available: Band structures of (a) three, (b) four, (c) five, (d) six, (e) seven, and (f) eight aryl groups adsorbed on an (8,0) tube. The addition of aryl groups is based on the ortho position. This information is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Iijima, S. Nature 1991, 354, 56. (2) (a) Zhang, G. Y.; Qi, P. F.; Wang, X. R.; Lu, Y. R.; Li, X. L.; Tu, R. S.; Bangsaruntip, Mann, D.; Zhang, L.; Dai, H. J. Science 2006, 314, 974. (b) Moyon, C. M.; Izard, N.; Doris, E.; Mioskowski, C. J. Am. Chem. Soc. 2006, 128, 6552. (c) Campidelli, S.; Meneghetti, M.; Prato, M. Small 2007, 3, 1672. (d) Kim, W.-J.; Usrey, M. L.; Strano, M. S. Chem. Mater. 2007, 19, 1571. (3) (a) Sun, Y. P.; Fu, K.; Lin, Y.; Huang, W. Acc. Chem. Res. 2002, 35, 1096. (b) Baughman, R. H.; Zakhidov, A. A.; de Heer, W. A. Science 2002, 297, 787. (c) Strano, M. S.; Dyke, C. A.; Usrey, M. L.; Barone, P. W.; Allen, M. J.; Shan, H.; Kittrell, C.; Hauge, R. H.; Tour, J. M.; Smalley, R. E. Science 2003, 301, 1519. (4) (a) Jeffrey, L. B.; Tour, J. M. J. Mater. Chem. 2002, 12, 1952. (b) Khabashesku, V. N.; Billups, W. E.; Margrave, J. L. Acc. Chem. Res. 2002, 35, 1087. (c) Niyogi, S.; Hamon, M. A.; Hu, H.; Zhao, B.; Bhowmik, P.; Sen, R.; Itkis, M. E.; Haddon, R. C. Acc. Chem. Res. 2002, 35, 1105. (d) Dyke, C. A.; Tour, J. M. J. Phys. Chem. A. 2004, 108, 11151. (e) Banerjee, S.; H-Benny, T.; Wong, S. S. AdV. Mater. 2005, 17, 17. (f) Tasis, D.; Tagmatarchis, N.; Bianco, A.; Prato, M. Chem. ReV. 2006, 106, 1105. (5) (a) Jeffrey, L. B.; Yang, J. P.; Kosynkin, D. V.; Bronikowski, M. J.; Sammlley, R. E.; Tour, J. M. J. Am. Chem. Soc. 2001, 123, 6536. (b) Dyke,

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