Covalently Functionalized Metallic Single-Walled Carbon Nanotubes

Oct 30, 2013 - School of Electrical & Electronic Engineering, Nanyang ... microscopy (DFM) are demonstrated to be very powerful tools of characterizin...
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Covalently Functionalized Metallic Single-Walled Carbon Nanotubes Studied Using Electrostatic Force Microscopy and Dielectric Force Microscopy Kang Zhang,†,‡ Nicola Marzari,†,§ and Qing Zhang*,†,‡ †

Advanced Materials for Micro- and Nano-Systems, Singapore-MIT Alliance, 4 Engineering Drive 3, Singapore 117576, Singapore School of Electrical & Electronic Engineering, Nanyang Technological University, 50 Nanyang Avenue, Singapore 639798, Singapore § Theory and Simulations of Materials (THEOS), Ecole Polytechnique Fédérale de Lausanne, CH-1015 Lausanne, Switzerland ‡

S Supporting Information *

ABSTRACT: Contactless electrostatic force microscopy (EFM) and dielectric force microscopy (DFM) are demonstrated to be very powerful tools of characterizing the electronic properties of individual single-walled carbon nanotubes (SWCNTs). Taking the advantages of the tools, we confirm that the metallicity of metallic SWCNTs can be largely preserved upon dichlorocarbene functionalization ([2 + 1] cycloaddition) in comparison with the SWCNTs subject to the Prato reaction ([2 + 3] cycloaddition). This work demonstrates the distinct difference between sp2 rehybridized and sp3 rehybridized covalent configurations on their influences to electronic properties of metallic SWCNTs and supports the hypothesis that [2 + 1] cycloaddition could recover the sp2 hybridization on the sidewall of metallic SWCNTs and preserve the intrinsic electronic properties of SWCNTs.

1. INTRODUCTION Single-walled carbon nanotubes (SWCNTs) are excellent candidates for nanoelectronic applications largely due to their extraordinary electrical and electronic properties.1−3 However, to make SWCNTs easily handled and processed, covalent functionalizations are generally employed. Moreover, for many applications such as solar energy, biology, and sensors, covalent modifications to their sidewalls are required to make them more amenable to rational and predictable manipulations and/ or sensitive to detecting species than pristine SWCNTs.42,44 Unfortunately, a severe drawback of most covalent functionalizations is that the conductance and electronic properties of SWCNTs could be largely degraded by sp3 hybridizations formed on their sidewalls.4−6,19−22,45 In sharp contrast, dichlorocarbene covalent functionalization ([2 + 1] cycloaddition) on SWCNTs was found not to degrade much their electrical conductances, even at a significant degree of the functionalization.7,8 This conductance preservation effect of [2 + 1] cycloaddition was attributed to the curvature-induced sidewall C−C bond cleavage upon the functionalization, recovering the SWCNT sidewall sp2 hybridizations.9−11 The schematic drawings in Figure 1 illustrate two rehybridizations formed by [2 + 1] cycloaddition and [2 + 3] cycloaddition functionalizations. Interestingly, the sp2 rehybridization originates from the sidewall C−C bond broken (red dashed line) after [2 + 1] cycloaddition, while sp3 rehybridization is formed with [2 + 3] cycloaddition. Moreover, this sp2 rehybridization recovery effect not only does preserve the quantum conductance but also tends to © XXXX American Chemical Society

Figure 1. Illustration of the effects of [2 + 1] cycloaddition (a) and [2 + 3] cycloaddition (b) on SWCNT sidewall C−C hybridization.

preserve the intrinsic electronic structure of SWCNTs.10 Simulation results by Zhao et al. show that even with dichlorocarbene addition at 12.5% ratio (which is considered extremely high experimentally), a (6,6) metallic SWCNT only Received: July 31, 2013 Revised: October 26, 2013

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subject to minor changes in its band structures and the band crossing at its Fermi level is still robust and retains its metallic character.10 In sharp contrast, many covalent functionalizations to SWCNTs with sidewall sp3 rehybridization are shown to largely destroy the electronic structure of metallic SWCNTs and result in apparent band gaps opening on metallic SWCNTs even at a small degree of the functionalizations.19−22,45 However, to the best of our knowledge, no experimental comparisons on the electronic properties of metallic SWCNTs influenced by covalent functionalizations with sp2 rehybridization and sp3 rehybridization have been reported. In this work, the local electronic properties (longitudinal polarizability, local conductivity, and band structure changes) of individual metallic SWCNTs influenced by [2 + 1] cycloaddition and [2 + 3] cycloaddition are studied utilizing contactless scanning probe technologies, i.e., electrostatic force microscopy (EFM) and dielectric force microscopy (DFM). EFM and DFM techniques eliminate sample contaminations/ damages during the analysis process, enable local characterization, and provide the information on the longitudinal polarizability, local conductivity, band gap, etc.24−29 It is worth to note that the longitudinal polarizability, ε∥, of SWCNTs studied in this work is controlled by their band gaps, Eg, i.e., ε∥ ∝ 1/Eg2.18 Therefore, [2 + 1] cycloaddition to metallic SWCNTs with sp2 rehybridization recovery (Figure 1a) is also expected to preserve the large longitudinal polarizability of metallic SWCNTs. The longitudinal polarizability of metallic SWCNT plays important roles in manipulation of SWCNTs under electrical fields for various applications.12−17 For example, with the ac dielectrophoresis (AC-DEP) method, one can place SWCNTs across desired electrodes by making use of the anisotropic dielectric properties of SWCNTs.15−17 Separation of semiconducting SWCNTs from metallic ones can be realized through the distinct difference in the longitudinal polarizabilities between two types of SWCNTs.12

Figure 2. Schematic illustrations of EFM and DFM working principle. The sample is imaged in a dual pass imaging process. In the first pass, the standard ac mode AFM imaging is performed to obtain a topographic scan line. In the second scan pass, the tip is lifted up to a constant height over the topographic profile obtained in the first scan. The tip is biased with a dc (EFM) or dc + ac (DFM) voltages only in the second scan pass.

phase shift which is recorded as the EFM signal.28 For a small vibration amplitude of the cantilever, a linear relationship between the electrostatic force gradient normal to the substrate and the cantilever phase shift could be established, i.e., Δφ ∝ ∂F/∂z, where z is the distance from the tip to the substrate.26,29 The electrostatic force between the sample and a dc biased AFM tip can be expressed as Fdc = 1/2dC/dz[(Vdc + Vcp + VQ)2], where C is the tip to sample capacitance, Vdc is the dc voltage applied to AFM tip, Vcp is the contact potential difference between the AFM tip and the sample, and VQ represents the response originating from the charges in the samples.28 Thus, the EFM signal of SWCNT under a constant dc bias, Vdc + Vcp, is determined by the transverse capacitance (i.e., transverse dielectric of the SWCNT) and the induced charges in the SWCNT which is related to its longitudinal polarizability (or band gap).24−26 As the transverse dielectric of a SWCNT is only dependent on its diameter,18 the EFM signal comparisons in this work, made on the same SWCNT in its pristine and functionalized state, should only be caused by the changes in the induced charges, i.e., the changes of the longitudinal polarizabilities. 2.2. Dielectric Force Microscopy (DFM). The limitation of conventional EFM technique is that it could not uncover the properties, such as the presence of band gaps, the carrier types, and the local conductivity of SWCNTs. Thus, a novel technique, dielectric force microscopy (DFM), has recently been demonstrated to be able to determine the gate modulated carrier response of SWCNTs.27 DFM imaging setup in this work is developed from EFM working principle. In the second scan pass of DFM, an ac voltage (with frequency ω), a gate voltage (dc voltage), and an offset voltage (dc voltage) are applied to the tip, and the 2ω component of the phase shift of the cantilever deflection signal is used as our DFM signal. In a conventional EFM with a dc + ac voltage (Vdc + Vac sin ωt) applied on the conductive tip, the 2ω component of the electrostatic force between the probe and the sample results from the interaction between the ac voltage and the induced dipoles.28−31 The force could be then be written as F(2ω) = −1/4dC/dzVac2 cos 2ωt, showing no dependence on the applied dc bias.28 Thus, in our DFM technique, the gate voltage (dc voltage) does not directly

2. ELECTROSTATIC FORCE MICROSCOPY AND DIELECTRIC FORCE MICROSCOPY TECHNIQUES 2.1. Electrostatic Force Microscopy. The EFM technique has been widely applied to characterize the electrical property of nanomaterials.23−27 It utilizes a conductive tip to scan over the sample surface at a constant height (tens of nanometers typically) with a voltage applied and detects the electrostatic forces between the probe and the charges and dipoles in the sample. EFM characterization has been employed to study the dielectric polarizabilities of SWCNTs as well as to distinguish between metallic and semiconducting SWCNTs.24−26,40 In those works, the longitudinal polarizability of SWCNT contributes to the EFM signal through the longitudinal distribution of induced charges in the SWCNT, resulting in a much stronger response of metallic SWCNTs than semiconducting SWCNTs.24−26,40 As illustrated in Figure 2, EFM measurements are conducted with a double-pass process. In the first pass of a scan line, a standard ac air mode (tapping mode) scan is performed to obtain the topographical morphology. In the second pass, the cantilever is lifted at a constant height over the topographical profile obtained in the first pass. A constant dc voltage is applied to the conductive tip only in the second scan pass. Thus, during the second scan path, the induced charges and dipoles in SWCNTs interact with the AFM tip so that the resonant vibration of the cantilever is affected, causing a slight B

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3.2. Dichlorocarbene Functionalizations ([2 + 1] Cycloaddition). In a typical dichlorocarbene functionalization, 5 g of potassium tert-butoxide is dissolved in 20 mL of tetrahydrofuran (THF) with 5 mg of phase transfer catalyst added. The solution is then stirred at 1000 rpm for 5 min to form a uniform solution. The SWCNTs/SiO2/Si sample is then dipped into the solution, and 5 mL of chloroform is added dropwise to the solution in a nitrogen environment. The reaction is exothermic, and thus the solution is kept under an ice bath throughout the experiment. Upon completion of the reaction, the sample is washed thoroughly with acetone, IPA, and DI water in sequence and then baked on a hot plate at 120 °C for 3 min to remove possible chemical residues. 3.3. Prato Reaction ([2 + 3] Cycloaddition). In a typical Prato reaction,38,39 10 mg of sarcosine is dissolved in 20 mL of toluene and 10 mL of formaldehyde (37%) solution and then stirred at 1000 rpm for 5 min. The SWCNTs/SiO2/Si sample is then dipped into the solution at 130 °C in a nitrogen environment. Upon completion of the reaction, the sample is washed thoroughly with acetone, IPA, and DI water in sequence and then baked on a hot plate at 120 °C for 3 min to remove possible chemical residues. 3.4. Raman Spectroscopy. The degree of the functionalizations in this work is determined by comparing the Raman scattering results, which are extensively studied in characterization of the structural properties of SWCNTs as well as covalent modifications to SWCNTs structures.32−36 Raman scattering is performed with excitation laser lines of 532 nm using a confocal Raman system (WITec) in ambient air. Raman intensity is calibrated with the silicon peak ∼520 cm−1. Raman excitation laser intensity is kept sufficiently low to avoid any possible damages to the SWCNTs. Raman scattering comparisons are performed in the same orientations of the SWCNTs before and after the functionalizations to avoid polarization-induced difference. The Raman G band (tangential vibrations modes of perfect SWCNT lattice at around 1500− 1600 cm−1) and D band (symmetry-breaking perturbation to SWCNT lattice at around 1300−1400 cm−1) intensity ratio is commonly used as a qualitative indication of the degree of SWCNT sidewall covalent modification.35,36 In addition, EDX analysis of a massive collection of SWCNTs under different degree of dichlorocarbene functionalizations is employed to calibrate the degree of functionalizations measured by Raman scattering (see Supporting Information). High degrees of functionalizations are not studied in this work because it is not easily achieved in our experiment. This is probably due to the relative small reaction interface of our SWCNTs sitting on the SiO2 substrate as compared to conventional functionalization schemes where the SWCNTs are suspended in solution phase.39,41 3.5. EFM Imaging. Cypher atomic force microscopy (AFM) MF3D (Asylum Research) is used for all EFM and DFM experiments. Conductive tips with a resonance frequency of about 75 kHz and a spring constant of about 2.8 N/m are used. As illustrated in Figure 2, EFM measurements are conducted with a double-pass process. In the first pass of a scan line, a standard ac air mode (tapping mode) scan is performed to obtain the topographical morphology. In the second pass, the cantilever is lifted at a constant height of 30 nm over the topographical profile obtained in the first pass. A constant dc voltage of 3 V with an offset voltage (typically ∼0.1 V) is applied to the conductive tip. The offset voltage is used to offset the contact potential difference between the probe and the

contribute to the 2ω component of the electrostatic force. Instead, it modulates the local carrier density of the SWCNTs and thus the local carrier motions (oscillates at ω frequency) induced by the ac voltage could interact with the ac voltage applied on the AFM tip, contributing to the 2ω component of the electrostatic force. Therefore, the ability of gate voltage to induce local charge carriers and the local carrier mobility in SWCNT provide additional contributions to the 2ω component of the electrostatic force. It is thus expected that in p-type semiconducting SWCNTs (holes are the majority carriers) the DFM signals at a negative gate voltage on the DFM tips should be stronger than the DFM signals at a positive gate voltage of the same magnitude (with the conditions that no contact potential difference between the tip and the tube, the gate voltages are not too large to cause a charge inversion, and a comparable mobility of electrons and holes). Since the dielectric response due to the charge carrier oscillation dominates over the polarization of the lattice,27 the DFM response is mostly determined by the carrier density and carrier mobility. The gate voltage-induced carrier density is determined by the band gap (or longitudinal polarizability) and the carrier type of the SWCNTs, while the carrier mobility is largely influenced by the scattering centers on the sidewall of SWCNTs. Therefore, the electronic properties of metallic SWCNTs influenced by covalent functionalizations could be indirectly imaged by our DFM technique. It is worth to note that our DFM implementation is different from ref 27. In our DFM scanning, the cantilever is oscillating at its resonant frequency while a bias voltage (Vg + Vac sin ωt) is applied. Thus, our DFM signal is obtained from the 2ω component of the phase shifts of the cantilever deflection signal. However, in ref 27, the DFM was performed with the resonant oscillation of the cantilever turned off, and thus the cantilever is purely driven by the applied voltage (Vg + Vac sin ωt). As a result, the DFM signal (2ω component of the electrostatic force) in ref 27 was obtained from the 2ω component of the oscillation of the cantilever deflection signal. However, the DFM signal obtained in both techniques represents the 2ω component of the electrostatic forces and thus provides the same information. It has been noticed that EFM and DFM measurements are very sensitive to conductive tips used.23−27 In this work, a particular SWCNT is scanned with the same tip in exactly the same setup (especially cantilever lift height which may cause DFM signal variations27) before and after the functionalizations to minimize the experimental errors. Furthermore, the functionalizations are carried out on half-length of several individual SWCNTs, while the other half leaves intact. We can thus directly compare EFM and DFM signals between the pristine and the functionalized SWCNTs in one scan, further minimizing experimental uncertainty.

3. EXPERIMENTAL SECTION 3.1. Sample Preparation. Pristine SWCNTs are grown on SiO2/Si substrates by the conventional thermal-CVD method.37 The catalyst (ferritin) density and growth conditions are carefully controlled so that high quality, low density isolated SWCNTs with a diameter range of about 1−2 nm and length from a few μm to tens of μm are produced. SiO2 markers are introduced for the purpose of tracking specifically selected SWCNTs. The SEM image of the as-grown SWCNTs is shown in the Supporting Information. C

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Figure 3. Demonstration of gate effect on the DFM signals. (a) Topography image of SWCNT-A. (b−d) 2ω component of DFM images of SWCNT-A at gate voltage of −1.5, 0, and +1.5 V, respectively. (e) Raman spectra (normalized to G band) of SWCNT-A. (f) Topography image of SWCNT-B. (g−i) 2ω component of DFM images of SWCNT-B at gate voltage of −1.5, 0, and +1.5 V, respectively. (j) Raman spectra (normalized to G band) of SWCNT-B. (k) Topography image of SWCNT-C. (l−n) 2ω component of DFM images of SWCNT-C at gate voltage of −1.5, 0, and +1.5 V, respectively. (o) Raman spectra (normalized to G band) of SWCNT-C.

substrate.30 During the second scan path, the induced charges and dipoles in SWCNTs interact with the AFM tip so that the resonant vibration of the cantilever is affected, and the phase shift of the cantilever deflection signal is recorded as our EFM signal. 3.6. DFM Imaging. DFM imaging setup is developed from EFM working principle described in above section. In the second scan pass of DFM, an ac voltage (3 V amplitude, ω = 500 Hz), a gate voltage (−3 to +3 V), and the offset voltage are applied to the tip and the 2ω component of the phase shift of the cantilever deflection signal is used as our DFM signal. The 2ω component of the phase shift is calculated offline from the phase image obtained from the cantilever deflection. The low pass filter (lock-in filter) used in the calculations has a cutoff frequency of ω/2 with an attenuation of 161 dB at 2ω.

(the same definition used in ref 27), SWCNT-B shows a modulation ratio of ∼0.8−1.2, while SWCNT-A and -C have modulation ratios of ∼1.8−5. Since the dielectric/dipole response of the DFM signal (2ω component) is insensitive to gate voltages,27 the modulation ratio is attributed to the charge carrier density variation induced by the gate voltages. It should be pointed out that although the DFM signal of an SWCNT partially depends on the SWCNT diameter (which determines the dipole response in the transverse direction), the modulation ratio comparisons in this work are made on the same SWCNT in its pristine state and functionalized state so that the uncertainty caused by diameter variations can be ruled out.27 4.2. EFM and DFM Image on Covalently Functionalized Metallic SWCNTs. 4.2.1. EFM and DFM Image on Covalently Functionalized Metallic SWCNTs. SWCNT-1 is carefully analyzed using AFM, EFM, DFM, and Raman spectra before (see Figures 4a−f) and after [2 + 3] cycloaddition functionalization (see Figures 4g−l). Pristine SWCNT-1 can be classified to be a metallic tube from its Raman scattering result in Figure 4f.34 The gate-independent DFM signals shown in Figures 4c−e also suggest its metallic behavior. Compared to Figure 4f, Figure 4l shows a sharp D band increase, suggesting that a successful [2 + 3] cycloaddition occurs. The degree of the functionalization is estimated to be ∼1.5% by comparing their Raman spectroscopy results before and after the functionalization (see Supporting Information). The EFM signal of SWCNT-1 is significantly weakened after the functionalization (compare Figure 4h with 4b). Normalized EFM signal strength at 10 sampling points along SWCNT-1 from the pristine and functionalized states (Figure 4m) clearly shows that the signal drop is more than 50% up on the Prato reaction, indicating a significant drop of the longitudinal polarizability for the functionalized SWCNT-1.

4. RESULTS AND DISCUSSION 4.1. Demonstration of DFM Technique on Pristine SWCNTs. First of all, three typical pristine SWCNTs (named as SWCNT-A, -B, and -C) are selected to examine the influences of the gate voltages on the DFM signals. Figure 3 shows the AFM morphologies (Figures 3a, 3f, and 3k), DFM images (Figures 3b−d, 3g−i, and 3l−n), and Raman spectra (Figures 3i, 3j, and 3o) of the three SWCNTs. Raman scattering results suggest that SWCNT-A and -C are semiconducting, while SWCNT-B is metallic. As seen from Figures 3g, 3h, and 3i, the DFM responses of SWCNT-B under different gate voltages from −1.5 to +1.5 V are almost identical, indicating no detectable gate effect or, in other words, no band gap exists in SWCNT-B.27 However, the DFM signals of both SWCNT-A and -C are largely modulated by the applied gate voltages from −1.5 to +1.5 V, suggesting the apparent gate effect and semiconducting characteristics, consistent with the Raman scattering results.32−36 If the DFM signal ratio between VG = −1.5 V and VG = +1.5 V is taken as the “modulation ratio” D

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Figure 4. Comparison of EFM and DFM signals of metallic SWCNT-1 before and after the Prato reaction. (a) Topography image of SWCNT-1 in pristine state. (b) EFM image of SWCNT-1 in pristine state. (c−e) DFM images of pristine SWCNT-1 at gate voltage of −1.5, 0, and +1.5 V, respectively. (f) Raman spectra (normalized to G band) of pristine SWCNT-1. (g) Topography image of SWCNT-1 after the Prato reaction. (h) EFM image of SWCNT-1 after the Prato reaction. (i−k) DFM images of SWCNT-1 functionalized with the Prato reaction at gate voltage of −1.5, 0, and +1.5 V, respectively. (l) Raman spectra (normalized to G band) of the SWCNT-1 functionalized with the Prato reaction. (m) Comparison plot of EFM signals of SWCNT-1 before and after the Prato reaction with 10 data points taken at equal distance of the EFM images shown in (b) and (h). (n) Comparison plot of DFM signals of SWCNT-1 before and after the Prato reaction at different gate voltages.

80%, while the modulation ratio almost remain unchanged, suggesting a large metallicity preservation of SWCNT-2. 4.2.2. Statistical Plots on EFM Signals and DFM Modulation Ratios. For a more statistical observation, six metallic SWCNTs are studied on dichlorocarbene functionalization, and another six metallic SWCNTs are studied on the Prato reaction. The degrees of the functionalizations are estimated to be ∼0.5−3% according to their Raman scattering data. The average EFM signal strength and modulation ratio changes before and after the functionalizations are plotted in Figure 6. As seen from the plot, the EFM signals of dichlorocarbene-functionalized metallic SWCNTs show a preservation of more than 80% as compared to their pristine states, while the Prato reaction-functionalized metallic SWCNTs shows a EFM signal drop of more than 50% typically. Dichlorocarbene-functionalized SWCNTs also show a relatively unchanged modulation ratio (∼0.9−1.1), while the Prato reaction-functionalized SWCNTs show a modulation ratio typically more than 1.5.

As seen in Figures 4i−k and the plot in Figure 4n, the DFM signal after the functionalization becomes strongly gatedependent, and the modulation ratio between Vg = −1.5 V and Vg = +1.5 V is approximately 5, falling in the modulation ratio range of pristine semiconducting SWCNTs. As DFM signal is determined by local conductivity and carrier density, the strong gate-dependent DFM signals observed suggest apparent band gap opening caused by the [2 + 3] cycloaddition. Moreover, the weakening of the DFM signal is probably due to the reduction of effective local conductivity induced by the highly scattering sp3 rehybridizations on the sidewall of SWCNT-1. The effect of ∼2% dichlorocarbene [2 + 1] cycloaddition (by comparing Raman scatterings in Figures 5f and 5l) of a metallic SWCNT (named as SWCNT-2) is shown in Figure 5. We could see that both EFM (Figures 5b and 5h) and DFM (Figures 5c−e and 5i−k) signals are largely preserved after the dichlorocarbene functionalization. Data plots in Figures 5m and 5n show that the EFM signal of SWCNT-2 is preserved up to E

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Figure 5. Comparison of EFM and DFM signals of metallic SWCNT-2 before and after dichlorocarbene functionalization. (a) Topography image of SWCNT-2 in pristine state. (b) EFM image of SWCNT-2 in pristine state. (c−e) DFM images of pristine SWCNT-2 at gate voltage of −1.5, 0, and +1.5 V, respectively. (f) Raman spectra (normalized to G band) of pristine SWCNT-2. (g) Topography image of SWCNT-2 after dichlorocarbene functionalization. (h) EFM image of SWCNT-2 after dichlorocarbene functionalization. (i−k) DFM images of dichlorocarbene-functionalized SWCNT-2 at gate voltage of −1.5, 0, and +1.5 V, respectively. (l) Raman spectra (normalized to G band) of dichlorocarbene-functionalized SWCNT-2. (m) Comparison plot of EFM signals of SWCNT-2 before and after dichlorocarbene functionalization with 10 data points taken at equal distance of the EFM images shown in (b) and (h). (n) Comparison plot of DFM signals of SWCNT-2 before and after dichlorocarbene functionalization at different gate voltages.

4.2.3. EFM and DFM Image of Partially Functionalized Metallic SWCNTs. To observe the effect of the covalent functionalizations in the same scan, the functionalizations are also realized on half-length of metallic SWCNTs, with the other half-length of SWCNTs covered with silicon nitride (∼200 nm by PECVD) prior to the functionalizations. The silicon nitride is etched away using dilute HF (HF:H2O = 1:100) upon successful functionalizations. Two metallic SWCNTs are selected for the experiments, and they are named as SWCNT-3 (see Figure 7a for its AFM image) and SWCNT4 (See Figure 7i). Figures 7b−e show the EFM and DFM image of SWCNT-3 whose lower portion is successfully functionalized with [2 + 1] cycloaddition while the rest part remains intact. The degree of the functionalization is estimated to be ∼2% (see Figures 7f− h). These EFM and DFM images suggest that both portions are almost identical, indicating that metallicity of the functionalized portion of SWCNT-3 is largely preserved.

Figure 6. Relative EFM strength variation and modulation ratio for six metallic SWCNTs subject to dichlorocarbene functionalization and six metallic SWCNTs subject to Prato reaction. All EFM signals are normalized to the pristine SWCNTs.

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Figure 7. EFM and DFM characterization of partially functionalized SWCNT-3 and SWCNT-4. (a) Topography image of SWCNT-3 with the lower portion (separated by the dashed line) functionalized with dichlorocarbene functionalization while the upper part intact. (b) EFM image of SWCNT-3. (c−e) DFM images of SWCNT-3 under gate voltage of −1.5, 0, and +1.5 V, respectively. (f) Raman mapping of G band intensity of SWCNT-3 and the typical Raman spectra taken from the pristine (g) and functionalized (h) portions of SWCNT-3. (i) Topography image of SWCNT-4 with lower portion (separated by the dashed line) functionalized by the Prato reaction. (j) EFM image of SWCNT-4. (k−m) DFM images of SWCNT-4 under gate voltage of −1.5, 0, and +1.5 V, respectively. (n) Raman mapping of G band intensity of SWCNT-4 and the typical Raman spectra taken from the pristine (o) and functionalized (p) portions of SWCNT-4.

Raman G+ peaks positions have limited shifts upon the functionalizations suggests the insignificance of the doping effect.34 Moreover, the Raman line shapes around G− peaks provide additional evidence on the metallicity changes of the metallic tubes. Detailed Raman scattering comparison and analysis are provided in the Supporting Information.

Figures 7j−m show the EFM and DFM images of a partially [2 + 3] cycloaddition-functionalized SWCNT-4. The Raman results comparison in Figures 7n−p indicate the successful of the functionalization. The degree of functionalization is ∼1.5%, comparable to that of [2 + 1] cycloaddition of SWCNT-3. However, Figure 7j shows a sharp contrast of the EFM signal between the functionalized and pristine portions. In addition, the DFM images of the functionalized portion of the SWCNT show a gate dependence which is consistent with previous observations from SWCNT-1 entirely functionalized by the Prato reaction. 4.3. Discussions. Clear evidence presented above suggests that at a low degree of functionalizations metallic SWCNTs could preserve their metallicity after dichlorocarbene functionalization ([2 + 1] cycloaddition). In contrast, they could lose their metallicity upon Prato reaction ([2 + 3] cycloaddition). These can be interpreted through C−C reconfigurations where [2 + 1] cycloadditions with sp2 rehybridizations could preserve the intrinsic electronic properties of metallic SWCNTs while [2 + 3] cycloaddition results in sp3 rehybridizations that largely degrade the metallicity of the functionalized metallic SWCNTs.9−11 It is worth to note that the sidewall C−C bond broken is largely dependent on the diameter of the SWCNTs as well as the orthogonal angles of the C−C bond with respect to the tube axis.9−11 Armchair metallic SWCNTs with diameter less than 2.4 nm are shown to have their sidewall C−C broken on the orthogonal C−C bonds.9 In this work, the diameters of the metallic SWCNTs are ∼1.7−1.9 nm (estimated by Raman scattering and AFM height measurement), and therefore thermodynamically the sidewall C−C broken should dominant in our samples although kinetically there are likely some dichlorocarbene groups attached to the nonorthogonal C−C bond that yield sp3 hybridizations.9 As the functional groups in this work are not strong electron acceptors/donors, direct doping from the attachment groups at such low coverage are treated as insignificant. The fact that the

5. CONCLUSION Contactless EFM and DFM are demonstrated to be a powerful tool for probing the local electronic properties of SWCNTs. With the aids of the techniques, we have confirmed that [2 + 1] cycloaddition could largely preserve the metallicity of dichlorocarbene-functionalized metallic SWCNTs. In sharp contrast, [2 + 3] cycloaddition functionalization degrades their metallicity significantly. This work demonstrates the large influence of the structural property (sidewall rehybridization upon covalent functionalization) on the electronic properties of SWCNTs, and possible sensing elements could be realized based on such mechanism.43

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ASSOCIATED CONTENT

S Supporting Information *

A detailed Raman scattering analysis; SEM image of the asgrown SWCNTs with markers; Raman scattering D band and G band intensities versus various degrees of dichlorocarbene functionalization examined by EDX. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected]; Tel (+65) 6790 5061 (Q.Z.). Notes

The authors declare no competing financial interests. G

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ACKNOWLEDGMENTS This work is supported in part by Singapore-MIT Alliance. The authors thank their colleagues at School of Electrical & Electronic Engineering, Nanyang Technological University for valuable discussions.

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