Investigating the Structural, Electronic, and Chemical Evolution of B

Nov 15, 2011 - Introduction of heteroatoms (dopants) changes the band struc- ture of MWCNTs at ... ABSTRACT: The evolution of B-doped multiwalled carb...
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Investigating the Structural, Electronic, and Chemical Evolution of B-Doped Multi-walled Carbon Nanotubes as a Result of Joule Heating Zabeada Aslam,* Rebecca Nicholls, Antal A. Koos, Valeria Nicolosi, and Nicole Grobert* Department of Materials, University of Oxford, Parks Road, Oxford OX1 3PH, United Kingdom ABSTRACT: The evolution of B-doped multiwalled carbon nanotubes in the presence of a passing current has been investigated using the in situ TEM/STM Nanofactory holder. The Joule heating results in the dopant being partially removed as the internal structure of the nanotube is altered, and significant changes in its electrical properties were observed. This is of great importance for electrical applications where doping is used to tune the Fermi level of carbon nanotubes.

’ INTRODUCTION Introduction of heteroatoms (dopants) changes the band structure of MWCNTs at the Fermi level; therefore, doping is considered a key tool for tuning the electronic properties of MWCNTs.1 11 Recent studies showed that the structure of pristine-MWCNTs changed and defects were removed due to Joule heating12 20 when an electrical current was applied to the nanotube. Moreover, for nitrogen-doped MWCNTs (N-MWCNTs) the dopant was also removed18 and significant changes in the electronic behavior were observed for current densities above (1.91 ( 0.73)  106 A cm 2. Hence, it is of vital importance to understand if and how the structure, chemistry, and resulting electronic properties of B-MWCNTs are affected by the current. Investigations of maximum current densities are crucial because for future applications doped-MWCNTs must remain intact within the limits of the maximum current density in order to maintain the required electronic properties of the doped-MWCNTs. ’ EXPERIMENTAL METHODS IV-curves of individual B-MWCNTs were taken in situ using a Nanofactory TEM-STM holder in the JEOL3000F operated at 300 kV. For these experiments, aerosol-CVD grown B-MWCNTs21 were mounted on a Au support wire. A second Au wire was employed as the probe. ’ RESULTS AND DISCUSSION B-MWCNTs possess a fairly disordered inner structure as compared with conventional MWCNTs. For example, conventional pristine-MWCNTs exhibit walls parallel to the length of the NT with very few defects, such as missing/extra atoms in the honeycomb carbon structure and a hollow core. When boron is introduced as a dopant, the walls of the tube are no longer parallel to the carbon nanotube (CNT) axis but are at an angle with r 2011 American Chemical Society

Figure 1. Transmission electron microscope bright field images of an as-produced B-MWCNT (a) before electrical contact was made and (b) after current induced restructuring occurred at an applied bias of 1600 mV and 250 μA.

respect to the axis. Consequently, the CNT consists of cones, and the center of the CNT is not always hollow (Figure 1a). In B-doped MWCNTs the inner structure consists of cones that have a range of different cone angles along the length of the tube ranging from 26° to stacked platelets. The outer walls encompassing the stacked cone arrangement of the CNT contain defects in the outer walls such as kinks, and the walls are not parallel along the length of the nanotube but are undulating. Once current is applied to the system, restructuring of the entire tube begins at an applied bias of 1500 mV and current of 87 μA as seen in Figure 1b. In contrast to N-MWCNTs,18 the restructuring of a B-MWCNT results in the CNT dividing into two sections as shown in Figure 2a,b and Figure 2e,f: an outer structure several Received: July 7, 2011 Revised: November 14, 2011 Published: November 15, 2011 25019

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The Journal of Physical Chemistry C nanometers thick of very disordered layers encapsulating an inner structure consisting mainly of cones of various cone angles. These two sections become more defined and segregated with increasing current as can be seen clearly when comparing parts a and b of Figure 2. As the bias is increased to 2200 mV (current

Figure 2. Transmission electron microscope bright field images showing the current-induced restructuring of two different B-MWCNTs as the bias is increased from (a) 0 mV, (b) to 2200 mV, (c) to 2300 mV. (d) the B-MWCNT after complete structural breakdown for one B-MWCNTs. Higher magnification TEM bright-field image of a B-MWCNT (e) before current and (f) after current is introduced. Parts a d are at the same magnification, and parts e and f are at the same magnification.

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increased to 180 μA), the outer structure begins to disappear leaving the inner cones exposed as shown in Figure 2c. Eventually, the CNT breaks into two sections (Figure 2d) at the center due to Joule heating.14 16 More importantly, apart from the drastic structural changes of the B-MWCNT, the current also has a significant impact on the chemistry of the tube which was determined by electron energy loss spectroscopy (EELS). For the B-MWCNT in Figure 1, ca. 3% B-doping is observed in the tube before current is applied. When the inner cones at the center of the tube are exposed the dopant is completely removed (Figure 3a). However, B is still present further along the tube in smaller quantities (ca. 1%), indicating the higher temperature expected at the center of the tube affects the removal of the B dopant. This is in contrast to N-MWCNT18 where the N was removed from the entire tube during current-induced restructuring. Control experiments in order to study the influence of the electron beam on the dopant concentration showed that prolonged exposure (5 min) to intense (focused beam) electron irradiation revealed drastic structural changes but did not result in the removal of the boron (Figure 3b). Also, the IV curves obtained without the electron beam illumination of the sample showed negligible difference to IV curves with the electron beam present. The B-MWCNT is initially metallic, but it is converted to a semiconducting tube once the outer walls are removed and inner cones are exposed, as indicated by the curvature in the IV plot in Figure 4. Once the outer section of the B-MWCNT is removed, current can only pass by tunneling between the cones, and this leads to the observed semiconducting behavior. In contrast to B-doped nanotubes, when current passes through pristine MWCNTs, structural defects are annealed and as the nanotube starts breaking down, layer by layer, at the center of the tube, the IV curves consistently show metallic behavior until the tube finally breaks down. The investigation of the IV characteristics showed quite clearly that large currents significantly alter the electrical properties of doped-MWCNTs. As a consequence, the maximum current densities need to be determined. The maximum current density

Figure 3. Electron energy loss spectra: (a) the B dopant (onset 186 188 eV) is present (ca. 3%) before current is introduced to the nanotube (gray) and after current induced restructuring results in complete structural breakdown of the B-MWCNT in Figure 1 indicating the dopant is removed by the current, (b) taken initially (gray) and after 5 min of electron beam irradiation (dashed) showing that the dopant is not removed. 25020

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The Journal of Physical Chemistry C

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An important point to consider when making these comparisons is the contact resistance between the MWCNT and the metal electrode. Since this setup is a two-probe method, the resistance calculated from the IV curves is the total resistance of the system, i.e., RCNT and RC from the two electrode contacts. Even though the contact resistance cannot be eliminated entirely, it can be significantly reduced23,24 by current annealing. In the present case, local Joule heating at the contacts is assumed to reduce the contact resistance considerably.

Figure 4. IV curves for the B-MWCNT in Figure 1 show the change in electronic behavior as current induced restructuring occurs.

’ CONCLUSIONS In summary, it was shown that B-MWCNTs can be restructured by annealing with sufficiently large current densities and removing the dopant in the process. This means that although doped-MWCNTs are still able to carry very large current densities (in range of 106 A cm 2) when compared with copper ∼0.6  106 A cm 2, if doped-MWCNTs are ever used in electronic applications, care should be taken to limit the maximum current density. ’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected] (Z.A.); nicole.grobert@ materials.ox.ac.uk (N.G.).

Figure 5. Maximum current density is plotted as a function of length for pristine MWCNTs, N-MWCNTs, and B-MWCNTs.

is defined as the point at which the structure begins to break down for pristine-MWCNTs and the point at which current-induced restructuring begins for the doped-MWCNTs. This is because, in pristine-MWCNTs, no chemical changes are expected since only defects are removed, whereas for doped-MWCNTs, current induced restructuring results in removal of the dopant. Therefore, the limits for the doped and undoped MWCNTs are different. From Figure 5 it was observed that for the pristine-MWCNTs short, thin tubes (less than 2 μm in length and 50 nm in diameter) exhibit a current carrying capacity of (19.16 ( 13.6)  106 A cm 2 which is greater compared to thicker, longer pristineMWCNTs (greater than 2 μm in length and 50 nm in diameter) (4.26 ( 3.21)  106 A cm 2. This is most likely due to the fact that, assuming the defect density is the same for all MWCNTs, the thinner tubes have fewer defects and so the electrons experience less scattering hence less Joule heating of the CNT. The B-MWCNTs have an average current carrying capacity of (3.52 ( 1.70)  106 A cm 2, slightly higher than N-MWCNTs18 which have an average of (1.91 ( 0.73)  106 A cm 2, while the p-MWCNTs is in the same region but with a greater spread. From ab initio calculations22 it was determined that B doping preserves the metallic characteristic of the SWCNT and the B-doping drops the current with same bias. It can be assumed that B also acts as a scattering center and reduces the maximum current flow.

’ ACKNOWLEDGMENT The authors are grateful to Dr. Andrew Watt for supplying the in situ TEM-STM holder and Ron Doole for technical EM expertise. We thank the European Commission under the 6 Framework Programme (STREP project BNC Tubes, Contract NMP4-CT-2006-33350) (N.G.), ERC Starting Grant (ERC2009-StG-240500) (NG), PIEF-GA-2008-220150 (V.N.), the John Fell Fund (N.G.), The Royal Society (N.G.), and The Royal Academy of Engineering (V.N.) for financial support. ’ REFERENCES (1) Xu, Z.; Lu, W.; Wang, W.; Gu, C.; Liu, K.; Bai, X.; Wang, E.; Dai, H. Converting Metallic Single-Walled Carbon Nanotubes into Semiconductors by Boron/Nitrogen Co-Doping. Adv. Mater. 2008, 20, 3615–3619. (2) Choi, H. J.; Ihm, J.; Louie, S. G.; Cohen, M. L. Defects, Quasibound States, and Quantum Conductance in Metallic Carbon Nanotubes. Phys. Rev. Lett. 2000, 84, 2917–2920. (3) Liu, K.; Avouris, Ph.; Martel, R.; Hsu, W. K. Electrical Transport in Doped Multiwalled Carbon Nanotubes. Phys. Rev. B 2001, 63, 161404(R). (4) Kaun, C. C.; Larade, B.; Mehrez, H.; Taylor, J.; Guo, H. CurrentVoltage Characteristics of Carbon Nanotubes with Substitutional Nitrogen. Phys. Rev. B 2002, 65, 205416. (5) Czerw, R.; Terrones, M.; Charlier, J. C.; Blase, X.; Foley, B.; Kamalakaran, R.; Grobert, N.; Terrones, H.; Tekleab, D.; Ajayan, P. M.; Blau, W.; Ruhle, M.; Carroll, D. L. Identification of Electron Donor States in N-Doped Carbon Nanotubes. Nano Lett. 2001, 1, 457–460. (6) Latil, S.; Roche, S.; Mayou, D.; Charlier, J.-C. Mesoscopic Transport in Chemically Doped Carbon Nanotubes. Phys. Rev. Lett. 2004, 92, 256805. (7) Latil, S.; Triozon, F.; Roche, S. Anomalous Magnetotransport in Chemically Doped Carbon Nanotubes. Phys. Rev. Lett. 2005, 95, 126802. (8) Adessi, Ch.; Roche, S.; Blase, X. Reduced Backscattering in Potassium-doped Nanotubes: Ab initio and Semiempirical Simulations. Phys. Rev. B 2006, 73, 125414. 25021

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dx.doi.org/10.1021/jp206424v |J. Phys. Chem. C 2011, 115, 25019–25022