Substitutional boron-doping of carbon nanotubes q R. Czerw a , P.-W. Chiu b , Y.-M. Choi c , D.-S. Lee c , D.L. Carroll a, * , S. Roth b , Y.-W. Park c a School of Materials Science and Engineering, Clemson University, Clemson, SC 29634, USA b Max-Planck-Institut f€ ur Festk€ orperforschung, Stuttgart, Germany c Department of Physics and Condensed Matter Research Institute, Seoul National University, Seoul, South Korea Received 8 May 2002; accepted 25 May 2002 The substitutional placement of boron within the lattice of carbon nanotubes yields quite different trans- port properties for single walled nanotubes (SWNTs) as compared to multi-walled nanotubes (MWNTs). Boron ‘‘doping’’ of the MWNTs results in an acceptor state in the local density of states (LDOS) that lies near the Fermi level and can be directly correlated with features in the thermoelectric power (TEP) of B-doped MWNT mats. Transport measurements of individual B-doped MWNTs exhibit features associated with variable range hopping. In contrast, B-doping of SWNTs results in features in the density of states further from the Fermi level, and transport of the SWNTs shows an unusual variability in rectification not observed in the MWNT case. This suggests that boron has been introduced into the lattice of these two morphologies of nanotubes in very different ways. Interest in the electrical transport properties of both single-walled and multi-walled carbon nanotubes stems primarily from potential applications in nanoelectronics [1]. Initially, nanoscale electronic architectures will be similar to those in use today. Thus, one expects metallic conduits along with heterojunctions formed from doped nanomaterials in analogy to ÔbulkÕ Si devices connected with metal interconnect lines. Further, it is clear that the effects of lattice impurities are of fundamental interest in understanding transport phenomena in these unusual topological objects. However, the direct substitutional doping of carbon nanotubes is quite difficult. Their low dimensional structure does not provide an energetically favorable environment for most impurity atoms. There are two promising candidates, boron [2] and nitrogen [3], both of which seem happy to reside within the car- bon lattice. The behavior of boron in SWNTs and in MWNTs, as evidenced through transport and tunnel- ing spectroscopy, appears to be quite different. In this work, we describe several important differences in B- doped MWNTs and B-doped SWNTs. For these studies arc growth methods were used ex- clusively. Pure carbon nanotubes were arc grown using methods described in detail elsewhere [4]. Transmission electron microscopy (TEM) showed a diameter distri- bution to be centered around 20 nm with tubes as small as 3 nm and as large as 40 nm with tube lengths typically 1 lm. The primary impurities were carbonaceous ma- terials and polyhedral particles and an scanning tun- neling microscopy (STM) image of a typical MWNT bundle is shown in Fig. 1a. B-doped MWNTs were also grown using arc methods as described in the literature [5]. TEM characterization showed these materials to have typical tube diameters of 20 nm with a range of 5–40 nm. Selected area diffraction confirms that these tubes possess predominantly zig-zag chiralities [6]. Tunneling microscopy and spectroscopy, coupled with electron energy loss spectroscopy (EELS), has been used to demonstrate that the boron is in- corporated into the lattice as islands of BC 3 [7]. The impurities in the growth materials were found to be polyhedral particles (also seen in Fig. 1b) and small concentrations of carbonaceous material. No catalysts were used in the growth of either of these MWNT, arc- produced materials. Finally, B-doped SWNTs were produced by using the same process of arc growth as outlined in the literature for pure SWNTs. However, in the case of B-doping, pure boron was mixed with Ni/Y catalysts and carbon and packed into the center of the graphite anode rod. The raw growth materials resulting from the boron-rich Current Applied Physics 2 (2002) 473–477 www.elsevier.com/locate/cap q Original version presented at QTSM & QFS Õ02 (Multi-lateral Symposium between the Korean Academy of Science and Technology and the Foreign Academies), Yonsei University, Seoul, Korea, 8–10 May, 2002. * Corresponding author. E-mail address: dcarrol@clemson.edu (D.L. Carroll). 1567-1739/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. PII:S1567-1739(02)00106-2