www.afm-journal.de FULL PAPER © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 1 www.MaterialsViews.com wileyonlinelibrary.com Adv. Funct. Mater. 2011, XX, 1–5 Zabeada Aslam,* Rebecca Nicholls, Antal Koós, Valeria Nicolosi, and Nicole Grobert* 1. Introduction The exceptional electrical properties of multi-walled carbon nano- tubes (MWCNTs) make them extremely promising for use in the electronics industry [1–5] and doping of such structures allows further control of such properties. [6–21] When a C atom is substi- tuted with a N atom, the dopant acts as a donor impurity in the nanotube, leading to n-type behavior. Moreover, recent studies showed that the structure of pristine MWCNTs changes with cur- rent. [22–29] Hence it is of vital importance to understand if and how the structure of doped-CNTs is affected by large currents. 2. Results and Discussion 2.2. Internal Structural Changes of N-doped MWCNTs Doped CNTs were grown using the aerosol chemical vapor deposition (CVD) method, [30] and supported on a gold wire that was fixed in the transmission electron microscopy/scanning tunneling micro- scopy (TEM/STM) holder. A gold tip was used as the moveable electrode. Generally, the internal structure of the synthesized N-doped MWNCTs (N-MWCNTs) was largely disordered, with some caplike structures (branching— often referred to as bamboo structure) across the inner channel, as shown in Figure 1a. Initially, no catalyst particles were observed along the tube. However, after current-induced annealing for ca. 6 min at an applied bias of 2000 mV and 47 μA, the walls began to restructure, becoming more ordered. The irregular caps between the inner walls were turned into ordered caps (Figure 1c and e). Capped inner tubes as well as (what appear to be) uncapped inner tubes (Figure 1d) were also observed within the same CNT. These uncapped inner tubes may possess dangling bonds (stabilized by lip–lip interactions [31,32] ) or may be termi- nated with N atoms. During the restruc- turing process, large fullerene-type structures migrate over the surface of the CNT towards the tip in the opposite direction to the current. Moreover, the formation of catalyst nanoparti- cles (NPs) was observed close to the negatively biased W tip (Figure 1b). The appearance of these catalyst particles is pos- sibly due to the agglomeration of extremely small NPs, trapped uniformly along the tube, that migrate during the restructuring of the CNT walls. [33–35] As current is passed through a sample, its temperature increases as a result of Joule heating and, for a nanotube of uniform thickness, the temperature of the tube is greatest at the center, [30,31] with the two electrodes acting as heat sinks. Therefore highly mobile, small, metal NPs present at the center of the tube agglomerate as they migrate to the lower- temperature region towards the electrode. The temperature can be estimated by observing the melting/evaporation of NPs and taking into consideration the size effects; [36] hence, we esti- mated the temperature of the doped-CNTs to be above 1000 °C during the restructing process. Control experiments showed that heat treatment of an ensemble of N-MWCNTs at 1200 °C for 30 min under vacuum resulted in restructuring as well as a reduction in the N-doping, which indicates that removal of the dopant results from the extremely high temperatures achieved. After some time, the current-induced restructuring results in removal of the outer layers and formation of cones at the center of the N-MWCNT, followed by a complete breakdown of the N-MWCNT at the center into two separate pieces. Current-Induced Restructuring and Chemical Modification of N-Doped Multi-walled Carbon Nanotubes DOI: 10.1002/adfm.201101036 Multi-walled carbon nanotubes (MWCNTs) have long been anticipated as candidates for electrical components in an increasingly miniaturized elec- tronics industry due to their inherent electrical properties. It is possible to manipulate and control these properties by introducing dopants such as N, B, and P. Although some current-induced structural changes in MWCNTs have been observed, no systematic study has been carried out to explore the correlation of changes in the internal structure with the electronic behavior of doped-MWCNTs in terms of the current densities present. In situ trans- mission electron microscopy (TEM) investigations are presented here of individual, N-doped MWNCT (N-MWCNTs) using the in situ TEM/scanning tunneling microscopy (TEM/STM) Nanofactory © holder. It is observed for the first time that N-MWCNTs not only undergo current-induced structural transformation; i.e., from the typical bamboo structure of N-MWCNTs to the stacked cones, but also—and most importantly—the complete removal of the dopant causes a significant change in the electronic behavior. This has serious implications for the use of doped CNTs as electronic components, especially since tremendous efforts are being made to synthesize CNTs with controlled dopant concentrations. Dr. Z. Aslam, Dr. R. Nicholls, Dr. A. Koós, Dr. V. Nicolosi, Prof. N. Grobert Materials Department Parks Road, Oxford, OX1 3PH, UK E-mail: zabeada.aslam@materials.ox.ac.uk; nicole.grobert@materials.ox.ac.uk