Appl. Phys. A 68, 267–273 (1999) Applied Physics A Materials Science & Processing  Springer-Verlag 1999 Microscopic growth mechanisms for carbon and boron-nitride nanotubes J.-Ch. Charlier 1 , X. Blase 2 , A. De Vita 3,4 , R. Car 4 1 Unit´ e de Physico-Chimie et de Physique des Mat´ eriaux, Universit´ e Catholique de Louvain, Place Croix du Sud 1, B-1348 Louvain-la-Neuve, Belgium 2 D´ epartement de Physique des Mat´ eriaux, U.M.R. n ◦ 5586, Universit´ e Claude Bernard, 43 bd. du 11 Novembre 1918, F-69622 Villeurbanne Cedex, France 3 Istituto Nazionale di Fisica della Materia (INFM) and Department of Material Engineering and Applied Chemistry, University of Trieste, via Valerio 2, I-34149 Trieste, Italy 4 Institut Romand de Recherche Num´ erique en Physique des Mat´ eriaux, PPH-Ecublens, CH-1015 Lausanne, Switzerland Received: 27 November 1998/Accepted: 18 December 1998 Abstract. The growth of carbon (C) and boron nitride (BN) nanotubes cannot be directly observed and the underlying microscopic mechanism is a controversial subject. Transi- tion metal catalysts are necessary to produce single-walled nanotubes (SWNT) of carbon, but they are not needed in the multi-walled (MWNT) case, suggesting different growth mechanisms. Here we report on the results of first-principles dynamical simulations of both single- and double-walled car- bon nanotube edges. We find that the open end of carbon SWNTs spontaneously closes by forming a graphitic dome in the 2500–3000 K temperature range of synthesis experi- ments. On the other hand, “lip–lip” interactions, consisting of chemical bonding between the edges of adjacent coax- ial tubes, trap the end of the double-walled carbon nanotube into a metastable energy minimum, preventing dome closure. The resulting end geometry is highly chemically active, and can easily accommodate incoming carbon fragments, thus al- lowing growth by chemisorption from the vapour phase. The growth mechanisms of boron nitride SWNTs is studied as well, and is compared to the case of pure carbon tubes. In the experimental temperature conditions, the behavior of growing BN nanotubes strongly depends on the nanotube network he- licity. In particular, we find that open-ended “zigzag” SWNTs close rapidly into an amorphous like tip, preventing further growth. In the case of “armchair” SWNTs, the formation of squares traps the tip into a flat cap presenting a large central even-member ring. This structure is metastable and able to re- vert to a growing hexagonal framework by incorporation of incoming atoms. These findings are directly related to frustra- tion effects, namely that B−N bonds are energetically favored over B−B and N−N bonds. PACS: 71.20.Tx; 68.65.+g Today, a few years after their discovery in 1991 [1, 2], carbon nanotubes are attracting much interest for their potential appli- cations in high performance nanoscale materials [3] and elec- tronic devices [4, 5]. Synthesis techniques for carbon nanotubes have recently achieved high production yields as well as good control of the tubes multiplicity, shape and size [6]. Carbon nanotubes typically grow in an arc discharge at a temperature of ∼ 3000 K. However, the mechanisms of nanotube formation and growth under such extreme conditions remain unclear [6]. The earliest models for growth of multi-walled nanotubes [7,8] were based on topological considerations and emphasised the role of pentagon and heptagon rings to curve inside or outside the straight hexagonal tubular network. The most debated issue, in later works, was whether these nanotubes are open- or closed- ended during growth. In favour of the closed-end mechanism, it was proposed that tubes grow by addition of atoms onto the reactive pentagons present at the tip of the closed structure [9, 10]. However, recent experimental studies suggest an open-end growth mechanism [11, 12]. In this model the atoms located at the open end of the graphitic structure provide active sites for the capture of carbon ions, atoms or dimers from the plasma phase. Since any capped configuration is more stable than the open-end geometry, it was proposed that the latter could be sta- bilized by the high electric field present at the tip [13]. How- ever, recent ab initio calculations [14,15] show that realistic electric fields cannot stabilize the growth of open-ended nano- tubes. Moreover, there is controversy about whether, in multi- walled nanotubes, the inner or the outer tubes grow first [8, 16] or whether different tubes may grow together [17, 18]. Finally, the growth of single-walled nanotubes requires the presence of metal catalysts, in contrast to the multi-walled case [12,19, 20]. To date no single model seems to be capable of explaining all the experimental evidence. 1 Microscopic growth mechanisms for carbon nanotubes In this work, we study the microscopic mechanisms underly- ing the growth of carbon (present section) and boron-nitride (next section) nanotubes by performing first-principles mo- lecular dynamics simulations of single- and double-walled nanotubes [21, 22]. In this approach [23], the forces acting on the atoms are derived from the instantaneous electronic ground state, which is accurately described within density functional theory in the local density approximation. In such a framework, the instantaneous electronic ground state is given within a formulation in which only valence electrons are explicitly taken into account. The interaction between valence electrons and nuclei plus frozen core electrons is de-