Properties of composite B x C y N z nanotubes and related heterojunctions X. Blase * D epartement de Physique des Mat eriaux, U.M.R. no. 5586, 43 Bd du 11 novembre 1918, 69622 Villeurbanne Cedex, France Abstract We review the stability and electronic properties of composite B x C y N z nanotubes, including BN-, BC 2 N- and B- doped carbon nanotubes. We show in particular that BC 2 N systems are driven towards the segregation of pure C and BN sections. The same process of segregation into BC 3 islands is evidenced in the case of B-doped carbon nanotubes. These spontaneous segregation processes lead to the formation of quantum dots or nanotube heterojunctions with potential important technological applications. Ó 2000 Elsevier Science B.V. All rights reserved. 1. Introduction In recent papers [1±5], the idea of making electronic switches with carbon nanotubes [6] was proposed. On the basis of experimental observa- tions which report the presence of odd-member rings in the hexagonal network of some carbon nanotubes, it was suggested that nanotubes with dierent metallicity [7±10] could be ``connected'' during the growth process. However, several problems could prevent such a motivating idea to turn into a practical way to realize nanometric size heterojunctions [11]. To date, it seems dicult to control the number and location of such odd- member carbon rings. In addition, carbon na- notubes are semiconducting only for the smallest ones. In practice, realistic nanotubes recover quickly (as a function of their diameter) the properties of graphite which is semi-metallic. This restricts the technological interest of such junc- tions. On the contrary, composite B x C y N z nanotubes oer a large variety of electronic properties. For example, the band gap of all BN nanotubes [12±14] is remarkably stable around 5.5 eV, in- dependently of radius, helicity and wall±wall interactions [15,16]. This is due to the ionic origin of the band gap. In addition, experimental evi- dences [17±22] suggest that synthesized BC 2 N compounds are unstable with respect to the phase segregation of pure graphite and BN domains, questioning thus the atomic model initially pro- posed [23] and subsequently adopted in several studies [24]. Further, a recent experimental and theoretical study [25] give support to the idea that doping of carbon nanotubes by boron leads to the formation of BC 3 nanodomains instead of a ran- dom network of isolated B atoms. This suggests that composite systems are spontaneously driven towards the realization of quantum dots or, e.g., C/BN nanotube heterojunctions with large band osets [26,27]. www.elsevier.com/locate/commatsci Computational Materials Science 17 (2000) 107±114 * Tel.: +33-04-72-43-26-48; fax: +33-04-72-43-12-03. E-mail address: xblase@dpm.univ-lyon1.fr (X. Blase). 0927-0256/00/$ - see front matter Ó 2000 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 7 - 0 2 5 6 ( 0 0 ) 0 0 0 0 6 - 9