Formation of linear Ni nanochains inside carbon nanotubes: Prediction from density functional theory Jurijs Kazerovskis a,⇑ , Sergei Piskunov a , Yuri F. Zhukovskii a , Pavel N. D’yachkov b , Stefano Bellucci c a Institute for Solid State Physics, University of Latvia, 8 Kengaraga Str., Riga LV-1063, Latvia b Kurnakov Institute of General and Inorganic Chemistry, Russian Academy of Sciences, Leninskii pr. 31, Moscow 119991, Russia c INFN-Laboratori Nazionali di Frascati, Via E. Fermi 40, 00044 Frascati, RM, Italy article info Article history: Received 21 February 2013 In final form 22 May 2013 Available online 29 May 2013 abstract First principles calculations have been performed to investigate the ground state properties of monope- riodic single-walled carbon nanotubes (CNTs) containing nanochain of aligned Ni atoms inside. Using the PBE exchange-correlation functional (E xc ) within the framework of density functional theory (DFT) we predict the clusterization of Ni filaments in (n,0) CNTs for n P 9 and for (n; n) CNTs for n P 6. The vari- ations in formation energies obtained for equilibrium defective nanostructures allow us to predict the most stable Ni@CNT compositions. Finally, the electronic charge redistribution has been calculated in order to explore intermolecular properties leading to stronger Ni–Ni bond formation. Ó 2013 Elsevier B.V. All rights reserved. 1. Introduction Carbon nanotubes with encapsulated monoatomic nanochains of magnetic metals (e.g., Ni) are technologically important one- dimensional (1D) Me@CNT nanostructures fabricated and studied in recent years [1]. Their mechanical, physical and chemical prop- erties can be applied in various nanodevices as well as magnetic data storage and drug delivery platforms. In addition, the CNT walls can provide an effective barrier against oxidation and, thus, ensure long-term stability of the encapsulated metals. Neverthe- less, the nanotubes filled with magnetic metals do not always dis- play designed properties because the amount and location of magnetic particles inside the tubes are difficult to be controlled. To guide reliable fabrication of Me@CNT, it is important to under- stand the formation mechanism of metal nanochains or separated nanoparticles in nanotubes. In this study, we consider monoatomic chains of nickel atoms encapsulated into single-walled (SW) CNTs of armchair-type ðn; nÞ; n ¼ 3; 4; 5; 6; 7, and zigzag-type ðn; 0Þ; n ¼ 7; 8; 9; 10; 11, chiralities. We determine the optimal size of CNT for encapsulating a single monoatomic chain, the most stable con- figuration of Ni atoms adopted by nanochain and its influence on CNT’s electronic structure. Recently, a number of both experimen- tal [2–4] and theoretical [5–7] studies of transition metal nanofil- aments encapsulated into CNTs of different morphologies were reported. However, we are not aware in any systematic study de- voted to monoatomic Ni nanofilament encapsulated inside CNT. The first principles method employed for this study allow us to describe 1D nanotubes in their original space form, unlike the stan- dard Plane-Wave (PW) methods, which are quite widespread now- adays for ab initio calculations on low-dimensional periodic systems, including carbon nanotubes [8,9]. Indeed, to restore the 3D periodicity in the PW NT calculations, the x  y supercell of nanotubes is artificially introduced: the NTs are placed into a square array with the intertube distance equal to 2–3 nm. At such separations the NT–NT interaction is found to be rather small; however, the convergence of results obtained in such calculations depends on the artificial intertube interactions demanding addi- tional computational efforts to ensure their negligibility. The meth- od that allows CNT formation starting from hexagonal graphite bulk and (0001) monolayer, in accordance with a model of struc- tural transformation (3D ? 2D ? 1D), is described elsewhere [10]. Using this approach we have constructed the monoperiodic unit cells for ideal carbon nanotubes of armchair (n ¼ 3; 4; 5; 6; 7) and zig-zag (n ¼ 7; 8; 9; 10; 11) types of CNT chiralities. Monoatom- ic Ni nanochain has been then incorporated into these nanotubes being fixed along the CNT axis (Figure 1). Off–center displacement of the incorporated Ni filament has been restricted by imposed rototranslational symmetry (rotation axis of nth order). The coordi- nates of all atoms in each of studied Ni@CNT nanostructures have been optimized along symmetry-preserved directions. The im- posed symmetry sufficiently reduces the computational efforts, however, it limits to some extent the flexibility of applied model. On the other hand, the CNTs under study have diameters less then 9 Å and, thus, the chosen model is very well consistent with recent HRTEM observation of Mo monoatomic nanochains in double- walled CNTs with inner diameter between 6 and 8 Å [2]. Within 0009-2614/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.cplett.2013.05.046 ⇑ Corresponding author. E-mail address: gio.gmbh@gmail.com (J. Kazerovskis). Chemical Physics Letters 577 (2013) 92–95 Contents lists available at SciVerse ScienceDirect Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett