The sensitivity to small changes in the electronic structure of carbon nanotubes (CNTs) from external perturbations limits their successful integration on electronic devices [1]. It is therefore necessary to develop experimental strategies for CNT synthesis that guarantee the formation of crystalline structures of carbon materials while ensuring a protection from the environment without affecting its electronic properties. Under this context, boron nitride nanotubes (BNNTs), due to their uniform electronic properties and their chemical inertness characteristics, is one of the most appropriated nanomaterials for achieving these goals. In fact, BNNTs are large band gap insulators exhibiting a resistivity to oxidation of up to 900 °C [2]. Here we report the synthesis and growth of crystalline carbon nanotubes inside a larger diameter BNNT via in-situ electron irradiation in a TEM [3].

Electron beam irradiation and HRTEM were performed using an imaging-side aberration-corrected FEI Titan-Cube microscope working at 80 kV, equipped with a Cs corrector. Complementary spatially-resolved EELS-STEM studies were also carried out using a FEI Titan Low-Base microscope, working at 80 kV, which is equipped with a Cs probe corrector. In both cases, particular attention was devoted to avoid contamination during acquisition. Single-walled (SW) BNNT were produced by laser vaporization technique [4]. Some of these BNNT can be partially filled by amorphous carbon [4]. Furthermore, density functional theory (DFT) simulations were conducted for determining the structural stability and electronic properties of such a hybrid system.

In Fig. 1, a six-frame HRTEM image sequence showing the evolution towards a nanotube structure of amorphous carbon enclosed within a BNNT. SR-EELS analyses confirm the presence of this amorphous carbon inside the NT, as displayed in Fig. 1 (g) where B-K, C-K and N-K edges are shown. In the HRTEM sequence defined by arrows, which occurs for a total cumulative dose of up to 1.8 10⁷ e^–/Ų and over a period of 380 seconds at room temperature, amorphous carbon is firstly observed in a straight BNNT and evolves over time to a crystalline structure. Simultaneously, a gradual shrinkage of the BNNT is observed. By the end of the process, the BNNT is broken and also disintegrated. We also observed the formation of an atomic-scale bridge between the tip of the carbon tube and the outer BN wall, and the subsequent reparation of the defect on both materials. Initially, a defect at the BNNT surface enhances the C-BN interaction by establishing a connection between both tubes.

These results show that the electron radiation stemming from the microscope supplies the energy required by the amorphous carbonaceous structures to crystallize in a tubular form in a catalyst-free procedure, at room temperature and high vacuum [3]. The structural defects resulting from the interaction of the shapeless carbon with the BN nanotube are corrected in a self-healing process throughout the crystallinization. Structural changes developed during the irradiation process such as defects formation and evolution, shrinkage, and shortness of the BN-NT were in situ monitored. The outer BN wall provides a protective and insulating shell against environmental perturbations to the inner C-NT without affecting their electronic properties, as demonstrated by first-principles calculations, see Fig. 1 (h)-(j).

[1] A. Jorio, G. Dresselhaus, M.S. Dresselhaus, Springer-Verlag: Berlin, 2008.

[2] R. Arenal, X. Blase, A. Loiseau, Advances in Physics 59, 101 (2010).

[3] R. Arenal and A. Lopez-Bezanilla, ACS Nano 8, 8419–8425 (2014).

[4] R. Arenal, O. Stephan, J.L. Cochon, and A. Loiseau, J. Am. Chem. Soc. 129, 16183 (2007).

[5] The research leading to these results has received funding from the EU under Grant Agreements 312483-ESTEEM2 and 604391 Graphene Flagship, from the Spanish Ministerio Economia y Competitividad (FIS2013-46159-C3-3-P) and from the EU under the Marie Curie Grant Agreement 642742 – Enabling Excellence.

Figures:

Figure 1. (a)-(f) Six-frame HRTEM image sequence displaying the formation process of a crystalline CNT from amorphous C encapsulated in a BNNT under electron beam irradiation (high doses for short periods of time, see Fig. 1 (c)) in a TEM. (g) EEL spectra recorded in 2 different areas (marked in the HAADF-STEM image showed below), displaying B-K, C-K and N-K edges. Scale bars in Fig. 1 (a) and (g) are 2 nm. (h) Electronic band structure of a double-walled hybrid nanotube formed by a inner (7,7) C-NT concentric to a (12,12) BN-NT. (i) Electronic band structure of a single-walled (7,7) C-NT. (j) Cross-sectional view of the concentric nanotubes.

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