Atomic Configuration of Nitrogen-Doped Single-Walled Carbon Nanotubes

Abstract

Abstract:Having access to the chemical environment at the atomic level of a dopant in a nanostructure is crucial for the understanding of its properties. We have performed atomicallyresolved electron energy-loss spectroscopy to detect individual nitrogen dopants in single-walled carbon nanotubes and compared with first principles calculations. We demonstrate that nitrogen doping occurs as single atoms in different bonding configurations: graphitic-like and pyrroliclike substitutional nitrogen neighbouring local lattice distortion such as Stone-Thrower-Wales defects. The stability under the electron beam of these nanotubes has been studied in two extreme cases of nitrogen incorporation content and configuration. These findings provide key information for the applications of these nanostructures. Doped carbon nanotubes (NT), notably nitrogen-doped (CN x -NT), have attracted much attention because of their interesting physical and chemical properties (1)(2)(3)(4). Knowledge of the atomic arrangement of the dopant atoms in such nanostructures is essential for a complete understanding of the material's electronic properties, e.g. field emission (5) or transport (6) properties. This requires precision measurements, combining high spatial resolution and high spectroscopic sensitivity. Several techniques have been deployed with this aim, but until now none of them have provided the required information in such nanostructures. Most characterization techniques, such as Raman spectroscopy and x-ray absorption spectroscopy (XAS), have relatively low spatial resolution, of the order of hundreds of nanometers, and in some cases only provide indirect information (1,2). Thus, local structural and analytical methods are needed. Scanning tunneling microscopy (STM) and spectroscopy (STS) are local techniques for studying the structural and electronic features/properties of materials. Indeed, some studies have been reported on CN x -NT (7-9) and recently also on N-doped graphene (10,11). However, chemical characterization cannot be unambiguously performed from the STM/STS analysis. Indeed, the interpretation of the results remains complicated, as different local structures may give rise to similar features (7-11). In this sense, high-resolution transmission electron microscopy (HRTEM) and scanning TEM (STEM) combined with electron energy-loss spectroscopy (EELS) have provided very valuable and rich information at the atomic scale (12).Great progress has been made in (S)TEM due to the development of aberration-corrected microscopes (12)(13)(14). Recent work has demonstrated that annular dark-field imaging performed in a C s -probe corrected STEM enables quantitative atom-by-atom analysis of 2D low-Z materials such as h-BN (13). Single atoms have also been investigated via STEM-EELS in these materials (12,(14)(15)(16)(17)(18)(19)(20)(21). Three recent works have demonstrated the possibility to identify by STEM-EELS the presence of N atoms in graphene (19)(20)(21). In addition, N atoms have been also detected in Nimplanted multi-walle...

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“…and the broad σ * band around 408.5 eV is commonly ascribed to the pyridinic N conformation, [ 43 ] which is recently demonstrated as the origin of active sites in nitrogen-doped carbon materials. [ 44 ] This type of EELS feature indicates the dominantly sp 2 -hybridized N C bonds in NGM.…”

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“…However, Raman spectroscopy presents some limitations for getting directly this chemical information from these materials at the local scale. Thus, in addition of this spectroscopic technique, electron energy-loss spectroscopy (EELS) performed in a (Scanning) transmission electron microscope ((S)TEM) is a complementary technique, which gives access to a wealth of structural, chemical and physical information with nanometer to sub-angstrom spatial resolution [41][42][43][44][45][46][47]. Analysis of core-loss EELS spectra yields qualitative and quantitative information on the elemental composition and on the local chemical environment (bonding configuration, valence state…) [48][49][50][51][52].…”

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“…Low-loss EELS spectra contain many excitation processes including volume and surface plasmons, interband transitions and semi-core states transitions which can confound the identification of specific features (Garcia de Abajo, 2010; Lajaunie et al, 2013;Moreau & Boucher, 2012). XAS and core-loss EELS spectra are also not simple -ionization edges can overlap (Hakouk et al, 2013;Panchakarla et al, 2015) and the physical phenomena behind the white line intensity ratio and near-edge fine structures are highly complex (Krüger, 2010;Mizoguchi et al, 2010;Arenal et al, 2014b;Lajaunie et al, 2015;). Different approaches have been proposed to tackle this (ideally a complete study should include multiple approaches (Danet et al, 2010;Arenal et al, 2014b;Lajaunie et al, 2015)).…”