Hi,

There are many recipes found in the literature, most of which could be considered reliable as such. However, getting exclusively p- or n-type intrinsic doping is still a significant challenge. In particular, the issue of the atomic bonding configuration of the dopants needs to be understood and considered.

Below is a condensed excerpt from an upcoming book chapter I have co-authored titled "Doping Carbon Nanomaterials with Heteroatoms", to be published by Wiley. We also have a review article coming out in the Beilstein Journal of Nanotechnology more on the detection of heteroatoms by x-ray photoelectron spectroscopy.

My PhD thesis discusses the synthesis of nitrogen-doped SWCNTs in more detail:

http://lib.tkk.fi/Diss/2011/isbn9789526041247/

Hope you will find this useful!

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Introduction

There are several possible ways to control the electronic and optical properties of graphene and nanotubes. A certain way to gain this control is via structural functionalization using different methods [1, 2]. Substitutional heteroatoms are ideal in this context. It should be noted from the outset that substitutional doping is distinct from other methods of donating charge to the nanotubes, such as from adsorbed species (chemical doping or intercalation), by electrochemical charging, or by simple electrical gating. The two natural substitutional candidates are the nearest neighbors to carbon in the periodic table, boron (B) and nitrogen (N) [2–6]. Nitrogen in particular has attracted much attention due to its suitable atomic size and additional electron compared to carbon. Another possible substitutional dopant is phosphorus (P) [7, 8], which like nitrogen has 5 valence electrons, but in the third electron shell instead of the second.

Nitrogen doping

Achieving the desired functionality or modification of the properties of the carbon materials discussed here depends on the local bonding of the dopants. For different heteroatoms, different challenges arise. For example, particularly in nanotubes, nitrogen tends to bond in structurally and electronically distinct configurations, whereas boron mainly stays in a direct-substitutional configuration. Since nitrogen has an extra electron compared to carbon, introducing N atoms into the graphitic lattice could be expected to contribute to n-type doping of the host. Indeed, theoretical works have shown that a direct substitution of a C atom by N results in localized states above the Fermi level [9–11]. This is because N uses three electrons in σ bonds and one in π, with the remaining fifth valence electron forced to occupy the π* donor state.

Since the size and electronic structure of nitrogen are different than those of carbon, N can also create a defect in the lattice. The most widely discussed of such defects is the so-called triple pyridinic vacancy configuration, where 3 N atoms each form sp2 bonds to two carbon atoms around a single vacancy [12]. This is energetically favorable since the site presents no dangling bonds, although other possibilities have also been considered. However, electronically these sites are p-type [11–13]. Thus the actual effect N doping has on the electronic structure of SWCNTs or graphene will also depend on the relative amounts of the different doping configurations incorporated into the lattice.

Nitrogen and boron doping of carbon nanotubes was proposed theoretically already in 1993 by Yi and Bernholc [3]. After pioneering work by Stephan et al. on BN co-doping using arc-discharge [14], first reports on the synthesis of N-MWCNTs are from 1997 by Yudasaka et al. [15] and Sen et al. [16], and from 1999 by Terrones et al. [17]. The first experimental work on N-SWCNT synthesis was by Glerup et al. in 2004 using arc discharge [18]. Successful synthesis was later achieved also by laser ablation [19]. In addition, there are a number of reports on N-SWCNTs synthesized using variations of chemical vapor deposition methods [20–32].

Boron doping

A simple substitution of B is the most favorable bonding configuration for this type of atom, resulting in p-type doping. However, attempts to synthesize B-doped material have reported the formation of B nanodomains along the nanotube structure [33, 34]. Only low concentrations of B atoms are expected to lead to the formation of a shallow acceptor state in the band gap. Early calculations [3] found an acceptor state located at 0.16 eV above the Fermi energy for an (8,0) nanotube with a B/C ratio of 1/80. Further studies discussed the evolution of this acceptor-like level with increasing levels of B, pointing out that low and high doping entail critical differences in the resulting physical properties [35].

Laser ablation was the first technique to produce B-SWCNTs by the introduction of boron into graphite-Co-Ni targets [36, 37]. Gai et al. [36] noted the difficulty of incorporating B in the graphitic network when characterizing the samples by transmission electron microscopy (TEM) and electron energy loss spectroscopy (EELS), only finding SWCNTs in the products when the B content in the target material was less than 3.5 at.%. Using chemical vapor deposition, boron-doped double-walled CNTs were first synthesized on bimetallic Fe/Mo catalysts supported in MgO in 2007 by Panchakarla et al. [38] by decomposing CH4 and Ar mixtures in presence of diborane. The first reported synthesis of B-SWCNTs involved the use of non-diluted precursors (triisopropyl borate) in a high vacuum CVD system [39]. In such a method, high quality SWCNTs could be formed in a wide temperature range, but it was again found that it is difficult to get B incorporated into the carbon nanotube network. Instead, more than 80 at.% of the B content in the samples was found to be boron carbide and boron oxide.

Phosphorus doping

Phosphorus has five valence electrons, but since they are on the third electron shell, P has a significantly larger atomic radius (107 pm), which is expected to cause it to protrude from the graphitic lattice [40, 41]. A higher curvature in small diameter SWCNTs is thought to mitigate the resulting stress [42]. As for the effect a P substitution will have on the electronic structure of SWCNTs, no consensus seems to have been reached in the available literature yet. It has been suggested that P will act as a net donor both in the substitutional configuration [43] or in a P analogue of the N pyridinic site (3 P atoms around a vacancy) [41]; however, Krstic et al. proposed that the substitutional site will be oxidized in ambient, converting it into a net acceptor [43]. Alternatively, Cruz-Silva et al. have suggested that the P atom will bond in sp3 hybridization creating a nondispersive localized state that does not act as a net donor [40, 42].

The production of phosphorus-doped SWCNTs has been reported only recently. CVD material has been produced using triphenylphosphine as the P precursor [42, 44, 45]. Krstic et al. reported on the synthesis of P-SWCNTs using arc discharge, with red phosphorus mixed into the anode rod acting as P precursor [43]. Recently, P-SWCNTs were synthesised by high vacuum CVD, purified by density gradient ultracentrifugation, which enabled the first direct measurement for the incorporation of P in the lattice via x-ray photoelectron spectroscopy [46].

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