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Evidence of Correlation between Catalyst Particles and the Single-Wall Carbon Nanotube Diameter: A First Step towards Chirality Control

Abstract : Controlling the structure of single-wall carbon nanotubes during their synthesis by chemical vapor deposition remains a challenging issue. Here, using a specific synthesis protocol and ex situ transmission electron microscopy, we perform a statistical analysis of the structure of the tubes and of the catalyst particles from which they grow. We discriminate two nucleation modes, corresponding to different nanotube-particle junctions, that occur independently of the particle size. With the support of tight binding calculations, we show that a direct control of the nanotube diameter by the particle can only be achieved under growth conditions close to thermodynamic equilibrium. Since their discovery, single-walled carbon nanotubes (SWNTs) have been produced in increasingly larger quan-tities using catalytic chemical vapor deposition (CCVD). The main hurdle in the development of a SWNT-based technology is to control their structure and more precisely their diameter and chirality. Although post-synthesis sort-ing techniques are very efficient and useful [1,2], these processes are complex and dramatically damage and shorten the tubes, which can be a serious obstacle for their use in optoelectronic applications. Therefore, it would be highly desirable to control the tube's structure directly during the growth. A limited control of the chiral selectiv-ity has been reported by empirically tuning experimental conditions [3–6], but the reasons for this selectivity are not well understood. This is not surprising as the number of variables involved in the synthesis (e.g., temperature, pres-sure, catalyst and substrate chemical composition, nature of feedstock) is large. It is a matter of fact that narrowing the nanotube diameter distribution and restricting diameter below 1 nm leads to a restricted set of chiralities. This has been successfully achieved with the CoMoCAT, a Co-Mo catalytic method that produces SWNTs, process [3,4]. According to several authors [7,8] a key parameter is the control of the catalytic nanoparticle (NP) size. However, the relationship between the nanotube diameter and that of the particles is not clear [9]. Transmission electron microscopy (TEM) is the most appropriate tech-nique to analyze this link. Observations can be done either ex situ after the synthesis [10,11] or in situ using environ-mental TEM [12–14]. A number of ex situ TEM studies addressed the NT-NP diameter issue, showing that the NT diameter is directly related to that of the NP from which it has grown, but with a variable ratio, found to range from 1:1 [11] to 1:1:6 [15] or more [16]. However, in situ experi-ments have shown nucleation situations where, most often, the NT and NP diameters are not correlated [13]. It should be pointed out, that beside these observations, most of the statistical measurements trying to relate NP and NT diam-eters were done separately on each population and do not provide a reliable insight on the NP-NT link. In this Letter, we present a systematic TEM study investigating the correlation between the sizes of the nano-tubes or nuclei and of the NP from which they grow, as a function of synthesis time. Ex situ investigation of a large number of samples obtained by stopping the synthesis at different stages was preferred to in situ studies. Indeed, although qualitatively very insightful, environmental TEM experiments focus on one particular event and are not necessarily representative of the general nucleation-growth mechanism. For this ex situ study, we used a procedure described in [17] to allow the direct observation of both NTs and their seed NPs at any stage of the growth without any post-synthesis manipulation. We first show that a threshold (around 5 nm) exists in the tube lengths, beyond which tubes can grow very long. We also identify two typical configurations of the graphenic wall of the tubes in contact with the particle (tangential and perpendicular) and show that their relative abundance depends on the synthesis time. With the support of tight binding Monte Carlo (TBMC) simulations, we suggest that tangen-tial growth is favored under slow growth conditions, close to local thermodynamic equilibrium, while the other requires a larger kinetic activation. In the former situation, tube and NP diameters are almost equal, indicating a
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M.-F. C. Fiawoo, Anne Marie Bonnot, H Amara, C Bichara, J Thibault-Pénisson, et al.. Evidence of Correlation between Catalyst Particles and the Single-Wall Carbon Nanotube Diameter: A First Step towards Chirality Control. Physical Review Online Archive (PROLA), American Physical Society, 2012, 108 (19), pp.195503. ⟨10.1103/PhysRevLett.108.195503⟩. ⟨hal-01075303⟩

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