Synthesis Methods and Growth Mechanisms

Multiwalled carbon nanotube dispersion methods affect their aggregation, deposition, and biomarker response

To systematically evaluate how dispersion methods affect the environmental behaviors of multiwalled carbon nanotubes (MWNTs), MWNTs were dispersed in various solutions (e.g., surfactants, natural organic matter (NOM), and etc.) via ultrasonication (SON) and long-term stirring (LT). The two tested surfactants [anionic sodium dodecyl sulfate (SDS) and nonionic poly(ethylene glycol)-poly(propylene glycol)-poly(ethylene glycol) (PEO-PPO-PEO) triblock copolymers (Pluronic)] could only disperse MWNTs via ultrasonication; while stable aqueous SON/MWNT and LT/MWNT suspensions were formed in the presence of the two model NOMs (Suwannee river humic acid and fulvic acid). Due to the inherent stochastic nature for both methods, the formed MWNT suspensions were highly heterogeneous. Their physicochemical properties, including surface charge, size, and morphology, greatly depended upon the dispersant type and concentration but were not very sensitive to the preparation methods. Aggregation and deposition behaviors of the dispersed MWNTs were controlled by van der Waal and electrostatic forces, as well as other non-DLVO forces (e.g., steric, hydrophobic forces, etc.). Unlike the preparation method-independent physicochemical properties, LT/NOM-MWNTs and SON/NOM-MWNTs differed in their fathead minnow epithelial cell metabolomics profiles.

In situ formation of carbon nanotubes encapsulated within boron nitride nanotubes via electron irradiation

We report experimental evidence of the formation by in situ electron-irradiation of single-walled carbon nanotubes (C-NT) confined within boron nitride nanotubes (BN-NT). 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. 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.

Identification of nitrogen dopants in single-walled carbon nanotubes by scanning tunneling microscopy

Using scanning tunnelling microscopy and spectroscopy, we investigated the atomic and electronic structure of nitrogen-doped single walled carbon nanotubes synthesized by chemical vapor deposition. The insertion of nitrogen in the carbon lattice induces several types of point defects involving different atomic configurations. Spectroscopic measurements on semiconducting nanotubes reveal that these local structures can induce either extended shallow levels or more localized deep levels. In a metallic tube, a single doping site associated with a donor state was observed in the gap at an energy close to that of the first van Hove singularity. Density functional theory calculations reveal that this feature corresponds to a substitutional nitrogen atom in the carbon network.

In situ monitoring of single-wall carbon nanotube laser assisted growth

Laser assisted catalytic chemical vapor deposition has recently emerged as an attractive method for locally growing carbon nanotubes (CNTs) in a cold wall reactor. So far, reported laser assisted CNT growth has been carried out without in situ process monitoring. This has made it difficult to control the growth process and limits the applicability of the method. Using a set of photodetectors with different spectral responses, we show that one can identify characteristic regimes of laser assisted CNT growth. More specifically, key process steps like catalyst activation, growth of CNTs, and amorphous carbon deposition are identified. Furthermore, the method allows optimization of growth conditions with respect to the quality of the growth products.