Chemistry of carbon nanotubes

Judicious application of site-selective reactions to non-aligned and aligned carbon nanotubes has opened a rich field of carbon nanotube chemistry. In order to meet specific requirements demanded by particular applications (e.g. biocompatibility for nanotube biosensors and interfacial strength for blending with polymers), chemical modification of carbon nanotubes is essential. The tips of carbon nanotubes are more reactive than their sidewalls, allowing a variety of chemical reagents to be attached at the nanotube tips. Recently, some interesting reactions have also been devised for chemical modification of both the inner and outer nanotube walls, though the seamless arrangement of hexagon rings renders the sidewalls relatively unreactive. This review provides a brief summary of very recent progress in the research on chemistry of carbon nanotubes.

Focused ion beam milling as a universal template technique for patterned growth of carbon nanotubes

Focused ion beam (FIB) milling system has been used to create nanosized patterns as the template for patterned growth of carbon nanotubes on Si substrate surface without predeposition of metal catalysts. Carbon nanotubes only nucleate and grow on the template under controlled pyrolysis of iron phthalocyanine at 1000 °C. The size, growth direction, and density of the patterned nanotubes can be controlled under different growth conditions and template sizes. Atomic force microscopy and electron microscopy analyses reveal that the selective growth on the FIB template is due to its special surface morphology and crystalline structure.

Patterned growth of carbon nanotubes on Si substrates without predeposition of metal catalysts

Aligned carbon nanotubes (CNTs) can be readily synthesized on quartz or silicon-oxide-coated Si substrates using a chemical vapor deposition method, but it is difficult to grow them on pure Si substrates without predeposition of metal catalysts. We report that aligned CNTs were grown by pyrolysis of iron phthalocyanine at 1000 °C on the templates created on Si substrates with simple mechanical scratching. Scanning electron microscopy and x-ray energy spectroscopy analysis revealed that the trenches and patterns created on the surface of Si substrates were preferred nucleation sites for nanotube growth due to a high surface energy, metastable surface structure, and possible capillarity effect. A two-step pyrolysis process maintained Fe as an active catalyst.

Nucleation and growth of carbon nanotubes in a mechano-thermal process

Separate nucleation and growth processes of carbon nanotubes were found in a mechano-thermal method in which carbon nanotubes are produced by first mechanical milling of graphite powder at room temperature and subsequent thermal annealing up to 1400 °C. The ball-milled graphite contains nucleation structures (nanosized metal particles and deformed (0 0 2) layers containing pentagons), and disordered carbon as a free carbon atom source. The subsequent annealing activates the growth of two types of multi-walled nanotubes in the absence of carbon vapor. Thin nanotubes (diameter <20 nm) are formed via crystallization of the disordered carbon with the preferred formation of the (0 0 2) basal planes. Thick nanotubes (diameter >20 nm) are formed through a metal catalytic solution–precipitation process (solid–liquid–solid). In both cases, carbon nanotubes grew out from disordered carbon particles with closed tips.

Carbon nanotubes formed in graphite after mechanical grinding and thermal annealing

Multi-walled carbon nanotubes with cylindrical and bamboo-type structures are produced in a graphite sample after mechanical milling at ambient temperature and subsequent thermal annealing up to 1400 °C. The ball milling produces a precursor structure and the thermal annealing activates the nanotube growth. Different nanotubular structures indicate different formation mechanisms: multi-wall cylindrical carbon nanotubes are probably formed upon micropores and the bamboo tubes are produced because of the metal catalysts. A two-dimensional growth governed by surface diffusion is believed to be one important factor for the nanotube growth. A potential industrial production method is demonstrated with advantages of large production quantity and low cost.

Demonstration of the advantages of using bamboo-like nanotubes for electrochemical biosensor applications compared with single walled carbon nanotubes

The modification of glassy carbon electrodes with random dispersions of nanotubes is currently the most popular approach to the preparation of carbon nanotube modified electrodes. The performance of glassy carbon electrodes modified with a random dispersion of bamboo type carbon nanotubes was compared with single walled carbon nanotubes modified glassy carbon electrodes and bare glassy carbon electrodes. The electrochemical performance of all three types for electrode were compared by investigating the electrochemistry with solution species and the oxidation of guanine and adenine bases of surface adsorbed DNA. The presence of edge planes of graphene at regular intervals along the walls of the bamboo nanotubes resulted in superior electrochemical performance relative to SWNT modified electrodes from two aspects. Firstly, with solution species the peak separation of the oxidation and reduction waves were smaller indicating more rapid rates of electron transfer. Secondly, a greater number of electroactive sites along the walls of the bamboo-carbon nanotubes (BCNTs) resulted in larger current signals and a broader dynamic range for the oxidation of DNA bases.

Scratching carbon nanotubes onto Si substrates

Large-scale, high-density, and patterned carbon nanotubes (CNTs) on both pure Si and quartz (SiO2) substrates have been produced using different approaches. The CNTs were synthesized by pyrolysis of the ball-milled iron phthalocyanine (FePc) in a tube furnace under a Ar-5% H2 gas flow. Because patterned CNTs are difficult to grow directly on smooth and perfect single-crystalline Si wafer surface, mechanical scratches were created to help the selective deposition and growth of CNTs on the scratched areas. This simple process does not require pre-deposition of any metal catalysts. For SiO2 substrates, which can be readily covered by a CNT film, patterned CNTs are produced using a TEM grid as mask to cover the areas where CNTs are not needed. The growth temperature and vapor density have strong influence on the patterned CNT formation. The scratch areas with a special structure and a higher surface energy help the selective nucleation of CNTs.

Dielectrophoretic separation of carbon nanotubes and polystyrene microparticles

The separation of multi-walled carbon nanotubes (MWCNTs) and polystyrene microparticles using a dielectrophoresis (DEP) system is presented. The DEP system consists of arrays of parallel microelectrodes patterned on a glass substrate. The performance of the system is evaluated by means of numerical simulations. The MWCNTs demonstrate a positive DEP behaviour and can be trapped at the regions of high electric field. However, the polystyrene microparticles demonstrate a negative DEP behaviour at a certain range of frequencies and migrate to the regions of low electric field. Experiments are performed on the microparticles at the frequencies between 100 Hz and 1 MHz to estimate their crossover frequency and select the range of separation frequencies. Further, experiments are conducted at the obtained range of separation frequencies to separate the MWCNTs and polystyrene microparticles.

Particle trapping using dielectrophoretically patterned carbon nanotubes

This study presents the dielectrophoretic (DEP) assembly of multi-walled carbon nanotubes (MWCNTs) between curved microelectrodes for the purpose of trapping polystyrene microparticles within a microfluidic system. Under normal conditions, polystyrene particles exhibit negative DEP behaviour and are repelled from microelectrodes. Interestingly, the addition of MWCNTs to the system alters this situation in two ways: first, they coat the surface of particles and change their dielectric properties to exhibit positive DEP behaviour; second, the assembled MWCNTs are highly conductive and after the deposition serve as extensions to the microelectrodes. They establish an array of nanoelectrodes that initiates from the edge of microelectrodes and grow along the electric field lines. These nanoelectrodes can effectively trap the MWCNT-coated particles, since they cover a large portion of the microchannel bottom surface and also create a much stronger electric field than the primary microelectrodes as confirmed by our numerical simulations. We will show that the presence of MWCNT significantly changes performance of the system, which is investigated by trapping sample polystyrene particles with plain, COOH and goat anti-mouse IgG surfaces.

Dielectrophoretically patterned carbon nanotubes to sort microparticles

This article compares the operation of a dielectrophoretic (DEP) platform before and after pattering carbon nanotubes (CNTs) between its microelectrodes. The diverse performance of the DEP system is assessed by separating 1 and 5 μm polystyrene particles. In the absence of CNTs, both particles can only be trapped by operating the system at low medium conductivities, (<10^-3 S/m) and frequencies (<75 kHz). Alternatively, applying CNTs to the system, some CNTs coat the surface of particles and increase their overall conductivity and permittivity, whereas the rest of them are patterned between the microelectrodes and induce strong DEP forces at their free ends, which can effectively trap the coated particles. The first development extends the range of medium conductivities and frequencies at which the trapping of both particles is achievable, whereas the second development facilitates the selective deposition of particles along the surface of curved microelectrodes. Setting the medium conductivity to 2×10^-3 S/m and the frequency to 20 MHz, most of 5 μm particles are trapped at the entry region of the first microelectrode pair, whereas most of 1 μm particles are trapped at the tips, and this distinction facilitates their separation. The trapping of 1 μm particles can be improved by decreasing the frequency to 1.5 MHz. This study demonstrates how the integration of CNTs into microfluidic systems enables them to operate beyond their capabilities.