Low temperature growth of carbon nanotubes on Si substrates in high vacuum

Carbon nanotube (CNT) growth was carried out on SiO₂/Si substrates using an alcohol gas source in a high vacuum without any carbon decomposition processes. In the Raman spectra of the grown CNTs, both the G/Si peak intensity ratio and G/D peak intensity ratio indicated that the optimum growth temperature became lower as the pressure decreased. By reducing the pressure to 1 × 10^− 4 Pa, CNTs could be grown at 400 °C, and the G/D ratio was about 16, indicating that the quality of the grown CNTs was good, taking into account the low growth pressure. In addition, the Raman spectra in the radial breathing mode (RBM) region showed that the diameter distribution of the grown CNTs was dependent on both the growth pressure and temperature, and the relative intensity of the RBM peaks from small-diameter CNTs increased as the growth pressure and/or temperature was reduced.     

Upscaling potential of the CVD stacking growth method to produce dimensionally-controlled and catalyst-free multi-walled carbon nanotubes

Multi-walled CNTs (MWCNTs) with structural characteristics optimised for bio-applications have been produced using a catalyst-supported chemical vapour deposition (CVD) method. The upscale potential of the process was demonstrated by combining classical semi-continuous and stacked-growth modes. The vertically aligned MWCNT films thus obtained were multi-layered with five continuous strata of well-structured nanotubes. Following gentle disentanglement, the stacks were converted to individual MWCNTs with short dimensions (a final length and diameter of ～1.2 μm and ～12 nm) and almost catalyst-free (<0.04%). Overall, our process produces dispersed, bio-tailored MWCNTs with an output growth-yield 20 times higher than a standard CVD setup and exempt of complex or destructive post-growth steps of purification and separation. These constitute key steps towards the mass production of MWCNTs with low toxicological risks, an essential prerequisite for biomedical applications.     

Self-assembling of multi-walled carbon nanotubes on a porous carbon surface by catalyst-free chemical vapor deposition

Synthesis of carbon nanotubes (CNTs) by catalyst-free chemical vapor deposition (CFCVD) is one of the most important challenges in nanotube science. Self-assembling multi-walled CNTs (MWCNTs) were produced on a porous carbon surface using carbon black (CB) as a substrate, at 800 °C by the decomposition of diluted ethylene. MWCNTs with an outer-diameter distribution of 20–80 nm, examined by scanning and transmission electron microscopy, could be self-assembled on pore structures of CB surface by CFCVD.     

Metal catalyst-free mist flow chemical vapor deposition growth of single-wall carbon nanotubes using C60 colloidal solutions

Metal catalyst-free mist flow chemical vapor deposition (CVD) growth of single-walled carbon nanotubes (SWCNTs) with C₆₀ fullerenes has been investigated by using an aqueous colloidal C₆₀ solution. Under the optimum reaction condition, relatively uniform SWCNTs with a mean diameter of 1.28 nm can be synthesized without any treatments of C₆₀ prior to CVD. Cap opening, nucleation and the growth of SWCNTs have been occurring almost simultaneously during the present CVD. C₆₀ can be used as the seeds (i.e., end-caps) of SWCNTs, in which oxygen atoms from water molecules provide etching of C₆₀ into caps. Furthermore, the coalescence of C₆₀ caps into a larger one leads to the growth of SWCNTs with larger diameters.     

The importance of catalyst oxidation for the growth of carbon nanotubes on Si substrates

Iron catalyst in the form of a thin layer was deposited directly onto silicon wafers in order to grow carbon nanotubes by a chemical vapor deposition process. The chemical and morphological transformations undergone during the process by the iron were monitored in situ by photoelectron spectroscopy and ex situ by SEM. We found that the growth of carbon nanotubes was successful only when the iron catalyst was previously oxidized. Metallic iron transformed to embedded iron silicide structures, unable to initiate nanotube growth. On the other hand, pre-oxidized iron led to the formation of superficial particles and showed only a partial conversion to silicide, which resulted in carbon nanotube growth, even at very low pressures.     

Mechanism growth of multiwalled carbon nanotubes on carbon black

The growth of multiwalled carbon nanotubes (MWCNTs) on carbon black has been studied using a combustion oxygen/acetylene flame method. Different types of carbon black and reaction temperatures were evaluated for the growth of carbon nanotubes. The reaction was stopped after different short duration times of deposition in an attempt to observe the growth of carbon nanotubes. The samples were characterized by Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM) and by Raman Spectroscopy. We have observed the transformation of the carbon black surface into graphitic sheets and the start formation of tubes from these graphitic sheets. The length of the tubes is increased but the diameter is decreased with increased deposition times. Carbon nanotubes with 10–20 nm of diameter and a length of about 50 µm are obtained after 1 min deposition. The growth of multiwalled carbon nanotubes on carbon black is a phenomenon that can not be fully explained by carbon nanotubes growth models currently known. Our results lead us to propose a mechanism for the solid-state transformation of the carbon black particles surface into nanotubes in the oxygen/acetylene flame.     

Low temperature, non-isothermal growth of carbon nanotubes

Non-isothermal growth of carbon nanotubes (CNTs) was studied in order to obtain the activation energy. CNTs were grown on an Fe-Si catalyst using microwave plasma-enhanced chemical vapor deposition of CH₄. During the growth, the substrate was heated by the plasma such that its temperature increased with the growth until equilibrium was reached. In other words, the CNT growth took place under simultaneously increased time and temperature. This has resulted in different growth kinetics from previously published studies. We have therefore examined the growth kinetics using an empirical function for CNTs grown on three different substrates and derived the relevant activation energies based on the empirical function.     

Methanol-mediated growth of carbon nanotubes

A new approach for the growth of carbon nanotubes (CNTs) is reported. In particular, the growth is mediated by adding methanol into n-hexane with dissolved ferrocene. The first mediating effect is that the thermal decomposition of n-hexane and consequent widespread formation of carbon particles can be suppressed. Thus, truly continuous production of CNTs can be realized at high temperature. The second mediating effect is that with increasing methanol addition the proportion of carbon which can be used for CNT growth is decreased. Therefore, CNTs with different diameters or different numbers of graphitic layers can be produced individually.     

Sandwich growth of carbon nanotubes

Direct formation of structures that comprise freestanding CNTs connected to two surfaces was, thus far, not possible. In this article we report a novel approach to grow structured, highly oriented carbon nanotubes that are vertically aligned between a substrate and a massive cover. Growth is feasible at pre-determined, e.g., lithographically defined sites on metallic, semiconducting, or glass substrates. A novel, sandwiched catalyst structure and microwave plasma chemical vapor deposition (CVD) led to the formation of freestanding, small diameter carbon nanotubes. Our new technology offers a simple and scalable pathway to create 3D structured nanotube-based two-terminal electronic devices, device arrays, sensors and corresponding electronic circuits.     

Enhanced field emission stability and density produced by conical bundles of catalyst-free carbon nanotubes

Self-assembled bundling and catalyst removal can enhance the field emission stability and density of vertically-aligned multiwalled carbon nanotubes (VA-MWCNTs). These catalyst-free, opened tip, VA-MWCNTs offered better emission stability than the as grown samples. Both the emission stability and density were further enhanced as the opened-tip MWCNTs self-assembled into arrays of conical bundles. Theoretical simulation suggests that higher emission density was due to the reduced screening effects. The simulated local fields at the tips of the bundles suggest for a two-order of magnitude lower electric field loading on MWCNTs and contribute to prolong emission stability needed for practical applications.     

Novel growth of carbon nanotubes on nickel nanowires

A novel growth phenomenon is presented in this paper where carbon nanotubes (CNT) were grown successfully on nickel (Ni) nanowire using chemical vapour deposition technique. The decomposed carbon from ethylene diffused through the surface of nanowires and precipitated into hollow cylindrical carbon structures. Nanotubes of various lengths are found to have grown along the length of the outer side of the nanowires and were firmly rooted to their walls. The presence of a thin layer of oxide (~ 3 nm) on the top surface of nanowires is believed to have promoted the growth of CNT. Raman, X-ray photoelectron (XPS) and electron energy loss spectroscopy (EELS) were conducted in order to understand the formation of nanotubes and verify their presence, their level of crystallinity and chemical bonding structure with nanowires. This hybrid nanostructure is also found to have ferromagnetic behaviour, which can be applied in devices such as magnetic sensors and spintronic devices that combine the unique properties of CNT and Ni nanowires.     

Lattice-directed growth of single-walled carbon nanotubes with controlled geometries on surface

How to control the orientations of single-walled carbon nanotubes (SWCNTs) on surface is the key point to controlling their geometries. In this work, we chose quartz (0 0 1), MgO (0 0 1) and layered mica with 3-, 4- and 6-fold symmetry, respectively as substrates to grow SWCNTs using gas-flow and lattice-directed modes. The produced SWCNTs were aligned along the symmetrical directions and displayed the homologous angles of 120°, 90° and 60° during growth on quartz (0 0 1), MgO (0 0 1) and mica surfaces, respectively. The obtained SWCNTs with controlled geometries would have wide applications in nanoelectronic devices in the future.     

Glucose biosensor based on multiwalled carbon nanotubes grown directly on Si

An amperometric biosensor based on covalent immobilization of glucose oxidase (GOx) on multiwalled carbon nanotubes (MWCNTs) with potassium ferricyanide as the redox mediator was developed. The MWCNTs were grown directly on a layered structure of Co/Ti/Cr on a SiO₂/Si substrate by microwave-heated chemical vapor deposition. The mediator helps to shuttle the electrons between the immobilized GOx and the MWCNT electrode, therefore operating at a potential of 0.25 V vs. the saturated calomel electrode. This potential precludes the interfering compounds from oxidization. The sensitivity of biosensors to glucose was found to depend on the acid pretreatment and GOx reaction times. The steady-state response of the optimized biosensor exhibits a sensitivity of 20.6 μA mM⁻¹ cm⁻², a linear range of up to 8 mM, and a response time of <5 s.     

Stable superhydrophobic surface of hierarchical carbon nanotubes on Si micropillar arrays

It is of great importance to construct a stable superhydrophobic surface with low sliding angle (SA) for various applications. We used hydrophobic carbon nanotubes (CNTs) to construct the superhydrophobic hierarchical architecture of CNTs on silicon micropillar array (CNTs/Si-μp), which have a large contact angle of 153° to 155° and an ultralow SA of 3° to 5°. Small water droplets with a volume larger than 0.3 μL can slide on the CNTs/Si-μp with a tilted angle of approximately 5°. The CNTs growing on planar Si wafer lose their superhydrophobic properties after exposing to tiny water droplets. However, the CNTs/Si-μp still show superhydrophobic properties even after wetting using tiny water droplets. The CNTs/Si-μp still have a hierarchical structure after wetting, resulting in a stable superhydrophobic surface.     

The features of the magnetoresistance of carbon nanotubes modified with Fe

Two specimens of modified multi-wall carbon nanotubes (MWCNT) with a mass fraction of iron of 0.20 and 0.29 have been obtained by the method of metal reduction from aqueous salt solution. According to structural and phase investigations, modified MWCNT contain Fe₃C, α-Fe (ferromagnetic phase), and iron oxides FeO and Fe₂O₃ (antiferromagnetic phases with a Néel temperature of 188 K and 260 K, respectively) particles up to 8 nm in size. It is experimentally found that for MWCNT modified simultaneously by several ferromagnetic phases with different coercive forces, a giant magnetoresistive effect is observed at room temperature. For MWCNT modified with only one ferromagnetic phase, the giant magnetoresistance has not been experimentally detected. With temperature decrease, the magnetoresistance dependence on magnetic field for such modified nanotubes acquires a specific butterfly-like form, which is characteristic of the magnetic ordering of ferromagnetic and antiferromagnetic phases due to exchange anisotropy. For MWCNT modified with only one ferromagnetic phase there has been an effect of asymmetric magnetoresistance, which is related to the presence in the specimen of inhomogeneous transverse Hall voltages due to the pronounced spin-orbit interaction of conduction electrons and magnetic moments of the magnetic phase, and an anisotropic magnetoresistive effect.     

Reinforcement of precursor-derived Si–C–N ceramics with carbon nanotubes

Nanocomposites consisting of precursor-derived Si–C–N ceramics incorporated with carbon nanotubes (CNTs) were successfully prepared by casting of a mixture of CNTs and a liquid precursor polymer followed by cross-linking and thermolysis. The effect of CNTs on the fracture toughness of these nanocomposites was investigated by a thermal loading technique. The results reveal a dependence of the fracture toughness on the type of the CNTs. One type shows a significant increase of the fracture toughness at CNT contents of only 1–2 mass%, whereas the other one exhibits no effect. The microstructural effects of CNTs observed at the fracture surfaces of the nanocomposites by scanning electron microscope (SEM) and transmission electron microscope (TEM) can be correlated with the observed fracture toughness behavior.     

Spontaneous, catalyst-free formation of nitrogen-doped graphitic carbon nanocages

A simple approach for spontaneous, catalyst-free formation of highly graphitic nitrogen-containing carbon nanocages has been demonstrated by using commercially available graphite rods as the initial materials. The resultant carbon nanocages have well-ordered graphitic shells with more compact graphite layer structure than that of conventional bulk graphite. The incorporation of nitrogen into the graphitic backbone of carbon nanocages opens the potential for metal-free catalysis of oxygen reduction reaction in fuel cells. It is believed that the formation of carbon nanocages were attributed to the incurvature and coalescence of graphite sheets shelled off from graphite rods. Thermal gravimetric analysis revealed the as-prepared carbon nanocages possessed excellent thermal stability in both N₂ (1200 °C) and air (700 °C) atmospheres promising for applications in high-temperature environments.     

Promotion of cell growth with magnetically enhanced single-walled carbon nanotubes

We demonstrate that purified and functionalized single walled carbon nanotubes (SWNTs) promote the growth of NIH3T3 mouse fibroblast cells under a magnetic field. The SWNTs are functionalized in acidic solutions by attaching carboxyl groups (–COOH) on their surfaces. Functionalized SWNTs (fSWNTs) exhibit a ferromagnetic property when dispersed in water. Cytotoxicity after the delivery of the fSWNTs into the cells is significantly reduced due to the complete removal of toxic metallic impurities during the functionalization process. The efficient uptake of the fSWNTs by the cells is confirmed through transmission electron microscopy (TEM). It is discovered that the growth of the NIH3T3 cells treated with the fSWNTs is enhanced by up to 25% than control cells when an external magnetic field is applied. Our findings may lead to the non-invasive and non-toxic drug delivery as well as targeted cell therapy with fSWNTs.     

Solid-liquid-solid growth mechanism of single-wall carbon nanotubes

Reasons are presented which suggest that the liquefaction of the catalytic particles is a decisive condition for formation of single wall carbon nanotubes (SWNTs) by physical synthesis techniques. It is argued that the SWNT growth mechanism is a kind of solid-liquid-solid graphitization of amorphous carbon or other imperfect carbon forms catalyzed by molten supersaturated carbon-metal nanoparticles. The assumption of low temperature melting of these nanoparticles in contact with amorphous carbon followed by its precipitation in the form of SWNTs allows to explain qualitatively the experimentally observed SWNT growth rates and temperature dependence of the SWNT yield. Guidelines for increasing SWNT yield are proposed.