Jia Choi, PhD, Yong Zhang, Ph.D
Carbon nanotubes (CNTs), sometimes referred to simply as "nanotubes," are the cylindrical carbon allotrope nanostructures fortuitously discovered by Japanese physicist Sumio Iijima while he was studying the surface of graphite electrodes in an electric arc discharge.1 Since their discovery, CNTs have held a fundamental role in the field of nanotechnology due to their unique structural, mechanical and electronic properties.1-3
CNTs have high conductivity and high aspect ratio which help them to form a network of conductive tubes. Their outstanding mechanical properties are derived from a combination of stiffness, strength, and tenacity.4 Incorporated within a polymer, CNTs transfer their mechanical load to the polymer matrix at a much lower weight percentage than carbon black or carbon fibers, leading to more efficient applications. CNTs have also been utilized for thermal protection as thermal interface materials. Their interesting electronic and mechanical properties can be used in numerous applications, such as field-emission displays,5 nanocomposite materials,6 nanosensors,7 and logic elements.8 CNTs have been extensively studied for utility in leading-edge electronic fabrication and also extended to pharmaceutical fields for treatment of several types of diseases.9
Single-walled carbon nanotubes (SWNTs) (Product No. 755710) are seamless cylinders comprised of a layer of graphene. They have unique electronic properties which can change significantly with the chiral vector, C = (n, m), the parameter that indicates how the graphene sheet is rolled to form a carbon nanotube.10
The dependence of SWNTs electrical conductivity on the (n, m) values is shown in Table 1. Depending on how they are rolled, SWNTs' band gap can vary from 0 to 2 eV and electrical conductivity can show metallic or semiconducting behavior.
Thermal and electrical conductivities of carbon nanotubes are very high, and comparable to other conductive materials as shown in Table 2.11
Multi-walled carbon nanotubes (MWNTs) (Product Nos. 755133, 755117) consist of multiple rolled layers of graphene. MWNTs have not been well-defined due to their structural complexity and variety when compared to SWNTs. Nonetheless, MWNTs exhibit advantages over SWNTs, such as ease of mass production, low product cost per unit, and enhanced thermal and chemical stability. In general, the electrical and mechanical properties of SWNTs can change when functionalized, due to the structural defects occurred by C=C bond breakages during chemical processes. However, intrinsic properties of carbon nanotubes can be preserved by the surface modification of MWNTs, where the outer wall of MWNTs is exposed to chemical modifiers.
Surface modification of CNTs is performed to introduce new properties to carbon nanotubes for highly specific applications which often require organic solvent or water-solubilization, enhancement of functionality, dispersion and compatibility or lowering the toxicity of CNTs.12 Common functionalized CNTs, such as MWNT-COOH (Product No. 755125), are obtained via oxidation using various acids, ozone or plasma, which creates other oxygen functional groups (e.g., -OH, -C=O). The presence of oxygen-containing groups promotes the exfoliation of CNT bundles, and enhances the solubility in polar media and the chemical affinity with ester containing compounds, such as polyesters. COOH groups on nanotube surfaces are useful sites for further modification. Various molecules, such as synthetic and natural polymers can be grafted through the creation of amide and ester bonds.13
Double-walled carbon nanotubes (DWNTs) (Product Nos. 755168, 755141) are a synthetic blend of both single-walled and multi-walled nanotubes, showing properties intermediate between the two types. DWNTs are comprised of exactly two concentric nanotubes separated by 0.35 – 0.40 nm, with sufficient band gaps for use in field-effect transistors.14 The inner and outer walls of DWNTs have optical and Raman scattering characteristics of each wall.15 Theoretically, if each wall behaves like a SWNT, DWNTs can consist of four combinations based on the electronic type (metallic or semiconducting) according to (n, m) values of their inner and outer walls, e.g., metallic-metallic (inner-outer), metallic-semiconducting, semiconducting-metallic, and semiconducting-semiconducting. Some experimental studies found that even though both walls are semiconducting, DWNTs may behave as a metal.16 This complication of their overall electrical behavior has limited the utility of DWNTs to applications such as thin film electronics. However, DWNTs also exhibit several beneficial properties observed from MWNTs, such as improved lifetimes and current densities for field emission and high stability under aggressive chemical, mechanical, and thermal treatments along with the flexibility observed with SWNTs.17 Selective functionalization of the outer wall has led to the use of DWNTs as core-shell systems made of a pristine carbon nanotube core and chemically-functionalized nanotube shells, which are applicable as imaging and therapeutic agents in biological systems.18 DWNTs can be utilized in gas sensors19 as sensitive materials for the detection of gases such as such as H2, NH3, NO2 or O2, dielectrics,20 and technically demanding applications, such as field-emission displays and photovoltaics.21
We offer high quality SWNTs, MWNTs and DWNTs, some of which are the most electrically conductive additives available today, for your innovative and advanced materials research needs. When indicated, these nanotubes are produced via the catalytic chemical vapor deposition (CCVD) technique, a proven industrial process well-known for its reliability and scalability, and are purified or functionalized to increase the performance for research applications where special chemical properties like high surface area, transparency, or high field emission characteristics are required.
Carbon nanotubes can be used for a wide range of new and existing applications:
Detail descriptions of carbon nanotubes available are shown in Table 3. Specification details provided will help you select the right material for your application.
*TEM images with reprints permission granted by Nanocyl SA.
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