Carbon is a critical component for creating complex structures used in numerous fields. Carbon nanotubes, zero-dimensional nanostructures discovered in 1993, are essential due to their unique magnetic and electrical properties, necessitating thorough study.
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What are Carbon Nanotubes?
Carbon nanotubes (CNTs) have gained considerable attention for their exceptional mechanical, thermal, and electrical properties, attributed to their hollow structure. They are one-dimensional ballistic conductors, meaning they efficiently support the movement of electrons.1
CNTs can be envisioned as cylindrical structures composed of various graphene layers, possessing high aspect ratios with nanometer-scale diameters and micrometer-scale lengths.2
CNTs: Magnetic Properties and Experimental Observations
Pure CNTs have negligible magnetic properties. However, researchers are fascinated by the ability to control CNT metallicity using external magnetic fields. CNTs can exhibit semiconducting or metallic behavior depending on the field strength, with their band gap oscillating as a function of the magnetic field.
The electronic spectrum of metallic or semiconducting carbon nanotubes (CNTs) is characterized by a set of van Hove singularities (VHSs), which reflect the quantized momentum component along the circumferential direction.
When applied parallel to the tube axis, the magnetic field affects the electronic wave functions through the Aharonov-Bohm effect, causing VHSs to shift and opening an energy gap in a metallic tube. This transformation can oscillate the system between metallic and semiconducting states.3
In contrast, a perpendicular magnetic field gradually transitions the band structure into a Landau-level spectrum as the field strength increases.
Researchers have studied the magnetic properties of multi-walled carbon nanotubes (MWCNTs) grown on silicon substrates with magnetic nanoparticles produced by microwave plasma generation. The ferromagnetic resonance observed was primarily due to these nanoparticles.4 Thermal fluctuations at high temperatures caused the magnetic behavior to shift from ferromagnetic to paramagnetic, even below the Curie temperature.
Researchers used a superconducting quantum interference device (SQUID) to measure low residual magnetization in virgin-grown MWCNTs. The results showed a low residual magnetization with a coercive force value of 2.1492×104 A/m. The small coercive field indicated small domain wall sizes in the CNTs. The small coercive field indicated small domain wall sizes, with embedded magnetic nanoparticles showing promise for biomedical applications.
CrI3 is a layered ferromagnetic insulator that has gained significant interest, as it is the first example of a stand-alone monolayer ferromagnet. This discovery has opened doors to research on two-dimensional magnetic materials.5
Scientists have recently synthesized and characterized a tubular one-dimensional van der Waals hetero-structure: CrI3 nanotubes encapsulated within multi-walled carbon nanotubes. Using advanced electron microscopy and theoretical calculations, they investigated the nanoscale magnetic properties of these encapsulated CrI3 nanotubes.
SQUID measurements revealed a ferromagnetic M(T) curve for the powder precursor, with a magnetic moment per Cr atom five times lower (0.6 Bohr magneton) than that of single crystals and a Curie temperature of 54 K.
Spectroscopic analysis indicated a lack of magnetic saturation, an effect even more pronounced in the CrI3 1D vdW heterostructures encapsulated within CNTs. Here, the spin effective moment was approximately eight times lower than the bulk value, indicating distinct magnetic behavior in these 1D structures.
These findings suggest that CrI3 nanotubes exhibit unique magnetic properties compared to their bulk and 2D counterparts. This discovery of new 1D magnetic van der Waals heterostructures encapsulated within CNTs will likely inspire new research, where local probes can be employed to validate and harness the predicted radial magnetic state.5
Applications of Magnetic CNTs
CNTs are utilized in various industrial applications, including biosensors, automotive components, and thin coatings.
Advancements in nanotechnology have driven the miniaturization of electronic devices, with significant interest in designing nanoscale electromagnets and inductors, where helicoidal electron flow through a nanowire generates a magnetic field.
Chiral nanotubes have been proposed as potential nano solenoids. Chemically modified hetero-nanotubes, composed of bonded spiral ribbons of carbon nanotubes twisted into spiral forms, are considered even more suitable for this purpose.6
Magnetic CNTs containing magnetic nanoparticles are used for contamination absorbents, electrochemical sensors, catalysts, magnetic storage media, and microwave absorbents. In biotechnology, MWCNTs are used prominently for drug delivery and gene therapy.7
Challenges and Future Directions
Despite their industrial potential, several challenges hinder the rapid commercialization of CNTs. Produced CNTs contain impurities, which are classified as metallic or carbonaceous. Carbonaceous impurities, similar in composition to CNTs, are difficult to detect and can adversely affect properties and toxicity.
The low affinity of CNTs for metals presents a significant challenge for metal-CNT structures. The chemically inert side walls of defect-free CNTs hinder strong bond formation with metals. Metals also struggle to wet the CNT surface due to significant differences in surface energies.
Application-specific challenges include controlled synthesis, transistor placement, and achieving uniform dispersion for bulk composites. Poor interfacial interaction between metals and CNTs universally impacts many applications.8
Despite these challenges, significant progress is being made in CNTs for future applications. Researchers have woven 1-mm long CNTs into a thread and impregnated it with epoxy resin to create a composite material with an impressive tensile strength of 1.6 GPa. These new CNTs may lead to stronger woven fibers.
One important application of CNT wires is in the novel structural components of super lightweight airplanes and automobiles, where they provide necessary strength and high performance. In aircraft like the Boeing 787, CNT composites help them withstand lightning strikes and reduce fuel consumption.
Due to their elasticity, which allows them to stretch up to 18 % and return to their original shape, CNTs can also be used in wearable technology, making the future of CNTs bright and profitable.9
The unique properties of CNTs and the continued interest of experts in discovering novel phenomena and applications ensure that they are truly the material of the future.
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References and Further Further Reading
[1] Gupta, N., et al. (2019). Carbon nanotubes: synthesis, properties and engineering applications. Carbon Lett. Available at: doi.org/10.1007/s42823-019-00068-2
[2] Liu, C., et al. (2013). Carbon nanotubes: Controlled growth and application. Materials Today. doi.org/10.1016/j.mattod.2013.01.019
[3] O’connell, M. (2018). Carbon nanotubes: properties and applications. doi.org/10.1201/9781315222127
[4] Chang, L., et al. (2009). Magnetic properties of multi-walled carbon nanotubes. Journal of nanoscience and nanotechnology. doi.org/10.1166/jnn.2009.441
[5] Caha, I., et al. (2024). Magnetic single-wall CrI3 nanotubes encapsulated within multiwall Carbon Nanotubes. arXiv. doi.org/10.48550/arXiv.2405.14967
[6] Popović, Z., et al. (2017). Current Distribution Dependence on Electric Field in Helically Coiled Carbon Nanotubes. Contemporary Materials. doi.org/10.7251/COMEN1702121P
[7] Samadishadlou, M., et al. (2018). Magnetic carbon nanotubes: preparation, physical properties, and applications in biomedicine. Artificial cells, nanomedicine, and biotechnology. doi.org/10.1080/21691401.2017.1389746
[8] Daneshvar, F., et al. (2021). Critical challenges and advances in the carbon nanotube–metal interface for next-generation electronics. Nanoscale advances. doi.org/10.1039/D0NA00822B
[9] Ghosh, R. (2023). Carbon nanotubes: methods, achievements, challenges, and future directions. Authorea Preprints. doi.org/10.36227/techrxiv.19502455.v2
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