Electromagnetic properties of single-walled carbon nanotubes investigated by microwave absorption

Due to their unique properties, single-walled carbon nanotubes (SWNT) are very interesting candidates for the development of new electronic devices. Some of these properties, e.g., a possible transition to a superconducting phase or the existence of ordered magnetic states, are still under investigation and intensively discussed. Macroscopic amounts of SWNT can hitherto only be obtained as mixtures of tubes of different electronic properties. Therefore researchers have always been interested in a simple, fast, and reliable screeningmethod to detect the signatures of metallic or semiconducting SWNT. It is assumed quite generally that these “standard” electronic properties can be identified rather easily. In contrast to this, the above mentioned “unconventional” properties, i.e., superconductivity and magnetism, are anticipated to arise only in a small fraction of the nanotubes. Furthermore these features might be influenced by impurities, topological defects, or intertube interactions. Due to this fact, the sought-after screening method should be able to resolve the correlated signatures selectively, even if they are masked by other constituents in the sample. This study invokes microwave absorption, both in its resonant (electron paramagnetic resonance) and in its non-resonant variant (cavity perturbation). This method represents a versatile and selective tool to characterize magnetic and electronic phases and occurrent transitions. Whereas metallic SWNT are intrinsically paramagnetic due to Pauli paramagnetism, ideal semiconducting tubes are diamagnetic and therefore not accessible to electron paramagnetic resonance (EPR). Nevertheless extrinsic and intrinsic temperature-activated defects can introduce paramagnetic states observable by EPR. In additional experiments, nitrogen encapsulated in C60 has been incorporated inside SWNT as a paramagnetic probe, forming so called peapods. The synthesis of these N@C60 peapods allows the examination of the electromagnetic properties of the SWNT “from the inside” by EPR. In early studies, the EPR signal of SWNT grown by the electric arc-discharge method was masked by spurious signals of the catalyst remaining in the sample. By using nanotubes grown by a special chemical vapor deposition (CVD) technique, samples could be investigated which were almost catalyst-free. Thus it was possible to study the electronic properties of different types of SWNT over a wide temperature range by EPR. The high-temperature signals are dominated by itinerant spins. They result from the temperature activated delocalization of shallow defect states. At low temperatures, these charge carriers get trapped at specific sites. This trapping leads to a strong magnetic resonance of localized electron spins. Furthermore, no indication of the existence of elements different than carbon can be detected in the sample. This was proven by continuous wave (c.w.) EPR and also by modern techniques of pulsed EPR. Non-resonant microwave absorption is introduced as a powerful tool to study the electronic conductivity of bulk samples of SWNT. A custom microwave bridge was constructed therefore. By evoking this method, the temperature dependence of the complex resistivity at T > 20 K could be attributed to the existence of pseudo-metallic or small-band-gap semiconducting tubes. At T ≈ 12 K the transition from a non-linear dissipative state at low temperature to a conventional Ohmic loss behavior is observed. This transition is taken as an indication for the formation of superconducting domains in small parts of the sample. Furthermore, the existence of a weak ferromagnetic signal is detected via alternating current (AC) magnetization measurements. The features of this ferromagnetism, i.e., weak magnetization, low saturation field, and the absence of hysteresis effects, exclude remaining iron catalyst as source of this observation. Instead, the cooperative magnetism might arise from an intrinsic exchange interaction in SWNT