Encapsulation of Magnetic Oxides inside Carbon Nanotubes

Abstract

Transition Metal Oxides (TMOs) are well-known to host a diverse range of exotic electronic and magnetic ground states. Carbon Nanotubes (CNT), on the other hand, possess exceptional electrical, thermal and mechanical properties. This thesis presents a novel approach of encapsulating and preserving functional TMOs inside CNT, thereby, bringing into fore new possibilities in the arenas of nanoscience and nanotechnology, and providing an opportunity for direct patterning of nano-electronic devices. With special focus on TMOs relevant to antiferromagnet (AFM) spintronics, we have explored the fundamental as well as application based aspects of TMOs encapsulated inside CNT. The transition metals and oxides, when confined and protected within CNT, can provide huge tunability in their functional properties as well as give rise to new emergent phenomena at the interface. In this context, we find that although Fe@CNT have been extensively investigated, encapsulation of other transition metals like Ni or Co, and their respective oxides, is rarely explored. Even though magnetic oxides exhibit a remarkably wide range of properties, oxide-based electronics applications are still scarce. In the first part of the thesis, we demonstrate the use of a single-zone furnace with a modified synthesis chamber design to obtain metal (Fe, Ni) as well as their respective oxides-filled carbon nanotubes with high filling efficiency and controlled morphology. We employ pyrolysis of metallocene, a technique which is well-known to successfully yield Fe-filled CNT, but does not result in well-formed Ni or Co-filled CNT. This is achieved by innovative use of a green compound camphor as a co-precursor with metallocene. Here the key result is an order of magnitude enhancement in the filling efficiency and controlled morphology in case of Ni@CNT. For AFM spintronics, our attention has been on the encapsulation of a multi- functional oxide, α-Fe2O3 (hematite), inside CNT. Bulk α-Fe2O3 is a well-known

Dzyaloshinskii-Moriya Interaction driven canted AFM and a piezomagnet. Previous studies have shown that these canted systems exhibit an unusually slow magnetization relaxation phenomenon, which leads to the observation of a time-stable remanence. The magnitude of this unique remanence scales up with the extent of spin canting, and bears an inverse correlation with the N ́eel temperature. Bulk α-Fe2O3 is a room temperature canted AFM which can be advantageous for technological applications. However, the extent of spin canting in α-Fe2O3 is small, owing to its high N ́eel tem- perature ∼ 950K, which limits its use in various applications. Through rigorous magnetization and remanence measurements using SQUID magnetometry, we show that the encapsulation of α-Fe2O3 inside CNT leads to a significant enhancement in the magnitude of this time-stable remanence, and therefore, in the extent of spin canting. More importantly, these effects exist at the room temperature which can have profound technological implications. Further still, the encapsulation of α-Fe2O3 gives rise to novel interface effects, which are evident from the temperature varia- tion of lattice parameters derived using Synchrotron X-ray diffraction. Investigation of these oxide/CNT hybrids through temperature variation of Raman Spectroscopy further confirms the presence of novel interface effects in these systems. For energy related applications, different TMOs@CNT (including α-Fe2O3, Fe3O4, NiO, and Co3O4) in various morphologies are scaled up, and tested as an anode ma- terial in Li–ion batteries for cyclic stability and electrochemical performance. Here the key result is that encapsulation inside CNT results in superior cyclic stability, irrespective of the type of the oxide-encapsulate. The oxides filled CNT maintain outstanding cyclic stability and high-reversible capacity, even at higher current den- sities. In addition, the electrochemical studies of various oxides-filled CNT also shed light on the role of the morphology well as filling efficiency of oxide@CNT. The facile environment-friendly synthesis approach and unique nano-structure design of oxide/CNT hybrids presented in this work can serve as potential materials for high- performance Li-ion batteries