|dc.description.abstract||Studies on charge transport properties in carbon nanotubes (CNTs) have been a subject of great interest for a long time not only as an important topic in fundamental science, but also as a basic requirement for the application of CNTs for nanoelectronics. CNTs show a wide range of transport behavior that varies from ballistic to hopping regime, depending on the dimensionality and nature of disorder in the system. Minute variations in disorder can lead from weak to strong localization, and this yields complex and intriguing features in the analysis of transport data. It is particularly important to carry out such a study for multiwall carbon nanotubes (MWCNTs), in which both dimensionality and disorder play an important role and the nature of localization is non-trivial as wave functions are extended along the tube or bundle of tubes.
A proper understanding of the mechanisms of charge transport and their quantitative knowledge is an essential requirement for any possible application of CNTs in nanodevices. Such studies not only yield information on the transport parameters crucial for applications but can also provide a test for any possible microscopic theories of transport. Main focus of the current thesis is to understand the mechanism of charge transport in iron-embedded MWCNTs and to gain more knowledge on the transport behavior. Magnetically functionalized CNTs, in particular the CNTs ﬁlled with ferromagnetic materials are of profound interest for the basic scientiﬁc research as well as for technological application. Iron-embedded MWCNTs are synthesized by one step pyrolysis method. This method gives a proper route to synthesize the magnetic particles encapsulated CNTs. Beyond the geometrical advantage of a cylinder-shaped nanostructure design, the carbon shells provide an effective protection against oxidation of magnetic nanoparticles. The iron-embedded MWCNTs exhibit excellent magnetic properties like the uniaxial magnetic anisotropy, and the high coercivity, which is larger than the coercivity of bulk iron. Thus, they have signiﬁcant potential for data storage devices and biomedical applications. Vertical alignment of CNTs is an important issue for device applications such as field electron emitters and flat-panel displays. Vertically aligned MWCNTs are grown on various substrates in the present work and the role of catalyst particles in vertical alignment is discussed. This thesis also reports the investigations on the magnetic properties including magnetotransport studies.
The thesis is organized in seven chapters and a brief summary of each chapter is given below.
Chapter 1 presents an introduction of the CNTs and its structural and electronic properties. Charge transport in CNTs is then discussed in terms of the fundamental aspects of conduction regimes and transport length scales.
The synthesis and characterization of iron-embedded MWCNTs is described in chapter 2. It is important to get good quality CNTs in a scalable way. The various methods available for CNT synthesis are arc discharge, laser ablation, chemical vapor deposition etc. A one-step thermally assisted pyrolysis method employed for synthesizing MWCNTs is a simple and cost-effective method. Benzene is used as a precursor and ferrocene as a catalyst in the present case. Good quality CNTs are obtained from this method, which are of multiwall in nature (outer diameter in the range of 10-25 nm). Vertically aligned mats of MWCNTs are also obtained on the quartz substrate. The thickness of the mats is several tens of microns. The prepared MWCNTs are characterized by electron microscopic studies for its structure and surface morphology. Many iron particles are seen inside the tubes. Energy dispersive x-ray (EDX) spectra taken from the small region of the sample under TEM show the presence of iron. Raman spectra of the sample suggest good quality of the tubes. Prominent G-peak in this spectrum shows that the sample is of well-graphitic nature. X-ray diffraction pattern of MWCNT material shows the presence of -Fe and Fe3C apart from the graphitic peak.
Chapter 3 describes the growth of vertically aligned MWCNTs (v-MWCNTs) on various substrates and role of catalyst particles in the alignment. The v-MWCNTs are grown on sapphire, quartz and thermally oxidized silicon substrates without pre-deposition of any catalyst. The grown MWCNT mats had a thickness of several tens of microns. Surface elemental analysis shows the presence of catalyst particles on the substrate which is essential for vertical alignment of the tubes. It is found that the order in which the precursor and the catalyst were introduced during chemical vapor deposition determines the orientation of the nanotubes. When there were no catalyst particles on the substrate in the beginning, random alignment of CNTs took place instead of vertical alignment. Base growth mode of CNTs is proposed in the present case from the results obtained.
Chapter 4 deals with the magnetic properties of the as-synthesized MWCNTs. The CNTs in pristine form are of diamagnetic in nature. The ferromagnetic-like behavior arises from the iron particles embedded in MWCNTs. These ferromagnetic particles are retained in the MWCNTs automatically, as the catalyst in this case contains iron. MWCNTs of different iron weight percentage are prepared by taking different amount of ferrocene as a precursor. These particles exhibit a magnetic moment up to 98 emu/g and coercivity in the range of 500–2000 Oe. Reduced magnetization is attributed to the formation of surface shell with spin disorder and to the presence of Fe3C phase. Large coercivity compared to the bulk vale of few orested is due to the complex state of interactions, which can create strong pinning centers for the core moments during the demagnetization. In addition the observed dependence of the magnetoresistance on the direction of applied ﬁeld, is correlated with the shape anisotropy of the Fe particles. The trend of saturation of magnetization at higher fields suggests that exchange coupling in the present case is one-dimensional.
The charge transport properties of MWCNT mats are discussed in chapter 5. Many of the transport parameters are often affected by the presence of magnetic field. In order to gain a deeper insight into the conduction mechanism, the study of the electrical transport in presence of magnetic field is highly useful. The temperature and magnetic field dependence of the conductivity of MWCNT mat is studied in the temperature range of 1.4-150 K in the magnetic field up to 10 T. The charge transport in the system is governed by Mott’s variable-range hopping (VRH) of three-dimensional type in the higher temperature range and two-dimensional type in the lower temperature range. Mott’s various parameters like localization length, hopping length, hopping energy, and density of states at the Fermi level are deduced from the VRH fit. The hopping length decreases from 13.2 to 12.2 nm, as temperature increases from 110 to 150 K. The obtained value of hopping length around ~13 nm is within the range of nanotube diameters of 10 to 25 nm. This is the main component of the hopping length, which indicates that VRH takes place on the tube scale. The localization lengths observed in the case of 3D VRH and 2D VRH conduction are well within the range of outer diameter of MWCNTs, which indicates that the localization takes place at the tube scale along the boundaries of the tubes. If the charges are localized at the tube boundaries, then the localization length gives an average diameter of the tubes and the results obtained supports this argument. It is also important to note that the defects present in the nanotubes in the form of structural defects and bad matching of chirality gives rise to localization.
There are not many reports on the effect of a magnetic field on the VRH process for MWCNT systems. The resistance of the sample decreases with the magnetic field in the direction of tube axis of the nanotubes. The magnetic field gives rise to delocalization of states as evident from the values of localization lengths at different fields. The application of magnetic field lowers the crossover temperature, at which three-dimensional VRH turns to two-dimensional VRH. The conductivity at the lower temperature side is governed by the weak localization (WL) give rise to positive magnetoconductance (MC). Here a phase diagram with temperature and magnetic field is proposed, showing different regions for different kind of transport mechanisms. This may be applicable for other class of disordered material as well.
Chapter 6 deals with the magnetotransport studies on disordered MWCNT mat. The electrical conductivity and MC data are analyzed in the temperature range of 1.4-150 K and in the magnetic fields up to 11 T. The system is in the critical regime obeying conductivity of metallic systems as suggested in weak localization-electron electron interaction model. The MC is positive for the whole temperature range except at temperature below 4.2 K. Results are analyzed in the terms of weak localization, electron-electron interaction and VRH. The
H 2 dependence at lower magnetic fields and H dependence at higher magnetic fields is
found supporting weak localization. Inelastic scattering lengths are also deduced from the low temperature MC data and its temperature dependence shows that the dominant dephasing mechanism in the present case is inelastic electron-electron scattering in the dirty limit.
Chapter 7 describes measurements on individual MWCNTs and subsequent charge transport studies. After many trials a suitable method was devised to isolate single tubes and to put contacts on it for the four probe measurement. For electrical measurements on isolated single tube, it is found that the joule heating due to excess current is an important issue. A current of the order of few µA burns the sample immediately. I-V characteristics of the MWCNTs show that the electrical contacts are ohmic and the resistance is few k. Initial electrical measurements show that there is slight decrease in resistance with increase of temperature and MR is approximately negative. This behavior suggests that signature of weak localization is present in the sample. Further studies are required in order to gain the insight into the transport mechanism for individual MWCNT.
Finally, the thesis concludes with a general conclusion and future directions for this