Growth of single-wall carbon nanotubes by chemical vapor deposition for electrical devices

Carbon emerges in di®erent forms. Diamond and graphite have been well known mate- rials for centuries. Moreover fullerenes and nanotubes were discovered only a few years ago. H. W. Kroto et al. depicted the fullerenes in 1985 [1]. A few years later, in 1991, S. Iijima described carbon nanotubes (CNTs) for the ¯rst time [2] (Figure 1.1). CNTs have a close relation to graphite, since a single-wall carbon nanotube is like a rolled-up graphite mono layer. However a nanotube has with its curved shape a higher chemical reacti- vity than a °at graphite layer. Both the side wall and the caps can be modi¯ed chemically [3]. Carbon nanotubes are regular carbon clusters with attractive mechanical and electronic pro- perties [4]. Nanotubes have a high mechanical strength due to a very large Young's modu- lus [5]. They can be used for the storage of hydrogen [5, 6], to store energy in electrochemical double layer capacitors [7] or to reinforce composite materials [3]. A single nanotube can be used as a sensor [8{12], a nanorelay [13], a vessel [14] or as a template [3, 15]. It is possible to produce light bulbs [16] and ¯bers [17] with carbon nanotubes. An array of CNTs can act as a °at panel display [3, 5] using their feature to act as ¯eld emitting devices [18{21]. CNTs are either metallic (1/3) or semiconducting (2/3). Nowadays it is not possible to select the desired characteristic of a nanotube in advance. It is only possible to separate metallic from semiconducting tubes by using an electrical ¯eld [22]. Metallic nanotubes with their diameter of a few nm represent the ultimate conducting wire whereas the semiconducting ones can be used as transistors [23{25] even on a transparent and °exible substrate [26]. The transistors can be optimized by the chemical control of the nanotube-electrode interface [27]. Quantum dots [28, 29] and spin valves [30{32] can be built alike simple logic gates [33] and a Y-junction recti¯er [34]. Carbon nanotubes have a very interesting property: they are "1-dimensional" molecules [35]. This has to be explained in a few words. In general, quantum con¯nement leads to a spacing of the allowed eigenenergies. Electrons cannot hop into a higher energy level if the thermal energy is much smaller than this energy di®erence. In a nanotube an electron is con¯ned in the directions perpendicular to the tube axis. The nanotube becomes a 1-dimensional conductor. For several years members of our research group are exploring the electrical properties of this very special conductor. The behavior of carbon nanotubes is investigated with electrical transport measurements at low temperatures (down to 50 mK) and in high magnetic ¯elds (up to 10 T). The raw material for the ¯rst experiments [36{38] were multi-wall carbon nanotubes ob- tained from L. Forr¶o (Ecole Polytechnique F¶ed¶erale de Lausanne) which were produced using laser ablation. The multi-wall carbon nanotubes were used to investigate the suppression of tunnelling [36, 39], multiple Andreev re°ection [28, 37], electrical spin injection [30{32] and quantum dots [37, 40{43]. The next step was to grow single-wall carbon nanotubes using chemical vapor deposition (CVD) [8,44{46]. This procedure has the advantage to be faster than an external collaboration and in addition the growth of the tubes directly on the device makes the samples ready for use without an additional treatment. It was veri¯ed that the CVD grown tubes are suitable of for electrical devices [47]. Vibrating nanotubes [48] and an ambipolar ¯eld-e®ect transistor [23] were studied. Kondo e®ect [49] and Fano-Resonances [50] were investigated as well. The latter experiments reveal one common de¯ciency. The grown tubes are often not sepa- rated but bundled [47] (Figure 6.10). Moreover it is not clear if they are multi- or single-wall tubes. This means for electronic transport measurements that several tubes are measured si- multaneously. Thus the tube with the best conductivity dominates the measurement, whereas the other tubes perturb the measured signal by there presence. The main focus of this thesis is the development of a growth process of single-wall carbon nanotubes by using CVD. The aim is to overcome the problem of bundling. The grown nanotubes have to be free of lattice defects and they need to have good electrode-nanotube contacts in order to make them suitable for electronic transport measurements. They have to lay °at, well separated and optimally distributed on SiO2 our standard substrate. On the one hand the tube density should not be too high since this would increase the probability of shortcuts between the electrodes due to nanotube-nanotube contacts. On the other hand it should not be too low since this would make the localization of an appropriate nanotube much more time consuming (Figure 1.2). Two ways to achieve this goal were tried. The single-wall nanotubes can be bought, dissolved in a solvent and spread after cleaning and separation [51{57], as in the thesis [46]. The second possibility is to grow the tubes directly on the device as presented in this thesis. Growing carbon nanotubes with CVD is very simple, at least in principle. There are only a few essential things needed: an oven, a substrate, a catalyst and a carbon feedstock. The main challenge is to acquire the right knowhow. The ¯rst step was to build up the CVD system. Afterwards the proper growth conditions and a simple method to check the demanded properties of the grown tubes had to be found. Scanning electron microscopy (SEM) is the standard characterization tool used in this thesis. Transmission electron microscopy (TEM) is a helpful mean in order to show that the tubes are separated and single-wall, since it allows the investigation of the tubes' internal structure. Atomic force microscopy (AFM) and Raman spectroscopy are used in addition. Outline of this thesis ² Chapter 2 gives a short overview with respect to the properties, the growth and the characterization of carbon nanotubes. ² The oven and the gas system are delineated in Chapter 3. Di®erent carbon feedstocks were used: ethylene/hydrogen, methane, methane/ethylene and methane/hydrogen. ² The steps towards a suitable catalyst are presented in Chapter 4. Evaporated and liquid based catalysts were tested. An iron molybdenum alumina catalyst dissolved in 2-propanol provides the best results. ² Chapter 5 gives a comparison of the results obtained utilizing di®erent growth processes, and describes the formation of amorphous carbon and the oxidation of nanotubes. ² Chapter 6 summarizes experiments on di®erent TEM grids (Au, Cu, Mo, Ni, stainless steel, Ti, quantifoils) and silicon nitride windows. ² The results from collaborations with other group members are presented in Chapter 7. These experiments show the good quality of the grown tubes

Electric Field Control of Spin Transport

Spintronics is an approach to electronics in which the spin of the electrons is exploited to control the electric resistance R of devices. One basic building block is the spin-valve, which is formed if two ferromagnetic electrodes are separated by a thin tunneling barrier. In such devices, R depends on the orientation of the magnetisation of the electrodes. It is usually larger in the antiparallel than in the parallel configuration. The relative difference of R, the so-called magneto-resistance (MR), is then positive. Common devices, such as the giant magneto-resistance sensor used in reading heads of hard disks, are based on this phenomenon. The MR may become anomalous (negative), if the transmission probability of electrons through the device is spin or energy dependent. This offers a route to the realisation of gate-tunable MR devices, because transmission probabilities can readily be tuned in many devices with an electrical gate signal. Such devices have, however, been elusive so far. We report here on a pronounced gate-field controlled MR in devices made from carbon nanotubes with ferromagnetic contacts. Both the amplitude and the sign of the MR are tunable with the gate voltage in a predictable manner. We emphasise that this spin-field effect is not restricted to carbon nanotubes but constitutes a generic effect which can in principle be exploited in all resonant tunneling devices.Comment: 22 pages, 5 figure