Partners and International Organizations
(English)
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TU Delft, (NL), Motorola (F), IWF Dresden (D), TU Dresden (D), LPMC-ENS (F), CSIC (E)
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Abstract
(English)
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The WP1 addresses the growth of carbon nanotubes (CNT), using both laser ablation and in?situ chemical vapour deposition (CVD), and its main objective is to provide high quality CNT material for the consortium activities. This year, a strong activity has been developed around this workpackage enhancing the collaborations within the whole consortium, especially with the WP5 devoted to the modelling. Indeed, the understanding of the growth mechanisms is needed to improve the quantity and quality of the produced CNT. Therefore, the systematic investigation of the process parameters has been refined to feed both the phenomenological model proposed within this workpackage and the more sophisticated ab-initio simulations of WP5. Another example in this perspective of improvement is given by the purification studies. Indeed, they have been performed in strong relationship with the WP2, as the main requirement is to finally work on µm-long (meaning with low damage) individual NT. WP2 concerns the technological studies on the necessary processes and techniques for achieving nanotube based device in strong relationship with the specific WP6, devoted to their fabrication. Therefore, it includes selective placement of CNTs based on the use of self-assembled monolayers (SAMs), as well as the chemical modification of the ends and sidewalls of CNTs. Moreover, some techniques of MWCNT coating have been considered. In 2001, significant progress was made in the further development of the selective placement of SWNTs via the chemical modification of the substrate surface. The method using the aqueous suspensions of SWNTs has been refined and a process using organic suspensions has been developed. During this year, we have preferentially used the first method for electrodes fabricated prior to selective placement, while the organic solution one has been employed in samples where the electrodes were deposited successively. This distinction, for sake of simplicity, will be respected in the following, even if the two processes are completely interchangeable with respect to the electrodes fabrication. CNTs can potentially be used as nanoscale wires to connect nanoparticles to objects with larger size scales (e.g., contact pads). To accomplish this, the ends of CNTs need to be modified with functional chemical groups that attaches to nanoparticles such as metallic nanocrystals and custom synthesized molecules. This task has been addressed this year. However, the very large length-to-diameter aspect ratio (> 1,000) of CNTs makes it very challenging to monitor the chemical steps needed to achieve the desired end modifications. Indeed, for long CNTs (> 100 nm), the length-to-end area ratio is sufficiently large that the sidewall-to-substrate binding energy will dominate over that between the ends and the substrate. Chemically modifying the sidewall to influence the binding energy (without affecting the transport properties of the CNT) is expected to allow extra control in the nanomanipulation of CNTs. During this second period (2001), we have started to work on this topic and we are currently exploring different ways of performing sidewall chemistry on SWNTs.The WP3 is devoted to the study of the conductive properties of nanotubes and their dependence on both the diameter and chirality of the hexagonal carbon lattice. Experimentally, this has been verified using the tip of a scanning tunnelling microscope (STM). Part of these data can be also found in the characterisation WP4. A particular interest has been also given to the study of junctions for devices. The understanding of this topic is crucial for the realization of a room-temperature transistor from a single semiconducting carbon nanotube. This year, room-temperature single-electron transistors have been realized within individual metallic single-wall carbon nanotube molecules. The devices feature a short (down to 20 nm) nanotube section that is created by inducing local barriers into the tube using an atomic force microscope. Moreover, to further explore the effect of strong gating on the screening of Luttinger liquid properties in metallic carbon nanotubes we prepared Al electrodes on Si/SiO2 substrates. These Al-electrodes are coated with naturally grown AlOx that is only 2nm to 3nm thick. By depositing tubes onto these electrodes we obtain an arrangement where the gate is very close to the tube and has a very strong effect. Using the same principle, some logic circuits with field-effect transistors based on single carbon nanotubes have been demonstrated for the first time. The objective of WP4 is to provide experimental input data leading to a better understanding of the property/structure relationships in carbon nanotubes, as well as to a full characterization of their electronic structure and optical properties. These activities feed into the synthesis drive on the one side (to lead to an improvement in the yield and selectivity of the growth process) and to modelling effort on the other hand. Three axis of characterization have been chosen: optical properties, structural characterization and transport properties. In the work performed this year, we started by using different experimental techniques to reliably evaluate the nanotube diameters. Among them one may quote optical absorption spectroscopy, Raman spectroscopy, electron energy loss spectroscopy and X-ray diffraction. Then, in collaboration with the WP5 theoretical predictions and experimental data are confronted. The goal of Saturn WP5 is to provide the support for the understanding both of growth mechanism and physical properties of CNT. This is done in direct contact with the experimental efforts. Concerning the growth, the main objective is the comprehension of the role of the catalyst particules in order to help the optimization of the growth process. This year we have tackled the problem to address the main questions raised in the experimental growth studies. In particular, we have done a detailed study of two of the processes which were found to be rate-limiting for the growth of SWCNT: the nucleation of fullerene-like structures in the carbon-catalyst drops, which controls the beginning of the growth process, and the diffusion of C atoms in the melt, which controls the growth process after nucleation. First principles Molecular Dynamics simulations were used in these studies, to determine the atomistic processes involved.WP6 deals with the devices fabrication and this work is related with the technological studies of WP2. As it is discussed in WP2, this year significant progress was made in the further development of the selective placement of SWNTs via the chemical modification of the substrate surface. The method using the aqueous suspensions of SWNTs has been refined and a process using organic suspensions has been developed.
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