Synthesis of carbon nanotubes and characterization of nanotubes as atomic force microscopy tips

This thesis describes the synthesis of carbon nanotubes (CNTs) by chemical vapor deposition (CVD) and the evaluation of them as tips of atomic force microscope (AFM) probes. Both arc-discharge and CVD grown nanotubes were investigated as AFM tips. Two methods were used to produce nanotube tipped AFM probes: (i) nanotubes were directly grown on the apex of AFM probes by CVD and (ii) pre-grown nanotubes were mechanically attached onto the apex of AFM probes. The first part of the thesis describes experiments that sought to control the growth of carbon nanotubes by chemical vapor deposition. The effects of various parameters e.g. catalyst, the carbon source, the addition of hydrogen and the growth temperature, on carbon nanotube growth were investigated. Ethylene was found to lead to better quality nanotubes than the acetylene. Grown nanotubes' quality was found to improve upon flow of hydrogen during the synthesis. Nanotubes synthesized on 3 nm iron film at 840°C using ethylene and hydrogen led to utilizable carbon nanotube tipped AFM probes. The second part of the thesis focuses on the mechanical production of nanotube tipped AFM probes in high resolution electron microscopes using nanomanipulators. Scanning electron microscopy (SEM) was found to be more convenient than transmission electron microscopy (TEM) to carry out the nanomanipulation process. Both arc-discharge and CVD grown nanotubes were successfully attached onto the AFM probes using a custom made nanomanipulator. Nanotubes were glued onto the probes by focused electron beam (FEB) induced amorphous carbon deposits. FEB irradiation deposited amorphous carbon performed more reliable attachment on CVD nanotubes than the arc-discharge nanotubes. The last part of the thesis describes tapping mode AFM measurements done using nanotube tipped AFM probes. Different nanotube tipped probes were investigated as AFM tips for scanning colloidal gold particles with different diameters and gratings from Digital Instruments™ AFMs. High resolution AFM images were obtained by arc-discharge nanotube tips. CVD grown nanotubes were shown to be more resilient to tip crashes than the arc-discharge grown nanotubes. However, all the AFM images acquired from the scans by CVD nanotubes had the same kind of ringing artifact, in good agreement with a previously reported data about the artifacts of the CVD nanotube tipped AFM probes. Open ends of the CVD nanotubes, lack of crystallinity and the defects in their structure are suggested to be the reason for these artifacts.

Etude par microscopie électronique de particules nanostructurées d'oxalate de cobalt précipitées en milieu aqueux

The understanding of material self-assembling and self-organization mechanisms, at mesoscale, is crucial for nanotechnologies development. Such structures, hierarchically organized in superlattices or colloïdal crystals, are observed in inorganic or organo-metallic precipitates. The classical characterization, using electron microscopy and X-ray diffraction, of a synthesized powder is inadequate to describe the hierarchically organized polycrystalline structure of the final particles. Here, we have highlighted the successive steps that lead to the spontaneous formation of a cobalt oxalate dihydrate (COD) colloïdal crystals that precipitate in aqueous media. The characterization of the final particles, made by X-ray and electronic diffraction, has allowed us to determine the controversial crystalline phase of the precipitated COD. The β and γ COD phases has been discriminated by comparing experimental results with simulations of X-ray diffractogram and electron diffraction patterns for these both structures. The precipitated COD corresponds to the γ phase indicating that it comes from solid dehydration of cobalt oxalate tetrahydrate. The final powder characterization, performed by X-ray diffraction (XRD), atomic force microscopy (AFM), low voltage high resolution scanning electron microscopy (LVHRSEM) and transmission electron microscopy (TEM), gives some diverging results about monocrystalline or polycrystalline nature of the precipitates. XRD and AFM analysis show a polycrystalline structure of particles whereas LVHRSEM, TEM and electron diffraction observations indicate a monocrystalline structure. This apparent contradiction has led us to elaborate a new characterization method allowing to follow the growth and the structural evolution of the precipitate as a function of time. The cryo- preparation techniques are well adapted as they allow freezing and direct observation of solid state suspension by electron microscopy. The freeze/drying method has been modified and used for both SEM and TEM, and has allowed us to study the morphological and structural evolution of the COD precipitate from nanoscale to mesoscale. In addition, TEM analysis of samples prepared by classical techniques has been performed to complement cryo-electron microscopy observations. The combination of these two methods show that COD precipitation is a complex multi-step process leading to the formation of a core/shell heterostructure. The anisotropic core is porous and partially crystalline. It is formed by agglomeration of isolated primary particles and agregated ones (15-20 nm sized secondary particles). The crystalline shell corresponds to the final particles faces and is made up by the layer by layer self-assembling and alignment of 5-7 nm sized crystalline primary particles. These nanoparticles are aligned and perfectly ordered in strings of nanograins in the layer structure. The self-assembling occurs in the last step of growth where we have a lower ionic strength and supersaturation inducing slower kinetics of growth and aggregation. Moreover, the crystalline order in the primary and secondary particles increase with the time of reaction consistent with a continuous process of primary particle nucleation and growth. Our results show that self-assembling occurs layer by layer with terraces, kinks and steps that are present on the faces. This reminds the layer by layer crystalline growth models but in this case, the buiding blocks are colloïdal instead of atomic or molecular. Based on these different results, we propose an original model describing the COD precipitate growth. In addition, we have studied the poly(methacrylic acid-sodium salt) effect on the COD precipitation. This additive (PMMA) appeared to be a very good growth inhibitor. The PMMA effect on the final particles morphology is dramatic. Its concentration variation in solution slows down the nanoparticles aggregation rates along a [101] preferential direction. The crystal habit can then be controlled and we have synthesized platelets, cubes and rods with tailored length. The use of cryogenic electron microscopy has been shown to be a powerful tool for the understanding of time dependent aspects of particle growth. The use of such techniques for the study of other systems, where the final precipitate appearance may hide particle substructures, will help to elucidate growth mechanisms and allow finer control of nanostructured materials for many applications. The complex self-assembling and self-organization processes could then be highlighted and studied on a nanometer to micrometer scale.

EPFL2004

Manipulation of individual carbon nanotubes

Mirko Milas

Carbon nanotubes the long, cylindrical carbon molecules are the most exciting material discovered lately. They offer the possibility for a large number of applications. My task was to explore the way how we can use multi-walled carbon nanotubes (MWNT) in scanning probe applications: Atomic Force Microscopy (AFM) and Scanning Nearfield Optical Microscopy (SNOM). Carbon nanotubes (CNTs) are potentially ideal atomic force microscopy probes because they can have diameters as small as one nanometer, they have robust mechanical properties, and can be specifically functionalized with chemical and biological probes at the tip ends. CNTs will not wear, because they buckle elastically under applied load and thus will stay sharp. They also show exceptional field emission properties, and remarkably enough, the field emission at high currents is accompanied by light emission. This phenomenon offers the possibility to use CNTs as nanometer-size light sources for SNOM. In order to test CNTs for AFM and SNOM applications a mounting method has been elaborated. Two nanomanipulators have been designed and constructed for operations inside a Transmission and Scanning Electron Microscopes (TEM and SEM). This latter one with large work space and available electrical contacts allowed versatile use of nanomanipulator, including in situ field emission, and photon counting measurements using a photodiode. Hundreds of individual MWNT and double-wall nanotubes (DWNT) bundles, produced by arc-discharge (AD) or chemical vapour deposition (CVD) method, have been mounted, one-by-one, on either commercial AFM probes or sharpened metallic wires (W, Au, Pt). The tests show that in both case, AFM and SNOM probes, the weakest point is the attachment of the nanotube to the support. Amorphous carbon deposit is good for attaching nanotubes for limited time and limited applications, but it deforms under the forces that it has been submitted to during AFM scans. This weak bonding of nanotubes is the source of failure in AFM. In field/light emission the high contact resistance, which can change during the mechanical stress exerted by the applied electric field, will lead to a Joule heating of the nanotube/contact area and results a thermal breakdown of the tip. Attempts were made to elaborate a good mechanical and electrical attachment of the carbon nanotubes to the Si/metallic cantilever by using indium as bonding agent.

EPFL2006

Transmission electron microscopy and atomic force microscopy characterisation of titanium-base alloys nitrided under glow discharge

Philippe Buffat

The microstructure of titanium alloys (Ti–1Al–1Mn, Ti–6Al–4V) nitrided under glow discharge was characterised using light microscopy, X-ray diffraction and analytical electron microscopy. The investigations revealed a complex multilayered microstructure produced by diffusion at 850 and 900 °C at the surface. The layers formed on the Ti–1Al–1Mn alloy were identified as δ-TiN (face-centred cubic NaCl type), δ′-Ti2N (tetragonal body-centred), ε-Ti2N (tetragonal primitive) phases and α (N) solid solution containing up to 19 at.% nitrogen. The layers formed on the Ti–6Al–4V alloy also contained, besides the phases mentioned above, Ti2AlN and other minor Ti–Al–N ternary phases. The layer topography of both nitrided alloys, examined by atomic force microscopy, exhibited pronounced surface development. A relationship between the microstructure of the alloys and their micromechanical and tribological properties has been established. It shows that the improved properties are related to the presence of nanocrystalline δ-TiN in the outermost sublayer on the top of the layers composed of δ′ + ε-Ti2N and Ti2AlN crystals.

2005

Etude par microscopie électronique de particules nanostructurées d'oxalate de cobalt précipitées en milieu aqueux

EPFL2004

Manipulation of individual carbon nanotubes

Mirko Milas

EPFL2006