Growth and optical, electrical and mechanical characterization of zinc oxide nanowires

The topic of this dissertation is embedded into the new-born field of inorganic nanowires. The research was focused on zinc oxide (ZnO) nanowires, in particular, which besides posing fundamental questions in physics, promise broad range of applications. ZnO nanowires were grown by chemical-vapour-deposition (CVD) using the vapour-liquid-solid (VLS) mechanism with gold particles as catalysts. Oxygen was introduced in the argon carrier gas and zinc was carboreduced from the ZnO/C mixture. We produced micrometer long ZnO nanowires and their diameter was controlled by the size of the gold catalyst particles. The diameter range of our ZnO nanowires is between 30 to 100nm. After growth, the alloyed particles were found at the top of the nanowires. ZnO nanowires are composed of single crystals of ZnO in the wurtzite phase. Growth direction is defined by their epitaxial relation with the substrate. From photoluminescence (PL) and cathodoluminescence (CL) characterizations, a deep impurity energy level at 2.3eV was found in a band gap of 3.3 eV, which could be related to oxygen off-stoichiometry. PL measurements showed that this energy level does not depend on the oxygen concentration used during the growth. Scanning-Tunnelling-Microscopy-based CL (STM-CL) measurements were made on single ZnO nanowires and showed that this energy level is likely to come from curved nanowires. Electrical measurements were performed on single nanowires, by using dielectrophoresis to place individual ZnO nanowires at a predefined location. E-beam lithography was used to deposit four metallic contacts on top of individual micrometer long ZnO nanowires. In order to avoid oxidation and the Shockley effect, we used chromium nitride (CrN) as electrical contacts. By measuring the temperature dependence on the resistance we extracted the activation energy of 2eV for charge transport, possibly arising from the same impurity level, as measured in PL. In the field-effect-transistor (FET) configuration, by using PMMA as dielectric layer and Cr as gate electrode, we extracted the charge carrier mobility of an individual ZnO nanowire as being 9.5 cm2/(Vs). In order to discover the origin of the energy level in the band gap, we measured the relaxation time of the current, after UV-illumination was switched off, and we found out that the surface states have significant effects. The elastic properties of individual ZnO nanowires were studied as a function of diameter and point defect/dislocation concentration. Using dielectrophoresis, nanowires were placed over a hole of a microfabricated Si3N4 membrane and, from bending measurements using an atomic force microscope (AFM) the Young's modulus was extracted. The diameter dependence of the Young's modulus showed a maximal value for a diameter of 50nm. Annealing under argon atmosphere showed an improvement of the Young's modulus which is explained by the removal of the defects/dislocations left during the growth. For comparison, the elastic properties of ZnS nanotubes produced by atomic layer deposition (ALD) were also measured. A clear dependence of the Young's modulus on the diameter and the wall thickness were found and attributed to the variation of the crystal structure of the nanotubes.

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

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

Yanki Keles

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.

EPFL2006

Synthesis, characterization and macroscopic manipulation of carbon nanotubes

Csilla Mikó

This thesis describes the synthesis, characterisation and macroscopic manipulation of carbon nanotubes. Two interconnected aspects were investigated: (i) growth of carbon nanotubes by catalytic chemical vapour deposition of acetylene and (ii) assembly and reinforcement of macroscopic carbon nanotube fibres. The first part describes a systematic study of the growth of carbon nanotubes by chemical vapour deposition. In particular, the effect of the growth parameters, such as catalyst composition, catalyst carrier, temperature, and retention time, on the yield and quality of CNTs was investigated. It was found that Fe2Co is the most active catalyst in the bimetallic family of Fe1-ξCoξ, whereas deviation from this ideal compound was identified as poisoning the catalytic reaction. Furthermore, the CaCO3 support was found to be an additional source of carbon. This chemical reaction was explained by the decomposition of CaCO3, which produces CO2 groups that react with acetylene to result in carbon for the growth of CNTs. Hence, it was discovered that carbonate supports are not only acting as an inert support to avoid the coarsening of catalyst particles but as an additional source of reactant during the growth. In the second part, macroscopic, oriented fibres were assembled from CNTs in order to exploit their excellent mechanical properties. Electrical as well as mechanical properties were measured. Although both semiconducting and metallic CNTs are present in the starting material, electrical measurements on the macroscopic ropes of CNTs revealed a hopping-like conductance of the fibres. Measurements of mechanical properties have pointed out that these fibres lack the strength of an individual nanotube. The main problem is the weak van der Waals interaction between the tubes which lead the tubes to slide easily along their axes. This problem was addressed by irradiation experiments: Pure covalent carbon-carbon bonds were established in CNT fibres by a low dose of electron and ultraviolet irradiation resulting in an enhancement of the shear modulus between the tubes. Moreover, electrical measurements have demonstrated that a low dose of electron irradiation can significantly increase the shear modulus via cross-linking of CNTs in the fibre, but a high electron dose disorders the graphitic structure and results in a loss of the intrinsically high Young's modulus of the individual tubes. In contrast to electrons, the ultraviolet photons do not displace carbon atoms in the nanotube, therefore the reinforcement can be achieved by keeping the graphitic structure of the tubes intact.

EPFL2005

Manipulation of individual carbon nanotubes

Mirko Milas

EPFL2006

Synthesis, characterization and macroscopic manipulation of carbon nanotubes

Csilla Mikó

EPFL2005

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

Yanki Keles

EPFL2006