444 research outputs found
Structural characterisation and Kerr magnetometry of Fe/Mo(110) epitaxial system
No full textTHESIS 7579This thesis describes the investigation of the growth and magnetic properties of ultrathin
Fe films on the Mo(110) surface. The surface magneto optical Kerr effect (SMOKE)
setup was designed and built by author and used in conjunction with the surface
characterisation techniques to study magnetic anisotropy and magnetic phase transition in
Fe/Mo(110) films. The emphasis is made on correlation between magnetic properties and
film morphology
STM study of the (001) and (110) surfaces of magnetite
No full textTHESIS 7623In this thesis the surface of single crystals and thin films of magnetite
(001) and (110) have been studied by scanning tunneling microscopy (STM),
scanning tunneling spectroscopy (STS), Auger electron spectroscopy (AES)
and low-energy electron diffraction (LEED). The use of STM tips made of
magnetic materials is highlighted in this work. A novel technique has been developed
by the author in order to provide STM tips made of paramagnetic,
ferromagnetic and antiferromagnetic materials. Special emphasis has been
placed on the study and characterization of tips made of antiferromagnetic
MnNi alloy. Chemical and magnetic characterization has been performed
by AES, X-ray diffraction (XRD), alternating gradient force magnetometer
(AGEM) and transmission electron microscope (TEM). Scanning electron
microscope (SEM) has also been employed for characterization of the tips.
The potential of the MnNi tips for SP-STM measurements has been confirmed
by STS/STM studies of Mn/Fe(001), a test sample whose magnetic
properties are well known. These tips are routinely used by the group for
SP-STM experiments on magnetite
Surface studies of Fe3O4(001), Fe/Ge(001) and Fe3O4(001)/Ge(001)
No full textTHESIS 7885This work consists of surface studies of three systems; firstly, the (001) surface of a Fe3 O4 single crystal, secondly, the initial nucleation of Fe on the Ge(001) surface, and finally, iron oxide formed on Ge(001). The magnetite (001) surface was studied using Auger electron spectroscopy (AES), low energy electron diffraction (LEED), scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS). The clean surface is shown to exhibit the (√2 x √2)R45° LEED mesh. The surface is
imaged with tips made from both antiferromagnetic MnNi and non-magnetic W. An enhanced atomic scale contrast is imaged with the MnNi tips, which is explained in terms of a spin polarised contribution to the tunneling current. A model of the surface termination is put forward
Controlled electromigration on vicinal surfaces
No full textTHESIS 9576The influence of an electric field on the dynamics of steps, on vicinal surfaces, during high temperature annealing was experimentally studied. An experimental setup was designed, constructed and employed to explicitly isolate, for the first time, the electromigration and thermal effects in the dynamics of the step-bunching process on the vicinal Si(111) surface. Unlike conventional experiments, the setup\u27s configuration allowed combined but independent control direct-current joule-heating and irradiative heating of samples
Novel transparent conducting oxides for application in solar cells
No full textTHESIS 10000In the present thesis deposition and characterization of transparent
conducting oxides (TCOs) have been explored. The coexistence of
transparency and conductivity in a material is often regarded as an unusual
property, since transparent materials are most likely to be insulating while
conductive materials are generally quite absorbing. This remarkable
property can be easily achieved in wide band gap semiconductors, for which
a band gap higher than 3 eV ensures transparency to the visible light while
conductivity can be induced by doping, either intrinsic or extrinsic. Among
the TCOs, n-type doping has been so far the most successful. In2O3:Sn,
SnO2:F and ZnO:Al are the most commonly used n-type TCOs in
commercial devices
Spectroscopic studies of self-assembled plasmonic nanoparticle arrays
No full textTHESIS 9509In this thesis an alternative self-assembled deposition technique able to
produce nanoparticle (NP) arrays ordered on the macroscale is described.
Stepped surfaces are produced by high temperatures annealing AI3O4 (0001)
and metals are evaporated at a glancing angle of incidence towards the
surface. In this way NPs are produced on a transparent and insulating
substrate. These structures show localised plasmon resonances whose energetic
positions depend on the polarization of the exciting light, the template
deposition, the NP size as well as the distance between the NPs
Plasmon resonance studies in silver nanoparticles arrays grown by Atomic Terrace Low Angle Shadowing (ATLAS)
No full textTHESIS 9485Nanotechnology refers to the study of nanomaterials aiming to understand the physical properties and phenomena that materials show at the nanometre scale. A key goal in nanotechnology is acquiring the ability to manipulate nanostructures in order to produce nanomaterials with desired properties. Many techniques have been explored and they can be divided into top-down and bottom-up approaches. Success in the formation of nanowires, planar or not, has been achieved but from the literature it stands there are difficulties in forming regular and well-ordered arrays on insulating substrates. A novel technique was developed in our laboratories called Atomic Terrace Low Angle Shadowing (ATLAS). The ATLAS approach uses the basic principle of shadowing of a glancing incidence flux of atoms by the step-bunched terraces of a vicinal single crystal substrate. The Atomic Terrace Low Angle Shadowing (ATLAS) technique is valuable in that it is not material-specific but rather universal in its application unlike many other processes for bottom-up fabrication of nanowires. The ATLAS technique requires the use of vicinal surfaces and, in order to fulfil these requirement, a study of the morphological surface evolution upon annealing of sapphire, a-Al2 O3, was carried out. Sapphire samples were annealed for different times at high temperature, such as 1100°C, to produce a step and terrace morphology. Lower temperatures did not produce any satisfactory results on any of the samples. Samples characterization was done using atomic force microscopy (AFM) and revealed that flat samples did not show any step bunching even for long annealing times. This can be due to the temperature which was too low to trigger the rearrangement of surface atoms, as confirmed by annealing of flat substrates at higher temperature. In contrast, vicinal samples provided good results since the high temperature annealing was able to induce the step bunching process. The surface morphology presents a number of coalescence points, i.e. local areas where two steps merge and form a higher step. The formation of step and terrace morphology makes C-plane oriented a-alumina a suitable substrate to act as a template for nanowire growth using the ATLAS technique. The nanostructures growth performed by ATLAS is affected by several critical parameters such as the deposition angle, the step orientations, the deposition rate etc. A detailed study was carried out in order to find out the morphological growth of the metal on the substrate. Silver nanoparticles arrays were routinely produced and it was found that each key parameter can heavily influence the morphology of the nanostructures. Also, the particle size was found to be related to the deposition time, increasing with longer times. Post-annealing of the nanoparticle samples induces a reorganization and gradually destroys the long-range order. As well as silver nanoparticles arrays, nanowires of iron and cobalt were routinely produced showing the same parameters dependence as observed for silver. Linear chains and two dimensional arrays of silver nanoparticles have applications in several fields and are of particular interest in optics and advanced photonics. A new sub-field has been suggested and called “plasmonics” with the aim to study potential applications of the Surface Plasmon Resonance
(SPR) phenomenon. SPR is due to the interaction between an external electromagnetic field and the electrons. The oscillating dipole field generated can have strong coupling with the light at a resonance frequency within the visible wavelength range and this can be exploited for optical and sensor applications, channeling of flow of electromagnetic energy over hundreds of nanometers without significant loss, electromagnetic energy transport, sub-wavelength photonic waveguiding etc. The optical properties of silver nanoparticles arrays were investigated by means of UV-vis spectroscopy which revealed a different plasmonic response of the samples depending on the light polarization. The dipole-dipole interaction between the particles causes a split of the plasmonic peak which might And applications in integrated photonics as transmission lines of electromagnetic energy. In the aligned nanostructures produced, the shift of the resonance peak proves that strong near-field coupling, which in turn is the mechanism for energy transport, is present. Also it was found that different morphologies can give rise to a similar optical response due to the local contributions to the overall response. A qualitative insight of the studied phenomena was given by using the discrete dipole approximation DDA as a numerical approximate method to calculate the interaction between electromagnetic radiation and objects of arbitrary geometry. DDA analysis suggests that the experimentally observed splitting is not due to shape effects but to the electrodynamic interaction between the particles. An enhanced collective behavior is responsible for the red-shift (longitudinal polarization) and blue-shifts (transversal polarization) of the resonance peak. Furthermore, volume effects resulted less strong than shape effects
Optical impedance matching studied with Scanning Near-Field Optical Microscopy
No full textTHESIS 7293This thesis is a study of reflection-mode Scanning Near-Field Optical Microscopy (SNOM). A microscope was built for non-invasive imaging of samples, with optical resolutions better than the diffraction limit. The microscope also incorporates a mechanism for imaging the topography of a sample with a resolution better than 30 nm. The newly implemented shear-force system is based on a tuning fork method and therefore does not rely on an additional laser close to the tip apex. The design of the SNOM and of a fibre pulling machine for producing optical fibre tips will be presented, along with results obtained on test, calibration and transmission line samples
Magnetic and transport properties of magnetite based nanostructures
No full textTHESIS 8739In this thesis, we investigate the magnetic and transport properties of
magnetite based nanostructures. We in particular investigate the magnetization
and surface Verwey transition of magnetite thin film, interlayer exchange
coupling (IEC) of Fe3O4 /MgO /Fe3O4 multilayers, and domain walls
of magnetite based nanowire arrays
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