Plasmon resonance studies in silver nanoparticles arrays grown by Atomic Terrace Low Angle Shadowing (ATLAS)

Abstract

Nanotechnology 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.