Nanofabrication and properties of gold and platinum nanostructures on graphite

Abstract

This Thesis presents studies of modern aspects of nanotechnology where the size and density of nanostructures play a huge role for further technological advances of our life. As an example of this is heterogeneous catalysis in which studies of catalytic properties of metallic nanostructures are of major interest. As a result the primary motivations of the work were to find methods for controlling size and density of nanostructures, and to investigate how these influence catalytic properties. The control over the catalytic properties of such structures allows for greater reaction selectivity and smaller amounts of waste byproducts in chemical processes. In order to improve the miniaturization effect one would require breaking and extending some well-known limitations that are related to nanofabrication. In fact, just such a possibility was found in the present work. In this Thesis, results are obtained from experimental, theoretical, and to some extent computational evidences. Correlation between experimental and theoretical analysis is presented. The systems studied include transition metals such as Pt and Au, while polycrystalline graphite and highly oriented pyrolytic polycrystalline graphite (HOPG) have been served as substrates. It is well-known that catalytic properties of Pt and Au dramatically change at the nanometer scale, while polycrystalline graphite and HOPG are known to prohibit a high degree of order. Hence, investigation of these systems is important for industry applications.

Self-assembled Pt and Au nanostructures have been initially formed by physical vapour deposition on graphite, while off-normal Ion Beam Sputtering (IBS) was used for post fabrication of these. The Pt and Au nanostructures were exposed to low energetic 500 eV Ar+ ion beam bombardment for various time periods. Additionally, many complementary techniques were used in the experimental work; X-ray Photoelectron Spectroscopy (XPS), Temperature Programmed Desorption (TPD), Scanning Electron Microscopy (SEM), Scanning Transmission Electron Microscopy (STEM) and Atomic Force Microscopy (AFM). The surface analysis was made by XPS for estimating the amount of studied materials as well as chemical states of those. TPD was used to study catalytic properties of studied materials for investigation of the behaviour of the nanoparticles towards gases (in our case CO) as a function of temperature. SEM, STEM and AFM were applied for surface features imaging. Depending on the task correlation between these tools has been made.

The evidence gained points out that depending on the time-regime and geometry of eroded materials IBS can be used as an innovative technique. It can reduce dimensions down to 2 nm in width and distribute nanostructures. It also can make quantum nanodots with size of about 1 nm out of self-assembled nanostructures proving to be a tool for application and enhancement of the levels of integration of nanocomponents. Theoretical analysis that is correlated to this experimental evidence was developed as well. It is based on controlling the effect of smoothing over roughening and describes a new model with correlations from previous studies. New terminology such as nanotransient kinetics has been introduced to describe the IBS process. From theoretical and experimental analysis follow that the main smoothing mechanism during IBS at normal temperatures is the ion-induced viscous laminar flow in the surface layer rather than the surface diffusion although this is also enhanced in the process. It was verified that a time associated with these mechanisms is important for the smoothing effect during IBS and it was recognized that low energy is not the only reason for smoothing domination over roughening. New terminology such as laminar, turbulent flow and associated momentum and concentration boundary layers were introduced, and applied to the nanoscale dimension. The option of controlling of smoothing over roughening was discussed and correlated to the experimental data determining the effects during IBS. In addition, to explain the distribution of nanostructures under IBS a new concept such as ioninduced island diffusion has been introduced. After applying IBS towards self-assembled Pt nanostructures it was found that ion-induced smaller Pt nanostructures result in lower desorption temperatures of carbon monoxide CO. This is valuable information for catalytic properties of Pt nanostructures.

The evolution of self-assembled Pt nanostructures that are formed by evaporation on polycrystalline graphite was studied as well. A new concept such as “arm diffusion” was introduced to explain the growth characteristics of the nanostructures. A new model that is based on nanotransient kinetics and describes self-assembly and post-growth relaxation of deposited materials was also introduced. According to the model, a convective mass transport is the main mechanism for an initial instability of the growth process that will eventually lead to complex pattern shapes. Another finding of the Thesis is a new model that reconstructs colourful 3D surfaces of the nanostructures from corresponding SEM images by using MATLAB software for better understanding of material and surface interactions.

I believe that the result of this PhD study has impact on further technological advances.