SURFACE PHYSICS OF NICKEL NANOPARTICLES ON GOLD
Abstract: The study of nanoparticles involves structures and processes that occur on the nanometer length scale, which often display unusual behavior compared to their macroscale counterparts. In this thesis, we study Ni nanoparticles grown on Au(111). The Au(111) surface forms the herringbone reconstruction that provides a template for the periodic growth of ordered Ni nanoparticles. The Ni ... read morenanoparticles grow radially until a fractional surface coverage of about 0.3 monolayers (ML), after which, subsequent Ni atoms contribute primarily to a second layer. In this thesis we report on the surface physics of these Ni nanoparticles grown on Au(111) substrates with several different experimental techniques. Surface resistivity, measurements can detect the presence of small numbers of adsorbates, and grant insight into their morphology and interactions. It arises from the diffuse scattering of conduction electrons off adsorbates, and there is typically a linear relation between the increase in resistivity of the substrate and the number of adsorbates in direct contact with the substrate. By growing in ordered clusters, the Ni nanoparticles break some assumptions of adsorbate-induced surface resistivity models, thus a non-linear dependence between surface resistivity and the size of the Ni nanoparticles was anticipated. Our results, however, show a linear dependence during first layer growth of the Ni nanoparticles. During second layer growth, above a fractional Ni coverage of about 0.3 ML, additional Ni atoms make no significant contribution to resistivity. Studies of CO adsorption to different surfaces can reveal otherwise hidden aspects of the surface. We investigate CO adsorption to Ni nanoparticles of different sizes grown on the Au(111) surface at 227 K with temperature programmed desorption (TPD) and Fourier transform infrared spectroscopy (FTIR). At this temperature, it is known through the literature and our measurements that CO does not bind to the Au surface. We find layer-dependent adsorption properties for CO binding to the Ni nanoparticles during first and second layer growth. For first layer Ni nanoparticle growth, we find normal CO saturation coverages of about 0.5 ML, but lower peak desorption temperatures than CO on pure Ni, and find CO primarily in the atop position. During second layer Ni growth, we find anomalously high CO saturation coverages near 1 CO/Ni, slightly higher peak desorption temperatures, but still primarily atop CO. Based on previous studies, we propose that in the first Ni layer, ligand effects from the Au substrate and possibly Au in the islands and strain due to the Ni/Au lattice mismatch affect the Ni-CO bonds. CO adsorption behavior on the two-layer islands is qualitatively explained by a decrease in Au nearest neighbors and the presence of a more expanded/corrugated structure.
Thesis (Ph.D.)--Tufts University, 2018.
Submitted to the Dept. of Physics.
Advisor: Roger Tobin.
Committee: Peggy Cebe, Hugh Gallagher, and Carl Ventrice.
Keywords: Condensed matter physics, Nanoscience, and Physics.read less