Date of Award
Program or Major
Doctor of Philosophy
Recent interest in novel physical properties of reduced dimensional systems is spurred by the advance of investigation methods at the nanoscale. Understanding bottom-up techniques for the growth of nanomaterials with novel physical, chemical, and mechanical properties require specialized investigation tools.
I am presenting a novel design and performance of an ultra high vacuum scanning tunneling microscope (STM) that allows for large scale (8 mum x 8 mum), fast scanning (3 s for a 100 A x 100 A frame), and atomically resolved studies of reduced dimensional systems on metallic surfaces. The STM proved excellent performance, allowing for variable temperature (100 K to 700 K) and high resolution (< 2 pm at 300 K) structural and dynamical studies on surfaces, as shown by the STM study of the general types of self-assembly processes on strained metallic interfaces.
With this new instrument, I performed a complete experimental investigation of the misfit dislocation network of one atom thick Ag films on Ru(0001) and of the restructuring induced by molecular sulfur adsorption when S filled Ag vacancy island are formed. The experiments suggest that the mechanism through which hundreds of Ag atoms are rear-ranging themselves following S deposition is driven by a process of threading dislocation pair annihilation and glide. The experimental observations are explained via an atomistic model, which is based on first-principles interaction parameters. I have found that the self-assembly process is driven by stress relaxation in the Ag film.
While the first part of my thesis is focused on the structural properties of low-dimensional metallic systems and instrumentation methods needed to access the nano-scale, in the secand part I investigated a novel low-energy electronic excitation of a metallic surface. I am presenting the first experimental measurement of an acoustic surface plasmon on metal surfaces. The experiment was performed using electron energy loss spectroscopy on Be(0001). This new mode is a collective excitation of the surface electrons. This discovery goes against the traditional wisdom that on metal surfaces only regular (optical) surface plasmons can exist. First-principles calculations show that this mode is caused by the coexistence of a partially occupied surface state band with the underlying bulk electrons.
Diaconescu, Bogdan, "Self -assembly on strained metallic interfaces, and, Novel collective excitations on metal surfaces" (2007). Doctoral Dissertations. 369.