Date of Award
Doctor of Philosophy (PhD)
Hani E. Elsayed-Ali
The surface superheating and phase transitions at the low-index surface of metal were investigated using conventional continuous and 100-ps time-resolved reflection high-energy electron diffraction. Three metal surfaces, In(111), Au(110) and Pb(111), have been investigated in this work. The high temperature behavior of the In(111) surface was investigated using reflection high-energy electron diffraction with 100-ps temporal resolution. The change of surface vacancy density on In(111) is observed from 300 K to near the bulk melting point. The vacancy-formation energy of In(111) surface is estimated from experimental results. The surface vacancy density is observed to increase with temperature; however, the average random step terrace width remains unchanged from its value at room temperature. When the indium surface is heated at a slow rate, the In(111) surface remains ordered up to the highest temperature studied, 4 K below the bulk melting point. When the surface is laser heated at a heating rate of ∼1012 K/s, the In(111) surface remains ordered up to 73 ± 9 K above its bulk melting pointing. The surface Debye temperature of indium is also obtained from the measurement. The top layer relaxation (or contraction) of In(111) surface is studied.
The vicinal Au(110) surface morphology throughout the (1 × 2)-(1 × 1) reconstruction is studied by quantitative reflection high-energy electron diffraction. As the surface is heated from 300 K, the average terrace width remains unchanged up to 635 ± 10 K. Above that temperature, the average terrace width increases and at 683 K has a value (34 ± 10)% more than at 300 K. At higher temperatures, the average terrace width decreases. The average string length at step terraces on Au(110) remains unchanged up to ∼650 K and decreases sharply at higher temperatures indicative of a step-induced roughening transition. Thermal-induced adatom/vacancy generation on terraces is shown to increase significantly above ∼680 K. At room temperature, the spacing between the topmost two layers of the Au(110) surface is contracted by 0.31 ± 0.03 Å or ∼22% of the bulk-terminated structure.
A laser-driven Photoemission electron microscope with 5 μm spatial resolution was developed and used to study surface morphology of laser heated Pb single crystal. The basic idea of this microscope is the use of μ50 ps ultraviolet laser pulse to photoemit electrons from the surface, while a well synchronized infrared laser pulse heats the surface and induces surface morphology changes
"Dynamics of Phase Transitions on Low-Index Metal Surfaces"
(2003). Doctor of Philosophy (PhD), dissertation, Electrical/Computer Engineering, Old Dominion University, DOI: 10.25777/c7t9-nc72