The second part of the talk will focus on atomic-scale engineering of semiconductor surfaces. Dangling bonds and surface states are inherent features of a semiconductor surface and have been detrimental to semiconductor devices since the Bardeen era. Techniques are being developed in our laboratory to terminate dangling bonds and remove surface states on semiconductor surfaces, following the concept of *valence mending*. When a monolayer of selenium is deposited on Si(100), the surface shows significantly-reduced surface states and the surface chemical reactivity is significantly suppressed. Important applications of this valence-mending technique include Si-based (not SiC) high-temperature electronics with operation temperature of 300*C, low-temperature and doping-free fabrication of semiconductor devices, and reduced contact resistance and reduced surface recombination in poly and single-crystalline Si solar cells for efficiency improvement.