Self assembled nano-structures of Pb on Si(111) studied by SPA-LEED: Quantum Size Effect driven Pb islands and the "Devil's Staircase"
An important goal in present day surface science is to grow uniform sized self-assembled nanostructures. One system which has displayed a number of interesting surface structures is Pb grown on a Si(111) substrate. The first part of the thesis discusses the "Devil's Staircase" (DS) of linear phases studied with Scanning Tunneling Microscopy (STM) and Spot-Profile Analysis Low Energy Electron Diffraction (SPA-LEED). The DS is one of the outstanding predictions in theoretical physics and is expected to exist in systems with two competing interactions. It was found that at low temperatures (<120>K) over 15 different DS phases can be observed by small Pb depositions (<0.01 ML). In addition, a DS phase diagram was mapped out with the transition temperatures of the DS phases as well as a family of hexagonal phases and a meandering phase. The second part of the thesis explores multiple structural features of uniform height Pb nanocrystals on Pb and In induced reconstructions of Si(111) using SPA-LEED and STM. Pb islands grown on these substrates grow with preferred heights do to the Quantum Size Effect (QSE). Pb islands of 2 and 4 layer height were grown on the Pb-alpha(√3x√3) phase.) phase. Using the observed corrugation pattern on the island tops, the rotation of the Pb crystals with respect to the silicon substrates was deduced. Also, the two types of corrugation patterns (arising from the two equivalent fcc stacking sequences of Pb(111) crystals) were used to determine the population of each stacking sequence with changing Pb coverage. At coverage 1 ML, the islands were preferentially aligned in the opposite stacking sequence as the Si substrate, while at higher coverages the islands were preferentially aligned with the Si substrate. Finally, Pb islands were grown on the anisotropic Si(111)-In(4x1) substrate. In addition to a preferred height of 4 ML, these islands grow as nanowires with a preferred width of 66 A due to strain driven growth from the anisotropic substrate. Islands grown on the In(4x1) substrate also retain their preferred height to room temperature in contrast to previously observed critical temperatures of 250 K or less for islands grown on other substrates.