Pb Adatom Diffusion on Pb(111) film

1. Quantum size effect on diffusion and growth morphology of Pb/Si(111)T. L. Chan, C. Z. Wang, M. Hupalo, M.Tringides, K. M. HoAmes Laboratory – USDOE & Department of Physics, Iowa State University, Ames, Iowa 50011 Quantum size effects (QSE) on the formation and stability of metallic nanostructures on semiconductor surfaces have attracted considerable attention recently. It has been shown that the intriguing and unexpected feature, i.e., formation of metal plateaus or islands of selective heights with flat tops and steep edges on the semiconductor surfaces, observed in the low temperature epitaxial growth of Ag films on GaAs, Ag islands on Si(111), and particularly Pb islands on Si(111) can be attributed to QSE due to electron confinement in the direction perpendicular to the film surface. Very recently, QSE is also shown to manifest itself in the superconductivity of ultra-thin Pb films. Nevertheless, the kinetics of such intriguing "quantum" growth, even the very basic question of how the islands grow so fast in the low temperature, is still much less understood. Using first-principles total energy calculations, we have studied the diffusion barriers of a Pb adatom on a free-standing Pb film as a function of film thickness [1]. We found that the hcp site is the lowest energy location for a Pb adatom on a Pb film, while the fcc site is the saddle point of the diffusion pathway. Diffusion of an adatom on the Pb film has very low barriers (less than 60 meV). A bi-layer oscillation in the diffusion barriers due to quantum size effect(QSE) is observed, with lower barrier on the odd-layered, relatively unstable Pb film. The diffusion barrier difference between the odd- and even-layered film is found to be as large as 40 meV. Such big difference in the diffusion barriers due to QSE can be used to explain the unusual growth morphology of Pb islands on Si(111) surface observed in the STM experiment where growth of a Pb layer on Pb islands with unstable heights starts from the periphery and moves towards the center, while the nucleation of the next layer on stable Pb islands starts near the center of the island. [1] T. L. Chan, C. Z. Wang, M. Hupalo, M. C. Tringides, and K. M. Ho, Phys. Rev. Lett. 96, 226102, (2006).

2. Pb Adatom Diffusion on Pb(111) film An intriguing growth morphology of Pb islands on Si(111):growth on Pb islands with unstable heights starts from the periphery and moves towards the center, while the nucleation of the next layer on stable Pb islands starts away from the periphery. Quantum size effect on the diffusion barriers and growthmorphology of Pb/Si(111)T.-L. Chan, C. Z. Wang, M. Hupalo, M. C. Tringides, K. M. Ho, PRL 96, 226102 (2006) Small diffusion barriers: 0.01 – 0.06 eV Higher diffusion barrier for stable height Lower diffusion barrier for unstable height Nucleate at the center due to high diffusion barrier and low step edge barrier Nucleate at the edge due to low diffusion barrier and high step edge barrier 6: stable 5: unstable Coverage=2.9 ML, T ~ 180K 5: unstable 6: stable 7: unstable Potential Energy Surface for the Diffusion & Nucleation