Surface relaxations in quantum-confined Pb

The relationship between electronic and atomic structure of quantum confined objects is a key issue in nanoscience. We performed low-energy electron diffraction to determine the surface relaxations of ultrathin Pb films on Si(111)7x7 with thicknesses ranging from four to nine atomic layers. The results indicate a contraction of the first interlayer spacing d(12) for all films. The d(12) contraction exhibits a small bilayer modulation as a function of thickness, indicative of a quantum size effect. The oscillatory relaxations furthermore suggest an interesting correlation with the work function oscillations predicted from density functional theory [Wei and Chou, Phys. Rev. B 66, 233408 (2002)] and can be understood qualitatively on the basis of the jellium model.