Magic heights in Pb nanodots induced by Quantum Size Effects

The search for methods to produce structures highly organized at the atomic scale during epitaxy is an active research area since it holds the promise for basic discoveries and novel technological applications. Along with the well known kinetic and thermodynamic parameters, it was recently found that Quantum Size Effects can play a crucial role during epitaxial growth [1-3]. For instance, metallic islands of uniform height with steep edges and flat tops, can be grown on both semiconductor [2] and metal surfaces [3,4]. In both cases, potencial barriers at the interfaces confine the electrons within the metal overlayers in the direction perpendicular to the layers. The energy associated to these Quantum Well States (QWS) might be large enough to stabilize preferentially certain shapes. We reported previously that the equilibrium height distribution of Pb nanoislands grown at elevated temperature on Cu(111), as obtained from STM images, showed "magic" heights, i.e. certain heights appeared much more frequently than others [3]. The QWS dictate the equilibrium shape of Pb nanocrystals by eliminating from the height distribution those nanodots whose heights would have a QWS at the Fermi level [3]. This effect is now used to stabilize at 300 K films of Pb with specific heights, which were deposited at low temperatures as flat films. The experiments have been performed in a UHV system equipped with LEED/AES and a variable temperature Scanning Tunneling Microscope (STM). The Cu(111) surface was cleaned by standard methods including ion sputtering and annealing. After cleaning, the simple displayed a sharp LEED pattern and atomically resolved high-quality STM images. Pb was evaporated from a Knudsen cell with the sample placed in the STM stage. The flux of Pb atoms, i.e. the deposition rate, was kept constant and the temperature of the Cu(111) substrate during and after the deposition was varied from 70 K up to 300 K. This in-situ experiment allows the direct observation of structural transformations on an atomic scale, as well as control and elucidation of the dynamic behavior as a function of sample temperature. Pb grows on Cu(111) at 300K in the Stranski-Krastanov mode of growth. After the completion of a single compact monolayer, 3D (111)-oriented Pb islands grow on the surface. Fig 1 shows a representative image of a Pb film deposited at 300 K. The surface is covered with 3D islands, most of them of similar height. Only the islands nucleated at step bunches are significantly …