Scanning Tunneling Microscopy Studies of Metal on Metal Epitaxy

Island Size and Environment Dependence of Adatom Capture: Cu/Co Islands on Ru(0001)

The rate of capture by stable Co islands on Ru(0001) of additionally deposited Cu atoms is quantified using scanning tunneling microscopy, kinetic Monte Carlo simulations, and diffusion equation analyses. We find strong dependence of the capture rates on Co-island size, larger islands showing larger capture rates, qualitatively distinct from self-consistent mean-field predictions. The observed size dependence is shown to reflect larger island-free areas surrounding bigger islands, i.e., a strong correlation between island sizes and separations neglected in mean-field treatments.

Entropically stabilized growth of a two-dimensional random tiling

The assembly of molecular networks into structures such as random tilings and glasses has recently been demonstrated for a number of two-dimensional systems. These structures are dynamically arrested on experimental time scales, so the critical regime in their formation is that of initial growth. Here, we identify a transition from energetic to entropic stabilization in the nucleation and growth of a molecular rhombus tiling. Calculations based on a lattice-gas model show that clustering of topological defects and the formation of faceted boundaries followed by a slow relaxation to equilibrium occur under conditions of energetic stabilization. We also identify an entropically stabilized regime in which the system grows directly into an equilibrium configuration without the need for further relaxation. Our results provide a methodology for identifying equilibrium and nonequilibrium randomness in the growth of molecular tilings, and we demonstrate that equilibrium spatial statistics are compatible with exponentially slow dynamical behavior.

Nanoscale "Quantum" Islands on Metal Substrates: Microscopy Studies and Electronic Structure Analyses

Confinement of electrons can occur in metal islands or in continuous films grown heteroepitaxially upon a substrate of a different metal or on a metallic alloy. Associated quantum size effects (QSE) can produce a significant height-dependence of the surface free energy for nanoscale thicknesses of up to 10–20 layers. This may suffice to induce height selection during film growth. Scanning STM analysis has revealed remarkable flat-topped or mesa-like island and film morphologies in various systems. We discuss in detail observations of QSE and associated film growth behavior for Pb/Cu(111), Ag/Fe(100), and Cu/fcc-Fe/Cu(100) [A/B or A/B/A], and for Ag/NiAl(110) with brief comments offered for Fe/Cu₃Au(001) [A/BC binary alloys]. We also describe these issues for Ag/5-fold i-Al-Pd-Mn and Bi/5-fold i-Al-Cu-Fe [A/BCD ternary icosohedral quasicrystals]. Electronic structure theory analysis, either at the level of simple free electron gas models or more sophisticated Density Functional Theory calculations, can provide insight into the QSE-mediated thermodynamic driving force underlying height selection.

The Importance of Threading Dislocations on the Motion of Domain Boundaries in Thin Films

Thin films often present domain structures whose detailed evolution is a subject of debate. We analyze the evolution of copper films, which contain both rotational and stacking domains, on ruthenium. Real-time observation by low-energy electron microscopy shows that the stacking domains evolve in a seemingly complex way. Not only do the stacking boundaries move in preferred directions, but their motion is extremely uneven and they become stuck when they reach rotational boundaries. We show that this behavior occurs because the stacking-boundary motion is impeded by threading dislocations. This study underscores how the coarse-scale evolution of thin films can be controlled by defects.

Direct observation of misfit dislocation glide on surfaces

Using scanning tunneling microscopy we have observed thermally induced dislocation glide in monolayer Cu films on Ru(0001) at room temperature. The motion is governed by a Peierls barrier that depends on the detailed structure of the dislocations, in particular upon whether the threading dislocations that terminate them are dissociated or not. Calculations based on the Frenkel-Kontorova model reproduce the threading dislocation structure and provide estimates of the Peierls barrier and dislocation stiffness which are consistent with experiment.

Facile Convergent Route to Molecular Caltrops

The convergent syntheses of molecular caltrops are described starting from tetraethyl orthosilicate and using organolithium additions and Pd/Cu-catalyzed coupling methods. The caltrop core is based on a tetrahedral silicon atom, and there are three legs each bearing sulfur-tipped feet for adhesion to metallic surfaces. The forth prong (arm) is non-sulfur-bearing for projection upward from the surface. Rigid phenyleneethynylene segments are used for the legs and arms. These organosilicon caltrops may have utility as scanning probe microscopy tips.