Enhanced Atom Mobility on the Surface of a Metastable Film

A remarkable enhancement of atomic diffusion is highlighted by scanning tunneling microscopy performed on ultrathin metastable body-centered tetragonal Co films grown on Fe(001). The films follow a nearly perfect layer-by-layer growth mode with a saturation island density strongly dependent on the layer on which the nucleation occurs, indicating a lowering of the diffusion barrier. Density functional theory calculations reveal that this phenomenon is driven by the increasing capability of the film to accommodate large deformations as the thickness approaches the limit at which a structural transition occurs. These results disclose the possibility of tuning surface diffusion dynamics and controlling cluster nucleation and self-organization.

Figure 1

(a) Co islands on Fe(001) for a nominal Co thickness equal to 0.25 ML. (b) Morphology of the second Co layer (thickness: 1.28 ML) on Fe(001). The insets show that the islands are 1 ML thick. (c) Percentage of layer completion as a function of the total coverage $\theta$, as measured from grain analysis of the STM images. Each symbol and color is associated with a different layer. All samples have been grown at room temperature. STM measurements have been performed in constant current mode ($I=1\mathrm{nA}$, $V=1\mathrm{V}$).

Figure 2

STM images collected at coverages for which the Co island saturation density is reached on the (a) first, (b) second, (c) third, and (d) fourth Co layers. The scale [indicated by the white bar in (c)] is the same for all panels. (e) Co island density as a function of the Co coverage $\theta$. The local maxima (hollow red dots) correspond to the saturation island density $n_s$ for a given layer. The previous results have been collected on Co films grown at room temperature. (f) Arrhenius plot of saturation island densities (expressed in units of islands per unit lattice cell) derived from STM measurements for islands belonging to the first, second, third, fourth, and fifth layers. The vertical error bars associated with a $\pm 5\%$ uncertainty in the determination of $n_s$ are of the same size of the dots. The lines are linear fits to the data points.

Figure 3

Structure of a 3 ML Co film on Fe(001). (a) Top view of the ideal film, with high-symmetry positions indicated: hollow ($H$), bridge ($B$), and top ($T$). (b) Optimized geometry for an adatom (1) occupying the bridge site defined by atoms 2 and 3. Atoms 4 and 5 are the nearest second-layer ones to the adatom.