Quantum Size Effect on Adatom Surface Diffusion

Using scanning tunneling microscopy, we demonstrate that the nucleation density of Fe islands on the surface of nanoscale Pb films oscillates with the film thickness, providing a direct manifestation of the quantum size effect on surface diffusion. The Fe adatom diffusion barriers were derived to be $204\pm 5$ and $187\pm 5\mathrm{meV}$ on a 21 and 26 monolayer (ML) Pb film, respectively, by matching the kinetic Monte Carlo simulations to the experimental island densities. The effect is further illustrated by the growth of Fe islands on wedged Pb films, where the Fe island density is consistently higher on the odd-layer films than on the even-layer films in the thickness range of 11 to 15 ML.

Figure 1

STM images of Fe islands grown at LT on (a) a 21 ML and (b) a 26 ML Pb film, respectively, up to the same coverage of 0.24 ML. Both images are $200\times 200{\mathrm{nm}}^2$ taken with a tip bias of $-2.67\mathrm{V}$ and tunneling current of 20 pA. (c) Counted Fe island density on a 21 ML (solid circles) and a 26 ML (open circles) Pb film.

Figure 2

Island nucleation density ($N$) as a function of surface diffusion barrier ($E_d$), obtained from kMC simulations. The solid line is the linear fit to the 3D island data (solid dots). The two dotted horizontal lines mark the experimental densities, from which the surface diffusion barriers were derived as indicated on the $x$ axis. For comparison, the dashed line is the fit to the 2D island data, which were not shown for better clarity.

Figure 3

A $1200\times 800{\mathrm{nm}}^2$ STM image (acquired at a tip bias of $-5.0\mathrm{V}$ and tunneling current of 20 pA) displays a density oscillation of Fe islands nucleated on a wedged Pb film at RT. The island density on the odd-layer films is consistently higher than that on the even-layer films on the given film thickness regime from 10 to 16 ML.