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Bulk-Surface Vacancy Exchange and Thermal Smoothing of Pt(111)
by Bene Poelsema, J. B. Hannon, N. C. Bartelt, and G. L. Kellogg







Motivation—Understanding the fundamental mechanisms by which a rough surface becomes smooth is a challenging problem with technological relevance to the growth and stability of nanometer-scale structures on surfaces. Depending on the particular material and temperature, rough surfaces can become smooth by mass-transport across the surface (surface diffusion), through the bulk (bulk diffusion) or through exchange with the gas phase (evaporation-condensation). Over the past decade, most experimental and theoretical research has focussed on smoothing by surface diffusion processes. In this case, adatoms or vacancies move between steps in regions of higher curvature to regions of lower curvature. This flow of material, in which small, two-dimensional islands decay while large islands grow, causes 3-D bumps on the surface to smooth out and 3-D valleys to fill in. By employing real-space imaging techniques such as scanning tunneling microscopy (STM) and low energy electron microscopy (LEEM) to follow the motion of steps, researchers have learned a great deal about the mechanisms and energetics of thermal smoothing due to mass transport across surfaces. More recently, McCarty et al. [Nature 412, 622 (2001)] have discovered that, at higher temperatures, bulk vacancy diffusion plays a dominant role in the smoothing of NiAl(110) surfaces.  This observation raises the question as to whether or not bulk-surface vacancy exchange takes place on the surfaces of other materials (e.g., pure metals) and whether bulk processes contribute to their thermal smoothing. 

Accomplishment—From LEEM measurements of 2-D island growth and decay, we have discovered that bulk-surface vacancy exchange contributes significantly to thermally induced morphology changes on the Pt(111) surface. We find that, like NiAl(110) [McCarty et al., Nature 412, 622 (2001)], two-dimensional islands on Pt(111) grow when the temperature is increased and shrink when the temperature is decreased indicating that the surface is a significant source and sink of bulk vacancies. The LEEM images shown in Fig. 1 below illustrate the effect.  When the sample is quickly heated from a base temperature of 1237 to a final temperature of 1256 K, the 2-D island at the center of the image increases in size (Fig. 1(a-c)). This change in area is reversible:  When the temperature is decreased by a similar amount, the island decreases in area (Fig. 1(d-f)).  Fig. 2 shows the area vs time curves for a series of base temperatures varying from 1054 to 1267 K. From the temperature dependence of the material transported to the surface (i.e., the saturation values of the plots), we determine a bulk vacancy formation energy of 1.54 ± 0.07 eV. This value is consistent with previous measurements and calculations.  LEEM measurements of island decay at constant temperature exhibits a time dependence and activation energy that suggest a smoothing mechanism controlled by the diffusion of both surface adatoms and bulk vacancies. This mechanism is qualitatively different than that found for NiAl(110) where bulk vacancies dominated the smoothing process.  A specific prediction of our model describing island decay on Pt(111) is that as the temperature becomes lower, there should be a crossover from bulk-vacancy- to surface-atom-limited decay.  This crossover was not observed because decay is too slow at the lower temperatures.  In future experiments we will explore other materials in an attempt to observe the crossover. 

Significance—Many important properties of surfaces depend on the details of surface morphology.  Thus, there have been many attempts to control (nanoscale) surface morphology to enhance useful surface properties.  However, these engineered structures can be thermally unstable to surface smoothing. The mechanisms of the smoothing of rough surfaces have been the subject of active research for at least fifty years.  A principal goal of this research has been to relate macroscopic smoothing rates to atomic properties.  Most recent work has emphasized the role of atomic surface diffusion in surfaces smoothing. Our results show that at high temperature there is a mechanism for surface smoothing that has been neglected in the literature:  bulk vacancy-surface exchange. Given the simplicity of the Pt system (and the observations of similar behavior in NiAl), it is unlikely that Pt is anomalous.  We expect that the same mechanism will occur in other systems.

 

Fig. 1  LEEM images showing the growth and decay of two-dimensional Pt islands on Pt(111) (field of view = 2.7mm). The dark circle is the perimeter of a 2-D island at the top of an island stack. (a-c) Area increases after the temperature is suddenly increased from 1237 to 1256 K. (d-f) Area decreases after the temperature is suddenly lowered from 1237 to 1219 K.
Fig. 2 Normalized area changes vs. time for temperature increases of ~20 K for base temperatures of 1267, 1237, 1208, 1171, 1139, 1059 and 1054 K (larger temperatures have larger area changes).  The vacancy formation energy obtained from the temperature dependence of the saturation values is 1.54 ± 0.07 eV
Publication: 
Bene Poelsema, J. B. Hannon, N. C. Bartelt, and G. L. Kellogg. Phys Rev. Lett. (submitted for publication)
Work supported by the U. S. Department of Energy, Office of Basic Energy Sciences, Division of Materials Science and Engineering
Last modified December 29, 2003
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