Vacancy-induced surface diffusion

Qualitative observations of vacancy-induced surface diffusion... 

THEORY OF TWO-DIMENSIONAL VACANCY-INDUCED TRACER DIFFUSION 3.1. Tracer diffusion on an infinite surface... 

The first qualitative observation of vacancy-induced motion of embedded atoms was published in 1997 by Flores et al. [20], Using STM, an unusual, low mobility of embedded Mn atoms in Cu(0 0 1) was observed. Flores et al. argued that this could only be consistent with a vacancy-mediated diffusion mechanism. Upper and lower limits for the jump rate were established in the low-coverage limit and reasonable agreement was obtained between the experimentally observed diffusion coefficient and a theoretical estimate based on vacancy-mediated diffusion. That same year it was proposed that the diffusion of vacancies is the dominant mechanism in the decay of adatom islands on Cu(00 1) [36], which was also backed up by ab initio calculations [37]. After that, studies were performed on the vacancy-mediated diffusion of embedded In atoms [21-23] and Pd atoms [24] in the same surface. The deployment of a high-speed variable temperature STM in the case of embedded In and an atom-tracker STM in the case of Pd, allowed for a detailed quantitative investigation of the vacancy-mediated diffusion process by examining in detail both the jump frequency as well as the displacement statistics. Experimental details of both setups have been published elsewhere [34,35]. A review of the quantitative results from these studies is presented in the next subsections. 

The effects of damage by ion implantation on the low-temperature diffusion of dopant can also be studied by implanting Si+ or Ge+ ions into predeposited layers in Si. Recently, Servidori et al. (58) studied the influence of lattice defects induced by Si+ implantation. Using triple crystal X-ray diffraction and TEM, they confirmed (1) that below the original amorphous surface-crystal interface, interstitial dislocation loops and interstitial clusters exist and (2) that epitaxial regrowth leaves a vacancy-rich region in the surface. 

Atomistically, both mechanisms entail the diffusion of ions from the grain boundary region toward the neck area, for which the driving force is the curvature-induced vacancy concentration. Because there are more vacancies in the neck area than in the region between the grains, a vacancy flux develops away from the pore surface into the grain boundary area, where the vacancies are eventually annihilated. Needless to say, an equal atomic flux will diffuse in the opposite direction, filling the pores. 

The imique resolving power of STM can provide important new information on the atomic-scale realm and on the dynamics of nanostructures. For example, the mobility of defects such as oxygen vacancies on Ti02 surfaces (which become mobile after O2 exposure) can be explored. For the diffusion of O2 molecules on rutile Ti02 (110) surfaces (which plays an important role in understanding (photo)catalytic activity), a charge-transfer-induced diffusion mechanism for the adsorbed O2 molecules was observed. 

There are many other defect-inducing processes that we may consider. We may have a system that forms a vacancy from diffusion of a zinc atom to the surface, followed by evaporation into the vapor. This process is described by the reaction ... 

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