Adatom-induced diffusion

Miinger, E.P. Chirita, V. Greene, J.E. Sundgren J.-E. Adatom-induced diffusion of two-dimensional close-packed Pty clusters on Pt(l 11). Sutf. Sci., 1996 355 L325-330. 

The plasma potential is the maximum value with which ions can be accelerated from the edge of the sheath towards the substrate, located at the grounded electrode. This may cause ion bombardment, which may induce ion-surface interactions such as enhancement of adatom diffusion, displacement of surface atoms, trapping or sticking of incident ions, sputtering, and implantation see Section 1.6.2.1. 

Fig. 3.2 The first picture at the upper left-hand comer is a 78 K He field ion micrograph of a W (112) surface with a W adatom on it. From top to bottom and then from left to right the rest of the images are 290 K He field ion micrographs of the same surface where diffusion of the adatom can be clearly seen. When the adatom is near the center of the plane, it performs a random walk. However, when it is slightly off the center it is driven toward the plane edge by a field gradient induced driving force, as will be discussed in Chapter 4. These are real time photos each one is separated by about 5 s.

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. 

Adsorbed atoms and molecules can also diffuse across terraces from one adsorption site to another [33]. On a perfect terrace, adatom diffusion could be considered as a random walk between adsorption sites, with a diffusivity that depends on the barrier height between neighbouring sites and the surface temperature [29]. The diffusion of adsorbates has been studied with FIM [14], STM [M, 35] and laser-induced thermal desorption [36]. 

A quite interesting alternative for the formation of an adsorbate -induced surface reconstruction is offered by the O/Cu(l 10) surface. Upon adsorption of O atoms on the clean (nonrecon-structed) 1x1 surface, a (2 x l)-0-Cu(l 10) phase is formed whose structure is depicted in Fig. 2.23c. This new "missing row" structure is, however, formed by condensation of mobile chemisorbed O atoms with Cu adatoms evaporating from steps and diffusing across the terraces of the substrate surface. This nucleation process is illustrated by the STM snapshots in Fig. 2.23a and b leading to the structure of Fig. 2.23c that is more appropriately described as "added row" than "missing row" phase [27]. 

Photochemistry of adsorbed O2 molecules results presumably from transient capture of an excited metal electron by the 3cr state of O2. After decay to the ground state, the excited molecules may rapidly diffuse, desorb, or dissociate [72]. By monitoring the coUision-induced desorption of coadsorbed noble gas atoms, a value of 0.7 eV for the kinetic energy of the fastest "hot" O adatoms was determined [73]. 

The experimental methods used to measure surface diffusion are manifold, and we restrict ourselves to a description of a few of the most relevant ones. Amongst the microscopy techniques, the field emission microscope (FEM) has been employed to monitor diffusion indirectly by the adatom density fluctuations it induces... 

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