Mobility of tungsten clusters on tungsten surfaces

We report on an accelerated molecular dynamics investigation of surface transport mechanisms on W surfaces. These mechanisms likely influence the formation of nano-fuzz in fusion plasma-facing conditions. The simulations use an embedded-atom potential to investigate the diffusion mechanisms of W clusters (Wn, n = 2–9) on W(1 1 0) and W(1 0 0) surfaces at 1000 K, reaching timescales beyond 10 microseconds. A number of fast moving adatom species, Wn (n = 2–6), were observed. In contrast, slightly larger clusters Wn (n = 7–9), especially W8, exhibit significantly lower diffusivities. The diffusion mechanisms of Wn clusters on the W(1 1 0) surface are complex, with the most common elementary steps involving hops of monomers and dimers. Moreover, we observe tetramer hopping in the diffusion processes of Wn (n = 4–7), trimer hopping in Wn (n = 5, 6, 8), and pentamer hopping in W5 on the W(1 1 0) surface. Wn (n ≤ 3) and metastable Wn (n > 3) clusters on the W(1 0 0) surface diffuse predominantly through monomer exchange with surface atoms. However, W clusters containing square tetramers are essentially immobile on the W(1 0 0) surface at 1000 K.