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 (W-n, 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, W-n (n = 2-6), were observed. In contrast, slightly larger clusters W-n (n = 7-9), especially W-8, exhibit significantly lower diffusivities. The diffusion mechanisms of W-n 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 W-n, (n = 4-7), trimer hopping in W-n (n = 5, 6, 8), and pentamer hopping in W-5 on the W(1 1 0) surface. W-n (n <= 3) and metastable W-n (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.