Foreign substitutional atoms attached on the surface act as trapping centers for adatoms and/or as obstacles preventing growth of an adsorbate, and affect behaviors of steps and surface morphology. It had been known that a step on crystal surface cannot advance when the period between impurities is shorter than the critical nucleus size[1]. Recently, spiral steps on monoclinic lysozyme crystal is observed and it is found that the interstep distance is about 50 times larger than the expected value[2]. This result suggests that the critical nucleus size in the experiment is much larger than that in pure system because of the impurity in the sample.
We have studied an impurity effect for the nucleus size from the thermodynamic point of view. When the free energy is taken account of the including impurities, the critical nucleus radius is large with increasing the concentration of impurities. The critical nucleus radius diverges when the concentration of the impurity is (L_impr_c  )−1 where L_imp is the periphery length of the impurity and r_c is the critical nucleus radius in the pure system. In order to confirm above result, we have also performed the Monte Carlo simulation of a simple square lattice model. The simulation result is in good agreement with that of the thermodynamical analysis[3]. The nucleus, which forms eventually on surface, cannot grow. This interpretation is different from the ordinary impurity effect[1].
References:
[1] N. Cabrera and D. A. Vermilyea, Growth and Perfection of Crystals, edited by R. H. Doremus, B. W. Roberts, and D. Turnbull, (Wiley, New York, 1958), p.393.
[2] G. Dai et al., J. Cryst. Growth, 311, 548 (2009).
[3] H. Katsuno, K. Katsuno, and M. Sato, Phys. Rev. E, 84, 021605 (2011).

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