Formation of ordered nanodroplet chains on a solid surface by enhanced surface diffusion and shadow effect

We report on a mechanism that under an off-normal incident ion beam, ordered nanodroplets may emerge spontaneously from a flat solid surface and line-up into chains perpendicular to the beam direction, forming a highly ordered hexagonal pattern. This behavior is in contrast to the general belief that an off-normal bombardment produces surface ripples, while the formation of dots requires either normal bombardment or off-normal bombardment with simultaneous sample rotation. The self-assembled nanodroplets reveal a mechanism different from the usual sputtering process. We propose a continuum theory which shows that the balance of mass flux plays an important role for the hexagonal pattern formation while the shadow effect causes the droplets to align into chains. The simulations suggest that the fundamental mechanism may be applicable to other systems, which may lead to an effective approach for nanofabrication.