Nanostructured surfaces

Controlled self-ordering in the dynamics of forming periodic quasi-one-dimensional 
Bereznykov O.V., Gorshkov V.N.
1 National Technical University of Ukraine, Igor Sikorsky Kyiv Polytechnic Institute, 37 Prospect Peremogy, Kiev 03056, Ukraine. 
E-mail: alexber36@gmail.com

The spontaneous formation of transversely modulated 1D- structures occurs either during the disintegration of nanowires into separate fragments or during the diffusion deposition of atoms onto their surface. In both versions, the main mechanism providing a fairly strict periodicity in the arrangement of the forming nanoclusters is surface diffusion of atoms. At a given temperature, T, the dynamics of the process corresponds to a decrease in the free energy, F, of the system: dF=dU-TdS<0. In our work, physical mechanisms are shown that can be controlled both in cases where the decrease in free energy is associated with a decrease in internal energy, U, and cases when this is achieved due to the dominant increase in entropy, S, despite the accompanying increase in U. The transition between these two regimes can be carried out by intensifying the surface diffusion of atoms by external irradiation of the nanowire (by electrons or ions but avoiding noticeable heating ), which leads to the forced formation of periodic perturbations on the wire surface as a result of the well-known roughening transition (RT) effect.
 The results obtained by us on the basis of the kinetic Monte Carlo model show that the indicated diversity in the dynamics of the system corresponds to the same diversity in their morphology at the final stage of evolution. We have demonstrated, for example, that for a given orientation of a nanowire relative to its internal crystal structure (variants of FCC, BCC and diamond-like crystal structures are considered), the period of its breakup can be significantly changed both upward and downward, up to the formation of super-short-wavelength "frozen in time" surface perturbations (unduloids), the occurrence of which corresponds to overcoming the threshold of the so-called energy instability of the surface. The variety of morphologies in the synthesis of one-dimensional systems and methods for controlling their parameters are demonstrated in the case of diffusion deposition of germanium or silicon atoms onto a silicon/germanium nanowire.