Abstract
We analyze the fundamental process of crystallization of silicon nanoclusters by means of molecular dynamics simulations, complemented by magnetron-sputter inert gas condensation, which was used to synthesize polycrystalline silicon nanoclusters with good size control. We utilize two well-established Si interatomic potentials: the Stillinger-Weber and the Tersoff III. Both the simulations and experiments show that upon cooling down by an Ar gas thermal bath, initially liquid, free-standing Si nanocluster can grow multiple crystal nuclei, which drive their transition into polycrystalline solid nanoclusters. The simulations allow detailed analysis of the mechanism, and show that the crystallization temperature is size-dependent and that the probability of crystalline phase nucleation depends on the highest temperature the cluster reaches during the initial condensation and the cooling rate after it.
Highlights
One of the key challenges in nanotechnology today is to control the size and crystallinity of silicon nanoclusters (Si NCs) with a high degree of accuracy, as these parameters have an important impact on uses in specific biomedical and optoelectronic applications [1,2,3]
We omitted the condensation process of Si NC for two reasons: (i) the previous studies [23,29,30,31,32,33,34,35] as well as our analytical estimation indicate that the NCs condensed in the plasma region, where the temperature is a few thousands degrees of Kelvin, are initially in the liquid phase; (ii) it is extremely slow to simulate sufficiently large NCs from the gas phase
The magnetron-sputter inert gas-condensation method is capable of producing Si NCs with good control over NC size and crystallinity
Summary
One of the key challenges in nanotechnology today is to control the size and crystallinity of silicon nanoclusters (Si NCs) with a high degree of accuracy, as these parameters have an important impact on uses in specific biomedical and optoelectronic applications [1,2,3]. The use of the Nose-Hoover thermostat [20,21] or its extension, the Anderson thermostat [22], can cause significant artifacts in the thermodynamic process of an NC system since these methods do not distinguish between surface and bulk atoms Such artificial scaling of velocities in the nanocluster can be avoided by applying the inert gas temperature control [23], which allows mimicking of the experimental conditions in a nanoparticle deposition chamber [24]. The condensation process of Si NCs can be describe properly, if the potential: (i) describes all three phases fairly well, (ii) yields the phase transition temperatures close to the experimental values, and (iii) reproduces the bond angle energy and bond angle distribution for solid Si correctly The latter is important for crystallization transition stage. To validate the simulation results and their interpretation, we compare them to the experimental results of polycrystalline Si NCs grown by magnetron sputter inert gas condensation method
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