Dopant-induced stabilization of silicon clusters at finite temperature

Abstract

With the recent advances in miniaturization, understanding and controlling the properties of technologically significant materials such as silicon in the nano regime assumes considerable importance. The silicon clusters in the size range of 15-20 atoms are known to be unstable upon heating. For example, Si₂₀ does not melt but fragments around 1250 K, whereas Si₁₅ has a liquidlike phase spread over a short temperature range and undergoes fragmentation at approximately 1800 K. In this paper, we demonstrate that it is possible to suppress such a fragmentation process by introducing the appropriate dopant (in this case Ti). Specifically, by using the first-principles density functional simulations we show that Ti-doped Si₁₆, having the Frank-Kasper polyhedral, remains stable till at least 2200 K and fragments only above 2600 K. Our calculations also indicate that the observed melting transition is a two-step process. The first step is initiated by the surface melting at approximately 600 K. In the second step, the destruction of the cage takes place at approximately 2250 K, giving rise to a peak in the heat capacity curve.