Thermal stability and melting mechanism of diamond nanothreads: Insight from molecular dynamics simulation

Abstract

In recent years, diamond nanothreads (DNTs) have attracted a wide attention of many experimental and theoretical studies. DNTs are composed of carbon polygons such as 5-, 6-, 7-, and 8-membered carbon rings. In this study, via molecular dynamics simulation we have studied the thermal stability and melting process of 15 lowest-energy DNTs into three classes; achiral, stiff-chiral, and soft-chiral, respectively. The achiral, stiff-chiral, and soft-chiral classes consist of 6, 4, and 5 molecular models, respectively. The results showed that DNTs have different melting points due to the various morphologies and different arrangements of carbon rings. Also, results showed that DNTs composed of purely 6-membered carbon rings have high thermal stability and melting point, wheras DNTs with dissimilar and distorted carbon polygons have lower melting point. In addition, the results showed that DNTs with Stone-Wales defects (SW) have lower thermal stability and melting point. Moreover, the results showed that the achiral, stiff-chiral, and soft-chiral DNTs have the same melting mechanism. During melting process, the increasing sp3 C-C bonds length and their dissociation occurs as the themperature increases. The broken bonds resulting in the formation of larger sp2-carbon rings. The higher temperatures lead to the dissociation of formed large rings and the formation of polyethylene-like molecular chains. Finally, at the melting point or after the melting point, these chains dissociate and the DNTs break down. In this study, the melting point of nanoparticles supported on 15 DNT substrates was also investigated via molecular dynamics simulation. The results showed that all DNTs increase the thermal stability and melting point of the nanoparticles. The role of DNTs in increasing the melting point obey the following trend: soft-chiarl > achiral > stiff-chiral. Soft-chiarl-DNTs with their helical morphologies show lowest deformation in the nanoparticle structure during the melting process.