High-temperature behavior of monolayer graphyne and graphdiyne

Molecular dynamics simulations with the adaptive intermolecular reactive bond order (AIREBO) potential are performed to study the high-temperature (T) behavior of monolayer graphyne and graphdiyne (MGY and MGDY). During the melting process, MGY and MGDY undergo three continuous phase transitions: (i) transformation into the initial amorphous graphene phase (AGP) starting from 2800 to 2500 K for MGY and MGDY, respectively, where broken bonds and large holes appear; (ii) transformation into AGP through a structural adjustment process until T reaches 3650 K. This AGP will remain relatively stable until T ≈ 5000 K; (iii) the AGP will then gradually transform into a nearly fluid state for T > 5000 K. Even prior to the first phase transition, some defects, such as 5, 7, 8 and 9-membered rings start appearing at 1700 and 1200 K for MGY and MGDY, respectively. In addition to the defects derived from severe thermal vibration, the sharp contraction and accompanying stretch of MGY and MGDY are decisive in the formation of the initial AGP. In contrast to the prediction of the Kosterlitz-Thouless-Halperin-Nelson-Young theory for the melting of two-dimensional materials, we don't observe a hexatic phase but rather an AGP as an intermediate phase.