Molecular-Dynamics Simulation Study On The Irregular Size Effects Of Gold Nanoclusters

Nanomaterial have become an active topic of material science. Cluster contains few atoms to hundreds of atoms. Its properties are different from a single atom and crystal. With the decreasing number of atom,the cluster may show irregular properties which we call irregular size-effect.Adding or cuting an atom can bring about radical structural change, accordingly lead to the change of properties. We present a detailed molecular-dynamics study of the melting , freezing of gold nanoclusters within the framework of the embedded-atom method . In order to be able to simulate clusters containing more than several hundred particles, it is necessary to resort to an empirical description of the interatomic forces. Here we choose to employ the embedded-atom method, an n-body potential with proven ability to model reliably various static and dynamic properties of transition and noble metals. The model is "semiempirical" in the sense that it approachs the total-energy problem from a local-electron-density view of point. But using a function form with parameters obtained from experiment(equilibrium lattice constant, sublimation energy , bulk modulus, elastic constants. etc)In this study fourteen clusters of different sizes are considered. First we should prepare various face-centered-cubic Au spherical clusters. Of course , the clusters cann't be perfectly spherical. The atoms follow the Maxwell distribution law. The cluster is heated up to 1000K, then cooled from the highest temperature to 200K. At every temperature the cluster should reach equilibration. Upon approaching the transition from either side, the temperature is changed in steps of 20-30K. In the simulation the change of temperature is achieved by changing energy and use a time step of 0.2fs. In the simulation we get potential energy, total energy and structure varying with time for each cluster. The melting-and-freezing simulations are carried out in the microcanonical ensemble.Our results show (1)the melting point of cluster does not increase monotonically with cluster size. This differs from what we have imagined. The smaller cluster may has higher melting temperature. (2)the microcanonical heat capacity for a greater cluster that contains more than 90 atoms can become negative. Every day experience tells us that if one adds energy to a system it will get warmer. But an increase of energy for nanoclusters may lead to a lower temperature near its solid to liquid transition. This has been observed in experiment for a cluster containing 147 sodium atoms. We try toexplain it in term of energy. (3)In the melt-quench cycle we observe the hysteresis . It indicates it is much easier for a cluster to go from an ordered state to a disordered state than the opposite. Namely the freezing temperature is lower than the melting temerature, however both are same for crystal. According to the simulated clusters,the greater the cluster is, the greater the difference is. Finally we discuss the factor that affects the simulation.