Molecular-Dynamics Simulation Study On The Thermodynamics Properties Of Au, Ag And Their Alloy Nanoclusters

The cluster has become an important field of nanomaterial science. It is also an active field for condensed matter physics. However its' many characters are still unclear. There are many studying for metal clusters, because they have some especial properties. When the size of clusters is small, the change of melting point is irregular. And negative heat capacity has been found in the cluster that is consisted of the one hundred and forty-seven sodium atoms. With these properties some new material can be developed.To study clusters more clearly, we present a detail molecular-dynamics simulation on the Au, Ag and Au-Ag clusters with the embedded-atom method and the function of Johnson for the interaction in the atoms. The method is good to study the noble metal. In our simulator, an empirical description of the interaction forces is adopted, therefore a lot of calculation will be reduced. This method is a semi-empirical model, because some parameter of the model is obtained from the experimental work (equilibrium lattice constant, sublimation energy, bulk modulus, elastic constant. etc.). Au and Ag clusters are chosen in the studying because there are many results for the two kinds of clusters.In this study, a lot of clusters were simulated, which contain AuN clusters (N=60~767), AgN clusters (N=50~767) and Aux-AgN-x clusters (N=80,144 and 276; X=0~N). Anneal is the most important process in the preparation of cluster models. Through the process, the structure of clusters is similar to the real cluster. Different melting and cooling speeds are taken in our simulation. Therefore we gain all kinds of experiment data that is included in the potential energy, total energy, temperature and structures of clusters. They are useful to our study on the clusters. With the basis of the analysis on the data, the result was obtained, which shows some irregular properties of clusters.Firstly, the change of melting point is irregular with the cluster size. Some small clusters have a higher melting point. The melting point of AgN cluster is higher than the melting point of AuN clusters when the N is from 276 to 767. Secondly, for the Aux-AgN-x clusters, the melting point descends quickly when the number of Au atoms is very little and is increase. When the number of Ag atoms is very little and is increase the melting point of alloy cluster is irregular. The Ag atoms is tend to occupy the surface of the alloy cluster and the Au atoms is tend to occupy the body of the alloy cluster. Thirdly, the negative heat capacity has been discovered in the AuN clusters (N=60~675), AgN clusters (N=80~276) and the alloy clusters (N=80, 144 and 276). The size is a vital factor for the negative heat capacity. Its' mechanism is changes of the structure. An increase of energy may lead to a lower temperature at the conversion from solid to liquid in nanoclusters. This character has been found in all the simulation system in our calculation. Fourthly, we find clusters can steadily exist in some structures respectively. Their distribution of atom also is different for all kinds of structures.