Calorimetry measurements for Al53+ clusters show the broad melting transition at 620K, well below the bulk melting point of 934K.

 

 

 

 

 

 

 

 

 

 

Studying Metal Nanoclusters with Mass Spectrometry

Metal clusters are of interest because they are fantastic catalysts, can improve electronic devices, and could eventually be incorporated into nano machines.  As devices are made smaller and are made from the bottom up instead of the top down, it is important to understand how the properties of materials change as they get smaller.  In studying the melting transitions of metal

Melting temperatures of aluminum clusters as a function the number of atoms in the cluster.
clusters, we have found gallium clusters (containing around 50 atoms) melt at temperatures hundreds of degrees higher than bulk gallium and aluminum clusters (containing 28-128 atoms) melt at temperatures sometimes hundreds of degrees lower than the bulk melting point of aluminum.  Furthermore, clusters that differ in size by only one atom can have melting temperatures over one hundred degrees apart. We are currently measuring the heat capacity of larger gallium clusters to determine phase transitions.

We are still understanding the unique properties of the nano size regime, but we would also like to understand the transition between nano properties and bulk properties.  For instance we have studied aluminum clusters that are calculated to have the geometric structure of an FCC fragment.  Bulk aluminum has an FCC geometry, but those aluminum clusters still exhibit completely different melting behavior than bulk aluminum.

Metal clusters also present a platform with which to study surface reactions.  In two recent studies, we investigated the interactions between Al100+ clusters and (1) CO2 molecules and (2) benzene molecules. Furthermore, metal clusters present a simple method for studying reactions on liquid metal surfaces.  We have found the reactivity of aluminum clusters can change drastically when the cluster melts.  In other instances, the biggest change in reactivity occurs when the cluster undergoes a solid to solid phase transition.