Calorimetry and reactivity studies of size-selected aluminum and gallium clusters

For decades, metal clusters have attracted significant interest due to their unusual physical and chemical properties. Fundamental properties of systems in the nanometer-size regime remain poorly understood. Generally, metal clusters have depressed melting temperatures compared to bulk due to the increased surface area-to-volume ratio. It has been well established that the addition of a single atom can result in drastically different melting temperatures. In this work, we investigated phase transitions and reactivity of unsupported aluminum and gallium clusters.;To determine phase transitions, heat capacities of size-selected metal clusters are measured on an in-house-built instrument comprised of a laser ablation source and tandem linear quadrupole mass spectrometers. Aluminum clusters (containing 150-342 atoms) melt hundreds of degrees lower than the bulk melting point of aluminum (934 K). Quite surprisingly, we have experimentally determined that small gallium clusters melt at temperatures hundreds of degrees above the corresponding bulk (303 K). As the sizes of clusters increase, the melting temperatures drop to roughly the bulk value.;Metal clusters also present a platform to study surface reactions. Furthermore, metal clusters present a simpler method for studying reactions on liquid metal surfaces. We have determined that the reactivity of aluminum clusters can change drastically upon melting. In two studies, we investigated the interactions between Al100+ clusters and (1) CO2 molecules and (2) benzene molecules. Generated products are controlled by thermodynamic phase of the aluminum clusters. In the case of benzene, dehydrogenation occurs as cluster temperature is increased.