Quantum mechanical modeling of structure evolution of transition metal clusters and metallocarbohedrenes

摘要

Ab initio quantum-mechanical modeling based on density functional theory (DFT) was used to study transition metal clusters and metallo-carbohedrenes (MetCars). Combined with the state-of-the-art spectroscopic experiments, DFT calculations are capable of yielding much insight into the structures, chemical bonding and growth mechanisms of these clusters. Two specific cluster systems were investigated in detail: one involves small chromium clusters and another contains titanium carbides. Exhaustive structural search was performed by fully optimizing a variety of cluster geometries. For the chromium clusters, we found that a tightly-bound Cr_2 dimer plays a key role in determining the cluster structures. A dimer growth route is discovered for clusters up to Cr_(11), at which a structural transition occurs from the dimer growth to a bulk-like body-centered-cubic structure. The uncovered structural evolution is consistent with the currently available experimental observations. For MetCars, we found that three factors, i.e., the C_2 dimer, cubic framework and layered structures, play an essential role in determining the structures and chemical bonding of the titanium carbide clusters. A growth pathway from Fi_3C_8 to Ti_(13)C_(22) with Ti_4C_8, Ti_6C_(13), Ti_7C_(13) and Ti_9C(15) as intermediates is thus proposed. Both theory and experiments suggest that the cubic layered growth with C_2 dimers can lead to a new type of highly stable one-dimensional quantum wires.