Energetics and structures of neutral and chargedSins(n⩽10) and sodium-dopedSinNa clusters

Energetics and structures of neutral and charged ${\mathrm{Si}}_{\mathrm{n}}$ (n⩽10) and sodium-doped ${\mathrm{Si}}_{\mathrm{n}}$ Na clusters have been investigated using local spin density functional electronic structure calculations and structural optimizations, with and without exchange-correlation gradient corrections. For the ${\mathrm{Si}}_{\mathrm{n}}$ clusters, the monomer separation energies show local maxima for n=4, 7, and 10. The vertical and adiabatic ionization potentials are smaller than the values for the Si atom and exhibit odd-even oscillations with values in agreement with experiments, and the adiabatic electron affinities show local minima for n=4, 7, and 10, with the value for the heptamer being the smallest, in agreement with the experimentally measured pattern. Binding of Na to ${\mathrm{Si}}_{\mathrm{n}}$ is characterized by charge transfer from the sodium resulting in the development of significant dipole moments for the ${\mathrm{Si}}_{\mathrm{n}}$ Na clusters. The binding energy of Na to ${\mathrm{Si}}_{\mathrm{n}}$ oscillates as a function of n, with local maxima for n=2, 5, and 9, and local minima for n=4, 7, and 10, with the value for n=7 being the smallest. A similar trend is found for the vertical and adiabatic ionization potentials of the doped clusters, correlating with the electron affinity trend exhibited by the ${\mathrm{Si}}_{\mathrm{n}}$ clusters, and in agreement with recent measurements. In the optimal adsorption geometry of ${\mathrm{H}}_2$ O on the ${\mathrm{Si}}_7$ Na cluster, the oxygen is bonded to the Na, with a hydration energy significantly higher than that of an isolated sodium atom. The vertical and adiabatic ionization potentials of ${\mathrm{NaH}}_2$ O are lower than those of ${\mathrm{Si}}_7$ ${\mathrm{NaH}}_2$ O, and the values for the latter are lower, by ≈0.2 eV, than those of the unhydrated ${\mathrm{Si}}_7$ Na cluster.