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The study of electronic excitations in hydrogen terminated silicon clusters is important for understanding the optical properties of confined systems such as quantum dots and porous silicon. Here we calculate the excitation energies and absorption spectra for SinHm clusters using linear response theory within the time-dependent local density approximation (TDLDA). We find the computed excitation energies and photoabsorption gaps agree with available experimental data. The TDLDA optical absorption spectra exhibit a substantial blue shift with respect to the spectra calculated within the time-independent local density approximation. This trend is consistent with other theoretical calculations for excited state properties, such as the Bethe-Salpeter technique.
Ab initio quasiparticle gaps. self-energy corrections. exciton Coulomb energies. and optical gaps of Si nanocrystals are calculated using the higher-order finite difference psendopotential method. The calculations are performed in real space on hydrogen-passivated Si clusters with diameters up to 30 A (> 1000 atoins). The size-dependent self-enerkgy correction is enhanced substantially compared to bulk. and quantum confinement and reduced electronic screening result in appreciable excitonic Coulomb energies. Calculated optical gaps are in very good agreement with absorption data from Si nanocrystals.
Determining the electronic and structural properties of semiconductor clusters is one of the outstanding problems in materials science. The existence of numerous structures with nearly identical energies makes it very difficult to determine a realistic ground state structure. Moreover, even if an effective procedure can be devised to predict the ground state structure, questions can arise about the relevancy of the structure at finite temperatures. Kinetic effects and non-equilibrium structures may dominate the structural configurations present in clusters created under laboratory conditions. We illustrate theoretical techniques for predicting the structure and electronic properties of small germanium clusters. Spefically, we illustate that the detailed agreement between theoretical and experimental features can be exploited to identify the relevant isomers present under experimental conditions.
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