A systematic study of the optical absorption of small silicon nanocrystals ͑Si-NCs͒ embedded in silicon dioxide is performed using real-time time-dependent density-functional theory. The modeled Si-NCs contain up to 47 Si atoms with the surrounding oxide being described by a shell of SiO 2 . The oxide-embedded Si-NCs exhibit absorption spectra that differ significantly from the spectra of the hydrogen-passivated Si-NCs. In particular, the minimum absorption energy is found to decrease when the Si-NCs are exposed to dioxide coating. Unexpectedly, the absorption energy of the oxide-embedded Si-NCs remains approximately constant for core sizes down to 17 atoms, whereas the absorption energy of the hydrogen-passivated Si-NCs increases with decreasing crystal size. This trend suggests a different mechanism for producing the lowest-energy excitations in these two cases.

We have calculated the absorption characteristics of different hybrid systems consisting of Ag, Ag 2 , or Ag 3 atomic clusters and poly͑methacrylic acid͒ using the time-dependent density-functional theory. The polymer is found to have an extensive structural-dependency on the spectral patterns of the hybrid systems relative to the bare clusters. The absorption spectrum can be "tuned" to the visible range for hybrid systems with an odd number of electrons per silver cluster, whereas for hybrid systems comprising an even number of electrons per silver cluster, the leading absorption edge can be shifted up to ϳ4.5 eV. The results give theoretical support to the experimental observations on the absorption in the visible range in metal cluster-polymer hybrid structures.

Optical absorption spectra have been calculated for a series of boron nitride fullerenelike cage structures B n N n of sizes n = 12-36. The method used is a real-time, real-space implementation of the time-dependent density-functional theory, involving the full time propagation of the time-dependent Kohn-Sham equations. The spectra are found to be a possible tool for distinguishing between different boron nitride fullerene species and isomers. The trends and differences in the spectra are found to be related to the general geometry of the molecules. Comparison between local-density and generalized-gradient approximations for electron exchange-correlation functionals shows that both of them produce essentially the same spectral characteristics.

Articles you may be interested in Photoabsorption in sodium clusters on the basis of time-dependent density-functional theoryThe photoabsorption spectra of a continuous series of Na n clusters ͑n ഛ 14, n = 20, n =40͒ have been calculated using a time-dependent density-functional scheme. Accordingly, we present these spectra and show that they are in very good agreement with other theoretically and experimentally obtained photoabsorption spectra. Furthermore, we discuss the influence of the cluster structure on the photoabsorption spectrum for some selected clusters and present for several cluster sizes photoabsorption spectra of different geometrical isomers. The spectra of clusters with five or more atoms are dominated by a few large peaks which can be interpreted as collective plasmon excitations.