Quasiparticle effects and optical absorption in small fullerenelike GaP clusters

Quasiparticle corrections to the electronic energies have been calculated for small GaP fullerenes, a class of nanoscaled materials recently predicted to be stable. These clusters have been also characterized by us for their optical absorption spectra using time-dependent density functional theory. The comparison between single-particle and optical absorption spectra supports the evidence of strong excitonic effects with bonding energy up to $3.5\mathrm{eV}$. The quasiparticle corrected highest-occupied-molecular-orbital–lowest-unoccupied-molecular-orbital energy gaps confirm the high stability predicted for such molecules using ground-state computational schemes. The present results will be useful to identify the successful synthesis of such systems via optical absorption and quasiparticle spectra.

Figure 1

(Color online) Ground-state geometries of the four GaP clusters studied in this work. The symmetry of ${\mathrm{Ga}}_8{\mathrm{P}}_{12}$ and ${\mathrm{Ga}}_{12}{\mathrm{P}}_8$, respectively, (a) and (b) in the figure, is $T_h$, which implies that they have only two inequivalent atoms, while ${\mathrm{Ga}}_{12}{\mathrm{P}}_{16}$ and ${\mathrm{Ga}}_{16}{\mathrm{P}}_{12}$, respectively, (c) and (d), belong to the $T_d$ symmetry point group and thus have three inequivalent atoms.

Figure 2

Calculated spectrum for ${\mathrm{Ga}}_{16}{\mathrm{P}}_{12}$. For every energy state, designed according to $T_d$ point group notation, the corresponding degeneracy is given in brackets. In the left panel the spectrum of the Kohn-Sham eigenvalues; in the right panel the quasiparticle one evaluated by rigidly moving up in energy the empty states by the gap correction of $3.88–1.12=2.76\mathrm{eV}$ (see Table ), maintaining fixed the filled ones. The corresponding values for the HOMO-LUMO gaps from Table are also given.

Figure 3

Optical absorption cross section $\sigma \left(E\right)$ expressed in megabarns $(1\mathrm{Mb}={10}^{-18}{\mathrm{cm}}^2)$ for the four molecules considered in this work; in each plot the asterisk marks the first intense electric dipole-permitted electronic transition.