Excitonic effects in the optical properties of a SiC sheet and nanotubes

By H. Hsueh, G. Guo, Steven G. Louie. 2011

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Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA(Dated: May 30, 2011)The quasiparticle band structure and optical properties of single-walled zigzag and armchair SiCnanotubes (SiC-NTs) as well as single SiC sheet are investigated by abinitiomany-body calculationsusing the GW and the GW plus Bethe-Salpeter equation (GW+BSE) approaches, respectively.Significant GW quasiparticle corrections of more than 1.0 eV to the Kohn-Sham band gaps fromthe local density approximation (LDA) calculations are found. The GW self-energy correctionstransform the SiC sheet from a indirect LDA band gap to a direct band gap material. Furthermore,the quasiparticle band gaps of SiC-NTs with different chiralities behave very differently as a functionof tube diameter, and this can be attributed to the difference in the curvature-induced orbitalrehybridization between the different chiral nanotubes. The calculated optical absorption spectraare dominated by discrete exciton peaks due to exciton states with large binding energy up to2.0 eV in the SiC sheet and SiC-NTs. The formation of strongly bound excitons is attributed tothe enhanced electron-hole interaction in these low dimensional systems. Remarkably, the excitedelectron amplitude of the exciton wavefunction is found to peak on the Siatoms near the hole position(which is on the C site) in the zigzag SiC-NTs, indicating a charge transfer from an anion (hole) toits neighboring cations by photoexcitation. In contrast, this pronounced peak structure disappearin the exciton wavefunction in the armchair SiC-NTs. Furthermore, in the armchair SiC-NTs, thebound exciton wavefunctions are more localized and also strongly cylindrically asymmetric. Thelarge excitation energy of ∼ 3.0 eV of the first bright exciton with no dark exciton below it, suggeststhat the small-radius armchair SiC-NTs be useful for optical devices working in the UV regime.On the other hand, the zigzag SiC-NTs have many dark excitons below the first bright exciton andhence may have potential applications in tunable optoelectric devices ranging from infrared to UVfrequencies by external perturbations.

Subfields: Materials Chemistry

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