We report the electronic structure and optical properties of the recently synthesized stable two-dimensional carbon allotrope-graphdiyne based on first-principles calculations and experimental optical spectrum. Due to the enhanced Coulomb interaction in reduced dimensionality, the band gap of graphdiyne increases to 1.10 eV within the GW many-body theory from a 0.44 eV within the density functional theory. The optical absorption is dominated by excitonic effects with remarkable electron-hole binding energy of over 0.55 eV within the GW-Bethe Salpeter equation calculation. Experimental optical absorption of graphdiyne films is performed and comparison with the theoretical calculations is analyzed in detail.

Opening a tunable and sizable band gap in single-layer graphene (SLG) without degrading its structural integrity and carrier mobility is a significant challenge. Using density functional theory calculations, we show that the band gap of SLG can be opened to 0.16 eV (without an electric field) and 0.34 eV (with a strong electric field) when properly sandwiched between two hexagonal boron nitride single layers. The zero-field band gaps are increased by more than 50% when the many-body effects are included. The ab initio quantum transport simulation of a dual-gated field effect transistor (FET) made of such a sandwich structure reveals an electric-field-enhanced transport gap, and the on/off current ratio is increased by a factor of 8.0 compared with that of a pure SLG FET. The tunable and sizeable band gap and structural integrity render this sandwich structure a promising candidate for high-performance SLG FETs. NPG Asia Materials (2012) 4, e6; doi:10.1038/am.2012.10; published online 17 February 2012Keywords: density functional theory; electric field; graphene; h-BN sheet; quasiparticle correction; transport properties INTRODUCTION Despite its extremely high carrier mobility (1. 5Â10 4 cm 2 V À1 s À1 for a SiO 2 -supported sample 1 and 2Â10 5 cm 2 V À1 s À1 for a suspended sample 2,3 ), pristine graphene cannot be used for effective roomtemperature field effect transistors (FET) because of its zero band gap. Opening and tailoring a band gap in graphene is probably one of the most important and urgent research topics in the graphene research currently. A large number of methods have been developed to open a band gap in graphene, and these methods can be classified into the following two types, depending on whether they preserve the integrity of the honeycomb structure: in a type I method, the honeycomb structure is destroyed, and in a type II method, the honeycomb structure of graphene is preserved. Typical type I methods include cutting graphene into nanoribbons, 4 making graphene nanomeshes, 5 and chemical functionalization. 6,7 The main disadvantage of the type I methods is that the carrier mobility and on-state current are greatly reduced because the destruction of the honeycomb structure introduces scattering centers, enhances the carrier effective mass and produces a non-tunable band gap.Unlike the type I method, high carrier mobility can be maintained in the type II method because the honeycomb structure is maintained. Typical type II methods include graphene-substrate interaction 8,9 and the application of strain. 10 The graphene band gap induced by a substrate is not tunable. The most effective type II method is the application of an external electric field to the graphene. Both

The electronic structures and magnetic properties of many rare-earth monopnictides are reviewed in this article. Possible candidate materials for spintronics devices from the rare-earth monopnictide family, i.e. high polarization (nominally half-metallic) ferromagnets and antiferromagnets, are identifi ed. We attempt to provide a unifi ed picture of the electronic properties of these strongly correlated systems. The relative merits of several ab initio theoretical methods, useful in the study of the rare-earth monopnictides, are discussed. We present our current understanding of the possible half-metallicity, semiconductor-metal transitions, and magnetic orderings in the rare-earth monopnictides. Finally, we propose some potential strategies to improve the magnetic and electronic properties of these candidate materials for spintronics devices.

No abstract