A new technique for the sp2^2/sp3^3 characterisation of carbon materials

We present a technique to determine the sp$^3$/sp$^2$ ratio of carbon materials which is based on the electron energy-loss spectroscopy and which uses the theoretical spectrum of graphite obtained from ab initio electronic structure calculations. The method relies on the separation of the $\pi^*$ and $\sigma^*$ components of the carbon K-edge of graphite. This $\pi^*$ spectrum is adopted and assumed to be transferable to other carbon systems given an appropriate parametrisation of the broadening. The method is applied on a series of Monte-Carlo generated amorphous carbon structures and is shown to be stable over a wide range of the energy windows for which spectral integration is performed. The results are found to be in good agreement with the sp$^3$ fraction obtained from a microscopic scheme which uses the $\pi-$orbital axis vector (POAV1) analysis. The technique was also applied on a series of experimental spectra of amorphous carbon and was found to be in good agreement with the results obtained from a functional fitting approach.Comment: 5 pages, 4 figures, two tables revtex4, Submitted for publication to Phys. Rev. Let

Attracting shallow donors: Hydrogen passivation in (Al,Ga,In)-doped ZnO

The hydrogen interstitial and the substitutional Al_Zn, Ga_Zn and In_Zn are all shallow donors in ZnO and lead to n-type conductivity. Although shallow donors are expected to repel each other, we show by first principles calculations that in ZnO these shallow donor impurities attract and form a complex, leading to a donor level deep in the band gap. This puts a limit on the n-type conductivity of (Al,Ga,In)-doped ZnO in the presence of hydrogen.Comment: 4 pages, 5 figure

First-principles study of possible shallow donors in ZnAl2O4 spinel

ZnAl2O4 (gahnite) is a ceramic which is considered a possible transparent conducting oxide (TCO) due to its wide band gap and transparency for UV. Defects play an important role in controlling the conductivity of a TCO material along with the dopant, which is the main source of conductivity in an otherwise insulating oxide. A comprehensive first-principles density functional theory study for point defects in ZnAl2O4 spinel is presented using the Heyd, Scuseria, and Ernzerhof hybrid functional (HSE06) to overcome the band gap problem. We have investigated the formation energies of intrinsic defects which include the Zn, Al, and O vacancy and the antisite defects: Zn at the Al site (Zn-Al) and Al at the Zn site (Al-Zn). The antisite defect Al-Zn has the lowest formation energy and acts as a shallow donor, indicating possible n-type conductivity in ZnAl2O4 spinel by Al doping

Origin of magnetism and quasiparticles properties in Cr-doped TiO2_2

Combining LSDA+$U$ and an analysis of superexchange interactions beyond DFT, we describe the magnetic ground states in rutile and anatase Cr-doped TiO$_2$. In parallel, we correct our LSDA+$U$ ground state through GW corrections ($GW$@LSDA+$U$) that reproduce the position of impurity states and the band gaps in satisfying agreement with experiments. Because of the different topological coordinations of Cr-Cr bonds in the ground states of rutile and anatase, superexchange interactions induce either ferromagnetic or antiferromagnetic couplings of Cr ions. In Cr-doped anatase, this interaction leads to a new mechanism which stabilizes a ferromagnetic ground state, in keeping with experimental evidence, without the need to invoke F-center exchange.Comment: 5<pages, 4 figure

First-principles study of the optoelectronic properties and photovoltaic absorber layer efficiency of Cu-based chalcogenides

Cu-based chalcogenides are promising materials for thin-film solar cells with more than 20% measured cell efficiency. Using first-principles calculations based on density functional theory, the optoelectronic properties of a group of Cu-based chalcogenides Cu$_2$-II-IV-VI$_4$ is studied. They are then screened with the aim of identifying potential absorber materials for photovoltaic applications. The spectroscopic limited maximum efficiency (SLME) introduced by Yu and Zunger is used as a metric for the screening. After constructing the current-voltage curve, the maximum spectroscopy dependent power conversion efficiency is calculated from the maximum power output. The role of the nature of the band gap, direct or indirect, and also of the absorptivity of the studied materials on the maximum theoretical power conversion efficiency is studied. Our results show that Cu$_2$-II-GeSe$_4$ with II=Cd and Hg, and Cu$_2$-II-SnS$_4$ with II=Cd and Zn have a higher theoretical efficiency compared to the materials currently used as absorber layer

Electronic Structure of Transparent Oxides with the Tran-Blaha Modified Becke-Johnson Potential

Abstract We present electronic band structures of transparent oxides calculated using the Tran-Blaha modified Becke-Johnson (TB-mBJ) potential. We studied the basic n-type conducting binary oxides In 2 O 3 , ZnO, CdO and SnO 2 along with the p-type conducting ternary oxides delafossite CuXO 2 (X = Al, Ga, In) and spinel ZnX 2 O 4 (X = Co, Rh, Ir). The results are presented for calculated band gaps and effective electron masses. We discuss the improvements in the band gap determination using TB-mBJ compared to the standard generalized gradient approximation (GGA) in density functional theory (DFT) and also compare the electronic band structure with available results from the quasiparticle GW method. It is shown that the calculated band gaps compare well with the experimental and GW results, although the electron effective mass is generally overestimated