We report on the discovery of a room-temperature ferromagnetic semiconductor in chalcopyrite (Zn1-xMnx)GeP2 with Tc = 312 K. We have also observed that, at temperatures below 47 K, samples for x = 0.056 and 0.2 show a transition to the antiferromagnetic (AFM) state, so that ferromagnetism is well defined to be present between 47 and 312 K. The observation that the AFM phase is most stable at low temperatures is consistent with the predictions of full-potential linearized augmented plane wave total energy calculations and has consequences for other chalcopyrite materials.

Density-functional methods are used to analyze the scaling of discrete oligomeric -electron conducting molecules towards idealized isolated polymer chains, treated in periodic boundary conditions. The band gaps of a series of conjugated oligomers of incrementally increasing lengths exactly fit a nearly-free-electron molecular-orbital picture and exhibit a smooth deviation from the classical empirical ''1/N'' trend for long oligomers and infinite polymers. The calculations also show a smooth convergence of bond lengths. The full band structures and densities of states of a polyacetylene, polypyrrole, polyfuran, and polythiophene show that band crossing, localized bands, and other effects cannot be accurately determined from simple extrapolation of oligomer electronic structures. Systematic comparisons of the electronic structure variations of the polymers investigated indicate that the electron affinity, rather than the electronegativity of the heteroatom or the bondlength alternation of the conjugated backbone, significantly affects the band gap of the resulting polymer as indicated by the presence of heteroatom states in the partial density of states of the conduction band, requiring revision of previous semiempirical analyses. Consequences for doping processes are also studied, along with a comparison of valence bandwidths, conduction bandwidths, and carrier effective masses as a function of heteroatom.

The nature and origin of ferromagnetism in magnetic semiconductors is investigated by means of highly precise electronic and magnetic property calculations on MnxGe1-x as a function of the location of Mn sites in a large supercell. Surprisingly, the coupling is not always ferromagnetic (FM), even for large Mn-Mn distances. The exchange interaction between Mn ions oscillates as a function of the distance between them and obeys the Ruderman-Kittel-Kasuya-Yosida analytic formula. The estimated Curie temperature is in good agreement with recent experiments, and the estimated effective magnetic moment is about 1.7mu(B)/Mn, in excellent agreement with the experimental values, (1.4-1.9)mu(B)/Mn.

First-principles calculations are used to explore the geometry, bonding, and electronic properties of MoS 2 /Ti 2 C and MoS 2 /Ti 2 CY 2 (Y = F and OH) semiconductor/metal contacts. The structure of the interfaces is determined. Strong chemical bonds formed at the MoS 2 /Ti 2 C interface result in additional states next to the Fermi level, which extend over the three atomic layers of MoS 2 and induce a metallic character. The interaction in MoS 2 /Ti 2 CY 2 , on the other hand, is weak and not sensitive to the specific geometry, and the semiconducting nature thus is preserved. The energy level alignment implies weak and strong n-type doping of MoS 2 in MoS 2 /Ti 2 CF 2 and MoS 2 /Ti 2 C(OH) 2 , respectively. The corresponding n-type Schottky barrier heights are 0.85 and 0.26 eV. We show that the MoS 2 /Ti 2 CF 2 interface is close to the Schottky limit. At the MoS 2 /Ti 2 C(OH) 2 interface, we find that a strong dipole due to charge rearrangement induces the Schottky barrier. The present interfaces are well suited for application in all-two-dimensional devices.

Density functional theory (DFT) calculations are employed to study H 2 O and CO dissociations on a set of CuNi bimetallic surfaces aiming at exploring the optimal Ni ensemble on Cu(111) for an efficient waterÀgas shift (WGS) process, i.e., splitting H 2 O with high reactivity and avoiding CO activation. We found that Ni additives in the Cu(111) surface layer including a Ni monomer can remarkably enhance water splitting. Meanwhile, H 2 O dissociation barriers (E act ) are strongly correlated with the H adsorption energies (E ad ): the larger E ad , the smaller E act . Moreover, H 2 O dissociation may be more practical via the dissociated H* rather than OH closing to Ni atoms. For the scission of the CÀO bond, the process is unfavorable on Ni monomers, though it is obviously promoted on Ni dimers, trimers, and other ensembles with higher Ni content. It is deduced that the selectivity of the CuÀNi bimetallic catalysts toward WGS would decrease with increasing Ni concentration. These findings suggest that the bimetallic CuNi catalysts with highly dispersed Ni ensembles containing lower Ni concentration should exhibit high performance for the WGS process.