The electronic structures, and static and dynamic magnetic properties of monolayer iron dioxide and iron dichalcogenides (FeX2 (X = O, S, Se, Te)) are investigated using first-principle calculations in conjunction with Monte Carlo (MC) simulation and atomic spin dynamics (ASD) simulation. In this rarely studied family of monolayer binary compounds a variety of possible phases are discovered, including narrow bandgap a semiconductor (FeO2), half-metal (FeS2) and metal (FeSe2 and FeTe2), and all the ground states are ferromagnetic (FM). Based on the magnetic exchange interactions, the temperature dependence of the average magnetic moment per unit cell and magnetic susceptibility of monolayer FeX2 are predicted. The Curie temperatures (TCs) are estimated and magnon dispersions as a function of temperature are demonstrated, revealing a new family of pristine monolayers with transition temperatures (96–168 K) above the liquid-nitrogen temperature.

The electronic transport properties of zigzag phosphorene nanoribbon (ZPNR) homostructures are investigated using density functional theory calculations and the nonequilibrium Green’s function technique. The proposed devices have simple constructions but exhibit robust negative differential resistance (NDR) as well as quite large current, which implies great potential for building nanoelectronic devices. Through the analysis of the electronic structures and transmission spectra, we give a clear physical picture of the NDR mechanism, in which such current–voltage (I–V) behaviors originate from the bias-dependent interaction between the discrete density of states (DOS) peaks of the electrodes and the narrow states around the Fermi level in the scattering region.

•Single crystalline ferromagnetic ZnO:Co nanocrystals were synthesized by polymer assisted deposition.•Enhancement of the room temperature ferromagnetism in ZnO:Co nanocrystals was achieved by post-annealing treatment in both oxygen and hydrogen atmospheres.•The improvement of ferromagnetism upon annealing is found to be attributed to the decrease of oxygen interstitials (Oi) and the increase of Zn1− vacancies in the nanocrystals.Co-doped ZnO thin films were prepared by polymer-assisted deposition (PAD), and then treated by post-annealing in different atmospheres. XRD, SEM, TEM and SQUID were employed to characterize the samples, which demonstrate that all the samples are composed of nanocrystals and the annealing shows a robust effect on the enhancement of ferromagnetism at room temperature. PL measurements show that the improvement of ferromagnetism upon annealing is related to the decrease of oxygen interstitials (Oi) and the increase of Zn1− vacancies. XPS measurements indicate that post-annealing also induces oxygen vacancies, which have no positive contribution to the ferromagnetic ordering. Our results may suggest an effective way for modulation of structure and magnetic properties of Co-doped ZnO nanocrystal thin films.

Ultrafine single crystal silicon nanowires (SiNWs) were grown along the <110> direction by thermal evaporation without catalyst, and were studied by electron microscopy and optical spectroscopy. Simulation of the first-order Raman scattering spectrum of the wires indicates that the Raman shift and asymmetric broadening line shape are due to phonon confinement and a narrow diameter distribution of the SiNWs, and confirms that the mean diameter of the wires is 2 nm, which is smaller than the excitonic Bohr radius of Si. Further analysis shows that the unique intense purple photoluminescence (PL) peak and blue-shift of absorption spectrum of the wires result both from the increase of the energy band gap due to quantum confinement of the carriers, and from the confinement of excitons in the ultrafine SiNWs, suggesting potential applications of the wires in optical and optoelectronic nanoscale devices.

Nonlinear concentration-dependent electronic and optical properties of the Si1–xGex substitutional alloy nanowires are investigated using first-principles calculations. The nonuniform distribution of Ge (or Si) atoms is found, and the resulting orbital hybridization of the inner Ge or Si atoms results in the nonlinear Ge concentration dependence of electronic properties in the Si1–xGex alloy NWs, which suggests an effective approach to modulate band-gap properties of the NWs along all three directions. Moreover, a strong adsorption of solar radiation and a high quantum yield is predicted in (110)-oriented alloy NWs, which implies a great potential of Si1–xGex alloy NWs in the optical electronics applications on nanoscale.