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Register a new userCu codoping control over magnetic precipitate formation in ZnCoO nanowires
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Using electrodeposition, we have grown nanowires of ZnCoO with Cu codoping concentrations varying from 4-10 at.%, controlled only by the deposition potential. We demonstrate control over magnetic Co oxide nano-precipitate formation in the nanowires via the Cu concentration. The different magnetic behavior of the Co oxide nano-precipitates indicates the potential of ZnCoO for magnetic sensor applications.
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Optical and magneto-optical properties of ZnCoO films grown at low temperature by Atomic Layer Deposition are discussed. Strong wide band absorption, with onset at about 2.4 eV, is observed in ZnCoO in addition to Co-related intra-shell transitions. This absorption band is related to Co 2+ to 3+ photo-ionization transition. A strong photoluminescence (PL) quenching is observed, which we relate to Co recharging in ZnO lattice. Mechanisms of PL quenching are discussed.
Precipitate strengthening of light metals underpins a large segment of industry.Yet, quantitative understanding of physics involved in precipitate formation is often lacking, especially, about interfacial contribution to the energetics of precipitate formation.Here, we report an intricate strain accommodation and free energy minimization mechanism in the formation of Omega precipitates (Al2Cu)in the Al_Cu_Mg_Ag alloy. We show that the affinity between Ag and Mg at the interface provides the driving force for lowering the heat of formation, while substitution between Mg, Al and Cu of different atomic radii at interfacial atomic sites alters interfacial thickness and adjust precipitate misfit strain. The results here highlight the importance of interfacial structure in precipitate formation, and the potential of combining the power of atomic resolution imaging with first-principles theory for unraveling the mystery of physics at nanoscale interfaces.
Using pulsed laser ablation with arsenic over pressure, the growth conditions for GaAs nanowires have been systematically investigated and optimized. Arsenic over pressure with As$_2$ molecules was introduced to the system by thermal decomposition of polycrystalline GaAs to control the stoichiometry and shape of the nanowires during growth. GaAs nanowires exhibit a variety of geometries under varying arsenic over pressure, which can be understood by different growth processes via vapor-liquid-solid mechanism. Single-crystal GaAs nanowires with uniform diameter, lengths over 20 $mu$m, and thin surface oxide layer were obtained and can potentially be used for further electronic characterization.
Density functional calculations are performed to investigate the room temperature ferromagnetism in GaN:Cu nanowires (NWs). Our results indicate that two Cu dopants are most stable when they are near each other. Compared to bulk GaN:Cu, we find that magnetization and ferromagnetism in Cu doped NWs is strongly enhanced because the band width of the Cu td band is reduced due to the 1D nature of the NW. The surface passivation is shown to be crucial to sustain the ferromagnetism in GaN:Cu NWs. These findings are in good agreement with experimental observations and indicate that ferromagnetism in this type of systems can be tuned by controlling the size or shape of the host materials.
We have studied the electronic structure and the magnetism of Cu-doped ZnO nanowires, which have been reported to show ferromagnetism at room temperature [G. Z. Xing ${et}$ ${al}$., Adv. Mater. {bf 20}, 3521 (2008).], by x-ray photoemission spectroscopy (XPS), x-ray absorption spectroscopy (XAS) and x-ray magnetic circular dichroism (XMCD). From the XPS and XAS results, we find that the Cu atoms are in the Cu$^{3+}$ state with mixture of Cu$^{2+}$ in the bulk region ($sim$ 100 nm), and that Cu$^{3+}$ ions are dominant in the surface region ($sim$ 5 nm), i.e., the surface electronic structure of the surface region differs from the bulk one. From the magnetic field and temperature dependences of the XMCD intensity, we conclude that the ferromagnetic interaction in ZnO:Cu NWs comes from the Cu$^{2+}$ and Cu$^{3+}$ states in the bulk region, and that most of the doped Cu ions are magnetically inactive probably because they are antiferromagnetically coupled with each other.
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