Multifunctional nanostructured Co-doped ZnO: Co spatial distribution and correlated magnetic properties.

In this report we present a systematic structural and magnetic analysis of Co-doped ZnO nanoparticles prepared via a microwave-assisted hydrothermal route. The structural data confirm the incorporation of Co ions into the wurtzite ZnO lattice and a Co concentration mainly near/at the surface of the nanoparticles. This Co spatial distribution is set to passivate the surface of the ZnO nanoparticles, inhibiting the nanoparticle growth and suppressing the observation of a ferromagnetic phase. Based on experimental and theoretical results we propose a kinetic-thermodynamic model for the processes of nucleation and growth of the Co-doped ZnO nanoparticles, and attribute the observed ferromagnetic order to a ferromagnetism associated with specific defects and adsorbed elements at the surface of the nanoparticle. Our findings give valuable contribution to the understanding of both the doping process at the nanoscale and the nature of the magnetic properties of the Co-doped ZnO system.

Kinetic effects in InP nanowire growth and stacking fault formation: the role of interface roughening.

InP nanowire polytypic growth was thoroughly studied using electron microscopy techniques as a function of the In precursor flow. The dominant InP crystal structure is wurtzite, and growth parameters determine the density of stacking faults (SF) and zinc blende segments along the nanowires (NWs). Our results show that SF formation in InP NWs cannot be univocally attributed to the droplet supersaturation, if we assume this variable to be proportional to the ex situ In atomic concentration at the catalyst particle. An imbalance between this concentration and the axial growth rate was detected for growth conditions associated with larger SF densities along the NWs, suggesting a different route of precursor incorporation at the triple phase line in that case. The formation of SFs can be further enhanced by varying the In supply during growth and is suppressed for small diameter NWs grown under the same conditions. We attribute the observed behaviors to kinetically driven roughening of the semiconductor/metal interface. The consequent deformation of the triple phase line increases the probability of a phase change at the growth interface in an effort to reach local minima of system interface and surface energy.

Evidence of space charge regions within semiconductor nanowires from Kelvin probe force microscopy.

We have studied the equilibrium electrostatic profile of III-V semiconductor nanowires using Kelvin probe force microscopy. Qualitative agreement of the measured surface potential levels and expected Fermi level variation for pure InP and InAs nanowires is obtained from electrical images with spatial resolution as low as 10 nm. Surface potential mapping for pure and heterostructured nanowires suggests the existence of charge transfer mechanisms and the formation of a metal-semiconductor electrical contact at the nanowire apex.