Surface modification of Co-doped ZnO nanocrystals and its effects on the magnetic properties

Abstract

A series of chemically prepared Co 2+ -doped ZnO colloids has been surface modified either by growing shells of ZnSe or by the in situ encapsulation in poly͑styrene͒. The surface modification effects using these two distinct chemical strategies on the magnetic properties of the nanocrystals were probed by electron paramagnetic resonance ͑EPR͒. Structural characterization by means of x-ray diffraction and transmission electron microscopy gave no evidence of second phase formation within the detection limits of the used equipment. The EPR analysis was carried out by simulations of the powderlike EPR spectra. The results confirm that in the core of these nanocrystals Co was incorporated as Co 2+ , occupying the Zn 2+ sites in the wurtzite structure of ZnO. Additionally we identify two Co signals stemming from the nanocrystals' shell. The performed surface modifications clearly change the relative intensity of the EPR spectrum components, revealing the core and shell signals.

Mn2+‐Doped CdSe Quantum Dots: New Inorganic Materials for Spin‐Electronics and Spin‐Photonics

“…[107][108][109] Experimentally, little evidence has yet emerged that specifically points to either of these proposed mechanisms as a major source of difficulties. Colloidal ZnO nanocrystals are among the most easily doped of the II-VI semiconductors despite their wurtzite lattice structure, [57,[110][111][112][113][114][115] and annealing causes Zn 2þ or Co 2þ impurity ions to enter colloidal wurtzite CdSe QDs rather than be extruded from them. [116,117] The practical importance of these two factors is therefore still an open question.…”

Chapter 9. Nano dimensional ZnO: new chemical insights from an old material

“…125,126,[130][131][132][133] Pure ZnO exhibits green emission when excited in the UV wavelength range. This emission has been assigned to oxygen vacancies or to interstitial zinc in excess, leading to intra gap electronic levels.…”

“…123,124 A closely related challenge is that of determining experimentally the exact environment of the dopants in the quantum dots. Some research has focused on specifically looking at the doping of II-VI semiconductor nanocrystals with transition metals [125][126][127][128] and rare-earth elements. [129][130][131] In general, these dopants influence the quantum dots not by introducing extra carriers, but rather by providing an impurity core that can interact with the quantum-confined electron-hole pair.…”

Magnetic and structural properties of transition metal doped zinc‐oxide nanostructures

“…Part of the original ZnO:Co NCs were subjected to chemical surface modification, either by encapsulation with polystyrene (PS) (sample NC4) or by forming ZnSe shells at the surface of the NCs (sample NC5), by means of a reaction with TOPSe and at the expense of the ZnO, as explained in detail in Ref. [15].…”