Room-temperature synthesis and characterization of highly monodisperse transition metal-doped ZnO nanocrystals

Abstract

Recent experimental verifications of predicted intrinsic room-temperature ferromagnetism in transition metal doped-ZnO nanostructures have increased their attractiveness as promising candidates for optoelectronic and spintronics- based devices. A control over dopant speciation and the determination of the size-dependence of functional properties at the nanoscale become then indispensable. We present here the results of our current efforts on size- controlled synthesis of bare and TM-doped ZnO nanocrystals. Stable suspensions of these materials were produced in ethanol solutions at room- temperature. XRD characterization of produced materials verified the fast and direct formation of ZnO nanocrystals after 10 minutes of contact time at room- temperature. Although the actual incorporation of TM atoms into the ZnO host structure was also evidenced by the XRD analyses, the presence of dopant ions in starting solutions delayed the formation of the oxide. Well-crystallized doped ZnO structures were produced only after aging of the nanocrystals in their mother liquors at ambient temperature. The length of this aging period was found to be dependent on the fraction of TM in the solids. HRTEM analyses of the suspensions revealed the high monodispersity and crystallinity of the 6-9 nm nanocrystals. UV-visible measurements confirmed not only the nanocrystalline nature of the samples but also evidenced the continuous growth of the nanocrystals when aged in their mother liquors. UV-visible analyses also confirmed the inhibition of crystal growth when the nanocrystals were coagulated by n-heptane and redispersed in fresh ethanol solutions. The application of this coagulation-redispersion process at different aging times allowed the production of highly monodisperse nanocrystals of different mean size. SQUID measurements indicated that doped ZnO nanocrystals were paramagnetic or ferromagnetic at room-temperature in contrast with the diamagnetic nature of bare ZnO.