(95k) Low Temperature Synthesis and Characterization of Highly Monodisperse Tm-Doped Zno Nanocrystals

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 progress on size-controlled synthesis of bare and Co-, Cu-, V-, Cu- and Fe- doped ZnO nanocrystals. Stable suspensions of these materials were produced in ethanol solutions at room-temperature. XRD characterization of the products verified the fast formation of ZnO wursite structure. The actual incorporation of dopants into the ZnO host was suggested by XRD analyses; however, their presence delayed the formation of the host structure. HRTEM analyses revealed the high monodispersity of the 5-8 nm nanocrystals. UV-visible and photoluminescence measurements evidenced not only the nanocrystalline nature of the samples but also the continuous growth of the nanocrystals when aged in their mother liquors. The estimated band gap energy of non-aged ZnO nanocrystals was 3.72eV and decreased down to 3.46eV after 48 hours of aging, when no further growth was observed. The UV-visible analyses also confirmed the inhibition of crystal growth when the nanocrystals were coagulated by n-heptane and redispersed in fresh ethanol. UV absorption measurements for Cu and Co doped ZnO evidenced the delay on the growth of the doped crystal when compared with bare ZnO. SQUID measurements indicated that as-synthesized doped ZnO nanocrystals were paramagnetic at room-temperature in contrast with the diamagnetic nature of bare ZnO. A weak ferromagnetism was evidenced in the Co-ZnO nanocrystals produced for x = 0.02. The effect of the processing temperature will also be presented.