Novel Magnetic and Optical Properties of Sn1-xZnxO2 Nanoparticles

In this work, we report on the effects of doping SnO2 nanoparticles with Zn 2+ ions. A series of ~23nm sized Sn1xZnxO2 crystallite samples with 0 ≤ x ≤ 0.18 were synthesized using a forced hydrolysis method. Increasing dopant concentration caused systematic changes in the crystallite size, oxidation state of Sn, visible emission and band gap of SnO2 nanoparticles. X-ray Diffraction (XRD) studies confirmed the SnO2 phase purity and the absence of any impurity phases. Magnetic measurements at room temperature showed a weak ferromagnetic behavior characterized by an open hysteresis loop. Their saturation magnetization Ms increases initially with increasing Zn concentrations, however for x > 0.06, Ms decreases. Samples with the highest Ms values (x = 0.06) were analyzed using an Inductively Coupled Plasma Mass Spectrometer (ICP-MS), looking for traces of any magnetic elements in the samples. Concentrations of all transition metals (Fe, Co, Mn, Cr, Ni) in these samples were below ppb level, suggesting the observed magnetism is not due to random inclusions of any spurious magnetic impurities and it cannot be explained by the existing models of magnetic exchange. A new visible emission near 490nm appeared in the Zn doped SnO2 samples in the photoluminescence (PL) spectra which strengthened as x increased, suggesting the formation of defects such as oxygen vacancies. X-ray Photoelectron Spectroscopy (XPS) confirmed the nominal Zn dopant concentrations and the 2+ oxidation state of Zn in the Sn1-xZnxO2 samples. Interestingly, the XPS data indicated the presence of a small fraction of Sn 2+ ions in Sn1-xZnxO2 samples in addition to the expected Sn 4+ , and the Sn 2+ concentration increased with increasing x. The presence of multi-valent metal ions and oxygen defects in high surface area oxide nanoparticles have been proposed as a potential recipe for weak ferromagnetism 1 .