Synthesis, Characterization And Magnetic Studies Of SnO₂ And SnO₂-based Nanomaterials

With the discovery of spin-related phenomena and its great application potential, tremendous efforts have been devoted to the research on spintronics around the world. Among various spintronic device systems, magnetic semiconductors are expected to be the dominant materials for next-generation micro-/nano-electronic devices with the degree of freedom of electron spins. The most attractive one is the dilute-magnetic semiconductors (DMSs), which exhibits ferromagnetism by replacing the nonmagnetic ions with magnetic transition-metal ions. However, there are a few challenges in current research on magnetic semiconductors:(1) The Curie temperatures of materials are generally lower than the room temperature; (2) It is still not clear about the origin of ferromagnetism. There are currently two theories to explain the observed ferromagnetism:One is the ferromagnetic exchange coupling mediated by carriers, such as RKKY and double exchange models; the other is the bound magnetic polaron (BMP) model, which is related to the defects in materials. Therefore, it is very important to understand the origin of ferromagnetism in DMSs. It was shown recently that a certain undoped semiconductors exhibit ferromagnetism at room temperature, the so-called d0 ferromagnetism. This discovery challenged the conventional theories and urged researchers to rejudge the real role that a doping may play in tailoring the magnetic properties of semiconducting. However, the mechanism governing the d0 ferromagnetism is still not clear yet. Therefore, it is very important to carry out detailed investigations along this line.In this thesis, we used sol-gel method, chemical vapor deposition and carbothermal reduction to fabricate SnO2 and SnO2-based nanomaterials, and carried out detailed systematic investigations on the fabrication techniques, the material morphology, optical and magnetic properties, based upon which we studied the essential mechanism of ferromagnetism. Our main results are as follows:A. V-doped SnO2 filmsBy the use of the sol-gel method and spin-coating technique and the efficient experimental controls, we fabricated the uniform and crackless Sn1-xVxO2 films. We carried out detailed investigation of theferromagnetism. Our conclusions are:1) All the Sn1-xVxO2 samples exhibit the rutile structure. The crystallite sizes decreases with increased doping level, indicating that the crystal growth was restrained by the doping of V; the lattice parameter decreases monotonously as the V content is increased, showing that V ions go into the SnO2 lattice. Combining with the result of XPS spectra, it is suggested that V4+ions substitute for Sn4+to form the Sn1-xVxO2 alloy.2) We obtained Sn1-xVxO2 samples with the room-temperature ferromagnetism. Based on the measurements of XRD, XPS and SEM, as well as the nonlinear relationship between log R and T-1/2, the presence of V secondary phase (oxides or clusters) is ruled out. Therefore, the ferromagnetism of the samples is intrinsic. We observed that the magnetic moment per V ion drops rapidly with the increase in V content. This behavior can be explained as follows. With the increase in V4+ions, some antiferromagnetic superexchange interaction takes place within the nearest V4+ ions through O2- ions (i.e., V4+-O2--V4+superexchange), which leads to the reduction in the average moment per V4+ ion.3) Our electrical characterizations show that the the carrier concentrations in the film decreases with the increase of V content. However, the quantitative carrier concentration of our Sn1-xVxO2 films is so small (1016Ω·cm as the largest) that the room temperature ferromagnetism is difficult to be interpreted by the carrier-mediated mechanism, where a large density of mobile carriers is required to induce FM.4) We firstly performed the annealing in Sn vapor for the Sn1-xVxO2 films, to increase the concentration of Sn interstitials, from which we investigated its impact on the ferromagnetism. The result shows that the Sn interstitials do not contribute to ferromagnetism.5) By carrying out the annealing experiments in vacuum or oxygen environment, we found that vacuum annealing is able to induce ferromagnetism, while the oxygen annealing reduces or eliminates ferromagnetism. This confirmed that oxygen vacancies play a very important role in the ferromagnetism of Sn1-xVxO2. Therefore, the BMP model mediated by oxygen vacancies is suitable for explaining the ferromagnetism in Sn1-xVxO2.B. Quasi-one-dimensional SnO2 nanomaterialsWe synthesized the quasi-one-dimensional SnO2 nanomaterials by a chemical vapor deposition using Au layers as catalyst, and firstly study the growth condition (the growth temperature, the flow rate of O2 and the carrier gas Ar, the thickness of catalytic Au layer and the substrate materials) dependences of morphology, structure and magnetic property. We explored in detailed the origin of d0 ferromagnetism. Our conclusions are as follows:1) The growth temperature has a direct effect on the product morphology. With increased temperature, the form of products changes from nanorods to nanowires, with a diameter increasing with increased temperature. The minimum temperature for producing nanowires is 700℃. All the samples exhibit room temperature ferromagnetism, whose magnetization decreases with increased temperature.2) The oxygen flow has a significant effect on the product morphology. With increased oxygen flow, the material morphology undergoes nanowires, nanobelts, nanoplates, and irregular microsheets. As a result, oxygen flow is a critical factor in fabricating SnO2 nanowires. The ferromagnetism decreases with increased oxygen flow, and disappear when the oxygen flow is up to 30sccm.3) The thickness of the catalytic gold layer has an impact on the nanowire diameters. The thinner the gold layer, the smaller the diameter. Consequently, the size of naowires can be controlled by changing the thickness of gold layer. Moreover, the thickness of gold layer also has an effect on the magnetism, which decreases with increased thickness of the gold layer.4) The flow rate of the carrier gas Ar and the substrate material have no effect on the morphology and magnetism of SnO2 nanomaterials. The flow rate of Ar has a direct effect on the nanowire growth rate, but the substrate only affects the smoothness of the nanowires surface.5) The characterization through XRD, HRTEM, and SAED excludes the existence of impurity phase, which proved that the observed ferromagnetism is intrinsic.6) The direct effect on ferromagnetism by the change of material morphology indicates that the latter has a certain relationship with the former. Based on the PL and XPS measurements, we proved that the ferromagnetism in SnO2 nanomaterials is induced by the oxygen vacancies on the surface.7) We performed the annealing under various temperature and environment to tune the concentration of oxygen vacancies and Sn interstitials. The results show that the ferromagnetism depends on oxygen vacancies but is independent of Sn interstitials.C. Cr-doped SnO2 nanowiresWe successfully fabricated Sn1-xCrxO2-δnanowires by a chemical vapor deposition using Au layers as catalyst. By comparing with the magnetic properties of pure SnO2 nanowires synthesized under the same experimental conditions, we explored the role of the doping on semiconductors. The main conclusions are as follows:1) All the Sn1-xCrxO2-δnanowires exhibit the intrinsic ferromagnetism at room temperature, and the magnetization decreases with the increase of the Cr content. This can be explained as:the distance between Cr3+decreases with increased Cr content, resulting in the antiferromagnetic superexchange through oxygen ions, which leads to the reduction in the magnetic moment per Cr3+.2) On doping the SnO2 nanowires with 1.8at.%Cr enhances the magnetization by 28%. Based on the PL and XPS measurements, we concluded that Cr doping can tailor the ferromagnetism of SnO2 nanowires. The ferromagnetism originates from two contributions:the oxygen vacancies on the sample surfaces, and the ferromagnetic coupling between 3d electrons of Cr ions mediated by the oxygen vacancies (i.e. BMP model).D. Cu-doped SnO2 nanowiresFor the first time, we fabricated Sn1-xCuxO2-δnanowires with different doping level by using Au-catalyst-assisted carbothermal reduction, and performed the investigation on the mechanism of its magnetism and growth. The conclusions are as follows:1) We firstly obtainedSn1-xCuxO2-δnanowires with the intrinsic ferromagnetism at room temperature. The magnetization decreases with increased Cu content. Its mechanism is similar to that of the Sn1-xCrxO2-δsystem.2) For the first time, we fabricated Sn1-xCuxO2-δnanowires, investigated their morphology variations under different reaction time and explored the growth mechanism. The results show that Cu not only functions as a copper source, but also acts as a catalyzer, following the VLS growth mechanism in the process of growing nanowires.