Synthesis, Growth Mechanism, Micro-structure And Magnetic Properties Of DMS Fe_xSn_1-xO₂ Nanowires

Diluted magnetic semiconductors (DMS) are formed by doping transition metals(TM) atoms in host semiconductor. Much attention has been paid to DMS by research workers because of DMS have the property of semiconductor and magnetism. The most important problem focuses on the origin of magnetism in DMS at present. Up to now most of the effort has been focused on the preparation and characterization of thin films and bulk materials, little work has been done on the one-dimensional DMS nanostructures. SnO₂ is a very interesting oxide semiconductor with a wide band gap of～3.6 eV. It can be used for solar cells, catalysis and gas-sensing applications and so on for its high optical transparency, electrical conductivity and chemical sensitivity.SnO₂ and Fe doped SnO₂ nanowires were successfully fabricated by chemical vapor deposition (CVD) method in this thesis. The X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), selected area electron diffraction (SAED), vibrating sample magnetometer (VSM) and superconductivity quantum interference magnetometer (SQUID) were employed to investigate the microstructure, morphology, magnetic properties of SnO₂ and Fe_xSn_1-xO₂ nanowires. The main results are as below:1. SnO₂ and Fe_xSn_1-xO₂ nanowires were synthesized on the Si wafer by a catalyst assisted CVD method. The diameters of the nanowires are between 70nm to 120nm and the length of the nanowires can reach hundreds of micron.2. The morphology of nanowires is sensitively influenced by the fabrication conditions, especially the flux of O₂.3. The growth mechanism of the SnO₂ and Fe_xSn_1-xO₂ nanowires are VLS and VS mechanism respectively.4. XRD, HRTEM and SAED results indicate that the Fe_xSn_1-xO₂ nanowires are tetragonal rutile structure with a growth direction along (020). There are no peaks of Fe cluster and oxide in XRD. HRTEM image shows clear lattice fringes indicating that the nanowire is single crystalline.5. Field-effect transistor (FET) based on an individual nanowire was fabricated by photolithography and the transport properties have been measured. The device shows typical n-type semiconductor behaviors with the carrier concentration estimated to be 7.6×1017/cm3.6. All of the Fe_xSn_1-xO₂ nanowires exhibit room temperature ferromagnetism (RTFM). The magnetic moment of Fe ion increases monotonically with x, reaches the maximum value 0.72μB at x=0.05 and then decreases. According to XRD, HRTEM and SAED results, it is believed that RTFM is the intrinsic property of the nanowires.