| Summary: | 博士 === 國立中正大學 === 化學工程所 === 97 === The research includes the studies of the synthesis of these?Fe2O3 nanowires (NWs) vertically/laterally and explores their applications. Fundamentals of ?Fe2O3 nanomaterial were reviewed in Chapter 1. The structure of ?Fe2O3 NWs and their anisotropic magnetroresistance, field emission, field effect transistor and visible light photodetector applications were systematically study and reported in chapter 2-5, respectively.
In Chapter 2, single–crystalline hexagonal ?Fe2O3 NWs were synthesized on a 50 nm iron thin film by thermal oxidation within 30 minutes at 250oC in air. SEM images show that the diameters of ?Fe2O3 NWs were about 8 – 25 nm, and the lengths were up to a few μm. TEM analysis revealed that the NWs had a hexagonal crystal structure with the growth direction of [110]. Magnetic force microscopy (MFM) analysis of a single ?Fe2O3 NW revealed that the direction of the magnetic domains was along the wire axis at room temperature. In addition, the results of magnetoresistance (MR) revealed that the anisotropic magnetoresistance (AMR) of ?Fe2O3 NWs reached 0.5% at 300 K.
In Chapter 3, ?Fe2O3 NWs were formed by the thermal oxidation of an iron film in air at 350 oC for 10 hours. The rhombohedral structure of the ?Fe2O3 NWs was grown vertically on the substrate with diameters of 8–25 nm and lengths of several hundred nm. It was found that the population density of the NWs per unit area (DNWs) can be varied by the film thickness. The thicker the iron film, the more NWs were grown. The growth mechanism of the NWs is suggested to be a combination effect of the thermal oxidation rate, defects on the film, and selective directional growth. The electrical resistivity of a single NW with a length of 800nm and a diameter of 15nm was measured to be 4.42×103 Ωcm using conductive atomic force microscopy. The field emission characteristics of the NWs were studied using a two–parallel–plate system. A low turn–on field of 3.3 V/μm and a large current density of 10-3 A/cm2 (under an applied field of about 7 V/μm) can be obtained using optimal factors of DNWs in the cathode.
In Chapter 4, an ?Fe2O3 nanobridge (NB) was laterally grown via the one–step thermal oxidation of 150 nm Fe film at 350 oC for 1hr in air atmosphere to form a NB field effect transistor (FET). The diameter of the as–grown NB was 7 nm, with a length of 170 nm. The electrical properties of the individual ?Fe2O3 NB were directly measured by microprobing the NB FET. The results show that the NB demonstrated n–type semiconductive behavior with a conductivity of 1.67 S/cm.
In Chapter 5, a single crystalline ?Fe2O3 nanobridge (NB) was laterally grown between two electrodes by one–step thermal oxidation of 100 nm Fe film at 350 oC in air atmosphere to form a NB photodector. The diameter of the as–grown NB was 8 nm, while the length of the NB was about 240 nm. The photocurrents of an individual ?Fe2O3 NB photodetector were invenstigated with the illumination of the visible light (wavelength: 300nm–800nm). With a light intensity of 0.5 mW/cm2, the photocurrent of the NB photodetector was increased by two orders of magnitude. The rapid photoresponse time (< 20ms), high gain (2.9×107) and high on/off ratio (>12) of an individual ?Fe2O3 NB photodetector can be attributed to the small diameter and high surface-to-volume of the NB. In addition, photocurrent measurements in various ambient (air, Ar and vacuum) demonstrate that the absorption of the oxygen at the surface of the NB can significantly influence the photoresponse of the ?Fe2O3 NB photodetector.
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