Effects Of Defects On The Optical And Electrical Properties For Several Nanomaterials

Nanomaterials and nanodevices, currently drawn great attention in condensed matter physics and material science, have a great of potential applicaitons in many fields such as optical and electrical devices. Generally speaking, defects can play a very important role in, or even determine, the morphology, structure and optical/electrical properties of nanomaterials. As a consequence, to deep understand the generation mechanism of the defects in nanomaterials as well as their impacts on the corresponding characteristics are prerequsites to make the nanodevices in practice. In this regards, with typical ZnO nanorods and layered material (MoS2 and Bi2Se3) for instances, we focused on the characterization of defects in nanomaterials and their influence on the optical and electrical properties of the nanomaterials. The main contents of the dissertation are outlined as follows:In chapter one, we firstly summaried the fundamental knowledge of defects, and briefly reviewed the structure, the preparation as well as the properties of one-dimensional ZnO and two-dimensional layered materials. Then we introduced the influence of the defects on the optical and electrical characteristics for the nanomaterials. Lastly, we presented the focuses and important resutls of our research.In chapter two, we fabricated ZnO nanorods and ZnO/Al2O3 core/shell nanorods through vapor phase condensation method and atomic layer deposition. We studied how the atomic layer deposition and annealing processes affect the structure and the photoluminescence properties of the nanorods. It is found that, after coverd with thin Al2O3 layer and annealed, both intensities from near band edge emission and defect level emission of the ZnO/Al2O3 core/shell nanorods are enhanced remarkably. More importantly, the salient behavior of negative thermal quenching is clearly observed in the annealed ZnO/Al2O3 nanorods. The result can be well interpreted by a multi-level model through introducing a Al shallow donor state, which is originated from the inter-diffusion between ZnO core and Al2O3 shell during the annealing process.In chapter three, we fabricated the multi-walled MoS2 nanotubes through chemical vapor deposition method and performed photoluminescence measurement for the first time. PL spectra of the nanotubes show the broad and blue shift features as comapred that from planar MoS2 nanosheet. More interestingly, under optical pumping with circularly and linearly polarized light, the nanotube displays high circular and linear polarization even at room temperature, demonstrating the stronger valley polarization and valley coherence in the nanotube than those in planar MoS2. With the help of high resulution TEM characterization, we find both the strain and the chirality existed in the nanotubes. Therefore, we contribute the peculiar optical properties of the nanotues to their uniquely interior strain and chirality structures. Our work gives more insights into the modulation of the band structure of the MoS2 nanotubes and makes them a new research platform for the spin and valley physics.In chapter four, we prepared both spiral-type and smooth Bi2Se3 nanoplates through a polyol synthesis method. It is inferred that the growth of the spiral-type nanoplates is driven by screw dislocation while the smooth ones is obtained in the layer by layer growth mode. The electrical properties of both kinds of the nanoplates were characterized in differernt temperature. It is observed that both nanoplates possess high conductivities and show metallic-like behavior. Compared to the smooth nanoplate, the spiral-type one exhibits the higher carrier concentration, lower electron mobility and higher surface activity. Moreover, with the CAFM measurement on the spiral-type nanoplate, we find that the conductance from the region near the screw dislocation is even higher than that on the terrace. The result implys that, even the dislocation can scatter the carriers and lessen the mobility, it might supply excess carriers to compensate the molibity decreasing and achieve the high conductance. Both the unique structure and electrical properties make the spiral-type Bi2Se3 nanoplates a good candidate for catalysts and gas sensors.In chapter five, we forecast the challenges and opportunities of the defect researches on one-dimensional ZnO and two-dimensional layerd materials. We also present a brief outlook for the ongoing work.