Study Of Ⅱ-Ⅵ Nanostructural Semiconductors By X-ray Absorption Fine Structure

This thesis presents the investigation on nanomaterials by X-ray absorption fine structure (XAFS), combined with X-ray diffraction (XRD), transmission electron microscopy (TEM). It involves three different kinds of nanomaterials: (1)â…¡-â…¥group CdSe semiconductor nanocrystals with controlled optoelectric property; (2) transition metal (cobalt) doped-ZnO nanocrystals with room temperature ferromagnetism; and (3) one-dimensional Se nanotubes with electrochemical hydrogen storage potential. This work provides structural aspect of nanomaterials, and will be helpful to controlled synthesis of nanomaterials as well as to better understanding of their properties.1.Structural evolutions of CdSe nanocrystals in ripening processThe structural evolutions of CdSe nanocrystals (NCs) in the ripening process were investigated by high-resolution transmission electron microscopy (HR-TEM), XRD, and XAFS. A size-selective separation method was used to synthesize CdSe NCs with different grain sizes. The XAFS results indicate that the structural disorderσ_S² of the first nearest neighbor coordination Se-Cd shell in CdSe NCs rises from 0.0005 to 0.0012 and 0.0034 (?) as the size of NCs increases from 1.9 to 2.7 and 3.1 nm. The smallσ_S² of the 1.9 nm NCs unambiguously reveals that at this stage the CdSe NCs are well crystallized and almost free of interior defects. The unusual increase ofσ_S² with size can only be interpreted by the interior defects rather than surface defects of NCs. The interior defects produced in the core of the CdSe NCs through the ripening process are accumulated. This leads to a rapid increase in their structural disorders for the large CdSe NCs.2. Structures and magnetism of Cobalt doped-ZnO nanocrystalsThe structures and magnetism of Cobalt-doped ZnO nanocrystals (Zn_1-xCo_xO, x=0.02, 0.05) were investigated by HR-TEM, XRD, XAFS and SQUID. The Zn_1-xCo_xO nanocrystals demonstrate strong room-temperature ferromagnetism (FM) behavior from SQUID measurement. Although neither secondary phase nor imbedded cluster was detected by XRD and HR-TEM, XAFS has clearly shown the coexistence of metallic Co phase and Co²⁺ ionic phase incorporated in to ZnO matrix. EXAFS fittings indicate that only 64% and 63% of Co has effectively incorporated into wurtzite ZnO crystal lattice for the Zn_1-xCo_xO (x=0.02, 0.05) samples, while the rest precipitated out as Co clusters with average size well below 10 nm. The incorporated Co²⁺ ions in the highly crystalline ZnO nanocrystals did not contribute to FM signals, while the FM was assigned to the Co clustering in the samples. The precipitate of Co nanoparticle would result in RT-FM due to the interaction with the coatings/matrix, even its size is within the critical size for superparamagnetism.3. Growth mechanism of Se nanotubesFluorescence XAFS and TEM were used to study the formation mechanism of trigonal selenium (t-Se) nanotubes synthesized by photothermally assisted solution method. The results demonstrate that the formation process of t-Se nanotubes contains three distinctive stages: (1) reduction of Se⁴⁺ to form amorphous Se (α-Se) phase, (2) partialα-Se crystallized into t-Se nanoparticles, (3) the crystallization of t-Se preferentially along [001] direction to form nanotubes. The formation of tubular structures is the outcome of competition between diffusion rate ofα-Se chains and reaction rate. This competition results in the cone-shaped hollow of inner tubular structures. Light irradiation plays an important role in the formation of tubular structures by accelerating the reaction rate of Se chains.