Chemical Synthesis And Structure Identification Of Two-Dimensional Colloidal Semiconductor Nanocrystals

Two dimensional (2D) nanomaterials, namely a single-crystalline structure with one or few atomic layers on their thickness direction and bulk-like extension on the lateral dimensions, have attracted extensive attention in recent years due to their unique properties. As an important part of the 2D namomaterials, ID semiconductor nanocrystals develop rapidly recently. Compared to the zero-dimensional (so-called quantum dots) and one-dimensional semiconductor nanocrystals (so-called quantum rods and wires), ID semiconductor nanocrystals only with one dimension----the thickness direction----in quantum confinement regime, and their diverse properties are usually determined by their thickness.The thesis shall discuss possibilities to synthesize both wurtzite and zinc-blende CdSe 2D nanocrystals with nearly identical thickness in the same simple reaction system. In literature, wurtzite CdSe 2D nanocrystals only could be synthesized in amine solvent, and we first demonstrate that they could also be obtained in non-amine solvent. Through the UV-Vis spectrum and transmission electron microscope, the formation mechanism of wurtzite CdSe 2D nanocrystals were revealed, and the growth of wurtzite 2D nanocrystals could be accelerated or suppressed under different reaction conditions. Furthermore, we also confirmed that formation of zinc-blende CdSe 2D nanocrystals could be strated with seeds, then extended laterally by continuous reaction of precursors.In the second part, we afford a method which could be universally applicable for the structure identification of 2D nanostructures. This quantitative simulation method can readily reveal their most important structural information, such as thickness, crystal orientation. Zinc-blende CdSe 2D nanocrystals with four different thicknesses were synthesized, and then characterized by the powder X-ray diffraction. Because of their discrete nature of thickness and flat basal planes, the simulations of diffraction patterns were carried out through the layer by layer fashion. Detailed analysis, including both distinction between contributions from different elements and monolayer pair interactions along the thickness direction, discovered the structure information along the thickness direction. Simulation revealed that the thickness direction of zinc-blende 2D nanocrystals should be [001] axis, [111] axis as the thickness direction is either non-existence or with a tiny fraction. The thickness of zinc-blende 2D nanocrystals possessed a half-integer model, the four thicknesses were 2.5MLs,3.5Mls,4.5MLs and 5.5MLs. Results also indicate that the very thin thickness yet atomically smooth basal planes may actually require adoption of a different unit cell from the corresponding bulk crystals with the same internal atomic packing. For instance, CdSe 2D nanostructures with their internal atomic packing similar to the bulk zinc-blende lattice (face-centered cubic) should be treated as tetragonal lattices with its lattice parameter c equals to the distance between two basal planes.This method can also be applied to reveal the thickness and orientation of wurtzite 2D nanocrystals both synthesized in amine solvent or non-amine solvent.