Lead Chalcogenide IV-VI Semiconductor Nanocrystal: Synthesis, Assembly And Properties

With the development of science and technology, more and more researchers are paying attention to the nano-sized materials. Their unique size-tunable physical and chemical properties differ greatly from the corresponding bulk materials because of quantum-confinement effect. In practice, to fulfill the extensive applications of these excellent nanomaterials, much effort tends to utilize them as ideal building blocks for self-assembling two and three dimensional (2D and 3D) superlattice structures. These novel"artificial solids"would provide tunable properties determined both by the properties of individual nanoparticle constituents and the collective physical properties of the superlattice. Well-defined ordered solids prepared from tailored nanocrystalline building blocks provide new opportunities to optimize and enhance the nanomaterial properties and performance. The achievement of this goal, initially requires the building block NCs to be synthesized with quite uniform size and shape. Although great progress on the synthesis of NCs has been made in the last two decades, the design and construction of these NCs superlattices is still a great challenge in material science. In this thesis, we focus on lead chalcogenide IV-VI semiconductor nanocrystals (including PbS, PbSe, and PbTe), discuss their synthesis methods, study the processe and mechanism of forming these NCs superlattices; we describe a facile method at room temperature to synthesize PbS NCs ,3D PbS nanoflowers, and PbS single crystal hollow nanostructures with tunable morphologies; study the stability of and molar extinction coefficient of PbSe semiconductor NCs.We presented a facile and efficient route to prepare single component NP superlattices. Mutual transformation between random PbSe NPs and their well-ordered superlattices was unified by a proposed model of ligand chain's configuration. When ligand chains were disordered at RT, PbSe NPs were random and monodisperse; comparatively, the ordered chains at RT would correspond to periodic superlattice structures obtained. The simulation results of classical molecular dynamics indicated that the configuration of capping ligand chains was sensitive to temperature; with the increase of temperature the chains would become more disordered. These theoretical results were consistent with our experimental phenomena. Employing the model and simulation presented here, we are extending our work to large-scale single and binary NPs superlattices.Using the experimental method and theoretical results above mentioned, size- and shape-tunable lead chalcogenide IV-VI (PbS, PbSe and PbTe) NCs could be synthesized by one-pot solvothermal method. The subsequent natural cooling process in the reaction vessel made these NCs capped with ordered ligand chains, thereby spontaneously assembling into highly regular superlattice patterns on TEM grids without any other post-synthetic procedure. This spontaneous assembly was attributed to entropy driven force and solvent evaporation-induced ligand-ligand VDW interaction. These results further proved our configuration model of ligand chains. It is also believed that this established method and ligand chains model can significantly be extended to control the growth of other NCs and fabricate their superlattices.We report a facile method at room temperature to synthesize monodisperse PbS nanoflowers, which is attributed to the coexistence of two types of amines with different-length alkyl chains and different steric hindrance. Furthermore, during the increase of reaction temperature and time, the intraparticle ripening drives these prepared nanoflowers to evolve into single crystal hollow nanostructures with different morphologies (sphere, cuboctahedron, cube, and tube/rod).The stability of PbSe semiconductor nanocrystals over several conventional physical conditions has been systematically studied. PbSe semiconductor nanocrystals under air exposure exhibited instability dependent on particle concentrations, particle sizes, and light conditions. These air-contacted instability trends were due to the These novel PbS nanostructures may have significant scientific and technological applications. destructive oxidation and the kinetically collisioninduced decomposition, which differed from the traditional thermodynamic mechanism. In contrast, under inert nitrogen gas, the absorption and photoluminescence properties of the PbSe semiconductor nanocrystals were preserved even with UV irradiation or upon being heated at mild temperatures.The molar extinction coefficients of PbSe nanocrystals at the first excitonic absorption peak have been determined by utilizing TEM, AA spectrometry, and UV-vis-NIR spectrophotometry. The large number particle statistics and our proposed calculation approach have allowed us to precisely find out the particle size and the total atom number for each standard PbSe nanocrystal sample, respectively. The experimental results have demonstrated that the individual PbSe nanoparticle is composed of a PbSe core terminated by an extra layer of Pb atoms. This property leads to a sizedependent Pb/Se atomic ratio in these nonstoichiometric PbSe nanocrystals. By taking account of the surfaceterminated Pb atoms, the size-dependent PbSe nanocrystal molar extinction coefficient is proportional to ~ 2.5 orders of the nanocrystal diameter.In the present work, the structure and optical properties of PbSe NCs were studied as a function of pressure. For PbSe NCs, both absorption and PL spectral exhibit a red-shift and structure transform form B1 to B2 with increasing pressure.