â…¡-â…¥ Semiconductor Nanowires: Solution Synthesis, Tunable Emissions And Luminescence Mechanisms

Since 1980 ^th, nanoscience and nanotechnology is fueled by the discovery and design of nanomaterials that can be systematically manipulated so as to exhibit unique properties toward numerous applications. Advances in chemistry and material science have led to the architectural control of nanobuilding blocks（nanoparticles, nanowires（NWs）, nanoribbons, nanoplates）. Semiconductor NWs, with cross-sectional dimensions that can be tuned from 2–200 nm, and lengths spanning from hundreds of nanometres to millimetres. These subwavelength structures represent a new class of semiconductor materials for investigating light generation, propagation, detection, ampli?cation and modulation. After more than a decade of research, NWs can now be synthesized and assembled with speci?c compositions, heterojunctions and architectures. The preparation methods of semiconductor NWs can be simply divided into “vapor phase route” and “solution route”. Compared with vapor phase route, solution-processed NWs have advantageous features of inexpensive, low-temperature（< 350°C）, surface passivation of surfactants（influence the solubility, luminescence characteristics, and electrical transport）, enabling the ability to achieve smaller-diameter NWs with quantum-confinement effects（typically < 10 nm）. However, there are several essential issues and challenges that have to be addressed to promote further progress. For example, the improvement and multifunctional optical properties of NWs is lacking, the research on fundamental optical properties of NWs is lagging behind its development, on the other hand, how to improve the photoluminescence quantum yields of semiconductor NWs is still a great challenge.In view of these problems and challenges, this dissertation focus on solution-liquid-solid（SLS） synthesis, tunable emissions and luminescence mechanism of 1D semiconductor NWs. The main results are summarized as follows:（1） Alloying is an effective strategy for tuning the optical properties of semiconductor materials. In this dissertation, solution synthesis of ultralong（several tens of micrometers） colloidal CdSe_xS_1-x NWs with homogeneously alloyed composition through a SLS method are reported. Colloidal semiconductor NWs are attractive materials with polarized light absorption and emissions. Solution synthesis of ultralong（several tens of micrometers） colloidal CdSe_xS_1-x nanowires with homogeneously alloyed composition through a solution–liquid–solid（SLS） method are reported. By varying the ratio of Se/S, the optical band and their corresponding photoluminescence spectra of the CdSe_xS_1-x NWs can be ?nely tuned from 508 nm（ca. 2.45 eV） to 628 nm（ca. 2.0 eV）. In comparison with the low quantum yields of pure CdS and CdSe NWs（less than 0.1%）, alloyed CdSe_xS_1-x NWs exhibit enhanced bright photoluminescence emission with a narrow spectral width of 25–31 nm and moderate quantum yields up to ca. 8.8%. Moreover, the randomly oriented ensembles on glass wafers show strong polarized emission with polarization values of ca. 0.6 due to the inherent larger aspect ratio. The combination of these characteristics in alloyed NWs sets the basis for the spectroscopic study of 1D excitons, as well as the fabrication of NWs based polarized photonic and optoelectronic devices. To understand the interesting photoluminescence enhancement in colloidal alloyed CdSe_xS_1-x NWs, low-temperature steady-state and time-resolved photoluminescence spectra were applied. In low temperature photoluminescence spectra, the band-edge emission and surface-defect emission of alloyed Cd Se_xS_1-x NWs show different changing trend with the variation of their composition. Moreover, the radiative lifetime for band-edge emission and surface-defect emission reveals an opposite changing trend with the variation of temperature. These ?ndings suggest that the variation of photoluminescence quantum yields with composition is determined by the competition between exciton move and localization. If the carriers are localized in the interior of NWs, the migration of photoinduced excitons to their surface will be prohibited, and more probability for radiative recombination at band edge occurred.（2） Doping is a controllable manner provides a flexible way to turn the optical, luminescent, magnetic, or other physical properties of semiconductor materials. However, because of self-purification effect of crystals, it is still a challenge to achieve doping in semiconductor materials, especially in NWs. Here we report a general and facile solution synthetic strategy to prepare colloidal diluted magnetic semiconductor NWs through SLS doping approach using single-source precursors. On the basis of this strategy, transition metal ions such as Mn²⁺ and Eu³⁺ doped CdS nanowires were successfully synthesized and characterized. The material characterizations demonstrated that the doping process is nucleation controlled. We further investigated the Mn²⁺doping effects on nanowire growth as well as their photoluminescence properties. The Mn²⁺ doped CdS nanowires exhibit photoluminescence emission related to the excitonic magnetic polaron in CdS, single Mn²⁺ ion and Mn-S-Mn centers as well as trap states, evidenced by the time-resolved photoluminescence spectra and magnetic measurements. With the increase of Mn precursor that used in the doping process, the Mn²⁺ related emission becomes more pronounced. By tuning the doping concentration, white emissive doped CdS nanowires were achieved.