Green Chemical Routes To Selenium Compound Semiconductor Quantum Dots

Over the past decade, due to quantum size effects, semiconductor quantum dots (QDs, also known as semiconductor nanocrystals) exhibit size- and shape-dependent physical properties, which greatly differ from those of the corresponding bulk materials, and those physical properties could be used in solar cells, light-emitting diodes (LED) and biological labels. Selenides QDs are a very important group of semiconductor QDs, which are widely used in solar cells, light-emitting diodes, biological labels, luminescence devices, laser or infrared detectors, infrared window materials, nonlinear optical materials and so on. There are many factors that will effectively influence the properties of semiconductor QDs, including size, shape, size distribution and phase structure. So we can modify the chemical and physical properties of semiconductor QDs through tuning those factors. The study on the control of size, shape, size distribution, and phase structure of selenide QDs are of great importance in both theory and practice. The most popular route to semiconductor QDs is organometallic precursor method. However, there are still some problems to be solved in organometallic precursor methods, for example, organometallic used as precursor, which is a hazardous, expensive, air-sensitive and pyrophoric material. The coordinating trioctylposhine oxide (TOPO) may generate unknown pollutants, and the trioctylposhine (TOP) is expensive, hazardous, unstable, not an environmentally friendly solvent also. In this dissertation, we reported the preparation of selenide QDs with high quality and good monodispersity through a new "green chemical method", the effects of reaction time, growth temperature, precursor molar ratios on the size, size distribution, shape, and structure of QDs have been investigated. The main achievements are listed as following:(1) A new green oleic acid/N-oleoyl-morpholine system was developed to prepare high quality monodisperse CdSe QDs, using N-oleoyl-morpholine as the solvent of Se powder, which eliminates the need for air-sensitive, toxic and expensive chemical TOP, cadmium oxide as sources, and oleic acid as capping ligands. They exhibit the emission of fluorescence is dominated by band-gap luminescence. The PL spectra are symmetrical, and their full-width at half-maximum (FWHM) is samll. The first absorption peak is very sharp. The synthesized CdSe QDs not only have a high monodisperse size distribution, but also exhibit high-quality optical properties. The optimization of reaction condition such as reaction time, synthesis temperature, and the ratio of Cd/Se were investigated in detail. Smaller CdSe QDs can be obtained at lower reaction temperature (200℃), shorter reaction time (8 s) and larger ratio of Cd/Se (3:1). In addition, high-quality CdSe quantum dots with zinc blende structure were successfully synthesized via this cheaper, greener phosphine-free route, using oleic acid (OA) as a primary capping ligand and benzophenone (BP) as a secondary ligand in the non-coordinating solvent. It has been found that the addition of BP can improve the size distribution (below 10%) of as-synthesized CdSe quantum dots greatly, and the nucleation and growth process can also be well-separated. The phosphine-free route enables us to obtain high-quality CdSe QDs with size ranging from 2.8 to 6.8 nm, and narrow full width of half-maximum between 27 and 35 nm by systematically varying the BP/OA molar ratio, which is comparable with the best values reported in other literature. It has been found that the BP has a neutral terminal oxygen similar to the phosphine oxide group, BP can strongly bind to Se vacancies, so that two or more Cd sites are bridged through the terminal oxygen atom in BP. Hence, BP could prevent the small particles from shrinking and favor the focusing of CdSe QDs sizes.(2) Size- (from 8 nm to 16 nm) and shape- (spherical, octahedral, and cubic) controlled monodisperse PbSe QDs were successfully synthesized by virtue of the green non-TOP route. The XRD and TEM measurement showed that the as-prepared PbSe QDs were rock salt structure, highly monodisperse and well crystalline. UV/VIS/NIR absorption spectroscopy measurement revealed that the band gap energy (0.73 eV) of the as-prepared PbSe QDs are blue shifted as compared to the band gap of bulk PbSe (0.28 eV), indicating the quantum confinement effect. The rock salt structure spherical, octahedral, and cubic PbSe QDs were successfully synthesized by varying the reaction from 100℃to 160℃in the non-coordination solvent, using PbO was used to Pb precursor, environmentally friendly N-oleoyl-morpholine as the solvent of Se, using oleic acid (OA) as capping ligands. The crystal size can be tuned in a range from 8 nm to 16 nm by varying the reaction time or growth temperature. Based on the evidence of TEM images, the mechanism of PbSe QDs evolution from spherical to cubic structure has also been discussed. We found that the growth temperature played an important role in the morphology of PbSe QDs. For rock salt structures PbSe QDs, the size-dependent sphere to cube transition is believed to occur in a high-temperature regime. The growth rate in the <111> direction is higher than that in the <100> direction because of the higher surface energy of {111} facets than {100} facets. This favors the growth of the {100} facets leading to a cubic morphology and lowest total surface energy.(3) Green chemical route to preparing monodisperse QDs was extended to ternary QDs. Chalcopyrite tetragonal phase CuInSe₂ QDs have been successfully synthesized by this way, using inorganic salt CuCl, InCl₃ as precursor, and oleylamine as capping ligands at low reaction temperature. It was proposed that the Ostwald ripening induced the morphology evolution of the CuInSe₂ QDs. The trigonal pyramidal CuInSe₂ nanocrystals have been synthesized by the precise control the reactants and reaction temperature, It was found that the surface facet polarity and stability of the (112) and (114) plane determined the nanocrystals shape. Oleylamine molecules can preferentially absorb on the {001} planes of hexagonal CuInSe₂ and the growth of the {001} planes is inhibited, resulting in the formation of CuInSe₂ nanodisk. CuInSe₂ hexagonal nanorings have been synthesized due to the etching effects of oleylamine on the {001} plane, the formation process of the CuInSe₂ hexagonal nanorings can be proposed as a coordination-assisted selective dissolution mechanism.