Investigation On Synthesis And Optical Properties Of Chalcogenides Nanocrystals

In this dissertation, the work is mainly focused on the synthesis and optical properties of chalcogenides nanomaterial fabricated in aqueous and oil phase. The nanomaterials are characterized by the measurement of TEM,HRTE2M,XRD,XPS,FTIR. We also studied the effect of the experimental conditions on optical properties of nanomaterials and the growth mechanism.1 CdTe nanocrystals was synthesized in aqueous solution with thioglycolic acid (TGA) as stabilizer, and the size is less than 10nm. The optical properties of products can be influenced by the reaction conditions. The emission peak shifts to long wavelength with increase in reaction time,PH value and the ratio of Te/Cd. Basing on the investigation on TGA-stabilized system, L-cysteine hydrochloride(L-Cys), 3-mercaptopropionic(MPA) and 1-thioglycerol(TG) were chosen as the stabilizers to investigate the effect of the stabilizers on the morphology, growth, stability and optical properties. CdTe nanocrystals capped with different stabilizers exhibit distinct UV-Vis absorption spectra and PL spectra. It is due to the different functional groups and stereo-hindrance effect. UV-Vis absorption spectra and PL spectra from CdTe nanocrystals obtained using L-Cys as the stabilizer shift to the longest wavelengths. That using TG as the stabilizer displays the shortest wavelengths. Besides sulfhydryl group (-SR) of all stabilizers can combine with Cd2+to form complexes, the other substituents of the stabilizer also affect the size and optical properties of CdTe nanocrystals. Moreover, the stability and morphology can also been effected by the property of stabilizer. The TGA-stabilized nanospheres are more stable than L-Cys-stabilized nanorods.2 We successfully synthesized ZnxCd1-xTe alloyed nanocrystals in aqueous solution with TGA as the stabilizer, at lower temperature by using a facile route. We studied the effect of the reaction conditions on optical properties of alloyed nanocrystals and probed into their application in photovoltaic devices.(?)ZnxCd1-xTe alloyed nanocrystals with different compositions were synthesized in aqueous solution with thioglycolic acid (TGA) as stabilizer. The synthesized nanocrystals were characterized by XRD, TEM, HR-TEM and XPS. The structure of alloyed nanocrystals was confirmed by Vegard law. The ZnxCd1-xTe alloyed nanocrystals have a good dispersity with typical size of 5.6nm. (?)Composition-dependent absorption and PL spectra shift to short wavelength with increasing Zn content due to the change of band gap. The highest PL quantum yield (QY) of ZnxCd1-xTe alloyed nanocrystals is achieved when the concentration of Zn is 30%. The particle size increases with raised refluxing time and synthesis temperature, which led to the red shift of absorption and PL spectra. All the samples display a narrow symmetric-band and few electronic defect PL properties.(?) We blend the alloyed nanocrystals (<10nm) with the MEH-PPV:C6o to prepare photovoltaic devices. The structure of device is ITO/PEDOT:PSS/MEH-PPV:C6o(+ZnCdTe)/Al. Comparing the spectral response of photocurrent of the MEHPPV:C6o(+ZnCdTe) nanocomposite device with that of the devices based on MEH-PPV:C6o and pristine MEH-PPV, one can find that the nanocomposite device exhibits an enhanced photocurrent. When the weight ratio is not higher than 40%, the short current density, the open-circuit voltage and the power conversion efficiency increased with the weight ratio of ZnCdTe nanocrystals raised. The characteristics of the devices degraded when the concentration of ZnCdTe nanocrystals is too high up to 70%. The power conversion efficiency is doubled and the short current density is close to triple by blending ZnCdTe nanocrystals with the concentration of 40%.3 Mono dispersed copper(I) sulfide (Cu2S) nanodisks with high quality were synthesized by using a simple one-pot colloidal process, in which no pre-prepared organometallic precursors are required.(?) We studied the growth process of Cu2S nanocrystals at low and high temperature, respectively. Early product was characterized by XRD and FTIR and confirmed by theoretical arithmetic. Copper thiolate forms at the beginning of the reaction which effectively acts as a precursor whose decomposition leads to further nucleation and growth of Cu2S nanocrystals. That tiny naodots(3-6 nm) can gradually turn into nanodiks with the reaction temperature or time increased.(?) The diameter, thickness, aspect ratio, and optical property of the Cu2S nanodisks can be adjusted systematically by changing the reaction time, the amount of surfactants, the concentration of the precursors and the frequency of multiple injections. The temperature can affect the stability of the intermediate complexes, the amount of monomer species, the mass transfer rate of the monomers to the surface of nanocrystals and the rates of both nucleation and growth. The size of nanocrystal is therefore various at different reaction temperature. Both TOPO and dodecanethiol are surfactants that direct the anisotropic growth of Cu2S nanodisks. As the amount of TOPO is increased, the size of nanodisks raised accordingly. Increasing the concentration of the precursors leads to still monodisperse samples with smaller size, which is consistent with the classic LaMer model. The adjustable range of size can be widened efficiently by multiple injections. The band gap of as-synthesized Cu2S nanodisks can be tuned in the range of 1.36-1.53 eV. The nanodisks show a broad absorption band, making them potential candidates for applications in photovoltaic devices.4 Wurtzite CuInS2 nanocrystals with different morphologies were synthesized by optimize the conditions containing the stabilizer, solvent, reaction temperature and precursors.(?) Using the inorganic metal salts as the precursors, the addition of TOPO before the injection of dodecanethiol is unfavorable for the formation of CuInS2 nanocrystals. Wurtzite CuInS2 nanocrystals can be synthesized with the addition of TOPO after the injection of dodecanethiol. CuInS2 nanoparticles can turn into large nanodisks with the increased injected temperature of dodecanethiol from 60℃to 160℃without the addition of TOPO, while the structure of CuInS2 nanocrystals can be changed from tetragonal to hexagonal pattern.(?) Using inorganic metal salts as the precursors, wurtzite CuInS2 nanodisks can be synthesized with oleylamine as the stabilizer and ODE as the solvent. The size of nanodisk increases with the raised reaction temperature.(?) Using the organic metal salts as the precursors, CuInS2 nanodisks can be achieved by the injection of dodecanethiol at 160℃with oleylamine as the stabilizer and solvent. But CuInS2 nanoparticles are synthesized with good dispersity when dodecanethiol is mixed with organic metal salts at room temperature.(?) Using organic metal salts as the precursors, the morphologies of CuInS2 nanocrystals transform in the orders of tadpole-like structure, nanowires and nanorods as the increase of temperature with dodecanethiol as the stabilizer and solvent. We can effectively control the morphology by change the reaction temperature. We studied the "oriented attachment" machanism at different temperature. All one-dimensional CuInS2 nanocrystals are assigned to hexagonal wurtzite pattern. The absorption spectra can cover from visible to infrared region, which make the materials have potential applications on photovoltaic devices.