Study On The Preparation And Composites Of CdTe Nanocrystals

Highly fluorescence (PL) semiconductor nanocrystals have wide applicationsin optoelectronic conversion, sensor, optical display, and especially biologicalimaging. Making these nanocrystals to real-world application, it is required tostabilize them in inert matrices. Attention is paid on water-soluble CdTenanocrystals in this thesis that the preparation and surface modification ofnanocrystals, and the assembly of nanocrystals with polymers for compositefilms, bulks, and solutions are investigated in detail. First, we develop a simple and cheap method to prepare water-soluble CdTenanocrystals by using the materials of Te powder and CdCl2. The Cd-SR/HTe-ratio, the concentration of precursor, the pH of precursor, and the structure ofstabilizer are important in the preparation of CdTe nanocrystals. Based on theseresults, we develop a method to high quality CdTe nanocrystals under the lowprecursor concentration, and middle hydrothermal condition. High temperatureaccelerates the growth rate and full width at half maximum (FWHM) narrowingof nanocrystals. Without any post-treatment, the quantum yields (QY) are up to30 % for green-yellow samples at room temperature. Our approach is a simpleand cheap method to produce high-quality nanocrystals with good biologicalcompatibility. We investigate the influence of surface modification on the spectral propertiesof CdTe nanocrystals for the further assembly of them with polymers. The IV张皓 吉林大学博士学位论文 2004popular method of constructing multilayer films is adopted as a tool tocharacterize surface composition of CdTe nanocrystals. By this method, theinterference of small molecular in solution is avoided without destroying theimportant surface composition of nanocrystals. Combined with XPS and opticalspectroscopy, the correlation between CdTe PL intensity and particle surfacecomposition is analyzed systematically. It is experimentally observed that Cd-SRcomplexes on the surface of the CdTe nanocrystals provide the crucial chemicalpassivation responsible for the high PL intensity of CdTe nanocrystals.Ploy(carboxylic acid) can also enhance the CdTe PL at proper pH range.Systematic pH-dependent photoluminescence and XPS studies havedemonstrated that carboxyl groups have a great influence on bothphotoluminescence and stability of the mercaptocarboxylic acid stabilized CdTenanocrystals. The coordination between the carboxyl groups and cadmium ionscan effectively improve the PL efficiency of the CdTe nanocrystals. But thisenhancement effect is only observed in certain acidic range, which suggests that,the interaction between carboxyl groups and the CdTe particle surface caneffectively diminish the nonradiative channel for electron-hole recombination. In this section, poly(acrylic acid) (PAA) acts as an important medium toenhance the interaction between nonionic copolymers and aqueous CdTenanocrystals. PAA has a strong coordinative interaction with CdTe nanocrystalsin acidic range via the interaction between the carboxyl in PAA and the cadmiumon the surface of CdTe. Meanwhile, the strong H-bonding interaction betweenPAA and pyridine groups in copolymers makes it possible to fabricate depositedfilm. As characterized by optical spectroscopy, IR spectroscopy and TEM, ourresults show that CdTe nanocrystals can be embedded into carbazole-containingcopolymers using layer-by-layer (LBL) assembly technique. Moreover, thephotoluminescence of the resulted films can be tuned by using copolymers withdifferent ratio of pyridine/carbazole and CdTe solutions with different PAAconcentration. A precursor film has been fabricated from CdTe nanocrystals andnitro-containing diazoresin (NDR) using electrostatic interactions and thestandard LBL assembly method, covalent bonds are formed under ultravioletirradiation. The long-term stability of these films has been d...