Optical Properties

UV and luminescence spectra of CdS nanoparticles synthesized in G4 PAMAM with hydroxyl and amino groups [98] showed that the dendrimer stabilizes nanoparticles so their optical properties differ from the CdS bulk. Although den-drimer macromolecules filled with nanoparticles are prone to aggregation, storage at low temperature (-10 °C) prevents the aggregation and solutions retain their photoluminescent properties. The dot-dendrimer nanocomposites can be captured in a SiO2 sol-gel matrix to yield a stable, bright blue-emitting glass [79].

Similar to dendrimers, Huang et al. [99] prepared CdS-containing nanocomposites using branched PEI as polymer stabilizer. These nanocomposites also demonstrated blue emission but with lower quantum yield. The authors believe that high local concentration of nitrogen atoms (in den-drimer or PEI) is responsible for formation of small well-defined particles.

Semiconductor nanoparticles can be doped with a second metal which strongly modifies the optical properties. Reference [100] reports synthesis of ZnS (doped with Mn and Tb) in microphase-segregated films of diblock copolymers containing carboxylic groups in one block. These films demonstrated emission which is characteristic for doping ions.

A remarkable nanocomposite was prepared by incorporation of CdTe nanoparticles in thin films made by layer-by-layer assembly of an anionic polyelectrolyte with poly(p-phenylene ethynylene) backbone (aPPE) and poly(allylamine hydrochloride) polycation [101]. Tethering of CdTe was accomplished by using poly(ethyleneglycol)

(PEG) chains with two reactive termini such as f-BOC-NH-PEG-COO-NHS. As found, luminescence spectra after incorporation of particles displays a new peak at 600 nm, identical to the emission peak of CdTe dispersions. Yet the excitation spectrum of CdTe-PEG-aPPE registered at 600 nm reveals a strong new band at 400 nm, which matches the absorption band of aPPE LBL films. This indicates that the light quanta emitted by the nanoparticles originate from the light absorption of the polymer film. The average separation distance between the nanoparticles tethered to the LBL films can be changed by altering the dielectric properties of the solvent affecting PEG tether coiling (water/alcohol mixture). CdTe-PEG-aPPE is an example of an organized nanoparticle system with tunable optical coupling.

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