Characterization and technical studies of semiconductor nanoclusters synthesized within polymer films

Abstract

Semiconductor nanoclusters were synthesized within diblock copolymer films containing carboxylic acid units on one of the blocks. Diblock copolymer films microphase separate during film casting, and yield many different morphologies (spherical lamellar/ cylindrical ) depending on the ratio of the lengths of the two blocks. Metals such as Zn or Pb were selectively sequestered into the acid-containing microdomains by treating the films with aqueous metal salt solutions or with suitable organometallic reagents. Subsequent treatment with H2S formed metal sulfide nanoclusters within the microdomains. Initial experiments focused on PbS nanoclusters synthesized using tetraethyllead as the organometallic reagent PbS is interesting on account of its small effective mass which results in a large quantum size effect. The composite films containing PbS nanoclusters were characterized by X-ray diffraction, X-ray fluorescence, electron microscopy, and UV­Vis spectroscopy. The cluster size and hence its optical properties could be controlled by varying processing parameters such as the reaction temperature, the presence of coordinating bases, and the time of exposure to H2S. A model was developed to explain the effect of these processing pm.meters on the cluster size. The experiments were also complemented by a theoretical study of the electronic structure of PbS nanoclusters. The semi-empirical tight binding method was used to study the dependence of the band gap of PbS nanoclusters on their size. 'The effects of spin-orbit coupling were included, and the calculated band gaps agreed well with previously published experimental values for PbS. Applications of semiconductor nanoclusters in devices have been hindered by surface defects, which greatly reduce the quantum yield for photoluminescence and electroluminescence. This difficulty can be overcome by using doped nanoclusters, where the emitter would be an impurity atom in the nanocluster. We have synthesized manganese-doped and terbium-doped ZnS nanoclusters within diblock copolymer films. The cations (zinc and the impurity atom) were successively loaded from aqueous solutions of their salts. The extent of loading was tracked using ICP discharge emission spectroscopy. Doped nanoclusters were formed by treating the ion-loaded films with H2S. The Mn-doped ZnS nanoclusters have been characterized by UV-Vis and fluorescence spectroscopy. The photoluminescence spectra show the characteristic manganese emission. Excitation spectra, in conjunction with UV-Vis spectra for doped ZnS nanoclusters of different sizes, demonstrated that manganese emission occurs by excitation of the ZnS nanocluster. To-doped ZnS nanoclusters also showed emission characteristic of terbium ions in the cluster.