Determination of volume fractions and ligand layer thickness of polymer/CdSe quantum dot blend films by effective medium approximations

Abstract

We investigate hybrid organic/inorganic films using different polymers and CdSe quantum dots (QD) and nanorods (NR) with hexanoic acid (HA)-treated hexadecylamine (HDA) or pyridine as the capping ligands. The volume ratios of the polymer:nanoparticle (NP) blends are studied by spectroscopic ellipsometry and transmission intensity data. Effective medium approximation based on the results of the pristine films is applied. With this routine, the polymer/NP volume ratio of the blend can be identified. In combination with the mass ratio of the components, the mass density of the NP including the inorganic crystalline core and the organic ligand layer is obtained. A geometrical model for QD and NR allows for the estimation of the ligand layer thickness. We find pyridine and HDA after HA treatment to be 0.9 and 0.6 nm on the QD surface, respectively. By contrast, the effective thickness of the organic ligand is 3.0 nm on the investigated NR. In both cases, the organic layer is thicker than a monolayer of the expected pyridine due to the presence of extant synthesis ligands as a result of incomplete ligand exchange. V C 2011 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 50: [75][76][77][78][79][80][81][82] 2012

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“…The calculated organic layers were thicker than the molecular size of pyridine or HA, which conrmed that the ligand treatment was not complete. 121 Considering incomplete ligand exchange, it might be bene-cial to have a thorough investigation of the effect of different ligands (including pyridine, amine, thiol, and acid) on the charge transfer process between the polymer and NCs. Different ligands have different binding affinities with NCs, and hence different abilities to replace the original bulky ligands, which affect the charge transfer process.…”

“…By collaborating with Krüger's group, Meerholz and coworkers further conrmed that ligand treatment is incomplete using a physical method. 121 Different from previous approaches, where experiments were performed on pure NCs, their experiments were performed on polymer:NC blends aer thermal annealing, making their results more relevant to nal devices. They employed spectroscopic ellipsometry and transmission intensity data to determine the volume ratios between polymers and NCs in polymer:NC blends, where an effective medium approximation based on single-component lms was applied.…”