On the Incorporation Mechanism of Hydrophobic Quantum Dots in Silica Spheres by a Reverse Microemulsion Method

Abstract

In this work, we show strong experimental evidence in favor of a proposed incorporation mechanism of hydrophobic semiconductor nanocrystals (or quantum dots, QDs) in monodisperse silica spheres (diameter ∼35 nm) by a water-in-oil (W/O) reverse microemulsion synthesis. Fluorescence spectroscopy is used to investigate the rapid ligand exchange that takes place at the QD surface upon addition of the various synthesis reactants. It is found that hydrolyzed TEOS has a high affinity for the QD surface and replaces the hydrophobic amine ligands, which enables the transfer of the QDs to the hydrophilic interior of the micelles where silica growth takes place. By hindering the ligand exchange using stronger binding thiol ligands, the position of the incorporated QDs can be controlled from centered to off-center and eventually to the surface of the silica spheres. The proposed incorporation mechanism explains how we can have high control over the incorporation of single QDs exactly in the middle of silica spheres. It is likely that the proposed mechanism also applies to the incorporation of other hydrophobic nanocrystals in silica using the same method. In conjunction with our findings, we were able to make QD/silica particles with an unprecedented quantum efficiency of 35%.

Solar Spectrum Conversion for Photovoltaics Using Nanoparticles

“…Another strategy could be the use of so-called type II QDs, as their Stokes' shift could be very large (~300 nm), however, but their stability and QE are not good enough yet. Stability could be improved using multishell QDs (Koole et al 2008), while interfacial alloying can be optimized to obtain type II QDs with desired properties, i.e., a Stokes' shift of ~50-100 nm, without spectral overlap (Chin et al 2007). An different approach was presented recently that employs resonance-shifting to circumvent reabsorption losses (Giebink et al 2011).…”

“…phase separation, agglomeration of particles leading to turbid plates, and luminescence quenching due to exciton energy transfer (Koole et al 2006). They have synthesized QDSCs using CdSe core/multishell QDs (Koole et al 2008) (QE = 60%) that were dispersed in laurylmethacrylate (LMA), see also Lee et al (2000) and Walker et al (2003). UV-polymerization was employed to yield transparent PLMA plates with QDs without any sign of agglomeration.…”

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“…The hydrophobic Fe 3 O 4 nanoparticles were silica coated by a modified reverse microemulsion procedure. [25][26][27] In a typical procedure, 10, 20, 60, and 300 mL of Fe 3 O 4 nanoparticles were dispersed in 60 mL of cyclohexane. The non-ionic surfactant TritonX-100 (1.12 mL), NH 4 OH (30 wt %, 152.8 μL), 1-octanol (400 μL), and tetraethyl orthosilicate (200 μL) were then added into each of the cyclohexane solutions containing dispersed Fe 3 O 4 nanoparticles.…”

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“…[44] The reverse microemulsion process was also used by other research groups. Meijerink et al [45] elucidated the mechanism of incorporating hydrophobic quantum dots into monodisperse silica spheres. In water-in-oil reverse microemulsion system, the hydrolyzed TEOS had a high affinity for the quantum dot surface for replacement of hydrophobic amine ligands, which enabled the transfer of the quantum dots to the hydrophilic interior of the micelles where silica growth occurred.…”