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Fluorescent Metal-Semiconductor Hybrid Structures by Ultrasound-Assisted In-Situ Growth of Gold Nanoparticles on Silica-Coated CdSe-Dot/CdS-Rod Nanocrystals Xiao Tang, Elvira Kröger, Andreas Nielsen, Simon Schneider, Christian Strelow, Alf Mews, and Tobias Kipp Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.8b04233 • Publication Date (Web): 06 Dec 2018 Downloaded from http://pubs.acs.org on December 6, 2018
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Chemistry of Materials
Fluorescent Metal-Semiconductor Hybrid Structures by Ultrasound-Assisted In-Situ Growth of Gold Nanoparticles on Silica-Coated CdSe-Dot/CdS-Rod Nanocrystals Xiao Tang, Elvira Kröger, Andreas Nielsen, Simon Schneider, Christian Strelow, Alf Mews, and Tobias Kipp*
Institute of Physical Chemistry, University of Hamburg, Grindelallee 117, 20146 Hamburg, Germany. *
[email protected] Abstract Gold nanocrystals (AuNCs) were grown on the surface of silica coated CdSe-dot/CdS-rod core/shell nanocrystals by reduction of Au3+ ions in polyethylene glycol under ultrasonic irradiation. The polyethylene glycol not only prevents the penetration of gold ions or precursor molecules into the silica shell, but it also acts as the reducing agent for Au3+ ions. The silica shell’s surface promotes the heterogeneous nucleation of gold nanocrystals, while the ultrasonic irradiation accelerates and enhances the gold nucleation on the silica surface, and ensures the formation of AuNCs with a relatively narrow size distribution. The plasmon-exciton interaction in these metal-semiconductor hybrid systems leads to decreased fluorescence lifetimes and strongly reduced fluorescence blinking of individual hybrid structures.
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Introduction In the past two decades, hybrid nanostructures consisting of fluorescent semiconductor nanocrystals (SNCs) and noble metal (e.g. Ag, Au, Pt, Al) nanocrystals (MNCs) have received extensive attention due to their unique properties and various potential applications.1–6 In general, these hybrid nanostructures can be divided into two categories with respect to the underlying interaction mechanisms between the SNCs and the MNCs, mainly determined by geometric parameters. First, when SNCs are directly attached to MNCs, a photo-induced charge separation can take place at the semiconductor-to-metal interface, which is of interest for the design of next generation photovoltaic and photo-catalyst devices. Here, the SNCs absorb energy that is then transferred into electric energy or catalytic driving forces.7–9 Second, when SNCs and MNCs are positioned in a close but finite distance, the coupling of the exciton dipole in the SNCs with the electromagnetic field of the localized surface plasmon resonance (LSPR) of MNCs can dominate the interaction. Such interaction has been demonstrated to result in several phenomena including increased recombination rates, enhanced absorption cross sections, and shifted emission spectra, all of them being of interest for applications in light-emitting devices and for biological imaging.10– 13
In light-emitting hybrid nanostructures, the interaction between SNCs and MNCs is strongly depending on the distance. For instance, when SNCs are placed in very close proximity to MNCs, due to the energy transfer the non-radiative recombination rate of the SNCs is increased more significantly compared to the radiative one, resulting in an efficient quenching of the fluorescence of the SNCs.9,14,15 On the other hand, when SNCs are placed far away from the MNCs, the coupling between SNCs and MNCs becomes ignorable since the intensity of the LSPRinduced electromagnetic field decays exponentially with the distance from the metal surface.16,17
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Chemistry of Materials
Consequently, the design and the synthesis of light-emitting hybrid semiconductor-metal nanostructures require a high accuracy. The distance between SNCs and MNCs can be adjusted by a transparent and dielectric spacer of controlled thickness. In previous publications, polymers, biomolecules, and SiO2 have been extensively used as the spacer in constructing various hybrid nanostructures.18–20 Typically, either SNCs or MNCs are first coated with the spacer material before they are conjugated via electrostatic forces with the corresponding pre-synthesized counterpart, i.e., with MNCs and SNCs, respectively. Such a method is promising in controlling both, the size and the number of the respective nanocrystals in each SNC/MNC hybrid structure, because detailed characterizations can be routinely performed on both kinds of readily synthesized nanocrystals before the conjugation process. However, these synthesis methods require very special conditions such as distinct concentration ratios or very narrow pH ranges
21,22
and often require additional ligand
transfer processes because the conjugation process can only be performed in water 21. The direct in-situ growth of MNCs onto predefined spacer-covered SNCs offers an alternative approach to the conjugation process for the fabrication of hybrid nanostructures. This approach has frequently been used for the synthesis of aforementioned first-category hybrid nanostructures with MNCs directly on SNCs by simply immersing SNCs in a metal-precursor solution to let MNCs in-situ grow in a continuous reaction.7–9 However, for the second-category (i.e. light-emitting) hybrid structures, the in-situ growth of MNCs on spacer-covered SNCs is more difficult. A complete in-situ growth has not been accomplished so far, whereas a combination of pre-synthesized metal clusters as seeds, and in-situ growth to form a continuous metal shell has been reported recently.4,23 Using this approach, it was shown that spherical 30-nm-diameter core/thick-shell SNCs (core diameter of 3 nm) covered with a thick silica shell of 35 nm can be
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additionally covered with a thick continuous gold shell of up to 20 nm to form a plasmonic resonator leading to non-blinking nanocrystals.4 For the synthesis, the silica shell was firstly functionalized via a sticky polymer (poly(1-vinylimidazole-co-vinyltrimethoxysilane), PVIS) to attach the gold seeds from which the gold shell could subsequently be grown upon reduction of HAuCl4.4,23 In this paper, we report on the in-situ preparation of gold MNCs (AuNCs) onto elongated semiconductor dot-in-a-rod nanocrystals (SDRs) covered with a thin (
