Plasmon enhanced fluorescence from quaternary CuInZnS quantum dots
Introduction
Colloidal semiconductor quantum dots (QDs) are considered extremely promising for applications in many different areas, such as biomedical labeling, solar cells, and photo-electronic devices [1], [2], [3]. To tune the band gap and improve the quantum yield (QY) of QDs, previous researches were mainly focused on the synthesis stage of the QDs by changing relative stoichiometries and reactivity of the various chemical species in QDs and growing a passivated shell over the QDs cores [4], [5], [6]. Other schemes such as QDs being excited by interaction with localized surface plasmons (LSPs) were also performed [7], [8], [9]. LSPs are charge density oscillations confined to metallic nanostructures, which is thought to lead to plasmonic fluorescence enhancement. Metallic nanostructures have long been studied due to their ability to manipulate incident light. It becomes more appealing when applying the QDs to random distributions of metallic nanoparticles or nanoscale roughness in metallic films. However, up to now, most of these methods were realized based on the binary QDs [7], [8], [9]. These QDs generally contain elements that are thought to be detrimental to health and the environment, such as cadmium, lead, etc. Few articles are about the ternary I–III–VI or quaternary systems. Herein, CuInZnS quaternary QDs according to the previous reports were fabricated [3], [4], [5], [10]. The plasmonic silver films (PSFs) on planar substrates with in-homogenous shapes and dimensions were also fabricated through vapor deposition with a post thermal treatment. SP coupling was realized by spin-coating the CIZS QDs to the PSFs without any spacer. Upon proper optimization, such an easy and reproducible method increases the excitation or emission rates of the QDs, thus resulting in a strong enhancement of their fluorescence, as high as 45 folds enhancement in emission intensity was achieved. Compared to other SP coupling strategies, this one is cheaper and easier to be fabricated. The metal-enhanced fluorescence (MEF) offers promise for a range of applications, including LEDs, sensor technology, microarrays and single-molecule studies.
Section snippets
Materials and synthesis of QDs
Octadecene (ODE, technical grade, 90%), indium(III) acetate (In(Ac)3, 99.99%), copper(I) iodide (CuI, purum ≥99.5%), 1-dodecanethiol (DDT, ≥98%), and zinc stearate (technical grade) were purchased from Sigma–Aldrich. Tri-noctylphosphine (TOP, min 97%) was purchased from Strem Chemicals. All the chemicals were used without further purification.
Due to the promising properties of CIZS, here we explored controlled synthesis of CIZS-alloyed QDs using a heat-up and solvent-free method following
Characteristics of the synthesized QDs
Fig. 1a shows the XRD patterns for the structural characterization of the representative CIS and CIZS QDs. There is an obvious shift to larger angles for the CIZS peaks compared to the CIS phase, indicating a smaller lattice constant for CIZS than that of CIS. The X-ray diffraction peaks for CIZS QDs at 28.119°, 46.739° and 55.127° correspond to (1 1 2), (2 0 4) and (3 1 2) planes. The crystal lattice constants are determined to be a = b = 5.505 Å, c = 10.935 Å. The crystallite size of 3.0 nm estimated from
Conclusions
CuInZnS quaternary QDs were fabricated in non-coordinating solvent ODE in the presence of the DDT by a two-step synthesis route. The size of the QDs is tuned via variation of the growth time of the CIS in the first step. The PSFs on silicon substrates with in-homogenous shapes and dimensions were also fabricated through vapor deposition with a post thermal treatment. Through direct spin-coating the CIZS QDs to the PSFs to form a SP coupling system, the QD/PSF hybrid emitter was realized. Such
Acknowledgments
This work was supported by National Key Basic Research Program of China (Grant No. 2011CB925603), Natural Science Foundation of China (Grant Nos. 61290305 and 11374259).
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