A popular application of Quantum Dots (QDs) is the coating of a thin layer of QDs over a surface to obtain surface luminescence at a specific wavelength or combination of wavelengths. When QDs are coated on a transparent planar substrate, blue or ultraviolet (uv) light can be illuminated on the backside of the substrate so that the QDs can absorb the light and down convert it to the longer specific wavelengths emitted by the QDs. In this way one can obtain illumination of high color quality at wavelength(s) of choice in the visible and near infrared regions of the spectrum. Also white light having many different color temperatures can be obtained by using a combination of QDs (e.g red and green) that will mix with light from a blue light source.
This down conversion application is often implemented by using a blue or uv light emitting diode (LED) light source for excitation and has been described in numerous journal articles. The following are examples of reviews or reports in this area:
Demir et al, Quantum dot integrated LEDs using photonic and excitonic color conversion, Nano Today (2011) 6, 632—647.
Yuan and Krüger, Polymer-Nanocrystal Hybrid Materials for Light Conversion Applications Polymers 2012, 4, 1-19
Song and Yang, Efficient White-Light-Emitting Diodes Fabricated from Highly Fluorescent Copper Indium Sulfide Core/Shell Quantum Dots, Chem. Mater. 2012, 24, 1961−1967
A second QD coating application is to pattern a QD coating on a transparent or opaque substrate. Illumination of the QD pattern with uv, blue or white light will produce a fluorescent pattern in virtually any color(s) of choice. Such marking can be decorative, used in signage, or very importantly can be used as an anti-counterfeiting measure to mark documents and product packaging. For example, many drivers licenses are marked in this way using fluorescent dyes. QD marking offers a wider range of colors including near infrared colors that only be detected using a sensitive camera to achieve more covert labeling.
How to Coat QDs on a Surface
A very good method to apply thin coatings of QDs on a surface is to blend the QDs into a polymer or a polymerizable mixture of monomers and to then spin coat or otherwise apply a thin layer of the blended material over the surface of interest. Monomer mixtures can be polymerized (cured) on the surface after application. Using this approach it is very important to assure that the polymer or monomer mixture is lipophilic (non-polar) if organic soluble QDs are being used. Conversely if water soluble QDs are being used, the polymer or monomers must be hydrophilic (water soluble). Polymer/monomer systems that can disperse QDs include but are not limited to: Poly(methyl methacrylate) (PMMA), Poly(lauryl methacrylate) (PLMA), Nylon, Poly(lauryl methacrylate-co-ethylene glycol dimethacrylate (EDGM)) (PLMA-co-EGDM), Cellulose triacetate (CTA), and biphenylperfluorocylobutyl polymer (BP-PFCB).
The review by Yuan and Krüger, cited above, provides references to experimental procedures for dispersion of QDs into several of these different polymers or monomer mixtures.
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