Photo-grade plasmonic colour printing: back to silver
In a paper titled “Ultrafast Light-Controlled Growth of Silver Nanoparticles for Direct Plasmonic Color Printing” published in the ACS Nano journal, the researchers detail the fabrication of plasmonic structures at large scale. The actual substrate consisted of a titanium dioxide (TiO2 photocatalyst) layered quartz plate, put in contact with a solution of silver nitrate.
engineering of silver nanoparticles on a TiO2-capped
quartz substrate.
In order to “reveal” specific plasmonic colours, the researchers used a 1920×1080 pixels digital micromirror device and specially-made projection optics to dynamically project predefined UV patterns on the photocatalyst layer, which initiated and regulated the reduction of silver salts into silver nanoparticles of predetermined sizes.
Because each pixel of the UV pattern can be individually controlled (both in the time and spatial domains), the researchers were able to create user-defined micrometer-scale patterns of precisely size-controlled silver nanoparticles, with each specific size distribution yielding a specific plasmonic colour.
The authors were able to obtain a whole palette of colours with a reflection spectra tuneable from light yellow (426nm with nanoparticles in the 50nm range), yellow, brown, magenta, dark blue, to royal blue (667nm with nanoparticles in the 200nm range) when different exposure doses were applied to print the silver nanoparticles.
In contrast with costly and low-throughput E-beam lithography, focused ion beam milling or direct femtosecond laser writing that have been used so far to create nanopatterns for plasmonic colour generation, the new method uses a readily available high-speed DMD (running at up to 20kHz) and only requires a very simple substrate preparation.
This allows the rapid fabrication of large-area plasmonic surfaces with multiscale customized structures (i.e., nanometer-scale dimension-controlled silver nanoparticles and micrometer-scale user-defined patterns) for direct colour printing.
As a demonstration, the authors converted a bitmap image (a photo of the Hong Kong Polytechnic University administration building) into a UV exposure dose map they could use to print a plasmonic colour image of the building. Interestingly, here the grey-scale equivalent of black-and-white photography translates into different hues, from faint yellow to dark blue, giving the image a Warholesque tone.
The AgNP-based colour images were proven to be very stable in air at room temperature, even after being stored unsealed for over a year. The researchers are looking forward to potential industrial collaborators to commercialize the technology.
Hong Kong Polytechnic University – www.polyu.edu.hk
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