The use of metal nanoparticles for surface colorization applications has gathered a lot of interest since its inception and development in the field of plasmonics. However, their commercial viability has been hampered due to current top-down coloring methods. In an attempt to improve their commercial potential, a team of Researchers from Ottawa, Canada, have developed a bottom-up approach using a picosecond laser pulse to produce a full palette of non-luminous colors onto silver, gold, copper and aluminum substrates.
Although recent advances in plasmonics has utilized metal nanoparticles as coloring agents, they have in fact been used for thousands of years to color the glass of, what are now, antiques- the most famous being the Lycurgus Cup, which is a Roman cage cup dating back to the 4th Century.
Upon exposure to optical radiation, metal nanoparticles are known to show scattering properties because of excited plasmons. The manifestation of color through plasmonic effects in metal nanoparticles has gathered interest within the scientific community as the color is long lasting, is an environmentally friendly approach, can be rendered down to the diffraction limit and can be used in any metal coloring application.
The top down approaches used today include laser interference lithography, electron beam lithography, ion beam lithography, milling, hot embossing and nano imprint lithography. Whilst current methods can produce a resolution of around 100,000 dots per inch, the ability to color a large surface has proved problematic and has subsequently stifled the method from reaching commercial continuity.
The team of Researchers have produced a high-throughput and deterministic method to produce an angle-independent, non-iridescent color palette composed of many colors. The Researchers used burst and non-burst picosecond laser methods on unpolished silver, gold, copper and aluminium surfaces, as well as on cm-scale topographic features. The Researchers also tested some silver coins produced by the Royal Canadian Mint, including large coins up to 5 kg with significant topographic features.
Throughout the experiments, the Researchers used a 1064 nm light from a 15 W Duetto (Nd:YVO4, Time-Bandwidth) mode-locked MOPA laser, with FlexBurst software to control the laser pulses between burst and non-burst modes.
The Researchers quantified and characterized the surface coloring using a Chroma meter (CR-241, Konica Minolta), high-resolution scanning electron microscopy (HR-SEM, JSM-7500F FESEM, JEOL) ultraviolet-visible near infrared (UV-Vis-NIR) spectroscopy (CARY7000, Agilent Technologies) and three-dimensional FDTD numerical simulations.
Through these methods, the Researchers identified the formation of random nanoparticles with varied size and separation distributions, of which, are controllable through parameter optimization. The Researchers also deduced this to be dependent upon a single parameter- the total accumulated fluence. The Researchers also found a significant increase in the quality of the coloration when the burst coloring mode compared to the non-burst method.
In (short-time spaced) burst mode, the color saturation (also known as the chroma) was found to increase by up to 70% against other methods and the color lightness range was found to extend by around 60%. The burst mode was also responsible for the introduction of color palettes on gold, copper and aluminum surfaces.
Using the non-burst method, the Researchers managed to demonstrate that a large set of parameters could produce a hue, i.e. a pure color with no tints, so long as the total accumulated fluence remained constant. In this, each individual color was linked to a specific total accumulated fluence value that could be used to decrease the coloring time, or fine-tune each specific hue.
To gain a better understanding of how the colors formed on the surface of the metal(s), the Researchers utilized a computational approach in the form of large-scale computational electrodynamics and large-scale finite-difference time-domain computations.
Using this approach, the Researchers simulated the scattering of different sized nanoparticles, utilizing different periodic distributions and geometrical parameters based on experimental and statistical analyzes. The studies showed that plasmonic resonance produced by the arrangement of the nanoparticles was the main driving force for the color formation. The colors themselves were found to originate from the surface embedment of small and medium sized nanoparticles occupying a random distribution.
In short, the Researchers have shown that the nanoparticle arrangements, particularly those with a medium size, are responsible for the plasmonic effects and subsequent coloring on the metal surface due to the interparticle distance possessing the ability to modify the plasmonic resonance conditions.
The utilization of this new bottom-up method, and its applicability with larger-scale operations has opened an avenue for industrial-scale laser coloring applications, such as coloring for anti-counterfeiting, biosensing, biocompatibility and the decoration of consumer products such as jewels, art, architectural elements and fashion items.
“Laser-induced plasmonic colours on metals”- Guay J-M., et al, Nature Communications, 2017, DOI: 10.1038/ncomms16095
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