Above -The nanospheres in a methanol suspension have different colors than when applied to a surface as a monolayer. The Kobe University researchers explain, “This is due to the multiple scattering, i.e., blue light subsides during consecutive scattering by absorption, while red light survives.” © FUJII Minoru (CC BY)
A scanning electron micrograph of the nanosphere monolayer shows almost perfectly round particles of uniform size and only small regions of voids or agglomerates. © FUJII Minoru (CC BY)
An object has color when light of a specific wavelength is reflected. With traditional pigments, this happens by molecules absorbing other colors from white light, but over time this interaction makes the molecules degrade and the color fades. Structural colors, on the other hand, usually arise when light is reflected from parallel nanostructures set apart at just the right distance so that only light of certain wavelengths will survive while others are cancelled out, reflecting only the color we see. This phenomenon can be seen in wings of butterflies or feathers of peacocks, and has the advantage that the colors don’t degrade.
But from an industrial point of view, neatly arranged nanostructures cannot be painted or printed easily, and the color depends on the viewing angle, making the material iridescent.
Kobe University material engineers FUJII Minoru and SUGIMOTO Hiroshi have been developing an entirely new approach to producing colors. They explain, “In previous work since 2020, we were the first to achieve precise particle size control and develop colloidal suspensions of spherical and crystalline silicon nanoparticles. These single silicon nanoparticles scatter light in bright colors by the phenomenon of ‘Mie resonance,’ which allows us to develop structural color inks.” With Mie resonance, spherical particles of a size comparable to the wavelength of light reflect specific wavelengths particularly strongly. This means that the color that mainly comes back from the suspension can be controlled simply by varying the size of the particles.
In their work now published in the journal ACS Applied Nano Materials, Fujii and Sugimoto demonstrate that the suspension can be applied to surfaces and will thus coat the underlying material in a form of structural color that does not depend on the viewing angle. This is because the color is not produced by the interaction of light reflected from neighboring structures as with “traditional” structural colors, but by its highly efficient scattering around individual nanospheres. Sugimoto explains another advantage: “A single layer of sparsely distributed silicon nanoparticles with a thickness of only 100-200 nanometers shows bright colors but weighs less than half a gram per square meter. This makes our silicon nanospheres one of the lightest color coats in the world.”
ACS Applied Nano Materials – Monolayer of Mie-Resonant Silicon Nanospheres for Structural Coloration
Abstract
Structural coloration of a monolayer of Mie-resonant silicon (Si) nanospheres (NSs) produced by a solution-based process is studied. It is shown by simulation that a monolayer of hexagonal close-packed Si NSs exhibits size-dependent structural color with a peak reflectance of ∼50%. The peak reflectance can be increased to over 90% by introducing spaces between the Si NSs. The high reflectance despite the small coverage is due to the very high scattering efficiency of Mie-resonant Si NSs. Monolayers of densely packed Si NSs are produced from Si NS suspensions by the Langmuir–Blodgett method. The monolayers exhibit size-dependent structural color with a peak reflectance of 30–50%. The color is very insensitive to the viewing angle, and the angle dependence of the reflectance spectra is very small. The peak reflectance is increased by increasing the distance between the NSs by partially oxidizing the layers. The results demonstrate that iridescence-free structural coloration of a substance is possible by a layer of Si NSs much thinner than the monolayer, i.e., by sparsely scattered Si NSs
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The Kobe University team used computational simulations to explore the properties of the ink under different circumstances, such as by varying the size of the particles and the distance between them, and then confirmed their results experimentally. They found that, contrary to intuition, the reflectance was highest when the individual particles were separated instead of when tightly packed. The authors explain, “This high reflectance despite small coverage of the surface by the nanospheres is due to the very large scattering efficiency. The requirement of a very small amount of silicon crystals for coloration is an advantage in the application as a color pigment.”
After further development and refinements, they are expecting interesting applications of their technology. Sugimoto explains, “We can apply it to the coating of, for example, airplanes. The pigments and coatings on an airplane have a weight of several hundreds of kilograms. If we use our nanosphere-based ink, we might be able to reduce the weight to less than 10% of that.”
Brian Wang is a Futurist Thought Leader and a popular Science blogger with 1 million readers per month. His blog Nextbigfuture.com is ranked #1 Science News Blog. It covers many disruptive technology and trends including Space, Robotics, Artificial Intelligence, Medicine, Anti-aging Biotechnology, and Nanotechnology.
Known for identifying cutting edge technologies, he is currently a Co-Founder of a startup and fundraiser for high potential early-stage companies. He is the Head of Research for Allocations for deep technology investments and an Angel Investor at Space Angels.
A frequent speaker at corporations, he has been a TEDx speaker, a Singularity University speaker and guest at numerous interviews for radio and podcasts. He is open to public speaking and advising engagements.
Fast forward 20 years…
“If you applied silicon nanoparticle-based coatings and developed silicosis or lung cancer, contact the attorneys of…”
Non-fading? Bring it on Lowes. Painting the house every few years is a pain.
Why say ‘structural’?
That would imply that the paint contributes one or more of toughness, rigidity, ductility, tensile/compressive strength attributes… or something?
Maybe just belly-aching… but please lets not throw around engineering terms without some kind of quantitative performance improvement…
Because that is the proper term and description. Look it up.
There are two ways our eyes can perceive the color of an object:
1. As Brian correctly states, the first way of perceiving light is the wavelength of light that is reflected off of an object. This is by far the most common way we as humans identify the color of an object.
2. In some instances, we perceive the color of an object by the light waves emitted from that object. LED lights emit different wavelengths of light. So do Televisions. So does the sun. We do not describe these from their reflected light, but by their emitted light.
3. I mainly bring this up to give a shout out to Japanese researchers. Their work with both reflective light colors and emitted light colors is quite impressive. The story behind ‘true’ blue LEDs (once thought impossible) is riveting. Hey, Shuji Nakamura, we haven’t forgotten you! Flat screen LED TVs owe you a debt of gratitude…
The paint on a Boeing 747 weighs about 1200 lbs. Even if the only purpose of paint on airplanes was for appearance, which it is certainly not, this would equate to a weight savings of about 1000 lbs. For an airplane that has a gross takeoff weight of 735,000 lbs, this doesn’t mean very much.
Paint can be a lot more, then just “paint”. It can be adaptive, and even communitive. Telling you what’s going on w/the surface it’s in contact with. Say w/a plane, it can alert you to damage, and perhaps provide some kind of physical repair. On a person, certain bio-organic paints can help repair wounds, in specific those that are antibiotic resistant. (Don’t believe in evolution? Tell that to bacteria) They love to evolve. Drug delivery not based on time, but when people need it.
Yes paint can be much more then gaudy covering over something you want to hide, or highlight.
Boeing usually ships “unpainted” airplanes with green primer. It takes a while to paint a jet, then it has to dry – all the while the jet is not making money. Paint decreases roughness and helps with laminar flow and protects against scratches which can cause stress fractures. Paint adds weight, so the less weight the better over the long term (about 3-4 passengers’ weight; so with 45,000 flights per day in the USA the fuel savings can add up). White paint reduces temperatures inside of a plane as well as the surface. It also cuts down on bird strikes. Polished aluminum can do that but the cost is MANY times higher than paint because of rivets and cracks spoiling the surface. Paint identifies a company, like a billboard, and is VERY helpful for airport personnel to identify planes more quickly. No paint creates a highly reflective surface making observations difficult. Unpainted surfaces tend not to wash off dirt as easily and surface bumps and cracks are harder to see with the eye. I don’t know if nano-color will cover all these areas or any I’ve missed pertinent factors.
Can say a few words about plastic jets? There are over 1500 carbon fiber wide bodies flying now. A-350’s and 78’s. They’re distinct from AI built aircraft. What about UV protection and etc..
The biggest use case for paint is protection.
Not sure if this gives much protection.
Is there a nanosphere paint that is a perfect reflector of ultra white (reflects all light from near UV down to infrared to cut down on heat loss from the airplane and heat gain from the sun?
This could be a fantastic roof paint for buildings, too. Maybe even work for rocket paint jobs and other more terrestrial-type applications.
This is quite a brekthrough!
OK very cool, but frankly how does this benifit anyone out there in the real world? You see, in order to make people understand why they should give a s*** about anything, is people need to connect the dots. Perhaps the most wonderful thing any human can experience is to see a person have that “ah ha”! moment in another person. That effect is not easy to create. Very good teachers try to teach kids how to think, and try to act rationally.
Hey gang, the issues of our day will change, What matters, is we know how to predict the future. How? By dealing w/what happens, right now. You don’t “predict” the future that way, you create it.
It benefits you if you fly airplanes, (Note, I didn’t say “in” airplanes.) or sell paint to airplane companies.
The benefit is pretty minor, though, because as “KGB” notes, you can just leave aluminum bare, and that’s lighter still.
Bare aluminum is smarter.
Bare aluminum does not repair itself when it has a fracture in its metal. By the way, who’s payroll are you on?
Neither does painted aluminum, I’d hope you’re aware.
Of course, Unless that “paint” is adaptive at the molecular level. Where that paint can react to physical deformations on an aluminum or other surface. Self repairing surfaces are the first step in developing self-repairing technologies. (Another very useful adjunct would be self-sterilizing surfaces. Very useful in medical situations)
Pretty cool!
Might be good for saving weight on structures too.