The science of black

"It's like, how much more black could this be? And the answer is none. None more black."

Nigel Tufnel's famously ungrammatical line from the mock rock documentary 'This is Spinal Tap' tries to infer depth from an entirely black album cover. It's perhaps more justifiable to apply Tufnel's observation to material reported earlier this year by scientists from the Rensselaer Polytechnic Institute (RPI) and Rice University in America.

Zu-Po Yang and colleagues have created an 'ultra black' coating, formed from vertical arrays of carbon nanotubes, which absorbs more than 99.9 per cent of visible light, making it (they believe) the world's blackest man-made material. As a comparison, black paint absorbs around 97.5 per cent of light. Previously, Richard Brown and his colleagues at the UK's National Physical Laboratory held the record for dark materials with a coating that absorbed around 99.65 per cent of light.

A black surface over the spectral range of interest absorbs all the light at all angles and reflects or scatters little or any of it: the more it absorbs and the less it scatters, the blacker it is. 'Black', to the human eye, is when light is absorbed across the entire visible light spectrum, typically 380-750nm. Some animals, birds and insects can also see ultra violet and near infrared, so if you're a certain breed of butterfly, 'none more black' may be quite colourful.

The Rensselaer team measured its black over wavelengths of 457-633nm and found the total reflectance was 0.045 per cent at the shorter wavelengths, rising to 0.07 per cent at 633nm. So, their material could be 'more black' although it seems slightly churlish to say so.

Creating new forms of blackness is a modern form of alchemy embraced, it seems, with particular enthusiasm on the American east coast.

Just up the road from the Rensselaer group (based in Troy), researchers at the University of Rochester, New York have been blackening metal surfaces using high intensity femtosecond lasers. In Boston, Eric Mazur's group at Harvard has been turning silicon black with lasers and sulphurous gas.

Surface irregularities

These groups are all playing with nanometre-scale surface structures in order to manipulate light. Surface regularities that are around the size of the wavelength of light cause particular wavelengths to diffract, interfere or scatter. We see this in nature, where highly regular nanometre-scale periodic structures are responsible for the iridescent blues and greens of bird and butterfly wings and the highly-coloured shells of certain beetles.

But the key to 'structural' black (and white) is surface irregularity. "Both black and white surfaces in nature have a disordered nanostructure," says Peter Vukusic, a physicist who researches into the optical effects found in animals and plants at Exeter University in the UK. "The difference in black systems is that they also contain an absorbing pigment. It's very elegant."

Rensselaer Polytechnic's super black material illustrates this combination of periodic structure and surface irregularity rather well: the carbon nanotubes are straight and vertically aligned except at the surface where they become entangled, forming a loosely connected network. Yang's team used chemical vapour deposition to make their film of nanotubes, which have diameters of 8-11nm with tube-to-tube spacing in the 10-50nm range. Their method allows them to deposit film thicknesses between 10-800µm, depending on growth time.

Eric Mazur's black silicon is cooked up in a different way (by shining short and intense laser pulses in a chamber of sulphur hexafluorine or chlorine gas) but features a similar surface structure of tiny spikes tens of microns high. Spikes trap light and give it a number of opportunities to get absorbed as it bounces between them.

Another factor that helps absorption in black silicon is the presence of sulphur. Silicon is usually transparent to infrared light, but black silicon absorbs nearly all light at wavelengths ranging from UV to infrared up to 2,500nm.

"The sulphur introduces additional absorption bands between the valence and conduction band so the silicon can start absorbing photons with lower energies than it would naturally be able to, which means ones with longer wavelengths. People have tried to dope silicon with sulphur before but no one has done it at the percentage we've achieved," explains Mazur.

Carbon and silicon, to a much lesser extent, are materials that naturally absorb a fair percentage of light before you start to fiddle about with their surfaces. Getting the same effect with something intrinsically as shiny as metal would appear to be close to sorcery.

But Chunlei Guo, associate professor of Rochester University's institute of optics, has achieved this by blasting metals with needle-sized femtosecond laser beams to form tiny pits, blobs and strands on their surfaces. On average, such a surface can absorb over 95 per cent of light, says Guo, including infrared wavelengths up to 2,500nm. "Over a single spot, we're pretty close to 100 per cent absorption."

Natural inspiration

An alternative way of making new forms of black is to recreate the nanostructured surfaces found in nature.

Darren Bagnall, senior lecturer in the Nano group of electronics and computer science at Southampton University, is working with Peter Vukusic at Exeter University to reproduce natural structural colours in silicon using electron beam lithography.

Bagnall's group has already copied moth eye structures, characterised by a surface of regular arrays of sub-wave-length-sized conical protuberances. Due out of the lab next are silicon samples that mimic black butterfly wings (from the Papilio Ulysses butterfly) featuring parallel lines with a honeycomb structure in the middle.

Computers powerful enough to handle the maths for describing 3D nanostructures and to simulate their interactions with light have made it easier to model and synthesise new types of black in this way. But it's still tricky.

"To do a meaningful computer simulation, you need to cover many square microns of area and that can involve many thousands of 3D nanometre scale structures. It can take days," explains Bagnall.

Also, unlike nature, computers struggle to generate randomness. "You can design a pseudo-random tile on a computer that works for a few microns, but, when you reproduce it on silicon, the interaction with light produces diffraction patterns because you're repeating the same structure, " he says.

So, while Bagnall is looking forward to seeing the black silicon modelled on a butterfly wing, if they haven't managed to achieve sufficient irregularity, it might appear iridescent blue rather than black because of the diffraction effects.

In order to effectively model the blackness of moths' eyes, Bagnall and his colleagues have had to create domains of nanostructures that form a pattern akin to crazy paving rather than regular hexagonal arrays. "Crazy paving avoids sharp iridescence features and ensures that light is scattered around 360°. This stops the moth-eye structures acting like a lighthouse at certain angles of incidence and certain azimuthal angles. We think that moths may have found an optimum average size for these paving stones but the jury is still out. "

From a biological perspective, black surfaces usually serve a deliberate purpose, such as stopping predators from finding you (if you're a moth), or regulating your temperature, or just making you look good.

"For instance, if a species has a black top surface, it can sit in the sun and warm up quickly compared to a coloured or white dorsal surface," says Vukusic. "Often one sees a very bright patch on a butterfly or bird wing but it turns out that, to make it more conspicuous, there's a black border around it. In the same way that a nice frame sets a picture off, one's eye is drawn to what the black edge is framing," he adds.

Black to the future

From a commercial point of view, blacker blacks may help to reduce the cost of solar cells or enable us to make ones that convert radiation to electricity more efficiently. High-contrast and anti-glare applications may also benefit, and they may lead to better photodetectors, as these should not reflect in order to minimise optical noise.

Vukusic's department is applying bio-mimetic principles to recreate ultra-black materials for optimising absorption in photovoltaic cells.

Bagnall's group is working on efficient light-absorbing surfaces that could be deposited on plastic or steel foil to make cheaper solar cells, funded through the ESPRC Supergen collaboration 'Photovoltaics for the 21st century'.

Eric Mazur has founded a company called Sionyx to commercialise his technology. Black silicon layers, for instance, could be evaporated onto plastic to make lightweight solar cells. A more unusual application is using the material to make low-cost detectors for surface enhanced raman spectroscopy (a technique for identifying the chemical composition and molecular structure of substances by their spectral fingerprint).

Guo and his team first made black metal in 2006. Since then, they have used the same technique to turn aluminium gold and titanium blue. Black metal surfaces, he says, could improve the efficiency of light-gathering devices such as thermal sensors and detectors that have metallic parts. They could boost the performance of metals used as catalysts because the blackening results in a greater surface area for reaction.

There's a big difference between 'black at whatever cost' and the kind of compromise, low-cost black found in nature and sought by industry.

So, the answer to 'how much more black could this be?' depends on many factors. Clearly, the Rensselaer Polytechnic Institute has achieved an almost perfect ten on their particular blackness scale, but no doubt some other group will take it to 11 (to borrow again from 'Spinal Tap') and make it 'one blacker'.