Out of step but in tune
(appeared in Nov 2014)

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Rhythms and part-rhythms in nature lend a helping hand in the fight against climate change, says S.Ananthanarayanan.

Being periodic and regular is everywhere in nature - sound waves, light waves, movement of the planets, the seasons, the generations of people. There are also regular patterns in art and architecture and in music, and in the way atoms combine to form crystals. And then, nature has examples of things that are precisely non-periodic, and useful for that very reason, that they prevent things from repeating.

A third form of regularity, which has been studied of late, is partial periodicity- things that are not periodic, but also not random, a form known as quasi-periodic, and this is found to have applications too, one area being in herding and sorting the components light, which is useful in studies of optical transmission, photoluminescence, laser action, and so on. Materials with this kind of part-periodicity are also found useful in light-trapping for solar energy. This last property would have great application, but fabricating the right patterns is expensive. Alexander J. Smith, Chen Wang, Dongning Guo, Cheng Sun and Jiaxing Huang, a multidisciplinary team from North Western University, Illinois, report in the journal, Nature Communications, that the now common Blu Ray movie disc carries a code that has a quasi-periodic character which is good for optimizing light-trapping in the solar spectrum.

Examples of things that are periodic are legion in nature and in life. A whole great part of electronics and communications depends on periodic electric or radio waves. The complex shades and patterns on the wings of birds or insects arise not from dyes or chemicals but from the effect of periodic striations, on waves of white light. Crystal structure of materials is the regular places of atoms, repeated unchanged and in three dimensions, and the regularity gives materials mechanical strength or useful electric or optical properties.

An example of being specifically non-periodic is the position of successive leaves around the stem of a plant as it grows. If there was anything periodic in the way the leaves sprouted, then sooner or later, one leaf would find itself directly below another, which is no good for getting the best of sunlight. It is found that the way leaves grow follows a remarkable pattern, reflected in a series of numbers, known as the Fiboncci series, in which the ratio of any pair of successive numbers is always different from that of other pairs. Because of this nature of the spiral growth of leaves, no leaf ever grows exactly above another one, which makes for the best benefit of sunlight for the plant. Another example is the way certain hibernating animals time the years when they emerge, for breeding. If there were a pattern in the way this happened, their predators would be there to get them. The animals keep predators guessing by varying the gap between successive breeding seasons, so that a pattern can never be discerned!

Quasi crystals

As opposed to such planned regularity, or the converse in the non-repeating series, we havethe quality of randomness or lack of any scheme or design. A crystal with specific periodicity would provide scattering centres which would specifically pick out some given frequencies of light and the crystal would reflect or transmit these frequencies. The same material in an amorphous, or non-crystaline form would be the case of randomness and it would have no selectivity. But yet another, a third form of crystal structure, has now been discovered, and this is the quasi-crystal, where atoms are packed in patterns, but such that they do not repeat themselves! For long, it was thought that matter simply could not exist in such a form, because the pockets of low energy, into which atoms tend to align themselves, all appeared in regular, repetitive structures. Daniel Shechtman, who discovered this structure in an aluminium-manganese alloy in 1982, and received the Nobel Prize in 2011, had to battle ridicule in the initial years – Linus Pauling is said to have commented: “There is no such thing as quasi-crystals, only quasi-scientists.”

A number of quasi-crystals have since been created in the lab and instances have been found in the natural world. Quasi-crystals that exist in some steels have been found to reinforce the material surface and commercial applications are being developed. Quasi-crystals, with their non-repeating regularity, or long-range order, show special properties of elasticity and propagating heat or sound. And in the field of photonics, or manipulation of light, these crystalline structures have shown the capacity to be efficient collectors of light, of all or a wide range of frequencies.

This last property is seen as having great potential to improve the efficiency of solar cells. With all the attention paid to this source of non-polluting energy, the best solar cell panels can do is about 20% efficiency, and many of them are at about 10%. If a quasi-crystal structure could be built onto the silicon thin film solar cell, this would maximize energy absorption and efficiency of the solar cell. The trouble, however, is that quasi-crystals for specific applications cannot be made to order, as there are no design or fabrication tools to create non-repeating patterns.

Quasi-random structure

A way around fabrication of actual quasi-crystals has been the development of other structures, like a plane with closely separated rulings placed above another plate with slightly different rulings, which are able to mimic the quasi-crystal ability to address a range of frequencies of light. The way quasi-crystals and structures seem to work is based on a principle that a complex wave can be represented as a series of simple waves, each of these component waves being weaker or stronger. A regular structure, like a crystal, then, corresponds to a small set of wavelengths, and a random structure, to all wavelengths. But quasi crystal, and equally the quasi-random structure, are found to map to a spectrum of wavelengths, and hence their value in channeling solar energy.

But creating artificial structures with architecture at the scale of wavelengths of light, which is routine in natural objects like crystals, is challenging and expensive. This difficulty may thus have been the limiting factor in the use of quasi-random nano-structures in the solar panel industry. At least, till the serendipitous discovery of the Illinois researchers.

Blu Ray movie discs

The Blu Ray is a convention of coding compact discs which superseded the DVD standard. The digital optical recording principle is to record data, audio or video, with the help of indentations known as ‘pits’, in a spiral track on a circular plastic disc. The DVD standard improved on the coding used in the CD, but it was limited by the wavelength of the red laser that was used for reading the code. The Blu Ray uses shorter wavelength, blue laser, which allows smaller ‘pits’ and a finer track pitch, along with improvements in data compression and checks to detect errors.

In the quest for more accessible quasi-random nanostructures, the research team in Illinois has discovered that the pattern of ‘pits’ and ‘lands’, which is where there are no ‘pits’, in the Blue Ray disc is a full-fledged quasi-random pattern, which is able to do for solar cells what was planned with the expensive original. The ‘pits’ in the Blu Ray disc are of the order of 150 nanometers, which compares well with the wavelengths in visible light – 390 to 700 nanometers. As Blu Ray discs are mass produced, the cost of their use in harnessing solar energy, would be little more than material cost, once the recordings have been marketed.

The Illinois group found that the quasi-random pattern of ‘pits’ on the Blu Ray discs, after encoding, regardless of the normal content of the discs, was well suited for photon management over the full solar spectrum. The team extracted the pattern on the discs and imprinted it on to polymer solar cells and found that there was increase in absorption and power conversion. The team was also able to prove that the same method would work with other photoactive materials. “This new insight opens up promising areas for repurposing a low-cost consumer product for a high-end, value-added application,” the team says in the paper in Nature Communications.

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