Nanocrystallography

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The colour blue of this butterfly is not due to any pigment but to the interaction of visible light with the microstructures of the scales of its wings. These microstructures that life has been creating for millions of years are what today we know as photonic crystals. Many other living beings colour their bodies making similar structures capable of diffracting visible light, emitting green-blue and iridescent colours. These are the so-called structural colours.

Do you know how these crystalline microstructures are formed? Would you like to find out how they are made and what photonic crystals are used for? Do you know any other examples of animal or plant that use this type of crystal?

Structural colours

The colour of butterfly wings, specifically the bluish-green colours of short wavelength and high energy, as well as iridescences, are not colours from pigments but from the dispersion of light by the microscopic periodic structure of chitin.

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The colour of the feathers of many birds is due to the same effect of the crystalline periodic structures of keratin.

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The famous headdress of Moctezuma is an explosion of structural colours of different bird feathers plus the absorption colour of gold and silver. The excellent preservation of the colours of these feathers over almost 500 years is due to the fact that their colour does not depend on pigments that can dissolve or oxidise, but on the interaction of light with a hard substance like chitin.

Diffraction occurs when a wave meets an obstacle of a size similar to its wavelength, or when it passes from one medium to a different one. When the wave is visible light, the phenomena associated with diffraction generate colours, because colours are the different wavelengths of light. When the obstacle is periodic and regular, as in crystals, multiple colours are generated as the wavelength of each one is affected differently by the spacing and size of the obstacle structure.

The microstructure of some parts in living beings (such as bird feathers or butterfly wings) makes up a natural diffraction lattice formed by the regular spacing of the materials that make it up (keratin and melanin in feathers, chitin in butterfly wings). This also occurs in other materials such as opal, where the silica molecules are regularly ordered in multiple layers submerged in water.

Iridescence is produced when this occurs simultaneously with more than one colour.

In feathers, it is the interplay between the structure of the keratin and the form in which the melanosomes are distributed that configures the colour range – in other words, the reflection plus the transmission in the different layers together with the interference.

The thickness of the different layers is what selects the primary colour. The blue-greens are mainly produced in the surface layers, while when the reflection is produced at greater depth (when the shorter wavelengths have already been absorbed) then the orange-red range of the spectrum is generated.

To find out more about these structural colours

Many of the colours of butterfly wings and bird feathers are due not to pigments but to the dispersion of light by the periodic structure of chitin. These are the photonic crystals that are nowadays obtained by sophisticated techniques for very different applications in industry.

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Photonic crystals

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The colours of the opal are also due to the same phenomenon. They are structural colours caused by the light dispersed by the structure of the spherical nanoparticles of silica.

Cosmetics

Since the days of the Egyptians minerals have been used for cosmetics. The famous black eyeliner “mesdemet” was obtained using lead sulphide and “Kohl” using galena with added cerussite, laurionite and phosgenite.

Nowadays the quality of cosmetics depends on the form and size of the crystals that are their base. Isometric forms are less desirable as they flow with sweat between the furrows of the skin – they “run” – compared to elongated crystalline forms in the shape of prisms or needles. Controlling the morphology of the base of a cosmetic is therefore critical. But it is also vital to control their size, because the size of the nanometric crystals of cosmetics is what controls their colour.

 

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Mineral pigments

A lovely example of the stability problems of mineral colours is the historical evolution of the colours of Raphael’s painting, Madonna and Child Enthroned with Saints.

rId13The initial azurite blue of the Madonna’s mantle has darkened with time due to its degradation into green malachite and now this mantle looks greenish. The degradation processes: the intensity of the colour blue is due to the presence of copper and the form in which it is chemically bonded with hydroxyl (OH) and carbonate groups (CO3). Azurite has good stability in oil and tempera mediums, although it is subject to processes of degradation to green or black. In fact, malachite, another natural mineral of copper, is only a more oxidised form of the mineral azurite. Therefore it is the increase in oxidation that causes the colour change from blue to green. The formula for this change includes the addition of a water molecule to two molecules of azurite, which releases one molecule of carbon dioxide and leaves three of malachite. The oxidation is continuous and so, therefore, is the slow transformation from blue to green. Azurite also suffers alteration to a black pigment, the copper oxide called tenorite

Azurite is a basic copper carbonate that is found in many parts of the world on the upper, oxidised parts of copper mineral deposits. In nature azurite is generally associated with malachite, the green basic copper carbonate which is much more abundant. It was used occasionally by the Egyptians, but it was not until the Middle Ages that its use increased, when the manufacture of the ancient synthetic pigment “Egyptian blue” fell into oblivion. It was produced artificially from the seventeenth century until it was substituted by “Prussian blue”, which was discovered in the eighteenth century. Azurite was the most important blue pigment in European painting during the Middle Ages and the Renaissance.

Malachite is a basic copper carbonate. It is a relatively stable pigment of variable colour, and is perhaps the oldest green-coloured pigment that exists. It is sensitive to acids and heat. It has been found in paintings on Egyptian tombs and in European paintings, having been particularly important in the fifteenth and sixteenth centuries.

 

Azurite Malachite Tenorite
From the Halpern Mineral Collection, San Francisco Modified by CombineZP rId16
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Unit Cell: 5.0110 5.8500 10.3530 90.000 92.410 90.000 Unit Cell: 9.5020 11.9740 3.2400 90.000 98.750 90.000 Unit Cell: 4.6837 3.4226 5.1288 90.000 99.540 90.000
Space Goup: P2_1/c Space Goup: P2_1/a Space Goup: C2/c
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Did you know that…

  • Colour gives insects camouflage and serves as a warning to predators of their disagreeable flavour, but to humans it most often attracts our attention due to its beauty.
  • The colour blue is not very common in food, and in general is associated with poisonous fruit or food in a bad state. However, the marble berry (Pollia condensata) not only has a brilliant metallic blue colour but also achieves this without using any blue pigments.
  • Structural colour can be found in soap bubbles, puddles, in the feathers of the peacock and in many other animal species.

To find out more

 

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