Since the dawn of humanity people have been fascinated by colour, and with good reason – it’s bloody confusing.
Amazingly, ancient Romans made “dichroic” glassware that could change its hue depending upon the direction it was lit from. It’s an example of lost technology, and whether they understood the optics or the manufacturing process is debated, as a modern understanding of nanotechnology is needed to explain it fully.
In school, we’re taught that the primary colours are red, blue, and yellow. Those of us young enough to have grown up glued to our computer screens may have objected: aren’t the primary colours red, blue, and green? After all, they’re the basic ingredients of colour we use when designing digital art or websites.
The answer, confusingly, is that both I and my teachers were right, depending on whether you’re using an additive or subtractive colour system. That is to say, are you projecting light onto a black background, adding colour to it, or are you covering a background that reflects all colours (such as white paper), only allowing some wavelengths though?
The secret to dichroic glass relies on a similar idea, but first it’s worth understanding why things have colour at all. When light hits an atom or molecule, it can either pass through (as in water or glass), bounce off, or be absorbed, giving its energy to an electron which will then jump up to a strictly-defined energy level. When the electron falls back down, it will emit light with a wavelength corresponding to the difference in energy levels.
In most molecules, these energy levels are so high that the corresponding wavelength is all the way up in the ultraviolet range. Thus, all light in the visible spectrum is reflected, hence why chemistry labs are full of so many suspicious white powders. Some molecules do absorb energy in the visible range, however, such as beta-carotene which absorbs blue light strongly, allowing wavelengths from the redder end of the spectrum to reflect. It is the presence of this molecule that gives carrots and peppers their colour.
Metals generally have so many energy levels that they absorb and reflect all colours, which is why we can use them as mirrors. In metal nanoparticles, however, the available energy levels are restricted, giving them colour. In the words of my nanomaterials lecturer, Professor Nic Harrison,
To explain why some objects absorb only particular colours we have to turn to quantum theory. It is the electrons in the object that absorb the energy of the light in discrete amounts, called “quanta”. What determines these discrete amounts in different objects? Electrons behave like waves and the waves are shared amongst the atoms in beautiful patterns: these are chemical bonds. This is what holds atoms together to make molecules, and molecules together to make, well, everything. In a metal the waves are shared between huge numbers of atoms: the electrons can move freely and so they tend to conduct electricity and reflect light. In dye molecules the electron waves are trapped in very specific bonds and so only very specific quanta of energy can be absorbed as the electrons jump between different bonds, so dye molecules are brightly coloured.
In nano-science we have another variable – the size of the object. We can now make particles of metal a few nanometres across, so small that the electron waves are strongly influenced. Even though the particle is made of metal the electrons are constrained to vibrate in very specific modes, much more like those confined to the bonds of a dye molecule. By varying the size of the particle we can change the light absorbed and generate every colour of the rainbow.
This behaviour illustrates beautifully the power of nano-science: historically we have been able to get new properties and advanced materials by changing the atoms a material is made from – carbon steels for strength, doped silicon for computer chips and solar cells, light emitting organic molecules for displays – now, we can change the size and shape of nanoparticles to trap electron waves and so get a much wider range of useful properties.
The artisans who made the Lycurgus cup added gold and silver during the glassmaking process. Though they weren’t aware of it, the metals formed nanoparticles, of which gold is characteristically red, leading to the cup’s amazing properties: when lit from the front we see the reflected green light, but when lit from behind, only the red light which is absorbed and re-emitted shines through.
The presence of silver as well as gold nanoparticles may explain why the cup looks so different from the red stained glass windows found in churches, which also use gold. Dichroic glass has been reinvented in the 20th century by NASA scientists, but the modern form, made from stacked thin layers of glass and metal, is an altogether different kind of material.
Keir spent last summer making silver nanoprisms, which are a lovely deep shade of blue. He now lives the life of a computational scientist, editing text files at a black-and-white terminal.