Tailored colours for photonic crystals

Korean and US scientists have permanently fixed the colour of block copolymer photonic crystals by swelling photonic gels and ’freezing’ them as they display the desired colour. The new study should help scientists better control the properties of photonic gels for potential applications including sensors, displays and photovoltaics, say the researchers.¹   

The new study builds on previous research into the colour tuning of these crystals across the light spectrum.  

In 2007, Chemistry World reported that Edwin Thomas and Youngjong Kang and colleagues had developed one-dimensional photonic gels made from self-assembling block copolymers that could rapidly change colour in response to stimuli such as temperature and pressure.² The gel consisted of two alternating layers of polystyrene and poly-2-vinyl-pyridine (P2VP). The space between the layers - controlled by swelling the P2VP layer with methanol - determined the wavelength of light reflected, and therefore the colour of the gel.

’But the problem was that even though we could control the photonic band gap by swelling, when the gel dried out, we lost that band gap and so it became transparent,’ says Kang at Hangyang University in Seoul, Korea.  Now, the team has taken things to the other extreme by developing a method to solidify the photonic gel and thus permanently fix the colour band. ’We had these very responsive, rather fragile gels, and now we’ve gone to fixed, mechanically robust, unresponsive solids,’ says Thomas at Massachusetts Institute of Technology, US. Although the colour is fixed, the material appears different colours when viewed from different angles.

Using off-the-shelf sol-gel chemistry, the team fixed the gel layers by infiltrating the P2VP domains with a silica precursor and catalysing the growth of silica using ammonium hydroxide. Colloidal silica then forms a network and solidifies, freezing the layer thickness and allowing the methanol to evaporate without losing the gel colour. 

Originality disputed

But Rudolf Zentel, professor of polymer chemistry at the University of Mainz, Germany, is unimpressed, highlighting that there are many well-known and easily made materials that show a photonic stop band. ’These all offer the potential to shift the stop band over the entire visible spectrum and to fix the desired stop band,’ says Zentel. ’The authors demonstrate this is now possible in block copolymers, however, much more complex block copolymer structures have been stabilised by sol-gel chemistry. So this stabilisation of stop bands is not new,’ Zentel asserts.

Thomas, however, focuses on the technique and materials in hand. ’The clever thing is to know that this simple chemistry exists and that it will work in this environment in a methanol-swollen P2VP layer,’ explains Thomas. ’Our main goal was to see how solid we can make this material. And that now means we can back off and explore quite a range of properties and applications in future work.’ 

Others are positive about the work. Photonic crystal pioneer Eli Yablonovich at the University of California Los Angeles, US, says that ’this use of block copolymers is completely original and it’s a very powerful chemical approach.’ Yablonovich suggests this research illustrates the way forward for creating sensors, screens and electronic newspapers.

Indeed, in another new study, Thomas and Kang and colleagues report making a prototype electronic screen that’s only one micron thick.³ Using their block copolymer photonic gel combined with an electrochemical cell, the P2VP layer thickness - and therefore the colour - can be controlled with an electric charge. The photonic gel may also be a cheap and scalable material for use in photovoltaic cells, adds Thomas.

James Urquhart

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