Biomimetic isotropic nanostructures for structural coloration

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Fall 2010 entry: Anna Wang

References

Biomimetic isotropic nanostructures for structural coloration J. D. Forster, H. Noh, S. F. Liew, V. Saranathan, C. F. Schreck, L. Yang, J-G Park, R. O. Prum, C. S. O'Hern, S. G. J. Mochrie, H.Cao, E. R. Dufresne Advanced Materials (2010)

Background

Nature has provided inspiration for methods of producing colour - and hence manipulating light - using not pigments but structural features. For instance, periodic biological structures such as iridescent beetle wings can be studied from the point of view of photonic bandgaps.

Forster et al explore the properties of aperiodic thick and thin self-assembled films of colloidal polymer nanoparticles, focussing in particular on isotropic films which exhibit structural colour but no iridescence.

Thin films

Figure 1. (a) film consisting of 226nm and 271nm diameter polystyrene spheres. Inset: a dried sessile droplet. (b) Side-view SEM of such a film. (c) Transmission spectra for the isotropic film pictured in a) [top] and a 226nm diameter polystyrene sphere crystalline film [bottom]. Measurements were taken at normal incidence, 30° and 80° as shown in the top left diagram. Inset: Side-view SEM of the crystalline film.

Thin films were made by spin-casting aqueous suspensions of spheres onto glass coverslips. Two types of thin films were made

  • anisotropic, crystalline films using monodisperse 226nm diameter spheres and
  • isotropic films using a mixture of 226nm and 271nm spheres

Side view SEM images of these can be seen in Figure 1.

Transmission spectra of the films from several different angles are shown in Figure 1. The crystalline film has a more pronounced transmission dip at 500nm at normal incidence but the position of this dip shifts and then disappears as the incidence angle is altered. The isotropic film has a shallower transmission dip at 500nm but the wavelength of the dip does not shift with change in incidence angle. This demonstrates a trade-off between how pronounced a signal is, and how angle-dependent it is when working with anisotropic and isotropic structures.

Thick films

Figure 2. (a) 5 films consisting of 226nm and 271nm diameter polystyrene spheres with varying concentrations of carbon black (units wt%). (b) Normalised (and inset: non-normalised) optical scattering spectra recorded at an angle of 20° for the five samples. (c) SEM of the interior region of sample 3, piece of carbon black indicated with an arrow. (d) The width of the spectra in b) at a reflectance of 0.9

The inset of Figure 1a) shows a film which was made by allowing a sessile droplet of colloidal suspension (of the same concentration as the solutions used in spin-casting) to dry. This was to demonstrate the critical impact of film thickness on the colour of isotropic films, which is an inconvenient property that may hinder the use of such films in many applications where the uniformity of colour (and thus thickness) is critical.

Carbon black of varying concentrations (and hence absorption lengths) was added to a suspension of 226nm and 265nm polystyrene spheres to introduce broadband absorption. Only the most strongly scattered photons will ‘escape’ the film and photons which do not get scattered within a small distance from the film-air interface are absorbed. This minimised the thickness dependence of the film’s colour. In Figure 2a), it can be seen that rather than a black solution and white solution combining to produce grey, a spectrum of blue to green was formed instead. The contrast between scattered intensity at the peak wavelength and shorter wavelengths was the best for sample 3, which also had the narrowest reflectance peak.

Biomimetics

Figure 3. (a) Crown feathers from L. coronata. (b) TEM of the beta-keratin's air spheres in the feathers. (c) The azimuthal averages of small angle X-ray scattering data from the feathers (dashed line) and a thick film created to mimic the data, sample 2 from above (solid line). Inset: SAXS pattern for the feathers' keratin air bubbles. (d) Comparison of optical scattering data of sample 2 and the feathers. The line colours are illustrative of the actual colours of the samples.

The optical properties of sample 2 above resembled that of the bird feathers of Lepidothrix coronata (the structure of beta-keratin air spheres is shown in Figure 3b)). The differences in small-angle X-ray scattering data and optical scattering data may be due to the ‘inversion’ in the two systems as the feathers have spheres of air in a high refractive index background, and the films are the opposite.

Conclusions

The properties of isotropic thin and thick films made from bidisperse mixtures of spheres was studied. The colour can be altered by manipulating film thickness, or the absorption length of the films (eg by adding carbon black). This knowledge was then used to mimic the structural colour properties of Lepidothrix coronata feathers.