Dichroic Colour-Changing Materials

E. G. Wrigglesworth, J.H. Johnston
Victoria University of Wellington,
New Zealand

Keywords: dichroic, dual colours, reflected and transmitted light, nanoparticles, microparticles, polymer composites

Summary:

When a material displays one colour in transmitted light, but a different colour in reflected light, it is known as the dichroic effect. The oldest and most famous example of this effect is the Lycurgus Cup, dated to the 4th century, which appears red in transmitted light but green in reflected light. This dichroism has been attributed to the presence of gold, silver and copper alloy nanoparticles in the glass of the cup. However due to its highly precious nature, no detailed studies have been carried out to characterise the nanoparticles or their distribution in the glass to further understand this effect. Through careful nanoparticle synthesis and thorough characterisation (electron microscopy, visible spectroscopy including CloudSpec, dynamic light scattering size analysis, etc.), our work has successfully traced the origin of the dichroic effect in metallic gold nanoparticles. Theoretical analysis of the interaction between light and metallic nanoparticles of different sizes and shapes has been achieved using Mie theory (solved analytically for spherical particles and numerically for non-spherical particles), and has been used for comparison with experimental results to further investigate the nature of scattering and absorption processes in these systems. The relationship between the size, shape and composition of the particles and the colours displayed in both transmitted and reflected light has been comprehensively deduced for this material. Further, in what we believe is a world-first, we have extended the dichroic effect into nano- and micro-particles made of alternative, non-metallic materials which we have extensively characterised and studied theoretically. Optimisation of our synthesis methodologies has resulted in the display of clean, bright and attractive colours. This further enhancement of our understanding of the features required to observe a dichroic effect has led to our production of a range of dichroic materials, producing a palette of different colours in reflected and transmitted light. Upon the proprietary incorporation of metallic and non-metallic particles into bulk materials, including into selected polymers, the colours displayed in both reflected and transmitted light have been retained as determined using visible spectroscopy accompanied by theoretical analysis. This process has effectively encapsulated the dichroic effect in the solid state, allowing us to utilise the unique optical effect displayed by these composite materials in design and security applications.