E.G. Wrigglesworth, J.H. Johnston
Victoria University of Wellington,
Keywords: dichroism, cuprous oxide, novel, 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. Since analysis of the Lycurgus cup, a small number of studies have examined the dichroic effect for gold and silver nanoparticles. In previous work we have presented gold nanoparticles that display dichroism and made correlations between the size and the shape of the particles and the observation of the effect. However further work is still necessary to determine what factors dichroism can be exclusively assigned to. It is therefore desirable to find alternative materials that display the effect to further understand it, whilst also increasing the colour range available for application. Cuprous oxide has attracted attention for many applications from optoelectronic devices to sensors, and the synthesis of Cu2O particles has been well studied. However the dichroic effect displayed by select Cu2O particles has been largely ignored. We present the aqueous synthesis and novel application of Cu2O particles that display the dichroic effect. By careful manipulation to control the particle shape and size we are directly able to control the colours displayed in transmitted and reflected light, producing samples that are bright yellow or orange in reflected light but pale blue or grey in transmitted light. We have further incorporated these particles into polymer systems with control of particle size and shape and the polymer-particle interaction, effectively producing solid-state materials where the colour displayed depends on the direction of the light source. The colours produced have been characterised using absorption and reflectance spectroscopy and linked to the particle size and shape observed via electron microscopy. Study via X-ray diffraction, infrared spectroscopy and hydrodynamic data measurements have complemented these results. To the best of our knowledge, this is the first time the origin of the dichroic effect has been studied for cuprous oxide particles, and further, the first example of the use of Cu2O particles to produce dichroic polymer materials. We foresee exciting potential applications for such materials in the areas of design and security.