Nano/Micro-Particles and Composites with Special Dichroic Properties

One of the most famous examples of the use of metallic nanoparticles to provide colour to a bulk material is the glass Lycurgus Cup, dated to the 4th century A.D., which appears red in transmitted light (when lit from within) but green in reflected light (when lit from outside). This dependence of colour on the direction of the light source has become known as the dichroic effect. In the Lycurgus Cup, these colours have been attributed to gold/silver/copper alloy nanoparticles 50-100 nm in size. Despite the dichroic effect being observed in metallic nanoparticles since then, the relationship between the observation of the dichroic effect and the size, shape, and composition of the particles has been poorly understood.

The presented research aimed to deduce what features are necessary to observe the dichroic effect, leading to improved control of the effect and the ability to utilise it. Four syntheses of gold nanoparticles that displayed the dichroic effect have been developed. Full characterisation of the resulting particles combined with theoretical studies of their interaction with light has enabled connections to be made between particle features and the colours observed. The final synthesis of highly monodisperse dichroic gold nanoparticles has proven conclusively that particle size is fundamentally responsible for the observation of the dichroic effect in gold nanoparticles.

A thorough literature review indicates that the dichroic effect has not been reported in non-metallic particles before, however this research programme has advanced beyond gold nanoparticles to synthesise non-metallic copper(I) oxide microparticles that display the dichroic effect in clean and attractive colours. This has proven that the localised surface plasmon resonance phenomenon responsible for metallic nanoparticle colour is not a requirement for the observation of the dichroic effect. This section of research has included the development of a highly novel methodology that reproducibly synthesises monodisperse copper(I) oxide microparticles with excellent size control. The characterisation of these particles, combined with theoretical results, has again indicated that size is the most important contributing factor to the observation of the dichroic effect.

The developed understanding of the parameters required to observe the dichroic effect has enabled the synthesis of dichroic nano/micro-particles in a range of different colours. The presented research has proceeded to incorporate both gold and copper(I) oxide particles into polymer systems with complete retention of the dichroic effect, effectively encapsulating it in the solid state. The production of these dichroic composite materials has important potential applications in security and design.