Colour

Colour is an optical property that behaves noticeably differently at the nanoscale compared with the bulk. Elemental gold, as everyone knows from jewellery and gold leaf, has a distinctive, shining yellow colour in its ordinary bulk form. But if only about 100 gold atoms are instead arranged together in a cube -- a genuinely nanoscale gold particle rather than a bulk sample -- its colour would instead be red, not yellow. The same chemical element, gold, therefore shows two visually different colours purely as a function of how many atoms are grouped together and at what size scale, which is a striking, easily-stated illustration of why nanoscale materials cannot be assumed to share the bulk material's familiar physical properties. This colour-shift phenomenon is also what makes colloidal gold nanoparticle solutions (Fig 16.4) visually distinctive, and it is the same underlying size-dependence that later gives rise to the historical ruby-red colour of ancient gold-nanoparticle-containing glass (Section 16.8). A related applied example, given in a c …