Nanoscale Manipulation of Responsive Colloidal Crystal Monolayers for Surface Enhanced Raman Scattering and Colloidal Patterning

Abstract

Colloidal crystal structures exhibit their characteristics not only from individual colloids but also from their ordered structures. They have been widely investigated in the fields of optical sensor, photonics, colloidal lithography, and solar energy applications. In particular, the morphology of individual particles and crystal structures of colloidal crystals composed of stimuli-responsive colloids change in response to environmental stimuli. In this article, temperature- and ion-responsive hydrogel colloids are used as building blocks for fabricating responsive colloidal crystal monolayers, to control nanoscale structure morphology and colloidal crystal structures of the colloidal crystal monolayers. First, a temperature- responsive colloidal crystal system is designed to control the nanoscale surface topology of the monolayers. It is found that the surface topology of the monolayer varies as the depth of the valleys between the colloids is controlled at the nanoscale during hydration−dehydration of the hydrogel colloids upon temperature variation. The temperature-responsive colloidal crystal system is exploited as surface enhanced Raman scattering (SERS) substrates in water. Au or Ag nanograin layers, which are further coated on the monolayers, can reversibly wrinkle and unwrinkle according to variations in water temperature. These structural changes are directed by surface structural changes in the monolayers. Further, the plasmonic nanowrinkles, formed through the intimate contact between the nanograins, can provide highly enhanced and reproducible SERS (both spot-to-spot and substrate-to-substrate) of toxic and biological molecules in water. Second, close-packed colloidal crystal monolayers can be easily transformed into non-close-packed crystal patterns by exposing them to salt aqueous solutions. The patterns can be directly achieved not only on various flat substrates but also on periodical micropatterns, leading to the formation of hierarchical crystal structures. These techniques can further be extended to the fabrication of non-close-packed inorganic−polymer hybrid colloidal crystal patterns where inorganic particles are spontaneously assembled on the surface of the colloids. The patterns described above exhibit tunable color change, anti-reflectivity, and enhanced transmittance. Moreover, such anti-reflective effect of the patterns is useful for solar energy harvesting.