Structure and plasmon coupling of gold-poly(N-isopropylacrylamide) core–shell microgel arrays with thermally controlled interparticle gap

Abstract

Core–shell Au@pNIPAM nanocomposites are assembled through a simple and inexpensive approach where drops from the stock nanoparticle solution are dried on ITO surfaces at fixed and finely tuned temperatures. Interparticle distances are determined by the length of the thermoresponsive PNiPAM shell. This process allows controlling the distance among gold cores as function of the PNiPAM shell thickness as it acts as mechanical spacer. The elasticity of the polymer shell at each temperature must be considered as the shell is more elastic at temperatures below PNIPAM's LSCT than at temperatures above this threshold. These two parameters, thermal PNiPAM response and shell elasticity, determine the interparticle gap and thus the structure of deposited arrays. Structural characterization is performed through electron and scanning force microscopy. We quantify the pair and orientational correlation functions characteristic to arrays deposited at different temperatures and resolve that deposited microgels exhibit predominant hexagonal structure. In a second step, Au@PNiPAM arrays are exposed to atmospheric plasma. Plasma reacts and degrades the PNiPAM shell, being non-invasive to Au cores which remain at their original array positions. 2D ordered arrays of bare gold nanospheres are achieved with the centre-to-centre distance previously tuned by temperature. Finally, plasmon coupling experiments are performed where vertical plasmon coupling between the Au particle cores and a sputtered gold substrate becomes apparent during the deswelling–drying process of the polymer shell, whereas no lateral coupling between the cores is detected.