Abstract: Self-assembled nanoparticle arrays at liquid interfaces provide a unique optical response which has opened the door to new materials and applications in sensing and tuneable optical metamaterials. Nanoparticles can spontaneously assemble at the liquid-liquid or liquid-air interface, forming a highly ordered, self-healing, low-defect 2D film. Our work aims to provide a greater understanding of the structure, assembly processes and surface enhanced Raman (SERS) properties of these nanoparticle films. Depending on the nanoparticle size, shape and spacing, the plasmon resonance of the film and resulting SERS enhancement can be tailored at will. We systematically tune the assembly and spacing between nanoparticles through increasing or decreasing the degree of electrostatic screening between the nanoparticles (Fig. 1a). The in situ modulation of nanoparticle positioning on the same sample allows us to monitor plasmon coupling and resulting SERS enhancement processes in real time with sub-nm precision (Fig. 1b). We further explore the assembly of shaped nanoparticles such as stars whose films could potentially provide a much enhanced Raman effect for sensing applications (Fig. 1c).
Figure 1 a) Schematic of 2D nanoparticle films formed at a liquid-liquid interface analysed by UV-Vis spectroscopy and Raman spectroscopy. b) Photo of NP film formed at the liquid-liquid interface and corresponding Raman spectra as interparticle spacing is reduced. A significant enhancement in the Raman signal is observed over short timescales, as well as a change in the colour of the film. c) Photos of films formed at an air-liquid interface using NPs of varying shape and composition. The films can be easily transferred onto substrates and dried for use in devices.