Approaching the liquid/air interface with scanning force microscopy

Abstract

Scanning force microscopy at both the air/solid and the liquid/solid interface is very common and understood accurately. Here we demonstrate that the scanning force microscope (SFM) is capable of investigating even at the liquid/air interface, i.e., the surface of a Langmuir film floating on the water subphase. We demonstrate the successful investigation of (a) a solid substrate surface, i.e., a polycrystalline gold sample under water, (b) a mechanically stabilized 2-docosylamino- 5-nitropyridine Langmuir–Blodgett (LB) film, and (c) a freely floating cadmium-arachidate (CdA) LB film. The SFM is constructed as a standalone, remote-controlled, and water-tight microscope that can be put upside down into the water subphase of a commercial LB trough. The SFM tip therefore approaches the air/water interface from underwater allowing the investigation of the LB chromophores. This force microscope complements our optical techniques (polarization and second-harmonic microscopy) that have already been set up on top of the LB trough and that achieve monolayer resolution on a floating Langmuir layer. Combined operation of all those techniques will provide correlated information on the optical and structural properties of the LB molecules.