Single component LB film investigated by five scanning probe microscopy methods

Abstract

Monolayers and multilayers from dipalmitoylphosphatidylethanolamine head labeled with nitrobenzoxadiazole (DPPE-NBD) were deposited as Langmuir–Blodgett (LB) films on silicon wafer or quartz glass plates at different deposition pressures at room temperature. The phase coexistence between the liquid and the solid phase was observed by atomic force microscopy, friction force microscopy, scanning surface potential microscopy (SSPM), scanning near-field optical microscopy (SNOM), and phase imaging by the tapping mode atomic force microscopy. The different methods reveal new features not imaged by a single method. There is no sharp boundary line between the liquid and the solid phase. Submicrometer solid domains are resolved above the phase transition together with multilayer needles. These needles are bilayer in thickness when deposition was carried below the equilibrium surface pressure of 19.6 mN m−1 and grow in height up to hundred nanometers upon increase of the deposition surface pressure. After some time, these very high needles equilibrate to bilayer height. SSPM and SNOM reveal big contrast between the liquid and the solid phase in the first case due to the large differences in the vertical component of the molecular dipoles in the two phases, and in the second case due to fluorescence self-quenching in the solid phase. Multilayer growth on top of a liquid phase in the center of the solid domains and liquid phase inside the solid domain could be resolved. Phase imaging of multilayers samples reveals features not seen in the AFM.