Scanning probe microscopies for molecular photodiodes

Photo-electric conversion molecular devices, i.e. molecular photodiodes based upon the charge separation mechanism of the primary process of natural photosynthesis, have been studied in our laboratory. Amphiphilic AS-D triad molecules can be oriented unidirectionally in monolayers at the air-water interface, The resulting oriented monolayer assemblies are fabricated into thin films on electrode surfaces by the Langmuir-Blodgett (LB) technique. The A-S-D triads for charge separation together with light harvesting antenna molecules can be assembled into sub-micron island domains by making use of phase separation of mixed monolayers of hydrocarbon (HC) and fluorocarbon (FC) amphiphiles. The structure and properties of the domains of HC-FC mixed monolayers were studied by various scanning probe microscopies (SPMs) such as atomic force microscopy, friction force microscopy, scanning surface potential microscopy (SSPM), and scanning near-field optical/atomic force microscopy. Some SPMs can be used not only to characterize the LB films, but also to drive the molecular devices. The photo-induced charge separation in unidirectionally oriented triad molecules embedded in alternate LB films was observed by SSPM.