Adsorption and light emission of a racemic mixture of [7]thiaheterohelicene-2,13-carboxaldehyde on au(111), cu(001), and nial(110) surfaces investigated using a scanning tunneling microscope

Abstract

The coverage-dependent adsorption of a racemic mixture of [7]thiaheterohelicene-2,13-carboxaldehyde on Au(111), Cu(001), and NiAl(110) surfaces was investigated using a scanning tunneling microscope (STM). At a low coverage, the adsorption process for helicene molecules on Au(111) was strongly affected by surface reconstruction. At monolayer saturation coverage, the dominant molecular structure observed on Au(111) characteristically had separated self-assembled twin rows aligned in ?112¯? directions. The helicene molecules within these separated twin rows are preferentialy arranged in a zigzag pattern with alternating (M)- A nd (P)-enantiomers. With increasing molecular coverage, the molecular structural transition from self-assembled twin rows to self-assembled single rows was observed. STM-induced light emission (STM-LE) investigation of helicene molecules on Au(111) showed the suppression of plasmon light emission over the molecules. The adsorption of helicene molecules on Cu(001) and NiAl(110) was quite different from that observed on Au(111). Neither the formation of self-assembled twin rows nor the molecular arrangement into different domains was observed. The formation of molecular clusters on Cu(001) and NiAl(110) was observed. STM-LE investigation of helicene molecules adsorbed on Cu(001) showed the suppression of plasmon light emission over these molecules. In contrast to metallic Au(111) and Cu(001) substrates, STM-LE investigations revealed the enhancement of light emission above the molecular clusters formed on metallic NiAl(110), suggesting plasmon-enhanced molecular light emission.