In this study we report our investigations of adsorption of a racemic mixture of 2,13-bis(aldehyde)-thiaheterohelicene (rac-TH-dial) on Au(111), Cu(001), and NiAl(110), as well as of detection of light emission/scattering from the molecules induced by a STM tip. Light emission from the rac-TH-dial molecules was investigated by STM-induced luminescence (STM-LE), while light scattering was investigated by STM tip-enhanced Raman spectroscopy (STM-TERS) techniques. Helicene molecule belongs to the class of chiral molecules revealing a characteristic helical shape, which comes from steric hindrance between adjacent aromatic rings. All STM-LE and STM-TERS measurements were carried by a low-temperature ultrahigh-vacuum spectrometer using Ag tip. At the beginning, the influence of the substrate on the self-assembly process of rac-TH-dial on Au(111), Cu(001), and NiAl(110) has been studied with the use of standard STM. Despite the same deposition conditions, only Au(111) governed molecular self-assembly formation. We observed a few different phases with highly ordered structures formed on a wide area of the Au(111) surface. The dominant observed structure was characterized by the presence of highly ordered domains ranging from tens to hundreds of nanometers adsorbed directly on the Au(111) surrounded by small disordered areas. The highly ordered domains consist of self-assembled zigzagged twin rows exhibiting self-assembled rac-TH-dial molecules with orientation and positional order were preferentially aligned parallel to the highly symmetric <1-10> or <11-2> directions. Despite the same deposition conditions, the adsorption of rac-TH-dial on Cu(001) and NiAl(110) surfaces was quite different from that observed on Au(111). The rac-TH-dial molecules on Cu(001) and NiAl(110) formed an amorphous layer with densely packed molecular stacking. As the amount of rac-THdial molecules decreased, the formation of randomly distributed clusters of similar sizes became dominant. What is more, we report our STM tip-induced light emission investigation on rac-THdial molecules and highlight the strong enhancement of light emission over rac-TH-dial clusters formed on NiAl(110), suggesting the molecular character of light. Additionally, we show evidence of nanoscale dehydrogenation occurring during STM-TERS measurements on rac-TH-dial molecules adsorbed on Au(111) surface. The near-field STM-TERS spectra obtained locally on a self-assembled monolayer of rac-TH-dial molecules showed vibrational frequencies in good agreement with that predicted by density functional theory calculations, except for the L-mode at ∼2000 cm−1, which was ascribed to a carbon—carbon triple bond, revealing that the benzene rings had been dehydrogenated during the experiments. We conclude that one side benzene ring was dehydrogenated as a result of pyrolysis, with the Ag tip acting as source of local heat and catalyst.
This work is supported by a Grant-in-Aid for Scientific Research (S) (No. 24221009) from the Ministry of Education, Culture, Sports, Science and Technology, Japan and National Science Centre (2017/26/E/ST8/00987), Poland.
5:00 PM–7:00 PM Apr 23, 2019 (US - Arizona)
PCC North, 300 Level, Exhibit Hall C-E