Abstract
Nowadays, reversible friction regulation has become the focus of scientists in terms of the flexible regulatory structure of photosensitive materials and theories since this facilitates rapid development in this field. Meanwhile, as an external stimulus, light possesses great potential and advantages in spatiotemporal control and remote triggering. In this work, we demonstrated two photo-isomerized organic molecular layers, tetra-carboxylic azobenzene (NN4A) and dicarboxylic azobenzene (NN2A), which were selected to construct template networks on the surface of the highly oriented pyrolytic graphite (HOPG) to study the friction properties, corresponding to the arrangement structure of self-assembled layers under light regulation. First of all, the morphology of the self-assembled layers were characterized by a scanning tunneling microscope (STM), then the nanotribological properties of the template networks were measured by atomic force microscope (AFM). Their friction coefficients are respectively changed by about 0.6 and 2.3 times under light control. The density functional theory (DFT) method was used to calculate the relationship between the force intensity and the friction characteristics of the self-assembled systems under light regulation. Herein, the use of external light stimulus plays a significant role in regulating the friction properties of the interface of the nanometer, hopefully serving as a fundamental basis for further light-controlling research for the future fabrication of advanced on-surface devices.
Highlights
Reversible friction control is a topic of great interest to scientists, especially as it can help realize the control of the interface friction characteristics of microscopic surfaces
Combined with previous research results, the assembly structure of NN4A on the highly oriented pyrolytic graphite (HOPG) surface forms a Kagomeimage in which two adjacent carboxylic groups form a pair of hydrogen bonds and make up the network structure, which included two cavities formed by six symmetry benzene rings and three NN4A molecules, respectively
The NN4A self-assembly layer on HOPG could isomerize from trans to cis isomers upon irradiation with UV light at 365 nm and reversibly isomerize from cis to trans when irradiation with visible light at 560 nm
Summary
Reversible friction control is a topic of great interest to scientists, especially as it can help realize the control of the interface friction characteristics of microscopic surfaces. Light-Controlled Friction by Carboxylic Azobenzene control of photochromic self-assembly due to its remote control, non-invasive, and cleaning properties. Self-assembly is capable of rendering the surface regular molecular arrangements and functional properties, which offers a useful way for the study of interfacial nano-tribology. The regulation of the self-assembled photo-inductive material coating by light irradiation on the surface interface, and using the properties of photo-isomerism to reversibly change the structure and properties of the coating, have broad application prospects. We utilized the supramolecular templates driven by different coupling systems, which are constructed by carboxylic azobenzene self-assembly molecules with different numbers of carboxylic groups on the HOPG substrate. We expected that the fundamental research of onsurface photo-chemistry at the level of a single molecule can considerably contribute to the future fabrication of advanced onsurface devices, simultaneously providing a pathway to fabricate certain molecular devices
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