Study on the properties of modified octadecyltrichlorosilane (OTS) anti-relaxation coatings for cesium atomic cell: a molecular dynamics simulation

To investigate the anti-relaxation performance of FOTS-modified OTS coatings on the inner walls of cesium (Cs) atomic cell, this study employs molecular dynamics (MD) simulations to explore the self-assembly process of FOTS-modified OTS molecular chains on the SiO₂ (001) surface and evaluates the effects of FOTS chain amounts, water molecule content, and temperature on the diffusion behavior of Cs atoms. Results show that the optimized interface model of the FOTS-modified OTS coating and SiO₂ substrate achieves thermodynamic and energetic equilibrium under the conditions of 25 °C and 2000 ps. The film formation process of FOTS-modified OTS chains on SiO₂ surfaces involves three distinct stages: initial anchoring, conformational rearrangement, and structural relaxation and equilibrium configuration. The molecular chains evolve from an initial perpendicular orientation into a final stable configuration parallel to the substrate surface. Increasing the FOTS content effectively reduces the diffusion coefficient of Cs atoms. The optimal OTS : FOTS blending ratio is identified as 20 : 8, yielding a minimum diffusion coefficient of 0.204 × 10^-6 cm² s^-1. Water molecules exhibit a significant influence on Cs diffusion dynamics. As the water content increases from 1 to 20, the diffusion coefficient rises from 0.271 × 10^-6 cm² s^-1 to 2.387 × 10^-6 cm² s^-1. Additionally, the mobility of Cs atoms displays a non-monotonic dependence on temperature, where the diffusion coefficient initially decreases and then increases with rising temperature, reaching a minimum value of 0.172 × 10^-6 cm² s^-1 at 80 °C. These findings provide theoretical guidance for the design, optimization, and fabrication of high-performance anti-relaxation coatings for Cs atomic vapor cell applications.