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Micro & Nano Letters

Study on desorption mechanism and thermal stability of OTS coating as an anti‐relaxation material

Guodong Liu, Xinxin He, Xiaoya Liu, Donghui Ma, Jun Tang, Zongmin Ma, Yan Jun Li, Jun Liu

Published Dec 16, 2024• 3 citations
Publisher

Abstract

Abstract This research explores the desorption mechanism and thermal stability of octadecyltrichlorosilane (OTS, CH 3 (CH 2 ) 17 SiCl 3 ) coating on quartz slides and in alkali‐metal vapour cells. The morphological thermal‐changes, energy dissipation diversity, and anti‐relaxation characteristic of OTS coatings before and after exposure to Cs atoms by Fourier transform infrared spectroscopy, water contact angle measurement, atomic force microscopy imaging, collision energy dissipation analysis, and free induction decay of Cs atoms are measured systematically. The results show that the OTS coatings exhibit the best thermal stability under the specific process conditions, and the homogeneous and dense structure makes the adsorption of alkali‐metal atoms more stable, which effectively reduces surface energy dissipation and prolongs the relaxation time of Cs atoms. The study provides certain reference for efficient anti‐relaxation coating fabrication and coated cell application.

DOI
10.1049/mna2.70006

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Abstract This research explores the desorption mechanism and thermal stability of octadecyltrichlorosilane (OTS, CH 3 (CH 2 ) 17 SiCl 3 ) coating on quartz slides and in alkali‐metal vapour cells. The morphological thermal‐changes, energy dissipation diversity, and anti‐relaxation characteristic of OTS coatings before and after exposure to Cs atoms by Fourier transform infrared spectroscopy, water contact angle measurement, atomic force microscopy imaging, collision energy dissipation analysis, and free induction decay of Cs atoms are measured systematically. The results show that the OTS coatings exhibit the best thermal stability under the specific process conditions, and the homogeneous and dense structure makes the adsorption of alkali‐metal atoms more stable, which effectively reduces surface energy dissipation and prolongs the relaxation time of Cs atoms. The study provides certain reference for efficient anti‐relaxation coating fabrication and coated cell application.
Paraffin and octadecyltrichlorosilane (OTS) coatings can alleviate collisions between alkali-metal atoms and cell walls and then prolong the atomic spin-polarization lifetime. The surface structure and collision effects of these antirelaxation coatings, as well as the methods to avoid antirelaxation invalidity, have been the focus of researchers. This study investigated the thermolability of coating surface structure and the collision interactions between alkali metal atoms and coatings, considering the influence of various coating preparation factors, where this collision interaction is indirectly analyzed by measuring the collision energy dissipation between an atomic force microscopy (AFM) probe and the atoms on the coating surface. We found that appropriate evaporation time, carbochain length, and postannealing process can enhance the thermostability of the paraffin coating and eliminate its morphological defects. Furthermore, the OTS/water concentration, the soaking time, and the type of solvent have different levels of influence on the cluster formation and the thermostability of the OTS coatings. Moreover, the antirelaxation performance of coatings has been shown to be characterized by counting the energy dissipated when the AFM probe collides with the antirelaxation coating, replacing the conventional light-atom interaction- based method for measuring the relaxation characteristics, but requiring specific coating preparation factors to be maintained.
In the process of miniaturizing atomic spin gyroscopes, the relaxation due to the wall collisions of the alkali vapor cell leads to a shorter transverse relaxation time, which increases the key index of the atomic spin gyroscope, the angle random walk, and reduces the gyroscope’s accuracy. This paper theoretically analyzes the thermal stability of the interaction between the new anti-relaxation coating and molecules of the vapor cell, as well as the surface energy of the anti-relaxation coating molecular groups. Based on the results, a new anti-relaxation coating material and its preparation process were proposed. A novel anti-relaxation coating was experimentally prepared. Compositional analysis confirmed its successful covalent bonding with hydroxyl groups on the cell inner-surface, demonstrating superior thermal stability. Through comparative experiments including surface morphology analysis, contact angle measurement, and transverse relaxation time measurement, the new anti-relaxation coating proposed in this paper exhibits lower surface energy. It also has more excellent anti-relaxation performance compared to the traditional OTS anti-relaxation coating.
Abstract Anti-relaxation coatings play an important role in enhancing the polarization lifetime of polarized alkali-metal atoms. In this study, octadecyltrichlorosilane (OTS) was coated in spherical rubidium vapor cells fabricated using borosilicate glass with two stems as anti-relaxation coatings. An orthogonal experiment method was employed to obtain the optimal conditions. The results showed that OTS concentration is the dominant factor in the number of bounces, followed by the soaking time and molar ratio/(H2O:OTS). The optimal parameters were 6 mM of OTS, 6:1 of molar ratio/(H2O:OTS), and 80 min of soaking time. The number of bounces can be up to 200. Atomic force microscope images showed that the number and coverage ratio of clusters on the anti-relaxation coating fabricated by the optimized parameters are significantly less than the number and coverage ratio of clusters formed by other conditions. This means that the clusters in the coatings significantly affect the anti-relaxation performance. In addition, five OTS-coated cells were fabricated by A3B3C5 combination. However, the variability from cell to cell using the exact same preparation procedure can be up to 43.6%. The variability may be caused by the different aging times of the OTS solution. Because only one beaker of OTS solution was prepared, the OTS solution was pumped into the cells in sequence, not at the same time.
To enhance the anti-relaxation performance of alkali metal vapor cells in quantum sensors, a synergistic optimization and quantitative evaluation strategy for surface parameters of octadecyl-trichlorosilane (OTS) anti-relaxation coatings is proposed. By systematically adjusting solution concentration and reaction time, both the polarization retention angle and the surface polarization retention cross-section are effectively optimized, achieving a maximum retention angle of 135°. A theoretical framework is subsequently established to quantify adsorption energy based on the inherent correlation between these two surface parameters. A correction method for the retention cross-section, developed through a non-uniform wetting model, enables in-situ mapping of the kinetic contribution to adsorption behavior. Through kinetic and thermodynamic parameter extraction, the adsorption energy and Gibbs free energy of the multilayer OTS coatings are determined to be 0.13 eV, which represents a substantial reduction compared to the adsorption energy of bare glass surfaces, previously reported as 2.01 eV. Furthermore, longitudinal relaxation time measurements are carried out inside vapor cells, yielding a maximum collision count of 593, which provides strong evidence of polarization preservation capability. This approach provides both theoretical guidance and an experimental pathway for optimizing coating performance and analyzing adsorption mechanisms, thereby facilitating the development of high-performance alkali metal vapor cell materials for quantum sensing applications. • Synergistic optimization of OTS coating parameters improved polarization retention, achieving a maximum retention angle of 135° by adjusting solution concentration (2-6 mM) and reaction time (60-120 min). • A heterogeneous wetting-based synergy quantification method is developed, enabling accurate evaluation of multilayer formation and local adsorption behavior by jointly analyzing retention angle and retention cross-section. • A combined thermodynamic-kinetic modeling framework establishes quantitative correlations between adsorption energy, Gibbs free energy, and coating surface characteristics, providing theoretical guidance for anti-relaxation material design. • Longitudinal relaxation tests in coated vapor cells demonstrate a maximum collision count of 593, confirming suppressed alkali-atom adsorption and strong polarization preservation consistent with the low adsorption energy of 0.13 eV.
Surface relaxation rates differ for spin-polarized alkali atoms interacting with monolayer or bilayer octadecyltrichlorosilane (OTS) coatings. The morphology and composition of Rb vapor-exposed films of OTS have been studied with atomic force microscopy (AFM) and x-ray photoelectron spectroscopy (XPS). On OTS monolayers, numerous small (<500 nm wide) Rb containing islands nucleate at the boundaries of the ∼1–2 μm wide organic domains. On OTS bilayers, singular large (∼3 μm wide) Rb containing islands were found. Alkali island formation mediated by surface structure could affect the antirelaxation behavior of organic coatings used in atomic magnetometer cells.