[论文解读] Understanding the anomalously low dielectric constant of confined water: an ab initio study

Thomas Dufils, Christoph Schran+4 more|arXiv (Cornell University)|Nov 25, 2022

Electrostatics and Colloid Interactions被引用 5

一句话总结

本研究采用从头算分子动力学（AIMD）和AIMD训练的机器学习势，揭示了石墨烯与hBN纳米狭缝中水的异常低面外介电常数源于前两层界面水分子中偶极子的反平行排列，从而诱导出净铁电序并抑制了极化涨落，且该效应与狭缝宽度和材料类型无关。

ABSTRACT

Recent experiments have shown that the out-of-plane dielectric constant of water confined in nanoslits of graphite and hexagonal boron nitride (hBN) is vanishingly small. Despite extensive effort based mainly on classical force-field molecular dynamics (FFMD) approaches, the origin of this phenomenon is under debate. Here we used ab initio molecular dynamics simulations (AIMD) and AIMD-trained machine learning potentials to explore the structure and electronic properties of water confined inside graphene and hBN slits. We found that the reduced dielectric constant arises mainly from the anti-parallel alignment of the water dipoles in the perpendicular direction to the surface in the first two water layers near the solid interface. Although the water molecules retain liquid-like mobility, the interfacial layers exhibit a net ferroelectric ordering and constrained hydrogen-bonding orientations which lead to much reduced polarization fluctuations in the out-of-plane direction at room temperature. Importantly, we show that this effect is independent of the distance between the two confining surfaces of the slit, and it originates in the spontaneous polarization of interfacial water. Our calculations also show no significant variations in the structure and polarization of water near graphene and hBN, despite their different electronic structures. These results are important as they offer new insight into a property of water that plays a critical role in the long-range interactions between surfaces, the electric double-layer formation, ion solvation and transport, as well as biomolecular functioning.

研究动机与目标

• 解决近期实验中观察到的纳米受限环境中水的异常低面外介电常数的起源。
• 通过采用能够精确捕捉电子效应和极化的从头算第一性原理方法，克服经典力场分子动力学（FFMD）的局限性。
• 探究尽管具有不同的电子结构，石墨烯与六方氮化硼（hBN）之间界面水的介电响应是否存在差异。
• 确定氢键网络拓扑结构与偶极子取向在抑制垂直于限制表面方向的介电涨落中的作用。
• 确立该效应是界面水的本征性质，且与限制壁之间的距离无关。

提出的方法

• 基于密度泛函理论（DFT）的从头算分子动力学（AIMD）模拟，用于计算受限水的电子结构和极化。
• 采用外场法，施加±1.0 Vnm⁻¹的电位移，以计算介电常数，确保线性响应与收敛性。
• 通过差分极化计算，通过减去裸限制表面的响应，分离出水的贡献。
• 开发并使用基于AIMD数据训练的Behler-Parrinello神经网络机器学习势（NNPs），实现在DFT精度下长时间尺度的模拟。
• 分析界面水层中偶极矩大小、取向分布及氢键网络的拓扑结构。
• 通过在分子层（0.4 nm宽度）上积分介电剖面，提取介电响应，与连续介质理论一致。

实验结果

研究问题

• RQ1 石墨烯与hBN纳米狭缝中水的异常低面外介电常数的微观起源是什么？
• RQ2 界面水层中偶极子排列与氢键模式如何影响极化涨落？
• RQ3 介电抑制效应是否依赖于限制壁之间的距离或限制材料的化学性质？
• RQ4 经典力场模型在多大程度上无法捕捉界面水的真实介电响应？
• RQ5 第一性原理模拟能否解决实验观测与先前FFMD模拟之间的差异？

主要发现

• 面外方向介电常数的降低（ε⊥ ≈ 2）主要源于前两层界面水分子中偶极子的反平行排列。
• 尽管具有液态流动性，这些界面层由于氢键取向受限，表现出净铁电序。
• 在室温下，垂直方向的极化涨落被强烈抑制，导致观测到的介电常数降低。
• 该效应与狭缝宽度无关，表明其为水-表面界面的本征性质，而非几何受限的伪影。
• 尽管石墨烯与hBN的电子结构不同，但未发现水结构或极化存在显著差异。
• 中心水层的介电响应接近体相性质，支持在差分计算方法中使用体相值。