以分子動力模擬探討二硫醇交聯之金奈米粒子薄膜力學性質及分子機制

Abstract

由奈米尺寸之金屬粒子和有機配體所組成的奈米粒子薄膜因為其特別的光電 和機械特性而被應用於多種微機電裝置(micro- and nanoelectromechanical, MEMS/NEMS devices)，例如:應變計(strain gauge)、觸控感測器(touch sensor)、氣 壓感測器(vapor sensor)等。過去有許多研究探討了奈米粒子薄膜在力學性質和導電 性上的可調控機制。然而，在實驗上較難觀察到奈米粒子薄膜的奈米尺度機制與這 些可調控特性間的關係。在本研究中，藉由分子動力學模擬(molecular dynamics)探 討短鏈二硫醇交聯之金奈米粒子薄膜的力學性質和分子機制。我們提供一套方法 構建金奈米粒子薄膜的分子動力模擬模型，並得以控制薄膜的二硫醇鍊長、核心金 粒子直徑以及二硫醇接枝密度。金奈米粒子在形成穩定的面心立方超晶格結構，並 且金奈米粒子薄膜模型中的奈米粒子間距和薄膜楊氏係數與實驗結果接近。結果 表明，較短的二硫醇、較大的核心金粒子或較高的二硫醇接枝密度皆會導致較高的 薄膜楊氏係數。除此之外，透過觀察奈米尺度下的配體結構，發現連接兩相鄰金粒 子的全反式構型二硫醇數量與薄膜勁度呈正相關。瞭解並觀察金奈米粒子薄膜的 機械性質與其微觀結構的關係可以使我們對該材料有更深入的認識。

Gold nanoparticle thin films, composed of gold cores and organic ligands, have been used for a variety of applications because of their exceptional electronic, optical, and mechanical properties. Numerous studies explored their tenability of the elastic and electronic transport properties. However, experimental studies did not indicate clear nanoscale mechanisms for their tunable properties. In this research, molecular dynamics simulations are used to explore the nanoscale features and mechanical responses of alkanedithiol cross-linked gold nanoparticle thin films. We employed an approach to construct the cross-linked thin film models with different ligand chain lengths, nanoparticle core sizes, and ligand grafting density. The thin films with 3-D fcc superlattice structure are stable and have interparticle distances and elastic moduli consistent with the experiment. The results show that shorter ligand length, larger core size, or dense ligand coverage lead to stiffer thin films. By observing the ligand structure at nanoscale, we conclude that the number of all-trans bridge linkers is positively correlated with the thin film’s stiffness. Understanding these nanoscale details can provide us deeper insights into the mechanical properties of the cross-linked gold nanoparticle thin film.