金属纳米薄膜在石墨基底表面的动力学演化

Dynamical evolution study of metal nanofilms on graphite substrates

采用分子动力学方法研究了金属Au和Pt纳米薄膜在石墨(烯)基底表面的动力学演化过程,探讨了金属薄膜和石墨(烯)基底间的相互作用对金属纳米薄膜在固态基底表面的去湿以及脱附的动力学演化的影响.研究结果表明,在高温下,相同层数的Au和Pt纳米薄膜在单层石墨基底表面上存在不同的去湿现象,主要表现为厚度较小的Pt纳米薄膜在去湿过程中有纳米空洞形成,而同样厚度的Au薄膜在去湿过程中没有形成空洞.Au和Pt两种金属薄膜在高温下都去湿形成纳米液滴,这些液滴最终都以一定的速度脱离基底.在模拟的薄膜厚度范围内(0.2—2.3 nm),Au和Pt纳米液滴脱离基底的速度随厚度增加表现出不同的变化规律.Pt纳米液滴的脱离速度随薄膜初始厚度的增加先增加后减少,而Au脱离速度随厚度的增加先减少,达到一个临界厚度后脱离速度突然迅速增加.利用薄膜与基底间相互作用的不同导致去湿过程中的黏滞耗散不同,定性分析了这种变化规律的原因.此外,进一步研究还发现金属液滴的脱离时间与薄膜厚度和模拟温度的依赖关系,发现脱离时间随薄膜厚度的增加而增加,随模拟温度的升高而减小.这些研究结果可以为金属镀膜、浮选、表面清洁、器件表面去湿等工业生产过程提供理论指导.

The dynamical evolution process of nanoscaled film on a solid substrate depends on many factors,such as the properties of thin film,the characteristics of the substrate,and the external environment.It is essential to elucidate the influences of these factors for our understanding self-organized growth of nanoparticles and the dewetting/detachment mechanism of nanofilm on a solid substrate.In the present paper,we investigate the dynamical dewetting/detachment of metal Au and Pt nanofilm on a graphene/graphite substrate at high temperature by using the molecular dynamics simulation technique.We discuss the influences of metal-substrate interaction,temperature and thickness of film on the dewetting dynamics.Our results reveal that the Au and Pt nanofilms with the same initial thickness on graphene substrates manifest different dewetting dynamical processes at high temperatures.Some nanoscale holes are formed randomly during the dewetting of Pt nanofilm with a thickness of less than 0.6 nm because of the strong interaction between the Pt films and substrate.In contrast,no hole is observed and a nanodroplet is formed directly by high temperature dewetting for Au nanofilm with the same initial thickness as that of Pt nanofilm.The resulting Au and Pt nanodroplets move in the vertical direction due to the surface tension and the constraint of the solid substrate.A high-temperature nanodroplet will be detached from the graphene substrate surface at a constant speed.Interestingly,the values of detachment velocity（v_d） of nanodroplets show different dependences on initial thickness for Au and Pt nanofilm,respectively.In a thickness range of 0.2 2.3 nm,the vd of Pt nanodroplet increases and then decreases as the thickness of nanofilm increases.However,the v_d of Au nanodroplet decreases gradually and then increases steeply as the Au nanofilm turns thicker.The different thickness dependences of v_d for Au and Pt nanofilms are analyzed qualitatively by considering different metal-substrate viscous dissipations.In addition,the detachment time（t_d） of a dewetting metal film is also related to the temperature and the thickness of substrate.Our results demonstrate that the t_d decreases monotonically with the decrease of film thickness and the raise of temperature.These results provide a theoretical guideline for industrial production processes,such as metal coating,flotation,and the surface cleaning.