直流电场下水中石墨烯定向行为研究

DC electric field induced orientation of a graphene in water

纳米颗粒在液体环境中的定向控制与系统物理性能的调控及新型纳米器件的制备等应用领域密切相关.本文使用分子动力学模拟方法,研究了水中单片不带电矩形石墨烯在直流电场下的定向行为.结果发现石墨烯平面趋向平行于电场方向且随着电场强度增大定向性增强,其主要原因在于极性水分子在电场下的响应以及水合作用;减小石墨烯长宽比,石墨烯法向矢量和长边矢量的定向性减弱,定量结果表明法向和长边定向度分别与绕长边和法向的转动扩散系数存在负相关关系.

Graphene, as a classical two-dimensional material, has various excellent physical properties, which can be further transferred into its nanocomposite. Under external fields, the nonspherical nanoparticles in liquid environment will exhibit various deterministic movements, among them is the orientation behavior. By realizing the orientation control of nanoparticles, we can, on one hand, increase the thermal conductivity of the system along the oriented direction, and on the other hand, fabricate novel nano-devices based on the nanoscale self-assembly, which may become the key components in NEMS and Lab-on-a-chip architectures. However, current studies mainly focus on the orientations of one-dimensional rod-shaped particles, like carbon nanotubes. For a two-dimensional nanoparticle, like graphene, the situation is more complex than the one-dimensional one, because two unit vectors should be defined to monitor the orientation behaviors.As far as we know, this part of research has not been extensively carried out. Thus, in this paper, the molecular dynamics method is used to study the orientation of a single uncharged rectangular graphene in water, induced by DC electric fields.We track the orientations of the normal and long-side vectors of graphene. The results show that at a relatively high electric strength of 1.0 V/nm, the graphene is preferred to orient its normal vector perpendicular and its long-side vector with a small angle（located between 0°and 30°） with respect to the electric direction, respectively. With the increase of the electric field strength, the orientation preference of the normal vector along the electric direction is increased.To explain this phenomenon, we calculate the orientation distribution of water molecules in the first hydration shell.The dipoles tend to be parallel to the electric direction, and the surfaces of water molecules tend to be parallel to the surface of graphene. These two combined effects result in the above orientation behavior of the normal vector. Another interesting phenomenon is that the decrease of the length to width ratio of graphene will cause both the orientation preferences of the normal vector and the long-side vector to decrease. By utilizing the Einstein relation, we can obtain the rotational diffusion coefficients of graphene around the normal vector and long-side vector. The qualitative results show that the orientation orders of the normal vector and long-side vector respectively have negative correlations with the rotational diffusion coefficients of the rotation around the long-side vector and the normal vector. The orientation behavior of the platelike graphene actually comes from the competing effects between its rotational Brownian motion and the external field. Increasing the strength of the external field or reducing the rotational diffusivity will both lead to an increased orientation order of the nonspherical nanoparticle.