School of Physics and Astronomy, University of Manchester, Manchester, UK.
National Graphene Institute, University of Manchester, Manchester, UK.
Nature. 2018 Jun;558(7710):420-424. doi: 10.1038/s41586-018-0203-2. Epub 2018 Jun 20.
Gas permeation through nanoscale pores is ubiquitous in nature and has an important role in many technologies. Because the pore size is typically smaller than the mean free path of gas molecules, the flow of the gas molecules is conventionally described by Knudsen theory, which assumes diffuse reflection (random-angle scattering) at confining walls. This assumption holds surprisingly well in experiments, with only a few cases of partially specular (mirror-like) reflection known. Here we report gas transport through ångström-scale channels with atomically flat walls and show that surface scattering can be either diffuse or specular, depending on the fine details of the atomic landscape of the surface, and that quantum effects contribute to the specularity at room temperature. The channels, made from graphene or boron nitride, allow helium gas flow that is orders of magnitude faster than expected from theory. This is explained by specular surface scattering, which leads to ballistic transport and frictionless gas flow. Similar channels, but with molybdenum disulfide walls, exhibit much slower permeation that remains well described by Knudsen diffusion. We attribute the difference to the larger atomic corrugations at molybdenum disulfide surfaces, which are similar in height to the size of the atoms being transported and their de Broglie wavelength. The importance of this matter-wave contribution is corroborated by the observation of a reversed isotope effect, whereby the mass flow of hydrogen is notably higher than that of deuterium, in contrast to the relation expected for classical flows. Our results provide insights into the atomistic details of molecular permeation, which previously could be accessed only in simulations, and demonstrate the possibility of studying gas transport under controlled confinement comparable in size to the quantum-mechanical size of atoms.
气体通过纳米级孔隙的渗透在自然界中无处不在，在许多技术中都具有重要作用。由于孔径通常小于气体分子的平均自由程，气体分子的流动通常用克努森理论来描述，该理论假设在限制壁处发生漫反射（随机角散射）。这一假设在实验中得到了惊人的验证，只有少数部分镜反射（镜面反射）的情况已知。在这里，我们报告了通过具有原子级平坦壁的埃级通道的气体传输，并表明表面散射可以是漫反射或镜反射，这取决于表面原子地形的细微差别，并且量子效应有助于室温下的镜反射。这些通道由石墨烯或氮化硼制成，允许氦气以比理论预期快几个数量级的速度流动。这可以通过镜面表面散射来解释，镜面表面散射导致弹道传输和无摩擦的气体流动。类似的通道，但具有二硫化钼壁，其渗透速度要慢得多，但仍符合克努森扩散的描述。我们将这种差异归因于二硫化钼表面更大的原子波纹，其高度与正在传输的原子及其德布罗意波长相似。这种物质波贡献的重要性得到了一个相反的同位素效应的观察结果的证实，即氢的质量流量明显高于氘的质量流量，与经典流动预期的关系相反。我们的结果提供了对分子渗透的原子细节的深入了解，这些细节以前只能在模拟中获得，并展示了在与原子量子力学尺寸相当的受控限制下研究气体传输的可能性。
