Journal Article

Ballistic molecular transport through two-dimensional channels

Haigh, S. J., Janardanan, A., Esfandiar, A., Dar, S. A., Rooney, A. P., Wang, F. C., Keerthi, A., Hu, S., Radha, B., Geim, A. K., Grigorieva, I. V.

2018

Overview

Gas permeation through nanoscale pores is ubiquitous in nature and has an important role in many technologies 1 , 2 . 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 3 – 7 . This assumption holds surprisingly well in experiments, with only a few cases of partially specular (mirror-like) reflection known 5 , 8 – 11 . Here we report gas transport through ångström-scale channels with atomically flat walls 12 , 13 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 10 , 14 , and demonstrate the possibility of studying gas transport under controlled confinement comparable in size to the quantum-mechanical size of atoms. Specular scattering of atoms of helium gas flowing through atomically flat, two-dimensional channels results in frictionless gas flow, which is much faster than expected assuming purely diffusive scattering.

• Publisher: Nature Publishing Group UK,Nature Publishing Group
• Subject: 142/126 / 142/136 / 639/301/119/544 / 639/766/119/544 / 639/766/119/995 / 639/925/927/1058 / Angle of reflection / Boron / Boron compounds / Boron nitride / Carbon / Channel pores / Channels / Confining / De Broglie wavelengths / Deuterium / Diffuse reflection / Diffusion rate / Gas flow / Gas permeation / Gas transport / Graphene / Graphite / Helium / Humanities and Social Sciences / Hydrogen / Isotope effect / Letter / Mass flow / Methods / Molecular chains / Molecular dynamics / Molecular modeling / Molybdenum / Molybdenum compounds / Molybdenum disulfide / multidisciplinary / Natural gas transmission / Nitrides / Penetration / Plasma etching / Pore size / Porosity / Properties / Scattering / Science / Science (multidisciplinary) / Silicon wafers / Specular reflection