It has been an aspiring goal to mimic biological channels like trans-membrane protein channels or aquaporin to achieve the ultimate size to make use of the exotic properties of these channels. Some efforts have been recognised to create a structure with dimensions approaching the size of small ions and water molecules but precise control on geometry and surface roughness makes it challenging to produce capillaries at this spatial scale. I have fabricated graphene and other 2D materials (h-BN and MoS2) based capillaries with heights ranging from a single atomic layer of graphene to a few layers (up to several tens of nanometers) by following nanofabrication protocols developed by Radha et al (Nature 2016). This fabrication method provides insight on removing the atomic layers through micromechanical cleavage and restacking these 2D layers through van der Waals heterostructures with no loss in atomic continuity. Such 2D capillaries use high-quality layered crystals (graphite, h-BN and MoS2) and basal planes of these 2D crystals act as walls (top and bottom). These novel atomic-scale capillaries are employed to study the interaction of molecules (gas, water and ions) inside these conduits for permeation and separation based on size exclusion. In the case of gas permeation, our results demonstrate that the frictionless surface of graphene induces enhanced gas flow and shows a 2-3 order difference compared to classical Knudsen flow. I have also checked the selectivity of different gases as compared to helium gas but no selectivity was found. Atomic-scale vacancies are demonstrated in monolayer tungsten disulfide (WS2) as the ultimate atomic limit of the pores, created by focused ion beam irradiation. The permeability of helium through this aperture validate Knudsen descriptions to quantify the relation between atomic-scale vacancies and gas flow. Atomic-scale vacancies proved to be mechanically robust and showed fast helium flow. Using angstrom scale capillaries, the response of ions flow under mechanical and electrical (pressure and voltage) forces are measured to explore water and ions transport coupling. Ionic motion in these ultimate scale conduits is affected by the channel walls and hydration shells of ions and transport strongly depends on wall 14 material (graphite and h-BN). Ionic flow is driven by pressure and applied electric force reveals a transistor-like electrohydrodynamic effect under such confinement i.e. slight increase (in fraction) in voltage significantly enhances the measured pressure-driven ionic transport up to 20 times. Overall, the nanofluidic structures in this thesis will provide a unique platform to study molecular transport at the ultimate atomic scale. The device structures used and the results of this thesis will help further 2D materials use to study the dynamics of molecules for their detection and separation.
Molecular transport through atomic-scale apertures and capillaries.
Dar, S. A. (Author). 1 Aug 2022
Student thesis: Phd