Advanced Chemical Characterization of Sub-Nanometer Pores in 2D Hexagonal Boron Nitride

Understanding and controlling the local chemistry and structure of vacancy defect clusters (i.e. nanopores) in 2D materials is essential for the development of advanced membranes that leverage the unique transport properties of these systems. In particular, nanopores in solid state 2D membranes can be designed to finely control ion selectivity and molecule permeability, and are thus of great interest for advanced applications in filtration and molecular sensing. Recent theoretical work and simulation has demonstrated strain-modulated ion transport behavior in graphene-embedded crown ethers, where oxygen termination of the pore edges forms C^δ+-O^δ-dipoles radially aligned toward the center of each pore [1]. To investigate this mechanosensitivity effect experimentally, we turn to the heteroatomic 2D material hexagonal boron nitride (hBN). This is because geometric (specifically, triangular) nanopores can be fabricated in 2D hBN with radially oriented dipoles by default, eliminating the need for additional chemical functionalization steps.<br/>To fabricate nanopores in 2D hBN with controlled shape and size, we use ion beam [2], electron beam [3], and combined ion and electron beam methods [4]. The ion irradiation is performed using an ion beam microscope, whereas the electron beam manipulation is carried out in a transmission electron microscope (TEM) with in-situ imaging at atomic resolution to track the pore growth process. Although we have control over nanopore size and shape using the TEM, dangling bonds present on the nanopore edges will be susceptible to chemical changes. Environmental effects are thus expected to alter the nanopore transport properties, since ion transport is heavily dependent on the electrostatic and steric interactions inside the pore. Therefore, it is essential that we elucidate the chemical structures of the nanopores in order to understand how they behave in a real system.<br/>We survey the local chemistry of our nanopores using monochromated electron energy loss spectroscopy (EELS) in the TEM, probing local bonding and chemical composition based on variations in the fine structure of the boron K-edge signals. We find that oxygen dopant defects are present in varying amounts in a range of hBN nanopore samples and that the local chemistry of the nanopores is susceptible to change both inside the vacuum environment of the microscopes as well as upon exposure to air. Furthermore, we find that nanopore stability in air is greatly affected by the presence of hydrocarbon surface contaminants. These results highlight the importance of thoroughly characterizing the chemical structure of 2D nanopores when they are exposed to different environments in order to enable accurate simulation and interpretation of ion transport data.<br/><br/>[1] Fang, A., K. Kroenlein, D. Riccardi, and A. Smolyanitsky. 2019. “Highly Mechanosensitive Ion Channels from Graphene-Embedded Crown Ethers.” Nature Materials 18 (1): 76–81. https://doi.org/10.1038/s41563-018-0220-4.<br/><br/>[2] Allen, Frances I. 2021. “A Review of Defect Engineering, Ion Implantation, and Nanofabrication Using the Helium Ion Microscope.” Beilstein Journal of Nanotechnology 12: 633–64. https://doi.org/10.3762/bjnano.12.52.<br/><br/>[3] Meyer, Jannik C., Andrey Chuvilin, Gerardo Algara-Siller, Johannes Biskupek, and Ute Kaiser. 2009. “Selective Sputtering and Atomic Resolution Imaging of Atomically Thin Boron Nitride Membranes.” Nano Letters 9 (7): 2683–89. https://doi.org/10.1021/nl9011497.<br/><br/>[4] Dana O Byrne, Archana Raja, Aleksandr Noy, Jim Ciston, Alex Smolyanitsky, Frances I Allen 2023. “Fabrication of Atomically Precise Nanopores in 2D Hexagonal Boron Nitride Using Electron and Ion Beam Microscopes.” Microscopy and Microanalysis, Volume 29, Issue Supplement_1 (August): 1375–1376. https://doi.org/10.1093/micmic/ozad067.707