Defect Engineering of 2D Hexagonal Boron Nitride Using Electron and Ion Beam Methods

<b>Vacancy defect engineering of 2D membranes is essential for advancing fundamental understanding of ion and molecule transport through subnanometer pores for applications in filtration and molecular sensing. Nanopores in 2D materials are thermally and mechanically stable, and are tunable in terms of their shape, chemistry, distribution, and size. Therefore, nanopores in solid state 2D membranes can have fine control over ion selectivity and molecule permeability and have become primary candidates for selective transport membranes. For example, recent theory and simulation has shown that ion transport behavior in graphene-embedded crown ethers and in sub-nanometer pores in MoS2 can be strain-modulated [1,2]. This behavior mimics that of mechanosensitive ion channels in biological membranes. In pursuit of a solid-state analog for biological mechanosensitivity, heteroatomic 2D materials such as MoS2 and hexagonal boron nitride (hBN) present the advantage of pre- “functionalized” nanopores by default, eliminating the need for additional chemical functionalization of the pore edges, as would be the case for graphene. </b><br/><br/><b>In our work we use electron beam and ion beam methods to fabricate nanopores in 2D hBN. We use hBN exfoliated from bulk and monolayer CVD-grown hBN, mechanically transferring the samples to holey substrates to produce suspended hBN. Following the electron beam method in a transmission electron microscope (TEM), we modulate the electron dose to nucleate vacancy defects and grow nanopores of desired size and shape [3]. This method enables in-situ nanopore fabrication and imaging at atomic resolution but lacks the high throughput necessary for large scale fabrication. In contrast, the ion beam method uses ion bombardment in a focused ion beam (FIB) microscope to nucleate defects and produce a large distribution of nanopores [4], offering higher throughput albeit with less control over the exact nature and size of the resulting pores. We investigate nanopores generated by irradiation with gallium, neon, and helium ions using a multibeam FIB microscope and also explore a combined ion and electron beam approach in which we seed vacancy defects using the the light helium ions and controllably grow those defects with the electron beam of the TEM. Nanopores are imaged at atomic resolution using a double-aberration-corrected (scanning)TEM (STEM).</b><br/><br/><b>Nanopore structure is also sensitive to environmental effects, which can alter the nanopore’s functional properties. We survey the local chemistry of the electron beam and ion beam fabricated nanopores using electron energy loss spectroscopy (EELS) in the TEM, probing the local bonding and chemical composition of the nanoporous regions based on variations in the core loss signals. We find that local chemistry of the nanopores is susceptible to change both inside the vacuum environment of the various microscopes as well as upon exposure to air. Hence, we argue that the investigation of the effect of the environment on the pore edge chemistry is essential for designing nanoporous 2D membranes for real life applications.<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.</b><br/><b>[2] Fang, A., K. Kroenlein, and A. Smolyanitsky. 2019. “Mechanosensitive Ion Permeation across Subnanoporous MoS2 Monolayers.” The Journal of Physical Chemistry C.</b><br/><b>[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.</b><br/><b>[4] 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.</b>