Dana Byrne1,2,Archana Raja2,Aleksandr Noy3,4,Jim Ciston2,Alex Smolyanitsky5,Frances Allen1,2
University of California, Berkeley1,Lawrence Berkeley National Laboratory2,Lawrence Livermore National Laboratory3,University of California, Merced4,National Institute of Standards and Technology5
Dana Byrne1,2,Archana Raja2,Aleksandr Noy3,4,Jim Ciston2,Alex Smolyanitsky5,Frances Allen1,2
University of California, Berkeley1,Lawrence Berkeley National Laboratory2,Lawrence Livermore National Laboratory3,University of California, Merced4,National Institute of Standards and Technology5
<b>The engineering of vacancy defects to create sub-nanometer pores in 2D materials is of great interest for the development of advanced membrane-based devices for ion and molecular transport and sensing applications. Recent theory and simulation work has shown that sub-nanometer pores in these materials can exhibit strain-dependent transport behavior that mimics mechanosensitivity in biological membrane channels [1,2]. In pursuit of a solid-state analog for biological mechanosensitivity, heteroatomic 2D materials such as MoS<sub>2</sub> and hexagonal boron nitride (hBN) present the advantage that nanopores produced in these materials are already “functionalized” by default, eliminating the need for additional chemical functionalization of the pore edges.</b><br/><b>Based on preliminary molecular dynamics simulations of mechanosensitive ion transport through hBN nanopores, we are specifically targeting the fabrication of tetravacancies in monolayer hBN of the type B<sub>3</sub>N<sub>1</sub>, i.e. triangular nanopores terminated with nitrogen atoms. A proven method for the fabrication of atomically-precise nanopores in hBN uses electron beam illumination in a transmission electron microscope (TEM) [3]. This technique employs dose-controlled and site-selective illumination to nucleate and grow vacancy defects as a result of elastic and inelastic scattering effects. Critically, the TEM-based method also enables in-situ imaging of the nanopores with atomic resolution during the nanopore growth process. An alternative fabrication method uses ion bombardment in a focused ion beam (FIB) microscope to nucleate defects and produce nanopores [4,5], offering higher throughput albeit with less control over the exact nature and size of the resulting pores.</b><br/><b>In our work we start with few-layer hBN flakes produced by mechanical exfoliation from bulk hBN crystals, which we then transfer to holey silicon nitride substrates. Using the TEM method, we further exfoliate the hBN flakes in-situ using the electron beam to produce monolayer regions, and then preferentially form boron vacancies and grow triangular nitrogen-terminated nanopores by appropriate tuning of the electron dose. We compare the results with the nanopores produced when irradiating samples with helium, neon and gallium ions using a multibeam FIB microscope (the Zeiss ORION NanoFab at UC Berkeley). Samples are imaged at atomic resolution using a double-aberration-corrected (scanning)TEM (STEM) (the TEAM I instrument at the Molecular Foundry, LBNL) and we use electron energy-loss spectroscopy to determine the elemental composition of the pore edges. Finally, we will discuss a combined electron- and ion-beam-based fabrication approach, in which we use the FIB to first nucleate individual vacancy defects and then the TEM for controlled growth and characterization. This combined approach paves the way to both high-precision and high-throughput vacancy defect engineering of sub-nanometer pores in 2D materials. </b><br/><br/><b>[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] Yoon, Kichul, Ali Rahnamoun, Jacob L. Swett, Vighter Iberi, David A. Cullen, Ivan V. Vlassiouk, Alex Belianinov, et al. 2016. “Atomistic-Scale Simulations of Defect Formation in Graphene under Noble Gas Ion Irradiation.” ACS Nano 10 (9): 8376–84. </b><br/><b>[5] Allen, Frances I. 2021. “A Review of Defect Engineering, Ion Implantation, and Nanofabrication Using the Helium Ion Microscope.” Beilstein Journal of Nanotechnology 12 (July): 633–64. </b>