Subatomic species transport through atomically thin membranes: Present and future applications

Description Selective subatomic separations Membranes are thin materials used to selectively separate gases or liquids and are used on a range of scales from benchtop experiments to industrial processes. Challenges arise in separating materials with very similar sizes or chemical properties, particularly at the smallest scales. Kidambi et al. review advances in using atomically thin two-dimensional materials such as graphene or hexagonal boron nitride for the separation of subatomic species, including electrons, hydrogen isotopes, and gases. The authors explore the scope to scale up the sizes of these membranes and their potential use in applications relating to energy, microscopy, and electronics. —MSL The uses of atomically thin, two-dimensional materials as selective membranes for electrons, protons, and deuterons are reviewed. BACKGROUND Membranes are thin physical barriers that allow for transport of certain species while limiting others. Atomically thin two-dimensional (2D) materials are crystalline solids with constituent atoms bonded in a planar 2D sheet and present opportunities to realize ultrathin membranes that allow for selective transport of subatomic species. The pristine lattice of monolayer graphene (honeycomb mesh of carbon atoms) and hexagonal boron nitride (h-BN, honeycomb mesh of alternating B and N atoms) is impermeable to small atoms such as helium (at room temperature) but allows for energy-dependent transmission of electrons and electric-field-driven transport of protons and deuterons. Energy-dependent electron transmission through graphene, coupled with its atomic thinness, impermeability to gases, high strength, and electrical conductivity, provides opportunities to overcome material challenges limiting the advance of electron microscopy and spectroscopy to sample environments beyond high vacuum. Selective proton transport through h-BN, graphene, and other 2D materials with impermeability to even small atoms can help advance energy storage and conversion processes by providing avenues to address persistent problems of crossover and poor cation selectivity in conventional proton exchange membranes that typically result in long-term efficiency degradation. The differences in the rates of selective proton and deuteron transport through h-BN and graphene offer transformative opportunities for advancing isotope separations over current processes that are extremely energy intensive. ADVANCES The impermeability of graphene and h-BN to helium and other gases was initially demonstrated by using monolayer flakes to seal micrometer-sized gas-filled cavities, forming atomically thin balloons. However, subatomic species such as electrons tunnel through the lattice of monolayer graphene and h-BN and show promise for applications in vertical transistors and tunnel barriers for spintronic devices. The transmission of energic electrons through graphene varies with their kinetic energy and offers transformative opportunities for advancing electron microscopy (EM). Graphene’s high electron transparency enables its use as an ultrathin, crystalline, conductive sample substrate for transmission electron microscopy (up to ~80 kV), minimizing background noise (compared with ~3- to 20-nm-thick holey amorphous carbon films) and also allowing the formation of uniform ice thickness for cryogenic electron microscopy. The use of graphene as an electron transparent molecularly impermeable barrier to isolate detectors from the sample environment (while still allowing for transmission of electrons to and from the sample) allows for advancing conventional in situ metrology techniques requiring high-vacuum environments (EM and spectroscopy) to new frontiers such as ambient pressures, liquid systems, and biology. Permeation of protons through h-BN and graphene with impermeability to atoms provides avenues to mitigate crossover of reactants and undesired species, and enhance selectivity of proton exchange membranes for improved efficiencies over a wide spectrum of energy generation and conversion processes, including (i) fuels cells for clean/green transportation and distributed or mobile auxiliary power generation, (ii) electrochemical hydrogen pumps for distributed hydrogen production and purification, and (iii) redox flow batteries for grid storage. The different rates of selective proton and deuteron permeation through h-BN and graphene (due to differences in vibrational zero-point energy) results in separation factors for H+/D+ of ~8 to 14 and offers transformational advances for isotope separations. OUTLOOK Selective transport of subatomic species through graphene and h-BN presents the potential for breakthrough advances in several fields. However, detailed insights into the mechanisms of transport are still emerging. Small-scale applications will be the first to emerge, e.g., transmission electron microscopy grids with graphene as the sample substrate are available commercially, and the use of graphene as an electron transparent barrier for imaging and spectroscopy is steadily rising. Scalable synthesis of high-quality 2D materials and facile cost-effective processes to integrate them into devices are imperative to enable large-area applications. However, understanding the influence of defects inevitably introduced during scalable synthesis and the resulting differences in transport characteristics compared to pristine materials will be essential. Studies are also needed to assess long-term 2D material durability under realistic application conditions. With the potential for substantial reduction in energy consumption compared to existing technologies, H+/D+ isotope separations are most likely to be explored in the nuclear industry. Other energy-related applications are expected to become viable, with scaled-up 2D material production offering economies of scale and by considering energy savings over the application life cycle. Subatomic species transport through atomically thin membranes. (Left) Electrons tunnel through the lattice of monolayer graphene and h-BN. The transmission of energetic electrons through graphene depends on their kinetic energy. (Middle) Transport of thermal protons and deuterons through the graphene and h-BN lattice occurs via pores in the electron cloud. (Right) The graphene and h-BN lattice is impermeable to helium atoms and other gases. Atomically thin two-dimensional materials present opportunities for selective transport of subatomic species. The pristine lattice of monolayer graphene and hexagonal boron nitride, although impermeable to helium atoms, allows for transmission of electrons and permits transport of thermal protons and its isotopes. We discuss advances in selective subatomic species transport through atomically thin membranes and their potential for transformative advances in energy storage and conversion, isotope separations, in situ electron microscopy and spectroscopy, and future electronic applications. We outline technological challenges and opportunities for these applications and discuss early adoption in imaging and spectroscopy that are starting to become commercially available, as well as emerging applications in the nuclear industry and future application potential in grid storage, clean/green transportation, environmental remediation, and others.