REVIEW SUMMARY
◥MEMBRANES
Subatomic species transport through atomically thin
membranes: Present and future applications
Piran R. Kidambi*, Pavan Chaturvedi, Nicole K. Moehring
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 (honey-
comb mesh of carbon atoms) and hexagonal
boron nitride (h-BN, honeycomb mesh of alter-
nating 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 opportu-
nities to overcome material challenges limiting
the advance of electron microscopy and spec-
troscopy 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 pro-
cesses by providing avenues to address per-
sistent 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 trans-
port through h-BN and graphene offer trans-
formative 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 ini-
tially 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 ener-
gic electrons through graphene varies with
their kinetic energy and offers transformative
opportunities for advancing electron micros-
copy (EM). Graphene’s high electron transpar-
ency enables its use as an ultrathin, crystalline,
conductive sample substrate for transmission
electron microscopy (up to ~80 kV), minimiz-
ing 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 transpar-
ent molecularly impermeable barrier to isolate
detectors from the sample environment (whilestill 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 pro-
vides avenues to mitigate crossover of re-
actants and undesired species, and enhance
selectivity of proton exchange membranes
for improved efficiencies over a wide spec-
trum of energy generation and conversion
processes, including (i) fuels cells for clean/
green transportation and distributed or mo-
bile auxiliary power generation, (ii) electro-
chemical hydrogen pumps for distributed
hydrogen production and purification, and
(iii) redox flow batteries for grid storage. The
different rates of selective proton and deuter-
on 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 ad-
vances for isotope separations.OUTLOOK:Selective transport of subatomic
species through graphene and h-BN pres-
ents 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 elec-
tron microscopy grids with graphene as the
sample substrate are available commercially,
and the use of graphene as an electron trans-
parent 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 scal-
able 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 dura-
bility 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 in-
dustry. 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.
▪RESEARCH
708 5 NOVEMBER 2021•VOL 374 ISSUE 6568 science.orgSCIENCE
The list of author affiliations is available in the full article online.
*Corresponding author. Email: [email protected]
Cite this article as P. R. Kidambiet al.,Science 374 , eabd7687
(2021). DOI: 10.1126/science.abd7687READ THE FULL ARTICLE AT
https://doi.org/10.1126/science.abd7687Graphene
h-BNSubatomic 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.