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Graphene Heterostructures by Self-Assembly:Top-down meets Bottom-up

Final Report Summary - 2D-HETEROSTRUCTURES (Graphene Heterostructures by Self-Assembly:Top-down meets Bottom-up)

The project objectives are as follows:
1. To study the 2D-materials which are atomically thin such as graphene and hBN in their pristine state as membranes for separation of sub-atomic particles.
2. To fabricate 2D-heterostructure based capillaries which are only few-atomic layer thin to study novel molecular transport phenomena.
3. To study the ultra-fast transport of 2D-water which has been proposed to have friction-free motion along surfaces via the fabricated 2D-heterostructure based atomic capillaries.

- a description of the work performed since the beginning of the project,
We investigated whether deuterons (D+) permeate through 2D crystals differently from protons (H+) studied previously. Two complementary approaches, electrical conductivity measurements and gas flow detection by mass spectrometry, were explored. In the first approach, graphene and hBN monocrystals were mechanically exfoliated and suspended over micrometer-sized holes etched in silicon wafers. To measure 2D crystals’ hydron conductivity σ, both sides of the resulting membranes were coated with a proton conducting polymer – Nafion – and electrically contacted using Pd electrodes that converted electron into hydron flow. The measurements were performed in either H2-Ar/H2O or D2-Ar/D2O atmosphere in 100% humidity at room temperature. The different atmospheres turned Nafion into a proton (H-Nafion) or deuteron (D-Nafion) conductor with little presence of the other isotope. For reference, similar devices but without 2D membranes were fabricated. The latter exhibited similar conductance, whether H- or D- Nafion was used, and it was typically 100 times higher than that found for devices incorporating 2D crystals.

2D-water with layered structure has been proposed to have friction-free motion along surfaces. The ultra-fast transport in confined water has been debated for many years now with hints of such fast transport observed in carbon nanotubes (CNT). In order to know the effect of confinement on water transport unambiguously, it would be necessary to study the water transport with systematic variation of the channel dimensions used for confinement. It is not only challenging to fabricate such nanocapillaries which can confine one to few layers of water by conventional top-down fabrication methods and materials, but also the resulting channels have high surface roughness limiting their use. To accomplish this task, 2D-materials such as graphene are highly suitable as the material itself is atomically smooth and infinitesimally thin.

- a description of the main results achieved so far,
One-atom-thick crystals are impermeable to atoms and molecules, but hydrogen ions (thermal protons) penetrate relatively easily through them. We show that monolayers of graphene and boron nitride can be used to separate hydrogen ion isotopes. Employing electrical measurements and mass spectrometry, we found find that deuterons permeate through these crystals much slower than protons, resulting in a large separation factor of ~10 at room temperature. The isotope effect is attributed to a difference of ~60 meV between zero-point energies of incident protons and deuterons, which translates into the equivalent difference in the activation barriers posed by two dimensional crystals. In addition to providing insight into the proton transport mechanism, the demonstrated approach offers a competitive and scalable way for hydrogen isotope enrichment.
A capillary device was developed, comprising hundreds of parallel channels, with unprecedented control on the channel thickness of up to one atomic layer. These are ultimately thin and smooth capillaries that can be viewed as if individual atomic planes were removed from a bulk crystal, leaving behind flat voids of a chosen height. The capillaries are fabricated by van der Waals assembly of atomically flat materials using two-dimensional crystals as spacers in between. To demonstrate the technology, we use graphene and its multilayers as archetypal two-dimensional materials and study water transport through channels ranging in height from a single atomic plane to many dozens of them.
The fabricated capillary devices are ideal for studying water confinement due to surface hydrophobicity of graphene walls of the channel. Water transport through graphene nanocapillaries was studied by means of weight loss experiments, where they were mounted on a metal container partially filled with water and the loss of water was monitored for several days. The unexpectedly fast flow (up to 1 m/s) is attributed by high capillary pressures (~1,000 bar) combined with large slip lengths. For channels that accommodate only a few layers of water, the flow exhibits a marked enhancement, which is associated with an increased structural order in nanoconfined water.

- the expected final results and their potential impact and use (including the socio-economic impact and the wider societal implications of the project so far).
The separation factor of proton and deuterons using 2D-atomic membranes compares favorably with sieving efficiencies of the existing methods for hydrogen isotope separation. Because graphene and boron nitride monolayers exhibit high proton conductivity, comparable to that of commercial Nafion films, this makes them potentially interesting for such applications. In this respect, the increasing availability of graphene grown by chemical vapor deposition (CVD) provides a realistic prospect of scaling up the described devices from micron sizes to those required for industrial uses. Indeed, while micromechanical cleavage allows 2D membranes of highest quality, the approach is not scalable. As a proof of concept, we repeated the mass spectrometry measurements using cm-size membranes made from CVD graphene and achieved the same . Importantly, this shows that macroscopic cracks and pinholes present in CVD graphene do not affect the efficiency because hydrons are electrochemically pumped only through the graphene areas that are electrically contacted. Furthermore, we estimate the energy costs associated with this isotope separation method as ~0.3 kWh per kg of feed water, significantly lower than costs of the existing enrichment processes. All this comes on top of the fundamentally simple and robust sieving mechanism, potentially straightforward setups and only water at the input without the use of chemical compounds.

We have also presented a novel device architecture for capillary devices, where the active device structure is composed of all-graphene walls. The present method gives a possibility to choose different functional 2D-materials for top and bottom layers of the capillary, instead of graphene. Thus this opens up huge range of choice for making capillaries with varying surface functionality, surface charges, and conducting/insulating nature etc. Our current devices transfer minute amounts of liquid, typical for nanofluidics, but it is feasible to increase the flow by many orders of magnitude using dense arrays of short (submicron) capillaries covering mm-size areas, which can be of interest for, e.g. nanofiltration.