Symposium FF02—2D Nanomaterials-Based Nanofluidics

It is an exciting period for nanofluidics, the field exploring the transport of fluids at the nanoscales. Routes now exist to fabricate individual channels with nanometric and even sub-nanometric dimensions, while new instruments have also been invented to probe transport across these channels. And indeed, a number of quite exotic properties for water and ion transport have emerged since.
In this presentation I will discuss the emergence of strongly non-linear transport in ultimate confinement. I will report experiments exploring ionic transport under combined voltage and pressure driving: both in 2D channels made of van der Waals nanostructures, and in sub-2nm carbon nanotubes. In both types of channels, strong couplings between voltage-driven and pressure-driven transport are highlighted. I will show how such phenomena can be rationalized theoretically in terms of the coupled ion dynamics and flow transport, emphasizing on the key role of surface friction on ionic transport. Beyond I will further show theoretically how the classical equivalent of Coulomb blockade does emerge for ionic transport in 1D systems, via a fractional Wien effect.
These various phenomena lead to tunable ionic transport under specific stimuli, and they allow to envision the design of artificial mechano-sensitive response, similar to those observed in biological channels.

This work was done in collaboration with T. Mouterde, A. Poggioli, A. Marcotte, N. Kavokine, A. Niguès, A. Siria, from Micromégas team, ENS Paris, and with A. Keerthi, S. Dar, R. Boya and A.K. Geim, from Manchester University.
References
[1] “Massive radius-dependent flow slippage in single carbon nanotubes ” E. Secchi, S. Marbach, A. Niguès, D. Stein, A. Siria and L. Bocquet, Nature 537 210 (2016)
[2] “Molecular streaming and voltage-gated response in Angström scale channels” T. Mouterde, A. Keerthi, A. Poggioli, S. Dar, A. Siria, A.K. Geim, L Bocquet and R. Boya, Nature 567 87 (2019)
[3] “Ionic Coulomb blockade as a fractional Wien effect ” N. Kavokine, S. Marbach, A. Siria, L. Bocquet, Nature Nanotechnology (2019)

Nanoporous atomically thin membranes, wherein nanopores in two-dimensional materials provide pathways for rapid and selective fluidic transport, represent the ideal limit of an ultrathin membrane and have potential for improving the efficiency, selectivity, productivity, versatility, and chemical resistance for a variety of membrane separations. I will discuss our work on understanding mass transport and developing membranes that employ nanoporous single-layer graphene as the selective layer. Through controlled nucleation of defects via ion bombardment and oxidative etching, sub-nanometer pores are created in the otherwise impermeable graphene placed on a porous support. We discuss strategies for the design of defect-tolerant membranes and show that design of the porous support is critical to exploiting the selectivity of nanoporous graphene, which enables molecular sieving of gases across centimeter-scale nanoporous graphene membranes. We investigate the role of defects, illustrate methods to rapidly assess nanoscale defects in graphene, and demonstrate that the impermeability of graphene can be exploited to selectively seal defects and greatly improve its selectivity. Through these developments, we are able to realize centimeter-scale graphene membranes that show high selectivity between ions, molecules, and proteins for nanofiltration and dialysis, and ultrahigh permeance in organic solvent nanofiltration. We also demonstrate that the membranes exhibit high mechanical strength and are able to withstand applied pressures of 100 bar. These studies illustrate the interplay between material structure and transport in nanoporous graphene and demonstrate its potential for the realization of next-generation membranes for water purification, bio/chemical separations, and other applications.

2D nanoslit devices¹, where the planes of two 2D materials such as hBN or graphene are separated by only a few nm, have attracted a lot of attention for their interesting water- and ion-transport behavior arising from their interesting confined geometry. Here, we investigate the passage of single DNA molecules through such nanoslits. These 2D nanoslits allow us to examine the entropic stretching behavior of DNA and its friction interactions of DNA with 2D materials as both are in close contact during translocation. Double-stranded (ds) DNA (which has a diameter of 2.2 nm) of 1-10kbp length in 2M LiCl salt is electrophoretically driven across a 2D nanoslit. The devices have a length of 1-2 microns, a width of 80-120nm and a height of only 3-6 nm. Upon entry into the channel, the dsDNA blocks the passage of ions leading to temporary current blockades – similar to nanopore translocations². Due to their small slit height, the entrance to the nanoslit presents a significant entropic barrier that the dsDNA must overcome in order to translocate into the slit³. Shortening the dsDNA length allows the entropic barrier to be overcome and clear dsDNA translocations to be resolved. Increasing the bias voltage allows for an increased signal-to-noise ratio and well resolved dsDNA blockade current signals. Ongoing experiments are underway to investigate the role of interaction and friction between the surface of different 2D materials and the translocating biomolecules.

References
1. Radha, B. et al. Molecular transport through capillaries made with atomic-scale precision. Nature 538, 222–225 (2016).
2. Dekker, C. Solid-state nanopores. Nat. Nanotechnol. 2, 209–215 (2007).
3. Levy, S. L., Mannion, J. T., Cheng, J., Reccius, C. H. & Craighead, H. G. Entropic Unfolding of DNA Molecules in Nanofluidic Channels. Nano Lett. 8, 3839–3844 (2008).

Nanoporous graphene membrane has shown great potentials for various promising applications such as water desalination, biochemical separation, energy management and biosensing, owing to the atomically 2-D structure of graphene and the fast molecular transport across graphene. These features of graphene also make nanoporous graphene membrane most desirable as nanoporous membrane evaporators. The atomic thickness of graphene can minimize hydraulic resistance during evaporation and the ultra-high mechanical strength of graphene can provide excellent mechanical stability of the membrane. Therefore, nanoporous graphene membrane is expected to achieve the ultimate performance of nanoporous membrane evaporator. However, there are two major challenges that inhibit the development of nanoporous graphene membrane evaporators: 1) evaporation kinetics from single graphene nanopore remains unknown and consequently there are no rational optimization guidelines for graphene nanoporous membrane evaporators; 2) there is still no mature method to fabricate scalable graphene nanoporous membranes and integrate them as evaporators. In this work, we aim to solve the first challenge by experimentally studying evaporation from graphene membranes with single nanopores and provide insights for solving the second challenge. We used a hybrid nanochannel-nanopore design that we recently invented to conduct water evaporation measurements. This design has been proved to be able to achieve kinetic-limited evaporation regime, where evaporation flux is only limited by evaporation kinetics at liquid-vapor interface at nanopore mouth instead of liquid/vapor transport toward/away from the interface. Furthermore, such design is capable to accurately measure evaporation flux from individual nanopores. Our results show that water evaporation flux from graphene nanopores increases as pore diameter decreases, exhibiting a similar trend but higher diameter dependence than water evaporation from silicon nitride nanopores. The highest measured flux from a graphene nanopore with diameter of 73 nm is 2.1 mm/s at room temperature, corresponding to the heat flux of 480 W/cm^2, reaching 80% of evaporation flux limit predicted by the classic Hertz-Knudsen theory. The enhanced evaporation from graphene nanopores compared to silicon nitride nanopores could be corroborated by a recent MD study which showed that the edge of graphene can facilitate evaporation due to the unique graphene-water interactions. Our findings provide new understanding of evaporation from graphene nanopores and consequently provide guidance for the design of nanoporous graphene membrane evaporators.

Recent experiments have confirmed that nanoscale mass transport of gases, water and ions can be modulated by external stimuli such as pressure, temperature and the external electric field (EEF). Compared to pressure and temperature, the EEF can be applied in-situ, with remarkable stability and a wide range in amplitude control. Here, I will present two of our recent work focused on the controlled transport of hydrated ion and gas in nanochannels through the response in ionic motion under the EEF. We first demonstrate the enforcement of near-surface ion diffusion under an EEF, where their hydration shells are distorted while making contact with the surface. We find from molecular simulations a trapped/hopping scenario of diffusive ionic dynamics, where the diffusivity does not depend on their hydration radius but the corrugation of the free energy surface. As a result, the diffusion-specific selectivity can be elevated to ~10 at the expense of reduction in the mobility. The second theoretical study on the ionic liquids (ILs) confined in graphene oxide membranes suggests that the spatial distribution of cations and anions can be displaced under the EEF, modifying the absorption/desorption process and gas solubility in the composite membrane, leading to significantly enhanced gas selectivity. The findings are validated by experimental data, demonstrating separation performance that is superior to the upper bound of state-of-the-art membranes.

Membrane technology promises to bring process intensification to various industrial processes such as chemical separation and resource purification. Membrane-participating processes can become more effective if selectivity, permeation and durability of membrane materials are improved. In this regard, mass transport phenomena under extreme confinement offered by low dimensional materials call for attention because understanding of their characteristics could help designing the membrane pores. Here, I present selective transport phenomena across 0D, 1D and 2D confined space that atomically thin orifices, nanotubes and 2D material lamellas provide, respectively. As the confinement dimension increases, mass permeation tends to decrease from ultimate permeation to fast transport to unimpeded diffusion, whereas chemical selectivity is prone to increase inherently. Nevertheless, one may tailor actual chemical selectivity through proper pore design and architectural modification. Thus obtained knowledge and pore architectures could lay the cornerstone of advancing membrane transport properties toward process intensification.

The movement of electrolyte solutions within nanometer-scale channels is central to understanding nanofluidic phenomena. Ion transport within small (nm) scale pores is also key to electrochemical energy storage, where the energy storage process is dependent on ionic ingress into porous carbon materials [1,2]. Although very significant advances in the understanding of ion movement within electrically charged pores have been made [3], a key barrier is that porous carbon materials contain a complex distribution of interconnected pores of varying size, making it difficult to de-convolute specific size effects. A further connection between the fields of nanofluidics and electrochemistry arises because electrochemical control, based on the phenomenon of electrowetting, may be used to drive liquids into small channels [4]. We have been able to overcome this restriction by applying the nanochannel technology developed by Radha et al [5,6] in the electrochemical context, where one “wall” of the graphene channel is used as an electrode. We are therefore able to observe the effects of differing ion sizes on electrical double-layer capacitance, additionally, the effects of such extreme confinement on electrochemical processes (both capacitive and Faradaic) can also be discerned.

References:
1. B.E. Conway, “Electrochemical Supercapacitors”, Springer, 1999.
2. Z. Chen et al, Adv. Mater., 23, (2011), 791
3 J. Chmiola, G. Yushin, Y. Gogotsi, C. Portet, P. Simon, P.L. Taberna, Science, 313, (2006), 1760
4 D.J. Lomax et al, Soft Matter, 12, (2016), 8798.
5 B. Radha et al, Nature, 538, (2016), 222
6 A. Esfandiar et al, Science, 358, (2017), 511

Single-digit nanopores, or those with characteristic dimensions less than 10 nm, can induce dramatic changes in phase behavior and transport of the fluids they confine. Isolated single-walled carbon nanotubes (SWNT) form well-characterized nanopores and have been shown to contribute to anomalous water transport and phase behavior. Here, we introduce a scanning method using Raman spectroscopy that we use to rapidly determine the filled state of isolated SWNT. By scanning large numbers of carbon nanotubes, we show that substantial changes (~3 cm^-1) in the Raman radial breathing mode occur only in situations consistent with fluid filling, demonstrate the relative success of wet and dry etching in opening carbon nanotube ends, and discuss the diameter dependence of water filling. Specifically, the proportion of carbon nanotubes that fill increases dramatically with nitric acid treatment, while variations in filled proportion are strong but non-monotonic with carbon nanotube diameter.

Currently basal planes of 2D materials like graphene (G) and hexagonal boron nitride (BN) are considered inert towards covalent addition chemistry in operando i.e. in aqueous conditions. However recent ionic transport experiments have unveiled a considerable electrification of the water-BN surfaces [1], with a contrasting response for its water-Graphite homologue [2]. This surface charging was conjectured to originate in the differential hydroxide adsorption at the surface of these twin materials, but the considerable challenge of simulating the elusive hydroxide ion in liquid water has precluded a proper explanation up to now.

In this talk I will first show by means of ab initio simulations that 2D materials do strongly interact with the hydroxide ion from water. As a key ingredient, our ab initio simulations involve the description of the quantum nature of both the layered material and liquid water, corresponding to Ab Initio Molecular Dynamics (AIMD). A full quantum treatment is indeed required to explore the targeted interfacial chemical reactivity of liquid water, in particular to properly include dynamical proton transfers between H-bonding water molecules, a process denoted as the Grotthuss mechanism. We highlight a strong (covalent) chemisorption of the OH^- anion on boron-nitride, while the OH^- ion is only physisorbed on the graphene layer. Furthermore in its physisorbed state, the OH^- species retains a large surface mobility thanks to the Grotthus mechanism, here confined at the graphene interface [3].
Building on these findings, we extended our exploration of liquid water at graphene-hBN heterojunctions. Counterintuitively, our ab initio simulations of liquid water do show that liquid water does dissociate on such borders [4], leading to the spontaneous formation of an adsorbed OH^- ion, while the proton is liberated in the liquid.

Finally I will extend the scope of water reactivity to activated 2D materials, like graphene oxide surfaces [5]. Our ab initio results point in particular a strong dynamical reactivity of the epoxide functions.

These different results are expected to impact strongly the ionic transport across such stacked graphitic membranes. In spite of their difficulty, quantum insights open unprecedented avenues for the use of intertwined 2D materials for nanofluidics.[6]


[1] A. Siria et al., Nature 494, 455 (2013).
[2] E. Secchi, Phys. Rev. Lett. 116, 154501 (2016).
[3] B. Grosjean, M.-L. Bocquet, R. Vuilleumier, Nat. Com. 10, 1656 (2019).
[4] B. Grosjean, A. Robert, R. Vuilleumier, M.-L. Bocquet submitted 2019.
[5] F. Mouhat, F.X. Coudert, M.-L. Bocquet, in preparation 2019.
[6] A. Siria, M.-L. Bocquet, L. Bocquet, Nat. Rev. Chem. 1, 0091 (2017)

Nanochannels based on carbon materials have been intensively studied in the last decade because of their promising application in nanofiltration and the novel physical phenomena arising between graphitic surfaces at the nanoscale. More recently, atomically-smooth graphitic channels with sub-nm heights have been fabricated and used to investigate the physics of ions and water molecules in slits comparable to the smallest ion sizes.

In this work, we investigated ionic flow in atomically-smooth graphitic channels with height ranging from 7Å to 35nm. We show that the mobilities of ions in such confinements do not scale with hydration shell in a simple fashion, and we further explored the role of the surface charge, physical confinement and chemical interactions. Engineering ionic mobilities within the graphitic channels, we could induce strong current driven by the salinity gradient. Such osmotic power generators – driven by mobility engineering, rather than surface charge – are more resilient on the variation of chemical environment and show orders of magnitude increase in osmotic power density compared to commercial membranes.

Controlling molecular transport on a molecular scale is important for applications ranging from membrane separations, to bioelectronic interface design. Living systems move ions and small molecules across biological membranes using protein pores that rely on nanoscale confinement effects to achieve efficient and exquisitely-selective transport. Carbon nanotube porins—pore channels formed by ultra-short carbon nanotubes assembled in a lipid membrane—can exploit similar physical principles to transport water, protons, and small ions with efficiency that rivals and sometimes exceeds that of biological channels. I will discuss the role of molecular confinement and slip flow in these pores and show how it can enhance water and proton transport efficiency and influence the mechanisms of ion selectivity in these pores. Overall, carbon nanotube porins represent simple and versatile biomimetic membrane pores that are ideal for studying nanoscale transport phenomena and building the next generation of separation technologies and biointerfaces.

Many simulations and experiments have investigated pressure-driven fluid flow in carbon nanotubes (CNTs) and demonstrated enormous transport rates through these channels. Comparatively little attention has been given so far to concentration-driven transport¹ in CNTs despite its importance in a large variety of fields. While a few studies assumed bulk/hindered diffusion for small molecules through nm-wide CNTs, other simulations have predicted self-diffusion coefficients several times larger than in the bulk, and NMR experiments supporting these claims are beginning to emerge. These large uncertainties in the magnitude of the diffusion rates through CNTs have hampered their full exploitation in nanofluidic devices.²
To obtain a precise quantification of the diffusive flow in CNTs, we have fabricated membranes with a large but known number of single-walled carbon nanotubes (SWCNT) as fluid transport pathways. Contrary to previous membrane systems, this platform enables us to minimize uncertainties in the calculation of the per-pore flow rate. A series of stringent control experiments confirms that these membranes are defect free and that transport occurs only through SWCNTs. Once corrected for the boundary layer resistance at the membrane/fluid interface, our measurements indicate that the transport diffusivity of small ions in single-walled carbon nanotubes is about an order of magnitude faster than in the bulk. The dependence of the flow enhancement on the ion chemico-physical properties is also discussed. These results suggest that CNT membranes could enable dialysis process with unprecedented efficiency, shed further light onto the unique transport properties of graphitic channels, and enable a more accurate design of CNT-based fluidic systems for a broad range of applications.

References
1. N. Bui, E. R. Meshot, S. Kim, J. Peña, P. W. Gibson, K. J. Wu, F. Fornasiero, Adv. Mater., 28 (2016) 5871.
2. S. Guo, E. R. Meshot, T. Kuykendall, S. Cabrini, F. Fornasiero, Adv. Mater. 28 (2015), 5871.


This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344.

Water exhibits unique properties under confinement that are distinct from properties of bulk water. Atomistic modeling can provide valuable insight into differences in nano-confined (e.g., by nm-scale carbon nanotubes) water and salt solutions. However, accurate, first-principles modeling of molecular interactions is necessary to predict emergent properties. To better understand the graphene-water interface in such systems, we performed ab initio molecular dynamics on representative confined systems. We investigate the nanoscale solvation behavior of selected ions on coronene and circumcoronenes as finite graphene models. We investigate the degree of charge transfer between the ion and water with range-separated hybrid density functional theory as well as how that charge transfer fluctuates during dynamics. We also investigate how defects alter these dynamics and solvation in model systems.

Extremely fast water transport and high ion selectivity in smaller-diameter carbon nanotubes (CNTs) have raised the prospect of the practical use of CNTs in membrane-based applications. Several experimental and molecular dynamics (MD) simulation studies have reported that the CNT wall behaves like a nearly frictionless surface, which results in a significant enhancement of the water flow rate through CNTs. The extent of water flow enhancement is typically quantified using the concept of a slip length, which mathematically represents the extrapolated distance relative to the wall of the CNT where the water velocity goes to zero. Although previous MD simulation studies have laid a solid foundation to uncover the molecular mechanisms responsible for the large slip lengths of water inside CNTs, the precise dependence of the slip length on the CNT diameter, chirality, and metallicity still remains unknown.

In this talk, we will discuss our results on the slip flow of water through 0.8 nm – 2 nm diameter CNTs, obtained using classical MD simulations. The classical MD simulations are carried out using quantum mechanically-derived polarizable force fields [1] to model water-CNT interactions. Our theoretical framework can accurately model the anisotropic polarizability tensors of CNTs, including self-consistently modeling the water-CNT polarization energy, which was neglected in previous MD simulation studies. By carrying out classical MD simulations, we investigated the dependence of the water density and friction coefficient on the CNT diameter and chirality. We find that contrary to common intuition, water behaves like a compressible fluid in nanoconfinement such that many-body polarization effects result in a pronounced enhancement in both the density and the structuring of water molecules confined inside CNTs. We also obtain an anomalous maximum in the water friction coefficient for ~ 1 nm diameter CNTs, which is in good qualitative agreement with previous experimental studies which have reported pronounced water ordering and freezing-point elevation inside such narrow CNTs. Further, by carrying out MD simulations using different water models, we will highlight the roles of water-CNT and water-water interactions on the water friction coefficient inside CNTs. Finally, we investigated the role of the CNT metallicity by carrying out a scaling analysis of the variation of the water density and friction coefficient as a function of the CNT longitudinal and transverse polarizabilities. Compared to previous MD simulation studies which have utilized a Lennard-Jones potential to model the water-CNT interactions, our findings highlight the necessity to utilize polarizable force fields to more realistically model the CNT/water interface.

[1] Misra, R. P. & Blankschtein, D. J. Phys. Chem. C 121, 28166–28179 (2017)

Recent nanofluidic experiments with single or few nanopores in graphene, molybdenum disulfide and hexagonal boron nitride have shown unique fluidic transport properties and the potential for electrokinetic energy conversion with unprecedented power densities. In such nanopores, the high-surface charge makes possible a diffusio-osmotic mechanism for ion-selective transport, distinct from the Donnan exclusion that is typical of conventional membranes. In particular, experiments with single boron nitride nanotubes (BNNTs) have reported large surface charge in aqueous solution, and osmotic power densities up to several kW/m² when extrapolated to macroscopic membranes. However, no such macroscopic BNNT membranes have ever been fabricated, and their performance at large scales and pore densities is unknown. Thus, we seek to devise scalable means to manufacture such large-area nanotube membranes, and to investigate their fundamental mechanisms and performance for ion selectivity and electrokinetic energy conversion.

Here, we describe the fabrication of the first-ever macroscopic vertically aligned- (VA-) BNNT membranes, and our study of their ion-selectivity mechanism and osmotic-power-generation performance. The membranes are fabricated with a unique solution-based technique in which BNNTs are aligned and concentrated in a liquid oligomer with an external field, and then locked in place by in-situ polymerization. With this scalable fabrication technique, we have made VA-BNNT membranes of size 1-10 cm² and having tube densities of 10⁸-10⁹ BNNTs/cm² and open pore densities on the order of 10⁷ pores/cm². We show that, due to the high surface charge in their pores, the BNNT membranes are highly cation-selective even when the Debye length is smaller than the inner radius of the nanotubes. Moreover, the membranes exhibit energy-conversion efficiencies of 30%, and have osmotic power densities (based on open pore area) comparable to and even exceeding that of single BNNTs, up to 7,500 W/m² at pH 11 and 1 M:1 mM KCl molarity difference. To elucidate the mechanisms for ion selectivity and osmotic-power generation, we compare the membrane performance for different salts and for few-wall and multi-wall BNNTs of different diameters. Notably, these scalably fabricated VA-BNNT membranes, with relatively large yet highly selective pores, have enhanced electrokinetic-energy-conversion performance, and may enable efficient energy harvesting from salinity gradients, as well as efficient desalination and other separations via a different mechanism than conventional membranes.

Atomic vacancy defects in single unit cell thick hexagonal boron nitride are of significant interest because of their photophysical properties, including single-photon emission, and promising applications in quantum communication and computation. The spectroscopic assignment of emission energies to specific atomic vacancies within the triangular lattice is confounded by the exponential scaling of defect candidates with the number of removed atoms. Herein, we collect more than 1000 spectra consisting of single, isolated zero-phonon lines between 1.69 and 2.25 eV, observing 6 quantized zero-phonon lines arising from hexagonal boron nitride vacancies. A newly developed computational framework for isomer cataloguing significantly narrows the number of candidate vacancies. Direct lattice imaging of hexagonal boron nitride, electronic structure calculations, and subsequent boric acid etching are used to definitively assign the 6 features. Systematic chemical etching supports the assignment by demonstrating the sequence of growth of successively larger vacancy centres from smaller, which include a single B vacancy and a 16-atom triangular defect. These features exhibit a range of emission lifetimes from 1 to 6 ns, and phonon sidebands offset by the dominant lattice phonon in hexagonal boron nitride near 1370 cm^-1. This assignment should significantly advance the solid-state chemistry and photophysics of such vacancy emitters.

Carbon nanopores underpin a large array of materials systems and technological applications, including supercapacitors and water desalination. In these devices, understanding of the ion behavior is essential for predicting and optimizing the performance; however, many mechanistic details remain enigmatic. These include transport and solvation properties of the ions, and how they are governed by the degree of confinement and interfacial interactions. Here, we employ first-principles simulations to unravel key features of the solvation structure of several common ions at the interface with graphitic interface, and under confinement within graphene slit pores as well as carbon nanotubes (CNTs). We find that polarizable ions generally exhibit a stronger adsorption at graphitic interfaces, and these effects are found to be significantly enhanced under confinement. In addition, we find that confinement significantly influences ion selectivity and transport, i.e., ions with a small radius are found to yield a notably larger energy barrier to reach the pore entrance. These results points to the complex interplay between confinement and specific ion effects, which has broad implications in optimizing nanopores for ion selectivity and energy storage. Comparison with available experimental data will also be discussed.

This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344.

Fast mass transport through carbon nanochannels has been extensively explored. However, only a few studies have investigated convective energy transport through nanochannels. The no-slip and temperature-jump conditions compete each other in nanoscale convection heat transport [1]. The fast mass transport provides an interesting opportunity to explore nanoscale convection heat transfer. Here, we report the experimental investigation of convective heat transport of air through the vertically-aligned multi-walled carbon nanotubes (VAMWNTs) [1]. The unit cell flow channel was defined as an interstitial space among four adjacent nanotubes (hydraulic diameter = 84.9 nm). The air flow was in the transition and creeping flow regime with two orders magnitude higher flow rates than the prediction of Hagen-Poiseuille theory [1]. Due to the ultrahigh microwave absorption cross-section of long nanotubes [2], rapid, selective, and volumetric heating of the VAMWNTs was realized by providing a constant heat flux using the single-mode microwave (2.45 GHz) heating system [1]. The convective heat transfer coefficient and Nusselt number were experimentally obtained [1]. The heat transfer coefficient was relatively small partly due to the temperature jump and small Reynolds number. However, the heat transfer coefficient increased significantly as Reynolds number increased. Furthermore, the surface area available for heat transfer could be increased by 4 orders of magnitude using the aligned nanotube structure [1]. Overall, a large number of nanochannels in a given cross-section of heat sinks may enhance the total heat flow rate significantly [1]. A recent progress in our lab will also be introduced. References: [1] Nanoscale, 10, 23103 (2018) [2] J. Phys. Chem. C, 118, 13757 (2014)

Interfaces, as well as water structuration at interfaces, have a major influence in nanofluidics. Hydrophilicity and charge of the interfaces are key parameters [1,2]. Here, we would like to introduce nanochannels that are extremely interesting but less well-known than those based on carbon or on boron nitride for instance. These are imogolite-like nanotubes, inspired from alumino-silicate imogolite nanotubes discovered in soils as early as 1962 [3]. They are synthesized by soft chemistry and they are monodisperse in diameter and in chirality [4]. Their hydrophilic inner surface can be rendered hydrophobic by replacing hydroxyl groups by methyl ones [5] and their surface charge is also tunable [4, 6]. Such properties obviously make imogolite-like nanotubes ideal model systems for fundamental studies about nanoconfinement. Imogolite-like nanotubes are also becoming appealing for a wide range of potential applications, from filtration to depollution of water [7].
Here, we focus on geo-inspired alumino-germanate imogolite-like nanotubes (Ge-INT) with nominal formula GeAl₂O₃(OH)₄, which are synthesized by our team at Orsay and whose atomic structure has been recently determined [4, 8]. By carrying out inelastic neutron scattering (INS) experiments coupled to ab-initio molecular dynamics simulations, we reveal a unique structure of the water layer adsorbed inside Ge-INT [8]. Water molecules form an ordered triangular lattice on the inner surface of the nanotube, each molecule having three H-bonds with germanol groups on the surface. To the best of our knowledge, this structure differs from that of any kind of two-dimensional water, either on planar surfaces [9] or on curve surfaces of nanochannels. Detailed analysis of the vibrational density of state measured by INS shows that the dynamics of INT and water are strongly interrelated, a fully original result with respect to the available experimental literature about water in one-dimensional or two-dimensional nanopores. For instance, enhanced diffusion of water in CNTs due to phonon-induced oscillation friction has been predicted from molecular dynamics simulations [10] but it has not been complemented by experimental evidence. A new, well-defined translation vibrational mode of water molecules with respect to the nanotube wall is finally observed. Due to the peculiar bonding of water molecules with germanol groups, the evolution of the mean square displacement of water molecules as a function of temperature in Ge-INT is found to differ markedly from the ones measured for CNT [11] which are associated with frictionless water slip [12, 13]. Notably, the dynamics of INT-confined water remains harmonic from 10K to 300K, with no phase transition towards a liquid state up to room temperature.

[1] Levitz P. et al. (2013) Soft Matter 9, 8654
[2] Siria A. et al. (2013) Nature 494, 455
[3] Yoshinaga N. and Aomine S. (1962) Soil Science and Plant Nutrition 8, 22
[4] Monet G. et al. (2018) Nature Comm. 9, 2033
[5] Amara M.S. et al. (2015) Chem. Mater. 27, 1488
[6] Poli E. et al. (2016) J. Phys.: Condens. Matter 28, 074003
[7] E. Paineau and P. Launois, ‘Nanomaterials From Imogolite: Structure, Properties, and Functional Materials’ in ‘Nanomaterials from Clay Minerals’, Eds. A. Wang and W. Wang, Elsevier, to be published on August 1, 2019
[8] Monet G., Chai Z., Amara M.S., Paineau E., Orecchini A., Jimenéz-Ruiz M.,Ruiz-Caridad A., Fine L., Liu L.M., Teobaldi G., Rols S. and Launois P., article in preparation
[9] Algara-Siller G. et al. (2015) Nature 519, 443
[10] Ma M. et al. (2015) Nature Nanotechno 10, 692
[11] Kolesnikov A.I. et al. (2004) Phys. Rev. Lett., 93, 035503
[12] Majumder M. et al. (2005) Nature 438, 44
[13] Holt et al. (2006) Science 312, 1034

Nanopore sensing based on the blockage of ionic current caused by the steric hindrance of nanoparticles translocation has shown the promise in the detection of a single molecule and its morphology. However, due to the fast translocation velocity of nanoparticles and complicated fold morphology, it is hard to distinguish one from several molecules in the nanopore and achieve the concrete morphology. Here, we present the method to detect single DNA molecule precisely with a special nanoliter-sized cage. The fabrication of this cage is based on the transfer of thin graphene on the nanopore membrane which is laid on the Si chip. On both sides of the cave are two nanopores, one in the graphene membrane and the other in the silicon nitride membrane. The preparation method for the former nanopore is dielectric breakdown and the counterpart for the latter one is the drilling of the focus ion beam. The length of the cave depends on the depth of the silicon nitride membrane which can be manipulated by etching or drilling methods. The graphene nanopore is 10 nm and the silicon nitrogen nanopore is 100 nm, which form a current circuit in series. Attributed to the difference in size, the resistance of the graphene nanopore is relatively larger, which means the potential drop across the 10 nm nanopore can be of two order of magnitude larger than that across the 100 nm nanopore. Hence, the DNA molecule will translocate from the bulk solution to the nano cage through the graphene nanopore under sufficient electrophoresis force but will not escape from the 100 nm nanopore owing to the weak potential drop. The single molecule will be trapped in the nano cage unless a reverse voltage is applied. This method can extend the duration of blockage event and increase the accuracy of detecting single molecule. According to the value of current drop, the morphology of the single molecule can be confirmed. This method and the structure of nano cage will promote the research in the field of single molecule detection and provide inspirations for nanopore sensing.

Energy harvesting has been widely recognized as an important issue. Among the novel energy-harvesting methods, liquid-flow-induced electricity generation stands out as a mechanically non-degradable strategy that targets a variety of natural body of water as a viable energy source, such as river, currents in ocean, rain, etc. The energy harvesting by flowing water on graphene has been studied. The physical view proposed to date is still not deterministic, however, surface conditions are key factors for the understanding of the mechanism and maximizing the performance. In our strategy, the surface conditions include electrical potential at the liquid-solid interface and wettability of graphene, both of which can be tuned by heteroatom doping. In this paper, we discuss the contribution of the bonding state of nitrogen atoms in graphene. In addition, the role of the surface condition of graphene on flow-induced electricity generation is investigated.

We studied two type of nitrogen-dopant atoms with specific bonding state in graphene, thus graphitic and pyridinic nitrogen. Graphitic nitrogen is located at in-plane of graphene and pyridinic nitrogen atoms are located either above point defect or the edge due to geometrical limitation. The power generation was investigated using CVD graphene and deionized water droplets by manually dropping the droplets (0.1 ml). Both pyridinic and graphitic nitrogen dopants enhance voltage generation by water-flow. On pristine graphene, a single water droplet induces approximately 80 mV (0.89 μW). In contrast, the voltage induced at both types of nitrogen-doped graphene is approximately three times greater in magnitude (~240 mV). However, the output power from pyridinic nitrogen is almost zero because of much larger resistance increase by doping compared with graphitic nitrogen. The geometric constraints of the pyridinic nitrogen degrade graphene’s electrical conductivity. Therefore, it is found the graphitic nitrogen dopant is superior for power generation (1.3 μW) [1].

The enhancement of voltage generation by nitrogen doping is explained as follows. The electronic state of the nitrogen-doped sites is considered to have an important role. Nitrogen doping of carbon in various applications acts as an electron donor by providing the unshared electron pair of nitrogen in graphene. The local charge of graphitic nitrogen is positive and that of the surrounding carbon atoms is negative. This is explained by the electron transfer from nitrogen atom. The inside of the droplet sitting on top of graphene is affected by the negative charge on the graphene surface, leading to potential formation at the interface. The induced negative charge reduces electric double layer at the interface, resulting in the formation of steep potential [2,3].

In summary, we have investigated the effect of graphene doping on flow-induced power generation between graphene and water interface and found that the nitrogen-doped graphene shows enhancement of power generation. The surface charge of the nitrogen-doped graphene reduces the electric double layer thickness, resulting in the formation of a steep potential, which contributes to higher power generation.

[1] T. Okada et al., Results in Physics, 12 (2019) 1291.
[2] T. Okada et al., Applied Physics Letters, 112 (2018) 023902.
[3] T. Okada et al., Advance Engineering Materials, 20 (2018) 1800387.

Over the past decade, ion sieving using carbon-based materials such as carbon nanotube, graphene and graphene oxide has been attracted attention in water purification and ion filtration. Among them, graphene oxide (GO) membranes have recently emerged as promising candidate in the industry due to their excellent mechanical property, inexpensive synthesis, controllable selectivity and easy processing. Especially, they naturally have sub-1nm interlayer spacing comparable to the dimension of molecules or ions, which is crucial factor for ion sieving. Since the ions and molecules are sieved out according to the interlayer spacing, it is important to narrow the interlayer spacing to enhance the filtration efficiency. Herein, we fabricated thermally reduced graphene oxide (rGO) membrane with different interlayer spacing ranging from 3.7 Å to 8.0 Å in a dry state. GO membranes have inherent property to swell in water, which enlarge the interlayer spacing by several angstroms in water. We found that the minimum spacing to allow the water molecules to permeate the channels was 5.7 Å. Once the GO membrane immersed in water, the interlayer spacing increased to 9.6 Å. While the untreated GO membrane with interlayer spacing of 8.0 Å in dry state (13.8 Å in water) have no selectivity in ion permeation, we found that the anions, such as SO₄^2-, and Fe(CN)₆^3-, was totally rejected to permeate through the thermally reduced GO membrane with interlayer spacing of 9.6 Å. On the other hand, the permeation of cations was exponentially decreased with the hydrated diameter of cations. We found that sub-nanometer interlayer spacing of reduced GO membrane can physically sieve the ions, and negatively charged oxygen-functional groups rejected the multivalent anions.

Molecular transport through atomically-thin, layered 2D materials is associated with unique physicochemical phenomena not observed in bulk solution. In particular, ion and water transport are affected by extreme confinement, and exhibit behaviors analogous to those observed in nanofluidic channels. Moreover, the surface chemistry of carbon nanomaterials such as graphene (oxide) results in distinct interfacial interactions, which may be further engineered by mechanical strain. Such controlled modulation of surface topography, colloquially referred to as wrinkling, could affect local electronic structure and chemical interaction by manipulation of lattice structure. The coupling of ionic and molecular transport with strain engineering in 2D material architectures could be leveraged for desalination, energy storage, molecular sensing and energy conversion. However, our fundamental understanding of physicochemical transport properties within strained 2D material nanofluidics remains limited. Here, we utilize strain engineering to modulate both in-plane and out-of-plane molecular transport within multilayered 2D material assemblies. We show that metal ions can intercalate between 2D materials, increasing the interlayer spacing. Furthermore, multilayer GO papers permit water and oxygen transport, but are impermeable to organic molecules, even when unfolded at large area strains. These mechanochemical mechanisms may enable new fundamental insights into molecular transport under extreme confinement, as well as stretchable and wearable devices.

New physical effects have been shown to be possible in “ultrasmall confined spaces” [1] such as higher viscosity or higher mobility among others. Here we present clay minerals as possible materials for the study of nanofluidics. Clays are nano-silicate layered materials with structural properties that can be modified and controlled for intercalation and transport of foreign molecules in their 2D nanospace. Intercalation of water in smectite clays occurs naturally and has been extensively studied with a wide range of techniques, among them neutron [2] and X-ray scattering [3]. Water dynamics in smectite clay interlayers has been studied extensively. Recent experiments and simulations have shown that CO₂ can intercalate and flow in smectite clay interlayers provided from external bulk gaseous,liquid or supercritical CO₂ [4]. We have demonstrated that under certain conditions of pressure and temperature, fluorohectorite smectite clays are able to capture a large amount of CO₂ in this way, depending on the type of charge compensating cation residing in the interlayer [5,6,7]. We have investigated fluorohectorite clays with three different cations (Na⁺, Ni⁺² and Li⁺), suggesting the formation of a stable CO₂ complexation with the clay interlayer cation. Thus, clays could also be used as a model for nanofluidics of CO₂.

References:
[1] Xu, Y. Nanofluidics: A New Arena for Materials Science. Adv. Mater. 2018, 30 (3), 1–17.
[2] M. L. Martins, W. P. Gates, L. Michot, E. Ferrage, V. Marry, and H. N. Bordallo, “Neutron scattering, a powerful tool to study clay minerals,” Appl. Clay Sci., vol. 96, pp. 22–35, 2014.
[3] E. L. Hansen et al., “Swelling transition of a clay induced by heating,” Sci. Rep., vol. 2, pp. 1–2, 2012.
[4] P. Giesting, S. Guggenheim, A. F. Koster van Groos, and A. Busch, “X-ray Diffraction Study of K- and Ca-Exchanged Montmorillonites in CO2 Atmospheres,” Environ. Sci. Technol., vol. 46, no. 10, pp. 5623–5630, May 2012.
[5] H. Hemmen, E. G. Rolseth, D. M. Fonseca, L. Elisabeth, J. O. Fossum, and T. S. Plivelic, “X-ray studies of carbon dioxide intercalation in Na- fluorohectorite clay at near-ambient conditions,” Langmuir, pp. 1–4, 2012.
[6] L. Michels et al., “Intercalation and retention of carbon dioxide in a smectite clay promoted by interlayer cations.,” Sci. Rep., vol. 5, p. 8775, 2015.
[7] L. P. Cavalcanti, G. N. Kalantzopoulos, J. Eckert, K. D. Knudsen, and J. O. Fossum, “A nano-silicate material with exceptional capacity for CO2 capture and storage at room temperature,” Sci. Rep., vol. 8, no. 1, p. 11827, 2018.

The effect of graphene conformal mapping on the wetting of a surface remains a controversial topic. Many groups have studied the wetting transparency of graphene coated surfaces by measuring the water contact angle on the surface, however, airborne contamination due to the exposure of the surface during contact angle experiment can alter the hydrophilicity/hydrophobicity of the surface. Here, we explore this controversy by comparing the interaction of water molecules with a surface through measurement of the adsorption energy of the water molecules on the surface with and without graphene coatings. We find that the isotherms of water adsorbing on graphene are strongly influenced by the substrate, with the surface energy only being reduced by 10% for Pt and Al and 30% for Au

Nanopores are nanoscale diameter holes in thin film membranes. They can deliver single-molecule sensing that provides concentrations while also profiling material properties of analytes from single molecules, to (bio)polymers such as DNA, proteins and sugars, and nanoparticles. Nanopores can be formed easily by controlled dielectric breakdown (CDB) to match their size to the single nanometer to tens or hundreds of nanometers size of analytes. Passage of a molecule through a pore immersed in electrolyte is achieved by application of a voltage across the membrane. Detection of this molecular passage is most commonly done by Resistive Pulse Sensing (RPS)—measuring the change in pore resistance caused by the presence of an analyte in the pore. The magnitude of the resistive pulse is determined by parameters such as the molecular size and charge density. Detection is inherently single-molecule, without the need for chemical labelling steps. Nanopore DNA sequencing has leveraged this sensitivity to native molecular structure and properties. We have extended this basic DNA sequencing concept to motivate the development of nanopores for profiling a much more variable set of biopolymers, oligo- and polysaccharides. With more than 100 naturally occurring monomers and with more variable charge than DNA—and with the added complication of branching—a more sophisticated set of nanopore tools and sensing mechanisms is required. We have begun to undertake this development, which spans preliminary measurements of a diverse and challenging array of biopolymers to correlate known molecular properties to nanopore signal patterns. We will discuss development of alternative fabrication strategies for the core nanopore element, device-design considerations including microfluidics integration, and how to exploit a wider range of electrokinetic phenomena than had been pioneered for nanopore DNA sequencing. In addition, we will discuss issues in streamlining signal processing. These topics will be presented in the context of developing nanopore science for glycomics.

Recently, a tremendous amount of attention has been paid to the technology of desalination and air purification to solve the problem caused by severe water shortage and air pollution. Membrane separation method is one of the key technologies to solve this problem due to its energy-saving and environmentally friendliness. In this field, Graphene oxide (GO) membrane is a prominent candidate with its exceptional properties of selective separation and high permeability as to specific ions or molecules. It also has the potential to overcome the limitations of traditional membranes due to its excellent mechanical properties, chemical stability and easy fabrication process. GO membrane forms a dense structure formed by layered flakes, so that it obtains a sieving-effect. Moreover, selective permeating and rejecting effect is also obtained with its intrinsic properties. However, at the same time permeation through numerous defects is one of the major permeation paths, so the precise mechanism is not known yet. In this study, using GO membranes fabricated and modified by various methods, we tried to clarify the permeating mechanism by observing the change of properties of membranes and the permeation of ions and molecules. GO films were characterized using SEM, AFM, XRD, XPS and Raman. With these results, we observed the correlation between the properties of GO membrane and permeation characteristics and tried to figure out of the possibilities to commercialize.

In the growing climate emergency the world finds itself in, sources of clean water and energy will be of critical importance in the near future. Efforts towards achieving this goal have been aplenty but a wholesome solution has yet to emerge. The potential of 2D materials are immense in this field given their thinness, mechanical strength and alterable surface chemistry. Most notably, anion selective 2D membranes - allowing passage of anions - are absent from literature. Membranes that are able to filter selectivity for both anions and cations are needed for applications ranging from blue energy to desalination. We present highly cation and anion permeable graphene and hexagonal boron nitride prepared with minimal overheads, achieving salt rejection ratios of up to 500. We characterise our membranes using HRTEM in order to elucidate some of the mechanisms for this selectivity. By using Goldman-Hodgkin-Katz (GHK) fitting we extract permeability factors, demonstrating degrees of selectivity. Anion filtration is achieved through the use of multivalent ions but the effect is recreated in monovalent species using a low concentration background of multivalent ions. This means that as grown chemical vapour deposition (CVD) membranes could be used as highly ion selective and high flux membranes, extrinsically controlling the ability to filter for anions or cations as needed.

With the ability to selectively control ionic flux, biological protein ion channels perform a fundamental role in many physiological processes. For practical applications that require the functionality of a biological ion channel, graphene provides a promising solid-state alternative, due to its atomic thinness and mechanical strength. Here, we demonstrate that nanopores introduced into graphene membranes, as large as 50 nm in diameter, exhibit inter-cation selectivity with a ~20x preference for K+ over divalent cations and can be modulated by an applied gate voltage. Liquid atomic force microscopy of the graphene devices reveals surface nanobubbles near the pore to be responsible for the observed selective behavior. Molecular dynamics simulations indicate that translocation of ions across the pore likely occurs via a thin water layer at the edge of the pore and the nanobubble. Our results demonstrate a significant improvement in the inter-cation selectivity displayed by a solid-state nanopore device and by utilizing the pores in a de-wetted state, offers an approach to fabricating selective graphene membranes that does not rely on the fabrication of sub-nm pores.
Nanoscale, 11, 9856 (2019)

Molecular sieving is of great importance to proton exchange in fuel cells, water desalination, and gas separation. Two-dimensional crystals emerge as superior materials showing desirable molecular permeability and selectivity. Here we demonstrate that the experimentally fabricated graphyne membrane shows superior proton conductivity and perfect selectivity thanks to its intrinsic nanomesh structure. The trans-membrane hydrogen bonds across graphdiyne serve as ideal channels for proton transport. The free energy barrier for proton transfer across graphdiyne membrane is ~2.4 kJ/mol, nearly identical to that in bulk water (2.1 kJ/mol), enabling “transparent” proton transport at room temperature (Fig. 1). This results in a proton conductivity of 0.6 S/cm for graphdiyne membrane, four orders of magnitude greater than graphene and ten times larger than commercial proton exchange membrane nafion. Considering its ultimate pore size of 0.55 nm, graphdiyne membrane blocks soluble fuel molecules and exhibits superior proton selectivity. These advantages endow graphdiyne a great potential as proton exchange material in fuel cells, sensors and other applications. If time allowed, results about water transport and water photosplitting on 2D materials will be also discussed.

Directly interrogating the mechanism of ion transport in sub-nanoscale pores is challenging. Thus, typically computational approaches are employed to help shed light on the processes contributing to ion translocation, which are many: steric hindrance due to van der Waal repulsion, dehydration, strong electrostatic interactions due to confinement and a reduced dielectric constant, the structural fluctuations of the pore and flexibility of the membrane, etc. Biological ion channels, for example, balance electrostatic and dehydration effects to yield large ion selectivities alongside high transport rates. These macromolecular systems are often interrogated through point mutations of their pore domain, limiting the scope of mechanistic studies. In contrast, we demonstrate that graphene crown ether pores afford a simple platform to directly investigate optimal ion transport conditions, i.e., maximum current densities and selectivity. Crown ethers are known for selective ion adsorption. When embedded in graphene, however, transport rates lie below the drift-diffusion limit. We show that small pore strains – 1 % – give rise to a colossal – 100 % – change in conductance. This process is electromechanically tunable, with optimal transport in a primarily diffusive regime, tending toward barrierless transport, as opposed to a knock-on mechanism. Measurements of mechanical current modulation will yield direct information on the local electrostatic conditions of the pore. These observations suggest a novel setup for nanofluidic devices while giving insight into the physical foundation of evolutionarily-optimized ion transport in biological pores.

Ion partition in nanopores, such as averaged in-pore ion concentration and ion distribution, is critical for many electrolyte related applications, for example, energy storage devices, water treatment, and biology-inspired sensor. A comprehensive model that can quantitatively describe the ion partition under nano-confinement conditions is highly desirable for us to go beyond the trial-and-error experiments to achieve quantitative prediction and design for the next generation of the electrolyte related applications. However, it remains a fundamental challenge, especially in the case of ion-specific effect.

In this talk, we will present molecular dynamics (MD) simulations and a new continuum model to describe the ion partition in nanopores with ion-specific effects quantitatively. Our model system is sodium tetrafluoroborate (NaBF₄) aqueous electrolyte (0.8 M) in graphene nanoslits (varying from 1.0 nm to 4 nm). The Na⁺ and BF₄^- ions exhibit different dispersion interaction with graphene surface, leading to a strong specific BF₄^- adsorption at graphene surface. Our MD simulations systematically studied how the nano-confinement effects in graphene nanoslits could affect the ion concentration and distribution. The ion concentration in graphene slits exhibits a non-monotonous variation to slit size. The ion distribution profiles in the slit also demonstrate significant dependence on slit size.

Aiming to develop practical device design tools at a large length scale, we also developed a quantitative continuum model to describe the ion-specific adsorption under nano-confinement condition. We modified the classic Poisson-Boltzmann (PB) equation by incorporating the ion-surface effective potential, the electrostatic correlation, and the steric correlation interactions. A new model for steric correlation interaction is proposed, which takes account of the solvent structures in the graphene nanoslits. Our model quantitatively reproduces our MD simulation results very well. Our new continuum model could serve as a general approach for quantitative description of ion local distribution in nanoslit with ion-specific effect. It would be much more efficient than the MD simulations to describe the ion-specific adsorption inside complex porous structures. Besides, it could also lay a ground for the development of continuum models to quantitatively describe the charging dynamics in nanoporous materials.

The last two decades have experienced rapid technological developments in the search of cheap and high accuracy devices for fast bio-molecular identification. In the realm of DNA and protein sequencing, there has been an increasing interest in the use of nanopores in solid-state materials because of their distinct advantage over biological pores in terms of flexibility in pore design and mechanical strength. Two-dimensional (2D) solid state materials such as graphene and Molybdenum di-sulphide (MoS2) in particular have attracted attention because of their atomically thin layered structure and electrically active characteristics, predisposing them to offer single base resolution and simultaneously multiple modalities of detecting biomolecular translocation. 2D nanopore devices promise seamless integration with semiconductor electronics and are poised to revolutionize a variety of technologies such as genomics, point-of-care diagnostics and digital data storage to name a few.
The past year has witnessed significant activity to experimentally realize nanopore Field Effect Transistors (FETs) and understand the fundamental sensing mechanism in such devices. Currently, the dominant consensus from theoretical calculations has involved the electrostatic modulation of the FET current due to the translocating biomolecules. In this talk, we review and provide insights into this sensing principle by modeling the electron flow through 2D material nanopore FETs. We describe a method to systematically characterize nanopores FETs by contrasting the changes in the FET behavior before-and-after nanopore drilling and DNA translocation. We outline measurable predictions of high-resolution FET based sensing of DNA-protein complexes and damaged DNA. We compare these FET signals to the corresponding ionic current signals calculated from all-atom Molecular dynamics simulations. Further, we also outline possible techniques to improve the detection SNR by augmenting pore and device design with statistical signal processing algorithms. Finally, we propose a scalable device design of nanopore FETs to detect and identify translocations of single-biomolecules in a massively parallel scheme.

Although a single-layer graphene membrane with nanopores can separate gases with high selectivity by molecular sieving, the pore size is difficult to control at sub-angstrom resolution. Here we discuss two approaches to mitigate this issue via molecular dynamics simulations. First, we demonstrate that a monolayer of ionic liquid on the porous graphene allows anions to gate the pores of significantly larger in size than the gas molecules while still achieves high selectivity. Second, we propose a novel design based on the bilayer nanoporous graphene membrane which allows the effective pore size of the membrane to be continuously tunable by the offset between the two single layers. We find that skinnier and shorter molecules tumble through the elliptic-cylinder-shaped nanopore of the bilayer membrane, leading to higher entropic selectivity.

Gas molecules permeating across nanopores of two-dimensional materials experience the least possible resistance due to the atomic thickness of the material. However, pristine two-dimensional materials, such as graphene, are impermeable even to the smallest gases making it unsuitable for gas separation applications. The potential of two-dimensional materials for gas separation can consequently only be harnessed if the material is perforated in a precise and controlled manner. In this work, we present a novel synthesis route of nanoporous graphene membranes with a facile and dry chemical etching approach. The pore growth is initiated in a first step within the graphene lattice selectively at previously introduced atomic defects. The second process step consists of dry etching graphene selectively at the lattice defects, resulting in well-controlled, parallel pore growth yielding membranes with narrow pore size distributions. The resulting membrane performances are investigated in the context of gas separation applications and demonstrate similar selectivity but orders of magnitude higher permeance compared to other perforation techniques of two-dimensional materials.

Nanoporous graphene have attracted much attention in nanofluidics due to their high permeation and low resistance to ions or molecules. They have been widely used in biomolecules sensing, material separation, and energy conversion. With the help of the surface charges on the rim of nanopores, the nanopores have strong ionic selectivity, which provides a perfect platform for seawater desalination with a low-energy cost, and for energy conversion with high efficiency. The major challenge is how to fabricate nanoscale pores on pristine graphene as it cannot be permeated by all atoms including the hydrogen atom in the world. Here we presented an efficient way to produce high-density nanopores on graphene membranes with tunable size in the sub-nanometer range by plasma etching. Besides the pore size, the nanopore density is also controllable through adjusting the exposure time of the sample to argon or oxygen plasma. The plasma-treated graphene membranes can selectively transport protons, Na⁺ and Cl^- ions. Density function theory calculations uncover that the sp³ and vacancy-type defects construct different energy barriers for different ions, which allow the defected graphene membrane to selectively transport ions.
In order to understand the mechanism of the ionic transport in graphene nanopores, we applied molecular dynamics (MD) to investigated the dynamic process of ion transport in graphene nanopores. With the advantage of the simulations, the duration time for each ion passing through the nanopore was analyzed. It is found that the formation of the contact ion pair plays a critical role in reducing the ion mobility inside a nanopore without surface charges. The mobility for both cations and anions decreases with the reduced pore size because it is easier to form the contact ion pairs inside the neutral nanopore with a smaller diameter. Inside a charged nanopore, besides the contact ion pair formation, the surface charges also play a significant role in reducing the counterion mobility through surface trapping. It is uncovered that the mobility of Na⁺ ions increases first and then decreases with the surface charge density, while Cl^- ions have the opposite trend. A modified first-passage time model is proposed to take into account the ion pair formation and the trapped ions inside a nanopore, which provides a clear picture in describing ion transport through a nanopore.

Structure, dynamics and transport of fluids in ultrathin membranes/nanopores can be vastly different from their macroscopic counterparts. For example, water transport through carbon nanotubes has been shown to exhibit flow rates which are many orders of magnitude larger compared to those obtained from classical theories. While classical theories are fast, they do not account for the microscopic mechanisms that are important in ultrathin membranes. Quantum and atomistic simulations can provide good accuracy, but they are limited in the length and time scales that can be dealt with. In this talk, we will present multiscale theories to describe fluidic behavior in ultrathin membranes. First, we will discuss multiscale approaches to calculate fluidic properties such as structure, interfacial friction, viscosity, permittivity, slip, etc. in ultrathin membranes. We will show that these properties can be vastly different when compared to their bulk counterparts. Second, we will discuss the development of nanofluidic theories incorporating microscopic physics and show that these theories can predict water flow rates which are consistent with experimental measurements. Finally, we will discuss the physics of electrical double layers (EDLs) and show that the structure and dynamical properties of ions and water in EDLs of ultrathin membranes can be different from those in larger pores/channels.

A major challenge in developing 2D membranes is to create a high density of sub-nm sized channels in the material. Carbon Nanomembranes (CNMs) are 2D films fabricated by crosslinking of self-assembled monolayers¹. This work will present a ~1.2 nm thin CNM made from terphenylthiol (TPT) precursors that combines high water permeance with high selectivity. TPT CNMs only allow water and helium to pass through, but stop most gases and liquids². In particular, water across with an extremely high permeance of ∼1.1×10⁻⁴ mol m⁻² s⁻¹ Pa⁻¹, 2,500 times faster than helium. Scanning probe microscopy revealed that these membranes consist of sub-nm channels with an areal density of 10¹⁸ m⁻². If assuming all channels are active for mass transport, we found a single-channel permeation coefficient of 66 water molecules s⁻¹ Pa⁻¹, as high as that observed for channels in carbon nanotubes and aquaporin proteins which support a single-file transport of water molecules. This suggests that the fast water flow through TPT CNMs is attributed to a dense network of sub-nm channels and a coordinated water movement per single channel. This encourages the potential use of these membranes for water purification. The versatile and flexible fabrication process³ also brings the possibility of tailoring CNM functions for usage in different separation systems.

1. Turchanin, A. & Gölzhäuser, A. Carbon Nanomembranes. Adv. Mater. 28, 6075–6103 (2016).
2. Yang, Y. et al. Rapid Water Permeation Through Carbon Nanomembranes with Sub-Nanometer Channels. ACS Nano 12, 4695-4701 (2018).
3. Angelova, P. et al. A Universal Scheme to Convert Aromatic Molecular Monolayers into Functional Carbon Nanomembranes. ACS Nano 7, 6489-6497 (2013).

Nanopore biosensor platforms, namely solid-state and biological nanopores, have been developed over the past two decades as label-free sensors of single molecules and biopolymers (DNA, RNA, and proteins). While biological pores are typically self-assembled and form a natural channel that penetrates an organic membrane, such as a lipid bilayer, in solid-state nanopores, pores are synthetically fabricated by focused electron beam ablation or by dielectric breakdown of an ultrathin solid-state membrane. Thus, solid-state nanopores are more tunable for arbitrary sizes and geometries, and further, the robust framework and mechanical stability of synthetic membranes offers the ability to perform experiments under harsh conditions (e.g., hydrostatic pressure, extreme pH, denaturing conditions, etc.). The advent of two-dimensional (2D) materials (graphene, MoS₂, WS₂, and BN, for example) as support membranes for solid-state nanopores, offers in principle the highest detection resolution of any pores, because the thickness of these pores is about one atom thick. To further investigate the potential of 2D materials as nanopore support membranes, we have studied novel solid-state nanopores based on new 2D materials known as MXenes. MXenes are an emerging family of 2D transition metal carbides and nitrides with a general formula of M_n+1X_nT_x (i.e. Ti₃C₂T_x), where M is a transition metal, X represents carbon or nitrogen (n=1, 2, and 3), and T_x indicates different functional groups (O, OH, F) present on MXene surface. Sensing, energy storage, and water purification are among the various applications that MXenes have shown promise towards, owing to their controllable composition and surface tunability. Our initial results have shown that MXenes provide high mechanical robustness, long-time stability, and low-noise ion current recordings that are suitable for single molecule detection. Through further improvement in the fabrication process of MXene membranes, as well as harnessing of some unique MXene characteristics, we show here how MXene membranes could potentially overcome some challenges that relate to limited resolution, such as access resistance limitations, that other solid-state and biological nanopores are facing.

The ferromagnetic state in van der Waals two-dimensional (2D) materials has been reported recently in the monolayer limit. Intrinsic CrI₃ and CrGeTe₃ semiconductors reveal ferromagnetism but the Tc is still low below 60K. In contrast, monolayer VSe₂ is ferromagnetic metal with Tc above room temperature but incapable of controlling its carrier density. Moreover, the long-range ferromagnetic order in doped diluted chalcogenide semiconductors has not been demonstrated at room temperature. The key research target is to realize the long-range order ferromagnetism, Tc over room temperature, and semiconductor with gate tunability. Here, we unambiguously observe a ferromagnetic hysteresis loop together with magnetic domains above room temperature in diluted V-doped WSe₂, while maintaining the semiconducting characteristic of WSe₂ with a high on/off current ratio of five orders of magnitude.

The understanding of strongly-correlated quantum matter has challenged physicists for decades. Such difficulties have stimulated new research paradigms, such as ultra-cold atom lattices for simulating quantum materials. In this talk I will present a new platform to investigate strongly correlated physics, based on graphene moiré superlattices. In particular, I will show that when two graphene sheets are twisted by an angle close to the theoretically predicted ‘magic angle’, the resulting flat band structure near the Dirac point gives rise to a strongly-correlated electronic system. These flat bands exhibit half-filling insulating phases at zero magnetic field, which we show to be a correlated insulator arising from electrons localized in the moiré superlattice. Moreover, upon doping, we find electrically tunable superconductivity in this system, with many characteristics similar to high-temperature cuprates superconductivity. These unique properties of magic-angle twisted bilayer graphene open up a new playground for exotic many-body quantum phases in a 2D platform made of pure carbon and without magnetic field. The easy accessibility of the flat bands, the electrical tunability, and the bandwidth tunability though twist angle may pave the way towards more exotic correlated systems, such as quantum spin liquids or correlated topological insulators.

I will review our group’s work on the molecular transport properties of angstrom-scale channels fabricated by der Waals assembly of 2D crystals. These channels can be viewed as if individual atomic planes were extracted from a bulk crystal leaving behind two edge dislocations with an empty space in between. Gas, water, ion and proton transport have been studied in capillaries down to one atom in height.

Two-dimensional (2D) materials have received widespread popularity over the past decade. Particular in the field of nanofluidic studies by reducing the dimensions from micro to sub-nano scale¹ system led to emergence of new fluidic phenomena. In the view of fundamental science passive ion transport under the strong confinement will provide basic information about water-ion-material interaction. Under the strong confinement, various parameters such as hydration shell, orientation of water molecule, surface charge influence the movement of solvent and solute through nanochannels. The hydration shell of ion is of great significance to understand ion permeation and it plays a crucial role in biological and aqueous chemical systems^2,3. We explore ions with increasing radii and study the influence of hydration shell on ionic transport in such confinement. Here our measurements of ion-transport through Å-scale sized channels allow us to study the ion mobility to discern the hard versus soft ball behaviour of ions. Surprisingly we found that similar hydrated diameter size ions show asymmetric mobility behaviour. Our observations may help to enhance the cutting-edge technology in the fields of ion separation, energy conversion and novel fluidic devices.

Permeation through nanometre-pore materials has been attracting unwavering interest due to fundamental differences in governing mechanisms at macroscopic and molecular scales, the importance of water permeation in living systems, and relevance for filtration and separation techniques. Latest advances in the fabrication of artificial channels and membranes using two-dimensional (2D) materials have enabled the prospect of understanding the nanoscale and sub-nm scale permeation behaviour of water and ions extensively. In particular, graphene oxide (GO) membrane containing 2D graphene capillaries shows unique permeation properties such as ultrafast permeation of water and molecular sieving. In my talk, I will discuss our recent results on molecular and ionic permeation properties of GO membranes and its prospect for several applications.

Electrolytes exhibit drastically different properties when confined in nanochannels. For example, in bulk solution, cations and anions simultaneously move along opposite directions to generate ionic current. However, in channels narrower than the Debye length of the electrolyte (a measure of how far electrostatic effects can persist), the surface charges on the inner walls repel ions of the same charge and attract the counterions, making them the dominating charge carriers. Such unipolar ionic transport can enhance ionic conductivity up to several orders of magnitudes. Nanochannels that enable such transport can be fabricated in bulk materials, but such “top-down” methods are rather expensive and difficult to scale up. The construction of nanofluidic channels with the 2D sheets, a “bottom up” approach, can be done simply by casting or filtration of 2D sheets dispersed in solution, as first demonstrated with graphene oxide sheets. The surface properties and the spacing of the 2D nanochannels can be conveniently controlled by modifying the starting sheets. A number of 2D materials are already available for this purpose.

The exfoliation-reconstruction strategy not only breaks down the experimental barrier of entry to study nanofluidics, but also offers a number of unprecedented opportunities. For example, one can also draw an analogy between the 2D nanofluidic films and extrinsically doped semiconductor wafers, because they both have a planar geometry and only one type of dominating charge carriers. Looking ahead, one can potentially use these lamellar films as “ionic wafers” to create ionic devices or even circuits mimicking the functions of their electronic counterparts. While the hard semiconducting wafers have to be patterned by lithography, these flexible ionic wafers can also be patterned by alternative methods such as Kirigami-like cutting techniques to define the functional elements. Some problems that should be addressed in the field will also be discussed.

Water conduction through porous materials and membranes plays a significant role in nature as well as water purification technologies. Flow through confined channels with dimensions below few nanometers displays a drastic enhancement in the observed water velocities. Most of these studies were with carbon surfaces (e.g., carbon nanotubes) and the enhanced flows were primarily attributed to frictionless transport of water molecules on atomically flat graphitic surface. Nevertheless, with another atomically flat material namely hexagonal boron nitride (hBN), a structurally similar but electronically dissimilar analogue of graphite, we test the water flow velocity. We investigate the link of the hydrodynamic flow to the surface friction and electronic properties of the confining materials, which remains unclear to date. Using precise angstrom scale capillaries made up of graphite and boron nitride walls, we study the water flow through them and address the effect of electronic properties and surface friction.
We employ precision gravimetry to study the water flow quantitatively. Unlike graphite capillaries, which show slip enhanced water flow, hBN capillaries showed at least two orders of magnitude lower water flux. While the velocities of water inside graphite capillaries reach upto 1 metre/second, for hBN capillaries the velocity can reach hardly 0.007 metre/second. Both the surfaces are atomically flat and even structurally similar. What contributes to this massive difference? We probe the possible reasons one by one. We have examined in detail the effects which can contribute to slip flow on two structurally similar atomically flat surfaces of graphite and hBN.

Structural and dynamic properties of ions confined in nanoslits are crucial to understand the fundamental mechanism underlying a wide range of chemical and biological phenomena. Recently, angstrom-scale channels were fabricated using 2-dimensional materials assembled into devices with controllable size, atomically smooth inner walls, and little surface charge, which provide us a new platform to study ion transport under strong confinement. In particular, this new confining geometry allows the size exclusion effect to be unambiguously decoupled from other mechanisms. Here, we investigate ion transport through two-dimensional slits using molecular dynamics simulations. K+ and Cl– show similar ion mobilities in a bulk aqueous solution, whereas they exhibit a remarkable difference when transporting through an angstrom-scale channel. Our molecular dynamics simulations uncover that such discrepancy originates from the subtle differences in their hydration structures and preferable locations across the channel. Opposite charge causes different water dipolar orientations around ions, mediating the distance and tribological interactions between hydrated ions and channel’s walls. Hydrated Cl– ions experience a remarkable larger friction force inside the channel and consequently a smaller mobility compared with K+ ions. Ion transport also shows clear size effect. As the slit size approaches the ultimate scale, dehydration at the entry impedes the ionic conductance significantly, and even induces a complete ion rejection. We demonstrate that energy barriers required to accomplish the ion permeation can be theoretically connected to the partial dehydration process. The proposed model is further validated by simulations. Our results offer insights into the atomistic details of ion permeation, which may also shed light on developing effective ways for water filtration and desalination.

Graphene oxide (GO) has been promising candidate material for separating water, ions and gases. Previous researchers have done great efforts to understand how GO membrane filtrates the ion and water molecules. The capability to separate the matters depends on the adsorption, diffusion and sieving diameter of the membranes. The transport mechanism through GO membrane with less 1nm interlayer spacing (wet state) becomes very complex to figure it out, because three effects should be considered. Using GO membrane with Angstrom channel size, separating results of ions, water and gases are presented and the plausible mechanisms are discussed.

Dipolar graphene-based interfaces are yet to be considered for the latest generation of ionic sieving membranes, while having remarkable potential related to their enhanced charge interactions, water acceptability and suppressed nonspecific adsorption of complex molecules on the solid-liquid interface (fouling). Such membrane bears resemblance to zwitterionic macromolecules that retain both positive and negative groups, which make them highly hydrophilic and electrokinetically stable at a broader range of pH and ionic strengths when compared to other conventional families of macromolecules or materials.

In this work, we investigated the ionic transport in dipolar laminar graphene oxide (GO)-based membranes, harboring both positive amid and negative covalently bonded charge groups. By controlling the content of different oxygen carrying groups, the overall ionic permeability can be tuned. Even with an enhanced hydrophilicity of the membrane, the membranes with higher degree of reduction showed ionic permeability values lowered by an order of magnitude. Moreover, a peculiar reversed ionic selectivity among mono- and divalent ions is observed, giving us general insight into molecular interactions within GO membranes. The scalability of these membranes, combined with their chemical stability, as well as antifouling properties make them strong candidates for applications such as, biochemical separation processes, desalination and osmotic power generation.

Atomic-defect engineering in 2D materials provides new opportunities from condensed
matter physics to molecular filtration. Introducing atomic and nm-scale vacancies in 2D materials changes their electrical and optical properties (for example the bandgap, as in antidot lattices). When 2D materials are suspended, vacancies make the membranes permeable to ions and molecules in liquid or gas phases, allowing transport studies at atomic scales. While Ohm’s law works well to describe ion current flow through nm-size holes, for sub-nm holes comprised of a handful of missing atoms, one measures non-linear current-voltage curves. Sub-nm holes allow the passage of smaller water molecules but block the larger hydrated salt ions which can be exploited for efficient water desalination. Raman and photoluminescence spectroscopy combined with aberration-corrected TEM, together with molecular-dynamics and DFT calculations, provide a comprehensive approach to characterize the holes and transport through them. Raman peak shifts can be used to quickly determine the density of vacancies, once those have been initially calibrated with electron microscopy data.
(1) P. Masih Das et al., “Centimeter-scale nanoporous 2D membranes and ion transport: porous MoS2 monolayers in a few-layer matrix”, Nano Letters 19 (1), 392, 2019.
(2) G. Danda et al., “Laser-induced fabrication of nanoporous WS2 membranes”, 2D Materials 5 (3), 2018.
(3) J.P. Thiruraman et al., “Angstrom-size defect creation and ionic transport through pores in single-layer MoS2”, Nano Letters 18 (3), 1651, 2018.
(4) P. Masih Das et al., “Periodic arrays of phosphorene nanopores as antidot lattices with tunable properties”, ACS Nano 11 (7), 7494, 2017.
(5) G. Danda et al., “Monolayer WS2 nanopores for DNA translocation with light-adjustable sizes”, ACS Nano 11 (2), 1937, 2017.
(6) P. Masih Das et al., “Controlled sculpture of black phosphorous nanoribbons”, ACS Nano 10 (6), 5687, 2016.
(7) W. Parkin et al., “Raman shifts in electron-irradiated monolayer MoS2”, ACS Nano 10 (4), 4134, 2016.

Thin membranes made from systems like graphene are emerging as essential building blocks for applications ranging from desalination, osmotic power production to molecular sensing. Here, we elucidate the physical interactions that govern trasport of ions and macromolecules in these systesm. Our quantitative measurements based on diffusion of positive and negative ions across porous 2D membranes of graphene and hexagonal boron nitride (h-BN) prove their cation selectivity in a wide pH range. The Goldman-Hodgkin-Katz equation allows us to extract the ion selectivity from current−voltage characteristics of graphene and h-BN monolayers separating reservoirs of different salt concentrations. Systematic measurements varying the Debye screening length as well as the type of ions and pH demonstrate that negative surface charge gives rise to cation selectivity. In addition, we show that the cation selectivity can be reversed into anion selectivity by the chosen ionic spcies. Surprisingly, h-BN and graphene membranes show very similar characteristics. The extrensic nature of the selectivity enables to create efficient power conversion for streaming currents. Finally, we will discuss our recent measurements on the transport of macromolecules like DNA through pores in graphene and hBN and efforts to measure the force during translocation using optical tweezers.

Nanopore technology using biological pores embedded in a lipid bilayer has emerged as a rapid and inexpensive alternative to conventional DNA sequencing methods. In a similar vein, artificial solid-state nanopores possessing the advantages of mechanical strength, modifiable geometry and stable chemical properties over the biological nanopores are expected to be more favorable for biomolecule detections. However, thus far the development of solid-state nanopore technology has encountered gigantic obstacles in both spatial and temporal resolutions hindering the fulfillment of practical sequencing applications. Comparing with the gap between each DNA nucleotide (~3.4 angstroms), the thickness of conventional silicon nitride membranes is approximately two orders of magnitude greater. In this regard, two-dimensional materials nanopores have been recently employed attempting to improve the spatial resolution, whose thicknesses coincide with the distance between each nucleotide (e.g. the thickness of a monolayer molybdenum disulfide is 6.5 angstroms). Nevertheless, although these ultrathin nanopores conceptually promise high spatial resolution, the reduction of the pore length concurrently brings up two major issues: (i) the augmentation of Joule heating effects in which the imposed electric field magnitude is inversely proportional to the pore length. In consequence, the superheated solution inside the nanopore not only can give rise to the denaturation of DNA molecule properties but deteriorates the signal-to-noise ratio due to the additional thermal noise; and (ii) the presence of transport-induced-charge effects resulting in a lack of translocation events at low applied electric field but excessive translocation speed at high applied electric field yielding poor temporal resolution.
Here, we propose a novel and simple approach to effectively resolve above issues originated from the external electric field. Instead of tracing the conduction current variation, we replaced the external electric field with a solute concentration difference across a 3 nm monolayer molybdenum disulfide nanopore and detected the diffusion current variation during the diffusiophoretic translocation events of 20-mer ssDNA molecules. In this diffusion current system, Joule heat generated from the conductive ionic flux and transport-induced-charge effects due to the nanopore ion concentration polarization were simultaneously avoided. As a result, we successfully obtained four levels of current signals which are considered as the evidence of four types of nucleotides on the DNA molecules. These promising results revive opportunities for direct DNA sequencing using solid-state nanopores.

Back in 2016 [1] we reported the first measurement of osmotic power across 2D membranes, and they were an important demonstration that nanomaterials may indeed represent a revolution in this domain, with direct application envisioned for renewable energy and small portable sources of energy. Power densities achieved in MoS2 nanopore outperformed the “conventional” thicker solid-state pores by orders of magnitude and make the whole concept of using 2D-pores extremely attractive. To reach the full potential, the osmotic power generator had to operate in an alkaline environment (pH 11), pH levels that are far from the values found in estuaries. High pH was required to increase the surface charge of the MoS2. Early on, we understood the importance of the surface charge in the power generation
of 2D membranes and we explored ways how to improve the performance of osmotic power generation. In this work, we demonstrate that the osmotic power can be increased five-fold using light. To enhance and modulate surface charges we have used low power densities of light (~1-3 W/m2), that should be attainable in real-world applications where the sun can act as the photon source. To exploit MoS2 photoexcitability, we selected laser sources operating at 475nm and 643nm, with photon energies above the bandgap of single-layer MoS2. Also, we investigate the role of the pore sizes and surface charges on the osmotic power generation. By introducing a formalism based on the Dukhin number [2], we can decipher the contributions to the osmotic power from the pore and surface conductions. At the high surface charge densities, the osmotic potential in small pores is reduced, since the dominant contribution originates from
the surface condition. The main message of our work concentrates on the elegant way how the two most abundant energy
sources can be efficiently married in the same device paving a way for next generation of tandem blue
energy/solar devices based on 2D materials.

1. Feng, J.D. et al. Single-layer MoS2 nanopores as nanopower generators. Nature 536, 197-+(2016).
2. Bocquet, L. & Charlaix, E. Nanofluidics, from bulk to interfaces. Chemical Society Reviews 39,1073-1095 (2010).

Two-dimensional (2D) Materials such as graphene,^1-4 molybdenum disulfide (MoS₂),^5-7 and tungsten disulfide (WS₂)⁸ have gained attention as alternatives to the protein nanopores for DNA sequencing applications, and as promising candidates for harvesting osmotic gradients for blue energy.⁹ From a device standpoint, such measurements are not viable unless a reliable scaled-up fabrication method of the 2D freestanding membranes is available.¹⁰ In addition, reducing the low frequency noise of these devices is critical for sensitive measurements of biomolecules.⁶ Among the 2D materials, MoS₂ has gained special attention due to its less hydrophobic nature as compared with graphene. Moreover, we have previously shown that unlike graphene, MoS₂ membranes can directly grow on apertures using chemical vapor deposition (CVD) method, which eliminates the time-intensive membrane transfer process.⁶ Here, we take advantage of this technique and demonstrate scaled-up fabrication of high quality MoS₂ membranes on apertures. The apertures with diameters in the range of 50 nm to 200 nm were fabricated in silicon nitride membranes in 4-inch wafers using electron beam lithography. By fine-tuning the growth conditions we have been able to grow freestanding MoS₂ membranes on apertures in 4-inch wafers with over 60% yield, which enables fabrication of hundreds of devices in a single CVD run. Understanding the temperature profile in such a large diameter CVD tube is essential in successful growth of the MoS₂ crystals. Our computational fluid dynamics simulation points to strong temperature gradients and density-driven flows that develop in the tube, which constrain the conditions under which growth occurs on the target wafer. By addressing this problem we have been able to reliably reproduce high-quality growth over large areas. We used a transmission electron microscope to drill nanopores in the freestanding membranes for conductance and noise measurements. The rms noise of the ionic current at 100 mV applied bias and 1MHz bandwidth was recorded to be typically less than 250 pA. Further, as the voltage increases the noise further increases, with large contributions from low-frequency components. Our wafer scale direct 2D materials growth method, combined with automated nanopore fabrication, for example, by feedback-controlled dielectric breakdown,¹¹ can address the device fabrication requirements for performing challenging single-molecule sensing of biomolecules.

References
[1] S. Garaj, W. H., A. Reina, J. Kong, D. Branton & J. A. Golovchenko. Graphene as a subnanometre trans-electrode membrane. Nature 467, 190–193 (2010).
[2] Merchant, C. A. et al. DNA translocation through graphene nanopores. Nano letters 10, 2915-2921 (2010).
[3] Schneider, G. F. et al. DNA translocation through graphene nanopores. Nano letters 10, 3163-3167 (2010).
[4] Waduge, P., Larkin, J., Upmanyu, M., Kar, S. & Wanunu, M. Programmed synthesis of freestanding graphene nanomembrane arrays. Small 11, 597-603 (2015).
[5] Liu, K., Feng, J., Kis, A. & Radenovic, A. Atomically thin molybdenum disulfide nanopores with high sensitivity for DNA translocation. ACS nano 8, 2504-2511 (2014).
[6] Waduge, P. et al. Direct and Scalable Deposition of Atomically Thin Low-Noise MoS2 Membranes on Apertures. ACS Nano 9, 7352-7359, doi:10.1021/acsnano.5b02369 (2015).
[7] Feng, J. et al. Identification of single nucleotides in MoS 2 nanopores. Nature nanotechnology 10, 1070 (2015).
[8] Danda, G. et al. Monolayer WS2 Nanopores for DNA Translocation with Light-Adjustable Sizes. ACS Nano 11, 1937-1945, doi:10.1021/acsnano.6b08028 (2017).
[9] Graf, M. et al. Light Enhanced Blue Energy Generation using MoS₂ Nanopores. arXiv preprint arXiv:1902.00410 (2019).
[10] Graf, M. et al. Fabrication and practical applications of molybdenum disulfide nanopores. Nat Protoc, 1 (2019).
[11] Feng, J. et al. Electrochemical reaction in single layer MoS2: nanopores opened atom by atom. Nano letters 15, 3431-3438 (2015).

Ionic transport in atomic-layer materials has demonstrated stimulating phenomena rising from its molecular interaction caused by spatial confinement parallel to that observed in biological proteins. Exploration of steric hinderance in ionic salt molecules is made possible thanks to the various device fabrication capabilities with 2D materials. In our study, we fabricate an array of nanoporous defects in TMDs ranging from single-atom to few-nm defects using ion irradiation and investigate the ionic-transport behavior of these devices. In addition, we create individual single-atom defects using advanced aberration-corrected scanning transmission electron microscopy (AC-STEM) to understand wetting behavior and therefore surface-charge density of the individual single-atom defects in monolayer TMDs. In this talk, I will cover the device fabrication of single-atom to few-nm defects and the governing effects of non-linearity observed in ionic current-voltage relationship through these defects.

Solid-state nanopores have been used as small molecule sensors due to its steady pore current [1]. Analytes can be detected by their translocation-based ionic blockade current through the nanopore. Biological pores have structural precision and high sensitivity to chemical and electronic modifications, but lack the necessary structural strength for good repeatability and stability of sensing which can be provided by the solid-state nanopores [1].
The drawbacks for both systems can be overcome by integrating the biological pore with the solid state pore to create a hybrid nanopore [2]. This can be achieved by insertion of a biological pore into a solid state nanopore. In this research, we have used both Silicon nitride, SiN_x and Molybdenum disulphide, MoS₂ (2D material) as membranes for solid state pores. MoS₂ thickness can be controlled to monolayer and hence the pore thickness can be controlled very precisely. This lower thickness translates to a higher resolution for molecular sensing. Here, we have used OmpG, a barrel protein having an outer diameter of 3.3 nm and inner diameter of 2 nm, for the biological pore. Single OmpG pore is pulled through by applying a bias to the solid state nanopore and it is prevented from going through the pore by choosing a correct size (3.5-4 nm diameter) solid state pore, fabricated of either SiN_x (50 nm thick) or MoS₂ (Monolayer: 0.65 nm thick) pores to form hybrid pores. One of the major advantages of OmpG nanopore is its capability of insertion in solid state pore without a guiding DNA. Additionally, the flexibility of its external loops can be fine- tuned, thus offering a good control over detection specificity. 40 pM solution of single stranded DNA oligos (each having 30 mixed nucleotides) were pulled through 2.5 nm solid state SiN_x and MoS₂ pores and 4 nm (outer diameter) hybrid SiN_x and MoS₂ pores, and their translocation statistics was compared.
Both solid state and hybrid nanopores created using SiN_x and MoS₂ along with OmpG were used for studying DNA translocation. In case of solid state nanopores, MoS₂ has the advantage of retarding nucleotide translocation by 26-fold as compared to SiN_x. This improves the signal to noise ratio (SNR) to 11.2 for MoS₂ as compared to 1.86 for SiN_x. MoS₂ also demonstrates a 12-32% of deviation around the average translocation current value for a particular nucleotide, hampering precise identification of individual species, especially those having low density. An improvement in current deviation to 2-5% is observed using the hybrid nanopores. Monolayer MoS₂ demonstrated a 2-8 fold reduction in deviation compared to solid state monolayer MoS₂, while preserving an SNR greater than 10.
Hybrid monolayer MoS₂/OmpG pore therefore is seen to sense individual DNA nucleotides from its picomolar concentration solution with 3-7 times improved resolution than SiN_x nanopore systems and greater than twice the specificity shown by solid state monolayer MoS₂ nanopores.
Current blockade values, signal-to-noise ratio, standard deviation of current and detected single nucleotide count will be reported for solid state and hybrid SiN_x and MoS₂ pores.

References

1. B. Luan, G. Stolovitzky, and G. Martyna, Nanoscale, 2012.
2. A. R. Hall, A. Scott, D. Rotem, K. K. Mehta, H. Bayley, and C. Dekker, Nat. Nano., 2010.

Dielectric breakdown (DBD) of solid-state membranes in solution has been proved to be a powerful method for fabricating nanopore, attracting plenty of research interests in solid-state nanopore sensing. Sub-2nm nanopores that are hard to process by FIB, TEM and laser can be fabricated with very high reliability and reproducibility by the DBD method, which enable the detection of the sizes and structures of small molecules. However, the position of fabricated nanopore via DBD method was stochastic.
Here in, we report a fascinating method called tip-controlled breakdown (TCBD) for fabricating single or multiple nanopores by combining atomic force microscopy (AFM) and DBD approach. During the fabrication process, a conductive AFM tip is firstly brought into contact with a graphene membrane positioned above an electrolyte reservoir filled with potassium chloride solution. Then, a voltage pulse is generated from AFM built-in voltage module to produce a local electrical field at the end of the AFM tip. Nanopores would form precisely beneath the tip due to the local electrical field effect with a complete suppression of multiple pore formation compared with DBD approaches. The diameter of nanopores was under control of voltage pulse frequency and amplitude. The experiments show that sub-2nm nanopore was fabricated with the voltage no more than 5V. Potassium chloride concentration, voltage pulse frequency and amplitude have been investigate to optimize the TCBD method. Our results demonstrate that the approach greatly accelerates the electric breakdown process, leading to an average pore fabrication time on the order of tens of milliseconds, at least two orders of magnitude shorter than achieved by classic DBD approaches. With this fast pore writing speed, over 100 pores can be fabricated within half an hour on the same membrane. Stable ionic-current blockades were observed when DNA passed through the nanopores created on the graphene membranes by TCBD approach.
Finally, combining the nano position advantages of AFM and the simplicity/low-cost of the DBD approach, the TCBD has been demonstrated to be a highly efficient, reliable and reproducible nanopore fabrication method. We anticipate TCBD has the potential to become a nanopore fabrication technique which, integrated into an optical/magnetic setup, could be used as a high-throughput and multichannel characterization technique.

Advances in nanofabrication are allowing miniaturization of Lab-On-a-Chip (LOC) units toward functional nanofluidic systems. The functionality of these multistage systems relies on the efficient transport of water solutions through nanopores integrating their active components. In such conduits, the extremely high surface to volume ratio leads to an augmented effect of the interfacial properties on the bulk of the fluid. As a result, the interfacial properties significantly increase the viscous friction reducing flow capacity. Hence, design of efficient nanofluidic platforms requires effective reduction of flow resistance within the channel network. Recently, the water-supported graphene interaction has widely been discussed highlighting the role of the underlying substrate on graphene wettability. These studies have reported wetting scenarios for supported monolayer graphene that reproduce from translucency to complete opacity to the substrate wettability. Nevertheless, experimental results have reported fast water flow in silica channels coated with graphene. Polymers are widely employed to fabricate nanostructured pores. Moreover, scalable and cost-effective techniques to deposit graphene on polymeric substrates have been developed. In the present study, we show that significant drag reduction can be achieved in a polyamide nanoslit pore by using graphene as wall coating. To this end, Molecular Dynamics (MD) simulations are conducted to study water flow through uncoated and coated polyamide nanoslit pores with different heights. We perform a characterization of the interfacial properties and evaluate the impact that the polymeric matrix has on water structure. The force fields employed to model the species are validated measuring the water contact angle and comparing values against experimental results. Using Non Equilibrium MD Simulations (NEMD), Poiseuille-like flow is studied. The flow is achieved by imposing body forces corresponding to values of 2.93, 5.87, 11.75 bar/nm. Viscous heating is transmitted to and conducted through the walls as in a real system. From the MD trajectories, we compute shear stress, density, temperature and velocity profiles across the channel. Using these observables, we analyze the correlation between local water structure, flow enhancement and slip length. The velocity profiles in the coated polymeric pore exhibit a parabolic shape with substantial wall slippage. Hence, our results indicate that water interactions with the underlying polyamide have a significant influence on the water flow rates in the pores. We compute flow enhancements ranging from 62 to 116 for the cases with coated pores. Equilibrium MD simulations (EMD) are performed to calculate the fluid-solid interfacial friction by using the Green-Kubo relation. The values of the friction coefficients calculated are 594 and 14.3 (kN s)/m³ for uncoated and coated nanoslits. We find that the flow enhancements computed from NEMD are in line with the friction coefficients computed from EMD. Our results show that graphene coatings can enable drag reduction in polymeric nanopores. The insights reported in this study may serve as a guide in designing strategies to achieve low friction transport of water solutions in nanostructured pores. These strategies will allow the development of more efficient LOC platforms and membranes for molecular sieving and nanofiltration.

Ultrathin membranes formed by stacking two-dimensional (2D) nanosheets, including graphene oxide (GO) and MoS₂ hold unique separation capabilities that enable their applications in the filtration of aqueous solutions as well as organic solvents. However, the separation capability of a layer-stacked membrane can be significantly affected by its natural tendency to swell in solutions by absorbing molecules into 2D nanochannels and forming an enlarged interlayer spacing (d-spacing). In this study, the d-spacing of a layer-stacked GO and MoS₂ membranes was experimentally quantified when the membranes were soaked in aqueous or organic solutions. We found that solvents with high dipole moments tends to cause significant swelling of GO, while non-polar solvents such as hexane and toluene only cause subtle swelling. To understand such unique swelling behavior, the solubility parameter of GO was experimentally determined to mathematically describe the affinity between GO and the testing solvents. Separation performance of the GO membrane in different organic solvents were tested to understand the mass transport in confined GO nanochannel. The layer-stacked GO membrane could achieve > 90% rejection of small organic dye molecules (e.g., rhodamine B and methylene blue) in ethanol and acetone without crosslinking due to insignificant swelling. Additionally, it was found that solvents with different polarity demonstrate very different transport resistance.

We have previously developed a soft chemistry approach to fabricate multilayered graphene membranes (MGMs) with their interlayer distance tuneable from 10 nm down to sub-nanometers [1]. In this talk, I will demonstrate how MGMs can provide a unique platform for nanofluidics, and particularly nanoionics research. My talk will include: (1) How experiments and simulations can be combined to quantify the nanostructure of the MGMs for nanoionics research [2]; (2) How supercapacitor and membrane experiments can be coupled to modulate ion diffusion through nano-confined electrical double layer; (3) How MGMs can serve as a model electrode to enable quantitative understanding of the ion transport in nanoporous for predictive design of capacitive energy storage devices.

References:

[1] Yang, X., Cheng, C., Wang, Y. Qiu, L. & Li, D., Science, 341, 534-537 (2013).
[2] Cheng, C., Jiang, G., Garvey, C. J., Wang, Y., Simon, G. P., Liu, J. L. & Li, D. Science Advances, 2, e1501272 (2016).
[3] Cheng, C., Jiang, G., Simon, G. P., Liu, J. L. & Li, D. Nature Nanotechnol. 13, 685-690 (2018).

There is great interest in exploiting van der Waals gaps in layered materials as continuous nanofluidic channels. Multilayer graphene oxide (GO) films with selective transport properties have been used for water desalination, oil-water separation, gas separation and pervaporation applications. Conventional GO membranes are often limited in practice by low permeability since the intersheet nanochannels align perpendicular to the desired overall transport direction, leading to highly circuitous fluid pathways that are orders of magnitude greater than the membrane thickness.
Here we demonstrate a novel approach to fabricate stable GO membranes with vertically aligned nanosheets spanning the membrane in the Z-direction. Compressive texturing of planar GO films tilts the nanosheets and nanochannels to near vertical directions, which are then stabilized imbedded in low-permeability polymer matrices and thin sectioned. The resulting membranes provide direct access to the entrances of intersheet nanochannels at both the top and bottom surfaces to create well-defined Z-directional fluid pathways whose length is on the same order as the micron-scale membrane thickness.
The compressive wrinkling and crumpling of graphene oxide films can also be used to control the release rates of molecular intercalants pre-loaded into GO gallery spaces. Experimental studies on rhodamine B dye, used as a model, show diffusive release rates in the rank order: planar > 1D wrinkled > 2D crumpled films. This type of fluidic-space manipulation should allow the intelligent design of 2D-material-based technologies such as time-release drug eluting coatings.

Laminar MoS₂ membranes show outstanding potential for practical applications in energy conversion/storage, sensing, and as nanofluidic devices. The re-stacking of exfoliated MoS₂ creates nanocapillaries between the layers of MoS₂ nanosheets. These MoS₂ membranes have been shown to possess a unique combination of ionic rejection properties, high water permeation rates, and long-term solvent stability, with no significant swelling when exposed to aqueous or organic solvents. Chemical modification of MoS₂ membranes has been shown to improve their ionic rejection properties, however the mechanism behind this improvement is not well understood. In this work, we elucidate the ion-sieving mechanism by the study of potential-dependent ion transport through functionalized MoS₂ membranes. The ionic permeability of the MoS₂ membrane is transformed by chemical functionalization with a simple naphthalene sulfonate dye (sunset yellow) and with a resultant attenuation of permeability by at least an order of magnitude, compared to the pristine MoS₂ membranes and permeability reported for graphene oxide and Ti₃C₂T_x (MXene) membranes. The effects of pH, solute concentration, and ionic size/charge on the ionic selectivity of the functionalized MoS₂ membranes are also reported. Understanding the mechanism of ionic sieving within functionalized MoS₂ membranes will enable future applications in electro-dialysis and ion exchange for water treatment technologies.

Separating CO₂ from other light gases is believed to have extremely practical significance and economic benefits due to the strong demand from the industrial processes and the green-house effect. Conventional adsorption and low temperature distillation have their intrinsic drawbacks while membrane separation shows great potential. However, the trade-off between the high selectivity and permanence is still a great challenge. In this report, we prepared a high-performance CO₂-philic membrane, two-dimensional nanosheets supported ionic liquid membrane, by confining [BMIM][BF₄] into the nanochannels of laminated graphene oxide membrane. The strong interaction between graphene oxide and [BMIM][BF₄] stratifies the cation and anion of [BMIM][BF₄], and the layered anion facilitates CO₂ transportation. The resulted membrane has an excellent performance with CO₂ high permeance and good selectivity of CO₂/H₂, CO₂/CH₄ and CO₂/N₂ respectively. Moreover, it has high temperature resistance, long-term durability and high-pressure stability, which shows great potential in CO₂ separation. In addition, we found that external electric field could further facilitate selectively CO₂ transportation through nanoconfined ionic liquid.

Graphene oxide (GO) has a heterogeneous nanostructure consisting of sp²-graphitic regions and sp³-oxidized regions. In general, the sp² domains of GO have hydrophobic nature, whereas sp³ domains represent hydrophilic nature. Oxygen moieties in GO act as a spacer to enlarge the interlayer distance between individual GO sheets within GO membrane. In general, the amount of oxygen moieties in GO sheets reaches above 80%, which acts as a hindrance for even small gas molecules, eventually leading to low gas permeability. To enhance the gas permeability in GO membranes, the sp² domains in GO sheets were precisely adjusted in this study. Commonly, sp²/sp³ ratio can be tuned by reducing GO using chemical, thermal, and electrochemical methods. However, these methods lead to much disadvantages of remaining chemical additives, structural deformation, difficulty in mass production. Above all, the sp²/sp³ ratio would not be precisely controlled using conventional reduction methods. Although the mild reduction methods have been reported to overcome these problems, there have been no systematic membrane studies using GO sheets with different sp²/sp³ ratio. Here we report a mild hydrothermal treatment to precisely control the graphitic regions in GO and also tune its electrical and molecular transport properties. The change in hydrothermal conditions, such as thermal temperature and time, significantly affects the gas separation performance as well as water and salt transport of the modified GO membranes. The high-resolution TEM images of hydrothermal treated GO membranes showed that this mild hydrothermal treatment leads to the gradual increase of sp²-graphitic regions and minimize the formation of structural defects during the reduction. Besides, the change in electrical conductivity as a function of sp² domain was correlated with gas permeability as well as water transport through the modified GO membranes, and also was elucidated using percolation theory. Then, we prepared the membrane using hydrothermal treated GOs to measure the gas permeability and selectivity, water permeability, and salt permeability and salt rejection. This work demonstrates the separation performance enhancement of GO membranes by redesigning the structure of GO and reveal the transport mechanisms through each domain in the modified GO membranes.

Nanoparticles are generally stabilized by ligands whose dimension is comparable to that of nanoparticle core. The surface ligands endow nanoparticles with unique physical and chemical properties which can modify collision dynamics and diffusive motion of nanoparticle when suspended in a confined medium. Here, we directly tracked trajectories of Au nanoparticles smaller than 5 nm in solution by graphene liquid cell transmission electron microscopy, and found that different modes of diffusion exist in the medium. Dynamics of the nanoparticles in graphene liquid cells display Brownian motion with stark deviation from the Stokes-Einstein equation. Detailed analyses of nanoparticle displacements exhibit that their distribution does not follow a Gaussian distribution, implying the existence of structural heterogeneity in liquid confined by graphene sheets. We also reveal that expansion of nanobubbles induces the superdiffusive motion of the nanoparticles at the bubble interface.

The ability to sieve ions through nanopores with high throughput addresses significant importance in seawater desalination and other separation applications. In this work, it is demonstrated that plasma etching process provides an efficient method to produce high density nanopores on graphene membranes with tunable size in sub-nanometer. Besides the pore size, nanopore density is also controllable through adjusting the exposure time of the sample to the argon or oxygen plasma. The plasma treated graphene membranes can selectively transport protons, Na⁺ and Cl^- ions. Density function theory calculations uncover that sp³ and vacancy-type defects construct different energy barriers for different ions, which allow the defected graphene membrane to selectively transport ions. Our study indicates that oxygen plasma etching can be used as a very convenient and efficient method for fabricating monolayer filtration graphene membrane with tunable sub-nanometer pores.