Observation and Isochoric Thermodynamic Analysis of Partially Water-Filled 1.32 and 1.45 nm Diameter Carbon Nanotubes.

Recent measurements of fluids under extreme confinement, including water within narrow carbon nanotubes, exhibit marked deviations from continuum theoretical descriptions. In this work, we generate precise carbon nanotube replicates that are filled with water, closed from external mass transfer, and studied over a wide temperature range by Raman spectroscopy. We study segments that are empty, partially filled, and completely filled with condensed water from -80 to 120 °C. Partially filled, nanodroplet states contain submicron vapor-like and liquid-like domains and are analyzed using a Clausius-Clapeyron-type model, yielding heats of condensation of water inside closed 1.32 nm diameter carbon nanotubes (3.32 ± 0.10 kJ/mol and 3.72 ± 0.11 kJ/mol) and 1.45 nm diameter carbon nanotubes (3.50 ± 0.07 kJ/mol) that are lower than the bulk enthalpy of vaporization and closer to the bulk enthalpy of fusion. Favored partial filling fractions are calculated, highlighting the effect of subnanometer changes in confining diameter on fluid properties and suggesting the promise of molecular engineering of nanoconfined liquid/vapor interfaces for water treatment or membrane distillation.

Discretized hexagonal boron nitride quantum emitters and their chemical interconversion.

Quantum emitters in two-dimensional hexagonal boron nitride (hBN) are of significant interest because of their unique photophysical properties, such as single-photon emission at room temperature, and promising applications in quantum computing and communications. The photoemission from hBN defects covers a wide range of emission energies but identifying and modulating the properties of specific emitters remain challenging due to uncontrolled formation of hBN defects. In this study, more than 2000 spectra are collected consisting of single, isolated zero-phonon lines (ZPLs) between 1.59 and 2.25 eV from diverse sample types. Most of ZPLs are organized into seven discretized emission energies. All emitters exhibit a range of lifetimes from 1 to 6 ns, and phonon sidebands offset by the dominant lattice phonon in hBN near 1370 cm-1. Two chemical processing schemes are developed based on water and boric acid etching that generate or preferentially interconvert specific emitters, respectively. The identification and chemical interconversion of these discretized emitters should significantly advance the understanding of solid-state chemistry and photophysics of hBN quantum emission.

Atomically Precise Control of Carbon Insertion into hBN Monolayer Point Vacancies using a Focused Electron Beam Guide.

Precise controlled filling of point vacancies in hBN with carbon atoms is demonstrated using a focused electron beam method, which guides mobile C atoms into the desired defect site. Optimization of the technique enables the insertion of a single C atom into a selected monovacancy, and preferential defect filling with sub-2 nm accuracy. Increasing the C insertion process leads to thicker 3D C nanodots seeded at the hBN point vacancy site. Other light elements are also observed to bind to hBN vacancies, including O, opening up a wide range of complex defect structures that include B, C, N, and O atoms. The ability to selectively fill point vacancies in hBN with C atoms provides a pathway for creating non-hydrogenated covalently bonded C molecules embedded in the insulating hBN.

Direct Observation of Charge Inversion in Divalent Nanofluidic Devices.

Solid-state nanofluidic devices have proven to be ideal systems for studying the physics of ionic transport at the nanometer length scale. When the geometrical confining size of fluids approaches the ionic Debye screening length, new transport phenomena occur, such as surface mediated transport and permselectivity. Prior work has explored these effects extensively in monovalent systems (e.g., predominantly KCl and NaCl). In this report, we present a new characterization method for the study of divalent ionic transport and have unambiguously observed divalent charge inversion at solid/fluid interfaces. This observation has important implications in applications ranging from biology to energy conversion.

Voltage gated ion and molecule transport in engineered nanochannels: theory, fabrication and applications.

Nanochannels remain at the focus of growing scientific and technological interest. The nanometer scale of the structure allows the discovery of a new range of phenomena that has not been possible in traditional microchannels, among which a direct field effect control over the charges in nanochannels is very attractive for various applications, since it offers a unique opportunity to integrate wet ionics with dry electronics seamlessly. This review will focus on the voltage gated ionic and molecular transport in engineered gated nanochannels. We will present an overview of the transport theory. Fabrication techniques regarding the gated nanostructures will also be discussed. In addition, various applications using the voltage gated nanochannels are outlined, which involves biological and chemical analysis, and energy conversion.

Gas Separations using Nanoporous Atomically Thin Membranes: Recent Theoretical, Simulation, and Experimental Advances.

Porous graphene and other atomically thin 2D materials are regarded as highly promising membrane materials for high-performance gas separations due to their atomic thickness, large-scale synthesizability, excellent mechanical strength, and chemical stability. When these atomically thin materials contain a high areal density of gas-sieving nanoscale pores, they can exhibit both high gas permeances and high selectivities, which is beneficial for reducing the cost of gas-separation processes. Here, recent modeling and experimental advances in nanoporous atomically thin membranes for gas separations is discussed. The major challenges involved, including controlling pore size distributions, scaling up the membrane area, and matching theory with experimental results, are also highlighted. Finally, important future directions are proposed for real gas-separation applications of nanoporous atomically thin membranes.

Direct Chemical Vapor Deposition Synthesis of Porous Single-Layer Graphene Membranes with High Gas Permeances and Selectivities.

Single-layer graphene containing molecular-sized in-plane pores is regarded as a promising membrane material for high-performance gas separations due to its atomic thickness and low gas transport resistance. However, typical etching-based pore generation methods cannot decouple pore nucleation and pore growth, resulting in a trade-off between high areal pore density and high selectivity. In contrast, intrinsic pores in graphene formed during chemical vapor deposition are not created by etching. Therefore, intrinsically porous graphene can exhibit high pore density while maintaining its gas selectivity. In this work, the density of intrinsic graphene pores is systematically controlled for the first time, while appropriate pore sizes for gas sieving are precisely maintained. As a result, single-layer graphene membranes with the highest H2 /CH4 separation performances recorded to date (H2 permeance > 4000 GPU and H2 /CH4 selectivity > 2000) are fabricated by manipulating growth temperature, precursor concentration, and non-covalent decoration of the graphene surface. Moreover, it is identified that nanoscale molecular fouling of the graphene surface during gas separation where graphene pores are partially blocked by hydrocarbon contaminants under experimental conditions, controls both selectivity and temperature dependent permeance. Overall, the direct synthesis of porous single-layer graphene exploits its tremendous potential as high-performance gas-sieving membranes.

Diameter Dependence of Water Filling in Lithographically Segmented Isolated Carbon Nanotubes.

Although the structure and properties of water under conditions of extreme confinement are fundamentally important for a variety of applications, they remain poorly understood, especially for dimensions less than 2 nm. This problem is confounded by the difficulty in controlling surface roughness and dimensionality in fabricated nanochannels, contributing to a dearth of experimental platforms capable of carrying out the necessary precision measurements. In this work, we utilize an experimental platform based on the interior of lithographically segmented, isolated single-walled carbon nanotubes to study water under extreme nanoscale confinement. This platform generates multiple copies of nanotubes with identical chirality, of diameters from 0.8 to 2.5 nm and lengths spanning 6 to 160 µm, that can be studied individually in real time before and after opening, exposure to water, and subsequent water filling. We demonstrate that, under controlled conditions, the diameter-dependent blue shift of the Raman radial breathing mode (RBM) between 1 and 8 cm-1 measures an increase in the interior mechanical modulus associated with liquid water filling, with no response from exterior water exposure. The observed RBM shift with filling demonstrates a non-monotonic trend with diameter, supporting the assignment of a minimum of 1.81 ± 0.09 cm-1 at 0.93 ± 0.08 nm with a nearly linear increase at larger diameters. We find that a simple hard-sphere model of water in the confined nanotube interior describes key features of the diameter-dependent modulus change of the carbon nanotube and supports previous observations in the literature. Longer segments of 160 µm show partial filling from their ends, consistent with pore clogging. These devices provide an opportunity to study fluid behavior under extreme confinement with high precision and repeatability.

Enhancing Lithium Insertion with Electrostatic Nanoconfinement in a Lithography Patterned Precision Cell.

The rapidly growing demand for portable electronics, electric vehicles, and grid storage drives the pursuit of high-performance electrical energy storage (EES). A key strategy for improving EES performance is exploiting nanostructured electrodes that present nanoconfined environments of adjacent electrolytes, with the goal to decrease ion diffusion paths and increase active surface areas. However, fundamental gaps persist in understanding the interface-governed electrochemistry in such nanoconfined geometries, in part because of the imprecise and variable dimension control. Here, we report quantification of lithium insertion under nanoconfinement of the electrolyte in a precise lithography-patterned nanofluidic cell. We show a mechanism that enhances ion insertion under nanoconfinement, namely, selective ion accumulation when the confinement length is comparable to the electrical double layer thickness. The nanofabrication approach with uniform and accurate dimensional control provides a versatile model system to explore fundamental mechanisms of nanoscale electrochemistry, which could have an impact on practical energy storage systems.

A nanofluidic ion regulation membrane with aligned cellulose nanofibers.

The advancement of nanofluidic applications will require the identification of materials with high-conductivity nanoscale channels that can be readily obtained at massive scale. Inspired by the transpiration in mesostructured trees, we report a nanofluidic membrane consisting of densely packed cellulose nanofibers directly derived from wood. Numerous nanochannels are produced among an expansive array of one-dimensional cellulose nanofibers. The abundant functional groups of cellulose enable facile tuning of the surface charge density via chemical modification. The nanofiber-nanofiber spacing can also be tuned from ~2 to ~20 nm by structural engineering. The surface-charge-governed ionic transport region shows a high ionic conductivity plateau of ~2 mS cm-1 (up to 10 mM). The nanofluidic membrane also exhibits excellent mechanical flexibility, demonstrating stable performance even when the membrane is folded 150°. Combining the inherent advantages of cellulose, this novel class of membrane offers an environmentally responsible strategy for flexible and printable nanofluidic applications.