Triangular Nanopores Research Articles

Synergistic pervaporation dehydration of ethanol/water mixture: Exploring the potential of a covalently designed hybrid membrane structure of polyacrylic acid grafted carbon nitride and polyvinyl alcohol

Polyacrylic acid (PAA) grafted CN sheet (P-g-CN) was synthesized to enhance the dispersive properties of carbon nitride (CN) in the membrane. A successful PAA grafting to the CN was confirmed from FTIR, TGA, and Zeta potential and XRD analyses. The A PVA membrane embedded P-g-CN, including a covalently constructed polymer-filler network, was developed to separate ethanol-water mixtures using pervaporation (PV). XPS study has confirmed a covalent attachment of P-g-CN sheets to the PVA matrix. Thereby, a defect-free membrane matrix was observed in the FESEM analysis. A 10 wt% loaded PVA-P-g-CN10 composite membrane was compared to the pristine PVA membrane, demonstrating improved PV dehydration performance. The flux decreased from 0.21 kg/m2h of pristine PVA membrane to 0.17 kg/m2h of PVA-P-g-CN10 membrane, while the separation factor improved from 49 to 176 in a 90/10 wt % ethanol/water feed at 40 °C. This improvement can be attributed to the selective diffusion of water through the P-g-CN interlayer spacing and tiny triangular nanopores in the s-triazine network, along with their dispersibility in the PVA matrix, resulting in well-ordered membrane morphology. Furthermore, PVA-P-g-CN10 exhibited higher water permeance (43.31–86.07 GPU) than ethanol (1.18–10.47 GPU) as the feed temperature increased from 30 to 70 °C, suggesting P-g-CN successfully inhibits swelling in the feed solution through proper interaction with PVA. In a long-term PV test lasting 250 h, the PVA-P-g-CN10 membrane displayed excellent structural stability and maintained its performance.

Nanoscale patterning on layered MoS2 with stacking-dependent morphologies and optical tunning for phototransistor applications

Nanoscale patterning has attracted considerable interest as a novel engineering technology for two-dimensional (2D) nanomaterials for various applications such as electronics, sensors, and catalysts. However, research on nanopatterning technologies that reflect the unique structural and electrical characteristics of 2D nanomaterials remains insufficient. In this study, we present a nanoscale patterning of two different polygons on layered molybdenum disulfide (MoS2), where the nanostructures are controlled by the stacking structure of MoS2. We investigate the dependence of nanopore edge stability on the stacking structure of MoS2 using density functional theory calculations and find the different geometric effects of the two nanopatterns on the band-structure modulation of semiconducting MoS2. Consistent with the theoretical calculations, we fabricate nanoporous MoS2 with hexagonal nanopores for AA’ and triangular nanopores for AB stacked MoS2 films. The AA’ MoS2 has narrow nanoribbons between the two hexagonal nanopores, resulting in a lateral quantum confinement effect, whereas the edge exposure effect is more dominant for the bandgap increase in the triangular nanoporous structure. Nanopatterns contribute to generating in-gap state of MoS2, significantly enhancing the photoelectrical characteristics of MoS2 phototransistors. Especially, the photogating effect is higher for AA’ MoS2 phototransistors with hexagonal nanoholes than AB MoS2 with triangular nanoholes. This study provides new insights into nanoscale patterning and band structure engineering of layered 2D materials for various optoelectrical applications.

Fabrication of wafer-scale nanoporous AlGaN-based deep ultraviolet distributed Bragg reflectors via one-step selective wet etching

In this paper, we reported on wafer-scale nanoporous (NP) AlGaN-based deep ultraviolet (DUV) distributed Bragg reflectors (DBRs) with 95% reflectivity at 280 nm, using epitaxial periodically stacked n-Al0.62Ga0.38N/u-Al0.62Ga0.38N structures grown on AlN/sapphire templates via metal–organic chemical vapor deposition (MOCVD). The DBRs were fabricated by a simple one-step selective wet etching in heated KOH aqueous solution. To study the influence of the temperature of KOH electrolyte on the nanopores formation, the amount of charge consumed during etching process was counted, and the surface and cross-sectional morphology of DBRs were characterized by Scanning electron microscopy (SEM) and atomic force microscopy (AFM). As the electrolyte temperature increased, the nanopores became larger while the amount of charge reduced, which revealed that the etching process was a combination of electrochemical and chemical etching. The triangular nanopores and hexagonal pits further confirmed the chemical etching processes. Our work demonstrated a simple wet etching to fabricate high reflective DBRs, which would be useful for AlGaN based DUV devices with microcavity structures.

Methane flow in nanopores: Analytical approximation based on MD simulations

In the current study by means of molecular dynamics (MD) computer simulation, we have investigated pressure-driven flow of methane at high temperature and pressures (398.15 K and 400–500 bar) in circular, square, and triangular nanopores of different size and different fluid–wall interaction strength. To generalize the results obtained in MD simulation, we have proposed an analytical approximation (ansatz) of fluid flow dependence on pore size, while the numerical coefficients have been derived from the fitting of computer simulation data. The formulae obtained take into consideration such nanoflow features as adsorption of fluid on the pore walls and high slip velocity, which are usually neglected in petrophysical simulators operating with continuum fluid approach. These analytical formulae can be then incorporated into various larger scale models used for calculation of fluid flow in nanoporous media. This study is aiming the investigation of general phenomena occurring near the fluid–wall interfaces at nanoscale.

Unveiling the Working Mechanism of g-C3N4 as a Protection Layer for Lithium- and Sodium-Metal Anode.

The practical application of lithium-/sodium-metal batteries is currently hindered by severe safety issues caused by uncontrolled continuous dendrite growth. Semiconductive nanoporous g-C3N4 film has been demonstrated to be an effective protection layer for lithium-/sodium-metal anode, which can suppress the growth of dendrite. However, the underlying mechanism of how this semiconductive flexible thin film works to suppress dendrite growth remains unclear. In this work, we investigate the detailed working mechanism of g-C3N4 protection layer employing both density functional theory calculations and ab initio molecular dynamics simulations. The calculation results indicate that g-C3N4 layers show strong adhesion toward the lithium-/sodium-metal surface. When contacting with lithium/sodium metal, the intrinsic triangular nanopores of g-C3N4 will be quickly filled with lithium/sodium atoms, turning the semiconductive g-C3N4 into a metallic material. Lithium/sodium atoms can migrate through the triangular nanopores of stacked g-C3N4 layers via a vacancy-mediated approach with moderate energy barriers of 0.42 and 0.61 eV, respectively. With a low current density, the newly deposited lithium/sodium atoms can permeate through the g-C3N4 protection layers, therefore resulting in a flat electrode surface with no dendrite; with a high current density, however, the newly deposited lithium/sodium atoms cannot transport across the protection layer timely, which will result in the aggregation of lithium/sodium atoms on the surface of the g-C3N4 protection layer.

Structural characterizations and electronic properties of CuSe monolayer endowed with triangular nanopores

CuSe monolayer possesses intrinsically patterned triangular nanopores with uniform size and can serve as a template for selective adsorptions for molecules and nanoclusters. Here, we prepare the CuSe monolayer on Cu(111) substrate by molecular beam epitaxy method and characterize CuSe monolayer in detail by bond-resolved scanning tunneling microscopy and non-contact atomic force microscopy. The results further confirm the honeycomb feature and triangular nanopores existence of CuSe monolayer. In addition, scanning tunneling spectroscopy measurements reveal the semiconducting features of CuSe monolayer with a band gap of 2.40 eV. This work helps to understand the structure and electronic properties of those intrinsically patterned two-dimensional materials.

Linear piezoresistive strain sensor based on graphene/g-C3N4/PDMS heterostructure

Here we report a novel hybrid material consists of 2D graphitic carbon nitride (g-C3N4) and graphene heterostructure that exhibits piezoresistivity superior to graphene and potentially being used as a strain sensor. The g-C3N4 that contains periodically spaced triangular nanopores is used for improving the piezoresistivity of the sensor imparting change in the polarization upon application of strain. In this work, we have investigated graphene/g-C3N4 interfaced materials and quantified its piezoresistive effects through experimental analysis and density functional theory (DFT) based computational studies provide insights into the electronic structures of the hybrid interfaces. We have observed a linear response in electrical resistance for a wide range of uniaxial strains up to ∼25%. The observed increase in resistance upon application of strain corroborates with our computational finding of strain-dependent band gap opening. Further, it has been realized that band-gap opening occurs exclusively in the graphitic layer of the composite materials under strain. However, the g-C3N4 bands remain intact at the interface. The linearity and a considerably small gauge factor (1.89) make graphene/g-C3N4 a promising heterostructure material unlike conventional metal gauge sensor in wide strain pressure sensor devices.

Large-scale synthesis of crystalline g-C3N4 nanosheets and high-temperature H2 sieving from assembled films.

Poly(triazine imide) (PTI), a crystalline g-C3N4, hosting two-dimensional nanoporous structure with an electron density gap of 0.34 nm, is highly promising for high-temperature hydrogen sieving because of its high chemical and thermal robustness. Currently, layered PTI is synthesized in potentially unsafe vacuum ampules in milligram quantities. Here, we demonstrate a scalable and safe ambient pressure synthesis route leading to several grams of layered PTI platelets in a single batch with 70% yield with respect to the precursor. Solvent exfoliation under anhydrous conditions led to single-layer PTI nanosheets evidenced by the observation of triangular g-C3N4 nanopores. Gas permeation studies confirm that PTI nanopores can sieve He and H2 from larger molecules. Last, high-temperature H2 sieving from PTI nanosheet-based membranes, prepared by the scalable filter coating technique, is demonstrated with H2 permeance reaching 1500 gas permeation units, with H2/CO2, H2/N2, and H2/CH4 selectivities reaching 10, 50, and 60, respectively, at 250°C.

Quantitative insights into the growth mechanisms of nanopores in hexagonal boron nitride

The formation of nanopores under electron irradiation is an ideal process to quantify chemical bonds in two-dimensional materials. Nowadays, high-resolution transmission electron microscopy (HRTEM) allows investigating such nucleation and growth phenomena with incomparable spatial and temporal resolution. Moreover, theoretical calculations are usually exploited to confirm characteristic features of these atomic-scale observations. Nevertheless, the full understanding of the ejection mechanisms of atoms requires a detailed investigation of the interplay between the very dynamic edge structure of expanding nanopores and the displacement energy of edge atoms $({E}_{D})$. Here, the dynamics of triangular nanopores in hexagonal boron nitride (h-BN) under various electron dose rates was followed by aberration-corrected HRTEM with high temporal resolution to provide new in situ insights into their growth processes. We reveal that the ejection of atomic pairs is an elemental mechanism that considerably speeds up the expansion of nanopores. Atomic-scale calculations were exploited to quantify the structure-dependent ${E}_{D}$ of all the ejected edge atoms. They revealed strong variations of this threshold energy during the growth processes. This quantitative study reconciles theoretical and experimental measurements of the ejection rate of atoms in h-BN under electron irradiation, which is essential for nanopore engineering in this atomically thin membrane.

Addressing the isomer cataloguing problem for nanopores in two-dimensional materials.

The presence of extended defects or nanopores in two-dimensional (2D) materials can change the electronic, magnetic and barrier membrane properties of the materials. However, the large number of possible lattice isomers of nanopores makes their quantitative study a seemingly intractable problem, confounding the interpretation of experimental and simulated data. Here we formulate a solution to this isomer cataloguing problem (ICP), combining electronic-structure calculations, kinetic Monte Carlo simulations, and chemical graph theory, to generate a catalogue of unique, most-probable isomers of 2D lattice nanopores. The results demonstrate remarkable agreement with precise nanopore shapes observed experimentally in graphene and show that the thermodynamic stability of a nanopore is distinct from its kinetic stability. Triangular nanopores prevalent in hexagonal boron nitride are also predicted, extending this approach to other 2D lattices. The proposed method should accelerate the application of nanoporous 2D materials by establishing specific links between experiment and theory/simulations, and by providing a much-needed connection between molecular design and fabrication.

Atomic-Scale Fluidic Diodes Based on Triangular Nanopores in Bilayer Hexagonal Boron Nitride.

Nanofluidic diodes based on nanochannels have been studied theoretically and experimentally for applications such as biosensors and logic gates. However, when analyzing attoliter-scale samples or enabling high-density integration of lab-on-a-chip devices, it is beneficial to miniaturize the size of a nanofluidic channel. Using molecular dynamics simulations, we investigate conductance of nanopores in bilayer hexagonal boron nitride (h-BN). Remarkably, we found that triangular nanopores possess excellent rectifications of ionic currents while hexagonal ones do not. It is worth highlighting that the pore length is only about 0.7 nm, which is about the atomic limit for a bipolar diode. We determined scaling relations between ionic currents I and pore sizes L for small nanopores, that are I ∼ L1 in a forward biasing voltage and I ∼ L2 in a reverse biasing voltage. Simulation results qualitatively agree with analytical ones derived from the one-dimensional Poisson-Nerst-Planck equations.

Atomic Structure and Dynamics of Self-Limiting Sub-Nanometer Pores in Monolayer WS2.

We reveal a self-limiting mechanism during the formation of a specific type of circular nanopore in monolayer WS2 that limits its diameter to sub-nm. A single W atom vacancy (triangular nanopore) is transformed into the self-limiting nanopore (SLNP) through the atomic restructuring of S atoms around the area, reducing the number of dangling bonds at the nanopore edge by shifting them further in-plane with W-W bonding instead. Bond rotations in WS2 help accommodate the electron beam induced atomic loss and ensure the stability of the SLNP. The SLNP shows significant improvement in diameter stability during electron beam irradiation compared to other triangular nanopores in WS2 that typically continue to expand in diameter during atom loss. The atomic structure of these SLNPs is studied using aberration-corrected scanning transmission electron microscopy with an in situ heating holder, revealing that the SLNPs are mostly formed at a temperature of ∼500 °C, which is a balance between thermally activated S vacancy diffusion and sufficient S vacancy density to initiate local atomic reconstruction. At higher temperatures ( i. e., 1000 °C), S vacancies quickly migrate away into long line vacancies, resulting in low S vacancy density and rapidly expanding holes generated at the edges of the line vacancies. At room temperature, S vacancy migration is low and vacancy density is very high, which limits atomic reconstruction, and instead many small holes open up. These results provide insights into the factors that lead to uniform sized nanopores in the sub-nm range in transition-metal dichalcogenides.

Fabrication of Subnanometer-Precision Nanopores in Hexagonal Boron Nitride

We demonstrate the fabrication of individual nanopores in hexagonal boron nitride (h-BN) with atomically precise control of the pore shape and size. Previous methods of pore production in other 2D materials typically create pores with irregular geometry and imprecise diameters. In contrast, other studies have shown that with careful control of electron irradiation, defects in h-BN grow with pristine zig-zag edges at quantized triangular sizes, but they have failed to demonstrate production and control of isolated defects. In this work, we combine these techniques to yield a method in which we can create individual size-quantized triangular nanopores through an h-BN sheet. The pores are created using the electron beam of a conventional transmission electron microscope; which can strip away multiple layers of h-BN exposing single-layer regions, introduce single vacancies, and preferentially grow vacancies only in the single-layer region. We further demonstrate how the geometry of these pores can be altered beyond triangular by changing beam conditions. Precisely size- and geometry-tuned nanopores could find application in molecular sensing, DNA sequencing, water desalination, and molecular separation.

Intrinsically patterned two-dimensional materials for selective adsorption of molecules andnanoclusters.

Two-dimensional (2D) materials have been studied extensively as monolayers, vertical or lateral heterostructures. To achieve functionalization, monolayers are often patterned using soft lithography and selectively decorated with molecules. Here we demonstrate the growth of a family of 2D materials that are intrinsically patterned. We demonstrate that a monolayer of PtSe2 can be grown on a Pt substrate in the form of a triangular pattern of alternating 1T and 1H phases. Moreover, we show that, in a monolayer of CuSe grown on a Cu substrate, strain relaxation leads to periodic patterns of triangular nanopores with uniform size. Adsorption of different species at preferred pattern sites is also achieved, demonstrating that these materials can serve as templates for selective self-assembly of molecules or nanoclusters, as well as for the functionalization of the same substrate with two different species.

Highly permeable and antifouling reverse osmosis membranes with acidified graphitic carbon nitride nanosheets as nanofillers

Ultrathin graphitic carbon nitride (gCN) nanosheets, due to their two-dimensional graphene-like structure, regularly distributed triangular nanopores and structural defects on the laminar network, have attracted great research attention in fabricating high-performance water selective membranes.

Electrochemically fabricated nanovolcano arrays for SERS applications

Nanovolcano arrays were fabricated by electrodepositing Ni/Cu alloys into a monolayer of self-assembled nanospheres and then electrochemically etching the deposited alloy film. The fabricated nanovolcano arrays feature highly ordered hexagonally arranged concave nanobowls decorated with triangular nanopores at their interstices. After coated with Ag, the nanovolcano arrays serve as high-performance substrates for surface enhancement Raman spectroscopy (SERS) measurements. The experimental study shows that the structural features of the nanovolcano arrays, including concave nanobowls, nanopores, and their long-range orders, all contribute to the observed strong SERS enhancement in a synergetic manner, which is further confirmed by the simulation results obtained using the finite-difference time-domain method. Copyright © 2012 John Wiley & Sons, Ltd.

Sub-100 nm Triangular Nanopores Fabricated with the Reactive Ion Etching Variant of Nanosphere Lithography and Angle-Resolved Nanosphere Lithography

Nanosphere lithography (NSL) is combined with reactive ion etching (RIE) to fabricate ordered arrays of in-plane, triangular cross-section nanopores. Nanopores with in-plane widths ranging from 44 to 404 nm and depths ranging from 25 to 250 nm are demonstrated. The combination of angle-resolved nanosphere lithography (AR NSL) and RIE yields an additional three-fold reduction in nanopore size.