Centimeter-Scale Nanoporous 2D Membranes and Ion Transport: Porous MoS2 Monolayers in a Few-Layer Matrix.

Two-dimensional nanoporous membranes have received attention as catalysts for energy generation and membranes for liquid and gas purification but controlling their porosity and facilitating large-scale production is challenging. We show the growth and fabrication of centimeter-scale molybdenum disulfide (MoS2) membranes with tunable porous areas up to ∼ 10% of the membrane and average nanopore diameters as large as ∼ 30 nm, controlled by the etch time. We also measure ionic conductance between 0.1 and 16 µS per µm2 through variably etched nanoporous membranes. Ensuring the mechanical robustness and large-area of the membrane, bilayer and few-layer regions form a strong supporting matrix around monolayer regions, observed by aberration-corrected scanning transmission electron microscopy. During etching, nanopores form in thin, primarily monolayer areas whereas thicker multilayer regions remain essentially intact. Atomic-resolution imaging reveals that after exposure to the etchant, the number of V1Mo vacancies increases and nanopores form along grain boundaries in monolayers, suggesting that etching starts at intrinsic defect sites. This work provides an avenue for the scalable production of nanoporous atomically thin membranes.

Angstrom-Size Defect Creation and Ionic Transport through Pores in Single-Layer MoS2.

Atomic-defect engineering in thin membranes provides opportunities for ionic and molecular filtration and analysis. While molecular-dynamics (MD) calculations have been used to model conductance through atomic vacancies, corresponding experiments are lacking. We create sub-nanometer vacancies in suspended single-layer molybdenum disulfide (MoS2) via Ga+ ion irradiation, producing membranes containing ∼300 to 1200 pores with average and maximum diameters of ∼0.5 and ∼1 nm, respectively. Vacancies exhibit missing Mo and S atoms, as shown by aberration-corrected scanning transmission electron microscopy (AC-STEM). The longitudinal acoustic band and defect-related photoluminescence were observed in Raman and photoluminescence spectroscopy, respectively. As the irradiation dose is increased, the median vacancy area remains roughly constant, while the number of vacancies (pores) increases. Ionic current versus voltage is nonlinear and conductance is comparable to that of ∼1 nm diameter single MoS2 pores, proving that the smaller pores in the distribution display negligible conductance. Consistently, MD simulations show that pores with diameters <0.6 nm are almost impermeable to ionic flow. Atomic pore structure and geometry, studied by AC-STEM, are critical in the sub-nanometer regime in which the pores are not circular and the diameter is not well-defined. This study lays the foundation for future experiments to probe transport in large distributions of angstrom-size pores.

Cross-Talk Between Ionic and Nanoribbon Current Signals in Graphene Nanoribbon-Nanopore Sensors for Single-Molecule Detection.

Nanopores are now being used not only as an ionic current sensor but also as a means to localize molecules near alternative sensors with higher sensitivity and/or selectivity. One example is a solid-state nanopore embedded in a graphene nanoribbon (GNR) transistor. Such a device possesses the high conductivity needed for higher bandwidth measurements and, because of its single-atomic-layer thickness, can improve the spatial resolution of the measurement. Here measurements of ionic current through the nanopore are shown during double-stranded DNA (dsDNA) translocation, along with the simultaneous response of the neighboring GNR due to changes in the surrounding electric potential. Cross-talk originating from capacitive coupling between the two measurement channels is observed, resulting in a transient response in the GNR during DNA translocation; however, a modulation in device conductivity is not observed via an electric-field-effect response during DNA translocation. A field-effect response would scale with GNR source-drain voltage (Vds), whereas the capacitive coupling does not scale with Vds . In order to take advantage of the high bandwidth potential of such sensors, the field-effect response must be enhanced. Potential field calculations are presented to outline a phase diagram for detection within the device parameter space, charting a roadmap for future optimization of such devices.

Gold nanorod translocations and charge measurement through solid-state nanopores.

We study translocations of gold nanoparticles and nanorods through silicon nitride nanopores and present a method for determining the surface charge of nanorods from the magnitude of the ionic current change as nanorods pass through the pore. Positively charged nanorods and spherical nanoparticles with average diameters 10 nm and average nanorod lengths between 44 and 65 nm were translocated through 40 nm thick nanopores with diameters between 19 and 27 nm in 1, 10, or 100 mM KCl solutions. Nanorod passage through the nanopores decreases ion current in larger diameter pores, as in the case of typical Coulter counters, but it increases ion current in smaller diameter nanopores, likely because of the interaction of the nanopore's and nanoparticle's double layers. The presented method predicts a surface charge of 26 mC/m(2) for 44 nm long gold nanorods and 18 mC/m(2) for 65 nm long gold nanorods and facilitates future studies of ligand coverage and surface charge effects in anisotropic particles.

Two-dimensional nanopores and nanoporous membranes for ion and molecule transport.

Two-dimensional (2D) materials from graphene to metal dichalcogenides and beyond, have interesting electrical, optical and mechanical properties. Recent advances in their growth, transfer and device physics have led to the shrinking of 2D material-based devices to the atomic scale and expanding their functionality. Single or multiple nanometer-scale holes, as small as single-atom vacancies, can be introduced in suspended atomically-thin 2D membranes, giving rise to nanopore and nanoporous devices, respectively. While engineering vacancies and holes is interesting for modulating optoelectronic properties, one equally fascinating research focus is ion and molecule transport through such pores in thin membranes. Here, we review the advancement of 2D nanopore science and technology for biomolecular detection and analysis, including DNA sequencing, and the largely parallel efforts towards development of 2D nanoporous membranes for ion selectivity and water desalination, both directions sharing similar fundamental principles.

Patterning Superconductivity in a Topological Insulator.

Topologically protected states in combination with superconductivity hold great promise for quantum computing applications, but the progress on electrical transport measurements in such systems has been impeded by the difficulty of fabricating devices with reliable electrical contacts. We find that superconductivity can be patterned directly into Bi2Se3 nanostructures by local doping with palladium. Superconducting regions are defined by depositing palladium on top of the nanostructures using electron beam lithography followed by in situ annealing. Electrical transport measurements at low temperatures show either partial or full superconducting transition, depending on the doping conditions. Structural characterization techniques indicate that palladium remains localized in the targeted areas, making it possible to pattern superconducting circuits of arbitrary shapes in this topological material.

Periodic Arrays of Phosphorene Nanopores as Antidot Lattices with Tunable Properties.

A tunable band gap in phosphorene extends its applicability in nanoelectronic and optoelectronic applications. Here, we propose to tune the band gap in phosphorene by patterning antidot lattices, which are periodic arrays of holes or nanopores etched in the material, and by exploiting quantum confinement in the corresponding nanoconstrictions. We fabricated antidot lattices with radii down to 13 nm in few-layer black phosphorus flakes protected by an oxide layer and observed suppression of the in-plane phonon modes relative to the unmodified material via Raman spectroscopy. In contrast to graphene antidots, the Raman peak positions in few-layer BP antidots are unchanged, in agreement with predicted power spectra. We also use DFT calculations to predict the electronic properties of phosphorene antidot lattices and observe a band gap scaling consistent with quantum confinement effects. Deviations are attributed primarily to self-passivating edge morphologies, where each phosphorus atom has the same number of bonds per atom as the pristine material so that no dopants can saturate dangling bonds. Quantum confinement is stronger for the zigzag edge nanoconstrictions between the holes as compared to those with armchair edges, resulting in a roughly bimodal band gap distribution. Interestingly, in two of the antidot structures an unreported self-passivating reconstruction of the zigzag edge endows the systems with a metallic component. The experimental demonstration of antidots and the theoretical results provide motivation to further scale down nanofabrication of antidots in the few-nanometer size regime, where quantum confinement is particularly important.

Transfer of monolayer TMD WS2 and Raman study of substrate effects.

A facile transfer process for transition metal dichalcogenide WS2 flakes is reported and the effect of the underlying substrate on the flake properties is investigated using Raman spectroscopy. The flakes are transferred from their growth substrate using polymethyl methacrylate (PMMA) and a wet etch to allow the user to transfer the flakes to a final substrate using a microscope and micromanipulator combined with semi-transparent Kapton tape. The substrates used range from insulators such as industry standard high-k dielectric HfO2 and "green polymer" parylene-C, to conducting chemical vapor deposition (CVD) grown graphene. Raman spectroscopy is used first to confirm the material quality of the transferred flakes to the substrates and subsequently to analyze and separate the effects arising from material transfer from those arising from interactions with the substrate. We observe changes in the Raman spectra associated with the interactions between the substrates in the flakes. These interactions affect both in-plane and out-of-plane modes in different ways depending on their sources, for example strain or surface charge. These changes vary with final substrate, with the strongest effects being observed for WS2 transferred onto graphene and HfO2, demonstrating the importance of understanding substrate interaction for fabrication of future devices.

Monolayer WS2 Nanopores for DNA Translocation with Light-Adjustable Sizes.

Two-dimensional materials are promising for a range of applications, as well as testbeds for probing the physics of low-dimensional systems. Tungsten disulfide (WS2) monolayers exhibit a direct band gap and strong photoluminescence (PL) in the visible range, opening possibilities for advanced optoelectronic applications. Here, we report the realization of two-dimensional nanometer-size pores in suspended monolayer WS2 membranes, allowing for electrical and optical response in ionic current measurements. A focused electron beam was used to fabricate nanopores in WS2 membranes suspended on silicon-based chips and characterized using PL spectroscopy and aberration-corrected high-resolution scanning transmission electron microscopy. It was observed that the PL intensity of suspended WS2 monolayers is ∼10-15 times stronger when compared to that of substrate-supported monolayers, and low-dose scanning transmission electron microscope viewing and drilling preserves the PL signal of WS2 around the pore. We establish that such nanopores allow ionic conductance and DNA translocations. We also demonstrate that under low-power laser illumination in solution, WS2 nanopores grow slowly in size at an effective rate of ∼0.2-0.4 nm/s, thus allowing for atomically controlled nanopore size using short light pulses.

Controlled Sculpture of Black Phosphorus Nanoribbons.

Black phosphorus (BP) is a highly anisotropic allotrope of phosphorus with great promise for fast functional electronics and optoelectronics. We demonstrate the controlled structural modification of few-layer BP along arbitrary crystal directions with sub-nanometer precision for the formation of few-nanometer-wide armchair and zigzag BP nanoribbons. Nanoribbons are fabricated, along with nanopores and nanogaps, using a combination of mechanical-liquid exfoliation and in situ transmission electron microscopy (TEM) and scanning TEM nanosculpting. We predict that the few-nanometer-wide BP nanoribbons realized experimentally possess clear one-dimensional quantum confinement, even when the systems are made up of a few layers. The demonstration of this procedure is key for the development of BP-based electronics, optoelectronics, thermoelectrics, and other applications in reduced dimensions.