Solvent-Induced Degradation of Electrochemically Exfoliated Vanadium Selenide Visualized by Electron Microscopy.

Recently discovered two-dimensional ferromagnetic materials (2DFMs) have rapidly gained much interest in the fields of spintronics and computing, where they may prove powerful tools for miniaturizing devices such as magnetic tunnel junctions and spin-transfer torque memory bits. In addition, heterojunctions and twisted bilayer stacks of such materials may yield exotic spin textures. However, preparation of such devices is complicated by the air sensitivity of many 2DFMs. Here, we report details on the preparation of few-to-monolayer flakes of vanadium selenide (VSe2) using electrochemical exfoliation in propylene carbonate. We also present a detailed study of the effects of air on the structure and magnetic properties of bare and passivated VSe2 after different concentrations of surface passivation treatment. We characterized the microstructure of holes in the VSe2 flakes and the formation of new compounds arising from air exposure, solvent exposure during the exfoliating process, and deliberate electron beam irradiation (sculpting). We sculpt VSe2 flakes while retaining the 1T-VSe2 lattice structure, opening the door for top-down patterned high-resolution 2DFM nanostructures. Additionally, investigation of the magnetic response of nanosheets using magnetic force microscopy (MFM) showed that the oxidation-induced damage only affects the surface fields locally and does not quench large-scale magnetic signal. The findings of this study pave the way toward practical incorporation of 2D ferromagnetic materials in nanoelectronics.

Silicon Nitride Nanopores Formed by Simple Chemical Etching: DNA Translocations and TEM Imaging.

We demonstrate DNA translocations through silicon nitride pores formed by simple chemical etching on glass substrates using microscopic amounts of hydrofluoric acid. DNA translocations and transmission electron microscopy (TEM) prove the fabrication of nanopores and allow their characterization. From ionic measurements on 318 chips, we report the effective pore diameters ranging from zero (pristine membranes) and sub-nm to over 100 nm, within 50 µm diameter membranes. The combination of ionic conductance, DNA current blockades, TEM imaging, and electron energy loss spectroscopy (EELS) provides comprehensive information about the pore area and number, from single to few pores, and pore structure. We also show the formation of thinned membrane regions as precursors of pores. The average pore density, about 5 × 10-4 pores/µm2, allows pore number adjustment statistically (0, 1, or more). This simple and affordable chemical method for making solid-state nanopores accelerates their adoption for DNA sensing and characterization applications.

Ultrafast Polymer Dynamics through a Nanopore.

Ultrathin nanopore sensors allow single-molecule and polymer measurements at sub-microsecond time resolution enabled by high current signals (∼10-30 nA). We demonstrate for the first time the experimental probing of the ultrafast translocation and folded dynamics of double-stranded DNA (dsDNA) through a nanopore at 10 MHz bandwidth with acquisition of data points per 25 ns (150 MB/s). By introducing a rigorous algorithm, we are able to accurately identify each current level present within translocation events and elucidate the dynamic folded and unfolded behaviors. The remarkable sensitivity of this system reveals distortions of short-lived folded states at a lower bandwidth. This work revisits probing of dsDNA as a model polymer and develops broadly applicable methods. The combined improvements in sensor signals, instrumentation, and large data analysis methods uncover biomolecular dynamics at unprecedentedly small time scales.

Deoxyribonucleic Acid Extraction from Mars Analog Soils and Their Characterization with Solid-State Nanopores.

Life detection on Mars is an important topic that includes a direct search for biomarkers. This requires instruments for in situ biomarker detection that are compact, lightweight, and able to withstand operations in space. Solid-state nanopores are excellent candidates that allow fast single-molecule detection. They can withstand high temperatures and be sterilized to minimize planetary contamination. The instruments are portable with low-power requirements. We demonstrate a few key results in advancing the use of nanopores for in-space applications. First, we developed modified deoxyribonucleic acid (DNA) extraction protocols to extract DNA from Mars analog soils. Second, we used silicon nitride nanopores to demonstrate the detection of extracted DNA and corresponding current characteristics. The yields and properties of extracted DNA (e.g., estimated diameters) varied somewhat by soil types, extraction methods, and nanopores used. The yields varied from a minimum of 0.9 ng DNA/g soil for a magnesium carbonate sample from Lake Salda to a maximum of 210 ng DNA/g soil for a calcium carbonate sample from Trona Pinnacles. For a given soil type, yields from different methods varied by a factor of up to 50. These observations motivate future studies with a broader range of Mars-like soils and improved instruments to increase signal-to-noise-ratio at higher measurement bandwidths.

Protein-enabled detection of ibuprofen and sulfamethoxazole using solid-state nanopores.

Enabled by proteins, we present an all-electrical method for rapid detection of small pharmaceuticals (ibuprofen and sulfamethoxazole [SMZ]) in aqueous media using silicon nitride pores. Specifically, we use carrier proteins, bovine serum albumin (BSA), and take advantage of their interactions with two small drug molecules to form BSA-drug complexes which can be detected by nm-diameter pores, thereby confirming the presence of small pharmaceuticals. We demonstrate detection of ibuprofen and SMZ at concentrations down to 100 nM (∼21 µg/L) and 48.5 nM (12 µg/L), respectively. We observe changes in electrical signal characteristics (reflected in event durations, rates, current magnitudes, and estimated particle diameters) of BSA-drug complexes compared to BSA-only, and differences between these two small pharmaceuticals, possibly paving a path toward developing selective sensors by identifying "electrical fingerprints" of these molecules in the future. These distinct electrical signals are likely a combined result of diffusion, electrophoretic and electroosmotic effects, interactions between the pore and particles, which depend on pore diameters, pH, and the resulting surface charges. The use of single-molecule-counting nanopores allows sensing of small pharmaceuticals, studies of protein conformational changes, and may aid in efforts to evaluate the impact of small drug molecules on aquatic and human life.

Mixed-Dimensional 1D/2D van der Waals Heterojunction Diodes and Transistors in the Atomic Limit.

Inverting a semiconducting channel is the basis of all field-effect transistors. In silicon-based metal-oxide-semiconductor field-effect transistors (MOSFETs), a gate dielectric mediates this inversion. Access to inversion layers may be granted by interfacing ultrathin low-dimensional semiconductors in heterojunctions to advance device downscaling. Here we demonstrate that monolayer molybdenum disulfide (MoS2) can directly invert a single-walled semiconducting carbon nanotube (SWCNT) transistor channel without the need for a gate dielectric. We fabricate and study this atomically thin one-dimensional/two-dimensional (1D/2D) van der Waals heterojunction and employ it as the gate of a 1D heterojunction field-effect transistor (1D-HFET) channel. Gate control is based on modulating the conductance through the channel by forming a lateral p-n junction within the CNT itself. In addition, we observe a region of operation exhibiting a negative static resistance after significant gate tunneling current passes through the junction. Technology computer-aided design (TCAD) simulations confirm the role of minority carrier drift-diffusion in enabling this behavior. The resulting van der Waals transistor architecture thus has the dual characteristics of both field-effect and tunneling transistors, and it advances the downscaling of heterostructures beyond the limits of dangling bonds and epitaxial constraints faced by III-V semiconductors.

Devices for Nanoscale Guiding of DNA through a 2D Nanopore.

We fabricate on-chip solid-state nanofluidic-2D nanopore systems that can limit the range of motion for DNA in the sensing region of a nanopore. We do so by creating devices containing one or more silicon nitride pores and silicon nitride pillars supporting a 2D pore that orient DNA within a nanopore device to a restricted geometry, yet allow the free motion of ions to maintain a high signal-to-noise ratio. We discuss two concepts with two and three independent electrical connections and corresponding nanopore chip device architectures to achieve this goal in practice. Here, we describe device fabrication and transmission electron microscope (TEM) images, and provide simulated translocations based on the finite element analysis in 3D to demonstrate its merit. In both methods, there is a main 2D nanopore which we refer to as a "sensing" nanopore (monolayer MoS2 in this paper). A secondary layer is either an array of guiding pores sharing the same electrode pair as the sensing pore (Method 1) or a single, independently contacted, guiding pore (Method 2). These pores are constructed parallel to the "sensing" pore and serve as "guiding" elements to stretch and feed DNA into the atomically thin sensing pore. We discuss the practical implementation of these concepts with nanofluidic and Si-based technology, including detailed fabrication steps and challenges involved for DNA applications in solution.

Computer vision AC-STEM automated image analysis for 2D nanopore applications.

Transmission electron microscopy (TEM) has led to important discoveries in atomic imaging and as an atom-by-atom fabrication tool. Using electron beams, atomic structures can be patterned, annealed and crystallized, and nanopores can be drilled in thin membranes. We review current progress in TEM analysis and implement a computer vision nanopore-detection algorithm that achieves a 96% pixelwise precision in TEM images of nanopores in 2D membranes (WS2), and discuss parameter optimization including a variation on the traditional grid search and gradient ascent. Such nanopores have applications in ion detection, water filtration, and DNA sequencing, where ionic conductance through the pore should be concordant with its TEM-measured size. Standard computer vision methods have their advantages as they are intuitive and do not require extensive training data. For completeness, we briefly comment on related machine learning for 2D materials analysis and discuss relevant progress in these fields. Image analysis alongside TEM allows correlated fabrication and analysis done simultaneously in situ to engineer devices at the atomic scale.

Engineering adjustable two-pore devices for parallel ion transport and DNA translocations.

We report ionic current and double-stranded DNA (dsDNA) translocation measurements through solid-state membranes with two TEM-drilled ∼3-nm diameter silicon nitride nanopores in parallel. Nanopores are fabricated with similar diameters but varying in effective thicknesses (from 2.6 to 10 nm) ranging from a thickness ratio of 1:1 to 1:3.75, producing distinct conductance levels. This was made possible by locally thinning the silicon nitride membrane to shape the desired topography with nanoscale precision using electron beam lithography (EBL). Two nanopores are engineered and subsequently drilled in either the EBL-thinned or the surrounding membrane region. By designing the interpore separation a few orders of magnitude larger than the pore diameter (e.g., ∼900 vs 3 nm), we show analytically, numerically, and experimentally that the total conductance of the two pores is the sum of the individual pore conductances. For a two-pore device with similar diameters yet thicknesses in the ratio of 1:3, a ratio of ∼1:2.2 in open-pore conductances and translocation current signals is expected, as if they were measured independently. Introducing dsDNA as analytes to both pores simultaneously, we detect more than 12 000 events within 2 min and trace them back with a high likelihood to which pore the dsDNA translocated through. Moreover, we monitor translocations through one active pore only when the other pore is clogged. This work demonstrates how two-pore devices can fundamentally open up a parallel translocation reading system for solid-state nanopores. This approach could be creatively generalized to more pores with desired parameters given a sufficient signal-to-noise ratio.

Rapid Growth of Monolayer MoSe2 Films for Large-Area Electronics.

The large-scale growth of semiconducting thin films on insulating substrates enables batch fabrication of atomically thin electronic and optoelectronic devices and circuits without film transfer. Here an efficient method to achieve rapid growth of large-area monolayer MoSe2 films based on spin coating of Mo precursor and assisted by NaCl is reported. Uniform monolayer MoSe2 films up to a few inches in size are obtained within a short growth time of 5 min. The as-grown monolayer MoSe2 films are of high quality with large grain size (up to 120 µm). Arrays of field-effect transistors are fabricated from the MoSe2 films through a photolithographic process; the devices exhibit high carrier mobility of ≈27.6 cm2 V-1 s-1 and on/off ratios of ≈105. The findings provide insight into the batch production of uniform thin transition metal dichalcogenide films and promote their large-scale applications.

Gas flow through atomic-scale apertures.

Gas flows are often analyzed with the theoretical descriptions formulated over a century ago and constantly challenged by the emerging architectures of narrow channels, slits, and apertures. Here, we report atomic-scale defects in two-dimensional (2D) materials as apertures for gas flows at the ultimate quasi-0D atomic limit. We establish that pristine monolayer tungsten disulfide (WS2) membranes act as atomically thin barriers to gas transport. Atomic vacancies from missing tungsten (W) sites are made in freestanding (WS2) monolayers by focused ion beam irradiation and characterized using aberration-corrected transmission electron microscopy. WS2 monolayers with atomic apertures are mechanically sturdy and showed fast helium flow. We propose a simple yet robust method for confirming the formation of atomic apertures over large areas using gas flows, an essential step for pursuing their prospective applications in various domains including molecular separation, single quantum emitters, sensing and monitoring of gases at ultralow concentrations.

Stochastic Ionic Transport in Single Atomic Zero-Dimensional Pores.

We report on single atomic zero-dimensional (0D) pores fabricated using aberration-corrected scanning transmission electron microscopy (AC-STEM) in monolayer MoS2. Pores are comprised of a few atoms missing in the two-dimensional (2D) lattice (1-5 Mo atoms) of characteristic sizes from ∼0.5 to 1.2 nm, and pore edges directly probed by AC-STEM to map the atomic structure. We categorize them into ∼30 geometrically possible zigzag, armchair, and mixed configurations. While theoretical studies predict that transport properties of 2D pores in this size range depend strongly on pore size and their atomic configuration, 0D pores show an average conductance in the range from ∼0.6-1 nS (bias up to 0.1 V), similar to biological pores. In some devices, the current was immeasurably small and/or pores could not be wet. Furthermore, current-voltage (I-V) characteristics are largely independent of bulk molarity (10 mM to 3 M KCl) and the type of cation (K+, Li+, Mg2+). This work lays the experimental foundation for understanding of the confinement effects possible in atomic-scale 2D material pores and the realization of solid-state analogues of ion channels in biology.

Spatial defects nanoengineering for bipolar conductivity in MoS2.

Understanding the atomistic origin of defects in two-dimensional transition metal dichalcogenides, their impact on the electronic properties, and how to control them is critical for future electronics and optoelectronics. Here, we demonstrate the integration of thermochemical scanning probe lithography (tc-SPL) with a flow-through reactive gas cell to achieve nanoscale control of defects in monolayer MoS2. The tc-SPL produced defects can present either p- or n-type doping on demand, depending on the used gasses, allowing the realization of field effect transistors, and p-n junctions with precise sub-µm spatial control, and a rectification ratio of over 104. Doping and defects formation are elucidated by means of X-Ray photoelectron spectroscopy, scanning transmission electron microscopy, and density functional theory. We find that p-type doping in HCl/H2O atmosphere is related to the rearrangement of sulfur atoms, and the formation of protruding covalent S-S bonds on the surface. Alternatively, local heating MoS2 in N2 produces n-character.

In Situ 2D MoS2 Field-Effect Transistors with an Electron Beam Gate.

We use the beam of a transmission electron microscope (TEM) to modulate in situ the current-voltage characteristics of a two-terminal monolayer molybdenum disulfide (MoS2) channel fabricated on a silicon nitride substrate. Suppression of the two-dimensional (2D) MoS2 channel conductance up to 94% is observed when the beam hits and charges the substrate surface. Gate-tunable transistor characteristics dependent on beam current are observed even when the beam is up to tens of microns away from the channel. In contrast, conductance remains constant when the beam passes through a micron-sized hole in the substrate. There is no MoS2 structural damage during gating, and the conductance reverts to its original value when the beam is turned off. We observe on/off ratios up to ∼60 that are largely independent of beam size and channel length. This TEM field-effect transistor architecture with electron beam gating provides a platform for future in situ electrical measurements.

Lifetime and Stability of Silicon Nitride Nanopores and Nanopore Arrays for Ionic Measurements.

Nanopores are promising for many applications including DNA sequencing and molecular filtration. Solid-state nanopores are preferable over their biological counterparts for applications requiring durability and operation under a wider range of external parameters, yet few studies have focused on optimizing their robustness. We report the lifetime and durability of pores and porous arrays in 10 to 100 nm-thick, low-stress silicon nitride (SiNx) membranes. Pores are fabricated using a transmission electron microscope (TEM) and/or electron beam lithography (EBL) and reactive ion etching (RIE), with diameters from 2 to 80 nm. We store them in various electrolyte solutions (KCl, LiCl, MgCl2) and record open pore conductance over months to quantify pore stability. Pore diameters increase with time, and diameter etch rate increases with electrolyte concentration from Δd/Δt ∼ 0.2 to ∼ 3 nm/day for 0.01 to 3 M KCl, respectively. TEM confirms the range of diameter etch rates from ionic measurements. Using electron energy loss spectroscopy (EELS), we observe a N-deficient region around the edges of TEM-drilled pores. Pore expansion is caused by etching of the Si/SiO2 pore walls, which resembles the dissolution of silicon found in minerals such as silica (SiO2) in salty ocean water. The etching process occurs where the membrane was exposed to the electron beam and can result in pore formation. However, coating pores with a conformal 1 nm-thick hafnium oxide layer prevents expansion in 1 M KCl, in stark contrast to bare SiNx pores (∼ 1.7 nm/day). EELS data reveal the atomic composition of bare and HfO2-coated pores.

Detection of single analyte and environmental samples with silicon nitride nanopores: Antarctic dirt particulates and DNA in artificial seawater.

Nanopore sensing is a powerful tool for the detection of biomolecules. Solid-state nanopores act as single-molecule sensors that can function in harsh conditions. Their resilient nature makes them attractive candidates for taking this technology into the field to measure environmental samples for life detection in space and water quality monitoring. Here, we discuss the fabrication of silicon nitride pores from ∼1.6 to 20 nm in diameter in 20-nm-thick silicon nitride membranes suspended on glass chips and their performance. We detect pure laboratory samples containing a single analyte including DNA, BSA, microRNA, TAT, and poly-D-lys-hydrobromide. We also measured an environmental (mixed-analyte) sample, containing Antarctic dirt provided by NASA Ames. For DNA measurements, in addition to using KCl and NaCl solutions, we used the artificial (synthetic) seawater, which is a mixture of different salts mimicking the composition of natural seawater. These samples were spiked with double-stranded DNA (dsDNA) fragments at different concentrations to establish the limits of nanopore sensitivity in candidate environment conditions. Nanopore chips were cleaned and reused for successive measurements. A stand-alone, 1-MHz-bandwidth Chimera amplifier was used to determine the DNA concentration in artificial seawater that we can detect in a practical time scale of a few minutes. We also designed and developed a new compact nanopore reader, a portable read-out device with miniaturized fluidic cells, which can obtain translocation data at bandwidths up to 100 kHz. Using this new instrument, we record translocations of 400 bp, 1000 bp, and 15000 bp dsDNA fragments and show discrimination by analysis of current amplitude and event duration histograms.

Ions and Water Dancing through Atom-Scale Holes: A Perspective toward "Size Zero".

We provide an overview of atom-scale apertures in solid-state membranes, from "pores" and "tubes" to "channels", with characteristic sizes comparable to the sizes of ions and water molecules. In this regime of ∼1 nm diameter pores, water molecules and ions are strongly geometrically confined: the size of water molecules (∼0.3 nm) and the size of "hydrated" ions in water (∼0.7-1 nm) are similar to the pore diameters, physically limiting the ion flow through the hole. The pore sizes are comparable to the classical Debye screening length governing the spatial range of electrostatic interaction, ∼0.3 to 1 nm for 1 to 0.1 M KCl. In such small structures, charges can be unscreened, leading to new effects. We discuss experiments on ∼1 nm diameter nanopores, with a focus on carbon nanotube pores and ion transport studies. Finally, we present an outlook for artificial "size zero" pores in the regime of small diameters and small thicknesses. Beyond mimicking protein channels in nature, solid-state pores may offer additional possibilities where sensing and control are performed at the pore, such as in electrically and optically addressable solid-state materials.