Logic-in-memory based on an atomically thin semiconductor.

The growing importance of applications based on machine learning is driving the need to develop dedicated, energy-efficient electronic hardware. Compared with von Neumann architectures, which have separate processing and storage units, brain-inspired in-memory computing uses the same basic device structure for logic operations and data storage1-3, thus promising to reduce the energy cost of data-centred computing substantially4. Although there is ample research focused on exploring new device architectures, the engineering of material platforms suitable for such device designs remains a challenge. Two-dimensional materials5,6 such as semiconducting molybdenum disulphide, MoS2, could be promising candidates for such platforms thanks to their exceptional electrical and mechanical properties7-9. Here we report our exploration of large-area MoS2 as an active channel material for developing logic-in-memory devices and circuits based on floating-gate field-effect transistors (FGFETs). The conductance of our FGFETs can be precisely and continuously tuned, allowing us to use them as building blocks for reconfigurable logic circuits in which logic operations can be directly performed using the memory elements. After demonstrating a programmable NOR gate, we show that this design can be simply extended to implement more complex programmable logic and a functionally complete set of operations. Our findings highlight the potential of atomically thin semiconductors for the development of next-generation low-power electronics.

Liquid-activated quantum emission from pristine hexagonal boron nitride for nanofluidic sensing.

Liquids confined down to the atomic scale can show radically new properties. However, only indirect and ensemble measurements operate in such extreme confinement, calling for novel optical approaches that enable direct imaging at the molecular level. Here we harness fluorescence originating from single-photon emitters at the surface of hexagonal boron nitride for molecular imaging and sensing in nanometrically confined liquids. The emission originates from the chemisorption of organic solvent molecules onto native surface defects, revealing single-molecule dynamics at the interface through the spatially correlated activation of neighbouring defects. Emitter spectra further offer a direct readout of the local dielectric properties, unveiling increasing dielectric order under nanometre-scale confinement. Liquid-activated native hexagonal boron nitride defects bridge the gap between solid-state nanophotonics and nanofluidics, opening new avenues for nanoscale sensing and optofluidics.

Synthesis of Fluorescent Cyclic Peptides via Gold(I)-Catalyzed Macrocyclization.

Rapid and efficient cyclization methods that form structurally novel peptidic macrocycles are of high importance for medicinal chemistry. Herein, we report the first gold(I)-catalyzed macrocyclization of peptide-EBXs (ethynylbenziodoxolones) via C2-Trp C-H activation. This reaction was carried out in the presence of protecting group free peptide sequences and is enabled by a simple commercial gold catalyst (AuCl·Me2S). The method displayed a rapid reaction rate (within 10 min), wide functional group tolerance (27 unprotected peptides were cyclized), and up to 86% isolated yield. The obtained highly conjugated cyclic peptide linker, formed through C-H alkynylation, can be directly applied to live-cell imaging as a fluorescent probe without further attachment of fluorophores.

Stress induced delamination of suspended MoS2 in aqueous environments.

Applying hydrostatic pressure with suspended 2D material thin membranes allows probing the effects of lateral strain on the ion and fluid transport through nanopores. We demonstrate how both permanent and temporary delamination of 2D materials can be induced by pressure and potential differences between the membrane, causing a strong mechanosensitive modulation of ion transport. Our methodology is based on in situ measurements of ionic current and streaming modulation as the supporting membrane is indented or bulged with pressure. We demonstrate how indirect measurements of fluid transport through delaminated MoS2 membranes is achieved through monitoring streaming current and potential. This is combined with TEM images of 2D material membranes before and after aqueous measurements, showing temporary delamination during mechanical or electrical stress. The obtained results allow a better understanding of measurements with supported 2D materials, i.e. avoiding misinterpreting measured data, and could be used to probe how the electrical field and fluid flow at the nanoscale influence the adhesion of supported 2D materials.

Single-layer MoS2 nanopores as nanopower generators.

Making use of the osmotic pressure difference between fresh water and seawater is an attractive, renewable and clean way to generate power and is known as 'blue energy'. Another electrokinetic phenomenon, called the streaming potential, occurs when an electrolyte is driven through narrow pores either by a pressure gradient or by an osmotic potential resulting from a salt concentration gradient. For this task, membranes made of two-dimensional materials are expected to be the most efficient, because water transport through a membrane scales inversely with membrane thickness. Here we demonstrate the use of single-layer molybdenum disulfide (MoS2) nanopores as osmotic nanopower generators. We observe a large, osmotically induced current produced from a salt gradient with an estimated power density of up to 10(6) watts per square metre--a current that can be attributed mainly to the atomically thin membrane of MoS2. Low power requirements for nanoelectronic and optoelectric devices can be provided by a neighbouring nanogenerator that harvests energy from the local environment--for example, a piezoelectric zinc oxide nanowire array or single-layer MoS2 (ref. 12). We use our MoS2 nanopore generator to power a MoS2 transistor, thus demonstrating a self-powered nanosystem.

Rhesus Blood Typing within a Few Seconds by Packing-Enhanced Nanoscattering on Individual Erythrocytes.

A method for the ABO and Rhesus (Rh) blood group typing from individual erythrocytes is proposed in this study. Blood-group-specific antibodies immobilized to gold nanoparticles (BG-AuNP) were utilized for the identification of blood groups from individual erythrocytes by objective-type dark-field microscopy (OTDFM). The scattering of free BG-AuNP and their Brownian motion as well as BG-AuNP attached on erythrocytes is easily observed by OTDFM. The strong scattering intensity caused by BG-AuNP packing-enhanced nanoscattering (PENS) on erythrocytes is first demonstrated. PENS combined with OTDFM allows us to identify blood groups within 5 s for all blood group antigens including A, B, D, C, c, E, and e. This was immediately identified by mixing with BG-AuNP without any washing step or waiting for hemoagglutination. Therefore, PENS combined with OTDFM demonstrates feasibility and advantages for use in emergency transfusions where the blood group of patients is unknown. Moreover, matching RhD+ in the case of emergency transfusions may also be beneficial in reducing the shortage of RhD- red blood cell concentrate in the case of a population with a high frequency in RhD-.

From Water Solutions to Ionic Liquids with Solid State Nanopores as a Perspective to Study Transport and Translocation Phenomena.

Solid state nanopores are single-molecular devices governed by nanoscale physics with a broad potential for technological applications. However, the control of translocation speed in these systems is still limited. Ionic liquids are molten salts which are commonly used as alternate solvents enabling the regulation of the chemical and physical interactions on solid-liquid interfaces. While their combination can be challenging to the understanding of nanoscopic processes, there has been limited attempts on bringing these two together. While summarizing the state of the art and open questions in these fields, several major advances are presented with a perspective on the next steps in the investigations of ionic-liquid filled nanopores, both from a theoretical and experimental standpoint. By analogy to aqueous solutions, it is argued that ionic liquids and nanopores can be combined to provide new nanofluidic functionalities, as well as to help resolve some of the pertinent problems in understanding transport phenomena in confined ionic liquids and providing better control of the speed of translocating analytes.

Wetting of nanopores probed with pressure.

Nanopores are both a tool to study single-molecule biophysics and nanoscale ion transport, but also a promising material for desalination or osmotic power generation. Understanding the physics underlying ion transport through nano-sized pores allows better design of porous membrane materials. Material surfaces can present hydrophobicity, a property which can make them prone to formation of surface nanobubbles. Nanobubbles can influence the electrical transport properties of such devices. We demonstrate an approach which uses hydraulic pressure to probe the electrical transport properties of solid state nanopores. We show how pressure can be used to wet pores, and how it allows control over bubbles or other contaminants in the nanometer scale range normally unachievable using only an electrical driving force. Molybdenum disulfide is then used as a typical example of a 2D material on which we demonstrate wetting and bubble induced nonlinear and linear conductance in the regimes typically used with these experiments. We show that by using pressure one can identify and evade wetting artifacts.

Pressure-Induced Enlargement and Ionic Current Rectification in Symmetric Nanopores.

Nanopores in solid state membranes are a tool able to probe nanofluidic phenomena or can act as a single molecular sensor. They also have diverse applications in filtration, desalination, or osmotic power generation. Many of these applications involve chemical, or hydrostatic pressure differences which act on both the supporting membrane, and the ion transport through the pore. By using pressure differences between the sides of the membrane and an alternating current approach to probe ion transport, we investigate two distinct physical phenomena: the elastic deformation of the membrane through the measurement of strain at the nanopore, and the growth of ionic current rectification with pressure due to pore entrance effects. These measurements are a significant step toward the understanding of the role of elastic membrane deformation or fluid flow on linear and nonlinear transport properties of nanopores.

Transverse Detection of DNA Using a MoS2 Nanopore.

Classical nanopore sensing relies on the measurement of the ion current passing through a nanopore. Whenever a molecule electrophoretically translocates through the narrow constriction, it modulates the ion current. Although this approach allows one to measure single molecules, the access resistance limits the spatial resolution. This physical limitation could potentially be overcome by an alternative sensing scheme taking advantage of the current across the membrane material itself. Such an electronic readout would also allow better temporal resolution than the ionic current. In this work, we present the fabrication of an electrically contacted molybdenum disulfide (MoS2) nanoribbon integrated with a nanopore. DNA molecules are sensed by correlated signals from the ionic current through the nanopore and the transverse current through the nanoribbon. The resulting signal suggests a field-effect sensing scheme where the charge of the molecule is directly sensed by the nanoribbon. We discuss different sensing schemes such as local potential sensing and direct charge sensing. Furthermore, we show that the fabrication of freestanding MoS2 ribbons with metal contacts is reliable and discuss the challenges that arise in the fabrication and usage of these devices.

Spatiotemporal Imaging of Water in Operating Voltage-Gated Ion Channels Reveals the Slow Motion of Interfacial Ions.

Ion channels are responsible for numerous physiological functions ranging from transport to chemical and electrical signaling. Although static ion channel structure has been studied following a structural biology approach, spatiotemporal investigation of the dynamic molecular mechanisms of operational ion channels has not been achieved experimentally. In particular, the role of water remains elusive. Here, we perform label-free spatiotemporal second harmonic (SH) imaging and capacitance measurements of operational voltage-gated alamethicin ion channels in freestanding lipid membranes surrounded by aqueous solution on either side. We observe changes in SH intensity upon channel activation that are traced back to changes in the orientational distribution of water molecules that reorient along the field lines of transported ions. Of the transported ions, a fraction of 10-4 arrives at the hydrated membrane interface, leading to interfacial electrostatic changes on the time scale of a second. The time scale of these interfacial changes is influenced by the density of ion channels and is subject to a crowding mechanism. Ion transport along cell membranes is often associated with the propagation of electrical signals in neurons. As our study shows that this process is taking place over seconds, a more complex mechanism is likely responsible for the propagation of neuronal electrical signals than just the millisecond movement of ions.

Wide-Field Spectral Super-Resolution Mapping of Optically Active Defects in Hexagonal Boron Nitride.

Point defects can have significant impact on the mechanical, electronic, and optical properties of materials. The development of robust, multidimensional, high-throughput, and large-scale characterization techniques of defects is thus crucial for the establishment of integrated nanophotonic technologies and material growth optimization. Here, we demonstrate the potential of wide-field spectral single-molecule localization microscopy (SMLM) for the determination of ensemble spectral properties as well as the characterization of spatial, spectral, and temporal dynamics of single defects in chemical vapor deposition (CVD)-grown and irradiated exfoliated hexagonal boron-nitride materials. We characterize the heterogeneous spectral response of our samples and identify at least two types of defects in CVD-grown materials, while irradiated exfoliated flakes show predominantly only one type of defects. We analyze the blinking kinetics and spectral emission for each type of defects and discuss their implications with respect to the observed spectral heterogeneity of our samples. Our study shows the potential of wide-field spectral SMLM techniques in material science and paves the way toward the quantitative multidimensional mapping of defect properties.

Facile Production of Hexagonal Boron Nitride Nanoparticles by Cryogenic Exfoliation.

Fluorescent nanoparticles with optically robust luminescence are imperative to applications in imaging and labeling. Here we demonstrate that hexagonal boron nitride (hBN) nanoparticles can be reliably produced using a scalable cryogenic exfoliation technique with sizes below 10 nm. The particles exhibit bright fluorescence generated by color centers that act as atomic-size quantum emitters. We analyze their optical properties, including emission wavelength, photon-statistics, and photodynamics, and show that they are suitable for far-field super-resolution fluorescence nanoscopy. Our results provide a foundation for exploration of hBN nanoparticles as candidates for bioimaging, labeling, as well as biomarkers that are suitable for quantum sensing.

Prospects of Observing Ionic Coulomb Blockade in Artificial Ion Confinements.

Nanofluidics encompasses a wide range of advanced approaches to study charge and mass transport at the nanoscale. Modern technologies allow us to develop and improve artificial nanofluidic platforms that confine ions in a way similar to single-ion channels in living cells. Therefore, nanofluidic platforms show great potential to act as a test field for theoretical models. This review aims to highlight ionic Coulomb blockade (ICB)-an effect that is proposed to be the key player of ion channel selectivity, which is based upon electrostatic exclusion limiting ion transport. Thus, in this perspective, we focus on the most promising approaches that have been reported on the subject. We consider ion confinements of various dimensionalities and highlight the most recent advancements in the field. Furthermore, we concentrate on the most critical obstacles associated with these studies and suggest possible solutions to advance the field further.

Orthogonal Tip-to-Tip Nanocapillary Alignment Allows for Easy Detection of Fluorescent Emitters in Femtomolar Concentrations.

Here we present the realization of a novel fluorescence detection method based on the electromigration of fluorescent molecules within a nanocapillary combined with the laser excitation through a platinum (Pt)-coated nanocapillary. By using the Pt nanocapillary assisted focusing of a laser beam, we completely remove the background scattering on the tip of the electrophoretic nanocapillary. In this excitation geometry, we demonstrate a 1000-fold sensitivity enhancement (1.0 nM to 1.0 pM) compared to the detection in microcapillaries with epifluorescence illumination and fluorescence spectrophotometry. Due to a significant electroosmotic flow, we observe a decelerating migration of DNA molecules close to the tip of the electrophoretic nanocapillary. The reduced DNA translocation velocity causes a two-step stacking process of molecules in the tip of the nanocapillary and can be used as a way to locally concentrate molecules. The sensitivity of our method is further improved by a continuous electrokinetic injection of DNA molecules followed by sample zone stacking on the tip of the nanocapillary. Concentrations ranging from 0.1 pM to 1.0 fM can be directly observed on the orifice of the electrophoretic nanocapillary. This is a 1000-fold improvement compared to traditional capillary electrophoresis with laser-induced fluorescence.

Imaging of Optically Active Defects with Nanometer Resolution.

Point defects significantly influence the optical and electrical properties of solid-state materials due to their interactions with charge carriers, which reduce the band-to-band optical transition energy. There has been a demand for developing direct optical imaging methods that would allow in situ characterization of individual defects with nanometer resolution. Here, we demonstrate the localization and quantitative counting of individual optically active defects in monolayer hexagonal boron nitride using single molecule localization microscopy. By exploiting the blinking behavior of defect emitters to temporally isolate multiple emitters within one diffraction limited region, we could resolve two defect emitters with a point-to-point distance down to ten nanometers. The results and conclusion presented in this work add unprecedented dimensions toward future applications of defects in quantum information processing and biological imaging.

Centimeter-Sized Single-Orientation Monolayer Hexagonal Boron Nitride With or Without Nanovoids.

Large-area hexagonal boron nitride (h-BN) promises many new applications of two-dimensional materials, such as the protective packing of reactive surfaces or as membranes in liquids. However, scalable production beyond exfoliation from bulk single crystals remained a major challenge. Single-orientation monolayer h-BN nanomesh is grown on 4 in. wafer single crystalline rhodium films and transferred on arbitrary substrates such as SiO2, germanium, or transmission electron microscopy grids. The transfer process involves application of tetraoctylammonium bromide before electrochemical hydrogen delamination. The material performance is demonstrated with two applications. First, protective sealing of h-BN is shown by preserving germanium from oxidation in air at high temperatures. Second, the membrane functionality of the single h-BN layer is demonstrated in aqueous solutions. Here, we employ a growth substrate intrinsic preparation scheme to create regular 2 nm holes that serve as ion channels in liquids.

Fluorescent Nanodiamonds as Versatile Intracellular Temperature Sensors.

Temperature is a widely known phenomenon, which plays an extremely important role in biological systems. Its behavior on the macro-scale has been quite well investigated and understood, thanks to the availability of reliable and precise thermometers such as thermocouples and infrared cameras. However, temperature measurements on the subcellular scale present an ongoing challenge due to the absence of universal nanoscale temperature sensors. Recent work on fluorescent nanodiamonds has revealed their unique ability to measure temperature with high spatial and temporal resolution, of particular importance in the intracellular environment. This review summarizes recent progress in the field and highlights the future directions for intracellular temperature sensing using fluorescent nanodiamonds.

Geometrical Effect in 2D Nanopores.

A long-standing problem in the application of solid-state nanopores is the lack of the precise control over the geometry of artificially formed pores compared to the well-defined geometry in their biological counterpart, that is, protein nanopores. To date, experimentally investigated solid-state nanopores have been shown to adopt an approximately circular shape. In this Letter, we investigate the geometrical effect of the nanopore shape on ionic blockage induced by DNA translocation using triangular h-BN nanopores and approximately circular molybdenum disulfide (MoS2) nanopores. We observe a striking geometry-dependent ion scattering effect, which is further corroborated by a modified ionic blockage model. The well-acknowledged ionic blockage model is derived from uniform ion permeability through the 2D nanopore plane and hemisphere like access region in the nanopore vicinity. On the basis of our experimental results, we propose a modified ionic blockage model, which is highly related to the ionic profile caused by geometrical variations. Our findings shed light on the rational design of 2D nanopores and should be applicable to arbitrary nanopore shapes.

Investigating Focal Adhesion Substructures by Localization Microscopy.

Cells rely on focal adhesions (FAs) to carry out a variety of important tasks, including motion, environmental sensing, and adhesion to the extracellular matrix. Although attaining a fundamental characterization of FAs is a compelling goal, their extensive complexity and small size, which can be below the diffraction limit, have hindered a full understanding. In this study we have used single-molecule localization microscopy (SMLM) to investigate integrin ß3 and paxillin in rat embryonic fibroblasts growing on two different extracellular matrix-representing substrates (i.e., fibronectin-coated substrates and specifically biofunctionalized nanopatterned substrates). To quantify the substructure of FAs, we developed a clustering method based on expectation maximization of a Gaussian mixture that accounts for localization uncertainty and background. Analysis of our SMLM data indicates that the structures within FAs, characterized as a Gaussian mixture, typically have areas between 0.01 and 1 µm2, contain 10-100 localizations, and can exhibit substantial eccentricity. Our approach based on SMLM opens new avenues for studying structural and functional biology of molecular assemblies that display substantial varieties in size, shape, and density.