Nanoscale Tip Research Articles

Atomic-crystal transition metal dichalcogenides Schottky triboelectricity nanogenerator with ultrahigh direct-current density

Direct-current triboelectric nanogenerators (DC-TENGs) have recently become more attractive to convert mechanical energy into electricity due to their high current density with no need for rectification. Interfacial charge transfer, induced by the sliding contact on semiconductor materials, is critical to generate DC output but usually limited by the interfacial properties. Here, we report Schottky DC-TENGs based on the atomic-crystal transition-metal dichalcogenides (TMDs) with single crystallinity, monolayer thickness and atomic flatness to enhance the interfacial charge transfer. A record-high current density of 1010 A/m2, two orders of magnitude higher than the state-of-the-art performance, can be directly generated by sliding a conductive-atomic force microscope tip on an atomic-crystal molybdenum disulfide. Density functional theory calculation and finite element simulation reveal that this ultrahigh current density can be attributed to the enhanced interfacial property owing to the atomic flatness of TMDs and strong local electrical field of nanoscale tip. We further demonstrate their excellent scalability by a high-crystalline monolayer film with sliding electrode. This work may guide and accelerate the development and application of high-performance DC-TENGs.

All-carbon heterostructures self-assembly during field electron emission from diamond nanotip

In this study we demonstrate formation of all-carbon heterostructures induced by field electron emission from diamond needle-shaped crystallites with nanoscale tips. We show that at certain experimental conditions a carbon nanoprotrusion can be formed at the apex of a diamond emitter. Staircase-like current-voltage curves observed for such emitters indicated the presence of the Coulomb blockade effect in the self-assembled all-carbon heterostructures. The mechanism of nanoprotrusion formation via the field-induced surface diffusion of carbon atoms is revealed by observing the structural transformation of the emitter material using transmission electron microscopy. We also explore how the properties of the formed heterostructures evolve with the field emission current, and show that the characteristic size of the formed nanoprotrusion depends on the dimensions of the diamond nanotip. The developed approach offers a way to reproducible fabrication of heterostructured emitters which can be applied as coherent single-electron sources in vacuum nanoelectronics and electron quantum optics.

A Smart Bacteria-Capture-Killing Vector for Effectively Treating Osteomyelitis Through Synergy Under Microwave Therapy.

Osteomyelitis caused by deep tissue infections is difficult to cure through phototherapy due to the poor penetration depth of the light. Herein, Cu/C/Fe3O4-COOH nanorod composites (Cu/C/Fe3O4-COOH) with nanoscale tip convex structures are successfully fabricated as a microwave-responsive smart bacteria-capture-killing vector. Cu/C/Fe3O4-COOH exhibited excellent magnetic targeting and bacteria-capturing ability due to its magnetism and high selectivity affinity to the amino groups on the surface of Staphylococcus aureus (S. aureus). Under microwave irradiation, Cu/C/Fe3O4-COOH efficiently treated S. aureus-infected osteomyelitis through the synergistic effects of microwave thermal therapy, microwave dynamic therapy, and copper ion therapy. It is calculated the electric field intensity in various regions of Cu/C/Fe3O4-COOH under microwave irradiation, demonstrating that it obtained the highest electric field intensity on the surface of copper nanoparticles of Cu/C/Fe3O4-COOH due to its high-curvature tips and metallic properties. This led to copper nanoparticles attracted more charged particles compared with other areas in Cu/C/Fe3O4-COOH. These charges are easier to escape from the high curvature surface of Cu/C/Fe3O4-COOH, and captured by adsorbed oxygen, resulting in the generation of reactive oxygen species. The Cu/C/Fe3O4-COOH designed in this study is expected to provide insight into the treatment of deep tissue infections under the irradiation of microwave.

Solution-Processed Formation of DNA-Origami-Supported Polyoxometalate Multi-Level Switches with Countercation-Controlled Conductance Tunability

We report a chemically programmed design and the switching characteristics of a functional metal–DNA-origami–polyoxometalate (POM) material obtained from the solution-processed assembling of biocompatible molecular precursors. The DNA origami is immobilized on the gold surface via thiolate groups and acts as a carrier (ad-layer) structure, ensuring the spatially controlled hybridization of the pre-defined six-helix bundle (6HB) positions with DNA-augmented, tris(alkoxo)-ligated Lindqvist-type polyoxovanadate (POV6) units. The DNA-confined POV6 units accept electrons in a stepwise fashion, allowing for a multi-logic function, which we directly probe using scanning tunneling electron microscopy and spectroscopy. Electron acceptance and injection into the originally non-conducting DNA structure and the subsequent release to the gold substrate depend upon the potential at the nanoscale tip and the oxidation state of POV6, as well as on the mechanism of action of POV6 countercations. By combining experiment and theory, we show that the bio-hybrid heterojunction has far-reaching potential to create a chemically controlled POM-based nano-environment with synaptic behavior.

Electromechanical resonances and field-emission-induced self-oscillations of single crystal diamond needles

Due to its outstanding mechanical characteristics, diamond is an ideal material for use in micro- and nano-electromechanical systems. In this paper, we report on the investigation of vibrational properties of singly clamped needlelike diamond microcrystallites with nanoscale tips. The single-crystal diamond needles were produced by selective oxidation of polycrystalline films grown using chemical vapor deposition. The study of resonant oscillations driven by the AC voltage indicated that the elastic modulus of such diamond needles is close to that of bulk single crystal diamond. A self-oscillation regime induced by the DC voltage during field emission from the apex of a diamond needle is also demonstrated. It is shown that this regime can be used for efficient DC–AC conversion in microdevices. The high structural quality of diamond needles, their remarkable mechanical properties, and the relative ease of their mass fabrication make them promising candidates for application in various electromechanical systems, field-emission devices, and scanning probe techniques.

Nanosecond bacteria inactivation realized by locally enhanced electric field treatment

Bacterial contamination in water is still a critical threat to public health; seeking efficient water disinfection approaches is of great significance. Here we show that locally enhanced electric field treatment (LEEFT) by electrodes modified with nanoscale tip structures can induce ultrafast bacteria inactivation with nanosecond electrical pulses. A lab-on-a-chip device with gold nanowedges on the electrodes is developed for an operando investigation. Attributed to the lightning-rod effect, the bacteria at the nanowedge tips are inactivated by electroporation. A single 20 ns pulse at 55 kV cm−1 has achieved 26.6% bacteria inactivation, with ten pulses at 40 kV cm−1 resulting in 95.1% inactivation. LEEFT lowers the applied electric field by about 8 fold or shortens the treatment time by at least 106 fold, compared with the system without nanowedges. Both Gram-positive and Gram-negative bacteria, including antibiotic-resistant bacteria, are inactivated with nanosecond pulses by LEEFT. According to simulation, when the membrane of the cell located at the nanowedge tip is directly charged by the concentrated charges at the tip, it is charged much faster and to a much higher level, leading to instant electroporation and cell inactivation. Efficient ways to disinfect water from bacterial contamination are essential for public health. Locally enhanced electric field treatment can be used to induce ultrafast bacteria inactivation with nanosecond electrical pulses.

Effects of Graphene Redox on Its Triboelectrification at the Nanoscale

Controlling triboelectrification behaviors of graphene-based materials plays a key role in the output performance and reliability of the graphene-based triboelectric nanogenerators. The electrical and mechanical properties of graphene-based materials are sensitive to their redox, and the study of the redox effects on their triboelectrification at nanoscale is the basis for regulating the triboelectrification performance. Here, the triboelectrification behaviors of graphene oxide (GO) and thermally and chemically reduced graphene oxide (RGO) were explored by scanning probe microscopy. It is found that the triboelectric charges on GO are localized in the rubbed region due to the sp3 hybridized structure. However, on RGO, triboelectric charges are distributed over the whole sheet due to the restored sp2 structure. In the rubbing process with insufficient contact, wrinkles in RGO, contact forces, and friction modes affect the triboelectric charging process dramatically. These influences are related to the resistance threshold of nanoscale tip–sample contact that is related to the present tip–sample potential difference, the contact force, the roughness, and the friction mode. When the tip–sample contact is sufficient enough, the charge transfer in the triboelectric charging process can reach equilibrium, and the roughness, contact force, and rubbing mode have no effect on the triboelectrification. Moreover, the triboelectric potential is linearly dependent on the initial potential, lifting (positive) with a negative initial potential and dropping (negative) with a positive initial potential. Tunneling triboelectrification on RGO is also affected by the initial potential, presenting a localized distribution of triboelectric charges in the rubbed area. Our findings would provide guidance for regulating the triboelectric charges on graphene-based materials and designing high-performance graphene-based triboelectric nanogenerators.

Specially Resolved Single Living Cell Perfusion and Targeted Fluorescence Labeling Based on Nanopipettes.

Targeted delivery and labeling of single living cells in heterogeneous cell populations are of great importance to understand the molecular biology and physiological functions of individual cells. However, it remains challenging to perfuse fluorescence markers into single living cells with high spatial and temporal resolution without interfering neighboring cells. Here, we report a single cell perfusion and fluorescence labeling strategy based on nanoscale glass nanopipettes. With the nanoscale tip hole of 100 nm, the use of nanopipettes allows special perfusion and high-resolution fluorescence labeling of different subcellular regions in single cells of interest. The dynamic of various fluorescent probes has been studied to exemplify the feasibility of nanopipette-dependent targeted delivery. According to experimental results, the cytoplasm labeling of Sulfo-Cyanine5 and fluorescein isothiocyanate is mainly based on the Brownian movement due to the dyes themselves and does not have a targeting ability, while the nucleus labeling of 4',6-diamidino-2-phenylindole (DAPI) is originated from the adsorption between DAPI and DNA in the nucleus. From the finite element simulation, the precise manipulation of intracellular delivery is realized by controlling the electro-osmotic flow inside the nanopipettes, and the different delivery modes between nontargeting dyes and nucleus-targeting dyes were compared, showcasing the valuable ability of nanopipette-based method for the analysis of specially defined subcellular regions and the potential applications for single cell surgery, subcellular manipulation, and gene delivery.

Additive effect of micro-damage zones from nano-scale crack tip and nano-grains for fracture toughness measurements of 3Y-TZP zirconia ceramics

A nano-scale crack tip around 500 nm wide introduced by femtosecond laser still affects the accuracy of fracture toughness KIC measurements of 3Y-TZP zirconia ceramics with average grain size G from 200 to 500 nm. A simple formula was proposed to estimate the additive effect of crack-tip damage zones from an infinitely sharp crack to a nano-scale blunt notch. The error in fracture toughness measurements is less than 8 % if the nano-scale crack-tip width < 0.5·G. The intrinsic KIC can be deduced from the simple formula if the nano-scale crack tip > 0.5·G. This study shows the same KIC was deduced from two different sets of 3Y-TZP measurements with nano- and micro-scale notches of 500 nm and 18 µm wide. Furthermore, the simple formula specifies the relation between the fracture toughness KIC and intrinsic strength ft via grain size G, which means KIC can also be estimated from ft and G without testing pre-cracked specimens. KIC values of 3Y-TZP from specimens with and without pre-cracks were compared.

Three-Dimensional Kelvin Probe Force Microscopy.

Traditional Kelvin probe force microscopy (KPFM) is mainly limited to the characterization of two-dimensional (2D) surfaces, and in situ surface potential (SP) imaging along 3D device surfaces remains a challenge. This paper presents a multimode 3D-KPFM based on an orthogonal cantilever probe (OCP) that can achieve SP mapping of 3D micronano structures. It integrates three working modes: a bending mode for 2D horizontal surface imaging, a torsion mode for vertical sidewall imaging, and a vector tracking-based 3D scanning mode. The customized OCP has a nanoscale tip protruding from the side and underside of the cantilever, rather than the front, and the extended tip makes the proposed approach universally applicable for 3D detection from the nanometer to micrometer scale. The spatial resolution of the proposed method is analyzed by simulation, which shows it can reduce the cantilever homogenization effect. Linearity and energy resolution measurements show that the proposed method has comparable performance to traditional methods. A comparative experiment using a gold-silicon interface verifies the accuracy of the reported method in its bending and torsion modes. Further, the imaging ability of the 3D scanning mode is confirmed in the 3D characterization of a step grating. This technique is applied to the in situ characterization of a microforce sensor with microcomb structures. The experiment results show that this method can excellently achieve the 3D quantitative characterization of topography and SP, including critical dimensions and SP along a 3D device surface. This novel 3D-KPFM technique has many potential applications in the further exploration of 3D micronano devices.

The Origin of Moiré‐Level Stick‐Slip Behavior on Graphene/h‐BN Heterostructures

AbstractFrictional behavior of a nanoscale tip sliding on superlattice of aligned graphene/(hexagonal boron nitride) h‐BN heterostructure is found to be strongly regulated by the moiré superlattices, resulting in long‐range stick‐slip modulation in experimental measurements. Through molecular dynamics simulations, it is shown that the origin of moiré‐level stick‐slip comes from the strong coupling between in‐plane deformation and out‐of‐plane distortion of the moiré superlattice. The periodicity of long‐range modulation decreases as the interfacial twist angle increases, once the periodicity of moiré becomes smaller than the contact region between the tip and graphene, the long‐range modulation becomes smooth and the stick‐slip behavior disappears. It is found that the contact trajectory of the tip during sliding is the key to reveal the underlying mechanism, based on which a modified Prandtl‐Tomlinson model is proposed considering deformation coupled effect to reproduce the frictional properties observed in molecular dynamics simulation. These findings emphasize the critical role of the moiré superlattice on frictional properties of van der Waals (vdW) heterostructures and open an avenue for the rational design of vdW devices with controllable tribological properties.

Electron transfer driven by tip-induced flexoelectricity in contact electrification

Contact electrification (CE) has been known for over 25 centuries, but the origin of the CE remains mysterious. Recent theoretical studies suggest that flexoelectricity may drive the CE, but experimental evidence is lacking. Here, the CE between a nanoscale tip and flat polymers is studied by using atomic force microscopy. The contributions of flexoelectricity to the CE are analyzed. We focus on the effect of the load, which is coupled to the strain gradient at the contact region. It is revealed that the flexoelectric polarization in general polymers can drive electron transfer, and even reverse the intrinsic polarity of electron transfer in the CE. It implies that the flexoelectricity is one of the driving forces for the CE. The flexoelectricity induced electric field is measured by applying a voltage between the tip and the sample, which counteracts the flexocoupling voltage. Further, a band structure model is proposed, in which the surface states of the solid are suggested to be shifted by the flexoelectric polarization.

Utilizing trapped charge at bilayer 2D MoS2/SiO2 interface for memory applications

In this work we use conductive atomic force microscopy (cAFM) to study the charge injection process from a nanoscale tip to a single isolated bilayer 2D MoS2 flake. The MoS2 is exfoliated and bonded to ultra-thin SiO2/Si substrate. Local current–voltage (IV) measurements conducted by cAFM provides insight in charge trapping/de-trapping mechanisms at the MoS2/SiO2 interface. The MoS2 nano-flake provides an adjustable potential barrier for embedded trap sites where the charge is injected from AFM tip is confined at the interface. A window of (ΔV ∼ 1.8 V) is obtain at a reading current of 2 nA between two consecutive IV sweeps. This is a sufficient window to differentiate between the two states indicating memory behavior. Furthermore, the physics behind the charge entrapment and its contribution to the tunneling mechanisms is discussed.

Nanoscale tip positioning with a multi-tip scanning tunneling microscope using topography images.

Multi-tip scanning tunneling microscopy (STM) is a powerful method to perform charge transport measurements at the nanoscale. With four STM tips positioned on the surface of a sample, four-point resistance measurements can be performed in dedicated geometric configurations. Here, we present an alternative to the most often used scanning electron microscope imaging to infer the corresponding tip positions. After the initial coarse positioning is monitored by an optical microscope, STM scanning itself is used to determine the inter-tip distances. A large STM overview scan serves as a reference map. Recognition of the same topographic features in the reference map and in small scale images with the individual tips allows us to identify the tip positions with an accuracy of about 20nm for a typical tip spacing of ∼1μm. In order to correct for effects such as the non-linearity of the deflection, creep, and hysteresis of the piezoelectric elements of the STM, a careful calibration has to be performed.

Bifurcation of nanoscale thermolubric friction behavior for sliding on MoS2

We present atomic force microscopy experiments of wearless sliding between nanoscale tips and both bulk and monolayer $\mathrm{Mo}{\mathrm{S}}_{2}$ in ultrahigh vacuum across a wide range of temperatures (150--450 K) and scanning speeds (5 nm/s to 500 $\ensuremath{\mu}\mathrm{m}/\mathrm{s}$). Atomic lattice stick-slip behavior is consistently resolved. However, a bifurcation of behavior is seen, with some measurements showing a strong decrease in friction with increasing temperature and others showing athermal and low friction under nominally identical conditions. The difference between thermal and athermal behavior is attributed to a change in the corrugation of the potential energy surface, possibly due to trace amounts of adsorbed contaminants. While the speed dependence at a given temperature is consistent with the thermal Prandtl-Tomlinson model for atomic-scale friction, that is not the case for the temperature dependence (when it is present), nor can the temperature dependence be described by other existing models. We discuss the limitations of these models considering the measured results.

Binary Spiky/Spherical Nanoparticle Films with Hierarchical Micro/Nanostructures for High-Performance Flexible Pressure Sensors.

Flexible pressure sensors have been widely explored for their versatile applications in electronic skins, wearable healthcare monitoring devices, and robotics. However, fabrication of sensors with characteristics such as high sensitivity, linearity, and simple fabrication process remains a challenge. Therefore, we propose herein a highly flexible and sensitive pressure sensor based on a conductive binary spiky/spherical nanoparticle film that can be fabricated by a simple spray-coating method. The sea-urchin-shaped spiky nanoparticles are based on the core-shell structures of spherical silica nanoparticles decorated with conductive polyaniline spiky shells. The simple spray coating of binary spiky/spherical nanoparticles enables the formation of uniform conductive nanoparticle-based films with hierarchical nano/microstructures. The two differently shaped particles-based films (namely sea-urchin-shaped and spherical) when interlocked face-to-face to form a bilayer structure can be used as a highly sensitive piezoresistive pressure sensor. Our optimized pressure sensor exhibits high sensitivity (17.5 kPa-1) and linear responsivity over a wide pressure range (0.008-120 kPa), owing to the effects of stress concentration and gradual deformation of the hierarchical microporous structures with sharp nanoscale tips. Moreover, the sensor exhibits high durability over 6000 repeated cycles and practical applicability in wearable devices that can be used for healthcare monitoring and subtle airflow detection (1 L/min).

Unveiling the impact of embedding resins on the physicochemical traits of wood cell walls with subcellular functional probing

As pressing needs for exploring molecular interactions in plants soar, conventional sample preparation methods come into question. Though resins used to embed plant tissues have long been assumed to bear no palpable effect on their properties, discrepancies in recent studies exploiting nanoscale microscopy suggest that their impact could be significant at small scales. By juxtaposing the traits of poplar sections prepared with and without embedding, we evaluate the diffusion (penetration depth) of acrylic and epoxy resins commonly used for embedding. Our results unveil critical quantitative differences when probing mechanical properties with a microscale nanoindentation indenter or a nanoscale tip. The latter resolves significant stiffness variations between the compound middle lamellae, the secondary cell wall layers S1 and S2, and the cell corner, not accessible with nanoindentation. Similar observations are drawn from comparing confocal Raman and nanoscale infrared spectroscopy. Our findings shed light on the effect of resin diffusion suggesting acrylic LR White to be the least diffusive for plant cell wall studies.

Effect of reduced graphene oxide overcoating on scratch resistance of Ag nanowire electrodes

In this study, the effect of reduced graphene oxide (rGO) overcoating on the scratch resistance of Ag nanowire electrodes was explored for the first time. By performing nanoscratch tests on the nanowire electrodes with and without the rGO overcoating, the lateral forces imposed on the samples were measured when the nanoscale tip was moved laterally; these forces were correlated to the scratch resistance. The lateral forces of the Ag nanowire electrode with an rGO overcoating were significantly higher than those of the uncoated electrode, indicating an increased resistance to the lateral movement of the tip and thus an enhanced scratch resistance. Furthermore, scanning probe microscope images showed that the Ag nanowire electrode without an rGO overcoating exhibited wider scratches after testing than those observed at the coated electrode. These results confirmed that rGO overcoating is effective for enhancing the scratch resistance of Ag nanowire electrodes.

Dynamic Exciton Funneling by Local Strain Control in a Monolayer Semiconductor.

The ability to control excitons in semiconductors underlies numerous proposed applications, from excitonic circuits to energy transport. Two dimensional (2D) semiconductors are particularly promising for room-temperature applications due to their large exciton binding energy and enormous stretchability. Although the strain-induced static exciton flux has been observed in predetermined structures, dynamic control of exciton flux represents an outstanding challenge. Here, we introduce a method to tune the bandgap of suspended 2D semiconductors by applying a local strain gradient with a nanoscale tip. This strain allows us to locally and reversibly shift the exciton energy and to steer the exciton flux over micrometer-scale distances. We anticipate that our result not only marks an important experimental tool but will also open a broad range of new applications from information processing to energy conversion.

Direct Chemical Imaging of Ligand-Functionalized Single Nanoparticles by Photoinduced Force Microscopy

Chemical characterizations of biochemically functionalized single nanoparticles are necessary to optimize the nanoparticle surface functionality in recently advanced nanobiological applications but have not yet been fully explored because of technical difficulties. Exploiting the photoinduced force exerted on a light-illuminated nanoscale tip, nanoscale mid-infrared hyperspectral images with a 10 nm spatial resolution of a monolayer ligand-functionalized single gold nanoparticle under ambient and environmental conditions are presented. We extend our study to the diagnosis of nanoscale heterogeneous chemical contaminants which come from a particle functionalization process but are undetectable in conventional ensemble-averaged imaging technique. High sensitivity and high spatial resolution are achieved via the strongly localized tip-enhanced force at the junction between the gold-coated tip and the functionalized nanoparticle in photoinduced force microscopy, which far exceeds the capability of the conventional methods. The present study paves a new way to directly detect heterogeneous nanochemicals at the single-component level, which is necessary to evaluate nanomaterial safety in biomedical applications.