AFM Tip Research Articles

Enhanced Exciton-Exciton Collisions in an Ultraflat Monolayer MoSe2 Prepared through Deterministic Flattening.

Squeezing bubbles and impurities out of interlayer spaces by applying force through a few-layer graphene capping layer leads to van der Waals heterostructures with the ultraflat structure free from random electrostatic potential arising from charged impurities. Without the graphene capping layer, a squeezing process with an AFM tip induces applied-force-dependent charges of Δn ∼ 2 × 1012 cm-2 μN-1, resulting in the significant intensity of trions in photoluminescence spectra of MoSe2 at low temperature. We found that a hBN/MoSe2/hBN prepared with the "graphene-capping-assisted AFM nano-squeezing method" shows a strong excitonic emission with negligible trion peak, and the residual line width of the exciton peak is only 2.2 meV, which is comparable to the homogeneous limit. Furthermore, in this high-quality sample, we found that the formation of biexciton occurs even at extremely low excitation power (Φph ∼ 2.3 × 1019 cm-2 s-1) due to the enhanced collisions between excitons.

Single-molecule force spectroscopy: A facile technique for studying the interactions between biomolecules and materials interfaces

Abstract The quantification of the interactions between biomolecules and materials interfaces is crucial for design and synthesis functional hybrid bionanomaterials for materials science, nanotechnology, biosensor, biomedicine, tissue engineering, and other applications. Atomic force spectroscopy (AFM)-based single-molecule force spectroscopy (SMFS) provides a direct way for measuring the binding and unbinding forces between various biomolecules (such as DNA, protein, peptide, antibody, antigen, and others) and different materials interfaces. Therefore, in this review, we summarize the advance of SMFS technique for studying the interactions between biomolecules and materials interfaces. To achieve this aim, firstly we introduce the methods for the functionalization of AFM tip and the preparation of functional materials interfaces, as well as typical operation modes of SMFS including dynamic force spectroscopy, force mapping, and force clamping. Then, typical cases of SMFS for studying the interactions of various biomolecules with materials interfaces are presented in detail. In addition, potential applications of the SMFS-based determination of the biomolecule-materials interactions for biosensors, DNA based mis-match, and calculation of binding free energies are also demonstrated and discussed. We believe this work will provide preliminary but important information for readers to understand the principles of SMFS experiments, and at the same time, inspire the utilization of SMFS technique for studying the intermolecular, intramolecular, and molecule-material interactions, which will be valuable to promote the reasonable design of biomolecule-based hybrid nanomaterials.

Robust quantum point contact operation of narrow graphene constrictions patterned by AFM cleavage lithography

Detecting conductance quantization in graphene nanostructures turned out more challenging than expected. The observation of well-defined conductance plateaus through graphene nanoconstrictions so far has only been accessible in the highest quality suspended or h-BN encapsulated devices. However, reaching low conductance quanta in zero magnetic field, is a delicate task even with such ultra-high mobility devices. Here, we demonstrate a simple AFM-based nanopatterning technique for defining graphene constrictions with high precision (down to 10 nm width) and reduced edge-roughness (+/−1 nm). The patterning process is based on the in-plane mechanical cleavage of graphene by the AFM tip, along its high symmetry crystallographic directions. As-defined, narrow graphene constrictions with improved edge quality enable an unprecedentedly robust QPC operation, allowing the observation of conductance quantization even on standard SiO2/Si substrates, down to low conductance quanta. Conductance plateaus, were observed at n × e2/h, evenly spaced by 2 × e2/h (corresponding to n = 3, 5, 7, 9, 11) in the absence of an external magnetic field, while spaced by e2/h (n = 1, 2, 3, 4, 5, 6) in 8 T magnetic field.

Modeling of eccentric nanoneedle in trolling-mode atomic force microscope.

Limitations on installation of a standard TR-AFM nanoneedle can have unpredictable effects on dynamics of system. Therefore, it is crucial to pay close attention to the position and geometry of mounted nanoneedle when deriving the mathematical model. During TR-AFM fabrication process, the nanoneedle may not always deposit precisely at the middle of AFM tip, which would result in coupled bending-torsion modes in the dynamical operation of system. In this paper, we investigate the effect of eccentric nanoneedle in dynamic response of TR-AFM. To address this issue, a continuous mathematical model is developed. This model accounts for eccentric nanoneedle which can address the couplings in nonlinear vibration. Hamilton's principle is used to derive equations of motion and then assumed mode method (AMM) is utilized. This model is capable of simulating the cantilever dynamics under complicated nanoneedle tip-sample interactions. Displacements of different components of system for various eccentricity are determined. It is found that nanoneedle eccentricity has noticeable effects on microbeam torsion angle and out of plane nanoneedle tip displacement.

Electrochemical preparation and physicochemical study of polymers obtained from carbazole and N-((methoxycarbonyl)methyl)carbazole

The polymerization of carbazole (Cz) and N-((methoxycarbonyl)methyl)carbazole (CzE) was carried out by electrochemical oxidation in acetonitrile solution. Different feed ratios of monomers were used to deposit polymer thin films with different compositions and study the influence of the ratio of monomers on the electrochemical, morphological, mechanical and adhesive properties of the polymer films. The electrochemical oxidation led to the deposition of a polymer film whatever the feed ratio but the thickness of the film increased with the portion of unmodified carbazole. On the contrary, the composition of the monomer mixture had a low influence on the AFM adhesion forces between the AFM tip and the polymer film. Depending on the feed monomer ratio, the morphology was either globular or columnar (when the portion of CzE was comprised between 10% and 70%). The stiffness of the polymer films was also found to be strongly dependent on the feed ratio since it varied from 2 to 115 GPa depending on the monomer ratio. The polymer films prepared from the ratios 90:10 and 70:30 in Cz:CzE monomer solutions were found to be the most interesting ones since they led to thick and conducting polymer films with a good adhesion and a low stiffness. Moreover, these coatings were uniform and exhibited an original columnar structure.

Nanoscale adhesion profiling and membrane characterisation in sickle cell disease using hybrid atomic force microscopy-IR spectroscopy

Sickle cell disease (SCD) presents a significant global health problem. At present there is no effective treatment, with most being supportive for its associated complications such as the vaso-occlusive crises that result from increased cell adhesion. Hypoxic sickle cells have previously shown greater phosphatidylserine (PS) exposure and oxidative damage, as well as being notably “stickier” suggesting that increased cell cohesion and adhesion to the blood vessel endothelium is a possible mechanism for vaso-occlusion. The present work uses the hybrid technique of atomic force microscopy nano-infrared spectroscopy (AFM-IR) to probe changes to the coefficient of friction and C-O IR intensity in SCD on a nanoscale for dried red blood cells (RBCs) fixed under conditions of hypoxia and correlates these observations with adhesive interactions at the membrane. Using functionalised AFM tips, it has been possible to probe adhesive interactions between hydrophilic and hydrophobic moieties exposed at the surface of the dried RBCs fixed under different oxygenation states and for different cell genotypes. The results are consistent with greater PS-exposure and oxidative damage in hypoxic sickle cells, as previously proposed, and also show strong correlation between localised oxidative damage and increased adhesion. A mechanistic explanation involving significant lipid tail disruption as a result of oxidative action, in combination with differing concentrations of externalised PS lipids, is proposed to explain the observed adhesion behaviour of each type of cell.

Tunable friction of monolayer MoS2 by control of interfacial chemistry

In this report, we utilize the heterostructures of monolayer MoS2 and self-assembled monolayers (SAMs) of organic molecules as a platform to understand how carrier density of nanomaterials affects their friction behaviors. Previous studies on the friction of two-dimensional materials have explored the effect of lattice structures and morphologies. Given the same normal force and scanning speed of the AFM tip, we observe the sliding friction of high-quality n-type monolayer MoS2 could be reduced by SAMs. The friction tunability is attributed to the charge transfer from MoS2to SAMs, which modulated the carrier density and hence the carrier-mediated friction behaviors in 2D MoS2. ab initio simulations and nanoscale mapping of work functions support this hypothesis. These SAMs-based heterostructures provide a potential tool to control friction in low-dimensional materials, and also an original perspective on the effect of electron–phonon coupling on friction at nanoscale.

Defect states contributed nanoscale contact electrification at ZnO nanowires packed film surfaces

Efficient conversion of mechanical energy in our surrounding environment into electric power has become a promising strategy for meeting the ever-increasing energy consumption of small and distributed electronics. The contact-electrification-based triboelectric nanogenerators are one of the emerging devices to achieve such energy conversion. However, conventional contact electrifications between two insulators are limited by their low current density and alternating current output. Here we report a nanoscale contact electrification induced direct current output based on the flow of electrons from the defect states of the ZnO nanowires-packed film to the contact sliding conductive AFM tip. Combining experimental materials characterization and density functional theory (DFT) calculations, the direct current output is closely related to the concentration of oxygen vacancy defect states on the surface of ZnO nanowires: the higher the oxygen vacancy concentration, the higher the current output. Under optimized conditions, we obtain an ultrahigh current density of ~108 A m-2, which is several orders of magnitude higher than that of the conventional contact electrification and other effects. This work provides a new route of utilizing defect states contributed contact electrification for realizing nanoscale mechanical energy scavenging.

Dry Two-Step Self-Assembly of Stable Supported Lipid Bilayers on Silicon Substrates.

Artificial membranes are models for biological systems and are important for applications. We introduce a dry two-step self-assembly method consisting of the high-vacuum evaporation of phospholipid molecules over silicon, followed by a subsequent annealing step in air. We evaporate dipalmitoylphosphatidylcholine (DPPC) molecules over bare silicon without the use of polymer cushions or solvents. High-resolution ellipsometry and AFM temperature-dependent measurements are performed in air to detect the characteristic phase transitions of DPPC bilayers. Complementary AFM force-spectroscopy breakthrough events are induced to detect single- and multi-bilayer formation. These combined experimental methods confirm the formation of stable non-hydrated supported lipid bilayers with phase transitions gel to ripple at 311.5 ± 0.9 K, ripple to liquid crystalline at 323.8 ± 2.5 K and liquid crystalline to fluid disordered at 330.4 ± 0.9 K, consistent with such structures reported in wet environments. We find that the AFM tip induces a restructuring or intercalation of the bilayer that is strongly related to the applied tip-force. These dry supported lipid bilayers show long-term stability. These findings are relevant for the development of functional biointerfaces, specifically for fabrication of biosensors and membrane protein platforms. The observed stability is relevant in the context of lifetimes of systems protected by bilayers in dry environments.

Cyclo[18]carbon as an ultra-elastic molecular O-ring with unique mechanical properties

Recent atomic manipulation by an AFM tip created the first sp-hybridized carbon material – cyclo[18]carbon. Herein, we revealed the unique mechanical properties of cyclo[18]carbon by the first principle calculations. Under uniaxial tension, cyclo[18]carbon is ultra-elastic as demonstrated by a small Young’s modulus of 0.11 TPa and a tiny specific tensile stiffness of 7.0 × 106 N m/kg, which are one and two orders of magnitude smaller than those of other carbon materials, respectively. The expansion and contraction of cyclo[18]carbon exhibit a relatively high specific stiffness of 9.95 × 108 N m/kg, and it would be broken with an energy requirement of 34.14 eV. While uniaxial tension enables the insulator-semiconductor transition of cyclo[18]carbon, expansion enlarges its energy gap. Furthermore, the strain energy was proposed as a function of bond angle and bond length for uniaxially-stretched and expanded/contracted cyclo[18]carbon respectively, which sheds light on predicting the mechanical properties of certain materials. The unique properties of cyclo[18]carbon are of great interest for its potential use as an ultra-elastic molecular O-ring in nanomechanical system and molecular electronics and devices.

Role of Exchange Interactions in the Magnetic Response and Intermolecular Recognition of Chiral Molecules.

The physical origin of the so-called chirality-induced spin selectivity (CISS) effect has puzzled experimental and theoretical researchers over the past few years. Early experiments were interpreted in terms of unconventional spin-orbit interactions mediated by the helical geometry. However, more recent experimental studies have clearly revealed that electronic exchange interactions also play a key role in the magnetic response of chiral molecules in singlet states. In this investigation, we use spin-polarized closed-shell density functional theory calculations to address the influence of exchange contributions to the interaction between helical molecules as well as of helical molecules with magnetized substrates. We show that exchange effects result in differences in the interaction properties with magnetized surfaces, shedding light into the possible origin of two recent important experimental results: enantiomer separation and magnetic exchange force microscopy with AFM tips functionalized with helical peptides.

Defect formation in a graphene overlayer on ruthenium under high pressure

Due to its highly corrugated moir\'e structure and the nanometer modulation of its electronic properties, graphene deposited on ruthenium substrates (G-Ru for short) is a versatile playground for molecular deposition and chemical reactivity. Under local ultrahigh pressure conditions, one can expect an even richer behavior, reinforced by the likely appearance of defects. We have theoretically investigated the formation of such defects on G-Ru by using density functional theory methods. We show that defects can be produced in the high areas of the moir\'e, either by generating reactive centers in graphene through the creation of vacancies or by modifying the relative carbon-ruthenium positions through conformational changes. The energetic cost to induce these defects is rather small, of the order of a few eV, so that defects are expected to appear when impacting a STM tip or applying high pressure by a diamond AFM tip in these regions. The different types of defects can be clearly distinguished from each other in our simulated STM images.

Conductive pattern, electrical and mechanical characterisation of gold nanorods inside porous alumina template

Nanowires and nanotubes composed of layered or nonlayered materials have been fabricated via different techniques, among which Anodic Aluminium Oxide (AAO) template-based electrodeposition technology provides a versatile technique for synthesizing one or two-dimensional nanostructured materials. Herein, Conductive Atomic Force Microscopy (CAFM) has been used to image and characterise the nanorods for nanorods with diameter of 50 nm ± 5. Given that only one previous study using CAFM on gold nanorods exists [1], recording the difficulties encountered with this technique will be of wider interest to the scientific community. This work details experiments on electrical response as a function of applied force on the AFM tip and contamination on the top surface on nanorods. We report a novel observation, revealing a hollow shaped conductive pattern of some of our gold nanorods. This is explained with reference to competing growth rates in horizontal and vertical directions inside the pores.

Local defect-enhanced anodic oxidation of reformed GaN nanowires

Understanding formation and distribution of defects in GaN substrates and device layers is needed to improve device performance in rf and power electronics. Here we utilize conductive atomic force microscopy (c-AFM) for studying defect-related leakage paths in an unintentionally doped GaN film formed by nanowire reformation. A nanoscopic Schottky contact is formed between the c-AFM probe and the GaN surface, which, under reverse-bias conditions, reveals local leakage currents at the positions of the nanowires. Cathodoluminescence shows these areas to be dominated by yellow-band luminescence, in contrast to the surrounding GaN matrix, which mainly shows near-band-gap luminescence. These results are attributed to a high density of native and residual defects, confined to the nanowires. In addition, we use anodic oxidation to map defect-related conductive paths through locally induced growth of gallium oxide. The oxide yield, which is known to depend on the local electric field strength between the AFM tip and the sample, correlates well with the level of reverse-bias leakage current. Local irregularities in oxide height reveal extended oxidation attributed to defect-related deep-level states. Thisis confirmed by controlled dissolution of the oxide in NaOH, showing that a deeper oxide film is grown over areas where defect-related conductive paths are formed. Finally, we demonstrate how this approach can be used as a quick and easy diagnostic tool for evaluating the influence of specific growth conditions and process steps on defect-induced leakage current levels and defect distribution in GaN structures, demonstrating its potential for accelerated test of leakage degradation at critical positions in GaN-based devices. (Less)

Quantitative Assessment of Tip Effects in Single‐Molecule High‐Speed Atomic Force Microscopy Using DNA Origami Substrates

AbstractHigh‐speed atomic force microscopy (HS‐AFM) is widely employed in the investigation of dynamic biomolecular processes at a single‐molecule level. However, it remains an open and somewhat controversial question, how these processes are affected by the rapidly scanned AFM tip. While tip effects are commonly believed to be of minor importance in strongly binding systems, weaker interactions may significantly be disturbed. Herein, we quantitatively assess the role of tip effects in a strongly binding system using a DNA origami‐based single‐molecule assay. Despite its femtomolar dissociation constant, we find that HS‐AFM imaging can disrupt monodentate binding of streptavidin (SAv) to biotin (Bt) even under gentle scanning conditions. To a lesser extent, this is also observed for the much stronger bidentate SAv–Bt complex. The presented DNA origami‐based assay can be universally employed to quantify tip effects in strongly and weakly binding systems and to optimize the experimental settings for their reliable HS‐AFM imaging.

Quantitative Assessment of Tip Effects in Single-Molecule High-Speed Atomic Force Microscopy Using DNA Origami Substrates.

High‐speed atomic force microscopy (HS‐AFM) is widely employed in the investigation of dynamic biomolecular processes at a single‐molecule level. However, it remains an open and somewhat controversial question, how these processes are affected by the rapidly scanned AFM tip. While tip effects are commonly believed to be of minor importance in strongly binding systems, weaker interactions may significantly be disturbed. Herein, we quantitatively assess the role of tip effects in a strongly binding system using a DNA origami‐based single‐molecule assay. Despite its femtomolar dissociation constant, we find that HS‐AFM imaging can disrupt monodentate binding of streptavidin (SAv) to biotin (Bt) even under gentle scanning conditions. To a lesser extent, this is also observed for the much stronger bidentate SAv–Bt complex. The presented DNA origami‐based assay can be universally employed to quantify tip effects in strongly and weakly binding systems and to optimize the experimental settings for their reliable HS‐AFM imaging.

First stages of plasticity in three-point bent Au nanowires detected by in situ Laue microdiffraction

The first stages of plasticity in three-point bent Au nanowires are investigated by in situ three-point bending tests in combination with Laue micro-diffraction. To separate the elastic and plastic deformation, loading–unloading cycles were performed with increasing load in each consecutive cycle. The storage of the first four geometrically necessary dislocations of [011¯](111) slip system is observed in the vicinity of both clamping points, which might be attributed to the local rotations induced by the rigid Si support. At later stages of the deformation, additional slip systems are activated either by the torsion of the nanowire or by unintentional indentation from the AFM tip. The cyclic loading–unloading approach combined with Laue microdiffraction thus allows to study the onset of plasticity in defect-scarce nanostructures deformed by bending, offering additional possibilities in studying the dislocation nucleation process in bent nano-objects, which are essential for future applications, e.g., in flexible electronics and nano-electromechanical systems.

Near-Field Spectroscopy of Cylindrical Phonon-Polariton Antennas.

Surface phonon polaritons (SPhPs) are hybrid light-matter states in which light strongly couples to lattice vibrations inside the Reststrahlen band of polar dielectrics at mid-infrared frequencies. Antennas supporting localized surface phonon polaritons (LSPhPs) easily outperform their plasmonic counterparts operating in the visible or near-infrared in terms of field enhancement and confinement thanks to the inherently slower phonon-phonon scattering processes governing SPhP decay. In particular, LSPhP antennas have attracted considerable interest for thermal management at the nanoscale, where the emission strongly diverts from the usual far-field blackbody radiation due to the presence of evanescent waves at the surface. However, far-field measurements cannot shed light on the behavior of antennas in the near-field region. To overcome this limitation, we employ scattering-scanning near-field optical microscopy (sSNOM) to unveil the spectral near-field response of 3C-SiC antenna arrays. We present a detailed description of the behavior of the antenna resonances by comparing far-field and near-field spectra and demonstrate the existence of a mode with no net dipole moment, absent in the far-field spectra, but of importance for applications that exploit the heightened electromagnetic near fields. Furthermore, we investigate the perturbation in the antenna response induced by the presence of the AFM tip, which can be further extended toward situations where for example strong IR emitters couple to LSPhP modes.

Effect of single-walled carbon nanotubes on the structural, physical, and mechanical properties of rat glial cell surface

Carbon nanotubes are a very promising class of nanomaterials for biological and medical applications. The work aims at studying the interaction of rat C6 glioma cells with single-walled carbon nanotubes in complex with DNA (DNA-SWCNTs) paying the special attention to the changes in the structural, mechanical, and physical properties of the cell surface layer and the parameters of the DNA-SWCNT accumulation in cells. The properties of cells incubated with DNA-SWCNTs for 1–24 h were studied by atomic force microscopy, patch-clamp method, and Raman spectroscopy. The surface of glioma cells exposed to DNA-SWCNTs for 1 h had invaginations of size similar to the length of nanotubes (100–250 nm). The parameters of sliding friction force measured between the AFM tip and cell surface and the resting membrane potential were significantly reduced during the initial phase of DNA-SWCNT uptake (1–2 h) by cells. With further increase in incubation time (18–24 h), when DNA-SWCNTs were accumulated in the cytoplasm, the friction force parameters increased and became higher than the corresponding parameters for control cells. Using the decision tree algorithm for classification, the difference in the spatial distribution of the cell surface mechanical properties for C6 glioma cells exposed to DNA-SWCNTs during 1 or 24 h was shown with high classification accuracy. Our results prove that endocytosis is the main mechanism of DNA-SWCNT uptake by C6 rat glioma cells. The reorganization of the cell surface layer accompanying nanotube endocytosis leads to changes in the mechanical and physical properties of the cell surface.

Measuring the interactions between carbon black nanoparticles and latex thin films in aqueous media using AFM force spectroscopy

AFM force spectroscopy was utilised to measure the interactions between latex and carbon black nanoparticles in neutral ultrapure water and basic ultrapure water with 0.7% ammonia (pH of 11.6 ± 0.05) by weight added. For the first time, carbon black nanoparticles were adhered to AFM tips with epoxy using force spectroscopy techniques and characterised using SEM and AFM. The carbon-functionalised tips were then utilised to interact with thin films (prepared from concentrated and field latex suspensions) in the two liquid media. The results demonstrated that both attractive (during tip approach) and adhesive (during tip retraction) forces were considerably greater between the latex and carbon nanoparticles when the experiments were carried out in ultrapure water compared to ultrapure water with 0.7% ammonia. This was because the basic ammonia solution increased the negative surface charges of the latex and carbon particles which was confirmed by zeta potential measurements. Therefore, in the ammonia solution, only repulsion was observed on the tip approach and only small amounts of adhesion were observed on the tip retraction. Furthermore, the results demonstrated that despite the different processing and treatment of the concentrated and field latex samples, their interactions with the carbon black nanoparticles were similar in each medium. This study directly measures the interactions between carbon black nanoparticles and natural rubber latex, which has a significance for the manufacturing of automotive tyres and other polymer/carbon composites.