Tip-sample Distance Research Articles

NMR spectroscopy with force-gradient detection on a GaAs epitaxial layer

We demonstrate nuclear magnetic resonance spectroscopy on 35μm3 of 69Ga in a GaAs epitaxial layer in vacuum at 5K, and 5T yielding a linewidth on the order of 10kHz. This was achieved by a force-gradient magnetic resonance detection scheme, using the interaction between the force-gradient of a Ni sphere-tipped single crystal Si cantilever and the nuclear spins to register changes in the spin state as a change in the driven cantilever’s natural resonant frequency. The dichotomy between the background magnetic field (B0) homogeneity requirements imposed by NMR spectroscopy and the magnetic particle’s large magnetic field gradient is resolved via sample shuttling during the NMR pulse encoding. A GaAs sample is polarized in a B0 of 5T for 3*T1. The sample is shuttled away from the magnetic particle to a region of negligible magnetic field inhomogeneity. A (π/2)x pulse rotates the polarization to the xy-plane, the magnetization is allowed to precess for 2–200μs before a (π/2)x or (π/2)y pulse stores the remaining spin along the z-axis that represents a single point of the free induction decay (FID). The sample is shuttled back to the established tip-sample distance. An adiabatic rapid passage (ARP) sweep inverts the spins in a volume of interest, causing the cantilever’s natural resonance frequency to shift an amount proportional to the spin polarization in the volume. By varying the delay between the first and second (π/2) pulses the entire FID is measured.

Lowest order in inelastic tunneling approximation: Efficient scheme for simulation of inelastic electron tunneling data

We have developed an efficient and accurate formalism which allows the simulation at the ab initio level of inelastic electron tunneling spectroscopy data under a scanning tunneling microscope setup. It exploits fully the tunneling regime by carrying out the structural optimization and vibrational mode calculations for surface and tip independently. The most relevant interactions in the inelastic current are identified as the inelastic tunneling terms, which are taken into account up to lowest order, while all other inelastic contributions are neglected. As long as the system is under tunneling regime conditions and there is no physisorbed molecule on the surface or tip apex, this lowest order in inelastic tunneling (LOIT) approach reduces drastically the computational cost compared to related approaches while maintaining a good accuracy. Adopting the wide-band limit for both tip and surface further reduces calculation times significantly, and is shown to give similar results to when the full energy dependence of the Green's functions is taken into account. The LOIT is applied to the Cu(111)+CO system probed by a clean and a CO contaminated tip to find good agreement with previous works. Different parameters involved in the calculations such as basis sets, $k$ sampling, tip-sample distance, or temperature, among others, are discussed in detail.

Optical image contrast enhancement in near-field optics induced by water condensation

In surface science, water adsorption on hydrophilic samples is usually invoked, addressing their nanoscale experimental effects in scanning probe microscopy, especially when water condensates between tip and sample. Here we study by means of a numerical hybrid method the effect of water bridge formation in near field imaging. We show how this nanometric water neck plays an important role not only in the optical image, producing a high contrast at hydrophilic patches, but also in the tip-sample distance control. This work contributes with a new methodology able to retrieve the original application of SNOM, using it as an instrument to study the optical properties of matter overcoming the diffraction limit. It extends the application of SNOM to study the hydrophilic character of polymeric and biological samples, taking advantage of ubiquitous effect of humidity when operating in ambient condition.

In situ SECM study on concentration profiles of electroactive species from corrosion of stainless steel

The corrosion behaviours of type 304 stainless steel in acidic chloride solutions were investigated by cyclic voltammetric (CV) measurements at a scanning electrochemical microscope (SECM) tip combined with SECM scan imaging techniques. The results showed that the detection of electroactive species (i.e. ferrous ions) originating from the corrosion process can be accomplished as a function of position of the SECM tip above the stainless steel sample. Moreover, the variation in the tip–sample distance would allow for the tip current to be quantitatively correlated with the concentration profiles of the electroactive species or the related electrochemical activities on the sample surface. In a word, both voltammetric characteristic and three-dimensional SECM imaging related to the corrosion behaviours of 304 stainless steel will be helpful to clarify the possible pathways and reaction mechanisms involved in the corrosion processes.

Visualization of hydration layers on muscovite mica in aqueous solution by frequency-modulation atomic force microscopy

A three-dimensional interaction force mapping experiment was carried out on a muscovite mica surface in an aqueous solution using a high-resolution and low-thermal drift frequency-modulation atomic force microscope. By collecting oscillatory frequency shift versus distance curves at the mica∕solution interface, complicated hydration structures on the mica surface were visualized. Reconstructed two-dimensional frequency shift maps showed dot-like or honeycomb-like patterns at different tip-sample distances with a separation of 0.2 nm with each other, which agree well to the water molecule density maps predicted by a statistical-mechanical theory. Moreover, site-specific force versus distance curves showed a good agreement with theoretically calculated site-specific force curves by a molecular dynamics simulation. It is found that the first and second hydration layers give honeycomb-like and dot-like patterns in the two-dimensional frequency shift images, respectively, corresponding to the lateral distribution function in each layer.

Dynamic electrostatic force microscopy technique for the study of electrical properties with improved spatial resolution

The need to resolve the electrical properties of confined structures (CNTs, quantum dots, nanorods, etc) is becoming increasingly important in the field of electronic and optoelectronic devices. Here we propose an approach based on amplitude modulated electrostatic force microscopy to obtain measurements at small tip–sample distances, where highly nonlinear forces are present. We discuss how this improves the lateral resolution of the technique and allows probing of the electrical and surface properties. The complete force field at different tip biases is employed to derive the local work function difference. Then, by appropriately biasing the tip–sample system, short-range forces are reconstructed. The short-range component is then separated from the generic tip–sample force in order to recover the pure electrostatic contribution. This data can be employed to derive the tip–sample capacitance curve and the sample dielectric constant. After presenting a theoretical model that justifies the need for probing the electrical properties of the sample in the vicinity of the surface, the methodology is presented in detail and verified experimentally.

Field ion microscopy characterized tips in noncontact atomic force microscopy: Quantification of long-range force interactions

Direct comparison of tip-sample forces obtained by dynamic force spectroscopy experiments with theoretical simulations is extremely difficult, since the precise tip shape and chemical identity of the apex atoms of the force sensing tip remain unknown in most experiments. Here, we present force curves measured with a tungsten tip on a Ag(111) surface obtained in a low-temperature atomic force microscope using tips that were analyzed by field ion microscopy down to atomic levels. The resulting van der Waals and electrostatic forces were found to be in quantitative agreement with analytical models, if the tip shape parameters from the field ion microscopy analysis were used. Furthermore, our analysis shows an additional long-range force interaction at tip-sample distances above 1.3 nm. We suggest that this unexpected force is related to patch charges arising from the inhomogeneous work function distribution on the surface of highly faceted sharp tips.

Nanoscale imaging of photoelectrons using an atomic force microscope

Photoemission current imaging at the nanoscale is demonstrated by combining an atomic force microscope with laser excitation. Photoelectrons emitted from the sample are collected by the tip while the tip-sample distance is precisely controlled by their van der Waals force interaction. We observe pronounced photoemission current contrast with spatial resolution of 5 nm on a cesium covered Au(111) surface. This high spatial resolution can be attributed to the strong dependence of the local potential barrier on the tip-sample distance. Our experiments provide a method for photoelectron imaging with high spatial resolution and extend the functionality of state-of-the-art scanning probe techniques.

Monotonic Damping in Nanoscopic Hydration Experiments

We present the first accurate damping profiles acquired in atomic-resolution hydration force spectroscopy, revealing a monotonic damping profile at the nanoscopic tip apex. Atomic resolution is confirmed by the observed inhomogeneity caused by random substitution of Si by Al on the mica surface, and two distinct damping regimes are identified. Damping only appears above our detection limit of 1 nNs/m once the tip interacts with the second hydration layer, and an onset of strong damping above 100 nNs/m appears upon direct interaction with the adsorbed layer and first hydration layer. These results are compared to various simulations to interpret the damping signals and determine the tip-sample distance.

Optical switching of protein interactions on photosensitive-electroactive polymers measured by atomic force microscopy.

The ability to switch the physico-chemical properties of conducting polymers opens up new possibilities for a range of applications. Appropriately functionalised materials can provide routes to multi-modal switching, for example, in response to light and/or electrochemical stimuli. This capability is important in the field of bionics wherein remote and temporal control of the properties of materials is becoming attractive. The ability to actuate a film via photonic stimuli is particularly interesting as it facilitates the modulation of interactions between host binding sites and potential guest molecules. In this work, we studied two different poly-terthiophenes: one was functionalised with a spiropyran photoswitch (pTTh-SP) and the second with a non-photoswitchable methyl acetate moiety (pTTh-MA). These substrates were exposed to several cycles of illumination with light of different wavelengths and the resulting effect studied with UV-vis spectroscopy, contact angle and atomic force microscopy (AFM). The AFM tips were chemically activated with fibronectin (FN) and the adhesion force of the protein to the polymeric surface was measured. The pTTh-MA (no SP incorporated) showed a slightly higher average maximum adhesion (0.96 ± 0.14 nN) than the modified pTTh-SP surface (0.77 ± 0.08 nN), but after exposure of the pTTh-SP polymer to UV, the average maximum adhesion of the pTTh-MC (merocyanine form) was significantly smaller (0.49 ± 0.06 nN) than both the pTTh-MA and pTTh-SP. In addition, the tip-sample separation distances of the adhesive interactions are indicative of the FN interaction occurring over a distance more closely related to the average dimensions of its compact conformation. The results suggest that surface energy and hydrophobic forces are predominant in determining the protein adhesion to the films studied and that this effect can be photonically tuned. By extension, this further implies that it should be possible to obtain a degree of spatial and temporal control of the surface binding behaviour of certain proteins with these functionalised surfaces through photo-activation/deactivation, which, in principle, should facilitate patterned growth behaviour (e.g. using masks or directional illumination) or photocontrol of protein uptake and release.

A theoretical study of the dynamical switching of a single spin by exchange forces

We demonstrate the possibility of dynamically switching the spin of a single atom or molecule with the magnetic tip of an atomic force microscope making use of the acting exchange forces. We choose single V-, Nb- and Ta-benzene molecules as model systems and calculate the exchange interaction with an Fe tip using density functional theory. The exchange energy displays a Bethe–Slater-type behavior with ferromagnetic coupling at large tip–sample distance and antiferromagnetic coupling at closer proximity. The exchange energies reach maximum values of a few tens of meV, which allows one to switch single spins by overcoming the energy barrier due to the magneto-crystalline anisotropy. The spin dynamics of the system was explored by solving the time-dependent Schrödinger equation with additional Landau–Lifshitz-like spin relaxation. We find that the distance dependence of the exchange interaction as well as the appearance of quantum tunneling results in different scenarios for the switching behavior, e.g. the tip can switch the adatom or lead to a stable superposition state with zero magnetization.

Non-contact characterization of printed resistors

In this work we demonstrate the use of a high resolution non-contact scanning microwave microscope for characterization of printed resistors. The resonant microwave probe operates at a frequency of 5.73 GHz and it is based on a dielectric resonator coupled to a gold-coated tungsten tip with radius of 200 μm protruding from a cavity wall. Direct print additive manufacturing was used to produce the resistive films. Non-contact measurements of the resonant frequency fr and quality factor Q of the resonant microwave probe at a standoff distance of 20 μm were performed. Quality factor images were obtained over a scan area of 160 μm × 1670 μm in steps of 10 μm. Measurements reveal that Q varies from 214 to 233 over the studied region. In this work, variations in Q are associated with non-uniformities on the resistor surface. The quality factor of the probe was also acquired as a function of the tip-sample distance and measured data was fitted to a polynomial equation. We converted Q images to sheet resistance images using the polynomial equation and the material resistivity (400 Ω/sq/mil). Using the proposed approach, we found that the average sheet resistance over the scan area is Rs = 1027 W/sq and that Rs variations up to 662 Ω/sq, due to non-uniformities in the resistor's thickness, were detected by the microwave microscope. The localized microwave characterization capability demonstrated by the non-contact microscope could be of interest for defect detection in printed microwave circuits.

Orbital-dependent electron tunneling within the atom superposition approach: Theory and application to W(110)

We introduce an orbital-dependent electron tunneling model and implement it within the atom superposition approach for simulating scanning tunneling microscopy (STM) and spectroscopy (STS). Applying our method, we analyze the convergence and the orbital contributions to the tunneling current and the corrugation of constant-current STM images above the W(110) surface. In accordance with a previous study [Heinze et al., Phys. Rev. B 58, 16432 (1998)], we find atomic contrast reversal depending on the bias voltage. Additionally, we analyze this effect depending on the tip-sample distance using different tip models and find two qualitatively different behaviors based on the tip orbital composition. As an explanation, we highlight the role of the real-space shape of the orbitals involved in the tunneling. STM images calculated by our model agree well with those obtained using Tersoff and Hamann's and Bardeen's approaches. The computational efficiency of our model is remarkable as the $k$-point samplings of the surface and tip Brillouin zones do not affect the computation time, in contrast to the Bardeen method.

Direct observation of the spatial distribution of charges on a polypropylene fiber via Electrostatic Force Microscopy

The spatial distribution of electrical charges along the longitudinal axes of a polypropylene electret fiber was determined using Electrostatic Force Microscopy (EFM). EFM mapping on highly curved surfaces, such as those of polymeric fibers, is a challenging endeavour and most work reported in the scientific literature has been limited to single line-scan analysis or flat specimens. Charged polymeric fibers, electrets, are extensively used in high performance filtration applications and methods to determine the amount and magnitude of the charges on these fibers remain elusive. Electrical charge maps of individual fibers were obtained by biasing the tip to -10 V and maintaining a constant tip-sample distance of 100 nm. Spatially dependant EFM phase and magnitude gradients were determined and the developed technique may provide a unique understanding into the heterogeneous charge distribution on electrets fibers. Direct mapping of the charge distribution in electrets fibers can offer new insights in the development of antistatic additives, new means to facilitate electrostatic self-assembly of nano-moieties on the surface of fibrous materials and a quantitative metrics capable of determining discharge dynamics and predicting the shelf-life of filtration media.

Tip-Induced Deformation of Graphene on SiO2 Assessed by Capacitance Measurement

Tip-induced deformation of graphene on a SiO2 substrate was probed through a combination of scanning capacitance microscopy (SCM) and dynamic force microscopy (DFM). Spectroscopic analysis revealed that the resonant frequency shift (Δf) of the probe tip oscillation and the modulated capacitance (ΔC) simultaneously measured on graphene depend on the externally applied bias voltage while keeping the tip–sample distance constant. This finding is interpreted as a result of a local displacement of the graphene surface caused by the electrostatic force between the probe tip and graphene. The approach curve of the SCM tip toward graphene can be used to calibrate the observed ΔC spectra, quantitatively yielding an average deformation of approximately 0.31 nm in trilayer graphene and 0.21 nm in single-layer graphene.

A three-dimensional finite element model of near-field scanning microwave microscopy

A three-dimensional finite element model of an experimental near-field scanning microwave microscope (NSMM) has been developed and compared to experiment on non conducting samples. The microwave reflection coefficient S11 is calculated as a function of frequency with no adjustable parameters. There is qualitative agreement with experiment in that the resonant frequency can show a sizable increase with sample dielectric constant; a result that is not obtained with a two-dimensional model. The most realistic model shows a semi-quantitative agreement with experiment. The effect of different sample thicknesses and varying tip sample distances is investigated numerically and shown to effect NSMM performance in a way consistent with experiment. Visualization of the electric field indicates that the field is primarily determined by the shape of the coupling hooks.

Quantitative Atomic Resolution Force Imaging on Epitaxial Graphene with Reactive and Nonreactive AFM Probes

Atomic force microscopy (AFM) images of graphene and graphite show contrast with atomic periodicity. However, the contrast patterns vary depending on the atomic termination of the AFM tip apex and the tip-sample distance, hampering the identification of the atomic positions. Here, we report quantitative AFM imaging of epitaxial graphene using inert (carbon-monoxide-terminated) and reactive (iridium-terminated) tips. The atomic image contrast is markedly different with these tip terminations. With a reactive tip, we observe an inversion from attractive to repulsive atomic contrast with decreasing tip-sample distance, while a nonreactive tip only yields repulsive atomic contrast. We are able to identify the atoms with both tips at any tip-sample distance. This is a prerequisite for future structural and chemical analysis of adatoms, defects, and the edges of graphene nanostructures, crucial for understanding nanoscale graphene devices.

Exploring atomic-scale lateral forces in the attractive regime: a case study on graphite (0001)

A non-contact atomic force microscopy-based method has been used to map the static lateral forces exerted on an atomically sharp Pt/Ir probe tip by a graphite surface. With measurements carried out at low temperatures and in the attractive regime, where the atomic sharpness of the tip can be maintained over extended time periods, the method allows the quantification and directional analysis of lateral forces with piconewton and picometer resolution as a function of both the in-plane tip position and the vertical tip–sample distance, without limitations due to a finite contact area or to stick-slip-related sudden jumps of tip apex atoms. After reviewing the measurement principle, the data obtained in this case study are utilized to illustrate the unique insight that the method offers. In particular, the local lateral forces that are expected to determine frictional resistance in the attractive regime are found to depend linearly on the normal force for small tip–sample distances.

Stability enhancement of an atomic force microscope for long-term force measurement including cantilever modification for whole cell deformation

Atomic force microscopy (AFM) is widely used in the study of both morphology and mechanical properties of living cells under physiologically relevant conditions. However, quantitative experiments on timescales of minutes to hours are generally limited by thermal drift in the instrument, particularly in the vertical (z) direction. In addition, we demonstrate the necessity to remove all air-liquid interfaces within the system for measurements in liquid environments, which may otherwise result in perturbations in the measured deflection. These effects severely limit the use of AFM as a practical tool for the study of long-term cell behavior, where precise knowledge of the tip-sample distance is a crucial requirement. Here we present a readily implementable, cost effective method of minimizing z-drift and liquid instabilities by utilizing active temperature control combined with a customized fluid cell system. Long-term whole cell mechanical measurements were performed using this stabilized AFM by attaching a large sphere to a cantilever in order to approximate a parallel plate system. An extensive examination of the effects of sphere attachment on AFM data is presented. Profiling of cantilever bending during substrate indentation revealed that the optical lever assumption of free ended cantilevering is inappropriate when sphere constraining occurs, which applies an additional torque to the cantilevers "free" end. Here we present the steps required to accurately determine force-indentation measurements for such a scenario. Combining these readily implementable modifications, we demonstrate the ability to investigate long-term whole cell mechanics by performing strain controlled cyclic deformation of single osteoblasts.

FM‐AFM constant height imaging and force curves: high resolution study of DNA‐tip interactions

Interaction of the atomic force microscopy (AFM) tip with the sample can be invasive for soft samples. Frequency Modulation (FM) AFM is gentler because it allows scanning in the non-contact regime where only attractive forces exist between the tip and the sample, and there is no sample compression. Recently, FM-AFM was used to resolve the atomic structure of single molecules of pentacene and of carbon nanotubes. We are testing similar FM-AFM-based approaches to study biological samples. We present FM-AFM experiments on dsDNA deposited on 3-aminopropyltriethoxysilane modified mica in ultra high vacuum. With flexible samples such as DNA, the substrate flatness is a sub-molecular resolution limiting factor. Non-contact topographic images of DNA show variations that have the periodicity of the right handed helix of B-form DNA - this is an unexpected result as dehydrated DNA is thought to assume the A-form structure. Frequency shift maps at constant height allow working in the non-monotonic frequency shift range, show a rich contrast that changes significantly with the tip-sample separation, and show 0.2 to 0.4 nm size details on DNA. Frequency shift versus distance curves acquired on DNA molecules and converted in force curves show that for small molecules (height < 2.5 nm), there is a contribution to the interaction force from the substrate when the tip is on top of the molecules. Our data shine a new light on dehydrated and adsorbed DNA behavior. They show a longer tip-sample interaction distance. These experiments may have an impact on nanotechnological DNA applications in non-physiological environments such as DNA based nanoelectronics and nanotemplating.