Force Modulation Microscopy Research Articles

Imaging chemical concentration pattern and early stages of spinodal decomposition in the AgxNa1−xBr system by scanning force microscopy

In quasi binary systems like AgBr–NaBr the demixing from the homogeneous high temperature phase into separated phases is a complex process that depends on the ageing temperature as well as on the quench rate. Spinodal decomposition and nucleation processes can be distinguished which lead, however, to the same final equilibrium state. Using frequency modulated scanning force microscopy we aimed to distinguish the demixed phases and their morphologies on a nm-scale at room temperature for samples with different concentration. While no contrast between the different phases are observed in the usual topographic mode, a modified evaluation of Kelvin experiments allows the distinction of silver- and sodium enriched phases. Moreover, direct evidence is found for concentration fluctuations which are characteristic for spinodal decomposition.

Force reconstruction from tapping mode force microscopy experiments

Fast, accurate, and robust nanomechanical measurements are intensely studied in materials science, applied physics, and molecular biology. Amplitude modulation force microscopy (tapping mode) is the most established nanoscale characterization technique of surfaces for air and liquid environments. However, its quantitative capabilities lag behind its high spatial resolution and robustness. We develop a general method to transform the observables into quantitative force measurements. The force reconstruction algorithm has been deduced on the assumption that the observables (amplitude and phase shift) are slowly varying functions of the tip–surface separation. The accuracy and applicability of the method is validated by numerical simulations and experiments. The method is valid for liquid and air environments, small and large free amplitudes, compliant and rigid materials, and conservative and non-conservative forces.

Investigation of the Orientational Dependence the Lateral Piesoresponse in Y–cut Periodically Poled Lithium Niobate Single Crystals

The angular dependence of the lateral component piesoresponse in Y−cut lithium niobate single crystals with a regular domain structure was investigated in piezoresponse force microscopy mode. It is shown that visualization of the domain structure is determined by the orientation of the X direction of the crystal relative to the scan direction: the greatest contrast between domains with opposite orientation of the polarization vectors takes place at locations along an X−direction line and the direction of scanning.

Characterization of topical film-forming systems using atomic force microscopy and Raman microspectroscopy.

Polymeric film-forming systems for dermal drug delivery represent an advantageous alternative to more conventional topically applied formulations. Their mechanical properties and homogeneity can be characterized with atomic force microscopy (AFM), using both imaging and nanoindentation modes, and Raman microspectroscopy mapping. Film-forming polymers, with and without a plasticizer and/or betamethasone 17-valerate (a representative topical drug), were dissolved in absolute ethanol. Polymeric films were then cast on glass slides and examined in ambient air using AFM imaging and Raman microspectroscopy. Using nanoindentation, the elastic moduli of various films were determined and found to decrease with increasing plasticizer content. Films with 20% w/w plasticizer had elastic moduli close to that of skin. AFM images showed little difference in the topography of the films on incorporation of plasticizer. Raman microspectroscopy maps of the surface of the polymeric films, with a spatial resolution of approximately 1 μm, revealed homogeneous distributions of plasticizer and drug within the films.

Bidirectional quantitative force gradient microscopy

Dynamic operation modes of scanning force microscopy based on probe resonance frequency detection are very successful methods to study force-related properties of surfaces with high spatial resolution. There are well-recognized approaches to measure vertical force components as well as setups sensitive to lateral force components. Here, we report on a concept of bidirectional force gradient microscopy that enables a direct, fast, and quantitative real space mapping of force component derivatives in both the perpendicular and a lateral direction. It relies solely on multiple-mode flexural cantilever oscillations related to vertical probe excitation and vertical deflection sensing. Exploring this concept we present a cantilever-based sensor setup and corresponding quantitative measurements employing magnetostatic interactions with emphasis on the calculation of mode-dependent spring constants that are the foundation of quantitative force gradient studies.

Impact of galactosylceramides on the nanomechanical properties of lipid bilayer models: an AFM-force spectroscopy study.

Galactosylceramides (GalCer) are glycosphingolipids bound to a monosaccharide group, responsible for inducing extensive hydrogen bonds that yield their alignment and accumulation in the outer leaflet of the biological membrane together with cholesterol (Chol) in rafts. In this work, the influence of GalCer on the nanomechanical properties of supported lipid bilayers (SLBs) based on DPPC (1,2-dipalmitoyl-sn-glycero-3-phosphocholine) and DLPC (1,2-didodecanoyl-sn-glycero-3-phosphocoline) as model systems was assessed. Phosphatidylcholine (PC):GalCer SLBs were characterized by means of differential scanning calorimetry (DSC) and atomic force microscopy (AFM), in both imaging and force spectroscopy (AFM-FS) modes. Comparing both PC systems, we determined that the behaviour of SLB mixtures is governed by the PC phase-like state at the working temperature. While a phase segregated system is observed for DLPC:GalCer SLBs, GalCer are found to be dissolved in DPPC SLBs for GalCer contents up to 20 mol%. In both systems, the incorporation of GalCer intensifies the nanomechanical properties of SLBs. Interestingly, segregated domains of exceptionally high mechanical stability are formed in DLPC:GalCer SLBs. Finally, the role of 20 mol% Chol in GalCer organization and function in the membranes was assessed. Both PC model systems displayed phase segregation and remarkable nanomechanical stability when GalCer and Chol coexist in SLBs.

Frictional behaviour of polycrystalline graphene grown on liquid metallic matrix

In this work friction characteristics of polycrystalline graphene grown on the metallic matrix from the liquid phase via new metallurgical method were investigated using atomic force microscopy under the range of applied normal loads. Polycrystalline graphene films were transferred onto silicon oxide using a modified polymer supported transfer process. Lateral force microscopy mode with both silicon and diamond-like carbon coated probes was used to measure the friction properties. Ogletree method was employed in order to calibrate the lateral force and estimate the average friction coefficient of polycrystalline graphene. As a comparison the friction of SiO2 surface, commercially available CVD grown graphene monolayers transferred onto SiO2 as well as highly oriented pyrolytic graphite were also measured under the same experimental conditions.

Surface topography, microstructure and magnetic domains in Al for Sn substituted metamagnetic Ni–Mn–Sn Heusler alloy ribbons

Heusler alloy Ni48Mn39.5Sn12.5−xAlx (x = 0, 1, 2,3) ribbons were prepared by melt spinning. The cross section of ribbons reveals a typical heterogeneous microstructure consisting of small equi-axed and columnar grains. The top and bottom ribbons sides are homogenous in composition but they differ in terms of surface structure. The wheel ribbon side appears smoother and shows some degree of macroscopic organization. On the other hand the free ribbon surface regardless of composition shows a more complex morphology consisting of small equi-axed grains, larger grains forming clusters and conical sub crystal structures. A characteristic twinned plate like microstructure confirming the presence of an increased amount of martensite has been observed for the Ni48Mn39.5Sn9.5Al3 ribbon. The martensitic plates are 70–100 nm in size. The phase shift of 0.5° was detected, while scanning the latter ribbon sample in magnetic force microscope mode, indicating the existence of a magnetic multi-domain structure in this sample.

Observation of elastic modulus inhomogeneities in thermosetting epoxies using AFM – Discerning facts and artifacts

Observations of a nanometer-scale nodular morphology on differently prepared surfaces of thermosets have frequently been interpreted as a sign for an inhomogeneous molecular network, which would result in an inhomogeneous modulus distribution within those thermosets. In order to test this hypothesis, the Peak-Force Tapping atomic force microscopy (AFM) mode was used on fracture surfaces and ultramicrotome cuts of epoxy and other polymers using differently sharp AFM probes.The nodular morphology is quite likely caused by an AFM artifact, which also seems to cause an apparently inhomogeneous modulus distribution; a variation in the tip–sample contact area could explain this effect. Smooth surfaces are necessary in order to reduce the contribution from this artifact. Ultramicrotome cutting currently seems to be the most appropriate surface preparation technique for the measurement of modulus distribution at the nanometer scale.All investigated materials seem to be homogeneous on a scale on the order of 10 nm–1000 nm. If modulus inhomogeneities are present, their amount or their lateral size is too small to be unambiguously measurable with this technique. From this data, it seems unlikely that epoxy exhibits an inhomogeneous molecular network.

Intermittent contact resonance atomic force microscopy

The intermittent contact resonance atomic force microscopy (ICR-AFM) mode proposed here is a new frequency modulation technique performed in scanning force controlled AFM modes like force volume or peak force tapping. It consists of tracking the change in the resonance frequency of an eigenmode of a driven AFM cantilever during scanning as the AFM probe intermittently contacts a surface at a controlled applied maximum force (setpoint). A high speed data capture was used during individual oscillations to obtain detailed contact stiffness–force curve measurements on a two-phase polystyrene/poly(methyl methacrylate) film with sub-micrometer size domains. Through a suitable normalization, the measurements were analyzed by linear fits to provide an improved quantitative characterization of these materials in terms of their elastic moduli and adhesive properties.

Investigation of tip current and normal force measured simultaneously during local oxidation of titanium using dual‐mode scanning probe microscopy

In this reported work, scanning probe microscopy is used to perform localised oxidation of Ti as a bottom-up nanofabrication method. The effects of normal force setpoint and oxidation voltage on oxide nanofeatures during atomic force microscopy (AFM) mode and scanning tunnelling microscopy (STM) mode oxidation are investigated. The normal force between the probe and sample and electrochemical current in the anodic oxidation process are measured using a custom control system, and the effects of these variables on the size and resistivity of oxidised nanopatterns are investigated. It is shown that a direct relation exists between oxidation voltage and written oxide resistivity as well as written feature width in the STM mode oxidation. In AFM mode oxidation, the normal force setpoint is shown to have a positive correlation with feature size, but an inverse relation with oxide resistivity. By leveraging the presented system's ability to operate in both AFM and STM mode, the dynamics and feature fabrication dependencies are shown to be a continuum of writing voltage and normal force. This reported work suggests that STM and AFM mode oxidation can be thought of as similar processes but working at different operating points in terms of normal force and tip-surface spacing.

Nanoscale characterization of the biomechanical properties of collagen fibrils in the sclera

We apply the PeakForce Quantitative Nanomechanical Property Mapping (PFQNM) atomic force microscopy mode for the investigation of regional variations in the nanomechanical properties of porcine sclera. We examine variations in the collagen fibril diameter, adhesion, elastic modulus and dissipation in the posterior, equatorial and anterior regions of the sclera. The mean fibril diameter, elastic modulus and dissipation increased from the posterior to the anterior region. Collagen fibril diameter correlated linearly with elastic modulus. Our data matches the known macroscopic mechanical behavior of the sclera. We propose that PFQNM has significant potential in ocular biomechanics and biophysics research.

High resolution imaging of native wheat and potato starch granules based on local mechanical contrast

We studied the nano-scale properties of dry native starch granules of wheat and potato by atomic force microscopy. Whereas at the macroscale the mechanical behavior of starch powders is known, its origin at sub-granule level has still to be understood. We observed fine morphological structures, such as the growth rings and blocklet domains, with minor differences between the two starches. The granules, embedded in resins with known stiffness, were analyzed with lateral-force, force-distance and force-modulation microscopy. Integer granules exhibited a similar friction coefficient to the tip, decreased with respect to the embedding resin, without occurrence of stick-slip. The compressive modulus measured was also similar for both starch types (∼1.4GPa in indentation and ∼2.0GPa in dynamic mode), with slightly higher values for potato starch. On sectioned granules, the effect of aging in air likely due to moisture produced in both starches a strong reduction in apparent modulus (∼0.2GPa).

Unravelling the biodiversity of nanoscale signatures of spider silk fibres

Living organisms are masters at designing outstanding self-assembled nanostructures through a hierarchical organization of modular proteins. Protein-based biopolymers improved and selected by the driving forces of molecular evolution are among the most impressive archetypes of nanomaterials. One of these biomacromolecules is the myriad of compound fibroins of spider silks, which combine surprisingly high tensile strength with great elasticity. However, no consensus on the nano-organization of spider silk fibres has been reached. Here we explore the biodiversity of spider silk fibres, focusing on nanoscale characterization with high-resolution atomic force microscopy. Our results reveal an evolution of the nanoroughness, nanostiffness, nanoviscoelastic, nanotribological and nanoelectric organization of microfibres, even when they share similar sizes and shapes. These features are related to unique aspects of their molecular structures. The results show that combined nanoscale analyses of spider silks may enable the screening of appropriate motifs for bioengineering synthetic fibres from recombinant proteins.

Peak forces and lateral resolution in amplitude modulation force microscopy in liquid

The peak forces exerted on soft and rigid samples by a force microscope have been modeled by performing numerical simulations of the tip motion in liquid. The forces are obtained by using two contact mechanics models, Hertz and Tatara. We present a comparison between the numerical simulations and three analytical models for a wide variety of probe and operational parameters. In general, the forces derived from analytical expressions are not in good quantitative agreement with the simulations when the Young modulus and the set-point amplitude are varied. The only exception is the parametrized approximation that matches the results given by Hertz contact mechanics for soft materials and small free amplitudes. We also study the elastic deformation of the sample as a function of the imaging conditions for materials with a Young modulus between 25 MPa and 2 GPa. High lateral resolution images are predicted by using both small free amplitudes (less than 2 nm for soft materials) and high set-point amplitudes.

Sensitivity of higher mode of rectangular atomic force microscope to surface stiffness in air environment

The sensitivity of the resonance frequencies and the amplitudes of the first four flexural vibration modes of atomic force microscopy with a rectangular cross-section cantilever in an air environment to variation of a sample's stiffness have been analysed. The cantilever has been modelled using Timoshenko beam theory, and the vertical and tangential forces between the tip and the sample in this simulation have been considered. The effect of tip position and the angle of the tip relative to the sample in changes of these sensitivities have been studied. Results indicate that for soft materials, the first mode compared with other modes is more sensitive to changes in the sample's elasticity, so the first mode is the best mode for supplying high-contrast images but by increasing the sample's stiffness, the higher mode will be sensitive, respectively. In addition, by increasing the angle between the cantilever and the sample's surface, the first mode is less sensitive and higher modes will be sensitive faster because of changes in material stiffness but in contrast, by increasing the distance between the tip's position and the free end of the cantilever, the sensitivity of the first mode increases and the higher mode will be sensitive for more stiff material.

Measurement of Viscoelastic Loss Tangent with Contact Resonance Modes of Atomic Force Microscopy

We show how atomic force microscopy techniques based on contact resonance (CR) can be used to measure the viscoelastic loss tangent tan δ of polymeric materials. The method does not require intermediate calculation of loss and storage moduli, calibration measurements, or use of the conventional CR tip shape parameter. We present the method’s physical concepts and sensitivity calculations for typical experimental parameters. In addition, CR experiments were performed on four homogeneous polymer samples (polystyrene, high-density polyethylene, and two commercial photostress polymers) with tan δ in the range from approximately 0.02 to 0.2. Results compare favorably to those obtained by microscale dynamic nanoindentation and macroscale dynamic mechanical analysis. These results show the potential of CR modes for nanoscale viscoelastic measurements of polymers and biomaterials.

Nanomechanical mapping of graphene layers and interfaces in suspended graphene nanostructures grown via carbon diffusion

Graphene's remarkable mechanical, electronic and thermal properties are strongly determined by both the mechanism of its growth and its interaction with the underlying substrate. Evidently, in order to explore the fundamentals of these mechanisms, efficient nanoscale methods that enable observation of features hidden underneath the immediate surface are needed. In this paper we use nanomechanical mapping via ultrasonic force microscopy that employs MHz frequency range ultrasonic vibrations and allows the observation of surface composition and subsurface interfaces with nanoscale resolution, to elucidate the morphology of few layer graphene (FLG) films produced via a recently reported method of carbon diffusion growth (CDG) on platinum-metal based substrate. CDG is known to result in FLG suspended over large areas, which could be of high importance for graphene transfer and applications where a standalone graphene film is required. This study directly reveals the detailed mechanism of CDG three-dimensional growth and FLG film detachment, directly linking the level of graphene decoupling with variations of the substrate temperature during the annealing phase of growth. We also show that graphene initially and preferentially decouples at the substrate grain boundaries, likely due to its negative expansion coefficient at cooling, forming characteristic “nano-domes” at the intersections of the grain boundaries. Furthermore, quantitative nanomechanical mapping of flexural stiffness of suspended FLG “nano-domes” using kHz frequency range force modulation microscopy uncovers the progression of “nano-dome” stiffness from single to bi-modal distribution as CDG growth progresses, suggesting growth instability at advanced CDG stages.

Direct Intermolecular Force Measurements between Functional Groups and Individual Metallic or Semiconducting Single‐Walled Carbon Nanotubes

Many electronic applications of single-walled carbon nanotubes (SWNTs) require electronic homogeneity in order to maximally exploit their outstanding properties. Non-covalent separation is attractive as it is scalable and results in minimal alteration of nanotube properties. However, fundamental understanding of the metallicity-dependence of functional group interactions with nanotubes is still lacking; this lack is compounded by the absence of methods to directly measure these interactions. Herein, a novel technology platform based on a recently developed atomic force microscopy (AFM) mode is reported which directly quantifies the adhesion forces between a chosen functional group and individual nanotubes of known metallicity, permitting comparisons between different metallicity. These results unambiguously show that this technology platform is able to discriminate the subtle adhesion force differences of a chosen functional group with pure metallic as opposed to pure semiconducting nanotubes. This new method provides a route towards rapid advances in understanding of non-covalent interactions of large libraries of compounds with nanotubes of varying metallicity and diameter; presenting a superior tool to assist the discovery of more effective metallicity-based SWNT separation agents.

Modification of high density polyethylene by gold implantation using different ion energies

High density polyethylene (HDPE) samples were modified by Au+ ion implantation at a dose of 5 × 1015 ions cm−2, using energies of 50, 100, 150 and 200 keV. The existence of implanted gold in the near-surface region of HDPE samples was confirmed by X-ray diffraction analysis. Surface roughness and Power Spectral Density analyses based on Atomic Force Microscopy (AFM) images of the surface topography revealed that the mechanism of HDPE modification during gold ion implantation depended on the energy of gold ions. Histograms obtained from phase AFM images indicated a qualitative change in the chemical composition of the surface during implantation with gold ions with different energies. Depth profiles obtained experimentally from cross-sectional Force Modulation Microscopy images and ones obtained from a theoretical simulation are in agreement for gold ions energies lower than 100 keV. The deviation that was observed for higher energies of the gold ions is explained by carbon precipitation in the near surface region of the HDPE, which prevented the penetration of gold ions further into the depth of the sample.