Mode AFM Research Articles

Practical Method to Limit Tip–Sample Contact Stress and Prevent Wear in Amplitude Modulation Atomic Force Microscopy

Amplitude modulation atomic force microscopy (AM-AFM) is one of the most popular AFM modes because of the reduced tip-sample interaction, compared to contact mode AFM, and the ability to acquire high-resolution images while interrogating the sample's material composition through phase imaging. Despite the reduced tip-sample interaction, tip and sample wear can occur through gradual atomic scale processes that can significantly accumulate due to the high frequency of the tip-sample interaction and through high intermittent contact stresses. Starting from existing analytical formulations, we introduce a method for selecting an appropriate probe and free oscillation amplitude that avoids exceeding a critical contact stress to minimize tip/sample damage. The approach is presented for the case of both a Hertzian- and a Derjaguin-Müller-Toporov-like tip-sample contact. Stress maps and related simplified formulas are provided that enable one to determine allowable free oscillation amplitudes to stay below a target contact stress for given cantilever and sample parameters (combined into a single "cantilever-sample constant" that we introduce). Experimental results show how sharp silicon tips, either uncoated or coated with diamond-like carbon and silicon nitride, interacting with a hard and wear-resistant sample (ultrananocrystalline diamond) can be preserved while attaining high-quality AM-AFM images by using our proposed scheme. We also show that using our analysis to select parameters that exceed the target contact stress indeed leads to significant tip wear. This method provides AM-AFM users with a better understanding of contact stresses and enables selection of AM-AFM cantilevers and experimental parameters that preserve the tip for long periods of use and prevents the sample from damage.

Fine DNA structure revealed by constant height frequency modulation AFM imaging

Few examples of sub nanometer resolution on biomolecules can be found in literature. In particular, AFM experiments on DNA mostly fail to show the double helical groove periodicity, and so far such a feat was only achieved in liquid. Here we describe a method that produces highly resolved images of DNA where contrast can be assigned to submolecular features such as DNA helix’s grooves. Two types of AFM experiments are presented: (1) We started by imaging DNA with Frequency Modulation (FM) AFM, where a shift in resonance frequency is used as a feedback signal and maintained constant during imaging. With FM-AFM we performed experiments in the non-contact (NC) mode, with z-feedback on, where only gentle (few tens of pN) non compressive force is applied between the tip and the sample. (2) Then we switched off the z-feedback and scanned the DNA sample at constant height measuring frequency modulation in order to acquire frequency shift maps of the sample. This was done at several separations including the distances where the frequency feedback is not stable due to the non-monotonicity of the frequency shift curve. At this distance sub-molecular resolution was reached and a calculation of the tip-sample van der Waals interaction at constant height, as well as the periodicity of the observed features that ranges from 3 to 5nm, suggest that these correspond to the DNA helix grooves. This is the first experimental study where submolecular features of single DNA molecules are observed in dry environment. The knowledge of DNA structure when dried and deposited on flat substrates is important for proper understanding of DNA’s electronic behavior and its eventual nanotechnology applications.

Subsurface imaging of carbon nanotube networks in polymers with DC-biased multifrequency dynamic atomic force microscopy

The characterization of dispersion and connectivity of carbon nanotube (CNT) networks inside polymers is of great interest in polymer nanocomposites in new material systems, organic photovoltaics, and in electrodes for batteries and supercapacitors. We focus on a technique using amplitude modulation atomic force microscopy (AM-AFM) in the attractive regime of operation, using both single and dual mode excitation, which upon the application of a DC tip bias voltage allows, via the phase channel, the in situ, nanoscale, subsurface imaging of CNT networks dispersed in a polymer matrix at depths of 10–100 nm. We present an in-depth study of the origins of phase contrast in this technique and demonstrate that an electrical energy dissipation mechanism in the Coulomb attractive regime is key to the formation of the phase contrast which maps the spatial variations in the local capacitance and resistance due to the CNT network. We also note that dual frequency excitation can, under some conditions, improve the contrast for such samples. These methods open up the possibility for DC-biased amplitude modulation AFM to be used for mapping the variations in local capacitance and resistance in nanocomposites with conducting networks.

The influence of alkyl substituents of POSS in polyethylene nanocomposites

The influence of the alkyl substituent of POSS containing Q8 cage in polyethylene (PE) – POSS nanohybrids was investigated in this paper. Octakis(dimethylsiloxy)octasilsesquioxanes bearing linear and branched alkyl substituents, chain length varying between C3 and C8 for the straight-chain derivatives and between C5 and C7 for the branched-ones have been prepared for this purpose. XRD analysis showed that POSS coalesced in amorphous aggregates even if the pristine POSS was crystalline and the amount of these aggregates decreases with the increased length of the alkyl substituent. Peak force QNM mode of AFM revealed the presence of high modulus poor-POSS-domains and low modulus rich-POSS-domains whose size was around 140 nm for the octapropyl substituted POSS composite and vary in the range 50–90 nm for the composites with longer alkyl substituents on POSS. Although improved compatibility and dispersion was highlighted for POSS with long alkyl substituents, short length linear alkyl substituents (e.g. propyl) for POSS are preferred for improvements in tensile strength and thermal stability.

Structure and Properties of Proton Exchanged Layers in +Z Cut and –Z Cut Lithium Niobate

The phase structure of planar and channel, un-annealed and annealed proton exchanged waveguides made in benzoic acid and benzoic acid + lithium benzoate melts on Z cut of congruent lithium niobate crystal was investigated using mode spectroscopy, IR spectroscopy, XRD, SEM, AFM, microindentation, and wet etching techniques. In un-annealed waveguides, three layers (probably β2, β1, and κ2 phases) can be distinguished. At least two different structural layers are present in annealed channel waveguides. Proton exchange (225oC, 28 h) in benzoic acid + 3 mol.% of lithium benzoate leads to formation of κ2 phase and a thin layer of α phase.

Conductivity measurement of individual SnS nanoparticles by Peak Force AFM

ABSTRACTPeak Force Atomic Force Microscope is a new technique to characterize fragile materials such as nanoparticles with high accuracy with only one measurement. Unlike the tapping mode AFM, Peak Force AFM operates at a frequency below the resonant frequency of the cantilever. This allows for a direct control of the forces and avoids lateral forces that may damage the sample as in contact mode AFM. Furthermore, the performance characteristics of Peak Force AFM are suitable to work also in Tunneling AFM (TUNA) mode, enabling the study of the electrical properties of materials.In this work SnS nanoparticles capped with tri-n-octylphosphine oxide (TOPO) have been characterized. By means of Peak Force AFM it is possible to measure simultaneously topography and current maps of nanoparticles, yielding information about the shape, size and the conductivity of even a single nanoparticle. The topography map clearly showed single nanoparticles with a size less than 5 nm and spherical shape. In the conductivity map it is possible to discern the same nanoparticles, the correlation with the topography map is evident. This confirms the conduction (though not calibrated) of SnS nanoparticles. This type of measurements has been repeated many times in order to check the reproducibility of this technique. Moreover, the same nanoparticles have been measured also by Torsional Resonant TUNA AFM in order to compare it with Peak Force AFM. By means of TR-TUNA it was possible to measure the topography of SnS nanoparticles capped with TOPO but not the current. Besides, the resolution of the topography map acquired by TR-TUNA AFM is inferior to Peak Force AFM. From this comparison it has been found that the conductivity of nanoparticles, even if they are capped with TOPO, can be measured by Peak Force AFM, a result that thus far has been difficult to achieve by other types of AFM.

液中周波数変調AFMを用いた固液界面構造の三次元計測

液中周波数変調AFMを用いた固液界面構造の三次元計測

Characterization of the Interphase in Carbon Fiber/Epoxy Composites Using Force Modulation Atomic Force Microscope

In order to describe directly interphase properties of composite, force modulation of atomic force microscopy is adopted to study the cross-section of unidirectional carbon fiber/epoxy composites systematically. Research results indicate that in force modulation mode of AFM, relative stiffness of various phases distinct in distribution, which is described by probability histogram of relative stiffness. By comparison of probability histogram of relative stiffness nearby interphase of untreated and oxidation treated by ozone composites, the relative stiffness change oxidation treated one was more obviously to be found than the one untreated. Indirect show that obvious interphase formed by oxidation treatment. This method plays a valuable role in assessment of interphase strength of carbon fiber/epoxy, as well as in instruction composite production technology.

Scanning ion conductance microscopy for imaging biological samples in liquid: A comparative study with atomic force microscopy and scanning electron microscopy

The present study was designed to show the applicability of scanning ion conductance microscopy (SICM) for imaging different types of biological samples. For this purpose, we first applied SICM to image collagen fibrils and showed the usefulness of the approach-retract scanning (ARS)/hopping mode for such samples with steep slopes. Comparison of SICM images with those obtained by AFM revealed that the ARS/hopping SICM mode can probe the surface topography of collagen fibrils and chromosomes at nanoscale resolution under liquid conditions. In addition, we successfully imaged cultured HeLa cells, with 15μm in height by ARS/hopping SICM mode. Because SICM can obtain non-contact (or force-free) images, delicate cellular projections were visualized on the surface of the fixed cell. SICM imaging of live HeLa cells further demonstrated its applicability to study the morphological dynamics associated with biological processes on the time scale of minutes under liquid conditions. We further applied SICM for imaging the luminal surface of the trachea and succeeded in visualizing the surface of both ciliated and non-ciliated cells. These SICM images were comparable with those obtained by scanning electron microscopy. Although the dynamic mode of AFM provides better resolution than the ARS/hopping mode of SICM in some samples, only the latter can obtain contact-free images of samples with steep slopes, rendering it an important tool for observing live cells as well as unfixed or fixed soft samples with complicated shapes. Taken together, we demonstrate that SICM imaging, especially using an ARS/hopping mode, is a useful technique with unique capabilities for imaging the three-dimensional topography of a range of biological samples under physiologically relevant aqueous conditions.

Improving AFM Images with Harmonic Interference by Spectral Analysis

Atomic force microscopy (AFM) is one of the most sensitive tools for nanoscale imaging. As such, it is very sensitive to external noise sources that can affect the quality of collected data. The intensity of the disturbance depends on the noise source and the mode of operation. In some cases, the internal noise from commercial AFM controllers can be significant and difficult to remove. Thus, a new method based on spectrum analysis of the scanned images is proposed to reduce harmonic disturbances. The proposal is a post-processing method and can be applied at any time after measurements. This article includes a few methods of harmonic cancellation (e.g., median filtering, wavelet denoising, Savitzky-Golay smoothing) and compares their effectiveness. The proposed method, based on Fourier transform of the scanned images, was more productive than the other methods mentioned before. The presented data were achieved for images of conductive layers taken in a contact AFM mode.

Electric-field-driven phase transition in vanadium dioxide

We report on local probe measurements of current-voltage and electrostatic force-voltage characteristics of electric-field-induced insulator to metal transition in VO2 thin film. In conducting AFM mode, switching from the insulating to metallic state occurs for electric-field threshold E~6.5\times10^7 Vm-1 at 300K. Upon lifting the tip above the sample surface, we find that the transition can also be observed through a change in electrostatic force and in tunneling current. In this noncontact regime, the transition is characterized by random telegraphic noise. These results show that electric field alone is sufficient to induce the transition; however, the electronic current provides a positive feedback effect that amplifies the phenomena.

電気化学周波数変調原子間力顕微鏡によるAu(111)電極表面構造の電極電位依存性の観察

電気化学周波数変調原子間力顕微鏡によるAu(111)電極表面構造の電極電位依存性の観察

Surface relief, Phase and Surface Potential Investigations of Composite Polymer Membranes Using AFM

Composite membrane consisting from two different polymers – SPEKK and PVDF was used as two-phase sample for investigations with atomic force microscope (AMF). Three AFM modes were applied to analysis – surface relief, phase and surface potential scanning. Theoretical advantage of all three methods used for same sample is possibility to provide more detailed information on different phases in material. AFM results of surface investigation for pure SPEKK showed small fluctuation in height and phase (only difference in some nm and about 5°), what means that material is homogeneous. We found that for composite polymer membrane sample the changes in both modes are much more significant: for height it reaches 600 nm, but for phase – 20°. If surface potential of composite polymer sample clearly identifies some areas as SPEKK polymer, than in topography and phase images these places are identified as PVDF polymer only. This can be explained with the fact that polymers dissipate charge not only from surface but also little in depth. Even with such small potential contrast the surface potential mode of AFM can be used to detect distinct polymer structures not only on surface but also in a volume under the surface.

Enhancement of Sensitivity for the Evaluation of Electrical Properties by Modifying the Nano Structure of Microwave AFM Probe

To confirm the sensitivity in the measurement of electrical properties affected by the nano structure of microwave AFM (M-AFM) probe, three kinds of M-AFM probe with a nano-slit on its tip in different width (75 nm, 120 nm and 160 nm) were investigated. Au and glass samples were measured by the probes working at a non-contact AFM mode. The M-AFM probe with the nano-slit having the width of 75 nm, by which the difference of the measured voltage between Au and glass samples is 55.1 mV, shows the highest sensitivity for detecting electrical properties of materials. As the result illustrated, the M-AFM probe with smaller width nano-slit on the tip can be considered to be an ideal nano structure.

Circular AFM Mode: A New AFM Mode for Investigating Surface Properties

ABSTRACTFor the first time, a new AFM mode is presented that simultaneously allows the measuring of adhesion and friction forces at different constant and continuous sliding velocities. Our methodology consists of implementing a circular relative displacement of the contact to reach a constant sliding velocity, with no stop periods. Some of the main advantages of performing a circular displacement is that continuous and high sliding velocities (more than 1 mm/s) can be reached compared to the low sliding velocities (up to 10 μm/s) available when using commercial AFM. Also, a stationary state is reached when doing measurements. Moreover, the circular mode can be coupled with the classical operating mode, for instance, force spectrum. Main applications of this circular mode are related to metrological measurements in physics that require high speed displacements. As an example, we report the evolution of friction and adhesive forces measured in air at different high sliding velocities.

Tip and sample flexibility effects on tapping mode (amplitude modulation) AFM measurements

This Letter is devoted to the investigation of the tip, substrate and particle flexibility effects on the elastic deformation, the maximum repulsive force and the topography images in tapping mode (amplitude modulation) atomic force microscopy (TM-AFM). Several quantitative comparisons among the different models are presented and the effects of the elastic deformations on TM-AFM measurement are investigated.

Stability and Dewetting of Metal Nanoparticle Filled Thin Polymer Films: Control of Instability Length Scale and Dynamics

We investigate the influence of gold nanoparticle addition on the stability, dewetting, and pattern formation in ultrathin polymer-nanoparticle (NP) composite films by examining the length and time scales of instability, morphology, and dynamics of dewetting. For these 10-50 nm thick (h) polystyrene (PS) thin films containing uncapped gold nanoparticles (diameter approximately 3-4 nm), transitions from complete dewetting to arrested dewetting to absolute stability were observed depending on the concentration of the particles. Experiments show the existence of three distinct stability regimes: regime 1, complete dewetting leading to droplet formation for nanoparticle concentration of 2% (w/w) or below; regime 2, partial dewetting leading to formation of arrested holes for NP concentrations in the range of 3-6%; and regime 3, complete inhibition of dewetting for NP concentrations of 7% and above. Major results are (a) length scale of instability, where lambdaH approximately hn remains unchanged with NP concentration in regime 1 (n approximately 2) but increases in regime 2 with a change in the scaling relation (n approximately 3-3.5); (b) dynamics of instability and dewetting becomes progressively sluggish with an increase in the NP concentration; (c) there are distinct regimes of dewetting velocity at low NP concentrations; (d) force modulation AFM, as well as micro-Raman analysis, shows phase separation and aggregation of the gold nanoparticles within each dewetted polymer droplet leading to the formation of a metal core-polymer shell morphology. The polymer shell could be removed by washing in a selective solvent, thus exposing an array of bare gold nanoparticle aggregates.

Bi-stability of amplitude modulation AFM in air: deterministic and stochastic outcomes forimaging biomolecular systems

The dynamics of the oscillating microcantilever for amplitude modulation atomicforce microscopy (AM AFM) operating in air is well understood theoretically butthe experimental outcomes are still emerging. We use double-stranded DNA onmica as a model biomolecular system for investigating the connection betweentheory and experiment. A demonstration that the switching between the twocantilever oscillation states is stochastic in nature is achieved, and it can be inducedby means of topographical anomalies on the surface. Whether one or the otherattractor basin is accessed depends on the tip–sample separation history used toachieve the imaging conditions, and we show that the behaviour is reproduciblewhen the tip is stable and well characterized. Emergence of background noiseoccurs in certain regions of parameter space regardless of whether two cantileveroscillation states coexist. The low state has been explored in detail and we notethat at low to intermediate values of the free amplitude, noise-free imaging isachieved. The outcomes shown here are general and demonstrate that a thorough andsystematic experimental approach in conjunction with standard modelling givesinsight into the mechanisms behind image contrast formation in AM AFM in air.

Determination and simulation of nanoscale energy dissipation processes in amplitude modulation AFM

We develop a theoretical framework that explains the use of amplitude modulation AFM to measure and identify energy dissipation processes at the nanoscale. The variation of the dissipated energy on a surface by a vibrating tip as a function of its amplitude has a shape that singles out the dissipative process. The method is illustrated by calculating the dynamic-dissipation curves for surface adhesion energy hysteresis, long-range interfacial interactions and viscoelastic processes. We also show that by diving the dissipated energy by its maximum value, the dynamic-dissipation curves become independent of the experimental parameters. In particular, for long-range dissipative processes we have derived an analytical relationship that shows the independence of the normalized dynamic-dissipation curves with respect the free amplitude, cantilever constant or quality factor.

Observation of Redox-State-Dependent Reversible Local Structural Change of Ferrocenyl-Terminated Molecular Island by Electrochemical Frequency Modulation AFM

Electroactive ferrocenylundecanethiol (FcC(11)H(22)SH) islands embedded in an n-decanethiol (C(10)H(21)SH) self-assembled monolayer (SAM) matrix on Au(111) were studied under potential control in 0.1 M HClO(4) aqueous solution using newly developed electrochemical frequency-modulation atomic force microscopy (EC-FM-AFM). The apparent height of the Fc islands from the surface of the matrix SAM increased by about 0.44 nm accompanied by the oxidation of the terminal Fc groups. This potential-dependent reversible change can be explained by formation of an ionic double layer where ClO(4)(-) ions are strongly bound on the Fc(+) groups. Simultaneous measurements of energy dissipation, which corresponds to the energy to keep the cantilever's vibrational amplitude constant, revealed distinct change in the magnitude by the oxidation state of the Fc groups. These results indicate that localized charge at an electrode/electrolyte solution interface can be identified, and microscopic information on the electric double layer at the interface is available by using EC-FM-AFM.