Conventional Tapping Mode Research Articles

Atomic resolution with high-eigenmode tapping mode atomic force microscopy

Atomic surface structure imaging is instrumental for the understanding of surface-related phenomena. Here, we show that conventional tapping mode atomic force microscopy with high cantilever eigenmodes and subnanometer amplitudes allow routine atomic imaging at atmospheric pressures. We identify the reasons for failure of atomic resolution imaging employing low eigenmodes. Strong tip-surface interactions cause significant differences between the oscillatory behaviors of the inclination of the cantilever as detected by conventional instruments and of the vertical position of the tip, which prevents correct functioning of instrumental feedback control loops. However, high effective spring constants of high eigenmodes make it possible to overcome the problem. Furthermore, the combination of high effective elastic constants of high cantilever eigenmodes with the high flexibility of the cantilever substantially enhances the imaging stability, thereby universally allowing atomic imaging of solid surfaces in gaseous environments and at elevated temperatures. Demonstrated imaging examples include single sulfur vacancies at the surface of ${\mathrm{MoS}}_{2}$ crystals imaged at temperatures ranging from room temperature to 250\ifmmode^\circ\else\textdegree\fi{}C and potassium ions on hydrophilic and highly adhesive muscovite mica surfaces. Moreover, the high imaging stability allows knocking atoms off the ${\mathrm{MoS}}_{2}$ surface by hard tapping, indicating the potential for ultrahigh resolution lithography.

Chaos: The speed limiting phenomenon in dynamic atomic force microscopy

This paper investigates the closed-loop dynamics of the Tapping Mode Atomic Force Microscopy using a new mathematical model based on the averaging method in Cartesian coordinates. Experimental and numerical observations show that the emergence of chaos in conventional tapping mode AFM strictly limits the imaging speed. We show that, if the controller of AFM is tuned to be faster than a certain threshold, the closed-loop system exhibits a chaotic behavior. The presence of chaos in the closed-loop dynamics is confirmed via bifurcation diagrams, Poincaré sections, and Lyapunov exponents. Unlike the previously detected chaos due to attractive forces in the AFM, which can be circumvented via simple changes in operation parameters, this newly identified chaos is seemingly inevitable and imposes an upper limit for the closed-loop bandwidth of the AFM.

Nematode Epicuticle Visualisation by PeakForce Tapping Atomic Force Microscopy.

The free-living soil nematode Caenorhabditis elegans has become an iconic experimental model animal in biology. This transparent animal can be easily imaged using optical microscopy to visualise its organs, tissues, single cells and subcellular events. The epicuticle of C. elegans nematodes has been studied at nanoscale using transmission and scanning (SEM) electron microscopies. As a result, imaging artefacts can appear due to embedding the worms into resins or coating the worms with a conductive gold layer. In addition, fixation and contrasting may also damage the cuticle. Conventional tapping mode atomic force microscopy (AFM) can be applied to image the cuticle of the dried nematodes in air, however this approach also suffers from imaging defects. Ideally, the nematodes should be imaged under conditions resembling their natural environment. Recently, we reported the use of PeakForce Tapping AFM mode for the successful visualisation and numerical characterisation of C. elegans nematode cuticle both in air and in liquid ( Fakhrullina et al., 2017 ). We imaged the principal nematode surface structures and characterised mechanical properties of the cuticle. This protocol provides the detailed description of AFM imaging of liquid-immersed C. elegans nematodes using PeakForce Tapping atomic force microscopy.

Fast Stiffness Mapping of Cells Using High-Bandwidth Atomic Force Microscopy.

The cytoskeleton controls cellular morphology and mediates the mechanical interactions between a cell and its environment. Atomic force microscopy (AFM) has the unique capability to map cytoskeletal mechanics and structures with nanometer resolution. However, whole-cell cytomechanical imaging with conventional AFM techniques is limited by low imaging speed. Here, we present fast nanomechanical mapping of cells using high-bandwidth AFM (HB-AFM), where >10(6) nanoindentation measurements were acquired in ∼10 min-a task that would take weeks to finish using conventional AFM. High-bandwidth measurements enabled capture of the entire tip-sample interaction for each tap on cells, engendering a new measurement ("force phase") that exceeds the contrast of conventional tapping mode and enabling spectral visualization of >10 harmonics. The abundance of measurements allowed discovery of subtle cytomechanical features, including the stiffness of fibers of the cellular spectrin network in situ. This approach bridges HB-AFM and high-harmonic imaging and opens future opportunities for measuring the dynamic mechanical properties of living cells.

An approach towards 3D sensitive AFM cantilevers

The atomic force microscope (AFM) tapping mode is a highly sensitive local probing technique that is very useful to study and measure surface properties down to the atomic scale. The tapping mode is mostly implemented using the resonance of the first bending mode of the cantilever and therefore provides sensitivity mainly along the direction of this oscillation. Driven by the semiconductor industry, there is an increasing need for accurate measurements of nanoscale structures for side wall characterization by AFM that requires additional sensitivity in the lateral direction. The conventional tapping mode has been augmented by various authors, for example by tilting the cantilever system (Cho et al 2011 Rev. Sci. Instrum. 82 023707) to access the sidewall or using a torsion mode (Dai et al 2011 Meas. Sci. Technol. 22 094009) of the cantilever to provide additional lateral sensitivity. These approaches however trade lateral sensitivity for vertical sensitivity or still lack sensitivity in the remaining lateral direction. We present an approach towards true 3D sensitivity for AFM cantilevers based on simultaneous excitation and optical detection of multiple cantilever resonance modes along three axes. Tuning the excitation of the cantilever to specific frequencies provides a mechanism to select only those cantilever modes that have the desired characteristics. Additionally, cantilever engineering has been used to design and create a substructure within the cantilever that has been optimized for specific resonance behavior around 4 MHz. In contrast to the conventional approach of using a piezo to actuate the cantilever modulation, we present results on photo-thermal excitation using an intensity modulated low-power laser source. By tightly focusing the excitation spot on the cantilever we were able to attain a deflection efficiency of 0.7 nm µW−1 for the first bending mode. The presented approach results in an efficient all optical excitation and deflection detection scheme where both the position of the spot to excite the cantilever and the spot position of the read-out beam provide additional parameters to fully control and optimize the multi-mode structure required for 3D AFM measurements.

Analysis of the contrast mechanism in bimodal atomic force microscopy combining amplitude modulation and band excitation

A new bimodal method of scanning probe microscopy was recently introduced in which one cantilever eigenmode is driven using the conventional tapping mode (amplitude modulation) technique while another one is driven using a broadband excitation signal, such that its transfer function can be measured throughout the full spectrum and used to infer material properties from changes in the resonance frequency and quality factor as the surface is scanned. This paper explores computationally the fundamental mechanisms underlying the observed contrast as well as the most relevant aspects of the cantilever dynamics. The objective is to place the new multifrequency method in the context of previous work in the field, describing in detail the type of information it provides, as well as the conditions that lead to meaningful measurements or measurements affected by artifacts.

Self-Sensing and –Actuating Probes for Tapping Mode AFM Measurements of Soft Polymers at a Wide Range of Temperatures

Self-sensing and –actuating probes optimized for conventional tapping mode atomic force microscopy (AFM) are described. 32-kHz quartz tuning forks with a chemically etched and focus ion beam (FIB) sharpened (curvature radii are 5-10 nm) tungsten tip are stable at air and liquid nitrogen atmosphere and at a wide range of temperatures. If driven at constant frequency, the scan speed of such sensors can be up to 3 Hz. AFM was performed on polymer samples in order to study the stability and applicability of these sensor for investigation of soft materials with high dynamical tendencies.

Micro-particle corrugation, adhesion and inhalation aerosol efficiency

Atomic force microscopy (AFM) was used to evaluate the particle adhesion and surface morphology of engineered particles for dry powder inhaler (DPI) respiratory therapy to gain a greater understanding of interparticle forces and the aerosolisation process. A series of spherical model drug particles of bovine serum albumin (BSA) was prepared with different degrees of surface corrugation. The particles were evaluated in terms of particle size (laser diffraction) and microscopic morphology (scanning electron microscopy). Conventional tapping mode AFM was used to evaluate the nanoscopic morphology and derive specific roughness parameters, while AFM colloid probe microscopy was used to directly measure the interaction of functionalised probes. The physical characterisation and AFM measurements were evaluated in terms of in vitro aerosolisation performance, using a conventional Rotahaler ® DPI and multistage liquid impinger. A direct relationship between the root mean square roughness, particle adhesion and in vitro aerosol performance (measured as fine particle fraction, FPF) was observed suggesting that as the degree of corrugation increased, particle adhesion was reduced which, resulted in a concomitant increase in FPF. This study demonstrates that AFM may be used to predict the aerosolisation performance micron sized particles for inhalation based on their morphological properties.

Feed-forward compensation of surface potential in atomic force microscopy

We introduce a feed-forward technique for lift-mode Kelvin probe force microscopy to minimize electrostatically induced errors in topography scans. Such errors typically occur when a grounded tip is scanned over a heterogeneous sample with differences in local work function or areas of local surface charging. To minimize electrostatic forces during the topography scan we bias the tip with the surface potential recorded in the previous Kelvin probe scan line. With this method we achieve an error free topography on samples with large variations in local surface potential. Compared to conventional tapping mode, we further observe a significant reduction of tip-induced surface charge modifications on charge patterns written in electret films.

Theory of phase-modulation atomic force microscopy with constant-oscillation amplitude

The recently introduced phase-modulation mode with constant oscillation amplitude is analyzed and compared with the conventional amplitude-modulation technique. We show that instabilities in the amplitude and phase versus distance curves are prevented by the additional feedback controlling the oscillation amplitude. Moreover, we demonstrate that the phase-modulation technique is capable to measure conservative and dissipative tip-sample forces by dynamic force spectroscopy without any jumps caused by instabilities. This feature is an important advantage compared to a spectroscopy technique based on the conventional tapping mode.

Nanoscale surface property estimation using proper orthogonal decomposition in atomic force microscopy

Atomic force microscopy (AFM) in a dynamic mode operation uses a resonating tip to measure the nanoscale surface topography and other properties. The dynamic response of the tip includes the complex tip–surface interactions due to the surface properties. We found that conventional tapping mode had a limitation in the accurate set-point control on soft and high adhesion surfaces. This study employed the proper orthogonal decomposition (POD)-based AFM microcantilever characterization method to estimate the surface property with more reliable control. The POD extracted the dominant empirical modes of the AFM microcantilever during tapping and scanning on the surface. Also the corresponding eigenvalues represented the significance of the empirical modes and the characteristic features of the nanoscale surface property. This experimental approach can offer a new insight on the novel application of nanoscale surface property estimation in dynamic AFM applications.

Theory of Q-Controlled dynamic force microscopy in air

The theory of dynamic force microscopy in air is developed with respect to the application of the Q-Control technique, which allows to increase or decrease the effective Q factor of the cantilever via an active external feedback. Analytical as well as numerical approaches are applied to solve the equation of motion describing the cantilever dynamics with and without Q-Control in the presence of a model tip-sample interaction force. Based on this analysis, the characteristics of Q-Controlled dynamic force microscopy are compared to conventional dynamic force microscopy carried out in amplitude modulation mode without active Q-Control (“tapping mode”). In the case of negligible tip-sample interaction (i.e., with the tip “far” from the surface), the theory describes how Q-Control alters the shape of the resonance curves of the cantilever by modifying the effective Q factor and shifting the resonance peak. Explicit consideration of tip-sample forces then permits insight into the imaging properties of Q-Controlled dynamic force microscopy. In particular, it is found that an increased Q factor prevents the oscillating cantilever to jump into a repulsive imaging regime during tip-sample approach, which often occurs during conventional tapping mode imaging in air. The analysis reveals in detail which parts of the tip-sample force curve are contributing to the contrast formation for the different imaging conditions if plotted as a function of the tip-sample distance. Based on these findings, we conclude that the restriction of the maximal tip-sample force to specific parts of the attractive regime, which is triggered by the activation of the Q-Control feedback, is the main reason for the enhanced imaging quality reported in several experimental studies if compared to conventional tapping mode imaging without Q-Control.

폴리머 표면측정을 위한 AFM 팁의 나노스케일 접촉-진동 해석

In tapping mode atomic force microscopy(TM-AFM), the vibro-contact response of a resonating tip is used to measure the nanoscale topology and other properties of a sample surface. However, the nonlinear tipsurface interactions can affect the tip response and destabilize the tapping mode control. Especially it is difficult to obtain a good scanned image of high adhesion surfaces such as polymers and biomolecules using conventional tapping mode control. In this study, theoretical and experimental investigations are made on the nonlinear dynamics and control of TM-AFM. Also we report the surface adhesion is an additional important parameter to determine the control stability of TM-AFM. In addition, we proved that it was adequate to use Johnson-Kendall-Roberts (JKR) contact model to obtain a reasonable tapping response in AFM for the soft and high adhesion samples.

Dynamics and Control of Tapping Tip in Atomic Force Microscope for Surface Measurement Applications

In tapping mode atomic force microscopy (TM-AFM), the vibro-contact response of a resonating tip is used to measure the nanoscale topology and other properties of a sample surface. However, the nonlinear tip-surface interactions can affect the tip response and destabilize the tapping mode control. Especially it is difficult to obtain a good scanned image of high adhesion surfaces such as polymers and biomolecules using conventional tapping mode control. In this study, theoretical and experimental investigations are made on the nonlinear dynamics and control of TM-AFM. Also we report the surface adhesion is an additional important parameter to determine the control stability of TM-AFM. In addition, we proved that it was adequate for the soft and high adhesion sample to be modeled with JKR contact to obtain a reasonable tapping response in AFM.

Height measurement of DNA molecules with lift mode AFM

The height of double-stranded DNA (dsDNA) is measured by lift mode AFM combined with conventional tapping mode AFM. While the tip scan height is raised step by step, the tip pressure on sample is decreased gradually. As a result, the deformation of the DNA strands decreases, and the height of double-stranded DNA (dsDNA) molecule can be deduced by the tip lift height. The measured height of dsDNA is 1.5± 0.2 nm in lift mode, but only 0.8±0.2 nm in conventional tapping mode. This demonstrates that the tip pressure is a key factor in soft sample height measurement resulting in artificating lower values via conventional tapping mode.

High- Q Dynamic Force Microscopy in Liquid and Its Application to Living Cells

We present a new dynamic force microscopy technique for imaging in liquids in the piconewton regime. The low quality factor ( Q) of the cantilever is increased up to three orders of magnitude by the implementation of a positive feedback control. The technique also includes a phase-locked loop unit to track the resonance of the cantilever. Experiments and computer simulations indicate that the tip-sample forces are below 100 pN, about two orders of magnitude lower than in conventional tapping mode atomic force microscopy. Furthermore, the spectroscopic ability is greatly enhanced. Either the phase shift or the resonant frequency shows a high sensitivity to variations in either the energy dissipation or conservative interactions between the tip and the sample, respectively. The potential of this technique is demonstrated by imaging living cells.

Tapping mode scanning force microscopy in water using a carbon nanotube probe

An improved technique for obtaining tapping mode scanning force microscopy (TMSFM) images of soft samples submerged in water is described. This technique makes use of a carbon nanotube several microns in length mounted on a conventional silicon cantilever as the TMSFM probe. The sample is covered by a shallow water layer and during imaging only a portion of the nanotube is submerged. This mode of operation largely eliminates the undesirable effects of hydrodynamic damping and acoustic excitation that are present during conventional tapping mode operation in liquids and leads to high-quality TMSFM images. Because of their low bending force constants, carbon nanotubes are ideal for gentle imaging of soft samples. Because of their small (5–20 nm) diameter and cylindrical shape they provide excellent lateral resolution and are ideal for scanning high aspect ratio objects.