Dynamic Mode Atomic Force Microscopy Research Articles

High-Resolution Imaging of Plasmid DNA in Liquids in Dynamic Mode Atomic Force Microscopy Using a Carbon Nanofiber Tip

To understand the motion of DNA and DNA complexes, the real-time visualization of living DNA in liquids is quite important. Here, we report the high-resolution imaging of plasmid DNA in water using a rapid-scan atomic force microscopy (AFM) system equipped with a carbon nanofiber (CNF) probe. To achieve a rapid high-resolution scan, small SiN cantilevers with dimensions of 2 (width) × 0.1 (thickness) × 9 µm (length) and a bent end (tip view structure) were employed as base cantilevers onto which single CNFs were grown. The resonant frequencies of the cantilever were 1.5 MHz in air and 500 kHz in water, and the spring constant was calculated to be 0.1 N/m. Single CNFs, typically 88 nm in length, were formed on an array of the cantilevers in a batch process by the ion-irradiation method. An AFM image of a plasmid DNA taken in water at 0.2 fps (5 s/image) using a batch-fabricated CNF-tipped cantilever clearly showed the helix turns of the double strand DNA. The average helical pitch measured 3.4 nm (σ: 0.5 nm), which was in good agreement with that determined by the X-ray diffraction method, 3.4 nm. Thus, it is presumed that the combined use of the rapid-scan AFM system with the ion-induced CNF probe is promising for the dynamic analysis of biomolecules.

Nanoscale liquid droplet deposition using the ultrasmall aperture on a dynamic mode AFMtip

We have developed a novel method for nanometer-sized droplet deposition of electricallyconductive liquids based on the dynamic mode atomic force microscopy (AFM). Wesucceeded in depositing liquid droplets with a volume of ten zeptoliters or smaller and inmaking arrays of the droplets with a spacing of several tens of nanometers using the AFMcantilever with a small aperture fabricated by a focused ion beam. The liquid droplets weredeposited through the aperture by applying an electric field between the liquid and aconductive surface.

Cantilever Tilt Causing Amplitude Related Convolution in Dynamic Mode Atomic Force Microscopy

It is well known that the topography in atomic force microscopy (AFM) is a convolution of the tip's shape and the sample's geometry. The classical convolution model was established in contact mode assuming a static probe, but it is no longer valid in dynamic mode AFM. It is still not well understood whether or how the vibration of the probe in dynamic mode affects the convolution. Such ignorance complicates the interpretation of the topography. Here we propose a convolution model for dynamic mode by taking into account the typical design of the cantilever tilt in AFMs, which leads to a different convolution from that in contact mode. Our model indicates that the cantilever tilt results in a dynamic convolution affected by the absolute value of the amplitude, especially in the case that corresponding contact convolution has sharp edges beyond certain angle. The effect was experimentally demonstrated by a perpendicular SiO(2)/Si super-lattice structure. Our model is useful for quantitative characterizations in dynamic mode, especially in probe characterization and critical dimension measurements.

Channel Modeling and Detector Design for Dynamic Mode High Density Probe Storage and Nano-imaging Applications

AbstractIn this paper, we provide a consolidated summary of the work reported in [1], [2], [3], [4] which is based on probe based high density storage and nano-imaging using atomic force microscopy (AFM) setup. In probe based high density storage, we consider a data storage system that operates by encoding information as topographic profiles on a polymer medium. A cantilever probe with a sharp tip (few nm radius) is used to create and sense the presence of topographic profiles, resulting in a density of few Tb per in.2. The high quality factor dynamic mode operation, that is less harsh on the media and the probe, is analyzed. The read operation is modeled as a communication channel which incorporates system memory due to inter-symbol interference and the cantilever state. Next, the solutions to the maximum likelihood sequence detection problem and maximum a posterior (MAP) symbol detection problem based on the Viterbi algorithm and BCJR algorithm respectively are devised. Experimental results demonstrate that the performance of the Viterbi and BCJR detectors are several orders of magnitude better than the performance of other existing schemes.Nano-imaging has played a vital role in basic sciences as it enables interrogation of material with sub-nanometer resolution. In this paper, we also present a high speed one-bit imaging technique using dynamic mode AFM with a high quality factor cantilever. Experimental results demonstrate that our proposed algorithm provides significantly better image resolution compared to current nano-imaging techniques at high scanning speed.

Development of High-Resolution Imaging of Solid–Liquid Interface by Frequency Modulation Atomic Force Microscopy

The high-resolution imaging technique used in liquid environments involving dynamic mode atomic force microscopy (AFM) with a frequency modulation (FM) detection technique has been newly developed on the basis of a commercial atomic force microscopy (AFM) apparatus, which is, generally, extremely difficult because of the large decrease in Q-factor caused by hydrodynamic damping in liquids. Through various improvements and optimization, the noise density of the improved deflection sensor was 29 fm/√Hz (f0 = 141 kHz) in liquid. In addition, the noise level and bandwidth of the FM detector were improved to 6 mHz/√Hz and 6 kHz, respectively. Thermal drift was successfully regulated at less than 1 nm/min in air for a sufficiently long time without any special air conditioning treatments. As a result, we succeeded in obtaining high-resolution images of polypropylene sheet, Au thin film, and DNA structures in a buffer solution. Therefore, liquid environments are proved to be suitable for high-resolution imaging by FM-AFM under ultrahigh vacuum (UHV) condition.

Implementation and characterization of a quartz tuning fork based probe consisted of discrete resonators for dynamic mode atomic force microscopy

The quartz tuning fork based probe {e.g., Akiyama et al. [Appl. Surf. Sci. 210, 18 (2003)]}, termed "A-Probe," is a self-sensing and self-actuating (exciting) probe for dynamic mode atomic force microscope (AFM) operation. It is an oscillatory force sensor consisting of the two discrete resonators. This paper presents the investigations on an improved A-Probe: its batch fabrication and assembly, mounting on an AFM head, electrical setup, characterization, and AFM imaging. The fundamental features of the A-Probe are electrically and optically characterized in "approach-withdraw" experiments. Further investigations include the frequency response of an A-Probe to small mechanical vibrations externally applied to the tip and the effective loading force yielding between the tip and the sample during the periodic contact. Imaging of an electronic chip, a compact disk stamper, carbon nanotubes, and Si beads is demonstrated with this probe at ambient conditions in the so-called frequency modulation mode. A special probe substrate, which can snap on a receptacle fixed on an AFM head, and a special holder including a preamplifier electronic are introduced. We hope that the implementation and characterization of the A-Probe described in this paper will provide hints for new scanning probe techniques.

Analysis and control of micro-cantilever in dynamic mode AFM

The dynamic behavior of micro-cantilever-sample system in atomic force microscopy (AFM) in tapping operational mode is studied. Micro-cantilever is modeled by single-mode approximation and subjected to sinusoidal force input. The interaction between tip and sample is modeled by Lennard–Jones potential. Using this dynamic model, the phase portrait of the system is obtained for a range of cantilever-sample distance, and the effect of cantilever parameters on predictability of the system is discussed. Then the nonlinear behavior of the cantilever-sample system is controlled by a robust sliding-mode controller. System output is designed to track the sinusoidal stimulation of the cantilever base under sample surface perturbation during scanning process. Results of SMC are simulated in MATLAB and compared to a stability that has been proved by PID controller which shows better controller effort and less cantilever tip position accuracy.

A new sample-profile estimation signal in dynamic-mode atomic force microscopy

In this paper, a design scheme is proposed that separates the issues of sample-profile estimation and amplitude regulation in dynamic-mode atomic force microscopy. In current AFM, the control signal for amplitude regulation is also used as the estimate for the sample-profile. Therefore, the sample profile estimation signal is accurate as long as the sample-profile signal perceived by the cantilever is well within the bandwidth of the control transfer function. In the proposed design scheme, maintaining a constant amplitude while scanning at high bandwidth does not impose limitations on the reconstruction of the sample topography. In fact, we analytically prove that the sample-profile signal estimation problem can be solved independently of the control design scheme for amplitude regulation. Therefore, accurate sample-profile estimations can be obtained even at frequencies near and beyond the closed-loop control bandwidths. However, we show that the robustness of estimation does depend on the control design for regulation and in fact, the robustness of estimation is described by the closed-loop sensitivity transfer function. The independence of the profile-estimation problem from the control design is another salient distinguishing characteristic of this work. The estimation bandwidths by this new scheme are improved significantly over commonly used signals. Comparison with the existing methods of using the control signal as the image is provided. The experimental results corroborate the theoretical development.

Spurious-free cantilever excitation in liquid by piezoactuator with flexure drive mechanism

We have developed a cantilever holder for spurious-free cantilever excitation in liquid by piezoactuator. In the holder, generation and propagation of an acoustic wave are suppressed by "acoustic barriers," i.e., boundaries between two materials having significantly different acoustic impedance while cantilever vibration is excited by "flexure drive mechanism" utilizing elastic deformation of a flexure hinge made of a material having a low elastic modulus. The holder enables to obtain amplitude and phase curves without spurious peaks in liquid using a piezoactuator, which ensures stability and accuracy of dynamic-mode atomic force microscopy in liquid.

T1601-1-2 ファンデルポール型自励発振方式FM-AFMによる液中観察(マイクロナノダイナミクスの計測と制御・マイクロナノメカトロニクス(1))

Atomic force microscopy (AFM) is recognized as indispensable for micro-electromechanical systems (MEMS), nanotechnology, and nano-biotechnology. Although observing bio-related samples in a liquid environment using AFM is increasingly important, deformable, uneven, and easily damaged surfaces of biological specimens require establishment of non-contact AFM observation. Contact mode and dynamic mode are AFM's two measurement methods. The latter reproduces the surface shape from variation appearing in the vibrating micro-cantilever's resonance, depending on change in the atomic force acting between the probe and the sample surface during measurement. Because soft irregular bumpy surfaces of biogenic samples are easily damaged, contact-mode AFM is unsuitable for samples of biological origin. Instead, non-contact observation using dynamic-mode AFM must be established. Toward observation of nanometer-scale biological samples in a liquid, this paper reports frequency-modulation atomic force microscopy (FM-AFM). It can avoid both collisions by the probe-cantilever with the sample surface and stops of oscillation by vibrating the probe-cantilever similarly to van der Pol-type self-excited oscillation.

Tunable optical properties of laser grown double‐structures with gold nanoparticlesand zinc oxide thin films

AbstractGold nanoparticles on zinc oxide thin films surfaces have been synthesized by means of two step pulsed laser deposition. Zinc and gold targets were subsequently submitted to pulses generated by an UV KrF* (λ = 248 nm, τFWHM ≅ 20 ns, ν = 2 Hz) excimer, as well as a frequency tripled Nd:YAG (λ = 355 nm, τ ∼ 10 ns) laser sources. The obtained structures surface morphology, crystalline quality, and chemical composition depth profile were investigated by acoustic (dynamic) mode atomic force microscopy, X‐ray diffraction, and wavelength dispersive X‐ray spectroscopy. The corresponding absorption and reflection spectra were recorded with the aid of a double beam spectrophotometer. The results proved the possibility to tailor the gold/zinc oxide nanostructures optical properties through the proper choice of the growth parameters, i.e. number of laser pulses used for the ablation of the gold targets as well as nature of the substrate material. The tunable optical features in the visible‐near infrared spectral region allow for the design of gold/zinc oxide nanostructures with pre‐defined optical characteristics. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Geometric Characterization of Carbon Nanotubes by Atomic Force Microscopy in Conjunction with a Tip Characterizer

An atomic force microscopy (AFM) probe tip characterizer with 14 line and space structures and two knife edges was fabricated by means of a superlattice technique. The shape of a probe tip both before and after AFM imaging was acquired by this tip characterizer with general variations <1.5 nm; depending on imaging conditions. The geometric structures of carbon nanotubes (CNTs) on a SiO2 substrate were studied by dynamic mode AFM in conjunction with this tip characterizer. Contact points between the tip and the CNTs were detected by observing changes in the AFM phase images. A modified CNT width correction model was established to calculate the estimated and upper-limit widths of two CNTs. The experimental results showed that imaging under a weak attractive force was suitable for obtaining accurate CNT height measurements, whereas a weak repulsive force provided the most accurate widths. Differing heights and widths between the two CNTs suggested that one CNT was double-walled, whereas the other had more than two walls; these results agree with transmission electron microscopy (TEM) measurements of the CNTs.

Hysteresis models of dynamic mode atomic force microscopes: analysis and identification via harmonic balance

A new class of models based on hysteresis functions is developed to describe the operation of dynamic mode atomic force microscopy. Such models can account for dissipative phenomena affecting the interaction between the probe and the sample. The model analysis, which is developed using frequency domain techniques, provides a insights into experimentally observed behavior. Experimental data corroborates the models developed.

Fabrication of 3D micro-cantilevers based on MBE-grown strained semiconductor layers

Strain-driven bending process proposed for the fabrication of small and precise cantilevers is reported. The process starts from the multi-layer structure grown by molecular beam epitaxy (MBE). The cantilevers consist of the two parts imitating the beam and the tip, each of which has different bending direction and radius. The shape determined by location of strained layer in multi-layer structure controlled with layer design and fabrication process including selective etching technique. We demonstrated cantilevers having a rolling up beam with 5 – 20 μ m length and a rolling down thinner tip with 3 μ m length. However the tips showed tortuous shape. The complex shape might originate from inhomogeneity dissolving of extra compounds generated in sacrifice layer etching process. Moreover, we measured resonant frequencies of the fabricated cantilevers by optical lever method. We obtained high resonant frequency of 2 MHz for the cantilever with 5 μ m length, 1 μ m width, and 100 nm thickness. This frequency seems suitable for video-rate scanning in dynamic mode atomic force microscopy (AFM). These results suggest that the strain-driven bending process is applicable to three-dimensional (3D) micro-cantilevers for video-rate scanning in dynamic AFM.

Suppression of spurious vibration of cantilever in atomic force microscopy by enhancement of bending rigidity of cantilever chip substrate

To improve the precision of dynamic atomic force microscopy (AFM) using cantilever vibration spectra, a simple but effective method for suppressing spurious response (SR) was developed. The dominant origin of SR was identified to be the bending vibration of the cantilever substrate, by the analysis of the frequency of SR. Although a rigid cover pressing the whole surface of the substrate suppressed SR, the utility was insufficient. Then, a method of enhancing the bending rigidity of the substrate by gluing a rigid plate (clamping plate, CP) to the substrate was developed. This chip can be used with an ordinary cantilever holder, so that the reproducibility of SR suppression when attaching and detaching the cantilever chip to the holder was improved. To verify its utility, the evaluation of a microdevice electrode was performed by ultrasonic atomic force microscopy. The delamination at a submicron depth was visualized and the detailed variation of the delamination was evaluated for the first time using clear resonance spectra. The CP method will particularly contribute to improving dynamic-mode AFM, in which resonance spectra with a low quality factor are used, such as noncontact mode AFM in liquid or contact resonance mode AFM. The effect of the CP can be achieved by fabricating a substrate with a thick plate beforehand.

Direct mechanical measurement of geodesic structures in rat mesenchymal stem cells

During numerous biological processes, cell adhesion, cell migration and cell spreading are vital. These basic biological functions are regulated by the interaction of cells with their extracellular environment. To examine the morphology and mechanical changes occurring in mesenchymal stem cells cultured on a mechanically rigid substrate, atomic force microscopy and fluorescence microscopy were employed. Investigations of the cells revealed both linear and geodesic F-actin configurations. No particular cell characteristics or intra-cellular location were implicated in the appearance of the geodesic structures. However, the length of time the cells were cultured on the substrate correlated with the percentage appearance of the geodesic structures. Calculating energy dissipation from cell images acquired by dynamic mode atomic force microscopy, it was observed that the vertices of the geodesic structures had significantly higher energy dissipation compared to the linear F-actin and the glass. This supports work by Lazarides [J. Cell Biol. 68, 202-219 (1976)], who postulated that the vertices of these geodesic structures should have a greater flexibility. Our results also support predictions based on the microfilament tensegrity model. By understanding the basic principles of cell ultrastructure and cell mechanics in relation to different extracellular environments, a better understanding of physiological and pathological process will be elicited.

A Review of the Systems Approach to the Analysis of Dynamic-Mode Atomic Force Microscopy

The atomic force microscope (AFM) is one of the foremost tools for imaging, measuring and manipulating matter at the nanoscale. This brief presents a review of the systems and control approach to analyzing the challenging dynamic-mode operation of the AFM. A Lure system perspective of the AFM dynamics facilitates the application of powerful tools from systems theory for the analysis. The harmonic balance method provides significant insights into the steady-state behavior as well as a framework for identifying the tip-sample interaction force. A simple piecewise-linear tip-sample interaction model and its identification using the harmonic balance method is presented. The dominant first harmonic is analyzed using multivalued frequency responses and the corresponding stability conditions. The ability of the simple tip-sample interaction model to capture the intricate nonlinear behavior of the first harmonic is demonstrated. This also points to the importance of studying the higher harmonics to obtain finer details of the tip-sample interaction. The suitability of the Lure system perspective for the analysis of the higher harmonics is demonstrated.

Effects of Nonlinear Forces on Dynamic Mode Atomic Force Microscopy and Spectroscopy

In this paper, we describe the effects of nonlinear tip-sample forces on dynamic mode atomic force microscopy and spectroscopy. The jumps and hysteresis observed in the vibration amplitude (A) versus tip-sample distance (h) curves have been traced to bistability in the resonance curve. A numerical analysis of the basic dynamic equation was used to explain the hysteresis in the experimental curve. It has been found that the location of the hysteresis in the A-h curve depends on the frequency of the forced oscillation relative to the natural frequency of the cantilever.

Unstable amplitude and noisy image induced by tip contamination in dynamic force mode atomic force microscopy

Liquid 1-decanethiol was confined on an atomic force microscope (AFM) tip apex and the effect was investigated by measuring amplitude-distance curves in dynamic force mode. Within the working distance in the dynamic force mode AFM, the thiol showed strong interactions bridging between a gold-coated probe tip and a gold-coated Si substrate, resulting in unstable amplitude and noisy AFM images. We show that under such a situation, the amplitude change is dominated by the extra forces induced by the active material loaded on the tip apex, overwhelming the amplitude change caused by the geometry of the sample surface, thus resulting in noise in the image the tip collects. We also show that such a contaminant may be removed from the apex by pushing the tip into a material soft enough to avoid damage to the tip.

Tunnelling charge injection into a pentacene layer using dynamic-mode scanning forcemicroscopy

Conductive atomic force microscopy has been utilized to study the charge injectionphenomenon on various materials. We proposed a gentle method for local charge injectionand observation based on self-excitation dynamic-mode atomic force microscopy, whichinvolves switching between amplitude modulation mode for tunnelling charge injection andfrequency modulation modes for surface potential measurement. We demonstrated that themethod was effective in investigating charge injection and dissipation processes withoutserious degradation of the tip and samples. To examine the method we measured goldnanoparticles on an insulator and an isolated pentacene island on a substrate.