Frequency Modulation Atomic Force Microscopy Research Articles

Black-Dye-Adsorbed TiO2(110) Electrodes Studied by Frequency-Modulation Atomic Force Microscopy

Single-crystalline TiO2(110) wafers were modified with black dye (BD) to simulate dye-sensitized, solar cell electrodes. The surface of the modified wafers was observed with a frequency-modulation atomic force microscopy in vacuum. Adsorbed BD was identified in a constant-frequency-shift topography. Mechanical energy that dissipated from the oscillating cantilever to the surface was enhanced in the presence of BD.

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.

Accurate formula for conversion of tunneling current in dynamic atomic force spectroscopy

Recent developments in frequency modulation atomic force microscopy enable simultaneous measurement of frequency shift and time-averaged tunneling current. Determination of the interaction force is facilitated using an analytical formula, valid for arbitrary oscillation amplitudes [Sader and Jarvis, Appl. Phys. Lett. 84, 1801 (2004)]. Here we present the complementary formula for evaluation of the instantaneous tunneling current from the time-averaged tunneling current. This simple and accurate formula is valid for any oscillation amplitude and current law. The resulting theoretical framework allows for simultaneous measurement of the instantaneous tunneling current and interaction force in dynamic atomic force microscopy.

Digitally tunable, wide-band amplitude, phase, and frequency detection for atomic-resolution scanning force microscopy

Frequency-modulation atomic force microscopy (FM-AFM) relies on an accurate tracking of the resonance frequency of a scanning probe. It is now used in environments ranging from ultrahigh vacuum to aqueous solutions, for slow and for fast imaging, with probes resonating from a few kilohertz up to several megahertz. Here we present a versatile experimental setup that detects amplitude, phase, and frequency of AFM probes for resonance frequencies up to 15 MHz and with >70 kHz maximum bandwidth for amplitude/phase detection. We provide generic parameter settings for variable-bandwidth frequency detection and test these using our setup. The signal-to-noise ratio of the frequency detector is sufficiently high to record atomic-resolution images of mica by FM-AFM in aqueous solution.

Noncontact microrheology at acoustic frequencies using frequency-modulated atomic force microscopy

We report an atomic force microscopy (AFM) method for assessing elastic and viscous properties of soft samples at acoustic frequencies under non-contact conditions. The method can be used to measure material properties via frequency modulation and is based on hydrodynamics theory of thin gaps we developed here. A cantilever with an attached microsphere is forced to oscillate tens of nanometers above a sample. The elastic modulus and viscosity of the sample are estimated by measuring the frequency-dependence of the phase lag between the oscillating microsphere and the driving piezo at various heights above the sample. This method features an effective area of pyramidal tips used in contact AFM but with only piconewton applied forces. Using this method, we analyzed polyacrylamide gels of different stiffness and assessed graded mechanical properties of guinea pig tectorial membrane. The technique enables the study of microrheology of biological tissues that produce or detect sound.

Potential dependent change in local structure of ferrocenyl-terminated molecular islands by electrochemical frequency modulation atomic force microscopy

Electroactive ferrocenylundecanethiol (FcC11H22SH) islands embedded in a n-heptanethiol (C7H15SH) self-assembled monolayer matrix on Au(111) were studied under potential control in 0.1M HClO4 aqueous solution using electrochemical frequency modulation atomic force microscopy. Signal of local structural change on the Fc islands upon oxidation from Fc0 to Fc+ was obtained in topography as a reversible process; however, the magnitude of the change was far less than expected in contrast to the case of the previous study for FcC11H22SH islands embedded in C10H21SH. Different degrees of freedom for the structures of the Fc islands originating from different molecular lengths for surrounding matrix molecules perhaps provide the distinct contrast between the two systems.

Frequency modulation atomic force microscope observation of TiO2(110) surfaces in water

Rutile titanium dioxide (TiO2) (110) surfaces were examined in water using a frequency modulation atomic force microscope. On the surfaces cleaned by Ar+ sputtering and annealing in ultrahigh vacuum, step-terrace structure was observed. The inlets at the steps and the pits on the terraces indicated erosion of the surface in water. Strings extended to the [001] direction were occasionally observed in the topography images and assigned to the clusters of the H2O molecules. The tip experienced a repulsive force when the vertical tip position of z was less than 6 nm from the surface, and the force oscillated at z at less than 2 nm. The repulsive force originated from the disruption of the hydrogen bonding network of H2O molecules formed on the hydrophilic sputter-annealed surface. The oscillatory force arose from structural alternate order-disorder transitions of the H2O molecules at the gap between the tip and the TiO2 surfaces. On the TiO2 surface annealed in air, no strings were observed in the topography images. The tip experienced an attractive force before experiencing a repulsive force in its approach to the surface. Oscillatory behavior was not observed in the force curve. The air-annealed TiO2 and tip surfaces were both hydrophobic and attracted to each other to expel the H2O molecules from their gap. Ordering the H2O molecules at the gap between the two hydrophobic surfaces was entropically unfavorable.

Molecular resolution investigation of tetragonal lysozyme (110) face in liquid by frequency-modulation atomic force microscopy

In order to determine the molecular structure by x-ray diffraction analysis, it is very important to grow high quality protein crystals. The molecular resolution imaging of soluble protein crystals such as the tetragonal lysozyme (110) face in saturated solution is demonstrated using frequency-modulation atomic force microscopy (FM-AFM). The surface structure of the (110) face and the crystallographic position of individual molecules were determined from molecular resolution images. For observation of protein crystals, FM-AFM is a favorable technique as an alternative to contact mode or amplitude-modulation AFM.

Three-electrode self-actuating self-sensing quartz cantilever: Design, analysis, and experimental verification

We present a novel quartz cantilever for frequency-modulation atomic force microscopy (FM-AFM) which has three electrodes: an actuating electrode, a sensing electrode, and a ground electrode. By applying an ac signal on the actuating electrode, the cantilever is set to vibrate. If the frequency of actuation voltage closely matches one of the characteristic frequencies of the cantilever, a sharp resonance should be observed. The vibration of the cantilever in turn generates a current on the sensing electrode. The arrangement of the electrodes is such that the cross-talk capacitance between the actuating electrode and the sensing electrode is less than 10(-16) F, thus the direct coupling is negligible. To verify the principle, a number of samples were made. Direct measurements with a Nanosurf easyPPL controller and detector showed that for each cantilever, one or more vibrational modes can be excited and detected. Using classical theory of elasticity, it is shown that such novel cantilevers with proper dimensions can provide optimized performance and sensitivity in FM-AFM with very simple electronics.

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.

A low noise all-fiber interferometer for high resolution frequency modulated atomic force microscopy imaging in liquids

We have developed a low noise all-fiber interferometer for use as the deflection sensor in liquid environment frequency modulated atomic force microscopy (FM-AFM). A detailed description and rationale for the choice of the critical components are provided along with the design of a simple alignment assembly. The optimization of the deflection sensor toward achieving the highest possible sensitivity and lowest deflection noise density is discussed in the context of an ideal interference cavity. Based on the provided analysis we have achieved deflection noise densities of 2 fm/square root(Hz) on commercially available cantilevers in both ambient and liquid environments. The low noise interferometer works without the need for differential detection, special focusing lenses, or polarization sensitive optics, dramatically simplifying measurements. True atomic resolution imaging of muscovite mica by FM-AFM in water is demonstrated using the developed deflection sensor.

Submolecular-scale Investigations on metal-phthalocyanine monolayers by frequency modulation atomic force microscopy

Copper-phthalocyanine (CuPc) monolayers and cobalt-phthalocyanine monolayers deposited on Au(111) surfaces were investigated by frequency modulation atomic force microscopy (FM-AFM). Submolecular-resolution topographic images were successfully obtained for both samples. Despite the similar molecular geometry of the two molecules, they showed clearly different contrasts in the topographic images. The origin of the contrast is discussed in terms of the relationship of the molecular orbitals and the chemical interaction between the tip and the molecules. In addition, a molecular-resolution surface potential (SP) image was obtained on CuPc monolayers using Kelvin probe force microscopy (KFM) utilizing FM-AFM. The molecular-scale SP contrast was explained by the electric dipole moment at the organic/metal interface. This result suggested the possibility of the detection of the single molecular dipole moment by KFM.

Direct observation of layered structures at ionic liquid/solid interfaces by using frequency-modulation atomic force microscopy

Presence of inhomogeneous layered structures of ionic liquid (IL) molecules at IL/HOPG and IL/mica interfaces was directly detected and imaged by using frequency-modulation atomic force microscopy. High stability of the layered structures may disturb their interface applications to catalysis and electrochemistry.

Molecular-Resolution Imaging of Hydration Layers on Model Biological Membranes by Frequency Modulation Atomic Force Microscopy

Molecular-Resolution Imaging of Hydration Layers on Model Biological Membranes by Frequency Modulation Atomic Force Microscopy

1P093 1F1325 液中周波数変調原子間力顕微鏡(FM-AFM)によるチューブリン表面構造の実空間観察(蛋白質-計測・解析の方法論,口頭発表,第48回日本生物物理学会年会)

1P093 1F1325 液中周波数変調原子間力顕微鏡(FM-AFM)によるチューブリン表面構造の実空間観察(蛋白質-計測・解析の方法論,口頭発表,第48回日本生物物理学会年会)

Probing the Local Conformation within π‐Conjugated One‐dimensional Supramolecular Stacks using Frequency Modulation Atomic Force Microscopy

Frequency-modulation atomic force microscopy is used to investigate the local conformation within 1D stacks obtained by the self-assembly of π-conjugated molecules from solution. The structural parameters extracted from the experimental data can be interpreted in terms of local molecular conformation, by comparison with models obtained by molecular mechanics and dynamics simulations.

Crystalline structure and squeeze-out dissipation of liquid solvation layers observed by small-amplitude dynamic AFM

Using frequency-modulation atomic force microscopy (FM-AFM) at sub-nanometer vibration amplitudes, we find in the system $n$-dodecanol/graphite that solvation layers may extend for several nanometers into the bulk liquid. These layers maintain crystalline order which can be imaged using FM-AFM. The energy dissipation of the vibrating tip can peak sharply upon penetration of molecular layers. The tip shape appears critical for this effect.

High-resolution molecular images of rubrene single crystals obtained by frequency modulation atomic force microscopy

Frequency modulation atomic force microscopy (FM-AFM) was employed to study molecular structures of rubrene single crystals in ultrahigh vacuum. Molecularly flat and extraordinarily wide terraces were extended over the width of more than a few micrometers with monomolecular steps. Molecular packing arrangements and internal structures were revealed by FM-AFM. The unit cell determined by FM-AFM was consistent with the lattice parameters of bulk crystal within the experimental error, suggesting that the surface structure of rubrene is not reconstructed.

Surface defects characterization with frequency and force modulation atomic force microscopy using molecular dynamics simulations

This paper is devoted to the characterization of the surface defects using a recently developed AFM technique called frequency and force modulation AFM (FFM–AFM). The simulated system includes a recently developed gold coated AFM probe which interacts with a sample including single-atom vacancy and impurities. In order to examine the behavior of the above system on different transition metals, the molecular dynamics (MD) simulation with Sutton–Chen (SC) inter-atomic potential is used. In this study, an online imaging simulation of the probe and sample is performed, and the effects of the horizontal scan speed, the effective frequency set-point, the cantilever stiffness, the tip-sample rest position and the cantilever quality factor on the resulting images are investigated. Using a proposed optimum controlling scheme for the excitation force amplitude, the cantilever horizontal speed can be increased.

Wideband digital frequency detector with subtraction-based phase comparator for frequency modulation atomic force microscopy

We have developed a wideband digital frequency detector for high-speed frequency modulation atomic force microscopy (FM-AFM). We used a subtraction-based phase comparator (PC) in a phase-locked loop circuit instead of a commonly used multiplication-based PC, which has enhanced the detection bandwidth to 100 kHz. The quantitative analysis of the noise performance revealed that the internal noise from the developed detector is small enough to provide the theoretically limited noise performance in FM-AFM experiments in liquid. FM-AFM imaging of mica in liquid was performed with the developed detector, showing its stability and applicability to true atomic-resolution imaging in liquid.