Frequency Modulation Atomic Force Microscopy Research Articles

The qPlus sensor, a powerful core for the atomic force microscope.

Atomic force microscopy (AFM) was introduced in 1986 and has since made its way into surface science, nanoscience, chemistry, biology, and material science as an imaging and manipulating tool with a rising number of applications. AFM can be employed in ambient and liquid environments as well as in vacuum and at low and ultralow temperatures. The technique is an offspring of scanning tunneling microscopy (STM), where the tunneling tip of the STM is replaced by using a force sensor with an attached tip. Measuring the tiny chemical forces that act between the tip and the sample is more difficult than measuring the tunneling current in STM. Therefore, even 30 years after the introduction of AFM, progress in instrumentation is substantial. Here, we focus on the core of the AFM, the force sensor with its tip and detection mechanism. Initially, force sensors were mainly micro-machined silicon cantilevers, mainly using optical methods to detect their deflection. The qPlus sensor, originally based on a quartz tuning fork and now custom built from quartz, is self-sensing by utilizing the piezoelectricity of quartz. The qPlus sensor allows us to perform STM and AFM in parallel, and the spatial resolution of its AFM channel has reached the subatomic level, exceeding the resolution of STM. Frequency modulation AFM (FM-AFM), where the frequency of an oscillating cantilever is altered by the gradient of the force that acts between the tip and the sample, has emerged over the years as the method that provides atomic and subatomic spatial resolution as well as force spectroscopy with sub-piconewton sensitivity. FM-AFM is precise; because of all physical observables, time and frequency can be measured by far with the greatest accuracy. By design, FM-AFM clearly separates conservative and dissipative interactions where conservative forces induce a frequency shift and dissipative interactions alter the power needed to maintain a constant oscillation amplitude of the cantilever. As it operates in a noncontact mode, it enables simultaneous AFM and STM measurements. The frequency stability of quartz and the small oscillation amplitudes that are possible with stiff quartz sensors optimize the signal to noise ratio. Here, we discuss the operating principles, the assembly of qPlus sensors, amplifiers, limiting factors, and applications. Applications encompass unprecedented subatomic spatial resolution, the measurement of forces that act in atomic manipulation, imaging and spectroscopy of spin-dependent forces, and atomic resolution of organic molecules, graphite, graphene, and oxides.

Analysis of Solid-Liquid Interface by Frequency Modulation Atomic Force Microscopy

Analysis of Solid-Liquid Interface by Frequency Modulation Atomic Force Microscopy

Frequency-Modulation Atomic Force Microscopic Observation for Ultralow Frictional Solid–Liquid Interface of Diamond-Like Carbon in an Environmentally Friendly Oil

The structure of the low friction solid–liquid interface of the diamond-like carbons (DLCs) in fatty acid was investigated by controlled frequency-modulation atomic force microscopy (FM-AFM) to elucidate the low frictional mechanism that arises under boundary lubrication. We focused on two factors in particular: the effect of the DLC type and the frictional energy in the structure of the adsorbents at the interface. The FM-AFM results indicate that the ultralow frictional tetrahedral amorphous carbon–fatty-acid interface was composed of a thick boundary film with a high molecular density, which may support the sliding surfaces and thereby reduce the friction.

Investigation of Local Hydration Structures of Alkanethiol Self-Assembled Monolayers with Different Molecular Structures by FM-AFM.

Hydration structures play crucial roles in a wide variety of chemical and biological phenomena. However, the key factors that determine a hydration structure remain an open question. Most recent studies have focused on the electrostatic interactions between the surface charges and dipoles of water molecules, which are determined by the atomic/ionic species of the outermost solid surface, as the dominating factor. The number of studies on the correlation between the hydration structure and the atomic-scale surface corrugation has been limited. In this study, we investigated the hydration structures of alkanethiol self-assembled monolayers terminated with a hydroxyl group using frequency-modulated atomic force microscopy. We observed two molecular structures, namely, the (√3 × √3) R30° structure and the c(4 × 2) superlattice structure, and found that their hydration structures are different mainly because of the slight differences in their molecular arrangements. This result suggests that a slight difference in the molecular/atomic arrangements as well as the atomic/ionic species in the outermost solid surface strongly influences the local hydration structures.

Hydration Layer Structure of Biofouling-Resistant Nanoparticles.

Hydrophilic surface chemistries can strongly bind water to produce surfaces that are highly resistant to protein adsorption and fouling. The interfacial bound water and its distinct properties have intrigued researchers for decades, yet the relationship between the water three-dimensional structure and function in antifouling coatings remains elusive. Here, we use hydrophilic, epoxy organosilane modified silica nanoparticles to demonstrate cheap, robust, and practically applied coatings that we discover have broad-ranging, ultralow fouling properties when challenged by various proteins, bacteria, and fungal spores. To understand their excellent antifouling properties, frequency modulation-atomic force microscopy is used to directly observe the interfacial water structure at subatomic resolution, which we validate using all-atom molecular dynamic simulations that strikingly predict similar structures of water layers on the original and ultralow fouling surfaces. The convergence of experimental and modeling data reveals that suitably spaced, flexible chains with hydrophilic groups interact with water molecules to produce a connective, quasi-stable layer, consisting of dynamic interfacial water, that provides a basis for antifouling performance of ultrathin, hydrophilic surface chemistries.

Energy Dissipation Detected by Frequency Modulation Atomic Force Microscopy

Energy Dissipation Detected by Frequency Modulation Atomic Force Microscopy

Direct Imaging of Atomic-Scale Surface Structures of Brookite TiO2 Nanoparticles by Frequency Modulation Atomic Force Microscopy in Liquid

Direct imaging of atomic-scale surface structures of TiO2 nanoparticles is a promising method for understanding the detailed mechanism of their photocatalytic activities. Atomic force microscopy (AFM) is one of the analytical tools used for direct imaging of surfaces, but atomic-scale AFM studies of TiO2 nanoparticles in liquid have not been reported. Here, we report two methods for fixing of TiO2 nanocrystals: an electrostatic method and a cross-linking method. Both methods enabled visualization of subnanoscale surface structures of brookite TiO2 nanoparticles in liquid by frequency modulation AFM (FM-AFM). The FM-AFM imaging results and density functional theory molecular dynamics calculations suggest that the subnanoscale structures in the FM-AFM images can be explained by the arrangement of the surface topmost atoms or their hydration structures on the brookite TiO2(210) surface. Our results open up various possibilities for studying atomic-scale surface structures of nanoparticles in liquids by AFM.

Atomic-Scale Imaging of Surface and Hydration Structures of Stable and Metastable Acetaminophen Crystals by Frequency Modulation Atomic Force Microscopy

In this paper, we have investigated hydration structures of the stable and the metastable acetaminophen crystals (Form I and Form II, respectively) by frequency modulation atomic force microscopy w...

Investigation of an n− layer in a silicon fast recovery diode under applied bias voltages using Kelvin probe force microscopy

We have imaged an n− layer of a commercial silicon fast recovery diode (Si-FRD) under applied bias voltages using frequency modulation atomic force microscopy and Kelvin probe force microscopy. It was possible to image the potential contrast of the exposed active region surface of the Si-FRD owing to the dominant behavior of carriers induced by applying the bias voltages. Under applied reverse bias voltages, potential contrasts at a pn− junction were recognized in potential images. At the applied forward bias voltage of 1.5 V, a potential drop at an n−n interface under conductivity modulation was clearly observed in a potential image. From the distance between the pn− junction and the n−n interface, the n− layer width of a power device was determined to be 36.6 µm.

Nanoscale investigation of bulk heterojunction organic solar cell by scanning capacitance force microscopy

We developed a frequency-modulation atomic force microscopy (FM-AFM) system combined with Kelvin probe force microscopy (KFM) and scanning capacitance force microscopy (SCFM). Using the developed system, we investigated the surface structure and electrical characteristics of a bulk heterojunction (BHJ) sample consisting of a mixture of [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) and poly(3-hexylthiophene) (P3HT). The BHJ structure consisted of PCBM and P3HT molecules. These materials are often used as the active and charge transport layers in an organic solar cell, respectively. The results suggest that we succeeded in visualizing the phase separation in the BHJ sample using SCFM measurements. We illustrated the energy band diagrams from the KFM results and showed that band bending occurs from the organic material side at the interface with the indium–tin oxide substrate. Furthermore, the KFM and SCFM results suggested that charge separation occurs in the BHJ structure.

Atomic-Scale 3D Local Hydration Structures Influenced by Water-Restricting Dimensions.

Hydration structures at solid-liquid interfaces mediate between the atomic-level surface structures and macroscopic functionalities in various physical, chemical, and biological processes. Atomic-scale local hydration measurements have been enabled by ultralow noise three-dimensional (3D) frequency-modulation atomic force microscopy. However, for their application to complicated surface structures, e.g., biomolecular devices, understanding the relationship between the hydration and surface structures is necessary. Herein, we present a systematic study based on the concept of the structural dimensionality, which is crucial in various scientific fields. We performed 3D measurements and molecular dynamics simulations with silicate surfaces that allow for 0, 1, and 2 degrees of freedom to water molecules. Consequently, we found that the 3D hydration structures reflect the structural dimensions and the hydration contrasts decrease with increasing dimension due to the enlarged water self-diffusion coefficient and increased embedded hydration layers. Our results provide guidelines for the analysis of complicated hydration structures, which will be exploited in extensive fields.

Quantitative comparison of wideband low-latency phase-locked loop circuit designs for high-speed frequency modulation atomic force microscopy.

A phase-locked loop (PLL) circuit is the central component of frequency modulation atomic force microscopy (FM-AFM). However, its response speed is often insufficient, and limits the FM-AFM imaging speed. To overcome this issue, we propose a PLL design that enables high-speed FM-AFM. We discuss the main problems with the conventional PLL design and their possible solutions. In the conventional design, a low-pass filter with relatively high latency is used in the phase feedback loop, leading to a slow response of the PLL. In the proposed design, a phase detector with a low-latency high-pass filter is located outside the phase feedback loop, while a subtraction-based phase comparator with negligible latency is located inside the loop. This design minimizes the latency within the phase feedback loop and significantly improves the PLL response speed. In addition, we implemented PLLs with the conventional and proposed designs in the same field programmable gate array chip and quantitatively compared their performances. The results demonstrate that the performance of the proposed PLL is superior to that of the conventional PLL: 165 kHz bandwidth and 3.2 μs latency in water. Using this setup, we performed FM-AFM imaging of calcite dissolution in water at 0.5 s/frame with true atomic resolution. The high-speed and high-resolution imaging capabilities of the proposed design will enable a wide range of studies to be conducted on various atomic-scale dynamic phenomena at solid–liquid interfaces.

Investigation of BMI-PF6 Ionic Liquid/Graphite Interface Using Frequency Modulation Atomic Force Microscopy

ABSTRACTStructural analysis on interfaces between ionic liquids (ILs) and solid substrates is an important study for not only the basic fundamental aspects but also many technological processes. In the present work, we utilized frequency modulation atomic force microscopy (FM-AFM) based on a quartz tuning fork sensor to elucidate the structure of interface between 1-butyl-3-methylimidazolium hexafluorophosphate (BMI-PF6) IL and highly ordered pyrolytic graphite (HOPG) surface. It was observed that this IL form solvation layers at their interface, with ∼0.5-0.57 nm thickness of each layer. We have compared our experimental results with previously reported results from molecular dynamics simulation study, and combination of classical molecular dynamics and density functional theory calculations in order to understand the IL/HOPG interface.

A Novel Method to Reconstruct the Force Curve by Higher Harmonics of the First Two Flexural Modes in Frequency Modulation Atomic Force Microscope (FM-AFM).

Atomic force microscope (AFM) is an idealized tool to measure the physical and chemical properties of the sample surfaces by reconstructing the force curve, which is of great significance to materials science, biology, and medicine science. Frequency modulation atomic force microscope (FM-AFM) collects the frequency shift as feedback thus having high force sensitivity and it accomplishes a true noncontact mode, which means great potential in biological sample detection field. However, it is a challenge to establish the relationship between the cantilever properties observed in practice and the tip-sample interaction theoretically. Moreover, there is no existing method to reconstruct the force curve in FM-AFM combining the higher harmonics and the higher flexural modes. This paper proposes a novel method that a full force curve can be reconstructed by any order higher harmonics of the first two flexural modes under any vibration amplitude in FM-AFM. Moreover, in the small amplitude regime, short range forces are reconstructed more accurately by higher harmonics analysis compared with fundamental harmonics using the Sader-Jarvis formula.

Morphology and Physical Properties of Hydrophilic-Polymer-Modified Lipids in Supported Lipid Bilayers.

Lipid molecules such as glycolipids that are modified with hydrophilic biopolymers participate in the biochemical reactions occurring on cell membranes. Their functions and efficiency are determined by the formation of microdomains and their physical properties. We investigated the morphology and properties of domains induced by the hydrophilic-polymer-modified lipid applying a polyethylene glycol (PEG)-modified lipid as a model modified lipid. We formed supported lipid bilayers (SLBs) using a 0-10 mol % range of PEG-modified lipid concentration ( CPEG). We studied their morphology and fluidity by fluorescence microscopy, the fluorescence recovery after photobleaching method, and atomic force microscopy (AFM). Fluorescence images showed that domains rich in the PEG-modified lipid appeared and SLB fluidity decreased when CPEG ≥ 5%. AFM topographies showed that clusters of the PEG-modified lipid appeared prior to domain formation and the PEG-lipid-rich domains were observed as depressions. Frequency-modulation AFM revealed a force-dependent appearance of the PEG-lipid-rich domain.

A High-Q AFM Sensor Using a Balanced Trolling Quartz Tuning Fork in the Liquid.

A quartz tuning fork (QTF) has been widely used as a force sensor of the frequency modulation atomic force microscope due to its ultrahigh stiffness, high quality factor and self-sensing nature. However, due to the bulky structure and exposed surface electrode arrangement, its application is limited, especially in liquid imaging of in situ biological samples, ionic liquids, electrochemical reaction, etc. Although the complication can be resolved by coating insulating materials on the QTF surface and then immersing the whole QTF into the liquid, it would result in a sharp drop of the quality factor, which will reduce the sensitivity of the QTF. To solve the problem, a novel method, called the balanced trolling quartz tuning fork (BT-QTF), is introduced here. In this method, two same probes are glued on both prongs of the QTF separately while only one probe immersed in the liquid. With the method, the hydrodynamic interaction can be reduced, thus the BT-QTF can retain a high quality factor and constant resonance frequency. The stable small vibration of the BT-QTF can be achieved in the liquid. Initially, a theoretical model is presented to analyze the sensing performance of the BT-QTF in the liquid. Then, the sensing performance analysis experiments of the BT-QTF have been performed. At last, the proposed method is applied to atomic force microscope imaging different samples in the liquid, which proves its feasibility.

Development of Local Analysis Technique of Electric Double Layer at Electrode Interfaces and Its Application to Ionic Liquid Interfaces

Abstract Local analyses of electrolyte/solid electrode interfaces at controlled electrode potentials are of fundamental importance to understanding the origin and properties of the electric double layer (EDL) at the interfaces, which is necessary for their application to electrochemical devices. This award account summarizes our recent achievements of such analyses by using the novel analytical tools of electrochemical frequency modulation AFM (EC-FM-AFM), which enables acquisition of information from the liquid molecules’ structuring as well as the atomic scale resolution of the solid side, both are often affected by the electrode potential. Potential and electrolyte dependent EDL structures at aqueous solution/graphite interfaces and strong substrate dependency on the structuring of interfacial ionic liquid are mainly discussed.

Hydration Structure of a Single DNA Molecule Revealed by Frequency-Modulation Atomic Force Microscopy.

Hydration interaction shapes biomolecules and is a dominant intermolecular force. Mapping the hydration patterns of biomolecules is therefore essential for understanding molecular processes in biology. Numerous studies have been devoted to this challenge, but current methods cannot map the hydration of single biomolecules, let alone do so under physiological conditions. Here, we show that frequency-modulation atomic force microscopy (FM-AFM) can fill this gap and generate 3D hydration maps of single DNA molecules under near-physiological conditions. Additionally, we present real-space images of DNA in which the double helix is resolved with unprecedented resolution, clearly revealing individual phosphate groups along the DNA backbone. FM-AFM therefore emerges as a powerful enabling tool in the study of individual biomolecules and their hydration under physiological conditions.

Composition-Dependent Alterations in Thickness and Physical Properties of Lipid Bilayer FILM Revealed by Frequency Modulation Atomic Force Microscopy

Assembly of microdomains and membrane viscosity due to these physical properties of membrane have been highlighted as important factors supporting proper functions of membrane proteins. Although advanced imaging technologies including scanning probe microscopy and super-resolution optical microscopy have revealed dynamics of microdomains and visco-elastic properties of cell membranes, it is still difficult to link modifications of nanoscale membrane structure and specific diseases. In this study, we aimed to obtain high-resolution structural information of supported planner membrane with different fatty acid lengths using liquid dynamic-mode AFM and FM-AFM (Frequency Modulation Atomic Force Microscope). In FM-AFM, the cantilever is moved in a non-contact state, maintaining the cantilever frequency shift (Δf) is constant. This enables a highly sensitive force detection, 20 times better S/N than with existing methods, thereby markedly improving the image resolution. We used supported phosphatidylcholine membranes with fatty acids ranged from PC 12:0 to PC 22:6. For saturated PCs, membrane thickness increased along with the number of carbon from 4.32 nm (PC12:0) to 6.37nm (PC18:0), but diarachidoylphosphatidylcholine (PC20:0) showed reduced membrane thickness: PC20:0 thickness had 6.09 nm, that is ≈0.3 nm thinner than PC18:0. Unsaturation of fatty acid further reduced membrane thickness to 4.12 nm for PC20:4, suggesting that structural arrangement of PCs with fatty acid chain length and unsaturation level occurred. These data demonstrated that our imaging modality can detect membrane thickness with minimal membrane compression by scanning probe while maintaining high detection sensitivity. We will also present modulations of mechanical properties of membrane in the functions of carbon chain length and the level of unsaturation.

A robust AFM-based method for locally measuring the elasticity of samples.

Investigation of the local sample elasticity is of high importance in many scientific domains. In 2014, Herruzo et al. published a new method based on frequency-modulation atomic force microscopy to locally determine the elasticity of samples (Nat. Commun. 2014, 5, 3126). This method gives evidence for the linearity of the relation between the frequency shift of the cantilever first flexural mode Δf1 and the square of the frequency shift of the second flexural mode Δf22. In the present work, we showed that a similar linear relation exists when measuring in contact mode with a certain load FN and propose a new method for determining the elastic modulus of samples from this relation. The measurements were performed in non-dry air at ambient temperature on three different polymers (polystyrene, polypropylene and linear low-density polyethylene) and a self-assembled monolayer of 1H,1H,2H,2H-perfluorodecyltrichlorosilane (FDTS) on a silicon oxide substrate perforated with circular holes prepared by polymer blend lithography. For all samples the relation was evidenced by recording Δf1, Δf2 and FN as a function of the Z-displacement curves of the piezoelectric scanner. The occurence of a plastic deformation followed by an elastic deformation is shown and explained. The necessary load FN for measuring in the elastic domain was assessed for each sample, used for mapping the frequency shifts Δf1 and Δf2 and for determining the elastic modulus from Δf22/Δf1. The method was used to give an estimate of the Young’s modulus of the FDTS thin film.