Frequency Modulation Atomic Force Microscopy Research Articles

Direct Imaging of Charge/Dopant Distribution in PANI/PSS Thin Films Using Advanced Frequency Modulation Electrostatic Force Microscopy.

This paper presents a detailed study that maps the surface charges and dopant distribution on the electropolymerized thin film of polyaniline-poly(styrenesulfonate) (PANI/PSS). The focus is on two distinct states of PANI/PSS: the fully doped emeraldine salt (ES/PSS) and the dedoped emeraldine base (EB/PSS). This investigation utilizes advanced frequency modulation electrostatic force microscopy (FM-EFM) and atomic force microscopy (AFM). The polymer film comprises polymer grains, and FM-EFM data suggest a non-uniform distribution of dopants on the grain surface, with a higher doped periphery than the core. Quantifying the charge at the periphery and core of ES/PSS and EB/PSS grains provides unique insight into the charge distribution within the polymer film. The charge density is estimated to be 10 times higher in the periphery region (∼120 μC/cm2) than in the core region (∼11 μC/cm2) and 100 times higher than EB/PSS (∼0.8 μC/cm2). We have directly observed the morphological changes of PANI/PSS from the ES/PSS state to the EB/PSS state using the AFM topographic profile. These findings provide a better understanding of the behavior of the charge/dopant distribution on the surface of the polymer films and pave the way for further research and development of PANI/PSS-based electronic devices.

Molecular-Resolution Imaging of Ionic Liquid/Alkali Halide Interfaces with Varied Surface Charge Densities via Atomic Force Microscopy.

When in contact with charged solid surfaces, ionic liquids (ILs) are known to form solvation structures consisting of alternating cation and anion layers. This phenomenon is considered to originate from the adsorption layer of counterions overcompensating the surface charge, so-called overscreening. However, the response of these layers to surfaces with near-zero or extremely high surface charge density (σ) remains inadequately understood. Here, we probe the solvation structure of ILs on alkali halide surfaces with varied surface orientations: nearly zero-charged RbI(100) and highly charged RbI(111), by employing frequency modulation atomic force microscopy with atomic resolution. Two commonly used ILs are examined in this study: 1-methyl-1-propylpyrrolidinium bis(trifluoromethylsulfonyl)imide ([C3mpyr][NTf2]) and 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([C2mim][NTf2]). On RbI(100) surfaces with near zero σ, we observe alternating cation and anion layers, diverging from the previously proposed monolayer model for IL/alkali halide(100) interfaces. These results support the argument that overscreening occurs under low σ, even approaching zero, and reconcile conflicting experimental conclusions about low σ systems. On RbI(111) surfaces with high σ, we identify solvation structures consisting of two consecutive counterion layers. This structure aligns with the theoretically predicted crowding; a phenomenon rarely observed in commonly used ILs due to typically unreachable σ in electrochemical IL/electrode systems. Our findings indicate that alkali halide(111) surfaces are potentially valuable for exploring the crowding phenomenon in ILs, addressing the current scarcity of experimental observations.

Spatially Resolved Dielectric Loss at the Si/SiO_{2} Interface.

The Si/SiO_{2} interface is populated by isolated trap states that modify its electronic properties. These traps are of critical interest for the development of semiconductor-based quantum sensors and computers, as well as nanoelectronic devices. Here, we study the electric susceptibility of the Si/SiO_{2} interface with nm spatial resolution using frequency-modulated atomic force microscopy. The sample measured here is a patterned dopant delta layer buried 2nm beneath the silicon native oxide interface. We show that charge organization timescales of the Si/SiO_{2} interface range from 1-150ns, and increase significantly around interfacial traps. We conclude that under time-varying gate biases, dielectric loss in metal-insulator-semiconductor capacitor devices is in the frequency range of MHz to sub-MHz, and is highly spatially heterogeneous over nm length scales.

Viscoelastic property enhancement of polymethylmethacrylate molecularly confined within 3D nanostructures

Owing to its applications in various fields, such as biomedical, microelectronics, sensors, and polymer composites, polymer nanoconfinement is a widely studied topic. This confinement changes the configuration of molecules compared with those of solids, which, in the case of polymeric films, decreases the glass transition temperature and mechanical properties of the polymer. In this study, nanostructured polymethylmethacrylate, presenting three-dimensional nanoscale confinement were evaluated using amplitude modulation–frequency modulation atomic force microscopy for the first time. The Young’s moduli and loss tangents were measured, and the results suggest that for cells smaller than approximately 39 nm, the Young’s modulus of the 3-D confined polymer enhances that of the raw solid owing to reduced molecular mobility. This research shows that the molecular mobility was reduced because polymer chains were confined within three-dimensional space.

Exploring the Molecular-Scale Structures at Solid/Liquid Interfaces of Li-Ion Battery Materials: A Force Spectroscopy Analysis with Sparse Modeling.

In this study, we clarify the liquid structure formed at the interface between LiCoO2 (LCO), the cathode material of Li-ion batteries, and propylene carbonate (PC), which is used as a solvent in the electrolyte, on a molecular scale. We apply sparse modeling-based modal analysis to force spectroscopy data measured by frequency modulation atomic force microscopy (FM-AFM) and show that each component in the FM-AFM force curve, such as oscillatory solvation force, background, and noise, can be automatically decomposed. Moreover, by combining detailed force curve analysis with solid/liquid interface simulations based on first-principles calculation, we have identified that there are distinct damped vibrational modes in the force curves at the LCO/PC interface with a period of about 0.57 nm and those with shorter periods, which likely correspond to the solvation forces associated with bulk-state PC molecules and those with PC molecules in "lying down" orientations.

Two-dimensional analysis of the interfacial solvation structure of an ionic liquid electrolyte on a hydrogen-terminated Si electrode by atomic force microscopy

Ionic liquids (ILs) have been intensively studied as new electrolytes for lithium-ion batteries (LIBs). Structural analysis of interfaces between an IL-based electrolyte and an LIB electrode would provide beneficial information for improving LIBs. In this study, we investigated the interfacial structures between an IL, 1-methyl-1-propyl-pyrrolidinium bis(trifluoromethanesulfonyl)imide, and a H-terminated Si(111) electrode in the presence and absence of Li salt by frequency modulation atomic force microscopy utilizing a quartz tuning fork sensor. Two-dimensional frequency shift mapping imaging of the solvation structure at the interface showed that the layered solvation structure was only observed in the absence of Li salts in the ILs, which was in good agreement with our previous studies performed on IL/lithium titanate interfaces. Combined with electrochemical measurements, the partial disappearance of the layered solvation structure in the Li salt-doped IL was strongly suggested to be due to the Li-ion insertion/extraction at the IL/Si interface.

Dynamics of Molecular Self-Assembly of Short Peptides at Liquid-Solid Interfaces - Effect of Charged Amino Acid Point Mutations.

Self-organizing solid-binding peptides on atomically flat solid surfaces offer a unique bio/nano hybrid platform, useful for understanding the basic nature of biology/solid coupling and their practical applications. The surface behavior of peptides is determined by their molecular folding, which is influenced by various factors and is challenging to study. Here, the effect of charged amino acids is studied on the self-assembly behavior of a directed evolution selected graphite-binding dodecapeptide on graphite surface. Two mutations, M6 and M8, are designed to introduce negatively and positively charged moieties, respectively, at the anchoring domain of the wild-type (WT) peptide, affecting both binding and assembly. The questions addressed here are whether mutant peptides exhibit molecular crystal formation and demonstrate molecular recognition on the solid surface based on the specific mutations. Frequency-modulated atomic force microscopy is used for observations of the surface processes dynamically in water at molecular resolution over several hours at the ambient. The results indicate that while the mutants display distinct folding and surface behavior, each homogeneously nucleates and forms 2D self-organized patterns, akin to the WT peptide. However, their growth dynamics, domain formation, and crystalline lattice structures differ significantly. The results represent a significant step toward the rational design of bio/solid interfaces, potent facilitators of a variety of future implementations.

Insight into the hydration friction of lipid bilayers.

Hydration layers formed on charged sites play crucial roles in many hydration lubrication systems in aqueous media. However, the underlying molecular mechanism is not well understood. Herein, we explored the hydration friction of lipid bilayers with different charged headgroups at the nanoscale through a combination of frequency-modulation atomic force microscopy and friction force microscopy. The nanoscale friction experiments showed that the hydration friction coefficient and frictional energy dissipation of a cationic lipid (DPTAP) were much lower than those of zwitterionic (DPPE) and anionic (DPPG) lipids. The hydration layer probing at the surfaces of different lipid bilayers clearly revealed the relationship between the charged lipid headgroups and hydration layer structures. Our detailed analysis demonstrated that the cationic lipid had the largest hydration force in comparison with zwitterionic and anionic lipids. These friction and hydration force results indicated that the difference of the lipid headgroup charge resulted in different hydration strengths which led to the difference of hydration friction behaviors. The findings in this study provide molecular insights into the hydration friction of lipid bilayers, which has potential implications for the development of efficient hydration lubrication systems with boundary lipid bilayers in aqueous media.

A Next-Generation qPlus-Sensor-Based AFM Setup: Resolving Archaeal S-Layer Protein Structures in Air and Liquid.

Surface-layer (S-layer) proteins form the outermost envelope in many bacteria and most archaea and arrange in two-dimensional quasicrystalline structures via self-assembly. We investigated S-layer proteins extracted from the archaeon Pyrobaculum aerophilium with a qPlus sensor-based atomic force microscope (AFM) in both liquid and ambient conditions and compared it to transmission electron microscopy (TEM) images under vacuum conditions. For AFM scanning, a next-generation liquid cell and a new protocol for creating long and sharp sapphire tips was introduced. Initial AFM images showed only layers of residual detergent molecules (sodium dodecyl sulfate, SDS), which are used to isolate the S-layer proteins from the cells. SDS was not visible in the TEM images, requiring more thorough sample preparation for AFM measurements. These improvements allowed us to resolve the crystallike structure of the S-layer samples with frequency-modulation AFM in both air and liquid.

Novel Environmentally Superior Tribomaterial with Superlow Friction: 100% Cellulose Nanofiber Molding

In this paper, we report on a novel, environmentally superior tribomaterial with superlow friction of 100% cellulose nanofiber (CNF) molding. Based on our experimental results, the CNF molding exhibited a superlow friction coefficient of approximately 0.01 under lubrication with a fatty acid: glycerin monooleate (GMO) diluted with poly-alfa-olefine. Attenuated total reflection Fourier-transform infrared spectroscopy and high-resolution frequency-modulation atomic force microscopy analyses demonstrated that superlow friction of the CNF molding was realized by GMO-assisted functionalization of the CNF surface, which effectively promoted the formation of a soft absorption film or soft swollen CNF layer. Our findings indicate that the in-situ functionalization of OH-terminated CNF surfaces during the friction process plays a crucial role in achieving superlow friction.Graphical

Are High Resolution Atomic Force Microscopy images proportional to the force gradient or the force maps?

Frequency modulation Atomic Force Microscopy using tips functionalized with carbon monoxide (also known as High Resolution AFM) shows an extraordinary resolution on molecules and it can disclose their inner structure on the frequency shift maps. When operating the microscope at low amplitudes, <1 Å, these frequency shift measurements have been traditionally interpreted as the force gradient between the probe and the sample. Here, we show that, for CO-terminated tips, this interpretation fails even for amplitudes as small as 0.2–0.3 Å. The main reason behind this failure is the very rapid variation in tip-sample interaction upon tip approach associated with the mobility of the CO probe, which deflects to reduce the Pauli repulsion with the sample. Besides, we propose a new and very simple approach for the frequency shift in terms of the force at the closest tip-sample separation. This force approximation reproduces the real frequency maps for amplitudes larger than ∼0.3 Å, independent of CO deflection.

Visualization of Electrolyte Reaction Field Near the Negative Electrode of a Lead Acid Battery by Means of Amplitude/Frequency Modulation Atomic Force Microscopy

The precise observation of a solid-liquid interface by means of frequency modulation atomic force microscopy (FM-AFM) was performed, demonstrating its applicability to a study on lead acid batteries using an electrochemical test cell for in-liquid FM-AFM embedded with a specialized cantilever holder. The consistency and reproducibility of each surface profile observed via amplitude modulation AFM and FM-AFM were verified properly in a strong acidic electrolyte. In terms of FM-AFM, the ability to observe remarkable changes in the force mapping is the most beneficial, especially near the negative electrode surface. The localization of lignosulfonate (LS) added into the electrolyte as an expander could be visualized since this characteristic force mapping was captured when LS was added to electrolyte.

Molecular Scale Structure and Kinetics of Layer-by-Layer Peptide Self-Organization at Atomically Flat Solid Surfaces.

Understanding the mechanisms of self-organization of short peptides into two- and three-dimensional architectures are of great interest in the formation of crystalline biomolecular systems and their practical applications. Since the assembly is a dynamic process, the study of structural development is challenging at the submolecular dimensions continuously across an adequate time scale in the natural biological environment, in addition to the complexities stemming from the labile molecular structures of short peptides. Self-organization of solid binding peptides on surfaces offers prospects to overcome these challenges. Here we use a graphite binding dodecapeptide, GrBP5, and record its self-organization process of the first two layers on highly oriented pyrolytic graphite surface in an aqueous solution by using frequency modulation atomic force microscopy in situ. The observations suggest that the first layer forms homogeneously, generating self-organized crystals with a lattice structure in contact with the underlying graphite. The second layer formation is mostly heterogeneous, triggered by the crystalline defects on the first layer, growing row-by-row establishing nominally diverse biomolecular self-organized structures with transient crystalline phases. The assembly is highly dependent on the peptide concentration, with the nucleation being high in high molecular concentrations, e.g., >100 μM, while the domain size is small, with an increase in the growth rate that gradually slows down. Self-assembled peptide crystals are composed of either singlets or doublets establishing P1 and P2 oblique lattices, respectively, each commensurate with the underlying graphite lattice with chiral crystal relations. This work provides insights into the surface behavior of short peptides on solids and offers quantitative guidance toward elucidating molecular mechanisms of self-assembly helping in the scientific understanding and construction of coherent bio/nano hybrid interfaces.

Accurate Electrostatic Force Measurements by Atomic Force Microscopy Using Proper Distance Control

Electrostatic force microscopy (EFM) enables us to examine the local electrostatic force between an AFM tip and sample surface, from which an electrical capacitance between them and surface potential can be evaluated, by applying direct current (DC) and alternating current (AC) voltages. From the dependences of the electrostatic force on the DC voltage and on the frequency of AC voltage in EFM, carrier density, carrier type, and deep level states in a semiconductor can also be investigated. Since EFM is basically operated as an atomic force microscopy (AFM), special care should be taken in an effect of the electrostatic force on a distance control between the tip and sample, and robust distance control is necessary even under the strong electrostatic force in order to realize proper measurements of both topography and electrostatic force. In this paper, we have examined the effectiveness of the usage of the oscillation amplitude of a cantilever as a feedback target for the distance control, which is referred to as Δ <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">A</i> -mode operation, while the cantilever oscillation frequency is always kept at its primary resonance, like the conventional frequency modulation AFM (FM-AFM) which uses a resonant frequency shift as the feedback target. First, we have verified that a distance change due to the strong electrostatic force was significantly reduced compared with FM-AFM. Secondly, we have confirmed that the Δ <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">A</i> -mode operation in EFM realized proper measurements of the dependance of a tip-sample capacitance including a surface depletion capacitance on the DC voltage and on the frequency of AC voltage, indicating good robustness of the method against the strong electrostatic force.

Molecular-Resolution Imaging of Interfacial Solvation of Electrolytes for Lithium-Ion Batteries by Frequency Modulation Atomic Force Microscopy.

Solvation structures formed by ions and solvent molecules at solid/electrolyte interfaces affect the energy storage performance of electrochemical devices, such as lithium-ion batteries. In this study, the molecular-scale solvation structures of an electrolyte, a solution of lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) in propylene carbonate (PC) at the electrolyte-mica interface, were measured using frequency-modulation atomic force microscopy (FM-AFM). The spacing of the characteristic force oscillation in the force versus distance curves increased with increasing ion concentration, suggesting an increase in the effective size of molecules at the interface. Molecular dynamics simulations showed that the effective size of molecular assemblies, namely, solvated ions formed at the interface, increased with increasing ion concentrations, which was consistent with the experimental results. Knowledge of molecular-scale structures of solid/electrolyte interfaces obtained by a combination of FM-AFM and molecular dynamics simulations is important in the design of electrolytes for future energy devices and in improving their properties.

&lt;i&gt;In Situ&lt;/i&gt; Observation of Hydration Structure in the Process of Phase Transition of Calcium Carbonate Crystal

We introduce the results of in situ observation of the surface and the solid-liquid interface of calcium carbonate in atomic-level by frequency modulation atomic force microscopy (FM-AFM) in the presence of additives. We have found that the calcite surface transformed into the aragonite structure by the cooperative effect of magnesium and a hydrophilic polypeptide. Furthermore, high-resolution imaging of solid-liquid interface revealed that these additives did not adsorb on the calcite surface but significantly changed the hydration structure in the vicinity of the surface. Especially magnesium ions contribute to change the hydration structure due to their hydrophilicity.

Hybrid mode atomic force microscopy of phase modulation and frequency modulation.

We propose hybrid phase modulation (PM)/frequency modulation (FM) atomic force microscopy (AFM) to increase the imaging speed of AFM in high-Q environments. We derive the relationship between the phase shift, the frequency shift and the tip-sample interaction force from the equation of motion for the cantilever in high-Q environments. The tip-sample conservative force is approximately given by the sum of the conservative force with respect to the phase shift in the PM mode and that with respect to the frequency shift in the FM mode. We preliminarily demonstrate that the hybrid PM/FM-AFM is a new and very promising AFM operation mode that can increase imaging speed.

Molecular Resolution Imaging of Adsorption of Dodecane on a Lignite Surface by Frequency Modulation AFM

The consumption of dodecane, a critical flotation collector, in the flotation of low-rank coal is particularly high; however, the adsorption mechanism of dodecane on coal still remains unclear. To shed light on this issue, the molecular-scale adsorption structure of dodecane on a lignite surface was investigated in situ by frequency modulation atomic force microscopy (FM-AFM) for the first time. The adsorption structure of dodecane on highly oriented pyrolytic graphite (HOPG) and mica has been detected in advance. Meanwhile, molecular dynamics (MD) simulations are utilized to elucidate and illustrate the adsorption structure on HOPG, mica, and lignite. Experimental results from FM-AFM show that a layered structure of dodecane molecules is formed on HOPG, while there is no stable adsorption structure on mica. Meanwhile, a few local monolayer structures of dodecane molecules are observed on the surface of lignite, and their thickness is approximately 1 nm, which is less than the molecular chain length of dodecane. It is speculated that the dodecane molecules are adsorbed on lignite selectively, with the capacity to form a stable adsorption structure. As indicated by MD simulations, the interaction energy between dodecane and the mentioned surfaces is in the order HOPG > lignite > mica. And the MD simulation results show a desirable match with direct observation by FM-AFM. In accordance with the characterization of the lignite sample surface, large amounts of polar oxygen-containing functional groups may hinder dodecane adsorption behavior. Through in situ molecular resolution imaging of the interface structure between dodecane and a raw lignite surface, we provide a new perspective for exploring the interaction mechanism between nonpolar collectors and a lignite surface.

Cell Adhesion Strength Indicates the Antithrombogenicity of Poly(2-methoxyethyl acrylate) (PMEA): Potential Candidate for Artificial Small-Diameter Blood Vessel

Poly(2-methoxyethyl acrylate) (PMEA) is a US FDA-approved biocompatible polymer, although there is insufficient work on human umbilical vein endothelial cells (HUVECs) and platelet interaction analysis on PMEA-analogous polymers. In this study, we extensively investigated HUVEC–polymer and platelet–polymer interaction behavior by measuring the adhesion strength using single-cell force spectroscopy. Furthermore, the hydration layer of the polymer interface was observed using frequency-modulation atomic force microscopy. We found that endothelial cells can attach and spread on the PMEA surface with strong adhesion strength compared to other analogous polymers. We found that the hydration layers on the PMEA-analogous polymers were closely related to their weak platelet adhesion behavior. Based on our results, it can be concluded that PMEA is a promising candidate for the construction of artificial small-diameter blood vessels owing to the presence of IW and a hydration layer on the interface.

Poly(2-Methoxyethyl Acrylate) (PMEA)-Coated Anti-Platelet Adhesive Surfaces to Mimic Native Blood Vessels through HUVECs Attachment, Migration, and Monolayer Formation

Confluent monolayers of human umbilical vein endothelial cells (HUVECs) on a poly(2-methoxyethyl acrylate) (PMEA) antithrombogenic surface play a major role in mimicking the inner surface of native blood vessels. In this study, we extensively investigated the behavior of cell–polymer and cell–cell interactions by measuring adhesion strength using single-cell force spectroscopy. In addition, the attachment and migration of HUVECs on PMEA-analogous substrates were detected, and the migration rate was estimated. Moreover, the bilateral migration of HUVECs between two adjacent surfaces was observed. Furthermore, the outer surface of HUVEC was examined using frequency-modulation atomic force microscopy (FM-AFM). Hydration was found to be an indication of a healthy glycocalyx layer. The results were compared with the hydration states of individual PMEA-analogous polymers to understand the adhesion mechanism between the cells and substrates in the interface region. HUVECs could attach and spread on the PMEA surface with stronger adhesion strength than self-adhesion strength, and migration occurred over the surface of analogue polymers. We confirmed that platelets could not adhere to HUVEC monolayers cultured on the PMEA surface. FM-AFM images revealed a hydration layer on the HUVEC surfaces, indicating the presence of components of the glycocalyx layer in the presence of intermediate water. Our findings show that PMEA can mimic original blood vessels through an antithrombogenic HUVEC monolayer and is thus suitable for the construction of artificial small-diameter blood vessels.