Advanced Atomic Force Microscopy Research Articles

Strain-engineered rippling at the bilayer-MoS2 interface identified by advanced atomic force microscopy

Strain-engineered rippling at the bilayer-MoS2 interface identified by advanced atomic force microscopy

Optical, conductive, and ferroelectric properties of the first layer of dip-coated BiFeO3 films from methoxyethanol and acetic acid-based chemical dissolvents

The overall performance of the multilayer resulting in a sol-gel bismuth ferrite (BiFeO3) film will be primarily determined by the properties of the first layer, but this has yet to receive much attention, even though chemical and morphological defects of this layer can accumulate as the number of layers increases. Here, we perform an optical, conductive, and ferroelectric study of first layer (L 1) dip-coating sol-gel BiFeO3 films using two routes that vary only in the dissolvent; the first one is based on 2-methoxyethanol (MOE), and the second one on acetic acid (AA) with some MOE (AA-MOE). Tauc plots reveal a band gap of 2.43 eV and 2.75 eV for MOE (30 ± 5 nm thick) and AA-MOE (35 ± 5 nm thick) films, respectively. MOE films showed a dielectric function with features at ∼2.5 eV, ∼3.1 eV, and ∼3.9 eV, which were associated with charge-transfer transitions, but such features are absent in AA-MOE films. Advanced atomic force microscopy techniques were used to identify the fine features or defects of the BiFeO3 films: The conductive maps show that the charge transport pathways in both film routes are controlled by nanometer defects rather than grain or grain boundary defects. Current-voltage curves reveal high conductive pathway at a lower voltage for the MOE films than for AA-MOE films. The piezoelectric coefficient for MOE films was ∼20% higher than AA-MOE films. Both deposition methods yield ferroelectric films with an electromechanical strain controlled by the piezoelectric effect and minimal contribution from electrostriction. An optimization for the AA-MOE-based route in the withdrawal speed results in a significant reduction of morphological defects and a more than twofold increase in the piezoelectric coefficient. Our results broaden the understanding of optical and ferroelectric BiFeO3 films based on a chemical solution by dip-coating.

Perovskite Solar Cell on Stainless Steel Substrate over 10% Efficiency for Building-Integrated Photovoltaics

This study investigated the integration of perovskite solar cells (PSCs) on stainless steel (SS) substrates for application in building-integrated photovoltaics (BIPV). Using advanced atomic force microscopy measurements, we confirmed that enhanced substrate roughness increased the reflectance along an interface. Consequently, a remarkable final efficiency of 11.9% was achieved. Notably, PSCs, known for their exceptional efficiency of 26.1%, can overcome the inherent efficiency limitations of SS-based thin-film solar cells. In this study, a PSC with an efficiency of 14% was fabricated on a flexible SS substrate. This study is a significant step towards advancing sustainable energy solutions for BIPV applications. The global shift towards renewable energy sources has catalyzed intensive research and development efforts, rendering the exploration of alternative materials and manufacturing processes a priority. The success of PSCs on SS substrates underscores their promise to achieve a balance between efficiency and versatility in BIPV solutions. Moreover, our findings reveal that controlling the substrate surface characteristics can significantly enhance the performance of PSCs, offering a pathway toward greater energy efficiency and sustainability in the construction industry.

Identification the Role of Grain Boundaries in Polycrystalline Photovoltaics via Advanced Atomic Force Microscope.

Atomicforce microscopy (AFM)-based scanning probing techniques, including Kelvinprobe force microscopy (KPFM) and conductive atomic force microscopy (C-AFM), have been widely applied to investigate thelocal electromagnetic, physical, or molecular characteristics of functional materials on a microscopic scale. The microscopic inhomogeneities of the electronic properties of polycrystalline photovoltaic materials can be examined by these advanced AFM techniques, which bridge the local properties of materials to overall device performance and guide the optimization of the photovoltaic devices. In this review, the critical roles of local optoelectronic heterogeneities, especially at grain interiors (GIs) and grain boundaries (GBs) of polycrystalline photovoltaic materials, including versatile polycrystalline silicon, inorganic compound materials, and emerging halide perovskites, studied by KPFM and C-AFM, are systematically identified. How the band alignment and electrical properties of GIs and GBs affect the carrier transport behavior are discussed from the respective of photovoltaic research. Further exploiting the potential of such AFM-based techniques upon a summary of their up-to-date applications in polycrystalline photovoltaic materials is beneficial to acomprehensive understanding of the design and manipulation principles of thenovel solar cells and facilitating the development of the next-generation photovoltaics and optoelectronics.

Quantification Characterization of Hierarchical Structure of Polyurethane by Advanced AFM and X-ray Techniques.

Polyurethane (PU) with microphase separation has garnered significant attention due to its highly designable molecular structure and a wide range of adjustable properties. However, there is currently a lack of systematic approaches for quantifying PU's microphase separation. To address this research gap, we utilized an atomic force microscopy (AFM) nanomechanical mapping technique along with Gaussian fitting to recolor and quantitatively analyze the evolution of PU's microphase separation. By varying the ratios of the chain extender to cross-linking agent, we observed the changes in the hydrogen bonding between the soft and hard segments. As the ratio of the chain extender to cross-linking agent decreases, the strength of the hydrogen bonding weakens, resulting in a reduction in the quantity and phase percentage of hard segment (HS) domains. Consequently, the degree of microphase separation between the soft and hard segments decreases, leading to specific alterations in the material's mechanical properties and dynamic viscoelasticity. To further investigate the hierarchical structure of PU, we employed various techniques, such as X-ray analysis, transmission electron microscopy (TEM), and AFM-based infrared spectroscopy (AFM-IR). Our findings reveal a spherulite pattern composed of lamellae within the HS domains, with the cross-linking density gradually increasing from the center to the periphery. Overall, our comprehensive characterization of PU provides valuable insights into its hierarchical structure and establishes a quantitative framework to explore the intricate relationship between the structure and properties.

Probing the charge state of threading dislocations in indium nitride through advanced atomic force microscopy

Indium nitride (InN) plays an imperative role in continuing the success of III-nitride technology and extending its footprint into new application fields. However, the development of InN-based devices is hindered by the unusually high residual electron concentration in unintentionally-doped InN, whose origin remains unresolved. In this work, pits are observed at regions around the InN island boundaries, which are assigned to threading dislocations (TDs). Since the surface band bending of InN is found to be intrinsically downward, the positive currents detected exclusively at the InN island boundary regions upon applying positive sample bias are leakage currents. This further points to the presence of TDs in these regions, which can act as localized conductive leakage paths. Interestingly, the surface potentials at the InN island boundary regions are found to be 35–45 mV more positive compared to the InN islands. This can be attributed to the presence of positively charged TDs due to the formation of donor-type point defects along their dislocation cores. Hence, TDs in InN can donate electrons and serve as one of the factors contributing to its high n-type conductivity.

Synthesis, Properties and Adsorption Kinetic Study of New Cross-Linked Composite Materials Based on Polyethylene Glycol Polyrotaxane and Polyisoprene/Semi-Rotaxane.

New composite materials were prepared via cross-linking of polyethylene glycol/2-hydroxypropyl-β-cyclodextrins polyrotaxane (PEG/HPβCD) and polyisoprene/HPβCD semi-polyrotaxane (PI/HPβCD SR) with 1,6-hexamethylene diizocyanate (HMDI). Advanced instrumental methods (such WAXS (wide angle X-ray scattering), AFM (atomic force microscopy), SEM (scanning electron microscopy), and thermal and dynamic vapor sorption) were employed for the structural, morphological and thermal characterization of the resulting composite materials. The roughness parameters calculated using AFM indicate a smoother surface for the composite material with 10 wt% of PI/HPβCD SR, denoting that a homogeneous film was obtained. SEM analysis reveals porous morphologies for both composite materials and the pore sizes increase with the increasing concentration of PI/HPβCD SR in the matrix. Dynamic vapor sorption/desorption measurements and type IV isotherms confirmed the hydrophilic and porous materials, which are in agreement with SEM analysis. The composite with a higher PI/HPβCD SR concentration in the matrix showed increased thermal stability than that of the pure cross-linked material. This material was further tested as a sorbent for methylene blue (MB) dye removal from an aqueous solution. The adsorption capacity of the composite film was found to be 2.58 mg g-1 at 25 °C.

Probing two-dimensional materials by advanced atomic force microscopy

Probing two-dimensional materials by advanced atomic force microscopy

Mechanism for the effects of surface chemical composition and crystal face on the wettability of α-quartz surface

Quartz is a ubiquitous matter in natural rock and industrial production, and the wetting property of quartz surface plays a critical role in determining numerous industrial processes. However, the key mechanism for the effects of surface chemical composition (based on surface pretreatment) and crystal face on the wettability of quartz surface remains vague. Herein, by applying an advanced atomic force microscopy (AFM), we measured the contact angles of liquids on three common crystal faces of α-quartz after different pretreatments. The time-of-flight secondary ion mass spectrometry (ToF-SIMS) analysis and molecular dynamics (MD) simulations were further conducted to uncover the microscopic mechanism involved. It is found that the surface pretreatment does significantly affect the wettability of α-quartz faces, mainly because of the variation in surface hydroxyl concentration after different surface pretreatments. The magnitude of change in contact angle after distinct pretreatments is also affected by the liquid property. After certain pretreatments, although there is no difference in the wettability of different faces, the inherent atomic arrangement of crystal faces still causes discrepancies in other interface properties, e.g., liquid structure, diffusion coefficient of liquid molecule. The reasonable pretreatments of α-quartz for a range of applications are recommended based on the current findings.

Single-molecular insights into the breakpoint of cellulose nanofibers assembly during saccharification

Plant cellulose microfibrils are increasingly employed to produce functional nanofibers and nanocrystals for biomaterials, but their catalytic formation and conversion mechanisms remain elusive. Here, we characterize length-reduced cellulose nanofibers assembly in situ accounting for the high density of amorphous cellulose regions in the natural rice fragile culm 16 (Osfc16) mutant defective in cellulose biosynthesis using both classic and advanced atomic force microscopy (AFM) techniques equipped with a single-molecular recognition system. By employing individual types of cellulases, we observe efficient enzymatic catalysis modes in the mutant, due to amorphous and inner-broken cellulose chains elevated as breakpoints for initiating and completing cellulose hydrolyses into higher-yield fermentable sugars. Furthermore, effective chemical catalysis mode is examined in vitro for cellulose nanofibers conversion into nanocrystals with reduced dimensions. Our study addresses how plant cellulose substrates are digestible and convertible, revealing a strategy for precise engineering of cellulose substrates toward cost-effective biofuels and high-quality bioproducts.

Effect of hydrophobic and hydrophilic surfaces on the morphological and physical properties of supported lipid membranes.

The characteristics of supported lipid bilayers (SLBs) have long been studied as SLBs are commonly accepted as basic model systems of cell membranes. Atomic force microscopy (AFM) has proved to be effective in characterising physical and thermodynamic properties of SLBs, owing to its high resolution and the ability of environmental control. Through study of these properties, information such as lipid phase transitions can be investigated. Lipid phase transitions, from solid-ordered to liquid-disordered states, are well known to vary depending upon temperature, pH and ionic strength etc., and play a substantial role into events such as membrane protein mobility. Although previously studied on hydrophilic substrates, knowledge on the impact of hydrophobic substrates, such as van der Waals (vdW) crystals, on lipid phase transition is still lacking. This is particularly important for developing new bio- and electrochemical sensing technologies, where conductive vdW materials are used as sensing electrodes. It is expected that the thermodynamic properties of SLBs affect their conductive and dielectric polarisation and in turn molecular charge distribution and dielectric properties are known to strongly influence hydration layers occurring at the interface of lipids. Here, we present the study of adsorption of lipid membranes on various substrates, where morphological and physical (i.e. thermodynamic, electrical, and interfacial) properties of the membrane were investigated using advanced AFM modes as a function of temperature. We find that the transition temperature of the lipid monolayers greatly differs depending upon the chosen substrate. We then probe interfacial properties of these electrically charged interfaces. Finally, we demonstrate that substrates with different chemical-physical properties used as a lipids support have a substantial effect on the physical properties of lipid monolayers.

Review: Advanced Atomic Force Microscopy Modes for Biomedical Research

Visualization of biomedical samples in their native environments at the microscopic scale is crucial for studying fundamental principles and discovering biomedical systems with complex interaction. The study of dynamic biological processes requires a microscope system with multiple modalities, high spatial/temporal resolution, large imaging ranges, versatile imaging environments and ideally in-situ manipulation capabilities. Recent development of new Atomic Force Microscopy (AFM) capabilities has made it such a powerful tool for biological and biomedical research. This review introduces novel AFM functionalities including high-speed imaging for dynamic process visualization, mechanobiology with force spectroscopy, molecular species characterization, and AFM nano-manipulation. These capabilities enable many new possibilities for novel scientific research and allow scientists to observe and explore processes at the nanoscale like never before. Selected application examples from recent studies are provided to demonstrate the effectiveness of these AFM techniques.

A universal substrate for the nanoscale investigation of two-dimensional materials

Since discovering two-dimensional materials, there has been a great interest in exploring, understanding, and taking advantage of their unique properties. Si/SiO2 is one of the most used substrates for the deposition and characterization of 2D materials due to its availability and optical contrast. This work goes beyond the conventional substrate and introduces highly-ordered pyrolytic graphite (HOPG) as universal support for investigating two-dimensional materials due to several unique properties such as chemical and temperature stability, intrinsic high flatness, reusability, electrical conductivity, ease of use, availability, and enhanced adhesion of two-dimensional materials. We demonstrate this by analyzing several 2D materials with advanced atomic force microscopy methods, Raman and photoluminescence spectroscopy with hyperspectral imaging, and scanning electron microscopy with elementary analysis imaging. The strong adhesion to HOPG allowed the instant deposition of different two-dimensional materials GaSe, MoS2, Zn2In2S5, talc, and h-BN. This feat is hard to accomplish on the conventional SiO2 substrate without polymer-assisted transfer. Moreover, this strong interaction can strain 2D materials deposited on HOPG, giving localized changes in reactivity, optical, and electronic properties. This effect is explored for selective Ag deposition on strained regions of 2D materials to activate photocatalytic reactions.

3D Generation of Multipurpose Atomic Force Microscopy Tips

In this work, 3D polymeric atomic force microscopy (AFM) tips, referred to as 3DTIPs, are manufactured with great flexibility in design and function using two‐photon polymerization. With the technology holding a great potential in developing next‐generation AFM tips, 3DTIPs prove effective in obtaining high‐resolution and high‐speed AFM images in air and liquid environments, using common AFM modes. In particular, it is shown that the 3DTIPs provide high‐resolution imaging due to their extremely low Hamaker constant, high speed scanning rates due to their low quality factor, and high durability due to their soft nature and minimal isotropic tip wear; the three important features for advancing AFM studies. It is also shown that refining the tip end of the 3DTIPs by focused ion beam etching and by carbon nanotube inclusion substantially extends their functionality in high‐resolution AFM imaging, reaching angstrom scales. Altogether, the multifunctional capabilities of 3DTIPs can bring next‐generation AFM tips to routine and advanced AFM applications, and expand the fields of high speed AFM imaging and biological force measurements.

(Invited) Multi-Frequency Atomic Force Microscopy for Nanoscale Infrared Spectroscopy

The emergence of tools at the forefront of advanced atomic force microscopy (AFM) holds great potential to bolster our fundamental understanding of heterogeneous and dynamic systems at the nanoscale, of prime interest for material science, chemistry and life sciences. Developments using multi-frequency AFM provide more sensitive detection and a wider frequency span to explore the morphology, mechanical and chemical properties of the sample volume. However how these performances compare to more conventional modes is not well established.Here, we will present the implementation of multifrequency AFM nanoscale infrared imaging and spectroscopy. We will first discuss how the nonlinear nature of the tip-sample interaction in AFM can be used as a sensitive channel to access infrared (IR) fingerprints of materials at the nanoscale. We will compare the IR signatures obtained with conventional nanoscale IR spectroscopy, both on homogeneous and heterogeneous materials. In doing so, we will estimate the volume probed as a function of the data acquisition parameters and materials properties. Finally, we will illustrate how the high sensitivity and spatial resolution of nanoscale IR spectroscopy provide new insights in the molecular arrangements of complex systems, especially when guided by macro- and micro-scale studies.

Size-dependent strain-engineered nanostructures in MoS2 monolayer investigated by atomic force microscopy

The strain has been employed for controlled modification of electronical and mechanical properties of two-dimensional (2D) materials. However, the thermal strain-engineered behaviors of the CVD-grown MoS2 have not been systematically explored. Here, we investigated the strain-induced structure and properties of CVD-grown triangular MoS2 flakes by several advanced atomic force microscopy. Two different kinds of flakes with sharp-corner or vein-like nanostructures are experimentally discovered due to the size-dependent strain behaviors. The critical size of these two kinds of flakes can be roughly estimated at ∼17 μm. Within the small flakes, the sharp-corner regions show specific strain-modified properties due to the suffering of large tensile strain. While in the large MoS2 flakes, the complicated vein-like nanoripple structures were formed due to the interface slipping process under the larger tensile strain. Our work not only demonstrates the size-specific strain behaviors of MoS2 flakes but also sheds light on the artificial design and preparation of strain-engineered nanostructures for the devices based on the 2D materials.

SARS-CoV-2 Unrevealed: Ultrastructural and Nanomechanical Analysis.

The ongoing outbreak of the severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2) started in late 2019 and spread across the world, infecting millions of people, with over 3.3 million deaths worldwide. To fight back the virus, it is necessary to understand how the main structures work, especially those responsible for the virus infectivity pathogenicity. Here, using the most advanced atomic force microscopy techniques, SARS-CoV-2 viral particles were analyzed, with a special focus on their ultrastructure, adsorption conformation, and nanomechanical behavior. The results uncovered the aspects of the organization and the spatial distribution of the proteins on the surface of the viral particles. It also showed the compliant behavior of the membrane and ability to recover from mechanical injuries. At least three layers composing the membrane and their thickness were measured, protecting the virus from external stress. This study provides new insight into the ultrastructure of SARS-CoV-2 particles at the nanoscale, offering new prospects that could be employed for mapping viral surfaces. The understanding of the viruses’ capacity to survive mechanical disruptions at any level and their ability to recover from such injuries can shed a light on the structure–function relationship and help us to find targets for drug action, especially for this virus that, to this day, has no course of treatment approved.

Strain-induced hierarchical ripples in MoS2 layers investigated by atomic force microscopy

Strain engineering plays a vital role in controlling the physical properties of two-dimensional (2D) materials. However, the nanomechanical behavior of atomically thin 2D crystals under strain has not been completely understood. Here, strain-induced hierarchical ripple nanostructures in triangular MoS2 flakes were investigated by advanced atomic force microscopy and optical spectral measurements. The hierarchical nanoripples exhibited a threefold radial pattern, and their mechanical, electronic, and optical spectra characteristics were significantly modified due to the suffering from large tensile strain. Structure evolution of these hierarchical nanoripples was further discussed based on the geometry and thickness of MoS2 flakes, and we attributed the curtain effect at the limit of a single atomic layer. Our study will be beneficial in designing nanomechanical structures and prototype electromechanical devices with 2D materials.

Atomic Force Microscopy (AFM) As a Surface Mapping Tool in Microorganisms Resistant Toward Antimicrobials: A Mini-Review.

The worldwide emergence of antimicrobial resistance (AMR) in pathogenic microorganisms, including bacteria and viruses due to a plethora of reasons, such as genetic mutation and indiscriminate use of antimicrobials, is a major challenge faced by the healthcare sector today. One of the issues at hand is to effectively screen and isolate resistant strains from sensitive ones. Utilizing the distinct nanomechanical properties (e.g., elasticity, intracellular turgor pressure, and Young’s modulus) of microbes can be an intriguing way to achieve this; while atomic force microscopy (AFM), with or without modification of the tips, presents an effective way to investigate such biophysical properties of microbial surfaces or an entire microbial cell. Additionally, advanced AFM instruments, apart from being compatible with aqueous environments—as often is the case for biological samples—can measure the adhesive forces acting between AFM tips/cantilevers (conjugated to bacterium/virion, substrates, and molecules) and target cells/surfaces to develop informative force-distance curves. Moreover, such force spectroscopies provide an idea of the nature of intercellular interactions (e.g., receptor-ligand) or propensity of microbes to aggregate into densely packed layers, that is, the formation of biofilms—a property of resistant strains (e.g., Staphylococcus aureus, Pseudomonas aeruginosa). This mini-review will revisit the use of single-cell force spectroscopy (SCFS) and single-molecule force spectroscopy (SMFS) that are emerging as powerful additions to the arsenal of researchers in the struggle against resistant microbes, identify their strengths and weakness and, finally, prioritize some future directions for research.

Multiparametric analytical quantification of materials at nanoscale in tapping force microscopy

Atomic force microscopy (AFM) is a powerful technique for accurate, reliable and non-destructive imaging and characterization of materials at the nanoscale. Among the numerous AFM methods, amplitude modulation or tapping mode AFM (AM-AFM) is an established method for imaging and characterization for most commercial AFM systems. Despite its high spatial resolution and sensitivity, quantitative characterization by AM-AFM lag behind other advanced AFM methods as far as quantification of materials properties is concerned. In this paper a fully analytical multiparametric approach for AM-AFM is proposed which simultaneously quantifies the Hamaker constant and viscoelastic properties of materials. The main advantage of the proposed method lies in the inclusion of adhesion to calculate viscoelasticity, which makes it superior to the current equations used in the AFM community. The accuracy of the proposed method is validated by several simulations and experiments and comparison with nanoindentation results, which strongly support its candidacy as a method of choice for material properties quantification by dynamic AFM.