Force-distance Spectroscopy Research Articles

Humidity-controlled interactions and lifting of MoS2 on SiO2

Abstract Fabrication of 2D material devices under ambient conditions faces challenges from humidity-dependent interfacial interactions and doping effects due to intercalated water layers. This study explores adhesion and force dynamics for a monolayer of molybdenum disulfide (MoS2) on silicon dioxide (SiO2) as a function of humidity, using atomic force microscopy (AFM). At low humidity (<10 %RH), capillary forces dominate due to a liquid bridge forming between the AFM tip and sample. At intermediate humidity 40 %RH, an additional water layer intercalates between the MoS2 and the SiO2 and the capillary force model alone is insufficient to describe the tip-sample interactions. Force-distance spectroscopy reveals additional force dynamics related to MoS2-lifting and dynamics in the intercalated water. Above 60 %RH, water condenses on the MoS2 and the capillary interactions are reduced. Understanding and identifying these interactions is crucial for optimising deposition of 2D materials under humid conditions.

Photo-crosslinked chitosan-gelatin xerogel-like coating onto “cold” plasma functionalized poly(lactic acid) film as cell culture support

This paper reports on biofunctionalisation of a poly(lactic acid) (PLA) film by surface activation through cold plasma treatment followed by coating with a chitosan-gelatin xerogel. The UV cross-linking of the xerogel precursor was simultaneously performed with the fixation onto the PLA support. This has a strong effect on surface properties, in terms of wettability, surface free energy, morphology and micromechanical features. The hydrophilic – hydrophobic character of the surface, determined by contact angle measurements, was tuned along the process, passing from moderate hydrophobic PLA to enhanced hydrophilic plasma activated surface, which favors coating adhesion, then to moderate hydrophobic chitosan-gelatin coating. The coating has a Lewis amphoteric surface, with a porous xerogel-like morphology, as revealed by scanning electron microscopy images. By riboflavin mediated UV cross-linking the chitosan-gelatin coating becomes high adhesive and with a more pronounced plasticity, as shown by AFM force-distance spectroscopy. Thus prepared surface-coated PLA supports were successfully tested for growth of dermal fibroblasts, which are known for their induction potential of chondrogenic cells, which is very important in cartilage tissue engineering.

Nanoarchitecture factors of solid electrolyte interphase formation via 3D nano-rheology microscopy and surface force-distance spectroscopy

The solid electrolyte interphase in rechargeable Li-ion batteries, its dynamics and, significantly, its nanoscale structure and composition, hold clues to high-performing and safe energy storage. Unfortunately, knowledge of solid electrolyte interphase formation is limited due to the lack of in situ nano-characterization tools for probing solid-liquid interfaces. Here, we link electrochemical atomic force microscopy, three-dimensional nano-rheology microscopy and surface force-distance spectroscopy, to study, in situ and operando, the dynamic formation of the solid electrolyte interphase starting from a few 0.1 nm thick electrical double layer to the full three-dimensional nanostructured solid electrolyte interphase on the typical graphite basal and edge planes in a Li-ion battery negative electrode. By probing the arrangement of solvent molecules and ions within the electric double layer and quantifying the three-dimensional mechanical property distribution of organic and inorganic components in the as-formed solid electrolyte interphase layer, we reveal the nanoarchitecture factors and atomistic picture of initial solid electrolyte interphase formation on graphite-based negative electrodes in strongly and weakly solvating electrolytes.

Detection of Nanobubbles on Lubricant-Infused Surfaces Using AFM Meniscus Force Measurements.

So far, the presence of nanobubbles on lubricant-infused surfaces (LIS) has been overlooked, because of the difficulty in detecting them in such a complex system. We recently showed that anomalously large interfacial slip measured on LIS is explained by the presence of nanobubbles [Vega-Sánchez, Peppou-Chapman, Zhu and Neto, Nat. Commun., 2022 13, 351]. Crucial to drawing this conclusion was the use of atomic force microscopy (AFM) force-distance spectroscopy (meniscus force measurements) to directly image nanobubbles on LIS. This technique provided vital direct evidence of the spontaneous nucleation of nanobubbles on lubricant-infused hydrophobic surfaces. In this paper, we describe in detail the data collection and analysis of AFM meniscus force measurements on LIS and show how these powerful measurements can quantify both the thickness and distribution of multiple coexisting fluid layers (i.e., gas and oil) over a nanostructured surface. Using this technique, thousands of force curves were automatically analyzed. The results show that the interfacial tension of the nanobubbles is reduced from 52 ± 9 mN m-1 to 39 ± 4 mN m-1 by the presence of the silicone oil layer.

Effect of annealing temperature on the magnetic domain structure and surface mechanical properties of Ni-C composite thin films: Magnetic and lateral force microscopy, and force-distance spectroscopy

Present study demonstrates the effect of annealing temperature on magnetic domain structure and surface mechanical properties of Ni-C composite thin films. The thin films with constant nickel and carbon concentration were synthesized by electrodeposition method at low voltage and at room temperature. The electrodeposited thin films were annealed at 80 °C, 100 °C, 120 °C and 140 °C for twenty minutes. Magnetic domain structures of varying width and contrast were observed in the films with change in annealing temperature by magnetic force microscopy (MFM). A qualitative study of the effect of annealing temperature on surface friction of the thin films were investigated by Lateral force microscopy (LFM). A decreasing trend is observed in the surface friction of the films with increase in annealing temperature. The Force-Distance spectroscopy was performed on the thin films annealed at different temperatures to explore the chemical and surface mechanical characteristics of the samples. Pull-off force or adhesive force of the thin films is decreased with increase in annealing temperature of the films.

Characterization of Ligand Adsorption at Individual Gold Nanocubes.

Cetyltrimethylammonium bromide (CTAB) is a widely used surfactant that aids the aqueous synthesis of colloidal nanoparticles. However, the presence of residual CTAB on nanoparticle surfaces can significantly impact nanoparticle applications, such as catalysis and sensing, under hydrated conditions. As such, consideration of the presence and quantity of CTAB on nanoparticle surfaces under hydrated conditions is of significance. Herein, as part of an integrated material characterization framework, we demonstrate the feasibility of in situ atomic force microscopy (AFM) to detect CTAB on the surface of Au nanocubes (Au NCs) under hydrated conditions, which enabled superior characterization compared to conventional spectroscopic methods. In situ force-distance (FD) spectroscopy and Kelvin probe force microscopy (KPFM) measurements support additional characterization of adsorbed CTAB, while correlative in situ AFM and scanning electron microscopy (SEM) measurements were used to evaluate sequential steps of CTAB removal from Au NCs across hydrated and dehydrated environments, respectively. Notably, a substantial quantity of CTAB remained on the Au NC surface after methanol washing, which was detected in AFM measurements but was not detected in infrared spectroscopy measurements. Subsequent electrochemical cleaning was found to be critically important to remove CTAB from the Au NC surface. Correlative measurements were also performed on individual nanoparticles, which further validate the method described here as a powerful tool to determine the extent and degree of CTAB removal from nanoparticle surfaces. This AFM-based method is broadly applicable to characterize the presence and removal of ligands from nanomaterial surfaces under hydrated conditions.

Evaluation of grain boundaries as percolation pathways in quartz-rich continental crust using Atomic Force Microscopy

Hydrous fluids play a vital role in the chemical and rheological evolution of ductile, quartz-bearing continental crust, where fluid percolation pathways are controlled by grain boundary domains. In this study, widths of grain boundary domains in seven quartzite samples metamorphosed under varying crustal conditions were investigated using Atomic Force Microscopy (AFM) which allows comparatively easy, high magnification imaging and precise width measurements. It is observed that dynamic recrystallization at higher metamorphic grades is much more efficient at reducing grain boundary widths than at lower temperature conditions. The concept of force-distance spectroscopy, applied to geological samples for the first time, allows qualitative estimation of variations in the strength of grain boundary domains. The strength of grain boundary domains is inferred to be higher in the high grade quartzites, which is supported by Kernel Average Misorientation (KAM) studies using Electron Backscatter Diffraction (EBSD). The results of the study show that quartzites deformed and metamorphosed at higher grades have narrower channels without pores and an abundance of periodically arranged bridges oriented at right angles to the length of the boundary. We conclude that grain boundary domains in quartz-rich rocks are more resistant to fluid percolation in the granulite rather than the greenschist facies.

Insights into Nanoscale Wettability Effects of Low Salinity and Nanofluid Enhanced Oil Recovery Techniques

In this study, enhanced oil recovery (EOR) techniques—namely low salinity and nanofluid EOR—are probed at the nanometer-scale using an atomic force microscope (AFM). Mica substrates were used as model clay-rich rocks while AFM tips were coated to present alkyl (-CH3), aromatic (-C6H5) and carboxylic acid (-COOH) functional groups, to simulate oil media. We prepared brine formulations to test brine dilution and cation bridging effects while selected concentrations (0 to 1 wt%) of hydrophilic SiO2 nanoparticles dispersed in 1 wt% NaCl were used as nanofluids. Samples were immersed in fluid cells and chemical force mapping was used to measure the adhesion force between polar/non-polar moieties to substrates. Adhesion work was evaluated based on force-displacement curves and compared with theories. Results from AFM studies indicate that low salinity waters and nanoparticle dispersions promote nanoscale wettability alteration by significantly reducing three-phase adhesion force and the reversible thermodynamic work of adhesion, also known as adhesion energy. The maximum reduction in adhesion energy obtained in experiments was in excellent agreement with existing theories. Electrostatic repulsion and reduced non-electrostatic adhesion are prominent surface forces common to both low salinity and nanofluid EOR. Structural forces are complex in nature and may not always decrease total adhesion force and energy at high nanoparticle concentration. Wettability effects also depend on surface chemical groups and the presence of divalent Mg2+ and Ca2+ cations. This study provides fresh insights and fundamental information about low salinity and nanofluid EOR while demonstrating the application of force-distance spectroscopy in investigating EOR techniques.

Engineering interfacial entropic effects to generate giant viscosity changes in nanoparticle embedded polymer thin films.

Thin polymer and polymer nanocomposite (PNC) films are being extensively used as advanced functional coating materials in various technological applications. Since it is widely known that various properties of these thin films, especially their thermo-mechanical behavior, can be considerably different from the bulk depending on the thickness as well as interaction with surrounding media, it is imperative to study these properties directly on the films. However, quite often, it becomes difficult to perform these measurements reliably due to a dearth of techniques, especially to measure mechnical or transport properties like the viscosity of thin polymer or PNC films. Here, we demonstrate a new method to study the viscosity of PNC thin films using atomic force microscopy based force-distance spectroscopy. Using this method we investigated viscosity and the glass transition, Tg, of PNC thin films consisting of polymer grafted nanoparticles (PGNPs) embedded in un-entangled homopolymer melt films. The PGNP-polymer interfacial entropic interaction parameter, f, operationally controlled through the ratio of grafted and matrix molecular weight, was systematically tuned while maintaining good dispersion even at very high PGNP loadings, φ. We observed both a significant reduction (low f) and giant enhancement (high f) in the viscosity of the PNC thin films with the effect becoming more prominent with increasing φ. Significantly, none of the established theoretical models for viscosity changes observed earlier in suspensions or polymer nanocomposites can explain the observed viscosity variation. Our results thus not only demonstrate the tunability of the interfacial entropic effect to facilitate a dramatic change in the viscosity of PNC coatings, which could be of great utility in various applications of these materials, but also suggest a new regime of viscosity variation in athermal PNC films indicating the possible need for a new theoretical model.

Investigation of the effects of ions on short-range non-DLVO forces at the calcite/brine interface and implications for low salinity oil-recovery processes

A primary question investigated in this study is the influence of brine chemistry on the behavior of short-range non-DLVO (e.g., hydration, discrete ion charge effects, and so on) forces at calcite surfaces. Specifically, how do wetting films containing Na+ differ from those containing Mg2+? Force-distance spectroscopy as measured by atomic force microscopy is used to probe short-range non-DLVO forces in various single-salt and multiple-salt electrolyte solutions. Experimental results reveal that, in single-component solutions, a greater concentration of Na+ ions decreases the decay length of short-range repulsion while a greater concentration of Mg2+ ions increases decay length. These results imply that Na+ ions reduce the affinity of calcite surfaces for water whereas Mg2+ ions make calcite more hydrophilic. Importantly, the relationship between the behavior of non-DLVO forces at small separations and concentrations of ions is not monotonic in multiple-component brines. Our observations support the hypothesis that Na+ ions disturb the interfacial water structure of calcite while Mg2+ ions are arranged farther away from the surface. Results obtained here may have implications for the design of so-called low salinity waterfloods where the composition of brines used for crude oil recovery is manipulated to increase oil recovery.

Investigation of CVD graphene as-grown on Cu foil using simultaneous scanning tunneling/atomic force microscopy.

Scanning tunneling microscopy (STM) and atomic force microscopy (AFM) images of graphene reveal either a triangular or honeycomb pattern at the atomic scale depending on the imaging parameters. The triangular patterns at the atomic scale are particularly difficult to interpret, as the maxima in the images could be every other carbon atom in the six-fold hexagonal array or even a hollow site. Carbon sites exhibit an inequivalent electronic structure in HOPG or multilayer graphene due to the presence of a carbon atom or a hollow site underneath. In this work, we report small-amplitude, simultaneous STM/AFM imaging using a metallic (tungsten) tip, of the graphene surface as-grown by chemical vapor deposition (CVD) on Cu foils. Truly simultaneous operation is possible only with the use of small oscillation amplitudes. Under a typical STM imaging regime the force interaction is found to be repulsive. Force–distance spectroscopy revealed a maximum attractive force of about 7 nN between the tip and carbon/hollow sites. We obtained different contrast between force and STM topography images for atomic features. A honeycomb pattern showing all six carbon atoms is revealed in AFM images. In one contrast type, simultaneously acquired STM topography revealed hollow sites to be brighter. In another, a triangular array with maxima located in between the two carbon atoms was acquired in STM topography.

Physicochemical Tools: Toward a Detailed Understanding of the Architecture of Targeted Radiotherapy Nanoparticles.

Targeted radiotherapy is proving to be an effective alternative to external beam radiotherapy for cancer treatment. Gold nanoparticles are biocompatible, commercially available, and readily functionalized, which makes them perfect candidates for the delivery of cytotoxic radionuclides labeled with antibodies to proteins abnormally expressed on cancer tissue. However, there is a lack of information regarding the efficacy of the successive modification steps involved in the functionalization process, as well as of the actual final state of the nanoparticles prior to preclinical tests, which results in a very inefficient screening and that will further impact on biological barriers, such as half-life interactions with serum proteins. Here, gold nanoparticles (15 nm diameter) were functionalized with linkers for antibody and radionuclide conjugation, following a well-stablished method. Successful coating of the gold nanoparticles was demonstrated using state-of-the-art physicochemical techniques, which include AF4-UV-ICPMS-MALS, Raman spectroscopy, and force-distance spectroscopy, which have led to an accurate description of the hydrodynamic diameter of the functionalized NPs and also about the adhesion energy and elastic properties of the modified NPs. Successive steps involved in the coating led to an organic shell of 12 nm diameter and no nanoparticle aggregation was observed. This may be a consequence of a decrease (or even the total absence) in water adsorption on the metal surface and/or of the organic labeling, that decreases the surface tension of the particles as estimated from the atomic force microscopy force-distance curves. Radiolabeling of gold nanoparticles prescreened using these physicochemical tools with 177Lu resulted in >75% efficiency.

Large Converse Piezoelectric Effect Measured on a Single Molecule on a Metallic Surface.

The converse piezoelectric effect is a phenomenon in which mechanical strain is generated in a material due to an applied electrical field. In this work, we demonstrate the converse piezoelectric effect in single heptahelicene-derived molecules on the Ag(111) surface using atomic force microscopy (AFM) and total energy density functional theory (DFT) calculations. The force-distance spectroscopy acquired over a wide range of bias voltages reveals a linear shift of the tip-sample distance at which the contact between the molecule and tip apex is established. We demonstrate that this effect is caused by the bias-induced deformation of the spring-like scaffold of the helical polyaromatic molecules. We attribute this effect to coupling of a soft vibrational mode of the molecular helix with a vertical electric dipole induced by molecule-substrate charge transfer. In addition, we also performed the same spectroscopic measurements on a more rigid o-carborane dithiol molecule on the Ag(111) surface. In this case, we identify a weaker linear electromechanical response, which underpins the importance of the helical scaffold on the observed piezoelectric response.

Correlative infrared nanospectroscopic and nanomechanical imaging of block copolymer microdomains.

Intermolecular interactions and nanoscale phase separation govern the properties of many molecular soft-matter systems. Here, we combine infrared vibrational scattering scanning near-field optical microscopy (IR s-SNOM) with force–distance spectroscopy for simultaneous characterization of both nanoscale optical and nanomechanical molecular properties through hybrid imaging. The resulting multichannel images and correlative analysis of chemical composition, spectral IR line shape, modulus, adhesion, deformation, and dissipation acquired for a thin film of a nanophase separated block copolymer (PS-b-PMMA) reveal complex structural variations, in particular at domain interfaces, not resolved in any individual signal channel alone. These variations suggest that regions of multicomponent chemical composition, such as the interfacial mixing regions between microdomains, are correlated with high spatial heterogeneity in nanoscale material properties.

Electrochemical and adhesion properties of hydrothermally deposited nano-ZrO2 coatings on oxide layers of stainless steel

ZrO2 coatings on pre-oxidized stainless steel (304L) surfaces have been developed by hydrothermal process and the coating adhesion and corrosion resistance properties of the coated specimens have been studied. The role of pre-oxidized SS surfaces composed of different oxides like spinels and α-Fe2O3 with different morphologies has been studied. The thicknesses of the ZrO2 coatings are in the range of 50nm to 350nm depending upon the compositions of the pre-oxidized layers. The coating is thick and compact when developed on α-Fe2O3 with high surface roughness and shows the best corrosion resistance properties. ZrO2 coatings developed on large size spinel oxides are thin and porous. The nano-mechanical properties and the local adhesion force have been studied by atomic force acoustic microscopy and force-distance spectroscopy. The adhesion force of ZrO2 coating on α-Fe2O3 is around 100nN while for the coatings developed on other pre-oxidized surfaces are around 50nN.

Electronic and Chemical Properties of Donor, Acceptor Centers in Graphene.

Chemical doping is one of the most suitable ways of tuning the electronic properties of graphene and a promising candidate for a band gap opening. In this work we report a reliable and tunable method for preparation of high-quality boron and nitrogen co-doped graphene on silicon carbide substrate. We combine experimental (dAFM, STM, XPS, NEXAFS) and theoretical (total energy DFT and simulated STM) studies to analyze the structural, chemical, and electronic properties of the single-atom substitutional dopants in graphene. We show that chemical identification of boron and nitrogen substitutional defects can be achieved in the STM channel due to the quantum interference effect, arising due to the specific electronic structure of nitrogen dopant sites. Chemical reactivity of single boron and nitrogen dopants is analyzed using force-distance spectroscopy by means of dAFM.

Radius ratio rule for surface hydrophilization of polydimethyl siloxane and silica nanoparticle composite

Polydimethyl siloxane (PDMS) and Silica (SiO2) nanoparticle composite blocks of three different batches (CB1–CB3) made by varying the size of SiO2 nanoparticles (NP), are studied for the degree of hydrophilization and retainability after oxidation by contact angle measurements (CA) and force distance spectroscopy (FDS) using Atomic Force Microscope (AFM). While CA measurements have shown high hydrophilization and retainability for CB3, F-D spectroscopy has reiterated the observation and has shown long range interactive forces and high Debye length of the electrostatic double layer formed. These results are in agreement with the radius ratio rule of binary sphere system for high density packing in the composite and thereby for strong hydrophilization and retainability due to reinforcement and restricted diffusion of uncured polymer.

A strategy for analyzing bond strength and interaction kinetics between Pleckstrin homology domains and PI(4,5)P2 phospholipids using force distance spectroscopy and surface plasmon resonance.

Phospholipids are important membrane components involved in diverse biological activities ranging from cell signaling to infection by viral particles. A thorough understanding of protein-phospholipid interaction dynamics is thus crucial for deciphering basic cellular processes as well as for targeted drug discovery. For any specific phospholipid-protein binding experiment, various groups have reported different binding constants, which are strongly dependent on applied conditions of interactions. Here, we report a method for accurate determination of the binding affinity and specificity between proteins and phospholipids using a model interaction between PLC-δ1/PH and phosphoinositide phospholipid PtdIns(4,5)P2. We developed an accurate Force Distance Spectroscopy (FDS)-based assay and have attempted to resolve the problem of variation in the observed binding constant by directly measuring the bond force. We confirm the FDS findings of a high bond strength of ∼0.19 ± 0.04 nN by Surface Plasmon Resonance (SPR) data analysis, segregating non-specific interactions, which show a significantly lower K(D) suggesting tight binding.

Conservative and Dissipative Interactions of Ionic Liquids in Nanoconfinement

By applying a small-amplitude (∼200 pm) oscillation to an atomic force microscopy probe during force–distance spectroscopy we are able to separate the “resistance to squeeze” of an ionic liquid nanoconfined between the probe and a mica sheet into its conservative and dissipative components. The interaction stiffness of the ionic liquid increases to a solid-like value as the mica is approached, while the “effective local viscosity” dramatically increases above the bulk value due to adjacent layers of ion-pairs interlocking as they slide over each other.

Investigations of the intermolecular forces between RDX and polyethylene by force-distance spectroscopy and molecular dynamics simulations.

The development of novel nanoenergetic materials with enhanced bulk properties requires an understanding of the intermolecular interactions occurring between molecular components. We investigate the surface interactions between 1,3,5-trinitro-1,3,5-triazacyclohexane (RDX) and polyethylene (PE) crystals on the basis of combined use of molecular dynamics (MD) simulations and force-distance spectroscopy, in conjunction with Lifshitz macroscopic theory of van der Waals forces between continuous materials. The binding energy in the RDX-PE system depends both on the degree of PE crystallinity and on the RDX crystal face. Our MD simulations yield binding energies of approximately 132 and 120 mJ/m(2) for 100% amorphous and 100% crystalline PE on RDX (210), respectively. The average value is about 36% greater than our experimental value of 81 ± 15 mJ/m(2) for PE (∼48% amorphous) on RDX (210). By comparison, Liftshitz theory predicts a value of about 79 mJ/m(2) for PE interacting with RDX. Our MD simulations also predict larger binding energies for both amorphous and crystalline PE on RDX (210) compared to the RDX (001) surface. Analysis of the interaction potential indicates that about 60% of the binding energy in the PE-RDX system is due to attractive interactions between HPE-ORDX and CPE-NRDX pairs of atoms. Further, amorphous PE shows a much longer interaction distance than crystalline PE with the (210) and (001) RDX surfaces due to the possibility of larger polymer elongations in the case of amorphous PE as strain is applied. Also, we report estimates of the binding energies of energetic materials RDX and octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX) with PE, propylene, polystyrene, and several fluorine-containing polymers using Lifshitz theory and compare these with reported MD calculations.