Scanning Polarization Force Microscopy Research Articles

Two-dimensional ice-like water adlayers on a mica surface with and without a graphene coating under ambient conditions.

Water tends to wet all hydrophilic surfaces under ambient conditions, and the first water adlayers on solids are important for a broad range of physicochemical phenomena and technological processes, including corrosion, wetting, lubrication, anti-icing, catalysis, and electrochemistry. Unfortunately, challenges in characterizing the first water adlayer in the laboratory have hampered molecular-level understanding of the contact water structure. Herein, we present the first ab initio molecular dynamics simulation evidence of a previously unreported ice-like adlayer structure (named as Ice-AL-II) on a prototype mica surface under ambient conditions. Calculation showed that the newly identified Ice-AL-II structure is more stable than the widely recognized ice-adlayer structure on mica surfaces (named as Ice-AL-I). Ice-AL-II exhibited a face-centered corner-cut tetragon (or a face-centered irregular pentagon) pattern of a hydrogen-bonded network. The center of the corner-cut tetragon was occupied by either a K+ cation or a water molecule with two H atoms pinned by the mica (100) via double hydrogen bonds. Our simulation also suggested that bilayer Ice-AL-II favors AA stacking rather than AB stacking. Interestingly, when a graphene sheet was coated on top of the ice-like adlayer, the stability of Ice-AL-II was further enhanced. In contrast, due to its strongly puckered structure, the Ice-AL-I structure could be crushed into a near-Ice-AL-II structure by the graphene coating. Ice-AL-II is thus proposed as a promising candidate for the ice-like structure on a mica surface detected by scanning polarization force microscopy and by atomic force microscopy between a graphene coating and a mica surface.

Soft matter analysis via atomic force microscopy (AFM): A review

Soft materials or soft condensed matter are being globally developed for various technological applications for chemical, consumer goods, pharmaceutical, agri-business, and petroleum industries. As such, there is a growing research interest in understanding the dynamics, structure, and morphology of soft materials, particularly in soft interfacial materials. While atomic force microscopy (AFM) is conventionally used for solid matter research studies, this review focuses on the emerging application of AFM as an effective analytical tool for investigating soft materials. By harnessing the capabilities of AFM, researchers can explore soft condensed matter in novel ways. Specifically, this review discusses the three AFM modes; contact, non-contact, and tapping modes, highlighting their working principles and recent applications in soft matter analysis. Moreover, derivatives of each principal AFM mode, such as Lateral Force Microscopy (LFM), nanolithography, force spectroscopy, Conductive AFM (CAFM), Scanning Polarization Force Microscopy (SPFM), and PeakForce Tapping (PFT) are included and discussed in detail. Critical analysis and comparison of these imaging modes are presented, shedding light on their advantages and disadvantages in soft matter analysis. This review aims to provide researchers with valuable insights for further advancements in the field by presenting a comprehensive overview of AFM modes in soft matter research.

Single-pass magnetic force microscopy technique, with topography feedback based on scanning polarization force microscopy

We propose the use of a hybrid scanning probe microscopy technique, which combines magnetic force microscopy with scanning polarization force microscopy, for rapid scans of relatively large areas. Topography feedback relies on maintaining a constant electric interaction between tip and sample, and is achieved by monitoring the off-resonance tip oscillation induced by electric modulation. Magnetic sensing is based on recording changes of the amplitude and phase of the cantilever’s mechanically modulated oscillation at resonance. By using electric interaction instead of van der Waals for topography feedback, the tip is held in feedback at relatively large distances from the surface – tens to hundreds of nanometers. This reduces the cross-talk between topography and magnetic signals, thus eliminating the need for two-pass scanning. Moreover, it allows imaging at higher speeds with lower risk of tip damage, opening this field to studies of dynamic phenomena. Advantages and disadvantages of this technique are discussed.

Formation and growth of two-dimensional ices withoutconfinement

<p indent="0mm">Confined and interfacial water/ice is ubiquitous in nature and plays an important role in a broad range of physicochemical phenomena and technological processes such as rock fracture, friction, and nanofluids. The highly confined environment at the nanoscale can disrupt the hydrogen bonding network in water, thereby increasing the number of possible polymorphs of two-dimensional (2D) water/ice. It is also known that water molecules next to a solid surface can undergo stratification that extends two to three molecular diameters. A number of recent studies have revealed the possibility of forming 2D ice without the need of nanoscale confinement. For example, the presence of an ice-like water layer has been detected in scanning polarization force microscopy on the hydrated muscovite surface at room temperature. A sum-frequency generation experiment revealed no free OH bonds in the adsorbed water layer on mica. Scanning tunneling microscopy imaging evidence of a helical ice monolayer in which every six water molecules are helically arranged along the normal to the basal plane has reported. In addition, the growth of 2D bilayer hexagonal ice (BHI) structures has been widely observed on different surfaces, such as graphene, Pt(111) substrate, Ru(0001) substrate and Au(111) substrate, without the need of confinement in the laboratory. In this paper, we review the recent theoretical progress on the formation and growth of 2D ices without confinement. In particular, we focus on the important role of surface wettability and surface structure in the formation of 2D ices. We find that BHI formation on various surfaces and the dependence of the 2D crystalline structure on the hydrophobicity and morphology of the underlying surface. The tendency toward the formation of BHI without confinement reflects a proper water-surface interaction that can compensate for the entropy loss during the freezing transition. Moreover, both experiments and simulations show that armchair-type edges coexist with the zigzag edges in BHI. In the case of zigzag edge of BHI whose growth processes follow a collective bridging mechanism, a periodic but unconnected array of pentagons is initially formed at the zigzag edge, and subsequent incoming water molecules collectively connect these pentagons, resulting in a 565-chain structure. The addition of further water molecules leads to the formation of a fully connected hexagon array. By contrast, armchair edge growth involves edge reconstruction into a periodic structure of 5756-type membrane rings. Addition of two water pairs converts the 575-type member rings to 656-type member rings. The 656-type member rings then grow laterally to form a 5656-type edge. Further, inserting one water pair into the hexagon of the 5656-type edge leads again to the formation of a 5756-type edge. At low temperature, BHI forms only on the hydrophilic surface and not on the hydrophobic surface. Because of the close lattice match between the BHI and the ice <italic>I</italic>_h basal face, the orientation of the ice crystal on the hydrophilic surface is dictated by the preexistence of the BHI. Ice growth is anisotropic. Among the three prism crystal faces of ice <italic>I</italic>_h, namely, basal face (BF), prism face (PF) and secondary prism face (SPF), the growth of the BF is the slowest. For the hydrophilic surface, because the SPF and PF are perpendicular to the solid surface, the ice crystal grows along the surfaces. By contrast, for the hydrophobic surface, an angle is observed between the solid surface and SPF, indicating that the ice crystal grows off the solid surface.

Humidity effects on scanning polarization force microscopy imaging

Scanning polarization force microscopy (SPFM) is a useful surface characterization technique to visually characterize and distinguish nanomaterial with different local dielectric properties at nanometer scale. In this paper, taking the individual one-atom-thick graphene oxide (GO) and reduced graphene oxide (rGO) sheets on mica as examples, we described the influences of environmental humidity on SPFM imaging. We found that the apparent heights (AHs) or contrast of SPFM imaging was influenced significantly by relative humidity (RH) at a response time of a few seconds. And this influence rooted in the sensitive dielectric constant of mica surface to the RH change. While dielectric properties of GO and rGO sheets were almost immune to the humidity change. In addition, we gave the method to determine the critical humidity at which the contrast conversion happened under different conditions. And this is important to the contrast control and repeatable imaging of SPFM through RH adjusting. These findings suggest a strategy of controllable and repeatable imaging the local dielectric properties of nanomaterials with SPFM, which is critically important for further distinguishment, manipulation, electronic applications, etc.

Direct imaging charge distribution in reduced graphene oxide sheets induced by isolated charges

In this paper, we directly visualized the charges distributed in one-atom-thick reduced graphene oxide (rGO) sheet induced by adjacent charged rGO using a sample-charged mode scanning polarization force microscopy. We found that electron carriers could be attracted to one side of the rGO sheet and leave holes on the other side. The induced charges were distributed inhomogeneously; that is, contrary to earlier reports, the free carrier concentration was neither distributed on the ends, nor distributed uniformly on the whole rGO sheet. When the surrounding rGO sheets were injected with electrostatic charges, the motion of the charge carriers happened in the target-neutral rGO sheet simultaneously. The charges induced in the rGO sheet by isolated charges on adjacent rGO sheets decayed rapidly with the increasing of their separated distance. In addition, fine control of the distribution of the induced charges in a single rGO sheet could be realized through placing more isolated charges in the surrounding areas. These findings suggest a feasible and precise strategy for the modulation and design of local-charge-sensitive functional graphene-based systems.

Humidity induced charge migration on single layer graphene oxide sheets

The charge migration on the surface of single layer graphene oxide (GO) sheets was investigated by scanning polarization force microscopy. Electrostatic charges were observed to transfer over 1 μm across the insulating single layer GO sheets surface with the help of water molecules condensed from the environment. Humidity-dependent charge migration on GO sheets was visualized on the nanometer scale and the migration velocity was plotted at a relative humidity range of 10%–40%. When the relative humidity reached 50% or higher, water film would form on the whole GO sheet surface, which made the GO sheet conductive to the electrostatic charges.

Sample-charged mode scanning polarization force microscopy for characterizing reduced graphene oxide sheets

A unique operation mode of scanning polarization force microscopy (SPFM) was developed for characterizing reduced graphene oxide (rGO) sheets that were individually charged, mainly by monitoring the change of the sample's apparent height along with its surface potential. The principles and features of this sample-charged mode SPFM (SC-SPFM) were introduced. By comparing with other scanning-probe based techniques that characterize the surface electrical properties, including the traditional tip-biased mode SPFM, electrostatic force microscopy, and Kelvin probe force microscopy, it was found that the SC-SPFM has higher sensitivity and lateral resolution. Furthermore, by monitoring charge transfer between two rGO sheets with SC-SPFM, the “good” or “bad” contacts related to junction geometry at the nanometer scale can be visualized clearly.

Wetting properties of glycerol on silicon, native SiO2, and bulk SiO2 by scanning polarization force microscopy

Scanning polarization force microscopy, a relatively new non-contact scanning probe microscopy technique, was applied in order to investigate the properties of liquid surfaces (droplets), such as: topography, microscopic contact angle θ, surface potential energy P(e), spreading coefficient S, and disjoining pressure П. Investigations were carried out on glycerol droplets deposited on surfaces of bare silicon, silicon covered with native oxide, and bulk silicon oxide. Contact angle values were determined from directly measured topography profiles of micro- and nanodroplets. Values of surface potential energy, spreading coefficient, and disjoining pressure were calculated based on a model of the dependence of contact angle on droplet height. The results of these experiments offer valuable insights into the mechanisms of wetting phenomena at the microscopic scale.

Scanning polarization force microscopy investigation of contact angle and disjoining pressure of glycerol and sulfuric acid on highly oriented pyrolytic graphite and aluminum

For liquid droplets of sub-micrometer dimensions, the study of wetting properties (quantified by contact angle, disjoining pressure, spreading coefficient, etc.) is possible using the relatively new technique known as scanning polarization force microscopy (SPFM). This non-contact scanning probe microscopy technique was successfully implemented in our laboratory in order to study the wetting properties of glycerol and sulfuric acid on the surface of highly oriented pyrolytic graphite (HOPG) and glycerol on aluminum film deposited on mica. An AC polarization bias of 3 V at 3 kHz frequency was applied between a conductive atomic force microscope tip and the substrate. The resulting polarization force was measured with high accuracy, allowing non-contact topography profile measurements of liquid micro- and nanodroplets. The dependence of the contact angle on droplet height was determined in order to calculate the values of the spreading coefficient and the disjoining pressure between the liquid and substrates. The calculated potential energies give disjoining pressure values of ∼0.4 atm for glycerol on HOPG, ∼0.47 atm for glycerol on aluminum and ∼13 atm for H2SO4 on HOPG. In the case of H2SO4 on HOPG the strength of the force appears to be thirty times bigger than that for glycerol on HOPG and aluminum.

MRT letter: Real time and in situ imaging the reversible evolution of ethanol vapor condensed on mica surface

The reversible adsorption and desorption of ethanol vapor on mica surface at ambient temperature were investigated in situ with time-evolution by Vibrating Scanning Polarization Force Microscopy (VSPFM). At temperature 20 °C and relative humidity 20%, ethanol vapor condensed and formed clusters on the freshly cleaved mica. These clusters expanded to a network structure and later formed a full film shown by continuous VSPFM imaging. The film broke into pieces and desorbed completely if in unsaturated condition. The film showed different apparent heights when different biases were used in VSPFM study, indicated polarized orientation of the ethanol molecules on the mica surface. It is a process of hours for the clusters or network structures to form a film on mica, which demonstrated that vapor ethanol molecule is not so easy to precipitate on mica as ethanol molecule in liquid stage.

Application of image alignment and time averaging methods in AFM detection for single DNA molecules

Enhancement of the SNR (signal to noise ratio) in single-molecule imaging is significantly important for improving image resolution and distinguishing the fine structures of single molecules at a higher precision level. Image processing techniques have demonstrated the remarkable capability to improve the SNR and the resolution level by breaking through some inherent limitations unresolved by instrument hardware optimization. In this paper, we focus on single-biomolecule imaging using atomic force microscopy (AFM), a unique method in separated single-biomolecule imaging, and there was few suitable image processing techniques reported to increase the SNR of a single molecule’s AFM image. With the typical samples of separately dispersed DNA molecules, we replaced the classified averaging method, which is usually used when the molecules’ structure can be easily and repeatedly prepared, with the time averaging method to improve the SNR in a single molecule’s AFM imaging. Combining the time averaging technique with the image alignment method for the series of time-lapse AFM images of a single DNA molecule, we were able to improve the image’s SNR and recover some buried signals from the background noises. Furthermore, the fine structures of single molecules can potentially be further resolved if other image processing techniques are applied collaboratively with some newly developed imaging methods like Vibrating Mode Scanning Polarization Force Microscopy (VSPFM), and such combination will also be advantageous for the manipulation of single-biomolecules by AFM. In addition, the proposed algorithms for the estimations of drift, distortion and SNR are applicable for quantitative status characterization of AFM imaging.

Evaluation of the Radial Deformability of Poly(dG)−Poly(dC) DNA and G4-DNA Using Vibrating Scanning Polarization Force Microscopy

Poly(dG)-poly(dC) DNA and G4-DNA are two types of synthetic molecules that are regarded as promising candidates for molecular nanodevices. In this work, vibrating scanning polarization force microscopy (VSPFM) was employed to study the radial deformability of these two molecules coadsorbed on a Ni(2+)-modified mica surface. The values of the radial compressive elastic modulus of these two types of molecules were found to be similar (approximately 5-10 MPa) when the external loading force was between approximately 0.04 and approximately 0.12 nN. However, G4-DNA molecules possessed higher stiffness than poly(dG)-poly(dC) DNA (approximately 20-40 vs approximately 10-20 MPa) when the loading force was larger than approximately 0.12 nN. The results will aid us in understanding these molecule's mechanical performances.

Ion Segregation and Deliquescence of Alkali Halide Nanocrystals on SiO2

The adsorption of water on alkali halide (KBr, KCl, KF, NaCl) nanocrystals on SiO2 and their deliquescence was investigated as a function of relative humidity (RH) from 8% to near saturation by scanning polarization force microscopy. At low humidity, water adsorption solvates ions at the surface of the crystals and increases their mobility. This results in a large increase in the dielectric constant, which is manifested in an increase in the electrostatic force and in an increase in the apparent height of the nanocrystals. Above 58% RH, the diffusion of ions leads to Ostwald ripening, where larger nanocrystals grow at the expense of the smaller ones. At the deliquescence point, droplets were formed. For KBr, KCl, and NaCl, the droplets exhibit a negative surface potential relative to the surrounding region, which is indicative of the preferential segregation of anions to the air/solution interface.

Influence of the macroscopic shape of the tip on the contrast in scanning polarization forcemicroscopy images

We demonstrate that a quantitative analysis of the contrast obtained in electrostatic forcemicroscopy images that probe the dielectric response of the sample (scanning polarizationforce microscopy (SPFM)) requires numerical simulations that take into account both themacroscopic shape of the tip and the nanoscopic tip apex. To simulate the SPFMcontrast, we have used the generalized image charge method (GICM), which isable to accurately deal with distances between a few nanometers and severalmicrons, thus involving more than three orders of magnitude. Our numericalsimulations show that the macroscopic shape of the tip accounts for most of the SPFMcontrast. Moreover, we find a quasi-linear relation between the working tip–sampledistance and the contrast for tip radii between 50 and 200 nm. Our calculations arecompared with experimental measurements of the contrast between a thermallygrown silicon oxide sample and a few-layer graphene film transferred onto it.

Mixing behaviour of a conjugated molecular wire candidate and an insulating fatty acid within Langmuir–Blodgett films

The miscibility of a conjugated “molecular wire” candidate and an “insulating” fatty acid within a thin film has been comprehensively analyzed. Specifically, Langmuir and Langmuir–Blodgett films containing mixtures of 4-[4′-(4″-hexyloxyphenylethynyl)-phenylethynyl]-aniline (HBPEB amine) and behenic acid (BA) have been prepared, characterized, and their miscibility compared with that of mixtures of 4-[4′-(4″-hexyloxyphenylethynyl)-phenylethynyl]-benzoic acid (HBPEB acid) and BA studied previously. Surface pressure vs. area per molecule isotherms were recorded, with negligible excess areas per molecule at high surface pressures suggesting either complete miscibility or immiscibility of the two materials. The Langmuir films were transferred onto solid supports to fabricate Langmuir–Blodgett (LB) films which were characterized by SPM probe techniques. Scanning Polarization Force Microscopy (SPFM) undoubtedly shows good miscibility of the two materials in the films with no evidence of phase separation, in contrast with the HBPEB acid–BA system where phase separation occurs for HBPEB acid mole fractions over the 0.1–0.8 range.

Effects of Ionic Side Groups Attached to Polydimethylsiloxanes on Lubrication of Silicon Oxide Surfaces

The boundary film formation and lubrication effects of low molecular weight silicone lubricant molecules with cationic side groups were studied. Poly(N,N,N-trimethylamine-3-propylmethylsiloxane-co-dimethylsiloxane) iodide was synthesized and deposited on silicon oxide surfaces to form a bound-and-mobile lubricant film. The bound nature was investigated with ellipsometry, water contact angle, and X-ray photoelectron spectroscopy for the polymers with cationic mole percent of 6, 15, and 30 mol % (monomer based). The bound layer thickness decreased as the cationic content increased. The quaternary ammonium cations in this layer were electrostatically bound to the substrate surface. The mobile nature of the multilayers was explored with scanning polarization force microscopy. The multilayer films exhibited characteristic topographic features due to ionic interactions within the polymer film. Contact scratching of these films altered the multilayer topography within the contact scanned area. Even after high load contact scanning, the bound layer was not removed from the scanned region. These results implied that the molecules in the first layer are strongly bound and the molecules in the multilayers are mobile. Both nanoscale and macroscale tribological tests of these films revealed that the polymer with 15 mol % cationic groups gives lower friction and adhesion than the 6 and 30 mol % cationic polymers as well as the polydimethylsiloxane control sample. This seems to be due to a synergistic effect between the bound and the mobile layers.

Thin water films grown at ambient conditions on BaF2(111) studied by scanning polarization force microscopy

The interaction of water with freshly cleaved BaF(2)(111) surfaces has been studied using scanning force microscopy operated in different modes at room temperature and under controlled humidity. The Kelvin probe microscopy (KPM) mode has been used to study the evolution of the surface potential differences (SPDs). In the 20%-50% relative humidity (RH) range, adsorbed water forms two-dimensional solidlike bilayers (islands). The SPD between water islands and the bare substrate surface exhibits a sign crossover from negative ( approximately -30 mV) at low RHs to positive ( approximately +50 mV) at higher RHs, evidencing a cooperative and irreversible flipping of the preferential orientation of water dipoles, from pointing toward the surface evolving into the opposite direction. The KPM results suggest that the classical hexagonal (I(h)) bilayer configuration is not the most favorable structure.

Two-Dimensional Ordering of Water Adsorbed on a Mica Surface at Room Temperature

Monte Carlo simulations of water films on muscovite mica at room temperature reveal the formation of a strongly bound and two-dimensionally ordered water layer dependent on the arrangement and concentration of adsorbed K+ ions. This first water layer has polygonal habits in accordance with those of mica crystals, and its formation within clusters with monolayer K+ ion coverage is proposed as a prerequisite for the nucleation and growth of the polygonal water islands observed recently with scanning polarization force microscopy (SPFM) on the muscovite surface at room temperature. The saturation of this water layer in the simulation corresponds very well to the threshold for the nucleation of polygonal water islands observed in experimental studies. The saturation of the first hydration shells of the K+ ions adsorbed within clusters with monolayer K+ ion coverage results in the formation of a water film with a water−air interface of ∼2.6 A above the first water layer in very good agreement with a height con...

Study of Radial Compression Elasticity of Single Xanthan Molecules by Vibrating Scanning Polarization Force Microscopy

Individual xanthan molecules were prepared on highly oriented pyrolytic graphite surface with a modified spin-casting technique. Then the radial compression elasticity of single xanthan molecules was investigated by vibrating scanning polarization force microscopy. The effective elastic moduli of xanthan molecules are estimated to be approximately 20-100 MPa under loads below 0.4 nN.