Electrostatic Force Microscopy Phase Research Articles

Probing Polymorphic Stacking Domains in Mechanically Exfoliated Two-Dimensional Nanosheets Using Atomic Force Microscopy and Ultralow-Frequency Raman Spectroscopy.

As one of the key features of two-dimensional (2D) layered materials, stacking order has been found to play an important role in modulating the interlayer interactions of 2D materials, potentially affecting their electronic and other properties as a consequence. In this work, ultralow-frequency (ULF) Raman spectroscopy, electrostatic force microscopy (EFM), and high-resolution atomic force microscopy (HR-AFM) were used to systematically study the effect of stacking order on the interlayer interactions as well as electrostatic screening of few-layer polymorphic molybdenum disulfide (MoS2) and molybdenum diselenide (MoSe2) nanosheets. The stacking order difference was first confirmed by measuring the ULF Raman spectrum of the nanosheets with polymorphic stacking domains. The atomic lattice arrangement revealed using HR-AFM also clearly showed a stacking order difference. In addition, EFM phase imaging clearly presented the distribution of the stacking domains in the mechanically exfoliated nanosheets, which could have arisen from electrostatic screening. The results indicate that EFM in combination with ULF Raman spectroscopy could be a simple, fast, and high-resolution method for probing the distribution of polymorphic stacking domains in 2D transition metal dichalcogenide materials. Our work might be promising for correlating the interlayer interactions of TMDC nanosheets with stacking order, a topic of great interest with regard to modulating their optoelectronic properties.

Stimuli-Responsive Electrospun Piezoelectric Mats of Ethylene-co-vinyl Acetate-Millable Polyurethane-Nanohydroxyapatite Composites.

Piezoelectric materials have gained interest among materials scientists as body motion sensors and energy harvesters on account of their fast responsiveness and substantial output signals. In this work, piezoelectric polymer mats have been fabricated from ethylene-co-vinyl acetate-millable polyurethane/nanohydroxyapatite (EVA-MPU/nHA) composite systems by employing the electrospinning technique. The ferro-piezoelectric features of the samples were confirmed from the butterfly loops of electrostatic force microscopy (EFM) amplitude signals as well as through the hysteresis curves of the EFM phase recorded with the assistance of dynamic-contact EFM. Piezoelectric responses of the samples to random finger tapping were evaluated after fabricating a simple device prototype connected to an oscilloscope. The efficacy of the mats to generate a voltage in response to activities such as mechanical bending, movement of throat muscles while drinking, movement of elbow joints, air blowing, and so forth has also been investigated. The results suggest the promising possibility of fabricating user-friendly piezoelectric mats out of the EVA-MPU/nHA system for physiological motion-sensing applications.

Quantitative characterization of dielectric properties of nanoparticles using electrostatic force microscopy

We use a new method based on Electrostatic Force Microscopy (EFM) to perform quantitative measurements of the dielectric constants of individual carboxylated as well as non-functionalized polystyrene nanospheres. The EFM data record the oscillation phase of an atomic force microscope (AFM) cantilever as a function of the AFM tip position. In our experiments, the relative dielectric constant of the sample is measured from the EFM phase shifts vs the tip–surface separation, according to a simple analytical model describing the tip–surface interactions. We perform a comprehensive study of how the dielectric constant depends on the sphere diameter for both types of nanospheres. Our results demonstrate that the experimental method has a high-resolution for measuring the dielectric constant of nano/microbeads and other nanoscale materials and is simple to implement on standard atomic force microscopes. This non-invasive technique can be applied to measure the electrical properties of colloidal particles, polymers, interphases, and polymer nanocomposites.

Quantitative characterization of dielectric properties of polymer fibers and polymer composites using electrostatic force microscopy

We use a new method based on electrostatic force microscopy (EFM) to perform quantitative measurements of the dielectric constants of individual electrospun nanofibers of poly(L-lactic acid) (PLLA), as well as composite fibers of PLLA with embedded multiwall carbon nanotubes (MWCNT-PLLA). The EFM data record the oscillation phase of an atomic force microscope (AFM) cantilever as a function of the AFM tip position. In our experiments the relative dielectric constants ϵ of the sample are measured from the EFM phase shifts vs. the tip-surface separation, according to a simple analytical model describing the tip-surface interactions. We perform a comprehensive study of how the dielectric constant depends on the fiber diameter for both electrospun PLLA and MWCNT/PLLA fiber composites. Our measurements show that EFM can distinguish between dielectric properties of PLLA fibers and fiber composites with different diameters. Dielectric constants of both PLLA and MWCNT-PLLA composite fibers decrease with increasing fiber diameter. In the limit of large fiber diameters (D > 100 nm), we measure dielectric constants in the range: ϵ = 3.4–3.8, similar to the values obtained for unoriented PLLA films: ϵfilm = 2.4–3.8. Moreover, the dielectric constants of the small diameter MWCNT-PLLA composites are significantly larger than the corresponding values obtained for PLLA fibers. For MWCNT-PLLA nanofiber composites of small diameters (D < 50 nm), ϵ approaches the values measured for neat MWCNT: ϵCN = 12 ± 2. These results are consistent with a simple fiber structural model that shows higher polarizability of thinner fibers, and composites that contain MWCNTs. The experimental method has a high-resolution for measuring the dielectric constant of soft materials, and is simple to implement on standard atomic force microscopes. This non-invasive technique can be applied to measure the electrical properties of polymers, interphases, and polymer nanocomposites.

Detection of charge around a nanoparticle in a nanocomposite using electrostatic force microscopy

In this work, electrostatic force microscopy (EFM) is used to investigate the interface around a nanoparticle in terms of its properties viz. electric charge density and dielectric constant. Nanodielectrics composed of barium titanate (BaTiO 3 ) nanoparticles embedded in epoxy matrix are used for the study. Phase shift images obtained from EFM experiments indicate the region occupied by an embedded particle in the matrix, as well as the presence of an interfacial region around it. In an earlier work [12], a finite element method (FEM) based 3-D electrostatic model was developed by the authors to computationally obtain the EFM phase shift in the interfacial region, and was used in conjunction with experimental data to estimate the dielectric constant and thickness of the interface. The possibility of charge on the particle surface and/or in the interfacial region around it, is investigated using the same model and methodology. Computational results indicate that the presence, magnitude and polarity of charge affect the EFM phase shift signal. Comparison of computational and experimental data, confirms the presence of an interfacial region around the particle with a dielectric constant higher than that of the bulk epoxy, and with negative charge present in it.

Effect of Surface Alignment on Connectivity in Phosphonium-Containing Diblock Copolymer Anion-Exchange Membranes

The performance of ion-exchange membranes in fuel cells is critically linked to their ion-conducting morphology. In this report, we analyzed the connectivity of anion-exchange membranes (AEMs) by electrostatic force microscopy (EFM). As a tapping-mode AFM-based technique, EFM probes height, phase, and electrostatic force gradient through two-pass interleave scans, which reveals the direct relationship between membrane surface morphology and connectivity of the ionic domain. The AEMs are diblock copolymers of a polyisoprene (PIp) block and a polystyrenic block containing the phosphonium ion (P(R3P+)MS). Different alignments of cylinders were observed in the bulk and on the surface for AEMs with different ion-exchange capacities (IECs) and preparation methods. The impacts of the cylinder alignment on the connectivity of the ionic domain were then investigated by EFM. For AEM 13 with IEC = 0.44 mmol/g and a hexagonal morphology, cylinders aligned parallel to the surface, which led to many disconnected regions observed throughout the membrane. In contrast, the perpendicularly aligned channels of AEM 16 with IEC = 0.87 mmol/g consisted only of a well-connected ionic phase throughout the membrane.

Electrostatic force spectroscopy revealing the degree of reduction of individual graphene oxide sheets.

Electrostatic force spectroscopy (EFS) is a method for monitoring the electrostatic force microscopy (EFM) phase with high resolution as a function of the electrical direct current bias applied either to the probe or sample. Based on the dielectric constant difference of graphene oxide (GO) sheets (reduced using various methods), EFS can be used to characterize the degree of reduction of uniformly reduced one-atom-thick GO sheets at the nanoscale. In this paper, using thermally or chemically reduced individual GO sheets on mica substrates as examples, we characterize their degree of reduction at the nanoscale using EFS. For the reduced graphene oxide (rGO) sheets with a given degree of reduction (sample n), the EFS curve is very close to a parabola within a restricted area. We found that the change in parabola opening direction (or sign the parabola opening value) indicates the onset of reduction on GO sheets. Moreover, the parabola opening value, the peak bias value (tip bias leads to the peak or valley EFM phases) and the EFM phase contrast at a certain tip bias less than the peak value can all indicate the degree of reduction of rGO samples, which is positively correlated with the dielectric constant. In addition, we gave the ranking of degree for reduction on thermally or chemically reduced GO sheets and evaluated the effects of the reducing conditions. The identification of the degree of reduction of GO sheets using EFS is important for reduction strategy optimization and mass application of GO, which is highly desired owing to its mechanical, thermal, optical and electronic applications. Furthermore, as a general and quantitative technique for evaluating the small differences in the dielectric properties of nanomaterials, the EFS technique will extend and facilitate its nanoscale electronic devices applications in the future.

Measuring the local electrical properties of individual vanadium-pentoxide nanowires by using electrostatic force microscopy

A nanoscale investigation of the local electrical properties of individual vanadium-pentoxide (V2O5) nanowires (NWs) deposited on a SiO2/p+ Si substrate was carried out by using electrostatic force microscopy (EFM) in ambient conditions. We found that the EFM phase shift as an electrostatic response of V2O5 NWs to the biased-tip decreases with the lift height due to the electrostatic force gradient. We also observed that the EFM phase shift of V2O5 NWs shows the quadratic voltage dependence for a biased-tip with a fixed lift height. Furthermore, it is observed that the crossed junctions or bundles of the NWs compared to the single NWs showed higher electrostatic response, resulting in a large phase shift in the EFM measurements. These results suggest that the local electrical properties of V2O5 NW surface can be characterized in the nanoscale range by using EFM.

Non-Contact Local Conductance Mapping of Individual Graphene Oxide Sheets during the Reduction Process

We used electrostatic force microscopy (EFM) to investigate local conducting states of atomically thin individual graphene oxide (GO) sheets and monitor the spatial evolution of their conducting properties during the reduction process. Because of the thinness of the GO sheets and finite carrier density, the electric field is partially screened in the reduced GO, which is manifested in the EFM phase signals. We found inhomogeneous oxidation states in as-prepared GO sheets and followed the evolution of reduction process in the individual GO sheets during both thermal and chemical reduction. We also compared the EFM measurement results with simultaneous IV characteristics to assess correlations between two measurements.

Interaction and dynamics of ambient water adlayers on graphite probed using AFM voltage nanolithography and electrostatic force microscopy

In this work, we report the impact of the interaction and dynamics of increasing ambient water adlayers on etch patterns on a hydrophobic highly oriented pyrolytic graphite (HOPG) surface obtained using atomic force microscopy (AFM) voltage nanolithography in contact mode by applying a positive bias to the sample. The changes in the dimensions of the etch patterns were investigated as a function of the increasing number of water adlayers present on the HOPG, which is varied by changing the time interval since HOPG cleavage. Changes in the width of the etch patterns and the surrounding water droplets were monitored with time, using intermittent-contact-mode AFM. Electrostatic force microscopy (EFM) has been employed to study the charged nature of the etch patterns and the neighboring water film with time. The width of the etch patterns made on freshly cleaved HOPG shows an increase of ∼33% over 48 h, whereas nine-day-old cleaved HOPG shows a 79% increase over the same period. No changes in the dimensions are observed while imaging in a nitrogen atmosphere soon after lithography. In ambient conditions, the EFM phase shift of the patterns shows a large change of ∼84–88% over 30 h. This study demonstrates the effect of the stored electrostatic energy of a polarized ice-like water adlayer, resulting in changes in the dimensions of the etch patterns long after lithography, whereas liquid-like water droplets do not affect the etch patterns.

Direct observation of the spatial distribution of charges on a polypropylene fiber via Electrostatic Force Microscopy

The spatial distribution of electrical charges along the longitudinal axes of a polypropylene electret fiber was determined using Electrostatic Force Microscopy (EFM). EFM mapping on highly curved surfaces, such as those of polymeric fibers, is a challenging endeavour and most work reported in the scientific literature has been limited to single line-scan analysis or flat specimens. Charged polymeric fibers, electrets, are extensively used in high performance filtration applications and methods to determine the amount and magnitude of the charges on these fibers remain elusive. Electrical charge maps of individual fibers were obtained by biasing the tip to -10 V and maintaining a constant tip-sample distance of 100 nm. Spatially dependant EFM phase and magnitude gradients were determined and the developed technique may provide a unique understanding into the heterogeneous charge distribution on electrets fibers. Direct mapping of the charge distribution in electrets fibers can offer new insights in the development of antistatic additives, new means to facilitate electrostatic self-assembly of nano-moieties on the surface of fibrous materials and a quantitative metrics capable of determining discharge dynamics and predicting the shelf-life of filtration media.

Mapping of Embedded Functionalized Carbon Nanotubes in Poly(vinyl alcohol)/Nanotube Composite Using Electrostatic Force Microscopy

In the present study, electrostatic force microscopy (EFM) was used as a nondestructive tool for mapping individual functionalized single-wall carbon nanotubes (SWCNTs) incorporated in a poly(vinyl alcohol) (PVA)/CNT composite. The nanocomposite film was prepared using a spin-coating technique. The FT-IR results show esterification between the PVA and the functionalized CNTs. Furthermore, AFM images show that SWCNTs are embedded inside the PVA film. The distribution and orientation of the embedded CNTs were mapped using EFM phase imaging through investigation of the polarizability signal elicited from the CNTs. Results show that polarized SWCNTs are dispersed and oriented in different directions at different depths in the PVA matrix.

Layer-dependent nanoscale electrical properties of graphene studied by conductive scanning probe microscopy

The nanoscale electrical properties of single-layer graphene (SLG), bilayer graphene (BLG) and multilayer graphene (MLG) are studied by scanning capacitance microscopy (SCM) and electrostatic force microscopy (EFM). The quantum capacitance of graphene deduced from SCM results is found to increase with the layer number (n) at the sample bias of 0 V but decreases with n at -3 V. Furthermore, the quantum capacitance increases very rapidly with the gate voltage for SLG, but this increase is much slowed down when n becomes greater. On the other hand, the magnitude of the EFM phase shift with respect to the SiO2 substrate increases with n at the sample bias of +2 V but decreases with n at -2 V. The difference in both quantum capacitance and EFM phase shift is significant between SLG and BLG but becomes much weaker between MLGs with a different n. The layer-dependent quantum capacitance behaviors of graphene could be attributed to their layer-dependent electronic structure as well as the layer-varied dependence on gate voltage, while the layer-dependent EFM phase shift is caused by not only the layer-dependent surface potential but also the layer-dependent capacitance derivation.

Mapping of Local Electrical Properties in Epitaxial Graphene Using Electrostatic Force Microscopy

Local electrical characterization of epitaxial graphene grown on 4H-SiC(0001) using electrostatic force microscopy (EFM) in ambient conditions and at elevated temperatures is presented. EFM provides a straightforward identification of graphene with different numbers of layers on the substrate where topographical determination is hindered by adsorbates. Novel EFM spectroscopy has been developed measuring the EFM phase as a function of the electrical DC bias, establishing a rigorous way to distinguish graphene domains and facilitating optimization of EFM imaging.

Application of electrostatic force microscopy on characterizing an electrically charged fiber

The application of Electrostatic Force Microscopy (EFM) was studied in analyzing the corona-charged polypropylene fiber. Electrostatic force gradient images were obtained from the phase shifts with the varied bias voltages applied to the EFM cantilever by the noncontact scans. A mathematical expression to model the EFM phase shifts as a function of the applied tip bias voltages was introduced. EFM analysis was used to test the hypothesis that the solvent-induced efficiency deterioration of electret filter media originates from charge deterioration. EFM investigation produced evidence that exposure to isopropanol in the liquid phase affected the electrostatic charges on fibers. Exposure to organic solvents in liquid phase is thought to increase chain and charge mobility in polypropylene fibers, thereby reducing the electrostatic charge and the particle capture ability.

SPM-based Electrical Characterization of Aged Waspaloy Microstructures

AbstractThe results of SPM-based localized electrical examination of precipitation hardened Waspaloy microstructures using Electrostatic Force Microscopy (EFM) and Current-Atomic Force Microscopy (I-AFM) are reported herein. The measurements were conducted on two differently etched specimens with the same initial microstructure. Selective etching by preferentially removing the γ′ or the γ phase resulted in a non-uniform surface topography leaving the less reactive phase standing in relief relative to the depressed phase. The presence of a non-uniform surface topography affected the measured EFM response by causing an inhomogeneous surface voltage distribution. A non-linear tip-surface interaction could have further complicated the measured EFM response by making it non-localized. The EFM phase obeyed a supplementary behavior upon reversing the polarity of the DC bias. Using I-AFM, the tip current was found to be the highest at γ-γ′ interphase boundaries, which was attributed to the relaxation of the lattice atoms in the relief zone formed upon etching.

Charge Characterization of an Electrically Charged Fiber via Electrostatic Force Microscopy

The charge of a corona charged electret fiber as well as an uncharged glass fiber was characterized via Electrostatic Force Microscopy (EFM). Electrostatic force gradient images were obtained by monitoring the shifts in phase between the oscillations of the biased EFM cantilever and those of a piezoelectric driver. EFM measurements were performed using noncontact scans at a constant tip-sample separation of 75 nm with varied bias voltages applied to the cantilever. A mathematical expression, based on the Coulombic and induced polarization effects, were used to model the EFM phase shifts as a function of the applied tip bias voltages. There was quantitative agreement between the experimental data and the mathematical expression, and the quantitative interpretation for charges on the fiber was made.