Electrostatic Force Microscopy Research Articles

Characterization of heterogeneity of CVD graphene on copper foil by scanning probe microscopy and Raman spectroscopy

The comparative analysis of the structural, morphological, and local electrical properties of CVD graphene films synthesized at atmospheric and low pressure CVD growth conditions was carried out. The use of electrical techniques of scanning probe microscopy, such as Kelvin probe force microscopy, scanning capacitance microscopy, electrostatic force microscopy, and conductive atomic-force microscopy for investigation of heterogeneous graphene coverage was discussed. A significant change in the surface potential values was observed indicating the formation of p-type multilayered graphene domains on single layer graphene at low pressure and n-type multilayered graphene films at atmospheric pressure CVD. The effect of Ar flow rate increasing during cooling stage in the temperature range of 700–200 °C causing graphite formation was described. The employment of surface potential spectroscopy allows to indicate the presence of nonlinear phenomena in heterogeneous graphene.

Structural, electrostatic force microscopy, work function, and optical characterization of pure and Al-doped ZnO nanoparticles

The electrical and optical characteristics of pure and Al-doped zinc oxide (ZnO) nanoparticles were analyzed. The work function of these nanoparticles was investigated using Kelvin Probe Force Microscopy (KPFM). The work function of the Al-doped ZnO nanoparticles was found to be lower than that of undoped ZnO nanoparticles. Electrostatic Force Microscopy (EFM) was employed to map the distribution of charges and conductivity in both ZnO and Al -doped ZnO, revealing enhanced charge trapping and increased conductivity in the Al-doped ZnO nanoparticles compared to the undoped ones. XRD analysis verified that both pure ZnO and Al-doped ZnO nanoparticles exhibited a hexagonal wurtzite crystal structure. Additionally, Raman spectroscopy revealed new vibrational modes at 572 cm−1, which were attributed to E1 (LO) in Al-doped ZnO. UV–visible spectroscopy indicated that the band gap of Al -doped ZnO nanoparticles is wider than that of pure ZnO nanoparticles, Additionally, photoluminescence spectroscopy demonstrated a blue shift in the emission spectrum of the Al-doped ZnO nanoparticles, accompanied by a reduction in green emission defects.

Nanoscale visualization of fast carrier dynamics in organic thin-film transistors by time-resolved electrostatic force microscopy

Fast carrier dynamics in organic thin-film transistors (OTFTs) was investigated by time-resolved electrostatic force microscopy (tr-EFM). We found that the carrier diffusion in the OTFTs proceeded in two stages: fast diffusion and slow diffusion. By applying the instantaneous frequency method to EFM, the temporal evolution of the spatial distribution of fast carriers in the channel region of the OTFTs, which took place on the timescale of several hundreds of nanoseconds, was evaluated. The inhomogeneous distribution of the local decay time constant showed that the carrier diffusion of the OTFTs was limited by the grain boundaries between each crystalline region. The quantitative capability of the method was verified by comparing the values of the carrier mobility estimated by the tr-EFM measurement and a numerical simulation. The mobility estimated from the experiment and the simulation showed good agreement, showing the possibility of the tr-EFM to evaluate the time evolution of dynamic phenomena in semiconductor devices.

Antibacterial and Antibiofouling Activities of Carbon Polymerized Dots/Polyurethane and C60/Polyurethane Composite Films.

The cost of treatment of antibiotic-resistant pathogens is on the level of tens of billions of dollars at the moment. It is of special interest to reduce or solve this problem using antimicrobial coatings, especially in hospitals or other healthcare facilities. The bacteria can transfer from medical staff or contaminated surfaces to patients. In this paper, we focused our attention on the antibacterial and antibiofouling activities of two types of photodynamic polyurethane composite films doped with carbon polymerized dots (CPDs) and fullerene C60. Detailed atomic force, electrostatic force and viscoelastic microscopy revealed topology, nanoelectrical and nanomechanical properties of used fillers and composites. A relationship between the electronic structure of the nanocarbon fillers and the antibacterial and antibiofouling activities of the composites was established. Thorough spectroscopic analysis of reactive oxygen species (ROS) generation was conducted for both composite films, and it was found that both of them were potent antibacterial agents against nosocomial bacteria (Klebsiela pneumoniae, Proteus mirabilis, Salmonela enterica, Enterococcus faecalis, Enterococcus epidermis and Pseudomonas aeruginosa). Antibiofouling testing of composite films indicated that the CPDs/PU composite films eradicated almost completely the biofilms of Pseudomonas aeruginosa and Staphylococcus aureus and about 50% of Escherichia coli biofilms.

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.

Probing the surface electrical properties of clay minerals with electrostatic force microscopy and kelvin probe force microscopy

Many surface processes of clay minerals require in-depth understanding of their surface electrical properties, such as surface charge density, surface potential distribution, etc In this paper, electrostatic force microscopy (EFM) and Kelvin probe force microscopy (KPFM) were used to study the surface charge densities, surface potentials, electric field intensities, and electric field force gradients of three common clay minerals: kaolinite, montmorillonite, and illite. The properties were directly imaged, and the average surface permanent charge densities of kaolinite, montmorillonite, and illite were obtained to be −0.0060, −2.136, and −5.456 μC m−2, respectively. In addition, a good linear relationship was found between the surface charge densities obtained by KPFM and the layer charges calculated from the mineral chemical formulas of three clay minerals.

Imaging the Quantum Capacitance of Strained MoS2 Monolayers by Electrostatic Force Microscopy.

We implemented radio frequency-assisted electrostatic force microscopy (RF-EFM) to investigate the electric field response of biaxially strained molybdenum disulfide (MoS2) monolayers (MLs) in the form of mesoscopic bubbles, produced via hydrogen (H)-ion irradiation of the bulk crystal. MoS2 ML, a semiconducting transition metal dichalcogenide, has recently attracted significant attention due to its promising optoelectronic properties, further tunable by strain. Here, we take advantage of the RF excitation to distinguish the intrinsic quantum capacitance of the strained ML from that due to atomic scale defects, presumably sulfur vacancies or H-passivated sulfur vacancies. In fact, at frequencies fRF larger than the inverse defect trapping time, the defect contribution to the total capacitance and to transport is negligible. Using RF-EFM at fRF = 300 MHz, we visualize simultaneously the bubble topography and its quantum capacitance. Our finite-frequency capacitance imaging technique is noninvasive and nanoscale and can contribute to the investigation of time- and spatial-dependent phenomena, such as the electron compressibility in quantum materials, which are difficult to measure by other methods.

Time-resolved electrostatic force microscopy under base-bias-level control

We propose a base-bias-level control method, in which the contact potential difference is always compensated in a similar way to Kelvin probe force microscopy, applicable to time-resolved electrostatic force microscopy using the pump–probe method. We experimentally acquired temporal waveforms of the electrostatic force signal on two SiO2/n-type Si samples, one of which was as-grown and the other was treated by forming gas annealing. Consequently, the effectiveness of the proposed method was confirmed.

Reaction-diffusion effects and spatiotemporal oscillations under SEM, STM and AFM-assisted charging in fiber-like and wire-like systems: From molecular and quantum wires to cooperative ferroelectric nanofibers and microfibers

This review addresses the problem of reaction-diffusion effects and spatiotemporal oscillations in fiber-like and wire-like systems under the electron beam in SEM and in the presence of electric field in some special AFM techniques, such as current sensing atomic force microscopy (CS-AFM)/conductive atomic force microscopy (C-AFM), electrostatic force microscopy (EFM) and Kelvin probe force microscopy (KPFM) also known as surface potential microscopy. Some similar reaction-diffusion effects also can be observed in scanning capacitance microscopy (SCM), scanning gate microscopy (SGM), scanning voltage microscopy (SVM) and piezoresponse force microscopy (PFM). At the end of this paper the authors provide analysis of their own results and approaches. In particular, the possibility of achieving the ion transfer controlled growth of cells along the ion concentration gradients in reaction-diffusion fibers and actuators is indicated. This fundamental idea is discussed within the framework of the implantable fiber “bioiontronics” and “neuroiontronics” controlled by acoustic and electrical signals that regulate the reaction-diffusion or chemical oscillation activity of such fiber structures as reaction-diffusion actuators and sensors. The literature review includes more than 130 references.

Enhancing interfacial Li+ transport and dielectric properties in poly(ethylene oxide)-based all-solid electrolytes via inactive g-C3N4 nanosheets filler incorporation

The advancement of all-solid-state Li metal batteries (ASSLMBs) faces a major challenge in the growth of lithium dendrites on the anode-electrolyte interface. In this study, we propose a dual-filler approach using poly(ethylene oxide) (PEO)-based solid polymer electrolytes (SPEs) that combine Li1.4Al0.4Ti1.6(PO4)3 (LATP) ion-conductive particles with graphitic carbon nitride (g-C3N4) nanosheets. Analysis through second harmonic resonance enhanced electrostatic force microscopy and critical current density (CCD) tests reveal that the g-C3N4 additives form nano-capacitors at the SPE-lithium interface, effectively reducing sudden changes in current densities. The distribution of relaxation time constant (DRT) measurements confirms that the g-C3N4 filler suppresses uncontrolled Li dendrite growth, effectively mitigating battery aging caused by anode interfacial degradation. Furthermore, X-ray photoelectron spectroscopy (XPS) analysis indicates that the nitrogen-containing organic groups in g-C3N4 are reduced to form a stable interfacial layer with lithium metal. As a result of these enhancements, the electrolyte demonstrates remarkable interfacial stability in Li/Li symmetrical cells at 0.65 mA/cm2 and delivers promising performance in assembled Li-LiFePO4 batteries, achieving a reversible capacity of 121.6 mAh/g at 1 C after 200 cycles. These findings highlight the potential of dual-filler PEO-based SPEs for promoting interfacial lithium-ion transport in all-solid-state Li metal batteries.

Local electrical characteristic of memristor structure in a high-resistance state obtained using electrostatic force microscopy: Fractal and multifractal dynamics of surface

A heterostructure BiFeO3/TiO2(Nt)Ti (BFOT) was obtained by the atomic layer deposition (ALD) method. After thermal treatment, the redistribution of Fe/Ti atoms forms an Aurivillius intermediate layered phase, and local charge capture centers are formed in the sample. Due to cationic non-stoichiometry, the BFO film exhibits p-type conductivity, while the nanotubes exhibit n-type conductivity due to oxygen vacancies. It was observed that lateral displacement of the sample can lead to ferroelectric switching, which can, in turn, affect the transition of the memristive structure from high-resistance (HRS) to low-resistance states (LRS). The hysteresis suppression tends to transition to an ohmic character and depends on the amplitude, frequency, and duration of the periodic signal. It has been found that compensation of static charge during resistive switching can affect the transport properties of the material. Fractal dimension analysis showed an acceleration of structure restructuring with increasing voltage, possibly contributing to the transition from an insulator to a metal in certain areas of the film volume. The joint analysis of piezoresponse force microscopy (PFM), electrostatic force microscopy (EFM), and fractal/multifractal dynamics showed a correlation between surface static charge and piezopotential. The new methodology described in this work can help understand the resistive switching processes in ferroelectric/semiconductor memristive structures.

Probing Energy-Level Alignment in Molecular Multilayers by Frequency-Modulation Electrostatic Force Microscopy under Tapping-Mode-Combined Fowler-Nordheim Tunneling Spectroscopy.

The alignment of molecular electronic levels in a molecular multilayer is of crucial importance to realize desired functions for molecular devices. Amplitude-modulation-feedback frequency-modulation electrostatic force microscopy combined with Fowler-Nordheim tunneling spectroscopy is utilized as a probe for the energy-level alignment in an organic multilayer. Bias-dependent electrostatic force spectra were examined for bilayers including a Ru complex as a benchmark multilayer system. Electrostatic properties in the low-bias region were captured well by a single-capacitor model, which indicates weak coupling at the bilayer interface between the Ru complex and self-assembled monolayer. In contrast, in the high-bias region, significant disagreement with the expected electrostatic force was recognized for the bilayers and evaluated as the loss of electrostatic energy through the Fowler-Nordheim tunneling process. Alignment of the lowest unoccupied molecular orbital (LUMO) level of the Ru complex was determined by Fowler-Nordheim emission through resonant tunneling. These results indicate an effective method to probe level alignment at interfaces inside multilayers and to provide the partition factor β that depicts a divided electric field.

Role of finite probe size in measuring growth exponent in film deposition

We use computer simulations to investigate the effects of the tip diameter of an electrostatic force microscope (EFM) operating at a constant force on the extraction of the growth exponent β during film growing in a one-dimensional substrate. Laplace’s equation is solved in the EFM simulation using the finite element method to determine the electrostatic force between the tip and the film interface. Importantly, for EFM tips with sufficiently large apex diameters, the topographies calculated with EFM and those computed with the transformed mean height profile (TMHP) method, where the interface is divided into bins of the same tip diameter size and the average height within each bin is used to transform the original interface, are almost identical. This was shown in the context of lattice models of the Kardar–Parisi–Zhang (KPZ) and Villain–Lai–Das–Sarma (VLDS) classes. The global roughness of the film surface, W, scales with the diameter of the EFM tip, ε, as W/a=(ε/a)αg[Ψ], where a is the lattice parameter, α is the KPZ/VLDS roughness exponent, and g is a universal scaling function of the argument Ψ≡t/(ε/a)z, where t and z are the reduced time of deposition and the KPZ/VLDS dynamic exponent, respectively. These results provide a limit for ε from which a KPZ/VLDS growth exponent can be reliably determined with EFM at a constant force. When the EFM tip diameter is larger than the surface correlation length, a misleading effective growth exponent consistent with uncorrelated growth is found.

The Effect of Filler Material on the Interface Around a Nanoparticle in a Nanocomposite

In this work, the change in the dielectric constant and thickness of the interfacial region around a nanoparticle in a composite is studied as a function of change in the composition of the filler material. An Electrostatic Force Microscopy (EFM) based method previously proposed by the authors, is used to examine the dependence of the interfacial parameters on the filler material. For this, two types of filler materials: barium titanate (BaTiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> ) and alumina (Al <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> ) are used with epoxy resin as host material. A 3-D finite element method (FEM) based model is used for the simulations. It is seen that change in filler material modifies the interface thickness and dielectric constant.

Quantitative Study of Charge Distribution Variations on Silica-Nafion Composite Membranes under Hydration Using an Approximation Model.

Understanding the ionic structure and charge transport on proton exchange membranes (PEMs) is crucial for their characterization and development. Electrostatic force microscopy (EFM) is one of the best tools for studying the ionic structure and charge transport on PEMs. In using EFM to study PEMs, an analytical approximation model is required for the interoperation of the EFM signal. In this study, we quantitatively analyzed recast Nafion and silica-Nafion composite membranes using the derived mathematical approximation model. The study was conducted in several steps. In the first step, the mathematical approximation model was derived using the principles of electromagnetism and EFM and the chemical structure of PEM. In the second step, the phase map and charge distribution map on the PEM were simultaneously derived using atomic force microscopy. In the final step, the charge distribution maps of the membranes were characterized using the model. There are several remarkable results in this study. First, the model was accurately derived as two independent terms. Each term shows the electrostatic force due to the induced charge of the dielectric surface and the free charge on the surface. Second, the local dielectric property and surface charge are numerically calculated on the membranes, and the calculation results are approximately valid compared with those in other studies.

Heterogeneity in Cation Exchange Ag+ Doping of CdSe Nanocrystals.

Cation exchange is becoming extensively used for nanocrystal (NC) doping in order to produce NCs with unique optical and electronic properties. However, despite its ever-increasing use, the relationships between the cation exchange process, its doped NC products, and the resulting NC photophysics are not well characterized. For example, similar doping procedures on NCs with the same chemical compositions have resulted in quite different photophysics. Through a detailed single molecule investigation of a postsynthesis Ag+ doping of CdSe NCs, a number of species were identified within a single doped NC sample, suggesting the differences in the optical properties of the various synthesis methods are due to the varied contributions of each species. Electrostatic force microscopy (EFM), electron energy loss spectroscopy (EELS) mapping, and single molecule photoluminescence (PL) studies were used to identify four possible species resulting from the Ag+-CdSe cation exchange doping process. The heterogeneity of these samples shows the difficulty in controlling a postsynthesis cation exchange method to produce homogeneous samples needed for use in any potential application. Additionally, the heterogeneity in the doped samples demonstrates that significant care must be taken in describing the ensemble or average characteristics of the sample.

Understanding the evolution of lithium dendrites at Li6.25Al0.25La3Zr2O12 grain boundaries via operando microscopy techniques

The growth of lithium dendrites in inorganic solid electrolytes is an essential drawback that hinders the development of reliable all-solid-state lithium metal batteries. Generally, ex situ post mortem measurements of battery components show the presence of lithium dendrites at the grain boundaries of the solid electrolyte. However, the role of grain boundaries in the nucleation and dendritic growth of metallic lithium is not yet fully understood. Here, to shed light on these crucial aspects, we report the use of operando Kelvin probe force microscopy measurements to map locally time-dependent electric potential changes in the Li6.25Al0.25La3Zr2O12 garnet-type solid electrolyte. We find that the Galvani potential drops at grain boundaries near the lithium metal electrode during plating as a response to the preferential accumulation of electrons. Time-resolved electrostatic force microscopy measurements and quantitative analyses of lithium metal formed at the grain boundaries under electron beam irradiation support this finding. Based on these results, we propose a mechanistic model to explain the preferential growth of lithium dendrites at grain boundaries and their penetration in inorganic solid electrolytes.

Doping C60(OH) to Regulate the Crosslink Network and Energy Band Structures of Epoxy Resin and Allow for Electronic Directional Drive in C60(OH)/EP

The interfacial effect of nanoparticles can effectively regulate electron migration in epoxy composites. In this article, hydroxylated C60 [C60(OH)] was prepared and added to epoxy resin (EP). The inhibition of electron motion in epoxy composites was realized, and the dielectric strength of the epoxy composites was improved. The results of dynamic mechanical analysis (DMA) showed that C60(OH) regulated the structure of the crosslinked EP network and increased the crosslink density by 16.5%. The results of a density functional theory (DFT) simulation showed that the doping of C60(OH) formed a potential energy barrier at the C60(OH)-EP interface, which was conducive to the directional migration and capture of electrons driven to C60(OH). Electrostatic force microscopy (EFM) and pulsed electroacoustic (PEA) experiments showed that C60(OH) had a strong ability to capture charge, and the charge migration rate of C60(OH)/EP was 43% lower than that of pure EP in the presence of 10 kV/mm when the EP was doped with 0.1wt% C60(OH). The breakdown strength and volume resistivity increased by 17.87% and 222%, respectively. Thus, this article provides a new method to improve the dielectric strength of epoxy composites.

Dielectric Properties of Polymer Nanocomposite Interphases Using Electrostatic Force Microscopy and Machine Learning.

Knowing the dielectric properties of the interfacial region in polymer nanocomposites is critical to predicting and controlling dielectric properties. They are, however, difficult to characterize due to their nanoscale dimensions. Electrostatic force microscopy (EFM) provides a pathway to local dielectric property measurements, but extracting local dielectric permittivity in complex interphase geometries from EFM measurements remains a challenge. This paper demonstrates a combined EFM and machine learning (ML) approach to measuring interfacial permittivity in 50 nm silica particles in a PMMA matrix. We show that ML models trained to finite-element simulations of the electric field profile between the EFM tip and nanocomposite surface can accurately determine the interface permittivity of functionalized nanoparticles. It was found that for the particles with a polyaniline brush layer, the interfacial region was detectable (extrinsic interface). For bare silica particles, the intrinsic interface was detectable only in terms of having a slightly higher or lower permittivity. This approach fully accounts for the complex interplay of filler, matrix, and interface permittivity on the force gradients measured in EFM that are missed by previous semianalytic approaches, providing a pathway to quantify and design nanoscale interface dielectric properties in nanodielectric materials.

Topography and electrostatic interactions of electret fibres in N95 facemasks

Facemasks have been a critical line of defense in the Covid-19 pandemic and have been used for decades to reduce the inhalation of sub-micron aerosols, including bacteria, viruses and nano-materials. These facemasks leverage electrostatic charge properties of non-woven electret fibre mats to remove sub-micron aerosols. However, the topographical and electrostatic properties of these mats have not been well characterized at the nano-scale. In this study, we use atomic force microscopy (AFM) to examine the surface topography of the electret fibres within a commercial respirator fibre mat, use electrostatic force microscopy to map the electrostatic interactions emanating from these fibres, and use synchrotron X-ray scattering to probe polymer morphology. We find that within these mats the fibres exhibit significant heterogeneity in surface topography, electrostatic interactions and polypropylene phase composition. The surface topographies ranged from nano-smooth surfaces to ordered spherulitic structures. The electrostatic interactions varied across fibre length and circumference, and are only weakly correlated with surface topographical features. Finally, we use AFM to characterize fibres after filtering a NaCl aerosol and find that the deposited particles are heterogeneous in size and distributed over the fibre surfaces.