Electrostatic Force Measurements Research Articles

Repulsive Force for Micro- and Nano-Non-Contact Manipulation

Non-contact positioning of micro-objects using electric fields has been widely explored, based on several physical principles such as electrophoresis, dielectrophoresis (DEP) or optical dielectrophoresis (ODEP), in which the actuation force is induced by an electric charge or an electric dipole placed in an electric field. In this paper, we introduce a new way to control charges in non-contact positioning of micro-objects using chemical functionalization (3-aminopropyl) triethoxysilane—APTES) able to localize charges on a substrate and/or on a micro-object. We demonstrate that this functionalization in a liquid with a low ionic strength is able to concentrate a significant amount of electric charges on surfaces generating an electric field over a long distance (about 10 microns), also called a large exclusion zone (EZ). A model is proposed and validated with electrostatic force measurements between substrate and microparticles (diameter up to 40 µm). We demonstrate that the magnitude of the force and the force range decrease rapidly when the ionic strength of the medium increases. Based on the proposed model, we show that this new way to localize charges on micro-objects may be used for non-contact positioning.

Accurate Electrostatic Force Measurements by Atomic Force Microscopy Using Proper Distance Control

Electrostatic force microscopy (EFM) enables us to examine the local electrostatic force between an AFM tip and sample surface, from which an electrical capacitance between them and surface potential can be evaluated, by applying direct current (DC) and alternating current (AC) voltages. From the dependences of the electrostatic force on the DC voltage and on the frequency of AC voltage in EFM, carrier density, carrier type, and deep level states in a semiconductor can also be investigated. Since EFM is basically operated as an atomic force microscopy (AFM), special care should be taken in an effect of the electrostatic force on a distance control between the tip and sample, and robust distance control is necessary even under the strong electrostatic force in order to realize proper measurements of both topography and electrostatic force. In this paper, we have examined the effectiveness of the usage of the oscillation amplitude of a cantilever as a feedback target for the distance control, which is referred to as Δ <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">A</i> -mode operation, while the cantilever oscillation frequency is always kept at its primary resonance, like the conventional frequency modulation AFM (FM-AFM) which uses a resonant frequency shift as the feedback target. First, we have verified that a distance change due to the strong electrostatic force was significantly reduced compared with FM-AFM. Secondly, we have confirmed that the Δ <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">A</i> -mode operation in EFM realized proper measurements of the dependance of a tip-sample capacitance including a surface depletion capacitance on the DC voltage and on the frequency of AC voltage, indicating good robustness of the method against the strong electrostatic force.

High-performance electrospun particulate matter (PM) filters embedded with self-polarizable tetragonal BaTiO3 nanoparticles

This study developed a novel transparent filter with a high particulate matter (PM) removal efficiency and low airflow resistance for use in windows. The main idea of this filter is to combine the ability of electrospun nanofiber (NF) structures to minimize airflow resistance with the unique self-polarizability of tetragonal BaTiO3 (t-BTO) nanoparticles, which enables the capture of PM through electrostatic interactions. Density functional theory (DFT) calculation predicted the self-polarization of t-BTO, and the presence of static electricity on the NF filters with t-BTO was confirmed by Kelvin probe force microscopy and electrostatic force measurement. Their PM removal efficiency and airflow resistance were dependent on the t-BTO loading and filter transparency. The filter with 20% t-BTO and 90% transparency exhibited a high PM removal efficiency (∼99.1% for PM10–2.5 and ∼98.9% for PM2.5) and a pressure drop of <70 Pa, indicating an extremely low airflow resistance. Moreover, its quality factor value reached ∼0.79 Pa−1, which far exceeds those of previously reported air filters (∼0.2 Pa−1). Further, accelerated long-term experiments for verifying the filter’s durability revealed that it exhibited a high PM removal performance and maintained a low airflow resistance despite a slight reduction in its performance. To enable the commercialization of our filter, the filter’s properties will be optimized based on the results of a field test under actual environments.

Single‐Molecule Interactions between Heme Proteins and Carboxylic Groups in Various Chemical Environments

AbstractNoncovalent chemical interactions, including electrostatic interactions, hydrogen bonding, and hydrophobic interactions, constitute crucial driving forces in the bottom‐up assembly of multifunctional molecular complexes and nanomaterials. Electrostatic interactions in chemical environments are generally considered as non‐specific interactions, which pose notable challenges for the control and direct measurement of electrostatic forces at the single‐molecule level. Here, we report a model system consisting of cytochrome c and molecules containing carboxyl groups, and use electrochemical AFM force spectroscopy to navigate the electrostatic forces of the system under various chemical conditions. The effects of solution pH, ion strength, and redox states of cytochrome c on electrostatic interactions as well as control over these parameters are systematically studied. This work provides a useful approach for investigating intermolecular electrostatic interactions at the single‐molecule level.

Quantitative electrostatic force measurement and characterization based on oscillation amplitude using atomic force microscopy

Measurement of electrostatic force at the micro-/nanoscale has a great scientific value and engineering significance. This paper develops a new determination method of electrostatic forces based on Kelvin probe force mode in atomic force microscopy (AFM). Applying DC voltage and AC voltage simultaneously, we measured the oscillation amplitudes of the probe at two specific frequencies. By the equivalent parallel-plate capacitor model and the vibration theory, we established quantitative relationship between electrostatic force and AFM raw data, and derived a complete and practical formula for calculating electrostatic force. Then, the fundamental characteristics of electrostatic force with time were revealed, and the changes of all components of electrostatic force with tip–sample distance and applied AC peak voltage were discussed in detail. The regulation effects of the distance and the voltage on the total electrostatic force were also compared. Furthermore, we pointed out the main advantages and disadvantages of this method and stated the applicable conditions of the conclusions according to the experimental results and theoretical analysis.

Vertical Phase Separation for Highly Efficient Organic Solar Cells Incorporating Conjugated‐Polyelectrolytes

AbstractHybrid organic solar cells are made through a simple one‐pot coating process with conjugated polyelectrolytes (CPEs), poly[9,9‐bis(4′‐sulfonatobutyl)fluorenealt‐thiophene‐doped (PFT‐D). The hybrid active layer incorporated with PFT‐D shows vertical phase separation by self‐assembled properties of PFT‐D, which result from a molecular dipole between the conjugated backbone and the side chain. The hybrid active layer with PFT‐D shows that homogeneous morphology, surface potential properties, and hydrophobic surface properties favor for enhancing photovoltaic performance. These results are identified by contact angle characteristics, X‐ray photoelectron spectroscopy (XPS) profiling, and X‐ray diffraction (XRD), atomic force measurement (AFM), and electrostatic force measurement (EFM) analyses. With fullerene based hybrid active layer, the power conversion efficiency (PCE) reaches to 8.7% with the enhanced short‐circuit current density (J SC), open‐circuit voltage (V OC), and fill factor (FF). The hybrid device with PFT‐D has a higher stability with a lower reduction ratio of 5.74% compared with only bulk‐heterojunction device (with reduction rate of 30.15%) and incorporating poly(3,4‐ethylene dioxythiophene)‐poly(styrene sulfonate) (PEDOT:PSS) device (with reduction rate of 10.84%). These results are also found in nonfullerene based system (PCE of 10.8%) and other conjugated polyelectrolytes system (PCE of 8.4%). These results have the potential of significantly contributing to the upsizing and commercialization of organic solar cells.

Determination of electrostatic force and its characteristics based on phase difference by amplitude modulation atomic force microscopy

Electrostatic force measurement at the micro/nano scale is of great significance in science and engineering. In this paper, a reasonable way of applying voltage is put forward by taking an electrostatic chuck in a real integrated circuit manufacturing process as a sample, applying voltage in the probe and the sample electrode, respectively, and comparing the measurement effect of the probe oscillation phase difference by amplitude modulation atomic force microscopy. Based on the phase difference obtained from the experiment, the quantitative dependence of the absolute magnitude of the electrostatic force on the tip-sample distance and applied voltage is established by means of theoretical analysis and numerical simulation. The results show that the varying characteristics of the electrostatic force with the distance and voltage at the micro/nano scale are similar to those at the macroscopic scale. Electrostatic force gradually decays with increasing distance. Electrostatic force is basically proportional to the square of applied voltage. Meanwhile, the applicable conditions of the above laws are discussed. In addition, a comparison of the results in this paper with the results of the energy dissipation method shows the two are consistent in general. The error decreases with increasing distance, and the effect of voltage on the error is small.

Atomic force microscopy developments for probing space charge at sub-micrometer scale in thin dielectric films

Charge injection and accumulation in dielectrics are phenomena at the origin of different kind of failure of devices. To improve the understanding of involved mechanisms, space charge measurement techniques have been successfully developed. However, their spatial resolution is incompatible with thin films or interfaces studies. In this paper, the contribution of techniques derived from Atomic Force Microscopy (AFM) to space charge measurement is investigated. First of all, localized charges injection is studied using classical Kelvin Force Microscopy (KFM). Secondly, we propose a new method called EFDC (Electrostatic Force Distance Curve) which allows direct measurement of electrostatic force induced by trapped charges. EFDC sensitivity to lateral localization of charges is demonstrated. Finally, comparing the results obtained by EFDC and KFM highlights the relative merit of each technique.

Large area scanning probe microscope in ultra-high vacuum demonstrated for electrostatic force measurements on high-voltage devices.

Background: The resolution in electrostatic force microscopy (EFM), a descendant of atomic force microscopy (AFM), has reached nanometre dimensions, necessary to investigate integrated circuits in modern electronic devices. However, the characterization of conducting or semiconducting power devices with EFM methods requires an accurate and reliable technique from the nanometre up to the micrometre scale. For high force sensitivity it is indispensable to operate the microscope under high to ultra-high vacuum (UHV) conditions to suppress viscous damping of the sensor. Furthermore, UHV environment allows for the analysis of clean surfaces under controlled environmental conditions. Because of these requirements we built a large area scanning probe microscope operating under UHV conditions at room temperature allowing to perform various electrical measurements, such as Kelvin probe force microscopy, scanning capacitance force microscopy, scanning spreading resistance microscopy, and also electrostatic force microscopy at higher harmonics. The instrument incorporates beside a standard beam deflection detection system a closed loop scanner with a scan range of 100 μm in lateral and 25 μm in vertical direction as well as an additional fibre optics. This enables the illumination of the tip–sample interface for optically excited measurements such as local surface photo voltage detection.Results: We present Kelvin probe force microscopy (KPFM) measurements before and after sputtering of a copper alloy with chromium grains used as electrical contact surface in ultra-high power switches. In addition, we discuss KPFM measurements on cross sections of cleaved silicon carbide structures: a calibration layer sample and a power rectifier. To demonstrate the benefit of surface photo voltage measurements, we analysed the contact potential difference of a silicon carbide p/n-junction under illumination.

Towards 3D charge localization by a method derived from atomic force microscopy: the electrostatic force distance curve

Charges injection and accumulation in the dielectric remains a critical issue, mainly because these phenomena are involved in a great number of failure mechanisms in cables or electronic components. Achieving a better understanding of the mechanisms leading to charge injection, transport and trapping under electrical stress and of the relevant interface phenomena is a high priority. The classical methods used for space charge density profile measurements have a limited spatial resolution, which prevents them being used for investigating thin dielectric layers or interface processes. Thus, techniques derived from atomic force microscopy (AFM) have been investigated more and more for this kind of application, but so far they have been limited by their lack of in-depth sensitivity. In this paper a new method for space charge probing is described, the electrostatic force distance curve (EFDC), which is based on electrostatic force measurements using AFM. A comparison with the results obtained using kelvin force microscopy (KFM) allowed us to highlight the fact that EFDC is sensitive to charges localized in the third-dimension.

Study on Multi-Frequency Method for Electrostatic Force Microscopy in Air

A detailed study of multi-frequency electrostatic force microscopy (MF-EFM) in air is presented. In the MF-EFM, the first eigenmode of the cantilever vibration is used for topographic imaging, while the second one is resonantly excited for the measurement of electrostatic force. Possible interferences between the two information channels are eliminated by using the lift mode and the two-pass mode. The lateral resolution of the microscopy is corroborated to be improved. The capabilities of the MF-EFM are demonstrated by characterizing a silicon wafer sample and by determining its local contact potential difference.

Electromechanical switching behavior of individual molecular complexes of Cu and Ni on NaCl-covered Cu(111) and Ag(111)

In a combined scanning tunneling microscopy (STM) and noncontact atomic force microscopy (nc-AFM) study, we characterize the reversible switching between two stable states of an organometallic complex, namely, bis-dibenzoylmethanato-copper [Cu(dbm)${}_{2}$] adsorbed on an insulating thin film. The switching is due to the transfer of an electron between the STM tip and the molecule, accompanied by a conformational change, i.e., a transition from a square-planar to a tetrahedral geometry. Evidence is given by STM topography and spectroscopy and nc-AFM electrostatic force measurements. Similar experiments conducted on bis-dibenzoylmethanato-nickel [Ni(dbm)${}_{2}$] show that this complex does not switch under comparable experimental conditions. We discuss these findings within the framework of crystal-field theory, stating that the occupation of the $d$ orbitals determines the favored coordination geometry of a complex. Accordingly, only the copper complex can undergo a conformational change that facilitates stable storing of the additional electron.

Multifrequency Atomic Force Microscopy: Compositional Imaging with Electrostatic Force Measurements

We demonstrate that single-pass Kelvin force microscopy (KFM) and dC/dz measurements in different environments expand the compositional imaging with atomic force microscopy. The KFM and dC/dz studies were performed in the intermittent contact mode with force gradient detection of tip-sample electrostatic interactions. Both factors contribute to sensitive measurements of the surface potential and capacitance gradient with nanometer-scale spatial resolution as it was verified on a broad range of materials: metal alloys, polymers, organic layers, and liquid-like objects. For many samples the surface potential and dC/dz variations complement each other in identification of individual components of heterogeneous materials. In situ imaging in different humidity or vapors of various organic solvents further facilitate recognition of the constituents of multicomponent polymer samples due to selective swelling of components.

Novel parallel plate condenser for single particle electrostatic force measurements in atomic force microscope

A combination of small parallel plate condenser with Indium Tin Oxide (ITO) glass slides as electrodes and an atomic force microscope (AFM) is used to characterize the electrostatic behavior of single glass bead microparticles (105–150 μm) glued to the AFM cantilever. This novel setup allows measurements of the electrostatic forces acting on a particle in an applied electrical field to be performed in ambient air conditions. By varying the position of the microparticle between the electrodes and the strength of the applied electric field, the relative contributions of the particle net charge, induced and image charges were investigated. When the microparticle is positioned in the middle of the electrodes, the force acting on the microparticle was linear with the applied electric field and proportional to the microparticle net charge. At distances close to the bottom electrode, the force follows a parabolic relationship with the applied electric field reflecting the contributions of induced and image charges. The method can be used for the rapid evaluation of the charging and polarizability properties of the microparticle as well as an alternative to the conventional Faraday's pail technique.

Electrostatic and van der Waals forces in the air contact between the atomic force microscope probe and a conducting surface

Electrostatic and van der Waals forces of interaction between commercial probes of atomic force microscopes (AFMs) and conducting surfaces under atmospheric conditions are measured using contact atomic force microscopy. An algorithm of statistical processing of the initial photocurrent-displacement dependences is developed, which makes it possible to transform these dependences into the force-distance dependences. The Hamaker constant at the platinum (probe)-graphite (sample) contact is determined. It is shown that the measurement of electrostatic forces makes it possible to determine geometrical parameters of the AFM probe and to independently calibrate the stiffness of the cantilever.

Electrostatic forces in micromanipulation: Experimental characterization and simulation including roughness

Manipulation by contact of objects between 1 μ m and 1 mm are often disturbed by adhesion between the manipulated object and the gripper. Electrostatic forces are among the phenomena responsible for this adhesive effect. Analytical models have been developed in the literature to predict electrostatic forces. Most models have been developed within the framework of scanning probe microscopy, i.e. for a contact between a conducting tip and a metallic surface. In our study, we developed a simulation tool based on finite elements modeling. The strength of this model lies in the fact that it integrates roughness parameters. Measurements of electrostatic forces in function of roughness were conducted by atomic force microscopy. The experimental results were compared with the simulation results showing very good correlation. This demonstrates the influence of surface topography on electrostatic forces, especially for very small separation distances and proves the utility of the simulation tool in designing surfaces with controlled adhesion. Some application fields to which these results can be applied are drug delivery devices and micromanipulation tools.

Quantitative force versus distance measurements in amplitude modulation AFM: a novelforce inversion technique

A new method for extracting quantitative data from amplitude modulation dynamicforce–distance measurements is developed. The method is based on the harmonic oscillatormodel of vibrating atomic force microscope cantilevers, and is capable of extracting boththe conservative and dissipative parts of the tip–sample interaction from a measurement ofoscillation amplitude and phase as a function of distance. Numerical simulations are usedto demonstrate the validity of the method. Further proof of the accuracy of this method isprovided by a measurement of electrostatic forces between an AFM tip and a graphitesample.

Topography and surface potential in Kelvin force microscopy of perfluoroalkyl alkanes self-assemblies

Self-assemblies of two perfluoroalkyl alkanes [F(CF2)n(CH2)mH with n=14, m=20 and n=12, m=8] on different substrates (mica, Si, and graphite) were examined with Kelvin force microscopy (KFM) by detecting either the electrostatic tip-sample force or its gradient. Both measurements were realized in the single pass operation using different frequencies for monitoring topography and surface potential changes. Furthermore, the KFM imaging was performed in the intermittent contact regime that allowed high-resolution mapping of surface potential changes without noticeable cross-talk between signals related to mechanical and electric forces. The use of force gradient for electrostatic force measurements led to high-resolution surface potential images of the perfluoroalkyl alkane adsorbates with strong contrast of the particular self-assemblies (spirals, ribbons, and toroids). The obtained potential data are close to ones recorded with macroscopic Kelvin probe and to theoretical estimates. The surface potential correlates with the orientation of molecular dipoles of –CF3 and –CF2–CH2– groups with respect to the substrate. Therefore, topography and surface potential data indicate that the fluorocarbon parts are mostly oriented vertically, whereas hydrocarbon segments are partially or fully spread on the substrate. In addition, the ability of KFM to monitor a transformation of the self-assemblies was demonstrated by studies of these objects in humid air.

On the reliability of scanning probe based electrostatic force measurements

The dimensions of semiconductor devices rapidly decreasing, the detection and control of spatial inhomogeneities of material properties on a sub-μm scale becomes essential. For mapping various electrical properties with nearly nm-resolution, scanning probe techniques appear to be ideally suited. However, data evaluation always involves a transfer from measured to real properties by a device depending convolution procedure. We report on our results for different modes of electrical measurements, namely scanning Kelvin probe microscopy (SKM) and scanning capacitance microscopy (SCM) on various sample systems discussing the influence of experimental parameters. In addition, results of finite element simulations on this topic are presented. It turns out that the averaging function correlating real and measured data may appear quite simple, thus making a reliable reconstruction possible. On the other hand, the existence of surface charges can drastically change the results.

Detection of Single-Electron Charging in an Individual InAs Quantum Dot by Noncontact Atomic-Force Microscopy

Single-electron charging in an individual InAs quantum dot was observed by electrostatic force measurements with an atomic-force microscope (AFM). The resonant frequency shift and the dissipated energy of an oscillating AFM cantilever were measured as a function of the tip-back electrode voltage, and the resulting spectra show distinct jumps when the tip was positioned above the dot. The observed jumps in the frequency shift, with corresponding peaks in dissipation, are attributed to a single-electron tunneling between the dot and the back electrode governed by the Coulomb blockade effect, and are consistent with a model based on the free energy of the system. The observed phenomenon may be regarded as the "force version" of the Coulomb blockade effect.