Surface potential extraction from electrostatic and Kelvin-probe force microscopy images

The ability of non-contact electric modes (Kelvin probe force microscopy (KPFM) and electrostatic force microscopy (EFM)) to provide quantitative information about the remnant polarization in a ferroelectric thin film is theoretically and experimentally investigated. The theoretical relationship between the microscopic signal and the polarization in the film is established: it is linear in the KPFM mode and parabolic in the EFM mode. The KPFM and EFM signals are predicted tip-shape, tip-surface distance, and poled area dimensions dependent so that an absolute value of the polarization cannot be extracted. Nevertheless provided these parameters are held constant, KPFM theoretically enables to investigate quantitatively the polarization dependence on any parameter relative to the film preparation conditions or to the poling process. It does not enable to investigate its dependence on poled area geometry. Based on these conclusions, experimental KPFM investigations have been carried out on a ferroelectric strontium barium niobate (SrxBa1−xNb2O6, SBN:100x) epitaxial thin film deposited on a platinum covered MgO substrate. Poled areas are created in the film by moving the biased tip in contact with the surface. KPFM images are dominated by the written polarization and not by injected or attracted extrinsic charges. The experimental decay of the KPFM signal with increasing tip-surface distance confirms the contribution of the volume of the lever/tip to the KPFM response. The spontaneous evolution of the written polarization as well as the polarization dependence on poling voltage and poling duration are investigated using KPFM, with the poled area dimensions as a parameter. For comparison, the macroscopic remnant polarization in the SBN film has been measured versus poling field from hysteresis cycles drawn using a macroscopic Pt dot as top contact. The significant difference observed between macroscopic and microscopic results can be accounted for by using a simple model of the inhomogeneous electric field generated by a poling tip. KPFM measurements do not alter the initial polarization of the film and can provide essential qualitative and quantitative information on the polarization in a ferroelectric thin film.

We provide in this article a comprehensive study of the role of ac cross-talk effects in Kelvin Probe Force Microscopy (KPFM), and their consequences onto KPFM imaging. The dependence of KPFM signals upon internal parameters such as the cantilever excitation frequency and the projection angle of the KPFM feedback loop is reviewed, and compared with an analytical model. We show that ac cross-talks affect the measured KPFM signals as a function of the tip-substrate distance, and thus hamper the measurement of three-dimensional KPFM signals. The influence of ac cross-talks is also demonstrated onto KPFM images, in the form of topography footprints onto KPFM images, especially in the constant distance (lift) imaging mode. Our analysis is applied to unambiguously probe charging effects in tobacco mosaic viruses (TMVs) in ambient air. TMVs are demonstrated to be electrically neutral when deposited on silicon dioxide surfaces, but inhomogeneously negatively charged when deposited on a gold surface.

The Kelvin Probe Force Microscopy (KPFM) is a method to detect the surface potential of micro- and nanostructured samples using a common Scanning Probe Microscope (SPM). The electrostatic force has a very long range compared to other surface forces. By using SPM systems the KPFM measurements are performed in the noncontact region at surface distances greater than 10 nm. In contrast to topography measurement, the measured data is blurred. The KPFM signal can be described as a convolution of an effective surface potential and a microscope intrinsic point spread function, which allows the restoration of the measured data by deconvolution. This paper deals with methods to deconvolute the measured KPFM data with the objective to increase the lateral resolution. An analytical and a practical way of obtaining the point spread function of the microscope was compared. In contrast to other papers a modern DoF-restricted deconvolution algorithm is applied to the measured data. The new method was demonstrated on a nanoscale test stripe pattern for lateral resolution and calibration of length scales (BAM-L200) made by German Federal Istitute for Materials Research and Testing.

The distributions of surface charges or surface potentials on biological molecules and electrodes are directly related to various biological functions and ionic adsorptions, respectively. Electrostatic force microscopy and Kelvin-probe force microscopy (KFM) are useful scanning probe techniques that can map local surface charges and potentials. Here, we report the measurement and analysis of the electrostatic and capacitive forces on the cantilever tip induced by application of an alternating voltage in order to discuss the feasibility of measuring the surface charge or potential distribution at solid/liquid interfaces in various liquid media. The results presented here suggest that a nanometer-scale surface charge or potential measurement by the conventional voltage modulation techniques is only possible under ambient conditions and in a non-polar medium and is difficult in an aqueous solution. Practically, the electrostatic force versus dc voltage curve in water does not include the minimum, which is used for the surface potential compensation. This is because the cantilever oscillation induced by the electrostatic force acting on the tip apex is overwhelmed by the parasitic oscillation induced by the electrostatic force acting on the entire cantilever as well as the surface stress effect. We both experimentally and theoretically discuss the factors which cause difficulties in application of the voltage modulation techniques in the aqueous solutions and present some criteria for local surface charge and potential measurements by circumventing these problems.

Astrocytes play an important role in the physiological functions of the central nervous system. In this study, contact potential differences (CPD) and capacitance gradients of the cell bodies and glial filaments of astrocytes were measured. Charge propagation properties in the astrocyte gap junctions were also studied using multimode Atomic Force Microscopy (AFM) at nanometre resolution. The CPD of the cell bodies and glial filaments were 324.2 ± 138.4 and 119.1 ± 31.7 mV, respectively. The measured capacitance gradients were 1.51 ± 0.31 and 1.98 ± 0.32 zFnm-1 , respectively. The gap junctions in the astrocytes showed no charge propagation and were not electrically sensitive. This furthers our understanding of astrocytes and other types of neuroglia. LAY DESCRIPTION: Neuroglia cells play important structural and functional roles in central nervous system (CNS). Neuroglia cells exceed the number of neurons by 10∼50 and can be divided into macroglia and microglia. Astrocytes are macroglia and are the largest and most abundant cells in the CNS. Astrocytes lack axons and dendrites and do not propagate action potentials. They have few cytoplasmic organelles, but possess abundant glial filaments, the main components of the cytoskeleton. Glial filaments are composed of the glial fibrillary acidic protein (GFAP). Astrocytes produce intercellular calcium waves in their gap junctions mediated through receptor activator (such as glutamate) to permit signal transduction._ENREF_5 In addition to their role in the support and nutrition of neurons, astrocytes are involved in various types of CNS activity including: (1) cytokine secretion for neuronal survival, growth and differentiation; (2) protection from brain injury; (3) modulation of the blood brain barrier; and (4) neuronal immunity. Bidirectional crosstalk between the astrocytes and neurons exists. Astrocytes can be activated by neurotransmitters released and can themselves release gliotransmitters to act upon neurons. Astrocytes are closely related to various disease states, including epilepsy and Alzheimer's disease. In this study, the electrical properties of astrocytes, including the contact potential difference (CPD) and capacitance gradients of the cell bodies and glial filaments, and charge propagation in the gap junctions were investigated at the nanometer level using quantitative Kelvin Probe Force Microscopy (KPFM) and Electrostatic Force Microscopy (EFM). The CPD of the cell bodies and glial filaments of the astrocytes were 324.2 mV and 119.1 mV, respectively. Capacitance gradients of the cell bodies and glial filaments of the astrocytes were 1.51 zF/nm and 1.98 zF/nm, respectively. Gap junctions in the astrocytes do not perform charge propagation functions and the astrocytes are not electrically sensitive. One should note that these results from KPFM and EFM were measured on dried cell and the situation might be different when studying in operando environment, still these findings aid our understanding of the electrical properties and functions of astrocytes, and further our knowledge of the electrical properties of the CNS.

The distance dependence and atomic-scale contrast recently observed in nominal contact potential difference (CPD) signals simultaneously recorded by Kelvin probe force microscopy (KPFM) using noncontact atomic force microscopy (NCAFM) on defect-free surfaces of insulating as well as semiconducting samples have stimulated theoretical attempts to explain such effects. Especially in the case of insulators, it is not quite clear how the applied bias voltage affects electrostatic forces acting on the atomic scale. We attack this problem in two steps. First, the electrostatics of the macroscopic tip-cantilever-sample system is treated by a finite-difference method on an adjustable nonuniform mesh. Then the resulting electric field under the tip apex is inserted into a series of atomistic wavelet-based density functional theory (DFT) calculations. Results are shown for a realistic neutral but reactive silicon nanoscale tip interacting with a NaCl(001) sample. Bias-dependent forces and resulting atomic displacements are computed to within an unprecedented accuracy. Theoretical expressions for amplitude modulation (AM) and frequency modulation (FM) KPFM signals and for the corresponding local contact potential differences (LCPD) are obtained by combining the macroscopic and atomistic contributions to the electrostatic force component generated at the voltage modulation frequency, and evaluated for several tip oscillation amplitudes $A$ up to 10 nm. For $A$ $=$ 0.1 \AA{}, the computed LCPD contrast is proportional to the slope of the atomistic force versus bias in the AM mode and to its derivative with respect to the tip-sample separation in the FM mode. Being essentially constant over a few volts, this slope is the basic quantity that determines variations of the atomic-scale LCPD contrast. Already above $A$ $=$ 1 \AA{}, the LCPD contrasts in both modes exhibit almost the same spatial dependence as the slope. In the AM mode, this contrast is approximately proportional to ${A}^{\ensuremath{-}1/2}$, but remains much weaker than the contrast in the FM mode, which drops somewhat faster as $A$ is increased. These trends are a consequence of the macroscopic contributions to the KPFM signal, which are stronger in the AM-mode and especially important if the sample is an insulator even at subnanometer separations where atomic-scale contrast appears.

A Kelvin probe force microscopy (KPFM) image is sometimes difficult to interpret because it is a blurred representation of the true surface potential (SP) distribution of the materials under test. The reason for the blurring is that KPFM relies on the detection of electrostatic force, which is a long-range force compared to other surface forces. Usually, KPFM imaging model is described as the convolution of the true SP distribution of the sample with an intrinsic point spread function (PSF) of the measurement system. To restore the true SP signals from the blurred ones, the intrinsic PSF of the system is needed. In this work, we present a way to experimentally calibrate the PSF of the KPFM system. Taking the actual probe shape and experimental parameters into consideration, this calibration method leads to a more accurate PSF than the ones obtained from simulations. Moreover, a nonlinear reconstruction algorithm based on total variation (TV) regularization is applied to KPFM measurement to reverse the blurring caused by PSF during KPFM imaging process; as a result, noises are reduced and the fidelity of SP signals is improved.

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.

Local rear contacts for silicon passivated emitter and rear contact solar cells can be established by point-wise treating an Al layer with laser radiation and thereby establishing an electrical contact between Al and Si bulk through the dielectric passivation layer. In this laser fired contacts (LFC) process, Al can establish a few μm thick p+-doped Si region below the metal/Si interface and forms in this way a local back surface field which reduces carrier recombination at the contacts. In this work, the applicability of Kelvin probe force microscopy (KPFM) to the investigation of LFCs considering the p+-doping distribution is demonstrated. The method is based on atomic force microscopy and enables the evaluation of the lateral 2D Fermi-level characteristics at sub-micrometer resolution. The distribution of the electrical potential and therefore the local hole concentration in and around the laser fired region can be measured. KPFM is performed on mechanically polished cross-sections of p+-doped Si regions formed by the LFC process. The sample preparation is of great importance because the KPFM signal is very surface sensitive. Furthermore, the measurement is responsive to sample illumination and the height of the applied voltage between tip and sample. With other measurement techniques like micro-Raman spectroscopy, electrochemical capacitance-voltage, and energy dispersive X-ray analysis, a high local hole concentration in the range of 1019 cm−3 is demonstrated in the laser fired region. This provides, in combination with the high spatial resolution of the doping distribution measured by KPFM, a promising approach for microscopic understanding and further optimization of the LFC process.

The presence of defect states, such as grain boundaries (GBs), can interfere with the charge transport properties of various semiconductor oxides. In this research, electrostatic force microscopy (EFM) and conducting atomic force microscopy (c-AFM) techniques were used to explore the nanoscale surface electrical properties of zinc oxide (ZnO) thin films deposited on a conductive fluorine-doped tin oxide (FTO) substrate. Films like these are often used as anode materials in photovoltaic and other optoelectronic devices. EFM measurements revealed the presence of charge trapping within the grain boundary region, suggesting localized band-bending effects. Furthermore, a current map obtained through c-AFM indicated that the grain regions exhibited higher conductivity, validating the observations made with EFM. By combination of c-AFM and Kelvin probe force microscopy (KPFM), it was possible to obtain experimental confirmation of band bending at grain boundaries. Data extracted from current–voltage (I–V) curves allowed the quantification of local saturation currents of 1.29 and 0.75 nA at the grain and GB. It was also possible to calculate the difference in potential barrier height between grain and GB as 50.40 meV. Urbach energy calculations identified the existence of defect states within the band gap. These defect states shifted the Fermi level toward the conduction band, reducing the local work functions to 3.93 and 3.89 eV for the grain and GB. These findings align with the thermionic emission (TE) model and Schottky–Mott theory, contributing to a deeper understanding of nanoscale charge transport within ZnO-based anodes and paving the way for the development of transparent conductive oxide-based optoelectronic devices and other applications.

Cancer cells produce extracellular vesicles (EVs) coated with an anionic sugar polymer, hyaluronan (HA), in the extracellular matrix. Hyaluronan is an established cancer biomarker in several cancer types. In this work, we thoroughly investigated the electrical properties of HA-coated EVs using advanced scanning probe microscopy (SPM) based nanoelectrical modes, which include EFM (electrostatic force microscopy), KPFM (Kelvin probe force microscopy), PFM (piezoresponse force microscopy) and C-AFM (conductive atomic force microscopy). Analyses revealed distinct properties for different sets of EVs regarding surface potential, charge distribution, and piezoelectric electro-mechanical response at the single-vesicle resolution. The typical electron transport capabilities are primarily driven by ions in sandwiched EV junctions. This correlative approach essentially could distinguish HA-coated cancer EVs (CEVs) from normal EV (NEVs) counterparts. The combined SPM-based nanoelectrical modes offered a multiplexed one-stop label-free solution for EV's electrical property assessments. This strategy is useful in developing EV-based bioelectronic sensors.

Field‐emission scanning electron microscopy (FE‐SEM) and surface potentiometry based on scanning probe microscopy, i.e. Kelvin probe force microscopy (KFM), have been applied to study microstructures consisting of organosilane self‐assembled monolayers (SAMs) terminated with ‐CH 3 or ‐CF 3 groups. Onto cleaned Si substrates covered with a thin oxide layer of 2 nm thick, SAMs were formed by chemical vapour deposition using n ‐octadecyltrimethoxysilane (CH 3 (CH 2 ) 17 Si(OCH 3 ) 3 , ODS) or fluoroalkylsilane (CF 3 (CF 2 ) 7 (CH 2 ) 2 Si(OCH 3 ) 3 , FAS) as a precursor. Through a photolithographic technique employing vacuum ultraviolet light at 172 nm, microstructures composed of ODS, FAS or both were fabricated. Micropatterns of the SAMs on SiO 2 /Si substrates were clearly imaged by FE‐SEM at an acceleration voltage of the electron beam below 1 kV, but image contrasts became faint with an increase in the acceleration voltage. At a voltage of 5 kV there was almost no detectable contrast. An FE‐SEM image of the ODS–FAS microstructure acquired at an acceleration voltage of 0.8 kV clearly demonstrated that the region covered with ODS was brighter and thus, emitted secondary electrons more efficiently than the region covered with FAS. Furthermore, as indicated in a KFM image, the region covered with FAS had a surface potential ∼180 mV lower than that of the region covered with ODS. The origin of these image contrasts between ODS‐ and FAS‐SAMs was the large difference in electronic states between ODS and FAS owing to the electron negativity of fluorine atoms. Copyright © 2003 John Wiley & Sons, Ltd.

Kelvin probe force microscopy (KFM) was applied to two-dimensional profiling of silicon pn-structures covered with a 2 nm-thick oxide layer. The surface potential contrast between the p- and n-type regions depended on the hydrophobicity of the oxide surface when KFM imaging was conducted in air with a relative humidity of more than 50%. By decreasing the density of surface hydroxyl groups on the oxide layer through thermal annealing, the potential contrast between the p- and n-type regions increased. While there was no detectable contrast on samples covered with hydrophilic oxide with a water contact angle of almost 0°, contrast increased to greater than 50 mV on the samples covered with hydrophobic oxide with a water contact angle of about 80°. However, when KFM imaging was conducted in a dry nitrogen atmosphere with relative humidity less than 0.6%, a clear potential contrast of about 50 mV could be acquired even on samples covered with the hydrophilic oxide layer. Since samples with less adsorbed water on their surface showed greater potential contrast, contrast degradation is attributed to a shielding effect of the adsorbed water layer.

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.

Recent advances in the development of micro-and nano-devices call for applications of thin nanocomposite dielectric films (thickness less than few tens of nanometers) with tuneable electrical properties. For optimization purposes, their behaviour under electrical stress needs to be probed at relevant scale, i.e. nanoscale. To that end electrical modes derived from Atomic Force Microscopy (AFM) appear the best methods due to their nanoscale resolution and non-destructive nature which permits in-situ characterization. The potentialities of electrical modes derived from AFM are presented in this work. The samples under study consist of plasma processed thin dielectric silica layers with embedded silver nanoparticles (AgNPs). Charge injection at local scale, performed by using AFM tip, is investigated by Kelvin Probe Force Microscopy (KPFM). Modulation of the local permittivity induced by the presence of AgNPs is assessed by Electrostatic Force Microscopy (EFM).