Kelvin Probe Force Microscopy Measurements Research Articles

Atomic force microscopy correlates mechanical and electrical properties of HepG2 cells with curcumin concentration

Hepatocellular carcinoma (HCC) is a highly prevalent cancer with a significant impact on human health. Curcumin, a natural compound, induces cytoskeletal changes in liver cancer cells and modifies the distribution of lipids, proteins, and polysaccharides on plasma membranes, affecting their mechanical and electrical properties. In this study, we used nanomechanical indentation techniques and Kelvin probe force microscopy (KPFM) based on atomic force microscopy (AFM) to investigate the changes in surface nanomechanical and electrical properties of nuclear and cytoplasmic regions of HepG2 cells in response to increasing curcumin concentrations. CCK-8 assays and flow cytometry results demonstrated time- and concentration-dependent inhibition of HepG2 cell proliferation by curcumin. Increasing curcumin concentration led to an initial increase and then decrease in the mechanical properties of nuclear and cytoplasmic regions of HepG2 cells, represented by the Young's modulus (E), as observed through nanoindentation. KPFM measurements indicated decreasing trends in both cell surface potential and height. Fluorescence microscopy results indicated a positive correlation between curcumin concentration and phosphatidylserine translocation from the inner to the outer membrane, which influenced the electrical properties of HepG2 cells. This study provides valuable insights into curcumin's mechanisms against cancer cells and aids nanoscale evaluation of therapeutic efficacy and drug screening.

Quantitative characterization of built-in potential profile across GaAs p–n junctions using Kelvin probe force microscopy with qPlus sensor AFM

The electrostatic potential distribution in materials and devices plays an important role in controlling the behaviors of charge carriers. Kelvin probe force microscopy (KPFM) is a powerful technique for measuring the surface potential at a high spatial resolution. However, the measured surface potential often deviates from the potential deep in the bulk owing to certain factors. Here, we performed KPFM measurements across the p–n junction, in which such factors were eliminated as much as possible by selecting the sample, force sensor, and measurement mode. The measured surface potential distribution agrees well with the line shape of the simulated bulk potential. Our results demonstrate that KPFM is capable of quantitatively characterizing potential distributions whose changes occur on the order of 10 nm.

Side charge propagation in simultaneous KPFM and transport measurement of humidity exposed graphene FET sensor

The surface diffusion (dissipation) of charge carriers enhanced by water molecules in solution-based biosensors and ambient operating gas sensors strongly influence their resistance response, sensitivity, and stability in time. Therefore, the information on the charge distribution at interfaces of conductive and insulating parts is essential for the operating sensors. This work presents the simultaneous measurement of the longitudinal macroscopic resistance response and local surface potential (SP) mapping by Kelvin probe force microscopy (KPFM) on a graphene Hall bar sensor. The results show the propagation of an electric charge from the main graphene channel onto the neighboring SiO2 surface. The charge propagation strongly increases with the relative humidity and can be controlled by a bottom-gate voltage used in most sensors based on a field effect transistor (FET) architecture. As proved by the longitudinal resistance measurements, the resulting side charge accumulation has a very small impact on the 2D resistivity of the graphene channel. It has been explained by an application of the Thomas-Fermi theory, proving an efficient screening of side accumulated charge potential caused by a redistribution of the charge inside the wide graphene channel. The combination of a transport resistance response and KPFM provides a deeper understanding of sensors/biosensors functionality and their design features than a simple resistance response usually observed.

Li3Bi/Li2O layer with uniform built-in electric field distribution for dendrite free lithium metal batteries

Lithium metal batteries have garnered significant attention as a promising energy storage technology, offering high energy density and potential applications across various industries. However, the formation of lithium dendrites during battery cycling poses a considerable challenge, leading to performance degradation and safety hazards. This study aims to address this issue by investigating the effectiveness of a protective layer on the lithium metal surface in inhibiting dendrite growth.The hypothesis is that continuous lithium consumption during battery cycling is a primary contributor to dendrite formation. To test this hypothesis, a protective layer of Li3Bi/Li2O was applied to the lithium foil through immersion in a BiN3O9 solution. Experimental techniques including kelvin probe force microscopy (KPFM) and density functional theory (DFT) calculations were employed to analyze the structural and electronic properties of the Li3Bi/Li2O layer.The findings demonstrate successful doping of Bi into the Li coating, forming Bi-Bi and Bi-O bonds. KPFM measurements reveal a higher work function of Li3Bi/Li2O, indicating its potential as an effective protective layer. DFT calculations further support this observation by revealing a greater adsorption energy of lithium on the Li3Bi/Li2O layer compared to the bulk material. Charge density analysis suggests that the adsorption of Li atoms onto the Li3Bi/Li2O layer induces a redistribution of charge, resulting in increased electron availability on the surface and preventing electrode–electrolyte contact.This study provides insights into the role of the Li3Bi/Li2O protective layer in inhibiting dendrite growth in lithium metal batteries. By mitigating dendrite formation, the protective layer holds promise for enhancing battery performance and longevity. These findings contribute to the development of strategies for improving the stability and reliability of lithium metal batteries, facilitating their wider adoption in energy storage applications.

Solution-Phase Synthesis of Vanadium Intercalated 1T′-WS2 with Tunable Electronic Properties

Metal ion intercalation into Group VI transition metal dichalcogenides enables control over their carrier transport properties. In this work, we demonstrate a low-temperature, solution-phase synthetic method to intercalate cationic vanadium complexes into bulk WS2. Vanadium intercalation expands the interlayer spacing from 6.2 to 14.2 Å and stabilizes the 1T' phase of WS2. Kelvin-probe force microscopy measurements indicate that vanadium binding in the van der Waals gap causes an increase in the Fermi level of 1T'-WS2 by 80 meV due to hybridization of vanadium 3d orbitals with the conduction band of the TMD. As a result, the carrier type switches from p-type to n-type, and carrier mobility increases by an order of magnitude relative to the Li-intercalated precursor. Both the conductivity and thermal activation barrier for carrier transport are readily tuned by varying the concentration of VCl3 during the cation-exchange reaction.

A simple KPFM-based approach for electrostatic- free topographic measurements: the case of MoS2 on SiO2

A simple implementation of Kelvin probe force microscopy (KPFM) is reported that enables recording topographic images in the absence of any component of the electrostatic force (including the static term). Our approach is based on a close loop z-spectroscopy operated in data cube mode. Curves of the tip-sample distance as a function of time are recorded onto a 2D grid. A dedicated circuit holds the KPFM compensation bias and subsequently cut off the modulation voltage during well-defined time-windows within the spectroscopic acquisition. Topographic images are recalculated from the matrix of spectroscopic curves. This approach is applied to the case of transition metal dichalcogenides (TMD) monolayers grown by chemical vapour deposition on silicon oxide substrates. In addition, we check to what extent a proper stacking height estimation can also be performed by recording series of images for decreasing values of the bias modulation amplitude. The outputs of both approaches are shown to be fully consistent. The results exemplify how in the operating conditions of non-contact AFM under ultra-high vacuum (nc-AFM), the stacking height values can dramatically be overestimated due to variations in the tip-surface capacitive gradient, even though the KPFM controller nullifies the potential difference. We show that the number of atomic layers of a TMD can be safely assessed, only if the KPFM measurement is performed with a modulated bias amplitude reduced at its strict minimum or, even better, without any modulated bias. Last, the spectroscopic data reveal that certain kind of defects can have a counterintuitive impact on the electrostatic landscape, resulting in an apparent decrease of the measured stacking height by conventional nc-AFM/KPFM compared to other sample areas. Hence, electrostatic free z-imaging proves to be a promising tool to assess the existence of defects in atomically thin TMD layers grown on oxides.

Surface Photovoltage Study of Metal Halide Perovskites Deposited Directly on Crystalline Silicon.

Perovskite (PVK) films deposited directly on n-type crystalline Si substrates were investigated by two operating modes of the surface photovoltage (SPV) method: (i) the metal-insulator-semiconductor (MIS) mode and (ii) the Kelvin probe force microscopy (KPFM). By scanning from 900 to 600 nm in the MIS mode, we consecutively studied the relatively fast processes of carrier generation, transport, and recombination first in Si, then on both sides of the PVK/Si interface, and finally in the PVK layer and its surface. The PVK optical absorption edge was observed in the range of 1.61-1.65 eV in good agreement with the band gap of 1.63 eV found from photoluminescence spectra. Both SPV methods evidenced an upward energy band bending at the PVK/n-Si interface generating positive SPV. Drift-diffusion modeling allowed us to analyze the shape of the wavelength dependence of the SPV. It was also observed that the intense illumination in the KPFM measurements induces slow SPV transients which were explained by the creation and migration of negative ions and their trapping at the PVK surface. Finally, aging effects were studied by measuring again SPV spectra after one-year storage in air, and an increase in the concentration of shallow defect states at the PVK/n-Si interface was found.

Probing depth-dependent inhomogeneous lithium concentration in thick LiNi0.88Co0.09Al0.03O2 cathodes for lithium-ion batteries

A high energy density is an essential requirement for commercial lithium-ion batteries (LIBs) in electric vehicles (EVs) because it directly affects driving range. In addition, the fast charging performance of LIBs, which determines the charging time, is critical in the EV industry. A common approach to enhance the energy density of LIBs is to increase the active material loading by increasing the electrode thickness. However, there is limited scope for increasing the thickness of LIB electrodes in fast charging operations because the power density characteristics of LIB cells are degraded with increasing electrode thickness. Here, we estimate the lithium concentration distribution within thick LIB cathodes after charging/discharging by studying the work function distribution within the thick electrodes using Kelvin probe force microscopy (KPFM). First, to determine the relationship between the work function and lithium content in LIB cathodes, we measure the work function of thin LiNi0.88Co0.09Al0.03O2 (NCA) electrodes under various state-of-charge (SOC) conditions. From these measurements, it is demonstrated that the work function of the NCA electrodes is proportional to the SOC. The results suggest that the work function measurements obtained using KPFM can be used to estimate the Li concentration of NCA electrodes. Next, we perform depth-dependent KPFM measurements in thick NCA samples, which can provide depth profiles of the work function depending on the C-rate conditions. The work function shows a monotonic increase along the depth direction of the thick NCA cathodes, implying a depth-dependent inhomogeneous distribution of lithium concentrations.

Surface structure and work function change of pentafluorobenzeneselenolate self-assembled monolayers on Au (111)

The formation and surface structure of pentafluorobenzeneselenolate (PFB-Se) self-assembled monolayers (SAMs) on Au(111) prepared by solution and vapor depositions were examined by scanning tunneling microscopy (STM). STM observations showed that solution-deposited PFB-Se SAMs on Au(111) had only a disordered phase containing many dark areas with irregular shapes, regardless of the deposition time. However, it was found that the degree of structural order of PFB-Se SAMs was markedly improved by the vapor deposition method at 348 K. After short deposition for 5 min, PFB-Se SAMs on Au(111) consisted of two distinct regions: a bright protruding region and a dark region with partial ordered domains. Interestingly, PFB-Se SAMs after deposition for 1 h showed various structural features with three different contrasts: an ordered dark region, an ordered and disordered bright region, and a very bright region. The ordered domain is assigned to a (5 × √3) structure. The relatively uniform and nicely ordered PFB-Se SAMs with a (14 × √3) structure were formed after longer deposition for 2 h, resulting from further adsorption and structural rearrangements. Furthermore, Kelvin probe force microscopy measurements (KPFM) exhibited a significant difference in the work function of pentafluorobenzenethiolate and the PFB-Se SAM-modified gold surface by 0.36 eV due to the large difference in the domain and packing structures of both SAMs. Herein, STM and KPFM results will give new insights into the formation and structures of PFB-Se SAMs on Au(111) and further the fundamental understanding of the interfacial electronic properties of fluorinated aromatic backbone-based thiolate and selenolate SAMs.

Probing the charge transfer and electron–hole asymmetry in graphene–graphene quantum dot heterostructure

In recent years, graphene-based van der Waals (vdW) heterostructures have come into prominence showcasing interesting charge transfer dynamics which is significant for optoelectronic applications. These novel structures are highly tunable depending on several factors such as the combination of the two-dimensional materials, the number of layers and band alignment exhibiting interfacial charge transfer dynamics. Here, we report on a novel graphene based 0D–2D vdW heterostructure between graphene and amine-functionalized graphene quantum dots (GQD) to investigate the interfacial charge transfer and doping possibilities. Using a combination of ab initio simulations and Kelvin probe force microscopy (KPFM) measurements, we confirm that the incorporation of functional GQDs leads to a charge transfer induced p-type doping in graphene. A shift of the Dirac point by 0.05 eV with respect to the Fermi level (E F) in the graphene from the heterostructure was deduced from the calculated density of states. KPFM measurements revealed an increment in the surface potential of the GQD in the 0D–2D heterostructure by 29 mV with respect to graphene. Furthermore, we conducted power dependent Raman spectroscopy for both graphene and the heterostructure samples. An optical doping-induced gating effect resulted in a stiffening of the G band for electrons and holes in both samples (graphene and the heterostructure), suggesting a breakdown of the adiabatic Born–Oppenheimer approximation. Moreover, charge imbalance and renormalization of the electron–hole dispersion under the additional influence of the doped functional GQDs is pointing to an asymmetry in conduction and carrier mobility.

Characterization of electronic charged states of high density self-aligned Si-based quantum dots evaluated with AFM/Kelvin probe technique

We have characterized charged states of a self-aligned Si quantum dot (QD) structure with an areal density as high as ∼1013 cm−2 by surface potential measurements by using atomic force microscopy (AFM)/Kelvin probe force microscopy (KFM). By line scanning with the electrically-biased AFM tip with respect to the substrate in a contact mode, the corresponding area was negatively charged caused by electron injection from the tip into QDs. Cross-sectional profiles of the surface potential were clearly changed, which depends on the tip bias voltages. Also, a stepwise shape in a relationship between the change of the surface potential (ΔV) and the tip bias voltages was observed, reflecting electron charging into the discrete energy levels due to the quantum confinement effect. By assuming a simple equivalent circuit model for KFM measurements of the self-aligned Si-QDs, calculated results for ΔV are in good agreement with the measured results. The agreements precisely clarified the charged states in Si-QDs.

Revealing of InP multi-layer stacks from KPFM measurements in the dark and under illumination

Solar cells are complex devices, being constituted of many layers and interfaces. The study and the comprehension of the mechanisms that take place at the interfaces is crucial for efficiency improvement. This paper applies Kelvin probe force microscopy (KPFM) to study materials and interfaces with nanometer scale imaging of the surface potential in the dark and under illumination. KPFM measurements are highly sensitive to surface states and to the experimental measurement environment influencing the atomic probe operating conditions. Therefore, in order to develop a quantitative understanding of KPFM measurements, we have prepared a dedicated structured sample with alternating layers of InP:S and InP:Fe whose doping densities were determined by secondary-ion mass spectroscopy. We have performed KPFM measurements and shown that we can spatially resolve 20 nm thick InP layers, notably when performed under illumination which is well-known to reduce the surface band-bending.

Retrieve the carrier population and built-in potential alignment in multi-quantum-well GaAs/InGaAs p-i-n photodiode

Quantum wells (QWs) are widely adopted in pn junctions to extend the cut-off wavelength for photodetection and solar energy conversion , in which the built-in electric field plays essential role in extracting the photo-carriers from QWs but inevitably affected by practical factors like the background doping. Herein, the carrier distribution and potential alignment in GaAs/InGaAs multi-quantum-well p-i-n photodiode were investigated by scanning capacitance microscopy (SCM) and Kelvin probe force microscopy (KPFM). Three of the five QWs were found injected by electrons with population density up to 7 × 10 16 cm −3 in maximum according to the numerical simulation. The unintentionally doping concentration was estimated to be 5 × 10 15 cm −3 in the absorption area, this led to a narrowed space charge region by 355 nm from expectation, which was well proved by the KPFM measurement. Nevertheless, the change of built-in potential could reach 0.577 V corresponding to a fairly high open-circuit voltage of the photodiode under illumination with modest intensity. Our study provides direct insight into p-i-n multi-quantum-well structures for high efficiency photoelectric conversion. • For multi-quantum-well GaAs/InGaAs p-i-n photodiode: • The actual carrier and built-in potential distributions were disclosed by KPFM and SCM. • The built-in potential was measured as 1.21 V which decreased under illumination indicating a high V OC . • The unintentionally doping in absorption area led to shrinkage of depletion region and high population of electrons in quantum wells according to numerical simulation.

Measurement of polarization effects in dual-phase ceria-based oxygen permeation membranes using Kelvin probe force microscopy.

In this study, a dual phase composite (CSO-FC2O) consisting of 60 vol % Ce0.8Sm0.2O1.9 as oxygen-conductive phase and 40 vol % FeCo2O4 as electron-conductive phase was synthesized. TEM measurements showed a relatively pure dual-phase material with only minor amounts of a tertiary (Sm,Ce)(Fe,Co)O3 perovskite phase and isolated residues of a rock salt phase at the grain boundaries. The obtained material was used as a model to demonstrate that a combination of polarization relaxation measurements and Kelvin probe force microscopy (KPFM)-based mapping of the Volta potential before and after the end of polarization can be used to determine the chemical diffusion coefficient of the ceria component of the composite. The KPFM measurements were performed at room temperature and show diffusion coefficients in the range of 3 × 10−13 cm2·s−1, which is comparable to values measured for single-phase Gd-doped ceria thin films using the same method.

The impact of Kelvin probe force microscopy operation modes and environment on grain boundary band bending in perovskite and Cu(In,Ga)Se2 solar cells

An in-depth understanding of the electronic properties of grain boundaries (GBs) in polycrystalline semiconductor absorbers is of high importance since their charge carrier recombination rates may be very high and hence limit the solar cell device performance. Kelvin Probe Force Microscopy (KPFM) is the method of choice to investigate GB band bending on the nanometer scale and thereby helps to develop passivation strategies. Here, it is shown that the workfunction, measured with amplitude modulation (AM)-KPFM, which is by far the most common KPFM measurement mode, is prone to exhibit measurement artifacts at grain boundaries on typical solar cell absorbers such as Cu(In,Ga)Se2 and CH3NH3PbI3. This is a direct consequence of a change in the cantilever–sample distance that varies on rough samples. Furthermore, we critically discuss the impact of different environments (air versus vacuum) and show that air exposure alters the GB and facet contrast, which leads to erroneous interpretations of the GB physics. Frequency modulation (FM)-KPFM measurements on non-air-exposed CIGSe and perovskite absorbers show that the amount of band bending measured at the GB is negligible and that the electronic landscape of the semiconductor surface is dominated by facet-related contrast due to the polycrystalline nature of the absorbers.

Electrochemical characteristics of grain boundaries in gadolinium and aluminum co-doped ceria and ceria-alumina composites

The influence of 1, 2, 4 and 10 cat.% Al2O3 addition on the electrochemical characteristics of commercially available 10 cat.% Gd-doped ceria and a 1:1 cat.% mixture of CeO2:Al2O3 were evaluated in our study. Analysis of the electronic conductivity and polarization-relaxation behavior of the samples showed that an Ohmic contribution to the total electronic resistance cannot only be attributed to the electrode contact resistance but is a combined contribution from the electrode interface and decreased pO2 dependence of the grain boundary resistivity of the material. The average electrical potential barrier at the grain boundaries was calculated from impedance spectroscopy data at 50–300 °C. Kelvin probe force microscopy (KPFM) measurements were used to analyze the local Volta potential distribution at the grain boundaries of pure Ce0.9Gd0.1O1.95 and of samples with 4 and 10 cat.% Al addition at room temperature. KPFM measurements showed clear evidence for an increased Volta potential difference between bulk and grain boundary cores and for a considerable broadening of the Volta potential gradient at the grain boundaries with increasing Al concentration.

Measuring and Tuning the Potential Landscape of Electrostatically Defined Quantum Dots in Graphene.

We use Kelvin probe force microscopy (KPFM) to probe the carrier-dependent potential of an electrostatically defined quantum dot (QD) in a graphene/hexagonal boron nitride (hBN) heterostructure. We show that gate-dependent measurements enable a calibration scheme that corrects for uncertainty inherent in typical KPFM measurements and accurately reconstructs the potential well profile. Our measurements reveal how the well changes with carrier concentration, which we associate with the nonlinear dependence of graphene's work function on carrier density. These changes shift the energy levels of quasi-bound states in the QD which we can measure via scanning tunneling spectroscopy (STS). We show that the experimentally extracted energy levels closely compare with wave functions calculated from the reconstructed KPFM data. This methodology, where KPFM and STS data are simultaneously acquired from 2D materials, allows the quasiparticle response to an electrostatic potential to be determined in a self-consistent way.

Interpreting Kelvin probe force microscopy on semiconductors by Fourier analysis

Kelvin probe force microscopy (KPFM) has become a popular surface scanning tool for functional materials and devices, and it has been widely interpreted by the contact potential difference (CPD) theory as the precedent Kelvin probe method. In this article, we developed a Fourier analysis framework for KPFM on the basis that the probe in KPFM is excited by a sinusoidal ac voltage, which is different from the classical Kelvin method. As a result, it was found that the KPFM signal will deviate from the CPD value if the sample charge quantity is not an odd function of the external bias, i.e., the CPD interpretation is invalid on those samples such as doped semiconductors. In order to further estimate the signal deviation from the CPD in the KPFM measurement on semiconductors, the tip–sample system was simulated as a one-dimensional metal–insulator–semiconductor capacitor using Fermi–Dirac statistics. The simulation results showed that the KPFM signals on doped semiconductors behave like those on an intrinsic one when the ac voltage is large, and therefore, the KPFM signal contrast on a pn junction could be flattened even if the sample surface has a clear CPD contrast without any Fermi level pinning due to surface states. Finally, possible ways for tuning KPFM operation parameters to suppress the signal deviation effect were also discussed.

Direct observation of potential phase at joining interface between p-MgO and n-MgFe2O4

Visualization of the depletion layer is a significant a guideline for the material design of gas sensors. We attempted to measure the potential barrier at the interface of core–shell microspheres composed of p-MgO/n-MgFe2O4/Fe2O3 from the inside out by means of Kelvin probe force microscopy (KPFM) as a first step to visualizing enlargement of the depletion layer. As determined by high-angle annular dark-field scanning transmission electron microscopy, ca. 70% of the microspheres were hollow with a wall thickness of ca. 200 nm. Elemental mapping revealed that the hollow particles were composed of ca. 20 nm of MgO, ca. 80 nm of MgFe2O4, and ca. 100 nm of Fe2O3. A difference of 0.2 V at the p-MgO/n-MgFe2O4 interface was clarified by KPFM measurements of the hollow particles, suggesting that this difference depends on the formation of a p–n junction. The potential barrier enlarged by the formation of a p–n junction was considered to increase the resistance in air (Ra), since the Ra of the core–shell hollow microspheres was higher than that of MgO, Fe2O3, MgO–Fe2O3, and MgO/MgFe2O4/Fe2O3 particles with irregular shapes. Measurement of the potential barrier height by KPFM is a promising potential approach to tuning the gas sensitivity of oxide semiconductors.

Stable and Highly Flexible Perovskite Solar Cells with Power Conversion Efficiency Approaching 20% by Elastic Grain Boundary Encapsulation

Here, we show that flexible perovskite solar cells (PSCs) with high operational stability and power conversion efficiency (PCE) approaching 20% were achieved by elastic grain boundary (GB) encapsul...