Local Surface Potential Distribution Research Articles

Atomic structure and electron distribution of Co atoms adsorbed on Si(111) surface by NC-AFM/KPFM at 78 K

The Co atoms adsorbed on a Si(111)-7 × 7 surface is investigated by NC-AFM/KPFM at 78 K. The Co atoms in the initial adsorption are distinguished from defects on the surface by line profile analysis and force spectroscopy. For the first time, Co clusters and their VLCPD is measured by NC-AFM/KPFM. Each Co cluster is found to have a ring-like structure after annealing a sample at 230℃, and its bond energy is lower than that of a Si adatom owing to the passivated dangling bonds. We propose a structure model to explain the atomic structure including three subsurface Co atoms and six Si adatoms. DFT calculations with the GGA mode are carried out to optimize the structure model and simulate the local surface potential distribution of ring-like Co cluster on Si(111)-7 × 7 surface.

Measuring Diffusion Coefficients of the Ceria Phase in a Dual-Phase Oxygen Permeation Membrane Using a Combined Polarization - Kelvin Probe Force Microscopy Method

Dual-phase membranes with FeCo2O4 or its iron-rich pendant Fe2CoO4 and Gd-doped ceria as an oxygen ion conductor have already been successfully applied as oxygen permeation membranes with high permeability in the temperature range above 800 °C [1-3]. Apart from Gd-doped ceria, Sm-doped ceria also can be an interesting alternative in this kind of composite due to its high ionic conductivity [4].As especially ceria or ceria-based composites have gained more and more interest for low-temperature applications, e.g. as catalyst materials, current research efforts are aiming to improve the composition and microstructure of such dual-phase membranes for application in a membrane reactor at considerably lower temperatures (below 600 °C) to perform partial oxidation reactions. For temperatures between 600-1200 °C the defect chemistry of these materials is well established, but experimental data for low temperature charge transport are limited and not fully understood, yet.Kelvin Probe Force Microscopy (KPFM) is an Atomic Force Microscopy (AFM)-based measurement method which can measure the local surface potential (or from the physics view: the Volta potential) of the sample [5, 6]. The surface potential is a measure for local changes of the defect chemistry, as it is directly related to the local Fermi niveau [7]. The implications of local oxidation and reduction at low temperatures for charge carrier mobility in pure ceria, and especially in dual-phase materials, are not very well understood so far, but KPFM can be used to measure the surface potential difference directly at the polarized contact area as well as to map the time-dependent relaxation process of the introduced gradient, also giving information about the local surface potential distribution, lateral extent of the gradient, variations of the shape of the gradient etc. [8, 9].In the present study, single ceria grains in a dual-phase Ce0.8Sm0.2O2-δ + FeCo2O4 oxygen permeation membrane were polarized at room temperature in ambient air using a Pt-coated AFM tip and a large silver paste back contact. Subsequently, the introduced defect concentration gradient (which is visible as a gradient of the surface potential) and the relaxation process over time was monitored by KPFM.By comparing the measurement results to previously acquired data of single phase ceria samples with different doping concentrations and crystallinity, we were able to show that by this technique, chemical diffusion coefficients of a single phase in a composite material are accessible. Acknowledgements: The work was funded by the German Research Foundation - project #387282673. Literature: Ramasamy, M.; Baumann, S.; Palisaitis, J.; Schulze‐Küppers, F.; Balaguer, M.; Kim, D.; Meulenberg, W. A.; Mayer, J.; Bhave, R.; Guillon, O. J Amer Ceram Soc 2016, 99 (1), 349-355.Ramasamy, M.; Persoon, E. S.; Baumann, S.; Schroeder, M.; Schulze-Küppers, F.; Görtz, D.; Bhave, R.; Bram, M.; Meulenberg, W. A. Journal of membrane science 2017, 544, 278-286.Lin, Y.; Fang, S.; Su, D.; Brinkman, K. S.; Chen, F. Nature Comm 2015, 6 (1), 1-9.Mori, T.; Drennan, J.; Lee, J.-H.; Li, J.-G.; Ikegami, T. Solid State Ionics 2002, 154-155, 461-466.Örnek, C.; Leygraf, C.; Pan, J. Corr Eng Sci Tech 2019, 54 (3), 185-198.Melitz, W.; Shen, J.; Kummel, A. C.; Lee, S. Surf Sci Rep 2011, 66 (1), 1-27.Zhang, Z.; Yates, J. T. Chem Rev 2012, 112 (10), 5520-5551.Neuhaus, K.; Schulze-Küppers, F.; Baumann, S.; Ulbrich, G.; Lerch, M.; Wiemhöfer, H.-D. Solid State Ionics 2016, 288, 325-330.Lee, W.; Lee, M.; Kim, Y.-B.; Prinz, F. B. Nanotechnology 2009, 20 (44), 445706.

Nanoscale charge transport and local surface potential distribution to probe the defect passivation in Cr-substituted earth abundant CZTS absorber layer

The non-stochiometric deviations in CZTS often results in certain detrimental point defects that act as trap centers causing non-radiative recombination and potential fluctuations, eventually results in large open circuit voltage (VOC) deficit in CZTS solar cells. Cationic substitution in CZTS layer is one of the leading approaches to capture the great potential of kesterite solar cells for future cost-effective photovoltaic technology. Here, chromium incorporation in CZTS is studied for cationic substitution in CZTS layer, as it renders multiple chemical states including +1, +2 and + 4 of Cu, Zn and Sn cations in CZTS. The CZTS films determine to have good crystallinity and morphology with no significant disparity among pristine and Cr-doped CZTS films, apart from slight shift of XRD (112) peak indicating partial cation substitution by smaller Cr atoms. However, the Zn content is found to be decreased with increasing Cr content in CZTS layer. Substantial enhancement in the absorption of Cr-doped CZTS films indicate intermediate band (IB) within the band gap. Photoluminescence (PL) emission spectra strongly suggests reduced non-radiative recombination and potential fluctuations in Cr-CZTS films. X-ray photoelectron spectroscopy investigations reveal Zn substitution by Cr in CZTS crystal geometry. Using nanoscale Kelvin Probe Force Microscopy (KPFM) and Conducting Atomic Force Microscopy (CAFM), we verify the surface potential variation and nanoscale electrical conductivity in pristine and Cr-CZTS films. Additionally, Zn substitution by Cr eventually lead to suppression of ZnSn deep level defects segregated at grain boundaries (GBs). CAFM strongly confirms the GB passivation in Cr-CZTS films. This work widens the opportunity of exploring potential cationic substitution in CZTS for developing high efficiency CZTS solar cells.

Nanoscale charge transport and local surface potential distribution to probe defect passivation in Ag doped Cu2ZnSnS4 absorbing layer

The performance of earth abundant Cu2ZnSnS4 (CZTS) material is limited by high deficit of open circuit voltage (VOC) which is mainly due to the easy formation of CuZn antisite defects. Suppression of CuZn defects is thus inevitably required for further developments in CZTS based solar cells. We studied systematic increase of Ag doping in CZTS thin film and investigated the nanoscale electrical properties using Kelvin probe force microscopy and current sensing atomic force microscopy (CAFM) to probe CuZn defects. Crystallographic analysis indicated the successful partial substitution of Cu+ ions by large size Ag+ ions. The considerable decrease in grain boundary potential from 66.50 ± 5.44 mV to 13.50 ± 2.61 mV with Ag doping, suggesting the substantial decrease in CuZn defects. Consequently, CAFM measurement confirms the remarkable increment in minority carrier current with Ag doping and their local mobility in CZTS layer. Finally, the lower persistent photoconductivity and fast decay response of photogenerated carriers for Ag doped CZTS photodetector further validate our results. This study provides a fresh approach of controlling deleterious CuZn defects in CZTS by tuning Ag content that may guide researchers to develop next generation high-performance CZTS based solar cells.

Domain structure, local surface potential distribution and relaxation of Pb(Zn1/3Nb2/3)O3–9%PbTiO3 (PZN–9%PT) single crystals

The local domain structure and surface potential of Pb(Zn1/3Nb2/3)O3–9%PbTiO3 (PZN–9%PT) single crystals were investigated by Kelvin probe force microscopy (KPFM) and piezoresponse force microscopy (PFM), respectively. The results confirm the existence of the screening layer on the surface of the material. The relaxation of the surface potential and its relation to polarization switching within the poled regions in air and argon gas were analyzed. It is found that the surface potential increases with the poling voltage to a limited range. Comparing the relaxation behavior in ambient air with that in argon gas, we find that the relaxation time in argon is much shorter than that in air, and the relaxation in air is dominated by the space charge transport phenomena. In addition, the results show that the injected charges have significant effects on the pinning of the polarization in PZN–PT single crystals.

Local surface potential distribution and its relaxation in ferroelectric poly(vinylidenefluoride-co-trifluoroethylene) thin films

Local surface potential distributions and their relaxation behavior in ferroelectric poly(vinylidene fluoride-co-trifluoroethylene) thin films were investigated using Kelvin-probe force microscopy. All surface potentials were negative regardless of the sign of the applied voltage because of the intrinsic negative charges that originated from the self-aligned region of dipoles at the ferroelectric/electrode interface. Importantly, the intrinsic charges would have a remarkable influence on the reliability of written data.

Control of selective adsorption behavior of Ti-binding ferritin on a SiO2 substrate by atomic-scale modulation of local surface charges

We have systematically investigated the influence of local surface potential distribution on selective adsorption of Ti-binding ferritin (TBF) on SiO2 substrates. We found that the TBF exhibits the adsorption ability onto Ti-decorated SiO2 surface of less than one atomic layer coverage. This experimental finding enabled us to propose a model in which the positively charged sites and their nanometer-scale interval are the dominant factors that determine bond strength between TBF and SiO2 surfaces. Furthermore, on the basis of this model, the adsorption behavior of TBF onto bare SiO2 surfaces was controlled by generation and/or recovery of electrical defects in SiO2.

AFM and KPFM Study of Effect of Cholesterol and Cortisol on Structure of Lipid Monolayers

The plasma membrane is a complex structure, composed primarily of phospholipids and other macromolecules, such as proteins, sterols and steroids, which define its complex functions. Phospholipid monolayers are widely used as convenient models to study plasma membrane. We investigated model lipid monolayers composed of dipalmitoylphosphatidylcholine (DPPC) in order to elucidate effect of cholesterol and cortisol on the structure and properties of these model systems. Supported DPPC monolayers were prepared with Langmuir-Blodgett monolayer technique and imaged by Atomic Force Microscopy (AFM) and Kelvin Probe Force Microscopy (KPFM). Frequency modulation KPFM technique (FM-KPFM) (1) based on electrostatic force gradient was used to measure local surface potential distribution in monolayers. This technique is performed by applying modulation frequency in addition to the resonance frequency of the cantilever and has been shown previously to have superior resolution for measuring surface potential in biomolecular films (2). We found that cholesterol and cortisol produced structural changes (nanoscale domains) both in topography and surface potential maps of the monolayers. In addition with Differential Scanning Calorimetry (DSC) we found that both steroids produce changes the phase transitions of the DPPC membrane. 1. U. Zerweck, C. Loppacher, T. Otto, S. Grafström, and L. M. Eng, Phys. Rev. B 71 (2005) 125424. 2. B. Moores, F.Hane, L. M. Eng, Z. Leonenko, Ultramicroscopy, (2010) pub. on line: http://dx.doi.org/10.1016/j.ultramic.2010.02.036.

Effect of local surface potential distribution on its relaxation in polycrystalline ferroelectric films

We have studied the effect of local surface potential distribution on its relaxation in the polycrystalline ferroelectric thin films. A lower surface potential region, i.e., potential pit, is generated near a grain boundary. The deep potential pit has a faster relaxation than the area far away from the potential pit due to the acceleration of the screen charge draining near the grain boundary and the domains formed by applying higher voltage have a faster relaxation due to the larger gradient of screen charge distribution. In addition, the surface potential and its relaxation depend on the sign of applying voltage. The result shows that the surface potential distribution may influence significantly to the reliability of bit signal on the memory devices.

Surface potential imaging of CNT-FET devices by scanning Kelvin probe microscopy

We investigated the surface potential around nanotube channel during CNT-FET operation by scanning Kelvin probe microscopy (SKPM). The results demonstrate that the local surface potential distribution depends on the fabrication process of FET devices. We also measured the transfer characteristic of CNT-FET and compared with the surface potential image. In addition, we investigate the specific FET device with the closed CNT loops into which channel CNT penetrates. The surface potential distribution is completely different with that of a simple single CNT channel. We find that the closed loop of CNT has a capability of trapping the charge inside the loop.

A resolution study for electrostatic force microscopy on bimetallic samples using theboundary element method

Electrostatic force microscopy (EFM) is a special design of non-contact atomic forcemicroscopy used for detecting electrostatic interactions between the probe tip and thesample. Its resolution is limited by the finite probe size and the long-range characteristicsof electrostatic forces. Therefore, quantitative analysis is crucial to understanding therelationship between the actual local surface potential distribution and the quantitiesobtained from EFM measurements. To study EFM measurements on bimetallic sampleswith surface potential inhomogeneities as a special case, we have simulated suchmeasurements using the boundary element method and calculated the force component andforce gradient component that would be measured by amplitude modulation (AM) EFMand frequency modulation (FM) EFM, respectively. Such analyses have been performed forinhomogeneities of various shapes and sizes, for different tip–sample separationsand tip geometries, for different applied voltages, and for different media (e.g.,vacuum or water) in which the experiment is performed. For a sample with asurface potential discontinuity, the FM-EFM resolution expression agrees with theliterature; however, the simulation for AM-EFM suggests the existence of an optimaltip radius of curvature in terms of resolution. On the other hand, for sampleswith strip- and disk-shaped surface potential inhomogeneities, we have obtainedquantitative expressions for the detectability size requirements as a function ofexperimental conditions for both AM- and FM-EFMs, which suggest that a largertip radius of curvature is moderately favored for detecting the presence of suchinhomogeneities.

Local surface potential distribution in oriented ferroelectric thin films

Using Kelvin force microscopy, the authors have investigated the potential distribution on ferroelectric films. The local distribution of potential was observed on downward, prepoled areas. The polarity of the potential corresponds to the screen charge. It was found that the electrical properties of the grain boundary affect the potential distribution. Most of the grain boundaries show a lower potential than the area inside the grain. The authors identified certain regions at the grain boundary with a very low potential. Such potential pits may act as efficient screen charge draining paths and may lead to important perturbations on the device level.

In situ observation of electrode surfaces using scanning Maxwell-stress microscopy under water

We present the results of a recent attempt to extend scanning Maxwell-stress microscopy (SMM) to aqueous environments. Scanning Maxwell-stress microscopy is a type of scanning probe microscopy designed to image microscopic electrical properties, such as the local surface potential distribution. In particular, the non-resonant behaviour of SMM makes it fundamentally suitable for underwater operations, in contrast to other resonant-type electric force microscopes. From underwater operations of SMM, the high viscosity of an aqueous solution can be made immaterial by making the driving frequency sufficiently low, whereas the large dielectric permitivity of water is always quite advantageous in enhancing the electric forces. Furthermore, the electrochemical potentials of stylus and sample are controlled independently and simultaneously during SMM by using a dual potentiostat. This makes it possible for in situ observation of charge on the electrode surface during an electrochemical reaction. Copyright © 1999 John Wiley & Sons, Ltd.

Structural Investigation of Spiropyran Containing Langmuir-Blodgett Films using Scanning Probe Microscope Technique

Scanning Maxwell stress Microscopy (SMM) which is a type of scanning probe microscopy designed to image microscopic electrical properties, have been applied to study the local surface potential distribution in photochromic spiropy ran dye Langmuir-Blodgett (LB) films. Using linear dependence between effective dipole moment and surface potential, structural change has been studied during photoisomerization.

Dynamics of nanodomains (clusters) and dielectric relaxation in disordered ferroelectrics

Dielectric relaxation in the disordered ferroelectric-type materials of different kind is under consideration. We defined the common features of the dielectric spectra and phase transitions of this materials and propose to consider observed phenomena from common position as an intrinsic feature of disordered ferroelectric materials of different kind. Dielectric relaxation, phase transition diffusing and peculiarities of dielectric spectra are a consequence of frequency and temperature dependent dipole dynamics of the polar phase microregions (clusters) and interphase boundaries. The evidence of the electrical heterogeneity of some disordered ferroelectric materials was obtained by the Scanning Force Microscopy method with simultaneous measurement of the local surface potential distribution.