Contact Potential Difference Measurements Research Articles

Topography and localized charge of steps on CeO2(111) investigated by AFM/KPFM

The configuration of step edges on metal oxide surface is significant for understanding fundamental reactivity and predicting the performance in catalysis. In this work, the topography feature and charge of steps on CeO2(111) are investigated by atomic force microscopy /kelvin probe force microscopy combined with density functional theory. Hexagonal islands and pits containing step I edge and step II edge have been observed at annealing 1100 K. The irregular structures with the addition of step III edge have been found at 1200 K. The measurement of contact potential difference reveals that step II displays the strongest localized charge state, followed by step III, and step I with the weakest localized charge state. The calculations of density functional theory have confirmed that step II possesses a stronger localized charge state. In addition, the Ce atoms at the step II edge exhibit greater positive and negative charge accumulation. These results provide the surface structure and electrical potential of three step edges on the CeO2(111), and establish both experimental and theoretical foundations for subsequent investigations on adsorption and catalysis.

Atomic force microscopy observation of surface morphologies and measurements of local contact potential differences of amorphous solid water samples deposited at 15 and 100 K.

We use an ultra-high vacuum cryogenic atomic force microscope to investigate the surface morphology of amorphous solid water (ASW) prepared by oblique deposition of water vapor onto Si(111)7 × 7 substrates at temperatures of 15 and 100 K. Height-height correlation function analysis of topographic images suggests that ASW at 15 K has a columnar structure and that the typical diameter of the column is 5-10 nm. At 100 K, the typical diameter is 10-30 nm, although columnar features are less prominent. The surface roughness (i.e., deviation of the height) is greater at 15 K than at 100 K, indicating that the surface at 100 K exhibits a relatively flat morphology. This result implies that transient diffusion of deposited water molecules affects the surface morphology at 100 K. In addition, measurements of the local contact potential difference between the tip and the ASW surface suggest that the magnitude of the negative surface potential at the microscopic scale, which is attributed to spontaneous polarisation, cannot simply be scaled by the thickness of ASW as predicted in previous experiments with Kelvin probes.

Flexoelectrically augmented triboelectrification enabled self-power wireless smart home control system

With the rapid development of the Internet of Things and artificial intelligence, smart home has emerged to fulfill the security, convenience, and energy-saving issues of modern life. Self-power, multifunctional, and sensitive sensors are one of the key components for the popularization of smart home systems. Herein, a flexoelectric mica crystal is shown to augment the finger touch-driven triboelectric output enough to operate a wireless and multichannel smart home controller. The smart home controller consists of concave mica films, a hand-grip-type vertical mouse, and an Arduino controller. Strain-gradient-dependent contact potential difference measurement and density functional theory calculation show that flexoelectric charge shifts the workfunction and concave bending reduces the effective electron mass in a mica. A portable hand-grip-type flexoelectrically augmented triboelectric nanogenerator (Flexo-TENG) demonstrates the wireless communication of message in real time and the selective operation of home appliances with various functions via a simple finger touch. This work provides important ingredients for the enhanced triboelectric output via flexoelectricity, and for the realization of a convenient, multifunctional, cost-effective, and adaptable smart home control system without batteries.

Charge state of steps on anatase TiO2(1 0 1) at 78 K by AFM/KPFM

Surface steps strongly influence the surface chemistry of metal oxides, a detailed picture of step charge and structure may help to understand reactivity and predict the performance of the overall material in catalysis. In this study, we report the existence of another type of step, step BII, on the anatase TiO2(101) surface using atomic force microscopy/Kelvin probe force microscopy combined with density functional theory (DFT). By in situ measurements of current and contact potential difference (CPD) information, we found that step B II exhibits the highest charge state, followed by step C and D, which have the same structure, and step A with the weakest charge state. This finding challenges the conventional understanding that step C possesses the highest charge state. By incorporating DFT calculations, we confirmed that step B II possesses a higher charge state. This discovery also supports the observation of higher charge states in step C than in step B I. In addition, our results indicate that the valence band is predominantly governed by oxygen atoms, while titanium atoms dominate the conduction band. At the step edge, oxygen atoms act as electron donors, with electrons mainly clustering near the titanium atoms and Ti-O bonds. The present study provides a complete characterization of the charge state of the step edges on the anatase TiO2(101) surface and plays an important role in investigating the molecular adsorption as well as charge transfer in catalytic reactions on metal oxides surface.

Unveiling localized electronic properties of ReS2 thin layers at nanoscale using Kelvin force probe microscopy combined with tip-enhanced Raman spectroscopy

Electronic properties of two-dimensional (2D) materials can be strongly modulated by localized strain. The typical spatial resolution of conventional Kelvin probe force microscopy (KPFM) is usually limited in a few hundreds of nanometers, and it is difficult to characterize localized electronic properties of 2D materials at nanoscales. Herein, tip-enhanced Raman spectroscopy (TERS) is proposed to combine with KPFM to break this restriction. TERS scan is conducted on ReS2 bubbles deposited on a rough Au thin film to obtain strain distribution by using the Raman peak shift. The localized contact potential difference (CPD) is inversely calculated with a higher spatial resolution by using strain measured by TERS and CPD-strain working curve obtained using conventional KPFM and atomic force microscopy. This method enhances the spatial resolution of CPD measurements and can be potentially used to characterize localized electronic properties of 2D materials.

Solution Combustion Synthesis and Characterization of Magnesium Copper Vanadates.

Magnesium vanadate (MgV2O6) and its alloys with copper vanadate were synthesized via the solution combustion technique. Phase purity and solid solution formation were confirmed by a variety of experimental techniques, supported by electronic structure simulations based on density functional theory (DFT). Powder X-ray diffraction combined with Rietveld refinement, laser Raman spectroscopy, diffuse reflectance spectroscopy, and high-resolution transmission electron microscopy showed single-phase alloy formation despite the MgV2O6 and CuV2O6 end members exhibiting monoclinic and triclinic crystal systems, respectively. DFT-calculated optical band gaps showed close agreement in the computed optical bandgaps with experimentally derived values. Surface photovoltage spectroscopy, ambient-pressure photoemission spectroscopy, and Kelvin probe contact potential difference (work function) measurements confirmed a systematic variation in the optical bandgap modification and band alignment as a function of stoichiometry in the alloy composition. These data indicated n-type semiconductor behavior for all the samples which was confirmed by photoelectrochemical measurements.

High-temperature contact potential difference measurement of surface work function using in vacuo Kelvin probe

Characterization of the work function of emitting surfaces represents an important contribution to thermionic emission research today. This is because materials that exhibit lower work functions are capable of thermionically emitting higher current densities at lower operating temperatures, which ultimately means longer lifetimes for the vacuum electron devices that make use of thermionic emitters. One widely used technique to characterize a material's work function is the measurement of contact potential difference using a Kelvin probe system. When applied in the traditional mode, this technique fails to produce meaningful results in situations where a sufficiently hot sample under inspection and the cooler Kelvin probe are in thermal disequilibrium. However, as this paper will outline, the standard application of a Kelvin probe system can be amended to facilitate meaningful asymmetric contact potential difference measurements, and consequently, obtain values of work function for samples at high temperatures, including relevant operating temperatures for thermionic emitters (>1000 °C).

Hybrid Cathodic/Anodic Electrosynthesis of Phase Pure Ag4V2O7 Thin Films

Here, we demonstrate a two-step electrosynthesis approach for the preparation of silver pyrovanadate, Ag4V2O7 in thin-film form. In the first, cathodic step, polycrystalline Ag was deposited on fluorine doped tin oxide (FTO) substrate from a non-aqueous bath. Aqueous pyrovanadate species were then generated by aging of a CO2-infused sodium orthovanadate (Na3VO4) solution for three weeks. Silver ions were subsequently generated in situ in this medium using anodic stripping of the Ag/ITO films from the first step. Interfacial precipitation of the Ag+ ions with the pyrovanadate species afforded the targeted product in phase pure form. The various stages of the electrosynthesis were monitored in situ via the combined use of voltammetry, electrochemical quartz crystal nanogravimetry (EQCN), and coulometry. The Ag4V2O7 thin films were characterized by a variety of experimental techniques, including X-ray diffraction, laser Raman spectroscopy, diffuse reflectance spectroscopy, scanning electron microscopy, and high-resolution transmission electron microscopy. Surface photovoltage spectroscopy, ambient-pressure photoemission spectroscopy, and Kelvin probe contact potential difference (work function) measurements afforded information on the energy band structure of the p-type Ag4V2O7 semiconductor. Finally, the electrochemical and photoelectrochemical properties of the electrosynthesized Ag4V2O7 thin films were studied in both aqueous and non-aqueous electrolytes.

Environmental sensitivity of GaN nanofins grown by selective area molecular beam epitaxy

Nanostructures exhibit a large surface-to-volume ratio, which makes them sensitive to their ambient conditions. In particular, GaN nanowires and nanofins react to their environment as adsorbates influence their (opto-) electronic properties. Charge transfer between the semiconductor surface and adsorbed species changes the surface band bending of the nanostructures, and the adsorbates can alter the rate of non-radiative recombination in GaN. Despite the importance of these interactions with the ambient environment, the detailed adsorption mechanisms are still not fully understood. In this article, we present a systematic study concerning the environmental sensitivity of the electrical conductivity of GaN nanofins. We identify oxygen- and water-based adsorbates to be responsible for a quenching of the electrical current through GaN nanofins due to an increased surface band bending. Complementary contact potential difference measurements in controlled atmospheres on bulk m- and c-plane GaN reveal additional complexity with regard to water adsorption, for which surface dipoles might play an important role besides an increased surface depletion width. The sensitive reaction of the electrical parameters to the environment and surface condition underlines the necessity of a reproducible pre-treatment and/or surface passivation. The presented results help to further understand the complex adsorption mechanisms at GaN surfaces. Due to the sensitivity of the nanofin conductivity on the environment, such structures could perform well as sensing devices.

Influence of relative humidity on CO2 interaction mechanism for Gd-doped SnO2 with respect to pure SnO2 and Gd2O3

The paper aims to identify the CO2 interaction mechanism for chemical sensors based on Gd-doped SnO2, SnO2 and Gd2O3 powders deposited as thick sensitive layers. The low reactivity of CO2 conferred by the thermodynamic stability and chemical inertia can be offset by the presence of relative humidity. The sensitive powders were prepared by wet chemical co-precipitation method. The Gd concentration was varied from 1% to 20 at% in order to determine the limit for Gd integration as a doping ion prior to chemical segregation as a secondary phase. Analytical transmission electron microscopy points to a homogeneous Gd doping of the nanostructured SnO2 powders for low doping concentrations and the formation of a nanocomposite based on SnO2 as main phase and cubic Gd2O3 as secondary phase for the highly doped samples. The electrical resistance is either influenced by the density of oxygen vacancies, or is the result of compensation for two opposite behaviours into the SnO2-Gd2O3 nanocomposite structures. The CO2 exposure to humid atmosphere determines distinct behaviours corresponding to SnO2 and Gd2O3 as constitutive elements. The associated CO2 interaction mechanism is based on simultaneous DC electrical resistance and Contact Potential Difference measurements, which allow decoupling the ionosorption from the dipolar processes, thus highlighting specific chemical interactions on the SnO2 and Gd2O3 surfaces.

Influence of gas adsorption on the surface photovoltage of Au nanorods embedded polymer coated ZnO nanorods under visible light irradiation

The synergism between the photosensitivity and gas sensitivity enabled the development of low powered highly selective gas sensors. In particular, organic/inorganic hybrid materials contribute to the coupling effect of light activated surface photovoltage modifications. In this report, the effect of visible light and volatile organic compounds (VOCs) on the hybrid sensing layer involving organic/inorganic/noble metal through scanning kelvin probe was disseminated. The sensor was fabricated by spin coating the layer-by-layer assembled poly (allylamine hydrochloride) (PAH) and dextran sulfate (DS) on hydrothermally grown ZnO nanorods followed by the incorporation of gold nanorods on ITO substrates. Structural and optical characterizations revealed the assembly of Au nanorods on the polymer coated ZnO nanorods and maximum absorbance in the visible light region, respectively. The optical nature of the hybrid sensor facilitated significant changes in contact potential difference (CPD) when irradiated with light. Changes in CPD values were measured with respect to alcohol, alkane and amine under illumination. A non-selective interaction was observed, yet the enhanced visible light response to triethylamine (TEA) corresponds to the formation of coordination bonds between the hybrid sensor and adsorbed species. The potential shift of ∼31 mV, ∼67 mV and ∼164 mV were observed for TEA by ZnO, ZnO/Poly and ZnO/Poly/Au, respectively. The simultaneous charge transfer/separation ability, surface plasmon resonance and inhibition of recombination of photogenerted carriers by Au promoted the interaction. Evidence of selective interaction can be elucidated through the selection of different VOCs under specific type of gas for CPD measurements.

Low-Temperature Mixing of Polar Hydrogen Bond-Forming Molecules in Amorphous Solid Water

The level of mixing of guest molecules within low-temperature amorphous solid water (ASW) (as a host) is of interest and relevance to model studies of photochemistry that take place in the interstellar medium (ISM). In this study, we explore how the physical properties of guest molecules, methanol and ammonia, which strongly interact with ASW films, affect the level of mixing and distribution of these molecules throughout the ASW film at adsorption temperatures of 35–100 K, while they are deposited on a Ru(0001) substrate under ultra-high vacuum conditions. Both methanol and ammonia have gas-phase dipole moments similar to that of water, and they are both capable of forming hydrogen bonds with water. The level of mixing and dispersion of these molecules in ASW films is explored through the isothermal noninvasive contact potential difference (ΔCPD) measurements and temperature programmed contact potential difference experiments (TP-ΔCPD). Upon adsorption at 35 K, both molecules form hydrogen bonds with the surrounding water molecules but do not move too far from their initial location in the ASW film. This is confirmed by the observation of an “inverse volcano” process, where guest molecules initially placed within the interaction range of the ruthenium substrate migrate to the substrate upon water crystallization instead of desorbing to the vacuum as expected in a typical “volcano” process. The adsorption temperature at which extensive mixing is achieved is correlated to the glass transition and crystallization temperatures of the guest molecules, which in this case is lower for both guest molecules than that of water. Homogeneous mixing is only achieved when the films are heated above the glass transition temperature of the host (water) molecules.

Рентгеновские фотоэлектронные спектры молекул кислорода, адсорбированных на нанопленках иттербия

The adsorption of oxygen molecules on ytterbium nanofilms of various thicknesses has been studied by X-ray photoelectron spectroscopy and contact potential difference measurements. It has been established that the maximum number of O2 molecules that can be adsorbed on the films increases with the film thickness. Such behavior is observed at thicknesses less than 10 monolayers. This size effect is ultimately due to the fact that an indispensable condition for the adsorption process at high doses of oxygen is the electron transfer from the film to molecules, and that the maximum number of electrons that can participate in this process depends on the nanolayer thickness. Localization of a negative charge on O2 molecules occurring in at high doses of adsorbate, causes a weakening of the bonding of 1s electrons in these molecules. In addition, it should lead to a weakening of the bond between oxygen atoms in O2 molecules.

X-ray photoelectron spectra of oxygen molecules adsorbed on ytterbium nanofilms

The adsorption of oxygen molecules on ytterbium nanofilms of various thicknesses has been studied by X-ray photoelectron spectroscopy and contact potential difference measurements. It has been established that the maximum number of O2 molecules that can be adsorbed on the films increases with the film thickness. Such behavior is observed at thicknesses less than 10 monolayers. This size effect is ultimately due to the fact that an indispensable condition for the adsorption process at high doses of oxygen is the electron transfer from the film to molecules, and that the maximum number of electrons that can participate in this process depends on the nanolayer thickness. Localization of a negative charge on O2 molecules occurring in at high doses of adsorbate, causes a weakening of the bonding of 1s-electrons in these molecules. In addition, it should lead to a weakening of the bond between oxygen atoms in O2 molecules. Keywords: oxygen, ytterbium, adsorbed molecules, nanofilms, X-ray photoelectron spectroscopy, work function.

Measurement of local contact potential difference of atomic scale Au/Si(111)-(7×7) delocalized adsorption state in room-temperature and ultra-high vacuum environment

The structural properties and local contact potential difference of Au on Si(111)-(7×7) surface are studied by the homemade ultra-high vacuum non-contact Kelvin probe force microscope. Although scanning tunneling microscopy has been widely used to study the metal- adsorbed semiconductor surfaces on an atomic scale, the tunnel current measured by scanning tunneling microscopy is easy to lead the charge states to accidentally switch in the measurement process, and it is limited only to the observation of metal and semiconductor surfaces. Kelvin probe force microscope allows us to directly measure the charges at different positions of various flat surfaces by local contact potential difference on an atomic scale, which has become a more convenient and accurate means of charge characterization. In this paper, the topography and local contact potential difference of Au adsorbed Si(111)-(7×7) surface are measured on an atomic scale by Kelvin probe force microscope at room temperature, and the corresponding adsorption model and first principle calculation are established. The differential charge density distribution of the stable adsorption position of Au/Si(111)-(7×7) is obtained, and the local contact potential energy difference relationship of the stable adsorption position of Au on Si surface is given, The mechanism of charge transfer between Au atom and Si(111)-(7×7) surface during adsorption is analyzed. The experimental results show that at room temperature, single Au atom will form triangular delocalized adsorption state in the half unit cell of Si(111)-(7×7). The delocalized adsorption state is due to the fact that the moving speed of a single Au atom in the HUC is faster than the scanning speed of Kelvin probe force microscope, and the local contact potential difference measurement of Au/Si(111)-(7×7) adsorbed surface can effectively identify Au and Si atoms. Obviously, this research is of great significance in promoting the development of surface charge precision measurement, and is expected to provide some insights into the charge properties of metal adsorbed semiconductor surfaces.

Investigation of UV light enhanced gas adsorption properties of CeO2 Nanoparticles by Scanning Kelvin Probe system

Cerium oxide (CeO2) nanoparticles (NPs) were synthesized by the precipitation method using poly(vinylpyrrolidone) as a precipitating agent. HRSEM and HRTEM confirmed the nano rocks like morphology with irregular shape and size. The synthesized CeO2 NPs particle size was ∼13.75 nm. SAED pattern revealed that the synthesized CeO2 NPs were polycrystalline in nature. The presence of oxygen vacancies in the as-prepared material was confirmed by Raman spectroscopy. The volatile organic compounds (VOCs) adsorption properties of CeO2 NPs were investigated using scanning Kelvin probe (SKP) system through contact potential difference (CPD) measurements both in the dark and ultraviolet (UV) light illumination. Interestingly, CeO2 NPs showed selective adsorption towards triethylamine vapours in comparison with other VOCs such as ethanol and n-hexane. These studies revealed that the CeO2 nanoparticles can be utilized for the development of a novel gas sensor for triethylamine detection at room temperature.

Quantitative amplitude-modulation scanning Kelvin probe microscopy via the second eigenmode excitation

Amplitude modulation scanning Kelvin probe microscopy (AM-SKPM) is widely used to measure the contact potential difference (CPD) between probe and samples in ambient or dry inert atmosphere. However, AM-SKPM is generally considered quantitatively inaccurate due to crosstalk between the cantilever and the sample. Here we demonstrate that the accuracy of AM-SKPM-based CPD measurements is drastically improved by exciting the SKPM probe at its second eigenmode. In the second eigenmode of oscillation, there exists a stationary node at the cantilever towards its free end, across which the displacement bears opposite signs; therefore driving the SKPM probe at its second eigenmode helps to partially cancel the virtual work done by the cantilever and reduce the crosstalk effect. The improvement in accuracy is experimentally confirmed with interdigitating electrodes calibration samples as well as practical samples such as the cross-section of wafer-bonded GaAs/GaN heterojunction.

Development of sensor for measuring the aircraft metal parts contact potential difference

The article is devoted to solving the problem of determining the electron work function of aircraft metal parts of aviation technology by the Kelvin probe method. The features of the application of the Kelvin probe method for contact potential difference measurement for aircraft metal parts are considered. Requirements for the Kelvin probe developed for measuring the contact potential difference of metal parts are formulated. A comparative analysis of materials for the manufacture of the measuring electrode of the sensor is carried out. A technique is proposed to ensure the stability of the measurement results of the contact potential difference, and the factors influencing it are assessed. Practical recommendations are given for the Kelvin probe development of the contact potential difference measuring on the aircraft metal parts surface.

DIGITAL CONTACT POTENTIAL PROBE IN STUDYING THE DEFORMATION OF DIELECTRIC MATERIALS

The paper reviews the results of a study on the surface electrostatic charges of dielectrics obtained using the contact potential difference (CPD) technique. Initially, the CPD technique was only applied to the study of metal and semiconductor surfaces. The conventional CPD measurement technique requires full compensation of the measured potential that, in the case of dielectrics, could reach very high values. Such high potentials are hard to compensate. Therefore, the conventional CPD method is rarely applied in the study of dielectric materials. Some important improvements recently made to the CPD measurement technique removed the need for compensation. The new method, which does not require compensation, has been implemented in the form of a digital Kelvin probe. The paper describes the principles of the non-compensation CPD measurement technique which was developed for mapping the electrostatic surface charge space distribution across a wide range of potential values. The study was performed on polymers such as low-density polyethylene (LDPE) and polytetrafluoroethylene (PTFE).

The Effect of Environmental Parameters on Nondestructive Control of Metal Machine Parts by the Contact Potential Difference Method

This article is devoted to improvement of a nondestructive control of metal machine parts by the contact potential difference method. The results of experimental studies of the effect of environmental parameters on the contact potential difference measurements with a Kelvin probe on the surface of metal samples from Al, Ti, and Ni are presented. The measured dependence of the contact potential difference on the surface of metals on the environmental parameters has been explained. Practical recommendations for measuring the contact potential difference on the surface of parts have been given. The results can be used in production, operation, and repair of metal machine parts.