Vibrating Capacitor Method Research Articles

Method for Measuring Surface Charge on Insulating Materials Based on the Vibrating Capacitor Method

The phenomenon of surface charging, known as contact electrification or tribocharging, has wide-ranging applications but also notable hazards. Precisely measuring surface charge density in insulating materials is crucial for optimizing tribocharging and mitigating adverse effects. Although the vibrating capacitor method is commonly used for this purpose, its principle, designed for conductive materials, limits direct application to insulating surfaces, leaving the relationship between measured surface potential and surface charge density unclear. To address this issue, this study simulated the process of measuring the surface potential of insulating materials using Comsol simulations. It analyzed the effects of charged area size, probe height, and probe position, and utilized the spatial distribution of potential measurement values of surface point charges to derive an integral relationship between the distribution of measured surface potential values and the distribution of surface charge density. The integral relationship of surface potential distribution under different forms of surface charge density distributions calculated from this formula largely matches the numerical simulation results. Based on this, a relationship between the distribution of surface charge density and surface potential measurement values was further derived. This relationship can be used for measuring the surface charge density of insulating materials.

Conduction-type control of fullerene films from n- to p-type by molybdenum oxide doping

Conduction-type control of fullerene (C60) films from n- to p-type by doping with molybdenum oxide (MoO3) was demonstrated. The energetic value of the Fermi level, 4.60 eV, for nondoped C60 films measured by the Kelvin vibrating capacitor method was positively shifted to 5.88 eV, and approached the valence band by the coevaporated doping of MoO3 at a concentration of 3300 ppm. The existence of upward band bending of the Schottky junction formed at the interface between a metal and a p-type C60 film formed by MoO3 doping was confirmed based on its photovoltaic properties.

Vibrating capacitor method in the development of semiconductor gas sensors

Adsorption usually results in work function shifts on catalytically active surfaces such as semiconductor gas sensors. The purpose of the present article is to summarise the capabilities of the vibrating capacitor from the simplest adsorption-induced work function tests to the scanning, vibrating, capacitor-yielded olfactory pictures and other chemical pictures. After a brief history and review of theoretical bases, the latest results will be discussed in detail. Olfactory pictures from semiconductor surfaces give a new chance to improve the selectivity of gas analysis. Chemical pictures from thin SnO 2 layers produced by atomic layer epitaxy reveal the inhomogeneities of the technology. CPD maps taken from Pd nanolayer (activator)-covered surfaces help to find the best layer-depositing parameters for the activation process of the thin semiconductor gas sensor films.

Low temperature work function variations of the Au/GaSb (110) interface: Experimental results and theoretical model

The interface formation between Au and cleaved (110) GaSb surfaces is investigated at low temperature (LT<140 K) with the vibrating capacitor method. Au depositions are made in the 10−4–1 Å range. On the lightly n-type doped sample, the work function varies by around 0.74 eV in three steps, as a function of Au coverage. We suggest possible surface state distributions which can explain the experimental curves. Our analysis shows that at least three acceptor states are formed at the very beginning of Au deposition, and before reaching one monolayer. We determine the energetic position of these states, and we propose a model to explain their density evolution.

Change in Work Function of Fatigue Damaged Surface and Its Relevance to Fatigue Failure(Student Poster Session)

As the initiation of fatigue cracks on smooth surfaces is predominantly a surface phenomenon, extensive efforts have been focused on characterizing a surface damage such as extrusion and intrusion during fatigue. Therefore careful surface analysis techniques such as measurement of the work function (have after denoted as WF) using a Kelvin Probe would have a potential to predict an initiation of fatigue cracks. The WF is very sensitive to changes on the surface and hence it is believed that the formation of extrusions and intrusions during fatigue would affect the work function of the sample and this can be easily detected by the Kelvin probe technique (vibrating capacitor method). Generally, the bare surface of a metal has an active work function following the general e.m.f series. In this present investigation, the Kelvin probe is employed to study the change in work function of a specimen surface after predetermined increments in the number of fatigue cycles. During the initial fatigue cycles, the work function becomes more active than that prior to fatigue loading. This can be attributed to the formation of new surfaces due to the generation of intrusions and extrusions during the fatigue test. When exposed to ambient conditions at room temperature, the work function increases towards a more noble direction with time as the new surface oxidised. Through the continuous measurement of the WF during fatigue, it was observed that the formation ratio of the new surface area rapidly decreased after N/N_f=0.02. On the other hand, the logarithmic rate law constant of oxidation of the new surface gradually decreased with the degree of damage. Furthermore, during the final stages of fatigue, a greater number of intrusions without oxides develop compared to that of extrusions. Oxides formed preferentially on extrusions as observed by the elemental mapping obtained by Time-Of-Flight Secondary Ion Mass Spectrometry (TOF-SIMS) analysis. More active development of intrusions can result in the initiation of a crack.

Determination of SiO 2–Si interface trap level density ( Dit) by vibrating capacitor method

The interface trap level density, D it, is one of the most important parameters of the SiO 2–Si layer structure. C– V methods are suitable for measuring D it. However, the importance of non-destructive, touch-free methods has recently been increasing. Contactless methods use capacitive coupling. Vibration (vibrating capacitor), temperature difference and light are used for surface excitation; interface trap level density can be determined from the change in the value of oxide surface potential (work function). The present article discusses a new method for determining SiO 2–Si interface trap level density, D it, by the vibrating capacitor method, focussing on theory, application and evaluation of the results. Measured potential maps are presented, and the results are converted into D it distributions as examples for the application of this method.

Surface potential studies using Kelvin force spectroscopy

A home-built Kelvin force microscope combined with a spectroscopic method is dedicated to local measure- ments of topography and contact potential differences (CPD). This technique is based on a simple modification of a non- contact atomic force microscopy (nc-AFM), where a bias voltage is applied between the sample and tip during the CPD measurement. The changes in CPD between the tip and vari- ous sample materials have been measured. Kelvin force spec- tra of Pd=Si and of self-assembling monolayer films (SAMs) are presented. The Kelvin method (1) is a well-established technique for measuring the contact potential differences (CPD) between a reference electrode and a sample. The CPD for clean metal surfaces is given by the difference in the work functions of the two materials. A common method of measuring the con- tact potential difference is the vibrating capacitor method or Kelvin method. In this method, the probe, which is made of a material with a known work function (commonly tungsten or platinum), is arranged close to the surface to be measured as a parallel plate capacitor with small spacing. In a sim- ple model, the contact potential between the two materials is VCPDD 1=e.2 1/ ,w here 1 and 2 are the work func- tions of the conductors, including changes due to adsorption layers on the surface. A periodic vibration between the two plates at frequency ! with amplitude d1 results in a current i.t/ given by i.t/D @C

Fermi Level Shift in Photoconductive Organic Pigment Films Measured by Kelvin Vibrating Capacitor Method

Fermi level shift in n-type perylene pigment film was measured by the Kelvin vibrating capacitor method. Purification of pigment by reactive sublimation under methylamine atmosphere caused negative shift of the Fermi level due to the effective removal of unknown but specific impurity acting as an acceptor. On the other hand, bromine doping of pigment film caused marked positive shift of Fermi level to near the valence band, resulting in reversal of the photocurrent flow arising from the Schottky junction with Au. This is a clear demonstration of the control of conduction type from n-type to p-type, which was observed for the first time in pigment-based organic semiconductors. This result enabled construction of an organic homojunction composed of p- and n-type perylene pigment films.

Capacitive air-fuel ratio sensor

A new mechanism for determining the stoichiometric air-fuel ratio (A.F) of the concentrations of oxidizing and reducing species in a gas phase is presented. The mechanism, which appears suitable for use in the automotive application, is the reversible change in the work function of Pt (by as much as 0.7V). This change occurs at elevated temperatures when the Pt is alternately exposed to dilute mixtures of oxidizing (oxygen) and reducing (hydrocarbons and related molecules ) species that are rich or lean of their stoichiometric ratio. The change can be electrically monitored by the Kelvin vibrating capacitor method or other capacitive techniques. The sensitivity to A/f is verified by comparing the Kelvin signal with that of a ZrO 2 electrochemical A/F sensor whose working Pt electrode is simultaneuosly the active surface in the Kelvin measurement. With the observation that O 2 pumping to or from the active Pt surface with the ZrO 2 cell ran also significantly affect the work function, the possibility exists for developing an extended-range A/F sensor using this mechanism.

Assessing Adsorption of Conventional Friction Modifying Molecules by Relative Contact Potential Difference Measurements

A contact potential difference device based on the vibrating capacitor method of Zisman (1932) was used to assess adsorption and film formation at metal-petroleum interfaces. Measurement of changes in contact potential due to material adsorbed on low surface area metallic substrates is an attractive and convenient method for obtaining information about the adsorbed layer such as the adsorption isotherm and energetics. The adsorption iostherms of methyl stearate, stearic, oleic, and elaidic acids on copper and steel have been determined by the vibrating capacitor method.

The absolute electrode potential of metal electrodes emersed from liquid electrolytes

Gold or platinum electrodes are withdrawn from various electrolyte solutions, at a constant potential, into a water-saturated nitrogen gas phase (“emersion” under potentiostatic control). The Volta potential difference between a vibrating SnO 2 surface (Kelvin's vibrating capacitor method) and either the emersed electrode or the surface of a solution of known redox potential is evaluated and subsequently used to determine the difference, if any, between the absolute electrode potential of the metal electrode in the immersed and emersed state. It is shown that, upon emersion of a metal electrode, the absolute electrode potential decreases by up to 0.32 V, and further, that the magnitude of this decrease is reduced for electrolytes containing specifically adsorbing anions. This change is attributed to the reorientation of the water molecules at the solution/gas interphase.

The surface state of solvated ion exchangers investigated with polymer-modified electrodes

Electrodes coated with ion-exchange polymer films are withdrawn from electrolytes under potential control (“emersed”). Apparent electron work functions are calculated from measurements of the Volta-potential differences between a vibrating SnO 2 surface (Kelvin's vibrating capacitor method) and both the emersed electrodes and the electrolytes. To analyze these results theoretical concepts are developed. It is shown that quantitative information on the Donnan potential and on the surface dipole potential of the solvated polymer surface in the emersed state can be derived. The determined surface dipole potential of an emersed polyvinylpyridinium polymer is proposed to be the result of an asymmetric ion-ion double layer. This conclusion is substantiated by infrared reflection-absorption spectra of the emersed coated electrodes.

Work function and UPS studies of propylene and cyclopropane adsorption on palladium surfaces

The Kelvin vibrating capacitor method was used to measure the work function changes of vacuum deposited thin palladium layers in propylene adsorption. The work function change found for the propylene-saturated surface is, within the experimental error, identical with the change observed in the adsorption of cyclopropane and equal -1.085 ± 0.07 eV. Propylene and cyclopropane adsorption leads in the region of higher surface coverages to similar difference UV photoelectron spectra, exhibiting emission bands located at 6.7, 8.2 and 9.3 eV below the Fermi level. It is obvious from the results that at T ≈ 300 K, the adsorption of propylene and cyclopropane leads at higher coverages with high probability to similar surface complexes.

Work function studies of propylene, acetylene, hydrogen and nitrogen chemisorption on molybdenum

The work function changes of vacuum deposited molybdenum films caused by the chemisorption of propylene, acetylene, hydrogen and nitrogen were measured using the Kelvin vibrating capacitor method. During the hydrocarbon chemisorption, the work function increased in a low surface coverage region but decreased at the higher surface coverages. The saturation values of the work function changes corresponding to complete coverage of the surface by chemisorbed propylene and acetylene equal -0.08 eV and -0.42 eV, respectively. The observed dependences of the work function change on surface coverage are interpreted by the dissociative chemisorption of hydrocarbons on a limited number of surface sites which are simultaneously the sites of preferred adsorption. The extent of dissociation decreases in the adsorption with the increasing surface coverage. The results of the study of the work function changes induced by the hydrogen and nitrogen chemisorption enabled to draw several conclusions on the surface topography of the used films.

The predominance of a surface plane of high work function on Ti films

The contact potential between clean Ti films and an Au reference surface have been determined by the Kelvin vibrating capacitor method. The majority of Ti films deposited on glass substrates had a high work function, which suggests that their surface was mainly (0001). Ti films on substrates of oxidised Ti had a much lower work function and probably had a surface of mainly (1011).

A study on a palladium-titanium oxide Schottky diode as a detector for gaseous components

It has been found that the current through a Schottky barrier formed at the interface between palladium film and n-type titanium oxide (TiO 2) single crystal is sensitive to hydrogen or other reducing gases in the ambient. This effect is explained by taking account of the diminished barrier height at the Pd and TiO 2 interface caused by the action of gases, changing the work function of Pd metal. The change in the Pd work function estimated from the result is confirmed by the direct measurements of the metal surface potentials by use of a vibrating capacitor method. Similar electrical properties have been studied for junctions of TiO 2 with Pt, Au, Ni, Al, Cu, Mg and Zn, and of ZnO, CdS, GaP and Si w ith Pd.

Diffusion of oxygen on tungsten (110)

The surface diffusion of oxygen on W(110) was studied in the temperature range 760–880°C and for coverages up to one monolayer. Coverage-distance plots after diffusion runs were determined via corresponding work function profiles measured by a newly developed, laterally resolving vibrating capacitor method. The coverage profiles were evaluated in terms of a concentration dependent diffusion coefficient (DC) using the Boltzmann-Matano analysis. The temperature dependence of the DC can be described by a constant activation energy of 27 ± 2 kcal/mole in the coverage range 0.4 < θ < 0.9. The concentration dependence of the DC shows a maximum near θ = 0.45and a decrease beyond θ = 0.6. This behavior is correlated with observed LEED structures and interpreted by a rather abrupt change of the chemical potential near θ = 0.5 and a decreasing hopping probability for higher coverages.

Electric charges on stainless steel surfaces: The effects of hydrogen, charged particles, illumination, and electric fields on the work function

The work function of stainless steel measured in ultra-high vacuum by the vibrating capacitor method was independent of hydrogen dosage to a pressure of 10 −6 torr. It was reversibly changed by illumination, incident charged particles, and applied electrostatic fields. The results are qualitatively consistent with charge trapping phenomena in surface states on or within an oxide film. Various models are examined to attempt to account for these observations.

Measurement of localized surface potential differences

There is considerable current interest in the detection and measurement of surface potential difference and charge density on metal oxide layers and on dielectrics. The Kelvin vibrating capacitor method has been developed so as to give a high degree of localization in the measurement of surface potential.

A two frequency vibrating capacitor method for contact potential difference measurements

Two frequencies are simultaneously applied to a vibrating capacitor for making contact potential difference measurements. Since the potential difference across the capacitor is usually a nonlinear function of the electrode spacing the resultant signal consists of one frequency modulated in amplitude by the other. By detecting only this amplitude modulation it is possible to observe contact potential differences while one of the surfaces, forming the capacitor, is emitting electrons or ions. Also the reproducibility of contact potential difference measurements is increased by a factor of ten over the more usual single frequency method. The system uses integral feedback to provide a self-balancing facility. The temperature coefficient of the minimum work function for a caesiated surface was found to be − 1·2±0·2×10−3 eV degK−1 over the temperature range 300 to 850°K. The work functions of tungsten (100) and (110) surfaces were measured to be 4·645±0·005 eV and 5·141±0·007 eV respectively, assuming that of the polycrystalline reference surface to be 4·550 eV.