Changes In Contact Potential Difference Research Articles

Work function analysis of photo-enhanced triethylamine adsorption impact on Au embedded CeO2 coated ZnO hybrid nanostructures: An investigation by scanning kelvin probe

This article utilizes a scanning kelvin probe to examine the impact of visible light and volatile organic compounds (VOCs) on an organic/inorganic/noble metal hybrid sensing layer. ZnO/Poly/CeO2/Poly/Au hybrid nanostructures were prepared on ITO substrate by layer-by-layer coating technique. In this case, PAH and DS polymers act as binders. The assembly of Au nanorods (NRs) on the polymer-coated ZnO NRs incorporated with CeO2 nanoparticles (NPs) and its maximum absorbance in the visible light region were studied by the structural and optical characterizations, respectively. The hybrid sensor's optical properties allowed for sizable changes in contact potential difference (CPD) when exposed to light. Although a non-selective interaction was observed, the increased responsiveness to Triethylamine (TEA) under visible light creates coordination bonds between the hybrid molecules. Notably, the surface charge was tuned in the surface of the sensing layer by varying the surfactant to explore the TEA responsiveness. Intriguingly, ZnO/Poly/CeO2 (+) has demonstrated improved adsorption capability towards TEA under irradiation due to the surface oxygen vacancies compared to ZnO/Poly/CeO2 (-). Also, ZnO/Poly/CeO2 (+) exhibited higher selectivity at adsorbing TEA than either ethanol or n-hexane.

Time-dependent measurement of plasmon-induced charge separation on a gold nanoparticle/TiO2 interface by electrostatic force microscopy

Plasmon-induced charge separation (PICS) is an efficient way to use the hot carriers generated by localized surface plasmon resonance. Although the ultrafast dynamics of hot carrier generation and annihilation itself are well understood, the slow dynamics of PICS are not, despite their importance for the use of hot carriers in chemical reactions. In this work, we directly observed the slow dynamics of PICS on an Au nanoparticle (NP)/TiO2 interface by using electrostatic force microscopy with time-resolved measurements obtained by sideband signal of frequency shift. The change in contact potential difference induced by PICS had a bias voltage dependence, indicating that the number of holes in the Au NPs ([{mathrm{h}}_{mathrm{AuNP}}^{+}]) accumulated by laser irradiation depended on bias voltage. The decay constant for the annihilation of the separated charge on the Au NPs at the Au NP/TiO2 interface was directly determined to be ca. 150 ms, and the annihilation process was discussed in a simple model based on the transient Schottky barrier.

(Invited) Surface Photovoltage Spectroscopy on BiVO4, Gallium Phosphide, and CuGa3Se5 Photoelectrodes in Contact with Aqueous Electrolytes

The quasi-Fermi level splitting energy (photovoltage) is important for the solar to fuel conversion efficiency of photocatalysts and photoelectrodes. While electrochemical measurements can provide the electrochemical potential of the majority carriers at the semiconductor back-contact, the photovoltage, and the potential of the minority carriers at the front semiconductor/electrolyte are more difficult to determine experimentally, due to the lack of a direct electrical connection. Here we show that surface photovoltage spectroscopy (SPS) on photoelectrode films in contact with aqueous electrolytes can provide photovoltage information. For the measurements the semiconductor films (BiVO4, CuGa3Se5) or wafers (n-GaP or p-GaP) are immersed in aqueous electrolytes and placed underneath a vibrating Kelvin probe to monitor the light induced contact potential difference change. We find that the photovoltage (quasi-Fermi level splitting) of all photoelectrodes is increased by the aqueous electrolyte, and strongly depends on the electrochemical potential of the redox couple (TEOA, Na2SO3, KI, K4[Fe(CN)6] in the electrolyte, the back contact, and the light intensity. Electrochemical potentials of the minority carriers at the solid-liquid interface can be estimated by combining the photovoltage data with open circuit potential measurements that yield the quasi-Fermi levels of the majority carriers. This study provides new insight on the factors that affect photochemical charge separation at solid-liquid interfaces. Figure 1

Exciton Transfer at Heterointerfaces of MoS2 Monolayers and Fluorescent Molecular Aggregates.

Integration of distinct materials to form heterostructures enables the proposal of new functional devices based on emergent physical phenomena beyond the properties of the constituent materials. The optical responses and electrical transport characteristics of heterostructures depend on the charge and exciton transfer (CT and ET) at the interfaces, determined by the interfacial energy level alignment. In this work, heterostructures consisting of aggregates of fluorescent molecules (DY1) and 2D semiconductor MoS2 monolayers are fabricated. Photoluminescence spectra of DY1/MoS2 show quenching of the DY1 emission and enhancement of the MoS2 emission, indicating a strong electronic interaction between these two materials. Nanoscopic mappings of the light‐induced contact potential difference changes rule out the CT process at the interface. Using femtosecond transient absorption spectroscopy, the rapid interfacial ET process from DY1 aggregates to MoS2 and a fourfold extension of the exciton lifetime in MoS2 are elucidated. These results suggest that the integration of 2D inorganic semiconductors with fluorescent molecules can provide versatile approaches to engineer the physical characteristics of materials for both fundamental studies and novel optoelectronic device applications.

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.

Elucidation of sensing mechanism through VOCs induced surface potential changes on graphene oxide/tin oxide nanocomposites

We report the use of a scanning Kelvin probe in measuring the gas adsorption induced change in contact potential difference (CPD) for the graphene-based binary nanocomposites towards various volatile organic compounds (VOCs) at room temperature. The binary nanocomposites (GO/SnO2) were prepared by one-step solvothermal (GS-I) and two-step mechanical mixing (GS-II) methods. The significant change in CPD upon visible light illumination for both the samples in the air medium enabled a room temperature interaction with VOCs. GS-I exhibited superior structural, optical and electrical properties in terms of lower crystallite size, defects, a higher percentage of surface atoms and visible light absorption. This appears to be consistent with the change in surface potential upon VOC interaction with a downward band bending for GS-I and upward bending for GS-II. These findings provided substantial details for understanding the interfacial modifications between graphene-based binary nanocomposites and assist in characterizing the mechanism of gas interaction.

Post-deposition annealing influences of gas adsorption on semi-vertical β-FeOOH nanorods at room temperature: A scanning kelvin probe analysis

An investigation on gas adsorption of β-FeOOH at room temperature was performed through kelvin probe analysis. The microscopic images revealed the semi vertically grown nanorods on conductive fluorine doped tin oxide (FTO) substrate prepared by hydrothermal method. The annealing temperature strongly influenced the crystallite size, lattice stress, grain size and boundaries estimated through X-ray diffraction analysis. The crystallite size increased from 57 nm to 67 nm with a proportional increase in annealing temperature from 350 °C to 450 °C. The UV absorption spectra indicated a red shift towards visible wavelength region with increasing annealing temperature elucidating a visible-light enhanced response for the activation of gas adsorption. Correspondingly, the energy band gap decreased with increasing annealing temperature. The adsorption behaviour of different types of volatile organic compounds (VOCs) were studied using scanning kelvin probe (SKP) to measure the contact potential difference (CPD) on the surface of the samples. The CPD changes for ethanol were higher (−264 mV) at lower temperatures (350 °C) and vice versa. However, an intriguing behaviour was found for other types of gases. This peculiar behaviour with respect to the annealing temperature was understood by SKP and attributed to the presence of numerous grain boundaries interfaces which were absent when annealed at low temperature due to higher crystallite size. These adsorption studies provides notion on the fundamental mechanism of gas interaction for the fabrication of selective gas sensors.

VOCs adsorption induced surface potential changes on phthalocyanines: A combined experimental and theoretical approach towards food freshness monitoring

Many organic molecules such as porphyrins, pthalocyanines, pyrene ligands and so on are being used to detect volatile organic compounds (VOCs). The richness in π-electrons, coordination metals and their structural integrity make them as potential candidates for chemical sensing applications. In this work, metal free (H2Pc) and iron (II) phthalocyanines (FePc) are used for detecting VOCs such as 1-hexanol, nonanal, triethylamine and so on. The structural, morphological and optical properties of as deposited films of H2Pc and FePc on fluorine doped tin oxide substrates are characterized by XRD, HR-SEM, UV–Vis adsorption and Raman spectroscopy studies. Their gas adsorption properties are investigated through scanning Kelvin probe by means of change in contact potential difference in dark as well as light illumination conditions. Among both pthalocyanines, FePc shows highly selective response towards 1-hexanol under visible light exposure. Interestingly, the one to one molecular computations have revealed that FePc show higher interaction towards VOCs when compared to H2Pc. Overall, being a marker for food freshness monitoring, 1-hexanol can be detected using FePc based photo enhanced room temperature gas sensors.

Contact Potential Difference as a Non-Destructive Testing of Steel

For steels of different grades, the effect of the chemical composition, structure, and surface etching on the contact potential difference is studied using the Kelvin probe method. It was shown experimentally that, with a change in the structure and chemical composition, the contact potential difference changes. Etching the surface of the steel with a 4% solution of nitric acid leads to a sharp decrease in the magnitude of the contact potential difference, which allows us to conclude that the value of the electron work function from the sample surface increases. The ability to control the composition and structure of the material by the Kelvin probe method is shown.

Probing the outermost layer of thin gold films by XPS and density functional theory

This work examines gold thin films of various thicknesses varying from 5 to over 100 nm along a 7 cm distance to investigate the effect on the surface sensitivity of the XPS Au4f signal. Increasing the relative intensity of the surface XPS signal can be useful for studying properties of the surface atoms in case of functionalized gold, and for shedding light on reactions occurring at the interface with the metal surface as well as adsorption geometry.The surface-to-bulk signal ratio increased with decreasing thickness for thicknesses greater than 15 nm. Below this value, the intensity of the surface signal decreased. The binding-energy shift of the surface component with respect to the bulk component was found to be in agreement with that calculated by density functional theory (DFT). The calculated shift was found to be essentially unaffected, whether or not final-state effects were included. KPFM was also performed to investigate correlations between the measured work function and the surface/bulk BE shift, revealing that the contact potential difference changes with thickness because of the influence of the substrate.Gradients were prepared by a novel evaporation method, by template stripping, and by photolithography, yielding stripes of increasing thickness in the same sample.

Indium content dependent VOCs interactions in monolithic InGaN/GaN multi quantum well structures grown by MOCVD

In this study, indium concentration dependence on Volatile Organic Compounds (VOCs) interaction were examined in InGaN-GaN multi quantum well structures (MQWs) grown on sapphire substrate at different quantum well (InGaN) growth temperatures. The periodicity, structural properties and indium dependent dislocation density was deduced from high resolution X-ray diffraction pattern. The variation of energy bandgap of InGaN quantum well clearly depicts that the bandgap decreases with increase in indium concentration in InGaN well layer. The interaction properties of the device with various VOCs such as ethanol, benzene, n-hexane and triethylamine were examined using Scanning Kelvin Probe (SKP) system and they are correlated with dislocation density. The change in contact potential difference (CPD) in dark and visible light illumination, as well as the variation of CPD with respect to various VOCs is observed. The difference in contact potential difference clearly suggests that the InGaN-GaN multi quantum well structure can be one of the suitable candidates for novel gas sensor applications.

Comparison of scanning Kelvin probe with SEM/EPMA techniques for fingermark recovery from metallic surfaces

Most traditional techniques to recover latent fingermarks from metallic surfaces do not consider the metal surface properties and instead focus on the fingermark chemistry. The scanning Kelvin probe (SKP) technique is a non-contact, non-destructive method, used under ambient conditions, which can be utilised to recover latent prints from metallic surfaces and does not require any enhancement techniques or prevent subsequent forensic analysis. Where a fingermark ridge contacted the metal, the contact potential difference (CPD) contrast between the background surface and the fingermark contact area was 10–50mV.Measurements were performed on the untreated brass, nickel-coated brass and copper metal surfaces and compared to traditional forensic enhancement techniques such as Vacuum Metal Deposition (VMD) using Au–Zn and Au–Ag. Using VMD, the CPD change ranged from 0 to 150mV between the dissimilar metal surfaces affected by the fingermark. In general, SKP worked best without additional enhancement techniques.Scanning Electron Microscope (SEM) scans were used to identify the fingermark contact areas through a sodium, chlorine and oxygen electron probe micro-analyzer (EPMA). The fingermark was observed in the backscattered electron image as the carbon deposits scattered the electrons less than the surrounding metal surface. The fingermark is shown clearly in a Cathodoluminescence scan on the copper sample as it blocks the photon emission at band gap (2.17eV) from the underlying copper oxide (Cu2O) surface. For the first time, SEM, EPMA and Cathodoluminescence techniques were compared to SKP data.Visible and latent fingermarks were tested with latent, eccrinous fingermarks more easily imaged by SKP. Results obtained were very encouraging and suggest that the scanning Kelvin probe technique, which does not need vacuum, could have a place as a first stage analysis tool in serious crime investigation.

Effect of dopant density on contact potential difference across n-type GaAs homojunctions using Kelvin Probe Force Microscopy

In this study, cross-sectional surface potential imaging of n+/semi-insulating GaAs junctions is investigated by using amplitude mode kelvin probe force microscopy. The measurements have shown two different potential profiles, related to the difference in surface potential between the semi-insulating (SI) substrate and the epilayers. It is shown that the contact potential difference (CPD) between the tip and the sample is higher on the semi-insulating substrate side than on the n-type epilayer side. This change in CPD across the interface has been explained by means of energy band diagrams indicating the relative Fermi level positions. In addition, it has also been found that the CPD values across the interface are much smaller than the calculated values (on average about 25% of the theoretical values) and increase with the electron density. Therefore, the results presented in study are only in qualitative agreement with the theory.

Challenges in hydrogen quantification using Kelvin probe technique at different levels of relative humidity

Kelvin probe-based contact potential measurements were conducted with in situ electrochemical hydrogen loading of a Pd membrane with a thickness of 100 μm. The theoretical basis of diffusion coefficient calculation based on measured response time of potential drop, arising from hydrogen arriving at the detection side of the Pd membrane, was discussed. In situ hydrogen loading was also utilized for insertion of different amounts of hydrogen into the Pd sample while monitoring the resulting contact potential difference changes. Measurement of contact potential difference at different final values of hydrogen concentration in the Pd membrane was performed at various atmospheric conditions, focusing in this work on increasing relative humidity (4 to 85% rH). Moreover, the effect of humidity changes on hydrogen effusion kinetics at room temperature and low oxygen content (<1%) was studied.Graphical abstractᅟ

Work Function Based CO2 Gas Sensing Using Metal Oxide Nanoparticles at Room Temperature

The reversible interaction of CO2 gas with sensing material is very difficult to achieve at room temperature. In this work we present copper oxide nanoparticles (CuO-NPs) as CO2 gas sensitive material. The CO2 interaction with CuO-NPs is investigated with the help of capacitance based Kelvin probe measurements, reflecting the adsorption induced changes in contact potential difference or work function alterations. Moreover, cross-sensitivity to humidity and the dependency of CO2 gas sensing on nanoparticles layer thickness has also been investigated. It is shown that CuO-NPs sensing layer depicts high sensitivity to CO2 gas exposure (400–4000 ppm) at room temperature and can be effectively used for the development of low cost CO2 gas sensors.

Investigation of stability of electrophysical parameters of Si-wafers before forming porous layer

The article studies the influence of Si-wafers processing in acid-peroxide and ammonia-peroxide solutions on the temporary stability of the electronic states of the crystal surfaces before formation of the nanostructured and porous silicon layers. A characteristic parameter of stability of the electrical properties of a semiconductor was selected the change in contact potential difference over time, which was measured by Kelvin. After alkaline treatments during the storage of Si-wafers in air, their contact potential difference decreases with time. The change in the contact potential difference for this type of treatments is small and most of them (up to120 hours) does not reach steady-state values in the studied interval of time. This fact indicates that Si surface is unstable after the treatment in ammonia-peroxide solutions and is characterized by long relaxation changes of the contact potential difference. In case of acid treatments or their combinations with alkaline ones, the changes of the contact potential difference over time is characterized by a sharp decrease during 20 ÷ 40 hours and subsequent attainment of steady-state values of the contact potential difference. This indicates more efficient processing of Si-wafers in acid-peroxide solutions than in ammonium peroxide ones. It was shown experimentally that the time necessary to reach the steady-state values ​​of the contact potential difference and the time of electron activity after Si-wafers surface treatment in acid-peroxide solutions is less than in ammonia-peroxide solutions. The results are treated by the formation on Si surface of oxide and hydroxyl layers that leads to the formation of macroporous, and microporous or mesoporous silicon layers

Electronic states and photoexcitation processes of titanium dioxide nanoparticle films dip coated from aqueous Degussa P25 photocatalyst suspension

The electronic properties of titanium dioxide (TiO2) nanocrystalline films, which were prepared by dip coating from Degussa P25 photocatalyst aqueous suspension, have been investigated by surface photovoltage spectroscopy (SPS). As indicated by the positive contact potential difference (CPD) change in the sub-band-gap region, SPS shows that the molecularly adsorbed H2O in the freshly prepared P25 film creates an empty electron state, which is distributed within 0.79eV below the conduction band edge, and acts as an electron trap and carrier recombination center. With film aging or under a drying atmosphere, the H2O-associated state diminishes, and the occupied electron state due to molecularly adsorbed oxygen, lying within 1.06eV above the valence band edge, is identified by the reversed polarity of the CPD change in the sub-band-gap region. This information is important in developing a better understanding of real photocatalyst behavior.

Studies on the Electrochromic Behavior of Lithium and Proton Based Solid State Devices

In the present investigation, two types of DC magnetron sputtered tungsten oxide (WO3) solid state electrochromic devices: one with lithium and the other using protons as intercalating ions have been prepared and analyzed. The lithium based device consisting of lithium aluminum fluoride (electrolyte ~ 500nm) and vanadium oxide (storage layer ~100 nm) has color/bleach cycle of about 10s and the transmittance variation of 40%. The proton device consisting of: Ta2O5 (electrolyte ~300nm) and NiOx (storage layer ~100nm) has a coloration time of few milliseconds and the transmittance variation is about 60%; the top electrode for these devices is semitransparent gold of thickness 10nm. The stability of the proton device has been found to be quite satisfactory for thermal cycling and X-ray dosage of 11.7R. The change in contact potential difference of WO3 during coloration has been measured using Kelvin probe technique for the first time to understand the electrochromic behavior.

Charge transfer at oxygen/zirconia interface at elevated temperatures: Part 6 Work function measurements

The present paper considers the application of the high temperature Kelvin probe for measurements of changes in work function (WF) of oxide materials during surface reactions at elevated temperatures and in controlled gas phase environments. The method is based on the determination of work function changes from measurements of the contact potential difference (CPD) between the Pt reference electrode and the studied specimen of a metal oxide. Experimental data on work function changes are reported for yttria-stabilised cubic zirconia (10 mol-%Y2O3) during oxidation and reduction experiments at 1173 K. It was found that changes in CPD exhibit good stability with time (2 mV drift over 15 h). The reproducibility of CPD changes during successive oxidation and reduction experiments remains within 0·5 mV. Good agreement was revealed between the obtained experimental data and the theoretical model describing the effect of oxygen partial pressure on the WF of both zirconia and the Pt electrode.

Charge transfer at oxygen/zirconia interface at elevated temperatures: Part 5: Reactivity models

Measurements of work function using the Kelvin method are based on determination of the contact potential difference (CPD) between the studied specimen and a reference electrode, typically Pt. The present work considers the physical meaning of the CPD, which is formed between the surfaces of a metal oxide and Pt in the presence of oxygen at elevated temperatures. The focus is on the reactivity of both metal oxide and Pt with oxygen and the associated electrical effects. The reactivity at the oxygen/zirconia interface is considered in terms of oxygen chemisorption, resulting in the formation of singly ionised molecular and atomic species, and oxygen incorporation into the zirconia lattice. The reactivity at the O2/Pt interface is considered in terms of oxygen incorporation into PtO2, which forms a thin layer covering the Pt. The derived reactivity models are used for quantitative assessment of CPD changes in terms of work function changes of yttria-stabilised zirconia (YSZ) during oxidation and reduction.