Contact Potential Difference Measurements Research Articles

Assemblies of “Hinged” Iron−Porphyrins as Potential Oxygen Sensors

Sequential self-assembly of a two-component system on a solid support is described with respect to structure and function. Two ligands, which bind to the semiconductor surface through one end and axially ligate a heme analogue at the other end, are described. Monolayer assemblies of complexes formed by these ligands and iron-porphyrin perform reversible binding of molecular oxygen. In the monolayer, a metalloporphyrin (the sensing unit) is held by the intervening ligand that serves as a “hinge”, away from the solid surface. Sensing events based on porphyrin chemistry are communicated via the ligand to the solid support. The transduction manifests itself as a change in the solid's surface electronic properties. Synthesis of the ligands and analysis of its complex formation with FeIII-porphyrin are described. The anisotropic orientation of the porphyrin ring within the ligand cavity, due to restricted rotation around the FeIII−N imidazole bonds, was probed by 1H NMR measurements in solution. We show that the porphyrin substituents stand as barriers for the free rotation even at room temperature. Molecular modeling supports the NMR evidence and reveals the stable conformations for the porphyrin's orientation relative to the solid support. The complexes were assembled as films on the (0001) surface of etched n-CdSe single crystals, and the films were characterized using transmission Fourier transform infrared (FTIR) and X-ray photoelectron (XPS) spectroscopies. Contact potential difference (CPD) and steady-state photoluminescence (PL) measurements of the derivatized CdSe show that the intervening ligands yield better conjugation and stronger binding of the sensing unit to the semiconductor surface, relative to direct adsorption of metalloporphyrins. Furthermore, the PL changes in the CdSe can be used to follow the interaction of the surface-bound FeIII-porphyrin−ligand complexes with molecular oxygen. A model is proposed to explain the electronic changes resulting from binding of O2 to the monolayer.

Self-polarization Mechanism in Textured Pyroelectric Pb(Ti1-xZrx)O3 films

AbstractSelf-polarization of Pb(Ti1−xZrx)O3 (PZT) thin films is explained on the basis of the formation of a TiO2−x-enriched interlayer close to the bottom electrode. Electrons provided by oxygen vacancies generate a n-type interface layer in the PZT at the electrode. A graded ferroelectric layer is then formed by electron injection from the bottom electrode and electron trapping on Ti4+ ions. A “built-in” difference in free energy forms which acts to pole the ferroelectric. Band bending was observed by contact potential difference measurements at a distance of 70 to 90 nm from the bottom electrode.

Direct measurement of minority carriers diffusion length using Kelvin probe force microscopy

We report on the use of Kelvin force microscopy as a method for measuring very short minority carrier diffusion length in semiconductors. The method is based on measuring the surface photovoltage between the tip of an atomic force microscope and the surface of an illuminated semiconductor junction. The photogenerated carriers diffuse to the junction, and change the contact potential difference between the tip and the sample as a function of the distance from the junction edge. The diffusion length L is then obtained by fitting the measured contact potential difference using the minority carrier continuity equation. The method is applied to measurements of electron diffusion lengths in GaP epilayers.

Kelvin Probe Force Microscopy on Surfaces: Investigation of the Surface Potential of Self-Assembled Monolayers on Gold

The local contact potential difference (CPD) between different linear alkanethiols self-assembled as monolayers on Au substrates was investigated with Kelvin probe force microscopy (KPFM). Our results demonstrate that KPFM simultaneously provides information on the sample topography and its contact potential down to a lateral resolution of 100 nm. The reported CPD measurements allow the distinction between regions in the monolayer comprising thiol molecules with chemically different terminal head groups down to a resolution of 3 meV. Furthermore, the CPD values measured in monolayers having one type of functionality vary with the chain length of the molecules in the film, which is consistent with a dipole-layer model. Variations in the length of the alkyl chain reveal a linear dependence as a function of the (−CH2−) units in the chain, with an increase in the CPD potential of 14.1 ± 3.1 mV per (−CH2−) unit.

Effect of shear horizontal leaky surface acoustic wave on selectivity for ethanol decomposition of a copper thin-film catalyst deposited on a positively polarized ferroelectric LiTaO 3 single crystal

The effect of dynamic surface lattice vibration on the selectivity of ethanol decomposition over a thin-film copper catalyst was studied by using a shear horizontal leaky surface acoustic wave (SH-LSAW) generated by radio frequency (RF) power on a positively polarized, 36° rotated, y-cut LiTaO 3 single crystal. The surface acoustic wave (SAW) caused a remarkably high enhancement of ethylene production but a small increase in acetaldehyde production, thus indicating the SAW's ability to dramatically change the selectivity. Laser Doppler measurements showed an irregular pattern of standing waves with different amplitudes up to 5 nm during SAW propagation. Contact potential difference measurements showed the generation of a negative voltage by the SAW propagation on the positively polarized ferroelectric surface. These changes are quite similar to those previously observed for a thickness–extensional mode resonance oscillation and are associated with the characteristic features of a leaky wave. Selective production of ethylene by the SAW is proposed to be related to the enhancement of the copper surface–oxygen atom interactions through dynamic lattice displacement.

Influence of electronic properties of Na2O/CaO catalysts on their catalytic characteristics for the oxidative coupling of methane

For Na2O/CaO catalysts of different sodium content the adsorption of oxygen and their electrical properties were studied by transient experiments and measurements of contact potential differences (CPD) as well as electrical conductivity. CPD results show a change of the mechanism of oxygen activation with increasing sodium concentration due to changing the type of ionic conductivity from cationic to anionic. Anion vacancies are formed by incorporation of sodium into the CaO lattice. As CPDs show, the cation conductivity promotes an accumulation of oxygen species on the catalyst surface resulting in a decrease of C2 product selectivity for the catalyzed oxidative coupling of methane. The anion conductivity favors a dissociation of molecular adsorbed oxygen and a subsequent incorporation into the oxide lattice, hereby, decreasing its concentration on the catalyst surface which favors in term selective formation of ethane and ethylene.

Study of Stress-Induced Leakage Current in Thin Oxides Stressed by Corona Charging in Air: Relationship to GOI Defects

ABSTRACTCorona charging in air combined with non-contact oxide charge measurement with a contact potential difference probe provides an unique possibility for fast monitoring of electron tunneling characteristics without preparation of MOS capacitors. It has also been found tha corona charging of thin oxides in the tunneling range is very effective in generating stressinduced leakage current. In this work we demonstrate the sensitivity of the corona stressinduced leakage current magnitude to gate oxide integrity defect density. The experimenta results cover three of the most common gate oxide integrity defects, namely: 1 – the defect induced by heavy metals (Fe.Cu) at a practically important low concentration range of 1×1010 to 1×1011 atoms/cm3: 2 – the defects originating from interface roughness and 3–the defects related to crystal originated particles.At low corona stress fluence, these defects play no role in the tunneling characteristics which follow ideal Fowler-Nordheim characteristics for oxides 50Å or thicker and a contribution from a direct tunneling current for thinner oxides. At high corona stress fluences, gate oxide integrity defects control the magnitude of stress-induced leakage current measured at constan oxide field. It is suggested that the gate oxide integrity role is associated with the enhanced rate of the trap generation during stress. It is noted that the present findings employ a novel methodology for gate oxide integrity monitoring based on corona charging and contact potential difference measurement.

Study on the surface nonstoichiometry of solid breeder blanket materials by means of work function measurement

The change of contact potential difference (CPD) between Li 4SiO 4 and Pt was measured in various gas compositions with a high temperature Kelvin probe. The change of the work function of Pt was estimated from the measurement of CPD between Pt and Pt, and that between Au and Pt. From the results, the effect of an oxygen deficient layer near the surface of Li 4SiO 4 was observed, and the processes of adsorption as H 2, and desorption as H 2O, were estimated to be dominant when hydrogen was introduced. Moreover, the effect of the oxygen deficient layer on the rate of tritium release as HTO was discussed.

Application of the contact potential difference technique for on-line rubbing surface monitoring (review)

The Kelvin technique for measuring contact potential differences has been adapted for continuous non-destructive monitoring of changes in the electron work function of a rubbing surface. The method can be used to investigate tribological materials for a wide range of conditions, including changes in load, sliding speed, and environment, with or without lubrication. It relies on the sensitivity of the work function to the various events which accompany friction, e.g., plastic deformation, creation of new surface material, adsorption, oxidation, phase changes and redistribution of alloy components. At present, this is the only method which is sensitive to both surface and near-surface defects and permits study of one of the two interacting surfaces during sliding. Current work emphasizes two aspects of sliding behavior: (1) critical points with respect to changes in normal load, with relevance to materials selection and optimization, and (2) the kinetics of friction processes, including periodic changes which may be related to those in fatigue.

Molecular electronic tuning of Si surfaces

Assembling quinolinium-based chromophores on silicon surfaces provides a new route to electronic control over such semiconducting surfaces. The two-step process by which the molecules are grafted on to the surface involves first coupling the organic functionality to silicon, followed by chromophore anchoring. These synthetic steps are monitored by XPS, UV-Vis and FTIR spectroscopies. Using contact potential difference measurements we found that the electron affinity of the modified silicon is a function of the molecule's dipole moment. The same technique shows a pronounced effect of the sub-nanometer siloxane-based, coupling-agent, layer by itself on the band bending and band-bending modification as function of chromophore adsorption.

Gas sensor characterization through both contact potential difference and photopotential measurements

Two different electrical measurement methods have been used to study p-Si/SiO 2/SnO 2/Pd structures which could be used in gas sensors. One of the methods is the contact potential difference measurement which characterizes the surface of the structure. A new method using a modulated light beam permits the measurement of the photopotential. These two methods are described. The relation between the surface potential and the photopotential is examined.

Controlling the Work Function of GaAs by Chemisorption of Benzoic Acid Derivatives

Control of the work function of GaAs single crystals, under ambient conditions, was achieved by chemisorption of a series of benzoic acid derivatives with varying dipole moments. Quantitative Fourier transform infrared spectroscopy shows that the benzoic acid derivatives bind as carboxylates, via coordination to oxidized Ga or As atoms, with a surface coverage of about one layer and a binding constant of 2.1 104 M-1 for benzoic acid. Contact potential difference measurements reveal that molecules affect the work function by changing the electron affinity while band bending is not affected significantly. The direction of the electron affinity changes depends on the direction of the dipole moments, and the extent of the change increases linearly with the dipole's magnitude. Investigation of the surface composition by X-ray photoelectron spectroscopy shows that the etched surface, onto which the molecules adsorb, is covered by an oxide layer. This may prevent the molecules from affecting band bending.

Kelvin Potential Measurement of Insulative Particles. 1. Mechanism of Metal Oxide Triboelectric Charging and Relative Humidity Sensitivity

A new method has been developed to measure the Kelvin contact potential difference (CPD) of insulating submicron particles as a function of relative humidity (RH). The insulative particles are dry blended onto the surface of conductive particles at a high relative humidity and the resultant mixture is conductive, thus, enabling a CPD measurement that is reflective of the insulative coating. A second modification to the standard CPD methodology is the use of water as an RH-independent reference. Using these two new modifications to the Kelvin measurement, the CPD of common xerographic toner additives such as silica, titania, and alumina particles have been determined as a function of RH. The CPD values of these insulative metal oxide particles are compared to the triboelectric values obtained for the same particles used as surface additives on a xerographic toner. A good linear correlation exists between the charge of the additive coated toner and the CPD measured as a function of RH. Both the charge level and the RH sensitivity of the charge with the metal oxides can be explained by changes in the metal oxide contact potential. Thus, the mechanism of charging in these metal oxides is dominated by the transfer of electrons between states that can be described in terms of work functions. Also concluded is that ion transport has a minor contribution in the mechanism of triboelectric charging with these metal oxides.

Influence of temperature on surface potential barrier of Cd0.99Mn0.01Te: Ga

Contact potential difference (cpd) measurements, between the illuminated surface of Cd 0.99 Mn 0.01 Te: Ga and vibrating metal reference electrode, have been carried out in the temperature range between 80 and 250 K at a pressure of 10 −4 Pa. In the temperature range (170–250) K linear changes of cpd were observed, while at a lower temperature an interesting effect of temperature hysteresis of cpd occurs. The observed effects have been correlated with the adsorption and desorption processes of oxygen chemisorbed on the surface sample.

Optimization of sub 3 nm gate dielectrics grown by rapid thermal oxidation in a nitric oxide ambient

The vertical scaling of oxide thickness in the ultra large scale integrated era places stringent requirements on oxide quality. In this letter we report optimization studies in the growth of ultrathin oxynitrides in the sub 3 nm range. The oxynitride growth technique used involved self-limiting growth in nitric oxide (NO) followed by reoxidation in oxygen or nitrous oxide (N2O) ambient. This method allows tight control of oxide thickness and resulted in consistently low leakage currents over a range of thicknesses from 2 to 3 nm. The reliability of the oxynitrides is characterized using QBD, stress-induced leakage and surface charge and contact potential difference measurements. Charge-to-breakdown (QBD) data indicate that the reliability of the oxide degrades with increasing nitridation times in an NO ambient. Increasing reoxidation times in O2 have a similar effect. It is found that an improvement in reliability can be obtained by reoxidation in an N2O ambient. Surprisingly, reoxidizing in N2O proceeds at a higher rate than in O2 and this enables the use of lower thermal budgets.

粉体の接触電位差の測定ならびに電子写真プロセス評価への応用

粉体の接触電位差の測定ならびに電子写真プロセス評価への応用

Effect of air annealing on the electronic properties of [formula omitted] solar cells

The effect of air annealing on state-of-the-art, solar-cell-quality CdS Cu(In,Ga)Se 2 heterojunctions has been studied using contact potential difference and surface photovoltage measurements. The annealing treatment is shown to have no significant effect on the band lineup of the heterojunction. However, the surface photovoltage spectral response increases markedly upon air annealing. These results can be reconciled if air annealing of the junctions leads mainly to elimination of recombination centers, rather than to changes in the built-in voltage or in the band lineup. We also show that ZnO deposition has an effect on the surface photovoltage that is similar to that of air annealing.

Electrical properties of diamond surfaces

An attempt is made to explain the presence of a low-resistivity layer on the surface of as-grown undoped CVD films and the unusual nature of the surface sensitivity to various surface treatments in terms of band bending. The experimental results of contact potential difference measurement by Kelvin probe and C 1s core level emission spectrum by X-ray photoelectron spectroscopy analysis are correlated reasonably with the band-bending scheme in combination with the Fermi-level position in the bulk and with the pinned surface Fermi level ( E v + 1.7 eV) in oxygenated samples. The energy band of the as-grown surfaces bends upwards to form an accumulation layer for holes owing to the existence of acceptor-type surface states well below the bulk Fermi level. On the other hand, on oxygen-terminated surfaces produced by oxygen-ambient annealing or oxygen-plasma treatment, there is a depletion layer for holes because of donor-type surface states existing about 1.7 eV above the valence band. The origin of these surface states is not known at present.

Improved Kelvin method for measuring contact potential differences between stepped gold surfaces in ultrahigh vacuum

A new mechanical system and a convenient piezoelectric driven Kelvin probe for the measurement of the contact potential difference (CPD) under ultrahigh vacuum are described. Charging effects on CPD measures are discussed. The probe is small (2.2 mm diam) and allows cartography of the surface. The mechanical system is frictionless so the distance between the sample and the probe can be maintained constant and at a low value (≊20 μm). By elimination of charging effects, reliable and reproducible results can be obtained within ±10 mV. The system is tested on gold vicinal surfaces.

Band diagram of the polycrystalline CdS/Cu(In,Ga)Se2 heterojunction

Contact potential difference measurements in the dark and under illumination are used to derive the conduction band offset (ΔEc) in a solar cell quality junction formed by chemical bath deposition of CdS on a polycrystalline thin film of Cu(In,Ga)Se2. Our experimental measurements and the estimates made for dipole contributions show that the junction is of type II, i.e., without a spike in the conduction band ( ΔEc=80 meV±100 meV). This is consistent with the high performance of the actual solar cell. However, it differs from most previous results on junctions based on single crystals and/or vacuum deposited CdS, which indicated the existence of a conduction band spike.