Contact Potential Changes Research Articles

Wavelength dependence of nanoscale photodegradation in poly(3-octylthiophene) thin films

The use of conducting polymers in optoelectronic devices is subject to the understanding of the electro-optical processes that take place at the nanoscale. One of the photo-induced processes that limit their application is the photodegradation, which reduces the device working life. In this work the photodegradation of poly(3-octylthiophene) thin films was studied by combining Kelvin probe microscopy and optical microscopy. In this way, a direct correlation between morphological and contact potential changes with optical density changes as a function of the irradiation wavelength and intensity can be made. These results, complemented with Raman spectra help to clarify the degradation processes that are taking place. We find that the photodegradation strongly depends on the irradiation wavelength blue light being much more aggressive than UV. In addition, the optical properties change abruptly before any substantial change in the morphology is observed.

A hybrid scanning tunneling–atomic force microscope operable in air

We describe a hybrid scanning tunneling–atomic force microscope (STM–AFM) capable of measuring current and force simultaneously under ambient conditions. In order to reduce meniscus forces, the microscope uses a sharp STM tip as a probe and an AFM cantilever as a sample substrate. This improvement allows use of correspondingly flexible cantilevers enhancing force detection sensitivity. Using the STM–AFM, we have been able to explore a number of phenomena that can occur in nanometer scale tunnel junctions in air, including a correlation between hysteretic changes in contact potential and rapid increases in current at large bias voltages.

Hysteretic contact potential changes in nanometallic tunnel junctions in air

We have used a hybrid scanning tunneling \char21{} atomic force microscope to measure current and forces generated between nanotunnel junction (NTJ) electrodes. Cyclic bias voltage scans performed in air reveal hysteretic increases in force above a \ensuremath{\sim}1 V threshold. Current measurements exhibit simultaneous increases in magnitude and noise. Our results are consistent with hysteretic changes in contact potentials and indicate that, in addition to tunneling and electrochemistry, molecules can mediate widespread occurrences of ion trapping and sudden detrapping in NTJ's.

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.

Induction of Superconductivity by the Inner Contact Potential

AbstractThe inner contact potential changes the density of electron states and increases the value of the electron–electron interaction. A theory of the inhomogeneous superconducting state in these systems is presented. The critical temperature and magnetic field are calculated.

Precise diode method for recording contact potential changes caused by gas adsorption

Precise diode method for recording contact potential changes caused by gas adsorption

Precise diode method for recording contact potential changes caused by gas adsorption

A low-temperature recording diode is described which allows the determination of adsorption-induced contact potential differences with an accuracy of better than 0.5 mV. Some experimental work function results of the hydrogen adsorption on platinum (111) and (997) single-crystal faces are presented which demonstrate the usefulness of the method.

A cryostat for measuring contact potential differences

A cryostat is described that is designed to measure changes in the contact potential difference between a normal metal and a superconductor as the superconductor passes through the transition temperature. The Kelvin-Zisman technique is used and the vibrating plate is driven with a commercially produced vibrator. Changes in contact potential of 0.2 mV can be detected.

Adsorption of oxygen and oxidation of CO on the ruthenium (001) surface

The adsorption of oxygen on the ruthenium (001) surface has been studied using a combination of techniques: LEED/Auger, Kelvin probe contact potential changes, and flash desorption mass spectrometry. Oxygen is rapidly adsorbed at 300 K, forming an ordered LEED structure having apparent (2 × 2) symmetry. Two binding states of oxygen are inferred from the abrupt change in surface work function as a function of oxygen coverage. LEED intensity measurements indicate that the oxygen layer undergoes an order-disorder transition at temperatures several hundred degrees below the onset of desorption. The order-disorder transition temperature is a function of the oxygen coverage, consistent with two binding states. A model involving the adsorption of atomic oxygen at θ < 0.5 and the formation of complexes with higher oxygen content at θ > 0.5 is proposed. The oxidation of CO to form CO 2 was found to have the maximum rate of production at a ruthenium temperature of 950 K.

Sodium adsorption on aluminum (100) and (111) surfaces: ELEED, Auger, and contact potential measurements

The adsorption of Na on sputter-cleaned Al(100) and Al(111) single-crystal surfaces has been investigated under ultrahigh vacuum conditions. Sodium was deposited with an apertured ion source permitting quantitative dosage determination. Low energy electron diffraction patterns show well-ordered coherent structures corresponding to 1 2 monolayer on both surfaces, and 1 3 monolayer on Al(111). A hexagonal pattern corresponding to 7 8 monolayer occurs at a nonconforming dosage on Al(100), indicating a reduction in effective sticking coefficient. This result is verified by Auger intensity measurements, which show saturation beginning at 1 2 monolayer on both surfaces. Changes in contact potential with exposure were inferred from shifts in the low-energy cutoff of secondary electrons. The initial dipole moment is in good agreement with that on transition metal substrates, while the saturation value of the contact potential corresponds closely to the difference between reported work functions for bulk Al and bulk Na. A true minimum in the work function was not observed. Present results are compared with results obtained from transition metal substrates. Essential differences are attributed to the more metallic character of the bonding in the Al:Na system.

Adsorption of CO on (001) Ruthenium at Temperatures ≥ 300 K

The adsorption of CO on Ru(001) has been studied using a combination of techniques: LEED/Auger, Kelvin probe contact potential changes, and flash desorption mass spectrometry. Adsorption of CO is reversible at temperatures and pressures as high as 700 K and 10-4 Torr, respectively. Two binding states of CO are identified, and isosteric heats determined both from work function changes and from LEED intensity measurements agree well with flash desorption energies. A disordering of the CO layer due to repulsive interactions between neighbors at high coverages is postulated. Electron beam-induced LEED pattern changes are characterized and found to have a high cross section (∼7×10-17 cm2) at ∼110 eV.

Stress-Dependent Contact Potential in Copper

We have measured the compressionally induced change in contact potential for the (100) face of a single crystal of copper. For a change in stress of about 34 atm we observed a negative contact-potential change which is interpreted as a decrease in work function. This change is (-2.0 \ifmmode\pm\else\textpm\fi{} 1.2) \ensuremath{\mu}V/atm, which gives an equivalent gravitationally induced field of (-1.8 \ifmmode\pm\else\textpm\fi{} 1.0) \ensuremath{\mu}V/m. The crystal face was cleaned by ${\mathrm{Ar}}^{+}$ bombardment and measurements made at 2\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}8}$ Torr. Some experiments on polycrystalline copper in different environments are also reported. A simple calculation based on the Gordy equation as interpreted by Steiner and Gyftopoulos gives, for a stress applied along the [001] cube axis, an increase of the work function with stress of 4.3 \ensuremath{\mu}V/atm for the (100) face but a decrease of 3.8 \ensuremath{\mu}V/atm for the (110) face.

Electron Beam Technique for Measuring Microvolt Changes in Contact Potential

A low voltage electron beam technique is reported which when coupled with ac detection is capable of measuring periodic changes in contact potential as small as 1.0μV. This sensitivity, which exceeds by a factor greater than 100 the reported sensitivities obtained by other methods, should enhance the usefulness of the contact potential measurement for detailed surface studies, especially in the case of semiconductors. The potential of the technique is demonstrated in the context of high resolution photovoltage measurements in n-type CdS which may allow for a sensitive form of surface state or surface barrier spectroscopy.

Contact potential changes produced by very small stresses

The stress dependence of contact potential has been measured in copper and aluminium at very low stress levels using a sensitive ac method. The results of two experiments are presented and in both cases the effect is found to be much smaller than the free electron estimates calculated from the theory of Dessler et al.

Contact-Potential Changes Produced on Metal Surfaces by Tensile Stresses

Contact-Potential Changes Produced on Metal Surfaces by Tensile Stresses

Chemisorbed β Phase of Nitrogen on Tungsten

The pertinent experimental information now available on the chemisorption of nitrogen in the β phase on tungsten is reviewed. The concept of two distinct, unrelated types of absorption site being responsible for the β1- and β2- desorption peaks is considered to be inadequate to explain many recently published experimental observations. A new model for the chemisorbed state is proposed based on the existence of a single type of site for β-phase adsorption and with emphasis on the role of surface diffusion on different types of planes. The model is then applied to the interpretation of a range of published observations on nitrogen adsorption on tungsten including desorption effects, isotopic mixing, contact potential changes, and field electron emission microscopy observations.

Boundary potentials and adsorption at the oil-metal interface

The adsorption of a number of different molecules at the interfaces between oxidized metals and aliphatic hydrocarbons has been examined by the measurement of changes in contact potential. The plots of the potentials against solute concentration closely resemble adSorption isotherms, and it has been shown that by assuming that the interaction between the adsorbed molecules can be neglected a self-consistent quantitative analysis of the results can be achieved. Several features of the results, including the magnitudes of the free energies of adsorption suggest that a layer of water approximately one molecule thick was present between the oxide surface and the hydrocarbon medium.

Contact potential changes on germanium surface channels

AbstractContact potential differences (c. p. d.) on surfaces are measured by the Kelvin method using a “point vibrating electrode”. Measurement of the c. p. d. distribution is shown to be a sensitive method of examining electrical surface inhomogeneity and also to facilitate the measurement of voltage distributions along thin samples.

Surface Electrical Changes Caused by the Adsorption of Hydrogen and Oxygen on Silicon

Measurements of conductance, lifetime, change in contact potential with light, and contact potential have been carried out on bombardment-cleaned silicon surfaces and during the adsorption of molecular oxygen and atomic hydrogen. In the case of oxygen adsorption, the work function increased linearly with coverage. A change of 0.35 ev was obtained in going from θ=0 to θ=1. Very small changes in the transport properties were observed. Hydrogen atoms produced an initial decrease in work function of 0.1 ev for coverages below θ=0.35. From θ=0.35 to θ=1.0 the work function was increased by 0.3 ev. The changes in the transport properties were substantial and indicated a downward movement of the energy bands at the surface by about 0.08 ev. In the clean condition, the valence band edge was 0.12–0.14 ev below the Fermi level at the surface compared to 0.36 ev in the interior. The effect of hydrogen adsorption is discussed in terms of the adsorption data previously obtained on this system.

Surface properties of vacuum cleaned silicon

Measurement of conductivity, photoconductivity and change in contact potential with light have been made on a sample of floating zone single crystal silicon cleaned by positive ion bombardment and annealed. The absence of a conducting p-film after such treatment has been demonstrated. The annealing after bombardment has been measured as a function of time at various temperatures. Changes in the bulk carrier concentration have been studied after various heating treatments. Changes in conductivity produced by argon ions indicate that in the clean condition the Fermi level is about 0.1–0.2 eV above the valence band edge. Adsorption of atomic hydrogen decreases the conductivity of both heated and bombarded and annealed samples. At the same time Δ CP becomes more negative indicating a downward movement of the energy bands at the surface.