Non-equilibrium Surface State Research Articles

Particle-Size-Dependent Triboelectric Charging in Single-Component Granular Materials: Role of Humidity

We carry out experiments to investigate the triboelectric charging due only to particle–particle collisions as a function of humidity. At low humidity, we find that large particles tend to charge positive and small particles tend to charge negative, in agreement with previous studies. However, at high humidity, we find no significant particle-size dependence for the particle charging. To explain these results, we apply a theoretical model based on charge carriers trapped in nonequilibrium surface states characteristic of electrically insulating materials. Monte Carlo simulations show that collisions between particles enable the charge carriers to reach lower energy states on other particles. These nonequilibrium dynamics lead to an accumulation of charge carriers on small particles, and if the charge carriers are negative (electrons or negative ions), the small particles would tend to charge negatively. We propose that humidity leads to conductive layers on the surface of particles that act as a sink for ...

Nanostructured cadmium oxide thin films for hydrogen sensor

Abstract Cadmium oxide (CdO) thin films have been deposited by reactive dc magnetron sputtering on to glass substrates at room temperature and subsequently annealed at 275 °C for 3 h in vacuum. The as-deposited and annealed films were characterized for their structural and optical properties. The films were also tested for hydrogen sensing at different operating temperatures. The annealed samples showed better response down to 50 ppm at a relatively low operating temperature of 100 °C. The role of non-equilibrium surface states as a possible sensing mechanism is explained.

Surface Structure of Sodium Dodecyl Sulfate Surfactant and Oil at the Oil-in-Water Droplet Liquid/Liquid Interface: A Manifestation of a Nonequilibrium Surface State

We present sum frequency scattering spectra on kinetically stabilized emulsions consisting of nanoscopic oil droplets in water, stabilized with sodium dodecyl sulfate (SDS). We have measured the interfacial structure of the alkyl chains of the surfactant molecules, the alkyl chain of the oil molecules, the weakly dispersive D(2)O response, and the interference between SDS and the oil. We find a big difference in chain conformation: SDS has many chain defects, whereas the oil has very few. Our spectra are interpreted to originate from a surface structure with oil molecules predominantly oriented parallel with respect to the plane of the interface. The SDS headgroup is surrounded by water molecules. The SDS alkyl tail is in a disordered state and partially in contact with water. Such a conformation of surfactant occupies a surface area of several hundreds of squared angstroms.

Electrochemical Investigation of Non-equilibrium Surface States of Matter.

High energy, non-equilibrium, surface states of metals, which are assumed to be associated with surface defect sites, have not received a great deal of attention in surface electrochemistry. Such states, which are assumed to be the basis of active site behaviour, are minority surface species which display redox (or faradaic) behaviour that is quite different from the conventional response of the majority surface state, i.e. similar atoms present in equilibrated form at the same surface. The effect of enhancing the coverage of the high energy surface state is illustrated, as is the role of such states in electrocatalysis, plating bath additive behaviour and the operation of metastable metal oxide electrodes such as ruthenium dioxide cathodes. Non-equilibrium surface states are difficult to control and investigate as they evidently involve quite low coverage, rather labile, surface species which are not easily imaged by STM.

Ablation of metals with picosecond laser pulses: Evidence of long-lived non-equilibrium surface states

Experiments on laser ablation of metals in air, in vacuum, and in similar irradiation conditions, revealed that the ablation thresholds in air are up to three times lower than those measured in vacuum. Our analysis shows that this difference is caused by the existence of a long-lived transient non-equilibrium surface state at the solid-vacuum interface. The energy distribution of atoms at the surface is Maxwellian-like but with its high-energy tail truncated at the binding energy. We find that in vacuum the rate of energy transfer from the bulk to the surface layer to build the high-energy tail, which determines the lifetime of this non-equilibrium state, exceeds other characteristic timescales such as the surface cooling time. This prohibits thermal evaporation in vacuum for which the high-energy tail is essential. In air, however, collisions between the gas atoms and the surface markedly reduce the lifetime of this non-equilibrium surface state allowing thermal evaporation to proceed before the surface cools. It was experimentally observed that the difference between the ablation depth in vacuum and that in air disappears at the laser fluencies 2–3 times in excess of the vacuum threshold value. The material removal at this level of the deposited energy density attains the features of the non-equilibrium ablation similar for both cases. We find, therefore, that the threshold in vacuum corresponds to non-equilibrium ablation during the pulse, while thermal evaporation after the pulse is responsible for the lower ablation threshold observed in air. This paper provides direct experimental evidence of how the transient surface effects may strongly affect the onset and rate of a solid-gas phase transition.

Ablation of metals with picosecond laser pulses: Evidence of long-lived nonequilibrium conditions at the surface

We report here experimental results on laser ablation of metals in air and in vacuum in similar irradiation conditions. The experiments revealed that the ablation thresholds in air are less than half those measured in vacuum. Our analysis shows that this difference is caused by the existence of a long-lived transient nonequilibrium surface state at the solid-vacuum interface. The energy distribution of atoms at the surface is Maxwellian-like but with its high-energy tail truncated at the binding energy. We find that in vacuum the time needed for energy transfer from the bulk to the surface layer to build the high-energy tail, exceeds other characteristic timescales such as the electron-ion temperature equilibration time and surface cooling time. This prohibits thermal evaporation in vacuum for which the high-energy tail is essential. In air, however, collisions between the gas atoms and the surface markedly reduce the lifetime of this nonequilibrium surface state allowing thermal evaporation to proceed before the surface cools. We find, therefore, that the threshold in vacuum corresponds to nonequilibrium ablation during the pulse, while thermal evaporation after the pulse is responsible for the lower ablation threshold observed in air. This paper provides direct experimental evidence of how the transient surface effects may strongly affect the onset and rate of a solid-gas phase transition.

Cyclic voltammetry responses of metastable gold electrodes in aqueous media

Gold is the noblest of metals and it is widely assumed that the surface of a gold electrode only begins to undergo oxidation in aqueous media at ca. 1.36 V (RHE) in acid or ca. 1.25 V in base. However, recent work in this laboratory has shown that, following thermal activation, gold atoms at the surface of a gold electrode may undergo oxidation in acid solution at potentials as low as 0.3 V. In the present work, in which the gold was allowed to cool rapidly from the molten state, oxidation responses were observed for gold in base at E<0.0 V. Such unusual behaviour is attributed to low gold-atom lattice stabilization energy plus the operation of a super-Nernstian E/pH effect. It appears, as outlined recently for silver, that a range of surface active states are available for metals; these give rise to a range of premonolayer oxidation and multilayer hydrous oxide reduction responses; such non-equilibrium surface states are often quite important with regard to electrocatalysis.

Surface phase morphology and composition of the casting films of PVDF–PVP blend

Films of a binary polymer blend comprising of hydrophobic poly(vinylidene fluoride) (PVDF) and hydrophilic poly(vinylpyrrolidone) (PVP) have been prepared by solution-casting. The dependence of surface structure and composition of the films on the PVP content in the blend was investigated by using atomic force microscope (AFM), XRD, XPS, SEM and differential scanning calorimeter (DSC). It has been found that the interaction between the two homopolymers prevents PVDF from crystallization in the blend, the net result of which has a primary effect on the surface properties of the films. PVP has a greater concentration at the surface than in the bulk as long as PVDF crystallizes in the bulk during the film formation process, which leaves a thermodynamically non-equilibrium surface state. On the other hand, with an increase in the PVP content, the interaction between segmental PVDF and PVP in the beginning transforms the crystalline state of PVDF from α to γ phase, and finally results in the disappearance of crystalline PVDF phases. A meager crystallization of PVDF segments could still carry on at the surface of a film with a miscible (or an amorphous) bulk; this occurrence makes the surface more hydrophobic than its bulk phase.

Surface segregation on Fe3%Si0.04%VC(100) single crystal surfaces

Surface segregation phenomena on (100) oriented single crystal surfaces of the ferritic Fe-3%Si-0.04%V-C alloy were investigated by AES and LEED. At temperatures below 635 °C vanadium and carbon cosegregation is observed after prolonged heating. At thermodynamic equilibrium the substrate surface is saturated with the binary surface compound VC. The two-dimensional VC is epitaxially arranged on the substrate surface as indicated by LEED investigations. Its structure corresponds to the (100) plane of the three-dimensional VC with rocksalt structure. Sharp above 635 °C the surface compound VC is dissolved into the bulk. At higher temperatures the substrate surface is covered with segregated silicon forming a c(2 × 2) structure. This surface phase transition is reversible. Because of the low concentration and slow diffusion of vanadium, non-equilibrium surface states are formed as intermediates upon segregation of silicon and carbon. Below 500 °C a disordered graphite layer with a characteristical asymmetrical C Auger peak is observed on the substrate surface. Above 500 °C carbon segregation leads to the formation of an ordered c(2 × 2) structure with a symmetrical C Auger peak being characteristic for carbidic or atomically adsorbed species. At increasing temperatures silicon segregation takes place leading to a c(2 × 2) structure. Between silicon and carbon site competition is effective.

Non-equilibrium surface state properties at clean cleaved silicon surface as measured by surface photovoltage

Surface photovoltage transients were measured at clean cleaved silicon (111) faces in ultrahigh vacuum. The temperature and doping of the samples, the intensity of the stimulating light pulses (energy less than band gap), and the surface coverage (clean and adsorbed water vapor) were varied systematically. The results yield information on the charge transfer at the surface and on surface recombination. The calculation of the surface photovoltage (using only the generation rates into and out of the surface states and data of thermal equilibrium) shows, that only one bulk band (conduction band for n-doped samples and valence band for p-doped samples) controls completely signal height and its relaxation via charge transfer to the surface states. The determined surface state parameters are: relaxation time constants, capture cross-sections for photons and transition probabilities. On the basis of the model all decay curves can be reproduced quantitatively.

Non-equilibrium surface state properties at clean cleaved silicon surface as measured by surface photovoltage

Non-equilibrium surface state properties at clean cleaved silicon surface as measured by surface photovoltage