513 research outputs found
On the Structure of the Helmholtz Layer and its Implications on Electrode Kinetics
Concepts and selected experiments on the structure of the Helmholtz double layer at the metal- and semiconductor - electrolyte phase boundary are reviewed. The widely used microcapacitor approach of the double layer and its limitations are assessed. Observations on the influence of the electrode potential on the energetic position of surface states at the Ag-electrolyte contact are compared to the predictions of classical charge transfer models that are based on transition state theory where adiabatic tunneling is assumed. Distance tunneling spectroscopy on Au(111) surfaces shows pronounced variations in tunneling barrier heights that are connected to the inner structure of the Helmholtz layer and implications on electrode kinetics are presented. At the semiconductor-electrolyte contact, the influence of the electrode potential on a charge injecting species that results in photocurrent doubling is reviewed for low- and higher doped Si(111) electrodes, showing that the complex that injects electrons into the conduction band is located outside the semiconductor surface. The observations are correlated with the search for low overpotential earth abundant electrocatalysts for solar fuel generation of solar fuels
Oscillations at the Si/electrolyte contact: Relation to Quantum Mechanics
The basic process at the surface of the Si electrode is characterized by a cyclic oxidation of a thin silicon layer and the subsequent removal of the oxide by etching. Here, the oxide thickness evolves not uniformly due to cracks and nanopores. The mathematical model used to describe the phenomenon is based on a sequence of time dependent (oxide thickness) oscillator density functions that describes the passing of the (infinitesimal) oscillators through their minimum at each cycle. Two consecutive oscillator density functions are connected by a second order linear integral equation representing a Markov process. The kernel of the integral equation is a normalized Greens Function and represents the probability distribution for the periods of the oscillators during a cycle. Both, the oscillator density function and the two-dimensional probability density for the periods of the oscillators, define a random walk. A relation between the oscillator density functions and solutions of the Fokker-Planck equation can be constructed. This allows a connection of the oscillations, originally considered only for the description of a photo-electrochemical observation, to the Schrodinger equation. In addition, if the trajectory of a virtual particle, located at the silicon oxide electrode surface, is considered during one oscillatory cycle, then it can be shown that the displacement of the particle measured at the electrode surface performs a Brownian motion
Quantum phases of a chain of strongly interacting anyons
We study a strongly interacting chain of anyons with fusion rules determined
by SO(5)2. The phase portrait is identified with a combination of numerical and
analytical techniques. Several critical phases with different central charges
and their corresponding transitions identified.Comment: 5 pages, 4 figure
Core-Level X-Ray Photoemission Satellites in Ruthenates: A New Mechanism Revealing the Mott Transition
Ru 3d core-level x-ray photoemission spectra of various ruthenates are
examined. They show in general two-peak structures, which can be assigned as
the screened and unscreened peaks. The screened peak is absent in a Mott
insulator, but develops into a main peak in the metallic regime. This spectral
behavior is well explained by the dynamical mean-field theory calculation for
the single-band Hubbard model with on-site core-hole potential using the exact
diagonalization method. The new mechanism of the core-level photoemission
satellite can be utilized to reveal the Mott transition phenomenon in various
strongly correlated electron systems, especially in nano-scale devices and
phase-separated materials.Comment: 4 pages, 5 figures, submitted to PR
Density of Superfluid Helium Droplets
The classical integral cross sections of large superfluid 4He_N droplets and
the number of atoms in the droplets (N=10^3-10^4) have been measured in
molecular beam scattering experiments. These measurements are found to be in
good agreement with the cross sections predicted from density functional
calculations of the radial density distributions with a 10-90 % surface
thickness of 5.7\AA. By using a simple model for the density profile of the
droplets a thickness of about 6-8\AA is extracted directly from the data.Comment: 27 pages, REVTeX, 5 postscript figure
Photoelectrochemical Conditioning of MOVPE p-InP Films for Light-Induced Hydrogen Evolution: Chemical, Electronic and Optical Properties
Homoepitaxial p-InP(100) thin films prepared by MOVPE (metallorganic vapor phase epitaxy) were transformed into an InP/oxide-phosphate/Rh heterostructure by photoelectrochemical conditioning. Surface sensitive synchrotron radiation photoelectron spectroscopy indicates the formation of a mixed oxide constituted by In(PO_3)_3, InPO_4 and In_(2)O_3 as nominal components during photo-electrochemical activation. The operation of these films as hydrogen evolving photocathode proved a light-to-chemical energy conversion efficiency of 14.5%. Surface activation arises from a shift of the semiconductor electron affinity by 0.44 eV by formation of In-Cl interfacial dipoles with a density of about 10^(12) cm^(â2). Predominant local In2O3-like structures in the oxide introduce resonance states near the semiconductor conduction band edge imparting electron conductivity to the phosphate matrix. Surface reflectance investigations indicate an enhanced light-coupling in the layered architecture
Probing the structure and dynamics of molecular clusters using rotational wavepackets
The chemical and physical properties of molecular clusters can heavily depend
on their size, which makes them very attractive for the design of new materials
with tailored properties. Deriving the structure and dynamics of clusters is
therefore of major interest in science. Weakly bound clusters can be studied
using conventional spectroscopic techniques, but the number of lines observed
is often too small for a comprehensive structural analysis. Impulsive alignment
generates rotational wavepackets, which provides simultaneous information on
structure and dynamics, as has been demonstrated successfully for isolated
molecules. Here, we apply this technique for the firsttime to clusters
comprising of a molecule and a single helium atom. By forcing the population of
high rotational levels in intense laser fields we demonstrate the generation of
rich rotational line spectra for this system, establishing the highly
delocalised structure and the coherence of rotational wavepacket propagation.
Our findings enable studies of clusters of different sizes and complexity as
well as incipient superfluidity effects using wavepacket methods.Comment: 5 pages, 6 figure
Monolithic Photoelectrochemical Device for Direct Water Splitting with 19% Efficiency
Recent rapid progress in efficiencies for solar water splitting by
photoelectrochemical devices has enhanced its prospects to enable storable
renewable energy. Efficient solar fuel generators all use tandem photoelectrode
structures, and advanced integrated devices incorporate corrosion protection
layers as well as heterogeneous catalysts. Realization of near thermodynamic
limiting performance requires tailoring the energy band structure of the
photoelectrode and also the optical and electronic properties of the surface
layers exposed to the electrolyte. Here, we report a monolithic device
architecture that exhibits reduced surface reflectivity in conjunction with
metallic Rh nanoparticle catalyst layers that minimize parasitic light
absorption. Additionally, the anatase TiO2 protection layer on the photocathode
creates a favorable internal band alignment for hydrogen evolution. An initial
solar-to-hydrogen efficiency of 19.3 % is obtained in acidic electrolyte and an
efficiency of 18.5 % is achieved at neutral pH condition (under simulated
sunlight)
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