The production of hydrogen through water splitting in a photoelectrochemical cell suffers from an overpotential that limits the efficiencies. In addition, hydrogen-peroxide formation is identified as a competing process affecting the oxidative stability of photoelectrodes. We impose spin-selectivity by coating the anode with chiral organic semiconductors from helically aggregated dyes as sensitizers; Zn-porphyrins and triarylamines. Hydrogen peroxide formation is dramatically suppressed, while the overall current through the cell, correlating with the water splitting process, is enhanced. Evidence for a strong spin-selection in the chiral semiconductors is presented by magnetic conducting (mc-)AFM measurements, in which chiral and achiral Zn-porphyrins are compared. These findings contribute to our understanding of the underlying mechanism of spin selectivity in multiple electron-transfer reactions and pave the way toward better chiral dye-sensitized photoelectrochemical cells.

We show that in an electrochemical cell, in which the photoanode is coated with chiral molecules, the overpotential required for hydrogen production drops remarkably, as compared with cells containing achiral molecules. The hydrogen evolution efficiency is studied comparing seven different organic molecules, three chiral and four achiral. We propose that the spin specificity of electrons transferred through chiral molecules is the origin of a more efficient oxidation process in which oxygen is formed in its triplet ground state. The new observations are consistent with recent theoretical works pointing to the importance of spin alignment in the water-splitting process.

Molecular spintronics or spin‐based electronics, which utilizes both the spin degrees of freedom and electron charge, has become a hot topic in modern science. Since the introduction of spintronics in 1988, many efforts have been devoted to controlling spin‐polarized current using an external magnetic field, leading to the implementation of commercial solid‐state devices based on the giant magnetoresistance effect. In molecular spintronics, much progress has been achieved with organic molecules, but the role played by chiral molecules is yet to be explored in detail, while it promises to play a role in the future. It has been proved that the interaction of electrons with chiral molecules is spin specific, as supported by several experimental tools, and by theoretical studies. This effect is named “chiral‐induced spin selectivity” (CISS). CISS is based on the fact that chiral molecules exhibit spin‐specific transport properties, and hence can be used as a substitute for ferromagnetic materials. Here, recent spin‐dependent electrochemistry results are highlighted, where chiral molecules are immobilized on a ferromagnetic electrode. Practical applications of the CISS effect, for spin control of charge transport in complex molecular architectures, and in the water‐splitting process are also reviewed.

No abstract

No abstract

a b s t r a c tTemperature dependent current-voltage (I-V) and capacitance-voltage (C-V) measurements have been performed on Pd/ZnO Schottky barrier diodes in the range 60-300 K. The room temperature values for the zero bias barrier height from the I-V measurements (F I-V ) was found to be 0.52 eV and from the C-V measurements (F C-V ) as 3.83 eV. From the temperature dependence of forward bias I-V, the barrier height was observed to increase with temperature, a trend that disagrees with the negative temperature coefficient for semiconductor material. The C-V barrier height decreases with temperature, a trend that is in agreement with the negative temperature coefficient of semiconductor material. This has enabled us to fit two curves in two regions (60-120 K and 140-300 K). We have attributed this behaviour to a defect observed by DLTS with energy level 0.31 eV below the conduction band and defect concentration of between 4 Â 10 16 and 6 Â 10 16 cm À 3 that traps carriers, influencing the determination of the barrier height.

We have systematically investigated the effects of high-temperature annealing on ZnO and ZnO devices using current voltage, deep level transient spectroscopy (DLTS) and Laplace DLTS measurements. Current-voltage measurements reveal the decrease in the quality of devices fabricated on the annealed samples, with the high-temperature annealed samples yielding devices with low barrier heights and high reverse currents. DLTS results indicate the presence of three prominent defects in the as-received samples.

Temperature dependent Hall (TDH) effect measurements have been performed on three virgin and hydrothermally grown ZnO samples with resistivities between ∼5 and ∼200 Ω cm at room temperature. The electrical conduction observed experimentally in the temperature range of 330–70 K can be accurately described by three donor levels with positions 41–48, 60–66, and ∼300 meV below the conduction band edge (EC) and an acceptor level in the lower part of the energy band gap (EG). Correlation of the TDH data with results from secondary ion mass spectrometry and admittance spectroscopy on the same samples suggests a rather firm association of the intermediate donor level with complexes involving Al impurities, while the shallowest one is tentatively ascribed to H-related centers. A large fraction of the deep donor remains nonionized in the temperature range studied and contributes substantially to the neutral-impurity-scattering of the conducting electrons. A detailed analysis of the TDH data, using the relaxation time approximation, reveals, however, that ionized-impurity-scattering and optical phonon scattering are the main mechanisms limiting the electron mobility which exhibits a maximum value of ∼125 cm2/V s at ∼200 K. The major reason for this modest value is the high concentration of compensating acceptors in the lower part of EG reaching values of ∼3×1017 cm−3 and where Li plays an important role. However, the Li content is not sufficient to account for all the acceptors and additional impurities, excluding group I elements, and/or intrinsic defects have to be considered.