Ernst S. Kooij

Contrast enhancement of rare-earth switchable mirrors through microscopic shutter effect

In contrast to the binary switchable mirror films (YHx, LaHx, REHx with RE:rare earth) which have a weak red transparency window in their metallic dihydride phase, rare-earth alloys containing magnesium are remarkable for the large contrast between their metallic dihydride and transparent trihydride phase. By means of structural, optical transmittance, and electrical resistivity measurements on a series of Y1−yMgyHx, films we show that this is due to a disproportionation of the alloy. While the yttrium dihydride phase is formed, Mg separates out, remaining in its metallic state. Upon further loading, insulating MgH2 is formed together with cubic YH3−δ. In this way Mg acts essentially as a microscopic optical shutter, enhancing the reflectivity of these switchable mirrors in the metallic state and increasing the optical gap in the transparent state.

In Situ Resistivity Measurements and Optical Transmission and Reflection Spectroscopy of Electrochemically Loaded Switchable YHx Films

We describe an experimental method for in situ resistivity measurements during the electrochemical hydrogen loading of thin, switchable metal hydride films. Using an oxygen-free electrolyte we are able to measure the hydrogen concentration in the films quantitatively and to determine the pressure-composition isotherms and the hydrogen concentration dependence of the resistivity. Furthermore, the optical properties can be investigated by simultaneous transmission and reflection spectroscopy. The power of these in situ measurements is discussed on the basis of results obtained on YHx films; possibilities for additional experiments are briefly described

Silicon Etching in HNO3/HF Solution: Charge Balance for the Oxidation Reaction

The potential dependence of the current and the etch rate of p- and n-type silicon electrodes in 6 M HNO3–6 M HF solution was investigated. Hydrogen evolved during etching at the open-circuit potential was also measured. These results give insight into the different processes occurring during etching. A detailed charge balance for silicon oxidation in the etchant is deduced.

Structural, optical and electronic properties of LaMgHx switchable mirrors

Structural, optical and electronic properties of lanthanum magnesium alloy thin films are studied in situ in real time during hydrogenation. X-ray data show that the as-deposited films contain the intermetallic phase LaMg with CsCl structure as well as fcc β-La and fcc LaHx. Hydrogenation initiates a transformation of β-La to β-LaHx, while the intermetallic phase disproportionates. The fcc structure stays intact during further uptake of hydrogen up to LaH3, and Mg transforms to the rutile structure of MgH2. During hydrogen absorption the alloys change from a good conducting, metallic state with high reflectance, to a transparent color neutral insulator with a band-gap of about 3 eV.

Electroluminescence from porous silicon due to electron injection from solution

We report on the electroluminescence from p‐type porous silicon due to minority carrier injection from an electrolyte solution. The MV+• radical cation formed in the reduction of divalent methylviologen is able to inject electrons into the conduction band of crystalline and porous silicon. The electrochemistry of this redox process at silicon electrodes is briefly described, and electroluminescence due to recombination of the injected electrons with holes from the substrate is described. The results are discussed in terms of a semiconductor model.

Highly reflecting Y/Mg-Hx multilayered switchable mirrors

Optical, structural and thermodynamic properties of Y/Mg–Hx multilayered switchable mirrors are investigated and compared with YMgHx-alloys and pure YHx. Multilayers clearly have superior reflectance in the low-hydrogen state over the whole range of photon energies, 0.72<hω<3.5 eV, investigated in this work. In the transparent high-hydrogen state the absorption edge of the multilayers with the same overall composition is shifted in energy on increasing the individual layer thicknesses. Thus, the absorption edge is non-trivially depending on the distribution of Mg in the film. Moreover, the optical contrast of multilayers is much higher than for pure YHx and their optical switching does not suffer from hysteretic effects. Finally, Mg and YH2 transform almost at the same time to MgH2 and YH3, respectively, with a heat of formation of −30.9 kJ/mol H in absorption and −36.6 kJ/mol H in desorption.

Luminescence studies of semiconductor electrodes

In this paper we review our recent results of in-situ luminescence studies of semiconductor electrodes. Three classes of materials are considered: single crystal compound semiconductors, porous silicon and semiconducting oxides doped with luminescent ions. We show how photoluminescence (PL) and electroluminescence (EL) measurements can give information about the optical and optoelectrical properties of the solid and about mechanisms of surface reactions. The relevance of time- and frequency-dependent measurements, which have been largely neglected, is stressed.

Infrared induced visible emission from porous silicon: the mechanism of anodic oxidation

The visible luminescence caused by anodic oxidation of p-type porous silicon has been studied. It is shown that similar luminescence can be observed in n-type material by illumination with near-infrared light. Addition of a suitable reducing agent to the electrolyte solution can both suppress the oxidation of the porous layer and quench its luminescence. These results confirm a previously suggested mechanism, in which the capture of a valence band hole in a surface bond of the porous semiconductor gives rise to a surface state intermediate capable of thermally injecting an electron into the conduction band.

Electrochemistry of porous and crystalline silicon electrodes in methylviologen solutions

From measurements using stationary and rotating disc and ring-disc electrodes, it is concluded that the reduction reactions of the divalent methylviologen cation MV2+ (to MV+· and MV0) proceed via the conduction band of both porous and crystalline silicon. The product of the second reduction step (MV0) forms a blocking layer on the electrode. The oxidation reactions of MV0 and MV+· take place by electron injection into the conduction band of n-type crystalline silicon. From the current-potential characteristics it follows that MV+· is also able to inject electrons into porous silicon formed on an n-type electrode. At p-type porous silicon, electroluminescence is observed as a result of radiative recombination of holes (majority carriers) with electrons (minority carriers) injected into the porous structure during oxidation of MV+·. Introducing ethanol into the solution leads to changes in the current-potential characteristics, which are explained by the solubility of MV0 in these solutions. The current-potential characteristics are considered on the basis of the position of the band edges, as deduced from impedance measurements, and differences between crystalline and porous silicon are discussed.

Analysis of rotational coupling in collisions of Li+ with Ne leading to double excitation of Ne

Electron angular distributions due to autoionization of Ne, doubly excited to the (2p43s2)1D state in collisions with Li+ in the energy range 1.2-2.2 keV, are measured in coincidence with Li+ scattered into a well defined direction ( Phi =0 degrees , Theta cm=10.8 degrees ). The experimental findings are analysed with the help of a collision model proposed earlier. In this model the initial excitation occurs by radial diabatic coupling to a molecular Sigma -state at small distances, followed by rotational coupling to Pi - and Delta -states at intermediate distances in the second half of the collision. The energy splitting between the Sigma -, Pi - and Delta -states is described by a model function. By adapting two parameters of this model function, the experimental findings can be reproduced within the experimental error in numerical calculations involving the relevant set of coupled differential equations.