S. J. van der Molen

Hydriding kinetics of Pd capped YHx switchable mirrors

The kinetics of the metal–insulator transition in polycrystalline, Pd-capped YHx switchable mirrors upon hydrogenation is investigated. Using the accompanying optical transition, we study switching of matrix-like samples with many (∼200) combinations of Pd and Y layer thicknesses. We find that: (i) With increasing Y thickness dY, the switching time τ increases for any constant Pd thickness dPd. (ii) With increasing dPd, there are three regimes. In regime I, it is impossible to switch a device. This can mainly be related to Pd–Y compound formation consuming all Pd within the UHV system, followed by surface oxidation in air. In regimes II and III switching is possible, but only in regime III does Pd form a closed cap layer. The Pd thickness needed for a closed cap layer depends on dY. (iii) An oxide buffer layer hinders Pd–Y interdiffusion, so that a thinner Pd cap layer is needed for switching than in the case without buffer layer. This is interesting for potential applications since it yields a higher opt...

Visualization of hydrogen migration in solids using switchable mirrors

Switchable mirrors made of thin films of the hydrides of yttrium (YHx), lanthanum (LaHx) or rare-earth metals exhibit spectacular changes in their optical properties as x is varied from 0 to 3. For example, α-YHx <0.23 is a shiny, hexagonally close-packed metal, β-YH2±δ is a face-centred cubic metal with a blue tint in reflection and a small transparency window at red wavelengths, whereas hexagonally close-packed γ-YHx >2.85 is a yellowish transparent semiconductor. Here we show that this concentration dependence of the optical properties, coupled with the high mobility of hydrogen in metals, offers the possibility of real-time visual observation of hydrogen migration in solids. We explore changes in the optical properties of yttrium films in which hydrogen diffuses laterally owing to a large concentration gradient. The optical transmission profiles along the length of the film vary in such a way as to show that the formation of the various hydride phases is diffusion-controlled. We can also induce electromigration of hydrogen, which diffuses towards the anode when a current flows through the film. Consequently, hydrogen in insulating YH3−δ behaves as a negative ion, in agreement with recent strong-electron-correlation theories,. This ability to manipulate the hydrogen distribution (and thus the optical properties) electrically might be useful for practical applications of these switchable mirrors.

The role of Joule heating in the formation of nanogaps by electromigration

We investigate the formation of nanogaps in gold wires due to electromigration. We show that the breaking process will not start until a local temperature of typically 400K is reached by Joule heating. This value is rather independent of the temperature of the sample environment (4.2–295K). Furthermore, we demonstrate that the breaking dynamics can be controlled by minimizing the total series resistance of the system. In this way, the local temperature rise just before breakdown is limited and melting effects are prevented. Hence, electrodes with gaps <2nm are easily made, without the need of active feedback. For optimized samples, we observe quantized conductance steps prior to the gap formation.

Feedback controlled electromigration in four-terminal nanojunctions

The authors have developed a fast, yet highly reproducible method to fabricate metallic electrodes with nanometer separation using electromigration (EM). They employ four terminal instead of two-terminal devices in combination with an analog feedback to maintain the voltage U over the junction constant. After the initialization phase (U≲0.2V), during which the temperature T increases by 80–150°C, EM sets in shrinking the wire locally. This quickly leads to a transition from the diffusive to a quasiballistic regime (0.2V≲U≲0.6V). At the end of this second regime, a gap forms (U≳0.6V). Remarkably, controlled electromigration is still possible in the quasiballistic regime.

Separating spin and charge transport in single-wall carbon nanotubes

We demonstrate spin injection and detection in single wall carbon nanotubes using a four-terminal nonlocal geometry. This measurement geometry completely separates the charge and spin circuits. Hence all spurious magnetoresistance effects are eliminated and the measured signal is due to spin accumulation only. Combining our results with a theoretical model, we deduce a spin polarization at the contacts alpha(F) of approximately 25%. We show that the magnetoresistance changes measured in the conventional two-terminal geometry are only partly due to spin accumulation.

Conductance switching in Ag2S devices fabricated by in situ sulfurization

We report a simple and reproducible method to fabricate switchable Ag(2)S devices. The alpha-Ag(2)S thin films are produced by a sulfurization process after silver deposition on an Si substrate. Structure and composition of the Ag(2)S are characterized using XRD and RBS. Our samples show semiconductor behaviour at low bias voltages, whereas they exhibit reproducible bipolar resistance switching at higher bias voltages. The transition between both types of behaviour is observed by hysteresis in the I-V curves, indicating decomposition of the Ag(2)S, increasing the Ag(+) ion mobility. The as-fabricated Ag(2)S samples are a good candidate for future solid state memory devices, as they show reproducible memory resistive properties and they are fabricated by an accessible and reliable method.

Transition voltage spectroscopy and the nature of vacuum tunneling.

Transition voltage spectroscopy (TVS) has been proposed as a tool to analyze charge transport through molecular junctions. We extend TVS to Au-vacuum-Au junctions and study the distance dependence of the transition voltage V(t)(d) for clean electrodes in cryogenic vacuum. On the one hand, this allows us to provide an important reference for V(t)(d) measurements on molecular junctions. On the other hand, we show that TVS forms a simple and powerful test for vacuum tunneling models.

Single atom adhesion in optimized gold nanojunctions

We study the interaction between single apex atoms in a metallic contact, using the break junction geometry. By carefully training our samples, we create stable junctions in which no further atomic reorganization takes place. This allows us to study the relation between the so-called jump out of contact (from contact to tunneling regime) and jump to contact (from tunneling to contact regime) in detail. Our data can be fully understood within a relatively simple elastic model, where the elasticity k of the electrodes is the only free parameter. We find 5<k<32 N/m. Furthermore, the interaction between the two apex atoms on both electrodes, observed as a change of slope in the tunneling regime, is accounted for by the same model.

Performance enhancement of metal-hydride switchable mirrors using Pd/AlOx composite cap layers

A drastic improvement of the optical properties and lifetime of switchable mirrors is obtained by placing a thin AlOx buffer layer between the Pd cap layer and the optically active, rare earth layer. The buffer layer lowers the minimum necessary Pd thickness to ∼1 nm, resulting in a ≈20% increase of the maximum transmittance. The optimal Pd and Al layer thicknesses are determined for the YHx and LaHx system using a powerful combination of optical and matrix film techniques. The AlOx buffer is shown to be superior to the native oxide layers YOx and LaOx. The buffer layer is essential for lanthanum, which is a particularly vulnerable, but fundamentally very important material. Using this composite cap layer, we have been able to switch LaHx films several times.

Stress development in thin yttrium films on hard substrates during hydrogen loading

The present drive to make munitions as safe as is feasible and to develop predictive models describing their constitutive response, has led to the development and production of plastic bonded explosives and propellants. There is a range of elastomers used as binder materials with the energetic components. One of these is known as Kel-F-800™ (poly-chloro-trifluroethylene) whose structure is in some ways analogous to that of poly-tetrafluoroethylene (PTFE or Teflon). Thus, it is of interest to assess the mechanical behavior of Teflon and to compare the response of five different production Teflon materials, two of which were produced in pedigree form, one as-received product, and two from previous in-depth literature studies. The equations of state of these variants were quantified by conducting a series of shock impact experiments in which both pressure-particle velocity and shock velocity-particle velocity dependencies were measured. The compressive behavior of Teflon, based upon the results of this study, appears to be independent of the production route and additives introduced.