J.W.J. Kerssemakers

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...

Epitaxial switchable yttrium-hydride mirrors

By means of x-ray scattering and scanning probe microscopy it is shown that high-quality epitaxial Y films can be deposited on (111)-CaF2 substrates. The films can reversibly be switched from metallic YH2 to transparent insulating YH3−δ. Although hydrogen absorption involves an expansion of the lattice and a symmetry change from hcp to fcc, the epitaxiality of the film remains intact during the switching process. The transparency and the insulating nature of the substrate opens unique possibilities to investigate electrically and optically these switchable mirror films in the single crystalline state.

Influence of spring stiffness and anisotropy on stick‐slip atomic force microscopy imaging

of detection that is used and is most prominent with optical lever detection. Another aspect is the influence of the scanning configuration or system itself. The latter concerns cantilever geometry, cantilever mechanical properties, sample orientation, and detector orientation. As far as the detection side is concerned, especially the optical lever system can be shown to be more sensitive to a lateral movement of the probe than a vertical deflection. 5 Keeping this in mind, the interpretation of the detected signal can be rather misleading. The forces in the perpendicular direction are often associated with the long-axis signal component of the cantilever, while the short-axis component is taken for the friction signal. 6,7 At an atomic scale, one should be cautious in making this assumption: along both axes the scan signal can be shown to follow friction-based behavior, as will be explained later. Even when the origin of the cantilever movement is unambiguously frictional, it is still not straightforward to interpret the signal in terms of the interaction force between probe tip and substrate. In various publications, attention was focused on the strong stick-slip nature of the signal. 4,8‐15 Here, the movement of the tip relative to the scan path resembles a relaxation oscillation, 16 i.e., static ~stick! phases alternate fast-moving slip-phases. The driving frequency in this picture is the periodicity of the lattice in combination with the scanning speed. Recently, the concept of atomicperiodicity stick slip is made more explicit in a twodimensional description. 15,17,18 The tip follows the forced scan path of the cantilever base by a series of discrete jumps from lattice point to lattice point. The difference between the continuous cantilever base route and the zig-zag motion of the tip, thus is the predominant cause for the well known images showing atomic resolution, especially of layered compounds ~Fig. 1!. In literature, this is clarified in various ways that, however, mainly differ in actual representations: l The perpendicular components of the tip displacement are usually associated with the ‘‘force’’ and ‘‘friction’’ signal. These signals were used to restore the original route of the tip. 18

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.

Mechanism of the structural phase transformations in epitaxial YHx switchable mirrors

The detailed mechanisms of the structural phase transformations that occur in epitaxial Y–hydride switchable mirrors are revealed with high resolution transmission electron microscopy (both cross sectional and plan view). The triangular ridge network that develops in Y prior to the α–β transformation is a result of {1012} deformation twinning. The basal plane that is originally parallel to the film/substrate interface is rotated by twinning over 5.6° and transformed into a prismatic plane and similarly the prismatic plane is transformed into a basal plane giving a final crystal reorientation for the ridge of 95.6°. After transformation to β, nearly vertical Σ3{111} twin boundaries arise in the ridges. In contrast, horizontal twin boundaries develop in the β domains to prevent macroscopic shape changes. Inbetween the two twin variants within the domains, Shockley partial dislocations are persistently present, which enable efficient reversible β–γ switching of the mirror.

Atomic force microscopy imaging of transition metal layered compounds: A two‐dimensional stick–slip system

Various layered transition metal dichalcogenides were scanned with an optical‐lever atomic force microscope (AFM). The microscopic images indicate the occurrence of strong lateral stick–slip effects. In this letter, two models are presented to describe the observations due to stick–slip, i.e., either as a static or as a dynamic phenomenon. Although both models describe correctly the observed shapes of the unit cell, details in the observed and simulated images point at dynamic nonequilibrium effects. This exact shape of the unit cell depends on cantilever stiffness, scan direction, and detector direction.

Probing the interface potential in stick/slip friction by a lateral force modulation technique

The influence of the shape of the interaction potential is investigated on details in stick/slip friction as encountered between an AFM tip and a substrate. Based on qualitative arguments of stick/slip systems, a novel technique is introduced in which the AFM tip is brought into a lateral resonance mode. In comparison to a direct measurement in the stick/slip signal, we suggest that the method is preferable to highlight these non-linear characteristics. In combination with the shape of the surface potential involved in stick/slip friction, this modulation diminishes the friction loop amplitude in a controlled way. Furthermore, a partial stick/slip behavior is observed above a certain threshold level of driving amplitude, where the tip alternates periodically between a zerofriction and a non-zero-friction state.

In situ generation and atomic scale imaging of slip traces with atomic force microscopy

We have designed, constructed, and tested a three-point bending system for in situ studies of slip in ionic crystals with an atomic force microscope (AFM). The work is aimed at developing a novel instrumental attachment for an in situ study of plastic deformation. The bending system is installed inside the optical head of the AFM on top of the piezoelectric scanner. Since the bending should not obstruct scanning, a piezocrystal is used for bending as well as an external stepper motor, which is connected with a screw in the bending system via a flexible shaft. The bending system performs over a relatively wide, continuous deflection range. The operation of the three-point bending system is illustrated by experiments on an ionic material in which the effect of macroscopic bending is demonstrated at an atomic scale.

Growth and hydrogenation of epitaxial yttrium switchable mirrors on CaF2

Rutherford backscattering (RBS) ion channeling measurements and X-ray diffraction experiments are performed to study the epitaxial nature of as-deposited yttrium on CaF2111 substrates and the effect of hydrogenation on the crystalline quality. The RBS and X-ray results clearly demonstrate the unique epitaxial relation between as-deposited films and the substrate, which is preserved upon loading with hydrogen. X-Ray diffraction reveals: (i) a remarkably large lattice expansion in the direction normal to the substrate, which decreases with increasing film thickness; and (ii) an in-plane compression of the lattice. This peculiar result is related to the difference in thermal expansion coefficients of film and substrate. RBS ion channeling measurements reveal a thickness dependence of the mismatch-induced stresses. As expected, the stresses relax with increasing distance from the film/substrate interface, but surprisingly, even with films as thick as 400 nm considerable dechanneling is still observed at the film surface. Film quality, i.e. the film/substrate mismatch as well as the induced stresses and their relaxation, are discussed in relation to atomic force microscopy (AFM) results on these epitaxial films.