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Item type:Publication, Access status: with files , Electrochemical tuning of the optical properties of nanoporous gold(2017-03-09); ; ; ; Using optical in-situ measurements in an electrochemical environment, we study the electrochemical tuning of the transmission spectrum of films from the nanoporous gold (NPG) based optical metamaterial, including the effect of the ligament size. The long wavelength part of the transmission spectrum around 800 nm can be reversibly tuned via the applied electrode potential. The NPG behaves as diluted metal with its transition from dielectric to metallic response shifted to longer wavelengths. We find that the applied potential alters the charge carrier density to a comparable extent as in experiments on gold nanoparticles. However, compared to nanoparticles, a NPG optical metamaterial, due to its connected structure, shows a much stronger and more broadband change in optical transmission for the same change in charge carrier density. We were able to tune the transmission through an only 200 nm thin sample by 30%. In combination with an electrolyte the tunable NPG based optical metamaterial, which employs a very large surface-to-volume ratio is expected to play an important role in sensor applications, for photoelectrochemical water splitting into hydrogen and oxygen and for solar water purification.Publicationtype: Journal ArticleScopus© Citations 29 756 669 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , Piezoelectric gold : strong charge-load response in a metal-based hybrid nanomaterialImpregnating the pores of nanoporous gold with aqueous electrolyte yields a hybrid nanomaterial with two separate and interpenetrating charge transport paths, electronic conduction in the metal and ionic conduction in the electrolyte. As the two paths are capacitively connected, space‐charge layers along the internal interfaces are coupled to electric potential differences between the paths and can be controlled or detected thereby. The present experiments show that the space charge couples to mechanical deformation of the hybrid material, so that external loading generates an electric current. The electric signal originates from charge displacement along the entire internal interface; the signal is particularly robust since the interface area is large. The charge transfer in response to load constitutes a piezoelectric response, yet the mechanism is quite different to classic piezoelectricity. The analysis in this work predicts links between electromechanical coupling parameters for strain sensing and actuation, which are in excellent agreement with the experiment.Publicationtype: Journal ArticleScopus© Citations 51 464 472 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , Nanostructured MWCNT/Polypyrrole Actuators with Anisotropic Strain Response(2016-04-01); ; ; ; Conducting polymers (CP) as an active material in electrochemical actuators are often likened to "artificial muscles". To transfer the isotropic change of volume in the CP component into an anisotropic actuation as in a natural muscle, different design approaches, mostly bilayer or triple layer bending devices, have been studied. Herein, we report a novel actuator design with an almost linear strain response, consisting of nanostructured aligned multi-walled carbon nanotube arrays in which each MWCNT is individually coated with a thin layer of polypyrrole.Publicationtype: Journal Article59 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , Electrically tunable functional nanomaterials for actuation and photonics(2012); Nanomaterials with tunable electronic structure exploit the large specific surface area of metal nanostructures along with the strategy of tuning the surface properties through the controlled introduction of space-charge regions. Then, materials with tunable macroscopic properties can be created. The present thesis work achieved a successful synthesis of metallic and carbon-based tunable nanomaterials and demonstrated novel functional behavior in two fields of application: actuation and photonics. The work also proposes concepts for the underlying fundamental processes. In more detail, the following aspects were explored: The growing interest in strain induced by capillary forces in porous materials motivates our search for the underlying mechanism. For the first time, an experiment was designed to illustrate the important distinctions in different capillary forces and their impact on the strain in porous materials. The strategy is to compare (1) dilatometry to probe macroscopic sample dimension change and (2) wide-angle x-ray diffraction to probe the lattice parameter variation of a gold crystal lattice. The resulting two strain measures on nanoporous gold show a significant difference. This observation confirms the fundamental distinction between the strain in response to the action of the surface stress at the solid surface and the strain in response to changes in the pressure in the fluid. This can be a correction of the previously reported works, which did not take into account the surface stress effect. Using nanoporous noble metals, one can get large strain and mechanical energy density in a stiff actuator material via an applied voltage in an electrolyte. A similar concept may be applied to carbon aerogel, which is a light-weight, low cost porous material with extremely large surface area. This work demonstrates the potential of this material for actuation, with an unusually large reversible volume strain, 6.6%. The mass-specific strain energy density even exceeds that of piezoceramics and of nanoporous metal actuators. On top of that, a fundamental electrochemical parameter potential of zero charge (pzc) of carbon aerogel is measured. The results demonstrate that the pzc does not necessarily coincide with the potential of the maximum surface stress. This confirms the above-mentioned distinction between the capillary forces. Another instance for the concept of tunable nanomaterials is electrical modulation of photon-ic metamaterials. Arrays of lithographical resonators are used to fabricate Metamaterials, from which one can achieve an unconventional optical response – in the extreme, the negative refrac-tive index. The novel concept presented here is that the space-charge at the surface of each reso-nator is modulated by the applied potential in electrochemical environment. In this way, this work achieves a large and reversibly tunable resonance. One can switch on/off the resonance ideally by an electric signal, which is attractive for applications as functional photonic metamaterial devices. While the underlying mechanism is not conclusively understood, it is natural to suspect a coupling between the space charge and the electric resistance. Experiments using Pb and Cu underpotential deposition were performed, and the trends in the results support the notion of a decisive impact of the resistance on resonance damping, while the electron density change appears to mainly influence the resonance frequency.Publicationtype: Doctoral Thesis1625 2008 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , Electrochemical restructuring of plasmonic metamaterials(American Institute of Physics (AIP), 2011-01); ; Wegener, MartinRecent electrochemical experiments on gold-based photonic metamaterials have shown reversible optical modulation as well as an irreversible reduction in the plasmonic damping. Here, we systematically study the latter aspect as a possible means of postprocessing plasmonic gold nanoantennae arrays aiming at loss reduction. We characterize the samples by optical spectroscopy, electron microscopy, and atomic-force microscopy. For sub-10 nm gold thicknesses, the measured damping decreases by factors exceeding 3; for 20-30 nm thin structures, the obtained loss reduction still amounts to about 30%. © 2011 American Institute of Physics.Publicationtype: Journal Article25