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  4. Electrically tunable functional nanomaterials for actuation and photonics
 
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Electrically tunable functional nanomaterials for actuation and photonics

Citation Link: https://doi.org/10.15480/882.1048
Other Titles
Elektrisch durchstimmbare funktionale Nanomaterialien zur Verwendung als Aktuatoren und in der Photonik
Publikationstyp
Doctoral Thesis
Date Issued
2012
Sprache
English
Author(s)
Shao, Li-Hua  
Advisor
Weissmüller, Jörg  
Title Granting Institution
Technische Universität Hamburg
Place of Title Granting Institution
Hamburg
Examination Date
2012-02-23
Institut
Werkstoffphysik und -technologie M-22  
TORE-DOI
10.15480/882.1048
TORE-URI
http://tubdok.tub.tuhh.de/handle/11420/1050
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.
Subjects
funktionale Nanomaterialien
Aktuatoren
Photonik
Elektrochemie
functional nanomaterials
actuation
photonics
electrochemistry
Lizenz
http://doku.b.tu-harburg.de/doku/lic_mit_pod.php
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