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Title: Electrohydrodynamic patterning of functional materials
Author: Goldberg Oppenheimer, Pola
Awarding Body: University of Cambridge
Current Institution: University of Cambridge
Date of Award: 2011
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This thesis explores an alternative route to induce and control the structure formation process in thin films by the use of strong electric fields. The objective of this thesis is to investigate, establish and apply the use of the electrohydrodynamic (EHD) lithography as a versatile patterning tool on the sub-micrometre and nanometre length scales for functional materials in thin films. Thin films are ubiquitous, they are found in nature and used in almost every aspect of daily life. While film instabilities are often undesirable in nature and technology, they can be utilized to produce structures by precisely controlling the destabilization of the film. EHD lithography utilizes instabilities induced by means of an electric field to fabricate periodic structures. While still in the developing phase, EHD patterning is set to become a competitive candidate for low-cost lithographic technology for a number of applications. Herein, the applied potential of this lithographic process is explored by expanding its applicability to a broad range of materials and material combinations and by a simultaneous patterning of multilayer systems or functional polymers yielding hierarchical architectures with novel functionalities. An intrinsic problem of the EHD patterning process is the rather long pattern generation times, which has to be overcome. A number of low-viscosity materials are exploited as high-speed resists to significantly reduce the patterning time of EHD to a few seconds. These results demonstrate the versatility of the EHD method and render it technologically appealing. Further work presents a route towards controlled reproducible alignment of carbon nanotubes (CNTs) during an EHD patterning process of nanocomposite. The degree of nanotube alignment is tuned by adjusting the EHD parameters and the degree of alignment is mirrored by the conductivity across the film. Patterned surfaces decorated by CNT brushes are also generated in this study. A method to create controlled self-organized hierarchical nanostructures using EHD instabilities is an additional topic of this thesis. Herein, pattern formation harnesses a sequential instability in multilayer thin films induced by an electric field to guide the layer material into design structures, allowing different materials to be patterned in a one-step procedure. This pattern formation enables the fabrication of multi-scale structured arrays as surface enhanced Raman scattering (SERS)-active platforms. Each of the formed structures is effectively tailored to provide high SERS enhancement. Furthermore, crystalline and conductive polymers are patterned using the EHD approach and the underlying structure formation mechanisms are discussed. This extension towards functional material systems offers interesting prospects for potential applications. Finally, the EHD formation of hierarchical structures spanning several length scales is demonstrated. Confining conductive block copolymers in presence of an electric field into fabricated microstructures gives rise to an internal vertical alignment of the copolymers nanometric domains with an additional molecular orientation. Inside the superstructure films ordered arrays of nanocrystals of the constituting block are aligned in a smectic phase giving rise to birefringence. The alignment of the nanodomians improves the charge conduction toward the electrodes. These findings are very promising for use in optoelectronic devices.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral