Atomic layer deposition of metal oxides on self-assembled peptide nanofiber templates for fabrication of functional nanomaterials

Abstract

There are mainly two basic approaches in nanostructured materials synthesis. The rst one is the top-down approach and requires material removal from a bulk substrate material by chemical, physical, mechanical or thermal means; acid etching, focused ion milling, and laser ablation are among these top-down synthesis techniques. It is a straightforward { albeit poor in material architecture control { method that has established its niche in today's high-volume CMOS transistor fabrication technology which already produces single-digit nanometerscale device features. On the other hand, bottom-up approach exploits ne-tuned materials assembly. Bottom-up approach is realized via direct self-assembly of target nanostructures or material growth on synthetic or natural nanotemplates. Bottom-up nanostructured materials synthesis o ers considerably wider spectrum of achievable material architectures and structural hierarchies. Synthesis of nanostructured materials on self-assembled soft nanotemplates is of signi cant importance because many biological systems utilize this very similar approach to construct complex biomolecule-templated materials. Peptide amphiphile (PA) molecules with their intrinsic property to self-assemble into nanostructures such as bers, present a versatile tool in inorganic material templating. PAs were used as soft templates in several studies for fabrication of nanoscale inorganic materials. Most of these studies are focused on in-solution material deposition on the surface of a template. Even though this approach allows successful material deposition, precise control over material thickness, uniformity, and high conformality is di cult to achieve in a repeatable manner. In order to circumvent this challenge, in this thesis, atomic layer deposition (ALD) technique was deployed for conformal coating of PA nanonetwork templates. ALD involves low-temperature iterative vapor-phase material deposition in a self-limiting fashion. In each deposition half-cycle, Ti- or Zn- containing volatile metalorganic complexes form a self-limiting uniform monolayer that consequently reacts with water vapor (H2O) as an oxygen precursor in the subsequent process half-cycle. As each half-cycle is separated with purge cycles, no gas-phase reactions occurs and material growth proceeds only with surface chemical ligand-exchange reactions. ALD approach allowed obtaining TiO2 or ZnO nanonetworks with tunable wall thickness and ultimate conformality. Obtained metal oxide-peptide hybrid materials were further treated di erently. In the case of TiO2, organic template was removed upon calcination at 450 C, a temperature at which amorphous titania transforms to anatase form. ZnO-peptide hybrid materials on the other hand, did not undergo any thermal processing, as ZnO already grows in wurtzite crystalline form during ALD process. In principle, nanostructured anatase TiO2 and wurtzite ZnO are wide bandgap semiconductors which can be used as photoanode materials. Nanostructured anodic materials still attract a great interest as the matter at nanoscale regimes can provide considerable enhancement in charge carrier separation, charge carrier transport, and active surface area. Here we demonstrate the fabrication of nanostructured TiO2 and ZnO on self-assembled soft templates. As a proof of principle, we utilized semiconducting TiO2 and ZnO in assembly of dye sensitized solar cells and studied material thickness e ect on device performance parameters such as open circuit voltage (Voc), short circuit current (Jsc), and ll factor. Three sets of nanostructured photoanodes with di erent TiO2 deposition cycles (100, 150, and 200) and ZnO deposition cycles (100, 125 and 150) were fabricated. TiO2 and ZnO nanonetworks in photoanodes form a system of interconnected nanotubes, which can facilitate electron transfer. Moreover, these networks are porous high-surface area materials and they can drastically increase number of sensitizer molecules attached to the semiconductor material surface.