Template Replication of Nanomaterials

Open Access
Author:
Hall, Anthony Shoji
Graduate Program:
Chemistry
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
May 08, 2014
Committee Members:
  • Tom Mallouk, Dissertation Advisor
  • Raymond Edward Schaak, Committee Member
  • John V Badding, Committee Member
  • Douglas Henry Werner, Special Member
Keywords:
  • Plasmonics
  • Porous Materials
  • Replication
  • Metal Organic Frameworks
  • Light Trapping
  • Nanocasting
  • Photonic Crystals
Abstract:
This thesis focuses on one central theme, the template replication of nanomaterials. The first half of this thesis focuses on studying new properties that emerge in nanostructured materials prepared by existing template replication strategies. The second half of this thesis focuses on the development of novel strategies to synthesize nanomaterials by replication of nanoscale templates. Chapter 1 of this thesis presents an introduction to the synthesis of nanomaterials by template replication. Top down and bottom up approaches are covered in this chapter. Chapter 2 of this thesis presents the different techniques used to characterize the materials studied in this thesis. Chapter 3 presents the optical properties of one-dimensional metallic gratings coupled to a one-dimensional photonic crystal. Light incident upon a periodically corrugated metal/dielectric interface can generate surface plasmon-polariton (SPP) waves. This effect is used in many sensing applications. Similar metallodielectric nanostructures are used for light trapping in solar cells, but the gains are modest because SPP waves can be excited only at specific angles and with one linear polarization state of incident light. Here we report the optical absorptance of a metallic grating coupled to silicon oxide/oxynitride layers with a periodically varying refractive index, i.e., a 1D photonic crystal. These structures show a dramatic enhancement relative to those employing a homogeneous dielectric material. Multiple SPP waves can be activated, and both s- and p-polarized incident light can be efficiently trapped. Many SPP modes are weakly bound and display field enhancements that extend throughout the dielectric layers. These modes have significantly longer propagation lengths than the single SPP modes excited at the interface of a metallic grating and a uniform dielectric. These results suggest that metallic gratings coupled to photonic crystals could have utility for light trapping in photovoltaics, sensing, and other applications. The metallic gratings used in this study were prepared by template replication process. Chapter 4 in this dissertation presents data which demonstrates that the resonance frequency of multiple SPP waves can be tuned by varying the periodicity of the metallic grating. In this study only p-polarized incident light was considered in the visible and near-infrared regimes. When the absorptance was plotted against the angle of incidence, the excitation of an SPP wave was indicated by an absorptance peak whose angular location did not change with the number of periods (beyond a threshold) of the photonic crystal. A decrease in the period of the metal grating resulted in shifting the excitation of the SPP waves to smaller wavelengths. The metallic gratings used in this study were prepared by template replication process. Chapter 5 in this dissertation presents a new method of fabricating wafer scale metallic gratings. By combining nanosphere lithography with template stripping, silicon wafers were patterned with hexagonal arrays of nanowells or pillars. These silicon masters were then replicated in gold by metal evaporation, resulting in wafer-scale hexagonal gratings for plasmonic applications. In the nanosphere lithography step, two-dimensional colloidal crystals of 510 nm diameter polystyrene spheres were assembled at the air-water interface and transferred to silicon wafers. The spheres were etched in oxygen plasma in order to define their size for masking of the silicon wafer. For fabrication of metallic nano-pillar arrays, an alumina film was grown over the nanosphere layer and the spheres were then removed by bath sonication. The well pattern was defined in the silicon wafer by reactive ion etching in a chlorine plasma. For fabrication of metal nano-well arrays, the nanosphere monolayer was used directly as a mask and exposed areas of the silicon wafer were plasma-etched anistropically in SF6/Ar. Both techniques could be used to produce sub-wavelength metal replica structures with controlled pillar or well diameter, depth and profile, on the wafer scale, without the use of direct writing techniques to fabricate masks or masters. Chapter 6 presents a new method of preparing microporous titania particles replicated from Metal-organic frameworks (MOFs) templates. Metal-organic frameworks provide access to structures with nanoscale pores, the size and connectivity of which can be controlled by combining the appropriate metals and linkers. Microporous titania replicas were made from the MOF template HKUST-1 by dehydration, infiltration with titanium isopropoxide, and subsequent hydrothermal treatment at 200 °C. Etching of the MOF with 1M aqueous HCl followed by 5% H2O2 yielded a titania replica that retained the morphology of the parent HKUST-1 crystals and contained partially ordered micropores as well as disordered mesopores. Interestingly, the synthesis of porous titania from the HKUST-1 template stabilized the formation of brookite, a rare titania polymorph.