title: Metal Nanohole Arrays : From Self-Assembly to Light-Harvesting
Creators: name: Stelling, Christian
Publisher: name: University of Bayreuth
Year: 2019
Subjects: subject: Colloids
subject: Self-Assembly
subject: Nanosphere Lithography
subject: Colloidal Crystals
subject: Surface Plasmon Polaritons
subject: 500
subject: 530
subject: 540

description: This thesis is dedicated to progress in the field of colloidal self-assembly and colloidal crystals as well as their potential application for the preparation of transparent, conducting electrodes. Colloidal crystals are periodic structures formed by particles with dimensions between 1 nm and 1 µm and are widely used for the preparation of functional surfaces with tailored physical properties. Their application requires the fast production of large-area, homogeneous structures with nanometer-sized features and minimal defect-densities. Colloidal self-assembly meets these expectations and offers a rapid and cost-effective fabrication of periodic patterns by employing the intrinsic properties of the single particles.
This thesis is divided into three parts, which present contributions to (I) the defined self-assembly of polymer particles and the properties of particle monolayers (chapters 3 - 5), (II) the directed self-assembly of polymer colloids into gold nanohole arrays (chapter 6), and (III) the fabrication of optical devices based on gold nanohole arrays (chapters 7 - 9). For this purpose, the self-assembly of spherical polystyrene particles with diameters between 0.1 µm and 2.5 µm was induced at the water/air interface, which yields purely two-dimensional colloidal crystals with large single-crystalline domains.
In chapter 3, I analyzed the optical properties of colloidal monolayers with subwavelength-scale particle diameters (&lt; 0.2 µm). These monolayers can be regarded as an effective medium and can act as single-layer antireflective coatings. Thereby, the effective refractive index of the coating and the wavelength of maximum transmittance can be adjusted independently. This supplements the optical characterization of colloidal particle arrays with the properties of monolayers with subwavelength-sized particles.
Based on the exceptionally high long-range order of the colloidal crystal, collective vibrational modes were detected for the first time across the entire Brillouin zone in a monolayer of large polystyrene spheres (1.5 µm) (chapter 4). An analytical model was developed, which well describes the experimental results, taking into account the particle-particle and particle-substrate contacts.
Usually, colloidal assembly methods yield hexagonal, close-packed particle monolayers limiting the variety of structures attainable with colloidal self-assembly. In chapter 5, I demonstrated the fabrication of non-close-packed particle arrays with symmetries of all possible two-dimensional Bravais lattices starting from hexagonal, close-packed monolayers floating at the water/air interface. As the monolayers are purely one-dimensionally stretched upon transfer onto hydrophobic substrates, this presents a scalable method for the preparation of colloidal crystals with arbitrary lattice symmetry.
Via colloidal lithography, the structures generated by colloidal particles can be replicated into various materials. In chapter 6, I used gold nanohole arrays, prepared by colloidal lithography, as templates for the directed self-assembly of polymer colloids. Exploiting a tailored surface charge contrast with feature sizes in the range of the single particles, negatively charged polymer particles were site-selectively trapped from the bulk dispersion, forming non-close-packed particle monolayers. Thus, hierarchical structures are accessible with high structural control over large areas.
Nanohole arrays in thin metal films, prepared by colloidal lithography, offer a versatile platform for optically active surfaces that support surface plasmon polaritons (SPP). These SPP resonances, collective oscillations of the conduction band electrons, efficiently couple to incident light due to the periodic perforation of the metal film and are easily tunable by adjusting the nanoscale geometry. Therefore, nanohole arrays are highly attractive as an electrode material for solar cells.
A drawback of employing colloidal lithography for the fabrication of metal nanohole arrays is that it is limited to few substrate materials due to the inherent plasma etching step. By introducing a plasma-stable sacrificial layer, the transfer of nanohole arrays onto arbitrary substrates via the water/air interface was shown (chapter 7). This broadens the fabrication flexibility considerably and enables the preparation of plasmonic metal-insulator-metal multilayers and free-standing nanohole arrays.
The optical properties of these multilayers were drastically altered compared to the single layer nanohole array, which was in good agreement with numerical and analytical models (chapter 8). This detailed analysis of the optical effects occurring in nanoscale materials is essential for the specific manipulation of light in potential applications.
Finally, I integrated gold nanohole arrays into polymer solar cells as transparent, conducting electrodes (chapter 9). A cavity SPP, confined between the nanohole array electrode and the silver back-electrode, was observed and increased the power conversion efficiency at the absorption edge of the photoactive polymer. However, the nanohole arrays exhibited high losses due to reflection and absorption in the gold layer leading to an overall lower efficiency compared to indium tin oxide reference electrodes. Altogether, the interface-assisted self-assembly of colloidal polystyrene particles was used to prepare functional surface patterns on a macroscopic scale. This thesis revealed fundamental optical and acoustic properties of self-assembled colloidal crystals and considerably extended the range of structures attainable with colloidal self-assembly. Furthermore, the complex optical properties of gold nanohole arrays, integrated into metal-insulator-metal absorbers as well as organic solar cells, were investigated.