Komla Nomenyo

Highly crystalline urchin-like structures made of ultra-thin zinc oxide nanowires

We report the synthesis of ultra-thin, monocrystalline, highly luminescent ZnO nanowires (NWs) on polystyrene (PS) beads as templates. The synthesis of these NWs on PS beads was conducted by the chemical bath deposition technique in the absence of any catalysts or additives. They have an average diameter of 15 nm (depending on the concentration of the solution) and an average length of 500 nm. Structural characterization reveals that these NWs are monocrystalline, with a hexagonal phase and grow along the [0001] direction. Photoluminescence measurements of these unannealed, ultra-thin NWs exhibit a strong ultra-violet emission at room temperature with an internal quantum efficiency of 23%. We show that the concentration of the aqueous solution plays a key role in controlling the size of the NWs.

High order symmetry interference lithography based nanoimprint

We report on soft nanoimprint lithography using masters obtained by high order symmetry interference lithography. The use of high order symmetry leads to the formation of three-dimensional structures with features smaller than 40 nm. Masters were realized in silicon in a two-step process without transfer layer. Pure silicon masters allow mechanical stability and potential surface functionalization. We further demonstrate the ability of these masters as mold for nanoimprint lithography. High fidelity replication in hybrid sol-gel and pure silica with conservation of both minute features and long distance organization is observed over large areas.

Topology assisted self-organization of colloidal nanoparticles: application to 2D large-scale nanomastering

Summary Our aim was to elaborate a novel method for fully controllable large-scale nanopatterning. We investigated the influence of the surface topology, i.e., a pre-pattern of hydrogen silsesquioxane (HSQ) posts, on the self-organization of polystyrene beads (PS) dispersed over a large surface. Depending on the post size and spacing, long-range ordering of self-organized polystyrene beads is observed wherein guide posts were used leading to single crystal structure. Topology assisted self-organization has proved to be one of the solutions to obtain large-scale ordering. Besides post size and spacing, the colloidal concentration and the nature of solvent were found to have a significant effect on the self-organization of the PS beads. Scanning electron microscope and associated Fourier transform analysis were used to characterize the morphology of the ordered surfaces. Finally, the production of silicon molds is demonstrated by using the beads as a template for dry etching.

Phenomenological modeling of light transmission through ZnO nanowires arrays (Conference Presentation)

Among alternative nanomaterials for energy related photonic applications, one-dimensional semiconductor nanowires are of a great interest due to their physical properties coming from electronic or quantum confinement. In particular, ZnO nanowires (or nanorods) has been widely investigated since ZnO has many unique properties such as wide direct band gap, large exciton binding energy and relatively high refractive index. Large optical gain also makes ZnO a well suited material for energy transfer in hybrid systems and especially optical energy transfer. There are however two issues remaining to be addressed, one is related to the control in size and dispersion in nanowires array and the other is related to the modeling of nanowires arrays. In this study, we report on a theoretical study on ZnO nanowires, in order to reach a better understanding of the mechanisms that govern the light propagation in nanowires arrays. A phenomenological model has been developed and discussed. The model is able to describe the experimentally measured light transmission nanowires arrays. A slab of nanospheres and rough layers with thickness waviness were combined to simplify the nanowires structure description. This phenomenological description was proved to be feasible by fitting the experimental data. As a conclusion, light transmitted by randomly distributed nanowires can be explained by the combination of Mie theory and a rough Fresnel reflection at the interfaces.

ZnO top-down structuring for UV photonic applications(Conference Presentation)

ZnO is a promising II-VI semiconductor for UV applications although p-type ZnO is not yet available. Nevertheless it remains an alternative material for GaN and its alloy InGaN. For example, the exciton binding energy of ZnO (60 meV) is higher than that of GaN (21 meV). This allows ZnO to emit light at ambient temperature and interestingly, it increases the device brightness. Besides promising intrinsic properties, light-matter control and especially in the UV relies on the ability of material nanostructuring. We present here two different kinds of top-down process in order to nanostructure ZnO. The first one relies on Electron Beam Lithography (EBL) combined with a lift-off process and inductively coupled plasma (ICP) reactive ion etching (RIE). Nickel (Ni) has been used as a mask in order to have a high selectivity in the presence of C2F6 and O2 ionized gases. The etching rate used was 26nm/s in order to avoid roughness. The second process is called Direct Holographic Patterning (DHP). ZnO thin films have been holographicaly patterned for the first time by direct photodissolution in NaCl solution using laser interference lithography. Application of an electrical potential strongly increases the dissolution rate and decreases the pattern formation time. Both processes will be discussed in terms of their respective potential for light confinement in the UV.

Enhanced luminescence excitation via efficient optical energy transfer (Presentation Recording)

Luminescent nanoscale materials (LNMs) have received widespread interest in sensing and lighting applications due to their enhanced emissive properties. For sensing applications, LNMs offer improved sensitivity and fast response time which allow for lower limits of detection. Meanwhile, for lighting applications, LNMs, such as quantum dots, offer an improved internal quantum efficiency and controlled color rendering which allow for better lighting performances. Nevertheless, due to their nanometric dimensions, nanoscale materials suffer from extremely weak luminescence excitation (i.e. optical absorption) limiting their luminescence intensity, which in turn results in a downgrade in the limits of detection and external quantum efficiencies. Therefore, enhancing the luminescence excitation is a major issue for sensing and lighting applications. In this work, we report on a novel photonic approach to increase the luminescence excitation of nanoscale materials. Efficient luminescence excitation increase is achieved via a gain-assisted waveguided energy transfer (G-WET). The G-WET concept consists on placing nanoscale materials atop of a waveguiding active (i.e. luminescent) layer with optical gain. Efficient energy transfer is thus achieved by exciting the nanoscale material via the tail of the waveguided mode of the active layer emission. The G-WET concept is demonstrated on both a nanothin layer of fluorescent sensitive polymer and on CdSe/ZnS quantum dots coated on ZnO thin film, experimentally proving up to an 8-fold increase in the fluorescence of the polymer and a 3-fold increase in the luminescence of the CdSe/ZnS depending of the active layer emission regime (stimulated vs spontaneous emission). Furthermore, we will discuss on the extended G-WET concept which consists on coating nanoscale materials on a nanostructured active layer. The nanostructured active layer offers the necessary photonic modulation and a high specific surface which can presumably lead to a more efficient G-WET concept. Finally, the efficiency as well as the observation conditions of the GWET will be discussed and compared with more conventional charge transfer or dipole-dipole energy transfer.

Highly efficient excitonic emission of CBD grown ZnO micropods (Presentation Recording)

Due to its wide direct band gap and large exciton binding energy allowing for efficient excitonic emission at room temperature, ZnO has attracted attention as a luminescent material in various applications such as UV-light emitting diodes, chemical sensors and solar cells. While low-cost growth techniques, such as chemical bath deposition (CBD), of ZnO thin films and nanostructures have been already reported; nevertheless, ZnO thin films and nanostructures grown by costly techniques, such as metalorganic vapour phase epitaxy, still present the most interesting properties in terms of crystallinity and internal quantum efficiency. In this work, we report on highly efficient and highly crystalline ZnO micropods grown by CBD at a low temperature (< 90°C). XRD and low-temperature photoluminescence (PL) investigations on as-grown ZnO micropods revealed a highly crystalline ZnO structure and a strong UV excitonic emission with internal quantum efficiency (IQE) of 10% at room temperature. Thermal annealing at 900°C of the as-grown ZnO micropods leads to further enhancement in their structural and optical properties. Low-temperature PL measurements on annealed ZnO micropods showed the presence of phonon replicas, which was not the case for as-grown samples. The appearance of phonon replicas provides a strong proof of the improved crystal quality of annealed ZnO micropods. Most importantly, low-temperature PL reveals an improved IQE of 15% in the excitonic emission of ZnO micropods. The ZnO micropods IQE reported here are comparable to IQEs reported on ZnO structures obtained by costly and more complex growth techniques. These results are of great interest demonstrating that high quality ZnO microstructures can be obtained at low temperatures using a low-cost CBD growth technique.