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.
The optical properties of ZnO has been widely investigated in detail. Typical photoluminescence (PL) of ZnO contains two parts of emission: near bandgap transition induced ultraviolet emission, and a relatively wide visible emission ranging from green to red, which is closely related to concentration of the structural defects. While the green luminescent has been reported to be associated with oxygen vacancies Vo. In this work, we report on an efficient technique namely desulfurization to increase the amount of oxygen vacancy in a ZnO nanowires array. In the case of the desulfurized sample the PL is increased by more than 1 order of magnitude as to compare with the sulfurized one and more than 2 orders of magnitude as to compare with the as grown sample. Structural analysis as well as morphological analysis confirm the origin of the green band emission enhancement in PL emission. Samples preparation as well an in-depth analysis including quantum efficiency will be presented and discussed within the frame of new rare-earth free phosphor material.
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.
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.
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.
We discuss here different strategies for making arrays of Au nanoparticles using copolymer templates. Top-down and
bottom-up routes are considered and the optical properties of as-prepared Au nanoparticles are discussed and compared
to numerical simulations. Potential for applications such as biosensors or strain sensors is also assessed.