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See detailLarge-scale and Flexible Nanogenerator based on ZnO conical nanostructures by Nano-Imprint Lithography and Atomic Layer Deposition
Spirito, David Mario André UL

Doctoral thesis (2018)

The present work stands in the context of the rapid growth of portable electronics and wireless sensors. This growth drives the request for advances in materials science and technology to harvest energy ... [more ▼]

The present work stands in the context of the rapid growth of portable electronics and wireless sensors. This growth drives the request for advances in materials science and technology to harvest energy from ambient sources. Piezoelectric nanogenerators, which convert vibrations into electrical energy, are considered as one of the promising building blocks for the design of low-cost and performant energy harvesting devices. Several demonstrations of PENGs have been reported throughout the last decade, many of them based on Zinc Oxide –ZnO– nanowires. Despite interesting performances, literature also suggests that the classical bottom-up approach of optimized ZnO-based PENGs performance approaches its limits and that top-down approaches merit further attention. This has motivated the present work, with the aim to conceptualize by models, to fabricate and to investigate a new type of piezoelectric nanogenerator based on ZnO nanostructures to overcome current limitations. To achieve this goal, we have used a top-down approach that allows an accurate control of the aspect ratio and density of tailored ZnO nanowires by using Nano-Imprint Lithography and Atomic Layer Deposition. In our work, we demonstrate that this approach enables the fabrication of large flexible piezoelectric nanogenerators with interesting properties. In the first part of this work, we have optimized the synthesis of crystalline N-doped ZnO films by ALD at a deposition temperature as low as T = 80 °C. We have particularly investigated the role of the time of purge with nitrogen as purge gas in each cycle of a ZnO monolayer of the ALD process. A thorough chemical and structural analysis illustrates that the time of purge allows tuning the N-doping-level which, despite being low, affects both the long-range and short-range structure. Raman and luminescence spectroscopy suggest a complex defect structure, characterized by nitrogen ions which substitute oxygen ions and by Zn cations on interstitial sites. Importantly, even the low level of nitrogen doping allows tuning the sheet resistivity of ZnO films by several orders of magnitude. The ability to obtain crystallized and tunable N-doped ZnO films down to 80 °C by ALD provides a critical building block to tune structural, optic and electric properties for a variety of applications. In the second part, we have designed, fabricated and characterized a new type of PENGs based on patterned nanostructures made of conical-shape ZnO pillars. First, we have used a finite element modeling to identify the optimization for the electromechanical performances of the ZnO nanowires, namely in terms of their aspect ratio and pitch. This has defined the stamps of the nano-imprint, which has then be combined with a low temperature conformal ALD to provide ZnO conical nanostructures. A thorough structural analysis of such nanostructures attests a high crystallinity, a polycrystalline growth and piezoelectric properties. This has been the necessary technological achievement for addressing in a next step functional patterned piezoelectric nanogenerators. We have produced small flexible devices with an active area of 4 x 4 mm2, using either a blocking electronic barrier with alumina, either a p-n junction with a conductive polymer (PEDOT:PSS). The different devices and architectures have then been characterized at matching impedance. The electric characterization of a device with a p-n junction exhibits a maximum output voltage of 0.2 V and a power density of 0.3 µW cm-2. An effective transverse piezoelectric coefficient value e31eff of -0.45 C m-2 is determined, which corresponds to the order of magnitude reported in literature. As a proof of concept for potential industrialization, we scaled the P-PENGs up to 20 cm² for large flexible substrates. The 3.7 billion pillars, sandwiched between electrodes, evidence the robustness of our process. The analysis of the constitutive piezoelectric equations has prompted us to pay a particular attention to an accurate setup for P-PENGs characterization, which turns out to be mandatory to compare accurately devices. For this, we setup an electromechanical actuator. Two devices with the same architecture but with different levels of N-doping are compared. Using an equivalent electrical model, the performance of the P-PENG with the higher N-doped ZnO shows a larger output voltage and power density than the device with a lower N-doping level. This comparison shows that the higher N-doping-level leads to an increase of 150 % of the power output. From this we calculate that the effective piezoelectric coefficient increases by about 60 % for a P-PENG based on higher N-doped ZnO. This latter demonstrates an energy conversion efficiency of 10 %, on top of PENG based on ZnO. [less ▲]

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