Gustavo Ardila

Photo-actuators coupled with mechanical energy harvesting systems feature a promising approach to solar energy conversion in the broad field of innovative energy harvesting technologies. In this study, we present a novel device capable of generating a 0.2 V peak-to-peak signal by sustaining the continuous oscillation of photomobile polymer (PMP) films in the range of 5-8 Hz when exposed to continuous steady illumination by effectively transferring mechanical motion to a piezoelectric transducer (PZL). An optomechanical energy-harvesting prototype based on PMPs was fabricated and experimentally characterized utilizing Xenon arc-lamp that closely mimics natural sunlight electromagnetic radiation. The analyzed harvesting system exhibits a distinctive dual-stage energy conversion process, from light to mechanical and subsequently from mechanical to electrical energy, highlighting its potential for diverse applications in next-generation solar energy systems.

Open-air manufacture of functional nanolaminates demands rapid and scalable methods that maintain nanoscale precision across individual layers. Conventional spatial atomic layer deposition (SALD) employs a single deposition head, and the concurrent exposure to mutually reactive precursors does not provide precise layer thickness accuracy nor sharp interfaces. This work addresses these challenges with an innovative multi-head SALD design. In this study, we introduced a novel multi-head SALD system comprising a uniform head for depositing 5 nm zinc oxide (ZnO) sublayers, while a combinatorial high-throughput head delivered wedge-like thickness gradient in aluminum oxide (Al₂O₃) sublayers, increasing gradually from 0 to approximately 6 nm along the substrate. Eight ZnO/Al₂O₃ bilayers (320 ALD cycles per material) were completed in open air at 200 °C. Transmission electron microscopy recorded highly sharp and reproducible layer-to-layer structures with thickness deviations of 0.3 nm; X-ray photoelectron spectroscopy verified the absence of undesirable aluminum contamination in the reference ZnO regions. With increasing Al₂O₃ thickness, surface roughness was reduced from 0.58 to 0.33 nm, ZnO (002) reflections broadened as crystallite size was confined, and Kelvin probe measurements showed a shift in work function values from 4.75 to 4.35 eV with Al₂O₃ passivation. This study demonstrates that the multi-head SALD method offers a robust platform for achieving precise multilayer control in open-air conditions, thereby paving the way for further innovations in materials research through optimized experimental conditions and combinatorial approaches.

As a biocompatible and non-critical material, ZnO, specifically in its nanowire morphology, holds great promise for integration into highly efficient mechanical energy transducers. However, controlling the density of the free charge carriers that drive the screening effect of the piezoelectric potential under mechanical solicitations is critical for enhancing the piezoelectric properties of ZnO nanowires. Hence, herein, the effects of several post-deposition treatments, including O₂ plasma, UV ozone, and thermal annealing under O₂ atmosphere, on the properties of ZnO nanowires grown via pulsed-liquid injection metal–organic chemical vapor deposition are thoroughly investigated and compared. The thermal annealing treatment at high temperature shows predominance over the other post-deposition treatments for the decrease in the density of free electrons (roughly estimated from 1.8–3.3 × 10¹⁸ to about 10¹⁷ cm⁻³ ), removal of carbon contamination inside the structure, and improvement in crystallinity. By proceeding with the thermal annealing treatment and increasing the temperature from 700 °C to 900 °C, time-resolved cathodoluminescence measurements further reveal the decrease in the density of surface traps from 7.7 to 3.0 × 10¹² cm⁻² owing to an increase in the amount of oxygen vacancies at the surfaces of the ZnO nanowires. The effective piezoelectric coefficient d^eff ₃₃, as measured by piezo-response force microscopy, eventually shows a significant enhancement of 47%, from 4.5 to 6.6 pm V⁻¹, as the annealing temperature and duration are increased. These findings reveal the trade-off to be optimized when using the post-deposition treatments, as supported by finite element method simulations, which shows that the reduction in the densities of free electrons and surface traps acts in an opposite manner on the piezoelectric response of the ZnO nanowires.

The question of the length dependence of the electromechanical response of semiconducting (SC) piezoelectric nanowires (NWs) was explored. We identified a new physical mechanism of piezoresponse saturation, which originates from the combination of the influence of interface traps and piezoelectric polarization in the depleted NW. Our results are in better qualitative agreement with experimental observations than presently existing theories. To do so, we used the finite element method to simulate the coupled set of equations describing mechanical, piezoelectric and SC properties. In order to reduce the number of parameters, simulations focused on the case of uniform ZnO NWs grown along the c-axis. Saturation was explained by the incapacity of surface traps to maintain depletion along the whole NW beyond a certain length, as a result of the electric potential shift induced by piezoelectric polarization. An analytical model was developed to support this analysis. It provided the dependence trends of saturation length and piezoresponse as a function of NW dimensions, doping level, surface traps density and crystal polarity, as well as with external pressure, in fair agreement with simulation results. Moreover, we discovered that one consequence of this mechanism was that crystal polarity had an impact on the smoothness of the radius-dependent transition between high and low piezoresponse under axial stress. These results have important implications for the optimization of electromechanical sensors and nanogenerators based on piezoelectric SC NWs and related composite materials.

ZnO nanowires are excellent candidates for energy harvesters, mechanical sensors, piezotronic and piezophototronic devices. These nanowires are usually non-intentionally n-doped during their growth. The essential role of doping, surface traps and surface Fermi level pinning in the actual response of piezoelectric semiconductors has already been demonstrated. In order to go further, this paper investigates the influence of the density and of the dynamics of surface traps on such important parameters as the output generated potential and the effective piezoelectric coefficient. We implemented numerical simulations based on the finite element method by combining the mechanical, piezoelectric, and semiconducting characteristic of ZnO nanowires array based nanocomposites (the so-called vertically integrated nanogenerator configuration) operated in compression. It was found that a certain amount of surface traps was required to obtain a usable generated output potential from the studied devices in the range of dimensions and doping level reported in most experimental results. Moreover, the surface traps influence was strongly dependent on their dynamics. As a first step towards the analysis of traps dynamics, we compared the two extreme cases of ultra-slow and ultra-fast traps. The symmetry and asymmetry of the piezoelectric response and a comparison to thin film was also discussed. This study demonstrates that the realistic modelling of the piezoelectric response of semiconductor based transducers should account for traps dynamics effects.

Zinc oxide (ZnO) nanogenerators have attracted increasing interest in the scientific community for use in energy harvesting and mechanical sensing applications. Understanding the interplay between piezoelectricity and semiconductor physics is fundamental to enhancing these devices’ performances, although direct characterization at the nanoscale is challenging. With this work, we present a new strategy to improve piezoresponse force microscopy (PFM) measurements and analysis. This strategy was applied to study the piezoelectric performances of ZnO nanowires grown on seed layers deposited by gravure printing onto flexible substrates. We demonstrate the influence of nanowire diameter and atomic force microscope (AFM) tip position on the piezoresponse amplitude. We also explain our results with simulations showing the importance of considering semiconducting properties in the analysis.

The formation process of ZnO thin films grown by pulsed-liquid injection metal–organic chemical vapour deposition (PLI-MOCVD) has a major impact on its morphological, structural, electrical and piezoelectric properties, but their correlation has not been elucidated yet nor decoupled from the thickness effects. In this work, we investigate the influence of the O₂ gas and diethylzinc (DEZn) solution flow rates on the properties of ZnO thin films with a given thickness and grown on silicon. We show that the O₂/DEZn flow rate ratio through the oxygen chemical potential significantly affects the O- and Zn-polarity domain distribution, their related size and shape, the chemical composition, and the incorporation of microstructural defects and residual impurities, resulting in a direct effect on the piezoelectric amplitude of ZnO thin films. In particular, the Zn-polarity domains are found to systematically exhibit a larger piezoelectric amplitude than the O-polarity domains, regardless of the O₂/DEZn flow rate ratio. A comprehensive description recapitulating the formation process of ZnO thin films through three different regimes depending on the O₂/DEZn flow rate ratio is further gained. These results demonstrate the crucial ability of the PLI-MOCVD system to tune the polarity of ZnO thin films along with other suitable properties for piezoelectric applications by carefully adjusting the O₂ gas and DEZn solution flow rates.