The piezoelectric nanowires (NWs) are considered as promising nanomaterials to develop high-efficient piezoelectric generators. Establishing the relationship between their characteristics and their piezoelectric conversion properties is now essential to further improve the devices. However, due to their nanoscale dimensions, the NWs are characterized by new properties that are challenging to investigate. Here, we use an advanced nano-characterization tool derived from AFM to quantify the piezo-conversion properties of NWs axially compressed with a well-controlled applied force. This unique technique allows to establish the direct relation between the output signal generation and the NW stiffness and to quantify the electromechanical coupling coefficient of GaN NWs, which can reach up to 43.4%. We highlight that this coefficient is affected by the formation of the Schottky nano-contact harvesting the piezo-generated energy, and is extremely sensitive to the surface charge effects, strongly pronounced in sub-100 nm wide GaN NWs. These results constitute a new building block in the improvement of NW-based nanogenerator devices.
The number of “Internet of Things” (IoTs), such as micro-devices (micro-sensors, small nomad electronic, medical implants…) is constantly on a rise both in our daily life and in high-tech applications. To deal with the critical increase of their associated energy consumption, but also to improve their condition of use, especially for those evolving in environment without electrical grid infrastructure or with restricted one, the question of their energetic autonomy is today a key worldwide challenge with strong economic and environmental repercussions.
With the recent miniaturization of the electronic devices resulting in the reduction of their energy consumption to mW and even µW, and thank to the micro-nano-fabrication progress, new perspectives appear to develop autonomous power systems based on the renewable energy harvesting.
Piezoelectric GaN NWs are promising nanostructures for fabricating efficient piezo-nanogenerators having the ability to convert mechanical deformations and vibrations into usable electrical energy. Thanks to their nanoscale dimensions, their large surface-to-volume ratio and their quasi-lattice perfection, NWs, in comparison with their bulk and 2D-film counterparts, are characterized by : 1) superior mechanical properties; 2) higher sensitivity to applied force; 3) higher piezoelectric coefficients; and 4) the apparition of novel properties, non-existing or non-significant at micrometric scales that can lead to a strong modulation of their characteristics. However, at the nanoscale, it is challenging to characterize these properties and thus fully take advantage of them to improve the performances of the NW-based devices.
We use an advanced nano-characterization tool derived from AFM, developed by colleagues from the Materials Team at GeePs, to quantify the piezo-conversion properties of NWs axially compressed with a well-controlled applied force. This unique technique allows deforming axially the NWs and then establishing the direct relation between their output signal generation and their dimensions. It also makes it possible to deform the NWs similarly to the ones integrated into the devices.
Then, we have highlighted the relationship between the NW deformation and their piezo-conversion properties. We have also demonstrated that the coupling coefficient is not only driven by the mechanical characteristics of the NWs. Two other parameters, often neglected, play a crucial role: 1) the formation of the Schottky nano-contact allowing increasing the piezo-generated energy harvesting; and 2) the strong influence of the surface charge effects, which under given conditions, are able to remarkably enhanced the electromechanical conversion efficiency of the GaN NWs.
Reference
Electromechanical conversion efficiency of GaN NWs: critical influence of the NW stiffness, the Schottky nano-contact and the surface charge effects
Noelle Gogneau1*, Pascal Chrétien2, Tanbir Sodhi1,2, Laurent Couraud1, Laetitia Leroy1, Laurent Travers1, Jean-Christophe Harmand1, François H. Julien1, Maria Tchernychevaa1, and Frédéric Houzéb2
Nanoscale, 2022, 14, 4965
DOI : 10.1039/d1nr07863a
Affiliations
1 Centre de Nanosciences et Nanotechnologies, Université Paris-Saclay, CNRS, UMR9001, Boulevard Thomas Gobert, 91120 Palaiseau, France
2 Université Paris-Saclay, CentraleSupélec, Sorbonne Université, CNRS, Laboratoire de Génie électrique et électronique de Paris, 11 rue Joliot-Curie, 91190 Gif sur Yvette, France
Figure : GaN nanowires for an efficient electromechanical conversion
