Pierre Gaffuri

Estelle Appert

Odette Chaix-Pluchery

Laetitia Rapenne

Mathieu Salaün

The Path of Gallium from Chemical Bath into ZnO Nanowires: Mechanisms of Formation and Incorporation

ZnO nanowires grown by chemical bath deposition (CBD) are of high interest, but their doping with extrinsic elements including gallium in aqueous solution is still challenging despite its primary importance for transparent electrodes and electronics, as well as mid-infrared plasmonics. We elucidate the formation mechanisms of ZnO nanowires by CBD using zinc nitrate and hexamethylenetetramine as standard chemical precursors, as well as gallium nitrate and ammonia as chemical additives. A complete growth diagram, revealing the effects of both the relative concentration of gallium nitrate and pH, is gained by combining a thorough experimental approach with thermodynamic computations yielding theoretical solubility plots as well as Zn(II) and Ga(III) species. The role of Ga(OH)4 - complexes is specifically shown as capping agents on the m-plane sidewalls of ZnO nanowires, enhancing their development and hence decreasing their aspect ratio. Additionally, the gallium incorporation into ZnO nanowires is investigated in details by chemical analyses and Raman scattering.

They show the predominant formation of gallium substituting for zinc atoms (GaZn) in as-grown ZnO nanowires and their partial conversion into GaZn-VZn complexes after post-deposition annealing under oxygen atmosphere. The conversion is further related to a significant relaxation of the strain level in ZnO nanowires. These findings reporting the physico-chemical processes at work during the formation of ZnO nanowires and the related gallium incorporation mechanisms offer a general strategy for their extrinsic doping and open the way for carefully controlling their physical properties as required for nanoscale engineering devices.

INTRODUCTION

As a low-cost, low-temperature, and easily implemented process, chemical bath deposition (CBD) has received, over the last fifteen years, an overwhelming interest to produce spontaneously-grown vertically aligned ZnO nanowires (NWs) with a controlled structural morphology over a large number of substrates. [START_REF] Xu | One-Dimensional ZnO Nanostructures : Solution Growth and Functional Properties[END_REF] Its additional combination with advanced lithography techniques has further shown an efficient way to form well-ordered ZnO NWs with a great structural uniformity on pre-patterned nucleation surfaces using selective area growth. [START_REF] Wei | Wafer-Scale High-Throughput Ordered Growth of Vertically Aligned ZnO Nanowire Arrays[END_REF][START_REF] Consonni | Selective Area Growth of Well-Ordered ZnO Nanowire Arrays with Controllable Polarity[END_REF] If the structural morphology of ZnO NWs is of high significance for enhancing the performances of the related nanoscale engineering devices, their extrinsic doping by using additional elements substituting for zinc or oxygen sites during CBD is crucial as well to carefully control their electronic, optical, and electrical properties (i.e. electron density, mobility, …). However, it has not been addressed in details yet despite its primary importance on the performances of the nanoscale engineering devices.

Joo et al. added ancillary metal sulfates forming ion complexes in the chemical bath, namely A-type (e.g. Cd, Cu, Mg, Ca) and B-type ions (e.g. Al, In, Ga) to favor attractive electrostatic interactions with the polar c-plane top facet and nonpolar m-plane sidewalls at the pH of 11, respectively. [START_REF] Joo | Face-Selective Electrostatic Control of Hydrothermal Zinc Oxide Nanowire Synthesis[END_REF] The aspect ratio of ZnO NWs was accordingly varied in the broad range of 0.1-100 in that way, but a possible incorporation and doping was excluded owing to the use of a low growth temperature. In the framework of that approach, Verrier et al. subsequently showed that the adsorption of Al(OH)4 -complexes as capping agents on the sidewalls of ZnO NWs results in the further incorporation of Al in their center following a thermal annealing from the low temperature of 200 °C. [START_REF] Verrier | Tunable Morphology and Doping of ZnO Nanowires by Chemical Bath Deposition Using Aluminum Nitrate[END_REF] The Al incorporation process was additionally found to be strongly related to the pH and a comprehensive diagram was reported as an efficient strategy to dope ZnO NWs by CBD. [START_REF] Verrier | Effects of the PH on the Formation and Doping Mechanisms of ZnO Nanowires Using Aluminum Nitrate and Ammonia[END_REF] Alternatively to Al, the Ga incorporation into ZnO NWs and its related doping have emerged as highly promising for a variety of nanoscale engineering devices, especially in the fields of transparent electrodes for photovoltaics, transparent electronics, and mid-infrared plasmonics. In particular, the doping efficiency of Ga into ZnO is much larger than its Al counterpart because the ionic radius of Ga 3+ is closer to the Zn 2+ radius than the Al 3+ radius, opening the way for higher doping levels in ZnO. [START_REF] Gabás | Differences in N-Type Doping Efficiency between Al-and Ga-ZnO Films[END_REF] The evolution of the structural morphology of ZnO NWs grown by CBD, when Ga(NO3)3 is used as a chemical additive in the chemical bath, has been reported. [START_REF] Wang | Microstructural and Optical Characteristics of Solution-Grown Ga-Doped ZnO Nanorod Arrays[END_REF][START_REF] Escobedo-Morales | Defect Annihilation and Morphological Improvement of Hydrothermally Grown ZnO Nanorods by Ga Doping[END_REF][START_REF] Lee | Synthesis of Ga-Doped ZnO Nanorods Using an Aqueous Solution Method for a Piezoelectric Nanogenerator[END_REF][START_REF] Pineda-Hernández | Morphology Evolution of Hydrothermally Grown ZnO Nanostructures on Gallium Doping and Their Defect Structures[END_REF][START_REF] Kim | Effects of Doping with Al , Ga , and In on Structural and Optical Properties of ZnO Nanorods Grown by Hydrothermal Method[END_REF][START_REF] Park | Effects of In or Ga Doping on the Growth Behavior and Optical Properties of ZnO Nanorods Fabricated by Hydrothermal Process[END_REF][START_REF] Park | Growth Behavior and Electrical Performance of Ga-Doped ZnO Nanorod/p-Si Heterojunction Diodes Prepared Using a Hydrothermal Method[END_REF][START_REF] Lim | Synergistic Effect of Indium and Gallium Co-Doping on Growth Behavior and Physical Properties of Hydrothermally Grown ZnO Nanorods[END_REF][START_REF] Li | Effects of Gax Zn1-xO Nanorods on the Photoelectric Properties of N-ZnO Nanorods/p-GaN Heterojunction Light-Emitting Diodes[END_REF][START_REF] Saravanakumar | Doping-Induced Electron Density Modification at Lattice Sites of ZnO:Ga Nanostructures: Effects on Vibrational and Optical Properties[END_REF][START_REF] Singh | Resistive Switching Behavior of Ga Doped ZnO-Nanorods Film Conductive Bridge Random Access Memory[END_REF] However, some of the findings are in apparent discrepancy because the critical parameters including the pH and Ga(NO3)3 concentration in the chemical bath as well as the Ga content into the ZnO NWs have not systematically been measured, nor varied on a broad range, nor combined with theoretical thermodynamic simulations. Recently, Lim et al. and Park et al. showed a decrease in the c-lattice parameter of ZnO NWs and an increase in their mean diameter when the Ga(NO3)3 concentration in the chemical bath was increased. [START_REF] Park | Growth Behavior and Electrical Performance of Ga-Doped ZnO Nanorod/p-Si Heterojunction Diodes Prepared Using a Hydrothermal Method[END_REF][START_REF] Lim | Synergistic Effect of Indium and Gallium Co-Doping on Growth Behavior and Physical Properties of Hydrothermally Grown ZnO Nanorods[END_REF] Conversely, Li et al. observed no effect on the c-lattice parameter, and a decrease in the mean diameter of ZnO NWs. [START_REF] Li | Effects of Gax Zn1-xO Nanorods on the Photoelectric Properties of N-ZnO Nanorods/p-GaN Heterojunction Light-Emitting Diodes[END_REF] The pH has not been reported in those three papers. In contrast, Pineda-Hernández et al. and Saravanakumar et al. investigated the spontaneous growth of ZnO nanostructures by CBD at a given pH of 12 following the addition of NaOH. [START_REF] Pineda-Hernández | Morphology Evolution of Hydrothermally Grown ZnO Nanostructures on Gallium Doping and Their Defect Structures[END_REF][START_REF] Saravanakumar | Doping-Induced Electron Density Modification at Lattice Sites of ZnO:Ga Nanostructures: Effects on Vibrational and Optical Properties[END_REF] They showed the shift of Raman lines in the spectra of wurtzite ZnO, as well as the occurrence of Ga related additional modes (AMGa). Those observations have been achieved on ZnO NWs homogeneously nucleated in the chemical bath, instead of vertically aligned ZnO NWs heterogeneously nucleated onto a substrate covered with a ZnO seed layer. In these papers, the regular additional modes (AMs) commonly observed in Ga-doped ZnO thin films [START_REF] Bundesmann | Raman Scattering in ZnO Thin Films Doped with Fe[END_REF] have surprisingly not been reported either.

In this article, the [Ga(NO3)3] / [Zn(NO3)2] ratio and the pH of the solution are systematically varied over a broad range of 0-10 % and 7.1-11.3, respectively, to elucidate their effects on the structural morphology of ZnO NWs. Speciation diagrams and theoretical solubility plots of Zn(II) and Ga(III) species are calculated to account for the experimental data. Moreover, the Ga incorporation into ZnO NWs is thoroughly investigated by energy-dispersive x-ray spectroscopy (EDS) analyses and temperature-dependent Raman scattering. The findings result in a comprehensive diagram reporting the physico-chemical processes in aqueous solution and the basic mechanisms during post-deposition annealing to simultaneously engineer the shape and dimensions of ZnO NWs while incorporating a given Ga content in their center.

METHODS

Deposition techniques. ZnO NW arrays were grown on c-axis oriented polycrystalline ZnO seed layers deposited on a (001) silicon wafer, using a two-step chemical synthesis process. The polycrystalline ZnO seed layers were deposited by the sol-gel process using dip coating. To form the sol, an equimolar mix of 375 mM of both zinc acetate dihydrate (Zn(CH3COO)2.2H2O, Emsure ACS) and monoethanolamine (MEA, Sigma-Aldrich) was added in pure ethanol. The solution was heated and stirred at 60 °C for 12 h, and then at room temperature for 12 h more. To form the xerogel film, the silicon wafer was dipped in the solution and withdrew at the speed of 3.3 mm/s in ambient atmosphere with a hygrometry smaller than 15 %. The xerogel film was annealed in air on a hot plate kept at 300 °C for 1 h to evaporate solvent and organic residues, and on another hot plate kept at 500 °C for 3 h to achieve its crystallization.

The growth of vertically aligned ZnO NWs was performed by CBD. The ZnO seed layer-coated silicon wafer was immersed face down in a sealed beaker containing an equimolar mix of 30 mM of zinc nitrate hexahydrate (Zn(NO3)2.6H2O, Sigma-Aldrich) and HMTA (C6H12N4, Sigma-Aldrich) in deionized water. The pH of the solution prior to heating (denoted here as pH0) was tuned by the addition of ammonia (NH3, VWR Chemicals). In order to extrinsically control the n-type doping of ZnO NWs, gallium nitrate hydrate (Ga(NO3)3.xH2O, Sigma-Aldrich) was used as a chemical additive with a concentration in the range of 0 to 3 mM. The sealed beaker containing the solution was placed in a regular oven heated at 85 °C for 3 h. Both the pH and temperature of the solution were monitored an in situ manner to follow their variation as CBD proceeds. The different CBD conditions used are summarized in Table 1. The first series of six samples consisted in achieving the CBD of ZnO NWs at a given [Ga(NO3)3] / [Zn(NO3)2] ratio of 5 % while varying the pH0 from 7.1 to 11.3 by the addition of NH3 with a concentration ranging from 11 to 1167 mM, respectively. The second series of six samples consisted in performing the CBD of ZnO NWs at a given pH0 of 10.9 using a NH3 concentration of 584 mM while varying the [Ga(NO3)3] / [Zn(NO3)2] ratio from 0 to 10 %. Ar + laser was used with a power on the sample surface lower than 3 mW. The laser beam was focused to a spot size of 1 μm 2 using a 50 times long working distance objective. The spectra were calibrated in wavenumber at room temperature by using a silicon reference sample and considering that the theoretical position of the silicon Raman line is set to 520.7 cm -1 . In situ post-deposition annealing was performed under an oxygen atmosphere from room temperature to 500 °C. The temperature was controlled using a commercial Linkam heating stage (THMS600) placed under the Raman microscope.

Thermodynamic Computations. Thermodynamic simulations were performed by using Visual MINTEQ software to determine the speciation diagrams of Zn (II) and Ga(III) species as well as the theoretical solubility plots of ZnO at 85 °C for each growth condition (i.e., with varying NH3, Zn(NO3)2, and Ga(NO3)3 concentrations). The two single metallic cations in aqueous solution (Zn 2+ and Ga 3+ ions) denoted as M x+ are capable of forming amine and/or hydroxide complexes with the two possible ligands (NH3 and HO -) denoted as L, according to the general reactions:

nM x+ + iL ↔ M n L i x+ ,

RESULTS AND DISCUSSION

Effects of the pH0 of the solution and of the Ga(NO3)3 Relative Concentration on the

Growth Mechanisms of ZnO Nanowires

Structural morphology of ZnO nanowires -ZnO NWs were grown on c-axis oriented polycrystalline ZnO seed layers with the same structural morphology. Their nucleation occurs on semipolar and polar c-plane ZnO nanoparticles composing the seed layer, [START_REF] Greene | General Route to Vertical ZnO Nanowire Arrays Using Textured ZnO Seeds[END_REF][START_REF] Cossuet | Ordered ZnO Nanowires with Controllable Inclination on Semipolar ZnO Surfaces by Chemical Bath Deposition[END_REF] which additionally transfer their O-or Znpolarity. [START_REF] Guillemin | Quantitative and Simultaneous Analysis of the Polarity of Polycrystalline ZnO Seed Layers and Related Nanowires Grown by Wet Chemical Deposition[END_REF] The morphology of ZnO NWs grown by CBD at a given [Ga(NO3 ZnO NWs grown at a pH0 of 7.1 have a very similar morphology as ZnO NWs grown by CBD using the standard chemical precursors (i.e., Zn(NO3)2 and HMTA) with no Ga(NO3)3. [START_REF] Parize | Effects of Hexamethylenetetramine on the Nucleation and Radial Growth of ZnO Nanowires by Chemical Bath Deposition[END_REF] They have a flat hexagonal tip and exhibit an aspect ratio of 12 ± 3 with an apparent number density around 60 µm -2 . In the pH0 intermediate range of 7.1-10.4, negligible heterogeneous growth proceeds, because the homogeneous growth in aqueous solution is strongly predominant. In the pH0 range of 10.4-11.0, all ZnO NWs have a pencil-like shape at their tip. From a pH0 of 10.4 to 10.9, the length and diameter of ZnO NWs strongly increase from 0.4 ± 0.15 µm to 3.1 ± 0.3 µm and from 90 ± 14 nm to 184 ± 13 nm, respectively. In contrast, their apparent number density strongly decreases from 170 ± 17 to 35 ± 4 µm - 2 , which is accounted for by highly pronounced coalescence effects. Correlatively, the deposited volume of ZnO NWs jumps from 0.4 ± 0.2 to 2.9 ± 0.7 µm 3 . As the pH0 is further increased from 10.9 to 11. The growth of ZnO NWs with the preferential elongated shape by CBD is governed by thermodynamic considerations related to surface energy minimization, but driven by kinetics.

Supersaturation is a key thermodynamic quantity to be determined because ZnO NWs typically develop over the metastable limit where both homogeneous and heterogeneous growths occur. Supersaturation was estimated here by the difference between the initial Zn(NO3)2 concentration of 30 mM and the equilibrium concentration of soluble Zn(II) species at 85 °C. In the low calculated pH range from 5.2 to 6.2 where most of the Zn(II) species are Zn 2+ ions in excess as compared to HO -ions acting as the limiting reactant following the hydrolysis of HMTA and NH3, only a few mM of NH3 drastically increase the pH and thus supersaturation is not straightforward to be monitored. Interestingly, supersaturation is found to progressively decrease in the calculated pH range of 8.0 to 9.5. In other words, as the pH0 is increased, the decrease in the supersaturation is favorable to strengthen the heterogeneous growth of ZnO NWs on the polycrystalline ZnO seed layer, which is at the expense of the homogeneous growth in aqueous solution. The present statement clearly accounts for the increase in Basically, the values of PZC and IEP strongly depend on the coordination of surface oxygen atoms and of their polarization by the zinc atoms, which are related to the nature of the crystallographic planes of ZnO. Since the concentration of H + ions needed for compensating the surface electrical charge is larger for the c-plane than for the m-plane, the values of PZC and IEP are expected to be higher for the mplane. The PZC was reported to be 10.2 ± 0.2 and 8.7 ± 0.2 for the nonpolar m-plane and polar c-plane, respectively. [START_REF] Joo | Face-Selective Electrostatic Control of Hydrothermal Zinc Oxide Nanowire Synthesis[END_REF][START_REF] Kunze | Self-Localization of Polyacrylic Acid Molecules on Polar ZnO(0001)-Zn Surfaces[END_REF][START_REF] Valtiner | Stabilization and Acidic Dissolution Mechanism of Single-Crystalline ZnO(0001) Surfaces in Electrolytes Studied by in-Situ AFM Imaging and Ex-Situ LEED[END_REF] It is thus significantly higher for the m-plane sidewalls of ZnO NWs than for their c- The scale bar is 250 nm.

Ga incorporation measurements by optical analyses

Following the chemical analysis, the Ga content into ZnO NWs was found to be high. Besides its presence on the surfaces of ZnO NWs, this suggests its massive incorporation in an homogeneous and controllable manner, provided that the pH0 lies in the range of 10.4-11. In the backscattering configuration with laser light propagation along the c-axis, only three Raman modes are characteristics of the wurtzite structure of ZnO NWs. The Raman lines at 100, 437-443 and 574 cm -1 are attributed to E2(low), E2(high), and A1(LO) modes, respectively. [START_REF] Cuscó | Temperature Dependence of Raman Scattering in ZnO[END_REF] The second-order Raman line at 333 cm -1 corresponds to the E2(high)-E2(low) mode. Notably, except for ZnO NWs grown at the pH0 of 7.1 and at the [Ga(NO3)3] / [Zn(NO3)2] ratio of 0 % in which no Ga atoms were revealed by chemical analyses, two Ga-related additional modes (AMGa) at 633.5 and 696.1 cm -1 occur in as-grown ZnO NWs, as revealed in Figure 8a,c. Those Raman lines have previously been reported in Ga-doped ZnO [START_REF] Pineda-Hernández | Morphology Evolution of Hydrothermally Grown ZnO Nanostructures on Gallium Doping and Their Defect Structures[END_REF][START_REF] Bundesmann | Raman Scattering in ZnO Thin Films Doped with Fe[END_REF][START_REF] Sohn | Surface Energy-Mediated Construction of Anisotropic Semiconductor Wires with Selective Crystallographic Polarity[END_REF][START_REF] Gabás | High Quality ZnO and Ga:ZnO Thin Films Grown onto Crystalline Si (1 0 0) by RF Magnetron Sputtering[END_REF][START_REF] Lung | Characterization of the Structural and Optical Properties of ZnO Thin Films Doped with Ga, Al and (Al+Ga)[END_REF] and are attributed to Ga-related point defects. In contrast, the peak at 633.5 cm -1 cannot be assigned to the Ag [START_REF] Wang | Microstructural and Optical Characteristics of Solution-Grown Ga-Doped ZnO Nanorod Arrays[END_REF] vibrational mode of the β-Ga2O3 phase at 630 cm -1 , because all other Ag modes of β-Ga2O3 are not observed. [START_REF] Onuma | Polarized Raman Spectra in β-Ga2O3 Single Crystals[END_REF][START_REF] Kranert | Raman Tensor Elements of Beta-Ga2O3[END_REF][START_REF] Rao | Blueshifted Raman Scattering and Its Correlation with the [110] Growth Direction in Gallium Oxide Nanowires[END_REF] The relative intensities of the AMGa at 633.5 and 696.1 cm -1 with respect to the E2(high) mode do not likely depend on the pH0 in the range of 10.4-11.0, as revealed in Figure 8a. In contrast, they continuously increase as the [Ga(NO3)3] / [Zn(NO3)2] ratio is increased from 0 to 10 % and thus as the Ga / Zn element ratio is increased from 0 to 4 %, as shown in Figure 8c and S4. It is additional strong evidence that these AMGa are directly attributed to Ga-related point defects.

Since Ga interstitials (GaI) typically have a high formation energy and thus are unlikely to form in ZnO, [START_REF] Demchenko | Impurity Complexes and Conductivity of Ga-Doped ZnO[END_REF] both AMGa are expected to be due to the incorporation of GaZn into ZnO NWs. Importantly, except for ZnO NWs grown at the pH0 of 7.1, the intensities of the AMGa strongly decrease after annealing under oxygen atmosphere as presented in Figure 8b,d when ZnO was doped with various elements such as N, Fe, Sb, Al, and Ga, despite their different mass and size. Accordingly, the standard AMs have been attributed either to dopant-induced defects in the host lattice (such as Zni [START_REF] Hu | Correlation between Defect-Related Photoluminescence Emission and Anomalous Raman Peaks in N-Al Co-Doped ZnO Thin Films[END_REF] , Zni clusters [START_REF] Gluba | Interstitial Zinc Clusters in Zinc Oxide[END_REF] , or Zni-AlZn complexes [START_REF] Momot | A Novel Explanation for the Increased Conductivity in Annealed Al-Doped ZnO: An Insight into Migration of Aluminum and Displacement of Zinc[END_REF] ) and/or to silent ZnO B1 modes which become Raman active due to a local breakdown of the translational symmetry [START_REF] Manjón | Silent Raman Modes in Zinc Oxide and Related Nitrides[END_REF][START_REF] Sundara | Silent Modes in Vertically Aligned Undoped ZnO Nanorods[END_REF] when the lattice is distorted. The AMGa still occur after annealing under oxygen atmosphere, but with a much lower intensity, indicating the presence of GaZn. Temperature-dependent Raman spectra of ZnO NWs grown by CBD for a pH0 of 10.9 and for a [Ga(NO3)3] / [Zn(NO3)2] ratio of 2.5 % are presented in Figure 9. Moreover, the position of the E2(high) line of as-grown ZnO NWs shifts towards higher wavenumber from 439 to 443.5 cm -1 as the [Ga(NO3)3] / [Zn(NO3)2] ratio is increased from 0 to 10 % for a given pH0 of 10.9, as shown in Figure 10a. After annealing under oxygen atmosphere, the E2(high) line shifts back near the position of undoped ZnO NWs (437.5 cm -1 ), as depicted in Figure 10b. The E2(high) mode is related to the vibration of the oxygen sub-lattice and is very sensitive to the strain, shifting towards higher wavenumber when the ZnO lattice is under a compressive stress state. [START_REF] Decremps | High-Pressure Raman Spectroscopy Study of Wurtzite ZnO[END_REF][START_REF] Harriman | Frequency Shifts of the E2 high Raman Mode Due to Residual Stress in Epitaxial ZnO Thin Films[END_REF] This shows that the Ga incorporation in the form of GaZn into as-grown ZnO NWs is responsible for the generation of a large strain in the ZnO lattice that further relaxes after annealing under oxygen atmosphere. The predominant AMGa at 633.5 and 696.1 cm -1 in as-grown ZnO NWs are attributed to the incorporation of GaZn, acting as shallow donors and further generating a drastic strain. Following the post-deposition annealing under oxygen atmosphere, the standard AMs at 277, 510, 581, 597, and 643 cm -1 in annealed ZnO NWs are assigned to the formation of GaZn-VZn complexes, acting as compensating acceptors and additionally relaxing the strain level. A significant part of GaZn in as-grown ZnO NWs is thus converted into GaZn-VZn complexes in annealed ZnO NWs. These findings report an in-depth understanding of the formation and Ga incorporation mechanisms of ZnO NWs by CBD, which can be applied to a vast number of extrinsic doping elements for carefully governing the physical properties as a further essential issue in nanoscale engineering devices.

  )3] / [Zn(NO3)2] ratio of 5 % while varying the pH0 from 7.1 to 11.3 by the addition of NH3 is presented in Figure 1a using FESEM imaging. The morphology of ZnO NWs grown by CBD at a given pH0 of 10.9 while tuning the [Ga(NO3)3] / [Zn(NO3)2] ratio in the range of 0-10 % is presented in Figure 1b. The length, diameter, and apparent number density of ZnO NWs were systematically measured over a large number of nanoobjects from such FESEM images, while their aspect ratio and deposited volume were deduced from these structural characteristics. The morphological properties of ZnO NWs as a function of the pH0 and of the [Ga(NO3)3] / [Zn(NO3)2] ratio are summarized in Figure 2a,b, respectively.

Figure 1 .

 1 Figure 1. (a) Cross-sectional view and top-view FESEM images of ZnO NWs grown by CBD with pH0 values in the range of 7.1-11.3 and for a given [Ga(NO3)3] / [Zn(NO3)2] ratio of 5 %. (b) Cross-sectional view and topview FESEM images of ZnO NWs grown by CBD with [Ga(NO3)3] / [Zn(NO3)2] ratio values in the range of 0-10 % and for a given pH0 of 10.9. The scale bar is 1 µm.

3 ,

 3 all the morphological characteristics of ZnO NWs, including their length, diameter, and deposited volume, fall. At a maximum pH0 of 11.3, the deposited volume is very low and ZnO NWs are very short, exhibiting a very small aspect ratio. ZnO NWs grown in the [Ga(NO3)3] / [Zn(NO3)2] ratio range of 0-2.5 % have a flat hexagonal tip. In contrast, they exhibit a pencil-like shape on their tip when grown in the [Ga(NO3)3] / [Zn(NO3)2] ratio range of 2.5-10 %, as shown in Figure S1. Also, ZnO NWs are strongly coalesced into a columnar film when grown at the [Ga(NO3)3] / [Zn(NO3)2] ratio of 10 %. The length of ZnO NWs strongly decreases from 5.3 ± 0.6 to 1.8 ± 0.3 µm, while their diameter increases from 77 ± 6 to 170 ± 30 nm in the [Ga(NO3)3] / [Zn(NO3)2] ratio range of 0-10 %. Correlatively, their aspect ratio decreases from 69 ± 20 to 10 ± 4. The apparent number density of ZnO NWs first increases in the [Ga(NO3)3] / [Zn(NO3)2] ratio range of 0-1 % from 40 ± 4 to 52 ± 5 µm -2 , and then decreases to reach a plateau around 40 ± 5 µm -2 . In contrast, the deposited volume increases from 1.0 ± 0.3 to 3.1 ± 1.5 µm 3 as the [Ga(NO3)3] / [Zn(NO3)2] ratio is increased from 0 to 5 %, and eventually decreases to 1.7 ± 0.8 µm 3 at the [Ga(NO3)3] / [Zn(NO3)2] ratio of 10 %.

Figure 2 .

 2 Figure 2. (a) Evolutions of the length, diameter, aspect ratio, apparent number density, and deposited volume of ZnO NWs as a function of the pH0 and for a given [Ga(NO3)3] / [Zn(NO3)2] ratio of 5 %. (b) Evolutions of the length, diameter, aspect ratio, apparent number density, and deposited volume of ZnO NWs as a function of the [Ga(NO3)3] / [Zn(NO3)2] ratio and for a given pH0 of 10.9.Effects of the supersaturation in the chemical bath -In order to deeply understand the evolution of

Figure 3 .

 3 Figure 3. In situ measurements of the pH during the CBD of ZnO NWs (a) for pH0 values in the range of 7.1-11.0 and for a given [Ga(NO3)3] / [Zn(NO3)2] ratio of 5 % as well as (b) for [Ga(NO3)3] / [Zn(NO3)2] ratio values in the range of 0-10 % and for a given pH0 of 10.9.Thermodynamic computations yielding theoretical solubility plot and speciation diagrams of Zn(II)

Figure 4 .

 4 Figure 4. (a) Theoretical solubility plot of Zn(II) species at 85 °C as a function of the calculated pH, each data point corresponding to the addition of 1 mM of NH3 in the chemical bath. The NH3 concentration was varied in the range of 0-1500 mM while the Zn(NO3)2 and Ga(NO3)3 concentrations were set to 30 and 1.5 mM, respectively. Supersaturation in Zn(II) species is illustrated by colored solid lines placed at the steady-state pH values as shown in Figure 3 and was estimated by the difference between the initial Zn(NO3)2 concentration of 30 mM and the equilibrium concentration of soluble Zn(II) species. (b) Theoretical solubility plots of Zn(II) species at 85 °C as a function of the calculated pH on a narrower range. The Zn(NO3)2 concentration was set to 30 mM, while the Ga(NO3)3 concentration was varied in the range of 0.15 to 3 mM. (c) Speciation diagram of Zn(II) species at 85 °C as a function of the calculated pH, each data point corresponding to the addition of 1 mM of NH3 in the chemical bath. The NH3 concentration was varied in the range of 0-1500 mM while the Zn(NO3)2 concentrations was set to 30 mM.

  the length and related deposited volume of ZnO NWs in the pH0 range of 10.4-11.0, as shown in Figure2a. In this range, Zn(NH3)4 2+ complexes are the predominant stable Zn(II) species and act as the limiting reactant, while HO -ions are strongly in excess following the hydrolysis of NH3. The addition of NH3 is thus a good mean to precisely control the morphology of ZnO NWs. As the pH0 is further increased to 11.3, Zn(II) species are highly soluble and hence supersaturation is very low, leading to a very low deposited volume of ZnO NWs.Importantly, the theoretical solubility plot of Zn(II) species depends on the [Ga(NO3)3] / [Zn(NO3)2] ratio. In particular, supersaturation is found to decrease as the [Ga(NO3)3] / [Zn(NO3)2] ratio is increased from 0 to 10 %. This further accounts for the increase in the deposited volume of ZnO NWs as the [Ga(NO3)3] / [Zn(NO3)2] ratio is increased from 0 to 5 % and for a given pH0 leading to a fairly constant steady-state pH value. As the [Ga(NO3)3] / [Zn(NO3)2] ratio is further increased to 10 %, supersaturation is much lower and thus the deposited volume of ZnO NWs is also lower. It should also be noted that the increase in the deposited volume follows the increase in the diameter of ZnO NWs, until a 2D layer is formed for the highest [Ga(NO3)3] / [Zn(NO3)2] ratio, where radial growth is spatially constrained.Effects of the nature of Ga(III) species in the chemical bath -To understand in more details the physico-chemical processes in the chemical bath, electrostatic interactions between the crystallographic planes of ZnO NWs and Ga(III) soluble species should be considered. The surface electrical charge of ZnO NWs switches from a positive to negative value at a given pH, denoted as the isoelectric point (IEP). The IEP represents the point for which the surface electrical charge is neutral and bares an equal number of positive and negative charges. Correlatively, the point of zero charge (PZC) represents the point for which the surface electrical charge is neutral and does not bare any individual charges.

  plane top facets. Except for the pH0 of 7.1, in the steady-state pH range that lies in between both PZC values as shown in Figure 3, the c-plane top facet and m-plane sidewalls of ZnO NWs thus have an opposite surface electrical charge. The c-plane top facet of ZnO NWs is negatively charged while their m-plane sidewalls are still positively charged. Theoretical speciation diagrams of Ga(III) species as a function of the calculated pH and for a given [Ga(NO3)3] / [Zn(NO3)2] ratio of 5 % were carried out using Visual MINTEQ software and are presented in Figure 5a.

Figure 5 .

 5 Figure 5. (a) Speciation diagram of Ga(III) species at 85 °C as a function of the calculated pH, each data point corresponding to the addition of 1 mM of NH3 in the chemical bath. The NH3 concentration was varied in the range of 0-1500 mM while the Ga(NO3)3 concentration was set to 1.5 mM. (b) Distribution of Ga(OH)3 and Ga(OH)4 -complexes as a function of the [Ga(NO3)3] / [Zn(NO3)2] ratio for a given pH0 of 10.9.The majority soluble Ga(III) species changes from Ga(OH)3 complexes in the low calculated pH range to Ga(OH)4 -complexes in the high calculated pH range. Other Ga(III) species are strongly in a

Figure 6 . 7 . 7 . 7 %

 6777 Figure 6. (a) FESEM-EDS spectra of ZnO NWs grown with pH0 values in the range of 7.1-11.0 and for a given [Ga(NO3)3] / [Zn(NO3)2] ratio of 5 %. (b) FESEM-EDS spectra of ZnO NWs grown with [Ga(NO3)3] / [Zn(NO3)2]ratio values in the range of 0-10 % and for a given pH0 of 10.9. All the spectra were normalized with respect to the Zn Kβ line pointed at 9.57 keV.Very interestingly, the intensity of the Ga Kα line at 9.25 keV in ZnO NWs progressively increases

Figure 7 .

 7 Figure 7. Evolution of the Ga / Zn element ratio in ZnO NWs as a function of the [Ga(NO3)3] / [Zn(NO3)2] ratio and for a given pH0 of 10.9, as inferred from FESEM-EDS and STEM-EDS measurements. The insets are brightfield TEM image and STEM-EDS elemental maps of the Zn (blue) and Ga (yellow) elements, respectively, collected on a single ZnO NW grown with a [Ga(NO3)3] / [Zn(NO3)2] ratio of 0.5 % and for a given pH0 of 10.9.

  3 and by varying the [Ga(NO3)3] / [Zn(NO3)2] ratio. However, to induce an efficient incorporation and related doping of ZnO NWs, these Ga atoms must be incorporated on dedicated sites in the ZnO lattice, namely they should substitute for Zn atoms to form GaZn. In order to investigate in more details the Ga incorporation mechanisms, Raman scattering spectra of ZnO NWs are presented in Figure 8.

Figure 8 .

 8 Figure 8. Raman spectra of ZnO NWs grown with pH0 values in the range of 7.1-11.0 and for a given [Ga(NO3)3] / [Zn(NO3)2] ratio of 5 %, (a) before and (b) after annealing at 500 °C for 1 h under oxygen atmosphere. Raman spectra of ZnO NWs grown with [Ga(NO3)3] / [Zn(NO3)2] ratio values in the range of 0-10 % and for a given pH0 of 10.9, (c) before and (d) after annealing at 500 °C for 1 h under oxygen atmosphere.

.

  This is not related to an exo-diffusion of Ga atoms, because the Ga / Zn element ratio retains a constant value as shown in Figure S5. Conversely, standard additional modes occur at 277 (AM1), 510 (AM2), 581 (AM3), 597 (AM4), and 643 (AM5) cm -1 after annealing under oxygen atmosphere as presented in Figure 8b,d. These AMs have widely been reported

Figure 9 .

 9 Figure 9. Raman spectra of ZnO NWs grown with a pH0 of 10.9 and a [Ga(NO3)3] / [Zn(NO3)2] ratio of 2.5 %, as collected from room temperature to 500 °C under oxygen atmosphere. The intensities of the AMGa are fairly constant from room temperature to 200 °C. As the temperature is

Figure 10 .

 10 Figure 10. (a,b) Raman spectra and (c,d) XRD patterns of ZnO NWs grown with [Ga(NO3)3] / [Zn(NO3)2] ratio values in the range of 0-10 % and for a given pH0 of 10.9; (a, c) before and (b, d) after annealing at 500 °C for 1 h under oxygen atmosphere. The position in the XRD patterns was precisely calibrated on the Si (400) peak pointing at 2θ = 69.13 °.Correlatively, the (0004) diffraction peak of as-grown ZnO NWs shifts from 72.55 to 72.8 ° as the

Table 1 .

 1 Conditions used during the CBD of ZnO NWs for the two series of six samples

	[Ga(NO 3 ) 3 ] [Zn(NO 3 ) 2 ]	pH0	[NH3] (mM)
	5 %	7.1	11
	5 %	10.4	350
	5 %	10.7	467
	5 %	10.9	584
	5 %	11	730
	5 %	11.3	1167
	0 %	10.9	584
	0.5 %	10.9	584
	1 %	10.9	584
	2.5 %	10.9	584
	5 %	10.9	584
	10 %	10.9	584
	Characterization techniques. The pH and temperature of the solution as CBD proceeds were
	monitored in an in situ manner using an InLab Versatile Pro pH electrode from Mettler Toledo. The
	morphology and structural properties of ZnO NWs were measured by top-view and cross-sectional view
	FESEM images using a FEI Quanta 250 field-emission-gun scanning electron microscope. X-ray
	diffraction patterns were collected with a Bruker D8 Advance diffractometer using Cu Kα1 radiation
	according to the Bragg-Brentano configuration. The Ga incorporation into ZnO NW arrays was
	investigated by EDS analyses and temperature-dependent Raman scattering measurements. FESEM-
	EDS spectra of ZnO NW arrays were recorded using a Bruker x-ray detector incorporated in the FEI
	Quanta 250 field-emission-gun scanning electron microscope operating at 20 kV. STEM specimens
	were prepared by scratching the surface of ZnO NW arrays using a diamond tip and spread on a copper
	(Cu) grid. STEM-EDS spectra and maps of single ZnO NWs were collected with a JEOL SDD Centurio
	detector having a large solid angle of up to 0.98 steradians incorporated in the JEOL 2100F field-
	emission-gun scanning electron microscope operating at 200 kV and having a 0.2 nm resolution in the
	scanning mode. Raman scattering spectra were recorded using a Horiba/Jobin Yvon Labram
	spectrometer equipped with a liquid-nitrogen-cooled CCD detector. The 488 nm excitation line of an

  where M n L i x+ is the complex considered, i is the coordination number, and x is the cation charge. The related stability constants β 𝑖 𝐿 associated to each reaction are given by: β 𝑖 𝐿 =

[𝑀 𝑛 𝐿 𝑖 𝑛𝑥+ ] [𝑀 𝑥+ ] 𝑛 [𝐿] 𝑖 . These constants were taken at 25 °C from NIST for Zn(II) and Ga(III) species. The constants at 85 °C were deduced from Van't Hoff relation. To calculate the theoretical solubility plots for each growth condition, ZnO(s) and the different phases of Zn(OH)2(s) were taken into account.

ACKNOWLEDGMENTS

The authors would like to thank Hervé Roussel from LMGP, Grenoble, France, for his assistance in the XRD experiments. The authors acknowledge the financial support by the French Research National Agency through the projects DOSETTE (ANR-17-CE24-0004) and ROLLER (ANR-17-CE09-0033). P.G. was supported by a doctoral fellowship from the CDP Eco-SESA (ANR-15-IDEX-02). The authors further acknowledge the facilities, and the scientific and technical assistance of the CMTC characterization platform of Grenoble INP, which is supported by the Centre of Excellence of Multifunctional Architectured Materials (LabEx CEMAM) under the contract ANR-10-LABX-44-01 funded by the "Investments for the Future" Program. Funding by the Carnot Institute Energies du Futur through the project ECOLED is also acknowledged.

to the undoped ZnO NWs, revealing the presence of a small residual strain in ZnO NWs after annealing under oxygen atmosphere. Notably, the standard AMs form during the strain relaxation, such that they are most likely caused by dopant-induced defects in the host lattice. [START_REF] Rao | Blueshifted Raman Scattering and Its Correlation with the [110] Growth Direction in Gallium Oxide Nanowires[END_REF][START_REF] Demchenko | Impurity Complexes and Conductivity of Ga-Doped ZnO[END_REF][START_REF] Hu | Correlation between Defect-Related Photoluminescence Emission and Anomalous Raman Peaks in N-Al Co-Doped ZnO Thin Films[END_REF][START_REF] Gluba | Interstitial Zinc Clusters in Zinc Oxide[END_REF][START_REF] Momot | A Novel Explanation for the Increased Conductivity in Annealed Al-Doped ZnO: An Insight into Migration of Aluminum and Displacement of Zinc[END_REF]

Ga incorporation mechanisms

These experimental observations suggest the following Ga incorporation mechanisms into ZnO NWs following the CBD and post-deposition annealing under oxygen atmosphere. Owing to the adsorption of Ga(OH)4 -complexes as capping agents on the sidewalls of ZnO NWs at a pH0 from 10.4, a large amount of Ga atoms as measured by chemical analyses occurs in ZnO NWs. It is expected that a significant part of Ga atoms is located on the surface of ZnO NWs, or more likely on the sub-surface of the m-plane sidewalls as shown in Ref. 26, promoting in turn their radial growth as shown in Figure 2b.

Additionally, another significant part of Ga atoms is most likely located in tetrahedral sites of the host lattice in the form of GaZn acting as shallow donors, inducing the presence of the AMGa in Raman spectra in Figures 8 and9 as well as generating a drastic strain level as revealed in Figure 10. The formation energy of GaZn is indeed low on a broad range of oxygen chemical potential, especially as compared to other native point defects such as zinc vacancies (VZn) or oxygen interstitials (Oi). [START_REF] Demchenko | Impurity Complexes and Conductivity of Ga-Doped ZnO[END_REF] Following the postdeposition annealing under oxygen atmosphere, GaZn result, to a significant part, in the formation of GaZn-VZn complexes acting as compensating acceptors, which is indicated by the decrease/vanishing of the AMGa modes at the benefit of the occurrence/increase of the standard AM modes in Figures 8 and9. Under oxygen-rich conditions (i.e. high oxygen chemical potential) and when the Fermi level approaches the conduction band minimum, the formation energy of GaZn-VZn complexes is indeed lower than the formation energy of GaZn in addition to any other native point defects. [START_REF] Demchenko | Impurity Complexes and Conductivity of Ga-Doped ZnO[END_REF] GaZn-VZn complexes are also expected to induce a significant lattice relaxation, in turn relaxing the strain level in ZnO NWs as shown in Figure 10.

CONCLUSIONS

A combined investigation involving a theoretical thermodynamic and experimental approach has been carried out to elucidate the formation and Ga incorporation mechanisms of ZnO NWs grown by CBD.

The addition of ammonia to tune the pH0 of the solution from 7.1 to 11.3 and of Ga(NO3)3 to tailor the

SUPPORTING INFORMATION

FESEM images of the tips of ZnO NWs (Figure S1); HRTEM and TEM images of ZnO NWs (Figure S2); XRD patterns of ZnO NWs (Figure S3); Evolution of the AMGa / E2(high) intensity ratio as a function of [Ga(NO3)3] / [Zn(NO3)2] ratio in as-grown ZnO NWs (Figure S4); FESEM-EDS spectra of ZnO NWs before and after annealing under oxygen atmosphere (Figure S5).

TABLE OF CONTENTS GRAPHIC SYNOPSIS

We show a general strategy for the intentional extrinsic doping of ZnO nanowires grown by chemical bath deposition, by investigating in detail the specific case of gallium. Controlling the gallium nitrate concentration and pH in the bath allows a fine tuning of both the morphology and gallium incorporation into ZnO nanowires, emphasizing the electrostatic origin of doping mechanisms in aqueous media. In addition, the post-deposition annealing under oxygen atmosphere modifies the nature of gallium-related defects.


Balthasar Fischer

Fabrizio Sarasini
email: fabrizio.sarasini@uniroma1.it

Jacopo Tirillò

Fabienne Touchard

Laurence Chocinski-Arnault

David Mellier

Nils Panzer

Ryan Sommerhuber

Pietro Russo

Ilaria Papa

Laurence Chocinski- Arnault

Valentina Lopresto

Romain Ecault

Impact damage assessment in

Keywords: Polymer-matrix composites, Natural fibres, Low-velocity impact, Non-destructive testing, Micro-CT

Introduction

Composite materials have gained wide acceptance in a broad range of both low-and hightechnology engineering applications driven by several advantages including higher specific strength and stiffness, superior corrosion resistance as well as improved fatigue properties compared to metals. However, composite materials are inherently prone to defects that can be caused by imperfect manufacturing (embedded foreign objects, regions of excessive porosity, mistakes in ply orientation or layup sequence, ply waviness, and delamination) or can occur in-service, mainly related to low velocity impacts, leading to non-visual damage on the surface but significant internal damage [START_REF] Richardson | Review of low-velocity impact properties of composite materials[END_REF].

As the use of composite increases, there is a greater need for non-destructive inspection procedures in both the manufacturing of the structures and in their subsequent in-service maintenance and repair. In particular, there is a strong need to develop testing techniques able to detect defects quickly and reliably, which has prompted the development of a number of methods and techniques over the years. The most promising techniques that meet these requirements and are commercially available include passive [START_REF] Sfarra | Improving the detection of thermal bridges in buildings via on-site infrared thermography: The potentialities of innovative mathematical tools[END_REF] and active [START_REF] Zhang | Enhanced Infrared Image Processing for Impacted Carbon/Glass Fiber-Reinforced Composite Evaluation[END_REF] thermography, shearography and ultrasonic [START_REF] Cawley | Inspection of Composites-Current Status and Challenges[END_REF][START_REF] Hsu | Nondestructive Inspection of composite structures: methods and practice[END_REF]. In this regard, ultrasonic inspection, due to its flexibility and ability to reveal internal defects such as delaminations or cracks, is probably one of the most widely used testing techniques, especially in the field of impact damage detection, which is generally recognized as one of the most severe threats to composite materials. However, current ultrasonic testing is conventionally performed using either liquid coupling (water, usually) or some type of gel or oil in contact-mode coupling.

Alternatives involve the coupling of acoustic energy via a localised water jet. These conventional systems are typically slow, require significant setup time for highly contoured parts, and are generally not suitable for in-service inspections where access is limited to a single side. In addition, immersion of some composite materials may result in moisture absorption and subsequent loss of mechanical integrity, which is particularly important for composites reinforced with natural fibres.

Natural fibre composites represent a family of composite materials that have been intensively investigated in the last two decades for benefits such as reductions in weight, cost, and CO2, less reliance on oil sources, and recyclability [START_REF] Pickering | A review of recent developments in natural fibre composites and their mechanical performance[END_REF][START_REF] Fortea-Verdejo | Plant fibre-reinforced polymers: where do we stand in terms of tensile properties?[END_REF]. Their global market is forecast to grow at a CAGR of 8.2% from 2015 to 2020, the major driver being the rise in demand for lightweight and environmentally sustainable composite materials in various applications, such as automotive, building and construction [START_REF]Growth Opportunities in the Global Natural Fiber Composites Market[END_REF]. Despite this growing interest, the applications of such materials are still mainly confined to the non-structural field, due to several technical and scientific challenges that include the variable fibres' mechanical properties, a full understanding of their mechanical behaviour and damage modes, fibre/matrix interfacial adhesion, moisture repellence and flameretardant properties. For these composites to be used in structural components, it is important that the designers and manufacturers understand how these materials behave under different loading conditions including fatigue and impact loading. Low velocity impact damage can cause a quite complex damage scenario where different failure modes, such as matrix cracking, delamination, fibre breakage and fibre pull-out, can be present. The impact response of natural fibre composites has been the subject of several investigations [START_REF] Liang | Impact behaviour of flax/epoxy composite plates[END_REF][START_REF] Huber | Flexural and impact properties of allcellulose composite laminates[END_REF][START_REF] Dhakal | Low-velocity impact response of non-woven hemp fibre reinforced unsaturated polyester composites: Influence of impactor geometry and impact velocity[END_REF] and the results, despite being promising, have highlighted peculiar damage mechanisms compared to traditional synthetic counterparts [START_REF] Dhakal | The post-impact response of flax/UP composite laminates under low velocity impact loading[END_REF][START_REF] Bensadoun | Low velocity impact properties of flax composites[END_REF][START_REF] Sarasini | Damage tolerance of carbon/flax hybrid composites subjected to low velocity impact[END_REF].

Therefore, it is of utmost importance to have a better understanding of the effects of various failure modes on the structural performance of natural fibre laminates in order to realize their full potential.

As a result of significant moisture absorption, it is not usually advisable to use standard contact ultrasonic methods. The development of air-coupled ultrasonic techniques has made ultrasonic methods more feasible for defect detection in several materials, ranging from wood [START_REF] Fang | Review of the use of air-coupled ultrasonic technologies for nondestructive testing of wood and wood products[END_REF][START_REF] Chimenti | Review of air-coupled ultrasonic materials characterization[END_REF], composite materials [START_REF] Imielińska | Air-coupled ultrasonic C-scan technique in impact response testing of carbon fibre and hybrid: glass, carbon and Kevlar/epoxy composites[END_REF], metals [START_REF] Waag | Air-coupled ultrasonic through-transmission thickness measurements of steel plates[END_REF] and food products [START_REF] Pallav | Air-coupled ultrasonic evaluation of food materials[END_REF]. The main disadvantage is that only a small fraction of the acoustic energy can be coupled into the specimen due to the large difference in impedance between air and solid materials, a problem that has been effectively faced by using increased emission energy and transducers with high sensitivity.

Another non-contact ultrasound testing uses optical excitation and detection of ultrasound with suitable lasers, the so-called Laser Ultrasonics (LUS) [START_REF] Green | Non-contact ultrasonic techniques[END_REF][START_REF] Park | Visualization of hidden delamination and debonding in composites through noncontact laser ultrasonic scanning[END_REF]. LUS is a non-contact method, without requiring any couplants, which can rapidly scan large areas and it is able to inspect at angles far off normal. The absorption of nanosecond-timescale laser pulses generates extremely short ultrasound transients, either via ablation or thermoelastic expansion and subsequent relaxation [START_REF] Davies | Laser-generated ultrasound: its properties, mechanisms and multifarious applications[END_REF][START_REF] Drake | Affordable NDE of Aerospace Composites with Laser Ultrasonics[END_REF]. These transients cause well-defined echoes, which can be separated with high temporal resolution, allowing for single-sided testing with a broadband detection method. Today, this is usually realized through the measurement of surface vibrations with suitable optical interferometers, with detection bandwidths in excess of 100 MHz.

A disadvantage of this LUS implementation is that its performance is strongly dependent on the optical properties as well as on the surface characteristics of the sample under test, both for excitation and for detection. For strongly scattering surfaces, high powered detection lasers are needed as well as complex interferometer set-ups, which may be costly and difficult to miniaturize.

In the present work, a novel method based on air-coupled detection of laser-generated ultrasound with a broadband optical microphone [START_REF] Fischer | Optical microphone hears ultrasound[END_REF] is discussed and implemented for impact damage assessment in composites based on flax fibres and poly (lactic acid) (PLA) modified with polypropylene (PP) to improve its ductility. The air-coupled detection mechanism has the main advantage of being nearly independent of surface characteristics and optical properties of the sample. The results are compared with those obtained with an airborne ultrasound testing system and micro-CT.

Optical microphone for air-coupled ultrasound detection -theoretical background

The optical microphone is an acoustic detector to measure sound pressure waves in the range of 5

Hz to 1 MHz in air and 20 MHz in fluids without any moving mechanical parts. Thereby, changes in the refractive index of the optical medium, caused by the incoming pressure wave, are measured using an interferometer. This so-called Fabry-Pérot etalon consists of two semipermeable mirrors which are arranged at a fixed distance of 2mm. To achieve constructive interference conditions this distance matches a multiple of the half laser`s wavelength. This condition for the wavelength is fulfilled at all times with the help of a slowly acting PID circuit, locking the laser to the cavity by tuning the laser current. While the incoming pressure wave changes the refractive index and thereby the optical path length of the miniaturized interferometer, the transmitted intensity of the laser decreases. This effect can be precisely detected by a photodiode; it corresponds directly to the acoustic pressure. The principle of operation is shown schematically in figure 1, while a photo of the sensor head is shown in figure 2.

Without any mechanically movable parts (such as membranes, deformable crystals, or similar), the optical microphone overcomes physical limitations of alternative solutions like piezoelectric transducers such as mechanical resonances and limited frequency bandwidth. The novel optoacoustic sensor enables a broadband, highly sensitive airborne pressure detection with a dynamic range expanding over 100 dB (5 decades). The detection principle is inherently linear and allows a high temporal resolution of less than 1 µs. Furthermore, the sensor head is very small having a size of a few millimeters only. This allows testing in confined spaces, nooks, or close to edge in an elbow-shaped element or a T-joint. Finally, the all-optical sensor head is immune to electromagnetic interferences (which might be present in the testing environment). On the other hand, the transmission of high ultrasonic frequencies through air is limited. While acoustic signals of frequencies in the human hearing range can easily propagate over hundreds of meters in air, a frequency of 1MHz is attenuated by as much as 160 dB/m [START_REF] Bass | Atmospheric absorption of sound: Update[END_REF]. Hence, the optical microphone has to be brought close to the surface of the specimen (millimeters to centimeters), and the setup is not suitable for remote sensing. However, given the small size of the fiber-coupled sensor head, this condition could be easily achieved in all the performed tests. It was found that, in laser-excited CFRP measurements, the acoustic energy centers in the lower few-hundreds kHz range (further investigations are necessary to derive exact values). While the lateral resolution of the scans using the optical microphone can be clearly below 1mm due to the small physical dimensions of the receiver, the depth resolution via time-of-flight is limited by the bandwidth of the receiver.

Materials and methods

Materials

Biotex Flax/PLA 400 g/m 2 2x2 Twill supplied by Composites Evolution Ltd has been used to manufacture the samples. These fabrics are commingled textiles made from flax fibre and polylactic acid (PLA) and are suitable for producing fibre reinforced thermoplastic composite parts. PLA is a biodegradable thermoplastic polyester with its high strength but low toughness and heat resistance.

These weaknesses can be tackled with several approaches, including plasticization, copolymerization, and melt blending with different tough polymers [START_REF] Krishnan | Toughening of Polylactic Acid: An Overview of Research Progress[END_REF][START_REF] Kfoury | Recent advances in high performance poly(lactide): from "green" plasticization to super-tough materials via (reactive) compounding[END_REF][START_REF] Nagarajan | Perspective on Polylactic Acid (PLA) based Sustainable Materials for Durable Applications: Focus on Toughness and Heat Resistance[END_REF]. In this study, polypropylene (PP) (MA712 by Unipetrol, Czech Republic; MFI at 230 °C, 2.16 kg = 12 g/10 min) was used to improve the toughness of such composite material via a film stacking process.

Composite materials manufacturing

Laminates have been manufactured by compression moulding (Collin GmbH (Edersberg, Germany) mod. P400E), using the film stacking method by alternating PP films with dry PLA/Flax layers with the following stacking sequence: (0/90)6s. During this process, the impregnation of the porous fabric by infiltration of a viscous polymer liquid is mainly carried out in the transverse direction rather than in-plane. This process is affected not only by the applied pressure but also by temperature boundary conditions used in heating and cooling in order to ensure (i) melting of the polymer without thermal degradation, (ii) impregnation of the reinforcing structure and (iii) consolidation of the laminate [START_REF] Mayer | Macro-and micro-impregnation phenomena in continuous manufacturing of fabric reinforced thermoplastic composites[END_REF]. Therefore, the compression moulding has been carried out in accordance with the pre-optimized temperature and pressure profiles reported in figure 3, to allow for correct impregnation of the fabric layers, which is driven by the rheological behaviour at melt processing temperature. Plates with a thickness of 4.1 ± 0.1 mm and a fibre volume fraction of 0.32 ± 0.02 have been cut to the required dimensions to perform the mechanical characterization. As a reference material, laminates without PP have been also manufactured with the same procedure.

Mechanical characterization of composites

Tensile tests have been carried out in accordance with ASTM D3039, with a gauge length of 50 mm and in displacement control with a cross-head speed of 2.5 mm/min. Three-point bending tests have been performed in accordance with ASTM D790 with a support span length of 80 mm and a cross-head speed of 2.5 mm/min. Both quasi-static mechanical tests have been carried out on a Zwick/Roell Z010 universal testing machine equipped with a 10 kN load cell.

Test coupons measuring 100 × 100 mm were at first impacted at room temperature up to perforation and then at target impact energies from 1 to 5 J to find the barely visible impact damage (BVID)

threshold. An instrumented drop-weight impact testing machine (CEAST Fractovis) was used to this purpose equipped with a hemispherical tip (diameter of 19.8 mm). A constant mass of 3.64 kg was used to deliver the required impact energy levels. Measurements were carried out on square samples simply supported on a steel cylindrical support with an internal hole 80 mm in diameter, according to the ASTM D5628. The force-time and force-displacement curves were recorded during each test by the DAS16K acquisition program. While BVID is subjective by nature, it is often defined as damage visible within a range of 1 m, or damage causing a specific permanent indentation. The depth of the residual indentation caused by BVID varies in literature and in this study 0.3 mm of dent depth was chosen as the threshold of detectability (as commonly adopted by aeronautical standards). Post-impact, the dent depth of each coupon was measured using a laser profilometer (Taylor-Hobson Talyscan 150) with a scanning speed of 8500 μm/s.

Non-destructive testing setup

The Laser Ultrasound setup consists of a fibre-coupled excitation laser, the optical microphone, a mechanical scanner and a control unit. The setup is shown in figure 4. The excitation laser generates a thermoelastic ultrasound wave within the probe, below the ablation threshold, in order not to damage the specimen. The generated ultrasound signal was measured in transmission mode, but might be also measured in a single sided approach -so called pitch-catch, or pulse-echo. A mechanical scanner unit is used to move the sensor and the excitation laser over the sample. The used stepping size is 0.2 mm, the sampling rate is 25 MHz. No averaging or post-processing methods were exploited.

Morphological and damage investigation

The fracture surfaces of specimens failed in tension were investigated by scanning electron microscopy (FE-SEM Zeiss Auriga). All specimens were sputter coated with chromium prior to FE-SEM observations. The volumetric damage visualization has been performed by X-ray microtomography (UltraTom CT scanner manufactured by RX Solutions) with a resolution of 20 μm/voxel. The system consists of a Hamamatsu sealed type microfocus X-ray tube operating at 40-150 kV/0-500 μA, within a maximum power of 75W.

Results and discussion

The potential of an air-coupled detection of laser-generated ultrasound using a broadband optical microphone in providing information about the probability of damage existence and its extension in natural fibre composites is discussed. For the sake of completeness, a quasi-static mechanical characterization of the material system under investigation will be described in the following before addressing the impact behaviour.

Quasi-static mechanical properties

The rationale for modifying the PLA with layers of PP was to explore the effect that a ductile polymer had upon the mechanical properties of flax fibre reinforced PLA composites. Table 1 summarizes the tensile and flexural properties, while figure 5 shows representative stress-strain curves obtained in tests at room temperature. The mechanical properties in tension compare quite favourably with those reported in the technical data sheet for the PLA/flax system. Despite a limited decrease in tensile modulus due to the addition of PP, as expected, no significant effect on the strength was noted. Flax fibres are known to show the presence of defects, kink bands, which act as stress concentration points in the matrix [START_REF] Hughes | Deformation and fracture behaviour of flax fibre reinforced thermosetting polymer matrix composites[END_REF]. A stress concentration that could have been partially relieved by the increased matrix ductility, the net effect of this being the slight increase in tensile strength of the modified composites. The bending tests highlighted a slightly different behaviour with a decrease in flexural strength, likely ascribed to the complex state of stress and the strong dependence of flexural properties on the stacking sequence (i.e., location of PP layers), even if in both configurations the failure was located on the tensile side. The results suggest that in order to exploit to a larger extent a marked improve in toughness of the resulting composites, an optimization of the location and amount of PP layers would be needed. It is worth mentioning that both tensile and flexural curves exhibited a marked non-linear behaviour at low strains, which cannot be only due to the presence of the ductile PP in the brittle PLA, because also the unmodified flax/PLA system showed the same trend. It is reasonable to ascribe such behaviour to the presence of flax fibres, as reported by many authors [START_REF] Poilane | Polymer reinforced by flax fibres as a viscoelastoplastic material[END_REF][START_REF] Berges | Influence of moisture uptake on the static, cyclic and dynamic behaviour of unidirectional flax fibre-reinforced epoxy laminates[END_REF][START_REF] Hughes | The fracture toughness of bast fibre reinforced polyester composites Part 1 Evaluation and analysis[END_REF], and it seems to be a characteristic of natural fibres, as it has been observed in wood [START_REF] Navi | Micromechanics of wood subjected to axial tension[END_REF][START_REF] Keckes | Cell-wall recovery after irreversible deformation of wood[END_REF], flax [START_REF] Mahboob | Tensile and compressive damaged response in Flax fibre reinforced epoxy composites[END_REF][START_REF] Baley | Analysis of the flax fibres tensile behaviour and analysis of the tensile stiffness increase[END_REF][START_REF] Aslan | Strength variability of single flax fibres[END_REF] and hemp [START_REF] Placet | Influence of environmental relative humidity on the tensile and rotational behaviour of hemp fibres[END_REF][START_REF] Placet | Characterization of the thermo-mechanical behaviour of Hemp fibres intended for the manufacturing of high performance composites[END_REF][START_REF] Placet | Nonlinear tensile behaviour of elementary hemp fibres. Part I: Investigation of the possible origins using repeated progressive loading with in situ microscopic observations[END_REF]. A conclusive explanation has not yet been proposed, even if recent studies have shed light on the possible mechanisms, including viscoelastic strain, cellulose microfibrils reorientation, shear strain-induced crystallisation of the amorphous paracrystalline components and degree of ellipticity of the fibre's cross-section [START_REF] Placet | Nonlinear tensile behaviour of elementary hemp fibres. Part I: Investigation of the possible origins using repeated progressive loading with in situ microscopic observations[END_REF][START_REF] Masto | Nonlinear tensile behaviour of elementary hemp fibres: a numerical investigation of the relationships between 3D geometry and tensile behaviour[END_REF]. As a general comment, all composite configurations exhibited a macroscopically brittle behaviour, which is confirmed also at the micro-scale (figure 6). Scanning electron microscopy of the fracture surface of the Flax/PLA-PP system revealed that there was a sufficient but non-homogeneous level of adhesion between the fibre and matrix as evidenced by the presence of limited fibre pull-out (figure 6a) and debonding (figures 6b and c) with fracture surface exhibiting a ''blocky'' appearance (figures 6b and c) with fibres fractured on the same plane. It is also possible to note the presence of ligaments connecting the fibres to the matrix (figure 6d), this would support the good mechanical behaviour exhibited by the biocomposites. From the micrographs, it is also evident that no significant phase separation occurred, despite the fact that PLA and PP are immiscible because of the high polarity difference between the component polymers [START_REF] Choudhary | Poly( L -lactide)/polypropylene blends: Evaluation of mechanical, thermal, and morphological characteristics[END_REF].

Low-velocity impact behaviour

Starting from the positive results of the quasi-static mechanical characterization, impact tests were performed under various impact energies ranging from 1 J to 5 J in order to examine damage evolution in Flax/PLA-PP laminates. These impact energies were selected according to the knowledge of the perforation threshold, which was found to be equal to 13.99 ± 1.63 J with a peak force of 2160.94 ± 90.98 N, in line with recent investigations [START_REF] Bensadoun | Low velocity impact properties of flax composites[END_REF][START_REF] Rubio-López | Compression after impact of flax/PLA biodegradable composites[END_REF]. The relatively low perforation threshold suggested the selection of impact energies in the range 1J -5 J in order to induce significant damage in the laminates while being sufficiently far from the penetration. Parameters like impact force, sample deflection, impact velocity, impact energy (Ei) and absorbed energy (Ea) were obtained by the data acquisition system. In particular, the absorbed energy in the impact event was calculated as the area enclosed in the load-deflection curve, which also includes some useful information for assessing damage process of composite structures [START_REF] Belingardi | Low velocity impact tests of laminate glass-fiber-epoxy matrix composite material plates[END_REF]. In impacts at perforation, not only the matrix but also the fibres play a significant role in the energy absorption process during an impact event. In the present case, flax fibres are on average weaker than glass or carbon fibres, and therefore the relative contribution of the matrix to the global composite impact response is much more important. Visual inspection of perforated samples, shown in figure 7, supports these results. It can be seen that samples failed with a sharp failure pattern ascribed to a predominantly brittle failure mechanism, as already observed in tensile and bending tests. During impact, parts of the samples broke away leaving a clean hole on the front face and a cross-shaped, pyramidal deformation on the back side.

Figure 8 shows typical force-displacement curves as a function of impact energy. The curves appear to be closed, with displacement returning toward the axis origin during unloading, thus meaning that some elastic energy is recovered by the laminate. These flax laminates exhibited a significant energy absorption capability, increasing with increasing impact energy, due to a slightly enhanced compliant behaviour but mainly due to the development of a significant internal damage, as confirmed by the presence of an extended plateau at the peak load, with the exception of 1J-impact.

In addition, the peak load was found to be very dependent on impact energy, with an increase as the energy level increased up to perforation. Key impact parameters like peak force, impact energy (Ei), absorbed energy (Ea) and damage degree (Ea/Ei) obtained from the transient response of each laminate are summarized in table 2. The damage degree [START_REF] Belingardi | Repeated impact response of hand lay-up and vacuum infusion thick glass reinforced laminates[END_REF] increased significantly from 1 J to 5 J, thus confirming the quasi-brittle behaviour of the laminates, which tend to absorb energy through the development of internal damage. In terms of impact damage, it was found that Flax/PLA-PP laminates reached indentation levels implying BVID already at 5 J, as can be clearly observed in table 2, while figure 9 shows the impacted surface of the samples as obtained from laser profilometry from which the dent depth was calculated.

Analysis of damage

Figure 10 shows the different damage patterns of laminates tested at increasing impact energy. By visual inspection, only a contact-induced indentation without edge delaminations around the impact point was detected at each impact energy on the front face. Damage was localized in the back face and exhibited a cross-shaped feature typical of woven laminates, following the warp and weft direction of the fabric and mainly caused by the failure of the flax yarns and the matrix [START_REF] Rubio-López | Compression after impact of flax/PLA biodegradable composites[END_REF]. This behaviour can be ascribed to the combined effect of the bending and membrane tensile forces. As impact energy increases, the extension of damage was found to increase in the back face. Hardly detectable damage was found in laminates impacted at 1 J. As regards the internal damage, two recent publications have highlighted different damage patterns in flax reinforced laminates impacted with a combination of masses and impactor geometry similar to the one used in the present study [START_REF] Bensadoun | Low velocity impact properties of flax composites[END_REF][START_REF] Rubio-López | Compression after impact of flax/PLA biodegradable composites[END_REF]. With the exception of inevitable differences in thickness and fibre volume fraction, in a Flax/PLA composite the main failure mechanism found was fibre failure without transverse cracks and delaminations [START_REF] Rubio-López | Compression after impact of flax/PLA biodegradable composites[END_REF]. Bensadoun et al. [START_REF] Bensadoun | Low velocity impact properties of flax composites[END_REF], when comparing the impact performance of epoxy and maleic anhydride polypropylene (MAPP) reinforced with flax fibres, emphasized the presence of extended through-the-thickness cracks with fibre failure and limited delaminations. In order to shed light into the damage modes of the Flax/PLA-PP laminates, all the samples were inspected by X-ray microtomography and representative images are reported in figures 11 and 12. Even with micro-CT, the specimen impacted at 1 J showed only a small dent on the impacted surface without evidence of additional damage mechanisms. Specimens impacted with energy levels in the range 2 J -5 J exhibited the typical "pine tree" damage pattern [START_REF] Shyr | Impact resistance and damage characteristics of composite laminates[END_REF], which includes matrix cracks due to shear and bending along with extensive delaminations. The damage had a conical appearance in the thickness direction with the in-plane damage area that increased from the impact surface to the rear one. This is a damage pattern usually found in thick laminates [START_REF] Cantwell | Detection of impact damage in CFRP laminates[END_REF]. In particular, the extent of transverse matrix cracks, ascribed to the relatively low strength of flax fibres, appears to be significant, thus potentially leading to a decrease in laminates' stiffness. Ultrasonic C-scan represents the standard approach for assessing the damage in composite laminates subjected to impact loading, but the need to use a coupling fluid prevents or at least limits its use in the field of natural fibre composites that are particularly prone to moisture absorption. In this regard, a novel system based on air-coupled detection of laser-generated ultrasound with an optical microphone has been used to inspect the Flax/PLA-PP laminates. The results are summarized in figure 13 along with a comparison with another non-contact method based on piezoelectric excitation in figure 14. It can be seen that laser-UT enables a better detection of low velocity impact damages compared to airborne ultrasound with a better-defined damaged area. It is also worth mentioning that laser-UT pointed out the presence of a small damaged area even in the specimens impacted at 1 J, which was not easy to be detected by micro-CT, visual inspection or aircoupled piezoelectric excitation. This innovative set-up allows for the implementation of compact, fibre-coupled NDT probes suited for the detection and generation of μs-timescale ultrasound transients with acquisition speed of 1 kHz, well suited for fast inline NDT inspection of composites and in particular of natural fibre composites.

Conclusions

The objective of this experimental work was to assess the impact damage in Flax/PLA-PP composites and propose a non-contact, non-destructive testing setup based on laser excitation of ultrasound and detection with a broadband, air-coupled optical microphone for its detection. In particular, a commercial Flax/PLA laminate modified with the addition of PP has been used as a reference material once subjected to low velocity impacts in the range 1 J -5 J. Damage mechanisms were detailed by visual inspection and micro-CT and complemented by the results provided by the laser-UT, which proved to be more sensitive to damages caused by low energy impacts. These preliminary results demonstrated that this set-up allows high-resolution imaging of defects in through-transmission mode. Future work will address single-sided setups, as well as an array configuration, where 8 optical microphones are arranged with a pitch of 2 mm, to increase scanning speed and allow for beamforming algorithms.

Tables

Figure 14

Figure captions Figure 1 .Figure 2 .Figure 3 .Figure 4 .Figure 5 .Figure 6 .Figure 7 .Figure 8 .Figure 9 .Figure 10 .Figure 11 .Figure 12 .Figure 13 .Figure 14 .

 captions1234567891011121314 Figure captions

Figure

  Figure 2

2

 2 Figure 2

Figure 3 Figure 4

 34 Figure 3

Figure 5 Figure 6

 56 Figure 5

  Figure 9

Table 1 .

 1 Summary of tensile and flexural properties for both configurations Flax/PLA 14.11±1.41 79.96±3.97 1.09±0.13 12.59±0.37 130.48±2.58 1.67±0.14 Flax/PLA-PP 12.48±1.17 86.42±1.24 1.36±0.07 12.43±0.48 124.96±5.17 1.69±0.05

		Tensile	Tensile		Flexural	Flexural
	Specimen ID	modulus	strength	ε f, T (%)	modulus	strength	ε f, F (%)
		(GPa)	(MPa)		(GPa)	(MPa)

Table 2 .

 2 Parameters obtained from impact tests on Flax/PLA-PP laminates

	Energy	Peak force (N)	E i (J)	E a (J)	Damage	Indentation
	level (J)				Degree	(μm)
	1	1371.61 ± 1.84	0.95 ± 0.01	0.05 ± 0.01	0.05 ± 0.01	35.08 ± 3.65
	2	1510.74 ± 26.54	1.98 ± 0.01	0.59 ± 0.08	0.30 ± 0.04	240.23 ± 9.85
	3	1668.80 ± 54.50	3.00 ± 0.01	1.13 ± 0.09	0.38 ±0.03	281.36 ± 10.21
	5	1877.86 ± 17.62	4.97 ± 0.02	2.32 ± 0.05	0.46 ± 0.03	415.02 ± 12.43

Data availability

The raw/processed data required to reproduce these findings cannot be shared at this time as the data also forms part of an ongoing study.


Mariana M Gil

Ahmed Maouche

Naz Serifoglu

Margarida Figueira

Miguel Godinho Ferreira

Non-cell autonomous roles of telomere shortening in zebrafish

Background:

Cancer incidence increases with age. Due to the end-replicative problem, telomeres shorten with each cell division throughout our lives. Telomere shortening has consequences beyond the cellular level. As we age, cells approach replicative senescence and DNA damage emanating by short telomeres initiate a cascade of events that expands to the extracellular environment. Senescent cells were shown to release a set of molecules including inflammatory cytokines, termed senescence-associated secretory phenotype (SASP). Indeed, an inflammatory environment generates a pro-tumorigenic environment that supports tumor invasiveness.

Aim of the study:

Investigate the causal relation between short telomeres and cancer incidence. Determine whether this effect has a non-cell autonomous component by using zebrafish embryo chimeras and tumor transplants.

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