Amine Khitous

Ching-Fu Lin

Farid Kameche

Hsiao-Wen Zan

Jean-Pierre Malval

Dominique Berling

Olivier Soppera
email: olivier.soppera@uha.fr

Plasmonic Au Nanoparticle Arrays for Monitoring Photopolymerization at the Nanoscale

Keywords: gold nanoparticles, Free radical photopolymerization, optical near-field radical photopolymerization, plasmonic nanosensors, hybrid nanoparticles

The localized surface plasmon resonance (LSPR) of Au nanoparticles (NPs) was used to monitor photopolymerization at the nanoscale, by in situ monitoring the optical response of AuNPs during the light-induced polymerization process. To show the interest of this approach, two configurations were used which correspond to a resonant and a non-resonant excitation regime between the photopolymer and the AuNPs used as nanoprobes. We show that not only this method enables the progress monitoring of the photopolymerization reaction at the nanometric scale but also can highlight the near-field coupling effect responsible for the acceleration of the photoinduced reaction. This methodology appears very interesting to study the photoinduced nanofabrication processes of metal/polymer hybrid nanoparticles and more globally as a methodology to study the photopolymerization reactions at the nanometric scale.

I. Introduction

Hybrid metal nanoparticles, combining a metallic core and an organic functional layer, possess novel properties that make them of significant interest in many fields such as energy, medicine and plasmonic nanosensors. [START_REF] El-Sayed | Surface Plasmon Resonance Scattering and Absorption of Anti-EGFR Antibody Conjugated Gold Nanoparticles in Cancer Diagnostics: Applications in Oral Cancer[END_REF][START_REF] Elghanian | Selective Colorimetric Detection of Polynucleotides Based on the Distance-Dependent Optical Properties of Gold Nanoparticles[END_REF][START_REF] Xu | Optical Refractive Index Sensors with Plasmonic and Photonic Structures: Promising and Inconvenient Truth[END_REF] Numerous methods for the nanofabrication of hybrid NPs have been proposed in the literature. [START_REF] Neoh | Functionalization of Inorganic Nanoparticles with Polymers for Stealth Biomedical Applications[END_REF][START_REF] Rangelov | Polymer and Polymer-Hybrid Nanoparticles: From Synthesis to Biomedical Applications[END_REF][START_REF] Froimowicz | Surface-Functionalized Particles: From Their Design and Synthesis to Materials Science and Bio-Applications[END_REF] These methods are often complex, and do not allow a fine control of the grafted organic layer, nor do they give a great flexibility on the nature of the organic component. Among these methods, the functionalization of metallic NPs by SAMs (Self Auto-assembled Monolayers) remains delicate and suppose the synthesis of corresponding precursors beforehand. [START_REF] Yang | A Facile One-Pot Synthesis of Versatile PEGylated Platinum Nanoflowers and Their Application in Radiation Therapy[END_REF] Optical near-field photopolymerization attracted recently much interest, as a smart approach to obtain hybrid NPs with a control at the nanoscale. [START_REF] Kameche | Plasmon-Triggered Living Photopolymerization for Elaboration of Hybrid Polymer/Metal Nanoparticles[END_REF][START_REF] Deeb | Plasmon-Based Free-Radical Photopolymerization: Effect of Diffusion on Nanolithography Processes[END_REF] The idea is to trigger radical photopolymerization reactions on the surface of metallic NPs by using two essential features of the optical near-field: i): the exaltation of the incident electromagnetic field by localized plasmon resonance phenomena, ii): the spatial localization of the exalted electromagnetic field at the vicinity of the metallic NPs.

Localization and enhancement effects allow to trigger radical photopolymerization reactions in the near-field region without affecting the far field if irradiation doses are chosen lower than the threshold dose for polymerization (E T ) at macroscale. The concept of this process has been described in several previous papers. [START_REF] Kameche | Plasmon-Triggered Living Photopolymerization for Elaboration of Hybrid Polymer/Metal Nanoparticles[END_REF][START_REF] Deeb | Plasmon-Based Free-Radical Photopolymerization: Effect of Diffusion on Nanolithography Processes[END_REF] Understanding the light interaction with metallic NPs in the context of optical nearfield photopolymerization is essential to move towards practical applications. Recently, advances in the understanding of mechanisms have been obtained by microscopy techniques such as atomic force microscopy (AFM), scanning electron microscopy (SEM) and transmission electron microscopy (TEM), [START_REF] Kameche | Plasmon-Triggered Living Photopolymerization for Elaboration of Hybrid Polymer/Metal Nanoparticles[END_REF][START_REF] Deeb | Plasmon-Based Free-Radical Photopolymerization: Effect of Diffusion on Nanolithography Processes[END_REF] by investigation of the changes in morphology of hybrid NPs synthesized under different conditions. However, there are still challenges and limitations in using these techniques in this context. For example, at the nanometric scale, AFM becomes inaccurate. SEM is efficient to demonstrate the presence of polymer but the precise size determination is difficult. TEM is the only technique that allows to perfectly visualize polymeric layers of a few nanometers, but unfortunately, the analysis is only possible on TEM grids, which limits the use of this technique. In this work, we propose a simple approach to follow optical near-field photopolymerization based on UV-vis spectroscopy.

The optical properties of metallic NPs are indeed largely dependent on the physical or chemical changes of the surrounding environment. [START_REF] Miller | Sensitivity of Metal Nanoparticle Surface Plasmon Resonance to the Dielectric Environment[END_REF][START_REF] Murphy | Anisotropic Metal Nanoparticles: Synthesis, Assembly, and Optical Applications[END_REF] This property explains their use as very sensitive sensors, either in thin film form (Surface Plasmon Resonance, SPR), or metallic nanoparticles (Localized Surface Plasmon Resonance, LSPR). [START_REF] Xu | Optical Refractive Index Sensors with Plasmonic and Photonic Structures: Promising and Inconvenient Truth[END_REF] In the quasistatic approximation, LSPR band of a metallic NPs is expressed by the following relation: [START_REF] Tang | Calculation Extinction Cross Sections and Molar Attenuation Coefficient of Small Gold Nanoparticles and Experimental Observation of Their UV-Vis Spectral Properties[END_REF] =

(1)

Where λ LSPR is the LSPR wavelength, is the real part of the dielectric function of the NP according to the interband model, and λ P is the resonant wavelength of the bulk. Equation 1 shows that the plasmonic band of metallic NPs depends directly on the refractive index of the surrounding medium (ε m = n 2 ). Theoretically, an increase in refractive index around the metallic NPs induces a red shift in the absorption spectrum.

Refractive index of the medium is known to change during photopolymerization.

According to F. Aloui et al, [START_REF] Aloui | Refractive Index Evolution of Various Commercial Acrylic Resins during Photopolymerization[END_REF] during a radical photopolymerization reaction, the refractive index increases by approximately 3.58 10 -2 (for a formulation close in composition to the one used in this work). This modification of the refractive index can be explained by the increase of the material density during the reaction from the monomer to the polymer, even if the consumption of the C=C bonds during the polymerization induces a decrease of the polarizability which has an opposite effect. [START_REF] Aloui | Refractive Index Evolution of Various Commercial Acrylic Resins during Photopolymerization[END_REF][START_REF] Matsuda | Optical Material of High Refractive Index Resin Composed of Sulfur-Containing Aliphatic and Alicyclic Methacrylates[END_REF] The objective of this paper is to demonstrate that the localized surface plasmon resonance shift can be used to monitor photopolymerization at the nanoscale, by in situ monitoring the optical response of gold nanoparticles (AuNPs) during the light-induced polymerization process. Because the spatial extend of LSPR is very limited, NPs should be sensitive to their direct environment, at the nanometric scale. The monitoring of the LSPR position is performed in a very simple way by using a low-cost fibered spectrophotometer.

Metal nanoparticles were obtained by thermal dewetting of Au thin films. This method is interesting because it is very simple to implement, reproducible and leads to nanoparticles with a high density, which allows simple transmission spectroscopy measurements. Gold is chosen here as a metal for its properties of stability, chemical inertness and its interesting exaltation factor. [START_REF] Kameche | Plasmon-Triggered Living Photopolymerization for Elaboration of Hybrid Polymer/Metal Nanoparticles[END_REF] Moreover, UV-visible allows an in situ characterization of the hybrid nanoparticles which is not possible by other techniques described previously. For example, AFM or electron microscopy can only be used "post-mortem", e.g. after development. Raman spectroscopy can not be used neither in situ since the high irradiance conditions in excitation may trigger photopolymerization of local heating.

Two configurations were used which correspond to a resonant and a non-resonant regime between the photopolymer and AuNPs used as nanoprobes. Indeed, we used a photopolymer sensitive in the UV range, excited at 365 nm (non-resonant with LSPR of AuNPs located between 532 and 552 nm) and a photopolymer sensitive at 532 nm, excited at 532 nm that can also photoactivate the plasmonic modes of the NP (resonant mode). Both photopolymers are generated through a free radical photopolymerization process whose mechanism is described elsewhere. [START_REF] Kameche | Plasmon-Triggered Living Photopolymerization for Elaboration of Hybrid Polymer/Metal Nanoparticles[END_REF][START_REF] Deeb | Plasmon-Based Free-Radical Photopolymerization: Effect of Diffusion on Nanolithography Processes[END_REF] These two configurations were chosen not only to

show that the plasmonic probes can be used to follow the photopolymerization of photopolymers with different wavelengths, but also to demonstrate that the technique is sensitive to probe optical near-field photopolymerization.

We first present the preparation of NPs arrays used in this work, from a morphological and spectroscopic point of view, as well as their optical response in media of different refractive index. The shift of LSPR is then followed by UV-visible spectroscopy to monitor the photopolymerization of a UV-sensitive photopolymer at 365 nm and a green-sensitive photopolymer at 532 nm, using the same nanoparticles with LSPR maxima between 532 and 552 nm. These results are also compared with FTIR spectroscopy characterizations in order to demonstrate that our method successfully address the monitoring of the photopolymerization process at the nanoscale, which remains a current fundamental challenge with potential feedbacks on nanofabrication and better knowledge on metal-organic molecules coupling.

II Experimental section

II.1 Preparation and characterization of gold nanoparticles (AuNPs)

AuNPs were prepared by thermal dewetting from a sputtered gold film. AuNPs were generated on this membrane using the same procedure as one glass substrates.

UV-visible spectra were recorded using a fibered spectrophotometer (AvaSpec-ULS2048CL-EVO from Avantes).

II.2 Preparation and characterization of photopolymers

II.2.1 Chemicals

The 532 nm sensitive formulation (F 532 ) was prepared by mixing 95.5 wt% pentaerythrilol-triacrylate (PETA, Sigma Aldrich) as a monomer, 0.5 wt% eosin Y (99%, Sigma Aldrich) as a photoinitiator and 4.0 wt% N-methyldiethanol-amine (MDEA, ≥ 99%, Sigma Aldrich) as a co-photoinitiator.

Another formulation (F 365 ) was prepared with the same monomer but a photoinitiator with an absorption band in the UV. There is thus no spectral overlap of F 365 with the extinction spectra of NPs used here. F 365 contains pentaerythrilol-triacrylate (PETA, Sigma Aldrich) as a monomer and phenylbis(2,4,6-trimethylbenzoyl) phosphine oxide (BAPO, Ciba, 0.1wt%) as a photoinitiator.

When irradiated within their absorption spectrum, each formulation undergoes a freeradical photopolymerization process. The scheme is summarized in

II.2.2 Characterization of the polymerization threshold

The polymerization threshold was characterized for both formulations. For a given power, it corresponds to the light dose (E T ) necessary to reach the gel point of the formulation and thus obtain polymer on the substrate after rinsing with a solvent (ethanol). For these formulations, the polymerization threshold is related to the inhibition time which is the time needed to consume the inhibitors present in the formulation and start the polymerization at the molecular scale. The main inhibitor is the oxygen dissolved in the formulation.

A Coherent Verdi 6 laser at 532 nm was used to irradiate F 532 . A spatial filter was used to generate an homogeneous spot with a diameter of 1.5 cm. Alignment marks were used to ensure that the sample is always placed in the same place during irradiation and characterization steps. The threshold energy was E T = 2.04 mJ/cm 2 for t T = 1.2 s at a power of P = 1.7 mW/cm 2 . For F 365 , a Hamamatsu LC8 L9588 UV lamp at 365 nm was used. The threshold dose for this formulation was E T = 1.8 mJ/cm 2 , t T = 0.9 s at a power of P = 2 mW/cm 2 .

II.2.3 FTIR

The polymerization kinetics were studied on the KBr pellets for both formulations with a Thermo Scientific Nicolet 6700/8700 FT-IR spectrometer. After synthesizing AuNPs on the glass slide, a drop of the photopolymerizable formulation was deposited on AuNPs and covered by another glass slide, to limit the effect of oxygen inhibition (Figure 2a). The thickness of the film (measured by digital microscope) was 24 µm. Figure 2 schematically depicts the general protocol for photopolymerization study.

II.3 Monitoring of photopolymerization by LSPR shift

The monitoring of the photopolymerization was performed by an AvaSpec-ULS2048CL-EVO fiber optic spectrometer (Avantes). This configuration allows the characterization to be performed in situ and thus the same location of the sample is analyzed, within the irradiated area. Each spectrum was averaged over 100 spectra for an integration time of 100 ms.

Photopolymerization was studied using the two protocols, both irradiated with doses between 0 and 200% E T . The "no-development method" (in situ monitoring) was performed by sequentially irradiating the sample and analyzing it after each irradiation (Figure 2 b-c). In the second method, the sample is opened and developed after each irradiation (Figure 2 b'c'). In both cases, the two formulations (F 532 and F 365 ) were used to compare their behavior.

III Results and discussion

III.1 AuNPs synthesis and their optical properties (∆λ LSPR )

AuNPs used in this work were synthesized by thermal dewetting. The spectral position of their LSPR band was controlled by the temperature, the time of the thermal treatment (400°C and 1 hour) and the initial thickness of the gold film. The interest of this technique compared to other colloidal AuNPs deposition techniques is its simplicity (no need for NPs synthesis step or surface functionalization for a homogeneous deposition, without aggregates). [START_REF] Lu | Polymer-Templated Gold Nanoparticles on Optical Fibers for Enhanced-Sensitivity Localized Surface Plasmon Resonance Biosensors[END_REF] Above all, this process allows to obtain a high surface density of AuNPs, which implies a good optical response in spectroscopy, an essential feature in this study.

The formation mechanisms of these films have been widely studied. The surface tension of gold decreases with increasing temperature. [START_REF] Mezger | Loi de Variation de La Tension Superficielle Avec La Température[END_REF] Therefore, heating of the gold film changes its properties and morphology in order to minimize its interfacial energy, this is done especially in particular according to its initial thickness. Holes appear first by dewetting effect in the film, then filaments and finally droplets, [START_REF] Ruffino | Formation of Nanoparticles from Laser Irradiated Au Thin Film on SiO2/Si: Elucidating the Rayleigh-InstabilityRole[END_REF] thanks to the Rayleigh instability phenomena. [START_REF] Powers | Pearling and Pinching: Propagation of Rayleigh Instabilities[END_REF] After cooling, these droplets turn into nanoparticles. Cross sections of AFM images and SEM images are given in S3.

Characterization of the synthesized AuNPs (Figure 3) shows that the synthesis of AuNPs by thermal dewetting confers good control over the shape and position of AuNPs LSPR band, with relatively narrow extinction spectra. By changing the initial thickness of the gold film from 3 to 5 nm, the LSPR band can be tuned from 532 to 552 nm, with a good reproducibility. From the AFM images (and SEM images in S3), we note that as the initial film thickness increases, the size and spacing between AuNPs increases. Their homogeneity is also reduced. This confirms what was demonstrated by Roffino et al, [START_REF] Ruffino | Formation of Nanoparticles from Laser Irradiated Au Thin Film on SiO2/Si: Elucidating the Rayleigh-InstabilityRole[END_REF] who showed that if the initial film thickness is increased, the nucleation-growth regime is favored and the homogeneity of AuNPs is lost.

From SEM and AFM images, AuNPs can be described as nanoislands which distribution depends on the gold film initial thickness, temperature and heat treatment time.

Assuming a circular section, the AuNPs diameter calculation gives a size of 15 ± 5, 22 ± 5 and 30 ± 5 nm for AuNPs with LSPR bands at 532, 542 and 552 nm, respectively. The cross sections of AuNPs (Figure S3) show that they are flattened and have a height of 13 ± 4 nm, 18 ± 3 nm and 20 ± 5 nm for AuNPs at 532, 542 and 552 nm, respectively. Analysis of the three types of AuNPs by UV-vis spectroscopy is consistent with AFM and SEM observation:

as the initial film thickness increases, larger (a red shift of LSPR band) and less homogeneous (a wider LSPR band) AuNPs are obtained.

Ultimately, this method does not result in a narrow dispersity size like those obtained by colloidal synthesis, but the spectral response is narrow enough to be exploited, as will be seen in the following. Moreover, the simplicity of the preparation, the reproducibility and the high density of NPs (allowing to obtain high extinction signal) completely justify the use of these NPs in this context to follow-up of the photopolymerization. To demonstrate this interest, the optical response of these NPs was validated by immersing them in media of different refractive index.

Mixtures of different ratio (Glycerin: H 2 O) were used, in order to prepare solutions with refractive indexes between 1.33 ≤ n ≤ 1.47. [START_REF] Kim | Improved Stability of Gold Nanoparticles on the Optical Fiber and Their Application to Refractive Index Sensor Based on Localized Surface Plasmon Resonance[END_REF][START_REF] Chen | Shape-and Size-Dependent Refractive Index Sensitivity of Gold Nanoparticles[END_REF] The calibration was done for the three NPs sizes. The initial spectrum of AuNPs was always recovered after three rinses with ethanol, demonstrating the absence of NPs changes from one solution to another. Figure 4 shows the variation in plasmonic shift versus air (∆λ LSPR ) for the three types of AuNPs.

Another point with a refractive index of 1.48 (pure PETA) was added to extend the calibration curve showing that the calibration curve can be applied to the photopolymer used in this study. Lu et al. [START_REF] Lu | Polymer-Templated Gold Nanoparticles on Optical Fibers for Enhanced-Sensitivity Localized Surface Plasmon Resonance Biosensors[END_REF] As we increase the density of AuNPs, by reducing their sizes, we get a better S RI .

These AuNPs show a better sensitivity (108 nm/RIU) than the spherical AuNPs studied by

Sun and al (60nm/RIU).[24]

III.2 Study of photopolymerization by LSPR band shift of AuNPs

III.2.1 Off-resonance photopolymerization (@ 365 nm, with F 365 )

The photopolymerization reaction is followed via the induced modulation of the refractive index that modifies the LSPR band position of NPs, following two experimental protocols: 1/ a first study is conducted in situ, i.e., the plasmonic shift is analyzed by UV-vis spectroscopy without rinsing the photopolymerizable formulation. 2/ In the second configuration, the photopolymerizable formulation is rinsed after each irradiation before analysis. Thus, in this case, the spectral shift is related to the polymer which remains attached to the surface of AuNPs.

We first consider the case of off-resonant irradiation, i.e. the chosen irradiation wavelength (365 nm) matches the absorption band of the F 365 photoinitiator but not the surface plasmon absorption band of AuNPs which is centered at 532 nm in air in our precise case (562 nm in the formulation F 365 ). The aim of this section is to prove that the proposed methodology is able to monitor the polymerization process of a reference photopolymer sensitive in the UV range.

Experimental results are plotted in Figure 5. In the first case (without development, Figure 5 a), we note that the extinction spectra of NPs do not evolve for doses lower than 100 % E T . We thus confirm that photopolymer modifications below the threshold dose do not appreciably affect the refractive index of the medium and thus do not induce a shift of the NPs resonance band. Upon an irradiation dose higher than E T , a slight shift of the resonance band is observed which is evaluated at 4 nm for 200% E T .

These results are confirmed for the experiments with development (Figure 5 b). In this case, the extinction spectrum of NPs obtained after irradiation and development is not modified for doses below 100 % E T . For such irradiation conditions, the initial spectrum of NPs in air is exactly recovered. This result also confirms that the rinsing procedure effectively removes the formulation before polymerization. Above 100 % E T , the spectra evolve significantly. The red-shift and the increase in absorbance show the appearance of a polymer layer on the nanoparticles. These first results demonstrate that the methodology developed here is able to detect the polymerization of the monomer when it is excited in the UV range. In the following, we consider resonant conditions.

III.2.2 Resonant irradiation (@ 532 nm, with F 532 )

In this section, the chosen irradiation wavelength (532 nm) matches both the absorption band of the photoinitiator of F 532 and the surface plasmon absorption band of AuNPs. AuNPs with LPSR in air centered at 532, 542 and 552 nm were chosen here. The same experimental strategies were used as before. In one case, monitoring was done in situ by UV-vis spectroscopy. In the second case, a rinsing step was used to remove the monomer, as before.

Figure 6 shows the evolution of the UV-visible spectra for the 3 types of NPs (LSPR in air at 532, 542 and 552 nm). This analysis was performed first without development. In the three cases, the spectra can be analyzed as the sum of two contributions: 1/ the spectrum of eosin Y (maximum at 532 nm) and 2/ the LSPR band of nanoparticles, which is shifted when nanoparticles are immersed in the photopolymer (respectively at 559, 573 and 580 nm). The shift of the LSPR band (∆λ LSPR around 28 nm) corresponds to what would be expected when changing from a medium of index n=1.00 to n=1.48 (refractive index of PETIA). In all three cases, the peak corresponding to the absorption spectrum of eosin Y decreases upon irradiation, as expected since eosin Y is consumed in the photopolymerization process. In the case of NPs with LSPR band at 532 nm, LSPR shift is observed from 30% E T (Figure 6 a). This means that the refractive index in the immediate vicinity of NPs increases well before the polymerization threshold which is, we remind, the dose for which cross-linked polymer is observed on the substrate after rinsing. This observation reveals an acceleration of the polymerization kinetics around these AuNPs that was not observed in the case of the nonresonant conditions. For the two other sizes of AuNPs (LSPR at 542 and 552 nm) the plasmonic shift is also observed before the threshold energy, but a slightly higher energy than

NPs with LSPR at 532 nm (above 50% E T , Figure 6 b-c).

The acceleration of the photopolymerization kinetics is thus observed in these three resonant cases, and with a maximum efficiency for the best overlap between the irradiation wavelength and the NPs spectrum. The polymerization before the threshold is explained by the exaltation of the electromagnetic field near the surface of NPs which makes the photoinduced reaction more efficient than in the far field, as led during the determination of the threshold energy. The plasmon resonance is thus more efficient for the best spectral match, which corresponds to a greater near-field exaltation, confirming that it is indeed the near-field polymerization that is observed. These first results confirm that the optical response of NPs can be used to probe the photopolymerization reactions.

In order to confirm that the shift of LSPR band corresponds to the photopolymerization around the nanoparticles, the same study is carried out with the three types of nanoparticles, with rinsing the sample between the irradiation and the spectroscopic analysis, to remove the uncross linked photopolymer. The results are shown in Figure 7, again with the three types of NPs. An interest of this approach is to get rid of the photoinitiator peak. This peak is no longer visible because the photopolymer containing eosin Y is eliminated before spectroscopic characterization. Note that the polymer layer around NPs may contain eosin Y but it is not visible because of the very small thickness of the layer and also because eosin Y is bleached during polymerization (Figure S18).

In all three cases, we can see the red-shift of LSPR, which means that the polymer is well formed around NPs. Note that without irradiation, a control experiment allowed us to verify that the rinsing conditions lead to recover the initial spectrum of NPs, demonstrating that the rinsing procedure allows to eliminate all the unreacted photopolymer and that the shift cannot be interpreted by a physisorption of some components of the photopolymer on the surface of NPs. For doses below the threshold dose, no polymer could be observed either with the naked eye or by optical microscopy, which also confirms that the spectral shift can only be explained by a very thin layer formed on the surface of NPs.

As previously indicated, the shift of the plasmon resonance is observed for values below the threshold dose, demonstrating the acceleration of the photopolymerization due to the presence of NPs. We also observe that the effect is sensitive from 30% of the threshold dose

for NPs resonating at 532 nm and 50% of the threshold dose for the other two types of particles, again in agreement with a near-field photopolymerization mechanism. In both cases, we notice that for 200% E T , we did not reach the maximum shift, which means that the reaction continues after this value. To better understand this phenomenon, the photopolymerization kinetics obtained by FTIR were recorded for both formulations excited at their respective sensitive wavelength (Figure 9), without nanoparticles. In this case, the consumption of C=C is followed and represented in conversion % of the monomer. The full spectra are given in Figure S19 and Figure S20. It can be seen that the conversion obtained at 100% E T is less than 10%. This value is very low compared to the final conversion, which is of the order of 40% for F 365 (blue dots) and 50% for F 532 (green dots). Note that these conversion values are in agreement with previously published results for this photopolymer in comparable irradiation conditions. [START_REF] Deeb | Plasmon-Based Free-Radical Photopolymerization: Effect of Diffusion on Nanolithography Processes[END_REF] The maximum conversion plateau is only reached for a dose of about 10 times the threshold dose, which explains why the LSPR shift curves do not yet saturate at 200% E T . These results

show that LSPR technique is very well adapted to characterize the near-field polymerization which corresponds to the zone probed by NPs.

Using the calibration curve established in Figure 4, it is possible to compile the results obtained with the 2 formulations and the 3 types of nanoparticles by plotting the variation of effective refractive index as a function of the dose used to irradiate the nanoparticles (Figure 10). Here, the data obtained in situ (without development) were used. We show that the monitoring of the NPs resonance is sensitive enough to follow the photopolymerization reaction. It is clearly seen that under resonance conditions, the photopolymerization kinetics is accelerated. This assumes a maximum match between the irradiation wavelength and the extinction spectrum of NPs. In the extreme case where there is no overlap, no near-field effect is observed and photopolymerization starts for an irradiation dose value close to the threshold energy. It is interesting to consider the value of the effective refractive index variations obtained at 100% E T and 200% E T . At 100 % E T , this value reaches 0.04, which is close to the value reported in the literature for measurements made in holography, with a similar formulation. [START_REF] Aloui | Refractive Index Evolution of Various Commercial Acrylic Resins during Photopolymerization[END_REF][START_REF] Espanet | Photopolymerization by Evanescent Waves: Characterization of Photopolymerizable Formulation for Photolithography with Nanometric Resolution[END_REF][START_REF] Dorkenoo | Time-Resolved Measurement of the Refractive Index for Photopolymerization Processes[END_REF][START_REF] Sainov | Wide Spatial Frequency Evanescent Wave Holographic Recording in Photopolymer[END_REF] We note that this value continues to increase between 100% E T and 200% E T . We can suppose that it is due to the increase of the polymerized volume around NPs which increases the effective refractive index value. For the green irradiation, this value seems to stabilize around 0.06. Note that for the UV irradiation, the plateau value is lower (0.03). As a thick layer of polymer surrounds NPs for 200 % E T in the UV, we can conclude that the polymeric network synthesized after far field UV irradiation is less dense than the one obtained in near-field with resonant irradiation. The exaltation of the electromagnetic field that accompanies the plasmon excitation of NPs can explain this difference. These conditions seem to demonstrate a faster and more complete cross-linking of the polymer network around NPs. Conversion measurements in surface-enhanced Raman spectroscopy (SERS) could be considered to investigate this point in the future.

Conclusion

By using both resonant and non-resonant excitation conditions, we showed that the LSPR bandshift of AuNPs observed during the irradiation is connected to the progress of the photopolymerization reaction which is concomitantly accelerated by plasmonic effects at metal NPs surfaces. Indeed, the shifts observed are caused by the sensitivity of gold nanoparticles to the refractive index variations during the progress of the photopolymerization reactions. By using UV-vis spectroscopy, this simple approach allowed us to monitor the kinetic of photopolymerization at the vicinity of the AuNPs. The spectral overlap between the gold nanoparticles and the photoinitiator was proved to be an important parameter, confirming the near-field photopolymerization. This study is important to confirm our understanding of the fabrication mechanisms of metal/polymer hybrid nanoparticles that was based on ex situ post-mortem analysis techniques. Here, the method can be applied to evidence the near-field photopolymerization in situ, without development. This can be applied to other photopolymer for the synthesis of new functional hybrid nanoparticles. The strategy used here can also be considered to follow in a more global way photopolymerization reactions in hyperconfined environments (under Deep-UV, EUV, two-photon absorption irradiation...). In this case, one could imagine using a single nanoparticle and follow the modification of its spectrum with photopolymerization.

Associated Content:

 UV-vis absorption spectra of formulations used and molecular structures of chemicals.

 AFM profiles of the sputtered Au thin films and AuNPs thin films. SEM images of AuNPs thin films.

 Raw absorption spectra of AuNPs with the two formulations, for different irradiation doses and with/without development.

 FTIR spectra of formulations during photopolymerization.

 Variation of Refractive index as a function of the irradiation threshold energy (% E T ) for both formulations (F 523 and F 365 ).

Figure 1 .

 1 The absorption spectra are given in Figure S1 and Figure S2.

Figure 1 .

 1 Figure 1. Scheme illustrating the molecular structures of the chemicals used and free-

Figure 2 .

 2 Figure 2. Schematic view of free radical photopolymerization monitoring protocol. Study

Figure 3 .

 3 Figure 3. Characterization of AuNPs prepared from different initial gold film thickness

Figure 4 .Figure 4

 44 Figure 4. Variation of LSPR shift of the 3 AuNPs (532, 542, 552 nm) with respect to air as a

Figure 5 .

 5 Figure 5. Absorption spectra of F 365 -AuNPs (LSPR at 532 nm) without (a) and with

Figure 5 c

 5 Figure 5 c displays the variation of the LSPR position for the two configurations (with

Figure 6 .

 6 Figure 6. Absorption spectra of F 532 -AuNPs without development, after each irradiation, for

Figure 7 .

 7 Figure 7. Absorption spectra of AuNPs -F 532 with development, after each irradiation, for the

Figure 8 .

 8 photopolymerization threshold. The shift of the NPs resonance is sensitive for lower doses

Figure 8 .

 8 Figure 8. Variation of plasmonic shift as function of percentage threshold energy (%E T ) for

Figure 9 .

 9 Figure 9. Conversion of PETA C=C bond (monitored by 1631 cm -1 band) during in situ photopolymerization of F 532 (λ irr: 532nm, 1.7 mW/cm 2 , green) and F 365 (λ irr: 365 nm, 2 mW/cm 2 , bleu) as function of percentage threshold energy (%E T ). (a): 0 ≤ % E T ≤ 10 5 , (b): 0

Figure 10 .

 10 Figure 10. Comparison of effective refractive index variations as a function of irradiation

Acknowledgements

The authors thank French Agence Nationale de la Recherche for fundings (project DYNABIO, grant ANR-19-CE19-0012, 3D-BEAM-FLEX, grant ANR-20-CE24-0020 and NIRTRONIC, grant ANR-18-CE24-0028). This work of the Interdisciplinary Institute HiFunMat, as part of the ITI 2021-2028 program of the University of Strasbourg, CNRS and Inserm, was supported by IdEx Unistra (ANR-10-IDEX-0002) and SFRI (STRAT'US project, ANR-20-SFRI-0012) under the framework of the French Investments for the Future Program.

Author contributions

A.K. carried out the experiments. C.-F.L. and F.K. contributed to the nanoparticles synthesis and analysis. All authors discussed the results and contributed to the final manuscript. O.S supervised the project.

TOC image


Ching-Fu Lin

Amine Khitous

Hsiao-Wen Zan

Olivier Soppera

Exploiting thermoplasmonic effects for laser-assisted preparation of Au nanoparticles/InZnO thin film with visible range photodetection properties

We propose a new method for preparing gold nanoparticles (Au NPs)/indium-zinc-oxide (IZO) nanocomposite thin films based on photothermal mechanisms with near-Infrared (NIR) laser-annealing, which allows integrating the nanomaterial on fragile substrates such as thin glass, plastic sheets, or 3D printed pieces. The Au NPs were first prepared by NIR laser dewetting of a thin Au layer.

Then, the Au NPs were used to locally cure the semiconductor material and provide suitable electronic properties owing to their efficient thermoplasmonic effects under our NIR laser annealing conditions. Finally, the electronic properties of the Au NPs/IZO thin films were characterized in the dark and under light excitation. Good photoresponsivity at 410 nm (UV, > 10 0 A/W) was demonstrated, but interestingly, the presence of Au NPs significantly improved the detection ability to a longer wavelength range, such as to 515 nm (green, ~ 5 × 10 -3 A/W), even extending to 630 nm (red, ~ 5 × 10 -4 A/W), and 780 nm (NIR, ~ 10 -4 A/W). In addition, with the critical evaluation of dynamic light detection and lifetime trace (> 22 days), the laser-annealed Au NPs/IZO photodetector (PD) demonstrated useful operating reliability and stability.

Introduction

Over the past few decades, thin-film oxide semiconductor devices have attracted considerable interest for various applications [START_REF] Yeh | Solution-Based Micro-and Nanoscale Metal Oxide Structures Formed by Direct Patterning for Electro-Optical Applications[END_REF], such as nanogenerators [START_REF] Waseem | Cu 2 O Heterostructured GaN Thin Film and GaN Nanowire Piezoelectric Nanogenerators[END_REF], sensors for biomedical [START_REF] Lin | NH 3 Plasma-Treated Magnesium Doped Zinc Oxide in Biomedical Sensors with Electrolyte-Insulator-Semiconductor (EIS) Structure for Urea and Glucose Applications[END_REF] or gas detection [4], photomemories [5,6], and photodetectors (PDs) [START_REF] Alema | Solar blind photodetector based on epitaxial zinc doped Ga 2 O 3 thin film[END_REF][START_REF] Ahmed | High-performance multicolor metal-semiconductor-metal Si photodetector enhanced by nanostructured NiO thin film[END_REF]. To achieve such excellent electrical properties, thin-film deposition by a high-vacuum system, such as sputtering, have been widely used [START_REF] Dong | Electrochromic properties and performance of NiOx films and their corresponding all-thin-film flexible devices preparedby reactive DC magnetron sputtering[END_REF][START_REF] Niang | Zinc tin oxide thin film transistors produced by a high rate reactive sputtering: effect of tin composition and annealing temperatures[END_REF][START_REF] Dawoud | Sputtered manganese oxide thin film on carbon nanotubes sheet as a flexible and binder-free electrode for supercapacitors[END_REF]. Such processes usually require high cost for device fabrication. In contrast, the use of solution-based materials, especially for metal precursors used in sol-gel chemistry, has attracted significant interest over the past few years [START_REF] Yu | Direct Laser Writing of Crystallized TiO 2 and TiO 2 /Carbon Microstructures with Tunable Conductive Properties[END_REF][START_REF] Han | Low-temperature, high-performance, solution-processed indium oxide thin-film transistors[END_REF][START_REF] Nomura | Room-temperature fabrication of transparent flexible thin-film transistors using amorphous oxide semiconductors[END_REF][START_REF] Leuschel | Deep-UV photoinduced chemical patterning at the micro-and nanoscale for directed self-assembly[END_REF][START_REF] Noor | A review of the Nd: YAG laser marking of plastic and ceramic IC packages[END_REF][START_REF] Ezra | The microsecond 1064 nm Nd: YAG laser as an adjunct to improving surgical scars following Mohs micrographic surgery[END_REF]. To achieve a thin-film oxide material based on the sol-gel method, a high-temperature thermal annealing process is usually necessary. It is necessary to remove the organic ligands and promote the condensation reaction to obtain a metal-oxide thin film from a xerogel structure. However, high-temperature treatment may limit further applications on flexible or plastic substrates. To date, a few techniques have been reported to overcome these drawbacks. Deep ultraviolet (DUV) or near infrared (NIR) annealing have already been proposed as the critical step for fabricating thin-film oxide semiconductor devices. Kim et al. [18], Bolat et al. [19], and Moon et al. [START_REF] Moon | Intense Pulsed Light Annealing Process of Indium-Gallium-Zinc-Oxide Semiconductors via Flash White Light Combined with Deep-UV and Near-Infrared Drying for High-Performance Thin-Film Transistors[END_REF] showed that DUV or NIR annealing at low environmental temperatures can be successfully used to obtain thin-film devices with good carrier mobility and electrical properties. Furthermore, we recently showed that the use of NIR dyes can significantly improve the NIR laser curing of sol-gel indium-zinc-oxide (IZO) materials and allow their use as gas sensors [START_REF] Chang | Near Infrared Laser-Annealed IZO Flexible Device as a Sensitive H2S Sensor at Room Temperature[END_REF].

One of the key parameters for the laser curing of metal-oxide precursors is to generate high absorption and efficient conversion to thermal energy. Because the sol-gel layers have an intrinsic low absorption in the NIR, it is necessary to add absorbers in the thin film to generate a local temperature increase. Highly conjugated organic molecules (NIR dyes), carbon black, and carbon nanotubes are potential candidates. Gold nanoparticles (Au NPs) have also proved their utility. Under light excitation, significant thermal effects can be generated, which are known as thermoplasmonic effects [START_REF] Webb | Emerging advances in nanomedicine with engineered gold nanostructures[END_REF][START_REF] Hodak | Spectroscopy and dynamics of nanometer-sized noble metal particles[END_REF][START_REF] Link | Laser photothermal melting and fragmentation of gold nanorods: energy and laser pulse-width dependence[END_REF][START_REF] Link | Shape and size dependence of radiative, non-radiative and photothermal properties of gold nanocrystals[END_REF][START_REF] Hirsch | Nanoshell-mediated near-infrared thermal therapy of tumors under magnetic resonance guidance[END_REF][START_REF] Govorov | Generating Heat with Metal Nanoparticles[END_REF]. They correspond to the damping of a plasmon resonance, which creates a temperature increase at the surface of NP that depends on several parameters such as irradiance of the incident light, irradiated area, light wavelength, nature and morphology, surface density, nature of the surrounding medium, and the substrate [START_REF] Baffou | Applications and challenges of thermoplasmonics[END_REF]. In specific conditions, temperature increases such as several hundreds of degrees are achievable and have been used to trigger several chemical reactions [START_REF] Fedoruk | Nanolithography by Plasmonic Heating and Optical Manipulation of Gold Nanoparticles[END_REF][START_REF] Stamplecoskie | Dual-Stage Lithography from a Light-Driven, Plasmon-Assisted Process: A Hierarchical Approach to Subwavelength Features[END_REF][START_REF] Walker | Photothermal Plasmonic Triggering of Au Nanoparticle Surface Radical Polymerization[END_REF]. In this paper, we demonstrate that this route is suitable for curing IZO thin films.

Moreover, when Au is used as nanoheaters in the film, the final material comprises Au NPs embedded in a MO thin film, which can be interesting for applications, such as PDs, with a sensitivity range that can span the UV and visible ranges owing to the extinction spectra of Au NPs. We demonstrated the potential of Finally, examples of Au NPs/IZO PDs were successfully fabricated on 3D printable polymer sheets, which firmly indicated that the novel NIR laser annealing technique can contribute to a breakthrough approach for integrating optoelectronic functions in the field of 3D printing technology.

Experimental section

Materials

Indium nitrate (indium(III) nitrate hydrate, 99.9%), zinc nitrate (zinc nitrate hexahydrate, 99.0%), and 2-methoxyethanol (2-MOE, 99.9%), which were used for forming photosensitive layer by sol-gel synthesis method, were purchased from Sigma Aldrich. The Au target for forming Au thin films (NPs) was purchased from Eloïse (France).

Synthesis of IZO using the sol-gel process:

To prepare the sol-gel IZO solution, indium nitrate and zinc nitrate precursors were dissolved in 2-MOE at a molar ratio of 5:4. The total concentration of indium: zinc in the solution was 0.25 M. After stirring for 24 h, the solution was used in the following PD fabrication steps.

Fabrication process of Au NPs/IZO PDs

Fig. 1(a) illustrates the fabrication steps for forming Au NPs/IZO PDs by an 808-nm-laser-annealing. The laser source was a high-power array of VCSELs (Innoptics) providing a maximum of 60 W over surfaces adjustable from 5 × 5 to 20 × 20 cm 2 . Initially, the substrates were prepared by a standard cleaning process, including dipping them in acetone and ethanol with ultrasonic baths. Next, the substrates were deposited in a sputtering machine (CRESSINGTON 108 auto) to form an Au film with a nominal thickness of 8 nm. The first laser-annealing step was used to generate Au NPs with no additional treatment.

The IZO precursor solution was deposited by spin-coating (SÜSS MicroTec spin-coater), thereby resulting in a continuous layer of IZO precursors covering the Au NPs. Then, a 2 nd laser-annealing process was adopted to convert the IZO precursors into amorphous IZO by removing solvent and organic residues, and inducing condensation. Finally, 20-nm-Au electrodes were deposited by sputtering through a shadow mask, which defined the active area of the PD as 2 × 2 mm 2 . Fig. 1(b) shows a schematic of the cross-section structure of the PD. The interval between the electrodes was set to 200 μm, which helped the device work as a miniaturized photosensitive resistor. In addition, Fig. 1(c) shows a real image of the experimental sample fabricated by an 808-nm-laser-annealing, including the Au NPs synthesized by the 1 st annealing process (28 W•cm -2 for 30 min) and Au NPs with IZO curing by the 2 nd annealing process (20 W•cm -2 for 1 h). As seen in these images, the generation of Au NPs by the 1 st laser annealing results in a color change from the olive to purple. After IZO was coated on the Au NPs during the 2 nd annealing process, the color of the treated part changed again to indigo.

For the comparison of Au NPs/IZO PDs fabricated by conventional thermal and the novel 808-nm-laser-annealing process, one PD with a similar structure was prepared. The fabrication process was the same as that in Fig. 1(a), except that the 1 st and 2 nd annealing processes were replaced by the thermal annealing processes. In all cases, the Au NPs arrays were characterized by UV-vis spectroscopy. With given fabrication process parameters, good reproducibility could be obtained, as proved by reproducible extinction spectra.

Device characterization

Atomic force microscopy (AFM) was performed using a Molecular Imaging PicoPlus (resonant mode). Au NP morphology was also studied by TEM with an ARM200F JEOL high-resolution transmission electron microscope. Silicon TEM grids with two rectangular slits (1500 × 100 µm) covered one face, with a 50 nm-thick Si 3 N 4 membrane (TEDPELLA). UV-visible spectra were recorded on a PerkinElmer Lambda 950 spectrophotometer, and XPS spectra were recorded on a ULVAC-PHI, PHI QuanteraII.

The light detection performance of the Au NPs/IZO PD was evaluated at 410 nm (UV), 465 nm (blue), 515 nm (green), and 630 nm (red) using commercial LEDs, and at 780 nm with an NIR laser diode. In addition, the PDs were connected to the electrical measurement system (KEITHLEY 2636B) to provide the sweeping voltage or fixed bias voltage with a dynamic monitoring function to collect the electrical properties.

Results and discussion

Surface morphology and LSPR characteristics of Au NPs/IZO film

In this study, Au NPs served as the key components of the device for their fabrication and final properties. The spectral properties of the PD are directly linked to the LSPR of the NPs, which are driven by their morphology. The synthesis of Au NPs on glass substrates was thus optimized. Fig. 2(a)-2(c) show the surface morphology obtained by AFM. Fig. 2(a) shows the original Au film with a thickness of 8 nm, with a smooth surface morphology (roughness: 1.27 nm). In Fig. 2(b), the sample was synthesized by a thermal annealing process (300 °C, 1 h). As shown in Fig. 2(c), the sample was synthesized by an 808-nm-laser-annealing process (28 W•cm -2 , 30 min). Both exhibited a discontinuous structure on their surfaces, demonstrating Au NPs can be synthesized by either thermal or laser treatment. In addition, the morphology of the Au NPs synthesized by laser-annealing process (average thickness: 29.6 nm, under 28 W•cm -2 , 30 min) could be tuned to similar results as those synthesized by the thermal annealing process (average thickness: 23.6 nm, under 300 °C, 1 h). For confirming the exact size distribution of Au NPs, TEM was used. Fig. 2(d) shows the image of Au NPs obtained by thermal annealing under the same conditions as Fig. 2(b), wherein well-disconnected Au NPs can be observed. The average distance between NPs appears to be more important in TEM analysis, which can be explained by the AFM tip aspect ratio in AFM images that artificially decrease this distance. Furthermore, from the TEM image analysis, the mean perimeter [Fig. S1(a)] and mean area [Fig. S1(b)] can be evaluated, which helps in evaluating the average diameter of Au NPs as approximately 21.9 nm (assuming a circular shape, which is a rough estimation). Interestingly, the NP structure remained unchanged after IZO coating (Fig. S1(c) to S1(e), average diameter: 20.2 nm). For all preparation conditions used in this study, it can be observed that the size and morphology of NPs is quite inhomogeneous. As shown later in this paper, such characteristic generates a broad extinction spectrum that is very interesting for fabricating PDs with broad spectral sensitivity. The electrical characteristic comparison of devices prepared by thermal annealed IZO (400 °C, 1 h) covered with Au film or Au NPs (300 °C, 1 h) and laser annealed Au NPs (28 W•cm -2 , 30 min)/IZO (20 W•cm -2 , 1 h) film.

To further investigate the correlation between the 808-nm-laser-annealing and thermal annealing process, Fig. 2(e) and 2(f) display the corresponding NP extinction spectra, which cover the range from 350 (UV) to 1200 nm (IR) for each fabrication step. From Fig. 2(e), the original spectrum of the 8-nm-Au film (red curve) shows absorption in the entire light spectrum from 350 to 1200 nm. Although the Au film can have stronger absorption in the NIR range, no obvious absorbing peak can be observed. After the 1 st thermal annealing process (300 °C, 1 h) for Au NP synthesis, an absorption peak at 586 nm (blue curve) can be observed owing to the LSPR effect.

The spectral position of this peak is related to the average size [START_REF] Link | Size and temperature dependence of the plasmon absorption of colloidal gold nanoparticles[END_REF]. The IZO film has an extremely weak absorption in the visible or NIR wavelength ranges (Fig. 2(e), black curve), which justifies the coupling of IZO with Au NPs. After the 2 nd annealing process (400 °C, 1 h), the extinction spectrum of Au NPs is red-shifted to 650 nm (purple curve). This is mainly owing to the increase in the effective refractive index of the medium surrounding the Au NPs. Indeed, it is well established that the refractive index of sol-gel layers increases during thermal treatment owing to the loss of organic functions and densification induced by condensation reactions, resulting in the shift of LSPR [START_REF] Rai | Surface enhanced absorption and transmission from dye coated gold nanoparticles in thin films[END_REF]. The absorption in the NIR range of the raw Au film with a thickness of 8 nm suggests that a photothermal treatment may be possible using NIR laser annealing (808 nm continuous laser) to replace the conventional thermal annealing process. Fig. 2(f) shows the spectra of Au NPs prepared by the 808-nmlaser-annealing process. The results are similar to those obtained by thermal treatment; the generation of Au NPs by the 1 st laser-annealing process (28 W•cm 2 , 30 min) produces a peak at 580 nm (blue curve). These results confirm the data obtained by AFM showing that continuous NIR laser annealing can produce metal NPs. This approach is beneficial because the second annealing step can also be conducted using the same laser. After IZO covering and 2 nd laser annealing (20 W•cm -2 , 1 h), the heat generated by the Au NPs is sufficient to red-shift the absorption peak to 645 nm (purple curve), which is close to the value obtained by thermal treatment.

To confirm the formation of disconnected Au NPs, the electrical properties of the thin films were characterized. Fig. 2(g) shows the dark current measured from the devices of an 8-nm-Au film coated with IZO (Au film/IZO) and IZO with Au NPs coupling (Au NPs/IZO) under an 808 nm laser or thermal annealing process. For the Au film, a high current was recorded (approximately 10 -3 A), which can be explained by the shortcut produced by the continuous Au film under the IZO layer. For Au NPs/IZO, the conductivity can be successfully lowered and the current reached around 10 -7 A. This result confirms the discontinuous structure of Au NPs and confirms the similar structures that can be obtained by both thermal and laser annealing.

Correlation of an 808 nm laser or thermal-annealed IZO with chemical analysis:

The curing of IZO under laser annealing was investigated by XPS and TEM. . This shows that the IZO layer fully covers the Au NPs and is also confirmed by chemical mapping of the Au NPs/IZO film achieved by TEM analysis (Fig. 3(c)). On these images, the homogeneous distribution of Zn and In within the thin film is also demonstrated.

Light-detecting performance of an 808-nm-laser-annealed Au NPs/IZO PD:

The light detection performance of Au NPs/IZO PDs was evaluated to demonstrate that these devices can serve as broadband PDs. Thermal-annealed IZO (400 °C, 1 h) PDs were used as reference devices for comparison with the devices prepared with an 808-nm-laser-annealing (1 st annealing: 28 W/cm 2 , 30 min, and 2 nd annealing: 20 W•cm -2 , 1 h). In addition, commercial LEDs with illumination peaks at 410 nm (UV, 38 mW•cm -2 ), 465 nm (blue, 750 mW•cm -2 ), 515 nm (green, 325 mW•cm -2 ), 630 nm (red, 325 mW•cm -2 ), and 780 nm laser diode (NIR, 275 mW•cm -2 ) were used in this section. It also shows that when Au NPs are discontinued, they do not contribute significantly to the sample conductivity. For blue light detection, the response is quite similar for devices with and without NPs. For longer wavelength detection, such as red light detection, the IZO PD was found with little light detection performance. However, with the assisting absorption of Au NPs, the detection of red light (red curves) is significantly improved when using laser-annealed Au NPs/IZO PDs. Apart from the light-detecting performance shown on Fig. 4(a), Fig. S3(a) to S3(g) offer the compilations of an 808-nm-laser-annealed Au NPs/IZO PD with keeping the 1 st annealing condition as 28 W•cm -2 for 30 min, but adjusting the 2 nd annealing power from 16 to 24 W•cm -2 , and increasing the treatment time from 10 to 60 min. To summarize the trend, the dark current level can be increased from 10 -8 to 10 -4 A by increasing the laser-annealing power and time. However, the higher dark current level can finally lead to the extinction of light-detecting ability because the difference between photo-induced and dark current cannot be well distinguished anymore. The encapsulation-free design also enhances the possibility for further applications on flexible surfaces.

Table.1:

The comparison with alternative nanocomposite PDs

Light-detecting mechanism of an 808-nm-laser-annealed Au NPs/IZO PD:

To realize the wide range (410-780 nm) light-detecting performance of our 808-nm-laser-annealed Au NPs/IZO PD, the inclusion of Au NPs is expected to play the most significant role. From Haifei et al. [START_REF] Haifei | Mapping Hot Electron Response of Individual Gold Nanocrystals on a TiO 2 Photoanode[END_REF] with the mechanism shown in

An 808-nm-laser-annealed Au NPs/IZO PD on plastic/flexible substrates:

In this last section, we demonstrate one of the most interesting features of our new fabrication process. To extend the field of applications, we produced some examples of full laser-processed Au NPs/IZO PDs on 3D polymer pieces (Dentona Optiprint Splint 385) substrates prepared by 3D printing. Owing to the tolerance of heat on such substrates cannot be as high as glass substrates, the low power of the 808-nm-laser-annealing process for both Au NPs synthesis and IZO curing is necessary.

Conclusion

In this study, a novel solution-processed Au NPs/IZO PD with a wide range of detection performance, fabricated by an 808 nm laser annealing process, was successfully achieved. First, we recognized that the strong absorption of the Au film with a thickness of 8 nm in the NIR range could generate heat from 808 nm laser irradiation (28 W•cm -2 , 30 min). Then, the Au NPs, which have similar morphology and absorption characteristics, could be successfully converted from a following

  our process through the proof-of-concept fabrication of a PD. Because visible-light detection [32,33] is interesting for biomedical detectors or colorimetric sensors in the Internet of Things (IoT), UV PDs [34] are important for chemical and biological analysis or flame detection and NIR PD [35,36] opens new possibilities in optical imaging for brain science, medical diagnostics, or remote sensing. Therefore, designing a PD under a low environmental temperature fabrication process with a wide range of detection ability and good operating lifetime without an extra encapsulation process is crucial. Herein, we aim to provide a fabrication process that is compatible with these features and with substrates that cannot withstand thermal annealing, even at moderate temperatures for such PDs. In this study, a PD with a wide detection range was successfully prepared from sol-gel IZO with Au NPs by full NIR laser annealing. ZnO was chosen because it is an environmentally friendly material [37] with a wide energy band gap (~ 3.3 eV) and stability in air. Its carrier mobility can be easily adjusted by ion doping, like indium ion doping, which is commonly used in thin-film transistors (TFTs) [38]. To fabricate ZnO-based PDs with an improvement in photoresponse of the long wavelength range, NIR laser annealing was first used to generate Au NPs by laser-induced dewetting of Au thin film [39]. This method was chosen because it can be performed at room temperature, and it is a convenient way to generate dense arrays of Au NPs. Indeed, the localized surface plasmon resonance (LSPR) effect generated from Au NP nanostructures coupled to IZO can extend the photosensitivity toward the visible range and NIR [40,41]. Furthermore, we discovered that using Au NPs as nanoheaters by low-power light excitation, we were able to ensure the curing of the thin-film semiconducting material by thermoplasmonic effects [42,43]. This novel approach allowed the preparation of Au NPs covered by an IZO thin layer with a wide range of light-detecting properties. The fabrication process is detailed, and we demonstrated the central role of the Au NPs in the fabrication process and the final photodetecting properties.

Fig. 1 :

 1 Fig. 1: (a) Schematic of the PDs fabrication process. (b) Cross-section structure of the PD device. (c) Full laser-processed Au NPs doped IZO PD as in steps 3 and 5 of the fabrication process.

Fig. 2 :

 2 Fig. 2: AFM images of (a) Au film (8 nm), (b) Au NPs synthesized by 300 °C thermal-annealing for 1 h, and (c) Au NPs synthesized by an 808 nm laser annealing at 28 W•cm -2 for 30 min. (d) TEM images of Au NPs synthesized by 300 °C thermal-annealing for 1 h; the absorption spectra of each corresponding fabrication process of Au NPs/IZO structure (e) by thermal-annealing (Au NPs (300 °C, 1 h)/IZO (400 °C,

Fig. 3 (Fig. 3 (

 33 Fig. 3(a) and 3(b) show the XPS analysis of the O(1s) peak for the Au NPs/IZO film fabricated by full thermal or 808-nm-laser-annealing. For each annealing process, we compared the results before and after IZO curing on glass substrates with Au NPs. In Fig. 3(a), the changes observed under thermal annealing are consistent with those reported previously [21]. A significant decrease in vacancies was observed with an increase in the M-O, which corresponds to the condensation reactions induced by thermal treatments. The moderate temperature leads to amorphous materials, which explain the relatively high level of vacancies in the final material. Remarkably, Fig. 3(b) shows a similar structure of the O(1s) peak after IZO curing by an 808-nm-laser-annealing under 20 W•cm -2 , 1 h. This confirms the possibility of using Au NPs as nanoheaters with a temperature increase that is sufficient to obtain the same material as thermal curing at 400 °C. Photoplasmonic effects have been the subject of extensive experimental and numerical studies recently [46-49], and we confirmed here that it can be used very efficiently to generate local heating to promote chemical reactions. The effect is quite efficient under our conditions owing to the choice of NIR wavelength for irradiation, and the high density of NP at the surface, which promotes collective effects. Furthermore, XPS confirms the structure of the Au NPs/IZO film, as schematized in Fig. 1(b) because the Au signal cannot be found from full range XPS scanning for

Fig. 3 :

 3 Fig. 3: XPS analysis for IZO before/after (a) thermal annealing (400 °C, 1 h) or (b) an 808 nm laser annealing (20 W•cm -2 , 1 h) on glass substrates with Au NPs; (c) The chemical mapping of thermal annealed Au NPs (300 °C, 1 h)/IZO (400 °C, 1 h) film.

Fig. 4 (

 4 Fig. 4(a) shows the dark and light-induced (blue and red LEDs) current for the thermally annealed IZO (without Au NPs) and laser-annealed Au NPs/IZO PDs. From the results, both PDs have similar dark current levels, which implies that the IZO film fabricated under two different annealing treatments can have similar carrier mobilities.

  To further point out the role of Au NPs, the normalized photoresponsivity calculated by the following function (1) [50] for both the thermally annealed IZO and laser-annealed Au-NP-doped IZO PDs are shown in Fig. 4(b) and 4(c). Because the PDs in our work were designed as photosensitive resistors, the photoresponsivity can exhibit symmetric performance by sweeping voltage measurements.

From Fig. 4 (

 4 Photoresponsivity =.

Fig. 4 (Fig. 4 (

 44 Fig. 4(e) shows the dynamic light detection performance of the laser-annealed Au NP/IZO PD. With repeated light irradiation (1 min) and recovery (6 min) for three rounds, almost all the light-induced responses, which are defined by the value of the current difference ( ) between the photo-induced current and dark current, can easily be found. Although the laser-annealed PD can already perform reliable light-detecting signals, on the other hand, the slow recovery time may come from the carrier persisting phenomenon [6] in IZO or the carrier blocking effect by the Schottky barrier in the Au/IZO surface [41].To evaluate the operating stability of an 808-nm-laser-annealed Au NPs/IZO PD, Fig.4(f) offers its lifetime tracing performance by collecting light-induced (green and red LEDs) and dark current. From the results, the dark and light-induced current can almost be maintained at the same level in nearly 1 month (> 22 d), and the operation stability of the PD can be determined.

Fig. 4 :

 4 Fig. 4: The discussion based on electrical properties with an 808 nm laser annealed Au NPs (28 W•cm -2 , 30 min)/IZO (20 W•cm -2 , 1 h) PD and thermal annealed IZO (400 °C, 1 h) PD (a) the comparison of light/dark current performance; the photoresponsivity of (b) thermal annealed IZO, (c) an 808 nm laser annealed Au NPs/IZO PD; (d) the calibration relation of 808 nm laser annealed Au NPs/IZO PD with different operating bias voltage (e) the dynamic light-detecting performance, and (f) the lifetime tracing performance of an 808 nm laser annealed Au NPs/IZO PD

Fig. 5 (

 5 Fig. 5(a), when the wavelength of incident light corresponds to the LSPR range of Au NPs, it can be absorbed by the NPs. Next, the charge distribution for forming dipoles in Au NPs [59] can be found, and the oscillation direction of plasmonic damping follows the amplitude direction of the incident electromagnetic wave. Afterwards, the energy of the surface scattering phenomenon, which comes from a part of plasmonic damping, can finally lead to the generation of hot electrons on the surface of Au NPs. Therefore, the hot electron transfer phenomenon happening at the interface of the M-O contact can dominate the light-induced current in the long wavelength range. Due to the use of five light sources in total to compare the photoresponsivity of thermally annealed IZO and an 808-nm-laser-annealed Au NPs/IZO PDs (Fig. 4(b) and (c)), Fig. 5(b) to 5(e) are the corresponding band diagrams for discussing the light-detecting mechanism. For 410 nm (UV) detection in Fig. 5(b), both of the 2 PDs can have similar and the best performance. This is mainly because the photon energy of 410 nm (~ 3.03 eV) is quite close to the band gap of IZO, leading to band to band excitation of carriers in IZO. When the irradiation wavelength of light becomes longer to 465 nm (blue), the photoresponsivity of Fig. 4(b) and 4(c) can still maintain almost consistent performance. Owing to the absorption decrease of IZO, the defect levels that exist in between valance (E v ) and conduction (E c ) band can provide assistance for light-induced carrier excitation. When the irradiation light is shifted to 515 nm (green), the different photoresponsivity of the two PDs can be observed. Although the IZO PD can still work for this light detection, the photoresponsivity of Au NPs/IZO PDs can be 10× higher, which proves the importance of Au NPs coupling. From Fig. 5(d), the defect levels in IZO can still work for generating photo-induced carriers; however, the hot electrons transferred from the Au NPs to IZO offer another route for forming light-induced current. Owing to the hot electrons generated from the LSPR effect on Au NPs, the temporary Fermi-Dirac distribution of hot electrons can be built, which finally results in the contribution of photo-induced current by thermionic emission. Finally, Fig. 5(e) corresponds to the detection of an extremely long wavelength range, such as 630 nm (red) and 780 nm (NIR). Because the IZO PD hardly demonstrates its photoresponsivity in this range, the Au NPs/IZO PD can still have the available detection ability, the hot electron transfer of Au NPs plays the most significant role in

Fig. 5 :

 5 Fig. 5: (a)The generation steps of hot electrons on Au NPs, and the light-detecting mechanism base on M-O contact with (b) 410 nm-UV, (c) 465 nm-blue, (d) 515 nm-green, and (e) 630 nm-red and 780 nm-NIR

Fig. 6 (Fig. 6 (

 66 Fig. 6(b), the Au NPs/IZO PD can be used to demonstrate UV detection because the repeated I value can be collected. Although the light-detecting performance still needs to be improved, the useful applications of Au NPs/IZO PDs on arbitrary 3D printed figures can be expected.

Fig. 6 :

 6 Fig. 6: An 808 nm laser annealed (a) Au NPs on photopolymerized resin substrate by 5.5 W•cm -2 , 10 min, and (b) an Au NPs/IZO PD for UV (365 nm, 81.1 mW•cm -2 ) dynamic detection.

[ 62 - 64 ]

 6264 photoresponsivity or response speed. Furthermore, from our work, the initial trials of fabricating Au NPs/IZO PD on 3D printable polymer substrates for UV (365 nm) detection were accomplished under a low power 808-nm-laser-annealing process. In the near future, we aim to combine nanocomposite Au NPs/IZO or other sol-gel based metal oxide PD with colorimetric sensing techniques for building low-cost and reliable biomedical sensing systems.

Acknowledgement:

The authors would like to acknowledge the support of Agence Nationale pour la recherche (Project ANR-18-CE24-0028), Région Grand-Est and Institut Carnot MICA, Institute and Ministry of Science and Technology, Taiwan (Projects 108-2923-E-009 -002 -MY3) and the Higher Education Sprout Project of the National Chiao Tung University and Ministry of Education (MOE), Taiwan. We also gratefully thank Benjamin Leuschel, Samar Garreau, and Loic Vidal for their technical assistance.