Po-Yi Chang

Ching-Fu Lin

Samer El

Khoury Rouphael

Ting-Hsuan Huang

Chang-Mao Wu

Dominique Berling

Ping-Hung Yeh

Chia-Jung Lu

Hsin-Fei Meng

Hsiao-Wen Zan
email: hsiaowen@mail.nctu.edu.tw

Olivier Soppera
email: olivier.soppera@uha.fr

Near Infrared Laser-Annealed IZO Flexible Device as a Sensitive H 2 S Sensor at Room Temperature

Keywords: gas sensor, IZO, sol-gel, laser annealing, NIR, flexible

à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Introduction

In recent years, with the emergence of the Internet of Things, the development of various types of sensors has become increasingly critical. Gas sensors in particular have drawn much attention because of their ability to give an alert in the presence of toxic or explosive gas for safety surveillance, [START_REF] Chatterjee | Graphene-metal oxide nanohybrids for toxic gas sensor: a review[END_REF] monitor air pollution conditions, [START_REF] Liu | Highly sensitive room temperature ammonia gas sensor based on Ir-doped Pt porous ceramic electrodes[END_REF] detect health conditions through analysis of human breath, [START_REF] Konvalina | Sensors for breath testing: from nanomaterials to comprehensive disease detection[END_REF][START_REF] Machado | Detection of lung cancer by sensor array analyses of exhaled breath[END_REF][START_REF] Chuang | Organic gas sensor with an improved lifetime for detecting breath ammonia in hemodialysis patients[END_REF][START_REF] Singh | Highly sensitive nitric oxide gas sensor based on ZnO-nanorods vertical resistor operated at room temperature[END_REF] and control the freshness of food stock. [START_REF] Timmer | Ammonia sensors and their applications-a review[END_REF][START_REF] Chang | One-minute fish freshness evaluation by testing the volatile amine gas with an ultrasensitive porous-electrode-capped organic gas sensor system[END_REF] Incorporating solid-state semiconductor gas sensors into plastic parts or wearable devices has also become attractive to facilitate easy handling and real-time detection. [START_REF] Park | In-Depth Studies on Rapid Photochemical Activation of Various Sol-Gel Metal Oxide Films for Flexible Transparent Electronics[END_REF][START_REF] Rieu | Fully inkjet printed SnO2 gas sensor on plastic substrate[END_REF] Metal-oxide semiconductor materials grown through a conventional vacuum process have been successfully used in many commercial gas detectors. [START_REF] Park | In-Depth Studies on Rapid Photochemical Activation of Various Sol-Gel Metal Oxide Films for Flexible Transparent Electronics[END_REF][START_REF] Jo | Highly Stable and Imperceptible Electronics Utilizing Photoactivated Heterogeneous Sol-Gel Metal-Oxide Dielectrics and Semiconductors[END_REF][START_REF] Yao | MOF Thin Film-Coated Metal Oxide Nanowire Array: Significantly Improved Chemiresistor Sensor Performance[END_REF] However, complex and expensive equipment and high-temperature operation are usually required in the fabrication of such sensors. [START_REF] Rieu | Fully inkjet printed SnO2 gas sensor on plastic substrate[END_REF][START_REF] Wang | A highly sensitive gas sensor based on CuO nanoparticles synthetized via a sol-gel method[END_REF][START_REF] Mirzaei | Highly stable and selective ethanol sensor based on α-Fe2O3 nanoparticles prepared by Pechini sol-gel method[END_REF] Along with the development of low-cost solution-processed metal-oxide semiconductor devices, many researchers have aimed to use a solution process to produce gas sensors on flexible or even stretchable substrates. For this, the operating temperature must be lower to preserve the properties of the plastic substrate. Room-temperature operation is still a challenging but attractive characteristic of the sensors. Myoung et al. developed micropatternable zinc oxide nanoflowers on polyimide substrate to detect nitrogen dioxide at 270°C (operating temperature). [START_REF] Kim | Micropatternable double-faced ZnO nanoflowers for flexible gas sensor[END_REF] Xu and team developed a special metal-organic coating and applied it on ZnO nanowires to realize a sensitive acetone sensor at 260°C. [START_REF] Yao | MOF Thin Film-Coated Metal Oxide Nanowire Array: Significantly Improved Chemiresistor Sensor Performance[END_REF] Zheng et al. used commercial ZnO nanoparticles together with an ultraviolet (UV)-light-controlling technique to form an ethanol sensor on a polyethylene terephthalate (PET) substrate at room temperature. [START_REF] Zheng | Light-controlling, flexible and transparent ethanol gas sensor based on ZnO nanoparticles for wearable devices[END_REF] Li et al. used an in situ chemical oxidation polymerization method to prepare polyaniline, a flower-like WO 3 nanocomposite, on a PET substrate to detect 500-ppb ammonia at room temperature. [START_REF] Li | The room temperature gas sensor based on Polyaniline@ flower-like WO3 nanocomposites and flexible PET substrate for NH3 detection[END_REF] Our previous work involved applying polymer-coating layers onto sputtered Indium-Gallium-Zinc-Oxide (IGZO) thin-film transistors to perform room-temperature ammonia and acetone sensing. [START_REF] Zan | Room-temperature-operated sensitive hybrid gas sensor based on amorphous indium gallium zinc oxide thin-film transistors[END_REF] Most successful metal-oxide-based gas sensors on flexible substrates exhibit nanostructures in their active layer. Few reports have demonstrated successful gas-sensing performance with the use of a solution-processed metal-oxide thin film. Jaisutti et al. reported the production of a sol-gel IGZO thin film on a glass substrate for detecting acetone at room temperature under UV irradiation. [START_REF] Jaisutti | Low-temperature photochemically activated amorphous indium-gallium-zinc oxide for highly stable room-temperature gas sensors[END_REF] The photochemically activated IGZO film did not require a thermal-annealing process; thus, it may also be applicable to flexible substrates.

For fabricating sensors on flexible substrates, the low curing temperature was also required. In prior studies including our prior reports, the decrease in the curing temperature relied on the use of UV treatment on metal-oxide sol-gel precursors.

Such conditions proved to be very effective in removing the organic ligands and cross-linking the material. [START_REF] Leuschel | Deep-UV photoinduced chemical patterning at the micro-and nanoscale for directed self-assembly[END_REF][START_REF] Park | Hydrothermally grown in-doped ZnO nanorods on p-GaN films for color-tunable heterojunction light-emitting-diodes[END_REF][START_REF] Stehlin | Room-temperature preparation of metal-oxide nanostructures by DUV lithography from metal-oxo clusters[END_REF][START_REF] Lin | Deep ultraviolet laser direct write for patterning sol-gel InGaZnO semiconducting micro/nanowires and improving field-effect mobility[END_REF][START_REF] Yeh | Chemical and structural investigation of zinc-oxo cluster photoresists for DUV lithography[END_REF] However, use of them is usually not sufficient for obtaining enough formation of a metal-oxide inorganic network with suitable electrical properties. In particular, the defect level is too high before thermal treatment. [START_REF] Park | Hydrothermally grown in-doped ZnO nanorods on p-GaN films for color-tunable heterojunction light-emitting-diodes[END_REF] For this reason, we decided to investigate near-infrared (NIR) laser curing.

NIR laser curing was introduced as an efficient technique with which to induce photothermal effects in materials absorbing the NIR. This effect has already been used in applications such as laser marking, [START_REF] Breaban | Nd: YAG Laser Beam-Material Interactions for Marking and Engraving: Application to Alumina and Granite[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] polymer melting for particle migration, [START_REF] Chang | Laser Polymer Tattooing: A Versatile Method for Permanent Marking on Polymer Surfaces[END_REF] and polymer-to-polymer fixation. In this work, we demonstrate that NIR laser irradiation can be used for curing metal-oxide precursor thin films prepared from solutions, with sol-gel chemistry. The NIR laser system with a wavelength of 808 nm and continuous irradiation was used to irradiate a sol-gel IZO xerogel thin film to activate its electrical property. In addition to the electrical characterization, material analysis methods such as X-ray photoelectron spectroscopy (XPS) and attenuated total reflection Fourier-transform infrared spectroscopy (ATR-FTIR) were used to investigate the NIR-laser-annealed IZO film. The low temperature process made it possible to integrate the IZO thin film onto the flexible polymer substrates of polycarbonate (PC), polyethylene (PE), and PET. Last, we determined that the IZO film can act as a gas sensor. The electrical properties were modified according to the presence of water in the atmosphere, allowing the use of the device as a humidity sensor. With a fixed humidity control, the IZO film also detected hydrogen sulfide gas down to 300 parts-per-billion (ppb) concentration at room temperature. Compared with prior works demonstrating flexible hydrogen sulfide sensors, [START_REF] Asad | High sensitive and selective flexible H2S gas sensors based on Cu nanoparticle decorated SWCNTs[END_REF][START_REF] Joshi | Flexible H2S sensor based on gold modified polycarbazole films[END_REF][START_REF] Mousavi | A room temperature hydrogen sulfide gas sensor based on electrospun polyaniline-polyethylene oxide nanofibers directly written on flexible substrates[END_REF] we are the first to realize the ppb-regime sensitivity on a flexible substrate, revealing its potential for application in wearable devices. According to the recommendation of the American Conference of Governmental Industrial Hygienists, the long-term (8 h time-weighted average) exposure limit for hydrogen sulfide is 1 ppm. The developed ppb-regime detection capacity is beneficial for the early alert of a dangerous environment. In this study, our proposed hydrogen sulfide sensor was sensitive, fast, and reversible, indicating promising applications in environmental surveillance.

Experimental [Device fabrication]

Figure 1a shows the schematic of device fabrication. The devices were produced using 3 cm 3 cm substrates, which can be either glass or flexible plastic materials such as PC, PE, or PET with 0. S1). IR-140 acts as a photoabsorber, provoking a photothermal effect in the sol-gel film, which was proved to be necessary to guarantee the stability of device performance. The absorption of the substrates was low at 808 nm as shown in Fig. S2 in supporting information, and thus no visible effect of the laser irradiation was observed on raw substrates in the range of power used here. This approach thus allowed thermal curing of the sol-gel layer without substantially affecting the properties of the substrate, which is the objective of this work.

After the coating process, the laser annealing treatment was applied (Figure 2b).

The irradiation area was 5 5 mm 2 (Figure 2c) using an NIR 808 nm continuous laser (DS3-11312-xxx-LD No., BWT Beijing). The distance between the laser head and the sample is 15 cm. After laser annealing, the substrate was baked at 50°C for 20 min. The thickness for IZO film after annealing was about 100 nm, confirmed by ET200 (SANPANY INSTRUMENTS CO., LTD.). Finally, the aluminum electrodes (60 nm) were deposited by thermal evaporation to form an IZO chemoresistor. The spacing between the two electrodes was 200 μm. The humidity detection system, as shown in Figure 1, delivered an air flow with a tunable relative humidity (RH). The RH was controlled by tuning the flow ratio between the "dry air" and the "wet air" settings, where the dry air is the ambient air passing through a NaOH tube to exhibit a RH of approximately 10% and the wet air is the ambient air passing through a bottle of water to deliver a RH of approximately 80%. To adjust the final RH, the flow rates of the dry and wet air were controlled by flow meters. With a pump in the outlet of the system, the mixed air with an adjustable RH was then injected into a glass tube, which served as the sensing chamber with the IZO chemoresistor.

For hydrogen sulfide (H 2 S) gas detection, as shown in Figure 1, the background air (with an RH set at approximately 62% by the aforementioned humidity system) was pumped into the sensing chamber at a flow rate of 500 mL/min to serve as the carrier gas. The analyte gas such as H 2 S gas was injected into the system to mix with the background air through a syringe pump system. Inside the syringe, 100-ppm H 2 S was placed. By tuning the speed of the syringe pump, the concentration of H 2 S in the mixed gas ranged from 300 to 1000 ppb. Inside the sensing chamber, the device current was recorded along with time by using an I-V analyzer (Agilent U2722A).

The XPS analysis was performed on two setups: a Gammadata Scienta (Uppsala, Sweden) SES 200-2 X-ray photoelectron spectrometer under ultrahigh vacuum conditions (P < 10 -7 Pa) to collect thermal annealing data and on a PHI Quanterall, ULVAC-PHI (P < 2.0 10 -7 Pa) to collect NIR laser annealing data, separately.

CASAXPS (Casa Software Ltd, Teignmouth, UK, www.casaxps.com) was used to fit all the peaks and area of each component of XPS data. ATR spectra were acquired using an IS50 spectrophotometer from Thermo Scientific. Diamond was supplied by Specac (Golden Gate diamond, with a 45° angle). Film roughness was characterized using AFM in tapping mode (Picoplus system from Agilent).

Results and Discussion

As depicted in Figure 2a, the key-step of the fabrication process relies on a photothermal effect that is triggered by the NIR laser irradiation and facilitated by the addition of the NIR dye. The local temperature increase allowed the classical condensation reactions to occur in the film and convert the material into metal oxide, with noteworthy electrical properties, as shown later.

The irradiation conditions were thus expected to have a strong effect on the final material properties. To investigate the effect of NIR laser annealing, different power densities were tested. Figure 2d illustrates the sheet resistance (R s ) of the IZO chemoresistor with different laser doses. With 60-s irradiation, laser power was set to 93 W/cm 2 , 105 W/cm 2 , 127 W/cm 2 , and 157 W/cm 2 . Notably, it was not possible to use a laser power lower than 93 W/cm 2 , because at a lower laser power, no stable electrical signal could be recorded. This was probably due to a low conversion of the material and the resulting high content of ionic species and solvent that can cause background electric signals and unstable device performance. As depicted in Figure 2d, when laser power was set to 93 W/cm 2 and 105 W/cm 2 , the R s remained almost constant at 25 MΩ. When laser power increased to 127 W/cm 2 and 157 W/cm 2 , the R s increased to 118 MΩ and 3950 MΩ, respectively.

We compared the R s of the NIR-laser-annealed IZO film with the R s of thermally annealed IZO film. Figure 1e demonstrates the R s of IZO films with different thermal annealing conditions. Thermal annealing was applied using a furnace tube with the temperature set to 150°C, 200°C, 250°C, and 300°C for 1 h. As expected, when the annealing temperature increased, the R s of the IZO film decreased, which confirmed the need for thermal curing to obtain suitable electrical properties.

The R s values of NIR-laser-annealed films are denoted by the red dashed lines in Figure 2e. The R s of NIR-laser-annealed IZO films with laser power from 93 W/cm 2 to 157 W/cm 2 (60 s) were comparable to those of thermally annealed films with annealing temperature from 200-300°C (1 h). The 60 s laser irradiation time, however, was much shorter than the 1 h thermal annealing time. In addition, laser irradiation only generated local heating, whereas thermal annealing heated the whole substrate, and thus it cannot be applied to a plastic substrate. We observed that the proposed NIR laser annealing method was advantageous to the formation of the IZO chemoresistor on a plastic substrate as a flexible device. The flexible device performance is discussed later. To confirm the laser-induced modification of the IZO thin film, we used ATR-FTIR analysis. As evident in Figure 3a, both thermal and laser annealing showed a substantial reduction at the peak of approximately 1400 cm -1 . This peak can be ascribed to nitrate ions, indicating that the nitrate ions are removed during both laser and thermal treatments with a similar yield. [START_REF] Miller | Infrared spectra and characteristic frequencies of inorganic ions[END_REF] The reduction of the absorbance of the broad peak at 3500 cm -1 indicates that the OH are also consumed in both conditions, which is consistent with condensation reaction between metal-oxide species.

XPS analysis was also used to investigate the chemical changes induced by NIR laser and thermal treatment of the IZO films. As shown in previous works [START_REF] Lin | Deep ultraviolet laser direct write for patterning sol-gel InGaZnO semiconducting micro/nanowires and improving field-effect mobility[END_REF] ,

condensation reaction of the sol-gel material thin film can be followed quantitatively by following the modification of the O 1s component. The original data are provided in supporting information S3, S4 and Table S1. The three peaks are 530.4 eV, standing for surface hydroxide and lattice hydroxide; 529.2 eV, standing for vacancies and lattice defect; and 528.6 eV standing for the metal-oxide bonding network. [START_REF] Lin | Deep ultraviolet laser direct write for patterning sol-gel InGaZnO semiconducting micro/nanowires and improving field-effect mobility[END_REF] We observed a slight difference in peak positions that may be ascribed to the charging effect or the difference in morphology detected by AFM measurements (Figure 3e).

The ratio between the intensity of the three specific peaks of thermal and laser annealing films are compared in Figures 3b and3c. We expect that the thermal or laser annealing provokes the condensation of the sol-gel thin film, promoting the conversion of defects (surface and lattice hydroxides and defects) into metal oxide network. For thermal annealing (Figure 3b), we observed, as expected, the condensation reaction of the metal-oxide network, with further conversion of the hydroxide species to metal-oxide lattice species. With increasing temperature, the proportion of vacancy and lattice defects decreased, which accounts for the improvement of the electrical properties. In Figure 3c, the effect of the NIR laser is plotted. Metal-oxide lattice was 50%, 48%, and 49% for samples with laser powers of 93 W/cm 2 , 107 W/cm 2 , and 157 W/cm 2 , respectively. Such metal-oxide lattice ratios are comparable to those of samples prepared under a 300°C thermal annealing condition (53%), as shown in Figure 3b. The finding of extremely similar metal-oxide lattice ratios for the laser-annealed samples also suggests that changing laser power from 93 W/cm 2 to 157 W/cm 2 does not substantially change the film composition. The XPS data are in agreement with the electrical properties presented in Figure 2,

showing that the 60-s irradiation is efficient enough to generate the same effect in the thin film as thermal heating at 200°-300°C during 60 min.

Laser irradiation time is another critical parameter. With a fixed laser power of 93 W/cm 2 , the irradiation time was tuned to 0 s, 30 s, 60 s, 90 s, and 120 s. The raw XPS analysis data are provided in supporting information. The ratio of hydroxide (530.4 eV) to the sum of the three peaks is plotted as a function of laser irradiation time, as shown in Figure 3d. This result suggests that a laser irradiation time of 60 s is sufficient to substantially remove the surface hydroxide defect. We also noticed that increasing irradiation time from 60 s to 120 s did not further improve the conductivity (data not shown), hence we chose 60 s as our optimal irradiation time.

To further investigate the reduction of conductivity with the increase of laser power, the film morphology was also studied by using AFM. The surface roughness of IZO film, as a function of laser power, is shown in Figure 3e. The AFM images of IZO film with the lowest laser power of 93 W/cm 2 and the highest laser power of 157 W/cm 2 are shown in Figures 3f and3g, respectively. The AFM images of samples with laser power of 105 W/cm 2 and 127 W/cm 2 are shown in supporting information Figure S4. It can clearly be observed in Figure 3f that the IZO film with high laser power (i.e., 157 W/cm 2 ) exhibits a very rough morphology (roughness as high as 77 nm). In Figure 2d, with increased laser power, the film roughness increases considerably. The high roughness may be explained by the rapid heating on the IZO film during the laser irradiation, which causes fast evaporation of the solvent and leads to the nonuniformity of the film. We therefore suggest that, as evident in Figure 2d, the degraded conductivity with high laser power is caused by the greatly increased film roughness. Notably, in Figure 3e, the surface roughness of the IZO film with low laser power is comparable to the roughness of the IZO film with 300°C thermal annealing for 1 h. Surface roughness of IZO film as a function of laser power. The irradiation time was 60 s. (f), (g) AFM images to compare low (93 W/cm 2 ) and high (157 W/cm 2 ) laser power differences.

A notable characteristic of the NIR laser curing process is its compatibility with plastic substrates that cannot stand thermal curing. For this reason, devices on plastic substrates were fabricated and their electrical properties were evaluated. We successfully used the NIR laser annealing method on flexible substrates such as PE, PET, and PC without damaging the substrates, as shown in Figure S5. Thermal annealing at 200°C for 1 h caused obvious deformation of these substrates. For example, normally, PC begins to flow at temperatures above 150°C. Such temperatures are too low to form thermally annealed IZO film with suitable electrical properties.

IZO semiconducting thin films are known to be sensitive to gas, and this material can thus be used in a chemoresistor material. After verifying that the film composition and the sheet resistance of NIR-laser-annealed IZO film are as good as their thermal annealed counterparts, we investigated the sensor performance of the fabricated IZO film. For sensor application, we like to have a large enough output current (i.e. 10 -8 ~ 10 -6 A) at a low operation voltage (i.e. 5-10 volts) to avoid the noise in the electrical measurement system. Hence, samples with 157 W/cm 2 60 sec are not very suitable due to the high resistance. For the following samples, if not particularly specified, the laser annealing condition is 127 W/cm 2 for 60 sec. We first noticed that the film's electrical response was very sensitive to humidity, hence humidity sensing results were demonstrated. Using the humidity control system described in the experimental section, we tested the real-time current change when modifying the humidity in the sensing chamber. As shown in Figure 4a, the relative humidity (RH) was variously 10%, 30%, 50%, and 60%. With increasing humidity, the current in the device elevated. Such an effect was also reported in prior works. Water molecules absorbed onto the surface of the metal oxide to form H + or H 3 O + through a dissociation effect. [START_REF] Traversa | Ceramic sensors for humidity detection: the state-of-the-art and future developments[END_REF][START_REF] Wang | Metal oxide gas sensors: sensitivity and influencing factors[END_REF] Then, H + or H 3 O + ions hopped between adjacent hydroxyl groups to form the current. Increasing RH led to an increase of such hopping current. [START_REF] Traversa | Ceramic sensors for humidity detection: the state-of-the-art and future developments[END_REF][START_REF] Wang | Metal oxide gas sensors: sensitivity and influencing factors[END_REF] Notably, to have a consistent current response, 10% humidity was used as a background reference condition. With this controlled background reference, the IZO film delivered a stable response to humidity change, as shown in Figure 4a. The current response is defined as the average current when switching the humidity for 1 min and is plotted as a function of RH in Figure 4b. The small deviation verifies the function of humidity sensing. In future application, the reference 10% RH can be produced by flowing the background air through the NaOH tube as reported in our prior works. [START_REF] Chuang | Organic gas sensor with an improved lifetime for detecting breath ammonia in hemodialysis patients[END_REF][START_REF] Chang | One-minute fish freshness evaluation by testing the volatile amine gas with an ultrasensitive porous-electrode-capped organic gas sensor system[END_REF][START_REF] Zan | Room-temperature-operated sensitive hybrid gas sensor based on amorphous indium gallium zinc oxide thin-film transistors[END_REF] In the second step, we demonstrated that the IZO chemoresistor device can be integrated by laser annealing on flexible PC substrate, without any noticeable modification of the substrate properties (Figure 4c). We then illustrated the hydrogen sulfide gas-sensing performance of the device on a flexible substrate. With a fixed humidity of 62%, the proposed IZO chemoresistor had a very good response to hydrogen sulfide (H 2 S) gas. It is noted that the humidity has to be fixed, otherwise the variation of humidity will cause an interference signal to the H 2 S sensing. The method to control the humidity in the sensing system was introduced in the experimental section. The real-time current response to H 2 S gas is illustrated in Figure 4d. In the orange regions, H 2 S gas with 1000, 500, and 300 ppb concentration was injected into the sensing chamber for 30 s. The responding current drops were very fast and were reversible after removing the H 2 S gas. The sensing response was defined as the current variation ratio, which is the current difference during the 30-s sensing period divided by the initial current level before injecting H 2 S gas. When the sensing response is defined by using the current variation ratio, the response will not be affected by the absolute value of the current. [START_REF] Singh | Highly sensitive nitric oxide gas sensor based on ZnO-nanorods vertical resistor operated at room temperature[END_REF][START_REF] Verma | A highly sensitive SnO2-CuO multilayered sensor structure for detection of H2S gas[END_REF] Hence, after the recovery period, even if the current level cannot reach its original value, the sensor delivers stable response to a fixed H 2 S concentration. (Figure S6). The slight decrease of the background current signal during measurement may be due to the increased defect density with continuous bias, which is known as the bias stress effect in oxide semiconductor devices [START_REF] Park | Improvement of bias stability of oxyanion-incorporated aqueous sol-gel processed indium zinc oxide TFTs[END_REF] . Figure 4e demonstrates the calibration curve by plotting the sensing response as a function of H 2 S concentration. For every H 2 S concentration, three sensing responses were recorded to calculate the average and standard deviation.

A linear dependence was obtained. As mentioned in Fig. 3(e), NIR laser intensity modulated the surface roughness. We compared the H 2 S sensing responses by using the IZO films annealed by 93 W/cm 2 and 157 W/cm 2 , with corresponding AFM images shown in Figs. 3(f) and 3(g), respectively. However, no significant difference was observed between the two sensors' responses (data not shown), indicating that the surface roughness is not the primary factor to control the H 2 S sensing. In prior reports, when using SnO 2 or Cu-doped ZnO to detect H 2 S, a current increase was observed upon exposure to H 2 S with ppm-regime concentrations. [START_REF] Verma | A highly sensitive SnO2-CuO multilayered sensor structure for detection of H2S gas[END_REF][START_REF] Shewale | H2S gas sensing properties of nanocrystalline Cu-doped ZnO thin films prepared by advanced spray pyrolysis[END_REF][START_REF] Manorama | Hydrogen sulfide sensor based on tin oxide deposited by spray pyrolysis and microwave plasma chemical vapor deposition[END_REF][START_REF] Gong | Micromachined nanocrystalline silver doped SnO2 H2S sensor[END_REF] Other recent reports [START_REF] Asad | High sensitive and selective flexible H2S gas sensors based on Cu nanoparticle decorated SWCNTs[END_REF][START_REF] Joshi | Flexible H2S sensor based on gold modified polycarbazole films[END_REF][START_REF] Mousavi | A room temperature hydrogen sulfide gas sensor based on electrospun polyaniline-polyethylene oxide nanofibers directly written on flexible substrates[END_REF][START_REF] Li | Room-temperature high-performance H2S sensor based on porous CuO nanosheets prepared by hydrothermal method[END_REF][START_REF] Wang | Low-temperature H2S detection with hierarchical Cr-doped WO3 microspheres[END_REF][START_REF] Tian | Hierarchical and hollow Fe2O3 nanoboxes derived from metal-organic frameworks with excellent sensitivity to H2S[END_REF][START_REF] Kim | Bimodally porous WO3 microbelts functionalized with Pt catalysts for selective H2S sensors[END_REF] for H 2 S sensing are compared in Table S2. to H 2 S gas. [START_REF] Stehlin | Room-temperature preparation of metal-oxide nanostructures by DUV lithography from metal-oxo clusters[END_REF] In the current work, we observed a current reduction in IZO upon H 2 S sensing at room temperature. In addition, the sensing capacity was not degraded by increasing the humidity. Finally, we evaluate sensing selectivity. Figure 4f shows the sensing response to ammonia (NH 3 ), acetone (denoted as ACE), nitrogen monoxide (NO), and hydrogen sulfide (H 2 S) with a gas concentration of 1 ppm. We noticed that the proposed IZO film was particularly sensitive to H 2 S. The high sensitivity and selectivity to H 2 S gas indicates that the NIR-laser-annealed IZO film is promising for application in industries or households for safety surveillance. As a preliminary test, we also compared the H 2 S sensing response using a just-fabricated sensor (noted as Day 0) and a sensor stored at a food-storage-quality nitrogen bag for 2 days (noted as Day 2)

in Fig. S7 in supporting information. There is no much difference between the Day 0 and Day 2 data. Also, within 2 hrs, the sensor delivers repeated sensing results even when the background current decreases due to the continuous bias effect (not shown).

In future, more studies will be conducted to investigate the optimal storage condition to obtain a good enough shelf-lifetime for real applications.

Conclusion

We demonstrated a new 808-nm NIR laser-based method to treat sol-gel IZO film that has a low-cost and fast annealing time. The performance of the laser-annealed device is comparable with that of a thermally annealed device with an annealing temperature above 200°C. The short laser irradiation time and the local heating phenomenon make the NIR laser suitable for fabrication on flexible substrates such as PC. The XPS analysis confirmed that the proposed NIR laser annealing method provides a suitable metal-oxide network for electric conduction. The NIR-laser-annealed IZO film was then used as a humidity sensor and hydrogen sulfide sensor. For humidity sensing, compared with the reference humidity, the IZO film delivered a stable sensing response to RH, varying from 10% to 60%. Then, with a fixed humidity, the IZO film exhibited an extremely sensitive response to H 2 S, ranging from 300 to 1000 ppb. The sensing mechanism does not rely on the interaction between H 2 S and the absorptive of oxygen

Figure 1 .

 1 Figure 1. The schematic diagram for the sensing system.

Figure 2 .

 2 Figure 2. (a) Schematic of the fabrication of IZO chemoresistor. (b) Photograph of the sample during NIR laser irradiation. (c) Photograph of the IZO chemoresistor device with Al electrodes on PC substrate, depicting the device in a bent state. (d) Sheet resistance (R s ) of the IZO thin film cured by NIR laser (60-s irradiation) as a function of laser power. (e) Comparison of thermal and laser annealing for R s values. (f) NIR photographs of temperatures measured by thermal imager for different NIR laser power.

Figure 2f is the

  Figure2fis the real-time thermal image taken by a thermal imager (Testo 875i -

Figure 3 .

 3 Figure 3. (a) ATR-FTIR analysis of the IZO thin film for laser and thermal annealing. The highlighted area corresponds to the removal of nitrate ions under both the thermal and laser annealing processes. (b), (c) Analysis of O1s peak from the XPS data for thermal and laser annealing, respectively. (b) shows the fluctuation of three main peaks, hydroxyl, vacancy and lattice defect, and metal-oxide lattice, for several temperatures. (c) shows the similar metal-oxide ratio of laser power at 93 W/cm 2 , 127 W/cm 2 , and 157 W/cm 2 .

Figure 4 .

 4 Figure 4. (a) Current response of NIR laser-annealed device. The NIR laser irradiation power was 127 W/cm 2 for 60 s on PC substrate. The operation voltage was 10 volts. (b) Calibration curve of NIR laser-annealed humidity sensor. With the change of humidity, we extracted and calculated the first 120 s of current data once it became stable. (c) Photograph of NIR-laser-annealed IZO chemoresistor on a bendable PC substrate. (d) The real-time current response and gas response of NIR-laser-annealed IZO chemoresistor when injecting H 2 S gas with different concentrations. The operation voltage was 5 volts. The NIR laser power density was 127 W/cm 2 for 60 s. (e) Calibration curves of NIR-laser-annealed IZO chemoresistor on PC substrate at RH = 62% and RH = 30%. The NIR laser power density was 127 W/cm 2 for 60 s. Three independent experiments were conducted to

  The H 2 S molecules then react with O -or O 2 -to form SO 2 and free electrons. Hence, a current increase is obtained when detecting H 2 S. However, in our work, we repeatedly observed a current drop when the NIR-annealed IZO chemoresistor was in contact with H 2 S. When we reduced the RH in the sensing chamber to 30%, as shown by the red line in Figure4e, the responses to H 2 S were almost the same as those with RH of 62%. Such a humidity effect on gas sensing in IZO differs from prior reports. In previous works, a degraded CO or H 2 S sensing response is observed when increasing the humidity. The elevated water absorption on the ZnO surface shields the reactive O -or O 2 -sites and hence suppresses the response

	temperatures.
			For H 2 S sensing on
	flexible substrate, we are the first to deliver sensitivity in ppb-regime. The sensing
	mechanism, particularly at operating temperatures above 140°C, is mostly explained
	by the reaction between H 2 S molecules and O -or O 2	-to generate additional free
	electrons. The high temperature greatly enhances the adsorption activity of oxygen
	onto the surface of the metal-oxide semiconductor. The conversion rate of adsorbed
	molecular oxygen (O 2	-) to atomic oxygen (2O -) is also increased at elevated

  It is thus plausible that the H 2 S detection of our sample does

	not rely on the reaction with O -or O 2	-. The detailed mechanism, however, requires
	further investigation. In this work, we aimed to propose a new annealing method to
	realize flexible IZO on a plastic substrate and to deliver an ultrasensitive ppb-regime
	H 2 S-sensing performance.

  (O -or O 2 -), which is only present at high temperatures. As a result, in our work, ppb-regime H 2 S sensing can be realized at room temperature on a bendable substrate. The H 2 S sensing selectivity among ammonia, nitric monoxide, carbon monoxide, and acetone is also extremely favorable. Preliminary stability test on the proposed H 2 S sensor revealed that the sensor delivered almost unchanged sensing response after being stored in a simple food-storage-quality nitrogen bag for 2 days. More investigation on long term stability and storage condition will be done in future work. The proposed low-cost bendable NIR-laser-annealed IZO chemoresistor is promising for use in wearable devices to detect environmental humidity or to give an alert of dangerous hydrogen sulfide levels.

Acknowledgements

The authors like to thank Philippe Fioux and Banjamin Leuschel for their assistance to run XPS and AFM experiments. This work was supported by funding from Agence Nationale pour la Recherche (Project PHOTOMOC, ANR-14-CE26-0039 and Project NIRTRONIC, ANR-18-CE24-0028), Institut Carnot MICA (project SENSORTEX) and Ministry of Science and Technology, Taiwan (Projects 108-2923-E-009-002-MY3 and 108-2221-E-009-114-).


Po-Yi Chang

Adèle Bruntz

Loïc Vidal

Pierre-Alain Vetter

Patrick Roudot

Laurent Bua

Jose Ortiz

Hsiao-Wen Zan

Olivier Soppera
email: olivier.soppera@uha.fr

P.-Y Chang

Prof Wen Hsiao-

Zan

Laser polymer tattooing (LPT): a versatile method for permanent marking on polymer surfaces

Keywords: Inkjet, nanoparticles, laser, polymer

A versatile and efficient method for the permanent marking of polymer surfaces that combines inkjet deposition and near-infrared (NIR) laser curing is investigated. The NIR laser treatment forces the ink particles to migrate into the upper layers of the polymer. This results in the fixation of a permanent grayscale image that can be applied to various polymers, such as polypropylene, which is widely used in industry but still difficult to mark. The physiochemical processes induced by laser curing are investigated by electronic and optical microscopy. The dependence of the thickness of the deposited ink and the laser power on the

contrast of marking are also studied. A mechanism implying fast laser-induced melting of the polymer surface followed by displacement of carbon nanoparticles by convection is proposed.

Finally, a comparison of the aging properties of samples prepared by our process and standard UV ink is proposed to illustrate the interesting nature of this new polymer marking process.

Integrating the ink under the surface of the polymer, as in a skin tattooing procedure, by laser curing is an efficient way to generate permanent images on polymer surfaces.

In the manufacturing of polymers, marking is an important final step with high added value.

Many manufactured polymer parts have markings for different purposes, such as decorating or personalizing objects, writing serial numbers or bar codes for identification, quality control, inscribing logo or product names, in both practical and cosmetic contexts. In most applications, the requirements are fast writing speed, writing directly on the finished product to avoid additional time-consuming and/or expensive steps, a reduced use of toxic or hazardous chemicals and ideally simple and versatile equipment. The process should also be easily digitalized to allow the direct writing of complex arbitrary shapes from a computergenerated file.

Inkjet printing has been increasingly used for polymer marking since the early 2000s. [START_REF] Hutchings | Inkjet Technology for Digital Fabrication[END_REF] However, this technique does not provide permanent marking due to the difficulty of ensuring good adhesion of the cured ink to the raw polymer surface. For instance, polyolefins require a specific surface treatment and an additional protective coating on top of the marking.

UV-curable inks were designed to improve the durability of markings in aggressive environments. UV curing is based on a photoinduced crosslinking reaction that solidifies the deposited ink. UV irradiation offers many advantages as it is fast and energy-efficient and limits the use and release of toxic solvents. The recent development of low-cost high-power UV-LED light sources has increased industrial interest in such techniques. UV inkjets also allow the design of color drawings and deposition of functional materials for applications in biology [START_REF] Murphy | [END_REF] or electronics, [ 3 ] which is impossible with many other marking processes.

However, UV inks present a major limitation in terms of adhesion and abrasion resistance, since the marking layer is added on top of the polymer surface and the adhesion of the cured inked to the polymer surface is thus crucial. Alternatively, e-beam curing can be used, but this technique requires sophisticated expensive equipment and is thus limited to very specific applications.

In this context, polymer marking by lasers has gained great attention during the last decade. [START_REF] Bäuerle | Laser Processing and Chemistry[END_REF] The technique is fast, cost-effective and versatile. It allows permanent and resistant marking. Marking is achieved by focusing a laser (usually a focused Nd-YAG pulsed laser emitting at 1064 nm) [START_REF] Breaban | [END_REF]6,7] at the surface of a polymer. The beam is displaced at the surface by galvanometric mirrors, allowing fast writing (a few m/s, which makes it possible to write more than 100 alphanumeric characters per second). The minimum pixel size is typically 25 microns, which makes it possible to obtain well-defined figures. [8] With this technique, no ink is used, but the laser modifies the polymer surface to create an image pixel by pixel.

Depending on the nature of the polymer and the laser power, several mechanisms can be involved. In this case, marking is the result of carbonization of the polymer surface under high laser power. At lower power, foaming or melting can be observed. [ 9 , 10 ] Most of the commonly used polymers are compatible with this marking method. [ 11 ] However, to guarantee an efficient process, the polymer needs to exhibit significant absorption at the laser wavelength. Poly(ethylene terephthalate) (PET), polycarbonate (PC), and polystyrene (PS) intrinsically exhibit some photosensitivity at 1064 nm and can be easily carbonized at high temperature so they can be marked without the use of additives. [12] However, even for these polymers, additives are usually added to increase the photosensitivity needed to reach a writing speed suitable for practical applications. Some polymers, such as polyolefins, are transparent in the NIR region (1000 to 1100 nm). [13,14] In this case, additives are required. These compounds can be either organic dyes (cyanine), metal complexes (copper complexes) or nano/microparticles (e.g., TiO 2 , [15,16] [15,19] black carbon, [20,21] and graphene [22]). Though the concentration of additives remains at a very low level (typically from 0.01 to 3.0 wt. %), the additives have to be included in the bulk polymer before the fabrication of the polymer part. Thus, they increase the complexity and cost of polymer development and may have undesired consequences on the final material properties. In particular, the dispersion of the absorber must be very homogeneous in the polymer matrix. Moreover, the thickness of the region in which the polymer and laser interact can reach 100 microns for marking with good contrast. For thin samples or applications where the mechanical properties of the material are important (aeronautic, health, automotive), it is necessary to check the properties of the objects after marking, which limits the versatility of the process.

Sb 2 O 3 ,[17] SnO 2 :Sb 2 O 3 ,[18] BiO 2 ,
Laser deposition is also involved in laser-induced forward transfer (LIFT), which has very interesting features for locally depositing materials. [START_REF] Piqué | Direct-write laser transfer and processing[END_REF] The nozzle-free setup in LIFT allows the use of highly viscous and/or functional inks with micrometric resolution. [24,25,26,27,28] However, in LIFT, the deposited material is transferred by a laser but remains at the surface of the substrate. Consequently, LIFT generally leads to low adhesion or abrasion properties.

In this context, a process that combines aspects of both the inkjet approach and laser curing was developed. [29] In the process, ink is deposited by an inkjet on a polymer surface. Laser irradiation is then focused on the ink to make it enter the upper polymer layer. After laser curing, pigments are thus inserted in the polymer, which guarantees the resistance of the marking, as shown later. The key point is the control of the local thermal effect induced by the NIR laser. The objectives of this paper are to investigate the processes involved and demonstrate the interesting aspects of the concept.

Results and discussion

The principle of laser polymer tattooing (LPT) is illustrated in Figure 1a. The image to print is first generated on a computer and sent to a conventional printer head. Ink is deposited at the desired location by an inkjet, and the NIR laser stabilizes the ink. The NIR laser diode is installed in close vicinity to the inkjet nozzles. The deposition and fixation process are thus achieved in a single pass. The laser power is adjusted according to the ink deposition speed to deliver the needed energy to activate the process. One of the interesting aspects of this approach is that there is no excess ink to eliminate from the polymer surface. Indeed, as shown later, with an optimized ink thickness and laser power, it is possible to fix all of the deposited ink into the polymer substrate. In this work, the ink is a dispersion of pigment Black 7 in diethylene glycol diethylether. The concentration of the black dye in the ink is 5 wt %.

The polymer substrate is held on a translation stage and displaced during inkjet marking and laser curing at a typical speed of 5 m/min. The schematic in Figure 1b provides details on the different parts of the setup, including 1) the translation stage that moves the sample, 2) the inkjet nozzles, 3) the ink tanks, 4) electronic control of the inkjet heads and 5) the NIR laser diode (808 nm). An example of a gray-level image printed by LPT is given in Figure 1c. In this case, the resolution of the image was 180 dpi. The gray level was controlled via control of the drop volume and pixel density. A polypropylene (PP) substrate was used here. The polymer surface was only cleaned with ethyl acetate before printing, without any other specific surface treatment.

To investigate what happens to the black carbon nanoparticles during laser irradiation, transmission electron microscopy (TEM) characterizations were conducted. In this set of experiments, to simplify the sample preparation, ink was deposited by spin-coating on PP substrates. Here, spin-coating allows deposition of a continuous layer of ink with a thickness that is directly linked to the rotation speed during deposition. The laser irradiation conditions were 5 m/min at 60 W. A thin section of the sample was then prepared by ultramicrotomy and observed by TEM. The results are shown in Figure 2. Figure 2a shows the first section of the raw polypropylene surface used in this study as a reference. In the sample shown in Figure 2b, laser treatment (5 m/min, 60 W) was conducted at the surface of the polymer, without any ink in this case. The morphology of the polymer was not modified by the laser. This is expected from the low absorption of PP at 808 nm, so the laser beam passes through the sample without any significant interaction, preserving the smooth surface. It is worth noting that none of the other polymers used in this study showed any significant intrinsic interaction with the NIR laser under the conditions needed to cure the ink. This justifies the choice of wavelength and the laser irradiation parameters. Indeed, it is important that the parts of the sample that are not covered by the ink do not interact with the laser to keep the polymer properties intact. Under such conditions, laser irradiation only needs to be performed on the areas covered by ink, which simplifies the laser irradiation step. Figure 2c shows a TEM cross-section after deposition by spin-coating. A section of the uniform ink layer can be easily visualized on top of the PP surface. The thickness of this layer was evaluated to be approximately 400 nm.

Single black carbon nanoparticles are visible. As expected, they exhibit a wide polydisperse size distribution, with diameters ranging from 10 to 60 nm. Figure 2d, Figure 2e and Figure 2f show the impact of the laser treatment on the PP surface covered with ink. The following can be observed. i) There is no longer ink on the surface of the polymer. ii) Carbon black nanoparticles can be observed inside the PP under the surface. Interestingly, the particles form large continuous aggregates with a width comparable to the initial thickness of the ink layer. This suggests that the ink layer initially present at the surface was transported by blocks inside the polymer. iii) The roughness at the polymer surface is significantly increased by the laser irradiation. iv) A separation line parallel to the surface (dashed line in Figure 2d) is observed.

The depth of this line is approximately 2000 nm. Such a separation was not observed in the polymer (Figure 2a and Figure 2b), even after laser irradiation. The difference in the contrast between the 2 zones in the TEM image reveals a difference in the density induced by laser melting.

Based on these observations, it is possible to propose a mechanism explaining the ink/laser/polymer interaction: NIR laser irradiation is absorbed by the C nanoparticles of the ink layer, which causes a significant increase in temperature. Indeed, as shown in Figure S1, a 400-nm-thick film absorbs 27% of the NIR light. The increase in temperature is high enough to provoke melting of the upper part of the polymer. We could not conduct a direct measurement of the temperature increase, but the observation by TEM of a change in density in the upper layer of the polymer proves that the first 2 microns under the surface of the PP melted. This is also supported by the presence of the carbon black nanoparticles under the surface of the PP. Obviously, the nanoparticles could not diffuse without melting of the PP.

The temperature reached under laser irradiation is thus between 160 °C (fusion temperature measured by DSC, see Supporting Information, Figure S2) and 250 °C, which is the degradation temperature of PP. The heating effect of the C nanoparticles is known and has already been reported in other contexts. [30,31,32] The heating of the polymer under laser irradiation to fusion temperature is in agreement with previous results from Fukumura et al.[ 33 , 34 , 35 , 36 ] The curved shape of the carbon nanoparticle aggregates strongly suggests that the nanoparticle movement was induced by the convective movement of the melted polymer.

Photothermal convection was recently proposed as a driving force for the directed selfassembly of colloidal plasmonic nanoparticles on surfaces. [ 37 ] In our case, the main difference is that the nanoparticles are already immobilized on the surface, but the heat generated by the nanoparticles can also provoke the melting of the polymer. High-temperature gradients are created, inducing convective movements when the polymer upper layer is in a liquid state. The convective movement of the melted polymer drives the displacement of the C nanoparticles in a continuous movement that explains the observation of blocks of nanoparticles inside the polymer. Since the viscosity of the melted polymer is high, the displacement of the C nanoparticles by diffusion cannot be proposed as a displacement mechanism. Photonic forces may also be invoked due to the strong absorption of the C nanoparticles that can result in repulsive forces applied to the C nanoparticles. It is difficult to state on the relative contribution of such forces in our experimental conditions, especially because the medium remains quite viscous. However, they are too weak to induce displacement in this viscous medium, and they would likely create a more chaotic distribution of the C nanoparticles inside the polymer matrix. The melted polymer convection contribution is also supported by additional experiments performed with a fixed laser source in equivalent irradiation conditions. In that case, we observed heating of the C nanoparticle film, but it did not result in the displacement of the ink into the polymer. This showed that the movement of the laser head over the sample played a role in the displacement of the nanoparticles. A schematic is proposed in Figure 2g to summarize the nanoparticle displacement process.

The impact of important parameters, such as ink thickness and laser power, was then investigated to confirm our proposed mechanism. The results are shown in Figure 3. In this section, the method was as follows. To obtain a homogeneous thin film, ink was deposited on PP circular substrates (diameter of 5 cm) by spin-coating, with spinning rates between 500 and 6000 rpm, leading to an ink thickness between 130 and 400 nm. This thickness was determined by atomic force microscopy by measuring the height of a scratch made on the ink layer. The laser conditions were then fixed to 88 W with a displacement of 5 m/min. Two lines, each with a width of 8 mm, were generated across the sample by passing the laser over the sample. Half the sample was then wiped by ethyl acetate to remove all nonfixed C nanoparticles on the surface. A typical result is shown in Figure 3a. Four different zones were generated and analyzed, as schematized in Figure 3b. Zone 1 corresponds to the deposited layer. The intensity of the black color can be correlated to the thickness of the deposited films, as shown in Figure S3. Zone 2 corresponds to the ink layer cured with the laser, without a wiping step. The difference between zones 1 and 2 is due to nanoparticles that may have been lost during laser irradiation. Zone 3 is the cured area after wiping with ethyl acetate. The difference between zones 2 and 3 provides information on the amount of C nanoparticles that penetrated into the polymer matrix with laser irradiation, since the nanoparticles that remain on the surface are easily removed by the solvent. Finally, zone 4 is the naked substrate. Zone 4 is used as a reference to analyze the images, providing the grayscale level of the naked substrate. All images were recorded using the same lighting conditions so that they could be compared with each other. The gray level was analyzed in each area using ImageJ software and normalized. The 0 value corresponds to complete black, and the 1 value corresponds to the white value of the PP substrate. The effects of the ink film thickness and laser power were investigated. The results are reported in Figure 3c and the intensity of the black color, as expected, increases accordingly with ink film thickness. For a lower thickness (< 250 nm), the gray levels after laser irradiation (zone 2red circles) and after washing (zone 3blue triangles) are very similar. This result means that for this range of thicknesses, most of the C nanoparticles are included in the PP substrate by laser irradiation and thus remain after washing. This result corresponds to efficient permanent marking. The slight difference between zone 1 and zone 2 may be attributed to a loss of contrast when the carbon nanoparticles are inside the polymer substrate and the appearance of surface roughness. However, at a low ink thickness, the initial gray level is limited. The resulting images cannot have much contrast. An increase in the contrast can be obtained only if the ink thickness is increased. In this case, as shown in Figure 3c, the initial gray value indeed increased with thickness (zone 1), but after laser irradiation, there was no improvement in the image contrast for an initial ink thickness greater than 250 nm. The light absorption varies from 14 to 22% for ink thicknesses in the range of 150 to 250 nm. Consequently, we can expect the temperature increase to be more pronounced with a thicker ink layer, which should favor the transfer of ink nanoparticles inside the polymer. As shown here, the results do not follow this trend. Indeed, the ink layer becomes too thick to provide efficient heating of the polymer for thicknesses greater than 250 nm. An appreciable proportion of light may be absorbed by the surface of the ink, but the absorbed energy is not efficiently coupled to the polymer. In the end, it appears that the optimum thickness to obtain the best performances is approximately 250 nm. These conditions correspond to a compromise in the absorption of the ink films and heat transfer to the polymer.

Another important parameter is the laser power. The effect of the laser power is reported in Figure 3d. In this set of experiments, the thickness was set to 260 nm (1500 rpm). The slight variation of the gray level in zone 1 (spin-coated film, without any irradiation) is related to the standard variation in the ink thickness from one sample to another. With laser treatment, three different regimes can be observed: i) For the lowest powers (less than 120 W), in zone 2, the gray value is almost constant, following the same trend as in zone 1. However, after washing the surface, a significant deviation from the initial gray value was observed. The marking efficiency decreases significantly, and the marking is not effective at the lowest power (15 W). This behavior can be explained by insufficient energy being delivered to the system by the laser, which does not allow the melting of the polymer and thus the displacement of the C nanoparticles.

ii) For intermediate powers (from 45 to 120 W), the result is quite stable over the whole range of power. The contrast loss after washing is limited, which means that most of the C nanoparticles have been included in the polymer after laser treatment. This range of power corresponds to the optimum laser conditions for this thickness.

iii) For the highest laser power (> 120 W), a decrease in the marking intensity is recorded. At the same time, fumes were observed, so this loss of contrast could be attributed to the vaporization of the C ink due to excessive laser power. This explains why the gray value is also lower directly after laser irradiation, even without a washing step.

In the end, this study allows us to define ranges of power and thickness that are the most suitable for an efficient process. It also demonstrates that under optimized conditions, the yield of the C nanoparticle fixation is almost maximum.

In the following section, we consider the ink deposited by the inkjet. As shown in Figure 4, on PP substrates, the drops of ink are circular in shape. A spacing of approximately 500 m between droplets was chosen here to observe laser-induced phenomena at the scale of a single droplet. We varied the volume of the deposited drop, using volumes of 14, 24 and 35 pL, to investigate the impact of the droplet volume on the marking process. As in the previous study with spin-coating, the results are compared before and after wiping with ethyl acetate to probe the displacement of the nanoparticles inside the polymer. With this configuration, lateral displacement can also be observed, which is an important parameter for inkjet printing (for resolution issues). Figure 4b refers to the droplet volume of 24 pL. For a lower power, we observed that the laser significantly increased the diameter of the drop. Heating of the drop by the laser decreases the viscosity of the drop, leading to spreading of the liquid over the surface.

At the same time, after wiping the irradiated zone, the ink was removed, which showed that the displacement of the nanoparticles into the polymer did not occur. Interestingly, on the washed sample, the imprint of the drop is visible. This means that the melting of the polymer occurred under the ink drop, but the heating was too low to induce displacement of the nanoparticles. Increasing the laser power from 25 W to 50 W did not improve the displacement of the nanoparticles inside the polymer. The only difference between these two last conditions is the wider spreading of the ink under laser irradiation. At a power of 88 W, laser irradiation is efficient enough to induce C nanoparticle fixation in the polymer. At a higher power (120 W), the result was similar. For smaller droplets (column a), the dependence on laser power follows a similar trend, but some differences were recorded. In particular, it was observed that the spreading of the drop under laser irradiation is less pronounced. This is also consistent with the thermal spreading of the initial drop. Interestingly, at 120 W, the increase in the diameter of the pixel is limited, and the contrast is similar after washing the surface. These conditions provide the best writing conditions to obtain the highest contrast and image resolution. As expected, when the volume of the droplet is increased, the diameter of the pixel increases without any improvement in the sensitivity of the ink to NIR irradiation.

In conclusion, small droplets (14 pL) are thus the best configuration to obtain high-quality marking.

Obviously, the results should be dependent on the polymer substrate. Indeed, the thermal parameters of the polymers strongly depend on their composition, and their surface energy will also drive the spreading of the ink droplet. The process is also highly dependent on the thickness of the deposited ink. The ink deposition and fixation processes were thus studied for the following polymers: PP (Tf=165 °C), PVC (Tf=180 °C), ABS (Tf=200 °C), PC (Tf=230 °C), and PA66 (Tf=258 °C). The results are displayed in Figure 5. First, the diameter of the drops after inkjet deposition varies greatly from one polymer to another. As previously mentioned, wetting is very limited on PP, resulting in a drop with the smallest diameter (and thus the highest height since the volume is constant). In contrast, the contact angle is zero for ABS and PC, which results in a significant spreading of the drop after deposition. For PVC and PA66, the drop spreading is intermediate. Note that at this stage, the most obvious impact of this spreading is the loss of resolution. We also emphasize that our objective was to evaluate the raw polymer surface without any chemical treatment or mechanical polishing. Spreading of the drop on ABS or PC can be modified by chemical treatment (plasma, corona or flaming). Additionally, the impact of the surface roughness of PET or PVC used here is clearly visible. The objective here is to demonstrate that marking can be effective, despite these surface defects.

The comparison of laser marking in these polymers highlights the importance of the fusion temperature on the efficiency of the process. Indeed, it appears that the marking process operates well for polymers with the lowest fusion temperature (less than 200 °C) for this set of parameters. PP, PVC and ABS were indeed well marked, which is consistent with the proposed mechanism, relying on the laser-induced fusion of the polymer surface. Some differences were noted between the polymers (Figure 5b). For PVC, a change in color was noticed after the laser treatment. Additionally, for a higher laser power, fumes were observed. They can be attributed to the thermal degradation of the polymer. However, the marking is quite efficient for this polymer with a limited loss of resolution during laser irradiation. ABS exhibited different behavior: the surface energy of the ABS surface provoked a significant spreading of the ink drop after deposition. However, laser irradiation resulted in a confined ink mark. As shown in Figure 3a, we believe that the ink thickness is too low in the periphery of the deposited drop to reach the threshold for fixing the ink into the polymer.

For ABS, the fusion temperature is more important than for PP, so the threshold is higher. This point was confirmed by experiments with larger deposited drops that resulted in local burning of the polymer, as revealed by fumes. In conclusion, the process can be adapted to ABS but is not as efficient as for PP or PVC. In terms of resolutions, this process can not be considered as high-resolution one. We can indeed observe that pixels with diameters of 100 microns can be obtained in some conditions, which does not degradate too much the resolution from inkjet deposition.

However, due to their high fusion temperatures, the process is more difficult to apply to PA66, PC and PET. Indeed, we observed that the ink could be partially removed by cleaning the polymer substrate with ethyl acetate in the cases of PA66 and PET. For PC, the ink was completely removed. Interestingly, in this last case, the imprint of the drop can be clearly seen. This corresponds to a fused polymer area that is well suited to the deposited drop. As mentioned before, the fusion of the polymer surface occurred under such conditions, but the temperature reached was not sufficient to induce displacement of the C nanoparticles. For these polymers, the laser power was increased. However, it appeared that this could not improve the results for these polymers, as the increased power resulted in a degradation of the ink. This resulted in fumes and showed that the ink was removed from the polymer surface during laser treatment.

The resistance of the ink after laser treatment was confirmed by conducting normalized aging tests. These tests are detailed in the experimental section. The ink deposited by LPT was compared to standard UV ink deposited by an inkjet on similar substrates (PP, ABS, PVC).

The stability of the marking was compared under various conditions. The results are summarized in Table 1. In all cases, the LPT marking showed very good stability with no significant modification noticed after aging. The most remarkable result is associated with the olefin substrate (PP), for which the UV inkjet marking exhibits low aging properties, whereas with LPT, the marking is very stable. This result confirms the displacement process of the C nanoparticles inside the polymer.

In conclusion, we have shown that the laser polymer tattooing (LPT) process can be successfully applied to various polymer substrates with fusion temperatures below 200 °C. This process is particularly interesting for marking olefin substrates such as PP. Indeed, for such polymer substrates, a stable marking could be obtained, with better aging properties than standard UV ink. The reason for this stability relies on the laser-induced process involved.

The NIR laser treatment forces the ink nanoparticles to migrate into the polymer substrate.

The optimization of the ink thickness and laser power allows integration of the major part of the deposited ink, so no further treatment after laser irradiation is needed. Such a process thus presents major advantages for polymer marking for potential applications in decoration, object personalization or identification and quality control. Our current works include the application of the same principle to colored inks.

  Figure S5. For the deposited layer (zone 1black squares), the gray level value follows the

Figure 1 .Figure 2 .

 12 Figure 1. a) Schematic view of the polymer tattooing process showing the image to be printed.

Figure 3 .

 3 Figure 3. a) Schematic of the sample preparation for the study of the effects of ink thickness

Figure 4 .

 4 Figure 4. Optical microscopy images of the LPT process on PP substrates. a), b) and c) correspond to deposited drop volumes of 14, 24 and 35 pL. The first row shows images of the drops as deposited, and each subsequent row corresponds to different laser powers (25, 50, 88 and 120 W). For each condition, images after laser irradiation and after washing with solvent (ethyl acetate) are displayed.

Figure 5 .

 5 Figure 5. Comparison of LPT on various polymer substrates. a) Images of drops as deposited after laser treatment (88 W) and after washing the surface with ethyl acetate. b) Zoomed images of the drops before and after laser irradiation (PP, PVC and ABS).

Table 1 .

 1 Comparison of ink aging after a standard UV inkjet processing and LPT for ABS, PVC, PA66 and PP.

			UV ink			LPT
	Polymer	ABS	PVC	PP	ABS	PVC	PP
	Adherence (DIN EN ISO
	2409) after aging:			Complete
		OK	OK	peeling-off	OK	OK	OK
	3 days, 120 °C (internal			ISO4
	method, cf ISO 3248)
	Adherence (DIN EN ISO
	2409) after aging:
	4 days, 70 °C, 95%RH	OK	OK	Peeling-off	OK	OK	OK
	(internal method, cf ISO
	4611)
	Abrasion
	Ethanol/water (60/40) (internal method, cf ISO	OK	OK	Slight peeling-off	OK	OK	OK
	2812)
	Immersion of ethanol/water
	(60/40) (internal method, cf	OK	OK	Peeling-off	OK	OK	OK
	ISO 175)

Acknowledgements

The authors thank the Fond Unique Interministeriel for its financial support (project LCOLOR3D), Moveo, Plastipolis, Vehicule du Futur and Agence National de la Recherche (grants ANR14CE26-003). Gautier Schrodj is acknowledged for DSC characterizations.

Received: ((will be filled in by the editorial staff))

Revised: ((will be filled in by the editorial staff))

Experimental Section

Chemicals

Black ink was prepared from a dispersion of pigment Black 7 (Aldrich, CAS Number: 1333-86-4) in diethylene glycol diethylether (Aldrich, CAS Number: 112-36-7). The concentration of the dye was 5 wt %.

The ink was deposited by two means. i) Homogeneous thin films were prepared by spincoating using a table-top spin-coater (SPIN150i). ii) Patterns were produced by an inkjet using a SEIKO 508GS printer head. The resolution was 180 and the volume of the drops delivered was tunable between 14 and 35 pL.

NIR light from a 500 W, 808 nm, continuous-wave laser diode (LDL500 from Laserline) was used to fix the ink. Irradiation surface was 0,16 cm 2 .

The printer head and laser diode were fixed, and the sample was translated by a mechanical stage with a linear speed of 5 m/min.

After laser fixation, the patterns were cleaned by firmly scrubbing the sample surfaces by hand with paper soaked with ethyl acetate.

Microscopy

Transmission electron microscopy was performed on a Philips CM200 microscope. Optical microscopy images were acquired with a binocular Olympus SZ and a Zeiss Axio Imager.M2m microscope.

Aging tests:

For the aging tests, the ink fixed by LPT was compared with standard UV ink deposited by an inkjet and stabilized by UV light. The printer was a DCS 1800z supplied by DirectColorSystems equipped with a UV LED (6 W, 395 nm). In this case, a protective UV curable ink was used. It served as an adhesion promoter and protective coating. The UV ink was UV-LED IRF6 (DirectColorSystems).

Aging tests were conducted following procedures adapted from the normalized tests. The procedures are summarized in Table 1. Adherence tests were carried out after aging i) 3 days at 120 °C and ii) 4 days at 70 °C with 95% RH, and peeling-off tests were conducted following the ISO2409 procedure. Abrasion tests were performed following the ISO2812 procedure using a mixture of ethanol/water (60/40). Immersion tests were performed with an ethanol/water (60/40) mixture following the ISO 175 procedure.

Supporting Information

Supporting Information is available from the Wiley Online Library or from the author.