F Stock

F Antoni

D Aubel

S Hajjar-Garreau

D Muller

Pure carbon conductive transparent electrodes synthetized by a full laser deposition and annealing process

Keywords: Pulsed Laser Deposition (PLD), Diamond-Like Carbon (DLC), Laser surface annealing, Transparent electrodes, graphitization

One of the biggest challenge that face optoelectronic and photovoltaic devices is the necessity to provide a reliable alternative to transparent conducting oxide (TCO) like Indium Thin Oxide (ITO). We recently published a study proposing a method to produce transparent conductive electrodes only based on carbon materials. In a first step, we use the Pulsed Laser Deposition (PLD) to produce high performance DLC. Those thin films own very interesting properties in kindship with diamond, like high transparency in the visible range, chemical inertness and biocompatibility. In addition, the DLC is a perfect electrical insulator and presents a relative high opacity in ultra-violet (UV). This particularity has a great interest to perform, in a second step, UV laser annealing over the DLC surface. The aim is to break the existing diamond bindings (sp 3 hybridization) on the surface and allow atoms being reorganized in graphitic bindings (sp 2 hybridization). We demonstrate that the increase of atomic graphitic bindings leads to a valuable surface conductivity. According to optimized annealing parameters, the surface conductivity reaches values comparable to ITO. We also show that the laser treatments only sparsely affects the DLC transparency. Moreover, this full laser-based process remains compatible with the standard microelectronic technological steps.

Introduction

One of the nowadays commonly used transparent conductive material in optoelectronic and photovoltaic devices is Indium Tin Oxide (ITO). This oxide combines a very high transparency in the visible range (nearby 90 %) and an important electrical conductivity (around 10 4 (Ωcm) -1 ) over wide areas [1][2][3]. ITO is mainly used as transparent conductive electrode deposited over photovoltaic cells or optoelectronic devices like capacitive touch panels or liquid crystal displays. However, the use of this material will face in important and increasing problem. Despite very interesting performances, the rarefaction of Indium combined to the impossibility of effectively recycle ITO will jeopardize the use of this technology. Therefore, a suitable alternative is needed to replace this metallic oxide in optoelectronic components. We propose an interesting and alternative solution using only pure carbon material elaborated by laser processes offering moreover a high compatibility level with standard opto and microelectronic technological steps.

An interesting carbon based material is the Diamond-Like Carbon (DLC). This material, already used as protective coating and as anti-reflexion layer in photovoltaic cells [START_REF] Pern | 31st IEEE PV specialists conference and exhibition[END_REF][START_REF] Lin | [END_REF][6] offers a very good potential for optoelectrical applications. This amorphous form of carbon share many properties with diamond, like high optical transparency in the visible range [7,8]. DLC is an homogeneous mixture of carbon sp 2 bindings (stable hybridization corresponding to graphitic carbon) and sp 3 bindings (metastable hybridization corresponding to diamond) [9,10]. Due to its proximity to diamond and the presence of sp 3 bindings, DLC is very high electrical insulator [7,[11][12]. Nevertheless, this form of carbon remains an interesting transparent material valuable in photovoltaic and optoelectronic devices. DLC properties are highly depending on the elaboration process (and the associated parameters) used to synthetize it [13][14][15]. In our approach, we use Pulsed Laser Deposition of graphite to obtain DLC thin films. This elaboration process while performed in high vacuum with a nuclear grade graphite target, PLD-DLC show a very high purity by offering hydrogen and oxygen-free layers. Very advantageous properties are obtained with this technique, allowing tuning the sp 3 and sp 2 bindings ratio by working at different wavelengths and energy densities [10,[16][17].

However, DLC being a strong insulator, other conductive forms of carbon have also our interest. Graphitic or graphenic forms of carbon show outstanding transport and conductivity properties. For example, graphene (a 2D material made of sp 2 bounded carbon atoms organized in a honeycomb structure) present a record electron mobility reaching 200 000 cm 2 V -1 s -1 while keeping a very low optical absorption (typical 2.3 % per atomic layer) [18][START_REF] Cho | Center of nanophysics and advanced materials[END_REF][START_REF] Bolotin | [END_REF]. Well studied in the literature, graphene is obtained by various elaboration methods like chemical vapour deposition, aerosol or graphite exfoliation. However, it remains nowadays very hard to grow graphene over large surfaces or to transfer it on wide areas. Multilayer graphene or graphitic thin films also keep interesting conductivity and optical transmission properties. Combining those properties with DLC transparency is an interesting approach to obtain transparent conductive layers.

We already published a study [21] using an original and innovative two-step process, composed of a DLC elaboration by PLD followed by a laser surface annealing of DLC layers. The first step consists in the growth of DLC by PLD allowing the synthesis of hydrogen free layers that permits laser annealing. The relative high opacity of DLC in the UV range also allows a possible energy absorption in the first atomic layer of the DLC. Therefore, a second step of UV laser annealing has the effect to convert existing and dominating sp 3 bounded atoms into a pure sp 2 graphitic surface layer. Sp 3 bindings are broken by the energy delivered by the successive laser shots permitting atomic reorganization in a stable graphitic form of carbon. This transformation affects the very first atomic layers and leads to the formation of a conductive film on the top of the deposited DLC film. The resulting structure is composed of a graphitic surface conductive layer (remaining highly transparent due to a very low thickness) set on the preliminary deposited DLC material. This approach allows obtaining interesting performances in term of transparency and conductivity.

Experimental details

Pulsed laser deposition of DLC

In this work, we use a standard PLD setup to perform DLC depositions. An excimer laser beam is focused with a 45° incidence angle on a high purity graphite target (99.999 %) placed into a high vacuum chamber (background pressure less than 10 -8 mbar, avoiding therefore any presence of oxygen inside the deposited films). As the laser hits the target, the matter is ablated perpendicularly to its surface, forming a plasma plume of the ablated species, which are collected on the substrate set in front of it. The target-substrate distance is set to 5 cm and kept constant in order to obtain a homogeneous sample coverage over 1 x 1 cm². To avoid crater formation by successive laser shots on its surface, the target is kept in rotation at 1 rpm for during the deposition time. The excimer laser (LAMBDA PHYSIK Compex 201) produces up to 600 mJ pulses (25 ns duration) at repetition rates from 1 to 10 Hz. By using a KrF gas mixture, the output ray is associated to a 248 nm wavelength and an ArF gas mixture leads to a 193 nm beam. With an optimal focus and considering all optical components energy losses, on-target fluences range is tuneable from 2 up to 15 J/cm² in KrF mode and 2 up to 7 J/cm² in ArF mode. All the deposition process is done at room temperature to obtain the highest adamant character as shown in previous studies [10,[21][22][23]. Previously cleaned (ethanol in ultrasonic bath) glass, quartz and SiO2/Si are used as substrates. Glass and quartz permits transmittance measurements while SiO2/Si substrates were used to measure conductivity as well as for X-ray Photoemission spectroscopy (XPS) analysis.

2.2.

Laser surface annealing

Laser treatments of DLC surfaces are performed with the same excimer laser as for deposition. The 248 nm excimer laser beam passes through an optical homogenizer in order to obtain a 1 x 1 cm² top hat beam (better than 1% homogeneity). A home-designed chamber, equipped with a quartz window (for the UV laser input) was specially built in order to anneal samples under vacuum or neutral gas atmosphere. In this study, annealing is done under 0.5 mbar of static argon pressure. With this setup and induced mechanical constraints, the laser beam reaches a 5 x 5 mm² homogeneous size, allowing working with fluences in the range of 0.05 up to 0.36 J/cm² (far under the carbon ablation threshold). To obtain homogeneity for large area treatments, the chamber is fastened on a computer controlled XYZ table . A special designed software commands the successive table positions, ensuring homogenous sample coverage by setting a 90 % beam overlap between each position (in X and Y directions).

XPS measurement

All XPS measurement presented in this study are perform with a 1486.6 eV photons beam. The analysed surface is 4 mm². Thanks to angle-resolved measurements, two different depths can be measured on each sample. Indeed, by using a 60° "grazing" incidence, a 4,8 nm (± 0.3 nm) depth is measured, where a 20° "normal" incidence leads to the measuring of a 9,1 nm (± 0.35 nm) in the layer depth. Combining both incidence measurements, an interesting set of data is obtained to investigate the structural modification in the DLC after laser annealing. To extract sp 2 and sp 3 binding percentages, we focus our analysis on the C1s peak representative of carbon atom bindings. This peak is deconvoluted in order to extract the sp 2 contribution (located around 284.5 eV) and the sp 3 contribution (located around 285.4 eV) [21,[24][25][26] as shown in Fig. 1. Measurements also reveal a very low oxygen surface contamination (less than 10 %). We highlighted that this oxygen presence comes from samples exposition to air during transfer (as depositions are performed under high vacuum PLD from an oxygenfree target). Therefore, we will not take it into account and consider DLC only made of pure carbon material. Moreover, in order to avoid an unexpected increase of sp² values in DLC, we strongly limited the desorption process at 100 °C for very short time. For the measurement set, we obtain less than 1 % peak shift for each of these contribution, guaranteeing a very fine precision of ± 0.5 % for each contribution.

Optical and electrical characterization setup

To evaluate the functional performances of our DLC layer after treatment, we mainly investigate transmittance and conductivity. For optical transmission measurements, we use an UV-visible spectrometer as type PERKIN-ELMER ® Lambda 19. Typical glass-deposited DLC spectra are acquired from 300 to 850 nm (due to the glass UV cut-off wavelength located around 280 nm) while quartz substrates allows to measure down to 200 nm. A deuterium lamp generates UV light source while visible and infrared light is provided by a halogen lamp. A low scanning speed (30 nm.min -1 ) is chosen to ensure high accuracy measurements. Blank glass and quartz signals are also acquired allowing the determination of DLC single response by subtracting the substrate contribution.

Conductivity measurements are performed with an ECOPIA HMS5000 apparatus. The setup is enhanced with 4 probes attached on printed circuit board in a Van der Pauw configuration. The 1 x 1 cm² sample is placed on the board and the probes are placed in the corners of the sample. A data set of at least 5 measures are made per sample and an average value is extracted.

In addition, a special designed testbench has been made to measure DLC surface resistance. The sample is embedded in a 3D printed plastic holder. Surface contact (on each side of the surface) is made with a single side copper epoxy board to obtain a 1 cm long contact on the sample. To optimize contact performances, a thin copper wire is soldered on the conductive face of the epoxy plate. The contacts are pressed on the sample surface with 3 springs (for each contact) offering a homogeneous pressure all over the sample length. This setup able a direct measurement of the surface treated-DLC equivalent resistance and permits the use of the layer in an electrical circuit.

Results

Evaluating best DLC properties to perform laser annealing

We compare the possibilities to perform laser annealing on ArF and KrF-made DLC. In order to obtain the best transparency performances, we first investigated laser annealing on ArF-deposited DLC. This DLC has a highest transparency as KrF-made DLC (as presented in Fig. 2). Therefore, starting from this material would have led to a better transparency on our layers. We use same treatment parameters for both DLC types. As conductivity is obtained on KrF-made DLC [21], we never able to obtain any conductivity on ArF-deposited DLC. We observe in that case an early DLC destruction even for low annealing fluences (nearby 0.2 J/cm²). By reducing this fluence to avoid DLC destruction, we never reach the energy threshold leading to conductivity apparition. XPS measurements show a slight graphitic binding increase after annealing. Nevertheless, the maximal sp² binding percentage never overpasses 20 %, which is not enough to obtain surface conductive layer. Therefore, KrF-produced DLC (due to initial higher amount of sp² bindings) is the best candidate to perform laser surface annealing. Moreover, as demonstrated in [10], we select KrF-DLC deposited at 5 J/cm² due to the highest adamant character of the obtained DLC at this fluence, leading to a highest initial optical transparency. For all the following study, we only consider DLC deposition performed at 5 J/cm² in KrF mode.

XPS analysis, structural changes modelling and graphitized depth estimation

In this paragraph, we investigate the structural changes induced by laser surface annealing on KrF-DLC. As reference, XPS measurements have been made on untreated KrF-DLC. The reference reveals a 60.9 % of sp 3 bindings and 39.1 % sp 2 bounded atoms homogenously distributed in the entire layer. Angle resolved measurements confirm the homogenous character of the DLC film after deposition. DLC were annealed with various sets of parameters (fluence and number of shots). To keep in the efficiency range as demonstrated in a previous study [21], the fluence is tuned from 0.1 up to 0.2 J/cm² for a maximum of 10000 shots. A clear increase of sp 2 bindings percentage is observed as soon as the annealing fluence is raised. The same behaviour is detected as the number of shots also increase [21]. A significant difference between normal and grazing incidence analyses is obtained, showing highest amount of sp² in the surface measurement.

We propose here a simple model in which we consider that the annealed DLC is composed of a fully-graphitized layer (pure sp 2 bindings) located at the surface of the film. As the relative opacity of the DLC at 248 nm has been demonstrated, we also believe that the in-depth DLC is not affected by laser annealing and keep the same structure (same amount of atomic bindings) than before annealing. We have then a structure composed of two layers, in which we will extract the thickness of the surface graphitized part. The XPS beam permits to estimate the DLC volume analysed. We approximate the analysed volume as a cylinder considering the dimension ratio between the analysis depth (a few nanometers) and the beam section (S) on the surface (in the mm² range). As the initial contained sp 2 in the depth (αI) is known such as the sp² bindings measured after annealing (αT), we can express the relative thickness (x) of the full graphitic layer as following:

𝑆𝛼 𝑇 = 𝑆(𝑥 + (1 -𝑥)𝛼 𝑇 )
As this equation is solved, we can calculated the absolute thickness of the full sp² part of the DLC film (dGRAPH) regarding the XPS beam penetration depth (dXPS), as given by the next formula :

𝑑 𝐺𝑅𝐴𝑃𝐻 = 𝑥 𝑑 𝑋𝑃𝑆
This modelisation also able to estimate the total amount of the full-graphitized layer in term of number of graphitic monolayers. Knowing the approached pure sp² depth and considering the thickness of one graphite 2D plan (Z = 3.4 Å), we can calculate the number of equivalent graphitic monolayers (nGML) that composes the film.

𝑛 𝐺𝑀𝐿 = 1 + 𝑑 𝐺𝑅𝐴𝑃𝐻 𝑍
We use this model to extract graphitic layer thicknesses from the XPS measurements. For each annealing parameters sets, we perform the calculation for grazing and normal analyses and calculate an average thickness between both results. Firstly, we observe (Tab. 1) that both measures lead to coherent and correlated results. For 0.1 J/cm², a typical graphitic thickness around 1.3 nm is calculated for 1000 and 10000 shots. As the fluence is raised to 0.2 J/cm², the graphitic layer thickness increased up to 1.74 nm for 1000 shots and reaches a 2.69 nm for 10000 shots. For this last one, we must add that the sample is sparsely affected by very small graphitic defects (which start to appear by annealing at the very high limit of the defined parameters range), which slightly boost the XPS measured sp² binding percentage.

It is also interesting to underline that no significant difference between 1000 and 10000 shots is observed for a 0.1 J/cm² fluence, meaning hereby that at low fluences, the DLC is not affected by an increase of the shots amount. It is then mainly the fluence, which controls the graphitized depth. This is confirmed by the increase of the graphitized layer depth for the 0.2 J/cm² annealing. This approximation seems to be very simple in a first approach, but it is important to notice that it shows very interesting and reliable results as correlated by other measurements (transmittance…) performed in this study.

Transmittance measurements

In this part, we focus on the optical properties of the DLC regarding the annealing parameters. It is also important to remember (Fig. 3) that for as deposited DLC, the optical transmittance is highly dependent of the film thickness, in the way that, when the layer thickness is raised, the optical transmittance is reduced (linear behaviour).

We perform DLC (KrF -5 J/cm² -20 nm) deposition on glass substrates, and we anneal those layers at various fluences for 1000 shots. For each measurement (Fig. 4), we observe the same transmittance behaviour. Indeed, for untreated DLC, the transmittance obey to a quasi-linear evolution over the visible range and the near UV. After annealing, this behaviour changes leading to a more constant value over the visible range and a harder sink down in the deep UV. This modification is also confirmed by optical visual observation showing a DLC appearing brown before annealing and presenting a greyer metallic colour after surface treatment. Furthermore, we clearly observe that despite various annealing fluences, we obtain equivalent transmittances after treatments, suggesting that the annealing fluence has a negligible impact on the transparency of the films. Moreover, it is also interesting to notice that in each case, a transmittance comparable to standard ITO [1] is obtained.

We also observe that, after annealing, the transmittance is higher for all the samples deposited on glass. This behaviour was not reported on quartz [21]. In this previous study, we measured the same change of transmittance shape before and after annealing, but globally keeping the same mean value. In fact, as we are being able to estimate a 6 pure sp² atomic layers on a 0.2 J/cm² (1000 shots) treated sample, we can justify the transmittance behaviour on quartz. At the infrared limit (800 nm), after annealing the transmittance is narrowing for about 14 %. Knowing, that one graphene layer has an optical absorption of 2.3 %, it means that the 6 graphitic layers top structure has an absorption of about 13.8 %, corresponding perfectly with the transparency loss in the near infrared domain. This point able us to validate the estimation of the graphitized thickness.

To shortly conclude on transmittance measurements and obtained performances, we show that laser annealing parameters have a reduced impact on the transmittance. Obtained results clearly demonstrate comparable performances to ITO, which are very promising for this technology.

Conductivity measurements

In a first approach, conductivity measurements are performed considering the total DLC thickness. Previously published results [21] present interesting conductivity especially for annealing made at 0.2 J/cm². Nevertheless, we here demonstrate that the DLC structure after laser annealing is converted in a conductive graphitic layer set on the unaffected DLC layer, remaining insulated. The measured conductivity is therefore mainly underestimated due to the conductive layer thickness overestimation.

As we have precise XPS measurements performed on samples processed in the same conditions (SiO2 substrate and 20 nm of KrF-DLC at 5 J/cm²), we are able to revaluate (proportionally) the real conductivity of the surface graphitic layer. As expected, results (Tab. 2) show very interesting values in term of conductivity. If annealing performed at 0.1 J/cm² for 10000 shots still remains very low (3.17 x 10 -2 (Ωcm) -1 ) and is unusable for the expected applications, samples annealed at 0.2 J/cm² show high conductivity values (3.35 x 10 4 (Ωcm) -1 for 1000 shots and 3.72 x 10 5 (Ωcm) -1 for 10000 shots). We conclude that annealing at 0.2 J/cm² for 1000 shots offers the best results (at 10000 shots, the surface presents some appearing defects).

We also investigated conductivity measurements on DLC deposited on glass substrate at various fluences for 1000 shots. It is important to notice, that results (Fig. 5) do not present the same values obtained on other substrates. Indeed, annealing at 0.2 J/cm² leads to high conductivity on SiO2, but shows lower value on quartz and on glass. We report that tuning the annealing fluence (for each substrate) permits to increase the conductivity. For example on glass, the best conductivity (1.08 x 10 3 (Ωcm) -1 ) is reach for a fluence of 0.15 J/cm². Values remains slightly under the performances obtained on SiO2.

These measurements show that the nature of the substrate is also affecting the conductivity. However, it is possible to refine annealing parameters to approach ITO conductivity. We also remember here that, the conductivity comparison between different substrates is made regarding a full DLC thickness (due to substrate effects on quartz and glass, we cannot guarantee that the same thickness of DLC is graphitized by laser annealing, so we keep the same thickness to obtain comparative measurements). Therefore, presented conductivity shows the worst case and are clearly underestimate compared to data presented previously (Tab. 2).

Integration as transparent resistor

We previously demonstrated that a very good transmittance and conductivity is obtained using a 0.2 J/cm² fluence with 1000 shots annealing parameter. This quartz deposited sample (Fig. 6) is placed in the resistor measurement testbench (described in 2. 3). An approximate resistance of 4.17 kΩ is measured thanks to a high precision ohmmeter. As sample is rotated in an other orientation, we obtain significantly the same value confirming the homogeneity of the surface conductive layer.

The testbench is integrated in an electrical circuit composed of a stabilized DC power supply and a red light emitting diode (LED). The DLC film will then play the role of LED current limiting resistor. Different supply voltages are set (10, 20 and 30 V) and corresponding currents are measured (respectively 1.99, 4.39 and 6.83 mA). In each case, the LED is on and current is stable in the circuit.

From this data, we calculated the power dissipated in the DLC layer. This power dissipation reaches 194 mW when a 30 V supply voltage is applied without DLC destruction. This simple test confirm that treated DLC layers are able to drive current and dissipate power. The layers are then finely usable as transparent resistor and show a promising potential for integration as transparent conductive electrodes.

Discussion

To evaluate the structural modification induced by the laser annealing, we consider a simple approach based on a structure made of two superposed layers, composed of a fully graphitized surface conductive layer set on an unaffected DLC insulated transparent material. Looking back on all the obtained results (conductivity, transmittance, XPS…), the validity of the model is demonstrated being in high accordance and correlation with structural and functional analyses.

Another point concerns the transmittance evolution. It is demonstrated that annealing parameters have no influence on the transmittance after treatment. It is the same for the change of the transmittance behaviour before and after annealing (which is in accordance with the colour change linked to the surface graphitization). The theoretical transparency after annealing has to be slightly reduced in correlation with the graphitization happening on the DLC surface (as it is observed on quartz substrates). Nevertheless, the increase of the transparency after annealing on glass substrates needs to be discussed. No clear explanation can be given yet for this effect. We can assume in the case of glass substrates (which are not transparent at 248 nm) that a part of the annealing laser beam is absorbed in the glass leading to more structural changes than expected. These changes could be responsible of reducing some DLC structural defects in the layer depth, driving then to the transparency increase. Nonetheless, this point is positive inducing a transparency increase which leads to better optical performances.

A similar substrate effect is highlighted by conductivity measurements, showing a lower conductivity on glass and quartz substrates than on SiO2(63nm)/Si. As known, quartz and glass do not have the same optical behaviour at 248 nm. As quartz keep a high transparency in the UV range, glass is fully opaque under 280 nm. It means that for a laser treatment performed on glass, the remaining part of the laser energy reaching the DLC-substrate interface is fully absorbed in the glass, leading therefore to a local thermal effect. This point leads to undesirable changes on DLC at the interface and in the layer depth, slightly modifying the DLC structure and leading to a conductivity variation. We also show that those effects could be countered with the tuning of the annealing fluence.

Conclusion

In this study, we demonstrate the possibility to elaborate pure carbon transparent conductive electrodes by a full laser process. We proved that KrF deposited DLC (despite lower transparency compared to ArF-deposited DLC) is the best material to perform laser annealing to graphitize the DLC surface (over less than 2 nm). A homogeneous surface graphitization happens on DLC when the film is annealed with a fluence set between 0.1 and 0.25 J/cm² as it has been demonstrated by XPS measurements. Combining XPS grazing and normal incidence measurements with a simple structure model, permits to estimate the thickness of the full-graphitized layer, typically around 1.74 nm for the best obtained sample (in term of conductivity and transparency).

In term of performances, 0.2 J/cm² and 1000 shots show the best results on SiO2 and quartz substrates, reaching a 3.35 x 10 4 (Ωcm) -1 conductivity with consideration of the a 1.74 nm full graphitized thickness. Higher conductivity is obtained for 10000 shots, but samples present in that case some surface defects affecting the transmittance. In term of transparency, DLC on glass shows systematically values in the same range than ITO. We demonstrate therefore that our process able us to produce pure carbon transparent conductive electrodes reaching ITO performances.

Finally, the integration of samples as transparent resistors proved the possibility to use treated DLC as functional electrical components and represent interesting perspectives for future applications. Following studies will investigate the substrate effects observed when annealing DLC on glass substrate and refine annealing parameters to obtain better performances in terms of transparency and conductivity.

Figures

Fig. 1. XPS C1s deconvoluted peak for a DLC deposited a 5 J/cm² (KrF) and annealed at 0.2 J/cm² for 1000 shots. This sample presents very good performances in term of conductivity and transparency.

Tables

Tab. 1. Graphitic thicknesses and equivalent atomic layer estimations for both angle XPS analyses and for various annealing parameters on DLC (KrF -5 J/cm² -20 nm). Tab. 2. Conductivity revaluation of SiO2 deposited DLC (KrF -5 J/cm² -20 nm) considering the real graphitized thickness as deduced from XPS analyses.

Highlights:

 Producing a pure carbon and laser based alternative to Indium Tin Oxide (ITO).

 KrF-deposited Diamond-Like Carbon (DLC) best candidate to UV laser surface annealing.

 UV laser surface annealing to fully graphitize DLC top layer over few nanometers.

 Conductivity and transparency performances tuneable by annealing parameters.

Annealing

Fig. 2 .

 2 Fig. 2. Transmittance of DLC (20 nm) deposited at 5 J/cm² for two different laser wavelengths.

Fig. 3 .

 3 Fig. 3. Transmittance of DLC (KrF) deposited at 5 J/cm² for different film thicknesses.

Fig. 4 .Fig. 5 .

 45 Fig. 4. Transmittance of DLC (KrF -20 nm -5 J/cm²) annealed at 1000 shots at various fluences. Fig. 5. Measured conductivity for DLC (KrF -5 J/cm² -20 nm) deposited on different substrates regarding laser annealing fluences.

Fig. 6 .

 6 Fig.6. Picture of a 20 nm of DLC deposited on quartz and annealed with 0.2 J/cm² for 1000 shots.

Acknowledgement

We specially acknowledge Mr ROQUES Stephane, Mr DIETRICH Florent and Mr MUGLER Florian for the technical and material support to carry on this study.


Marc Comet
email: marc.comet@isl.eu

Cedric Martin

Fabien Schnell

Denis Spitzer

Cédric Martin

Energetic nanoparticles and nanomaterials for future defense applications

Keywords: Nanomaterials, Explosives, Nanothermites, Pyrotechnics

The integration of nanostructured materials in defense systems is expected to improve their performance in terms of power, safety, and reliability. That is why considerable research effort has been undertaken by major military powers worldwide in this domain. The first important step was to develop the production capacities of organic explosives in the state of fine powders with submicron to nanosized particle size distributions. The Spray Flash Evaporation (SFE) process, which is a unique method for producing such materials, was developed at industrial scale. Explosive nanopowders obtained by this process were subsequently mixed with nanosized pyrotechnic compositions such as nanothermites, to prepare hybrid detonating materials able to replace lead-based primary explosives. Composite propellants can also be prepared by SFE which allows mixing their components in a single step with better homogeneity. The ultimate challenge is to move from powder to object, in order to integrate energetic nanomaterials in operational systems. Although the research in this last domain is still in its infancy, several ways of preparation of objects from nanothermites have been recently reported in scientific literature. The focus will be on two examples studied in our laboratory. The first one is the preparation of nanothermites in the state of solid, porous foams; the second one is the use of nanothermites for coating grains of propulsive powder to change their combustion properties.

Introduction

In the domain of energetic materials, the research has mainly focused for one century and a half on the synthesis of new molecules. The most glaring exception to this trend was the invention of dynamite by Alfred Nobel, who stabilized nitroglycerine into different porous nanomaterials and thus became the first scientist to have a materials approach of explosives [START_REF] Nobel | Improved explosive compound[END_REF].

The era of nanomaterials has open new horizons to the science of explosives and pyrotechnic compositions and is deeply changing the ways to think and to do in this field. The future energetic substances will be Bsmart materials^with high performances, high safety and reliability of use, and minimized impact on environment. The first challenge to overcome was to have particles with size distributions below 1 μm. For this purpose, a revolutionary process called the Spray Flash Evaporation (SFE) process for nanostructuring explosives and the components of energetic compositions was developed over the last decade [START_REF] Risse | Continuous formation of submicron energetic particles by the flash-evaporation technique[END_REF][START_REF] Risse | Préparation de nanoparticules par évaporation flash WO2013117671 A1[END_REF]. The second challenge was to use these elementary bricks to prepare hybrid composite nanomaterials, by mixing the explosive nanopowders with nanothermites. These new detonating compositions, which were called NSTEX (NanoStructured Thermites and Explosives), have unconventional properties in comparison to those of classical explosives and pyrotechnic compositions, which make them promising candidates for replacing primary explosives containing heavy metals such as lead or cobalt [START_REF] Comet | Energetic nanocomposites for detonation initiation in high explosives without primary explosives[END_REF]. The last challenge is to move from nanopowders to objects, in order to integrate energetic nanomaterials in real systems produced at industrial scale. This specific aspect will be first discussed through the example of the chemical synthesis of nanothermites in the form of combustible foams from aluminum nanopowder and orthophosphoric acid (H 3 PO 4 ) [START_REF] Comet | Nanothermite foams: from nanopowder to object[END_REF][START_REF] Martin | Nanothermite with meringue-like morphology: from loose powder to ultra-porous objects[END_REF]. A second example dealing with the coating of a propulsive powder by a nanothermite will be given [START_REF] Berthe | Propellants reactivity enhancement with nanothermites[END_REF].

Preparation of Fine Explosive Powders

The preparation of high explosives in the state of fine powders (0.05-1 μm) is difficult owing to the specific properties of these compounds. First, most of organic explosives are molecules which cannot be polymerized: it is therefore impossible to prepare them by a sol-gel approach, unless using a gelling substance as template [START_REF] Gash | Nanostructured energetic materials with sol-gel methods[END_REF][START_REF] Tillotson | Nanostructured energetic materials using sol-gel methodologies[END_REF]. For this reason, explosive nanopowders prepared by this technique are never chemically pure. The milling technology is hazardous to carry out with explosives for obvious safety reasons. Samples must be prepared by operating in successive small batches. Furthermore, in a milling process, large particles are broken in smaller ones, which are afterward agglomerated in larger aggregates by the pressure exerted by the impact of milling beads. An intrinsic problem of the milling technique is the contamination of samples by the matter coming from different sources: beads, grinding bowl, and liquid used to desensitize the energetic material during the process. The anti-solvent method consists to induce the fast precipitation of an explosive dissolved in a solvent, by gradually pouring the solution into an anti-solvent of the explosive [START_REF] Kumar | Optimized synthesis of HMX nanoparticles using antisolvent precipitation method[END_REF][START_REF] Lee | 2 0 0 9 ) S u p e r c r i t i c a l a n t i s o l v e n t m i c r o n i z a t i o n o f cyclotrimethylenetrinitramin: influence of the organic solvent[END_REF]. This technique generally leads to micron-sized powders, with inclusions of solvent. The spray techniques are the most efficient for producing explosives in fine powders. They differ by the technique used to evaporate the solvent. In spray-drying, the aerosol is dried progressively by a hot gas flow [START_REF] Anandharamakrishnan | Spray drying techniques for food ingredient encapsulation[END_REF][START_REF] Qiu | Investigation of the crystallization of RDX during spray drying[END_REF]. The particles formed have spherical shapes and typical size of 1 μm. The rapid expansion of supercritical solution (RESS process) lead to fine powders with submicron-sized to nanosized particles. The main drawback of this technique is the use of important amounts of supercritical fluid (CO 2 ) for preparing limited quantities of sample [START_REF] Stepanov | Production of nanocrystalline RDX by rapid expansion of supercritical solutions[END_REF][START_REF] Essel | Formation and characterization of nano-sized RDX particles produced using the RESS-AS process[END_REF]. None of these techniques can be used for preparing explosive fine powders at industrial scale.

The SFE process is the first technique that allows preparing fine and pure explosive powders, in amounts of typically 100 g per hour. The underlying principle is to spray in a chamber maintained under dynamic vacuum an aerosol of preheated solvent containing one or several explosives dissolved in it. The ultrafast evaporation of the solvent induces the rapid crystallization of the explosive into small particles which do not have time to grow, and have typical sizes ranging from 50 nm to less than 1 μm. The most used solvent for preparing explosive nanopowder by SFE is acetone, whose physicochemical properties are particularly well suited for this technique. To date, trinitrotoluene (TNT), pentaerythritol tetranitrate (PETN), hexogen (RDX), octogen (HMX), hexanitrohexaazaisowurtzitane (CL-20), ammonium dinitramide (ADN) were prepared by SFE (Fig. 1) [START_REF] Risse | Continuous formation of submicron energetic particles by the flash-evaporation technique[END_REF][START_REF] Klaumünzer | Production of energetic nanomaterials by Spray Flash Evaporation[END_REF][START_REF] Séve | Trinitrotoluene nanostructuring by Spray Flash Evaporation process[END_REF][START_REF] Risse | Synthesis and desensitization of nano-β-HMX[END_REF].

One of the advantages of SFE process relies in the possibility to produce fine powders with different particle morphology. For instance, core-shell particles of hexolite, in which RDX is the core and TNT is the shell, were prepared in a single step [START_REF] Deckert-Gaudig | Highresolution Raman spectroscopy for the nanostructural characterization of explosive nanodiamond precursors[END_REF]. A co-crystallized structure (1:2 mol/mol) was obtained from HMX and CL-20, which confirmed that molecular recognition occurs in short times [START_REF] Spitzer | Continuous engineering of nano-cocrystals for medical and energetic applications[END_REF]. These examples show the outstanding versatility of this process.

Composite LOVA propellants were also prepared by SFE from solutions of nitrocellulose, a plasticizer (2,4-dinitro-2,4diazahexan: DNDA6) and hexogen, in ethyl acetate [START_REF] Brize | Plasticization of submicron-structured LOVA propellants by a linear dinitramine[END_REF]. In this domain, SFE makes possible the preparation of propulsive powders with a better homogeneity, in a single step.

Use of Fine Explosive Powders for Preparing Hybrid Energetic Nanocomposite Materials

Fine explosive powders prepared by SFE process were mixed with nanothermites to obtain hybrid energetic nanocomposite materials, which were called NSTEX (NanoStructured Thermites and Explosives). This concept originates from the research of Comet et al., who have proposed to solidify hexogen in a porous matrix of chromium (III) oxide firstly synthesized by the combustion of ammonium dichromate, and then to use the resulting RDX@Cr 2 O 3 material as gas generating oxidizer in aluminothermic compositions to improve their reactivity [START_REF] Comet | Preparation of explosive nanoparticles in a porous chromium(III) oxide matrix: a first attempt to control the reactivity of explosives[END_REF]. In the most classical sense, nanothermites are energetic materials made up of a metallic oxide such as Fe 2 O 3 , MnO 2 , WO 3 , MoO 3 , CuO, and Bi 2 O 3 in the form of submicron-sized powder and of an aluminum nanopowder with typical particle size distribution ranging from 50 to 120 nm [START_REF] Lafontaine | Nano-thermites[END_REF]. The thermochemical properties of the most representative thermite compositions were reported by Fischer and Grubelich [24]. In an extended meaning, nanothermites are any combustible compositions containing a significant amount of metallic species, used as fuels or/and oxidizers, in form of fine powders. In the field of nanothermites, oxygenated metallic salts are more and more used as oxidizers instead of metallic oxides to improve reactivity [START_REF] Wang | Nanocomposite thermites with calcium iodate oxidizer[END_REF][START_REF] Zhou | Reaction mechanisms of potassium oxysalts based energetic composites[END_REF][START_REF] Brusnahan | Periodates as potential replacements for perchlorates in pyrotechnic compositions[END_REF][START_REF] Wang | Metal iodate-based energetic composites and their combustion and biocidal performance[END_REF][START_REF] Wu | Performance of iodine oxides/iodic acids as oxidizers in thermite systems[END_REF].

The nanothermite part of NSTEX is prepared by dispersing its components in a liquid phase (e.g., acetonitrile) by ultrasonic agitation. The mixture is recovered by evaporating the liquid under reduced pressure. Nanothermites should be handled with precaution, owing to their particularly high sensitivity, especially to electrostatic discharge (ESD).

The formulation of a NSTEX is achieved by the dry mixing of the nanothermite with the fine explosive powder. This delicate operation is carried out by a vortex mixing and a manual crushing of aggregates with a spatula, which are repeated till obtaining a material with a good homogeneity. No liquid should be used for mixing the components of NSTEX, in order to avoid the dissolution and the growth of the explosive particles.

The main NSTEX feature is their ability to detonate by a transition from deflagration to detonation (TDD). This phenomenon is only observed in loose or slightly pressed NSTEX powders. The high power delivered by the fast combustion of the nanothermite activates the detonation of the fine explosive powder, which occurs in short distances (10-20 mm). The detonation wave then propagates in the NSTEX charge: Its propagation velocity depends both on the density of the explosive in the composition and on the resistance opposed by the nanothermite particles. In other words, the velocity can be adjusted through the ratio nanothermite/explosive and the density of the NSTEX. The detonation wave produced by the reaction of a NSTEX (e.g., nano-Al/nano-WO 3 /nano-RDX:12.4/27.6/60.0 wt.%) can be transmitted to a PETN secondary charge. This shock to detonation transition is only observed when the detonation velocity of the NSTEX is higher than 3 km/s [START_REF] Comet | Energetic nanocomposites for detonation initiation in high explosives without primary explosives[END_REF]. Figure 2 illustrates the functioning of an experimental detonating device, in a transparent tube, observed with a high speed camera operating at 840000 fr/s. The system comprises of a layer of ignition nanothermite (5 mm), a NSTEX in loose powder state (20 mm), a compacted layer of the same NSTEX (5 mm) and finally a 1 g PETN charge (20 mm).

The detonation characteristics of NSTEX make them suitable for the replacement of primary explosives which are salts of heavy metals such as lead or cobalt. Lead azide (PbN 6 ) is the most used primary explosive in detonators: It is synthesized by the Curtius' method from soluble lead salts. On the other hand, the detonation of lead azide produces small lead particles which are dispersed in environment. The persistence of the pollution by lead and its devastating effects on human health, has led European Chemical Agency to put a series of lead compounds on the list of the Substances of Very High Concern (REACH regulation). It was therefore necessary to develop Bgreener^compositions for replacing traditional primary explosives. From this standpoint, NSTEX are particularly promising, as they can be prepared from benign compounds such as potassium or calcium sulfates [START_REF] Comet | Sulfates-based nanothermites: an expanding horizon for metastable interstitial composites[END_REF], aluminum and pentaerythritol tetranitrate.

Fabrication of Macroscopic Materials from Nanoscopic Powders

The extremely fast flame propagation velocities (FPV) is only observed in nanothermites, in the state of loose powders. The pressing of nanothermites decreases their internal porosity, which considerably slows down the FPV. This effect was observed on Al/WO 3 nanothermites by Prentice et al. [START_REF] Prentice | Combustion wave speeds of sol-gel-synthesized tungsten trioxide and nano-aluminum: the effect of impurities on flame propagation[END_REF] and on Al/CuO nanothermites by Apperson et al. [START_REF] Apperson | Generation of fast propagating combustion and shock waves with copper oxide/aluminum nanothermite composites[END_REF] It is attributed to the fact that the convection of hot gases is made more difficult in denser materials. Moreover, the compression of aluminothermic compositions could slow down the fast spreading of the combustion by the melt dispersion mechanism (MDM) of aluminum [START_REF] Levitas | Mechanochemical mechanism for fast reaction of metastable intermolecular composites based on dispersion of liquid metal[END_REF][START_REF] Levitas | Melt dispersion mechanism for fast reaction of nanothermites[END_REF]. On the other hand, the mixtures of nanopowders of different natures are metastable from a physical point of view, because the phases tend to separate along time by the effect of gravity. The settling changes the local stoichiometry, which alters the pyrotechnics properties of the composition.

Having a high porosity is essential to keep a fast FPV in nanothermites and the most prominent challenge in the forthcoming years will be to develop efficient processes for shaping nanothermite powders into solid, porous objects with good mechanical properties. Various approaches have been explored for this purpose: Tillotson et al. synthesized nanothermites as aerogel monoliths [START_REF] Tillotson | Nanostructured energetic materials using sol-gel methodologies[END_REF], Yan et al. used the electrospinning technique with an energetic polymer as binder to prepare nanothermite mats [START_REF] Yan | Electrospun nanofiber-based thermite textiles and their reactive properties[END_REF] and Yang et al. simply used filtration to shape nanothermites in membranes [START_REF] Yang | Hierarchical MnO 2 /SnO 2 heterostructures for a novel free-standing ternary thermite membrane[END_REF]. One of the most promising ways was reported by Comet et al. who transformed an aluminothermic composition into solid, highly porous objects, by a chemical foaming process [START_REF] Comet | Nanothermite foams: from nanopowder to object[END_REF][START_REF] Martin | Nanothermite with meringue-like morphology: from loose powder to ultra-porous objects[END_REF].

The principle of the foaming process is based on the reaction of aluminum nanopowder which is introduced in excess in an aluminothermic mixture (Al/WO 3 ) with an aqueous solution of orthophosphoric acid (H 3 PO 4 ). The reaction occurs in two exothermic stages: The acid solution first reacts with the oxide shell of aluminum nanoparticles; then, it oxidizes the aluminum core of particles. Water vapor and hydrogen are respectively released by these reactions: In escaping through the reaction medium, these gases create the final porosity of the foam. On the other hand, aluminum phosphate (AlPO 4 ) which is produced by the reaction acts as a strong binder (cement) towards the nanoparticles from which the nanothermite is formed. It is worth noting that phosphates and sulfates, which are considered as inert compounds from a pyrotechnic point of view, behave as oxidizers at high temperatures, especially when they are mixed with aluminum nanopowders [START_REF] Comet | Nanothermite foams: from nanopowder to object[END_REF][START_REF] Comet | Sulfates-based nanothermites: an expanding horizon for metastable interstitial composites[END_REF]. Special caution is required to handle the pastes prepared from concentrated inorganic acids such as H 3 PO 4 and H 2 SO 4 and fine aluminum powders, due the high risk of hydrogen explosion [START_REF] Martin | Aluminum nanopowder: a substance to be handled with care[END_REF]. The induction time of the foaming reaction and the maximal temperature of the medium can be controlled through the concentration of the aqueous orthophosphoric acid solution (Fig. 3).

Nanothermites foams have the appearance of concrete pieces; they are quite insensitive to impact, friction and electrostatic discharge (ESD). They can be sawn or drilled without activating their combustion. Once ignited, nanothermite foams combust violently, with a blinding fireball and the release of abundant fumes. The reaction mechanism consists, first in the reduction of aluminum phosphate in phosphorus, which in turns burns in contact with atmospheric oxygen.

Nanothermites in the state of loose powders can be stabilized in thin layers deposited on the surface of the grains of a propulsive powder. For instance, Berthe et al. have coated Vectan A1 powder flakes by a layer of a sulfate-based nanothermite (Al/Na 2 SO 4 ) with a thickness of 50 μm. The addition of only some percents (10 wt.%) of nanothermite totally changed the ignition and the combustion regime of the propulsive powder. The effect observed was tuned by mixing coated and uncoated Vectan A1 grains in different mass ratios [START_REF] Berthe | Propellants reactivity enhancement with nanothermites[END_REF]. In these energetic systems, the density of nanothermite layer and the thermal conductivity of the substrate on which it is coated are crucial parameters. Sullivan et al. have deposited Al/CuO nanothermite dense layers by electrophoresis on platinum electrodes in miniaturized systems. However, the propagation velocity is smaller than 50 m/s owing too high density of the nanothermite deposits (≈ 29% of the TMD) [START_REF] Sullivan | Electrophoretic deposition and mechanistic studies of nano-Al/CuO thermites[END_REF].

Conclusions

The use of nanomaterials in the science of pyrotechnics and explosives will lead to important breakthroughs in this domain. Future energetic materials will be greener and safer [START_REF] Comet | Nanothermites: a short review. Factsheet for experimenters, present and future challenges[END_REF]. Moreover, they will have intrinsic high performance, owing to the fact that they will be designed in such a way to perfectly match with precise needs. Although this trendy research area has its roots in past, black powder and dynamites are indeed historical examples of energetic nanomaterials, much academic and applicative research remains to be done to integrate nanomaterials in operational systems.

The first step was to have the components of energetic systems in the state of nanopowders. The research carried out over the past two decades has led to the commercialization of fine and stable aluminum nanopowders, which are a major component of most of energetic nanomaterials. On the other hand, the Spray Flash Evaporation (SFE) process developed in our laboratory has been used for preparing numerous high explosives in the state of fine or ultrafine nanopowders in quantities which are still compatible with industrial applications.

The fine explosive powders prepared by the SFE process, were mixed with nanothermites to prepare a new family of detonating substances, which were called Nanostructured Thermites and Explosives (NSTEX). The detonation velocity of NSTEX can be set at a desired value, by playing on their composition and on their porosity. NSTEX have similar properties as those of primary explosives, which are molecules comprising toxic metals, such as lead or cobalt. Furthermore, NSTEX can be prepared from benign compounds from a toxicological standpoint, which makes them promising candidates for replacing classical primary explosives.

The last challenge to overcome for the integration of nanomaterials in pyrotechnic systems is the transformation of loose nanopowders, which are metastable in nature, into porous, solid and stable objects. Much research remains to be done in this domain, but recent scientific literature reports very promising results for future defense applications [START_REF] Lafontaine | Nano-thermites[END_REF][START_REF] Comet | Nanothermites: a short review. Factsheet for experimenters, present and future challenges[END_REF].

Fig. 2 Fig. 1

 21 Fig. 2 Curve representing the distance traveled by the flame front according to time, in an experimental device (length = 50 mm): burning of the nano-Al/nano-CuO nanothermite used for ignition (1), deflagration of a NSTEX composition (2), detonation of the NSTEX (3), acceleration of the detonation wave in a denser NSTEX layer (4), transmission and detonation of a PETN charge[START_REF] Comet | Nanothermite foams: from nanopowder to object[END_REF]

Fig. 3

 3 Fig. 3 Curves representing the induction time of the foaming reaction (squares) and the maximum temperature reached during the reaction (circles) depending on the orthophosphoric acid concentration

Acknowledgments Authors are thankful to the Greener and Safer Energetic and Ballistic Systems committee for having given the opportunity of presenting our work in the 3rd GSEBS conference held in Brest.

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