Emmanuel Scorsone
email: emmanuel.scorsone@cea.fr

Nouha Gattout

Lionel Rousseau

Gaelle Bazin Lissorgues

Porous diamond pouch cell supercapacitors

ouverte pluridisciplinaire HAL, est destinée au dépôt

Introduction

The beginning of the 21 st century has seen a radical change in our way of living with the arrival of multifunction portable electronic devices including smartphones, tablets, etc. In such mobile equipment, wireless communication, touch pads or embedded sensors are functions that all contribute to the need for both compact and efficient energy storage.

Nowadays this is achieved mostly by thin film technologies based rechargeable micro-

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*Corresponding author. Email: emmanuel.scorsone@cea.fr (Emmanuel Scorsone) batteries. Nevertheless, these batteries generally suffer from poor cycle lifetime and poor low temperature kinetics [START_REF] Tarascon | Issues and challenges facing rechargeable lithium batteries[END_REF]. Thus the design of miniaturized energy-storage devices, with the capacity for autonomous recharging through energy harvesting, along with the ability to deliver high energy for the lifetime of the equipment being powered, remains a real challenge. Supercapacitors or electric double layer capacitors (EDLC) are energy storage devices with high specific energy when compared to conventional capacitors [START_REF] González | Review on supercapacitors: Technologies and materials[END_REF]. Thin film supercapacitors are seen as a promising alternative to batteries in portable electronic devices, because in supercapacitors energy is stored and released by transferring surface charges, meaning that they can be charged and discharged rapidly. Because the electrode materials are not involved in any chemical reactions, they can be cycled hundreds of thousands of times [START_REF] Simon | Materials for electrochemical capacitors[END_REF]. Three types of supercapacitors may be distinguished: carbon/carbon [START_REF] Zhang | Carbon-based materials as supercapacitor electrodes[END_REF][START_REF] Frackowiak | Carbon materials for supercapacitor application[END_REF][START_REF] Frackowiak | Carbon materials for the electrochemical storage of energy in capacitors[END_REF], metal oxide [START_REF] Lokhande | Metal oxide thin film based supercapacitors[END_REF] and electronically conducting polymers [START_REF] Sun | Preparation and characterization of polypyrrole films for threedimensional micro supercapacitor[END_REF]. Metal oxide supercapacitors are generally expensive or feature low conductivity whereas conductive polymers have been reported to exhibit short cycling life [START_REF] Wang | A review of electrode materials for electrochemical supercapacitors[END_REF]. In contrast, carbon electrodes are low cost, and they possess superior combination of chemical and physical properties including high conductivity, high temperature stability, high versatility in structure/texture, and high specific surface area [START_REF] Frackowiak | Carbon materials for supercapacitor application[END_REF].

For these reasons, most of the currently available supercapacitors on the market are based on activated carbon electrodes in an organic electrolyte. But although symmetrical carbon/carbon electrodes in organic electrolyte is now mature, it can still be improved by developing better active carbon electrodes and/or electrolyte formulations.

An important feature in supercapacitors is their potential window, since both energy storage and power release depend on this parameter, according to the well-known equations E=1/2CV 2 and P=V 2 /4ESR, where C, V and ESR are the capacitance, the potential window, and the equivalent series resistance of the device, respectively [START_REF] Frackowiak | Carbon materials for supercapacitor application[END_REF]. When more active material is deposited onto the charge collector, more energy can be stored by the device, but then energy become more difficult to release [START_REF] Kötz | Principles and applications of electrochemical capacitors[END_REF]. Thus a good compromise needs to be found in terms of active material thickness. But generally speaking, an efficient way to increase both energy and power consists in enlarging the potential window, according to the above equations.

One way to open the potential window is to use organic solvents, typically acetonitrile or propylene carbonate (PC), the practical opening voltage of organic electrolytes in symmetric carbon/carbon supercapacitors being around 2.7-2.8 V [START_REF] Azaïs | Causes of supercapacitors ageing in organic electrolyte[END_REF][START_REF] Ue | Electrochemical properties of organic liquid electrolytes based on quaternary onium salts for electrical double-layer capacitors[END_REF]. However, this involves the need to ensure that no water traces are present in the solvent or the active electrode material, which brings some constrains in the fabrication process [START_REF] Kurzweil | Electrochemical stability of organic electrolytes in supercapacitors: Spectroscopy and gas analysis of decomposition products[END_REF]. Moreover the low flash point of some solvents such as acetonitrile (below 5 o C) may cause some safety

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*Corresponding author. Email: emmanuel.scorsone@cea.fr (Emmanuel Scorsone) concerns [START_REF] Lin | Capacitive energy storage from -50 to 100 o C using an ionic liquid electrolyte[END_REF]. Additionally, the low conductivity and low ionic mobility in such liquids lead to poor charge propagation inside the active electrode's pores. Ionic liquids have also been considered as a good alternative because of their low toxicity and flammability, high thermal stability, and wide voltage window typically around 4.5 V [START_REF] Burke | R&D considerations for the performance and application of electrochemical capacitors[END_REF]. Unfortunately their high price and fairly low conductivity cannot satisfy the requirements for their use for large-scale supercapacitor applications [START_REF] Galiński | Ionic liquids as electrolytes[END_REF]. This issue could be overcome using aqueous electrolytes but most carbon electrodes exhibit a narrow potential window in such media, where water starts to decompose from 0.7 to 0.8 V in typical acidic or basic electrolytes [START_REF] Toupin | Performance of experimental carbon blacks in aqueous supercapacitors[END_REF]. It was shown that neutral aqueous electrolytes such as alkali sulfates could achieve higher voltages with carbon/carbon supercapacitors than generally obtained in KOH or H 2 SO 4 based electrolytes.

For instance a practical voltage of 1.6 V was reached in symmetric carbon/carbon system using 0.5 M Na 2 SO 4 [START_REF] Demarconnay | A symmetric carbon/carbon supercapacitor operating at 1.6 V by using a neutral aqueous solution[END_REF], but this is still 1 V less than with organic electrolytes.

Boron doped diamond (BDD) is also considered as a promising alternative because of its significantly superior electrochemical properties when compared to other forms of carbon, including a wide potential window in aqueous media, and long term response stability [START_REF] Fujishiman | Diamond Electrochemistry[END_REF].

These properties have recently led to some research to attempt to apply diamond in energy storage devices such as supercapacitors. Indeed, with a potential window exceeding 3 V in water, diamond is offering hope to make energy efficient aqueous based supercapacitors thus benefiting from the high ionic mobility in water. The high corrosion resilience even in harsh acidic or alkaline media is also expected to offer long cycling lifetime of such energy storage devices. Unfortunately, planar polycrystalline diamond electrodes exhibit low surface area and low intrinsic double layer capacitance, in the order of 5 µF.cm -2 , which limit drastically energy storage. Nevertheless several successful developments have been reported recently to increase the surface area of diamond surfaces [START_REF] Yang | Diamond electrochemistry at the nanoscale: A review[END_REF]. Some approaches have used top down processes in order to etch diamond and increase its porosity [START_REF] Smirnov | Aligned diamond nano-wires: Fabrication and characterisation for advanced applications in bio-and electrochemistry[END_REF][START_REF] Kondo | Porous boron-doped diamond electrodes fabricated via two-step thermal treatment[END_REF][START_REF] Mehedi | Etching mechanism of diamond by Ni nanoparticles for fabrication of nanopores[END_REF][START_REF] Yoshimura | Electrochemical characterization of nanoporous honeycomb diamond electrodes in non-aqueous electrolytes[END_REF][START_REF] Zhuang | Diamond Network: Template-Free Fabrication and Properties[END_REF]. However, the relatively low conductivity of diamond, even when highly doped, generally results in electrodes with high series resistance. Therefore bottom up approaches have been preferred where diamond is grown on a porous template, ideally a good current collector. One approach has consisted of growing BDD on silicon nanowires. Such substrate was assess for supercapacitor application in ionic liquid [START_REF] Gao | Diamond-coated silicon wires for supercapacitor applications in ionic liquids[END_REF]. A double layer capacitance of 105 µF.cm -2 was reported. Here the series resistance was fairly high, in the order of 200, most probably because of the high resistivity of the silicon nanowires and the electrolyte used. High surface area silica spheres [START_REF] Gao | Highly porous diamond foam as a thin-film micro supercapacitor material[END_REF] or fibers [START_REF] Gao | Diamond-based supercapacitors: realization and properties[END_REF] template were also used. Both substrates led to similar results in terms of electrochemical performances. If the silica sphere template seemed more promising for ACCEPTED MANUSCRIPT *Corresponding author. Email: emmanuel.scorsone@cea.fr (Emmanuel Scorsone) integration into small electronic devices, there was no data on the series resistance of the films, which affects drastically the performances of supercapacitor. A pouch cell device with silica fibers scaffold was built and showed some highly interesting performances in aqueous based electrolyte, despite the non-conducting template. The double layer capacitance of such films was in the order of 0.6 mF.cm -2 with a series resistance of 1.3. Recently alternative material consisting of BDD grown on highly porous carbon nanotubes [START_REF] Hébert | Boosting the electrochemical properties of diamond electrodes using carbon nanotube scaffolds[END_REF] or polypyrrole templates [START_REF] Hébert | Porous diamond with high electrochemical performance[END_REF] were reported. The latter diamond-coated template showed the best results in terms of double layer capacitance, up to 3 mF.cm -2 , which is the highest value reported for diamond films to the author's knowledge. Moreover, in contrast with other types of porous diamond based materials, it can be processed homogeneously on 4 inches surfaces and thus offers a clear advantage in terms of manufacturing. However, it is known that 3electrode cell systems tend to overestimate the electrochemical capacitance of thin films, and this material was never tested in symmetrical configuration. Therefore in this paper, such porous diamond film, known as SPDia TM , was integrated into a pouch cell supercapacitor and tested. The main objective of this work is to investigate this material for its supercapacitor performances in comparison with other diamond based electrode materials. The performances of the device were compared when organic and aqueous based electrolytes were used, since the main advantage of diamond for such application is the potentiality to use aqueous based electrolytes.

Material and methods

Porous diamond film preparation

All chemicals were purchased from Sigma Aldrich, France, unless specified, and used as received. Highly porous polypyrrole film was initially prepared as follows onto a highly conductive (<0.005 .cm -1 ) 4 inches <100> silicon wafer, previously coated with a boron doped diamond thin film (thickness approx. 300 nm, doping level 2.10 21 .cm -3 ) by Microwave Plasma Chemical Vapour Deposition (MPCVD) using a process described elsewhere [START_REF] Kiran | Boron doped diamond electrodes for direct measurement in biological fluids: an in situ regeneration approach[END_REF].

Such substrate was used for convenience, and because it is compatible with the diamond growth conditions, but it could be replaced by a less resistive metal collection provided that it is also appropriate for diamond growth.

Firstly, a stock solution of 4 M FeCl 3 dissolved in polyethylene glycol was prepared and stored in the dark at 4 o C when not used. This solution was spin coated over the BDD coated

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*Corresponding author. Email: emmanuel.scorsone@cea.fr (Emmanuel Scorsone) silicon wafer at 3000 rpm for 20 sec. The substrate was then placed in a sealed chamber and exposed to saturated pyrrole vapour in dry nitrogen at room temperature for 4 hrs. This way, pyrrole was polymerized in situ over the silicon substrate using FeCl 3 as oxidizing agent/dopant [START_REF] Ansari | Polypyrrole Conducting Electro Active Polymers: Synthesis and Stability Studies[END_REF]. Following polymerization, the samples were rinsed several times gently in acetonitrile until the wash solution became colourless, in order to remove the excess FeCl 3 and polyethylene glycol. Finally the substrate was dried under nitrogen flow.

Next the substrate was dipped alternatively three times into an aqueous solution of poly(diallyldimethylammonium chloride) (PDDAC) and three times in a colloidal solution of nanodiamond (0.1 wt% prepared in deionized water from Syndia SYP 0-0.02 GAF, Van Moppes, Switzerland) to electrostatically immobilize the nanoparticles onto the polymer template [START_REF] Hébert | Porous diamond with high electrochemical performance[END_REF]. Boron doped diamond was then grown over the seeded substrate in a MPCVD reactor Seki AX6500 reactor in a gas mixture of 1% CH 4 in H 2 with trimethylboron as the source of dopant, at microwave power of 3 kW and gas pressure of 20 mBar for 3 hrs. A highly porous BDD film was obtained as described and characterized in ref [START_REF] Hébert | Porous diamond with high electrochemical performance[END_REF]. A SEM image of the resulting polypyrrole film after diamond growth recorded using an in-lens Field Emission Scanning Electron Microscope ZEISS Supra-40, operating with an acceleration voltage of 5 kV, is shown in Fig. 1.a. The thickness of the porous diamond film was measured to be approximately 15 µm. The weight of the porous film was measured gravimetrically and was found to be 31 mg.cm -2 .

The highly textured nature of the diamond film combined with its hydrogen terminated surface following CVD diamond growth result in a highly hydrophobic surface. Contact angle was measured to be in the order of 135 o after growth. The porous diamond film was therefore oxidized through a photochemical treatment under oxygen in order to increase wettability.

During this process, low pressure oxygen (400 mbar) is exposed to UV light using a xenon excimer lamp (172 nm). This step involves oxygen radicals and ozone active species that react with the diamond surface. Following this oxidation treatment, contact angle was measured again and was found to be around 24 o (contact angle _ suppl.mat).

Pouch cell fabrication

The substrate was cleaved into 3 cm 2 pieces. Electrical contacts were taken from the back side of two 3 cm 2 SPDia TM coated silicon substrates by polishing gently the silicon substrate to remove the oxide layer and then fixing stainless steel strips to the substrate using silver loaded

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*Corresponding author. Email: emmanuel.scorsone@cea.fr (Emmanuel Scorsone) epoxy paste. Finally the contact area was covered with fast epoxy adhesive (Araldite TM ) in order to strengthen mechanically the contact and insulate any residual silver paste from the electrolyte solution. The 2 plates were then placed face to face and separated by glass microfiber filter (Whatman, Grade GF/D) soaked with electrolyte solution. The electrolyte was either aqueous 1 M H 2 SO 4 or 1 M tetraethylammonium tetrafluoroborate (TEABF 4 ) in propylene carbonate (PC). The electrodes assembly was finally packaged in soft plastic enclosure. A photograph of the pouch cell before and after assembly is shown in Fig. 1b and Fig. 1c, respectively.

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*Corresponding author. Email: emmanuel.scorsone@cea.fr (Emmanuel Scorsone)

Results and discussions

Cyclic voltammetry was initially performed to assess the potential window of the two devices with either organic or aqueous electrolytes. Diamond is known to be one of the materials that feature the highest potential window in water, typically above 3 V. However, when grown over some substrates including carbon based substrates such as CNTs, this window is generally reduced. This is most probably due to the specific growth conditions used not to damage the substrate that possibly increase the sp2 carbon ratio, which tends to reduce the potential window. Here cyclic voltammetry was performed at 100 mV.s -1 with both devices, containing either PC or aqueous based electrolyte solution, while gradually opening the window from 0.6 V to 3 V in steps of 0.2 V (Fig. 2). In the case of aqueous electrolyte (Fig. 2a), at small potentials below typically 1.5 V, no faradic process seems to be observed as CVs are almost symmetrical. Then water splitting starts to increase gradually. At 2.6 V the ratio of faradic current over capacitive current is still < 4, due to the absence of electro catalytic processes at the diamond surface. Water decomposition behavior is comparable to symmetrical pouch cell made of BDD coated glass fiber electrodes [START_REF] Gao | Diamond-based supercapacitors: realization and properties[END_REF] and occurs at slightly higher potential than symmetrical activated carbon supercapacitor using non acidic or alkaline electrolyte, which is known to widen the potential window overall [START_REF] Demarconnay | A symmetric carbon/carbon supercapacitor operating at 1.6 V by using a neutral aqueous solution[END_REF]. As expected, no significant faradic process due to solvent decomposition seems to occur in the case of PC within the 3 V window, as shown in Fig. 2b for comparison purpose. Cyclic voltammetry was also investigated in the potential window of 1 V to assess the performances of the pouch cells, while scanning at different scan rates from 10 mV.s -1 up to 100 V.s -1 (Fig. 3). The CV curves of both symmetric capacitors retain a nearly rectangular shape and a rapid current response to voltage at each potential end and show low distortion even at high scan rates up to 100 V.s -1 . This suggests a good capacitive behavior and fairly fast ionic diffusion into the porous diamond layer [START_REF] Liu | Flexible and conductive nanocomposite electrode based on graphene sheets and cotton cloth for supercapacitor[END_REF]. The small distortion observed at high scan rates may be due to the slightly high series resistance, as observed later on from EIS data, possibly attributed to the semiconducting nature of diamond, although highly doped. It may also imply some faradic processes, in particular in aqueous electrolyte. The areal capacitance for both electrolytes was derived from the CVs and graphed as a function of scan rates in Fig. 4. The highest capacitance values obtained for the aqueous and PC based electrolytes at 10 mV.s -1 are 1.31 mF.cm -2 and 0.42 mF.cm -2 , respectively. The maximum areal capacitance in aqueous electrolyte is in good agreement with the 3 mF.cm -2 value reported in 0.1M LiClO 4 . The slightly higher value in the latter case may be explained by the fact that three-electrode measurement configuration has been reported to overestimate the

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*Corresponding author. Email: emmanuel.scorsone@cea.fr (Emmanuel Scorsone)

capacitance value [START_REF] Stoller | Best practice methods for determining an electrode material's performance for ultracapacitors[END_REF] and also by the use of different ionic salts. Also some variation may be observed according to the thickness of the diamond layer grown over polypyrrole [START_REF] Hébert | Porous diamond with high electrochemical performance[END_REF]. In PC, the areal capacitance is significantly lower. This is attributed firstly to the higher dielectric constant as well as higher size of solvated ions in PC compared to water [START_REF] Frackowiak | Carbon materials for supercapacitor application[END_REF]. The capacitance values in aqueous and organic electrolytes are reported in Table 1 and compared to other porous diamond materials reported to date in the literature. The highest capacitance value (1.83 mF.cm -2 ) was measured on diamond honeycombs by Yoshimura and coworkers. Here the exceptionally high capacitance measured despite the fairly large pore size was explained by the needle-like structure at the bottom of the honeycomb pores as a result of the etching process. However, this result was obtained in a three-electrode configuration, which, as mentioned before, tends to overestimate the areal capacitance. The second highest value was obtained with SPDia TM (1.31 mF.cm -2 ) this time in a symmetrical electrodes configuration.

This value is significantly higher than diamond coated glass fibers, also in symmetrical electrode configuration (0.7 mF.cm -2 ), and much higher than any other reported porous diamond electrode materials even in aqueous electrolytes, to the author's knowledge.

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*Corresponding author. Email: emmanuel.scorsone@cea.fr (Emmanuel Scorsone) Three electrodes measurements [START_REF] Gao | Highly porous diamond foam as a thin-film micro supercapacitor material[END_REF] Porous BDD by thermal treatment 1M aqueous Na 2 SO 4 CV (10 mV.s -1 ) 140 µF.cm -2 Three electrodes measurements [START_REF] Kondo | Porous boron-doped diamond electrodes fabricated via two-step thermal treatment[END_REF] BDD on CNTs Phosphate Buffer Saline CV (50 to 200 mV.s -1 ) 580 µF.cm -2 Three electrodes measurements [START_REF] Hébert | Boosting the electrochemical properties of diamond electrodes using carbon nanotube scaffolds[END_REF] The complex impedance plane plots (Cole-Cole) are shown in Fig. 5a and Fig. 5c for aqueous and organic electrolytes, respectively. The complex impedance of the capacitor is equivalent to an RC circuit with Z=ESR+(1/jC). At low frequencies, impedance tends to behave as a pure capacitor, whereas at high frequencies, it behaves like a pure resistor. The transition between capacitive (quasi-vertical line) and resistive behavior (Warburg at approx.45 o ) is observed at the "knee frequency" [START_REF] Taberna | Electrode surface treatment and electrochemical impedance spectroscopy study on carbon/carbon supercapacitors[END_REF]. In the case of 1 M H 2 SO 4 electrolyte, this transition frequency is clearly observed at around 5.8 Hz. The transition is much less pronounced in the case of PC, but seems to appear at around 200 Hz. The more vertical curve at low frequencies for the aqueous based electrolytes indicates that this device behaves more closely like an ideal capacitor than in the case of the organic electrolyte. The equivalent series resistances ESR were deduced from when the imaginary part equals zero. It was found to be 11  for the aqueous based electrolyte and 20  for PC. This resistance value corresponds to the intrinsic resistance of the porous diamond layer along with the current collector, the contact resistance between the active layer and the current collector, and the electrolyte. Since all components are the same here apart from the electrolyte, the higher ESR value here is attributed to the lower conductivity of the organic electrolyte over the aqueous one. Indeed, the conductivity of TEABF 4 in PC is reported to be in the order of 14.5 mS.cm -1 whereas that of H 2 SO 4 solution is around 750 mS.cm -1 [START_REF] Zhong | Electrolytes for Electrochemical Supercapacitors[END_REF]. Since the power released by the super capacitor is inversely proportional to the ESR value, it is expected that this value shall be as low as possible, and in commercial carbon based capacitors usually below 1  [START_REF] Boukhalfa | Atomic layer deposition of vanadium oxide on carbon nanotubes for high-power supercapacitor electrodes[END_REF].

Hence the values recorded in this work are considered to be fairly high, and attributed both to the intrinsically low conductivity of diamond, although highly doped, and possibly poor electrical interfacing with the substrate. However when compared to other porous diamond active electrodes, it is still reasonably low. For instance the series resistance of diamond on

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*Corresponding author. Email: emmanuel.scorsone@cea.fr (Emmanuel Scorsone) silicon nanowires in mixtures of ionic liquids and PC was reported to be above 100  [START_REF] Gao | Diamond-coated silicon wires for supercapacitor applications in ionic liquids[END_REF],

and also much above 100 .cm -1 in the case of diamond honeycomb electrodes [START_REF] Yoshimura | Electrochemical characterization of nanoporous honeycomb diamond electrodes in non-aqueous electrolytes[END_REF]. The equivalent distributed resistance (EDR), obtained from the linear projection of the almost vertical portion of the Cole-Cole plot to the real axis gives some indication regarding the ion diffusion through the diamond pores [START_REF] Kötz | Principles and applications of electrochemical capacitors[END_REF]. It was assessed to be 38  for the aqueous electrolyte and 120  for the organic one, respectively, which reflect the much larger cations radius in the case of the organic electrolyte. The Cole-Cole plots do not allow easy extraction of frequency information that can better be described by complex capacitance modelling of impedance data [START_REF] Taberna | Electrode surface treatment and electrochemical impedance spectroscopy study on carbon/carbon supercapacitors[END_REF]. Thus the evolution of the real part C'() and imaginary part C"() of the capacitance are plotted in Fig. 5b and Fig. 5d for both capacitors, respectively. The real capacitance values at 0.1 Hz are in fairly good agreement with the results from CV measurements (0.96 mF.cm -2 versus 1.31 mF.cm -2 in 1 M H 2 SO 4 and 222 µF.cm -2 versus 420 µF.cm -2 in 1 M TEABF 4 in PC). In aqueous electrolyte, a frequency peak is observed in C"

() at approx. 0.5 Hz corresponding to the relaxation time  0 of the device (2s). In the case of PC, the imaginary part plot seems also to reveal a peak centered at 50 Hz ( = 20 ms).  0 defines the limit between predominately resistive behavior at frequencies higher than 1/ 0 and capacitive behavior at lower frequencies. Low relaxation times are associated with low energy loss and fast response time of the supercapacitor (fast charge/discharge). In the case of organic electrolyte, the relaxation time is close to that measured e.g. with diamond coated glass fibers (31.7 ms) [START_REF] Gao | Diamond-based supercapacitors: realization and properties[END_REF]. In the case of acid based electrolyte, the time constant is much higher. Generally speaking, aqueous electrolytes have much higher ionic conductivity, and thus provide lower resistance, as observed in this study, therefore they should also feature lower time constants than organic electrolytes [START_REF] Huggins | Energy storage -Fundamentals, Materials and Applications[END_REF]. Nevertheless, the larger the capacitance, the greater the time constant, which may explain the higher  0 value found here in acidic electrolyte.

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*Corresponding author. Email: emmanuel.scorsone@cea.fr (Emmanuel Scorsone) Galvanostatic charge discharge (GCD) was carried out in acid medium (Fig. 6a) and propylene carbonate (Fig. 6b) in the voltage range of 2 V at various current densities. The CDC curves are fairly linear at low current densities, which reveals a good capacitive behavior.

Nevertheless, a clear ohmic drop at the turning point of charge-discharge as well as slight polarization of the electrodes can be observed. The ohmic drop increases with current density and indicates some energy loss due to internal DC resistance. The ohmic drop seems to be more pronounced for the organic electrolyte, as expected from previous ESR measurements, and results from the lower ionic mobility in PC. Overall the shape of the GCD curves is in agreement with the efficiency profiles of the devices as shown in fig. 2c, suggesting moderate charge-discharge kinetics, presumably due to highly porous diamond network with somehow limited conductivity, and Faradic processes in the case of the aqueous based electrolyte, since dissociation of water was shown to appears from typically 1.5 V. GCD was repeated over 10,000 cycles at current density of 500 µA.cm -2 in 1M H 2 SO 4 electrolyte and revealed no significant difference in charge-discharge profile, thus confirming the good stability of the device.

Ragone plot relating energy density to power density was calculated from the GCD measurements using the equations from reference [START_REF] Liu | Flexible and conductive nanocomposite electrode based on graphene sheets and cotton cloth for supercapacitor[END_REF] and shown in Fig. 6c. The data were compared to that of diamond coated silicon nanowire thin films measured in ionic liquid as reported elsewhere [START_REF] Gund | Diamond-coated silicon nanowires for enhanced micro-supercapacitor with ionic liquids[END_REF]. The estimated maximum energy and power densities for PC are 0.17 µWh.cm -2 and 200 µW.cm -2 , respectively. Those values are very similar to the values obtained for diamond coated silicon nanowires in ionic liquid. In the case of the aqueous based electrolyte, the highest energy and power values are close to 1 µWh.cm -2 and 700 µW.cm -2 , respectively. This is typically one order of magnitude larger in terms of energy stored and more than 5 times better in terms of power released when compared to diamond coated nanowires. The better performances of SPDia TM in water over SPDia TM in PC and diamond coated nanowires in ionic liquid reflects the lower ESR value and higher specific capacitance of the aqueous based electrolyte. including also that for diamond coated silicon nanowires in ionic liquid (measured in 3electrode cell configuration) as reported in [START_REF] Gund | Diamond-coated silicon nanowires for enhanced micro-supercapacitor with ionic liquids[END_REF] 4 Conclusions

The first symmetrical thin film diamond supercapacitor on silicon mounted in pouch cell configuration was reported, using porous boron doped diamond grown over polypyrrole template as energy storage electrodes. This material was chosen because it was reported to be among the porous diamond coatings with the highest areal double layer capacitance, around 3 mF.cm -2 , and because it can be processed homogeneously over 4 inches silicon wafer with

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*Corresponding author. Email: emmanuel.scorsone@cea.fr (Emmanuel Scorsone) good control. Using diamond porous electrodes in supercapacitors allows taking advantage of their large potential window in water while benefiting from low ionic conductivity, low cost and low environmental impact of aqueous based electrolyte. Here the performances of the 3 cm -2 symmetrical pouch cell diamond supercapacitor were compared in the same configuration when an acidic and organic electrolytes were used. The large potential window in water, typically above 2 V was confirmed, along with a reasonably low series resistance in the low ohm range. The porous diamond material was grown over silicon for practical reasons, but this may not be optimum in the long term since the silicon substrate contributes to increasing the ESR and thus reduces the supercapacitor overall energy storage performances. A simple way to overcome this limitation would be to grow the porous diamond layer directly onto the charge collector, made e.g. of stainless steel, or any carbide forming metal compatible with low temperature diamond growth. Energy and power values close to 1 µWh.cm -2 and 700 µW.cm -2 were recorded in water, thus surpassing by almost one order of magnitude the energy storage performances of similar carbon based devices using organic or ionic liquid electrolytes. Although such porous diamond coating are probably not competitive in terms of cost efficiency against other carbon materials when it comes to design high power bulk supercapacitors, they are certainly promising for thin film applications, where they could when combined with lithium batteries improve the power performances of the energy source for portable electronics applications.

Fig. 1 a

 1 Fig.1 a, SEM images of the surface of SPDia TM material used as energy storage electrodes b, 3 cm -2 pouch cell supercapacitor before and c, after assembly

  Fig.2cshows theACCEPTED MANUSCRIPT*Corresponding author. Email: emmanuel.scorsone@cea.fr (Emmanuel Scorsone) Coulombic efficiency of the device, calculated via efficiency = Q discharging /Q charging where Q discharging and Q charging are the amount of charge during charging and discharging process while running cyclic voltammetry. The efficiency at low voltages is approximately 85 % and remains constant up to 3 V for PC. In the case of the aqueous electrolyte, its starts to decrease from typically 1.5 V, in agreement with the observations made previously on the CVs and resulting from solvent decomposition. The non-rectangular shape of the CVs for low potential windows, as well as the fairly low efficiency at low voltages for water and across the whole potential window range for PC, both indicate a moderate charge-discharge process.

Fig. 2 .

 2 Fig.2. Opening window test from 0.6 to 3V in steps of 0.2V, at a scan rate of 100 mV.s -1 for diamond pouch cell supercapacitor with a. 1 M H 2 SO 4 in water and b. 1M TEABF 4 in PC, as electrolytes. c. plot of the Coulombic efficiency of the device against the potential window

Fig. 3 .Fig. 4 .

 34 Fig.3. Cyclic voltammetry results from 0 to 1 V for scan rates varying from 10 mV.s -1 to 100 V.s -1 for diamond pouch cell supercapacitor with 1 M H 2 SO 4 in water (a and b) and 1M TEABF 4 in PC (b and c)

Fig. 5 .

 5 Fig.5. Cole-Cole plots and complex capacitance (real part C' and imaginary part C" are calculated according to equations from ref. [35]) for diamond pouch cell supercapacitor with 1 M H 2 SO 4 in water (a and b) and 1M TEABF 4 in PC (c and d) electrolytes, respectively.

Fig. 6 .

 6 Fig.6. Examples of Galvanostatic Charge-Discharge (GCD) curves of diamond pouch cell supercapacitor with a. 1 M H 2 SO 4 in water and b. 1M TEABF 4 in PC electrolytes, respectively, within a 2 V window; c. Ragone plot for the acidic and organic electrolytes,

Table 1 .

 1

		Electrolyte	Measurement type	Areal	Comments	references
				capacitance
	SPDia TM	0.5M aqueous H 2 SO 4		1.31 mF.cm -2	Symmetrical	This work
			CV (10 mV.s -1 )		electrode system
		0.5M TEABF 4 in PC		0.42 mF.cm -2
	BDD coated	Bis(trifluoromethylsulfo	CV	317 µF.cm -2	Three electrodes	[39]
	Silicon nanowire	(ionic liquid) nyl) imide	(5 mV.s -1 )		measurements
	BDD coated	3 M aqueous NaClO 4	Low frequency EIS	0.7 mF.cm -2	Symmetrical	[28]
	Glass fibers				electrode system
	Nanoporous	1M aqueous H 2 SO 4		1.83 mF.cm -2	Three electrodes	[24]
	honeycomb diamond	0.5M TEABF 4 in PC	Low frequency EIS	187 µF.cm -2	measurements Pore size 60 x 500 nm

Comparison of areal capacitance values of our device with other state-of-the-art supercapacitor using porous diamond electrodes from the literature. *Corresponding author. Email: emmanuel.scorsone@cea.fr (Emmanuel Scorsone)

Acknowledgement

The authors would like to thank Guillaume Berthet for contact angle measurements.

Highlights

 A symmetrical pouch cell supercapacitor was fabricated using a highly porous diamond electrode material, grown over a polymer template on four inches substrates, and featuring a capacitance of 3 mF.cm -2  The performances of the device were compared when using either an aqueous or organic based electrolytes  Energy and power values close to 1 µWh.cm -2 and 700 µW.cm -2 were recorded in water, thus surpassing by almost one order of magnitude the energy storage performances of similar carbon based devices using organic or ionic liquid electrolytes.


N Vaissiere
email: nicolas.vaissiere@cea.fr

S Saada

M Bouttemy

A Etcheberry

P Bergonzo

Jean-Charles C Arnault

Heteroepitaxial diamond on iridium: New insights on domain formation

Introduction

Compared with other heterosubstrates (Si, 3C-SiC), the nucleation mechanism of diamond on iridium appears to be a unique pathway. This mechanism involves the formation of domains [1,2] that are produced during bias-enhanced nucleation (BEN). For these particular areas, epitaxial diamond is obtained after CVD growth. However, in contrast to observations on silicon or silicon carbide, observations on diamond crystal are not possible prior to the growth step. This suggests that diamond nuclei are somehow protected from the CVD plasma during the BEN step. A pseudo-2D growth mode was subsequently reported during the growth step [3,4]. This finding may explain the higher crystalline quality of the epitaxial diamond layers that were grown on iridium [1,5,6]. Previous studies have demonstrated that surface modifications of iridium simultaneously occur during BEN, namely, roughening [5,[7][8][9] or carbon dissolution [10].

Several important questions remain regarding this particular nucleation pathway that occurs on Ir: Does surface roughening play a role in domain formation? Would it be possible to detect diamond nuclei prior to growth using SEM? What are the topography and surface chemistry of domains? This study combines two experimental approaches: first, an investigation of the evolution of surface morphology during the nucleation process was performed in the same localized areas at the nanoscale (SEM and AFM); second, the surface chemistry was probed within the domains using nano-Auger. New experimental findings are reported concerning the localization of nuclei after the BEN step, such as the role of furrows in domain formation. The topography and surface chemistry of domains were also examined.

Experimental

Iridium/STO samples

Iridium buffer layers with thicknesses of 200 nm were grown at 900 °C by e-beam evaporation (M-EV) on single crystals of SrTiO 3 (001), which were purchased from CrysTec Inc. The iridium deposition rate was 2 nm per min under a pressure of 9.0 × 10 -7 mbar. The full widths at half maximum (FWHM) of the rocking curve (ω-scan) and the azimuthal curve (ϕ-scan) for the Ir(200) and Ir(220) peaks were measured by X-ray diffraction, are 0.22°and 0.26°, respectively.

BEN and growth

The Ir/SrTiO 3 (001) substrates (5 × 5 mm 2 ) were transferred to a microwave plasma-enhanced chemical vapor deposition (MPCVD) reactor, which was equipped with a 2 kW SAIREM microwave generator that operated at 2.45 GHz. This reactor was connected to an ultra-high vacuum (UHV) Omicron system for surface sequential analysis. The base pressure of the reactor was 10 -9 mbar and gases of high-purity research grades were employed: N55 for methane and N90 for hydrogen (a catalytic purifier is present on the gas line). The substrate holder was electrically insulated from the metallic walls of the reactor, which were maintained at ground potential; thus, it was possible to apply a negative voltage to the substrate holder during plasma exposition.

To induce diamond epitaxial nucleation on the iridium surface, the BEN step was applied. As previously detailed [11], the entire nucleation process consists of three successive steps: (i) a pure hydrogen plasma is applied to clean the surface; (ii) a methane/hydrogen plasma exposure enables stabilization of the sample temperature; and (iii) the BEN treatment occurs, with a duration of 30 or 60 min. During the BEN step, a negative bias voltage of -307 V was applied between the sample holder and the walls. At the end of the BEN step, the plasma exposition and the bias voltage were stopped simultaneously.

A growth step with a duration of either 5 or 15 min was applied to reveal the diamond nanocrystals. The sample was cooled under a secondary vacuum after growth, without the presence of plasma and process gases. The nucleation process and growth experimental parameters are listed in Table 1.

SEM and AFM investigations of surface topography

The surface topography was observed by field emission scanning electron microscopy (FE-SEM) performed with a Zeiss Supra 40 microscope. By using an in-lens detector, a high lateral resolution of approximately 3 nm was obtained. Pictures were recorded for two acceleration voltages of the incident primary electron beam (20 kV and 0.5 kV). A localization technique, which consists of observing an area on a nanometric scale prior to the MPCVD process and imaging the same area after plasma exposure, was developed for SEM observations. By maintaining a constant brightness, contrast and working distance during the SEM analyses regardless of the voltage, an image treatment was applied on the SEM images to accurately compare the gray level distributions. Threshold tools were employed to associate specific colors with some of the gray levels observed after the nucleation process and short growth period. To enhance the image contrast, an FFT bandpass filter was applied prior to an autoscale gray level using the ImageJ software. The FFT bandpass filter adjusts large structures (shading correction) and small structures (smoothing) through Gaussian filtering in Fourier space. The surface topography of the samples was also investigated by AFM (Molecular Imaging PicoLE), which was operated in non-contact mode.

Surface chemistry

The first two steps of the nucleation process were previously analyzed by XPS [10]. The main results are summarized in part 3.1. Assuming a homogeneous layer, the XPS data were used to extract an equivalent thickness for the carbon overlayer that was detected on iridium after the BEN step [10,11]. Complementary nano-AES spectroscopy is currently used to investigate the chemistry of specific areas on a localized scale. Auger spectroscopy is extremely sensitive to the carbon-binding state because two valence electrons are involved in the C KVV transition. Surface analyses were achieved using the JEOL Jamp 9500F, which is a new generation of Auger nanoprobe. The following optimized conditions were employed: acceleration voltage of 10 kV; beam current of 5 nA (lateral resolution equivalent to a spot size of 15 nm), and tilt of 40°. The energy resolution was dE / E = 0.5% at 2000 eV. Wide scans were acquired with a 50 ms dwell time and an energy step of 0.25 eV. The resolution of the elemental maps and the positioning stability of the scanned area were controlled using a probe-tracking control tool. For Auger references, a single-crystal diamond was cleaned using an acid and plasma hydrogenation pre-treatment to eliminate or minimize the inherent carbon contamination. Concerning the HOPG sample, a fresh surface was produced by cleavage using a tape. Wide-scan AES spectra (not presented in this study) revealed the absence of O-KLL signals, which indicated that neither surfaces exhibited oxygen and carbon contamination.

Results

During the successive steps of the process (cleaning with pure hydrogen plasma, stabilization under CH 4 /H 2 plasma and BEN), the surface evolves morphologically and chemically. A sequential SEM study was performed on the same localized areas on a nanometric scale. First, the surface evolution (prior to BEN) during the stabilization step is presented (part 3.1). Second, the effects induced by the BEN step (parts 3.2, 3.3, and 3.4) are discussed. Last, a short growth step, which was applied to reveal heteroepitaxial diamond crystals, is examined (part 3.7).

Morphology and surface chemistry evolution of iridium before BEN

Fig. 1 shows the SEM images of the iridium surface for the two steps preceding BEN: after H 2 plasma and after H 2 /CH 4 plasma. The image contrast was enhanced by an FFT bandpass filter and an autoscale gray level using ImageJ software. SEM revealed a granular morphology with oxygen and carbon surface contaminations of 2.5 and 33.0 at.%, respectively, on the initial iridium surface, as revealed by XPS (Fig. 1a). The H 2 plasma causes an efficient surface cleaning of initial carbon and oxygen contaminations, as evidenced by XPS (Fig. 1b). These concentrations are below the detection limit (0.5 at.%) [10,12]. Similar granular morphology is evident after H 2 plasma (Fig. 1b). A loss of granular morphology is observed after H 2 /CH 4 with a surface softening in the same area. Previous XPS investigations revealed the presence of a carbon overlayer with an equivalent thickness of 1.4 nm [10]. Fig. 1c provides a comparison of the XPS spectra that was recorded in the 280-305 eV binding energy range before and after H 2 /CH 4 plasma. The formation of this carbon layer is most likely due to segregation during the cooling of carbon solubilized at high temperatures in the iridium, as previously shown [10]. The XPS results demonstrate that the iridium surface is most likely free from any carbon overlayer once the BEN begins directly after the stabilization step. SEM observations in the same area indicate that there is no evolution of surface morphology, with the exception of a smoothening of the surface. According to SEM and XPS experiments, no trace of diamond was detected on the surface at this stage.

Surface morphological changes induced by the BEN step

The BEN step leads to significant changes in surface morphology, as illustrated in Fig. 2. Different nanosized structures are observed on the iridium surface. The SEM image reveals white spots, iridium balls (spherical nanoislands), and furrows, as well as various contrasts that are named domains [8].A l ls u r f a c em o d ifications are associated with ion bombardment that was induced by the bias.

Among these structures, the domains are located at the origin of the heteroepitaxial diamond growth, as shown in Fig. 3 and in accordance with the literature [6].

SEM investigation of domains

A SEM study was conducted using the localization area technique and the sequential approach of the MPCVD process. This sequential approach was conducted in parallel using two acceleration voltages: 20 kV was utilized to enhance the lateral resolution because the outlines of the domains are very thin, and a low acceleration voltage of 0.5 kV was employed to enhance surface sensitivity.

After the BEN step, homogeneous bright areas corresponding to domains were observed in the surface image at 20 kV, as shown in Fig. 4a. Image treatment was performed to isolate these bright areas and obtain a better observation. First, a threshold was applied on the grayscale to select furrows and domains. Second, furrows were separated from domains based on size and shape criteria. Third, a red layer mask was used to color the domains.

After a short growth step of 5 min, the corresponding new gray level displays a green color. This image analysis is displayed in Fig. 4b. The green areas are confined to locations that previously contained domains. No morphological modifications are present on the iridium surface: furrows and iridium balls do not appear to evolve.

At a very low acceleration voltage (0.5 kV), another area was observed on the same sample after the BEN step, in which a heterogeneous contrast appears inside the domains: bright spots appear inside the red domains. The same treatment that was previously applied is used to distinguish the blue domains, as shown in Fig. 4c. After a short growth step of 5 min and after the image treatment, two characteristic gray level ranges were discernible, as shown in Fig. 4d, namely, the presence of domains (colored red) and the appearance of green spots. A distinct similarity was detected between the heterogeneities observed after the BEN step, which appear blue in Fig. 4c, and the location of the green areas after the short growth, as illustrated in Fig. 4d.

Considering that domains lead to the formation of heteroepitaxial diamond, we can assume that these green areas correspond to the presence of diamond nanocrystals after this short growth step. Concerning the blue spots after the BEN step, the presence of some novel precursors or diamond nuclei, which appear prior to the growth step, should be considered.

Furrows

SEM observations also highlight the presence of furrows on the iridium surface just after the BEN step (Fig. 2). The topography of these surface patterns is dependent on the plasma parameters, namely, the carbon concentration and the bias voltage, as previously reported in the literature [5,13]. However, the furrows do not evolve during the growth step. Their morphology also changes with the duration of BEN. Fig. 5 shows that the different shapes of these iridium structures are predominantly oriented in the [100] and [010] directions, as shown in Figs. 7a and 8 [6,11].

Due to the contrast adjustment in Fig. 6, furrows are visible within the domains. According to our SEM statistical observations, no domain was observed without furrows. So, furrows are likely to be required to form domains on the iridium surface.

After the BEN step, a 15 min growth step confirms the formation of heteroepitaxial diamond within the domains in which furrows are present. Furrows remain at the surface without morphological modifications after the growth step. Subsequently, diamond crystals grow at 45°with respect to the direction of the furrows, as shown in Fig. 7b.

Temporal evolution of domain formation

The temporal evolution of the domains was investigated by SEM (Fig. 8). For two samples which underwent the BEN steps for 30 and 60 min, the coverage of the iridium surface by domains increased from 8% to 86%, respectively. This observation suggests that two possible schemes are crucial to domain expansion. First, the nucleation of new domains may occur during the BEN step. Second, domains may grow by propagation mechanisms, which may lead to expansion and coalescence. The temporal evolution of domains may be a combination of these two phenomena that occur during the BEN step.

The domain expansion is limited by the topography of the iridium surface, namely, by the steps between terraces (appearing as dark lines in Fig. 8b). This occurrence prevents total coverage of the surface by domains.

As observed in the SEM images (Fig. 9), a high magnification of these dense domains after 60 min of the BEN step reveals structures that resemble cubic nanoislands.

As described in part 3.4, furrows are present all over the iridium surface, namely, within domains; they exhibit a temporal evolution during BEN. These cubic forms are associated with the roughening of the iridium surface, which produces other furrow shapes.

Topography of dense domains

The topography of domains was investigated using AFM measurements. The same area that was partially covered by domains was observed with SEM and AFM (Fig. 10). The black areas in both images correspond to the iridium surfaces that are not covered by domains. After several line scans, an average height of 8.5 ± 0.4 nm was extracted with an RMS of 0.68 nm. This height corresponds to the total height of the domain and furrows, as shown in Fig. 15. Among the domains, a mean height of 1.7 ± 0.3 nm was measured for the furrows (Fig. 11). From previously measured thicknesses, a height of 6.8 nm can be deduced for the thin film deposited on the roughened iridium surface. This value is confirmed by a comparison with the equivalent thickness deduced by XPS, which is discussed in further detail in the next section. appears very similar to the diamond reference (Fig. 12b). Fine Auger structures in the two pointed areas of Fig. 12a are demonstrated by considering d 2 N(E)/dE 2 . In this case, inflexion points, namely, A 1 , A 2 ,A 3 and A 0 , can be accurately determined (Fig. 13a), and their energy positions are compared with the principal inflexion point A 0 . These points correspond to KVV, A 0 -ŋw and π → π * transitions as described in the literature [14]. The energy differences versus the A 0 point are reported in Fig. 13b for each inflexion point. This histogram demonstrates that the energy values are equivalent inside as well as outside the domains. In addition, these values do not correspond exactly to the value determined on the diamond (C*) or HOPG references but correspond instead to the values reported by Constant et al. [14], which were obtained on an amorphous carbon reference.

After a short growth step

Fig. 14a shows the secondary electron image (SEI) of a sample after a short growth step of 5 min. Nano-Auger cartography was performed in this area to differentiate diamond and iridium. The bi-color image in Fig. 14b is the combination of the iridium and diamond scans. The red areas that correspond to the bright areas of the SEI image contain the same Auger signature as the diamond reference (C*). This observation is supported by the spectra shown in Fig. 14c.

Discussion

Several studies have focused on the BEN step on iridium to facilitate the understanding of heteroepitaxial diamond growth [1,9]. However, some uncertainty remains regarding the unique nucleation pathway. In this study, modifications of the evolution of surface morphology and surface chemistry induced by BEN are combined on a nanometric scale.

Our SEM study supports the presence of diamond nuclei after the BEN step. At a low acceleration voltage (0.5 kV), the secondary emission (SE) signal of the extreme surface is greater than the SE signal at 20 kV, and contrast heterogeneities can be detected within the domains (Fig. 4c). After growth, diamond nanocrystals, which are limited to the contrast heterogeneities previously observed after BEN (Fig. 4d), were identified by nano-Auger (Fig. 14). Contrast heterogeneities within the domains can be correlated to the presence of several carbon phases with different SE yields at the surface. According to the literature, a factor of 300 was reported between the SE yield of diamond and the one of amorphous carbon at 0.5 kV [START_REF] Joy | A Database on Electronic-Solid Interactions[END_REF].

The colocalization between the heterogeneities observed after BEN in the domains and diamond nanocrystals after a short growth strongly suggests the presence of diamond nuclei prior to the growth step. These nuclei are most likely located at the interface with iridium. Their presence is a required condition to obtain a registry of heteroepitaxial diamond with iridium lattice (Fig. 3). This assumption can be further corroborated by the nano-Auger results. This hypothesis is consistent with a previous study by Schreck et al. After a maximum annealing of 1000 °C, dark regions were observed by the in-lens detector instead of the bright domains that were observed after the BEN step [START_REF] Gsell | [END_REF].Th is reduction in contrast may indicate the thermal transformation of diamond nuclei into carbon onions. Such a temperature causes a surface graphitization of diamond detonation nanoparticles of 5 nm [17]. Graphite plays an important role in the electron emitting process by limiting the field emission properties of diamond nanocrystals [18]. This finding may explain the faded contrast.

In this study, domains exhibit a particular topography after the BEN step compared with the topography in other studies. In our case, the domains are composed of a thin film of amorphous carbon deposited on furrows with diamond nuclei at the interface, as represented by Fig. 15.

In part 3.6, the height of the dense domains and the height of the furrows were measured successively as 8.5 ± 0.4 nm and 1.7 ± 0.3 nm, respectively, in the same area by AFM analysis. After subtraction, a thin film of approximately 6.8 nm is deposited on the furrows. This value deduced by calculation was confirmed by XPS analysis. The ratio of C 1s and Ir 4d 5/2 XPS peaks leads to the determination of an equivalent carbon thickness d eq , assuming a homogeneous carbon overlayer [10,11]. This estimated equivalent thickness of carbon from XPS corresponds to 6.7 nm deposited on the iridium surface.

In the literature, the height of the furrows reported by Schreck et al. is approximately 3 nm [8]. Our height of 1.7 ± 0.3 nm is of the same order of magnitude. Schreck et al. reported the domain topography corresponding to a 1 nm deep depression compared with the surface of the surrounding area [19]. Our investigations indicate that the domains are composed of an overlayer that is 8.5 ± 0.4 nm thick. This difference in domain topography is most likely due to different BEN conditions employed by both groups.

To complete the topography description of the domain, a nano-AES analysis was performed on the dense domains. The analysis did not reveal any chemical composition equivalent to diamond or HOPGraphite. However, the Auger signature of the domains resembles an amorphous carbon layer [14] (Fig. 13).

For the heterogeneities observed by SEM analysis after the BEN step, we cannot detect any difference in the nature of the carbon phases using nano-Auger. The depth probed by this technique is low compared to the carbon thickness measured by XPS analysis. Depths in the range of 1-2 nm are estimated compared to a depth of 6.7 nm. This result supports the presence of diamond nuclei at the interface between the iridium roughening and the deposition of a thin film of amorphous carbon, as previously described. The observation of heteroepitaxial diamond crystals on iridium after a short growth step strongly supports this hypothesis (Fig. 3).

The last point to be discussed is the relation between furrow and domain formation. Most studies on diamond heteroepitaxy from the iridium surface report this specific substrate roughening after the BEN step [5,7,9,13]. In this study, SEM analyses demonstrate that the presence of roughening is closely related to furrows. This type of roughening appears only during the BEN step. According to SEM observations, morphologies of furrows are not modified after the growth step.

After 30 min of the BEN step, parallel patterns are observed along [010] and [100] directions of the iridium surface (Fig. 7a). This morphological surface evolution of iridium can be attributed to the ion beam bombardment during the BEN step. Similar patterns of metallic surfaces after focused ion beam (FIB) exposure have been reported in the literature. This exposure, which is named the 'ion sandblasting' phenomenon [20], is analogous with the action of wind on sand dunes. If there is a defect at the surface, these patterns change their direction of propagation along the steps of terraces, as shown in Fig. 7 and described in the literature. The form, lateral periodicity and orientation of ripples are dependent on the flux, energy, and incidence angle of the ions, as well as the surface temperature.

Bauer [5] demonstrated that when the methane ratio injected in plasma is modified, these ripples can be created preferentially at the surface. The bias voltage can also control these patterns [13]. Domains without furrows were never observed during multiple SEM observations. This absence may indicate a strong correlation between furrow and domain formation. The presence of furrows is required but not sufficient for the appearance of domains. In the literature, furrows are always observed in SEM images with domains, whether parallel or not parallel with other forms (cubic morphology and dots). By tuning the bias voltage [13], the optimum surface coverage of diamond after the growth step corresponds to the surfaces of domains with furrows equivalent to dots. Subsequently, the TEM analysis of cross sections indicates that there is no interface layer between the iridium and the diamond after the growth step [7]. The XPD analysis demonstrated that there is a diamond signature after the BEN step inside the domains [21].T h i s finding was obtained by Hoffman et al. using HR-EELS analysis. In the latter case, the detected nuclei are covered by a thin and weakly bonded amorphous or highly defective hydrogenated carbon layer after the BEN step [22].

This study has highlighted that the formation of domains requires furrows. We believe that they can act as channels to facilitate carbon super-saturation to induce the nucleation sites of diamond. The localization of these diamond nuclei are observed at significantly low acceleration voltages by SEM analysis prior to the growth step. A similar hypothesis was proposed by Kawarada et al. on β-SiC substrate [23]. The presence of β-SiC protrusions that resemble furrows, which are named grooves, was assumed to initiate the growth of heteroepitaxial diamond from the Volmer-Weber mode.

Conclusions

The topography and surface chemistry of iridium surfaces were simultaneously monitored after each step of the nucleation process and growth step. Observations of localized areas were performed on a nanoscale using SEM, AFM and nano-Auger. This multi-technique approach provides new experimental data that improve the knowledge of nucleation pathways on iridium.

SEM imaging at a low acceleration voltage enables the detection of diamond nuclei or stable precursors after BEN. After a short growth step, diamond nanocrystals were probed at similar locations by SEM and nano-Auger. Domains were composed of an amorphous carbon overlayer with a thickness of 6.8 nm, as confirmed by AFM and XPS data. Diamond nuclei probed after the BEN step are most likely located at the interface with iridium. It is a required condition to get highly oriented diamond after growth (Fig. 3). This assumption is confirmed by the nano-Auger experiment performed after the BEN step. The analyzed depths (1-2 nm) are insufficient to probe diamond nuclei. Consequently, the Auger C KVV signal obtained is similar to the amorphous carbon reference that corresponds to the carbon overlayer.

Our SEM observations also suggest a close relationship between the furrows created under ion bombardment and the domains created under our conditions. No domain was found without the presence of furrows. In the furrows, carbon super-saturation, which is required to form diamond nuclei may be easily achieved. According to these results, a sketch was proposed that describes the topography and surface chemistry of the domains (Fig. 15).

Fig. 2 .

 2 Fig. 2. SEM image of iridium surface after the BEN step. The scale of the images inside the circles is 100 nm.

Fig. 3 .

 3 Fig. 3. SEM images of domains (a) after the BEN (30′) step and (b) after the growth step (15′).

Fig. 4 .

 4 Fig. 4. Two areas observed after 30 min of BEN (first line) and after a short CVD growth of 5 min (second line) by FEG-SEM at different acceleration voltages: 20 kV for (a) and (b), 0.5 kV for (c) and (d), respectively.

Fig. 5 .

 5 Fig. 5. SEM images of different shapes of furrows.

3. 7 .Fig. 6 .

 76 Fig. 6. SEM image of the iridium surface after BEN with the furrows highlighted. The brightness and contrast adjustments signify the presence of furrows in the domains.

Fig. 7 .

 7 Fig. 7. SEM images of a domain (a) after BEN (30′) and (b) after 15′ of growth: the presence of epitaxial diamond on the iridium surface at 45°of the furrows.

Fig. 8 .Fig. 9 .

 89 Fig. 8. SEM images of the domain evolution according to the duration of the BEN step: (a) isolated domains after 30 min and (b) domain coalescence after 60 min.

Fig. 10 .Fig. 11 .

 1011 Fig. 10. Correlation between (a) SEM image a (b) AFM image of the same areas exhibiting domains after BEN; (c) line scans from AFM image.

Fig. 12 .

 12 Fig. 12. a) Localizations of nano-Auger analysis outside the domains and inside the domains and b) comparison of C KVV Auger spectra for the HOPG sp 2 reference (•••), the Diamond sp 3 (C*) reference (line), and outside (--) and inside (•••) the domains after BEN using the derivative mode (dN(E)/dE normalized to [0,1]).

Fig. 13 .

 13 Fig. 13. a) Comparison of C KVV Auger spectra for the HOPG sp 2 reference (•••), diamond sp 3 (C*) reference (line) and outside (--) and inside (•••) the domains after BEN using the derivative mode (d 2 N(E)/dE 2 normalized to [0,1]) and b) difference in energy of each inflexion point versus A 0 pointmeasuredond 2 N(E)/dE 2 of AES spectra acquired in the domains, outside the domains, on diamond (C*), on HOPG graphite and on amorphous carbon [14] references.

Fig. 15 .

 15 Fig.[START_REF] Joy | A Database on Electronic-Solid Interactions[END_REF]. Drawing of domain (carbon thin film + furrows) at the iridium surface with thicknesses extracted from AFM, diamond nuclei which are represented at the interface with iridium according to SEM and nano-Auger.

Fig. 14 .

 14 Fig. 14. Nano-Auger analysis after 5 min of growth with (a) SEI image, (b) chemical mapping of C*/Ir with C* AES diamond signal and (c) spectra of diamond (line), HOPGraphite (•••••) references and sample (---).

Table 1

 1 Experimental CVD parameters for the nucleation process and growth. τ CH4 = [CH 4 ]/ ([CH 4 ]+[ H 2 ]) corresponds to the mole fraction of methane in the reactor.

	Steps	Nucleation process		Growth
		H 2 cleaning	H 2 /CH 4 stabilization	BEN	Short/long
	τ CH4 (%)	0	4	4	0.6
	Pressure (mbar)	20	20	20	20
	Power (W)	400	400	400	400
	U (V)	0	0	-307	0
	T (°C)	650	650	700/750	650/700
	t (min)	10	10	30/60	5/15

Fig 1. SEM images and XPS spectra of iridium surface after each step: (a) as received, (b) after H 2 cleaning and (c) after H 2 /CH 4 stabilization. The two XPS C1s spectra recorded after H 2 (dotted gray line) and H 2 /CH 4 (solid black line) are compared.

Acknowledgments

The authors would like to thank B. Vilquin for the XRD measurements and J. Faerber and H. A. Girard for their helpful discussions.