A composite material made of carbon nanotubes partially embedded in a nanocrystalline diamond film

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A composite material made of carbon nanotubes partially embedded in a nanocrystalline diamond film Cle´ment He´bert a,b,*, Se´bastien Ruffinatto b, David Eon a, Michel Mermoux c, Etienne Gheeraert a, Franck Omne`s a, Pascal Mailley d a

Institut Ne´el, CNRS et Universite´ Joseph Fourier, BP 166, F-38042 Grenoble Cedex 9, France CEA/INAC/SPrAM/CREAB, 17 rue des Martyrs, 38054 Grenoble Cedex 9, France c Laboratoire d’Electrochimie et de Physicochimie des Mate´riaux et des Interfaces (LEPMI), UMR 5279, CNRS, Grenoble INP, Universite´ de Savoie, Universite´ Joseph Fourier BP75, 38402 Saint Martin d’He`res, France d Laboratoire de stockage de l’e´lectricite´, LITEN-DTS, CEA, 50 avenue du lac Le´man, 73370 le Bourget du lac CEDEX, France b

A R T I C L E I N F O

A B S T R A C T

Article history:

A composite material, made of carbon nanotubes (CNTs) partially embedded in a nanocrys-

Received 27 April 2012

talline diamond film was produced. The diamond film was first decorated with palladium

Accepted 21 September 2012

or nickel nanoparticles. An array of nanopores was drilled in the film in a hot filament CVD

Available online 1 October 2012

(HFCVD) reactor thanks to the anisotropic etching that takes place under the nanoparticles. During this etching process, the metallic particles penetrate the diamond film to a controlled depth, thus remaining at the bottom of the nanopores. The buried nanoparticles remain catalytically active and are used to grow a multiwall carbon nanotube forest using HFCVD in the same reactor without breaking the vacuum. The quality of the CNTs was assessed by scanning electron microscopy and Raman spectroscopy. The interface between the carbon nanotubes and the diamond was characterized by ultrasonication, lateral force microscopy, cyclic voltammetry and electrochemical impedance spectroscopy. As a result of these characterizations, we demonstrate that the buried carbon nanotubes exhibit higher mechanical stability and improved electrical behavior compared to CNTs directly grown on the diamond surface.  2012 Elsevier Ltd. All rights reserved.

1.

Introduction

The fascinating properties of carbon nanotubes (CNTs) have been widely studied over the last 20 years, leading to many outstanding results and applications that are on the verge of reaching the marketplace [1,2]. All these major breakthroughs were possible thanks to many cutting edge technologies, from connection to characterization, that were developed simultaneously. Now scaling up those technologies to take benefit of the CNT properties for designing new functional materials represents another challenging issue [3,4]. While the parallel connection of a single nanotube for mass

transistor production is still far from the market, CNT forests have already been implemented in many devices [5,6]. However electrical and mechanical connectivity of the forests are still to be improved to design fully reliable devices for long-term applications [7–9]. Among the panel of ongoing applications, two particular ones can be distinguished as really needing both good mechanical stability and good electrical connectivity of the CNTs, namely sensors for in vivo applications and field emission. In particular, Mattson et al. [10] showed in 2000 that carbon nanotubes are a very promising coating for neural interfacing. A huge number of articles followed this

* Corresponding author at: Institut Ne´el, CNRS et Universite´ Joseph Fourier, BP 166, F-38042 Grenoble Cedex 9, France. E-mail address: [email protected] (C. He´bert). 0008-6223/$ - see front matter  2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.carbon.2012.09.051

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breakthrough in neuroscience so that carbon nanotubecoated electrodes are now recognized as one the most interesting material for neural interfacing [11–14]. However, recent studies highlighted toxicity issues of the CNTs that might preclude the development and use of such composite electrodes [15,16]. On the other hand one study has stressed that those issues can be easily overcome by strongly anchoring the individual nanotubes on the electrodes since they are not considered harmful as long as they are not dispersed in single entities [17]. Field emission, with the development of field emission displays, is also demanding for very reliable CNT connection [18,19]. Poor adhesion of the nanotube can lead to the covering of the anode or a vacuum breakdown, resulting in a dramatic decrease of the devices efficiency. Many methods such as electrophoretic deposition [20], screen printing [21] microwave treatment [22] where developed to anchor properly the carbon nanotube to the substrate, but reliable devices are still difficult to obtain. Thus, the need for a strong adhesion of the nanotubes on their substrates is highlighted in view of these two applications. Such a strong anchoring implies an important role of the substrate material. Indeed, if the carbon nanotubes can be deposited or directly grown on many substrates, only a few of them have the necessary properties to be used in those devices. Diamond is certainly one of the materials that can fulfill the required properties for the applications described above. It is intrinsically insulating and can be made highly conductive when heavily doped with boron [23]. It can be placed in harsh environments thanks to its chemical and extreme mechanical and thermal properties. Diamond is also considered to be biocompatible [24]. Diamond thin layers can be transferred on polymers to comply with flexible technologies [25]. Another important feature makes diamond a very special substrate for CNT growths. It can be catalytically etched by many metallic nanoparticles such as nickel [26], platinum, cobalt, molybdenum or others that remain in the bottom of the pore after the etching process. In the present study, we used this last feature with the aim of creating a very strong bond with a good electrical contact between individual carbon nanotube and a diamond substrate. To reach this goal, the diamond thin film was drilled with nickel and palladium nanoparticles. The metallic particles which remained at the bottom of the pores were then used as a catalyst for the subsequent growth of a carbon nanotube forest in the same growth chamber without breaking the vacuum. The quality of the CNTs was assessed by scanning electron microscopy and multiwavelength Raman Spectroscopy. The interface between the carbon nanotubes and the diamond substrates was characterized by ultrasonic treatment, lateral force microscopy, cyclic voltammetry and electrochemical impedance spectroscopy.

2.

Experimental

A nanocrystalline diamond film grown on silicon was used as a substrate. The diamond nucleation was performed in a 5200 Seki technotron microwave plasma chemical vapor deposition (MPCVD) reactor using the bias enhanced nucleation (BEN) method [27]. During this nucleation step, a 280 V

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voltage was applied on <1 0 0>silicon substrates that were heated at about 700 C. The microwave power was held at 1400 W with a methane concentration of 5% in H2. The duration of the nucleation step has been fixed to 7 min. Then either a non-intentionally doped (NID) or a heavily boron doped (BDD) diamond film was grown from the diamond nuclei. The NID films were grown in the same MPCVD reactor at 900 C. The methane concentration was 0.4 vol.% in H2, and the gas pressure was 30 Torr. The BDD films (boron source: B2H6) were grown in a homemade NIRIM-type reactor at 890 C at 30 Torr, with a methane concentration of 0.48% in H2 and a boron to carbon ratio (B/C) of 6000 ppm. Diamond individual grains were in the 100–200 nm size range and exhibited mainly <1 1 1> and <1 0 0> surface orientations. Both types of films were about 600 nm in thickness. The following step consisted in forming pores into the diamond films thanks to metallic nanoparticles. For this purpose, a 3 nm thick palladium or nickel film was first deposited on the diamond films by e-beam evaporation. Then, the sample was transfered in a hot filament CVD reactor (HFCVD) specially designed for CNT growth in which it was heated by a temperature-controlled molybdenum substrate holder and a tungsten filament located 7 mm above the sample. The etching was performed during 10 min under a H2 atmosphere held at 60 Torr. The temperature of the filament and the substrate were respectively set to 1950 and 850 C. No specific study of the dewetting of the metallic film that generated the nanoparticles was performed. It is thought to occur in the first seconds of the process when the temperature at the surface of the sample reached the required value for the dewetting of the metallic film. The CNT growth was carried out in the same HFCVD reactor, after the dewetting/etching process. The tungsten filament was first carburized in a H2/CH4 atmosphere during 1 h to lower carbon adsorption on the wire during the growth. The growth duration was 10 min with CH4 as a carbon source. The CH4/H2 ratio was of 9%. The substrate was held at 800 C and the tungsten filament temperature was kept at about 1930 C. The filament has been placed at 12 mm above the substrate. Scanning electron microscopy (SEM) and Raman spectroscopy were carried out to assess the quality of the CNTs. Raman measurements were performed using an InVia Renishaw or a UV-dedicated Jobin Yvon T64000 spectrometer. These spectrometers were already described in Ref. [28]. 785, 632, 514, 244 and 229 nm were used as excitation wavelengths to discriminate between the contributions of the diamond substrate and CNTs. The interface between the carbon nanotube and diamond was characterized by simple ultrasonication, lateral force microscopy (LFM), cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). Atomic Force Microscopy (AFM) measurements, in the lateral force mode, were performed using a VEECO Dimension 3100 instrument. A 2 lN normal load was applied on the sample with NT-MDT CSG contact tips. 70 lm · 5 lm areas were investigated. To perform the electrochemical measurements, Ferrocene was covalently grafted on the CNTs in a dimethylformamide solution containing 50 mM of aminated ferrocene synthesized as already described [29], and 5 mM BOP coupler (Sigma

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Aldrich) during 12 h at room temperature. Prior to the functionalization step, the CNTs were slightly oxidized to introduce carboxylic groups. This oxidation step was carried out using an oxygen plasma (plasma generator Femto from Diener, oxygen pressure 0.6 bar, polarization 20 W, duration 10 s). Infrared spectroscopy was performed on the samples all along the functionalization steps. Cyclic voltametry and electrochemical impedance spectroscopy were performed using a computer driven (EC-lab software) VSP 300 potentiostat from Biologic (Claix France). CV experiments were conducted at a sweep rate of 100 mV s1 within a 0.2 M aqueous LiClO4 solution. EIS spectra were recorded in a 0.2 M aqueous LiClO4 solution at a bias potential of 0.38 V/Ag/AgCl using a modulated sinusoidal function of 10 mV rms in the 200 mHz–200 kHz frequency range. This potential corresponds to the half-wave potential of the Fc/Fc+ redox couple. The electrodes were connected from the rear side.

3.

Result and discussion

The catalytic etching of a diamond surface with metallic nanoparticles under hydrogen was first studied for the patterning of diamond surfaces [30,31]. More recently the same catalytic processes were thought to offer a new way to create diamond porous membranes [26]. However, the depth of etching was too small to cross a membrane or even embed completely the nanoparticles inside the diamond. Here an enhanced catalytic etching with hot filament under hydrogen flux was performed on both a boron-doped and an insulating diamond layer deposited on silicon substrates. The main mechanisms of the catalytic etching were already described by Smirnov et al. [26]. Schematically, the carbon is first dissolved in the metallic nanoparticle and then evacuated into the gas phase with the aid of atomic hydrogen. The hot filament is thought to enhance the etching by providing heat and atomic hydrogen which respectively help the dissolution of the diamond by the catalyst and its evacuation in the gas phase. The catalytic etching was first analyzed by SEM. Fig. 1 shows some of the results obtained when the etching process was performed using a 3 nm thick palladium film during 500 s. From the Fig. 1a, it is seen that the etching was rather homogenous. All the crystallographic orientations were etched without any distinction. This was observed both for nickel- and palladium-coated nanocrystalline diamond films. However, a few metallic particles did not penetrate the

diamond, in particular when h1 0 0i faces were concerned. This suggests that the etching rate is slower on h1 0 0i faces. SEM image were also used to determine the nanoparticle and the pore mean sizes. For both nickel and palladium, the dewetting creates metallic nanoparticles whose mean size are about 28 nm, with a standard deviation of about 10 nm. The mean pore size is around 40 nm in diameter with a standard deviation of 14 nm. The difference between the size of the holes and the particles arises from the fact that the etching occurs simultaneously around and under the catalytic particles by diffusion of the carbon species towards the metal nanoparticles. Finally, the cross-sectional SEM images (Fig. 1b) revealed that some holes have a ‘‘saw tooth’’ like shape. The angle formed by the teeth is about 108–110. It is assumed that this angle corresponds to anisotropic etching conditions that selectively reveals the h1 1 0i-oriented diamond planes. Note that the drilling can be performed down to about 500 nm, a value that fully opens the way for the development of porous membranes. All these observations unambiguously show that the hot filament-enhanced catalytic etching makes porous the diamond surface. The growth of carbon nanotubes on a porous surface has been studied for long but none of the methods presented so far could lead to one nanotube per pore. Moreover, none of those methods have been performed on conductive substrates. Here, those issues can be easily solved since conductive diamond pores embed only one particle. To perform the subsequent carbon nanotube growth, the porous diamond loaded with catalytic particles was maintained in the same HFCVD reactor. Cross-sectional SEM images of the resulting composite materials exhibited a dense carbon nanotube (MWNT) forest, see Fig. 2. For the particular growth duration used in this study, the mean length of the tubes is in the 5–8 lm range for palladium and around 700 nm for nickel. Those lengths are shorter than the ones obtained when CNTs are directly grown on diamond using the same growth conditions, but without the drilling step. This might be due to the reduction of the amount of carbon species that can reach the catalyst once the pore is filled by the nanotube. However, the possibility to grow nanotube clearly emphasizes that the catalytic properties of nickel and palladium were not impaired during the etching process. The tube diameters are in the 15–30 nm range, which corresponds to the catalytic nanoparticles diameter range. This is in good agreement with works which conclude that the diameter of the CNTs is controlled by the diameter of the

Fig. 1 – SEM image of pores in nanocrystalline diamond after an etching made with a 3 nm thick palladium film. (a) Top view of the surface of the diamond etching on h1 0 0i and h1 1 1i orientated crystals, (b) ‘‘saw tooth’’-like shape of a pore.

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Fig. 2 – (a, b) Cross-section SEM views of the CNT forest partially embedded in diamond. (c, d); Cross-section views recorded with energy selective backscattered EsB detector showing the base growth mode.

catalyst [32,33]. Finally no tubes were found to grow when the catalytic particles are buried at a depth larger than 250 nm below the diamond surface. This suggests that for such depths, the carbon species that are responsible of the growth of the tubes cannot reach the catalytic particles anymore. Generally two different growth mechanisms may lead to CNTs formation, they are referred as base and tip growth [34]. For a base growth, the catalytic particles remain on the substrate at the base of the nanotube whereas for a tip growth, the catalytic particles are located at the tip of the tubes. SEM images (Fig. 2c and d) recorded with the energy selective backscattered detector clearly show that the base growth mechanism prevails during the growth inside the pores for both catalysts since it can be clearly seen that all the metallic particles remain inside the diamond, at a mean depth of about 200 nm. Most probably, this is an interesting result for future potential biological applications. Such a base growth mechanism may contribute to prevent the interaction of any biological medium with the metal nanoparticles, thus reducing toxicity issues. The cristallinity and quality of CNTs partially embedded in diamond was assessed by Raman spectroscopy at several excitation wavelengths. First, whatever the excitation radiation we used, no low frequency radial breathing modes were detected, see Fig. 3a and b. This confirms that the forest is only composed of multi-walled carbon nanotubes. The spectra recorded at 514.5 nm are clearly dominated by the CNTs response. These spectra are given in Fig. 3c. The well-known D (disorder-induced breathing mode), G band (Raman-allowed E2g mode) are clearly observed, as well as the second-order features peaking in the 2400–3200 spectral range [35].

Comparison of the CNTs spectra obtained for tubes grown with nickel or palladium immediately shows that the quality of the tubes is better when they were grown using nickel as a catalyst. Indeed, the G band is peaking at the expected 1580 cm1 frequency, and the shoulder located at about at 1620 cm1 is clearly resolved. The D band peaks at about 1350 cm1 and exhibits a shoulder at 1333 cm1, which corresponds to the Raman peak of the diamond substrate. Moreover, the second-order lines peaking at about 2700 and 2927 cm1 are clearly resolved. Looking at the tubes grown using palladium as a catalyst, the G band is observed at 1595 cm1 and the D band at 1355 with a clear diamond peak at 1333 cm1. The second-order spectrum only exhibits broad features also located in the 2400–3200 spectral range. The comparison of the full width at half maximum (FWHM) of the E2g mode also highlights the better crystallinity of the tubes grown using nickel as a catalyst. The value of these line widths (55 and 85 cm1, respectively) suggests a greater rate of disorder for the films grown using palladium as a catalyst. The comparison of the line widths of the D bands confirms this trend. The spectra recorded using excitations in the deep UV range are dominated by the nanocrystalline diamond substrate signature. This is an indication that the CNTs are out of resonance conditions when such excitations wavelengths are used. For both films, the diamond signature is clearly observed, now allowing a detailed analysis of its frequency and line profile. Finally, some attempts have also been performed to compare CNTs spectra when they are grown on diamond or partially embedded in diamond (Fig. 3d). Spectra were recorded

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Fig. 3 – Multi-wavelength Raman spectrum of CNTs embedded in diamond grown with Ni (a) and palladium catalysts (b). The excitation wavelengths are given in the figures. The broad peak observed at about 2800 cm1 using the excitation at 785 nm is due to the silicon substrate. (c) Comparison of the Raman spectra of tubes grown with Ni and palladium as a catalyst. Excitation wavelength was 514.5 nm. (d) Comparison of the Raman spectra of tubes recorded inside and outside the pores. Excitation wavelength was 632 nm.

on CNTs grown using nickel as a catalyst, excitation wavelength was at 632 nm. Spectra obtained inside diamond pores seem to present a wider D band which may indicate higher defect density of the CNTs inside the pores. Again, this difference in crystallinity is assumed to arise from local differences in the gas phase composition inside and outside the pores. However, this trend has still to be confirmed. This growth path was considered to create better adhesion conditions between the tubes and the diamond substrates. Actually the base of the nanotube was seen to be embedded in the diamond substrate. Thus, it is expected that the resistance to tearing should be increased as compared to that of CNTs directly grown on a diamond substrate surface. Moreover, as suggested from Fig. 4, the ‘‘saw-tooth’’ shape of the pores should increase both mechanical and electrical contacts because it gives a higher contact surface between the carbon nanotubes and the pores. To test this hypothesis three different kinds of experiments were conducted. First samples with CNTs grown inside pores were simply sonicated (0.7 W/ml, 37 kHz) for 30 min in dimethylformamide (DMF) medium. DMF was chosen to ensure a complete wetting of the tubes. This basic technique provides important information on the mechanical stability of the whole forest under high mechanical stresses. A SEM

Fig. 4 – Sketch and SEM image of the compliance of the nanotube inside a pore with a ‘‘saw tooth’’ shape. view of the sample surface after such a sonication is given in Fig. 5. The SEM image reveals that all the carbon nanotubes

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were cut at the diamond surface, while the base of the tubes remained in the pores. It is assumed that the tangential force that was applied in the ultrasonic bath induced a high stress on the nanotubes at the top of the pore leading to the rupture of the nanotubes. This is a strong indication that the growth process investigated here effectively led to a strong adherence between the base of the tube and the diamond substrate, most probably via the catalytic particles which remain embedded in the diamond substrate. Second, we used Atomic Force Microscopy (AFM) in friction mode (lateral force microscopy or LFM) to compare the adhesion of CNTs grown at the diamond surface and CNTs grown inside diamond pores. LFM consists in recording the deflection of an AFM tip due to the lateral force induced by the scratching of a batch of nanotubes. It is thus possible to get information on the adhesion of a small batch of CNTs. In the case investigated here, this lateral force will depend in part on the strength of the bond between the CNTs and their substrate, but also on the quality of the CNTs themselves and their density. Actually, when the CNTs present a lot of defects, their mechanical properties are impaired, leading to a decrease of the lateral force which is necessary to tear out the tubes from their substrates. In the same way when the density of CNTs is very low, only a few CNTs bound to the tip so that the resistance of the batch of nanotubes to the lateral force decreases. Thus, samples of carbon CNTs grown on diamond and inside diamond pores with roughly the same density and quality (according to SEM and Raman spectroscopy) are to be compared. Fig. 6a shows typical curves obtained for the friction measurement on CNTs. The shape of the resulting curves can be described as follows. First, the tip binds to a batch of tubes because of Van der Waals interactions, and it keeps on bending as long as it is in contact with the batch of CNTs. When the batch is strongly bonded to the substrate, the tip bends a lot before the bond breaks. So high peak amplitudes on the curve corresponds to high strengths to tear out nanotubes. In the Fig. 6a, the tip deflection is given in terms of voltage, which is directly proportional to the lateral force for a given tip. Fig. 6b gives the distribution of the peak amplitude measured for CNTs

Fig. 5 – SEM picture of CNTs cut at the top of the pores after 30 min of sonication.

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on diamond and inside diamond pores. Results show that the peak amplitude distribution on the CNTs grown inside diamond pores is three times higher than the one of the nanotube grown on diamond. This means that it is more difficult to tear out the nanotubes form the substrate when they are grown from pores made on diamond surface. Again, these results suggest a strong adhesion between the nanotubes and their diamond substrates. Finally, the electrical and electrochemical properties of the nanotubes-diamond composite electrodes were investigated. For that purpose, ferrocene groups functionalized by an aminated polyethylene glycol chain were grafted on carboxylic groups using classical peptidic chemistry [29]. The carboxlylic groups were generated at CNTs walls owing to a mild oxygen plasma treatment. Thus, CNTs only serve as electrical wires between the grafted ferrocene mediators and the borondoped diamond substrate. We first verified that the ferrocene groups were covalently grafted and not simply physisorbed on the CNTs wall through the comparison of infrared spectra of oxidized nanotubes and grafted nanotubes shown on Fig. 7. The spectrum of the oxidized nanotubes is used as the background reference for the recording of the grafted nanotubes. We notice the presence of two peaks in the region of the aliphatic CHx bonds at 2938 and 2871 cm1. They are assigned to the asymmetric and symmetric stretch of the CH2 located at the alpha position of the oxygen atoms of the ferrocene spacer. The wavenumbers related to the methylene group stretch of aliphatic alcanes chains (2925 and 2855 cm1), differ from those of the aliphatic ethers (located in the alpha position of the oxygen atom) by a shift towards the high wave numbers. The presence of the amide group, formed during the coupling, is highlighted by the presence of peaks at about 1650 and 1560 cm1 [36,37]. The first one is related to the C@O stretching of the amide function, the second one is linked to the acyclic monosubstitutes amides (trans). This peak is related to the combined effects of the C–N stretch and the N–H bend. In parallel a negative peak, located at 1730 cm1, indicates a decrease of carboxylic acid groups at the surface of the sample. Finally a peak at 1100 cm1 associated to the aliphatic ethers asymmetric O–C–O stretching can be observed. Besides several cyclic voltametry experiments were performed for different potential scan rate (Fig. 8a). The intensity of the reduction and oxidation versus the scan rate are reported on Fig. 8b. The linear dependency of the intensity to the potential scan rate immediately indicates the immobilization of the ferrocene at the surface of the electrode. From the integration of the voltammograms of the oxidation and reduction the density of ferrocene grafted on CNT modified diamond is estimated to 4 · 109 mol cm2. This is 20 times higher than the value obtained on diamond surface for a compact monolayer (2.5 · 1010 mol cm2) [38]. This result demonstrates that the ferrocene is mainly grafted on the nanotubes. We compared the electrochemical response of both kinds of carbon nanotubes-diamond composite electrodes, with the CNTs grown on and inside diamond pores. Between two successive potential cycles, the electrodes were thoroughly rinsed with deionized water. Fig. 9 presents the results obtained for four successive cyclic voltammograms performed on the ferrocene derivatized composite electrodes. First, the ferrocene immobilization was effectively demonstrated for

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Fig. 6 – lateral force microscopy using AFM. (a) Typical deflection shape (in Volt) obtained on a nanotube carpet and sketch of the tip deflection during the scan. (b) Distribution of the peak amplitude (in Volt) recorded for CNTs grown on diamond and partially embedded in diamond.

Fig. 7 – Infrared spectra (with ATR) performed on (a) MWCNTs oxidized by plasma treatment and (b) MWCNTs grafted with ferrocene.

both composites electrodes according to the linear evolution of the peaks current densities with the voltammetric scan rate. One can point out the higher amplitude of the ferrocene redox signal for the CNTs partially embedded in diamond. However, such a phenomenon is undoubtedly due to the larger thickness of the CNTs forest grown on diamond that

brings a larger number of derivatization sites. Examining the effects of the washing cycles, it is clear from Fig. 9a that the intensity measured for the CNTs grown on diamond decreased after each washing step while it remained perfectly constant in the case of CNTs grown inside the porous diamond layer (Fig. 9b). Considering that the intensity depends

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Fig. 8 – (a) voltammograms of ferrocene graphted on MWCNTs in LiClO4 (0.2 M) for several scan rate. (b) Scan rate dependency of the current density recorded of ferrocene grafted on MWCNTs (reference electrode (Ag/AgCl).

Fig. 9 – Cyclic voltamogrammes (CV) and EIS spectra, associated to their electrical equivalent circuit, of ferrocene grafted on nanotubes recorded with carbon nanotube/nanocrystalline diamond electrodes in LiCLO4 0.2 M (100 mV s1) (counter electrode Ag/AgCl). (a) CV of CNTs grown on diamond, (b) CV of CNTs grown inside diamond. (c) EIS spectrum of CNTs grown on diamond (insight: enlarged view of the high frequency part of the EIS spectrum), (d) EIS spectrum of CNTs grown inside diamond.

on the number of molecules grafted on the CNTs, the drop of intensity highlights a loss of carbon nanotube after every washing step when the CNTs are grown at the diamond surface, while the washing process does not lead to a loss of nanotubes when they are grown in pores made below the diamond surface. This result nicely confirmed the sonication and LFM experiments.

To assess the gain in electrical connectivity brought by the CNTs embedding, both ferrocene modified composite materials were characterized by electrochemical impedance spectroscopy (EIS). Main results are given in Fig. 9c and d. Table 1 summarizes the values of the different components used to draw the electrical equivalent circuit of both electrochemical interfaces. First of all, one can note the large series

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Table 1 – Values of the electrical components used for the fitting the CNT-diamond composites EIS spectra. Elements Rs (O) R1 (O) Q1 (F.s(a1)) a1 C2 (F) R2 (O) v2

CNTs deposited « on » the diamond surface

CNTs grown inside porous diamond layer

662.2 28877 9.1 · 104 0.86 5.8 · 103 1100 3 · 104

421.1 22186 7.87 · 104 0.92 – – 5 · 104

resistance of both electrode materials, which was already observable on the CV curves, with a slightly higher value for the CNTs grown on diamond composite. Typically, through the characterization of the diamond substrates before any physical modification with CNTs, it was already seen that this series resistance comes essentially from the silicon-diamond interface Indeed, during the nucleation phase, which used a BEN step, some carburation of the silicon interface could be generated [39] that impede the overall transverse conductivity of the electrode substrate. We believe that such a drawback could be easily removed by using substrate seeding in place of BEN. More interestingly, both impedance spectra were associated to an electrical equivalent circuit. Results showed that the spectrum of the composite electrode fabricated with partially embedded carbon nanotubes could be interpreted using the classical Randles model through the implementation of a constant phase element (CPE) in place of a pure capacitance, according to the large surface roughness brought by the nanotubes array. The insight in Fig. 8d represents this equivalent circuit in which R1 and CPE represent the charge transfer resistance between the CNTs and the anchored ferrocene moieties and the constant phase element associated to the capacitive behavior of the CNTs modified interface respectively. One can note here the absence of a Warburg component within this model. This is physically dictated by the anchoring of the ferrocene redox system on the electrode itself, which suppresses any diffusion phenomenon in the redox mechanism. Conversely to the CNTs grown inside diamond pores, the CNTs grown on diamond interface spectrum (Fig. 9c) can be accurately described only through the addition of a series RC circuit. Such a behavior could be readily related to the electrical interface between the carbon nanotubes and the diamond substrate. This clearly highlights, in agreement with the increased mechanical stability, the improved electrical connection of the CNTs grown inside diamond pores.

4.

Conclusion

Carbon nanotubes were grown inside pores catalytically drilled in diamond films. The metallic particles (nickel and palladium) that were used for the etching process were simultaneously used as catalysts for the subsequent growth of the nanotubes, creating a simple and quick way to grow them inside the pores, at about 200 nm below the sample surface. The whole process was performed in a HFCVD reactor without breaking the vacuum. The CNTs crystallinity was assessed

using multi-wavelength Raman spectroscopy. Tuning the excitation wavelength from the visible to the deep UV spectral ranges allowed probing selectively the multiwall carbon nanotubes or the diamond substrate. The improvement in the adhesion and in the electrical connection between carbon the nanotubes and the boron-doped diamond substrates has been evaluated with simple sonication experiments, more sophisticated AFM measurements in the lateral force mode and finally with electrochemical analysis, including cyclic voltammetry and electrochemical impedance spectroscopy. In particular, the electrochemical measurements unambiguously showed that the composite electrode did not lose its properties after several washing steps. Obviously, thanks to the increase in the electrode apparent surface, and thanks to the diamond and carbon nanotube electrochemical properties, such electrodes can be attractive for biological applications, in particular neural interfacing or biosensing. Using this growth process, the metal nanoparticles are buried below the conductive diamond substrate, which reduces both their interaction with biological media and the dispersion of the CNTs, thus preventing toxicity issues. Any applications where CNTs must be strongly bonded to the substrate with a good electrical contact such as field emission devices could be improved thanks to this technique. The first electrochemical tests of these composite electrodes were ongone and appeared promising.

Acknowledgements The authors wish to acknowledge Alexandre Crisci (SIMAP, Grenoble) for technical support during the deep-UV Raman experiments and Charles Agne`s for the fruitful discussion.

R E F E R E N C E S

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