Temperature effect on hydrogenated amorphous carbon leading to hydrogenated graphene by pulsed laser deposition

Accepted Manuscript Title: Temperature effect on hydrogenated amorphous carbon leading to hydrogenated graphene by pulsed laser deposition Authors: R.H. Benhagouga, S. Abdelli-Messaci, M. Abdesselam, V. Blondeau-Patissier, R. Yahiaoui, M. Siad, A. Rahal PII: DOI: Reference:

S0169-4332(17)32117-7 http://dx.doi.org/doi:10.1016/j.apsusc.2017.07.125 APSUSC 36659

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Received date: Revised date: Accepted date:

13-2-2017 6-7-2017 15-7-2017

Please cite this article as: R.H.Benhagouga, S.Abdelli-Messaci, M.Abdesselam, V.Blondeau-Patissier, R.Yahiaoui, M.Siad, A.Rahal, Temperature effect on hydrogenated amorphous carbon leading to hydrogenated graphene by pulsed laser deposition, Applied Surface Sciencehttp://dx.doi.org/10.1016/j.apsusc.2017.07.125 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Temperature effect on hydrogenated amorphous carbon leading to hydrogenated graphene by pulsed laser deposition R. H. Benhagouga1*, S. Abdelli-Messaci. 2, M. Abdesselam3, V. Blondeau-Patissier4, R. Yahiaoui5, M. Siad 6, A. Rahal1

1

Université des Sciences et des Technologies Houari Boumediene (USTHB), Laboratoire de Physique

des Matériaux, B.P. 32, El Alia, Bab Ezzouar, Alger, Algeria 2

Centre de Développement des Technologies Avancées (CDTA), Cité 20 Août 1956, B.P. 17, Baba

Hassan, Alger, Algeria. 3

Université des Sciences et des Technologies Houari Boumediene (USTHB), Laboratoire de Sciences

Nucléaires et Interactions Rayonnements-Matière, B.P. 32, El Alia, Bab Ezzouar, Alger, Algeria 4

FEMTO-ST, Time and Frequency Department, Chemin de l’Epitaphe 27, 25000 Besançon, France

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Institut FEMTO-ST, UMR CNRS 6174, Department MN2S, 26, Chemin de l'Epitaphe, 25030

Besançon cedex, France 6

Centre de Recherche Nucléaire d’Alger 2 Bd Frantz Fanon, B.P. 399 Algiers, Algeria

*

Corresponding author:

E-mail address: [email protected]

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Highlights  One-step synthesis of hydrogenated graphene by pulsed laser deposition.  The substrate temperature rise induce a structural change from hydrogenated amorphous carbon toward hydrogenated graphene.  The ERDA analysis shows a decrease in the hydrogen amount in the synthesized thin films.  The XPS reveals the amount of the Csp2 and Csp3 along with the substrate temperature increase.

Abstract A direct synthesis of hydrogenated graphene at a reduced temperature of 400° C using pulsed laser ablation technique is reported. Here, we have investigated the effect of the substrate temperature. Therefore, hydrogenated carbon thin films were elaborated under the same conditions of laser fluence and CH4 pressure but at different substrate temperatures ranging from room temperature to 600°C. The Raman and X-ray photoelectron spectroscopy revealed a structural change induced by the substrate temperature increase. Indeed the elaborated thin films evolved from amorphous carbon to hydrogenated graphene as the substrate temperature increases. The elastic recoiled detection analysis and the Rutherford backscattering measurements indicated a decrease in the hydrogen content as well as a thickness reduction along with the substrate temperature increase. We have concluded that the structural change can be ascribed to the increase in the species mobility during the growth. Keywords: Pulsed laser deposition, hydrogenated amorphous carbon, hydrogenated graphene, Raman spectroscopy, X-ray photoelectrons spectroscopy.

1. Introduction Hydrogenated amorphous carbon thin films a-C:H are used in several field of applications in mechanical and optical coatings, electronic devices, field emitters and biomedical devices [1, 2]. This wide range of applications is due to their attractive properties. Indeed, a-C:H thin films present high hardness, chemical inertness, very low electrical conductivity, optical transparency over a wide spectral range and biocompatibility [1, 3, 4]. It has been shown that the properties of a-C:H depend on both of the sp2: sp3 ratio and the hydrogen content in the films [5, 6] [7]. Furthermore, the amount of hydrogen incorporated strongly depends on the 2

deposition technique and the parameters used. Hydrogenated amorphous carbon have been synthesized by several techniques among them pulsed laser deposition (PLD).This wellestablished technique can provides high energy particles. Moreover, when the ablation is carried out in hydrocarbon reactive ambiance, the particles interact with the background gas forming CnHm radicals that condense on a substrate[8]. Together with the increase of the substrate temperature during the a-C:H synthesis may lead to an arranged carbon structure, which is graphane. Graphane can be the alternative for electronics application where the graphene has found its limits. Graphene is a two dimensional (2D) carbon based material packed in a honeycomb lattice. It was discovered by Geim and Novoslov in 2004 [9]. This 2D material is a zerooverlap semimetals, tuning his electronics properties by opening a band gap would enhance his applications in electronics devices [10]. One way to achieve this is the hydrogenation of graphene, which has led to the graphane. The Graphane is a graphene layer bonded to hydrogen atom. In graphane, the carbon atoms are in sp3 hybridization. Graphane exhibits interesting semiconductor properties that make it the material for the next generation electronics devices for instance as transistors.[11] It was predicted theoretically by Sofo et al. in 2007 [12]. Experimentally, the hydrogenation of graphene was achieved first by Elias et al. [13] by exposing graphene layer prepared by micromechanical cleavage to low pressure hydrogen plasma. In their study, they have shown the change induced by the hydrogenation of the graphene in the electronic properties. Moreover, this change is reversible with temperature. Poh et al. [14] have succeeded in preparation of hydrogenated graphene by thermal exfoliation of graphite oxide in high hydrogen gas and high temperature atmosphere. In their work, those authors have set the optimal condition leading to the synthesis of graphane by investigating the effect of the temperature at a fixed hydrogen gas pressure and vice versa. Hydrogenated graphene layers was also elaborated by Ray et al. [15] by annealing a hydrogenated diamond-like carbon (HDLC) films in vacuum at a fixed temperature of 1050° C during 30 min. In the above works, the hydrogenation is achieved in two steps; the preparation of the material (graphene or HDLC) and then the hydrogenation or the annealing. Here we report a direct synthesis of hydrogenated graphene at a reduced temperature of 400° C using pulsed laser ablation technique. The effect of the substrate temperature on the hydrogenated graphene synthesis is reported.

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2. Experimental The thin films were synthesized on silicon (100) substrate by PLD using a KrF laser (248 nm, 25 ns, 5 Hz) at an incident angle of 45°. A rotating graphite target of 99.99% purity was ablated with a laser fluence of 12 J/cm2 during 15 min. The substrates were cleaned ultrasonically in a bath of acetone and methanol respectively then pasted on a heated substrates holder placed at a distance of 4 cm from the target. Prior to deposition, the vacuum chamber was initially evacuated at a base pressure of 1.5×10-5 mbar and the deposition process take place in methane atmosphere at a fixed pressure of 0.3 mbar. To investigate the effect of the temperature, the substrate holder has been heated at varying temperature starting from room temperature to 600°C. The Rutherford backscattering (RBS) and elastic recoiled detection analysis (ERDA) measurements were used to quantify the amount of carbon and hydrogen in the thin films. The backscattered particles were detected at 160° and the recoiling hydrogen at a scattering angle of 20°, with respect to the incident beam direction. The He+ incident energy was kept at 2 MeV and RBS and ERDA spectra were collected simultaneously. The Mylar foil placed in front of the ERDA detector stops the scattered He+ particles and only recoiling hydrogen from the sample reaches the detector. The amount of carbon and hydrogen is deduced from the simulation of the RBS and ERDA spectra respectively. This simulation is achieved using the SIMNRA [16] program. Note that the parameters used for the simulation must fit at the same time both the RBS and ERDA spectra.

The structure of the different elaborated thin films was investigated with micro-Raman spectroscopy. The Raman spectra were recorded at room temperature with a Jobin-Yvon spectrophotometer. The power is kept at 2 mW to avoid heating. The excitation energy is HeNe 1.96 eV (wavelength 632.8 nm). In order to drawn more information from the Raman spectra, the PL base line was subtracted and a deconvolution with Gaussian functions was performed as reported in references [17, 18]. The G peak position and the ID/IG ratio were estimated. The Raman spectroscopy provides bonding information of the whole film thickness. Furthermore, when the measurement is performed with visible light, the Raman scattering is more sensitive to the C sp2 since Csp3 has much smaller cross-section than Csp2 [1].

The carbon hybridizations are controlled by the X-ray photoelectrons (XPS) technique. The XPS measurements were carried out at room temperature in a Multilab 2000-Thermo 4

Scientific spectrometer. The different spectra were recorded with a non-monochromatized Al Kα source at an incident angle of 45° with the sample surface. The decomposition of the C 1s peak of the different spectra is carried out with Avantage software. The decomposition was achieved considering four contributions C-O , C=O , Csp2 and Csp3 with a constant difference binding energy between sp2 and sp3 contributions [C1s BE(sp3) - BE(sp2) = 0.77 eV], as reported by Zebda et al. [19] The XPS technique was used to probe the C1s core level of all carbon atoms located within a few nm at the layer surface. It is worthwhile to mention that the Raman and XPS results are not directly comparable but complementary to further study the films structure.

3. Results and Discussion When the PLD is carried out in the presence of reactive background gas, physicochemical processes are involved inducing collisions and chemical reactions between the plasma species and the background gas molecules. As reported by Budai et al.[8], when the a-C:H films are synthesized at room temperature in reactive methane atmosphere, the interaction of the plasma species with the background gas results in the formation of CnHm radicals. Moreover, the variation of the CH4 pressure influences the films properties. At low pressures (p ≤ 1Pa), tetrahedral amorphous carbon is produced while at a pressure of 30 Pa polymer-like carbon films are synthesized, using ArF excimer laser at a fluence of 8J/cm2.

3.1. ERDA/RBS Analysis We have performed The RBS/ERDA analysis were performed on the samples deposited at three different substrate temperatures. We have chosen The choice of those specific temperatures (RT, 400°C, 600°C) according to a structural change observed by the Raman spectroscopy, as we it will be seen further. Fig. 1a shows the RBS spectra recorded by the detector on which an oxygen contamination of the samples is seen. This contamination could occur during or after the synthesis of the thin films. The hydrogenated carbon thin films thickness represents the Si-edge shift. From Fig. 1a it is noticed that the substrate temperature increase induces a film thickness decrease. Fig. 1b represents the ERDA spectra.

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The amount of carbon and hydrogen is deduced from the simulation of the RBS and ERDA spectra respectively. This simulation is achieved using the SIMNRA [16] program with The simultaneous simulation of RBS/ERDA spectra was achieved considering a film structure composed of two layers, a bulk hydrogen-depleted layer (Layer 2 of the Table 1) capped by a hydrogen-rich (Layer 1 of the Table 1) overlayer. Note that the parameters used for the simulation must fit at the same time both the RBS and ERDA spectra. Fig. 2 represents an example of RBS/ERDA spectra simulation, the areal densities of carbon and hydrogen are reported in the Table 1. The two layers structure is in good coherence with the Robertson model [20] of diamond/graphite/Si. This model has the minimal lattice mismatch and interfacial energy when diamond is grown on Si substrate. On the other hand, the two layers structure may be explained by the growth mechanism. Indeed, when a plasma deposition technique is used for the growth of hydrogenated carbon two processes are involved [2]. The first process is the penetration of the energetic ionized or neutral carbon in the film according to the subplantation model[21].The second process is the preferential sputtering of the hydrogen [22], thus, the penetration of carbon ions in the film cause the dehydrogenation of the C-H bonds leaving a layer with only carbon. Moreover, from Table 1 it is observed that the hydrogen amount in the films decreases when the temperature increases. This decrease can be ascribed to the exo-diffusion of the hydrogen induced by the substrate temperature increase [23]. 3.2. Raman spectroscopy On Fig. 3 is plotted the Raman spectra of the hydrogenated carbon thin films deposited at different substrate temperatures. These spectra reveal the presence of photoluminescence (PL) background that decreases along with the increase of the substrate temperature. The PL is due to recombination of electron hole pairs within sp2 bonded clusters in an sp3 bonded hydrogenated carbon matrix [15, 24]. This reduction in the PL background at relatively high substrate temperatures suggests a decrease in the hydrogen content, which is confirmed by the ERDA measurements. Indeed, Bousa et al [25] have observed this PL background in the Raman spectra for the hydrogenated graphene. Moreover, those authors found that the PL background depends on

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the degree of hydrogenation since highly hydrogenated graphene exhibits strong PL background. When the substrate temperature increases from room temperature to 200°C, a typical Raman spectrum of a hydrogenated carbon thin film is observed. It shows an asymmetrical broad G peak with an obscure D peak shoulder [26]. The D and G peaks are situated around 1330 cm-1 and 1530 cm-1 respectively. The D peak is attributed to the breathing modes of sp2 atoms in rings whereas the G peak corresponds to the bond stretching of all pairs of sp2 atoms both in rings and in chains [27]. The low position of the G peak for the a-C:H deposited at room temperature and 100°C can be related to the degree of the bond disorder and the presence of nonsixfold rings in the films[28]. When the substrate temperature increases to 400°C and higher, the Raman spectra obtained present a significant change. They evolve from Raman spectra of amorphous hydrogenated carbon to that of hydrogenated graphene. The ordered structure observed at 400°C may originate from the hydrogenate top layer. Indeed, the film elaborated in the same conditions (PLD, 400°C) but in vacuum presents a different structure as it can be seen from the insert of Fig. 3 d. Ray et al. [15] reported the same observation the synthesis of hydrogenated graphene when they annealed a hydrogenated carbon thin film at 1050° C. In Fig. 3 (d and e), the spectra exhibit a well separated D and G peaks situated around 1320 cm-1 and 1590 cm-1 respectively. This separation is ascribed to the clustering of the aromatic rings leading to the crystallization of the amorphous phase [29]. In addition, the spectra reveal two more peaks 2D and (D+G). The 2D peak is situated around 2650 cm-1 and is the graphene signature; it is the second order of zone-boundary phonons [30]. The (D+G) or S3 peak situated around 2915 cm-1 is the combination of D and G modes [31]. In order to drawn more information from the Raman spectra, the PL base line was subtracted and a deconvolution with Gaussian functions was performed as reported in references [29,30]. The G peak position and the ID/IG ratio were estimated. In the range of low deposition temperatures below 400 ° C, the ID/IG ratio increases indicating an increase of the Csp2 in rings. Furthermore for this range of temperature, a shift toward the higher wavenumber of the D and G peaks is observed along with the substrate temperature increase. The evolution of the ID/IG ratio and the position of the G peak with the substrate temperature can be used for the interpretation of the Raman spectra. Thus according to the amorphization trajectory model proposed by Ferrari et al. [32],we can say that our thin films evolved from

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amorphous to nanocrystalline structure when the substrate temperature increases from room temperature to 400°C. At a substrate temperature of 400°C and above, the D and G became sharper and the ID/IG can be used to estimate the crystallite size La according to equation (1) [33]. I L a (nm)  2.4  10 10   4laser  G ID

(1)

Where IG and ID are the intensities of the G and D bands,  laser is the wavelength of the laser in nm. A decrease of 10 nm in La is found when the substrate temperature increases from 400°C to 600°C, thus indicating the presence of more defects. Those defects could be due to the release of the hydrogen from the film leaving dangling bonds. Elias et al. [13] have observed a structural disorder induced by the dehydrogenation of a graphene by annealing the sample at 450°C in Ar atmosphere during 24 hours. Furthermore, the spectrum of the film deposited at 600°C presents a D peak with higher intensity that can be due to the conversion of the sp3 bond to sp2 as well as to desorption of the hydrogen [26]. 3.3. XPS analysis The X-ray photoelectron spectroscopy probes the chemical state of the surface of the material. The XPS survey spectra of the hydrogenated carbon thin films and hydrogenated graphene films show the presence of carbon C 1s peak in addition to the contamination oxygen O peak. The inset figure in fig. 4 represents an XPS survey of a film deposited at 400°C. A particular attention is paid to the C 1s peak represented in Fig. 4 It can be observed from Fig. 4a, that when the substrate temperature increases from room temperature to 400° C the C 1s peak width narrowed and its position slightly shifted toward the highest binding energy (BE). This slight shift is ascribed to the increase of the oxygen contamination in the films. When the substrate temperature keep increasing to 600°C, the C 1s peak width narrowed became asymmetric and its position shifts toward the lowest BE, this behavior can be due to the increase of the sp3 as we will see later. The decomposition of the C 1s peak of the different spectra is carried out with Avantage software. The decomposition was achieved considering four contributions C-O , C=O , Csp2 and Csp3 with a constant difference binding energy between sp2 and sp3 contributions [C1s BE(sp3) - BE(sp2) = 0.77 eV], as reported by Zebda et al.[33] 8

The results of the decomposition are represented on Fig. 4 b-d. Those results show the C sp2 and C sp3 peaks located at the lowest and the highest BE respectively. The C-O and C=O peaks situated respectively at 286.4 eV and 288.2 eV are due to the contamination of the thin films by the oxygen during or after the synthesis. A quantitative analysis of the C sp2 and C sp3 can be drawn from their XPS peaks obtained from spectra decomposition. The area of each peak represents their amount in the thin film. Table 2 summarize the results of the XPS spectra decomposition. From Table 2, it is observed that the C1s peak of the films deposited at room temperature and 400°C is mostly due the C-O bond. Rising the substrate temperature induced the desorption of the oxygen contamination leaving a clean surface with high amount of Csp3. This result does not match with the Raman analysis where a graphitization is observed at high temperature (600°C).This mismatch is due to the fact that the XPS characterizes a few nm at the external surface while the Raman spectroscopy investigates the whole film thickness. Furthermore, the visible Raman is more sensitive to the vibration of Csp2 than Csp3 so even a few amount of Csp2 will contribute considerably in the Raman signal. Pulsed laser deposition has been attracted a lot of interest for the synthesis of graphene layers this last decade. Indeed, each laser shot provides a nanostructure carbon species that condensed on a substrate placed in front of the target [31, 34, 35]. Here we have investigated the effect of the substrate temperature on the synthesis of hydrogenated graphene layer in a methane atmosphere. From, the different characterizations we have performed, we propose an explanation attempt. The ERDA/RBS measurements indicated a thickness decrease as well as a hydrogen amount decrease along with the substrate temperature increase The substrate temperature increase induced a structural change clearly evidenced by the Raman spectroscopy. This change can be explained by the increase of the arriving species mobility at the substrate surface during the synthesis. Indeed, at room temperature the arriving species have no sufficient energy to diffuse on the substrate surface to find a thermodynamically stable site. The result is an amorphous structure. Rising the substrate temperature, increases the mobility of the arriving carbon species. Thus, the carbon species have sufficient energy to diffuse on the substrate surface and rearrange to form a hydrogenated graphene domain. As seen from the features of the Raman spectrum. At higher substrate temperature, the quality of the hydrogenated graphene drops. This can be due to the graphitization of the films as well as to the release of the hydrogen.

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Conclusion In summary, we have elaborated hydrogenated graphene thin film by pulsed laser deposition technique in one-step. We have investigated the effect of the substrate temperature on the thin films synthesis. The structural characterization revealed the evolution from hydrogenated amorphous carbon thin films to hydrogenated graphene thin film. We have attributed this structural change to the increase of the arriving species mobility during the synthesis.

Acknowledgment We would like to thank Kamel MIROUH from Laboratoire de couches minces et interfaces, Département de Physique, Université Mentouri Constantine-1-,Constantine, Guillaume HERLEM and Fabien PICAUD from Nanomedicine Lab , Université de FRANCHE-COMTE, Besançon for their valuable help.

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Figures caption: Fig. 1 (a) RBS (b) ERDA spectra of hydrogenated carbon thin films deposited at different substrate temperatures. Fig. 2 (a) RBS (b) ERDA spectra simulation of hydrogenated carbon thin film deposited at a substrate temperatures of 400°C. Fig. 3 Raman spectra of hydrogenated carbon thin films deposited at different substrate temperatures (a) room temperature, (b) 100°C, (c) 200°C, (d) 400°C and (e) 600°C. Fig. 4 XPS spectra of hydrogenated carbon thin films deposited at (a) different substrate temperatures (b) room temperature (c) 400°C (d) 600°C. Table caption: Table 1: RBS/ERDA quantification results Table 2: XPS decomposition results

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700

800

900

Binding energy (eV)

282 283 284 285 286 287 288 289 290 291 292 293

Binding Energy (eV) 4800 (b) RT 2 Csp 3 Csp C-O C=O

4600

Count/s

4400 4200 4000 3800 3600

282 283 284 285 286 287 288 289 290 291 292 293

Binding energy (eV)

6

4000 (c) 400°C 2 C sp 3 C sp C-O C=O

3800

Count/s

3600 3400 3200 3000 2800 282

283

284

285

286

287

288

289

290

291

292

293

Binding energy (eV) 5000 (d) 600°C 2 Csp 3 Csp C-O C=O

4800 4600

Count/s

4400 4200 4000 3800 3600

3400 282 283 284 285 286 287 288 289 290 291 292 293

Binding energy (eV) FIG. 4. XPS spectra of hydrogenated carbon thin films deposited at (a) different substrate temperatures (b) room temperature (c) 400°C (d) 600°C. 7

Tables captions Table 1 RBS/ERDA quantification results

RT

400°C

600°C

Thickness H

Thickness C

Total thickness

(E15 at./cm2)

(E15 at./cm2)

(E15 at./cm2)

Layer 1

104

905

1040

Layer 2

3

482

500

Layer 1

57

470

547

Layer 2

2

260

264

Layer 1

27

373

413

Layer 2

-

105

150

Table 2 XPS decomposition results Csp2

Csp3

C-O

C=O

(At. %)

(At. %)

(At. %)

(At. %)

RT

5.1

13.7

63.3

17.7

400°C

9.0

6.9

65.2

18.7

600°C

2.8

57.0

27.0

13.0

14