Physics Procedia 32 ( 2012 ) 696 – 705

1875-3892 © 2012 Published by Elsevier B.V. Selection and/or peer review under responsibility of Chinese Vacuum Society (CVS).doi: 10.1016/j.phpro.2012.03.620

Researches of Mechanical properties and Biocompatibility on Nanocrystalline Diamond

J. M. Hunga, C. Z. Wua, K. L. Oub,c,d**, C. M. Liub,e,f, P. W. Pengb,c,g,* aSchool of Dentistry, Taipei Medical University, Taipei 110,Taiwan

bResearch Center for Biomedical Implants and Microsurgery Devices, Taipei Medical University, Taipei 110, Taiwan cResearch Center for Biomedical Devices, Taipei Medical University, Taipei 110, Taiwan

dGraduate Institute of Biomedical Materials and Engineering, Taipei Medical University, Taipei 110, Taiwan eDepartment of Chemical and Material Engineering, LungHwa University of Science and Technology, Taoyuan 306, Taiwan

fGraduate School of Engineering Technology, LungHwa University of Science and Technology, Taoyuan 306, Taiwan gSchool of Dental Technology, Taipei Medical University, Taipei 110, Taiwan

Abstract

Nanocrystalline diamond (NCD) films were synthesized using microwave plasma chemical vapor deposition procedure with the CH4/Ar/H2 gas mixture on silicon substrate at moderate temperatures in the present study. The characteristics of NCD films were evaluated using scanning electron microscopy, Raman spectroscopy, transmission electron microscopy, optical emission spectroscopy and optical contact angle meter. The analytical results revealed that C2 radial was the dominant species in the deposited process. From TEM observation the NCD films was formed via the etching of hydrocarbons and a small amount of hydrogen content additive into gas mixture could improve the aggregation of the nucleation film to form the NCD films. A (crystalline amorphous-like nanocrystalline structure) transition occurred during formation of NCD films. The more hydrophobic surfaces implied that NCD films were the potential biomaterial in the application of article heart valve or stent.

© 2009 Published by Elsevier B.V.

PACS: Type pacs here, separated by semicolons ; Keywords: nanocrystalline diamond; microwave plasma; chemical vapor deposition; hydrogen

1. Introduction

Nanocrystalline diamond (NCD) films possess marvelous physical and chemical properties, such as high hardness, negative electron affinity, marvelous thermal, optical and chemical properties [2, 3]. Therefore, they were great potential materials for the application fields of protective coating, field emission devices and cutting tools [3, 4]. Recent studies focused on their bio-inertness and non-cytotoxicity attracting more and more interests in biomedical application [5-8].

NCD films are usually synthesized using a chemical vapor deposition (CVD) concerning the decomposition procedure of gas mixture. The gas decomposition can be activated by a hot metal filament, combustion flame or microwave plasma discharge [9, 10]. Recently, microwave plasma chemical vapor deposition (CVD) has been widely employed for synthesis of NCD films, which adopts Ar/CH4 gas mixture [11]. Avoiding instability process

* Corresponding author. ** Co-corresponding author. Tel.: 886-2-27361661 ext.5140 E-mail address:apon@tmu.edu.tw.

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© 2012 Published by Elsevier B.V. Selection and/or peer review under responsibility of Chinese Vacuum Society (CVS). Open access under CC BY-NC-ND license.

Open access under CC BY-NC-ND license.

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and deposit contamination, molecular hydrogen is added to achieve a pronounced improvement in grain size and surface roughness [12, 13].

Although it is generally evidenced that the microstructures and properties of NCD films are influenced by varying the primary composition of the Ar/H2 ratio [14, 15], the fundamental mechanism of the NCD film deposition is still quite debatable. Zhou et al [16] reported that C2 radical was a significant role in the growth mechanism for NCD film, while some investigations suggested that CH3 and C2H species might be the growth precursors more important than C2 [15, 17]. The effect of the constituents of H2/Ar/CH4 gas mixtures on the formation of NCD films is still under systemic discussion.

In the present study, NCD films were synthesized using microwave plasma CVD procedure with the CH4/Ar/H2 gas mixture on silicon substrate at moderate temperatures. The effects of varying the hydrogen content on plasma composition, and characteristics and crystallite size of NCD films were studied and discussed. The growth mechanism of NCD formation was also be elucidated.

2. Experimental Procedures

The plasma ball was generated for diamond growth using ASTeX microwave plasma equipment with a quartz tube reactor. NCD films were synthesized on nucleated silicon wafers prepared by ultrasonic alcohol bath with diamond powders of 1 m for 30min. During NCD deposition, the varying amount of gas mixture containing CH4, H2 and Ar was fed into the reactor with a microwave power of 800 W at constant pressure of 100 Torr for 2h. The concentration of CH4 was retained constant (1 %), whereas Ar flow rate was altered with the different H2 addition to maintain the total gas flow rate at 50 sccm. The varying concentrations of hydrogen were 0, 5, 15, 25 and were denoted as D0, D5, D15 and D25, respectively.

The resulting spherical plasma was detected by optical emission spectroscopy (OES). Raman spectrometer (JOBIN-YVON T64000) was used to evaluate the quality and the purity of NCD films. The microstructures and grain sizes were observed by scanning electron microscopy (SEM, JEOL-7500). The surface roughness was performed with an atomic force microscopy (AFM) in the tapping mode. The nanocrystalline microstructure of NCD films was obtained by high-resolution transmission electron microscopy (HRTEM, JEOL-80). The degree of wettability of NCD films was utilized to examine the interaction between proteins and deposited films with the sessile drop method using a CAM 200 optical contact angle meter.

3. Results and Discussions

Fig. 1 illustrated the surface morphologies of diamond films synthesized by microwave plasma with different H2 additive in 1 % CH4 diluted by Ar. The ordinary polycrystalline diamond film composed of well-facetted surface emerged in the case of 25 % H2 addition as shown in Fig. 1(a). The microstructure of diamond films change extremely with decreasing the concentration of hydrogen. In the case of D5, the faceted crystals disappeared and the diamond clusters composing of nano-sized particles were deposited as shown in Fig. 1(b).

Although the optical emission spectroscopy could not provide a quantitative information, the emission spectra was still carried out to confirm the relationship between the change of C2 radical density and the formation of NCD films under H2/ Ar/ CH4 gas mixture as shown in Fig. 2. From the optical emission spectra, the quality of diamond film was strong correlation with the intensities of carbon dimmers and atomic hydrogen. The optical emission from C2 Swan band (know as 619.1, 536.6, 516.5, 473.7, and 438.3 nm) were dominant accompanying the strong peak at 387.2 nm which was featured as CH emission in the case of D5. CH radicals usually responded to trans-polyactylene composition in the grain boundaries of NCD films [3]. The intensities of C2 radicals and CH decreased and with the increase of hydrogen concentration. Even though methyl was the important depositing species in the formation of normal diamond films, C2 radicals were regarded as a critical species for secondary nucleation enhancement and nanocrystalline diamond formation [18]. The renucleation rate of the NCD film growth was influenced by C2 [22]. Depositions of NCD films resulted from high secondary nsucleation rates in hydrogen-poor environment were observed from SEM images. The main reaction of C2 radicals production was C2H2+ Ar C2+ H2+ Ar [3]. In the hydrogen-rich environment, C2 radicals might be lost via C2+ H2 C2H+ H [23]. Based on the above explanations, the intensity of C2 radicals decreased with the hydrogen addition.

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Raman spectra from deposited films as a function of H2 flow ratio were shown in Fig. 3. It was evident that the ratio of H2 to Ar strongly influenced the spectra of NCD films. The sharp peak in sp3-diamond bonding centered at 1332 cm-1 [12] was observed in the case of D25. The diamond band became broaden as the hydrogen concentration decreased. In the case of D15, the little Raman scattering at 1332 cm-1 illustrated the primarily microcrystalline diamond was still present. Diamond band at 1332 cm-1 was so broadened to identify in the case of D10. In the mean time, the peak centered at 1138 cm-1 was more clearly observed which was ascribed to trans-polyacetylene [1]. This Raman shift was resulted from the size effect of nano-scale grains or disorder in the tetrahedrally bonded carbon network, providing the evidence that NCD films was formed [18, 19]. All specimens were revealed the clear peaks at 1354 and 1587 cm-1 assigned for D and G band of disordered sp2-bonded carbon, revealing stretching modes of the -bonded amorphous carbon was present at the grain boundaries in the nanocrystalline films [20]. Based on the results of SEM images and Raman spectra in this study, the microcrystalline structures were observed with sharp sp3-bonding related peak at 1332 cm-1 at the concentration of hydrogen higher than 15 %. On the other hand, in the case of D15, the nanocrystalline diamond films were featured with peaks located at 1138, 1354 and 1587 cm-1. These observations were similar to Ref. [12] and [21], that microstructure of the diamond films was transited from microcrystalline structure to nanocrystalline structure as the concentration of hydrogen decreased.

The further investigation by TEM images were conducted to obtain the informations on the mechanism of the diamond growth as show in Fig. 4. The continuous nuclei film was sequentially formed as shown in area A of Fig. 4. The selected area diffraction pattern (SADP) taken form area A, consisting of diffraction rings and discrete diffraction spots was shown in the bottom left corner. The SADP taken form area B, consisting of diffraction rings, was shown on the right top corner. These ring patterns demonstrated that the nanocrystalline diamond formed on the nucleation film. Tang et al reported that high nucleation barrier needed to be overcome to create a nucleus by abrading the substrates with diamond pastes [24]. The nucleation barrier could not remove efficiently in the gas mixture without hydrogen. Therefore, the discrete diamond islands were observed on the substrate. Little hydrogen additive in Ar/CH4 gas mixture efficiently etched the nucleation barriers on the silicon substrate, and accelerated the discrete nuclei aggregation. As stated above, the structures transition occurred from crystalline to amorphous-like and then turn to nanocrystalline.

The wettability of NCD films were measured by the contact angle of water and displayed in Fig. 5. It could be found NCD film possessed a highly hydrophobic surface. Hydrogen addition was enhanced the hydrophobic property. The contact angle decreased as the concentration of hydrogen was higher than 15%. These phenomena occurred because the surface sp2-bond carbon and hydrogen inclusion decreased the surface energy and gave more hydrophobic films [25]. Furthermore, the nanocrystalline structure improved the hydrophobic property. Surface characteristics of anti-coagulating material were based on the interface energy, surface charge and surface micro-heterogenous domains [26]. Surface wettability influenced the degree of conformational changes of the adsorbed proteins and then affected surface blood compatibility. Based on the above reasons, NCD films were the potential biomaterial in the application of article heart valve or stent.

4. Conclusion

The influences of hydrogen concentration on the information of nanocrystalline diamond were investigated in the present study. NCD films could be deposited in the case of 5 % hydrogen additive in the CH4/Ar gas mixture. C2 radial was the dominant species in the deposited process. From TEM observation the NCD films was formed via the etching of hydrocarbons onto the diamond clusters and a small amount of hydrogen content could improve the aggregation of the NCD films. A (crystalline amorphous-like nanocrystalline structure) transition occurred during formation of NCD films. The surface energy and wettability of NCD films could be controlled. The more hydrophobic surfaces implied that NCD films were the potential biomaterial in the application of article heart valve or stent.

5. Acknowledgements

The authors highly appreciated Wei-Hung Chang for participation in experiments. We highly acknowledge financial support of research by Center of Excellence for Clinical Trial and Research in Neurology and Neurosurgery, Taipei Medical University-Wan Fang Hospital under the grand number of DOH99-TD-B-111-003.

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Figure Captions

Fig. 1. SEM images of the diamond film under condition of (a) D25, and (b) D5 Fig. 2. Raman shift of the diamond film with various CH4/Ar/H2 gas mixtures Fig. 3. Optical emission spectra of the diamond film with various CH4/Ar/H2 gas mixtures Fig. 4. Cross-section TEM image of the NCD film in the case of D5 Fig. 5. The contact angle measurements with various hydrogen content.

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Fig. 1(a) K.L.Ou

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Fig. 1(b) K.L.Ou

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Fig. 2 K.L.Ou

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Fig. 3 K.L.Ou

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Fig. 4 K.L.Ou

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Fig. 5 K.L.Ou