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nanosilica composite films by an electrochemical method
Electrochemistry Communications 6 (2004) 1159–1162 www.elsevier.com/locate/elecom
Synthesis of diamond-like carbon/nanosilica composite films by an electrochemical method Xing-bin Yan a
a,b
, Tao Xu a, Gang Chen
a,b
, Qun-ji Xue a, Sheng-rong Yang
a,*
State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, China b Graduate School of the Chinese Academy of Sciences, Beijing 100083, China Received 16 August 2004; received in revised form 7 September 2004; accepted 9 September 2004 Available online 29 September 2004
Abstract Diamond-like carbon/nanosilica composite films have been deposited on silicon substrates, making use of the electrolysis of methanol–dimethylethoxydisilane (DDS) solution at low temperature. The electrodeposited composite films were characterized by Raman spectroscopy, X-ray photoelectron spectroscopy and transmission electron microscopy, respectively. Moreover, the growth mechanism of the composite films in liquid phase was discussed as well. As the results, the films are diamond-like carbon films containing polycrystalline SiO2 nanoparticles. The introduction of DDS contributes to promote the growth of the carbon films and leads to the formation of the crystalline SiO2 grains. 2004 Elsevier B.V. All rights reserved. Keywords: Diamond-like carbon; Electrochemical deposition; Composite films; Silica; Nanoparticles
1. Introduction Extensive interests have been focused on the diamond-like carbon (DLC) that is considered as promising material for electronic, optical, and wear protection applications, owing to their similarities of properties to those of diamond. These similarities ensure superior features such as excellent chemical inertness, extremely high hardness, very low friction coefficients, atomic-level smoothness, good optical transparency, and high thermal conductivity [1,2]. However, a high level of residual compressive stress of the DLC films has strongly limited the practical applications of these materials. The residual compressive stress leads to a reduction in the film/ substrate bonding strength and fracture and delamination of the films at a high contact load. In order to reduce the film stress without sacrificing the hardness, many efforts have been made to improve the nanostruc*
Corresponding author. Fax: +86 931 8277088. E-mail address: [email protected] (X.-b. Yan).
1388-2481/$ - see front matter 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.elecom.2004.09.012
ture of the DLC films. With a view to the DLC films of modified nanostructures, two particular classes of modified DLC composite films, that is, the diamond-like nanocomposite (DLN) films composed of an amorphous carbon network (a-C:H) and amorphous silica network (a-Si:O) [3,4], and the DLC nanocomposite films composed of an amorphous carbon matrix and SiC, Si3N4, and SiOx (x 6 2) nanoparticles [1] are of a great significance. These composite films possess excellent adhesion to various substrate materials (i.e., metals, plastics, ceramics, semiconductors, etc.), high hardness, low internal residual stress, excellent thermal stability, and excellent wear resistance. Thus they have significant advances over the conventional DLC films. These composite films can be deposited using various chemical and physical vapor deposition techniques such as ion beam assisted deposition [5], multicascade remote plasma vacuum deposition [6], inductively coupled plasma CVD, etc., [1]. However, their applications have been limited owing to the complicated and costly equipment and rigorous preparation conditions.
X.-b. Yan et al. / Electrochemistry Communications 6 (2004) 1159–1162
2. Experimental A simple electrolytic cell system similar to that reported in [7] was used to prepare the DLC/nanosilica composite films. The silicon (1 0 0) substrate with a sheet resistance about 7–13 X/cm2 was mounted on the negative electrode. A graphite plate was used as the counter electrode and kept 6 mm away from the negative electrode. DDS ((CH3)2–Si–(OC2H5)2) was dissolved in analytically pure methanol (CH3OH), with the volume ratio of DDS to methanol to be 1:4. The cleaned substrates were placed into the above solution, with an area of 1.0 · 1.5 cm2 to be immerged therein, to allow the deposition of the composite films at a temperature of (50 ± 2) C, a DC power-supply voltage of 1200 V, and for deposition duration of 5 h. As compared, the pure carbon films were deposited by the electrolysis of methanol at the same conditions. The micro-Raman backscattering spectra of the films were recorded on a Jobin Yvon T64000 spectrometer, operating with a 514.5-nm Ar laser as the excitation source. X-ray photoelectron spectroscopic (XPS) measurements were performed on a Perkin–Elmer PHI5702 system to investigate the chemical composition of the films. The film structure was characterized on a JEM-1200EX transmission electron microscopic (TEM) at an accelerating voltage of 100 kV.
The nano-hardness of the DLC/nanosilica composite film is also much larger (about 12 GPa) than that (about 8 GPa) of the pure carbon film, measured by nanoindentation. The hydrogen content of the pure carbon film and the carbon composite film is close to 30% and 24%, respectively, measured by an elemental analyzer. The Raman spectra of the pure carbon film and of the composite film are shown in Fig. 1(a) and (b), respectively. The both Raman spectra can be classified into three regimes: the D peak, the G peak, and the peak around 1470 cm 1 which may be assigned to the symmetrical deformation frequency of C–CH with sp2 hybridized C–C bonding [11] or a sp3-bonded diamond precursor phase [10,12]. In spectrum of pure carbon film, the position of the D and G peaks is 1340 and 1582 cm 1, respectively. While the position of the D and G peaks is 1338 and 1577 cm 1, respectively, in spectrum of carbon/nanosilica film. Furthermore, it is seen that the relative intensity ratio of the D peak to G peak (ID/IG) of the carbon/nanosilica composite film (about 0.8) is considerably smaller than that of the pure carbon film (about 1.7), and the G peak of the composite film shifts to a lower wave number and the full width at half maximum (FWHM) of the G peak is also larger than that of the pure carbon film. Ferrari et al. [13] thought that the D peak would decrease with increasing disorder in amorphous carbon, that the width of the G peak was proportional to the bond-angle disorder at sp2 sites, and that the mixing with sp3 modes would help to lower the G peak in hydrogenated amorphous carbon (a-C:H). They also found the relationships between visible Raman spectra (514-nm) and the sp3 fraction in
(a)
Intensity (a.u.)
Recently, much attention has been paid to the deposition of DLC films by the electrolysis of organic liquids [2,7–10]. It has been found that the liquid deposition techniques have many advantages such as availability for large area deposition on intricate surfaces, low deposition temperature, low consumption of energy, and simplicity of the set up, over the vapor deposition techniques, with a view to the practical use. However, no report has been reported so far on the deposition of DLC nanocomposite films by electrochemical method. In this letter, it has been attempted to prepare the DLC nanocomposite films containing polycrystalline SiO2 nanoparticles, making use of the electrolysis of methanol–dimethylethoxydisilane (DDS) solution. This novel method could provide a new deposition strategy for DLC nanocomposites, making use of electrochemical deposition. Furthermore, the SiO2 nanocrystals incorporated in a suitable carbon matrix may have potential applications in microelectronic devices and optical apparatus.
1100
1200
1300
1400
1500
1600
1700
1800
1600
1700
1800
-1
Raman Shift (cm ) (b)
Intensity (a.u.)
1160
3. Results and discussion 1100
1200
1300
1400
1500 -1
During the deposition for composite film, we found that the current density decreased slightly from 25 mA/cm2 to less than 16 mA/cm2 with deposition time.
Raman Shift (cm ) Fig. 1. Raman spectra of: (a) pure carbon film and (b) DLC/nanosilica composite film.
X.-b. Yan et al. / Electrochemistry Communications 6 (2004) 1159–1162
a-C:H, which was that the sp3 content increased with the simultaneous decrease of G-peak position and ID/ IG ratio in a-C:H [13]. Therefore, the DLC/nanosilica composite films should have higher concentrations of sp3 carbon than the pure carbon films [12,13]. The Si/C ratio in the DLC/nanosilica composite film was measured to be about 0.12 by XPS analysis. Fig. 2 shows the XPS spectra of the Si2p and C1s of the DLC/ nanosilica composite film. It is seen that the Si2p photoelectron spectrum exhibits a main peak at 102.6 eV, which is assigned to Si–O bond [14]. The C1s main peak of the DLC/nanosilica composite film appears at 285.1 eV, which is the same as that of the as-deposited pure carbon film. The C1s spectrum corresponds to the C– C and C–H units and is consistent with that of the DLC films prepared by CVD [15]. The shoulder of the C1s spectrum at a higher-binding-energy side indicates that some carbon atoms are bonded to oxygen. The bright-field TEM image and electron diffraction pattern of the DLC/nanosilica composite film are shown in Fig. 3. It is seen that the nanoparticles are embedded in the amorphous carbon matrix and the nanoparticles have a size of 20–50 nm, while the selected area electron diffraction (SAED) pattern consists of well-marked diffraction rings, which corresponds to the crystalline nature of the nanoparticles. Such an electron diffraction pattern of the composite film is consistent with that of the tetragonal SiO2 phase from JCPDF: 86-2326 (the identification of the electron diffraction rings is given in Table 1). It can thus be concluded that the composite film has an amorphous carbon structure, with a stable SiO2 nanoparticles phase therein. The electrodeposition technique in our experiments is different from traditional electroplating. The molecules of CH3OH and DDS both contain electron-donating methyl group (CH3) and electron-withdrawing groups (OH in methanol and OC2H5 in DDS). The molecules
Si2p
Intensity (a.u.)
C1s
96
98
100 102 104 106
280 282 284 286 288 290 292
Binding Energy (eV) Fig. 2. Core level Si2p and C1s XPS spectra of DLC/nanosilica composite film. The C1s peak is minified (1.8·) as compared to the Si2p peak.
1161
Fig. 3. The bright field TEM image and electron diffraction pattern of the DLC/nanosilica composite film.
Table 1 The experimentally measured and theoretically predicted lattice spacing (d), and the corresponding crystal planes of silicon oxide Experimental Ring
˚) d (A
Theoretical ˚) d (A
(h k l)
1 2 3 4 5 6
2.924 1.964 1.580 1.421 1.246 1.154
2.956 1.980 1.531 1.478 1.235 1.159
(1 1 0) (1 1 1) (2 1 1) (2 2 0) (3 0 1) (3 2 0)
will be polarized under a high voltage. The methyls at the end of their molecules display somewhat positive charge due to their low electro-negativity. Consequently, the methyls in the two molecules are liable to be adsorbed on the negative electrode. On one hand, when the supplied energy reaches a certain value, the C–O covalent bonds in the polarized methanol will be preferentially broken to release methyl group and to allow the formation of DLC film on the surface of the cathode [8– 10]. On the other hand, the DDS molecules consists of C–H, C–Si, Si–O, C–O and C–C bonds which can be ranked according to the bond strength as Si–O (422.5 kJ/mol) > C–H (413.3 kJ/mol) > C–O (359.5 kJ/ mol) > C–C (346.9 kJ/mol) > C–Si (334.7–242.7 kJ/ mol). Therefore, the Si–C, C–C, and C–O bonds will be preferentially dissociated in the presence of a high electric field, while it is more difficult to break the C– H and Si–O bonds. Subsequently, the –O–Si–O– bond tends to form SiO2 phase due to the higher bond strength of Si–O and hence the DLC/nanosilica composite film is generated by the deposition of the SiO2 nanoparticles in the amorphous carbon matrix. It is thus proposed that the growth of the DLC/nanosilica composite film follows both the polarization–adsorption and decomposition–electrochemical reaction processes, which is schematically shown in Fig. 4. It is interesting to note that the DDS molecule contains two equivalent methyl groups, which contributes
1162
X.-b. Yan et al. / Electrochemistry Communications 6 (2004) 1159–1162
Acknowledgements CH 3-OH CH 3
CH 3
polarization
adsorption
Si
OC 2 H 5 OC 2 H 5
SiO 2 nanoparticle Electron
The authors are grateful to the National Natural Science Foundation of China (Grant Nos. 50172052, 50375151 and 50175105), 863 program (Grant No. 2003 AA305670) and ‘‘Top Hundred Talents Program’’ of Chinese Academy of Sciences for financial support.
References decomposition
electrochem ical reactions Composite film Silicon substrate
Fig. 4. Schematic of the two steps involved in the growth of DLC/ nanosilica composite film.
to speed the growth of the carbon films since more methyl active groups are able to react to form carbon network. Consequently, the composite films have higher concentrations of sp3 carbon than the pure amorphous carbon films deposited in methanol.
4. Conclusion In summary, the DLC films containing polycrystalline SiO2 nanoparticles were deposited by the electrolysis of the methanol–DDS solution. The introduction of DDS contributes to promote the growth of the DLC films and leads to the formation of the crystalline SiO2 grains under a high electric field.
[1] L.Y. Chen, F.C.N. Hong, Appl. Phys. Lett. 82 (2003) 3526. [2] E. Shevchenko, E. Matiushenkov, D. Kochubey, D. Sviridov, A. Kokorin, A. Kulak, Chem. Commun. 4 (2001) 317. [3] V.F. Dorfman, Thin Solid Films 212 (1992) 267. [4] Z.Y. Rong, M. Abraizov, B. Dorfman, M. Strongin, X.Q. Yang, D. Yan, F.H. Pollak, Appl. Phys. Lett. 65 (1994) 1379. [5] X.Z. Ding, F.M. Zhang, X.H. Liu, P.W. Wang, W.G. Durrer, W.Y. Cheung, S.P. Wong, I.H. Wilson, Thin Solid Films 346 (1999) 82. [6] F. Ruiz, W.D. Sun, F.H. Pollak, C. Venkatraman, Appl. Phys. Lett. 73 (1998) 1802. [7] H. Wang, M.R. Shen, Z.Y. Ning, C.B. Cao, H.Y. Dang, H.S. Zhu, Appl. Phys. Lett. 69 (1996) 1074. [8] K. Cai, C.B. Cao, H.S. Zhu, Carbon 37 (1999) 1860. [9] D. Guo, K. Cai, L.T. Li, Y. Huang, Z.L. Gui, H.S. Zhu, Carbon 39 (2001) 1395. [10] A.I. Kulak, A.I. Kokorin, D. Meissner, V.G. Ralchenko, I.I. Vlasov, A.V. Kondratyuk, T.I. Kulak, Electrochem. Commun. 5 (2003) 301. [11] Y.F. Lu, S.M. Huang, C.H.A. Huan, X.F. Luo, Appl. Phys. A 68 (1999) 647. [12] Y.F. Lu, S.M. Huang, Z. Sun, J. Appl. Phys. 87 (2000) 945. [13] A.C. Ferrari, J. Robertson, Phys. Rev. B 61 (2000) 14095. [14] A.M. Wrobel, A. Walkiewicz-Pietrzykowska, J.E. KlembergSapieha, Y. Nakanishi, T. Aoki, Y. Hatanaka, Chem. Mater. 15 (2003) 1749. [15] T.Y. Leung, W.F. Man, P.K. Lim, W.C. Chan, F. Gaspari, S. Znkotyski, J. Non-Cryst. Solids 254 (1999) 151.
Synthesis of diamond-like carbon/nanosilica composite films by an electrochemical method Xing-bin Yan a
a,b
, Tao Xu a, Gang Chen
a,b
, Qun-ji Xue a, Sheng-rong Yang
a,*
State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, China b Graduate School of the Chinese Academy of Sciences, Beijing 100083, China Received 16 August 2004; received in revised form 7 September 2004; accepted 9 September 2004 Available online 29 September 2004
Abstract Diamond-like carbon/nanosilica composite films have been deposited on silicon substrates, making use of the electrolysis of methanol–dimethylethoxydisilane (DDS) solution at low temperature. The electrodeposited composite films were characterized by Raman spectroscopy, X-ray photoelectron spectroscopy and transmission electron microscopy, respectively. Moreover, the growth mechanism of the composite films in liquid phase was discussed as well. As the results, the films are diamond-like carbon films containing polycrystalline SiO2 nanoparticles. The introduction of DDS contributes to promote the growth of the carbon films and leads to the formation of the crystalline SiO2 grains. 2004 Elsevier B.V. All rights reserved. Keywords: Diamond-like carbon; Electrochemical deposition; Composite films; Silica; Nanoparticles
1. Introduction Extensive interests have been focused on the diamond-like carbon (DLC) that is considered as promising material for electronic, optical, and wear protection applications, owing to their similarities of properties to those of diamond. These similarities ensure superior features such as excellent chemical inertness, extremely high hardness, very low friction coefficients, atomic-level smoothness, good optical transparency, and high thermal conductivity [1,2]. However, a high level of residual compressive stress of the DLC films has strongly limited the practical applications of these materials. The residual compressive stress leads to a reduction in the film/ substrate bonding strength and fracture and delamination of the films at a high contact load. In order to reduce the film stress without sacrificing the hardness, many efforts have been made to improve the nanostruc*
Corresponding author. Fax: +86 931 8277088. E-mail address: [email protected] (X.-b. Yan).
1388-2481/$ - see front matter 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.elecom.2004.09.012
ture of the DLC films. With a view to the DLC films of modified nanostructures, two particular classes of modified DLC composite films, that is, the diamond-like nanocomposite (DLN) films composed of an amorphous carbon network (a-C:H) and amorphous silica network (a-Si:O) [3,4], and the DLC nanocomposite films composed of an amorphous carbon matrix and SiC, Si3N4, and SiOx (x 6 2) nanoparticles [1] are of a great significance. These composite films possess excellent adhesion to various substrate materials (i.e., metals, plastics, ceramics, semiconductors, etc.), high hardness, low internal residual stress, excellent thermal stability, and excellent wear resistance. Thus they have significant advances over the conventional DLC films. These composite films can be deposited using various chemical and physical vapor deposition techniques such as ion beam assisted deposition [5], multicascade remote plasma vacuum deposition [6], inductively coupled plasma CVD, etc., [1]. However, their applications have been limited owing to the complicated and costly equipment and rigorous preparation conditions.
X.-b. Yan et al. / Electrochemistry Communications 6 (2004) 1159–1162
2. Experimental A simple electrolytic cell system similar to that reported in [7] was used to prepare the DLC/nanosilica composite films. The silicon (1 0 0) substrate with a sheet resistance about 7–13 X/cm2 was mounted on the negative electrode. A graphite plate was used as the counter electrode and kept 6 mm away from the negative electrode. DDS ((CH3)2–Si–(OC2H5)2) was dissolved in analytically pure methanol (CH3OH), with the volume ratio of DDS to methanol to be 1:4. The cleaned substrates were placed into the above solution, with an area of 1.0 · 1.5 cm2 to be immerged therein, to allow the deposition of the composite films at a temperature of (50 ± 2) C, a DC power-supply voltage of 1200 V, and for deposition duration of 5 h. As compared, the pure carbon films were deposited by the electrolysis of methanol at the same conditions. The micro-Raman backscattering spectra of the films were recorded on a Jobin Yvon T64000 spectrometer, operating with a 514.5-nm Ar laser as the excitation source. X-ray photoelectron spectroscopic (XPS) measurements were performed on a Perkin–Elmer PHI5702 system to investigate the chemical composition of the films. The film structure was characterized on a JEM-1200EX transmission electron microscopic (TEM) at an accelerating voltage of 100 kV.
The nano-hardness of the DLC/nanosilica composite film is also much larger (about 12 GPa) than that (about 8 GPa) of the pure carbon film, measured by nanoindentation. The hydrogen content of the pure carbon film and the carbon composite film is close to 30% and 24%, respectively, measured by an elemental analyzer. The Raman spectra of the pure carbon film and of the composite film are shown in Fig. 1(a) and (b), respectively. The both Raman spectra can be classified into three regimes: the D peak, the G peak, and the peak around 1470 cm 1 which may be assigned to the symmetrical deformation frequency of C–CH with sp2 hybridized C–C bonding [11] or a sp3-bonded diamond precursor phase [10,12]. In spectrum of pure carbon film, the position of the D and G peaks is 1340 and 1582 cm 1, respectively. While the position of the D and G peaks is 1338 and 1577 cm 1, respectively, in spectrum of carbon/nanosilica film. Furthermore, it is seen that the relative intensity ratio of the D peak to G peak (ID/IG) of the carbon/nanosilica composite film (about 0.8) is considerably smaller than that of the pure carbon film (about 1.7), and the G peak of the composite film shifts to a lower wave number and the full width at half maximum (FWHM) of the G peak is also larger than that of the pure carbon film. Ferrari et al. [13] thought that the D peak would decrease with increasing disorder in amorphous carbon, that the width of the G peak was proportional to the bond-angle disorder at sp2 sites, and that the mixing with sp3 modes would help to lower the G peak in hydrogenated amorphous carbon (a-C:H). They also found the relationships between visible Raman spectra (514-nm) and the sp3 fraction in
(a)
Intensity (a.u.)
Recently, much attention has been paid to the deposition of DLC films by the electrolysis of organic liquids [2,7–10]. It has been found that the liquid deposition techniques have many advantages such as availability for large area deposition on intricate surfaces, low deposition temperature, low consumption of energy, and simplicity of the set up, over the vapor deposition techniques, with a view to the practical use. However, no report has been reported so far on the deposition of DLC nanocomposite films by electrochemical method. In this letter, it has been attempted to prepare the DLC nanocomposite films containing polycrystalline SiO2 nanoparticles, making use of the electrolysis of methanol–dimethylethoxydisilane (DDS) solution. This novel method could provide a new deposition strategy for DLC nanocomposites, making use of electrochemical deposition. Furthermore, the SiO2 nanocrystals incorporated in a suitable carbon matrix may have potential applications in microelectronic devices and optical apparatus.
1100
1200
1300
1400
1500
1600
1700
1800
1600
1700
1800
-1
Raman Shift (cm ) (b)
Intensity (a.u.)
1160
3. Results and discussion 1100
1200
1300
1400
1500 -1
During the deposition for composite film, we found that the current density decreased slightly from 25 mA/cm2 to less than 16 mA/cm2 with deposition time.
Raman Shift (cm ) Fig. 1. Raman spectra of: (a) pure carbon film and (b) DLC/nanosilica composite film.
X.-b. Yan et al. / Electrochemistry Communications 6 (2004) 1159–1162
a-C:H, which was that the sp3 content increased with the simultaneous decrease of G-peak position and ID/ IG ratio in a-C:H [13]. Therefore, the DLC/nanosilica composite films should have higher concentrations of sp3 carbon than the pure carbon films [12,13]. The Si/C ratio in the DLC/nanosilica composite film was measured to be about 0.12 by XPS analysis. Fig. 2 shows the XPS spectra of the Si2p and C1s of the DLC/ nanosilica composite film. It is seen that the Si2p photoelectron spectrum exhibits a main peak at 102.6 eV, which is assigned to Si–O bond [14]. The C1s main peak of the DLC/nanosilica composite film appears at 285.1 eV, which is the same as that of the as-deposited pure carbon film. The C1s spectrum corresponds to the C– C and C–H units and is consistent with that of the DLC films prepared by CVD [15]. The shoulder of the C1s spectrum at a higher-binding-energy side indicates that some carbon atoms are bonded to oxygen. The bright-field TEM image and electron diffraction pattern of the DLC/nanosilica composite film are shown in Fig. 3. It is seen that the nanoparticles are embedded in the amorphous carbon matrix and the nanoparticles have a size of 20–50 nm, while the selected area electron diffraction (SAED) pattern consists of well-marked diffraction rings, which corresponds to the crystalline nature of the nanoparticles. Such an electron diffraction pattern of the composite film is consistent with that of the tetragonal SiO2 phase from JCPDF: 86-2326 (the identification of the electron diffraction rings is given in Table 1). It can thus be concluded that the composite film has an amorphous carbon structure, with a stable SiO2 nanoparticles phase therein. The electrodeposition technique in our experiments is different from traditional electroplating. The molecules of CH3OH and DDS both contain electron-donating methyl group (CH3) and electron-withdrawing groups (OH in methanol and OC2H5 in DDS). The molecules
Si2p
Intensity (a.u.)
C1s
96
98
100 102 104 106
280 282 284 286 288 290 292
Binding Energy (eV) Fig. 2. Core level Si2p and C1s XPS spectra of DLC/nanosilica composite film. The C1s peak is minified (1.8·) as compared to the Si2p peak.
1161
Fig. 3. The bright field TEM image and electron diffraction pattern of the DLC/nanosilica composite film.
Table 1 The experimentally measured and theoretically predicted lattice spacing (d), and the corresponding crystal planes of silicon oxide Experimental Ring
˚) d (A
Theoretical ˚) d (A
(h k l)
1 2 3 4 5 6
2.924 1.964 1.580 1.421 1.246 1.154
2.956 1.980 1.531 1.478 1.235 1.159
(1 1 0) (1 1 1) (2 1 1) (2 2 0) (3 0 1) (3 2 0)
will be polarized under a high voltage. The methyls at the end of their molecules display somewhat positive charge due to their low electro-negativity. Consequently, the methyls in the two molecules are liable to be adsorbed on the negative electrode. On one hand, when the supplied energy reaches a certain value, the C–O covalent bonds in the polarized methanol will be preferentially broken to release methyl group and to allow the formation of DLC film on the surface of the cathode [8– 10]. On the other hand, the DDS molecules consists of C–H, C–Si, Si–O, C–O and C–C bonds which can be ranked according to the bond strength as Si–O (422.5 kJ/mol) > C–H (413.3 kJ/mol) > C–O (359.5 kJ/ mol) > C–C (346.9 kJ/mol) > C–Si (334.7–242.7 kJ/ mol). Therefore, the Si–C, C–C, and C–O bonds will be preferentially dissociated in the presence of a high electric field, while it is more difficult to break the C– H and Si–O bonds. Subsequently, the –O–Si–O– bond tends to form SiO2 phase due to the higher bond strength of Si–O and hence the DLC/nanosilica composite film is generated by the deposition of the SiO2 nanoparticles in the amorphous carbon matrix. It is thus proposed that the growth of the DLC/nanosilica composite film follows both the polarization–adsorption and decomposition–electrochemical reaction processes, which is schematically shown in Fig. 4. It is interesting to note that the DDS molecule contains two equivalent methyl groups, which contributes
1162
X.-b. Yan et al. / Electrochemistry Communications 6 (2004) 1159–1162
Acknowledgements CH 3-OH CH 3
CH 3
polarization
adsorption
Si
OC 2 H 5 OC 2 H 5
SiO 2 nanoparticle Electron
The authors are grateful to the National Natural Science Foundation of China (Grant Nos. 50172052, 50375151 and 50175105), 863 program (Grant No. 2003 AA305670) and ‘‘Top Hundred Talents Program’’ of Chinese Academy of Sciences for financial support.
References decomposition
electrochem ical reactions Composite film Silicon substrate
Fig. 4. Schematic of the two steps involved in the growth of DLC/ nanosilica composite film.
to speed the growth of the carbon films since more methyl active groups are able to react to form carbon network. Consequently, the composite films have higher concentrations of sp3 carbon than the pure amorphous carbon films deposited in methanol.
4. Conclusion In summary, the DLC films containing polycrystalline SiO2 nanoparticles were deposited by the electrolysis of the methanol–DDS solution. The introduction of DDS contributes to promote the growth of the DLC films and leads to the formation of the crystalline SiO2 grains under a high electric field.
[1] L.Y. Chen, F.C.N. Hong, Appl. Phys. Lett. 82 (2003) 3526. [2] E. Shevchenko, E. Matiushenkov, D. Kochubey, D. Sviridov, A. Kokorin, A. Kulak, Chem. Commun. 4 (2001) 317. [3] V.F. Dorfman, Thin Solid Films 212 (1992) 267. [4] Z.Y. Rong, M. Abraizov, B. Dorfman, M. Strongin, X.Q. Yang, D. Yan, F.H. Pollak, Appl. Phys. Lett. 65 (1994) 1379. [5] X.Z. Ding, F.M. Zhang, X.H. Liu, P.W. Wang, W.G. Durrer, W.Y. Cheung, S.P. Wong, I.H. Wilson, Thin Solid Films 346 (1999) 82. [6] F. Ruiz, W.D. Sun, F.H. Pollak, C. Venkatraman, Appl. Phys. Lett. 73 (1998) 1802. [7] H. Wang, M.R. Shen, Z.Y. Ning, C.B. Cao, H.Y. Dang, H.S. Zhu, Appl. Phys. Lett. 69 (1996) 1074. [8] K. Cai, C.B. Cao, H.S. Zhu, Carbon 37 (1999) 1860. [9] D. Guo, K. Cai, L.T. Li, Y. Huang, Z.L. Gui, H.S. Zhu, Carbon 39 (2001) 1395. [10] A.I. Kulak, A.I. Kokorin, D. Meissner, V.G. Ralchenko, I.I. Vlasov, A.V. Kondratyuk, T.I. Kulak, Electrochem. Commun. 5 (2003) 301. [11] Y.F. Lu, S.M. Huang, C.H.A. Huan, X.F. Luo, Appl. Phys. A 68 (1999) 647. [12] Y.F. Lu, S.M. Huang, Z. Sun, J. Appl. Phys. 87 (2000) 945. [13] A.C. Ferrari, J. Robertson, Phys. Rev. B 61 (2000) 14095. [14] A.M. Wrobel, A. Walkiewicz-Pietrzykowska, J.E. KlembergSapieha, Y. Nakanishi, T. Aoki, Y. Hatanaka, Chem. Mater. 15 (2003) 1749. [15] T.Y. Leung, W.F. Man, P.K. Lim, W.C. Chan, F. Gaspari, S. Znkotyski, J. Non-Cryst. Solids 254 (1999) 151.