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C:H:N films prepared by planar magnetron deposition
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Thin Solid Films 516 (2008) 4581 – 4586 www.elsevier.com/locate/tsf
Composite Ag/C:H:N films prepared by planar magnetron deposition P. Hlídek ⁎, J. Hanuš, H. Biederman, D. Slavínská, J. Pešička Charles University, Faculty of Mathematics and Physics, Praha, Czech Republic Available online 14 June 2007
Abstract Composite Ag/C:H:N films were deposited by means of an unbalanced magnetron operated in a gas mixture of nitrogen and n-hexane. Composition of the films was controlled by electric power delivered to the magnetron and by ratio of nitrogen and n-hexane in the working gas mixture. The films were characterized using transmission electron microscopy, by the absorption spectra in visible and near infrared regions and by Fourier transform infrared spectroscopy. Immediately after film deposition and without breaking vacuum (in situ) corresponding vibration infrared spectra were scanned and their evolution during ageing of the films was monitored. Wettability as determined from water contact angle was improved with raising nitrogen contents, i.e. with increasing the electric power and the ratio of nitrogen/n-hexane in the working gas mixture. The increased wettability is likely caused by presence of NHx groups in Ag/C:H:N films. The incorporation of nitrogen effectively prevents the formation of carboxylate groups on the silver inclusions surfaces during the aging in the open air. In addition, the oxidation mechanism of the polymer matrix is modified. © 2007 Elsevier B.V. All rights reserved. Keywords: Composite films; Magnetron deposition; Fourier transform infrared spectroscopy; Electron microscopy
1. Introduction Nanometer-sized particles of silver in various matrixes have been investigated extensively in recent decades because of their interesting properties depending strongly on the specific methods of preparation. Their application was attempted as e.g. catalysts, antibacterial coating and use of surface enhanced Raman scattering in spectroscopy etc. Recently we reported different properties of Cu/C:H and Ag/C:H films [1]. Properties of the films depend on the filling factor (i.e. volume fraction ratio of metallic phase in a composite) and on shape and size of inclusions. In our previous paper [2] we used a gradual cover of the target by reaction products (i.e. so called “target poisoning”) to deposit Ag/C:H nanocomposite films with decreasing rate of silver sputtering. Relatively intensive infrared absorption bands caused by oxygen containing structural groups were observed in these films after aging in ambient air. It has been found that the spectra of the aged Ag/C:H films differ from the spectra of the aged C:H films (without a metal) and the aged (oxidized) polymers such as polyethylene and polypropylene. In the ⁎ Corresponding author. E-mail address: [email protected] (P. Hlídek). 0040-6090/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2007.05.085
present paper, basic properties of composite films Ag/C:H:N are reported and compared with the Ag/C:H films. Both types of films were prepared using plasma polymerization of n-hexane in a low-pressure direct current (DC) discharge in gas mixtures of argon/n-hexane or nitrogen/n-hexane with simultaneous silver sputtering by means of an unbalanced magnetron. The films were characterized using transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), wettability, optical and infrared spectroscopy. Various methods of infrared (IR) spectroscopy have been reported for investigation of surfaces and thin films [3]. One of the methods, namely reflectivity measurement of films deposited on metallic surface with a high angle of incidence (reflection–absorption infrared spectroscopy RAIRS or IR RAS), has an advantage of experimental simplicity and good sensitivity. The method RAIRS was used for measurement of IR spectra in situ without breaking vacuum immediately after the film deposition, and for ex situ investigation during a long time aging in the air. Some specific features should be taken into account when discussing IR vibrational spectra of materials containing metallic components. Namely, the metal-surface selection rules for infrared absorption cause that some absorption bands are strengthened and others are absent or weakened when molecules are adsorbed
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Fig. 1. TEM micrographs of the films deposited from working gas argon/n-hexane and nitrogen/n-hexane, respectively. The flow of hexane: 0.7 sccm, the flow of argon (or nitrogen): 3.3 sccm, electric current: 100 mA, time of deposition: 20 s.
conditions for obtaining high filling factor were preferred. Before each deposition the silver target was cleaned mechanically by an emery paper and then sputtered by argon ions. The chamber was provided with KRS5 windows for in situ infrared measurements. Using an external globar IR source focused on the sample inside the chamber the infrared spectra were measured by Fourier transform infrared (FTIR) spectrometer IFS66s (Bruker) without breaking vacuum. Further investigation was done ex situ. The thicknesses as well as the morphology of the films were surveyed using an atomic force microscope Quesant 350. The silver inclusions were observed by a transmission electron microscope JEOL 2000 FX on samples deposited on carbon foils. X-ray photoelectron spectroscopy has been performed using an ADES 400 (VG Scientific, UK) spectrometer equipped with Al Kα as excitation radiation (1486.6 eV) and a hemispherical electron energy analyzer. Contact angles of water were determined for drops resting on a solid surface and imaged by a CCD camera. A water drop 2 mm in diameter was created on the surface and its shape after 1 min was processed by means of self-built computerized apparatus. 3. Results and discussion
on the metal surface. Only those vibrations with a component of their dipole moment perpendicular to the metal surface are infrared active. This rule holds not only for flat surfaces and large particles but for small metal particles of nanometers size as well [4]. In this case, an additional effect of surface enhanced IR absorption (SEIRA) [5–7] emphasizes absorption due to vibrations in structural groups placed tightly at surfaces of small metallic particles of a high surface curvature. Especially, outstanding enhancement is observed near metal percolation threshold owing to extreme strong local fields in the narrow dielectric gaps between metallic particles caused by plasmons at metal surfaces [8]. 2. Experimental details The films were prepared in a small vacuum chamber (inner diameter 190 mm) where plasma polymerization and cosputtering of the target material took place. Unbalanced planar magnetron with circular target (diameter 75 mm) was equipped with ring ceramic permanent magnets and a mild steel magnetic circuit. A detailed study of plasma excited by this type of magnetron is reported in [9]. The glow discharge was powered by a DC power supply (MDX500 Advanced energy) in the regime of constant current. Typical parameters were in the ranges: total gas flow 3–4 sccm, total pressure 2 Pa, current 20–160 mA (corresponding power 5–70 W). The substrate holder was placed at a distance 80 mm from the magnetron target. The silver coated glass substrates were used for deposition of samples intended for IR RAS measurements. In addition, the films were deposited on silicon wafers for ex situ IR transmittance measurement, uncoated glass for atomic force microscopy and for spectral measurements in visible range and copper grids with carbon foil for transmission electron microscopy. We focused our attention on interaction of plasma polymer matrix with the silver inclusions. Therefore the experimental
Electron microscopy (Fig. 1) reveals that silver inclusions of size about 10 nm are created in plasma polymer matrix both in the case of discharge in argon and discharge in nitrogen. The heavier argon ions are more efficient projectiles for the sputtering than nitrogen. It results in a slightly higher deposition rate (e.g. 67 nm/ min versus 55 nm/min for I = 100 mA, hexane 0.7 sccm, argon or nitrogen 3.3 sccm) and in a higher filling factor for the Ag/C:H (Ar) films. Results of XPS analysis (Table 1) show that nitrogen is effectively built in the films. Hydrogen is not detectable by this XPS measurement. A remarkable oxidation during aging is realized in both types of films. The deposition rate grows sublinearly with the discharge current (Fig. 2) and the deposition rate increases with the relative n-hexane flow rise (increase of precursor concentration for plasma polymerization) as shown in Fig. 3 and as expected from the theory. The contact angle is decreased by the rise of the discharge current (Fig. 4). In addition, the contact angle becomes higher due to increased content of hydrophobic groups when during the deposition of the films the relative nhexane concentration in the working gas is increased (Fig. 5). A very broad absorption band in the visible spectral region was observed. It is similar to the band observed in Ag/C:H (Ar) films [2] and originates from the absorption by surface plasmons excited in metallic inclusions. FTIR spectra of the gas products of the discharge in the working gas n-hexane/argon/nitrogen in our chamber measured Table 1 Relative composition from XPS analysis of the films after 1 month aging in the air
Ag/C:H:N Ag/C:H (Ar)
C [at.%]
O [at.%]
Ag [at.%]
N [at.%]
63 67
12 13
9 20
16 0
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Fig. 2. Deposition rate as a function of discharge current.
previously [10] at slightly higher pressures above 10 Pa show that the main substances created during the discharge are hydrogen cyanide HCN and ammonia NH3. FTIR spectra of deposited films (Fig. 6) show that polymerlike CH matrix (with only a tiny fraction NHx groups appearing near 3300 cm− 1) is created during the low power discharge (minimum used power was 6 W), while only a minor content of CHx groups (bands near 2900, 1450 and 1375 cm− 1) is observed for the high electric power input (55 W at current 140 mA). It means that CHx radicals (with a higher deposition rate) survive in the case of low excitation, while they are destroyed in the case of high power excitation. In this case NHx groups are created more readily. A higher concentration of hydrophilic, nitrogencontaining groups in the films deposited at higher currents causes the observed decrease of water contact angle. Nevertheless, there are some remarkable differences between the films deposited at low discharge current in nitrogen and those deposited in argon as can be seen from the comparison of the top curve in Fig. 6 (high content of hydrocarbon groups in the Ag/C:H:N film) and the bottom curve in Fig. 7 (a typical spectrum of Ag/C:H(Ar)). The most noticeable band near 1620 cm− 1 is not observed in the latter case and it can be attributed to double bonded nitrogen (–N = containing groups). Another interesting spectral region of remarkable differences is
Fig. 3. Deposition rate as a function of the ratio n-hexane/nitrogen flow rates in the working gas. The total flow rate was 3.5 sccm, total pressure 2 Pa. The silver sputtering at zero hexane concentration took place.
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Fig. 4. Dependence of water contact angle on discharge current. Full circles for samples deposited on glass, open circles for films deposited on Ag/glass substrates.
above 2000 cm− 1, where triple bonds (like –C≡N) or conjugated double bonds (like –N=C=N–) are expected in the case of nitrogen deposited films. Therefore, the nitrogen atoms are chemically bonded even in the case of low powered discharge, although it is not in the form of NH groups. Some of possible assignments of the respective bands are given in Table 2. The assignment is based on well-known characteristic vibration frequencies of the respective structural groups [11]. However, the adsorption of structural groups to the silver inclusions can cause a considerable shift of typical vibration frequencies: e.g. absorption due to carbonyl C=O in organic compounds is usually observed in the range 1700– 1800 cm− 1, while for (Agsolid)=C=O the wavenumber is shifted to 1950–2180 cm− 1 region with a dependence on a quality of the silver surface and geometry of bonding. Moreover, the effect SEIRA (surface enhanced infrared absorption) emphasizes just the absorption due to vibration of dipoles placed tightly on the metal surface with their dipole moment oriented perpendicularly to the surface. Vice versa, the absorption due to dipole moment vibrations parallel to the metal surface becomes weaker. A wellknown example is the carboxylate group (e.g. [12,13]): many
Fig. 5. Dependence of the water contact angle on the n-hexane concentration in the working gas. Full circles for samples deposited on glass, open circles for films deposited on Ag/glass substrates.
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P. Hlídek et al. / Thin Solid Films 516 (2008) 4581–4586 Table 2 Infrared absorption peak assignment
Fig. 6. IRRAS spectra of Ag/C:H:N deposited at different currents measured in situ immediately after the deposition without breaking vacuum.
types of molecules are adsorbed on silver surfaces by the two oxygen atoms of their carboxylate COO− end group. In many cases, the axis of the carboxylate group is oriented almost
Fig. 7. Infrared spectra and their evolution during ageing in open air for the film deposited in discharge in working gas argon/n-hexane.
Wavenumber [cm− 1]
Possible assignment
N3000 2960 2930 2870 2190–2200 2150–2160 2130 1960–1980 1610–1635 1550–1580 1450–1460 1400–1420 1380–1395 1375–1380 1040 900–910, 970
Stretch OH and/or NHx Asymmetrical stretch CH3 Asymmetrical stretch CH2 Symmetrical stretch CH3 Stretch –N=C=N–, –C≡N Stretch NC=O adsorbed on Ag inclusions Stretch –N=N=N– or (C≡N)− in Ag(CN)x complexes Stretch NC=O adsorbed on Ag inclusions Stretch NC=N– Asymmetrical stretch CO−2 Deformation of CH2, CH3 Stretch C–N, N–O Symmetrical stretch CO−2 adsorbed on Ag inclusions Deformation of CH3 Stretch C–O Deformation of –CH=CH2, –CH=CH–
perpendicularly to the surface. Consequently, the symmetrical stretching of the carboxylate group with dipole moment perpendicular to the surface reveals much stronger absorption than antisymmetrical stretching with dipole moment parallel to the surface. In contrary, the antisymmetrical stretching causes much more intensive absorption than symmetrical stretching in the case of the same molecules (ions) in solution. Huge changes of the Ag/C:H(Ar) spectra during the oxidation in the open air can be seen in Fig. 7, first of all in the spectral range near 1390 cm− 1. We believe that the formation of
Fig. 8. Spectra of the Ag/C:H:N films measured after 1 month ageing in open air.
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carboxylate and carbonate groups placed on silver inclusions during oxidation of the Ag/C:H(Ar) films causes the remarkable changes of the absorption near 1390 and 1550 cm− 1 shown in Fig. 7. It is not the case of the Ag/C:H:N films (Fig. 8) where the changes of IR spectra induced by ageing are subtle in this spectral region even for samples deposited at low electric current with relatively high content of CHx groups. Only a slight modification of the spectra during ageing can be identified comparing spectra of the films deposited from working gas with lower and higher concentration of hexane (Figs. 9 and 10). Nevertheless, the changes in the spectral region of symmetrical carboxylate stretching (1390 cm − 1) are more significant for the film with a higher content of hydrocarbon groups. The presence of nitrogen atoms seems to prevent the formation of the carboxylate groups on the silver surfaces very effectively. In the case of Ag/C:H:N, the most intensive peak is shifted during the aging from 1620 to 1570 cm− 1 . It can be hardly interpreted in details because it is apparently a complex band with various contributions at least from NHx deformation, C=N stretching and OCO antisymmetrical stretching vibrations. Oxidation of Ag/C:H(Ar) films gives rise to absorption bands near 2000 cm− 1 assigned to C=O groups bonded to silver inclusions by carbon atoms. The remarkable bands and their changes in spectral region above 1950 cm− 1 are also observed in the Ag/C:H:N films. We suppose that groups containing triple bonded nitrogen and/or conjugated double bonds are responsible for the absorption in this region in as-deposited films. The
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Fig. 10. Evolution of the spectra for the sample deposited in working gas mixture n-hexane/nitrogen = 2:5.
observed distinguished changes during the aging are due to oxidation (and/or hydrolysis) of these species resulting in CO groups bonded to silver surface by carbon atoms (bands at 1960 and 2150 cm− 1). It is interesting that no sample reveals a remarkable absorption band due to carbonyl C=O in the polymer matrix near 1700 cm− 1. Such a band is usually observed as a consequence of oxidation in many hydrocarbon polymers without a metallic inclusions [14] including those materials prepared by plasma polymerization [15] or by sputtering [16]. 4. Conclusion
Fig. 9. Evolution of spectra during ageing of film deposited in working gas mixture n-hexane/nitrogen = 1:5.
Nanocomposite films Ag/C:H:N can be prepared by means of plasma polymerization in a nitrogen/hydrocarbon gas mixture with co-sputtering of silver target. These films are more hydrophilic than similar films Ag/C:H deposited in the argon/hydrocarbon gas mixture and aging (oxidation mechanisms) of these films is different. The increased wettability in the former case is likely caused by the presence of NHx groups in Ag/C:H:N films. However, nitrogen is also bonded to carbon as may be seen from the infrared active C=N and C≡N groups. The incorporation of nitrogen effectively prevents the formation of carboxylate groups on the silver inclusion surfaces during the aging in the open air. In addition, the oxidation mechanisms of the polymer matrix are modified as demonstrated by the fact that carbonyl C=O absorption band at 1700 cm− 1 is missing in the FTIR spectra of Ag/C:H:N films.
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Acknowledgement This work is a part of the research plan MSM0021620834 that is financed by the Ministry of Education of Czech Republic. The authors would like to thank for XPS analyses to Dr. Josef Zemek from the Institute of Physics, Academy of Sciences of the Czech Republic.
[5] [6] [7] [8] [9] [10]
References [11] [1] P. Hlídek, H. Biederman, H. Boldyryeva, D. Slavínská, in: A. Bouchoule, J. M. Pouvesle, A.L. Thomann, J.M. Bauchire, E. Robert (Eds.), Proceedings of 15th International Symposium on Plasma Chemistry, vol. VI, Orleans, France, July 9–13 2001, p. 2373. [2] H. Boldyryeva, P. Hlídek, H. Biederman, D. Slavínská, A. Choukourov, Thin Solid Films 442 (2003) 86. [3] Y.J. Chabal, Surf. Sci. Rep. 8 (1988) 211. [4] R.G. Greenler, D.R. Snider, D. Witt, R.S. Sorbello, Surf. Sci. 118 (1982) 415.
[12] [13] [14] [15] [16]
M. Osawa, Top. Appl. Phys. 81 (2001) 163. J. Yang, S.H. Chen, Appl. Spectrosc. 55 (2001) 399. J.A. Seelenbinder, C.W. Brown, Appl. Spectrosc. 56 (2002) 295. Y. Suzuki, H. Seki, T. Inamura, T. Tanabe, T. Wadayama, A. Hatta, Surf. Sci. 427–428 (1999) 136. H. Biederman, V. Stundžia, D. Slavínská, J. Glosík, Vacuum 52 (1999) 415. P. Hlídek, H. Biederman, D. Slavínská, J. Kousal, in: R. d'Agostino, P. Favia, F. Fracassi, F. Palumbo (Eds.), Proceedings of 16th International Symposium on Plasma Chemistry, Taormina, Italy, June 22–27 2003, (paper PO 2.29). G. Socrates, Infrared Characteristic Group Frequencies, John Wiley &Sons, Bristol, 1980. Y.T. Tao, J. Am. Chem. Soc. 115 (1993) 4350. S.J. Lee, S.W. Han, H.J. Choi, K. Kim, J. Phys. Chem., B 106 (2002) 2892. T.R. Gengenbach, Z.R. Vasic, R.C. Chatelier, H.J. Griesser, J. Polym. Sci., A, Polym. Chem. 32 (1994) 1399. A. Hallil, B. Despax, Thin Solid Films 358 (2000) 30. J. Kousal, J. Hanuš, A. Choukourov, P. Hlídek, H. Biederman, D. Slavínská, J. Zemek, Surf. Coat. Technol. 200 (2005) 472.
Thin Solid Films 516 (2008) 4581 – 4586 www.elsevier.com/locate/tsf
Composite Ag/C:H:N films prepared by planar magnetron deposition P. Hlídek ⁎, J. Hanuš, H. Biederman, D. Slavínská, J. Pešička Charles University, Faculty of Mathematics and Physics, Praha, Czech Republic Available online 14 June 2007
Abstract Composite Ag/C:H:N films were deposited by means of an unbalanced magnetron operated in a gas mixture of nitrogen and n-hexane. Composition of the films was controlled by electric power delivered to the magnetron and by ratio of nitrogen and n-hexane in the working gas mixture. The films were characterized using transmission electron microscopy, by the absorption spectra in visible and near infrared regions and by Fourier transform infrared spectroscopy. Immediately after film deposition and without breaking vacuum (in situ) corresponding vibration infrared spectra were scanned and their evolution during ageing of the films was monitored. Wettability as determined from water contact angle was improved with raising nitrogen contents, i.e. with increasing the electric power and the ratio of nitrogen/n-hexane in the working gas mixture. The increased wettability is likely caused by presence of NHx groups in Ag/C:H:N films. The incorporation of nitrogen effectively prevents the formation of carboxylate groups on the silver inclusions surfaces during the aging in the open air. In addition, the oxidation mechanism of the polymer matrix is modified. © 2007 Elsevier B.V. All rights reserved. Keywords: Composite films; Magnetron deposition; Fourier transform infrared spectroscopy; Electron microscopy
1. Introduction Nanometer-sized particles of silver in various matrixes have been investigated extensively in recent decades because of their interesting properties depending strongly on the specific methods of preparation. Their application was attempted as e.g. catalysts, antibacterial coating and use of surface enhanced Raman scattering in spectroscopy etc. Recently we reported different properties of Cu/C:H and Ag/C:H films [1]. Properties of the films depend on the filling factor (i.e. volume fraction ratio of metallic phase in a composite) and on shape and size of inclusions. In our previous paper [2] we used a gradual cover of the target by reaction products (i.e. so called “target poisoning”) to deposit Ag/C:H nanocomposite films with decreasing rate of silver sputtering. Relatively intensive infrared absorption bands caused by oxygen containing structural groups were observed in these films after aging in ambient air. It has been found that the spectra of the aged Ag/C:H films differ from the spectra of the aged C:H films (without a metal) and the aged (oxidized) polymers such as polyethylene and polypropylene. In the ⁎ Corresponding author. E-mail address: [email protected] (P. Hlídek). 0040-6090/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2007.05.085
present paper, basic properties of composite films Ag/C:H:N are reported and compared with the Ag/C:H films. Both types of films were prepared using plasma polymerization of n-hexane in a low-pressure direct current (DC) discharge in gas mixtures of argon/n-hexane or nitrogen/n-hexane with simultaneous silver sputtering by means of an unbalanced magnetron. The films were characterized using transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), wettability, optical and infrared spectroscopy. Various methods of infrared (IR) spectroscopy have been reported for investigation of surfaces and thin films [3]. One of the methods, namely reflectivity measurement of films deposited on metallic surface with a high angle of incidence (reflection–absorption infrared spectroscopy RAIRS or IR RAS), has an advantage of experimental simplicity and good sensitivity. The method RAIRS was used for measurement of IR spectra in situ without breaking vacuum immediately after the film deposition, and for ex situ investigation during a long time aging in the air. Some specific features should be taken into account when discussing IR vibrational spectra of materials containing metallic components. Namely, the metal-surface selection rules for infrared absorption cause that some absorption bands are strengthened and others are absent or weakened when molecules are adsorbed
4582
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Fig. 1. TEM micrographs of the films deposited from working gas argon/n-hexane and nitrogen/n-hexane, respectively. The flow of hexane: 0.7 sccm, the flow of argon (or nitrogen): 3.3 sccm, electric current: 100 mA, time of deposition: 20 s.
conditions for obtaining high filling factor were preferred. Before each deposition the silver target was cleaned mechanically by an emery paper and then sputtered by argon ions. The chamber was provided with KRS5 windows for in situ infrared measurements. Using an external globar IR source focused on the sample inside the chamber the infrared spectra were measured by Fourier transform infrared (FTIR) spectrometer IFS66s (Bruker) without breaking vacuum. Further investigation was done ex situ. The thicknesses as well as the morphology of the films were surveyed using an atomic force microscope Quesant 350. The silver inclusions were observed by a transmission electron microscope JEOL 2000 FX on samples deposited on carbon foils. X-ray photoelectron spectroscopy has been performed using an ADES 400 (VG Scientific, UK) spectrometer equipped with Al Kα as excitation radiation (1486.6 eV) and a hemispherical electron energy analyzer. Contact angles of water were determined for drops resting on a solid surface and imaged by a CCD camera. A water drop 2 mm in diameter was created on the surface and its shape after 1 min was processed by means of self-built computerized apparatus. 3. Results and discussion
on the metal surface. Only those vibrations with a component of their dipole moment perpendicular to the metal surface are infrared active. This rule holds not only for flat surfaces and large particles but for small metal particles of nanometers size as well [4]. In this case, an additional effect of surface enhanced IR absorption (SEIRA) [5–7] emphasizes absorption due to vibrations in structural groups placed tightly at surfaces of small metallic particles of a high surface curvature. Especially, outstanding enhancement is observed near metal percolation threshold owing to extreme strong local fields in the narrow dielectric gaps between metallic particles caused by plasmons at metal surfaces [8]. 2. Experimental details The films were prepared in a small vacuum chamber (inner diameter 190 mm) where plasma polymerization and cosputtering of the target material took place. Unbalanced planar magnetron with circular target (diameter 75 mm) was equipped with ring ceramic permanent magnets and a mild steel magnetic circuit. A detailed study of plasma excited by this type of magnetron is reported in [9]. The glow discharge was powered by a DC power supply (MDX500 Advanced energy) in the regime of constant current. Typical parameters were in the ranges: total gas flow 3–4 sccm, total pressure 2 Pa, current 20–160 mA (corresponding power 5–70 W). The substrate holder was placed at a distance 80 mm from the magnetron target. The silver coated glass substrates were used for deposition of samples intended for IR RAS measurements. In addition, the films were deposited on silicon wafers for ex situ IR transmittance measurement, uncoated glass for atomic force microscopy and for spectral measurements in visible range and copper grids with carbon foil for transmission electron microscopy. We focused our attention on interaction of plasma polymer matrix with the silver inclusions. Therefore the experimental
Electron microscopy (Fig. 1) reveals that silver inclusions of size about 10 nm are created in plasma polymer matrix both in the case of discharge in argon and discharge in nitrogen. The heavier argon ions are more efficient projectiles for the sputtering than nitrogen. It results in a slightly higher deposition rate (e.g. 67 nm/ min versus 55 nm/min for I = 100 mA, hexane 0.7 sccm, argon or nitrogen 3.3 sccm) and in a higher filling factor for the Ag/C:H (Ar) films. Results of XPS analysis (Table 1) show that nitrogen is effectively built in the films. Hydrogen is not detectable by this XPS measurement. A remarkable oxidation during aging is realized in both types of films. The deposition rate grows sublinearly with the discharge current (Fig. 2) and the deposition rate increases with the relative n-hexane flow rise (increase of precursor concentration for plasma polymerization) as shown in Fig. 3 and as expected from the theory. The contact angle is decreased by the rise of the discharge current (Fig. 4). In addition, the contact angle becomes higher due to increased content of hydrophobic groups when during the deposition of the films the relative nhexane concentration in the working gas is increased (Fig. 5). A very broad absorption band in the visible spectral region was observed. It is similar to the band observed in Ag/C:H (Ar) films [2] and originates from the absorption by surface plasmons excited in metallic inclusions. FTIR spectra of the gas products of the discharge in the working gas n-hexane/argon/nitrogen in our chamber measured Table 1 Relative composition from XPS analysis of the films after 1 month aging in the air
Ag/C:H:N Ag/C:H (Ar)
C [at.%]
O [at.%]
Ag [at.%]
N [at.%]
63 67
12 13
9 20
16 0
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Fig. 2. Deposition rate as a function of discharge current.
previously [10] at slightly higher pressures above 10 Pa show that the main substances created during the discharge are hydrogen cyanide HCN and ammonia NH3. FTIR spectra of deposited films (Fig. 6) show that polymerlike CH matrix (with only a tiny fraction NHx groups appearing near 3300 cm− 1) is created during the low power discharge (minimum used power was 6 W), while only a minor content of CHx groups (bands near 2900, 1450 and 1375 cm− 1) is observed for the high electric power input (55 W at current 140 mA). It means that CHx radicals (with a higher deposition rate) survive in the case of low excitation, while they are destroyed in the case of high power excitation. In this case NHx groups are created more readily. A higher concentration of hydrophilic, nitrogencontaining groups in the films deposited at higher currents causes the observed decrease of water contact angle. Nevertheless, there are some remarkable differences between the films deposited at low discharge current in nitrogen and those deposited in argon as can be seen from the comparison of the top curve in Fig. 6 (high content of hydrocarbon groups in the Ag/C:H:N film) and the bottom curve in Fig. 7 (a typical spectrum of Ag/C:H(Ar)). The most noticeable band near 1620 cm− 1 is not observed in the latter case and it can be attributed to double bonded nitrogen (–N = containing groups). Another interesting spectral region of remarkable differences is
Fig. 3. Deposition rate as a function of the ratio n-hexane/nitrogen flow rates in the working gas. The total flow rate was 3.5 sccm, total pressure 2 Pa. The silver sputtering at zero hexane concentration took place.
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Fig. 4. Dependence of water contact angle on discharge current. Full circles for samples deposited on glass, open circles for films deposited on Ag/glass substrates.
above 2000 cm− 1, where triple bonds (like –C≡N) or conjugated double bonds (like –N=C=N–) are expected in the case of nitrogen deposited films. Therefore, the nitrogen atoms are chemically bonded even in the case of low powered discharge, although it is not in the form of NH groups. Some of possible assignments of the respective bands are given in Table 2. The assignment is based on well-known characteristic vibration frequencies of the respective structural groups [11]. However, the adsorption of structural groups to the silver inclusions can cause a considerable shift of typical vibration frequencies: e.g. absorption due to carbonyl C=O in organic compounds is usually observed in the range 1700– 1800 cm− 1, while for (Agsolid)=C=O the wavenumber is shifted to 1950–2180 cm− 1 region with a dependence on a quality of the silver surface and geometry of bonding. Moreover, the effect SEIRA (surface enhanced infrared absorption) emphasizes just the absorption due to vibration of dipoles placed tightly on the metal surface with their dipole moment oriented perpendicularly to the surface. Vice versa, the absorption due to dipole moment vibrations parallel to the metal surface becomes weaker. A wellknown example is the carboxylate group (e.g. [12,13]): many
Fig. 5. Dependence of the water contact angle on the n-hexane concentration in the working gas. Full circles for samples deposited on glass, open circles for films deposited on Ag/glass substrates.
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P. Hlídek et al. / Thin Solid Films 516 (2008) 4581–4586 Table 2 Infrared absorption peak assignment
Fig. 6. IRRAS spectra of Ag/C:H:N deposited at different currents measured in situ immediately after the deposition without breaking vacuum.
types of molecules are adsorbed on silver surfaces by the two oxygen atoms of their carboxylate COO− end group. In many cases, the axis of the carboxylate group is oriented almost
Fig. 7. Infrared spectra and their evolution during ageing in open air for the film deposited in discharge in working gas argon/n-hexane.
Wavenumber [cm− 1]
Possible assignment
N3000 2960 2930 2870 2190–2200 2150–2160 2130 1960–1980 1610–1635 1550–1580 1450–1460 1400–1420 1380–1395 1375–1380 1040 900–910, 970
Stretch OH and/or NHx Asymmetrical stretch CH3 Asymmetrical stretch CH2 Symmetrical stretch CH3 Stretch –N=C=N–, –C≡N Stretch NC=O adsorbed on Ag inclusions Stretch –N=N=N– or (C≡N)− in Ag(CN)x complexes Stretch NC=O adsorbed on Ag inclusions Stretch NC=N– Asymmetrical stretch CO−2 Deformation of CH2, CH3 Stretch C–N, N–O Symmetrical stretch CO−2 adsorbed on Ag inclusions Deformation of CH3 Stretch C–O Deformation of –CH=CH2, –CH=CH–
perpendicularly to the surface. Consequently, the symmetrical stretching of the carboxylate group with dipole moment perpendicular to the surface reveals much stronger absorption than antisymmetrical stretching with dipole moment parallel to the surface. In contrary, the antisymmetrical stretching causes much more intensive absorption than symmetrical stretching in the case of the same molecules (ions) in solution. Huge changes of the Ag/C:H(Ar) spectra during the oxidation in the open air can be seen in Fig. 7, first of all in the spectral range near 1390 cm− 1. We believe that the formation of
Fig. 8. Spectra of the Ag/C:H:N films measured after 1 month ageing in open air.
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carboxylate and carbonate groups placed on silver inclusions during oxidation of the Ag/C:H(Ar) films causes the remarkable changes of the absorption near 1390 and 1550 cm− 1 shown in Fig. 7. It is not the case of the Ag/C:H:N films (Fig. 8) where the changes of IR spectra induced by ageing are subtle in this spectral region even for samples deposited at low electric current with relatively high content of CHx groups. Only a slight modification of the spectra during ageing can be identified comparing spectra of the films deposited from working gas with lower and higher concentration of hexane (Figs. 9 and 10). Nevertheless, the changes in the spectral region of symmetrical carboxylate stretching (1390 cm − 1) are more significant for the film with a higher content of hydrocarbon groups. The presence of nitrogen atoms seems to prevent the formation of the carboxylate groups on the silver surfaces very effectively. In the case of Ag/C:H:N, the most intensive peak is shifted during the aging from 1620 to 1570 cm− 1 . It can be hardly interpreted in details because it is apparently a complex band with various contributions at least from NHx deformation, C=N stretching and OCO antisymmetrical stretching vibrations. Oxidation of Ag/C:H(Ar) films gives rise to absorption bands near 2000 cm− 1 assigned to C=O groups bonded to silver inclusions by carbon atoms. The remarkable bands and their changes in spectral region above 1950 cm− 1 are also observed in the Ag/C:H:N films. We suppose that groups containing triple bonded nitrogen and/or conjugated double bonds are responsible for the absorption in this region in as-deposited films. The
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Fig. 10. Evolution of the spectra for the sample deposited in working gas mixture n-hexane/nitrogen = 2:5.
observed distinguished changes during the aging are due to oxidation (and/or hydrolysis) of these species resulting in CO groups bonded to silver surface by carbon atoms (bands at 1960 and 2150 cm− 1). It is interesting that no sample reveals a remarkable absorption band due to carbonyl C=O in the polymer matrix near 1700 cm− 1. Such a band is usually observed as a consequence of oxidation in many hydrocarbon polymers without a metallic inclusions [14] including those materials prepared by plasma polymerization [15] or by sputtering [16]. 4. Conclusion
Fig. 9. Evolution of spectra during ageing of film deposited in working gas mixture n-hexane/nitrogen = 1:5.
Nanocomposite films Ag/C:H:N can be prepared by means of plasma polymerization in a nitrogen/hydrocarbon gas mixture with co-sputtering of silver target. These films are more hydrophilic than similar films Ag/C:H deposited in the argon/hydrocarbon gas mixture and aging (oxidation mechanisms) of these films is different. The increased wettability in the former case is likely caused by the presence of NHx groups in Ag/C:H:N films. However, nitrogen is also bonded to carbon as may be seen from the infrared active C=N and C≡N groups. The incorporation of nitrogen effectively prevents the formation of carboxylate groups on the silver inclusion surfaces during the aging in the open air. In addition, the oxidation mechanisms of the polymer matrix are modified as demonstrated by the fact that carbonyl C=O absorption band at 1700 cm− 1 is missing in the FTIR spectra of Ag/C:H:N films.
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Acknowledgement This work is a part of the research plan MSM0021620834 that is financed by the Ministry of Education of Czech Republic. The authors would like to thank for XPS analyses to Dr. Josef Zemek from the Institute of Physics, Academy of Sciences of the Czech Republic.
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