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Fabrication and tribological properties of a self-assembled silane bilayer on silicon
Thin Solid Films 592 (2015) 105–109
Contents lists available at ScienceDirect
Thin Solid Films journal homepage: www.elsevier.com/locate/tsf
Fabrication and tribological properties of a self-assembled silane bilayer on silicon Peng Wang ⁎, Shu-Hua Teng School of Materials Science and Engineering, China University of Mining and Technology, Xuzhou 221116, China
a r t i c l e
i n f o
Article history: Received 11 September 2014 Received in revised form 30 August 2015 Accepted 3 September 2015 Available online 5 September 2015 Keywords: Self-assembly Octadecyltrichlorosilane Bilayer films Surface morphology Tribological property
a b s t r a c t A self-assembled bilayer film of 1,2-bis(triethoxysilyl) ethane–octadecyltrichlorosilane (BTSE–OTS) was prepared on single-crystal silicon substrates by utilizing BTSE as an intermediate layer. The surface morphology of the film was characterized by small round protuberances, with a surface roughness of about 1.82 nm. Moreover, the BTSE– OTS bilayer film displayed comparable hydrophobicity and nano-scale friction behaviors to the OTS monolayer owing to their identical surface composition and similar surface morphology. The investigation of the microtribological properties revealed that the BTSE–OTS bilayer film possessed much higher anti-wear durability and load-carrying capacity than the OTS monolayer while maintaining the low friction coefficient (~ 0.05). The BTSE–OTS bilayer film may find expanded applications in the lubrication of micro-/nano-electromechanical systems as compared to the conventional OTS monolayer. © 2015 Elsevier B.V. All rights reserved.
1. Introduction As a preferred kind of boundary lubricant, self-assembled monolayers (SAMs) have gained tremendous growth over the past few decades due to their effectiveness in improving the reliability and stability of the micro-/nano-electromechanical systems (MEMSs/NEMSs) by reducing adhesion and friction on the surface of the devices [1,2]. Moreover, they provide the ability to tailor the tribological properties of the silicon surface at the molecular level by varying the molecular composition or structure [3,4]. Especially, the monolayers of organosilicon derivatives on silicon wafers have attracted more attention because of their superior tribological properties, among which the octadecyltrichlorosilane (OTS) SAM is the first reported one and has undergone the most extensive studies [5–7]. The OTS SAM is a densely packed and well-ordered monolayer film, covalently bound to the silicon substrate. Nevertheless, despite being known for its reputably low friction coefficient, the OTS SAM exhibited poor anti-wear durability and load-carrying capacity due to its monolayer structure and flexibility, which will greatly limit its wider applications [8]. The mechanical durability and load-carrying capacity of the organic monolayers when subjected to a tribological load can be greatly enhanced by constructing self-assembled bilayer or multilayer films [9,10]. The formation of the layered films usually requires the molecules in the first layer of SAMs to provide reactive terminal groups capable of mediating the deposition of a second layer. Various types of selfassembled bilayer and multilayer films have been fabricated in an effort ⁎ Corresponding author. E-mail address: [email protected] (P. Wang).
http://dx.doi.org/10.1016/j.tsf.2015.09.007 0040-6090/© 2015 Elsevier B.V. All rights reserved.
to improve the tribological properties of the organic monolayers. However, until recently there has been little research conducted on the layered films for improving the tribological performance of the OTS SAMs. In the present paper, the 1,2-bis(triethoxysilyl) ethane (BTSE) films were firstly prepared on single-crystal silicon substrates via hydrolysis and subsequent self-assembly processes. Thereafter, the residual hydroxyl groups (\\OH) on the surface of the BTSE films may play a similar role as the hydroxylated silicon substrate to further chemically adsorb another self-assembled monolayer onto the hydroxyl-terminated surface, thus making it possible to generate the BTSE–OTS bilayer films. The surface morphology, wettability, adhesive behavior and tribological properties of the as-obtained BTSE–OTS films were investigated and compared with those of the BTSE films and OTS monolayers. It is expected that the introduction of the BTSE intermediate layer would endow the BTSE–OTS films with improved tribological properties as compared to the OTS SAMs. 2. Experimental procedure 2.1. Preparation of the self-assembled films Both BTSE (96%) and OTS (≥90%) were obtained from Sigma-Aldrich Corporation (USA) and used as received. The hydrolysis of BTSE was performed according to the procedures reported in the literatures [11,12]. Briefly, the BTSE solution was prepared in a Teflon beaker by mixing BTSE, distilled water and methanol (5:5:90 by vol.%). The pH of the silane solution was adjusted by adding acetic acid to a value of 4.5. Afterwards, the beaker was sealed and the solution was gently stirred at room temperature for 2 days to achieve a homogeneous BTSE hydrolysate.
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The P-type polished (100) Si wafers (Zhiyan Electronic Technology Co., Ltd, China) were sufficiently cleaned and then hydroxylated in a piranha solution (7:3 volume ratio of concentrated H2SO4 and 30% H2O2) at 90 °C for 30 min. After being rinsed with distilled water and dried, the wafers were immersed in the as-prepared BTSE solution for 60 s. Then, the wafers were taken out and ultrasonically washed, followed by thermal treatment at 100 °C for 1 h to generate stable BTSE films. Subsequently, the Si substrates deposited with the BTSE layers were placed in 1 mM OTS-toluene solution for 24 h under nitrogen atmosphere to allow the formation of the BTSE–OTS bilayer films. After being ultrasonically washed and dried, the substrates were heat-treated at 100 °C for 1 h. In addition, the OTS monolayer was also prepared as control by immersing the hydroxylated wafers into the OTS-toluene solution under the same conditions.
2.2. Characterization of the films The films were characterized by X-ray photoelectron spectroscopy (XPS, Thermo ESCALAB 250Xi, USA) to analyze their chemical composition. The surface morphology of the films was observed in tapping mode with an atomic force microscope (AFM, Nanoman VS, Veeco, USA), and their surface roughness was also evaluated. The contact angles of the films were measured with a JGW-360A contact angle meter (Chenghui Testing Machine Company, China). The thickness of the films was determined by a L116-E ellipsometer (Gaertner, USA) equipped with a He–Ne laser (632.8 nm).
2.3. Evaluation of tribological properties of the films The nano-tribological behavior of the self-assemble films was investigated at an applied normal load of 10 nN and a scanning speed of 2 μm/s by using AFM in a contact mode. Silicon nitride cantilevers with a normal force constant of 0.3 N/m were employed. The output voltages were directly used as relative frictional force. At least five separate locations on each sample surface were measured and the average value was calculated. To obtain the adhesive force between the AFM tip and the film surface, the force–distance curve was recorded and the pull-off force was considered as the adhesive force. The micro-tribological tests of the films were performed in a ball-onplate contact configuration using a UMT-2MT tribometer (CETR, USA). The specimens slid reciprocally against Si3N4 balls (φ = 4 mm, Ra = 0.014 μm) at the sliding speed of 2 mm/s and the sliding distance of 5 mm. The normal load was ranged from 0.2 to 1 N. The friction coefficient-versus-time curves were recorded automatically. The surface morphology of the worn films was observed with a scanning electron microscope (SEM, S-3000N, Hitachi, Japan).
3. Results and discussion 3.1. Formation of the BTSE–OTS bilayer films Fig. 1 schematically shows the formation process of the BTSE–OTS bilayer film on the hydroxylated single-crystal silicon wafer. Firstly, the hydrolysis of each BTSE molecule yielded six silanols (Si\\OH). Those silanols in the adjacent BTSE molecules were capable of further condensation, forming small BTSE clusters (step 1). When the hydroxylated silicon wafer was immersed in the BTSE hydrolysate, the BTSE clusters will be chemisorbed onto the silicon substrate through the reaction between silanols and hydroxyl groups, and thus generate the stable BTSE film as the first layer (step 2). Subsequently, the OTS layer was chemically grafted to the surface of the BTSE film via the hydrolysis of the OTS molecules and the following reaction with the silanols on the other side of the BTSE clusters, eventually leading to the formation of the BTSE–OTS bilayer film (step 3). 3.2. Characterization of composition, surface morphology, thickness and contact angles The chemical states of the elements on the surface of the films were detected by XPS. It was observed in Fig. 2 that, the three types of films displayed almost the same chemical compositions because the deposition of the OTS layer on the BTSE film did not introduce any new elements. However, the BTSE–OTS films exhibited an enhanced intensity of the carbon element as compared to both BTSE and OTS monolayers (inset of Fig. 2), which could be induced by the increase in the C carbon content with the self-assembly of the OTS molecules as the second layer. Furthermore, no peaks related to the chlorine element were detected in the spectrum of the BTSE–OTS films, indicating that the trichlorosilane groups of the OTS molecules reacted completely with the hydroxyl groups on the BTSE films. The AFM images of the silicon substrate and different types of selfassembled films were displayed in Fig. 3. Their surface roughness (Rq) was also calculated from the entire 5 × 5 μm2 area of the images and listed in Table 1. It was observed in Fig. 3a that the bare silicon substrate exhibited a quite smooth surface. However, after deposition of the BTSE films on the Si substrates, the surface was characterized by a grainy topography due to the homogeneous formation of the BTSE nano-clusters (Fig. 3b), with the surface roughness of about 1.31 nm. Furthermore, the chemical absorption of the OTS layers onto the BTSE films resulted in a surface that was covered with some round protuberances (Fig. 3c). It was quite similar to the surface morphology of the OTS monolayer shown in Fig. 3d, indicating that the OTS self-assembled film was formed on the BTSE layer. Additionally, the surface roughness of the BTSE–OTS bilayer films was estimated to be 1.82 nm over an area of 5 × 5 μm2, which was slightly higher than 1.68 nm of the OTS
Fig. 1. Schematic drawing of the formation of the BTSE–OTS films on hydroxylated silicon wafers.
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Table 1 Surface roughness (Rq), contact angle and thickness of the self-assembled films. All the data were obtained from at least 5 average measurements. Samples
Surface roughness (Rq, nm)
Water contact angle (°)
Thickness (nm)
BTSE BTSE–OTS OTS
1.31 ± 0.18 1.82 ± 0.24 1.68 ± 0.08
27.2 ± 1.0 98.0 ± 2.1 99.8 ± 1.6
4.7 ± 0.2 2.2 ± 0.1a 2.4 ± 0.1
a
Fig. 2. Typical XPS survey scan spectra of the films and the C1s regions (the inset).
monolayer. This was probably attributed to the relatively loose packing structure of the OTS molecules formed in the outer layer of the BTSE– OTS bilayer films, as manifested by the larger protuberances in Fig. 3c than those in Fig. 3d. The thickness data for the self-assembled films were reported in Table 1. The BTSE film showed an average thickness of 4.7 nm, suggesting that it may consist of several stacked layers of BTSE molecules. This seemed to partially confirm our former statement (Fig. 1) that the BTSE molecules existed in the form of small clusters after hydrolysis. The average thicknesses of the OTS layer in the BTSE–OTS film and the OTS self-assembled monolayer were determined by the ellipsometer to be about 2.2 and 2.4 nm, respectively, even though some protuberances with a height of not more than 5 nm were observed in their AFM images. Their approximately equal thickness values indicated that the OTS molecules had been deposited onto the surface of the BTSE layer and formed a well ordered monolayer.
Fig. 3. AFM morphologies of (a) silicon substrate, (b) BTSE films, (c) BTSE–OTS films, and (d) OTS films over a scanning area of 1 × 1 μm2. For all images, the linear Z-scale is 20 nm.
The value represents the thickness of the OTS layer.
The water contact angles of the self-assembled films, which indicate the chemical composition of solid surfaces, were demonstrated in Fig. 4, and the corresponding values were listed in Table 1. Firstly, the hydroxylated silicon possessed a highly hydrophilic surface (figure not shown), with a water contact angle of ~0°. After hydrolysis and deposition of the BTSE films, the surface was still hydrophilic due to the exposure of many hydroxyl groups on the surface, as previously noted in Fig. 1. In contrast, the BTSE–OTS and OTS films exhibited hydrophobic surfaces with contact angles of 98.0° and 99.8°, respectively, which was consistent with the reported value of the OTS self-assembled film by other authors [13,14]. The hydrophobic behavior can be ascribed to the long alkyl chain of the OTS molecules and the similar mastoid microstructure with lotus leaves formed on the surface of both OTS and BTSE–OTS self-assembled films (Fig. 3c and d). In addition, a high contact angle indicates a low solid surface energy or chemical affinity [4]. Thus, the above four samples can be arranged in order of decreasing interfacial energy: Si\\OH N BTSE N BTSE–OTS N OTS. 3.3. Characterization of adhesive and tribological behaviors in nanoscale The adhesion forces between the AFM tip and the sample surfaces were determined from the AFM force–distance curves, and the results are shown in Fig. 5. Strong adhesion of about 94 nN was observed for the hydroxylated silicon substrate, which can be mainly ascribed to the capillary adhesion caused by the highly hydrophilic surface [15]. After formation of the BTSE, BTSE–OTS and OTS self-assembled films on silicon, the adhesion forces were decreased to about 52, 45 and 33 nN, respectively. It was found that the variation tendency of the adhesion force was opposite to that of the water contact angle shown in Table 1, that is, adhesion force gradually decreased as the hydrophobicity of the surface increased. This was mainly because that the adhesion force and hydrophobicity of the self-assembled films were both closely related to the chemical characteristics of their terminal groups on the surface. The more hydrophobic the film was, the lower the capillary force between the tip and the surface will be, and the better the adhesion-resistance performance of the surface will be [10]. In addition, despite possessing the same terminal groups with the OTS monolayer, the BTSE–OTS bilayer film exhibited a higher adhesion force, probably due to the relatively loose packing of the OTS molecules in the outer layer. The nano-tribological behaviors of the silicon wafer and three kinds of self-assembled films were evaluated by AFM. The friction forces shown in Fig. 6 are given in the form of voltage signal (mV), which is proportional to the real friction force. Among the four samples, the uncoated silicon wafer exhibited the highest friction of about 288.3 mV because of its high interfacial energy indicated by the low water contact angle. The nano-friction forces of the self-assembled films were greatly decreased in the order of BTSE N BTSE–OTS N OTS, with the values of 88.9, 45.2, and 42.3 mV, respectively. It was inferred that the presence of the OTS outer layers made the BTSE–OTS bilayer films more efficient in reducing the friction force on silicon than the BTSE films. The good friction-reducing behavior of the BTSE–OTS films could be mainly attributed to the long-chain OTS molecules in the outer layer. Under the normal load applied by the AFM tip, these OTS molecules with one end chemically attached to the BTSE layer can swing freely and rearrange themselves along the sliding direction, therefore yielding
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Fig. 4. Pictures showing contact angles of deionized water on different types of self-assembled films: (a) BTSE, (b) BTSE–OTS, and (c) OTS.
a smaller resistance. Furthermore, the high hydrophobicity of the OTS layer endowed the BTSE–OTS films with low interfacial energy and adhesive force, which correspondingly reduced the friction force.
3.4. Characterization of tribological behaviors in microscale The micro-tribological behaviors of the films were investigated in a ball-on-plate contact configuration using a UMT-2MT tribometer. Fig. 7 shows the friction coefficient versus time curves of the selfassembled films under various loads. It was assumed that the lubrication failure of the films occurred when the friction coefficient rose abruptly to a stable value of ~0.6 (a typical value of friction coefficient for the silicon wafer). As illustrated in Fig. 7a, the BTSE films had an initial friction coefficient of ~0.16 and no obvious lubrication failure of the films occurred in 1 h under a load of 0.2 N. Although the OTS monolayer displayed a low friction coefficient of ~0.05, it was easily destroyed after ~1500 s. Comparatively, the BTSE–OTS bilayer films displayed a greatly improved tribological properties characterized by a much lower friction coefficient (~ 0.05) than the BTSE films and a greatly longer wear life (over 1 h) than the OTS monolayers. Moreover, even at an applied load of 0.5 N, the BTSE–OTS films still retained their good lubricity after 1 h of reciprocal slide against the counterpart, while the BTSE films were destroyed after ~2000 s (Fig. 7b). The lubrication failure of the BTSE–OTS bilayer films occurred until ~ 1200 s of sliding under 1.0 N, as observed in Fig. 7c. The morphology of the worn surface of the BTSE–OTS films after lubricant failure is shown in Fig. 7d. It was observed that the films were damaged after sliding against Si3N4 balls under 1.0 N for 1300 s, and some small particles were generated on the surface, which may account for the abrasive wear of the BTSE–OTS films. The remarkable improvement of the BTSE–OTS films in durability and load-carrying capability as compared to the BTSE films and the OTS monolayers could be primarily correlated with their stable bilayer structure. As displayed in Fig. 1, the interlayer combinations of the BTSE–OTS films as well as their adhesion to the Si substrates were all
Fig. 5. Nano-scale adhesion forces between the AFM tip and the surfaces of the tested samples.
accomplished by chemical bonding, which will contribute to the enhancement of the mechanical stability and wear-resistance capacity of the films. Moreover, the good anti-wear durability of the BTSE films that had been demonstrated above can ensure the mechanical stability of the bilayer films to withstand normal wear. Under shear stress, the relatively stiff, thick BTSE film was also expected to serve as a transitional layer to efficiently buffer the impact of the external forces and accordingly postpone the lubrication failure of the BTSE–OTS films. In addition, the bilayer structure of the BTSE–OTS films may permit a transfer of the films from the Si substrates to the surface of the Si3N4 balls even though the outer OTS layer was completely destroyed. Afterwards, the friction would occur at the interface of the transferred OTS film and the BTSE layer, which was desirable to significantly extend the wear life of the films. Mechanical stability or the ability of a thin film to withstand a normal load while maintaining beneficial lubrication is an important property for MEMS/NEMS applications. Therefore, it could be anticipated that the BTSE–OTS bilayer film with excellent anti-wear durability and load-carrying capacity as well as good lubricity would serve as an appropriate boundary lubricant for MEMSs/NEMSs.
4. Conclusions In this paper, the BTSE–OTS bilayer film was developed on singlecrystal silicon substrates via a self-assembled process. The surface morphology, wettability, adhesive force and tribological properties of the film were investigated. The results showed that the BTSE–OTS bilayer film had a surface roughness of about 1.82 and was hydrophobic with a contact angle of about 98°. Moreover, the BTSE–OTS film possessed good adhesive resistance and greatly reduced nano-friction force. In micro-scale, the film exhibited a low friction coefficient of ~ 0.05 as well as much better anti-wear durability and load-carrying capacity than the OTS monolayer. The OTS monolayer was easily destroyed after ~ 1500 s of sliding under 0.2 N, whereas the BTSE–OTS film sustained their good lubricity for over 1 h under both 0.2 and 0.5 N,
Fig. 6. Nano-scale friction values of the tested samples at an applied normal load of 10 nN.
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Fig. 7. Curves of friction coefficient vs time for three types of self-assemble films under different loads and the SEM image showing the wear track of the BTSE–OTS films after sliding against Si3N4 balls under 1.0 N for 1300 s.
and lubrication failure occurred until ~1200 s of sliding under 1.0 N. The results suggested the effectiveness of the BTSE intermediate layer in improving the tribological performances of the OTS self-assembled monolayer on silicon. Acknowledgments The authors are grateful to the Fundamental Research Funds for the Central Universities (No. 2013QNA06) for financial support. References [1] Z. Kang, W. Liu, Y. Liu, Preparation and micro-tribological property of hydrophilic selfassembled monolayer on single crystal silicon surface, Wear 303 (2013) 297–301. [2] B. Lu, Y. Cai, Molecular tilting and its impact on frictional properties of n-alkane self-assembled monolayers, Langmuir 27 (2011) 5953–5960. [3] B.D. Booth, S.G. Vilt, J.B. Lweis, J.L. Rivera, E.A. Buehler, C. MaCabe, G.K. Jennings, Tribological durability of silane monolayers on silicon, Langmuir 27 (2011) 5909–5917. [4] H.-S. Ahn, P.D. Cuong, S. Park, Y.-W. Kim, J.-C. Lim, Effect of molecular structure of self-assembled monolayers on their tribological behaviors in nano- and microscales, Wear 255 (2003) 819–825. [5] J. Sagiv, Organized monolayers by adsorption. 1. Formation and structure of oleophobic mixed monolayers on solid surfaces, J. Am. Chem. Soc. 102 (1980) 92–98.
[6] S. Ren, S. Yang, Y. Zhao, J. Zhou, T. Xu, W. Liu, Friction and wear studies of octadecyltrichlorosilane SAM on silicon, Tribol. Lett. 13 (2002) 233–239. [7] Y. Wang, L. Wang, Y. Mo, Q. Xue, Fabrication and tribological behavior of patterned multiply-alkylated cyclopentanes (MACs)–octadecyltrichlorosilane (OTS) dual-component film by a soft lithographic approach, Tribol. Lett. 41 (2011) 163–170. [8] S. Ren, S. Yang, Y. Zhao, Micro- and macro-tribological study on a self-assembled dual-layer film, Langmuir 19 (2003) 2763–2767. [9] J. Pu, L. Wang, Y. Mo, Q. Xue, Preparation and characterization of ultrathin dual-layer ionic liquid lubrication film assembled on silica surfaces, J. Colloid Interface Sci. 354 (2011) 858–865. [10] J. Ou, J. Wang, S. Liu, J. Zhou, S. Yang, Self-assembly and tribological property of a novel 3-layer organic film on silicon wafer with polydopamine coating as the interlayer, J. Phys. Chem. C 113 (2009) 20429–20434. [11] A. Franquet, H. Terryn, P. Bertrand, J. Vereecken, Use of optical methods to characterize thin silane films coated on aluminium, Surf. Interface Anal. 34 (2002) 25–29. [12] A. Franque, H. Terryn, J. Vereecken, Composition and thickness of non-functional organosilane films coated on aluminium studied by means of infra-red spectroscopic ellipsometry, Thin Solid Films 441 (2003) 76–84. [13] Y. Cheng, B. Zheng, P. Chuang, S. Hsieh, Solvent effects on molecular packing and tribological properties of octadecyltrichlorosilane films on silicon, Langmuir 26 (2010) 8256–8261. [14] K.-H. Cha, D.-E. Kim, Investigation of the tribological behavior of octadecyltrichlorosilane deposited on silicon, Wear 251 (2001) 1169–1176. [15] B. Yu, L. Qian, J. Yu, Z. Zhou, Effects of tail group and chain length on the tribological behaviors of self-assembled dual-layer films in atmosphere and in vacuum, Tribol. Lett. 34 (2009) 1–10.
Contents lists available at ScienceDirect
Thin Solid Films journal homepage: www.elsevier.com/locate/tsf
Fabrication and tribological properties of a self-assembled silane bilayer on silicon Peng Wang ⁎, Shu-Hua Teng School of Materials Science and Engineering, China University of Mining and Technology, Xuzhou 221116, China
a r t i c l e
i n f o
Article history: Received 11 September 2014 Received in revised form 30 August 2015 Accepted 3 September 2015 Available online 5 September 2015 Keywords: Self-assembly Octadecyltrichlorosilane Bilayer films Surface morphology Tribological property
a b s t r a c t A self-assembled bilayer film of 1,2-bis(triethoxysilyl) ethane–octadecyltrichlorosilane (BTSE–OTS) was prepared on single-crystal silicon substrates by utilizing BTSE as an intermediate layer. The surface morphology of the film was characterized by small round protuberances, with a surface roughness of about 1.82 nm. Moreover, the BTSE– OTS bilayer film displayed comparable hydrophobicity and nano-scale friction behaviors to the OTS monolayer owing to their identical surface composition and similar surface morphology. The investigation of the microtribological properties revealed that the BTSE–OTS bilayer film possessed much higher anti-wear durability and load-carrying capacity than the OTS monolayer while maintaining the low friction coefficient (~ 0.05). The BTSE–OTS bilayer film may find expanded applications in the lubrication of micro-/nano-electromechanical systems as compared to the conventional OTS monolayer. © 2015 Elsevier B.V. All rights reserved.
1. Introduction As a preferred kind of boundary lubricant, self-assembled monolayers (SAMs) have gained tremendous growth over the past few decades due to their effectiveness in improving the reliability and stability of the micro-/nano-electromechanical systems (MEMSs/NEMSs) by reducing adhesion and friction on the surface of the devices [1,2]. Moreover, they provide the ability to tailor the tribological properties of the silicon surface at the molecular level by varying the molecular composition or structure [3,4]. Especially, the monolayers of organosilicon derivatives on silicon wafers have attracted more attention because of their superior tribological properties, among which the octadecyltrichlorosilane (OTS) SAM is the first reported one and has undergone the most extensive studies [5–7]. The OTS SAM is a densely packed and well-ordered monolayer film, covalently bound to the silicon substrate. Nevertheless, despite being known for its reputably low friction coefficient, the OTS SAM exhibited poor anti-wear durability and load-carrying capacity due to its monolayer structure and flexibility, which will greatly limit its wider applications [8]. The mechanical durability and load-carrying capacity of the organic monolayers when subjected to a tribological load can be greatly enhanced by constructing self-assembled bilayer or multilayer films [9,10]. The formation of the layered films usually requires the molecules in the first layer of SAMs to provide reactive terminal groups capable of mediating the deposition of a second layer. Various types of selfassembled bilayer and multilayer films have been fabricated in an effort ⁎ Corresponding author. E-mail address: [email protected] (P. Wang).
http://dx.doi.org/10.1016/j.tsf.2015.09.007 0040-6090/© 2015 Elsevier B.V. All rights reserved.
to improve the tribological properties of the organic monolayers. However, until recently there has been little research conducted on the layered films for improving the tribological performance of the OTS SAMs. In the present paper, the 1,2-bis(triethoxysilyl) ethane (BTSE) films were firstly prepared on single-crystal silicon substrates via hydrolysis and subsequent self-assembly processes. Thereafter, the residual hydroxyl groups (\\OH) on the surface of the BTSE films may play a similar role as the hydroxylated silicon substrate to further chemically adsorb another self-assembled monolayer onto the hydroxyl-terminated surface, thus making it possible to generate the BTSE–OTS bilayer films. The surface morphology, wettability, adhesive behavior and tribological properties of the as-obtained BTSE–OTS films were investigated and compared with those of the BTSE films and OTS monolayers. It is expected that the introduction of the BTSE intermediate layer would endow the BTSE–OTS films with improved tribological properties as compared to the OTS SAMs. 2. Experimental procedure 2.1. Preparation of the self-assembled films Both BTSE (96%) and OTS (≥90%) were obtained from Sigma-Aldrich Corporation (USA) and used as received. The hydrolysis of BTSE was performed according to the procedures reported in the literatures [11,12]. Briefly, the BTSE solution was prepared in a Teflon beaker by mixing BTSE, distilled water and methanol (5:5:90 by vol.%). The pH of the silane solution was adjusted by adding acetic acid to a value of 4.5. Afterwards, the beaker was sealed and the solution was gently stirred at room temperature for 2 days to achieve a homogeneous BTSE hydrolysate.
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The P-type polished (100) Si wafers (Zhiyan Electronic Technology Co., Ltd, China) were sufficiently cleaned and then hydroxylated in a piranha solution (7:3 volume ratio of concentrated H2SO4 and 30% H2O2) at 90 °C for 30 min. After being rinsed with distilled water and dried, the wafers were immersed in the as-prepared BTSE solution for 60 s. Then, the wafers were taken out and ultrasonically washed, followed by thermal treatment at 100 °C for 1 h to generate stable BTSE films. Subsequently, the Si substrates deposited with the BTSE layers were placed in 1 mM OTS-toluene solution for 24 h under nitrogen atmosphere to allow the formation of the BTSE–OTS bilayer films. After being ultrasonically washed and dried, the substrates were heat-treated at 100 °C for 1 h. In addition, the OTS monolayer was also prepared as control by immersing the hydroxylated wafers into the OTS-toluene solution under the same conditions.
2.2. Characterization of the films The films were characterized by X-ray photoelectron spectroscopy (XPS, Thermo ESCALAB 250Xi, USA) to analyze their chemical composition. The surface morphology of the films was observed in tapping mode with an atomic force microscope (AFM, Nanoman VS, Veeco, USA), and their surface roughness was also evaluated. The contact angles of the films were measured with a JGW-360A contact angle meter (Chenghui Testing Machine Company, China). The thickness of the films was determined by a L116-E ellipsometer (Gaertner, USA) equipped with a He–Ne laser (632.8 nm).
2.3. Evaluation of tribological properties of the films The nano-tribological behavior of the self-assemble films was investigated at an applied normal load of 10 nN and a scanning speed of 2 μm/s by using AFM in a contact mode. Silicon nitride cantilevers with a normal force constant of 0.3 N/m were employed. The output voltages were directly used as relative frictional force. At least five separate locations on each sample surface were measured and the average value was calculated. To obtain the adhesive force between the AFM tip and the film surface, the force–distance curve was recorded and the pull-off force was considered as the adhesive force. The micro-tribological tests of the films were performed in a ball-onplate contact configuration using a UMT-2MT tribometer (CETR, USA). The specimens slid reciprocally against Si3N4 balls (φ = 4 mm, Ra = 0.014 μm) at the sliding speed of 2 mm/s and the sliding distance of 5 mm. The normal load was ranged from 0.2 to 1 N. The friction coefficient-versus-time curves were recorded automatically. The surface morphology of the worn films was observed with a scanning electron microscope (SEM, S-3000N, Hitachi, Japan).
3. Results and discussion 3.1. Formation of the BTSE–OTS bilayer films Fig. 1 schematically shows the formation process of the BTSE–OTS bilayer film on the hydroxylated single-crystal silicon wafer. Firstly, the hydrolysis of each BTSE molecule yielded six silanols (Si\\OH). Those silanols in the adjacent BTSE molecules were capable of further condensation, forming small BTSE clusters (step 1). When the hydroxylated silicon wafer was immersed in the BTSE hydrolysate, the BTSE clusters will be chemisorbed onto the silicon substrate through the reaction between silanols and hydroxyl groups, and thus generate the stable BTSE film as the first layer (step 2). Subsequently, the OTS layer was chemically grafted to the surface of the BTSE film via the hydrolysis of the OTS molecules and the following reaction with the silanols on the other side of the BTSE clusters, eventually leading to the formation of the BTSE–OTS bilayer film (step 3). 3.2. Characterization of composition, surface morphology, thickness and contact angles The chemical states of the elements on the surface of the films were detected by XPS. It was observed in Fig. 2 that, the three types of films displayed almost the same chemical compositions because the deposition of the OTS layer on the BTSE film did not introduce any new elements. However, the BTSE–OTS films exhibited an enhanced intensity of the carbon element as compared to both BTSE and OTS monolayers (inset of Fig. 2), which could be induced by the increase in the C carbon content with the self-assembly of the OTS molecules as the second layer. Furthermore, no peaks related to the chlorine element were detected in the spectrum of the BTSE–OTS films, indicating that the trichlorosilane groups of the OTS molecules reacted completely with the hydroxyl groups on the BTSE films. The AFM images of the silicon substrate and different types of selfassembled films were displayed in Fig. 3. Their surface roughness (Rq) was also calculated from the entire 5 × 5 μm2 area of the images and listed in Table 1. It was observed in Fig. 3a that the bare silicon substrate exhibited a quite smooth surface. However, after deposition of the BTSE films on the Si substrates, the surface was characterized by a grainy topography due to the homogeneous formation of the BTSE nano-clusters (Fig. 3b), with the surface roughness of about 1.31 nm. Furthermore, the chemical absorption of the OTS layers onto the BTSE films resulted in a surface that was covered with some round protuberances (Fig. 3c). It was quite similar to the surface morphology of the OTS monolayer shown in Fig. 3d, indicating that the OTS self-assembled film was formed on the BTSE layer. Additionally, the surface roughness of the BTSE–OTS bilayer films was estimated to be 1.82 nm over an area of 5 × 5 μm2, which was slightly higher than 1.68 nm of the OTS
Fig. 1. Schematic drawing of the formation of the BTSE–OTS films on hydroxylated silicon wafers.
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Table 1 Surface roughness (Rq), contact angle and thickness of the self-assembled films. All the data were obtained from at least 5 average measurements. Samples
Surface roughness (Rq, nm)
Water contact angle (°)
Thickness (nm)
BTSE BTSE–OTS OTS
1.31 ± 0.18 1.82 ± 0.24 1.68 ± 0.08
27.2 ± 1.0 98.0 ± 2.1 99.8 ± 1.6
4.7 ± 0.2 2.2 ± 0.1a 2.4 ± 0.1
a
Fig. 2. Typical XPS survey scan spectra of the films and the C1s regions (the inset).
monolayer. This was probably attributed to the relatively loose packing structure of the OTS molecules formed in the outer layer of the BTSE– OTS bilayer films, as manifested by the larger protuberances in Fig. 3c than those in Fig. 3d. The thickness data for the self-assembled films were reported in Table 1. The BTSE film showed an average thickness of 4.7 nm, suggesting that it may consist of several stacked layers of BTSE molecules. This seemed to partially confirm our former statement (Fig. 1) that the BTSE molecules existed in the form of small clusters after hydrolysis. The average thicknesses of the OTS layer in the BTSE–OTS film and the OTS self-assembled monolayer were determined by the ellipsometer to be about 2.2 and 2.4 nm, respectively, even though some protuberances with a height of not more than 5 nm were observed in their AFM images. Their approximately equal thickness values indicated that the OTS molecules had been deposited onto the surface of the BTSE layer and formed a well ordered monolayer.
Fig. 3. AFM morphologies of (a) silicon substrate, (b) BTSE films, (c) BTSE–OTS films, and (d) OTS films over a scanning area of 1 × 1 μm2. For all images, the linear Z-scale is 20 nm.
The value represents the thickness of the OTS layer.
The water contact angles of the self-assembled films, which indicate the chemical composition of solid surfaces, were demonstrated in Fig. 4, and the corresponding values were listed in Table 1. Firstly, the hydroxylated silicon possessed a highly hydrophilic surface (figure not shown), with a water contact angle of ~0°. After hydrolysis and deposition of the BTSE films, the surface was still hydrophilic due to the exposure of many hydroxyl groups on the surface, as previously noted in Fig. 1. In contrast, the BTSE–OTS and OTS films exhibited hydrophobic surfaces with contact angles of 98.0° and 99.8°, respectively, which was consistent with the reported value of the OTS self-assembled film by other authors [13,14]. The hydrophobic behavior can be ascribed to the long alkyl chain of the OTS molecules and the similar mastoid microstructure with lotus leaves formed on the surface of both OTS and BTSE–OTS self-assembled films (Fig. 3c and d). In addition, a high contact angle indicates a low solid surface energy or chemical affinity [4]. Thus, the above four samples can be arranged in order of decreasing interfacial energy: Si\\OH N BTSE N BTSE–OTS N OTS. 3.3. Characterization of adhesive and tribological behaviors in nanoscale The adhesion forces between the AFM tip and the sample surfaces were determined from the AFM force–distance curves, and the results are shown in Fig. 5. Strong adhesion of about 94 nN was observed for the hydroxylated silicon substrate, which can be mainly ascribed to the capillary adhesion caused by the highly hydrophilic surface [15]. After formation of the BTSE, BTSE–OTS and OTS self-assembled films on silicon, the adhesion forces were decreased to about 52, 45 and 33 nN, respectively. It was found that the variation tendency of the adhesion force was opposite to that of the water contact angle shown in Table 1, that is, adhesion force gradually decreased as the hydrophobicity of the surface increased. This was mainly because that the adhesion force and hydrophobicity of the self-assembled films were both closely related to the chemical characteristics of their terminal groups on the surface. The more hydrophobic the film was, the lower the capillary force between the tip and the surface will be, and the better the adhesion-resistance performance of the surface will be [10]. In addition, despite possessing the same terminal groups with the OTS monolayer, the BTSE–OTS bilayer film exhibited a higher adhesion force, probably due to the relatively loose packing of the OTS molecules in the outer layer. The nano-tribological behaviors of the silicon wafer and three kinds of self-assembled films were evaluated by AFM. The friction forces shown in Fig. 6 are given in the form of voltage signal (mV), which is proportional to the real friction force. Among the four samples, the uncoated silicon wafer exhibited the highest friction of about 288.3 mV because of its high interfacial energy indicated by the low water contact angle. The nano-friction forces of the self-assembled films were greatly decreased in the order of BTSE N BTSE–OTS N OTS, with the values of 88.9, 45.2, and 42.3 mV, respectively. It was inferred that the presence of the OTS outer layers made the BTSE–OTS bilayer films more efficient in reducing the friction force on silicon than the BTSE films. The good friction-reducing behavior of the BTSE–OTS films could be mainly attributed to the long-chain OTS molecules in the outer layer. Under the normal load applied by the AFM tip, these OTS molecules with one end chemically attached to the BTSE layer can swing freely and rearrange themselves along the sliding direction, therefore yielding
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Fig. 4. Pictures showing contact angles of deionized water on different types of self-assembled films: (a) BTSE, (b) BTSE–OTS, and (c) OTS.
a smaller resistance. Furthermore, the high hydrophobicity of the OTS layer endowed the BTSE–OTS films with low interfacial energy and adhesive force, which correspondingly reduced the friction force.
3.4. Characterization of tribological behaviors in microscale The micro-tribological behaviors of the films were investigated in a ball-on-plate contact configuration using a UMT-2MT tribometer. Fig. 7 shows the friction coefficient versus time curves of the selfassembled films under various loads. It was assumed that the lubrication failure of the films occurred when the friction coefficient rose abruptly to a stable value of ~0.6 (a typical value of friction coefficient for the silicon wafer). As illustrated in Fig. 7a, the BTSE films had an initial friction coefficient of ~0.16 and no obvious lubrication failure of the films occurred in 1 h under a load of 0.2 N. Although the OTS monolayer displayed a low friction coefficient of ~0.05, it was easily destroyed after ~1500 s. Comparatively, the BTSE–OTS bilayer films displayed a greatly improved tribological properties characterized by a much lower friction coefficient (~ 0.05) than the BTSE films and a greatly longer wear life (over 1 h) than the OTS monolayers. Moreover, even at an applied load of 0.5 N, the BTSE–OTS films still retained their good lubricity after 1 h of reciprocal slide against the counterpart, while the BTSE films were destroyed after ~2000 s (Fig. 7b). The lubrication failure of the BTSE–OTS bilayer films occurred until ~ 1200 s of sliding under 1.0 N, as observed in Fig. 7c. The morphology of the worn surface of the BTSE–OTS films after lubricant failure is shown in Fig. 7d. It was observed that the films were damaged after sliding against Si3N4 balls under 1.0 N for 1300 s, and some small particles were generated on the surface, which may account for the abrasive wear of the BTSE–OTS films. The remarkable improvement of the BTSE–OTS films in durability and load-carrying capability as compared to the BTSE films and the OTS monolayers could be primarily correlated with their stable bilayer structure. As displayed in Fig. 1, the interlayer combinations of the BTSE–OTS films as well as their adhesion to the Si substrates were all
Fig. 5. Nano-scale adhesion forces between the AFM tip and the surfaces of the tested samples.
accomplished by chemical bonding, which will contribute to the enhancement of the mechanical stability and wear-resistance capacity of the films. Moreover, the good anti-wear durability of the BTSE films that had been demonstrated above can ensure the mechanical stability of the bilayer films to withstand normal wear. Under shear stress, the relatively stiff, thick BTSE film was also expected to serve as a transitional layer to efficiently buffer the impact of the external forces and accordingly postpone the lubrication failure of the BTSE–OTS films. In addition, the bilayer structure of the BTSE–OTS films may permit a transfer of the films from the Si substrates to the surface of the Si3N4 balls even though the outer OTS layer was completely destroyed. Afterwards, the friction would occur at the interface of the transferred OTS film and the BTSE layer, which was desirable to significantly extend the wear life of the films. Mechanical stability or the ability of a thin film to withstand a normal load while maintaining beneficial lubrication is an important property for MEMS/NEMS applications. Therefore, it could be anticipated that the BTSE–OTS bilayer film with excellent anti-wear durability and load-carrying capacity as well as good lubricity would serve as an appropriate boundary lubricant for MEMSs/NEMSs.
4. Conclusions In this paper, the BTSE–OTS bilayer film was developed on singlecrystal silicon substrates via a self-assembled process. The surface morphology, wettability, adhesive force and tribological properties of the film were investigated. The results showed that the BTSE–OTS bilayer film had a surface roughness of about 1.82 and was hydrophobic with a contact angle of about 98°. Moreover, the BTSE–OTS film possessed good adhesive resistance and greatly reduced nano-friction force. In micro-scale, the film exhibited a low friction coefficient of ~ 0.05 as well as much better anti-wear durability and load-carrying capacity than the OTS monolayer. The OTS monolayer was easily destroyed after ~ 1500 s of sliding under 0.2 N, whereas the BTSE–OTS film sustained their good lubricity for over 1 h under both 0.2 and 0.5 N,
Fig. 6. Nano-scale friction values of the tested samples at an applied normal load of 10 nN.
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Fig. 7. Curves of friction coefficient vs time for three types of self-assemble films under different loads and the SEM image showing the wear track of the BTSE–OTS films after sliding against Si3N4 balls under 1.0 N for 1300 s.
and lubrication failure occurred until ~1200 s of sliding under 1.0 N. The results suggested the effectiveness of the BTSE intermediate layer in improving the tribological performances of the OTS self-assembled monolayer on silicon. Acknowledgments The authors are grateful to the Fundamental Research Funds for the Central Universities (No. 2013QNA06) for financial support. References [1] Z. Kang, W. Liu, Y. Liu, Preparation and micro-tribological property of hydrophilic selfassembled monolayer on single crystal silicon surface, Wear 303 (2013) 297–301. [2] B. Lu, Y. Cai, Molecular tilting and its impact on frictional properties of n-alkane self-assembled monolayers, Langmuir 27 (2011) 5953–5960. [3] B.D. Booth, S.G. Vilt, J.B. Lweis, J.L. Rivera, E.A. Buehler, C. MaCabe, G.K. Jennings, Tribological durability of silane monolayers on silicon, Langmuir 27 (2011) 5909–5917. [4] H.-S. Ahn, P.D. Cuong, S. Park, Y.-W. Kim, J.-C. Lim, Effect of molecular structure of self-assembled monolayers on their tribological behaviors in nano- and microscales, Wear 255 (2003) 819–825. [5] J. Sagiv, Organized monolayers by adsorption. 1. Formation and structure of oleophobic mixed monolayers on solid surfaces, J. Am. Chem. Soc. 102 (1980) 92–98.
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