- Email: [email protected]
Nanopatterning of metallic features over uniformed arrays of microbowl structures
Microelectronic Engineering 98 (2012) 266–269
Contents lists available at SciVerse ScienceDirect
Microelectronic Engineering journal homepage: www.elsevier.com/locate/mee
Nanopatterning of metallic features over uniformed arrays of microbowl structures A. Mohammadkhani ⇑, H. Hassanin, C. Anthony, K. Jiang School of Mechanical Engineering, University of Birmingham, Birmingham, B15 2TT, UK
a r t i c l e
i n f o
Article history: Available online 19 June 2012 Keywords: Nanopattern Nanosphere lithography Soft lithography PDMS Microbowl
a b s t r a c t There is a growing interest for formation of periodic nanostructures in high-throughput and low-cost processes, due to their wide range of applications. This paper demonstrates a facile and efficient method to fabricate ordered gold nanopatterns over uniformed arrays of poly (dimethylsiloxane) (PDMS) microbowl structures by combining nanosphere lithography (NSL) and soft lithography. A monolayer of selfassembled polystyrene (PS) spheres is used as a template to create the PDMS microbowl structures. Another self assembly is processed on the PDMS mould followed by gold deposition, resulting in periodic pillars on the surface of microbowl structure. After removal of the microspheres, an array of highly ordered gold nanopatterns over hexagonal distributed pillars with perpendicular bisector of 210 nm and height of 50 nm in average are achieved. The geometry of the nanopillars with curvilinear triangle surface can be tuned by changing the size of spheres. This approach can be also extended to fabricate ordered periodic nanostructures of a wide range of metals (Pt, Ag, Cu, etc.) with controlled size. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction Advances in periodical nanostructure with different patterns are widely studied for various applications. For instance, ordered nanohole and nanodisk arrays are useful for nanopatterning, data storage, and potentially for nanoscale waveguides [1,2]. In recent years, uniformed array of triangular gold or silver nanoparticles on glass substrates have been demonstrated as biosensors owing to their remarkable sensitivity using localized surface plasmon resonance [3]. One of the constant challenges is to find cost effective methods to fabricate nanostructures. Nanoimprint Lithography (NIL), soft lithography and nanosphere lithography are commonly preferred to those high costs and time consuming fabrication techniques like electron beam lithography and focused ion beam lithography [4]. NIL method is straightforward to be implemented, but it needs nanoimprint templates which require the same resolutions as the patterns. Among these fabrication techniques, nanosphere lithography and soft lithography are both extensively applied because of their potential to create large area and complicated shapes of nanostructure. The conventional NSL process begins with self-assembly of sub-micron size spheres as a mask onto a substrate which can be followed by either deposition of a material or etching of a substrate through the mask [5]. In soft lithography, an elastomeric ⇑ Corresponding author. E-mail addresses: [email protected], (A. Mohammadkhani), [email protected] (K. Jiang).
[email protected]
0167-9317/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.mee.2012.05.064
stamp based on PDMS is usually used as a mould with relief structures on its surface to fabricate components with nano features [6]. Fabrication of nanostructures based on NSL or soft lithography has been widely studied. A large area of well ordered closely packed spheres with defect-free area up to 1 cm2 has been reported by Li et al. [7]. A large area nanoscale gold hemisphere pattern was fabricated by nanosphere lithography combined with electroplating [8]. A fabrication procedure that can produce periodic and size-tuneable silicon nanopillar arrays via NSL has been presented by Kuo et al. [9]. Arrays of silicon nanopillars with aspect ratio of 10 were fabricated using a combination of the deep reactive ion etching and NSL techniques [10]. A sequential replica moulding based on soft lithography combined with colloidal selfassembly has been applied to construct arrays of hemispherical microlens with conformal surface [11]. Ren et al. demonstrated a formation process to produce PDMS microwells with nonclosely-packed arrangements via moulding of colloidal crystals surface [12]. In this paper, a facile and efficient process to fabricate arrays of three dimensional (3D) nanostructures by a combination of nanosphere lithography and soft lithography techniques is presented. The method begins with fabrication of the master mould by selfassembly of microspheres onto a substrate followed by formation of PDMS replica microbowls. The fabricated PDMS soft mould is then considered as the secondary substrate for another self assembly process of microspheres. Gold particles are then deposited through PS spheres mask. The uniformed arrays of triangular gold nanopatterns on microbowl structures are revealed after mask removal.
A. Mohammadkhani et al. / Microelectronic Engineering 98 (2012) 266–269
2. Experimental details Fig. 1 schematically shows the fabrication process. A 1.5 1.5 cm silicon substrate is firstly treated by piranha solution (1:3, H2O2:H2SO4) at 80 °C for 30 min. Once cooled, the substrate is rinsed with copious amounts of distilled water. The substrate is then sonicated in a mixture solution (1:1:5, NH3:H2O2:H2O) for 40 min to increase a hydrophilicity of surface and followed by rinsing repeatedly with distilled water. Next, a micropipette is used to drop 10 ll of 2 wt.% 1.1 lm polystyrene microspheres in aqueous suspensions onto the silicon substrate. After a monolayer of PS microspheres is obtained on the silicon substrate, the structures are used as a template for soft lithography process. The template is placed in 20 20 5 mm plastic box and PDMS is used to produce a soft mould. PDMS is prepared by first mixing Sylgard 184 precursors (1:10, curing agent: elastomer) in a glass beaker and followed by de-airing of the mixture under vacuum. The PDMS slurry is then poured onto the plastic box to cover entire surface of template and placed in a vacuum chamber to remove all residual bubbles. Next, it is cured at 60 °C for 2 h and cooled down. The cured PDMS is gently peeled off from the template. The fabricated mould is then sonicated in ethanol to be cleaned of any residual colloidal spheres. This soft mould is then considered as a secondary
267
substrate to reassemble a monolayer of microspheres with the same mean diameter onto it. In the course of self-assembly of 10 ll of PS aqueous solution onto PDMS microbowls, due to hydrophobicity of PDMS surface, a megasonic system with submersible transducer is applied. Subsequently, thermal evaporation system is used to get a 50 nm gold film over PDMS microbowls with self-assembled microspheres on top. This process is finally followed by removal of PS spheres from the surface of soft mould by sonicating in ethanol bath for a short time to get uniformed arrays of gold nanopatterns over PDMS microbowl structures.
3. Results and discussion Drop coating and spin coating are known as effective methods to form uniformed arrays of colloidal spheres. In this research, area of several hundred square micrometers with uniform arrays of spheres was obtained by using micropipette deposition. A scanning electron microscope (SEM) was applied to inspect the result of each step described above. A monolayer of self-assembled PS spheres over silicon substrate is shown in Fig. 2. The uniformity of the monolayer is highly affected by the concentration of PS solution, the substrate preparation and the environmental condition.
Fig. 1. Schematic of fabrication process: (a) PS spheres assembly on silicon substrate, (b) covering spheres with PDMS, (c) peeling PDMS off and clean it, (d) Reassembly of spheres on PDMS mould, (e) gold deposition over structure and (f) revealing of gold nanopatterns after PS spheres removal.
268
A. Mohammadkhani et al. / Microelectronic Engineering 98 (2012) 266–269
Fig. 2. SEM image of self-assembled microspheres on silicon substrate.
Fig. 5. A monolayer of self-assembled PS spheres over PDMS mould.
Fig. 6. Effect of the sphere size on the curvilinear triangle profile. Fig. 3. SEM image of PDMS microbowl structures.
Fig. 4. Arrays of curvilinear triangular pillars slightly above microbowl structures.
Since PDMS has many features including flexibility, thermally and electrically insulating, optically transparent and relatively inert, it can be used as a popular elastomer material to render more
complex structures. Fig. 3 illustrates PDMS microbowl structures after being peeled off from the template. It can be clearly observed that the microbowl structures are uniformly patterned with smooth concave surface. When the PDMS slurry is prepared by adding curing agent into the elastomer and poured onto template, it is essential to evacuate all trapped gas in the sample to let the PDMS penetrate into existing spaces between spheres. The sample was also left in vacuum chamber for two hours to make sure that all residual voids are filled and cross link is initiated. The curing process has been done at a temperature lower than normally used to cure PDMS to prevent any deformation of the PS spheres as the glass transition temperature of polystyrene is about 95 °C and the melting point is about 240 °C. The morphology of the microbowls is included the reverse profile of the top half part of microspheres and the distributed pillars at the corners of each hexagonal structure. As it is shown in Fig. 4 the vertices of each hexagonal pattern are curvilinear triangular in shape and they are placed slightly above other parts. These pillar arrays are formed by the interstitial spaces between the PS spheres. To create gold nanopatterns over these distributed pillars, another self assembly process is required. A monolayer of self-assembled PS spheres onto microbowl structures is shown in Fig. 5. As it is clearly observed from this figure, the voids between spheres over PDMS mould are narrower in comparison of the voids between spheres over silicon substrate in Fig. 2, due to the existence of
A. Mohammadkhani et al. / Microelectronic Engineering 98 (2012) 266–269
269
on the curvilinear triangle profile is illustrated in Fig. 6. As can be seen, the surface area of curvilinear triangle increases exponentially with the size increase of PS spheres. The following geometric calculation also shows the relationship between the surface area of curvilinear triangle, Act, and the microsphere radius, r.
Act ¼
hpffiffiffi pi 3 r2 2
ð1Þ
After gold deposition, the PS sphere mask is removed by sonicating the entire sample in a solvent, leaving behind the deposited gold over PDMS pillars with curvilinear triangle in shape. Figs. 7a and b show the SEM images of top-view and side-view of the gold nanofeatures, respectively. In Fig. 7a, arrays of highly ordered gold nanopatterns over hexagonal pillars with perpendicular bisector of 210 nm in average are observed. The height of the gold nanopatterns can also be tuned by controlling the thickness of metal deposition while its surface area can be controlled by utilizing different spheres in size. Fig. 7a. A top-view of the gold nanopatterns above curvilinear triangular pillars.
4. Conclusions An efficient nanofabrication technique based on NSL and soft lithography is developed to produce highly ordered arrays of gold nanopattern over PDMS microbowl structures. The dimensions of the microbowl structures are adjustable by changing the spheres diameter. Since the smaller interstitial spaces between spheres can been achieved by self assembly of colloidal spheres over PDMS microbowls, the size of metallic patterns deposited on the PDMS surface can be tuned by either controlling the thickness of metal deposition or utilizing different spheres in size. Gold nanoparticles with perpendicular bisector of 210 nm in average and height of 50 nm were obtained in curvilinear triangular shape. This approach can also be extended to fabricate uniformed arrays of nanostructures of a wide range of metals such as platinum and silver with controlled size. The proposed nanofabrication process is suitable for volume nanopattern production at low costs. Acknowledgment The authors acknowledge the partly support to The work by Chinese National Science Foundation under project 90923001. References
Fig. 7b. A side-view of highly ordered gold nanopattern arrays.
underneath patterns since the same spheres in diameter have been used. Following self-assembly of spheres, gold is then deposited by thermal evaporation. In fact, the spheres over microbowl structures act as a protective mask to prevent the deposition of gold particles on the entire surface. Hence, the gold nanopatterns are only formed on the tips of pillars after removal of the PS mask. One remarkable advantage of the presented approach is that the geometry of the structure can be controlled by changing the size of spheres in the self assembly process. The effect of the sphere size
[1] J. Prikulis, P. Hanarp, L. Olofsson, D. Sutherland, M. Kall, Nano Lett. 4 (2004) 1003–1007. [2] J. Dintinger, S. Klein, T.W. Ebbesen, Adv. Mater. 18 (2006) 1267–1270. [3] C.L. Haynes, R.P. Van Duyne, J. Phys. Chem. B. 105 (2001) 5599–5611. [4] L. Li, T. Zhai, H. Zeng, X. Fang, Y. Bando, D. Golberg, J. Mater. Chem. 21 (2011) 40–56. [5] J. Zhang, Y. Li, X. Zhang, B. Yang, Adv. Mater. 22 (2010) 4249–4269. [6] Y. Xia, G.M. Whitesides, Angew. Chem. Int. Ed. Engl. 37 (1998) 550–575. [7] H. Li, J. Low, K.S. Brown, N. Wu, IEEE Sensors J. 8 (2008) 880–884. [8] H. Li, N. Wu, Nanotechnology 19 (2008) 275301. [9] C. Kuo, J. Shiu, P. Chen, G.A. Somorjai, J. Phys. Chem. B 107 (2003) 9950–9953. [10] C.L. Cheung, R.J. Nikolic´, C.E. Reinhardt, T.F. Wang, Nanotechnology 17 (2006) 1339–1343. [11] H. Nam, D. Jung, G. Yi, H. Choi, Langmuir 22 (2006) 7358–7363. [12] Z. Ren, X. Li, J. Zhang, W. Li, X. Zhang, B. Yang, Langmuir 23 (2007) 8272–8276.
Contents lists available at SciVerse ScienceDirect
Microelectronic Engineering journal homepage: www.elsevier.com/locate/mee
Nanopatterning of metallic features over uniformed arrays of microbowl structures A. Mohammadkhani ⇑, H. Hassanin, C. Anthony, K. Jiang School of Mechanical Engineering, University of Birmingham, Birmingham, B15 2TT, UK
a r t i c l e
i n f o
Article history: Available online 19 June 2012 Keywords: Nanopattern Nanosphere lithography Soft lithography PDMS Microbowl
a b s t r a c t There is a growing interest for formation of periodic nanostructures in high-throughput and low-cost processes, due to their wide range of applications. This paper demonstrates a facile and efficient method to fabricate ordered gold nanopatterns over uniformed arrays of poly (dimethylsiloxane) (PDMS) microbowl structures by combining nanosphere lithography (NSL) and soft lithography. A monolayer of selfassembled polystyrene (PS) spheres is used as a template to create the PDMS microbowl structures. Another self assembly is processed on the PDMS mould followed by gold deposition, resulting in periodic pillars on the surface of microbowl structure. After removal of the microspheres, an array of highly ordered gold nanopatterns over hexagonal distributed pillars with perpendicular bisector of 210 nm and height of 50 nm in average are achieved. The geometry of the nanopillars with curvilinear triangle surface can be tuned by changing the size of spheres. This approach can be also extended to fabricate ordered periodic nanostructures of a wide range of metals (Pt, Ag, Cu, etc.) with controlled size. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction Advances in periodical nanostructure with different patterns are widely studied for various applications. For instance, ordered nanohole and nanodisk arrays are useful for nanopatterning, data storage, and potentially for nanoscale waveguides [1,2]. In recent years, uniformed array of triangular gold or silver nanoparticles on glass substrates have been demonstrated as biosensors owing to their remarkable sensitivity using localized surface plasmon resonance [3]. One of the constant challenges is to find cost effective methods to fabricate nanostructures. Nanoimprint Lithography (NIL), soft lithography and nanosphere lithography are commonly preferred to those high costs and time consuming fabrication techniques like electron beam lithography and focused ion beam lithography [4]. NIL method is straightforward to be implemented, but it needs nanoimprint templates which require the same resolutions as the patterns. Among these fabrication techniques, nanosphere lithography and soft lithography are both extensively applied because of their potential to create large area and complicated shapes of nanostructure. The conventional NSL process begins with self-assembly of sub-micron size spheres as a mask onto a substrate which can be followed by either deposition of a material or etching of a substrate through the mask [5]. In soft lithography, an elastomeric ⇑ Corresponding author. E-mail addresses: [email protected], (A. Mohammadkhani), [email protected] (K. Jiang).
[email protected]
0167-9317/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.mee.2012.05.064
stamp based on PDMS is usually used as a mould with relief structures on its surface to fabricate components with nano features [6]. Fabrication of nanostructures based on NSL or soft lithography has been widely studied. A large area of well ordered closely packed spheres with defect-free area up to 1 cm2 has been reported by Li et al. [7]. A large area nanoscale gold hemisphere pattern was fabricated by nanosphere lithography combined with electroplating [8]. A fabrication procedure that can produce periodic and size-tuneable silicon nanopillar arrays via NSL has been presented by Kuo et al. [9]. Arrays of silicon nanopillars with aspect ratio of 10 were fabricated using a combination of the deep reactive ion etching and NSL techniques [10]. A sequential replica moulding based on soft lithography combined with colloidal selfassembly has been applied to construct arrays of hemispherical microlens with conformal surface [11]. Ren et al. demonstrated a formation process to produce PDMS microwells with nonclosely-packed arrangements via moulding of colloidal crystals surface [12]. In this paper, a facile and efficient process to fabricate arrays of three dimensional (3D) nanostructures by a combination of nanosphere lithography and soft lithography techniques is presented. The method begins with fabrication of the master mould by selfassembly of microspheres onto a substrate followed by formation of PDMS replica microbowls. The fabricated PDMS soft mould is then considered as the secondary substrate for another self assembly process of microspheres. Gold particles are then deposited through PS spheres mask. The uniformed arrays of triangular gold nanopatterns on microbowl structures are revealed after mask removal.
A. Mohammadkhani et al. / Microelectronic Engineering 98 (2012) 266–269
2. Experimental details Fig. 1 schematically shows the fabrication process. A 1.5 1.5 cm silicon substrate is firstly treated by piranha solution (1:3, H2O2:H2SO4) at 80 °C for 30 min. Once cooled, the substrate is rinsed with copious amounts of distilled water. The substrate is then sonicated in a mixture solution (1:1:5, NH3:H2O2:H2O) for 40 min to increase a hydrophilicity of surface and followed by rinsing repeatedly with distilled water. Next, a micropipette is used to drop 10 ll of 2 wt.% 1.1 lm polystyrene microspheres in aqueous suspensions onto the silicon substrate. After a monolayer of PS microspheres is obtained on the silicon substrate, the structures are used as a template for soft lithography process. The template is placed in 20 20 5 mm plastic box and PDMS is used to produce a soft mould. PDMS is prepared by first mixing Sylgard 184 precursors (1:10, curing agent: elastomer) in a glass beaker and followed by de-airing of the mixture under vacuum. The PDMS slurry is then poured onto the plastic box to cover entire surface of template and placed in a vacuum chamber to remove all residual bubbles. Next, it is cured at 60 °C for 2 h and cooled down. The cured PDMS is gently peeled off from the template. The fabricated mould is then sonicated in ethanol to be cleaned of any residual colloidal spheres. This soft mould is then considered as a secondary
267
substrate to reassemble a monolayer of microspheres with the same mean diameter onto it. In the course of self-assembly of 10 ll of PS aqueous solution onto PDMS microbowls, due to hydrophobicity of PDMS surface, a megasonic system with submersible transducer is applied. Subsequently, thermal evaporation system is used to get a 50 nm gold film over PDMS microbowls with self-assembled microspheres on top. This process is finally followed by removal of PS spheres from the surface of soft mould by sonicating in ethanol bath for a short time to get uniformed arrays of gold nanopatterns over PDMS microbowl structures.
3. Results and discussion Drop coating and spin coating are known as effective methods to form uniformed arrays of colloidal spheres. In this research, area of several hundred square micrometers with uniform arrays of spheres was obtained by using micropipette deposition. A scanning electron microscope (SEM) was applied to inspect the result of each step described above. A monolayer of self-assembled PS spheres over silicon substrate is shown in Fig. 2. The uniformity of the monolayer is highly affected by the concentration of PS solution, the substrate preparation and the environmental condition.
Fig. 1. Schematic of fabrication process: (a) PS spheres assembly on silicon substrate, (b) covering spheres with PDMS, (c) peeling PDMS off and clean it, (d) Reassembly of spheres on PDMS mould, (e) gold deposition over structure and (f) revealing of gold nanopatterns after PS spheres removal.
268
A. Mohammadkhani et al. / Microelectronic Engineering 98 (2012) 266–269
Fig. 2. SEM image of self-assembled microspheres on silicon substrate.
Fig. 5. A monolayer of self-assembled PS spheres over PDMS mould.
Fig. 6. Effect of the sphere size on the curvilinear triangle profile. Fig. 3. SEM image of PDMS microbowl structures.
Fig. 4. Arrays of curvilinear triangular pillars slightly above microbowl structures.
Since PDMS has many features including flexibility, thermally and electrically insulating, optically transparent and relatively inert, it can be used as a popular elastomer material to render more
complex structures. Fig. 3 illustrates PDMS microbowl structures after being peeled off from the template. It can be clearly observed that the microbowl structures are uniformly patterned with smooth concave surface. When the PDMS slurry is prepared by adding curing agent into the elastomer and poured onto template, it is essential to evacuate all trapped gas in the sample to let the PDMS penetrate into existing spaces between spheres. The sample was also left in vacuum chamber for two hours to make sure that all residual voids are filled and cross link is initiated. The curing process has been done at a temperature lower than normally used to cure PDMS to prevent any deformation of the PS spheres as the glass transition temperature of polystyrene is about 95 °C and the melting point is about 240 °C. The morphology of the microbowls is included the reverse profile of the top half part of microspheres and the distributed pillars at the corners of each hexagonal structure. As it is shown in Fig. 4 the vertices of each hexagonal pattern are curvilinear triangular in shape and they are placed slightly above other parts. These pillar arrays are formed by the interstitial spaces between the PS spheres. To create gold nanopatterns over these distributed pillars, another self assembly process is required. A monolayer of self-assembled PS spheres onto microbowl structures is shown in Fig. 5. As it is clearly observed from this figure, the voids between spheres over PDMS mould are narrower in comparison of the voids between spheres over silicon substrate in Fig. 2, due to the existence of
A. Mohammadkhani et al. / Microelectronic Engineering 98 (2012) 266–269
269
on the curvilinear triangle profile is illustrated in Fig. 6. As can be seen, the surface area of curvilinear triangle increases exponentially with the size increase of PS spheres. The following geometric calculation also shows the relationship between the surface area of curvilinear triangle, Act, and the microsphere radius, r.
Act ¼
hpffiffiffi pi 3 r2 2
ð1Þ
After gold deposition, the PS sphere mask is removed by sonicating the entire sample in a solvent, leaving behind the deposited gold over PDMS pillars with curvilinear triangle in shape. Figs. 7a and b show the SEM images of top-view and side-view of the gold nanofeatures, respectively. In Fig. 7a, arrays of highly ordered gold nanopatterns over hexagonal pillars with perpendicular bisector of 210 nm in average are observed. The height of the gold nanopatterns can also be tuned by controlling the thickness of metal deposition while its surface area can be controlled by utilizing different spheres in size. Fig. 7a. A top-view of the gold nanopatterns above curvilinear triangular pillars.
4. Conclusions An efficient nanofabrication technique based on NSL and soft lithography is developed to produce highly ordered arrays of gold nanopattern over PDMS microbowl structures. The dimensions of the microbowl structures are adjustable by changing the spheres diameter. Since the smaller interstitial spaces between spheres can been achieved by self assembly of colloidal spheres over PDMS microbowls, the size of metallic patterns deposited on the PDMS surface can be tuned by either controlling the thickness of metal deposition or utilizing different spheres in size. Gold nanoparticles with perpendicular bisector of 210 nm in average and height of 50 nm were obtained in curvilinear triangular shape. This approach can also be extended to fabricate uniformed arrays of nanostructures of a wide range of metals such as platinum and silver with controlled size. The proposed nanofabrication process is suitable for volume nanopattern production at low costs. Acknowledgment The authors acknowledge the partly support to The work by Chinese National Science Foundation under project 90923001. References
Fig. 7b. A side-view of highly ordered gold nanopattern arrays.
underneath patterns since the same spheres in diameter have been used. Following self-assembly of spheres, gold is then deposited by thermal evaporation. In fact, the spheres over microbowl structures act as a protective mask to prevent the deposition of gold particles on the entire surface. Hence, the gold nanopatterns are only formed on the tips of pillars after removal of the PS mask. One remarkable advantage of the presented approach is that the geometry of the structure can be controlled by changing the size of spheres in the self assembly process. The effect of the sphere size
[1] J. Prikulis, P. Hanarp, L. Olofsson, D. Sutherland, M. Kall, Nano Lett. 4 (2004) 1003–1007. [2] J. Dintinger, S. Klein, T.W. Ebbesen, Adv. Mater. 18 (2006) 1267–1270. [3] C.L. Haynes, R.P. Van Duyne, J. Phys. Chem. B. 105 (2001) 5599–5611. [4] L. Li, T. Zhai, H. Zeng, X. Fang, Y. Bando, D. Golberg, J. Mater. Chem. 21 (2011) 40–56. [5] J. Zhang, Y. Li, X. Zhang, B. Yang, Adv. Mater. 22 (2010) 4249–4269. [6] Y. Xia, G.M. Whitesides, Angew. Chem. Int. Ed. Engl. 37 (1998) 550–575. [7] H. Li, J. Low, K.S. Brown, N. Wu, IEEE Sensors J. 8 (2008) 880–884. [8] H. Li, N. Wu, Nanotechnology 19 (2008) 275301. [9] C. Kuo, J. Shiu, P. Chen, G.A. Somorjai, J. Phys. Chem. B 107 (2003) 9950–9953. [10] C.L. Cheung, R.J. Nikolic´, C.E. Reinhardt, T.F. Wang, Nanotechnology 17 (2006) 1339–1343. [11] H. Nam, D. Jung, G. Yi, H. Choi, Langmuir 22 (2006) 7358–7363. [12] Z. Ren, X. Li, J. Zhang, W. Li, X. Zhang, B. Yang, Langmuir 23 (2007) 8272–8276.