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Cite this: Chem. Commun., 2014, 50, 362
Capturing by self-assembled block copolymer thin films: transfer printing of metal nanostructures on textured surfaces†
Received 13th August 2013, Accepted 30th October 2013
Hidenori Mizuno,* Tetsuya Kaneko, Isao Sakata and Koji Matsubara
DOI: 10.1039/c3cc46198j www.rsc.org/chemcomm
A method to fabricate metal nanostructures by transfer printing, applicable to textured surfaces, is described. The key is the use of self-assembled polystyrene-block-poly-2-vinylpyridine thin films as binding layers. The plasmonic properties of the obtained metal (Ag) nanostructures showed the potential of this method in the design of novel devices.
The application of the intriguing physical and chemical properties derived from nanostructured metals is an actively ongoing area of research in a broad range of technological fields.1–4 For example, the use of the near-field electromagnetic effect and far-field light scattering of noble metal nanoparticles have been explored in sensing,2 light-emitting diodes,3 and solar cell applications.4 Developing a handy strategy to integrate functional metal nanostructures with various material/device architectures is thus increasingly important to allow novel designs for improved or innovative performances. Continuous efforts are underway to this end, including the traditional photolithographic approach, soft and nanoimprint lithography, block copolymer templating, and many others.5–8 In this respect, stamp-based transfer printing has also been regarded as a simple but powerful technique to create, in particular, metal nanostructures on various materials.9 Accurate nanoscale pattern transfer has been demonstrated on flat surfaces, such as single-crystalline semiconductors (Si and GaAs) and glass, using thiol-containing self-assembled monolayers (SAMs) as binding layers.10,11 In terms of the practical device applications, however, transfer printing should be compatible with non-flat (textured) surfaces as well, since some of the technologically relevant materials, such as transparent conductive oxides (TCOs), are not always flat when processed,12,13 and additional polishing Research Center for Photovoltaic Technologies, National Institute of Advanced Industrial Science and Technology, Central 2, 1-1-1 Umezono, Tsukuba, Ibaraki 305-8568, Japan. E-mail: [email protected]; Fax: +81-29-861-3367; Tel: +81-29-861-3264 † Electronic supplementary information (ESI) available: Experimental procedures and supporting figures (AFM and SEM images). See DOI: 10.1039/c3cc46198j
362 | Chem. Commun., 2014, 50, 362--364
processes are required to make the surfaces flat. In order to meet this challenge, we herein propose the exploitation of selfassembled block copolymer thin films as binding layers to facilitate transfer printing of metal nanostructures not only on flat but also on textured surfaces. The stamps used in this study were prepared from poly(dimethylsiloxane) (PDMS) using a commercially available nanoimprinted plastic film (Scivax corp.) as a master mold. The parent structure was a hexagonally close-packed round-hole array with a diameter of 230 nm, depth of 500 nm, and hole-to-hole spacing of 460 nm. In order to replicate this structure as precisely as possible, a double-layered, hard/soft-PDMS composite was employed.14 The final PDMS stamp had an inverse structure of the parent master (i.e., a hexagonally close-packed round-pillar array), with a slightly smaller diameter (200 nm) because the holes in the master were somewhat tapered. The choice of the plastic mold made this study efficient, since the fabrication of expensive and time-consuming parent masters by electron beam lithography was avoided. Fig. 1 outlines the process for the transfer printing of metal nanostructures using self-assembled block copolymer thin films as binding layers. The surface of a nanopatterned PDMS stamp was first deposited with a metal (Ag, Au or Cu) using electron beam evaporation. In parallel, the surface of a target substrate (e.g., semiconductor wafers or oxide films on glasses) was spin-coated with an o-xylene solution of polystyrene-blockpoly-2-vinyl-pyridine (PS-b-P2VP).15 The same spin conditions (5000 rpm, 40 s) were applied regardless of the surface roughness to afford self-assembled thin films (the thickness was B15 nm in the case of flat surfaces). Next, the metal-deposited PDMS stamp was manually placed on top of the PS-b-P2VP-coated surface, which was wet with ethanol. Due to the surface tension of the ethanol upon evaporation, the metals on the raised region of the PDMS stamp spontaneously made intimate contact with the PS-b-P2VP layer; therefore, no additional force (pressing) was required. The contact was continued for several minutes under reduced pressure until the ethanol completely evaporated away from the contact interface. The PDMS stamp was then removed
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Fig. 1
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General procedure for the transfer printing of metal nanostructures using self-assembled PS-b-P2VP as a binding layer.
from the substrate to leave a metal nanostructure on the PS-b-P2VP layer. Finally, brief Ar plasma treatment was applied to the transfer-printed sample to remove the PS-b-P2VP layer and expose the underlying material.16 The process mentioned above was first tested using an oxide-capped flat Si wafer (Fig. 2). The root mean square roughness (Rrms) of the corresponding surface was 0.2 nm. PS-b-P2VP, with molecular weights of 133 and 132 kg mol 1 (for PS and P2VP, respectively), was employed. A typical atomic force microscopy (AFM) image of the obtained self-assembled thin film composed of P2VP domains and the surrounding PS network is provided in the ESI† (Fig. S1). On this surface, transfer printing of evaporated Ag (thickness: 40 nm) was carried out. Nearly complete transfer (yield: >99%)17 was achieved over a large area (Fig. 2a), as confirmed by scanning electron microscopy (SEM, Fig. 2b and Fig. S2, ESI†). Similar transfer printing results were also obtained with Au and Cu. The thickness (defined by the thickness of the evaporated films on the PDMS stamps) of the metal nanostructures (nanodisks, hereafter) could be increased up to 100 nm, although burr formation along the disk edges was inevitable as
the thickness increases because the round-pillars on the PDMS stamp are tapered as mentioned above (Fig. S3, ESI†). This PS-b-P2VP-mediated approach was subsequently applied to textured surfaces with much larger roughness (Fig. 3). The sample was F-doped SnO2 (SnO2:F),13 a typical TCO material industrially produced on glass (Asahi Glass Company). The SEM image of such a SnO2:F surface is shown in Fig. 3a. A relatively steep morphology composed of many random pyramids and cuboids was confirmed, and the Rrms of the corresponding surface was 29.5 nm. On this rough surface, transfer printing of Ag nanodisks (thickness: 40 nm) was carried out again. High-yield transfer (>95%)17 was achieved regardless of the roughness of the surface (Fig. 3b and Fig. S4, ESI†). The cross-sectional SEM image of the same sample revealed that the printed Ag nanodisks were attached to the underlying PS-b-P2VP/ SnO2:F with various angles and contact points (Fig. 3c). Nevertheless, strong adhesion was confirmed; for instance, no significant loss of Ag nanodisks was observed upon long-time ultrasonication in water (15 min, Fig. S5, ESI†). It should be emphasized that no transfer printing occurs when the textured SnO2:F surface was treated with 3-mercaptopropyltrimethoxysilane (a typical SAM-forming molecule used for transfer
Fig. 2 (a) Digital camera and (b) SEM images of transfer-printed Ag nanodisks on a flat Si wafer (thickness: 40 nm).
Fig. 3 SEM images of (a) an original SnO2:F surface and (b and c) transferprinted Ag nanodisks on the SnO2:F surface (thickness: 40 nm).
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Fig. 4 (a–c) UV-visible extinction spectra of Ag nanodisks on SnO2:F/ glass substrates. The thicknesses of nanodisks were 20, 40, and 60 nm, respectively. (d) UV-visible extinction spectra of a reference SnO2:F/glass substrate.
printing on flat oxide surfaces),10 ensuring the advantage of the use of self-assembled PS-b-P2VP thin films. In addition, a series of control experiments was also performed on the textured surface to further support the usefulness of the proposed approach. First, the use of a PS homopolymer resulted in transfer printing of Ag nanodisks with very low yield (o5%). This result indicated the necessity of coordinative pyridyl groups to capture the Ag upon printing. Secondly, inhomogeneous transfer printing was observed when the P2VP homopolymer was used. This result was attributed to the high solubility of P2VP with ethanol. Since P2VP is completely soluble with ethanol, transfer printing proceeded locally where P2VP remained on the textured surface. Thirdly, the use of a highly diluted PS-b-P2VP solution resulted in low-yield transfer printing again, possibly due to the lack of the formation of the self-assembled film upon spin-coating. Based on these findings, the mechanism for the PS-b-P2VPmediated transfer printing of metals was envisioned as follows. The presence of ethanol at the metal/polymer interface plays a critical role. It has been reported that ethanol induces the surface reconstructions of self-assembled PS-b-P4VP thin films due to its selective affinity to the P4VP polymer blocks.18 A similar phenomenon was observed in our case (P2VP instead of P4VP, Fig. S4, ESI†), and such dynamic transformation would allow the effective interaction between pyridyl groups and metal surfaces, leading to the formation of metal–pyridyl coordination bonds.19,20 On the other hand, the reconstructed film is able to stay on the textured surface because PS blocks do not dissolve in ethanol (i.e., the dissolution problem observed in the case of P2VP homopolymer is mitigated). As a result, metal nanodisks on PDMS stamps are firmly captured by the P2VP polymer block and successfully released from the PDMS stamp. Finally, UV-visible extinction spectra of the transfer-printed Ag nanodisks on the textured SnO2:F are shown in Fig. 4. The spectrum of the original SnO2:F on glass is also included as a reference. The strong extinction (i.e., reflection and absorption) signatures were clearly confirmed over a wide wavelength range
364 | Chem. Commun., 2014, 50, 362--364
Communication
(400–1000 nm) depending on the thickness of the Ag nanodisks (20, 40, or 60 nm). These unique optical properties of Ag21 and other metals22 especially in the visible and near infrared region are of high interest in optoelectronics, such as plasmonic solar cells.4 The device application and the detailed plasmonic properties of the transfer printed Ag, Au, and Cu nanodisks are currently under investigation in our group. In summary, we have demonstrated transfer printing of metal nanostructures on both flat and textured surfaces using self-assembled PS-b-P2VP thin films as binding layers. In addition to the surface roughness tolerance, the simplicity (quickness) of the spin-coating process compared to the previous SAM-formation process is another benefit of this approach. The plasmonic properties observed using UV-Vis spectroscopy were promising for potential device applications, thus, novel opportunities would be available through this approach. This work was supported by New Energy and Industrial Technology Development Organization (NEDO) under Ministry of Economy, Trade, and Industry (METI), Japan.
Notes and references 1 Nanostructured metals and alloys: Processing, microstructure, mechanical properties and applications, ed. S. H. Whang, Woodhead Publishing, Cambridge, 2011. 2 L. Guerrini and D. Graham, Chem. Soc. Rev., 2012, 41, 7085. 3 X. Gu, T. Qiu, W. Zhang and P. Chu, Nanoscale Res. Lett., 2011, 6, 199. 4 H. A. Atwater and A. Polman, Nat. Mater., 2010, 9, 205. 5 A. Pimpin and W. Srituravanich, Eng. J., 2011, 16, 38. 6 B. D. Gates, Q. Xu, M. Stewart, D. Ryan, C. G. Willson and G. M. Whitesides, Chem. Rev., 2005, 105, 1171. 7 Y. Lei, S. Yang, M. Wu and G. Wilde, Chem. Soc. Rev., 2011, 40, 1247. 8 D. J. Lipomi, R. V. Martinez and G. M. Whitesides, Angew. Chem., Int. Ed., 2011, 50, 8566. 9 A. Carlson, A. M. Bowen, Y. Huang, R. G. Nuzzo and J. A. Rogers, Adv. Mater., 2012, 24, 5284. 10 Y.-L. Loo, R. L. Willett, K. W. Baldwin and J. A. Rogers, J. Am. Chem. Soc., 2002, 124, 7654. 11 Y.-L. Loo, J. W. P. Hsu, R. L. Willett, K. W. Baldwin, K. W. West and J. A. Rogers, J. Vac. Sci. Technol., B, 2002, 20, 2853. 12 A. Hongsingthong, T. Krajangsang, A. Limmanee, K. Sriprapha, J. Sritharathikhun and M. Konagai, Thin Solid Films, 2013, 537, 291. 13 R. Kykyneshi, J. Zeng and D. P. Cann, in Handbook of Transparent Conductors, ed. D. S. Ginley, Springer, New York, 2011, ch. 6, pp. 171–191. 14 T. W. Odom, J. C. Love, D. B. Wolfe, K. E. Paul and G. M. Whitesides, Langmuir, 2002, 18, 5314. 15 S. Krishnamoorthy, R. Pugin, J. Brugger, H. Heinzelmann and C. Hinderling, Adv. Funct. Mater., 2006, 16, 1469. 16 J. Chai, D. Wang, X. N. Fan and J. M. Buriak, Nat. Nanotechnol., 2007, 2, 500. 17 The yields were determined by averaging the ratios of the number of transferred Ag nanodisks/the number of pillars on the stamp, calculated from 5 different SEM images. 18 S. Park, J.-Y. Wang, B. Kim, J. Xu and T. P. Russell, ACS Nano, 2008, 2, 766. 19 D.-Y. Wu, X.-M. Liu, S. Duan, X. Xu, B. Ren, S.-H. Lin and Z.-Q. Tian, J. Phys. Chem. C, 2008, 112, 4195. 20 S. Malynych, I. Luzinov and G. Chumanov, J. Phys. Chem. B, 2002, 106, 1280. 21 V. Germain, A. Brioude, D. Ingert and M. P. Pileni, J. Chem. Phys., 2005, 122, 124707. 22 A. Murry and W. L. Barnes, Adv. Mater., 2007, 19, 3771.
This journal is © The Royal Society of Chemistry 2014
Published on 30 October 2013. Downloaded by Drexel University on 26/10/2014 11:25:40.
COMMUNICATION
View Article Online View Journal | View Issue
Cite this: Chem. Commun., 2014, 50, 362
Capturing by self-assembled block copolymer thin films: transfer printing of metal nanostructures on textured surfaces†
Received 13th August 2013, Accepted 30th October 2013
Hidenori Mizuno,* Tetsuya Kaneko, Isao Sakata and Koji Matsubara
DOI: 10.1039/c3cc46198j www.rsc.org/chemcomm
A method to fabricate metal nanostructures by transfer printing, applicable to textured surfaces, is described. The key is the use of self-assembled polystyrene-block-poly-2-vinylpyridine thin films as binding layers. The plasmonic properties of the obtained metal (Ag) nanostructures showed the potential of this method in the design of novel devices.
The application of the intriguing physical and chemical properties derived from nanostructured metals is an actively ongoing area of research in a broad range of technological fields.1–4 For example, the use of the near-field electromagnetic effect and far-field light scattering of noble metal nanoparticles have been explored in sensing,2 light-emitting diodes,3 and solar cell applications.4 Developing a handy strategy to integrate functional metal nanostructures with various material/device architectures is thus increasingly important to allow novel designs for improved or innovative performances. Continuous efforts are underway to this end, including the traditional photolithographic approach, soft and nanoimprint lithography, block copolymer templating, and many others.5–8 In this respect, stamp-based transfer printing has also been regarded as a simple but powerful technique to create, in particular, metal nanostructures on various materials.9 Accurate nanoscale pattern transfer has been demonstrated on flat surfaces, such as single-crystalline semiconductors (Si and GaAs) and glass, using thiol-containing self-assembled monolayers (SAMs) as binding layers.10,11 In terms of the practical device applications, however, transfer printing should be compatible with non-flat (textured) surfaces as well, since some of the technologically relevant materials, such as transparent conductive oxides (TCOs), are not always flat when processed,12,13 and additional polishing Research Center for Photovoltaic Technologies, National Institute of Advanced Industrial Science and Technology, Central 2, 1-1-1 Umezono, Tsukuba, Ibaraki 305-8568, Japan. E-mail: [email protected]; Fax: +81-29-861-3367; Tel: +81-29-861-3264 † Electronic supplementary information (ESI) available: Experimental procedures and supporting figures (AFM and SEM images). See DOI: 10.1039/c3cc46198j
362 | Chem. Commun., 2014, 50, 362--364
processes are required to make the surfaces flat. In order to meet this challenge, we herein propose the exploitation of selfassembled block copolymer thin films as binding layers to facilitate transfer printing of metal nanostructures not only on flat but also on textured surfaces. The stamps used in this study were prepared from poly(dimethylsiloxane) (PDMS) using a commercially available nanoimprinted plastic film (Scivax corp.) as a master mold. The parent structure was a hexagonally close-packed round-hole array with a diameter of 230 nm, depth of 500 nm, and hole-to-hole spacing of 460 nm. In order to replicate this structure as precisely as possible, a double-layered, hard/soft-PDMS composite was employed.14 The final PDMS stamp had an inverse structure of the parent master (i.e., a hexagonally close-packed round-pillar array), with a slightly smaller diameter (200 nm) because the holes in the master were somewhat tapered. The choice of the plastic mold made this study efficient, since the fabrication of expensive and time-consuming parent masters by electron beam lithography was avoided. Fig. 1 outlines the process for the transfer printing of metal nanostructures using self-assembled block copolymer thin films as binding layers. The surface of a nanopatterned PDMS stamp was first deposited with a metal (Ag, Au or Cu) using electron beam evaporation. In parallel, the surface of a target substrate (e.g., semiconductor wafers or oxide films on glasses) was spin-coated with an o-xylene solution of polystyrene-blockpoly-2-vinyl-pyridine (PS-b-P2VP).15 The same spin conditions (5000 rpm, 40 s) were applied regardless of the surface roughness to afford self-assembled thin films (the thickness was B15 nm in the case of flat surfaces). Next, the metal-deposited PDMS stamp was manually placed on top of the PS-b-P2VP-coated surface, which was wet with ethanol. Due to the surface tension of the ethanol upon evaporation, the metals on the raised region of the PDMS stamp spontaneously made intimate contact with the PS-b-P2VP layer; therefore, no additional force (pressing) was required. The contact was continued for several minutes under reduced pressure until the ethanol completely evaporated away from the contact interface. The PDMS stamp was then removed
This journal is © The Royal Society of Chemistry 2014
View Article Online
Communication
Published on 30 October 2013. Downloaded by Drexel University on 26/10/2014 11:25:40.
Fig. 1
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General procedure for the transfer printing of metal nanostructures using self-assembled PS-b-P2VP as a binding layer.
from the substrate to leave a metal nanostructure on the PS-b-P2VP layer. Finally, brief Ar plasma treatment was applied to the transfer-printed sample to remove the PS-b-P2VP layer and expose the underlying material.16 The process mentioned above was first tested using an oxide-capped flat Si wafer (Fig. 2). The root mean square roughness (Rrms) of the corresponding surface was 0.2 nm. PS-b-P2VP, with molecular weights of 133 and 132 kg mol 1 (for PS and P2VP, respectively), was employed. A typical atomic force microscopy (AFM) image of the obtained self-assembled thin film composed of P2VP domains and the surrounding PS network is provided in the ESI† (Fig. S1). On this surface, transfer printing of evaporated Ag (thickness: 40 nm) was carried out. Nearly complete transfer (yield: >99%)17 was achieved over a large area (Fig. 2a), as confirmed by scanning electron microscopy (SEM, Fig. 2b and Fig. S2, ESI†). Similar transfer printing results were also obtained with Au and Cu. The thickness (defined by the thickness of the evaporated films on the PDMS stamps) of the metal nanostructures (nanodisks, hereafter) could be increased up to 100 nm, although burr formation along the disk edges was inevitable as
the thickness increases because the round-pillars on the PDMS stamp are tapered as mentioned above (Fig. S3, ESI†). This PS-b-P2VP-mediated approach was subsequently applied to textured surfaces with much larger roughness (Fig. 3). The sample was F-doped SnO2 (SnO2:F),13 a typical TCO material industrially produced on glass (Asahi Glass Company). The SEM image of such a SnO2:F surface is shown in Fig. 3a. A relatively steep morphology composed of many random pyramids and cuboids was confirmed, and the Rrms of the corresponding surface was 29.5 nm. On this rough surface, transfer printing of Ag nanodisks (thickness: 40 nm) was carried out again. High-yield transfer (>95%)17 was achieved regardless of the roughness of the surface (Fig. 3b and Fig. S4, ESI†). The cross-sectional SEM image of the same sample revealed that the printed Ag nanodisks were attached to the underlying PS-b-P2VP/ SnO2:F with various angles and contact points (Fig. 3c). Nevertheless, strong adhesion was confirmed; for instance, no significant loss of Ag nanodisks was observed upon long-time ultrasonication in water (15 min, Fig. S5, ESI†). It should be emphasized that no transfer printing occurs when the textured SnO2:F surface was treated with 3-mercaptopropyltrimethoxysilane (a typical SAM-forming molecule used for transfer
Fig. 2 (a) Digital camera and (b) SEM images of transfer-printed Ag nanodisks on a flat Si wafer (thickness: 40 nm).
Fig. 3 SEM images of (a) an original SnO2:F surface and (b and c) transferprinted Ag nanodisks on the SnO2:F surface (thickness: 40 nm).
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Fig. 4 (a–c) UV-visible extinction spectra of Ag nanodisks on SnO2:F/ glass substrates. The thicknesses of nanodisks were 20, 40, and 60 nm, respectively. (d) UV-visible extinction spectra of a reference SnO2:F/glass substrate.
printing on flat oxide surfaces),10 ensuring the advantage of the use of self-assembled PS-b-P2VP thin films. In addition, a series of control experiments was also performed on the textured surface to further support the usefulness of the proposed approach. First, the use of a PS homopolymer resulted in transfer printing of Ag nanodisks with very low yield (o5%). This result indicated the necessity of coordinative pyridyl groups to capture the Ag upon printing. Secondly, inhomogeneous transfer printing was observed when the P2VP homopolymer was used. This result was attributed to the high solubility of P2VP with ethanol. Since P2VP is completely soluble with ethanol, transfer printing proceeded locally where P2VP remained on the textured surface. Thirdly, the use of a highly diluted PS-b-P2VP solution resulted in low-yield transfer printing again, possibly due to the lack of the formation of the self-assembled film upon spin-coating. Based on these findings, the mechanism for the PS-b-P2VPmediated transfer printing of metals was envisioned as follows. The presence of ethanol at the metal/polymer interface plays a critical role. It has been reported that ethanol induces the surface reconstructions of self-assembled PS-b-P4VP thin films due to its selective affinity to the P4VP polymer blocks.18 A similar phenomenon was observed in our case (P2VP instead of P4VP, Fig. S4, ESI†), and such dynamic transformation would allow the effective interaction between pyridyl groups and metal surfaces, leading to the formation of metal–pyridyl coordination bonds.19,20 On the other hand, the reconstructed film is able to stay on the textured surface because PS blocks do not dissolve in ethanol (i.e., the dissolution problem observed in the case of P2VP homopolymer is mitigated). As a result, metal nanodisks on PDMS stamps are firmly captured by the P2VP polymer block and successfully released from the PDMS stamp. Finally, UV-visible extinction spectra of the transfer-printed Ag nanodisks on the textured SnO2:F are shown in Fig. 4. The spectrum of the original SnO2:F on glass is also included as a reference. The strong extinction (i.e., reflection and absorption) signatures were clearly confirmed over a wide wavelength range
364 | Chem. Commun., 2014, 50, 362--364
Communication
(400–1000 nm) depending on the thickness of the Ag nanodisks (20, 40, or 60 nm). These unique optical properties of Ag21 and other metals22 especially in the visible and near infrared region are of high interest in optoelectronics, such as plasmonic solar cells.4 The device application and the detailed plasmonic properties of the transfer printed Ag, Au, and Cu nanodisks are currently under investigation in our group. In summary, we have demonstrated transfer printing of metal nanostructures on both flat and textured surfaces using self-assembled PS-b-P2VP thin films as binding layers. In addition to the surface roughness tolerance, the simplicity (quickness) of the spin-coating process compared to the previous SAM-formation process is another benefit of this approach. The plasmonic properties observed using UV-Vis spectroscopy were promising for potential device applications, thus, novel opportunities would be available through this approach. This work was supported by New Energy and Industrial Technology Development Organization (NEDO) under Ministry of Economy, Trade, and Industry (METI), Japan.
Notes and references 1 Nanostructured metals and alloys: Processing, microstructure, mechanical properties and applications, ed. S. H. Whang, Woodhead Publishing, Cambridge, 2011. 2 L. Guerrini and D. Graham, Chem. Soc. Rev., 2012, 41, 7085. 3 X. Gu, T. Qiu, W. Zhang and P. Chu, Nanoscale Res. Lett., 2011, 6, 199. 4 H. A. Atwater and A. Polman, Nat. Mater., 2010, 9, 205. 5 A. Pimpin and W. Srituravanich, Eng. J., 2011, 16, 38. 6 B. D. Gates, Q. Xu, M. Stewart, D. Ryan, C. G. Willson and G. M. Whitesides, Chem. Rev., 2005, 105, 1171. 7 Y. Lei, S. Yang, M. Wu and G. Wilde, Chem. Soc. Rev., 2011, 40, 1247. 8 D. J. Lipomi, R. V. Martinez and G. M. Whitesides, Angew. Chem., Int. Ed., 2011, 50, 8566. 9 A. Carlson, A. M. Bowen, Y. Huang, R. G. Nuzzo and J. A. Rogers, Adv. Mater., 2012, 24, 5284. 10 Y.-L. Loo, R. L. Willett, K. W. Baldwin and J. A. Rogers, J. Am. Chem. Soc., 2002, 124, 7654. 11 Y.-L. Loo, J. W. P. Hsu, R. L. Willett, K. W. Baldwin, K. W. West and J. A. Rogers, J. Vac. Sci. Technol., B, 2002, 20, 2853. 12 A. Hongsingthong, T. Krajangsang, A. Limmanee, K. Sriprapha, J. Sritharathikhun and M. Konagai, Thin Solid Films, 2013, 537, 291. 13 R. Kykyneshi, J. Zeng and D. P. Cann, in Handbook of Transparent Conductors, ed. D. S. Ginley, Springer, New York, 2011, ch. 6, pp. 171–191. 14 T. W. Odom, J. C. Love, D. B. Wolfe, K. E. Paul and G. M. Whitesides, Langmuir, 2002, 18, 5314. 15 S. Krishnamoorthy, R. Pugin, J. Brugger, H. Heinzelmann and C. Hinderling, Adv. Funct. Mater., 2006, 16, 1469. 16 J. Chai, D. Wang, X. N. Fan and J. M. Buriak, Nat. Nanotechnol., 2007, 2, 500. 17 The yields were determined by averaging the ratios of the number of transferred Ag nanodisks/the number of pillars on the stamp, calculated from 5 different SEM images. 18 S. Park, J.-Y. Wang, B. Kim, J. Xu and T. P. Russell, ACS Nano, 2008, 2, 766. 19 D.-Y. Wu, X.-M. Liu, S. Duan, X. Xu, B. Ren, S.-H. Lin and Z.-Q. Tian, J. Phys. Chem. C, 2008, 112, 4195. 20 S. Malynych, I. Luzinov and G. Chumanov, J. Phys. Chem. B, 2002, 106, 1280. 21 V. Germain, A. Brioude, D. Ingert and M. P. Pileni, J. Chem. Phys., 2005, 122, 124707. 22 A. Murry and W. L. Barnes, Adv. Mater., 2007, 19, 3771.
This journal is © The Royal Society of Chemistry 2014
