Highly Transparent and Conductive Films of Densely Aligned Ultrathin

Letter pubs.acs.org/NanoLett

Highly Transparent and Conductive Films of Densely Aligned Ultrathin Au Nanowire Monolayers Ana Sánchez-Iglesias,†,§ Beatriz Rivas-Murias,‡ Marek Grzelczak,†,§,∥ Jorge Pérez-Juste,† Luis M. Liz-Marzán,†,§,∥ Francisco Rivadulla,*,‡ and Miguel A. Correa-Duarte*,† †

Departamento de Química Física, Universidade de Vigo, 36310, Vigo, Spain Centro de Investigación en Química Biolóxica e Materiais Moleculares (CIQUS), Universidad de Santiago de Compostela, 15782 Santiago de Compostela, Spain § CIC biomaGUNE, Paseo de Miramón 182, 20009 Donostia - San Sebastián, Spain ∥ Ikerbasque, Basque Foundation for Science, 48011 Bilbao, Spain ‡

S Supporting Information *

ABSTRACT: The combination of low electrical resistance and high optical transparency in a single material is very uncommon. Developing these systems is a scientific challenge and a technological need, to replace ITO in flexible electronic components and other highly demanding applications. Here we report a facile method to prepare single layers of densely aligned ultrathin Au-nanowires, homogeneous over cm2 areas. The asdeposited films show an electrical/optical performance competitive with ITO and graphene-based electrodes. Moreover, the Au-films show a good stability under ambient conditions, and the large aspect ratio of the ultrathin nanowires makes them perfect for deposition in flexible substrates. KEYWORDS: Gold nanowires, transparent electrodes, self-assembly, solution process extraordinary Rs/Ts ratio: Rs ≈ 125 Ω□−1 and T ≈ 97%.15 This material presents the best performance for any non ITO-based film reported to date and renders graphene an excellent candidate for the replacement of ITO in highly demanding applications. The only drawback may be the need for hightemperature postdeposition annealing to obtain the highquality films required for this excellent performance. Hence, to be competitive with ITO and the currently developing graphene-based technology, a material for conductive transparent electrodes should obey the following characteristics: (i) its production must be chemically simple and cost-effective; (ii) it must be chemically stable under ambient conditions, and it must show a good thermal-cycling stability; (iii) there must be simple and scalable methods for deposition of monolayers on different substrates; (iv) the transparency must be >95% and the sheet resistance of a few tens of Ω□−1; (iv) if possible, these properties must be intrinsic to the as-deposited material, without further treatment (annealing, etc.) after deposition. To fulfill these requirements, wet-chemistry production of metallic nanowire networks has emerged as a promising route.

ndium tin oxide (ITO) and fluorinated indium tin oxide (FTO) are the base materials for most of the transparent and conductive films fabricated nowadays. Their excellent optical transparency (T ∼ 90%) and low sheet resistance (Rs < 100 Ω□−1) have extended their use as electrodes in highly demanding applications, such as organic photovoltaic (OPV) cells.1,2 However, indium scarcity, complex processing requirements, and its low resistance to mechanical stress (lack of flexibility) have somehow limited its widespread application and encouraged the search for novel materials that can be used for transparent and conductive electrodes. In this context, different materials have been proposed as an alternative to ITO, like carbon nanotubes,3−6 metal grids,6,7 and thin films of different conducting materials, including graphene.8−11 Random networks of CNTs on plastic substrates show typical Rs ≈ 100−1000 Ω□−1 and T ≈ 80−90%, whereas graphene films prepared through a wet transfer process show Rs ≈ 350−2100 Ω−1 and T ≈ 90−97.9%. These relatively high resistance values are not problematic for voltage-driven devices, such as capacitive touch screens, electrowetting displays, and liquid crystal displays.12,13 However, compared with the 10−50 Ω□−1 of the best ITO on plastic, they would limit the practical application of transparent CNT or graphene electrodes in current based devices such as organic light-emitting diodes and solar cells.14 Recently, Bae and co-workers have developed a roll-to-roll process for the production of graphene films with an

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© XXXX American Chemical Society

Received: June 7, 2012 Revised: November 6, 2012

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Figure 1. Gold nanowire dispersion. (a−c) Transmission electron microscopy (TEM) images of the as-prepared Au-NWs deposited on a TEM-grid. The inset in panel a shows a photograph of a typical Au-NW solution; the arrows in b point to places that evidence the high flexibility of the ultrathin nanowires. A constant distance of 3 nm between aligned Au-NWs is observed (c).

provides them with an extraordinary flexibility (Figure 1b). Homogenous coating with oleylamine used as a stabilizer prevents interconnections between different wires, maintaining a constant interwire separation of ≈3 nm throughout the whole sample (Figure 1c). Au-DANW-films were produced by the self-assembly of Au-NWs on a liquid (diethyleneglycol, DEG) air interphase.25 Thus, the convective flow guides their spontaneous formation that takes place as soon as a hexane dispersion of Au-NWs is spread on diethyleneglycol (DEG) surfaces (Figure 2).26,27 Using a Teflon container the Au-NW

Different strategies have been developed for the fabrication of metallic (mostly Cu and Ag) nanowire networks as transparent conductors with comparable performance to ITO films. Spraydeposited silver nanowires (Ag-NWs) produce large-area films with T and Rs of 87−94% and 45−3000 Ω□−1, respectively.16 A postprocessing step, treating a Ag-NW film with TiO2 sol− gel and PEDOT:PSS solutions, fuse-crosses the nanowires and improves Rs up to 15 Ω□−1, although T remains lower than the desirable ≈83%.17 In general, although the performance of such metallic nanowires is reasonably good, they present serious disadvantages such as their instability against rapid oxidation of Ag and Cu, which damages severely their electrical and optical properties.18 On the other hand, Au-NWs present a higher chemical stability and are not sensitive to oxidation under common ambient conditions, making them excellent a priori candidates for the development of transparent conductive films.19 Fabrication of transparent and conductive films (≈47 nm thick) based on networks of ultrathin Au-NWs (≈2 nm) was recently reported. The values of T = 83−87% and Rs = 49−200 Ω□−1 are promising, but the transparency is still too low.20,21 This is due to the impossibility to prepare dense, homogeneous monolayers of Au-NWs over large surfaces. Notwithstanding, we report here a simple, one-step method for the production of highly transparent (T = 96.5%) and conductive (Rs = 400 Ω□−1) films of self-assembled ultrathin Au-NWs (4 × 103 Ω□−1, showing the dramatic role played by wire−surface interaction for the fabrication of competitive transparent and conducting films. To increase the current density transported in the film, we tried to prepare thicker films by successive deposition of Aumonolayers. However, we found that this resulted in a disrupting of the first layer of wires and hence a poor coverage of the surface with an even lower electrical conductivity (3 kΩ for two layers and 20 kΩ for three layers) than in the single layer films (see Figure S1 in the Supporting Information). The resistance of the films was measured from 77 K up to room temperature (Figure S2, Supporting Information). The wires present a slightly decreasing resistance with decreasing temperature and an excellent thermal stability at cryogenic temperatures. This shows that surface stabilization is not detrimental for their performance at room temperature. Moreover, the conductivity is completely isotropic, because although parallel to each other, the wires do not keep a single orientation over the typical distance between source/drain electrodes (100 μm). Thus, measurements of the resistance difference between parallel and perpendicular orientation present similar values and are within experimental error. The main drawback of the as-prepared Au-DANW film stems from the reported fracture of the Au-NW into smaller pieces upon exposure to elevated temperature (150 °C, 1 h)29 or under intense electron beams (120 kV, 10 s).30 However, the thermal stability of the Au-NW could be easily improved by coating with a protective layer of polymer, SiO2, and so forth, according to the requirements of each particular application. The excellent Rs vs T properties (Figure 4c) of the AuDANW were retained in samples stored at room temperature and ambient conditions for at least three months. The constant values obtained for these films reveal the chemical stability of the Au-NW (e.g., against the typical oxidation that takes place in silver or copper materials). Comparing their performance with other relevant materials (Figure 4d), they stand as advantageous materials for the fabrication of conductive and transparent films. The as-prepared Au-films show a transmittance similar to that recently reported in graphene films (>95%), though the Rs of graphene films is still better. However, the as-prepared films present remarkable advantages such as the simplicity of production, cost effectiveness, no a priori limit for scale-up production, and no need for any further treatment after deposition. It is also worth pointing out that the Au-DANW were obtained through a simple handassisted process, so there is still much room for the improvement of these materials (especially their Rs) and a promising future can be envisioned for them. Experimental Section. Unless otherwise stated, all chemicals were purchased from Aldrich and used without further purification. Synthesis of Au-NWs. Au-NWs were synthesized according to procedures described in the literature.22−24 In a typical synthesis, 100 μL oleylamine was mixed with 3 mg HAuCl4 in 2.5 mL hexane, followed by the addition of 150 mL triisopropylsilane to form a yellow solution. Then the reaction proceeded at room temperature without stirring for 4−5 h until the color gradually changed to dark red. The final products were centrifuged (4000 rpm, 5 min), washed twice in methanol and finally redispersed in hexane.

substrates used in this work, although in principle larger areas could be coated by this method. To preserve the self-assembled structure of the Au-DANW network on the solid substrate, it is crucial to consider the interaction of the functionalized Au-NWs with the surface of the substrate. A hydrophobic substrate is a priori more suitable for the preparation of ordered films, due to the functionalization of the nanowire surface with hydrophobic oleylamine. For a comparison we used a carbon-coated glass substrate with a water contact angle of 40° and a nonfunctionalized glass, with a water contact angle of 30°. Figure 3 shows typical TEM and

Figure 3. Gold NW film. Electron microscopy images of a selfassembled monolayer thick Au-NW deposited on a carbon-coated Ni TEM grid (TEM images a and b) and on carbon-coated glass (SEM images c and d). SEM images deposited on nonmodified glass (e) and on silicon (f).

SEM images of Au-DANW films deposited on the carboncoated glass. The nanowires are densely aligned and homogenously cover the entire glass surface (2.5 × 2 cm2), preserving the self-assembled structure of the liquid−air interface. SEM images show that the wires are well-oriented over typical lengths of hundreds of nanometers. Moreover, surface interactions keep them parallel to each other over their whole length. This clearly evidences the successful transfer of the wires from the liquid subphase to the substrate by the simple and fast hand-assisted strategy used. On the other hand, the morphology of the films changes completely when the nanowires are deposited on nonmodified glass. Repulsion between the substrate and the oleylamine increases as the hydrophilic character of the substrate increases, which leads the nanowires to group creating ropes or networks (Figure 3e), thereby perturbing the homogeneous coating of the substrate. These bundles have a strong impact on the transparency and conductivity of the films (Figure 4). The more uniform distribution of the Au-NWs, obtained using a carbon-modified glass substrate, leads to 96.5% transmittance at 550 nm, as compared to 93.5% for unmodified glass, probably due to the lateral plasmon coupling on agglomerated nanowires (Figure 4a). Room temperature I/V curves measured from both types of films are shown in Figure 4b. Monolayers of densely aligned C

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Figure 4. Optical and electrical characterization of the Au-DANW films. Transmittance (a) and current−voltage characteristics (b) of Au-DANW monolayer films deposited onto glass and carbon-coated glass substrates, measured at room temperature. Sheet resistance vs transmittance of AuDANW films deposited on carbon and carbon-coated glass (c) and compared to the best reported materials in the literature (d). In the inset (b), digital picture of the carbon-coated glass with the DANW film deposited, covering the whole surface (see Figure S3 of the Supporting Information for a higher resolution image). The shiny pads on top are gold contacts evaporated trhough an stencil mask to measure electrical conductivity (10 channels 2000 μm long by 200 μm wide).

wires deposited) was always higher than 5 × 105 Ω□−1 (Figure S4 of the Supporting Information). TEM images were obtained using a JEOL JEM 1010 transmission electron microscope operating at an acceleration voltage of 100 kV. SEM images were acquired with a fieldemission scanning electron microscope (FESEM ULTRA Plus) operating at an acceleration voltage of 3 kV for secondary electron imaging (SEI). UV−vis spectra were recorded in a Cary 5000 UV−vis−NIR spectrophotometer.

Synthesis of Densely Aligned Au NWs Films. To prepare Au-NWs assembled on liquid substrates,27 1 mL of AuNWs ([Au] = 0.3 mM) was spread on the DEG surface inside a Teflon well. The well was covered with a glass slide, and hexane was allowed to evaporate (within ca. 10 min). To transfer the assemblies, the superlattice film on the DEG surface was lightly touched by the solid substrate (carbon-coated nickel TEM grids or glass slide) and gently lifted. The substrates were dried on filter paper. Characterization. Electrical characterization of the samples by current−voltage (I−V) measurements was carried out using a Keithley 4200 Semiconductor Characterization System and a four-point probe-method. Gold contacts were evaporated over the sample through a stencil mask, defining 2 mm long × 100 μm wide channels. At least six different channels were tested for each sample, so as to check the homogeneity of the film over the substrate. I/V curves were measured from 77 K up to room temperature (Figure S1 of the Supporting Information). The average resistivity of the carbon-coated bare substrates (no Au-

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ASSOCIATED CONTENT

* Supporting Information S

SEM images of the AuNW films and their UV−vis spectra, current−voltage characteristics of Au-DANW at different temperatures, image of carbon-coated glass with the DANW film deposited, and current−voltage characteristics of the carbon-coated glass without Au-DANW. This material is available free of charge via the Internet at http://pubs.acs.org. D

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(20) Lyons, P. E.; et al. High-Performance Transparent Conductors from Networks of. J. Phys. Chem. Lett. 2011, 2, 3058−3062. (21) Azulai, D.; Belenkova, T.; Gilon, H.; Barkay, Z.; Markovich, G. Transparent Metal Nanowire Thin Films Prepared in Mesostructured Templates. Nano Lett. 2009, 9, 4246−4249. (22) Huo, Z.; Tsung, C.-k.; Huang, W.; Zhang, X.; Yang, P. Sub-Two Nanometer Single Crystal Au 2008. Nano Lett. 2008, 8, 2041−2044. (23) Pazos-Pérez, N.; et al. Synthesis of Flexible, Ultrathin Gold Nanowires in Organic Media. Langmuir 2008, 24, 9855−9860. (24) Feng, H.; et al. Simple and rapid synthesis of ultrathin gold nanowires, their self-assembly and application in surface-enhanced Raman scattering. Chem. Commun. 2009, 1984−6. (25) Dong, A.; Chen, J.; Vora, P. M.; Kikkawa, J. M.; Murray, C. B. Binary Nanocrystal Superlattice Membranes Self-assembled at the Liquid-air Interface. Nature 2010, 466, 474−477. (26) Santhanam, V.; Liu, J.; Agarwal, R.; Andres, R. P. Self-Assembly of Uniform Monolayer Arrays of Nanoparticles. Langmuir 2003, 19, 7881−7887. (27) Sánchez-Iglesias, A.; Grzelczak, M.; Pérez-Juste, J.; Liz-Marzán, L. M. Binary Self-Assembly of Gold Nanowires with Nanospheres and Nanorods. Angew. Chem. 2010, 122, 10181−10185. (28) Bigioni, T. P.; et al. Kinetically driven self assembly of highly ordered nanoparticle monolayers. Nat. Mater. 2006, 5, 265−70. (29) Wang, C.; Sun, S. Facile synthesis of ultrathin and singlecrystalline Au nanowires. Chem. Asian J. 2009, 4, 1028−34. (30) Lu, X.; Yavuz, M. S.; Tuan, H.-y.; Korgel, B. A.; Xia, Y. Ultrathin Gold Nanowires Can Be Obtained by Reducing Polymeric Strands of Oleylamine-AuCl Complexes Formed via Aurophilic Interaction. J. Am. Chem. Soc. 2008, 130, 8900−8901.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] and [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Xunta de Galicia (PGIDIT06PXIB31479PR; 2008/077; INCITE09-209-101-PR).

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