Thin Solid Films 520 (2012) 5161–5164

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Au nanopatterns on glass substrate using block copolymer and their applications intransparent conducting electrode

Md. Mahbub Alam a, Jin-Yeol Kim b, Woo-Gwang Jung b,⁎a Department of Arts and Sciences, Ahsanullah University of Science and Technology, 141–142 Love Road, Tejgaon, Dhaka-1208, Bangladeshb Department of Advanced Materials Engineering, Graduate School of Kookmin University, 861-1 Jeongneung-dong, Seongbuk-gu, Seoul, 136-702, Republic of Korea

⁎ Corresponding author. Tel.: +82 2 910 4643; fax: +E-mail address: wgjung@kookmin.ac.kr (W.-G. Jung

0040-6090/$ – see front matter © 2012 Elsevier B.V. Alldoi:10.1016/j.tsf.2012.04.006

a b s t r a c t

a r t i c l e i n f o

Article history:Received 27 March 2011Received in revised form 20 March 2012Accepted 1 April 2012Available online 6 April 2012

Keywords:Transparent conducting electrodesBlock copolymersPatteringTransmittanceResistivityScanning electron microscopyGold

High density Au nanostructures were fabricated using polystyrene-block-polymethylmethacrylate (PS-b-PMMA) copolymer on glass substrate for the preparation of electrode materials with good stability, hightransparency and excellent conductivity. A 1 wt.% polymer solution in toluene was spin coated on glass sub-strate. Samples were baked for 48 h at 200 °C with a continuous flow of Ar. Patterned polymer film wasobtained by removing the PMMA region through exposing ultraviolet irradiation and rinsing in acetic acid.Au thin films with several thicknesses were then deposited onto the patterned glass substrates by thermalevaporation or sputtering. Removing PS cylinders by sonicating in acetone resulted in Au nanopattern onglass substrates. The connecting gold film acts as conductor while the holes allow light pass through it andhelps to be transparent. The transmittance with Au film thickness of 7 nm and 4 nm was found to be about63% and 70%, respectively. The resistivity was in the range 10−5 Ω cm–10−6 Ω cm which is comparable withITO (10−3 Ω cm–10−4 Ω cm).

© 2012 Elsevier B.V. All rights reserved.

1. Introduction

Block copolymer self-assembly has attracted significant interest inrecent years, since the resultant bulk and thin film morphologies offerideal platforms for the generation of nanoscopically ordered patternsin a range of the promising applications [1–5]. The self-assembly ofblock copolymer is a versatile way of preparing nanoparticles andcontrolling size, shape and location [6–8]. As for thin films, however,interfacial interactions play an important role in influencing the mor-phology and orientation of the block copolymer microdomains [9,10]allowing the desired nanopatterns or arrays [11–14]. One commonlystudied diblock copolymer is polystyrene-b-polymethyl-methacrylate(PS-b-PMMA), which is valued for having both blocks present at thesurface in the film [15–17]. It is generally accepted that perpendiculardomain orientation in PS-b-PMMA films requires elimination of thepreferential segregation of PMMA on the substrate–film interface. Itwas found that perpendicular and hybrid [10] morphologies can beobtained for certain film thickness ranges.

There is a need in a number of different areas of fundamental sci-ence and applied technology for thin films those are both electricallyconducting and optically transparent. Almost all practically electro-chromic devices must incorporate optically transparent electrodes.Indium-tin-oxide (ITO) is the most widely known and commonly

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usedmaterial of this kind because of its properties to offer transparen-cy in the visible range of the electromagnetic spectrum and also elec-trical conductivity [18–21]. Advantages of ITO are its low resistivity(10−3 Ω cm–10−4 Ω cm depending on the deposition method) andits high transparency in the visible region (80%–90% between 400 nmand 800 nm). Due to unique properties, ITO films have many applica-tions, such as solar cells [22], light emitting electrochemical cells [23]and flat panel displays [24,25]. A stable supply of ITO may be difficultto achieve for the recently expandingmarket for optoelectronic devicesbecause of the cost and scarcity of indium, the principal material of ITO.

The search for electrodematerials with good stability, high transpar-ency and excellent conductivity is therefore a crucial goal for optoelec-tronics [26]. In our work, we have tried to make gold patterned glasselectrode,where the gold pattern is connected throughout the substrateto act as a conductor. At the same time, the holes in the patterns helppassing light through it to increase the transmittance of the substrate.

2. Experimental details

Two different compositions of PS-b-PMMA block copolymer wereused for patterning. One with Mn: PS (46100), PMMA (21000) andMw/Mn: 1.09, having a PS volume fraction of 0.69 and in another Mn:PS (25000), PMMA (40000) andMw/Mn: 1.07, having a PS volume frac-tion of 0.38 (Polymer Source Inc.). The substrates were ordinary glassmicroscope slides, of dimensions 2.5 cm×7.5 cm and thickness 1 mm.A 1 wt.% polymer solution in toluene was spin coated on glass sub-strates. The samples were then baked for 48 h with a continuous flow

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of Ar gas at approximately 200 °C, which is well above the glass transi-tion temperatures (Tg) of both PS (100 °C) and PMMA (115 °C). Duringbaking, the polymer components self-assemble into a phase separatedlayer on the substrate surface. The PMMA regions from the patternedpolymer films were removed selectively by exposing to ultraviolet(UV) irradiation for 5 min followed by rinsing in acetic acid whichgave cylindrical PS patterns on glass substrates. In a fewcases sonicationwas used instead of simply rinsing to ensure the complete removal ofPMMA chains. Au thin films with several thicknesses were then depos-ited onto those patterned glass substrates by both thermal evaporationand sputtering methods. The remaining PS domains on gold depositedsubstrate surface could be removed by sonicating in acetone to giveAu patterned glass substrates. The surface properties were examinedby scanning electron microscopy (FE-SEM, JEOL JSM-7401F) with

Fig. 1. PS patterns on glass substrates using a 1 wt.% (PS:PMMA=69:31) solution afterremoval of PMMA domains; (a) without sonication, (b) with sonication for 1 min intoluene after baking and 1 min in acetic acid after UV exposure, (c) with sonicatonfor 4 min in toluene after baking and 4 min in acetic acid after UV exposure.

operating voltage 15 kV, the resistivity of the samples was measuredby 4 point probe (CMT-SR 2000N), and UV–VIS-NIR spectrometer (Shi-madzu UV 3150) was used to measure transmittance of the samples.

3. Results and discussion

A 1 wt.% PS-b-PMMA polymer solution in toluenewas used tomakepatterns on glass substrates. PS and PMMA have significantly differentphotodegradation properties. PMMA is known to be a negative photoresist, i.e., with ultraviolet (UV) or electron beam irradiation, the poly-mer degraded via chain scission [27]. A typical value necessary for deg-radation of PMMA is 3.4 J/cm2 [27]. After spin coating and baking at200 °C the samples were exposed to UV irradiation. Deep UV exposureof ordered PS-b-PMMA should, therefore, lead to a degradation of the

Fig. 2. PS patterns using a 1 wt.% block copolymer (PS:PMMA=38:62) solution;(a) without sonication, (b) with sonication for 1 min in toluene after baking and1 min in acetic acid after UV exposure, and (c) using a 2 wt.% polymer solution, withsonication for 1 min in toluene after baking and 1 min in acetic acid after UV exposure.

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PMMA block, whereas the PS matrix becomes insoluble. The degradedPMMA chains could be then easily removed by rinsing in acetic acid.

The SEM image of the PS pattern on glass by so doing can be seenin Fig. 1(a) which shows wirelike PS patterns. In this case the compo-sition of PS-b-PMMA block copolymer was PS:PMMA=69:31. If thethickness of polymer layer on glass substrate is sufficiently high, acertain amount of polymer chains cannot be grafted onto the sub-strate surface and remain unreacted. The wirelike PS regions areprobably unreacted PS chains on the surface which could be removedby sonicating in toluene after baking the samples. But for thinnerpolymer layers, no unreacted polymer chains could be found and thecomponents of the block copolymer self-assemble homogeneouslyon the substrate surface. Fig. 1(b) shows the PS hole pattern foundby sonicating the samples in acetic acid for 1 min after baking andagain after UV exposure the samples were sonicated for another min-ute to ensure the complete removal of PMMA chains.

But, in order to get transparent conducting electrodes, we need thepatterns where deposition of gold film and subsequent removal of PSwill result in connected gold with holes. For this purpose, this timewehave sonicated the samples in those two different steps for longertime which gave random PS pattern on glass substrate as shown inFig. 1(c).

At second approach, we have used PS-b-PMMA polymer solution intoluene with PS:PMMA=38:62 to make polymer films on glass sub-strates. Without sonication a random pattern of PS (Fig. 2(a)) could befound while with sonication for small time in both steps (after bakingin toluene and after exposing to UV in acetic acid), it gives PS cylindricalpatterns with large spaces between the cylinders (Fig. 2(b)).

Those large spaces are not suitable as the large area of gold filmwill reduce the transmittance of the substrate. So, a 2 wt.% polymersolution was used and with all other similar experimental conditions,

Fig. 3. The substrate surface; (a) after deposition of 12 nm Au film by thermal evapora-tion on PS patterns, (b) Au pattern on glass substrates after removing PS cylinders.

PS cylindrical patterns with higher density were found shown as inFig. 2(c). The spaces between PS cylinders were small but connectedthroughout the substrate surface.

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Fig. 4. (a) The transmittance of Au patterned glass substrates with Au film thickness;(i) 15 nm, (ii) 12 nm and (iii) 9 nm, where Au film is deposited by thermal evapora-tion. (b) and (c) show the change in transmittance and resistivity of the sampleswith the Au film thickness, respectively.

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Fig. 5. Transmittance of Au patterned glass substrate with Au film thickness; (a) 7 nmand (b) 4 nm, where Au film is deposited by sputtering.

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Au films with different thicknesses ranging from 9 nm to 15 nmwere then deposited with thermal evaporator on the PS cylindricalpatterned glass substrates. The PS chains are soluble in acetone andcould be removed by sonication after Au deposition. Fig. 3(a) showsthe surface of the glass substrate after deposition of Au film and thesurface morphology after removal of PS chains can be seen in Fig. 3(b).

Removal of PS cylinders from the substrate surface leaves con-nected gold patterns with holes. The connected gold on the surfaceacts as conductor while the holes help the light passing through it.As detailed below, with thicker Au films, the resistivity was lower,but the transmittance was also lower. For 15 nmAu film, the transmit-tance was only just above 60%, for 12 nm Au film, it showed about 70%transmittance and about 76% transmittance was recorded for 9 nm Aufilm thickness.

Fig. 4(a) shows the transmittance of Au patterned glass for differ-ent Au film thicknesses. As transmittance increased with reducingthe Au film thickness, the resistivity also was found to increase. With15 nm Au film, the resistivity was very low (1.96×10−4 Ω cm)which increased to 83.33 Ω cm for 9 nm Au film thickness. Fig. 4(b)and (c) shows the change in transmittance and resistivity of the sam-ples with the change in the Au layer thickness. For 12 and 15 nm thickAu films, the resistivity was near to zero, while it rises sharply for 9 nmof Au film. This might be because of the film was not thick enough tomake a connecting layer over the substrate surface. Our target wasto get the maximum transmittance with minimum resistivity. Duringour experiment, in the thermal evaporator, the target substrate wasnot perpendicular to the source rather there was a particular anglebetween the Au source and the substrate holder. So the Au particlesmay not be deposited directly to the glass surface due to the cylindri-cal PS in it. To avoid this problem, we have deposited the Au film usingsputtering instead of thermal evaporator. This brought surprising im-provement in resistivity, i.e. with the same thickness of gold films de-posited by sputtering method, the resistivity was significantly lower.

The resistivity of the Au patterned glasswas very lowevenwith verythin Au film thickness deposited by sputtering. The resistivity for 7 nmand4 nmAufilmswas9.80×10−6 Ω cmand3.14×10−5 Ω cm, respec-tively, while for ITO glass the resistivity is in the range 10−3 Ω cm–

10−4 Ω cm.The transmittance for the Au film thickness of 7 nm was found to

be about 63% and it reaches to about 70% for Au film thickness of4 nm. The spectrum for the transmittance for the different Au filmthickness can be seen in Fig. 5. Thus, with the Au films deposited bysputtering, the samples showed lower resistivity, almost similar trans-mittance over larger range of wavelength.

4. Conclusion

Various types of PS patterns could be made on glass substratesusing diblock copolymer polystyrene block polymethylmethacrylate(PS-b-PMMA) by a very easy and simple process. The patterns weredifferent for different composition and could be controlled using son-ication as well. The samples with Au film deposited by sputteringshowed lower resistivity even with much thinner films than thosedeposited by thermal evaporation. The resistivity was in the range10−5 Ω cm–10−6 Ω cm which is even lower in comparison with ITO(10−3 Ω cm–10−4 Ω cm). Although, we still need to work to improvethe transmittance.

Acknowledgments

This work was supported by the ERC (Center for Materials andProcesses of Self-Assembly) program of MOST/KOSEF (R11-2005-048-00000-0) and the Research Program 2011 of Kookmin Universityin Korea.

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