Plasmonic refraction-induced ultrahigh transparency of highly conducting metallic networks

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ORIGINAL PAPER

Abstract We demonstrate a high optoelectronic performance and application potential of our random network, with subwavelength diameter, ultralong, and high-quality silver nanowires, stabilized on a substrate with a UV binder. Our networks show very good optoelectronic properties, with the single best figure of merit of 1686, and excellent stability under harsh mechanical strain, as well as thermal, and chemical challenge. Our network transparency strongly exceeds the simple shading limit. We show that this transmission enhancement is due to plasmonic refraction, which in an effective medium picture involves localized plasmons, and identify the inhomogeneous broadening as the key factor in promoting this mechanism. Such networks could become a ba- sis for a next generation of ultrahigh-performance transparent conductors.

Plasmonic refraction-induced ultrahigh transparency of highly conducting metallic networks

Ruopeng Li

¹

, Qiang Peng

¹

, Bing Han

¹

, Yuanyu Ke

¹

, Xin Wang

^3,1

, Xubing Lu

¹

, Xueyuan Wu

²

, Jiantao Kong

²

, Zhifeng Ren

⁵

, Eser Metin Akinoglu

⁶

, Michael Giersig

⁶

, Guofu Zhou

³

,

Jun-Ming Liu

^1,4

, Krzysztof Kempa

^2,1,^∗

, and Jinwei Gao

^1,^∗

1. Introduction

Discontinuous thin metallic films [1–6] belong to a class of emerging transparent conductors (TCs). One of the leading systems in that class are 2D nanowire networks [7–14], which combine good optoelectronic and mechanical performance with inexpensive processing [9, 15, 16], and thus provide an attractive alternative to conventional materials, like indium tin oxide (ITO). Silver (Ag) nanowire networks have been investigated for applications in touch-screen displays [8, 17, 18], flexible transparent heaters [19–23], and photovoltaic devices [10, 15, 16, 24, 25], but their electro- optic performance has been so far limited, in part due to problems with wire quality [12, 26], contacts between the wires [1, 7, 8, 15, 18], and poor network integrity [27, 28].

Here, we report a 2D random network made of ultralong and high-quality Ag nanowires, stabilized and strengthened on a substrate with the UV binder. This network displays a very high performance as a TC electrode for various applications, with the statistical figure of merit (F) of 1493, and

1Institute for Advanced Materials and Laboratory of Quantum Engineering and Quantum Materials, South China Normal University, Guangzhou 510006, China

2Department of Physics, Boston College, Chestnut Hill, Massachusetts 02467, USA

3Electronic Paper Displays Institute, South China Normal University, Guangzhou 510006, China

4Laboratory of Solid State Microstructures, Nanjing University, Nanjing 210093, China

5Department of Physics and TcSUH, University of Houston, Houston Texas, 77204, USA

6Institute of Experimental Physics, Freie University of Berlin, Berlin 14195, Germany

R. P. Li, K. Kempa, and J.W.Gao: Plasmonic refraction induced ultra-high transparency of highly conducting metallic networks

∗Corresponding author(s): e-mail: [email protected], [email protected]

the single bestF=1686 (with transmittance96%, and sheet resistance5.4/sq), and excellent chemical, thermal and mechanical stability. We also find that surprisingly, the transmission through this network exceeds the simple shading limit, and we perform a comprehensive theoretical and experimental study of its optoelectronic parameters, demonstrating that this effect is caused by the plasmonic refraction.

2. Experimental section

2.1. Synthesis of the silver nanowires

Preparation of the AgNO₃/PVP/EG mixture: 0.36 g of PVP (Mw=58 000; Aladdin, 9003-39-8) and 0.4 g AgNO3(Xi Long, Guangdong, China, 7761-88-8) were individually added to two containers with 25 mL of EG, and completely dissolved by using magnetic stirring at room temperature.

Mixing these solutions together led to the AgNO₃/PVP/EG

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2 R. Li et al.: Plasmonic refraction-induced ultrahigh transparency of highly conducting metallic networks mixture. 50 mL of ethylene glycol (EG) (Da Mao, Tian-

jin, China) in a glass flask was preheated to 160 °C, and kept for30 min under continuous magnetic stirring. Sub- sequently, 140 μL of 6 mM FeCl₃ (Aladdin, 7705-08-0) solution (in EG) was added into the flask. After a two- minute stirring, the AgNO3/PVP/EG mixture was added to the flask by pipette over 20 min with continuous stirring.

Then the reaction temperature was controlled at 135 °C and held for 3 h (without stirring) until the reaction was complete. The nanowires form, and by choosing the reaction temperature and reaction duration, their length can be roughly controlled (normally lower reaction temperature and longer duration lead to longer nanowires). The synthesized silver nanowires are rinsed and centrifuged (1000 rpm), and finally redispersed in ethanol.

2.2. Network fabrication

The as-synthesized silver nanowires (dispersed in ethanol), were deposited on a substrate with the Meyer Rod. Subse- quently, the mixture of the UV binder (UVTM 1019, UVTM Company, Guangzhou, China) and ethanol (volume ratio is 1:100) was spray deposited on the nanowires, followed by UV light irradiation for 20 s in a vacuum chamber.

2.3. Fabrication of the a-Si PV cell

Glass substrates were either sputter coated (AJA Interna- tional ATC Orion 8, USA) with ITO, or covered with the ULW/binder network as described in Section 2.2, and in addition covered by a sputtered AZO buffer layer (80 nm thick). These substrates were installed in the vacuum chamber (5×10⁻⁸Torr) of the PECVD system (Solasta, Boston, USA), The growth of thea-Si junction layers began with heating a substrate to 260 °C, and exposing it to a gas mixture consisting of B2H6/SiH4/CH4/H2, resulting in the 10-nm thick p-doped layer. Subsequently, the mixture of SiH4/H2/CH4 was used to make the 10-nm thick buffer layer, and the mixture of SiH₄/H₂ to make the530-nm thick i-layer. Subsequently, the mixture of PH₃/SiH₄/H₂ was used to produce the50-nm thick n-layer. Finally, the junction layers were capped by the 80-nm thick buffer layer of AZO, followed by 300 nm of Ag film as the back contact.

Both were deposited by sputtering.

2.4. Microscopy

Sample morphology was characterized using the SEM (JEOL JCM-5700, Tokyo, Japan), HRTEM (JEOL JEM- 2100 HR, Japan, operating at an acceleration voltage of 200 kV), and AFM (Cypher, Asylum Research, USA, operating in AC mode). The photographs were taken by using an optical microscope (MA 2002, Chongqing Optical &

Electrical Instrument Co., Ltd). Cross-sectional images of the solar cell were obtained using the high-resolution SEM (ZEISS Ultra 55, Germany).

2.5. Optoelectronic property measurements

Optical transmittance was measured in an integrating sphere system (Ocean Optics, USA). All transmittance measurements were normalized to the absolute transmittance through the substrate (PET or glass). The method for the sheet resistance measurements was the same as in Refs. [3] and [5].

2.6. Transparent heater

The current measurements of the LW network-based transparent heater were made in a two-terminal, side-contact configuration (Fig. S1 in Supplementary Materials). The temperature of the film was recorded using a thermocouple and data acquisition system (PCI-6221, NI, USA). The infrared image was recorded with the infrared thermal imager (NEC San-ei Instruments, Ltd, Japan).

2.7. I–V characteristics

Solar cells were characterized under simulated AM1.5 sun- light (101 mW cm⁻² irradiance) generated by the solar simulator (Newport 92193H-1000, USA).

2.8. Software used in simulations

The MEEP package is an open-source software from Mas- sachusetts Institute of Technology [29]. The data for metals were taken from Ref. [30].

3. Results

3.1. Nanowire and network fabrication

The key factor in improving the nanowire arrays is to base them on ultralong nanowires, because this minimizes the number of wire-to-wire contacts. We have synthesized our nanowires by a modified polyol process [31, 32]. These have length L ranging from 50μm to 300 μm, and the diameter D in the 100 nm to 200 nm range (Fig. S2 in Supplementary Materials). The corresponding aspect ratio of these wires (L/D) ranges from 300 to 1500. Details of the nanowire synthesis are given in the Experimental section. The high resolution transmission electron microscopy (HRTEM) image in Fig. 1a shows the excellent structural quality of the nanowires, with good crystallinity and low defect density. We prepared two monodisperse suspensions of these nanowires in ethanol, one with L/D=500 (long wires, LW), and the other withL/D=1000 (ultralong wires, ULW). The SEM image of a small area of the network with a UV binder is shown in Fig. 1b, and the inset in this figure shows a schematic of the nanowire junction, with the UV binder indicated in green color. The network fabrication

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Figure 1 Nanowire and network morphology. (a) HRTEM of individual nanowires. Insets are the electron diffraction pattern and the lattice-resolved TEM image. (b) SEM image of a small area of the nanowire network cured by the UV binder. Inset shows a schematic of the wire junction, with UV binder shown in green color. (c) AFM image (top) and the height profile (bottom) of a selected fragment of the nanowire network with the UV binder. The AFM scan was taken along the line indicated in the top panel. (d) Photograph of the completed network on a flexible PET substrate.

steps are as follows. Firstly, the nanowires were deposited from the suspensions onto a substrate with the Meyer Rod [18, 21, 33]. Secondly, the film is dried in air, and subsequently spray coated with a UV-curable polymer dispersed in ethanol, which acts as an adhesion binder (between wires and the substrate). The final step in the network preparation involves drying it in air, with the UV binder shrinking un- derneath the nanowires (see the inset in Fig. 1b), and then curing the binder in a vacuum chamber by exposing it to the UV light. Details of the network fabrication are described in the Experimental section, and theSupplementary Mate- rialsFig. S3. An AFM image and the height profile of the network fragment coated with the UV binder are shown in Fig. 1c, and the photograph in Fig. 1d shows a sample of the polyethylene terephthalate (PET) substrate coated with the completed ULW network, demonstrating its excellent transparency and structural flexibility, required by some applications.

3.2. Optoelectronic performance

Figure 2a shows the light transmittanceTmeasured at the wavelengthλ=550 nm, versus the measured sheet resis- tanceR_s, compiled for our ULW and LW networks, as well as for other leading Ag nanowire networks reported in the literature. Clearly, our networks are slightly better than the

others, with the corresponding data points (red triangles, and black circles), having the very good combined transmission and conductivity. It is also clear that longer wires promote better networks, as expected. Figure 2b shows plots ofTversusλin the visible range, for two our selected networks (LW and ULW). As compared to the standard ITO, our networks are superior, with almost constant spectral response, higher transparency and lower resistance. We note that the contribution of the UV binder to the total transmission is negligible (Fig. S4 inSupplementary Materials).

3.3. Mechanical, thermal, and chemical stability

Our networks outperform ITO also in the mechanical flexibility. Figure 2c illustrates the tape “peeling-off” test [9]

showing that the adhesion to the PET substrate is much better when the UV binder is used. The single cycle of the test involves attaching the 3M tape to the network, and subsequently peeling it off. While the network without the binder detaches from the substrate after only a single cycle, the one with the binder remains attached during multiple cycles, and the sheet resistance increases only slightly, even after more than 100 cycles. Figure 2d plotsR_s(green solid line) for one of our networks subjected to large-angle, multiple bending events (the bending radius is 4 mm),

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4 R. Li et al.: Plasmonic refraction-induced ultrahigh transparency of highly conducting metallic networks

Figure 2 Optoelectronic performance and stability of nanowire networks. (a)T (at wavelengthλ=550 nm) vs.Rsfor our networks, and other networks reported in the literature. (b)T vs.λfor LW and ULW networks and ITO. (c)Rsversus the number of peeling-off test cycles for our ULW network. The insets show photographs of the network without (inset left) and with (inset right) the UV binder, before and after the tape peeling-off test. (d)Rsversus the number of bending cycles (green solid line) for a ULW network, and the ITO film (red dots). Inset shows an enlarged view of the bending test of a ULW network.

showing excellent stability (the amplitude of the recover- able Rs oscillations is 3%) after hundreds of bending events. In contrast, the ITO film subjected to the same treatment completely disintegrates after only a few events (red dots), and the ULW network without binder shows a poorer bending resistance than the ULW network with binder (Figs. S5 and S6 inSupplementary Materials). The performance of our UV binder-stabilized network is excellent also under sonication (Table S1 inSupplementary Ma- terials). Our networks also have excellent thermal (Fig. S7 inSupplementary Materials)and chemical stability, without obvious change in resistance when exposing to water, ethanol, acetone, etc., for 12 h, even after a subsequent peeling-off test (Fig. S8 inSupplementary Materials).

3.4. Examples of applications

Figure 3 demonstrates two applications of our networks:

an amorphous silicon (a-Si) solar cell, and a flexible transparent heater. Figure 3a shows an SEM image of the so-

lar cell cross section, with the glass substrate, ULW/binder stabilized network, the p-i-na-Si junction sandwiched between AZO films, and the Ag top contact film clearly visible. The ULW/binder network before the solar cell processing hadT92% atλ=550 nm, andR_s4.82/sq (Fig. S9 inSupplementary Materials). The solar cell fabrication process is described in detail in the Experimental section. The inset in Fig. 3a shows a photograph of the cell with dimensions of 50 mm×40 mm. As a control we used the same structure and processing steps, but with the ULW/binder film replaced with the lab-made 150-nm thick ITO film. The I–V characteristics of the resulting cells, under one sun (AM1.5) illumination, and in the dark, are shown in Fig. 3b and the solar cell parameters are given inSupplementary MaterialsTable S2. These demonstrate that the solar cell based on our network, with an efficiency of 6.57%, outperforms the one based on the lab-made ITO, with an efficiency of 4.95%. As evidenced by the I–V characteristics, this is due to the much better optoelectronic performance of our ULW network. Note that the relatively low overall efficiency of the cells is a result of missing

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Figure 3 Examples of Ag nanowire network applications: (a) SEM image of the cross section of the solar cell based on a ULW/binder network. Inset shows a photograph of the cell. (b) The corresponding illuminated (AM1.5) and dark I–V characteristics of the solar cell based on the ULW/binder electrode (red line), compared to the ITO-based cell (black line). (c) The IR image of the transparent heater based on the LW network (heated area size: 30 mm×30 mm). (d) Temperature versus time plot for the heater subjected to 240-s long, square voltage pulses. The applied voltage values are indicated at the plateaus of each curve. The inset shows the temperature response to multiple voltage pulses.

trapping schemes, omitted intentionally to emphasize the electrode performance. Figure 3c shows an infrared (IR) image of our LW/binder network havingT85.5% (atλ

=550 nm) andR_s=3.6/sq, subjected to a constant current flow. Clearly, our network can be used as a transparent heater, with high temperature uniformity. The corresponding measurement set-up, and further demonstration of the uniformity of the network during bending, is shown in the Supplementary Materials Fig. S10. Figure 3d shows the excellent response time of the heater, with the plateau of constant temperature reached in40 s, and a very good stability of the thermal response to multiple current pulses, as shown in the inset.

4. Discussion

4.1. Localized plasmonic refraction

While an excellent performance of our networks is expected, and results from the very high quality and very large

length of the nanowires, stabilized and strengthened on a substrate with the UV binder, their unusually high transparency needs explanation. Figures 4a and b show SEM images of two selected ULW networks, one with lower (a), and the other with higher (b) wire density. While for the higher-density network, the classical, shading limited transmittance isTg=1 –ν=0.71 (νis the metal coverage obtained from the SEM image), the corresponding measured T=0.84. The corresponding relative transmission increase, given byη=(T −Tg)/Tg, for this higher-density network isη=18%. The lower-density network, shown in Fig. 4a, hasTg=0.90, and its measuredT=0.96, i.e.η=7%. The transmission enhancement is not unusual, because the wire diameters are clearlynotin the geometric optics limit (D λ). However, the strength of this enhancement is unusual, which suggests a nonclassical, plasmonic refraction, which in extreme circumstances can lead to the phenomenon of the extraordinary optical transmission (EOT) [34]. To un- derstand this effect, we assume that a metallic nanowire network can be adequately modeled as a uniform effective film of thickness tf λ, made of an effective metallic

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Figure 4 Study of plasmonic refraction. SEM images of the ULW network of low (a) and high (b) nanowire density. (c) Plot ofT vs.

R_s⁻¹of the ULW (red circles) and the LW (black squares) networks. (d) Transmittance spectrum of the copper based nanonetwork: red solid line (experiment), black dashed line (simulation). Inset shows an SEM image of the nanonetwork.

medium described by a dielectric functionε(ω). For a normal incidence of an electromagnetic wave on this effective film, the transmission coefficienttis given by [35]

t =1−α/2, (1)

where

α=i2π(1−ε) (tf/λ). (2)

For large imaginaryε,tis real, and then simply T =t²≈1−α=1−i2π(1−ε)(t_f/λ)

=1−β(RS)⁻¹, (3) where β =Z₀/F, Z₀ is the vacuum impedance, F= σ(0)/σ(ω) is the figure of merit, often used as one of the measures of a network optoelectronic performance, and σ(ω)=iω[1−ε(ω)]/4π. The sheet resistance is given by R_s=ρ/t_f =1/t_fσ(0). Figure 4c shows plots ofTvs.R_s⁻¹ for our ULW and LW networks, and the solid lines represent the best second-order polynomial least square fits to the respective curves. These are dominated by the linear parts,

and thus demonstrate that our networks obey Eq. (3). From the slopes of the curves we extract figures of merits, and findF=572 for our LW array (black squares), and much higherF=1493 for our ULW network (red circles), which is much larger thanF=500 for the network of Ref. [13].

The fact that our networks obey Eq. (3) implies that the ε of our networks is large and imaginary, which can be explained by employing the Drude–Lorentz form ofε(ω), which is valid for an arbitrary medium [36]

ε(ω)=1+

N

f=1

ω²_pf

ω_rf² −ω(ω+iγ), (4)

whereγis the electron scattering rate with lattice imperfec- tions (phonons or defects),ωrfare frequencies of localized plasmonic resonances, andωpfare their resonator strengths.

In the nanowire networks the sum in Eq. (4) contains many terms, implying many resonances due to the multiplicity of local plasmonic oscillations resulting from the structural randomness of a nanowire network. Changing the sum into an integral, and assuming a constant spectral weight of each resonance type, leads to an analytical expression forε

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[37], which forωγ leads to Re(ε)/Im(ε)→0, and soε indeed becomes imaginary. The imaginary part ofεorigi- nates from imaginary parts of the resonant terms in Eq. (4), which in the effective medium model represent localized plasmonic resonances.

4.2. Delocalized plasmonic refraction

In periodic networks only a few terms exist in the sum of Eq. (4). Then, theTspectrum is expected to consist of a se- ries of dips, located at the localized plasmonic resonances of the film, separated by regions of very large T. To test this effective medium idea, we have made a copper-based, periodic network on glass as shown in the SEM image (inset in Fig. 4d). The network was obtained by masking a glass substrate (during copper sputtering) with an array of nanospheres of 750 nm diameter, which subsequently have been reduced in size by ion beam etching, while maintain- ing the initial intersphere distance [38–40]. The resulting structure is a copper film (60 nm thick), with a periodic (hexagonal) array of circular holes (diameter300 nm). Its measured transmittance spectrum (Fig. 4d) shows the fea- tures predicted with the simple effective medium model. In particular, there is a rather broad maximum at the wavelength of 1100 nm, withT_max=88%, far exceedingT_g= 38% obtained from analysis of the SEM image in the inset, leading toηmax =130%. Such dramatic transparency im- provement is desired for TCs, since this allows “loading”

the film with metal, and thus lowering its resistance. The simulated results are shown in Fig. 4d as a dashed line, and are in very good agreement with the experiment. In another system of a periodic nanoribbon array [41], we foundη= 30% in the visible frequency range. These results illustrate the great potential of delocalized plasmonic refraction for TC application, but obviously the structural and material parameters must be adjusted to shift the transparency maximum into the desired visible range. The parameters must include the substrate dielectric response, as demonstrated in Ref. [42].

Note that the terms “localized” and “delocalized” used in describing the plasmonic refraction cases above are cor- rect only within themacroscopic,effective mediummodel.

In amicroscopicmodel with individual nanowires explic- itly included, the “localized” plasmonic refraction condi- tion would be represented bypropagatingsurface plasmon modes of individual nanowires, and the “delocalized” plasmonic refraction by coupledpropagatingsurface plasmon modes of interacting nanowires.

5. Conclusions and outlook

We have developed a random network of ultralong silver nanowires, with subwavelength diameters, stabilized on a substrate with a UV binder. This network shows excellent optoelectronic performance, as demonstrated by the reported figure of merit F = 1493 (the single best F =

1686), as well as mechanical flexibility, and excellent stability under harsh mechanical strain, as well as thermal and chemical challenge. We also show the network’s application potential by incorporating it into an a-Si solar cell (as the window electrode), and showing that it outperforms conventional ITO. A flexible transparent heater based on our network also shows excellent performance.

Most importantly, our network shows, surprisingly, unusual transparency well exceeding the simple shading limit. We show that this enhancement is due to plasmonic refrac- tion, which involves localized plasmons (in the effective medium picture), and identify the inhomogeneous broadening as the key factor in promoting this mechanism in the random networks. Following this analysis we show that the plasmonc refraction can be further enhanced with propa- gating plasmonic modes. This leads to a dramatic 130%

increase of transparency above the shading limit in a periodic network of circular holes in a copper film. Nanowire networks used as transparent conductive electrodes have shown advantages in both optoelectonic performance (es- pecially in transparency) and cost (inexpensive wet chemical methods and fully compatible with large-scale roll to roll coating methods). However, these problems, such as interwire contact resistance, surface flatness, etc. still need to be thoroughly solved before practical applications can be realised.

Supporting Information

Additional supporting information may be found in the online ver- sion of this article at the publisher’s website.

Acknowledgements.This work is supported by NSFC grant No.

51571094, Guangdong province grant Nos. 2014B090915005, 2014A030313447, 2013KJCX0056, and HD14CXY010. K. K.

thanks the Boston College Ignite Program for additional financial support. J. M. thanks the NSFC grant No. 51431006. This work has been partially supported from the “International Laboratory for Optical Information Technologies (LOIT)”, the “China National Undergraduate Innovation Experiment Program” and the “SCNU Golden Seed Platform”.

Received:26 October 2015,Revised:26 March 2016, Accepted:26 March 2016

Published online:26 April 2016

Key words: metallic nanonetworks, discontinuous metallic films, silver nanowires, transparent conductors, plasmonic refraction.

References

[1] L. Hu, H. Wu, and Y. Cui, MRS Bull.36, 760–765 (2011).

[2] H. Wu, L. Hu, M. W. Rowell, D. Kong, J. J. Cha, J. R.

McDonough, J. Zhu, Y. Yang, M. D. McGehee, and Y. Cui, Nano Lett.10, 4242–4248 (2010).

[3] B. Han, Y. Huang, R. Li, Q. Peng, J. Luo, K. Pei, A. Her- czynski, K. Kempa, Z. Ren, and J. Gao, Nature Commun.5, 5674 (2014).

(8)

LASER

& PHOTONICS REVIEWS

[4] C. F. Guo, T. Sun, Q. Liu, Z. Suo, and Z. Ren, Nature Com- mun.5, 3121 (2014).

[5] B. Han, K. Pei, Y. Huang, X. Zhang, Q. Rong, Q. Lin, Y.

Guo, T. Sun, C. Guo, D. Carnahan, M. Giersig, Y. Wang, J. Gao, Z. Ren, and K. Kempa, Adv. Mater.26, 873–877 (2014).

[6] D. S. Hecht, L. Hu, and G. Irvin, Adv. Mater.23, 1482–1513 (2011).

[7] D. Langley, G. Giusti, C. Mayousse, C. Celle, D. Bel- let, and J. P. Simonato, Nanotechnology 24, 452001 (2013).

[8] S. Ye, A. R. Rathmell, Z. Chen, I. E. Stewart, and B. J. Wiley, Adv. Mater.26, 6670–6687 (2014).

[9] Y. Jin, D. Deng, Y. Cheng, L. Kong, and F. Xiao, Nanoscale 6, 4812–4818 (2014).

[10] R. Zhu, C.-H. Chung, K. C. Cha, W. Yang, Y. B. Zheng, H.

Zhou, T.-B. Song, C.-C. Chen, P. S. Weiss, G. Li, and Y.

Yang, ACS Nano5, 9877–9882 (2011).

[11] B.-T. Liu, and H.-L. Kuo, Carbon63, 390–396 (2013).

[12] J. Lee, P. Lee, H. Lee, D. Lee, S. S. Lee, and S. H. Ko, Nanoscale4, 6408–6414 (2012).

[13] S. De, T. M. Higgins, P. E. Lyons, E. M. Doherty, P. N.

Nirmalraj, W. J. Blau, J. J. Boland, and J. N. Coleman, ACS Nano3, 1767–1774 (2009).

[14] C. Sachse, N. Weiß, N. Gaponik, L. M¨uller-Meskamp, A.

Eychm¨uller, and K. Leo, Adv. Energy Mater. 4, 1300737 (2014).

[15] W. Gaynor, G. F. Burkhard, M. D. McGehee, and P. Peumans, Adv. Mater.23, 2905–2910 (2011).

[16] S. Nam, M. Song, D. H. Kim, B. Cho, H. M. Lee, J. D.

Kwon, S. G. Park, K. S. Nam, Y. Jeong, S. H. Kwon, Y. C.

Park, S. H. Jin, J. W. Kang, S. Jo, and C. S. Kim, Sci. Rep.

4, 4788 (2014).

[17] A. R. Madaria, A. Kumar, and C. Zhou, Nanotechnology22, 245201 (2011).

[18] J. Lee, P. Lee, H. B. Lee, S. Hong, I. Lee, J. Yeo, S. S. Lee, T.-S. Kim, D. Lee, and S. H. Ko, Adv. Funct. Mater.23, 4171–4176 (2013).

[19] C. Celle, C. Mayousse, E. Moreau, H. Basti, A. Carella, and J.-P. Simonato, Nano Res.5, 427–433 (2012).

[20] S. Choi, J. Park, W. Hyun, J. Kim, J. Kim, Y. B. Lee, C.

Song, H. J. Hwang, J. H. Kim, and T. Hyeon, ACS Nano9, 6626–6633 (2015).

[21] Q. Huang, W. Shen, X. Fang, G. Chen, J. Guo, W.

Xu, R. Tan, and W. Song, RSC Adv. 5, 45836–45842 (2015).

[22] T. Kim, Y. W. Kim, H. S. Lee, H. Kim, W. S. Yang, and K.

S. Suh, Adv. Funct. Mater.23, 1250–1255 (2013).

[23] H.-Y. Lu, C.-Y. Chou, J.-H. Wu, J.-J. Lin, and G.-S. Liou, J.

Mater. Chem. C3, 3629–3635 (2015).

[24] J.-W. Lim, D.-Y. Cho, K. Jihoon, S.-I. Na, and H.-K. Kim, Sol. Energy Mater. Sol. Cells107, 348–354 (2012).

[25] J. Y. Lee, S. T. Connor, Y. Cui, and P. Peumans, Nano Lett.

8, 689–692 (2008).

[26] J. H. Lee, P. Lee, D. Lee, S. S. Lee, and S. H. Ko, Cryst.

Growth Des.12, 5598–5605 (2012).

[27] S. M. Bergin, Y.-H. Chen, A. R. Rathmell, P. Charbonneau, Z.-Y. Li, and B. J. Wiley, Nanoscale4, 1996–2004 (2012).

[28] S. De, P. J. King, P. E. Lyons, U. Khan, and J. N. Coleman, ACS Nano4, 7064–7072 (2010).

[29] A. F. Oskooi, D. Roundy, M. Ibanescu, P. Bermel, J. D.

Joannopoulos, and S. G. Johnson, Comput. Phys. Commun.

181, 687–702 (2010).

[30] E. D. Palik, Handbook of Optical Constants of Solids. (Aca- demic Press, 1998).

[31] Y. Sun, B. Mayers, T. Herricks, and Y. Xia, Nano Lett.3, 955–960 (2003).

[32] J. Jiu, T. Araki, J. Wang, M. Nogi, T. Sugahara, S. Nagao, H.

Koga, K. Suganuma, E. Nakazawa, and M. Hara, J. Mater.

Chem. A2, 6326–6330 (2014).

[33] J. Wang, Z. Fang, H. Zhu, B. Gao, S. Garner, P. Cimo, Z.

Barcikowski, A. Mignerey, and L. Hu, Thin Solid Films556, 13–17 (2014).

[34] T. W. Ebbesen, H. J. Lezec, H. F. Ghaemi, T. Thio, and P. A.

Wolff, Nature391, 667–669 (1998)

[35] J. A. Kong and New York, Theory of Electromagnetic Waves (Wiley-Interscience, New York, USA, 1975).

[36] Y. Wang, E. Plummer, and K. Kempa, Adv. Phys.60, 799–

898 (2011).

[37] K. Kempa, Phys. Rev. B74, 033411 (2006).

[38] N. Denkov, O. Velev, P. Kralchevski, I. Ivanov, H.

Yoshimura, and K. Nagayama, Langmuir 8, 3183–3190 (1992).

[39] A. Kosiorek, W. Kandulski, P. Chudzinski, K. Kempa, and M. Giersig, Nano Lett.4, 1359–1363 (2004).

[40] P. Patoka and M. Giersig, J. Mater. Chem.21, 16783–16796 (2011).

[41] P. Patoka, T. Sun, M. Giersig, Z. Ren, and K. Kempa, Adv.

Mater.24, 3042–3045 (2012).

[42] T. Sun, E. M. Akinoglu, C. Guo, T. Paudel, J. Gao, Y. Wang, M. Giersig, Z. Ren, and K. Kempa, Appl. Phys. Lett.102, 101114 (2013).