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Responsive Monochromatic Color Display Based on Nanovolcano Arrays Bin Ai, Ye Yu, Helmuth Möhwald, and Gang Zhang* In view of remarkable advantages and broad application prospects, many efforts have been made to develop novel types of nanostructures that show plasmonic structural color, such as nanohole arrays (NAs) in metal films,[1a,3] bull’s eye structures,[4] metal–insulator–metal (MIM) nanoresonators[5] and metallic resonant waveguide gratings (MRWG).[6] In particular, nanohole arrays exhibiting extraordinary optical transmission (EOT)[7] proved to possess outstanding properties[8] and have been the focus of much research activity about the plasmonic structural color. For example, free standing Ag NAs have been fabricated to show the three primary red–green–blue (RGB) colors in transmission,[1a] and then colored letters ‘hν’ were generated.[3a] Inoue’s group fabricated aluminum nanohole arrays confirming the function as RGB color filters.[3d] Recently, panchromatic plasmonic color patterns have been obtained by adjusting the Ag nanohole arrays from embedded to elevated ones.[3f ] Although investigations have progressed rapidly, challenges in this field still exist. Good purity of the color is vital for lots of applications, but to achieve monochromatic colors, traditional substrate-supporting nanohole arrays need an index-matching layer always covering the whole structure to keep the environment on both sides symmetrical.[3a,b,d,9] This makes nanohole arrays inaccessible to other environments so that the color is limited to be tuned (inefficiently) by changing lattice constants or morphologies. Although the free-standing structure can solve this problem, the fabrication process is difficult and it is usually impractical.[1a] For more efficient application, it is of high priority to find a novel substratesupporting nanohole array not only with monochromatic color but also with facile manufacturing and low-cost tunability. In this paper, a novel nanohole array with volcano shaped holes is reported to successfully achieve the above goal. Ag nanovolcano arrays are fabricated on glass substrates with excellent stability by a low-cost, large-area and well-controlled colloidal lithography (CL) method. By controlling the hole size and height, Ag nanovolcano arrays can be designed to provide optimal transmission spectra to yield structural colors possessing only one single sharp transmission peak with effective suppression of undesired wavelength light, while which doesn’t need any index-matching layers. Numerical simulations are performed to understand and control this unique filtering process, demonstrating excellent agreement with experimentally
Novel nanohole arrays with volcano-shaped holes (truncated cones) are reported to obtain structural colors with excellent purity without using index-matching layers. They further show efficient sensitive environment responses. The novel structures are fabricated via a low-cost, large-area colloidal lithography method. Numerical simulations demonstrate that each nanovolcano, comprising one upper hole and one lower hole, excites two surface plasmon resonances (SPRs), generating only one single transmission maximum at a tunable position (monochromatic color), and calculations show excellent agreement with the measured spectra. Three primary red–green–blue (RGB) monochromatic colors are obtained by adjusting the lattice constant. Moreover, the colors can be tuned easily and inexpensively across the whole visible range, retaining a single sharp transmission peak, showing an efficient responsive color display. Because of the simple fabrication process and remarkable properties, Ag nanovolcano arrays are believed to be greatly advantageous for sensors, optical devices, color displays, and nanoantennas. Furthermore, the novel structures can be used as versatile substrates for cell biology, analysis of single protein molecules, and surfaceenhanced Raman scattering.
1. Introduction With the development of nanofabrication and characterization techniques, surface plasmons (SPs) and related plasmonic nanostructures have generated considerable interest in recent years.[1] In particular, one type of attractive application is plasmonic structural color which is generated by efficiently controlling the conversion between free-space photons and confined SPs at a certain wavelength through exploiting plasmonic nanostructures. In contrast to the color produced by ordinary electronic absorption, plasmon-based structural color provides several advantages: more compaction, lower power, higher reliability, higher durability and better conductivity, opening up a colorful future for the next generation high-resolution displays, spectral imaging, and other relevant applications.[2] B. Ai, Y. Yu, Prof. G. Zhang State Key Lab of Supramolecular Structure and Materials College of Chemistry Jilin University Changchun, 130012, PR China E-mail:
[email protected] Prof. H. Möhwald Max Planck Institute of Colloids and Interfaces D-14424, Potsdam, Germany
DOI: 10.1002/adom.201300224
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measured characteristics. Three primary RGB monochromatic colors are obtained by adjusting the period. Moreover, just by changing the surrounding environments the colors can be tuned facilely and inexpensively retaining a single sharp transmission peak across the whole visible range, showing an efficient responsive color display, which demonstrates that the Ag nanovolcano arrays can be used for novel color-displaying sensors and which enables optics applications with low-cost and high-throughput. Furthermore, the novel nanovolcano structures are believed to offer opportunities to be used as nanoantenna as well as versatile substrates for cell biology, analysis of single protein molecules, surface enhanced Raman scattering (SERS).
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Scheme 1. Outline of the process for fabricating Ag nanovolcano arrays. The central schematic shows the cross section of a single nanovolcano and the main structural parameters.
can be well controlled by the diameter of PS spheres, thickness of photoresin films, deposition process and RIE process, respectively. Figure 1 shows scanning electron microscopy (SEM) images of the structure formed in different steps. Owing to the masking effect of PS spheres, as shown in Figure 1(A–C), the photoresin film is etched to circular truncated cones with different feature sizes. Besides, the heights of the three circular truncated cones are the same. Figure 1(D–F) shows SEM images of the Ag nanovolcano arrays corresponding to the asprepared ones shown in Figure 1(A–C). It can be seen that the samples show a well-defined volcanic shape and keep undamaged although with a sonication procedure. The diameters of the top and bottom holes are both decreased by prolonging the etching duration. It’s also observed that the top hole possesses a wide edge, which is against the expected sharp edge. This result is considered to be partly owing to the scatter of the Ag vapor in the deposition process. Another reason is that the top platform of the etched photoresin film is slightly larger than the hole diameter due to different etching rates of photoresin and PS (details are shown in Figure S1). Furthermore, various Ag nanovolcano arrays with different periods, hole diameters and heights are also fabricated in this work, of which the structural parameters are shown in Table 1. Hence through this low-cost and controllable fabrication process Ag nanovolcano arrays with two types of holes can be precisely prepared.
2.2. Optical Properties of Ag Nanovolcano Arrays To quantify the novelty in spectral evolution of Ag nanovolcano arrays based on the EOT from the traditional nanohole arrays, the transmission spectra of traditional nanohole arrays with hole diameter of 450 nm and period of 700 nm are also shown in Figure 2A. The EOT effect manifests itself as a series of intense peaks in the optical transmission spectra that are
2. Results and Discussion 2.1. Fabrication of Ag Nanovolcano Arrays Highly ordered Ag nanovolcano arrays with large area were fabricated based on welldeveloped CL approaches.[10] Scheme 1 shows the fabrication process of Ag nanovolcano arrays and cross sectional schematic of a single nanovolcano. In brief, ordered PS sphere monolayers were first prepared onto a substrate coated by a photoresin film. Then RIE with increasing duration was carried out to etch the photoresin film. Next, the samples were vertically deposited with 100-nm Ag. Finally after PS spheres and photoresin were removed by toluene and ethanol, respectively, Ag nanovolcano arrays were formed. In the fabrication process the period (P), height (H), thickness (h), and the hole diameters (d/D)
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Figure 1. Cross sectional SEM images of PS spheres and circular truncated cones etched for (A) 270 s, (B) 300 s and (C) 330 s. Typical SEM images from 45° tilting views of the corresponding Ag nanovolcano arrays with diameters of (D) 300/600 nm, (E) 200/500 nm and (F) 150/450 nm. The inset in (D) shows the cross section of a nanovolcano. The scale bar applies to all the images.
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www.MaterialsViews.com Table 1. Structural parameters of Ag nanovolcano arrays fabricated in this work. Sample
Period [nm]
Bottom diameter (D) [nm]a)
Top diameter (d) [nm]a)
Height [nm]a)
1
700
600
300
400
2
700
500
200
400
3
700
450
150
400
4
700
500
300
200
5
700
400
200
200
6
700
350
150
200
7
580
520
220
400
8
580
420
120
400
9
500
500
200
400
10
500
380
80
400
a)The
structure parameters correspond to those described in Figure 2 and have an error of 20 nm.
stronger than predicted by classical diffraction theory. Generally, the spectral position of the hexagonally packed NAs can be approximated with the following equation:[11] 8max (i, j ) = a0
4 3
i2 + i j + j 2
− 1/2
εm εd εm + εd
(1)
Where a0 is the periodicity of the array, the integers (i, j) represent the Bragg resonance orders, εd and εm are the dielectric functions of the dielectric and metal, respectively. According to Equation (1) and previous theoretical calculations,[12] two sets of transmission peaks observed in Figure 2A can be ascribed to the different SPR modes at the Ag/air (A1, A2) and Ag/ glass (G1, G2) interface. Moreover, the G2 (1, 1) transmission peak and the A1 (1, 0) transmission peak overlap at ∼800 nm, resulting in a broader resonance. The two peaks and broader resonances in the visible range degrade the purity of the filtered color, which greatly limits practical applications. For the transmission spectra of Ag nanovolcano arrays shown in Figure 2B, the A1 and G2 transmission peaks no longer overlap, and the A2 (1, 1) Ag/air transmission peak newly appears. Besides, the Ag/air (A1, A2) peaks get stronger
and Ag/glass (G1, G2) peaks become weaker relative to the ones of the Ag nanohole arrays. Furthermore, the spectra of Ag nanovolcano arrays are strongly affected by the hole size. As shown in Figure 2 (B∼D), the peaks (G1, G2) and A2 (1, 1) are getting weaker as the hole diameter decreases and finally disappear, while the A1 (1, 0) Ag/air transmission peak is getting stronger. Eventually, only a single sharp A1 (1, 0) Ag/air transmission peak located at ∼636 nm is left generating the red color as shown in Figure 2D. This peak has a half-bandwidth of ∼100 nm, indicating the purity of the nanovolcano array. While the transmission efficiency of the A1 peak in Figure 2D is still low due to the low refractive index (RI) of air for the Ag/air resonance mode. In conclusion, the result from Figure 2 demonstrates that monochromatic color can be obtained only by Ag nanovolcano arrays with small enough holes. Beyond that the results are the same no matter the white light vertically illuminates from the bottom or top side. The traditional arrays display various peaks (Figure 2A), and this is also the case for most volcano arrays (Figure 2B and Figure 2C). However, one may be able to find conditions where the nanovolcano arrays exhibit only one sharp peak (Figure 2D). These changes of transmission spectra from traditional Ag nanohole arrays to Ag nanovolcano arrays and the effect of the hole diameter on transmission spectra can be demonstrated by the simulation of SP energy distributions. Figure 3A shows the calculated transmission spectra of Ag nanovolcano arrays with the corresponding structures measured in Figure 2 (B∼D), which can be directly compared to the experimental data. The overall qualitative agreement between experimental and simulated profiles is excellent, and the remaining discrepancy can be attributed to extra losses in the metal due to increased surface scattering, the rough Ag surface, and the inhomogeneity of the inter-nanovolcano separation. Figure 3B and C present the electric-field intensity for the z direction (⎢Ez ⎢ (abs)) in the x-z plane simulated for the peak wavelength indicated by the corresponding red and green dots in Figure 3A. Each nanovolcano can enable SPRs at two locations which are respectively positioned at the bottom and top hole. For the bottom hole with different environment on both sides, relatively strong Ag/glass and weak Ag/air SP modes are excited as shown in Figure 3B, resulting in two sets of Ag/air and Ag/glass transmission peaks. Hence this is the exact mechanism and the spectra are
Figure 2. (A) Measured and calculated zero-order light transmission spectrum and the SEM image of Ag nanohole arrays with the hole diameter of 450 nm and a period of 700 nm. The scale bare in the inset corresponds to 1 μm. Measured transmission spectra of the Ag nanovolcano arrays with hole diameters of (B) 300/600 nm, (C) 200/500 nm and (D) 150/450 nm. A1 and A2 indicate the (1, 0) and (1, 1) Ag/air transmission peak, respectively. G1 and G2 indicate the (1, 0) and (1, 1) Ag/glass transmission peak respectively. The inset in (D) shows the microscopic optical image of a red Ag nanovolcano array sample.
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FULL PAPER Figure 3. (A) FDTD calculated transmission spectra for Ag nanovolcano arrays with decreasing hole diameters. The green and red dots indicate the (1, 0) Ag/air and (1, 0) Ag/glass transmission peak, respectively. Electric-field intensity for the z direction (⎪Ez⎪ (abs)) in the x-z plane simulated at the peak wavelength indicated by the corresponding (B) red and (C) green dots in (A). (D) Schematic process of getting monochromatic color for the Ag nanovolcano array with the hole diameter of 150/450 nm and height of 400 nm.
produced by traditional Ag nanohole arrays. For the top hole shown in Figure 3C, only one strong Ag/air SP mode is excited, which will greatly weaken the Ag/glass transmission peaks and enhance the Ag/air ones. The unique filtering process of twice SPRs leads to the differences between traditional Ag nanohole arrays and Ag nanovolcano arrays. Furthermore, the simulations of Ag nanovolcano arrays with reducing hole diameters in Figure 3B and C present apparent differences in the energy density. Figure 3B illustrates that the energy at the bottom holes is getting stronger with decreasing hole diameters, which means the absorbance of light at the main Ag/glass transmission peak wavelength grows. Then through the second SPR at the top hole, the Ag/glass transmission peaks are getting weak until disappearing completely. However, the energy associated with the corresponding main Ag/air transmission peaks in Figure 3C shows the opposite trend, resulting in a relative enhancement of the A1 (1, 0) Ag/ air transmission peak. Other Ag/air transmission peaks will disappear gradually due to the lower light transmission. Finally, monochromatic color without index-matching layers is successfully generated through the unique filtering process which is described clearly in Figure 3D. In addition to the hole diameter, the effect of the height (H) on the transmission spectra of Ag nanovolcano arrays has also been investigated. Figure 4A and B show the SEM images and transmission spectra of the Ag nanovolcano arrays with heights of 200 nm. The top hole diameters are the same as the corresponding ones with heights of 400 nm while the bottom hole
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diameters are slightly smaller, which is considered to have little effect on the spectra according to previous work.[13] So the effect of the small changed hole diameters can be neglected relative to that of the heights. In Figure 4B, it can be observed that, unlike the Ag nanovolcano arrays with 400 nm heights, the spectral
Figure 4. (A) SEM images and (B) transmission spectra of the Ag nanovolcano arrays with heights of 200 nm and different hole diameters of 300/500 nm, 200/400 nm and 150/350 nm. The dotted line is the calculated spectrum of the one with hole diameters of 150/350 nm. The SEM image is taken from the 45° tilting views. The scale bar corresponds to 500 nm and applies to all the SEM images.
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Besides, slightly rough edges are observed for the samples with smaller periods, which is believed to have little effect on the spectra. In Figure 6A, the peak positions are tuned to 636 nm, 510 nm and 441 nm retaining a single transmission maximum with periods of 700 nm, 580 nm and 500 nm, respectively, which is in excellent agreement with the calculation results. The peaks keep small half bandwidths of ∼100 nm. Figure 6B shows the macro-micro optical images of three primary RGB colors obtained by Ag nanovolcano arrays with different periods. The result completely conforms to the corresponding relationship between the Figure 5. Near-field electric field profile of Ag nanovolcano arrays with heights of (C) 200 nm color and peak wavelengths according to the and (D) 400 nm. color ring shown in Figure 6C. Further investigations in Figure 6D reveal that the peak position shows a strictly linear response to the period, which is shape of ones with 200 nm heights is similar to the spectrum of in good accord with the linear function in Equation (1). In additraditional nanohole arrays and shows very small changes with tion, Ag nanovolcano arrays with periods of 500 nm and 580 nm different hole diameters, resulting in mixed colors. show a spectral evolution with decreasing hole diameters The effect of the height can be understood by means of the similar to that of the Ag nanovolcano array with the period of SP energy distribution. As shown in Figure 5A, the electric 700 nm, demonstrating the same effect of hole diameters on fields of Ag nanovolcanoes with height of 200 nm are mostly the filtering process of Ag nanovolcano arrays with different located in the glass, which indicates that the SPR at the top hole periods (Details are shown in Figure S2 in the supporting inforis much affected by the glass substrate due to the low height, mation). In a word, the desired color can be easily achieved by resulting in strong Ag/glass SPR modes for the whole Ag nanousing a nanovolcano array with the corresponding period. volcano array. So there exist still two sets of transmission peaks for the nanovolcano arrays with heights of 200 nm, failing to make the monochromatic color. For the ones with high enough 2.4. Responsive Monochromatic Color Display height (400 nm in this experiment) the SPR at the top holes is independent from the substrate, which then leads to a redisWithout using index-matching layers, the color of Ag nanotribution in electric field enhancement. This then produces an volcano arrays presents remarkable advantages of a display electric field that exhibits maxima exclusively in the air above responsive to the surrounding environment. Figure 7A shows the planar substrate, which is shown in Figure 5B. The electric the peak shifts of the Ag nanovolcano array with the period of field in air is responsible that the Ag nanovolcano array obtains 500 nm upon immersion into different liquids. The measured a color with excellent purity. transmission spectra and optical images reveal that the color Consequently, features in transmission spectra of the Ag of the Ag nanovolcano array can be tuned to the colors of blue, nanovolcano arrays can be well demonstrated by the effect of green, yellow-green, yellow, orange and red by increasing the EOT. Furthermore, proper structural parameters are the essenRI of the liquids. In the process the bandwidths of the peaks tial conditions for Ag nanovolcano arrays to get monochromatic become larger due to the increasing RI, while the colors still color without any index-matching layers. retain high purity and have an exact relationship to the peak wavelength according to Figure 6C. Moreover, Figure 7B shows 2.3. Color Evolution of Ag Nanovolcano Arrays a strictly linear dependence between transmission peak and RI, with a sensitivity of ∼314 nm per refractive index unit (nm RIU−1). The linear relation is considered to result from the According to the previous results and theoretical calculations, if redistributed electric fields in Figure 5B. The electric fields in the surrounding environment remains the same, the transmisair make it accessible to other species and are not affected by sion peak wavelength would be proportional to the period. Therethe glass substrate. Figure 7C shows the macroscopic optical fore the color of the samples can be tuned through precisely images of the process where a blue Ag nanovolcano array controlling the period of the array. To achieve this, Ag nanovolsample was immersed into ethanol. As the sample was going cano arrays with periods of 580 nm and 500 nm were also fabto touch the liquid, the blue color changed to green. When ricated and studied with respect to color display. According to immersed partly, the sample obviously showed two colors the fabrication process the hole diameter is decided by the RIE (blue, green) and had a clear boundary. When wholly in ethduration. For Ag nanovolcano arrays with periods of 500 nm and anol the sample changed to green entirely. Extrapolated from 580 nm, oxygen RIE is carried out for 270 s and 300 s, respecthe transmission peak of Ag nanovolcano arrays in ethanol, the tively, to get small enough holes. SEM images in Figure 6A substrate should appear yellow-green. The discrepancy can be show 45° views of the Ag nanovolcano arrays with different attributed to the low transmission that makes the yellow part periods. The heights (H) of the three samples are all 400 nm.
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keeps a single transmission maximum. The result indicates that the substrate plays an indispensable role to decide the peak position while it doesn’t affect the purity, which exactly fits to the explanation of the process to get monochromatic color. Besides, the samples keep excellent stability in the test processes. With the novel facile tuning of the color with high purity, Ag nanovolcano arrays can be converted into spectra filters with different filter wavelengths by a much more low-cost and simple-operating process.
3. Conclusion Novel Ag nanovolcano arrays comprised of two different types of holes are fabricated by a low-cost, large-area and well-controlled colloidal lithography technique, which provides the remarkable property of efficient Figure 6. (A) SEM images and transmission spectra of Ag nanovolcano arrays with periods of responsive monochromatic color display. FDTD simulations demonstrate that only 700 nm, 580 nm and 500 nm. The hole diameters of the ones with 500 nm and 580 nm period are ∼80/380 nm and ∼120/420 nm, respectively. The dotted lines are the corresponding calcu- one single sharp transmission peak in the lated spectra. The SEM images are taken from the 45° tilting views. The scale bar corresponds visible range is achieved by the interacto 200 nm and applies to all the SEM images. (B) The macro-microscopic optical images of tions of two different SPRs excited in each the ∼1.5 cm × 1.5 cm Ag nanovolcano array. The RGB colors are generated by Ag nanovolcano substrate-supporting nanovolcano while arrays with periods of 700 nm, 580 nm, and 500 nm, respectively. The scale bar corresponds to needing no index-matching layers, which 1 cm. (C) Schematic color ring relating the peak wavelength to color. (D) Linear fit of the peak is a great progress for fabricating structural wavelength with the period. The R2 is 0.99794. The numbers indicate the peak wavelengths. color based on EOT. Excellent agreement is achieved between measured spectra and difficult to be identified. Therefore, the sample appears green results of FDTD simulations, leading to a rational design of in ethanol. Besides, the sample will return to the original structures and properties. In this work three primary RGB blue color after being taken out of ethanol and complete solcolors with excellent purity are obtained by adjusting the lattice vent evaporation. The sensitive environment responses enable constant. Moreover, without index-matching layers, the color Ag nanovolcano arrays to be used as novel sensors with welldisplays sensitive response to environmental changes, and can identified monochromatic color displays, and hence with a be tuned across the whole visible range with excellent stability sensitivity of better than 0.1% of a refractive index unit. It also and purity by various facile ways. This indicates novel and effiindicates that the Ag nanovolcano arrays provide opportunities cient applications for sensors and spectral filters. Ag nanovofor practical applications like anti-counterfeit tags and rewritlcano arrays are believed to offer great potential for practical able photonic papers. applications such as sensors, imaging, spectral filtering, highBeyond immersing into liquids, Figure 8 indicates two resolution color display, nanoantenna and SERS substrates. We other facile ways to tune the color of Ag nanovolcano arrays. have concentrated here on their optical properties, but should Figure 9A shows the transmission spectra of Ag nanovolcano mention that they also provide excellent model substrates to arrays coated by polyvinyl alcohol (PVA) and poly-p-vinylphenol deposit films or cells that then are addressable through the (P4VP). With different polymer films the transmission spectra holes as well as through the wells, and further can be used as display good purity and apparent peak shifts from 441 nm to versatile substrates for cell biology and analysis of single pro457 nm and 520 nm. The other tuning way is changing the RI tein molecules. of substrates. The Ag nanovolcano arrays can be transferred to different substrates through the process designed in our earlier 4. Experimental Section work.[14] In brief, the samples were inserted slowly into deionized water with tilt to make Ag nanovolcano arrays disengage Materials: In all experiments deionized water was ultrapure (18.2 MΩ cm) from a Millipore water purification system. The glass from the substrate after immersion in HF solution for 10 s. slides (15 mm × 30 mm) used as substrates were cleaned in an O2 Then the free-standing films were lifted up by other substrates. plasma cleaner for 2 min to create a hydrophilic surface. Polystyrene Figure 8B shows the transmission spectra of the Ag nanovo(PS) spheres with the diameter of 580 nm were prepared by emulsion lcano array with a 580-nm period on glass and polyethylene polymerization, as mentioned previously.[15] PS spheres (500 nm) terephthalate (PET) substrate. The transmission peak presents were purchased from Sigma−Aldrich, and 700 nm PS spheres were a significant red shift from 510 nm to 536 nm with the subobtained from Wuhan Tech Co., Ltd. 1-Methoxy-2-propanol-acetate (MPA) was purchased from Aldrich. Photoresist (BP212-37 positive strate changing from glass to PET and the each of the spectra
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www.MaterialsViews.com Fabrication of the Ag Nanovolcano Arrays: For the Ag nanovolcano array with the height of 400 nm and period of 700 nm, 50 wt% photoresist (diluted with MPA) was spin-coated onto the glass substrate and cured at 88 °C for 2 h. Next the PS sphere (700 nm) monolayers were deposited on the as-prepared substrate by the interface method.[16] Oxygen reactive ion etching (RIE), performed with a Plasmalab Oxford 80 Plus system (ICP 65) (Oxford Instrument Co., UK), was applied for 270 s, 300 s and 330 s, generating PS spheres and circular truncated cones with different feature sizes. The RIE procedure was operated at a pressure of 10 mTorr, a flow rate of 50 sccm, a radio-frequency (RF) power of 100 W and an inductively coupled plasma (ICP) power of 100 W, After that the samples were mounted in a thermal evaporator to vertically deposit 100-nm Ag (99.9%). Finally the PS spheres were removed by sonication in toluene and the photoresist was washed away by ethanol, forming an Ag nanovolcano array. For the Ag nanovolcano array with the height of 200 nm, 40 wt% photoresist was used; for the Ag nanovolcano arrays with periods of 500 nm and 580 nm, PS spheres with diameters of 500 nm and 580 nm were used, and the RIE process was carried out for 270 s and 300 s, respectively. The other materials and procedures are the same. Finite-Difference Time-Domain (FDTD) Simulations: A commercial software package (FDTD Figure 7. (A) Measured transmission spectra and microscopic optical images of the Ag nano- Solutions, Lumerical Solutions Inc.) was used to simulate the optical responses of Ag nanovolcano volcano array with the period of 500 nm in air (1.00, blue), methanol (1.33, green), ethanol (1.36, yellow-green), dichloromethane (1.42, yellow), toluene (1.50, orange) and styrene (1.55, array structures. The structure was excited by a red). (B) Linear fit of the peak wavelength with the increasing RI. The R2 is 0.99306. The num- normally incident, unit magnitude plane wave propagating in the z direction with an electric field bers indicate the RI and peak wavelengths. (C) Macroscopic optical images of the process where a blue Ag nanovolcano array substrate is immersed in ethanol to be changed to green. polarization along the x-axis. Periodic boundary conditions were imposed at x and y boundaries of The period of the Ag nanovolcano array is 500 nm and the hole diameters are 80/380 nm. the x–y plane. Monitors were placed to calculate the amount of reflected, transmitted, absorbed power photoresist, Kempur (Beijing) Microelectronics, Inc.) was diluted with as a function of wavelength. The optical parameter of Ag and SiO2 were MPA before use. Poly-p-vinylphenol (P4VP) was purchased from Sigmataken from Palik’s handbook.[17] Aldrich. The silver (99.9%) powder for vapor deposition was purchased Characterization: Scanning electron microscopy (SEM) images from Sinopharm Chemical Reagent Co. Ltd. Polyvinyl alcohol (PVA), were taken with a JEOL JSM 6700F field emission scanning electron polyethylene terephthalate (PET), toluene, methanol, dichloromethane, microscope with a primary electron energy of 3 kV, and the samples styrene and ethanol were purchased from Beijing Chemical Works, and were sputtered with a layer of Pt (ca. 2 nm thick) prior to imaging to were used as-received. improve conductivity. A Maya 2000PRO optics spectrometer, and a model DT 1000 CE remote UV/vis light source (Ocean Optics) was used to measure the transmission spectra. All microscopic optical images were captured with a camera mounted on the microscope (Olympus microscope BX51) in transmission mode with a total magnification of 500× and the exposure time fixed at 30 ms. The sample was illuminated by a condenser. When taking photos, one had to adjust the aperture stop on the condenser to obtain high quality images.
Supporting Information Supporting Information is available from the Wiley Online Library or from the author.
Acknowledgements This work was supported by the National Natural Science Foundation of China (51073070, 51173068). Figure 8. (A) Transmission spectra of Ag nanovolcano arrays with the period of 500 nm coated by PVA (RI ≈ 1.50) and P4VP (RI ≈ 1.44). (E) Transmission spectra of Ag nanovolcano arrays with the period of 580 nm on glass (RI ≈ 1.50) and PET (RI ≈ 1.65) substrate.
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Received: May 25, 2013 Revised: June 27, 2013 Published online: July 22, 2013
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