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Optical properties of ordered Dot-on-Plate nano-sandwich arrays
Microelectronic Engineering xxx (2014) xxx–xxx
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Optical properties of ordered Dot-on-Plate nano-sandwich arrays Zhenxing Li a,⇑, Thang Duy Dao b,c,1, Tadaaki Nagao b,c,1, Masahiko Yoshino a a
Department of Mechanical and Control Engineering, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguroku, Tokyo 152-8552, Japan International Center for Materials Nanoarchitectonics, National Institute for Materials Science, 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan c CREST, Japan Science and Technology Agency, 4-1-8 Honcho, Kawaguchi, Saitama 332-0012, Japan b
a r t i c l e
i n f o
Article history: Received 23 January 2014 Received in revised form 21 March 2014 Accepted 31 March 2014 Available online xxxx Keywords: 3D nanostructure array Nano-sandwich Double plasmon resonances SERS Nanoplastic forming
a b s t r a c t A cost-effective fabrication method for realizing ordered Dot-on-Plate nanostructure array is presented. This fabrication method is a combination of Nanoplastic forming (NPF) and thermal dewetting, which does not involve costly and complicated processes such as lithography. The fabricated Au–SiO2–Au sandwich nanostructures exhibit double plasmon resonances and strong magnetic response, which makes it a potential candidate for metamaterials. It is also found that the Dot-on-Plate nanostructure shows good performance as surface-enhanced Raman scattering (SERS)-active substrate. The milling of SiO2 spacer layer by dry etching is studied and the relation between the plasmonic properties and SERS enhancement is discussed. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction Three-dimensional metal-dielectric composite nanostructures have found broad promising applications from biosensing, photocatalysis, metamaterials, to light harvesting devices due to their unique plasmonic properties [1–7]. Cut-wire pairs and plate pairs were investigated as magnetic atoms for optical metamaterials which shows the potential to be an alternative design of the well-established split-ring resonator [8,9]. A tuneable plasmon resonance in arrays of nano-sandwiches suggests its applications for biosensing [10]. As for photovoltaic cells, vertical-stacked metal disks with broadband interaction of incoming light show the possibility to achieve efficient light harvesting [11]. By combining noble metal with catalytically active transition metals (Pt or Pd), the sandwich nanostructure offers a promising way to drive chemical reactions for photocatalysis [12,13]. Metal-dielectric-metal composite nanostructures have also been extensively investigated in the field of near-field enhanced spectroscopy and surfaceenhanced Raman spectroscopy [14,15]. Lithographic techniques are the widely implemented top-down methods in the production of such sandwiched nanostructures. The most commonly utilized technique is electron-beam lithography (EBL) [4,5,8,9,11]. The procedures include electron-sensitive ⇑ Corresponding author. Tel.: +81 3 5734 2640; fax: +81 3 5734 2506. E-mail addresses: [email protected] (Z. Li), [email protected] (T.D. Dao), [email protected] (T. Nagao), [email protected] (M. Yoshino). 1 Tel.: +81 29 860 4746; fax: +81 29 860 4706.
resist coating, pattern writing by electron beam and following lift-off processes. Another approach referred to as focused ion beam (FIB) [16] typically utilizes a gallium ion beam to directly sputter away unwanted parts of a continuous film and fabricate desired nanostructures. While both of these techniques can produce nanostructures with well-controlled size, shape and alignment, they need costly facilities, stringent processes and cannot address the problem of low throughput. Another drawback of these techniques is that nanostructure adhesion to the substrate usually requires an intermediate layer such as chromium or titanium. Nanoimprint lithography (NIL) is an alternative top-down method, and it shows the advantage of high resolution and high efficiency [17–19]. Self-assembly methods based on chemical synthesis [1,2] are difficult to obtain ordered nanostructures immobilized on the substrates. Other unconventional approaches such as porous anodic alumina (PAA) method [3] and colloidal lithography [6,10,12–14] are able to produce layered nanostructures based on the PAA or nanosphere templates; however, the limitation is the geometrical constraint of the template. In order to address the problems and supplement limitations for the current fabrication methods, authors developed an alternative approach by using Nanoplastic forming and following thermal dewetting processes to fabricate two-dimensionally ordered nanodot array structures [20,21]. This approach possesses a number of advantages including low-cost, low-emission, low stringency and without any chemical disposals which causes environmental pollutions. By combining NPF with other techniques such as roller imprinting process [22], it is possible to increase the throughput
http://dx.doi.org/10.1016/j.mee.2014.03.045 0167-9317/Ó 2014 Elsevier B.V. All rights reserved.
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of the fabrication process. Three-dimensional (3D) regular nanodot array were further developed based on thermal dewetting induced self-organization [23]. The objective of this research is to develop a new fabrication process to produce ordered sandwich nanostructure arrays based on Nanoplastic forming. By controlling the experimental conditions, 3D internal structure with tandem plate-dot architecture by Au/SiO2/Au stacking was successfully fabricated. The effects of the selective etching of the spacer layer were studied. Numerical simulations for electric and magnetic field were conducted to study the origin of the spectral feature of the plasmon resonance, and the SERS performance of the nano-sandwich arrays was also investigated. 2. Experimental processes The proposed fabrication procedures are shown in Fig. 1(a). Briefly, nano-plate array pattern was firstly fabricated by
Fig. 1. (a) Schematic illustration of the proposed fabrication process and (b) the dry etching process.
Nanoplastic forming (NPF) followed by lower-temperature thermal dewetting. Then, SiO2 was deposited as a spacer layer with controllable thickness. The functions of SiO2 spacer involve protection of the first layer nanostructure and separation of double layers during the following thermal dewetting process, also the plasmonic properties can be tuned by varying the spacer thickness [10]. Second layer nanodot array was regularly formed by Au deposition and higher-temperature thermal dewetting. A newly developed double layer nanostructure was fabricated. We named it ‘‘Dot-on-Plate’’ (DoP) nanostructure. This technique is developed primarily based on solid-state dewetting and has the advantage of creating periodic nanostructure arrays without involving other complicated procedures, such as lithography or template. Fig. 1(b) shows the schematic illustration of the proposed dry etching process. Reactive ion etching was conducted to selectively etch the SiO2 spacer layer.
2.1. Fabrication details In this experiment, gold (Au) was used as dot material, and quartz glass was used as substrate material. The substrate was made from a quartz slide glass, and cut into size of 12 12 1 mm. Its surface was finished to optical flat by the maker. The substrate was cleaned by ultrasonic in an acetone bath before it was applied to the experiment. In the step (1) and (5), Au was deposited on the substrate by using a DC sputter coater. Sputter gas was argon (Ar), and pressure was 15 Pa. The distance between the specimen and the Au target was 35 mm. In the experiment the thickness of the Au film was controlled by adjusting the sputtering time. A Nanoplastic forming (NPF) equipment was used for the grid patterning on a deposited metal film. Details of the equipment were explained in papers [20–23]. The graph of the equipment and knife edge tool are given in Fig. 2(a). The Nanoplastic forming process is illustrated in Fig. 2(b). Firstly, a series of parallel nanogrooves was fabricated on the metal film by indenting a diamond tool. Indentation load was controlled so that the tool was indented only to the metal film layer and it does not damage the quartz glass substrate. Then, the substrate was rotated laterally for 90°, and another series of parallel grooves was indented again on the prepatterned grooves. Distance of the grooves (pitch) can be controlled in the experiment, and a square groove grid was fabricated on the metal film. The NPF tester was placed on a vibration isolation table. All of the equipment was placed inside a clean chamber to prevent contamination by dust.
Fig. 2. (a) Nanoplastic forming equipment and knife-edge diamond tool, (b) the schematic for the procedures of Nanoplastic forming process.
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Thermal dewetting was conducted by annealing in an electric furnace under ambient atmosphere. The figure of the equipment is supplied in the Supplementary materials. Two levels of annealing temperature were used in the experiment, 500 °C and 600 °C. Annealing time was 10 min. A SiO2 spacer layer was deposited on the first layer nanodot array by using an RF-sputter coater. The sputtering gas was Ar. The gas pressure was 10 Pa. In order to calculate the coating rate of the spacer layer, atomic force microscope (AFM) was used to measure the thickness of several samples with different coating times. The process conditions of first layer are summarized in Table 1. As for the second layer, the condition parameters are shown in Table 2. Reactive Ion Etching (RIE) was conducted using a high-density plasma etching system (CE-300I, ULVAC, Inc.). The gases flow rate was 15 sccm (Standard Cubic Centimeter per Minutes) for CHF3, and 25 sccm for O2. The pressure was 0.3 Pa. The plasma power and bias power was 150 and 2 W respectively. The etching rate was 0.2 nm/s. The etching depth can be controlled by adjusting the etching time. 2.2. Morphology characterization and spectra measurement The morphology of the dot array was characterized by using a field-emission scanning electron microscope (FE-SEM, JSM-6301F JEOL or Hitachi SU8000). To investigate the optical property of the fabricated nanostructure array, ultraviolet–visible–near infrared (UV–vis–NIR) transmittance spectra were measured by using a spectrometer (Stellar Net EPP2000). The light source was a broadband light source combining Deuterium and Halogen (Mikropack DH-2000-BAL). The wavelength range of the spectrum was selected from 400 to 1100 nm. All Raman spectra were recorded using a confocal Raman microscope (WITec alpha-300). A 20 lL droplet of probe molecule solution (10 lM) was spread on the surface of the samples and dried naturally in air. Raman measurements were performed with laser excitation wavelength of 532 nm. The treated sample surfaces were scanned over a 100 lm wide square frame, with a unit pixel mesh of 2 2 lm and an accumulation time of 0.3 s/pixel. The Raman spectra were obtained after averaging over the scanned area through the intensity mapping data. All of the SERS spectra in this paper are overall SERS effects of many hot-spots in an incident laser spot.
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method (Fullwave, Rsoft). The dielectric function of Au and SiO2 was taken from literature [24] and the literature [25], respectively. The periodic boundary conditions were adopted by choosing the same periodicity as the fabricated structure (e.g., 200 nm) with the discretized mesh size. The resolution was set to 1 nm. A free software (ImageJ) was used to analyze the FE-SEM images of the nanostructure array. The size of the nanostructure used for simulation was based on the ImageJ data. The excitation field Ex and Hy of the incident light lies in the xy plane (parallel to the substrate) with incident plane wave injected from the top of the DoP array in the zdirection. 3. Experimental results and discussions 3.1. Morphology of the fabricated nanostructure The fabricated typical nanostructure arrays are shown in Fig. 3. Fig. 3(a) shows FE-SEM image of the single layer nano-plate array. Fig. 3(b) shows the FE-SEM image of double layer nanostructure array. It is confirmed that the ‘‘Dot-on-Plate’’ 3D plasmonic array is successfully fabricated. The regularity and uniformity of dots is also demonstrated by using the proposed method. The typical etched sample is shown in Fig. 3(c and d). It can be found that the size of the dot on the second layer is slightly reduced after etching. 3.2. Plasmonic properties of the DoP structures Fig. 4(a) shows the transmittance spectra of the DoP nanostructure with different etching thickness. Double valleys are observed representing the surface plasmon resonance. The two distinctive features are considered as a high-energy resonance where valley at around 670 nm and a low-energy resonance with a minimum at around 1100 nm. For comparison, the spectrum from single purely metallic nano-plate array shows a valley at around 760 nm. It is considered that the resonance features of the double layer nanodot array cannot be identified as elementary contributions from its individual layer nanodot. In contrast, the double resonance valleys are due to dynamic coupling between the two layer components [10,26]. When the sample was etched 10 nm, the valleys blue-shifted indicating the SiO2 layer was etched out, and the environmental media of the first layer nano-plate changed into air.
2.3. Simulation The transmittance spectra of the DoP plasmonic array structure was calculated by using the rigorous coupled-wave analysis (RCWA) method (Diffractmod, Rsoft). The electric and magnetic field distribution in the vicinity of a DoP plasmonic structure was simulated by using the finite-difference time-domain (FDTD)
Table 1 Experimental conditions of the first layer. Thickness of gold layer [nm]
Grid size [nm]
Annealing temperature [°C]
Annealing time [min]
20
200
500
10
Table 2 Experimental conditions of the second layer. Thickness of SiO2 spacer [nm]
Thickness of gold layer [nm]
Annealing temperature [°C]
Annealing time [min]
10
15
600
10
Fig. 3. FE-SEM image of the morphology of the nanostructure array. (a) Single layer nano-plate array, (b) double layer nanostructure array without etching, (c) double layer nanostructure array after etching 10 nm, (d) double layer nanostructure array after etching 30 nm. Grid size is 200 nm. Thickness of SiO2 spacer is 10 nm.
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Fig. 4. Transmittance spectra of the plasmonic nanostructure array. (a) Measurement and (b) Simulation. Transmittance of single layer nano-plate array (gray line), double layer nanostructure array without etching (red line), with 10 nm etching (cyan line) and with 30 nm etching (blue line). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
When the etching thickness was 30 nm, the size and thickness of second layer Au dot became smaller due to over-etching, and the second valley of the transmittance spectra became shallower. Numerical simulations were conducted to study the origin spectral characteristics. The experimental transmittance spectra are compared with the spectra simulated based on RCWA method. Fig. 4(b) shows simulated transmittance spectra of single-layer structure and double-layer plasmonic structures in their evolution changing of SiO2 layer by etching process. The agreement between experiment and theory is good in terms of the valley positions. In the case of single layer nano-plate array (gray curve), there is a broaden valley around 760 nm due to the plasmon resonance of the squared Au disk array. Nevertheless, in the case of double layer nanostructure, there are two resonance valleys corresponding to the electric dipolar (parallel, high-energy mode at wavelength of about 650 nm–700 nm) and magnetic dipolar (unparallel, lowenergy mode at wavelength of about 1000 nm–1150 nm) [26,27]. As shown in Fig. 4(a), the valley positions in both electric dipolar and magnetic dipolar modes are slightly blue-shifted by etching process. These blue-shifts were confirmed by simulated results as shown in Fig. 4(b) due to the change of the dielectric function around bottom layer surface from SiO2 to air and the size reduction of the metal objects during etching process. Some quantitative variations between the experimental results and numerical results are expected because the numerical structure does not precisely represent the real structure (e.g., including metal and dielectric permittivity, the shape, etc.). It is thus likely that the large experimental valley widths are a result of inhomogeneous broadening, due to a dispersion of nanostructure morphology for the measured samples. To clarify the optical properties of the DoP double-layer nanostructure, the electric and magnetic fields distribution around
Fig. 5. Simulated electrical field (Ex) and magnetic field (Hy) distribution of double layer nanostructure array (10 nm etching). (a) Ex and (b) Hy fields distribution under excitation at electric dipole mode (675 nm); (c) Ex and (d) Hy fields distribution under excitation at magnetic dipole mode (1040 nm).
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structure were simulated as shown in Fig. 5 for the case of 10 nm SiO2 etching. Fig. 5(a) and (b) shows the simulated electric and magnetic fields distribution around DoP structure under highenergy mode excitation at 675 nm. The regions of intense electromagnetic field indicate the ‘‘hot spot’’. The electric field enhancement was found to be 11.5 at the hot spots and placed at the sides of top and bottom objects with the same phase (positive electric field) of electric dipoles oscillation, while there was no enhancement of magnetic field, indicating the symmetric and parallel nature of the electric dipoles. In contrast, Fig. 5(c) and (d) shows that clear enhancements in both electric and magnetic fields were found in the vicinity of the structure under low-energy mode excitation at 1040 nm, to be 25.5 and 7.5, respectively. Furthermore, in this case, the electric dipoles at the sides of top and bottom objects are oscillated in opposite phases (electric field at top object is positive, while electric field at bottom object is negative), resulting a loop-like charge motion which generates a strong magnetic field in between top and bottom objects at the center of the object. The field distribution of these two resonances resembles the electric dipolar and magnetic dipolar resonances of the previously reported tandem-disk or nano-sandwich structure although the line widths and the spectral features are not identical. This indicates that our present architecture can be used as promising building block as plasmonic metamolecules, for example for the negative refraction, enhanced magneto-optical effects by nonmagnetic components, and magnetic field enhancement effect in light emission [28]. 3.3. SERS performance of the DoP nanostructure Raman measurements were conducted by using nile blue A (NBA) as the probe molecule. Representative SERS spectra are plotted in Fig. 6(a). Each Raman spectrum was obtained after subtracting the background baseline and was normalized to the acquisition time. The peak positions in the SERS spectra were accurately assigned to the vibrational bands of NBA molecules reported in literatures [29,30]. For comparison, the intensity of Raman signal from the DoP nanostructure is more than 10 times larger than that from the planar nano-plate array while under the same conditions. The Raman spectrum by using Au film is also shown for comparison. It is apparent that the DoP nanostructures effectively increase the Raman signal of NBA molecules. In our work, the enhanced factor (EF) of the SERS results was estimated based on the following formula [31,32],
EF ¼
ISERS C REF PREF T REF IREF C SERS PSERS T SERS
where ISERS and IREF is the measured Raman intensity, CREF and CSERS is the concentration of NBA solution dropped, PSERS and PREF is the excitation power, and TSERS and TREF is the exposure time on DoP substrate and the reference substrate, respectively. The reference substrate here was 5 103 M NBA on quartz substrate without SERS effect. The concentration of NBA solution used for the sample was 1 105 M. The excitation power was 0.5 mW and the exposure time was 0.3 s. The intensity for the 1643 cm1 band of NBA molecules was used for the calculation. In this work, the EF was estimated to be about 1 105. In addition, the effect of pitch setting and spacer thickness on SERS intensity was studied. The details of the discussion are supported in the Supplementary materials. To make the SERS signals of the double-layer nanostructure more visible, some part of the patterned area was peeled off by mechanically scratching for comparison. The FE-SEM image in Fig. 6(b and c) shows the difference between the peeled off area and the patterned area. The enlarged image shows the boundary region of the patterned area. The overall appearance of the SERS intensity map in Fig. 6(d) shows a distinct enhancement of Raman
Fig. 6. (a) Raman spectra of Nile blue A molecules using the DoP nanostructure array as a substrate, the reference Au film on glass substrate and single layer nanoplate array are also shown for comparison. (b) FE-SEM image of the region showing patterned and non-patterned area (c) FE-SEM image for the enlarged part, (d) Raman intensity mapping image of the corresponding area for comparison. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
signal from the DoP arrayed region. This intensity map presenting the spatial distribution of the Raman intensity integrated over frequency shifts from 1300 to 1700 cm1. Even though in the intensity map, the individual DoP nanostructures are not distinguishable due to the limitation of lateral resolution, the yellow/orange and black colored regions in the map correspond well with the patterned area and peeled-off area, respectively. The etching process is an important factor influencing the SERS effect. Fig. 7 shows the dependence of the Raman signals on the etching thickness of spacer layer. The Raman intensity increased when the etching thickness was 10 nm, which was almost the same as the spacer layer thickness. The Raman intensity decreased when the etching thickness was 30 nm. The Raman intensity increased at first and then went down when the etching time was extended. It is expected that the controlled etching process will remove the SiO2 spacer layer on the first layer nanodot array and expose the first layer dot. More analyte molecules can be effectively directly attached on the hot spots on the first layer Au surface. As the etching process continued, the Au was etched out and the gaps between nanodots became widen, thus the electromagnetic coupling and Raman scattering decreased. According to the experimental and simulated results shown in Figs. 4 and 5, major absorption feature from the two plasmon resonances are located around 670 nm and 1040 nm respectively. The laser excitation wavelength used in the present SERS measurement is off resonance from these frequencies. It means that SERS
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Fig. 7. (a) Raman spectra of a set of double layer nanostructure arrays with different etching thickness. The spacer thickness of the samples is 10 nm. (b) Dependence of SERS intensity of the 595 cm–1, 1492 cm–1 and 1643 cm–1 band on etching thickness.
enhancement effect is not optimized. If the laser source with excitation wavelength longer than the current one is adopted, even higher Raman signal is expected from the present SERS substrate. Inversely, the plasmon resonance frequency can be tuned by varying the lateral size of the DoP structures, and the SERS substrates with different resonance frequencies suitable for various laser excitation wavelengths can be fabricated with the current method by varying the periodicity. It is also known that silver nanoparticles exhibit much higher SERS enhancement under a 532 nm laser excitation. It is expected that combining silver and gold to fabricate hybrid DoP nanostructures will further improve the SERS performance with higher sensitivity. 4. Conclusion An efficient fabrication method to produce a large-scale array of plasmonic metamolecules was successfully developed. The optical properties of the nanostructure array were studied. The structure was also utilized for SERS application and showed good SERS performance, proving its promising use as a low-cost SERS substrate. The effect of dry etching was studied, and the relation between the plasmonic properties and SERS enhancement was discussed. Combining with our numerical simulation for the electric and magnetic fields, the presence of magnetic dipolar mode as well as electric dipolar mode were clarified. It suggests that the fabricated nanostructures have potential use in metamaterials, plasmonenhanced magneto-optical device, and light emitters. Acknowledgements Part of this research is supported by the Grant-In-Aid for Scientific Research (B) (No. 23360065) and (No. 20671002) of JSPS. Some of FE-SEM observation was carried out at the Center for Advanced Materials Analysis in Tokyo Institute of Technology. RIE was conducted in MANA foundry. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.mee.2014.03.045.
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Optical properties of ordered Dot-on-Plate nano-sandwich arrays Zhenxing Li a,⇑, Thang Duy Dao b,c,1, Tadaaki Nagao b,c,1, Masahiko Yoshino a a
Department of Mechanical and Control Engineering, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguroku, Tokyo 152-8552, Japan International Center for Materials Nanoarchitectonics, National Institute for Materials Science, 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan c CREST, Japan Science and Technology Agency, 4-1-8 Honcho, Kawaguchi, Saitama 332-0012, Japan b
a r t i c l e
i n f o
Article history: Received 23 January 2014 Received in revised form 21 March 2014 Accepted 31 March 2014 Available online xxxx Keywords: 3D nanostructure array Nano-sandwich Double plasmon resonances SERS Nanoplastic forming
a b s t r a c t A cost-effective fabrication method for realizing ordered Dot-on-Plate nanostructure array is presented. This fabrication method is a combination of Nanoplastic forming (NPF) and thermal dewetting, which does not involve costly and complicated processes such as lithography. The fabricated Au–SiO2–Au sandwich nanostructures exhibit double plasmon resonances and strong magnetic response, which makes it a potential candidate for metamaterials. It is also found that the Dot-on-Plate nanostructure shows good performance as surface-enhanced Raman scattering (SERS)-active substrate. The milling of SiO2 spacer layer by dry etching is studied and the relation between the plasmonic properties and SERS enhancement is discussed. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction Three-dimensional metal-dielectric composite nanostructures have found broad promising applications from biosensing, photocatalysis, metamaterials, to light harvesting devices due to their unique plasmonic properties [1–7]. Cut-wire pairs and plate pairs were investigated as magnetic atoms for optical metamaterials which shows the potential to be an alternative design of the well-established split-ring resonator [8,9]. A tuneable plasmon resonance in arrays of nano-sandwiches suggests its applications for biosensing [10]. As for photovoltaic cells, vertical-stacked metal disks with broadband interaction of incoming light show the possibility to achieve efficient light harvesting [11]. By combining noble metal with catalytically active transition metals (Pt or Pd), the sandwich nanostructure offers a promising way to drive chemical reactions for photocatalysis [12,13]. Metal-dielectric-metal composite nanostructures have also been extensively investigated in the field of near-field enhanced spectroscopy and surfaceenhanced Raman spectroscopy [14,15]. Lithographic techniques are the widely implemented top-down methods in the production of such sandwiched nanostructures. The most commonly utilized technique is electron-beam lithography (EBL) [4,5,8,9,11]. The procedures include electron-sensitive ⇑ Corresponding author. Tel.: +81 3 5734 2640; fax: +81 3 5734 2506. E-mail addresses: [email protected] (Z. Li), [email protected] (T.D. Dao), [email protected] (T. Nagao), [email protected] (M. Yoshino). 1 Tel.: +81 29 860 4746; fax: +81 29 860 4706.
resist coating, pattern writing by electron beam and following lift-off processes. Another approach referred to as focused ion beam (FIB) [16] typically utilizes a gallium ion beam to directly sputter away unwanted parts of a continuous film and fabricate desired nanostructures. While both of these techniques can produce nanostructures with well-controlled size, shape and alignment, they need costly facilities, stringent processes and cannot address the problem of low throughput. Another drawback of these techniques is that nanostructure adhesion to the substrate usually requires an intermediate layer such as chromium or titanium. Nanoimprint lithography (NIL) is an alternative top-down method, and it shows the advantage of high resolution and high efficiency [17–19]. Self-assembly methods based on chemical synthesis [1,2] are difficult to obtain ordered nanostructures immobilized on the substrates. Other unconventional approaches such as porous anodic alumina (PAA) method [3] and colloidal lithography [6,10,12–14] are able to produce layered nanostructures based on the PAA or nanosphere templates; however, the limitation is the geometrical constraint of the template. In order to address the problems and supplement limitations for the current fabrication methods, authors developed an alternative approach by using Nanoplastic forming and following thermal dewetting processes to fabricate two-dimensionally ordered nanodot array structures [20,21]. This approach possesses a number of advantages including low-cost, low-emission, low stringency and without any chemical disposals which causes environmental pollutions. By combining NPF with other techniques such as roller imprinting process [22], it is possible to increase the throughput
http://dx.doi.org/10.1016/j.mee.2014.03.045 0167-9317/Ó 2014 Elsevier B.V. All rights reserved.
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of the fabrication process. Three-dimensional (3D) regular nanodot array were further developed based on thermal dewetting induced self-organization [23]. The objective of this research is to develop a new fabrication process to produce ordered sandwich nanostructure arrays based on Nanoplastic forming. By controlling the experimental conditions, 3D internal structure with tandem plate-dot architecture by Au/SiO2/Au stacking was successfully fabricated. The effects of the selective etching of the spacer layer were studied. Numerical simulations for electric and magnetic field were conducted to study the origin of the spectral feature of the plasmon resonance, and the SERS performance of the nano-sandwich arrays was also investigated. 2. Experimental processes The proposed fabrication procedures are shown in Fig. 1(a). Briefly, nano-plate array pattern was firstly fabricated by
Fig. 1. (a) Schematic illustration of the proposed fabrication process and (b) the dry etching process.
Nanoplastic forming (NPF) followed by lower-temperature thermal dewetting. Then, SiO2 was deposited as a spacer layer with controllable thickness. The functions of SiO2 spacer involve protection of the first layer nanostructure and separation of double layers during the following thermal dewetting process, also the plasmonic properties can be tuned by varying the spacer thickness [10]. Second layer nanodot array was regularly formed by Au deposition and higher-temperature thermal dewetting. A newly developed double layer nanostructure was fabricated. We named it ‘‘Dot-on-Plate’’ (DoP) nanostructure. This technique is developed primarily based on solid-state dewetting and has the advantage of creating periodic nanostructure arrays without involving other complicated procedures, such as lithography or template. Fig. 1(b) shows the schematic illustration of the proposed dry etching process. Reactive ion etching was conducted to selectively etch the SiO2 spacer layer.
2.1. Fabrication details In this experiment, gold (Au) was used as dot material, and quartz glass was used as substrate material. The substrate was made from a quartz slide glass, and cut into size of 12 12 1 mm. Its surface was finished to optical flat by the maker. The substrate was cleaned by ultrasonic in an acetone bath before it was applied to the experiment. In the step (1) and (5), Au was deposited on the substrate by using a DC sputter coater. Sputter gas was argon (Ar), and pressure was 15 Pa. The distance between the specimen and the Au target was 35 mm. In the experiment the thickness of the Au film was controlled by adjusting the sputtering time. A Nanoplastic forming (NPF) equipment was used for the grid patterning on a deposited metal film. Details of the equipment were explained in papers [20–23]. The graph of the equipment and knife edge tool are given in Fig. 2(a). The Nanoplastic forming process is illustrated in Fig. 2(b). Firstly, a series of parallel nanogrooves was fabricated on the metal film by indenting a diamond tool. Indentation load was controlled so that the tool was indented only to the metal film layer and it does not damage the quartz glass substrate. Then, the substrate was rotated laterally for 90°, and another series of parallel grooves was indented again on the prepatterned grooves. Distance of the grooves (pitch) can be controlled in the experiment, and a square groove grid was fabricated on the metal film. The NPF tester was placed on a vibration isolation table. All of the equipment was placed inside a clean chamber to prevent contamination by dust.
Fig. 2. (a) Nanoplastic forming equipment and knife-edge diamond tool, (b) the schematic for the procedures of Nanoplastic forming process.
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Thermal dewetting was conducted by annealing in an electric furnace under ambient atmosphere. The figure of the equipment is supplied in the Supplementary materials. Two levels of annealing temperature were used in the experiment, 500 °C and 600 °C. Annealing time was 10 min. A SiO2 spacer layer was deposited on the first layer nanodot array by using an RF-sputter coater. The sputtering gas was Ar. The gas pressure was 10 Pa. In order to calculate the coating rate of the spacer layer, atomic force microscope (AFM) was used to measure the thickness of several samples with different coating times. The process conditions of first layer are summarized in Table 1. As for the second layer, the condition parameters are shown in Table 2. Reactive Ion Etching (RIE) was conducted using a high-density plasma etching system (CE-300I, ULVAC, Inc.). The gases flow rate was 15 sccm (Standard Cubic Centimeter per Minutes) for CHF3, and 25 sccm for O2. The pressure was 0.3 Pa. The plasma power and bias power was 150 and 2 W respectively. The etching rate was 0.2 nm/s. The etching depth can be controlled by adjusting the etching time. 2.2. Morphology characterization and spectra measurement The morphology of the dot array was characterized by using a field-emission scanning electron microscope (FE-SEM, JSM-6301F JEOL or Hitachi SU8000). To investigate the optical property of the fabricated nanostructure array, ultraviolet–visible–near infrared (UV–vis–NIR) transmittance spectra were measured by using a spectrometer (Stellar Net EPP2000). The light source was a broadband light source combining Deuterium and Halogen (Mikropack DH-2000-BAL). The wavelength range of the spectrum was selected from 400 to 1100 nm. All Raman spectra were recorded using a confocal Raman microscope (WITec alpha-300). A 20 lL droplet of probe molecule solution (10 lM) was spread on the surface of the samples and dried naturally in air. Raman measurements were performed with laser excitation wavelength of 532 nm. The treated sample surfaces were scanned over a 100 lm wide square frame, with a unit pixel mesh of 2 2 lm and an accumulation time of 0.3 s/pixel. The Raman spectra were obtained after averaging over the scanned area through the intensity mapping data. All of the SERS spectra in this paper are overall SERS effects of many hot-spots in an incident laser spot.
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method (Fullwave, Rsoft). The dielectric function of Au and SiO2 was taken from literature [24] and the literature [25], respectively. The periodic boundary conditions were adopted by choosing the same periodicity as the fabricated structure (e.g., 200 nm) with the discretized mesh size. The resolution was set to 1 nm. A free software (ImageJ) was used to analyze the FE-SEM images of the nanostructure array. The size of the nanostructure used for simulation was based on the ImageJ data. The excitation field Ex and Hy of the incident light lies in the xy plane (parallel to the substrate) with incident plane wave injected from the top of the DoP array in the zdirection. 3. Experimental results and discussions 3.1. Morphology of the fabricated nanostructure The fabricated typical nanostructure arrays are shown in Fig. 3. Fig. 3(a) shows FE-SEM image of the single layer nano-plate array. Fig. 3(b) shows the FE-SEM image of double layer nanostructure array. It is confirmed that the ‘‘Dot-on-Plate’’ 3D plasmonic array is successfully fabricated. The regularity and uniformity of dots is also demonstrated by using the proposed method. The typical etched sample is shown in Fig. 3(c and d). It can be found that the size of the dot on the second layer is slightly reduced after etching. 3.2. Plasmonic properties of the DoP structures Fig. 4(a) shows the transmittance spectra of the DoP nanostructure with different etching thickness. Double valleys are observed representing the surface plasmon resonance. The two distinctive features are considered as a high-energy resonance where valley at around 670 nm and a low-energy resonance with a minimum at around 1100 nm. For comparison, the spectrum from single purely metallic nano-plate array shows a valley at around 760 nm. It is considered that the resonance features of the double layer nanodot array cannot be identified as elementary contributions from its individual layer nanodot. In contrast, the double resonance valleys are due to dynamic coupling between the two layer components [10,26]. When the sample was etched 10 nm, the valleys blue-shifted indicating the SiO2 layer was etched out, and the environmental media of the first layer nano-plate changed into air.
2.3. Simulation The transmittance spectra of the DoP plasmonic array structure was calculated by using the rigorous coupled-wave analysis (RCWA) method (Diffractmod, Rsoft). The electric and magnetic field distribution in the vicinity of a DoP plasmonic structure was simulated by using the finite-difference time-domain (FDTD)
Table 1 Experimental conditions of the first layer. Thickness of gold layer [nm]
Grid size [nm]
Annealing temperature [°C]
Annealing time [min]
20
200
500
10
Table 2 Experimental conditions of the second layer. Thickness of SiO2 spacer [nm]
Thickness of gold layer [nm]
Annealing temperature [°C]
Annealing time [min]
10
15
600
10
Fig. 3. FE-SEM image of the morphology of the nanostructure array. (a) Single layer nano-plate array, (b) double layer nanostructure array without etching, (c) double layer nanostructure array after etching 10 nm, (d) double layer nanostructure array after etching 30 nm. Grid size is 200 nm. Thickness of SiO2 spacer is 10 nm.
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Fig. 4. Transmittance spectra of the plasmonic nanostructure array. (a) Measurement and (b) Simulation. Transmittance of single layer nano-plate array (gray line), double layer nanostructure array without etching (red line), with 10 nm etching (cyan line) and with 30 nm etching (blue line). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
When the etching thickness was 30 nm, the size and thickness of second layer Au dot became smaller due to over-etching, and the second valley of the transmittance spectra became shallower. Numerical simulations were conducted to study the origin spectral characteristics. The experimental transmittance spectra are compared with the spectra simulated based on RCWA method. Fig. 4(b) shows simulated transmittance spectra of single-layer structure and double-layer plasmonic structures in their evolution changing of SiO2 layer by etching process. The agreement between experiment and theory is good in terms of the valley positions. In the case of single layer nano-plate array (gray curve), there is a broaden valley around 760 nm due to the plasmon resonance of the squared Au disk array. Nevertheless, in the case of double layer nanostructure, there are two resonance valleys corresponding to the electric dipolar (parallel, high-energy mode at wavelength of about 650 nm–700 nm) and magnetic dipolar (unparallel, lowenergy mode at wavelength of about 1000 nm–1150 nm) [26,27]. As shown in Fig. 4(a), the valley positions in both electric dipolar and magnetic dipolar modes are slightly blue-shifted by etching process. These blue-shifts were confirmed by simulated results as shown in Fig. 4(b) due to the change of the dielectric function around bottom layer surface from SiO2 to air and the size reduction of the metal objects during etching process. Some quantitative variations between the experimental results and numerical results are expected because the numerical structure does not precisely represent the real structure (e.g., including metal and dielectric permittivity, the shape, etc.). It is thus likely that the large experimental valley widths are a result of inhomogeneous broadening, due to a dispersion of nanostructure morphology for the measured samples. To clarify the optical properties of the DoP double-layer nanostructure, the electric and magnetic fields distribution around
Fig. 5. Simulated electrical field (Ex) and magnetic field (Hy) distribution of double layer nanostructure array (10 nm etching). (a) Ex and (b) Hy fields distribution under excitation at electric dipole mode (675 nm); (c) Ex and (d) Hy fields distribution under excitation at magnetic dipole mode (1040 nm).
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structure were simulated as shown in Fig. 5 for the case of 10 nm SiO2 etching. Fig. 5(a) and (b) shows the simulated electric and magnetic fields distribution around DoP structure under highenergy mode excitation at 675 nm. The regions of intense electromagnetic field indicate the ‘‘hot spot’’. The electric field enhancement was found to be 11.5 at the hot spots and placed at the sides of top and bottom objects with the same phase (positive electric field) of electric dipoles oscillation, while there was no enhancement of magnetic field, indicating the symmetric and parallel nature of the electric dipoles. In contrast, Fig. 5(c) and (d) shows that clear enhancements in both electric and magnetic fields were found in the vicinity of the structure under low-energy mode excitation at 1040 nm, to be 25.5 and 7.5, respectively. Furthermore, in this case, the electric dipoles at the sides of top and bottom objects are oscillated in opposite phases (electric field at top object is positive, while electric field at bottom object is negative), resulting a loop-like charge motion which generates a strong magnetic field in between top and bottom objects at the center of the object. The field distribution of these two resonances resembles the electric dipolar and magnetic dipolar resonances of the previously reported tandem-disk or nano-sandwich structure although the line widths and the spectral features are not identical. This indicates that our present architecture can be used as promising building block as plasmonic metamolecules, for example for the negative refraction, enhanced magneto-optical effects by nonmagnetic components, and magnetic field enhancement effect in light emission [28]. 3.3. SERS performance of the DoP nanostructure Raman measurements were conducted by using nile blue A (NBA) as the probe molecule. Representative SERS spectra are plotted in Fig. 6(a). Each Raman spectrum was obtained after subtracting the background baseline and was normalized to the acquisition time. The peak positions in the SERS spectra were accurately assigned to the vibrational bands of NBA molecules reported in literatures [29,30]. For comparison, the intensity of Raman signal from the DoP nanostructure is more than 10 times larger than that from the planar nano-plate array while under the same conditions. The Raman spectrum by using Au film is also shown for comparison. It is apparent that the DoP nanostructures effectively increase the Raman signal of NBA molecules. In our work, the enhanced factor (EF) of the SERS results was estimated based on the following formula [31,32],
EF ¼
ISERS C REF PREF T REF IREF C SERS PSERS T SERS
where ISERS and IREF is the measured Raman intensity, CREF and CSERS is the concentration of NBA solution dropped, PSERS and PREF is the excitation power, and TSERS and TREF is the exposure time on DoP substrate and the reference substrate, respectively. The reference substrate here was 5 103 M NBA on quartz substrate without SERS effect. The concentration of NBA solution used for the sample was 1 105 M. The excitation power was 0.5 mW and the exposure time was 0.3 s. The intensity for the 1643 cm1 band of NBA molecules was used for the calculation. In this work, the EF was estimated to be about 1 105. In addition, the effect of pitch setting and spacer thickness on SERS intensity was studied. The details of the discussion are supported in the Supplementary materials. To make the SERS signals of the double-layer nanostructure more visible, some part of the patterned area was peeled off by mechanically scratching for comparison. The FE-SEM image in Fig. 6(b and c) shows the difference between the peeled off area and the patterned area. The enlarged image shows the boundary region of the patterned area. The overall appearance of the SERS intensity map in Fig. 6(d) shows a distinct enhancement of Raman
Fig. 6. (a) Raman spectra of Nile blue A molecules using the DoP nanostructure array as a substrate, the reference Au film on glass substrate and single layer nanoplate array are also shown for comparison. (b) FE-SEM image of the region showing patterned and non-patterned area (c) FE-SEM image for the enlarged part, (d) Raman intensity mapping image of the corresponding area for comparison. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
signal from the DoP arrayed region. This intensity map presenting the spatial distribution of the Raman intensity integrated over frequency shifts from 1300 to 1700 cm1. Even though in the intensity map, the individual DoP nanostructures are not distinguishable due to the limitation of lateral resolution, the yellow/orange and black colored regions in the map correspond well with the patterned area and peeled-off area, respectively. The etching process is an important factor influencing the SERS effect. Fig. 7 shows the dependence of the Raman signals on the etching thickness of spacer layer. The Raman intensity increased when the etching thickness was 10 nm, which was almost the same as the spacer layer thickness. The Raman intensity decreased when the etching thickness was 30 nm. The Raman intensity increased at first and then went down when the etching time was extended. It is expected that the controlled etching process will remove the SiO2 spacer layer on the first layer nanodot array and expose the first layer dot. More analyte molecules can be effectively directly attached on the hot spots on the first layer Au surface. As the etching process continued, the Au was etched out and the gaps between nanodots became widen, thus the electromagnetic coupling and Raman scattering decreased. According to the experimental and simulated results shown in Figs. 4 and 5, major absorption feature from the two plasmon resonances are located around 670 nm and 1040 nm respectively. The laser excitation wavelength used in the present SERS measurement is off resonance from these frequencies. It means that SERS
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Fig. 7. (a) Raman spectra of a set of double layer nanostructure arrays with different etching thickness. The spacer thickness of the samples is 10 nm. (b) Dependence of SERS intensity of the 595 cm–1, 1492 cm–1 and 1643 cm–1 band on etching thickness.
enhancement effect is not optimized. If the laser source with excitation wavelength longer than the current one is adopted, even higher Raman signal is expected from the present SERS substrate. Inversely, the plasmon resonance frequency can be tuned by varying the lateral size of the DoP structures, and the SERS substrates with different resonance frequencies suitable for various laser excitation wavelengths can be fabricated with the current method by varying the periodicity. It is also known that silver nanoparticles exhibit much higher SERS enhancement under a 532 nm laser excitation. It is expected that combining silver and gold to fabricate hybrid DoP nanostructures will further improve the SERS performance with higher sensitivity. 4. Conclusion An efficient fabrication method to produce a large-scale array of plasmonic metamolecules was successfully developed. The optical properties of the nanostructure array were studied. The structure was also utilized for SERS application and showed good SERS performance, proving its promising use as a low-cost SERS substrate. The effect of dry etching was studied, and the relation between the plasmonic properties and SERS enhancement was discussed. Combining with our numerical simulation for the electric and magnetic fields, the presence of magnetic dipolar mode as well as electric dipolar mode were clarified. It suggests that the fabricated nanostructures have potential use in metamaterials, plasmonenhanced magneto-optical device, and light emitters. Acknowledgements Part of this research is supported by the Grant-In-Aid for Scientific Research (B) (No. 23360065) and (No. 20671002) of JSPS. Some of FE-SEM observation was carried out at the Center for Advanced Materials Analysis in Tokyo Institute of Technology. RIE was conducted in MANA foundry. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.mee.2014.03.045.
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