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Amino-terminated ionic liquid assisted multiphoton photoreduction (IL-MPR) was developed for the direct writing of subwavelength gold nanostructures in AuCl4- ions aqueous solution by femtosecond laser. It was revealed that the carbon chain length was crucial for morphology and size control of gold nanostructures. A 228 nm width of gold nanostructure, which was beyond the optical diffraction limit, was fabricated by the matching between IL and the power and scanning speed of the laser beam. The measured conductivity is of the same order as that of bulk gold. Furthermore, we successfully fabricated a U-shaped terahertz planar metamaterial whose spectral response is consistent with the theoretical expectation. The IL-MPR nanofabrication protocol is expected to play an important role in the fabrication of fine metallic micro/nanostructures for applications in microelectromechanical systems, nanoelectronics and nanophotonics.
© 2013 Optical Society of America
1. Introduction
The burgeoning field of nanotechnology, such as microelectromechanical systems (MEMS) [1–3], microelectronics [4] and nanophotonics [5], has led to rapid growing advances in the patterning of highly conductive metallic structures with micro and nanoscale resolution. Conventional photolithography processes combined with vacuum metal deposition are widely used for high-resolution metal fabrication in integrated circuits [6] and surface plasmon optics [7]. However, conventional photolithography approaches require high vacuum facility, complicated overlay steps, corrosive chemicals to fabricate, and expensive unchangeable photomask [8,9]. For these reasons, the development of alternative maskless, high-resolution and direct writing technique to fabricate micro and nanoscale metallic structures without harsh vacuum environment has attracted extensive attention in recent years.
Multiphoton absorption nanofabrication has demonstrated its great potential for applications in preparation of complex three-dimensional (3D) nanostructures at micro and nanoscales [10–15]. Nanoscale resolution has been achieved for photoresists based on polymers [16–19] through multiphoton absorption polymerization. However, the non-conductivity of the polymeric nanostructures hampered the applications in electronic and photonic devices whose properties strongly rely on the conducting free electron in metal [7,20]. Thus, the investigation on the metallic nanostructures with excellent conductivity by using multiphoton direct writing has emerged as a new challenge. The fabrication of metallic nanostructures by direct laser writing (DLW) technique has been widely investigated in polymeric films and aqueous solutions. Examples include the fabrication of metallic structures inside a dielectric matrix [21], self-supporting structures using a non-commercially available polymer polyvinylcarbazole nanocomposite materials [22–24], conductive metallic structures in N-methyl pyrrolidone and metal ions solution [25]. In fact, the obstacle for the polymer-metal systems is the lower metal loading capability, which would result in the aggregations of isolated metal particles in the metal structures and consequently bad conductivities. Prasad and associates demonstrated that simultaneous metal reduction and photopolymerization could improve the metal loading (~40 wt% precursor can be incorporated into polymer films) [26], but the continuities between gold nanoparticles in the obtained composite structures couldn’t be guaranteed when the feature size was reduced to nanoscale.
One of the most promising alternatives is the femtosecond (fs) DLW in the metal ions solution based on multiphoton photoreduction (MPR). The critical point to improve the resolution of the metallic micro/nanostructures is to reduce the size of the metallic nanoparticles. In 2006, Tanaka et al. reported metallic nanostructures through two-photon induced reduction of silver ions aqueous solution [27,28]. The conductivity was only 3.3 times lower than that of bulk silver, while the resolution was at micrometer scale [27]. Recently, silver nanostructures with smooth surface and 120 nm resolution have been reported by using surfactant as a particle-growth inhibitor [29] or trisodium citrate as a photoreducing agent [30]. However, it is important to note that oxidization or sulfurization of the silver nanostructures is more liable to take place, which would degrade their properties. The advantage of corrosion and oxidation resistance of gold makes it the better choice for applications in humid or corrosive environment and in use for devices with a high failure cost. Recently, Sun et al. reported the gold nanodots as the precursory source to form micronanopatterns under pinpoint scanning by a tightly focused fs beam. The wire of about 560 nm wide was achieved [31]. Writing gold nanostructures through MPR of the gold ions aqueous solution without the involvement of polymer would be promising for realizing the application of metallic micro/nanostructures on MEMS, microelectronics and nanophotonics, which is necessary to be extensively explored.
Here, we report a new approach to fabricate 2D gold nanostructures with excellent electrical and optical properties by MPR in the aqueous solution of gold ions with the assistance of ionic liquid (IL). As a kind of newly developed stabilizer, IL is promising for achieving nanoparticles with controlled sizes [32,33] since it enables the preparation of metal nanoparticles without any additional surfactants or capping agents. Previously, we have successfully achieved gold nanoparticles with sizes less than 5 nm by using IL as stabilizer, inhibitor and photoreducing promoter for the growth of gold nanoparticles [34], which could not only provide the resolution beyond the optical diffraction limit, but also decrease the photoreduction threshold. In this study, we found that IL with longer anion carbon chain could confine the gold nanoparticles in a smaller size and achieve smoother gold lines with high crystallinity. The linewidth was reduced with the increasing of the laser scanning speed and reducing the laser power. The obtained gold nanostructure has a minimum linewidth of 228 nm. Furthermore, we fabricated and characterized a planar electromagnetic metamaterial made of U-shaped split ring resonators (SRRs). To our best knowledge, it is the first demonstration that the measured spectra are consistent with theoretical predictions for the metamaterial fabricated by DLW. This DLW technique promises a simple and low-cost method for patterning micro and nanoscale conductive gold structures.
2. Experimental
Tetrachloroauric acid (HAuCl4) and sodium chloride (NaCl) were of analytical grade and purchased from Beijing Chemical Reagent Company. Choline hydroxide (46 wt% aqueous solution) and glycine were purchased from Alfa Aesar. ((2-hydroxyethyl)trimethylammonium 5-aminobutyric (C4), (2-hydroxyethyl)trimethylammonium 5-aminopentanoic (C5) or (2-hydroxyethyl)trimethylammonium 5-aminocaproic (C6)) were synthesized according to the reference [34,35] and the molecular structure was confirmed by 1H NMR (Fig. 7 in the Appendix below). All of the aqueous solutions were prepared with ultra-pure water (18 MΩ) from Milli-Q Plus system (Millipore). All of the experiments were carried out in room temperature.
The three sample solutions for MPR fabrication were prepared by mixing ultra-pure water with different ILs (C4, C5, C6), respectively, and then mixed with HAuCl4. The concentrations of the ILs and HAuCl4 in all of the sample solutions were 0.56 and 0.08 M, and the molar ratio for ILs with HAuCl4 was 7. We realized the MPR nanofabrication experiments by employing a mode-locked Ti:Sapphire laser system (Tsunami, Spectra-Physics) with a center wavelength of 780 nm, a pulse width of 80 fs, and a repetition rate of 80 MHz. The laser beam was tightly focused by a 100 × oil-immersion objective lens with a high numerical aperture (N.A. = 1.45, Olympus). The laser beam was focused into the mixed aqueous solution of gold ions and IL which was placed on the cover glass above a computer aided 3D piezostage (P-563.3CL, Physik Instrumente). The desired structures were obtained after washing out the unreacted solution with ethanol.
1H NMR spectra were recorded on a Brucker 400 spectrometer using D2O as solvent and tetramethylsilane as reference. Cyclic voltammetry scans were recorded using an electrochemical workstation (IM6, Zahner Elektrik, Germany) equipped with a conventional three-electrode electrochemical cell, wherein the boron-doped diamond served as the working electrode, a platinum wire and Ag/AgCl (sat. NaCl) were used as the counter and reference electrodes, respectively. UV-Vis absorption spectra were recorded on a UV 3100PC Scanning Spectrophotometer (Shimadzu, Japan) with a slit width of 5 nm and sample quartz cuvette of 1 cm × 1 mm × 4.5 cm. The images of the fabricated structures were recorded by a field-emission scanning electron microscopy (SEM, S-4300, Hitachi, Japan) and (SEM, S-4800, Hitachi, Japan). The gold nanostructures were also characterized by transmission electron microscopy (TEM, JEOL JEM-2100) and high-resolution TEM (HR-TEM, JEOL JEM-2100F). A dimension-3100 (Veeco Metrology Group, USA) Atomic Force Microscope (AFM) was used to measure the height profile of the gold micro/nanostructures. Optical transmission and reflection spectra of the metamaterials for linear polarized incident light are measured with a homebuilt Fourier Transform infrared spectroscopy (FTIR).
3. Results and discussion
The schematic illustration of the MPR fabrication is shown in Fig. 1(a). The IL assisted gold ions aqueous solution was placed on a cover slip and irradiated with a 780 nm fs laser. Materials for photoreduction consisted of HAuCl4 as the gold source and amino-terminated IL with different anion carbon chain lengths C4, C5, and C6 as the stabilizers, as illustrated in the inset of Fig. 1(c). The photoreduction process of the sample solution occurs near the focal point of the incident beam, which produces the IL-capped spherical gold nanoparticles (Fig. 1(b)). Figure 7 in the Appendix below presents the evolution of 1H NMR spectra with the varied molar ratios for HAuCl4/IL, which indicates that the AuCl4- is coordinated with amino of the anions and formed [-OOC-CHn-NH2]-Au(III) (n = 3, 4, 5) complex. UV-Vis absorption spectra of pristine C5, HAuCl4, and their mixture aqueous solution were shown in Fig. 1(c). The absorption peak of AuCl4- at 302 nm in the aqueous solution of HAuCl4 was not observable when mixed with C5, resulting from the formation of the complex, which was in agreement with the results of 1H NMR (Fig. 7). The absorbance of the mixed aqueous solution was extremely weak at around 450 nm and there was almost no absorption at the wavelength of 780 nm. These results suggest that the reduction of [-OOC-R-NH2]-Au(III) complex can be initiated through the MPR process. The electrical properties were investigated by cyclic voltammograms. As shown in Fig. 8 in the Appendix below, the aqueous solution of HAuCl4 showed a reduction peak at 0.373 V, while the peak was shifted to the lower potential of 0.253 V when mixed with IL at a molar ratio of 7 for C5/HAuCl4, which verified the generation of the stable complex.
Fig. 1 (a) Scheme for femtosecond laser direct writing the mixture aqueous solution of HAuCl4 and ionic liquid. (b) Schematic illustration of the MPR process for the formation of gold micro/nanostructures. (c) The structure of ionic liquid C4, C5 and C6; and UV-Vis absorption spectra of HAuCl4, C5 and the mixture aqueous of C5 and HAuCl4 (C5/HAuCl4 = 7, the concentration of HAuCl4 is 0.08 M).
Patterned gold nanostructures are obtained by MPR process through moving the computer-controlled 3D piezostage. The intensive optical field near the focal point induced MPR of the [-OOC-R-NH2]-Au(III) complex, which produced the IL-capped spherical gold nanoparticles (Fig. 1(b) and Fig. 9 in the Appendix below). The IL was absorbed on the surface of the gold nanoparticles through the strong binding of trimethylammonium in the cation and gold nanoparticles. Here, we expect to use IL to control the morphology and size of gold micro/nanostructures in MPR nanofabrication. Actually, we have also investigated some surfactants such as hexadecyltrimethylammonium bromide (CTAB), alkyl carboxylate (n-decanoylsarcosine sodium, NDSS), fatty salts (sodium valerate, sodium caprylate) and cetylpyridinium chloride (CPCl) assisted MPR of gold ions aqueous solution. Large laser power was needed for the reduction of gold ions and it was impossible to obtain a continuously complete gold line due to the occurrence of bubbles. Compared to surfactants, ionic liquid is able to forming complex with gold ions [34], and thus reduces the laser intensity in the photoreduction, resulting in the smooth gold nanostructures.
We first investigate the effect of the length of the anion carbon chain, and the laser irradiation condition, i.e. the laser power and the scanning speed, on the morphology and size of gold nanolines in the MPR process. Figure 2(a) shows the SEM images of gold lines fabricated with the assistance of ILs C4, C5, and C6, respectively, under the same scanning speed of 3 μm/s. The threshold power was 0.65 mW for C4, 0.84 mW for C5, and 1.02 mW for C6 assisted MPR, which increased with the carbon chain length. Figure 10 in the Appendix below presents the similar tendency that the width of the gold lines decreased while reducing the laser power under the same scanning speed for all of these three cases. Besides, the magnified SEM images show that the lines were composed of many small nanoparticles and the size became smaller with the carbon chain length from C4 to C6. Therefore, smoother lines have been achieved for the IL with longer carbon chain lengths, which was consistent with the results that obtained from fatty salts surfactant assisted MPR [29,36]. The result suggests that the carbon chain length of IL plays an important role in the inhibition of gold nanoparticle growth. The sizes of the gold nanoparticles for the gold nanostructure were well controlled and the surface smoothness was also improved for the IL with longer anion carbon chain.
Fig. 2 (a) SEM images of gold lines fabricated with assistance of ILs C4, C5, and C6, respectively, under the same scanning speed of 3 μm/s. The used laser powers were shown on the images. (b) SEM images of aqueous solution of HAuCl4 assisted by C5 after MPR at the laser power of 1.57 mW (the scanning speeds were 2, 3, 4, 5 μm/s for the image with red, blue, bright-red and green, respectively.); and the dependence of the linewidth upon laser power for the aqueous solution of C5 and HAuCl4 under the different laser scanning speed (2, 3, 4, 5 μm/s). (c) SEM image of aqueous solution of HAuCl4 assisted by C6 after MPR on the laser power of 1.57 mW (the scanning speeds were 2, 3, 4, 5 μm/s for the image with red, blue, bright-red and green, respectively.); and the dependence of the linewidth upon laser power for the aqueous solution of C6 and HAuCl4 under different laser scanning speed (2, 3, 4, 5 μm/s). (IL/HAuCl4 = 7, the concentration of HAuCl4 is 0.08 M).
The scanning speed is a critical factor for obtaining a fine feature size and improving the surface smoothness of metallic nanostructures. The SEM images in Figs. 2(b) and 2(c) present the gold nanostructures fabricated in sample solutions of HAuCl4 with IL of C5 and C6, respectively. The laser power was 1.57 mW and the scanning speed changed from 2 to 5 μm/s. It was confirmed that the surface morphology and linewidth were varied with the laser irradiation duration for the sample solution with C5. Higher laser scanning speed produced larger gold nanoparticles, and thus induced the loose and rough surface morphology of metallic nanostructures. This is because the reduction speed is not fast enough to produce more gold nanoparticles at higher scanning speed. Compared to the solution with C5, the sizes of gold lines for that of C6 were smaller while changing the scanning speed, which formed the relative compact and smooth surface. This is ascribed to the fact that IL with longer carbon chain length (for example, C6) has larger hydrophobic force, which inhibits the growth of the gold nanoparticles more effectively. Therefore, the relatively compact and smooth line surface can be obtained for C6 compared with the loose and rough surface for that of C5 under the same experimental conditions.
Moreover, we have summarized the relationship between the laser irradiation condition and the average width of the gold lines fabricated using C5 and C6 assisted sample solutions, respectively (Figs. 2(b) and 2(c)). Obviously, the linewidth was reduced with the increase of the scanning speed under the same laser power. It is reasonable since the shorted laser irradiation duration would lead to the less photoreduction of gold ions. Therefore, the amount of the reduced gold nanoparticles decreased and consequently reduced the width of gold lines. The result is more practical for C6 than that of C5, owing to the loosely structures for the sample solution with C5 under the larger scanning speeds. In this study, the minimum linewidth achieved was 228 nm (4 μm/s, 0.8 mW) for C5 and 325 nm (2 μm/s, 0.92 mW) for C6, respectively. These results indicate that the matching between IL and the optimum irradiation condition including laser power and scanning speed can effectively reduce the size of gold nanoparticles and improve the surface smoothness of the gold nanostructures.
The crystalline morphology of the fabricated metallic nanostructures has been evaluated by TEM. Figure 3 shows the typical TEM images of the gold nanostructure for the sample solution with C5 under the laser power of 1.57 mW and the scanning speed of 2 μm/s. The gold line was composed of aggregated gold nanoparticles with the average size of less than 10 nm. From HR-TEM image (Figs. 3(c)-(e)), we can see that the gold nanostructures obtained were composed of crystalline gold nanoparticles with the {111} and {110} facets, possessing either single or joint crystalline structures. The highly crystalline gold nanoparticles composed for the gold nanostructures are expected to provide better electrical and optical properties.
Fig. 3 (a) TEM image of gold nanoline fabricated using C5 assisted aqueous solution of HAuCl4. (b) The magnified TEM images selected from the square region of (a). (c), (d) and (e) were the magnified HR-TEM images from the marked region of (b).
Additionally, the conductivity of the gold nanostructures has been further characterized. This is of great importance for the metallic nanostructures because the level of conductivity will directly determine the ranges of the application in electronics, photonics and so on. We use a metal mask plate to cover the individual line that prefabricated on a glass substrate and then deposit gold electrodes on the ends of the line. Figure 4(a) is the typical SEM image of gold nanostructure between two gold electrodes. The line was fabricated under the laser power of 2.74 mW and the scanning speed of 3 μm/s. The corresponding AFM image and the profile of cross section are presented in Figs. 4(b) and 4(c), respectively. The conductivity measurement was performed using the electrochemical workstation and the current versus applied voltage was plotted as shown in Fig. 4(d). The length and cross section of the fabricated gold nanostructure was 75 μm and 0.05308 μm2, respectively. As a result, the calculated electrical resistivity was 16.5 × 10−8 Ω m, which is of the same order as that of bulk gold. The ability to write subwavelength conductive gold structures through MPR could exhibit significant development for the micro/nanotechnology. The highly crystalline gold nanostructures exhibit excellent electrical performance, which holds important potential for applications in nanoelectronics and nanophotonics.
Fig. 4 Electrical characterization of gold nanoline: (a) SEM image of gold nanoline between two Au electrodes. (b) AFM image of the gold nanoline. (c) Height profile of the gold nanoline. (d) Current-voltage curve of the gold nanoline.
Based on the understanding of the optimum fabrication conditions for gold nanolines, we have further designed and fabricated some typical gold micro/nanostructures to confirm the feasibility to direct writing microelectronic, micromechanical and photonic devices through IL-MPR (Figs. 5(a)-5(d)). The microgears, microcircuit diagram, coil and microantennas have been successfully fabricated under a laser power of 1.57 mW and a proper scanning speed using the sample solution with C5, which may be further explored to realize the fabrication of functional devices.
Finally, we have successfully fabricated planar THz metamaterials with the developed IL-MPR technique. Metamaterials are artificially engineering nanostructured metal-dielectric composites with exotic electromagnetic properties, which have demonstrated the potential benefits in various applications such as negative index of refraction, superlens and cloaking [20,37–39]. As shown in Fig. 6(a), we fabricated 2D arrays of U-shaped gold SRRs with the laser power of 1.57 mW and scanning speed of 3 μm/s. The magnified images (top and bottom) showed the uniformity of the gold SRRs in different areas. Figure 11 in the Appendix below shows the designed metamaterials and the height profile of the fabricated structures. The periodicity P of a unit cell is 2 μm, the width W is 633 nm, the height H is 150 nm, and the arm length of the SRR L is 1.9 μm. We have characterized the optical transmission and reflection spectra for linear polarized incident light by FTIR (Fig. 6(b)), which clearly shows an electric resonance around 63 THz for x-polarized wave, but no resonances are observed for y-polarization (Fig. 12 in the Appendix below). Theoretically, we have further numerically calculated the transmission and reflection spectra using finite element method. There is a good qualitative agreement between numerical simulations and measurements (Fig. 6(c)). The electric field and current distributions at the resonant frequency shown in Fig. 6(d) indicate that the observed resonance is excited by the electric field and strong coupling effect occurs between two closed arms in the adjacent unit cell. These results indicate that the direct multiphoton photoreduction of metal ions with the assistance of IL could provide high potential for numerous photonic and electronic devices.
Fig. 6 (a) SEM images of the U-shape gold resonance rings on glass substrate, which was fabricated under the laser power of 1.57 mW and the scanning speed of 2 μm/s using the sample solution with C5, L = 1.9 μm, H = 150 nm, W = 640 nm, P = 3 μm. Magnified images in the edge (top, blue) and center (bottom, red) of the resonance rings. Measured (b) and numerical calculated (c) transmission and reflection spectra for the metamaterials with x-polarized illumination. (d) Simulated electric field and current density at the resonant frequency for x-polarized illumination.
4. Conclusion
In summary, we have demonstrated a new nanofabrication approach for fabricating functional metallic micro/nanostructures. The morphology and size dependence of gold nanostructures on the anion carbon chain lengths and fabrication condition were investigated in the process. By matching the IL with the optimum laser fabrication parameters, not only the feature size can be reduced, but also the surface smooth can be improved. The obtained minimum linewidth was 228 nm, which overcame the classical optical diffraction limit. The TEM measurement indicated that the gold nanostructure was composed by crystalline gold nanoparticles, which denoted that the electrical conductivity was in the same order as that of the bulk gold. Moreover, various functional gold nanostructures have been prepared, in which the U-shaped metamaterials have been experimentally and numerically confirmed an electric resonance around 63 THz for x polarization. The IL-MPR technique could provide an efficient approach for fabricating metallic micro/nanostructures with fine size and smooth surface, which would be used in electronic and photonic devices.
Supplemental Information
1. The 1H NMR data
For better understanding of the interaction between HAuCl4 and IL, 1H NMR measurements have been done on the pristine IL and the mixture of HAuCl4 and IL at different molar ratios. Figure 7 presented the evolution of spectra with the varied molar ratios for HAuCl4 to IL of 0, 1/112, 1/56, 1/28, 1/14, 1/9, and 1/7, respectively. The 1H NMR data of the ionic liquid (IL) (2-hydroxyethyl)trimethylammonium 5-aminobutyric (C4), (2-hydroxyethyl)trimethylammonium 5-aminopentanoic (C5) and (2-hydroxyethyl)trimethylammonium 5-aminocaproic (C6) was shown as follows:
Fig. 7 The chemical structure of the C4-C6 and 1H NMR spectra of the pristine C5, and HAuCl4 mixed with C5 at different molar ratios (HAuCl4/C5: 0, 1/112, 1/56, 1/28, 1/14, 1/9 and 1/7, respectively.). The solvent used here was D2O.
C4: 1H NMR δ/ppm (400 MHz, D2O): 1.76 (2H, m, CH2CH2CH2NH2), 2.24 (2H, t, CH2COO-), 2.73 (2H, t, CH2NH2), 3.18 (9H, s, (CH3)3N), 3.41 - 3.44 (2H, m, CH2CH2N), 3.95 - 3.98 (2H, m, CH2OH).
C5: 1H NMR δ/ppm (400 MHz, D2O): 1.44 (4H, m, CH2CH2CH2CH2NH2), 2.09 (2H, t, CH2COO-), 2.62 (2H, t, CH2NH2), 3.18 (9H, s, (CH3)3N), 3.41 - 3.44 (2H, m, CH2CH2N), 3.95 - 3.98 (2H, m, CH2OH).
C6: 1H NMR δ/ppm (400 MHz, D2O): 1.38 (2H, m, CH2CH2CH2CH2CH2NH2), 1.61 (4H, m, CH2CH2CH2CH2CH2NH2), 2.22 (2H, t, CH2COO-), 2.88 (2H, t, CH2NH2), 3.18 (9H, s, (CH3)3N), 3.41 - 3.44 (2H, m, CH2CH2N), 3.95 - 3.98 (2H, m, CH2OH).
The chemical shift at 1.44, 2.09, and 2.62 ppm was indexed as the H from CH2 of 5-aminobutyric anion for the pristine C5, whereas it was shifted to the low field when mixed with HAuCl4. Especially, for the H of CH2 directly linked to the NH2, the peak from 2.62 ppm changed to 3.46 ppm with the molar ratio of 1/7. This was ascribed to the interaction between HAuCl4 and the 5-aminobutyric anion, which reduced the shielding action of electrons around the anion. However, there was no change for the chemical shift peaks of H from cation for the mixture aqueous solution.
2. Cyclic voltammetry (CV) spectra
CV scans were recorded to study the reaction of the IL and HAuCl4 as shown in Fig. 8. The concentration of HAuCl4 and IL was 0.001 and 0.007 M, respectively. The reduction peak shifted from 0.373 to 0.253 V for the IL C5 (0.241 V for C4; 0.289 V for C6) when HAuCl4 was added into the aqueous solution of IL. The lower potential shift indicated the generation of complex and it was more difficult to be reduced than that of the pristine aqueous solution of HAuCl4. Combined with the results from Fig. 1, it was confirmed that the [-OOC-R-NH2]-Au(III) complex were formed.
Fig. 8 Magnified CV curves for pristine HAuCl4 (black) and the solution of HAuCl4 mixed with IL (IL/HAuCl4 = 7) (C4, blue; C5, red; C6, green.), respectively. Inset showed the CV cycles in original range. The solvent used was 0.1 M NaCl aqueous solution. Scan rate: 0.05 V/s.
3. Interaction between IL and AuNPs
4. The threshold of C4-C6 assisted HAuCl4 aqueous solution
The MPR threshold power of IL C4-C6 assisted HAuCl4 aqueous solution were investigated for these three kinds of sample solutions under the same laser scanning speed of 3 μm/s but with different laser powers. The threshold was 0.65 mW for C4, 0.84 mW for C5, and 1.02 mW for C6 assisted MPR, respectively. It indicated that the larger reduced threshold was required for IL with longer carbon chain.
Fig. 10 The dependence of the linewidth upon laser power for the aqueous solution of C6 and HAuCl4 under the scanning speed of 3 μm/s.
5. The designed planar metamaterial based on U-shaped split ring resonators (SRRs)
Fig. 11 (a) Sketch of the designed planar metamaterial based on U-shaped split ring resonators (SRRs), P is defined as the period, L is the length, W is the width, and H is the height of the ring. (b) AFM image of a small region form the array which was fabricated under the laser power of 1.57 mW and the scanning speed of 3 μm/s using the sample solution with C5. (L = 1.9 μm, H = 150 nm, W = 640 nm, P = 3 μm). (c) Height profile of the gold resonant rings.
6. The measured and numerical calculated transmission and reflection spectra for the metamaterials with y-polarized illumination
Acknowledgments
We are grateful to the National Natural Science Foundation of China (Grant Nos. 91123032, 61205185, 61205194, 61275171, 61275048), the National Basic Research Program of China (2010CB934103), International Cooperation Program of MOST (2010DFA01180), and the Opening Project of Key Laboratory of Microelectronics Devices & Integrated Technology, Institute of Microelectronics, Chinese Academy of Sciences for financial support.
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