Large-area gold nanoring arrays were fabricated using interference lithography and metallic transformation through annealing of colloidal gold nanoparticles. The strong surface tension of the suspension solution and the molten gold, as well as the effective distance of these interaction mechanisms, is responsible for the creation of gold nanorings. The size and shape of the gold nanorings can be controlled by adjusting the size of the holes in the template photoresist grating, which is accomplished in the stage of interference lithography. Furthermore, the concentration of the colloidal gold nanoparticles and the annealing temperature can be utilized to achieve further optimization of the gold nanoring structures. Optical spectroscopic measurements show unique plasmonic response of the nanoring arrays in the visible and in the infrared spectral ranges, which agrees well with the theoretical simulation. This fabrication method provides a simple and low-cost route for achieving metallic nanoring arrays in a large scale for practical applications.
© 2013 OSA
Plasmonic nanostructures for constructing metamaterials have been patterned into a variety of shapes, including nanoparticles, nanowires, nanorods, nanoholes, nanotriangles, nanodiscs, and nanorings [1–6], which may be extensively applied in sensors , optical switches , photovoltaic diodes , and antennas . The metallic nanoring structures exhibit unique properties that are particularly suitable for applications in high-sensitivity sensors , surface enhanced Raman spectroscopy (SERS) [11, 12], and surface enhanced infrared absorption spectroscopy (SEIRA) .
Regular nanoring arrays have been produced by electron beam lithography (EBL)  and responsive block copolymer template . Nanoring arrays with regular and irregular ensembles have been fabricated by nanosphere lithography (NSL) . Among these methods, EBL enables fabrication of metallic ring arrays with precisely controllable shapes and sizes, however, the structures can be achieved only in a relatively small area. In particular, high costs restrict possible applications of EBL in the mass fabrication of large-scale nanostructures. It is difficult to create regular or periodically arranged metallic ring structures without defects in a large area using the template of the responsive block copolymer micelles. Furthermore, the film of the block copolymer underneath the structures will bring inconvenience or disturbance in further applications. The morphology and the period of the nanoring structures are also difficult to be well controlled in large scale by the NSL method, because of the inevitable defects from self-assembly of nanospheres. Thus, fabrication of regular arrays of the metallic nanorings in a large area with low costs is still a challenge in practice.
In this work, we introduce a solution-processible technique to achieve gold nanoring arrays with a large area but low costs. Although interference lithography is a very conventional technique for nanofabrication, it is difficult for this method to achieve arrays of ring structures, in particular when the structures need to be transformed into metallic patterns. However, solution-processed gold nanoparticles may be employed to solve this challenge. Making use of the interaction between the solution-processible gold nanoparticles and the patterned template grating structures, the large surface tension of the molten gold and its effective distance, the removal of the photoresist template at a temperature above 450 °C, we achieved fabrication of large-area arrays of gold nanorings. The interesting spectroscopic properties of such structures in the visible and the infrared spectra, corresponding to the plasmonic response of the inner and outer shells of the metallic nanorings, paves the way for practical applications.
2. Fabrication and microscopic characterization of the gold nanoring arrays
The photoresist (PR) mask grating is first fabricated using interference lithography through double-exposure with orthogonal orientations of the substrate, producing nano-hole arrays with a period of 1 μm and a modulation depth of about 200 nm. The positive photoresist of S1805 (from Rohm and Haas Electronic Materials Ltd.) is used as the recording medium and He-Cd laser at 325 nm is used as the UV laser source. The separation angle between the two laser beams is adjusted to be about 17 degrees before they are overlapped onto the PR film, leading to a separation of about 1 μm between the interference fringes. The exposure time is controlled between 9 and 11 s by a remote shutter. Thus, holes with a diameter in the range from 500 to 600 nm are obtained in the photoresist film after the development process. Some more details about the fabrication of the photoresist grating structures are described in our previous reports in Ref [16,17]. The substrate consists of 1-mm-thick glass coated with 200-nm indium tin oxide (ITO) and is cut into a square piece as large as 20 × 20 mm2.
In the subsequent stage of metallization, the chemically synthesized gold-nanoparticle colloids  with a concentration of 100 mg/ml in xylene are spin-coated onto the grating structures at a speed of 2000 rpm for 30 s. The sample is then annealed at 250 °C on a hot plate for 20 s. This intends to remove the ligands as modifications on the surface of the gold nanoparticles and to melt the gold nanoparticles (Au NPs). As a result, the precursor of the gold nanoring arrays are produced, as shown in the scanning electron microscopic (SEM) image in Fig. 1, where Fig. 1(a) shows the large-area patterns and Fig. 1(b) shows an enlarged image of a single hole. Clearly, most of the gold nanoparticles are confined into the holes and they prefer to aggregate to the walls of the photoresist holes, forming confusion circles consisting of gold nanoparticles. After being annealed by the hotplate, small gold nanoparticles have been aggregated into larger ones, as verified by Fig. 1(b).
In the last stage, the precursor sample is annealed further in a furnace at 450 °C for about 20 minutes. This process is crucial for finalizing the gold nanoring fabrication, where the gold nanoparticles are molten again to be fused together into homogeneous entities and the photoresist template grating is removed completely due to the thermal effect . After being cooled down to the room temperature, the gold nanorings arrays are produced. The SEM and AFM images of the gold nanorings are shown in Figs. 2(a) and 2(b), respectively. Both microscopic images show excellently arranged gold nanorings with high quality in their shapes and bodies. The gold rings have a height of about 80 nm, an inner diameter of 270 nm, and an outer diameter of about 650 nm.
3. Mechanisms for the formation of the gold nanoring arrays
Figure 3 illustrates schematically the mechanisms for the formation of the gold nanorings. After the spin-coating process, the colloidal gold nanoparticles fill the holes and cover the surface of the photoresist template grating in the form shown in Fig. 3(a). Due to the large diameter of the photoresist holes or the large duty cycle of the template grating, the high concentration of the colloidal solution, the majority of the gold nanoparticles are confined into the holes, whereas, a small portion stays outside. Since the colloidal solution wets both the ITO and the S1805 photoresist, the majority of the colloidal gold nanoparticles aggregate to the corner formed by the photoresist wall and ITO substrate. After the annealing process at about 250 °C, the ligands covering the gold nanoparticles were sublimated and the gold nanoparticles are molten to join together. Thus, the whole volume of the colloidal gold nanoparticles is reduced dramatically. The large surface tension of the molten gold pulls the gold inside and outside the hole to the most aggregated area in the hole, which is actually located on the outer shell and on the bottom corner of the hole, as shown in Fig. 3(b). As the annealing temperature is increased further to 450 °C, the aggregated gold nanoparticles become completely molten and tend to aggregate together into a more condensed and more spherically shaped entity. Meanwhile, the photoresist template grating is evaporated at such a high temperature, leaving gold nanorings on the previous sites of the template holes, as shown in Fig. 3(c). Figure 3 demonstrates the simplicity how the gold nanoring arrays are produced through annealing and confining the gold nanoparticles into the interference-lithography- patterned holes with a diameter larger than 600 nm. Furthermore, it is understandable that the shape and size of the nanorings depend strongly on the size of the holes in the photoresist template grating. This can be controlled by adjusting the exposure and development time in the stage of interference lithography. If the holes are so small that they cannot hold all of the gold nanoparticles spin-coated onto the grating, the excessive amount will be aggregated to the hole sites, forming regularly shaped mounds standing above the grating surface. After the annealing process, all of the gold nanoparticles will become molten and fused together into homogeneous entities, as shown in Fig. 4(a). However, when the holes are even larger, the total volume of the holes is even larger than that of the spin-coated gold nanoparticles, so that the holes cannot be fully filled. Since the colloidal solution wets both the S1805 photoresist and the ITO glass, the gold nanoparticles tend to aggregate to the outer shell and lower corner of the holes, leaving a much thinner center than the edge corners. Since the size of the hole is still within the effective distance of the strong surface tension of the molten gold, during the annealing process the gold nanoparticles are pulled from the center to the outer shell and body of the gold nanoparticles in the hole is broken in the center. Thus, gold nanorings form on the hole sites, as illustrated in Fig. 4(b). As the size of the holes is increased further, even less gold nanoparticles stay in the center of the hole after the spin-coating process. However, during the annealing process the surface tension of the molten gold is not strong enough to pull all of the gold from the center to the edge corner due to the large distance across the hole radius, leaving a small amount of gold in the center area with a gold ring form on the outer shell, as shown in Fig. 4(c). It should be noted that the photoresist has been evaporated completely during the annealing process at 450 °C.
Figure 5(a) and 5(b) show the SEM images for the fabricated structures, corresponding to the mechanisms illustrated in Figs. 4(a) and 4(c), respectively. When an exposure time 6~8 s is used, holes with a diameter of about 360 nm can be obtained, whereas, when the exposure time was increased to 12~15 s, holes with a diameter larger than 700 nm can be produced. Figure 5(a) shows the SEM image of the metalized structures fabricated using a mask with a hole-diameter less than 360 nm. Clearly, arrays of gold nanoislands were produced, where the average diameter of the island is smaller than 400 nm. The SEM image in Fig. 5(b) shows that when the hole diameter is increased to larger than 700 nm, gold nanorings with an outer diameter of about 680 nm and an inner diameter of about 440 nm were produced. However, the holes are so large that the molten gold cannot pull all of the gold in the center to the edge and some gold remains in the center area. Thus, after the annealing process, hybrid structures consisting of gold rings with small islands in the center were produced.
4. Optical spectroscopic properties
The optical extinction spectra of the gold nanoring arrays shown in Fig. 2 and Fig. 5(b) were measured by an infrared spectrometer and are plotted in Fig. 6 by the black and red curves, respectively. Two extinction peaks can be observed for both measurements with one centered at 600 nm and the other at 1800 nm.
To understand the plasmonic resonance modes shown in Fig. 6, electromagnetic near-field distribution around the gold nanorings was calculated using the finite-difference time-domain (FDTD) method . The Au nanoring is assumed to be illuminated by normally incident light polarized along the Y-axis (Fig. 7).The nanorings are assumed to be located in air. Figures 7(a) and 7(b) show the calculated optical electric field distribution for two resonance wavelengths at 600 and 1800 nm, respectively. Clearly, the resonance mode in the visible centered around 600 nm actually corresponds to the strongly enhanced electric field on the inner shell of the ring. However, that in the infrared at 1800 nm induces enhanced electric fields on the outer shell of the ring. For more direct comparison between theoretical and experimental results, the extinction spectrum was calculated for the gold nanoring arrays in Fig. 2 over the studied spectral band from 400 nm to 2600 nm, as shown by dashed curve in Fig. 6. Basically, the calculated spectrum agrees well with the measurement result shown by the solid black curve, in particular, both results demonstrate spectral peaks in the infrared at almost the same wavelength of about 1800 nm. An extinction peak is also observed in the visible spectrum for the calculation, which is located at a shorter wavelength than 500 nm and is shorter than the measured value of about 600 nm. This deviation is understandable if considering that the parameters of the nanorings in the modeling differ from those of the practical structures. Thus, the simulation results not only accomplished the spectroscopic characterization of the gold nanoring arrays, but also verified further our proposed photophysics in such nanostructures.
The gold nanorings in Fig. 2 have an outer diameter of about 650 nm and an inner one of 270 nm. However, those in Fig. 5(b) have an outer and inner diameter of 680 and 440 nm, respectively. The larger outer diameter of rings in Fig. 5(b) than that in Fig. 2 leads to a red shift of the extinction spectrum from 1840 to 1950 nm, as shown in Fig. 6. As for the inner-shell resonance modes, although the rings in Fig. 2 have a smaller inner diameter than those in Fig. 5(b), the optical extinction spectra are centered almost at the same wavelength of 600 nm. This can be understood by considering that the small gold nanoparticles inside the larger rings actually reduce the effective inner diameter of the larger rings in Fig. 5(b) and counter-balance the red-shift effect. Some small structures may be observed in the optical extinction spectra in Fig. 6, which result from the coupling between the plasmonic resonance and different orders of the waveguide resonance modes based on the configuration of gold nano-structure arrays sitting on an ITO waveguide [16, 17].
5. Shape and size control of the gold nanorings
The shape and size of the gold nanorings are mainly determined by the shape and size of the photoresist template hole gratings. However, the thickness and the height of them are finally fixed in the metallization process, which may still be modified by controlling the concentration, the spin-coating speed, and the annealing temperature of the colloids of gold nanoparticles.
Figure 8(a) shows the SEM image of elliptical gold nanoring arrays. In the interference-lithography stage of the fabrication process, we slightly modified the grating period and used different exposure dose in X and Y directions, so that elliptical hole arrays were produced in the template grating. In the metallization process, the same concentration of colloidal gold nanoparticles, the same spin-coating speed, and the same annealing temperature have been employed as in the above fabrications. Thus, elliptical gold nanorings with a different X to Y ratio of about 0.7 were produced, as shown in Fig. 8(a). The grating period is about 960 nm in X direction and 1060 nm in Y direction. The outer diameter of the elliptical ring is about 490 nm in X (DX) and 696 nm in Y (DY) directions, whereas, the inner diameter is about 258 and 490 nm, respectively. Therefore, the thickness of the elliptical gold nanorings is about 100 nm, which is smaller than that of those structures shown in Fig. 2 and Fig. 5. Basically, the elliptical nanorings were produced with high qualities, although some of them are broken and some small gold nanoparticles can be observed inside the rings.
The optical extinction spectra of the elliptical gold nanorings shown in Fig. 8(a) were measured for different polarization directions (X and Y), as shown in Fig. 8(b). For X polarization, the incident light excites plasmon resonance across the shorter axis of the nanorings. The corresponding optical extinction spectrum (black curve) is peaked at about 1440 nm. For Y polarization and plasmon resonance across the longer axis of the elliptical nanorings, the optical extinction spectrum is measured to peak at about 1900 nm, as shown by the red curve in Fig. 8(b). The spectroscopic response in the infrared results from the plasmon resonance in the outer shell of the nanorings. As for the inner-shell resonance modes, the spectral features can still be observed in the visible, as compared with Fig. 6. However, also due to the small gold nanoparticles that remain inside the rings, the effective inner diameter is reduced in both the X and the Y directions. Furthermore, more nanoparticles remain along the Y than the X axis, resulting in stronger reduction of the inner diameter in Y than in X direction. This explains why the spectroscopic features are located at almost the same position of about 600 nm for X and Y polarizations.
The comparison between the fabrication results in Fig. 2, Fig. 5(b), and Fig. 8(a) shows that there is a limited range of the size of “clean” nanoring structures without any small gold nanoparticles inside. Actually, this can be controlled by adjusting the concentration of the colloidal solution, the surface-energy properties of the substrate, and the height of the photoresist walls in the template PR hole arrays. However, the small gold nanoparticles did not influence much the spectroscopic response in the infrared by the plasmon resonance on the outer shell of the nanorings. Therefore, this fabrication method is flexible and stable for achieving plasmonic devices for applications in the infrared.
We demonstrated fabrication of large-area nanoring arrays using interference lithography and solution-processed gold nanoparticles. The size and the shape of the gold nanorings can be controlled by adjusting the exposure time during the interference lithography stage, which has actually changes the size of the holes in the photoresist template grating. The strong surface tension of the colloidal solution and the molten gold, as well as the effective distance of these interaction mechanisms, is crucial for achieving this fabrication. Comparison between the optical spectroscopic investigation and the theoretical simulation shows two resonance modes in the visible and in the infrared spectral range, corresponding to the collective oscillation of electrons on the inner and outer shells of the gold nanoring structures.
We acknowledge the 973 program (2013CB922404), the National Natural Science Foundation of China (11274031, 11104007), and the Beijing Natural Science Foundation (4133082, 1132004) for the support.
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