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This paper presents a new method for fabricating periodic arrays of metallic nano-particles on flexible substrates. This method is based on metallic film contact transfer method and high-power pulsed laser annealing. Experiments have been carried out to produce arrayed metallic nano-particles oriented in a hexagonal pattern. The nano-particle size is 70 nm in diameter and the center-to-center pitch of the hexagonal array is 400 nm. Large-area patterning and fabrication of these arrayed nano-particles can be easily achieved up to an area size of few cm2. Besides, composite or compounded metallic nano-particle arrays can also be produced using different metal materials. The localized surface plasmon resonance (LSPR) effects induced by the fabricated arrays of nano-particles are experimentally observed and quantitatively measured. Numerical simulation on these LPSR effects is performed and the simulation results are in good agreement with experimental data.
©2013 Optical Society of America
1. Introduction
Due to the development of advanced nano-fabrication technologies, researches on physical behaviors of nano-structures have gained a lot of attention recently. One example is the phenomenon of localized surface plasmon resonance (LSPR) [1,2], which is formed by charge density oscillation inside metallic nano-particles. The charge density oscillation is come from a specific incident light or electromagnetic wave which causes the resonance of an electron field. LSPR is often to apply in physical and optical applications such as surface enhanced Raman scattering (SERS) [3–9] to enhance the sensitivity of Raman spectrum measurement, the photo-luminescence enhancement of light emitting diodes [10–16], and the photovoltaic conversion efficiency enhancement of organic solar cell [17–21].
For the fabrication of nano-metallic particles to induce LSPR, various methods have been developed and can be divided into three main categories: laser ablation method [22–30], metal vapor synthesis [31], and chemical reduction method [32]. However, these methods cannot accurately control the particle sizes nor the distribution of particle sizes of produced nano-particles, and therefore will broaden the spectrum bandwidth of LSPR and reduce the resonance effect. It is also very difficult, if not impossible, for these methods to arrange the locations of produced nano-particles to form a desired pattern, which can be very useful in obtaining specific characteristics of resonance effects.
Instead of using randomly patterned nano-particles, several research groups utilize periodic and arrayed metallic nano-particles to induce LSPR, and to control the wavelength range of localized surface plasmon resonance. Conventional methods for fabricating these periodic arrays of metallic nano-particles are advanced photolithography and electron beam lithography, but they usually need very complicated and expensive equipments. Furthermore, e-beam lithography can only produce nano-particle samples with a very limited area size at an extremely high cost. Recently, nano-sphere lithography [33–37] has also been applied for fabricating periodic and arrayed metallic nano-patterns. However, successful arrangement of nano-spheres over large patterning area is still very challenging.
In this work, we propose a new method which can better implement the fabrication of periodical arrays of metallic nano-particles on a flexible template. The basic idea is using metal-film contact transfer lithography and high-power pulsed laser annealing. The advantages of this proposed method are straightforward, easy to implement, large-area patterning, and with the potential for mass production. In addition, it can be used to prepare arrayed metallic nano-particles on flexible templates which are bio-compatible, and hence be used for biomedical and diagnostic applications of LSPR.
2. Experiments for fabricating arrayed nano-particles
The fabrication of arrayed nano-particles starts with replicating a soft PDMS or h-PDMS mold from a master silicon mold, as shown in Fig. 1 . The silicon mold is prepared by standard photolithography using a 5 × stepper. On the surface of the silicon mold, several hexagonally oriented hole-array structures are fabricated with a hole diameter ranging from 200 nm to 2μm, and the center-to-center pitch between holes is kept as twice the diameter of holes. The silicon molds is first coated with an anti-adhesion layer of 1H, 1H, 2H, 2H-perfluorooctyltrichlorosilane by using the vapor-deposition method [38,39]. The anti-adhesion layer has good adhesion to the silicon mold and in the mean time forms a surface with very low surface energy to assist de-molding. A negative replica of the Si mold is obtained using a polyurethane acrylate (PUA) resin through UV light (365 nm in wavelength) exposure and curing processes. The reason of using PUA mold as an intermediate step for mold replication is to reduce the risk of damaging the master silicon mold.
The PDMS molds used in this work are prepared by pouring a PDMS material (Sylgard 184, Dow Corning) on top of the PUA mold. The PDMS material is mixed with the exclusive curing agent with a ratio of 10:1. A vacuum step is first performed to remove the air bubbles of the PDMS material, and the PDMS material is then cured at 70 °C. By directly peeling off from the PUA mold, a PDMS template with the same hexagonal hole-array structures on the silicon mold is completed.
As for preparing the h-PDMS molds, h-PDMS material is prepared by mixing trimethylsiloxy-terminated vinylmethylsiloxane-dimethylsiloxane copolymers (VDT-731, Gelest), Pt-divinyltetramethylsiloxane (SIP 6831.1, Gelest), 2,4,6,8-tetramethyltetravinylcyclotetrasiloxane (Fluka, 87927), and methylhydrosiloxane-dimethylsiloxane copolymers (HMS-301, Gelest). A layer of h-PDMS material is first applied to the PUA mold surface. After thermal curing of the h-PDMS layer, the same PDMS material for making a PDMS mold is subsequently applied on the h-PDMS layer. After removing air bubbles by vacuum and thermal curing, a PDMS substrate is formed and firmly adhered to the h-PDMS layer. After peeling off from the PUA mold, an h-PDMS mold is then obtained
The reason for having two different kinds of soft molds, PDMS and h-PDMS, is that the detailed profiles of the formed hole-array structures are quite different. This difference will result in quite different characteristics of the fabricated metallic nano-particles as well as their LSPR effects, which will be discussed later on.
The procedures for fabricating metallic nano-particles are shown in Fig. 2 . First of all, the PDMS or h-PDMS templates are evaporated with an Au film using an E-beam thermal evaporator (VT1-10CE, ULVAC, Kanagawa, Japan). The thickness of this Au film is ranging from 10 nm to 30 nm. A PET layer is then placed on top of the PDMS or h-PDMS soft template, as shown in Fig. 2(b), and a pressure is exerted to force the PET layer and the soft template to be in close contact with each other. In the mean time, the PET/template is heated up to the glass transition temperature (80°C) of PET. Under the action of both exerted contact pressure and high temperature heating, the Au film which is in touch with the PET layer can develop a stronger bonding force with the PET layer and hence can be removed from the PDMS of h-PDMS template after separating the PET layer form the template. In another word, the Au film on the top surface of the template is transferred to the PET layer, which is known as metal film contact transfer. In the soft PDMS or h-PDMS template, the remaining Au films are on the concave surfaces of the hole-arrayed structures as shown in Fig. 2(c).
Fig. 2 Schematic diagrams for fabricating periodical arrays of metallic nano-particles on a flexible template.
In our previous work [40], metallic nano-particles were formed by applying pulsed laser annealing to the metallic film remaining at the bottom surfaces of the holes after metal film contact transfer. To avoid the particles being escaping from the holes, a transparent plate was used to cover the holes during laser annealing. However, it is found that some of the produced nano-particles are attached to the transparent cover plate and therefore cause certain defects to the arrayed nano-particles. In this work, a new approach is adopted by spin-coating a thin PMMA layer on top of the PDMS or h-PDMS template after metal-film contact transfer, as shown in Fig. 2(d). After that, a Nd:YAG pulse laser (LS-2137U, LOTIS TII, Minsk, Republic of Belarus) which has a wavelength of 1064 nm and a pulse duration of 6~7 ns is used as a heating source. The Nd:YAG laser pulses can pass through the PMMA to anneal the residuary Au film remaining inside the holes. The Nd:YAG laser is operated at an energy of 140 mJ/pulse and is scanning across over the whole template surface. Estimated laser fluence radiated on the sample surface is 2 J/cm2 by assuming a Gaussian beam profile.
The Au films which are confined in the concave surfaces of hole-arrayed structures and subjected to pulsed laser heating is melted and them form into a spherical nano-particle due to surface energy. A wet etching process with acetone is used to remove the PMMA layer. Finally, the metallic nano-particles are fabricated inside the surface cavities of PDMS or h-PDMS templates.
3. Experimental Results and Discussions
Figure 3 shows the scanning electron microscope (SEM, JSM-7000, JEOL, Tokyo, Japan) images of the remaining Au films are on the concave surfaces of the hole-arrayed structures before using laser annealing processes. Figure 4 shows the Au particles formed on a PDMS template after using metallic film contact transfer and laser annealing processes. In this experiment, the deposited Au film is 15 nm thick. Four different hole-diameters of 300 nm, 400 nm, 800 nm, and 2 μm are tested, and the SEM images of their experimental results are shown with two different magnification factors in Fig. 4(a) and 4(b), 4(c) and 4(d), 4(e) and 4(f), and 4(g) and 4(h), respectively. From Fig. 4, it is quite clear that, when the hole-diameter is not too much larger that the film thickness, a single metallic nano-particle is successfully formed within each individual hole. When the hole-diameter is getting larger in comparison with the metal film thickness, there will be several nano-particles co-formed within a hole. This phenomenon can be understood by considering the formation of liquid drops on a large area of hydrophobic surface.
Fig. 3 SEM image shows the remaining Au films are on the concave surfaces of the hole-arrayed structures of a PDMS template before using laser annealing processes.
Fig. 4 SEM images with two different magnification factors of Au nano-particles formed in arrayed holes of a PDMS template; the diameters of holes are: (a) and (b)300 nm, (c) and (d) 400 nm, (e) and (f) 800 nm, and (g) and (h) 2 μm.
Besides forming nano-particles of one single metal element, it is also possible to form compound or alloy type of metallic nano-particles which contain two or more metal elements. This is simply done by evaporating multiple films of different metal materials on top of the PDMS or h-PDMS template in the step shown in Fig. 2(a). For demonstration, a PDMS template with a hole-diameter of 400 nm is subsequently deposited with Au (5 nm), Ag (10 nm), and Au (5 nm) films, and is then used for metallic film contact transfer and laser annealing to produce compound metallic nano-particles. The SEM image of obtained Au/Ag nano-particles is shown in Fig. 5(a) . The material properties of these Au/Ag nano-particles are analyzed by using the line mapping of energy dispersive spectrometer (EDS) (INCA Energy, Oxford, UK). The measured EDS line mappings for Au and Ag elements from these fabricated nano-particles are shown in Fig. 5(b) and 5(c), respectively. It shows that the formed nano-particles indeed contain both Au and Ag elements. However, the detailed material micro-structures and compositions are subjected to further investigation.
Fig. 5 The formation of compound Au/Ag nano-particles: (a) an SEM image, (b) and (c) the line mappings of Au and Ag elements, respectively, from an EDS measurement on the nano-particles.
Since we are more interested in the applications of LSPR in visible light range, a silicon master mold with a hole-diameter of 200 nm and a center-to-center pitch of 400 nm is used in the following experiments. Figure 6 shows the SEM images of this silicon mold with two different magnification factors. Both PDMS and h-PDMS templates are replicated from this Si mold following the procedures shown in Fig. 1. A 15 nm and a 25 nm Au film are deposited on the PDMS and h-PDMS templates, respectively. The SEM images of the fabricated Au nano-particles on the PDMS and h-PDMS templates are displayed in Fig. 7 and Fig. 8 , respectively.
Fig. 6 SEM images of a Si mold at two different magnification factors; the mold surface contains hexagonally close-packed holes with a diameter of 200 nm and a center-to-center pitch of 400 nm.
By comparing the SEM images in Fig. 7 and Fig. 8, it is observed that the Au nano-particles formed on a PMDS template are more close to a spherical shape and more uniform in their particle size as in comparison with their counterparts on an h-PDMS template. We believe one of the reasons comes from the different shapes of micro-structures being formed on the PDMS and h-PDMS templates. Figures 9(a) and 9(b) reveal the detailed profiles of the holes on PDMS and h-PDMS templates, respectively. Since the h-PDMS material has higher fluidity, it can more faithfully maintain the structure profiles of holes during the replication process. On the other hand, since the PDMS material is less easily in forming micro-structures in sub-micrometer scale, the replicated surface structures tend to become a bowl shape profile instead of a cylindrical hole. During and after the laser heating, the molten metal is easily concentrated on the lowest position of the bowl-shape structures on a PDMS template to form a spherical particle. As for the h-PDMS templates, the hole-structures with a cylindrical profile could have some metal film deposited on the side wall of the holes, and not so easy to join the molten metal on the bottom part of the hole to form a sphere.
Fig. 9 SEM images of (a) a cross-section view of the hole-shaped structures on a h-PDMS template, and (b) a tilt view of the bowl-shaped structures on a PDMS template.
Furthermore, to achieve a complete and successful metal film contact transfer, the thickness of deposited metal films on top of the soft templates cannot exceed certain limits. For a PDMS template with a hole-diameter of 200 nm, experimental tests show the thickness of an Au film must be less than 15 nm. Otherwise the metal film deposited on the top flat surface of a PDMS template cannot easily separated from the metal film deposited on the concave surface of a bow-shaped cavity during the metal film contact transfer process. On the other hand, for the case of an h-PDMS template, the metal film can be much thicker because of the well-defined corners and side-walls of the holes.
In this work, totally five samples of arrayed nano-particles are prepared based on the silicon mold which has a hexagonal array of holes with a 200 nm hole-diameter and a 400 nm center-to-center pitch. Two samples are fabricated on PDMS templates with an evaporated Au film thickness of 10 nm and 15 nm, respectively. Three samples are prepared on h-PDMS templates with an evaporated Au film thickness of 20 nm, 25 nm, and 30 nm, respectively. After laser annealing and etching off of the PMMA layer, the samples are investigated under a SEM and the distributions of particle sizes are determined by analyzing the obtained SEM images.
Figures 10(a) and 10(b) show the measured distributions of particle sizes of the two PDMS-template samples and the three h-PDMS-template samples, respectively. As expected, the size of nano-particles is proportional to the thickness of evaporated Au film. When using a PDMS template, the average particles sizes are 75 nm and 83 nm for evaporating Au films with a thickness of 10 nm and 15 nm, respectively. When using an h-PDMS template, the average particles sizes are 103 nm, 119 nm, and 129 nm, respectively, corresponding to Au film thickness of 20 nm, 25 nm, and 30 nm. Table 1 summarized the average diameters of fabricated Au nano-particles for the five samples. Furthermore, from Fig. 10, it is obvious that the PDMS templates produce nano-particles with a more uniformly distributed particle size that the h-PDMS templates do.
Fig. 10 The distributions of the Au particles sizes fabricated on (a) a PDMS template with evaporated Au film thicknesses of 10 and 15 nm, and (b) an h-PDMS templates with evaporated Au film thicknesses of 20, 25, and 30 nm.
Table 1. The average sizes of Au nano-particles using PDMS and h-PDMS templates and different evaporated Au film thicknesses.
4. Optical measurements and analysis
The optical behaviors of these fabricated nano-particles as well as possible LSPR phenomena will be investigated using a UV/VIS spectrometer (V-600, JASCO, Tokyo, Japan). The spectral transmittance of the arrayed nano-particles along with their PDMS or h-PDMS templates will be measured at the optical wavelength from 300 nm to 900 nm. For the purpose of comparison, the PDMS or h-PDMS templates right after the metal contact transfer process but before laser annealing are also measured by the same UV/VIS spectrometer. The measured spectral transmittance for these templates with metal films deposited within their concave surface structures sever as a good reference for indicating the LSPR effects produced by nano-particles after laser annealing.
Figure 11(a) shows the spectral transmittance of the two PDMS templates right after metal film contact transfer but before laser annealing. The two PDMS templates are coated with a 10 nm and 15 nm thick Au film, respectively, on the concave surfaces of bowl-shaped micro-structures. The spectral transmittance can be viewed as a background signal for light transmission and absorption by the PMDS templates as well as these distributed Au films contained within these arrayed structures. In Fig. 11(a), the spectral transmittance does show a little curvature between the wavelength from 650 nm to 750 nm, and this curvature is more obvious when the thicknesses of Au film is increased from 10 nm to 15 nm. Therefore, we believe the main reason that there is no LSPR-related absorption band appeared in the spectra is due to the very small thickness of Au films. After laser annealing, Au nano-particles are formed with the particle size distribution shown in Fig. 10(a) and the corresponding spectral transmittance are measured and displayed in Fig. 11(b). In Fig. 11(b), clear LSPR effects induced by Au arrayed nano-particles are observed for both PDMS samples. The lowest transmittances are 0.72 and 0.56 at the wavelengths of 550 nm and 570 nm, respectively, for the 10 nm thick and 15 nm thick Au film samples.
Fig. 11 The spectral transmittance measured from PDMS templates, (a) before laser annealing so that metal film with a thickness of either 10 nm or 15 nm is deposited on the concave surface of bowl-shaped micro-structures, and (b) after laser annealing so that Au nano-particles are formed.
The measured spectral transmittances are from the three samples, which are prepared before and after laser annealing on h-PDMS templates, as shown in Figs. 12(a) and 12(b), respectively. In Fig. 12(a), all three samples exhibit LSPR effect even before nano-particles are formed by laser annealing. These LSPR can be attributed to the disk-like metal films sitting at the bottom surfaces of the holes. For three different film thicknesses of 20 nm, 25 nm, and 30 nm, the lowest transmittances are 0.17, 0.15, and 0.125, respectively, at the wavelength of 840 nm, 760 nm, and 705 nm, which indicates a blue shift for increasing film thickness. After laser annealing, nano-particles are formed and the lowest transmittances are now 0.73, 0.56, and 0.44 at the wavelengths of 578 nm, 587 nm, and 596 nm, respectively. As shown in Fig. 12(b), with increasing Au film thickness and hence increasing particle size, the LSPR effect becomes more pronounced.
Fig. 12 The spectral transmittance measured from h-PDMS templates, (a) before laser annealing so that metal film with a thickness of 20, 25, and 30 nm is deposited on the concave surface of hole-shaped micro-structures, and (b) after laser annealing so that Au nano-particles are formed.
5. Optical simulations and analysis
To simulate and analyze the observed LSPR effects, a commercial software OptiFDTD (Optiwave Systems Inc., Ottawa, Canada) based on finite difference time domain (FDTD) analysis is used to simulate the spectral transmittance of the Au nano-particles on PDMS or h-PDMS templates. The optical refraction indexes (n) and extinction coefficients (κ) of the two kinds of templates of PDMS and h-PDMS are approximately the same, hence there is no distinctions being made in the simulation. However, the surface microstructures are quite different and are taken into simulation. Figures 13(a) and 13(b) show the schematics used in the FDTD simulation of the arrayed nano-particles and their surface microstructures in PDMS and h-PDMS templates, respectively. In the PDMS template, the profile of arrayed cavities is modeled as a semi-ellipsoid with a circular opening with a diameter of 200 nm and a depth of 200 nm. The metallic particles are sitting at the bottom of the semi-ellipsoid. In the h-PDMS template, cylindrical holes with a hole-diameter of 200 nm and a depth of 200 nm are modeled. The optical refraction indexes and extinction coefficients are 1.45 and 0, respectively, for both PDMS and h-PDMS. The sizes of Au particles in the simulation are 80 nm, 100 nm, 120 nm, and 140 nm in diameter.
Fig. 13 Schematic diagram of the two kinds of PDMS templates and these templates are put an Au particle of 120 nm diameter on their holes structure; (a) Au particle is on a PDMS template with bowl structure (b) Au particle is on a PDMS template with cylindrical hole structure.
The simulated transmittance for PDMS template with semi-ellipsoidal surface cavities are shown in Fig. 14(a) , while the simulated results for h-PDMS template with cylindrical holes are shown in Fig. 14(b). In both cases, the intensity of LSPR effects is increasing with increasing size of nano-particles, as indicated by the decreasing of the lowest transmittance. The wavelength corresponding to the lowest transmittance is also shifting to the right along with increasing particle size. For example, in Fig. 14(a), the wavelength of minimum transmittance of PDMS templates is shifting from 550 nm to 625 nm when the particle size is increasing from 80 to 120 nm. Similarly, for the h-PDMS cases, the wavelength is shifting from 530 nm to 570 nm. These results are in good agreements with the experimental results shown in Fig. 11(b) and 12(b).
Fig. 14 The relationship between simulated transmittance of the incidence light and the incidence wavelength for the assumed Au particles of diameter of 80 nm, 100 nm, 120 nm, and 140 nm; (a) the spectral transmittance on PDMS template (b) the spectral transmittance on h-PDMS template.
6. Conclusion
We have successfully produced arrayed metallic nano-particles on both PDMS and h-PDMS templates by using metal film contact transfer method and a high-power pulse laser annealing approach. The minimum diameter of these metallic nano-particles can be well controlled around 70 nm using a silicon mold with arrayed hole-structures at a 200 nm hole-diameter and a 400 nm center-to-center pitch. The distribution of particle size can be very uniform, and the location of particles can be easily and precisely arranged when designing surface patterns of the silicon mold. The fabricated metallic particles can be made of a single element or composed of several different metallic elements.
We utilize the produced Au particles to induce LSPR, and find out the relationship between the Au particles size and the resulted spectral transmittance. When Au particle size is increasing, the intensity of transmittance will reduce, and the wavelength corresponding to minimum transmittance exhibits a red shift. The phenomena are observed in experimental measurements as well in theoretical simulation.
In the future, we can utilize the LSPR effects in many applications such as SERS, the photo-luminescence enhancement of light emitting diodes, and the photovoltaic conversion efficiency enhancement of organic solar cell. For further increasing the effect of LSPR and creating better performances on the aforementioned applications, it requires reducing the diameter and pitch of the hole-array structures on the PDMS and h-PDMS templates.
Acknowledgments
This work is supported by the National Science Council (NSC) of Taiwan.
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