- Email: [email protected]
Three-dimensional metallic opals fabricated by double templating
Thin Solid Films 517 (2009) 5166–5171
Contents lists available at ScienceDirect
Thin Solid Films j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / t s f
Three-dimensional metallic opals fabricated by double templating Qingfeng Yan a,b,c,1, Pavan Nukala d, Yet-Ming Chiang c, C.C. Wong b,⁎ a
Singapore-MIT Alliance, N3.2-01-36, 65 Nanyang Drive, Singapore 637460 School of Materials Science and Engineering, Nanyang Technological University, Nanyang Avenue, Singapore 639798 c Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA d Department of Metallurgical and Materials Engineering, Indian Institute of Technology Madras, Chennai, India 600036 b
a r t i c l e
i n f o
Available online 20 March 2009 Keywords: Metallic photonic crystal Self-assembly Colloidal crystal Template Electrochemical deposition
a b s t r a c t We report a simple and cost-effective double templating method for fabricating large-area three-dimensional metallic photonic crystals of controlled thickness. A self-assembled polystyrene opal was used as the first template to fabricate a silica inverse opal on a gold-coated glass substrate via sol–gel processing. Gold was subsequently infiltrated to the pores of the silica inverse opal using electrochemical deposition. A highquality three-dimensional gold photonic crystal was obtained after removal of the secondary template (silica inverse opal). The effects of template sphere size and deposition current density on the gold growth rate, and the resulting morphology and growth mechanism of the gold opal, were investigated. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Photonic crystals (PCs) are structured materials with spatially periodic variations in dielectric constant that possesses unique characteristics in the manipulation and control of the flow of light [1,2]. Compared to dielectric and semiconductor PCs, metallic photonic crystals have the advantages of very large full bandgaps that extends well into infrared and high reflectance from relatively small number of stacking layers due to the large refractive index of metals [3–5]. It was found that around the band-edge of a metallic photonic crystal, some interesting electromagnetic phenomena such as enhancement of absorption [6] can arise. Recently, metallic PCs have shown their potential in applications as efficient thermal emitters and photovoltaic devices [7–9]. However, the fabrication of three-dimensional (3D) metallic PCs remains problematic because of challenges in microfabrication at optical scales. Although lithography techniques such as photolithography [9–11], E-beam lithography [12], X-ray lithography [13], and soft lithography [14] can be used to fabricate 3D metallic PCs, these methods are expensive or limited to small areas or only several layers. Self-assembly of monodisperse microspheres has been demonstrated to be a simple, flexible and costeffective approach to fabricate 3D ordered structures including photonic crystals [15,16]. Based on colloidal self-assembly, various approaches have been reported to fabricate 3D metallic [17–21] or metallodielectric [22–23] photonic crystals. However, most of these metallic photonic crystals possess a metal inverse opal structure [17– 19], which has a relatively small filling ratio (0.26 for face-centered cubic structure). Although directly assembling of metallic colloidal
⁎ Corresponding author. Tel.: +65 67904595. E-mail address: [email protected] (C.C. Wong). 1 Current address: Department of Chemistry, Tsinghua University, China 100084. 0040-6090/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2009.03.094
particles provides an alternate to metallic photonic crystal, the difficulty to synthesize monodisperse metal colloidal particles with such range of size limits its wide implementations. Herein we report a simple and low-cost double templating strategy to fabricate large-area 3D metallic photonic crystals with an opal structure. A self-assembled polystyrene (PS) opal was used as the first template to fabricate a silica inverse opal on a gold-coated glass substrate via sol–gel process. The silica inverse opal was then employed as the secondary template, into which gold was infiltrated by electrochemical deposition. Highquality 3D gold opal was obtained after the silica inverse opal template was removed by wet chemical etching. 2. Experiment Fabrication of PS opal. Polystyrene spheres with size ranging from 200 nm to 800 nm were synthesized by using the emulsifier-free emulsion polymerization technique [24]. Glass substrates (microscope glass slides, 22 × 22 × 0.3 mm3, Marienfeld, Germany) were cleaned with a “Piranha” solution (a 3:1 volume ration mixture of concentrated sulfuric acid with 30% hydrogen peroxide) to obtain a hydrophilic surface. After rinsing with deionized water, the glass substrates were dried in a nitrogen gas flow before use. The colloidal crystal film was fabricated at 35 °C by using a flow-controlled vertical deposition method [25]. Aqueous PS colloids with a volume concentration of 1–2% were employed. The as-prepared PS opal film was then transferred to a gold-coated glass substrate by using a recently developed layer transfer technique [26], followed by annealing at 100 °C for 5 min. Fabrication of silica inverse opal. Silica inverse opal was prepared via sol–gel processing. A silica precursor solution was prepared by mixing tetraethylorthosilicate (98%, Acros Organics), ethanol (99.95%, Aldrich), and 0.1 M HCl (Merck) solution (volume ratio = 1:10:1)
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Fig. 1. Schematic illustration of the fabrication process of 3D gold opal.
under stirring for 4 h. Then the solution was infiltrated into the PS opals using a spin coater (Laurell, WS-400B-6NPP/LITE) operating at 1200 rpm for 15 s, followed by drying at room temperature for 4 h. Six to nine cycles of infiltration and drying were performed to achieve a full infiltration. Subsequently, the PS opal template was removed by heating at 450 °C in air for 5 h with a temperature ramp rate of 1 °C/ min. Electrochemical deposition of gold. Backfilling of metal into the silica inverse opal was achieved by using electrochemical deposition. For gold deposition, the above silica inverse opal was dipped into a gold plating solutions (Techni-Gold 25 ES, Technic, Inc.) with a platinum mesh as a counter electrode. Electrochemical plating was carried out under a constant current density (1.6, 3.2 or 4.8 mA/cm2). After a desired thickness was achieved, the gold-infiltrated silica inverse opal was immersed in diluted HF solution (1 wt. %) for 2 h to remove the silica inverse opal template together with the glass substrate, leaving a free-standing gold opal film. Characterization. Microstructures of the PS opal, silica inverse opal and gold opal films fabricated above were imaged with a field emission scanning electron microscope (FESEM, JEOL JSM-6340F). The optical photographs were captured using a digital camera (Nikon Coolpix3700). 3. Results and discussion 3.1. Fabrication of gold opals The fabrication process is schematically illustrated in Fig. 1. A final free-standing gold 3D opal film floating on the surface of water can be seen in Fig. 2. In this work, PS colloidal spheres with three different
sizes (328 nm, 498 nm, and 777 nm) were used to fabricate the first template. It was found out that conventional convective self-assembly does not work well on gold-coated glass substrates because of the poor wettability of the substrates. Therefore, colloidal crystal films were first grown on hydrophilic glass substrates. Then, using our recently developed layer transfer technique [26], PS opal films grown on hydrophilic glass substrates were easily transferred onto a conductive gold-coated glass substrate. As an example, SEM images in Fig. 3a,b shows the top-view and cross-sectional view of the PS opal film with sphere size of 498 nm fabricated by using the improved vertical deposition method [25]. It can be seen clearly that the colloidal spheres were organized into an ordered close-packed facecentered cubic (fcc) structure with the (111) planes parallel to the substrate. Once transferred onto a gold-coated glass substrate, annealing was applied to the PS opal film to enhance its mechanical integrity. More importantly, annealing causes sintering, whereby the contacts between the neighboring spheres, as well as those between the bottom-layer spheres and the gold substrate, grow in area. These contacts or sinter-necks are preserved as open channels after sol–gel infiltration processing of the silica and removal of PS sphere templates, making the subsequent gold electrochemical deposition feasible. After infiltrating with the silica precursor and burning out the PS sphere template, silica inverse opals with “air” spheres interpenetrating each other were formed on the gold substrates, as seen in Fig. 3c,d (templated from PS 498 nm opal). Here it should be pointed out that the diameters of the interconnecting pores between the air spheres are dependent on not only the diameter of PS template spheres but also the sintering conditions (temperature and time). Fig. 3e and f shows the gold opal fabricated by using the silica inverse opal (Fig. 3c and d) as the secondary template. Ordered and interconnected gold spheres inheriting the original PS opal fcc structure were observed. The diameter of the gold spheres was about 446 nm and the size distribution remained remarkably narrow after the double templating process. The size of the gold spheres is nearly identical with that of the air spheres of the silica inverse opal. Comparing with the original PS template spheres (498 nm), the gold spheres shrank about 10% in diameter, which was likely caused by the alkoxide hydrolysis condensation during the first templating. Nevertheless, the long-range ordering of the gold spheres is obvious. Fig. 4a and b show the gold opal fabricated by using the PS opal with sphere sizes of 328 nm and 777 nm as the first template, respectively. Similar interconnected gold spheres with an fcc crystalline structure is seen. 3.2. Growth mechanism of inter-connected gold spheres
Fig. 2. An optical photograph of a free-standing gold opal film floating on water surface.
Two strategies have been widely used to backfill a porous template, i.e. dry physical deposition (chemical vapor deposition or atom layer deposition) and wet chemical method (sol–gel, polymerization, or
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Fig. 3. SEM images of (a) and (b) a PS opal (sphere size 498 nm), (c) and (d) a silica inverse opal, and (e) and (f) a gold opal. (a)(c)(e) are top views and (b)(d)(f) are cross-sectional views. It should be noted that the silica inverse opal in (c) and (d) and the gold opal in (e) and (f) are fabricated from PS opals with different thickness as shown in (a) and (b).
electrochemical deposition). For dry physical deposition methods, it is challenging to completely fill a complex porous template like an inverse opal. The reason is that the narrowest pore channels tend to pinch off before the interstitial voids are fully filled [27]. In contrast, wet chemical methods are able to achieve maximal filling when inverse opals are used as templates. Among various strategies of wet chemical infiltration, electrochemical deposition is unique in its ability to completely fill the voids of the template with the plated material. No shrinkage happens during the removal of the template. More importantly, electrochemical deposition allows precise control over the thickness of the infiltrated material due to its bottom-up growth mechanism. The bottom-up growth of gold was confirmed by checking the cross-section of a gold-infiltrated silica inverse opal during electrochemical deposition. As an example, Fig. 5a shows a cross-sectional
SEM image of a gold-partially-infiltrated silica inverse opal templated from PS spheres of 498 nm. In the bottom gold-infiltrated region, interconnected gold spheres which fully fill the original air spheres within the silica inverse opal can be clearly seen. At the same time, some nodules and half spheres (highlighted by white and black arrows in Fig. 5a, respectively) can be observed at the interface between the unfilled and gold-infiltrated silica inverse opal. The gold nodules can be seen more clearly in a top-view SEM image of the gold opal obtained after removal of the silica inverse opal template, as highlighted by the white arrows in Fig. 5b. Three nodules appear on the surface of a sphere because the height of plated gold just approaches the position where the original air sphere interconnects with the upper three air spheres inside the silica inverse opal. With continued deposition, the sizes of the gold nodules increase and three nodules on three neighboring gold spheres gradually merge into one
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Fig. 6. A cross-sectional view of the crack area of a gold-partially-infiltrated silica inverse opal.
half sphere, the morphology of which is similar with that shown in Fig. 5c and d. Given time, another layer of gold spheres can be grown. These observations confirm the bottom-up growth of the gold opal.
3.3. Thickness control of the gold opal
Fig. 4. SEM images of gold opals templated from (a) PS opal with sphere size of 328 nm and (b) PS opal with sphere size of 777 nm.
3.3.1. Thickness uniformity Electrochemical deposition is a reliable approach for preparing gold films of uniform thickness, as proven in the microelectronics industry. However, when it is used to fill gold into a complex porous structure like a silica inverse opal in our case, it was found that the thickness uniformity of the resultant gold opal was affected by the
Fig. 5. (a) Cross-section view of a gold-partially-infiltrated silica inverse opal, (b) top view of the gold opal in (a) after removal of the silica template (1.6 mA/cm− 2, 25 min), (c) and (d) SEM images of a gold opal with less gold infiltration time (1.6 mA/cm− 2, 13 min). The white arrows in (a) and (b) highlight the presence of gold nodules. The black arrows in (a) highlight the formation of gold semispheres.
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deposition is limited by electrolyte diffusion rather than the reaction rate at the growth site [28]. At the same time, one may observe that the gold deposition rate is higher for silica inverse opals templated from larger PS colloidal spheres. We believe larger PS spheres result in silica inverse opals with larger pores which facilitate the transport of the electrolyte, leading to a higher gold growth rate. Fig. 7b shows the thickness of the gold opal obtained at different current densities. The deposition time for gold in silica inverse opals templated from PS 328 nm, 498 nm, 777 nm are 20 min, 13 min, and 25 min, respectively. It is obvious that the gold growth rate is higher under a larger current density. Similarly, larger PS spheres resulted in silica inverse opals with a more open structure. Therefore electrolyte diffusion in silica inverse opals templated from larger PS spheres is much easier, resulting in a higher gold growth rate. 4. Conclusions A double templating strategy based on colloidal self-assembly has been demonstrated for fabricating large-area three-dimensional gold opal photonic crystals of controlled thickness. A polystyrene colloidal crystal was used as the first template to fabricate a silica inverse opal on a gold-coated glass substrate via sol–gel processing. The silica inverse opal was then employed as the secondary template to backfill with gold using electrochemical deposition. High-quality threedimensional gold opal photonic crystal was obtained after removing the silica inverse opal template. The morphology and bottom-up growth mechanism of the gold opals were revealed by means of SEM observation. The effects of template sphere size and current density on the gold deposition rate were investigated. The fabrication approach is feasible for other metallic opals as well due to the generality of the electrochemical deposition technique. The optical characterization of the gold opal is still under progress and these gold opals are expected to find applications in photonic crystals devices, sensors, metallic thermal emitters and other photovoltaic devices. Acknowledgment Fig. 7. (a) The thickness of gold opals of different deposition time at a fixed current density of 1.6 mA/cm− 2. (b) The thickness of gold opals when applied different current density. The deposition time for gold in silica inverse opals templated from PS 328 nm, 498 nm, 777 nm are 20 min, 13 min, and 25 min, respectively.
cracks introduced during templating process. Fig. 6 shows a crosssectional view of a silica inverse opal with a crack which was partially infiltrated with gold by electrochemical deposition. The dashed arrows in Fig. 6 highlight the gold-infiltrated part around the crack in a silica inverse opal. It can be observed that the thickness of the gold opal near the crack is larger than that of the area far from the crack. Most likely the cracks serve as high diffusivity paths for the transport of the gold electrolyte to maintain growth. Therefore the electrodeposition of gold is faster around the crack area, leading to a thickness variation in the resultant gold opal film. Nevertheless, our observation shows that the thickness of the gold opal far from the crack area is relatively uniform. 3.3.2. Thickness control Since the interconnected gold spheres grow in a bottom-up manner, one may fabricate gold opals of desired thickness by manipulating the electroplating conditions, even using a much thicker silica inverse opal template. We have investigated the effects of current density and template sphere size on the growth rate of the gold opals. Fig. 7a shows the thickness variation of the resultant gold opal film when applying different deposition time under a fixed current density of 1.6 mA/cm2. It can be seen that the gold growth rate is not linear but keeps decreasing with time. The nonlinear growth implies that gold
The authors thank the Singapore-MIT Alliance for financial support. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13]
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Contents lists available at ScienceDirect
Thin Solid Films j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / t s f
Three-dimensional metallic opals fabricated by double templating Qingfeng Yan a,b,c,1, Pavan Nukala d, Yet-Ming Chiang c, C.C. Wong b,⁎ a
Singapore-MIT Alliance, N3.2-01-36, 65 Nanyang Drive, Singapore 637460 School of Materials Science and Engineering, Nanyang Technological University, Nanyang Avenue, Singapore 639798 c Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA d Department of Metallurgical and Materials Engineering, Indian Institute of Technology Madras, Chennai, India 600036 b
a r t i c l e
i n f o
Available online 20 March 2009 Keywords: Metallic photonic crystal Self-assembly Colloidal crystal Template Electrochemical deposition
a b s t r a c t We report a simple and cost-effective double templating method for fabricating large-area three-dimensional metallic photonic crystals of controlled thickness. A self-assembled polystyrene opal was used as the first template to fabricate a silica inverse opal on a gold-coated glass substrate via sol–gel processing. Gold was subsequently infiltrated to the pores of the silica inverse opal using electrochemical deposition. A highquality three-dimensional gold photonic crystal was obtained after removal of the secondary template (silica inverse opal). The effects of template sphere size and deposition current density on the gold growth rate, and the resulting morphology and growth mechanism of the gold opal, were investigated. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Photonic crystals (PCs) are structured materials with spatially periodic variations in dielectric constant that possesses unique characteristics in the manipulation and control of the flow of light [1,2]. Compared to dielectric and semiconductor PCs, metallic photonic crystals have the advantages of very large full bandgaps that extends well into infrared and high reflectance from relatively small number of stacking layers due to the large refractive index of metals [3–5]. It was found that around the band-edge of a metallic photonic crystal, some interesting electromagnetic phenomena such as enhancement of absorption [6] can arise. Recently, metallic PCs have shown their potential in applications as efficient thermal emitters and photovoltaic devices [7–9]. However, the fabrication of three-dimensional (3D) metallic PCs remains problematic because of challenges in microfabrication at optical scales. Although lithography techniques such as photolithography [9–11], E-beam lithography [12], X-ray lithography [13], and soft lithography [14] can be used to fabricate 3D metallic PCs, these methods are expensive or limited to small areas or only several layers. Self-assembly of monodisperse microspheres has been demonstrated to be a simple, flexible and costeffective approach to fabricate 3D ordered structures including photonic crystals [15,16]. Based on colloidal self-assembly, various approaches have been reported to fabricate 3D metallic [17–21] or metallodielectric [22–23] photonic crystals. However, most of these metallic photonic crystals possess a metal inverse opal structure [17– 19], which has a relatively small filling ratio (0.26 for face-centered cubic structure). Although directly assembling of metallic colloidal
⁎ Corresponding author. Tel.: +65 67904595. E-mail address: [email protected] (C.C. Wong). 1 Current address: Department of Chemistry, Tsinghua University, China 100084. 0040-6090/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2009.03.094
particles provides an alternate to metallic photonic crystal, the difficulty to synthesize monodisperse metal colloidal particles with such range of size limits its wide implementations. Herein we report a simple and low-cost double templating strategy to fabricate large-area 3D metallic photonic crystals with an opal structure. A self-assembled polystyrene (PS) opal was used as the first template to fabricate a silica inverse opal on a gold-coated glass substrate via sol–gel process. The silica inverse opal was then employed as the secondary template, into which gold was infiltrated by electrochemical deposition. Highquality 3D gold opal was obtained after the silica inverse opal template was removed by wet chemical etching. 2. Experiment Fabrication of PS opal. Polystyrene spheres with size ranging from 200 nm to 800 nm were synthesized by using the emulsifier-free emulsion polymerization technique [24]. Glass substrates (microscope glass slides, 22 × 22 × 0.3 mm3, Marienfeld, Germany) were cleaned with a “Piranha” solution (a 3:1 volume ration mixture of concentrated sulfuric acid with 30% hydrogen peroxide) to obtain a hydrophilic surface. After rinsing with deionized water, the glass substrates were dried in a nitrogen gas flow before use. The colloidal crystal film was fabricated at 35 °C by using a flow-controlled vertical deposition method [25]. Aqueous PS colloids with a volume concentration of 1–2% were employed. The as-prepared PS opal film was then transferred to a gold-coated glass substrate by using a recently developed layer transfer technique [26], followed by annealing at 100 °C for 5 min. Fabrication of silica inverse opal. Silica inverse opal was prepared via sol–gel processing. A silica precursor solution was prepared by mixing tetraethylorthosilicate (98%, Acros Organics), ethanol (99.95%, Aldrich), and 0.1 M HCl (Merck) solution (volume ratio = 1:10:1)
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Fig. 1. Schematic illustration of the fabrication process of 3D gold opal.
under stirring for 4 h. Then the solution was infiltrated into the PS opals using a spin coater (Laurell, WS-400B-6NPP/LITE) operating at 1200 rpm for 15 s, followed by drying at room temperature for 4 h. Six to nine cycles of infiltration and drying were performed to achieve a full infiltration. Subsequently, the PS opal template was removed by heating at 450 °C in air for 5 h with a temperature ramp rate of 1 °C/ min. Electrochemical deposition of gold. Backfilling of metal into the silica inverse opal was achieved by using electrochemical deposition. For gold deposition, the above silica inverse opal was dipped into a gold plating solutions (Techni-Gold 25 ES, Technic, Inc.) with a platinum mesh as a counter electrode. Electrochemical plating was carried out under a constant current density (1.6, 3.2 or 4.8 mA/cm2). After a desired thickness was achieved, the gold-infiltrated silica inverse opal was immersed in diluted HF solution (1 wt. %) for 2 h to remove the silica inverse opal template together with the glass substrate, leaving a free-standing gold opal film. Characterization. Microstructures of the PS opal, silica inverse opal and gold opal films fabricated above were imaged with a field emission scanning electron microscope (FESEM, JEOL JSM-6340F). The optical photographs were captured using a digital camera (Nikon Coolpix3700). 3. Results and discussion 3.1. Fabrication of gold opals The fabrication process is schematically illustrated in Fig. 1. A final free-standing gold 3D opal film floating on the surface of water can be seen in Fig. 2. In this work, PS colloidal spheres with three different
sizes (328 nm, 498 nm, and 777 nm) were used to fabricate the first template. It was found out that conventional convective self-assembly does not work well on gold-coated glass substrates because of the poor wettability of the substrates. Therefore, colloidal crystal films were first grown on hydrophilic glass substrates. Then, using our recently developed layer transfer technique [26], PS opal films grown on hydrophilic glass substrates were easily transferred onto a conductive gold-coated glass substrate. As an example, SEM images in Fig. 3a,b shows the top-view and cross-sectional view of the PS opal film with sphere size of 498 nm fabricated by using the improved vertical deposition method [25]. It can be seen clearly that the colloidal spheres were organized into an ordered close-packed facecentered cubic (fcc) structure with the (111) planes parallel to the substrate. Once transferred onto a gold-coated glass substrate, annealing was applied to the PS opal film to enhance its mechanical integrity. More importantly, annealing causes sintering, whereby the contacts between the neighboring spheres, as well as those between the bottom-layer spheres and the gold substrate, grow in area. These contacts or sinter-necks are preserved as open channels after sol–gel infiltration processing of the silica and removal of PS sphere templates, making the subsequent gold electrochemical deposition feasible. After infiltrating with the silica precursor and burning out the PS sphere template, silica inverse opals with “air” spheres interpenetrating each other were formed on the gold substrates, as seen in Fig. 3c,d (templated from PS 498 nm opal). Here it should be pointed out that the diameters of the interconnecting pores between the air spheres are dependent on not only the diameter of PS template spheres but also the sintering conditions (temperature and time). Fig. 3e and f shows the gold opal fabricated by using the silica inverse opal (Fig. 3c and d) as the secondary template. Ordered and interconnected gold spheres inheriting the original PS opal fcc structure were observed. The diameter of the gold spheres was about 446 nm and the size distribution remained remarkably narrow after the double templating process. The size of the gold spheres is nearly identical with that of the air spheres of the silica inverse opal. Comparing with the original PS template spheres (498 nm), the gold spheres shrank about 10% in diameter, which was likely caused by the alkoxide hydrolysis condensation during the first templating. Nevertheless, the long-range ordering of the gold spheres is obvious. Fig. 4a and b show the gold opal fabricated by using the PS opal with sphere sizes of 328 nm and 777 nm as the first template, respectively. Similar interconnected gold spheres with an fcc crystalline structure is seen. 3.2. Growth mechanism of inter-connected gold spheres
Fig. 2. An optical photograph of a free-standing gold opal film floating on water surface.
Two strategies have been widely used to backfill a porous template, i.e. dry physical deposition (chemical vapor deposition or atom layer deposition) and wet chemical method (sol–gel, polymerization, or
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Fig. 3. SEM images of (a) and (b) a PS opal (sphere size 498 nm), (c) and (d) a silica inverse opal, and (e) and (f) a gold opal. (a)(c)(e) are top views and (b)(d)(f) are cross-sectional views. It should be noted that the silica inverse opal in (c) and (d) and the gold opal in (e) and (f) are fabricated from PS opals with different thickness as shown in (a) and (b).
electrochemical deposition). For dry physical deposition methods, it is challenging to completely fill a complex porous template like an inverse opal. The reason is that the narrowest pore channels tend to pinch off before the interstitial voids are fully filled [27]. In contrast, wet chemical methods are able to achieve maximal filling when inverse opals are used as templates. Among various strategies of wet chemical infiltration, electrochemical deposition is unique in its ability to completely fill the voids of the template with the plated material. No shrinkage happens during the removal of the template. More importantly, electrochemical deposition allows precise control over the thickness of the infiltrated material due to its bottom-up growth mechanism. The bottom-up growth of gold was confirmed by checking the cross-section of a gold-infiltrated silica inverse opal during electrochemical deposition. As an example, Fig. 5a shows a cross-sectional
SEM image of a gold-partially-infiltrated silica inverse opal templated from PS spheres of 498 nm. In the bottom gold-infiltrated region, interconnected gold spheres which fully fill the original air spheres within the silica inverse opal can be clearly seen. At the same time, some nodules and half spheres (highlighted by white and black arrows in Fig. 5a, respectively) can be observed at the interface between the unfilled and gold-infiltrated silica inverse opal. The gold nodules can be seen more clearly in a top-view SEM image of the gold opal obtained after removal of the silica inverse opal template, as highlighted by the white arrows in Fig. 5b. Three nodules appear on the surface of a sphere because the height of plated gold just approaches the position where the original air sphere interconnects with the upper three air spheres inside the silica inverse opal. With continued deposition, the sizes of the gold nodules increase and three nodules on three neighboring gold spheres gradually merge into one
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Fig. 6. A cross-sectional view of the crack area of a gold-partially-infiltrated silica inverse opal.
half sphere, the morphology of which is similar with that shown in Fig. 5c and d. Given time, another layer of gold spheres can be grown. These observations confirm the bottom-up growth of the gold opal.
3.3. Thickness control of the gold opal
Fig. 4. SEM images of gold opals templated from (a) PS opal with sphere size of 328 nm and (b) PS opal with sphere size of 777 nm.
3.3.1. Thickness uniformity Electrochemical deposition is a reliable approach for preparing gold films of uniform thickness, as proven in the microelectronics industry. However, when it is used to fill gold into a complex porous structure like a silica inverse opal in our case, it was found that the thickness uniformity of the resultant gold opal was affected by the
Fig. 5. (a) Cross-section view of a gold-partially-infiltrated silica inverse opal, (b) top view of the gold opal in (a) after removal of the silica template (1.6 mA/cm− 2, 25 min), (c) and (d) SEM images of a gold opal with less gold infiltration time (1.6 mA/cm− 2, 13 min). The white arrows in (a) and (b) highlight the presence of gold nodules. The black arrows in (a) highlight the formation of gold semispheres.
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deposition is limited by electrolyte diffusion rather than the reaction rate at the growth site [28]. At the same time, one may observe that the gold deposition rate is higher for silica inverse opals templated from larger PS colloidal spheres. We believe larger PS spheres result in silica inverse opals with larger pores which facilitate the transport of the electrolyte, leading to a higher gold growth rate. Fig. 7b shows the thickness of the gold opal obtained at different current densities. The deposition time for gold in silica inverse opals templated from PS 328 nm, 498 nm, 777 nm are 20 min, 13 min, and 25 min, respectively. It is obvious that the gold growth rate is higher under a larger current density. Similarly, larger PS spheres resulted in silica inverse opals with a more open structure. Therefore electrolyte diffusion in silica inverse opals templated from larger PS spheres is much easier, resulting in a higher gold growth rate. 4. Conclusions A double templating strategy based on colloidal self-assembly has been demonstrated for fabricating large-area three-dimensional gold opal photonic crystals of controlled thickness. A polystyrene colloidal crystal was used as the first template to fabricate a silica inverse opal on a gold-coated glass substrate via sol–gel processing. The silica inverse opal was then employed as the secondary template to backfill with gold using electrochemical deposition. High-quality threedimensional gold opal photonic crystal was obtained after removing the silica inverse opal template. The morphology and bottom-up growth mechanism of the gold opals were revealed by means of SEM observation. The effects of template sphere size and current density on the gold deposition rate were investigated. The fabrication approach is feasible for other metallic opals as well due to the generality of the electrochemical deposition technique. The optical characterization of the gold opal is still under progress and these gold opals are expected to find applications in photonic crystals devices, sensors, metallic thermal emitters and other photovoltaic devices. Acknowledgment Fig. 7. (a) The thickness of gold opals of different deposition time at a fixed current density of 1.6 mA/cm− 2. (b) The thickness of gold opals when applied different current density. The deposition time for gold in silica inverse opals templated from PS 328 nm, 498 nm, 777 nm are 20 min, 13 min, and 25 min, respectively.
cracks introduced during templating process. Fig. 6 shows a crosssectional view of a silica inverse opal with a crack which was partially infiltrated with gold by electrochemical deposition. The dashed arrows in Fig. 6 highlight the gold-infiltrated part around the crack in a silica inverse opal. It can be observed that the thickness of the gold opal near the crack is larger than that of the area far from the crack. Most likely the cracks serve as high diffusivity paths for the transport of the gold electrolyte to maintain growth. Therefore the electrodeposition of gold is faster around the crack area, leading to a thickness variation in the resultant gold opal film. Nevertheless, our observation shows that the thickness of the gold opal far from the crack area is relatively uniform. 3.3.2. Thickness control Since the interconnected gold spheres grow in a bottom-up manner, one may fabricate gold opals of desired thickness by manipulating the electroplating conditions, even using a much thicker silica inverse opal template. We have investigated the effects of current density and template sphere size on the growth rate of the gold opals. Fig. 7a shows the thickness variation of the resultant gold opal film when applying different deposition time under a fixed current density of 1.6 mA/cm2. It can be seen that the gold growth rate is not linear but keeps decreasing with time. The nonlinear growth implies that gold
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