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Lithography-free fabrication of large-area plasmonic nanostructures using colloidal gold nanoparticles
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Photonics and Nanostructures – Fundamentals and Applications 8 (2010) 131–139 www.elsevier.com/locate/photonics
Lithography-free fabrication of large-area plasmonic nanostructures using colloidal gold nanoparticles Hongmei Liu a, Xinping Zhang a,*, Zhihua Gao b a
College of Applied Sciences, Beijing University of Technology, 100 Ping Le Yuan, Chao Yang District, Beijing 100124, People’s Republic of China b College of Materials Science and Engineering, Beijing University of Technology, Beijing 100124, People’s Republic of China Received 15 October 2009; received in revised form 15 February 2010; accepted 22 February 2010 Available online 4 March 2010
Abstract We report simple and efficient fabrication of large-area gold nanostructures using solution-processible gold nanoparticles, where lithography and vacuum evaporation techniques are not involved in the fabrication processes. These gold nanoisland structures exhibit strong particle plasmon resonance that is characterized by optical extinction spectroscopy in the visible spectral range. The tunability of the optical response is realized by controlling the annealing temperature and by changing the concentration of the colloidal solutions of gold nanoparticles. This enables a low-cost route for exploiting new photonic devices, biosensors, and optoelectronic devices with localized field-enhancement. # 2010 Elsevier B.V. All rights reserved. Keywords: Lithography-free fabrication; Colloidal gold nanoparticles; Plasmonic nanostructures; Annealing temperature
1. Introduction Large-area metallic photonic crystals can be fabricated using solution-processible gold nanoparticles [1–3], which enables simple and efficient construction of plasmonic nanodevices that are important for sensors and optoelectronic applications [4,5]. Combination of the UV-laser interference lithography, solution-processed spin-coating, and the designed annealing processes is the central technique, where the gold has been confined completely into the one- or twodimensional photoresist grating grooves through selfassembly of the colloidal gold nanoparticles during the spin-coating process and further confinement of the molten gold in the annealing process. The success and
* Corresponding author. E-mail address: [email protected] (X. Zhang). 1569-4410/$ – see front matter # 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.photonics.2010.02.002
the advantages of this technique have been demonstrated by our previous fabrications and by the related applications [1,2,6–8]. However, in some applications, for example, in the plasmonic enhanced solar cell device [9–11] or in surface-enhanced Raman spectroscopy [12], the periodic distribution of the photonic structures are actually not necessary, instead, the gold nanostructures are simply required to provide the locally enhanced field due to the particle plasmon resonance. Thus, the lithography procedure may be bypassed, consequently simplifying significantly the fabrication procedures. Furthermore, without the limitation by the lithography technology, the dynamic range of the fabrication area of the plasmonic nanostructures can be considerably enlarged. In this paper, we report a very simple technique for the fabrication of large-area plasmonic nanoisland structures using colloidal gold nanoparticles, where the continuous gold film breaks into isolated nanoislands
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under high-temperature annealing and no lithographyrelated processes are required. By carefully controlling the annealing temperature and the concentration of the colloidal solution, the size (the diameter and height) of the gold nanoislands can be controlled and the corresponding particle plasmon resonance can be tuned in the visible spectral range. 2. Fabrication methods The gold nanoparticles were synthesized using the procedures as reported in Ref. [13]. However, hexanethiol instead of hexylthiol has been used as the functional ligands to cover the gold nanoparticles in this work, so that they can be dispersed very well in organic solvents like xylene and toluene. The length of the ligands is approximately 0.5 nm. Additionally, small modifications have been made to the processing of the aqueous phase. The gold nanoparticles have excellent uniformity and a mean diameter of about 5 nm with a distribution range from 2 to 8 nm, which is larger than that reported in Ref. [13]. In this work, the obtained gold nanoparticles were dissolved in xylene to produce gold colloid solution with a concentration as high as 100 mg/ ml. The gold nanoparticle colloidal solution is then spin-coated onto the fused silica or indium–tin-oxide glass substrate with an area of 10 mm 10 mm and a thickness of 1 mm at a speed of 2000 rpm for 30 s. The sample is finally annealed at different temperatures, so that the gold nanoparticles become melted and form nanostructures with different morphologies. A muffle furnace was used to perform the annealing process, where the samples were heated for 5 min before being cooled down to room temperature in air. 3. Dependence on the annealing temperature Fig. 1(a)–(f) shows the scanning electron microscopic (SEM) images of the samples of gold nanostructures that have been annealed at 200, 250, 300, 350, 450, and 550 8C, respectively. As can be observed, the completely isolated gold nanoisland structures form at an annealing temperature above 300 8C. Clearly, the size of the gold nanoislands increases in both the diameter and the height with increasing the annealing temperature further from the threshold temperature of 300 8C. This could have been based on the strong surface tension of the molten gold and the mechanism that the total interfacial energy could be minimized, which equals gliquid–air + gliquid– solid, where ‘‘liquid’’ and ‘‘solid’’ denote the molten gold and the substrate, respectively, gliquid–air and gliquid–
Fig. 1. (a)–(f) SEM images of the samples of gold nanostructures that have been fabricated using an annealing temperature of 200, 250, 300, 350, 450, and 550 8C, respectively. (g) The corresponding optical extinction spectra.
denote the interfacial energies between the molten gold and air and that between the molten gold and the substrate, respectively. To minimize this total interfacial energy, the molten gold tends to shape into spheres to reduce the contact area of the gold with both the substrate and the air. Fig. 1(g) shows the optical extinction spectra of the gold nanoislands at different
solid
H. Liu et al. / Photonics and Nanostructures – Fundamentals and Applications 8 (2010) 131–139
annealing temperatures, indicating that strong optical extinction due to the particle plasmon resonance of the gold nanoislands can be observed only when the sample is heated to temperature higher than 300 8C. The peaks of the optical extinction spectra are located at 558, 554, 564, and 556 nm for an annealing temperature of 300, 350, 450, and 550 8C, respectively, as shown in Fig. 1(g). However, we did not observe a monotonic increase of the peak wavelength of the optical extinction spectra of particle plasmon resonance as shown in Fig. 1(g). This is mainly due to the fact that the spectral position of particle plasmon resonance is strongly dependent on the shape and the aspect ratio (height/
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diameter) of the gold nanoislands, as well as on how the gold nanoislands contact the environmental medium. With increasing the annealing temperature, the shape, the mean diameter, and the mean height change differently, as will be demonstrated in the following sections, resulting in an unmonotonic shift of the resonance spectrum. Furthermore, higher annealing temperature allows melting of larger gold nanoparticles, thus, facilitates formation of larger gold nanoislands. The formation of fewer and larger gold nanoislands actually reduces the total surface area of the whole device, although the total surface area of each individual gold nanoisland is
Fig. 2. (a)–(c) Characterization of the samples of gold nanoisland structures that have been annealed at 350, 450, and 550 8C, respectively. Left panel: AFM height images; mid-panel: the cross-section images of gold that nanoislands corresponding to the green lines marked in the left panel; right panel: the statistic evaluation of the mean diameter of the gold nanoislands using the histograms produced by the Image-Pro1 Plus software from Media Cybernetics Inc.
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increased, which is allowed and enhanced at higher temperatures. Fig. 2(a)–(c) shows further characterization of the gold nanoisland structures that have been fabricated using an annealing temperature of 350, 450, and 550 8C, respectively. The left panel shows the atomic force microscopic (AFM) height images; the mid-panel evaluates the variation of the height of the gold nanoislands with changing the annealing temperature using the profile positioned by the green line marks on the left panel; whereas, the right panel evaluates the mean diameter of the gold nanoislands at different annealing
temperatures using the histograms produced by the Image-Pro1 Plus software from Media Cybernetics Inc. As can be measured approximately from Fig. 2, the gold nanoislands have a mean height of about 45, 55, and 65 nm and a mean diameter of about 60, 80, and 90 nm at an annealing temperature of 350, 450, and 550 8C, respectively. Thus, changing the annealing temperature enables tuning the size of the gold nanoislands and thus tuning the corresponding spectroscopic properties of particle plasmon resonance, which can be confirmed by the optical extinction spectra as shown in Fig. 1(g).
Fig. 3. (a)–(c) SEM (left panel) and AFM (mid-panel) images of Au nanoisland structures prepared on a fused silica glass substrate at the colloidal concentration of 60, 80, and 100 mg/ml, respectively, and at an annealing temperature of 450 8C. The corresponding statistic evaluation on the mean diameter of the gold nanoislands using the histograms is shown in the right panel.
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Furthermore, as the height of the gold nanoislands increases with increasing the annealing temperature, the intensity of the optical extinction spectra of particle plasmon resonance is enhanced, while the corresponding bandwidths become smaller. This indicates that larger gold nanoislands induce strong absorption and scattering of the incident light due to the particle plasmon resonance, and these nanostructures become more homogeneous at higher annealing temperatures. 4. Dependence on the concentration of the colloidal solution Experiments also showed a strong dependence of the size of the gold nanoislands on the concentration of the colloidal solution of gold nanoparticles. Fig. 3(a)–(c) shows the microscopic characterization of the gold nanoisland structures prepared on a fused silica glass substrate at a colloidal concentration of 60, 80, and 100 mg/ml, respectively, and at an annealing temperature of 450 8C, where a strong dependence of the size of
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the gold nanoislands on the concentration of the gold nanoparticle colloids can be observed. The left panel of Fig. 3 shows the SEM images of the gold nanoisland structures fabricated using different colloidal concentrations; the mid-panel shows the corresponding AFM images; whereas, the right panel evaluates the mean diameter of the gold nanoislands using the statistic histograms. Fig. 3 indicates convincingly that both the mean diameter and the mean height of the gold nanoislands increase with increasing the concentration of the gold nanoparticle colloid. A mean diameter of 35, 75, and 90 nm has been measured for gold nanoisland structures fabricated using a concentration of 60, 80, and 100 mg/ml, respectively. The corresponding mean height is approximately 20, 50, and 70 nm. Considering the size of the tip used in the tapping mode AFM measurements, the height of the Au nanoislands is actually larger than the measured value. As has been discussed above, the gold nanoislands tend to shape into spheres to minimize the total interfacial area and thus minimize the total interfacial energy. This is also
Fig. 4. (a) Optical extinction spectra of the gold nanoisland structures before (solid) and after (dashed) PMMA coating for the colloidal concentration of 40, 60, 80, and 100 mg/ml. (b) Comparison between the particle plasmon resonance of the four samples in (a) before (l1) and after (l2) PMMA coating with the modification due to the PMMA coating evaluated by Dl = l2 l1.
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confirmed by a faster increase in the height than in the diameter of the gold nanoislands. Increasing the concentration while keeping the spin-coating speed has actually increased the amount of gold that has been spin-coated onto the substrate. Thus, more gold needs to be held in each spherically shaped nanoisland and larger nanoislands are produced at higher concentration of the colloidal solution. The optical properties of the gold nanoislands that have been prepared with different concentrations of the colloidal suspensions are shown in Fig. 4(a) by the solid curves. An obvious red-shift of the optical extinction spectrum from 535, 539, 553, to 565 nm can be observed as the colloidal concentration increases from 40, 60, 80, to 100 mg/ml. Meanwhile, the corresponding amplitude of the optical extinction spectrum also increases from 0.13 to 0.35 OD. We attribute the redshift of the particle plasmon resonance to the increase of sizes of the gold nanoislands with increasing the colloidal concentration. The spectral position of the particle plasmon resonance depends not only on the shape and the height/diameter ratio of the gold nanoislands, but also on the dielectric constant of environmental materials. Although the mean height of the gold nanoislands increases faster than the mean diameter, which should have induced blue shift of the particle plasmon resonance, larger gold nanoislands have significantly increased the contact area with the glass substrate. Larger contact area with higher refractive index materials leads to larger red-shift of the plasmonic resonance spectrum. Thus, we have observed a red-shift of the extinction spectrum as a result of multifold effects. Therefore, the red-shift rate is even higher when the environmental medium has a higher refractive index, because the larger gold nanoislands have not only much larger contact area with the substrate but also much larger total surface area than the smaller ones. This can be confirmed by the experimental results shown in Fig. 4, where the nanoisland devices are coated with poly(methyl methacrylate) (PMMA) that has higher refractive index (1.49) than air. As shown by the dashed curves in Fig. 4(a), the peak of the extinction spectra shift to 566, 573, 589, and 604 nm for the samples that have been fabricated using the colloidal concentration of 40, 60, 80, and 100 mg/ml, respectively, and have been coated with PMMA. Meanwhile, the corresponding optical extinction increases from 0.3 to 0.55 OD. Fig. 4(b) shows a comparison between the tuning properties of the particle plasmon resonance of the 4 samples characterized in Fig. 4(a) before and after being coated with PMMA. Coating with PMMA has not only shift the
tuning range from 535–565 to 566–604 nm, but also induced a higher tuning rate as increasing the fabrication concentration of the colloidal solution, which is indicated by the open circles in Fig. 4(b) and is identified by the larger values of Dl with increasing the concentration of the colloidal solution. A series of experiments have been performed to investigate the full ranges of the allowed annealing temperatures and the possible colloidal concentrations for the successful fabrication of gold nanoisland structures. It has been found that the threshold or the lower limit of the annealing temperature of 300 8C for the formation of isolated nanoislands is very critical. However, the upper limit of the annealing temperature is actually determined by the material of the substrate in this work, because the glass substrate tends to become destroyed at a temperature higher than 600 8C. This is why we did not demonstrate any results with an annealing temperature above 550 8C. As for the colloidal concentration, we have even prepared a colloidal solution with a concentration as high as 200 mg/ml. However, we could spin-coat continuous and homogeneous films of gold nanoparticles within a concentration range from 20 to 140 mg/ml. If using the colloidal concentration as high as 160 mg/ml, the spincoated film of gold nanoparticles becomes obviously inhomogeneous and is not suitable for the fabrication of large-area gold nanostructures. Whereas, if using the colloidal concentration lower than 20 mg/ml, we can still produce gold nanoisland structures with excellent particle plasmon resonance that takes place in the visible spectral range. However, the density of the gold nanostructures is extremely low and the optical response becomes very weak. Thus, the produced devices may not be suitable for practical applications. Therefore, a concentration ranging from 40 to 120 mg/ ml is recommended for the large-area fabrication of excellent gold nanoisland structures with the optical response resonant in the visible spectral range. 5. Scattering by the gold nanoisland structures To understand better the physical meaning of the optical extinction induced by particle plasmon resonance of gold nanostructures, we investigate experimentally the scattering of light by the gold nanoislands. In the experiment, we have selected two samples that have been fabricated using an annealing temperature of 450 8C, where colloidal concentrations of 60 and 100 mg/ml have been used to prepare samples 1 and 2, respectively. Furthermore, sample 2 has been coated with PMMA, so that the peak wavelength of particle
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plasmon resonance of the gold nanoislands shifts to about 600 nm. Thus, we obtain largely different optical extinctions at 532 and 633 nm using sample 2. This is demonstrated in Fig. 5(a), which shows the optical
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extinction measurements on both samples 1 and 2. Using the small circles on the optical extinction spectra, we evaluate how these two samples behave differently in the optical extinction response at 532 and 633 nm.
Fig. 5. Light scattering effects by the gold nanoislands due to particle plasmon resonance: (a) Optical extinction spectra of samples 1 and 2 that have been prepared at an annealing temperature of 450 8C and the colloidal concentration of 60 and 100 mg/ml, respectively. Sample 2 is coated with PMMA so that the particle plasmon resonance becomes red-shifted to match the laser wavelength of about 633 nm. (b) Light scattering experiment using a pure glass substrate and a laser beam at 633 nm with an illustration of the geometry of the experimental setup: the laser beam is incident through a hole on the white screen onto the surface of the sample and is then reflected back through the same hole. The scattered light in the reflection space is collected by the white screen that is about 11 cm in front of the sample. (c) and (d) The light scattering patterns by sample 1 at 633 and 532 nm, respectively. (e) and (f) The light scattering patterns by sample 2 at 633 and 532 nm, respectively.
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The laser beam is incident onto the gold nanostructure device through a small hole on a large white screen. The laser light will experience scattering and absorption by the particle plasmon resonance of the gold nanoislands in addition to the reflection and transmission processes. The sample is positioned so that the reflected laser beam goes back again through the hole on the white screen. Thus, the scattering in the reflection can be observed with very little disturbance by the reflected laser beam. The white screen is about 11 cm far away from the sample in the reflection space. The corresponding geometry is also depicted schematically in Fig. 5(b). We have found that the scattering pattern in the reflection is less affected by other radiations from the laser source. For example, there is strong emission from the discharging tube in addition to the laser beam in a He–Ne laser. Therefore, we demonstrate the experimental results of the scattering patterns in the reflection space, as shown in Fig. 5(b)–(f). In the experiment, a power of 2 mW and a beam diameter of about 2 mm have been used for both the laser beams at 532 and 633 nm. In Fig. 5(b), a pure glass substrate is used to demonstrate that almost no scattering of the incident laser at 633 nm can be observed. Fig. 5(c) and (d) demonstrated the scattering patterns by sample 1 at 633 and 532 nm, respectively. As shown in Fig. 5(a), the amplitude of the optical extinction is less than 0.25 OD at the peak, therefore, very weak scattering to the light can be observed at both 532 and 633 nm. However, it can be resolved that the scattering at 532 nm is stronger than that at 633 nm when their respective scattering patterns on the white screen is compared. This agrees with the optical extinction characterization by the blue curve in Fig. 5(a). Fig. 5(e) and (f) shows the scattering patterns by sample 2 at 633 and 532 nm, respectively. Strong scattering of the light can be observed for both the red and the green laser beams. This can be understood by considering the much larger amplitude of the optical extinction, which is about 0.35 OD at 532 nm and 0.67 OD at 633 nm, as shown by the red curve in Fig. 5(a). However, the scattering at 633 nm is much stronger than that at 532 nm, which agrees well with the large difference in the optical extinction at these two wavelengths. It should be noted that all of the pictures in Fig. 5 have been taken under the same conditions, so that these pictures enable objective evaluations and convincing comparison of different scattering effects. We can conclude from Fig. 5 that the scattering of light should be taken into account in the evaluation on
the optical extinction of the gold nanoislands. In the optical extinction measurements, the detection head of the spectrometer (USB4000 from Ocean Optics), which consists of an optical fiber with a diameter of about 400 mm, is positioned about 20 cm away from the studied sample, meaning a considerably small acceptance angle. This implies possibly large contribution of light scattering to the optical extinction. As has been demonstrated theoretically [9,14], the scattering of light becomes dominant in the optical extinction of the plasmonic nanospheres with a diameter around 100 nm. This agrees well with our experimental observations as shown in Fig. 5. 6. Conclusions In conclusion, we demonstrate simple lithographyfree techniques for the fabrication of large-area plasmonic gold nanoisland structures. Changing the annealing temperature and the concentration of the colloidal solution can be used to tune the size of the gold nanoislands and to tune the plasmonic resonance. This simple spin-coating-plus-direct-annealing technique also lowers significantly the requirements on the uniformity of the gold nanoparticles, thus improving the efficiency and reproducibility of the fabrication processes and lowering further the costs of the resultant device. Acknowledgments We acknowledge the High-Tech Research and Development Program of China (2007AA03Z306), the Natural Science Foundation of China (10774011), the Project sponsored by SRF for ROCS, SEM, and the Beijing Educational Commission (KZ200810005004) for the financial support. References [1] X.P. Zhang, B.Q. Sun, R.H. Friend, H.C. Guo, D. Nau, H. Giessen, Nano Lett. 6 (2006) 651. [2] X.P. Zhang, B.Q. Sun, R.H. Friend, H.C. Guo, N. Tetreault, H. Giessen, R.H. Friend, Appl. Phys. Lett. 90 (2007) 133114. [3] X.P. Zhang, H.M. Liu, S.F. Feng, Nanotechnology 20 (2009) 425303. [4] J.C. Yang, J. Ji, J.M. Hogle, D.N. Larson, Nano Lett. 8 (2008) 2718. [5] E.B. Namdas, M.H. Tong, P. Ledochowitsch, S.R. Mednick, J.D. Yuen, D. Moses, A.J. Heeger, Adv. Mater. 21 (2009) 799. [6] X.P. Zhang, H.M. Liu, J.R. Tian, Y.R. Song, J.Y. Song, L. Wang, Nano Lett. 8 (2008) 2653.
H. Liu et al. / Photonics and Nanostructures – Fundamentals and Applications 8 (2010) 131–139 [7] X.P. Zhang, H.M. Liu, J.R. Tian, Y.R. Song, L. Wang, J.Y. Song, G.Z. Zhang, Nanotechnology 19 (2008) 285202. [8] X.P. Zhang, B.Q. Sun, J.M. Hodgkiss, R.H. Friend, Adv. Mater. 20 (2008) 4455. [9] K.R. Catchpole, A. Polman, Opt. Express 16 (2008) 21793. [10] A.J. Morfa, K.L. Rowlen, T.H. Reilly III, M.J. Romero, J. Lagemaat, Appl. Phys. Lett. 92 (2008) 013504. [11] D.M. Schaadt, B. Feng, E.T. Yu, Appl. Phys. Lett. 86 (2005) 063106.
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[12] A.M. Schwartzberg, C.D. Grant, A. Wolcott, C.E. Talley, T.R. Huser, R. Bogomolni, J.Z. Zhang, J. Phys. Chem. B 108 (2004) 19191. [13] M.J. Hostetler, J.E. Wingate, C.J. Zhong, J.E. Harris, R.W. Vachet, M.R. Clark, J.D. Londono, S.J. Green, J.J. Stokes, G.D. Wignall, G.L. Glish, M.D. Porter, N.D. Evans, R.W. Murray, Langmuir 14 (1998) 17. [14] P.K. Jain, K.S. Lee, I.H. El-Sayed, M.A. El-Sayed, J. Phys. Chem. B 110 (2006) 7238.
Photonics and Nanostructures – Fundamentals and Applications 8 (2010) 131–139 www.elsevier.com/locate/photonics
Lithography-free fabrication of large-area plasmonic nanostructures using colloidal gold nanoparticles Hongmei Liu a, Xinping Zhang a,*, Zhihua Gao b a
College of Applied Sciences, Beijing University of Technology, 100 Ping Le Yuan, Chao Yang District, Beijing 100124, People’s Republic of China b College of Materials Science and Engineering, Beijing University of Technology, Beijing 100124, People’s Republic of China Received 15 October 2009; received in revised form 15 February 2010; accepted 22 February 2010 Available online 4 March 2010
Abstract We report simple and efficient fabrication of large-area gold nanostructures using solution-processible gold nanoparticles, where lithography and vacuum evaporation techniques are not involved in the fabrication processes. These gold nanoisland structures exhibit strong particle plasmon resonance that is characterized by optical extinction spectroscopy in the visible spectral range. The tunability of the optical response is realized by controlling the annealing temperature and by changing the concentration of the colloidal solutions of gold nanoparticles. This enables a low-cost route for exploiting new photonic devices, biosensors, and optoelectronic devices with localized field-enhancement. # 2010 Elsevier B.V. All rights reserved. Keywords: Lithography-free fabrication; Colloidal gold nanoparticles; Plasmonic nanostructures; Annealing temperature
1. Introduction Large-area metallic photonic crystals can be fabricated using solution-processible gold nanoparticles [1–3], which enables simple and efficient construction of plasmonic nanodevices that are important for sensors and optoelectronic applications [4,5]. Combination of the UV-laser interference lithography, solution-processed spin-coating, and the designed annealing processes is the central technique, where the gold has been confined completely into the one- or twodimensional photoresist grating grooves through selfassembly of the colloidal gold nanoparticles during the spin-coating process and further confinement of the molten gold in the annealing process. The success and
* Corresponding author. E-mail address: [email protected] (X. Zhang). 1569-4410/$ – see front matter # 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.photonics.2010.02.002
the advantages of this technique have been demonstrated by our previous fabrications and by the related applications [1,2,6–8]. However, in some applications, for example, in the plasmonic enhanced solar cell device [9–11] or in surface-enhanced Raman spectroscopy [12], the periodic distribution of the photonic structures are actually not necessary, instead, the gold nanostructures are simply required to provide the locally enhanced field due to the particle plasmon resonance. Thus, the lithography procedure may be bypassed, consequently simplifying significantly the fabrication procedures. Furthermore, without the limitation by the lithography technology, the dynamic range of the fabrication area of the plasmonic nanostructures can be considerably enlarged. In this paper, we report a very simple technique for the fabrication of large-area plasmonic nanoisland structures using colloidal gold nanoparticles, where the continuous gold film breaks into isolated nanoislands
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under high-temperature annealing and no lithographyrelated processes are required. By carefully controlling the annealing temperature and the concentration of the colloidal solution, the size (the diameter and height) of the gold nanoislands can be controlled and the corresponding particle plasmon resonance can be tuned in the visible spectral range. 2. Fabrication methods The gold nanoparticles were synthesized using the procedures as reported in Ref. [13]. However, hexanethiol instead of hexylthiol has been used as the functional ligands to cover the gold nanoparticles in this work, so that they can be dispersed very well in organic solvents like xylene and toluene. The length of the ligands is approximately 0.5 nm. Additionally, small modifications have been made to the processing of the aqueous phase. The gold nanoparticles have excellent uniformity and a mean diameter of about 5 nm with a distribution range from 2 to 8 nm, which is larger than that reported in Ref. [13]. In this work, the obtained gold nanoparticles were dissolved in xylene to produce gold colloid solution with a concentration as high as 100 mg/ ml. The gold nanoparticle colloidal solution is then spin-coated onto the fused silica or indium–tin-oxide glass substrate with an area of 10 mm 10 mm and a thickness of 1 mm at a speed of 2000 rpm for 30 s. The sample is finally annealed at different temperatures, so that the gold nanoparticles become melted and form nanostructures with different morphologies. A muffle furnace was used to perform the annealing process, where the samples were heated for 5 min before being cooled down to room temperature in air. 3. Dependence on the annealing temperature Fig. 1(a)–(f) shows the scanning electron microscopic (SEM) images of the samples of gold nanostructures that have been annealed at 200, 250, 300, 350, 450, and 550 8C, respectively. As can be observed, the completely isolated gold nanoisland structures form at an annealing temperature above 300 8C. Clearly, the size of the gold nanoislands increases in both the diameter and the height with increasing the annealing temperature further from the threshold temperature of 300 8C. This could have been based on the strong surface tension of the molten gold and the mechanism that the total interfacial energy could be minimized, which equals gliquid–air + gliquid– solid, where ‘‘liquid’’ and ‘‘solid’’ denote the molten gold and the substrate, respectively, gliquid–air and gliquid–
Fig. 1. (a)–(f) SEM images of the samples of gold nanostructures that have been fabricated using an annealing temperature of 200, 250, 300, 350, 450, and 550 8C, respectively. (g) The corresponding optical extinction spectra.
denote the interfacial energies between the molten gold and air and that between the molten gold and the substrate, respectively. To minimize this total interfacial energy, the molten gold tends to shape into spheres to reduce the contact area of the gold with both the substrate and the air. Fig. 1(g) shows the optical extinction spectra of the gold nanoislands at different
solid
H. Liu et al. / Photonics and Nanostructures – Fundamentals and Applications 8 (2010) 131–139
annealing temperatures, indicating that strong optical extinction due to the particle plasmon resonance of the gold nanoislands can be observed only when the sample is heated to temperature higher than 300 8C. The peaks of the optical extinction spectra are located at 558, 554, 564, and 556 nm for an annealing temperature of 300, 350, 450, and 550 8C, respectively, as shown in Fig. 1(g). However, we did not observe a monotonic increase of the peak wavelength of the optical extinction spectra of particle plasmon resonance as shown in Fig. 1(g). This is mainly due to the fact that the spectral position of particle plasmon resonance is strongly dependent on the shape and the aspect ratio (height/
133
diameter) of the gold nanoislands, as well as on how the gold nanoislands contact the environmental medium. With increasing the annealing temperature, the shape, the mean diameter, and the mean height change differently, as will be demonstrated in the following sections, resulting in an unmonotonic shift of the resonance spectrum. Furthermore, higher annealing temperature allows melting of larger gold nanoparticles, thus, facilitates formation of larger gold nanoislands. The formation of fewer and larger gold nanoislands actually reduces the total surface area of the whole device, although the total surface area of each individual gold nanoisland is
Fig. 2. (a)–(c) Characterization of the samples of gold nanoisland structures that have been annealed at 350, 450, and 550 8C, respectively. Left panel: AFM height images; mid-panel: the cross-section images of gold that nanoislands corresponding to the green lines marked in the left panel; right panel: the statistic evaluation of the mean diameter of the gold nanoislands using the histograms produced by the Image-Pro1 Plus software from Media Cybernetics Inc.
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increased, which is allowed and enhanced at higher temperatures. Fig. 2(a)–(c) shows further characterization of the gold nanoisland structures that have been fabricated using an annealing temperature of 350, 450, and 550 8C, respectively. The left panel shows the atomic force microscopic (AFM) height images; the mid-panel evaluates the variation of the height of the gold nanoislands with changing the annealing temperature using the profile positioned by the green line marks on the left panel; whereas, the right panel evaluates the mean diameter of the gold nanoislands at different annealing
temperatures using the histograms produced by the Image-Pro1 Plus software from Media Cybernetics Inc. As can be measured approximately from Fig. 2, the gold nanoislands have a mean height of about 45, 55, and 65 nm and a mean diameter of about 60, 80, and 90 nm at an annealing temperature of 350, 450, and 550 8C, respectively. Thus, changing the annealing temperature enables tuning the size of the gold nanoislands and thus tuning the corresponding spectroscopic properties of particle plasmon resonance, which can be confirmed by the optical extinction spectra as shown in Fig. 1(g).
Fig. 3. (a)–(c) SEM (left panel) and AFM (mid-panel) images of Au nanoisland structures prepared on a fused silica glass substrate at the colloidal concentration of 60, 80, and 100 mg/ml, respectively, and at an annealing temperature of 450 8C. The corresponding statistic evaluation on the mean diameter of the gold nanoislands using the histograms is shown in the right panel.
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Furthermore, as the height of the gold nanoislands increases with increasing the annealing temperature, the intensity of the optical extinction spectra of particle plasmon resonance is enhanced, while the corresponding bandwidths become smaller. This indicates that larger gold nanoislands induce strong absorption and scattering of the incident light due to the particle plasmon resonance, and these nanostructures become more homogeneous at higher annealing temperatures. 4. Dependence on the concentration of the colloidal solution Experiments also showed a strong dependence of the size of the gold nanoislands on the concentration of the colloidal solution of gold nanoparticles. Fig. 3(a)–(c) shows the microscopic characterization of the gold nanoisland structures prepared on a fused silica glass substrate at a colloidal concentration of 60, 80, and 100 mg/ml, respectively, and at an annealing temperature of 450 8C, where a strong dependence of the size of
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the gold nanoislands on the concentration of the gold nanoparticle colloids can be observed. The left panel of Fig. 3 shows the SEM images of the gold nanoisland structures fabricated using different colloidal concentrations; the mid-panel shows the corresponding AFM images; whereas, the right panel evaluates the mean diameter of the gold nanoislands using the statistic histograms. Fig. 3 indicates convincingly that both the mean diameter and the mean height of the gold nanoislands increase with increasing the concentration of the gold nanoparticle colloid. A mean diameter of 35, 75, and 90 nm has been measured for gold nanoisland structures fabricated using a concentration of 60, 80, and 100 mg/ml, respectively. The corresponding mean height is approximately 20, 50, and 70 nm. Considering the size of the tip used in the tapping mode AFM measurements, the height of the Au nanoislands is actually larger than the measured value. As has been discussed above, the gold nanoislands tend to shape into spheres to minimize the total interfacial area and thus minimize the total interfacial energy. This is also
Fig. 4. (a) Optical extinction spectra of the gold nanoisland structures before (solid) and after (dashed) PMMA coating for the colloidal concentration of 40, 60, 80, and 100 mg/ml. (b) Comparison between the particle plasmon resonance of the four samples in (a) before (l1) and after (l2) PMMA coating with the modification due to the PMMA coating evaluated by Dl = l2 l1.
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confirmed by a faster increase in the height than in the diameter of the gold nanoislands. Increasing the concentration while keeping the spin-coating speed has actually increased the amount of gold that has been spin-coated onto the substrate. Thus, more gold needs to be held in each spherically shaped nanoisland and larger nanoislands are produced at higher concentration of the colloidal solution. The optical properties of the gold nanoislands that have been prepared with different concentrations of the colloidal suspensions are shown in Fig. 4(a) by the solid curves. An obvious red-shift of the optical extinction spectrum from 535, 539, 553, to 565 nm can be observed as the colloidal concentration increases from 40, 60, 80, to 100 mg/ml. Meanwhile, the corresponding amplitude of the optical extinction spectrum also increases from 0.13 to 0.35 OD. We attribute the redshift of the particle plasmon resonance to the increase of sizes of the gold nanoislands with increasing the colloidal concentration. The spectral position of the particle plasmon resonance depends not only on the shape and the height/diameter ratio of the gold nanoislands, but also on the dielectric constant of environmental materials. Although the mean height of the gold nanoislands increases faster than the mean diameter, which should have induced blue shift of the particle plasmon resonance, larger gold nanoislands have significantly increased the contact area with the glass substrate. Larger contact area with higher refractive index materials leads to larger red-shift of the plasmonic resonance spectrum. Thus, we have observed a red-shift of the extinction spectrum as a result of multifold effects. Therefore, the red-shift rate is even higher when the environmental medium has a higher refractive index, because the larger gold nanoislands have not only much larger contact area with the substrate but also much larger total surface area than the smaller ones. This can be confirmed by the experimental results shown in Fig. 4, where the nanoisland devices are coated with poly(methyl methacrylate) (PMMA) that has higher refractive index (1.49) than air. As shown by the dashed curves in Fig. 4(a), the peak of the extinction spectra shift to 566, 573, 589, and 604 nm for the samples that have been fabricated using the colloidal concentration of 40, 60, 80, and 100 mg/ml, respectively, and have been coated with PMMA. Meanwhile, the corresponding optical extinction increases from 0.3 to 0.55 OD. Fig. 4(b) shows a comparison between the tuning properties of the particle plasmon resonance of the 4 samples characterized in Fig. 4(a) before and after being coated with PMMA. Coating with PMMA has not only shift the
tuning range from 535–565 to 566–604 nm, but also induced a higher tuning rate as increasing the fabrication concentration of the colloidal solution, which is indicated by the open circles in Fig. 4(b) and is identified by the larger values of Dl with increasing the concentration of the colloidal solution. A series of experiments have been performed to investigate the full ranges of the allowed annealing temperatures and the possible colloidal concentrations for the successful fabrication of gold nanoisland structures. It has been found that the threshold or the lower limit of the annealing temperature of 300 8C for the formation of isolated nanoislands is very critical. However, the upper limit of the annealing temperature is actually determined by the material of the substrate in this work, because the glass substrate tends to become destroyed at a temperature higher than 600 8C. This is why we did not demonstrate any results with an annealing temperature above 550 8C. As for the colloidal concentration, we have even prepared a colloidal solution with a concentration as high as 200 mg/ml. However, we could spin-coat continuous and homogeneous films of gold nanoparticles within a concentration range from 20 to 140 mg/ml. If using the colloidal concentration as high as 160 mg/ml, the spincoated film of gold nanoparticles becomes obviously inhomogeneous and is not suitable for the fabrication of large-area gold nanostructures. Whereas, if using the colloidal concentration lower than 20 mg/ml, we can still produce gold nanoisland structures with excellent particle plasmon resonance that takes place in the visible spectral range. However, the density of the gold nanostructures is extremely low and the optical response becomes very weak. Thus, the produced devices may not be suitable for practical applications. Therefore, a concentration ranging from 40 to 120 mg/ ml is recommended for the large-area fabrication of excellent gold nanoisland structures with the optical response resonant in the visible spectral range. 5. Scattering by the gold nanoisland structures To understand better the physical meaning of the optical extinction induced by particle plasmon resonance of gold nanostructures, we investigate experimentally the scattering of light by the gold nanoislands. In the experiment, we have selected two samples that have been fabricated using an annealing temperature of 450 8C, where colloidal concentrations of 60 and 100 mg/ml have been used to prepare samples 1 and 2, respectively. Furthermore, sample 2 has been coated with PMMA, so that the peak wavelength of particle
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plasmon resonance of the gold nanoislands shifts to about 600 nm. Thus, we obtain largely different optical extinctions at 532 and 633 nm using sample 2. This is demonstrated in Fig. 5(a), which shows the optical
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extinction measurements on both samples 1 and 2. Using the small circles on the optical extinction spectra, we evaluate how these two samples behave differently in the optical extinction response at 532 and 633 nm.
Fig. 5. Light scattering effects by the gold nanoislands due to particle plasmon resonance: (a) Optical extinction spectra of samples 1 and 2 that have been prepared at an annealing temperature of 450 8C and the colloidal concentration of 60 and 100 mg/ml, respectively. Sample 2 is coated with PMMA so that the particle plasmon resonance becomes red-shifted to match the laser wavelength of about 633 nm. (b) Light scattering experiment using a pure glass substrate and a laser beam at 633 nm with an illustration of the geometry of the experimental setup: the laser beam is incident through a hole on the white screen onto the surface of the sample and is then reflected back through the same hole. The scattered light in the reflection space is collected by the white screen that is about 11 cm in front of the sample. (c) and (d) The light scattering patterns by sample 1 at 633 and 532 nm, respectively. (e) and (f) The light scattering patterns by sample 2 at 633 and 532 nm, respectively.
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The laser beam is incident onto the gold nanostructure device through a small hole on a large white screen. The laser light will experience scattering and absorption by the particle plasmon resonance of the gold nanoislands in addition to the reflection and transmission processes. The sample is positioned so that the reflected laser beam goes back again through the hole on the white screen. Thus, the scattering in the reflection can be observed with very little disturbance by the reflected laser beam. The white screen is about 11 cm far away from the sample in the reflection space. The corresponding geometry is also depicted schematically in Fig. 5(b). We have found that the scattering pattern in the reflection is less affected by other radiations from the laser source. For example, there is strong emission from the discharging tube in addition to the laser beam in a He–Ne laser. Therefore, we demonstrate the experimental results of the scattering patterns in the reflection space, as shown in Fig. 5(b)–(f). In the experiment, a power of 2 mW and a beam diameter of about 2 mm have been used for both the laser beams at 532 and 633 nm. In Fig. 5(b), a pure glass substrate is used to demonstrate that almost no scattering of the incident laser at 633 nm can be observed. Fig. 5(c) and (d) demonstrated the scattering patterns by sample 1 at 633 and 532 nm, respectively. As shown in Fig. 5(a), the amplitude of the optical extinction is less than 0.25 OD at the peak, therefore, very weak scattering to the light can be observed at both 532 and 633 nm. However, it can be resolved that the scattering at 532 nm is stronger than that at 633 nm when their respective scattering patterns on the white screen is compared. This agrees with the optical extinction characterization by the blue curve in Fig. 5(a). Fig. 5(e) and (f) shows the scattering patterns by sample 2 at 633 and 532 nm, respectively. Strong scattering of the light can be observed for both the red and the green laser beams. This can be understood by considering the much larger amplitude of the optical extinction, which is about 0.35 OD at 532 nm and 0.67 OD at 633 nm, as shown by the red curve in Fig. 5(a). However, the scattering at 633 nm is much stronger than that at 532 nm, which agrees well with the large difference in the optical extinction at these two wavelengths. It should be noted that all of the pictures in Fig. 5 have been taken under the same conditions, so that these pictures enable objective evaluations and convincing comparison of different scattering effects. We can conclude from Fig. 5 that the scattering of light should be taken into account in the evaluation on
the optical extinction of the gold nanoislands. In the optical extinction measurements, the detection head of the spectrometer (USB4000 from Ocean Optics), which consists of an optical fiber with a diameter of about 400 mm, is positioned about 20 cm away from the studied sample, meaning a considerably small acceptance angle. This implies possibly large contribution of light scattering to the optical extinction. As has been demonstrated theoretically [9,14], the scattering of light becomes dominant in the optical extinction of the plasmonic nanospheres with a diameter around 100 nm. This agrees well with our experimental observations as shown in Fig. 5. 6. Conclusions In conclusion, we demonstrate simple lithographyfree techniques for the fabrication of large-area plasmonic gold nanoisland structures. Changing the annealing temperature and the concentration of the colloidal solution can be used to tune the size of the gold nanoislands and to tune the plasmonic resonance. This simple spin-coating-plus-direct-annealing technique also lowers significantly the requirements on the uniformity of the gold nanoparticles, thus improving the efficiency and reproducibility of the fabrication processes and lowering further the costs of the resultant device. Acknowledgments We acknowledge the High-Tech Research and Development Program of China (2007AA03Z306), the Natural Science Foundation of China (10774011), the Project sponsored by SRF for ROCS, SEM, and the Beijing Educational Commission (KZ200810005004) for the financial support. References [1] X.P. Zhang, B.Q. Sun, R.H. Friend, H.C. Guo, D. Nau, H. Giessen, Nano Lett. 6 (2006) 651. [2] X.P. Zhang, B.Q. Sun, R.H. Friend, H.C. Guo, N. Tetreault, H. Giessen, R.H. Friend, Appl. Phys. Lett. 90 (2007) 133114. [3] X.P. Zhang, H.M. Liu, S.F. Feng, Nanotechnology 20 (2009) 425303. [4] J.C. Yang, J. Ji, J.M. Hogle, D.N. Larson, Nano Lett. 8 (2008) 2718. [5] E.B. Namdas, M.H. Tong, P. Ledochowitsch, S.R. Mednick, J.D. Yuen, D. Moses, A.J. Heeger, Adv. Mater. 21 (2009) 799. [6] X.P. Zhang, H.M. Liu, J.R. Tian, Y.R. Song, J.Y. Song, L. Wang, Nano Lett. 8 (2008) 2653.
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