J. Phys. Chem. C 2008, 112, 13917–13921
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Fabrication of Twin-Linked Gold Nanoparticles and Their Linear/Nonlinear Optical Properties Shigemasa Matsubara,† Tomokatsu Hayakawa,*,†,§ Yong Yang,† Masayuki Nogami,†,§ Sigeru Okamoto,‡,§ and Naokiyo Koshikawa§ Ceramic DiVision and Organic Materials DiVision, Department of Materials Science and Engineering, Nagoya Institute of Technology, Showa, Nagoya 466-8555, Japan, and ISS AdVanced Materials Research and Applications Center (ISAAC), International Space Station Applied Research Partnership Program, Japan Aerospace Exploration Agency (JAXA) ReceiVed: April 3, 2008; ReVised Manuscript ReceiVed: June 15, 2008
We developed a new method for the preparation of connected-spherical gold nanoparticles (GNPs), which exhibited surface plasmon resonance properties different from those of spherical GNPs. In this method, citrate-stabilized GNPs in spherical form were connected to each other by use of cetyltrimethylammonium bromide, and then the progressing connection of GNPs in solution was restrained by the coverage of thin silica layer on them. The connectivity of GNPs was analyzed by transmission electron microscopy images and optical absorption spectra. The modulus of the third-order nonlinear optical susceptibility of the twin-linked GNPs film was estimated as 1.61 × 10-9 esu (the real and imaginary parts were 1.55 × 10-9 and -4.24 × 10-10 esu, respectively). 1. Introduction The development of nonlinear optical materials with large third-order optical nonlinearities and ultrafast response times is technologically important in a number of future applications in optical telecommunications, data storage, optical computation, and information processing.1-4 It is well-known that nanometersized gold and silver particles embedded in dielectric mediums exhibit large third-order optical nonlinearities because of the local field enhancement near a wavelength of surface plasmon resonance (SPR). However, these large third-order optical nonlinear responses are observed only at their SPR wavelength and decrease drastically at wavelengths far from the SPR position,5,6 which is supposed to limit the above-mentioned applications of this kind of materials. A large optical nonlinearity and fast response speed at multiple wavelengths need to be investigated more to establish novel multimode optical systems. Recent developments in the chemical synthesis of metal nanostructures have exploited various shapes and connection of metal nanoparticles,7-11 but connected gold nanoparticles (GNPs) were unstable and easily changed time after time (or finally aggregated to be precipitated/clustered) in solution, while drying and/or when connected GNPs were fixed on substrates. Thus, we propose here a simple strategy for synthesizing twinlinked or more-connected gold nanoparticles and stabilizing the state of connectivity in solution. The linear optical properties can be changed over an extended wavelength range by connecting the particles. The twin-linked gold nanoparticles with silica thin coating were self-assembled on a glass substrate for the nonlinear optical characterizations. Furthermore, we found that greatly enhanced third-order nonlinear responses were observed at an extended wavelength over SPR. * To whom correspondence should be addressed. E-mail: hayatomo@ nitech.ac.jp. † Ceramic Division, Nagoya Institute of Technology. ‡ Organic Materials Division, Nagoya Institute of Technology. § JAXA.
Figure 1. Optical absorption spectrum and TEM image of GNP sol.
2. Experimental Section Gold colloids were prepared by the citrate thermal reduction method. One milliliter of 1 wt % HAuCl4 aqueous solution and 2 mL of 38.8 mM sodium citrate aqueous solution were added into 90 mL of boiling water. After that, the solution color turned wine red within 3 min. The citrate ions acted as both a reductant and a stabilizer. The solution was boiled for 7 min after we added sodium citrate and cooled quickly in an ice bath (referred to as GNP sol). This resulted in a control of reduction of HAuCl4 and a stable monodisperse GNP sol. Forty milliliters of as-prepared GNP sol was reacted with 350 µL of 0.1 mM cetyltrimethylammonium bromide (CTAB) aqueous solution at room temperature (referred to as GNP-1 sol).12 Thirty-six milliliters of as-prepared GNP sol reacted with 4 mL of 1 wt % HAuCl4 aqueous solution and 350 µL of 0.1 mM CTAB aqueous solution at room temperature (referred to GNP-2 sol). To cover GNPs with a thin silica layer, 1 mL of 0.02 vol % 3-mercaptopropyltrimethoxysilane (MPTMS) methanol solution and 1 mL of 0.54 wt % Na2SiO3 aqueous solution were added to the
10.1021/jp803091v CCC: $40.75 2008 American Chemical Society Published on Web 08/19/2008
13918 J. Phys. Chem. C, Vol. 112, No. 36, 2008
Figure 2. Optical absorption spectra (a) of GNP-1 sol with different aging time after the addition of CTAB. (b) Changes in the area of the spectrum of GNP-1 sol separated by Gaussian functions by least-squares fitting.
Figure 3. TEM image of GNP-1 sol reacted for 30 min and the GNP number distribution of connected GNPs in GNP-1 sol.
GNP-2 sol (reacted for 90 min).13 The silica-coated GNPs were self-assembled on a poly(diallyldimethylammoniumchloride) (PDDA)-modified glass substrate by immersing a soda-lime glass plate for 5 h in the solution.14 The absorption optical spectra of GNP sols and GNP film were recorded using a UV-vis-NIR spectrophotometer (Jasco, V-570), and the microstructures were observed with transmission electronic microscope (TEM; JEOL, JEM-2010HR) and atomic force microscope (AFM; SII, SPI3800N-SPA300HV). The thirdorder nonlinear optical susceptibilities of the film samples were measured by Z-scan technique,15,16 where the femtosecond pulse laser from a regenerative Ti:sapphire laser system (Spectra
Matsubara et al.
Figure 4. Optical absorption spectra (a) of GNP-2 sol with different aging times after the addition of CTAB. (b) Changes in the area of the spectrum of GNP-2 sol separated by Gaussian functions.
Figure 5. TEM image of GNP-2 sol reacted for 30 min and the GNP number distribution of connected GNPs in GNP-2 sol.
Physics, Harricane) operating at a wavelength of 805 nm with a 1 kHz repetition rate and approximately 170-fs pulse duration was used. The incident Gaussian beam had a diffraction length of zR ) 9.12 mm and a waist radius of ω0 ) 53 µm. The beam intensity I0 was typically 0.2 GW/cm2 at the focal position. Open and close Z-scan measurement using a femtosecond laser was applied to estimate third-order nonlinear optical susceptibilities for the film. A ZnSe plate was also examined as a reference to confirm the preciseness of our Z-scan measurement. 3. Results and Discussion 3.1. Linear Optical Properties and Connection of Spherical GNPs. Figure 1 shows the optical absorption spectrum and TEM image of monodisperse GNPs prepared without CTAB.
Twin-Linked GNPs and Their Optical Properties
Figure 6. Optical absorption spectra of GNP sol before and after silica coating. Broken lines show separated peaks by Gaussian functions.
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Figure 8. AFM images of GNP film. Section analysis data is also given for the solid line in the image.
Figure 7. Optical absorption spectrum of GNP film. Dotted lines show peaks separated by Gaussian functions.
The absorption spectrum has one narrow peak due to the SPR band of monodisperse GNPs at 519 nm. The particle shape is almost spherical, and the average diameter is 15.5 nm. Figure 2 shows optical absorption spectra of GNP-1 sol and the changes in the integrated intensities of the decomposed spectra, which were separated with Gaussian functions by a least-squares fitting. The dotted line shows an absorption spectrum of GNP sol before the addition of CTAB. The solid lines show absorption spectra of GNP-1 sol reacted for 30, 60, and 120 min after the CTAB addition. The new absorption peaks were prominent at 583 and 667 nm 30 and 60 min after the addition. These absorption spectra indicate that the monodisperse GNPs were connected and aggregated with increasing reaction time. The absorption peak at 583 nm was attributed to the SPR coupling band of twin-linked GNPs. The absorption peak at 667 nm was attributed to the SPR coupling band of connected GNPs over three particles and red-shifted/broadened with increasing reaction times. It indicates that GNPs were successively connected by CTAB with increasing reaction time. The area of the first peak at 519 nm decreased immediately after the addition of CTAB because single GNPs were connected via CTAB. The area of the second peak at 583 nm reached maximum value when GNP-1 sol reacted for 30 min. It seemed that the twinlinked GNPs were efficiently synthesized in the sol. Figure 3 shows TEM images of GNP-1 sol reacted for 30 min and the distribution of GNP number in connection for GNP-1 sol. The TEM image elucidates the presence of connected GNPs, and the average connection number of GNPs is 2.3 and the standard deviation is 3.1. In the case of GNP-1 sol reacted for 60 min,
Figure 9. Z-scan results of the twin-linked GNP film (a) data of open aperture and (b) data of close aperture divided by that of open aperture. The dotted line is the theoretical fit in both panels.
the average connection number of GNPs is increased to 2.6 and the standard deviation is 2.3. These show that the optimized reaction time is 30 min to prepare the twin-linked GNPs in GNP-1 sol. Figure 4 shows optical absorption spectra of GNP-2 sol and changes in the integrated intensities of the decomposed spectra, separated with Gaussian functions with different reaction times. The dotted line shows an absorption spectrum of GNP sol before the addition of CTAB. The solid lines show absorption spectra of GNP-2 sol reacted for 60, 90, and 120 min after the CTAB addition, where the new absorption peaks appeared at 573 and 670 nm after the reaction for 60 and 90 min, respectively. The
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TABLE 1: Third-Order Nonlinearities at 805 nm of Twin-Linked GNPs Self-Assembling Film nonlinear refractive index, γ/cm2 W-1
Re(χ(3))/esu
nonlinear absorption coefficient, β/cm W-1
Im(χ(3))/esu
third-order nonlinear susceptibility χ(3)/esu
(χ(3)/R)/esu cm
3.07 × 10-11
1.55 × 10-9
-1.32 × 10-6
-4.24 × 10-10
1.61 × 10-9
2.49 × 10-13
absorption peak at 573 nm was attributed to the SPR coupling band of twin-linked GNPs. The absorption peak at 670 nm was attributed to the SPR coupling band of connected GNPs over three particles and red-shifted with increasing reaction times. The area of the first peak at 524 nm was found to be constant until 60 min. When GNP-2 sol was reacted for 90 min after the addition of CTAB, the area of the first peak decreased suddenly and the second peak at 573 nm reached a maximum. It was presumed that the twin-linked GNPs were efficiently synthesized in GNP-2 sol. Figure 5 shows a TEM image of GNP-2 sol reacted for 90 min and the GNP number distribution in GNP-2 sol. The TEM image shows the presence of connected GNPs, and the average connection number of GNPs is 2.1 and the standard deviation is 1.3. It can be seen that the twin-linked GNPs were more efficiently synthesized in GNP-2 sol reacted for 90 min than that in GNP-1 sol. This difference was explained by the different role of CTAB in sols. In GNP-1 sol, when a few drops of CTAB were added, CTAB was selectively adsorbed on different crystal facets, (111), (100), or (110), of GNPs soon and GNPs were reacted gradually to be connected to each other. On the other hand, in GNP-2 sol, CTAB was combined with AuCl4- electrically,17 and they were used for the crystal growth in the particular crystal facet (111) of GNPs. AuCl4- was decreased by reduction, and then the connection occurred rapidly when GNP-2 sol reacted for 90 min. Thus, the fact that the crystal facet with high concentration of CTAB was formed caused the GNPs to connect in twin-linked formation. 3.2. Silica Coating on Twin-Linked GNPs. Figure 6 shows optical absorption spectra before and after the addition of MPTMS MeOH solution and Na2SiO3 aq. into GNP-2 sol reacted for 90 min. Before the addition (shown in dotted line), the absorption spectrum had two peaks at 522 and 578 nm. After the addition (referred to as GNP-3 sol), the two peaks disappeared while another absorption peak appeared around 540 nm. The new absorption peak was composed of these two peaks, and it was found that the first peak attributed to single GNPs at 522 nm was red-shifted to 537 nm. The second peak due to the SPR coupling band of twin-linked GNPs was found at 605 nm. These indicate that the twin-linked GNPs were coated with a silica layer, because a silica layer has a high refractive index. The spectral change of GNP-3 sol was not observed except the red-shift of the absorption peak due to silica coating. This means that the progressive connection of GNPs was restrained by silica coating. Figures 7 and 8 show an optical absorption spectrum and AFM images of GNP film deposited on a PDDA-modified glass substrate. The absorption spectrum of GNP film has one absorption band around 570 nm, composed of two peaks at 534 and 609 nm. The latter peak, due to the SPR coupling band of twin-linked GNPs, indicates the presence of twin-linked GNPs in the GNP film. Indeed, elliptic particles, which are the twinlinked GNPs coated with silica layer as analyzed in the cross section data, could be observed in the AFM image (see the particle marked with the white dotted line). Conclusively, for the first time to our knowledge, the twin-linked GNPs were selfassembly fixed on PDDA-modified glass substrate, which opens a new avenue for the development of nonlinear optical devices in nanoparticle plasmonics.
3.3. Nonlinear Optical Properties Measured in Z-Scan Technique. In Z-scan measurements using a femtosecond laser (λ ) 805 nm), sufficient Z-scan signal was observed for this film as shown in Figure 9a,b, even though no Z-scan signal was observed from silica-coated single-GNP film. The obtained signals for open (Figure 9a) and closed (Figure 9b) aperture Z-scan setting were analyzed with Sheik-Bahae et al.’s formulas15 to have nonlinear refractive index coefficient γ from Figure 9b and nonlinear absorption coefficient β from Figure 9a. These coefficients are defined by the following equations:
n ) n0 + γI
(1)
R ) R0 + βI
(2)
where n0 and R0 are linear refractive index and absorption coefficient, respectively. I is an incident power of an order of watts per square centimeter. The real and imaginary parts of the third-order nonlinear optical susceptibility χ(3) is evaluated from γ and β, respectively, using the relation defined as follows:18
Re[χ(3)(esu)] ) 10-4
ε0c2n02 γ(cm2 W-1) π
(3)
and
Im[χ(3)(esu)] ) 10-2
ε0c2n02λ 2
4π
β(cm W-1)
(4)
The linear refractive index n0 was 1.41. Finally, the modulus of the third-order nonlinear optical susceptibility χ(3) ) {[Re(χ(3))2 + Im(χ(3)]2}1/2 was estimated as 1.61 × 10-9 esu, as shown in Table 1. According to the literature,19-21 typical χ(3) value was of an order of 10-10 to 10-12 esu for uniformly dispersed gold nanoparticles in dielectric media. Recent studies reported higher χ(3) values of 10-9 esu4 and 10-7 esu22 in resonant wavelength with SPR at 532 nm. Most works derived such a high χ(3) in SPR wavelength from high-density packing of gold nanoparticles. Our work on GNP twins elucidates comparatively high χ(3) of 10-9 order even at the off-resonant wavelength of SPR due to interparticle coupling effect23 of gold nanoparticle twins in addition to the effect of high densification of gold nanoparticles, as shown in the inset in Figure 8. The novel finding is the negative Im(χ(3)), which means more transparency in strong irradiation of incident light.5,24 It must be a decomposition of coupled SPR mode in twin-linked GNPs in the 805-nm region. This effect will be discussed elsewhere in more detail. 4. Conclusions The twin-linked GNPs were prepared and stabilized in solution. Using HAuCl4 and CTAB, we efficiently prepared the twin-linked GNPs in GNP sol and found the optimized reaction time to be 90 min for GNP-2 sol. The excessive connection of connected-GNPs by CTAB was restrained by silica coating of GNPs with MPTMS MeOH solution and Na2SiO3 aq. The twinlinked GNPs coated with silica layer were successfully selfassembled on PDDA-modified glass substrate. The obtained third-order nonlinear susceptibilities were 1.55 × 10-9 and
Twin-Linked GNPs and Their Optical Properties -4.24 × 10-10 esu for the real and imaginary parts of χ(3), respectively, at 805 nm, which is an off-resonant wavelength of SPR. The modulus of χ(3) was 1.61 × 10-9 esu. Conclusively, it was found that this methodology with interparticle coupling GNPs (twins) would be useful to develop nonlinear optical devices applied in an extended spectral region from SPR wavelength of GNPs to near-infrared regions around 800 nm. Acknowledgment. A part of this work was performed as the International Space Station Applied Research Partnership Program of the Japan Aerospace Exploration Agency and Nagoya Institute of Technology. References and Notes (1) Hamanaka, Y.; Kuwabata, J.; Tanahashi, I.; Omi, S.; Nakamura, A. Phys. ReV. B 2001, 63, 104302. (2) Hayakawa, T.; Usui, Y.; Bharathi, S.; Nogami, M. AdV. Mater. 2004, 16, 1408–1412. (3) Selvan, S. T.; Hayakawa, T.; Nogami, M.; Kobayashi, Y.; LizMarza´n, L. M.; Hamanaka, Y.; Nakamura, A. J. Phys. Chem. B 2002, 106, 10157–10162. (4) Selvan, S. T.; Hayakawa, T.; Nogami, M.; Mo¨ller, M. J. Phys. Chem. B 1999, 103, 7441–7448. (5) Yang, Y.; Nogami, M.; Shi, J.; Chem, H.; Ma, G.; Tang, S. Appl. Phys. Lett. 2006, 88, 081110. (6) Uchida, K.; Kaneko, S.; Omi, S.; Hata, C.; Tanji, H.; Asahara, Y.; Tokizaki, T.; Nakamura, A.; Ikushima, A. J. J. Opt. Soc. Am. B 1994, 11, 1236–1243. (7) Sau, T. K.; Murphy, C. J. J. Am. Chem. Soc. 2004, 126, 8648– 8649. (8) Kim, F.; Connor, S.; Song, H.; Kuykendall, T.; Yang, P. Angew. Chem., Int. Ed. 2004, 3673–3677.
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