Optical properties of Au/Ag core/shell nanoshuttles

Optical properties of Au/Ag core/shell nanoshuttles

M. Li, Z. S. Zhang, X. Zhang, K. Y. Li and X. F. Yu*

Department of Physics, Key Laboratory of Acoustic and Photonic Materials and Devices of Ministry of Education, Wuhan University, Wuhan, 430072, People’s Republic of China

*Corresponding author: [email protected]

Abstract: Au/Ag nanoshuttles with sharp tips at both ends have been synthesized in glycine solution by chemically depositing silver on gold nanorods. Strong local field in the Au/Ag nanoshuttles enhanced by longitudinal surface plasmon resonance (LSPR) were investigated by theoretical calculations and experimental measurements. At the corresponding LSPR wavelengths, the extinction cross section and nonlinear refraction of the Au/Ag nanoshuttles are about 1.5 and 8.0 times of those of the original Au nanorods, respectively.

©2008 Optical Society of America

OCIS codes: (160.3900) Metals; (240.6680) Surface plasmons; (190.7110) Ultrafast nonlinear optics.

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#97680 - $15.00 USD Received 24 Jun 2008; revised 8 Aug 2008; accepted 26 Aug 2008; published 28 Aug 2008

(C) 2008 OSA 1 September 2008 / Vol. 16, No. 18 / OPTICS EXPRESS 14288

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1. Introduction

Surface plasmon resonance (SPR) excitation within the gold and silver nanostructures greatly enhances the local electric field [1-3]. This intense local field has been used to enhance the surface Raman scattering [4-7] and multiphoton luminescence [8-11], which have important applications in nanospectroscopy, biosensors and tissue imaging [12-15]. The frequency and intensity of SPR can be tuned by controlling the composition, size and shape of the nanostructures. It’s well known that the gold nanorods (Au NRs) and Ag nanobars have transverse and longitudinal SPRs (TSPR and LSPR), and their LSPR peaks can be easily tuned to the near-infrared (NIR) region for biological applications [13-15].

In general, the field enhancement can be significantly improved by the nanostructures with sharp corners [3,16,17], which motivated the studies on the synthesis and optical properties of metal nanocubes [18] and nanobipyramids [19,20]. Recently, the laser polarization dependent nonlinear field enhancement of single Ag nanobars and nanorice was also reported [5]. On the other hand, the composite metallic nanostructures with multiple elements are of significant interests from both nanotechnological and scientific points of view for improving catalytic activity and optical properties [21-23]. Especially, the bimetallic Au/Ag nanostructures have aroused much attention very recently due to their special optical properties for plasmon applications [24,25].

Here we report the synthesis of a novel Au/Ag nanostructure with sharp tips at both ends. Such nanostructure looks like the shuttle that pulls the thread of weft between the threads of warp, so we call it nanoshuttle [26]. The Au/Ag nanoshuttles have blue-shifted extinction peak and larger extinction cross section in comparison to the original Au NRs. The great field enhancement of the nanoshuttles was comparatively investigated by theoretical calculations of



field distribution and experimental measurements on the nonlinear absorption and refraction. Though other “sharp” metal nanostructures also generate great localized field enhancement, these Au/Ag nanoshuttles can be easily adjusted the LSPR peak by changing the aspect ratio of the original Au NRs.

2. Synthesis and characterization

The original Au NRs were prepared through the seed-mediated growth method reported by El-Sayed et al. previously [27]. To synthesize Au/Ag nanoshuttles, 1 mL of Au NRs (2.9 pmol) were mixed with 1 mL of 0.2 M glycine acid and 30 µL of 2 M NaOH. After adding with 16 µL of 0.1 M AgNO3, the mixture were incubated at room temperature without stirring for 10 h. Finally, the solution was subjected to three centrifuge/wash cycles to stop the growth. We noted that previous studies also demonstrated that the silver atoms could be deposited onto the Au NRs to result in the formation of various Au/Ag core/shell nanostructures, such as I-shaped, corn-shaped, and dumbbell-shaped [28,29]. In our experiment, the Au/Ag nanoshuttles were efficiently shaped by controlling the concentration of Ag+ (0.78 mM), the pH (9-10) and the incubation time (around 10 h), and the sharpness of the tips could be controlled by just adjusting the incubation time. The transmittance electron microscopy (TEM) and high resolution TEM (HRTEM) images were measured with a JEOL 2010 HT and JEOL 2010 FET transmission electron microscope, respectively (operated at 200 kV). The extinction spectra were measured by using a Varian Cary 5000 UV-Vis-NIR spectrophotometer.

The TEM images in Fig. 1 clearly show that the Au/Ag nanoshuttles have sharp tips at both ends in comparison to the original Au NRs. The average height and width of the Au NRs measured from their TEM images including 110 particles (not shown) are 31±3 nm and 6.4±1 nm, respectively. The thickness of the silver shell of the Au/Ag nanoshuttle is about 1.6 nm. The HRTEM image shown in Fig. 1(c) reveals that the Ag atoms here prefer grow on the {111} facet of the Au NRs and consequently form sharp tips. In general, the competition of the overgrowth on different surface facets of the Au NRs results in highly anisotropic overgrowth pattern [29]. Relative to the {111} and {100} surfaces, the {110} facets of the Au NRs are less accessible for the overgrowth due to the stronger interactions with CTAB. This anisotropic overgrowth leads into the formation of the Au/Ag nanoshuttles.

Fig. 1. (a), (b) TEM images of Au NRs and Au/Ag nanoshuttles, respectively. (c) HRTEM image of one Au/Ag nanoshuttle.

3. Extinction cross section

We calculated the extinction spectra of the averagely sized Au NRs and Au/Ag nanoshuttles under longitudinally polarized light by using discrete-dipole approximation (DDA) method (Fig. 2(a)). In our calculation, the dielectric functions of gold and silver were generated from Ref. 30 and the medium around the NRs was assumed to have a refractive index of 1.33 corresponding to that of water. Compared with the corresponding experimental extinction spectra shown in Fig. 2(b), the tendency of the extinction peak in the NIR region are precisely the same. In both the calculated and experimental spectra, the LSPR peak of the Au NRs is about 875 nm, and that of the Au/Ag nanoshuttles blue-shifts to about 755 nm, which is



attributed to the silver coating [24]. Moreover, the extinction cross section at LSPR of the nanoshuttles increases about 50% comparing to that of the Au NRs, which indicates stronger local field enhancement in the nanoshuttles at the LSPR band. It should be noted that the experimental spectra show a broader LSPR band than calculated ones. The inhomogeneous broadening from the size and shape distribution is the main reason, while the possible contribution from additional scattering by boundaries should also be addressed [22].

Fig. 2. Calculated (a) and experimental (b) extinction spectra of Au NRs and Au/Ag nanoshuttles with insets giving TEM images, respectively.

4. Field enhancement

We further performed finite-difference time-domain (FDTD) calculations [31] on an averagely sized Au NR and Au/Ag nanoshuttle to evaluate their local electric field enhancements. Fig. 3(a) and 3(b) show that the maximal field enhancement of the Au/Ag nanoshuttle is about 5.1 times that of the Au NR. It is obvious that the tips of the nanoshuttle are much sharper than the ends of the NR, thus the local electric field enhancement associated with the nanoshuttle is much larger than that associated with the NR, according to the lightning-rod effect [1]. Such tip effect from other nanostructures with sharp tips has been investigated previously [3,19]. To further investigate the tip effect, we have also performed the FDTD calculations on an Au nanoshuttle with the average size of the Au/Ag sample (data not shown), almost the same enhancement can be obtained. The results make the nanoshuttles highly attractive for the applications in designing substrates for surface-enhanced Raman scattering with the added advantage that the hot spots are completely open to the surrounding medium in this geometry. Moreover, because the LSPR wavelength of the Au/Ag nanoshuttles can be easily adjusted to the NIR region, where absorbance of light by biological tissues is at a minimum, these nanoshuttles can potentially be used in deep tissue applications.



Fig. 3. Resonant enhancement of local fields obtained from FDTD calculations of the averagely sized Au NR (a) and Au/Ag nanoshuttle (b) at a mesh size of 0.2 nm. (c) Electric field enhancement profiles along the longitudinal direction for (a) Au NR and (b) Au/Ag nanoshuttle. The origin is at their centers. They are excited at their respective LSP peak wavelengths and polarization parallel to the long axis.

The electric field enhancements in the Au NRs and Au/Ag nanoshuttles can be indirectly

measured through third-order susceptibility χ(3). Further, the real and imaginary parts of χ(3)

are proportional to the nonlinear refraction (NLR) index γ and the nonlinear absorption (NLA) coefficient β with the relationships: Reχ(3) = 2n0

2ε0cγ, and Imχ(3) = n02ε0cλβ/2π, where n0 is the

linear refractive index, ε0 and c are the dielectric constant and the speed of light in the vacuum, respectively. The values of β and γ were measured by open- and closed-aperture Z-scans [32-35]. The laser pulses used in the Z-scan measurements were generated using a Ti: sapphire laser (Mira 900, Coherent) with a pulse width of 2.5 ps and a repetition rate of 76 MHz. The Au NRs and Au/Ag nanoshuttles were scanned around the waist of the focused Gaussian laser beam along Z axis and the irradiance on the samples varied as a function of Z position. The NLA coefficient β and the NLR index γ can be calculated from the normalized TOP and TCL by using the following relationships:

∑∞

= ++−

=0m

2320

2

0

)1()1(

)(

mzz

qT

m

m

OP (1)

( )( ) ( )( )19

41

20

20

00

++Δ+=

zzzz

zzTT OPCL

φ (2)

, where q0 = βI0Leff and effLkI00 γφ =Δ , I0 is the peak irradiance incident on the sample, Leff is

the effective thickness of the sample, k = 2π/λ, λ is the excitation wavelength, and z0 is the Rayleigh length of the Gaussian incident beam.

The measurements reveal that Reχ(3) >> Imχ(3) (shown in Fig. 4(a) and 4(b)) and the values of Reχ(3) reach the maxima at the excitation wavelength λexc(Au) = 860 nm and λexc(Au/Ag) = 760 nm (shown in Fig. 4(c)), which are very close to the corresponding LSPRs of the Au NRs and Au/Ag nanoshuttles, respectively. Fig. 4(a) and 4(b) also show the normalized open-aperture transmittance (TOP) and the closed-aperture transmittance normalized by the open-aperture transmittance (TCL/TOP) of the Au NRs and the Au/Ag nanoshuttles at the maximal χ(3). By using the above-mentioned two formulas, we get that γmax(Au) = -1.4×10-3 cm2/GW and χ(3)

max(Au) = 9.4×10-11 esu for Au NRs at λexc = 860 nm, and γmax(Au/Ag) = -1.1×10-2 cm2/GW and χ(3)

max(Au/Ag) = -7.2×10-10 esu for Au/Ag



nanoshuttles at λexc = 760 nm. Thus, χ(3)max(Au/Ag) is about 8 times that of χ(3)

max(Au). These results indicate that the field enhancement factor of the Au/Ag nanoshuttles is about 2 times that of the Au NRs. It should be noted that the different nanoshuttles and NRs in the Z-scan measurements were excited at arbitrary polarization of the light, while the polarization of the excitation was parallel to their long axis in FDTD calculations. Such factor induced the discrepancy between the Z-scan measurements and the FDTD calculations on the field enhancement.

Fig. 4. Enhanced third-order optical nonlinearity of Au NRs and Au/Ag nanoshuttles: (a), (b) Open- and closed-aperture Z-scans of Au NRs and Au/Ag nanoshuttles when their NLR reaches the maxima around the corresponding LSPRs (λexc(Au) = 860 nm, λexc(Au/Ag) = 760 nm). The scatter open circles are experimental data while the solid lines are the fitting curves by using the standard Z-scan theory. (c) The value of χ(3) vs the excitation wavelength.

5. Conclusion

In conclusion, the synthesized Au/Ag nanoshuttles have sharp tips at the both ends and their linear absorption is blue-shifted and enhanced. By using the FDTD simulations and experimental measurements on the NLR and NLA, the Au/Ag nanoshuttles exhibit stronger local field enhancements than the original Au NRs. Such bimetallic nanostructures are expected to find use in plasmonic applications such as biosensors and tissue imaging.

Acknowledgments

This work was supported by the Natural Science Foundation of China (10534030) and the National Program on Key Science Research (2006CB921500, 2007CB935300).



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Optical properties of Au/Ag core/shell nanoshuttles