Surface plasmon resonance of gold nano-sea-urchin

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Surface plasmon resonance of gold nano-sea-urchin

Yen Hsun Su,

^a^兲

Wei Hao Lai,

^b^兲

Wei-Yu Chen,

^c^兲

and Min Hsiung Hon

^d^兲

Department of Materials Science and Engineering, National Cheng Kung University, Tainan 70101, Taiwan Shih-Hui Chang

^e兲

Institute of Electro-Optical Science and Engineering, National Cheng Kung University, Tainan 70101, Taiwan

共Received 30 November 2006; accepted 30 March 2007; published online 30 April 2007兲

The authors synthesized high-quality gold nano-sea-urchin in aqueous solution with an environment-friendly method. They found that the gold nano-sea-urchin can induce the interaction of surface plasmon resonance 共SPR兲 mode with substrate. The SPR peak splits and blueshifts from 630 to 440 nm and the result has potential application for enhanced-Raman scattering, optical communications, and solar cells. © 2007 American Institute of Physics. 关DOI: 10.1063/1.2732823兴

Gold nanoparticles 共NPs兲 exhibit surface plasmon reso- nance 共SPR兲 causing optical extinction at visible wavelength

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due to special physical properties based on different sizes,

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shapes,

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and environments.

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The synthe- sis of branched quantum dots

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could enable studies of en- tangled quantum states and quantum information processing within individual NPs.

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Metallic NPs also benefit from the formation of complex structures.

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Gold complex nanostruc- tures are a major research area in the field of unusual physi- cal properties, including work on biological imaging labels, nanophotonics, tunable magnetic properties, and potential applications in solar cells and biosensors.

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Many synthetic methods have been developed to control the shape and size of NPs,

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such as the photoinduced method.

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As more is understood about the properties and the mechanism of the formation of these NPs, better control of their size, shape, and applications can be achieved.

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There are two convenient syntheses for the preparation of Au NPs, the Brust-Schiffrin method

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and chemical reduction.

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Au NPs are synthesized and stabilized by thiol legend in the Brust-Schiffrin method or by trisodium citrate in chemical reduction. The Brust-Schiffrin method is, however, a toxic approach, with associated environmental toxicity or biologi- cal hazards. The synthesis of gold complex nanostructures by using citrate as a chemical reducing agent is an environment- friendly method, although more research needs to be done into their formation. The unusual physical and chemical properties of the Au complex nanostructure are based on the quantum size and shape effects.

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The development of Au NPs synthesized by the chemical reduction method in a water-based solution is limited because of size and shape problems.

In this study, we investigate the synthesis and optical properties of gold nano-sea-urchin. Due to the symmetric face-centered cubic lattice of Au NPs,

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the growth of Au NPs during photoinduced method

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will have an isotropic com- plex structure. Unlike the photoinduced method, we synthe- size gold nano-sea-urchin in a dark field by using a friendly-

environmentally method. Then optical properties of gold nano-sea-urchin are recorded not only in the solution via UV-vis spectrum but also on the indium tin oxide 共ITO兲 glass substrate via ellipsometry. We found that the SPR of gold nano-sea-urchin interacts with the substrate strongly. Com- pared with the SPR of spherical Au NPs, the SPR of gold nano-sea-urchin splits one mode to two modes and blueshifts evidently.

Au NPs were prepared by chemical reduction method with HAuCl

₄_共aq兲

and Na-cit solution. 1.5 ml of freshly pre- pared 0.34M trisodium-citrate

_共aq兲

solution was added to 15 ml of 0.5 mM HAuCl

_4共aq兲

solution at room temperature under vigorous stirring. Furthermore, 45 ml of 1M NaCl

_共aq兲

a兲FAX:共886兲62380208; electronic mail: kistic@mail.mse.ncku.edu.tw

b兲FAX:共886兲62380208; electronic mail: jeffrey@mail.mse.ncku.edu.tw

c兲FAX:共886兲28222548; electronic mail: hector2714@yahoo.com.tw

d兲Author to whom correspondence should be addressed; FAX:

共886兲62380208; electronic mail: mhhon@mail.ncku.edu.tw

e兲FAX:共886兲62747995; electronic mail: gilbert@mail.ncku.edu.tw FIG. 1. TEM images of Au NPs synthesized with 87.5% of de-ionized-water addition without photoinduced conversion.

APPLIED PHYSICS LETTERS 90, 181905共2007兲

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solution were added to gold colloid, and then this was irra- diated for 3 h under visible light to photoinduce. The other sample was not irradiated to synthesize gold nano-sea- urchin. The ITO glass substrate was immersed in Au colloid for 6 h. The morphologies of Au NPs were observed by transmission electron microscopy 共TEM兲. The absorption of Au NPs was observed by UV-vis spectrum. The Au colloid was directly injected into a quartz tube for UV-vis analysis.

The absorption coefficient of Au particles on the ITO glass substrate was measured by ellipsometry.

The size of Au NPs has recently been synthesized to about 10 nm 共Ref. 14兲 by the chemical reduction method.

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Morphologies of Au NPs were observed by TEM. Au NPs synthesized with HAuCl

₄_共aq兲

and trisodium citrate

_共aq兲

were uniform and spherical, as shown in Fig. 1. Syntheses of Au NPs by using trisodium citrate can be highly reproducible and have narrow distributions.

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The Au particles synthesized with 87.5% 1M NaCl

_共aq兲

addition without being photoin- duced are nano-sea-urchin in high quantity. 关Fig. 2 共a兲兴. The size of the Au nano-sea-urchin is about 210 nm in Fig. 2共b兲.

Meanwhile, Au NPs synthesized with 87.5% 1M NaCl

_共aq兲

with photoinduction have the appearance of isotropic growth, as in Fig. 3. The shapes of the smaller sized Au NPs, less than 20 nm, are spherical and ellipsoid. The shapes of the bigger Au NPs, larger than 20 nm, are triangular, rodlike, and hexangular. Au nanoparticles are aggregated together in the water-evaporating process on the Cu grid due to the high surface tension of water on the Cu grid.

The morphology of the Au nano-sea-urchin is shown in Fig. 2共b兲. The radial symmetrical body of Au particles with nanoscaled “spines” gives it the appearance of a nanoscaled

“sea urchin.” The diffraction pattern of nanoscale spines on the gold nano-sea-urchin is shown in Fig. 2 共d兲 . The structure of the spines on the nano-sea-urchin belongs to the face-

centered cubic phase and it has a polycrystalline structure.

The size of nanoscaled spines is 23 nm in diameter and 81 nm in length in Fig. 2 共c兲 .

According to the Mie theory,

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the SPR is related to the onset of quantum size and shape effects of Au NPs.

Figure 4共a兲 shows that the intensity of SPR for Au NPs 共540 nm兲 decreases during photoinduced process. Also, the SPR peak appears at 760 nm and redshifts to 970 nm as the time increases. Au NPs grow isotropically after the addition of NaCl

_共aq兲

with the photoinduced method. The SPR of un- spherical Au NPs exhibits two or more bands.

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The red- shift of the surface plasmon occurs as the particle size increases.

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The SPR peak of Au particles is diverged and broadened because the size increases and the shape becomes more dispersive over time. Figure 4共b兲 shows that the inten- sity of SPR for Au nano-sea-urchin 共grown from Au NPs兲 increases without being photoinduced. The SPR peak red- shifts from 540 to 560 nm as the size of Au nano-sea-urchin increases, and is thus enhanced by the Au nano-sea-urchin.

Oscillators on the surface of Au NPs are in various direc- tions, which makes the direction of Au NP growth isotropic.

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Otherwise the direction of Au NP growth is an-

FIG. 3. TEM images of Au particles synthesized with 87.5% of 1M NaCl_共aq兲 addition with photoinduced conversion.

FIG. 2.共a兲 Morphology and 共b兲 zoom-in image of Au nano-sea-urchin and 共c兲 nanoscaled spines on Au nano-sea-urchin and 共d兲 diffraction pattern.

FIG. 4.共Color online兲 Time-dependent spectra showing the conversion of Au NPs to 共a兲 polygons with photoinduced method and 共b兲 nano-sea-urchin without photoinduced method.

181905-2 Su et al. Appl. Phys. Lett. 90, 181905共2007兲

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isotropic without irradiating. The radial symmetrical growth occurs and is observed as a nanoscaled sea urchin.

The absorption coefficient of Au nanostructure on ITO glass substrate is shown in Fig. 5. The dielectric constants of air, thanol, and ITO are 1, 1.88, and 2.89, respectively. SPR of Au NPs in the air/Au NP/ITO system has two peaks 共512.1 and 598.8 nm兲. This is different from the absorption spectra of Au NPs in Fig. 4共a兲, which has only one peak at 540 nm. By adding the ethanol to the Au NP/ITO system, the SPR shows a peak at 582.7 nm. Similar results are also ob- served in the Au nano-sea-urchin system. The SPR of the air/Au nano-sea-urchin/ITO system has two peaks 共436.8 and 607.0 nm兲. This also different from the absorption spectra of Au nano-sea-urchin in Fig. 4共b兲, which only has one peak at 560 nm. By adding the ethanol to the Au nano-sea-urchin system, the SPR shows two peaks 共488.9 and 551.3 nm兲. The appearance of these two peaks is due to the asymmetric di- electric environment experienced by Au NPs. For the upper interface, the gold nanostructure is in contact with air or ethanol. For the bottom interface, the gold nanostructure is in contact with the ITO glass substrate. Ethanol has a dielectric index close to that of the ITO substrate, so the splitting is reduced. This is evidence of the SPR splitting caused by the ITO substrate. Furthermore, due to the circular shape of Au NPs, the splitting of peaks is less predominant.

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When the Au NPs are on the surface of the ITO glass substrate, the splitting peak of SPR is not predominant due to the weak interaction of SPR mode with the substrate.

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Au nano-sea- urchin can thus enhance the interaction of SPR mode with the substrate.

In our research, we use only trisodium citrate with the addition of sodium chloride to synthesize a high quantity of gold nano-sea-urchin, which is thus an environment-friendly technology. Moreover, we demonstrate that isotropic SPR makes the direction of Au NP growth isotropically in the photoinduced mechanism in a water-based solution. The in- teraction of SPR with the substrate is reinforced by the gold

nano-sea-urchin. These results have potential applications for the enhanced-Raman scattering, optical communications, and solar cells.

This work was financially supported by the National Sci- ence Council of Taiwan, the Republic of China, Grant No.

NSC 94-2120-M-006-006, which is gratefully acknowl- edged. The authors also thank the Center for Micro/Nano Technology Research, National Cheng Kung University, Taiwan, for providing equipment support.

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FIG. 5. 共Color online兲 Absorption spectra of 共a兲 Au NPs and 共b兲 Au nano-sea-urchin in ethanol and air environment.

181905-3 Su et al. Appl. Phys. Lett. 90, 181905共2007兲