Organization

This thesis was organized as follows: First, I will briefly review the development on nano-structured metal material or devices in chapter 2. In Chapter 3, the theoretical background was described due to the Mie theory. Chapter 4 mentioned the first work in this study. The topic is “Real time absorbance spectra due to optical dynamics of silver nano-particles film”. In this chapter, we proposed an optical method to observe the dynamic behavior of silver nano-particles film under a heating

treatment. The method exhibit the real time optical information differed from the analysis of SEMs. Chapter 5 focuses on the fast and simple fabrication of metal nanoparticles. By laser pulse ablation, the metal nano-particles can be obtained immediately from a film structure and the nano-particles can be applied in optical nano-device and high order nano-particles structure. Chapter 6 shows an extra diffraction band of cholesteric liquid crystal grating induced by the surface plamons effect. It was noted that the periodic localized surface plasmons were excited due to the CLCs grating environment and to affect the diffraction behavior of intrinsic CLCs grating. The summary of this study is finally concluded in Chapter 7.

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Nanoparticle Enhanced Transmission Surface Plasmon Resonance Spectroscopy”, J. Phys. Chem. B 107, 6497( 2003)

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Chapter 2

Study and application of metallic nanoparticles system

Collective electron oscillation, localized surface plsmons, can be easily excited by light. The particular optical property due to the surface palsmon effect has been explored based on the electrodynamics of conduction electrons in metal nanoparticles.

Since wavelength dependent scattering varies with the shape, size, and even environment dielectric property of metal nanoparticles, a wild application, such as photonics, sensors, signal enhancement of local electromagnetic filed, are rapidly studied and developed. Recent studies and applications of localized surface plasmons are reviewed here. In this chapter, a brief survey of fundamental characteristics of LSP is introduced first. Many outstanding experimental and theoretical works present the dependence of the geometry and dielectric property of metal nanoparticles on the localized surface plasmon resonance. In the following section, I will mention various applications of metal nano-structure system, such as surface enhanced Raman scattering, light propagating channel, mirror, condenser, biosensor, and so on. Finally, a future development of LSPR will be given.

2.1 Fundamental optical property of metal nano-particles

It is well known that the localized surface plasmon resonance is dependent on the size, shape, dielectric property of metal nanoparticles. The collective electron oscillation confined in the metal nanoparticles determines the particular optical property differed from bulk material.

2.1.1 Geometry effect on localized surface plasmons

Rongchao Jin shows that the metal nanoparticles can exhibit various bright colors due to different geometry. The bright colors of noble metal nanoparticles (silver and gold) are due to the excitation of resonant collective oscillation of the free

electrons in the particles. Figure 2.1 shows that the gold and silver nanonparticle scattered light in particular band and various color dots can be observed in dark field microscopy. First, to compare A with D Ag nanoparticle, the particle excites red and wathet-blue colors, respectively, under the shape effect (triangular and sphere). In the other set of B and C particle (Au spherical particle with size of 100 and 50 nm, respectively), exhibited yellow and green colors, referred to the size effect. Since the collective electron oscillation is strongly dependent the size and shape of metal nanoparticles (geometry determines the oscillating behavior of electron in metal particles) and resulting the variation of the electron damping relation, thus the resonance wavelength was altered by these factors. The interesting dependent of particle size and shape on localized surface plasmon resonance will be detail discussed in next chapter.

Figure2.1: Rayleigh light-scattering of particles deposited on a microscope glass slide. The slide is used as a planar waveguide, which is illuminated with a tungsten source. The image was taken with a digital camera. [2]

In addition, the shape effect also determines the optical scattering spectra of metal nanoparticles. As shown in Figure 2.2, the particle in various shapes of sphere, triangular, and pentagon lead to a big shift of optical resonance wavelength. The wavelength shift of 325 nm is exhibited between the spherical and triangular metal particles.

Figure 2.2: Dependence of color on shape lead to a big variation of resonant wavelength. [3]

2.1.2 Dielectric effect on localized surface plasmons

Furthermore, the extinction scattering due to the excitation of localized surface plasmons is also strongly dependent on the dielectric property of surrounding environment. Figure 2.3 presents that the scattered particular light band can be observed be immersing gold nanoparicle with a diameter ~16 nm in the solution of refractive index (from left to right, 1.336, 1.407, 1.481, 1.525, and 1.583). It is noted that the scattering light shifted to longer wavelength when the index is increasing.

Figure 2.3: Extinction scattering of gold nanoparticles was sensitive to the dielectric constant of solution environment. [4]

2.1.3 Localized Surface Plasmon of Metal Nanorod

At present, so many factors, such as size, shape, and dielectric effect, affected localized surface plasmon resonance were rapidly reviewed. However, the other factor of aspect ratio of metal nanorod also leads to an interesting optical property of metal nanopart. In S. Link’s work, he presents that the aspect ratio of gold nanorod significantly affects the optical property of localized surface plasmon. Two different excited plasmons modes, transverse and longitudinal mode, are found on the metal nanorod. In Figure 2.4 (a), the absorption spectrum shows two absorption maxima at 525 and 740 nm corresponding to two different modes. This is because the absorption of visible light both along the direction of the nanorod length (the longitudinal plasmon band) and along the direction of the nanorod width (the transverse plasmon band). Figure 2.4 (b) is the transmission electron micrograph (TEM) image of nanorods with average aspect ratio of ~3.3. A well control aspect ratio of nanorod has be done in this work.

Figure 2.4: (a) absorption spectrum of a gold nanorod sample with an average aspect ratio of 3.3. The band at 525 nm is referred to as the transverse plasmon resonance, while the one centered at 740 nm is the longitudinal plasmon absorption. (b) TEM image [6]

Although two different surface plasmon modes are excited due to the geometry of nanorods, the absorbance property due to dielectric variation seems to be the same

with that of metal spherical nanoparticles. The resonance wavelength shifts to red when increasing the dielectric constant of surrounding environment. In Figure 2.5 (a) and (b), the theoretical dielectric and aspect-ratio dependent absorbance spectra of metal nanorod are shown. By increasing the environment dielectric constant and the aspect ratio of metal nanorod, the maximum absorbance in the spectra shifts to red band. This characteristic determined the macroscopic optical property and that provides a potential to externally modulate the localized surface plasmon effect.

Figure 2.5: Theoretical absorbance spectra of metal nanorod with various aspect ratios and surrounded in environment with various dielectric constants. [6]

Besides the above theoretical analysis, Catherin J. Murphy and Nikhil R. Jana proposed an experimental observation of aspect-ratio dependent optical absorbance spectra. In this experiment, a seed-mediated growth approach is used to make metallic nanorods and nanowires in homogeneous solution. Figure 2.6 (a) shows the evolution begins with the synthesis of metallic nanospheres by chemical reduction of a metal salt with a strong reducing agent such as sodium borohydride. Citrate is employed as a capping agent to prevent particle growth. The gold or silver spheres with diameter of 3-5 nm are generated and become as seeds to grow more anisotropic nanostructures.

Figure 2.6 (b) is the transmission electron micrograph (TEM) of the gold nanorods with an aspect ratio of 18.

Figure 2.6: (a) A seed-mediated growth for gold and silver nanorods and nanowires. (b) TEM of gold nanorods, aspect ratio 18, made by the seed-mediated growth approach. [7]

This seed-method provides a very good control of the aspect ratio of metal nanorod. As shown in figure 2.7 (a), the maximum absorbance shifted to longer wavelength as increasing the aspect ratio of metal nanorods. This variation of absorbance spectra lead to various scattering light due to aspect ratio. The result confirms the theory prediction.

Figure 2.7: Aqueous solutions of silver nanorod show (A) absorbance spectra and (B) a beautiful variation in visible color depending on the aspect ratio of the suspended nanoparticles. : (a)-(f) silver nanorods of aspect ratio 1-10 [7].

In summary, there are so many factors, such as geometry and dielectric property of surrounding environment, significantly affect the excitation of localized surface plasmons on the metallic nanoparticle and leading to the particular optical property.

On the base of these advantages, high potential of sensor and classifying applications, can be accomplished simply.

2.2 Preparation and Arraying of metal material

There are two general ways available to produce nano-structure materials. As shown in Figure 2.8, the first way is to start with a bulk material and then piece it into smaller fragments via using mechanical, chemical or other form of energy (top-down).

The opposite approach is to synthesise the material from atomic or molecular species by chemical reactions, allowing for the precursor particles to grow and become bigger size (bottom-up). Most of the researchers are interested to well control of particle size, shape, size distribution, particle composition, and the degree of particle agglomeration.

In this section, we will review three previous works included the fabrication of metal nanoparticles and metal nano-array.

Figure 2.8: Two basic approaches to nanomaterials fabrication: top-down (from left to the right) and bottom-up (from right to the left).

2.2.1 Photoinduced conversion of silver naonspheres to nanoprisms

In general, spherical silver particles are prepared by injecting NaBH4 solution to an aqueous solution of AgNO3, with a presence of trisodium citrate.

Bis(p-sulfonatophenyl) phenylphosphine dihydrate dipotassium salt solution (BSPP) is subsequently added by drop-wise addition to the solution as a particle stabilizing agent. After the radiation of the conventional fluorescent light, the metal nanoparticle can be produced in the solution.

Rongchao Jin presented a time-dependent spectroscopic observation correlated with TEMs of silver nanoparticles. In Fig. 2.9 (a), we can find that the initial silver particles (~8 nm) are converted from spherical to triangle structure. During the initial stages of growth, both spheres and prisms exist at the same time. After 70 hours, most of the initial spheres (.99%) are converted to the prismatic structures.

In this growth reaction, they also observed a set of color changes of the sample.

Initially, the solution is yellow and then turned green and finally blue in whole growth of 70 hours. From the analysis time dependent extinction spectrum in Fig. 2.10 (B), a decrease in intensity of the surface plasmon band at λmax = 400 nm (for the spherical particles) was turned to a growth of three new bands of λmax = 335 (weak), 470 (medium), and 670 nm (strong), respectively. After 70 hours, the band at 400 nm completely disappeared. This optical response also implied the growth evolution of silver nanoparticle from spherical to triangle shape.

Figure 2.9: (A): Three distinctive stages are identified in nanoprism formation: induction, growth, and termination. (B) Time-dependent UV-Vis spectra showing the conversion of silver nanospheres to nanoprisms (a) before irradiation and after (b) 40, (c) 55, and (d) 70 hours of irradiation. [2]

In Figure 2.10 (a) and (d), TEM pictures shows the whole growth of nanoparticles. Size and population of the Ag prisms increase with time and a concomitant decrease in the number of spherical particles can be found. Figure 2.10 (D) shows the most of nanoparticles turned to triangle shape. These data clearly show

that the silver nanoprisms evolve from the initial spherical nanoparticles. This work proposes a detail time dependent observation on the evolution of photoinduced chemical synthesis and that leads to a advance understanding of chemical synthesis mechanism.

Figure 2.10: TEM images (reverse print) mapping the morphology changes (A) before irradiation and after (B) 40, (C) 55, and (D) 70 hours of irradiation. Except for the inset in (A), the scale bar is 200 nm for all four images. [2]

2.2.2 Electron beam lithography (nano-array)

Electron beam lithography is employed a beam of electrons to scan in a pattern a resistant surface and the pattern can be transferred on the sample surface by the selectively removing either exposed or non-exposed regions of the resist. The purpose is to create very small structures and to create very nano-scale electronic devices.

The primary advantage of electron beam lithography is to beat the diffraction limit of light and make features in the nano-scale regime. This form of maskless lithography has found wide usage in mask-making for photolithography, low-volume production of semiconductor components, and research and development.

Figure 2.11 shows a two-dimensional photoresist dot array covered by a thin silver film. The pattern was prepared by an e-beam lithography system. This approach provides a precise control of array structure in and a good band gap property can be obtained as this photonic device. By altering the factors, such as spacing and size of the metal dots, of this nano-array, the band gap can be modulated for certain purpose in optical photonic application.

Figure 2.11: (a) A SEM of hexagonal array of dots. (b) The energies of the upper and lower branches of the SPP energy gap as a function of the propagation direction. [9]

2.2.3 Focused ion beam (FIB)

Focused ion beam (FIB) is inherently destructive to the specimen. When a high-energy gallium ion-beam strikes the sample, the atoms are sputtered from the surface. FIB, like e-beam lithography, is often used in the semiconductor industry to patch or modify an existing semiconductor device. For example, in an integrated circuit, the gallium beam could be used to cut unwanted electrical connections, or to deposit conductive material for a connection.

In the surface plasmon study area, FIB is a good tool to fabricate a metal nano-structure or metal array. V. Vlasko-Vlasov uses FIB system to fabricate Ag circular nanoslits with width of 140 nm and diameter of 7.55 um, as shown in Fig.

12(a). The circular nanoslits excite strong surface plasmon polaritons (SPPs), resulting a very interesting interference patterns due to plasmons emitted from opposite sides of the circle. The sample is illuminated from the bottom by a tunable gas laser generating a series of sharp lines between 476 and 676 nm. The resulting SPP interference patterns were imaged by a near-field scanning optical microscope (NSOM) with a 50 nm fiber aperture (Figure 2.12 (b)).

Figure 2.12: (a) SEM image (aken at an angle of 52°) of circular nanoslits of 7.55 um diameter and 140 nm width. (b) Schematic of the NSOM setup. (c) NSOM image of the SPP interference pattern at λlight=568 nm. [10]

Figure 2.12 (b) illustrates a typical NSOM picture of the plasmon intensity. The obvious interference pattern presents the existence of SPP. NSOM images are largely a representation of the in-plane electric field intensity on the surface of the film. The interference pattern has a period equal to half the SPP wavelength, λ^SPP/2, the usual period of the standing wave intensity. With decreasing frequency the period increases, however, remaining always smaller than λ^light /2. The FIB approach to precisely fabricate metal nano-structure leads to an advance study for localized surface plasmon effect.

A precise hole-array can also be fabricated by FIB. Figure 2.13 shows a gold hole-array with a diameter and spacing of 300 nm and 400 nm, respectively.

Figure 2.13: A silver nano-hole array fabricated by FIB system

In Figure 2.14, the transmission spectra of hole-arrays display peaks that can be tuned by adjusting the period and the symmetry. For the blue, green and red arrays, the periods were 300, 450 and 550 nm, respectively, the hole diameters were 155, 180 and 225 nm and the peak transmission wavelengths 436, 538 and 627 nm. The arrays were made in a free standing 300 nm thick silver film.

Figure 2.14: (a) Normal incidence transmission image and (b) spectra for the subwavelength hole-arrays. [11]

In summary, the development of nano-technology promotes the advance of metal nano-structure system. Since the nano-structure can be designed and fabricated in demanded nano-optics device, the optical phenomena of reflection, transmission, propagation, and diffraction, are duplicated in marco-vision optics. The preparations of metal nanoparticle system mentioned in above system provide a easy way to fabricate nano-device for certain purpose. A well control of metal nanoparticle shape can be achieved by chemical synthesis. However, for a complicate metal nanoparticles system, E-beam lithography and FIB is proper to the fabrication of device. Actually, no matter which way to manufactory of optical metal nano-device, a creative idea is essential to advance study.

2.3 Application and development of localized surface plasmon resonance (LSPR) In this section, I will introduce the previous works which present the potential application and development of metal nano-device, such as sensor, light channel, condenser, bio-detector, and so on. The realization of these devices not only exhibits interesting physical and chemical phenomena but also leads to a potential for nano-techonology.

2.3.1 Surface plasmon enhanced Raman scattering [12, 13]

Due to the weak signal from the molecular structure, it’s difficult to detect the information from the molecular bound. The surface plasmon effect excited on the metallic nano-particles can enhance the signal, known as the surface plasmon enhanced Raman scattering. For this purpose, metallic nanoparticles were widely applied to the observation of weak signal exhibited from molecules.

Shuming Nie proposed the Raman spectra of a single rhodamine 6G molecules adsorbed on the selected nanoparticles, the intrinsic Raman enhancement factors were on the order of 10¹⁴ to 10¹⁵, much larger than the ensemble-averaged values of conventional measurements. This strong enhancement leads to a vibrational Raman signals with more intense and more stable than single-molecule fluorescence. The enhancement of localized electromagnetic field of molecule due to the exciting surface plasmons improves the sensitivity for detection of individual molecule or individual metal nano-particle.

Figure 2.14 show the surface-enhanced Raman spectra of R6G obtained with a linearly polarized confocal laser beam from two Ag nanoparticles. The R6G concentration was 2x10^-11 M, corresponding to an average of 0.1 analyte molecule per particle. By changing the polarization direction of laser corresponding to the expected particle orientation, the enhanced spectra can be obtained when the polarization parallel to the long axis of particle. All spectra were plotted on the same intensity