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Self-organized Ag nanorings antenna were formed on quartz glass wafers by a simple chemistry reaction without any template. By using absorption measurements and three-dimensional finite-difference time-domain (3D-FDTD) calculations, the dipole and quadrupole plasmon resonances of Ag nanorings antenna were investigated experimentally and theoretically. Calculations have shown that large electric fields are confined at the quadrupole of the Ag nanoring, leading to quadrupole plasmon resonances. Compared the electric enhancement factor of the exterior surfaces of Ag nanoring, the electric enhancement factor of the interior surface is about six times excited by an incident light with 514.5 nm wavelength. Furthermore, the highest electric-field intensity of Ag nanorings is around four times larger than that for Ag nanodome with the same condition. These results pave the way to design plasmonic nanostructures for practical applications that require metallic nanoparticles with enhanced electric fields.
© 2015 Optical Society of America
1. Introduction
Recent scientific investigations that allow noble metal to be structured and characterized on the nanoscale have attracted much interest in surface plasmons (SPs) [1, 2]. When the oscillating electric field of the incident light resonantly couples to the conduction electrons, making them collectively oscillate at the same frequency, an intense band in the extinction spectrum will be shown, which is named the surface plasmon resonance (SPR) [3]. In this interaction, the free electrons respond collectively by oscillating in resonance with the light wave. Recent scientific studies in the field of plasmonics have focused on noble metal nanoparticles substantially smaller than the wavelength of light [4]. The properties of SPs can be tailored through changing the structure of a metal’s surface, which offers the potential applications for developing new types of photonic device. SPs can help us to concentrate and channel light using subwavelength structures, which could lead to miniaturized photonic circuits with length scales much smaller than those currently achieved [5–7]. Consequently, the optical response reflects the properties of the local electric field at the nanoparticle and has a predominantly dipole character that can be optimized by manipulating the system geometry. The SPR spectra of noble metal nanocrystals have been demonstrated to strongly depends on the shape, size and dielectric environment, arrangement configuration and separation of particles [8–10]. Detailed information on how all of these parameters can influence the optical properties of metallic structures is important to use the SPR of these metallic structures for various applications such as those developed in negative refractive index metamaterials, biosensor, photonic waveguide, and surface-enhanced Raman scattering (SERS) and medical diagnostics [11–13]. Through increasing the size of nanoparticle, the optical excitation of higher order harmonics, multipolar plasmon resonances, becomes possible and significantly modifies the optical properties [14]. Multipolar SP modes were demonstrated spectrally in the visible and near-infrared regions; meanwhile, correlated theoretical approaches were developed [15–17]. In the long wavelength regime where the surface plasmon dispersion is close to the light line, the combined excitation consisting of an SP and an electromagnetic wave is called a surface-plasmon polariton (SPP). SPPs have attracted considerable attention due to their potential applications in optical circuits and optical computers [18, 19].
Ring shaped nanostructures with various diameter and wall thickness have been synthesized by several groups. They showed that the SPR spectra of these nanoparticles are highly sensitive to the parameters such as size and dielectric environment [20]. The plasmon frequencies of nanorings are highly tunable and depend both on the diameter and the wall thickness of the ring [21,22]. This tunability of nanorings has recently been exploited in the design of plasmonic waveguides in the optical telecommunication band [23]. Ring shaped nanostructures are particularly attractive for sensing applications due to their ability to contain high volumes of molecules and provide uniform electric fields inside the cavity [24, 25]. These properties of nanoring have been used in applications which including optical antennas, plasmonic waveguide, optical confinement, data storage and SERS [25–27].
Here, in this paper, we provide a convenient and low cost way for the large-area self-organized synthesis of Ag nanorings through heat treatment of Ag+/PVA/PVP composite film on quartz glass (PVA is Polyvinyl Alcohol, PVP is Polyvinylpyrrolidone). We investigated the dipole and quadrupole plasmon modes of Ag nanorings antenna using absorption measurements and three-dimensional finite-difference time-domain (3D-FDTD) calculations. In addition, we calculated the maximal field enhancement the Ag nanoring and Ag nanodome excited by an incident light with 514.5 nm wavelength. These results show that Ag ring nanostructures are good plasmonic nanostructures which could be used in applications in near-infrared surface-enhanced spectroscopy and sensing.
2. Experimental and theoretical approach
2.1 Synthesis
The desired Ag nanorings antenna may be synthesized from a simple and inexpensive approach based on heat-treatment of Ag+/PVA/PVP composite film on quartz glass reported in our previous paper [27]. In the experiment, at first, we prepared an aqueous mixture containing 50 ml of AgNO3 (100 mM), 50 ml of PVP (10 mM) and 50 ml of PVA (100 mM). Secondly, we spin-coated the mixture at 3000 rpm for 3 min on quartz glass wafer. And then we dried these films in a vacuum overnight. Thirdly, we reduced prepared Ag+/PVA/PVP film by aqueous H2O2, or calcined Ag+/PVA/PVP film without reduction. In the former experiment, we immersed the prepared Ag+/PVA/PVP film into 10 mM aqueous H2O2 for 10 min. And then the film was rinsed with deionized water, dried in vacuum overnight finally. At last, we calcined these spin coated films at a rate of 1°C/min from room temperature to 700°C. In this process, we kept at 700°C for 4 hours under Ar gas ambience. And finally we allowed these films to cool. The details of the preparation processes are not the main concern in this paper, and here we center on the plasmon resonance characteristic of Ag nanorings.
2.2 Instrumentation and Measurements
Scanning electron microscopy (SEM) images are recorded using a Leica Cambridge S440 field emission scanning electron microscope with an accelerating voltage of 5.0 kV. The transmission electron microscopy (TEM) graphs were performed by using a JEOL 2010HT TEM machine operated at 200 kV. The absorption spectra were recorded by an ultraviolet-visible-near-infrared (UV-vis-NIR) spectropho-tometer (PerkinElmer Lambda950). In SERS spectra measurements, first of all, by using an accurate pipette, we dropped a 10 μL droplet of melamine aqueous (1 × 10−4 M) solution on the samples. Secondly, let the samples dried in air at ambient temperature, in order to gain a uniform molecule membrane over an area of about 10 mm2. In this experiment, we prepared three samples with the same about SERS-active substrates. In addition, each sample was used to select ten different points in order to detect the melamine probes which confirm the reproducibility and stability of these samples. Raman measurements were conducted with a Renishaw 2000 laser Raman microscope equipped with a 514.5 nm laser of 2 μm spot size in diameter for excitation. All the spectra were acquired for10 s with the laser power measured at the sample being 2.5 mW.
2.3 Finite difference time domain simulations
We used a commercial FDTD calculations package (FDTD solution) for 3D-FDTD simulations [28]. The boundary conditions of the simulation domain are perfectly matched layer absorbing boundaries. The calculation region is 1.3 × 1.3 × 0.3 μm3, and the cell size is 1 × 1 × 1 nm3. In our simulation, the dielectric function of Ag nanorings was described by the Lorenz-Drude model [29]. The refractive index of surrounding medium was set to be 1.0 for air and the dispersion of quartz glass substrate was considered. The dielectric constants of the glass have been taken from Palik [30]. We installed polarization directions of plane waves parallel with the x axis. Figure 1 shows the schematic geometries of the structures studied. The scheme of Ag nanoring with outer radius r1 (r1 = 250 nm), inner radius r2 (r2 = 150 nm) and thickness h (h = 125 nm) is shown in Fig. 1.
Fig. 1 Geometry of the Ag ring used in the simulations: outer radius (r1), inner radius (r2), height (h).
3. Results and discussion
The SEM images of Ag nanorings are shown in Fig. 2(a). The low magnification SEM image obviously shows homogenously distributed self-organized Ag nanorings covering large areas. The inset picture of Fig. 2(a) shows a high magnification SEM image that the products are all ring shape with average outer radius (~250 nm) and inner radius (~150 nm). Figure 2(b) gives the 3 D image of the Ag nanorings. The image confirms the Ag nanorings with an average thickness of 100~150 nm. The detailed morphology analyses have been reported in our previous paper [27].
Further details of the Ag nanorings are obtained by the TEM micrograph and the EDS spectrum (see Fig. 3). Figure 3(a) is the TEM image of one individual Ag nanoring. The element analysis by energy-dispersive X-ray spectroscopy (EDS) corroborates that the main component of the Ag nanorings is metallic Ag (Fig. 3(b)). The element Si and O are from the quartz glass wafer.
Fig. 3 (a) A representative TEM image of one individual Ag nanoring; (b) EDS spectrum of the Ag nanorings on quartz glass wafer.
Figure 4 shows the absorption spectra of Ag nanoparticles on quartz glass wafer with different experiment conditions. Black and red curves correspond to the Ag nanorings and Ag nanodomes, respectively. The preparation method and SEM image of Ag nanodomes have been reported in our previous paper [27]. Some groups [31, 32] have reports that this noble metal with ring shape could red shifted SPR absorption into NIR because of the special shape. As shown, the red curve exhibits the typical UV-Vis absorption spectra of Ag spherical nanoparticles. However, two distinct bands could be distinguished in the black curve: the one located at 515 nm and the other appearing at 990 nm. It should be noted that as the sample is the Ag nanoring (black curve), the strongest SPR absorption is appeared at 990 nm. Also, it should be noted that the absorption band located at about 515 nm comes from the quadrupole plasmon mode of Ag nanoring. At this research, through the SEM and UV-Vis absorption spectra studies, we think the resonance wavelength in 990 nm is as a result of the Ag ring nanostructures.
Fig. 4 UV-Vis absorption spectra of Ag nanoparticles on quartz glass wafer with different experiment conditions. Black and red blue lines correspond to the Ag nanorings and Ag nanodomes, respectively.
In order to clearly distinguish the plasmon mode of Ag nanoring, we calculated the electric field distributions of Ag nanoring at 515 nm (Fig. 5(a)) and 990 nm (Fig. 5(b)) using the 3D-FDTD method. During the calculation, the excitation laser beam was set to be perpendicular incidence (z-axis) and the electric field was parallel (x-axis) to Ag nanoring. From Fig. 5(a) (at 515 nm), field maximum points are located at the interior surface of the Ag nanoring while the minimum field points appear at the exterior surface. Very interestingly, four field relative maximum points along the exterior surface of nanoring (points at a, b c and d) can be seen, which implies the existence of quadrupole plasmon mode. In Fig. 5(b) (at 990 nm), field intensity of point at the exterior surface are the strongest spot of the nanoring, while that in Fig. 5(a) is weaker than that of points at interior surface. This field distribution shows the presence of dipole plasmon mode at this excitation wavelength.
Fig. 5 Electric ðeld distributions of Ag nanorings: (a) the quadrupole plasmon mode at 515 nm; (b) the dipole plasmon mode with an incident wavelength of 990 nm.
Here, to evaluate the SERS enhancement ability of Ag nanorings prepared by the present method, we compare a SERS spectrum of melamine on Ag nanorings with that on Ag nanodomes. Figure 6(a) and 6(b) compare the SERS spectrum of melamine on the Ag nanorings and Ag nanodomes, respectively. Compared to the response of the Ag nanodomes, the Ag nanorings displays a much stronger SERS response. To determine the enhancement effect (EF) of melamine on the nanoparticles quantitatively, the EF values of melamine in the nanoparticles are calculated with the following expression [33]:
ISERS is the enhanced intensity of the adsorbed melamine molecules on the SERS substrate. The value of ISERS mainly arises from a single molecule layer covering a nanoparticle array, from which other additional molecule layers of analytes on the SERS substrate, as previously reported [33], do not contribute to Raman gain and can be neglected. IRef is the spontaneous Raman scattering intensity from the bulk melamine molecules under the laser spot on the blank glass substrate. NSERS is the number of the single-layer molecules covering the SERS substrate under the laser spot. NRef is the number of the bulk molecules excited by laser on the surface of the regular substrate. In order to obtain the values of these four parameters, we follow the same procedures based on the published literatures [33]. Then the EF for the Ag nanorings and Ag nanodomes were roughly estimated by comparing the peak intensity at 682 cm−1 to 1.6 × 107 and 9.7 × 105, respectively. The SERS signal of melamine on the Ag nanorings is about 16 times stronger than that on the Ag nanodomes. We believe the SERS enhancement is mainly due to the local electromagnetic field enhancement. Since Ag nanorings display different electric field characteristic compared Ag nanodomes, which may lead to large SERS enhancement. The simulated maximal field enhancement ((|E|/|E0|)2) the Ag nanoring and Ag nanodome excited by an incident light with 514.5 nm wavelength is shown in Fig. 7. As shown the Fig. 7(d), the average diameter of Ag nanodome is about 500 nm. The detailed morphology analyses have been reported in our previous paper [27]. Compared the electric enhancement factor of the exterior surfaces ((|E|/|E0|)2 = 11), the electric enhancement factor of the interior surface ((|E|/|E0|)2 = 61) is about six times. Furthermore, the highest electric-field intensity of Ag nanorings is around four times larger than that for Ag nanodome with the same condition. The strong field enhancement suggests that the interior surface of Ag nanorings can afford suitable active sites for detection of molecular binding, even for biosensor applications. These biomolecules in the surrounding media can be generally accessible to the region inside of the nanorings, unlike in conventional nanoshells. The result can make nanorings advantageous for using in SERS applications.Fig. 6 Typical SERS spectra of melamine (1 × 10−4 M) adsorbed on (a) Ag nanorings, (b) Ag nanodomes. The excitation wavelength is 514.5 nm.
Fig. 7 (a) and (d) SEM observation of Ag nanorings and Ag nanodome. E-field amplitude patterns from theoretical calculations at the excitation wavelength of 514.5 nm for (b) Ag nanoring, (e) Ag nanodome. (c) Electric enhancement factor along the red line in Fig. 7(b). (F) Electric enhancement factor along the red line in Fig. 7(e).
Conclusion
In conclusion, we have prepared Ag nanorings with average outer radius (~250 nm) and inner radius (~150 nm), respectively. From UV-vis-NIR spectrophotometer spectra and FDTD calculations, we experimentally and theoretically achieved the dipole and quadrupole plasmon resonance in the Ag nanorings, which is located at about 515 and 990 nm and caused large electric field enhancements. The electric enhancement factor of the interior surface of Ag nanoring is about six times than the electric enhancement factor of the exterior surfaces excited by an incident light with 514.5 nm wavelength. Furthermore, the highest electric-field intensity of Ag nanorings is larger (>4 times) than those of Ag nanodome with the same condition. These results show that Ag ring nanostructures are good plasmonic nanostructures which could be used in applications in near-infrared surface-enhanced spectroscopy and sensing.
Acknowledgments
The work is supported by the National Natural Science Foundation of China (No. 10804101; 60908023; 11375159), Science and Technology Development Foundation of Chinese Academy of Engineering Physics (No. 2010B0401055; 2013B0302052), Open Foundation of Joint Laboratory for Extreme Conditions Matter Properties, Southwest University of Science and Technology and Research Center of Laser Fusion, CAEP (No. 12zxjk07; 12zxjk01; 13zxjk03), Scholarship Award for Excellent Doctoral Student granted by Ministry of Education (1343-76140000014), Hunan Provincial Innovation Foundation for Postgraduate (No. CX2012B114), and the Open-End Fund for the Valuable and Precision Instruments of Central South University (CSUZC2012032).
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