Multi-Level Surface Enhanced Raman Scattering Using AgOx Thin Film

Ag nanostructures with surface-enhanced Raman scattering (SERS) activities have been fabricated by applying laser-direct writing (LDW) technique on silver oxide (AgOx) thin films. By controlling the laser powers, multi-level Raman imaging of organic molecules adsorbed on the nanostructures has been observed. This phenomenon is further investigated by atomic-force microscopy and electromagnetic calculation. The SERSactive nanostructure is also fabricated on transparent and flexible substrate to demonstrate our promising strategy for the development of novel and low-cost sensing chip. ©2013 Optical Society of America OCIS codes: (250.5403) Plasmonics; (240.6695) Surface-enhanced Raman scattering; (310.3840) Materials and process characterization; (220.4241) Nanostructure fabrication; (140.3390) Laser materials processing.


Introduction
Localized surface plasmon resonance (LSPR) can induce highly intense and localized electromagnetic fields near the surfaces of metallic nanostructures under illumination [1][2][3]. In molecular spectroscopy, the Raman scattering cross section of molecules near these "hotspots" [2] will be dramatically enhanced, known as surface-enhanced Raman scattering (SERS) [3]. SERS is very useful for sensing and characterizing DNA molecules [4], cancer cell [5], explosives vapors [6], and food toxins [7]. These applications are very important for genetics, pathogen identification, and public security, etc. Therefore, many approaches for fabricating LSPR-active nanostructures and substrates have been developed [8][9][10][11][12]. Among the many micro-and nanofabrication techniques, laser-direct writing technique (LDW) [13][14][15][16] has been proven to be very useful in fabrication of SERS-active structures [17][18][19]. For example, using laser pulses to treat the metal surfaces, people can make metallic nanostructures with SERS activity on the surface [17]. Moreover, recent experiments have shown that the LSPR-active Ag nanostructures can be locally fabricated on the laser-treated silver AgO x thin film with the laser-induced chemical reduction of the AgO x material [19], leaving the unprocessed AgO x thin film acted as dielectric and an optically transparent background. This property makes laser-treated AgO x thin film very promising in the integration of SERS nanostructures with various optical components [20][21][22]. In comparison with other LDW-based methods for SERS-active nanostructures, using AgO x thin film can make the SERS-active layer thinner. People can also make SERS-active nanostructures into specific pattern by controlling the laser raster path. In our previous work, the SERS capability of the laser-treated AgO x thin film is found to be varied with the processing laser power [19]. However, the origin of this experimental result is not well understood. In this paper, fabrication of Ag nanostructures from AgO x thin film with different plasmonic enhancements is carried out with different laser powers. SERS from different Ag nanostructures is investigated using Raman spectroscopy to probe the Raman vibration signals of organic molecules located on the structures' surface. Atomic force microscopy (AFM) and electromagnetic simulation are employed to understand the relation between the Ag nanostructure morphologies and the corresponding SERS efficiencies. Moreover, many novel SERS sensors are needed to be fabricated on the flexible substrate for sensing applications on curved samples [23][24][25][26], such as human bodies and foods. The Ag nanostructure is fabricated on the transparent and flexible polycarbonate substrate for demonstrating the potential of our proposed method.

Experimental
AgO x thin films are reactively sputtered on transparent BK7 substrates (thickness = 1.5 mm) by RF-magnetron sputtering machine (Shibaura Mechatronics Corp.) in an Ar/O 2 (flow ratio = 10/25) mixed-gas atmosphere (pressure of the gas mixture = 5 × 10 −1 Pa). In the fabrication of Ag nanostructures, the as-deposited AgO x thin film is mounted on the computer-controlled three-dimensional stage (Mad City Lab. Inc.) of the fs-laser system. A Ti:Sapphire fs-laser oscillator (Coherent Inc.) emitting at 800 nm, with a repetition rate and a pulse width of 80 MHz and 140 fs, respectively, is focused by an oil-immersion objective lens (Zeiss Plan-Apochromat, 100 × , working distance = 0.17 mm, NA = 1.4) through the substrate and illuminated on the AgO x thin film. The incident laser power is adjusted by an attenuator. Before enter the objective lens, the laser beam is expended to a diameter of 6 mm, and is made circularly polarized using a λ/4 waveplate. In this work, the applied powers on the thin film are 21 mW, 11 mW, and 7 mW, which the corresponding fluences are 18.9 mJ/cm 2 , 9.9 mJ/cm 2 , and 6.3 mJ/cm 2 , respectively.
Characterization of the Ag nanostructures is carried out using an atomic force microscope (Asylum Research, MFP-3D) for surface morphology. For Raman spectroscopy and imaging, a WITec CRM200 scanning confocal Raman microscope with 532 nm-wavelength semiconductor laser for excitation is employed. The excitation laser beam is focused with a 100 × objective lens (NA = 0.95) on a Nikon Plan microscope. In Raman measurement, Rhodamine 6G (R6G) is used to evaluate the SERS efficiencies of the samples. In the literature, R6G has been widely utilized for studying different SERS-active structures previously, and the Raman vibration of R6G has been studied comprehensively [8,[27][28][29][30]. Drops of 10 −5 M R6G solution are put on the sample by a dropper, and purged by pure N 2 gas. The sample is subsequently mounted on the piezostage of the Raman system and point-bypoint scanned (step size = 1μm, exposure time = 1s) under excitation, while the corresponding Raman spectrum of each point is acquired in the scanning. The laser power on the sample is kept at 0.1 mW to avoid undesired laser-induced reduction of AgO x and sample damage. Figure 1(a) is the optical reflection image of the laser-processed AgO x thin film. Three rectangular zones on an as-deposited AgO x thin film are treated by fs-laser beam in the form of raster scanning with various fs-laser powers. The applied powers on the thin film are 21 mW, 11 mW, and 7 mW, respectively, to write parallel lines with a separation of 1 µm (scanning rate ~33.3 μm/s). In the optical image, optical reflectance of the processed area is apparently raised in comparison with the untreated one, indicating the obviously metallic property of the processed area. No obvious difference in reflectivity is observed among the regions processed with laser powers of 21 and 11 mW, and the reflectivity at the 7-mW region being slightly smaller. Even the reflections are not seen to vary with laser powers significantly, the Raman enhancements in the three regions are rather different. Figure 1(b) is the corresponding Raman intensity map image of R6G adsorbed on the area. In Raman intensity map, the regions displayed in brighter color are with higher intensity of selected Raman peak. The Raman image of intensity map shows the spatial distribution of Raman intensity integrated over the peak in the regime of 598-623 cm −1 , which is associated with the in-plane bending of the xanthene ring in the R6G molecule [30]. Four obvious levels of R6G Raman intensity can be observed in the image: Raman intensity at the 21-mW processed region is brighter than those at 11-mW and 7-mW, and the Raman intensity recorded from laser-processed regions are all stronger than the unprocessed one. Figure 1(c) shows the average Raman spectra obtained from various regions on processed AgO x thin film. Peaks at 611, 770, 1358, 1507, and 1647 cm −1 corresponding to R6G Raman vibration modes can be identified at the spectra acquired from laser processing regions [30]. The average intensities of Raman vibration peak at the wave number 611 cm −1 acquired from regions processed with laser of 21 mW, 11 mW, 7 mW, and from unprocessed region are found to be around 15900, 12120, 600, and 340 CCD counts (arbitrary unit), respectively. In comparison with the unprocessed region, the Raman signal of R6G is enhanced more than 46fold in the 21-mW laser-processed region. The enhancements of the other main vibrational modes of R6G at 770, 1358, 1507, and 1647 are 42-, 43-, 40-, 39-fold, respectively. The Raman intensities of R6G molecules are increased with increasing processing powers, indicating that SERS capability of processed AgO x thin film depends significantly on the incident laser power.  On the other hand, for observing the evolution of height of the nanoparticles in the 3D-AFM images, the maximum of the height scale is set as 120 nm. Interestingly, the Ag nanoparticles generated with 21-mW fs-laser beam are obviously smaller than the ones generated with 7-mW fs-laser beam. This result is similar to the ones reported by Sugiyama et al. [31], from which the sizes of the lasergenerated Au NPs (generated in AuCl 4 aqueous solution with laser-induced chemical reduction) decrease with the rise of laser power. The size reduction of the generated metallic nanoparticles can be explained by the laser-induced ablation and re-shaping in the processing [31]. The size distributions of the Ag nanoparticles shown in Figs. 2(a)-2(c) are analyzed by using Image J software. This image analysis method has been reported in our previous works [32]. By counting the particle number of each size, the relation between laser power and particle diameter becomes much clearer (Figs. 2(g)-2(i)). When irradiating the sample with higher laser power at 21 mW, the particle size distributed like Poisson distribution and the average particle size is located at about 30 nm. The same tendency occurs as the incident power changed to 11mW; however, the average particle size shifts to 40nm, which indicates the energy provided by laser would affect the occurrences of small particles. When the laser power is 7 mW, the average particle size is 50 nm, which is largest among these three irradiating laser power.

Figures 3(a) and 3(b)
show the simulation electrical-field energy distribution which are obtained using MEEP (an electromagnetic simulation software package) [33]. The morphologies of Ag nanoparticles in simulation are built in accordance with the lower left corner of AFM images in Figs. 2(d) and 2(f) with the image analysis software IGOR Pro (version 6.3). The simulation region are set to be 1.1 μm × 1.1 μm × 130 nm along x, y and z direction, respectively. In the simulation process, the dimensions of grid cell are set as 1 nm in three directions, which is enough to resolve the fields at the metal-dielectric interface. All the boundary conditions in x, y and z directions are set as the perfectly matched layers that can truncate computational regions in numerical methods to simulation problems with open boundaries. The relative permittivity of Ag are given by the Lorentz-Drude model which can be expressed in the form as 2 2 where ω p is the plasma frequency, k is the number of oscillators with resonance frequency ω j , strength f j , and lifetime 1/Γ j [34]. While p Ω = 0 p f ω is the plasma frequency associated with interband transitions of oscillator strength f 0 and damping constant Γ 0 . All the fitting parameters of Ag can be found in Ref [34]. The substrate material is BK7 (ε = 2.3088). According to the definition of electric-field energy E*• D/2, the region with positive intensity value shown in Figs. 3(a) and 3(b) indicate that the space is filled with air. Note that the areas with negative intensity indicate the region of metal. The regions with black green color (value = 0) present the internal part of the Ag nanoparticles which weakly interact with the incident wave.
Comparison between the simulation results in Figs. 3(a) and 3(b) shows that more plasmon-active sites (shown in yellow and red colors) can be observed in Fig. 3(a). On the fixed probing area, SERS of molecules adsorbed on the nanostructures will be strongly related to the area of plasmon-active sites in the laser spot. Also, since the hotspots are formed at the gaps between the nearby nanoparticles, the structure with higher surface densities of nanoparticles has more plasmon-active sites, providing stronger SERS efficiency in measurement [3].

Fabrication of SERS surface on transparent and flexible substrate
The SERS-active Ag nanostructures can be made on the optical transparent and flexible substrate by our proposed strategy. AgO x thin film (thickness: 15 nm) is reactively sputtered on the polycarbonate substrate (thickness = 0.6mm, refractive index~1.584). Here a 20 × Zesis Epiplan lens is utilized. For avoiding the laser-induced damage of the polycarbonate substrate, a transparent dielectric ZnS-SiO 2 film (composition ratio: ZnS 80% and SiO 2 20%) as protective layer is employed. ZnS-SiO 2 film has been widely used in the fields of optical data storage for protecting the recording media because of its high flexibility, optical transparency, low thermal conductivity, and thermal stability [35][36][37][38]. Stacked films of 200nm-thick ZnS-SiO 2 and 15-nm-thick AgO x are sputtered on the polycarbonate substrate. This layered structure is highly transparent and flexible. After laser processing (power: 11mW, scanning rate: 55 μm/s, spacing between scanning lines: 250 nm) Ag nanostructures are formed on the surface. As shown in Fig. 4, obvious Raman enhancement for R6G is obtained at the laser-generated Ag nanostructure on flexible substrate.

Conclusion
We have reported an efficient method to fabricate SERS-active Ag nanostructures by using laser-direct writing to treat sputtered AgO x thin films. The multi-level Raman enhancements of R6G molecules observed in our experiments have their origins from the different average sizes of the generated Ag nanoparticles on the surfaces. These sizes can be controlled by the laser power, leading to different plasmon-active areas on the fixed probing area. In addition, proof-of-principle demonstration of making SERS-active structures on the flexible substrate is also presented. The present methodology is thus very promising for future applications of SERS to sensing and fabrication of lab-on-chip systems.