Surface Enhanced Infrared Silver Antenna Arrays for Polymer Sensing


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Infrared spectroscopy is widely studied and applied due to its label-free and accurate molecular identification capabilities. However,the small absorption cross-section of the molecule limits its sensitivity. To overcome this problem, the surface-enhanced absorption effect of metal nanoantennas is used to increase the sensitivity when detecting trace target molecules. In the design optimization of metal nanoantenna arrays,silver(Ag) is often used as an alternative to gold (Au) due to its similar properties and lower price. Although much progress has been made in the study of antenna structures,these structures are generally smaller in size,generally less than 100 nm. Relying on advanced technology,the preparation cost is expensive. In this paper,Ag and ultraviolet exposure techniques are used to realize low-cost, large-area metasurface design. First,to study the impact of nanorod size and guide experiments,hollow cross- shaped antenna arrays and hexagonal antenna arrays metasurface are simulated with the help of FDTD software. The simulation results show that the hollow cross-shaped nanoantennas has an extinction peak at 547.7 cm(-1), and its extinction coefficient is 64.8%. The extinction coefficient of the hexagonal nanoantennas at 448.3 cm(-1) is 85.5%. Compared with hollow cross-shaped nanoantennas,hexagonal nanoantennas have a redshift in resonant wavelength and a larger extinction coefficient. The parameters of length and width of the hollow cross-shaped and hexagonal antennas are swept separately. For hollow cross-shaped nanoantennas,with the increase of length,the formant is significantly redshifted,and with the increase of width,the formant is slightly blue-shifted. For hexagonal nanoantennas,with the increase of length,the formant is significantly redshifted,and with the increase of width,the formant is slightly redshifted. The influence of length on the position of the formant is much greater than the effect of width. The electric field enhancement of the two antenna structures at 547.7 cm(-1) and 448.3 cm(-1) is 576 and 1 335,respectively. Hot spots are distributed near the edge of the antenna element, especially at the tip. This indicates that the electric field is significantly enhanced around the edge of the metal antennas. Then, the two optimized patterns of metasurface are prepared. To characterize their shape, the substrates are observed with scanning electron microscope. The result shows that the tip of the deposited metal is relatively rounded and not strictly rectangular,which will lead to a decrease in the electric field enhancement of the tip. The Ag nanorod unit is larger in size (error less than 1 mu m) relative to the theoretically designed nanorod cell. Compared with the size of the nanorod unit with the hollow crosss-haped structure,the nanorod cell size of the hexagonal structure is smaller. Antenna size is related to the resolution of the lithography machine,the shape of the structure and other factors. To measure the position and shape of the extinction peaks,the transmission spectra of the prepared substrates are measured with a Fourier transform infrared spectrometer. The results show that the hollow cross-shaped antenna substrate has an extinction peak at 525 cm(-1),and the extinction coefficient is about 20%. The extinction peak of the hexagonal antenna is located at 474.4 cm(-1),and its extinction coefficient is about 24.5%. The extinction coefficient of both structures is smaller than that of the two structures in the simulation, which is caused by the excessive deviation in the transmittance of the silicon used in the experiment and simulation. Compared with the simulation of the extinction peak of the hollow cross-shaped antenna, the extinction peak shows a red-shift of 22 cm(-1). And the extinction peak of the hexagonal antenna shows a blue-shift of 26 cm(-1) compared with the simulation result. This is due to the fact that the size of the antenna element structure in the prepared substrate deviates from the theoretical design. To verify the performance of designed silver antenna arrays for polymer sensing, Polymethyl Methacrylate (PMMA) was chosen as the analyte. The enhanced absorption effect of hexagonal Ag-Surface-Enhanced Infrared Absorption(Ag-SEIRA)substrate is studied. Both the silicon and hexagonal AgSEIRA substrates coated with PMMA have extinction peaks at an absorption peak of PMMA(483.14 cm(-1)). The extinction coefficients are 3.15% and 8.99%, respectively. The hexagonal antenna arrays achieve 2.85-fold enhancement in PMMA absorption with an enhancement factor of 1995.
Infrared spectroscopy,Surface plasmons,Surface enhanced infrared,Antenna arrays,Metasurface
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