Growth of Highly Oriented Crystalline Silver Nanostructures on MgO (001) Substrates By Pulsed Laser Ablation and Application to SERS Substrates
Authors: Takeda Naoki , Kurumi Satoshi 
Published: December 2024

Surface enhanced Raman scattering (SERS) substrates are expected to be applied to some functional devices such as high-sensitive biosensors and photocatalysts.[1-2] In order to produce metal nanostructures, electron beam lithography and chemical method have mainly been used so far. Although these methods are effective for nano-level microfabrication, there are problems such as the need for multiple processes, unsuitability for large-area processing, poor mass production, and susceptibility to surface oxidation. For clearing these problems, we have fabricated SERS substrates by depositing metal nanostructures on MgO(001) substrates using the pulsed laser ablation (PLA) method,[3] which is a one-step fabrication process. In previous research, we produced gold nanostructures by using PLA[4]. The wavelength of plasmon resonances due to the gold nanostructures exhibited in the infrared region, and therefore laser beam with 785 nm was utilized to SERS and Raman spectroscopy. We observed significant strong SERS signals after applying the gold nanostructures to SERS substrates. However, we also observed strong Raman signals due to the MgO(001) substrate, resulting in a spectrum where the SERS signals and the Raman signals interfering. To reduce the Raman signal due to the MgO(001) substrate, it is necessary to decrease a wavelength of an excitation laser which is equipped with the Raman system because the penetration depth of the laser beam into the MgO(001) substrate becomes shallower. Therefore, we focused on silver nanostructures because the wavelength of plasmon resonances was shorter than those of gold. The SERS substrates, which consists in silver nanostructures on MgO(001) substrate, expects to resonate with visible light. Consequently, the SERS signals will be generated to irradiate a laser beam with 532 nm, leading to reduce the Raman signals due to MgO(001) substrate. In this study, silver nanostructures with plasmon resonance wavelengths in the visible light region were fabricated by the PLA method. Silver nanostructures are deposited on a MgO(001) substrate (Furuuchi; size: 10×10mm2 and thickness: 0.5mm) by PLA. A Silver target (Furuuchi; diameter: 13mm, 99.99%) and MgO(001) substrate were located on a vacuumed chamber. The chamber was pumped down to 10-5 Torr and the substrate was treated by outgas (350℃, 30min) and cleaning (800℃, 30min) using a silicon-carbide heating system. After cleaning, the deposition temperature was set to 350 ℃, 450 ℃ and 550 ℃. Argon or oxygen gas was flowed in the vacuum chamber. A pulsed laser beam (LOTIS TII, LS-2147: wavelength: 355 nm, pulse width: 10 ns, laser fluence: 0.8 J/cm2, repetition frequency: 4 Hz, and number of laser pulses: 7000) was irradiated through a laser insertion window into the installed silver target. Silver nanostructures were fabricated by ablation plumes and depositing them on a MgO(001) substrate. Optical transmittance measurements of the silver nanostructures fabricated by the PLA method showed that they have an optical absorption peak in the visible light region around 600 nm. Surface observation by using an atomic force microscopy (AFM: Hitachi High-Tech, SPA 300, dynamic force mode) showed self-grown nanoparticles with edges and silver nanostructures was observed Ag (200) by a high angle x-ray diffraction (XRD: Rigaku, RINT2000). SERS measurements were carried out using 4-MBA, and a SERS substrate with an enhancement factor of 5.51×105 was successfully developed.

References

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[2] M. G. Albrecht et al., J. Am. Chem. Soc., vol. 99 No. 15 p. 5215 (1977).

[3] B. L. Ong, et al., Nano Letter, vol. 27, p. 7448 (2021).

[4] S. Kurumi et al., Appl. Phy. Lett., vol. 123, p. 053502 (2023)