Host: The Japan Society of Vacuum and Surface Science
Name : Annual Meeting of the Japan Society of Vacuum and Surface Science 2023
Location : [in Japanese]
Date : October 31, 2023 - November 02, 2023
Introduction
Sum frequency generation (SFG) vibrational spectroscopy, which detects the light of the sum frequency of near-infrared (NIR) and mid-infrared (MIR) which resonates with molecular vibrations, is a powerful technique to elucidate structures and orientation of molecules on surfaces [1]. However, far-field SFG spectroscopy does not have enough spatial resolution, down to sub-μm, due to the diffraction limit of the light. Therefore, it has been difficult to observe the molecular behaviors that govern the reactivity of heterogeneous catalysts with nm-resolution. To tackle this problem, we have been developing the technique of tip-enhanced (TE)-SFG vibrational spectroscopy that induces the second-order nonlinear optical processes in the nanogap between a scanning tunneling microscope (STM) tip and a substrate [2]. In our previous developments, we have dealt with systems in which gold (Au), a coinage metal, is used for both the tip and the substrate, in order to obtain a large electric field enhancement effect of plasmons. On the other hand, in many cases, non-coinage materials, such as platinum (Pt) or nickel (Ni), were used in the field of heterogeneous catalysis [3]. In this study, we have attempted TE-SFG vibrational spectroscopy of adsorbed molecules on the surface of Pt, a typical non-coinage metal.
Methods
Pt-deposited mica substrates were immersed in 4-methylbenzenethiol (MBT) ethanolic solution, and an MBT-adsorbed Pt substrate (MBT/Pt) was prepared as a sample. An Au-deposited mica substrate was also used as a reference sample. The STM tip was fabricated by electrochemical etching of Au wire. The light source was a Yb-fiber laser whose output was split into a narrow-band filter and an optical parametric oscillator to generate narrow-band NIR light and tunable MIR light, respectively. The coaxially aligned NIR and MIR beams were focused on the tip-substrate gap, and the SFG signals were detected by a spectrometer. The contrast between the spectra acquired during the tip's approach to the sample in the constant-current mode of the STM and its retraction from the sample by 30 nm was extracted as the tip-enhanced signal. In addition, we estimated the intensity of the electric field enhancement on Au and Pt substrates by an electromagnetic simulation based on the finite difference time domain (FDTD) method [2].
Result & Discussion
In the case that vibrationally non-resonant MIR light (3000 cm-1) was employed, we observed non-resonant TE-SFG signals from the MBT/Pt with the same shape as from the Au. These signals were observed only when the tip was approached to the substrate in the tunnel current region of STM. In a previous study of tip-enhanced Raman spectroscopy (TERS) [4], no signals were detected from the Pt substrate in the tunnel current region. The result suggests the existence of an enhancement mechanism unique to the SFG process, which was not observed in TERS. The result validated by the FDTD simulation can be explained by the comparable enhancement for both the Au and Pt substrates in the region of the NIR to MIR.
In the MIR region around 2900 cm-1, where the CH resonance is situated, a dip structure was observed in the spectrum from the MBT/Pt, which is not found in Au. This dip is caused by the interference of the resonant SFG signal of methyl groups with the non-resonant signal of the Pt. Thus, we succeeded in obtaining the vibrational resonance TE-SFG signals originating from molecules adsorbed on Pt substrates. The applicability of TE-SFG vibrational spectroscopy to molecular systems on non-coinage surfaces has been demonstrated. This opens the way to the observation of heterogeneous catalytic reaction systems.
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