Sum-frequency generation (SFG) spectroscopy is known as a powerful technique that can selectively observe molecular structure and orientation information at surfaces. However, its spatial resolution has been limited to the micrometer scale due to the diffraction limit of light (the focus size limit).
A research group consisting of Research Assistant Professor Shota Takahashi, Assistant Professor Atsunori Sakurai, Graduate Student Tatsuto Mochizuki, and Associate Professor Toshiki Sugimoto from the Institute for Molecular Science, along with Graduate Student Koichi Kumagai (at the time), Assistant Professor Tomonori Hirano, and Professor Akihiro Morita from Tohoku University, has succeeded in detecting SFG signals from surface molecules with an extremely high spatial resolution of approximately 10 nanometers, which was previously difficult to achieve, by irradiating a femtosecond pulse laser onto the tip of the metal nanoprobe of a scanning tunneling microscope (STM) that can control probe position at the atomic level. Their results were published online in The Journal of Physical Chemistry C.
b. Magnified view of a nanoscale sharp gold tip apex used in the experiments. The curvature radius of the apex is ∼50 nm.
c. Schematic depiction of tip-enhanced SFG (TE-SFG) experiment. The incident mid- and near-infrared femtosecond laser pulses are confined and enhanced by the optical near-field generated within the STM nanogap, enabling the detection of SFG signals originating from a nanoscale molecular system beneath the tip apex.
d. SFG spectra obtained with the tip-substrate separation of ∼30 nm (green) and 0.7 nm (red). While almost no signal is observed at a separation of ∼30 nm, reducing the gap to ∼0.7 nm induces strong near-field enhancement and yields large SFG signals. The distortion in the spectral shape observed around ∼2930 cm-1 region represents vibrationally resonant SFG signals arising from molecules within the nanogap.
Provided by the Institute for Molecular Science
The key to achieving spatial resolution two orders of magnitude higher than conventional methods was the application of an STM equipped with an extremely sharp metal probe. A minute gap of less than 1 nanometer is formed between the STM's metal nanoprobe and the metal substrate. When light is irradiated into this nanogap, the light is strongly confined beyond the diffraction limit, forming a special light called a "near field" that is localized and enhanced in the nanoscale microscopic space. The research group applied this near-field light and selectively triggered the SFG process in the microscopic space directly below the probe, thereby obtaining tip-enhanced SFG signals from a small number of molecular systems existing in nano-regions sufficiently smaller than the diffraction limit of light.
Furthermore, by introducing high-precision theoretical calculations and analyzing the shape of the experimentally obtained nano-SFG spectra, the group also demonstrated that information on absolute orientation—whether molecules existing in the microscopic space directly below the nanoprobe are adsorbed facing upward or downward relative to the surface—can be obtained for each nanosize surface domain. Additionally, they established for the first time a theoretical framework for appropriately interpreting SFG signals generated from directly below the STM probe.
This achievement represents the world's first example of extending SFG spectroscopy to a nano spatially-resolved spectroscopy scheme beyond the diffraction limit of light. It is expected to become an important tool for elucidating how nanoscale local molecular orientations at heterogeneous surfaces and interfaces are closely related to molecular functions and reaction dynamics.
The research group stated that, building upon the unique foundational technologies and basic scientific principles established in this study, they aim to elucidate in detail the diverse and complex surface phenomena created by surface molecules, and will focus not only on atomically flat ideal surfaces but also on actual material surfaces and functional material surfaces, which are inherently highly heterogeneous.
Journal Information
Publication: The Journal of Physical Chemistry C
Title: Tip-Enhanced Sum-Frequency Vibrational Nanoscopy beyond the Diffraction Limit
DOI: 10.1021/acs.jpcc.5c05411
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