Design and fabrication of a hybrid metamaterial scanning probe for tunable tip-enhanced nanospectroscopy

Research Project

Project/Area Number	23K13640
Research Category	Grant-in-Aid for Early-Career Scientists
Allocation Type	Multi-year Fund
Review Section	Basic Section 28030:Nanomaterials-related
Research Institution	Institute of Physical and Chemical Research
Principal Investigator	OGUCHI MariaVanessa 国立研究開発法人理化学研究所, 光量子工学研究センター, 研究員 (00775083)
Project Period (FY)	2023-04-01 – 2025-03-31
Project Status	Granted (Fiscal Year 2023)
Budget Amount *help	¥4,030,000 (Direct Cost: ¥3,100,000、Indirect Cost: ¥930,000) Fiscal Year 2024: ¥910,000 (Direct Cost: ¥700,000、Indirect Cost: ¥210,000) Fiscal Year 2023: ¥3,120,000 (Direct Cost: ¥2,400,000、Indirect Cost: ¥720,000)
Keywords	field enhancement / metamaterials / plasmon / Raman spectroscopy / tip-enhanced Raman / plasmonics
Outline of Research at the Start	During the course of this research, I plan to design using FDTD simulations and fabricate a hybrid scanning probe made of metal nanoparticles and hyperbolic metamaterials that will provide high enhancement for tip-enhanced Raman spectroscopy applications.
Outline of Annual Research Achievements	In the first year, finite-difference time-domain (FDTD) calculations using a commercial software (Ansys Lumerical) were conducted to simulate the electric field distribution and field enhancement of a hyperbolic metamaterial (HMM) on a silicon (Si) substrate. The HMM consists of gold/alumina (Au/Al2O3) thin layer pairs and a top layer of spherical nanoparticles made of Au, Ag or titanium nitride (TiN). The initial thicknesses used for the Au/Al2O3 pair were 9 nm/9 nm and the diameter of the nanoparticles was 60 nm. A gap distance of 10 nm was used for the nanoparticles. These parameters were based on literature to test if our model is working. A comparison was done between the noble metals and TiN to determine how much field enhancement could be obtained at the gap between two nanoparticles. It was found that relatively high enhancement can be obtained using the noble metals at wavelengths less than 600 nm, whereas for wavelengths longer than 600 nm, higher enhancement can be obtained when using TiN. Hence, Ag nanoparticles can be used for 532 nm laser illumintaion while Au particles can be used for 633 nm laser illumination and TiN can be used for 785 nm, depending on the target application. FDTD simulations were also conducted to compare the enhancement of a metallic tip and substrate junction at varying distances, with that of a tip using HMM and metallic nanoparticles. The results for the FDTD simulations for the enhancement at a Au metallic tip-substrate junction was part of one paper that was just accepted to the Journal of Applied Physics and is currently in press.
Current Status of Research Progress	Current Status of Research Progress 2: Research has progressed on the whole more than it was originally planned. Reason Having much experience in FDTD simulations, I was able to set up a working simulation FDTD model for the proposed hybrid metamaterial scanning probe tip. The FDTD simulation software (Lumerical) was readily available and I was able to start doing the simulations right away. The initial FDTD simulations are almost completed. While the simulations were running, I also conducted preliminary tip-enhanced Raman spectroscopy experiments for a gap-mode configuration, wherein a pure metallic tip and a pure metallic substrate are used. This system would serve as a reference point for comparison with the hybrid metamaterial scanning probe tip. Before using an actual Si tip to make the hybrid metamaterial probe, metal deposition experiments on a Si test substrate were conducted to see the performance of the electron beam evaporation equipment and the quality of the metal layers (observed using a scanning electron microscope). The preparation of the Si substrates and the experiments for the deposition of the target metals (Au/Al2O3) are also on track. There were some initial difficulties in the metal deposition, due to some problems with the evaporation equipment, but the problems were eventually solved and the experiments were able to move forward.
Strategy for Future Research Activity	The next phase of the project is to fabricate the proposed HMM on an actual Si atomic force microscope tip (HMM-probe) and determine the performance of the HMM-probe as compared to a typical metal-coated Si tip used for tip-enhanced Raman spectroscopy experiments. The deposition parameters to be used when depositing the metals onto the Si tip will be based on the experimental parameters determined when using a Si substrate. The metals to be used for the nanoparticles will be Ag and Au. The methodology for the deposition of TiN using a RF magnetron sputtering machine will be considered, particularly the resulting stoichiometry of Ti and N and how this affects the enhancement of the electric field. The planned target sample will be the very Raman active carbon nanotubes (semi-conducting, Ag and 532 nm, and metallic, Au and 633 nm). FDTD simulations will continue, particularly to simulate the actual condition of the HMM-probe. The actual condition of the HMM-probe will be determined through observations using a scanning electron microscope. Afterwards, the findings will be summarized and will be reported in both journal publications and conferences.